(82d) N-Butane Partial Oxidation in a Fixed Bed: A Local Selectivity Study By Detailed Numerical Simulations

N-butane
partial oxidation in a fixed bed:

A
local selectivity study by detailed numerical simulations

Behnam
Partopour, Anthony G. Dixon

Department
of Chemical Engineering, Worcester Polytechnic Institute,

Worcester,
MA, USA, 01609

Maleic
anhydride (MA) is one of the most important chemical intermediates which is
mainly produced by partial oxidation of n-butane over VPO catalyst. The
reaction is fast and highly exothermic and therefore, a highly diluted inlet
flow (n-butane < 2%) is used to avoid the reaction runaway limit. Likewise,
unreacted n-butane is not recycled. The fixed bed process is carried out in
multitubular reactors with narrow tubes (<
6) cooled in a molten salt bath. All these factors lead to a non-optimal
process with an overall yield less than 65%.¹

            Recent
studies on MA production have been mostly focused on the catalyst active phase
nature while there are few reactor scale investigations. Therefore, we have
conducted a multi-scale CFD simulation of a packed bed of 807 spherical
particles to investigate the interactions of the fixed bed configuration with
the reaction, transport and the flow field. In this study a semi-empirical
surface kinetics² is coupled with the transport and momentum
equations in both the fluid and solid phases. It is shown that the temperature
increases very rapidly along the tube while due to the cooled tube wall a
strong radial temperature gradient forms in the bed. The particles are mainly
isothermal and the temperature gradient has the most significant impact on
species and reaction profiles along the bed. However, it could be observed that
compared to the species mass fractions, the weakly adsorbed oxygen surface
coverage is highly affected by the cooled tube wall, and a strong gradient is
formed inside each particle. Furthermore, the local selectivity patterns in the
bed follow similar patterns to the weakly adsorbed oxygen coverage. These
trends are shown in Figure 1.

A
statistical approach shows that the correlation is not an injective function
and there are more factors contributing to the local selectivity. A similar
study for the surface oxygen shows that reduced catalyst (higher surface oxygen
coverage) contributes to a lower local MA selectivity as is extensively
discussed in the literature, but again that is not the only contributor.
Therefore, it could be concluded that while the 3D detailed numerical
simulation of the bed shows that all local reaction conditions and particle
positions are affecting the local MA selectivity in the bed, temperature and
weakly adsorbed oxygen are the dominant factors in shaping the local selectivity
patterns.

Figure
1. Contours are shown on a mid-tube plane along the fixed bed

References

1- Trifirò, F., Grasselli, R.K. How the yield of
maleic anhydride in n-butane oxidation, using VPO catalysts, was improved over
the years. Top. Catal. 2014; 57 (14): 1188–1195.

2- Gascon, J., Valenciano, R., Tellez, C., Herguido,
J., Menendez, M. A generalized kinetic model for the partial oxidation of
n-butane to maleic anhydride under aerobic and anaerobic conditions. Chem Eng
Sci. 2006; 61:6385-6394