An assembly for deaeration of liquids

Abstract: A deaeration assembly for removal of air or other gases dissolved in a liquid. The assembly includes a deaeration element having a gas-channel-forming component enclosed and sealed within an envelope formed of a nonporous fluoropolymer film. The assembly also incudes a liquid-channel- forming component which can be positioned on the outside of the element, or can be enclosed with the gas-channel-forming component within the element. The assembly can be formed in spiral-wound or folded configurations for installation in a deaeration module or apparatus. The deaeration assembly is useful for removal of gases dissolved in chemically aggressive liquids, high-purity liquids, and other special liquids.

Description: The present invention relates to an apparatus for deaeration of liquids, more particularly, to an assembly for removing a gas that is dissolved in a liquid. That corrosive, oxidative, reactive, and contaminating properties harmful to certain products and equipment is associated with air or other gases dissolved in liquid is well known. To reduce or minimize these harmful effects are among many reasons why it is sometimes desirable to remove air and/or other gases dissolved in a liquid. Known apparatus for performing such deaeration or degassing operations are modules which use a porous polymeric membrane material through which the dissolved gas can permeate as the means for removing the gas from the liquid. Typically, in this type of apparatus, deaeration is accomplished by having one side of the membrane contact the liquid to be deaerated, and on the other side of the membrane provide a gas channel, usually under reduced pressure, to draw away the gas permeating through the membrane. Such systems are quite effective in deaerating water of normal purity. However, a problem with the above-mentioned deaeration apparatus in which a porous polymeric membrane material is used is that, when the liquid to be deaerated is a solvent, a liquid fat or oil, or an aqueous liquid that contains a surfactant, the liquid tends to wet the membrane material and penetrate through the pores, thus precluding deaeration. In an effort to solve this problem, there have been proposals for a deaeration apparatus that makes use of a non-porous membrane material, for example, one obtained by coating the surface of a porous polymeric support membrane with a silicone resin, or other polymer resin through which gases can permeate at acceptable rates. While this apparatus does indeed allow deaeration to be performed when the liquid to be deaerated is a relatively mild solvent, liquid fat or oil, or aqueous liquid that contains a surfactant, they are not successfully used when the liquid to be deaerated is of exceptionally high purity or is chemically aggressive. For example, liquids such as the deionized water required for semiconductor processing, or special liquids such as photoresist liquids or developing fluids used in the manufacture of semiconductor products. Such liquid tend to leach substances from the separation membranes which then contaminate the liquids; or the liquids may cause the membranes to degrade and fail. EP-A-0448973 (Toray Industries, Inc.) describes a gas permeable membrane module for deaeration of liquids, having an envelope-like, gas-permeable membrane wrapped around a mandrel in which gas flows, with the outer surface of the membrane being positioned adjacent a feed water spacer. The presence of a porous membrane adjacent the mandrel is not suggested. It is an object of the present invention to obviate or mitigate at least one of these disadvantages. This may be achieved by providing an assembly for use in a deaeration apparatus with which such special high purity and aggressive liquids can be deaerated. The deaeration assembly for use in a deaeration apparatus may resist attack by high purity or aggressive liquids, and may minimize leaching of materials from the assembly which could harmfully contaminate the liquid to be deaerated. According to a first aspect of the present invention, there is provided a deaeration element for a deaeration assembly, said element having a gas-channel-forming component with a porous membrane laminated to one or both sides thereof, the gas channel forming component being enclosed and sealed within an envelope formed of a non-porous fluoropolymer film, said envelope having inward-facing and outward-facing surfaces and at least one port leading from the inside to the outside of said envelope for passage of gases permeating into the element to a location external to the element; and at least one outward-facing surface of said fluoropolymer film envelope being adapted to be contiguous with a liquid-channel-forming component. In use liquid to be deaerated is passed over the outward-facing surface of the element at a pressure higher than the pressure inside the element. The gas channels formed by the gas-channel-forming component provide pathways through the inside of the envelope