Seawater membranes test conditions

FIGURE 30 Performance of some commercial reverse osmosis membranes for (a) seawater desalination (test conditions 56 bar 25°C 3.5% NaCI feed) (b) low-pressure desalination (15 bar 25°C 1500 mg/liter NaCI feed) and (c) ultralow-pressure nanofiltration applications (7.5 bar, 25°C 500 mg/liter NaCI feed). 

Seawater membranes are used to treat high-salinity (35,000 to 50,000 ppm total dissolved solids (TDS)) feed waters. These membranes can operate at pressures up to 1,500 psi. Typical membrane test conditions are as follows ... 

Based on a simple economic analysis, it appears that, when seawater is used as the salt solution, a membrane with a water flux in PRO of about 1 x 10 2cm3/cm2-sec ( <200 gal/ft2-day) is required to make the process economically viable in today s economy, even if the installed membrane cost is as low as 100/m2. The highest flux we projected under these PRO conditions among the RO membranes tested was only somewhat greater than 1 x 10 cm3/cm -sec ( 2 gal/ft -day), for a power output of 1.6 watt/m2. 

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. 

As a result of prior field experience with the furan system, a qualifying test has been employed at Albany International Research Co. to measure the durability. Table III displays data of typical samples tested against synthetic seawater at 1000 psig maintained at a temperature of 50°C. Samples of cellulosic seawater membrane and polyamide membrane were found to fail in several hours of challenge by these conditions. 

As discussed in Chapter 4.2.2, DuPont introduced linear aromatic polyamide membranes in hollow fine fiber form as the B-9 (brackish water) and B-10 (seawater) Permeators. These Permeators were available in 4-, 8- and 10-inch diameter models. The 4-, 8-, and 10-inch B-9 Permeators were capable of producing 4,200, 16,000, and 25,000 gallon per day of permeate, respectively, at 75% recovery (standard test conditions 1,500 ppm NaCl at 400 psig and 25°C).28 Permeators ranged from about 47 inches to 53 inches in length. DuPont discontinued these modules in 2001. 

As with seawater membranes, there is no one uniform test condition for all brackish water membranes of the same type. Thus, a direct comparison between manufacturers requires a close look at the test conditions. 

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. 

The NS-100 membrane is capable of giving salt rejections in excess of 99% in tests on salt solutions simulating seawater (18 gfd, 3.5% synthetic seawater, 1,500 psi, 25°C). If the polyurea interfacial reaction step is omitted, and the polyethylenimine-coated polysulfone film is heat-cured as usual, a crosslinked polyethylenimine semipermeable barrier film is generated. This membrane gives 70% salt rejection and 55 gfd water flux under the same test conditions as above. Also, if the fully formed NS-100 membrane is dried at 75°C, which is too low a temperature to effectively crosslink the amine layer, the resulting film will exhibit a salt rejection of 96% or less. 

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. 

The most outstanding property of the NS-200 membrane was its extremely high salt rejection. In simulated seawater tests, salt rejection levels of 99.8 to 99.9% were rountinely observed. Typical performance in a 1,500 psi test on 3.5% synthetic seawater at 25°C was 20 gfd and 99.9% salt rejection. In a few instances, laboratory samples of NS-200 membranes with fluxes of 40 to 50 gfd under these same test conditions were obtained. 

When furfuryl alcohol was added as a comonomer to the THEIC, water fluxes were increased tenfold. In addition, the extremely high salt rejections characteristic of NS-200 were obtained, while the high organic rejections characteristic of the isocyanurate moiety were retained. A typical patent example of membrane fabrication uses a water solution of 1 2 2 1 weight percent THEIC fur-furyl alcohol sulfuric acid dodecyl sodium sulfate, deposited on microporous polysulfone and cured at 150°C for 15 minutes. This membrane, possessing a thin active layer 100 to 300 angstroms thick, showed 99.9% rejection and 12 gfd flux under seawater test conditions at 1,000 psi. 

Figure 5.16, adapted from Kurihara,79 80 shows a comparison of several types of commercial reverse osmosis membranes in terms of salt rejection and permeate flow rate under seawater test conditions (35,000 ppm, 800 psi, 25°C). This chart emphasizes the capability of PEC-1000 to provide complete single-stage seawater desalting. In a test at Toray s Ehime desalination test facility on 42,000 ppm seawater (equivalent to Red Sea salinity), PEC-1000 spiral elements operated at 35% recovery produced a permeate having an average salinity of only 220 ppm, well below WHO standards. Average salt rejection was 99.5%. 

Figure 16. Permeate flow rate per unit membrane area (gallons/day/fl ) and NaCl rejection of seawater membranes offered by GE (K>), FilmTec/Dow (x), Koch (o), Toray (n), and Nitto Denko/Hydranautics (Us). All values taken from the manufacturers web sites. Test conditions for all membranes were 32,000ppm NaCl feed concentration, 800 psi feed pressure, and 77 °F feed temperature. Permeate recovery varied slightly in the tests from 7-10% andfeed pH variedfrom 6.5-8. Note that a 32,800 ppm NaCl feed was used to obtain the Koch values normalization to 32,000 ppm NaCl increases permeability by 2%.

Test conditions are important to take note of as these are the conditions xmder which rated performance is based. Operating under different conditions will result in performance that differs from the rated performance. (Chapter 9 discusses the effect of varying operating conditions on the performance of RO membranes). Notice that there is not one uniform test condition to which all membrane manufacturers adhere. Therefore, because of the difference in pH and recovery xmder such membranes are tested, the rated performance of seawater membranes from different manufacturers cannot be directly compared. 

The first aim of this work was to study the influence of an unwashed membrane filter on the cadmium, lead, and copper concentrations of filtered seawater samples. It was also desirable to ascertain whether, after passage of a reasonable quantity of water, the filter itself could be assumed to be clean so that subsequent portions of filtrate would be uncontaminated. If this were the case, it should be possible to eliminate the cleaning procedure and its contamination risks. The second purpose of the work was to test the possibility of long-term storage of samples at their natural pH (about 8) at 4 °C, kept in low-density polyethylene containers which have been cleaned with acid and conditioned with seawater. 

The PEC-1000 membrane of Toray Industries, Inc., has been described by Kurihara et al (21). This membrane was characterized as a thin-film composite type made by an acid catalyzed polymerization on the surface. Membrane performance reported for seawater tests was 99.9 percent TDS rejection at fluxes of 5.0 to 7.4 gfd (8.3 to 12.3 L/sq m/hr) when tested with 3.5 percent synthetic seawater at 800 psi (5516 kPascals). The membrane was stable in 1500-hour tests in spiral-wrap elements and exhibited stability in a temperature range of 25 to 55°C and in a pH range from 1 to 13. High organic rejections were reported for the PEC-1000 membrane rejection of dimethylformamide from a 10 percent solution was 95 percent and similar tests with dimethylsulfoxide showed 96 percent rejection. The composition and conditions for preparation of PEC-1000 membrane is not disclosed in Reference 21. Apparently it is a dip-cast membrane related to compositions described by Kurihara, Watanaba and Inoue in Reference 18. 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

It is recommended that pilot testing be used in developing design criteria for the site-specific conditions and MF/UF product selection. Both pressure-type MF/UF systems, where the membranes are encased in pressiue vessels, and vacuum-type systems, where the membrane are immersed in tanks open to atmosphere and use fUtrate/permeate pumps to create the driving force, may be used for seawater pretreatment. 

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