Salt rejection osmosis

In other areas, POD has been used to improve the wear resistance of a mbber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this appHcation a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Some data iEustrating the effect of pressure on the water and salt fluxes and the salt rejection of a good quaUty reverse osmosis membrane are shown ia Figure 34 (76). 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

Useful PRO membranes do not require the very high permselectivity necessary in reverse osmosis, and a trade-off between flux and salt rejection in conventional RO membranes is possible. If the salt rejection is too low, however. Internal concentration polarization due to excessive salt leakage can limit the water flux. 

Membrane Properties. The reverse osmosis performance of the bentonite-doped membrane under brackish water conditions is compared to that of the reference membrane in Figure 5 (I, reference membrane II, with organophilic bentonite). At low salt rejection the bentonite membrane again shows a higher initial flux than the reference membrane, the performance of the two becoming identical at the high rejection limit. 

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

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. 

If an asymmetric membrane is reversed, and the reverse osmosis experiments are carried out, the degree of salt rejection will be quite different from the results obtained for the normal experiments. In this case, the salt rejection is given by ... 

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 ... 

NS-300 Membrane. The NS-300 membrane evolved from an effort at North Star to form an interfacial poly(piperazine Isophthala-mide) membrane. Credali and coworkers had demonstrated chlorine-resistant poly(piperazineamide) membranes in the asymmetric form (20). The NS-lOO, NS-200, and PA-300 membranes were all readily attacked by low levels of chlorine in reverse osmosis feedwaters. In the pursuit of a chlorine-resistant, nonbiodegra-dable thin-fiim-composite membrane, our efforts to develop interfaclally formed piperazine isophthalamide and terephthalamide membranes were partially successful in that membranes were made with salt rejections as high as 98 percent in seawater tests. 

This membrane demonstrated a vastly improved flux compared with the poly(piperazine isophthalamide) membrane, but its seawater salt rejection was low — in the range of 60 to 70 percent. A reverse osmosis test with a magnesium sulfate feedwater showed greater than 99 percent salt retention, however, dispelling the possibility that low sodium chloride rejections were due to defects in the polyamide barrier layer. The piperazine polyamide was soon concluded to have the following structure (see Reaction 111). 

This thin-film-composite membrane has been found to have appreciable resistance to degradation by chlorine in the feed-water. Figure 2 illustrates the effect of chlorine in tap water at different pH values. Chlorine (100 ppm) was added to the tap water in the form of sodium hypochlorite (two equivalents of hypochlorite ion per stated equivalent of chlorine). Membrane exposure to chlorine was by the so-called "static" method, in which membrane specimens were immersed in the aqueous media inside closed, dark glass jars for known periods. Specimens were then removed and tested in a reverse osmosis loop under seawater test conditions. At alkaline pH values, the FT-30 membrane showed effects of chlorine attack within four to five days. In acidic solutions (pH 1 and 5), chlorine attack was far slower. Only a one to two percent decline in salt rejection was noted, for example, after 20 days exposure to 100 ppm chlorine in water at pH 5. The FT-30 tests at pH 1 were necessarily terminated after the fourth day of exposure because the microporous polysul-fone substrate had itself become totally embrittled by chlorine attack. 

Reverse osmosis is essentially the same process as ultrafiltration with low salt rejection. Kopfler and co-workers (12) used reverse osmosis in combination with solvent extraction and XAD adsorption. Solvent extraction with pentane and methylene chloride was used to remove organics from the reverse-osmosis concentrate (i.e., for desalting), and XAD was used for adsorption of the intractables from the extracted... 

Reverse osmosis membranes were also prepared from polyamides with pendant carboxamide groups 90). For example, 4,4 -diaminodiphenylmethane-3,3 -dicarbox-amide-isophthaloyl chloride copolymer 33 was dissolved in DMF containing LiCl, cast to 250 p thickness, dried at 100 °C for 15 min, and gelled in ice water to give a membrane with the water flux permeability of 900 1/m2 day and salt rejection of 80% (0.5% NaCl aqueous solution, 30 kg/cm2). After heating the membrane in... 

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

R.L. Riley, H.K. Lonsdale, C.R. Lyons and U. Merten, Preparation of Ultrathin Reverse Osmosis Membranes and the Attainment of Theoretical Salt Rejection, J. Appl. Polym. Sci. 11, 2143 (1967). 

A number of membrane materials and membrane preparation techniques have been used to make reverse osmosis membranes. The target of much of the early work was seawater desalination (approximately 3.5 wt% salt), which requires membranes with salt rejections of greater than 99.3 % to produce an acceptable permeate containing less than 500 ppm salt. Early membranes could only meet... 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

The goal of most of the early work on reverse osmosis was to produce desalination membranes with sodium chloride rejections greater than 98 %. More recently membranes with lower sodium chloride rejections but much higher water permeabilities have been produced. These membranes, which fall into a transition region between pure reverse osmosis membranes and pure ultrafiltration membranes, are called loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes. Typically, nanofiltration membranes have sodium chloride rejections between 20 and 80 % and molecular weight cutoffs for dissolved organic solutes of 200-1000 dalton. These properties are intermediate between reverse osmosis membranes with a salt rejection of more than 90 % and molecular weight cut-off of less than 50 and ultrafiltration membranes with a salt rejection of less than 5 %. 

Figure 9.4 Reverse osmosis membrane salt rejection as a function of temperature. Assumes constant applied feed pressure and less than 45°C temperature.

Figure 9.7 Reverse osmosis membrane salt rejection as a function of pressure.

Reverse Osmosis recovery affects the overall water flux and salt rejection as shown in Figures 9.10 and 9.11 respectively. As the recovery increases, the water flux decreases slowly until the recovery is so high that the osmotic pressure of the feed water is as high as the applied pressure, in which case, the driving force for water through the membrane is lost and the flux ceases. 

Ultraviolet graft polymerization of arylamide onto cellulose acetate reverse osmosis membranes yielded grafted membranes with higher salt retention and lower water flux compared with pristine cellulose acetate [152]. Acid-catalyzed grafting of styrene on cellulose acetate reverse-osmosis membranes imparted a higher salt rejection rate (92.4%) to the membrane than those of ungrafted membranes (80.8%) and heat-shrunk membranes (90.2%) [153]. 

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