Adsorption and Ion-Exchange

In mass transfer operations involving a solid porous phase, the resistance in the majority of cases resides predominantly in the solid phase. This brings 

Conservative Parameter Values for Batch Extraction Efficiencies of E 0.8 

In principle, mass transfer in solid particles is distributed in both time and distance, calling for the use of a PDE (Pick s equation) for a rigorous description of the process. In an elegant study conducted in the 1950s, it was shown by Glueckauf that the results of the formal treatment can be approximated by a volumetric solid-phase mass transfer coefficient, given by 

The amount of adsorption is limited by the available surface and pore volume, and depends also on the chemical natures of the fluid and solid. The rate of adsorption also depends on the amount of exposed surface but, in addition, on the rate of diffusion to the external surface and through the pores of the solid for accessing the internal surface which comprises the bulk of the surface. Diffusion rates depend on temperature and differences in concentration or partial pressures. The smaller the particle size, the greater is the utilization of the internal surface, but also the greater the pressure drop for flow of bulk fluid through a mass of the particles. 

In ion exchange equipment, cations or anions from the fluid deposit in the solid and displace equivalent amounts of other ions from the solid. Suitable solids are not necessarily porous the ions are able to diffuse through the solid material. A typical exchange is that of H + or OH ions from the solid for some undesirable ions in the solution, such as Ca++or SOa. Eventually all of the ions in the solid are replaced, but the activity is restored by contacting the exhausted solid with a high concentration of the desired ion. for example, a strong acid to replace lost hydrogen ions. 

For economic reasons, saturated adsorbents and exhausted ion exchangers must be regenerated. Most commonly, saturation and regeneration are performed alternately and intermittently, but equipment can be devised in which these processes are accomplished continuously by countercurrent movement of the solid and fluid streams. Only a few such operations have proved economically feasible. The UOP and Toray processes for liquid adsorption are not true continuous processes but are effectively such. 

Adsorption and ion exchange are considered together because they share many common features, including 

The use of instrumental chromatographic methods was described earlier in the role of hyphenated or hybridized techniques when combined with infrared spectroscopy. In this case, the chromatographic front-end acts as a sophisticated sample preparation system for isolation of specific chemical species in a mixture. In cases in which a particular component needs to be removed or separated, it is not necessary to resort to an instrumental method. In such cases, the sample may be passed through a simple column containing the solid-separation phase. A convenient approach for liquids is to prepare small columns of adsorbent or ion-exchange resin in a Pasteur or dropper pipette. This is ideal as a method for sample cleanup or for selectively removing contaminants or specific chemical components. 

In such cases, the eluted material can be analyzed directly and can be compared to the sample prior to elution. If a specific additive or chemical compound is removed by the solid-phase material, then its spectrum may be generated by subtraction of the spectra recorded before and after the column separation. Alternatively, the separated component may be eluted from the column by the use of an appropriate solvent. 

This approach is not limited to liquids, and solid-phase adsorbents may be used for selective removal of components from gas streams. In cases in which molecular sieve compounds are used, the separated components may be studied following thermal desorption of the adsorbed component(s). A variant of this approach can be used for the analysis of particulates or dust in gas streams (or in the environment). In this case, the sample is drawn through a suitable membrane. Membranes made from materials such as PVC have sufficiently good infrared transmission so that separated components may be measured directly on the filter material. Membrane separations may also be applied to liquid systems. One convenient approach is to use a porous silver membrane and to measure any separated components (particulates or other insoluble matter) retained on the surface of the membrane directly via a reflection measurement. 

If appropriate adsorbents are chosen, e.g. activated charcoal, aluminium 

Adsorption is carried out either with the so-called batch method or by filtration through a column. It is possible to conduct fractional desorption in many cases, for example by using acids or organic solvents. Certain substances in water can also be concentrated by means of ion exchange resins. By using an appropriate ion exchanger, e.g. cation or anion exchangers of certain types, it is possible not only to separate cations 

Ion exchange is also a good method of determining the total dissolved mineral substances in a water (see Section 3.1). 

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T. Vermeulen, M. D. LeVan, N. K. Hiester, and G. Kleia, "Adsorption and Ion Exchange," Section 16 of Perry s Chemical Engineers Handbook, 6th ed., McGraw-Hill Book Co., New York, 1984. 

Giorgio Carta, Ph.D., Professor, Department of Chemical Engineering, University of Virginia Member, American Institute of Chemical Engineers, American Chemical Society, International Adsorption Society (Section 16, Adsorption and Ion Exchange)... 

Vermeuleu, LeVau, Hiester, and Klein, Adsorption and Ion Exchange in Perry, R. H. and Green, D. W. (eds.), Peny s Chemical Engineeis Handbook (6th ed.), McGraw-Hill, New York, 1984. 

Adsorption and ion exchange share so many common features in regard to apphcation in batch and fixed-bed processes that they can be grouped together as sorption for a unified treatment. These processes involve the transfer and resulting equilibrium distribution of one or more solutes between a fluid phase and particles. The partitioning of a single solute between fluid and sorbed phases or the selectivity of a sorbent towards multiple solutes makes it possible to separate solutes from a bulk fluid phase or from one another. 

Table 16-1 classifies sorption operations by the type of interaction and the basis for the separation. In addition to the normal sorption operations of adsorption and ion exchange, some other similar separations are included. Applications are discussed in this section in Process Cycles. ... 

Many models have been proposed for adsorption and ion exchange equilibria. The most important factor in selecting a model from an engineering standpoint is to have an accurate mathematical description over the entire range of process conditions. It is usually fairly easy to obtain correcl capacities at selected points, but isotherm shape over the entire range is often a critical concern for a regenerable process. 

Separation Factor By analogy with the mass-action case and appropriate for both adsorption and ion exchange, a separation factor / can be defined based on dimensionless system variables [Eq. (16-10)] by... 

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