A METHOD OF REGENERATING A DYNAMIC MEMBRANE IN A SEPARATION MEANS FOR A LIQUID MEDIUM AND DEVICE FOR CARRYING OUT THE METHOD

Abstract: A method and means for regenerating a dynamic mem­ brane in a separation means for a liquid medium containing undissolved constituents comprises a chamber (10) with at least one dynamic membrane (11,12) arranged therein, said membrane consisting of a porous support matrix (11a,12a) or other support matrix with through-holes, and a layer (11b,12b) of small-sized particle material generated thereon in the chamber. In such a separation means the flow of the liquid medium is split into two streams, one constituting the permeate stream which penetrates through the membrane and the other constituting the reject stream which flows past the membrane. To regenerate the membrane, the deposit of said undissolved constituents formed on the membrane dur­ ing operation is according to the invention removed from the support matrix, with the aid of one or more movable devices (21b,22,23) brought into mechanical contact with the layer of particle material and moved along the support matrix by a rotor (21) arranged in the chamber, while at least a portion of the particle material is allowed to remain on the support ma­ trix.

Description: The invention relates to a method of regenerating a dynamic membrane in a separation means for a liquid medium according to the precharacterising part of claim 1. The layer of small-sized particle material is porous and has finer pores than the holes or pores in the support matrix. The invention also relates to a means for carrying out the method. During use of the separation means a layer of undissolved constituents in the liquid medium which is deposited on the membrane gradually increases in thickness, thereby gradually reducing the permeate stream. After some time of operation, therefore, the deposit must be removed and the layer of the small-sized particle material must be renewed or its original properties restored in some other way. This regeneration of the membrane should be performable without the need to dismantle the separation means. Hitherto this has been effected by back flushing with liquid or compressed air or by dissolving or washing away the deposit and the particle material and thereafter supplying a slurry of new particle material to the chamber under such conditions that a layer of the particle material is deposited on the support matrix, including in its through-holes or pores. From the "Encyclopedia of Chemical Technology" page 316 a drum filter is known fitted with a relative thick layer (50 - 150 mm) of precoat whereby the entire liquid to be filtered has to pass through the precoat layer. Under operation continuously a thin surface portion with a thickness of a few hundredth of a millimeter of the precoat is shaped off by a steadily advancing precoat scraper. The CH-A-424 724 discloses a washing apparatus for fibres. The substances which are to be washed away from the fibres are passed at a rate of 100 per cent through a filter sieve. A washing liquid is added to the fibre mass to be washed which liquid passes through the sieve taking along with it the substance to be washed away from the fibres. Inside the cylindrical sieve a constantly rotating scraper is arranged, which scrapes along the sieve in order to ensure that the fibres are brought into a position perpendicular to the surface of the sieve and to pull back fibres that have partly penetrated into the apertures of of the sieve. In order to reduce the amount of fibres sucked into the sieve measures are taken to equalize the pressure on both sides of the sieve. The invention aims at developing a method of the above-mentioned kind which is considerably simpler and achieves superior reproducibility than the known methods. The invention also aims at a means for carrying out the method. To achieve this aim the invention suggests a method of regenerating a dynamic membrane in a separation means according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the method are characterized by the features of the additional Claims 2 and 3. A means for carrying out the method according to the invention is characterized by the features of Claim 4. Further developments of said means are characterized by the features of the additional Claims 5 to 9. According to the invention a simple and effective regeneration of the dynamic membrane is achieved while still in the separation chamber. One feasible explanation of the excellent results achieved with the invention is that the material is removed from the support matrix in such a manner that the pore structure of the dynamic membrane remains essentially intact. Particle material present in the pores or through-holes of this support matrix or their entrances will remain in place during the treatment. With previous regeneration methods it has been impossible to avoid altering the pore structure of the dynamic membrane. Back flushing causes local clogging of pores and uncontrolled movement of particles in other pores and entrances to other pores in the support matrix. If the membrane is washed, the deposit may prevent sufficient contact between wash fluid and particle material and the treatment may cause both chemical and physical alteration of the particle layer. Chemical alterations are even more prevailing if the deposit is removed by dissolving. A support matrix of porous type may consist in conventional manner of a polymer material such as polyamide, polypropene, cellulose acetate and polysulphone, in the form of a semi-permeable, porous film, of a thin porous sheet of metal such as stainless steel or a nickel alloy, or of a thin porous sheet of ceramic material such as aluminum oxide. The support matrix may also consist of a woven or felted product with through-holes, composed of fibres of a polymer material such as those exemplified above, or of polyethylene glycol terephthalate or polytetrafluoroethylene, or fibres of a metallic material, such as stainless steel, or of a natural fibre, such as cotton. The pore size in a porous support matrix is preferably 0.01-10 micrometers, and the holes in a woven or felted support matrix are preferably sized 0.5-20 micrometers. The thickness of the support matrix is preferably 0.1-10 mm. The small-sized particle material may in conventional manner consist of an inorganic oxide, such as silica, titanium dioxide or magnetite, of glass or of a polymer material, such as polystyrene. The particles in