Reverse osmosis spiral-wound module

Spira.1- Wound Modules. Spiral-wound modules were used originally for artificial kidneys, but were fuUy developed for reverse osmosis systems. This work, carried out by UOP under sponsorship of the Office of Saline Water (later the Office of Water Research and Technology) resulted in a number of spiral-wound designs (63—65). The design shown in Figure 21 is the simplest and most common, and consists of a membrane envelope wound around a perforated central coUection tube. The wound module is placed inside a tubular pressure vessel, and feed gas is circulated axiaUy down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals toward the center and exits through the coUection tube. 

Hollow-fiber designs are being displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. Also, thin-film interfacial composite membranes, the best reverse osmosis membranes available, have not been fabricated in the form of hoUow-fine fibers. 

Reverse Osmosis. This was the first membrane-based separation process to be commercialized on a significant scale. The breakthrough discovery that made reverse osmosis (qv) possible was the development of the Loeb-Sourirajan asymmetric cellulose acetate membrane. This membrane made desalination by reverse osmosis practical within a few years commercial plants were installed. The total worldwide market for reverse osmosis membrane modules is about 200 million /yr, spHt approximately between 25% hoUow-ftber and 75% spiral-wound modules. The general trend of the industry is toward spiral-wound modules for this appHcation, and the market share of the hoUow-ftber products is gradually falling (72). 

Reverse osmosis seawater Both hollow-fibres and spiral-wound modules... 

Reverse osmosis industrial Spiral-wound modules used almost exclusively fine fibres too... 

Configurations used include tubes, plate-and-frame arrangements and spiral wound modules. Spiral wound modules should be treated to remove particles down to 20 to 50. im, while hollow fiber modules require particles down to 5 im to be removed. If necessary, pH should be adjusted to avoid extremes of pH. Also, oxidizing agents such as free chlorine must be removed. Because of these restrictions, reverse osmosis is only useful if the wastewater to be treated is free of heavy contamination. The concentrated waste material produced by membrane processes should be recycled if possible but might require further treatment or disposal. 

Data of WRPC. The Water Reuse Promotion Center(WRPC) in JAPAN has been engaged in development of sea water desalination by reverse osmosis since 197. At IDEA meeting at Mexico city 1976,the first redults were reported with two types of modules, du Dont hollow fine fiber module B-10 and UOP s cellulose triacetate ultrathin spiral wound module,tested at their laboratory at Chigasaki beach. Then the WRPC has adopted two types of modules made in Japan, Toray new type of spiral wound module made from cellulose acetate and Toyobo s cellulose triacetate hollow fine fiber module. 

In current practice, turbulence promoters most often take the form of a net or screen material which also serves as a feed channel spacer between two membranes. For example, the familiar spiral wound modules (Figures 29) used extensively in reverse osmosis and to a lesser extent in ultrafiltration use a plastic screen material as the feed channel spacer. This is also used in some plate and frame systems (Figure 30). 

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...

Figure 3.44 Schematic of a spiral-wound module [115] installed in a multimodule pressure vessel. Typically four to six modules are installed in a single pressure vessel. Reprinted from Reverse Osmosis Technology, B.S. Parekh (ed.), Marcel Dekker, New York (1988), p. 81, by courtesy of Marcel Dekker, Inc.

Reverse osmosis seawater Spiral-wound modules. Only one hollow fiber producer remains... 

Y. Kamiyama, N. Yoshioka, K. Matsui and E. Nakagome, New Thin-film Composite Reverse Osmosis Membranes and Spiral Wound Modules, Desalination 51, 79... 

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... 

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

Table 5.5 Advances in spiral-wound module reverse osmosis performance...

Reynolds number reverse osmosis coefficient Schmidt number Sherwood number filament surface area of spacer specific surface area of spacer spiral wound module bubble rising velocity mean azimuthal velocity time... 

Flat-sheet membranes for reverse osmosis are usually used in spiral-wound modules. The membrane is folded over a porous spacer sheet, through which... 

Spiral-wound modules of the type used for reverse osmosis (see Fig. 26.19) are widely used for UF. They are not as prone to plugging as hollow-fiber units, since the entrance is a narrow slit about 1 mm wide, but prefiltration of the feed solution is recommended. The velocity in the feed channels corresponds to laminar flow, but the flow disturbances caused by the spacers make the pressure drop and the mass transfer greater than for true laminar flow. 

A variation of the basic plate-and-frame concept is the spiral-wound module, which is widely used today in reverse osmosis, ultrafiltration, and gas separation. Its basic design is illustrated in Figure 1.33 (c). The feed flow channel spacer, the membrane, and the porous membrane support are rolled up and inserted into an outer tubular pressure shell. The filtrate is collected in a tube in the center of the roll. 

Hollow fibers and spiral wound modules have been constructed for this application. A module 18 inches in diameter and 5 feet long can produce up to 1,500 GPD. Plants now exist in the Florida Keys, the Bahamas, and the Cayman Islands ranging in size from 10,000 to 60,000 GPD. They have the highest capacity per unit volume for evaporation plants now in existence. It is claimed that these plants are more cost effective than multiple-effect evaporators, but in the author s view, it is doubtful they can compete with reverse osmosis. 

The Permutit Liberator 1 reverse osmosis unit contains two spiral wound modules of 0.96 m each with Filmtec BW30 membranes, a crosslinked aromatic polyamide. These were selected based on low adsorption and high rejection characteristics. 

Kawada I., Inoue K., Kazuse Y., Ito H., Shintani T., Kamiyama Y. (1987), New thin-film composite low pressure reverse osmosis membranes and spiral wound modules. Desalination, 64, 387-401. 

Membrane Process for Recovery of Alkanesulfonates. Many attempts have been made over the years to reduce the wastewater load—which represents a loss of product—by a number of different methods. These include evaporation, extraction, reverse osmosis, and ultrafiltration. All of these processes have the disadvantage of high equipment cost and high energy requirements, and the space-time yield is low. The first breakthrough came with the development of new types of membrane with a definite separating efficiency and a large surface area, so-called spiral-wound modules. 

A reverse osmosis process developed by Mobil for this separation is illustrated in Fig. 7.4(b). Polyimide membranes formed into spiral-wound modules are used to separate up to 50% of the solvent from the dewaxed oil. The membranes have a flux of 10-20 gal/ft day at a pressure of 450 to 650 psi. The solvent filtrate bypasses the distillation step and is recycled directly to the incom-... 

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