Multi-component adsorption

Crittenden. J.C., Luft, P.. Hand. D.W., Oravitz, J.F., Loper, S.W., and Ari, M. Prediction of multi-component adsorption equilibria using ideal adsorbed solution theory. Environ. Sci Technol, 19(11) 1037-1044,1985. 

There have been few studies reported in the literature in the area of multi-component adsorption and desorption rate modeling (1, 2,3., 4,5. These have generally employed simplified modeling approaches, and the model predictions have provided qualitative comparisons to the experimental data. The purpose of this study is to develop a comprehensive model for multi-component adsorption kinetics based on the following mechanistic process (1) film diffusion of each species from the fluid phase to the solid surface (2) adsorption on the surface from the solute mixture and (3) diffusion of the individual solute species into the interior of the particle. The model is general in that diffusion rates in both fluid and solid phases are considered, and no restrictions are made regarding adsorption equilibrium relationships. However, diffusional flows due to solute-solute interactions are assumed to be zero in both fluid and solid phases. 

During a discharge, the composition of the gas at the outlet is a consequence of the multi-component adsorption equilibria between the previously adsorbed phase and the gas phase. The ratio profiles are necessarily linked to the dynamic working mode of the vessel (like the flow rates during charge and discharge) and the heat and mass transfer properties of the composite adsorbent. Nevertheless, general tendencies can be observed. 

For safety reasons, the cycling operation is interrupted during the weekend. After an interruption, always happening at the end of a discharge phase, several cycles are necessary to join the continuous curve representative of the evolution of the behaviour of the tank. This phenomenon tends to demonstrate the importance of the kinetics of the multi-component adsorption equilibrium in the case of a complex mixture of gas. 

The Langmuir equation can be easily extended to multi-component adsorption, for example, for a binary mixture of components 1 and 2 ... 

Simulation data on mixture adsorption can be used to screen zeolites as adsorbents, but experimental data are necessary to validate the simulations and to accurately design the separation process. The first step of the process design is to obtain such data. However, the experimental assessment of multi-component adsorption equilibria and kinetics is not straightforward and is highly time-consuming. As a result, some theories have been developed that predict adsorption behaviour for a mixture based on the pure component equilibria [1,3]. The isotherm data have to be correlated before their use in a design model... 

SINGLE AND MULTI COMPONENT ADSORPTION OF VOLATILE ORGANIC COMPOUNDS ONTO HIGH SILICA ZEOLITES - DISCUSSION OF ADSORBED... 

The multi-component adsorption equilibrium was predicted by the LRC model and the sorption rate into an adsorbent pellet was described by the following modified LDF model. 

It is well-known that the nature of NG composition has a negative impact on ANG technology due to the deterioration of the adsorbent capacity on extended cyclic operation. Although this problem has been addressed by some authors, either experimentally or theoretically, the single- and multi-component adsorption data reported to date, as they apply to natural gas, are still rather scarce. 

Quantum chemistry approaches to zeolites are complemented by an active research community that uses classical force-field methods to study molecular adsorption and diffusion in zeolites and similar materials. This topic was comprehensively reviewed by Keil, Krishna, and Coppens in 2000.262 For more recent examples of activity in this area, see References 263-270. Examples of impressive agreement between adsorption isotherms and molecular dilfusivities predicted with calculations of this type and experimental data are available.271,272 There appear to be many future opportunities for linking the detailed understanding of multi-component adsorption and diffusion that is now emerging from this area with detailed quantum chemistry approaches to reactivity at active sites inside zeolites. 

Sievers, W. and Mersmann, A. Single and multi-component adsorption equilibria of C02, N2, CO, and CH4 in hydrogen purification process. Chemical Engineering Technology, 1997, 17, 325. 

VI. Validation of the accuracy of the perturbation peak method for determination of multi-component adsorption isotherm parameters in LC. 

The FACP and ECP methods cannot be used to determine adsorption isotherm parameters from multicomponent mixtures. By contrast FA can be used to determine multi-component adsorption data but it is a complex and time-consuming process [124, 125],... 

Single-component isotherm parameters cannot always predict elution profiles with satisfied accuracy [122, 123], Therefore, to be able to predict accurate overloaded multi-component elution profiles where competition occurs competitive adsorption isotherm parameters are often necessary. Measurement of isotherms from a mixture is also often necessary because the pure enantiomers are not always accessible in large quantities. However, there exist only a small number of reports on the determination of multi-component adsorption isotherm parameters. FA can be used to determine binary isotherm data but it is time-consuming. The PP method is an alternative method to determine isotherm parameters from binary mixtures. It has been reported that the PP method works well up to weakly non-linear conditions [118, 119],... 

