Dear ChE 212 students,

Have a good class tomorrow with Dr. Ashish Jha. He finished his Ph.D. here at URI very recently and is helping out with class while I'm away.

If you have questions for me, you can stop by on Wednesday during office hours (11-noon). I will not be on campus on Tuesday.

(1) Homework 5 clarification

Hwk 5 (problem 1) contains a poor phrasing that matches a poor phrasing that was present on quiz 3. The problem begins with

"95 mass% of C leaves the process in stream 4" ...

A better phrasing would be something like

"Of all the C that enters the system, 95% of it leaves in the stream flowing at 175 kg/hr, and the mass fraction of C in that stream is unknown."

My intent is that the 95% is a specification, not a mass fraction.

The quiz 3 solution uses that 95% (90.3% on the quiz) as a specification. When I graded the quiz, it was fine to use the 0.903 as a specification or as a mass fraction, since the wording was unclear.

(2) Quiz 3 comments

Performance on the quiz was generally good. I had to revise my grading scheme because of the unclear specification, so a few of you will have comments that are crossed out.

The most common mistake was writing all 4 balances and not clarifying that only three of them could be used.

Another common mistake was not using the fact that the mass fractions in stream 4 add up to 1. I encourage you to use that whenever you can! It will help you reduce the number of variables. For example, in the complicated homework problem with 2 distillation columns it is possible to label the input at .300 B, .250 T, and .450 X, rather than using an unknown for the mole fraction of X.

(3) The next quiz will focus on specifications and material balances.

For example, you will need to be able to write the equation for a specification, given the verbal description and a flow chart.

Prof. Greenfield

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