Catalyst Performance Testing in Multiphase Systems: Implications of Using Small Catalyst Particles in Hydrodesulfurization

Abstract

Three different gas–liquid–solid reactor configurations have been used to investigate the performance of a P-doped NiMo/Al2O3 catalyst in the hydrodesulfurization of dibenzothiophene. The commonly used millipacked bed reactor with 250–500 μm catalyst particles diluted with 125 μm inert particles, a micropacked bed reactor with 55–90 μm catalyst particles, and a slurry reactor with 150–250 μm catalyst particles were used in the catalyst performance testing program. It appeared that the inherently small particle size in the packed beds causes the hydrodynamics to be dramatically different compared to the industrially applied trickle-bed reactors. For particles smaller than typically 2 mm the capillary forces predominate over the viscous and gravitational forces, in sharp contrast to large-scale industrial reactors. Since the gas flow follows preferential pathways through beds consisting of small particles, the poor radial dispersion of the gaseous components can cause mass-transport limitations, even for a rather slow reaction such as the hydrodesulfurization (HDS) of dibenzothiophene, as a result of the strong inhibition by the reaction product H2S. An adapted criterion is proposed for estimation of the contribution of poor radial dispersion in catalyst performance testing.

Solvent-free semihydrogenation of acetylene alcohols in a capillary reactor coated with a Pd–Bi/TiO 2 catalyst

“…In addition, liquid tends to form preferential pathways between the catalyst particles leading to poor radial dispersion and wide residence time distribution, which in the case of semihydrogenation results in low alkene yield [42][43][44]. Magnetically-recoverable catalysts offer a convenient way of catalyst introduction and good reusability, but the price of the catalysts and complexity of reactor operation seem to be a disadvantage for industrial semihydrogenation at the current stage.…”

Performance Testing of Hydrodesulfurization Catalysts Using a Single‐Pellet‐String Reactor

“…In the evaluation it is assumed that external mass transport is not limiting. In earlier work we compared packed‐bed microreactors with larger‐scale trickle‐bed reactors . We found that external diffusion for the model reaction considered (hydrodesulfurization of dibenzothiophene) was not rate limiting in any of the discussed reactors.…”

“…4. It should be noted that the LHSV values shown only apply to those reactors containing a full catalyst bed (reactors[1][2][3][4][5][6][7][8]. For the other reactors, containing smaller bed volumes, proportionally higher LHSV values were applied.…”

Automated measurements of gas‐liquid mass transfer in micropacked bed reactors

“…For this reason, there have been several applications of µPBRs in highly exothermic or mass transfer limited three phase reactions, such as hydrogenation and oxidation where the small length scales allow for rapid heat removal and approaches isothermal operation. Additionally, micropacked beds are often applied in fast catalyst screening . Compared to typical bench scale screening systems that require 1–10 g of catalyst, such microreactors can use only 10–100 mg catalyst for rapid and efficient catalyst screening.…”

“…Micropacked beds exhibit liquid holdups in the range of 0.65∼0.85, which is substantially higher than 0.05∼0.25 observed in large‐scale trickle beds under similar superficial flow conditions . Alsolami et al found that in µPBRs, gas flow follows preferential pathways through beds consisting of small particles, allowing for poor radial dispersion of the gaseous components, which can lead to mass transport limitations. There have been several mass‐transfer studies in µPBRs using the hydrogenation of α‐methylstyrene to probe the transport, indicating the overall (gas–liquid–solid) volumetric mass‐transfer coefficients are in the range of 0.5∼6 s −1 .…”