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
Treated
Natural
Gas
LNG
M M
Pre Cooling Liquefaction
G
Sub Cooling
M
74
constant, it results in a significant temperature rise which has to be
dissipated, typically in a seawater heat exchanger as shown in the figure
above (indicated by the blue wavy line). It then goes though one or more
heat exchangers/cold boxes before it expands, either though a valve or a
turbo-expander, causing the temperature to drop significantly. It is then
returned to cool its cold box before going on to the compressor.
The pre-cooling stage cools the gas to a temperature of about -30 to -50 ºC
in the precooling cold box. The cooling element is generally propane or a
mixture of propane and ethane and small quantities of other gases. The precooling cold box also cools the cooling medium for the liquefaction and sub
cooling stage.
The liquefaction process takes the gas down from -30 ºC to about -100-125
ºC, typically based on a mixture of methane and ethane and other gases. It
cools the LNG stream as well as the refrigerant for the final stage.
Sub-cooling serves to bring the gas to final stable LNG state at around 162
ºC. The refrigerant is usually methane and/or nitrogen.
The ConocoPhillips optimized cascade process was developed around
1970. It has three cycles with a single refrigerant gas (propane, ethylene and
methane) in each.
Figure 1
5. Optimized cascade process
75
The dual cycle mixed refrigerant (DMR), developed by Shell and others, may
look simpler but the overall design will be similar in complexity as multistage
compressors are typically needed. It is shown on the left with the C3MR on
the right for comparison.
Figure 16. Shell DMR and APLI C3MR designs
For small and micro LNG, single cycle designs are often preferred. There are
literally hundreds of patented solutions, but only a handful of mainline
licensors, that have solved the challenge of achieving single cycle
refrigeration. However, this means multiple internal stages in the process
flow and the heat exchanger itself. The PRICO SMR is shown on the left and
the Linde LIMUM®
(on the right).
Figure 17. PRICO SMR and Linde LIMUM®
76
5.5.2 Storage, transport and regasification
Storage at the terminals and on LNG carriers is done in cryogenic tanks at
atmospheric pressure or slightly above, up to 125 kPa. The tanks are
insulated, but will not keep LNG cold enough to avoid evaporation. Heat
leakage will heat and boil off the LNG. Therefore LNG is stored as a boiling
cryogen, which means that the liquid is stored at its boiling point for its
storage pressure (atmospheric pressure), i.e., about -162 ºC. As the vapor
boils off, heat of vaporization is absorbed from and cools the remaining
liquid. The effect is called auto-refrigeration. With efficient insulation, only a
relatively small amount of boil-off is necessary to maintain temperature. Boiloff gas from land-based LNG storage tanks is compressed and fed to natural
gas pipeline networks. On LNG carriers, the boil-off gas can be used for fuel.
Figure 18. LNG terminal process overview
At the receiving
terminal, LNG is
stored in local
cryogenic tanks. It is
regasified to ambient
temperature on
demand, commonly in
a sea water heat
exchanger, and then
injected into the gas
pipeline system.
Cove point LNG terminal
LNG ship
LNG discharge
line
Gas return line Compressor
LNG tank
Recondenser
Vaporizer
Gas
pipelines
Sea water
T ambient
(10°C ÷ 25°C)
Discharge sea water
(5°C ÷ 20°C)
77
6 Refining
Up to the early 1970s, crude oil prices were kept reasonably stable by major
international oil companies and industrialized nations. Less value was
created in the upstream production operations and relatively more profits
were generated in refining and distribution operations. With the 1973 oil
crisis and rising crude oil prices, more of the value was created upstream.
Now, the success of a modern refinery depends more on economies of scale
and the ability to process a wide range of crudes into the maximum quantity
of high value fuels and feedstock. A refinery that is able to handle multiple
types from heavy to light crude is said to have to have a large swing. Trade
specifications such as ”West Texas Intermediate” (WTI) API 38.3°, “Brent
Blend” API 38.3°, “Heavy Arab Crude” API 27.7° or “Grane” API 18.7° are
examples of such crudes.
Medium light crudes can be used directly in early engines and burners.
Modern consumers, such as gas and diesel engines, aviation turbojet
engines and ship bunkers need fuels manufactured to precise specifications.
This includes removing contaminants and pollutants, such as sulfur.
