MEMBRANE MODULE FOR HYPERFILTRATION OR ULTRAFILTRATION OF CONTAMINATING LIQUID FLOWS

Abstract: A membrane envelope (1) is spirally wound around a tube (4) provided with openings and the membrane layers of which are alternately kept at a distance from each other by retentate spacers and permeate spacers (2), which retentate spacers comprise relatively rigid rods or wires (3) extending parallel to the centre line of the module. In order to obtain a very constant and defined geometry of the retentate flow channels and a favorable ratio between membrane surface and module volume, the width dimension of the rods measured perpendicular to the membrane layers is 0.5 to 2.5 mm and the distance between the rods is 4 to 12 times said width dimension. Said width dimension of the rods measured perpendicular to the membrane layers should be at least three times the diameter of the largest particles in the liquid flow to be filtered.

Description: The invention relates to a membrane module for hyperfiltration (reverse osmosis) or ultrafiltration of liquid flows containing particles up to 0.8 mm, comprising a membrane envelope which is spirally wound around a tube provided with openings and the membrane layer of which are alternately kept at a distance from each other by retentate spacers and permeate spacers, which retentate spacers comprise relatively rigid rods or wires extending parallel to the centre line of the module. Such a membrane module is described in EP-A-0,146,298. In applying reverse osmosis or ultrafiltration to liquid flows which could contaminate the membrane considerably, in particular, flows which contain solid particles and/or colloidal material, use is usually made of tubular membrane modules having a diameter in the range between 5 and 15 mm. An important advantage of said tubular membrane modules is that they are not susceptible to blockage. By using high liquid velocities along the membrane surface, a turbulent liquid flow is obtained, as a result of which contamination of the membrane surface is limited. Tubular membrane modules have, however, an unfavorable ratio between membrane surface and module volume (between 50 and 150 m²/m³). The liquid capacity is also relatively large and this presents a problem if the modules have to be cleaned: large quantities of cleaning agents have to be used. With strongly contaminating flows, daily cleaning of the membrane modules is quite normal. The flows of waste which are the consequence thereof often present an environmental problem. In tubular modules, the costs of cleaning materials may amount to 20% of the operating costs. The module according to EP-A-0,146,298 is intended for processing viscous liquids which in principle contain no solid particles and/or colloidal material. The rods of these known modules have a diameter of 2.5 mm and a spacing between the rods of approximately 5 cm. The geometry resulting therefrom of the retentate flow channels is undefined in view of the normal flexibility of the membranes. In fact, the membrane layers succeeding each other in the winding will be closer to each other in the winding at the midpoint between two rods than at the position of the rods. The object of the known module is to reduce the pressure drop across the module. Use of said module for filtration of liquids containing solid particles and/or colloidal material would have the result that said particles or said colloidal material could settle in the central region between the rods and could cause blockage. In addition, the packing density of this known module is too low and the energy consumption during filtration too high. The object of the invention is to provide a membrane module described in the introduction which has a very constant and defined geometry of the retentate flow channels, which is not susceptible to blockage and of which the ratio between membrane surface and module volume is not unfavourable. According to the invention the module is characterized in that the width dimension of the rods measured perpendicular to the membrane layers is 0.5 to 2.5 mm and the distance between the rods is 4 to 12 times said width dimension, and that the rods are anchored outside the wound membrane envelope. If the width dimension of the rods measured perpendicular to the membrane layers (in the case of round rods, that is the diameter) is less than 0.5 mm there is too great a chance of blockage. A width dimension greater than 2.5 mm results in a too unfavorable ratio between membrane surface and module volume. A too large distance between the rods (greater than twelve times the rod width or rod diameter) entails, as stated in the discussion of EP-A-0,146,298, an undefined flow and a risk of blockage. Too small a distance between the rods again results in a poor surface:volume ratio. Spirally wound membrane modules, the spacers of which, on the retentate side of the membrane, comprise a fabric, web or gauze are also unsuitable for treating strongly contaminating liquids containing particles, since contamination readily occurs in the dead corners of the structure of the spacers. If wires or rods placed parallel to the flow direction are used, parallel channels having smooth walls are produced in which the chance of blockage is extremely small. It has been measured that no blockage of the module occurs if the width of the gap between the membrane layers in which the liquid to be treated flows is kept by rods or wires at a