Separator Design Guide.doc

Separator Design Guide

7-Feb-2000

Worley Resources & Energy ACN 001 279 812 Worley Limited Level 2, 80 Albert Street Brisbane Queensland 4000 Australia PO Box 81, Albert Street Brisbane Queensland 4002 Australia Tel: +61 7 3221 7444 Fax: +61 7 3221 7791 Web: http://www.worleylimited.com © Copyright 2000 Worley Limited

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CONTENTS RESOURCES & ENERGY...................................................................................................................... I APPENDICES........................................................................................................................................ II A SEPARATOR SIZING SPREADSHEET............................................................................................. II B RECOMMENDED LIQUID RESIDENCE TIME..................................................................................II 1. INTRODUCTION............................................................................................................................... 1 1. INTRODUCTION............................................................................................................................... 1 2. DESIGN CONSIDERATIONS............................................................................................................ 2 2. DESIGN CONSIDERATIONS............................................................................................................ 2 3. HORIZONTAL VS VERTICAL SEPARATOR SELECTION.............................................................3 3. HORIZONTAL VS VERTICAL SEPARATOR SELECTION.............................................................3 3.1

Theory of Horizontal Separators.............................................................................................. 3

3.2

Theory of Vertical Separators.................................................................................................. 4

3.3

Theory of Separators Applied To V-L Systems........................................................................4

3.4 Advantages & Disadvantages of Horizontal / Vertical Separators............................................5 4. SETTLING VELOCITY CALCULATIONS.......................................................................................... 7 4. SETTLING VELOCITY CALCULATIONS.......................................................................................... 7 4.1 Conventional Approach........................................................................................................... 7 4.2 Alternative Approach................................................................................................................. 8 5. SEPARATOR SIZING PRINCIPLES .............................................................................................10 5. SEPARATOR SIZING PRINCIPLES .............................................................................................10 5.1

Vertical Vapour-Liquid Separators.........................................................................................10 5.1.1 Sizing Diameter.......................................................................................................... 10 5.1.2 Sizing Vessel Length.................................................................................................. 10

5.2 Sizing Demister Pads.............................................................................................................. 11

Appendices A

Separator Sizing Spreadsheet

B

Recommended Liquid Residence Time

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1.

INTRODUCTION

Separators play an important role in the chemical industry and are critical to the success of almost all processes. The usage of phase separators range from functions as diverse as ensuring that final product quality meets often stringent standards to protecting downstream equipment from undesirable impurities. In the petrochemical industry the most common uses include:

•

Separation of liquid hydrocarbon from a hydrocarbon vapour (2-phase vapour – liquid separation)

•

Separation of a two liquids differing in their respective densities (2-phase, liquid – liquid separation)

•

Separation of a feed stream consisting of vapour, liquid hydrocarbon and water into 3 separate components. (3-phase separation).

Some examples of separators are as follows:

2-Phase Separators

3-Phase Separators

Fuel gas KO drum

Glycol Separator

Compressor KO drum

Hydrotreater high pressure separator

Relief gas KO drum

Wet column reflux drum

Crude unit desalter

Production separator

Amine absorber KO drum Coalescer units This design guide presents the basic principles and methods involved with sizing vertical and horizontal separators for both 2-phase and 3-phase separation. It is important to note, however, that this guide does not cover the separation of solids from either liquids or vapour.

Whilst theoretically, it may be important to take into consideration dynamic operating conditions of the separation process, such as variation in fluid properties with time; and the transient start-up and shut-down characteristics of separation operation when undertaking the design, this presents a near impossible task in reality. As such, this design guide uses steady-state operation of the separator as its basis.

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2.

DESIGN CONSIDERATIONS

As is the case with the design of any vessel, the associated system properties and process requirements must first be defined. In the case of separator design, these may include:

System Properties •

Flow rates of each phase to be separated.

•

Physical properties of each phase (viscosity, density, etc.)

•

Identification of continuous phase(s) (see following table) .

System

Continuous / discontinuous phase distinction

Vapour – Liquid

The vapour is the continuous “light” phase, with liquid being the heavy, discontinuous phase which settles out as droplets.

