GB2223690A - Filter tubes - Google Patents

Abstract

Filtration systems which improve the retentate, permeate flow rates and surface area of tubular crossflow filtration systems use filter tubes comprising at least one layer of a porous membrane and a stellated inner surface. The filter tubes may have a membrane comprising two layers, the inner one (23) having finer pores than the outer one (21). In a preferred form the stellated inner surface has a spiral configuration which may have up to 500 turns per metre. The stellated surface provides an increase in surface area of about 25 percent over cylindrical tubes having the same diameter allowing a reduction in flow rate of about 50 percent over prior cylindrical filter tubes. The induced turbulence caused by the spirals further reduces the necessary flow rate.

Description

FILTRATION SYSTEMS This invention relates to filtration systems and particularly to methods of improving the retentate, permeate flow rates and surface area of tubular crossflow filtration systems.

Tubular crossflow filtration systems are known in which a fluid containing dissolved and suspended solids, called the retentate, is made to flow across the suface of a membrane or layer of porous media whose pore size is insufficient to permit passage of the particles but which will allow the fluid to flow outwards and be collected, called the permeate. In such systems the fluid is generally caused to flow at a high rate to create surface turbulence minimising clogging (blinding) of the membrane by deposited particles.

The membranes used in such tubular crossflow filtration systems have a pore size in the nanometre range up to 100 micrometres (microns). These membranes are currently manufactured from porous metal, particulate stone, ceramic or synthetic polymer compositions. The ceramic compositions are frequently based on aluminium and/or zirconium oxides. The polymer compositions may be polyamides, such as nylons, cellulosic, such as cellulose esters, polysulphones, polyethylenes and polytetrafluroethylene (PTFE).

The membranes may comprise a single layer or two or more superposed layers. Particularly useful membranes of this type are manufactured in tubular form either as single tubes or banks of tubes.

According to the present invention there is provided a filter tube comprising at least one layer of a porous membrane and a stellated inner surface.

Preferably the tube comprises an outer layer of a porous material with an inner coating of a second porous material being the active membrane surface. The inner coating is may be a porous ceramic or a synthetic polymer in a porous form such as a layer of PTFE. Additionally or alternatively the outer and inner porous tube may be covered or have attached a coating, such as a silicone or a synthetic polymer, to minimise adsorption of the permeate to the membrane and the membrane supporting material.

In a most preferred form of the invention the stellated inner surface is fabricated in the form of a spiral having up to 500 turns per metre.

The tubes may be prepared by extruding the coarse porosity ceramic outer membrane in paste form and thereafter drying and sintering the extruded tube. Alternatively the tubes may be slip cast in a porous mould from a slurry. The cast tube is then dried and fired to sinter the casting.

A fine porosity ceramic inner membrane may be prepared by slip casting on the inside surface of the coarse membrane.

In the case of a fine porosity polymer membrane it can be solvent cast on the inner surface of the ceramic membrane by known means. Such methods for preparing polymeric membranes are described in 'Synthetic Membranes : Science, Engineering and Applications' : P M Bungay et al, Reidel Publishing Company 1986.

In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of a filter tube of known type, Figure 2 is a perspective view of a multi-channel filter tube of known type, and Figure 3 is a cross-sectional view of a single filter tube of known type and filter tubes according to the invention.

A filter tube of known type, see Figure 1, consists of a single tubular membrane 1 having an inner membrane layer 2.

Fluid travels along the tube in the direction shown by the arrows 3 and the permeate, free from solids, passes out of the side of the membrane 1 as shown by the arrows 4. It is most important that the flow shown by the arrows 3 is turbulent or the fine membrane layer 2 will become clogged (surface blinded) with deposited solids and become totally or partially impermeable.

A multi-channel filter of known type, see Figure 2, consists of a tubular membrane block 11 pierced by a number of cylindrical channels 12. Each channel 12 has an inner membrane layer 13. Fluid travels along the channels in the direction shown by the arrows 14 and the permeate, filtered material, passes through the block of membrane 11 as shown by the arrows 15. As with the filter tubes it is most important that the flow shown by the arrows 14 is turbulent or the fine membrane layers 13 will become clogged with deposited solids.