for gases which have permeated from the liquid through the film forming the envelope, to the port. The port provides a means for transfer of gases to a location external to the apparatus. The liquid-channel-forming component provides pathways for liquid to pass over and to contact the outward-facing surface of the element. When the element is formed into a spiral or folded structure, the liquid-channel-forming component also serves as a spacer between adjacent layers of the element. According to a further aspect of the present invention, there is provided a deaeration assembly for removal of a gas from a liquid comprising a deaeration element in accordance with the first aspect of the invention, and a liquid-channel-forming component contiguous with at least one outward-facing surface of said fluoropolymer film envelope. Further aspects of the invention are described in the dependent claims. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a cross-sectional view of the element containing a gas-channel-forming component enclosed by an envelope having an unreinforced edge seal; Figure 2 is a partial cross-sectional view of the element with an envelope having a reinforced edge seal; Figure 3 is a cross-sectional view of a gas-channel-forming component; Figure 4 is a cross-sectional view of a deaeration element containing a gas-channel-forming component; Figure 5 is a cross-sectional view of an example of a connection between a gas removal tube and the envelope at a port in the envelope; Figure 6 shows a spirally-wound element with a liquid-channel-forming component interposed between adjacent outer surfaces of the element; Figure 7 shows a folded element with a liquid-channel-forming component interposed between adjacent outer surface of the element; Figure 8 is a cross-section view of a module containing an embodiment of the assembly of the invention; With reference to the figures, the invention will be described in detail. To facilitate understanding, the same numerical identifiers for elements common to the figures will be used through the figures. The term "non-porous" is used herein simply to describe a material which is essentially free of pores or voids, and which is a barrier to bulk flow of liquids or gases. While a material may be non-porous, it may still be "permeable" to liquids or gases. The term "permeable", (and correspondingly "impermeable"), or a variation thereof, is used herein to describe the property of a material to transport (or not transport) a particular species, such as gas or water-vapor, through the material. The term describes the overall property of mass transfer by diffusion at a molecular level, and in no way implies any particular scientific mechanism by which this occurs. In Figure 1 is shown a cross-section of a deaeration element 1 which includes an envelope 4 made of a single sheet of non-porous fluoropolymer film. The envelope encloses a porous gas-channel-forming material 2 on each side of which is laminated a porous polymeric membrane 3. The overlapping edges of the fluoropolymer film envelope 4 are sealed at an edge region 5 which extends along the length and across the ends of the deaeration element 1. Non-porous fluoropolymer films are used to make the envelope 4 of the deaeration element 1 due to their well known chemical inertness, i.e., they are highly resistant to attack by aggressive chemicals, solvents, oils, and high purity water or other aqueous liquids, and thereby minimize contamination of liquids in contact with them. Many fluoropolymers can be used so long as non-porous films made of them have sufficient chemical resistance to the liquids to which they will be exposed and are sufficiently permeable by the gases dissolved or entrained in the liquids. Preferably, the fluoropolymers are melt-processible thermoplastic fluoropolymers such as tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-(perfluoroalkyl) vinyl ether copolymer (PFA), amorphous fluoropolymers, such as TEFLON AF® amorphous fluoropolymer, and the like. Such fluoropolymers are well know in the art and are readily available in sheet and film form from a number of suppliers. The thickness of the non-porous fluoropolymer film influences properties such as gas permeation rate, strength, processability, durability in use, etc., and may necessitate some compromise or trade-offs between desired properties. The film used to make the envelope 4 should be in the range 5 to 100 micrometers thick, and preferably in the range 10 to 40 micrometers thick. If the film is less than about 5 micrometers thick, it will be difficult to handle and will lack pressure resistance and durability in use. On the other hand, if the film is thicker than about 100 micrometers, gas permeation rates through the film may be too low to be useful. There are no particular limitations on the length and width dimensions of the envelope 4 except as dictated by deaeration performance desired and availability of materials. Typically, the envelope 4 should be in the range 10 