ChE 471 Fall 2005
Exam 1
Closed Book, Closed Notes, One crib sheet allowed
Problem 1
The decomposition of acetaldehyde is assumed to follow a multi-step mechanism listed below.
On the basis of the above mechanism, derive a rate expression for the rate of disappearance of acetaldehyde.
Can the rate be expressed in terms of a power law model? If so, when, and state the apparent order of reaction.
Assume all the above steps are irreversible.
Hint: This is a catalytic chain mechanism. Use the PSSA and set the rate of the two active intermediates to be zero and remember that the rates of propagation steps (steps 2 and 3) are many times larger than those of initiation (step 1) or termination (step 4), so the latter ones can be neglected sometimes. However, the difference in rates of the propoagation steps cannot be neglected compared to termination or initiation rates. .
Problem 2
For a liquid phase reaction products you have measured reactant concentration as a function of time in an isothermal, batch reactor and the data are given below:
The reaction is now to be carried out in a back-mixed reactor (CSTR) of 2m3 volume and the feed rate is 0.1m3/sec. The inlet concentration of A is 500 mol/m3. Find the conversion in this reactor.
To improve the conversion, a second reactor of equal volume is added in series. What is the conversion now at the end of second reactor?
Problem 3
Gas-phase dimerization reaction , 2A P , was studied in a batch autoclave at constant volume and at constant temperature of 440°C. The information obtained for the total pressure in the reactor as a function of time is shown below.
Determine the order of reaction and the rate constant. Assume the starting mixture is pure A.
For a feed with 50% A and 50% inerts at a total initial pressure of 82.7 kPa, calculate the final pressure.
Problem 4
The reaction, AP, is autocatalytic with rate form given as:
We want to design a reactor to make FP = 100 (mol/min) of P at 90% conversion of A of the feed that contains CA0= 1 (mol/L).
You want to minimize the reactor volume needed to achieve the above production rate at the conversion specified above. To gain insight, plot the rate (or its reciprocal) as a function of conversion in the conversion range from 0 to 0.9.
a) What reactor type or reactor combinations do you recommend and why?
b) What is the needed total reactor(s) size?
The production rate is FP = FAo XAf where XAf is the exit (or final) conversion.