Binary and Ternary Adsorption of n-Alkane Mixtures on Activated Carbon

Liquid adsorption and immersion of two alcohol–water mixtures on carbon molecular sieves

The adsorption excess isotherms of ethanol–water and propanol–water mixtures are studied on a series of carbon molecular sieves with well-separated micro- and mesoporosity at 298.15 K. The preferential adsorption of one component from a mixture is measured by using vibration densitometry for the concentration analysis. Microcalorimetrically measured enthalpies, which are released upon immersion of the carbon materials in the binary mixtures, complement the adsorption excess data. It is shown that (i) density measurements are well applicable for studying liquid-phase adsorption, (ii) liquid-adsorption isotherms are sensitive to smallest chain length differences of the adsorptives, (iii) the calculated separation diagrams depend strongly on the assumptions about the adsorbed phase, and (iv) the combined determination of gas, vapor and liquid adsorption isotherms and immersion enthalpies offers advantages for the analysis of complex adsorption systems.

Analysis of Binary Adsorption Excess Isotherms at the Liquid/Solid Interface

Adsorption excesses of binary liquid mixtures are the primary experimental quantity in liquid-phase adsorption onto porous or disperse solids. Binary liquid adsorption isotherms can be easily measured and allow statements to be made about the characteristics of solid and liquid mixtures, immersion behaviour at the liquid/solid interface, multi-component adsorption, etc. In this paper, a brief summary is given of different ways of interpreting and analysing binary isotherms over the whole concentration range. Experimental data and calculation examples show that thermodynamics provides a powerful tool for obtaining relevant information from such isotherms.

Further advancements in predicting adsorption equilibria using excess formalism: calculation of adsorption excesses at the liquid/solid interface

Prediction of adsorption equilibria for ternary liquid mixtures on solid surfaces by means of adsorption data for the corresponding three binary liquid mixtures can be improved by combining the thermodynamic excess formalism with geometrical models. This new strategy for the prediction of excess adsorption isotherms is examined for four ternary adsorption systems ranging from ideal to highly non-ideal ternary mixtures. The predicted isotherms are discussed and compared with experimental ones as well as with those obtained for a model based on the absolute quantities. The results confirm: (i) superiority of predicting adsorption in terms of excess quantities, and (ii) utility of geometrical models for constructing ternary molar compositions on the basis of binary ones to predict equilibria not only for liquid mixtures alone but also for adsorption of liquid mixtures on solid surfaces.

Liquid adsorption of n-octane/octanol/ethanol on SBA-16 silica

SBA-16 silica samples with cubic Im3 m symmetry were synthesized according to methods reported in literature and characterized by nitrogen adsorption and X-ray diffraction. The adsorption behavior of the n-octane(1)/octanol(2), n-octane(1)/ethanol(3), and octanol(2)/ethanol(3) binary liquid mixtures on SBA-16 was studied over the whole concentration range at 25 degrees C. Inverted U-shape isotherms were found and described by mathematical functions. The experimental binary data were verified by using a consistency test for the specific free wetting energies on the liquid/solid interface. The ternary adsorption excess isotherms of n-octane(1)/octanol(2)/ethanol(3) at 25 degrees C were predicted from the binary data. The work presents the first complete and consistent tabular set of binary liquid adsorption data for creating ternary data on ordered mesoporous silica.

On numerical classification of solution adsorption isotherms

To numerically classify solution adsorption isotherms, a difference or deviation measure, DS(mc) (the relative difference between two sums of the adsorption maximum's characteristics of selectivity isotherms x 100), is derived. The measure is applicable to completely miscible binary solutions on solids. This quantity evaluates the difference between an adsorption system and the ideal adsorption system (ideal adsorbed and bulk phases, homogeneous surface, and equal molar area solution components) at the point of maximum adsorption. For model systems, DS(mc)s are calculated at several levels of surface heterogeneity (Gaussian distribution of surface energy) and for different signs of phase nonideality (regular solution phases) on a homogeneous surface and on a simple two-site-type heterogeneous surface. All heterogeneous surfaces have negative DS(mc) values, but nonideal phases have DS(mc)s with signs opposite to the sign of deviation from Raoult's law. DS(mc)s from both U- and S-shape isotherms are reported for 16 experimental systems consisting of hydrocarbon mixtures and both alcohol + hydrocarbon and alcohol + water solutions or acetone + carbon tetrachloride on several silica gels and a variety of carbons.

