Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

SAPO-34 membranes on stainless steel, tubular supports separated CO 2 from CH 4 at feed pressures up to 3.1 MPa. The highest CO 2 permeance was 2.4 × 10 -7 mol/(m 2 s Pa) for a 50/50 feed mixture at a pressure drop of 0.14 MPa. For a pressure drop of 3 MPa, the CO 2 /CH 4 separation selectivities at 253 K were 140-150; at lower pressure drops, the highest selectivity was 270. The highest CO 2 flux was 21 kg/(m 2 h) at 295 K and a pressure drop of 3 MPa. Separation selectivity decreased as temperature increased because separation was partly based on competitive adsorption. As transmembrane pressure drop increased, both CO 2 flux and CO 2 permeate concentration increased for a 50/50 mixture. The flux pressure dependence was modeled by Maxwell-Stefan diffusion for mixtures. Methane decreased the CO 2 diffusion rate and, thus, decreased the CO 2 flux. The CH 4 flux was also lower in a mixture because CO 2 inhibits CH 4 adsorption.

Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.

A graphite fluid-wall aerosol flow reactor heated with concentrated sunlight has been developed over the past five years for the solar-thermal decarbonization of methane. The fluid-wall is provided by an inert or compatible gas that prevents contact of reactants and products of reaction with a graphite reaction tube. The reactor provides for a low thermal mass that is compatible with intermittent sunlight and the graphite construction allows rapid heating/cooling rates and ultrahigh temperatures. The decarbonization of methane has been demonstrated at over 90% for residence times on the order of 10 milliseconds at a reactor wall temperature near 2000 K. The carbon black resulting from the dissociation of methane is nanosized, amorphous, and ash-free and can be used for industrial rubber production. The hydrogen can be supplied to a pipeline and used for chemical processing or to supply fuel cell vehicles.

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Transient permeation of binary mixtures through zeolite membranes was modeled by Maxwell−Stefan diffusion and extended Langmuir adsorption. The permeate response of the faster-diffusing component overshoots its steady-state permeance, even when adsorption properties of the two components are identical. For zero permeate pressure and negligible support resistance, the adsorption equilibrium constants only affect the transient response by determining feed occupancies. Both higher permeate pressures and more support resistance decrease the overshoot in the permeate response of the faster-diffusing component. In some cases, molecules transport through the membranes against their concentration gradient under transient conditions because of the influence of slower-diffusing molecules. At steady-state, slower-diffusing molecules can cause faster-diffusing molecules to transport against their concentration gradient for high permeate pressures or substantial support resistance.

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