Oil and gas production handbook An introduction to oil and gas production

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 
Treated
Natural
Gas
LNG
M
M
Pre Cooling
Liquefaction
G
Sub Cooling
M

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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 pre-
cooling 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.