GAS-PHASE FORMATION OF HYDROGEN-CHLORIDE BY A SOLAR-DRIVEN CHLORINE STEAM REACTION

Net H-2 can be obtained from HCI electrolysis if the product Cl2 can be reconverted to HCI other than by reaction with H-2. This can be achieved by reacting the Cl2 with steam, producing HCl PIUS O2 (The Reverse Deacon Process). We are investigating a method of carrying out the reaction using sunlight; the process can be described as ''photothermal'', since both high temperature and a high photon flux appear relevant. Experiments have been carried out over a wide range of temperatures and at different flow rates Of Cl2 ranging from 4 cm3 min-1 to 25 cm3 min-1; the flow rate of steam was 25 cm 3 min-1. Using a 6:1 mixture of H2O:Cl2, the percentage conversion ranged from 40% at 342-degrees-C to 90% at 704-degrees-C. The conversion efficiency, as compared with calculations based on DELTAG and K(eq), is ca 96%. Purely thermal reaction over the same temperature range averages ca 80% of the equilibrium expectation. The experimentally determined DELTAH value from the van't Hoff plot of log(K(p)) vs 1/T is somewhat lower than the value determined from standard thermochemical tables. Improvement under solar conditions is probably due to the photodissociation of the Cl2 molecule to Cl atoms which react with water vapor initially yielding HCI and OH radicals; a complex reaction sequence follows forming HCl and O2 as its major products.