Spring 2000                                             ChE 250  Exam I                             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 stapled on top. 

Problem 1  (35 points)

            The spent acid from a nitrating process contains 40 wt% H₂SO₄ and 28 % HNO₃ (balance water). This dilute acid is to be reused after restoring it to the desired composition of 42 % H₂SO₄ and 40 % HNO₃. You have available supplies of fresh, concentrated acids (one is 92 % H₂SO₄, another is 87 % HNO₃). The spent acid stream flows to the restoration mixer at 40 lb_m/min.

a)   Show a flowsheet, labeling all known streams.

b)      Show that the problem is fully specified. You may use degree of freedom analysis, or may explicitly identify appropriate equations and variables.

c)   Find the flowrates of the makeup acid streams, and the final flowrate of the restored acid.

Problem 2 (30 points)  Short Answer:

a)   A process stream containing ethylene and benzene is stated to be at 30 wt% ethylene.  What are the mole fractions of ethylene and benzene?

b)   Define the term ‘Basis.’  How do you choose what value to assign, and which process stream to assign it to?

c)   When filtering a slurry to obtain a wet filter cake (solid + entrained liquid) and the clear filtrate (solution), we can usually describe the composition of the entrained liquid in terms of another stream composition.  Give that relation, and explain why it usually is valid.

Problem 3 (35 points)

            Seawater is to be desalinated by reverse osmosis using the scheme indicated below. Determine:

a)   The rate of waste brine removal (B).

b)   The rate of desalinized water (called potable water) production (D).

c)   The fraction of the brine leaving the osmosis cell that is recycled. Why might this recycle be used?