Page 315 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.5 Design illustration e fractionator 317

D ¼ (495.8 kg mol Benzene þ 26.1 kg mol Toluene) ¼ 521.9 kg mol distillate and
x D ¼ 0:95=0:05=0:00, mole fraction.
By balance, composition of B:
B ¼ (165.3 kg mol Benzene þ 446.1 kg mol Toluene þ 755.6 kg mol) ¼ 1367 kg mol

x B ¼ 0:1209=0:3263=0:5528; mole fraction
Deciding the column operating pressure
Reflux drum will have D at its bubble point. If cooling water (Supply @33 C, Return @47 C) is

used in condenser, the reflux drum temperature (condenser outlet temperature) is expected to be
around 60 C. The other option is to use air cooled condenser and have condenser outlet temperature

about 90 C. Estimated bubble pressure of D with possible options of drum temperature is

T( C) 60 90

Bubble pressure of D (kPa) 51.5 134.8

If cooling water is used and the reflux drum temperature is 60 C, the drum will be at subatmo-

spheric pressure e a potentially hazardous condition for air-leakage into a hydrocarbon system. The
second option will operate the reflux drum at slightly above atmospheric pressure. So an air-cooled
condenser is selected to obtain the outlet temperature of 90 C.

Based on pressure drop across the condenser and the overhead vapour line pressure drop of 0.3 kg/
2
cm , the column pressure is now estimated to be
P col ¼ 134:8 þ 101:325 0:3=1:0332 ¼ 164:2 kPa
Column top temperature is the dew point of D at column pressure 164.2 kPa, estimated to be
98.6 C. The condenser inlet and exit temperatures are therefore 98.6 and 90 C.

A small pressure drop is assumed across the column, which is guessed to have w10 trays.
Column bottom temperature is estimated to be 137.9 C, the bubble point of B at column bottom

pressure (164.2 þ 5.2 ¼ 169.4 kPa).
Estimation of relative volatility
Pure component vapour pressure, kPa

Temperature ( C) B T O-x

Column top 98.6 173.17 70.97 25.17
Column bottom 137.9 450.77 207.33 84.98

a LK HK top ¼ 173:1=70:97 ¼ 2:439
a LK HK bottom ¼ 450:77=207:33 ¼ 2:174
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2:439 2:174 ¼ 2:303
a ¼