Sorbates and sorbent

The variation of the energy of attraction attributed to the van der Waals force as a function of distance between sorbate and sorbent may be described graphically with a hypothetical plot of potential energy vs. distance. 

As expected, the total interaction energies depend strongly on the van der Waals radii (of both sorbate and sorbent atoms) and the surface densities. This is true for both HK type models (Saito and Foley, 1991 Cheng and Yang, 1994) and more detailed statistical thermodynamics (or molecular simulation) approaches (such as Monte Carlo and density functional theory). Knowing the interaction potential, molecular simulation techniques enable the calculation of adsorption isotherms (see, for example, Razmus and Hall, (1991) and Cracknell etal. (1995)). 

Based on the principles of n-complexation, we have already developed a number of new sorbents for a number of applications. These include sorbents for (a) olefin/paraffin separations [9-12], (b) diene/olefin separation or purification (i.e., removal of trace amounts of dienes from olefins) [13], and (c) aromatics/aliphatics separation and purification (i.e., removal of trace amounts of aromatics from aliphatics [14]. Throughout this work, we have used molecular orbital calculations to obtain a basic understanding for the bonding between the sorbates and sorbent surfaces, and further, to develop a methodology for predicting and designing n-complexation sorbents for targeted molecules (e.g. Ref 11). 

One example of an intervening condition is the exhaustion of one reactant in a nonfirst-order reaction. Sorption reactions by nature must be second-order, depending on both sorbate and sorbent in the system. This introduces an unwanted complexity to sorption kinetics research, but it can be circumvented. Sorption sites on any given sorbent are relatively constant, whereas sorbate can be easily varied. Therefore, if the sorbate present in the system is substantially less than the number of sorption sites, the reaction will be directly dependent on sorbate concentration and reaction will occur as if it is first-order. This type of condition is usually referred to as pseudo-first-order. ... 

Sorbates and Sorbent Characteristics. K-clinoptilolite, obtained from natural clinoptilolite from Olygocene 3 horizon (10), is used as a sorbent. The natural sample chosen is with a fairly high pota sium content, because K-form has high selectivity towards Nj (10) -the following ion-exchange increases K+ content in the sorbent up to 7.83%. 

The slope dnjdp) of the isotherm at the origin, reflecting the strength of the interaction between sorbate and sorbent, depends most strongly on the chemical nature of the sorbate, besides also on the structure of the sorbent and on the cations in the solid. Values of (dnjdp) at 0 = 0 and for the standard temperature 298°K are given in column 9 of Table I. 

Gas cleaning by sorption by a liquid or solid sorbent is one of the most widely applied operations in the chemical and process industries (Table 23.1). Some processes have the potential for sorbent regeneration, but, in a few cases, the process is applied in a nonregenerative manner. The interaction between sorbate and sorbent may either be physical in nature or consist of physical sorption followed by chemical reaction. Other gas stream treatments use the principle of chemical conversion of the contaminants with the production of harmless (non-contaminant) products or to substances, which can be removed much more readily than the impurities from which they are derived (Mokhatab et al., 2006 Speight, 2007, 2008). 

The extent of r-complexation between the sorbate and sorbent depends, for a given sorbent, on the density of the tt-electrons in the sorbate molecule. Thus, very strong bonds can be formed with molecules with more than two double bonds (e.g., dienes), triple bonds, and polynuclear aromatics. At the same time, for a given sorbate, the sorbent can be tailored to yield a desired bond strength, by choosing the appropriate cation. 

The Kelvin-equation-based methods were found to perform reasonably well for macro-porus and some mesoporous materials. However, it was foimd that the classical approach does not hold trae for micropores, in which case the intermolecular attractive forces between the sorbate and sorbent molecules predominate over bulk fluid forces such as surface tension. The potential energy fields of neighboring sorbent surfaces are known to overlap when the pores are only a few molecular dimensions wide. This results in a substantial increase in the interaction energy of an adsorbed molecule [12], which is not accounted for by simple classical thermodynamic models such as the Kelvin equation. 

A number of physical parameters pertaining to both the sorbate as well as sorbent are required as inputs for the HK models described. These include the size of the sorbate and sorbent molecules... 

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