Adsorption from liquid phase

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... 

Polymer adsorption from liquid phase onto solid surfaces has been investigated by numerous authors. A good summary of these investigations was given earlier [13]. 

Adsorption from liquid phase can take place at any of the three interfaces liquid-solid, liquid-liquid, or liquid-vapor. In practice, however, more attention has been directed and more is known about the liquid-solid interface. This is due to the fact that purification of liquids such as water, wine, and oils, and their decolorization and detoxification have been carried out for centuries using charcoals and active carbons. With the expansion of chemical, pharmaceutical and food industries the range of substances to be purified by carbons has increased enormously. 

Activated carbon adsorption from liquid phase has found wide applications in several areas of food production and processing industries. The activated carbons remove undesirable odors, colors, and unwanted components of the solution, and improve the quality and consumability of the food material. The use of active carbons in food processing is continuously on the increase because the food processing and production industries in which the use of active carbon is well established are always expanding. Furthermore, the use of activated carbons is also being explored and expanding to areas of the food processes, where it was not previously used. 

The following are some of the typical industrial applications for liquid-phase carbon adsorption. Generally liquid-phase carbon adsorbents are used to decolorize or purify liquids, solutions, and liquefiable materials such as waxes. Specific industrial applications include the decolorization of sugar syrups the removal of sulfurous, phenolic, and hydrocarbon contaminants from wastewater the purification of various aqueous solutions of acids, alkalies, amines, glycols, salts, gelatin, vinegar, fruit juices, pectin, glycerol, and alcoholic spirits dechlorination the removal of... 

Also, the Kataoka correlation has been used in adsorption systems from liquid phase with... 

Apart from liquid phase adsorption on a solid adsorbent such as bauxite, the early processes for sweetening and desulfurization were of a chemical nature. Some are in operation today in substantially their original forms, some have been greatly improved, and new processes performing similar functions have been developed. It is beyond the scope of this paper to cover them all, even in outline, therefore a comparative selection has been made to illustrate the advances achieved. The division, which is on a rather arbitrary basis, is given in Table V. 

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). 

As a preparation to the following sections, we briefly discuss some aspects of measuring adsorption from fluid phases, including dilute solutions. For the sake of systematics, we divide the treatment into two parts (1) adsorption on disperse systems, sometimes poorly defined, and (ii) the same on well-defined, mostly smooth model surfaces. In case (1) adsorption is almost exclusively determined from solution analysis, i.e. by depletion, so that problems arise with the separation of liquid from solid and the accurate bulk composition determinations. In case (ii), adsorbed amounts can often be determined directly using typical surface analytical techniques. 

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. 

The main differences between adsorption from the gas phase and that from liquid phase are as follows [3]. First, adsorption from solution is essentially an exchange process, and hence, molecules adsorb not only because they are attracted by solids but also because the solution may reject them. A typical illustration is that the attachment of hydrophobic molecules on hydrophobic adsorbents from aqueous solutions is mainly driven by their aversion to the water and not by their attraction to the surface. Second, isotherms from solution may exhibit nonideality, not only because of lateral interactions among adsorbed molecules but also because of nonideality in the solution. Third, multilayer adsorption from solution is less common than from the gas phase, because of the stronger screening interaction forces in condensed fluids. 

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. 

The less well known temperature scanning stream swept reactor (TS-SSR) has features that are particularly well suited to the study of fluid/solid interactions, such as the study of ore roasting or adsorption. The TS-SSR can be constructed in two variants the TS-PF-SSR based on the PFR and the TS-CST-SSR based on the CSTR. Since the data from liquid phase TS-PF-SSR is easier to understand and interpret, we will consider this type of TS-SSR first. 

The metal complex was immobilized on the mesoporous support by adsorption in liquid phase. FTIR and UV/vis analyses were performed at different stages of the preparation process in order to monitor the preparation of the materials. In particular, the residual solutions obtained from the immobilization process were analyzed before and after Soxhlet extraction. 

Secondary Solvent Effects from Liquid Phase Hydrogen Bonding in Adsorption on Silica [data of Klouwen et al. (15)]... 

Although GAC was used almost exclusively for adsorption from gas phase, whereas PAG was used as a material for purification of liquids these distinctions have become blurred in recent years. Thereby, the use of PAG increased in a limited number of gas phase applications, while GAC is now often used in many liquid phase applications. The summary of practical applications of activated carbon is presented in Table 3. 

In the two other cases (1 and 2) the adsorption is much slower due to adsorption from one phase only and moreover due to the loss of adsorbed molecules via desorption into the second liquid phase. It was emphasized in Refs 113-115 that when dealing with liquid/liquid interfaces one always faces the problem that surfactant molecules are soluble in both adjacent liquids and hence adsorption from one phase generally leads to a transfer across the interface. Experiments particularly dedicated to this transfer are discussed in Sec. V. 

It should be noted that regeneration of gas adsorption AC is very different from liquid-phase adsorption AC. The granular material used in gas-phase operations has a very long life provided that it is... 

On the other hand, the difference will be accentuated in cases where there is a weak specific uptake of one component into the adsorbed phase because the contribution of n to n will be small and the second term, on the right-hand side of Equation (5), will contribute more to the amount adsorbed. Then approximation (6) cannot be used and Equation (5) must be utilized and therefore, a definition of the adsorbed phase, (n ), must be specified. This is commonly done by the acceptance of the monolayer concept of adsorption from liquid solutions. It is assumed that only one layer of molecules covering the solid surface is affected by the solid and hence only this monolayer differs from the bulk liquid phase. This concept allows the number of molecules present in the adsorbed phase to be calculated based on a known specific area of solids and known cross-sectional areas of molecules of the adsorbate and the solvent ... 

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 theoretical basis for the chromatographic analysis of adsorption phenomena in gas and/or liquid phase was given by Don DeVault (ref. 1) and Glueckauf (ref. 2, 3). The mathematical procedure developed by these authors enables one to determine the adsorption isotherm of a solute from its elution profiles in column chromatography. The experimental procedure required for this method is far less laborious than those for the conventional static methods of adsorption measurement, and many experimental works have appeared since (ref. 4, 5). Many of these works, however, dealt with adsorption from gaseous phases, and applications to liquid phases are scarce (ref. 6, 7). 

Adsorption of phenol or benzoic acid from liquid phase is also used for determining the number of basic sites. 

Adsorption from liquid solution is more complicated than that from the gas phase. In a two-component liquid solution, both the solvent and the solute will be adsorbed to different extents. Usually the adsorption of the solute is of interest. The experimental procedure and expression for the amount adsorbed both differ from those used for gas adsorption. The basis for expressing the amount adsorbed from liquids is the concept of surface excess given by Gibbs in 1878. Gibbs surface excess is the difference in the amount of a given component in the surface layer (per unit surface area) over that in the bnlk Uqnid. 

In adsorption by solids from liquid phases, substances in solution at very low concentrations are often preferentially adsorbed the presence of trace quantities of water and other impurities in the solution may therefore have an effect on adsorption. Harkins and Dahlstrom [31], for example, reported that extremely small quantities of water in benzene increas the energy of immersion of oxides to about three times the value obtained wiUi pure benzene. In order to obtain liquids of sufficient purity it may be necessary to fractionate and then store over metallic sodium or other drying agent such as silica gel, calcium sulphate, alumina and so on. 

Most earlier theories considered adsorption from gas phase. In principle, the gaseous phase isotherms should be applicable to liquid systems when capillary condensation is neglected. Figure 8.1 show shapes of various isotherms described below. 

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