WO2008127098A1

WO2008127098A1 - Crossflow filter with backflushing device

Info

Publication number: WO2008127098A1
Application number: PCT/NL2008/050202
Authority: WO
Inventors: Jan Vos; Petrus Johannes Martinus Adriaans
Original assignee: Fluxxion B.V.
Priority date: 2007-04-11
Filing date: 2008-04-11
Publication date: 2008-10-23
Also published as: EP2144682A1; NL1033669C2

Abstract

A filtering device provided with a filter module (26), having a crossflow chamber and a permeate chamber with a microsieve (22) there between. The filtering device is provided with a crossflow supply conduit (27) and a crossflow discharge conduit (28), both coupled to the crossflow chamber of the filter module. The permeate chamber of the filter module is coupled to a permeate discharge conduit (29). The filter module is provided with a buffer, having a surface (248) resiliently displaceable in the permeate chamber under the influence of permeate pressure. Preferably, the filtering device comprises a vertical stack of such filter modules which are connected to a common supply and discharges. The buffer counterbalances volume changes of the amount of permeate in the permeate chamber upon pressure reversals across the microsieve for the purpose of cleaning. Also, pressure differences between different filter units in the stack can be counterbalanced. In addition a buffer (30) can also be forseen on the feed side.

Description

Title: Filtering device

This invention relates to a filtering device and in particular to a filtering device comprising a multiplicity of microsieves.

PCT patent application WO05/075134 describes the operation of a sieve whereby the pressure across the sieve is frequently reversed by interruption of crossflow on the side of the sieve where unsieved liquid flows. Here, use can be made of a stack of sieves.

Known from U.S. 5,753,014 is a microsieve which is manufactured by etching holes in a very thin layer. WO05/075134 describes a filtering device in which such a sieve is operated in crossflow. This is to say that a crossflow stream is led along one side of the microsieve, whereby a fraction of the crossflow stream, the permeate, drains through the microsieve. WO03/084649, further, also mentions stacking of a few microsieves without further elaborating on the operation of the microsieve under dynamic conditions.

The filtering device of WO05/075134 provides frequent pressure reversal across the microsieve, because otherwise the permeate flow would soon come to a standstill due to clogging of the microsieve. Because use is made of a microsieve with a thin layer, the allowable pressure difference across the microsieve is limited, for instance to less than 100 kPa. This applies basically both to the filtering and to the pressure reversal.

In a number of applications, as in the filtering of fermented liquids or milk, a high frequency of pressure reversals is required. The required frequency may be around 1 Hz (1 pressure reversal per second) or even higher. Pressure reversal at such a frequency can be realized by dynamic interruption of the supply of the crossflow stream.

Known from U.S. 5,690,829 is a filtering device for water, in which the water is filtered through a reverse osmosis membrane and in which also use is made of periodic interruptions of the crossflow, albeit of a much lower frequency. No stacking of filters is involved here. The membrane of this device is tubular and forms an inner tube in an outer tube with a crossflow stream. In this filtering device, the permeate (purified water) is normally discharged to the inner tube via a discharge.

In generating the pressure reversal across the membrane, in addition to the interruption of the crossflow, also a valve in the discharge for the purified water is closed. In this way, it is possible to maintain the pressure in the inner tube on the permeate side of the membrane. In addition to the shutoff valve in the discharge, the device comprises a reservoir for pure water, which allows pure water to be delivered back to the permeate side of the membrane during the pressure reversal. In this way, under move or less static conditions, a reversed permeate flow is kept going upon the pressure reversal. In either case, a pressure decrease on the permeate side cannot be prevented or can be prevented to a limited extent only.

Known from WO 01/10540 is a crossflow filtering device in which the filter surface is cleaned by backwash which is generated by raising the pressure on the permeate side of a sieve. In one embodiment, there is provided a buffer for supplying permeate to a permeate chamber during the backwash. In another embodiment, the pressure on the permeate is applied by pressurizing a separate backwash medium and transferring this pressure to the permeate via a flexible wall.

