632 ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 632–639. © Pleiades Publishing, Ltd., 2016. Original Russian Text © V.M. Khanaev, E.S. Borisova, P.N. Kalinkin, O.N. Kovalenko, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 5, pp. 636–644. Effect of Micropores on the Effective Diffusion Coefficient V. M. Khanaev a, b, *, E. S. Borisova a , P. N. Kalinkin a, c , and O. N. Kovalenko a a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia b Novosibirsk State Technical University, Novosibirsk, 630073 Russia c Novosibirsk State University, Novosibirsk, 630090 Russia *e-mail: VMKhanaev@gmail.com Received September 14, 2015 Abstract—The effective diffusion coefficient for catalysts differing in their porous structure has been derived from experimental data on H 2 S conversion in the Claus reaction. The effective diffusion coefficient increases under conditions of catalyst deactivation due to sulfur condensation in micropores. A mathematical model is suggested to describe the micropore effect on the effective diffusion coefficient. Keywords: Claus reaction, bidisperse catalyst, micropores, effective diffusion coefficient, modeling DOI: 10.1134/S0023158416050116 The effective diffusion coefficient is an important parameter for calculating catalyst grain effectiveness factor [1, 2]. It characterizes matter transfer inside the porous grain and must account for textural properties of the catalyst, such as grain porosity, pore radius, and capillary tortuosity. There are various methods of experimental determination of the effective diffusion coefficient. They can conventionally be divided into the following two groups: methods used under chemi- cal reaction conditions and physical methods involv- ing no chemical reaction. The physical methods are based on measuring the rate of matter diffusion through a porous medium [3–5]. Under chemical reaction conditions, the effective diffusion coefficient is determined from experimental reaction rate data obtained for a fine-particle catalyst fraction, when there are no internal diffusion limitations, and from reaction rates observed for a coarse catalyst fraction, when these limitations exist. In this case, the effective diffusion coefficient is the diffusion coefficient appearing in the diffusion equation describing mass balance for a catalyst grain under chemical reaction conditions [6]. Approximate methods of solving this equation typically reduce to calculation of the Thiele modulus. Knowing its value, one can determine the catalyst grain effectiveness factor via familiar formulas for a given grain geometry. Experimental data on cat- alyst grain effectiveness factor are used to calculate the Thiele modulus and, accordingly, the effective diffu- sion coefficient. The effective diffusion coefficient under chemical reaction conditions may differ from the same coeffi- cient determined when there is no chemical reaction [6, 7]. This is particularly true for catalysts with a non- uniform porous structure, including bidisperse ones, because reactant transport takes place in large pores, while chemical reactions occur mostly in small pores. Formulas for the effective diffusion coefficient in bidisperse structures involve two characteristic pore sizes, which are typically the average radii of macrop- ores and mesopores. At the same time, in addition to having mesopores and macropores, bidisperse porous structures may include micropores, which increase the reactive surface area. Under chemical reaction conditions, micropores and mesopores may behave differently because of the difference between the phys- ical processes taking place therein. For example, in the Claus reaction, there can be condensation of the reac- tion product (sulfur) in micropores. For this reason, in some cases the micropore and mesopore effects in the determination of the effective diffusion coefficient should be considered separately. Here, we report estimation of the micropore effect on the effective diffusion coefficient under conditions of the Claus reaction. EXPERIMENTAL The H 2 S conversion in the Claus reaction was determined for a fine-particle catalyst fraction with an average particle radius of 0.075 mm and for real grains with an average radius of 2.25 mm. The fine-particle fraction was obtained by crushing the real grains. Three commercial alumina catalysts and four labo- ratory-made ones were used in experiments. The chemical composition of all catalysts was as follows (wt %): Al 2 O 3 , ≥90: Na + K, ≤0.14; Si ≤0.3. Their phase composition was 80–85 wt % χ-Al 3 O 3 , + 20– 15 wt % γ-Al 2 O 3 .