IL34681A - Membrane tube process for the manufacture of a reinforced semipermeable - Google Patents

Description

Process for the Manufac ure of a Beinforced Semipermeable Membrane Tube ΐ?ττποι n¾ns ττη »anaap max ms»» y nti This invention is concerned with a process for the manufacture of a reinforced serai-permeable membrane in tube form, which process can be continuously operated and yields a flexible membrane tube of any desired length.

It is known that very thin films of film-forming substances are semi-permeable and on account of this property can be used for technical purposes, e.g. for water desalination by pressure osmosis and for ultra-filtration.

Such film-forming substances may be organic compounds of high molecular weight, e.g. cellulose esters, water-insolujole cellulose ethers, polyamides, or other polymers.

Further it is known tnat membranes suitable for technical purposes can be produced from a solution of a film-forming substance, by applying the solution to a base material* evaporating part of the solvent from the surface of the coated layer, and then precipitating the film-forming substance from the residual solution forming the coated layer. In most cases, precipitation is caused by the action of a liquid which is miscible with the solvent contained in the solution of the ilm- orming substance, but in which the film-forming substance itdelf is either insoluble or only very sparingly soluble. By the partial evaporation of the solvent contained in the coated solution, an extremely thin skin is formed on the surface of the. coating, which skin is presumably the only semipermeable element of the membrane, whereas the layer formed beneath the skin, by precipitation, is probably permeable and represents only a supporting element for the semipermeable membrane.

Further, it is known how to influence the semipermeability conditions of the prevailing atmosphere, the volatility of the solvent used for the solution, and the composition of the solution may he of influence. Further, it is known that in addition to the film-forming substance, the solution may contain a salt of swelling action. Further, it is known that the permeability of the membrane may be varied by heating it in water or in air. These and other known proposals may be applied in the case of the membranes of the present invention. The fundamentals of the manufacture of membranes are described, e.g. in U.S.Patent Specifications Nos.3»290j286f and 3,250,701.

When semipermeable membranes are to be used for technical purposes, it is very often advantageous for the semipermeable membranes to be in the form of tubes. Therefore, it has already been suggested to produce semipermeable membrane tubes, but their manufacture has been somewhat complicated. In particular, it has hitherto only been possible to produce semipermeable membrane tubes of relatively short length, e.g. up to approximately 3 meters. In addition, the tubular sections have been of low mechanical strength, so that they have had to be reinforced by a permeable outer tube for storage and transport and in practical use. Semipermeable membrane tubes reinforced by an outer tube have already been prepared by producing stiff porous tubes from glass fibres bonded xiith synthetic resins and providing these tubes on their inner wall with a semipermeable membrane. However, membrane tubes of this type can also only be produced in limited lengths, and they require much space during storage and transport.

The present invention provides a process for the manufacture of a reinforced semipermeable membrane tubet wherein a coherent web of permeable material, while being conveyed in a downward direction, is continuously converted into a tube with a longitudinal seam, a layer of a solution of a film-forming substance suitable for the production of a semipermeable membrane is applied to the inside wall of the resulting tube, part of the solvent is evaporatedtfrom the surface of the said layer and the a fluid precipitating' agent is caused to act on said layer £ -l-a-£ozQai»g-substance._ia..than, pre i pitateri from -the-Teeidual thereby precipitating the film forming substance being solution, dissolved in said solution.

The tube formed from the permeable web in the process of the invention serves as support for the layer from which the semipermeable membrane is produced and as a permanent reinforcement of the resulting semi-permeable membrane. The permeability of the web should be such that it can be penetrated by a flowing liquid, such as water. The web may consist of a fibre fleece, a fabric of natural or synthetic fibres, or of a perforated plastics film. The web is bent to form a tube, and its edges are either butted and joined to form a longitudinal seam, or they overlap one another and are united by gluing or welding.

The application of the layer to the inside wall of the tube is advantageously performed by means of an annular slot die. The die is fixedly mounted in such a manner that it is in the interior of the tube, while its mounting projects sideways from the tube where the web has not yet been formed into a tube. In the course of its downward movement, the 3^681/2 tube carrying an inner coating reacheu a precipitating liquid which may be inside or outside of the tube? preferably, both an internal and an external precipitating liquid are provided.

Between the coating of the tube and the action of a precipitating bath, the tube passes through an evaporation zone of suitable length. While the tube passes through this zone, solvent evaporates from. the surface o the layer into the interior of the tube, which is filled with a mixture of gas and solvent vapours. Advantageously, the gas mixture present in the tube when it passes through the evaporation zone is agitated by blowing air into the lower portion of the evaporation zone. If desired* the air blown into the tube may already contain a proportion of the vapours which evaporate from the solvent up to the point of being saturated therewith. In this manner, the speed of evaporation may be reduced. The manner in which the speed of evaporation or the quantity of evaporated solvent, or other details of the evaporation process, may be varied, depends mainly on the performance desired of the resulting semipermeable membrane.

Precipitation of the film-forming substance from the solution is caused by the action of a fluid agent capable of precipitating te substance in question. It shall be understood that a fluid precipitating agent is a liquid which is miscible with or soluble in te solution in which the film forming substance is dissolved, the latter substance being insoluble in said liquid. Frequently, water can be used, e.g. when the membrane 1H produced from a solution of cellulose acetate in acetone. In the course of the process, the water or other liquid used for precipitation becomes mixed with the solvent replenish precipitating liquid also contained in the interior of the tube. When a precipitating liquid is contained in the interior of the tube, it is of advantage to allow such liquid to flow continuously through the respective section of the tube.

The process of the invention is illustrated by way of example by reference to the accompanying drawing, the single figure of which is a diagrammatic sectional view of an apparatus for performing the process of the invention.

