Spatially resolved probing of diffusion and reaction in porous catalyst pellets
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Technische Universität Hamburg (2020)
In the (petro)chemical industry, catalytic fixed-bed reactors packed with porous catalyst pellets are amongst the most widely employed types of reactors. The performance of these reactors in terms of product yield at the reactor outlet is determined by the interplay between diffusion and reaction inside each individual catalyst pellet. For decades, researchers in industry and academia alike have strived to optimize - through analytical and numerical simulation results and integral measurements - pellet size, shape, pore network, and orientation to the flow; and spatial distribution of the active component. To this date, operando measurements of concentration profiles inside porous catalyst pellets were not available. Industrial catalyst pellets are generally non-transparent and thus inaccessible by optical methods; gas spin densities are too low for magnetic resonance imaging, and traditional sampling methods are inadequate when considering the minute diffusional fluxes. In this work, a method was developed to directly measure spatially resolved concentration profiles inside a single catalyst pellet under reaction conditions, by employing a capillary sampling method. CO oxidation on a platinum-coated, porous alumina cylinder was chosen as a test reaction system. CFD simulations were conducted to ascertain the invasiveness of the proposed method; it was hence deemed acceptable for practical applications. Spatially resolved mole fraction profiles of products and educts, inside and in the boundary layer of the particle, are presented for different reaction conditions. Furthermore, phenomena that result from the interplay between diffusion and reaction such as boundary layers, bifurcation, multiple steady states, and kinetic oscillations are shown. Additionally, the possibility of coupling this method with Raman microscopy to gain spatial concentration and temperature profiles from the gas phase was explored. Overall, the developed method could allow the knowledge-based optimization of many industrial fixed-bed processes, without requiring major changes to the process layout.
Porous catalyst pellet