SEPARATION OF GASEOUS MIXTURES USING A SEMI-PERMEABLE MEMBRANE

Abstract: A process for the separation of a gaseous mixture containing at least one of carbon dioxide, oxygen and hydrogen sulfide into a fraction being enriched in at least one of these and a fraction being correspondingly depleted in at least one of these, comprises contacting the gaseous mixture with a semi-permeable membrane which is constructed from at least one ring-substituted poly(arylene oxide) polymer. For example, carbon dioxide is efficiently separated from a gaseous mixture of carbon dioxide and methane, such as natural gas, by contacting it with a membrane constructed from a ring-brominated poly(2,6-dimethyl-p-phenylene ox­ ide) polymer.

Description: SEPARATION OF GASEOUS .¯MIXTURES USING A SEMI PERMEABLE 11E#IBRANE This invention relates to gas separation utilizing a semi-permeable membrane. The use of a semi-permeable membrane to effect the separation of a gaseous mixture into its various component parts (or at least enriched fractions of same) has long been known. The development of semipermeable membranes for this purpose is well summarized at columns 4-6 of US-A- 4,230,463. In addition to those membranes there described, others are known for this purpose including those described in US-A- 3,350,844 and US-A- 3,899,309. While most, if not all, of these membranes show effectiveness in their intended use, none are completely satisfactory. Generally, the membranes used for these separations lack either one or more of desirable flux, selectivity and life and thus industry continues to search for alternatives. Of particular interest to the chemical processing industry are membranes showing good characteristics for separating carbon dioxide and/or hydrogen sulfide from hydrocarbon streams (typically natural gas) and oxygen enrichment from air. Commercially, these processes do not employ membranes but rather absorption techniques in the former and cryogenic techniques in the latter. Both of these techniques are energy intensive and consequently expensive to operate. Accordingly, there exists the need for identifying and implementing alternative processes and those employing membrane technology appear at this time to offer significant economic advantages. However, to date no membrane has been identified as completely suitable for either one of these processes. According to this invention, a process for separating a gaseous mixture containing at least one of carbon dioxide, oxygen and hydrogen sulfide into two fractions, one fraction being enriched with at least one of the carbon dioxide, oxygen and hydrogen sulfide and the other fraction being depleted in same, is improved by using as the separation vehicle a semipermeable membrane containing at least 25 weight percent, based upon the total weight of the membrane, of at least one substituted poly(arylene oxide) polymer containing at least 50 mole percent of structural units of the formula EMI2.1 where each R is independently an aliphatic, cycloaliphatic or aromatic radical or an inertly-substituted derivative of any one of them, each X is independently a radical at least the size of a chloride radical, and a is 0 or 1. This process operates in substantially the same manner as known processes, i.e. the gaseous mixture is contacted with the semi-permeable membrane in such a manner that a portion of the gaseous mixture selectively passes through the membrane resulting in the enriched fraction being on one side of the membrane while the depleted fraction remains on the other side of the membrane. This process is particularly useful for separating carbon dioxide and hydrogen sulfide from natural gas streams, as tolley are constituted at the wellhead, and for producing an enriched oxygen stream from air. The process of this invention is characterized by excellent flux and selectivity, particularly in relation to separating carbon dioxide from methane and oxygen from nitrogen. The invention will be further described under appropriate headings: Gaseous Mixtures: The process of this invention is suitable for separating any one of a number of different gases from mixtures containing same. This invention is particularly useful for separating carbon dioxide and/or hydrogen sulfide from natural gas and oxygen from air. The relative amount of the gases comprising the gaseous mixture can vary widely and thus this invention finds applicability in a wide variety of separation activities. Semi-permeable Membrane: The membrane here used according to the invention contains at least 25 weight percent of at least one substituted poly(arylene oxide) polymer containing structural units of the formula EMI3.1 where R, X and a are as previously defined. Preferably, each R is independently a C1-C6 aliphatic (more preferably alkyl) radical, C5-C7 cycloaliphatic (more preferably cycloalkyl) radical or an aryl (more preferably phenyl) radical or an inertlysubstituted derivative of any one of these. As here used, the term "inertly-substituted" means the radical can contain one or more substituents, such as halogen, alkyl, nitro, alkoxy, etc., that do not substantially reduce the selectivity characteristics (for its intended use) of the membrane into which the polymer is constructed. Typical R radicals include methyl, ethyl, n-propyl, isopropyl, butyl, t-butyl, cyclohexyl, phenyl, tolyl and bromomethyl. As here used, the term "independently" means that each substituent can be the same or different on any given structural unit, e.g. each R can be methyl or one can be methyl while the other ethyl. The X radicals of formula I serve, at least in part, as steric inhibitors and are thus bulky and/or polar in nature. By "steric inhibitors" is meant that these radicals tend to act as spacers between adjacent polymer chains. Although these radicals may also serve other functions as well, such as enhancing the overall polarity of the membrane, the total gamut of their function is not fully understood at this time. These radicals are preferably at least the size of a bromide radical. Representative of these radicals are such groups as halide (chloride, bromide or iodide); azide; hydroxyl; thiol; ether; thioether; ester; thioester; a phosphoric acid, ester or salt; nitro; nitrate; nitrite; alkyl; aryl; acyl; and thioacyl. Preferred among these radicals are the halide radicals, particularly the bromide radical. In those structural units of formula I where the subscript a is zero, the ring valence otherwise satisfied by the X radical is satisfied by a hydrogen atom. The substituted poly(arylene oxide) polymers which are used can be either home- or copolymers and in the latter case will contain at least 50 mole percent, preferably at least 75 mole percent, structural units of formula I. If the polymer is a copolymer, then the comonomers other than units of formula I can consist of essentially any structural unit, such as bisphenol A, bisphenol sulfone, etc., that can be copolymerized with the structural units of formula I. Preferably, the substituted poly(arylene oxide) polymers used here consist essentially of structural units of formula I although not each structural unit may contain the same substituents (e.g. the same R or X radicals). Ring-brominated poly(2,6-dimethyl-pphenylene oxide), also known as ring-brominate#d poly (2, 6-dimethyl-l,4-phenylene oxide) and ring-brominated poly(xylylene oxide) or poly(2,6-xylenol) is an especially preferred substituted polyarylene polymer. Practical considerations, such as economy, ease of membrane fabrication, etc., are the only significant limitations on the molecular weight of these substituted poly(arylene oxide) polymers. The substituted poly(arylene oxide) polymers used here are generically known materials and thus the preparation of many of them is known in the art. For example, ring-brominated poly(2 , 6-dimethyl-p-phenylene oxide) can be readily prepared by first polymerizing 2,6-xylenol as taught by A. S. Hay in the Journal of Polymer Science, 58, 581 (1962) entitled "Polymerization by Oxidative Coupling" and subsequently brominating the polymer under ionic conditions as taught by White and Orlando in Polyethers, 12, 178 (1974) entitled "Brominated Poly(Phenylene Oxide)s. II Bromination of Poly(2,6 dimethyl-l,4-Phenylene Oxide)." By analogous steps, many of the other substituted polyarylene oxide polymers here described can be prepared. The semipermeable membranes used in this invention are constructed from at least one of the substituted poly(arylene oxide) polymers containing structural units of formula I. Any polymer that is compatible with the substituted poly(arylene oxide) polymer(s) can also be used in the construction of the membrane and typical polymers include poly(aryl sulfone), poly(phenylene oxide), poly(maleic anhydride) and its various copolymers. While the molecular weight of these polymers can vary widely, they are usually within a fifty percent range of the molecular weight of the substituted poly(arylene oxide) polymer. Although some blending of polymers may be desirable to impart certain physical characteristics to the membrane, such as strength, durability and flexibility, generally the incorporation of other polymers into the membrane is preferably kept to a minimum to avoid diluting the flux and selectivity characteristics imparted to it by the substituted poly(arylene oxide) polymer. Accordingly, the substituted poly(arylene oxide) polymer constitutes at least 25, preferably at least 50 and more preferably at least 75, weight percent of the membrane. These membranes are fabricated by the conventional methods used to fabricate other membranes and in one