to 100 centimeters wide and in the range 2 to 20 meters long. As the element 1 is typically operated at a pressure differential between the outside and inside of the element, the inside being at a lower pressure, there are some practical limitations imposed due to pressure drop across the element walls or through the interior of the element. If the envelope is too long, it will be difficult to maintain the desired pressure differential across the walls of the element over its full length. On the other hand, if the envelope is too short there may not be sufficient surface area available to achieve the desired gas permeation rate. Nevertheless, even with these considerations, there is considerable flexibility in choosing suitable length and width dimensions for the envelope 4. The non-porous fluoropolymer film envelope 4 can be made from a single sheet as shown in Figure 1, or can be made using two sheets of non-porous fluoropolymer film, in which case the seal region 5 extends around the entire periphery of the element. Alternatively, thin-walled tubes of a melt-processible thermoplastic fluoropolymer can be extruded or blow-molded and flattened to form the envelope 4. In this case, only the ends will have a seal region 5. Sealing the open edges of an envelope 4 made of a melt-processible thermoplastic fluoropolymer film can be readily accomplished by application of heat and pressure at seal regions 5 where the film overlaps the porous gas-channel-forming component 2. Many heat sealing methods are known in the art and can be used. An alternative seal region 15 is illustrated in Figure 2. In this configuration reinforcing strips 6 of porous polytetrafluoroethylene (PTFE) film are placed on the outward-facing surfaces of the envelope 4 at the seal region 15 prior to heat sealing. When heat sealing is performed some of the melt-processible fluoropolymer film penetrates into the porous PTFE film and the strength and reliability of the seal are enhanced. The porous PTFE film also serves as an excellent release material which prevents contact between melted thermoplastic fluoropolymer and equipment surfaces applying heat and pressure to the seal region 15. Preferably, the porous PTFE film is porous expanded polytetrafluoroethylene film. The interior of the deaeration element 1 contains a gas-channel-forming component 2 which provides pathways through the inside of the element for gases which have permeated from the liquid through the film forming the envelope 4, to at least one port through the film envelope 4 which is connected to means for transfer of gases to a location external to the apparatus. The gas-channel-forming component must be able to withstand the compressive forces exerted on it, be compatible with the gases to be removed from the liquid, and, within these limitations, have a structure as open or porous as possible so as to minimize the pressure drop through the interior of the element 1. Preferably the gas-channel-forming component 2 is made of a synthetic polymer, although other materials can also be used. Suitable polymeric material and forms are known in the art and are available commercially. Suitable materials include polymers such as polyolefins, polyesters, nylons, polyurethanes, polycarbonates, polystyrenes, polyvinyl chloride, polyvinylidene chloride, and the like; or fluoropolymers such as PTFE, FEP, PFA, polyvinylfluoride, polyvinylidene fluoride, and the like. Suitable forms include nonwoven fabric, knit fabric, woven fabric or mesh, open-cell foams, porous membranes, and the like. The thickness of the gas-channel-forming component 2 is preferably in the range of about 0.3 millimeters to about 2 millimeters, and should have length and width dimensions somewhat less, about 2 millimeters to about 10 millimeters less, than the length and width dimensions of the envelope 4 in order to provide sufficient area to form the seal region 5. It can be desirable to laminate a porous membrane 3 to both sides of the gas-channel-forming component 2, to form a subassembly as shown in Figure 3; or to one side of the gas-channel-forming component 2, as shown in Figure 4, where the subassembly is shown positioned in the fluoropolymer envelope 4 of the element 1. The porous membrane 3 provides support to the non-porous fluoropolymer film forming the envelope 4 and helps to more uniformly distribute the compressive load on the gas-channel-forming component 2 encountered during operation. By providing such support to the fluoropolymer film, a thinner film can be used to form the envelope 4, thereby increasing the gas permeation rate from the liquid outside the element to the gas channels inside the element. Preferably the porous membrane 3 is also made of a synthetic polymer and its selection is subject to the same constraints listed above for the gas-channel-forming component. Most preferred are porous membranes of polytetrafluoroethylene. Porous polytetrafluoroethylene sheet or film suitable for use in the invention can be made by processes known in