GB2403167A

GB2403167A - High flux filtration system

Info

Publication number: GB2403167A
Application number: GB0315195A
Authority: GB
Inventors: Mark Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-06-28
Filing date: 2003-06-28
Publication date: 2004-12-29
Also published as: GB0315195D0

Abstract

A filtration system with reduced blocking by captured material and with improved flux and size discrimination is formed by exciting a flexible micro-structured membrane 27 into vibrational motion. Typically the vibration will be at ultrasonic frequencies and in one embodiment the excitation is in the form of a transverse standing or travelling wave. The transverse wave motion is effective in fluidising filter-cakes which form on the surface of the membrane. The membrane should be microstructured with pores formed lithographically, by ablation or by the track-etch process using an additional masking step, typically providing porosity only in certain areas of the membrane away from the supported edges. The high pore density gives a high initial flow-rate, while the vibration maintains the high flow essentially indefinitely. The vibration allows smaller particles to pass though the filter, along with a larger volume of the carrier fluid, while still retaining the larger particles. In this way the volume of fluid that can be screened is greatly increased, typically by 10-100 times, allowing more particles to be captured for analysis.

Description

1 24031 67 Title: HIGH FLUX FILTRATION SYSTEM

TECHNICAL FIELD OF THE INVENTION

The invention relates to the design of filtration systems for separating particulate materials from fluids, such as those used in chemical, hydraulic, micro-biological and air-quality analysis processes.

BACKGROUND OF THE INVENTION

In many methods of the life sciences and industrial diagnostics it is necessary to filter particulate material from liquid or gaseous fluids, and membrane filters are used for this. For example, many millions of screening tests for micro-biological pathogens in drinking water are made each year by filtering a known volume of water through a membrane filter with well-defined pore size, afterwards culturing the captured species at an optimised temperature in a nutrient medium. In a similar manner airborne particles are commonly filtered from air samples using such membranes, with the captured particles being weighed, investigated visually, or passed to further chemical and physical analyses. Further, drinking water can be processed and effectively disinfected by filtration, in both small and large-scale applications. The primary discrimination criterion is the size of the particle. Smaller-than-pore particles should pass through the filter, while larger particles should be captured.

The sharpness of this size discrimination can be helped by the provision of membranes with a narrow pore-size distribution, such as those manufactured by the track-etc* process. However, these membrane filtration applications suffer from two fundamental problems. First, in all these processes a cake of filtered particles builds up on the membrane, which entraps further particles, including smaller-than-pore ones. As soon as this "secondary-membrane" forms, flow-rate through the filter is greatly reduced, and size fractionation is no longer determined by the filter pore-size. - 2 -

In fact filtration performance is then defined by the captured material, not by the membrane.

This blocking process is depicted in Figure 1. We aim to pass a high volume of the particle-laden fluid 1 through the membrane 2 supported by a structure 3. Initially the small particles 4 pass through, but as soon as larger particles 5 are captured, they themselves capture smaller particles, forming a cake which blocks further significant flow.

Flux can be restored and size discrimination helped by operating under cross-flow conditions, but this is inconvenient or unacceptable for many quantitative processes.

I have discovered that the total flux of liquid can be greatly increased, and size fractionation performance improved, by subjecting the membrane to standing or travelling wave vibrations. This is common in large sieve operations, but appears to be novel in membrane filtration. Figure 2 shows the same membrane 2 subjected to a transverse standing-wave vibration 6. This fluidises the particle cake near to the membrane, allowing smaller particles and fluid to pass. This only appears to function with surface-filters, which have a hard, flat surface on which particles are captured, not with depth-f lters which capture particles within the membrane structure. The track-etch membranes mentioned above perform well, although there may exist other effective types.

The second problem with membrane filters in general, and with track-etch types in particular, is the low pore-density (1-15%), restricting flowrates even with the purest liquids. Recently, synthetically microstructured filters have been formed using patterning, etching and ablation processes borrowed from the semiconductor industry.

In one technique lithographic processes are used to pattern and micromachine a thin, rigid silicon nitride membrane. In Figure 3 a semiconductor membrane 7 has been micro-machined with a dense array of holes 8. The much higher pore-densities have lead to the description "micro-sieves". They offer greatly superior initial flows with pure liquids, but when used under normal dead-end filtration conditions these sieves block just as do conventional filters, and any advantage is rapidly lost. - 3

The combination of a new type of high pore-density, micro-structured sieve machined from a thin, flexible material and excited with flexural vibrations will provide the high initial flows, which are additionally maintained for long periods. This is an almost ideal filter process. The design freedom of engineered sieves, with holes whose size, shape, location and density can be chosen within wide bounds, opens up a number of new applications.

