Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2002-02-05
2003-06-03
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S045000, C095S096000, C095S230000
Reexamination Certificate
active
06572680
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the separation of gases from hydrocarbon gas mixtures. In particular, the invention relates to the separation of carbon dioxide from hydrocarbons. The separation is carried out using hydrocarbon-resistant membranes, and is useful in natural gas processing 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 in which membranes have difficulty delivering and maintaining adequate performance is the removal of carbon dioxide from natural gas. 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. In particular, about 10% of gas contains excess carbon dioxide. Membrane technology is attractive for removing this carbon dioxide, because many membrane materials are very permeable to carbon dioxide, and because treatment can be accomplished using the high wellhead gas pressure as the driving force for the separation.
However, since carbon dioxide readily sorbs into and interacts strongly with many polymers, and most natural gas contains at least some C
3+
hydrocarbons, the expectation is that the gas components will have a swelling or plasticizing effect, thereby adversely changing the membrane permeation characteristics. These issues are discussed, for example, in J. M. S. Henis, “Commercial and Practical Aspects of Gas Separation Membranes,” Chapter 10 of D. R. Paul and Y. P. Yampol'skii,
Polymeric Gas Separation Membranes,
CRC Press, Boca Raton, 1994. This reference gives upper limits on various contaminants in streams to be treated by polysulfone membranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation of aromatics, 25% saturation of olefins and 11° C. above paraffin dewpoint (pages 473-474).
In the past, cellulose acetate, which can provide a carbon dioxide/methane selectivity of about 10-20 in gas mixtures at pressure under favorable conditions, has been the membrane material of choice for this application, and about 100 plants using cellulose acetate membranes are believed to have been installed. Nevertheless, cellulose acetate membranes are not without problems. Natural gas often contains substantial amounts of water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. However, contact with liquid water can cause the membrane selectivity to be lost completely, and exposure to water vapor at relative humidities greater than only about 20-30% can cause irreversible membrane compaction and loss of flux. The presence of hydrogen sulfide in conjunction with water vapor is also damaging, as are high levels of C
3+
hydrocarbons. These problems are presented in more detail in U.S. Pat. No. 5,407,466, columns 2-6, which patent is incorporated herein by reference.
Thus, the need remains for membranes that will provide and maintain adequate performance under conditions of exposure to organic vapors, particularly C
3+
hydrocarbons, in conjunction with high concentrations of acid gas and water vapor that are commonplace in natural gas treatment.
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-1,3-dioxole/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 carbon dioxide from a gaseous hydrocarbon in a gas mixture. Such a mixture might typically, but not necessarily, be encountered during the processing of natural
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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