Patent Application
20190247806
The invention relates to polyimide membranes (PMs) to separate gases. In particular the invention relates to a method for producing PMs with improved selectivity.
Membranes are widely used for the separation of gases and liquids, including for example, separating acid gases, such as CO2 and H2S from natural gas, and in particular the removal of 02 from air. Gas transport through such membranes is commonly modeled by the sorption-diffusion mechanism. Polymeric membranes polyimide membranes are well known for separating gases such as those described in U.S. Patent Re. 30,351 and U.S. Pat. Nos. 4,705,540 and 4,717,394.
Polyimides as well as other polymeric membranes have incorporated solubilized small molecules to improve the selectivity of the gas separation membranes that are films or hollow fibers, but this invariably leads to a concomitant reduction in permeability or productivity (see, for example, Effect of Antiplasticization on Selectivity and Productivity of Gas Separation Membranes, Y. Maeda and D. R. Paul, J. Mem. Sci., 30 (1987) 1-9 and U.S. Pat. No. 4,983,191).
It would be desirable to provide a method to make a polyimide membrane that avoids the aforementioned problem. Likewise it would be desirable to provide a polyimide membrane that is able to viably separate other gases and in particular smaller gas molecules (e.g., hydrogen from methane, ethane, ethylene, propylene or propane).
A first aspect of the invention is a method of making a polyimide containing halogen membrane comprising,
The method of the invention may realize a polyimide gas separation membrane with an improved combination of selectivity and permeance. Illustratively, the method allows for a polyimide membrane having good selectivity for similar sized gas molecules (e.g., hydrogen/ethylene) while still having high permeance of the target permeate gas molecule (e.g., hydrogen). That is, the selectivity is substantially improved compared to a polyimide membrane that has not been exposed to the treating atmosphere with hardly any loss in permeance of the hydrogen.
A second aspect is a process for separating a gas molecule from a gas feed comprised of the gas molecule and at least one other gas molecule comprising
A third aspect is a gas separating module comprising a sealable enclosure comprised of: a plurality of polyimide membranes, comprising at least one polyimide containing halogen membrane of the first aspect, contained within the sealable enclosure; an inlet for introducing a gas feed comprised of at least two differing gas molecules; a first outlet for permitting egress of a permeate gas stream; and a second outlet for egress of a retentate gas stream.
A fourth aspect is a polyimide containing halogen membrane comprised of a polyimide, a halogen compound that is solubilized in the polyimide and a hydrocarbon having 2 to 5 carbons in the polyimide halogen membrane.
The gas separation method is particularly useful for separating gas molecules in gas feeds that have very similar molecular sizes such as hydrogen/ethylene, ethane/ethylene and propane/propylene. It may also be used to separate gases from atmospheric air such as oxygen or separating gases (e.g., methane) in natural gas feeds.
The polyimide containing halogen membrane for separating gases may be in any useful form such as a thin film or asymmetric membrane and in particular a hollow fiber having a thin dense layer on the outer surface of the fiber and a wider larger microporous/mesoporous/macroporous layer on the inner surface of the fiber. Desirably, the hollow fibers are substantially defect-free. “Defect-free” means that selectivity of a gas pair, typically oxygen (O2) and nitrogen (N2), through a hollow fiber membrane is at least 90 percent of the selectivity for the same gas pair through a dense film prepared from the same composition as that used to make the polymeric precursor hollow fiber membrane. By way of illustration, a 6FDA/BPDA(1:1)-DAM polymer has an intrinsic O2/N2 selectivity (also known as “dense film selectivity”) of 4.1.
When making the membrane, conventional procedures known in the art may be used (see, for example U.S. Pat. Nos. 5,820,659; 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; 4,512,893 and 4,717,394). Exemplary methods include coextrusion procedures including such as a dry-jet wet spinning process (in which an air gap exists between the tip of the spinneret and the coagulation or quench bath) or a wet spinning process (with zero air-gap distance) may be used to make the hollow fibers.
