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ExxonMobil Proprietary FRACTIONATING TOWERS

DEVICE SELECTION AND BASIC CONCEPTS DESIGN PRACTICES

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CONTENTS Section

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SCOPE ............................................................................................................................................................4 REFERENCES.................................................................................................................................................4 BACKGROUND...............................................................................................................................................4 KEY STEPS INVOLVED IN TOWER DESIGN - AN OVERVIEW ...........................................................4 TYPES OF CONTACTING DEVICES AVAILABLE .................................................................................5 CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE.........................................................................7 CROSS FLOW DEVICES - HARDWARE DEFINITIONS ..............................................................................11 CROSS FLOW DEVICES - PROCESS DEFINITIONS..................................................................................24 VAPOR HANDLING LIMITATIONS .......................................................................................................24 LIQUID HANDLING LIMITATIONS........................................................................................................27 OTHER PROCESS CONSIDERATIONS ..............................................................................................28 CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONS................................................................33 CROSS FLOW DEVICES - GENERAL CONCLUSIONS ..............................................................................34 COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) ....................................................................................................................................34 EQUIPMENT TYPES AND APPLICATIONS .........................................................................................34 COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS......................................................................38 VAPOR / LIQUID CAPACITY LIMITATIONS.........................................................................................38 EFFICIENCY AND TURNDOWN ..........................................................................................................38 HEAT TRANSFER.................................................................................................................................39 OTHER CONSIDERATIONS.................................................................................................................39 COUNTER-CURRENT DEVICES - GENERAL CONCLUSIONS ..................................................................39 NOMENCLATURE.........................................................................................................................................40 COMPUTER PROGRAMS ............................................................................................................................40

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CONTENTS (Cont) Section

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TABLES Table 1 Table 2 Table 3A Table 3B Table 3C Table 3D Table 4 Table 4A Table 4B Table 4C Table 4D Table 5A Table 5B Table 5C Table 6A Table 6B Table 7 Table 8

Trays - A Summary Of Characteristics ..........................................................................41 Counter-Current Devices - A Summary Of Characteristics............................................42 Tower Internals Selection For New Towers ...................................................................43 Tower Internals Selection For New Towers ...................................................................44 Tower Internals Selection For New Towers ...................................................................45 Relative Fouling Resistance Of Common Fractionation Devices...................................46 Tower Internals Selection For Revamps........................................................................47 Application Guidelines For Debottlenecking Fractionation Towers................................48 Tower Internals Selection For Revamps........................................................................49 Tower Internals Selection For Revamps........................................................................50 Tower Internals Selection For Revamps........................................................................51 Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....52 Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....53 Tower Internals Selection For Heat Transfer Service (New Towers And Revamps) .....54 Tower Internals Selection For Entrainment Removal Service........................................55 Tower Internals Selection For Entrainment Removal Service........................................56 Tower Design Checklist (Trays).....................................................................................57 Tower Design Checklist (Packing).................................................................................58

FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6A Figure 6B Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25 Figure 26

Cross Flow Vs. Counter-Current Device Operation .........................................................6 Types Of Trays (Schematic) ............................................................................................7 Types Of Moveable Valves..............................................................................................8 Some High Capacity Tray Designs..................................................................................9 Enhanced Downcomer Trays ........................................................................................10 Typical Sieve Tray Tower ..............................................................................................12 Typical Tray Layout .......................................................................................................13 Pass Arrangement On Multi-Pass Trays........................................................................15 Stepped Vs. Sloped Downcomers .................................................................................16 Modified Arc Vs. Arc Type Downcomers .......................................................................16 Sulzer Cartridge Trays And Their Envelope Downcomers.............................................17 Downcomer Sealing Techniques ...................................................................................18 Bubble Area Definitions .................................................................................................20 Free Area Definitions .....................................................................................................21 Sketches Of Some Hardware Devices ..........................................................................23 Jet Flooding: Its Impact On Entrainment And Tray Efficiency........................................24 Froth Regime Vs. Spray Regime Operation ..................................................................25 Flow Regime Within Normal Operating Range ..............................................................25 Generating Entrainment.................................................................................................26 Downcomer Filling Components (Static Pressure Balance) ..........................................27 Effect Of Liquid Rate On Sieve Tray Turndown.............................................................29 Effect Of Weeping On Efficiency ...................................................................................32 Typical Sieve Tray Performance Diagram .....................................................................33 Random (Dumped) Packings.........................................................................................35 Structured Packing (By Koch-Glitsch, Inc.)....................................................................35 Various Types Of Grids .................................................................................................36 Different Types Of Baffles..............................................................................................37

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Revision Memo 12/01

This update represents a major revision effort. Highlights of this revision are: (1) Major updates to discussions and figures throughout the entire section, to reflect the merging of ExxonMobil fractionation technologies. (2) Updated Figure 4, “Some High Capacity Tray Designs”, to include Provalve deck, UOP Multiple Downcomer Sieve Trays, and the Sulzer HiFi Tray. (3) Updated the definition of waste area to account for three different types of wasted tower area. (4) Included discussion and sketches of picket fence and swept back weirs. (5) Major modifications to Vapor Handling Limitations section include: updated hierarchy of design considerations, included emulsion regime in discussion of flow regimes. (6) Major modifications to Liquid Handling Limitations section include: updated hierarchy of primary and secondary design considerations, expanded and updated discussions of all design considerations. (7) Major modifications to Other Process Considerations section include: updated hierarchy of design considerations, included discussion of fouling. (8) Inserted Table 3D, “Relative Fouling Resistance of Common Fractionation Devices”. (9) Major modifications to Tables 1, 2, 3, 4A, 7, 8 to include newer fractionation devices and debottlenecking strategies. (10) Inserted NOMENCLATURE section.

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SCOPE This section presents guidelines which are useful for selecting the optimum contacting device for a given service. Also included are discussions on the various process limitations which restrict the operating range of a given device. The detailed design procedures for the various fractionating devices are covered in subsequent sections. A detailed Tower Design Checklist is included as Table 7 (for trays) and Table 8 (for packing). They are intended for use after the design has been completed. For information on mechanical requirements see GP 5-2-1. For fractionator process information, refer to the Separations Guide (TMEE 058).

REFERENCES DESIGN PRACTICES All Other Sections of Section III

GLOBAL PRACTICE GP 5-2-1

Internals for Towers and Drums

OTHER REFERENCES 1. 2. 3. 4.

➧ ➧ ➧

5. 6. 7. 8. 9.

Atmospheric Pipestill Guide, ExxonMobil Engineering Technical Manual, EEPE-0006. Fractionating Tower Troubleshooting Guide, ExxonMobil Engineering Technical Manual, TMEE 021. Fuels Vacuum Pipestill Guide, ExxonMobil Engineering Technical Manual, EETD 076 (EE.20E.96). Maloney, D.P., Fuels Vacuum Pipestill Wash Zone Reliability, New Section of the Fuels Vacuum Pipestill Guide, Technical Manual Update, EE.53E.98. Mobil Tower Internals Program (MoTIP) User’s Manual, Mobil Research and Development Corporation. Refinery Distillation Practices, Mobil Technology Company, May, 1996. Separations Guide, ExxonMobil Engineering Technical Manual, TMEE 058. Trayed Tower Internals, MTC Process Design Practices, Mobil Technology Company, Practice No. II, Volume No Attached to the Table of Contents for Section Ill - Fractionating Towers, is a list of ExxonMobil FRACTIONATION SPECIALISTS who can provide help on a broad range of fractionation issues.

BACKGROUND KEY STEPS INVOLVED IN TOWER DESIGN - AN OVERVIEW The purpose of this Subsection is to define the Basic Concepts needed to understand how the various contacting devices work and to help select the best internal for a given service. Once the contacting device has been selected, the designer should then read the particular Subsection that covers that device. Before beginning, however, it is important to look at all the steps that must be taken in order to achieve an optimum tower design. To do this, the eight key steps in designing a distillation tower are briefly reviewed below. 1. Define the key separations. In many cases, where individual components can be identified, the amount of impurity permitted in the products is given. This is needed in order to set the yield and purity level of the products being produced. 2. Obtain the appropriate vapor-liquid equilibrium (VLE) and enthalpy data method. This can be done by referring to the ExxonMobil Data Library Manual or by contacting your FRACTIONATION SPECIALIST. The use of an inappropriate data method could lead to off-spec products or an inoperable design. 3. Calculate the theoretical trays required at different reflux rates. By running parametric cases on a plate to plate computer program with differing numbers of trays and reflux rates, the shape of the theoretical trays vs. reflux ratio curve can be determined. This curve will assist in determining the most economical tower height and diameter combination.

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BACKGROUND (Cont) 4.

5.

6.

➧

7.

8.

Estimate the tray efficiency. This step assumes that the contacting internal will be sieve or valve trays; however, a similar process should be followed with other internals, such as packing. For most services, towers similar to the one being designed have been built in the past and operating efficiencies have been determined. These are tabulated in Section IIII, Tray Efficiency, and are suitable for preliminary scoping purposes. For truly new systems, and for checking the efficiency during the tower design, the efficiency should be calculated via Section III-I (or the appropriate computer program), since the final efficiency is also influenced by the tray hardware design. The liquid flow path length (lfp) is the most important hardware parameter with respect to tray efficiency, provided normal hydraulics (that is, weeping and entrainment below allowable limits, and Ao/Ab and weir heights within allowable ranges). The flow path length determines the number of mixing pools on the tray, which in turn has a direct influence on the tray efficiency. Tray efficiency increases rapidly from the minimum allowable flow path length (16 in [400 mm] for conventional trays) up to about 30 in (760 mm). For flow path lengths greater than 30 in. (760 mm), liquid flow on the tray is essentially plug flow, and efficiency is independent of flow path length. However, for flow path lengths greater than about 70 in. (1780 mm), tray efficiency starts to decrease due to the presence of stagnant regions on the tray, depending on tray geometry. Define the maximum and minimum feed rates that the tower must handle. In general, a turndown ratio of 2/1 is adequate for most services. If greater turndown than this is needed, the turndown ratio itself becomes one of the key items in selecting both the tray type and how it should be designed. Another term commonly used is "flexibility”, which is the ability of a device to operate efficiently over a range of vapor and liquid loadings. Selecting the best tower internal. Subsequent discussions plus Tables 1-3 will facilitate internals selection. For most trayed columns, sieve or valve trays will be the first choice whereas for packed towers a dumped packing of the 2 in. (50 mm) size will normally suffice. Of course, such items as fouling tendency, allowable pressure drop, turndown requirements, and in the case of revamps, required capacity and/or efficiency will have an important influence on the internal finally selected. As a design technique, zoning the tower into different sections can increase flexibility and reduce tower height. However, most new tower designs should not be divided into more than two sections (with the feed zone being the line of demarcation the two sections), due to diminishing returns on overly complex design. Contact a FRACTIONATION SPECIALIST if a design with more than two sections is being considered for a simple tower. Tower sizing and tray hydraulic calculations. The key parameters to consider in the sizing and hydraulics areas are discussed in subsequent pages of this Subsection for the various devices available. Various computer programs are available in PEGASYS to facilitate calculations and permit rapid optimization via parametric cases. It is strongly recommended, however, that the engineer's first internals design be done with the close guidance of a FRACTIONATION SPECIALIST, following the steps in the detailed design procedure, in order to develop the necessary feel for how the many variables interrelate. Subsequent designs can then be done via computer with confidence. Process Control. In order to get optimal performance from any distillation tower, it must be controlled properly. As the tower diameter and number of trays increases, the lag time in the system can become quite large. The designer should see Section XIl - Instrumentation and consult with the appropriate SYSTEMS ENGINEERING SPECIALIST to ensure that the correct process control system has been specified.

TYPES OF CONTACTING DEVICES AVAILABLE A contacting device must have good liquid and vapor handling capacities, good contacting efficiency, reasonable pressure drop, predictable turndown characteristics and be economical. The devices available fall into two broad categories - cross flow and counter-current. They are shown conceptually on Figure 1. Subsequent paragraphs will discuss how each device works.

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BACKGROUND (Cont) ➧

FIGURE 1 CROSS FLOW VS. COUNTER-CURRENT DEVICE OPERATION Cross Flow

Liquid In

Counter-Current

Contacting Zone Liquid

Downcomer

Tray

Vapor DP3Af01

Vapor

Cross Flow Devices (Trays). The liquid flows horizontally across a flat, level plate (called the tray) that contains a contacting device selected by the designer that intimately disperses the vapor into the liquid. In addition, the dispersion process must produce sufficient interfacial area and maintain the phases in contact with each other long enough to promote adequate mass transfer between the phases. As the liquid flows across the tray, it is contacted by the ascending vapor. At the far side of the tray, the liquid enters a downcomer which carries it to the tray below where the contacting process is repeated. Obviously, the contacting area must be large enough to handle the required liquid and vapor rates while promoting the desired mass transfer. Likewise, the downcomer must be large enough to handle the liquid being processed. The various process limitations that define the operating constraints for the contacting area and the downcomer will be discussed later under CROSS FLOW DEVICES PROCESS DEFINITIONS. Counter-Current Devices (Packing, Grids, Baffle Sections, and Dualflow Trays). With these devices, the liquid flow is counter-current to the vapor flow. The efficiency of contact is again dependent on the area available for mass transfer. In trays, this is provided by bubbling vapor through the liquid, thereby producing sufficient interfacial area for mass transfer. With such counter-current devices as packing and baffle trays, however, the interfacial area needed for mass transfer is provided by the surface area of the device, or by forcing the vapor to flow through descending curtains of liquid (thereby breaking them up into droplets). Generally speaking, as the surface area of the device goes up, the efficiency goes up. However, as the surface area increases, the capacity falls while its cost rises. Thus, the final choice involves optimizing the capacity, efficiency, cost, and other process considerations of the various internals available. Subsequent paragraphs will discuss how this is done.

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CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE Some trays used widely within the ExxonMobil circuit are sketched in Figure 2 and discussed briefly in subsequent paragraphs. The pertinent Subsection for designing each device is also noted next to the figure in parentheses. FIGURE 2 TYPES OF TRAYS (SCHEMATIC) Sieve (III-B)

Valve (III-E) Closed

Fully Open

Pulsating (Floating)

Jet (III-D)

Bubble Cap (III-C) Standard Cap

1.

➧

2.

