Azeotropic Separation of HCl-Water by Pressure Swing Distillation: Steady State Simulation and Economic Optimization

Abstract

Copper-Chlorine (Cu-Cl) thermochemical cycle for hydrogen production involves decomposition of water into hydrogen and oxygen with the use of intermediate copper and chlorine compounds and all other reactants are regenerated and recycled to create a closed loop. Our ongoing investigations on copper-chlorine (Cu-Cl) cycle involve electrochemical and thermal reactions constituting six steps. Hydrolysis of cupric chloride to copper oxide and dry hydrogen chloride (HCl) gas is one of the crucial steps. This involves non-catalytic reaction of cupric chloride and steam between 350 and 400oC which needs excess steam for complete conversion of cupric chloride thereby resulting formation of dilute hydrochloric acid rather than dry HCl gas which needs to be recycled to hydrogen generation. Therefore, azeotrope between the HCl and water needs to be eliminated. This work has proposed design and optimization of pressure swing distillation (PSD) for separation of HCl -water by using heat integration of two columns operating at different pressures and temperatures. The first column operating at low pressure concentrates the feed up to the azeotropic point and second column operating at high pressure eliminates the azeotrope and generates dry HCl gas. The reboiler heat of high pressure column (HPC) is reutilized to heat the low pressure column (LPC) and HPC feed which reduces the total annual cost (TAC). Heat integration saves an energy cost by 11.03% of and total annual cost by 6.83 % as compared to non-heat integrated PSD system. Feed composition has strong impact on energy and ultimately economics.