Patent Application
20130337708
This invention relates to permeate carrier fabrics for membrane filters, such as reverse osmosis filters, ultrafiltration filters and other types of membrane filters. Such filters are now in use in many applications for high-efficiency liquid filtration. Such membrane filters appear in spiral wound elements and are used with reverse osmosis filtration systems, microfiltration systems, nanofiltration systems, as well as ultrafiltration systems.
As stated above, such membrane filters generally comprise spiral wound elements made of three components, the membrane, the permeate carrier fabric, and the feed spacer. The membrane is the part of the element where the separation occurs and can be either a reverse osmosis, nanofiltration, ultrafiltration, or microfiltration type membrane. The feed spacer, on the other hand, separates two adjacent membrane faces and acts as a spacer and also a turbulence promoter. The permeate carrier fabric is placed between layers of the membrane and acts as a pipe to allow the permeate to flow between the adjacent membranes and exit from the element. A composite is made which consists of a first membrane layer, an intermediate permeate carrier fabric, and a second membrane layer. These three components are glued or sonically welded together on three sides, and numerous layers of these three-part composites are each glued around a perforated filter core. The spacer fabric is used between each layer of composite. The layers are rolled around the core to a certain diameter based on the size/diameter of the filter. This element is then placed in a cylindrical container.
As stated above, the purpose of the permeate carrier fabric is to provide direction for and channel the flow of the liquid. It is therefore important that the yarns in the fabric be sufficiently stiff or firm to prevent collapse.
The permeate carrier fabric is generally a knitted polyester tricot fabric. This fabric, as stated above, is placed between permeable membranes. Tricot knitted fabric has been found to be a particular desirable structure for supporting the membrane material due to the porous knitted structure and raised rows of stitches which define between them long, continuous passageways akin to corrugation through which the liquid being filtered flows. However, other types of fabric may also be used as the permeate carrier, including other types of knitted fabric or even, woven fabric. The permeate carrier fabric should have a low pressure drop (high permeability) for the permeate flow while being able to withstand the high pressures exerted by the liquid being filtered without compaction.
In the past, there have generally been two types of permeate carrier fabrics. One type are knit fabrics of multifilament polyester yarns, which yarns are then coated with a resin to add firmness to the fabric. The other type of permeate fabric are fabrics made with bi-component yarns having a regular polyester core and a low melt polyester sheath. This sheath part of the yarn is melted during finishing to give the yarn its necessary firmness. Resin applied to add firmness to the resin coated yarns also tends to partially block the fabric channels thereby restricting part of the permeate flow. When the yarns in the bi-component fabrics are melted, again, the same result occurs, the channels are again partially blocked restricting the permeate flow.
The inventors here then have determined there is a need for a permeate carrier fabric which will be sufficiently stiff and firm to withstand the filter pressure without collapsing, however will reduce blockage of the permeate flow. The use of monofilament yarns gives the permeate carrier fabric exceptional firmness with improved flow. Since monofilament yarns used in the permeate carrier fabric are sufficiently firm, the monofilament yarns do not necessarily need to be coated or physically changed, the permeate channels will be wider for better flow. The permeate carrier fabric comprising at least some monofilament polyester (or nylon) yarns will be used as a channeling fabric between two layers of the membrane. It is important that the yarns forming the channels in the fabric be firm enough to withstand the filter pressure without collapsing. The addition of a monofilament yarn provides the necessary firmness needed to withstand filter pressure conditions.
Such a permeate carrier fabric optimizes the design, cost and efficiency of the finished filter. As a result of the monofilament yarns used in the permeate carrier fabric, there is less further processing required, a firmer permeate carrier fabric results, with a lower cost, and the channels in the fabric are wider for better permeate flow.
It is therefore one aspect of the invention to provide a permeate carrier fabric that comprises at least some monofilament yarn ends in polyester or nylon and the monofilament yarn size is at least 10 denier. The permeate carrier fabrics according the present invention may have varying wale counts, thickness, and weights. They may be made with (a) 100% monofilament yarns, (b) monofilament yarns and other spun or multifilament yarns, (c) monofilament yarns partnered with bi-component yarns, (d) 100% monofilament yarns cross-linked with epoxy resin, or (e) monofilament yarns partnered with other yarns and coated with an epoxy resin.
While the permeate carrier fabric of the present invention may be knit or woven, the preferred approach is a warp knit, preferably tricot, in which at least one of the yarns is a monofilament yarn of at least 10 denier. The wale and course count may vary based on filter performance, but the wale count should be at least 20 per inch and the course count should be at least 40 per inch. The monofilament yarn is preferably formed of polyester, but could possibly be nylon. Where the fabric is made on a warp knitting machine, the machine may be either a 2, 3, or 4 bar machine.
The monofilament concept can be present in a range of fabrics including (1) 100% monofilament yarns, (2) monofilament yarns and other spun or multi-filament yarns, (3) monofilament yarns partnered with bi-component yarns, (4) 100% monofilament yarns cross-linked or coated with epoxy resin, and (5) monofilament yarn partnered with other yarns and coated or cross-linked with epoxy resins.
A trial sample of permeate carrier fabric was prepared using a combination of (1) 70 denier 24 filament bi-component, polyester yarn ends and (2) 20 denier monofilament polyester yarn ends. The bi-component yarn ends were set up on the top bar of a 2 bar tricot knitting machine and the monofilament yarn ends were set up on the bottom bar of the machine. The top bar used a 2/3, 1/0 pattern and the lower bar used a 1/0, 1/2 pattern. Each beam included 1,340 ends with final fabric targets of 60 wales per inch, 50 courses per inch and a weight of 3.87 ounces per square yard. The resulting fabric was spit into two 40″ panels and weighed 4.16 ounces per square yard.
A second trial fabric was made using the same yarns as Example 1 but the finished fabric targeted a wale count of 46 wales per inch. The resulting fabric weighed 3.23 oz/yd2.
A third trial fabric was made; again using the same yarns as Example 1, but the set-up used a targeted wale count of 35 wales per inch. The resulting fabric had a weight of 2.45 oz/yd2.
Another trial run was conducted using a slightly different multi-filament yarn in the fabric. In this example, the multi-filament yarn was 50 denier 24 filament bi-component polyester yarn. Otherwise, the setup was the same. In this example, the resulting fabric weighed 3.07 ounces/yard square.
A test was run in which the fabric of Example 1 was pressure tested against a conventional tricot knitted fabric formed with all 70 denier 24 filament bi-component polyester and 50 denier 24 filament bi-component polyester. The 70 denier yarns ends were set up on the top beams of a tricot knitting machines in which the top bar used a 2/3, 1/0 patters, and the 50 denier yarn ends were set up on the bottom bar using a 1/0, 1/2 pattern. The two fabrics were placed between adjacent reverse osmosis membranes and tested at various water pressures. The lateral permeability of the two fabrics were measured at pressure differentials; and the fabric of Example 1 showed an improvement in permeability of approximately 30% depending on the pressure differential applied. The two samples were both effective to support the membranes.
