Modeling of Volatile Organic Compound Removal in Rotating Drum Biofilters
|Keywords||Biofiltration Biomass distribution Biomass Accumulation Rate Model Numerical Methods Rotating Drum Biofilter Volatile Organic Compounds|
Rotating Drum Biofilters (RDBs) could effectively remove volatile organic compounds (VOCs) from waste gas streams, which have overcome the disadvantages including the uneven distribution of organic loadings, nutrients, liquid and biomass of conventional biofilters or biotrickling filters, and to an extent released the biomass excess accumulation or clogging in the filter bed. It would be important theory and practical significance in the research and application of bioreactors for VOC removal to simulate the complex processes, such as the multi-layer RDB for VOC removal at different operating conditions, the dynamic kinetics of biomass growth, the spatial and temporal changes in biomass distribution, and the biomass accumulation rate.A mathematical model was developed on the basis of mass transport and mass balance equations in an RDB, the two-film theory, and the Monod kinetics. This model took account of mass transfer and biodegradation of VOC in the gas-water-biofilm three-phase system in the biofilter, and could simulate variations of VOC removal efficiency with a changing specific surface area and porosity of the media due to the increasing of biofilm thickness in the biofilter. This model was further simplified by introducing a coefficiency of the gas velocity and neglecting the water phase due to the complexity of operating conditions. The equations for the biofilm phase, gas phase, and biofilm accumulation in this model were solved using collocation method, analytic method, and the Runge-Kutta method separately. A computer program was written down as MATLAB to solve this model.Toluene, diethyl ether, hexane were used as the model VOC respectively. Results of numerical solutions showed that the effect of VOC loadings and gas empty bed contact times (EBCTs) on the removal efficiency and porosity of the filter bed has a dynamic diversification even at the same operating conditions but different medium position. Generally, the removal efficiency increases with the decreasing VOC loadings or the increasing gas EBCTs, and the bed porosity decreases with the increasing VOC loadings or the increasing gas EBCTs. The removal efficiency in the porous medium of the outer layers is larger than the inner layers. At the same operating conditions, the removal efficiency of diethyl ether is highest, toluene second, and hexane lowest. The removal efficiency would be decrease with an increasing Henry’s constant of the organics. The biomass volumes and biomass accumulation rates in different medium layers are decreased with decreased organic loadings or decreased gas EBCTs. The biomass accumulations are high in the outer layers, but low in the inner layers. A more uneven distribution of biomass is existed in the different medium layers when at a small organic loading or a small gas EBCTs.The dynamic removal efficiencies from this model correlated reasonably well with experimental results for toluene removal in a multi-layered RDB. It provides the basis for the spreading and application of biofiltration on the waste gas treatment.