JPH0364175B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to gas separation. In one aspect, the present invention relates to the separation of gases through the use of semipermeable membranes, and in another aspect, the invention relates to a method for separating gases through the use of semipermeable membranes made from at least one substituted poly(arylene oxide) polymer. It is related to. The use of semipermeable membranes to separate gas mixtures into their various component parts (or at least enriched fractions of their components) has long been known. The development of semipermeable membranes for this purpose was published in a U.S. patent.
It is well summarized in columns 4 to 6 of No. 4230463. In addition to the membranes described therein, U.S. Pat.
Other membranes are known for this type of purpose, including those described in Nos. 3,350,844 and 3,899,309. Although most, if not all, of these membranes have shown effectiveness for their intended use, none are completely satisfactory. Generally, the membranes used for this type of separation lack one or more of the desirable flux, selectivity, or lifetime, so the industry continues to look for alternatives. Of particular interest to the chemical process industry is the separation of carbon dioxide and/or hydrogen sulfide from hydrocarbon streams (typically natural gas);
The membrane also exhibits good properties for concentrating oxygen from air. Commercially, these methods do not use membranes, the former using absorption techniques and the latter using cryogenic techniques. Both techniques require energy and are therefore expensive to operate. Therefore, there is a need to identify and implement alternative methods, and methods using membrane technology currently appear to offer significant economic advantages. However, to date, no membrane has been verified to be fully suitable for either of these processes. According to the invention, a gas mixture containing at least one of carbon dioxide, oxygen, and hydrogen sulfide is divided into two fractions.
In other words, a method of separating into one fraction enriched with at least one of carbon dioxide, oxygen, and hydrogen sulfide and the other fraction depleted in the above is a separation peak method.
The following structural formula at least one substituted poly(arylene oxide) containing at least about 50 mole percent structural units of
at least about 25% by weight of the polymer based on the total weight of the membrane
improved by using a semipermeable membrane containing provided that each R is independently hydrogen, an aliphatic, alicyclic, or aromatic group, or an inert substituted derivative thereof;
Each X is independently a group at least the size of a chloro group, and a is 0 or 1. This method operates in substantially the same manner as known methods. That is, a gas mixture is passed through a semipermeable membrane in such a way that a portion of the gas mixture selectively passes through the membrane such that the enriched fraction is on one side of the membrane and the depleted fraction remains on the other side of the membrane. bring into contact. When configured in a wellhead, this method is particularly useful for the separation of carbon dioxide and hydrogen sulfide from natural gas streams and for the production of enriched oxygen streams from air. Particularly for the separation of carbon dioxide from methane and the separation of oxygen from nitrogen, this process is characterized by excellent flux and selectivity. The method of the invention is suitable for separating any one of a number of different gases from a mixture containing different gases. The present invention is particularly useful for separating carbon dioxide and/or hydrogen sulfide from natural gas and oxygen from air. The relative amounts of gases that make up the gas mixture can vary widely, so the invention is applicable to a wide variety of separation activities. The membrane used in the present invention has the following structural formula: at least about 25% by weight of at least one substituted poly(arylene oxide) polymer containing structural units of. However, R, X, and a are as defined above. Preferably, each R is independently C ₁ -C ₆
Aliphatic (more preferably alkyl) group, C ₅ -
_C7 alicyclic group (more preferably cycloalkyl)
or an aryl (more preferably phenyl) group, or an inert substituted derivative thereof.
