WO2004112945A1

WO2004112945A1 - Spacer for use in a membrane separation device and a membrane separation device comprising such a spacer

Info

Publication number: WO2004112945A1
Application number: PCT/NL2004/000429
Authority: WO
Inventors: Fahong Li; André Banier de HAAN; Geert Wytze Meindersma; Thomas Reith
Original assignee: Stichting Voor De Technische Wetenschappen; Universiteit Twente
Priority date: 2003-06-25
Filing date: 2004-06-17
Publication date: 2004-12-29
Also published as: NL1023742C2

Abstract

A spacer (1) for use in a membrane separation device, wherein the same is at least partly formed by an assembly of bands (3, 4) having a width t and a height Dm, wherein t ≠ Dm, which bands are subdivided in a first series of bands (3) disposed substantially parallel to one another in a first plane and a second series of bands (4) disposed substantially parallel to one another in a second plane, wherein the first plane and the second plane are oriented parallel to one another and wherein the longitudinal direction of the bands in the first series runs at an angle to the longitudinal direction of the bands in the second series, and wherein the bands are twisted around the longitudinal axis.

Description

Spacer for use in a membrane separation device and a membrane separation device comprising such a spacer

The present invention relates to a spacer for use in a membrane separation device. The invention also relates to a membrane separation device comprising such a spacer.

Spacers for use in membrane separation devices are generally known in the art. Such spacers serve to keep membranes, of which generally several are used in a membrane separation device, or which in some cases are used rolled up, separate from each other. This ensures that the medium to be separated sustains a good flow over the membrane walls. Such spacers' often consist of nets made of filaments, which nets are composed of intersecting threads that may be woven as well as superposed.

It has been shown that these prior art spacers can be improved. In particular it has been shown, that the energy dis-_. sipation caused by the spacers can be reduced.

It is an object of the invention to provide an improved spacer.

It is a particular object of the invention, to pro- vide a spacer with a reduced energy dissipation.

It is a further object of the invention to provide a spacer that affords a good feed flow to the membrane surfaces and produces a low energy dissipation.

In accordance with the invention, an improved spacer is provided thanks to the fact that the same is at least partly formed by an assembly, which assembly comprises bands having a width t and a height Dm, wherein t ≠ Dm, which bands are subdivided in a first series of bands disposed substantially parallel to one another in a first plane and a second series of bands disposed substantially parallel to one another in a second plane, wherein the first plane and the second plane are oriented parallel to one another, wherein the bands in the first series run at an angle to the bands in the second series, and wherein the angle formed by the latitudinal plane of at least some of the bands in relation to the principal plane, in which the bands are disposed, varies over the longitudinal direction of said bands. The spacer in accordance with the invention provides a lower energy dissipation than the spacers generally used in the art. Despite the lower energy dissipation the mass transfer stayed at least the same as that of the known spacers. According to a preferred embodiment, the spacer in accordance with the invention is characterised in that at least one side of the assembly is provided with a net-like material layer of filaments, preferably filaments having a substantially circular cross section. Such a layer of material on both sides of the above mentioned assembly greatly improves the mass transfer in comparison with the known spacers. In addition, the energy dissipation obtained with such a multi- layered spacer is much lower than with prior art spacers. It is especially advantageous when both sides of the assembly are provided with such a net-like layer of material. In accordance with a particular preferred embodiment, the spacer in accordance with the invention is characterised in that at least part of the bands is twisted around the longitudinal axis. In this way the flowing medium is changed regularly. As a result, the medium to be separated will over the longitudinal direction of the bands be directed upward or downward or even slightly sideways. This greatly improves the feed flow to the membranes at both sides of the spacer. Possibly this twisted configuration of the bands also causes the reduction of the energy dissipation. Advantageous preferred embodiments of the spacer in accordance with the invention are mentioned in the dependent claims 2-14 and 17.

As already mentioned, the invention also relates to a membrane separation device comprising at least one membrane and at least one spacer abutting to the membrane, which spacer is characterised in that it is at least partly formed by an assembly of the kind mentioned in claim 1. Such a membrane separation device has a very low energy dissipation.

