Thermal hazard analysis of a dehydrogenation system involving methylcyclohexane and toluene
Journal of Thermal Analysis and Calorimetry
Thermal hazard analysis of a dehydrogenation system involving methylcyclohexane and toluene
Jo Nakayama 0 1 2
Hiroyuki Aoki 0 1 2
Tomohiro Homma 0 1 2
Nana Yamaki 0 1 2
Atsumi Miyake 0 1 2
Atsumi Miyake 0 1 2
0 Institute of Advanced Sciences, Yokohama National University , 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501 , Japan
1 Graduate School of Environment and Information Sciences, Yokohama National University , 79-7 Tokiwadai, Hodogayaku, Yokohama, Kanagawa 240-8501 , Japan
2 Center for Creation of Symbiosis Society with Risk, Yokohama National University , 79-5 Tokiwadai, Hodogayaku, Yokohama, Kanagawa 240-8501 , Japan
The development of hydrogen infrastructure is important because its commercialization will help reduce carbon dioxide emissions significantly. The construction of hydrogen fueling stations will increase the demand for fuel cell vehicles. While the risk associated with various types of fueling stations has been assessed, and appropriate safety regulations have been proposed, there have been few studies on hydrogen fueling stations with on-site dehydrogenation systems that use methylcyclohexane (MCH). This is because such stations are very new. In particular, the thermal hazards associated with such systems must be analyzed because they could lead to equipment damage. The purpose of the present study was to identify the thermal hazards of such systems using various thermal analysis methods. Thermal analyses were performed while assuming spontaneous ignition and oxidation under normal and abnormal conditions, in order to identify the thermal hazards associated with the storage of MCH, toluene, and heat carriers in underground storage tanks as well as their use in the dehydrogenation reactor. In addition, the thermal safety of the tank and the reactor was estimated based on the results of the thermal analyses. It was found that the underground storage tanks for MCH and toluene have a lower thermal risk because the process conditions are mild, and the thermal hazards related to the chemicals are low. Further, in the case of the dehydrogenation reactor, the risk of the spontaneous ignition of the heat carrier is low under quasi-adiabatic conditions and moderate air ventilation, in case the heat carrier leaks from damaged piping and equipment. However, it is important to regularly inspect the reactor to prevent any issues that may arise from an exothermic reaction of the heat carrier.
Hydrogen fueling station; Thermal hazard; On-site hydrogen production system; Methylcyclohexane; Toluene
Division for Environment, Health and Safety, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan
Hydrogen shows significant promise as an energy carrier.
An effective use of hydrogen is as a fuel in fuel cell
vehicles (FCVs), which emit water and oxygen instead of
carbon oxides, nitrogen oxides (NOx), and sulfur oxides
(SOx). This characteristic of FCVs should aid the
realization of a sustainable society, because such vehicles can
help significantly reduce the emissions of the greenhouse
gases responsible for climate change. Building hydrogen
fueling stations all over the world is essential for the
commercialization of FCVs. Hydrogen fueling stations
have been constructed in several countries, including the
USA, Germany, and Japan. However, safety issues have
limited their wide adoption, because these stations store a
large amount of hydrogen compressed to 82 MPa in
pressurized storage tanks. Hydrogen is inherently hazardous,
owing to its explosiveness and embrittlement [
factors significantly increase the risk of accidents.
Therefore, improving the safety of hydrogen fueling stations is
the key to their wider acceptability.
Hydrogen fueling stations can be categorized into two
types: off-site stations, which require the transport of
compressed or liquified hydrogen from other places, and
on-site stations, which involve the local production of
hydrogen by chemical reactions. These stations consist of
common components, such as a hydrogen compressor,
pressurized storage tanks that store hydrogen at 82 MPa,
and dispensers for the delivery of hydrogen to FCVs. The
presence of a hydrogen production system is the only
difference between on-site and off-site hydrogen fueling
stations. Off-site stations are the most common type
throughout the world and have had several safety issues,
which have been investigated. For example, the leakage
and dispersion of compressed or liquified hydrogen have
been analyzed both experimentally and using simulations
]. Further, consequence analysis and risk assessment
studies of off-site hydrogen fueling stations have been
performed to determine the necessary safety measures
]. In addition, the results of these assessments have led
to the development of risk assessment software such as
FLACS, HyRAM, and SUSANA [
]. In contrast, the
safety issues related to on-site hydrogen fueling stations
have not been studied widely, even though doing so could
improve the safety of on-site stations significantly. A
qualitative risk analysis was performed using HAZID for a
hydrogen fueling station with an on-site hydrogen
production system involving methylcyclohexane (MCH) [
A simulation-based safety investigation of the domino
effect was also performed [
]. The results of these studies
helped with risk reduction and provided suggestions for
safety measures. However, they did not produce concrete
safety data for on-site hydrogen production systems,
because the studies focused on the holistic risks of the
station. Further, because the system studied was at the
conceptual design stage, detailed design information such
as data related to the equipment size, material, flow rate in
pipes, and safety measures was not available. On-site
systems carry the risk of temperature and pressure
increases, which can accelerate the dehydrogenation of MCH into
hydrogen and toluene. However, these thermal hazards
have not been studied in detail. These risks can lead to fires
or explosions at the stations, causing major injury and
property damage, not only to the FCV drivers and
passengers using the stations but also to the station
maintenance personnel and neighborhood residents. Thermal
analysis is a useful technique for identifying and evaluating
the thermal hazards of dangerous substances [
], lithium-ion batteries [
], ionic liquids
], and reactive monomers [
]. Thus, the purpose of this
study was to analyze the thermal hazards of
dehydrogenation systems that involve the use of MCH, toluene, and
heat carriers, based on thermal analysis equipment.
