Chapter 4 - Separator Design

4-1

CHAPTER 4

DESIGN FOR THE TWO-PHASE SEPARATOR (V-101)

4.1

Introduction In the petrochemical production, a separator is a large drum designed to separate

production fluids into their constituent components of oil, gas and water. In the event that water is not present, the bottom output would consist of only oil. It works on the principle that the three components have different densities, therefore allowing them to stratify when moving slowly with gas on top, water at the bottom and oil in the middle. Any solids such as grit and sand will also settle at the base of the vessel.

Separators may cater to the separation of all kinds of phase combinations, whether it be liquid-liquid, vapour-vapour and vapour-liquid, the latter being the kind that we are designing as an example for this 1-propanol plant. Vapour-liquid separators are the most common types of process equipment. They may be oriented either vertically or horizontally, depending on which one is more economically feasible according to the plant design. The operation principle is rather basic. Once the oil and other fluids have been separated the oil will leave the vessel at the bottom through a dump valve that is controlled by the level controller. The separated gas rises to the top, leaves through the top and is passed through a meter run for measurement purposes.

The degree of separation between gas and liquid depends on the separator operating pressure, the residence time of the fluid mixture and the type of fluid flow. All three of these parameters will be accounted for in the calculations.

4-2 4.2

Process Description

The purpose of the calculations in this chapter is to size the two phase separator V-101 that performs the separation of the incoming vapour from the first catalytic reactor R-101 into a waste vapour stream and liquid propanal that would later enter the second reactor, R-102. This separation therefore involves only vapour and heavy liquid. The absence of a light liquid distinguishes this type of separator from the more conventional three-phase one. This separator operates under high pressure but low temperature, at 1990 kPa and 10oC. Figure 2.1 below exhibits the schematic (not to scale) diagram of the proposed two phase separator.

9

8

10

Figure 4.1: Schematic diagram of a horizontal two phase separator

4-3 4.3

Chemical Design

4.3.1

Steps Taken for Separator Design

Below are the steps taken to determine separator chemical design specification: 1.

Calculate the design flow.

2.

Determination of section 1 sizing.

3.

Determination of section 2 sizing.

4.

Vapour Liquid Separation. Check gas available area.

4.3.2

Types of Separator

A separator can be either horizontal or vertical. Spherical separators may also be used for high pressure and high liquid hold-up systems like storage of light hydrocarbons etc. The choice between horizontal or vertical types of separator primarily depends upon the following process requirements: 

relative liquid and vapour load,



availability of plot area,



economics,



special considerations.

Table 4.1: Selection guideline for separator types System Characteristics

Type of Separator

Large vapour, less liquid Load (by volume)

Vertical

Large liquid, less vapour Load (by volume)

Horizontal

Large vapour, large liquid Load (by volume)

Horizontal

Liquid-liquid separation

Horizontal

Liquid-solid separation

Vertical

The horizontal three-phase separator is the most conventional and versatile type of process in the three phase industry. Design procedures of this type of separator can also be incorporated into the simpler one of the two-phase separator.

4-4

Vapour-liquid disengagement section

Liquid section

Figure 4.2: Sections in the separator Section 1 is basically the liquid division of the separation system where heavy liquid propanal is most prevalent at. Section 2 covers the full length of the vessel and is where vapour and liquid disengagement occurs.

4.3.3

Design Data

4.3.3.1

Calculation for Gas Mixture Density

The critical temperatures and pressures are needed to determine the densities for gas mixture. These critical properties as displayed in Table 4.3 are used to find the compressibility factor Z, which can be estimated from a generalised compressibility plot.

Table 4.2: Molecular weights of each component Component

Formula

Molecular weight (kg/mol)

Carbon Monoxide

CO

28.0

Hydrogen

H2

2.02

Propanal

CH3CH2CHO

58.08

Ethylene

C2H4

28.05

Ethane

C2H6

30.07

4-5 Table 4.3: Critical properties for each component Critical temperature, Tc (K)

Critical pressure, Pc (bar)

Critical volume, Vc 3 (m /mol)

Carbon Monoxide

133.2

35.0

0.089

Hydrogen

33.2

13.0

0.065

Propanal

496.5

47.6

0.223

Ethylene

282.9

50.3

0.129

Ethane

305.4

48.8

0.148

Component

Table 4.4: Separator inlet and outlet data Stream

8 (feed)

9 (liquid out)

10 (gas out)

Pressure (kPa)

1990

1990

1990

Temperature (oC)

10

10

10

Mass flow (kg/h)

35400

16400

19030

Mole flow (kmole/h)

1509

283.9

1225

Vapour fraction

0.562

0

1

Carbon monoxide

0.4016

0.0036

0.4938

Hydrogen

0.3996

0.0026

0.4916

Propanal

0.1948

0.9925

0.01

Ethylene

0.002

0.0006

0.0023

Ethane

0.002

0.0008

0.0023

Component mole fractions

4-6 8

𝑃𝑐,𝑚 =

𝑃𝑐,𝑖 𝑦𝑖 𝑛=1 8

𝑇𝑐,𝑚 =

𝑇𝑐,𝑖 𝑦𝑖 𝑛=1

Where,

Pc = critical pressure, Tc = critical temperature, y = mole fraction, suffixes, m = mixture, i = component.

