Method and apparatus for the reliquefaction of a vapour

Abstract

Boiled off liquefied natural gas flows from a storage tank and is compressed in vapour compression stages. The resulting compressed vapour is condensed in a condenser and the condensate returned to the tank. The condenser is cooled by a working fluid, for example nitrogen, flowing in a Brayton cycle. The Brayton cycle includes a heat exchanger which removes heat of compression from the compressed natural gas vapour upstream of its passage through the condenser. In addition a part of the working fluid is withdrawn from a region of the Brayton cycle intermediate the working fluid outlet from the condenser and the working fluid inlet to the heat exchanger and the withdrawn working fluid flows through another heat exchanger in which it removes heat of compression from the natural gas vapour intermediate the compression stages. The withdrawn working fluid is returned to the Brayton cycle.

1 . A method of reliquefying vapour boiled off from at least one volume of liquefied natural gas held in at least one storage tank, comprising compressing the vapour in first and second vapour compression stages in series, condensing the compressed vapour in a condenser by heat exchange with a working fluid flowing in a main endless refrigeration cycle, and returning at least some of the resulting condensate to the said storage tank, wherein in the main working fluid cycle the working fluid is, in sequence, compressed in at least one working fluid compressor, cooled in a first heat exchanger, expanded in an expansion turbine, employed in the condenser to perform the condensation of the natural gas vapour, warmed in the said first heat exchanger in heat exchange with the working fluid being cooled and returned to the said working fluid compressor, characterised in that in the main working fluid cycle intermediate the passage of the working fluid through the condenser and its passage through the first heat exchanger, the working fluid is employed to pre-cool in a second heat exchanger the compressed natural gas vapour downstream of the second vapour compression stage but upstream of the condenser, and in that a flow of working fluid is diverted from a region of the main working fluid cycle where the working fluid is flowing from the condenser to the second heat exchanger and is passed through at least one third heat exchanger so as to cool the natural gas vapour intermediate the first and second vapour compression stages, the diverted working fluid being returned to the main working fluid cycle at a region where the working fluid is flowing from the second heat exchanger to the first heat exchanger. 
     
     
         2 . The method according to  claim 1 , in which the proportion of the working fluid that is diverted from the main working fluid cycle to the third heat exchanger is controlled in response to the temperature at the inlet to the second vapour compression stage. 
     
     
         3 . The method according to  claim 1 , wherein when the said storage tank is fully laden with liquefied natural gas the condenser is operated such that sub-cooled liquefied natural gas exits from it. 
     
     
         4 . The method according to  claim 1 , wherein the vapour upstream of the first vapour compression stage is pre-cooled by having mixed therewith a stream of condensed natural gas taken from the condenser. 
     
     
         5 . The method according to  claim 4 , wherein the flow rate of the stream of condensed vapour is controlled in response to the temperature at the inlet to the first compression stage. 
     
     
         6 . An apparatus for reliquefying natural gas vapour comprising at least one storage tank for holding at least one volume of liquefied natural gas, first and second vapour compression stages in series for compressing boiled-off natural gas vapour communicating with at least one vapour space in the said storage tank, a condenser for condensing the compressed vapour having a natural gas inlet communicating with a second vapour compression stage and an outlet communicating with the said storage tank, wherein the condenser is arranged so as to be cooled, in use, by a working fluid, the condenser forming part of an endless main working fluid cycle comprising, in sequence, (a) at least one working fluid compressor for compressing a flow of the working fluid, (b) a cooling path through a first heat exchanger for cooling the working fluid flow, (c) an expansion turbine for expanding the flow of working fluid, (d) the condenser, (e) a warming path through the first heat exchanger for warming the working fluid, and (f) an inlet to the said working fluid compressor, characterised in that the main working fluid cycle comprises the second heat exchanger for cooling the natural gas by heat exchange with the working fluid, the second heat exchanger having a natural gas vapour path therethrough intermediate the second vapour compression stage and the condenser and a working fluid path therethrough intermediate the working fluid outlet from the condenser and the inlet to the warming path through the first heat exchanger, and in that there is a third heat exchanger for cooling the natural gas vapour intermediate the first and second natural gas vapour compression stages by heat exchange with working fluid diverted from the main working fluid cycle, the third heat exchanger having a working fluid path therethrough communicating at its inlet with a region of the working fluid cycle intermediate the working fluid outlet from the condenser and the working fluid inlet to the second heat exchanger and at its outlet with a region of the working fluid cycle intermediate the working fluid outlet from the second heat exchanger and the inlet to the warming path through the first heat exchanger. 
     
     
         7 . The apparatus according to  claim 6 , wherein there is a valve for controlling the proportion of the working fluid that is diverted from the main working fluid cycle to the third heat exchanger in response to the temperature at the inlet to the second vapour compression stage. 
     
     
         8 . The apparatus according to  claim 6 , wherein the first and second vapour compression stages are driven by a single plural speed motor. 
     
     
         9 . The apparatus according to  claim 6 , further comprising a mixer upstream of the first vapour compression stage in which the natural gas vapour is able to be cooled, the mixer having an inlet for condensed natural gas communicating with the condenser. 
     
     
         10 . The apparatus according to  claim 9 , further comprising a valve for controlling the flow of condensate to the mixer and operable to maintain constant the temperature at the inlet to the first compression stage. 
     
     
         11 . The apparatus according to  claim 6 , wherein an outlet for condensate from the condenser is able selectively to be placed through an expansion valve in communication with a phase separator having an outlet for returning liquid to the storage tank and an outlet for passing vapour to a combustion unit.