NL1033669C2 - Filtering device. - Google Patents

Description

Title: Filtering device

The invention relates to a filtering device and in particular to a filtering device comprising a plurality of microsieves.

PCT patent application WO05 / 075134 describes the operation of a screen, wherein the pressure across the screen is frequently reversed by interruption of cross-flow on the side of the screen where unsieved liquid flows. A stack of sieves can be used for this.

From US 5,753,014 a microsieve is known which is manufactured by etching holes in a very thin layer. WO5 / 075134 describes a filtering device in which such a screen is operated in cross-flow. That is, a cross-flow stream is passed along one side of the microsieve, with a fraction of the cross-flow stream, the permeate, flowing through the microsieve. WO03 / 084649 further mentions stacking of a pair of microsieves without being further detailed about operating the microsieve under dynamic conditions.

The filtering device of WO5 / 075134 provides for frequent pressure reversal over the microsieve, because otherwise the permeate flow would soon stop due to clogging of the microsieve. Because use is made of a micro-screen with a thin layer, the permissible pressure difference over the micro-screen is limited, for example to less than 100 kPa. This essentially applies to both filtering and pressure reversal.

In a number of applications, such as when filtering fermented liquids or milk, a high frequency of pressure inversions is required. The required frequency can 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 cross-flow flow.

US 5,690,829 discloses a water filtering device in which the water is filtered through a reverse osmosis membrane and in which use is also made of periodic interruptions of the cross-flow, albeit at a much lower frequency. There is no question of stacking filters here. The membrane of this device is tubular and forms an inner tube in an outer tube with a cross-flow flow. In this filtering device, the permeate (purified water) is normally discharged via a drain to the inner tube.

When generating the pressure reversal across the membrane, in addition to the interruption of the cross-flow, a valve in the outlet for the purified water is also shut off. Thus, it is possible to maintain the pressure in the inner tube on the permeate side of the membrane. In addition to the shut-off valve in the outlet, the device comprises a reservoir for pure water, with which pure water is returned to the permeate side of the membrane during the pressure reversal. As a result, a reverse permeate flow is maintained under more or less static conditions during the pressure reversal. In both cases, it is not possible, or only limited, to prevent the permeate side pressure from falling.

It is inter alia an object of the invention to provide a compact filtering device with a large number of microsieves that provides for frequent pressure reversal without thereby exceeding a permissible pressure drop across the microsieves.

A filtering device according to claim 1 is provided. The device comprises a filter module which is provided with a buffer, with a surface that is resiliently displaceable in the permeate. In this way the pressure in the permeate in the filter modules is substantially maintained with a pressure reversal. Thus, the permeate discharge line does not determine the pressure. The permeate discharge line can remain connected to the filter chamber without it being necessary to reverse the flow direction in the permeate discharge line.

In one embodiment use is made of a vertical stack of filter modules with common cross-flow supply and / or discharge and common permeate discharge. By using buffers with a resiliently displaceable surface in each of the filter modules, the connection to the permeate discharge line can thereby be maintained at the interruption of the cross-flow flow without the dynamic pressures in the filter modules thereby significantly influencing each other. The flow direction in the permeate discharge line does not have to be reversed.

In one embodiment the buffer comprises a bellows for being able to move the surface with a wide stroke. In a further embodiment, the bellows is expressed with a spring, so that the permeate pressure is determined at least in part by the spring. Use can also be made of gas pressure in the buffer to contribute to the resilient displacement. When using a stack, differences in the expression of the springs in the different modules absorb hydrostatic pressure differences. When the cross-flow is interrupted, the springs dynamically compensate for the backflowing volume of permeate in the permeate chamber.

In one embodiment, the movable surface is also used to close the permeate chamber by pressing the surface against an outlet of the permeate chamber. This can be achieved, for example, by increasing a gas pressure in the buffer.

These and other objects and advantages will further become apparent from the following description of exemplary embodiments with reference to the following drawings, in which Figure 1 shows a stack of filter modules 4

Figure 2 shows a filter module

Figure 3 shows a filter module

Figure 1 shows schematically a filter device with a stack of filter modules (not to scale). The device comprises a cross-flow supply line 12, a cross-flow discharge line 14, a permeate discharge line 16, pressure regulator 17, a cross-flow pump 18 and a circuit breaker 19. The cross-flow drain is also referred to in the prior art as concentrate drain or retentate drain. The filter modules 10 are stacked vertically, i.e. with 10 consecutive filter modules 10 in the direction of gravity. Although for the sake of clarity only a few modules are shown, in practice ten to one hundred and preferably around fifty stacked modules can be used. Filter modules 10 are each, for example, 30 mm thick.

