Apparatus and method for degassing liquids

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APPARATUS AND METHOD FOR DEGASSING LIQUIDS

Field of the Invention

The present invention is directed to degassing liquidsusing microporous hollow fiber membrane contactors.

Background of the Invention

The degassing of liquid refers to the complete orcontrolled removal of dissolved or entrained gasses from aliquid. Liquids, used in some industrial processes, arerequired to be ultrapure. These ultrapure liquids are freeor substantially free from: minerals, for example, iron;ions; and gas. The removal of the minerals and ions is mostoften accomplished through a reverse osmosis process. Thereverse osmosis process, however, does not remove thedissolved or entrained gasses. The most common dissolved orentrained gas is air which has as its major components.nitrogen, oxygen, and carbon dioxide.

In the past, the dissolved or entrained gasses have been.removed by the use of membrane contactors, for example, hollow fiber membrane contactors. These contactors used polypropylene microporous hollow fiber membranes with wallthicknesses of less than or equal to about thirty microns.These contactors, however, could only degas a liquid fromabout 1°C to about 60°C. For example see: Hoechst CelaneseCorporation's LIQUI-CEL® Extra-Flow 4" X 28" membranecontactor. While removal of gases at this temperature isadequate, increased demand for ultrapure liquid, in someindustries, has required that more aggressive degassingtechniques be investigated. Using conventional contactors,degassing liquids was limited to these temperatures becauseof the inability of the conventional contactors to withstandgreater temperatures. At temperatures of about 60-85°C,where more agressive degassing may occur, the hollow fiberswould collapse, rendering the contactor ineffective.Accordingly, there is a need for a contactor that willoperate at higher temperatures.

In the semiconductor industry, ultrapure water isneeded because the ultrapure water is used to clean the surfaces of the silicon wafers used to make the semiconductor chip. Any contaminate, even the dissolved orentrained gasses, typically air, can have a detrimentaleffect on the chip. Accordingly, there is a need to have asource of ultrapure water. 2

Japanese Patent Application No. 52-143213 filedNovember 28, 1977 discloses a polyolefin (e.g., polyethyleneand polypropylene) hollow fiber with wall thicknesses of 30and 27 microns, respectively. The hollow fibers may be usedas, among other things, a gas separation membrane (which isa process for separating one gas from another gas). Thepolyolefin is crystalline and is nucleated, one nucleatingagent is sodium benzoate. The hollow fiber is described ashaving "shape stability" which is understood to meanrelatively small shrinkage.

Summary of the Invention

The present, invention is directed to a microporousmembrane contactor for degassing liquids at temperaturesgreater than (&gt;) 60°C and at pressures greater than or equalto (&gt;) 40 psig. The contactor has a microporous hollowfiber membrane adapted to withstand collapse and resistappreciable pore shrinkage or pore closure when subjected toliquid temperatures greater than (&gt;) 60°C and liquidpressures greater than or equal to (&gt;) 40 psig for a periodof greater than or equal to (&gt;) 30 days. A housing encloses the membrane. 3

Description of the Drawings

For the purpose of illustrating the invention, thereshown in the drawings a form which is presently preferred;it being understood, however, that this invention is notlimited to the precise arrangement and instrumentalities shown.

Figure 1 is a schematic illustration of a liquiddegassing process.

Figure 2 is a sectional view of a contactor.

