Polysulfone membrane having high selectivity

Abstract

A spin dope composition produces a polymeric fiber useful in non-cryogenic gas separation. The composition includes polysulfone as the polymeric component, two solvents, in which the polymer is soluble, and a non-solvent, in which the polymer is insoluble. The solvents preferably include N-methyl-pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAC), and the non-solvent preferably includes triethylene glycol (TEG). Fibers made from the present composition have been found to exhibit superior properties of gas flux and selectivity, as compared with fibers made from spin dopes having only one solvent component.

Claims

1. A spin dope composition for making a polymeric fiber membrane for use in non-cryogenic gas separation, the composition comprising 35 to 55 wt. % polysulfone, a solvent comprising a mixture of N-methyl-pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAC) in a 60/40 weight ratio respectively, and a non-solvent comprising triethylene glycol (TEG).

2. The composition of claim 1, wherein a ratio, by weight, of solvents to non-solvents is in a range of about 1.6-2 respectively.

3. A composition for use as a spin dope for forming a polymeric fiber membrane for use in non-cryogenic gas separation, the composition comprising polysulfone, two solvents which dissolve polysulfone, said solvents including N-methyl-pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAC), and a non-solvent comprising triethylene glycol (TEG), wherein NMP and DMAC are present in the composition in a ratio of 60/40 by weight respectively, and wherein a ratio of a weight of the two solvents to a weight of the nonsolvent is 2.2, respectively.

4. The composition of claim 1, wherein the weight ratio of solvents to non-solvent is 2.2, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention comprises a composition of matter, the composition being used to make a polymeric gas-separation membrane, wherein the membrane has both an enhanced gas flux and an enhanced selectivity.

(2) In general, a spin dope formulation comprises 1) a polymer, 2) one or more solvents, and 3) one or more non-solvents. As used herein, the term “solvent” means a solution which dissolves the polymer, and a “non-solvent” means a solution which does not dissolve the polymer.

(3) The composition which has been found to work best has been a composition containing about 35-55% polymer (by weight), a mixed solvent comprising N-methyl-pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAC), and a non-solvent comprising triethylene glycol (TEG). The ratio, by weight, of solvent to non-solvent is in a range of about 1.6:1 to 2.5:1. The ratio, by weight, of NMP to DMAC is in a range of 80/20 to 20/80.

(4) The preferred polymer in the present invention is polysulfone.

(5) The spin dope of the present invention is used to produce a hollow fiber by extruding the spin dope through a die, and passing the resulting fiber through appropriate quench and leach baths so as to cool the fiber, and so as to remove residual solvents and non-solvents. The fibers so produced are formed into a bundle, using a loom to weave the fiber tows into a cloth. The cloth then goes through an additional hot extraction bath and then into an oven where it is dried and heat-treated. This process is described in U.S. Pat. No. 5,598,874, the disclosure of which is incorporated by reference herein. The fiber can then be assembled into a module, as described above, for use in gas separation.

(6) The composition of the present invention has been found to produce a polymeric fiber having flux and selectivity which are substantially better than what is obtained with conventional fibers. The composition of the present invention has been shown to provide improvements of up to about 50% in both the oxygen flux and oxygen/nitrogen selectivity, when the fiber is used to separate air.

(7) An important aspect of the present invention is the use of a mixed-solvent system. The preferred composition includes two solvents, and it has been found that this mixed-solvent composition provides much better fiber than that produced with spin dopes having only one solvent component.

(8) Single-solvent spin dope compositions, such as compositions having NMP as the single solvent, in a concentration of greater than about 30%, typically produce fiber having poor transport properties. These compositions also exhibit high shrink rates and welded fibers in the downstream process, due to the high temperature, of the order of 100° C., used to dry the fibers and to remove residual solvent. In terms of the ratio of solvent to non-solvent (i.e. the ratio of NMP to TEG), the above formulation would translate into a maximum ratio of about 1.6-1.7. Above this ratio of solvent to non-solvent, in a single-solvent composition, the fiber produced could not be sampled.

