Question

Reducing the reaction rate rA,obs improves the effectiveness of mass transfer aimed at increasing the reaction rate.

(a) True

(b) False

I got this question in a job interview.

This interesting question is from Minimising Mass Transfer Effects in chapter Heterogenous Reactions of Bioprocess Engineering

Answer 1

Correct choice is (a) True

The best explanation: When the catalyst is very active with a high demand for substrate, mass transfer is likely to be slow relative to reaction so that steep concentration gradients are produced. However, limiting the reaction rate by operating at sub-optimum conditions or using an organism or enzyme with low intrinsic activity does not achieve the overall goal of higher conversion rates. Because rA,obs is the reaction rate per volume of catalyst, another way of reducing rA,obs is to reduce the cell or enzyme loading in the solid. This reduces the demand for substrate per particle so that mass transfer has a better chance of supplying it at a sufficient rate. Therefore, if the same mass of cells or enzyme is distributed between more particles, the rate of conversion will increase.