Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-04-12
2003-07-01
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S054000, C096S004000, C096S008000, C518S722000
Reexamination Certificate
active
06585802
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mixed matrix membranes, including sieve particles, with the ability to separate a desired gaseous component from gaseous mixtures.
BACKGROUND OF THE INVENTION
Fischer-Tropsch synthesis is used to convert methane to higher molecular weight hydrocarbons. The first step involves converting the methane to synthesis gas, which is a mixture of carbon monoxide and hydrogen. The synthesis gas is contacted with a Fischer-Tropsch catalyst under conditions of increased temperature and pressure. In addition to desired products, the reaction can also produce methane and carbon dioxide. The methane can be recycled through the synthesis gas generator, but over time, each recycle can lead to increased levels of carbon dioxide. The increased levels of carbon dioxide can adversely affect the Fischer-Tropsch synthesis. It would be advantageous to separate and remove the carbon dioxide from the methane.
Polymeric membrane materials have been proposed for use in gas separation. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen
itrogen separation (See, for example, Koros et al.,
J. Membrane Sci.,
83, 1-80 (1993), the contents of which are hereby incorporated by reference, for background and review).
The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater concentration of one of the components in the feed mixture than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained at a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., P
A
/P
B
, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Various materials (fibers, porous supports, etc.) have been incorporated into polymeric membranes to provide mechanical strength to the membranes. Such materials and the resulting composites are described, for example, in U.S. Pat. Nos. 3,457,170; 3,878,104; 3,993,566; 4,032,454; and 4,341,605. Some composite materials show improved liquid separation properties over the polymer material themselves. For example, U.S. Pat. No. 3,862,030 discloses a composite material that includes inorganic fillers within a polymeric matrix that shows enhanced filtering capabilities for microscopic, submicron particles.
Membrane materials with incorporated materials within a polymeric matrix have also been used for gas separations (see, for example, Zimmerman et al.,
J. Membrane Sci.,
137, 145-154 (1997)). Paul and Kemp (
J. Polymer Sci.,
Symposium No. 41, 79-93, (1973)) disclose a composite material including Type 5A zeolites in silicone rubber. This material purportedly caused a time lag in achieving steady-state permeation by “immobilizing” various gases due to adsorption to the zeolites. However, after saturation of adsorption sites, permeation of the gases reached steady state. Later, Jia et al. (
J. Membrane Sci.,
57, 289 (1991)) showed that incorporating silicalite (a hydrophobic crystalline silica-based zeolite, described in U.S. Pat. No. 4,061,724) into silicone rubber provided improved selectivities for oxygen
itrogen separation. The oxygen
itrogen selectivity (P
O2
/P
N2
) increased from the original 2.1 for the silicone rubber to 2.7. Later, Kulprathipanja et al. (U.S. Pat. Nos. 4,740,219 and 5,127,925) introduced silicalite into cellulose acetate and showed slightly enhanced oxygen
itrogen selectivity (from 2.99 to 3.63). Süer et al. (
J. Membrane Sci.,
91, 77 (1994)) and Third International Symposium on Separation Technology, Antwerp, Belgium, Elsevier Science B. V., (1994) incorporated Type 4A zeolite into a glassy polymer, polyethersulfone, and also showed enhanced oxygen
itrogen selectivity (from 3.7 to 4.4). Additional studies have examined zeolites in other polymer matrices and/or have explored surface modification techniques (silane coupling, etc.) to improve adhesion between the dispersed zeolite and polymer matrix: Gür (
J. Membrane Sci.,
93, 283 (1994)) and Duval et al. (
J. Membrane Sci.,
80, 189 (1993) and
J. Appl. Polymer Sci.,
54, 409 (1994)). Modest or no improvement in oxygen
itrogen separation was observed.
The art cited above demonstrates the incorporation of various zeolites into various rubbery and glassy polymers for oxygen
itrogen separation. Although several studies show some enhanced oxygen
itrogen selectivity, these mixed matrix membranes still do not exhibit the anticipated improvement necessary for commercial application. The most significant work to date was performed by the French Petroleum Institute, where significant selectivity enhancement was reported for methane gas mixtures using flat-sheet membranes containing equal weights of the polyetherimide Ultem® and zeolite 4A (U.S. Pat. No. 4,925,459).
It would be desirable to provide additional devices, materials and processes for separating gaseous components. The present invention provides such devices, materials and processes.
SUMMARY OF THE INVENTION
The present invention is directed to carbon molecular sieve particles and methods of making the same, and to mixed matrix membranes (hereinafter “mixed matrix membrane” or “mixed matrix film”) and methods of making the same. The membrane comprises carbon molecular sieving entities or particles incorporated into a polymeric membrane. Methods of separating gaseous mixtures using the mixed matrix membrane are also within the scope of this invention. The molecular sieving entities increase the effective permeability of a desirable gas component through the polymeric membrane (and/or decrease the effective permeability of the other gas components), and thereby enhance the gas separation (selectivity) of the polymeric membrane material. Hereinafter, “enhanced” permeation properties or “enhanced” selectivity refers to this phenomenon. In a preferred embodiment, the membranes are useful for separating carbon dioxide from a gaseous mixture that includes carbon dioxide and methane.
The mixed matrix membrane includes a polymer and small, discrete molecular sieving particles encapsulated in the polymer. The mixed matrix membrane is preferably in the form of a dense film, tube or hollow fiber.
The polymer forming the membrane permits passage of desired gaseous components, in one embodiment carbon dioxide and methane, but at different diffusion rates, such that one of the components, for example either carbon dioxide or methane, diffuses at a faster rate through the polymer. The polymer is generally a rigid, glassy polymer (having high glass transition temperatures, i.e., temperatures a
Koros William J.
Mahajan Rajiv
Miller Stephen J.
Vu De Q.
Schulte Richard J.
Spitzer Robert H.
The University of Texas System
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