Thesis for Doctor of Philosophy

Cláudia Isabel Oliveira e Carmo de Silva Maia

“Gas dispersion and mass transfer in fermenters”
Ph. D. Chemical Engineering, IST, 2002
Supervisors: J.M.T. Vasconcelos; S.S. Alves

Abstract

Gas dispersion and mass transfer were studied in a stirred tank equipped with two Rushton turbines, in coalescing (tap water) and non-coalescing (Na2SO4 solution and PEG solution) media.

Local average bubble diameter was measured with a capillary photoelectric suction probe. Bubble size increases from the stirrer tip along the discharge stream, soon to reach a value representative of the bulk of the tank. Bubble sizes in non-coalescing media are smaller and more sensitive to power input.

Local gas holdup was determined by volumetric method. Specific interfacial area distributions were calculated from holdup and average bubble diameter data.

Volumetric mass transfer coefficient was measured using the peroxide decomposition steady-state technique. Film coefficient was calculated from experimental results of specific interfacial area. It was found that film coefficient is lower for non-coalescing media, probably due to the immobilization of bubble’s surface.

A compartment model was established for gas dispersion, that takes into account bubble breakage and coalescence. Reasonable agreement between experimental and simulated values of local average bubble size and gas holdup was achieved.