Because of the anticipated decline in the supply of petroleum in the future, natural gas recovery and storage activities have become more important in recent years. High-pressure adsorption processes have been utilised due to the commercial availability of a wide variety of microporous adsorbents, such as activated carbon. Therefore, it is important to pursue a theoretical and experimental study on understanding high-pressure adsorption of methane and other components in natural gas. Theoretical understanding of high pressure adsorption has been improved due to the applicability of molecular simulation, which has helped greatly to minimise the experimental effort in the prediction of adsorption isotherms and isosteric heats of adsorption over a wide range of conditions. This work will also involve experimental work to support theoretical findings. Although methane adsorption on carbonaceous solids has been extensively studied, understanding the fundamentals of high pressure adsorption is still a challenging field for the development of theories. The main objective of this project is to extend the knowledge of adsorption of methane and other components in natural gas on carbonaceous materials under subcritical and supercritical conditions and reconcile the differences between the experimental data and simulation results.
This thesis focused on the fundamental aspects of methane adsorption that is related to natural gas storage. To this end, choosing a proper interaction potential model for methane is crucial in the correct description of bulk fluid equilibria and adsorption on a surface and in a confined space of pores. Since high pressure and temperature above ambient are common operating conditions in natural gas storage, the proper potential for methane pair-wise interaction must account the repulsion between molecules. Among the many families of potentials that are available in the literature, the Lennard-Jones (LJ) model and the Buckingham Exponential-6 (Exp-6) model are selected because they have different degrees of stiffness in the repulsion, and it was found that both models can describe well the bulk phase behaviours and the adsorption data on graphite surface and in slit-like pores. This means that the extent of repulsion in these models is not critical to describe adsorption of methane at ambient temperatures, but it is expected that repulsion can play an important role at very low temperatures, which is outside the scope of this thesis. The LJ model was chosen in this thesis because its molecular parameters models are available in the literature for a very wide range of fluids. To gain a deeper understanding of how physical adsorption occurs, an adsorption process were analysed by means of microscopic tools, effectively carried out with a computer simulation, which is not possible experimentally. To this end, several microscopic analyses were introduced in this thesis, and we have shown that these analyses have helped to shed better light on the adsorption behaviours, such as the 2D transition, the compressibility of the adsorbed phase, the contribution of different regions of the adsorbed layer to the isosteric heat, and the energetic behaviour of the adsorbed phase such as the number of neighbouring molecules and the evolution in the arrangement of molecules.
Since high pressure adsorption process is required for the storage of natural gas under supercritical conditions, predicting the adsorption data using molecular simulation is more important than ever. A variety of steps were carefully taken to reconcile the differences between the experimental data and the simulation results. It was found that it can be accomplished by using a proper potential model for methane and other components that can describe the bulk phase behaviours and the vapour-liquid equilibria, and by having our own accurate experimental data because many data in the literature are sometimes inaccurate. This is because of the principle failure in getting accurate void volume as a small error in this could lead to a significant error in the calculation of the excess density. Finally to have a good molecular model for describing adsorption of natural gas, a study on adsorption of mixtures involving methane on carbonaceous surface was carried out and the simulation results were compared with experimental data. It was found that it is important to determine correctly the cross interaction parameters between the gas and the solid surface and between the various components in the mixture. We have successfully tested our molecular model with mixtures of methane-ethane and propane-propylene adsorption in carbonaceous solids.