Reverse-osmosis membranes specifications

In the literature, there are many transport theories describing both salt and water movement across a reverse osmosis membrane. Many theories require specific models but only a few deal with phenomenological equations. Here a brief summary of various theories will be presented showing the relationships between the salt rejection and the volume flux. 

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... 

Because NMR spectroscopy is a nuclei-specific technique and has the ability to distinguish between similar compounds, it is an excellent method for identifying similar species in complex matrices. Thus, 31P FT-NMR spectroscopy is ideal for the identification and characterization of the hydrosphere DOP. Even so, NMR spectroscopy is fairly insensitive and requires high sample concentrations. Low DOP concentrations are increased to 31P FT-NMR detection limits by using ultrafiltration and reverse osmosis membranes. Not only is the DOP concentrated, but it is fractionated according to its molecular size. Compared to other concentration and molecular size fractionation techniques for DOP, ultrafiltration and reverse osmosis are relatively rapid and easy. 

In some applications, the interaction of the polymer with a specific "solvent" and/or with certain molecules carried by that solvent is not a detrimental event, but an essential aspect of the operation of the polymer. Reverse osmosis membranes and swollen hydrogels used in applications such as the desalination of water, kidney dialysis, soft contact lenses and surgical implants [17] are among such polymers. 

Applications for FT-30 membrane have appeared in all reverse osmosis fields from seawater desalination to home tapwater systems operating on line pressure. At this date, it is the only commercial reverse osmosis membrane other than cellulose acetate that has specific FDA approval for food contact usage.63 Versions of this membrane, manufactured under license to FilmTec, are available in tubular form (ZF-99, Patterson Candy International) and plate-and-frame design (HR-95, HR-98, De Danske Sukkerfabrikker).64 65... 

Numerous kinds of polymers in combination with solvents and other additives are used for preparation of membranes for specific purposes. In reverse osmosis, for example, composite membranes have become very important and have been studied intensively. These membranes are composed of two layers, each with a specific function, such as mechanical support or selective separation. Table 2 shows some examples of polymers used for preparation of reverse osmosis membranes. 

A. E. Yaroshchuk, S. S. Durkhin. Phenomenological theory of reverse osmosis in macroscopically homogeneous membranes and its specification for the capillary charged model. J Memb Sci 79 133, 1993. 

Applications RO is primarily used for water purification seawater desalination (35,000 to 50,000 mg/L salt, 5.6 to 10.5 MPa operation), brackish water treatment (5000 to 10,000 mg/L, 1.4 to 4.2 MPa operation), and low-pressure RO (LPRO) (500 mg/L, 0.3 to 1.4 MPa operation). A list of U.S. plants can be found at www2.hawaii.edu, and a 26 Ggal/yr desalination plant is under construction in Ashkelon, Israel. Purified water product is recovered as permeate while the concentrated retentate is discarded as waste. Drinking water specifications of total dissolved solids (TDS) < 500 mg/L are published by the U.S. EPA and of < 1500 mg/L by the WHO [Williams et ak, chap. 24 in Membrane Handbook, Ho and Sirkar (eds.). Van Nostrand, New York, 1992]. Application of RO to drinking water is summarized in Eisenberg and Middlebrooks (Reverse Osmosis Treatment of Drinking Water, Butterworth, Boston, 1986). 

As summarized above, there are many transport models and flow mechanisms describing reverse osmosis. Each requires some specific assumptions regarding membrane structure. In general, membranes could be continuous or discontinuous and porous or non-porous and homogeneous or non-homogeneous. One must be reasonably sure about the membrane structure before he analyzes a particular set of experimental data based on one of the above theories. Since this is difficult, in many cases, it would be desirable to develop a model-independent phenomenological theory which can interpret the experimental data. 

An exact mathematical relationship is obtained between the salt rejection and total volume flux in reverse osmosis based on a purely phenomenological theory assuming constant salt permeability. This approach does not require a specific membrane model ... 

In summary, water can be a source of contaminants. If the raw material (drinking water) complies with the quahty parameters established by authorities, contaminants still present can be eliminated by usual water purification processes available to the pharmaceutical industry. While distillation and reverse osmosis provide water with the quality specifications for purified water and highly purified water, WFI is generally obtained by membrane filtration (associated with another purification process) not only because of chemical contamination but mainly because of sterility requirements. 

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

Ultrafiltration can be utilized to separate the emulsion and dissolved oil from water. The specific ultrafiltration membrane polymer and pore-size requirement are determined by the oil chemistry however/ the oil can typically be concentrated up to 60 - 80%, and in some cases, incinerated to recover energy in the form of heat. The permeate stream may be pure enough to be re-used, or may require treatment with reverse osmosis prior to re-use. 

HOLLOW-FIBER MEMBRANES. A hollow-fiher membrane is a capillary having an inside diameter of - inn and an outside diameter < I mm and whose wall functions as a semipermeahlc membrane. The fibers can he employed singly or grouped into a bundle which may contain tens of thousands of fibers and up to several million libers as in reverse osmosis (Fig. 11. In most eases, hollow fibers are used as cylindrical membranes that permit selective exchange of materials across (heir walls. However, they can also he used as containers to effect the controlled release of a specific material, or as reactors to chemically modify a permeate as il diffuses through a chemically activated hollow-liher wall. e g., loaded with immobilized enzyme. 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

The membranes under study are thin-film composite membranes composed of two layers as illustrated in Fig. 3 a thin polyamide film as active layer and a large mesoporous polysulphone as the support layer. The three studied membranes are 2 NF membranes, noted NF90, NF270 and a low-polarization reverse osmosis (LPRO) membrane, noted BW30. All membranes were purchased from Filmtec (DOW, USA) the specifications of the membranes are given in Table 2. The chemical structures of the support and active layer materials are reported in Fig. 4 [86], Polyamide material is the more used but some authors have reported results... 

The performance of reverse osmosis is directly dependent on the properties of the membrane material.11 More specifically, the chemical nature of the membrane polymer and the structure of the membrane are what determines the rejection and flux properties... 

There is another type of membrane that is conceptually different from the membranes prepared according to the above methods. It is called dynamic membranes. They are formed, during application, on microporous carriers or supports by deposition of the colloidal particles or solute components that are present in the feed solution. This in-situ formation characteristic makes it possible to tailor them for specific applications in ultrafiltration and reverse osmosis (hyperfiltration). 

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