BACKGROUND OF THE INVENTION

The present invention relates to the non-cryogenic separation of gas mixtures. The invention provides a composition of matter comprising a polymeric membrane which exhibits superior properties when used for the non-cryogenic separation of gases such as air.

It has been known to use a polymeric membrane to separate air into components. Various polymers have the property that they allow different gases to flow through, or permeate, the membrane, at different rates. A polymer used in air separation, for example, will pass oxygen and nitrogen at different rates. The gas that preferentially flows through the membrane wall is called the "permeate" gas, and the gas that tends not to flow through the membrane is called the "non-permeate" or "retentate" gas. The selectivity of the membrane is a measure of the degree to which the membrane allows one component, but not the other, to pass through.

A membrane-based gas separation system has the inherent advantage that the system does not require the transportation, storage, and handling of cryogenic liquids. Also, a membrane system requires relatively little energy. The membrane itself has no moving parts; the only moving part in the overall membrane system is usually the compressor which provides the gas to be fed to the membrane.

A gas separation membrane unit is typically provided in the form of a module containing a large number of small, hollow fibers made of the selected polymeric membrane material . The module is general ly cyl indrical , and terminates in a pai r of tubesheets which anchor the hol low fibers. The tubesheets are impervious to gas . The fibers are mounted so as to extend through the tubesheets , so that gas fl owing through the interior of the fibers (known in the art as the bore side) can effectively bypass the tubesheets. But gas fl owing in the region external to the fibers (known as the shel l side) cannot pass through the tubesheets .

In operation, a gas is introduced into a membrane module, the gas being di rected to fl ow through the bore side of the fibers . One component of the gas permeates through the fiber wal l s , and emerges on the shel l side of the fibers, while the other, non- permeate, component tends to flow straight through the bores of the fibers. The non -permeate component comprises a product stream that emerges from the bore sides of the fibers at the outlet end of the module.

Alternatively, the gas can be introduced from the shel l side of the module. In this case, the permeate is withdrawn from the bore side, and the non -permeate is taken from the shel l side.

An example of a membrane -based ai r separation system is given in U.S. Patent No. 4,881,953, the discl osure of which is incorporated by reference herein.

Other examples of fiber membrane modules are given in U.S. Patent Nos. 7,497.894, 7, 517,388, 7, 578.871, and 7, 662 ,333, the disclosures of which are al l hereby incorporated by reference.

The effectiveness of a membrane in gas separation depends not only on the inherent selectivity of the membrane, but al so on its capabil ity of handl ing a sufficiently l arge product flow. A membrane module is therefore evaluated according to flux, i .e. the fl ow rates of various components through the membrane, as wel l as according to sel ectivity. A polymeric fiber for use in the module described above is made from a mixture known as a "spin dope" . The spin dope includes a polymeric material or precursor, a sol vent which dissol ves the polymer, and a non- sol vent in which the polymer is insol uble. The spin dope comprises the material which is spun into a fiber. In general , one tailors the spin dope to control the formation of the fiber. That is, the composition of the spi n dope determi nes when the pol ymeri c materi al wi l l undergo a transformation from a state in which it is intermixed and homogeneous in the spin dope, to a state in which the polymer is the principal component.

The fiber formul ation is defined by three parameters, namely 1) the percentage of polymer in the spin dope, 2) the type of polymer solvent(s) , and 3) the non- sol vent in the mixture. The terms "sol vent" and "non- solvent" are used to mean, respectively, the components of the spin dope in which the polymer is sol uble and insol uble.

The present invention provides a composition for use in making a gas- separati on membrane i n whi ch both the gas fl ux and sel ecti vi ty are improved. The polymeric membrane of the present composition therefore reduces the capital cost for gas -separation systems.

SUMMARY OF THE INVENTION

The present invention comprises a spin dope composition, for use in making a polymeric fiber membrane for non -cryogenic gas separation. The composition comprises polysul fone as the polymeric component, two solvents in which the polymer is sol uble, and a non -sol vent in which the polymer is insoluble. The preferred sol vents are N -methyl -pyrrol i done (NMP) and N. N- dimethyl acetamide (DMAC) , and the preferred non-solvent is triethylene glycol (TEG) . In a preferred embodiment, the ratio of weights of NMP/DMAC is in a range of about 80/20 to 20/80. In a more preferred embodiment , this ratio is about 60/40. The preferred range of the ratio of solvents to non- sol vents is in a range of about 1.6-2.5. In a more preferred embodiment, the l atter ratio is about 2.2.

The composition described above has been found to exhibit superior flux and selectivity properties, as compared with fiber made from spin dopes having a single sol vent.

The present invention therefore has the primary object of providing a composition for making a polymeric fiber for use in non -cryogenic gas separation.

