Dynamics of a Fixed Bed Adsorption Column in the Kinetic Separation of Hexane Isomers in MOF ZIF-8

A fixed bed adsorption mathematical model has been developed to describe the kinetic separation of hexane isomers when they flow through a packed bed containing the microporous Metal-Organic Framework (MOF) ZIF-8 adsorbent. The flow of inert and adsorbable species through the fixed bed is modeled with fundamental differential equations according to the mass and heat conservation laws, a general isotherm to describe adsorption equilibrium and a lumped kinetic mass transfer mechanism between bulk gas phase and the porous solid. It is shown that a proper combination of two characteristic times (the residence time of the gas in the fixed bed, \(\tau _{fb}\) and the characteristic time of diffusion of solutes into the pores \(\tau _{dif}\)) can lead to very different dynamics of fixed bed adsorbers where in a limiting case can gives rise to a spontaneous breakthrough curves of solutes. The numerical simulations of an experimental breakthrough curve with the developed mathematical model clearly explain the complete separation between linear n-Hexane (nHEX) and the respective branched isomers: 3-Methyl-Pentane (3MP) and 2, 2-Dimethyl-Butane (22DMB). The separation is due to significant differences in the diffusivity parameters \(\tau _{dif}\) between 3MP and 22DMB and the residence time of the gas mixture \(\tau _{fb}\) within the fixed bed. This work shows the importance of mathematical modelling for the comprehension and design of adsorption separation processes.