To make the polyimide membrane a dope solution is used comprised of a polyimide, halogen compound and solvents. Typically, when making a thin film membrane a dope solution comprised of a solvent that dissolves the polyimide is used, for example, when casting onto a flat plate and the solvent removed. When making a hollow fiber, typically a dope solution that is a mixture of a solvent that solubilizes the polyimide and a second solvent that does not solubilize (or to a limited extent solubilizes) the polyimide, but is soluble with the solvent that solubilizes the polyimide are used. Exemplary solvents that are useful to solubilize the polyimide include N-Methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethylacetamide (DMAc) and dimethylformamide (DMF). Exemplary solvents that do not solubilize the polyimide, but are soluble with the solvents that do solubilize the polyimide include methanol, ethanol, water, and 1-propanol.
The polyimide may be any polyimide such as the aromatic polyimides described by U.S. Pat. No. 4,983,191 from col. 2, line 65 to col. 5, line 28. Other aromatic polyimides that may be used are described by U.S. Pat. Nos. 4,717,394; 4,705,540; and re30351. Desirable polyimides typically contain at least two different moieties selected from 2,4,6-trimethyl-1,3-phenylene diamine (DAM), oxydianaline (ODA), dimethyl-3,7-diaminodiphenyl-thiophene-5,5′-dioxide (DDBT), 3,5-diaminobenzoic acide (DABA), 2,3,5,6-tetramethyl-1,4-phenylene diamine (durene), meta-phenylenediamine (m-PDA), 2,4-diaminotolune (2,4-DAT), tetramethylmethylenedianaline (TMMDA), 4,4′-diamino 2,2′-biphenyl disulfonic acid (BDSA); 5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]-1,3-isobenzofurandion (6FDA), 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), and benzophenone tetracarboxylic dianhydride (BTDA), with two or more of 6FDA, BPDA and DAM being preferred.
A particularly useful polyimide, designated as 6FDA/BPDA-DAM, may be synthesized via thermal or chemical processes from a combination of three monomers: DAM; 6FDA, and BPDA, each commercially available for example from Sigma-Aldrich Corporation. Formula 1 below shows a representative structure for 6FDA/BPDA-DAM, with a potential for adjusting the ratio between X and Y to tune polymer properties. As used in examples below, a 1:1 ratio of component X and component Y may also abbreviated as 6FDA/BPDA(1:1)-DAM.
A second particularly useful polyimide, designated as 6FDA-DAM, lacks BPDA such that Y equals zero in Formula 1 above. Formula 2 below shows a representative structure for this polyimide.
A third useful polyimide is MATRIMID™ 5218 (Huntsman Advanced Materials), a commercially available polyimide that is a copolymer of 3,3′,4,4′-benzo-phenonetetracarboxylic acid dianhydride and 5(6)-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane (BTDA-DAPI).
It should be noted, that the polyimide may be provided as the precursor monomers in the dope solution and polymerized after shaping by application of heat if desired, which is also described in the prior art cited above, but this is not preferred.
The halogen compound may be any halogen compound that contains a halogen and is soluble in the polyimide used. Generally, this means that at least about 0.5% of the halogen compound is soluble in the polyimide. Likewise, it is understood that soluble means that the membrane that is formed has the halogen homogeneously within the formed polyimide containing halogen membrane. Desirably, the halogen compound is an aromatic epoxide. Preferably, the halogen compound has at least one bromine and even more preferably, all of the halogens in the halogen compound are bromines. Generally, the aromatic epoxide has a molecular weight from 50 to 50,000, but desirably the molecular weight is from 500 to 5000.