the art, for example, by stretching or drawing processes, by papermaking processes, by processes in which filler materials are incorporated with the PTFE resin and which are subsequently removed to leave a porous structure, or by powder sintering processes. Preferably the porous polytetrafluoroethylene membrane is porous expanded polytetrafluoroethylene film having a structure of interconnected nodes and fibrils, as described in US-A-3,953,566 and 4,187,390 which describe the preferred material and processes for making them. The porous membrane should have a pore volume in the range of about 30 to 95 percent, a nominal pore size in the range of about 0.1 to 100 micrometers, and be about 5 to about 100 micrometers thick. Lamination of the porous membrane 3 to the gas-channel-forming component 2 can be done using conventional methods and equipment, for example, by adhesive bonding. The adhesive can be applied to the surface to be bonded of either layer, and should be applied in a non-continuous pattern. A non-continuous pattern of adhesive is used herein to indicate a layer of adhesive which is applied to a surface so as to not form a non-porous continuous film. For example, a layer applied to a surface as a pattern of discrete dots, a porous non-woven web or mesh, or the like. The adhesive may be selected from many known in the art. The adhesive can be a thermoplastic, thermosetting, or reaction curing type, in liquid or solid form, selected from the classes including, but not limited to, polyamides, polyacrylamides, polyesters, polyolefins, polyurethanes, and the like. The adhesive should be applied so that it forms a porous (non-continuous) gas-permeable layer which minimizes resistance to air flow while adhering the porous membrane 3 to the gas-channel-forming component 2. Preferably, the adhesive is applied so as to cover about 30 percent or less of the surface. Suitable application means include gravure printing, spray coating, powder coating, interposing a non-woven web of adhesive, and the like. Lamination of the porous membrane 3 to the gas-channel-forming component 2 can be also be done using conventional heat-fusing methods and equipment, for example, by application of heat and pressure in the nip between rolls, or by a heated platen press. One or more ports, or openings, are provided in the element 1 for passage of gases out of the element. Transfer means, preferably a tube of the same melt-processible thermoplastic fluoropolymer used to form the envelope 4, for removal of gases from inside the element to a location external to the module or apparatus in which the element 1 is positioned is connected to the port by any convenient method. The outside diameter of the tube should be about 4 millimeters to about 10 millimeters, and the wall thickness about 0.5 millimeters to about 1 millimeter. By way of example only, one such connection is illustrated in Figure 5. In Figure 5 is shown a section of one wall of the melt-processible thermoplastic fluoropolymer film envelope 4 in which an opening 22 leading to the inside of the element has been made. An end of a tube 23, preferably of the same fluoropolymer as the envelope, is positioned around the opening 22. A fluoropolymer ring 27 and porous PTFE film 26 are pressed into place around the end of the tube 23 so that, when finally positioned, the film forming the envelope 4 and the PTFE film 26 overlap the end of the tube 23, and the edge of the ring 27 is approximately even with the end of the tube 23. The portion of the envelope 4 overlapping the end of the tube 23 is heat sealed to the end of the tube by application of a heated plate (not shown) to the PTFE film portion overlapping the end of the tube, and a strong air-tight seal is formed. The fluoropolymer ring 27 serves as a strain relief for the connection and also prevents distortion or contraction of the envelope film during the heat sealing step. The porous PTFE film 26 serves to reinforce the seal region and serves as a release material to prevent melted fluoropolymer from sticking to the heated plate. To most efficiently use the element, it may be positioned in a module or apparatus in a spiral-wound configuration, as shown in Figure 6; or in a folded configuration, as shown in Figure 7. In such configurations a space between adjacent element layers must be provided to permit the liquid to be deaerated to contact the outer surface of the element and to permit the liquid to flow through the module. This space can be provided by interposing a liquid-channel-forming component 12 between adjacent layers of the element 1, as shown in Figures 6 and 7. The void size and distance between element layers provided by the liquid-channel-forming component 12 should be in the range 50 to 1000 micrometers, preferably in the range 100 to 400 micrometers. If the spacing is larger than 1000 micrometers, deaeration performance will suffer as the diffusion distance for the gas through the liquid will become too great. If the spacing is less than 50 micrometers, the