André Lavanchy

Water adsorption in active carbons described by the Dubinin–Astakhov equation

It is shown that the adsorption of water by a variety of active carbons can be described within the framework of Dubinins theory. Owing to the low values of the characteristic energy, E(0.8–2.5 kJ mol–1), the Dubinin–Astakhov equation becomes S-shaped in the range 0.3 < p/po <0.7 and provides a good basis for the fit of the adsorption branch of type V isotherms near room temperature. The parameters of the equation are almost temperature invariant and consequently a good agreement is also found in many cases for the enthalpies of immersion into water, as predicted by the extension of Dubinins theory.

The adsorption of water by active carbons, in relation to their chemical and structural properties

An interesting letter has recently been published in this total micropore volume of the solid, the thermal expansion Journal by Lodewyckx and Vansant [1], introducing the coefficient of the adsorptive and its molar volume in the idea of an affinity coefficient for water with respect to liquid state. G(1 1 1/n) is the tabulated ‘Gamma’ function, benzene, the usual reference in Dubinin’s theory. This which takes values between 0.89 and 0.92 when n varies concept is valid for carbons with relatively low oxygen from 1.5 to 5. This means that the enthalpy of immersion contents, where the water adsorption isotherm is of type V depends essentially on the characteristic energy E 5 bE . o and corresponds to a Dubinin-Astakhov equation, as Since active carbons also possess an external (nonshown earlier [2,3]. microporous) surface S , the experimental enthalpy of e We wish to show that Lodewyckx’s idea can be included immersion, Dh (J /g) is i exp in a more general approach based essentially on the Dh (J /g) 5 Dh (J /g) 1 h S (3) i exp i mi i e comparison of the enthalpies of immersion of carbons into water and benzene and taking into account the chemistry where h , a negative quantity, represents the wetting of the i of the surface through an excess enthalpy of immersion. surface. For benzene and water at 293 K, 2 h corresponds i Our approach follows two recent studies dealing with the 2 to 0.114 and 0.030 J /m [3–5] and the last term of Eq. (3) interaction of water [4] and of methanol and ethanol [5] is only a fraction of the total enthalpy of immersion. with active carbons containing variable amounts of oxygen From Eqs. (2) and (3), it follows that the enthalpy of and basic groups. The starting point is the DA equation immersion of a given adsorptive can be calculated from the [3,6] parameters of the DA isotherm. A good agreement has

Dynamic adsorption of vapour mixtures in active carbon beds described by the Myers-Prausnitz and dubinin theories

Abstract The adsorption of binary vapour mixtures from air by active carbon beds can be predicted with a good accuracy by adapting a model developed earlier for single vapours. This model is based on a semi-implicit finite difference scheme. Multiple adsorption of vapours is described by the combined theories of Myers-Prausnitz and of Dubinin (MPD), which have already been applied successfully to static adsorption. A good agreement is found between the predicted and the experimental breakthrough curves of the systems 2-chloropropane + chlorobenzene and carbon tetrachloride + chlorobenzene on two activated carbons at 298 K and under different experimental conditions.

Binary Adsorption of Vapours in Active Carbons Described by the Dubinin Equation

It is shown that the use of the Dubinin–Radushkevich isotherm within the framework of the Myers–Prausnitz theory provides a satisfactory description of the adsorption of mixtures of chlorobenzene + carbon tetrachloride vapours by active carbons. This approach can be extended to other systems and full use can be made of the advantages of Dubinins theory. The experimental data was obtained by Headspace-GC, a rapid and reliable technique.

Dynamic adsorption, in active carbon beds, of vapour mixtures corresponding to miscible and immiscible liquids

Abstract It is shown that the combined Myers–Prausnitz–Dubinin theory (MPD) can be extended to the adsorption of ternary mixtures from an air stream. When combined with a computer model developed for dynamic adsorption, it provides a satisfactory agreement with the experimental breakthrough curves in active carbon beds. On the other hand, as illustrated by a mixture of water and 2-chloropropane vapours, MPD is no longer valid when the corresponding liquids are not miscible. In this case, binary adsorption can be described with a relatively good precision by assuming independent co-adsorption of the vapours. This, in turn, leads to satisfactory previsions for dynamic adsorption.

Binary Adsorption of Vapours in Active Carbons Described by the Combined Theories of Myers-Prausnitz and Dubinin (II)

The new MPD method, described recently, has been applied to the adsorption of benzene +1,2-dichloroethane vapours by a typical active carbon at 293 K. A total of 102 experiments have been carried out for adsorbed mole fractions between 0.06 and 0.94. Good agreement was found between the experimental and calculated values of the selectivity of the carbon for benzene, which decreased as the mole fraction of benzene increased. This variation reflects the heterogeneity of the carbon, which is taken into account by the DR equation. Although the liquid mixture is ideal, deviations from Raoults law are observed in the adsorbed state, indicating a clear preference for benzene. The values of the activity coefficients obtained from the liquid/solid equilibrium improve the fit of the MPD method.

The Non-ideality of the System Benzene + 1,2-Dichloroethane Adsorbed in Microporous Carbons at 293 K

In the case of simple vapour mixtures adsorbed by active carbons, the activity coefficients seem to depend essentially on the composition of the adsorbed phase, rather than on the degree of micropore filling. Consequently, the liquid–solid adsorption equilibrium of benzene + 1,2-dichloroethane mixtures has been investigated at 293 K, using a typical active carbon and following earlier work for adsorption from the vapour phase. This system has the advantage that the mixture is ideal in the liquid state, which provides a convenient reference for the study of the adsorbed phase. The activity coefficients, as well as the excess enthalpy of immersion of the carbon into the liquid mixtures, provide information on the modifications in the adsorbed state with respect to the ideal mixture. It is also shown that the introduction of the activity coefficients derived from the solid–liquid equilibrium increases considerably the accuracy of the Myers–Prausnitz–Dubinin model for the adsorption of the vapour mixtures.

Static adsorption, by activated carbons, of vapour mixtures corresponding to immiscible liquids

It is shown that the static adsorption of benzene and water vapours by a typical industrial active carbon can be described by the model of independent co-adsorption proposed for compounds which are not miscible in the liquid state. The model assumes that the vapours are adsorbed according to their respective DA isotherms, each component using the micropore volume left unoccupied by the other. A good agreement is found between the calculated and the experimental adsorption, but in the case of water the model must take into account the decrease of the characteristic energy E(H2O) with increasing co-adsorption of benzene. This behaviour is confirmed by earlier studies of water adsorption and of immersion calorimetry into water, following variable pre-adsorption of n-nonane.

The Myers–Prausnitz–Dubinin Theory and Non-Ideal Adsorption in Microporous Solids:

The adsorption of vapour and liquid mixtures of benzene + 1,2-dichloroethane, chlorobenzene + carbon tetrachloride, chlorobenzene + cyclohexane and 1-bromo, 2-chloroethane + 1,2-dichloroethane on two activated carbons and on two zeolites (UC13X and ZSM-5) was examined near room temperature. It was shown that the combination of the recent Myers–Prausnitz–Dubinin (MPD) theory with the activity coefficients of the corresponding solid–liquid equilibrium leads to a good correlation between the calculated and the experimental mole fractions for binary vapour adsorption by microporous solids. This confirms that the approach based on an ideal adsorbed state (IAS) can be improved by using these activity coefficients as a first and good approximation.