WO1994021361A1

WO1994021361A1 - Ultrasonic cleaning

Info

Publication number: WO1994021361A1
Application number: PCT/GB1994/000620
Authority: WO
Inventors: Antony Nigel Reed
Original assignee: Leatherhead Food Ra
Priority date: 1993-03-25
Filing date: 1994-03-25
Publication date: 1994-09-29
Also published as: GB9306174D0

Abstract

A method of cleaning a filter membrane (10) comprises generating an ultrasonic standing or pseudo-standing wave by means of one or more piezo devices (18), the standing wave dislodging foulant which is then carried away in the flow of retentate.

Description

ULTRASONIC CLEANING

The present invention relates to the cleaning of membranes used in membrane filtration. Membrane filtration is used in many applications. In one type, a liquid containing suspended solids to be removed is simply passed through a filter to remove the suspended solids and this system is widely used. Another type, cross-flow membrane processing, is used for a variety of food industry applications, including processing of milk and whey products and whey protein concentrates, and clarification of apple juice. Cross-flow membrane processing also has non-food industrial applications, such as water/oil separation, recovery of paints in the car industry, water treatment/purification for the electronics industry, etc., and recovery and purification of cell cultures and enzymes, drug production/purification, dialysis machines (for renal patients) , etc. in the medical industry. The types of processes used in the cross-flow membrane processing usually include one of the following: ultra- filtration, reverse osmosis, ultra-osmosis, nanofiltration, microfiltration, dialysis or diafiltration.

The term "cross-flow" indicates that the liquid flow is in a direction parallel to the plane of the filtering membrane. These processes are all driven by pressure, ranging from 1 or 2 bar up to 1,000 bar. The driving force for the filtration process is the trans-membrane pressure and this is dependent on the membrane system, the process liquor, and the geometric configuration of the membrane.

Membrane geometry describes the types of system being used. Many arguments and discussions have arisen over the subject of which membrane system is the best. The end user will have many reasons for choosing one system in preference to another. The types of systems can generally be thought of in three different classifications: 1. a) Plastics membranes - e.g. typically manufactured from polysulfone, cellulose acetate, PVDF, nylon, butyl rubber, etc. b) Composite membranes - plastic membranes with a surface coating; e.g. in "ultra-osmosis", a polysulfone membrane is coated with a charged amine group layer. c) Hollow-fibre membranes - cast from any of the above. 2. a) Ceramic membranes - typically manufactured from silicates with an active membrane layer of zirconium or aluminium oxide, b) Cast carbon membranes - typically manufactured from compressed carbon powder. 3. Metallic membranes - typically manufactured from aluminium or steel.

The types of membranes most commonly encountered in cross-flow membrane processing are configured geometrically as: a) Plate and frame systems b) Hollow-fibre c) Tubular d) Spiral-wound

The most common problem with all membrane filtration systems is surface layer fouling. This is also known as concentration polarisation fouling. There are various theories on the mechanisms of this, e.g. Van der Waal's forces, hydrophobicity or hydrophilicity of the membrane surface, etc. Such fouling causes a flux decline, which, after minutes or hours, renders the plant unusable until it is cleaned. In most systems, a complete cleaning cycle can take from 45 minutes up to 4 hours or more.

In Figure 1, the solid curved line A shows the normal expected flux decay. Most of the fouling occurred in the first 1-2 hours. After about 5 hours of run time, the membrane in this case would have to be cleaned. A system of gas-backflushing has been proposed and this gives the saw-tooth flux pattern B. However, this technology is limited to the hollow-fibre type membranes because of the necessity to have an inherent strength in the membrane to prevent delamination and wall rupture during the gas-backflush. This method also necessitates stopping the plant for 1 minute or so in every 10 or 30 minutes, dependent on the rate of fouling, but has the advantage of maintaining a high flux value. According to a first aspect of the present invention, there is provided a method of cleaning or reducing fouling on a filter, the method comprising the step of: creating an ultrasonic standing or pseudo-standing wave across the fouled surface of the filter by means of an ultrasonic signal.

According to a first aspect of the present invention, there is provided a filter system comprising a housing; a filter; and, a piezo device located on the surface of the housing for generating an ultrasonic wave across the filter.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a graph showing the flux decline of various systems;

Fig. 2 is a cross-sectional view through a membrane structure according to the present invention;

Figs. 3 and 4 are diagrammatic cross-sectional views, at spaced points, through a tubular or sheet filter used in cross-flow membrane processing of conventional design and according to the present invention respectively;

Fig. 5 is a perspective view of a hollow fibre or tubular membrane structure used in cross-flow membrane processing according to the present invention; Fig. 6 is a perspective view of a tubular membrane structure used in cross-flow membrane processing according to the present invention; and, Fig. 7 is a diagrammatic cross-sectional view of a "dead-end" type filter according to the present invention. It is known that small particles can be made to accumulate at the nodes of a standing wave. In the present invention, ultrasound is used to dislodge particles and foulants from the membrane and, in particular in a membrane used in cross-flow membrane processing, from the interfacial area of the membrane and fouling layer.

