Reverse Osmosis Section

Once the pretreatment study had been completed, it will be possible to decide on the type of elements to be used in the reverse osmosis unit. If the SDI of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine fiber elements can be used. The choice will depend on economics (element price) and desalination characteristics (flux and rejection). If the pretreated feed SDI is more than 3.0, then the spiral wound element should be used. When the decision as to element type is made, then it is appropriate to forward a copy of the pretreated feed water analysis to reverse osmosis element manufacturers to obtain a prediction of product water quality, recommended type of element, total number of elements required, possible problems with sparingly soluble compounds in the feedwater, allowable recovery, and price and delivery. 

The quality of the product water is a function of the rejection as shown in the following equation  

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  

The average concentration of the feed and reject streams as represented by the following equation is not truly representative  

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Chemical treatment is applicable usually to raw, mains and softened water, but is also used to treat the storage and distribution systems of distilled and deionized water and of water produced by reverse osmosis (section 3.5). 

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. 

Section 4.15 describes membranes and introduces a range of membrane separation options. Molecular geometry is exploited in separations of gases via gas permeation. Section 4.16. Dialysis and electrodialysis are considered in Sections 4.17 and 4.18 respectively. Other methods to separate species in liquids are given in Section 4.19, pervaporation Section 4.20, reverse osmosis Section 4.21, for nanofiltration Section 4.22, for ultrafiltration Section 4.23, for microfil-tration and Section 4.24 for chromatographic separations. Separations of larger sized species are considered heterogeneous systems and are considered in Chapter 5. 

Consider now a porous membrane for the process of ultrafiltration separation of a protein solution (Section 3.4.2.S). Suppose the regions on the two sides of the membrane have become filled with liquid (from the feed region 1 to product region 2). If the membrane does not reject the protein (a macrosolute) completely, the solute protein will slowly diffuse from region 1 to region 2. Ultimately, both sides will have the same protein concentration. The separation achieved initially will be lost Therefore, in practice, it is necessary to have an open system so that any permeate appearing from the feed side into the permeate side is immediately withdrawn. The same eugument is equally valid if the membrane in reverse osmosis (Section 3.4.2.1) has some permeability of the microsolute present in the feed side. 

Distillation is implemented almost universally for the recovery of higher concentrations of volatile bioproducts such as alcohol, acetone and butanol. However, this step consumes almost 50% of the energy cost of the alcohol production plant (Shuler and Kargi, 2002). As a result, lower energy consuming techniques, such as pervaporation (Section G.3.3.4) (Thongsukmak and Sirkar, 2007), reverse osmosis (Section 7.2.1.2), etc., are being explored. On the... 

Fig. 10. Composite hoUow-fiber membranes (a) polysulfone boUow fiber coated witb fiiran resin. A and B denote fiiran resin surface and porous support, respectively (b) cross section of composite boUow fiber (PEI/TDI coated on polysulfone matrix). C, D, and E denote tightly cross-linked surface, "gutter" gel layer, and porous support, respectively. Both fibers were developed for reverse osmosis appHcation (15).

In most industrial applications, it is rare that a single RO module can be used to address the separation task. Instead, a reverse-osmosis network (RON) is employed. A RON is composed of multiple RO modules, pumps and turbines, llie following sections describe the problem of synthesizing a system of RO modules and a systematic procedure for designing an optimal RON. Once a RON is synthesized, it can be incorporated with a mass integration framework (see Problem 11.6). 

Additional water purification steps, such as reverse osmosis may be required (Section 8.17), depending on the source and condition of the water. 

Figure 13. Cross-section of the skin zone of a reverse osmosis membrane consisting of cellulose acetate-polyfbromophenylene oxide phosphonate)...

Figure 18. Cross-section scheme of a composite reverse osmosis membrane...

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

As we discussed in Section 3.2, samples of solution and solvent separated by a semipermeable membrane will be at equilibrium only when the solution is at a greater pressure than the solvent. This is the osmotic pressure. If the solution is under less pressure than the equilibrium osmotic pressure, solvent will flow from the pure phase into the solution. If, on the other hand, the solution is under a pressure greater than the equilibrium osmotic pressure, the pure solvent will flow in the reverse direction, from the solution to the solvent phase. In the last case, the semipermeable membrane functions like a filter that separates solvent from solute molecules. In fact, the process is referred to in the literature by the terms hyperfiltration and ultrafiltration, as well as reverse osmosis (Sourirajan 1970) however, the last term is enjoying common use these days. 

One can also recognize that application of sufficient pressure (above the equilibrium osmotic pressure n) to the right-hand chamber in (7.67) must cause the solvent flow to reverse, resulting in extrusion of pure solvent from solution. This is the phenomenon of reverse osmosis, an important industrial process for water desalination. Reverse osmosis is also used for other purification processes, such as removal of H20 from ethanol beyond the azeotropic limit of distillation (Section 7.3.4). Reverse osmosis also finds numerous applications in wastewater treatment, solvent recovery, and pollution control processes. 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

Hollow fiber membranes are primarily homogeneous. In use, their lower permeability is compensated for by large surface per unit volume of vessel. Fibers are 25-250 pm outside dia, wall thickness 5-50 pm. The cross section of a vessel for reverse osmosis may have 20-35 million fibers/sqft and a surface of 5500-9000 sqft/cuft of vessel. Recently developed hollow fibers for gas permeation processes have anisotropic structures. 

In this section the solution-diffusion model is used to describe transport in dialysis, reverse osmosis, gas permeation and pervaporation membranes. The resulting equations, linking the driving forces of pressure and concentration with flow, are then shown to be consistent with experimental observations. 

Figure 3.42 Exploded view and cross-section drawings of a spiral-wound module. Feed solution passes across the membrane surface. A portion passes through the membrane and enters the membrane envelope where it spirals inward to the central perforated collection pipe. One solution enters the module (the feed) and two solutions leave (the residue and the permeate). Spiral-wound modules are the most common module design for reverse osmosis and ultrafiltration as well as for high-pressure gas separation applications in the natural gas industry...

The cross-, co- and counter-flow schemes are illustrated in Figure 4.17, together with the concentration gradient across a median section of the membrane. It follows from Figure 4.17 that system performance can be improved by operating a module in an appropriate flow mode (generally counter-flow). However, such improvements require that the concentration at the membrane permeate surface equals the bulk concentration of the permeate at that point. This condition cannot be met with processes such as ultrafiltration or reverse osmosis in which the permeate is a liquid. In these processes, the selective side of the membrane faces the... 

Reverse osmosis also serves some of the waste management and resource recovery needs in the metals and metal finishing industry. Effluent streams from mining and plating operations containing heavy metals, acids, and other chemicals can be treated with reverse osmosis to recover both the metal as its salt, and purified water for reuse. For metal ion recovery from dilute solutions, however, reverse osmosis faces competition from conventional solvent extraction, membrane-based solvent extraction, and its variant, coupled transport (see Section V.F.3). 

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