Pc,m of gas out: 5

𝑃𝑐,𝑚 =

𝑃𝑐,𝑖 𝑦𝑖 𝑛=1

= (35.0 x 0.4938) + (13.0 x 0.4916) + (47.6 x 0.0100) + (50.3 x 0.0023) + (48.8 x 0.0023) = 18.37773 bar

Tc,m of gas out: 8

𝑇𝑐,𝑚 =

𝑇𝑐,𝑖 𝑦𝑖 𝑛=1

= (133.2 x 0.4938) + (33.2 x 0.4916) + (496.5 x 0.0100) + (282.9 x 0.0023) + (305.4 x 0.0023) = 89.41347 K Pr = P/Pc,m Where,

Pr = reduced pressure Pc,m = critical pressure Tr = T/Tc,m

Where,

Tr = reduced temperature Tc,m = critical temperature

4-7 Pr of gas out: Pr = P/Pc,m = 19.9 bar / 18.37773 bar = 1.08283 bar Tr of gas out: Tr = T/Tc,m = 283.5 K / 89.41347 K = 3.17066 With Pr = 1.08283 bar and Tr = 3.17066 K, the value of the compressibility factor, Z is 1.0.

Specific volume of outlet gas: V/n = Z (RT/P) Where,

P = absolute pressure, bar V = volume, m3 n = moles of gas T = absolute temperature, K Z = compressibility factor R = universal gas constant, 0.083 bar.m3/kmol

V/n = 1 [(0.083 bar.m3/kmol)(313.15 K)/1.5 bar] = 17.3276 m3/kmol

Density of gas mixture going out of the separator: Pv = A MWi,gas / (V/n) Therefore, Pv = (57.7668 kg/kmole) / (17.3276 m3/kmol) = 3.334 kg/m3

4-8 Using the above calculations, the densities of the other streams are also computed and tabulated in Table 4.5 below:

Table 4.5: Stream densities Stream

10

11

12

*Density (kg/m3)

24.12

804.1

3.334

4.3.4

Design Flow Rates

A flow rate is defined by; Q=

𝑚 𝜌

Where, Q = Volumetric flow rate (m3/min) 𝜌 = Gas phase density (kg/m3) 𝑚 = Mass flow rate (kg/hr)

Volumetric flow rate for vapor phase, 𝑄𝑔 =

𝑚𝑔 𝜌𝑔

=

19030 kg/h 3.334 kg m3 x 60 min

= 95.131 m3/min Volumetric flow rate for liquid phase, 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 = = =

𝑚𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝜌𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙

16400 kg/h 804.1 kg m3 x 60 min 0.3399 m3/min

Zero margins are added to separator flow or design.

4-9 So, design flows are; 𝑸𝒈 = 95.131 m3/min 𝑸𝒑𝒓𝒐𝒑𝒂𝒏𝒂𝒍 = 0.3399 m3/min

4.3.5

Assumptions

1.

Vessel dished end volumes are ignored to simplify calculation and add margin.

2.

No vessel margin shall be added to maximum flow rate.

3.

No design margin shall be added to separator sizing.

4.

Residence time for two phase separator is 5 to 30 minutes.

4.3.6

Calculation of Section 1 Sizing

4.3.6.1

Volume of Cylinder Section

The separator is required to have residence time of 30 minutes. Therefore the required volume operating volume is: Vpropanal = 0.339 m3/min x 30 mins = 10.17 m3 = Total Liquid Operating Volume

The vessel Normal Liquid Level (NLL) is intended to be more than 50% of the vessel diameter; this is equivalent to 50% of the vessel volume. Cylinder volume, Vcyl = Liquid operating volume/0.5 = 10.17 m3/ 0.5 = 20.34 m3

4.3.6.2

Diameter and Length of Vessel

In the design of a horizontal separator, the vessel diameter cannot be determined independently of its length. The length to diameter ratio is in the range 2.5 to 5.0, the smaller diameter at higher pressure and for liquid settling. A rough dependence on pressure is based Table 4.6 below.

4-10 Table 4.6: L/D ratio dependence on pressure P (kPa)

0 ≤ P ≤ 1724

1731 ≤ P ≤ 3447

3454 ≤ P

L/D

3

4

5

(Source: Sinott et al, 2005)

The suitable L/D ratio for 1990 kPa is 4 Lv / Dv = 4 Lv = 4Dv Volume of vessel, 𝑉𝑐𝑦𝑙

𝜋𝐷𝑣 2 𝐿𝑣 = 4

Where, Vcyl = Cylinder volume (m3) Dv = Vessel diameter (m) Lv = Vessel length (m)

Subtitute Lv = 4Dv into equation above, Therefore 𝑉𝑐𝑦𝑙 = 𝜋𝐷𝑣 3 Rearrange equation above. So that diameter of the vessel is

𝐷𝑣 =

𝐷𝑣 =

3

𝑉𝑐𝑦𝑙 𝜋

3

20.34 𝑚3 𝜋

= 1.8638 m Select standard separator diameter = 2.1336 m (7 ft) Length of the vessel, L1 = 4Dv = 4 x 2.1336 = 8.5344 m

4-11 Pseudo-weir Section Sizing

This section is the volume to the right of where the weir would be if this separator was a three phase one. It is a nominal length to allow for the heavy liquid propanal outlet nozzle. This length is typically 0.3 of the vessel diameter. Vessel diameter, Dv = 2.1336 m Typical weir section length, L2 = 0.3 Dv = 0.3 (2.1336) m = 0.7 m Total Vessel Length

= L1 + L2 = (8.5344 + 0.7) m = 9.2344 m

4.3.6.3

New Volume Cylinder Section

Volume for selected separator size is, Vcyl

πDv 2 L1 𝜋 2.13362 𝑚 × (9.2344 𝑚 ) = = 4 4 = 33.016 m3

Operating volume of separator = Vcyl x 0.5 = 33.016 m3 x 0.5 = 16.508 m3

4-12 4.3.6.4

Liquid Section Level Setting

The partial volumes within the vessel are calculated using the following equation for the area of the segment of a circle. (Perry, 1997)

A

H Figure 4.3: Vessel cross-section

𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟 2 𝑐𝑜𝑠 −1

𝑟−𝐻 − 𝑟

𝑟−𝐻

2𝑟𝐻 − 𝐻 2

Where, Asegment = Area of the segment (m2) r = Radius of the vessel (m) H = Height of the liquid above the vessel base (m) There area of the segment can then be multiplied by the length of the section to determine the partial volume. From the process design philosophy, level settings should be as minimum as specified in Table 4.7 below.

