Blend Polymer Gas Separation Membrane

PATENT
Blend Polymer Gas Separation Membrane
United States Patent Application 20120322646
Inventors:
Liu, Chunqing (Schaumburg, IL, US)
Bowen, Travis C. (Crystal Lake, IL, US)
Harbert, Emily G. (Chicago, IL, US)
Minkov, Raisa (Skokie, IL, US)
Faheem, Syed A. (Huntley, IL, US)
Osman, Zara (Niles, IL, US)
Application Number:
13/278545
Publication Date:
12/20/2012
Assignee:
UOP LLC (Des Plaines, IL, US)
Abstract:
The present invention discloses a new type of polyimide membrane with high permeances and high selectivities for gas separations and particularly for CO2/CH4 and H2/CH4 separations. The polyimide membranes have CO2 permeability of 50 Barrers or higher and single-gas selectivity for CO2/CH4 of 15 or higher at 50° C. under 791 kPa for CO2/CH4 separation. The polyimide membranes have UV cross-linkable functional groups and can be used for the preparation of UV cross-linked polyimide membranes having CO2 permeability of 20 Barrers or higher and single-gas selectivity for CO2/CH4 of 35 or higher at 50° C. under 791 kPa for CO2/CH4 separation.

BACKGROUND OF THE INVENTION
This invention relates to a new type of polyimide membrane with high permeances and high selectivities for gas separations and more particularly for use in natural gas upgrading and hydrogen purification.

In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies are a low capital cost solution and provide high energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including N2 enrichment from air, carbon dioxide removal from natural gas and from enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate spiral wound polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.

Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore allow smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. High performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne), and polytriazole have been developed to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising intrinsic properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8).

The membranes most commonly used in commercial gas and liquid separation applications are asymmetric polymeric membranes and have a thin nonporous selective skin layer that performs the separation. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as permeability or PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.

One of the components to be separated by a membrane must have a sufficiently high permeance at the preferred conditions or an extraordinarily large membrane surface area is required to allow separation of large amounts of material. Permeance, measured in Gas Permeation Units (GPU, 1 GPU=10−6 cm3 (STP)/cm2 s (cm Hg)), is the pressure normalized flux and equals to permeability divided by the skin layer thickness of the membrane. Commercially available gas separation polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. However, it is very complicated and tedious to make such asymmetric integrally skinned membranes having a defect-free skin layer. The presence of nanopores or defects in the skin layer reduces the membrane selectivity. Another type of commercially available gas separation polymer membrane is the thin film composite (or TFC) membrane, comprising a thin selective skin deposited on a porous support. TFC membranes can be formed from CA, polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. Fabrication of TFC membranes that are defect-free is also difficult, and requires multiple steps. Yet another approach to reduce or eliminate the nanopores or defects in the skin layer of the asymmetric membranes has been the fabrication of an asymmetric membrane comprising a relatively porous and substantial void-containing selective “parent” membrane such as polysulfone or cellulose acetate that would have high selectivity were it not porous, in which the parent membrane is coated with a material such as a polysiloxane, a silicone rubber, or a UV-curable epoxysilicone in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids. The coating of such coated membranes, however, is subject to swelling by solvents, poor performance durability, low resistance to hydrocarbon contaminants, and low resistance to plasticization by the sorbed penetrant molecules such as CO2 or C3H6.

Many of the deficiencies of these prior art membranes are improved in the present invention which provides a new type of polyimide membrane with high permeances and high selectivities for gas separations.

SUMMARY OF THE INVENTION

A new type of polyimide membrane with high permeances and high selectivities for gas separations has been made.

The present invention generally relates to gas separation membranes and, more particularly, to high permeance and high selectivity polyimide membranes for gas separations. The polyimide membranes with high permeances and high selectivities described in the current invention have CO2 permeability at least 50 Barrer (1 Barrer=10−10 cm3 (STP) cm/cm2 s (cm Hg)) and single-gas CO2/CH4 selectivity at least 15 at 50° C. under 791 kPa feed pressure.

The present invention provides a new type of polyimide membranes with high permeance and high selectivity for gas separations. One polyimide membrane described in the present invention is fabricated from poly(3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride-2,4,6-trimethyl-m-phenylenediamine) polyimide (abbreviated as NPI-1), which is derived from the polycondensation reaction of 3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride (DSDA) with 2,4,6-trimethyl-m-phenylenediamine (TMPDA). Tests showed that this NPI-1 polyimide membrane has an intrinsic CO2 permeability of 73.4 Barrers and single-gas CO2/CH4 selectivity of 25.3 at 50° C. under 791 kPa for CO2/CH4 separation. This membrane also has intrinsic H2 permeability of 136.6 Barrers and single-gas H2/CH4 selectivity of 47.1 at 50° C. under 791 kPa for H2/CH4 separation. This NPI-1 polyimide membrane contains UV cross-linkable sulfonic groups.

