Method of cooling boil off gas and an apparatus therefor

Abstract

The disclosure relates to a method and apparatus for cooling, preferably liquefying a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, said method comprising at least the steps of: -compressing a boil off gas stream (01) from said liquefied cargo in three or more stages of compression comprising at least a first stage (65), a second stage (70) and final stage (75) to provide a compressed discharge stream (06), wherein intermediate compressed BOG streams (02, 04) are provided between consecutive stages of compression; -cooling the compressed discharge stream (06) to provide a cooled compressed discharge stream (07); -heat exchanging an expanded, optionally further cooled, portion of the cooled compressed discharge stream (07), with (i) one or more intermediate compressed BOG streams (04) from consecutive stages selected from between the second and final stages (75) of compression to provide one or more cooled intermediate compressed BOG streams (05) and optionally (ii) one or more portions (07a, 108a), optionally after further cooling, of the cooled compressed discharge stream (07); and -passing the one or more cooled intermediate compressed BOG streams (05) to the next stage of compression (75).

Description

TECHNICAL FIELD[0001]This disclosure relates to a method for the cooling, particularly the reliquefaction, of a boil off gas (BOG) from a liquefied cargo, such as liquefied petroleum gas (LPG), on a floating transportation carrier, and an apparatus therefor.BACKGROUND OF THE DISCLOSURE[0002]Floating transportation carriers, such as liquefied gas carrier vessels and barges, are capable of transporting a variety of cargoes in the liquefied state. In the present context, these liquefied cargoes have boiling points of greater than −110° C. when measured at 1 atmosphere and include liquefied petroleum gas, liquefied petrochemical gases such as propylene and ethylene, and liquefied ammonia. For instance, liquefied petroleum gas is a useful fuel source, such as for heating appliances and vehicles, as well as being a source of hydrocarbon compounds. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and eth...

AI Technical Summary

Benefits of technology

[0008]The present disclosure utilises a method of heat exchanging one or more intermediate BOG streams between the second and further stages of multiple stages of compression with part of the cooled compressed discharge from the final compressor stage. This can provide an improvement in the co-efficient of performance for the reliquefaction of the boil off gas.

[0050]The apparatus and method disclosed herein are applicable to any floating transportation vessel for a liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, such as an LPG carrier. The apparatus and method disclosed herein may be utilized in floating transportation vessels where the liquefied cargo storage tanks are fully refrigerated to maintain the cargo in liquid phase at approximately atmospheric pressure by lowering the temperature, as well as in those vessels in which the cargo in the storage tanks is maintained in the liquid phase by a combination of reduced temperature and increased pressure.

[0052]The method and apparatus disclosed herein utilizes three or more stages of compression. Heat exchangers such as economizers can be placed between consecutive stages of compression selected between the second and final stages. For instance, an economizer can be situated between at least the second and third stages, optionally in combination with an economizer between the first and second stages. In such an economizer, an expanded, optionally further cooled, portion of the cooled compressed discharge stream can be heat exchanged with (i) an intermediate compressed BOG stream and / or (ii) an optionally further cooled continuing portion of the cooled compressed discharge stream. This leads to further improvements in the coefficient of performance and increased cooling, particularly reliquefaction, capacity.

Problems solved by technology

The size of the carrier imposes limitations on the space available for the reliquefaction system.

Furthermore, size restrictions may also preclude the use of a closed refrigeration system to cool the condenser for the compressed BOG stream, such that the cooling duty may be supplied by seawater.

Images