Isothermal and non-isothermal tests were performed for
accident scenarios while focusing on the spontaneous
ignition of the chemicals involved, in order to elucidate the
potential hazards of the system. Based on the analysis
results, safety measures for developing safer systems are
Accident scenarios for on-site hydrogen production systems
Figure 1 shows the MCH dehydrogenation reaction, while
Fig. 2 shows a schematic of an on-site hydrogen
production system installed at a hydrogen fueling station. The
production system consists of an underground MCH
storage tank, dehydrogenation reactor, heat exchanger, gas–
liquid separator, hydrogen refinery, and underground
toluene storage tank. MCH stored in the tank is transferred
to the reactor, where toluene and hydrogen are produced by
the dehydrogenation reaction in the presence of a catalyst
at ca. 400 C under 1.0 MPa. This is followed by the
separation of the toluene and hydrogen in the separator.
The toluene is stored in the tank before being sent to a
toluene hydrogenation plant, while the hydrogen is purified
and transferred to a pressurized hydrogen tank.
Accident scenarios exist for each component of the
system. Three scenarios likely to cause unexpected
reactions involving rapidly increasing temperatures and
pressures were identified, and experiments were performed
based on these scenarios while assuming both normal and
The first scenario involves the spontaneous ignition of
MCH or toluene in the underground storage tanks. Normal
tank conditions include ambient temperature and pressure
and are similar to those for underground gasoline storage
tanks. The consumption of MCH, which produces toluene,
requires the storage of toluene over a period of weeks.
Oxygen enters the tank when the MCH is delivered, and the
toluene is recovered. Introducing inert gases into the tank is
not practical. On the other hand, oxygen can oxidize the
MCH or toluene, resulting in a thermal distribution because
the large size of the tanks results in adiabatic conditions.
These events are likely to accelerate the thermal
decomposition process and trigger spontaneous ignition in the
tank, resulting in a fire. Moreover, if the MCH is stored for
long periods, it slowly oxidizes, producing organic
], which can decompose violently. Therefore, if a
small amount of the organic peroxides derived from MCH
were to be produced in the tank, the heat from the
exothermic decomposition reaction could lead to a fire
involving the MCH.
The second scenario involves the oxidation of MCH,
toluene, and the heat carriers within the dehydrogenation
reactor. Normal reactor conditions include a temperature of
ca. 400 C and a pressure of less than 1 MPa. Therefore,
the oxidation of MCH, toluene, and the heat carriers should
not occur. However, MCH containing dissolved oxygen or
the oxygen introduced into the system owing to material
corrosion or equipment failure can result in oxidation.
The third scenario also involves oxidation but that of the
heat carriers in and around the heat exchanger. When
passing through a pipe, the heat carriers can leak because
of corrosion and erosion, soaking into the adiabatic
materials under atmospheric conditions. This situation could
inhibit heat release and promote heat accumulation,
accelerating the oxidation of the heat carriers and causing
Pressurized hydrogen tanks
Gas liquid separator
Toluene underground storage tank
The MCH and toluene samples used in this study were
obtained from Wako Pure Chemical Industries, Ltd. Two
industrial heat carriers, heat carrier_T and heat carrier_N,
were also used.
The experimental conditions used were based on the
three above-mentioned scenarios. With respect to the first
scenario, isothermal tests were conducted using a thermal
activity monitor (TAM IV/Q200, TA Instruments). For the
TAM IV measurements, ca. 550 mg of MCH or toluene
was loaded into a sealed SUS ampoule with a glass inner
vessel in air, and the vessel was stored at 30 C for a week.
