Polyamide composite membranes improvements

There have been several improvements made to polyamide, composite membranes that have enhanced their performance. Perhaps the most... 

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. 

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. 

The use of chlorine dioxide is not recommended for use with polyamide, composite membranes. This is because free chlorine is always present with chlorine dioxide that is generated on site from chlorine and sodium chlorate (see Chapter 8.2.1.1). (Note that other formation techniques have been developed that do not rely on chlorine, which may improve on the compatibility of the membranes with chlorine dioxide, (see chapter 8.5.2.1.3))... 

Chlorine is usually added upstream of a clarifier to oxidize organics, to improve the removal of color in the clarifier, and to control microbial growth in the clarifier and downstream equipment. Chlorine along with an alum feed at pH 4.5 to 5.5 is optimum for color removal. This is important for RO pretreatment, as color can irreversibly foul a polyamide composite membrane (see Chapter 8.2.1.1 and 8.5.2.1.1 for a more detailed discussions about chlorine for RO pretreatment). 

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. 

The thin film composite membrane exhibited superior overall rejection performance in these tests, with ammonia and nitrate rejection showing an outstanding improvement. It has also been reported that silica rejection by the thin film composite membranes is superior to that of cellulose acetate. While the above data indicates a marginal improvement in the rejection of chemical oxygen demand (COD), which is an indication of organic content, other tests conducted by membrane manufacturers show that the polyurea and polyamide membrane barrier layers exhibit an organic rejection that is clearly superior to that of cellulose acetate. Reverse osmosis element manufacturers should be contacted for rejection data on specific organic compounds. ... 

Yu, S., Liu, M., Lii, Z., Zhou, Y., and Gao, C. 2009. Aromatic-cycloaliphatic polyamide thin-film composite membrane with improved chlorine resistance prepared from m-phenylenediamineA-methyl and cyclohexane-1,3,5-tricar-bonyl chloride. Journal of Membrane Science 344 155-164. 

Although the hydrophobic surface of nylon membranes may be improved by hydrolysis or chemical modification, composite polyamide membranes are preferred in chromatographic applications. Klein et al. [42] prepared polyamide microporous membranes by modification of terminal amino groups. Covalent binding of a polyhydroxyl-containing material to the polyamide membrane increases the density... 

The first generation of the NF membranes can be traced back to the early 1970s when most of the membranes were made of cellulose acetate (CA) and other cellulose esters. These cellulose-based membranes, however, severely limited the range of industrial applications due to their poor chemical and biological resistances coupled with insufficient water permeation. This consequently resulted in the development of a second generation of noncellulosic NF-composite membranes made of polyamide (PA) and polyurea (PU) with the aim to improve water permeability and selectivity, together with better pH and solvent stability (Schafer et al. 2003). This section does not intend to provide an exhaustive review of all the NF membranes developed to date. It simply aims to give the latest development of NF membranes in the past decade. Attention is paid to the research and development of NF prepared from two different fabrication techniques. 

Another preparation method for composite membrane is an in situ monomer condensation method using the monomeric amine and monomeric acid halide, which was also invented by Cadotte. Then, many companies succeeded in developing composite membranes using this method, and the membrane performance has been drastically improved up to now. Now, composite membrane of cross-linked fully aromatic polyamide is regarded as the most popular and reliable material in the world. Permeate flow rate and its quality have been improved 10 times greater than that of the beginning (Kurihara et al., 1987, 1994b). 

The key to the reverse osmosis (RO) process is a suitable semipermeable membrane. Improvements in membrane technology now mean that the process can apply to industrial-scale plants. Common contemporary membrane selections are made of cellulose-based polymer or a polyamide layer applied to a microporous poljmier fllm. This membrane is bonded to a porous polyester sheet for structural stiffiiess. This composite is rolled into a spiral. Spun hollow fine fibers are the finished product. The semipermeable layer is on the outside of the fibers. The total thickness of the composite is about 24 pm. The outside diameter of the tube is about 95 pm, making for a large surface area for rejecting salt. The fibers are made into bundles that are sealed with epo in a fiberglass pressure container. 

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. 

Zhou, Y., Yu, S., Gao, C. and Feng, X. 2009. Surface modification of thin composite polyamide membranes by electrostatic self-deposition of polycations for improved fouling resistance. Sep. Purif. Technol. 66 287-294. 

It should be noted that Car et al. [10,11] (see also Chapter 12 of this volume) worked independently on similar blend manbranes, which were made of Pebax 1657, a grade of commercial poly(amide-b-ether) block copolymer with six polyamide blocks, and free PEG. Those membranes were also shown to exhibit high selectivity and permeability performances, which were attribnted to changes in both the chemical composition (i.e. higher EO content) and the morphological stmctuie (i.e. lower material crystallinity). On the contrary, Jaipurkar [12] observed CO2 permeability and selectivity improvements for the blend of Pebax 2533 with 25% of PEG 10000, bnt not for the blends with other PEG molecular weights or composition. 

One of the more interesting fronts of development includes the search for improved membrane materials. While no new polymeric RO membranes have been introduced commercially over the last 20 to 30 years, there have been developments in performance (see Figure 1.5). These improvements in performance were achieved via modifications to the membrane itself (surface modifications made possible due to more advanced membrane characterization techniques) and closer tolerances in the interfacial polymerization reaction to make the membrane, and enhancements of the module design. Membranes with these improvements are commercially available today. While work is continuing with modifications to the current thin-film composite polyamide membranes, researchers are looking toward additional materials that might be suitable for use as RO membranes. 

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