Single Component Sorption

Contrary to condensation or crystallization the process of accumulation of molecules is called adsorption when the fluid concentration c, is smaller than the saturation concentration c. In the case of gas adsorption the partial pressure /r, of the adsorptive is smaller than the vapor pressure at the temperature of the system. Dealing with the adsorption of a liquid component the concentration y. is smaller than the saturation concentration y. As a rule the relative saturation p = p/p or (p = y/y, respectively., is plotted against the loading of the solid adsorbent, see Fig. 2.4-1. The cirrve a is representative for an adsorption isotherm which 

The process of adsorption takes place when the concentration of the adsorptive is greater than the equiUbrium value vahd for the given temperature however, desorption requires a fluid concentration of the adsorptive which is smaller than the equilibrium concentratiom An adsorption isotherm favorable for adsorption is unfavorable for desorption and vice versa. Condensation of gases or vapors and solidification or crystalhzation will start when the relative supersaturation becomes 1. In the case of adsorbents with capillary or very narrow pores, capillary condensation is observed for relative saturations Pi . Based on (2.1-28) the hysteresis of the adsorption isotherm vahd for adsorption and desorption can sometimes be ejqrlained, see Fig. 2.4-2. Sohd materials exposed to drying (see Chap. 10) often show such hysteresis behavior which can sometimes be explained by the ciu-vature of the liqttid sttrface in capillaries The radius of this surface is greater in the case of adsorption in comparison to the radius valid for a desorption process, see Fig. 2.4-2. 

The relationship between the relative saturation q , the loading X, and the temperature T can be described in diagrams in which the temperature T (isotherms), the partial pressure (isobars), and the loading X (cmves of constant loadings) are the parameter when the other parameters are plotted as ordinates against abscissa, see Fig. 2.4-3. 

Type I isotherms are favorable for adsorption and the loading assumes a final value for 1. Inorganic and organic gases or vapors adsorbed on rrricroporous adsorbents like active carbon or zeolites often lead to isotherms of this type. 

Type II isotherms are typical for adsorberrts with wide or no pores and applicable for one and multilayer adsorptioa 

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A chromatographic method has been used to measure single component sorption isotherms of O, N / CH, ... 

The prim purpose behind these binary mbcture sorption studies was to develop a theoretical model which was able to predict binary sorption isotherms from the respective single component sorption data with an accuracy which is sufficient for the design of pressure swing adso tion (PSA) separation systems. These studies will, hopefully, also lead to a better understanding of the fundamentals of adsoibent/adsoibate and a oAate/adsoibate interactions. 

ISOTHERMAL SINGLE-COMPONENT SORPTION MICROPORE DIFFUSION CONTROL... 

Figure 2 presents a comparison of concentration dependences of sorption enthalpy values for various gases being typical components of natural gas, as measured calorimetrically for their single component sorption by a NaCaA zeolite... 

The calculation of the sorption equilibrium proceeds by means of eqs. (16) and (17). The characteristic parameters, q, E and AS, are obtained by fitting to data that were either experimentally obtained or taken from literature. Although utilization of eq. (16) with single component sorption equilibrium data may lead to extreme deviations, its applicability had been proven [63]. 

For a gas that contains 0.5 vol.% CO2 the breakthrough time of H2S shortens from 150 h to 13 h. The roll-up in concentrations for several components is due to their replacement by the other ones (concentrations at the column outlet are referred to those at the column inlet). First, N2 is replaced by CH4, then CO2 by H2S and, of course, by H2O, and, finally, H2S by H2O of the feed gas. Sorption of H2O is strongest, due to its large dipole moment, and it is able to replace all the other gas components. For this reason, H2O breakthrough time changes only a little as compared with single component sorption. If the feed gas CO2 content is increased to 10 vol. %, cf.. Figure 18, the result of model calculations becomes even... 

Choudhary, V.R. Nayak, V.S., and Choudhary, T.V., Single-component sorption/diffusion of cyclic compounds from their bulk liquid phase in H-ZSM-5 zeolite. Ind Eng. Chem. Res.. 36(5), 1812-1818 (1997). 

The basic idea of direct measurement of sorption isosteres for microporous sorption systems was first expressed by Serpinsky in 1967 [6] and published by Bering et al. in 1969 [7]. Fundamental thermodynanric features related to sorption isosteres and their direct measurement were discussed frequently by Bering, Serpinsky, Fomkin et al, e.g., in [8-11]. The first direct measurement of single-component sorption isosteres was carried out by the Schirmer school for n-paraffin compounds on FAU- and LTA-type zeolites, reported in 1969 and published in 1971 [12]. Extended basic research performed by that school, specifically for hydrocarbon-zeolite systems, utilized SIM in close connection with other techniques, e.g., calorimetry, and theoretical methods such as Monte Carlo and statistical thermodynamics [13-17]. 

Hi) An apparent sorption isostere may deviate from linearity due to in(de)creasing desorbed amount with T in(de)crease to an extent that is specific for a considered sorption system, over a given T range. A correction of such a de(ad)sorbed amount can be applied to a single-component sorption isostere based on considerations below. An analogous correction is practically impossible for the mixture-sorption case, because of exact knowledge needed for mixture-sorption isotherms, which is very difficult to obtain. 

MOF material can selectively capture and separate Kr from Kr/Xe mixtures at moderate temperature via a temperature gating mechanism on a copper-based fluorinated MOF [65], MOFs have shown great application potential in the gas capture and separations area, which will help in evaluating the MOFs for sorption, separation-related applications, and selective single-component sorption/separation applications, respectively. 

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