Referring to the drawing, in the apparatus shown, a web of permeable material, e.g. a paper web, is continuously wound off from a roll 1 and guided around two deflector rollers 2, leaving the latter of these in a downward direction.

By a number of folding devices 3» of which only two are shown for the sake of simplicity, the web, on its downward path, is gradually converted into a tube with an overlapping longitudinal seam which may be closed, e.g., by gluing. The folding devices 3 are attached to a frame 11. While it is being formed into a tube, the web gradually envelops a die tube 5b held by a mounting 5a which, in turn, is attached to the frame 11. An annular die 5 is provided at the lower end of the die tube 5b. The resulting tube is passed through a guide sleeve 4 which serves the purpose of shaping the tube in such a manner that its cross-section is as closely circular as possible. Gluing of the longitudinal seam may be performed in a manner known per se, not shown in the drawing.

The die tube 5b not only supports the annular die 5» but also serves for the supply of the solution with which the inside wall of the tube is coated. Further, the die tube b holds three additional conduits which will be referred to further down.

After the tube has been given an exactly circular cross-section by means of the guide sleeve 4» it is passed through an annular gap formed between the annular die 5 and a calibrating ring 6. The annular die 5 has a lateral orifice which is in the shape of a ring slot. This slot is surrounded by the calibrating ring 6. The coating solution - which may consist of a solution of 1 part by weight of cellulose acetate* 3 parts by weight of acetone and 0*5 part by weight of a 10$ aqueous solution of "magnesium perchlorate - is supplied to the upper end of the die tube 5b through a supply pipe 5c, passes through the die tube ¾ to the annular die 5 rom which it emerges laterally, and is forced upon the inside wall of the downwardly moving tube* The thickness of the layer formed on the inside wall of the tube is substantially determined by the width of the clearance of the calibrating ring 6* the outer diameter of the annular die 5» and the extrusion pressure and viscosity of the coating solution.

Upon leaving the annular die 5 and before it reaches a precipitating liquid, described below, the coated tube passes through a zone in which it is filled with air in which solvent vapours evaporated from the coated layer are contained.Evaporation of the solvent may be expedited and controlled by. the introduction of air through a ventilating pipe 8 fitted into the die tube 5b.

After having passed through the evaporation zone,the tube reaches a precipitating liquid 12 which is not only contained in a trough but also in the interior of the tube.

The precipitating liquid may consis of water. The pre-cipitating liquid contained in the interior of the tube is replaced by means of a feed pipe 9 through which fresh or regenerated precipitating liquid is supplied, and a draw-off pipe 10, through which spent precipitating liquid is drained off. In the case of the precipitating liquid outside the tube, a feed line 13 and an overflow device 14 are employed for the purpose of renewal.

Advantageously, the level of the liquid within the tube is adjusted such tha it corresponds to the level of the liquid outside the tube. When the tube enters the precipitating liquid, it passes through a second guide sleeve 7. This sleeve serves the purpose of maintaining the circular cross-section imparted to the tube by the first guide sleeve 4 and of protecting the longitudinal seam of the tube, which at this point may be subjected to a certain amount of pressure.

Within the precipitating liquid, the tube is guided around three guide pulleys 15, and then, after it has left the precipitating liquid, passes over a fourth guide pulley 15«i When it passes over the first guide pulley 15, the tube is collapsed and, at the same time, sealed off from precipitating liquid that was contained in the interior of the tube during the previous stage of the process. After leaving the fourth guide pulley.15 , the f nished semipermeable membrane tube reinforced by the web is wound into a roll 16.

The process of the invention has the advantage that it can be performed continuously. One of its further advantages is that it enables the production of very long semipermeable membrane tubes. The membrane tubes thus produced are distinguished by a very low weight per unit length. Due to their flexibility, they can be handled very easily and require only very little space during transport and storage. Surprisingly, the membrane tubes are of such mechanical strength that full use can be made of their flexibility without damaging the tubes. Due to their high mechanical strength, the membrane tubes may be used in combination with relatively coarsely permeable pressure vessels, which can be fabricated relatively easily. Thus, it may be sufficient to use vessels of perforated sheet metal instead of vessels made of sintered metal.

Claims (7)

WHAT IS CLAIMED IS

1. A process for the manufacture of a reinforced semi -permeable Riembrane tube, wherein a coherent web of permeable material , while being conveyed 1n a downward direction , is continuously converted into a tube with a longitudinal seam, a layer of a solution of a film-forming substance suitable for the production of a semipermeable membrane Is applied to the inside wall of the resulting tube, part of the solvent 1s evaporated from the surface of the said layer and then a fluid precipitating agent Is caused to act on said layer thereby precipitating the film-forming substance being dissolved in said solution.

2. A process as claimed in claim 1 , wherein a gas Is caused to f ow through that part of the tube In which evaporation of the solvent occurs .

3. A process as claimed in claim 2, wherein the gas 1s air containing vapour of the solvent.

4. A process as claimed In any one of claims 1 - 3, wherein the said fluid precipitating agent in the precipitation Is caused by the action ^6 ^a liquid capable of precipitating the fi lm-forming substance Into which liquid the tube such is passed and by the passage of liquid through the interior of the tube.

5. A process as claimed in claim 4, wherein the tube Is col lapsed in the bath , whereby the Interior liquid is confined to a portion of the tube above a co lapsing means for the tube.

6. A process as claimed In claim 1 , substantial ly as described herein with reference to the accompanying drawing.

7. A tubular semipermeable membrane produ of any one of claims 1 to 6. P.O.Box 1169 Tel Aviv Attorneys for Applicants.