MEMBRANE SEPARATION APPARATUS AND METHOD

Abstract: Continuous flow membrane separation is accomplished by apparatus adaptable for osmosis, dialysis, and electrodialysis. For electrodialysis to separate from one another, in a single step, multiple differing molecular weight components of fluids, a series of adjoining flow chambers are all separated from one another by semipermeable membranes and at least one of the chambers is separated from each adjoining chamber by a membrane having a molecular weight cutoff different from that of the membrane separating it from the other chamber adjoining it. To insure that neither the crude to be processed nor the desired product comes into contact with an electrode which could be deleterious, there is provided a minimum of four flow chambers adjoining one another in sequence. The apparatus employs a tubular membrane encircled by a flexible reinforcing mesh fabric with pressure in the fluid within the membrane sufficient to bulge the membrane radially and outward into the interstices of the mesh fabric to eliminate dead spots in fluid flow along the exterior of the membrane and sufficient to stiffen and straighten the membrane tube into essentially the form of a right cylinder. For electrodialysis, one or more additional, similar, successively larger reinforced membrane assemblies, surrounds(s) the first assembly. A housing encloses the membrane(s) to form a cartridge. The membranes are spaced to avoid contact with electrodes used in electrodialysis.

Description: Membrane separation is a process by which one or more components carried by a crude or feed fluid which is in contact with one side of a semipermeable membrane are separated from other components in the crude when a wash or rinse fluid is in contact with the other side of that membrane. Examples of membrane separation are direct osmosis, usually called simply osmosis, dialysis and electrodialysis. In continuous flow membrane separation there has existed the problem of reinforcing the membrane, whether it be of the tubular type or the flat sheet type used in plate and frame apparatus. To maximize the permeate flux through the membrane, such membrane is usually made as thin as possible. To reinforce such a thin membrane against rupture under the pressure involved in producing the continuous flow, a reinforcing means in contact with the membrane is needed which perforce covers a relatively large portion of area of the membrane if the transmembrane fluid pressure difference is relatively large. The presence of such a reinforcing means thus significantly reduces the effective bare area of the membrane in contact with fluid on the low pressure side of the membrane, hence undesirably reducing the portion of the membrane from which permeate can flow unobstructedly away from the membrane and rinse fluid can flow against the membrane. Additionally, such a reinforcing means typically provides "dead" spots where fluid flow rate is reduced causing concentration polarization. Both of these phenomena thus reduce the effective flux through the membrane. Typical prior art electrodialysis apparatus has three chambers, the chamber through which the crude or feed fluid flows being separated by membranes from each of the other two chambers, each of which latter contains an electrode. In such an arrangement there is unavoidable contact between an electrode and the fluids, which may contain desired products, flowing in the chambers adjoining the single intermediate chamber containing crude. Such contact of electrodes with desired products is undesirable since it may well alter such products detrimentally. Also the products may detrimentally affect the electrodes by causing deposits thereon or the like. By way of example, pertinent prior art is shown in US-A-2,739,938 issued to S.G. Wiechers illustrating a three-chamber electrodialysis apparatus and in an article by Jan Al and S.G.Wiechers entitled "Production of an Artificial Human Milk", in the January 1952 issue of the journal "Research" subtitled "Science and its Application in Industry", published by Butterworths Scientific publications, London, England. The article shows and describes the equipment of which the apparatus of the patent forms a part. According to the invention, there is provided a membrane separation apparatus for use in osmosis, dialysis and electrodialysis with an assembly comprising an enclosure of thin flexible, semipermeable membrane and reinforcing means, made of supple, soft material and provided with interstices surrounding said membrane enclosure, characterised in that the size of said interstices is such that said membrane bulges into the interstices to a distance equal to at least the thickness of the elements of said material bounding the interstices when inflated by a fluid pressure creating a transmembrane pressure difference of a magnitude substantially less than the minimum at which any significant ultrafiltration occurs with said membrane. Preferably the membrane enclosure is a membrane tube and said membrane tube bulges generally radially outwards into the interstices to a distance equal to at least the radial thickness of said elements when inflated by said internal fluid pressure. By "radial thickness" of an element bounding an interstice of the reinforcing material is meant the distance measured on a prolonged radius of the cylindrical membrane between the point of intersection of that radius with the bounding element where the element