Abstract
The present invention discloses membrane modules utilizing innovative geometries of net-type feed spacers for improved performance in separations and spacer-fabrication methods therein. Membrane modules include: a central tube having perforations along its length for collecting a permeate; at least two filtration envelopes, each filtration envelope having two membrane sheets and a porous, fabric-type insert in between the two membrane sheets for facilitating the permeate to flow, wherein each envelope is closed along three edges of the two membrane sheets, and wherein each envelope is configured to allow the permeate to exit from a fourth, open edge attached to the central tube; and a respective feed-spacer sheet in planar contact with an outer membrane surface of a respective filtration envelope; wherein the respective feed-spacer sheet is in the form of net having nodes connected by flexible segments such that the net forms a regular planar net.
Claims
1. A spiral-wound membrane (SWM) module, the SWM module comprising: (a) a central tube having perforations along its length for collecting a permeate; (b) at least two filtration envelopes, each said filtration envelope having two membrane sheets and a porous, fabric-type insert in between said two membrane sheets for facilitating said permeate to flow, wherein each said envelope is closed along three edges of said two membrane sheets, and wherein each said envelope is configured to allow said permeate to exit from a fourth, open edge attached to said central tube; and (c) a respective feed-spacer sheet in planar contact with an outer membrane surface of a respective said filtration envelope; wherein said respective feed-spacer sheet is in the form of a single-layer net having nodes symmetrically distributed in said net such that node centers of said nodes are on a common plane which is defined as forming a single layer, said common plane being the planar plane of symmetry of said respective feed-spacer sheet, and neighboring centers of four nearest nodes form apices of a parallelogram, and wherein all said nodes are connected by flexible segments such that said net forms a regular planar net, wherein symmetry axes of said flexible segments are on the same plane as the plane of said nodes, wherein said net is symmetric in relation to the planar plane of symmetry of said respective feed-spacer sheet, wherein a ratio of a mean node diameter of said nodes over a mean segment diameter of said flexible segments varies between 1.5 and 5.0, and wherein a symmetric gap, on either side of said flexible segments, between said flexible segments and each of said two membrane sheets varies between 0.167 of said mean node diameter and 0.4 of said mean node diameter.
2. The SWM module of claim 1, wherein said respective feed-spacer sheet is positioned in between two adjacent said filtration envelopes, thereby forming a feed-flow channel such that a main feed-flow direction is along a bisector of said parallelogram.
3. The SWM module of claim 1, wherein an angle, defined by the intersection of symmetry axes of two neighboring said flexible segments of said feed-spacer sheet, varies between about 30 and 150 .
4. The SWM module of claim 3, wherein said angle has an angle bisector coinciding with a main feed-flow direction.
5. The SWM module of claim 1, wherein a node shape of said nodes is approximately spherical or oblate spheroidal.
6. The SWM module of claim 1, wherein a segment shape of said flexible segments is such that their cross-section is approximately circular or elliptical.
7. The SWM module of claim 1, wherein a ratio of the distance between said node centers over a mean segment diameter of said flexible segments varies between about 5 and 14.
8. The SWM module of claim 1, wherein a ratio of a mean node diameter of said nodes over a mean segment diameter of said flexible segments is equal to about two, and wherein symmetry axes of said flexible segments and of said node centers of said nodes lie substantially on said common plane such that said net is symmetric.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The present invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
(2) FIG. 1 is a simplified perspective diagram of a typical SWM module having a standard mesh-type spacer used in water desalination and membrane water treatment/separation, according to the prior art;
(3) FIG. 2 is a simplified perspective diagram of a typical net-type spacer, according to the prior art;
(4) FIG. 3 is a simplified perspective diagram of a novel net spacer, according to embodiments of the present invention;
(5) FIG. 4 is a simplified top view of the geometrical configuration of the net spacer of FIG. 3, according to embodiments of the present invention;
(6) FIG. 5 is a simplified cross-sectional view of the net spacer of FIG. 3, according to embodiments of the present invention;
(7) FIG. 6 is a simplified top view of a spacer fabrication mold, according to embodiments of the present invention;
(8) FIG. 7 is a simplified side view of a fitting assembly for spacer fabrication, according to embodiments of the present invention;
(9) FIG. 8 is a simplified side view and enlarged sectional view of a spacer fabrication mold, according to alternative embodiments of the present invention;
(10) FIG. 9 is a simplified end view of the spacer fabrication mold of FIG. 8 with the melt feed, according to embodiments of the present invention;
(11) FIG. 10 is a graph depicting the computed flow shear-stresses on membrane surfaces employing the net spacer of FIG. 3 with a -value of 105, according to embodiments of the present invention;
(12) FIG. 11 is a graph depicting the computed flow shear-stresses on membrane surfaces employing the net spacer of FIG. 3 with a -value of 120, according to embodiments of the present invention.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
(13) The present invention relates to membrane modules utilizing innovative geometries of net-type feed spacers for improved performance in separations and spacer-fabrication methods therein. The principles and operation for providing such modules and methods, according to the present invention, may be better understood with reference to the accompanying description and the drawings.
