Adsorption from the Liquid Phase

Despite its industrial importance, adsorption from the liquid phase has been studied much less extensively than adsorption from the vapor phase. There is no difference in principle between adsorption from liquid and vapor phases since, thermodynamically, the adsorbed phase concentration in equilibrium with a liquid must be precisely the same as that which is in equilibrium with the saturated vapor. The differences arise in practice because in adsorption from the liquid phase one is almost invariably concerned with high adsorbed phase concentrations close to the saturation limit. The simple model isotherms, developed primarily to describe adsorption from the vapor phase, are at their best at low sorbate concentrations and become highly unreliable as saturation is approached. Such models are therefore of only very limited applicability for the correlation of liquid phase adsorption data. 

The classical thermodynamic approach has been applied to liquid phase adsorption by Larionov and Myers and by Minka and Myers. It was shown that for sorption of carbon tetrachloride-isooctane and benzene-carbon tetrachloride on aerosil the adsorbed solutions show approximately ideal behavior whereas adsorbed mixtures of benzene, ethyl acetate, and cyclohexane on activated carbon showed appreciable deviations from ideality. However, it is shown that the activity coefficients and hence the adsorption equilibrium data for the ternary systems may be successfully predicted, by classical methods, from data for the constituent binaries. 

In unpublished work the generalized statistical model [Eq. (4.17)] has been successfully applied to the correlation of liquid phase adsorption equilibrium data for Cg aromatics on faujasite zeolites. For these systems the saturation limit corresponds to approximately three molecules/cage, and at equilibrium with the liquid the adsorbent is essentially saturated so that each cage can be assumed to contain three sorbate molecules. This simplifies the model since only the terms corresponding to / + y = 3 in Eq. (4.17) need be retained, and the expression for the separation factor, assuming an ideal binary fluid phase, becomes 

Schirmer, G. Friedrich, A. Grossman, and H. Stach. in Molecular Sieves, Proceedings of the First International Conference on Molecular Sieves, London, 1967. Society of Chemical 

Fowler and E. A. Guggenhiem, Statistical Thermodynamics. Cambridge University Press, Cambridge, 1939. 

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Adsorption from the liquid phase is experimentally much easier (cheaper), but requires, even for highest-purity solvents, chemical inert substrates, e.g., hopg, or selective adsorbate systems, like thiols on gold. 

Several general monographs on carbonaceous adsorbents have been published in the last decade. The ones on activated carbons devote much more space to gas-phase adsorption than to liquid-phase adsorption, and they discuss surface physics in more detail than surface chemistry. Thus, for example, Jankowska et al. [34] devote less than 10% of their book to adsorption from the liquid phase, but they do mention that the process is also affected by the nature of the adsor-... 

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. 

Denoyel, R. and Rouquerol, F. (2002). Adsorption from the liquid phase. Handbook of Porous Solids. Wiley-VCH, Chapter 2.6. 

The foregoing equations appear to apply whether or not inert carrier gases are present. Regardless of the total pressure (so long as an ideal gas mixture is approximated), only the partial pressure of adsorptive affects the equilibrium. In adsorption from the liquid phase, it is generally necessary to assume that the solid is wetted by all the components of the liquid phase, and that all such components are adsorbed to an appreciable extent. For a two-component liquid, designating by n the mole-fraction of the component under consideration, isotherms similar t( can be used on an empirical basis ... 

Adsorption from the liquid phase is used to remove organic components from aqueous wastes, colored impurities from sugar solutions and vegetable oils, and water from organic liquids. Adsorption can also be used to recover reaction products that are not easily separated by distillation or crystallization. Some of the same types of solids are used for both vapor-phase and liquid-phase adsorption, though often adsorbents with larger pores are preferred for use with liquids. 

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. 

Part 111 Adsorption from the Liquid Phase onto Nanoparticles and Porous Solids... 

In contrast to investigations of adsorption from the gas phase, the number of methods applicable to adsorption from the liquid phase is very small. On the one hand this is caused by the fact that not all methods using either electrons or ions can be applied in situ. In addition the adsorbents are normally powders with no plane surfaces. As a consequence the results of quantitative adsorption measurements are usually calculated from the difference between the liquid concentrations before and after the adsorption process. In principle, any analytical method may be used provided it has sufficient sensitivity pH measurements with a glass electrode and atomic adsorption spectroscopy (AAS) are standard, but complexometry and ion-selective electrodes can also be used. Radiochemical methods are useful in the case of small final concentrations. If electrochemical methods are used, one has to consider that activities, not concentrations, are obtained. In the case of partially soluble adsorbents, such as transition aluminas, their concentration should also be determined, as well as those of all other constituents of the solution, e.g., CO3. ... 

Chromatographic methods using adsorption from the liquid phase have been developed. When a large enough sample of silica is available it can be packed into a column 2 mm in diameter. The length of the chromatographed colored zone of methyl red in benzene is measured and related to spedflc surface area (97). Adsorp tion of C1-C4 alcohols from various concentrations in benzene was measured by Hoffman et al. (98), from which the surface areas covered by the physically adsorbed molecules on silica were calculated as follows ... 

Powdered activated carbons (PAC) have very small particle sizes (usually less than 100 pm in diameter). Their advantage over large particles is that there is less diffusional resistance to adsorption and, hence, much higher adsorption rates are attained. Powdered carbons are generally prepared by chemical activation from sawdust. They are preferably used for adsorption from the liquid phase and their application is simple. PAC is added to the solution directly, agitated, left in contact for a short time, and subsequently separated by filtration. 

The carbons used, for these two studies, were prepared from polyvinylidene chloride (PVDC) by carbonizing to 850 °C in nitrogen and activating in carbon dioxide at 850 °C to 24, 41, 70 and 85wt% bum-off. A commercial charcoal (Sutcliffe Speakman Co. Limited, No, 112 (UVD/20) prepared for adsorption from the liquid phase) was also used. Characterizations were made by adsorptions of nitrogen at 77 K and of carbon dioxide at 273 and 195 K, using the Brunauer-Emmett-Teller (BET) and Dubinin-Radushkevich (DR) equations of adsorption. The saturation vapor pressures of CO2 at 273 and 195 K are taken as 3.44 and 0.186 MPa, respectively. The carbons are described in Table 8.1. Thus, a... 

The simplest solution of a fixed-bed adsorption problem is provided by isothermal equilibrium nondispersive operation of the fixed hed. For single solute adsorption from the liquid phase to the adsorbent particles (where p = 0) under isothermal conditions,... 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

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