US3396103A - Method and apparatus for mounting membrane filters on tubular supports without laterally stressing the active surface - Google Patents

Description

Aug. 6;, 1968 M. G. HUNTINGTON 3,396,103

METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS ON TUBULAR SUPPORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE Filed April 1, 1.965 5 Sheets-Sheet l 6 t I E E Q3 E 112% NM "Q '0 is l E N A N L. N N m N 838% Q luuq Q Q 0.

INVENTOR Morgan Gf'Hwmh'rgtam j Am Aug. 6. 1 68 M. e. HUNTINGTON 3,396,103

METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS ON TUBULAR SUPPORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE 5 Sheets-Sheet 2 Filed April 1, 1.965

INVENTOR Morgan GI fi/urzhryi'om r A RNEYIS' g- 6, 1968 M. G. HUNTINGTON 3,396,103

METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS ON TUBULAR SUPPORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE Filed April 1, 1965 5 Sheets-Sheet 3 IN VENTOR 2/6" Morgan 6. Hami A ORNEYS Aug. 6, 1968 M. G. HUNTINGTON 3,396,103

METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS ON TUBULAR SUPPORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE Filed April 1, 1965 5 Sheets-Sheet A 3,396,103 METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS 0N Aug. 6, 1968 M. G. HUNTINGTON TUBULAR SUPPORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE 5'Sheets-Sheet 5 Filed April 1, 1965 INVENTOR Marya 6T f/wziz'rgq 50m A A ORNEYS United States Patent METHOD AND APPARATUS FOR MOUNTING MEMBRANE FILTERS ON TUBULAR SUP- PORTS WITHOUT LATERALLY STRESSING THE ACTIVE SURFACE Morgan G. Huntington, Galesville, Md., assignor to Waterdrink, Inc., Salt Lake City, Utah, a corporation of Nevada Filed Apr. 1, 1965, Ser. No. 444,751 6 Claims. (Cl. 210-23) ABSTRACT OF THE DISCLOSURE An apparatus and method for extracting solvents from pressurized solutions and suspensions by flow through a semipermeable membrane. A membrane is supported on a surface of a tubular support, which surface is composed of planar surface segments and has fluid flow passageways spaced along the vertices of the angles formed by said planar segments. The membrane and support are rotated about the longitudinal axis of the support while a pressurized solution is maintained in contact with the membrane. Solvent diminished in solute content passes through the membrane by reverse osmosis and is withdrawn from the interior of the tubular support.

This invention relates to improved methods and apparatus for extracting solvents from pressurized solutions and from suspensions by means of osmotic membranes and ultrafine filters. This invention particularly relates to methods and apparatus for mounting membrane filters on and within tubular supports in such a manner that the necessary imposed solution pressure subjects the membrane filter to a single compressive stress normal to the supporting surface and which method of support prevents any lateral stress and strain on the working areas of the membrane filter. This invention also relates to the isolation of unavoidably stressed areas and to the identification and sealing of damaged and/or physically imperfect parts of desalinizing membranes and membrane filters.

In my copending application, Ser. No. 418,574, filed Oct. 22, 1964, entitled Centripetal Acceleration Method and Apparatus (whose disclosure is hereby incorporated herein by reference), novel methods and apparatus are described for extracting solvents from solutions and from suspensions by means of reverse osmotic and ultrafiltration techniques. The novel methods to which that application is directed involve a rotating assembly by which the solution and the membrane are subjected to centripetal acceleration greater than the acceleration of gravity and in a direction away from the membrane surface. The effect of such acceleration is to promote convection Within the solution so that any increase of volumetric density over that of the main body of solution results in instantaneous mass transfer away from the membrane surface.

The rotating apparatus described in said copending application as being preferred for carrying out said process is comprised of one or more perforated tubular supports having cylindrical or conical surfaces upon which membrane filters are mounted over laterally permeable solvent collecting media. These membranes and supports are concentric within one or more solution-filled pressure shells from which the solvent is to be selectively extracted by passage through the membrane filters.

