Spiral wound membrane modules cleaning

Figure 33.4 Comparison between the permeation flux relative to that for an unfouled membrane (upper data and left ordinate) and the normalized arrival time shift factor (lower data and right ordinate) as a function of operating time for a sequence consisting of pure water feed (0-30 h), fouling with a 1.6-g/L aqueous solution of calcium-sulfate (30-100 h), and cleaning with a pure water feed (100-171 h) in a Koch 2521 spiral-wound membrane module operating at 0.7 MPa and 20°C.

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. 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

Figure 8.12 Block diagram and photograph of a contained in the horizontal pressure vessels, membrane fuel-gas conditioning unit (FGCU) The unit produces 0.5-1.0 MMscfd of clean gas. used for a field gas compressor engine (the unit Reproduced with permission from Ind. Eng. uses silicone rubber membranes in spiral-wound Chem. Res. 2008, 47(7), 2109-2121. Copyright modules). The membrane modules are 2008 American Chemical Society [17].

Spiral wound modules can be difficult to clean. There are dead spaces within the module where high velocity cannot scour the surface of the membrane, and cleaning solution does not mix well to remove debris. 

Membranes are typically made of cellulose acetate or aromatic polyamides because of their high permeability to water and low permeability to salts. They are normally produced in tubular or spiral-wound modules, which are then packed inside a reaction vessel. Their fouling can be minimized by either pretreating the influent streams or diluting them with the clean water produced. 

The two most common RO membrane configurations used in water treatment today are spiral-wound and hollow fiber. The spiral-wound elements can operate at a higher pressure and at a higher silt density index (SDI) than the hollow fiber type, and thus may require less pretreatment (and are more tolerant of pretreatment upsets). They also are easier to clean than the hollow fiber type. The main advantage of the hollow fiber configuration is that it has the highest amount of membrane area per unit volume, thus requiring less space. Since there is only one hollow fiber element per pressure vessel, it is easier to troubleshoot, and it is easier to replace membrane modules. 

Fortunately, hollow fibers may be cleaned by back-washing which tends to compensate for their propensity to foul. Manufacturers of tubes, plate and frame units, and spiral wound modules do not recommend back-washing due to problems with membrane delamination and glue line seal rupture. Because hollow fibers are self-supporting and hold up well under the compression force of a reverse transmembrane pressure drop, they can easily withstand back-wash pressures of 15 to 20 psi. However, the back-wash fluid should be filtered to remove any particles which would tend to lodge in the porous wall of the fiber. 

The membrane shapes described are usually incorporated into compact commercial modules and cartridges. The four more common types of modules are (1) plate-and-frame, (2) spiral-wound, (3) tubular, and (4) hollow-fiber. Table 9.2 is a comparison of the characteristics of these four types of modules. The packing density refers to the surface area per unit volume of module, for which the hollow-fiber modules are clearly superior. However, hollow-fiber modules are highly susceptible to fouling and very difficult to clean. The spiral-wound module is very popular for most applications because of its low cost and reasonable resistance to fouling. 

The commercial RO modules that have appeared in the literature since 1977 are summarized in Table 3.2 together with the typical operating conditions and performance. Some of them might no longer exist in the market. From the table, it is clear that the majority of high-salt-rejection RO membranes are spiral-wound aromatic PA-TFC membranes with a noted exception of Toyobo that is based on CTA hollow fiber. The reason might be easier cleaning of the spiral-wound module, on one hand, and chlorine resistance of CA, on the other. 

Flat sheet membranes in a plate-and-frame unit offer the greatest versatility but at the highest capital cost (P6). Membranes can easily be cleaned or replaced by disassembly of the unit. Spiral-wound modules provide relatively low costs per unit membrane area. These units are more prone to foul than tubular units but are more resistant to fouling than hollow-fiber units. Hollow-fiber modules are the least resistant to fouling when compared to the three other types. However, the hollow-fiber configuration has the highest ratio of membrane area per unit volume. 

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