Reverse osmosis interfacial composite

Although these composite fibers were developed for reverse osmosis their acceptance in the desalination industry has been limited due to insufficient selectivity and oxidative stabiUty. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerisation (20) have gained wide industry approval. HoUow fibers using this technique to give equivalent properties and life, yet to be developed, should be market tested during the 1990s. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

Hollow-fiber designs are being displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. Also, thin-film interfacial composite membranes, the best reverse osmosis membranes available, have not been fabricated in the form of hoUow-fine fibers. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

UF Membranes as a Substrate for RO An important use of UF membranes is as a substrate for composite reverse-osmosis membranes. After the UF membrane (usually polysulfone) is prepared, it is coated with an aqueous solution of an amine, then dipped in an organic solution of an acid chloride to produce an interfacially polymerized membrane coating. 

Considerable activity has been generated on composite reverse osmosis membranes by Japanese researchers. Patent applications were recently published, for example, covering research at Teijin Ltd. on interfacially formed membranes prepared from polydiallylamines (17) and from amine adducts of trls-(glycidyl) isocyanurate (18). Both types of membranes were formed on micro-porous polysulfone supports. Kurihara and coworkers have developed a composite membrane, designated PEC-1000, which is formed by an... 

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. 

They fabricated another two kinds of composite membranes through the interfacial reaction of triethylenetetramine 106,107). The one was the (3,(3 -dichloroethylether-triethylenetetramine-isophthaloyl chloride-trimesoyl chloride copolymer membrane, which had the water permeation rate of 2400 1/m2 day and desalination rate of 96.8 %. The other was the adipic-triethylenetetramine-isophthaloyl chloride copolymer membrane, which showed the water flux 95.8 1/m2 day and NaCl rejection 99.8 % on the reverse osmosis of a 0.5% aqueous solution at 25 °C and 42.5 kg/cm2. These characteristics for both membranes did not decrease during the continuous operation for 100 500 hr. 

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

Currently, approximately one billion gal/day of water are desalted by reverse osmosis. Half of this capacity is installed in the United States, Europe, and Japan, principally to produce ultrapure industrial water. The remainder is installed in the Middle East and other desert regions to produce municipal drinking water from brackish groundwater or seawater. In recent years, the interfacial composite membrane has displaced the anisotropic cellulose acetate membrane in most applications. Interfacial composite membranes are supplied in spiral-wound module form the market share of hollow fiber membranes is now less than... 

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. 

During the 1960s and 1970s the Office of Saline Water sponsored development of noncellulosic reverse osmosis membranes. Many polymers were evaluated as Loeb-Sourirajan membranes but few matched the properties of cellulose acetate. Following the development of interfacial composite membranes by Cadotte, this line of research was abandoned by most commercial membrane producers. 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

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 5.18 Organic solute rejection as a function of solute molecular weight for three representative reverse osmosis membranes [45] the interfacial composite membranes, (O) PA300 (UOP) and (A) NTR 7197 (Nitto), and the crosslinked furfuryl alcohol membrane ( ) PEC-1000 (Toray). Reprinted from R. Rautenbach and R. Albrecht, Membrane Processes, Copyright 1989. This material is used by permission of John Wiley Sons, Inc.

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. 

Composite Nonporous barrier on microporous substrate Flat-sheet, hollow fiber Direct coating, interfacial polymerization, plasma polymerization Reverse osmosis, gas separation, perstraction... 

There are several potential routes to the preparation of composite reverse osmosis membranes, whereby the ultrathin semipermeable film is formed or deposited on the microporous sublayer.1 2 The film can be formed elsewhere, then laminated to the microporous support, as was done in the earliest work on this membrane approach. Or it can be formed in place by plasma polymerization techniques. Alternatively, membrane polymer solution or polymer-forming reactants can be applied in a dipcoating process, then dried or cured in place. The most attractive approach from a commercial standpoint, however, has been the formation of the semipermeable membrane layer in situ by a classic "non-stirred" interfacial reaction method. Several examples of membranes made by this last approach have reached commercial status. 

The NS-100 membrane (initially designated as NS-1) was the first noncellu-losic composite membrane to appear in the published literature and have an impact on the reverse osmosis scene.22/23 This membrane, invented by Cadotte,24 consisted of a microporous polysulfone sheet coated with polyethylenimine, then interfacially reacted with either 2,4-toluenediisocyanate (TDI) or with isophthaloyl chloride (IPC). In the first case, a polyurea is formed in the second case, a polyamide. The chemistry of this membrane is as follows ... 

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. 

J.E. Cadotte, R.S. King, R.J. Majerle and R.J. Pedersen, Interfacial synthesis in the preparation of reverse osmosis membranes, J. Macromol. Sci. Chem., 1981, A15, 727-755 M. Kurihara, Molecular and morphological designs of high performance polymeric membranes, Pure Appl. Chem., 1994, A31, 1791-1805 R.J. Petersen, Composite reverse osmosis and nanofilteration membranes, J. Membr. Sci., 1993, 83, 81-150. 

Figure 6. Formation of a composite reverse osmosis membrane via in-situ interfacial polycondensation.

State of the art composite membranes for reverse osmosis consist of three layers 1) the discriminating surface layer (commonly a polyamide produced by interfacial polymerization of m-phenylenediamine - trimesoyl chloride), 2) a supporting ultrafiltration layer (commonly polysulfone), and 3) a non-woven fabric that provides the majority of the mechanical strength [28, 30]. This trilayer composite reduces the resistance to permeation in the supporting layers without compromising mechanical integrity. 

Truly chlorine-resistant interfacial composite reverse osmosis membrane finaliy deveioped... 

In aromatic polyamide polymers, aromatic rings are connected by an amide linkage, -CONH-. While the aromatic ring attached to -NH- is metasubstituted, the ring attached to -CO- is the mixture of meta- and parasubstitutions, which gives more flexibility to the polymeric material. Aromatic polyamide remains one of the most important materials for reverse osmosis membranes since the thin selective layer of composite membranes is aromatic polyamide synthesized by interfacial in situ polymerization. 

These various methods have been discussed in chapter III. Since reverse osmosis membranes may be considered as intermediate between porous ultrafiltration membranes and very dense nonporous pervaporation/gas separation membranes, it is not necessary that their structure to be as dense as for pervaporation/gas separatipn. Most composite reverse osmosis and nanofiltration membranes are prepared by interfacial polymerisation (see chapter in. 6) in which two very reactive bifunctional monomers (e.g. a di-acid chloride and a di-amine) or triiunctional monomers (e.g. trimesoyicbloride) are allowed to react with each other at a water/organic solvent interface and a typical rietwork structure is obtained. Another example of monomers used for interfacial polymerisation are given in table VI.6 (see also table m.1). 

---