+ All Categories
Home > Documents > Chapter 4 - Separator Design

Chapter 4 - Separator Design

Date post: 20-Oct-2015
Category:
Upload: john-murtagh
View: 439 times
Download: 29 times
Share this document with a friend
Description:
Separator design guidelines for oil gas hyudrocarbons
Popular Tags:
39
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.
Transcript
Page 1: 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.

Page 2: Chapter 4 - Separator Design

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.

8

9

10

Figure 4.1: Schematic diagram of a horizontal two phase separator

Page 3: Chapter 4 - Separator Design

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.

Page 4: Chapter 4 - Separator Design

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

Page 5: Chapter 4 - Separator Design

4-5

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

Page 6: Chapter 4 - Separator Design

4-6

𝑃𝑐 ,𝑚 = 𝑃𝑐 ,𝑖𝑦𝑖

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:

𝑃𝑐 ,𝑚 = 𝑃𝑐 ,𝑖𝑦𝑖

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

Page 7: Chapter 4 - Separator Design

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

Page 8: Chapter 4 - Separator Design

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 =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

Zero margins are added to separator flow or design.

Page 9: Chapter 4 - Separator 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.

Page 10: Chapter 4 - Separator Design

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

2𝐿𝑣

4

Where, Vcyl = Cylinder volume (m3)

Dv = Vessel diameter (m)

Lv = Vessel length (m)

Subtitute Lv = 4Dv into equation above, Therefore

𝑉𝑐𝑦𝑙 = 𝜋𝐷𝑣3

Rearrange equation above. So that diameter of the vessel is

𝐷𝑣 = 𝑉𝑐𝑦𝑙

𝜋

3

𝐷𝑣 = 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

Page 11: Chapter 4 - Separator Design

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

2L1

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

Page 12: Chapter 4 - Separator Design

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)

Figure 4.3: Vessel cross-section

𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−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

A H

Page 13: Chapter 4 - Separator Design

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 = 𝑟2𝑐𝑜𝑠−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

Page 14: Chapter 4 - Separator Design

4-14

At Level Alarm Low (LAL)

Internal level = 0.200 m

Cumulative level = 1.00 m

𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−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.

Page 15: Chapter 4 - Separator Design

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

Page 16: Chapter 4 - Separator Design

4-16

At Level Alarm High High (LAHH)

Internal level = 0.202 m

Cumulative level = 1.579 m

𝐴𝑠𝑒𝑔𝑚𝑒𝑛𝑡 = 𝑟2𝑐𝑜𝑠−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−1𝑟 − 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%

= 64.06%

Page 17: Chapter 4 - Separator Design

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)

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:

𝑉𝑐 = 𝐾 𝜌𝑙 − 𝜌𝑔

𝜌𝑔 𝐿𝑣

10

0.56

Page 18: Chapter 4 - Separator Design

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

Vc = 0.101 (804.1 𝑘𝑔/𝑚3−3.334𝑘𝑔/𝑚3

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.

Page 19: Chapter 4 - Separator Design

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 = 𝜋𝐷2

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.

Page 20: Chapter 4 - Separator Design

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

Page 21: Chapter 4 - Separator Design

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,

Pdesign = (10

100 × 19.9 bar) + 19.9 bar

= 21.89 bars

= 2.189 N/mm2

Page 22: Chapter 4 - Separator Design

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 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.

Page 23: Chapter 4 - Separator Design

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

Tensile

Strength

Design stess at temperature oC (N/mm2)

(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)

Design stress, f = 135 N/mm2, Tensile stress = 360 N/mm2

Page 24: Chapter 4 - Separator Design

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,

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 = 2.189 × 2133.6

2 ×1 ×135 − 2.189 + 2

= 19.4394 mm ≈ 20 mm

Page 25: Chapter 4 - Separator Design

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 = 𝑃𝑖𝐷𝑖

2𝑆𝐸−0.2𝑃𝑖

Where, S = maximum allowable stress

E = joint efficiency

Page 26: Chapter 4 - Separator Design

4-26

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

Page 27: Chapter 4 - Separator Design

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 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

Page 28: Chapter 4 - Separator Design

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

Page 29: Chapter 4 - Separator Design

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.

Page 30: Chapter 4 - Separator Design

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

Page 31: Chapter 4 - Separator Design

4-31

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

Page 32: Chapter 4 - Separator Design

4-32

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

= 8000𝑘𝑔

𝑚𝑠 2 /11.748𝑘𝑔

𝑚3

= 680.967

= 26.095 m/s

Page 33: Chapter 4 - Separator Design

4-33

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

= 1500

3.334

= 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

= 0.067 m = 66.76 mm

Page 34: Chapter 4 - Separator Design

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 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).

Figure 4.6: Standard flange design dimensions

Page 35: Chapter 4 - Separator Design

4-35

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 ≈

67 75.7 194 19 46 130 3 M16 4 14 149 83 9 5

Page 36: Chapter 4 - Separator Design

4-36

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

Type of support Saddle support

Page 37: Chapter 4 - Separator Design

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

Page 38: Chapter 4 - Separator Design

4-38

For pressure vessel, when vessel thickness, ,003.0 mtvessel

𝐹𝑃,𝑣𝑒𝑠𝑠𝑒𝑙 =

𝑃 + 1 𝐷2[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:

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 622.6

397

= $ 175 237.11

= RM 529 917.01

Page 39: Chapter 4 - Separator Design

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.,

<http:www.monarchseparators.com>


Recommended