HC liquid – Water.

The HC liquid is the continuous phase, with water settling out as droplets

Water – HC liquid

The water is the continuous, “heavy” phase, with HC liquid being the light discontinuous phase rising up through the water as droplets.

Process Requirements •

The required throughput and composition of feed mixture to be purified will ultimately determine the size and type of separator selected.

•

Degree of separation required. The minimum droplet sizes required to be separated from each phase need to be specified. This is generally set by factors such as purity of product required for sale or purity required to avoid upsets to downstream equipment / processes. For example:

•

In HC liquid – vapour separation, efficient liquid separation from vapour is needed, particularly in situations where the vapour subsequently flows downstream to a compressor. Separators for this purpose often include mist eliminators to enhance their separation performance.

•

In water – HC liquid separation, efficient separation of water from the HC is needed to avoid over-loading downstream water treatment facilities. This is particularly important if the liquid HC is then fed to a downstream distillation column.

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3.

HORIZONTAL VS VERTICAL SEPARATOR SELECTION

Before a judgement is made regarding the type of separator most suitable for use in a specific application, there are number of factors that need to be taken into consideration. The following sections outline the general characteristics of both horizontal and vertical separators and where they are most useful. It is important to note, however, there are exceptions to these recommendations and each specific case should be evaluated before any decision is made.

For ease of reading, when comparing the performances of vertical and horizontal separators in this section, the light phase will be referred to as “vapour” and the heavy phase as “liquid”, even in the case of liquid-liquid systems.

3.1

T heor y of Horizontal Separator s

In terms of equivalent vapour flow areas, horizontal separators are more efficient than are their vertical counterparts, due largely to the fact that the liquid settles out perpendicular to the direction of vapour flow, rather than in a direction countering the vapour flow (as is the case with vertical separators).

If the residence time 1 of the vapour is greater than the time taken for the liquid droplets to reach the liquid surface, then the liquid will undergo satisfactory disengagement from the vapour. It is this relationship which ultimately determines the maximum allowable vapour velocity through the separator, as follows:

vmax ≤

Where L

v s .L H

Equation 3.1

= distance between feed inlet and vapour outlet.

Vs = liquid droplet settling velocity (details of calculation method to follow in section 4) H

= distance between top of drum and Normal Liquid Level (NLL)

Vmax = maximum allowable vapour velocity

Thus, for horizontal separators with a L/H ratio greater than 1, the maximum velocity can exceed the liquid settling velocity without affecting the vessel’s ability to achieve satisfactory separation. As will be shown in section 3.2, however, this is not the case with vertical separators.

Horizontal separators are most efficient where large volumes of liquids are present with the vapour or alternatively, when large volumes of vapour are dissolved in the liquid phase(s). They are generally

1

The vapour residence time refers to the time taken for vapour to flow from the feed inlet to vapour outlet nozzle.

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used in applications such as 3-phase separators, high-pressure vapour-liquid separators, relief KO drums and liquid-liquid separators, although there are frequent exceptions.

3.2

T heor y of Ver tical Separator s

In the case of a vertical separator, the liquid droplets settle out in a direction opposite to the direction of vapour flow. Therefore, the liquid will not disengage from the vapour unless the vapour velocity is lower than the liquid settling velocity. That is, v max ≤ v s

Equation 3.2

Despite the fact that vertical separators are a less efficient alternative than horizontal ones with an appropriate L/H ratio, they do offer some distinct advantages which have seen them favoured for applications as fuel gas and compressor suction KO drums. Such advantages are detailed in section 3.4.

3.3

T heor y of Separator s Applied To V-L Systems

In reality, the maximum allowable vapour velocity for liquid-vapour systems is highly dependent on the specific separator application and indeed is vastly different, for example, in cases where a demister pad is installed and cases where one is not. Such flexibility is not catered for using Equations 3.1 and 3.2, where it is not possible to effectively take into consideration any variation in separator characteristics. Instead, a more appropriate equation to use for calculating the maximum allowable vapour velocity for vapour-liquid systems is as follows:

v max = K Where

( ρl − ρv ) ρv

Equation 3.3

vmax = Maximum allowable vapour velocity, m/s K

= Empirical constant, m/s

ρl = Density of liquid phase, kg/m3 ρv = Density of vapour phase, kg/m3

A range of values for K are available for use in the above equation, as illustrated in the following table (Table-3.1) taken from figure 7-9 of GPSA (SI units) Volume 1, Section 7, page 7-7.

