CO2 aided H2 recovery from water splitting processes

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Solar energy can be utilized to produce H2 via photocatalytic water splitting. One major drawback of the one-step approach is the co-production of H2 and O2 in the same reactor environment creating a potentially hazardous scenario. This obstacle can be avoided by utilizing CO2 as a flammability suppressant which is proven to be more effective than N2. In this case study, several membrane cascade designs were implemented to recover the H2 while maintaining compositions outside the flammability range. The optimizations are based on the use of commercially available composite polymer membranes from Membrane Technology and Research, Inc. (MTR) in the spiral wound architecture. Both the H2-selective membrane (Proteus™) and CO2-selective membrane (Polaris™) were explored in three layouts. The process optimization is solved by nonlinear programming. The aim of the study was to minimize the present value of all outgoing cash flow (no income) for the separation process and achieving 99% product (H2) purity. Optimization results showed that utilizing three membrane units of Proteus™ material with one recycle stream is the optimum layout over a wide range of recovery values. Incorporating the CO2-selective membrane (Polaris™) leads to more expensive process due to higher recycle flow rates to compensate for the low selectivity of this material. Overall, the best economic results for this process were obtained at 85% recovery rate with 99% product purity at a cost of 6.40 $/kg. Comparing to our previous study using a N2 diluent, higher purity product with lower specific cost can be achieved with CO2 diluent system but with slight decrease in recovery rate. As a final element to this study, comparative simulations were executed to demonstrate the potentially added value of using hollow fiber membranes versus spiral wound for this separation process.