NS-300 membrane

Figure 3c. SEM photomicrograph of composite membranes surface view of the NS-100 membrane.

Figure 3.20 Schematic of the interfacial polymerization process. The microporous film is first impregnated with an aqueous amine solution. The film is then treated with a multivalent crosslinking agent dissolved in a water-immiscible organic fluid, such as hexane or Freon-113. An extremely thin polymer film forms at the interface of the two solutions [47]. Reprinted from L.T. Rozelle, J.E. Cadotte, K.E. Cobian, and C.V. Knopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada (1977) by permission from NRC Research Press...

Figure 3.21 Idealized structure of polyethyleneimine crosslinked with toluene 2,4-diiso-cyanate. This was called the NS-100 membrane. The chemistry was first developed by Cadotte to make interfacial reverse osmosis membranes with almost twice the water flux and one-fifth the salt leakage of the best reverse osmosis membranes then available. Even better membranes have since been developed by Cadotte and others [47]...

L.T. Rozelle, J.E. Cadotte, K.E. Cobian and C.V. Kopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada, pp. 249-262 (1977). 

Figure 4.8 Interfacial polymerization, using polyethyleneimine (PEI) crosslinked with toluene diisocyanate (TDI) to from the NS-100 membrane as patented by Cadotte (U.S. Patent 4039440, August 2,1977).

Two types of reactions involved in the preparation of NS-100 membranes are illustrated in Figure 2. The structural representation of polyethylenimine (PEI) is simplified to show only the reactive primary and secondary amine groups. In the first step the amine groups react rapidly with isophthaloyl chloride at the interface to produce a polyamide surface skin, while amine groups below... 

The low ethanol rejection and the instability of the hollow-fiber NS-100 membranes preclude the use of this membrane for practical ethanol enrichment. Nevertheless, for the purpose of demonstrating the concept of CCRO using hollow-fiber membranes, CCRO experiments were conducted at the reduced ethanol concentration of 10 vol%. The permeate fluxes of NS-100 modules were measured at 250 psi in the absence and presence of recirculation with a 10-volX ethanol solution. The results were varied recirculation brought about flux increases ranging from 5% to about 20%. The limited flux increase may again be explained in terms of the formation of a polyamine gel during NS-100 membrane fabrication. Nevertheless, the flux increase shows that the hollow-fiber geometry is a viable one for CCRO operation. 

During the period of 1965 to 1972, the best data on flux and salt rejection for cellulose acetate membranes were exhibited by the composite membranes. However, these membranes never reached commercial viability efforts on them died out completely by 1975. Reasons for this appear to be threefold. First, composite cellulose acetate membranes were technically difficult to scale up. Second, the advent of noncellulosic composite membranes in 1972 (the NS-100 membrane) offered much more promise for high performance (salt rejection and water flux), especially for seawater desalination. Third, continual improvements in asymmetric cellulose acetate membrane casting technology (such as the development of swelling agents and of blend membranes) brought the performance of asymmetric membranes to full equality with composite cellulose acetate membranes. 

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 ... 

Figure 5.6 illustrates the preparation of the NS-100 membrane. A micro-porous polysulfone sheet is saturated with a water solution of the polymeric amine (0.5 to 1.0% solution of polyethylenimine). Excess solution is drained off... 

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. 

This membrane is highly sensitive to chemical attack by hypochlorite ion and hypochlorous acid in chlorinated feedwaters. Wrasidlo prepared a variation of the NS-100 membrane in which the primary amine groups of polyethylenimine were cyanoethylated before reaction with isophthaloyl chloride.27 This membrane was claimed to be chlorine-resistant, but was probably not so. 

Optimization studies on the NS-100 membrane were carried out by other groups in addition to North Star Research Institute. Fang and Chian used a statistically designed set of 33 experiments to produce a membrane with... 

The NS-100 membrane, being the first of a kind, was by no means optimum in chemistry and performance. Flux was only marginally attractive for desalination, sensitivity to chlorine was extreme, and the barrier surface was thin and brittle. In commercial fabrication trials, overcoating of the membrane surface with a layer of water-soluble polyvinyl alcohol was practiced in order to overcome its brittleness and susceptibility to abrasion damage during handling and spiral element fabrication. 

In other respects, it also shared the favorable characteristics of the NS-100 membrane. That is, it was resistant to pH 3 to 12, and showed far better compaction resistance than cellulose acetate. Also, it possessed the capability to operate at elevated temperatures, though some irreversible flux decline could still occur.34 Rejections of various organics were also good, as shown in Table 5.1 These were in sharp contrast to organic rejection data on cellulose acetate membranes. Initially, PA-300 was also postulated to possess good chlorine resistance.31 Subsequent experience showed it to be equally sensitive to chlorine as NS-100. 

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. 

A thin polymer him can be formed in situ on the surface of a porous substrate membrane by an interfacial polycondensation process. A classical example of this method was described by Rozelle et al. in detail for the formation of the North Star NS-100 membrane 30]. According to their description, the polysulfone support films are placed, shiny surface upwards, into a 0.67% aqueous polyethyicnimine (PEI) solution in an aluminum tray. After 1 min, the PEI solution is poured off, and the tray held in a vertical position for 1 min to allow the excess solution to drain from the surface of the him. Then the wet surface is contacted with a 0.5% solution of toluene 2,4-diisocyanate (TDI) for 1 min at room temperature. After draining the excess TDI solution, the u y is placed horizontally at 11S C for 10 min. After the heat curing, the composite membrane is easily peeled off from the aluminum surface. 

The NS-100 membrane so prepared is believed to consist of three layers depicted by Figure 3.1. 

Figure 3.1. Structure of NS-100 membrane. (Reproduced from I30J with permission.)...

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