Patent Description:
Membranes are widely used for the separation of gases and liquids, including for example, separating acid gases, such as CO<NUM> and H<NUM>S from natural gas, and the removal of O<NUM> from air. Gas transport through such membranes is commonly modeled by the sorption-diffusion mechanism. Currently, polymeric membranes are well studied and widely available for gaseous separations due to easy process-ability and low cost. CMS membranes, however, have been shown to have attractive separation performance properties exceeding that of polymeric membranes.

CMS membranes are typically produced through thermal pyrolysis of polymer precursors. For example, it is known that defect-free hollow fiber CMS membranes can be produced by pyrolyzing cellulose hollow fibers (<NPL>)). In addition, many other polymers have been used to produce CMS membranes in fiber and dense film form, among which polyimides have been favored. Polyimides have a high glass transition temperature, are easy to process, and have one of the highest separation performance properties among other polymeric membranes, even prior to pyrolysis.

<NPL>, relates to "Aging of carbon membranes under different environments". <CIT> relates to a supported nanoporous carbon membrane (SNPCM) exhibiting improved gas separation performance and a novel method for preparation thereof. <CIT> relates to a high carbon content membrane and method for making the same. <NPL>, relates to "Effect of air oxidation on gas separation properties of adsorption-selective carbon membranes". <NPL>, relates to "Gas permeation and micropore structure of carbon molecular sieving membranes modified by oxidation",.

<CIT>, describes a method of synthesizing CMS membranes. In particular, a polyimide hollow fiber was placed in a pyrolysis furnace with an evacuated environment, with a pyrolysis pressure of between <NUM> and <NUM> Hg air. <CIT> also discloses a method of using CMS membranes to separate CO<NUM> from a methane stream containing <NUM>% CO<NUM>, at <NUM> MPa (<NUM> psia) and <NUM>. , with a selectivity of approximately <NUM>, a selectivity that is much higher than typical commercial polymeric membranes. Other patents that describe processes for producing carbon membranes (both asymmetric hollow "filamentary" and flat sheets), and applications for gas separation, include <CIT>, and <CIT>.

Prior research has shown that CMS membrane separation properties are primarily affected by the following factors: (<NUM>) pyrolysis precursor, (<NUM>) pyrolysis temperature, (<NUM>) thermal soak time, and (<NUM>) pyrolysis atmosphere. For example, Steel and Koros performed a detailed investigation of the impact of pyrolysis temperature, thermal soak time, and polymer composition on the performance of carbon membranes. ) Membranes were produced in an air atmosphere at <NUM> Hg pressure. The results showed that increases in both temperature and thermal soak time increased the selectivity but decreased permeance for CO<NUM>/CH<NUM> separation. In addition, Steel et al showed that a precursor polymer with a rigid, tightly packed structure tends to lead to a CMS membrane having higher selectivity compared with less rigid precursor polymers.

The impact of pyrolysis atmosphere has been researched only to a limited extent. Suda and Haraya disclosed the formation of CMS membranes under different environments. ) CMS dense films were prepared from polyimide KAPTON at <NUM> in either argon or in vacuum. According to their gas separation properties, the results of an O<NUM>/N<NUM> separation were almost the same between <NUM> membranes formed under the different atmospheres. Suda and Haraya did not disclose the effects of atmosphere on CO<NUM> separation from natural gas, nor disclose how separation properties vary with ability and low cost. Similarly, Geiszler and Koros disclosed the results of CMS fibers produced from pyrolysis of fluorinated polyimide in helium and argon for both O<NUM>/N<NUM> and H<NUM>/N<NUM> separations. That paper disclosed a slightly higher selectivity with vacuum pyrolysis than the purged pyrolysis processes. In addition, Geiszler and Koros showed that the flow rate of the purge gases impacted performance. Geiszler and Koros, however, did not disclose the effects of atmosphere on CO<NUM> separation from natural gas, or the effects of oxygen concentration on separation properties. None of the aforementioned describe the long term use of the CMS membranes and the stability of the membranes to maintain the permeance and selectivity for particular gas molecules of interest. The aforementioned also fail to describe methods of optimizing and improving the selectivity and permeance for a desired retentate gas molecule such as hydrogen with improved stability of the same.

