Numerical study of sulfur trioxide decomposition in bayonet type heat exchanger and chemical decomposer with porous media zone and different packed bed designs

Abstract

The Department of Energy (DOE) Nuclear Hydrogen Initiative was investigating thermochemical cycles for hydrogen production using high temperature heat exchangers. The present work was concerned with use of bayonet type heat exchanger as silicon carbide integrated decomposer (SID) which produces sulfuric acid decomposition product – sulfur dioxide. The product can be used within the sulfur–iodine thermochemical cycle and hybrid sulfur process. A two-dimensional axis-symmetric geometry of the bayonet heat exchanger has been created using GAMBIT software. Fluid, thermal and chemical reaction analyses were performed using FLUENT software. The working fluids in the model were sulfur trioxide, sulfur dioxide, oxygen and water vapor. Silicon carbide with 1 wt% of platinum was used as a catalyst for the chemical reaction. The form of pellets packing was simple cubical packing and porous media approach was used. The chemical reaction for the two-dimensional model was carried out in two cases (ways): (1) constant outer wall temperature and (2) by applying the measured values obtained from the thermocouples placed along the outer wall of the lab scale model of the bayonet heat exchanger in Sandia National Lab (SNL). The decomposition of sulfur trioxide for the two-dimensional model was calculated and the obtained results were compared with the experimental results. But, practically the flow of fluid between the pellets in the decomposer region of the bayonet heat exchanger may have many swirling and recirculation. Hence a further study on three-dimensional model of the decomposer with different arrangement of the pellets in the packed bed region was carried out. The chemical decomposition that occurred in packed bed was of the decomposer. The engineering design of the packed bed was very much influenced by the structure of the packing matrix, which was governed by the shape, dimension and the loading of the constituent particles. The investigations of different types of catalyst in the packed bed region and the decomposition of sulfur trioxide were calculated and the results obtained were consistent with the experimental results.