(9) It was attempted to make spin dope compositions having DMAC as the single solvent, again with solvent concentrations of greater than about 30%. However, the resulting transport properties of the fibers produced were again poor. The hollow fiber membranes produced from these compositions had better resistance to the downstream process temperatures, and did not exhibit the welded fibers seen in the case where NMP was the single solvent.

(10) But the use of NMP and DMAC together, as a mixed solvent, made it possible to use ratios of solvent to non-solvent as high as about 2.5 without encountering the downstream process problems described above, in the case of the use of NMP alone. This mixed solvent spin dope also produced significantly improved membrane transport properties, as detailed in the following Example.

EXAMPLE

(11) The Example consisted of three tests. In Test No. 1, the spin dope contained a single solvent, comprising NMP. In Test No. 2, the spin dope contained a single solvent, comprising DMAC. In Test No. 3, the spin dope contained a mixed solvent comprising both NMP and DMAC, in which the ratio NMP/DMAC, by weight, was 60/40. In all three tests, the non-solvent was TEG, and the polymer was polysulfone.

(12) The following Table 1 summarizes the parameters of each test:

(13) TABLE-US-00001 TABLE 1 Test No. 1 Test No. 2 Test No. 3 Solvent: NMP DMAC NMP/DMAC: 60/40 Melt pump: 68 gm/min 65 gm/min 65 gm/min Non-solvent: TEG TEG TEG S/NS ratio: 1.50 2.2 2.2 Quench temp: 4.0 C. 6.0 C. 5.3 C. Leach temp: 70 C. 41 C. 36 C. Line rate: 120 ft/min 130 ft/min 130 ft/min Solids: 50% 50% 47% Spin temp: 68 C. 53 C. 60 C.

(14) For Test 2, the fibers were assembled into a bundle which was about 35 cm long, and which had 90 fibers. The bundle was soaked in water at 90° C. for 2-7.5 hours followed by air drying at 50° C. for 40 minutes.

(15) For Tests 1 and 3, the fibers were assembled using the loom process described in the above-cited U.S. Pat. No. 5,598,874, with a loom oven drying temperature of 50° C.

(16) The results of the three tests are summarized in Table 2.

(17) TABLE-US-00002 TABLE 2 Test No. 1 Test No. 2 Test No. 3 O2 Flux GPU 0.70 2.00 3.05 (×10−6 scc/ sec-cm2-cm Hg) N2 Flux GPU 0.25 1.74 0.74 O2/N2 selectivity: 2.8  1.15 4.12 OD × ID (microns): 296 u × 203 u 250 u × 140 u 250 u × 135 u Shrinkage (inches) 2.5 inches No loom 0.75 inches @ Loom oven temp: @ 50 C. samples @ 50 C.

(18) The above tests show that the mixed-solvent spin dope produces a membrane having significantly improved transport properties, as compared to the single-solvent spin dope membranes of the prior art. The mixed-solvent membranes of the present invention showed improvements, in both oxygen flux and selectivity, of about 50%. As noted previously, the addition of DMAC allowed the NMP to be used at the higher solvent ratios (greater than 1.6) without the associated downstream process and sampling problems.

(19) The parameters used in making the spin dope composition of the present invention can be varied. The melt pump may operate in a range of about 45-120 gm/min. The ratio of NMP to DMAC may be in the range from 80/20 to 20/80. The S/NS ratio (solvent to non-solvent ratio) can be in a range of about 1.6 to 2.5. The quench temperature can be in a range of 4-30° C., and the leach temperature can be in a range of 25-85° C. The line rate can be in a range of about 100-350 ft/min. The percentage of solids can be about 35-55%. The spin temperature (i.e. the temperature of the spin dope as it is extruded through the die) can be about 55-85° C.

(20) The invention can be modified in other ways that will be apparent to those skilled in the art. For example, the ratio of solvent to non-solvent can be varied, within the range specified above. The exact choice of solvent and non-solvent, as well as the choice of polymer, may be varied. These and other variations should be considered within the spirit and scope of the following claims.