Reverse osmosis flux characteristics, membrane

This paper has provided the reader with an introduction to a class of polymers that show great potential as reverse osmosis membrane materials — poly(aryl ethers). Resistance to degradation and hydrolysis as well as resistance to stress Induced creep make membranes of these polymers particularly attractive. It has been demonstrated that through sulfonation the hydrophilic/hydrophobic, flux/separation, and structural stability characteristics of these membranes can be altered to suit the specific application. It has been Illustrated that the nature of the counter-ion of the sulfonation plays a role in determining performance characteristics. In the preliminary studies reported here, one particular poly(aryl ether) has been studied — the sulfonated derivative of Blsphenol A - polysulfone. This polymer was selected to serve as a model for the development of experimental techniques as well as to permit the investigation of variables... 

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. 

The influence of metal oxide derived membrane material with regard to permeability and solute rejection was first reported by Vernon Ballou et al. [42,43] in the early 70s concerning mesoporous glass membranes. Filtration of sodium chloride and urea was studied with porous glass membranes in close-end capillary form, to determine the effect of pressure, temperature and concentration variations on lifetime rejection and flux characteristics. In this work experiments were considered as hyperfiltration (reverse osmosis) due to the high pressure applied to the membranes, 40 to 120 atm. In fact, results reproduced in Table 12.3 show that these membranes do not behave as h)qjerfiltra-tion membranes but as membranes with intermediate performances between ultra- and nanofiltration in which surface charge effect of metal oxide material plays an important role in solute rejection. 

We believe that for reverse osmosis new membranes with high chemical stability, high temperature resistance, and improved performance rates in respect to rejection characteristics and flux rates are coming on the market very soon in the form of improved thin-film composite membranes. 

Preparation Procedures of Asymmetric Membranes. The development of the first asymmetric phase inversion membranes was a major breakthrough in the development of ultrafiltration and reverse osmosis. These membranes were made from cellulose acetate and yielded fluxes 10 to 100 times higher than symmetric structures with comparable separation characteristics. Asymmetric phase inversion membranes can be prepared from cellulose acetate and many other polymers by the following general preparation procedure 27... 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

A new variation related to the FT-30 membrane is being developed-the NF-50 composite membrane-which would appear to occupy a unique place in membrane technology. The NF-50 membrane has approximately the same characteristics as NTR-7250 and NF-40, but possesses an extremely high water flux. Reverse osmosis operation in large systems at a pressure of 35 to 50 psi is possible. The NF-50 membrane thus becomes the first example of a reverse osmosis membrane capable of operation at ultrafiltration membrane pressures. 

Semipermeable membranes and hollow fibers are produced from cellulose acetate. Dry-jet wet-spinning techniques are described to provide asymmetric and homogeneous hollow fiber membranes. Manipulation of spinning conditions leads to morphologies that permit higher rejection and higher fluxes. The excellent balance of the hydrophobic-hydrophilic characteristics for cellulose acetate makes this polymer useful for reverse osmosis [89-93]. Cellulose acetate membranes and hollow fiber membranes are commercially available for hemopurification. [94], for ultrafiltration [95], and for other commercial separation processes. 

Thin film composite membranes Reverse osmosis characteristics Water flux (gfd) Salt rejection (%) Surface roughness, Ra (nm) Surface area (tun )... 

Three key elements determine the potential and applications of a hollow-fiber membrane (1) pore size and pore size distribution, (2) selective layer thickness, and (3) inherent properties (chemistry and physics) of the membrane material. Pore size and its distribution usually determine membrane applications, separation factor, or selectivity. The selective layer thickness determines the membrane flux or productivity. Material chemistry and physics govern the intrinsic permselectivity for gas separation and pervaporation, fouling characteristics for RO (reverse osmosis), UF (ultrafiltration), and MF (microfiltration) membranes, chemical resistance for membranes used in harsh environments, protein and drug separation, as well as biocompatibUity for biomedical membranes used in dialysis and biomedical and tissue engineering. 

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