Journal of Catalysis 198, 355–358 (2001)
RESEARCH NOTE
The Equilibrium Constant for the Methylcyclohexane–Toluene System
T. Schildhauer,1 E. Newson, and St. Mu¨ller
Department of General Energy Technology, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
Received September 15, 2000; revised November 20, 2000; accepted November 20, 2000; published online February 13, 2001
In previous kinetic work on the system (3) supported by
experimental data, the equilibrium constant determined in
the literature (4), Keq(T=650 K) = 4.61 109 kPa3, was used in
preference to an older literature value based on API data
(5), Keq(T=650 K) = 2.03 109 kPa3. Subsequent kinetic mea-
surements in an isothermal microreactor with high nitrogen
dilution in the feed still revealed a lack of fit if the equilib-
rium constant from (4) was used. For example, the value
of the activation energy appeared to vary with the nitrogen
dilution in the reactants.
The equilibrium constant for the methylcyclohexane–toluene
system has been experimentally determined due to a lack of
fit appearing in kinetic evaluations when using literature values
for the equilibrium constant. The most recent literature value
of Keq(T=650 K) = 4.61 0.04 109 kPa3 due to J. Akyurtlu and
W. E. Stewart (J. Catal. 51, 101 (1978)) was redetermined to a
value of Keq(T=650 K) = 3.60 0.05 109 kPa3 with isothermal ex-
periments using methylcyclohexane and toluene feeds separately.
c
2001 Academic Press
Key Words: methylcyclohexane; toluene; equilibrium constant;
wall reactors.
The purpose of this Research Note is to experimen-
tally determine the equilibrium constant for the methyl-
cyclohexane–toluene system to resolve this lack of fit.
INTRODUCTION
METHODS
The hydrogenation–dehydrogenation of toluene and
methylcyclohexane (MCH) has been studied for seasonal
hydrogen energy storage for both mobile (1) and station-
ary (2) systems. The efficiency of the system is strongly de-
pendent on the kinetics of the endothermic, equilibrium-
limited dehydrogenation reaction of methylcyclohexane
(C7H14) to toluene (C7H8)
The experimental setup consisted of a PC-controlled re-
actor system, allowing continuous operation while vary-
ing reaction parameters such as temperature, pressure,
concentration of reactants, and residence time. Hydrogen
and nitrogen gases (purity > 99.995% ) with liquid MCH
(>99.75% ) and toluene (>99.5% ) were monitored and fed
to a preheater and reactor system located in a heated, flu-
idized sand bath. Reaction took place isothermally in three
tube reactors in series (length 3 200 mm, inner diameter
4 mm). Their inner walls were coated with a 100- m
washcoat layer of alumina (BET area = 197 m2/g),which
was impregnated using the incipient-wetness method, re-
sulting in a 2.75 wt.% platinum catalyst. The prepara-
tion of these reactors has been described (6). On-line
temperature and concentration measurements (GC-FID)
were made with product separation and further analyses
(GC-MSD) for byproduct formation and carbon balances
over the unit. At different temperatures (578–643 K),
pressures (707–742 kPa), and feed inlet hydrogen-to-
hydrocarbon ratios between 17 and 27, the residence
time was increased from 20 to 66.5 s (MCH feed 1.26–
0.44 ml/h or toluene feed 1.56–0.9 ml/h), until no further
change in conversion occurred to attain equilibrium in the
reactor.
C7H14 ⇔ C7H8 + 3H2 1Hor = 205 kJ/mol,
[1]
where 1Hor is the endothermic heat of reaction under stan-
dard conditions. The equilibrium limitation of the reaction
rate can be described as
r = k pMCH
1
pToluene pH3 Keq pMCH
,
[2]
2
where r means the reaction rate, k the reaction rate con-
stant, pi the different partial pressures, and Keq the equilib-
rium constant, which depends on the temperature:
Keq = Keq(T =650 K) exp 1Hro R (1/T 1/650 K) . [3]
1 To whom correspondence should be addressed. Fax: ++41-56-310 26
24. E-mail: tilman.schildhauer@psi.ch.
355
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Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.