(395d) Predicting Residence Time Distribution in Gas-Phase Exothermic Reactors with Non-Reacting CFD Model

Residence time distribution (RTD) provides a quantified characterization of non-ideal flow in reactor, and can be used in reactor/process design, analysis and optimization. RTD in a gas-phase reactor was measured with radioactive tracers. Computational fluid dynamics (CFD) provides an efficient alternative to obtain RTD in a reactor, using stimulus-response technique on top of the flow field solved with CFD. However, unlike in liquid-phase reactor, the flow field in a gas-phase exothermic reactor is coupled with kinetics and temperature field, and solving such flow field requires reacting flow simulation, which is complicated, difficult and time-consuming. A simple and effective modeling approach has been developed to predict RTD in gas-phase reactors where the temperature gradient is significant and the isothermal flow simplification is inappropriate. The reactor flow field is solved through the CFD simulation of non-reacting flow with a heat-source patch. The patch mimics the effect of heat generation due to exothermic reactions in the reactor through a rather simple source-term, which is applied to the heat equation in the patch zone. The flow field predicted with this approach was found reasonably close to the prediction using the complex reacting flow model with large eddy simulation (LES). The heat source patch provides a rather simple way to account for effect of exothermic reactions without involving the complexity associated with the reactions and avoid unrealistic isothermal assumption. This approach has been validated by the good agreement between the predicted RTD and the plant measurements. The sensitivity study showed that the developed approach is not only simple but also robust in providing flow field for RTD calculation.