More recently, CMS membranes have been discovered to undergo substantial aging that deleteriously affects the performance as described by <NPL>. For example, the permeance of a desired gas retentate molecule may be reduced by a factor of <NUM> to <NUM> within <NUM> days of cooling to room temperature with only a very small increase in selectivity (e.g., <NUM>% or so). <CIT> has described CMS membranes being treated to improve the permeance of olefins from paraffins by exposing the CMS membranes shortly after pyrolysis to a light olefin such as propylene at a temperature of <NUM>.

It would be desirable to provide a method to make a CMS membrane and CMS membrane made by the method that addresses one or more of the problems of the prior art such as one described above such as improving the selectivity for select gases such as hydrogen without substantially decreasing its permeance. It would also be desirable to have such CMS membrane maintain the same selectivity and permeance whether being stored for use or while being used (i.e., stable).

A first aspect of the invention is a method of making a carbon molecular sieve membrane comprising,.

The method of the invention may realize a CMS that has an improved combination of selectivity and permeance particularly for the separation of hydrogen such as from methane, or streams from natural gas steam methane reformers, or light hydrocarbon streams such as found in olefin cracker gas streams or propane dehydrogenation unit streams. In addition it has been discovered that the method may improve the stability of the CMS membrane (i.e., substantially retains the permeance and selectivity over time during use).

Also disclosed is a carbon molecular sieve made by the process of the first aspect.

Also disclosed is a method for separating hydrogen from a gas feed comprised of hydrogen and at least one other gas molecule comprising.

The gas separation method is particularly useful for separating hydrogen from gases such methane, or streams from natural gas steam methane reformers, or light hydrocarbon gas streams.

The precursor polymer is a polyimide. The polyimide may be a conventional or fluorinated polyimide. Desirable polyimides typically contain at least two different moieties selected from <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-phenylene diamine (DAM), oxydianaline (ODA), dimethyl-<NUM>,<NUM>-diaminodiphenyl-thiophene-<NUM>,<NUM>'-dioxide (DDBT), <NUM>,<NUM>-diaminobenzoic acid (DABA), <NUM>,<NUM>,<NUM>-tetramethyl-<NUM>,<NUM>-phenylene diamine (durene), meta-phenylenediamine (m-PDA), <NUM>,<NUM>-diaminotolune (<NUM>,<NUM>-DAT), tetramethylmethylenedianaline (TMMDA), <NUM>,<NUM>'-diamino <NUM>,<NUM>'-biphenyl disulfonic acid (BDSA); <NUM>,<NUM>'-[<NUM>,<NUM>,<NUM>-trifluoro-<NUM>-(trifluoromethyl)ethylidene]-<NUM>,<NUM>-isobenzofurandion (6FDA), <NUM>,<NUM>',<NUM>,<NUM>'-biphenyl tetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), <NUM>,<NUM>,<NUM>,<NUM>-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 <NUM> 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 <NUM>:<NUM> ratio of component X and component Y may also abbreviated as 6FDA/BPDA(<NUM>:<NUM>)-DAM.

A second particularly useful polyimide, designated as 6FDA-DAM lacks BPDA such that Y equals zero in Formula <NUM> above. Formula <NUM> below shows a representative structure for this polyimide.

A third useful polyimide is MATRIMID™ <NUM> (Huntsman Advanced Materials), a commercially available polyimide that is a copolymer of <NUM>,<NUM>',<NUM>,<NUM>'-benzophenonetetracarboxylic acid dianhydride and <NUM>(<NUM>)-amino-<NUM>-(<NUM>'-aminophenyl)-<NUM>,<NUM>,<NUM>-trimethylindane (BTDA-DAPI).

