ChE 471Fall 2000

Exam 2

(Closed book, closed notes)

  1. Chemicals A & B react in an isothermal continuous flow stirred tank reactor (CSTR) as follows:

2A = U

A + B = D

2B = W

Product D is the desired product and you need to maximize the overall selectivity

S = CD//(CU + CD + CW).

a)What is the optimal ratio CB/CA that you should maintain in your CSTR to achieve maximum selectivity S? [Hint: Write the expression for point selectivity s = RD/(RU + RD+ RW) and relate it to overall selectivity, S. Use CB/CA as the independent variable]. What is the optimal selectivity?

b)If the feed contains CAo = 1 (mol/L) and no U, D, or W, and you operate at conversion of A of xA = 0.98, while maintaining the optimal CB/CA ratio determined in part a), what is the concentration of D, CD . that will be produced?

c)How much should the feed concentration of B be (CBo = ?) so that you can attain and maintain the conditions in the reactor determined in part a and b?

  1. A reversible first order exothermic reaction A = R takes place in a continuous flow stirred tank reactor (CSTR). The rate is

RR = k1CA-k2CR(mol/L min)

with

where activation energies are in (cal/mol). Hence R = 1.987 (cal/mol K).

The feed is at CAo = 5 (mol/L) and To = 300 K. We want to maximize the production rate of

R from a V= 2000 (L) CSTR when the reactor operates at 70% conversion of A.

a)At what temperature should the CSTR operate for optimal performance?

b)What is the reaction rate at these optimal conditions and what is the production rate of R?

c)What are the heating or cooling requirements for this reactor? (

d)You would like to operate a plug flow reactor (PFR) adiabatically for this reaction and get the exit conversion of 70%. Sketch on a xA vs T diagram what such adiabatic operation would look like. Describe clearly in what range of temperature would you search for an optimum feed temperature for your PFR.. Document your answer.

[Note: The kinetic parameters for the above reaction are almost identical to the ones for the reaction for which the enclosed chart from your book is appended. You can use the chart to get your answers].

  1. An irreversible liquid phase dimerization is conducted under pressure. The stoichiometry is:

2A = D

and the second order rate of disappearance of A is given by

where R = 1.987 (cal/mol K). The heat of this exothermic reaction is written as and the liquid heat capacity is . The feed rate to be processed is Q = 1.3 (L/s) and the feed concentration is CAo = 16 (mol/L).

We would like to operate a continuous flow stirred tank reactor (CSTR) adiabatically at a conversion of xA = 0.9.

a)If the feed is preheated to 39˚C (312 K) find the needed operating adiabatic temperature for the reactor.

b)Find the required reactor volume needed to process the above feed to the desired conversion. What is the production rate of D?

c)Is your operating condition above the only possible steady state in the reactor of the

size determined in part b? Explain clearly how you would check and sketch your answer.

  1. The desired product R is produced by the autocatalytic liquid phase reaction of reactant A

A + R = R + R

with the rate of production of R given by:

RR = k1CACR (mol/Lmin)

In parallel a competitive reaction occurs resulting in unwanted product W:

2A = W

with the rate of production of W given by:

RW = k2CA2 (mol/L min).

The feed is available at CAo = 1 (mol/L) CWo = CRo = 0 and we need to process

Q = 10,000(L/min) of the feed. At the selected temperature for isothermal operation of T = 60˚C the rate constants are k1 = 1 (L/mol min) and k2 = 0.5 (L/mol min). We are told to limit our reactor sizes to V = 100,000 (L) if choosing a CSTR, and to 1000 (L) if selecting a PFR, since for larger sizes than those the capital costs are prohibitive.

a)Your goal is to maximize the overall selectivity CR/CW! What reactor type (contacting flow pattern) would you choose? Explain your reasoning.

b)What reactor size would you need and what is your overall selectivity, conversion of A and production rate of R?

c)If you know that the activation energy for the first (desired) reaction is

E1 = 20,000 cal/mol and the activation energy for the undesired reaction is

E2 = 40,000 cal/mol, and you can operate isothermally at any temperature in the range from 40˚C to 80˚C, which temperature of operation would you pick if your main goal is: i) to further increase the overall selectivity, ii) to increase the production rate of R. Explain and document your answer fully.

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