The basis of my claims of inventive steps is described now.

ESSENTIAL TECHNICAL FEATURES

According to the present invention there is provided a fluid filtration system comprising a flexible membrane filter capturing particles substantially on rather than within the membrane, substantially free of supporting contact over its area, under tension and excited into standing or travelling transverse vibration waves.

A large number of experimental investigations has been carried out to support this novel idea. Some of these will be described now. One very convenient way to couple the vibration energy into the membrane is to immerse it, supported by a suitable support, in an ultrasonic cleaning bath. Although ultrasonic energy has been widely used to clean filters by scouring and cavitation, the increase in flux produced is only modest. I have found that much superior performance is obtained if the membrane is free of contact over much of its area and under tension. Absence of a contacting support allows transverse vibration of the membrane, while by operating the membrane under tension, the vibration wavelength is controlled and damage is reduced. The wavelength of the vibrations is determined by the membrane material properties, especially mass, Young's modulus, thickness and tension, and can be quite different from the wavelength of ultrasonic waves in the surrounding fluid. For example, if as in Figure 4 the membrane 2 bonded to its support structure 3 is tensioned via a pressure difference and hence flow 9, a typical ultrasonic bath 10 with its 40kHz transducer l l will excite standing waves in the membrane with a wavelength of about 0.5-l.Omm, coupled in via the liquid 12.

The photomicrograph 13 shows particles captured on a track-etch membrane at the - 4 standing-wave vibration nodes, supporting our understanding of the key processes.

The use of two or more, variable or broadband vibration frequencies, or variation of the membrane tension can be used to shift the location of nodes. This is useful to avoid unfluidised regions, and to generate travelling waves which drive material on the membrane surface, for example to a collection point. Although the discussion here describes essentially planar membranes, other forms such as cylindrical, conical, spherical or of more complex shape are useful in some applications.

Alternative excitation methods will be obvious to those skilled in these technologies, such as direct drive of the membrane edge using peripheral piezoelectric or magnetostrictive transducers 14 (Figure 5) to excite standing waves 6 or even asymmetric transducers 15 to excite travailing waves 16. This is even possible with ultrasonic-bath excitation using edge absorbers 17 to give an asymmetric system. We have used this method to transport particulate material to one side of the system.

Transport can also be arranged as in Figure 6 by combining the membrane vibration with a second force, such as an asymmetric in-plane jerk 18 or centrifugal force.

Figure 7 shows mass-flow through a lcm2 unsupported 2pm pore size tracketch membrane both without excitation 19 and with excitation 20. As the receiving container is filled by the flux it is repeatedly emptied 21 and allowed to fill again.

This process was continued for a total flux of 58 litres without visible change in flow- rate, in comparison with the fully blocked 0.4 liter flux of the unexcited membrane.

In order to control the location of captured particles it is highly desirable to structure the membrane with regions containing pores surrounded by impervious regions. By vibrating both porous and non-porous regions we can reduce the deposition of particles around the membrane's supported edges, which is advantageous for many subsequent analysis steps. This can be arranged using a number of different fabrication technologies, either by occluding pores in unwanted regions of a filter membrane having pores initially covering its whole area, or by forming pores only where needed. One fabrication procedure for the latter approach is shown in Figure 8, in which an opaque mask 23 with macroscopic, typically millimeter- to several centimeter-dimension holes simply defines the porous regions in an otherwise conventional track-etch fabrication process. A source 24 directs energetic particles 25 - s - through a polycarbonate or other polymer film 22 causing random damage tracks which are subsequently etched in an alkaline solution. The mask defines the porous regions, while the pores themselves are determined by the random track-etch process.

Where the highest throughput is required, the location and size of the pores themselves must be controlled; lithography and ablation through a mask are suitable large-scale manufacturing techniques for the latter need. Below we describe one preferred embodiment for filter and membrane fabrication.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 9 shows a preferred embodiment of the method in which a high poredensity, micro-machined, flexible membrane filter 27 with pores present in the central region only is vibrated in an out-of-plane wave-motion of a few particle diameters by immersion in a conventional ultrasonic cleaning bath 10. A controlled trans- membrane pressure ensures tension of the membrane, and hence an optimised vibration wavelength and amplitude. The membrane is fabricated using the track-etch process with the addition of a microscopic masking step to produce patches of well- defined holes where needed. The mask aperture dimensions are of the order of the pore sizes needed. This has the additional advantage of allowing a higher radiation dose without formation of double-pores, allowing much higher pore densities. A source 24 directs energetic particles 25 through a mask 26 to form damage tracks in the polycarbonate, polyester or other membrane material 27, which are subsequently etched in an alkaline solution to form holes. Filtered fluid passes out via an overflow 28.