Preferred polymeric precursor hollow fiber membranes, the hollow fibers as produced but not pyrolyzed, are substantially defect-free. "Defect-free" means that selectivity of a gas pair, typically oxygen (O<NUM>) and nitrogen (N<NUM>), through a hollow fiber membrane is at least <NUM> 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(<NUM>:<NUM>)-DAM polymer has an intrinsic O<NUM>/N<NUM> selectivity (also known as "dense film selectivity") of <NUM>.

The precursor polymers are formed into hollow fibers or films. Conventional procedures to make these may be used. For example, 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.

Pyrolysis conditions influence carbon membrane physical properties and, accordingly, are chosen with care. Any suitable supporting means for holding the CMS membranes may be used during the pyrolysis including sandwiching between two metallic wire meshes or using a stainless steel mesh plate in combination with stainless steel wires and as described by <CIT> at col. <NUM> line <NUM> to col. <NUM>, line <NUM>.

Precursor polymers may be pyrolyzed to form the CMS membranes (i.e., carbonize the precursor polymer) under various inert gas purge or vacuum conditions, preferably under inert gas purge conditions, for the vacuum pyrolysis, preferably at low pressures (e.g. less than <NUM> MPa (<NUM> millibar)). <CIT> describes a heating method for pyrolysis of polymeric fibers to form CMS membranes. For either polymeric films or fibers, a pyrolysis temperature of between about <NUM> to about <NUM> is advantageously used. The pyrolysis temperature may be adjusted in combination with the pyrolysis atmosphere to tune the performance properties of the resulting CMS membrane. Optionally, the pyrolysis temperature may be between about <NUM> and about <NUM>. The pyrolysis soak time (i.e., the duration of time at the pyrolysis temperature) may vary (and may include no soak time) but advantageously is between about <NUM> hour to about <NUM> hours, alternatively from about <NUM> hours to about <NUM> hours, alternatively from about <NUM> hours to about <NUM> hours. An exemplary heating protocol may include starting at a first set point of about <NUM>, then heating to a second set point of about <NUM> at a rate of about <NUM> per minute, then heating to a third set point of about <NUM> at a rate of about <NUM> per minute, and then a fourth set point of about <NUM> to <NUM> at a rate of about <NUM> per minute. The fourth set point is then optionally maintained for the determined soak time. After the heating cycle is complete, the system is typically allowed to cool while still under vacuum or in a controlled atmosphere.

Precursor polymers may be carbonized under various inert gas purge or vacuum conditions, preferably under inert gas purge conditions, for the vacuum pyrolysis, preferably at low pressures (e.g. less than <NUM> MPa (<NUM> millibar)). In one embodiment the pyrolysis utilizes a controlled purge gas atmosphere during pyrolysis in which low levels of oxygen are present in an inert gas. By way of example, an inert gas such as argon is used as the purge gas atmosphere. Other suitable inert gases include, but are not limited to, nitrogen, helium, or any combinations thereof. By using any suitable method such as a valve, the inert gas containing a specific concentration of oxygen may be introduced into the pyrolysis atmosphere. For example, the amount of oxygen in the purge atmosphere may be less than about <NUM> ppm (parts per million) O<NUM>/Ar. Alternatively, the amount of oxygen in the purge atmosphere may be less than <NUM> ppm O<NUM>/Ar. Embodiments include pyrolysis atmospheres with about <NUM> ppm, <NUM> ppm, or <NUM> ppm O<NUM>/Ar.

After pyrolyzing, the CMS membrane that has formed is cooled to temperature around ambient, below <NUM>. The cooling may be at any useful rate, such as passively cooling (e.g., turning off the power to furnace and allowing to cool naturally). Alternatively, it may be desirable to more rapidly cool such as using known techniques to realize faster cooling such as cooling fans or employment of water cooled jackets or opening the furnace to the surrounding environment.

After cooling, the carbon molecular sieve membrane is reheated to a temperature from <NUM> to <NUM> (reheating temperature). Temperatures less than <NUM> tend to be impractically long to have any substantial change in the selectivity and permeance desired. Temperatures above <NUM> even for short times result in hydrogen permeances and selectivities from gas molecules such as ethylene or propylene that are not desirable. These higher reheating temperatures may, however, be useful for other gas separations such as olefin/paraffin separations due to higher permeances of these gas molecules. Desirably, the reheating temperature is at least <NUM>.

