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COLUMN OPTIMIZATION

BASED ON A PONCHON-SAVARIT METHOD

 

 

View: Process Flow Diagram

 

View: Simulation Results

 

 

Introduction

This example shows the optimization of a de-propanizer using the XPSIM column optimizer based on a Ponchon-Savarit type method.

This method is rigorous for the simulation of binary systems only, but may be applied successfully to a larger category of systems.

 

This sample is quite interesting: it is derived from an engineering study for the revamping of a large LPG recovery plant with numerous separation column.

In this study the existing de-propanizer column was analyzed in detail.

 

The de-propanizer column is simulated twice.

The first calculation is performed using the configuration of the original design. Column optimization is applied to these results obtaining a set of possible configurations (lower no of trays and different feed location).

The optimizer generates, for a set of possible reflux ratios, the number of theoretical trays and the feed location required to obtain the same component separation.

One result is selected and verified with a second rigorous simulation.

 

Base case

English units of measure are used.

The Peng-Robinson (PR) equation is used for vapor-liquid equilibrium and the Lee-Kesler (LK) for enthalpy, entropy and density calculations.

 

Column feed is stream S1, its temperature is 151F and the pressure is set at 310 psia.

Flowrate is about 2658 lbm/h.

The feed composition on molar percent basis is shown on the next table:

No

Components

Molar composition, %

1

C1

0.00

2

C2

0.4878

3

C3

54.7627

4

IC4

9.9486

5

NC4

21.6828

6

IC5

4.8852

7

NC5

4.9738

8

NC6

3.2444

9

NC7

0.0041

10

NC8

0.0099

11

NC9

0.00005

12

NC10

0.00

 

The first column simulation is done with 33 theoretical trays plus a condenser and a reboiler.

The feed stream is inserted on tray 17. Condenser pressure is 295 psia, and its pressure drop is 4 psi. Column pressure drop is specified as 0.2 psi per tray.

The column is operated with a reboiler duty of 30000 kBtu/h . Condenser is sub-cooled with a return liquid temperature set at 125F.

A weight flow rate specification is applied to the overhead stream: propane recovery must be 64200 lb/h.

 

Optimization

The column optimization is performed by defining C3 as the light key component and IC4 as the heavy key component.

Top and bottom products specifications are:

 

Top product

C3

Max heavy comps molar fraction: 0.15.

Bottom product

S3

Max light comps molar fraction: 0.0008

 

The optimizer finds that the minimum reflux ratio is 1.0331 corresponding to a condenser duty value of -18697.42 kBtu. The related reboiler minimum duty is 23429.11 kBtu.

A number of calculations are performed using duty multipliers ranging from 1.1 to 1.5.

 

No

Duty Multiplier

Reboiler duty, kBtu

No of trays

Feed location

1

1.10

0.2530E+05

29

5

2

1.16

0.2637E+05

27

5

3

1.21

0.2744E+05

26

5

4

1.27

0.2850E+05

25

4

5

1.32

0.2957E+05

24

4

6

1.38

0.3064E+05

23

4

7

1.44

0.3171E+05

23

4

8

1.50

0.3278E+05

23

4

 

 

Solution no. 5 which has reboiler duty near the original value of 30000 kBtu is selected.

This configuration is verified by including a second column C4 in the pfd.

This column has 22 theoretical trays, condenser and reboiler. The same recovery specification of propane in the overhead product is applied.

The solution shows that the same column could be operated with a significant lower no of trays.

 


 

Generated Keyword Input File

 

<XPSIM> ...generated by XpsimWin v.1.07 ...

*

RUN ID=DE-C3 CUSTOMER=STAFF PROJECT='LPG RECOVERY - DE C3'

DESC DEPROPANIZER COLUMN OPTIMIZATION

DESC LPG RECOVERY PLANT

DIMENSION INPUT ENG

*

*

System Data

*

*

CHEMCOMP C1 / C2 / C3 / IC4 / NC4 / IC5 / NC5 / NC6 / NC7 / NC8 / NC9 +

/ NC10

THERMSET UID=M1

METHODS K=PR HS=LK

*

*

Flowsheet Data

*

*

*

STREAM=S1 TEMP=151.174 PRES=310 RATE(M)=2657.489 XBASIS=M

COMP C1:0 / C2:0.4878 / C3:54.7627 / IC4:9.9486 / NC4:21.6828 / +

IC5:4.8852 / NC5:4.9738 / NC6:3.2444 / NC7:0.0041 / NC8:0.0099 / +

NC9:0.00005 / NC10:0.00

*

COLUMN IN S1 OUT C3 S3 UID=C1

PARA TRAY=33 CON REB NITER=20

FEED STR=S1 TRAY=17

DRAWOFF STR=C3 TRAY=0 PHASE=LIQ

DRAWOFF STR=S3 TRAY=34 PHASE=LIQ RATE(VAR)=1200

PSPEC PTOP=295 TRAY=0 DP=4 / TRAY=1 DP=0.2

PROFILE TRAY=0 LIQ=5000 TEMP=125 / TRAY=34 TEMP=260

HEAT TRAY=0 VAR HXID=COND

HEAT TRAY=34 DUTY(FIX)=30000 HXID=REB

CONDENSER TRET=125

SPEC RATE(W)=64200 STR=C3 COMP=C3

COLOPT LKEY=C3 HKEY=IC4

SPEC FRAC(M)=0.15 HEAVY OVERHEAD

SPEC FRAC(M)=0.0008 LIGHT BOTTOM

CALC MDMIN=1.1,1.5 POINTS=8

*

COLUMN IN S1 OUT C3-A S3-A UID=C4

PARA TRAY=22 CON REB NITER=30

FEED STR=S1 TRAY=4

DRAWOFF STR=C3-A TRAY=0 PHASE=LIQ

DRAWOFF STR=S3-A TRAY=23 PHASE=LIQ RATE(VAR)=1200

PSPEC PTOP=295 TRAY=0 DP=4 / TRAY=1 DP=0.2

PROFILE TRAY=0 LIQ=5000 TEMP=156 / TRAY=23 TEMP=270

HEAT TRAY=0 VAR HXID=COND

CONDENSER TRET=125

HEAT TRAY=23 DUTY(FIX)=30000 HXID=REB

SPEC RATE(W)=64200 STR=C3-A COMP=C3

PRINT TRACE=PART STREAM=1

*

*

END