(14) Referring again to the drawings, FIG. 3 is a simplified perspective diagram of a novel net spacer, according to embodiments of the present invention. The geometric details of the spacer are shown with nodes 9 of spherical or spheroidal shape having a diameter H, which is equal to the gap (or height) of the flow channel for the feed-liquid. Nodes 9 are symmetrically distributed in the mesh-like pattern such that their centers are on the same plane (i.e., the planar plane of symmetry of the spacer), and form the apices of parallelograms. Nodes 9 are connected during production with flexible cylindrical or nearly-cylindrical segments/filaments 10 of length L and diameter D. In some embodiments, D has a range up to half the node diameter (i.e., H/2) of nodes 9 so that a regular net is formed. Nodes 9 and cylindrical filaments 10 are aligned along lines depicted as E-E.
(15) FIG. 4 is a simplified top view of the geometrical configuration of the net spacer of FIG. 3, according to embodiments of the present invention. The symmetry axes of connecting segments 10 form equilateral parallelograms with an obtuse angle . The unit parallelograms (or cells of the mesh) shown in FIG. 4 can be square or have a non-orthogonal geometry with crossing angle which may vary over a fairly large range of values. A diagonal 11 of the unit parallelogram is also depicted. In typical applications, in SWM RO and NF modules, the values can vary between about 30 and 150, depending on the intended purpose of the particular water treatment.
(16) FIG. 5 is a simplified cross-sectional view of the net spacer of FIG. 3, according to embodiments of the present invention. FIG. 5 shows how the symmetry axes of cylindrical filaments 10 are on the same plane with the centers of nodes 9 along a symmetry plane depicted as line E-E, resulting in a symmetrical mesh. However, achieving a perfect symmetry (i.e., axes and centers of symmetry on exactly the same plane) is not necessary for spacer production. In such a configuration, the spacers only contact the membranes at contact points (or narrow circular areas), not along contact lines which are undesirable as with conventional spacers.
(17) In one embodiment of the present invention, the net-type pattern is formed of equilateral squares, with a distance between centers of neighboring nodes 96 mm and a ratio of diameters (i.e., H/D)2.0. In such an embodiment, the spacer is arranged within the SWM module so that the main (axial) flow direction (e.g., feed flow 5 of FIG. 1) is along diagonal 11 of the unit parallelogram, shown in FIG. 4.
(18) The geometric configuration of the net spacer shown in FIG. 3 results in the elimination of areas of reduced or zero flow when implemented in an SWM module. Such areas are the regions in which undesirable phenomena of increased CP and MF are encountered. Furthermore, in such a geometric configuration, flow constrictions between spacer filaments and membranes are doubled, compared with conventional spacer geometry. As described above, such regions of flow constriction have high shear stresses and mass-transfer coefficients due to locally-increased flow velocities. Such an increase in transport coefficients significantly benefit SWM-module performance regarding effectiveness and degree of separation (see Koutsou et al. cited above).
(19) In the embodiments of FIGS. 3-5, typical values of the geometric parameters L, H, and can be used. Previous studies using conventional spacer geometry (see Koutsou et al. cited above and Koutsou et al. in Desalination and Water Treatment, 18, 139-150 (2010)) show that with values of these parameters varying within certain ranges (e.g., L2.4-4.8 mm, H0.7-1.2 mm, and 60-120, one can achieve almost optimum results regarding relatively high mass-transfer coefficients with an acceptable pressure drop within the filled flow channels. To attain the best SWM performance possible with the spacer configuration of FIGS. 3-5, it is possible to broaden the range of its parameter values, especially if other conditions or limitations (arising in the fabrication process of the SWM modules) so dictate, or if such conditions point in the direction of a broader range of spacer geometric parameters.
