The catalyst packing in the Single-String-Pellet Reactors (SPSR) is straightforward and does not require special procedures. A single string of catalyst particles is loaded in the reactors with an internal diameter (ID) that closely matches the particle average diameter. This applies to single catalyst systems, as well as stacked-bed systems.
Extrudates loaded in the reactor tubes automatically align as a string of extrudates. The use of a narrow reactor avoids any maldistribution of gas and liquid over the catalyst bed, thereby eliminating catalyst-bed channeling and incomplete wetting of the catalyst.
To enhance hydrodynamics, an inert nonporous diluent material (with a defined average particle size distribution) is used as a filler. Before doing the final loading in a steel reactor tube, a trial loading in quartz reactors is often done to confirm the packing.
Technology on a Reactor Level
Fixed bed reactors are designed to approach the ideal plug flow regime. Rule of thumb criteria have been developed that are based on the ratio of the particle diameter to bed diameter, and the number of particles making up the bed length.
At the reactor level, the main aspects to consider are the axial dispersion, radial dispersion and wetting characteristics (trickle-flow reactor case), the flow velocity profile, and pressure drop across the catalyst bed.
General principles of down-scaling:
- Well understood heat- and mass-transfer characteristics
- Well understood hydrodynamics
- As close as you can get to operation under intrinsic kinetic conditions