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, kmolMole fraction
Benzene114.240.51
Cumene106.370.475
DIPB3.360.015
TOTAL223.971
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 ID123456
FromB1 B1 PUMP B2 B2
 To B1PUMPB2
LIQUIDLIQUIDLIQUIDLIQUIDLIQUIDLIQUID
Substream: MIXED
Mole Flow   kmol/hr
  BENZENE114.25113.10751.14251.14251.14251.26E-10
  CUMENE106.37891.063789105.3151105.3151105.20980.105315
  DIPB3.3602221.26E-053.3602093.3602093.36E-033.356849
Total Flow  kmol/hr223.9891114.1713109.8178109.8178106.35563.462164
Total Flow  kg/hr22255.98963.10113292.813292.812735.4557.3907
Total Flow  l/min443.8993183.2093309.9853310.0228284.70913.76414
Temperature K323.15353.5526453.6059453.687423.4744511.4289
Pressure    atm1.512312
Vapor Frac000000
Liquid Frac111111
Solid Frac000000
Enthalpy    cal/mol1898.70413455.42-898.834-892.978-2394.85-12894.5
Enthalpy    cal/gm19.10905171.394-7.42567-7.3773-19.9998-80.0926
Enthalpy    cal/sec1.18E+054.27E+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/cc8.41E-030.0103865.90E-035.90E-036.23E-034.19E-03
Density     gm/cc0.8356210.8153790.71470.7146140.7455220.674931
Average MW99.3615178.50573121.0441121.0441119.7436160.9949
Liq Vol 60F l/min425.7355169.3162256.4193256.4193245.53210.8873
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 columnCumene column
Number of stages1010
Key component recoveryLight keyHeavy keyBenzene :0.99Cumene: 0.01Cumene: 0.999DIPB: 0.001
PressureCondenserReboiler1 atm2 atm1 atm2 atm
PumpDischarge pressure 3 atmEfficiency : 0.7
Distillation column design for separation of benzene, cumene and Diisopropyl benzene obtained from the aspen plus simulation
Design SpecificationsBenzene columnCumene column
Minimum reflux ratio:0.22505770.33920087
Actual reflux ratio:0.388122830.46788461
Minimum number of stages:4.738518649.52894138
Number of actual stages:1021
Feed stage:5.4136422610.6995836
Number of actual stages above feed:4.413642269.69958359
Reboiler heating required:cal/sec617909.761339717.734
Condenser cooling required:cal/sec336736.127395629.835
Distillate temperature:K353.5526423.474389
Bottom temperature:K453.605864511.428876
Distillate to feed fraction:0.509718060.96847357