posted on 2024-01-02, 16:46authored byRajamani Krishna
This article has the objective of
elucidating the variety of factors
that quantify influences of intracrystalline diffusion on mixture
separations in fixed bed devices packed with microporous crystalline
adsorbents such as metal-organic frameworks (MOFs) and zeolites. Such
diffusional influences may act either synergistically or anti-synergistically
to the mixture adsorption equilibrium, providing the ratio of the
diffusivities Đ1/Đ2 ≫1. Experimental data on transient mixture uptake
inside single crystals display overshoots in the loadings of the more
mobile guest species; this overshoot can be quantitatively captured
by use of the Maxwell–Stefan (M-S) diffusion formulation that
takes proper account of thermodynamic coupling influences; if such
thermodynamic influences are ignored, as is done in the Linear Driving
Force (LDF) model, overshoots are not realizable. The use of the M-S
formulation to simulate transient breakthroughs in fixed bed adsorbers
provides a quantitative match with experiments; the match is significantly
poorer if thermodynamic coupling effects are ignored. For a fixed
bed of length L, packed with adsorbent particles
of radius rc and operating with an interstitial
gas velocity, v, the diffusional influences are quantified
by two separate parameters: (a) diffusional time constant, Đ1/rc2, and gas–particle contact time L/v. The transient breakthroughs are uniquely dependent on
the product (Đ1/rc2)(L/v);
this result is of practical importance for scaling up from small scale
laboratory units to large scale industrial units that use different
particle sizes, bed dimensions, and gas velocities.