Figure 2 shows a section of a plate and frame system used for cross-flow membrane processing in which membranes 10 are separated by vexar plastic support material 12. End plates 14 are provided on the outermost sides of the system. A permeate valve 16 is provided between alternate pairs of membranes 10 to control the flow of permeate out of the system, the retentate passing through the other alternate pairs of membranes 10 as shown.

Piezo devices 18 are mounted on end plates 14. In order to effect removal of particulates and fouling materials, the ultrasonic emissions from the piezo devices 18 need to be established such that a resonant wave is set up perpendicular to the plane of the membranes 10. This may be in the form of a standing wave or a pseudo-standing wave.

In a classical standing wave, the incident wave is reflected back such that the nodes of the incident wave and reflected wave coincide. In this system, the nodes represent areas of low pressure, where denser particles accumulate, and the antinodes are areas of high pressure, where less dense particles accumulate. In a pseudo-standing wave, two waves of slightly different frequencies meet to form a beat wave or a modulated output. This beat wave can be regarded as a standing or pseudo-standing wave and causes accumulation of particulate materials in the same manner as in a classical standing wave.

A pulse of ultrasound will dislodge foulants. However, the dislodged foulants may immediately re-foul the system and the dislodged fouling material may have to be moved away from the membrane/product interface. In order to achieve this in this example, the standing wave or pseudo-standing wave can be progressively and incrementally moved. Ultrasound is applied for, perhaps, 30 seconds or so and, at the same time, a phase change is progressively induced in the sound wave by progressively inducing a phase change in the output wave produced by the piezo device or devices 18 in Figure 2. The standing wave or pseudo- standing wave accordingly moves away from the membrane surface into the liquid or product such that particles are carried away from the surface of the membrane for sufficient time, so that cleaning is effected and foulant 13 is carried out of the system with the retentate as shown.

In some systems, Stokes forces may be greater than the standing wave force; in the normal cross-flow membrane filtration process, Stokes forces will act in a direction perpendicular to the membrane, which is the same as the direction of flow of permeate through the membrane, and may overcome the effect of the standing wave force and reduce or nullify its ability to remove fouling materials from the membrane surface.

This problem may be overcome by removal of the Stokes forces, and this may be done by closing the permeate outlet valves 16, so that there is no permeate flow. The fouling materials 13 will then be carried away in the product flow retentate stream.

This process is also illustrated by Figures 3 and 4. As shown in Figure 3, which shows a filter of conventional design, an inner membrane 22 in the form of a tube or sheet is surrounded by a layer of vexar plastics material 24, and an outer housing 26. The product fluid flows axially through the tubular or plate and frame membrane 22 and the permeate passes through the membrane 22 to be collected at the end faces of the plastics material 24. In the invention, as shown in Figure 4, several piezo devices 28 are fixed to the exterior of the outer housing 26, the piezo devices 28 being connected to an appropriate power supply (not shown) . Depending on the attenuation of the wave transmitted from a particular device 28 through the tubular or plate and frame system, and the wave reflected from the opposite face of the outer housing, it may be necessary to provide opposed pairs of piezo devices 28, as shown in Figure 4, transmitting towards each other, in order to have sufficient wave strengths to form a standing wave which is powerful enough to remove foulants.

As foulant builds up on the inner face of the membrane

22, it is removed by closing a permeate outlet valve 30 and then energising the piezo devices 28 on the outer housing to produce a standing ultrasound wave 32 which dislodges the foulant which is then carried away in the flow of retentate 34. By appropriate control of the power supplied to the piezo devices 28, the phase of the transmitted waves may be progressively changed so that the standing waves or pseudo-standing waves, progress normal to (i.e. out of) the membrane 22. Such progressive waves help to lift and carry off foulant from the membrane 22. A backpressure control valve 36 is also provided.

It is preferred to use ultrasound which is at a resonant frequency substantially matched to that of the fluid stream. Failure of the fluid to resonate will cause cavitation and this may be deleterious to the membrane filtration element. This is because the plastics inner membrane 22 may become de-laminated from its backing material, or the membrane 22 may rupture and subsequent mechanical failure may result.

In order to set up a standing wave or pseudo-standing wave, careful adjustment of the frequency is needed such that the standing waves coincide at the nodes. Failure to set up this standing wave pattern may cause the fouling material not to be removed. A typical design of a hollow-fibre or tubular system is shown in Figure 5. Fibres 50 are potted in an epoxy resin or similar food grade chemically-inert material at each end of a hexagonal outer case or housing 52. The housing 52 is usually manufactured from a polycarbonate translucent type material and is provided with an exit tube for the permeate which passes through the walls of the fibres 50 from the product flow passing axially through the fibres 50. In this invention, the material for the housing 52 is preferably stainless steel.

