(95f) Statistical Activity Variations in Diluted Catalyst Beds: Influence on Reactor Behavior

In multitubular fixed bed reactors for heterogeneously catalyzed highly exothermic gas phase reactions the catalyst bed is usually diluted with inert particles for improving the heat management. In order to distribute the heat generated by the reaction more evenly along the reactor length often a graded activity profile in axial direction is applied. This graded activity profile is realized by loading the tube segment by segment. Each of these segments features a different catalytic activity, which is commonly realized by different dilution levels. To help balancing the heat generation and heat removal out of the reactor with the concept of catalyst dilution, the active catalyst particles and the inert material must be perfectly mixed in the diluted zones [1, 2]. Otherwise the thermal stability of the reactor cannot be guaranteed and, e.g., catalyst deactivation may take place because of the formation of local hot spots. Since loading a mixture of active and inert particles into a reactor tube is a statistical process, non-uniform activity profiles in reactor tubes based on statistical variations cannot be avoided.

As basis for the investigation of these statistical activity fluctuations in diluted catalyst beds and their influence on reactor behavior, a numerical generation of random packings consisting of spherical particles [3, 4] is applied. The statistical process of loading a mixture into a reactor tube is mimicked by using a random number generator for the definition of active and inactive particles. The results were analyzed with regard to the statistical activity fluctuations in axial and radial direction. Additionally, the analysis of statistical activity variations in detail reveals significant non-idealities caused by accumulation of active particles in local clusters. From the geometrical point of view the extent of the non-idealities depends on the characteristics of the packing (e.g. tube-to-particle diameter ratio).

In order to study the influence of statistical activity variations on the reactor behavior, the partial oxidation of n-butane to maleic anhydride was investigated as case study. The simulation studies were carried out using 1D and 2D pseudo-homogeneous reactor models. The results illustrate significant deviations compared to the ideal case and a strong influence of operating conditions, especially for the inlet and coolant temperature. It is demonstrated that a direct link between the calculated activity profiles in axial and radial direction and the occurrence of local hot spots in the tube exists. By introducing sensitivity coefficients for reaction rates with respect to activity fluctuations based on the ideal case the extent of the deviations for the non-ideal case can be identified and described.

References

[1] E.M. Calverley, P.M. Witt, J.D. Sweeney, Reactor runaway due to statistically driven axial activity variations in graded catalyst beds, Chem. Eng. Sci., 80 (2012) 393-401.

[2] E.M. Calverley, P.M. Witt, J.D. Sweeney, Reactor runaway due to statistically driven axial activity variations in graded catalyst beds: Loading from pre-measured single tube aliquots, Chem. Eng. Sci., 90 (2013) 170-178.

[3] H. Freund, T. Zeiser, F. Huber, E. Klemm, G. Brenner, F. Durst, G. Emig, Numerical simulations of single phase reacting flows in randomly packed fixed-bed reactors and experimental validation, Chem. Eng. Sci., 58 (2003) 903-910.

[4] H. Freund, J. Bauer, T. Zeiser, G. Emig, Detailed Simulation of Transport Processes in Fixed-Beds, Ind. Eng. Chem. Res., 44 (2005) 6423-6434.