Gas separation: processes – Compressing and indirect cooling of gaseous fluid mixture to...
Reexamination Certificate
2002-03-18
2003-06-17
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Compressing and indirect cooling of gaseous fluid mixture to...
C095S047000, C095S049000, C095S052000, C095S054000
Reexamination Certificate
active
06579341
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the separation of nitrogen gas from hydrocarbon gas mixtures. The separation is carried out using hydrocarbon-resistant membranes, and is useful in natural gas fields, refineries, petrochemical plants, and the like.
BACKGROUND OF THE INVENTION
Polymeric gas-separation membranes are well known and are in use in such areas as production of oxygen-enriched air, production of nitrogen from air, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from air or nitrogen.
The preferred membrane for use in any gas-separation application combines high selectivity with high flux. Thus, the membrane-making industry has engaged in an ongoing quest for polymers and membranes with improved selectivity/flux performance. Many polymeric materials are known that offer intrinsically attractive properties. That is, when the permeation performance of a small film of the material is measured under laboratory conditions, using pure gas samples and operating at modest temperature and pressure conditions, the film exhibits high permeability for some pure gases and low permeability for others, suggesting useful separation capability.
Unfortunately, gas separation in an industrial plant is seldom so simple. The gas mixtures to which the separation membranes are exposed may be hot, contaminated with solid or liquid particles, or at high pressure, may fluctuate in composition or flow rate or, more likely, may exhibit several of these features. Even in the most straightforward situation possible, where the gas stream to be separated is a two-component mix, uncontaminated by other components, at ambient temperature and moderate pressure, one component may interact with the membrane in such a way as to change the permeation characteristics of the other component, so that the separation factor or selectivity suggested by the pure gas measurements cannot be achieved.
In gas mixtures that contain condensable components, for example C
3+
hydrocarbons, it is frequently, although not always, the case that the mixed gas selectivity is lower, and at times considerably lower, than the ideal selectivity. The condensable component, which is readily sorbed into the polymer matrix, swells or, in the case of a glassy polymer, plasticizes the membrane, thereby reducing its selective capabilities. Carbon dioxide is also known to swell or plasticize many membrane materials. As a result of these effects, a technique for predicting mixed gas performance under real conditions from pure gas measurements with any reliability has not yet been developed.
A good example of an application that is very difficult for membranes is the separation of nitrogen from other gases by selectively permeating nitrogen. First, unless the gas to be separated from the nitrogen is very condensable or has very large molecules (for example volatile organic compounds), essentially no membrane materials are available that will permeate the nitrogen preferentially over the second gas with anything more than the most meager selectivity. In addition many materials that are somewhat nitrogen-selective have very low gas permeability. Secondly, such materials as are known to be nitrogen-selective, such as certain polyimides, are swelled or plasticized to such an extent in the presence of hydrocarbons or carbon dioxide that their use in any real-world industrial nitrogen/methane separation is not practical.
One specific case where nitrogen-selective membranes with adequate properties would be useful is in natural gas treatment. Natural gas provides more than one-fifth of all the primary energy used in the United States, but much raw gas is “subquality”, that is, it exceeds the pipeline specifications in nitrogen, carbon dioxide and/or hydrogen sulfide content. For example, fourteen percent of known U.S. natural gas reserves contain more than 4% nitrogen. Currently, the principal technology available to remove nitrogen from natural gas and other hydrocarbon gas streams is cryogenic separation. Cryogenic plants are expensive and complicated, however, and the feed gas must be subjected to extensive pretreatment, so their economic feasibility is limited.
Other processes that have been considered for performing this separation include pressure swing adsorption, lean oil absorption, and membrane separation using methane-selective, nitrogen-rejecting membranes, as taught in U.S. Pat. Nos. 5,669,958 and 5,647,227. One problem of using methane-selective membranes to treat natural gas, however, is that the methane-rich product is retrieved as a low-pressure permeate and must be recompressed. Also, the membranes must be operated at very low temperature.
U.S. Pat. No. 3,616,607 to Northern Natural Gas Company, discloses membrane-based separation of nitrogen from methane for natural gas treatment, using nitrogen-selective membranes. The patent reports extraordinarily high nitrogen/methane selectivities up to 15 and 16. These numbers are believed to be erroneous and have not been confirmed elsewhere in the literature. Also, the membranes with these alleged selectivities were made from polyacrylonitrile, a material with extremely low gas permeability of the order 10
−4
Barrer (ten thousandths of a Barrer) that would be impossible to use for a real process.
Thus, the need remains for membranes that will provide adequate nitrogen-selective separation performance, and that will be able to maintain such performance under the conditions of exposure to organic vapors, and particularly C
3+
hydrocarbons, that are commonplace in gas fields, chemical plants and the like.
Films or membranes made from fluorinated polymers having a ring structure in the repeat unit are known. For example:
1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclose various perfluorinated polymers having repeating units of perfluorinated cyclic ethers, and cite the gas-permeation properties of certain of these, as in column 8, lines 48-60 of U.S. Pat. No. 4,910,276.
2. A paper entitled “A study on perfluoropolymer purification and its application to membrane formation” (V. Arcella et al.,
Journal of Membrane Science,
Vol. 163, pages 203-209 (1999)) discusses the properties of membranes made from a copolymer of tetrafluoroethylene and a dioxole. Gas permeation data for various gases are cited.
3. European Patent Application 0 649 676 A1, to L'Air Liquide, discloses post-treatment of gas separation membranes by applying a layer of fluoropolymer, such as a perfluorinated dioxole, to seal holes or other defects in the membrane surface.
4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patent also discloses comparative data for membranes made from perfluoro(2-methylene-4-methyl-1,3-dioxolane) polymer (Example XI).
5. A paper entitled “Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3dioxole/tetrafluoroethylene” (I. Pinnau et al.,
Journal of Membrane Science,
Vol. 109, pages 125-133 (1996)) discusses the free volume and gas permeation properties of fluorinated dioxole/tetrafluoroethylene copolymers compared with substituted acetylene polymers. This reference also shows the susceptibility of this dioxole polymer to plasticization by organic vapors and the loss of selectivity as vapor partial pressure in a gas mixture increases (FIGS.
3
and
4
).
Most of the data reported in the prior art references listed above are for permanent gases, carbon dioxide and methane, and refer only to measurements made with pure gases. The data reported in item 5 indicate that even these fluorinated polymers, which are characterized by their chemical inertness, appear to be similar to conventional membranes in their inability to withstand exposure to propane and heavier hydrocarbons.
SUMMARY OF THE INVENTION
The invention is a process for separating nitrogen from a gaseous hydrocarbon in a gas mixture. Such a mixtu
Amo Karl D.
Baker Richard W.
Da Costa Andre R.
Daniels Ramin
He Zhenjie
Farrant J.
Membrane Technology and Research, Inc.
Spitzer Robert H.
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