Vacuum Cap

DP3AF2

Sieve Trays. For most fractionation services in refining and petrochemicals, seive trays will be the first choice. The contacting area consists of flat plates containing perforations, usually 1/2 in. (13 mm) in diameter. They are the simplest trays to fabricate and are therefore the cheapest. They also exhibit good capacity, efficiency, and turndown characteristics (about 2/1 to 3/1). Their flat surface facilitates maintenance; thus, they may be used in fouling services provided the hole size is increased to 3/4 - 1 in. (19-25 mm). Valve Trays. The valve size, shape weight and other parameters vary from vendor to vendor (Figure 3). Valve trays should be used in lieu of sieve trays when the turndown ratio exceeds 3/1. These trays contain proprietary devices manufactured by Koch-Glitsch Inc. (formerly two companies, Koch and Glitsch), Saint-Gobain NorPro Corporation (formerly Norton), Sulzer Chemtech (formerly Nutter Engineering and Sulzer), and other vendors. Valve trays should be used in lieu of sieve trays when the turndown ratio exceeds 3/1; in the upstream sector, valve trays are the preferred choice due to this high turndown capability. For design purposes their capacity and efficiency are assumed to be about equal to that of a sieve tray, but their cost is roughly 10% higher. Floating (pulsating) valve trays are not recommended for fouling service; however, there are cases where fixed valve trays would be recommended for fouling service (see number 6 below). All else being equal (cost / delivery time, etc.), rectangular valves are preferred due to lack of rotation. Rotation eventually erodes the holes and legs, causing the valves to pop out. This occurs even if dual rotation stops are included in the valve specification.

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CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont) FIGURE 3 TYPES OF MOVEABLE VALVES

Koch-Glitsch Type A

Koch-Glitsch Type T

Koch-Glitsch V-1

Sulzer (Nutter) BDH DP3AF3

3.

➧

4.

5.

➧

➧

6.

Jet Trays. Jet trays, an Exxon development in the 1950's, are now used primarily in heavy hydrocarbon services that handle high liquid rates. The directional effect of the vapor as it leaves the inclined tabs helps push the liquid across the tray, thereby increasing its capacity. Unfortunately, this directional effect also reduces liquid residence time on the tray such that efficiency becomes low for most light ends and other towers with low to moderate liquid loads. For higher liquid rates typical of pumparounds, the efficiency in heat transfer service is comparable to other cross flow devices. Bubble Cap Trays. These trays are used infrequently within ExxonMobil today because they cost about 100% more, and have a higher pressure drop, than a sieve trays. They should be considered when flexibility needs exceed 3/1 and valve trays cannot be used because fouling is a problem. While even bubble caps will foul over time their large physical clearances (compared to valve trays) provide a longer run length. For applications that require low pressure drop, vacuum caps should be considered instead of standard caps. Cartridge Trays. These trays derive their name from the fact that they are really a collection of trays, held together by rods, to form bundles or “cartridges.” These cartridges are then lowered into a top-flanged vessel. For convenient installation each cartridge usually contains 10-15 trays. The contacting device is usually a sieve or valve tray. They are used in small diameter columns (i.e., < 3 ft or 900 mm) where inaccessibility prevents the use of standard trays. Most cartridge trays in use by ExxonMobil were made by Sulzer Chemtech, since Sulzer’s tray has the only commercially proven peripheral sealing device. Fixed Valve Trays. These devices can best be described as valve trays whose valve units are fixed in the fully open position. Sulzer Chemtech's Nutter Small V-Grid (SVG) trays on triangular pitch are considered to be an alternative to sieve trays and generally have somewhat better turndown ratio (20% higher). Larger fixed valve units such as the Nutter SVG or the NorPro Provalve may be useful for extending run lengths in some fouling services (but not where sticky material is entrained from below). Nutter Large V-Grid (LVG) and fixed valve trays on square pitch have lower capacity and should only be used upon consultation with a FRACTIONATION SPECIALIST. Mini Fixed Valve Trays: Sulzer Mini V-Grid (MVG), Figure 4, and Koch-Glitsch VG-0. At pressures under 50 psia (345 kPa) a tray with mini valves has an estimated 10-15% jet flood capacity advantage when compared to a conventional (0.5 in. [13 mm] hole) sieve tray with approximately the same efficiency. The capacity gain is similar to what is expected with a small hole sieve tray in low liquid rate services. V-Grid trays have the same directional effect as the NorPro Provalve tray. Koch-Glitsch's Bi-FRAC should not be considered for application. Contact your FRACTIONATION SPECIALIST for more details should an application of fixed mini valve trays exist.

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CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont) ➧

FIGURE 4 SOME HIGH CAPACITY TRAY DESIGNS SULZER CHEMTECH'S NUTTER MVG TRAY

NORPRO PROVALVE DECK

Liquid Flow Liquid Flow

0.5" (13mm) or 0.67" (17mm)

1.5" (38mm)

UOP MULTIPLE DOWNCOMER SIEVE TRAYS

SULZER HI-FI TRAY

Downcomers

Downcomers

Tray Deck

Downcomer orientation alternates from tray to tray

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CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont) ➧

7. Enhanced Downcomer Trays. Enhanced downcomer trays currently available include Koch-Glitsch’s Nye Trays and MaxFrac trays, Saint-Gobain NorPro’s Triton trays (see 8d below) and the Sulzer MVGT Tray. By utilizing the area under the downcomer for additional vapor flow, an increase in the vapor handling capacity is achievable (Figure 5). Nye trays and recently, to a more limited extent, Triton trays have successfully been applied in tower revamps. An estimated 10-15% additional jet flood capacity can be expected with similar efficiency when compared to a conventional sieve tray. It should be noted that the increase in capacity is approximately proportional to the ratio of the downcomer outlet area (downcomer bottom area) to the tower cross-sectional area. Contact your FRACTIONATION SPECIALIST should an application of an enhanced downcomer tray exist.

➧

FIGURE 5 ENHANCED DOWNCOMER TRAYS CONVENTIONAL TRAY

KOCH-GLITSCH NYE TRAY

NORPRO TRITON TRAY

Orifice Plate

Provalve Deck

DP3AF5

➧

➧

Downcomer

8. Other Trays. a. Multiple Downcomer Trays: UOP MD (Multiple Downcomer) and ECMD (Enhanced Capacity Multiple Downcomer) Trays, Sulzer Hi-fi Tray. These proprietary trays are particularly useful for fractionation services handling high liquid rates. Although they have about 20% higher capacity than a conventional sieve tray, their efficiency is less than that of sieve trays while their cost is greater. These characteristics make them unattractive for new tower designs of less than 15 ft (4570 mm) diameter. For high pressure, light ends towers greater than 15 ft (4570 mm) diameter, MD, ECMD or Hi-Fi trays should considered in the scoping phase, with total tower cost (internals plus shell plus installation) compared to the conventional 4-pass design cost. In revamp situations, these trays can be installed at low tray spacings, and may therefore provide more theoretical stages for a given tower height. Because of the many downcomers used in these trays, tray to tray access is limited. Thus, the entire tray may have to be removed if full access to lower trays is required. This may be a significant maintenance debit for some applications. These trays are not recommended in fouling services because of the inability to fully inspect and clean the trays in a reasonable amount of time. The small hole size typically used, 3/16 in. (4.76 mm), is a further concern in fouling services. Capacity increases of 10% relative to the standard MD tray are achievable with the ECMD tray. The ECMD tray is the highest capacity tray used by ExxonMobil. The first installation of ECMD trays within ExxonMobil occurred in 1996. The Sulzer Hi-fi Tray is an alternate multiple downcomer tray with similar performance. Contact your FRACTIONATION SPECIALIST if an application of multiple downcomer trays is being considered. b. Koch-Glitsch SuperFrac Tray. Koch-Glitsch uses the brand name SuperFrac to describe a variety of tray technologies developed to improve the maximum vapor handling capacity of trays relative to standard sieve, valve, and bubble cap trays. The technologies incorporated into a SuperFrac tray may include any of the following: (1) small (mini-) valves, either fixed or movable depending on turndown requirements, (2) a patented downcomer design to minimize its size which maximizes the space for vapor flow, (3) an inlet weir to seal the downcomer and/or distribute liquid to the tray, (4) special aerated washers, used to fasten tray panels, that distribute vapor to relatively unaerated areas of the tray deck, (5) bubble promoters to provide rapid aeration of the liquid flowing from the inlet downcomers, (6) directional orientation of some valves to distribute liquid uniformly over the tray deck, and (7) a calming box with orifices at the bottom (also called a relief downcomer) upstream of each downcomer to unload the downcomer. There are a few applications of SuperFrac trays within ExxonMobil. EMRE has evaluated limited commercial data on this technology, including a high pressure C3 splitter. Based upon the sparse data available to EMRE, the SuperFrac tray has features that can allow 15% to 25% increased jet flood capacity versus standard sieve, valve, or bubble cap

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trays, when the system is non-fouling and non-foaming. Contact your FRACTIONATION SPECIALIST if a KochGlitsch SuperFrac application is being considered.

CROSS FLOW DEVICES - TYPES OF TRAYS AVAILABLE (Cont) ➧

c.

➧

d.

➧

e.

NorPro Provalve Tray. This tray uses large valves that are wider on the upstream end of the valve than on the downstream end, thereby deflecting vapor and directing liquid. This directional effect promotes uniform vapor distribution throughout the bubbling area of the tray, resulting in better performance than conventional valve trays. In addition, the large size and unique shape of the fixed Provalve valves make them highly resistant to fouling. ExxonMobil has used Provalve trays in such services as debutanizers, coker fractionators, amine regenerators, and C8/C9 splitters. Contact your FRACTIONATION SPECIALIST for assistance regarding potential application of this technology. NorPro Triton Tray. This tray uses proprietary Provalve valves, with truncated downcomers to increase tray bubbling area. FRI has verified NorPro claims of significantly higher capacity (about 20% higher) compared to conventional valve trays, with comparable mass transfer efficiency. The results of FRI tests also suggest about 10% higher capacity compared to the Nutter MVG tray. Triton trays also exhibit lower pressure drop than conventional fixed valve and MVG trays. Contact your FRACTIONATION SPECIALIST for assistance regarding potential application of this technology. Other Devices. For trays not mentioned in this Subsection please contact your FRACTIONATION SPECIALIST for guidance.

CROSS FLOW DEVICES - HARDWARE DEFINITIONS In order to understand how a tray functions, it is important to define various hardware and process parameters. The hardware parameters will be described first. Figure 6A shows a typical sieve tray tower. The particular tower in Figure 6A has six single-pass sieve trays, with the sieve holes on a triangular pitch. Inlets include a reflux inlet nozzle to the top tray, a reboiler return distributor below the bottom tray, and a liquid distributor to the third tray (from the top). There is a total drawoff following the second tray (from the top), a reboiler drawoff after the bottom tray, and a third liquid outlet at the bottom of the tower (not shown) for the bottom product. The arrows in the figure depict the direction of liquid flow. The hardware parameters in Figure 6B are common to all the cross flow trays.

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➧

FIGURE 6A TYPICAL SIEVE TRAY TOWER Typical Reflux Inlet Arrangement for Single-Pass Trays

Reflux Inlet Nozzle

False Downcomer Tray Outlet Weir

Downcomer Apron Total Drawoff with no Provision for Overflow to Tray Below

Inlet Distributor Cutaway Shows Holes Directing Liquid Against Downcomer Apron

No Sieve Holes in Downcomer Floor

Liquid Flows Through Clearance Under Downcomer

Reboiler Return Distributor Cutaway Shows Slots Seal Pan

Reboiler Drawoff Overflow is Bottom Product Reboiler Drawoff Box DP03Af06a

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➧

FIGURE 6B TYPICAL TRAY LAYOUT

Note (1)

Tray Support Ring

Downcomer Inlet Area (Adi)

Perforated, Valved, Tabbed or Capped Area (Ab)

1/2 Outlet Weir Length (Io)

r Diameter DT

Downcomer Width (Rise)

Weir Height (hwo)

Downcomer Clearance (c) Liquid Flow

H

Tray Spacing

Downcomer Apron

Note: (1) Any portion of these areas within 3 in. (75 mm) of a hole is included in the tray’s bubble area (Ab). DP3Af06b

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)

➧

➧

➧

As shown, a typical tray layout contains certain key items: 1 . Tower Diameter and Tray Spacing. These are the two major parameters that set the capacity of the tower. As the distance between trays is increased (tray spacing, H), the tower capacity will increase. The most economic spacing usually falls between 18-24 in. (450-600 mm) for most services. Spacings above 36 in. (900 mm) provide little incremental capacity advantage and are therefore not usually recommended. Likewise, tray spacings as low as 12 in. (300 mm) can also be used, but this increases the tower diameter (DT) required to handle a given set of vapor and liquid loadings. In addition, low spacings also make maintenance much more difficult. Each tray’s Section III (B, C, D and E) contains a table of minimum acceptable tray spacings for maintenance as a function of tower diameter and service (i.e., whether it is clean or fouling). 2. Contacting [bubble] area (Ab, shown shaded). The bubble area is defined as the tray deck area where the liquid/vapor contacting occurs. It is calculated during the detailed design of the tray. 3. Downcomer Area. This is the area (Ad) that must be devoted to handling the liquid as it flows from a given tray to the tray below. A distinction is made between three different areas associated with a given downcomer geometry, because tray performance depends on each of these values. The downcomer inlet area (Adi) is the horizontal area available for liquid flow into the downcomer. The downcomer outlet area (Ado), also called the downcomer bottom area, represents the corresponding horizontal area at the bottom of the downcomer. The area under the downcomer (Aud), also called the downcomer escape area, is the downcomer clearance (c) multiplied by the downcomer outlet chord length. The edge of the downcomer is usually chordal in shape and its maximum width is called the downcomer width or downcomer rise (r). The difference between the width at the top and the width at the bottom of a sloped downcomer is referred to as the "downcomer kick". Section III-K contains the necessary tables to calculate downcomer areas and other geometric parameters. Alternatively, the PEGASYS computer program’s “Segment of Circles” program located within the Geometry Menu can also be used. 4. Downcomer Clearance. This is the vertical clearance (c) between the tray floor and the bottom edge of the downcomer apron. This clearance varies with liquid rate and is discussed in each tray’s Subsection under BASIC DESIGN CONSIDERATIONS. 5. Outlet Weir Height and Outlet Weir Length. As the liquid leaves the contacting area, it flows over the outlet weir as it enters the downcomer. The height of the outlet weir (hwo) is set by the designer to provide liquid holdup on the tray to promote efficient liquid/vapor contacting and to seal the downcomer upstream of the weir. The outlet weir length (Io) is the same as the downcomer inlet chord length. Note: The "inlet" and "outlet" terminology used for weirs and downcomers can be confusing. For weirs, "inlet" refers to the upstream weir on a given tray. For downcomers, "inlet" refers to the upstream (top) part of the downcomer. Therefore, the outlet weir is located on the downcomer inlet chord. 6. Multi-pass Trays. As the liquid rate on a tray increases, the capacity of the tower can usually be increased if the liquid flow is split into more than one path (see arrows labeled L on Figure 7). Such split-flow trays are called multi-pass trays. While single pass trays are the most common, double pass (2 pass) trays are frequently used. Three and four pass tray designs are also available, but their use comprises less than 5% of the total trays used in ExxonMobil towers, with four pass designs being more common than three pass. Since higher pressure towers have a higher volumetric ratio of liquid to gas, multi-pass trays with greater total downcomer area are common in these towers.