contacts the exterior surface of the membrane cylinder and the point of intersection of that prolonged radius with an imaginary circle concentric with the membrane cylinder and passing through the radially outermost point of that bounding element. The invention also includes the method of membrane separation using the aforedescribed apparatus, including the step of subjecting fluid located in said apparatus for participating in said separation to fluid pressure to produce transmembrane pressure differences substantially less than the minimum at which any significant ultrafiltration occurs with said membrane(s) but sufficient to bulge said membrane(s) into the interstices of said reinforcing means to a distance equal to at least the thickness of the elements of said material bounding said interstices and to generally straighten and stiffen said membrane(s). Where a membrane tube is used, if the nominal diameter of the tube exceeds the inside diameter of the reinforcing means bulging of the membrane to the minimum required level will be achieved at a lower pressure than if the nominal diameter of the membrane tube is less than the inside diameter of the reinforcing means. This bulging effect takes out slack membrane material which in turn causes stiffening and straightening of the tube. In Fig. 1 there is shown, schematically, a tubular membrane 2 surrounded by a netting 4 constituting one preferred embodiment of the reinforcing means. The material chosen for the reinforcing means must be strong enough to resist the stresses it will encounter and also must be inert to the fluids and other products it will encounter. It may be in the form of essentially any type of fabric: woven (included braided) knitted, pressed, molded, or created from originally imperforate plastic sheeting or the like by making holes in it to thereupon constitute patent interstices, or by slitting it and thereafter stretching it to form patent interstices in the manner used to form expanded plastic sheets of the product know variously as vented plastic wrap, lettuce wrap, or serrated wrap. The fabric can either be originally fabricated in tubular form or, if fabricated as a sheet, can be sewn or otherwise joined together to form a tube to surround the membrane tube. Netting or mesh fabrics have been found to be especially useful. When the assembly of membrane tube and netting is used in membrane separation apparatus the tube 2 is fastened at its ends, as by means such as are well known in the prior art, to pipe stubs or the like, not shown, serving as an inlet and outlet for fluid to flow through the tube. Since the tube is thin and delicate it is prudent to provide, for example, a soft sleeve of rubber or similar elastomer over the pipe stub end. The end of the membrane assembly tube is drawn over the soft sleeve, a similar sleeve is then drawn over the outside of the assembly encircling the pipe stub and clamping means is then applied to clamp the assembly in fluid-tight connection to the pipe stub. When the membrane tube 2 is not inflated it is flaccid and lies flat as it comes from the manufacturer. When the uninflated assembly is mounted on pipe stubs the cross-section of the membrane tube appears approximately as a flat collapsed oval. With the outside diameter of the membrane tube slightly smaller than the inside diameter of the netting, then when filled with liquid under a pressure of about 6.89 KPa gauge (one psig), the cross-section appears generally circular as seen in Fig.2 and the membrane, typically made of regenerated cellulose from 0.0254mm to 0.763mm (1 to 3 mils) thick, has not noticeably deformed outwardly in the radial direction. Under such a liquid load, with the hydraulic pressure of approximately 34.45 KPag (5 psig) a 1.83m (six foot) length of membrane assembly using 2.54 cm (1 inch) diameter tubular membrane and oriented horizontally will not remain straight, as shown in Fig.1, but will sag and exhibit about a 10.16cm (4 inch) deflection from the horizontal at its midpoint. According to the method of this invention the fluid in the tube is subjected to sufficient pressure to bulge the membrane radially outward substantially into the interstices of the reinforcing material and to straighten and stiffen the membrane tube so that it becomes generally a right cylinder. When the fluid pressure in the illustrated embodiment reaches about 48.23 KPag (7 psig) the required bulging occurs, as seen in Fig.3 and the assembly appears straight i.e., essentially without sag. In its flaccid, unstressed condition, as shown in Fig. 4, the preferred netting of this embodiment exhibits mesh openings in the shape of a symmetrical hexagon with two parallel sides 6 and 8 about 0.635cm (1/4 inch) long and four others each about 0.3175cm (1/8 inch) long, those aforementioned parallel sides being spaced apart about 0.4763cm (3/16 inch). The average width of the knitted strands constituting the elements bounding the mesh opening, namely the sides of the hexagon, is about 0.1191cm (3/64 inch). This netting leaves about 70% of the membrane bare. When the membrane tube is subjected to an internal hydraulic pressure insufficient to bulge the membrane 2 substantially into the interstices, i.e the mesh openings of the netting, then undesirable "dead" spots such as 10 in Fig.2A are created in depression or cavities such as that shown in Fig.2 where the floor of the cavity is constituted by that portion of the membrane 2 which spans the six sides