Polyamide composite membranes characteristics

Kurihara and coworkers at Toray Industries prepared several aminated derivatives of polyepichlorohydrin, then formed composite polyamide membranes by interfacial reaction with isophthaloyl chloride.38 Polyepichlorohydrin was converted to polyepiiodohydrin, then reacted with either4-(aminomethyl)piperidine, 3-(methylamino)hexahydroazepine, or 3-(amino)hexahydroazepine. Also, poly-epiaminohydrin was prepared by reduction of the azide derivative of polyepiiodohydrin. Best salt rejections were obtained if the polymeric amine formulation contained a substantial proportion of the monomeric amines as coreactants in the interfacial reaction. In tests on 3.5% sodium chloride at 800 psi and 25 C, salt rejections of 99.5% at fluxes of 8 to 9 gfd were characteristic. A three-zone barrier layer was produced, consisting of a heat-crosslinked polyamine gel (as in NS-100), a polyamide layer incorporating both the polymeric... 

Membrane material. During the early days, membranes were usually made of cellulose acetate. At present, membranes can also be made from aromatic polyamide and thin-filmed polymer composites. Different membrane materials have their own distinctive characteristics, such as hydraulic resistance, pH range, temperature range, chlorine tolerance, and biodegradation tolerance. 

A commercial composite polyamide/polysulfone membrane for reverse osmosis (HR95) from DSS (Denmark). The characteristic membrane parameters, such as total thickness, hydraulic permeability, and rejection, are Ax = (165 5) pm, = 8.5 x 10 m/(sec Pa), and a = 99.5%, respectively (Jonsson and Benavente 1992). 

Table 4.2 lists the predominant characteristics of polyamide, composite membranes. 

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. 

Ramon et al [17] compiled a list of commercial and prototype osmotic membranes and evaluated the resulting theoretical power densities according to their characteristics. The analysis showed that prototype lab-cast thin-film composite membranes based on a selective polyamide active layer can achieve water permeability in the range of 5-7 m/s-Pa, which result in theoretical power density in the range of 5-6 W/m with seawater feed and over 15 W/m in plant fed with I.IM brine discharged from RO-based desalinization plants. Theoretical power densities achievable with commercial membranes are below 3 W/m with seawater feed. 

In order to correlate chemieal analysis of aetivated membrane surface with other characteristic parameters, tangential streaming potential measurements at a constant NaCl concentration (5x10 " M) but different pHs were carried out with the DT200 sample and analysed using the local dissociation model [68-69], which allows the determination of the pKa and the munber of acid sites accessible on the membrane surface (N ), and their comparison with the same parameter for PAO polyamide/polysulfone composite membrane (the un-modified base membrane). The obtained values and surface roughness are ... 

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