Physical adsorption from mixtures of gases-Thesis submitted for the Degree of Doctor of Philosophy In the faculty of Science

Abstract

The thesis deals with a study of the adsorption from binary mixtures of gases on solid surfaces. A major part of the investigations is carried out on silica gel, while a part of the work consists of an investigation on molecular sieves. The thesis is divided into four chapters. Chapter 1 forms a general introduction on the adsorption from gas mixtures and reviews the results of the earlier studies on the subject and the current theories of mixture adsorption. Chapter 2 deals with the experimental techniques generally employed for studies on mixture adsorption. A static volumetric apparatus employed in the present investigations is described, some of the important components of the apparatus being a gas?circulation pump and a thermal conductivity meter for in situ analysis of the mixture. The method of experimentation is described. Chapter 3 deals with the study on the adsorption from argon–oxygen and nitrogen–carbon monoxide mixtures on silica gel at ?184°C. Silica gel samples of varying surface polarity obtained by activating the gel at 100°, 600° and 1000°C have been employed. Oxygen is preferentially adsorbed from argon–oxygen mixtures while carbon monoxide is the preferentially adsorbed component from nitrogen–carbon monoxide mixtures. The experimental data obtained in the present work have been analysed in the form of the adsorbed?phase–gas?phase composition diagrams (G–A diagrams). It is noticed that these diagrams are unique for a given adsorption system, and are independent of the total pressure and the total adsorption. A shift in the G–A diagram shows the diminution in the selective adsorption with decreasing surface polarity. The effect of surface polarity on the adsorption from mixtures in relation to the saturation pressure and the molecular size of the adsorbates has been discussed in detail. It has been observed that on a predominantly non?polar gel the selectivity in adsorption from argon–oxygen mixtures is due only to the differences in the saturation pressures and molecular sizes of the two gases. The significance of this observation has been discussed. The present data has been examined against a few of the current theories of mixture adsorption.

The Mixed BET equation has been applied to the present data. The theory has been successful in the case of the adsorption from argon–oxygen mixtures. In the case of adsorption from nitrogen–carbon monoxide mixtures, the applicability of the theory is less satisfactory.

The Ideal Adsorbed Solution Theory (IAST) is satisfactorily obeyed by argon–oxygen mixtures and it has been shown that the theory holds good even in the multilayer region. However, the prediction by the IAST is not satisfactory for the nitrogen–carbon monoxide system. The deviations from the theory have been attributed to the interactions between the unlike molecules in the adsorbed phase.

Certain improvised methods for the application of Mixed Langmuir and BET equations have been suggested and applied to the data in literature and also to the present work. The modifications consist in calculating the adsorbed?phase and gas? phase compositions using the ratio of the isotherm constants calculated in the low?pressure regions.

Conditions for the dependence of the adsorbed?phase compositions on total pressure have been discussed on the basis of various theories. It has been shown on the basis of Mixed BET theory that when the surface–adsorbate interactions are very different for the two gases, the adsorbed?phase composition would depend on the total pressure. Such a prediction of the pressure dependence has been verified by determining the adsorption from mixtures of nitrogen and argon on silica gel at ?184°C. Selectivity coefficients for various systems have been calculated from theories and compared with experimental results. Chapter 4 deals with the studies on the adsorption from mixtures of gases by molecular sieves. Adsorption from argon–oxygen, argon–nitrogen and nitrogen–carbon monoxide mixtures has been determined at ?184°C on molecular sieve 13X. Preferential adsorption of oxygen has been noticed from argon–oxygen mixtures, a high preference has been observed for nitrogen from nitrogen– argon mixtures and for carbon monoxide from nitrogen–carbon monoxide mixtures. The Mixed Langmuir equation has been applied to the data on the adsorption from argon–oxygen mixtures on 13X. The oxygen adsorption has been found to be higher than predicted. The preferential adsorption of oxygen has been correlated with the number of cations in the molecular sieve. It has been suggested that the preferential adsorption of oxygen occurs on the cationic sites due to the slightly higher polarisation interaction of the oxygen molecule. This correlation has been supported by studies on the molecular sieves 10X and 5A. It has been inferred from the high preference for nitrogen from nitrogen–argon mixtures that the dominant factor in the adsorption on molecular sieves is the surface–adsorbate interactions and not the saturation pressure of the adsorbate. The high preference for carbon monoxide observed in the case of nitrogen–carbon monoxide mixture adsorption has been attributed to the presence of a dipole in the carbon monoxide molecule. The adsorption behaviours from oxygen–argon, argon–nitrogen and nitrogen–carbon monoxide mixtures are compared and the importance of even small differences in the interactions in mixture adsorption has been emphasised. The present work further illustrates the role of cations and surface properties of molecular sieves in the separation and purification of gases.