Reverse osmosis description

Transport Models. Many mechanistic and mathematical models have been proposed to describe reverse osmosis membranes. Some of these descriptions rely on relatively simple concepts others are far more complex and require sophisticated solution techniques. Models that adequately describe the performance of RO membranes are important to the design of RO processes. Models that predict separation characteristics also minimize the number of experiments that must be performed to describe a particular system. Excellent reviews of membrane transport models and mechanisms are available (9,14,25-29). 

M. Williams, "Measurement and Mathematical Description of Separation Characteristics of Ha2ardous Organic Compounds with Reverse Osmosis Membranes," dissertation. University of Kentucky, Lexiagton, Ky., 1993. 

Retention Rejection and Reflection Retention and rejection are used almost interchangeably. A third term, reflection, includes a measure of solute-solvent coupling, and is the term used in irreversible thermodynamic descriptions of membrane separations. It is important in only a few practical cases. Rejection is the term of trade in reverse osmosis (RO) and NF, and retention is usually used in UF and MF. 

Process Description Reverse osmosis (RO) and nanofiltration (NF) processes utilize a membrane that selectively restricts flow of solutes while permitting flow of the solvent. The processes are closely related, and NF is sometimes called loose RO. They are kinetic processes, not equilibrium processes. The solvent is almost always water. 

Membrane filtration processes, such as reverse osmosis, and micro and ultra filtration, are used to filter out dissolved solids in certain applications see Table 10.9. These specialised processes will not be discussed in this book. A comprehensive description of the techniques used and their applications is given in Volume 2, Chapter 8 see also Scott and Hughes (1995), Cheryan (1986), McGregor (1986) and Porter (1997). 

The structure of the so-called "composite" membranes used in reverse osmosis is also much more complex than the conventional, simplistic description of the ultrathin semipermeable film deposited on and supported by a porous substrate. Most of these membranes which exhibit high flux and separation are composed of an anisotropic, porous substrate topped by an anisotropic, ultrathin permselective dense layer which is either highly crosslinked, or exhibits a progressively decreased hydrophilicity toward the surface. The basic difference between the conventional anisotropic (asymmetric) membrane and the thin film composite is that the latter might be... 

The mathematical description of this process is identical to that of reverse osmosis given in Equations (2.37) and (2.44) and leads to expressions for the solute and solvent fluxes... 

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

Figure 4.4 Salt concentration gradients adjacent to a reverse osmosis desalination membrane. The mass balance equation for solute flux across the boundary layer is the basis of the film model description of concentration polarization...

For truly high rejection reverse osmosis membranes, the solution-diffusion description of this process is the most popular and probably the most realistic. In this case, the high osmotic pressure difference between the... 

Process Descriptions Selectively permeable membranes have an increasingly wide range of uses and configurations as the need for more advanced pollution control systems are required. There are four major types of membrane systems (1) pervaporation (2) reverse osmosis (RO) (3) gas absorption and (4) gas adsorption. Only membrane pervaporation is currently commercialized. 

Sucrose in a concentration of 1,000 mg/L has an osmotic pressure of 7.24 kNa (kiloNewtons absolute). Thus, the reverse pressure to be applied must be, theoretically, in excess of 7.24 kNa for a sucrose concentration of 1,000 mg/L. For NaCl, its osmotic pressure in a concentration of 35,000 mg/L is 2744.07 kNa. Hence, to reverse the flow in a NaCl concentration of 35,000 mg/L, a reverse pressure in excess of 2744.07 kNa should be applied. The operation just described (i.e., applying sufficient pressure to the tip of the tube to reverse the flow of water) is the fundamental description of the basic reverse osmosis process. 

Bourns, W.T. and Le, V.T., The Reverse Osmosis Plant in CRNL Waste Treatment Centre-Description Design and Operation Principles, Report CRNL-2352, Atomic Energy of Canada Ltd., Chalk River, 1984. 

REVERSE OSMOSIS PILOT PLANT 41.3.1 Description of the Plant... 

It is to this topic of solute preferential sorption in reverse osmosis that this paper is dedicated. Specifically, this discussion will involve a description of solute preferential sorption, an overview of the literature in the area, and finally a presentation of some recent work on the removal of aromatic hydrocarbons from water. The significance of this work is at least two-fold. From a practical point of view the classes of solutes which demonstrate preferential attraction to the membrane material tend to be organic compounds and the removal and recovery of these solutes from water is environmentally and economically important. From a theoretical point of view an understanding of the phenomena involved is essential to the achievement of a fundamental description of the RO process. Although this paper deals solely with aqueous solutions and cellulose acetate membranes, it Is important to recognize that the concepts discussed can be extended to Include other membrane materials and non-aqueous systems. 

In conclusion, several Important points of this work should be reiterated. An understanding and quantitative description of solute preferential sorption Is Imperative to the advancement of a fundamental knowledge of the separation mechanism and to the application of reverse osmosis. For the systems studied,... 

Cake filtration could be used for some of these materials, but a cake of l-/rm particles would have a high resistance to flow, and the filtration rate would be very low. Ultrafiltration (UF) covers a wider size range, from 1-pm particles down to molecules about 10 /rm in size (Af= 300). The term hyperfiltration is sometimes used for separation of small molecules or ions, but reverse osmosis is a more descriptive term, because the osmotic pressure has a major effect on the flux. Furthermore, the separation in reverse osmosis occurs by a solution-diffusion mechanism in the dense polymer rather than by a screening action at the membrane surface (see Chap. 26). 

Description Reverse osmosis Nanofiltration Ultrafiltration Microfiltration Macrofiltration... 

Reverse osmosis units are used for molecular separations such as the removal of salt from seawater. Nanofiltration is another variant of membrane filtration whose separation characteristics fall between reverse osmosis and ultrafiltration. Their description is beyond the scope of this text. 

The description given here can be applied in general. However, a distinction must be made for pressure-driven processes such as microfiltration, ultrafiliration and reverse osmosis. Here the feed consists of a solvent (usually water) and one or more solutes. In general, the concentration of the solute(s) is low and the separation characteristics of the membrane are always related to the solute(s). On the other hand, in liquid separation (pervaporation) and gas separation the terms solvent and solute are best avoided. 

As can be seen from the above description, there are many variables involved in the phase-inversion technique. Among others the composition of the polymer solution, the solvent evaporation temperature and evaporation period, the nature and the temperature of the gelation media, and the heat treatment temperature are the primary factors affecting the reverse osmosis performance of the membrane. When polymers other than cellulose acetate are used, solvents and nonsolvent additives appropriate to prepare membranes from the particular polymer must be found. Depending on the combination of variables, membranes of different polymeric materials with different pore sizes can be prepared. 

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