HP March 2022 eBook—Energy Transition - 57

Hydrogen
FIG. 2. Process flow showing how H2
within an organic carrier (LOHC).
CH3
- 3H2
+ 3H2
MCH
process.
TOL
FIG. 3. Chemical reaction formula used
for the hydrogena
position and obvious improvements to
health, the environment and quality of
life. Benefits on a more global basis would
be commensurately similar.
Options for hydrogen deployment.
While the future for H2
availlooks
bright, production,
transportation, distribution and
a host of associated infrastructural issues
need to be considered. Assuming H2
ability, how to get it from where it is made
to where it is most needed (and at a reasonable
cost) is of paramount interest.
Historically, H2
has been transported
either as a compressed gas in tube trailers
or as a cryogenic liquid. Both methods
have pros and cons, but transportation selection
and cost are ultimately determined
by how much H2
must be transported and
over what distance.
Prior studies undertaken for localized
distribution have considered and compared
these conventional methodologies
with newer, more promising options of
transporting H2
bound by a chemical carrier,
known generically as a liquid organic
H2
carrier (LOHC).
FIG. 2 shows how H2
is liquefied to 20K
(-253°C/-423°F) for transport as a cryogenic
liquid. This is an energy-intensive
operation that uses around 30% of the contained
H2
tion process. The cryogenic H2
energy to complete the liquefacis
then
CH3
∆H = 205 kJ/mol
is liquefied for transport as a cryogenic liquid, is transported as a compressed gas or is transported
transported from the liquefaction point to
the point of use by truck. As per U.S. Department
of Transportation (DOT) limitations
on truck transport of liquefied H2
approximately 4,000 kg of H2
,
can be transported
per truckload. At the delivery site,
the liquid can be cryo-pumped to the dispensing
point for subsequent vaporization.
At present, transportation of liquid H2
is
available only by truck; therefore, the scale
of transportation is limited, and further
technology development is required for
large-scale liquid transportation by ship.
Alternatively, H2
at approximately 540
bar (7,830 psi) can be transported by tube
trailer from a terminal to a dispensing
point where final compression (to around
700 bar/10,000 psi) is accomplished prior
to ultimate utilization. U.S. DOT regulations
also limit the amount of H2
that can
be transported per tube-trailer truckload
to approximately 1,050 kg of H2
per truck.
Lastly, the LOHC option may be considered
as a viable alternative. LOHC is a
suitable technology for large-scale, longdistance
H2
scale or daily to seasonal H2
transportation, or for largestorage,
as will
be discussed in more detail in this article.
In all of these cases, the distance of transport,
the volume of material being transported
and the distribution particulars
will drive the best choice for the particular
H2
application at hand.
Advances in hydrogen storage/
transport technology. A trademarked
hydrogena
process is an H2 storage and
is
the reaction shown in FIG. 3. Methylcyclohexane
is a convenient carrier for H2
,
as it is easy to store and transport under
ambient temperature and atmospheric
pressure. In the process, H2
is stored and
transported in large-scale quantities at a
competitive cost, since cryogenic liquefaction
or compression to very high pressures
is not required.
The process is based on a simple process
configuration. Methylcyclohexane
and toluene react in fixed-bed, tubulartype
reactors in the vapor phase. FIG. 4
shows simplified process flow diagrams of
the hydrogenation and dehydrogenation
processes for the overall process. In the
hydrogenation process, toluene feed is
vaporized in the vaporizer (1) and mixed
with H2
, including recycle gas. The mixed
feed is superheated to the reaction temperature
(2) and then enters the top of the
reactor (3), which is a fixed-bed, tubular
reactor charged with a semi-conventional
hydrogenation catalyst. In the reactor
tubes, toluene reacts with H2
to produce
methylcyclohexane.
Hydrogenation of toluene is an exotransportation
technology for large-scale
and long-distance transportation. H2
chemically fixed to toluene and converted
to methylcyclohexane, according to
thermic reaction. The generated heat is
removed by cooling water to control the
reaction temperature, and the heat is subsequently
recovered as medium-pressure
steam, which can be further utilized as
needed. The generated steam is clean energy
without carbon emissions. The effluent
gas from the tubular reactor is cooled,
the condensed methylcyclohexane is separated
(6) from the recycle gas, and the liquid
product (methylcyclohexane) is sent to
storage tanks. Recycle gas is then returned
to the reactor after being mixed with fresh
H2
Hydrocarbon Processing | AUGUST 2021 | HydrocarbonProcessing.com
feed. Very high product yield is achieved

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HP March 2022 eBook—Energy Transition

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