value which is at least three times the diameter of the largest particles in the liquid flow fed in. For many applications, the rods have a diameter of at least 1 mm while the spacing of the rods is approximately six times their diameter. The favorable surface/liquid capacity ratio of the module results in such high shear-plane velocities in the module that a low contamination of the membrane surface occurs, accompanied by a laminar flow pattern. The laminar flow pattern also has a great advantage in energy consumption compared with the tubular module with turbulent operation. An advantage of spacing the wires or rods is, furthermore, that the loss of membrane surface due to the spacer itself is low. Where the spacer presses against the membrane surface, the membrane does not function. By anchoring the rods outside the wound membrane envelope, the membrane windings of the envelope cannot slide out of each other under the influence of high axial flow velocities of the retentate, while the anchoring cannot cause any blockage. For example, end parts of the rods projecting outside the wound membrane envelopes are embedded in a disc. The invention further relates to a filtration device for hyperfiltration or ultrafiltration of liquid flows, which device comprises a housing with at least one cylindrical cavity in which an above-described membrane module is accommodated, which housing is provided with a feed-stock inlet, a permeate discharge and a retentate discharge. A plastic sheath, the ends of which are sealed with 0-rings with respect to the housing can be provided around the wound module. A further possibility is that the outermost membrane winding is kept at a small distance from the housing by the rod-type retentate spacers to form a gap through which flow can take place. Several modules can be placed behind each other in a housing. In this case the anchoring bodies of the rods are provided with openings through which flow can take place and the housing furthermore has a feed-stock inlet, a permeate discharge and a retentate discharge. Finally, the invention relates to a method for purifying, by hyper- and ultrafiltration, contaminating liquid flows containing solid particles, in which the liquid to be purified is fed into the feed-stock inlet of the above-described filtration device and flows in the gaps between the membrane layers, the gap width of which is kept by the said rods or wires at a value which is at least three times the diameter of the largest particles in the liquid flow fed in. The rods preferably have a diameter of at least 1 mm and the centre-to-centre spacing of the rods is approximately six times the diameter of the rods. The invention will now be explained in more detail with reference to the figures. Figure 1 shows a longitudinal section of a filter device according to the invention. Figure 2 shows a cross section along the line II-II in figure 1. Figure 3 shows a longitudinal section of a second embodiment. Figure 4 shows a section along the line IV-IV in figure 3. The shown devices are intended for ultrafiltration or hyperfiltration of contaminating liquid flows. Examples of liquids to be treated are blood plasma, skimmed milk, cheese wey, paint suspensions and slurries in general. The device is suitable, in particular, for liquids which contain solid particles and/or colloidal material. An envelope 1 of flat membranes is glued at three sides while the fourth side remains open. Inside the envelope closed at three sides a compression-resistant permeate spacer in the form of impregnated tricot 2 is provided, while a retentate spacer 3 in the form of a multiplicity of parallel rods situated at a distance from each other is placed or securely glued outside the envelope. This entity is wound onto a perforated tube 4 (for example, of stainless steel) in a manner such that the open side of the envelope is joined to the tube 4. The possibility exists for arranging for the gluing of the longitudinal edges of the membrane envelope to be carried out at the same time as the winding. In the construction according to figures 1 and 2, a thin sheath 5 (for example, having a thickness of 1 mm) composed of glass-fibre reinforced plastic is fitted around the wound module. The thickness of said sheath is increased at the ends (for example, to approximately 9 mm). Situated in these thickened regions are 0-rings 6 which serve to seal the module with respect to a glass fibre reinforced plastic or stainless steel outer sheath 7, the so-called housing. One end of said outer sheath 7 is closed off with a stainless steel plate 8 which is likewise sealed with an 0-ring 9 and is secured by a Seeger ring 10. The other end of the outer sheath 7 is closed off by an epoxy disc 11 in which the ends of the rods or wires of the retentate spacer are embedded. This disc is also provided with an 0-ring 12. The epoxy disc holds the rods in position. The disc is pressure-loaded during operation. If there may be a risk of the disc bursting, a stop plate for the rods may be placed at the other end of the rods. Instead of embedding in an epoxy disc, other plastics may be used and it is also possible to mount the rods on a metal disc, for example on the end plate of a module tube. Figures 3 and 4 show a construction in which no plastic sheath reinforced with glass fibre is used around the module, but in which the rod-type retentate spacers themselves from a seal, through which flow can take