Table- 3.1 Typical K & C Factors for Sizing Woven Wire Demisters

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Separator Type

K Factor (m/s)

C Factor (m/hr)

Horizontal

0.12 to 0.15

420 to 540

Vertical

0.05 to 0.11

200 to 400

Spherical

0.05 to 0.11

220 to 400

Wet Steam

0.076

270

Most vapours under vacuum

0.061

220

Salt & Caustic Evaporators

0.046

160

Adjustment of K & C Factor for Pressure - % of design value. Atmospheric

100

1000 kPa

90

2000 kPa

85

4000 kPa

80

8000 kPa

75

 For glycol and amine solutions, multiply K by 0.6 to 0.8.  Typical use one-half of the above K or C values for approximate sizing of vertical separators without wire demisters.  For compressors suction scrubbers and expander inlet separators multiply K by 0.7 to 0.8.

3.4

Advantages & Disadvantages of Horizontal / Ver tical Separator s

There are many factors Horizontal Separators - Advantages: •

High separation efficiency due to higher vapour space volumes and vapour residence times.

•

Lower nozzle outlet elevations

•

More applicable for use as reflux accumulators, which can be more readily attached to and supported by horizontal separators.

•

More suitable than vertical separators for handling large total liquid volumes.

•

May be used in applications requiring 3-phase separation.

Horizontal Separators – Disadvantages: •

Larger footprint area required than for vertical separators.

•

Become less economical than vertical separators for high vapour / liquid ratios.

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Vertical Separators – Advantages •

Less potential for entrainment of liquid in vapour stream. Unlike a horizontal separator, the area available for vapour flow is not reduced when the liquid level rises during operation.

•

Less footprint area required.

•

Easier installation and operation of level alarms and shutdown control systems.

•

Generally a lower cost option than horizontal separators.

•

More efficient than horizontal separators for high vapour / liquid ratios.

Vertical Separators – Disadvantages •

Not generally recommended for three-phase separation unless the total liquid fraction present in the feed stream is very low (10 – 20% by weight).

•

Less suitable for a feed with a high liquid/vapour ratio.

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4.

SETTLING VELOCITY CALCULATIONS

Perhaps the most important criterion used in sizing phase separators is the velocities at which entrained droplets of a discontinuous, dispersed phase settle out in a continuous medium. The settling velocity of such droplets is dependent upon factors such as droplet size and shape, phase densities and the droplets’ resistance to settling. Unfortunately, due to the fact that the droplet’s resistance to settling is in fact a function of the settling velocity itself, various approximations are required in any solution method and a precise determination of the settling velocity is not possible. In order to minimise the errors associated with any approximations made, however, two independent approaches to calculating the settling velocity are used in this design guide.

4.1 Conventional Approach The conventional approach taken by the majority of literature sources in the past to calculate settling velocities has been to use one of three laws, namely “Stokes Law”, the “Intermediate Law” or “Newton’s Law”. These laws are given below, referenced from figure 7-4 of GPSA (SI Units), Volume 1, Section 7, page 7-4:

1000 * g * D p ( ρ l − ρ v ) 2

Vt =

Vt =

18µ

3.54 * g 0.71 * D p

ρv

Vt = 1.74

Where:

0.29

1.14

Stokes Law

( ρ l − ρ v ) 0.71

µ 0.43

g * D p * (ρl − ρv )

ρv

(Equation 4.1)

Intermediate Law

Newton’s Law

(Equation 4.2)

(Equation 4.3)

Vt

=

Settling velocity of droplet (m/s)

g

=

Gravitational acceleration constant (9.81 m/s 2)

Dp

=

Droplet diameter (metres)

