Polymers spiral wound membrane

Membranes are manufactured in a diverse range of geometries they include flat, tubular, and multi-tubular, hollow-fiber, and spiral-wound membranes. The type of geometry the membrane is manufactured into depends on the material the membrane is made from. Ceramic membranes, generally, come in tubular, multi-tubular and flat geometries, whereas spiral-wound and hollow-fiber membranes seem, for the most part (with a few notable examples), to be made from polymers. 

The spiral wound membrane packaging configuration is shown in Figure 4.8. Basically, the spiral wound element consists of two sheets of membrane separated by a grooved, polymer reinforced fabric material. This fabric both supports the membrane against the operating pressure and provides a flow path... 

Two types of membranes are used in hydrogen recovery service the spiral wound membrane and the hollow fiber membrane. The spiral wound membrane is made from flat sheets of polymer. It is illustrated in Figure 6 [9]. Hollow fiber membranes, such as those manufactured by UOP (UOP POLYSEP ) and Air Products and Chemicals (PRISM ), are illustrated in Figure 7 [lOJ. 

A membrane filtration plant suitable for process water from reactive dyeing and printing of cotton is a two step plant Pre-filtration by ultrafiltration and a final treatment by reverse osmosis. Pre-treatment technologies for RO spiral wound membrane filtration have focused on flat sheet polymer UF membranes in a high cross-flow filter. The quality of water produced by this plant will go beyond what most dyehouses use today and will be well suited for all processing, including reactive dyeing of cotton." ... 

Not all membrane materials can be made into a thin selective layer on a porous substrate in a hollow fiber form. Consequently, spiral-wound membranes, which can be made from a wider range of materials, usually have higher permeation rates. However, this is offset by the much higher packing density of hollow fiber modules, resulting in similar overall productivity per unit module volume for the two configurations. This situation could change if developments in polymer science lead to more effective thin films in a hollow fiber form. 

Membrane modules have found extensive commercial appHcation in areas where medium purity hydrogen is required, as in ammonia purge streams (191). The first polymer membrane system was developed by Du Pont in the early 1970s. The membranes are typically made of aromatic polyaramide, polyimide, polysulfone, and cellulose acetate supported as spiral-wound hoUow-ftber modules (see Hollow-FIBERMEMBRANEs). 

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

Whereas the liquid-solid filtration processes described so far can separate particles down to a size of around 10 xm, for smaller particles that need to be separated, a porous polymer membrane can be used. This process, known as microfiltration, retains particles down to a size of around 0.05. im. A pressure difference across the membrane of 0.5 to 4 bar is used. The two most common practical arrangements are spiral wound and hollow fiber. In the spiral wound arrangement, flat membrane sheets separated by spacers for the flow of feed and filtrate are wound into a spiral and inserted in a pressure vessel. Hollow... 

The geometries for asymmetric mixed-matrix membranes include flat sheets, hollow fibers and thin-fihn composites. The flat sheet asymmetric mixed-matrix membranes are formed into spirally wound modules and the hollow fiber asymmetric mixed-matrix membranes are formed into hollow fiber modules. The thin-film composite mixed-matrix membranes can be fabricated into either spirally wound or hollow fiber modules. The thin-film composite geometry of mixed-matrix membranes enables selection of different membrane materials for the support layer and low-cost production of asymmetric mixed-matrix membranes utilizing a relatively high-cost zeolite/polymer separating layer on the support layer. 

The method of impregnating liquid membranes has become more and more popular. By impregnating fine-pore polymer films with a suitable membrane liquid, relatively stable heterogeneous solid-liquid membranes are obtained. These membranes are shaped as thin, flat barriers or hollow fibers. Usually they are manufactured from oleophilic polymers, wettable by membrane liquid. The two interfaces, F/M and M/R, have equal or close areas which can be made very large by employing modules of spirally wounded flat membrane or bundles of hollow fibers. 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

Microfiltration units can be configured as plate and frame flat sheet equipment, hollow fiber bundles, or spiral wound modules. The membranes are typically made of synthetic polymers such as Polyethersulfone (PES), Polyamide, Polypropylene, or cellulosic mats. Alternate materials include ceramics, stainless steel, and carbon. Each of these come with its own set of advantages and disadvantages. For instance, ceramic membranes are often recommended for the filtration of larger particles such as cells because of the wider lumen of the channels. However, it has been shown that spiral wound units can also be used for this purpose, provided appropriate spacers are used. 

Microfiltration membranes are similar to UF membranes but have larger pores. Microfiltration membranes are used to separate particles in the range of 0.02-10 pm from liquid or gas streams. Commercial MF membranes are made from a wide variety of materials including polymers, metals, and ceramics. A wide variety of membrane module designs are available including tubular, spiral wound, pleated sheet, hollow fiber, and flat sheet designs. Some modules are best suited for crossflow filtration, and others are designed for dead-end filtration. In dead-end filtration, the feed liquid flows normal to the surface of the membrane, and retained particles build up with time as a cake layer on the membrane surface or within the pores of the membrane. 

Membrane separating modules can be (a) flat sheets (such as continuous column, supported liquid, or polymer film), (b) tabular, (c) capillary, (d) hollow fibers (either coated fibers or supported liquid) and (e) spiral wound. The hollow fiber is similar to the tubular mounting except that hollow fibers typically have a much smaller diameter. 

Dorr-Oliver began to search for other polymers suitable for casting asymmetric UF membranes. By 1965, the first laboratory-scale UF membranes and cells appeared on the market. The ten-year period between 1965 and 1975 was a period of intense development where chemically and thermally resistant membranes were made from polymers like polysulfone (PS) and even polyvinylidene difluoride (PVDF) in molecular weight cut-offs (MWCO) from 500 to 1,000,000. Hollow fibers were also developed during this decade and a whole host of module configurations. Tubes, plate and frame units, and spiral-wound modules became available. 

Most applications of GP use dense membranes of cellulose acetates and polysulfones. For high-temperature applications where polymers cannot be used, membranes of glass, carbon, and inorganic oxides are available, but they are limited in their selectivity. Almost all large-scale applications of GP use spiral-wound or hollow-fiber modules, because of their high packing density. 

Seawater or brackish water can be purified by reverse osmosis. To maximise the flow of water through a polymer membrane, the polymer must have a high water permeability, yet a low permeability for the salts. To maximise efficiency, the membrane area must be large and its thickness as small as possible consistent with a lack of pinholes. A high pressure is applied to the salt water side of the membrane. Because it is thin, a cellulose triacetate membrane is supported on a porous cellulose nitrate-cellulose acetate support structure to resist the pressure. To make the unit compact the composite membrane is spirally wound on to an inner cylinder, and the edges glued together. When a pressure of 70 bar is applied to the seawater side, NaCl rejection levels in excess of 99.7% can be achieved. 

Most commercial membrane separations use natural or synthetic, glassy or rubbery polymers. To achieve high permeability and selectivity, nonporous materials are preferred, with thicknesses ranging from 0.1 to 1.0 micron, either as a surface layer or film onto or as part of much thicker asymmetric or composite membrane materials, which are fabricated primarily into spiral-wound and hollow-fiber-type modules to achieve a high ratio of membrane surface area to module volume. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

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