Hydrocarbon Processing - May 2022 - 50

Carbon Capture/CO2
Mitigation
The generally accepted rule, however,
is that for each km of depth of injection,
~1,150 psi of gas pressure is required.
Since many of the geological formations
presently under consideration for CO2
storage are relatively shallow, injection
pressures below 2,000 psi should be expected
to occur frequently. A typical carbon
separation and storage pressure application
requires CO2
to be compressed
from below 50 psia to above 2,100 psia,
as shown in FIG. 1.
Many viable thermodynamic path
options, including refrigeration and
liquid pumping, near-isothermal, and
high-pressure ratio compression, exist
to move this compression process from
its start to its endpoint. Put differently,
the available options are to compress the
CO2
and remain in the gas state on the
and pump it in the liquid state
right side of the vapor dome, refrigerate
the CO2
on the left side of the dome, or utilize
some combination of these methods.
Cooler
Compressor
Compressor
CO2
sequestration and storage. For
most power plant carbon capture and
sequestration applications, the following
new compression duties are required:
1. Pipeline header injection and
recompression transport
2. Injection into geological storage
reservoirs for sequestration
3. Separation processes
(membrane, thermal or chemical)
4. Power plant cycle compression.
According to industry convention,
CO2 as a supercritical (dense phase) fluid
above 2,100 psi should be transported in
pipelines. At 2,100 psi, CO2
is well above
its critical point and will be supercritical
at almost all ambient temperatures. Fluids
in a dense phase share some physical
properties of liquids, such as a very low
compressibility; they also share some
physical properties of gases and will expand
in space to fill voids. The advantage
of transporting CO2
at supercritical pressures,
therefore, is that its density does
not change much with pressure: from a
thermodynamic perspective it is essentially
pumped rather than compressed.
This significantly reduces the power demand
for the pumping stations along a
CO2
pipeline.
There are two disadvantages, however:
Cooler
Electric
motor
Cooler
Compressor
CO2 out
2,100 psia
Compressor
CO2 in
50 psia
FIG. 3. Integrally geared compressor with
interstage cooling.
the added injection compression ratio required
at the pipeline header station and
the significantly higher material costs
when building a pipeline designed for a
maximum allowable operating pressure
above 2,100 psi. Since the CO2
available
from separation is at low, near-atmospheric
pressures (< 100 psi), the pipeline
header station must always use a compressor;
for a 2,100-psi CO2
pipeline, a highpressure
ratio header compressor with
many intercooled stages will be needed
to handle the significant volume reduction.
In such a case, however, beyond the
header station the gas is simply pumped.
Fortunately, transport at 2,100 psi is
not required for all applications: the actual
transport pressure of CO2
depends
must be
on the separation process outlet starting
pressure, the distance the CO2
transported and the geological sequestration
injection pressure (which is often
well below 2,100 psi). If a lower-pressure
CO2
pipeline is utilized, conventional
compressors are preferred for the header
station and for recompression along the
line. The transport pressure is selectable
depending on the carbon sequestration
application; it is not always advantageous
to go with supercritical CO2
.
Purely from a compression stage thermodynamic
perspective, CO2
gas but relatively easy to compress. That
ease notwithstanding, CO2
presents several
technical challenges that must be
addressed to make its compression or
pumping process efficient and reliable.
These include:
* Most equations of state for CO2
are still inaccurate at high pressures
and temperatures.
* CO2 is a heavy gas, resulting in
amplified rotor dynamic and
impeller-dynamic forces.
* CO2 has a strong thermodynamic
path dependence and multi-phase
behavior.
* CO2 forms carbonic acid in
the presence of water, which
then drives corrosion.
* CO2 is soluble in elastomeric
materials, which can lead to rapid
decompression failures.
* When rapidly expanded, CO2
quickly forms liquids and dry
ice, which can be a problem
at the shaft seals.
* CO2 has a low sonic speed, which
results in higher shock losses and
a reduced operating range.
* CO2 selectively leeches certain
elements from common metals
and has a very low viscosity at
high pressures.
All of these represent manageable, if complex,
engineering and design challenges.
Compression and pumping options.
CO2
has a high pressure ratio per comFIG.
4. An 8-impeller-stage, barrel-type,
centrifugal compressor with multiple nozzles
for intercooling or side streams.
50 MAY 2022 | HydrocarbonProcessing.com
FIG. 5. A multi-stage, horizontally split
compressor with nozzles for two intercoolers
driven by an electric motor through a gearbox.
pressor impeller stage. Because of this,
it also has a significant specific volume
decrease with pressure along with a very
high heat of compression. This means
that CO2
heats up when compressed and
requires stage intercooling to maintain
is a heavy
Gear
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