Competitive adsorption and chromatography at supercritical conditions

Abstract

Measuring supercritical adsorption is difficult and represents a scientific and technical challenge. Compared to the adsorption of pure fluids, multicomponent adsorption is still less understood. The aim of this thesis is therefore to deepen our understanding of pure and competitive adsorption at near- and supercritical conditions, both from a fundamental point of view as well as in the light of the applications CO2 storage by Enhanced Coalbed Methane Recovery (ECBM) and Supercritical Fluid Chromatography (SFC). For this purpose, a high-pressure setup to measure competitive adsorption based on a gravimetric chromatographic technique has been successfully designed and built. The core of this setup is a magnetic suspension balance and the reliability of adsorption measurements at near-critical conditions depends on the homogeneity of the density in the measuring cell. Therefore, the impact of changing the way it is being thermostated has been investigated and an optimum configuration has been identified. The improved setup has subsequently been used to measure near- and supercritical adsorption isotherms of CO2 on activated carbon, of CO2 on 13X zeolite and of N2O on silica gel. The obtained results have been carefully discussed in terms of effects observed in the vicinity of the critical point, namely critical adsorption and critical depletion. The recent interest in geological CO2 storage is an area where the need for high-pressure competitive adsorption data on coal is increasingly felt. The possibility of enhancing the recovery of the trapped methane in these coal seams through the injection of CO2, has added a commercial value to the storage of CO2. However, the gases involved do not only adsorb in the coal pores, but also dissolve in the coal structure causing it to swell. Therefore, the first step towards a better knowledge of the ECBM process is to have reliable data on the adsorption equilibria of both single- and multicomponent mixtures of CO2, CH4 and N2 on coal and its swelling behavior. This study was performed on a dried coal sample from the Sulcis Coal Province (SW, Italy) and the CO2 uptake at reservoir conditions was found to be about 10% of its mass. The competitive adsorption isotherms showed preferential adsorption of CO2 over CH4 and N2. They have been described by a model based on lattice density functional theory (DFT), which is able to predict the ternary behavior based on the pure and binary adsorption equilibria. Finally, a highpressure view cell has been used to measure swelling, showing a maximum swelling of 4% under CO2 and 2% under CH4. The obtained results and the developed techniques are the firm basis for the design and implementation of future ECBM field tests. The second application studied in this thesis is SFC, where the solubility of supercritical CO2 is enhanced by the addition of small fractions of organic modifiers. Since preparative separations are usually performed under overloaded conditions, the adsorption isotherm usually becomes non-linear. Therefore, the parameters of a binary Langmuir isotherm and their dependence on density and modifier concentration were determined with SFC for the separation the enantiomers of 1-phenyl-1-propanol on Chiralcel-OD.