WO2023111023A1

WO2023111023A1 - Spiral-wound element

Info

Publication number: WO2023111023A1
Application number: PCT/EP2022/085861
Authority: WO
Inventors: Eugen Reinhardt; Walter LAMPARTER
Original assignee: UNISOL Membrane Technology GmbH
Priority date: 2021-12-16
Filing date: 2022-12-14
Publication date: 2023-06-22
Also published as: DE102021133420A1

Abstract

The invention relates to a spiral-wound element (1) comprising a winding that comprises a permeate tube (3) and a spiral-wound membrane-spacer sandwich (2) wrapped with one or more loops of a trim material, and an outer shell (4), characterized in that the outer shell is executed as an injection molding.

Description

Title: Spiral-wound element

Description

The present invention relates to a spiral-wound filter cartridge, called a spiral-wound element, which can be used for fluid separation applications, for example ultrafiltration, micro- and nanofiltration, and reverse osmosis.

Spiral-wound elements are known in the art, as is the use thereof for the abovementioned applications; in this regard see, for example, US 5,985,146 and the prior art documents cited on the title page of the patent specification.

A spiral-wound element consists of multiple membrane cushions that are sealed by an adhesive bond on three sides and stuck to a perforated tube on the fourth side. The permeate cleaned by the filtering operation flows into this tube. The membrane cushions in turn consist of two membranes laid against one another as mirror images, with a spacer material (the permeate spacer) positioned in the middle thereof, which assures the outflow of permeate to the perforated permeate tube (membrane-spacer sandwich).

The membrane cushions are spiral-wound together with a further spacer material, called the feed spacer. In order to impart stability and dimensional integrity to the resultant winding, the resultant winding is provided with an additional sheath. For this sheath, various materials are proposed in the prior art, for example a GFRP sheath (GFRP = glass fiber- re info reed plastic).

These spiral-wound filter cartridges or spiral-wound elements, the standard format of which is an 8” (inches) spiral-wound element, are typically positioned in a glass-fiber or stainless-steel pressure housing that provides sufficient mechanical strength in order to withstand the high feed pressures required for the filtration operation.

The membrane elements mentioned have long been considered to be efficient devices for separation of constituents of fluid mixtures using, for example, ultrafiltration, microfiltration, nanofiltration and reverse osmosis methods. It is possible here to contact a pressurized fluid mixture with a membrane surface. On account of a different chemical potential and on account of different mass transfer rates through the membrane, only some portions of a fluid mixture can pass through the membrane, and a separation into constituents is achieved. The fluid mixture or the charge enters at one end of the cylindrical cartridge and migrates through feed spacers and permeate spacers that are disposed parallel to and between the membrane sheets. Separation is effected at the membrane-liquid interface. A portion of the liquid, called the permeate, passes through the membrane layer, while the rest of the mixture remains on the opposite side of the membrane as a highly concentrated feedstock. The permeate stream moves in a radial direction in the form of an inward spiral until it crosses the walls of the central tube (see US 4,235,723, US 3,367,504, US 3,504,796 and US 3,417,870).

For a general module description see e.g. US 10,123,255.

For the standard applications of the spiral-wound elements (or filter cartridges) described, the filter material itself and the surface of the sheath need not meet any hygiene demands.

Other demands, by contrast, are applicable to spiral-wound elements for hygiene purposes, as used, for example, in the foods industry and pharmaceutical production. Important factors here are not only questions of material but in particular also hygiene demands and the associated flow-related demands. Since the processes here are generally concentration processes, there is a need for a hygienic feed side or retentate side. This is achieved by configuring the interspace that exists between the spiral-wound element (filter cartridge) and pressure tube, which has to be kept very small or narrow on account of maximization of performance (throughput) of the filtering device, such that it can be purged and cleaned. This is accomplished by enabling forced flow through this interspace, which itself in turn has to be kept very small. A further measure is, for example, brine seals that are used on the feed inlet side, which seal the outside of the filter cartridge against the inside of the pressure housing. In the annular space beyond these seals, however, a region with standing water may be formed, in which bacteria can grow.