For the abnormal situation, the thermal degradation
products of MCH in air were synthesized; these are referred to
as MCH-air. The synthesis conditions involved the storage
of MCH at 100 C in an air atmosphere in a pressurized
vessel at 0.5 MPa. After the measurements, the MCH-air
compounds were condensed in an N2 atmosphere for 31 h
for purification. Non-isothermal screening tests were
performed using differential scanning calorimetry (DSC)
(Q200, TA Instruments) to evaluate the thermal hazards of
MCH-air. During the DSC measurements, ca. 4 mg of
MCH and MCH-air was loaded into a SUS303 cell, which
was sealed in an Ar atmosphere and heated from 50 to
300 C at a heating rate of 10 K min-1.
For the second scenario, non-isothermal tests were
performed using DSC (HP DSC 827e, Mettler Toledo, and
Q200, TA Instruments). During the DSC measurements,
ca. 2 mg of MCH, toluene, and the heat carriers was loaded
into a SUS303 cell, which was sealed either in air or in an
Ar atmosphere and then heated from 30 to 400 C at a
heating rate of 5 or 10 K min-1. The tests did not compare
the thermal behaviors of the different samples, as the
purpose of the tests was to elucidate the thermal behavior
of each sample using screening methods. Therefore, the use
of different experimental conditions was not an issue.
For the third scenario, non-isothermal tests were
performed using thermogravimetry (TG) (STA2500, Netzsch)
measurements. For the measurements, ca. 3–4 mg of the
heat carriers was loaded into an alumina cell, which was
heated from 30 to 500 C at 5 K min-1 in air or a He
atmosphere. In addition, non-isothermal tests were
performed using high-sensitivity heat flux calorimetry (C80,
Setaram). Approximately 300 mg of the heat carriers was
loaded into a sealed SUS304 vessel with an inner glass
vessel in an air atmosphere at 0.1 MPa. The samples were
heated from the ambient temperature to 300 C at a heating
rate of 1 K min-1. After the first measurement, the vessel
lid was opened, and the vessel was ventilated to increase
the amount of oxygen in it. Then, the MCH was reheated
under the same conditions, in order to analyze the
exothermic reaction related to oxidation. This procedure
was performed twice. A chemical composition analysis
was performed using Raman spectroscopy (RamanRxn2,
Kaiser Optical Systems, Inc.), in order to investigate the
causes of oxidation.
Results and discussion
Figure 3 shows the thermal behaviors of MCH and toluene
as determined from the isothermal tests performed using
TAM IV. Exothermic and endothermic reactions were not
observed under the test conditions. The dehydrogenation
system is generally operated daily to supply hydrogen to
FCVs. Even though the experimental conditions were
selected based on generous assumptions, the thermal
behavior remained stable for a week. Therefore, it can be
concluded that the spontaneous ignition of MCH and
toluene in the tanks is highly unlikely under normal
Figure 4 shows the DSC curves of MCH and MCH-air
as determined in Ar. While the thermal behavior of MCH
remained unchanged, MCH-air underwent exothermic
reactions, with the onset temperature for the first
exothermic reaction being ca. 100 C. Thus, it can be concluded
that the thermal hazard is low because the temperature
within underground tanks is maintained at ca. 10–20 C
and remains stable throughout the year; exothermic
reactions are unlikely to occur under these real-life conditions.
In addition, the scenario involving the production of
MCHair and the decomposition of MCH-air is an extreme one
and unlikely to occur. Therefore, although the process
appears to be dangerous, the risk of a fire in the tank would
be very low under both normal and abnormal conditions.
The DSC curves of MCH, toluene, and heat carriers
under normal conditions are shown in Fig. 5. Toluene,
MCH, heat carrier_T, and heat carrier_N did not undergo
exothermic reactions under normal conditions. Therefore,
1 W g–1
there are no thermal hazards associated with toluene,
MCH, heat carrier_N, and heat carrier_T under normal
Figure 6 shows the thermal behaviors of toluene, MCH,
heat carrier_T, and heat carrier_N in air. Exothermic
reactions were observed in every case, with the onset
temperatures being ca. 200, 175, 150, and 150 C,
respectively. The thermal hazard from MCH was very low
under these abnormal conditions because the exothermic
reaction of MCH was not violent and thus probably will not
damage the process equipment and piping. In addition, the
dehydrogenation of MCH under abnormal conditions
would not produce hydrogen, owing to the presence of the
oxidation products of MCH.