Another polyimide membrane described in the present invention is fabricated from poly(3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride-3,3′,4,4′-biphenyl tetracarboxylic dianhydride-2,4,6-trimethyl-m-phenylenediamine-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) polyimide (abbreviated as NPI-2), which is derived from the polycondensation reaction of DSDA and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) with 3,3′,5,5′-tetramethyl-4,4′-methylene dianiline (TMMDA) and TMPDA (DSDA:BPDA:TMMDA:TMPDA=3.06:1.02:2.00:2.00 (molar ratio)). Pure gas permeation results showed that this NPI-2 membrane has an intrinsic CO2 permeability of 57.5 Barrers and single-gas CO2/CH4 selectivity of 20.2 at 50° C. under 791 kPa for CO2/CH4 separation. This membrane also has intrinsic H2 permeability of 109.9 Barrers and single-gas H2/CH4 selectivity of 38.6 at 50° C. under 791 kPa for H2/CH4 separation. This NPI-2 membrane contains UV cross-linkable sulfonic groups.

Yet another polyimide membrane that is a part of the present invention is fabricated from poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-pyromellitic dianhydride-2,4,6-trimethyl-m-phenylenediamine) polyimide (abbreviated as NPI-3), which is derived from the polycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA) with TMPDA (BTDA:PMDA:TMPDA=2.04:2.04:4.00 (molar ratio)). Pure gas permeation results showed that this NPI-3 membrane has an intrinsic CO2 permeability of 179 Barrers and single-gas CO2/CH4 selectivity of 15.8 at 50° C. under 791 kPa for CO2/CH4 separation. This membrane also has intrinsic H2 permeability of 256.5 Barrers and single-gas H2/CH4 selectivity of 22.7 at 50° C. under 791 kPa for H2/CH4 separation. This NPI-3 membrane contains UV cross-linkable carbonyl groups.

Yet another polyimide membrane that is a part of the present invention is fabricated from poly(3,3′,4,4′-benzophenone tetracarboxylic dianhydride-pyromellitic dianhydride-2,4,6-trimethyl-m-phenylenediamine-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) polyimide (abbreviated as NPI-4), which is derived from the polycondensation reaction of BTDA and PMDA with TMPDA and TMMDA (BTDA:PMDA:TMPDA:TMMDA=2.04:2.04:2.00:2.00 (molar ratio)). Pure gas permeation results showed that this NPI-4 membrane has an intrinsic CO2 permeability of 97.0 Barrers and single-gas CO2/CH4 selectivity of 17.1 at 50° C. under 791 kPa for CO2/CH4 separation. This membrane also has intrinsic H2 permeability of 159.5 Barrers and single-gas H2/CH4 selectivity of 28.2 at 50° C. under 791 kPa for H2/CH4 separation. This NPI-4 membrane contains UV cross-linkable carbonyl groups.

In another embodiment of the invention, this invention pertains to high performance polyimide membranes that have undergone an additional crosslinking step, by chemical or UV crosslinking or other crosslinking process as known to one skilled in the art. A cross-linked polyimide membrane can be prepared by UV cross-linking of the polyimide membrane via exposure of the membrane to UV radiation. The polyimide polymers used for the preparation of the polyimide membranes described in the current invention have UV cross-linkable sulfonic (—SO2—) or carbonyl (—C(O)—) functional groups. The cross-linked polyimide membranes comprise polymer chain segments where at least part of these polymer chain segments are cross-linked to each other through possible direct covalent bonds by exposure to UV radiation. The cross-linking of the polyimide membranes provides the membranes with improved selectivities and decreased permeances compared to the corresponding uncross-linked polyimide membranes.

The membrane dope formulation for the preparation of polyimide membranes with high permeances for gas separations in the present invention comprises N-methylpyrrolidone (NMP) and 1,3-dioxolane which are good solvents for the polyimide polymer. In some cases, the membrane dope formulation for the preparation of polyimide membranes with high permeances and high selectivities for gas separations in the present invention also comprises acetone and isopropanol (or methanol) which are poor solvents for the polyimide polymer. The new polyimide membranes with high permeances and high selectivities for gas separations described in the current invention have either flat sheet (spiral wound) or hollow fiber geometry. In some cases, the selective skin layer surface of the polyimide membranes is coated with a thin layer of material such as a polysiloxane, a fluoropolymer, a thermally curable silicone rubber, or a UV radiation cured silicone rubber.

The invention provides a process for separating at least one gas from a mixture of gases using the new polyimide membranes with high permeances and high selectivities described herein, the process comprising: (a) providing a polyimide membrane with high permeance and high selectivity described in the present invention which is permeable to said at least one gas; (b) contacting the mixture on one side of the polyimide membrane to cause said at least one gas to permeate the membrane; and (c) removing from the opposite side of the membrane a permeate gas composition comprising a portion of said at least one gas which permeated said membrane.

The new polyimide membranes with high permeances and high selectivities are not only suitable for a variety of liquid, gas, and vapor separations such as desalination of water by reverse osmosis, non-aqueous liquid separation such as deep desulfurization of gasoline and diesel fuels, ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, CO2/CH4, CO2/N2, H2/CH4, O2/N2, H2S/CH4, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations, but also can be used for other applications such as for catalysis and fuel cell applications.
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