Cross-flow pump 18 is connected to all filter modules 10 via successively circuit breaker 19 and 15 cross-flow supply line 12. The supply of cross-flow pump 18 can be connected to a storage vessel (not shown). In the embodiment shown, the circuit breaker 19 has a bypass output, to which the liquid flow is directed when the flow is interrupted to filter modules 10. The bypass output can also be connected to the storage vessel (not shown). Cross-flow discharge line 14 connects all filter modules 10 to pressure regulator 17. Permeate discharge line 16 connects all filter modules 10 to a permeate output of the filter device. In an embodiment, the device comprises a further pressure regulator (not shown) on the permeate discharge line 16.

Figure 2 schematically shows a filter module 10. The filter module 10 comprises a microsieve 20 mounted on a cross-flow permeate partition 22, a buffer 24, outer walls 26 and connections 27, 28, 29 for cross-flow supply line 12, cross-30 flow discharge line, respectively. 14, and permeate discharge line 16 (not shown). Cross-flow permeate partition wall 22 and microsieve 20 separate the space between outer walls 26 in a cross-flow chamber and a permeate chamber. Microsieve 20 is preferably arranged horizontally, so that there is no hydrostatic pressure gradient over the surface of microsieve 20. The permeate side of microsieve in all modules is preferably arranged facing upwards. This has the advantage that the module is easier to get rid of air / gas inclusions. Connections 27.28 for cross-flow supply line 12, cross-flow discharge line 14 are arranged on opposite sides of microsieve 20 on the cross-flow chamber. Connection 29 for permeate discharge line 16 is provided on the permeate chamber.

Buffer 24 is mounted between cross-flow permeate partition 22 and outer wall 26. Buffer 24 comprises an air chamber 240, a bellows 242, a spring 244 and an air connection 246. Bellows 242 comprises an extensible wall protruding into the permeate chamber of the intermodule 10 15 and is sealed with a surface 248 of bellows 242 disposed on the extensible wall. The extensible wall and surface 248 isolate air chamber 240 from the permeate chamber of the intermodule 10. The air chambers 240 of all filter modules 10 are connected to an air supply via air connections 246. The air supply is preferably arranged to supply an adjustable air pressure, and even more preferably to supply an air pressure that is adjustable per filter module. This has the advantage that the expansion of bellows 242 can be easily adjusted. 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. Furthermore, the expansion of the bellows can also be driven mechanically, for example with an electric motor in the filter unit which is coupled to the bellows via a spring. By expressing the bellows with pressure from the air or liquid supply it is also possible to generate pressure inversions over the microsieve without interruption of the cross-flow reversals. This is also the case if one motor is provided per filter unit. Because of the required pressure 6, use is preferably made for this purpose of liquid supply under pressure to the air chamber 240 (which in this case is a liquid chamber), or mechanical drive. Mechanical drive also has the advantage that a very fast response can easily be realized, so that the pressure reversal can be maintained for a maximum time.

Spring 244 is between the surface of bellows 242 and the wall of air chamber 240. Spring 244 causes the surface of bellows 242 to be compressed with an elastic force into the permeate chamber of intermodule 10, whereby depending on the pressure of the permeate in the permeate chamber the surface of bellows 242 is pushed back a larger or smaller distance.

In operation, cross-flow pump 18 pumps cross-flow to cross-flow feed line 12. Therefore, in all filter modules 10, cross-flow from the cross-flow feed line 12 is pumped through the cross-flow kainer along microsieve 20. Microsieve 20 allows permeate to pass through to the permeate chamber. The permeate flows out of the filter modules 10 via permeate discharge line 16. Interrupter 19 interrupts the supply of cross-flow periodically through cross-flow supply line 12. As a result, the pressure falls in the cross-flow chambers, with the result that a pressure reversal occurs over microsieves 20. This leads to a flow reversal through microsieves 20 whereby they are cleaned. The permeate chambers of the different filter modules then remain connected to the common permeate discharge line 16.

Use is preferably made of a fairly high frequency of interruption, for example 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 for example 15 Hertz . In an embodiment in which microsieves 20 have a diameter of 150 mm, the volume of backflow permeate per microsieve is for example 1 to 2 cubic centimeters. Without buffers 24 this would lead to problems because permeate cannot be supplied to all filter modules 10 sufficiently quickly via the common 7 permeate discharge line 16.

Buffers 24 compensate permeate volume changes due to the reverse flow at the pressure reversals and thereby 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 are (substantially) hydrostatic pressure differences between the liquid in corresponding chambers of different filter modules 10. That is, the pressure difference between different filter modules 10 corresponds to the height difference between the filter modules times the liquid mass density and the gravity 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, a balance is established between the expression 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 expressed so that the difference in resilience between different filter modules corresponds to difference in hydrostatic pressure. If the bellows 242 also has some spring force, this applies to the assembly of the spring force of spring 244 and bellows 242. Instead of a spring, other resilient, for example elastic, elements can also 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, force differences of the spring force exerted on the surface can be realized which correspond to hydrostatic pressure differences between the filter units. Also, the spring constant is preferably chosen such that the volume loss of the permeate in the permeate chamber upon pressure reversal over the microsieve is compensated by resilient expansion without the permeate pressure becoming lower than the pressure on the cross-flow side of the microsieve.