The reheating time is generally from <NUM> hour to <NUM> hours, with the time being dependent on the temperature, and may be any sufficient to realize the improved CMS membrane characteristics desired such as further described below and may vary depending on the particular CMS membrane (e.g., type of precursor polymer and particular pyrolysis conditions). Generally, the amount of time is from several hours to several days or even a week. Typically, the time is from about <NUM> hours to <NUM>, <NUM> or <NUM> days.

The time between the cooling until reheating may be any suitable time and may be several minutes to several days or weeks or longer. Illustratively, the reheating desirably occurs within <NUM> days of cooling to ambient temperature. Even though the exposing may occur within <NUM> days, it may be desirable to expose the CMS membrane in as short as possible a time after cooling from pyrolysis such as within <NUM> days, <NUM> days, <NUM> day,
<NUM> hours, <NUM> hours or even <NUM> hour. The CMS membranes when being reheated do not need to be fabricated into a separation module (apparatus capable of flowing gas through the CMS membrane), but may be reheated upon cooling in the same chamber of the furnace used to make the CMS membrane.

The atmosphere, during the reheating ("reheating atmosphere), may be static, flowing or combination thereof. Desirably, the atmosphere is static at least a portion of the time during the exposing and preferably is static the entire time of the exposing. Generally, the gas may be any including dry or wet air, inert gas (e.g., noble gas), nitrogen or vacuum. In an embodiment, at least a portion of the gas within the conditioning atmosphere flows through the CMS membrane walls. The atmosphere desirably is air, nitrogen or argon with air being preferred.

The pressure of the reheating atmosphere may be any useful and may range from a pressure below atmospheric pressure (vacuum) to several hundred pounds per square inch (psi). Desirably, the pressure is from atmospheric pressure to about <NUM> to <NUM> MPa (<NUM> to <NUM> psi) above atmospheric pressure. The pressure may also be varied during the exposing. When reheating the CMS membrane, where at least a portion of the gas in the atmosphere flows through the walls of the CMS 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 <NUM>, <NUM> or <NUM> to <NUM>, <NUM> or <NUM> MPa (<NUM>, <NUM> or <NUM> to <NUM>, <NUM> or <NUM> psi).

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 (<NUM> Barrer=<NUM>-<NUM> [cm<NUM> (STP) cm]/[cm<NUM> 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) (<NUM> GPU=<NUM>-<NUM> [cm<NUM> (STP)]/[cm<NUM> 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 CMS membrane produced by the method enables a CMS membrane that has a permeance of at least <NUM> and preferably at least <NUM>, <NUM> or even <NUM> GPU for hydrogen (permeate) and a selectivity of at least about <NUM> and preferably at least <NUM> or even <NUM> and a stability such that said permeance and selectivity varies less than <NUM>% after being continuously separating a feed gas comprised of hydrogen gas molecule for <NUM> days. Desirably, the permeance and selectivity varies less than <NUM>%, recovering the fibers and drying them under vacuum at a set point temperature of <NUM> for one hour or drying under vacuum at <NUM> for <NUM> hours.

Prior to pyrolyzing the fibers, a sample quantity of the above fibers (also known as "precursor fibers") were tested for skin integrity. One or more hollow precursor fibers were potted into a <NUM> (¼ inch) (outside diameter, OD) stainless steel tubing. Each tubing end was connected to a <NUM> (¼ inch) stainless steel tee; and each tee was connected to <NUM> (¼ inch) female and male NPT tube adapters, which were sealed to NPT connections with epoxy. Pure gas permeation tests were performed in a constant-pressure system maintained at <NUM>. For each permeation test, the entire system and leak rate was determined to ensure that the leakage was less than <NUM> percent of the permeation rate of the slowest gas. After evacuating, the upstream end was pressurized (end closest to feed source) of the tube with feed gas (e.g. pure oxygen or pure nitrogen) while keeping the downstream end (end furthest from feed source) under vacuum. The pressure rise was recorded in a constant, known downstream volume over time using LABVIEW software (National Instruments, Austin, TX) until reaching steady state. The permeance of each gas was determined through the membrane by the rate of pressure rise, the membrane area and the pressure difference across the membrane. The selectivity of each gas pair as a ratio of the individual gas permeance was calculated.