NL 1020180 shows a filtering device with which backwash is generated by pressure from a magnetically operated piston or with a crankshaft rotating against a hose squeezed flat. EP 588348 shows a filtering device with which backwash is generated by contacting a pressure medium alternately with the filtrate chamber and the permeate chamber. It is one object of the invention to provide a compact filtering device with a largo number of microsieves, which provides frequent pressure reversal without thereby exceeding an allowable pressure drop across the microsieves. There is provided a filtering device according to claim 1. The device includes a filter module which is provided with a buffer, having a surface which is resiliently displaceable in the permeate. With this, responsive to pressure change at pressure reversal, the pressure in the permeate in the filter modules is virtually maintained. In this way, the permeate discharge conduit is not determinative of the pressure. The pressure change can for instance be generated via the filtrate chamber, with the buffer in the permeate chamber responding. The pressure change can for instance be generated t hrough momentary interruption of the crossflow. The buffer then ensures thiit the change of the permeate-side pressure remains minimal, so as to allow the pressure reversal to be utilized as optimally as possible. The permeate discharge conduit can remain connected to the filter chamber without recμiiring that the direction of flow in the permeate discharge conduit be reversed.

In an embodiment, use is made of a vertical stack of filter modules with common crossflαw supply and/or discharge and common permeate discharge. By the use of buffers with resiliently displaceable surface in each of the filter modules, the connection with the permeate discharge conduit can then be maintained upon the interruption of the crossflow stream without the dynamic pressures in the filter modules thereby influencing each other significantly. The direction of flow in the permeate discharge conduit does not need to be reversed.

In an embodiment, the buffer comprises a bellows to allow the surface to be displaced with an ample stroke. In a further embodiment, the bellows is expanded with a spring, so that the permeate pressure is determined at least in part by the spring. Also, use can be made of gas pressure in the buffer to contribute to the resilient displacement. When a stack is used, differences in extension of the springs in the different modules counterbalance hydrostatic pressure differences. Upon interruption of the crossflow, the springs dynamically compensate the back-flowing volume of permeate in the permeate chamber.

In an embodiment, the movable surface is additionally used to allow the permeate chamber to be closed by pressing the surface against an exit of the permeate chamber. This can for instance be effected by increasing a gas pressure in the buffer.

These and other objectives and advantages will further become clear from the following description of exemplary embodiments with reference to the following drawings, in which

Figure 1 shows a stack of filter modules Figure 2 shows a filter module

Figure 3 shows a filter module.

Figure 1 schematically shows a filtering device with a stack of filter modules 10 (not on scale). The device comprises a crossflow supply conduit 12, a crossflow discharge conduit 14, a permeate discharge conduit 16, a pressure controller 17, a crossflow pump 18, and an interrupter 19. In the prior art, the crossflow discharge is also referred to as concentrate discharge or retentatf discharge. The filter modules 10 are stacked vertically, that is, with successive filter modules 10 in the direction of gravity. Although for the sake of simplicity only a few modules are shown, in practice ten to a hundred and preferably around fifty stacked modules can be used. Filter modules 10 are each, for instance, 30 mm thick.

Crossflow pump 18 is connected with all filter modules 10 via, in succession, interrupter 19 and crossflow supply conduit 12. The supply of crossflow pump 18 may be connected to a storage vessel (not shown). In the embodiment shown, interrupter 19 has a bypass exit to which the liquid flow is guided upon interruption of the flow to filter modules 10. The bypass exit may also be connected to the storage vessel (not shown). Crossflow discharge conduit 14 connects all filter modules 10 with pressure controller 17. Permeate discharge conduit 16 connects all filter modules 10 with a permeate exit of the filtering device. In an embodiment, the device comprises a further

controller (not shown) on the permeate discharge conduit 16.

