||Regenerative oxidation is an economic and effective means of controlling volatile organic compounds (VOCs) with concentrations exceeding 1,200 mg/m3 in gas streams. Regenerative catalytic oxidizers (RCO) and regenerative thermal oxidizers are two main applications for the regenerative oxidation. However, factors influencing the performance of regenerative oxidation when treating VOCs in gas streams have seldom been addressed. Therefore, this study presents a convection-dispersion model with an effective thermal diffusivity (αe ) as a parameter to simulate the performance of regenerative beds. To verify the effectiveness of the proposed model, a pilot-scale RCO was constructed with two 20-cm x 200-cm (ID x H) regenerative beds. Gravel was used as the thermal regenerative solid material. Experimental results indicated that the model with an αe of 2.0-3.8 x 10-6 m2/s can be used to describe the time variation of solid temperatures with the packing height at superficial gas velocities (Ug ) of 0.080-0.382 Nm/s . Values ofαe for the bed are closer to those for the gravel solids (αs = 1.0 x 10-6 m2/s) than for air (αg = 54 x 10-6 m2/s). Those results demonstrate that the conductive heat transfer in the solid material in the axial direction of the bed is a major controlling factor for the performance of the RCO and the convective one is a minor factor in the present case.|
The above pilot RCO was then used to treat methyl ethyl ketone (MEK) and toluene, respectively, in air streams. The catalyst bed temperature was kept around 400oC and the Ug was operated at 0.234 Nm/s. This investigation measured and analyzed distributions of solid and gas temperatures with operating time and variations of VOC concentrations in the regenerative beds. The overall VOCs removal efficiency exceeded 98% for MEK of around 800 ppm as methane and 95% for toluene of around 400 ppm as methane. Degradation of MEK was believed to occur on the surface of solid material (gravel) in the temperature range of 330-400oC, which is much lower than its autoignition point, and toluene did not exhibit this phenomenon. The calculated energy conservation presents that RCO is an economic approach to treat VOCs, and it should be much further applied to industrial fields.
Furthermore, based on the earlier empirical results of RCO, a series of plant scale low temperature regenerative oxidizers (LTRTOs) equipped with heating wires were constructed to treat VOC-laden gas streams. The regenerative beds were still packed with the same gravel which was applied to the above pilot RCO. Gas streams for performance tests were exhausted from manufacturing sections of varnishing, semiconductor packing, and petrochemical plants, respectively. Components of tested VOCs were comprised of several commercial solvents (e.g. ketone, toluene, iso-propanol, methanol, ethanol, formaldehyde, dimethylamine, and others). Results indicate that exceeding 98% of single or multiple VOCs with concentrations of less than 100 and increasingly to 7,000 ppm as methane would be effectively destroyed. Gas temperature variations with time at various bed depths were analyzed, and results confirm that the degradation of VOCs exists in the gravel beds at the temperatures ranging from 300 to 440oC, which are much lower than auto-ignition points of tested compounds. Moreover, the residence time for a gas stream passing through the main oxidation zone (Tg >300oC) in the regenerative beds is an essential criteria for LTRTO design and 1.0 s is recommended. These findings demonstrate that LTRTO is an effective approach to treat VOCs.