Hydrocarbon Processing - February 2022 - GP-21

GREEN TECHNOLOGIES
Innovative technology for re-liquefying evaporated
LNG to meet net-zero emissions targets
A. RAZA, KBR-AMCDE, Al-Khobar, Saudi Arabia
In the current era, environmental challenges do not exist in
isolation. The protection of the environment is one of the greatest
challenges facing the world.
Many industries are swiftly moving toward the green environment,
and many companies are adopting more stringent environmental
regulations that require the provision for a vapor
control system to reduce the emissions of volatile organic compounds
(VOCs). These investments are to ensure improvements
in air quality, resulting in a healthier environment and
improving the health and well-being of its inhabitants.
Vapor recovery for LNG. In countries like Australia, vapor recovery
has become mandatory in major urban areas. One technology
that has evolved is re-liquefaction of evaporated LNG-
boil-off gas (BOG)-on LNG carriers. The BOG released from
LNG storage vessels on carriers can be used to power the steam
turbines to run the ships, all the while reducing greenhouse gas
(GHG) emissions. Advancements in turbo-expander technologies
have led to the development of new re-liquefaction systems.
LNG usually comprises methane, ethane, propane, butane
and nitrogen. LNG is sold on an energy basis-therefore, the
composition of the product is of great importance. Nitrogen is
normally removed from the natural gas stream through a nitrogen
rejection unit to increase LNG's energy value. The energy
value of LNG is measured based on the higher heating value
(HHV), which is the amount of heat obtained by the complete
combustion of a unit quantity of a material.
LNG is stored at approximately -163°C in tanks close to atmospheric
pressure. The pressure in the tanks is normally less
than 7 psig. LNG cryogenic storage tanks are equipped with
extremely efficient insulation; however, heat is still transmitted
due to the surrounding atmosphere. For the LNG to stay liquefied,
it must stay at or near constant temperature, which can be
achieved if the tank is held at a constant pressure.
To remain at a constant pressure, the LNG vapor boil-off is
allowed to leave the tank, which will keep the pressure constant.
BOG from the cryogenic tank is of high value; therefore, it is
compressed and used as a fuel gas (not flared).
BOG may also be produced when LNG is pumped into
LNG carriers. Due to the temperature of the empty tanks, BOG
forms quickly. The rate that the BOG is produced begins to slow
down quite substantially as the LNG and the tank temperatures
reach equilibrium. BOG can be converted to a fuel source, or
it can be re-liquefied in the stream and exported as LNG. Reliquefaction
systems can be optimized based on the following:
* Arrangements of the turbo expander
* The pressure of the BOG liquefaction system
* The refrigerant selection.
With the optimization of the turbo-expander arrangements
(the refrigerant selection and the BOG feed pressure), energy
savings of approximately 30% can be achieved.
Nitrogen liquefaction systems. There is a constraint on refrigerant
at a pressure of 2 bara due to the necessity of latent
heat at -155°C. Nitrogen is the only refrigerant that can efficiently
liquefy the BOG at 2 bara.
FIG. 1 is a generic process flow diagram required for the liquefaction
of BOG at 2 bara. The flow through the Joule-Thomson
(JT) valve provides the latent heat required to liquefy the process
fluid. The purpose of the JT valve is to provide liquid nitrogen
expansion, which provides the latent heat required at the cold
end of the system. The discharge pressure must be controlled to
have an efficient system. When nitrogen is used as a refrigerant,
the discharge pressure from the JT valve is acceptable. Because of
the boiling temperatures of the components, a methane refrigerant
has a discharge pressure near atmospheric pressure.
The LNG is cooled using liquid nitrogen, which is cooled
by the nitrogen in the cold and warm turbo expanders. FIG. 1
shows the use of a JT valve for latent heat cooling and a turbo
expander in re-compressor mode to provide sensible heat for
warm-end cooling. The following are the main points of this
study (FIGS. 2 and 3):
* Sufficient heat is between ambient temperature and -155°C.
* Nitrogen refrigerant can provide latent heat at -155°C
at realistic operating conditions (methane at 160 kPaA).
* The nitrogen and carbon system is more efficient.
With a combined refrigerant case, a turbo-expander loop
will cool, using a nitrogen feed, and a second turbo-expander
FIG. 1. Process flow diagram required for BOG liquefaction at 2 bara.
Gas Processing & LNG | JANUARY/FEBRUARY 2022 21
Hydrocarbon Processing - February 2022

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