Identification of mass transfer parameters in three-phase biofilm reactors

Abstract

This work presents the analysis of dynamic models describing oxygen transfer and reaction in three-phase biofilm reactors. The modelling is mainly aimed at designing experiments for the estimation of mass transfer parameters in these systems. Systems with and without reaction are investigated separately and distinct approaches are used to simplify the model equations in the two cases. In order to show their limitations, simplified models are compared to a mechanistic model describing oxygen gas–liquid transfer, liquid–solid transfer, and diffusion and reaction within the biofilm. Model reliability in the estimation of the parameters is established through a sensitivity analysis. In the case of plain oxygen absorption in the liquid phase and adsorption without reaction in the biofilm layer two limiting cases are considered for slow and fast diffusion within the biofilm. When assuming fast diffusion, the simplified model has an error of less than 4% (relative to the mechanistic model) in the estimation of volumetric gas–liquid mass transfer coefficient ( k L a) in the case of low solid hold-up or low value of k L a. In the case of a reacting system, half-order and zero-order expressions for the biochemical reaction are used to calculate explicitly the oxygen flux at the biofilm surface without remarkable deviation from the values calculated through the mechanistic model. On the basis of this analysis, criteria are given to design experiments for the estimation of mass transfer parameters in three-phase biofilm reactors.