A networks-of-zones analysis of mixing and mass transfer in three industrial bioreactors

Abstract

The original version of the networks-of-zones (N-o-Z) model developed for the description of gas–liquid flow in stirred-vessel reactors (R. Mann, Gas–liquid stirred vessel mixers: towards a unified theory based on network of zones, Transactions of the Institution of Chemical Engineers 64 (1986) 23–34) has been extended and enhanced to cover distributed bubble sizes, gas–liquid mass transfer, bioreaction kinetics and multiple-impeller operation. In addition, a modified version of the N-o-Z model for bubble columns has been simply derived from the impeller version, assuming the existence of a two-loop axisymmetrical circulation pattern induced by the non-uniform distribution of gas holdup in bubble columns. The liquid circulation velocity has been expressed as a function of gas flow rate and the density difference between the gas and liquid phases, based on Zehner's circulation model (P. Zehner, G. Schuh, A concept for the description of gas phase mixing in bubble columns, German Chemical Engineering 5 (1985) 282–289). These two variants of the N-o-Z model have been used for modelling three different industrial fermenters: 3 and 31 m 3 triple-impeller stirred reactors, and a 236 m 3 bubble column reactor. The performance of these three reactors, typical of the fine chemicals, bioprocessing and pharmaceutical process industries was evaluated and compared in terms of geometry/size, gas flows, power inputs, pressure, liquid mixing, oxygen mass transfer, reaction speed and spatial variability of behaviour. This provides potentially valuable insights into the relative factors influencing the selection of an appropriate reactor type.