place, between the outermost winding and the outer sheath. For sanitary applications, such an embodiment through which cleaning liquids can also flow is to be preferred. In addition, the membrane surface/volume ratio of the module is thereby increased still further. Figure 4 shows the possibility of using a body through which flow can take place as a lattice for anchoring the rods instead of a plastic or metal disc 11, as a result of which a number of modules can be placed in series inside an outer sheath 7 in a simple manner. The outer sheath 7 is provided with an inlet nozzle 13. The central tube 4 has a discharge 15 for discharging permeate while a discharge 14 is provided for discharging retentate. The liquid supplied via the nozzle 13 is kept under pressure, for example a pressure of 1 or 2 x 10⁶ Pa, in the gaps between the membrane envelope layers kept at a distance by the rods 3. Permeate gets into the Permeate spacer inside the membrane envelope, follows the spiral winding of said envelope inwards and eventually flows into the central tube 4 where it is discharged via the nozzle 15. The most important advantage of the device described is that the ratio between surface and volume is very favorable (between 400 and 600 m2/m3 with respect to 50-150 m2/ml for the existing device). As a consequence of the construction of the retentate spacer in the form of the relatively rigid rods or wires, a good flow parallel to the rod direction along the membrane surfaces can be obtained with very little risk of blockage. The gap width should be at least three times the diameter of the largest particles in the liquid. To summarize, a very low degree of contamination is achieved despite the fact that the device is suitable for treating liquids containing relatively large particles (up to at least 0.8 mm). The compression-resistance (especially important for reverse osmosis) is satisfactory. It is possible to ensure good thermal stability. The operating costs of a system with spirally wound modules are at least 10% lower than those of a system containing tubular modules. Various modifications of the module shown and described are possible within the scope of the claims. Essential to the inventive idea is that the use can be made of a spirally wound module for hyperfiltration or ultra filtration without there being a risk of blockage if the geometry of the retentate channels is such that the chance of blockage is low. This geometry is achieved by means of retentate spacers which comprise axially extending, relatively rigid rods or wires.

Claims: Membrane module for hyperfiltration (reverse osmosis) or ultrafiltration of liquid flows containing particles up to 0.8 mm, comprising a membrane envelope which is spirally wound around a tube provided with openings and the membrane layers of which are alternately kept at a distance from each other by retentate spacers and permeate spacers, which retentate spacers comprise relatively rigid rods or wires extending parallel to the centre line of the module, characterized in that the width dimension of the rods measured perpendicular to the membrane layers is 0.5 to 2.5 mm and the distance between the rods is four to twelve times said width dimension, and that the rods are anchored outside the wound membrane envelope. Membrane module according to claim 1, characterized in that end parts of the rods projecting outside the wound membrane envelope are embedded in a disc. Filtration device for hyperfiltration or ultra filtration of liquid flows, characterized in that the device comprises a housing with at least one cylindrical cavity in which a membrane module according to one of the claims 1 or 2 is accomodated, which housing is provided with a feed-stock inlet, a permeate discharge and a retentate discharge. Filtration device according to claim 3, characterized in that a plastic sheath, the ends of which are sealed with 0-rings with respect to the housing, is provided around the wound module. Filtration device according to claim 3, characterized in that the outermost winding of the module is kept at some distance from the housing by said rod-type retentate spacers to form a gap through which flow can take place. Filtration device for hyperfiltration or ultrafiltration of liquid flows, characterized in that the device comprises a housing with at least one cylindrical cavity, a multiplicity of modules according to claim 1 being provided in series in said cavity, the anchoring bodies of the rods being provided with flow openings and the housing further having a feed-stock inlet, a permeat discharge and a retentate discharge. Method for purifying, by hyper- or ultrafiltration, contaminating liquid flows containing solid particles, characterized in that the liquid to be purified is fed under pressure into the feed stock inlet of the filtration device according to one of the claims 4 to 6 incl. and flows in the gaps between the membrane layers, the gap width of which is kept by the said rods or wires at a value which is at least three times the diameter of the largest particles in the liquid flow fed in. Method according to claim 7, characterized in that the rods have a diameter of at least 1 mm and the centre-to-centre spacing of the rods is approximately six times the diameter of the rods.

Inventors: VAN WIJK, HENDRIK FREDERIK
Applicants: NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1988
Main classification: B01D61
IPC codes: B01D63, B01D61