In a particular embodiment, the aromatic epoxide is an oligomeric or polymeric residue having at least one halogen substituent represented by:
where Ar represents a divalent aromatic group of the form:
where R1 is a direct bond or anyone of the following divalent radicals:
Desirably Ar is substituted with at least one halogen and preferably more than one halogen with it being most desirable for the halogen to be bromine. In a particular embodiment, each aromatic ring of the aromatic epoxide is substituted with a halogen ortho to the glycidyl ether end groups of the aforementioned. A particular aromatic epoxide is an oligomer or polymer having repeating units represented by:
The value of n may be any value, but generally is a value that realizes the aforementioned molecular weight for the aromatic epoxide described above.
After the dope solution is formed, the solution is shaped into a membrane as described above. After shaping, the solvents are removed by any convenient method such as application of heat, vacuum, flowing gases or combination thereof and include those known in the art.
After removing the solvent, the formed or untreated membrane is exposed to a treating atmosphere to a treating atmosphere comprising a hydrocarbon having 2 to 5 carbons for a time sufficient to make the polyimide containing halogen membrane. The time may vary depending on the particular hydrocarbon, polyimide or halogen compound used and the amount of halogen compound used. The polyimide containing halogen membranes when being exposed do not need to be fabricated into a separation module (apparatus capable of flowing gas through the polyimide membrane), but may, for example, merely be exposed to the treating atmosphere in a vessel.
The treating atmosphere, during the exposing, may be static, flowing or combination thereof during the exposing. Desirably, the treating atmosphere is flowing at least a portion of the time during the exposing and preferably is flowing the entire time of the exposing. Even though the polyimide membrane may be intermittently exposed to the treating atmosphere (e.g., the treating atmosphere is intermittently substituted with another gas or vacuum), it is desirable that polyimide containing halogen membrane is continuously exposed to the treating atmosphere. In an embodiment, at least a portion of the gas within the conditioning atmosphere flows through the polyimide membrane walls.
The pressure of the conditioning atmosphere may be any useful and may range from a pressure below atmospheric pressure to several hundred pounds per square inch (psi) or more. Desirably, the pressure is from about 10 to 300 psi. The pressure may also be varied during the exposing. When exposing the membrane, where at least a portion of the gas in the conditioning atmosphere flows through the walls of the hollow fiber membrane, the pressure differential across the wall may be any useful such as several psi to several hundred psi. Desirably, the pressure differential is from about 1, 5 or 10 to 25, 50 or 100 psi.
The time of exposing, may be any sufficient to realize the improved polyimide membrane characteristics desired such as further described below and may vary depending on the particular membrane (e.g., type of polyimide and halogen compound). Generally, the amount of time is from several hours to several days or even a week or 10 days. Typically, the time is from about 4 hours to 4, 3 or 2 days.
The treating atmosphere is comprised of a hydrocarbon having 2 to 5 carbons. Typically, the hydrocarbon is an alkane or alkene, which generally is linear. Preferably, the hydrocarbon is an alkene. Exemplary hydrocarbons include ethane, ethylene, propane, propylene, butane, butylene or mixture thereof. Illustratively, the conditioning atmosphere desirably is comprised of at least a majority of the hydrocarbon. Preferably, the conditioning atmosphere is comprised of at least 75%, 90%, 99% or even essentially 100% of the hydrocarbon. When using a conditioning atmosphere having less than 99% of the permeate molecule, it is desirable that the other gas molecules in the conditioning atmosphere are smaller than the hydrocarbon such as hydrogen.
The gas permeation properties of a membrane can be determined by gas permeation experiments. Two intrinsic properties have utility in evaluating separation performance of a membrane material: its “permeability,” a measure of the membrane's intrinsic productivity; and its “selectivity,” a measure of the membrane's separation efficiency. One typically determines “permeability” in Barrer (1 Barrer=10−10 [cm3 (STP) cm]/[cm2 s cmHg], calculated as the flux (ni) divided by the partial pressure difference between the membrane upstream and downstream (Δpi), and multiplied by the thickness of the membrane (l).
