Modeling of Axial and Recycle Backmixing Effects in a Biological Packed Bed Loop Reactor

A theoretical model for a packed bed biological loop reactor is presented by considering both external and internal resistances for the general case of Monod kinetics. Numerical solutions have been obtained by the method of orthogonal collocation for a wide range of saturation parameters to cover the two limiting cases of zero-order and first-order kinetics. The numerical solutions for the limiting cases were found to show good agreement with the analytical solutions. The effects of recycle ratio and axial dispersion on the performance of the reactor were studied parametrically. The results show that for low recycle ratios the conversion increases with decrease in Peclet number for first-order and Monod type kinetics; for zero-order kinetics, however, the conversion is independent of both Peclet number and recycle ratio. The effectiveness factor profiles along the length of the reactor were compared for Monod kinetics. It was found that an increase in recycle ratio tends to flatten the profile. The effect of axial dispersion on the concentration profiles at higher recycle ratio was found to be negligible. The dynamics of how steady state conditions are achieved in the reactor is also presented.