Elucidating adsorption mechanisms in carbon molecular sieves and development of new methodologies for fast screening of gas and liquid adsorption systems

Abstract

The term “Separation Processes” refers to processes that transform a stream containing a mixture of two or more components into pure or concentrated single component product streams. During the last decades, due to the development of adsorption processes and emerging new porous materials, the adsorption processes are considered to be effective and economic means for separation of different gas and liquid mixtures at different operating conditions. The heart of the adsorption process is the adsorbent, whose performance determines the efficiency of the separation system. As a result, selecting the appropriate adsorbent is crucial for the specific gas and liquid mixture separation in terms of selectivity, capacity, recovery, and life time. In order to determine the optimum adsorbent for a particular separation of interest, thorough characterization is carried out involving the following measurements and analysis: surface (textural) characterization, adsorption equilibria, and detailed kinetics, e.g., diffusivity. This study provides systematic investigations of measurement techniques that were developed to determine, kinetics, equilibrium and the governing mass transfer mechanism for single and multi-component adsorption in nanoporous adsorbents. The main objective is focused on development and application of new and fast screening methods for adsorbents’ selection, and new as well as extend traditional methodologies to measure equilibrium, and selectivity of gas mixtures and liquid solutions. One of the screening techniques, for liquid adsorption developed in this study, is based on gas chromatography (GC) headspace technique. A new methodology was developed using liquid calibration technique. In addition, the earlier developed vapor calibration headspace technique was extended to non-ideal solutions. The newly developed technique was shown to provide significant advantages, e.g., more time effective as a result of direct liquid composition determination from the sampled vapor phase and using the simpler experimental setup by eliminating the injection of separate vapor streams required by the vapor calibration technique. Binary and multi-component solutions were also analyzed by applying both GC headspace techniques. Another important fast screening technique (differential column technique) was developed in this study for binary gas mixture analysis, e.g., adsorption of binary mixtures of CO2, CO and C2H4 on NaY zeolite. The novel technique measures binary mixture isotherms accurately and time effectively utilizing a simple experimental setup based on the fractional desorption measurements. The proposed techniques were compared to standard characterization techniques, e.g., gravimetric and chromatographic, to prove reliability of the proposed methods. A number of adsorption systems, e.g., CO2, CO, light hydrocarbons, ethylene and xylene isomers on zeolites and carbon molecular sieves, were chosen for the development and application of these methods. In addition, an in depth study was conducted to elucidate sorption mechanisms of adsorption systems where separation is controlled by complex kinetic-steric effects, as in a case of carbon molecular sieves (CMS) for separation of smaller molecules.