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) ➧

FIGURE 7 PASS ARRANGEMENT ON MULTI-PASS TRAYS Single Pass

Double Pass (2 Pass)

L

L

Three Pass

L

L

L

L

Four Pass

Center (Inboard) Downcomer

Side (Outboard) Downcomer

L

L

L

L

L

L

L

L

L

L

L

L

L

L

Off-center Downcomer DP3Af07

On multi-pass trays, the downcomer(s) nearest the tower centerline are referred to as “inboard” or “center” downcomers, while those farthest away are called “outboard” or “side” downcomers. 7.

Sloped and Stepped Downcomers. For a given tower diameter, a certain amount of the available cross-sectional area is needed for liquid handling (downcomer area) with the remainder available for vapor flow (bubble area). Therefore, any steps that can be taken to reduce the area consumed by the downcomer(s) will provide additional area for vapor flow. This goal can be achieved on heavily liquid loaded trays by using stepped or sloped downcomers as shown on Figure 8. Typical design for a downcomer “step” is for the step to be located at 33% of the tray spacing. Stepped downcomers are preferred in small diameter towers [DT < 48 in. (1220 mm)] due to easier weld-in installation. Otherwise, sloped downcomers are preferred. When a sloped downcomer is specified, often suppliers offer a “semi-sloped” design. This design has better mechanical features, since the upper vertical portion is often used as a support beam. However, the associated reduction in average free area results in a slight loss in capacity.

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) ➧

FIGURE 8 STEPPED VS. SLOPED DOWNCOMERS Stepped Downcomer

Straight Downcomer

Sloped Downcomer

hv,hi

hv,hi

hv,lo

Tray Deck

hv,lo Dashed Lines represent semi-sloped design DP3AF8

8.

Modified Arc and Envelope Downcomers. The chord length at the bottom of a downcomer must be sufficient to provide uniform liquid flow distribution onto a tray, because uniform liquid distribution results in good tray efficiency. The appropriate Subsection in Section III provides the minimum suggested chord length for a specific type of tray. In some services where very low liquid rates must be handled, minimizing the downcomer area can result in a downcomer chord length that is smaller than that suggested in the appropriate Subsection. In these cases, a modified arc (also known as segmental, or swept back) downcomer can be specified (see Figure 9). A modified arc downcomer has a smaller area than a chordal downcomer, when the chord length of the chordal downcomer is equal to the “projected” weir length of the modified arc downcomer. The modified arc downcomer thereby enables a reduction in downcomer area without sacrificing uniform liquid flow distribution onto the tray below. Modified arc downcomers can also be used to help balance weir lengths for four-pass tray designs. Some pre-1960 towers may contain a full arc-type downcomer. This style downcomer functions in the same manner as a modified arc but can be more expensive to build; therefore, smooth arc-type downcomers are not specified in new designs. However, they can be retained if their area is sufficient for revamp conditions. Section III-K provides geometric data for chordal and modified arc downcomers. FIGURE 9 MODIFIED ARC VS. ARC TYPE DOWNCOMERS Modified Arc

Arc

Straight Line Segments

Projected Weir Length

Smooth Arc

Ri se

➧

DP3AF9

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DESIGN PRACTICES

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) For towers under 36 in. (900 mm) in diameter, cartridge or package trays are frequently used. These trays are installed in sets (or bundles) held together by rods, and can be lowered into a top-flanged vessel. Since the trays are removable the downcomer must be an integral part of the tray. These downcomers are called envelope type downcomers (See Figure 10). Standardized downcomer sizes for these trays can be found in Section III-E, Table 4A and 4B. However, custom sizes can be provided if one of the standard sizes does not meet process requirements. FIGURE 10 SULZER CARTRIDGE TRAYS AND THEIR ENVELOPE DOWNCOMERS Nutter Cartridge Tray

Envelope Downcomer (Plan View)

DP3AF10

9.

Downcomer Sealing Techniques. In order to prevent some of the vapor from bypassing the contacting region by flowing upward through the liquid in the downcomer, the downcomer must be “sealed" by liquid. In most tray designs with an outlet weir, the liquid holdup (or clear liquid height) will sufficiently seal the downcomer clearance without additional hardware devices (See discussion on Downcomer Seal under Liquid Handling Limitations). When this is not possible, however, ways to provide a seal via hardware techniques are shown in Figure 11. Both the recessed inlet box and the inlet weir seal use mechanical means to help seal the downcomer. In the case of the recessed box, the seal is obtained because the bottom of the downcomer apron and the tray below are at the same elevation. With an inlet weir, the weir height plus the liquid head over the inlet weir must be at least equal to the downcomer clearance. By using a shaped downcomer lip, the head loss under the downcomer is reduced, allowing a smaller downcomer clearance to be used. This reduced downcomer clearance can provide a process seal for the downcomer, although the corresponding increase in the velocity under the downcomer may cause it to exceed the specified limit (See Downcomer Seal and Velocity under downcomer in CROSS FLOW DEVICES-PROCESS DEFINITIONS).

➧

Recessed inlet boxes are susceptible to trapping solids, which may affect tray performance. If this is a problem in an existing tower, the boxes should be covered or removed and the edge of the downcomer panel will need to be modified in order to properly seal the downcomer. If the recessed inlet boxes are covered, then a side vent and bottom drain are required. Consult GP 5-2-1 regarding drain hole specifications for recessed inlet boxes.

➧

For shaped lip downcomers, the radius tip (shaped downcomer lip) may be in the range of 1 to 2 in. (25 or 50 mm), although 1 in. (25 mm) is typical. Remember to check the location of sieve holes, valves, or jet tabs relative to the edge of the shaped lip to avoid getting vapor into the downcomer. Do not use shaped downcomer lips on top reflux distributors (false downcomers), or with a recessed inlet box, an inlet weir, or a bubble cap tray. Most importantly, the presence of a radius tip in conjunction with a seal pan or inlet weir creates an undesirable restriction for the liquid flow. In addition, the presence of these downstream devices induces turbulence and defeats the purpose of streamlining the flow with a shaped downcomer lip.

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) ➧

FIGURE 11 DOWNCOMER SEALING TECHNIQUES Recessed Inlet Box

Inlet Weir

Shaped Downcomer Lip

See Detail A

C

C

Detail A

DP3AF11

10. Contacting Area Definitions. During the design of a given device, such terms as bubble area, free area, hole area, and waste area are used. They are explained below. a. Bubble area (Ab). This is the area between the downcomers where vapor/liquid contacting occurs. See Figure 12. b. Hole/valve/cap/tab area (Ao). This is the open area or hole area provided within the bubble area to permit vapor to enter, contact, and pass through the liquid on the tray. In the case of a sieve tray, it will be equal to the total area of all the holes on a given tray. The hole area is usually expressed as a fraction or percentage of the bubble area (Ao/Ab). This ratio is determined by various correlations discussed in each tray's Subsection.

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) ➧

c.

➧

d.

Free area (Af). Test data have shown that as the vapor flows through and leaves the bubble area (Ab) it expands over the downcomer(s) and its velocity drops. Thus, an area greater than the bubble area is available for vapor flow. This larger area is known as the free area (Af) and is defined in Figure 13. Data analysis has shown that a combination of bubble area and free area help determine a tray’s capacity at jet flood. For trays with sloped or stepped downcomers the average free area is used. It is defined at the bottom of Figure 13. Waste area (Aw). Waste area is defined as any area of a tower that cannot be used to enhance tray capacity or efficiency, thereby representing "wasted" tower area. There are three general types of waste area:

•

Tray deck waste area refers to any area in the bubble area that is further than 3 in. (75 mm) from the edge of a contacting device. For rating calculations, tray deck waste area reduces both the bubble area and the free area of a tray. Tray blanking, recessed inlet boxes, inlet weirs, shaped downcomer lips, and rectangular bubble area blanking patterns all contribute to tray deck waste area.

•

Volumetric waste area results from any obstruction that inhibits vapor flow through the free area above a tray deck. Wasted volume may be represented as an effective waste area in the same way free volume is represented as an effective free area. For rating calculations, volumetric waste area reduces the free area only. Splash baffles, distributor pipes, beams and drawoff boxes may contribute volumetric waste area.

•

Downcomer waste area exists wherever there is an obstruction in the downcomer, reducing the downcomer capacity. For rating calculations, downcomer waste area reduces the effective downcomer inlet area, outlet area, and/or mean downcomer area, depending on where in the downcomer the obstruction exists.

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➧

FIGURE 12 BUBBLE AREA DEFINITIONS Single-Pass Trays

Two-Pass Trays

Adi

A*di

Ado

A*do Adi

Adi

2

2

Ado 2

Ado 2

Plan View Shows Inboard Tray Bubble Area lfp

Ab

Ab = As - Adi - Ado - Aw (if any)

lud

lo

lfp

lfp

Ab

Ab

lo

lo*

For Side Downcomer Tray Ab = As - Adi - A*do - Aw (if any) For Center Downcomer Tray Ab = As - A*di - Ado - Aw (if any)

* Terms with asterisks refer to center (inboard) downcomer, those without asterisks refer to side (outboard) downcomer.

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DP3AF12

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont) FIGURE 13 FREE AREA DEFINITIONS Single-Pass Trays

Adi

Adi Af

Af

Ado

Af = As -

Af = As - Adi - Aw (if any)

Ado

Adi + Ado 2

- Aw (if any)

Two-Pass Trays

A*di

A*di

Adi

Af

Af

Af

2

2

2

A*do

2

Af 2

A*do

Adi

Adi

Adi

2

2

2

Af

Ado

Af

Ado 2

2 For Side Downcomer Tray Af = As - A*di - Aw (if any)

For Side Downcomer Tray Af = As -

A*di + A*do 2

- Aw (if any)

For Center Downcomer Tray Af = As - Adi - Aw (if any) For Center Downcomer Tray Af = As -

Adi + Ado 2

- Aw (if any)

* Terms with asterisks refer to center (inboard) downcomer, those without asterisks refer to the side (outboard) downcomer. DP3AF13

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CROSS FLOW DEVICES - HARDWARE DEFINITIONS (Cont)

➧

➧

➧

➧

11. Other Hardware Definitions a. Anti-jump baffle. On multi-pass trays, a baffle is centered above the center and off-center downcomer to prevent liquid collision in the center of the downcomer. The specific need for this baffle varies with liquid rate and the type of tray used and is discussed fully under DETAILED DESIGN CONSIDERATIONS in each tray’s Subsection. The typical height of the anti-jump baffle is 14-16 in. (350-400 mm). If this criteria cannot be satisfied due to height constraints, then the anti-jump baffle should extend from the top of the outlet weir (or tray deck if no outlet weir is present) to a minimum of 2 in. (50 mm) below the underside of either the tray deck or beam located above the baffle. An isometric sketch of an anti-jump baffle is shown in Figure 14A. Other mechanical details are contained in GP 5-21. b. Picket fence weirs. The use of picket fence weirs to increase the effective liquid height above the tray is the most common way to reduce blowing or spray regime operation. Picket fence weirs are used for low liquid rate applications, commonly for low liquid rate wash tray (large hole sieve tray) design in fouling service. Picket fence weirs may also be used to balance weir loading on 4-pass trays. The effective outlet weir length (that is, the cumulative length of weir over which liquid flows into the downcomer) should be no less than 30% of the actual weir length. If less than 30% is desired, a FRACTIONATION SPECIALIST should be consulted. See Figure 14B for a typical detail of a picket fence weir. c. Swept back weirs. Use of swept back weirs with a given tray design has the effect of increasing liquid handling capacity slightly, while lowering the pressure drop. Swept back weirs have little effect on efficiency, except for low liquid load systems where there is a slight decrease in efficiency caused by the reduction in bubble area. Swept back weirs can be used for sloped downcomers, where a modified arc is not feasible. For design or rating calculations, the projected weir length (that is, the chord that connects the two points where the weir intersects the tower wall) is used, and the tray deck area between the swept back weir and the downcomer is stictly regarded as tray deck waste area. Remember to correctly account for the waste area caused by the swept back weir. See Figure 14C for a typical detail of a swept back weir. d. Minimum width (rise) for center (inboard) downcomers. To ensure good downcomer performance on center downcomers, the minimum width (rise) of the center downcomer is 8 in. (200 mm) at the inlet and 6 in. (150 mm) at the outlet. These dimensions must be maintained even if anti-jump baffles are used. e. Splash baffle. When operating at low liquid rates excessive entrainment may occur. In addition, clear liquid height becomes lower and could result in an unsealed downcomer or poor fractionation efficiency. The installation of a vertical splash baffle above the tray deck at the entrance to the downcomer is an alternative to picket fence weirs for increasing the clear liquid height. Splash baffles (Section III-B, Figure 15) provide additional liquid head on the tray by providing additional resistance to flow into the downcomer and by acting as an entrainment deflector when operating in the spray regime. Splash baffles are not to be used in fouling services.

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➧

FIGURE 14 SKETCHES OF SOME HARDWARE DEVICES A. Anti-Jump Baffle Top edge of baffle should be positioned a minimum of 2" (50 mm) below the underside of the tray deck or beam located above the baffle

Baffle Height 14 -16" (350 - 400 mm) Typical

Bottom edge of baffle should be positioned flush with top of outlet weir, if any; otherwise, with top of tray deck. See discussion in Subsections III, B, C, D and E.

Picket Fence Weir

B. Picket Fence Weir

Pickets should be no more than 6" (150 mm) wide, and between (1/2) H and (3/4) H tall.

Downcomer

Tray Deck

Swept Back Weir

C. Swept Back Weir

Waste Area Tray Deck

Downcomer Waste Area

DP3AF14

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CROSS FLOW DEVICES - PROCESS DEFINITIONS The purpose of this section is to provide the designer with insight into the many parameters that influence tray design. This will be achieved by providing both a verbal description and a drawing (wherever possible) for each definition. Unless obvious or noted otherwise, each of the items discussed below apply to all of the cross-flow trays. They will be discussed under three broad categories: a) those affecting vapor capacity, b) those affecting liquid capacity, and c) other limitations/considerations.

VAPOR HANDLING LIMITATIONS 1.

