ADSORPTION AND ION EXCHANGE S preferential …

1 15 ADSORPTION AND ION EXCHANGE eparation of the components of a fluid can be effected by contacting them with a solid that has a preferential attraction for some of them. Such S processes are quantitatively significant when the specific surfaces of the solids are measured in hundreds of m*/g. Suitable materials are masses of numerous fine pores that were generated by expulsion of volatile substances. The most important adsorbents are activated carbon, prepared by partial volatilization or combustion of a carbonaceous body, and activated alumina, silica gel, and molecular sieves which are all formed by expulsion of water vapor from a solid.

2 The starting material for silica gel is a coagulated silicic acid and that for molecular sieves is hydrated aluminum silicate crystals that end up as porous crystal structures. Porous glasses made by leaching with alkai have some application in chromatography. Physical properties of common adsorbents are listed in Tables and Representative manufacturing processes are represented on Figure 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.

3 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 SO.

4 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.

5 Temperature, or reducing the pressure, or by washing with a suitable reagent. The desorbed material may be recovered as valuable product in concentrated form or as a waste in easily disposable form. Adsorbent carbons used for water treating often must be regenerated by ignition in a furnace. Relatively small amounts of adsorbents that are difficult to regenerate are simply discarded. Desorption is accomplished by elevating the ADSORPTION EQUILIBRIA The amount of adsorbate that can be held depends on the concentration or partial pressure and temperature, on the chemical nature of the fluid, and on the nature, specific surface, method of preparation, and regeneration history of the solid.

6 For single adsorbable components of gases, the relations between amount adsorbed and the partial pressure have been classified into the six types shown in Figure Many common systems conform to Type I, for example, some of the curves of Figure ADSORPTION data are not highly reproducible because small contents of impurities and the history of the adsorbent have strong influences on their behavior. One of the simplest equations relating amount of ADSORPTION and pressure with some range of applicability is that of Freundlich, w =UP" ( ) and its generalization for the effect of temperature w=aP"exp(-b/T).

7 ( ) The exponent n usually is less than unity. Both gas and liquid ADSORPTION data are fitted by the Freundlich isotherm. Many liquid data are fitted thus in a compilation of Landolt-Bornstein (11/3, Numerical Data and Functional Relationships in Science and Technology, Springer, New York, 1956, pp. 525-528), but their gas data are presented in graphical form only (LB IV 4/b, 1972, pp. 121-187). The effect of temperature also is correlated by a theory of Polanyi, whereby all data of a particular system fall on the same curve; Figure is an example. For isothermal data, a combination of the Freundlich and Langmuir equations was developed by Yon and Turnock (Chem.)

8 Eng. Prog. Symposium Series 117, 67, 1971): w = kP"/(l+ kP"). ( ) Individuals of multicomponent mixtures compete for the limited space on the adsorbent. Equilibrium curves of binary mixtures, when plotted as x vs. y diagrams, resemble those of vapor-liquid mixtures, either for gases (Fig. ) or liquids (Fig. ). The shapes of ADSORPTION curves of binary mixtures, Figure , are varied; the total adsorptions of the components of the pairs of Figure would be more nearly constant over the whole range of compositions in terms of liquid volume fractions rather than the mol fractions shown. Higher molecular weight members of homologous series adsorb preferentially on some adsorbents.

9 The desorption data of Figure attest to this, the hydrogen coming off first and the pentane last. In practical cases it is not always feasible to allow sufficient time for complete removal of heavy constituents so that the capacity of regenerated adsorbent becomes less than that of fresh, as Figure indicates. Repeated regeneration causes gradual deterioration 495 TABLE Physical Properties of Adsorbents External Effective Bulk Void External Specific Reactivation Particle Mesh Diameter Density Fraction Surface Heat Tempera! ure Form* Size D1,, ft. pb, F, a,, C,, Btullb OF OF Examples A Activated Carbon.

10 P 4x6 30 310 20@-1000 Columbia L I, *I P 6x8 30 446 P 8X 10 30 645 G 4x 10 30 460 G 6X 16 30 720 G 4X 10 28 450 G 6X 16 45 720 S 4x8 50 300 Activated G 4x8 52 380 G 8X 14 52 480 G 14x28 54 970 S (1/4*) 52 200 S (1/8") 54 400 Molecular . G 14 x 28 30 970 P (1/16') 45 650 P (1/8") 45 400 S 8X 12 45 565 I* ., Pittsburgh BPL Witco 256 Davison 03 ., I, ,I I. ,I I, 250-450 300-450 Mobil Sorbead R Silica .. G 3x8 45 230 II 1, Alcoa Type F 350-600 ,I *I I. ,* I, I. ,I I, I, I. 350-1000 Alcoa Type H 300-600 Davison. Linda *# It I. I. ,. I..I P.