Page 69 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

Most processes use a mixed refrigerant (MR) design. The reason is that the
gas has a heat load to temperature (Q/T) curve that must be closely
matched to improve stability and efficiency, see the figure below. The curve
tends to show three distinct regions, matching the pre-cooling, liquefaction
and sub-coiling stages. The refrigerant gas composition will vary based on
the individual design, as will the power requirement of each stage, and is
often a patented, location-specific combination of one or two main
components and several smaller, together with careful selection of the
compressed pressure and expanded pressure of the refrigerant, to match
the LNG gas stream.

Typical LNG train power use is about 28 MW per million tons of LNG per
annum (mtpa), corresponding to typically 200 MW for the largest trains of
7.2 mtpa, or 65 MW per stage. In addition other consumers in gas treatment
and pre-compression add to total energy consumption and bring it to some
35-40 MW per mtpa.

For each train, the cooling medium is first passed though its cooling
compressor. Since Pressure times Volume over Temperature (PV/T)
remains 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 (as 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 -50ºC
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 -30ºC to about -100-
125ºC 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
162ºC. The refrigerant is usually methane and/or nitrogen.

5.2.2 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

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