Jet Flooding. For the vast majority of cases, this is the limitation that sets the vapor handling capacity of all cross-flow trays. As its name implies, the liquid is projected or “jetted" to the tray above by the vapor as it leaves the tray’s orifice. If sufficient liquid is carried to the tray above (i.e., entrained), it will overload the downcomers, and the tray will flood. When flooding occurs, the liquid begins to back up on the tray until the inter-tray space is filled with a dense froth. See Figure 15. This causes the next higher tray to flood and moves progressively up the tower until the liquid is carried out the top of the tower or is removed through a liquid drawoff nozzle. When flooded, the tower fractionates poorly and is very difficult to control. Tray capacity at jet flood is largely a function of tray geometry. Namely, tray spacing, hole area, hole size, bubble area, free area / bubble area ratio, and downcomer inlet area / tower area ratio are important factors in determining capacity at jet flood. Since jet flooding sets the maximum capacity of the tower, it must not be exceeded. Furthermore, as the percent of the jet flood velocity moves from 85% to 100%, the entrainment rate increases exponentially and the tray efficiency falls off sharply. Thus, it is essential that the designer stay within the jet flood limitations discussed in each tray’s Subsection. FIGURE 15 JET FLOODING: ITS IMPACT ON ENTRAINMENT AND TRAY EFFICIENCY

50% Flood

% Jet Flood vs. Efficiency

100% Flood

Entrainment

Tray Efficiency

➧

0 Inter-Tray Space and Downcomer Operating Normally

2.

Inter-Tray Space and Downcomer Full (Flooded)

50

85

100

Percent of Jet Flooding DP03Af15

Ultimate Capacity. Is the highest vapor load the tower can handle. Unlike jet flooding capacity, ultimate capacity cannot be increased with hardware modifications. It represents the velocity at which the liquid is broken into such small droplets that most are entrained and cannot fall back to the tray via Stokes Law. For most systems, however, the Jet Flooding velocity will limit before ultimate capacity is reached. Normally, ultimate capacity will only be a limitation in some high pressure, light ends fractionators (such as some demethanizers, deethanizers, depropanizers, etc.) It occurs on these units because the surface tension is low [< 5 dynes/cm (mN/m)] and the liquid can be shattered more easily into small droplets. Ultimate capacity can also limit deep cut vacuum towers packed with high capacity grid or structured packing. Nevertheless, it should be checked on all designs regardless of operating pressure, to ensure that no problem exists. When ultimate capacity is reached (new designs), the only practical way to increase capacity is by increasing tower diameter. For revamps, there may be no way to avoid ultimate capacity and a new tower may be needed. Discuss such cases with your FRACTIONATION SPECIALIST. ExxonMobil Research and Engineering Company – Fairfax, VA

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont) Flow regimes - spray, froth and emulsion. Different flow regimes can exist on a tray (see Figure 16). The first, and most common, is known as the froth regime. In this regime, vapor passes through the liquid on the tray as discrete bubbles of irregular shape. The bubbles are formed at the tray perforations and are swept away by the froth. The froth surface is mobile and not level. As the vapor rate decreases, the flow regime crosses from the froth regime to the emulsion regime, where the vapor is dispersed as small bubbles in the liquid. As the vapor rate increases from the froth regime, jets and bubbles of rapidly changing shape are observed. If the vapor rate is raised still further, a gas jet issues from the orifice and some of the liquid is shattered into droplets. This latter regime is called the spray regime. In the spray regime, the vapor phase is continuous whereas in the froth and emulsion regimes, the liquid phase is continuous (see Figure 16). Spray regime operation occurs primarily at high vapor velocities and low liquid rates, whereas emulsion regime operation occurs primarily at low vapor velocities and high liquid rates (see Figure 17). FIGURE 16 FROTH REGIME VS. SPRAY REGIME OPERATION Spray

Froth

DP03Af16

FIGURE 17 FLOW REGIME WITHIN NORMAL OPERATING RANGE

Uppe

Vapor Rate

3.

Spray Regime

rO pe

rat i

ng

Li m it

Froth (or Mixed) Regime Emulsion Regime Lower Operating limit

Liquid Rate

DP03Af17

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

4.

Operation in the spray regime can be very detrimental to good tower performance, with tray efficiency dropping sharply. This occurs primarily because the liquid and vapor residence times as well as the interfacial area generated on the tray are reduced. (See later discussion on Tray Efficiency.) While spray regime operation has been observed on all the widely used trays discussed earlier, it has been investigated primarily with sieve trays. Under spray regime conditions, the vapor rate is sufficient to “blow through" the liquid, thereby making the vapor phase continuous. This is where the term "blowing” originated which is another term often used to describe the spray regime. Since the liquid rate is usually set by the process itself and can not be increased, the most effective way to suppress the spray regime is to dissipate the jet leaving the orifice as quickly as possible. The common method for avoiding spray regime operation is through use of a picket fence weir. Another way to avoid spray regime operation is to increase the open area on the tray, thereby reducing jet velocity. A third way is to use smaller sized orifices [say 1/8 in. (3 mm)] holes vs. the standard 1/2 in. (12 mm) holes used on sieve trays. Since the distance to dissipate a jet is a function of the orifice diameter, the smaller the orifice the faster the jet will dissipate. A fourth way is through the use of valve trays. Since the vapor leaves the valve element almost horizontally, its vertical velocity component is greatly reduced and its jet more quickly dissipated. Other, less frequently used techniques are mentioned in Section III-B (Sieve Trays). Their use, however, will require contacting your FRACTIONATION SPECIALIST for guidance. Entrainment. Is defined as liquid that is carried by the vapor from a given tray to the tray above. As the vapor rate in the contacting area is increased, the amount of energy being dissipated also increases. This energy creates the interfacial area needed to provide good contacting between the liquid and the vapor. It also expands the froth or spray height on the tray, thereby decreasing the distance between the top of the spray and the tray above. As this distance decreases further, some of the liquid is carried (entrained) to the tray above as droplets. As Figure 18 indicates, the smallest drops will be entrained to the tray above while the largest drops will fall back to the entrainment generation tray. As the quantity of entrainment increases, the tray above becomes overloaded and “floods" and the tray’s efficiency drops sharply (see above under Jet Flooding). FIGURE 18 GENERATING ENTRAINMENT

Drops Uniform Vapor Velocity Profile Vapor Bubbles In Liquid Initial Projection Velocity Of The Drops

DP03Af18

The quantity of entrainment generated is dependent on vapor rate, liquid rate, and certain hardware parameters. Correlations for predicting entrainment have only been developed for sieve trays. One correlation predicts entrainment under low liquid loading conditions while the other is used for the balance of the liquid rate range (see Section III-B Sieve Trays). In general, tray designs shall not allow entrainment to exceed 10% of the tray’s liquid flow rate.

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont) ➧

LIQUID HANDLING LIMITATIONS 1.

Downcomer flood. Percent of downcomer flood is the criterion that determines how close a tower is to flooding as a result of excessive liquid height in the downcomer. Downcomer filling (hd) is defined as the clear liquid height in the downcomer. As Figure 19 shows, it is composed of the tray pressure drop (ht) across the tray immediately upstream of the downcomer being considered, the head loss under the downcomer (hud), the inlet head (hi) on the tray, and the head loss due to two-phase flow through the downcomer (hdc). If there is no inlet weir on the tray, the inlet head will be the same as the clear liquid height (hc) on the tray. If an inlet weir is present, downcomer filling will increase due to the weir height, the crest over the weir, and added pressure drop of the liquid flowing between the downcomer apron and the inlet weir. The tray pressure drop (ht) is composed of the dry tray pressure drop (hed) and the clear liquid height (hc). Each of the pressure drops (or heads) is expressed in inches (mm) of hot clear liquid.

➧

FIGURE 19 DOWNCOMER FILLING COMPONENTS (STATIC PRESSURE BALANCE)

hc hed

ht

ht = hed + hc

hdc hud hd ht

hi

hc

Inlet Weir

hi = hc where there is no inlet weir

DP3AF19

The above calculation procedure expresses the downcomer filling in inches (mm) of clear liquid. Since the liquid enters the downcomer as a froth, the actual fluid level in the downcomer will be higher than the calculated clear liquid filling. The exact height is dependent on the average froth density in the downcomer. As the liquid travels downward in the downcomer, the vapor disengages and escapes out the top of the downcomer. If the downcomer is sized properly, the liquid leaving should be essentially clear liquid. Thus, there is a froth density gradient down the downcomer that ranges from the froth density on the tray (at the top) to partially to totally clarified clear liquid at the bottom.

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

2.

➧

100% of downcomer flood and 100% downcomer froth backup both occur when the froth height in the downcomer is equal to the distance from the bottom of the downcomer to the top of the outlet weir. At this point, an incremental increase in downcomer filling would result in downcomer froth backing up to the tray above. Percent downcomer froth backup is the froth height in the downcomer divided by the distance from the bottom of the downcomer to the top of the outlet weir. Percent downcomer flood represents the ratio of the actual vapor and liquid feed rates to the feed rates that would result in 100% downcomer froth backup. Therefore, percent downcomer froth backup is simply a measure of downcomer froth filling, whereas percent of downcomer flood implicitly takes into account the effect of both liquid rate and vapor rate as well. If percent downcomer froth backup or percent of downcomer flood exceeds the design limit, then the designer must take steps to reduce ht, hud, hi, or hdc. While many variables determine the values of these head losses, downcomer clearance, hole area, downcomer inlet and outlet chord lengths, and outlet weir height are the most significant hardware parameters that can be modified to meet downcomer flood and downcomer backup requirements. If this cannot be done, then either multi-pass trays should be considered or the tray spacing should be increased. If neither of these steps corrects the problem, consider using packing and consult your FRACTIONATION SPECIALIST. Secondary limitations. While downcomer flood and downcomer backup limits should always be met to ensure successful tower operation, there are additional criteria that should be met whenever possible. Consult your FRACTIONATION SPECIALIST if any of the following secondary criteria can not be met. a. Liquid rate per inch of outlet weir. The accuracy of the jet flood and downcomer flood correlations can only be ensured within the range of operating conditions used to develop the correlations. For sieve and bubble cap trays when jet flood is the limiting flood mechanism, the liquid rate should be in the range 1.5 - 15 gpm/inch of weir (3.7 - 37 3 dm /s/m of weir). When downcomer flood is the limiting flood mechanism, the liquid rate should be in the range 2.3 3 17.5 gpm/inch of weir (5.7 - 43.5 dm /s/m of weir). b. Downcomer choking. If the downcomer inlet area is too small and the froth on the tray cannot readily enter the downcomer, the froth height will increase in the contacting area. This height will continue to increase until there is sufficient head to “force” the froth into the downcomer or until the froth reaches the tray above, causing flooding. When the downcomer is bridged over at the entrance with froth, it is called downcomer choking. Since downcomer choking can cause flooding, it must be avoided. Data analysis has also shown that downcomer backup may work in combination with downcomer choking to cause flooding. For this reason, a limit is also placed on the geometric mean (square root of the product) of percent downcomer backup and percent downcomer choking. c. Velocity under downcomer. A high downcomer outlet velocity produces a channeling effect, in which liquid flow from the downcomer is unevenly distributed on the tray deck. Instead of generating a uniform liquid velocity profile over the entire bubble area, the sides of the tray deck hold stagnant liquid due to the overwhelming velocity component in the center of the liquid flow path. Rising vapor from the tray below flows through the sides of the tray deck, which represent the path of least resistance to vapor flow. This vapor and liquid maldistribution results in poor vapor-liquid contact on the tray and poor separation efficiency. This may also lead to premature weeping in the middle of the tray, which further decreases efficiency. Additionally, high velocity under the downcomer with high vapor loading has been shown to produce a rooster tail effect (froth height is much greater at the outlet side of the tray) that promotes outlet side flooding. If the velocity under the downcomer exceeds the specified limit, increase downcomer clearance or increase the downcomer bottom area. d. Downcomer seal. If a downcomer is not sealed, some vapor will bypass the tray and flow upward through the downcomer, resulting in a drop in tray efficiency. To avoid this problem, the designer should make certain that the downcomer is sealed at minimum rates (turndown conditions at minimum loaded tray) for new designs and revamps. For multi-pass trays, the downcomer seal must be checked for every tray pass. While a downcomer may be sealed by mechanical means (see Figure 11), it can also be sealed by process means. That is, if the sum of the clear liquid height (hc) and the head loss under the downcomer (hud), plus 1/4 of an inch (6 mm) for allowable unseal, equals or exceeds the downcomer clearance (c) the downcomer is said to be sealed [hc + hud + 1/4 in. (6 mm) ≥ c]. For most normal designs, with 50% turndown, if the downcomer is sealed at the minimum rates it is usually sealed at design rates. However, if the minimum rates have a different liquid to vapor ratio (L/V) than the design rates, it is advisable to check for both cases since hc (for sieve trays) is dependent on both liquid and gas rates. For liquid rates < 1.5 gpm/in. 3 of weir/pass (3.7 dm /s/m of weir/pass) an inlet weir should be considered. If downcomer sealing criteria can’t be met, consult your FRACTIONATION SPECIALIST to determine the impact on your particular design\

OTHER PROCESS CONSIDERATIONS 1.

Tray Efficiency. The purpose of the brief discussion below is to acquaint the designer with the key variables that affect tray efficiency. A more fundamental discussion is provided in Section III-I, Tray Efficiency. Tray capacity and tray efficiency are the two most important criteria in tray design. While the diameter of a tower is mainly determined by capacity considerations, tray efficiency determines the number of actual trays, and therefore the height, of the tower. ExxonMobil Research and Engineering Company – Fairfax, VA

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

FIGURE 20 EFFECT OF WEEPING ON EFFICIENCY

Good Operation Tray Efficiency

Fractional Weepage (%)

2.

To achieve good efficiency the vapor must be dispersed in the liquid phase intimately and kept in contact long enough for sufficient mass transfer to occur. This means that we must be able to calculate the liquid and vapor residence times on the tray as well as the amount of interfacial area generated. The vapor residence time is the time required for the vapor to flow through the volume of froth on the tray. Likewise, the liquid residence time is the time required by the liquid to flow through the volume of froth on the tray. Both of these variables are dependent on liquid and gas rates as well as the weir height and bubble area on the tray. Efficiency is also affected by physical properties, such as the vapor and liquid diffusivities, but such parameters cannot be changed through tray hardware changes. To achieve good efficiency, the designer must optimize the weir height, open area, bubble area, liquid flow path length, number of liquid passes, and other variables. Likewise, excessive weeping, entrainment and operation in the spray regime must be avoided. The only practical way to perform this optimization is by using a computer program, such as the PEGASYS - Sieve Tray Design and Rating Program, which is accessible from the PEGASYS Fractionating Tower Menu. Tray turndown. Turndown (or flexibility) is the term used to define the range of loadings over which acceptable tray performance is achieved. This usually means the range over which the tray efficiency stays at or above the design value (see Figure 20). Turndown ratio is defined as the ratio of the capacity factor at design conditions to the capacity factor at minimum turndown conditions. As the efficiency curve in Figure 20 shows, there is a relatively flat portion of the efficiency curve where design (or better) efficiency is obtained. At low vapor rates, however, excessive weeping decreases efficiency whereas at high vapor rates (above 85% of flood) excessive entrainment decreases efficiency. These falloff points generally correspond to 20% fractional weeping and 10% entrainment. An alternative to changing trays to obtain turndown capability is to operate the distillation column at higher reflux or boilup rates at the turndown conditions to compensate for the loss in efficiency. This can be particularly advantageous if operation at turndown conditions is anticipated to occur fairly infrequently (which is typically the case).