ρv

=

Density of continuous phase (kg/m3)

ρl

=

Density of discontinuous phase (kg/m3)

µ

=

Viscosity of continuous phase (Pa.s)

Note: Maximum settling velocity shall not exceed 0.0042 m/s. The law that is selected for use in a specific situation depends upon the size range of settling droplets. The maximum droplet size for which each respective law applies is given by the following:

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1

D p , max

Where:

 3 µ2 = K CR   g * ρ * (ρ − ρ )   v l v  

(Equation 4.4)

Dp,max is the maximum droplet size for which each respective law applies (metres). KCR is a dimensionless constant.

It is seen then from the above equation that K CR is in fact the quantity which determines which of the three laws is applicable. Values for KCR are as follows:

Applicable Settling Law

Value of KCR

Stoke’s Law

0.025

Intermediate Law

0.334

Newton’s Law

18.13

The law to be used is then selected by substituting the KCR values shown above sequentially into equation 4.4 for each law and determining for which law the actual droplet size is lower than the value of Dp,max calculated.

4.2

Alter native Approach

An alternative approach which can be used to calculate the settling velocity of the droplets was developed by Haider & Levenspiel, (1989). This approach introduces two useful quantities – a dimensionless droplet size, dp* and a dimensionless droplet settling velocity, ut*, defined as follows: 1

dp

*

 ρv ( ρl − ρv ) g 3   = dp  µ2  

(Equation 4.5)

1

ut

*

2  3 ρv  = ut   µ( ρ − ρ ) g  l v  

(Equation 4.6)

For the direct evaluation of ut*, Haider & Levenspiel present the following approximation (valid for spherical droplets), which expresses ut* as a function of dp*. ut

*

 18 =  *   dp

(

)

2

 0.591  + 0 . 5  *  dp 

(

−1

(Equation 4.7)

)

Given the appropriate physical properties of the system under consideration, d p* is first calculated (using equation 4.5) followed by ut (using back substitution in equation 4.8), as follows:

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 18 0.591  ut =  +  *2 * 0.5  d d p  p 

( ) ( )

−1

  ρv    µ (ρ − ρ )g  l v   2

1 −3

(Equation 4.8)

Due to the fact that the “conventional” methods described in section 4.1 are more widely used and trusted by industry world-wide than the “alternative” method, it is the method of choice used in the calculation of droplet settling velocities for the purposes of actually sizing the separator in this design guide. While the droplet settling velocities are calculated using both methods, this is merely to ensure that the values calculated for the settling velocities do not differ substantially from one another. This is designed to ensure that the potential errors associated with the approximations made in each method are reduced.

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5. SEPARATOR SIZING PRINCIPLES 5.1 5.1.1

Ver tical Vapour-Liquid Separator s Sizing Diameter

For a vertical separator, the vessel diameter required is determined by first calculating the maximum allowable vapour velocity, (using equation 3.3) before using the following equation:

d min =

Where

4.Q π.vmax

Equation 5.1

dmin

= Minimum Vessel Diameter (m)

Q

= Vapour Flowrate (m3/s)

vmax

= Maximum Vapour Velocity (m/s)

The vessel diameter should be selected as the next largest pipe/drum size to the d min value calculated, where deemed practical. 5.1.2

Sizing Vessel Length

Amongst the most important factors required for consideration when designing the length of a proposed vertical separator, are: •

The location of various liquid levels inside the vessel (LLSD, LLL, NLL, HLL, HLSD, etc.)

•

The location of inlet and outlet nozzles

•

The surge volume required to permit corrective action to be taken by plant operators in times of unsatisfactory plant operation. Such information is usually provided by the vendor, or based upon experience gained in sizing similar vessels in the past.

•

Whether or not a demister pad is to be installed.

The following recommendations can be made regarding vessel shell lengths which will assist the process engineer in addressing the important issues listed above.

•

The low liquid level (LLL) should be a minimum of 300 mm above the bottom vessel tangent line. The precise level height may depend on the nature of level control instruments available.

