Three-dimensional computational fluid dynamics (CFD) models were developed to investigate the flow distribution and chemical reactions in a partially packed vessel similar to a catalytic sulfur converter used to sweeten sour gas, known as a Claus converter. A gas feed enters the vessel and reacts over the catalyst before leaving as products. Simulations of the flow in a Claus converter showed that the packed bed acts as a good flow distributor. However, some fluctuations of flow were observed in the packed bed, especially in the zones corresponding to the incoming feed jet impingement locations. Near the outlet of the bed, the flow becomes significantly better distributed. The chemical reactions were simulated using a finite rate model. Results of the complete CFD model (including flow, chemical reactions, and heat transfer) of each of two Claus converters placed in series showed good agreement with the available industrial data. The numerically predicted conversion matched the available value closely, and so did the detailed composition of the gas leaving each of the three converters. The predicted and measured values of the temperature also showed good agreement.