embodiment, the polymer is dissolved in a suitable solvent to form a 1-20, preferably 5-10 weight percent solution. Generally any nonpolar solvent can be employed examples of such solvents being chloroform, toluene, chlorobenzes (e.g. o-dichlorobenzene) and the chlorinated hydrocarbons (e.g. perchloroethylene). These nonpolar solvents can be used in combination with polar solvents, such as dimethylformamide, dimethyl sulfoxide, dimethyl acetamide, acetone and methylethyl ketone but in such combinations, the nonpolar solvent(s) constitutes at least 50 weight percent of the mixture. If a crosslinker and/or another polymer is used in the manufacture, then a 1-20, preferably 5-10, weight percent solution of these materials is prepared, generally in the same solvent used to prepare the substituted poly(arylene oxide) polymer solution, and then the two or more solutions are mixed at room temperature. The resulting mixture can then be poured over a clean glass plate and spread evenly to a uniform thickness with the aid of some instrument, e.g. a doctor blade. The membrane is then air dried, removed from the glass plate and further dried in air under ambient conditions for a suitable period of time, generally in excess of 24 hours. Crosslinking, if any, and solvent evaporation (removal) occur concurrently throughout the drying step. In other embodiments, these membranes can be manufactured by the various laboratory and commercial techniques known in the art. These membranes can also be manufactured into structures other than films, such as hollow fibers. Moreover, these membranes or films can be used in composite formulations, such as a coating on a substrate, laminates, etc. The membranes used in this invention can be fabricated at any desired thickness although membranes having a thickness less than about 25 mils (1 mil equals 25 micrometers), preferably less than about 10 mils, tend to be the most useful. Becuase the flux of a membrane tends to increase with a decrease in the thickness of the membrane, generally the thinner the membrane, the more desirable the membrane. Of course, the ultimate thickness of the membrane is determined by a whole host of factors of which flux is just one and thus the preferred membrane thickness will vary from application to application. Process Operation: The semipermeable membranes of this invention are used in the same manner as conventional membranes, i.e. the gaseous mixture is contacted with one side of the membrane, typically under pressure, and one or more gaseous constituents of the mixture are selectively passed through the membrane while the remaining gaseous constituents are rejected by the membrane. This results in an enriched fraction of the desired gas forming on one side of the membrane while a depleted fraction of the same gas is formed on the other side of the membrane. Generally the desired gas is passed through the membrane, i.e. in separating carbon dioxide from the other gaseous constituents of natural gas, carbon dioxide passes through the membrane while the majority of the other gaseous constituents are rejected. However, certain gases will generally not be rejected by these membranes and these are generally molecules of small size, such as hydrogen and helium. Likewise, in the separation of oxygen from the other gaseous components of air, oxygen permeates through the membrane while nitrogen and various other gaseous components are selectively rejected. Operating temperatures employed in the process of the invention can vary widely and are generally those used under similar separation processes. Any temperature at which the membrane is both physically and chemically stable can be used while the pressure will vary with, among other parameters, the physical strength of the membrane. The following examples illustrate the invention and unless indicated to the contrary, all parts and percentages are by weight. SPECIFIC E#"3ODIj'IENTS Polymer Preparation: The polymers here used to manufacture the membranes were all based upon poly(2,6-dimethyl-p-phenylene oxide) (subsequently "polymer") having a weight average molecular weight of about 40,000 (sold by Aldrich Chemical Company). A typical preparation of a ring-brominated polymer commenced with contacting the polymer (60 g) dissolved in chloroform (600 g) at room temperature with bromine (94 g) in a glass reactor. Contacting or addition time was approximately 45 minutes. The reaction mixture was then further stirred at room temperature for an additional hour. The final mixture was poured into stirred methanol to precipitate the polymer, the product polymer was further washed with methanol and dried under vacuum to give 97 g of product. Elemental analysis showed the bromide content of the polymer to be about 40 weight percent and proton NMR spectroscopy showed that the bromide