IL30907A - Disposable membrane,structure,method of making membrane structure and apparatus for removing dissolved material from a liquid by reverse osmosis - Google Patents

Description

DISPOSABLE MEMBRANE* STRUCTURE, METHOD OF MAKING MEMBRANE STRUCTURE AND APPARATUS FOR REMOVING DISSOLVED MATERIAL FROM A LIQUID BY REVERSE OSMOSIS n**BN>> no*© n jn ( runs oo) n*m-ip ^ΛΪ» >Ti30 o^osioi: ο'τβιη n«sin> i rim n'onpn njao nsiDn ntiao wa The presai t invention resides in an improved tubular membrane structure for use in reverse osmosis purification equipment.

More specifically, the invention provides ;a new and improved membrane structure that may be easily inserted arid re¬ moved from equipment in the field, is economical in cost,' does not require costly processing at the field site, and which may be easily shipped o The' invention also resides in a method of making a membrane structureo Because of the ever-increasing water shortage problem* considerable' effort has been expended to develop economical methods and apparatus for removing the salt from seawater or for, purifying brackish water. Thus far, most success has been achieved by distillation methods or apparatus or by electrodi-alysis methods and apparatus. However, considerable promise also exists in the process of reverse osmosis for economically providing potable water.

In order to purify salt water or brackish water by reverse osmosis, the salt containing water in contact with a semi-permeable membrane must be subjected to pressure in excess of the osmofcic pressure of the salt water. For a typical concentration of salt in seawater, the osmotic pressure is on the order of 350p3ig and accordingly, a pressure differential in excess of that figure must be maintained across the membrane. In this respect, it is not unusual to provide a pressure differential on the order of about 1000 psig and accordingly, pressures.

Typically the membranes, which are formed of well-known materials, are about 0.004 Inch thick and are supported by a strong porous backing which does not readily compress under pressure and close its pores. or capillaries so that the potable water may flow therethrough to be collected for use. Because, in practice, the membrane Itself may tend to compress after use periods on the order of two to three months, the membranes must be periodically replaced in order that the pro-cess may continue at an economical rate.

Currently, most equipment for purifying water using the reverse osmosis principle utilize tubular membranes which are supported in rigid tubes and the membranes are formed by two basic methods. One such method requires that the membrane be cast as an integral part of the supporting tube while the other requires that the membrane be formed in a casting tube. The membrane thus formed is removed and its central portion is wrapped in a porous cloth. The resulting structure is pulled into a supporting tube and cured in place. The unwrapped ends of the membrane are then flared for sealing purposes.

In the case of the first method, in order to replace the membranes, the used membrane must first be dissolved out of the supporting tube and a new membrane be cast internally, quenched in cold water and cured in hot water to a very exact-ing degree. Such operations are extremely involved, controlled processes and do not lend themselves to be performed in the field at the site of a reverse osmosis desalinlzation plant. Thus, such desalinlzation membrane modules must either be scrapped or shipped back to the factory for the replacement process, either being a costly procedure. method, the membranes themselves can only be replaced by removing the old cloth and membrane structures from the tubes, wrapping new membrane structures in the cloth, pulling them into the support structures and heat curing the resultant structures by carefully controlled process. Again, such a process is not Conducive to field operations.

Furthermore, because membranes would require shipping to the site, another disadvantage presents itself. More specifically, the membranes are very fragile, are limp in nature, and are difficult to handle especially in long lengths, andean easily be damaged. As a result, to replace membranes formed according to the second method, the membrane modules must be shipped to a ffactory for replacement.

The present invention provides a disposable semipermeable membrane structure for use within a rigid support tube in reserve osmosis liquid mixture separating equipment, comprising an elongate, thin-walled tube havinga^membrane film of material semi-permeable to the liquid mixture bonded to the internal surface thereof, the tube being formed of a porous material capable of supporting the membrane film during transit of the membrane structure but incapable of withstanding differential pressure across its internal and external surfaces of the order of the osmotic of the liquid mixture to be processed.

The present invention also provides a method of making a semi-permeable membrane structure for use in removing dissolved material from a liquid solution by reverse osmosis, face and an external surface from a porous, semi-rigid material, bonding a film of a material semi-permeable to the liquid solution to the internal surface, and forming a plurality of grooves in the external surface along the length thereof.

Other objects and advantages will become apparent from the following specification taken in conjunction with the accompanying drawings in which: Figure 1 is a perspective view of the membrane module made according to the invention with parts shown in section; Figure 2 is a fragmentary sectional view of a portion of a desalinization device operative on the principle of reverse osmosis and including a membrane module made according to the invention; Figure 3 is a fragmentary sectional view taken approximately along the line 3-3 of Figure 2; Figure 4 is an enlarged, fragmentary, cross section of a modified membrane module; Figure 5 is a rear elevation of the modified membrane module undergoing a forming step during its manufacture; and Figure 6 is a fragmentary vertical section taken approximately along the line 6S6 of Figure 5.