The hollow fibers were pyrolyzed to form the CMS membranes by placing the precursor fibers on a stainless steel wire mesh plate each of them bound separately to the plate using stainless steel wire. The combination of hollow fibers and mesh plate were placed into a quartz tube that sits in a tube furnace. The fibers were pyrolyzed under an inert gas (argon flowing at a rate of <NUM> standard cubic centimeters per minute (sccm)). Prior to pyrolyzing, oxygen was eliminated by evacuating and then purging the tube furnace for a minimum of four hours to reduce the oxygen level to less than <NUM> ppm. All of the fibers were heated at a ramp rate of <NUM>/minute up to <NUM>, then heated at <NUM>/min to <NUM> and finally heated at <NUM>/min to <NUM> and held at that temperature for <NUM> hours (soak time). After the soak time, the furnace was shut off, cooled under the flowing argon (passively cooled), which typically cooled in about <NUM> to <NUM> hours.

For reheating below <NUM>, the newly formed cooled CMS fibers were removed from the pyrolysis furnace, placed upon an aluminum foil and placed into a preheated convection oven at the desired reheating temperature, the atmosphere being atmospheric air. For reheating to above <NUM> the fibers were left in the quartz tube of the pyrolysis furnace, but upon cooling to room temperature, the tube was removed from the furnace and the furnace reheated to the desired reheating temperature. The sealed end plates were removed from the quartz tube and the tube was placed back in the pyrolysis furnace for the desired time, with the atmosphere being ambient air. After the reheating, the CMS hollow fiber membranes were removed from the furnace and potted into modules as described above. The modules were allowed to set over night (e.g., about <NUM> to <NUM> hours) before being loaded into the permeation testing system.

All permeation tests were determined using pure hydrogen and ethylene as <NUM> MPa (<NUM> psia) upstream and downstream vacuum at <NUM> using the constant volume method, similar to the precursor fiber testing. For the hydrogen tests, the system was evacuated and then hydrogen was fed on the shell side while downstream was kept under vacuum for ~ <NUM> to ensure a steady state was obtained before data recording. For ethylene tests, ethylene was fed and maintained overnight before data recording. The tests were repeated <NUM>-<NUM> times. The average rate of pressure rise was then used to calculate permeance of gas through the hollow fibers, and selectivity was calculated as the ratio of the permeances of hydrogen and ethylene. The results of the tests are shown in Table <NUM>.

From the results, it is readily apparent that the Examples employing reheating in the claimed range realize a surprising increase in selectivity that may be an order of magnitude greater than Comparative Example <NUM>, where no reheating was performed, while still maintaining high hydrogen permeance. In addition, the range of reheating temperatures must not be too high or the selectivity decreases below even those for Comparative Example <NUM> (see Comparative Examples <NUM> and <NUM>).

Claim 1:
A method of making a carbon molecular sieve membrane comprising,
(i) providing a precursor polymer, wherein the precursor polymer is a polyimide, wherein the precursor polymer is formed into hollow fiber or film form;
(ii) heating said precursor polymer in hollow fiber or film form to a pyrolysis temperature of <NUM> to <NUM>, where the precursor polymer undergoes pyrolysis to form the carbon molecular sieve membrane;
(iii) cooling the carbon molecular sieve membrane to a cooling temperature less than or equal to <NUM>; and
(iv) after the cooling, heating the carbon molecular sieve membrane to a reheating temperature of at least <NUM> to at most <NUM> for a reheating time from <NUM> minutes to <NUM> hours under a reheating atmosphere and then
(v) cooling back to below <NUM>.