(20) The net spacer of FIGS. 3-5 can be incorporated as a feed-spacer sheet into standard SWM modules (such as depicted in FIG. 1). In such an assembly, the net spacer of FIG. 3 is employed in place of mesh-type spacer 1 of FIG. 1. Given that three sides/edges of membrane sheets 2 are closed (as described above with regard to FIG. 1), the combined sheet structure takes the form of an envelope. In embodiments of the present invention, each envelope, formed from two membrane sheets, encloses a fabric-type insert, which allows flow of the permeate along transverse direction 3. A feed-spacer sheet, having the geometric configuration of FIG. 3, is placed between adjacent membrane envelopes, thus forming the flow channels for the fluid, which is fed for treatment in an axial flow direction (as in feed flow 5).
(21) The desalinated or clean fluid of membrane permeate 3 flows toward central perforated tube 4 in a direction substantially normal to feed flow 5. As the feed fluid exits the SWM module along feed flow 5, the feed fluid is reduced in volume due to removal of clean or desalinated filtrate, and proportionately increases in the concentration of rejected salts and other undesirable species. Such a concentrate possesses an increased osmotic pressure compared to that of the initial feed-fluid.
(22) FIG. 6 is a simplified top view of a spacer fabrication mold, according to embodiments of the present invention. The mold has two, practically-identical slabs 12 with very smooth surfaces in contact. On each surface of slabs 12 (which is essentially the plane of symmetry of the resulting net), narrow parallel channels are engraved having a semi-circular lateral cross-section (which, when filled with polymer melt or other appropriate material, essentially form half of the spacer net of FIG. 3 with respect to its planar symmetry plane 13). On the smooth surface of slabs 12, larger side channels 14 and 15 are engraved for facilitating the injection of polymer melt, and the filling of the engraved net during production. Additional side channels 16 are engrave to facilitate the removal of air during the injection process (particularly with regard to the filling of the small net channels with polymer melt), thus facilitating total filling of the engraved channels and nodes with material.
(23) FIG. 7 is a simplified side view of a fitting assembly for spacer fabrication, according to embodiments of the present invention. The fitting assembly of FIG. 7 facilitates the precise assembly of slabs 12 of FIG. 6 under pressure. Side channels 16 are also shown in FIG. 7. The surface pattern of one of slabs 12 forms a minor image with the engraved pattern in the surface pattern of the other slab 12. Special fittings are embedded in appropriate locations of each slab 12 using known machining techniques. Depending on the size of the mold, an appropriate means of exerting pressure is employed (e.g., a hydraulic press) to ensure a tight fit that is leak-free. During melt injection, slabs 12 are maintained at an appropriate temperature, depending on the melting point and other relevant physicochemical properties of the injected material. After injection and filling the engraved net pattern with polymeric or other appropriate material, the mold is sufficiently cooled and disassembled after which the solidified net is removed.
(24) FIG. 8 is a simplified side view and enlarged sectional view of a spacer fabrication mold, according to alternative embodiments of the present invention. Two cylinders 17 and 20 of the mold are shown having very smooth surfaces before engraving the pattern leading to fabrication of the net spacer. On the surface of cylinders 17 and 20, helical channels 18 of small width and semi-circular, lateral cross-section are engraved, which each correspond to half of the net structure. The two sets of channels 18 together form the geometric configuration of the net spacer of FIG. 3. Therefore, when the two cylinders are rotated at practically the same rotational speed in opposite directions toward their line of contact, a net spacer 19 is formed (corresponding to FIG. 3). Channels 18 have a semi-circular, lateral cross-sectional diameter D, which corresponds to half of connecting filaments 10 in FIG. 3 with respect to the main symmetry plane. Such parallel helical engravings on the surface of cylinders 17 and 20 are configured such that the projection of the engraved pattern on a plane surface forms parallel small channels (i.e., connecting filaments 10 of the spacer).
(25) In some embodiments of the present invention, the distance between parallel filaments 10 is L, and is equal to the side of an equilateral square of the net. In other embodiments, it is possible for the engraved pattern to lead to formation of a net comprised of non-orthogonal parallelograms of various crossing angles 0 and length L (as shown in FIG. 4).
