FT-30 composite membrane

Cadotte discovered that aromatic diamines, interfacially reacted with triacyl halides, gave membranes with dramatically different reverse osmosis performance characteristics than membranes based on aliphatic diamines. 56 Before that time, the area of aromatic amines in interfacial membrane formation had been neglected because of two factors (a) the emphasis on chlorine-resistant compositions, which favored use of secondary aliphatic amines such as piperazine, and (b) poor results that had been observed in early work on interfacial aromatic polyamides. The extensive patent network in aromatic polyamide (aramid) technology may also have been a limiting factor. 

A typical recipe for an interfacially formed aromatic polyamide composite membrane comprised a 2.0% aqueous solution of the aromatic diamine and a 0.1% nonaqueous solution of trimesoyl chloride. This recipe was extraordinarily simple, and ran quite contrary to experience with piperazine-based membranes. For example, surfactants and acid acceptors in the aromatic diamine solution were generally not beneficial, and in many cases degraded membrane performance by lowering salt rejection. In contrast, surfactants and acid acceptors were almost always beneficial in the NS-300 membrane system. In the nonaqueous phase, use of isophthaloyl chloride as a partial replacement for trimesoyl chloride had relatively little effect on flux, but tended to decrease salt rejection and increase susceptibility to chlorine attack. 

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  

Some hydrolysis of the trimesoyl chloride takes place during membrane fabrication. ESCA studies indicated that approximately one-sixth of the carboxyl groups ere present as ionic carboxylate and five-sixths of the carboxyl groups are present as amides, leading to the above structure. The FT-30 barrier layer is insoluble in sulfuric acid and in all organic solvents, in agreement with the crosslinked nature indicated above. Its chemical structure is somewhat similar to the composition of the duPont Permasep B-9 hollow fiber polyamide, believed to be approximately as follows  

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. 

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Membrane Rejection. Both cellulose acetate and FT-30 composite membranes were evaluated for rejection of solutes. Sodium chloride rejections were confirmed and listed in Table III. Typical organic rejections of model compounds are listed in Tables IV and V for cellulose acetate and FT-30 composite membranes, respectively. Rejections were measured during screening and concentration tests solute levels were in the parts-per-billion range. Measurement of feed and permeate stream solute concentrations provided the necessary information to calculate solute rejection. Eq 1 was used to calculate rejection values. 

Rejections of all solutes studied were substantially higher for the FT-30 composite membrane (Table V) than for the cellulose acetate membrane (Table IV). This trend is predictable on the basis of the cross-linked nature and chemical inertness of the barrier layers. 

Table V. Model Compound Rejection for the FT-30 Composite Membrane from Screening and Concentration Test...

The overwhelming conclusion supported by data is the superiority of the FT-30 composite membrane for the majority of organic compounds tested. From arguments presented earlier, improved recovery of organic compounds on the basis of these higher rejection properties would be expected. Data from selected literature sources (6, 10-20) on membrane rejections of organics in water at parts-per-million levels were reviewed. Results are presented by chemical class in Table VI. Data are compiled for cellulose acetate and a cross-linked NS-1-type composite membrane. Differences in the rejection of various compound classes by the two membrane types determined at higher solute levels are similar to those observed and reported here at parts-per-billion levels. 

Tables VIII, IX, and X present mass balance data for three concentration tests. Two tests (one with each membrane type) were completed without humic acid, and the third was a repeat of the FT-30 composite membrane test including humic acid. Referral to Figure 6 will help clarify the first six columns in Tables VIII-X. Column 1 (Aff) and the sum of the components (column 6) were used to calculate mass accountability values (column 7). Although scattered, accountability of mass was good, considering the complexity of the system and the trace levels at which this work was performed.

In summarizing Tables VIII-XI, several points should be highlighted (1) Recoveries were higher, as expected, for the FT-30 composite membrane than for the cellulose acetate membrane. (2) Compounds exhibiting negative rejections with the cellulose acetate membrane were not recovered. (3) Many compounds were not recovered at their predicted levels in tests with either type of membrane. 