A preferred embodiment of a membrane separation de- vice is characterised in that a netted layer is placed between the spacer and the membrane, formed of filaments, preferably filaments having a substantially circular cross section. This greatly improves the mass transfer of the membrane separation device while maintaining the low energy dissipation. Hereinbelow an extensive description of the invention will be given by way of several figures. The figures and the following detailed description merely serve to illustrate a number of preferred embodiments and must on no account be understood as limitation of the invention. Fig. 1 shows a perspective view of a portion of a spacer in accordance with a preferred embodiment of the invention.

Fig. 2 shows a top view of an assembly of bands forming part of the spacer in accordance with Fig. 1. Fig. 3 shows a component detail of the spacer in accordance with the invention.

Fig. 4 shows a top view of a component of the spacer in accordance with a preferred embodiment of the invention. Fig. 5 shows a cross section of a component of the spacer in accordance with the preferred embodiment of the invention.

Fig. 1 shows a perspective view of a multi-layered spacer 1 in accordance with the invention. This spacer 1 reduces the negative effect of concentration polarisation thanks to the considerable improvement of the mass transfer of the bulk of the medium to be separated to the membrane surface (not shown) and vice versa. Because of the innovative shape of the intermediate layer 2, formed here by the twisted bands 3, 4, the energy dissipation is low. Fig. 1 shows a so-called unit cell of the spacer 1 in accordance with a preferred embodiment of the invention. The term "unit cell" signifies that a spacer 1 is comprised of a plurality of such unit cells which, joined together, form the complete spacer. The unit cell shown in Fig. 1 is therefore the smallest unit of a spacer but possessing all the spacer's characteristics .

Hereafter, with reference to the unit cell in Fig. 1, the general term spacer will be used.

The spacer 1, as shown in Fig. 1, comprises an assem- bly 2 of bands 3, 4 arranged to cross each other. Both at the top side and the bottom side, this assembly 2 is covered by a net-like layer of material 5 and 6, respectively. In the embodiment shown, the two net-like layers of material 5, 6 are identical. Hereafter, reference will be made to the net-like. layer of material, meaning both the bottom and the top layer. In practice, however, it is possible that these layers 5, 6 are different from each other. It is also possible that only one net-like layer of material 5 or 6 is provided.

The net-like layer of material 5, 6, also referred to as netted layer 5, 6, consists of thread-like elements (filaments) arranged so as to intersect. These thread-like elements preferably have a circular cross section, as shown in Fig. 5. The thickness of these thread-like elements is indicated with D_t (for the top layer) and D_b for the bottom layer. The inter- secting elements may be superposed, i.e. arranged on top of each other, or they may exhibit a somewhat coherent structure to the use of a weaving technique.

The assembly 2 shown in Fig. 1 consists of bands twisted around their longitudinal axis. These bands have a width t and a height D_m, wherein t ≠ D_m. The bands are twisted about their longitudinal axis. This causes the medium to be separated and flowing in the direction of the bands to be deflected, preferably at an angle , as shown in Fig. 2. Owing to the fact that the bands 3, 4 are twisted about their longi- tudinal axis, the medium will be deflected upward, downward, to the left or to the right, depending on the position of the medium's flow approach to the bands 3, 4. An unhindered flow of the medium will only be possible at the positions where the medium approaches a portion of the bands 3, 4 where the band is disposed horizontally, i.e. in the plane of the crossing bands .

Fig. 2 shows a top view of an assembly from Fig. 1. This clearly shows that the bands 3, 4 cross each other at an angle of approximately 90 A The feed flow direction of the me- dium to be separated indicated with the arrow A is such that the flow approaches the two bands 3, 4 at approximately the same angle. In this top view it can be clearly seen that the bands are twisted about their longitudinal axis. At a position 7, the band 3 is disposed substantially in a horizontal posi- tion. At position 8, band 4 is substantially disposed horizon- tally. Drawn in Fig. 2 to imply that the crossover point of the bands 3, 4 is on a vertical portion of the bands, it is of course equally possible for the bands to cross at the positions 7, 8. This will reduce the total thickness of the assem- bly at those positions. It is also possible that the bands 3, 4 cross at a position 7 of the band 3 and a vertical portion of band 4.