As used herein, "inert substitution" refers to substituents such as halogen, alkyl, nitro, alkoxy, etc., in which the group does not substantially reduce the selectivity properties (for its intended use) of the membrane containing the polymer. This means that it can contain one or more. Typical R groups include methyl, ethyl, n-propyl, isopropyl, butyl, tert-butyl, cyclohexyl, phenyl, tolyl, bromomethyl, and the like. The term "independently" as used in the present invention means:
It is meant that each substituent can be the same or different in a given structural unit, e.g. each R
can be methyl, or one can be methyl and the other ethyl. The X group of structure () serves at least in part as steric inhibitors and is therefore bulky and/or polar. By "sterically inhibited" is meant that these groups tend to act as spacers between adjacent polymer chains. These groups may serve other functions as well, such as increasing the overall polarity of the membrane, but the full range of their functions is currently not fully understood. These groups are preferably at least the size of a bromo group. Representative examples of these groups are halogen (chloro, bromo, or iodo), azide, hydroxyl, thiol, ether, thioether, ester, thioester, phosphoric acid, ester or salt thereof, nitro, nitrate, nitrite, alkyl , aryl, acyl, thioacyl, etc. Preferred among these groups are halogen groups, especially bromo groups. In the structural unit of formula () where the subscript a is zero,
Ring valencies other than those filled by the X groups are filled by hydrogen atoms. The substituted poly(arylene oxide) polymers used in the present invention can be homopolymers or copolymers, in the latter case at least about 50 mol%, preferably at least about 75 mol% of the formula () Contains structural units of If the polymer is a copolymer, the comonomer other than the unit of formula () may consist essentially of any structural unit such as bisphenol A, bisphenol sulfone, etc. that can be copolymerized with the structural unit of formula (). You can also do it. Preferably, the substituted poly(arylene oxide) polymers used in the present invention consist essentially of structural units of formula (), but each structural unit does not contain the same substituents (e.g., the same R or X groups). It's okay. Cyclobrominated poly(2,6-dimethyl-1,4-phenylene oxide) and also known as cyclobrominated poly(xylylene oxide) or poly(2,6-xylenol) , 6-dimethyl-p-phenylene oxide) are particularly preferred substituted polyarylene polymers. Practical considerations such as economics, 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 in the present invention are generally known materials, and many methods of making them are known in the art. For example, 2,6-xylenol is first polymerized and then White, Orlando, Polyethers, vol. 12, p. 178 (1974), entitled ``Brominated poly(phenylene oxide), bromination of poly(2,6-dimethyl-1,4-phenylene oxide)''. By brominating the above polymer under ionic conditions as described above, ring brominated poly(2,6-dimethyl-p-phenylene oxide)
can be easily manufactured. Many other substituted polyarylene oxide polymers described herein can be made by similar processes. The semipermeable membrane used in the present invention is composed of at least one substituted poly(arylene oxide) polymer containing the structural unit of formula (). Any polymer that is compatible with the substituted poly(arylene oxide) polymer can be used in membrane construction; typical polymers include poly(aryl sulfone), poly(phenylene oxide), poly(maleic anhydride), and It includes various copolymers thereof. The molecular weight of this polymer can vary widely, but is usually within 50% of the molecular weight of the substituted poly(arylene oxide) polymer.
Some blend of polymers may be desirable to impart certain physical properties to the membrane, such as strength, durability, flexibility. However, to avoid compromising the flux and selectivity properties provided by the substituted poly(arylene oxide) polymer, it is desirable to keep the incorporation of other polymers into the membrane to a minimum. Thus, the substituted poly(arylene oxide) polymer comprises at least about 25%, preferably at least about 50%, and more preferably at least about 75% by weight of the membrane. The membranes are prepared by conventional methods used in the manufacture of other membranes, and in one embodiment, the polymer is dissolved in a suitable solvent to form a solution of about 1 to 2% by weight, preferably 5 to 10% by weight. form. Generally non-polar solvents can be used, examples being chloroform, toluene, chlorobenzenes (eg o-dichlorobenzene), chlorinated hydrocarbons (eg perchlorethylene). These non-polar solvents can be used in combination with polar solvents such as dimethylformamide, dimethyl sulfoxide, dimethylacetamide, acetone, methyl ethyl ketone, where the non-polar solvent constitutes at least about 50% by weight of the mixture. do. When crosslinking agents and/or other polymers are used in the preparation, they are generally mixed in 1 to 20 of the above materials in the same solvent used to make the substituted poly(arylene oxide) polymer solution.
% by weight, preferably 5-10% by weight solution,
Mix the two or more solutions at room temperature.
