This paper presents the results of numerical calculations of the hydrogen iodide decomposition process under different temperature conditions, at different hydroiodic acid mass flow rates, in the presence of active coke acting as a catalyst. The numerical studies were aimed to reproduce the conditions of laboratory work carried out in an experimental hydrogen iodide decomposition reactor. The key element of the laboratory installation was a U-shaped tubular reactor made of quartz glass with a catalytic bed inside. Laboratory tests were carried out to determine the hydrogen iodide conversion rate as a function of process temperature. The aim of this paper is to compare and discuss the results obtained from the numerical calculations with those from the laboratory tests. Both the numerical calculations and the laboratory tests confirmed that the conversion rate increases with increasing temperature. In addition, the analysis of the results showed the influence of hydrogen iodide mass flow rate on the conversion rate of the decomposition process. The validated numerical model may become a useful tool for optimizing the future designs of chemical loop test stands and prototypes.