Gas-liquid mass transfer enhancement by catalyst particles, a modelling study
J.A.A. Hoorn1, J.A.M. Kuipers2, N.G. Deen2,*
1 DSM Ahead, Advanced Chemical Engineering Solutions (ACES),
P. O. Box 18, 6160 MD Geleen, The Netherlands
2 Multiphase Reactors Group, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
*Corresponding author: N.G.Deen@tue.nl
In chemical industry the importance of multiphase reactors is eminent already for decades. Still today there are enough challenges left to further improve performance of these reactors to maintain profitability and also to meet more strict (future) environmental targets. Especially for reactor systems in large bulk chemical plants every small fraction in reactor performance increase can be a substantial overall improvement. Not only in increased reactor yield as such but also separation downstream can be positively affected. Studies to support the fundamental understanding of the mechanisms taking place in the reactors and at the same time extending engineering skills are still very worthwhile to do. One of the more complicated multiphase systems is the so-called gas-liquid-solid reactor where gaseous and liquid components are allowed to react by means of a solid (supported) catalyst. A proper understanding of the hydrodynamics, mass transfer, heat transfer, and kinetics is required for the proper design of such a reactor. For slurry reactors, where the small catalyst particles are dispersed throughout the liquid phase, the gas to liquid mass transfer is one of the critical design parameters.
It is known that the rate of gas-liquid mass transfer can be significantly influenced by the presence of small (catalyst) particles in the liquid phase, either being gas-absorbing or reactive catalyst particles 1-6. The small particles near the gas-liquid interface can reduce the concentration of the dissolved gaseous component, thus increasing the driving force for mass transfer. Depending on the lyophobicity the particles can actually adhere to the gas-liquid interface7-9, reducing the diffusion distance. The enhancement of mass transfer due to catalyst particles has already been discussed in a number of papers, typically using a continuum approaches. Also, if internal diffusion limitation for the catalyst is taken into account this is typically done via the effectiveness factor approach. However, none of these models takes into account concentration inhomogeneities due to the presence of the catalyst particles in space nor the effect of diffusion limitation within the catalyst particle itself.
In this study, the gas-liquid mass transfer enhancement was studied using 3D unsteady state simulations. The diffusion of the gaseous component through a suspension of catalyst particles has been studied for different cases, viz. what is the effect of:
Randomly distributed particles throughout the liquid versus closely packed particles near the gas-liquid interface.
2) Catalyst particle diameter
3) Catalyst particle concentration
4) Rate of reaction, this also directly relates to the degree of internal diffusion limitation.
5) Gas-liquid diffusivity.
In addition to the 3D simulations a homogeneous model was derived which takes into account the effect of the catalyst particles and of internal diffusion limitation on the rate of gas-liquid mass transfer through the gas-liquid interface. The results of the homogenous model have been compared to those of the 3D simulations.
The results of the 3D simulations show that the enhancement factor increases with increasing catalyst concentration and increasing rate of reaction, as expected. As the particle diameter is increased, the enhancement factor decreases due to an increasing degree of diffusion limitation. The results of the homogeneous model match with the results of the numerical simulations within a 10% error. Therefore it is concluded that the significantly less complex and time consuming homogeneous model can be used to predict the enhancement of the mass transfer due to the presence of catalytic particles.
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