Question

When the boundary layer is present in the shell mass transfer then?

(a) CAs = CAb

(b) CAs ≠ CAb

(c) CAs < CAb

(d) CAs > CAb

The question was posed to me in a job interview.

The question is from External Mass Transfer in portion Heterogenous Reactions of Bioprocess Engineering

Answer 1

Right option is (d) CAs > CAb

The best I can explain: The term CAs is the concentration of substrate A at the external surface of the catalyst. This term made its way into the analysis in the boundary conditions used for solution of the shell mass balance.

Reduction in substrate concentration from CAb in the bulk liquid to CAs at the catalyst surface occurs across the boundary layer surrounding the solid. In the absence of the boundary layer, CAS = CAb, which is easily measured. When the boundary layer is present, CAs takes some value less than CAb.