20% Weepage

Vapor Rate

DP3Af20

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

3.

4.

5.

The left hand sketch in Figure 21 shows the fractional weepage and entrainment curves for a typical sieve tray with a moderate to high liquid rate. The range of vapor rates over which the weeping is below 20% and entrainment is below 10% provide a second way to look at tray turndown. This is also referred to as the tray's operating window. Sieve trays can usually be designed to provide a turndown ratio of 2/1 to 3/1. If the liquid rate on a tray is low [say below 1.5 gpm/in. of weir/pass (3.7 dm3/s/m of weir/pass)], the operating window on the tray is extremely small or non-existent. This is shown on the right hand sketch in Figure 21. Designing trays to fractionate under these conditions is extremely difficult and your FRACTIONATION SPECIALIST should be consulted for help. Also see Section III-B, BASIC DESIGN CONSIDERATIONS - Froth to Spray Regime Transition for further background. If a sieve tray cannot be designed to meet the required turndown, then valve trays with their higher turndown capabilities should be evaluated (greater than 3/1). Since these trays are only marginally more expensive than sieve trays (by 0-10%) their use can frequently be justified for high turndown cases. Foaming. Foaming can occur in distillation towers via several different mechanisms including: a. The presence of surface active materials b. The presence of solids c. Entrainment of hydrocarbon liquids into aqueous systems d. Condensation of hydrocarbon vapors into aqueous systems e. When a second liquid phase is about to form. Foam is different than a froth primarily because it takes far longer for the foam to collapse. Foam also typically has a higher amount of vapor compared to a froth. For example, in a laboratory foaming test, the collapse time of a froth is low, usually less than 5 seconds. On the other hand, a foam can persist much longer, sometimes for minutes or even hours. Thus, a foaming system in a tower will begin to entrain at lower loadings and will not readily disengage in the downcomers. To design for foaming, the tray is usually oversized by using a lower percent of jet flooding and downcomer flood, a low dry tray pressure drop, a low downcomer entrance velocity, and a reduced allowable downcomer filling. A radius tip and large downcomer clearance are often specified. “Standard" foaming guidelines are provided for various tray types in their respective Subsections in this manual. Since the degree of foaminess varies and is generally unpredictable, experience in similar towers may be used instead to set some or all of these hydraulic criteria. In some severely foaming situations, flooding can still occur even when the above guidelines are followed. Operating engineers should then consider process changes to eliminate the foaming or determine the cause and remove the source of impurity causing the foam. If these changes do not eliminate the problem, the use of anti-foam agents may provide the only solution. Although usually effective, especially for short-term relief, users should be aware that anti-foam agents are often costly and may adversely affect product purity. Fouling. Fouling is the buildup of any type of solid deposit on a tower internals device. Fouling eventually plugs or reduces the effective size of an orifice or opening, resulting in diminished performance (efficiency, capacity, etc.) or even complete inoperability. Tower internals for fouling services must be specified to minimize fouling and to provide easy accessibility for periodic cleaning of the device. Refer to Table 3D for relative fouling resistance of fractionation devices. Since fouling is strongly system dependent, the designer should contact the FRACTIONATION SPECIALIST for past experience and guidance. Additional hydraulic considerations: a. Weeping. At low vapor velocities, the dry tray pressure drop of the tray is insufficient to support the liquid head on the tray (hc) and some liquid begins to flow intermittently through the vapor openings. The point at which this liquid bypassing begins is called the "weep point." As the vapor rate is decreased further, more liquid pours through the holes and continuous weeping occurs. While the total quantity of liquid that weeps is constant at a given vapor rate, the weep rate per hole is sporadic. That is, some holes are in the weeping mode while others are in the vapor bubbling mode. Thus, at any instant in time a given hole may be bubbling, weeping, or doing neither. This occurs on a random basis across the contacting area of the tray. While weeping can occur on all tray types, the designer is concerned about its effect primarily on sieve tray performance, since this is the most widely used tray in ExxonMobil plants. Since weeping occurs only at reduced rates, it is the major factor in determining tray turndown (i.e., the range of vapor loadings over which acceptable fractionation is achieved. See Tray Turndown discussion). For sieve trays, this usually means a ratio between 2/1 to 3/1. As Figure 20 shows, as vapor rate decreases, weeping increases very rapidly. Data have shown that above 20% fractional weepage, tray efficiency begins to decrease sharply.

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont)

b.

c.

d.

e.

Therefore, for most designs, the lower operating limit is reached when fractional weepage exceeds 20%. By using the correlations presented in Section III-I (Tray Efficiency) the falloff in efficiency can be predicted for any value of fractional weepage. However, the designer should reduce weeping as much as possible by reducing hole area (new designs) or by blanking holes (on revamps). Dumping. When all the liquid flows through the holes on a tray (i.e., no liquid flows over the weir) dumping is said to occur. Because the efficiency is extremely poor and the products produced will be off-spec, trays should not be operated in the dumping region. Dry tray pressure drop (hed). This represents the energy expended by the vapor as it flows through the contacting device (sieve hole, valve element, etc.) as if no liquid were present, i.e., the tray is “dry.” To increase this pressure drop, the area available for vapor flow (hole area) must be decreased. To lower the pressure drop, the hole area must be increased. Since the dry tray pressure drop is proportional to the square of the vapor velocity, small changes in open area can produce large changes in pressure drop. The dry tray pressure drop must be high enough to provide good contacting between the vapor and the liquid at both design and turndown rates. However, it must be low enough at design rates to prevent excessive entrainment, operation in the spray regime, or excessive downcomer filling. On the other hand, if the designer chooses a hole area which is too large, there may be insufficient pressure drop at turndown conditions to prevent liquid from weeping through the holes. Thus, the designer must select a hole area which satisfies both turndown and design rate operations. The usual approach is to design the tray for maximum rates and then check it for turndown conditions. However, if changes in the dry tray pressure drop cannot be made that will satisfy all operating conditions, a more flexible device (such as valve trays) may be required. Tray pressure drop (ht). As the vapor flows through the contacting zone on a given tray, it must overcome two resistances in series. The first is the dry tray pressure drop (hed) and the second is the clear liquid height (hc). The tray pressure drop (ht) is the sum of hed + hc. The tray pressure drop is critical in tray design since it is one of the major components of downcomer filling and therefore downcomer flood. It is especially critical, however, for vacuum systems where minimum pressure drop is required. Clear liquid height (hc). This is the height of liquid on a tray, expressed in inches (mm) of hot liquid. It is the second resistance the vapor must overcome as it passes through the tray. The clear liquid height is a function of the liquid rate and outlet weir height. The clear liquid height must be high enough to provide sufficient contact time between the liquid and the vapor for mass transfer to occur. Excessive clear liquid heights should be avoided, however, since they increase the tray’s pressure drop, increase downcomer filling, and may cause premature weeping. If the tower is heavily liquid loaded and hc is too high, consider increasing the number of liquid passes to reduce the liquid rate per pass.

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CROSS FLOW DEVICES - PROCESS DEFINITIONS (Cont) FIGURE 21 EFFECT OF LIQUID RATE ON SIEVE TRAY TURNDOWN No Operating Window (Low Liquid Rate)

40 30

Good Operating Range (Good Turndown)

20 10

No Operating Range (Poor Turndown) Fractional Entrainment (%)

50

Fractional Weepage (%)

60

Fractional Entrainment (%)

Fractional Weepage (%)

Broad Operating Window (Mod. to High Liquid Rate)

60 50 40 30 20 10

Vapor Rate

Vapor Rate

Note: 20% fractional weepage and 10% entrainment is the maximum allowed to achieve good efficiency. DP3Af21

6.

7.

Heat Transfer. The most frequent application for trays in heat transfer service is in heavy hydrocarbon fractionators such as atmospheric pipestilIs, cat fractionators, steam cracker primary fractionators, etc. Since these applications usually entail handling high liquid rates, jet trays are the tray of preference for these pumparound circuits. While single pass jet trays would be adequate for most applications, two pass jet trays may be required to avoid costly transitions to accommodate the trays above and below the pumparound. If the liquid rate is low [say < 4 gpm/in. of diameter/pass (10 dm3/s/m of diameter/pass)] sieve trays or valve trays should be considered. After using Table 5A to select the tray type, the appropriate Subsection should be used to design the tray. Then, Section III-F, Heat Transfer should be used to determine how many actual trays are required. For services where pressure drop is critical, packing is usually used because of its much lower pressure drop. This is especially true in vacuum pipestills and other vacuum distillation applications. This subject is discussed further under COUNTER-CURRENT DEVICES - PACKING. Overall Flood. In reality, flooding mechanisms do not act independently. Instead, flooding is usually caused by a combination of effects, resulting in a continuous flooding curve (Figure 17 and Figure 22). For this reason, an "overall flood" correlation has been developed for cross flow fractionation devices. Overall flood is a combination of jet flood, downcomer flood, and ultimate capacity, and depends primarily on the limiting flood mechanism (that is, the flood mechanism with the highest percent of flood). If overall flood exceeds the design limit, actions must be taken to reduce the limiting flood mechanism. It is important to note that overall flood does not take into account every possible flooding mechanism, so all other design criteria must also be met to ensure successful design.

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CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONS As discussed earlier a fractionating tray must be operated within a certain range of vapor and liquid rates to give optimum performance and an economical design. Outside this range, efficiency drops off and/or the tower becomes inoperable. The effects of vapor and liquid rates on tray performance are depicted schematically on Figure 22. These performance limitations are summarized below. ➧

FIGURE 22 TYPICAL SIEVE TRAY PERFORMANCE DIAGRAM

o ent Flo ntrainm (E d o Jet Flo

Vapor Rate

iv e es s Exc

rain E nt

m en

d)

t Do wn co m er

Fl oo d

Area of Normal Operation o Weep P

int

Moderate Weeping

Dum p

ing

Heavy Weeping

0 0

Liquid Rate

DP03Af22

Maximum Vapor Rate Considerations. A very high vapor rate may cause: 1. Jet flooding, excessive entrainment, spray regime operation, or exceeding the ultimate capacity of the tray. 2. High pressure drop across the tray, resulting in excessive downcomer filling and subsequent downcomer flooding. Minimum Vapor Rate Considerations. A very low vapor rate may cause: 1. 2.

Weeping or dumping. Poor contacting and tray efficiency because of inadequate vapor/liquid mixing. These conditions can result from insufficient vapor loading or from excessive open area on the tray, both of which produce a low vapor velocity through the tray openings. Maximum Liquid Rate Considerations. High liquid rates may cause: 1 . Tray flooding (due to insufficient disengaging in the downcomers), excessive tray pressure drop, and excessive downcomer filling. 2. Tray flooding due to excessive downcomer entrance or exit velocity and downcomer bridging. Minimum Liquid Rate Considerations. Low liquid rates may cause: 1. 2.

Spray regime operation at high vapor rates. Vapor bypassing up the downcomers, if the downcomer is not sealed. ExxonMobil Research and Engineering Company – Fairfax, VA

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CROSS FLOW DEVICES - BASIC DESIGN CONSIDERATIONS (Cont) 3.

Poor contacting and low tray efficiency, because of inadequate liquid residence time on the tray due to operation in the spray regime. To avoid these problems, the correct combination of tray type, tower diameter, tray spacing, free area, downcomer size, hole area and number of liquid passes must be utilized. This optimum can be achieved with correct use of the PEGASYS design program.

CROSS FLOW DEVICES - GENERAL CONCLUSIONS For most applications, sieve trays will be the first choice. If high turndown (>3:1) is needed, valve trays should be considered. For most heat transfer services (pumparounds) in atmospheric pressure columns, jet trays are preferred. A summary of the key parameters for each of the major tray types used by ExxonMobil is provided in Table 1. Table 2 provides a similar summary for the counter-current devices to be discussed later in this Subsection. In addition, to aid the designer in selecting the best internal for a given application, a series of "Decision Trees” (in the form of Tables) have been prepared for the design objectives outlined below. Table 3 - Tower Internals Selection for New Towers Table 4 - Tower Internals Selection for Revamps Table 5 - Tower Internals Selection for Heat Transfer Service Table 6 - Tower Internals Selection for Entrainment Removal Devices Once the design has been completed, the engineer should review Table 7, Tower Design Checklist (Trays) to be sure that no major point has been overlooked.

COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) EQUIPMENT TYPES AND APPLICATIONS 1.

PACKING (See Section III-G for more details.) Although a packed tower design may result in a smaller diameter tower, the total cost of the installation with packing, packing supports, and distributors/re-distributors is more expensive than a trayed tower. In addition, packed towers are more sensitive to liquid and vapor maldistribution, and packing and liquid distributors are less tolerant to fouling than trays. New high capacity trays should be considered first when revamping an existing tower as the revamp costs and time are much lower than converting a trayed vessel into a packed tower. Therefore, packing is usually justified only for: •

Applications where pressure drop across the internals is critical, such as in vacuum distillation or in some gas cleanup units handling recycled gas that requires compression.

•

Corrosive but non-fouling services where ceramic packings are more economical than alloy trays.

•

Towers less than 3 ft (900 mm) in diameter where packing and tray costs are nearly equal.

•

Sidestream strippers where increased efficiency can reduce steam consumption and provide ENCON credits.

•

Revamps where an acceptable tray design cannot be achieved.

•

Foaming systems, such as demethanizers, TEG Contactors, and some amine units.

•

➧

Applications for which a reduction in height, weight, or footprint is desired (such as offshore towers, or towers susceptible to swaying conditions) a. Random packings (also called dumped packings) are the most frequently used counter-current devices. Their name derives from the fact that they are dumped into the column and orient randomly. Random packings come in a number of different shapes, sizes and materials of construction (Figure 23). Basically, as the packing size increases, the capacity increases while the pressure drop, cost, and efficiency decrease. Thus, for a given design, there is an optimum economic combination of packing size, tower diameter and tower height. For Pall rings, past studies have shown that the 1.5 to 2 in. (38 - 50 mm) ring size usually provides the optimum design. Several other packings provide improved performance characteristics. These include Koch-Glitsch’s Cascade Mini Ring (CMR), Saint-Gobain NorPro's Intalox Metal Tower Packing (IMTP) also known as Metal lntalox, Sulzer Chemtech's Nutter Ring, and UOP Raschig Super-Rings. Some of these are depicted in Figure 23. There are a number of other packing types available but they are not widely used in ExxonMobil towers and therefore have not been included here. ExxonMobil Research and Engineering Company – Fairfax, VA

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COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) (Cont) FIGURE 23 RANDOM (DUMPED) PACKINGS PALL RING

KOCH-GLITSCH METAL CMR

NOR PRO METAL INTALOX (IMTP)

NUTTER (SULZER) RING

DP3AF23

➧

b.