Detailed Description of the Invention

Referring to Figure 1, there is shown a liquiddegassing process 10. Process 10.utilizes a membranecontactor 12 (discussed in further detail below). Process10 has a counterflow configuration, however, the inventionis not so limited. Contactor 12 typically has a shell sideand a tube side. Liquid I.4 which contains entrained ordissolved gas is preferably introduced into the shell sideof contactor 12. A sweep gas 16 (or vacuum or both sweepgas/vacuum) is preferably introduced into the tube side of 4 «λ the contactor 12. Degassed water 18 is discharged fromcontactor 12 and sweep gas 16 is also removed from thecontactor 12. One of ordinary skill will readily appreciatethat by varying the flow rate (or pressure), temperature,and composition of the materials on the shell and tube sidesthat the transport of gas across the membrane can betailored as desired. In the instant invention, water is oneliquid that can be degassed. Typically, the liquid 12, whenit is water, has a minimum temperature of greater thanapproximately 60°C. The liquid has a maximum temperature of less than the liquid's boiling point, but preferably no greater than 80°C (if the liquid is water). The pressure ofthe liquid into the contactor should be greater than about40 psig up to approximately 120 psig, but preferably nogreater than about 85 psig. The hollow fiber membraneshould have good dimensional stability at or above 60°C and40 psig. Good dimensional stability refers to, at least,being adapted to withstand collapse and/or resistappreciable pore shrinkage or pore closure. Resistappreciable pore shrinkage or pore closure refers to anydiminution or blockage of a significant number of the pores,so that the ability of the membrane to transport gas ismaterially reduced (i.e, by at least 50%). 5 120779/2

Referring to Figure 2, an exemplary contactor 12 isshown in greater detail. Contactor 12 may be made accordingto U.S.. Patent Nos. 5,264,171; 5,284,584; and 5,352, 361,each is incorporated herein by reference. Contactor 12generally comprises a shell 30 and a plurality of tubes 32.Tubes 32 are preferably microporous hollow fibers. Thecenter tube 34 is located along the longitudinal axis ofshell 30. Center tube 34 is a perforated tube, so thatliquid may ingress and egress therefrom. A baffle 36 may be affixed to the center tube 34 intermediate the ends thereof.

Shell 30, tube sheets 38, and the exterior surfaces of thetubes 32 define shell side 40. Shell side 40 is provided with an inlet 42 and an outlet 44. The flow of material through the shell side 40 is indicated by arrows 46. Theinterior or lumenae of the tubes 32 define, in part, tubeside 48. Tube side 48 is provided with an inlet 50 and anoutlet 52. Contactor 12 is not limited to the foregoingconfiguration.

Microporous hollow fibers 32- preferably: are made of apolyolefin material having a wall thickness of greater than30 micron (more preferably greater than 35 microns, and mostpreferably greater than or equal to (&gt;) 50 microns); have aporosity of less than 80% (more preferably less than 60%,and most preferably about 20%); have a Gurley number of 6 ......... t- Ά * *

' Ϊ fc* J greater than 1 (more preferably greater than 100, and mostpreferably about 300); have a bubble point of greater thanor equal to 25 psig (more preferably greater than 100 psig,and most preferably greater than 200 psig); and have ashrinkage, under no load, of less than 5% at 90°C for 60minutes (preferably about 2% or less).

Polyolefin refers a class group of thermoplasticpolymers derived from simple olefins. Exemplary polyolefinsinclude, but are not limited to, polyethylene, polypropylene, polymethylpentene, copolymers of , polyethylene, copolymers of polypropylene, copolymers ofpolymethylpentene, and combinations thereof. Preferably,the polyolefin is polypropylene which will be discussed hereinafter.

The polypropylene is preferably crystalline and has acrystallization temperature of greater than or equal to125°C. To obtain this crystalline polypropylene, it ispreferably nucleated. Nucleated or having a nucleatingagent refers to the promotion of crystal nucleation which isoften initiated by a material added to the polymer.

Preferred nucleating agents may be selected from the group of sodium benzoate or sorbital acetate or combinations thereof. The preferred nucleating agent is sodium benzoate'. 7

Preferably, about 2400 ppm of nucleating agent is added tothe polymer. The nucleated material is beneficial from atleast two stand points. First, the better crystaluniformity provides greater heat stability. Second, thebetter crystal uniformity reduces the polymer's tendency to"creep" or close or shrink the pore size.

