Chlorine composite membrane

Considerable research and development effort is being placed on a chlorine-resistant membrane that wiU maintain permeabUity and selectivity over considerable time periods (years). This polymer activity is not limited to hoUow fibers, but the thick assymetric skin of hoUow-fiber constmction might offer an advantage in resolving the end use need as opposed to the ultrathin dat-sheet composite membranes. 

Thin film composite (TFC) is an ultrathin barrier membrane on polysulfone support layer, of good chemical stability. It has a wide operating pH range of 2.0 to 12.0 at 0 to 40 °C, but cannot tolerate chlorine. TFC membranes are better at rejecting silica than CA membranes. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

By contrast, membranes U-1, A-2 and X-2 are all chlorine sensitive, each responding in a unique manner. U-1 is a thin film composite membrane, the active layer consisting of cross-linked poly(ether/urea) polymer. A-2 is a homogeneous aromatic polyamide containing certain polyelectrolyte groups. X-2 is a thin film composite membrane of proprietary composition. 

NS-300 Membrane. The NS-300 membrane evolved from an effort at North Star to form an interfacial poly(piperazine Isophthala-mide) membrane. Credali and coworkers had demonstrated chlorine-resistant poly(piperazineamide) membranes in the asymmetric form (20). The NS-lOO, NS-200, and PA-300 membranes were all readily attacked by low levels of chlorine in reverse osmosis feedwaters. In the pursuit of a chlorine-resistant, nonbiodegra-dable thin-fiim-composite membrane, our efforts to develop interfaclally formed piperazine isophthalamide and terephthalamide membranes were partially successful in that membranes were made with salt rejections as high as 98 percent in seawater tests. 

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. 

This thin-film-composite membrane has been found to have appreciable resistance to degradation by chlorine in the feed-water. Figure 2 illustrates the effect of chlorine in tap water at different pH values. Chlorine (100 ppm) was added to the tap water in the form of sodium hypochlorite (two equivalents of hypochlorite ion per stated equivalent of chlorine). Membrane exposure to chlorine was by the so-called "static" method, in which membrane specimens were immersed in the aqueous media inside closed, dark glass jars for known periods. Specimens were then removed and tested in a reverse osmosis loop under seawater test conditions. At alkaline pH values, the FT-30 membrane showed effects of chlorine attack within four to five days. In acidic solutions (pH 1 and 5), chlorine attack was far slower. Only a one to two percent decline in salt rejection was noted, for example, after 20 days exposure to 100 ppm chlorine in water at pH 5. The FT-30 tests at pH 1 were necessarily terminated after the fourth day of exposure because the microporous polysul-fone substrate had itself become totally embrittled by chlorine attack. 

In summary, the FT-30 membrane is a significant improvement in the art of thin-film-composite membranes, offering major improvements in flux, pH resistance, and chlorine resistance. Salt rejections consistent with single-pass production of potable water from seawater can be obtained and held under a wide variety of operating conditions (ph, temperature, pressure, and brine concentration). This membrane comes close to being the ideal membrane for seawater desalination in terms of productivity, chemical stability, and nonbiodegradability. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

Two different RO membrane types were evaluated in this study. The first was a standard cellulose acetate based asymmetric membrane. The second type, a proprietary cross-linked polyamine thin-film composite membrane supported on polysulfone backing, was selected to represent potentially improved (especially for organic rejection) membranes. Manufacturer specifications for these membranes are provided in Table III. Important considerations in the selection of both membranes were commercial availability, high rejection (sodium chloride), and purported tolerance for levels of chlorine typically found in drinking water supplies. Other membrane types having excellent potential for organic recovery were not evaluated either because they were not commercially... 

Results of this study confirm the expected improved recoveries of trace organics with membranes more selective and more highly cross-linked than the classical cellulose acetate membrane. Improved recoveries were predicted from literature data reported for similar membrane types. In light of these results, cellulose acetate should no longer be considered for applications such as these. Further improvements in recovery can be expected as developmental membranes with more highly selective barriers are brought into commercial use. Each new membrane type considered for use on disinfected waters should be evaluated for sensitivity to common disinfectants (oxidants). Both decreased selectivity and potentially troublesome chemical breakdown products should be considerations under these conditions. Although the cellulose acetate and FT-30 composite membranes did not prove to be particularly sensitive to chlorine, many commercially available... 

In pursuit of a chlorine-resistant, non-biodegradable thin-film-composite membrane, Cadotte et al. 97 )03,104 fabricated interfacially the poly(piperazineamide) membrane (NS-300). The interfacially formed piperazine isophthalamide and terephthalamide membranes exhibited high salt rejection (98 %) in sea water tests but their flux was low (Table 8). The replacing of the isophthaloyl chloride with its triacyl chloride analog, trimesoyl chloride improved vastly the flux of the membrane but its seawater salt rejection was low — in the range of 60 70 % (55). The trimesoyl... 

Kawaguchi et al.105) in Teijin Ltd. prepared a similar polyamide composite membrane from piperazine, trimesoyl chloride, and isophthaloyl chloride on a polysulfone support. The membrane exhibited high chlorine-resistance and excellent pressure-resistance. When used for reverse osmosis of an aqueous solution of 0.5% NaCl and NaOCl (available Cl 4 5 ppm) at pH 6.5 7.0, 25 °C, and 42,5 kg/cm2, the water permeation was 1400 and 13301/m2 - day and desalination was 93.4% and 95.7% after 2 and 100 hr, respectively. 