Structured packings. These devices are fabricated in bundles from crimped sheet metal and installed in the tower in layers having a fixed orientation. Structured packing provides more effective surface area per unit volume and is more efficient than random packing. Due to competitive pressures, costs are now similar between random and structured packing. Since the crimp height can be changed, the capacity, efficiency, pressure drop and cost can also be varied. Thus, the optimum choice must be determined by an economic study. Of all the contacting devices available, structured packing provides the lowest pressure drop per theoretical stage of contacting as well as the best capacity / efficiency combination. This feature makes it especially attractive in vacuum towers and as a high capacity revamp option in other low pressure towers. Structured packing is not recommended 2 3 2 for use in high pressure distillation applications or for liquid rates above 20 gpm/ft (13.6 dm /s/m ), unless the application is a high pressure aqueous system. See Section III-G for details. There are several suppliers including: Flexipac and Gempak by Koch-Glitsch, Intalox Structured by Saint-Gobain NorPro, Montz-Pak by Montz, and Mellapak and MellapakPlus by Sulzer Chemtech. The Sulzer MellapakPlus series structured packings have comparable efficiency and higher capacity than the Mellapak series. ExxonMobil experience with MellapakPlus is limited, and a FRACTIONATION SPECIALIST should be consulted for specific applications. An example of structured packing, by Koch-Glitsch, Inc., is shown in Figure 24. FIGURE 24 STRUCTURED PACKING (BY KOCH-GLITSCH, INC.)

DP3Af24

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COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) (Cont) 2.

➧

GRIDS (also see Section III-G) Grids are similar to structured packing in that they are fabricated in panels and installed in an ordered manner. However, their efficiency characteristics are much poorer due to their high open area and low surface area per unit of volume. The first grid to appear on the market (circa 1961) was Glitsch grid. It was intended for use in services where entrainment removal was critical but where fouling was too severe to use crinkled wire mesh screens. This made it ideal for use in wash zones of vacuum pipestills, cat fractionators, etc. Because of the grid's large physical openings, it has demonstrated good run lengths in these fouling services. However. these large physical openings, plus a relatively low specific surface area, give it the performance characteristics of a very large size packing. Therefore, it has a very high capacity and low pressure drop. Its efficiency, however, is very low (about 50% of that provided by the 2 in. [50 mm] Pall ring). In recent years, several new grids have come on the market. They are Flexigrid #2 and #3 (and newer styles) by KochGlitsch, SNAPGRID #3 and Mellagrid by Sulzer Chemtech, and Intalox grid by NorPro. Intalox grid is considered functionally equivalent to Glitsch Grid EF-25A. ExxonMobil experience with Mellagrid is limited, and a FRACTIONATION SPECIALIST should be consulted for specific applications. Pictures of these major grids are shown in Figure 25.

➧

FIGURE 25 VARIOUS TYPES OF GRIDS KOCH-GLITSCH FLEXIGRID STYLE 2

KOCH-GLITSCH EF-25A

SULZER MELLAGRID

SULZER SNAPGRID

Single Sheet

As Installed DP3AF25

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COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) (Cont)

3.

Because of their high capacity and low pressure drop grids have also been used in heat transfer sections (pumparounds) of vacuum pipestills and other heavy hydrocarbon fractionators. The liquid is introduced on the top layer of grid via spray nozzles. In revamping pumparounds, a split bed may be required. That is, the grid is used in the bottom portion of the bed for capacity reasons while a dumped or structured packing is used on the top where the loadings are lower, to maximize efficiency. BAFFLE SECTIONS (also see Section III-J) There are two basic types of baffle sections - sheds, and disc and donuts. These devices operate differently than grids or packing. In baffle sections, the liquid cascades from baffle to baffle in the form of liquid curtains. As the vapor flows through these curtains, the liquid is broken up into droplets and mass transfer occurs. However, this is a very inefficient liquid/vapor contacting mechanism and produces very low efficiency. These devices are sketched in Figure 26. FIGURE 26 DIFFERENT TYPES OF BAFFLES Sheds

Disc and Donut

Disc Donut

DP3AF26

Because of their high open area and large physical dimensions they are ideally suited for:

•

Slurry sections of cat fractionators and coker scrubbers where high temperatures and solids are present, a severely fouling service.

•

➧

4.

Condensable blowdown drums and water quench towers where large volumes of liquids must be handled and solids are sometimes present. For severe fouling services, baffle sections are about the only internal available if long run lengths are required. Because of their high open area, they have high capacity but very poor efficiency. Thus, baffle sections require a disproportionate amount of tower straightside for the functions they perform. DUALFLOW TRAYS (also see Section III-L) Dualflow trays are basically sieve trays without downcomers; thus, the entire cross-sectional area of the tower is available for vapor and liquid flow. Unlike sieve trays, however, the vapor and liquid flow through the same holes on a periodic basis. It is very important to ensure that dualflow trays are installed level to the ground to prevent vapor / liquid bypass. Dualflow trays are useful primarily for revamping heavily liquid loaded towers. Although they have high capacity, their poor turndown and low efficiency (vs. sieve trays) make them unattractive for new designs. In addition, they can exhibit poor stability in large diameter towers.

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COUNTER-CURRENT DEVICES TYPES AVAILABLE (PACKING, GRIDS, BAFFLE SECTIONS, DUALFLOW TRAYS) (Cont) There are several types of dualflow trays on the market. The most common consists of a flat tray deck containing round sieve holes. The Ripple tray, developed by Stone and Webster is a flat, perforated tray that has been crimped to form a sine wave in the tray panel. The third device is the proprietary Shell Oil Turbogrid Tray which is a flat-surfaced tray whose openings are long, rectangular slots. These devices were all developed to maximize tower throughput. Since the industry only has reliable data for the FRI-type dualflow tray, that is the one currently being recommended for ExxonMobil revamps although Ripple trays are applied in some ExxonMobil towers. ExxonMobil has not used the Shell Turbogrid Tray. If a tower to be revamped contains any type of dualflow tray, please contact your FRACTIONATION SPECIALIST for advice.

COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS VAPOR / LIQUID CAPACITY LIMITATIONS 1.

Flooding. Dumped and structured packings as well as grids have similar capacity characteristics. Their vapor handling capacity is determined by the specific packing or grid type, size, liquid loading, and system physical properties (surface tension, density, and viscosity). a. Packed towers. Unlike trays, flooding is harder to define in a packed tower. There is no tray spacing or downcomer to fill with liquid. What does occur is that liquid begins to accumulate in the packing and the pressure drop begins to rise more sharply. This is known as the load point. With further increases in vapor rate, the pressure drop rises almost vertically and liquid begins to “pile up” on the top of the packing. This has been observed visually at Fractionation Research Incorporated (FRI). Like trays, however, as liquid begins to accumulate in the packing and backmixing occurs (analogous to entrainment on trays), the efficiency (HETP) becomes poorer (see efficiency discussion below). b. Baffle sections. As the vapor rate is increased, the curtain of descending liquid is steadily raised until it is nearly horizontal. Simultaneously, liquid begins to pile up on the top surface of the shed or disc and donut element. Further increases in rate cause entrainment to occur from a given element to the element above and flooding begins. The capacity of these devices is likewise dependent on tray spacing, percent open area, curtain area, liquid rate, and system physical properties. These variables are discussed more fully in Section III-J. c.

2.

Dualflow trays. These trays flood in a manner similar to sieve trays; that is, the intertray space becomes completely filled with a dense froth which is entrained from tray to tray. For a given diameter, their capacity is a strong function of tray spacing and percent hole area and a weak function of hole size and liquid physical properties. Ultimate capacity (see discussion under CROSS-FLOW DEVICES - PROCESS DEFINITIONS). All of the countercurrent devices can also flood due to ultimate capacity. This is especially true for baffle sections and dualflow trays, even at low operating pressure. This occurs because these devices inherently have a very high capacity and thus operate closer to an ultimate capacity limitation to start with. Each device should be checked for this limitation by using the equations given in the appropriate Subsection of Design Practice III.

EFFICIENCY AND TURNDOWN 1.

2.

3.

Dumped and structured packings. These devices provide the highest efficiency per unit of pressure drop (i.e., lowest pressure drop per theoretical stage). Small packing sizes have the highest efficiency, pressure drop and cost, but have the lowest capacity. Thus the designer must choose the optimum combination for a given design case. To ensure optimum efficiency at all rates, a high quality liquid distributor must be used. In fact, the distributor’s turndown usually limits before that of the packing. For most applications, a turndown of 2/1 is specified. Higher turndown, say 4/1, will result in a more expensive distributor. The selection of a liquid distributor is critical in the design of a packed bed. This topic is discussed extensively in Section III-G. For most applications, dumped packings should be considered first because they are lower cost. If a low pressure drop per theoretical stage and/or a short column height is required, then structured packings should be evaluated as an option. Grids. Because of their high open area and relatively small amount of surface area per unit volume, their efficiency is quite low [about 50% of 2 in. (50 mm) Pall rings]. Since they are normally only used in fouling services (wash zones) or in pumparounds, their low efficiency can usually be accepted. For most grid applications, long run length and/or low pressure drop are the two key features required. Turndown is usually limited by the spray nozzle distributors to about 2/1. Baffle sections. The efficiency of these devices is usually quite poor. They provide little interfacial area for vapor/liquid contacting. Their only redeeming feature is resistance to fouling and thus good run length. Their turndown characteristics are poor (about 1.25/1). However, acceptable operation can usually be maintained in pumparound service when the liquid rate can be kept at or near design flow.

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COUNTER-CURRENT DEVICES - PROCESS DEFINITIONS (Cont) 4.

Dualflow trays. This device has a peak efficiency only about 80% as high as that provided by a sieve tray. In addition, they have a low turndown ratio of about 4/3, or 1.33/1. Their primary use is in debottlenecking relatively small diameter columns where turndown is not a major concern. However, they are very infrequently used.

HEAT TRANSFER The heat transfer characteristics of each of the counter-current devices parallels that of their mass transfer capabilities (see discussion under EFFICIENCY AND TURNDOWN immediately above). For a more thorough discussion on heat transfer, refer to Section III-F. Section III-F also provides the necessary correlations for each device to design a heat transfer section.

OTHER CONSIDERATIONS 1.

➧

2.

3.

Liquid, vapor, and mixed phase inlets. When working with any of the packings or grids, great care must be taken to ensure adequate vapor and liquid distribution. This is true because there is very little pressure drop to help correct any vapor maldistribution problem. Likewise, the liquid will not be redistributed since most of the newer packings have poor spreading characteristics. For today's packings, a high quality liquid distributor is essential. Furthermore, if there is a mixed phase inlet present, the two phases must be separated before the liquid is distributed to the packing. Otherwise, adverse kinetic effects and disengaging problems will almost guarantee that the packed bed will perform poorly. Baffle sections usually employ a perforated pipe to distribute the liquid to the various shed elements. In the past this has not posed a problem, but it should be remembered that the fractionation requirement (and thus the need for good liquid distribution) is not critical for services where these devices are normally used. Dualflow trays require good liquid distribution on the top tray deck; otherwise, vapor/liquid maldistribution will occur. Dualflow trays also require good leveling and the decks must be flat. Fouling. The relative fouling resistance of common fractionation devices is shown in Table 3D. Since fouling is strongly system dependent, the designer should always contact the FRACTIONATION SPECIALIST for past experience and guidance. Corrosive services. For many years, dumped ceramic packings have been used in highly corrosive services because of their low cost. Today, plastic packings are finding increasing use in these services. Since the amount of corrosion is strongly system dependent, the designer should always contact the FRACTIONATION SPECIALIST for guidance on internals selection first. Then, the appropriate MATERIALS SPECIALIST should be consulted to optimize the materials of construction as needed.

COUNTER-CURRENT DEVICES - GENERAL CONCLUSIONS For most fractionation applications, sieve or valve trays remain the best overall internals choice. However, if packing is needed for process reasons, dumped packings of the 2 in. (50 mm) size will usually prove most economical. When low pressure drop per theoretical stage and/or column height restrictions apply, structured packing should be considered. Structured packing should also be considered for high capacity revamps of hydrocabon distillation towers with operating pressures of less than 100 psia (690 kPa). For highly fouling services, baffle sections or one of the grids should be considered. For highly corrosive services, a packing made of ceramic or plastic material should be considered. To aid the designer in selecting the best internal for a given application, a series of "Decision Trees" (in the form of Tables) have been prepared for the objective outlined below. Table 1 - Trays - A Summary of Characteristics Table 2 - Counter-Current Devices - A Summary of Characteristics Table 3 - Tower Internals Selection for New Towers Table 4 - Tower Internals Selection for Revamps Table 5 - Tower Internals Selection for Heat Transfer Service Table 6 - Tower Internals Selection for Entrainment Removal Service Once the design has been completed, the engineer should review Table 8, Tower Design Checklist (Packing) to be sure that no major point has been overlooked.

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➧

NOMENCLATURE ft2

(m2)

Ab

=

Bubble area,

Ad Adi

= =

Downcomer area, ft2 (m2) Total downcomer inlet area, ft2 (m2)

(see Figure 12)

A *di

=

Total downcomer inlet area on inboard tray, ft2 (m2)

Ado

=

Total downcomer outlet area (downcomer bottom area), ft2 (m2)

* A do

=

Total downcomer outlet area (downcomer bottom area) on inboard tray, ft2 (m2)

Af

=

Free area, ft2 (m2) (superficial area minus arithmetic average of inlet and outlet area of downcomer(s) above the tray minus the waste area) (see Figure 13)

Ao As Aud Aw c

= = = = =

Hole/valve/cap/tab area, ft2(m2) Superficial (total) tower area, ft2 (m2) Area under the downcomer (downcomer escape area), ft2 (m2) Waste area, ft2 (m2) Downcomer clearance between tray and downcomer apron at tray inlet, in. (mm) (see Figure 3)

DT H HETP hc hd hdc hed hi ht hud hv, hi

= = = = = = = = = = =

Tower diameter, ft (mm) Tray spacing, in. (mm) Height equivalent to a theoretical plate, in. (mm) Clear liquid height on tray, in. (mm) of hot liquid Downcomer filling, in. (mm) of hot liquid Head loss due to two-phase flow through the downcomer, in. (mm) of hot liquid Effective dry tray pressure drop, in. (mm) of hot liquid Tray inlet head, in. (mm) of hot liquid Total tray pressure drop, in. (mm) of hot liquid Head loss under downcomer or splash baffle, in. (mm) of hot liquid Distance between downcomer step (for stepped design), or straight / sloped transition (for semi-sloped design), and the tray above, in. (mm) (see Figure 8)

hv, lo

=

hwo lfp lo

= = =

Distance between downcomer step (for stepped design), or straight / sloped transition (for semi-sloped design), and the bottom of the downcomer apron, in. (mm) (see Figure 8) Outlet weir height, in. (mm) (see Figure 3) Flow path length (distance between inlet and outlet downcomers), ft (mm) (see Figure 12) Outlet weir length, in. (mm) (see Figure 12)

P QL r

= = =

Pressure, psia (kPa abs) Liquid rate, gpm (dm3/s) at conditions Downcomer inlet rise (width) for chordal downcomers or downcomer inlet width for inboard downcomers, in. (mm)

COMPUTER PROGRAMS Once the contacting device has been selected, the designer should read the corresponding Section of Design Practice III. Each Subsection lists the various computer program(s) available under the heading COMPUTER PROGRAMS. Supporting documentation is also provided. For the various contacting devices, the PEGASYS computer program can be used for equipment design and rating.

