MOLECULAR SIMULATION STUDY OF THE SURFACE-BARRIER EFFECT - DILUTE GASLIMIT

The mass transfer resistance associated with penetrating the mouth of a very small pore is evaluted using classical molecular dynamics simul ation techniques. The effects of temperature, pore size, and thermal m otion of the adsorbent atoms are studied for a slit pore mouth model. Adsorption followed by surface diffusion to the pore mouth makes a sig nificant contribution to the mass transfer when the temperature is low or, equivalently, when the adsorptive potential is strong. Thermal vi brations of the adsorbent atoms have little effect on the adsorption/s urface diffusion mechanisms but cause fluctuations in the effective po re mouth area which can significantly affect transport rates. Perhaps the most important observation is that when the pore size approaches t he kinetic diameter of the gas molecules, changes of a few percent in the pore size cause order-of-magnitude changes in the resistance. Ther efore, it is possible that the surface barrier effect observed in zeol ites and carbon molecular sieves is governed by highly localized (sing le atomic layer) structural details.