Spiral wound modules applications

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

Modules and Housings Modern gas membranes are packaged either as hollow-fiber bundles or as spiral-wound modules. The former uses extruded hollow fibers. Tube-side feed is preferable, but it is limited to about 1.5 MPa. Higher-pressure applications are usually fed on the shell side. A large industrial permeator contains fibers 400 pm by 200 pm i.d. in a 6-inch shell 10 feet long. Flat-sheet membrane is wound into spirals, with an 8- by 36-inch permeator containing 25 of membrane. Both types of module are similar to those illustrated in Background and Definitions. Spiral modules are useful when feed... 

Membrane gas-separation systems have found their first applications in the recovery of organics from process vents and effluent air [5]. More than a hundred systems have been installed in the past few years. The technique itself therefore has a solid commercial background. Membranes are assembled typically in spiral-wound modules, as shown in Fig. 7.3. Sheets of membrane interlayered with spacers are wound around a perforated central pipe. The gas mixture to be processed is fed into the annulus between the module housing and the pipe, which becomes a collector for the permeate. The spacers serve to create channels for the gas flow. The membranes separate the feed side from the permeate side. 

In 1968 we started investigations of RO applications for desalting brackish water. In the course of the investigations, we have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. 

In some applications of reverse osmosis and ultrafiltration spiral-wound modules in the food industry, it may be desirable to allow a small portion of the feed solution to bypass the module to prevent bacteria growing in the otherwise stagnant fluid. One way of achieve this bypass is by perforating the ATD as illustrated in Figure 3.45 [115]. 

Figure 3.42 Exploded view and cross-section drawings of a spiral-wound module. Feed solution passes across the membrane surface. A portion passes through the membrane and enters the membrane envelope where it spirals inward to the central perforated collection pipe. One solution enters the module (the feed) and two solutions leave (the residue and the permeate). Spiral-wound modules are the most common module design for reverse osmosis and ultrafiltration as well as for high-pressure gas separation applications in the natural gas industry...

Table 3.5 Typical membrane area and number of membrane envelopes for 40-in.-long industrial spiral-wound modules. The thickness of the membrane spacers used for different applications causes the variation in membrane area...

Figure 3.45 By perforating the antitelescoping device, a small controlled bypass of fluid past the module seal is achieved to eliminate the stagnant area between the reverse osmosis module and the pressure vessel walls. This device is used in food and other sanitary applications of spiral-wound modules [115], Reprinted from Reverse Osmosis Technology, B.S. Parekh (ed.), Marcel Dekker, New York (1988), p. 359, by courtesy of Marcel Dekker, Inc.

In reverse osmosis, the commonly used modules are spiral-wound. Plate-and-frame and tubular modules are limited to a few applications in which membrane fouling is particularly severe, for example, in food applications or processing heavily contaminated industrial wastewater. The hollow fiber reverse osmosis modules used in the past have now been almost completely displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. 

For ultrafiltration applications, hollow fine fibers have never been seriously considered because of their susceptibility to fouling. If the feed solution is extremely fouling, tubular systems are still used. Recently, however, spiral-wound modules with improved resistance to fouling have been developed these modules are increasingly displacing the more expensive tubular systems. This is particularly the case with clean feed solutions, for example, in the ultrafiltration of boiler feed water or municipal water to make ultrapure water for the electronics industry. Capillary systems are also used in some ultrafiltration applications. 

Spiral-wound modules are much more commonly used in low-pressure or vacuum gas separation applications, such as the production of oxygen-enriched air or the separation of organic vapors from air. In these applications, 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 rubbery polymers used to make them both work against hollow fine fiber modules for such applications. 

Pervaporation operates under constraints similar to those for low-pressure gas separation. Pressure drops on the permeate side of the membrane must be small, and many pervaporation membrane materials are rubbery, so both spiral-wound modules and plate-and-frame systems are in use. Plate-and-frame systems are competitive in this application despite their high cost, primarily because they can be operated at high temperatures with relatively aggressive feed solutions, conditions under which spiral-wound modules might fail. 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

Currently, approximately one billion gal/day of water are desalted by reverse osmosis. Half of this capacity is installed in the United States, Europe, and Japan, principally to produce ultrapure industrial water. The remainder is installed in the Middle East and other desert regions to produce municipal drinking water from brackish groundwater or seawater. In recent years, the interfacial composite membrane has displaced the anisotropic cellulose acetate membrane in most applications. Interfacial composite membranes are supplied in spiral-wound module form the market share of hollow fiber membranes is now less than... 

The investment in time and equipment to develop a new membrane material in a high-performance hollow fine fiber or capillary form is far larger than that required to develop flat-sheet membranes, and many materials cannot be formed into fiber modules at all. For this reason, flat-sheet membranes, formed into spiral-wound modules, are used in many niche applications which cannot support the development costs associated with fiber modules. Spiral-wound modules are also competitive in the natural-gas processing area, where their general robustness is an asset. 

Hollow fibers are generally used for submerged applications, although flat sheets, and more recently, spiral wound modules have come into use in submerged systems. In general, pressurized and submerged systems compare favorably, with the submerged... 

Typical membranes in NF are thin film composite membranes and their surface layers are mostly proprietary and not very easily identified by the user. Some industrial use of sulfonated polyethersulfone membranes has also been seen (see Section 35.6.2. Linpac recycle paper mill in Cowpens, South Carolina). Tubular and spiral wound modules are the dominating module type in NF and RO. Both membrane processes are used in industrial-scale applications in the pulp and paper industry (Table 35.1). When the membranes are installed in spiral wound modules, water pretreatment is often important to prevent plugging of the modules and fouling of the membranes. 

Filters are available in several constructions, effective filtration areas, and configurations. Depending on the individual process, the filter construction and setup will be chosen to fit its purpose best. Most commonly used for RO filters are tubular devices, so-called spiral wound modules due to the spiral configuration of the membrane within the support construction of such device. UF systems can be found as a spiral wound module, a hollow fiber, or a cassette device. The choice of the individual construction depends on the requirements and purposes towards the UF device. Similar to the different membrane materials, UF device construction has to be evaluated in the specific applications to reach an optimal functioning of the unit. Microfilters and depth filters can be lenticular modules or sheets but are mainly cylindrical filter elements of various sizes and filtration areas, from very small scale of 300 cm to large scale devices of 36 m. A 10-inch high cylindrical filter element can be seen in Fig. 6. 

Treatment of contaminated wastewater by pervaporation is superficially attractive, however, only a few commercial installations have been built. Phenol, which azeotropes with water at around 10wt%, is typically recovered by extraction in large plants, where measures to eliminate extractant loss can be economically applied. Pervaporation is an attractive and cost competitive alternative for small plants. Organophilic membranes in spiral wound modules are used in these applications. 

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