CL 351                                   Homework # 10

 

Due date: 28 October 2004

 

  1. One of our processes produces a waste stream of 45,000 kg of a 59 wt% methanol/ethanol mixture (per batch). The sales department believes it could sell methanol recovered from this waste stream, provided that its concentration is increased to 89 wt%. The plant manager would like you to make a preliminary study of the feasibility of recovering methanol of the desired purity.  A batch rectifier is available for the solvent recovery operation. It takes 2.5 hours to charge, preheat, discharge and clean the column for each batch (the column is used for other separations between solvent recovery batches). The column is normally operated at a constant boilup rate of 50kmol/hr. The total fractionating capacity is equivalent to 6 theoretical stages (including the reboiler). The mixture can be considered to have a constant relative volatility of 1.72. The sales office suggests that you evaluate the column productivity, P, by dividing the weight of on specification methanol by distillation time plus changeover time (2.5 hr).  The operating reflux ratio is 1.5 times the minimum.  Assume negligible hold up in the column.

 

  1. Do tutorial problem discussed in the class.

 

  1. A distillation column with a partial reboiler and a total condenser is being used to separate a mixture of benzene (B), toluene (T) and cumene (C).  The feed is 40 mole% B, 30mole% T and 30mole% C.  The feed is input as a saturated vapour.  We desire a recovery of 99% for toluene in the bottoms and 98% for benzene in the distillate.  The reflux is returned as a saturated liquid, constant molal overflow can be assumed and relative volatilities are constant.  Choosing T as reference component, relative volatilities of B and C are 2.25 and 0.21, respectively.  Use Fenske equation to find number of stage at total reflux and the recovery fraction of cumene in the bottoms.  Find the minimum reflux ratio by Underwood equation.  If R = 1.25 Rmin, find number of stages and optimal feed location by Gilliland correlation.  Photocopies of text given to you contain the correlation.

 

  1. We want to batch distill a mixture of ethanol and water using a column with seven plates and a reboiler.  The feed is charged into the reboiler.  The feed is 520 kmol and contains 18 mole% ethanol and 82 mole% water.  It is desired to operate the column at variable reflux and maintain a constant product composition of 80 mole% ethanol.  How long will it take to reduce still composition to 4 mole% ethanol?  How much product would have been produced in this time?