the layer applied are preferably of a size of 0.001-10 micrometers. They are applied from a slurry supplied on the support matrix in the separation chamber, a pressure difference being maintained between the chamber and the permeate side of the membrane during application. The pore size in the layer of particle material formed on the support matrix is preferably 0.001-0.5 micrometers and the thickness of the layer is preferably 0.01 micrometer - 0.2 mm. In some cases it may be suitable for the particle layer to be formed of two or more layers. In this case, the particles in the layer nearest the support matrix should be larger than in the layer(s) more distant from the support matrix. During removal of the deposit to regenerate the dynamic membrane, a pressure difference of 0.03-0.4 MPa is preferably maintained over the dynamic membrane. If particle material is removed simultaneously with the deposit to such an extent as to essentially alter the function of the membrane, fresh particle material should be applied on the support matrix. According to one embodiment of the invention the devices moved by the rotor for removal of deposits collected on the dynamic membrane, consist of scrapers fitted on the rotor. These may be movably secured to the rotor and arranged to be brought into mechanical contact with the layer of particle material in only one direction of rotation of the rotor, while contact with said layer is prevented in the other direction of rotation. According to another embodiment of the invention the shaft of the rotor is perpendicular to the active surface of the dynamic membrane and the movable devices are brought into contact with the layer of particle material by the rotor being displaced relatively to said surface in the longitudinal direction of the shaft. In the embodiments described above the movable devices may comprise brushes, felt pads, blades or shutters arranged on the rotor and consisting of a material which will not cause any damage to the matrix, preferably a material softer than the material in the support matrix. The material may be removed from the support matrix in a separate step between separation phases or during a separation phase. In the latter case the rotor is rotated continuously or intermittently while separation of undissolved constituents from the liquid is in progress. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 an embodiment of a separation means according to the invention in a section through the rotor shaft perpendicular to the plane of the dynamic membrane, Figure 2 an embodiment of a dynamic membrane and the rotor seen from inside the separation means in the direction of the rotor shaft, Figures 3 and 4 different embodiments of the rotor in the same sectional view as in Figure 1, Figure 5 an embodiment of the rotor in a section perpendicular to the radial extension of the rotor. The separation means according to Figures 1 and 2 comprises a chamber 10, cylindrical in shape and housing two circular dynamic membranes 11 and 12, one at each end surface of the chamber. The edges of the membranes are sealed to the chamber walls (seals not shown). Each dynamic membrane consists of a porous support matrix 11a and 12a, respectively, consisting of polyamide film with a central circular aperture 11a₁,12a₂, and of a layer 11b,12b, respectively, of a small-sized particle material in the form of silica having a particle size of 0.01 micrometer. The pore size in the material of the support matrix is about 1 micrometer, and in the material in the layers 11b and 12b about 0.05 micrometer. Each polyamide film 11a,12a, is 0.2 mm thick and each layer 11b,12b, is 20 micrometers thick. At one point on the envelope surface the chamber is provided with an inlet 13 for liquid medium to be treated in the separation means. An outlet 14 is provided in the envelope surface diametrically opposite the inlet 13 for removal of that portion of the liquid medium, the so-called reject, that flows past the dynamic filters. The inlet 13 and outlet 14 are arranged in the side walls 15 and 16 of the chamber. The portion of liquid passing through the dynamic membrane, the so-called permeate, enters channels 17 arranged in the end walls 18 and 19 of the chamber. The channels 17 communicate with an outlet 20 for the permeate. Layers 11b and 12b are generated in the chamber by an aqueous suspension of silica having a silica concentration of 200 ppm being conducted into the chamber under gradually increasing pressure until the pressure difference over the dynamic membranes is 0.1 MPa. A rotor 21 is arranged in the chamber 10, in the example shown with a shaft 21a, its centre line coinciding with the symmetry axis of the cylindrical chamber. The rotor carries two radially extending vanes or blades 21b. The rotor shaft is sealingly journalled in the walls of the chamber (seals not shown). When the separation means is in operation, the liquid medium with undissolved constituents is introduced continuously through inlet 13 in chamber 10 of the separation means. The liquid may consist of water containing drops of oil and solid particles, such as oily water from oil refineries and oil platforms or waste water from workshops performing machining by detachment of cuttings. Most of the water passes through the dynamic membranes 11 and 12 as a permeate stream and can usually be conducted via outlet 20 to a recipient. The remainder, the reject stream, is removed via outlet 14 and either returned to the polluted starting material or subjected to some other treatment, generally a rather simple treatment. During the process described a pressure difference is maintained, preferably of 0.03-0.3 MPa between the medium on both sides of a dynamic membrane. The undissolved constituents which gradually accumulate on the dynamic membranes will be removed by the rotor 21 when the deposit of undissolved constituents offers such resistance to the permeate stream that the separation can no longer be performed with commercially satisfactory results. As is clear from the above, regeneration of the dynamic membrane is effected by the movable devices on the rotor being brought into contact with the layer of particle material and moved along the support matrix. In the case shown in Figure 3 the rotor 21 is provided with brushes 22 arranged on the vanes 21b, or mats of textile material such as polythene or polypropene fibres or some other polymer or cotton fibre which will remove the deposits when the rotor is turned