Note: Descriptions are shown in the official language in which they were submitted.
CA 02655037 2008-12-11
WO 2007/144078 PCT/EP20071/ 004902
PROCESS FOR LIQUEFYING HYDROGEN
DESCRIPTION
The invention relates to a method for liquefying
hydrogen.
Hydrogen in particular is currently increasingly
gaining in importance as energy carrier due to the
growing energy demand and increased environmental
consciousness. Trucks, buses, passenger cars and
locomotives are thus already powered by means of
engines which are operated by natural gas or hydrogen
as well as by means of combinations of fuel cell and
electric motor. In those cases, the most sensible form
of storage of the hydrogen "on board" the
aforementioned means of transportation is the liquid
form. Even though, for this purpose, the hydrogen must
be cooled to approximately 25 K and maintained at this
temperature - which is only achievable by using
appropriate insulation measures on the storage
containers or storage tanks -, owing to the low density
of GH2, storage in gaseous form in the aforementioned
means of transportation is, as a rule, less favorable,
since, in this case, storage has to take place in
large-volume and heavy storage tanks under high
pressures.
Hydrogen liquefaction processes normally comprise two
process steps, namely the so-called precooling step as
well as the subsequent liquefaction step. In the
above processes, hydrogen must be cooled to below its
upper Joule-Thomson inversion temperature - this is
understood to be the temperature below which an
expanding gas cools down - before it can be liauefied.
The hydrogen must therefore usually be precooled to a
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tem-cerature of at least -1500C before it can be
supplied to the subsequent liquefaction process.
Gaseous hydrogen is usually composed of approximately
750 ortho-hydrogen and approximately 2501 para-hydrogen.
For this reason, during the liquefaction process -
since the liquefied hydrogen is normally to be
intermediately stored over a longer period of time -,
the ortho-hydrogen must be converted into para-
hydrogen. Typically, a proportion of at least 990
para-hydrogen is aimed for. If such a conversion is not
performed, a quicker evaporation of the liquefied
hydrogen will be the result. The conversion from ortho-
hydrogen to para-hydrogen takes place by means of
suitable conversion catalysts.
A large number of methods for liquefying hydrogen are
known from the literature, in which the precooling of
the gaseous hydrogen takes place against a coolant
circuit or coolant mixture circuit. Nitrogen is often
used as coolant in this case. Hydrogen liquefaction
methods are known from the international patent
application WO 2005/080892 as well as from the European
patent application 1 580 506, where the precooling of
the hydrogen stream to be liquefied takes place in
indirect heat exchange with a pressurized LNG (Liquid
Natural Gas) stream. The LNG evaporating during this
process transfers its cold to the gaseous hydrogen
stream to be precooled. The evaporation of LNG is an
issue in particular in LNG terminals. This evaporation
normally takes place by means of suitable natural gas
burners which are immersed in water baths and are
operated with a small partial stream of the LNG.
It is the object of the present invention to
provide a method for liquefying hydrogen, which,
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compared to the methods which form part of the
state of the art, has a lower specific energy
consumption.
The method according to the invention for liquefying
hydrogen comprises the following method steps:
a) precooling of the hydrogen stream by indirect heat
exchange against a pressurized LNG stream to a
temperature of between 140 and 130 K,
b) precooling of the hydrogen stream by indirect heat
exchange against a coolant to a temperature of
between 85 and 75 K,
C) with the precooling of the coolant taking place
against a pressurized LNG stream, and
d) cooling and at least partial liquefaction of the
precooled hydrogen stream by indirect heat
exchange against a further hydrogen stream which
is circulated in a closed cooling circuit,
e) with the precooling of the compressed hydrogen
stream, which is circulated in a closed cooling
circuit, taking place against a pressurized LNG
stream.
The method according to the invention for liquefying
hydrogen will be explained in more detail below with
reference to the exemplary embodiment illustrated in
the figure.
The hydrogen stream to be liquefied is supplied via
line 1 with a pressure of 2200 kPa and a temperature of
300 K to the heat exchanger El. In the latter, the
hydrogen stream is cooled to a temperature of 135 K
against an LNG stream, which is conducted via line A
through the heat exchanger El and has a temperature of
125 K and a pressure of 7800 kPa.
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It should be emphasized that all of the heat
exchangers shown in the figure represent in each case
one or also more, if necessary different heat
exchangers or heat exchanger types.
The precooled hydrogen stream is now supplied via line
2 to a further heat exchanger E2 and cooled therein to
a temperature of 80 K against a nitrogen cooling
circuit, which will be described in more detail below.