As expected, compounds demonstrating consistent negative rejections with cellulose acetate membranes (dichlorophenol, biphenyl, furfural, chloroform) were not recovered to any extent. Compounds with the best rejections (>90%) were the better recovered substances. The FT-30 composite membrane clearly demonstrated superior performance to the cellulose acetate membrane for organic rejection, concentration, and recovery. Sodium chloride rejection was no indicator of potential organic rejection. 

Table XII. Comparison of Volumetric Concentration, Organic Compound Concentration, and Trace Organic Mass Recovery Obtained in a Laboratory RO Concentration with the FT-30 Composite Membrane...

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... 

Figure 5.13 Effect of pH on the chlorine resistance of the FT-30 composite membrane.

Figure 5.12 Topside and underside of the FT-30 composite reverse osmosis membrane (a) topside showing well-developed ridge-and-valley structure, and also an area of membrane barrier layer folded over upon itself (b) underside of the barrier layer (foldover zone) showing the network of micropores inside the ridge-and-valley structure.

Membranes from various manufacturers A, Hollosep-cellulose triacetate hollow fibre membrane (Toyobo) B, sulphonated polysulphone composite hollow fibre membrane (Albane International) C, BlO-aromatic polyamide hollow fibre membrane (Du Pont) D, PEC-1000-composite flat-sheet membrane (foray) E, NS-200-composite polyfurfuryl alcohol membrane F, FT-30-composite polyamide flat-sheet membrane (Film Tec/Dow) G,... 

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. 

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. 

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

Petersen, R.J. Larson R.E. Majerle, R.J. "Development of the FT-30 Thin-Film Composite Membrane for Desalting Applications," Technical Proceedings, 8th Ann. Conf. National Water Supply Improvement Assn., San Francisco, CA, July 6-10, 1980. 

The importance of proper RO membrane selection has already been discussed. A review of commercially available RO membranes revealed five different basic membranes that could provide organic recovery. Cellulose acetate and cellulose acetate blends, aromatic polyamide, polyamide thin-film composite, cross-linked polyimine thin-film composite (FT-30), and polybenzimidazole were available when this work was performed. Only the first four types were commercially available. All membranes were available with excellent salt rejection (>97 sodium chloride). Two types of membranes, cellulose acetate and FT-30, have shown short-term (<2-months intermittent use) resistance... 

Cadotte et al. 108) discovered a new thin-film composite membrane (FT-30), but its composition has not been disclosed. 

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. 

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). 

Figure 1. Schematic representation of the structure of FT-30 thln-fllm composite membrane.

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. 

A new variation related to the FT-30 membrane is being developed-the NF-50 composite membrane-which would appear to occupy a unique place in membrane technology. The NF-50 membrane has approximately the same characteristics as NTR-7250 and NF-40, but possesses an extremely high water flux. Reverse osmosis operation in large systems at a pressure of 35 to 50 psi is possible. The NF-50 membrane thus becomes the first example of a reverse osmosis membrane capable of operation at ultrafiltration membrane pressures. 

Second, insoluble crosslinked barrier layer compositions are possible, and, in fact, are almost universal in the composite membrane approach. Optimum reverse osmosis performance and chemical stability can be achieved, in part, due to preparation of crosslinked compositions. This is readily possible by the composite membrane approach, but not so simple by the asymmetric membrane approach. The PA-300, FT-30, and PEC-1000 barrier layer compositions, for example, are simply not feasible to prepare by asymmetric film casting techniques. The composite approach, therefore, is far more versatile. 

Figure 6.15 (a) Cross-section of a thin-film composite polyamide RO membrane (FT-30), (b) chemical structure of FT-30 polymer, and (c) Micrograph of a thin-film composite membrane-polyamide layer on polysulphone support. Source P.A. Pacheo et al, J. Memb. Sci. 358 (2010), 51-59. Copyright (2010), with permission from Elsevier. 

Figure 4.10. Electron micrographic picture of a composite FT>30 membrane. (Reproduced from [64] with permission.)...

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