Spiral wound membranes

Radioactive waste management plant Selective actinide extraction Supported liquid membrane Spiral-wound reverse osmosis Solvent extraction Tri-n-butyl phosphate Total dissolved solids Tri-n-octylamine... 

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). 

Table 2.10 Typical specifications of RO and NF membrane spiral wound elements ...

The quality of reclaimed water from secondary treated sewage is comparable to the potable water supply from the data in Table 3.12. Although the type ofRO membranes is not revealed, most likely it is low-fouhng TFC membranes (spiral wound, 20 cm X 100 cm nominal element) with neutral or hydrophilic surface. These membranes are less prone to fouling due to their lower affinity to adsorption of dissolved organics and bacterial growth. The reclaimed water proved to be a reliable source of feed water for the UPW plant. 

The membranes currently used for seawater desalination are aromatic polyamide membrane (hollow fiber, Du Pont), cellulose acetate membranes (hollow fiber, Toyobo), thin-flim composite membranes (spiral-wound, UOP). thin-film composite membranes (spiral-wound. Dow), and thln-fllm composite membranes (spiral-wound. Toray). 

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). 

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. 

Fig. 22. Multileaf spiral-wound module, used to avoid excessive pressure drops on the permeate side of the membrane. Large, 30-cm diameter modules may have as many as 30 membrane envelopes, each with a membrane area of about 2 m. ...

In reverse osmosis, most modules are of the hollow-fine fiber or spiral-wound design plate-and-frame and tubular modules are limited to a few appHcations in which membrane fouling is particularly severe, for example, food appHcations or processing of heavily contaminated industrial wastewater. 

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. 

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

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). 

Four configurations for membranes are plate, hoUow fine fiber, spiral wound, and tubular (32). With a variety of shapes, sizes, and materials many options exist for meeting the various needs in the dairy industry. 

Solids nd Colloids. Suspended soHds can accumulate at the membrane surface, creating an additional resistance to flow through the membrane as well as a possible feed channel, such as that for a spiral-wound module plugging and subsequently a decrease in flux. Prevention of this type of fouling lies in the removal of the suspended soHds, which can be accompHshed using filters and screens prior to arrival at the RO unit. 

Spiral Wound. A spiral-wound cartridge has two flat membrane sheets (skin side out) separated by a flexible, porous permeate drainage material. The membrane sandwich is adhesively sealed on three sides. The fourth side of one or more sandwiches is separately sealed to a porous or perforated permeate withdrawal tube. An open-mesh spacer is placed on top of the membrane, and both the mesh and the membrane are wrapped spirally around the tube (Fig. 16). 

Spiral-wound cartridges are inserted ia series into cylindrical pressure vessels. Feed flows parallel to the membrane surfaces ia the channel defined by the mesh spacer which acts as a turbulence promoter. Permeate flows into the center permeate-withdrawal tube which is sealed through the housing end caps. 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

The earhest reverse osmosis and ultrafiltration units were based on flat membrane sheets ia arrangements similar to that of a plate and frame filter press. Siace then, mote efficient membrane configurations, ie, tubular, spiral wound, and hoUow fiber, have emerged (96—98). 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

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. 

Basically, the spiral-wound configuration consists of a jelly roll-like arrangement of feed transport material, permeate transport material and membrane material. At the heart of the wall is a perforated permeate collector tube. Several rolls are usually placed end to end in a long pressure vessel. 

Common types of membrane materials used are listed in Table 3. This gets us into the concept of geometry. There are three types of modules generally used, namely Tubular, Spiral wound, and Hollow fiber. A comparison of the various geometries is given in Table 4. 

---