Because membrane filters and their laterally permeable linings over the perforated supports ordinarily used in reverse osmosis and ultrafiltration processes are always somewhat compressible when mounted upon cylindrical and conical convex surfaces, the membrane filters 3,396,103 Patented Aug. 6, 1968 are also subject to lateral compressive stress and strain (as a direct function of 21r times the change in radial distance from the axial center) in addition to the compressive stress normal to the supporting surface. Such lateral compressive strain of membrane filters which are cylindrically supported is evidenced by random longitudinal folds and creases which deleteriously change the filtering and osmotic characteristics of the membrane filter.

Similarly, when membrane filters are placed on the inside of restraining perforated cylinders the hydraulic pressure of the solution exerts radially compressive forces which consequently cause the membrane filter to stretch and to split longitudinally at random because of the 21r relation of peripheral length to radial distance. Such tensile stresses also result in undesirable changes of membrane filter performance in separating solvent from the pressurized solution and/or liquid from the suspension.

In accordance with the present invention, a membrane filter support structure has now been developed which effectively overcomes the shortcomings of the cylindrical and conical mounts described above, yet makes feasible the use of tubular membrane supports.

It is, accordingly, a primary object of this invention to provide convenient and economical tubular mounting structures which have a series of adjacent planar surfaces, as opposed to cylindrical and conical surfaces, which thereby eliminate all destructive lateral stresses in membrane filters, except at the dihedrals formed by the junction of adjacent planar surfaces.

In addition to the foregoing, cylindrical mounts as disclosed in my copending application Ser. No. 418,574 are less satisfactory than the multiplanar, tubular membrane filter supports of the present invention because in the former, the centripetal acceleration is constant at every point throughout the cylindrical surface. The multiplanar supports of the present invention add the very desirable effect of imparting greater centripetal acceleration to the solution at the dihedrals than at the longitudinal median axes of the planar surfaces, thereby causing forced transverse lateral circulation of the solution over the membrane filter surface from the longitudinal median axis to the dihedral which fluid flow is markedly helpful in lowering boundary layer concentration of solute and in reducing the physical blinding rate of ultrafilters.

In addition, it has been found impractical to perforate the supporting tubular structures on the planar membrane support surfaces because of the difficulty of bridging the perforations with laterally permeable material so that the membrane shows no depression, and therefore, lateral strain under the necessary hydraulic pressure of the solution.

It is, therefore, also an object of this invention to make all support perforations at the dihedrals along lines where the flexed and stressed membrane is deliberately made impervious, as will be hereinafter described. Thus, the major areas of the membrane filter support of this invention are comprised of very smooth, flat, uninterrupted multiplanar surfaces.

Laterally permeable materials suitable as a membrane base covering the tubular support exhibit, in varying degrees, a gasketing effect and reduction of lateral permeability under pressure. (A gasketing effect becomes apparent when the pressure-water flux curve departs from the initial straight line is viscous flow.) This pressure gasketing effect with reduction of lateral permeability is in inverse function of the peripheral length of the total apertures and a direct function of the compressibility of membranes and of the laterally permeable support materials.

It is therefore, also an object of this invention to miniice mize the pressure gasketing effect of laterally permeable materials used as membrane backing, through connection of all perferations along the dihedrals by a longitudinal groove no deeper than the maximum distance between chord and are so as to maximize the peripheral length of the bleedings apertures and with weakening the structural resistance to deformation.

In the use of semipermeable membranes such as have been described above, such membranes may frequently have pin-holes or other imperfections in their body. Unless such imperfections are sealed, it is apparent that the membrane containing them will be ineffective to the extent of such imperfections and, if they are great enough, the membrane will not be useful in carrying out its intended function in the process and apparatus of the present invention.

It is, accordingly, a further object of the present invention to provide a procedure for and means by which such imperfections can be located and eliminated.