pressure drop of the liquid through the liquid-channel-forming component may become too high. The length of the liquid-channel-forming component should be roughly the same as the length of the deaeration element. The width of the liquid-channel-forming component should be at least as wide as the deaeration element, and may be somewhat wider (about 1-2 centimeters) so as to extend beyond and protect the longitudinal edges of the fluoropolymer film envelope. As with the gas-channel-forming component described earlier, suitable materials include polymers such as polyolefins, polyesters, nylons, polyurethanes, polycarbonates, polystyrenes, polyvinyl chloride, polyvinylidene chloride, and the like, or fluoropolymers such as PTFE, FEP, PFA, polyvinylfluoride, polyvinylidene fluoride, and the like, in forms such as nonwoven fabric, knit fabric, woven fabric or mesh, and the like. In systems in which chemically aggressive and high purity liquids, or special liquids in which contamination must be minimized, it is preferred that the liquid-channel-forming component be made of a fluoropolymer such as PTFE, PFA, FEP. A cross-sectional view of a deaeration apparatus or module containing a spiral-wound assembly of the invention is shown in Figure 8. The module 41 has a cylindrical casing body 41a, and end caps 41b at each end. A spirally-wound element 1, with a liquid-channel-forming component 12 interposed between adjacent layers of the element, is disposed in the casing body. A gas removal tube 23 is connected to the element 1 and exits the module through a fitting in an end cap 41b. An inlet opening 46 for a liquid to be deaerated, and a vent opening 48 for removal of air during initial filling of the module with the liquid, are provided in one end cap. In the opposing end cap is an outlet opening 47 for liquid which has been deaerated. At each end of the spiral-wound assembly is a porous spacer element 49 which serves both to space the assembly within the module and provides passages for liquid. The module shown is a conventional type, well known in the art, as are the materials and constructions methods to make it, which are selected according to the fluids and operating conditions which will be encountered in the projected end-use. Again, in systems in which chemically aggressive and high purity liquids, or special liquids in which contamination must be minimized, it is preferred that the module be made of a fluoropolymer such as PTFE, PFA or FEP, or the liquid-wetted surfaces be lined with a fluoropolymer. After the module is initially filled with a liquid to be deaerated, and trapped air exhausted through the vent opening 48, liquid flow through the liquid-channel-forming component is begun and the interior of the element 1 is operated at a pressure lower than the pressure of the liquid flowing over the outer surface of the element, for example, by drawing a vacuum through the gas removal tube 23. Gas dissolved in the liquid, driven by the pressure differential between the liquid and the interior of the element, diffuses out of the liquid and permeates through the fluoropolymer film forming the envelope of the element, passes through the gas channels formed inside the element to the gas removal tube, and thence out of the module. At the same time, the dissolved gas concentration in the liquid becomes progressively lower as the liquid flows over and past the surface of the deaeration element. The deaeration assembly of the invention accomplishes the efficient removal of gases dissolved not only in ordinary liquids such as water or aqueous solutions, but also in chemically aggressive, high purity, and other special liquids, while contributing virtually no contaminants to the liquids. Example 1 On each side of a 250 micrometers thick polyester knitted fabric (Stock No. 2020, 20-denier staple fiber, made by Toray Co) was laminated a 30 micrometers thick porous expanded PTFE membrane (made by Japan Gore-Tex, Inc.) having a pore volume of 82%. The laminate thus produced was cut to make a gas removal subassembly 20 centimeters wide and 9 meters long of the type shown in Figure 3. The subassembly was placed on a 12.5 mm thick FEP film (made by Daikin Industries) which was then folded over the subassembly, and the open ends and longitudinal edge heat-sealed to form an element about 20 cm wide and 9.3 m long of the type shown in Figure 1. An FEP tube, 6 mm outside diameter and 4 mm inside diameter, was connected to one end of the element to serve as a gas removal tube. The tube was connected and heat-sealed to the element by the method shown in Figure 5. A woven mesh of PFA (made by Gunze Co.), about 8 mesh/cm (20 mesh/inch) and 0.51 mm thick, was cut to a width of 24 cm and a tenth of 9.3 m to serve as a liquid-channel-forming component. The liquid-channel-forming component was placed on and aligned over the element, and the element, with the liquid-channel-forming component interposed between layers, was rolled up (as shown in Figure 6) to form a spiral-wound