Experimental and theoretical comparison of flow modes in a catalytic

KERLEAU Philippe, GUINOT Stéphane, PALLIER Stéphanie, SCHWEICH Daniel,
PITAULT* Isabelle, HEURTAUX1 Fabien.
Laboratoire de Génie des Procédés Catalytiques, CNRS, CPE-LYON, 43, bd du 11 Novembre
1918, BP 82207, 69616 VILLEURBANNE, FRANCE.
Renault s.a.s., DTAA, 1 Avenue du golf, 78288 GUYANCOURT, FRANCE.
[email protected]
In this work, we have investigated hydrogen storage in and release from liquid hydrocarbons
for subsequent fuel-cell applications. This paper focuses on the hydrogen production step
using cycloalkanes as hydrogen carrier (weight storage capacities higher than 6wt-%).
Cycloalkane dehydrogenation reactions are highly endothermic. Heat can be provided by the
combustion of a small fraction of the aromatics, produced by cycloalkane dehydrogenation. A
catalytic structured plate heat exchanger reactor (HER) has been thus designed to couple both
the dehydrogenation and combustion reactions in a compact and efficient device that can
work under auto-thermal conditions. The HER is a stack of plate-like reactors. It is made of
two dehydrogenation reactors between three combustion reactors. This stainless steel device
is 5 cm long, 5 cm wide and 2.5 cm high. Each dehydrogenation reactor is a catalytic fixed
bed that contains 4.5 g powder catalyst (300 µm diameter pellets of Pt/Al2O3) and is designed
in order to maximise catalyst weight and minimize radial thermal gradients. Each combustion
reactor is a catalytic wall reactor wash-coated on its two faces. The latter reactors contain a
thin layer made of 0.7 g Pt/Al2O3 catalyst. The internal structures and arrangement of plates
have been designed in order to promote heat transfer between dehydrogenation and
combustion catalysts while ideal plug flow prevails in each reactor. The HER is equipped
with ten thermocouples. Four of them measure inlet and outlet gas temperatures. The others
are inserted in the walls between dehydrogenation and combustion reactors. They give an
indication on temperatures along reactors (inlet, middle and outlet). Methyl-cyclohexane
(MCH) is used as a model hydrogen carrier, and toluene (TOL) is used for combustion. MCH
and TOL-air mixture can be fed either co-currently or counter-currently. Autothermal
conditions are successfully achieved both in co-current and counter-current flow modes.
Lower temperature excursions along the reactor and higher conversions are achieved in cocurrent flow (MCH conversions resp. 59% and 78%, temperature differences along the reactor
resp. 50K and 20K for counter-current and co-current modes for 2.3 ml.min-1 MCH, 0.25
ml.min-1 TOL and 3 l.min-1 (STP) air inlet flows). This behaviour has been numerically
simulated using a 2-D pseudo-homogeneous plug flow model (dehydrogenation) and a 1-D
heterogeneous plug-flow model (combustion). In this HER, performing two highly endo and
exothermic reactions, the minimum entropy loss is obtained with the co-current flow mode,
contrary to heat exchangers. Finally, this HER is operated autothermally with a hydrogen
production of 5gH2.s-1.mHER-3.
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