Table 4.7: Level setting in the separator Level type Level Alarm High High (LAHH) Level Alarm High (LAH) Normal Alarm Level (NAL) Level Alarm Low (LAL)

Level Alarm Low Low (LALL)

Level setting 30 – 60 seconds or 200 mm whichever is greater 30 – 60 seconds or 200 mm whichever is greater 60% of horizontal separator 30 – 60 seconds or 200 mm whichever is greater 30 – 60 seconds or 200 mm whichever is greater Should be at least 200 mm above the vessel bottom or maximum interface level

4-13 4.3.6.5

Residence Time for Propanal

Vessel radius, r = D/2 = 2.1336 m / 2 = 1.0668 m Section length, L = 9.2344 m 1 minute of heavy liquid propanal hold up = operating volume for propanal = 10.17 m3 Liquid section volume = 16.508 m3 Propanal hold up

= 30 min

At Normal Liquid Level (NLL)

Internal level = 0.067 m Cumulative level = 1.067 m 𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟 2 𝑐𝑜𝑠 −1

𝑟−𝐻 − 𝑟

𝑟−𝐻

1.0668 −1.067 1.0668

= (1.06882) cos-1

2𝑟𝐻 − 𝐻 2 – [(1.0668-1.067)

2 × 1.0668 × 1.067 − 1.0672 ] = 1.7877 m2 Cumulative volume, V = Asegment x L = 1.7877 m2 x 9.2344 m = 16.5083 m3

4-14 At Level Alarm Low (LAL)

Internal level = 0.200 m Cumulative level = 1.00 m 𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟 2 𝑐𝑜𝑠 −1

𝑟−𝐻 − 𝑟

𝑟−𝐻

1.0668 −1 1.0668

= (1.06682) cos-1

2𝑟𝐻 − 𝐻 2

– [(1.0668-1)

2 × 1.0668 × 1 − 12 ]

= 1.6452 m2 Cumulative volume, V = Asegment x L = 1.6452 m2 x 9.2344 m = 15.1924 m3

Internal volume at NLL = Cumalative volume at NLL – Cumulative volume at LAL = 16.5083 m3 - 15.1924 m3 = 1.3159 m3 Internal hold-up time for heavy liquid propanal; t = V /1 minutes of heavy liquid propanal – up = 1.3159 m3 / 10.17 m3 = 0.13 mins

These calculations were repeated for LAL, LALL, LIAHH, LIAH, NIL, LIAL, LIALL and vessel bottom. Table 4.8 below displays the summary of the level calculations for the separator.

4-15

Table 4.8: Liquid levels Level

Internal level (m)

Cumulative level (m)

Cumulative volume (m3)

Internal volume (m3)

Internal hold-up time -propanal (minutes)

NLL

0.067

1.067

16.5083

1.3159

0.13

LAL

0.200

1.000

15.1924

3.8874

0.38

LALL

0.200

0.800

11.3050

2.4729

0.24

LIAHH

0.150

0.600

8.8222

3.7508

0.37

LIAH

0.100

0.450

5.0714

1.5364

0.15

NIL

0.100

0.350

3.5350

1.3677

0.13

LIAL

0.100

0.250

2.1673

1.1448

0.11

LIALL

0.150

0.150

1.0225

1.0225

0.10

Vessel Bottom

0.000

0.000

0.0000

0.0000

0.0000

Residence time for heavy liquid propanal, tpropanal = time from Vessel Bottom to NLL = (0.13 + 0.38 + 0.24 + 0.37 + 0.15 + 0.13 + 0.11 + 0.10) mins = 96.6 seconds

4.3.7

Vapour-Liquid Disengagement Section

This section contains the oil high level alarm and high level trip. The volumes are calculated in the same way as for the liquid section, but the whole vessel length can be used. Vessel radius, r = =

𝐷 2 2.1336 𝑚 2

= 1.0668 m Vessel length, L = 9.2344 m 1 min of heavy liquid propanal hold-up = operating volume for heavy liquid propanal = 10.17 m3

4-16 At Level Alarm High High (LAHH)

Internal level = 0.202 m Cumulative level = 1.579 m 𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟 2 𝑐𝑜𝑠 −1

𝑟−𝐻 − 𝑟

2𝑟𝐻 − 𝐻 2

𝑟−𝐻

1.0668−1.579 1.0668

= (1.06682) cos-1

– [(1.0668-1.579)

2 × 1.0668 × 1.579 − 1.5792 = 2.8365 m2 Cumulative volume, V = Asegment x L = 2.8365 m2 x 9.2344 m = 26.1934 m3 Level % of Vessel diameter = (Cumulative level / Vessel diameter) x 100% = (1.579 m / 2.1336 m) x 100% = 74.00% At Level Alarm High (LAH)

Internal level = 0.30 m Cumulative level = 1.367 m 𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟 2 𝑐𝑜𝑠 −1

𝑟−𝐻 − 𝑟

2𝑟𝐻 − 𝐻 2

𝑟−𝐻

1.0668−1.367 1.0668

= (1.06682) cos-1

– [(1.0668-1.367)

2 × 1.0668 × 1.367 − 1.3672 = 2.4192 m2 Cumulative volume, V = Asegment x L = 2.4192 m2 x 9.2344 m = 22.3399 m3 Level % of Vessel diameter = (Cumulative level / Vessel diameter) x 100% = (1.367 m / 2.1336 m) x 100% = 64.06%

4-17 Internal volume at LAHH = Cumulative vol. at LAHH – Cumulative vol. at LAH = 26.1834 m3 – 22.3399 m3 = 3.8435 m3 Internal hold-up time for heavy liquid propanal, t = V/1 minute of heavy liquid proanal hold-up = 3.8435 m3 / 10.17 m3 = 0.3779 mins

These calculation steps were repeated for LAH and NLL. Table 4.9 below shows the summary of the level calculations for the vapour section of the separator.

