Chapter 4 - Separator Design

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 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.

8

9

10

Figure 4.1: Schematic diagram of a horizontal two phase separator

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.

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

Table 4.3: Critical properties for each component Component Critical temperature, Tc (K) Critical pressure, Pc (bar) Critical volume, Vc (m3/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

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 Component mole fractions 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

𝑃_𝑐,𝑚 = 𝑃_𝑐,𝑖𝑦_𝑖 8 𝑛=1 𝑇𝑐,𝑚 = 𝑇𝑐,𝑖𝑦𝑖 8 𝑛=1

Where, Pc = critical pressure, Tc = critical temperature, y = mole fraction, suffixes, m = mixture, i = component. Pc,m of gas out: 𝑃_𝑐,𝑚 = 𝑃_𝑐,𝑖𝑦_𝑖 5 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: 𝑇𝑐,𝑚 = 𝑇𝑐,𝑖𝑦𝑖 8 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

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

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,

𝑄_𝑔 =𝑚 𝑔 𝜌𝑔

= 19030 kg/h 3.334 kg m3 _{x 60 min}

= 95.131 m3/min

Volumetric flow rate for liquid phase, 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 = 𝑚 𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝜌𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 = 16400 kg/h 804.1 kg m3 _{x 60 min} = 0.3399 m3/min

So, design flows are;

𝑸_𝒈 = 95.131 m3/min 𝑸_{𝒑𝒓𝒐𝒑𝒂𝒏𝒂𝒍}= 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.

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 = 𝜋𝐷_𝑣2𝐿_𝑣 4

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

Subtitute Lv = 4Dv into equation above, Therefore 𝑉_𝑐𝑦𝑙 = 𝜋𝐷_𝑣3

Rearrange equation above. So that diameter of the vessel is

𝐷_𝑣= 𝑉𝑐𝑦𝑙 𝜋 3 𝐷_𝑣= 20.34 𝑚3 𝜋 3 = 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

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,

V_cyl =πDv 2_L 1 4 = 𝜋 2.13362 _{𝑚 × (9.2344 𝑚 )} 4 = 33.016 m3

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

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)

Figure 4.3: Vessel cross-section

𝐴_{𝑠𝑒𝑔𝑚𝑒𝑛𝑡} = 𝑟2_𝑐𝑜𝑠−1𝑟 − 𝐻

𝑟 − 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2

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 setting

Level Alarm High High (LAHH) 30 – 60 seconds or 200 mm whichever is greater

Level Alarm High (LAH) 30 – 60 seconds or 200 mm whichever is greater

Normal Alarm Level (NAL) 60% of horizontal separator

Level Alarm Low (LAL) 30 – 60 seconds or 200 mm whichever is greater

Level Alarm Low Low (LALL)

30 – 60 seconds or 200 mm whichever is greater

Should be at least 200 mm above the vessel bottom or maximum interface level

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 𝐴_{𝑠𝑒𝑔𝑚𝑒𝑛𝑡} = 𝑟2_𝑐𝑜𝑠−1𝑟 − 𝐻 𝑟 − 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2 = (1.06882) cos-11.0668−1.067 1.0668 – [(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

At Level Alarm Low (LAL) Internal level = 0.200 m Cumulative level = 1.00 m 𝐴_{𝑠𝑒𝑔𝑚𝑒𝑛𝑡} = 𝑟2_𝑐𝑜𝑠−1𝑟 − 𝐻 𝑟 − 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2 = (1.06682) cos-11.0668−1 1.0668 – [(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.