My copending application Ser. No, 418,574 relates to a rotatable apparatus generating centripetal acceleration and thereby forcing convective mass transfer of soluteenriched solvent away from the membrane filter surface. In cases in which the solute has greater density than the solvent, the filter membranes can be usefully mounted upon the outer surface of the tubular support contained within its pressure shell. When the reverse is true, a similar effect is achieved by mounting the semipermeable membrane on the inside of the tubular support.

When the solution from which solvent is being extracted contains a relatively low concentration of solute, and particularly in the case that all particulate matter and basteria have been previously removed (as, for example, the first stage product of the desalinizing process described in said copending application), it may be sufiiciently economical to depend upon solution flow over the membrane surface without centripetally induced convection to augment the diffusion of solute away from the osmotic membrane surface. In such case, the necessity of a solution pressure shell can be eliminated by mounting an osmotic desalinizing membrane on inner and outer surfaces of a plurality of generally concentric tubular supports.

It is, therefore, another object of this invention to provide a nonrotatable assembly of a plurality of concentric tubular membrane supports with membranes mounted on the inside of one tube and on the outside of the other tube, which requires no solution pressure shell and thereby reduces the necessary assembly structure weight by approximately half.

These and other important objects and advantages of the present invention will become more apparent in accordance with the ensuing description and claims, as well as in connection with the appended drawings wherein:

FIGURE 1 is a longitudinal section of a rotating apparatus incorporating two single stage membranes arranged in parallel relationship in accordance with the present invention;

FIG. 2 is a transverse section along line 2-2 of FIG. 1;

FIGS. 3 and 4 are enlarged fragmentary sections of membrane supports having membranes respectively mounted externally and internally on the planar surfaces of said supports;

FIG. 5 is an enlarged fragmentary section of a membrane support having an externally mounted membrane and which is adapted to permit the identification of imperfections in such membrane;

FIG. 6 is a longitudinal section through apparatus which is non-rotatable but which incorporates novel features in accordance with the present invention; and

FIG. 7 is a transverse section along line 77 of FIG. 6.

To facilitate the description of the inventions to which this application is directed, brief reference will be made to said copending application S.N. 418, 574. FIG. 1 of that application sets forth a flow sheet diagram illustrating the sequence of fluid flow utilizing the novel centripetal accelerator to which that case is directed in connection with the desalination of sea water. The structure illustrated in FIGS. 1 and 2 of this application is capable of being directly substituted for the rotating membrane assembly 18 illustrated in FIG. 1 of said copending application and, as may be seen by the construction of the apparatus of FIGS. 1 and 2 in this case, the latter apparatus is capable of being substituted for the structure of FIGS. 4 and 5 in said copending application, it being understood that the various operating conditions set forth in said copending application for the structure of FIGS. 4 and 5 therein are equally applicable to the structure of FIGS. 1 and 2 in this application.

With the foregoing considerations in mind and with the further consideration that the structure of FIGS. 1 and 2 herein is to be described in connection with the extraction of potable water from salt water and/or organically polluted water, these figures illustrate a rotating apparatus incorporating a plurality of single stage osmotic membranes, both of which reject a common concentrated solution and extract a common product of reduced solute content from a comm-on feed of solution. In this apparatus, pressurized solution enters rotating shaft injection gland 10 to flow axially under self-aligning bearing 12, then through a radial feed injection manifold 14. From radial injection manifold 14, the solution enters the concentric chambers 16 and 18 through openings 20 and 22. Water substantially reduced in solute content by passage through osmotic membranes 24 and 26 is forced respectively through laterally permeable backing material 28 and 30, through apertures 32 and 34 in membrane supports 36 and 38, into concentric water chamber 40 and into central Water chamber 42. Product Water concentrated in solute content from concentric water chamber 40 is conducted around impervious concentric separator 44 by product water by-pass 46. The product water is bled from the assembly through axial port 48, then through shaft outlet gland 50.

