Thermodynamic modelling of adsorption isotherms and kinetics for gas storage and cooling applications
Date of Issue2015
School of Mechanical and Aerospace Engineering
Adsorption occurs due to force interactions between a solid adsorbent surface and gaseous phase molecules. With an effect of low or negligible carbon footprint, it is capable of performing gas storage and adsorptive chilling. Extensive works have been done in the literature to establish the knowledge of adsorption isotherms, kinetics, isosteric heat of adsorption and adsorbent + adsorbate thermodynamic properties. All these works laid a foundation for industrial applications of adsorption science. Nevertheless, all existing classical adsorption models, as proposed by Langmuir, Toth, Dubinin, Henry and others, are limited by their individual deficiencies from being valid within the complete temperature and pressure conditions. As a result, it is necessary to develop a consistent adsorption thermodynamic framework, for understanding its behaviours ranging from the Henry’s region to the saturated pressures. From this motivation, this thesis presents the theoretical and experimental studies of the adsorbate + adsorbent thermodynamic frameworks, with prospective applications to natural gas storage and adsorptive cooling purpose. Firstly, novel adsorption isotherm and kinetics equations are developed from the rigor of the partition distribution function of each adsorbate adsorptive site on adsorbents, and the condensation approximation of adsorptive molecules. The proposed models are thermodynamically connected with the pore structures of adsorbent materials. The kinetics theory is formulated with the analogy of Langmuir kinetics. It is found that the proposed models are thermodynamically consistent from the Henry’s region to the saturated pressure. Secondly, the thermo physical properties of adsorbents, including AQSOA-type zeolites, HKUST-1 and MIL-101(Cr) metal-organic frameworks (MOFs), are verified and characterised by X-ray diffraction (XRD), scanning electron microscope (SEM), N2 ad/desorption isotherms and thermo gravimetric analyzer (TGA). A constant volume variable pressure apparatus is designed, developed and fabricated to measure the amount of adsorbate uptakes for various pressures and temperatures onto these materials. The experimental setup is extensively utilised to measure the amount of methane uptake for the temperatures ranging from 120 K to 303 K and pressures up to 10 bar. The proposed isotherm model is validated with the experimental data of silica gel + water and AQSOA-Z01, Z02, Z05 + water systems, and explains the S-shaped isotherms within acceptable error ranges (±5%), as compared with Langmuir and Toth isotherm equations. Thirdly, the derived kinetics model is verified with the experimental data of silica gel + water systems, with respect to various size and grain layers configuration. The results are compared with Langumirian kinetics model and the Linear Driving Force model (LDF). It is found that the derived kinetics model provides better results when they are compared with the experimental data. Besides the analytical and experimental studies, the Grand Canonical Monte Carlo simulation is performed and verified with the experimental results, in order to give an computational insight of CH4 adsorption onto MOFs unit cells. From the computational simulations, it can be found that the guest CH4 molecules favour the metal atom sites at lower pressure range, and then start to capture larger porous channels of the MOFs cell. Therefore, the adsorption of CH4 inside MOFs can be concluded to be a heterogenous process.
DRNTU::Science::Physics::Heat and thermodynamics