Spiral wound elements

FT-30 Membrane. FT-30 is a new thin-film-composite membrane discovered and developed by FilmTec. Initial data on FT-30 membranes were presented elsewhere (23). It was recently introduced in the form of spiral-wound elements 12 inches long and 2 to 4 inches in diameter (24). The barrier layer of FT-30 is of proprietary composition and cannot be revealed at this time pending resolution of patentability matters. The membrane shares some of the properties of the previously described "NS series of membranes, exhibiting high flux, excellent salt rejection, and nonbiodegradability. However, the response of the FT-30 membrane differs significantly from other noncellulosic thin-film-composite membranes in regard to various feedwater conditions such as pH, temperature, and the effect of chlorine. 

Spiral-wound elements, as shown in Figure 2, consist primarily of one or more membrane "leaves, each leaf containing two membrane layers separated by a rigid, porous, fluid-conductive material known as the "permeate channel spacer." The permeate channel spacer facilitates the flow of the "permeate", an end product of the separation. Another channel spacer known as the "high pressure channel spacer" separates one membrane leaf from another and facilitates the flow of the high pressure stream through the element. The membrane leaves are wound around a perforated hollow tube, known as the "permeate tube", through which the permeate is removed. The membrane leaves are sealed with an adhesive on three sides to separate the feed gas from the permeate gas, while the fourth side is open to the permeate tube. 

Figure 3. Separex spiral-wound element assembly.

SEPAREX delivered a hydrogen recovery system, utilizing 4-in. diameter spiral-wound elements, in early 1982 The system will recover hydrogen from the off-gas of a unit utilizing UOP s ButamerR process in a LPG processing complex. 

The SEPAREX system will recover over 90% of the hydrogen at a purity of 96+% for recycle, while increasing the heating value of the fuel gas from -550 BTU/SCF to -950 BTU/SCF. The projected flow rates and gas purities for the membrane separation are shown in Table II. Under the bone-dry feed conditions the cellulose acetate membrane is not affected by HCl. Special materials of construction and adhesives have been used in the fabrication of the spiral-wound elements to ensure their resistance to HCl in the gas streams. 

MF (cross-flow filters, 0.2 pm) and RO (high salt rejection spirally wound elements)... 

Marinas, B.J. and Urama, R.I., Modeling concentration-polarization in reverse osmosis spiral wound elements, J. Environ. Eng., 122(4), 292, 1996. 

Properties of FT-30. The properties of FT-30 membranes have been reviewed in several publications. Therefore, only the salient features that relate to the chemistry of the barrier layer will be considered here. Reverse osmosis performance of FT-30 under seawater and brackish water test conditions was described by Cadotte et al (48) and by Larson et al (51). In commercially produced spiral-wound elements the FT-30 membrane typically gives 99.0 to 99.2 percent salt rejection at 24 gfd (40 L/sq m/hr) flux in seawater reverse osmosis tests with 3.5 percent synthetic seawater at 800 psi (5516 kPascaJJand 25°C. 

A recommended pH range for FT-30 spiral-wound elements was set at 3 to 11 for feedwaters at ambient temperatures. The membrane will tolerate a much wider range of pH for limited periods, and strong mineral acids such as phosphoric or nitric acids can be used for cleaning. Also, dilute solutions of strong bases such as sodium hydroxide or trisodium phosphate can be used with anionic surfactants for cleaning at high pH. 

Spiral wound elements have been used by a number of companies in the water purification area since 1968. Osmonics has been unique in that we have used spirals on a very large number of non-water purification applications, including oil concentration, latex concentration and of course, the concentration and fractionation of cheese whey. In fact, our installed capacity on cheese whey is approximately 324,000 sq. ft. (29,160 m2) of membrane or 20 miles (32 km) of membrane as it comes off of our machinery 3 ft. (1 meter) wide. Or, if you prefer the installed capacity is capable of handling 800,000 lbs (360,000 kg) of cheese whey per hour. 