Calculation of immersion enthalpy data from adsorption isotherms

The thermodynamic equations for the calculation of binary and ternary immersion data in excess formalism are presented. Immersion enthalpies and entropies of the n-hexane/n-octane, n-octane/n-tetradecane and n-hexane/n-tetradecane binary mixtures as well as the n-hexane/n-octane/n-tetradecane ternary mixture on activated carbon are calculated from the temperature dependence of adsorption isotherms. In order to evaluate the quality of the calculations, the calculated immersion enthalpies of the binary mixtures on activated carbon are compared with those that were measured calorimetrically. It is shown that phenomenological thermodynamics can be used successfully to predict calorimetric data on the basis of adsorption excess isotherms.

Prediction of multicomponent liquid adsorption using excess quantities

The utilization of excess quantities as the basis for a thermodynamic approach can simplify the prediction of multicomponent liquid adsorption from binary data. A new method for predicting liquid adsorption on solids is suggested, which is different from the existing equations with respect to the theoretical background and formulation. The applicability of the new model is tested with three ternary adsorption systems. The predicted surface excesses are discussed and compared with experimental ones and with those of other prediction models in the literature. The accordance between measured and predicted ternary data is convincing.

Adsorption behavior of zinc and cadmium ion on granular activated carbon in singular and binary systems and the influence of nitrilotricetic acid as a complexing agent

Adsorption characteristics of zinc and cadmium ion on granular activated carbon have been studied in dynamic and equilibrium aspects and singular and binary adsorption behavior were compared. Based on the van't Hoff equation, reaction orders and rate constants were observed to increase as initial concentration of adsorbate was raised for both metal ions. Adsorption of Zn and Cd ion was found to increase with temperature, and this endothermic nature of adsorption reaction was further verified by thermodynamic calculation. Taking nitrilotriacetic acid (NTA) as a ligand, the influence of complexing agent was examined and the variation of adsorption percentage was found to closely relate with the change in ionic form of NTA depending upon pH conditions and amount of complexing agent used. Features of binary adsorption are discussed for several influential parameters and experimental observations for both ions were correlated with a predicted adsorption isotherm based on a Langmuir multi-component model.

Calculation and Prediction of Adsorption Excesses on the Ternary Liquid Mixture/Air Interface from Surface Tension Measurements

The surface tensions of the n-propanol/ethylene glycol/water ternary mixture and its binary mixtures are measured at 293 K and described by mathematical functions. The experimental data and their descriptions are shown in the three-dimensional space by means of coordinate transformation. A thermodynamic model to calculate and predict ternary adsorption excess data on the ternary liquid mixture/air interface from surface tensions is presented. The calculated adsorption excess data in the ternary triangle are discussed.

Prediction of Ternary Liquid Adsorption on Solids from Binary Data

The adsorption behavior of the ternary n-hexane/n-octane/n-tetradecane mixture and the binary n-hexane/n-octane, n-octane/n-tetradecane, n-hexane/n-tetradecane mixtures on the well-characterized TA 95 activated carbon has been studied. Measured binary data at 25 degrees C are used to predict the ternary data which are compared with measured ones. With the measured binary data at 15 degrees C, a prediction of ternary data is proposed without ternary experimental support. To predict ternary data from the binary ones, a thermodynamical model combining the methods of Myers and Price and Danner is used. Copyright 2000 Academic Press.