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➧

TABLE 1 TRAYS - A SUMMARY OF CHARACTERISTICS TRAY TYPE

CAPACITY

EFFICIENCY

COST PER UNIT AREA

FLEXIBILITY *

REMARKS

Sieve

Medium to High

High. Equal to or better than other tray types.

Lowest of all trays with downcomers.

Medium. 23/1 can usually be achieved.

First choice for most applications; extensive design data available.

Moveable Valve

Medium to high; as good as sieve trays or better.

High. As good as sieve trays.

Medium. About 10% greater than sieve trays.

High. Possibly up to 5/1.

Not recommended for fouling services.

Fixed Valve

As good as sieve trays or slightly better for SVG on triangular pitch; LVG and fixed valve trays on square pitch have lower capacity. See Table 4A for capacity of other devices mentioned.

High. As good as sieve trays.

About the same as sieve trays.

Medium.

Do not use in fouling services when foulant material is “sticky” and entrained in vapor phase. See Table 3D.

Sulzer Mini V-Grid (MVG)

Medium to high; approximately 10-15% higher than sieve trays at pressures below 50 psia (345 kPa).

High. As good as sieve trays.

At least 5% higher than sieve trays.

Medium. Slightly higher than sieve trays.

Good alternative to sieve trays at low liquid rate where higher capacity is needed.

Koch-Glitsch Nye

High; approximately 10-15% higher than a sieve tray.

High. Slightly lower than sieve trays.

Medium; About 25% higher than sieve trays.

Medium. Almost as good as sieve trays.

Good alternative to sieve trays in tower revamps where higher capacity is needed.

Jet

Highest at low pressures and high liquid rates.

Low to medium.

Low to medium. About 5% higher than sieve trays.

Low. 1.5 or 2/1.

Consider only when liquid rate exceeds 4.0 gpm/in. of diameter per pass (10.0 dm3/s/m of diameter per pass).

Bubble Cap

Medium to high, except low to medium at high liquid rate.

Medium to high.

High. At least twice the cost of sieve trays.

3/1 to 4/1

Use for high flexibility or low liquid rate application where fouling of valve trays may be a problem.

UOP Multiple Downcomer (MD)

Very high. Estimated to be 30-40% higher than a conventional sieve tray for high liquid rate services.

Low to medium.

Higher than sieve or valve trays.

Low. (< 2/1)

Can be installed on very low tray spacings. Consider for revamps where no other device is acceptable. Not recommended for fouling services. Limited inspection access.

Sulzer SVG, LVG Koch-Glitsch V-0 NorPro Provalve MVG Others

NorPro Triton MVGT

UOP Enhanced Capacity Multiple Downcomer (ECMD) Sulzer Hi-Fi

* Ratio of maximum to minimum vapor loads at which tray efficiency remains above about 90% of its design value.

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➧

TABLE 2 COUNTER-CURRENT DEVICES - A SUMMARY OF CHARACTERISTICS DEVICE

CAPACITY

EFFICIENCY

COST PER UNIT AREA

FLEXIBILITY *

REMARKS

Random Packing (Pall Rings, Metal Intalox, Nutter Rings, etc.)

Medium.

Medium.

Medium to low, depending on material of construction.

> 3/1.

Good efficiency per unit of pressure drop. Mainly used in high liquid rate absorbers.

Structured Packing (Flexipac, Montz Gempak, Mellapak Intalox, etc.)

High to very high (large crimp).

High - depends on surface area.

Medium; depends on material.

> 3/1.

Best efficiency per unit of pressure drop. Selection of packing size allows trade-off of capacity and efficiency.

Glitsch Grid

Very high.

Poor as fractionation device. Good for entrainment removal and heat transfer.

Medium to high.

Low; less than 2/1.

Good for high vaporlow liquid service to minimize effect of entrainment. Used in wash zones of heavy hydrocarbon fractionators where moderate coking occurs.

Sheds and Disc-andDonuts

Very high.

Poor as fractionation device.

Medium.

Low. less than 1.5/1.

Used in severe fouling service; e.g., slurry pumparound in cat fractionator.

Downcomerless (Dualflow or Ripple) Trays

Highest in some instances.

Medium to good, at design liquid and vapor rates.

Lowest to medium; royalty on Ripple Tray, none on FRI Dualflow types.

Low.

Of interest for revamps if poor flexibility is tolerable.

Flexigrid Snapgrid Intalox Grid

*

Ratio of maximum to minimum vapor loads at which efficiency remains above approximately 90% of its maximum value. Packing flexibility is typically limited by distributor turndown, and can be up to 10/1 with appropriately designed distributor.

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➧

TABLE 3A TOWER INTERNALS SELECTION FOR NEW TOWERS FOULING TENDENCY?

SEVERE

NONE

SEE TABLE 3B

LIQUID RATE?

MODERATE

SEE TABLE 3B

≤1.5 gpm/inch*

>1.5 gpm/inch*

EFFICIENCY

EFFICIENCY

CRITICAL?

CRITICAL?

YES OR NO

YES OR NO

∆P CRITICAL?

NO

∆P CRITICAL?

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

NO

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 3/1

> 3/1

ALL CASES

≤ 3/1

> 3/1

SIEVE-POSSIBLE HIGH ENTRAINMENT - USE PICKET FENCE WEIRS

VALVE - IF < 0.25 gpm/in. (0.62 dm3/s/m)

STRUCTURED PACKING SPECIAL LIQUID DISTRIBUTOR REQUIRED

SIEVE

VALVE

CONSULT FRACTIONATION SPECIALIST

ALL CASES

PACKING **

DP3AT3A

* **

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems; otherwise use random packing.

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➧

TABLE 3B TOWER INTERNALS SELECTION FOR NEW TOWERS

FOULING TENDENCY?

SEVERE

MODERATE

NONE

ALL LIQUID RATES

LIQUID RATE?

SEE TABLE 3A

SHEDS

≤ 1.5 gpm/inch *

> 1.5 gpm/inch *

EFFICIENCY CRITICAL?

SEE TABLE 3C

NO

YES

LARGE HOLE SIEVE WITH PICKET FENCE WEIR, OR FIXED VALVE

∆P CRITICAL?

NO

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 2/1

> 2/1

≤ 2/1

> 2/1

GRIDS PLUS CONSULT FRACTIONATION SPECIALIST

CONSULT FRACTIONATION SPECIALIST

GRIDS

CONSULT FRACTIONATION SPECIALIST

DP3AT3B

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass

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TABLE 3C TOWER INTERNALS SELECTION FOR NEW TOWERS

FOULING TENDENCY?

MODERATE

LIQUID RATE?

> 1.5 gpm/inch *

≤ 1.5 gpm/inch *

EFFICIENCY CRITICAL?

SEE TABLE 3B

YES OR NO

∆P CRITICAL?

NO

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 2/1

> 2/1

LARGE HOLE DIAMETER SIEVE OR JET TRAY OR SVG OR PROVALVE

V-GRID OR PROVALVE TRAY

≤ 2/1

LARGE HOLE DIAMETER SIEVE OR SVG OR PROVALVE OR GRIDS

> 2/1

LARGE SIZE STRUCTURED PACKING OR DEEP GRID BED DP3AT3C

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass

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➧

TABLE 3D RELATIVE FOULING RESISTANCE OF COMMON FRACTIONATION DEVICES For a given fractionation device, a larger limiting hole / lift / path size corresponds to higher fouling resistance. Therefore, the list below is not universally valid, but instead represents a general guideline for the relative fouling resistance of devices based on the "standard" size of the specified device. In addition to the fractionation devices listed in Table 3D, the fouling resistance of other tower internals such as liquid distribtors must be considered. RELATIVE FOULING RESISTANCE Lowest

FRACTIONATION DEVICE * Caged valve tray with heavy foulant flowing on deck Floating valve trays Fixed valve trays (when sticky material entrained from below) Structured packings - low crimp height [0.5 in. (13 mm) or less] Dumped packings - small diameter [1.5 in. (38 mm) or less] UOP MD and ECMD trays, Sulzer Hi-fi tray Mini-valve trays or fixed valves with 0.25 in. (6.4 mm) lift or less Bubble cap trays - small [0.375 in. (9.5 mm) or less] skirt clearance Caged valve trays Sieve trays [0.5 in. (13 mm) hole diameter] Bubble cap trays - large [greater than 0.375 in. (9.5 mm)] skirt clearance

Moderate

Large fixed valve trays, such as Sulzer SVG (no sticky material entrained from below) NorPro ProValve tray deck Large hole sieve trays [≥ 0.75 in. (19 mm)] Jet trays Dumped packings - large diameter [~3.5 in. (90 mm)] Structured packings - large crimp height [~2 in. (50 mm)] Koch-Glitsch Flexigrid Style 3 or EF 25A or equivalent Koch-Glitsch Flexigrid Style 2 or equivalent Baffle sections (sheds; disc and donuts)

Highest

Open spray chamber

* Nye tray inserts can be designed to have the same fouling resistance as the associated tray deck.

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TABLE 4 TOWER INTERNALS SELECTION FOR REVAMPS

REVAMP OBJECTIVE?

MAXIMIZE CAPACITY LOSS IN EFFICIENCY TOLERABLE

SEE TABLE 4B

* **

INCREASE CAPACITY SAME OR BETTER EFFICIENCY

SAME CAPACITY IMPROVE SEPARATION

LIQUID RATE?

SEE TABLE 4C

≤ 1.5 gpm/inch *

> 1.5 gpm/inch *

STRUCTURED PACKING

∆P CRITICAL?

NO

YES

TURNDOWN RATIO REQUIRED?

PACKING **

≤ 3/1

> 3/1

SEE TABLE 4A

MULTIPASS VALVE TRAYS

DP3AT4

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems; otherwise use random packing.

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TABLE 4A APPLICATION GUIDELINES FOR DEBOTTLENECKING FRACTIONATION TOWERS(1) (2) RELATIVE CAPACITY INCREASE @ CONSTANT SEPARATION EFFICIENCY (3) 0 - 10%

PRESSURE

Under 50 psia (345 kPa)

Low

10 - 20%

• MVG

• Triton

• ProValve

• 2-Pass MVG

• SuperFrac (6)

• 2-Pass SuperFrac (6)

•2-Pass Nye (4)

• 2 Pass Nye (4)

•2-Pass Trays

• Random Packing

• Random Packing

• Structured Packing

20 - 30%

• Structured Packing

30% +

• Structured Packing

• Structured Packing 50 psia (345 kPa) Moderate

to 165 psia (1140 kPa)

High

Above 165 psia (1140 kPa)

• Nye (4) • ProValve • 2-Pass Trays • MD Trays • Random Packing

• 2-Pass Nye & Superfrac Trays (6) • Triton • 4-Pass Trays • MD Trays

• MD Trays • Hi-fi Trays

• Hi-fi Trays

• ECMD Trays

• ECMD Trays

• Structured Packing (5)

• Structured Packing (5)

• Random Packing

• Nye (4)

• 2-Pass Nye & SuperFrac Trays (6)

• 4-Pass Nye & Superfrac Trays (6)

• 4-Pass Trays

• MD Trays

• MD Trays

• ECMD Trays

• Random Packing (5)

• Hi-fi Trays

• ProValve • 2-Pass Trays • MD Trays • Random Packing (5)

• Structured Packing (5)

• Hi-fi Trays • ECMD Trays

Notes: (1)

Stainless steel is assumed for all internals. The cost of MD trays and random packing are generally very close. However, if stainless steel MD trays are required, random packing will generally be less expensive.

(2)

If the application is in fouling or corrosive service, consult your FRACTIONATION SPECIALIST.

(3)

Approximate capacity increase relative to single-pass sieve tray.

(4)

Installing a Nye tray above the feed tray has been shown to successfully debottleneck feed trays. Also consider installing a Nye tray to debottleneck a tray above a reboiler return, where excessive waste area due to distributor pipes may reduce effective bubble area. Consult your FRACTIONATION SPECIALIST for details of this application.

(5)

Structured packing is not recommended for pressures greater than 100 psia (690 kPa) or liquid loading greater than 20 gpm/ft2 (13.6 dm3/s/m2), unless the application is a high pressure aqueous system.

(6)

SuperFrac trays require FRACTIONATION SPECIALIST involvement for all applications.

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TABLE 4B TOWER INTERNALS SELECTION FOR REVAMPS

REVAMP OBJECTIVE?

MAXIMIZE CAPACITY LOSS IN EFFICIENCY TOLERABLE

LIQUID RATE?

≤

> 1.5 gpm/inch *

1.5 gpm/inch *

CONSULT

TURNDOWN

FRACTIONATION

RATIO

SPECIALIST

REQUIRED?

≤ 2/1

> 2/1

∆P CRITICAL?

∆P CRITICAL?

NO

YES

NO

YES

DUAL FLOW, UOP MD OR ECMD, SULZER HI-FI OR JET TRAYS FOR HIGH LIQUID RATES

LARGER

MULTIPASS

LARGER

SIZE

SIEVE OR

SIZE

PACKING OR

PACKING OR

PACKING ** OR

GRID

GRID

GRID DP3AT4B

* **

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems; otherwise use random packing.

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TABLE 4C TOWER INTERNALS SELECTION FOR REVAMPS REVAMP OBJECTIVE?

SAME CAPACITY IMPROVE SEPARATION

LIQUID RATE?

≤ 1.5 gpm/inch *

> 1.5 gpm/inch *

SPARE CAPACITY IN TOWER?

SPARE CAPACITY IN TOWER?

YES

NO

YES

NO

INCREASE REFLUX AND/OR DECREASE OPERATING PRESSURE PLUS SEE FRACTIONATION SPECIALIST

STRUCTURED PACKING

TURNDOWN RATIO REQUIRED?

SEE TABLE 4D

≤ 2/1

> 2/1

∆P CRITICAL?

∆P CRITICAL?

NO

YES

YES

NO

INCREASE REFLUX AND/OR DECREASE PRESSURE, REDUCE NUMBER OF PASSES, OR LOWER TRAY SPACING

PACKING **

PACKING **

INCREASE REFLUX OR DECREASE OPERATING PRESSURE OR PACKING DP3AT4C

* **

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems; otherwise use random packing.