The above embodiments are described by way of example only. While the embodiments have emphasized liquid filtration systems, it will be clear to anyone skilled in the art that similar advantages accrue for airborne particle capture and size discrimination.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. A membrane filter 2 and support structure 3 captures large particles 5, which then build into a "filter cake", which itself entraps smaller particles 4. The filtration - 6 mechanism, which rapidly blocks the membrane filter 2, is then almost independent of the membrane filter pore size, and the flow-rate and total flux of fluid I is greatly restricted.

Figure 2. By accelerating the membrane, for example with a transverse standing wave 6, trapped particles may be fluidised and moved away from the membrane. This allows smaller particles to pass, greatly increasing the flux of fluid and improving size-discrimination.

Figure 3. We combine this flux-enhancing mechanism with the high potential throughput of a high pore-density membrane 7 lithographically- formed in a flexible material. Higher pore density can be achieved using micro-fabrication techniques such as lithography, ion-beam etching and ablation to form precise and precisely located hole-arrays 8 in the membrane 7.

Figure 4. A convenient way to excite transverse waves of an effective wavelength in the membrane is to immerse the membrane 2 held in a support structure 3 in a conventional ultrasonic cleaning bath 10. Vibration is then coupled from a transducer 11 via the liquid 12. The membrane can be advantageously tensioned with a controlled trans-membrane pressure and hence flow 9. The photograph 13 shows captured material arranged on the nodes of the membrane standing wave pattern.

Figure 5. Excitation of standing waves 6 and travelling waves 16 is also possible using edge-drive with electro-mechanical transducers 14, 15. Travelling waves which transfer captured material can also be arranged using an asymmetric drive, for example using an absorbing impedance match 17.

Figure 6. Transverse vibrations can also be combined with material transport motion such as produced using stick-slip drive 18 or centrifugal forces.

Figure 7. Experimental mass versus time plot of filtration through a track-etch membrane. Without excitation 19 the flow quickly drops to zero as the membrane blocks. High flow-rate can be restored with the membrane agitation switched on 20.

Each time the receiving container is filled, it is emptied automatically 21.

Figure 8. Simple masking by a hole-mask 23 can be used to define the porous and impervious regions of a track-etch membrane 22, by aperturing the particle beam 25 emanating from source 24.

Figure 9. In a preferred embodiment the membrane 27 is structured with pores in only part of its area, being formed in a modified track-etch process whereby the energetic source 24 shines through a microscopicholed mask 26 to pattern the particle beam 25 and hence individually define each pore of the membrane 27 after development of the damage tracks. An ultrasonic transducer 1 I couples energy to excite membrane vibration via the fluid 12 of an ultrasonic bath 10. Filtrate can be passed over a suitable overflow 28. "Out-to-in" flows are equally possible and effective for fluid extraction. - 8

Claims

What is claimed is: 1. A fluid filtration system comprising a flexible membrane filter capturing particles substantially on rather than within the membrane, substantially free of supporting contact over its area, under tension and excited into standing or travelling transverse vibration waves.
2 A system as claimed in Claim 1 in which the membrane is a standard membrane filter manufactured by the track-etch process.
3. A system as claimed in Claim 1 in which the membrane pores are defined by micro-machining an essentially non-porous sheet using a masked tracketch process.
4. A system as claimed in Claim 1 in which the membrane pores are micromachined from an essentially non-porous sheet by a process of laser ablation.
5. A system as claimed in Claim 1 in which the membrane pores are micromachined from an essentially non-porous sheet by a lithography and etching process.
6. A system as claimed in Claims 1 to 5 in which the membrane is porous only in one or more selected areas while vibration is excited over substantially the full membrane area.
7. A system as claimed in Claim 6 in which the areas of membrane porosity are defined by macroscopic masking in the track-etch process, the location and density of the pores themselves being determined by the random tracks.
8. A system as claimed in Claim 6 in which the porous membrane areas are defined by occluding pores of a membrane previously micro-machined over substantially its whole area.
9. A system as claimed in Claims 1 to 8 in which vibration is excited by fluid coupling to a vibrating transducer - 9 -
10. A system as claimed in Claims 1 to 8 in which vibration is excited by direct mechanical bonding to a vibrating transducer.
11. A system as claimed in Claims I to 10 in which the membrane vibration is combined with an in-plane jerk or centrifugal force causing captured particles to be transported substantially in a desired direction.