Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production

Abstract

As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration and separation must cope with the pseudo-azeotropy of HIx (HI-I2-H2O) and excess iodine in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distillation is a validated method of HI separation from HIx solution in the SI process, the iodine deposition, resulting from changes in temperature and HI molality in HIx solution, can lead clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column. The energy duty and required input flow rate per mole HI was investigated in this study using a process flowsheet simulation. A decrease in iodine concentration in the streams to EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar concentration in the feed of the distillation column exhibited an appreciable influence on the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate.