Abstract
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.
Highlights
The gasoline used by automotive cars has a high Research-Octane Number (RON above 95) and is mainly obtained from a stream fraction of the crude distillation column called Light Naphtha which in turn has a very low RON content
The mathematical model developed contains important time dependent group parameters that influence the overall dynamics of the fixed bed namely: (i) the residence time measured by a characteristic time L/vi; and (ii) the Linear Driving Force (LDF) mass transfer coefficient kLDF equal to 15Dc/rc2
The term L/vi can be viewed as the residence time τfb of the gas in the column, that is, the time that gas and the adsorbent within the fixed bed are in contact and the reciprocal of 15Dc/rc2 as the time τdif that the adsorbable species spent needs to diffuse into the pores of the adsorbent
Summary
The gasoline used by automotive cars has a high Research-Octane Number (RON above 95) and is mainly obtained from a stream fraction of the crude distillation column called Light Naphtha which in turn has a very low RON content (average RON of 63). Some of these additives were forbidden due to their toxicity (the case of lead) to make it usable by cars They increase significantly the final price of gasoline (MTBE and ETBE). Since this final TIP output stream still contains compounds such as the linear nHEX (RON 24.8), mono-branched 3MP (RON 74.5) and di-branched 22DMB (RON 94.0), there is nowadays a significant effort to discover new adsorbents to separate in a clean way the low from the high RON paraffins of the final TIP output stream (the separation by distillation is very energy consuming due to the close boiling point of the isomers) to reduce the use of additives
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