Abstract
Packed bed reactors with diameters below 1 cm, millipacked beds hereafter, are often used to test catalysts. The prospect of using these reactors for catalytic kinetic measurements is quite attractive, but it requires a better knowledge of the flow characteristics of these reactors. This study focuses on the main hydrodynamic features of millipacked bed reactors in single phase gas flow. The effects of the bed length, gas velocity, reactor/particle diameter ratio (δ) and the use of fine inert powder as porosity filler are investigated via residence time distribution measurements and reactive tests. When using spherical particles, for values of reactor/particle diameter ratios between 1 < δ < 3, the behavior of the Peclet number, Pe, is not monotonous. Lower values of the Pe number for specific values of δ corresponding to large void spaces between the particles and/or between the particles and the walls, have been observed. For values of δ > 3 higher Pe numbers are observed, in particular at high gas velocities, due to the transition to a more uniform packing where the walls are no longer the major constraint. When using cylindrical particles, higher dispersion is observed when the cylinders align on each other along the reactor axis, as compared to when they are randomly arranged in the reactor. A criterion to calculate the maximum conversion that can be achieved neglecting dispersion effects has been proposed, highlighting the situations where it has to be used with caution. The dispersion in these reactors can often be neglected. In case the criterion is not fulfilled, filling the porosity of the beds with powder reduces the dispersion. Using porosity fillers in reactors with internal diameters between 2 and 4 mm showed an improvement of the conversions for n-heptane reforming and methylcyclohexane dehydrogenation, which is attributed to external mass transfer improvement. Fine and spherical powders fill better the porosity and yield lower dispersion and higher mass transfer.
Highlights
The development cycle of a catalyst begins with a broad screening of many catalyst formulations, which is followed by a detailed investigation of a selection of the best candidates aiming at gathering sufficient knowledge to design an industrial process and guarantee its performance
This study focuses on the main hydrodynamic features of millipacked bed reactors in single phase gas flow
The analysis of the temporal concentration profiles of the residence time distribution (RTD) measurements has confirmed that the axial dispersion model is adequate for the description of the hydrodynamics of millipacked beds with small values of d (Fig. 4)
Summary
The development cycle of a catalyst begins with a broad screening of many catalyst formulations, which is followed by a detailed investigation of a selection of the best candidates aiming at gathering sufficient knowledge to design an industrial process and guarantee its performance. The industrial catalyst development is preferably based on the final millimetric size catalyst pellet rather than the powder sample for several reasons. A powder catalyst may perform differently in terms of conversion and selectivity in presence of internal and/or external mass and heat transfer limitations so that a model coupling kinetic and mass and/or heat transport is necessary to predict the millimetric size catalyst performances (Sie, 1996), introducing more uncertainty in the process design.
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