Hydrocarbon Processing - February 2022 - GP-22

GREEN TECHNOLOGIES
FIG. 2. LNG heating curve.
FIG. 4. LNG heating curve for combined refrigerant case.
FIG. 3. Enthalpy change-temperature.
FIG. 5. Enthalpy change-temperature for combined refrigerant case.
loop will cool the warm end with a methane feed. The intermediary
section-which requires latent heat-will be cooled with
the use of liquid methane. The methane system at the cold end
will use latent heat to provide the required cooling. The limitation
is that methane will begin to condense in the temperature
range between -110°C and -143°C (FIGS. 4 and 5). The details
of this case include:
* The requirement of latent heat is -110°C.
* Nitrogen refrigerant can provide sensible cooling
at -155°C.
* Latent heat is provided with methane at -110°C
within realistic operating conditions.
The turbo-expander system is only able to follow a straight
path; therefore, there will be areas of inefficiency at the significant
points of curvature. Looking at systems at 18 bara and 2 bara,
there is no real curvature. The changes in the curves are sharp,
and this can be modeled using a turbo-expander cycle. At 50 bara,
a mixed refrigerant could be used to follow the curves more efficiently
(FIG. 6). The varying composition of methane, ethane and
nitrogen means that the evaporation of the refrigerant will occur
over a larger range of temperatures. The accurate selection of the
mixed-refrigerant composition is critical to follow the cold end
of the LNG heating curve. The problem with this system is that
methane will still condense at low-end temperatures. The result
22 JANUARY/FEBRUARY 2022 | GasProcessingNews.com
will be a larger gap of inefficiency, which is significant. The process
flow diagram and enthalpy change are shown in FIGS. 7 and 8.
Mixed-refrigerant liquefaction systems. The LNG heating
curves play an important role in the selection of the refrigerant.
Mixed refrigerant would be unable to achieve cooling along the
flat sections of the heating curves. When the BOG is at 50 bara,
there is more curvature in the heating curve. This means that
the process fluid is condensing through a wider range. For this
reason, a mixed refrigerant is suitable, since the components will
also condense at different temperatures. A mix of the three components
(methane, ethane and nitrogen) will result in a curve
that will justifiably provide cooling at the cold end of the system.
The warm end of the system can be cooled with the use
of heat. Therefore, the use of turbo expanders is appropriate.
The turbo-expander cycle can use nitrogen, methane or mixed
refrigerant as a refrigerant. As proved in the previous models,
methane is a more effective refrigerant due to its molar enthalpy.
In addition, ethane should be a better refrigerant than methane;
however, its range of operation is limited.
The mixed refrigerant will be more efficient at the cold end
due to the latent heat of vaporization that it possesses. If mixed
refrigerant is only being used at the cold end, heavier hydrocarbons
(such as propane and pentane) should not be used.
http://www.GasProcessingNews.com
Hydrocarbon Processing - February 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - February 2022
