Engineering analysis of a packed-bed biofilter for removal of volatile organic compound (VOC) emissions

This study dealt with removal of VOC emissions from airstreams using an evolving new technology which is known as biofiltration. The basis of this technology is biodegradation of VOCs in biofilms formed around porous solids which are placed in packed-bed reactors. A detailed model describing steady state biofiltration of single and mixed VOCs was developed, and experimentally validated. The model takes into account biodegradation kinetics, the effect of oxygen, kinetic interactions among structurally similar compounds, and mass transfer from the gas phase to the biolayer. It was found that oxygen (a factor neglected in all previous studies) plays a very important role in biofiltration of VOCs, especially those which are hydrophilic. It was also found that the kinetics of biodegradation are complex, and that assumptions of zero or first order kinetics made by other researchers are invalid, and can lead to significant errors in biofilter design. Sensitivity studies with the model have shown that some of the kinetic parameters, and the biofilm surface area per unit volume of biofilter bed are important in all cases. For hydrophilic solvent vapors, sensitivity studies indicate that oxygen availability in the biolayer is also extremely important. The model was experimentally validated. In the case of single VOCs, methanol, benzene, and toluene were the model compounds. Methanol data were obtained from another study, while benzene and toluene data were generated during the course of this study from a unit 75cm-high and 10cm in diameter. For benzene removal, the residence time was varied from 2.7 min to 4.7 min, and the concentration in the inlet air from 0.07 gm-3 to 0.56 gm-3. During the experiments for toluene vapor removal, the residence time was varied from 2.7 min to 8.6 min, and the inlet concentration from 0.62 gm-3 to 2.81 gm-3. Validation of the model for the case of mixed VOCs was done with experiments involving mixtures of benzene and toluene. The unit was a three-stage glass column specifically designed during the course of this work. Each segment was 15.2cm in diameter and 30.5cm in height. Residence times varied from 0.9 min to 3.1 min, inlet benzene concentrations from 0.13 gm-3 to 0.37 gm-3, and inlet toluene concentrations from 0.21 gm-3 to 0.52 gm-3. In all cases, there was excellent agreement between model predictions and experimentally obtained concentrations. The experimental columns were continuously operated for periods over six months for single VOCs, while for mixed VOCs the column operated continuously for a year and a half. Except at start-up, in no case were additional nutrients added to the columns, while the pressure drop never exceeded 0.25 water/m of biofilter bed. Peat and perlite mixtures (2:3 volume ratio before packing) were used in all columns as solid porous support for the biofilm. Transient operation of biofilters involves, in addition to the mass transfer and reaction processes occurring at steady state, reversible adsorption of VOCs onto the packing material. This extra process was taken into account in developing a model which describes transient biofiltration of airstreams containing a single VOC. This model was experimentally validated with data for transient removal of toluene vapor. Good agreement was found between theory and experiments. The experimentally validated models developed in this study, can be used in (at least preliminary) scale-up and design of industrial biofilters