Spring 2000                                           ChE 250  Exam II                              Name _____________________

This is a closed-book exam. State any assumptions explicitly to ensure adequate credit.
Write only on the front of each page, and turn in all pages in order with the exam on top. 

Problem 1 (20 points) 

            A heater is designed to burn 50 lb_m/hr of propane (C₃H₈) in 20% excess air.

a)   Determine the required air feed stream mass flowrate (MW_air = 29.2).  

b)   Find the product gas stream flowrate and composition.

Problem 2 (20 points)  Short Answer:

a)   Define “selectivity” and “yield.”

b)   When an equilibrium constant has units, what does this indicate about the chemical reaction? Illustrate your answer with one or more examples.

Problem 3 (30 points)

            Methyl acetate, an important chemical in film manufacture, is produced in the following reversible reaction between acetic acid and methanol.

                                                CH₃COOH + CH₃OH <=> CH₃COOCH₃ + H₂O

                                                      (AA)            (M)                  (MA)           (W)

The reaction is conducted over a catalyst in the gas phase at 120°C, and the equilibrium constant is
K_eq = 2.5 at this temperature.

a)   For the case where reactants are fed in the stoichiometric ratio, find the equilibrium conversion and the outlet stream composition.

b)   Feeding excess methanol can increase the acetic acid conversion. Determine the feed ratio of methanol to acetic acid required to obtain 90% conversion of acetic acid. Calculate the outlet composition for this case.

c)   Identify advantages and disadvantages of each strategy (a and b). What other operating conditions could be changed to increase the methyl acetate concentration in the reactor outlet?

Problem 4  (30 points)

            Operators of a computer chip ‘fab’ (fabrication plant), wish to recover a valuable compound from a wastewater stream using the flowsheet shown. You are to investigate the possibility that the proprietary compound (C) can be extracted into diethyl ether (E), since the distribution coefficient K = (g C / kg organic stream) / (g C / kg aqueous stream) = 6.  Since C has low volatility, it is readily recovered from the ether in a small distillation column that produces a distillate stream of pure ether for recycle, and a bottoms stream of 30 wt% C in ether. It is desired to operate this process to recover 95% of the compound C. The initial design uses a single simple mixer-settler extractor. [For now, treat the solubility of ether in water as negligible.]

a)   Analyze the Degrees of Freedom for the overall system, the extractor, and the still. Where do you recommend starting calculations?

b)   Determine the flowrates of the extract stream, the C product stream and the fresh ether stream.

c)   Why is so much ether circulation needed? What could you change to decrease the solvent inventory?