Control of diffusive time scales in zeolitic imidazolate frameworks for the kinetic separation of light hydrocarbons
Pimentel, Brian R.
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The goal of this thesis is to investigate new sorbent materials for the kinetic separation of light hydrocarbons and incorporate them into a viable mass transfer contactor for use in a pressure swing adsorption unit. Three objectives are set to achieve this goal. i) The fundamental measurement of adsorption and diffusion phenomena within zeolitic imidazolate frameworks, and the investigation of how the structural flexibility of these affect guest transport. ii) Determining the role of diffusive time scales in the kinetically-selective uptake of a multicomponent mixture and the manipulation of those time scales for the separation of such a mixture. iii) The operation and evaluation of a kinetic pressure swing adsorption system for the separation of light hydrocarbons using structured Metal-Organic Framework fiber sorbent beds. The first objective demonstrated that adsorbent material flexibility is inherently linked with guest transport phenomena. Moreover, different structures are found to not be equally flexible; topological configurations can affect the flexibility of a pore based on its number of ring members. Furthermore, the flexibility of the structures is found to impart a unique temperature dependence on diffusion, where the changing pore rigidity leads to non-Arrhenius behavior. Exploring the second objective, idealized batch separations of multicomponent mixtures allowed for the tuning of the diffusive time constants in a kinetically-controlled adsorptive separation. It was demonstrated that differences in mass transfer may only be exploited at the relevant time scale of the process, which may be manipulated by changing the sorbent size or diffusion rates of the guest species via temperature changes. The transition from equilibrium-control to kinetic-control in a ethane/propane breakthrough system was demonstrated by only changing the crystal size, reversing the selectivity of the sorbent. The third objective culminated in the synthesis of ZIF-8/cellulose acetate fiber sorbent composites up to 60%, which were assembled into modules and employed in the kinetic separation of propane/propylene. Equimolar mixtures were separated up to an 81 mol% propane product, demonstrating the viability of the materials as kinetic sorbents for hydrocarbon separations.