(532b) Analysis of the Copper Sulfate Cycle for the Thermochemical Splitting of Water for Hydrogen Production

The Copper Sulfate cycle has a long history ? original work and experiments were performed in the late 1970's. This cycle involves two major steps: (1) hydrogen production from the reaction of water, SO2(g) and CuO(s) at room temperature and (2) the thermal decomposition of the products of the first phase to form oxygen and to regenerate reagents for the first step. The first step has been performed electrolytically and the second step appears to be possible at a temperature of about 700 °C. More complex versions of the Copper Sulfate cycle, called H-5 and H-7, involve 4 and 6 reactions, respectively.

In this paper, estimates of the process efficiency based on heats of reactions and sensible heats as well as on equilibrium yield considerations are reported. All of these estimates were in the neighborhood of 47%.

Additionally, results that are based on an Aspen Plus simulation of a proposed flowsheet for cycle H-5 are presented. This cycle can be simulated using standard Aspen Plus features and the Peng-Robinson equation of state for all separations involving oxygen and sulfur oxides (this system represents a significant departure from ideality). The flowsheet contains a significant section devoted to the separation of oxygen from sulfur oxides. A sizeable portion of the plant costs are associated with these separation processes.

Pinch technology was used to optimize heat integration. A sensitivity analysis revealed that it is economically attractive to use a 10 °C approach temperature (rather than a 20 °C one) since significantly more energy can be recovered in the heat exchange processes while maintaining process realizability.

A cost analysis of the designed plant (to deliver 100 million kmol/y of hydrogen) indicates a total major equipment cost of about $35 million. This translates (using a rule of thumb factor of 8) to a turnkey plant price of approximately $280 million. Based on a selling price for hydrogen of $5 per kg, the yearly plant profit could be on the order of $1 billion. Using a more conservative estimate based on the current price of natural gas, yearly plant profitability would be about $200 million. In either case, the plant has a very short pay back period.