Fixed-bed reactor technology at the lab scale has experienced a strong evolution during the past half century. Lab reactor volumes have been substantially reduced for sustainability reasons. This significant reduction has been accompanied by the development of new generations of catalytic formulations with improved activity, selectivity and stability. Reactor downscaling coupled with increasing chemical reaction rates have triggered many studies on fixed-bed hydrodynamics and mass transfer phenomena. As a matter of fact, these phenomena can become limiting at lab scale, and as a consequence, they can prevent from a reliable catalyst performances scale-up.

Some approaches for reducing fixed-bed reactor effects on kinetics measurements can be distinguished. Bed dilution with inert particles is an efficient way to improve reactor hydrodynamics and heat transfer. Several studies in literature show that adding an inert material to fill the catalytic bed porosity strongly reduces axial dispersion effects and prevents from preferential pathways^[1-3]. This technique intensifies heat transfer providing that the inert material thermal conductivity is sufficiently high, as with SiC or non-porous ceramics (alumina, zirconia, etc)^[4]. Literature on catalyst dilution for the improvement of mass transfer within fixed-bed reactors at the lab scale is poor. Different ways of diluting catalyst can be considered. The inert material can be uniformly distributed along the entire bed. Different dilution factors can be applied at different reactor locations depending on the expected kinetic rate evolution. Another approach for catalyst dilution is to alternate catalytic and inert beds. What is the best technique to be applied for a specific case? Where should the inert material be placed? In what proportion?

The aim of this work is to study the effect of different catalyst dilution techniques on kinetics measurements related to chemical systems limited by gas-liquid mass transfer. More particularly, this study must provide catalysts suppliers and experts from the field of heterogeneous catalysts kinetic modeling with specific guidelines for improving catalyst testing methods.

The methodology, based on global phenomena and dilution modeling to assess and optimize reactor loading techniques, was applied to the oil residue hydrodemetallation and to the benzene hydrogenation. A kinetic parameters regression with available experimental data was carried out for both the applications prior to the evaluation of the different dilution approaches. Splitting consecutive catalytic beds with an inert bed of SiC particles with the same size as the catalyst pellets poorly improves gas-liquid mass transfer. Uniform dilution consisting on directly mixing both the catalyst and the inert material allows to strongly intensify gas-liquid mass transfer. This is due to the reduction of the reactant consumption per reactor unit volume, whereas the gas-liquid mass transfer rate is kept constant. Combinations of both abovementioned techniques can be useful for high reaction rate systems operated at conversions higher than 70%. The results obtained in this work allowed the establishment of a criterion on the minimum dilution factor to be applied for neglecting gas-liquid mass transfer effects on kinetics measurements. This criterion depends on conversion degree, external mass transfer rate as well as on global pseudo-second order kinetics. The dilution factor, defined as the ratio between the volume of catalyst and the total volume occupied by both the catalyst and the inert particles, decreases with the decrease of conversion and with the increase of the pseudo-second order kinetic constant. The agreement between rigorous modeling and criterion results is excellent, as illustrated in the figure below.

References

[1] Sie S.T. (1996) Miniaturization of hydroprocessing catalyst testing systems: Theory and practice, AIChE J. 42, 12, 3498-3507.

[2] Mears D.E. (1971) The role of axial dispersion in trickle-flow laboratory reactors, Chem. Eng. Sc. 26, 1361-1366.

[3] Gierman H. (1988) Design of laboratory hydrotreating reactors: Scaling Down of Trickle-flow Reactors, Appl. Catal. 43, 277-286.

[4] Taniewski M., Lachowicz A., Skutil K., Czechowicz D. (1996) The effect of dilution of the catalyst bed on its heat-transfer characteristics in oxidative coupling of methane, Chem. Eng. Sc. 51, 18, 4271-4278.