radical was attached to the aromatic ring portion of the polymer. The polymer was also used to prepare a control polymer, i.e. benzylic-brominated poly(2,6-dimethyl-pphenylene oxide). This preparation commenced by contacting the polymer (70 g) dissolved in 1500 g of carbon tetrachloride with N-bromosuccimide (220 g) and benzoyl peroxide (3 g) . The reaction mixture was then heated with stirring to 76 C for approximately 6 hours under a nitrogen atmosphere. Isolation and purification of the product polymer were identical to those used for preparing the ring-brominated material and a yield of 102 g of product polymer was recovered. Elemental analysis showed that the bromide content of the product polymer to be approximately 35.5 weight percent and the proton NMR spectrum showed the bromination occurred primarily on the benzylic (methyl) position of the polymer. Membrane Preparation: The membranes tested here were prepared from the brominated poly (2, 6-dimethyl-p-phenylene oxide) polymers prepared by the above procedure although those of Examples 2 and 3 were prepared from a blend containing another polymer. The membrane was prepared by mixing a dilute 7 to 8 weight percent) solution of brominated polymer or blend in a suitable solvent, typically chloroform, poured over a clean glass plate and spread evenly to a uniform thickness with the aid of a doctor blade, air dried, removed from the glass plate, and further dried in air at ambient conditions for at least 24 hours. Apparatus and Procedure: A modified Gilbert cell was used to test the permeation of the films. The test side as exposed to a carbon dioxide/methane/nitrogen mixture in a mole ratio of 2.99:32:65. The plermeant was picked up by a carrier gas, helium, and injected intermittently through a sample valve into a GC column for analysis. The experiments were conducted at 230 C, the partial pressure of the test gas on the feed side was 29.8 psi (2.09 kg/cmê) and the partial pressure of the product gas on the permeant side was about 0 and purged with 29.8 psi (2.09 kg/cmê) helium at a flo-rate much in excess of the permeation rate. The area of the test 2 membrane was 7.1 square inches (47.3 cm thickness was between 1-2 mils. The carbon dioxide permeability and carbon dioxide/methane selectivity figures are reported in Table I. The polymer(s) from which the membrane was constructed is given in the column captioned "Polymer" and the bromide content of each of these polymers is given in the parentheses following its identification. RB-PPO means ring-brominated poly(2,6-dimethyl-p- phenylene oxide), PPO means poly(2,6-dimethyl-p- phenylene oxide), BB-PPO means benzylic-bromThated poly(2,6-dimethyl-p-phenylene oxide), and PAS means poly(aryl sulfone). The separation factor was calculated by the following formula: Separation Factor = Carbon Dioxide Permeation Factor Methane Permeation Factor The permeation factor is expressed as the quotient of the product of product gas volume (cc at STP) times membrane thickness (cm) divided by the product of 2 membrane surface area (cm ) times the pressure differential across the membrane (cm Hg) times the period of separation (sec). The factor 10-10 is used simply for convenience. TABLE I SEPARATION CHARACTERISTICS OF SEMI-PERMEABLE MEMBRANES PREPARED FROM VARIOUS SUBSTITUTED POLY(2,6-DIMETHYL-p-PHENYLENE OXIDE) POLYMERS C02 Permeation 3. Ex. Polymer Separation Factor cm cm ¯ ¯10 cm cm2 cm(Hg) sec 1 RB-PPO (40 %) 22 180 2 2:1 Blend 1 RB-PPO(40%):PAS(7%)1 31 30 3 1:1 Blend RB-PPO(40%) :PXO 20 154 20 4 RB-PPO (32%) 20 149 A PPO 18 87 B BB-PPO (36%) 15.3 22 lPoly(aryl sulfone) (weight average molecular weight 30,000, containing about 7% sulfur, sold by Aldrich Chemical Company). By weight. The data of Table I clearly demonstrate the general superiority of membranes formed from ring-brominated poly (2, 6-dimethyl-p-phenylene oxide) polymers over membranes formed from similar polymers but without an X group. The separation factor was much greater with a ring-brominated polymer than for either the benzylic brominated or unbrominated polymers as was the carbon dioxide permeation factor. The above procedure for determining carbon dioxide permeability and carbon dioxide/methane selectivity was repeated except a mixture of oxygen and nitrogen at a mole ratio of 21.3:78.7 was used. The results of this testing are reported in Table II. TABLE II SEPARATION CHARACTERISTICS OF SEMI-PE#4EABLE MEMBRANES PREPARED FROM VARIOUS SUBSTITUTED POLY(2,6- DIMETHYL-p-PHENYLENE OXIDE) POLYMERS CO2 Permeation Factor Ex. Polymer Separation cm3 lO#l0 cm cm2 cm(Hg) ¯ sec 5 RB-PPO (40g) 3.86 23 C BB-PPO (36%) 4.0 3.2 D PPO 4.3 13.1 The data of Table II again shows the superiority of a ring-brominated poly(2,6-dimethyl-p-phenyiOne oxide) polymer over its benzylic brominated and unbrominated counterparts.

