ADSORPTION FROM LIQUIDS

Adsorption from liquids is less well understood than adsorption from gases. In principle the equations derived for gases ought to be applicable to liquid systems, except when capillary condensation is occurring. In practice, some offer an empirical fit of the equilibrium data. One of the most popular adsorption isotherm equations used for liquids was proposed by Freundlich 21-1 in 1926. Arising from a study of the adsorption of organic compounds from aqueous solutions on to charcoal, it was shown that the data could be correlated by an equation of the form  

C is the concentration of solute in solution in equilibrium with that on the solid. a2 and n are constants, the latter normally being greater than unity. 

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Fig. XI-11. Relation of adsorption from binary liquid mixtures to the separate vapor pressure adsorption isotherms, system ethanol-benzene-charcoal (n) separate mixed-vapor isotherms (b) calculated and observed adsorption from liquid mixtures. (From Ref. 143.)...

The same relation has been observed in adsorption from liquid solutions, but is not absolute. 

The data which are plotted as isotherms in the case of adsorption from liquid solutions on solid adsorbents are different in nature from those of gas (or vapor) adsorption on the same adsorbents. In fact, while the isotherm for adsorption of a single gas by a solid represents directly the quantity (weight or volume under standard conditions) of gas adsorbed per unit weight of the solid, the experimental measurement in adsorption from solution is the change in concentration of the solution which results from adsorption. The fact that a change in concentration is measured emphasizes that there are at least two components in the solution [13]. 

For 0.0015 < Re < 55, the Wilson-Geankoplis correlation has been used in adsorption from liquid phase (Perry and Green, 1999 Xiu and Li, 2000 Chen and Wang, 2004). 

In this chapter, we have so far discussed the adsorption of gases in solids. This section gives a brief description of the adsorption process from liquid solutions. This adsorption process has its own peculiarities compared with gas-solid adsorption, since the fundamental principles and methodology are different in almost all aspects [2,4,5], In the simplest situation, that is, a binary solution, the composition of the adsorbed phase is generally unknown. Additionally, adsorption in the liquid phase is affected by numerous factors, such as pH, type of adsorbent, solubility of adsorbate in the solvent, temperature, as well as adsorptive concentration [2,4,5,84], This is why, independently of the industrial importance of adsorption from liquid phase, it is less studied than adsorption from the gas phase [2],... 

Isotherms for the Description of Adsorption from Liquid Phase... 

Adsorption from liquid solution is almost a new world in comparison with adsorption from the gas phase the fundamental principles and methodology are different in almost all respects (Gregg, 1961). 

F. Riedo and E. Kovats, Adsorption from liquid mixtures and hquid chromatography, /. Chromatogr. 239 (1982), 1-28. 

H. L. Wang, U. Duda, and C. J. Radke, Solution adsorption from liquid chromatography,/. Colloid Interface Sci. 66 (1978), 152-165. 

Adsorption from liquid mixtures with H bonding between components. 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

Three carbon samples showing differences in pore structure are chosen to study the effect of porous texture on adsorption from liquid solutions. The benzene adsorption/desorption isotherms are applied to determine the properties of geometrical surface structure of investigated carbons. The liquid adsorption data are analyzed in terms of the theory of adsorption on heterogeneous solids. The relation between parameters of porous structure of the activated carbon samples and parameters of adsorption from the liquid phase is discussed. 

Earlier investigations presented in the papers [4,14] suggest a higher selectivity of adsorption in respect to preferentially adsorbed component for adsorbents containing narrower mesopores. Similar effect should be expected for microporous materials In order to study the influence of porosity type on adsorption from liquids the measurements of specific surface excess isotherms were performed for binary liquid mixtures benzene + n-heptane / carbon AC-2 and AC-4 benzene + 2-propanol / carbon AC-2 and AC-4. 

Richardeau et al. investigated thiophene adsorption from liquid solutions containing hydrocarbons over HFAU zeolites in a stirred batch system at room temperature.144 They found that the maximum number of thiophene molecules adsorbed per gram of zeolites is equal to their concentration of acidic sites and considered that the acidic sites are the adsorption sites. They further found that the presence of toluene causes a large decrease in the removal of thiophene, and when the concentration of thiophene is high (27.7 wt%), an acid-catalyzed condensation of thiophene occurs to form dimers, trimers, and tetramers, which remain trapped on the zeolite. They concluded that thiophene removal by adsorption on acidic zeolites could only be carried out from diluted solutions containing no olefinic compounds. 