An exemplary embodiment .of an expendable membrane structure, generally designated 10, made according to the invention is illustrated in Figure 1 and is seen to comprise an outer, generally cylindrical, semi-rigid, thin-walled tube 11 having bonded to its interior surface, a tubular film of semi bonded to the tube 11 by conventional casting techniques.

The tube 11 Is formed of any suitable porous material such as paper or porous plastic having as a desirable characteristic the ability to withstand compression to maintain poro- sity over a substantial period when subjected to relatively high pressures. While the material forming the tube 11 should not compress under high pressures, the tube 11 itself is of a nature such that it, alone, is incapable of withstanding high pressures and would rupture when subjected to the same, The ends of the tube 11 are dipped in a liquid impervious plastic of any suitable composition so as to provide a liquid impervious coating 14 at the ends prior to the bonding of the film 12 to the tube 11 for sealing purposes. The resulting tube structure will permit potable water diffusing through the tubular film 12 to flow not only radially through the pores of the tube 11, but also in a direction parallel to the longitudinal axis of the tube 11 until such time as it reaches a barrier in the form of the liquid impervious coatings 14.

Turning now to Figure 2, one form of an apparatus for utilizing the expendable membrane structure 10 is illus--trated. The apparatus comprises a plurality of rigid tubes 16 capable of withstanding the relatively high pressures present in liquid purifying equipment utilizing the reverse osmosis principle and which support the expendable membrane structure 10 along the major portion of the length of the latter to preclude the same from rupturing.

The rigid support tubes 16 have their ends in turn received and supported within bores 18 in headers 20 (only one of which is shown) . The bores 18 at their outer extremity a manifold for conducting potable water that has diffused through the film 12 to a point of collection for subsequent use. More specifically, the expendable membrane structures 10 are received within the support tube 16 such that the ends of the former including the coating 14 and an uncoated portion 24 adjacent thereto extend from the support tube and outwardly from the annular groove 22 such that the innermost edge of the coating 14 is located outwardly of the outer edge 26 of the header 20. As a result of this construction, it will be appar-ent that a liquid to be purified within the membrane structure 10 will, when subjected to pressure in excess of its osmotic pressure, by the process of reverse osmosis, diffuse through the film 12 into the porous tube 11 to flow parallel to the longitudinal axis of the membrane structure 10 to be received within the annular groove 22 .

In order to direct a liquid to be purified to the internal surface of the membrane structure 10, each header 20 is provided with an end cap 28 which may Include channels 30 for conducting the liquid to be purified through various ones of the membrane structures 10 in their respective support tubes 16 . The end caps 28 are maintained in place on the headers 20 by means of bolts 32 (only one of which is shown) extending through a bore 34 in the end cap and a complementary bore 36 in the header 20. A nut 38 is threaded upon the bolt 32 to tightly clamp the end cap 28 to the header 20. In order to seal the interface between the header 20 and the end cap 28, a suitable gasket 40 may be provided. The gasket 40 not only seals the interface as mentioned, but additionally is arranged in conjunction with washers 42 to provide a product channel for the purified liquid. flat side 44 embracing the end cap 28 and a grooved side 46 having a plurality of radial grooves 48 separated by lands 50 (Figure 3 ) . The washers 42 surround the projecting end of the membrane structure 10 adjacent the Interface between the coat- lng 14 and the uncoated portion 16 so that purified liquid received within the annular groove 22 will flow through the grooves 48 into a space 52 between the end cap 28 and the header 20 for subsequent collection.

In order to insure that undesirable mixing of product liquid and liquid to be purified does not take place through fluid communication between the channel 30 which contains the liquid to be purified and the space 52, the end cap 28, near the point of emergence of the channel 30, includes an annular groove 54 which receives an 0-ring 56 or the like. The 0-ring 56 bears against the coating 14 on the end of the membrane structure 10 to provide a seal. As a result, the only liquid received within the space 52 is product liquid which diffuses through the film 12 and flows laterally through the porous, thin walled tube 11 when reverse osmosis is taking place.

The space 52 may be common to several of the support tubes 16 and their corresponding membrane structures 10 to serve as a manifold for collection of the purified liquid. In order to deliver the purified liquid received within the space 52 to a point of use, a bore 58 in the end cap 28 is provided which communicates with the space 52.

The just-described construction provides several advantages over those heretofore used. For example, by removal of the end cap 28,' new, unused membrane structures 10 may be easily placed by simple insertion into the support tube 16 at the field site, there being no need to remove the support tube thereon. In a like manner, the thin walled, semi-rigid tube 11 supports the membrane film 12 during transit and renders the handling thereof a simple matter in contrast to prior art membranes wrapped in; a cloth.

