Reverse osmosis elements membrane

An industrial reverse osmosis plant usually will consist of three separate sections which are shown in Figure 4.2. The first section is the pretreatment section in which the feedwater is treated to meet the requirements of reverse osmosis element manufacturers and the dictates of good engineering practice. Following pretreatment, the feedwater is introduced into the reverse osmosis section where the feedwater is pressurized and routed to the reverse osmosis elements which are in pressure vessels. The feedwater flows across the membrane surface where product water permeates through the membrane and a predetermined amount remains behind as reject. The reject is discharged to waste while the product water is routed to the posttreatment section. The third or posttreatment section treats the product water to remove carbon dioxide and adds chemicals as required for industrial use of the product water. 

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. ... 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

Fig. 13. A hoUow-fibet reverse osmosis membrane element. Courtesy of DuPont Permasep. In this twin design, the feedwater is fed under pressure into a central distributor tube where half the water is forced out tadiaUy through the first, ie, left-hand, fiber bundle and thus desalted. The remaining portion of the feedwater flows through the interconnector to an annular feed tube of the second, ie, right-hand, fiber bundle. As in the first bundle, the pressurized feedwater is forced out tadiaUy and desalted. The product water flows through the hoUow fibers, coUects at each end of the element, and exits there. The concentrated brine from both bundles flows through the concentric tube in the center of the second bundle and exits the element on the right.

Peivaporation is a relatively new process that has elements in common with reverse osmosis and gas separation. In peivaporation, a liquid mixture contacts one side of a membrane, and the driving force for the process is low vapour pressure on the permeate side of the membrane generated by cooling and condensing the permeate vapour. The attraction of peivaporation is that the separation obtained is proportional to the rate of permeation of the components of the liquid mixture through the selective membrane. Therefore, peivaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult to separate by distillation... 

Spiral-wound reverse osmosis membrane element, 26 75 Spiramycin... 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

R.E. Larson, J.E. Cadotte and R.J. Petersen, The FT-30 Seawater Reverse Osmosis Membrane-element Test Results, Desalination 38, 473 (1981). 

Fig. 12. A spkal-wound reverse osmosis membrane element (a) schematic depiction (b) cross section of a spiral-wound thin-film composite RO Fikntec...

Properties of FT-30. The properties of FT-30 membranes have been reviewed in several publications. Therefore, only the salient features that relate to the chemistry of the barrier layer will be considered here. Reverse osmosis performance of FT-30 under seawater and brackish water test conditions was described by Cadotte et al (48) and by Larson et al (51). In commercially produced spiral-wound elements the FT-30 membrane typically gives 99.0 to 99.2 percent salt rejection at 24 gfd (40 L/sq m/hr) flux in seawater reverse osmosis tests with 3.5 percent synthetic seawater at 800 psi (5516 kPascaJJand 25°C. 

In 1969, I founded Osmonics, Inc. to carry the technology of reverse osmosis and ultrafiltration to the marketplace. We originally purchased membrane from Eastman Kodak Company and made our own spiral elements. We continued purchasing membrane until Kodak decided not to remain in the membrane business and we decided to begin the manufacture of membrane. By 1973, we were in full production manufacturing cellulose acetate membrane using the Loeb-Sourirajan approach. One year later, we were manufacturing polysulfone membrane for ultrafiltration. Last year, 1979, Osmonics manufactured over one million square feet of RO/UF membrane. 

However, with more stringent guidelines and since Whitestone was discharging Its waste after treatment directly to a stream, it was decided to install an RO system on the final aeration pond. A pilot test of approximately 1000 hours showed the usefulness of reverse osmosis and allowed Osmonics to specify the membrane elements and system layout with sufficient certainty to guarantee a reasonable life and operating characteristics to Whitestone. 

This equation produces accurate results for a membrane sample or a small element with a low recovery, e.g., 2% or less. However, a practical reverse osmosis system is designed to recover from 25 to 90% of the feedwater. This means that the concentration of the feed varies throughout the membrane system. At 90% recovery, the initial membranes will have a feed which is about 10 times less concentrated than the feed to the final membranes and the quality of the product water will vary incrementally throughout the system. The product water from the first membrane elements will be less concentrated than the product water from the last elements. The product water from the practical reverse osmosis system is combined in the product water manifold and its concentration is usually represented as the average product water concentration. The average product water concentration is determined by the following formula ... 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

The initial projections of 20 years ago have proven to be unrealistic in that reverse osmosis has not caused deserts to bloom, nor does every household contain a reverse osmosis unit to improve the tap water. Yet, the process has been of economic value in providing process water to industry, potable water to high income arid regions and a method of reclaiming municipal and industrial wastes. As of 1985, it was estimated that the worldwide market for reverse osmosis membrane elements (not total systems) was about 50 million. 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

Figure 5.16, adapted from Kurihara,79 80 shows a comparison of several types of commercial reverse osmosis membranes in terms of salt rejection and permeate flow rate under seawater test conditions (35,000 ppm, 800 psi, 25°C). This chart emphasizes the capability of PEC-1000 to provide complete single-stage seawater desalting. In a test at Toray s Ehime desalination test facility on 42,000 ppm seawater (equivalent to Red Sea salinity), PEC-1000 spiral elements operated at 35% recovery produced a permeate having an average salinity of only 220 ppm, well below WHO standards. Average salt rejection was 99.5%. 

Wydeven and his coworkers examined the plasma polymerization of several amine monomers and obtained the best reverse osmosis membranes using allyl-amine.90/91 Reverse osmosis performances of 98 to 99% salt rejection and 4 to 8 gfd flux were achieved at test conditions of 1.0% sodium chloride, 600 psi, 20 C. ESCA spectra showed the nitrogen groups in the plasma-formed polymer to be nitrile or imine groups, but not amine groups. Oxygen was also present in the ESCA analysis. Elemental analysis showed a membrane stoichiometry of C3H 3 8N 0.9O0.1. 

Yasuda obtained membranes with good reverse osmosis properties by polymerizing gas plasma mixtures of acetylene/water/nitrogen and acetylene/wa-ter/carbon monoxide.92 9J Elemental analysis of the acetylene/water/nitrogen membrane was not given, but it was likely approximately equivalent to the allyl-amine-derived composition. Yasuda has observed membrane performance levels as high as 99% salt rejection and 38 gfd, tested on 3.5% sodium chloride at 1,500 psi. 

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