The invention has the further object of providing a polymeric fiber composition which exhibits superior properties of gas flux and selectivity.

The invention has the further object of enhancing the efficiency of a non -cryogenic gas separation process .

The i nventi on has the further object of reduci ng the cost of separating gases by use of polymeric membranes.

The reader skil led in the art wil l recognize other objects and advantages of the present invention, from a reading of the fol lowing detailed description of the invention, and the appended cl aims.

DETAILED DESCRIPTION OF THE INVENTION

The present i nventi on compri ses a composi ti on of matter , the composition being used to make a polymeric gas -separation membrane, wherein the membrane has both an enhanced gas fl ux and an enhanced selectivity.

In general , a spin dope formulation comprises 1) a polymer, 2) one or more solvents , and 3) one or more non- sol vents. As used herein, the term "sol vent" means a solution which dissolves the polymer, and a "non- sol vent" means a solution which does not dissol ve the polymer.

The composi ti on whi ch has been found to work best has been a composition containing about 35-55* polymer (by weight) , a mixed sol vent comprising -methyl -pyrrol i done (NMP) and Ν , Ν-dimethyl acetamide (DMAC) , and a non-sol vent comprising triethylene glycol (TEG) . The ratio, by weight, of sol vent to non -sol vent 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.

The preferred polymer in the present invention is polysul fone.

The spin dope of the present invention is used to produce a hol low 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 sol vents and non -sol vents. 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. Patent 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.

The composition of the present invention has been found to produce a polymeric fiber having fl ux and selectivity which are substantial ly 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 fl ux and oxygen/nitrogen selectivity, when the fiber is used to separate ai r.

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

Single- sol vent spin dope compositions , such as compositions having NMP as the single solvent , in a concentration of greater than about 30% , typi cal l y produce fi ber havi ng poor transport properti es . These compositions al so exhibit high shrink rates and wel ded 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 sol vent to non -sol vent (i .e. the ratio of NMP to TEG) , the above formul ation would transl ate into a maximum ratio of about 1.6- 1.7. Above this ratio of solvent to non -sol vent, in a single -sol vent composition, the fiber produced could not be sampled.

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 agai n poor . The hol l ow fi ber 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 sol vent.

But the use of NMP and DMAC together, as a mixed solvent, made it possible to use ratios of solvent to non -sol vent as high as about 2.5 without encountering the downstream process problems described above, in the case of the use of NMP al one. This mixed solvent spin dope al so produced significantly improved membrane transport properties , as detailed in the fol lowing Example.

Example

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 sol vent comprising both NMP and DMAC, in which the ratio NMP/DMAC, by weight, was 60/40. In al l three tests, the non- sol vent was TEG, and the polymer was polysul fone.

The fol lowing Table 1 summarizes the parameters of each test:

Table 1

Test No. 1 Test No. 2 Test No. 3

Sol vent : NMP DMAC NMP/DMAC: 60/40

Melt pump: 68 gm/min 65 gm/min 65 gm/min

Non -sol vent: 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/mi n 130 ft/mi n 130 ft/mi n

Sol ids : 50* 50* 47*

Spin temp: 68 C 53 C 60 C For Test 2, the fibers were assembled into a bundle which was about 35 cm l ong, and which had 90 fibers. The bundle was soaked in water at 90° C for 2-7.5 hours fol lowed by air drying at 50° C for 40 minutes.

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

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

Table 2

Test No. 1 Test No. 2 Test No. 3

02 Flux GPU 0.70 2.00 3.05

(x 10-6 sec/sec -cm2- cm Hg)

N2 Flux GPU 0.25 1.74 0.74

02/N2 selectivity: 2.8 1.15 4.12

OD X ID (microns) : 296u x 203u 250u x 140u 250u x 135u Shrinkage (inches)

@ Loom oven temp: 2.5 inches @ 50C No loom samples 0.75 inches

The above tests show that the mixed -sol vent spin dope produces a membrane having signi ficantly improved transport properties, as compared to the single- sol vent spin dope membranes of the prior art. The mixed-sol vent membranes of the present invention showed improvements, in both oxygen fl ux and selectivity, of about 50*. As noted previously, the addition of DMAC al lowed the NMP to be used at the higher solvent ratios (greater than 1.6) without the associated downstream process and sampl ing problems.

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 -sol vent 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 l ine rate can be in a range of about 100-350 ft/mi n. The percentage of sol ids 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.

The invention can be modified in other ways that wil l be apparent to those skil led in the art . For example, the ratio of solvent to non -sol vent can be varied, within the range specified above. The exact choice of sol vent and non -sol vent , as wel l as the choice of polymer, may be varied. These and other variations should be considered within the spi rit and scope of the fol lowing cl aims.