Fuel Processing for Fuel Cells - Institut für Technische Chemie und ...

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50 Torsten Kaltschmitt and Olaf Deutschmann
The Beretta group (Politecnico di Milano) has recently been able to
show by a quartz capillary sampling technique that olefins are formed
even in the initial catalyst section over a Rh/alumina catalysts in CPOX of
propane (Donazzi et al., 2011), which clearly reveals that the application of
two-zone model is risky. In these models, it is assumed that, in the first
section of the catalyst, only catalytic conversion is significant, and then,
further downstream, only gas-phase chemistry needs to be considered.
In the experiment, the downstream part of the catalyst is coked-up;
here, the Rh surface cannot act as a sink for radicals. The study also
numerically simulated the coverage of the Rh catalyst as a function of
the axial coordinate, revealing a sufficient number of unoccupied surface
sites at lean (C/O¼0.8) conditions (Figure 15 (left)). At rich conditions
(C/O ¼1.2; Figure 15 (right)), however, the surface further downstream is
completely covered by carbon. This study (Hartmann et al., 2010) also
revealed that the applied chemical models—even the most detailed ones
available—need further improvement, in particular regarding the formation
of minor by-products at rich conditions.
7.2 Predicting and controlling coking in fuel reformers
7.2.1 Catalyst deactivation due to coking
The model discussed above predicts a surface completely covered by
carbon, C(s), in the downstream section of the catalyst, z>1 mm. Since
the model does not include direct interactions between gas-phase species
and carbon on the surface, the gas-phase conversion practically proceeds
Surface coverage
1E+00
Rh (s)
1E+00
1E-01
CO (s)
Rh (s)
1E-01
CO
1E-02
H (s)
1E-02
1E-03
1E-03
H O
C (s)
1E-04
1E-04
O (s)
C
1E-05
1E-05
OH (s)
1E-06
OH (s)
1E-06
1E-07
H 2 O (s)
1E-07
H 2 O (s)
1E-08
1E-08 Fuel-rich conditions—enlarged
1E-09
1E-09
CO 2 CO 2 (s)
1E-10
1E-10
0.0 2.5 5.0 7.5 10.0
0.0 0.3 0.5 0.8 1.0
Axial position [mm]
Axial position [mm]
Figure 15 Numerically predicted surface coverage as a function of axial position along
the honeycomb catalyst channel in CPOX of iso-octane over a Rh/alumina-coated
monolith. Conditions: C/O¼0.8, 1359 K (left), C/O¼1.2, 1076 K (right); taken from
Hartmann et al. (2010).
Surface coverage