Adsorption-based fluorocarbon separation in zeolites and metal organic frameworks
Wanigarathna, Juwan Arachchillage Darshika Kumari
Date of Issue2018
Interdisciplinary Graduate School
Fluorocompounds have important applications in industry, but is environmentally unfriendly, which can cause ozone depletion and contribute to the global warming with long atmospheric lifetime and high global warming potential. Therefore, reclamation of used fluorocompounds via energy efficient adsorption-based capture and separation shall greatly contribute to control their environmental release while reducing the impact on the environment. R32 (difluoromethane), R22 (chlorodifluoromethane), R125 (Pentafluoroethane), R134a (1,1,1,2-Tetrafluoroethane) and R143a (1,1,1-Trifluoroethane) are important fluorocarbons that are widely being used in different types of air conditioning and refrigeration systems either as single components or as blends of two or more fluorocarbons. In this study, the choices of porous materials for the separation of blends of these fluorocarbons were systematically studied. During this study, absolute separation of R32, R22 and R125 fluorocarbon blend was achieved using 4A molecular sieve zeolite under ambient conditions. Steric effects were responsible for the separation of R32 and R22 from R125 while both steric and kinetics effects (due to the larger molecular size of R22 compared to R32) facilitated the successful separation of R22 from R32. The regular pore structure, excellent match of pore size of 4A zeolite with the molecular sizes of the fluorocarbon make the product gases very pure, to the extent of direct industrial applications. Further, a facile method for the adsorption separation of fluorocarbon blends containing R134a, R125 and R143a refrigerants into their pure components using commercial zeolite 13X and pore size modified 5A zeolite was introduced. The mixed gas breakthrough experiments reveal that zeolite 13X selectively adsorbs R134a over R125 and R143a. By running two adsorption cycles, it is possible to obtain R134a with ultrahigh purity. Through chemical modification of tetraethyl orthosilicate (TEOS), the pore size of 5A zeolite could be successfully narrowed to the extent to just adsorb R125 while excluding R143a. The modified 5A zeolite was utilized to separate refrigerant mixtures containing R125 and R143a into their pure components. Among the various types of adsorbents, emerging Metal Organic Frameworks (MOFs) displays excellent gas capture and separation performances thanks to the designability and adjustability of their structures and thereby the functions. The metal-organic frameworks UiO-66(Zr) and HKUST-1 are demonstrated to have excellent performance characteristics to separate fluorocarbon mixtures at room temperature. Breakthrough data obtained for binary R22/R32 and R32/R125 mixtures reveal high selectivities and capacities of UiO-66(Zr) and HKUST-1 for the separation and recycling of these fluorocarbon mixtures. Furthermore, the UiO-66(Zr) and HKUST-1 saturated with R22 and R125 can be regenerated at temperatures as low as 120 oC with excellent desorption-adsorption cycling stabilities.