Water is concentrated in solute content leaves the con centric chambers 16 and 18 through manifold 52, then through axial outlet gland 54. The concentration of solute in the rejected solution is controlled by regulating the discharge flow through the concentric membrane assembly by adjusting valve 56. Valve 56 reacts to a salinity meter (not shown) reading product salinity and limits the solution concentration ratio.

A suitable cylindrical pressure shell 58 contains the apparatus described above, this pressure shell and the associated elements described above being maintained in fluidtight relationship within a main body section indicated generally at 60 by means of a plurality of O-rings 62.

As is most clearly illustrated in FIG. 2, membrane supports 36 and 38 have an exterior configuration which is prismatic in nature, the specific prism illustrated in FIGS. 1 and 2 being octagonal. A plurality of through-bores 32 are longitudinally spaced along each lateral edge 64 of the prismatic support 36, such bores, as clearly illustrated in FIG. 2, being radially extending. A similar series of through-bores 34 are provided in a longitudinally spaced manner at the lateral edges 66 of prismatic support 38, also in a radially extending manner. These through-bores may suitably be about of an inch in diameter and spaced longitudinally along each of the lateral edges at which they are positioned at approximately one inch intervals.

Also provided at each of the lateral edges 64 and 66 of prismatic supports 36 and 38, respectively, are trenchlike channels 68 and 70 which are about A inch wide, which serve to connect each of the through-bores along a given lateral edge of a prismatic support and which also serve to accommodate the inwardly folded membrane and its laterally permeable backing material. These channels preferably should be no deeper than the distance between the chord and are of the planar surfaces of supports 36 and 38 so as to maintain maximum structural strength in the support. Since the maximum distance between chord and are is 0.0192 times the radius of the tubular support, this will serve as a limit on the depth of said channels.

The operation of the structure described above may best be illustrated by reference to FIG. 3, which illustrates an enlarged fragmentary section of a membrane support having a membrane mounted on the external planar surfaces of the support. In this figure, the membrane support is identified by the numeral 200, the semipermeable membrane by 202, the laterally permeable backing material by 204, the through-bores at the dihedrals by 206, and the longitudinally extending channels connecting through bores 206 along the dihedrals by 208. The subjecting of the membrane mounted on its support to moderate pressures causes an initial compression ridge 210 (shown by broken lines in FIG. 3) to form in the membrane at each of the lateral edges (dihedrals) with which it is in contact. As pressure on the membrane is increased, this compression ridge folds inwardly into the accommodating channels 208, which phenomenon is clearly shown in full lines in FIG. 3.

As will be apparent, the size of channels 208 must be sufficient to accommodate the inwardly folded membrane 202 and its underlying backing material 204. If channels 208 are not too deep, the membrane portions at the compression ridge fold lines will not be placed in tension but, on the contrary, will remain in slight compression. As a result, no damage to the membrane as a consequence of the formation of the compression ridges or the folding of such ridges into channels 208 will result. If, on the other hand, channels 208 are too deep, the membrane portions at these compression ridge fold lines may be damaged from the standpoint of their filtering capacity, and it therefore will be desirable to repair such damage to avoid leaks at such points. This may readily be accomplished by removing the membrane support from the assembly in which it and its membrane is mounted following an initial application of hydraulic pressure suiiicient to cause the membrane 202 and backing material 204 to fold into channels 208. Following such removal, the stretched (and thus damaged) portions of the membrane directly overlying each dihedral and now inwardly folded are capped with an impervious material 211, such as ladies fingernail lacquer or any plastic cement which is compatible with the material of the membrane and impervious to the fluid with which it will come in contact, to preclude the possibility of leaks at such points. The thus treated membrane assembly is then replaced in its pressure shell and operation of the entire assembly resumed under normal operating conditions.

As shown in FIG. 3, the edges 213 of channels 208 should be rounded to prevent possible perforation or other damage to the membrane 202 as it folds downwardly into the channels.

