(411d) CFD for the Study of the Multiphase Flow and Mass Transfer in a Stirred Tank Reactor for the Screening of Shaped Catalysts

Many reactions in the petrochemical, refining and biofuel industries occur in the presence of shaped solid catalysts, in the form of beads or extrudates. These particles have typical dimensions of the order of 1 to 3 mm, catalysing reactions occurring in liquids or gas-liquid mixtures. Before the selection of a catalyst for an industrial process, primary screening tests need to be performed previously in laboratory-scale reactors to assess the catalyst efficiency in its commercial shape, preserving the spatial distribution of active sites. These catalyst testing reactors need to fulfil some essential requirements to allow for a correct assessment of the kinetics of the reactions [1]: isothermal operation; appropriate flow distribution and mixing rates; and the absence of interphase mass transfer limitations. If these requirements are not met, the measured rate will not be equal to the reaction rate but instead to the rate of the limiting transport phenomenon, i.e., mixing or mass transfer. With the continuous development of new and more efficient catalysts, and the growing interest in the use of complex fluids like heavy crude feedstock or biomass-derived, catalysts testing and selection in lab-scale reactors become often hindered by interphase mass transport limitations or by hydrodynamics.

One of the existing types of reactors used for catalyst testing are stirred tank reactors with annular stationary catalytic baskets, also known as Robinson-Mahoney (RM) catalyst testing reactors [2]. In RM reactors, the shaped catalyst is introduced in an annular basket made from a metallic net or a perforated metal sheet, and immersed in the liquid. An axial flow impeller is then placed in the center of the annular basket to promote simultaneously the flow through the catalyst walls and particles, and the mixing of the bulk. Although RM reactors have been reported to offer less mass transfer limitations comparatively to laboratory fixed bed reactors [3], its way of operation is not yet completely understood and mass transfer limitations can still occur. Small changes in catalytic basket reactor operations (rotation speed, liquid viscosity, foam formation,…) may greatly reduce the reactor performance, and recommend measuring mass transfer coefficients for the reaction conditions before performing the kinetic measurements [3]. In addition, geometrical changes to the basket or impeller designs, or the selection of a different catalyst filling, of different shape or size, can also lead to different reactor performances, even when similar operating conditions are chosen. Measurements of mass transfer rates on RM reactors are not abundant in the literature, and are always specific for reactor geometry, working fluid and catalyst shape and size [4-6]. This happens because the extensive experimental study of RM reactors at the laboratory becomes neither practical nor economical when a large number of geometrical or operational parameters needs to be studied. 

In this work, Computational Fluid Dynamics (CFD) is used to simulate the gas-liquid flow in a commercial lab-scale RM reactor. A multiphase 3D CFD model of gas-liquid flow in the reactor with the stationary catalytic basket has been developed with ANSYS Fluent. The studied reactor is a cylindrical, flat-bottomed tank with a diameter of 58 mm and a total volume of 300 cm³, from Top Industrie, designed for the kinetic testing of shaped catalysts. Mixing is promoted by a stainless steel radial flow self-inducing impeller, which simultaneously promotes de induction and dispersion of the gas phase in the liquid. A static annular basket is placed inside the tank around the impeller to support the shaped catalyst particles. The basket walls are made from a perforated steel sheet with a thickness of 0.5 mm and perforations with a diameter of 0.8 mm, corresponding to an open area of 45%. The radial flow turbine promotes the flow across the basket annular section, around the catalyst particles, from the impeller site until the tank wall. Four baffles with a width of 10 mm and a thickness of 1 mm are placed above the basket. The Eulerian-Eulerian multiphase flow model was chosen for the simulation of the gas-liquid flow. Turbulence in the flow, exteriorly to the basket, was simulated with the Standard Multiphase k-εModel, and the Multiple Reference Frame (MRF) approach was chosen to deal the impeller motion. The flow in the basket filling was simulated as a homogeneous and unconsolidated porous medium with the Brinkman-Forchheimer equations, and the perforated basket walls were simulated as semi-permeable zero-thickness surfaces with a pressure drop. The numerical results were compared with experimental data from Particle Image Velocimetry, a bubble tracking imaging technique and gas absorption measurements.

The CFD simulations allow knowing the velocity field and gas volume fraction in each point of the domain. This information is used to estimate the mass transfer coefficients between the solid-liquid and gas-liquid phases, k_S and k_La, respectively, and compare it to the characteristic time of the catalysed reaction. Simulations for different operational conditions are used to identify the operational regimes and the range of conditions that allow a correct assessment of reaction rates with the existing setup. The developed CFD flow model will be used as well in the future as an optimisation tool for the design of enhanced reactor configurations.

References

[1] C. Perego, S. Peratello, Experimental methods in catalytic kinetics, Catalysis Today, 52 (1999) 133-145.

[2] J.A. Mahoney, K.K. Robinson, E.C. Myers, Catalyst evaluation with the gradientless reactor, Chemtech, 8 (N12) (1978) 758-763.

[3] I. Pitault, P. Fongarland, M. Mitrovic, D. Ronze, M. Forissier, Choice of laboratory scale reactors for HDT kinetic studies or catalyst tests, Catalysis Today, 98 (2004) 31-42.

[4] M. Mitrovic, I. Pitault, M. Forissier, S. Simoens, D. Ronze, Liquid-solid mass transfer in a three-phase stationary catalytic basket reactor, AIChE Journal, 51 (2005) 1747-1757.

[5] I. Pitault, P. Fongarland, D. Koepke, M. Mitrovic, D. Ronze, M. Forissier, Gas–liquid and liquid–solid mass transfers in two types of stationary catalytic basket laboratory reactor, Chemical Engineering Science, 60 (2005) 6240-6253.

[6] M. Braga, Étude des phénomènes de transfert et de l'hydrodynamique dans des réacteurs agités à panier catalytique, in:  Ph.D. Dissertation, Claude Bernard University, Lyon, 2013.