Table 4.9: Vapour section liquid levels

Level

Internal level (m)

LAHH

0.202

1.579

26.1934

3.8535

Internal hold-up time – propanal (mins) 0.38

LAH

0.300

1.367

22.3399

5.8316

0.57

NLL

1.067

1.067

16.5083

0.0000

0.0000

Cumulative Cumulative level volume (m) (m3)

Internal volume (m3)

The LAH volume is 5.83 m3 as calculated and tabulated above. Therefore, the surge volume can be accommodated within the LAH volume.

4.3.8

Vapour Liquid Separator

Most separators that employ mist extractor are sized using equations that are derived from gravity setting equation. The most common equation used is the critical velocity equation:

𝑉𝑐 = 𝐾

𝜌𝑙 − 𝜌𝑔 𝐿𝑣 𝜌𝑔 10

0.56

4-18 Where, Vc = Critical gas velocity necessary for particle to drop or settle (m/s) 𝜌𝑙 = density of liquid (kg/m3) ρg = density of vapour (kg/m3) Lv = Vessel length (m) K = 0.101 (refer to table 2.10) ρl = 804.1 kg/m3 ρg = 3.334 kg/m3 Lv = 9.2344 m 804.1 𝑘𝑔/𝑚 3 −3.334𝑘𝑔/𝑚 3 9.2344 𝑚 ) ( 10 )0.56 3.334 𝑘𝑔 /𝑚 3

Vc = 0.101 (

= 1.5308 m/s

Table 4.10: Typical K factors for the sizing of wire mesh demisters Separator type

K factor (m/s)

Horizontal (with vertical pad)

0.122 to 0.152

Spherical

0.061 to 0.107

Vertical or horizontal (with horizontal pad)

0.055 to 0.107

At atmospheric pressure

0.107

At 2100 kPa

0.101

At 4100 kPa

0.091

At 6200 kPa

0.082

At 10300 kPa

0.064

Wet steam

0.076

Most vapours under vacuum

0.061

Salt and caustic evaporators

0.046

(Source: IPS-E-PR-880, 1997) Note that the preferred orientation of the mesh pad in horizontal separators is in the horizontal plane, and it is reported to be less efficient when installed in vertically.

4-19 4.3.8.1

Area for Vapour

4.3.8.1.1

Area Required for Vapour Flow

Vs = 1.5308 m/s Qg = 95.131 m3/min = 1.5855 m3/s Area required for gas flow, Ag = Qg / Vs = (1.5855 m3/s) / (1.5308m3/s) = 1.03573 m2

4.3.8.1.2

Vapour Height

Liquid height at liquid mixture LAHH, HLAHH = 1.579 m Vapour height, Hv = Dv - HLAHH = 2.1336 m – 1.579 m = 0.555 m

4.3.8.1.3

Area Available for Vapour

Total Vessel Area, Av =

𝜋𝐷 2 4

= 3.5753 m2

Area of liquid, Al = Area at LAHH = 2.8365 m2 Area of available gas = Total Area – Liquid Area = 3.5753 m2 – 2.8365 m2 = 0.7388 m2 Therefore, the area available for gas is acceptable.

4-20 4.3.9

Mist Extraction Section

Wire mesh pads are frequently used as entrainment separators for the removal of very small liquid droplets and therefore a higher overall percentage removal of liquid. Most installation will use a 150 mm thick pad with 150kg/m3 bulk density. Minimum recommended pad thickness is 100 mm. The pad length recommended is 0.348 to be installed0.0508 m from the roof of the vessel. (Sinnot et al, 2005)

4.3.10

Conclusion

Chemical design specifications: Table 4.11: Summary of the chemical design for this separator Item

Value

Diameter of vessel, D

2.1336 m

Length of vessel, L

9.2344 m

Volume of vessel, V

33.016 m3

Critical velocity, Vc

1.5308 m/s

Area of vessel, Av

3.5753 m2

Area of liquid, Asegment

2.8365 m2

Area of vapour, Ag

0.7388 m2

4-21 4.4

Mechanical Design

4.4.1

Steps Taken for Separator Design

Below are the steps taken to determine mechanical design specification for a two-phase horizontal separator: 1. Determination of separator design pressure. 2. Determination of separator design temperature. 3. Determination of suitable material for construction. 4. Determination of separator design stress. 5. Determination of cylindrical wall thickness. 6. Determination of head and closure. 7. Determination of weight loads. 8. Determination and selection of a suitable separator support. 9. Determination of nozzle size. 10. Determination of flanges.

