Oil and gas production handbook An introduction to oil ... - ABB Group

Typical LNG train power use is about 28 MW per million tons of LNG per
annum (mtpa), corresponding to typically 200 MW for a large trains of 7.2
mtpa, or 65 MW per stage for three cycles. In addition, other consumers in
gas treatment and pre-compression add to total power consumption and
bring it to some 35-40 MW per mtpa, and over 50 for small LNG facilities well
under 1 mtpa capacity.
Some examples are given here. (Please note that these process flow
diagrams are simplified to illustrate the principle and do not give a complete
design.) All designs are shown with heat exchangers to the sea for
comparison. This is generally needed for high capacity, but for smaller plants
air fin heat exchangers are normally used.
A triple cycle mixed refrigerant cascade claims to have the highest energy
efficiency. It is represented here by the Linde design, co-developed with
Statoil.
Treated
Natural
Gas
Pre Cooling
Liquefaction
Sub Cooling
LNG
G
M
M
M
Figure 14. A triple cycle mixed refrigerant cascade
The actual design varies considerably with the different processes. The most
critical component is the heat exchanger, also called the cold box, which is
designed for optimum cooling efficiency. Designs may use separate cold
boxes, or two or three cycles may combine into one complex common heat
exchanger. This particular deign uses the patented Linde coil wound heat
exchanger, also called the “rocket design,” due to its exterior resemblance to
a classic launch vehicle.
For each train, the cooling medium is first passed through its cooling
compressor. Since pressure times volume over temperature (PV/T) remains
73