Claims: CLAIMS: 1. A process for separating a gaseous mixture containing at least one of C02, 02 and H2 S into two fractions, one fraction being enriched with at least one of CO2, 92 and H2S and the other fraction being depleted in same, which process comprises contacting the gaseous mixture with a semi-permeable membrane in such a manner that a portion of the gaseous mixture selectively passes through the membrane resulting in the enriched fraction being on one side of the membrane and the depleted fraction being on the other side of the membrane, characterised in that there is used a membrane containing at least 25 weight percent, based upon the total weight of the membrane, of at least one substituted poly(arylene oxide) polymer containing at least 50 mole percent of structural units of the formula EMI13.1 where each R is independently an aliphatic, cycloaliphatic or aromatic radical or an inertly substituted derivative of any one of them, each X is independently a radical at least the size of a chloride radical, and a is O or 1. 2. A process as claimed in claim 1 characterised in that each R is independently a C1-C6 alkyl radical. 3. A process as claimed in claim 2 characterised in that each R is a methyl radical. 4. A process as claimed in any of claims 1 to 3 characterised in that each X is a halide radical. 5. A process as claimed in claim 4 characterised in that each X is a bromide radical. 6. A process as claimed in any of claims 1 to 5 characterised in that the substituted poly(arylene oxide) polymer contains at least about 75 mole percent of the structural units of formula I. 7. A process as claimed in claim 6 characterised in that the substituted poly(arylene oxide) polymer consists essentially of structural units of formula I. 8. A process as claimed in any of claims 1 to 7 characterised in that the membrane contains at least 50 weight percent, based upon the total weight of the membrane, of at least one substituted poly(arylene oxide) polymer. 9. A process as claimed in any claims 1 to 7 characterised in that the membrane contains at least 75 weight percent, based upon the total weight of the membrane, of at least one substituted poly(arylene oxide) polymer. 10. A process as claimed in claim 7 characterised in that the membrane is constructed essentially from one or more substituted poly(xylylene oxide) polymers. 11. A process as claim in claim 7 characterised in that the membrane contains essentially ring-brominated poly (2, 6-dimethyl-p-phenylene oxide). 12. A process as claimed in claim 8 characterised in that the membrane contains at least one substituted poly(xylylene oxide) polymer in combination with a compatible polymer. 13. A process as claimed in claim 12 characterised in that the compatible polymer is pclyjarl sulfone) or poly (2, 6-dimethyl-p-phenylene oxide). 14. A process as claimed in any of claims I to 13 characterised in that the ,membrane is a hollow fiber. 15. A process as claimed in claim 14 characterised in that the membrane has a thickness less than 10 mils. 16. A process as claimed in any of claims 1 to 13 characterised in that the membrane is cast as a coating on a substrate. 17. A process as claimed in claim 16 characterised in that the coated substrate is a hollow fiber. 18. A process as claimed in any of claims 1 to 17 characterised in that the gaseous mixture is natural gas. 19. A process as claimed in any of claims 1 to 17 characterised in that the gaseous mixture is air.

Inventors: SU-HSIANG LI, GEORGE
Applicants: THE STANDARD OIL COMPANY
Application year: 1983
Main classification: B01D53
IPC codes: B01D71