A cross section of a filter tube of known type, see Figure 3a, is shown on an enlarged scale. The tube has an outer membrane 21 having a coarse porosity and an inner channel 22 having a circular cross section. The inner surface of the membrane 21 carries a second membrane layer 23 having a finer pore size. Turbulence in the fluid passing down the axis of the tube is ensured by pumping the fluid at a high rate of flow.

In a-filter tube according to the invention the inner# channel 22 of the coarse membrane-has a stellated cross section, see Figures 3b and 3c. The stellated cross section increases the effective surface area of the inner flow channel and decreases the cross section of the flow channel thus reducing the mass retentate flow whilst maintaining the velocity of the retentate at the membrane surface 23. Furthermore the stellated contour increases the tendency for turbulent flow to take place so that lower flow rates can be used than with the known filters without clogging.

In known filter tubes it is generally recommended that a surface velocity at the membrane should be between 4 to 6 metres per second. For a filter tube having 4 mm internal diameter this represents a flow rate of 0.27 cubic meters per hour; such a tube has a surface area of about 0.012 square metres per metre length. In the case of filter tubes according to the invention, such as those described with reference to figures 3b and 3c, the stellated surface provides an increase in surface area of about 25 percent over cylindrical tubes having the same diameter. This will allow a reduction in flow rate of about 50 percent.

The induced turbulence caused by the stellations further reduces the necessary flow rate. A membrane surface velocity as high as 6 meters per second is not required when the turbulence is high.

In a preferred embodiment the stellated inner surface is fabricated in the form of a spiral. The spiral may have up to 500 turns per metre. The spiral shape together with the stellated cross section makes the filter tube most efficient and provides a high rate of permeate flow in conjunction with a lower rate of retentate flow than that required by known filters to reduce clogging.

The single filter tubes according to the invention may be up to 2 metres in length, or even longer, and 3 to 10 mm in average diameter. The peripheral area provided by the stellated inner surface of the tubes is greater than the area available with a conventional tube having a circular inner surface.

A filtration system may be formed by arranging one or more filter tubes, according to the invention, to be supplied continuously with a flow of retentate and arranging collecting means to receive the permeate. The permeate is directed to an appropriate collecting vessel. The invention also provides a method of removing fluid from a liquid containing suspended solids and comprising the fluid wherein the liquid is caused to flow through such a filtration system so that the fluid is separated as the permeate.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims (11)

1. A filter tube comprising at least-one layer of a porous membrane and a stellated inner surface.

2. A filter tube according to claim 1 in which the stellated inner surface has a spiral configuration.

3. A filter tube according to claim 2 in which the stellated inner surface is fabricated in the form of a spiral having up to 500 turns per metre.

4. A filter tube according to any of the preceding claims in which the membrane comprises two layers, the inner one having finer pores than the outer one.

5. A filter tube according to claim 4 in which the membrane comprises an outer layer of a porous ceramic material with an inner coating of a second porous material.

6. A filter tube according to claim 5 in which the inner coating is formed from a synthetic polymer in a porous form such as a layer of PTFE.

7. A filter tube according to any of the preceding claims in which the membrane comprises sintered alumina and/or zirconia or other porous material.

8. Filter tubes as claimed in claim 1 and as herein described with reference to the accompanying drawings.

9. A filtration system comprising one or more filter tubes, as claimed in any of the claims 1 to 8, adapted to be supplied continuously with a flow of retentate and having collecting means to receive and provide a flow of permeate.

10. A method of removing fluid from a liquid containing suspended solids and comprising the fluid wherein the liquid is caused to flow through a filtration system as claimed in claim 9 so that the fluid is separated as the permeate.

11. Methods of removing fluid from a liquid containing suspended solids, using filter tubes according to any of the 1 to 8, as herein described