of the hexagon, of which only the two sides labelled 6 and 8 are shown for clarity. The walls of the cavity are constituted by those six sides of the hexagon. In such a cavity fluid contacting the exterior of membrane 2 tends to form a pool in which the flow tends to stagnate or slow down. This makes for concentration polarization which undesirably reduces the permeate flux. According to the invention the membrane 2 is to be subjected to sufficient internal pressure to bulge it into the interstices to a distance equal to at least the radial thickness of the elements bounding the interstices. In Fig.2A, this would mean that the membrane would have to bulge into the interstices up to at least the level of the surfaces 12 and 14 of the sides 6 and 8 of the hexagon (as well as the corresponding surfaces of all the remaining sides of the hexagon, including those omitted from the drawing for clarity). In this instance the radial thickness of the bounding elements is the same as the thickness of the netting material, i.e about 0.041 cm (0.016 inches). If the reinforcing material were of the expanded plastic sheet type, similar to lettuce wrap, then the radial thickness of the bounding elements would not be defined as the thickness of the plastic sheet but rather would be the width of the strips of the slit plastic standing on edge to form the interstices. In Fig.3 the membrane 2 is shown bulged into the interstices to a distance greater than the radial thickness of the bounding elements, such as 6 and 8. In practice, this can commonly occur and there will be no rupture of the membrane as long as the allowable tensile stress of the membrane material is not exceeded. When the membrane is inflated, the reinforcing fabric is stressed so that the hexagonal mesh openings convert to essentially circular openings. Assuming for the simplicity of calculations that the interstices are circular, then it can be shown that, for a membrane of given thickness subjected to a given hydrodynamic pressure differential across the membrane, the stress induced in the membrane is proportional to the diameter of the interstices. Thus if a hydrodynamic pressure is chosen to be applied to the fluid in the membrane tube to achieve a desired flow rate through the tube and a pressure is chosen to be applied to the fluid contacting the exterior surface of the tube (which latter fluid is always present in membrane separation apparatus), then the maximum allowable diameter for the interstices can be calculated, knowing the thickness of the membrane and the maximum allowable tensile stress in the membrane. According to the invention, by configuring the reinforcing means to provide what amounts to large diameter interstices commensurate with allowable stress in the membrane fabric, there is achieved a highly favourable ratio of bare membrane surface to total membrane surface. If the maximum allowable diameter for each single interstices is used then the maximum total area of the membrane is left uncovered by the entire reinforcing material. With membranes of the order of 0.0254mm to 0.0762mm (1 to 3 mils) thick the aforedescribed mesh fabric shown in Fig.4 leaves in excess of 60% of the membrane bare, exposed to view in the interstices, with no risk of rupture at fluid pressures of about 192.92 KPa g (28 psig). Using similar fabrics according to the teaching of this invention bare areas of 90% have been achieved. Commonly the bulges of the membrane which enter into the interstices are formed by the hydrodynamic pressure of the fluid in the membrane tube. However, the invention includes the concept of membranes with preformed deformations, such as exist in one of the two bonded sheets of plastic constituting a piece of "bubble wrap". Such deformations can enter the interstices and, when inflated sufficiently by the fluid pressure differential to reach the prescribed radially outward distance, will constitute the bulges required by the inventive concept by serving to eliminate dead spots and pools aforementioned. In Figs. 5 and 6 there is shown a membrane separation apparatus embodying the inventive assembly of Fig. 1. This apparatus is a four-chamber electrodialysis unit with its longitudinal axis preferrably oriented in a generally horizontal direction. To avoid a multiplicity of figures in the drawings the illustration in Figs. 5 and 6 will also be used, by simply envisioning the omission of certain structural elements, to describe apparatus using the inventive assembly for osmosis and for dialysis. As seen in Fig. 5, there is a head unit generally indicated at 16, having four separate passageways for fluids with separate nipples, or the like, 18, 20, 22, 24 for leading fluids into or out of each passageway. A similar head unit, generally indicated at 26, is at the opposite end of the apparatus and has nipples, or the like, 28, 30, 32, 34 for leading fluids separately into or out each of four separate passageways in that unit. A reinforced membrane tube assembly 36, in some preferred embodiments from 0.914m to 1.828m (3 to 6 feet) or more in length, similar to the assembly of Fig. 1, is slipped over the end of and attached in fluid-tight relation to pipe stub 37 or the like on head unit 16. A group of additional assemblies, in this example only two, namely, 38 and 40, similar to the assembly of Fig. 1, are also attached, respectively, to pipe stubs 39 and 41 or the like on head unit 16. Thus the three aforementioned assemblies are connected