The features of construction including the porous nature of the semi-rigid, thin walled tube 11, which allows lateral flow of purified water together with the presence of the liquid impervious coating 14 allows the purified liquid to be collected internally of the basic apparatus including the support tubes 16 of the header 20 and the end caps 28. Accordingly, the purified liquid is less subject to contamination after it is diffused through the film 12 than would be the case in prior art apparatus wherein the support tubes 16 are apertured or porous to permit exit of the purified liquid therefrom for subsequent external collection.

Furthermore, an expendable membrane structure made according to the invention when used in the above -described apparatus can materially reduce the capital costs of a de-sallnization plant insofar as product liquid pumps need not be required. More specifically, in the prior art devices, support tubes for the membrane structure are customarily perforated to allow the diffused, purified liquid to exit from the supporting tubes for collection. In order that liquid thus collected may be conveyed to a point of use, some sort of pumping means are required.

In contrast, with an apparatus made according to the invention, since all diffused, purified liquid will be collected in one or more of the confined spaces 52 and access may be had to the same through the bore 58, by suitable piping the liquid flowing from the latter may be directed, without the sure applied to the liquid to be purified will cause the liquid to be purified to diffuse through the membrane and as the same accumulates after diffusion, a pressure will build up, which pressure will serve to cause the purified liquid to flow . through such piping as may be provided to the point of use.

While those skilled in the art will be aware- of many materials that may be used for fabricating the thin walled tube 11, the membrane film 12 and the coating 14, it has been found that satisfactory results may be obtained in brackish water desalinizatlon if the thin walled tube 11 is formed of a paper in which the cellulose fibers have been coated with a polycarbonate or polystyrene resin. For that matter, any paper in which the cellulose fibers have been coated with a plastic material without rendering it impervious to liquid flow may be used. Alternatively, a material composed of plastic fibers oriented in a random fashion and bonded together by conventional lay up techniques to form a porous structure that will permit the flow of water may be used. Additionally, the thin walled tube 11 may be formed of insoluble particles that have been bonded or cemented together by heat, physical processes or by chemical processes may be used. Finally, material which has been rendered porous to permit water to flow through its structure by means of selective solvent action of chemical or the release of volatile components due to aging in the atmos-phere, or the application of heat or a partial or full vacuum treatment may be used.

Preferably, such a structure will have a wall thickness in the range of 0.025 to 0.050 inches. Similarly, the membrane film 12 may be formed of cellulose acetate by conven-tional, well-known casting techniques to have a wall thickness Finally, as mentioned previously, the coating 14 may be formed by simply dipping the ends of the membrane structure 10 into a liquid impervious plastic such as polystyrene, polycarbonate or epoxy resins, A modified embodiment of a membrane to be made according to the invention is illustrated in Figure 4 and is seen to comprise a thin walled, semi-rigid tube, generally designated 70 which is formed of three distinct layers 72, 74 and 76 of a paper whose fibers have been coated with a plastic such as polycarbonate. As in the case of the membrane structure 10, the inner wall of the semi-rigid tube 70 has a semipermeable membrane 78 bonded thereto by conventional casting techniques. The material of the membrane film 78 may be identical to that used in forming the membrane structure 10.

The outer surface 80 of the semi-rigid tube 70 is provided with a plurality of grooves 82 which will be located at the interface between the tube 70 and the rigid support tube, such as the tube 16, within which the tube 70 is received for performance of a reverse osmosis purification method. De-pending upon the diameter of the semi-rigid tube 70, any number of grooves 82 may be provided. In one embodiment wherein the diameter, of the semi-rigid tube 70 is about one -half inch, sixteen such grooves are provided.

The primary purpose of the grooves 82 is to provide for increased longitudinal flow of purified water between the semi-rigid tube 70 and the support tube, such as tube 16 in Figure 2, in the reverse osmosis apparatus with which the membrane structure is to be used. Additionally, the presence of the grooves 82 in the outer surface of the semi-rigid tube 70 tend to provide the membrane structure with better composite The method by which the membrane structure illustrated in Figure 4 is formed, with the exception of the formation of the grooves 82 is identical to the method by which the membrane structure 10 (Figure ) is formed. The manner in which the grooves 82 are formed in the semi-rigid tube 70 (Figure 4) can best be understood by reference to Figures 5 and 6.

As illustrated in Figures 5 and 6, a circular die, generally designated 84 is provided and is seen to include a center cavity 86 through which the semi-rigid tube 70 may be drawn. The inner cavity 86 is defined by a generally circular wall 88 which is continuous in nature except for the presence of Inwardly extending embossing ridges 90. The number of embossing ridges 90 may be selected to provide the desired number of grooves 82.