4.4.2

Design Pressure

In order to allow for possible surges in operating, it is customary to raise the maximum operating pressure by 10%. Operating Pressure, Pi = 19.9 bar (absolute value) By considering 10% safety factor for internal pressure, the design pressure, Pdesign is, 10 100

Pdesign = (

× 19.9 bar) + 19.9 bar

= 21.89 bars = 2.189 N/mm2

4-22 4.4.3

Design Temperature

T = 10oC = 50oF Tmax = T + 50oF = 50oF + 50oF = 100oF = 37.78oC

4.4.4

Material of Construction

Many factors need to be considered when selecting engineering materials, but for a chemical process plant the overriding consideration is usually the ability to resist corrosion. The material selected must have sufficient strength and easily operated. The most economical material that satisfies both process and mechanical requirements should be selected; this would be the material that gives the lowest cost over the working life of the plant, allowing for maintenance and replacement. Other factors such as product contamination and process safety must also be considered.

Table 4.12 shows some criteria to be considered in selecting the material to be used in constructing the separator. The melting points and corrosion resistance towards the components in the separator are the main criteria that will affect the system. Table 4.12: Construction material characteristics Criteria Melting point (oC) Density (kg/m3) Corrosion resistance

Aluminium

Stainless steel 304

Carbon steel

Lead

Copper

660

1371- 1399

1540

327

1084

2700

8300

7900

11340

8940

Low

High

High

Low

Low

From the criteria above, it can be concluded that Carbon Steel is the best material to be used in constructing our separator.

4-23 4.4.5

Design Stress

The material to be used is carbon steel. The design stress for a design temperature of 37.8oC is obtainable from Table 4.13 below.

Table 4.13: Typical design stresses

Material

0 to 50

100

150

200

250

300

350

400

360

135

125

115

105

95

85

80

70

460

180

170

150

140

130

115

105

100

450

180

170

145

140

130

120

110

110

550

240

240

240

240

240

235

230

510

165

145

130

115

110

105

540

165

150

140

135

130

175

150

135

120

115

(N/mm2) Carbon steel (semikilled or silicon killed) Carbonmanganese steel (semikilled or silicon killed) Carbonmolybdenum steel 0.5% Mo Low alloy steel (Ni, Cr, Mo, V) Stainless steel 18Cr/8Ni unstabilised (304) Stainless steel 18Cr/8Ni Ti stabilised (321) Stainless steel 18Cr/8ni

Design stess at temperature o C (N/mm2)

Tensile Strength

450

500

220

190

170

100

100

95

90

130

125

120

120

115

110

105

105

100

95

1

Mo 22 % (316) 520 (Source: Sinnott, 2005)

Design stress, f = 135 N/mm2, Tensile stress

= 360 N/mm2

4-24 4.4.6

Vessel Thickness

4.4.6.1

Minimum Practical Wall Thickness

There will be a minimum wall thickness required to ensure that any vessel is sufficiently rigid to withstand its own weight and any incidental loads. As general guide the wall thickness of any vessel should not be less than the values given in Table 4.14 below. The values include a corrosion allowance of 2mm.

Table 4.14: Minimum thickness according to vessel diameter Vessel diameter (m)

Minimum thickness (mm)

1

5

1.0 to 2.0

7

2.0 to 2.5

9

2.5 to 3.0

10

3.0 to 3.5

12

(Source: Sinnott, 2005)

Minimum wall thickness required is given by, 𝑃𝐷

t = 2𝑗𝑓𝑖− 𝑖𝑃 + c 𝑖

Where, t = minimum thickness required (mm) Pi = operating pressure (N/mm2) Di = internal diameter (mm) f = design stress (N/mm2) J = joint factor, (taken as 1) c = corrosion allowance, (taken as 2 mm) Pi = 2.189 N/mm2 Di = 2133.6 mm = 135 N/mm2

f t=

2.189 × 2133 .6 2 ×1 ×135 − 2.189

+2

= 19.4394 mm ≈ 20 mm

4-25 The thickness is of the separator wall is ideal.

4.4.7

Design of Heads and Closure

Heads and closures are used at the end of a cylindrical vessel. The heads come in various shapes and the principal types used are hemispherical heads, ellipsoidal heads and torispherical heads. For this design, an ellipsoidal head design is chosen as it is the most commonly used as end closures for high pressure vessel and as well as being economically effective for vessels with an operating pressure above 15 bar. (Sinnott, 2005)

4.4.7.1

Ellipsoidal Heads

Most standard ellipsoidal heads are manufactured with a major and minor axis ratio of 2:1. For this ratio, the following equation can be used to calculate the minimum thickness required:

t=

𝑃𝑖 𝐷𝑖 2𝑆𝐸−0.2𝑃𝑖

Where, S = maximum allowable stress E = joint efficiency

4-26 Table 4.15: Weld Joint Efficiencies

Joint Type

Degree of Radiographic Examination

Acceptable Joint Categories

Full

Spot

None

1

0.85

0.7

1

A, B, C, D

2

A, B, C, D (See ASME Code for limitations)

0.9

0.8

0.65

3

A, B, C

NA

NA

0.6

4

A, B, C (See ASME Code for limitations)

NA

NA

0.55

5

B, C (See ASME Code for limitations)

NA

NA

0.5

6

A, B (See ASME Code for limitations)

NA

NA

0.45

Table 4.16: ASME Maximum Allowable Stress ALLOWABLE STRESS IN TENSION FOR CARBON AND LOW ALLOY STEEL Spec. Nominal No Grade Composition Carbon Steel Plates and Sheets SA-515 55 C-Si 60 C-Si 65 C-Si 70 C-Si SA-516

55 60 65 70

C-Si C-Mn-Si C-Mn-Si C-Mn-Si

Low Alloy Steel Plates SA-387 2 Cl.1 1/2Cr - 1/2/Mo 2 Cl.2 1/2Cr - 1/2Mo 12 Cl.1 1Cr - 1/2Mo 12 Cl.2 1Cr - 1/2Mo 11 Cl.1 1 1/4Cr - 1/2Mo-Si 11 Cl.2 1 1/4Cr - 1/2Mo-Si 22 Cl.1 2 1/4Cr - 1Mo 22 Cl.2 2 1/4Cr - 1Mo

P-No.