Figure 2 schematically shows a filter module 10. The filter module 10 comprises a microsieve 20 mounted on a crossflow-permeate partition 22, a buffer 24, outer walls 26, and connections 27, 28, 29 for, respectively, crossflow supply conduit 12, crossllow discharge conduit 14, and permeate discharge conduit 16 (not shown). CiOSsflow-permeate partition 22 and microsieve 20 separate the space between outer walls 26 into a crossflow chamber and a permeate chamber. Microsieve 20 is preferably arranged horizontally, so that there is no hydrostatic pressure drift over the surface of microsieve 20. The microsieve permeate side in all modules is then preferably disposed to face up. This provides the advantage of making it easier to remove air/gas inclusions from the module. Connections 27, 28 for crossflow supply conduit 12 and crossflow discharge conduit 14 are provided on opposite aides of microsieve 20, on the crossflow chamber. Connection 29 for permeate discharge conduit 16 is arranged on the permeate chamber.

Buffer 24 is mounted between crossflow-permeate partition 22 and outer wall 26. Buffer 24 compi'ises an air chamber 240, a bellows 242, a spring 244 and an air connection 246. Bellows 242 comprises a stretchable wall which projects into the permeate chamber of the filter module 10 and is closed with u surface 248 of bellows 242, which is arranged on the stretchable. wall. The stretchable wall and the surface 248 close off air chamber 2*10 from the permeate chamber of the filter module 10. The air chambers 240 of all filter modules 10 are connected via air connections 246 to an air supply. The air supply is preferably arranged to provide a settable air pressure, and more preferably to provide an air pressure that is settable per filter module. This provides the advantage that the expansion of bellows 242 can be simply set. Alternatively, the air supply can be connected to an air reservoir or to the outside air, or to a supply of another gas, or hydraulic fluid. Further, the expansion of the bellows may also be driven mechanically, for instance with an electric motor in the filter unit which is coupled to the bellows by way of a spring. By pressing out the bellows with pressure from the air or fluid supply, pressure reversals across the micvosieve cm be generated also without interruption of the crossflow. This is also the case when a motor is provided per filter unit. Because of the required pressure, for this purpose preferably use is made of liquid supply under pressure to the air chamber 240 (which in this case is a liquid chamber), or mechanical drive. Mechanical drive further provides the advantage \ hat a very fast response can be realized in a simple manner, so that the pressure reversal can be maintained for a maximum time.

Spring 244 is arranged between the surface of bellows 242 and the wall of air chamber 240. Spring 244 provides that the surface of bellows 242 is pressed with an elastic force into the permeate chamber of filter module 10, so that, depending on the pressure of the permeate in the permeate chamber, the surface of bellows 242 is pressed back over a larger or smaller distance.

In operation, crossflow pump 18 pumps crossflow to crossflow supply conduit 12. Aa a result, in all filter modules 10, crossflow is pumped from the crossflow supply conduit 12 through the crossflow chamber along microsieve 20. Microsieve 20 then allows permeate to pass to the permeate chamber. The permeate drains from the filter modules 10 via permeate discharge conduit 16. Interrupter 19 periodically interrupts the supply of crossflow through crossflow supply conduit 12. As a result, the pressure in the crossflow chambers drops, with the result that a pressure reversal occurs across micvosieves 20. This leads to a reversal of flow through microsieves 20, so that these are cleaned. The permeate chambers of the different filter modules then remain in communication with the common permeate discharge conduit 16. Preferably, use is made of a fairly high frequency of interruption, for instance in a range of 1-50 Hertz (1 to 50 pressure reversals per second), more preferably between 5 Hz and 50 Hz or 10-20 Hz, and being for instance 15 Hertz. Tn an embodiment in which microsieves 20 have a diameter of 150 mm, the volume of back -flowing permeate per microsieve is for instance 1 to 2 cubic centimeters. Without buffers 24, this would lead to problems because via the common permeate discharge conduit 16 permeate cannot be supplied sufficiently fast to all filter modules 10.

