(617gx) Resolved Particle Fixed-Bed CFD with Microkinetics for Methanol Oxidation on Silver

Resolved
Particle Fixed-bed CFD with Microkinetics for
Methanol Oxidation on Silver

Behnam
Partopour, Anthony G. Dixon

Department
of Chemical Engineering, Worcester Polytechnic Institute,

Worcester,
MA, USA, 01609

Formaldehyde
is one of the highest volume chemicals with annual production of 46.4 million
tons as of 2012.  One of the major production processes for formaldehyde
is methanol oxidation on silver. The reaction occurs at high temperature while
carbon dioxide is produced as a dominant by-product. A microkinetics
model was proposed and validated by Andreasen et al.¹, to describe the reaction mechanism. The
reaction parameters were highly affected by the temperature, species
concentrations, and reactor configuration. Therefore, it is desirable to study
the mechanism under realistic operating conditions. Recently, we have developed
two approaches to couple the microkinetics model with
computational fluid dynamics. Unlike the general approaches that couple CFD
with microkinetics by setting the reaction as
boundary condition for the fluid phase², here we integrate the
kinetics and transport by defining scalars inside the solid particles. The
first method is using the reduced microkinetics with
general rate expressions which are generated using the conventional reaction
engineering assumptions. The second approach is to couple the detailed kinetics
without any assumption about the elementary steps using splines regression with
look-up tables.³In this approach the microkinetics model is solved under the steady-state
condition initially. Then the reaction rates are mapped into quadratic splines
which are used for reaction rate evaluation during the CFD simulations. The
simulations are carried out for a randomly packed bed of several hundreds of
particles. It is observed that the reduced microkinetic
model deviates from the original full microkinetic
model under some extreme operating conditions. Moreover, diffusion limitations,
axial and radial heat and transport profiles and their effects on the
selectivity are studied. It is shown that the combustion is favorable at lower
temperatures. Therefore, formaldehyde mass fraction increases along the tube
while inside of the upstream particles (closer to the inlet) formaldehyde
concentration decreases. It is also observed that when the oxygen mass fraction
approaches zero inside the particles the overall reaction rate becomes zero.
However, in the regions where methanol concentration becomes very low but
oxygen and formaldehyde are present, combustion speeds up. These observations
can be used to guide operation of the reactor to improve 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.
       Wehinger GD, Eppinger T, Kraume M. Detailed
numerical simulations of catalytic fixed-bed reactors: Heterogeneous dry
reforming of methane. Chem Eng Sci. 2015;122:197-209.

3.
       Partopour B,
Dixon AG. Computationally efficient incorporation of microkinetics
into resolved-particle CFD simulations of fixed-bed reactors. Comput Chem Eng.
2016;88:126-134.