It is also possible that these crossover points are positioned alternately on one band 3 or 4, respectively on a horizontal portion and a vertical portion of the respective bands. The crossover point may also be located on an intermediate portion.

Reverting to Fig. 1, it is clearly shown that the mutual distance between the elements forming the netted layers 5, 6 are smaller than the mutual distance between the different bands 3, 4. In practice, however, these mutual distances may vary within a wide range.

In Fig. 3A a detail of a band 3, 4 in accordance with the invention is shown. The distance over which the band 3 makes a complete, i.e. 360° turn, is indicated with w. The horizontal positions 7 are indicated.

Fig. 3B shows a cross section through the band 3 in accordance with the invention, with the band being in a vertical position. The broken line designates the outer perimeter line of the band 3 when twisted. The width t of the band is considerably smaller than the height D_m. The ratio t/D_m preferably lies in the range from 0.1-0.8. It preferably lies in the range from 0.15-0.6. Even more preferably the ratio lies in the range from 0.2-0.4. Fig. 4 shows a top view of the net-like layer of material. This layer, also referred to as netted layer, comprises a first series of thread-like elements disposed substantially parallel in relation to one another, and a second series of elements disposed substantially parallel in relation to each other. The first series 9, 9', 9'', is disposed at an angle to the second series 10, 10', 10''. The angle at which these elements 9, 10, etc., are disposed in relation to each other is β. The mutual distance between the filaments 10, 10', 10'', etc., is indicated with 1₃, while the mutual distance between the filaments 9, 9', 9'', etc., is indicated with 1₄. Every time reference is made to Fig. 1 in combination with Fig. 2, it is obvious that the feed flow direction of the medium to be separated (indicated with the arrow A) is such that it is substantially the same for the filaments 9, 9', 9'', etc., and 10, 10', 10'', etc. In this way the medium is evenly distributed. An embodiment in which β + 2α = 180° is therefore preferred.

Fig. 1 shows that the height h of the multi-layer spacer in accordance with a preferred embodiment of the inven- tion is formed by combining the height h_b of a bottom netted layer and a height h_t of a top netted layer with the height h_m of the assembly. Studies have shown that the ratio h_t/h or h_b/h preferably lies in the range from 0.15-0.3. This provides a good relationship between the flow variation caused by the assembly and the mass transfer between medium and membrane surface .

It has also been shown that the feed flow angle of the medium to the bands preferably lies in the range from 20- 80°, preferably in the range from 25-60°. The angle β between the bands 3, 4 lies, for example, in the range from 45-150°, preferably from 60-130°. This produces very favourable deflections of the medium wherein the energy dissipation stays low. The ratio of the length of the unit cell lχ in relation to the thickness of the assembly lies, for example, in the range from 2-10, preferably from 2-8, more preferably from 3.0-6.0. The same applies for the length 1₂ in relation to the height h_m.

Although therefore an embodiment is preferred wherein li = 1₂, this is not necessarily so. These distances may vary. The turning ratio li/b (or l₂/b) is also referred to as twist ratio. The twist ratio preferably lies in the range from 0.2-10, preferably in the range from 0.4-5 and more preferably in the range from 0.5-2.

Generally speaking, a large number of twists over a small distance, i.e. a small value of the li/b or l₂/b ratio will cause much turbulence in the medium. This may reduce the energy dissipation. However, if this ratio is very high, there will be insufficient deflection of the medium. An optimal value of the lχ/b or l₂/b ratio is therefore obtained in the above-mentioned range. As shown in the preferred embodiment of Fig. 1, the filaments 9, 10 forming the netted layers 5, 6 are disposed parallel to the bands 3, 4 of the assembly 2. In practice their orientation does not need to be parallel. The distance between the elemenls forming the netted layers 5, 6 is preferably designated as ratio of the distance 1₃ or 1₄, respectively, in relation to the thickness h_t or h_b, respectively, of the top or bottom netted layers 6, 5, respectively. The ratio l₃/h_t and l₄/h_t (wherein h_t may be replaced by h_b) preferably lies in the range from 2.0-10.0, preferably 2.5-8.0, more preferably 3.0-6.0.