At one end of the die ,84 the embossing ridges 90 are tapered outwardly as at 92 to terminate in a manner such that the corresponding end of the cavity 86 has a diameter no less than the outer diameter of the semi-rigid tube 70 to facilitate introduction of the semi-rigid tube into the cavity 76. Pref-erably, the same end of the die 84 and the cavity 86 thereof includes relieved portions 92 which may be semi-circular in cross section and which taper to the substantially continuous circular portion to facilitate introduction of the tube into the die cavity 86.

To provide the grooves 82 in the semi-rigid tube 70, the die 84 is heated and the tube 70 introduced therein and drawn through the cavity 86. The heat applied to the die 84 is transmitted thereto to the embossing ridges 92 and thence to the semi-rigid tube 70 to heat the plastic coated paper fibers forming the semi-rigid tube 70. As a result, the plas heat and flows to the walls of the grooves 82 being formed by the embossing ridges 90. Subsequent cooling fixes the plastic and causes the membrane structure to be provided with the membrane grooves as Illustrated In Figure 4 .

As a result of the method, the grooves 82 are provided and It has been found that the same do not collapse even under extended exposure to the typical 1, 000 pslg pressure to which the membrane structure is exposed during operation of reverse osmosis apparatus with which it may be used.

In one embodiment, each of the layers 72, 74 and 76 forming the semi-rigid tube 70 were formed of a 7-9 mil porous paper impregnated with a polycarbonate plastic and grooves as deep as 18 mils in a 27 mil overall tube wall thickness were formed. Die temperatures in the range between about 400° to 700° Fahrenheit were utilized and tube velocity during the drawing operation was varied to obtain smooth drawing and optimum flow of the softened plastic dependent upon die temperature .

After the grooves 82 are formed, the ends of the tube 70 may be coated with a liquid Impervious coating as the coating 14 for sealing purposes in an apparatus such as that illustrated in Figures 2 and 3 .

From the foregoing, it will be appreciated that the modified membrane structure illustrated in Figures 4-6 pro-vides significant advantages in that the grooves 82 permit fE>ee longitudinal flow of the product water within the supporting tubes of the reverse osmosis apparatus with which the membrane structure is to be used and, like the membrane structure 10, obviates the need for costly drilling operations to provide perforations in the supporting tubes. Additionally, structure tend to increase the flow of product water through the membrane to a rate of and above obtainable with the membrane structure 10 and accordingly, the grooved structure is preferable to the former. Finally, the secondary benefit pro-vided by the presence of the grooves 82, that of strengthening the membrane structure, provides significant advantages in terms of minimizing the number of membrane structures damaged in transit because of the greater strength of the membrane structure.

Having described a specific embodiment of the invention, we do not wish to be limited to the details set forth, but rather, to have the invention construed broadly in accordance with the following claims.

Claims (8)

1. 30907/2 '.: 1. A disposable semi-permeable membrane structure for use within a rigid support tube in reverse osmosis liquid mixture separating equipment, comprising an elongate, thin-walled tube having a membrane film of material semi-permeable to the liquid mixture bonded to the internal surface thereof, the tube being formed ,of a porous material capable of sup-porting the membrane film during transit of the membrane structure but incapable of withstanding differential pressure across its internal and external surfaces of the order of the osmotic pressure of th liquid mixture to be processed „

2. The membrane structure according to claim 1, wherein said porous material comprises paper, paper in which the cellulose fibers have been coated with a plastics material, or ith a polycarbonate or polystyrene resin, insoluble particles bonded together,, or material rendered porous by means of selective solvent action of a chemical.

3. The membrane structure according to claim 1 or 2, wherein said film covers the entire internal surface of the tube.

4. „ The membrane structure according to claim 1, 2 or 3, wherein the ends of the tube are coated with a water impervious material.

5. The membrane structure accprding to any one of the preceding claims, wherein the external surface of the tube includes a plurality of grooves extending along the length thereof and adapted to conduct liquid longitudinally of the tube. sow ?

6. A method of making a semi -permeable membrane structure for use in removing dissolved material from a liquid solution by reverse osmosis , comprising the steps of forming a tube having an internal surface and an external surface from a porous , semi -rigdid material, bonding a film of a material semi-permeable to the liquid solution to the internal surface, and forming a plurality of grooves in the external surface along, the length thereof .

7. The method according to claim 6, including the steps of coating the ends of the tube with a liquid impervious coating.

8. The method according to claim 6 or 7, wherein the material from which the tube is formed is a plastic coated paper and said grooves are formed by providing a die having an internal cavity vi h a plurality of embossing ridges, heating the die to a temperature in the range of about 400 to 700°p. , arid drawing the tube through the internal cavity of the die.