ChE 128, Fall 2014 Prof. S. Scott

Homework 6 Due in class on Wednesday, December 3

If you miss handing your work in during class, you may put it in the TA mailbox. A late penalty of 25% will be assessed. No late assignments will be accepted after Friday, December 5, at 9 am.

1. An ethanol-water mixture will be separated by batch distillation at P = 1 atm, using a column with 10 equilibrium stages and a total condenser. The charge to the still pot is 500 kg of a solution that contains 12 mol% EtOH. The distillation will be conducted at a constant external reflux ratio of 0.75 until the still pot contains 1 mol% EtOH. Find the amount of distillate and its EtOH concentration. Equilibrium data: xE 0 0.019 0.0721 0.0966 0.1238 0.1661 0.2337 0.2608 yE 0 0.170 0.3891 0.4375 0.4704 0.5089 0.5445 0.5580 xE 0.3273 0.3965 0.5198 0.5732 0.6763 0.7472 0.8943 1 yE 0.5826 0.6122 0.6599 0.6841 0.7383 0.7815 0.8943 1

Note: Work smart! Observe what happens when you step off stages for your first operating line, and apply a reasonable shortcut for all subsequent operating lines.

2. An equimolar mixture of toluene (MVC) and ethylbenzene (50 kmol total)

will be separated in a simple batch still at P = 1 atm. Assume relative volatility is constant at 2.15.

a. If the distillation stops when the final concentration of toluene in the still pot is 20 mol%, find the amount and concentration of toluene in the distillate.

b. If the distillation stops when the average concentration of toluene in the distillate is 65 mol%, find the amount and concentration of toluene left in the still pot. (Hint: you can solve the mass balances for Wfinal/F for use in the integrated Rayleigh equation.)