A solution in order to enable controlled bypass flow between filter cartridge (spiral-wound element) and pressure housing is, for example, that of using what is called a mesh plane (for example made of washable PVC weave with a mesh-like structure), which, as a hose, forms the outer shell or outside of the actual spiral-wound element. The mesh structure enables more or less controlled bypass flow. A development of this hygienic outer shell of the filter cartridge (spiral-wound element) is described in US 5,985,146. The outer shell of this filter cartridge consists of a polypropylene (PP) tube of exact dimensions which is shrunk onto the actual spiral-wound element, i.e. the PP tube is heated and pushed over the actual spiral-wound element and shrunk on. As a result, it contracts and forms a close-fitting outer shell for the spiral-wound element. Before the shrink-fitting, a spiral-shaped structure is machined into the outer surface of the PP tube. This is a thread groove that runs around the circumference and along the length of the outer surface of the PP tube. This outer structure enables a bypass flow between pressure housing and filter cartridge, which is much smaller than in the mesh variant, which increases the performance of the filter device; cf. fig. 1 from US 5,985,146 and the accompanying description.

However, spiral-wound elements thus produced and configured are still in need of improvement, especially with regard to the following aspects: the spiral-wound membrane cushions comprising permeate spacers and feed spacers do not form an interspace-free sealing interface or connection to the inner smooth surface of the outer shell of the spiralwound element, for example with the PP tube described that has a thread groove on the outside. Cavities are formed, which are often insufficiently purged, and in which soil, residues of the liquid to be treated and bacteria can accumulate. This can lead to problems with regard to hygiene demands, for example in the foods or medical sector. Most cavities occur here between the outer windings (outer windings also referred to as trim material) of the spiral winding and the outer shell of the spiral-wound element. Furthermore, it is not always certain that the actual spiral winding (filter element without the outer shell, for example PP- polypropylene tube) has a sufficiently tight fit in a PP or polymer tube utilized as outer shell.

Also desirable would be a way of producing the spiral-wound element, including the outer shell thereof, that encompasses simpler and fewer separate production steps, and hence a corresponding reduction in production costs.

These objects are achieved in accordance with the invention by an improved process for producing the outer shell of a spiral-wound element. For this purpose, the winding (the actual spiral-wound element without the outer shell) is inserted into an injection mold and provided or subjected to insert molding with the desired outer structure in one operation. The cavities mentioned between the winding and the inner surface of the outer shell that has been applied by injection molding in accordance with the invention are filled here with the injection molding material and no longer exist.

The injection molding process of the invention is rapid, requires fewer steps and offers higher flexibility in terms of design than the production processes known in the art and results in a product having a tight fit of the actual spiral winding in the outer shell (e.g. PP tube).

However, for the inventive manufacture of the outer shell of the spiral-wound element, it was not possible to make use of known standard injection molding methods. According to the known prior art, the insert molding of the winding would not be possible on account of the lack of a cooling surface on the winding side, the injection temperature and the injection pressure, without having an adverse effect on the membranes and the wound structures.

The filter cartridges available on the market that have the tube sheath described, for example, are trimmed prior to the ensheathing and are cut to size at both ends after the PP tube has been applied by shrink-fitting.

It is necessary to choose a different course of action for the injection molding process of the invention. The winding to be subjected to insert molding has to be trimmed and cut to size before the injection molding.

In this context, high demands are already placed on the trimming of the windings, since it is necessary to compensate for the winding tolerance that arises in the trimming by virtue of the injection molding. This is important for the dimensional integrity of the finished filter cartridge in relation to the diameter of the pressure housing as well as for the avoidance of unwanted cavities.

Various methods of trimming are known in the art. A suitable method is described, for example, in US patent US 8,668,828. This involves providing the winding that has been finished per se with a further ensheathing film material which is bonded to one of the feed spacer films and is wound under tension around the winding at a force of at least 300 N/m. As a result, the films are put under tension in radial compression. For this purpose, at least 1.5 windings of the ensheathing film material are required in order to create an essentially cylindrical element with a selected external diameter within tight tolerances. By means of this ensheathing film, the outer shell is then inserted in the shape of the PP tube. In the process of the invention for creating the outer injection-molded shell of a spiral-wound element that imparts the necessary stability to the spiral-wound element, the injection molding material meets a relatively large thermally sensitive and pressure-sensitive body. For the insert molding, a very small volume of injection molding material is required relative to the body to be subjected to insert molding. This constitutes a high demand on the distribution of the injection molding material. The injection molding material is injected at up to about 500 bar at about 200°C. By contrast with conventional injection molded products, the winding to be subjected to insert molding has only very limited means, if any, of removing the temperature, in order to cause the injection molding material to solidify. This means that, by virtue of very exact feeding of the injection molding material through a matched injection nozzle system, exact filling of the injection mold with defined flow distances has to be achieved in order to avoid temperature- related damage to the spiral-wound element or winding to be subjected to insert molding, and simultaneously to limit the filling to the outer end of the feed spacer. The temperature must be removed unilaterally toward the mold. The winding to be subjected to injection molding does not withstand any external pressure acting at a particular point. For that reason, the number of injection nozzles and the arrangement thereof in the mold must be chosen such that the feed pressure does not act directly on the winding to be subjected to insert molding.

