Process Simulation Of Open-loop Sulfur-iodine Cycle For H₂ Production And Experimental Research On Two-phase Separation Of Bunsen Reaction

Hydrogen energy, which is considered as an ideal regenerative energy carrier has many advantages such as clean, high efficiency, safety, advantageous storage, regeneration and various source. Large-scale and low-cost hydrogen production is the foundation of hydrogen economy. Compared with other candidate hydrogen production processes, the sulfur-iodine (SI) thermochemical cycle has a number of attractive features including high thermal efficiency, few flowsheet steps and available industrialization. It is the most promising thermochemical cycle for mass hydrogen production.The Bunsen reaction is the first step of SI cycle. The efficient separation of HI and H2SO4 which are the product of Bunsen reaction is a technical challenge needs to be solved. In this paper, the Iodine Excess Addition method is used to investigate the two-phase separation characteristics of Bunsen reaction products. The phenomenon of two-phase separation and excess iodine solidification is first studied in wide temperature range. Then, characteristics of two phase density, impurity in each phase, and component of sulfuric acid phase as well as HI concentration in HI phase influenced by excess iodine are discussed. The influences of temperature and excess water on two phase purification are also investigated. Finally, an optimal operating condition for the Bunsen reaction of SI thermochemical cycle is recommended. The results have indicated that:The solubility of iodine in HI/H2O solvents is increased by temperature while decreased by H2O molar ratio; The two-phase region of the product of Bunsen reaction is widened while the temperature increasing; The two-phase separation time is less than 2 minutes which is slightly effected by solution components; The two-phase separation characteristics are significantly improved by the excess iodine; Based on the results, the allowable window ranges 2.4-4 mol for the excess iodine and 345-358K for the temperature when the pressure is 1 atm and the molar ratio of H2SO4, HI and H2O is 1:2:12. The optimal operating point is represented by 2.5 mol of excess iodine and 12 mol of excess water at temperature of 351K. At this optimal operating point, the HI concentration in HI phase is over-azeotropic which reaches 27.35% while the impurities in two phases keep in low level.A flowsheet of open-loop sulfur-iodine thermochemical cycle for the production of hydrogen and sulfuric acid is designed and simulated by Aspen Plus. The production rate of H2 is fixed at 1 mol/s. The heat and mass balance as well as thermal efficiency of this process are calculated. Effects of several operating conditions on the thermal efficiency are also evaluated. The results have indicated that:The thermal efficiency of the total process is 66.79%; The molar flow rate of HI phase and the reflux ratio at HI distillation column are the main factors which influence the thermal efficiency; The optimization of Bunsen reaction process can increase the HI concentration in HI phase, decrease the heat and electric energy demand in HI concentration and distillation process which increase the total process thermal efficiency.