A control strategy of flow reversal with hot gas withdrawal for heat recovery and its application in mitigation and utilization of ventilation air methane in a reverse flow reactor

Abstract

A control strategy of flow reversal with hot gas withdrawal for heat recovery was developed and verified for the mitigation and utilization of ventilation air methane (VAM) in a reverse flow reactor. The flow reversal strategy was based on the thermal front propagation velocity and a logic-based controller was built to avoid catalyst deactivation and reaction extinction caused by prolonged rich or lean feed conditions; the utilization of heat released from VAM combustion was considered through heat extraction by withdrawing part of hot gas from the middle of the reactor. By means of simulation using a one-dimensional heterogeneous model, the results revealed that the flow direction should be reversed before the temperature peak passing through the middle of the reactor to reduce the discharge of unreacted methane as well as to enhance the heat recovery efficiency; a fixed (relatively short) switching time is reasonable for the reactor operation in the case of hot gas withdrawal. The stability and autothermicity of reverse flow reactor are jointly influenced by the feed concentration and hot gas withdrawal fraction; the reactor can only be run stably in a narrow operating window (stability area). Heat recovery and bed temperature regulation are achieved mainly by hot gas withdrawal, while air dilution and extra methane injection are the auxiliary manipulated variables to sustain the reaction in some extreme circumstances. An extra catalyst bed along the way to the boiler is effective to enhance the overall methane conversion by oxidizing the unreacted methane. A case study under modified real ventilation conditions shows that the control strategy developed in this work is feasible to mitigate and utilize VAM if the methane content is higher than 0.2vol.%.