Upon the pressure revei'sals, buffers 24 compensate permeate vohime changes resulting from the reversed flow and then keep the pressure in the permeate chambers of the different filter modules 10 virtually constant.

As a result of the vertical stacking of filter modules 10, there exist (virtually) hydrostatic pressure differences between the liquid in corresponding chambers of different filter modules 10. That is to say that the pressure difference between different filter modules 10 corresponds to the height difference between the filter modules times the liquid mass density and gravitational acceleration. Thus, the pressure drop across the microsieves in the different filter modules is virtually the same in each filter module 10. During normal filtering, an equilibrium is established between the extension of the springs 244 in buffers 24 and the different pressures in the permeate chambers of the different filter modules 10. The springs 244 are then extended so far that the difference in spring action between different filter modules corresponds to difference in hydrostatic pressure. If the bellows 242 also has some spring action, this holds for the composite of the spring action of spring 244 and bellows 242. Instead of a spring, also other resilient, for instance elastic, elements may be used.

The spring constant of these resilient elements is preferably chosen such that with displacements within the available range of displacements of the surface of the bellows, differences in force of the spring action exerted on the surface can be realized thai correspond to hydrostatic pressure differences between the filter units. Also, the spring constant is preferably chosen such that the loss by volume of the permeate in the permeate chamber upon pressure reversal across the microsieve is compensated by resilient expansion without the permeate pressure becoming lower than the pressure on the crossflow side of the microsieve.

Upon the fast and momentary pressure reversal across microsieves 20, it is not possible pass permeate back through the permeate discharge conduit 16 to the filter modules 10 sufficiently fast to compensate for permeate discharge resulting from the reversed liquid flow through the microsioves 20. Consequently, without buffers 24, the permeate pressure would strongly fall back to a point where the pressure reversal across microsieves hardly leads to reversed flow anymore. This would virtually undo the cleaning action of the pressure reversal. Buffers 2Λ expand in response to the onset of the pressure drop in the permeate. The expansion is proportional to the pressure drop but at the same time the pressure drop is counteracted because the expansion reduces the available volume in the permeate chamber, which largely compensates the volume of permeate that flows back through microsieve 20. As a result, the pressure drop is reduced by a factor. The extent to which the pressure drop is reduced is proportional to the surface of the bellows 242 that moves counter to the permeate and the proportionality constant between volume and pressure changes in the permeate, and inversely proportional to the spring constant of spring 244. By using a sufficiently large surface, in this way the pressure drop can bo limited sufficiently to preserve the cleaning action of the pressure reversal. In the embodiment in which the volume of back-flowing permeate per microsieve is between 1 to 2 cubic centimeters, for instance use can be made of a surface of ten to twenty square centimeters. Thus, a limited stroke of the bellows suffices. The surface is proportional in order of magnitude to the required volume.

Because of the short duration of the pressure reversal and because the volume flowing back is completely, or virtually completely, counterbalanced by the buffer, there is no reversal of flow on the discharge conduit for the permeate. It has been found that with a centrally arranged storage vessel or buffer for all filter modules, the volume cannot be counterbalanced at the frequencies used.

It will be clear that in this manner a relatively simple filtering device can be realized, with common supply and discharge conduits for a multiplicity of filter modules. The stack of filter modules is modular in construction. By closing the permeate discharge of a module, a module can be simply set out of operation if it does not function properly, without requiring that the filtering process in the other filter modules be stopped. For establishing improper functioning of filter modules, preferably pressure sensors are arranged in the filter modxiles to monitor the pressure drop across the microsieve. Further, the use of the buffers makes it possible to deploy each filter module at any height in the stack without further adjustment.

In a further embodiment, buffer 24 is disposed opposite an exit of the filler module 10 for the permeate to permeate discharge conduit 16. Buffer 24 is then so arranged that the surface of bellows 242 closes off the exit at a maximum expansion of the buffer. Further, in this embodiment, an air pressure switch is provided to switch the air pressure in the chamber of buffer 24, by hand or under control of a control computer, to an increased pressure so that the bellows 242 closes off the exit. In this manner, the permeate discharge of the filter module 10 can be closed without separate shutoff.