to, respectively, the innermost passageway, the next outer passageway, and the still further out passageway in head 16. The outermost passageway terminates within pipe stub 43 or the like. Similar connections are made between the assemblies and the head unit 26. Through the center of the apparatus extends a rod 50, serving as an electrode, in fluid-tight relation with the innermost passageway of each head unit. A piece of pipe 52, or the like, fastened by any conventional means to the head units 16 and 26, forms a fluid-tight housing or shell around the several membrane assemblies. The illustrated fastening means includes nipples 53, threaded collars 54, and rubber gaskets or the like 56 constituting compression fittings to establish fluid-tight connections between pipe 52 and heads 16 and 26. Pipe 52 is typically many lengths longer than nipples 53 and is appreciably shorter than the distance between head 16 and head 26 in the final assembled configuration. With nipples 53 and intermediate collars 54 slid toward the midpoint of the length of pipe 52 and with pipe 52 centered between heads 16 and 26 sufficient exposed gaps are provided between the ends of pipe 52 and heads 16 and 26 to give access to the ends of the membrane assemblies to fasten them to their respective pipe stubs on the heads. Pipe 52 may serve as an electrode. Within the housing there is thus formed a series of four chambers 58, 60, 62, and 64, as seen in Figs. 5 and 6. Chamber 58 is connected to nipples 18 and 28, chamber 60 to nipples 20 and 30, chamber 62 to nipples 22 and 32, and chamber 64 to nipples 24 and 34. The structural elements, rod 50, pipe 52, and the three membrane assemblies are shown as concentric but they need not necessarily be concentric. Although it is efficient to employ rod 50 and pipe 52 as electrodes it would suffice, in principle, for electrodialysis to simply have the tip of a piece of wire as an electrode, dipping into fluid in chamber 58 and a similar wire tip dipping into the fluid in chamber 64. To use this apparatus for electrodialysis, crude material is flowed through chamber 62. The crude may, for example, be a mixture of a desired protein contaminated with say calcium chloride, bacteria, and other components. The membrane of assembly 38 is chosen to have a molecular weight cutoff large enough, say 20000, to allow permeation of the desired protein into product chamber 60, but not the undesired bacteria, etc., which are larger. The membrane of assembly 36, forming the other boundary of product chamber 60, is chosen to have a molecular weight cutoff small enough, say 3500, to retain the desired protein in chamber 60 but still large enough to permit calcium ions, for example, to permeate into chamber 58. Rinse fluids comprised of water with some electrolytes in solution are flowed through chambers 58 and 64. With pipe 52 connected as anode and rod 50 connected as cathode to a source of direct current and with the molecular weight cutoff, say 3500, of the membrane of assembly 40 chosen to allow permeation of chloride ions but retain in chamber 62 any larger solutes or suspended components destined in this case for disposal, then the desired positively charged protein will be pulled through membrane 38 into chamber 60 as the purified product to be flowed out through nipple 20. The embodiment in its entirety shown in Figs. 5 and 6 is thus seen to illustrate two of the aforementioned novel concepts of this invention: (1) By having a series of adjoining chambers all separated from one another by membranes and in which at least one, in this case 60, is separated from each chamber which adjoins it by a membrane having a molecular weight cutoff different from that separating it from the other chamber adjoining it the invention provides a compact, simple, unitary apparatus to accomplish the separation from one another in a single step process of multiple components of fluids when these components have different molecular weights. This concept is applicable to plate and frame apparatus as well as to tubular apparatus. (2) The inventive provision of a tubular membrane apparatus with a minimum of four chambers adjoining one another in sequence insures that neither the crude nor the desired product is in contact with an electrode which could be deleterious to ingredients in these fluids. An osmosis apparatus is made by simply omitting from the construction in Figs. 5 and 6 the rod 50 and membrane assemblies 38 and 40, for example. This leaves only two chambers 58 and 64 and it is assumed that the head unit passageways for the omitted chambers are blocked or omitted. Fruit juice to be concentrated can be flowed rapidly through chamber 58 and concentrated sugar solution through chamber 64, now bounded by membrane assembly 36 and pipe 52. By osmosis, water from the fruit juice permeates into the sugar solution. The membrane assembly of this invention allows for rapid turbulent flow, without excessive drop, and a relatively long region of contact with the membrane without fouling or concentration polarization even with fluids containing 75% or more of dissolved solutes and 35% suspended matter. Dialysis can be carried out in apparatus similar to that described for osmosis except that in place of a concentrated sugar solution an appropriate rinse fluid of water or dilute salt solution is used and the membrane chosen is one with the proper pore size to allow permeation of the salts or the like to be extracted from the crude.

