The Effect of Diffusion Limitations on the NH3–SCR Process over a Shaped Fe-Modified Clinoptilolite Catalyst

Abstract

Research was carried out on the catalytic effect of Fe-modified clinoptilolite in the selective catalytic NO_x reduction by NH₃ (NH₃–SCR) in the real tail gas stream, taken from the Łukasiewicz – INS pilot plant for testing the ammonia oxidation process. To obtain a catalyst with the desired textural parameters and increase the acidity of a zeolite and the content of Fe³⁺ active sites, clinoptilolite was modified with iron using a two-stage method, including dealumination and wet impregnation. Catalytic tests were performed over the shaped Fe-modified clinoptilolite to study the effect of diffusion limitations on the overall NH₃–SCR reaction rate. The effectiveness factor of the catalyst shaped into pellets was determined based on the Weisz modulus. It was found that diffusion limitations influence the rate of the contact process to a greater extent, the larger the size of the shaped catalyst body. The contact process over the pellets 5.0 mm in diameter and 4.8 mm high occurred in the diffusion regime, and the diffusion limitations were influenced more by Knudsen diffusion than by molecular diffusion. Simulation calculations of the $X_{{\mathrm{NO}}_x}=f(T,\tau )$ relationship were performed for various sizes of the catalyst pellets. It was found that the temperature had an insignificant effect on the shaped catalyst’s performance within the considered “temperature window”. The pellet size and contact time proved to be significantly more critical for achieving high conversion.