Figure 3 shows an embodiment in which a further buffer is arranged in the crossflow chamber. In the embodiment shown, the further buffer comprises a buffer chamber 30, a bellows 32, a surface 33, a spring 34, an air supply 36 and a stop 38. Bellows 32 and the top surface 33 connected therewith enclose buffer chamber 30, to which air supply 36 is connected. Spring 34 is so arranged that the bellows is expanded by it. Stop 38 limits the expansion of bellows 32. In operation, the further buffer serves to prevent pressure peaks in the crossflow chamber. It has been found that upon interruption of the crossflow, pressure peaks occur, so that in the absence of measures, a higher pressure arises in the crossflow chamber than during normal crossflow. Also when the interruption is undone, a pressure peak can occur. The further buffer limits these pressure peaks. During crossflow, spring 34 presses the surface 33 against stop 38 (condition shown in the figure). The spring force of spring 34 and the pressure in buffer chamber 30 are set such that extra pressure in the crossflow chamber presses surface 33 off stop 38. Due to the restating volume increase in the crossflow chamber, the extra pressure is limited. By setting the air pressure on air supply 36, the desired crossflow pressure is set at which the further buffer comes into action. Instead of air, use can be made of gas or liquid. It will further be clear that the buffer in the crossflow chamber may bo of different design with the same effect of volume change upon presstire peaks. In principle, every filter module may be provided with such a further buffer, or a part of the filter modules may be provided therewith, in filter modules where the pressure peak is greater than in the other filter modules, depending on the distance to the interruption. By providing the further buffers in the crossflow chamber, the pressure peak can be counteracted more effectively than by measures in the common crossflow supply or discharge. The further buffer may be combined with the buffer in the permeate chamber, for instance by increasing the volume (and hence the pressure) of the permeate chamber at the same time when the pressure in the crossflow chamber increases. Also, a common air supply can be used, although it is preferred that the pressure in the two buffers is independently settable. In another embodiment, the further buffer can be included in the crossflow chamber without buffer in the permeate chamber, so that pressure peaks are limited. In the permeate chamber, sufficient pressure can then be provided for in a different manner. Use of buffers in both the permeate chamber and the crossflow chamber provides the advantage of preventing an undesired pressure drop across the mierosieve locally in the filter module.

Although a specific embodiment has been shown, it will be clear that alternatives to it are possible. Thus, in buffer 24, instead of a bellows and spring, use may also be made of an elastic membrane, or of a cylinder having therein a resiliently displaceable piston. It is simpler, however, to effect sufficiently large volume changes with a bellows or a piston. A cylinder needs to be sealed, which may lead to contamination. For that reason, the use of the bellows is preferred. Further, use may also be made of permeate reservoirs for the different filter modules, with each permeate reservoir comprising a relatively wide column of permeate with a height that corresponds to the hydrostatic pressure of the permeate in the filter modulo 10. However, such a solution renders the modular construction of the filtering device more difficult. It also renders exchanging or repairing a filter module more difficult. This is simpler with a buffer in the filter modulo.

Further, naturally, per filter module 10, instead of one mierosieve 10, multiple microsicves can be placed parallel. This increases the capacity per filter module. Then, preferably, also the capacity of buffer 24, or the number of buffers 24, per filter module is proportionally increased. Further, the pressure reversal can be effected in a different manner, for instance with an interrupter in the discharge conduit or a pump in the supply conduit, and so forth.

Also, multiple filter modules 10 may be stacked horizontally, that is, next to each other, optionally without further vertical stacking. In this case, there are no hydrostatic pressure differences, but due to effects of flow, pressure differences between the filter modules may yet occur. The buffers compensate these pressure differences. Also, the microsieves may be arranged vertically, instead of horizontally, as shown, for instance by placing the filter modules in an orientation rotated through a quarter turn. Horizontal arrangement with the permeate chamber at the top, however, provides the advantage that air inclusions can be removed more easily.

