WO1988005330A1

WO1988005330A1 - Composite membrane

Info

Publication number: WO1988005330A1
Application number: PCT/GB1988/000016
Authority: WO
Inventors: Emil Gyorgy Arato; Andrew Green
Original assignee: The British Hydromechanics Research Association
Priority date: 1987-01-13
Filing date: 1988-01-12
Publication date: 1988-07-28
Also published as: GB8700709D0

Abstract

A filter device comprises a fluid passage constituted by a plurality of porous layers, one layer being a substrate (2) and another layer being a film (1) of metal, metal alloy or combination of metals on only the surface region of the substrate (2).

Description

COMPOSITE MEMBRANE

This invention relates to membrane systems used for cross-flow microfiltration, ultrafiltration and reverse-osmosis.

The essence of membrane science is relatively simple, involving the physical separation of specific compounds from liquid or gaseous mixtures, but in practice there are many complications. Pressure driven membrane processes can be divided into three distinct areas; microfiltration, ultrafiltration and reverse osmosis.

Microfiltration is used to remove colloids or micro-organisms from liquids and the typical pore size of a micro filtration membrane is in the range 0.1 microns to 5 microns.

Ultrafiltration is mainly used to separate extremely fine part- iculate matter or large molecules (e.g. proteins) from liquid while reverse osmosis is employed to remove dissolved matter from a solvent. Typical pore sizes range from 0.002 to 0.2 microns.

The membrane used in all three areas of application is generally made of polymeric material, although in recent years some ceramic based micro and ultrafiltration membranes have become commercially available. Both of these materials have inherent problems. Polymeric materials cannot be operated above 80°c. This means that they cannot be used in many process applications or be steam sterilised. Ceramic materials can be steam sterilised, but with these it is very difficult to accurately control the pore size. Such prior art membranes are usually mounted on a porous support, and the rnembrane/support system assembled into a module, to provide a large surface area within a small volume and operated in a cross-flow mode. Membranes can be used in a wide number of areas such as effluent treatment, concentration of dilute suspensions, separation of emulsions, water purification, separation of cells and micro-organisms and separation of product molecules from impurities.

According to the invention there is provided a filter device constituted by a plurality of porous layers, one layer being a non-polymeric substrate and another layer being a film of metal or combination of metals or a metal alloy on only the surface region of the substrate. If required an electrical charge of either polarity can be applied to the film surface to reject charged particles and reduce fouling. The composite of materials in the filter device can withstand elevated temperature (e.g. steam sterilization) .

The film can be deposited onto the substrate using a high vacuum evaporative technique or sputtering to produce an even, porous layer. This technique is usually used to give a thin impermeable layer for corrosion protection. However, by careful setting of the operating parameters, a thin permeable layer of controlled pore size and porosity can be deposited onto a surface. Porosity can be controlled by in-filling the pores of the surface region of the substrate with a single metal or by a co-deposition technique with a plurality of metals.

Suitable non-polymeric substrates include ceramic materials, sintered glass, metal, and carbon.

This composite membrane system has a number of advantages:-

1. It has the ability to withstand high temperature process operations. This is because both materials are resistant to heat when heated individually, and any forces that could cause fracture of the material due to differential heat expansion are very small due to the microscopic thickness of the film. It is believed that during heating, the pores within the film distort slightly to take up differential expansion forces. 2. The pore size can be controlled by controlling the deposition parameters. Because the film is very thin (a few microns) flux rates will be high.

3. The substrate can be manufactured with high porosity and relatively cheaply, because pore size does not need to be controlled accurately for efficient operation of the composite rnembrane system.

4. The film can be electrically charged with either positive or negative potential. Thus particles of either polarity can be rejected. In certain applications this can reduce fouling of the membrane. For example, with ultrafiltration of washing residues following electro-deposition of paints the fouling tendency of the membrane is determined by the nature of its surface charge in relation to the charge on the paint droplets.

5. The membrane is robust and use of abrasive cleaning techniques and backwashing to remove foulants will be possible.

6. Damage to the membrane surface can be monitored from the conductivity of the film. As the pores become damaged and the pore size increases, then the conductivity of the film will be affected. Condition monitoring of membrane filters can be very important in certain industries, and is very difficult with current membrane systems.

