Adsorption of Gas Mixtures

Selective adsorption so-called competitive adsorption occurs when a gas containing more than one component is in interaction with a solid in such a way that the 

XAand xb mole fractions in the adsorbed phase Solid = adsorbent 

The adsorption equilibrium of binary mixture A + B on a solid can be written  

There are seven thermodynamics parameters characterizing this equilibrium (p, T, rr, ya, yB, xa, andxB). In that case, the application of the phase rule of Gibbs leads to a variance equal to 3. The representation of the binary adsorption equilibrium is obviously more complicated than for the adsorption of a single component. Coadsorption equilibrium could be represented in a three dimensional space by plotting for example the amounts adsorbed as a function of the pressure in one direction and the composition of the adsorbate in another one. However, this kind of plot is not common because it requires a lot of data measured at different composition and pressure. We prefer to represent the coadsorption equilibria in a two-dimension space by maintaining constant certain variables. The representations that are the more often used and easily understandable are  

The adsorption selectivity is the more appropriate parameter to describe the preferential adsorption of one component with respect to another one. For the adsorption equilibrium of a binary mixture. 

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Isotherm Models for Adsorption of Mixtures. Of the following models, all but the ideal adsorbed solution theory (lAST) and the related heterogeneous ideal adsorbed solution theory (HIAST) have been shown to contain some thermodynamic inconsistencies. References to the limited available Hterature data on the adsorption of gas mixtures on activated carbons and 2eohtes have been compiled, along with a brief summary of approximate percentage differences between data and theory for the various theoretical models (16). In the following the subscripts i and j refer to different adsorbates. 

Another group of theoretical approaches is based on a comparison with the adsorption of gas mixtures. In principle, one may consider adsorption from a binary fluid mixture as an extrapolation of the adsorption from a binary gas mixture to saturation of the two components. This extrapolation covers the least accessible part of the Isotherms, because non-ideality in adsorbate and adsorptive and adsorbate-bulk phase interaction have to be accounted for. General rules are not readily derived, other than those that are so formal as to be of little use in practice. For more simple systems useful limiting laws can be derived. We refer to the literature l. It may be noted that Myers and Sircar developed a consistency test by establishing a relation between surface pressures for the two gases and for the fluid mixture. For some binary mixtures of benzene, cyclohexane, n-heptane and 1,2 dlchloroethane on silica gel they found this relation to apply. 

Adsorption of gas mixtures on solid heterogeneous substrates has received an increasing interest in the last decades, due to its importance in relation with new technological developments, like gas-separation and purification [1,2]. 

The IAS theory was later extended to account for the adsorption of gas mixtures on heterogenous surfaces [52,53]. It was also extended to calculate the competitive adsorption isotherms of components from hquid solutions [54]. At large solute loadings, the simplifying assumptions of the LAS theory must be relaxed in order to account for solute-solute interactions in the adsorbed phase. The IAS model is then replaced by the real adsorbed solution (RAS) model, in which the deviations of the adsorption equilibrium from ideal behavior are lumped into an activity coefficient [54,55]. Note that this deviation from ideal beha dor can also be due to the heterogeneity of the adsorbent surface rather than to adsorbate-adsorbate interactions, in which case the heterogeneous IAS model [55] should be used. 

IAS model for dilute liquid solution The IAS method was first proposed to accoimt for the adsorption of gas mixtures. It was later extended to multisolute adsorption from dilute liquid solutions [54]. Assuming that both the solution and the adsorbed phase are ideal, the following equation can be derived to calculate multi-solute equilibriirm composition [54]. 

The adsorption of gas mixtures has been extensively studied. For example, Wendland et al. [64] applied the Bom—Green—Yvon approach using a coarse grained density to study the adsorption of subcritical Lennard-Jones fluids. In a subsequent paper, they tested their equations with simulated adsorption isotherms of several model mixtures [65]. They compared the adsorption of model gases with an equal molecular size but different adsorption potentials. They discussed the stmcture of the adsorbed phase, adsorption isotherms, and selectivity curves. Based on the vacancy solution theory [66], Nguyen and Do [67] developed a new technique for predicting the multicomponent adsorption equihbria of supercritical fluids in microporous carbons. They concluded that the degree of adsorption enhancement, due to the proximity of the pore... 

Bakaev, V.A. and Steele, W.A. (1997). Hard rods on a line as a model for adsorption of gas mixtures on homogeneous and heterogeneous surfaces. Langmuir, 13, 1054-63. 

The most commonly used methods for gas adsorption systems are manometric or volumetric, gravimetric, and different chromatographic methods. Combinations of two methods can be used for measuring adsorption of gas mixtures [8]. 

This equation can also be derived by formally applying the lAST, using the Sips equation, Eq. (47), to describe pure component data. Hence, for adsorption of gas mixtures onto activated carbon, the use of Eq. (50) is generally justified. 

Clearly, the factors determining the isosteric heats of adsorption of gas mixtures are complex. Many more binary experimental data have to be gathered and analyzed before any acceptable theory to explain the observed behavior emerges. There is also an urgent need for generating a comprehensive and systematic database for multicomponent gas (/ > 2) isosteric heats of adsorption for aiding practical adsorptive process designs. 

Jaroniec, M., and J. Toth. 1976. Adsorption of gas mixtures on heterogeneous solid surfaces I. Extension of T6th isotherm on adsorption from gas mixtures. Colloid and Polymer Science [Kolloid Zeitschrift Zeitschrift fur Polymere] 254, no. 7 643-649. doi 10.1007/BF01753693. 

Jaroniec. M. Borowko, M., and Rudzinski, W., Gonzalez-Holland model for adsorption of gas mixtures. AlChE... 

Cracknell, R.F., and Nicholson, D Adsorption of gas mixtures on solid surfaces, theory and computer simulation. Adsorption, 1 (1), 7-16 (1995). 

Glanz, P., and Findenegg, G.H., Adsorption of gas mixtures of propene and propane on graphitized carbon black, Adsorpt. Sci. Technol., 1(1), 41-50 (1984). 

Measurement of adsorption of gas mixtures including inert components on zeolite. Report of IFT- USI, Thermodynamics Series, 2003. 

Liu Y, Liu H, Hu Y, Jiang J Density functional theory for adsorption of gas mixtures in metal — organic frameworks, J Phys Chem B 114(8) 2820-2827, 2010b. 

In the same way, the Dubinin-Astakhov isotherm has been extended to the adsorption of gas mixtures [176] ... 

The alternative approach to the adsorption of gas mixtures has been proposed by Jaroniec [188], who utilized the earlier kinetic considerations of Crickmore and Wojciechowski [189]. As the result, he proposed the following linear relation ... 

The application of Langmuir and lAST/RAST to adsorption from liquid solutions is limited, thus we concentrate our comparative evaluation on the adsorption of gas mixtures on solids. 

This book covers all aspects of gas phase adsorption processes ranging from thermodynamics to the modelling and design of cyclic processes, including pressure swing adsorption. The equilibrium adsorption of gas mixtures is... 

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

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