Page 344 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

346 Chapter 11 Distillation

Solution 1A: Separation using a simple batch still
Since the charge is small, a batch distillation process is considered. The process involves
inflammable hydrocarbon and so the operating pressure is set to slightly above atmospheric
(¼ 1.1 101.325 ¼ 111.5 kPa) to avoid ingress of air.

P ¼ 111:5kPa; x D;avg ¼ 0:4; x F ¼ 0:25
Checking the feasibility of using a simple batch still with no column and reflux
Bubble point of the feed calculated at P based on vapour pressure data of the components, gives
T bub_feed ¼ 136.5 C.

Pure component vapour pressures for toluene and O-xylene at 136.5 C are 200.4 and 81.77 kPa,

respectively.
Relative volatility, a ¼ 200:4=81:77 ¼ 2:45
a x F
¼ 0:449 w¼ 0:45.
Initial composition of vapour leaving the still ¼
1 þða 1Þ x F
Initial composition is higher than 0.4 and hence it is possible to continue the process and obtain
distillate with average composition x D;avg ¼0.4.
Based on Eqs. 11.45 & 11.46:
B 1=ða 1Þ 1 x F a=ða 1Þ x F ðB=FÞ x B
x B
F ¼ x F 1 x B & x D;avg ¼ ð1 B=FÞ

Case 1 Case 2 Case 3 (Desired operation) Case 4 Case 5 Case 6
x B 0.225 0.200 0.175 0.150 0.125 0.100
0.88 0.79 0.67 0.57 0.48 0.39
ðB=FÞ
x D;avg 0.433 0.416 0.400 0.382 0.364 0.346
a x B 0.4157 0.3799 0.3420 0.3019 0.2590 0.2140
x D ¼
1 þ x B ða 1Þ
The desired operation is for x D;avg ¼ 0.4.
At start of distillation, x D ¼ 0.45, equilibrium composition w.r.t. x B ¼ 0:25.
At end of distillation, x D ¼ 0.56, equilibrium composition w.r.t. x B ¼ 0:3420.
Checking for a value at end of operation: Bubble point of B calculated at P at end of operation,
gives T bub_end ¼ 139.7 C and corresponding value of a ¼ 216:6=89:29 ¼ 2:43. This is not much

different from the value (2.45) used and no revised calculation is required.
Distillate collection starts at still temperature of 136.5 C and ends at 139.7 C.

0:25 ðB=FÞ x B 0:25 0:67 0:175
¼ 53:1%
Recovery of toluene ¼ 100 ¼ 100
ð1 B=FÞ x D;avg 0:25
Design of the physical system
This requires information on the initial charge volume and temperature.
Still: A steam coil heated still is envisaged. Its design involves estimation of the charge volume.
The estimated diameter and height must ensure minimum 150 mm liquid depth above the heating coils
at end of operation. The vessel design may be guided by available information in Chapter 17. Heating
coil details are to be based on heat transfer rate with sufficient extra cushion for which Chapter 2 may
be referred to.