•

The high liquid shutdown (HLSD) level should be no less than 300 mm below the inlet feed distributor / nozzle.

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•

The shell length between the HLL and LLL should be set so as to provide sufficient liquid surge volume.

•

The shell length between the HLSD and HLL should be made sufficient to provide the operator with sufficient time to shut down the separator before the performance and operation of downstream processes and equipment is compromised.

•

The minimum T/T length of the vessel should be 2500 mm.

•

The clearance between the bottom of the demister pad and the top of the feed distributor should be set at 70% of the vessel diameter or 750 mm, whichever of the two is greater.

•

The clearance between the upper tangent line and the top of the demister pad should be 10% of the vessel ID or 300 mm, whichever of the two is greater.

Following figure-5.1, adapted from GPSA (SI Units) Volume 1, Section 7, depicts an example of a typical vertical separator containing a wire mesh mist extractor, in which the above-mentioned recommendations have been adhered to.

Figure – 5.1

150 mm Dv or 600 mm (min)

Inlet Di

2 Di 300 mm (min) LSH (S/D)

Dv

300 mm (min) LG/LC Slug Capacity

5.2

300 mm (min)

Sizing Demister Pads

Mist extractors of the mesh-type are typically constructed of a pad of compact stainless steel wire designed to capture entrained liquid droplets or those droplets too small to settle by gravity. The liquid droplets impinge on the mesh pad, coalesce and fall downward as larger droplets, back through the rising vapour. Demister pads are generally aligned horizontally in both vertical and horizontal

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separators, with the vapour and entrained liquid passing vertically upward through the pad. In fact, the pad performance is found to be adversely affected if tilted at an angle of greater than 30 degrees from the horizontal.

As reported by GPSA (SI) volume 1, most installations use a 150 mm thick pad with the minimum recommended thickness being 100 mm. In order to size the diameter of a demister pad, the following correlation may be used:

4.Q d= π .K

Where:

 ρl − ρ v   ρ v 

   

−

1 2

(Equation 5.4)

Uv

=

Superficial velocity of vapour through the demister pad (m/s).

K

=

Empirical constant, provided by vendor, but usually in the range of 0.1 – 0.12 m/s.

Q

=

Vapour flowrate (m3/s)

ρl

=

Density of liquid phase (kg/m3)

ρv

=

Density of vapour phase (kg/m3)

The diameter of the mist extractor is usually substantially less than the diameter of the separator. In the case of vertical separators with diameters less than 1000 mm, a mesh diameter equivalent to the vessel diameter should be used. In addition, the demister pads are typically located 300 mm below the vapour outlet nozzle of a horizontal separator.

For any variety of service, the material of construction for the mesh pad should be at least 304 ss. For application in corrosive service, either 316 ss or Monel mesh pads should be used.

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Appendix A – Separator Sizing Spreadsheet Instructions In order to allow for designs of separators to be generated rapidly, an Excel ’97 spreadsheet file (Separator Sizing.xls) has been developed. This spreadsheet allows for the following types of separator to be sized:

Horizontal Separators

Vertical Separators

2-phase, vapour-liquid

2-phase, vapour-liquid

2-phase, liquid-liquid

2-phase, liquid-liquid

3-phase, liquid-liquid-vapour (with weir, no boot) 3-phase, liquid-liquid-vapour (with boot, no weir) In the sections that follow, a brief outline is given for each type of separator on each of the following topics: •

Inputs required by the spreadsheet,

•

Final outputs provided by the spreadsheet,

•

Warnings provided by the spreadsheet, and

•

A chronological procedure for using the spreadsheet.

A.1 Vertical Liquid-Vapour Separator A.1.1 Inputs Required The following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:

1. Physical properties of both phases •

Maximum flowrates (kg/hr)

•

Actual Densities (kg/m3)

•

Viscosities (cP)

2. Separator vessel dimensions – •

Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable vapour velocity and specified liquid residence times are not exceeded.

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•

Liquid level heights (mm) – heights of LLSD, LLL, NLL, HLL and HLSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are encountered.

