Polyamide composite membranes, flux

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. 

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 surface of a membrane can be modified by chemical reactions. For example, when the surface of a polyamide composite membrane is brought into contact with a strong hydrofluoric acid solution, the top polyamide layer becomes slightly thinner by a chemical reaction with hydrofluoric acid. As a result the flux increases considerably while fhe rejecfion of sodium chloride is rmchanged or slighfly increased. ... 

A further requirement of a CCRO membrane is that it should have an open, microporous sublayer structure. Such membranes allow effective diffusion of ethanol into the membrane from a recirculation solution supplied on the permeate side of the membrane. In our survey of various flat-sheet and hollow-fiber membranes, a monomer-derived polyamide composite membrane designated 3N8 was identified which satisfied this requirement. Other membranes tested either exhibited small or no measurable flux increases with permeate-side recirculation and are thus not suited to CCRO applications. 

Figure 6. Typical flux behavior with PA-300 polyamide composite membranes. Feed white must previously ultrafiltered through BMR-021006 modulus T = lO C.

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. 

There have been several improvements made to polyamide, composite membranes that have enhanced their performance. Perhaps the most important improvement has come through advanced manufacturing techniques, which have allowed for thinner membranes with few imperfections. Thinner membranes exhibit higher flux rates at the same operating pressure than their thicker counterparts. 

Fig. 1. Water flux and NaCl rejection of several membrane types (10), where (D) represents seawater membranes, which operate at 5.5 MPa and 25°C ( ), brackish water membranes, which operate at 1500 mg/L NaCl feed, 1.5 MPa, and 25°C and (SSI) nanofiltration membranes, which operate at 500 mg/L NaCl feed, 0.74 MPa, and 25°C. A represents cellulose acetate—cellulose triacetate B, linear aromatic polyamide C, cross-linked polyether D, cross-linked fully aromatic polyamide E, other thin-film composite membranes F, asymmetric membranes G, BW-30 (FilmTec) H, SU-700 (Toray) I, A-15 (Du Pont) J, NTR-739HF (Nitto-Denko) K, NTR-729HF (Nitto-Denko) L, NTR-7250 (Nitto-Denko) M, NF40 (FilmTec) N, NF40HF (FilmTec) O, UTC-40HF (Toray) P, NF70 (FilmTec) Q, UTC-60 (Toray) R, UTC-20HF (Toray) and S, NF50 (FilmTec). To convert MPa to psi,...

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. 

Like RO membranes, many NF membranes are polyamide thin film composite membranes. These membranes can be prepared by interfacial reaction of piperazine with 1,3,5-benzenetricarbonyl trichloride and/or isophthaloyl dichloride, or by treating polyamide thin film composite RO membranes with compounds such as mineral acids, to increase their flux and lower their salt rejection. A few ceramic NF membranes have also been developed. New methods... 

Homogeneous asymmetric CAs and polyamides made by the phase inversion process and cross-linked TFC polyamides have been the workhorse of RO plants for more than 30 years [21], Both CA and PA membranes possess an economically viable combination of high rejection and water flux [8]. However, TFC membranes now dominate the RO/NF market with CA membranes a distant second. For example, with the exception of Toyobo CTA polymer, all new seawater RO desalination plants deploy interfacial composite membranes of the fuUy aromatic type manufactured by Dow, Hydranautics (Nitto Denko), Tri-Sep and Toray. Along with the ability to remain stable over a greater pH range than cellulose-based membranes, TFC membranes exhibit much higher intrinsic water permeabilities because of their extremely thin ( 100 nm) polyamide-selective layers [21]. A typical spectrum of TFC membranes for various applications is given in Table 1.8. 

Membranes UF membranes consist primarily of polymeric structures (polyethersulfone, regenerated cellulose, polysulfone, polyamide, polyacrylonitrile, or various fluoropolymers) formed by immersion casting on a web or as a composite on a MF membrane. Hydrophobic polymers are surface-modified to render them hydrophilic and thereby reduce fouling, reduce product losses, and increase flux [Cabasso in Vltrafiltration Membranes and Applications, Cooper (ed.). Plenum Press, New York, 1980]. Some inorganic UF membranes (alumina, glass, zirconia) are available but only find use in corrosive applications due to their high cost. 

These membranes have exceptional properties, including seawater salt rejections of up to 99.6 % and fluxes of 23 gal/ft2 day at 800 psi. Unfortunately, they are even more sensitive to oxidants such as chlorine or dissolved oxygen than the polyamide/polyurea interfacial composites. The membranes lose their excellent properties after a few hundred hours of operation unless the feed water is completely free of dissolved chlorine and oxygen. A great deal of work was devoted to stabilizing this membrane, with little success. 

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