Another term, “permeance,” is defined herein as productivity of asymmetric hollow fiber membranes and is typically measured in Gas Permeation Units (GPU) (1 GPU=10−6 [cm3 (STP)]/[cm2 s cmHg]), determined by dividing permeability by effective membrane separation layer thickness.
Finally, “selectivity” is defined herein as the ability of one gas's permeability through the membrane or permeance relative to the same property of another gas. It is measured as a unitless ratio.
In a particular embodiment the method creates a polyimide containing halogen membrane comprised of a polyimide, a halogen compound that is soluble in the polyimide and a hydrocarbon having 2 to 5 carbons. Generally, the halogen compound is homogeneously distributed within the polyimide throughout the membrane. The hydrocarbon may be adsorbed, or solubilized into the polyimide or combination thereof. Surprisingly, the polyimide containing halogen membrane may substantially improve, for example, the selectivity of hydrogen in a hydrogen/ethylene gas mixture without any substantial reduction in the hydrogen permeance, whereas the same polyimide in the absence of the halogen compound does not. In a particular embodiment, the polyimide containing halogen membrane has a selectivity of hydrogen of at least 40 from a hydrogen/ethylene gas mixture and a hydrogen permeance of at least 250 GPU at 35° C. Preferably, the polyimide containing halogen membrane has a selectivity of hydrogen of at least 50 from a hydrogen/ethylene gas mixture and a hydrogen permeance of at least 300 GPU at 35° C.
The polyimide containing halogen membranes are particularly suitable for separating gases that are similar in sizes such as described above and involve flowing a gas feed containing a desired gas molecule and at least one other gas molecule through the membrane. The flowing results in a first stream that has an increased concentration of the desired gas molecule and second stream that has an increased concentration of the other gas molecule. The process may be utilized to separate any number of gas pairs and in particular is suitable for separating hydrogen from ethylene, ethane, propylene, propylene or mixture thereof or hydrogen from any low molecular weight hydrocarbon, nitrogen, oxygen, CO2 or air. When practicing the process, the membrane is desirably fabricated into a module comprising a sealable enclosure comprised of a plurality of polyimide membranes that is comprised of at least one polyimide membrane produced by the method of the invention that are contained within the sealable enclosure. The sealable enclosure having an inlet for introducing a gas feed comprised of at least two differing gas molecules; a first outlet for permitting egress of a permeate gas stream; and a second outlet for egress of a retentate gas stream.
Polyimide Membrane Preparation without Halogen (PM):
The membranes were made using 6FDA:BPDA-DAM polymer. The 6FDA:BPDA-DAM was acquired from Akron Polymer Systems, Akron, Ohio The polymer was dried under vacuum at 110° C. for 24 hours and then a dope was formed. The dope was made by mixing the 6FDA:BPDA-DAM polymer with solvents and compounds in Table 1 and roll mixed in a Qorpak™ glass bottle sealed with a polytetrafluoroethylene (TEFLON™) cap and a rolling speed of 5 revolutions per minute (rpm) for a period of about 3 weeks to form a homogeneous dope.
The homogeneous dope was loaded into a 500 milliliter (mL) syringe pump and the dope was allowed to degas overnight by heating the pump to a set point temperature of 50° C. using a heating tape.
Bore fluid (80 wt % NMP and 20 wt % water, based on total bore fluid weight) was loaded into a separate 100 mL syringe pump and then the dope and bore fluid were co-extruded through a spinneret operating at a flow rate of 100 milliliters per hour (mL/hr) for the dope; 100 mL/hr bore fluid, filtering both the bore fluid and the dope in line between delivery pumps and the spinneret using 40 am and 2 am metal filters. The temperature was controlled using thermocouples and heating tape placed on the spinneret, dope filters and dope pump at a set point temperature of 70° C.