The enlarged fragmentary section illustrated in FIG. 4 shows the technique of mounting a semipermeable membrane on the interiorly located planar surfaces of a membrane support. The assembly illustrated in FIG. 4 includes a semipermeable membrane 212 mounted on interiorly located planar surfaces 214 of membrane support 216 with laterally permeable membrane backing material 218 being interposed between the two. As was the case with the structure of FIG. 3, through-bores 220 are provided at the dihedrals though, in this embodiment, connecting channels 221 connecting through-bores at a given dihedral are provided in an internal bead 223 formed at each internal dihedral. Each of such channels 221 is only as deep as the bead 223 in which it is formed so as not to unduly weaken the membrane support structure. Since compression ridges such as were formed in the externally mounted membrane structure of FIG. 3 do not form with the use of the structure of FIG. 4, it is not necessary for channels 221 to be sufficiently large to accommodate the membrane and its underlying backing material.

For ease of application of the membrane, the membrane 212 and backing material 218 are cut respectively into individual strips 212' and 218', 212" and 218", 212" and 218", etc., each the approximate size of the planar surface they are to overlie. The junctures between such membrane strips are sealed with pressure sensitive adhesive vinyl tape or other impervious material 222 to prevent leaks. When the membrane is roughly in place and the hydraulic pressure is applied, each strip Will lie fiat against its respective planar surface because the pressure sensitive adhesive has insuflicient strength to stress the membrane surface in tension.

In addition to the functions previously described, the longitudinal channels provided at each of the lateral edges also serve the important function of increasing the permeability to fluids passing through the membrane to the collecting side of the prismatic support. Increased drainage from each lateral face of the prismatic supports results from decreasing the pressure drop on the underside of the membrane, as a result of the presence of the longitudinal channels. This provides a significant additional advantage in connection with the apparatus of the present invention since all pressure drop is reflected by an increased pumping energy requirement.

In addition to eliminating the damaging lateral strain associated with the mounting of semipermeable membranes on cylindrical supports as described above, the use of prismatically configured membrane supports serves another significantly useful purpose. More particularly, when a particle is rotated about an axis, it is accelerated perpendicularly away from the axis. Such centripetal acceleration, as is clearly set forth in said copending application Ser. No. 418,574, is expressed by where:

a is centripetal acceleration in feet per second;

r equals particle distance away from the axis of rotation in a direction perpendicular to such axis; and

w is the angular velocity in radians per second.

Since the radial length between the lateral edges of a given prismatic, tubular support and the longitudinal axis of the support, on the one hand, and a line drawn from said axis to the median longitudinal axis of a given lateral face, on the other hand, are unequal to one another (the former being the greater of the two), the centripetal acceleration of the solution will be greater at the lateral edges than at any other portion of the lateral faces of the prismatic support. As a result, there will be a tendency for the denser solution to flow from the centers of the lateral faces toward the lateral edges of the prismatic support, which aids in the dispersal of solute back into the solution. As will be clear, this induced flow across the membrane will further serve to remove undesirable concentration of salts and the like from the phase interface and thereby increase the efiiciency of the mem brane separative process.

It was previously pointed out that membranes such as may be used in practicing the present invention may frequently have pin-holes or other imperfections which may limit or impair their elfectiveness. This difiiculty may be eliminated in accordance with an embodiment of the present invention such as is illustrated in FIG. 5.

The fragmentary and enlarged structure shown in FIG. 5 is similar to that shown in FIG. 3 except that the membrane layer and its supporting layers have not yet been subjected to pressure and have, accordingly, not been stressed. As shown in this figure, a membrane support 230 is provided with through-bores 232 at each of its dihedrals, the through-bores at a given dihedral being connected by a channel 234. Supported by the planar faces 236 of support 230 is a semipermeable membrane 238. Interposed between membrane 238 and support 230 is the laterally permeable backing material 240.

