Membrane processes spiral membranes

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. 

Another factor is the ease with which various membrane materials can be fabricated into a particular module design.1618 Almost all membranes can be formed into plate-and-frame, spiral-wound and tubular modules, but many membrane materials cannot be fabricated into hollow fine fibres or capillary fibres. Finally, the suitability of the module design for high-pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can be important factors.4-11 The types of module generally used in some of the major membrane processes are listed in Table 16.2. 

Membrane processes. Conventional filtration processes can separate particles down to a size of around 10 xm. If smaller particles need to be separated, a porous polymer membrane can be used. Microfiltration retains particles down to a size of around 0.05 xm. A pressure difference across the membrane of up to 4 bar is used. The two most commonly used arrangements are spiral wound and hollow fiber, as discussed in Chapter 8. 

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. 

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. 

Figure 5.28 Simplified flow schemes of (a) a conventional and (b) Mobil Oil s membrane solvent dewaxing processes. Refrigeration economizers are not shown. The first 3 million gallon/day commercial unit was installed at Mobil s Beaumont refinery in 1998. Polyimide membranes in spiral-wound modules were used [41-43]...

HERO—High-Efficiency RO Process Boosts Recovery Ratios, Cuts Cleaning Frequency," GE Water Process Technologies, www. gewater.com/products/equipment/spiral membrane/HERO.jsp... 

FIG. 22-53 Spiral -wound module used in many membrane processes. Permeate collection material is wound on a perforated permeate pipe. A membrane sandwich is constructed over the permeate carrier using glue seams as seals. Membrane sandwiches are separated by feed-channel spacers, through which the feed stream is passed. Courtesy Koch Membrane Systems. )... 

Membrane processes are classified according to the nature of the membrane and the driving force employed. A number of different types of polymeric/inorganic membranes have been developed in recent times in basically four types of modular assemblies, namely, plate and frame, spiral, tubular, and hollow fine fiber. Table 29.4 gives the application of various membrane processes for the separation of different types of contaminants encountered in effluents. 

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. 

Membrane processes are used to filter liquids. Instead of conventional filter materials (e.g. filter cloth, filter candles,) microporous membranes are employed with molecular size pores. First the industry had to learn how to manufacture membranes with controlled pore sizes. To optimise the filtration capacities specific filter structures had to be designed in which the liquid followed well defined flow patterns on one side of the membrane. Many different systems were developed for the varied applications, all having their advantages and also disadvantages, i.e. plate modules, tubular modules, spiral wound membranes, etc. Research and development in this field is far from being exhausted. Today membrane systems are available which are sufficiently resistant to chemical, mechanical and thermal stress. They are produced from plastic... 

Abolmaali, B. Yassine, I. Capone, P. Water recovery from an aluminum can manufacturing process using spiral wound membrane elements. In Membrane Technologies for Industrial and Municipal Wastewater Treatment and Reuse Water Environment Federation Alexandria, VA, 2000 51-56. 

Because a blocatalyst-blndlng membrane works as a specific, heterogeneous catalyst that can be used repeatedly. It finds many applications In chemical processes. Several types of reactors have been designed to Incorporate blocatalyst-blndlng membranes. A spiral module reactor has an advantage that pressure-drop Is extremely retarded (22)- The chemical engineering of membrane bioreactors has been reviewed elsewhere (30,31). 

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. 

Commercial applications of membrane processes have been achieved by using four types of modules namely spiral wound, hollow fibers, tubular, and plate-and-frame modules. In recent years, the number of membrane manufacturers has increased from a few to several dozens in many countries around the world. Partial list of membrane manufacturers and suppliers is provided by Rautenbach and Albrecht (1989), and Gutman (1987). 

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