In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:

“Standard” Inputs Required (ρmvm2)max – inlet mixture feed nozzle

Typical / Suggested Values 1000 kg/ms2 – where no inlet device present 1500 kg/ms2 – where half-open pipe inlet present)

(ρmvm2)max – vapour outlet nozzle

3750 kg/ms2

vmax – liquid outlet nozzle

1 m/s

Droplet size (liquid in gas)

150 µm – without demister pad installed 500 mm – with demister pad installed

Liquid Residence Times

See Appendix-B.

LLSD height above bottom tangent line

150 mm (minimum)

HLSD height above bottom tangent line

A value such that the HLSD is no less than 150 mm below the inlet feed nozzle.

Thickness of Demister Pad

100 mm (minimum), but usually 150 mm.

K-value

1. May be supplied by vendor and manually inputted into spreadsheet. 2. If not supplied by vendor, may be obtained using information from Fig 7-9, GPSA (SI) Volume 1, Section 7, Page 7-7.

A.1.2 Final Outputs Provided Once all required input values have been entered, the spreadsheet calculates values for the following:

1. Fluid Properties •

Feed mixture density

•

Maximum allowable vapour velocity

•

Actual maximum vapour velocity

•

Liquid droplet settling velocity & the settling law used in the calculation

•

Liquid residence times between specified liquid levels.

2. Vessel Dimensions

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•

Minimum vessel diameter

•

Vessel height (T/T) – taken as a minimum of 2500 mm.

•

Minimum inlet nozzle ID∅

•

Minimum vapour outlet nozzle ID∅

•

Minimum liquid outlet nozzle ID∅

•

Clearance between inlet feed nozzle and bottom of demister pad (if fitted) – shall be taken as the greater of 0.7D or 750 mm.

•

Clearance between top of demister pad (if fitted) and upper tangent line – taken as the greater of 0.1D or 300 mm.

A.1.3 Warnings Provided

The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:

1. The calculated vapour velocity exceeds the maximum allowable vapour velocity calculated by the K-factor method. 2. The specified liquid level heights are inadequate to provide the required liquid residence times. 3. The vessel height / diameter ratio does not fall in the range of 2 – 5.

An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an “OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.

A.1.4 Procedure for Spreadsheet Use. The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows: 1. Enter all the required input data listed in A.1.1, except for the vessel ID and liquid level elevations. 2. Set the vessel diameter to the calculated value for the “minimum vessel diameter” as a starting point. 

The minimum value for any nozzle diameter is considered to be 50 mm.

 

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3. Enter arbitrary values for the various liquid level elevations and vary them to meet the required liquid residence times. 4. Examine the vessel height / diameter ratio to ensure that it lies between 2 – 5. (i)

If the ratio is greater than 5, a more economical ratio can be achieved by gradually increasing the vessel ID from the value selected in step 2, while decreasing one or all of the LLSD, LLL, NLL, HLL and HLSD elevations specified in step 3. If the latter approach is taken, it must be ensured that all required liquid residence times remain satisfied and that the LLSD elevation is a minimum of 150 mm above the bottom tangent line.

(ii)

If the ratio is less than 2, on the other hand, the elevations of the LLSD, LLL, NLL, HLSD may be increased to extend the height of the vessel and increase the ratio. The vessel diameter may not be decreased from the value selected in step 2. In step 2, the minimum vessel diameter was selected, which cannot be decreased in order to increase the H/D ratio without the maximum allowable vapour velocity being exceeded.

A.2 Vertical Liquid-Liquid Separator A.2.1 Inputs Required The following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:

1. Physical properties of both phases -

2.

•

Maximum flowrates (kg/hr)

•

Actual Densities (kg/m3)

•

Viscosities (cP)

Separator vessel dimensions – •

Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable light liquid velocity and specified liquid residence times are not exceeded.

•

Liquid level heights (mm) – heights of LILSD, LIL, NIL, HIL, HILSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are encountered.

