Hydrocarbon Processing - February 2022 - GP-23

GREEN TECHNOLOGIES
FIG. 6. LNG heating curve of turbo-expander system.
FIG. 8. Enthalpy change-temperature of turbo-expander system case.
FIG. 7. Process flow diagram of turbo-expander system case.
Warm-end cooling can be achieved by using heat provided
by expansion. Mixed refrigerant is more efficient than nitrogen
at the warm end, as it possesses a higher molar heat capacity.
Therefore, mixed refrigerant is normally used in the warmest
turbo expander to provide the greatest efficiency within the
operating limitations of the expanders.
With the optimization of the turboexpander arrangements,
the refrigerant selection and the BOG feed pressure, energy
savings of approximately 27% can be achieved.
The system's efficiency improves with increasing the BOG
feed pressure. When the BOG feed pressure is increased, the
heating curve becomes more curved. Condensation does not
take place over a limited area, but rather across a larger area.
The BOG will liquefy more efficiently at a higher feed pressure.
This is clearly presented by the respective LNG heating
curves in FIG. 9. As the feed pressure of the BOG is increased,
the curvature in the LNG heating curve begins to increase. The
heating curves also show the required cooling.
This means that methane is more efficient than nitrogen
due to the molar heat capacity it possesses. This was a significant
advantage, and methane was used for all warm-end refrigeration.
The challenge with methane was its ability to provide
sensible heat at cold temperatures below -110°C. Due to methane
having a warmer boiling point than nitrogen, it was unable
to effectively provide cooling below these temperatures, using
a turbo-expander cycle. At 18 bara, the evaporation of methane
to provide cooling was still more effective than the combined
nitrogen and methane system. At 50 bara, methane was inefficient
due to its inability to follow the curve at the cold end. In
this case, it was advantageous to have combined nitrogen and
methane systems.
Further advantages included using ethane as a refrigerant,
which has an even higher molar heat capacity than methane.
The use of a mixed refrigerant with ethane then became quite
attractive. Ethane has a higher boiling point and begins to conFIG.
9. BOG will liquefy more efficiently as a higher feed pressure.
dense at warmer temperatures. This restricts its use in turbo
expanders, where the discharge stream must have less than
10% liquid flow. In the most efficient modeled system, the
mixed refrigerant only passes through the warmest turbo expander
to produce no liquid on the outlet of the expander. This
still provides a significant energy savings of approximately 3%
vs. methane being passed through the expander.
Therefore, methane was a more effective refrigerant than
nitrogen since methane has a higher molar heat capacity. This
makes the methane system more efficient than the nitrogen
system. The same was found with the mixed refrigerant, which
condensed at a higher temperature due to the ethane content.
The mixed refrigerant has an even higher molar heat capacity
than the methane due to the presence of ethane. This justified
why the mixed refrigerant system-with mixed refrigerant
flowing through the warmest expander-was the most
efficient system.
A large amount of energy could be efficiently recovered,
and maximum efficiency of the whole process system could be
achieved by applying the best engineering approach. Simultaneously,
it would also contribute to reducing the exposure of
GHG and to improving the air quality, resulting in advancements
toward achieving net-zero emissions targets. GP
Gas Processing & LNG | JANUARY/FEBRUARY 2022 23
Hydrocarbon Processing - February 2022

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