Claims: A membrane separation apparatus for use in osmosis, dialysis and electrodialysis with an assembly comprising an enclosure of thin flexible, semipermeable membrane and reinforcing means, made of supple, soft material and provided with interstices surrounding said membrane enclosure, characterised in that the size of said interstices is such that said membrane bulgess into the interstices to a distance equal to at least the thickness of the elements of said material bounding the interstices when inflated by a fluid pressure creating a transmembrane pressure difference of a magnitude substantially less than the minimum at which any significant ultrafiltration occurs with said membrane. An apparatus according to claim 1, wherein said membrane enclosure is a membrane tube, and said membrane tube bulges generally radially outwards into the interstices to a distance equal to at least the radial thickness of said elements when inflated by said internal fluid pressure. Apparatus as claimed in claim 2 wherein the internal diameter of the reinforcing means is less than the nominal outside diameter of the membrane tube. Apparatus as claimed in claim 2 or 3 wherein the material of said reinforcing means is configured to leave uncovered, a portion constituting in excess of about 60% of the area of the exterior surface of that segment of the membrane tube which is to be subjected to contact with liquid flowing within said membrane tube. Apparatus as claimed in any of claims 2 to 4 including an inner electrode placed to contact fluid located within said membrane tube; a group of additional assemblies, generally similar to said original assembly but each succeeding assembly having an inner diameter greater than the outer diameter of the preceding assembly of said group; the first of said group encircling and spaced from said original assembly and each succeeding assembly of said group encircling and spaced from the preceding assembly of said group when the membrane tubes of all of said assemblies are inflated; a housing encircling the outermost of said group of assemblies thereby to provide a series of at least four chambers in said apparatus, through each of which chambers fluid, when present, is confined by the surfaces of the structural components of said apparatus to flow longitudinally in a channel generally in the form of a cylinder; an outer electrode placed to contact fluid located in the outermost of said chambers when fluid is located in said outermost chamber; each membrane of at least one pair of adjacent membranes of said assemblies having a molecular weight cutoff different from the molecular weight cutoff of the other membrane of that pair; and fluid inlet and outlet means for each of said chambers separate from those for the other chambers so that fluid for each chamber can enter and leave that chamber without flowing in the same conduit in which the fluid of another chamber is simultaneously flowing. The apparatus of claim 5 wherein said inner electrode is encircled by said originally mentioned assembly and, when the membrane tube of said originally mentioned assembly is inflated, is spaced from said membrane tube and extends generally in the direction of the longitudinal axis of said membrane tube essentially coextensive with the length of that segment of the membrane tube in contact with fluid when fluid is present in said tube. A method for effecting membrane separation using an apparatus according to any of claims 1 to 6, the method including the step of subjecting fluid located in said apparatus for participating in said separation to fluid pressure to produce transmembrane pressure differences substantially less than the minimum at which any significant ultrafiltration occurs with said membrane(s) but sufficient to bulge said membrane(s) into the interstices of said reinforcing means to a distance equal to at least the thickness of the elements of said material bounding said interstices and to generally straighten and stiffen said membranes. A membrane separation apparatus for separating from one another components of different molecular weights carried by fluids, said apparatus comprising a series of sequentially adjoining chambers through each of which fluid can flow, each chamber being separated from each adjoining chamber by a semipermeable membrane, said membrane forming a barrier common to the two adjoining chambers which can be contacted on one of its surfaces by fluid in one of the two adjoining chambers and can be contacted on the other of its surfaces by fluid in the other of said two adjoining chambers, any two membrane barriers that serve as two separate boundaries for the same chamber constituting a pair of adjacent membrane barriers, and each membrane barrier of at least one pair of adjacent membrane barriers having a molecular weight cutoff value different from that of the other membrane barrier of said pair, reinforcing means being provided in the form of a supple, soft material with interstices engaging said membranes characterised in that the size of said interstices is such that said membranes bulge into the interstices to a distance equal to at least the thickness of the elements of said material bounding the interstices when inflated by fluid pressures difference creating a transmembrane pressure difference of magnitude substantially less than the minimum at which any significant ultrafiltration occurs with said membranes.

Inventors: CARO, RICARDO F., SALTER, ROBERT J., OSMOTEK INCORPORATED
Applicants: CARO, RICARDO F., SALTER, ROBERT J.
Application year: 1988
Main classification: B01D61
IPC codes: B01D61, B01D63