Using modeling to select catalyst dilution methods for mass transfer intensification in lab gas–liquid fixed-bed reactors

Alberto Servia

doi:10.2516/ogst/2020071

Abstract

Heterogeneous catalyst testing methodology at the lab scale must provide intrinsic kinetics data for reactor design purposes as well as the intrinsic activity ranking during catalyst screening in the field of refining and petrochemistry. The significant downscaling of the past century coupled with the increasingly active catalytic formulations may introduce considerable momentum, mass and heat effects in experiments at small scale. Catalyst dilution has emerged as one versatile and robust way to reduce the impact of momentum and heat effects on heterogeneous catalyst testing. This paper presents a methodology based on global phenomena and catalyst dilution modeling to assess and optimize reactor loading techniques for specific problems. More particularly, the aim is to provide catalysts developers and kinetics experts with concrete guidelines for intensifying gas-liquid mass transfer in lab gas–liquid fixed-bed reactors through catalyst dilution. The methodology is applied to the kinetics determination of the oil residue Hydrodemetallation (HDM) and to the screening of catalysts for benzene hydrogenation. Layered dilution, consisting on the split of the catalyst in two beds separated by an intermediate bed containing an inert material of the same size as the catalyst, poorly improves gas–liquid mass transfer. Uniform dilution, based on the direct mixture of catalyst and inert material of the same size, significantly enhances gas–liquid mass transfer as the reactant local consumption per reactor unit volume is strongly reduced. Combinations of both abovementioned dilution techniques can be used with fast and/or high stoichiometric factor chemical systems operated at conversions higher than 70%. A new criterion is proposed to calculate the minimum dilution factor to guarantee negligible gas–liquid mass transfer limitations as a function of conversion, external mass transfer and global pseudo second-order kinetics.

Highlights

The exponential growth of the demand for petrochemical intermediates and the increasing market volume of heterogeneous catalysts for refinery applications ensure the attractiveness of this sector for the main world catalysts suppliers [1, 2]
Lab reactor volumes have been substantially reduced from some liters to a few milliliters or even less with the high throughput experimentation [3], with highly parallelized fixed-bed reactors containing less than 100 catalyst particles
The 520 °C+ liquid concentration is practically equal to the 520 °C+ liquid interface concentration, which means that gas–liquid mass transfer is not limiting for this lump

Summary

Introduction

The exponential growth of the demand for petrochemical intermediates and the increasing market volume of heterogeneous catalysts for refinery applications ensure the attractiveness of this sector for the main world catalysts suppliers [1, 2]. Lab reactor volumes have been substantially reduced from some liters to a few milliliters or even less with the high throughput experimentation [3], with highly parallelized fixed-bed reactors containing less than 100 catalyst particles. This downscaling coupled with the increasing chemical reaction rate of new catalytic formulations has triggered many studies on reactor performances, i.e. hydrodynamics, heat and mass transfer. Poor reactor performance may significantly affect the results and prevent from a reliable up-scaling

Methods

Results

Discussion

Conclusion