Evaluation of the IS process featuring membrane techniques by total thermal efficiency

The IS process to produce hydrogen from water requires efficient separation procedures. Effects of three typical membrane techniques (an electro-electrodialysis (EED), an electrochemical cell (EC), a hydrogen permselective membrane reactor (HPMR)) on total thermal efficiency were evaluated by heat/mass balance calculations based on the experimental data. The EED to concentrate HI solution is the most important membrane technique to obtain high thermal efficiency among the three techniques. The maximum thermal efficiency was 40.8% at 12.5molkg-H2O-1 of HI molality after the EED. The second important technique is the EC at the reaction of H₂O, SO₂ and I₂. The maximum thermal efficiency was 38.9% at 15.3molkg-H2O-1 of H₂SO₄ molality after the EC. The HPMR at the decomposition reaction of HI was effective to improve one pass conversion of HI to 76.4%, and the amounts of recycled HI was reduced by 91.5% using this membrane technique. The required heat at the reactor was small compared with that at the EED or at the EC. Total thermal efficiency was improved only 0.7% by the application of the HPMR.