Spiral wound membrane modules pressure vessel

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... 

Focusing on spiral wound membrane modules as the most common type of membrane modules used in industry today, an RO array or "skid" or "train" consists of a number of pressure vessels arranged in specific patterns. Figure 5.1 shows an array of 3 pressure vessels. The pressure vessels are arranged into 2 sets, with 2 pressure vessels in parallel followed by 1 single pressure vessel. The 2 sets of pressure vessels are in series. Each set of pressure vessels in parallel (even if there is only 1 vessel) is called a STAGE. 

Spiral wound membrane modules are well-known in water desalination and are better than plate frame and tubular modules due to their high water flux, lower salt permeability and lower operational cost. Several membrane elements are connected with each other in a spiral mode and wrapped around a centrally installed permeate tube (Fig. 4.5). The whole setup is placed inside a pressurized tubular vessel where the feed water passes through the membranes axially down the module. 

Recovery through individual membrane modules changes, based on the position of the module in the pressure vessel. Most spiral wound membrane modules operate with individual module recoveries ranging from 10% to 15%, with an average of 11% to achieve 50% recovery in a single, 6-module pressure vessel stage. The module at the feed end of the pressure vessel typically exhibits the lowest recovery of all modules in the pressure... 

The typical spiral-wound membrane, as shown in Figure 4.20, consists of four layers wrapped around a central collection pipe membrane, spacer (providing a permeate channel), membrane, and a new spacer (providing a feed channel). The feed-side spacer acts as a turbulence promotor, whereas on the permeate side the flow is directed toward the central pipe. The spiral-wound membrane will typically be a polymeric composite material, and is much used also for liquid separation. The packing density of this type of module will depend on the channel height, but is usually within the range of 300-1000 m /m [1]. Several modules may be assembled in one pressure vessel. 

The whole membrane module is composed of six racks, each of them consisting of eleven pressure vessels, operated in parallel, designed to house one spiral wound membrane each. The pressure vessels were originally equipped with cellulose acetate membranes specifically developed for PRO. During the first period, the plant operation has been optimized and the performance has been monitored, resulting in a power density lower than 0.5 W/m. Next, thin film composite membranes developed for PRO were installed in early 2011. So far the measured power density has reached nearly 1 W/m, which is a major improvement compared to the cellulose acetate membranes originally installed. Based on this preliminary experience, Statkraft plans to build a full-scale 25 MW osmotic power plant by 2015 [20]. 

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. 

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].

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. 

If the normalized salt rejection is low or the normalized permeate flow is high, the integrity of the membrane may be in question. The vacuum decay test is a direct test for the integrity of a spiral wound RO membrane module. The test is best used to identify leaks within the membrane modules rather than leaks due to chemical attack. The test requires the isolation of an individual membrane module or the entire pressure vessel. A vacuum is then pulled on the membrane(s) and the rate of decay in pressure is observed. A decay of greater than 100 millibar per minute is indicative of a leaky membrane. Refer to ASTM Standards D39235 and D69086 for a more detailed review of the technique. 

In the spiral-wound mounting, a porous hollow tube is spirally wrapped with a porous sheet for the feed flow, and a membrane sheet and a porous sheet for the product flow to give a spiral sandwich-type wrapping. The spiral module is encased in a pressure vessel, and the feed flow through the porous sheet is in an axial direction to the porous tube. As the feed flow passes through the porous sheet, a portion of the flow passes through the membrane into the porous sheet for the product. From there, the product flows spirally to the porous center tube. The retentate stream is discharged from the downstream end of the porous sheet for the feed flow. 

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. 

Seperation Unit. The elements of a separation process. Separation units of a separation process could be, for example, a gas centrifuge, a membrane module, a tray of distillation column or the evaporator of a multiple effect plant. It should be kept in mind, however, that in plate-and-frame modules as well as in modules of the spiral-wound-type usually every block or pressure-vessel contains more than one separation unit ... 

The module assembly is placed within a case that possesses manifolds to direct feed water along the membrane surface, withdraw product water from the collection tube, and remove the rejected water. A rectangular case might be used but a more compact unit is obtained by rolling the envelope and feed spacer around the permeate collection tube the feed spacer defines a channel for feed water flow between successive layers of the envelope. A cylindrical pressure vessel is used to hold the spiral wound module thus produced. 

The membrane modules, which are commonly used in organic vapor separation, are spiral-wound modules or the envelope-type GKSS GS modules. Gapillary or hollow-fiber modules are only used in small-scale laboratory applications. The spiral-wound module and the envelope module are based on flat-sheet membranes. Spiral-wound modules are compact and cheaper in comparison to installed membrane area, but there are limitations in mass transfer on both sides of the membrane. The packing density - the ratio of installed membrane area over pressure vessel housing volume - of a spiral-wound module varies from approx. 300 to 1000 m /m (Fig. 1.3). 

Figure 2.25 A typical RO skid showing a 4 2 two-stage membrane array, high-pressure pump, instruments and control panels. The high-pressure RO pump is a horizontal, multi-staged submersible type. Each pressure vessel contains six spiral-wound modules, 20 cm dia. x 100 cm long. Source USFilter.

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