Useful ensheathing film materials (outer windings or trim material) include various materials, for example the material for the feed spacers, a weave having larger pores than the material, or else a film (also referred to as tape) having no pores. These materials are preferably identical to the feed spacer material in terms of their chemical composition and are preferably welded to one or else more than one feed spacer by means of ultrasound. Cavities can arise between the membrane cushions and the ensheathing film material used for trimming. These are not shown explicitly in Figure 1, since this figure also does not show wrapping of the spiral winding with trim material. Trimming with open-pore material can assure controlled bypass flow.

For the production of the outer injection-molded shell of the spiral-wound element of the invention, preference is given to using polypropylene (PP), polyethylene, acrylonitrile- butadiene-styrene, polystyrene, polyamide, polybutylene terephthalate, polymethylmethacrylate etc., especially polypropylene. In principle, it is possible to process any thermoplastics, thermosets and elastomers for the inventive application by injection molding. Additives that improve stability or other properties, such as fibers, pigments etc., may be added in the amount and with the characteristics that are customary in injection molding processes.

PP was selected because it can be processed at relatively low temperatures and pressures compared to other plastics. PP also has the necessary stability and the necessary certifications for use in the food industry. The use of blowing gases in order to lower viscosity and hence to further reduce pressure and temperature is possible. In the case of PP, this results in a lowering of pressure and temperature to temperatures of preferably not more than 200°C (at the injection point preferably 200°C) and a preferred pressure of not more than 100 bar. In general, the injection pressure used in the injection moulding process according to the invention is max. 150 bar, where the pressure on commencement of injection moulding for filling of the injection mould is relatively low, and the maximum value is attained in completing the filling of the injection mould. The more exact the dimensions of the spiral winding after the trimming, the less casting material has to be used in the completion of the filling. This too shows the significance of most possible ideal trimming (i.e. an ideal cylinder shape) for the invention described here. The temperature of the injection moulding material during the wrapping of the winding is preferably between 80°C and 160°C (to 160°C especially in the completion of the filling).

For the injection molding process of the invention, preference is given to using a 2-plate injection molding machine (Engel E-Duo) from Engel Austria GmbH, Austria.

In addition to various quality monitoring systems such as Clamp Control, Flow Control, Melt Control, Process Observer and Vibration Control, IQ Weight Control is also used on the machine. IQ Weight Control permits optimization of the injection weight during the shot. Different shot weights required can be adjusted to the situation during the shot. The shot procedure can be optimized such that the preferred flow distances can be defined. The positioning of the openings in the injection mold and the number thereof can be used to achieve optimal filling conditions at minimum pressures and temperatures.

The injection mold is preferably configured such that 2 injection nozzles in each case are positioned at distances of 25 cm, which are arranged at the top and bottom and inject in a mutually opposite manner. Preference is given to using flat nozzles for injection that have a lower injection pressure than conventional round nozzles. The hot runner system is designed for the relatively low injection weight. Different regions in the injection mold assist heating and/or cooling. Experiments have also shown that, depending on the size of the spiral winding and in order to assure complete filling of the cavity between spiral winding and injection mould, a higher number (for example up to 6 injection nozzles) of injection nozzles in the injection mould may also be appropriate. In addition, the use of flat nozzles having a rectangular cross section may be advantageous over the use of conventional round injection nozzles, since this can reduce the pressure to be expended for the injection moulding by about 20-30%.

As stated, by contrast with known prior art methods, in the injection molding process of the invention, the trimming and cutting-to-size of the spiral-wound element must precede the injection molding, i.e. the application of the outer shell.

The injection molding of the invention should be effected within a minimum period of time, and at minimum temperature and pressure. The insert molding is undertaken in a tailored manner on the outside. It is possible here for the outer shell applied by injection molding to assume any desired outer surface shape. For the purposes of the present invention, the outer surface of the filter cartridge of the invention is preferably executed as described in US 5,985,146 (thread groove).

