Improved solvation routes for the Bunsen reaction in the sulphur iodine thermochemical cycle: Part II – Molecular solvent properties

Operating the Bunsen reaction in the presence of a molecular solvent has the potential to decrease the large excesses of water and iodine currently required and therefore to increase the efficiency of the sulphur iodine cycle. Tri-n-butyl phosphate (TBP) has been shown to extract HI into the solvent phase, however the TBP breaks down when treated thermally to recover the HI. Alternative solvents which also show a high extraction of HI, but that are stable in the presence of HI at high temperature, are needed. This paper investigates dibutylbutylphosphonate (DBBP) and Cyanex® 923 as alternatives to TBP. Both DBBP and Cyanex® 923 were shown to perform as well as or better than TBP against metrics including: HI extraction, H2SO4 extraction, SO2 solubility and I2 solubility. Using TGA, thermal recovery of the HI was not successful from DBBP, as the solvent broke down in a similar way to TBP. Small amounts of HI were recovered through distillation of Cyanex® 923, however the yield was low. Partial success was achieved by backwashing the HI loaded Cyanex® 923 with water. Further work is needed to assess the performance of the actual Bunsen reaction in each of these solvents and to find ways of recovering the HI from the solvent.