Patent number 8617293 is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "This invention generally relates to membranes, and their use in separating a gas from a gas stream. In some specific embodiments, the invention relates to the preferential separation of hydrogen from synthesis gas mixtures, and related power generation systems.
"Membranes are selectively permeable barriers that can be used to separate gases. One exemplary application for membranes is to separate gases in power generation, specifically integrated gasification combined cycle (IGCC) power plants. These plants generate electricity from carbonaceous fuel such as coal, petcoke, or biomass, through a series of steps, including gasification of the solid fuel to form a mixture of hydrogen (H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), water vapor, and trace impurities. The mixture is commonly known as 'synthesis gas' or 'syngas'. Impurities are removed from the syngas mixture, through a series of clean-up operations. The cleaned gas is then combusted to produce electricity in a combined cycle.
"IGCC plants offer advantages in efficiency because the clean-up of impurities is performed on high pressure gas streams before combustion. Membranes can be used in the IGCC clean-up process to separate the syngas into a fuel-rich stream that can be used to generate electricity, and a CO.sub.2-rich retentate stream to enable 'carbon capture'. The use of a membrane for carbon capture can involve the selective permeation of CO.sub.2 through the membrane, separating it from the rest of the gas stream, or can involve the selective permeation of hydrogen, the primary fuel gas. In an ideal situation for some power generation systems, gas separation is carried out at high temperature and pressure, so as to minimize the necessity for compressing the CO.sub.2 prior to sequestration. In some cases, hydrogen-selectivity (as compared to CO.sub.2 selectivity) is a key parameter in a gas separation system. In addition, a water-gas-shift reactor is usually employed upstream of the membrane. The water-gas-shift reactor converts carbon monoxide into hydrogen and carbon dioxide, to maximize the overall thermal efficiency of the plant.
"Many types of membrane structures are available for gas separation at relatively high temperatures. Most are based on metallic or ceramic materials. While dense metallic membranes are useful for some gas separation processes, they are also deficient in some respects. For example, the metals in such membranes are often intolerant of sulfur. Therefore, in separating gas mixtures which may include compounds like hydrogen sulfide (e.g., gas streams produced from sulfur containing feedstocks such as low rank coal, petcoke, or biomass), metallic membranes can suffer irreversible degradation.
"Porous ceramic membranes can also be used for gas separation processes, provided the pore size can be sufficiently controlled to enable high selectivity.
"In the case of membranes with pores larger than about 2 nm, but smaller than the mean free path for a gas, the transport mechanism is predominantly Knudsen diffusion. Knudsen diffusion has a different temperature dependence than activated transport, with the flux decreasing with the square root of temperature, as the temperature increases. In membranes where transport is dominated by Knudsen diffusion, the ideal membrane selectivity for gases is the inverse square root of the ratio of their molecular masses. For example, Knudsen H.sub.2/CO.sub.2 selectivity is about 4.7.
"In general, the formation of microporous membranes which have fine pores and high flux characteristics (i.e., flow capacity) can be difficult. As an example, since the flux through a membrane can decrease with decreasing pore size, it is often desirable to employ membrane layers which are as thin as possible. However, it can be difficult to manufacture thin, porous layers which have uniform pores, and which are also mechanically robust.
"Silica-based membranes are well-known in the art for use in gas separation processes. The manufacture of the silica membranes is a relatively straightforward and economical process, and in some situations, the membranes are very effective for gas separation. State-of-the-art silica membranes often consist of a thin silica layer, on top of a supported, porous aluminum oxide layer, which provides mechanical strength. Unlike the metallic-based membranes, silica-based membranes are somewhat more tolerant to the presence of sulfur-based compounds.
"However, there are considerable drawbacks associated with silica membranes. For example, in some cases, there is poor reproducibility in the fabrication process, which can result in large fluctuations in performance, e.g., separation properties. Moreover, under elevated temperature conditions, silica can be very sensitive to steam, which adversely affects the microstructure and gas separation performance of the membrane structure.
"With some of these concerns in mind, new membranes and membrane structures, based in part on porous ceramic materials, would be welcome in the art. The membranes should exhibit good hydrogen selectivity. The membranes should also be relatively tolerant of harmful gases like hydrogen sulfide, and in general, should be suitable for use in corrosive atmospheres. Moreover, the membranes should be capable of economic fabrication, and should generally be compatible with a variety of power generation and gasification systems that utilize fossil fuels, or biomass."
In addition to the background information obtained for this patent, NewsRx journalists also obtained the inventors' summary information for this patent: "An embodiment of this invention is directed to a porous membrane structure, comprising:
"a) a porous substrate;
"b) a mesoporous, aluminum oxide layer disposed on the substrate, comprising a plurality of unconnected pores; and
"c) a relatively thin, continuous, microporous barrier layer disposed on the mesoporous aluminum oxide layer, comprising aluminum oxide, and capable of improving hydrogen selectivity within a gas stream contacting the membrane structure.
"Another embodiment of the invention relates to a porous, tubular membrane structure, comprising:
"(I) a tubular, porous substrate having an inner surface and an outer surface;
"(II) a tubular, mesoporous, aluminum oxide layer which comprises a plurality of unconnected pores, having an inner surface and an outer surface, wherein the outer surface of the mesoporous aluminum oxide layer is in contact with the inner surface of the porous substrate; and
"(III) a thin, continuous, microporous barrier layer which comprises aluminum oxide, having an outer surface which contacts the inner surface of the mesoporous aluminum oxide layer; and an inner surface which defines a cavity;
"wherein fluid communication between the outer surface of the substrate and the cavity is capable of occurring through layers I (substrate), II, and III. Membrane supports which include at least one of these tubular membrane structures represent another embodiment of the invention.
"A gas separation module is also an inventive embodiment. The module comprises at least one membrane support, as described herein, extending through a length of the module. The module can be positioned within an enclosure to allow the structure to be exposed to a fluid stream containing hydrogen and other components. It is capable of allowing hydrogen to be preferentially transported across the membrane, thereby separating hydrogen from the other components.
"An additional embodiment is directed to a power plant, comprising
"I) a gasification unit which converts carbonaceous fuel into synthesis gas;
"II) a water-gas-shift reactor in flow-communication with the gasification unit, and configured to receive the synthesis gas, and to produce a gaseous product mixture comprising hydrogen and carbon dioxide;
"III) a membrane unit in flow-communication with the water-gas-shift reactor; and capable of separating hydrogen from the gaseous product mixture; wherein the membrane unit includes at least one porous membrane structure, as described herein; and
"IV) a power generation unit in communication with the membrane unit, so as to accept the hydrogen separated in the membrane unit as a fuel source, wherein the power generation unit is configured to produce electricity.
"A method for separating hydrogen from a fluid stream is also described herein. The method comprises the step of contacting the fluid stream with a porous membrane structure, to preferentially transport hydrogen across the structure. The membrane structure is described below in detail."
URL and more information on this patent, see: Eadon,
Keywords for this news article include: Gases, Sulfur, Elements, Chemistry, Chalcogens, Light Metals, Aluminum Oxide, Carbon Dioxide, Carbon Monoxide, Hydrogen Sulfide, Aluminum Compounds, Inorganic Chemicals,
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