COLUMN OPTIMIZATION
BASED ON A PONCHON-SAVARIT METHOD
View: Process Flow Diagram
View: Simulation Results
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.
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 151°F 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 125°F.
A weight flow rate specification is applied to the overhead stream: propane recovery must be 64200 lb/h.
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.
<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