The solution obtained is then poured onto a clean glass plate and spread evenly to a uniform thickness with the aid of some kind of equipment, for example a doctor blade. The membrane is then air dried, removed from the glass plate, and further dried in air under normal conditions for a suitable period of time, generally 24 hours or more. Cross-linking and solvent evaporation (removal) occur simultaneously during the drying process. In other embodiments, these membranes can be manufactured by a variety of laboratory and commercial techniques known in the art. These membranes are
It can also be manufactured into structures other than films, such as hollow fibers. Additionally, these membranes or films can be used in composite formulations such as coatings on substrates, laminates, etc. Although the membranes used in the present invention can be manufactured to any desired thickness, membranes having a thickness of about 25 mils (1 mil equals 25 μm) or less, preferably about 10 mils or less are most useful. have a tendency. Thinner membranes are generally more desirable since membrane flux tends to increase with decreasing membrane thickness. Of course, the final thickness of the membrane is determined by a number of factors, one of which is flux, so the preferred membrane thickness will vary depending on the application. The semipermeable membrane of the present invention can be used in the same manner as a conventional membrane. That is, a gas mixture is typically brought into contact with one side of the membrane under pressure, allowing one or more gaseous components of the mixture to selectively pass through the membrane while the remaining gaseous components are rejected by the membrane. This results in an enriched fraction of the desired gas being produced on one side of the membrane, while
A depleted fraction of the same gas is produced on the other side of the membrane.
Generally, the desired gas is passed through the membrane. That is, in the separation of carbon dioxide from other gaseous components of natural gas, carbon dioxide passes through the membrane while most of the other gaseous components are rejected. However, certain gases are generally not rejected by these membranes, and these are generally small molecules such as hydrogen, helium. Similarly, in the separation of oxygen from other gaseous components of air, oxygen passes through the membrane while nitrogen and various other gaseous components are selectively rejected. The operating temperatures used during the practice of this invention can vary widely. This is generally the temperature used in similar separation methods. Temperature can also be employed, such that the membrane is physically and chemically stable, while pressure (among other parameters) varies with the membrane's physical strength. The following examples are illustrative of specific embodiments of the invention and all parts and percentages are by weight unless otherwise indicated. EXAMPLE The polymers used here for the production of the membranes were all poly(2,6-
Dimethyl-p-phenylene oxide (hereinafter referred to as "polymer") (sold by Aldrich Chemical Company)
It was based on A typical preparation of a ring brominated polymer began by contacting the polymer (60 g) dissolved in chloroform (600 g) with bromine (94 g) in a glass reactor at room temperature. The contact or addition time was approximately 45 minutes. The reaction mixture was then stirred for an additional hour at room temperature. The final mixture was poured into stirred methanol to precipitate the polymer, and the product polymer was further washed with methanol and dried under reduced pressure to yield 97 g of product. Elemental analysis shows that the bromine content of this polymer is approximately 40% by weight, with proton
NMR spectroscopy showed that the bromo group was attached to the aromatic ring moiety of the polymer. The above polymer was used to prepare a control polymer, benzyl brominated poly(2,6-dimethyl-p-phenylene oxide). This production is carbon tetrachloride
It started by contacting the polymer (70 g) dissolved in 1500 g with N-bromosuccinimide (220 g) and benzoyl peroxide (3 g). The reaction mixture was then stirred and heated to 76°C for approximately 6 hours under a nitrogen atmosphere. The isolation and purification of the product polymer was the same as that used in the preparation of the ring brominated polymer, and 102 g of product polymer was recovered. Elemental analysis showed that the bromine content of the product polymer was about 35.5% by weight, and proton NMR spectra showed that bromination occurred primarily on the benzyl (methyl) moiety of the polymer. The membranes tested here were made from the brominated poly(2,6-dimethyl-p-phenylene oxide) polymer prepared above, except that the membranes of Examples 2 and 3 contained other polymers. Made from a blend. A dilute solution of the brominated polymer or blend (approximately 7 to 8
% by weight), pour it into a clean glass plate, spread it to a uniform thickness with the help of a doctor blade, air dry it, remove it from the glass plate, and further dry it in air under normal conditions for at least 24 hours. I made a membrane. A modified Gilbert tank was used for the membrane permeation test. The test side was exposed to a carbon dioxide/methane/nitrogen mixture in a molar ratio of 2.99:32:65. The permeate was picked up by the carrier gas helium and intermittently injected through the sample valve into a gas chromatography column for analysis. The experiment was conducted at 23 °C, the partial pressure of the test gas on the feed side was 29.8 psi, the partial pressure of the product gas on the permeate side was approximately 0, and the helium was purged at 29.8 psi at a flow rate far in excess of the permeation rate. did. The figures for carbon dioxide permeability and carbon dioxide/methane selectivity are shown in Table 1. The polymers constituting the membrane are shown under the column heading "Polymer", and the bromine content of each of these polymers is shown in brackets following the abbreviation. RB-PPO means ring brominated poly(2,6-dimethyl-p-phenylene oxide); BB-
PPO means benzyl brominated poly(2,6-dimethyl-p-phenylene oxide) and PAS means poly(arylsulfone). The separation factor was calculated using the following formula. Separation coefficient = carbon dioxide permeation coefficient / methane permeation coefficient The permeability coefficient is the product of product gas volume (cc at STP) times membrane thickness (cm), membrane surface area (cm ² ) times pressure difference across the membrane (cmHg). Separation time to be multiplied (seconds)
It is expressed as the quotient divided by the product of The factor 10 ^-10 is used merely for convenience.

