Tortuosity and mass transfer limitations in industrial hydrotreating catalysts: effect of particle shape and size distribution

Abstract

The tortuosity factor of γ-alumina supports and catalysts used in hydrotreating applications was evaluated in liquid phase with three different techniques: Pulse Field Gradient-NMR (PFG-NMR), Inverse Liquid Chromatography (ILC) and catalytic experiments in a batch reactor. In order to satisfy the specific experimental constraints associated to each technique, tortuosity factor values were evaluated in a wide range of operating conditions: temperature was varied from 25°C to 340°C, catalyst particles were used as-synthesized (trilobe extrudates) or crushed, the liquid composition was either pure toluene, n-heptane/squalane mixtures, or squalane/2,5-bis-(octadecyl)thiophene mixtures. It is demonstrated that not taking into account both the shape factor and the size distribution of the particles can lead to significant errors on the tortuosity factor value. For both parameters, the optimal mean spherical radius depends on the contribution of internal diffusion to the overall performance. When internal diffusion is the limiting step, a new expression for the mean radius of distributed particle is proposed and validated by comparison between ILC and PFG-NMR results. This expression is transposable to any catalytic system, making it possible to measure tortuosity factors using either microscopic or macroscopic methodologies. Moreover, the tortuosity factors obtained with ILC and NMR are in good agreement with the one estimated from the catalytic experiments, showing that mass transfer parameters can be extrapolated from non-reactive to reactive conditions.

Scaleable Hydrotreating of HTL Biocrude to Produce Fuel Blendstocks

“…Three catalyst geometries were simulated: (a) catalyst extrudates (cylindrical) with a radius of 0.5 mm and a length of 5 mm, (b) catalyst extrudates (cylindrical) with a radius of 1 mm and a length of 5 mm, and (c) crushed catalyst (spherical) with a radius of 0.05 mm. The catalyst properties inputs were as follows: catalyst bulk density of 330 kg/m 3 , catalyst thermal conductivity of 0.25 W/m/K, catalyst internal surface area of 11m 2 /g, catalyst pore volume of 4.8 x 10 7 m 3 /g, and catalyst pore tortuosity of 5. , …”

“…The catalyst properties inputs were as follows: catalyst bulk density of 330 kg/m 3 , catalyst thermal conductivity of 0.25 W/m/K, catalyst internal surface area of 11m 2 /g, catalyst pore volume of 4.8 x 10 7 m 3 /g, and catalyst pore tortuosity of 5. 12,35 This evaluation simulated the concentration of hydrogen in the pores assuming it was the limiting reactant. The solubility of hydrogen was estimated to be 0.00145 g/g based on the solubility of hydrogen in creosote oil at 400°C.…”

Tailoring morphology of hierarchical catalysts for tuning pore diffusion behaviour: a rational guideline exploiting bench-top pulsed-field gradient (PFG) nuclear magnetic resonance (NMR)

“…A powerful tool to probe diffusion inside porous materials is the pulsed-field gradient (PFG) NMR technique. [29][30][31][32][33][34][35] Amongst notable work done in the area of zeolites, Kortunov et al 36 have investigated the effect of introducing mesopores in microporous zeolites showing that if the introduced pores form as isolated cavities, little or no increase in diffusion coefficient is observed. Conversely, the formation of a newly formed interconnected pore network is expected to lead to significant changes in diffusion coefficients.…”

Effect of Intraparticle Mass Transfer on the Catalytic Site Formation in the Preparation of Silica‐Supported Metallocene Catalysts

“…However, the diffusion coefficient has been observed to change with diffusion time, [60] metal loading, [61,62] and silica particle tortuosity. [63] Both MAO and metallocene are expected to interact considerably with the pore wall leading to a lower diffusivity value compared to the bulk phase. [52] Ideally, the supported catalysts would have a uniform distribution of Al and Zr throughout the particle.…”