Reverse osmosis governing

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

From the point of view of the science of reverse osmosis, the fundamental question is "what governs reverse osmosis separations ". This is an intensely practical question because, to the extent this question is answered correctly, precisely, and completely, to that extent - and, to that extent only - the applications and technology of reverse osmosis can be made effective. Further, this overriding question becomes specially significant when one considers the obvious potential applications of reverse osmosis, and their immense social relevance in the context of today. 

According to the above mechanism, reverse osmosis separation is governed by two distinct factors, namely (i) an equilibrium effect which is concerned with the details of preferential sorption in the vicinity of the membrane surface, and (ii) a kinetic effect which is concerned with the mobilities of solute and solvent through membrane pores. While the former (equilibrium effect) is governed by repulsive and attractive potential gradients in the vicinity of the membrane surface, the latter (mobility effect) is governed both by the potential gradients (equilibrium effect) and the steric effects associated with the structure and size of molecules relative to those of pores on the membrane surface. 

The solute-solvent-polymer (membrane material) interactions, similar to those governing the effect of structure on reactivity of molecules (20,21,22,23,24) arise in general from polar-, sterlc-, nonpolar-, and/or ionic-character of each one of the three components In the reverse osmosis system. The overall result of such interactions determines whether solvent, or solute, or neither is preferentially sorbed at the membrane-solution Interface. 

The parameters AVg (acidity), AVg (basicity), pK, and Zo represent properties of solute in the bulk solution phase. If reverse osmosis separation is governed by the property of solute in the membrane-solution interface, the existence of unique correlations between data on reverse osmosis separations and those on the above parameters, means that the property of solute in the bulk solution phase and the corresponding property of solute in the membrane-solution interface are also uniquely related. This leads one to the development of interfacial free energy parameters (-AAG/RT) for both nonionized solute molecules and dissociated ions in solution for reverse osmosis systems where water is preferentially sorbed at the membrane-solution interface. 

The relevance of LSC data to reverse osmosis stems from the physicochemical basis (adsorption equilibrium considerations) of liquid-solid chromatography (52), and the principle that the solute-solvent-membrane material (column material) Interactions governing the relative retention times of solutes in LSC are analogous to the interactions prevailing at the membrane-solution Interface under reverse osmosis conditions. The work already reported in several papers on the subject (53-58) indicate that the foregoing principle is valid, and hence LSC data offer an appropriate means of characterizing interfacial properties of membrane materials, and understanding solute separations in reverse osmosis. 

Materials Science of Reverse Osmosis Membranes - Factors Governing Porous Structure of Membranes... 

All symbols are defined at the end of the paper. Equation 10 defines the pure water permeability constant A for the membrane which is a measure of its overall porosity eq 12 defines the solute transport parameter D /K6 for the membrane, which is also a measure of the average pore size on the membrane surface on a relative scale. The Important feature of the above set of equations is that neither any one equation in the set of equations 10 to 13, nor any part of this set of equations is adequate representation of reverse osmosis transport the latter is governed simultaneously by the entire set of eq 10 to 13. Further, under steady state operating conditions, a single set of experimental data on (PWP), (PR), and f enables one to calculate the quantities A, Xy 2> point... 

The economics of Reverse Osmosis Process will be highly favourable provided the desalination industry is taken up in a big way bringing down the capital investment. Water management and distribution particularly the water supply in the rural areas must be given top priority and should be under the direct control of central and federal government agencies and in this endeavour reverse osmosis has a potential... 

The ionic concentration to be treated is an overriding consideration governing the cost of plant of a given design and therefore for very high ion concentrations it is foreseeable that membrane pretreatments such as reverse osmosis and electrodialysis will continue to fulfil an important role as might a more widespread revival of continuous countercurrent ion exchange. 

Estimation of Interfacial Forces Governing the Reverse-Osmosis System Nonionized Polar Organic Solute-Water-Cellulose Acetate Membrane... 

Pure water can be obtainnd from brackish water by permention through a reverse osmosis membrane. Consider the stendy laminar flow of a salt solution in a thin channel between two walls composed of a membrane that rejects salt. Derive the governing equations for the salt distribution in die transverse direction for a given water peuneaiion flux (see Fig. 2.2-4),... 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

Ion exchange is an adsorptive process governed by equilibration. However, separation between ions and water may be effected also by a diffusive process through ionically charged membranes, as in the case of dialytic water softening24 or electrodialysis44. Membranes of this kind may also be used for desalination by reverse osmosis, where a combined mechanism of coulombic ion-exclusion and water diffusion under pressure is involved. 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

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