Study On Reverse Flow Reactor For Decontaminating Lean Volatile Organic Compounds In Waste Gas

Reverse flow reactor (RFR) is an integration of regenerative heat exchanger and fixed bed, in which flow direction is periodically changed. Applying such forced unsteady state operation (FUSO), a hot zone is trapped in the central catalytic section, which suits for exothermal catalytic reaction with low inlet concentration or reversible exothermal reaction with mild reaction heat release. Thus an increase in reactant conversion or product selectivity can be expected. This paper presents experimental and numerical results of the behavior of a bench-scale RFR, which is considered as a proper technology for decontaminating lean volatile organic compounds (VOCs) in waste gas. Effects of operation conditions on reactor behavior and corresponding control strategies when reactor stability is impaired are discussed.A bench-scale RFR is built up, on which pressure drop through the reactor is studied. The results show that with periodic flow reversal, pressure drop of the reactor changes periodically. It takes several seconds to get a stable flow condition in the reactor soon after the flow reversal. However, effect of such temporary states on reactor behavior is rather since it is very short comparing to flow reversal time. Total pressure drop through the reactor can be calculated by Ergun equation very well, based on regression of experimental data, which means the pressure drop shows a quadratic increase of superficial velocity and a liner increase of bed depth.Effects of operation parameters on reactor behavior are discussed, taking methane or benzene as model reactant. Temperature in the reactor varies periodically with flow reversal, while conversions of VOCs are all above98%if an auto-thermal operation is maintainded. Little NOx is detected. Basically, increasing inlet combustibles concentration or shortening flow reversal time will result in a higher thermal level and wider temperature plateau in reactor, which increase the VOCs conversion as well. Effect of superficial velocity is co-effect of heat release and convection. When binary mixture of methane and benzene is combusted in RFR, thermal level in reactor increases with the increasing of methane portion. Conversion of benzene is above95%. To eliminate the possibility of reactor thermal run away, decreasing inlet concentration or lengthening flow reversal time are all feasible ways. While using electrical heater or adding auxiliary fuel can increase reactor thermal lever, avoiding reactor extinction. But if methane is added as auxiliary fuel, large amount of methane is needed.A one-dimensional heterogeneous is built up. Good accordance between simulation and experimental results are acquired.Effect of reactor wall, transport parameters and reaction kinetics are discussed, based on numerical simulation. Moving range of heat wave is chosen as criterion when comparing effects of various operation parameters on reactor behavior. With parameters applied in this study, following optimum range of operation parameters are proposed, with superficial velocity of0.2～0.4m·s-1, moving range of heat wave taking20～40%of catalytic bed length and a inert packing taking20～40%of total reactor. Within this range, reactor operates more stable and lower minimum inlet concentration is required in maintaining auto-thermal operation.Effect of periodic inlet concentration is studied. The results show that reactor stability is impaired in such situation, decreasing of reactor thermal level may even cause reactor extinction. Adjusting flow reversal time turns out to be a proper way in maintaining reactor stability. Based on experimental and numerical results, flow reversal time is regarded as characteristic response time of the reaction system. The reason why synchronizing between periodic inlet variation cycle and flow reversal cycle leads to dramatic maximum temperature variation is explained adopting harmonic analysis. According to this theory, the feasibility of changing system characteristic response time in maintaining stable operation is verified by numerical simulation.With the results acquired in experiments and numerical simulation, a industrial scale reverse flow reactor for purifying waste gas flow of60000m·3h-1is designed, including system feasibility and economical analysis.