To cause ultrasound waves to set up a resonant standing wave, or pseudo-standing wave, to remove the fouling material, a plurality of piezo devices 56 are attached to the housing 52. When the piezo devices 56 are driven with the correct frequency, standing or pseudo- standing waves 58 will be generated at 90° to the walls of the housing 52, i.e. across from one side of the housing 52 to the opposite side. This will cause foulant to be removed from the product/membrane interface. As described above, the standing or pseudo-standing wave can be made to progress by appropriate control of the power supply to the piezo devices 56 to induce a progressive phase change in the generated wave, thus improving the efficiency of removal of the foulant from the surface of the membrane. The foulant thus removed is carried away with the flow of retentate.

A tubular membrane filter which uses ceramics is shown in Figure 6. Basically the same technique as described above with reference to Figure 5 is used to remove foulants. The tubular membrane filter has a central ceramic filtration module 60 which is surrounded by a housing 62. Both the central module 60 and the housing 62 are of hexagonal cross-sectional shape. The central module 60 can typically consist of one element or many elements; for example, one, seven, or nineteen ceramic filtration elements are common arrangements in industry. A plurality of piezo devices 64 are attached to the housing 62. As above, when the piezo devices 64 are driven with the correct frequency, standing or pseudo-standing waves 66 will be generated at 90° to the walls of the housing 62, i.e. across from one side of the housing 62 to the opposite side. This will cause foulant to be removed from the product/membrane interface as described above. The standing or pseudo-standing wave can be made to progress by appropriate control of the power supply to the piezo devices 64 to induce a progressive phase change in the generated wave. The foulant thus removed is again carried away with the flow of retentate.

The use of a hexagonal housing 52, 62 in the examples shown in Figures 5 and 6, or other polygonal housing (preferably with parallel sides) , helps to overcome wave reflection problems which may occur if a circular cylindrical housing is used since the piezo devices can be fixed to a flat side and the standing or pseudo-standing waves can more easily be set up. Also, the method of operation can be more varied with this type of arrangement. For example, the output wave from one face of the hexagon can be reflected back on itself to form a standing wave, provided that the attenuation of the reflected wave is not too great, and this standing wave can be made to move to dislodge fouling material. This would avoid having to use opposed pairs of piezo devices.

Where two piezo devices on opposite faces of the hexagonal housing 52,62 are used (as shown in Figures 5 and 6) , they can be driven at slightly different frequencies to form the pseudo-standing or beat wave, rather than having to vary the phase of each individual piezo device.

A "dead-end" type filter is shown in Figure 7. In this type of filter, product is fed into a chamber 70 through an inlet 72. The product passes across a filter 74, often a cloth, which is supported by a filter support 76. The resultant filtrate can then pass out of the chamber 70 through an outlet 78. A pressure gauge 80 and backpressure valve 82 are used to prevent the pressure within the chamber 70 becoming to high, which can occur especially when the filter 74 becomes clogged.

Piezo devices 84 are fixed in opposed pairs to the outside of the chamber 70 so that the standing or pseudo- standing waves thus produced pass across the filter 74 in order to lift foulant 86 from the filter 74. The use of a progressive wave, induced by shifting the phase of the waves output by the piezo devices 84, helps to ensure that the foulant 86 is properly lifted from the filter 74 and prevented from refouling the filter 74.

The use of a hexagonal housing or other polygonal housing, preferably with parallel sides, is also preferred for the dead-end type filter. An indication of the improved flux rates achievable with the present invention is shown by the curve C in Figure 1, from which it can be seen that the fall-off in flux is much reduced compared to simple backflushing.

Claims

1. A method of cleaning or reducing fouling on a filter, the method comprising the step of: creating an ultrasonic standing or pseudo-standing wave across the fouled surface of the filter by means of an ultrasonic signal.

2. A method according to claim l, wherein said wave is made to progress at right angles to the surface of the membrane to move foulant from the surface.

3. A method according to claim 2, wherein a phase change is induced in the ultrasonic signal to create the progressing wave.

4. A method according to any of claims 1 to 3, wherein the pseudo-standing wave is created by interference of two waves of similar, but not identical, frequency.

5. A method according to any of claims 1 to 4, wherein the filter is of the cross-flow membrane type.

6. A method according to claim 5, wherein the filter has an outlet for permeate and an outlet for fluid with retentate from which the permeate has passed, the method including the step of stopping the flow of permeate to or through the permeate outlet whilst maintaining the flow of fluid with retentate during cleaning of the filter.

7. A method according to any of claims 1 to 4, wherein the filter is of the dead-end type.

8. A filter system comprising a housing (26,52,62); a filter (22,50,60); and, a piezo device (28,56,64) located on the surface of the housing for generating an ultrasonic wave across the filter.

9. A system according to claim 8, wherein the filter is of the dead-end type.

10. A system according to claim 8, wherein the filter system is of the cross-flow membrane type.

11. A system according to any of claims 8 to 10, wherein the housing (26,52/62) is cylindrical, the filter (22,50,60) being contained within the cylindrical housing.

12. A system according to claim 11, wherein the housing has a polygonal cross-section.

13. A system according to claim 11 or claim 12, wherein the housing has at least two parallel sides.

14. A system according to any of claims 8 to 13, comprising at least two piezo devices (28,56,64) on opposed side surfaces of the housing.