In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:

“Standard” Inputs Required (ρmvm2)max – inlet mixture feed nozzle

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Typical / Suggested Values 1000 kg/ms2 – where no inlet device present

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1500 kg/ms2 – where half-open pipe inlet present vmax – light liquid outlet nozzle

1 m/s

vmax – heavy liquid outlet nozzle

1 m/s

Droplet size (heavy liquid in light liquid)

600 µm

Droplet size (light liquid in heavy liquid)

1000 µm

Liquid Residence Times

See Appendix-B

LILSD height above bottom tangent line

150 mm (minimum)

HILSD height above bottom tangent line

A value such that the HILSD is no less than 150 mm below the inlet feed nozzle.

A.2.2 Final Outputs Provided Once all required input values have been entered, the spreadsheet calculates values for the following:

1. Fluid Properties

2.

•

Feed mixture density

•

Maximum allowable light liquid velocity (assumed equal to 0.85 * settling velocity of heavy liquid droplets in the light liquid phase)

•

Actual maximum light liquid velocity

•

Light liquid droplet settling velocity & the settling law used in the calculation

•

Heavy liquid droplet settling velocity & the settling law used in the calculation

•

Liquid residence times between specified liquid levels.

Vessel Dimensions •

Minimum vessel diameter

•

Vessel height (T/T) – taken as a minimum of 2500 mm.

•

Minimum inlet nozzle ID∅

•

Minimum light liquid outlet nozzle ID∅

•

Minimum heavy liquid outlet nozzle ID∅

A.2.3 Warnings Provided



The minimum value for any nozzle diameter is considered to be 50 mm.

 

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The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:

1. The calculated light liquid velocity exceeds the maximum allowable light liquid velocity. 2. The residence time of the heavy liquid between the vessel bottom and the NIL is less than the light liquid droplet “fall” time in the heavy liquid medium. 3. The specified liquid level heights are inadequate to provide the required liquid residence times. 4. The vessel height / diameter ratio does not fall in the range of 2 – 5.

An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an “OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.

A.2.4 Procedure for Spreadsheet Use. The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows: 1. Enter all the required input data listed in A.2.1, except for the vessel ID and elevations of LILSD, LIL, NIL, HIL and HILSD. 2. Set the vessel diameter to the calculated value for the “minimum vessel diameter” as a starting point. 3. Enter arbitrary values for the various liquid level elevations (i.e. LILSD, LIL, NIL, HIL and HILSD) and vary them to meet the required liquid residence times. Also ensure in specifying these levels that the residence time of the heavy liquid between the vessel bottom and the NIL is less than the light liquid droplet “fall” time in the heavy liquid medium. 4. Examine the vessel height / diameter ratio to ensure that it lies between 2 – 5. (i)

If the ratio is greater than 5, a more economical ratio can be achieved by gradually increasing the vessel ID from the value selected in step 2, while decreasing one or all of the LILSD, LIL, NIL, HIL and HILSD elevations specified in step 3. If this approach is taken, it must be ensured that all required liquid residence times remain satisfied and that the LILSD elevation is a minimum of 150 mm above the bottom tangent line.

(ii)

If the ratio is less than 2, on the other hand, the elevations of the LILSD, LIL, NIL, HILSD may be increased to extend the height of the vessel and increase the ratio. In doing so, it is again vital to ensure that the light liquid droplet fall time is less than the heavy liquid residence time between the vessel bottom and the NIL. The vessel diameter may not be decreased from the value selected in step 2. In step 2, the minimum vessel diameter was selected, which cannot be decreased in order to increase the H/D ratio without the maximum allowable light liquid velocity being exceeded.

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A.3 Horizontal Vapour-Liquid Separator A.3.1 Inputs Required The following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:

1. Physical properties of both phases •

Maximum flowrates (kg/hr)

•

Actual Densities (kg/m3)

•

Viscosities (cP)

2. Separator vessel dimensions – •

Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable vapour velocity and specified vapour and liquid residence times are not exceeded.

•

Vessel length (T/T) (mm) – adjusted such that the minimum length is 2500 mm and that the length / diameter ratio is between 2 – 4 for the final design. The vessel length must also be adjusted such that the required vapour and liquid residence times are not exceeded. Of particular importance in this regard is ensuring that the vapour residence time provided is sufficient to allow a liquid droplet to fall from the top of the vessel to the NLL.