In this chapter, we discuss first a number of models that have been used to accoimt for competitive isotherm data. Although the multi-component extension of many of these models is straightforward, most of them have been used almost exclusively with binary mixtures. In the second part of this chapter, we describe the methods of determination of competitive isotherms. Finally, we discuss the methods of data acquisition for multi-component adsorption and we present a few examples. 

In the presence of s.a. ions that form surface complexes with the proton sites multi-component adsorption occurs on reference sites Sj and Eqs. (77) and (79), in general, no longer hold. Moreover, for the local adsorption the competitive or multicomponent form of Eq. (55) should be used, i.e. Eq. (55) in combination with Eq. (59). For adsorption of a proton on site Sj, on which also s.a. ions C and A can adsorb Eq. (83) results ... 

Rynders, R.M. Rao, M.B., and Sircar, S. (1997). Isotope exchange technique for measurement of pure and multi-component adsorption equiUbria and kinetics. 

Isotherms other than Langmuir s are not readily generalized for multi-component adsorption and are therefore rarely used in practice in kinetics of heterogeneous systems. An exception is the use of the Temkin isotherm in ammonia synthesis [31]. 

Mohan D., Singh K.P., (2002), Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse-an agricultural waste, Water Research, 36,2304-2318. 

The set of r integral equations is linked together, for example via a multi-component adsorption isotherm. In the simplest case a set of r independent relations of the type of Eq. (4.22) results when linear adsorption isotherms Eq. (2.40) are assumed for each of the surfactants (Miller et al. 1979) ... 

The system of adsorption isotherms (2.93) allows for the discussion of the critical phenomena in the multi-component adsorption layer. When there is an interaction between die adsorbed particles and the adsorbate segregation occurs. For the sake of clarity, as an example we will present below the derivation for a two-component layer, however the conclusions are qualitatively the same for this case and for the cases where more than two components form the adsorbed layer. 

DYNAMICS OF MULTI-COMPONENT ADSORPTION WITH INTERACTIONS A MEAN - FIELD APPROACH... 

There are serious difficulties in the derivation of the multi-component adsorption (which is typical of the reforming scenario), taking into account the role of adsorbate interactions. The only mathematically tractatable way to do it is, by following the Bragg-Williams lattice -gas Mean field field Approach (MFA). In this approach, the pair and higher-order correlations between the adsorbed molecules are ignored. With these assumptions, the adsorption isotherms within the MFA format can be easily written(8-l 1,17)... 

The adsorption window is demarcated by the adsorption equilibrium within the boundaries of the imposed pressure cycle and is determined by the multi-component adsorption isotherm and the pressure range. The Langmuir type isotherm is assumed. 

The set of r integral equations is interlinked via a multi-component adsorption isotherm. 

A brief qualitative evaluation of CO2 adsorption selectivity in MOFs is the direct observation on differences in uptakes between separated gases under given conditions, usually based on the single-component adsorption isotherms. These isotherms can also be used to quantitatively estimate the adsorption selectivity. If adsorption species are presented at low loadings, namely within the Henry s regime, the adsorption selectivity for an equimolar mixture is close to the ratio of the Henry s constants for each species. At non-dilute loadings, however, more information is required to estimate multi-component adsorption. One common approach is to use ideal adsorbed solution theory (lAST) to predict multi-component adsorption isotherms and selectivity based on the single-component adsorption isotherms [60], This approximate theory is known to work accurately in many porous materials. 

The statistical models are based on thermodynamics arguments. But these models are normally specific to adsorbents with well-defined structures such as zeolites [12] or carbon black [13]. There was also a statistical model reported for the multi-component adsorption equilibria of vapors on activated carbon, but it was validated only for a particular system [10]. 

Systematic and reliable experimental data of single adsorbates and binary and ternary mixtures on samples of activated carbon with various pore structures and surface chemistry are badly needed for the critical evaluation of models of multi-component adsorption equilibria. 

It is also unfortunate that Eq. (11) cannot be easily integrated for multicomponent adsorption as in the case of pure gas adsorption [Eq. (28)] because no multi-component adsorption experiment can be designed to generate p, as functions of T at constant n ii = 1.2,..., N). Consequently, there is no easy way to estimate multicomponent (i > 2) isosteric heats of adsorption by measuring rif as functions of P. r, and. ... 

The next step is the modelling of multi-component adsorption and diffusion inside the pores (Fig. 8). 

Filippov, L.K., Coherent and incoherent frontal patterns for multi-component adsorption dynamics Nonuniqueness of frontal patterns in the isothermal case for convex isotherms of two-component mixtures, Chem. Eng. Sci., 47(5), 1211-1218 (1992). 

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