Study On The Effect Of Internal Capillary Condensation On The Effective Diffusivity Of Gases In A Porous Pellet

The low-boiling volatile substances in multi-phase exothermic reaction were easy to appear capillary condensation in catalyst pores so as to reduce the diffusion rate. This research attempted to measure internal wetting fraction and the effective diffusivity in order to characterize the effect of capillary condensation on the gas diffusion in the catalyst, also provided an important basis for the catalyst preparation, the choice of carrier and the optimization of reaction conditions.The adsorption isotherms of three adsorbents, cyclohexane, acetone and methanol respectively, on an industrial catalyst(Ni/γ-Al2O3) bed was measured through gravimetric method. The experiments were carried out in the range from 373 K to 413 K and 0.4 MPa to 1.0 MPa. It showed that two steady states were found multiple hysteresis loops in a certain range of relative pressure. Shrinking of the hysteresis loops of the adsorption isotherms was observed with the increase of temperature. Under the same condition, the starting points of hysteresis loops would move towards the direction of relative vapor pressure decreasing with the polarity of adsorbent molecules increasing. Taking advantage of the D-R model to simulate the adsorption amount in the catalysts bed, the uncertain parameter could be confirmed according data linear fitting, which confirms the applicability and the accuracy of the model.The effective diffusivity of hydrogen in the catalyst pellet was measured by an optimized Wicke-Kallenbath steady-state diffusion cell at the same condition with adsorption experiments. Through changing the flow rate of adsorbent in a sequence from low to high to a maximum and then vice versa, the effect of relative vapor pressure of adsorbent on the effective diffusivity of hydrogen was investigated in the presence of capillary condensation within the catalyst pores. The effective diffusivity of hydrogen also displayed multiple hysteresis loops with respect to the relative vapor pressure of adsorbent, and the hysteresis loops also moved back and narrowed in width with the temperature rising. The plots of the effective diffusivity against the internal wetting fraction(fw) were drawn afterwards. It was shown that the less fw was, the more sensitive the effective diffusivity would be to changes in operating conditions. A correlation between the effective diffusivity of hydrogen and fw was proposed, which showed good agreement between the calculation and the experimental data.