(586c) First Principles Based Design of 3D Pyrolysis Reactors Using LES

Pyrolysis
is the main process for the production of many valuable organic building block
chemicals such as ethylene, propylene and vinyl chloride. The free-radical gas
phase reactions are accompanied by secondary reactions leading to the formation
of a carbonaceous coke layer on the inner walls of the reactor tubes. This
layer leads to an increased pressure drop over the reactor causing a loss in
selectivity. Additionally, the insulating effect of the layer forces a higher
furnace firing rate and higher tube metal temperatures to maintain the same
conversion. In industrial plants regular decoking procedures are hence
inevitable.

Because
of this detrimental effect on the overall economics, the design of novel
reactor geometries exhibiting improved heat transfer and reduced metal
temperatures with the aim of increasing run lengths has received quite some
attention [1]. In order to assess the effect of these
geometries however, one must account for the chemical kinetics, as well as the
complex flow phenomena in the reactor. While reliable microkinetic models with
thousands of reactions and hundreds of intermediates are widely applied,
implementing these efficiently in a Computational Fluid Dynamics (CFD) code
remains a challenge [2]. Commercial packages such as ANSYS Fluent
for example do not even allow kinetic models with more than 50 species.

Figure
1: Streamwise periodic temperature field for a
spirally corrugated pipe.

In
the present contribution, detailed kinetics were incorporated in the
open-source CFD code OpenFOAM by reduction of a butane pyrolysis reaction
mechanism, automatically generated with Genesys [3]. The quasi-steady-state approximation
is applied in order to reduce the stiffness of the system and the computational
time. A large eddy simulation (LES) methodology is employed in order to capture
the turbulent-chemistry interaction on a smaller scale than the classical Reynolds-averaged
Navier-Stokes (RANS) turbulence models. The length of the computational domain
is limited by extending on the periodic methodology of Patankar et al. [4] for fully developed velocity and
temperature fields. This novel approach is validated by comparison with
full-scale simulations for a variety of geometries and Reynolds numbers. Additionally,
a pilot plant setup with a standard tubular reactor and a spirally corrugated
tube is simulated and compared with the experimental results. The advantages
and disadvantages of using three-dimensional reactor configurations instead of
conventional tubular reactors are discussed.

Citations

[1] C.M. Schietekat, D.J. Van Cauwenberge, K.M. Van Geem,
G.B. Marin, Computational fluid dynamics-based design of finned steam cracking reactors,
AlChE J., 60 (2013).

[2] K. He, M.G. Ierapetritou,
I.P. Androulakis, Integration of on-the-fly kinetic reduction with
multidimensional CFD, AlChE J., 56 (2010) 1305-1314.

[3] N.M. Vandewiele, K.M. Van
Geem, M.-F. Reyniers, G.B. Marin, Genesys: Kinetic model construction using
chemo-informatics, Chem. Eng. J., 207?208 (2012) 526-538.

[4] S.V. Patankar, C.H. Liu, E.M. Sparrow, Fully Developed
Flow and Heat Transfer in Ducts Having Streamwise-Periodic Variations of
Cross-Sectional Area, J. Heat Transfer, 99 (1977) 180-186.

Acknowledgements

DVC
gratefully acknowledges financial support from the combined IWT and BASF
Antwerpen N.V. Baekeland program. PAR acknowledges financial support from a
doctoral fellowship from the Fund for Scientific Research Flanders (FWO). The
authors also acknowledge the financial support from the Long Term Structural
Methusalem Funding by the Flemish Government ? grant number BOF09/01M00409. The
computational work was carried out using the STEVIN Supercomputer
Infrastructure at Ghent University, funded by Ghent University, the Flemish
Supercomputer Center (VSC), the Hercules Foundation and the Flemish Government
? department EWI.