To this point, the structure described does not differ from the previously described exteriorly mounted membrane assemblies. The structure of FIG. does difler from such other structures, however, in that interposed between membrance 238 and backing material 240 is a layer of filter paper 242 whose porosity is normal to its surface. This filter paper layer, it fine enough, serves to permit the location of imperfections in the membrane as follows: During the initial application of pressure to the membrane assembly and before such assembly is removed from its pressure shell for the application of the impervious capping over the stressed portions of the membrane at the dihedrals (as previously described with respect to FIG. 3), fine particles of a visible material (such as carbon black particles about millimicrons in diameter) are included in the solvent stream which is flowing under pressure into the pressure shell. So long as the porosity of the filter paper 242 is such as to retain such fine particles (viz., in the case of the carbon black mentioned previously the pores of the filter paper should be less than 25 millimicrons), the filter paper will serve to identify any imperfections (such as pin-holes) in the membrane since the fine particles will pass through such imperfections and, being visible, will leave a visible identifying trace on the filter paper at that point. When the membrane assembly is removed from its pressure shell for the application of the impervious capping material at the dihedrals, the same impervious material can be employed to patch up the imperfections identified by the darkened filter paper. The patched membrane assembly can then be remounted in its pressure shell and the normal operation of the device begun. As will be apparent, of course, the size of channels 234 must be such as to accommodate all three of the layers 238, 240 and 242.

In the description set forth above, as well as in said copending application Ser. No. 418,574, the various structures described were all adapted for rotation to provide the centripetal acceleration which forms the essence of a broad inventive concept developed by applicant. In FIGS. 6 and 7 of the present application, however, a structure is illustrated which is not adapted to be rotated but which is made feasible through the use of the planar surfaced support previously described. in this application, as will be made more clear as this description proceeds. The structure of FIGS. 6 and 7 is particularly adapted to situations where the quantity of matter in the fluid to be treated is not great and where, as a result, the membrane blinding problems which otherwise would be created with a non-rotating apparatus to be employed do not present a serious obstacle to its operation.

Once again, for ease of description, the structure and operation of the apparatus of FIGS. 6 and 7 will be set forth in connection with the desalination of sea water. The apparatus of FIGS. 6 and 7 essentially comprises a saline water injection port which is integrally connected to end wall 102. An exit port 104 in turn is integrally connected to another end Wall 106, the two end walls 102 and 106 being joined together with the outer tubular membrane support 108 by means of suitable nuts and bolts 110. Mounted concentrically between end walls 102 and 106 are a plurality of spaced tubular members of progressively larger diameter 112, 114, 116 and 108, respectively.

Tubular members 112 and 116 are constructed similarly to the prismatic members 36 and 38 in the structure of FIGS. 1 and 2. Thus, prismatic support 112 is provided with through-bores 118 spaced longitudinally at each of its lateral edges, longitudinal channels 120 connecting the through-bores 118 along the lateral edges, laterally permeable backing material 122 mounted about the prismatic support 112 and semipermeable membrane 124 mounted about laterally permeable backing material 122. In a similar manner, prismatic support 116 is provided with through-bores 126, longitudinal channel 128, laterally permeable backing material and semipermeable membrane 132. Tubular support 114, on the other hand, has an outer periphery which is a circular cylinder in section and which, While provided with through-bores 134, contains a laterally permeable backing material layer 136 and a semipermeable membrane 138 on its inner periphery rather than on its outer periphery. Tubular support 108 is similar to support 114 in that it has an outer periphery which is a circular cylinder in section and which contains a laterally permeable backing material layer and semipermeable membrane 137 on its inner periphery, through-bores 139 being provided at the dihedrals of support 108. Tubular supports 112, 114, 116 and 108 are secured to end walls 102 and 106 by bolts 110 and sealed in a fluid-tight manner by means of suitable O-rings 140.

