Polyamide composite membranes effect

Exposure to high temperature at pH extremes can hydrolyze the membranes, leading to loss of membrane integrity. (See Chapter 4.2.1, Table 4.2, and Chapters 9.2,9.8, and 13.2 for more detailed discussions on the effect of temperature and pH on polyamide composite membranes.) Hydrolysis also tends to involve the entire RO skid rather than focus on only the lead membranes. Just as with oxidation of the membrane, feed water will pass into the permeate resulting in an increase in permeate flow and decrease in the product quality. 

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. 

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. 

Improvement of Water Permeability (UTC-70L) In our past experiments of various polyamide composite membranes, introduction of end acids is preferable to obtain better water permeability and decrease of end amines is preferable to obtain better tolerance to chloride. From the view point, we tried to improve water permeability of UTC-70. Our strategy for introduction of end acids and decrease of end amines is an improvement of acid chlorides reactivity by using catalyst for in-situ interfacial polycondensation. Thus, we found common catalysts for acylation worked effectively as we had expected, and water permeability of UTC-70 were increased without severe decrease of membrane selectivity. This type of membrane are commercialized as "UTC-70L", and membrane performance is shown Figure 7. 

Monochloramine is approximately 200 times less effective than free chlorine as a disinfectant. However, it is still used as an alternative to chlorine because chloramines do not react as readily with organic materials to form trihalomethanes (THMs). In theory, due to the less aggressive nature of chloramines, the tolerance of polyamide composite membranes to chloramine is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

Microporous membranes are used to effect the separation by MF and UF processes. These microporous membranes differ from polyamide composite RO membranes in that they are not composites of two different polymeric materials they are usually constructed using a single membrane polymeric material. In simple terms, both UF and MF technologies rely on size as the primary factor determining which... 

Dalwani, M., Benes, N. E., Bargeman, G., Stamatialis, D., and Wessling, M. 2011. Effect of pH on the performance of polyamide/polyacrylonitrile based thin film composite membranes. Journal of Membrane Science 372 228-238. 

The incorporation of positive charges has decreased the fouling susceptibility of membranes even more effectively. This is the principle of the aromatic polyamide membrane series commercialized by Hydranautics as low fouling composite membranes (LFC). Cationic charge-modified nylon membranes are also commercially available from CUNO 3M, under the trademark Zeta Plus . Pall Corp. sells cationic charge-modified nylon membranes under the trademark Ngg Posidyne. There are different ways to make the membrane positively charged. A patent from Millipore [148] describes the surface modification of hydrophobic membranes by contacting them with a solution of polyamine epichlorohydrin... 

Ariza, MJ Canas, A MaUeito, J Benavente, J. Effect of pH on electrokinetic and electrochemical parameters of both sub-layers of composite polyamide/polysulphone membranes. Desalination, 2002, 148,377-382. 

Spiral woimd module of 1 m effective separation area of thin film composite (TFC) Polyamide RO membrane was fabricated with the help of Permion-ics Membranes Pvt. Ltd., Vadodara, India, /w-phenylenediamine (MPD) and trimesoyl chloride (TMC) obtained from AVRA Synthesis Pvt. Ltd (Hyderabad, India) were used without further purification. Piperazine was purchased from Sigma-Aldrich, USA. Groimd water samples collected from two different places of Andhra Pradesh (A.P.), India was used for experimental trials. Potassium dichromate, ferrous ammonium sulfate, mercuric sulfate, sulfuric acid, ferroin indicator for COD analysis, sodium thiosulfate, Wrinkler s reagent, MnSO, potassimn iodide, starch indicator for BOD analysis, citric acid, HCl, EDTA, NaOH, and sodium metabisulphite (SMBS) for washing the membranes were purchased from SD Fine Chemicals Ltd., Mumbai, India. Deionized water for cleaning and feed preparation was generated from the same RO system. BOD incubator (RCI-S.NO-313 India), COD incubator... 

In this pilot plant the UF pretreatment system is arranged in 2 trains, each housing 3 modules (PAN HF membranes, nominal pore size of 0.02 (im, MWCO 50 000, total effective surface area of 30 m2). Raw seawater (samples from Qingdao Jiaozhou Bay, the Yellow Sea of China) was first passed into a cartridge sand filter and successively feed to UF system, the UF permeate was then pumped to the RO system (spiral-wound composite polyamide) (Figure 12.1). 

Goosen M.F.A., Sablani S., Dal Cin M., Wilf M., Jackson D., Al-Belushki R., and AlMaskri S., Effect of high temperature and pressure on permeation properties of composite polyamide seawater reverse osmosis and brackish water nanofiltration membranes. Journal of Membrane Science (Submitted 2008). 

Goosen, M.F.A., Sablani, S., Del-Cin, M., and Wilf, M. 2010. Effect of cyclic changes in temperature and pressure on permeation properties of composite polyamide spiral wound reverse osmosis membranes. Sep Sci Technol. 46 14-26. 

Usually, the casting solution for aromatic polyamide reverse osmosis membranes includes the electrolyte additive, and the composition is near the critical concentration. Then the radius of macromolecular spheres should be nearly equal to 52 X, as indicated by Table 2, and their packing fa.shion is close to the cubic packing. Approximating the square-shaped interstitial void area generated between four neighboring spheres (see Figure 4.11) by a circle of equal area, the effective radius of such interstitial void spaces is 27 A. 

Chu et al. [70] demonstrated a simple and effective route for the hydrophilic surface modification of ceramic-supported PES membranes by synthesizing a polyfvinyl alcohol) (PVA)/polyamide (PA) composite thin sinface layer with an interfacial polymerization method (IP) method. The reaction of the interfadal polymerization is schematically shown in Figure 2.7. A prepared tubular ceramic-supported PES membrane (both ends sealed) was immersed in a terephthaloyl chloride solution in benzene and... 

Table 7.1 Effect of feed composition on the graft yield of surface modified polyamide membranes using plasma polymerization method at 60X for 2 hours...

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