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Title:
FEED SPACER FOR CROSS-FLOW MEMBRANE ELEMENT
Document Type and Number:
WIPO Patent Application WO/2020/169883
Kind Code:
A1
Abstract:
This invention relates to a feed spacer, used in a cross-flow membrane element (1) comprising a feed spacer element (2) equipped with a feed channel input end (la) and a feed channel output end (lb), which feed spacer element (2) is arranged to keep the feed channel (4a) open for a feed flow (4) between the membrane surfaces (3a, 3b). The feed spacer element (2) is formed in such a way that the volume taken in the space between the membrane surfaces (3a, 3b) by the feed spacer element (2) increases in the direction of the feed flow (4) from the feed channel input end (la) to the feed channel output end (1b).

Inventors:
AULANKO ESKO (FI)
MUSTALAHTI JORMA (FI)
Application Number:
PCT/FI2020/050097
Publication Date:
August 27, 2020
Filing Date:
February 17, 2020
Export Citation:
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Assignee:
EMP INNOVATIONS OY (FI)
International Classes:
B01D63/06; B01D61/02; B01D61/14; B01D65/08; B01D69/04; C02F1/44
Domestic Patent References:
WO2004080577A22004-09-23
WO2015153116A12015-10-08
WO2019220886A12019-11-21
WO2019117479A12019-06-20
Foreign References:
JP2004089763A2004-03-25
Attorney, Agent or Firm:
SALOMAKI OY (FI)
Download PDF:
Claims:
CLAIMS

1. Feed spacer, used in a cross-flow membrane element (1) com prising a first surface (3a) , a second surface (3b) and a feed spacer element (2) arranged to support the membrane surfaces (3a, 3b) , keeping them at a distance from each other, thus forming a feed channel (4a) with an input end (la) and output end (lb) and having a structure allowing a feed flow (4) in the feed channel (4a) between the membrane surfaces (3a, 3b) , characterized in that the volume taken in the space between the membrane surfaces (3a, 3b) by the feed spacer element (2) increases in the direction of the feed flow (4) from the feed channel input end (la) to the feed channel output end (lb) .

2. Feed spacer according to claim 1, characterized in that the feed spacer element (2) comprises filaments (2d) and apertures (2e) , and that width of the filaments (2d) of the feed spacer element (2) is arranged to increase towards the output end (lb) of the feed channel .

3. Feed spacer according to claim 1 or 2, characterized in that the number of mesh apertures (2e) of the feed spacer element (2) is arranged to increase towards the output end (lb) of the feed channel.

4. Feed spacer according to claim 1, 2 or 3, characterized in that the feed spacer element (2) comprises more than one sec tions (2a, 2b, 2c) each having a different fill ratio which is arranged to increase section by section towards the output end (lb) of the feed channel so that the volume taken in the space between membrane surfaces (3a, 3b) by the feed spacer element (2) increases section by section in the direction of the feed flow (4) from the feed channel input end (la) to the feed chan nel output end (lb) .

5. Feed spacer according to claim 1, 2 or 3, characterized in that the fill ratio of the feed spacer element (2) is arranged to increase gradually towards the output end (lb) of the feed channel so that the volume taken in the space between membrane surfaces (3a, 3b) by the feed spacer element (2) increases gradually in the direction of the feed flow (4) from the feed channel input end (la) to the feed channel output end (lb) .

6. Feed spacer according to any of the claims above, characterized in that the thickness of the feed spacer element (2) is substantially unchanged from the feed channel input end (la) to the feed channel output end (lb) .

7. Feed spacer according to any of the claims above, characterized in that the feed channel (4a) formed by the feed spacer element (2) has a substantially uniform thickness. 8. Feed spacer according to any of the claims above, characterized in that the feed spacer element (2) is compatible with existing cross-flow membrane element (1) arrangements.

Description:
FEED SPACER FOR CROSS-FLOW MEMBRANE ELEMENT

The object of the invention is a feed spacer for a cross-flow membrane element according to claim 1.