Group No.

Min. Yield (ksi)

Min. Tensile (ksi)

1 1 1 1

1 1 1 2

30 32 35 38

55 60 65 70

1 1 1 1

1 1 1 2

30 32 35 38

55 60 65 70

3 3 4 4 4 4 5 5

1 2 1 1 1 1 1 1

33 45 33 40 35 45 30 45

55 70 55 65 60 75 60 75

4-27 Table 4.17: ASME Maximum Allowable Stress (cont’d) ALLOWABLE STRESS IN TENSION FOR CARBON AND ALLOY STEEL Maximum Allowable Stress, ksi for Metal Temperature oF, Not Exceeding 650

700

750

800

850

900

950

1000 1050 1100 1150 1200

Spec. No

Carbon Steel Plates and Sheets 13.8 13.3 12.1 10.2 15

14.4

13

10.8

16.3 15.5 13.9 11.4 17.5 16.6 14.8

12

13.8 13.3 12.1 10.2 15

14.4

13

10.8

16.3 15.5 13.9 11.4 17.5 16.6 14.8

12

8.4

6.5

4.5

2.5

SA-515

8.7

6.5

4.5

2.5

SA-515

9

6.5

4.5

2.5

SA-515

9.3

6.5

4.5

2.5

SA-515

8.4

6.5

4.5

2.5

SA-516

8.7

6.5

4.5

2.5

SA-516

9

6.5

4.5

2.5

SA-516

9.3

6.5

4.5

2.5

SA-516 Low Alloy Steel Plates (Cont'd)

13.8 13.8 13.8 13.8 13.8 13.3

9.2

5.9

SA-387

17.5 17.5 17.5 17.5 17.5 16.9

9.2

5.9

SA-387

13.8 13.8 13.8 13.8 13.4 12.9 11.3

7.2

4.5

2.8

1.8

1.1

SA-387

16.3 16.3 16.3 16.3 15.8 15.2 11.3

7.2

4.5

2.8

1.8

1.1

SA-387

15

14.6 13.7

9.3

6.3

4.2

2.8

1.9

1.2

SA-387

18.8 18.8 18.8 18.8 18.3 13.7

9.3

6.3

4.2

2.8

1.9

1.2

SA-387

8

5.7

3.8

2.4

1.4

SA-387

7.8

5.1

3.2

2

1.2

SA-387

15

15 15

15 15

15 15

14.4 13.6 10.8

17.7 17.2 17.2 16.9 16.4 15.8 11.4

Based on Table 2.16, the chosen type of carbon-steel plate for the separator’s ellipsoidal head is SA-515 Gr. 60. With a design temperature of 37.78 oF (not exceeding 600oF), the maximum allowable stress, S, is 15 ksi = 15 000 psi . Based on Table 2.15, the joint efficiency, E, is 1. Therefore, with Pi = 2.189 N/mm2 = 473.1 psig and Di = 2.1336 m = 85.3 in; 473.1 × 85.3 − 0.2 ×473.1 ]

t = [ 2 ×15000 ×1 = 1.35 in

= 3.38 cm ≈ 𝟑𝟒 𝐦𝐦 For convenience, the thickness of the vessel is taken to be the same as the head thickness = 34 mm

4-28 4.4.8

Weight Loads

4.4.8.1

Weight of Shell

For preliminary calculations, the approximate weight of a cylindrical vessel with ellipsoidal heads and uniform thickness all around, can be estimated from the equation below: Wv = 240CvDm(Hv + 0.8Dm)t Where, Wv = total weight of the shell, excluding internal fittings such as plates (N) Cv = a factor to account for the weight of nozzles, manways and internal supports. (for separator = 1.08) Hv = height or length of the cylindrical section (m) Dm = mean diameter of vessel = Di + t x 10-3 (m) t = wall thickness, (mm) Mean diameter, Dm = Di + t × 10-3 = 2.1336 + 34 × 10-3 = 2.1676 m Therefore, Wv = 240(1.08)(2.1676)[9.2344 + (0.8 × 2.1676)](34) = 209.53 kN

4.4.8.2

Weight of Insulation

Mineral wool is chosen due to its characteristics that make it a great insulator at absorbing heat. Mineral wool density

= 130kg/m3

Thickness of insulation = 75 mm Approximate value of insulation; Vi = π × Dm × Hv × thickness of insulation Vi = π × 2.1676 m × 9.2344 m × 0.075 m = 4.72 m3

4-29 Weight of insulation; Wi = Vi × ρ × g = 4.72 m3 × 130kg/m3 × 9.81m/s2 = 6.02 kN Double this value to allow for fitting, therefore W i = 12.04 kN

4.4.8.3

Weight of Demister Pad

In this separation, stainless steel pads around 100mm thick and with a nominal density of 150kg/m3 is to be used. Demister pad density = 150 kg/m3 Demister pad thickness = 100 mm Pad area, A = (0.348 m)2 = 0.696 m2 Weight of pad; Wp = A × ρ × thickness× g = 0.696 m2 × 150 kg/m3 × 0.1 m × 9.81 m/s2 = 0.11 kN Therefore, total weight; WT = Wv +Wp + Wi = 209.53 kN + 0.11 kN + 6.02 kN = 215.66 kN

4.4.9

Wind Loads

Wind loads are only important and considered when designing tall columns to be installed outdoors. Since our separator is horizontal with a diameter of only 2.1336m, wind loads are therefore insignificant.

