Development and modeling of a radial-flow vapor phase bioreactor for the removal of para-xylene

The work presented investigated the experimental and theoretical development of a radial-flow, vapor phase bioreactor for the biodegradation of para-xylene. A vapor phase bioreactor which utilizes porous metal membranes in a cylindrical design and employs radial flow as opposed to traditional axial flow for the vapor stream was developed. The system was evaluated for the biodegradation of para-xylene from a water-saturated air stream by Pseudomonas putida ATCC 23973 immobilized onto sand. The biocatalyst was placed in the annular space between two cylindrical, porous stainless steel membranes. Details of the reactor system are presented along with biological experimental results verifying reactor performance. The operating parameters that were examined and allowed to vary were the influent p-xylene concentration and the flow rates of the influent vapor streams. The influent concentrations were in the range of 15 to 200 ppm at flow rates between 60 and 130 cm3/min. Continuous reactor operation was maintained for 80 to 200 hours with fractional removal of p-xylene achieving 80 to 95%. The effluent concentration curves were compared to determine the operating range of the bioreactor. As the influent para-xylene concentration increased from 50 to 100 to 150 ppm, the fractional removal successively decreased while still remaining as high as 80% for the 50 ppm case and 60% for the 100 ppm case for up to 80 hours. The system did appear to become saturated after 60 hours of operation with an influent para-xylene concentration of 147 ppm. A mathematical model for reactor operation suitable for design purposes was developed. The Galerkin finite element model was applied to the solution of the developed transport and constitutive equations. The effluent concentration curves of para-xylene for 15 ≤ C_o ≤ 50 ppm could be closely correlated with the predictive model consisting of the transport and constitutive equations combined with Monod kinetics. For the concentration range 50 ≤ C_{o,/sub> ≤ 150 ppm, a modified model incorporating a cell death as well as growth with the original Monod kinetics was successful in describing the data. The biomass radial profiles developed from the computational models indicate two different types of behaviour within the reaction zone: a growth region, termed type I, where B≥Bo at all radial nodes and biomass continues to increase with time; and a death region, type H, where B< Bo at all radial nodes and biomass decreases as time increases. Profiles representative of type I occurred with the growth kinetics model, whereas the growth/death kinetic model resulted in profiles of type II.}