H2Tech - Q1 2021 - 19

ADVANCES IN HYDROGEN TECHNOLOGY
from steam reforming H2 plants and existing PEM electrolyzers. This pressure
affects the overall power requirement of
the LH2 plant.
It has been assumed that seawater at
20°C will cool a closed circuit of treated
water cooling the process coolers and
heat exchangers within the LH2 plant.
Methane precooling H2 liquefaction. A
schematic of the overall liquefaction process is shown in FIG. 1. The illustrated concept integrates a dual-expander methane
precooling process with a cold-end H2 expander design. The precooling, however,
could be combined with other cold-end
arrangements using, for example, helium
and/or neon refrigerants.
The precooling circuit, shown in red,
was originally developed and patented
for LNG application. The cold-end H2
cycle is shown in green.
Methane expander cycle. The methane make-up gas to the precooling system
is assumed to be pipeline-quality natural
gas. Given the limited volume of the initial fill and make-up quantities, molecular sieve pretreatment provides a practical solution to remove carbon dioxide
(CO2 ), water vapor and residual hydrocarbons that would freeze in the liquefaction system. LH2 plants proximate to
LNG liquefaction or regasification facilities and with access to pretreated LNG
feed gas or regasified LNG may not require pretreatment facilities.
The methane cycle operates as a
closed circuit with the methane refrigerant compressed and circulated by the
electrically driven recycle compressor,
CP1. The compressed gas then flows to
the cold box comprising multi-passage
brazed aluminium heat exchangers,
which cool the H2 feed and the circulating H2 refrigerant streams to approximately -160°C. After preliminary chilling in the cold box, the compressed
methane flows to the expander wheels
of the two expander-compressors, EC1/
EC2. The high-temperature expander,
EC1, typically discharges at 25 bar/-
50°C, and the low-temperature expander,
EC2, discharges at 10 bar/-125°C. The
cold outlet streams from EC1/EC2 return to the cold box, where they cool the
H2 feed and the circulating refrigerant
streams to approximately -120°C. EC1/
EC2 are loaded by compressor wheels in
series with the main recycle compressor,

as illustrated in FIG. 2; they compress the
circulating methane to 70 bar-100 bar at
the inlet to the cold box.
The low-temperature expander, EC2,
operates in partially liquefying mode
and efficiently converts latent heat into
mechanical work, improving cycle efficiency. This partial liquefaction (FIG. 2),
which uses well-proven expander technology, reduces the total power demand
of the H2 liquefaction process and is a
distinctive feature.
The condensed liquid formed in EC2
is separated from the gas phase in separator SP1 and is then flashed to near-atmospheric pressure. The resulting two-

SPECIAL FOCUS

phase methane stream is evaporated and
reheated by further heat exchange with
the H2 feed and the circulating H2 refrigerant streams, which are thereby cooled
to -155°C. The evaporated and reheated
flashed gas stream is recaptured to the
system by the flash gas compressor, CP3,
and routed together with the outlet gas
stream from SP1 to CP1 suction for recompression and return to the expanders
for further cooling duty.
The typical impact on precooling power demand of increasing liquids content at
the outlet of EC2 is shown in FIG. 3. Power
consumption is reduced as the liquid content of the expander outlet increases.
AD2
Ortho-para catalyst

Cold box
H2 feed gas

AD1

LH2

CP2

CP1

CP3

EH1

Flash to low
pressure

SP1
EC1

EC2

EH2

Liquefying expander

FIG. 1. Overall process configuration for methane precooling H2 liquefaction.

CP1

CH4 recycle
compressor
Flash gas
compressor

CP3

Flashed
gas

Compressor wheel

Recycle
gas

H2

EC1

EC1

Expander-compressor
Compressor
wheel

Liquefying
SP1

EC2

EC2

Expander-compressor

Liquid

FIG. 2. Methane cycle power recovery and liquefying expander configuration.
H2Tech | Q1 2021 19
H2Tech - Q1 2021

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