1. Field of the Invention
This invention relates to polyethersulfone asymmetric hollow fibers with improved gas permeability and selectivity. In another aspect, the invention relates to formulating polymer dopes containing the polymer, an organic solvent, and a suitable additive that is a polar compound ("nonsolvent-additive"). Polymer dopes of the invention have moderate polymer concentrations and viscosities as well as low coagulation valves. In yet another aspect, the invention relates to the process of producing asymmetric hollow fibers for gas separation.
2. Description of Related Art
References of the scientific periodical and patent literature are cited throughout this specification. Each such literature reference is hereby incorporated in its entirety by such citation.
There are three key parameters that determine the commercial viability of a membrane for gas separation. The first is its separation factor towards the gases to be separated and this directly controls the degree of the separation and indirectly determines the membrane area requirement. The second parameter is membrane permeation flux which simply dictates the membrane area requirement. The third is the working life of membrane. The separation factor depends mainly on the membrane materials. It has been shown that polyethersulfone exhibits superior selectivity compared to polysulfone and cellulose acetate which have been used to produce commercial gas separation membranes. Polyethersulfones also have good thermal resistance and mechanical strength, but display only moderate permeability. The challenge to the production of polyethersulfone gas separation membranes suitable for industrial applications is the fabrication of this polymer into membranes having high permeation flux. The gas permeation rate through a dense polymer membrane is proportional to the pressure difference across the membrane, its membrane area and the permeability coefficient of the membrane material, and inversely proportional to the membrane separating layer thickness. Of these parameters, the permeability coefficient depends on the nature of polymer material; and pressure difference is the operating condition. The goal in membrane making is therefore, to prepare membranes with an ultra thin separating layer and enhancement of membrane area and mechanical strength.
To provide maximum membrane area and minimum separating layer thickness, asymmetric hollow fiber membranes are the favorite choices. Hollow fiber membranes which can provide the maximum area per unit packing volume is the most important membrane configuration. A process for producing asymmetric hollow fibers typically includes the following steps: (1) formulating a polymer dope; (2) extruding from the orifice of a tube-in-orifice spinnerette suitable for forming a hollow fiber configuration; (3) meanwhile, an internal coagulant (a nonsolvent for polymer) is injected into the tube of the spinnerette in order to maintain the bore configuration; (4) the nascent hollow fiber passes through an air-gap; (5) the hollow fibers are immersed into the coagulation bath so as to leach out the solvent or additive in the nascent fibers to produce an asymmetric structure by forming a thin dense skin supported by a thick and more porous sublayer; (6) removing the fibers from the coagulation bath; and (7) drying of the fibers.
Asymmetric membranes have good possibility in forming very thin separating layers. The method for producing this kind of membrane by the phase inversion process was first invented by Loeb and Sourirajan (U.S. Pat. No. 3,133,132). These asymmetric membranes were soon used in industrial liquid separation processes such as reverse osmosis and ultrafiltration. However, these membranes prepared from non-cellulose polymers often exhibit poor selectivity for gas separation due to the presence of bigger pores on the membrane surface and because the transport of gas is largely due to Knudsen flow and viscous flow.
U.S. Pat. No. 4,230,463 issued to Henis and Tripodi describes a method to seal the big pores by coating a thin silicone rubber film on the surface of asymmetric membranes. The separation properties of these composite asymmetric membranes are generally determined by the material of the asymmetric membrane instead of the material of the coating. Development of this kind of membrane allowed large-scale commercial applications of gas separation using asymmetric membranes.
Unlike flat-sheet membranes which require a solid support, hollow fibers are self-supporting. The polymer dope used for spinning of hollow fibers must be of sufficiently high viscosity and polymer concentration in order to produce a self-support extrusion prior to a coagulation process. However, too high a viscosity of the polymer dope is undesirable as it causes difficulty in spinning. The spinning process involves many variables which affect the structure of the membrane and its gas separation characteristics. These variables include polymer dope composition, spinning conditions and coagulation conditions. The nature of polymer dope is highly influential in determining the morphology of the hollow fiber membrane and its gas separation properties.
U.S. Pat. No. 4,871,494 issued to Kesting et al. describes a process for forming asymmetric gas separation hollow fiber membranes having graded density skins. This process comprises dissolving a hydrophobic polymer in Lewis acid/Lewis base complexes wherein the Hildebrand parameters of the solvent system and the polymer are less than 1.5 cal.sup.0.5 /cm.sup.1.5. The useful acids employed as additives must have the Gutman acceptor number (AN) of 47&lt;AN&lt;63, and the infra-red frequency shifts (.DELTA..nu.) of a complex with N-methyl-2-pyrrolidone falling within the range of -25&lt;.DELTA..nu. &lt;-38 cm. The solubility parameters of useful acids have been found to have values of 12&lt;.delta.&lt;12.5 cal.sup.0.5 /cm.sup.1.5. The polymer dope has a high polymer concentration (35 wt %-40 wt %), high viscosity (&gt;100,000 cp) and a low coagulation value (0&lt;G.nu.&lt;1.5 g). A suitable choice of acid (e.g. propionic acid) results in the hollow fibers exhibiting high permeabilities and good potential for high separation factors. The development of this kind of membrane has led to the production of the commercial gas separator Perme-.alpha..
U.S. Pat. No. 4,992,221 issued to Malon et al. discloses a process for preparing asymmetric polymers hollow fibers with improved separation factor and mechanical strength. The membranes were produced from a process utilizing membrane forming dopes of solvent systems formulated from two nonsolvents and one solvent. The nonsolvents were chosen according to the nonsolvent strength, i.e. one strong nonsolvent and one weak nonsolvent which were combined with solvent in an acid:base complex solvent system. A strong nonsolvent is defined as one having a .DELTA..delta.(.delta..sub.nonsolvent -.delta..sub.polymer).gtoreq.6 cal.sup.0.5 /cm.sup.1.5. A weak nonsolvent is defined as one with a .DELTA..delta.&lt;6 cal.sup.0.5 /cm.sup.1.5.
Pinnau et al. (U.S. Pat. No. 4,902,422) discloses a process to prepare "defect-free" asymmetric flat-sheet membranes by the dry/wet phase inversion process. Highly permeable asymmetric membranes were prepared by selecting suitable polymer dopes and coagulants as well as controlling conditions of the drying process so as to form a dense skin separating layer.
It is known in the art to make polyethersulfone hollow fiber gas separation membranes from 1:1 molar mixtures of N-methyl-2-pyrrolidone (NMP): proprionic acid, a Lewis acid:base complex and high polymer concentration. Such a process has been patented (U.S. Pat. No. 4,871,494). However, the use of solvent systems containing a polar liquid or a mixture of polar liquids as a nonsolvent-additive for improved gas separation performance of polyethersulfone hollow fibers has not previously been known.