(773c) Coupling Detailed Heterogeneous and Homogeneous Kinetics with Mass and Heat Transfer In Catalytic Reforming of Logistic Fuels

Coupling Detailed Heterogeneous and
Homogeneous Kinetics with Mass and Heat Transfer in Catalytic Reforming of Logistic
Fuels

Lubow Maier¹, Marco Hartmann²,
Steffen Tischer¹, Olaf Deutschmann^1,2*

¹Institute for Nuclear and Energy
Technologies, Karlsruhe Institute of Technology (KIT), Campus North,
Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344 (Germany)

²Institute for Chemical Technology and Polymer
Chemistry, Karlsruhe Institute of Technology (KIT), Campus South, Kaiserstraße
12, Karlsruhe 76128 (Germany)

*lubow.maier@kit.edu

Introduction

A considerable long term interest in hydrogen as a fuel in relation to
the scarcity of fossil fuels and the associated pollution problems excites the
development of systems using catalytic partial oxidation and steam reforming
for the production of hydrogen-rich synthesis gas from conventional fuels. Two
examples of such systems being currently of great technological interest are
the Solid-Oxide Fuel Cell (SOFC) [1] when operated with non-pure hydrogen fuels, e.g. partially reformed
logistic fuels, and short-contact time reactors for reforming gasoline and
diesel fuels [2], e.g., as first stage of an on-board auxiliary power unit (APU). The
non-linear coupling of complex homogeneous and heterogeneous chemical reaction
kinetics with heat and mass transfer in such systems matters for reactor
behavior, often even superimposed by transient modifications of the active
catalytic phase, e.g. by oxidation and coking

In this work, we will present an experimental, modeling, and simulation
study on a catalytic reformer for the production of hydrogen-rich synthesis gas
from the gasoline surrogate iso-octane. Reforming of this model fuel exhibits
all features mentioned above: complex homogeneous and heterogeneous reaction
schemes, mass and heat transfer effects, catalyst deactivation. Present work is
related to the coupling of models of these phenomena, and their computational
implementation to explain the impact of residence time on fuel conversion and
hydrogen production and to optimize the reactor performance.

Experimental and Modeling Methods

                The model couples elementary-step based
reaction mechanisms with a two-dimensional parabolic description of the flow
field in a representative number of monolith channels and heat transport in the
entire solid structure of the reactor including catalyst, heat shields,
insulation, and reactor wall (Fig. 1). This approach is realized in the computer code DETCHEM^MONOLITH,
which uses the code DETCHEM^CHANNEL for the simulation of the
individual channels [3].

The concept is applied to analyze
conversion, selectivity, and temperature profiles in partial oxidation of
iso-octane, a gasoline surrogate, over a rhodium/alumina monolithic catalyst. Although more complex fuels have already been studied experimentally
[4], data from our experimental
study of a single-component reference fuel iso-octane (2,2,4-trimethylpentane) [5] are chosen as reference because detailed
reaction mechanisms of CPOX of iso-octane over Rh has recently been developed
and coupled with homogeneous reaction schemes [4, 6]. Both homogeneous und heterogeneous
mechanisms are applied in the current study without further modifications.

Results and Discussion

The numerical
simulation predicts the two-dimensional temperature profile of the three
monoliths of the reactor as function of axial and radial position as well as
species profiles and product distribution in the single channel of monoliths for
the different flow rates (2 ? 6 slpm) studied at 0.8 and 1.0 C/O ratios.

It was found that the major objective of the reactor, i.e.
production of high hydrogen yields at minimal formation of coke-precursors, can
be achieved at C/O ratios close to 1.0 and sufficiently but not extremely high
flow rates. The counter-intuitive flow rate effect on hydrogen yield is
explained by the ratio of chemical heat release to physical heat loss. Coking
propensity is related to the C/O ratio, the flow rate, and the occurrence of
homogeneous fuel conversion.

The implementation of detailed chemical
kinetics in a two-dimensional parabolic flow model for individual monolith
channels and the coupling with heat balances of the catalytic monolith as well
as heat shields, insulation, and reactor wall provides a simulation tool that
is able to analyze the behavior of structured CPOX reactors in great detail.
The simulation can provide guidance to reactor design and optimization of the
operating conditions such as flow rate and fuel/oxygen ratio.

Figure 1.
Sketch of the catalyst section of experimental setup
with two heat shields simulated (top) and numerically predicted steady-state
monolith temperature at C/O = 1.0 and at flow rates of 2 slpm and 6 slpm
(bottom). The symmetry axis of the monolith is at radial dimension of zero.

References

1.       
Hecht, E.S., Gupta, G.K.,
Zhu, H.Y., Dean, A.M., Kee, R.J., Maier, L., Deutschmann, O. Appl. Catal. A.
General  295, 40 (2005).

2.       
Thormann, J., Maier, L., Pfeifer, P.,
Kunz, U., Deutschmann, O., Schubert, K. Int. J. Hydrogen Energy  34,
5108 (2009).

3.       
Deutschmann, O., Tischer, S. et al.
DETCHEM^TM vers. 2.3 (2010)www.DETCHEM.com

4.       
Hartmann, M., Maier, L., Deutschmann,
O. Combustion and Flame 157, 1771 (2010).

5.       
Hartmann, M., Maier, L., Deutschmann,
O. Appl. Catalysis A: General 391 144 (2011) .

6.       
Kaltschmitt, T., Maier, L., Deutschmann,
O. Proceedings of the Combustion Institute 33 3177 (2011).