Unformatted text preview: can be solved with the batch distillation model of Chemcad.
To do this, the Wilson equation might be used to compute vapor-liquid equilibrium. Also, a
decision must be made as to whether a constant distillate rate or a constant boilup rate should be
specified. Based on the above findings, it is probable that a nearly constant reflux ratio can be
used. Assuming this is so, either the distillate rate or boilup rate can be held constant. To verify
this, a run was made with a constant distillate rate of 12.8 lbmol/h, as computed above in Part
(c). The result shown below shows that the reflux ratio varies only slightly from approximately
2.3 to 2.5, except for the last hour, where it increases dramatically. The higher reflux ratios,
compared to the value of 1.98 computed in Part (b) above are probably due to differences in the
vapor-liquid equilibrium data used and the inaccuracy of the graphical determination in Part (b)
of Rmin and the use of an R = 1.2Rmin. Exercise 13.15
Subject: Batch rectification of an ethanol-water mixture under conditions of constant boilup
rate and constant distillate composition.
Given: Charge of 1,000 kmol of 20 mol% ethanol and 80 mol% water. Operation in a column
with 6 theoretical plates plus a partial reboiler (7 equilibrium stages) at 101.3 kPa with a boilup
rate of 100 kmol/h to give a constant mole fraction of 0.80 for ethanol in the distillate. Vaporliquid equilibrium data in Exercise 7.29.
Assumptions: Negligible liquid holdup on trays and in the condenser. No pressure drop.
Perfect mixing in the reboiler.
(d) Time in hours for the residue to reach an ethanol mole fraction of 0.05.
Kmol of distillate produced at conditions of Part (a).
Minimum and maximum reflux ratios during the rectification.
Variation of the distillate rate during the rectification. Analysis: This exercise can be solved by the procedure outlined in Section 13.3, with the
McCabe-Thiele construction shown in Fig. 13.7, where here the calculations are made for
ethanol with xD = 0.8. From a series of operating lines through that point with slopes L/V, values
of xW are determined by stepping off 7 equilibrium stages. With W0 = 1,000 kmol/h, V = 100
kmol/h, and x0 = 0.20, Eq. (13-16) is numerically integrated to the final xW = 0.05. This is a
straightforward, but tedious procedure. Alternatively, this exercise is solved rapidly with the
batch distillation model of Chemcad based on the following input:
BATC (the column with reboiler, condenser and reflux line) connected to TANK.
Components: Ethanol (1) and water (2)
Thermodynamics: Wilson equation with built in binary interaction parameters of:
λ 12 − λ 11 = 276.760 cal / gmol and λ 21 − λ 22 = 975.490 cal / gmol
Charge: 200 kmol of 1 and 800 kmol of 2 at 101.3 kPa and the bubble point.
Operation Step 1: Start from total reflux. 8 stages (includes total condenser). Simultaneous
correction method with damping factor of 0.7 because nonideal system. Distillate sent to TANK.
First specification of 0....
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