FT-30 membrane

The predominant RO membranes used in water applications include cellulose polymers, thin film oomposites (TFCs) consisting of aromatic polyamides, and crosslinked polyetherurea. Cellulosic membranes are formed by immersion casting of 30 to 40 percent polymer lacquers on a web immersed in water. These lacquers include cellulose acetate, triacetate, and acetate-butyrate. TFCs are formed by interfacial polymerization that involves coating a microporous membrane substrate with an aqueous prepolymer solution and immersing in a water-immiscible solvent containing a reactant [Petersen, J. Memhr. Sol., 83, 81 (1993)]. The Dow FilmTec FT-30 membrane developed by Cadotte uses 1-3 diaminobenzene prepolymer crosslinked with 1-3 and 1-4 benzenedicarboxylic acid chlorides. These membranes have NaCl retention and water permeability claims. 

In 1977 the North Star membrane research group was spun off by Midwest Research Institute, forming FilmTec Corporation. Two new thin-film-composite reverse osmosis membranes have been under development at FilmTec Corporation since that time, the NS-300 and the FT-30 membranes. 

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. 

The FT-30 membrane was found to be resistant to swelling or salt rejection losses at high feedwater temperatures. In simulated seawater tests, the membrane had stabilized at about 99 percent salt rejection at temperatures of 40°C and higher. 

In trials at different feedwater concentrations, the FT-30 membrane showed single-pass seawater desalting capabilities at up to 6.0 percent synthetic seawater. Basically, any combination of pressure and brine concentration at room temperature that gave a membrane flux of 15 gfd also resulted in a 99 percent level of salt rejection. 

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 a related case, FT-30 membrane elements were placed on chlorinated seawater feed at OWRT s Wrightsville Beach Test Facility. Flux and salt rejection were stable for 2000 hours at 0.5 to 1.0 ppm chlorine exposure. Chlorine attack did become noticeable after 2000 hours, and salt rejection had dropped to 97 percent at 2500 hours while flux increased significantly. Long term laboratory trials at different chlorine levels led to the conclusion that the membrane will withstand 0.2 ppm chlorine in sodium chloride solutions at pH 7 for more than a year of continuous exposure. 

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. 

Figure 2. Exposure of FT-30 membranes to 100 ppm chlorine in water at different pH levels. Effect on salt refection in simulated. seawater reverse osmosis tests (0) pH 1 Cn pH 5 (O) pH 8 (A) pH 12.

Figure 3f. SEM photomicrograph of composite membranes surface view of the FT-30 membrane.

Reverse osmosis for concentrating trace organic contaminants in aqueous systems by using cellulose acetate and Film Tec FT-30 commercial membrane systems was evaluated for the recovery of 19 trace organics representing 10 chemical classes. Mass balance analysis required determination of solute rejection, adsorption within the system, and leachates. The rejections with the cellulose acetate membrane ranged from a negative value to 97%, whereas the FT-30 membrane exhibited 46-99% rejection. Adsorption was a major problem some model solutes showed up to 70% losses. These losses can be minimized by the mode of operation in the field. Leachables were not a major problem. 

Table IX. Mass Balance Data and Estimates of Solute Recovery (Test Run 618-51) FT-30 Membrane Concentration without Humics...

Figure 2.9 Flux and rejection data for a model seawater solution (3.5 % sodium chloride) in a good quality reverse osmosis membrane (FilmTec Corp. FT 30 membrane) as a function of pressure [10]. The salt flux, in accordance with Equation (2.44), is essentially constant and independent of pressure. The water flux, in accordance with Equation (2.43), increases with pressure, and, at zero flux, meets the pressure axis at the osmotic pressure of seawater 350 psi...

The chemistry and properties of some of the important interfacial composite membranes developed over the past 25 years are summarized in Table 5.1 [10,12,29,30], The chemistry of the FT-30 membrane, which has an all-aromatic structure based on the reaction of phenylene diamine and trimesoyl chloride, is widely used. This chemistry, first developed by Cadotte [9] and shown in Figure 5.9, is now used in modified form by all the major reverse osmosis membrane producers. 

Figure 5.9 Chemical structure of the FT-30 membrane developed by Cadotte using the interfacial reaction of phenylene diamine with trimesoyl chloride...

FT-30 membrane patented and assigned to FilmTec (now owned by Dow Chemical Company, Midland, MI). 

Figure 4.1 Flux and rejection data for a seawater FilmTec FT-30 membranes operating on 35,000 ppm (350 psi osmotic pressure) sodium chloride solution.2...

The PAs in the prefixes for the naming of the membranes stand for polyamide. Thus, the membranes referred to are polyamide membranes. PA membranes contain the amide group [NH2] it is for this component that they are called polyamide membranes. The formulas for the NS-100, NS-300, and the FT-30 membranes contain the amide group, thus, they are polyamide membranes. The NS-200 is not a polyamide membrane. 

Universal Oil Products (UOP) developed reverse osmosis equipment for demineralization of brackish and seawater using composite membranes with a polyamide as the functional coating. The UOP products carry a "TFC" registered trademark. Another good example of a thin-film composite membrane involving a thin film of polyamide as the functional coating is the FilmTec FT-30 membrane for RO (21). 

Reaction Mechanism. Electron micrographs of the face of FT-30 membrane show an unusual appearance. Unlike other interfacially formed membranes such as the NS-lOO which appear smooth and featureless in electron micrographs, the FT-30 has a rough surface with protuberances coming out of the plane of the membrane (Figure 3). This appearance of FT-30 seems to be related to the mechanism of the interfacial reaction. As stated previously, Morgan presented evidence that the reaction takes place primarily on the solvent side of the interface that is, the amine migrates from the water phase to... 

Figure 3. Electron micrograph showing surface and fractured edge of FT-30 membrane.

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. 

Chlorine resistance tests on FT-30 membranes, whether static-jar storage tests or dynamic tests with chlorine added to the feed-water showed a much lower rate of oxidation compared to other polyamide membranes such as the NS-lOO. A peculiar property of the FT-30 membrane in regard to chlorine attack was that the rate of oxidation was lowest in an acid pH range of 5 to 6 and higher in the... 

This has been observed both at FilmTec and by Glater and co-workers (53). Because of the low rate of oxidation of FT-30 membrane by chlorine it can tolerate an accidental exposure to chlorine. Shock chlorinations, if used with care, are possible, but not generally recommended. 

Later, a group of people from the North Star Research Institute founded the company FilmTec, and in 1979 the FT-30 membrane was introduced (15), and this membrane seems to be the best of thin-film composite membranes till now. 

FT-30 membrane is made from one of the simplest aromatic diamines 1,3-benzenediamine. The final chemical structure of the membrane is believed to be as follows ... 

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