deaeration assembly of 101.6 mm diameter and 24 cm length which provided 3.72 m 2 of surface area for liquid contact. The deaeration assembly thus made was installed in a module of the type shown in Figure 8 which had a casing body and end caps made of PTFE. The deaeration assembly was tested by flowing tap-water having an initial dissolved oxygen concentration of 8.2 ppm through the module at a rate of 300 cm 3 /minute at 25°C. The pressure inside the element was reduced and kept at about 100 Torr by a vacuum pump. The dissolved oxygen concentration of the deaerated liquid at the outlet of the module was measured and found to be 2.9 ppm, which is a good value. A second test under the same conditions except that tap-water containing 9% neutral detergent (with surfactant) was used. The dissolved oxygen content of the deaerated liquid at the outlet was found to be 2.95 ppm, which is also a good value. After the second test, the module was emptied of water and filled with 98% ethyl alcohol. The pressure inside the element was reduced to 100 Torr. No alcohol was detected in the gas sampled from the gas removal tube, confirming that the alcohol liquid did not permeate through the FEP deaeration film. Hot water (90°C) was also passed through the module, after which the apparatus was disassembled and the various components inspected. No change whatsoever was seen in any of the materials. It is apparent from the above that the deaeration assembly of the invention permits deaeration of a liquid containing a surfactant, and that there is no problem with washing the assembly with 90°C hot water. Furthermore since the deaeration apparatus used in this example makes use of FEP, PFA, or PTFE for all its liquid-wetted parts, it can also be used with chemically aggressive liquids which are strongly acidic or alkaline. Example 2 A deaeration assembly was made as described in Example 1, except that the FEP film was 25 micrometers thick. The deaeration assembly was installed in the module and the same tests performed as described in Example 1, except that the liquid flow rate was 200 cc/minute. The dissolved oxygen content of the deaerated liquid at the module outlet was found to be 2.95 ppm, which is a good value. The results of the other tests were the same as described in Example 1. Example 3 A deaeration assembly having the same structure and dimensions as described in Example 1 was made, the only difference being that the FEP film was 25 micrometers thick and was extruded in tubular form. A jig was prepared and the gas removal subassembly was drawn into the tube, thus only the ends of the tube required sealing. This procedure greatly simplified and shortened the production process to form the deaeration assembly. The deaeration assembly thus made was installed in a module and tested as described in Example 1. The dissolved oxygen content of the deaerated liquid at the module outlet was found to be 2.95 ppm, which is a good value. The results of the other tests were the same as described in Example 1.

Claims: A deaeration element (1) for a deaeration assembly, said element having a gas-channel-forming component (2) with a porous membrane (3) laminated to one or both sides thereof, said gas-channel-forming component (2) being enclosed and sealed within an envelope (4) formed of a non-porous fluoropolymer film, said envelope having inward-facing and outward-facing surfaces and at least one port (22) leading from the inside to the outside of said envelope (4) for passage of gases permeating into the element (1) to a location external to the element; and at least one outward-facing surface of said fluoropolymer film envelope (4) being adapted to be contiguous with a liquid-channel-forming component (12). A deaeration assembly for removal of a gas from a liquid comprising: a) a deaeration element (1) in accordance with claim 1; and b) a liquid-channel-forming component (12) contiguous with at least one outward-facing surface of said fluoropolymer film envelope (4). A deaeration assembly as claimed in claim 2, wherein said envelope (4) is formed of a non-porous film of tetrafluoroethylene-hexafluoropropylene copolymer. A deaeration assembly as claimed in either of claims 2 or 3, wherein said liquid-channel-forming component (12) is formed of a fluoropolymer material. A deaeration assembly as claimed in any of claims 2 to 4, in which said gas-channel-forming component (2) has continuous interconnected pores and passageways throughout its structure to facilitate the passage of gases therethrough. A deaeration assembly as claimed in any of claims 2 to 5, wherein each channel-forming component (2, 12) comprises at least one synthetic polymeric porous material in the form of a nonwoven fabric, knit fabric, woven fabric, woven mesh, or a porous membrane.

Inventors: HAMASAKI SADAKATSU, KOBAYASHI MASAYUKI, HAMASAKI, SADAKATSU, KOBAYASHI, MASAYUKI
Applicants: JAPAN GORE TEX INC, JAPAN GORE-TEX, INC.
Application year: 1997
Main classification: B01D19
IPC codes: B01D19, B01D71, B01D63