Multiple Reactions and Selectivity: Example Problems

Try to solve these problems before watching the solutions in the screencasts.

Example Problem 1

The reaction A + B → 2D is accompanied by an undesired side reaction where
A + B → U1 + U2. Expressions for the rates of the desired and undesired reactions are given below. If the reaction is run at 70% conversion, would you expect the selectivity toward the desired product to be higher in a PFR or in a CSTR?

Example Problem 3

The selective oxidation of ethylene (A) to ethylene oxide (D) has an activation energy of 15 kcal/mol over a certain catalyst.  The complete combustion of ethylene to CO2 (U1) and water (U2) has an activation energy of 20 kcal/mol over the same catalyst.  Both reactions are exothermic and reversible.  It is specified that the feed has a fixed composition, temperature, and pressure, and that the ethylene conversion will be 20%.  To achieve a high selectivity to the desired ethylene oxide product, would it be better to run the reaction isothermally or adiabatically?  Which reactor type would require a larger amount of catalyst? 

Example Problem 2

A CSTR is used to carry out a liquid-phase reaction 2A + B → D. A large stoichiometric excess of A is fed to the reactor. A parallel reaction A + 2B → U also occurs. Both reactions follow elementary rate laws. The selectivity of D to U at the CSTR outlet is 2.0. If a second CSTR of equal volume is added in series to the original reactor, will the selectivity at the exit of the second reactor be greater than, less than, or equal to 2.0? Explain.

Example Problem 4 (optional)

Reactant A can isomerize to produce the desired product, D (A → D) or dimerize to produce the undesired product, U (2A → U). Both reactions are elementary as written. Consider the following isothermal reactor scenarios, each of which utilize the same inlet stream (at a pressure of 18 bar) and a tubular fixed bed reactor. Which scenario is likely to lead to the highest selectivity of the desired product, D?
A. a reactor that achieves a 70% conversion with minimal pressure drop
B. a reactor that achieves a 70% conversion with a pressure drop of 15 bar
C. a reactor that achieves a 20% conversion with a pressure drop of 15 bar
D. The selectivity will be the same for at least two of the three cases.

Selectivity for Parallel Reactions in Fixed Bed Reactor (Interactive)