By Virtue of the respective arrangement of the tubular supports 112, 114, 116 and 108, annular chambers 142, 144 and 146 are created, and a central collecting chamber 148, as well. Connecting entry port 100 with annular chambers 142 and 146 are, respectively, ports 150 and 152. A manifold 154 is connected to annular chamber 144 and central chamber 148 by means of outlet ports 156 and 158, respectively. A manifold 160 is similarly connected to annular chamber 144 and central chamber 148 by means of ports 162 and 164. Desalinated water leaves the apparatus respectively through outlets 166 and 168 and passes into a colecting trough 141, which also serves to catch the output from through-bores 139 in membrane support 108. Of course, the assembly may also be operated in a vertical position.

The operation of the apparatus of FIGS. 6 and 7 involves the injection of pressurized saline water through inlet 100 and into annular chambers 142 and 146 through ports 150 and 152, respectively. Desalinated water is forced through membranes 132, 137, 124 and 138 and their respective underlying laterally permeable backing material and tubular supports into annular chamber 144 and central chamber 148 and out through-bores 139. Desalinated water from the latter chambers 144 and 148 enters manifolds 154 and 160 and leaves the apparatus through outlets 166 and 168. Saline water. leaves the apparatus through ports 161 and 163 and, ultimately, through outlet port 104.

The prismatic outer configuration of membrane sup ports 112 and 116 prevents membranes 124 and 132 from being stressed undesirably as indicated previously in connection with the structure of FIGS. 1 and 2. Membrane supports 114 and 103 are prismatically configured on their inner peripheries, however, in view of the fact that membranes 138 and 137 are supported on the interior of the tubular structures forming membrane supports 114 and 108, in which case the stressing problems associated with externally supported membranes in a high pressure system are reversed and arise from tension rather than from compression.

The structure of FIGS. 6 and 7 is particularly effective, notwithstanding its lack of rotation and the creation of the centripetal acceleration effect which is so significant to the success of the structure of FIGS. 1 and 2, with feeds which are both colloid free and low in their solute content (viz., in the case of saline water, the salt content should be less than about 10,000 ppm. of salt). Feeds of this type will not create such serious boundary layer concentration problems which otherwise would be involved with feeds containing high salt concentration. Whatever solute buildup occurs at the membrane interface can be reduced sufficiently by circulating solution across the membrane surface.

Typical operating data for an exemplary run using the nonrotatable apparatus of FIGS. 6 and 7 as applied to the desalination of brackish water are set forth in Table 1. A semipermeable membrane which may be effectively employed under the conditions set forth below is UCLA membrane No. 11, described in UCLA Report No. 63-22, Department of Engineering, University of California,

9 Water Resources Center, Contribution No. 74 (May 1963).

Table 1 Salinity of feed, p.p.m. 2,625 Salinity of discharge, p.p.m. 1 10,000

Salt transport, grains/ft. day 1,100 Salinity of product, p.p.m 1 500 Water flux per ft. day

. P.s.i. Osmotic pressure 92 Osmotic overpressure due to boundary layer concentration 184 Distilled Water pressure drop through membrane at 20 gal/ft. day 375 Total pressure 651 1 The salinity of the rejected solution varies somewhat as a result of holding the product water salinity to 500 p.p.m.

The potable water yield per square foot of the aboveidentified membrane under the foregoing conditions will be about 20 gallons per 24 hrs. The estimated net energy requirement, with optimum pumping energy recovery effected through the use of impulse turbines, is less than k.w.h. per 1000 gallons of potable Water. The estimated capital requirement (when power for pumping is purchased from existing facilities) is about $0.50 per daily gallon of potable water output.