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

At Level Alarm High High (LAHH) Internal level = 0.202 m Cumulative level = 1.579 m 𝐴_{𝑠𝑒𝑔𝑚𝑒𝑛𝑡} = 𝑟2_𝑐𝑜𝑠−1𝑟 − 𝐻 𝑟 − 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2 = (1.06682) cos-11.0668−1.579 1.0668 – [(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 = 𝑟2𝑐𝑜𝑠−1 𝑟 − 𝐻 𝑟 − 𝑟 − 𝐻 2𝑟𝐻 − 𝐻2 = (1.06682) cos-11.0668−1.367 1.0668 – [(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%

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) Cumulative level (m) Cumulative volume (m3) Internal volume (m3) Internal hold-up time – propanal (mins) LAHH 0.202 1.579 26.1934 3.8535 0.38 LAH 0.300 1.367 22.3399 5.8316 0.57 NLL 1.067 1.067 16.5083 0.0000 0.0000

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: 𝑉_𝑐 = 𝐾 𝜌𝑙− 𝜌𝑔 𝜌_𝑔 𝐿𝑣 10 0.56

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 Vc = 0.101 ( 804.1 𝑘𝑔/𝑚3_{−3.334𝑘𝑔/𝑚}3 3.334 𝑘𝑔/𝑚3 ) ( 9.2344 𝑚 10 )0.56 = 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) At atmospheric pressure At 2100 kPa At 4100 kPa At 6200 kPa At 10300 kPa 0.055 to 0.107 0.107 0.101 0.091 0.082 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.

4.3.8.1 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 = 𝜋𝐷2

4 = 3.5753 m

2

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

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

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, Pdesign = (₁₀₀10 × 19.9 bar) + 19.9 bar

= 21.89 bars = 2.189 N/mm2

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 Aluminium

Stainless steel 304

Carbon

steel Lead Copper

Melting point

(oC) 660 1371- 1399 1540 327 1084

Density

(kg/m3) 2700 8300 7900 11340 8940

Corrosion

resistance 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.

4.4.5 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

Tensile

Strength

Design stess at temperature o_{C (N/mm}2₎ (N/mm2) 0 to 50 100 150 200 250 300 350 400 450 500 Carbon steel (semi-killed or silicon killed) 360 135 125 115 105 95 85 80 70 Carbon-manganese steel (semi-killed or silicon killed) 460 180 170 150 140 130 115 105 100 Carbon-molybdenum steel 0.5% Mo 450 180 170 145 140 130 120 110 110 Low alloy steel (Ni, Cr, Mo, V) 550 240 240 240 240 240 235 230 220 190 170 Stainless steel 18Cr/8Ni unstabilised (304) 510 165 145 130 115 110 105 100 100 95 90 Stainless steel 18Cr/8Ni Ti stabilised (321) 540 165 150 140 135 130 130 125 120 120 115 Stainless steel 18Cr/8ni Mo 21 2 % (316) 520 175 150 135 120 115 110 105 105 100 95 (Source: Sinnott, 2005)

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,

t = 𝑃𝑖𝐷𝑖 2𝑗𝑓 − 𝑃𝑖 + c

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

f = 135 N/mm2

t = _{2 ×1 ×135 − 2.189}2.189 × 2133.6 + 2 = 19.4394 mm ≈ 20 mm

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 = 𝑃𝑖𝐷𝑖 2𝑆𝐸−0.2𝑃𝑖

Where, S = maximum allowable stress E = joint efficiency

Table 4.15: Weld Joint Efficiencies

Joint Acceptable Joint

Degree of Radiographic Examination

Type Categories _{Full Spot None}

1 A, B, C, D 1 0.85 0.7

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. No Grade Nominal Composition P-No. Group No. Min. Yield (ksi) Min. Tensile (ksi)

Carbon Steel Plates and Sheets

SA-515 55 C-Si 1 1 30 55 60 C-Si 1 1 32 60 65 C-Si 1 1 35 65 70 C-Si 1 2 38 70 SA-516 55 C-Si 1 1 30 55 60 C-Mn-Si 1 1 32 60 65 C-Mn-Si 1 1 35 65 70 C-Mn-Si 1 2 38 70

