The effect of chemical reaction on the effective diffusivities within mono-and bi-distributed porous catalyst was investigated experimentally and theoretically. An experimental method was developed to study the adsorption and diffusion of reactant and product with chemical reaction by simultaneously analyzing the responses of both product and reactant to small perturbation in the inlet of a recycle reactor.
Zeolite Y is used as catalyst, of which three types, LiY, NaY, and KY, are prepared by exchanging the cation. For each type, powder and binderless spherical pellets are used to study the contribution of micro and macropore diffusion to the overall mass transfer. The reaction used is the isomerization of cyclopropane in which molar volume does not change.
To study the effect of reaction on the effective diffusivity, the knowledge of precise adsorption and diffusion characteristics is necessary under both nonreactive and reactive conditions. The first experimental part deals with the measurements of equilibrium adsorption coefficients and diffusivities under nonreactive condition. In the second part of experiments adsorption coefficient effective diffusivities in macro- and micropore, and reaction rate constant are measured under reaction condition using recycle reactor and compared with the values obtained under nonreactive condition.
The equilibrium adsorption constant obtained ranged from 49.6 to 8995 $cm^3$/g in the range of temperature between 30℃ and 200℃. The isosteric heat of adsorption was also found to increase in the order, LiY ＜ NaY ＜ KY, and ranged from 7.5 to 8.7 Kcal/g-mole.
The intraparticle diffusivities are measured by constant volumetric dynamic sorption method at temperatures between 30℃ and 70℃. The values obtained ranged from 3.75x$10^{-13}$ to 2.39x$10^{-11}cm^2$/sec and increased in the order, KY＜NaY＜LiY. The activation energy varied from 12.1 to 13.5 Kcal/mole, increasing in the order, LiY＜NaY = KY. The results show that the diffusivity varies with surface concentration.
The experiments under reaction condition are carried out in a recycle reactor by injecting square pulse of cyclopropane using catalyst in powder and binderless spherical pelletforms at temperatures, 175℃ to 250℃. A mathematical model was obtained for the concentration changes of both reactant and product which made it possible to obtain the equilibrium adsorption constant, intra- and interparticle diffusivity, and reaction rate constant.
The equilibrium adsorption constant of the product, propylene, was smaller than that of cyclopropane, but the heat of adsorption and the activation energies of intraparticle diffusion were larger than those of cyclopropane.
For nonactive KY, the equilibrium constant and intraparticle diffusivity obtained in the recycle reactor are nearly the same as the values obtained under nonreactive condition. For active catalysts, NaY and LiY, in which the ativation energies were 27.5 and 20.5 Kcal/g-mole, the intraparticle diffusivities measured under reaction condition were lowered by one order of magnitude from the values obtained under nonreactive condition. This may be partially attributed to the counterdiffusion of reaction product, but is due mainly to geometric factors not sufficiently defined by commonly used pore volume, pore size, and surface area alone.
Four kinds of experimental methods widely used for evaluation of the diffusivity are simulated with a simple micro- and macropore model to show that the calculated effective diffusivity under inert condition and the difference between them increases as the fraction of micropore increases to one order or more.
Therefore, application of the effective diffusivities obtained in non-reacting system to reacting system as a packed bed reactor may give significantly errorneous results.
다공성 촉매를 사용하는 화학반응기의 효율을 나타내는 effectiveness factor 를 예측하기 위하여 많은 연구가 행해져 왔으며, 그것은 일반적으로 촉매내의 확산에 의해 크게 좌우된다. 다공성 촉매내의 확산은 촉매내의 기공의 구조가 복잡하여 쉽게 예측할 수 없으며 실험에 의하여 유효확산계수를 구하게 되며 그것은 촉매내의 기공의 구조에 의하여 크게 영향을 받게 된다. 다공성 촉매의 effectiveness factor 를 예측하는데 보통 반응을 수반하지 않는 경우에서 effective diffusitivity 를 구하여 반응을 수반하는 경우에 적용을 하여 왔으나 그의 타당성에 대해서는 의문시 되어 왔다.
본 연구에서는 단일 혹은 이중구조를 가진 제올라이트 촉매내에서 확산, 흡착 표면화학반응이 일어날 때 각각의 영향을 실험적으로 구하고 반응이 일어나지 않는 경우에 흡착 및 확산 실험을 통하여 비교하였다.
Faujasite형태의 제올라이트를 각기 양이온 교환에 의하여 서로 다른 성질의 KY, NaY, LiY 촉매를 제조하여 흡착 및 촉매내부의 확산 및 반응에 대해 연구하였다. 먼저 반응이 없는 상황에서 흡착 평형실험을 통하여 각기 촉매에 대한 cyclopropane 의 흡착평형 계수를 결정하였으며 그때 얻어진 흡착열은 $LiY