•

Liquid level heights (mm) – heights of LLSD, LLL, NLL, HLL and HLSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are encountered. The NLL is in fact one of the first inputs required as it is needed to calculate the minimum vapour flow area and subsequently the vapour residence time (once vessel length is specified).

In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:

“Standard” Inputs Required (ρmvm2)max – inlet mixture feed nozzle

Typical / Suggested Values 1000 kg/ms2 – where no inlet device present 1500 kg/ms2 – where half-open pipe inlet present

(ρmvm2)max – vapour outlet nozzle

3750 kg/ms2

vmax – liquid outlet nozzle

1 m/s

Droplet size (liquid in gas)

150 µm – regardless of whether or not a demister pad is installed

Liquid Residence Times

See Appendix-B

LLSD height above vessel bottom

150 mm (minimum)

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HLSD height above vessel bottom

A value such that the HLSD is no less than 150 mm below the inlet feed nozzle.

Thickness of Demister Pad

100 mm (minimum), but usually 150 mm.

K-value

1. May be supplied by vendor and manually inputted into spreadsheet. 2. If not supplied by vendor, may be obtained using information from figure 7-9, GPSA (SI) Volume 1, Section 7, Page 7-7..

A.3.2 Final Outputs Provided Once all required input values have been entered, the spreadsheet calculates values for the following:

1. Fluid Properties •

Feed mixture density

•

Maximum allowable vapour velocity

•

Actual maximum vapour velocity

•

Liquid droplet settling velocity & the settling law used in the calculation

•

Liquid droplet fall time (from top of vessel to NLL)

•

Vapour residence time (from top of vessel to NLL)

•

Liquid residence times between specified liquid levels.

2. Vessel Dimensions



•

Minimum vessel diameter

•

Minimum inlet nozzle ID∅

•

Minimum vapour outlet nozzle ID∅

•

Minimum liquid outlet nozzle ID∅

•

Clearance between inlet feed nozzle and bottom of demister pad (if fitted) – shall be taken as the greater of 0.7D or 750 mm.

•

Clearance between top of demister pad (if fitted) and upper tangent line – taken as the greater of 0.1D or 300 mm.

The minimum value for any nozzle diameter is considered to be 50 mm.

 

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A.3.3 Warnings Provided

The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:

1. The calculated vapour velocity exceeds the maximum allowable vapour velocity calculated by the K-factor method. 2. The liquid droplet fall time (from top of vessel to NLL) exceeds the vapour residence time (also from top of vessel to NLL). 3. The specified liquid level heights are inadequate to provide the required liquid residence times. 4. The vessel length / diameter ratio does not fall in the range of 2 – 4.

An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an “OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.

A.3.4 Procedure for Spreadsheet Use. The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows: 1. Enter all the required input data listed in A.3.1, except for the vessel ID, vessel length and elevations of LLSD, LLL, NLL, HLL and HLSD. 2. As a starting point, set the vessel diameter to an arbitrary value and the vessel length initially as 3 times this quantity. 3. Enter arbitrary values for the various liquid level elevations (i.e. LLSD, LLL, NLL, HLL and HLSD). (i)

(ii)

If the liquid droplet fall time exceeds the vapour residence time (warning number 2), the vapour residence time may be increased by: •

Increasing vessel diameter as needed – the most successful measure if the vapour residence time is significantly lower than the liquid droplet fall time (i.e. >25% lower).

•

Increasing the length of the vessel – the most successful measure if the vapour residence time is only slightly lower than the liquid droplet fall time (i.e. 25% lower).

•

Increasing the length of the vessel – the most successful measure if the light liquid residence time is only slightly lower than the heavy liquid droplet fall time (i.e. 25% lower).

•

Increasing the length of the vessel – the most successful measure if the vapour residence time is only slightly lower than the liquid droplet fall time (i.e. 25% lower).

•

Increasing the length of available setting section between the inlet feed nozzle and the weir – the most successful measure if the vapour residence time is only slightly lower than the liquid droplet fall time (i.e.