DENSE MEMBRANES FOR SEPARATION OF H_2 FROM CO_2 IN HIGH-PRESSURE WATER-GAS SHIFT REACTORS

摘要

Studies were performed to determine the feasibility of using various dense hydrogen transport membranes for economical separation of hydrogen from CO_2 in high-pressure water-gas shift reactors. As an alternative to burning fuels directly in air, all carbonaceous materials, in principle, can be steam reformed into a mixture of H_2 + CO. The CO can be further reacted with steam in water-gas shift reactors operating above 30 bar and 340-440EC to form CO_2 and additional H_2. If membranes were commercially available to separate CO_2 from H_2 in water-gas shift reactors, the hydrogen could be utilized as a clean fuel, and the CO_2, remaining at high pressure and undiluted by nitrogen, would be in a very concentrated form desirable for economic sequestration. Dense membranes have an advantage over porous membranes in that they possess essentially 100% selectivity for hydrogen. Proton conducting ceramic membranes, thin palladium membranes supported on porous ceramic substrates, cermet membranes, and dense metallic membranes were all investigated in this program. Membranes made from metals and alloys of Group ⅣB and ⅤB elements (i.e. Nb, Ta, V, Zr) exhibited the best overall performance. These materials have long been used as hydrogen separation membranes in the nuclear industry and possess hydrogen permeabilities 10 to 100 times better than palladium in the desired temperature range of 340-440℃. Preliminary investigations showed that some of these metals were susceptible to hydrogen embrittlement at water-gas shift reactor temperatures and desired partial pressures of hydrogen of up to 13.1 bar. However, select Group IVB and VB metals and their alloys, when utilized appropriately, were capable of record hydrogen flux, at essentially 100% selectivity. Free-standing, unsupported metal disks, 16 mm in diameter, were found to resist a target differential pressure of 31 bar with a partial pressure of hydrogen in the feed of 13.1 bar, while yielding a hydrogen flux of 280 mL min~(-1) cm~(-2) (STP) (2.1 mol m~(-2) s~(-1)) at 440℃. Under an ideal hydrogen/helium atmosphere at 34 bar partial pressure of hydrogen in the feed and 33 bar differential pressure, a record flux of 423 mL min~(-1) cm~(-2) (STP) (3.1 mol m~(-2) s~(-1)) at 440℃ was achieved. It was concluded that the all-metal membranes appear superior for separation of H_2 from CO_2 and steam at high pressure.