Cellulose acetate was the first high-performance reverse osmosis membrane material discovered. The flux and rejection of cellulose acetate membranes have now been surpassed by interfacial composite membranes. However, cellulose acetate membranes still maintain a small fraction of the market because they are easy to make, mechanically tough, and resistant to degradation by chlorine and other oxidants, a problem with interfacial composite membranes. Cellulose acetate membranes can tolerate up to 1 ppm chlorine, so chlorination can be used to sterilize the feed water, a major advantage with feed streams having significant bacterial loading. 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

For a few years after the development of the first interfacial composite membranes, it was believed that the amine portion of the reaction chemistry had to be polymeric to obtain good membranes. This is not the case, and the monomeric amines, piperazine and phenylenediamine, have been used to form membranes with very good properties. Interfacial composite membranes based on urea or amide bonds are subject to degradation by chlorine attack, but the rate of degradation of the membrane is slowed significantly if tertiary aromatic amines are used and the membranes are highly crosslinked. Chemistries based on all-aromatic or piperazine structures are moderately chlorine tolerant and can withstand very low level exposure to chlorine for prolonged periods or exposure to ppm levels... 

Sterilization of a membrane system is also required to control bacterial growth. For cellulose acetate membranes, chlorination of the feed water is sufficient to control bacteria. Feed water to polyamide or interfacial composite membranes need not be sterile, because these membranes are usually fairly resistant to biological attack. Periodic shock disinfection using formaldehyde, peroxide or peracetic acid solutions as part of a regular cleaning schedule is usually enough to prevent biofouling. 

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

The key short-term technical issue is the limited chlorine resistance of interfacial composite membranes. A number of incremental steps made over the past 10-15 years have improved resistance, but current chlorine-resistant interfacial composites do not have the rejection and flux of the best conventional membranes. 

In addition to the monopolar membrane described above a large number of special property membranes are used in various applications such as low-fouling anion-exchange membranes used in certain wastewater treatment applications or composite membranes with a thin layer of weakly dissociated carboxylic acid groups on the surface used in the chlorine-alkaline production, and bipolar membranes composed of a laminate of an anion- and a cation-exchange layer used in the production of protons and hydroxide ions to convert a salt in the corresponding acids and bases. The preparation techniques are described in detail in numerous publications [13-15]. 

Since the late 1970 s, researchers in the US, Japan, Korea, and other locations have been making an effort to develop chlorine-tolerant RO membranes that exhibit high flux and high rejection. Most work, such as that by Riley and Ridgway et.al., focuses on modifications in the preparation of polyamide composite membranes (see Chapter 4.2.2).11 Other work by Freeman (University of Texas at Austin) and others involves the development of chlorine-tolerant membrane materials other than polyamide. To date, no chlorine-resistant polyamide composite membranes are commercially available for large-scale application. 

Polyamide, composite membranes are very sensitive to free chlorine (recall from Chapter 4.2.1 that cellulose acetate membranes can tolerate up to 1 ppm free chlorine continuously). Degradation of the polyamide composite membrane occurs almost immediately upon exposure and can result in significant reduction in rejection after 200 and 1,000-ppm hours of exposure to free chlorine (in other words after 200-1,000 hours exposure to 1 ppm free chlorine). The rate of degradation depends on two important factors 1) degradation is more rapid at high pH than at neutral or low pH, and 2) the presence of transition metals such as iron, will catalyze the oxidation of the membrane. 

Chloramines also pose a risk to polyamide, composite membranes (see Chapter 8.2.1.1). Chloramines are virtually always in equilibrium with free chlorine. Although the tolerance of the FilmTec FT30 membrane to chloramines is 300,000 ppm-hrs, FilmTec still recommends that influent water with chloramines be dechlorinated prior... 

The use of chlorine dioxide is not recommended for use with polyamide, composite membranes.4 This is because free chlorine is always present with chlorine dioxide that is generated on site from chlorine and sodium chlorate (see Chapter 8.2.1.1). 

Initially, polyamide composite membranes that have been degraded due to chlorine attack will exhibit a loss in flux.4 This drop in flux is followed by an increase in flux and salt passage. 

The sum of chlorine gas, sodium hypochlorite, calcium hypochlorite, hydochlorous acid, and hypochlorite ion is known as the free or free available chlorine. Most polyamide composite membranes have little tolerance for free chlorine they can tolerate about 200 - 1,000 ppm-hrs of exposure (e.g., 200 hours at 1 ppm of free chlorine) before rejection drops to unacceptable levels. While the pretreatment to RO should have a free chlorine residual of about 0.5 to lppm, the influent to the RO must be dechlorinated to bring the free chlorine concentration down to less than 0.02 ppm. 

In theory, the tolerance of polyamide composite membranes to chloramines is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

Although chloramines are generally not recommended by membrane manufacturers for use with polyamide composite membranes, there is some anecdotal support for the use of chloramines if the ammonia is naturally occurring in the water to be treated.8 In such cases, there usually is an excess of ammonia. Difficulties arise when ammonia is added to chlorine to make the chloramines. These systems tend to have more free chlorine present in equilibrium with the chloramines (see Chapter 7.11 for more discussion on this topic). 

Generally, the salt rejections observed for these membranes in seawater reverse osmosis tests did not exceed 80 percent. This process was applied at Albany International to form composite membranes on hollow polysulfone fibers (25). Salt rejections on the hollow fiber membranes were above 98 percent at an average flux of about 1.5 gfd (2.5 L/sq m/hr) in a 12 000-hour test using 30 000 ppm seawater at 1000 psi. In other 5000-hour tests using 3500 ppm brackish water at 400 psi with addition of 100 ppm chlorine at pH 8 flux and salt rejection remained constant at 1 gfd and 98 percent respectively (Figures 8 and 12 in Reference 25 a). 

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