The polypropylene has a melt flow index (ASTM D1238-85)of greater than 0.1 gram per 10 minutes, preferably greaterthan 1 gram per 10 minutes and most preferably in a range of0.1 to 20 grains per 10 minutes.

With regard to polyolefin resin selection, one should consider the additive package or stabilizer that customarily is present in all commercially available resins because some of these stabilizers can cause pore closure or pore shrinkage and/or skin formation on the fiber surface. For example, a stabilizer consisting of: 0.05% of BHT (butylated hydroxytoluene or 2,6-di-t-butyl-4-methyl phenol); 0.12% (Irganox 1010 from Ciba Geigy Corp.) tetrakis[methylene (3,5-di-butyl-r-hydroxyhydrocinnamate)] methane; and 32 ppm calcium sterate, caused complete pore blockage and a skin formed on the fiber surface. It is believed that this stabilizer leached from the polymer to block the pores and* to skin the fiber surface.. On the other hand, a stabilizer 8 consisting of: 600 ppm hinder phenolic (Ethyl 330 fromEthyl Corp) and 1000 ppm phosphite (Orgafox from Ciba CiegyCorp.), had no effect on the pores or fiber surface. Basedupon the foregoing, the following stabilizer selectioncriteria is proposed: a stabilizer that will not migrate ispreferred, such stabilizer may have high molecular weightsand/or side chains consisting of long hydrocarbons (e.g.,nonpolar, chemically more compatible with polyolefins)/ and a lower amount of stabilizer (so that there is less material to migrate).

Modules made according invention and subjected totemperatures above about 60°C and pressures above 40 psigremained operation for greater than 30 days.

The following is a preferred procedure by which amicroporous hollow fiber according to the instant inventionis made. The resin is polypropylene (Fina PP3362 from FinaCo) . The extrusion temperature was 210°C, the extrusionspeed was 100 m/min, and spinning tension was 21 grams.

Rapid quenching was not necessary in view of the nucleatingagent (about 2400 ppm sodium benzoate). After fiberspinning, the fiber was drawn. The spun fiber had a 332 micron OD and a 55 micron, wall thickness. It was annealed at 150°C. Then, it was subjected to a 10% cold draw, a 50% 9 hot draw (at 142°C), and thereafter to a 20% relax (at142°C). The resulting fiber had the following properties:325 micron OD; 215 micron ID; 55 micron wall thickness; 250sec/m2 - Gurley number; 250 psi - bubble point; 1.2% -shrinkage; 1.02 - shape ratio; 460g - tensile strength, and220% - elongation at break.

All measured values set forth herein are measured according to conventional industry standards (e.g. appropriate ASTM procedures), the following values arediscussed in greater detail.

Gurley - "Gurley" refers to a measure of the resistance toair flow through the wall of the microporous hollow fiber.The resistance to airflow, as measured by a Gurleydensometer, is the time in seconds required to pass lOcc ofair through one square inch of product at a constantpressure of 12.2 inches of H2O. The measure is reported in"sec/in2" and is normalized in the one square inch value.

Bubble point - "Bubble point" is a measure for determiningthe large pore diameter and the general pore diameter of the hollow fiber. About one foot of hollow fiber is tested using nitrogen pressure gas in a methanol bath. The initial pressure is 15 psig and is increased at about 5-10 psig per 10 second. The measurement is calculated when 15 streamers of bubbles appear. The bubble point, herein, is reported inpsig and correlates to pore size in microns by: (micron)= 6.56/(psig in methanol bath).

Porosity - "Porosity" is a measure of the interior porevolume and the apparent pore diameter distribution of thehollow fiber. Porosity is measured in accordance with theprocedures set forth in ASTM D-2873-89.

The present invention may be embodied in other formswithout departing from the spirit and essential attributesthereof and, accordingly, reference should be made to theappended claims, rather to the foregoing specification, asindicating the scope of the invention 11