Expressing variable mass transfer coefficients for gas fermentation in trickle bed reactor

Abstract

Gasification of lignocellulosic biomass and syngas biomethanation are promising technologies for producing biomethane. Trickle bed reactors (TBR) are efficient for syngas fermentation allowing for higher concentration of microbial cells and high surface area for mass transfer. Optimized scaling up and dimensioning of the reactor is crucial when it comes to industrial applications. The first step towards this goal and the scope of the present study is to develop a model able to simulate the mass transfer of gases to the liquid phase as a function of the operating conditions and reactor geometry. Experiments were performed in a lab scale TBR with co-current flow of syngas and water, representing the growth medium, to determine the volumetric mass transfer coefficient (kLa) of each gas compound and its dependence on the flowrate of syngas and water and reactor structural characteristics. The kLa achieved were in the range of 6.18–13.6 hr−1 for H2, 2.6–5.79 hr−1 for CO and 1.53–3.43 hr−1 for CO2. A dimensionless equation was fitted to the experimental data and a model was developed to predict kLa of a specific gas at various gas and liquid flowrates, and reactor configurations. A kinetic model was also developed to simulate the concentrations of syngas in gas phase and liquid phase by applying the kLa values predicted by the dimensionless correlation. Finally, the model was validated using experimental data obtained by mass transfer coefficient determination in pilot scale TBR with pure argon gas and literature data where the predicted kLa had a coefficient of variance of ± 15% and ± 25% respectively within a realistic range of liquid and gas flowrates.