(712a) Diffusion of 1,3,5-Triisopropylbenzene in a Serie of Al2O3-Y Zeolite Particles By Zero Length Column Method: From Effective Diffusion to Pore Diffusion Coefficient

The kinetics of desorption of 1,3,5-triisopropylbenzene (TiPB) in a series of Al₂O₃-Y zeolite pellets using ZLC method is reported. In this work, Five adsorbents are involved, that is, Al₂O₃, Y zeolite and three pellets synthesized by different ratios of Al₂O₃, Y zeolite and carbon black. The three pellets are named 20Y, 30Y and 40Y by the percentage of Y zeolite respectively.

In the ZLC experiment, a small sample of 20-24 mesh adsorbent pellets (equivalent spherical radius is about 0.55 ) are mounted in ZLC unit, except that Y zeolite is pressed to a very thin layer of crystal. Therefore, for granular materials, it’s effective diffusion coefficients that are obtained in the diffusion experiments. The experiments are first carried out on Al₂O₃ materials at three different purge flow rates—100, 110, 120 sccm. The results show that the diffusion coefficients are consistent under the three purge flow rates, and thus the experimental processes are confirmed to be kinetically controlled. Therefore, ZLC experimental conditions are set as follows: Purge flow rate is 120 sccm and temperature ranges from 90 to 210 °C.

Effective diffusion coefficients calculated from ZLC desorption curve (D_eff) can be transformed into the effective diffusion coefficients in the pore (εD_p). The experimental result shows that under the experimental temperature conditions, the influence of the surface diffusion on εD_p can be neglected so that εD_p could be related to long straight channel diffusion coefficients (D_l) and D_l is composed of molecular diffusion (D_m) in larger channel and Knudsen diffusion (D_kn) in smaller channel. In this way, the correlation between D_eff and D_kn is established. The diameter of the pores in the Knudsen diffusion formula (d_kn) is defined to illustrate the relation between D_eff and D_l. As temperature increases, d_kn of the four samples tends to be constant. This result indicates that the diffusion processes of TiPB in the particle samples change from mesoporous control to macroporous control with increasing temperature.