(550a) 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.