The semipermeable membranes employed in connection with the practice of the present invention can be films of cellulose acetate, prepared as described in US. Patents 3,133,132 and 3,133,137. A sheet of such membrane is mounted on the planar surfaced periphery of the tubular support with a layer of laterally permeable backing material between them, the exposed longitudinally extending edges of said membrane being sealed together by taping or the like. The layer of backing material interposed between the semipermeable membranes and their respective supports may suitably be made of any very fine woven, previous, relatively smooth material which is reasonably resistant to crushing. For example, such material may be made of nylon parchment, a suitable such material being sold as clothing interliner. Best results are obtained using at least three plies of such backing material.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

1 claim:

1. A membrane assembly for use in decreasing the concentration of matter in a fluid containing said matter comprising a tubular support having inner and outer peripheral surfaces, at least one of said peripheral surfaces being comprised of a plurality of planar surface segments; a semipermeable membrane supported by said planar surface segments; said support having fluid flow passages spaced along the vertices of the angles formed by the intersection of said planar surface segments establishing fluid flow communication between said membrane and the peripheral surface other than that by which said membrane is supported.

2. A membrane assembly as defined in claim 1 wherein the portions of said semipermeable membrane coinciding with the vertices of the angles formed by the intersection of said planar surface segments are provided with means rendering said membrane impermeable to said fluid at said portions.

3. A membrane support for use in decreasing the concentration of matter in a fluid containing said matter comprising a tubular support; said tubular support having an outer peripheral surface which is comprised of a plurality of planar surface segments; said support having fluid flow passages through its walls spaced along the vertices of the angles formed by the intersection of said planar surface segments establishing fluid flow communication between said outer peripheral surface and the interior of said support; a plurality of said flow passages along at least one of said vertices being connected by a hollow formed in the outer peripheral surface of said support along the vertex in question.

4. Apparatus for carrying out a process in which the concentration of matter in a fluid is decreased by bringing said fluid into contact with one side of a semipermeable membrane, comprising: a tubular support having inner and outer peripheral surfaces, at least one of said peripheral surfaces being comprised of a plurality of planar surface segments; a semipermeable membrane supported by said planar surface segments; said support having fluid flow passages establishing fluid flow communication between said membrane and the peripheral surface other than that by which said membrane is supported; means for bringing a matter-containing fluid under pressure into contact with said membrane; means for withdrawing fluid diminished in its content of said matter from said other peripheral surface; means for withdrawing fluid increased in its content of said matter away from said membrane; and means for rotating said tubular support and the membrane supported thereby about the longitudinal axis of said tubular support.

5. Apparatus as defined in claim 4 wherein said flow passages are spaced along the vertices of the angles formed by the intersection of said planar surface segments and wherein a plurality of said flow passages along at least one of said vertices are connected by a hollow formed in the peripheral surface of said support along the vertex in question.

6. A method of decreasing the concentration of matter in a fluid containing such matter, comprising (1) providing a tubular support having inner and outer peripheral surfaces, at least one of said peripheral surfaces being comprised of a plurality of planar surface segments, a semipermeable membrane supported by said planar surface segments, said support having fluid flow passages establishing fluid flow communication between said membrane and the peripheral surface other than that by which said membrane is supported; (2) rotating said support and. the membrane supported thereby about the longitudinal axis of said support; (3) bringing a matter-containing fluid under pressure into contact with said semipermeable mem brane; (4) withdrawing fluid diminished in its content of said matter from said other peripheral surface; and (5) withdrawing fluid increased in its content of said matter away from said membrane.

References Cited UNITED STATES PATENTS 1,427,031 8/ 1922 Stepp 209406 2,674,440 4/ 1954 Donovan 2l0-321 X 3,055,504 9/1962 Schultz 2l032l X 3,266,630 8/1966 Litt 210-24 X FOREIGN PATENTS 84,684 4/ 1958 Denmark. 211,180 6/1909 Germany. 216,286 11/ 1909 Germany. 376,375 11/ 1939 Italy.

OTHER REFERENCES Reverse Osmosis Unit Desalts Water for City Mains, Chemical Engineering, Aug. 2, 1965, p. 62.

SAMIH N. ZAHARNA, Primary Examiner.

W. S. BRADBURY, Assistant Examiner.

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