In this system, LNG is heated by propane in a closed loop and the propane is heated by seawater. In situations where the seawater is too cold to supply all the required heating energy, additional heat can be introduced. The cascade concept is recommended instead of directly heat exchanging with seawater. LNG heat exchanged directly with seawater increases the risk of freezing the seawater in the heat exchanger. Propane as a secondary medium is suggested because of its thermodynamic properties with a low freezing point.
LNG enters a cryogenic pump capable of producing the required send-out pressure (e.g. up to 130 bar has been studied). LNG at the required discharge pressure is heated in two stages.
In the first stage LNG is heated from -160°C to -10°C in a compact printed circuit heat exchanger with propane as a heating medium. In the second stage, LNG can be heated further using seawater as a heating medium in a shell and tube heat exchanger.
In the LNG/Propane heat exchanger, heat is exchanged against propane circulating in a closed loop. The propane enters the heat exchanger at approx. 0°C at 4.7 bar as gas. In the heat exchanging process propane is condensed, and leaves the exchanger in a liquid state at approximately -5°C. The propane in the closed loop is then pumped by the circulating pump and heated against seawater in titanium semi welded plate heat exchangers. In these heat exchangers, the propane is evaporated and heated to 0°C before returning as gas to the printed circuit heat exchanger.
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