Select thermodynamic models for process simulation
A Practical Guide to a Three Steps Methodology

Example 2.7: Distillation column

A mixture of light hydrocarbons is processed in a distillation column. Compositions of distillate obtained from the total condenser and of the residue at the bottom of the column are as follows (see figure 1):

image Figure 1: VL equilibria in a distillation column
Table 1: Distillate and residue composition for example
Component Distillate Residue
Propane 0.23
iso-Butane 0.67 0.02
n-Butane 0.10 0.46
iso-Pentane 0.15
n-Pentane 0.37

Use the Scheibel and Jenny diagram (Figure 2) to evaluate the distribution coefficients.

image Figure 2: Scheibel and Jenny nomograph for light hydrocarbons.

Analysis:

Components are all light hydrocarbons and we are told to use the Scheibel and Jenny procedure. This procedure assumes an ideal mixture (equilibrium coefficients are independent of composition) and is more suitable than Raoult's law at expected working pressure (10 bar).

All calculations are relative to vapour-liquid equilibrium and none of the HC in the mixture is at supercritical conditions.

Solution:

See complete results in file (xls):

Some help on nomenclature and tips to use this file can be found here.

Table 2 Results of bubble pressure computing
Component image image image image image image
Propane 0.23 16.5 1.68 0.387 1.70 0.391
iso-Butane 0.67 6.3 0.81 0.543 0.82 0.594
n-Butane 0.10 4.6 0.62 0.062 0.62 0.062
Sum/Result 8.48 0.991 1.002
Table 3: Results of example of the dew temperature
Component image image image image image image
Propane 0.23 1.7 0.14 0.12 1.85 0.12
iso-Butane 0.67 0.82 0.82 0.73 0.91* 0.73
n-Butane 0.10 0.26 0.16 0.14 0.72 0.14
Sum/Result 1.11 1.00

* calculated using (K=0.67/0.73=0.91)

Table 4: Composition of reboiler
image image image image image image image
iC4 0.02 0.91 0.018 0.037 1.78 1.70 0.034
nC4 0.46 0.72 0.331 0.671 1.46* 1.37 0.629
iC5 0.15 0.32 0.048 0.097 0.77 0.74 0.111
nC5 0.37 0.26 0.096 0.195 0.66 0.63 0.233
Sum/ 0.493 1.000 1.007

* calculated using (K=0.67/0.46 = 1.46)

The temperature of the reboiler is found to be 197°F.