With the rapid and short-term pressure reversal over microsieves 20, it is not possible to return permeate through the permeate discharge line 16 to the filter modules 10 sufficiently quickly to compensate for permeate discharge through the reverse liquid flow through the microsieves 20. As a result, without buffers 24, the permeate pressure would fall sharply to a point that the pressure drop across microsieves hardly led to a reverse flow. As a result, the cleaning effect of the pressure reversal would be virtually undone.

Buffers 24 expand in response to the onset of 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 for the volume of permeate that flows back through microsieve 20. This reduces the pressure drop by a factor. The extent to which the pressure drop is reduced is proportional to the area of the bellows 242 that moves against 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 having a sufficiently large area Thus, the pressure drop can be sufficiently limited to maintain the cleaning effect of the pressure reversal. In the embodiment in which the volume of backflow permeate per microsieve is between 1 and 2 cubic centimeters, use can be made, for example, of a surface area of ten to twenty square centimeters. A limited stroke of the bellows is therefore sufficient. The surface area is of the order of magnitude proportional to the volume required.

Due to the short duration of the pressure reversal and because the backflowed volume is completely, or almost completely, collected by the buffer, no current is reversed on the discharge conduit for the permeate. With a centrally arranged storage tank or buffer for all filter modules, it appears that the volume at the frequencies used cannot be collected.

It will be clear that in this way a relatively simple filtering device can be realized, with common supply and discharge lines for a plurality of filter modules. The stack of filter modules has a modular structure. By closing off the permeate discharge of a module, a module can simply be put out of operation if it does not function properly without the filtering process having to be stopped in the other filter modules. For detecting malfunctioning of filter modules, pressure sensors are preferably fitted in the filter modules to check the pressure drop across the microsieve. Furthermore, the use of the buffers makes it possible to use any intermodule at any height in the stack without further adjustment.

In a further embodiment, buffer 24 is arranged opposite an outlet of the permeate to permeate discharge line 16 intermodule 10. Thereby buffer 24 is arranged such that the surface of bellows 242 closes the outlet at a maximum expansion of the buffer. Furthermore, in this embodiment, an air pressure switch is provided to manually switch, or under the control of a control computer, the air pressure in the chamber of buffer 24 to an increased pressure whereby the bellows 242 closes the outlet. In this way the permeate discharge of the filter module 10 can be closed without a separate valve.

Figure 3 shows an embodiment in which a further buffer is arranged in the cross-ilow 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 upper surface 33 connected thereto exclude a buffer chamber 30 to which air supply 36 is connected. . Spring 34 is arranged such 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 cross-flow chamber. It has been found that when the cross-flow is interrupted, pressure peaks occur, whereby a lack of measures results in a higher pressure in the cross-flow chamber than during normal cross-flow. A pressure peak can also occur when the interruption is reversed. The further buffer limits these pressure peaks. During cross-flow, spring 34 presses the surface 33 against stop 38 (state shown in the figure). The spring force of spring 34 and the pressure in buffer chamber 30 are set such that additional pressure in the cross-flow chamber prints surface 33 of stop 38. The additional pressure is limited by the resulting volume increase in the cross-flow chamber. By adjusting the air pressure on air supply 36, the desired cross-flow pressure is set whereby the further buffer comes into action. Gas or liquid can be used instead of air. It will further be understood that the buffer in the cross-flow chamber can be designed differently with the same effect of volume change at pressure peaks.

In principle, each filter module can be provided with such a further buffer, or a part of the filter modules can be provided with it, in filter modules in which the pressure peak is larger than in the other filter modules, depending on the distance to the interruption. By providing the further buffers in the cross-flow chamber, the pressure peak can be combated more effectively than by measures in the common cross-flow supply or discharge. The further buffer can be combined with the buffer in the permeate chamber, for example by simultaneously increasing the volume (and thereby the pressure) of the permeate chamber as the pressure in the cross-flow chamber increases. A common air supply can also be used, although it is preferred that the pressure in both buffers is independently adjustable. In another embodiment, the further buffer can be included in the cross-flow chamber without buffer in the permeate chamber, thereby limiting pressure peaks. Sufficient pressure can then be provided in another way in the permeate chamber. Use of buffers in both permeate chamber and the cross-flow chamber has the advantage that an undesired pressure drop across the microsieve is locally prevented in the filter module.