[Table] From the data in Table 1, ring brominated poly(2,6-
It can be clearly seen that membranes formed from dimethyl-p-phenylene oxide) polymers are generally superior to membranes formed from similar polymers without the X group. Similar to the carbon dioxide permeability coefficient, the separation factor was much greater for the cyclobrominated polymer than for the benzyl brominated or unbrominated polymer. The above operation to determine the carbon dioxide permeability and carbon dioxide/methane selectivity was repeated, but
A mixture of oxygen and nitrogen with a molar ratio of 21.3 to 78.7 was used. The test results are shown in Table 2.

[Table] From the data in Table 2, we also found that ring brominated poly(2,
It can be seen that the 6-dimethyl-p-phenylene oxide) polymer is superior to its benzylic brominated and unbrominated versions. Although the present invention has been described in considerable detail by the above examples, they are given for illustrative purposes only and are not intended to limit the invention.

Claims (1)

[Claims] 1. Passing a portion of the gas mixture selectively through a semipermeable membrane in such a way that a concentrated fraction is on one side of the membrane and a depleted fraction is on the other side of the membrane. CO ₂ , O ₂ , H ₂ S by contacting them with a semipermeable membrane.
One fraction is enriched with at least one of CO ₂ , O ₂ , H ₂ S and the other fraction is enriched with at least one of CO ₂ , O ₂ , H ₂ S. In the method of separating into two fractions in which (wherein each R is independently a hydrogen, aliphatic, or aromatic group, or an inert substituted derivative thereof, each X is independently a group at least the size of a chloro group, and a is 0 or 1) ) of at least about 25% by weight, based on the total weight of the membrane, of at least one substituted poly(arylene oxide) polymer containing at least about 50% by mole of structural units of . 2. The method of claim 1, wherein each R is independently a _C1 - _C6 alkyl group. 3. The method of claim 2, wherein each R is a methyl group. 4. The method of claim 3, wherein each X is a halogen group. 5. The method of claim 4, wherein each X is a bromo group. 6. The method of claim 5, wherein the substituted poly(arylene oxide) polymer contains at least 75 mole percent of structural units of formula (). 7. The method of claim 5, wherein a is zero. 8. The method of claim 7, wherein the substituted poly(arylene oxide) polymer consists essentially of structural units of formula (). 9. The method of claim 8, wherein the membrane includes at least about 50% by weight of at least one substituted poly(arylene oxide) polymer, based on the total weight of the membrane. 10. The method of claim 8, wherein the membrane contains at least about 75% by weight of at least one substituted poly(arylene oxide) polymer, based on the total weight of the membrane. 11. The method of claim 8, wherein the membrane consists essentially of one or more substituted poly(xylylene oxide) polymers. 12. The method of claim 8, wherein the membrane essentially comprises ring-brominated poly(2,6-dimethyl-p-phenylene oxide). 13. The method of claim 9, wherein the membrane comprises at least one substituted poly(xylylene oxide) polymer in combination with a compatible polymer. 14 Compatible polymer is poly(arylsulfone)
or poly(2,6-dimethyl-p-phenylene oxide). 15. The method of claim 1, 9, 11, 12, or 13, wherein the membrane is a hollow fiber. 16. The method of claim 15, wherein the membrane has a thickness of about 10 mils or less. 17. The method of claim 1, 9, 11, 12, or 13, wherein the membrane is cast as a coating onto a substrate. 18. The method of claim 17, wherein the coated substrate is a hollow fiber. 19. The method of claim 6, wherein the gas mixture is natural gas. 20 Claim 6 in which the gas mixture is air
the method of. 21. The method of claim 17, wherein the gas mixture is natural gas. 22 Claim 1 in which the gas mixture is air
7 ways.