Large areas in the world suffer from poor or non-usable drink ing water. Also, in many areas of the world tap water is not suitable for drinking or cooking. The most problematic areas are densely populated areas, such as China, India, Indonesia, Africa but the problem is very common also in Southern Europe, America and in the rest of Asia.

Several techniques to solve the clean water problem has been introduced. One of the most promising water treatment technique is reverse osmosis (RO) system.

Reverse osmosis (RO) systems are effective for suspended and dissolved solids, microbes, toxins and carcinogens. However, low cost RO systems of prior art suffer from poor water econo my. Typically, about 500 litres of water is needed to produce 100 litres of purified water. More effective RO systems need electric power and are more expensive. In spite of their disad vantages, RO systems are the most common Point-Of-Use systems, as they are most effective, removing practically all the con taminants from the water.

However, increasing volume of domestic POU systems using RO methods according to the prior art solutions can be a future threat to the whole mankind because the water consumption would tremendously grow. The recovery rate of prior art POU systems is only 10-30%, which means that 70-90% of the used tap water is wasted.

The operation of a membrane filter, which can be also called a membrane separator, can be described in a simplified manner as follows: The permeable membrane to be used, which possesses certain properties, allows a substance to be purified, e.g. water, to pass through it and retains other materials. In prac- tice, the separation of membrane filters is not perfect, in which case the result of the separation is the permeate that has passed through the membrane and the concentrate, or corre sponding, obtained from the material to be filtered.

A generally used membrane filtration method for liquids is a so-called cross-flow filtering. The cross-flow filtering is in use in many different types of membrane filtering methods, e.g. in the reverse osmosis filtration, nanofiltration, ultrafiltra tion and microfiltration. In the cross-flow filtering the mate rial to be purified is brought to flow in the direction of the surface of the filtering membrane in which case, although some of the material filters through the membrane, the material to be purified rinses the surface of the membrane and in this way keeps the membrane permeable while preventing clogging or a large local concentration on the surface of the membrane. The driving force in the membrane filtration is a pressure differ ence across the membrane. Energy is also needed, apart from overcoming the resistance caused by the membrane, for moving and conducting the liquids in the different phases of the pro cess as well as for raising the pressure of the liquid to the operating pressure needed.

Membrane filter devices and their components are generally available for various purposes. Membrane filter plants and devices are fabricated from commercially available components and are based on generally known techniques. For example, re verse osmosis filters are commercially available for various applications and suitably also for the treatment of various solutions and for the separation of many types of substances, and, inter alia, filters for salt removal can be optimized and also the salt content ratio of the water to be treated can be selected.

A membrane filter device typically comprises an enclosure with a tap water inlet and outlets for a permeate and concentrated water, a central tube for the permeate, a reverse osmosis mem- brane and a feed spacer. The feed spacer serves as a passageway through which raw water is led.

One important parameter in the cross-flow filtering is the membrane recovery ratio = ratio between the part of the feed flow which permeates the membrane to the inputted feed flow. The higher the recovery ratio, the simpler the systems can be made, and in cases where it is not possible to increase recov ery by the purifying system structure, the better is the water economy i.e. less water wasted as rejected concentrate.

A maximum recommended recovery ratio of a typical spiral wound membrane element is usually about 20%. Reason for this is con centrate polarization and foulants collecting on the membrane surface and/or the feed spacer, blocking the feed water flow through the feed channel and blocking the membrane surface. To reduce the polarization and fouling the flow velocity in the feed channel needs to be high enough and preferably turbulent to flush away the foulants and to reduce the polarization lay er .

The primary purpose of the feed spacer is to form a feed chan nel and to keep the feed channel open. Other purposes of the feed spacer are to make the flow turbulent and to have as small flow resistance as possible.

