Hollow fiber membrane contactor

#This invention relates to a hollow fiber membrane contactor for phase separations and other phase contact applications. The contactor is made from perfluorinated thermoplastic polymeric materials, has a high packing density providing high useful contacting area, and the ability to operate with liquids of low surface tension.

#Liquid-gas contactors are used to transfer one or more soluble substances from one phase to another. Examples of conventional contactors are packed towers, plate columns and wetted wall columns. In these systems, gas absorption of one or more components from the gas stream is accomplished by dispersing the gas as bubbles in packed towers and plate columns in a countercurrent flow to the liquid stream. Absorption efficiency is controlled apart from solubility considerations by the relative rate of the flows and the effective surface area of the gas flow bubbles. In wetted wall contactors the gas stream flows past a downward flow of liquid on the inside wall of a vertical tube. Gas stripping is used to transfer a gas dissolved in liquid into a gas stream. Similar contactors are used for gas stripping.

#Conventional contactors have several deficiencies. Primary among these is the fact that the individual gas and liquid flows cannot be varied independently over wide ranges. Tray columns are prone to such problems as weeping at low gas flows and flooding at high liquid flows. Packed towers can flood at high flow rates. The use of low liquid flow rates In a packed tower would lead to channeling and reduced effective surface area. Excessive frothing or foam formation can lead to process inefficiency. Wetted wall contactors have inherently low mass transfer coefficients, and can flood at high gas flow rates. The development of membrane contactors has overcome these deficiencies.

#FIG. 1. is an illustration of a hollow fiber membrane contactor.

#FIG. 2a. is an illustration of hollow fiber membrane wrapped around a frame.

#FIG. 2b. is an illustration showing a flat rectangular coil of hollow fiber membrane.

#The asymmetric skinned hollow fiber membrane is produced by the process described in concurrent U.S. patent application Ser. No. 60/117,854 filed Jan. 29,1999, the disclosure of which is incorporated by reference. That process is based on the Thermally Induced Phase Separation (TIPS) method of making porous structures and membranes. A mixture of perfluorinated thermoplastic polymer pellets, usually ground to a size smaller than supplied by the manufacturer, from about 100 to about 1000 microns, preferably about 300 microns, more preferably supplied or ground to a powder, and an solvent, such as chlorotrifluoroethylene oligimer, is first mixed to a paste or paste-like consistency. The polymer comprises between approximately 12% to 75%, preferably 30% to 60%, by weight of the mixture. Preferred polymers are perfluorinated thermoplastic polymers, more specifically poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) (poly (PTFE-CO-PFVAE)) or poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP), or blends of these polymers, which are dissolved in a solvent to give a solution having an upper critical solution temperature, and which when the solution is cooled, separates into two phases by liquid-liquid phase separation. PFA Teflon® is an example of a poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) in which the alkyl is primarily or completely the propyl group. FEP Teflon® is an example of poly (tetrafluoroethylene-co-hexafluoropropylene). Both are manufactured by DuPont. Neoflon™ PFA (Daikin Industries) is a polymer similar to DuPont's PFA Teflon®. A poly (tetrafluoroethylene-co-perfluoro (alkylvinylether)) polymer in which the alkyl group is primarily methyl is described in U.S. Pat. No. 5,463,006. A preferred polymer is Hyflon® POLY (PTFE-CO-PFVAE) 620, obtainable from Ausimont USA, Inc., Thorofare, N.J.

#The solvent is chosen so the membrane formation occurs by liquid-liquid, rather than solid-liquid phase separation when the solution is extruded and cooled. Preferred solvents are saturated low molecular weight polymers of chlorotrifluoroethylene. A preferred solvent is HaloVac® 60 from Halocarbon Products Corporation, River Edge, N.J. Choice of the solvent is dictated by the ability of the solvent to dissolve the polymer when heated to form an upper critical solution temperature solution, but not to excessively boil at that temperature. Fiber extrusion is referred to as spinning and the extruded fiber length from the die exit to the take-up station is referred to as the spin line. The paste is metered into a heated extruder barrel where the temperature raised to above the upper critical solution temperature so that dissolution occurs. For inside skinned hollow fiber membranes, the homogeneous solution is then extruded through an annular die directly into a liquid cooling bath with no air gap. The lumen diameter is maintained with a constant pressure of gas. The liquid cooling bath is maintained at a temperature below the upper critical solution temperature of the polymer solution. The preferred bath liquid is not a solvent for the thermoplastic polymer, even at the extrusion temperature. Upon cooling, the heated and shaped solution undergoes phase separation and a gel fiber results. The die tip is slightly submerged for vertical spinning, i.e.; the spin line falls downward, in the direction of a freely falling body. For horizontal spinning, where the spin line exits directly in the horizontal attitude, and is maintained more or less in that plane until at least the first guide roll, a specially design die is used. The die is firmly positioned against an insulated wall with the die tip penetrating through an opening having a liquid-tight seal in the insulator wall. A trough for cooling liquid flow is placed in a recess in the opposite side of the insulating wall, in a manner that will maintain the die nose outlet in a submerged condition. Cooling liquid flows in the trough and overflows in a region of the trough of lesser depth, keeping the die nose outlet submerged with a flow of cooling liquid. In both the vertical and horizontal methods, a booster heater and temperature control means is used to briefly raise the solution temperature at the die tip to prevent premature cooling. In a subsequent step, the dissolution solvent is removed by extraction and the resultant hollow fiber membrane is dried under restraint to prevent membrane shrinkage and collapse. Optionally, the dried fiber may be heat set at 200° C. to 300° C. Preferably the fiber will be heat set or annealed under restraint at a temperature near the melting temperature of the fiber, which for the preferred polymer of this invention is within a range of from about 270° C. to about 290° C., preferably from about 275° C. to about 285° C., with the most preferred range from about 278° C. to about 282° C. In order to minimize shrinkage during potting, a second, unrestrained, annealing step at similar temperatures is a preferred step. Annealing times for these steps if from about 6 to about 48 hours, more preferably, from about 18 to about 30 hours.

#In the invention described in U.S. Ser. No. 60,117,854, controlled evaporation of solvent from at least one surface of the hollow fiber as it exits the die tip is combined with higher polymer solids solutions and the submerged extrusion process to produce inner diameter skinned asymmetric hollow fiber porous membranes from perfluorinated thermoplastic polymers. For this embodiment, the lumen is maintained with a constant pressure of a gas continuously fed into the inner diameter of the lumen. In this embodiment the superheated solvent evaporates inside the lumen as soon as it emerges from the die. The loss of solvent causes a superficial increase in solids concentration on the inner lumen surface. As the melt is quenched, a very thin skin is formed on the lumen surface, while the rest of the membrane forms a microporous structure due to its being submerged in a cooling or quenching bath which prevents the porogen from flashing off the outer surface and prevents the formation of a skin on the outer surface.