Modeling and simulation of biological CO conversion in trickle bed reactor and comparison with CSTR

Abstract

Syngas biomethanation in a trickle bed reactor (TBR) by mixed microbial consortia has reached a technology readiness level of 4–5. Scaling up the TBR requires investigating the operating conditions under which mass transfer or growth kinetics becomes the rate-limiting step. The novelty of the current work is the first-of-its-kind model development that can simulate microbial growth and biofilm formation together with the hydraulic and mass transfer behavior of TBR. The focus is on carboxydotrophic hydrogenogens, which biologically convert CO to CO₂ and H₂, reducing gas phase CO levels. Model validation showed excellent predictive capacity for TBR of different volumes and height-to-diameter ratios. CO conversion efficiency was compared between 5000 ml (packed bed) semi-pilot-scale TBR, 220 ml (packed bed) lab-scale TBR, and 5000 ml (liquid volume) CSTR. Considering suspended growth, the semi-pilot-scale TBR outperformed the lab-scale TBR and CSTR due to a higher volumetric mass transfer coefficient. When biofilm growth was considered for the TBRs, CO conversion efficiency increased 1.03 times and 4 times for the semi-pilot- and lab-scale TBRs, respectively. Notably, the lab-scale TBR, with biofilm growth, surpassed the 22 times higher working volume CSTR. CO conversion efficiency in the lab-scale TBR increased from 23.3 % (suspended growth) to 86.3 % (biofilm growth) due to an increase of microbial cell mass from 0.081 to 0.29 g cells. Finally, simulations at varying liquid recirculation rates and constant gas flow rates for the TBR revealed a threshold at a liquid Reynolds number 35, where mass transfer limitations shift to microbial growth limitations.