4-30 4.4.10

Design of Saddle Support

The method used to support a vessel depends on the size, shape and weight of the vessel; the design temperature and pressure; the vessel location and arrangement; and the internal and external fittings and attachments. For a horizontal vessel, it is commonly mounted with two saddle supports (Sinnot, 2005).

Figure 4.4: Horizontal cylindrical vessel on saddle supports

Figure 4.5: The dimensions of the saddle support

4-31 Table 4.18: The dimensions of the saddle support

Dvessel (m)

Max. weight

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.6

Dimensions (m)

(mm)

(kN)

V

Y

C

E

J

G

230 330 380 460 750 900 1000 1350 1750 2000 2500

0.88 0.98 1.08 1.18 1.28 1.38 1.48 1.58 1.68 1.78 1.98

0.20 0.20 0.20 0.20 0.23 0.23 0.23 0.25 0.25 0.25 0.25

1.24 1.41 1.59 1.77 1.95 2.13 2.30 2.50 2.64 2.82 3.20

0.53 0.62 0.71 0.8 0.89 0.98 1.03 1.10 1.18 1.26 1.40

0.305 0.350 0.405 0.500 0.529 0.565 0.590 0.025 0.665 0.730 0.815

0.140 0.140 0.140 0.140 0.150 0.150 0.150 0.150 0.150 0.150 0.150

t2

t1

12 10 12 10 12 10 12 10 16 12 16 12 16 12 10 12 16 12 16 12 16 12

Dbolt 24 24 24 24 24 2733 2733 2733 2733 2733 2733

Bolt holes 30 30 30 30 30 33 33 33 33 33 33

From Table 4.18 above, the dimensions of the saddles suitable for our separator are extracted and displayed in Table 4.19 below. The diameter used to obtain the dimensions the dimensions is 2.2 m (diameter of the vessel). The saddle’s material is concrete.

Table 4.19: Selected dimensions for the saddle supports

Dvessel (m) 2.134

Max. weight (kN) 750

4.4.11

Dimensions (m) V

Y

1.28 0.225

C 1.95

(mm) t2

t1

Dbolt

Bolt holes

0.89 0.520 0.510 16

12

24

30

E

J

G

Nozzle Sizing

The sizing of nozzles shall be based on the maximum flow rates, including the appropriate design margin. Nozzles shall be sized according to the following criteria (PTS,2002).

4-32 For inlet ρV2 < 1400.0 kg/ms2

No inlet device:

Half pipe inlet device: ρV2 < 2100.0 kg/ms2 ρV2 < 8000.0 kg/ms2

Inlet vane:

For outlet ρV2 < 2100.0 kg/ms2

Gas outlet:

V2 < 2.0 m/s

Liquid outlet

4.4.11.1

Inlet Nozzle Sizing

The volumetric flow for all; Qg

= 95.131 m3/min

Qpropanal = 0.3399 m3/min Qtotal

= Qg + Qpropanal = 95.131 m3/min + 0.3399 m3/min = 95.4709 m3/min = 1.5912 m3/s

The density, ρg

= 3.334 kg/m3

ρpropanal = 804.1 kg/m3 ρmixture =

𝜌 𝑔 𝑄𝑔 + 𝜌 𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝑄𝑔 + 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙

= 11.748 kg/m3

Assume inlet vane pack, therefore; Allowable ρV2 = 8000.0 kg/ms2 Allowable velocity, v = =

𝜌𝑉 2 /𝜌 𝑘𝑔

𝑘𝑔

8000 𝑚𝑠 2 /11.748 𝑚 3

= 680.967 = 26.095 m/s

4-33 So, the nozzle area, A = Qtotal / v =

1.5912 m 3 /s 26.095 m/s

= 0.061 m2 4𝐴/𝜋

Required nozzle diameter, dnozzle-in = =

4 0.061 𝜋

= 0.28m = 280 mm

4.4.11.2

Vapour Outlet Nozzle Sizing

The volumetric flow for gas outlet; Qg = 95.131 m3/min = 1.586 m3/s Gas outlet density; ρg = 3.334kg/m3 Allowable ρV2 = 1500 kg/ms2 𝜌𝑉 2 /𝜌

Allowable velocity, v = =

1500 3.334

= 449.91 m/s

So, the nozzle area, A = Qg/v =

1.586 𝑚 3 /𝑠 449.91 𝑚 /𝑠

= 0.0035 m2

Required nozzle diameter, dnozzle-out = =

4𝐴/𝜋 4 0.0035 𝜋

= 0.067 m = 66.76 mm

4-34 4.4.11.3

Heavy Liquid Propanal Outlet Nozzle Sizing

The volumetric flow for heavy liquid propanal outlet; Qpropanal = 0.3399 m3/min = 0.0057 m3/s Heavy liquid propanal outlet density; ρpropanal = 804.1 kg/m3 Allowable velocity, v = 2 m/s

So, the nozzle area, A = Qpropanal/v =

0.0057 𝑚 3 /𝑠 2 𝑚 /𝑠

= 0.0029 m2

Required nozzle diameter, dnozzle-propanal = =

4𝐴/𝜋 4(0.0029)/𝜋

= 0.0061 = 61 mm

4.4.12

Standard Flanges

Flanged joints are used for connecting pipes and instruments to vessels, for manhole covers and for removable vessel heads when ease of access is required. Figure 4.6 below shows the typical standard flange design (Sinnott, 2005).