Typically, the feed spacer is a diamond shaped mesh formed by filaments with apertures between the filaments. Usually the feed spacer is made of plastic, where the mesh nodes, where the filaments cross, contact the membrane surfaces and support the membranes, while in other parts of the mesh, where there is just a single filament in the channel, or the mesh eye, water can flow. The "zig-zag" filaments cause turbulence intensifying the flush. Typically, the mesh is positioned so that the mesh filaments are at about 45 degrees angle to the flow direction.

As the feed water is entered in the feed channel from first end of the channel and a part of the water permeates the membrane, the flow velocity in the feed channel reduces in the flow di rection as the feed channel cross section is constant. The reduced flow velocity causes increasing fouling, concentrate polarization and clogging on the surface of the membrane and thus remarkable reduction of the membrane element recovery rate .

In this regard, there is a need for an improved feed spacer capable of improving the membrane element recovery rate by maintaining or even increasing the feed flow velocity and miti gating the concentration polarization.

The problem can be solved with different, more complicated membrane element structures, such as Pentair GRO membranes, which are spiral wound membranes, but with the radial feed flow, or by making the element from a stack of round discs with a radial feed flow. However, these are quite expensive struc tures, and the invention is also applicable in these structures to improve them further. One solution trying to solve the prob lem is presented in a patent publication EP3415224A1, which however has not a solution for increasing the feed flow veloci ty and improved membrane element recovery rate.

The problem which the present invention solves is increasing the membrane element recovery ratio by making the feed spacer such that the cross section of the feed channel reduces in the flow direction, thus keeping the feed flow relatively constant or even increasing towards the output end of the feed channel. This means that the amount of water permeating the membrane can be increased without increasing fouling or concentrate polari zation and thus having a higher recovery rate. The reduction of the cross section of the feed channel means in this connection the same as increasing the space taken by the feed spacer, thus reducing the cross section open for the water flow in the direction of the feed flow from the feed channel input end to the feed channel output end. The aim of this invention is to achieve an effective, inexpen sive, reliable and well-functioning feed spacer for the cross- flow membrane element. More particularly the invention aims for reverse osmosis filtration solutions, nanofiltration solutions, ultrafiltration solutions and microfiltration solutions and especially the type of solutions that are suitable for use in the reverse osmosis enrichment or reverse osmosis purification of aqueous solutions. Further, one aim is to achieve a filter ing solution in a membrane filtration by which water consump tion can be reduced as close as possible to minimum.

The feed spacer for the cross-flow membrane element according to the invention is characterized by what is presented in the characterization part of claim 1. Other embodiments of the invention are characterized by what is presented in the other claims .

An aspect of the invention is to provide a feed spacer for the cross-flow membrane element, which membrane element comprises a first surface, a second surface and a feed spacer element ar ranged to support the membrane surfaces, keeping them at a distance from each other, thus forming a feed channel with an input end and output end and which feed spacer element having a structure allowing a feed flow in the feed channel between the membrane surfaces. To compensate the feed flow reduced by the flow penetrating the membrane the feed spacer structure is made such that the volume taken in the space between the mem brane surfaces by the feed spacer element increases in the direction of the feed flow from the feed channel input end to the feed channel output end.

The substance to be filtered, for example pressurized tab water is later called as input substance. The clean substance that has passed through the membrane filter is later called as per meated substance or permeate. The part of the input substance, which is not passing through the membrane filter is later called as unpermeated substance or concentrate. The membrane filtration unit comprises an enclosure or pressure keeping structure, which is closed and sealed with a cap or by another way. The enclosure cap or the enclosure itself comprises one inlet for the input substance, one outlet for the permeated substance and a second outlet for the unpermeated substance or concentrate that is led, for instance, to the drain.

The basic idea of this invention is that the feed spacer is made such that its fill ratio increases towards the output end of the feed channel. The feed spacer fill ratio in this context means the surface area of the filaments in the specific area of the feed spacer element compared to the surface area of the mesh apertures in the same specific area of the feed spacer element. In other words, the fill ratio here means the volume taken by the material of the feed spacer in relation to the total volume taken by the feed spacer.