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. 

FIGURE 20,1-5 Spiral wound elements and assembly. The use of mullilenf elemenl avoids long pathways Ter permeeal throngh the porons backing material to minimize pressure buildup in the permeant channel. 

The abovedescribed tests are conducted at a low recovery rate to minimize the effects of concentration polarization which is described below. For example, membrane tests above are conducted at less than 1% recovery and tests with spiral wound elements are conducted at recoveries from 5 to 10%. 

The spiral wound membrane packaging configuration is shown in Figure 4.8. Basically, the spiral wound element consists of two sheets of membrane separated by a grooved, polymer reinforced fabric material. This fabric both supports the membrane against the operating pressure and provides a flow path... 

The advantages of the spiral wound elements are the high packing density and high flux which makes it one of the most cost effective elements. The disadvantage of the element is that a moderate amount of pretreatment is required for some feedwaters to prevent fouling of the mesh brine spacers. 

Figure 4.9 Spiral wound element pressure vessel assembly.

Manufacturers of hollow fine fiber elements usually require that the pretreated feedwater have an SDI of 3.0 or below In order for the element warranty to be effective. One manufacturer of spiral wound element requires that the pretreated feedwater have a turbidity of less than 1.0 turbidity unit to maintain the element warranty. As a general rule, if hollow fine fiber elements are to be used, the pretreated feedwater should have an SDI of 3.0 or less and, if spiral wound elements are to be used, an SDI of 5.0 or less. However, spiral wound elements have been used to recover municipal wastes with an SDI in excess of 5.0 after pretreatment. 

The spiral wound element requires less pretreatment than the hollow fine fiber element or, stated in another manner, it is less susceptible to fouling. For instance, the hollow fine fiber has been tested and found to foul excessively on municipal wastewater reclamation applications while the spiral wound element has been operated successfully. The spiral wound element must be cleaned with chemical solutions, but there are no mechanical means available to clean this element. 

The hollow fine fiber element, with the great number of close packed fibers, is an effective filter in itself. Consequently, it is the most easily fouled membrane configuration and requires the most pretreatment. The hollow fine fiber element can be cleaned with chemical cleaning solutions, but it is not amenable to mechanical cleaning. It is also more difficult to clean than the spiral wound element. 

Once the pretreatment study had been completed, it will be possible to decide on the type of elements to be used in the reverse osmosis unit. If the SDI of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine fiber elements can be used. The choice will depend on economics (element price) and desalination characteristics (flux and rejection). If the pretreated feed SDI is more than 3.0, then the spiral wound element should be used. When the decision as to element type is made, then it is appropriate to forward a copy of the pretreated feed water analysis to reverse osmosis element manufacturers to obtain a prediction of product water quality, recommended type of element, total number of elements required, possible problems with sparingly soluble compounds in the feedwater, allowable recovery, and price and delivery. 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

Figure 6 Exploded view of spiral wound element.

C. Bartels, M. Hirose, S. Rybar, R. Franks, Optimum RO system design with high area spiral wound elements. In EDS/ EuroMed Conference, Dead Sea, Jordan, April 2008. 

ES10 operating conditions (2-inch spiral-wound element) ... 

Figure 2.18 Multiple spiral wound elements connected in series in a pressure vessel. Permeate tubes of each element are connected by module inter-connectors. Up to seven elements are connected for RO applications, and two to four elements in the case of UF and MF applications. Source Cheryan, Copyright 1998 from UltrafUtration/Taylor Francis Group, LLC.

Each pressure vessel contains six polyamide TFC seawater (Dow/Fihn-Tec 30) spiral-wound elements, 20 cm diameter x 100 cm long, producing 1000 re /d. The feed water TDS is about 38,000 mg/1. Since 45% is recovered as product water, the TDS of the RO reject is 69,000 mg/1, and the osmotic pressure of the reject brine stream is nearly 54 bar. 

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