Simulation and optimization on the regenerative thermal oxidation of volatile organic compounds

Abstract

Volatile organic compounds (VOCs) are involved in the generation of ozone via reaction with atmospheric NO x in the atmosphere through a photochemical pathway. One of the popular treatment processes for VOCs is thermal oxidation, since it shows high destruction and removal efficiency (DRE). A regenerative thermal oxidizer (RTO) is commonly used because of its high heat exchange efficiency. The process uses ceramic beds to capture heat from gases exiting the combustion zone. Steady and unsteady flow field, distributions of temperature, pressure and compositions of flue gas inside an RTO were simulated by computational fluid dynamics (CFD). The DRE of VOCs and the pollutant concentrations of CO and NO emitted from the RTO are estimated by incorporating two-step incomplete combustion reaction of VOCs and Zeldovich’s NO formation mechanism. The model system was the oxidation of benzene, toluene and xylene by the RTO, which was composed of three beds packed with ceramic beads to exchange heat. The height of the ceramic beds was varied and the effect of change of height was investigated in terms of DRE and pressure drop. The objective of the study was to simulate the performance of the RTO and optimize the height of ceramic beds. In a preliminary calculation, more than 95% of DRE is obtained when the premixed flow velocity of VOCs–air is less than about 3 m/s at the steady state at a constant fuel rate of 0.469 m/s under the given conditions. A weak recirculation zone above the center bed appeared and the intensity of recirculation decreases when no fuel is added in the normal direction. This region represents the putative site for the formation of NO, while most of the CO produced is converted to CO 2 in the recirculation zone. The results show that a level of 1.0% of VOCs is sufficient to provide energy for the oxidation when heat is exchanged through the ceramic bed. A ceramic bed of 0.2 m in height is sufficient to operate properly at these conditions and 5 s is recommended as the stream switching time. The importance of this simulation study is that the performance of RTO can be calculated at any operating conditions without the need for a pilot stage. In addition, the results can provide insight and practical responses involved in the design of an industrial RTO unit.