Although a specific embodiment has been shown, it will be clear that alternatives to this are possible. Thus, in buffer 24 instead of a bellows and spring, use can also be made of an elastic membrane, or of a cylinder with a resiliently displaceable piston therein. However, it is easier to achieve sufficiently large volume changes with a bellows or a piston. A cylinder must be sealed, which can lead to contamination. Therefore, the use of the bellows is preferred. Furthermore, use can also be made of permeate reservoirs for the different filter modules, wherein each permeate reservoir comprises a relatively wide column of permeate with a height corresponding to the hydrostatic pressure of the permeate in the filter module 10. However, such a solution complicates the modular construction of the filter device. It also makes changing or repairing a filter module more difficult. This is easier with a buffer in the filter module.

Furthermore, of course, per filter module 10, instead of one microsieve 10, a plurality of microsieves can be placed in parallel. As a result, the capacity per filter module is increased. In addition, the capacity of buffer 24, or the number of buffers 24, per filter module is preferably also increased proportionally. Furthermore, the pressure reversal can be effected in a different way, for example with a breaker in the discharge line or a pump in the supply line and so on.

12

A plurality of filter modules 10 can also be stacked horizontally, i.e. side by side, possibly without further vertical stacking. In this case there are no hydrostatic pressure differences, but due to flow effects pressure differences between the filter modules 5 can still occur. The buffers absorb these pressure differences. The microsieves can also be arranged vertically, instead of horizontally, as shown, for example by placing the filter modules a quarter turn. Horizontal arrangement with the permeate chamber above, however, has the advantage that air inclusions can be discharged more easily.

Furthermore, use can also be made, for example, of buffers with a sealed inner chamber instead of a chamber which is coupled to a reservoir. Instead of a spring, another resilient element can be used, or a resilient bellows will suffice. The bellows need not protrude into the permeate chamber, but can also protrude into the cross-flow chamber, within air chamber 240, with which more or less extra space can be made for permeate depending on the permeate pressure. In this case, the space inside the bellows is part of the permeate chamber. As intended herein, the surface 248 of the bellows is also displaceable in the permeate chamber in this case.

20 1033669

Claims (12)

1. Filtering device provided with an intermodule, wherein the intermodule is provided with a cross-ilow chamber and a permeate chamber with a microsieve in between, wherein the filtering device is provided with a cross-flow supply line and a cross-flow discharge line, both coupled to the cross-flow chamber of the intermodule, and a permeate discharge line coupled to the permeate chamber of the intermodule, and in which the intermodule is provided with a buffer, with a surface resiliently displaceable in the permeate chamber under the influence of permeate pressure. A filtering device according to claim 1, provided with a stack of identical filter modules, including said intermodule, and wherein the cross-flow chambers of the filter modules in the stack are connected to each other via the cross-flow supply line and / or the cross-flow discharge line and the permeate chambers of the filter modules in the stack are connected to each other via the permeate discharge line. 3. Filtering device as claimed in claim 2, wherein the identical filter modules are stacked vertically with respect to each other. A filtering device according to any one of the preceding claims, wherein the buffer comprises a bellows on which said surface is located. The filtering device of claim 4, wherein the buffer comprises a spring tensioned between the surface and a wall of the buffer. 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. 7. Filtering device as claimed in claim 6, wherein the intermodule comprises a permeate discharge opening and wherein the buffer of the filter module is arranged opposite the permeate discharge opening, so that the surface of the buffer closes off the permeate discharge opening at maximum expansion. 1033669 A filtering device according to claim 7, including a switch valve configured to individually increase a gas pressure in the inner chamber for the filter module. 9. Filtering device as claimed in any of the foregoing claims, provided with a circuit breaker coupled to the common cross-flow supply line or the common cross-flow discharge line and adapted to periodically interrupt a cross-flow flow through the cross-flow chambers of the modules. 10. Filtering device as claimed in any of the foregoing claims, wherein at least one of the filter modules is provided with a further buffer with a surface that is resiliently displaceable in the cross-flow chamber at peaks in a pressure of the cross-flow in the cross-flow chamber . A method for filtering fluid through a microsieve comprising 15. creating a fluid flow along a first surface of the microsieve; - discharging permeate from a permeate chamber adjacent to a second surface of the microsieve opposite the first surface; - periodically applying an interruption in the liquid flow; 20. reducing a volume of the permeate chamber upon application of the interruption, with a surface that is resiliently displaceable in the permeate chamber under the influence of permeate pressure. 12. Method as claimed in 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 cross-flow chamber adjacent to the first surface of the microsieve in the relevant filter module, wherein the cross-flow chambers of the filter modules in the stack are connected to each other via a cross-flow supply line and / or a cross-flow discharge line, and the permeate chambers of the filter modules in the stack via a permeate discharge line are connected to each other. 1033669