Distillation column design for separation of benzene, cumene and Diisopropyl benzene

Multicomponent distillation column design is done for separation of benzene, cumene and diisopropyl benzene mixture. In the cumene production process via benzene alkylation, after the reaction unreacted benzene from the reactor products to be separated and sent for recycle. The better recovery of benzene assures the improvement in yield, conversion and purity of cumene product. Benzene used as excess component in alkylation process to obtain complete conversion of propylene and the excess is recycled continuously.

After the reactor the product stream is cooled and sent to flash separator. Unreacted propylene and propane are removed in gaseous form. The liquid mixture from  the bottom of the separator contains of benzene, cumene, and DIPB, it is to be separated in two distillation columns. First column separates the benzene and the second column separates cumene and DIPB. Due to large variation in the boiling points, the benzene column can be operated at 1 atm pressure to obtain top product benzene with 98.1 mole% purity.  Similarly cumene column with top product 99.9 mole% of pure cumene.

Model diagram of two distillation columns for benzene, cumene and diisopropyl benzene mixture separation operation

Distillation column design 2D diagram

Distillation columns for benzene, cumene and diisopropylbenzene separation

 

Using Aspen plus process simulator the distillation column system developed and simulated to find the design parameter like

  • Minimum reflux ratio
  • Actual reflux ratio
  • Minimum number of stages
  • Number of actual stages
  • Feed stage
  • Number of actual stages above feed
  • Reboiler heating required
  • Condenser cooling required
  • Distillate temperature
  • Bottom temperature
  • Distillate to feed fraction

300 ton/day cumene production plant is taken as basis for design simulation and the feed composition to the benzene column is assumed as,

FEED, kmol Mole fraction
Benzene 114.24 0.51
Cumene 106.37 0.475
DIPB 3.36 0.015
TOTAL 223.97 1
Simple aspen plus simulation process flowsheet developed for distillation column design.
Distillation column design Aspen plus model diagram

Aspen plus flowsheet diagram of two distillation columns

Aspen simulation process flowsheet stream table
 Stream ID 1 2 3 4 5 6
From B1  B1  PUMP  B2  B2
 To  B1 PUMP B2
LIQUID LIQUID LIQUID LIQUID LIQUID LIQUID
Substream: MIXED
Mole Flow   kmol/hr
  BENZENE 114.25 113.1075 1.1425 1.1425 1.1425 1.26E-10
  CUMENE 106.3789 1.063789 105.3151 105.3151 105.2098 0.105315
  DIPB 3.360222 1.26E-05 3.360209 3.360209 3.36E-03 3.356849
Total Flow  kmol/hr 223.9891 114.1713 109.8178 109.8178 106.3556 3.462164
Total Flow  kg/hr 22255.9 8963.101 13292.8 13292.8 12735.4 557.3907
Total Flow  l/min 443.8993 183.2093 309.9853 310.0228 284.709 13.76414
Temperature K 323.15 353.5526 453.6059 453.687 423.4744 511.4289
Pressure    atm 1.5 1 2 3 1 2
Vapor Frac 0 0 0 0 0 0
Liquid Frac 1 1 1 1 1 1
Solid Frac 0 0 0 0 0 0
Enthalpy    cal/mol 1898.704 13455.42 -898.834 -892.978 -2394.85 -12894.5
Enthalpy    cal/gm 19.10905 171.394 -7.42567 -7.3773 -19.9998 -80.0926
Enthalpy    cal/sec 1.18E+05 4.27E+05 -27418.9 -27240.3 -70751.6 -12400.8
Entropy     cal/mol-K -92.3321 -55.0925 -109.684 -109.671 -112.52 -161.902
Entropy     cal/gm-K -0.92925 -0.70176 -0.90615 -0.90604 -0.93967 -1.00563
Density     mol/cc 8.41E-03 0.010386 5.90E-03 5.90E-03 6.23E-03 4.19E-03
Density     gm/cc 0.835621 0.815379 0.7147 0.714614 0.745522 0.674931
Average MW 99.36151 78.50573 121.0441 121.0441 119.7436 160.9949
Liq Vol 60F l/min 425.7355 169.3162 256.4193 256.4193 245.532 10.8873
Models used in the process flowsheet
  • Distillation column: DSTWU model selected for shortcut analysis
  • Pump: Pressure changer
  • Property methods selected: NRTL

Input data entered:

Benzene column Cumene column
Number of stages 10 10
Key component recoveryLight keyHeavy key Benzene :0.99Cumene: 0.01 Cumene: 0.999DIPB: 0.001
PressureCondenserReboiler 1 atm2 atm 1 atm2 atm
Pump Discharge pressure 3 atm Efficiency : 0.7
Distillation column design for separation of benzene, cumene and Diisopropyl benzene obtained from the aspen plus simulation
Design Specifications Benzene column Cumene column
Minimum reflux ratio: 0.2250577 0.33920087
Actual reflux ratio: 0.38812283 0.46788461
Minimum number of stages: 4.73851864 9.52894138
Number of actual stages: 10 21
Feed stage: 5.41364226 10.6995836
Number of actual stages above feed: 4.41364226 9.69958359
Reboiler heating required:cal/sec 617909.761 339717.734
Condenser cooling required:cal/sec 336736.127 395629.835
Distillate temperature:K 353.5526 423.474389
Bottom temperature:K 453.605864 511.428876
Distillate to feed fraction: 0.50971806 0.96847357