slowly, preferably while simultaneously supplying water or the aqueous medium being treated at inlet 13 and removing the liquid at outlet 14. According to Figures 4 and 5 scrapers 23 may be used instead of brushes, in the form of blades or shutters of a material such as polypropene or some other polymer, which will not damage the membrane. The scrapers are secured by joints 24 to the rotor vanes 21b so that they are in operative position when the rotor turns in one direction but are inoperative when the rotor turns in the opposite direction. The scrapers can thus be turned down during cleaning and turned up during regeneration. Yet another method of effecting contact between the dynamic membranes and active parts of the rotor is to utilize the rotor vanes 21b as scrapers. In this case they should be made of a material which will not damage the dynamic membranes, and be arranged movable in relation to the membranes in a direction perpendicular to the plane of the membranes. This can be achieved either by making the rotor shaft 21a with the rotor displaceable in a stationary chamber 10 or by making the chamber displaceable along the rotor shaft while the rotor is stationary in its longitudinal direction. In all the examples described above if required the silica particles are replenished as described above after removal of the deposits which generally cause a part of the layers 11b and 12b to be removed at the same time. In the examples described it is assumed that regeneration of the dynamic filters is performed as a separate process between separation phases. However, it is also possible, using the equipment and material exemplified, to perform the regeneration intermittently or continuously during the separation process, i.e. without the process being interrupted. In this case the rotor is turned intermittently or continuously during the separation process, bringing the exemplified movable devices on the rotor into contact with particle layers 11b,12b and moving the devices along these layers. The drawings show a separation means with only one chamber. However, it is usually advisable to use a group of several such means stacked close together with the inlets 13 connected to a first common main pipe, the outlets 14 connected to a second common main pipe and the outlets 20 connected to a third common main pipe. The invention is also suitable for use in separating various types of slurries such as coal-water slurry, peat-water slurry and waste water from the production of cellulose.

Claims: A method of regenerating a dynamic membrane in a separation means for a liquid medium containing undissolved constituents, said separation means comprising a chamber (10) with at least one dynamic membrane (11,12) arranged therein, said membrane consisting of a layer (11b,12b) of a small-sized particle material generated on a support matrix (11a,12a) with through-holes, such as a porous support matrix in the chamber, a liquid medium being supplied to the separation means and conveyed past the dynamic membrane, the liquid medium thereby being split into two streams one constituting the permeate stream which penetrates through the dynamic membrane and the other constituting the reject stream which flows all the way past the dynamic membrane, characterized in that the deposit formed on the dynamic membrane during use of the separation means is removed from the support matrix with the aid of one or more movable devices (21b,22,23) mounted on a rotor (21) arranged in the chamber, whereby said movable devices (21b,22,23) are brought into mechanical contact with the layer of particle material by rotating said rotor (21) in only one specific direction of rotation, the movable devices (21b,22,23) thereby passing along the support matrix in such a way that at least a portion of the particle material is removed from the support matrix at the same time. A method according to claim 1, characterized in that after the removal of the particle material, fresh particle material is applied on the support matrix. A method according to claim 1 or 2, characterized in that removal of material from the support matrix (11a,12a) is effected while the separation means is being used to separate undissolved constituents from a liquid medium, the rotor (21) being rotated continuously or intermittently. A separation means with a dynamic membrane for a liquid medium containing undissolved constituents, said separation means comprising a chamber (10) with at least one dynamic membrane (11,12) arranged therein, said membrane consisting of a layer (11b,12b) of a small-sized particle material generated on a support matrix (11a,12a) with through-holes, such as a porous support matrix in the chamber, and with means to split a liquid medium into two streams one constituting the permeate stream which is intended to penetrate through the dynamic membrane and the other constituting the reject stream intended to flow all the way past the dynamic membrane, characterized in that there are one or more movable devices (21b,22,23) mounted on a rotor (21) arranged in the chamber, whereby said movable devices (21b,22,23) are adapted to be brought into mechanical contact with the layer of particle material by rotating said rotor (21) in only one specific direction of rotation, the movable devices (21b,22,23) being adapted to pass thereby along the support matrix in such a way that at least a portion of the particle material is removed from the support matrix. A separation means according to claim 4, characterized in that the devices (21b,22,23) moved by the rotor (21) consist of scrapers fitted on the rotor. A separation means according to claim 5, characterized in that the scrapers (23) are movably secured to the rotor (21) in such a way that they are brought into mechanical contact with the layer of particle material only when the rotor rotates in said one direction of rotation. A separation means according to any of claims 4 to 6, characterized in that the movable devices (21b,22,23) comprise brushes or felt pads arranged on the rotor and consisting of a material softer than the material in the support matrix. A separation means according to any of claims 4 to 6, characterized in that the movable devices (21b,22,23) comprise blades or shutters.

Inventors: AKERBLOM, BERTIL, DAHLQUIST, ERIK, FLINK, SUNE, STRIDSBERG, SOEREN, TEPPLER, MILAN, Stridsberg, Sören
Applicants: AB ASEA-ATOM
Application year: 1987
Main classification: B01D37
IPC codes: B01D69, B01D29, B01D65