Methanol Partial Oxidation on Silver in a Fixed Bed of Raschig Rings: An Integrated CFD with Microkinetics Study

Methanol partial oxidation on silver in a fixed bed of Raschig rings:

An integrated CFD with microkinetics study

Behnam Partopour, Anthony G. Dixon

Department of Chemical Engineering, Worcester Polytechnic Institute,

Worcester, MA, USA, 01609

 

           

            Formaldehyde is extensively used in the chemical industry both as an intermediate and as a final product. Fixed bed partial oxidation of methanol over a silver catalyst is one of the main production processes for formaldehyde. The silver catalyst is usually placed on cylindrical or ring shaped pellets. The reaction is highly exothermic with carbon dioxide as a by-product. Andreasen et al., proposed a microkinetic model to explain the reaction mechanism which was validated by experimental data¹. It is desired to study such detailed kinetics under realistic reacting conditions to understand the interaction of the fluid flow and transport with the kinetics, and how different parameters impact the overall performance of the fixed bed.

            In this work we have coupled the microkinetics model with resolved particle fixed bed CFD simulations using a spline mapping method that we have recently developed². Then the code is implemented for a packed bed geometry of 250 Raschig rings with tube to particle diameter ratio equal to five (i.e. N= 5 and d_p=8 mm). Previous resolved particle studies were all conducted in a packed bed of spherical particles³, and this is the first time such a simulation is done for bed of Raschig rings.

            The study is separated into two parts: 1- effect of surface species and operating conditions on local selectivity, 2- comparison between packed bed of Raschig rings and a packed bed of spherical particles.

            Preliminary results show that the temperature gradient is dominantly controlling the reaction and species profiles in the bed. Since the temperature profile is correlated to the fixed bed configuration it could be shown that the species profile in the bed is affected by fixed bed configuration and so is the selectivity. Furthermore, the comparison between spheres and rings shows that due to higher contact area between fluid flow and catalyst surface, the local selectivity is improved in the case of Raschig rings. The results show that particles with higher surface to volume ratio, and thus higher rates of heat removal, are cooler, which prevents the further oxidation of the products. Furthermore, in such particles reactant species diffuse faster to the particle center which also helps to improve the local selectivity.

References

1- Andreasen,  A., Lynggaard, H, Stegelmann, C, Stoltze, P. A microkinetic model of the methanol oxidation over silver. Surf.  Sci. 2003;544(1):5-23.

2- Partopour, B., Dixon, AG. Computationally efficient incorporation of microkinetics into resolved-particle CFD simulations of fixed-bed reactors." Comp. Chem. Eng. 2016; 88:126-134.

2- Maffei, T., Gentile, G., Rebughini, S., Bracconi, M., Manelli, F., Lipp, S., Cuoci, A., Maestri, M. A Multiregion operator-splitting CFD approach for coupling microkinetic modeling with internal porous transport in heterogeneous catalytic reactors. Chem. Eng. J. 2016; 283:1392.