Low Alloy Steel Plates

SA-387 2 Cl.1 1/2Cr - 1/2/Mo 3 1 33 55 2 Cl.2 1/2Cr - 1/2Mo 3 2 45 70 12 Cl.1 1Cr - 1/2Mo 4 1 33 55 12 Cl.2 1Cr - 1/2Mo 4 1 40 65 11 Cl.1 1 1/4Cr - 1/2Mo-Si 4 1 35 60 11 Cl.2 1 1/4Cr - 1/2Mo-Si 4 1 45 75 22 Cl.1 2 1/4Cr - 1Mo 5 1 30 60 22 Cl.2 2 1/4Cr - 1Mo 5 1 45 75

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 8.4 6.5 4.5 2.5 SA-515 15 14.4 13 10.8 8.7 6.5 4.5 2.5 SA-515 16.3 15.5 13.9 11.4 9 6.5 4.5 2.5 SA-515 17.5 16.6 14.8 12 9.3 6.5 4.5 2.5 SA-515 13.8 13.3 12.1 10.2 8.4 6.5 4.5 2.5 SA-516 15 14.4 13 10.8 8.7 6.5 4.5 2.5 SA-516 16.3 15.5 13.9 11.4 9 6.5 4.5 2.5 SA-516 17.5 16.6 14.8 12 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 15 15 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 15 15 15 15 14.4 13.6 10.8 8 5.7 3.8 2.4 1.4 SA-387 17.7 17.2 17.2 16.9 16.4 15.8 11.4 7.8 5.1 3.2 2 1.2 SA-387

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;

t = 473.1 × 85.3

[ 2 ×15000 ×1 − 0.2 ×473.1 ]

= 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

4.4.8 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

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 Wi = 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.

4.4.10 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

Table 4.18: The dimensions of the saddle support Dvessel Max. weight Dimensions (m) (mm) (m) (kN) V Y C E J G t2 t1 Dbolt Bolt holes 1.4 230 0.88 0.20 1.24 0.53 0.305 0.140 12 10 24 30 1.6 330 0.98 0.20 1.41 0.62 0.350 0.140 12 10 24 30 1.8 380 1.08 0.20 1.59 0.71 0.405 0.140 12 10 24 30 2.0 460 1.18 0.20 1.77 0.8 0.500 0.140 12 10 24 30 2.2 750 1.28 0.23 1.95 0.89 0.529 0.150 16 12 24 30 2.4 900 1.38 0.23 2.13 0.98 0.565 0.150 16 12 2733 33 2.6 1000 1.48 0.23 2.30 1.03 0.590 0.150 16 12 2733 33 2.8 1350 1.58 0.25 2.50 1.10 0.025 0.150 10 12 2733 33 3.0 1750 1.68 0.25 2.64 1.18 0.665 0.150 16 12 2733 33 3.2 2000 1.78 0.25 2.82 1.26 0.730 0.150 16 12 2733 33 3.6 2500 1.98 0.25 3.20 1.40 0.815 0.150 16 12 2733 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

4.4.11 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). Dvessel Max. weight Dimensions (m) (mm) (m) (kN) V Y C E J G t2 t1 Dbolt Bolt holes 2.134 750 1.28 0.225 1.95 0.89 0.520 0.510 16 12 24 30

For inlet

No inlet device: ρV2 < 1400.0 kg/ms2 Half pipe inlet device: ρV2 < 2100.0 kg/ms2

Inlet vane: ρV2 < 8000.0 kg/ms2

For outlet

Gas outlet: ρV2 < 2100.0 kg/ms2

Liquid outlet V2 < 2.0 m/s

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 = 𝜌𝑔𝑄𝑔 + 𝜌𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 𝑄𝑔 + 𝑄𝑝𝑟𝑜𝑝𝑎𝑛𝑎𝑙 = 11.748 kg/m3

Assume inlet vane pack, therefore; Allowable ρV2 = 8000.0 kg/ms2 Allowable velocity, v = 𝜌𝑉2_/𝜌

= 8000_𝑚𝑠𝑘𝑔₂/11.748_𝑚𝑘𝑔₃ = 680.967

So, the nozzle area, A = Qtotal / v

= 1.5912 m3/s

26.095 m/s

= 0.061 m2

Required nozzle diameter, dnozzle-in = 4𝐴/𝜋

= 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 Allowable velocity, v = 𝜌𝑉2_/𝜌

= _3.3341500 = 449.91 m/s

So, the nozzle area, A = Qg/v

= 1.586 𝑚3/𝑠

449.91 𝑚/𝑠

= 0.0035 m2

Required nozzle diameter, dnozzle-out = 4𝐴/𝜋

= 4 0.0035

𝜋

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 𝑚3/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).