With respect to mass transfer and energy dissipation, the multi-layered spacer described above by way of the figures is more effective than the existing spacers. A multi-layered spacer having the geometrical parameters mentioned in the following table is very effective: with an equivalent energy dissipation, the average mass transfer coefficient is approximately 30% higher than that obtained using prior art spacers. If the mass transfer coefficient is kept equal, the energy dissipation is reduced by approximately 40% compared with a spacer generally used in the art.

Table 1. Geometrical parameters of a multi-layered spacer in accordance with the invention

Although .the invention has been described above by way of a preferred embodiment, this must not be understood as a limitation to the invention. For example, the bands may have different shapes than those shown in the figures. The inven- tion is not restricted to a spacer comprising bands that twist around their longitudinal axis. It is also possible to use band-like elements, which in their longitudinal direction have an optionally gradually changing shape. For example, portions of these band-like elements may be positioned so as to incline upward and to incline downward. Intermediate positions are, of course, also possible.

Claims

1. A spacer for use in a membrane separation device, characterised in that the same is at least partly formed by an assembly of bands having a width t and a height Dm, wherein t ≠ Dm, which bands are subdivided in a first series of bands disposed substantially parallel to one another in a first principal plane and a second series of bands disposed substantially parallel to one another in a second principal plane, wherein the first principal plane and the second principal plane are oriented parallel to one another and wherein the longitudinal direction of the bands in the first series runs at an angle to the longitudinal direction of the bands in the second series, and regarding at least part of the bands, wherein the angle formed by the latitudinal plane of those bands in relation to the principal plane in which those bands are disposed, varies in the longitudinal direction of those bands .

2. A spacer according to claim 1, characterised in that the first and the second principal plane are substantially the same plane.

3. A spacer according to claim 1 or 2, characterised in that at least some of the bands are twisted around the longitudinal axis .

4. A spacer according to claims 1 - 3, characterised in that at least one side of the assembly is provided with a net-like material layer of filaments, preferably filaments having a substantially circular cross section.

5. A spacer according to claim 4, characterised in that the net-like material layer is formed from a plurality of intersecting filaments.

6. A spacer according to claim 4 or 5, characterised in that a first series of substantially parallel-oriented filaments is disposed at an angle in relation to a second series of substantially parallel-oriented filaments.

7. A spacer according to claims 1 - 6, characterised in that the bands make a 360° turn over a distance w.

8. A spacer according to claims 2 - 7, characterised in that the height h of the spacer is formed by the height h_m of the assembly and the height h_t or h_b, respectively, of at least one of the top netted layer and a bottom netted layer, respectively, and wherein the ratio h_τ/h or h_b/h lies in the range from 0.05-0.50, preferably in the range from 0.15-0.30.

9. A spacer according to claims 1 - 8, characterised in that the distance lχ or 1₂ between adjacent bands of the first or second series of bands, respectively, lies in the range from 0.2 w to 10 w, preferably from 0.4 w to 5 w and more preferably from 0.5 w to 2 w.

10. A spacer according to claims 1 - 9, characterised in that the second series of bands is placed on the first series of bands.

11. A spacer according to claims 6 - 10, characterised in that the second series of filaments is placed on the first series of filaments.

12. A spacer according to claims 6 - 10, characterised in that the second series of filaments is interwoven with the first series of filaments.

13. A spacer according to claims 1 - 12, characterised in that the same is comprised of a plurality of unit cells having a dimension of li * 1₂, each consisting of a portion of a first band and, crossing over this, a portion of a second band.

14. A spacer according to claims 1 - 13, character- ised in that the feed flow angle of a medium to be separated to the bands lies in the range from 25° - 65°.

15. A membrane separation device, comprising at least one separating membrane and at least one spacer abutting to the separating membrane, characterised in that the spacer is at least partly formed by an assembly in accordance with one of the claims 1 - 14.

16. A membrane separation device according to claim 15, characterised in that between the assembly and the membrane a netted layer is provided, formed from filaments having a substantially circular cross section.

17. A spacer according to claim 1, characterised in that the latitudinal plane of all the bands forms an angle in relation to their principal plane, which angle varies in the longitudinal direction of those bands.