University of Saskatchewan

Department of Chemical Engineering ChE 411.3 – Chemical Reaction Engineering

FINAL EXAMINATION

DATE: Monday, December 9, 2002 INSTRUCTOR: Professor A.K. Dalai TIME: 9:00 a.m. - 12:00 (3 hours) Closed Book. Answer any five questions. Question #1 (20 Marks) (Answer five of six.)

Define and discuss the following concepts as applied to catalytic/noncatalytic reactions/reactors. Give examples to illustrate your point where necessary. (a) Weisz-Prater Criterion for Internal Diffusion. (b) Langmuir-Hinshelwood Kinetics. (c) Estimation of speed of reactions. (d) Straight-through and re-circulating transport reactors. (e) Rate controlling step. (f) Non-isothermal effectiveness factor.

Question #2 (20 Marks)

The exothermic reaction A → B + C was carried out adiabatically and the following data recorded:

X 0 0.2 0.4 0.5 0.6 0.8 0.9 -rA(mol/l · min) 10 16.67 50 50 50 12.5 9.09

The entering molar flow rate of A was 300 mol/min. (a) What are the PFR and CSTR volumes necessary to achieve 40% conversion? (b) Over what range of conversions would the CSTR and PFR reactor volumes be identical? (c) What is the maximum conversion that can be achieved in a 10.5 l CSTR? (d) What conversion can be achieved if a 7.2 l PFR is followed in series by a 2.4 l CSTR?

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Ch E 411.3 December 2002

Question #3 (20 Marks)

The thermal decomposition of isopropyl isocyanate was studied in a differential packed bed reactor. From the data below, determine the reaction-rate-law parameters (k0 E and α).

Run Rate Concentration Temperature

(mol/s·l) (mol/l) (K)

1 4.9 x 10-4 0.2 700

2 1.1 x 10-4 0.02 750

3 2.4 x 10-3 0.05 800

4 2.2 x 10-2 0.08 850

5 1.18 x 10- 0.1 900

6 1.82 x 10-

20.06 950

Assume: - rA = koe –ERT CAα

Question #4 (20 Marks) Ethyl Benzene (A) dehydrogenation reaction to styrene (B) and hydrogen (C) takes place in the presence of Shell 105 catalyst at 10 atm and 630oC. Given: The reaction is independent of the hydrogen partial pressure, and surface reaction is irreversible and rate controlling, the following mechanism holds:

A + S A ● S (adsorption)

A ● S → B ● S + C (surface reaction) B ● S B + S (desorption) Determine the rate of conversion of ethyl benzene, and evaluate the rate constants using the following data:

Experiment PA (atm) PB (atm) -rA1 (gmol/min. g cat.)

1 1.0 0.0 1.415 * 10-3 2 0.01 0.0 0.214 * 10-3 3 1.0 1.0 0.116 * 10-3

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Ch E 411.3 December 2002

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Question #5 (20 Marks) The following chain reaction has been proposed for the ethane pyrolysis reaction:

(a) C2H6 2CH→ 1k3 ●

(b) CH3 ● + C2H6 CH→ 2k

4 + C2H5 ● (c) C2H5 ● C→ 3k

2H4 + H ● (d) H ● + C2H6 H→ 4k

2 + C2H5 ● (e) 2 C2H5 ● C→ 5k

4H10 Define an expression for the rate of appearance of ethylene.

Question #6 (20 Marks)

The conversion (x) for externally mass transfer-limited reaction in a packed-bed reactor can be given by the equation:

ln LU

ak - x

c=

11

where kc = Mass transfer coefficient, m/s a = surface area of catalyst per unit reactor volume, m2/m3 L = length of the packed-bed of the catalyst in the reactor, m U = superficial linear velocity, m/s Derive this correlation giving all pertinent assumptions. State the importance of this correlation in the design

of a heterogeneous catalytic reactor.

END OF EXAMINATION