H2Tech - Q1 2021 - 20

SPECIAL FOCUS

ADVANCES IN HYDROGEN TECHNOLOGY

H2 cooling and liquefaction. The H2
cooling, liquefaction and refrigerant
flows are shown schematically in FIG. 1.
The two separate systems include the H2
feed stream and the H2 expander refrigeration cycle.
Feed gas H2 is pretreated in molecular
sieve adsorbers (AD1) to remove impurities, such as traces of CO2 and water
vapor, and fed to the cold box inlet. After precooling by the methane expander
cycle to -150°C, the H2 feed is routed
to parallel, second-stage, regenerable
cryogenic adsorbers (AD2) to remove
residual trace impurities, mainly nitrogen and hydrocarbons. The adsorbers
accomplish several key functions:2
* Provide protection to the
downstream ortho-H2 to
para-H2 conversion catalyst
* Prevent freezing of the trace
impurities in the downstream
low-temperature regime
* Produce an ultra-high-quality LH2
product, as required for certain
industrial applications.
Multiple stages of ortho-H2 to paraH2 catalyst are provided, integrated with
the cold end of the brazed aluminium
exchanger system. Near-complete conversion of the ortho-H2 to the para-allotrope is essential prior to LH2 storage, as
residual ortho-H2 to para-H2 conversion
in storage is exothermic, causing significant boil-off losses.
After ortho-H2 to para-H2 conversion and further expansion, the H2
feed leaves the cold box as liquid H2 at

2 bar/-250°C. It is depressurized, if required, and routed to storage.
The H2 refrigerant system comprises
a separate closed-cooling circuit with
multiple expander stages that provide
low-temperature refrigeration for liquefaction of the feed H2. The H2 in this
second H2 system consists essentially of
unequilibrated " normal H2 " (75% orthoH2 plus 25% para-H2).
The H2 recycle compressor (CP2)
delivers precooled H2 to the expanders
at 50 bar, with the first expander (EH1)
typically discharging at 4.5 bar/-216°C
and the last expander (EH2) discharging
at 2.7 bar/-249°C. The expander outlet
streams provide counter-current cooling
of the H2 feed gas and are then routed to
the CP2 suction and returned to EH1/
EH2 for further cooling duty. EH1/EH2
recover power, which is used to reduce
the overall power demand of the system.
The H2 expanders are both expected
to require more than one impeller in series due to enthalpy drop limitations.
Design issues. In developing the overall system design, the authors have noted
the difficulty in reconciling the available
data on the properties of ortho-H2. This
has been addressed by modifying the reference state of ortho-H2 in the National
Institute of Standards and Technology's
REFerence fluid PROPerties (REFPROP) database.3
Alternative configurations. A number

of variants to the described system have

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0

0.10

0.20
Liquid fraction from LP expander

FIG. 3. Relative power vs. liquid fraction from low-pressure expander.

20 Q1 2021 | H2-Tech.com

0.30

0.40

been investigated, as described in the following sections.
Nitrogen precooling refrigerant.
The use of nitrogen (or a nitrogen/methane mixture) in place of methane in the
precooling system has been investigated.
With nitrogen, a lower precooled temperature of approximately -190°C may
be attained, but the overall specific power of the liquefaction process was found
to be up to 10% higher than with methane. However, due to the lower attainable
final temperature of the precooling stage
(around -190°C), there is a reduction in
the power requirement of around 20%
for the relatively expensive H2 compressor CP2, which may be significant in
terms of capital cost.
Low-temperature H2 recycle compression. FIG. 4 shows a variant of the H2
liquefaction cycle that operates the H2
recycle compressor with a significantly
sub-ambient suction temperature. The
recycled H2 enters the first part of compressor CP2 typically at a temperature of
-120°C. Alternatively, the inlet stream to
compressor CP2 may be taken directly
from the outlet stream of the H2 liquefier
unit at a lower temperature.
Depending on the inlet temperature of
compressor CP2, its power consumption
may be reduced by approximately 50%,
relative to the configuration with a nearambient inlet temperature, as shown in
FIG. 1. While this leads to an approximately equivalent increase in the power demand for the precooling methane compression, operation of the H2 compressor
with a sub-ambient inlet temperature has
advantages for H2 liquefaction:
* H2 compression generally requires
use of reciprocating compressors,
as the density of H2 is low for use
in centrifugal compressors. Taking
into consideration the relatively
high investment and operational
costs of reciprocating machines,
the reduction in power
requirement and cost of
reciprocating compressors arising
from the use of a sub-ambient inlet
temperature may be significant.
Furthermore, the differential
power requirement imposed on
the methane precooling circuit is
then provided, in the configuration
shown in FIG. 4, by relatively
inexpensive centrifugal machines.
* Operation of the H2 compressor

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