Modeling sulfuric acid decomposition in a bayonet heat exchanger in the iodine-sulfur cycle for hydrogen production

Accurate simulations of sulfuric acid decomposition are important for the design and control of sulfuric acid decomposers, which are key equipments for nuclear hydrogen production with the iodine-sulfur cycle driven by heat from a very high temperature gas cooled reactor. The present study included sulfuric acid decomposition experiments with measurements of the decomposition fraction and temperature distribution to verify a user defined boiling model that relaxes the coupling between the phase transition and the chemical reactions modeled using a species transport model based on computational fluid dynamics. The heat transfer in the bayonet heat exchanger was simulated numerically including the liquid sulfuric acid phase change, the reversible sulfuric acid decomposition reaction, and the sulfur trioxide decomposition into sulfur dioxide in the catalyst zone. The results show that the proposed method agrees well with experimental data. The sulfuric acid needs to completely decompose before entering the catalyzer to make full use of the catalyst. The decomposition fraction of sulfuric acid decreases significantly with increasing flow rate due to the inadequate retention time in the catalyst zone. The sulfuric acid decomposition rate increases and then tends towards a stable rate with increasing velocity. This study provides a useful model for engineering designs of high-performance sulfuric acid decomposers.