Advantageously, the height or thickness of the feed spacer element is equal throughout the width and length of the feed spacer element. This means that the thickness of the feed spac er is substantially uniform or unchanged throughout the width and length of the feed spacer element.

The fill ratio increase can be based on a gradual fill ratio increase, or the fill ratio can be formed by connecting togeth er a group of sections having different fill ratio. The effect can be made in numerous ways, e.g. if the feed spacer is a diamond shaped mesh aperture by:

- decreasing the mesh aperture size towards the output end of the feed channel,

- increasing the filament width towards the output end,

- shaping the filament to take more space towards the output side, or

- a combination of the above

Benefit is gained from the improved feed spacer at least in many ways :

- the membrane recovery is improved, - the water economy is increased,

- smaller flow losses, as the feed spacer can be with smaller fill ratio at the input side of the channel,

- low cost effects, and

- fits in existing manufacturing process.

In addition, upgrading existing cross-flow membrane elements with improved feed spacer to existing water purification units, is very simple and inexpensive. With the invention, both large and small plants can be upgraded energy-efficiently without pressure exchangers or corresponding energy recovery devices being needed. With the solution according to the invention an increased membrane recovery ratio and better water economy can be achieved, which is important e.g. in water purification, in which pretreatment and/or the untreated water has a significant proportion in the total costs.

The invention is particularly advantageous in water purifica tion applications, e.g. in desalination, in making process water, boiler water or plant water, in preparing irrigation water or drinking water, and even for the purification of waste water. The invention is also suited to the enrichment of dis solved substances, e.g. in the food industry or in the mining industry.

The primary conceived use of the invention is the application of it in reverse osmosis separation. Many effects or technical solutions are very similar in other membrane separation tech niques. Within the scope of applicability, the inventive solu tions can therefore be extended to be used in other membrane separation procedures also, although the description and expla nations of the technical solutions are presented here in this context most often in a reverse osmosis device environment.

In the following the invention will be described in more detail by the aid of examples of its embodiments with reference to the attached simplified drawings, wherein Fig. 1 presents in an oblique top view an opened cross-flow membrane element and a feed flow applicable to the in vention,

Fig. 2 presents one embodiment of a feed spacer element of the invention, and

Fig. 3 presents another embodiment of the feed spacer element of the invention.

Fig. 1 presents in an oblique top view and in a simplified way an opened cross-flow membrane element 1 and a feed flow 4 applicable to the invention, and Fig. 2 presents one embodi ment of a feed spacer element 2 of the invention. The feed flow 4 is fed into the cross-flow membrane element 1 from its first end, also called a feed channel input end la.

The cross-flow membrane element 1 of a membrane filter device comprises at least a permeate channel 8, the feed spacer ele ment 2 and a permeate pouch 3. The feed spacer element 2 and the permeate pouch 3 are rolled as a spiral around the permeate channel 8. The first surface or outer surface 3a and side sur faces of the permeate pouch 3 are impermeable, and the second surface or the inner surface or penetration surface 3b of the permeate pouch 3 comprises a membrane film that allows a perme ate 7 penetration 5 from the feed flow 4 to the hollow channel of the permeate pouch 3.

The permeate 7 flows spirally inside the permeate pouch 3 to wards the permeate channel 8. An unpermeated substance or the concentrate 6 of the feed flow 4 not penetrating to the perme ate pouch 3 flows out from the second end of the cross-flow membrane element 1, also called a feed channel output end lb.

The feed flow 4 flows in the space formed by the feed spacer 2 between the outer surface 3a of the permeate pouch 3 and the penetration surface 3b of the permeate pouch 3. The space is called a feed channel 4a. The feed flow 4 is substantially perpendicular to the spirally wound feed spacer element 2 and permeate pouch 3 of the cross-flow membrane element 1 and flows in the feed channel 4a from the feed channel input end la to the feed channel output end lb.

Preferably, the thickness of the spacer element 2 is substan tially equal or uniform from the feed channel input end la to the feed channel output end lb. For that reason, also the thickness of the feed channel 4a is substantially equal or uniform from the feed channel input end la to the feed channel output end lb.

