OPTIMAL DESIGN OF CH4 CATALYTIC PARTIAL OXIDATION REFORMERS ON Rh
BASED CATALYSTS FOR SHORT-CONTACT-TIME SYNGAS PRODUCTION
Matteo Maestri, Alessandra Beretta*, Pio Forzatti,
Gianpiero Groppi, Enrico Tronconi, Dion Vlachos§
Laboratory of Catalysis and Catalytic Processes – Dipartimento di Energia
Politecnico di Milano – 20133 Milano, Italy
§
Center for Catalytic Science and Technology – Department of Chemical Engineering
University of Delaware, 19716, Newark, DE (USA)
*corresponding author. Email: alessandra.beretta@polimi.it
TOPIC 4: Modeling and scale-up of structured reactors and microreactors
Temperature [°C]
mmol/s
The possibility to produce H2/CO mixtures without external energy input in small reactors with reduced heat
capacity makes the catalytic partial oxidation (CPO) of methane and higher hydrocarbons a promising
solution for on-board and in situ generation of syngas for energy applications. Though very flexible, the
process is very complex: CH4 CPO short-contact-time reactors work under severe conditions, with very high
gas hourly space velocity and temperatures, complex fluid patterns and a strong coupling of heat and mass
transfer with surface kinetics. These severe operating conditions pose a challenge [1] for process stability
requiring a suitable trade-off between low hot spot temperature and high syngas productivity. In this work,
we have coupled the previously derived 1D dynamic model of the adiabatic CH4 CPO reformer [2] with our
recently developed microkinetic model for CH4 activation on Rh based catalysts [3] for the model-based
analysis of spatially resolved CH4 CPO experimental data on foams in adiabatic conditions [4]. This
improved model allows us to analyse in detail the behaviour of the reactor (including transport phenomena at
the macroscale and surface chemistry at the microscale) and propose strategies for process optimization.
Figure 1 compares spatially
1100
2
100 ppi
resolved experimental data [4] and
1050
H
2
model predictions for a typical
1.5 CH4
70 ppi
1000
CPO experiment on foam. The
950
model reveals that along the entire
1
O
900 40 ppi
reactor the most abundant reactive
CO
2
50 ppi
HO
intermediates (MARI) are CO*
850
2
0.5
and H* and the surface turns out to
800
CO
2
be reasonably clean. Under those
750
0
conditions of surface coverages,
0
2
4
6
8
10
-2
0
2
4
6
8
10
Axial length [mm]
Axial length [mm]
methane consumption occurs via
Figure 1. Species flows along the axial Figure 2. Effect on catalyst temperature pyrolysis followed by oxidation of
length. Solid lines: model predictions – of decreasing the transport properties of C* species by OH* and O2
Square symbols: experimental data [4]
the bed, by changing the cubic cell
consumption is always mass
length of the foam.
transfer controlled. In line with
previous results [2], this is confirmed by the analysis of the species concentration at the catalyst interface,
that points out that O2 concentration at the surface interface is nearly zero. Consequently, the heat release on
the catalyst surface is strictly governed by mass transfer. On the other hand, methane consumption has a
mixed chemical-mass transfer character and consequently the catalyst loading and distribution along the
reactor affect the rates of the endothermic reactions. Overall, the final temperature profile is determined by
the combination of several physical phenomena in addition to the chemical reactions (including quenching
effect of the feed cold gas, the rate of the O2 mass transfer, and the axial thermal dispersion). As an example,
we investigated (Figure 2) the effect of varying the pore foam cell density, while keeping constant the Rh
load: the increase in interphase mass transfer resistances results in a more gradual dosing of O 2 along the
axial coordinate and consequently in a reduction of the hot-spot temperature (from 1080°C for a 100 ppi
foam to 950°C for a 40 ppi foam).
The great sensitivity of the reactor behaviour upon those characteristics offers a large number of degrees of
freedom for reactor design and optimization, by proper tailoring of the type, the geometry and the material of
the catalyst support with a twofold purpose: reduction in hot spot temperature and enhanced production of
syngas. Variation of other parameters leading to reactor optimization will also be discussed. Experimental
validation is currently on-going.
References
[1] I. Tavazzi, A. Beretta, G. Groppi, M. Maestri, E. Tronconi and P. Forzatti, Catalysis Today, 129 (2007) 372-379
[2] M. Maestri, A. Beretta, G. Groppi, E. Tronconi and P. Forzatti, Catalysis Today, 105 (2005) 709-717
[3] M. Maestri, D.G. Vlachos, A. Beretta, G. Groppi and E. Tronconi, AIChE J., doi:AIC11767 (2009)
[4] A. Bitsch-Larsen, R. Horn and L.D. Schmidt, Applied Catalysis A: General, 348 (2008) 165-172