Table 4.20: Standard flange design specifications

Nom. Pipe Flange Raised face Drilling Neck Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r

d1 ≈ 200 219.1 340 24 62 268 3 M20 8 22 295 235 16 10 250 273 395 26 68 320 3 M20 12 22 350 292 16 12 300 323.9 445 26 68 370 4 M20 12 22 400 344 16 12 350 355.6 505 26 68 430 4 M20 16 22 460 385 16 12 400 406.4 565 26 72 482 4 M24 16 25 515 440 16 12 450 457.2 615 28 72 532 4 M24 20 26 565 492 16 12 500 508 670 28 75 585 4 M24 20 26 620 542 16 12 600 609.6 780 28 80 685 5 M27 20 30 725 642 18 12 700 711.2 895 30 80 800 5 M27 24 30 840 745 18 12 800 812.8 1015 32 90 905 5 M30 24 33 950 850 18 12 900 914.4 1115 34 95 1005 5 M30 28 33 1050 950 20 12 1000 1016 1230 34 95 1110 5 M33 28 36 1160 1052 20 16 1200 1220 1455 38 115 1330 5 M36 32 39 1380 1255 25 16 1400 1420 1675 42 120 1535 5 M39 36 42 1590 1460 25 16 1600 1620 1915 46 130 1760 5 M45 40 48 1820 1665 25 16 1800 1820 2115 50 140 1960 5 M45 44 48 2020 1868 30 16 2000 2020 2325 54 150 2170 5 M45 48 48 2230 2072 30 16

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. Pipe Flange Raised face Drilling Neck Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r

d1 ≈

210 230 351 24 66 278 3 M20 9 22 306 246 16 10

Table 4.22: Values for bolt and flange of the vapour outlet nozzle Nom. Pipe Flange Raised face Drilling Neck

Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r

d1 ≈

Table 4.23: Values for bolt and flange for the heavy liquid propanal outlet nozzle Nom. Pipe Flange Raised face Drilling Neck

Size o.d. D b h1 d4 f Bolting No. d2 k d3 h2 r

d1 ≈

61 69.3 187 18 45 123 3 M16 4 13 142 77 9 4

4.4.13 Conclusion

Table 4.24: Summary of mechanical design

Item Value

Design pressure 2.189 N/mm2

Design temperature 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 221.68 kN

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):

log

C

= K

+ K

log

(A) + K

[log

(A)]

Where,

A = capacity or size parameter for the equipment

K

, K

, K

= 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 m

From Table A.1 (Appendix A);

K

= 3.5565

K

= 0.3776

K

= 0.0905

Therefore,

log

C

= 3.5565+ (0.3776) log

(33.016) + (0.0905) [log

(33.016)]

= 4.3387

C

= $ 21 812.23

Pressure factors for process vessels:

t

= 0.034 m

For pressure vessel, when vessel thickness,

t



0

.

003

m

,

𝐹

=

𝑃 + 1 𝐷

2[850 − 0.6 𝑃 + 1 ] + 0.00315

0.003

=

2.189 + 1 2.1336

2[850 − 0.6 2.189 + 1 ]

+ 0.00315

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:

C

= C

F

= C

(B

+ B

F

F

)

From Table A.4 (Appendix A), B

= 1.49, B

= 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, F

= 1.0

And so,

C

= 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 C

= $ 111 739.69 x

622.6

397

= $ 175 237.11

= RM 529 917.01

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., <http:www.monarchseparators.com>