Diffusion Measurements of Ethane in Activated Carbon

Abstract

Increasing demand for energy in the world has made industries to look for economically efficient methods to produce energy. One possible approach is to increase natural gas productions, due to its cleanliness and lower price. However, energy consumption of the cryogenic processes is high in natural gas processing plants. Adsorptive separations are becoming widespread in various industries such as oil and gas as a promising alternative to the conventional cryogenic processes. Activated carbons are among the most attractive porous materials used in adsorption processes, due to their high internal surface areas and ease of availability in the market. They exhibit bimodal pore size distributions, comprising microporous structure throughout a network of larger macropores. This makes them interesting and challenging adsorbents in adsorption studies. The separation of ethane from natural gas is one of the important processes in gas processing plants. Ethane is highly needed as feedstock for ethylene production plants. In this thesis, adsorption kinetics of ethane on activated carbon was studied using Zero Length Column (ZLC) technique. ZLC is a useful chromatographic method to study equilibrium and kinetics of adsorption. It is known as a fast and easy lab-scale technique for adsorbent screenings and diffusion studies. In ZLC, external heat and mass transfer resistances and dispersion are eliminated by the use of low adsorbate concentration, small amount of adsorbent and high flow rates. In the experiments, the adsorbent sample is first pre-equilibrated with the test gas for a sufficient time. The kinetics and equilibrium information can be obtained from the desorption curve when the flow is switched to pure purge gas under controlled conditions. The experimental set-up was developed during this project. System characterization experiments such as dead volume measurements, detector selection, detector's response time calculations were performed and are discussed in detail. ZLC measurements were carried out to study the controlling diffusion mechanism and obtain the diffusivity values. By performing low concentration experiments, diffusion in macropores found to be the controlling resistance. Among the various mechanisms, molecular and Knudsen diffusion were identified to be important.