Further, for instance, use may also be made of buffers with closed inner chamber, instead of a chamber which is coupled to a reservoir. Instead of a spring, a different resilient element may be used, or a resilient bellows may suffice. The bellows does not need to project into the permeate chamber but may also project into the crossflow chamber, within air chamber 240, allowing more, or less, extra space for permeate to be created depending on the permeate pressure. In this case, the space within the bellows is part of the permeate chamber. Λs intended here, in this case too, the surface 248 of the bellows is displaceable in the permeate chamber.

Claims

1. A filtering device provided with a filter module, wherein the filter module is provided with a crossflow chamber and a permeate chamber with a microsieve therebetween, wherein the filtering device is provided with a crossflow supply conduit and a crossflow discharge conduit, both coupled to the crossflow chamber of the filter module, and a permeate discharge conduit coupled to the permeate chamber of the filter module, and wherein the filter module is provided with a buffer, which buffer has a surface responsively resiliently displaceable in the permeate chamber, which is displaceable responsively under the influence of permeate pressure changes resulting from pressure reversal across the microsieve.

2. A filtering device according to claim 1, provided with a flow interrupter coupled to the common crossflow supply conduit or the common crossflow discharge conduit and arranged to periodically interrupt a crossflow flow through the cvossflow chambers of the modules, for generating the pressure reversal.

3. A filtering device according to claim 1 or 2, provided with a stack of identical filter modules, among which said filter module, each having a buffer of its own, which has a surface resiliently displaceable in the permeate chamber responsively under the influence of permeate pressure change, and wherein the crossflow chambers of the filter modules in the stack are in communication with each other via the crossflow supply conduit and/or the crossflow discharge conduit, and the permeate chambers of the filter modules in the stack are in communication with each other via the permeate discharge condxxit.

4. A filtering device according to claim 3, wherein the identical filter modules are stacked vertically with respect to each other.

5. A filtering device according to any one of the preceding claims, wherein the buffer comprises a bellows on which said surface is located.

6. A filtering device according to claim 5, wherein the buffer comprises a spi'ing, tensioned between the surface and a wall of the buffer.

7. A filtering device according to any one of the preceding claims, wherein the buffer comprises an inner chamber which is coupled to a fluid supply.

8. A filtering device according to claim 7, wherein the filter module comprises a permeate discharge opening and wherein the buffer of the filter module is disposed opposite the permeate discharge opening, so that at maximum expansion the surface of the buffer closes off the permeate discharge opening.

9. A filtering device according to claim 8, provided with a switching valve configured to increase for the filter module individually a gas pressure in the inner chamber.

10. A filtering device according to any one of the preceding claims, wherein at least one of the filter modules is provided with a further buffer with a surface resiliently displaceable in the crossflow chamber, at peaks in a pressure of the crossflow in the crossflow chamber.

11. A method for filtering liquid through a microsieve, comprising:

- effecting a liquid flow along a first surface of the microsieve;

- discharging permeate from a permeate chamber contiguous to a second surface of the microsieve which is opposite the first surface;

- periodically providing an interruption in the liquid flow as a result of which permeate pressure changes occur in the permeate chamber;

- reducing a volume of the permeate chamber when providing the interruption, with a surface resiliently displaceable in the permeate chamber responsively under the influence of the permeate pressure changes.

12. A method according to claim 11, wherein use is made of a vertical stack of filter modules, each provided with a microsieve and a permeate chamber with a surface resiliently displaceable in the permeate chamber, and a crossflow chamber contiguous to the first surface of the microsieve in the respective filter module, wherein the crossflow chambers of the filter modules in the stack are in communication with each other via a crossflow supply conduit and/or a crossflow discharge conduit, and the permeate chambers of the filter modules in the stack are in communication with each other via a permeate discharge conduit.