Figure 4.6: Standard flange design dimensions

4-35 Table 4.20: Standard flange design specifications Nom. Size 200 250 300 350 400 450

Pipe o.d. d1 219.1 273 323.9 355.6 406.4 457.2

Flange D b h1

Raised face d4 f

340 395 445 505 565 615

24 26 26 26 26 28

62 68 68 68 72 72

268 320 370 430 482 532

3 3 4 4 4 4

M20 M20 M20 M20 M24 M24

8 12 12 16 16 20

22 22 22 22 25 26

500 600 700 800 900

508 609.6 711.2 812.8 914.4

670 780 895 1015 1115

28 28 30 32 34

75 80 80 90 95

585 685 800 905 1005

4 5 5 5 5

M24 M27 M27 M30 M30

20 20 24 24 28

1000 1200 1400 1600 1800

1016 1220 1420 1620 1820

1230 1455 1675 1915 2115

34 38 42 46 50

95 115 120 130 140

1110 1330 1535 1760 1960

5 5 5 5 5

M33 M36 M39 M45 M45

2000

2020

2325

54 150

2170

5

M45

Bolting

Drilling No. d2 k 295 350 400 460 515 565

Neck d3 h2 ≈ 235 16 292 16 344 16 385 16 440 16 492 16

10 12 12 12 12 12

26 30 30 33 33

620 725 840 950 1050

542 642 745 850 950

16 18 18 18 20

12 12 12 12 12

28 32 36 40 44

36 39 42 48 48

1160 1380 1590 1820 2020

1052 1255 1460 1665 1868

20 25 25 25 30

16 16 16 16 16

48

48

2230

2072

30 16

r

Interpolation of table 4.20 was done by using D nominal of 280 mm of the inlet pipe and 67 mm, and 61 mm for the outlet pipes. The following values were obtained for bolt and flange designs for the separator.

Table 4.21: Values for bolt and flange for the inlet nozzle Nom. Size 210

Pipe o.d. d1 230

Flange D b h1 351

24

66

Raised face d4 f 278

3

Bolting M20

Drilling No. d2 k 9

22

306

Neck d3 h2 r ≈ 246 16 10

Table 4.22: Values for bolt and flange of the vapour outlet nozzle Nom. Size 67

Pipe o.d. d1 75.7

Flange D b h1 194

19

46

Raised face d4 f 130

3

Bolting M16

Drilling No. d2 k 4

14

149

Neck d3 h2 ≈ 83 9

r 5

4-36 Table 4.23: Nom. Size 61

Pipe o.d. d1 69.3

Values for bolt and flange for the heavy liquid propanal outlet nozzle

Flange D b h1 187

4.4.13

18

45

Raised face d4 f 123

3

Bolting M16

Drilling No. d2 k 4

13

142

Conclusion

Table 4.24: Summary of mechanical design Item Design pressure Design temperature

Value 2.189 N/mm2 27.8 oC

Material used

Carbon steel

Design stress

135 N/mm2

Tensile stress

369 N/mm2

Wall thickness

34 mm

Ellipsoidal head thickness

34 mm

Weight loads Type of support

221.68 kN Saddle support

Neck d3 h2 ≈ 77 9

r 4

4-37 4.5

Separator Costing

The material cost of the equipment is calculated using the equation below (Turton et al., Analysis, Synthesis, and Design of Chemical Processes, 3rd Edition, page 906): log10 Cp° = K1 + K2 log10 (A) + K3 [log10 (A)] 2 Where, A = capacity or size parameter for the equipment K1, K2, K3 = constants in Table A.1 (Appendix A)

Process vessel (horizontal):

Material of construction = carbon steel Diameter, D = 2.1336 m Length, L = 9.2344 m Volume, V = 33.016 m3 From Table A.1 (Appendix A); K1 = 3.5565 K2 = 0.3776 K3 = 0.0905 Therefore, log10 Cp° = 3.5565+ (0.3776) log10 (33.016) + (0.0905) [log10 (33.016)]2 = 4.3387 Cp° = $ 21 812.23 Pressure factors for process vessels: tvessel = 0.034 m P = 2.189 N/mm2

4-38

For pressure vessel, when vessel thickness, t vessel  0.003m,

𝐹𝑃,𝑣𝑒𝑠𝑠𝑒𝑙

=

𝑃 +1 𝐷 + 0.00315 2[850 − 0.6 𝑃 + 1 ] = 0.003 2.189 + 1 2.1336 + 0.00315 2[850 − 0.6 2.189 + 1 ]

0.003

= 2.39 The bare module factor for this process vessel (Turton et al., Analysis, Synthesis, and Design of Chemical Processes, 3rd Edition, page 927) is: CBM

= Cp°FBM = Cp°(B1 + B2FMFp)

From Table A.4 (Appendix A), B1 = 1.49, B2 = 1.52 From Table A.3 (Appendix A), the identification number for carbon steel horizontal process vessels is 18. Hence, from Figure A.18 (Appendix A), material factor, FM = 1.0 And so, CBM

= 21 812.23 [1.49 + (1.52)(1.0)(2.39)] = $ 111 739.69

Correlation: CEPCI for year of 2010 is 622.6 CEPCI for year of 2001 is 397 Therefore, New CBM = $ 111 739.69 x = $ 175 237.11 = RM 529 917.01

622.6 397

4-39 REFERENCES

Sinnot, R.K., Coulson, J.M., Richardson, J.F., (2005), Chemical Engineering

Design,

4th Edition, Vol. 6, UK: Butterworth-Heinemann. Perry, R.H., Green, D.W., (1997), Chemical Engineer’s Handbook, 7th

Edition, McGraw-

Hill Book Company. API 12J, (1989), Specification for Oil and Gas Separators, 7th Edition,

Washington DC:

American Petroleum Institute.

IPS-E-PR-880, (1997), Engineering Standard for Process Design og

Gas(Vapour)-

Liquid Separators, Original Edition.

Monarh,

D.,

Separators:

Gas/Oil,



Monarch

Separators

Inc.,