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TABLE 4D TOWER INTERNALS SELECTION FOR REVAMPS

REVAMP OBJECTIVE?

SAME CAPACITY IMPROVE SEPARATION

LIQUID RATE?

> 1.5 gpm/inch *

SPARE CAPACITY IN TOWER?

* **

NO

YES

TURNDOWN RATIO REQUIRED?

SEE TABLE 4C

≤ 2/1

> 2/1

∆P CRITICAL?

∆P CRITICAL?

NO

YES

YES

NO

UOP MD OR ECMD TRAYS, OR SULZER HI-FI AT LOWER TRAY SPACING OR PACKING

PACKING **

PACKING **

PACKING **

DP3AT4D

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass Apply structured packing if P < 100 psia (690 kPa) and QL / As < 20 gpm/ft2 (13.6 dm3/s/m2), or for high pressure aqueous systems; otherwise use random packing.

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TABLE 5A TOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE (NEW TOWERS AND REVAMPS)

VACUUM SERVICE?

YES

NO

FOULING?

SEE TABLE 5B

MODERATE

NONE

SEVERE

∆P CRITICAL?

∆P CRITICAL?

SHEDS

YES

NO

YES OR NO

GRID

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

< 2/1

2-3/1

> 3/1

INDEPENDENT

LARGE HOLE DIAMETER SIEVE TRAYS

SEE FRACTIONATION SPECIALIST

PACKING OR GRID

GRID

DP3AT5A

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TABLE 5B TOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE (NEW TOWERS AND REVAMPS)

VACUUM SERVICE?

NO

FOULING?

NONE

MODERATE

SEE TABLE 5C

LIQUID RATE?

≤ 1.5 gpm/inch*

SEVERE

SHEDS

> 1.5, ≤ 4 gpm/inch*

SEE FRACTIONATION SPECIALIST

> 4 gpm/inch*

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 3/1

> 3/1

≤ 2/1

> 2/1

LARGE HOLE DIAMETER SIEVE OR JET TRAYS

SEE FRACTIONATION SPECIALIST

JET OR LARGE HOLE DIAMETER SIEVE TRAYS

MULTIPASS LARGE HOLE DIAMETER SIEVE OR BUBBLE CAP DP3AT5B

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass ** 4 gpm/in. of weir/pass = 10 dm3/s/m of weir/pass

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TABLE 5C TOWER INTERNALS SELECTION FOR HEAT TRANSFER SERVICE (NEW TOWERS AND REVAMPS)

VACUUM SERVICE?

NO

FOULING?

NONE

LIQUID RATE?

≤

1.5 gpm/inch *

>1.5, ≤ 4 gpm/inch*

> 4 gpm/inch *

TURNDOWN

TURNDOWN

STRUCTURED PACKING

RATIO

RATIO

REQUIRED?

REQUIRED?

≤ 3/1

> 3/1

≤ 2/1

> 2/1

SIEVE TRAY OR STRUCTURED PACKING

VALVE TRAY

JET TRAYS

OR PACKING

MULTIPASS SIEVE OR VALVE TRAY OR RANDOM PACKING DP3AT5C

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass ** 4 gpm/in. of weir/pass = 10 dm3/s/m of weir/pass

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TABLE 6A TOWER INTERNALS SELECTION FOR ENTRAINMENT REMOVAL SERVICE

FOULING?

SEVERE

MODERATE

NONE

SHEDS

LIQUID RATE?

SEE TABLE 6B

≤ 1.5 gpm/inch *

> 1.5 gpm/inch *

CONSULT FRACTIONATION SPECIALIST

EFFICIENCY CRITICAL?

YES OR NO

∆P CRITICAL?

NO

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 3/1

> 3/1

≤ 2/1

> 2/1

LARGE HOLE DIAMETER SIEVE TRAY OR SVG OR PROVALVE

CONSULT FRACTIONATION SPECIALIST

GRID OR LARGE CRIMP STRUCTURED PACKING

DEEPER GRID BED OR LARGE CRIMP STRUCTURED PACKING DP3AT6A

* 1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass

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TABLE 6B TOWER INTERNALS SELECTION FOR ENTRAINMENT REMOVAL SERVICE(1)

FOULING?

NONE

LIQUID RATE?

≤ 1.5 gpm/inch *

> 1.5 gpm/inch *

CONSULT FRACTIONATION SPECIALIST

EFFICIENCY CRITICAL?

NO

YES

∆P CRITICAL?

∆P CRITICAL?

NO

YES

NO

YES

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

TURNDOWN RATIO REQUIRED?

≤ 3/1

> 3/1

≤ 2/1

SIEVE TRAY OR STRUCTURED PACKING

VALVE TRAY OR STRUCTURED PACKING

GRID OR LARGE CRIMP STRUCTURED PACKING

> 2/1

≤ 3/1

> 3/1

STRUCTURED PACKING

SIEVE TRAY OR STRUCTURED PACKING

VALVE TRAY OF STRUCTURED PACKING

≤ 2/1

> 2/1

STRUCTURED STRUCTURED PACKING PACKING

DP3AT6B

*

1.5 gpm/in. of weir/pass = 3.7 dm3/s/m of weir/pass

(1)

Crinkled wire mesh screens (CWMS) or vane-type mist eliminators can also be used in many situations. See Sections III-H and V-A.

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Section

DEVICE SELECTION AND BASIC CONCEPTS DESIGN PRACTICES

III-A

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➧

TABLE 7 TOWER DESIGN CHECKLIST (TRAYS) The purpose of this checklist is to ensure the designer has addressed all major items involved in the design, revamp, or rating of tower internals. After reviewing this list, the designer should check the pertinent Subsection for specific guidelines. If unresolved questions remain, your FRACTIONATION SPECIALIST should be consulted. HYDRAULICS

•

Are the percents of jet flood, downcomer flood, and ultimate capacity within the allowable values?

•

Is the entrainment rate greater than 10%? If so, consult your FRACTIONATION SPECIALIST to determine its impact on efficiency.

•

Is the weeping rate greater than 20%? If so, did you check the impact on tray efficiency?

•

Has the correct valve type and valve count been specified for valve trays using conventional valves? If valves are being considered which do not have pressure drop coefficients, has the correct dry tray pressure drop been specified?

•

Is pressure drop critical for process reasons (vacuum operation, etc.)? If so, would structured packing have been a better choice?

•

Have anti-jump baffles and picket fence weirs been specified if required?

•

Is downcomer backup within acceptable limits?

•

Is downcomer entrance velocity at or below the allowable value?

•

Is the downcomer sealed? Does it meet the choking criteria and other secondary criteria?

•

If a jet tray design, is the liquid rate between the limits of 4 and 24 gpm/in. of diameter/pass (10 and 60 dm3/s/meter of diameter/pass)?

•

Have multi-pass trays been considered if the liquid rate limits tower capacity? (Sieve and valve trays only)

•

Have 4-pass trays been properly balanced? (Balancing 4-pass trays involves adjusting effective weir lengths, panel hole areas and downcomer clearances so that the liquid to vapor ratio on each panel is approximately equal. See the associated tray section for more details and guidance on balancing 4-pass trays.)

TRAY EFFICIENCY

•

If the chemical system is new (not used elsewhere in ExxonMobil), did you review the final value with your FRACTIONATION SPECIALIST?

•

Does the calculated efficiency look reasonable when compared with values listed in Section III-I, Table 2 (Overall Efficiencies Recommended in Past Designs)?

•

Was the Fluidity graph (Section III-I, Figure 8) used to set the efficiency for heavy hydrocarbon systems?

BLANKING

•

For revamps - does the amount and distribution of blanking meet the criteria given in the pertinent tray Subsection and GP 5-2-1?

•

For new designs - has the vendor been given enough input to provide the correct amount and distribution of blanking for the tray in question? (See pertinent tray Subsection and GP 5-2-1)

•

Would valve trays have been a better choice?

•

For cases requiring excessive blanking, has a rectangular bubble area / vertical baffle design (Section III-I) been considered?

SPECIAL CONSIDERATIONS

•

Is the final design as "balanced” as possible? (See Balanced Design discussion in the Subsection for each device).

•

If a drawoff box is present, was the free area and bubble area calculated correctly? In many cases, these reduced areas may require higher tray spacing to keep within flooding limits. See discussion in Section III-H.

•

Is there sufficient clearance between the drawoff box and the tray below to avoid tray flooding and/or downcomer entrance problems?

•

For flashing feeds or reflux, have the correct internals been specified? Is there sufficient room to install a perforated pipe distributor? Has adequate open area been provided to avoid entrainment generation? Is there sufficient tray

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spacing to compensate for the waste area of the distributor as well as the total vapor rate (vapor to tray from tray below plus vapor in feed)? TABLE 7 TOWER DESIGN CHECKLIST (TRAYS) (Cont)

•

Does the reboiler drawoff box design meet the criteria given in Section III-H?

•

Make sure anti-vortex baffles are provided at all liquid drawoffs.

•

For all internals that the vendor must supply, does the Design Specification contain a table that lists the necessary vapor / liquid rates and physical properties?

•

Are clear, unambiguous drawings (preferably to scale) provided for all ExxonMobil designed internals? Are all critical process dimensions clearly shown on these drawings?

•

Is tray spacing sufficient for maintenance purposes or for cleaning if fouling is expected? Does the affiliate have local specifications that may be more restrictive than those in the ExxonMobil Design Practices Manual?

•

Has the correct material of construction been chosen for the tower internals? The default material should be stainless steel.

•

If a fouling service, are the internals acceptable? (NH4Cl deposition, coking, polymerization, “imported” solids in the feed, etc.)

•

If a foaming service, have the guidelines for foaming systems been followed?

•

For steam stripping sections, have individual tray loadings been calculated via guidelines in Section III-I?

•

If minimum overhead entrainment is critical, has a deentrainment device been provided?

•

Are all process nozzles correctly sized?

•

Are vessel manholes appropriately specified?

•

Are all tower internals designed to be accessible for inspection and cleaning?

TURNDOWN CONSIDERATIONS

•

If greater than 3/1, have valve trays been considered?

•

If system has a liquid rate lower than 1.5 gpm/in. of weir/pass (3.7 dm3/s/meter of weir/pass), does the tray meet the weeping, entrainment, spray / froth (primarily sieve trays), and turndown criteria? Have picket fence weirs been considered?

➧

TABLE 8 TOWER DESIGN CHECKLIST (PACKING) The purpose of this checklist is to ensure the designer has addressed all major items involved in the design, revamp, or rating of tower internals. After reviewing this list, the designer should check the pertinent Subsection for detailed guidelines. If questions arise, your FRACTIONATION SPECIALIST should be consulted.

•

For debottlenecking a trayed column, were tray alternatives considered?

PACKING TYPE If a vacuum system or a distillation system less than 100 psia (690 kPa) and 20 gpm/ft2 (13.6 dm3/s/m2), was structured packing considered because of its low pressure drop characteristics and high capacity and efficiency versus random packing?

•

Does the packing size specified fall within the range given in Table 2, Section III-G?

•

Is the packing choice consistent with the system's fouling tendency or would grid have been a better choice?

•

Is the material of construction appropriate for the fluids being handled? Packing would not be a good choice where ammonia chloride deposition could occur. (If ceramic packing was chosen, remember some breakage will occur and chips may cause plugging of the reboiler or packed bed/support plate. Metal packings must have virtually no corrosion rate due to the thin material used in their fabrication.)

HYDRAULICS

•

Is the percent of flood within the range provided in Table 3 of Section III-G? Also, is ultimate capacity at or below 85%?

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Section III-A

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•

If tower pressure drop is critical, has allowance been made for the pressure drop of support plates, chimney trays, etc. as well as for the packing itself? Have design vapor pressures (and therefore volumetric vapor rates) been adjusted to reflect the actual pressure drops?

•

Do the support plate and liquid distributor have enough open area to avoid flooding before the packing floods? This is especially critical for revamps.

•

If fouling is expected, has its adverse effect on capacity and run length been considered? TABLE 8 (Cont) TOWER DESIGN CHECKLIST (PACKING)

EFFICIENCY CONSIDERATIONS

•

Has the type and size of packing been considered from an efficiency, pressure drop and capacity standpoint, or would the optimum choice consist of a smaller size or different type?

•

Is the type of liquid distributor chosen adequate for the design liquid rate and turndown needed?

•

Does the distributor chosen have the turndown needed for your service?

•

For reviewing vendor drawings or evaluating existing distributors, does the pour point distribution meet the criteria in Section III-G, APPENDIX A? Does the distributor design meet the other requirements of this APPENDIX? Are all metering orifices shown with size? Is a table of liquid heads at minimum, design, and overflow rates included?

•

Are clear, unambiguous drawings (preferably to scale) provided for all ExxonMobil designed internals? Are all critical process dimensions clearly shown on these drawings?

•

For revamps, if the existing liquid distributor was built prior to 1983, it is probably of poor design. Consider replacement with a newer model if improved separation is one of the revamp objectives.

•

Ensure that the Design Specification requires that all liquid distributors, except spray nozzle distributors, for towers with a diameter of 3 ft (900 mm) or greater be water tested in the presence of a representative of the owner.

•

Attach Flow Test Requirements (Section III-G, Table 7) and Liquid Distributor Guidelines (Section III-G, Table 6), as well as minimum and design liquid rates for the flow test.

•

Ensure that the design specification requires each spray distributor to be water tested in place prior to startup to ensure optimum distribution. Consult Section III-G, SPECIAL DESIGN CONSIDERATION FOR GRIDS, GRID SPRAY NOZZLE CONSIDERATIONS for details concerning this procedure.

AUXILIARIES

•

Are external strainers with correct mesh size specified? Is piping material downstream of the strainer upgraded as required to prevent pipe scale from fouling the distributor?

•

For flashing feeds, ensure that the mixed phase will disengage completely before it enters the liquid distributor. (Provide a chimney tray (preferred) or a flashbox pre-distributor to achieve this goal.)

•

Do reboiler returns and vapor-containing feeds have a perforated pipe distributor with adequate open area?

•

If a low overhead entrainment rate must be met, has a deentrainment device been provided?

•

For all internals that the vendor must supply, does the Design Specification contain a table that lists the necessary vapor/liquid rates and physical properties?

•

Are all process nozzles correctly sized?

•

Are vessel manholes appropriately specified?

•

Is vapor distribution taken into account (as in Section III-G) in the design of collector trays and feed pipes?

•

Is selection and installation of internals consistent with the guidelines provided in Section III-G, under SPECIAL DESIGN CONSIDERATIONS FOR PACKING?

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SPECIAL CONSIDERATIONS

•

Has the potential for pyrophoric fires been assessed? Have proper cleaning procedures been used to remove pyrophoric materials (such as pyrophoric iron sulfide deposits) before the column is opened up?

•

Have venting and draining of internals been taken into account?

•

Are all tower internals (most importantly, distributors) designed to be accessible for inspection and cleaning?

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