An example of the invention will now be described with reference to the accompanying drawings in which:

Figure 1(a) illustrates a substrate with a surface-region in-filling of a single metal;

Figure Kb) illustrates a substrate with a structured coating;

Figure 2 shows a metal matrix deposited onto a ceramic tube;

Figure 3 shows how a number of membrane tubes may be mounted together to form a module to provide large surface area within a small volume.

Figure 1(a) illustrates microscopically sintered particles 2 of the substrate material. Although the particles are shown completely spaced from one another, they will in fact be joined at a few points with gaps between the joins. The particles 2 have had a top coating 1 of metal, which tends to close up the gaps between the particles to the dimension 3. Most of the deposition occurs on the top layer of particles but some deposition occurs on the immediately succeeding layers within the surface region. Controlling the deposition of the coating controls the gaps, that is, the pore size.

In Figure 1(a) the deposition is of a single metal. In Figure Kb) there has been co-deposition of two metals (e.g. from two separate sputter sources) which has caused wafers of metal 4 to be firmly bonded on the particles 2. There are gaps between the wafers themselves and between the wafers and the next particles and these gaps control the effective pore size. Nickel chromium alloy and aluminium are suitable metals for co-deposition.

The sheet of filter material can be used for many applications. Conveniently it is mounted in a module, with an inlet (and possibly an outlet) for unfiltered fluid and with an outlet for filtered fluid. By locating the outlet for the filtered fluid adjacent the inlet, cross-flow is achieved.

Figure 2 shows the sheet of filter material made into a tube. Fluid to be filtered surrounds the tube, and filtered fluid is taken from its bore, after passing through the outer surface of the tube 11 on which is deposited the metal film 12, then through the ceramic material 13 to the bore 14 leading to the outlet. Figure 3 shows a cross-flow filter system, where fluid enters at an inlet 22 and passes between the exteriors of a number of tubes 11 similar to that of Figure 2 mounted in a cylindrical chamber 21 and out to an unfiltered outlet 20, flowing in a first direction.

Some fluid passes through the walls of the tubes 21 and flows in cores 14 in the opposite direction through a manifold 25 to a filtered outlet 23, the other ends of the tubes 21 being closed. The tubes are supported on a grid 24 across the cylindrical chamber 21. Flow could be in the opposite direction to that shown in which case the metal film 12 would be deposited on the internal surface of the tubes 11.

One application for this apparatus would be the cold sterilisation of beer and wine. Typically the micro-organisms that must be removed are about 1 micron or less in size. . For effective removal of micro-organisms a membrane pore size of 0.2 to 0.4 microns is required for the filter material. The pore size is typically 0.2 micron in the metal film 12 and 1-2 micron in the ceramic annulus 13.

The invention could also operate in a flat plate module, with flow across the surface of a composite membrane in the form of a flat plate.

Claims

1. A filter device comprising a fluid passage constituted by a plurality of porous layers, one layer being a substrate and another layer being a film of metal, metal alloy or combination of metals on only the surface region of the substrate.

2. A filter device as claimed in claim 1 wherein the substrate is ceramic.

3. A filter device as claimed in claim 1, wherein the metal film has a pore size in the range 0.1 - 5 microns.

4. A filter device as claimed in claim 1 wherein the metal film has a pore size in the range 0.002 to 0.2 microns.

5. A device as claimed in any one of Claims 1 to 4 wherein the layers are cylindrical and concentric.

6. A method of producing the filter device as claimed in any one of claims 1 to 5 comprising depositing the film onto the substrate by a vacuum deposition technique.

7. Filter apparatus comprising an inlet chamber, an outlet chamber and a filter device as claimed in any one of claims 1 to 5 separating the inlet and outlet chambers.

8. Apparatus as claimed in claim 7 comprising an inlet for the inlet chamber and an outlet for the outlet chamber arranged so that fluid flows in one direction from the inlet to the filter device and in the opposite direction from the filter device ro the outlet.

9. A fluid filtering method comprising passing the fluid from an inlet chamber through a filter device as claimed in any one of claims 1 to 5 to an outlet chamber.

10. A method as claimed in Claim 6 comprising using an ion plating deposition technique.

11. A method as claimed in Claim 6 comprising using a sputtering process.