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

This patent describes a method and apparatus for improving the efficiency of liquefied gas reliquefaction on a floating transportation vessel. The method utilizes multiple stages of compression and heat exchangers between stages to cool and further compress the gas. This results in a higher efficiency of the reliquefaction process and increased cooling capacity, particularly in the case of a fully refrigerated vessel. The technical effects include improved performance and capacity of the liquefied gas reliquefaction process on a floating transportation vessel.

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.

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