(26) At a linear distance L along the small engraved channels, semi-spherical cavities are machined or formed which correspond to half the volume of nodes 9 of FIG. 3 with respect to the plane of symmetry depicted as line E-E (as shown in FIG. 5). Such small channels and semi-spherical cavities are engraved on the surface of cylinders 17 and 20 in such a way that their projection on a flat plane comprises half of net spacer 19 with respect to its plane of symmetry. The angle formed in such a projection between the narrow channels and the generator of cylinders 17 and 20 equals the angle /2 (as shown in FIG. 4) which is characteristic of the elementary unit parallelogram of the net spacer. Channels 18 in cylinders 17 and 20 are virtually identical with regard to the distance of the parallel small channels and distance L between the centers of the semi-spherical cavities along each cylinder's channels 18.
(27) Cylinders 17 and 20 are assembled on a special metal frame with the necessary precision such that their axes of symmetry are parallel (normal with respect to the direction of the gravitational field), and are in contact along their surface generator (i.e., a line parallel to their axes). Moreover, their relative position (with respect to engraved channels 18 and cavities) is precisely adjusted so that net spacer 19 is produced when these small channels and cavities are filled with polymer melt upon the cylinders' rotation.
(28) FIG. 9 is a simplified end view of the spacer fabrication mold of FIG. 8 with the melt feed, according to embodiments of the present invention. Cylinders 17 and 20 are rotated at the same rotational speed and in opposite directions toward their line of contact, while molten polymer is provided by a special feeding device 21 at a controlled rate along the contact line of rotating cylinders 17 and 20. Such operation enables engraved channels 18 of FIG. 8 to fill with polymer melt, resulting in net spacer 19 being formed. Upon exiting counter-rotating cylinders 17 and 20, net spacer 19 is sufficiently cooled to be solidified, and subjected to controlled tension. Net spacer 19 can then be rolled around a reel in order to form a final marketable product.
EXAMPLES
(29) The following examples show that the novel spacer geometric configuration depicted in FIGS. 3-5 leads to improved SWM-module performance in comparison to that of conventional spacers. In these examples, basic geometrical parameter values are used which are defined with regard to FIGS. 3-5. For the purpose of comparison, the distribution of the flow shear-stresses on the membranes is presented, which is an important feature of membrane operation. In general, increased values of shear stresses are associated with improved performance of SWM modules (see Koutsou et al. cited above).
(30) FIG. 10 is a graph depicting the computed flow shear-stresses on membrane surfaces employing the net spacer of FIG. 3 with a -value of 105, according to embodiments of the present invention. FIG. 11 is a graph depicting the computed flow shear-stresses on membrane surfaces employing the net spacer of FIG. 3 with a -value of 120, according to embodiments of the present invention. The computed flow shear-stresses (see Koutsou et al. cited above) in FIGS. 10 and 11 correspond to common spacer geometry (labeled conventional geometry) in comparison to the spacer geometry of FIG. 3 (labeled novel geometry). In both FIGS. 10 and 11, the ratio of distance L between nodes 9 over diameter D of connecting filaments 10 (shown in FIG. 4) is the same (i.e., L/D=10); whereas, the crossing angle equals 105 in the data of FIGS. 10 and 120 in the data of FIG. 11.
(31) The range of Reynolds numbers in the results of FIG. 10 corresponds to the superficial velocity range employed in practice (i.e., 10-30 cm/s). In both data sets (conventional and novel geometry) of FIG. 10, the ratio of the distance between the center of neighboring nodes over the nominal diameter of filaments D is L/D=10; whereas, their crossing angle is =105. The main flow direction is parallel to the line bisecting the angle. The shear stresses induced by the novel geometry are increased by approximately 50%. Such an increase demonstrates the potential for improvement of SWM-module performance when the novel geometry is employed because the increased shear stresses tend to mitigate the undesirable phenomena of CP and MF.
(32) In both data sets (conventional and novel geometry) of FIG. 11, the ratio of the distance between the center of neighboring nodes over the nominal diameter of filaments D is L/D=10; whereas, their crossing angle is =120. Again, as with FIG. 10, the main flow direction is along the line bisecting the angle. FIG. 11 shows that the shear stresses induced when the flow channels are filled with the novel geometry are increased by approximately 50% with the expected significant improvement in SWM-module performance due to the mitigation of effects related to CP and MF.
(33) While the present invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications, and other applications of the present invention may be made.
