Anthony Dixon

Department of Chemical Engineering, Worcester Polytechnic Institute

"Selective oxidation in fixed bed reactors – insights from CFD simulations"

Abstract:

Fixed-bed reactors are widely used for large-scale, heterogeneously-catalyzed gas-phase reactions, such as selective hydrogenations and partial oxidations. These reactions have strong heat effects and can exhibit “hot-spots” which can reduce selectivity. Computational fluid dynamics (CFD) is a valuable tool in the modeling of fixed bed reactors. In particular, resolved-particle CFD in which a bed of particles is simulated directly, without invoking a porous media representation, is being used to probe reaction systems and understand the complexities of reactions in heterogeneous systems. This presentation will use three case studies to illustrate the information that these simulations can provide about the selectivity loss in fixed bed partial oxidations.

Maleic anhydride (MA) is one of the most important chemical intermediates which is mainly produced by partial oxidation of n-butane over VPO catalyst. The reaction is fast and highly exothermic and therefore, a highly diluted inlet flow (n-butane < 2%) is used to avoid the reaction runaway limit. Unreacted n-butane is not recycled. The fixed bed process is carried out in multitubular reactors with narrow tubes (N < 6) cooled in a molten salt bath. These factors lead to a non-optimal process with an overall yield less than 65%. A multi-scale CFD simulation using a semi-empirical surface kinetics in a packed bed of spheres will be reported to show the interactions of the fixed bed configuration with the reaction, transport and the flow field.

Formaldehyde is one of the highest volume chemicals with annual production of 46.4 million tons as of 2012 A major production process for formaldehyde is methanol oxidation on iron molybdate. The reaction occurs at moderate temperature, carbon monoxide is produced as a dominant by-product. A detailed surface chemistry model is used to describe the reaction mechanism. Particle shape is one of the most important parameters in the design and optimization of methanol partial oxidation reactors. Packings of spheres, cylinders, rings and trilobes were numerically generated and used to carry out resolved particle CFD simulations under industrial conditions. Pressure drop, voidage and velocity profiles, radial heat transfer, and local and overall conversion and selectivity are presented. The advantages and drawbacks of rings over other particle shapes are discussed.

Steady-state reactor analysis does not give the whole picture; when dynamics are considered then hot-spots, extinction waves and “wrong-way” behavior (increased downstream temperature following inlet feed temperature decrease) can result. The third case in this talk presents the interactions between the local pellet-scale dynamic responses, and the bed-scale global fields, using particle-resolved computational fluid dynamics. A microkinetic model of ethylene epoxidation over silver supported on spherical catalyst particles is used. We will show local dynamic hot-spot formation and wrong-way responses to inputs and their connections to bed-scale flow and transport.

Biography

Anthony Dixon joined the Chemical Engineering faculty of Worcester Polytechnic Institute (WPI) in 1980 after obtaining his Ph.D. in chemical engineering from the University of Edinburgh, and spending two years as an Assistant Scientist at the University of Wisconsin-Madison. He is currently a full professor, and served as interim department chair 1994/5. His research interests lie in chemical reaction engineering, including computational fluid dynamics (CFD), heat transfer in chemical reactors, mathematical modeling and simulation of fixed bed reactors, inorganic membrane reactors and diffusion in porous catalysts. He has contributed research in recent years in particular on CFD in fixed bed reactors, and has co-authored approximately 125 journal papers in this and other reaction engineering areas. He has consulted in industry and collaborated with industrial groups, most notably with Johnson Matthey Catalysts on using CFD for catalyst design. He has received several awards for his educational work and research, including the 2009 WPI Board of Trustees’ Award for Outstanding Research and Creative Scholarship, and the 2001 William H. Corcoran Award (with colleagues) for best paper appearing in Chemical Engineering Education. He is a Fellow of AIChE, and has been a member of various conference organizing committees and scientific committees. He is the Executive Editor for Catalysis and Reaction Engineering for Chemical Engineering Science, a Member of the Editorial Board for the International Journal of Chemical Reactor Engineering, a Member of the Editorial Board for Advances in Chemical Engineering, and an Editorial Board Member for Membranes.