Typical Membrane Modules

The choice of module configuration as well as the arrangement of the modules in a system are based on economic considerations with correct engineering parameters being employed to achieve this economy, which include the type of separation problem, ease of cleaning, ease of maintenance, ease of operation, compactness of the system, scale, and the possibility of membrane replacement (13). Next, we will discuss these typical membrane modules. 

Typical Membrane Modules 6.2.1. Plate-and-Frame Module 

A schematic drawing of a plate-and-frame module is given in Fig. 9. This type of module appeared in the earliest stage of industrial membrane applications. The structure is simple and the membrane replacement is easy. As illustrated, sets of two membranes are placed in a sandwich-like fashion with their feed sides facing each other. In each 

The spiral-wound module is in fact a plate-and-frame system wrapped around a central collection pipe, similar to a sandwich roll. The basic structure of this module is illustrated in Fig. 10. Membrane and permeate-side spacer material are then glued along three edges to build a membrane envelope. The feed-side spacer separating the top layer of the two flat membranes also acts as a turbulence promoter. The feed flows axial through the cylindrical module parallel along the central pipe and the permeate flows radially toward the central pipe. In order to make the membrane length shorter, several membrane envelopes are wound simultaneously. The spiral-wound module is featured by 

Usually, a number of spiral-wound modules are assembled in one pressure vessel and are connected in series via the central permeate tubes. Some industrial-scale spiral-wound modules are shown in Fig. 11. 

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A comprehensive presentation of all membrane types, modules and geometries is beyond the scope of this chapter, reference available membrane books for details [12,17, 55, 60, 71, 77,90]. The examples in Figure 16.2 are an illustration of a typical membrane module and installation. The most widespread FS membrane system is mounted as a spiral-wound (SW) unit. In the SW example the actual membrane module is shown together with how they are mounted inside a pressure vessel. A typical installation is shown where several pressure vessels are subsequently mounted in a stack. Pressurized HF units are typically operated as a crossflow system. In the example shown the HF modules are mounted vertically and arranged in a skid. Several variations of the theme can be found depending on the type of module and the manufacturer, where Figure 16.2 is not specific to a particular item. 

Pervaporation is used to separate water-organic and organic-organic mixtures that form azeotropes and may be difficult to separate by enhanced distillation. Typical membrane modules cost 30/ft of membrane surface area. 

Ultrafiltration uses a microporous polymer membrane, which allows water and molecules of less than some cut-off molecular weight to pass through, depending on the pore diameter, while retaining larger molecules. A typical membrane module may contain 30 ft of membrane surface area at a cost of from 8 to 20 /ft of surface area. 

Figure 10.23. Schematic drawing of a typical membrane module for helium recovery. (Reproduced from [324] with permission.)...

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). 

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Four to six spiral-wound membrane modules are normally connected in series inside a single pressure vessel (tube). A typical 8-in.-diameter tube containing six modules has 100-200 m2 of membrane area. An exploded view of a membrane tube containing two modules is shown in Figure 3.44 [115]. The end of each module is fitted with an anti-telescoping device (ATD) which is designed to... 

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...

Table 4.1 Representative values of the concentration polarization modulus calculated for a variety of liquid separation processes. For these calculations a boundary layer thickness of 20 im, typical of that in most spiral-wound membrane modules, is assumed...

In coupled transport and solvent dehydration by pervaporation, concentration polarization effects are generally modest and controllable, with a concentration polarization modulus of 1.5 or less. In reverse osmosis, the Peclet number of 0.3-0.5 was calculated on the basis of typical fluxes of current reverse osmosis membrane modules, which are 30- to 50-gal/ft2 day. Concentration polarization modulus values in this range are between 1.0 and 1.5. 

Figure 5.24 Flow schematic of a typical brackish water reverse osmosis plant. The plant contains seven pressure vessels each containing six membrane modules. The pressure vessels are in a Christmas tree array to maintain a high feed velocity through the modules...

Application Typical membrane material Selectivity (a) Average pressure-normalized flux [10-6 cm3(STP)/ cm2 s cmHg] Module design commonly used... 

Table 9.2 Typical silicone rubber membrane module pervaporation separation factors (VOC removal from water)...

Hollow fiber dialyzers typically contain 1 -2 m2 of membrane in the form of fibers 0.1-0.2 mm in diameter. A typical dialyzer module may contain several thousand fibers housed in a 2-in.-diameter tube, 1-2 ft long. Approximately... 

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

In Table 16.2 some of the typical characteristics of the various types of membranes and configurations are given. In MBR systems SW membrane modules are not used as the channels within the spiral are prone to clogging when the feed water has high suspended-solids concentrations. Tubular membrane systems are not common either as they tend to become very expensive due to the low area to volume ratio. Commercial MBR systems today are normally based on immersed FS configurations or H F/CT configurations. 

Plasmapheresis typically employs a membrane module of similar configuration as a high-flux hemodialyzer. Alternatively, a rotating membrane separation element is used in which the tendency of the blood cells to deposit on the membrane surface is counteracted with hydrodynamic lift forces created by the rotation. The membrane element and the associated plasmapheresis circuitry are shown in Fig. 49. Worldwide, about 6 million plasmapheresis procedures are performed annually using this system, making this one of the largest biomedical membrane applications after hemodialysis. 

Plate and frame RO modules are typically used for specialty, high suspended solids applications and are not generally found in water-purification facilities. These modules consist of flat sheets of membrane that are modularized into plates, typically two membranes placed back to back per plate. The plates are then stacked within a framework for support. There are patterned spacers materials that are used to keep the membranes from sticking to each other and providing open channels for the feed and product water to flow through. Figure 4.12 shows a typical plant-and-frame membrane module. 

Most tubular membrane modules are used for specialty microfiltration (MF) and ultrafiltration (UF) applications rather than RO due to the lower packing density of this type of module and because MF and UF typically treat higher-solids feed water (see Chapter 16.1). 

Automated manufacturing of the membrane modules has allowed for more membrane area per unit volume and for higher-quality modules. This is because automation allows for more precise glue line application on the membrane leaves. A typical industrial module that is 8-inches in diameter and 40-inches long can hold up to 440 ft2 of membrane area when automated manufacturing is employed (see Chapter 4.4.2.5). 

Low-differential-pressure membrane modules can be considered a subset of low-fouling membranes. These low-differential-pressure membrane modules typically have a thicker feed spacer. Instead of the standard 28-mil thick spacer, these low-differential-pressure membranes have 31- or 34-mil thick spacers. There is less resistance to flow through the feed channels, resulting in lower pressure drops through the membrane modules. Furthermore, the feed channels will not plug as quickly with suspended solids, foulants, or scale. Examples of low-differential-pressure membrane modules are the FilmTec BW30-400-34i (with a 34-mil feed spacer) and the Hydranautics CPA3-LD (with a 31-mil feed spacer). 

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