On the inside of the outer shell applied by injection molding (toward the actual spiral-wound element), significant changes arise with respect to the prior art filter elements as described, for example, in the two US patents cited. The winding here is wound commencing with the permeate tube with the membrane cushions and the feed spacers. On the outside of the winding, two additional orbits are then undertaken exclusively with the feed spacers. This region in the prior art spiral-wound elements enables bypass flows through the feed spacer windings.

With regard to the present invention, the outer winding may preferably likewise be provided with 2 windings with the feed spacer material. In principle, however, it is also possible to undertake just one winding or else more than 2 windings, e.g. 3-5 windings. The aim here is to provide as few feed spacer windings as possible since the possible membrane area of the winding is otherwise unnecessarily reduced. The further outer windings (trim material) mentioned serve for trimming of the winding and hence also for subsequent fixing of the winding in the injection mold. In the spiral-wound elements of the invention or in the windings (without the outer shell), the above-described outer region of the winding is filled (by injection molding) via control of the shot of the injection molding material, while the feed spacer within the winding remains clear, which ultimately assures optimal flow through the spiral-wound element of the invention.

Depending on factors including the temperature used in the injection moulding, the pressure and the viscosity of the polymer used (PP here has a lower viscosity at relatively low temperature), the polymer penetrates into the spiral winding to different extents. Cavities between the trimmed spiral winding and the outer injection-moulded shell applied are completely or largely non-existent. This is the main difference from the prior art spiral winding elements, in which cavities regularly arise between trimmed spiral winding and outer shell (e.g. overfitted PP tube), which can give rise to uncontrolled bypass flow and result in accumulation of residues of the starting material to be cleaned or filtered. If materials having pores (for example the feed spacer material or a plastic square mesh) are used for trimming, depending on the process parameters described, the injection moulding material may also penetrate into this ensheathing material. By virtue of suitable choice of parameters, depending on the respective spiral winding, it is possible to completely or largely prevent penetration of the injection moulding material into the actual feed spacers of the filter. This does not impair the size of the filter area and hence the filter performance. If a pore-free impervious tape, for example, was used for trimming, the injection moulding material will encroach up to this tape, and can push it into cavities beneath and hence either wholly or largely fill or close them. In other words, in the most favourable case, the spiral-wound elements according to the invention largely have no unwanted bypass flows, if any (either between the spiral winding and trim material or between the trim material and outer injection- moulded shell).

The sheath applied by injection molding preferably receives a smooth annular end face at the top end in each case, which concludes the sheath itself and the spacer subjected to injection molding.

The core of the present invention lies in the applying of the outer shell for a spiral-wound element by an injection molding process. In this case, a relatively large component is manufactured with precision for a small component (for example a plastic Lego brick), wherein the sensitive insert part to be subjected to insert molding (the actual spiral-wound element) has to be protected. In relation to excessive thermal stress on the spiral winding during the injection moulding operation, for example, the end faces of the element to be insert-moulded may be provided with air inlet and outlet channels and air for cooling may be passed therethrough, and hence through the filter zone of the spiral winding.

The cycle time for an 8" filter element according to the invention is about 120s and could be brought down to about 20s, with parallel preparation, insertion, discharge etc. It is thus possible to produce about 700 filter elements with one machine per day. This is a distinct increase in production capacity with a corresponding saving on manufacturing costs compared to the prior art.

The present invention can be used with or in the case of any spiral-wound membrane device that uses a flat membrane. For example, this invention may be used with membranes that use reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), gas separation or pervaporation methods. Membranes for RO, NF, UF, MF, gas separation and pervaporation are well known in the art. Both anisotropic (asymmetric) membranes with a single and a double barrier layer (skin) and isotropic membranes are currently produced in flat film form for RO, NF, UF, MF, gas filtration and pervaporation (see US patent no. 3,615,024; 3,597,393; and 3,567,632). The membranes may consist of a single polymer or a copolymer, may be laminated or else may consist of a composite structure, wherein a thin and charged barrier layer or a film, charged or uncharged, is formed atop a thicker substrate film, the latter being either porous or nonporous (capable of diffusion). The polymers that are suitable for such membranes range from highly stable hydrophobic materials such as polyvinylidene fluoride, polysulfones, modified acrylic copolymers, polychloroethers and the like (normally used for UF, MF, gas filtration and pervaporation, and as substrates for RO and NF composites) up to the hydrophilic polymers such as cellulose acetate and various polyamides (see US patent nos. 4,399,035; 4,277,344; 3,951,815; 4,039,440; and 3,615,024). In the case of low-pressure applications (e.g. 2-10 atmospheres) such as ultrafiltration, nanofiltration, microfiltration and low-pressure RO, the spiral-wound element may optionally be permanently mounted in a dedicated pressure vessel or a cartridge with suitable connections for connection to the filtration systems.