Radlce, C. J., Prausnitz, J. M. Thermodynamics of multi-solute adsorption from liquid solutions, AIChE J., 1972, 18, 1, 761-768. 

Figure 7 shows an example of type IV isotherms for adsorption of trace water from toluene and p-xylene mixtures on Alcoa H-152 alumina at 22° C [22], The abcissa of the plot represents relative saturation of water (xi/x ), where x is the molar fraction of water at the solubility limit in the hydrocarbon liquid. Equilibrium isotherm models analogous to those used for pure water vapour adsorption can be derived for describing trace water adsorption from liquid mixtures [20-22],... 

Adsorption from liquid mixtures on solids underlies a number of extremely important processes and plays a significant role in many fields of the natural sciences. Studies of this phenomenon were reported in several monographies and reviews e.g. [1-3]. The formulation of a unified theory of adsorption from solutions is difficult because there are liquid mixtures with extremely different physicochemical properties. Usually, existing theoretical descriptions are limited to the following cases ... 

This is one of the oldest empirical adsorption isotherms, developed by Freundlich in 1907. It is useful for adsorption from liquid solutions and also for chemisorption isotherms. 

Larionov and co-workers (Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (308-312) carried out systematic theoretical and experimental investigations of the adsorption from liquid solutions of nonelectrolytes on silica adsorbents. They studied the adsorption of individual substances and binary liquid solutions (benzene/carbon tetrachloride, carbon tetrachloride/isooctane, benzene/isooctane, etc.) on Si02 samples with different degrees of porosity but identical surface chemical properties. The experimental results were compared with the theoretical calculations carried out by the Gibbs method. This procedure made it possible to calculate the dependence of the enthalpy, entropy, and free energy of wetting on the concentration and to obtain expressions describ-... 

With liquids the experience is somewhat different. After a brief rapid initial rate, the subsequent rate diminishes noticeably, especially when large size molecules are to be adsorbed. Granular carbon (8 x 30 mesh) may require five or more hours to utilize an appreciable portion of the total potential adsorptive capacity. The rate of adsorption from liquids is much accelerated by pulverizing the carbon so as to expose more of the interior surface.6 With powdered carbons, if mixing is adequate, the major portion of the adsorption is complete within an hour (Table 2 1). Adsorption will continue... 

Adsorption from liquids. An important example of adsorption from the liquid phase is the use of activated carbon to remove pollutants from aqueous wastes. Carbon adsorbents are also used to remove trace organics from municipal water supplies, which improves the taste and reduces the chance of forming toxic compounds in the chlorination step. For these uses the carbon beds are many feet in diameter and up to 30 ft (10 m) tall, and there may be several beds operating in parallel. Tall beds are needed to ensure adequate treatment, because the rate of adsorption from liquids is much slower than from gases. Also the spent carbon must be removed from the bed for regeneration, and so relatively long periods between regeneration are desirable. 

Adsorption from liquids onto solid adsorbents is widely utilized in liquid chromatography (HPLC, see Chapter 11,4), described in detail in physical chemistry and instrumental methods textbooks, e.g. in [16]. The separation by HPLC is based on adsorption-desorption kinetics, i.e. on how long various dissolved compounds remain present in the adsorbed state. The average time, ta, during which molecules are present in an adsorption layer is... 

Ion-exchange resins are insoluble in water, but they swell in aqueous solution to an extent determined by the degree of cross-linking, the concentration of fixed charges, and the concentration of electrolytes in solution. Some swelling is desirable to increase diffusion rates inside the particles, but swelling also decreases the capacity of the resin per unit volume of the bed. Resins are available as spherical beads between 0.3 and 1.2 mm, and are generally used in packed beds similar to those used for adsorption from liquids (McCabe et al., 2005). 

The mathematical models that have been applied to the physical adsorption from liquid solutions are generally extensions of the theories that have been developed to describe the sorption of gases on solid surfaces with modifications to account for the competition between the solute and solvent for the adsorption sites. Two of these models have been applied to the adsorption isotherms of nonelectrolytes from solution they are the Langmuir model and the Brunauer, Emmett, and Teller (BET) model in addition the Freundlich empirical equation has also been used. In the Langmuir model it is assumed that the adsorbed species forms a monolayer on the surface of the adsorbent, that the adsorbed molecules... 

Ash et al. (1973) extended the preceding theory to the adsorption from liquid mixtures. It follows very similar lines to the gas/solid situation and so, in the interests of concision, it will not be reproduced here. For a binary mixture of solute 2 in solvent 1, the analogue of the gas/solid equation (17.17) is... 

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