After passing through a two centimeter (cm) air gap, the nascent fibers that were formed by the spinneret were quenched in a water bath (50° C.) and the fibers were allowed to phase separate. The fibers were collected using a 0.32 meter (M) diameter polyethylene drum passing over TEFLON guides and operating at a take-up rate of 5 meters per minute (M/min).
The fibers were cut from the drum and rinsed at least four times in separate water baths over a span of 48 hours. The rinsed fibers were placed in containers and solvent exchanged three times with methanol for 20 minutes and then hexane for 20 minutes before recovering the fibers and drying them under UHP argon purge at a set point temperature of 100° C. for two hour to form the polyimide membranes.
The same above procedure was followed except that the dope solution composition was as shown in Table 2 and the spinning conditions were as listed below. F-2016 (catalog number) is a brominated epoxy oligomer having a molecular weight of 1600 available from ICL Industrial Products (Beer Sheva, Israel). The structure of F-2016 is shown below where n is approximately 2.7.
The spinning temperature, quench bath temperature and the air gap were set at 50° C., 35° C., and 15 centimeters, respectively.
One or more hollow fibers were potted into a ¼ inch (0.64 cm) (outside diameter, OD) stainless steel tubing. Each tubing end was connected to a ¼ inch (0.64 cm) stainless steel tee; and each tee was connected to ¼ inch (0.64 cm) female and male NPT tube adapters, which were sealed to NPT connections with epoxy. An Argon sweep gas was used as sweep gas in the permeate side. The flow rate of the combined sweep gas and permeate gas was measured by a Bios Drycal flowmeter, while the composition was measured by gas chromatography. The flow rate and composition were then used for calculating gas permeance. The selectivity of each gas pair as a ratio of the individual gas permeance was calculated. The gas permeation was tested in a constant pressure permeation system maintained at 35° C., and the feed and permeate/sweep pressures were kept at 52 and 2 psig, respectively, if not noted specifically. The CO2/N2 (10 mol %/90 mol %) feed gas was pre-mixed and supplied by Airgas. The H2/C2H4 mixture feed gas was mixed using mass flow controllers. Retentate flow was set to keep the stage cut (ratio of permeate to feed flow rate) below 1%.
The PMCH fiber was exposed to a gas containing 50% ethylene and 50% hydrogen as described above with the hydrogen permeance shown in
Example 1 was repeated except that the PM fiber was used.
From the graphs (
In this Example, prior to exposing the PMCH fiber a baseline hydrogen permeance and selectivity of hydrogen/nitrogen gas mixture was first performed. The gas mixtures were all 50%/50% by mole mixtures with same exposure testing criteria described above. After the baseline was established ˜2.2 hours of exposure/testing, the fiber was then exposed to a hydrogen/ethane gas mixture for 2 hours followed by exposing the fiber to a hydrogen/ethylene mixture for 66.2 hours. Afterwards, the hydrogen permeance in nitrogen and ethane were again determined.
From the results shown in Table 3, it is readily apparent that the PMCH after being exposed to the ethane and ethylene has not decreased the hydrogen permeance and the selectivity is improved in both the hydrogen/nitrogen and hydrogen/ethane gas mixtures.
In this Example, prior to exposing the PMCH fiber to a hydrocarbon having 2 to 5 carbons a baseline carbon dioxide permeance and selectivity of carbon dioxide/nitrogen gas mixture was first performed followed by a baseline for a carbon dioxide/methane gas mixture. The gas mixtures were all 50%/50% by mole mixtures with same exposure testing criteria described above. After the baselines were established, the fiber was then exposed to a hydrogen/ethylene gas mixture for 68.4 hours. Afterwards, the carbon dioxide permeance in methane and nitrogen was again determined.
From the results shown in Table 4, it is readily apparent that the PMCH after being exposed to the ethylene has decreased the carbon dioxide permeance somewhat yet improved its selectivity in carbon dioxide/nitrogen and carbon dioxide/methane gas mixtures.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/053151 | 9/25/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62417557 | Nov 2016 | US |