Toluene, which is produced by the dehydrogenation of
MCH, is recovered to an underground storage tank. Shortly
after the reaction, the toluene, which is at an elevated
temperature, is cooled and transferred to the tank through a
pipe. If a process shutdown occurs owing to a blackout or
equipment failure, the toluene stays in the pipes. In such a
case, an exothermic reaction may occur under the abnormal
conditions. This may prevent the ready release of the heat
of the reaction. If this were to occur repeatedly, the
resulting thermal stress could damage the pipes at various
positions over time, causing fatigue cracks that could allow
toluene to leak from the damaged areas. The thermal
hazards of toluene are lower than those related to other
dangerous substances. However, adequate safety measures,
such as periodic inspections, must be taken for the early
and timely identification of the hazards.
Although heat carriers are oxidized when air is present
in the heat exchange system, they usually do not pose any
hazard. However, this can change if a heat carrier leaks
from a pipe in the heat exchanger, causing heat
accumulation owing to the oxidation of the heat carrier that has
seeped into the adiabatic material swaddled around the
piping. Spontaneous ignition could occur eventually,
because the leakage of the heat carriers from the pipes is
difficult to detect through simple inspections. Therefore,
investigating the thermal hazards associated with the heat
carriers in detail is essential, since DSC is merely a
screening tool. Thus, in this study, we also performed TG
measurements for analyzing the thermal hazards related to
the heat carriers.
Figure 7 shows the TG and differential thermal analysis
(DTA) curves of heat carrier_T and heat carrier_N in air
and a He atmosphere, while Table 1 lists the onset and end
temperatures for mass loss for each heat carrier. The TG
curves indicate that the onset and end temperatures in air
are higher than those obtained under He. The DTA curves
in air indicate that each heat carrier underwent exothermic
reactions, while the DTA curves in a He atmosphere are
indicative of endothermic reactions. These results suggest
that a heat carrier leaking from a damaged pipe does not
evaporate readily under atmospheric conditions, in contrast
to the case under inert conditions, and that it decomposes
through an exothermic reaction. Therefore, if the heat
carriers were to leak continuously, their thermal
1 mV g–1
decomposition and heat accumulation would occur
interdependently, and spontaneous ignition would likely occur.
The results of the TG tests raised the question as to how
many times the heat carriers can leak and get oxidized by
air. To determine the answer, repeated thermal analyses
were performed using C80.
The thermal behaviors of pure heat carriers as well as
the reheated heat carriers are shown in Fig. 8. The
exothermic onset temperatures of heat carrier_T and heat
carrier_N were ca. 180 C and ca. 170 C, respectively.
These results are in keeping with those of the DSC and TG
measurements, namely that the thermal behavior of heat
carrier_N is similar to that of heat carrier_T. The
usefulness of the thermal analysis instruments used in this study
is limited to closed vessels containing very small amounts
of oxygen and may underestimate the extent of oxidation in
the absence of oxygen. Therefore, repeated measurements
were performed using C80. The thermal behaviors of the
heat carriers heated two and three times indicated that they
underwent exothermic reactions, showing onset
temperatures of ca. 160 and 170 C, respectively. This suggests
that the air continues to oxidize the heat carriers when a
large amount of oxygen is present in it. The heated heat
carriers turned from colorless to clear yellow. Figure 9
shows the Raman shifts of heat carrier_N and the heated
heat carrier_N; however, the reason for the color change of
the heat carriers could not be determined. These
measurement results indicate that spontaneous ignition can
occur under quasi-adiabatic conditions and moderate air
ventilation when a heat carrier leaks from damaged piping
and equipment. The detection of such leaks by inspection
may be difficult; however, the change in the color of the
oxidized heat carrier to yellow can aid in leak detection
Reheating heat carrier_N_2 times
Reheating heat carrier_N_1 time
Heated heat carrier_N
Pure heat carrier_N
Fig. 9 Raman shifts of heat carrier_N and heated heat carrier_N
In this study, we attempted to estimate the thermal hazards
related to the MCH dehydrogenation system using TAM
IV, DSC, TG, and C80 measurements. The results obtained
can be summarized as follows:
Under normal operating conditions, the thermal
hazards of MCH, toluene, and the heat carriers are very
low, and the dehydrogenation system can be operated
Under abnormal operating conditions, thermal hazards
exist, and minor incidents can occur. However, fatal
accidents are less likely. The leakage of the heat
carriers can be likely to cause the spontaneous ignition
of the heat carrier system. Therefore, regular and
thorough inspections of the system are essential.
Process safety assessments of the dehydrogenation
system must be included at the design stage based on
the thermal hazard data obtained in this study.
Acknowledgements This work was supported by the Council for
Science, Technology and Innovation (CSTI) through its
Cross-ministerial Strategic Innovation Promotion Program (SIP), ‘‘Energy
Carrier’’ [Funding agency: Japan Science and Technology Agency
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
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