Dynamic Simulation of a Binary Distillation Column

step magnitude in percent

9

distillate composition

bottom composition

Consider an ideal equimolar binary mixture of benzene and toluene at

760mmHg

. This mixture, with a thermal quality equal to 0.5, is fed to a 10-stage column with a total condenser and partial reboiler. The feed stage is stage 7. The feed flow rate is taken equal to

10kmol/hr

. Up to time

t=100hr

, the reflux and reboil ratios are

R=3

and

s=2.5

, respectively. At times above

100hr

, the column is subject to a step in the reflux ratio,

R

, while

s

is kept equal to

2.5

.

This Demonstration computes the dynamic behavior of the distillate and bottom compositions (e.g. benzene mole fractions). It can be seen that an increase in the reflux ratio causes a simultaneous increase in distillate purity at the expense of a decrease in bottom composition purity.

Similarly, one can easily extend this study to see the effect of a step in the reboil ratio,

s

.

For the purpose of simplicity, the molar holdups of the condenser, reboiler, and plates are assumed constant, and equal to

5kmol

,

10kmol

, and

1kmol

, respectively. As an alternative route, one could use the Francis weir formula in order to compute time-dependent molar holdups.