Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H2O to I2 in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H2, with a Section energy requirement of 367 kJ/mol H2, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.

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► Process simulation of S–I HIx with reactive distillation and heat pumps.
► Optimal is 10-stage RD column at 12 bar with reflux ratio of 0.75.
► Minimum S–I energy of 367 kJ/mol H2 for HIx feed rate of 126 to 140 mol/mol H2.
► High feed rate lowers reboiler temperature, reduces water in H2 product.