In English

Studie av återkondenseringsanläggningar för LNG-tankers - Jämförelse mellan nitrogenanläggningar och kaskadanläggningar

Study of religuefaction plants for LNG carriers - nitrogen and cascade plants

Johan Elander ; Simen Rempling
Göteborg : Chalmers tekniska högskola, 2014. 56 s.
[Examensarbete på grundnivå]

This paper is a comparison study of nitrogen and cascade reliquefaction plants, for re-liquefaction of LNG on-board LNG carriers. LNG is transported in a liquefied state in gravity tanks with temperatures about -160 °C. During a sea voyage, parts of the cargo will be vaporized due to heat ingress in the cargo tanks. This results in an increase in cargo tank pressure. To control the tank pressure during a sea voyage, some type of vapour handling system need to be installed on-board the ship. Either an incineration plant can be used to process the excess gas or the operator can choose to re-liquefy the vapour in a reliquefaction plant. Two different systems are studied in this paper, nitrogen systems which use liquid nitrogen as a refrigerant in the cargo condenser and cascade systems which use several different refrigerants in different stages, every stage uses its refrigerant to cool the next stage refrigerant to achieve a temperature low enough to liquefy the LNG vapour. Example of refrigerants in a cascade system is seawater, propane and ethylene/ethane. Both types of systems use a principal of compression and expansion cycles to lower the temperature of the refrigerant, same type of principal is also applied to the LNG vapour. The study is concentrated towards determine which of the two systems are most suitable for reliquefaction of LNG vapour on-board a LNG carrier. By comparing data from calculations, previous research and identification of important variables the study concluded which system is a more beneficial choice for a LNG carrier. By studying the energy efficiency of both systems, conclusions could also be drawn about energy saving possibilities. The result of the study clearly identifies the cascade system as the more efficient one. But other important variables such as area efficiency, flexibility and operation problems, made the nitrogen system the logical choice of system for reliquefaction of LNG on-board LNG carriers.

Publikationen registrerades 2014-10-10.

CPL ID: 204092

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