The hydrogen stream precooled to 80 K is subsequently
supplied via line 3 to a purification device 4 working
preferably adsorptively, in which final traces of
contamination are removed from the hydrogen stream to
be liquefied. The purification device 4 usually
comprises at least two adsorbers arranged in parallel
so that a continuous purification process can be
realized by switching.
The hydrogen stream to be liquefied withdrawn from the
purification device 4 via line 5 is supplied to the
heat exchanger E4 and cooled therein against the still
to be described closed hydrogen cooling circuit to a
temperature of 26 K. A pressure reduction to
approximately 200 kPa takes place in the expansion
device 8 downstream of the heat exchanger E4, which
results in a partial liquefaction of the cooled
hydrogen stream. Subsequent to the completed
liquefaction of the gas phase in the heat exchanger E7,
a liquid hydrogen product stream is withdrawn via line
9 and supplied to its further use and/or intermediate
storage.
Alternatively, the expansion device 8 can also be
realized by a combination consisting of an expansion
valve and an ejector following the expansion valve. In
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= s case, ~asaous hydrogen produced during the
intermediat~ storage of the liquid hvdrogen product
stream can be supplied to the ejector.
S The open hydrogen cooling circuit is composed of the
line sections 17, 11, 13, 15 and 16, the heat
exchangers E4, E5, E6 und E7, at least one expansion
device 12, and a preferably multi-stage compressor 14.
Hydrogen is first supplied via line 17 to the heat
exchanger E4 and cooled therein. It is subsequently
supplied via line 11 to the expansion device 12 and
expanded in it for the purpose of providing the peak
cold necessary for the liquefaction of the hydrogen.
Next, the evaporation takes place in the heat exchanger
E7 and a heating of the expanded hydrogen stream in the
heat exchanger E4 in indirect heat exchange with the
hydrogen stream to be cooled and liquefied in line 17.
The heated hydrogen stream is supplied via line 13 to
the heat exchanger ES and heated against itself
therein, prior to being compressed to the desired
circuit pressure in the compressor unit 14.
The compressed hydrogen stream is supplied via line 15
to a heat exchanger E6 and cooled therein against a
further partial LNG stream, which is supplied to the
heat exchanger E6 via line C. This cooled hydrogen
stream is subsequently supplied via line 16 to the heat
exchanger E5, cooled against itself therein and
thereafter supplied again via the line sections 17 to
the already described heat exchanger E4.
For reasons of clarity, several expansion devices are
not shown in the figure; they are being supplied in
each case with cooled partial hydrogen streams from the
line sections 17 and 11 and, subsequent to the
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completed cooling expansion, supplied again to the
cooling circuit 13 shown, located upstream of the
expansion device 12 (before and/or after E4).
5?'he aforementioned nitrogen cooling circuit used for
precooling the natural gas stream to be liquefied by
means of the heat exchanger E2, has in addition to the
line regions 20, 21, 23 and 24 a further heat exchanger
E3, an expansion device 25, as well as a preferably
multi-stage compressor unit 22.
The nitrogen stream expanded in the expansion device 25
and having a cooling effect in the process is supplied
via line 20 to the aforementioned heat exchanger E2 and
heated therein against the hydrogen stream to be
cooled, and evaporated. The evaporated nitrogen stream
is then supplied via line 21 to the compressor unit 22
and compressed therein to the desired circuit pressure.
The compressed nitrogen stream is supplied via line 23
to the heat exchanger E3 and cooled therein against a
further LNG stream, which is supplied to the heat
exchanger E3 via line B. The cooled nitrogen stream is
then supplied via line 24 to the aforementioned
expansion device 25.
According to the invention, the LNG being available in
the hydrogen liquefaction process environment is now
used for precooling the hydrogen stream to be liquefied
(heat exchanger El), for cooling the compressed
nitrogen in the nitrogen cooling circuit (heat
exchanger E3), as well as for cooling the compressed
hydrogen stream (heat exchanger E6) circulating in the
open hydrogen cooling circuit.
For reasons of clarity, the catalysts and/or catalyst
mountings required for the desired or possibly required
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ortho-para conversion of the hydrogen are not shown ir_
the figure. Generally, a first ortho-para conversion
will be provided downstream of the purification device
4. In this -jurification device 4, an increase of the
para-hydrogen content from approximately 25 to
approximately 43% can take place. The following ortho-
para conversion takes place preferably by way of
catalysts arranged in the passages of the heat
exchanger E4. Preferably, the liquid hydrogen product
stream withdrawn via line 9 should consist of at least
9901 para-hydrogen.
