Reverse osmosis membrane principle

Not all membranes and membrane structures are covered by the classification given in figure III - 1. This approach is used for the sake of simplicity so chat the basic principles can be understood more readily. There is distinct transition from one type to the other. Reverse osmosis membranes, for example, can be considered as being intermediate between porous and nonporous membranes. 

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

The above equation indicates that the selectivity of the composite membrane is controlled by a barrier component whose resistance is far greater than the other. This is the principle underlying the development of the composite reverse osmosis membrane where an active surface layer is supported by a porous support layer (Figure 5.7). The resistance to the permeant flow is contributed almost... 

Complete ion removal is also offered by reverse osmosis, in principle, although the permeation of ions through even the tightest membrane is not zero, so there will be a finite, if very low, metal salt content in the purified water. If extremely low salt contents are required, then the most cost-effective method is probably reverse osmosis followed by a deionization process. 

Membrane Filtration. Membrane filtration describes a number of weU-known processes including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, and electro dialysis. The basic principle behind this technology is the use of a driving force (electricity or pressure) to filter... 

An alternative method of purifying water is by reverse osmosis. Under normal conditions, if an aqueous solution is separated by a semi-permeable membrane from pure water, osmosis will lead to water entering the solution to dilute it. If, however, sufficient pressure is applied to the solution, i.e. a pressure in excess of its osmotic pressure, then water will flow through the membrane from the solution the process of reverse osmosis is taking place. This principle has been... 

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. 

In bioprocesses, a variety of apparatus that incorporate artificial (usually polymeric) membranes are often used for both separations and bioreactions. In this chapter, we shall briefly review the general principles of several membrane processes, namely, dialysis, ultrafiltration (UF), microfiltration (MF), and reverse osmosis (RO). 

We can use the same filtration principle for the separation of small particles down to small size of the molecular level by using polymeric membranes. Depending upon the size range of the particles separated, membrane separation processes can be classified into three categories microfiltration, ultrafiltration, and reverse osmosis, the major differences of which are summarized in Table 10.2. 

In principle, pollution control should be a major application for reverse osmosis. In practice, membrane fouling, causing low plant reliability, has inhibited its widespread use in this area. The most common applications are special situations... 

X. J. Chai, G. H. Chen, P. L. Yue, and Y. L. Mi, Pilot scale membrane separation of electroplating waste water by reverse osmosis. Journal of Membrane Science 123,235-242 (1997). J. D. Seader and E. J. Henley, Separation Process Principles, John Wiley Sons, New York, 1998. 

FIGURE 5.4 Principles of operation of two types of membrane water desalination units reverse osmosis and electrodialysis. (From Pryde [22], and reprinted courtesy of Cummings Publishing Co.)... 

The field of membrane separations is radically different from processes based on vapor-liquid or fluid-solid operations. This separation process is based on differences in mass transfer and permeation rates, rather than phase equilibrium conditions. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of multi-component membrane separation are discussed for membrane modules in various flow patterns. Several applications are considered, including purification, dialysis, and reverse osmosis. 

The study of gas transport in membranes has been actively pursued for over 100 years. This extensive research resulted in the development of good theories on single gas transport in polymers and other membranes. The practical use of membranes to separate gas mixtures is, however, much more recent. One well-known application has been the separation of uranium isotopes for nuclear weapon production. With few exceptions, no new, large scale applications were introduced until the late 1970 s when polymer membranes were developed of sufficient permeability and selectivity to enable their economical industrial use. Since this development is so recent, gas separations by membranes are still less well-known and their use less widespread than other membrane applications such as reverse osmosis, ultrafiltration and microfiltration. In excellent reviews on gas transport in polymers as recent as 1983, no mention was made of the important developments of the last few years. For this reason, this chapter will concentrate on the more recent aspects of gas separation by membranes. Naturally, many of the examples cited will be from our own experience, but the general underlying principles are applicable to many membrane based gas separating systems. 

Most importantly non-porous membranes such as ion exchange membranes, membranes for reverse osmosis, pervaporation, etc. should not be used in systems in which insoluble compounds precipitate on and in the membranes because this will destroy them and their functionality will be lost. Secondly all separation membranes, including ion exchange membranes, can achieve excellent performance by use of an appropriate apparatus and under optimum operation. For example, because solute and solvent transport speeds in the membrane phase are different from those in the solution, membrane-solution interfaces play an important role in separation, which depends on the structure of the apparatus and its operation. In this chapter, many examples of applications of ion exchange membranes are explained together with the principles on which they rely to achieve separation. 

In this chapter, membrane filtration in water treatment is reviewed. The aim is to assess the current status and reveal gaps in knowledge firom the wealth of literature. The background on models and principles is summarised for the relevant processes microfiltration (MF), ultrafiltration fUFJ, and nanofiltration (NF). Reverse osmosis is brifily considered to put NF, which is often described as a process "in between" UF and RO, in perspective. 

While reverse osmosis and ultrafiltration were being established in several applications, there was a lack of available membranes with cutoffs between 400 and 4000 g/mol. Increasing interest in NF membranes developed in the last decade. An extensive review on principles and applications of nanofiltration has been published recently [38]. Nanofiltration is important for water softening [39] and removal of organic contaminants. In the food industry, nanofiltration can be applied for concentration and demineralization of whey, concentration of sugar and juice. Nanofiltration also finds application in the pulp and paper industry, in the concentration of textile dye effluents and in landfill leachate treatment. The improvement of solvent stabihty of available NF membranes opens a wide range of potential applications in the chemical and pharmaceutical industry as weU as in metal and acid recovery. 

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