The present invention is elucidated in detail by the drawings that follow and the working example, without the broad applicability of these being limited in any way. Fig. 1 shows the general construction of a spiral-wound element in cross section.

The spiral-wound element 1 comprises a winding comprising a permeate tube 3 and a spiralwound membrane-spacer sandwich 2, wherein the winding is enclosed by an outer shell 4 executed according to the present invention as an injection molding (injection-molded shell 4). Cavities 5 are formed between the shell 4 and the outer region (edge) of the winding in the prior art spiral-wound elements; these are filled with the injection molding composition in the inventive spiral-wound element 1 . It is possible here for cavities to be formed in the prior art spiral-wound elements both between the actual winding and trim material (trim material not shown here) and between the trim material and the outer shell (e.g. PP tube).

Fig. 2 shows an inventive injection mold 6 in cross section.

This consists of an upper half 7 and a lower half 8 that are form-fittingly bonded to one another. The spiral winding 9 (without the outer shell 4) is inserted into the injection mold, and a cavity 10 is formed, which is filled with the injection molding composition. As a result, the spiral winding 9 is fully insert-molded (i.e. including the top ends) with the injection molding composition and fully encased thereby (outer injection-molded shell 4). The arrows 11 show the shooting direction of the injection molding composition through mutually opposite injection molding nozzles (injection nozzles) 12.

Working example

An 8" or 3.8" spiral winding is placed into the injection mold by means of a supply robot. Centering is effected via the permeate tube. The mold reliably covers the two top ends of the spiral-wound element. After closure, the mold has an exact distance of about 5 mm from the spiral winding. The gap is filled by the injection molding shot and thereafter forms the sheath.

The finished element is removed by the robot and deposited. List of reference numerals

1 spiral-wound element (or filter cartridge)

2 membrane-spacer sandwich

3 permeate tube

4 outer injection-molded shell

5 cavities

6 injection mold

7 upper half

8 lower half

9 spiral winding

10 cavity

11 shot direction of injection molding composition

12 injection nozzles for injection molding

Claims

1. A spiral-wound element (1) comprising a winding that comprises a permeate tube (3) and a spiral-wound membrane-spacer sandwich (2) wrapped with one or more loops of a trim material, and an outer shell (4), characterized in that the outer shell is executed as an injection molding.

2. The spiral-wound element (1) as claimed in claim 1 , characterized in that the outer injection-molded shell (4) consists of a thermoplastic, thermoset or elastomer.

3. The spiral-wound element (1) as claimed in claim 1 or 2, characterized in that the outer injection-molded shell (4) consists of polypropylene, polyethylene, acrylonitrile- butadiene-styrene, polystyrene, polyamide, polybutylene terephthalate or polymethylmethacrylate, preferably of polypropylene.

4. The spiral-wound element (1) as claimed in any of claims 1 to 3, characterized in that the injection-molded shell (4) has a thread groove on its outside.

5. The spiral-wound element (1) as claimed in any of claims 1 to 4, characterized in that the spiral-wound element is free or largely free of cavities between the winding and trim material and/or between the trim material and outer injection-moulded shell (4), especially free or largely free of cavities between the trim material and outer injection- moulded shell (4).

6. A process for producing a spiral-wound element (1) as claimed in any of claims 1 to 5, characterized in that the outer shell (4) of the spiral-wound element is applied by injection molding.

7. The process as claimed in claim 6, characterized in that the outer shell (4) of the spiral-wound element (1) is produced from polypropylene applied by injection molding. The process as claimed in claim 6 or 7, characterized in that the injection-molding material is injected at about 500 bar at about 200°C. The use of an injection molding process for production of the outer shell (4) of a spiral-wound element (1). The use of a spiral-wound element (1) as claimed in any of claims 1-5 as a filter cartridge or filter element.