The feed spacer element 2 has a diamond shaped mesh with fila ments 2d and apertures 2e between the filaments. The mesh fila ments 2d are at about 45 degrees angle to the flow direction. The substance feed flow 4 flows in the feed channel 4a in the space formed by the feed spacer element 2 at a certain pressure from the feed channel input end la of the cross-flow membrane element 1 to the feed channel output end lb of the cross-flow membrane element 1.

Because the feed spacer element 2 has a certain thickness the filaments 2d and the apertures 2e of the filaments 2d have a certain volume. The filaments 2d surrounding an aperture 2d and the thickness of the feed spacer 2 form together the volume of the aperture 2e.

According to the invention the volume of the apertures 2e of the feed spacer element 2 decreases in the direction of the feed flow 4 from the feed channel input end la to the feed channel output end lb. In other words, this means that a fill ratio of the feed spacer element 2 increases towards the output end lb of the feed channel 4a.

The feed spacer element 2 comprises three sections 2a, 2b and 2c each having a different fill ratio. In the first section 2a, the filament 2d width is small and the aperture 2e volume is high, which gives more space to the feed flow 4 substance and the feed spacer element 2 fill ratio is low. In the second section 2b, the filament 2d width is medium the aperture 2e volume is medium, which gives less space to the feed flow 4 substance and the feed spacer element 2 fill ratio is medium. In the third section 2c, the filament 2d width is high and the aperture 2e volume is small, which gives even less space to the feed flow 4 substance and the feed spacer element 2 fill ratio is high. The increased fill ratio of the feed spacer element 2 towards the output end lb of the feed channel is achieved by increasing the filament 2d width of the feed spacer element 2 from section 2a to section 2b to section 2c. At the same time when the width of the filaments 2d is increased the volume of the apertures 2e is decreased.

Fig. 3 presents another embodiment of a feed spacer element 2 of the invention. The decreased volume of the apertures 2e of the feed spacer element 2 or the increased fill ratio of the feed spacer element 2 is achieved by increasing the number of mesh apertures 2e towards the output side of feed channel. This means that the mesh apertures 2e are smaller and the number or filaments 2d increases which makes the total volume of the filaments 2d bigger and the total volume of the apertures 2e smaller .

Also, other methods increasing the fill ratio of the feed spac er element 2 can be done, for example by shaping the filaments 2d to take more area towards the output side lb of feed channel 4a .

When using feed spacer elements 2 comprising filaments 2d and apertures 2e like presented in figures 2 and 3 the fill ratio can also be determined as the volume taken by the filaments 2d of the feed spacer element 2 in relation to the total volume taken by the feed spacer element 2.

A low feed spacer element 2 fill ratio or a high total volume of the apertures 2e of the feed spacer element 2 in the first end la of the membrane element 1 provides a high permeating rate and smaller flow losses, since the free surface area of the penetration surface 3b, is high.

Increasing the spacer element 2 fill ratio towards downstream or the output end lb maintains or even increases the feed flow 4 speed, improving the membrane recovery rate. The amount of the permeate 7 is increased without increasing a fouling or the concentrate 6 polarization and thus having a higher recovery rate and increasing the overall water economy.

The cross-flow membrane element 1 with improved feed spacer element 2 according to this invention can be very simply and inexpensively upgraded to existing water purification units.

It is obvious to the person skilled in the art that the in vention is not limited to the examples described above, and it may be varied within the scope of the claims presented below as well as of the description and the drawings present ed. For example, the shape or width of the feed spacer ele ment can be different from that presented in above. Also, the feed spacer element fill ratio can be increased steplessly or gradually instead of sectorially. The most important tech nical effect of this invention is, however, that the fill ratio of the feed spacer element is increasing towards the downstream of the cross-flow membrane element.

Also, the membrane element type can be different from the examples described above. For example, the membrane element may comprise more than one membrane pouches and feed spacer elements connected to the permeate channel.