Integrally-geared compressors for precooling in lng applications

Abstract

A natural gas liquefaction system is disclosed, which comprises at least a pre-cooling loop, through which a first refrigerant is adapted to circulate. The pre-cooling loop comprises at least one compressor for pressurizing the first refrigerant; at least one prime mover for driving the compressor; at least one condenser for removing heat from the first refrigerant; at least a first expansion element for expanding the first refrigerant; at least a first heat exchanger for transferring heat from natural gas to the first refrigerant. The system further comprises at least a cooling loop, downstream of the pre-cooling loop, where through a second refrigerant circulates. The natural gas is adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop. The compressor of the pre-cooling loop is an integrally-geared turbo-compressor comprising a plurality of compressor stages.

What is claimed is: 
     
         1 . A natural gas liquefaction system comprising:
 at least a pre-cooling loop, through which a first refrigerant is adapted to circulate, the pre-cooling loop comprising:   at least one compressor for pressurizing the first refrigerant;   at least one prime mover for driving said compressor;   at least one condenser for removing heat from the first refrigerant;   at least a first expansion element for expanding the first refrigerant;   at least a first heat exchanger for transferring heat from natural gas to the first refrigerant;   and at least a cooling loop, downstream of said pre-cooling loop, where through a second refrigerant circulates, the natural gas being adapted to be sequentially cooled in the pre-cooling loop and in the cooling loop;   wherein said compressor is an integrally-geared turbo-compressor comprising a plurality of compressor stages each one being provided with independent set of movable inlet guide vanes for autonomously regulating flows entering in the compressor stages.   
     
     
         2 . The system of  claim 1 , wherein the pre-cooling loop is configured to divide the first refrigerant into two or more side streams returned at respective compressor stages of said integrally-geared turbo-compressor. 
     
     
         3 . The system of  claim 1 , wherein said prime mover comprises an electric motor. 
     
     
         4 . The system of  claim 1 , wherein said prime mover comprises a gas turbine. 
     
     
         5 . The system of  claim 1 , wherein the pre-cooling loop further comprises: a plurality of sequentially arranged first expansion elements configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of first heat exchangers arranged and configured for receiving respective portions of said first refrigerant expanded through said plurality of sequentially arranged first expansion elements and for transferring heat from the natural gas to the first refrigerant; a plurality of return paths returning side streams of the first refrigerant from each said first heat exchangers to a respective compressor stage of the integrally-geared turbo-compressor. 
     
     
         6 . The system of  claim 1 , wherein the pre-cooling loop further comprises: at least a first auxiliary expansion element and at least a first auxiliary heat exchanger receiving a portion of said first refrigerant expanded through said first auxiliary expansion element and configured for transferring heat from the second refrigerant circulating in the cooling loop to the first refrigerant circulating in the pre-cooling loop. 
     
     
         7 . The system of  claim 1 , wherein the pre-cooling loop further comprises: a plurality of sequentially arranged second expansion elements configured for expanding the first refrigerant at a plurality of decreasing pressure levels; a plurality of second heat exchangers arranged and configured for receiving respective portions of said first refrigerant expanded through said first auxiliary expansion elements and for transferring heat from the second refrigerant to the first refrigerant; a plurality of return paths returning the portions of first refrigerant from each said second heat exchangers to a respective compressor stage of the integrally-geared turbo-compressor. 
     
     
         8 . The system of  claim 1 , wherein said first refrigerant comprises a gas with a molecular weight greater than 35. 
     
     
         9 . The system of  claim 1 , wherein said second refrigerant is a mixed refrigerant, or ethylene, or methane. 
     
     
         10 . The system of  claim 1 , wherein said integrally-geared turbo-compressor comprises: a central gear driven into rotation by said prime mover; a plurality of driven shafts, each driven shaft comprising a pinion meshing with the central gear and driven into rotation by said central gear; at least one respective compressor impeller mounted on each shaft. 
     
     
         11 . (canceled) 
     
     
         12 . The system of  claim 1 , further comprising at least one intercooler between at least two serially arranged compressor stages of said integrally-geared turbo-compressor. 
     
     
         13 . The system of  claim 1 , wherein the integrally-geared turbo-compressor compresses the first refrigerant so that the pressurized first refrigerant is delivered from the last stage of the integrally-geared turbo-compressor at a pressure ranging from about 45 to about 65 bar absolute. and more particularly between about 50 and about 60 bar absolute; and wherein the pressure of the first refrigerant at the inlet of the first stage of the integrally-geared turbo-compressor ranges from about 2.5 to about 10, and more particularly from about 3 to about 5 bar absolute. 
     
     
         14 . The system of  claim 1 , wherein the integrally-geared turbo-compressor compresses the first refrigerant so that the pressurized first refrigerant is delivered from the last stage of the integrally-geared turbo-compressor at a pressure ranging from about 10 to about 30 bar absolute, and more particularly between about 15 and about 25 bar absolute; and wherein the pressure of the first refrigerant at the inlet of the first stage of the integrally-geared turbo-compressor ranges from about 1 to about 2.5, and more particularly from about 1.5 to about 2 bar absolute. 
     
     
         15 . The system of  claim 1 , wherein the integrally-geared turbo-compressor delivers a first refrigerant flow at a flow rate ranging from about 10,000 actual m3/h to about 70,000 actual m3/h. 
     
     
         16 . The system of  claim 1 , wherein the integrally-geared turbo-compressor absorbs a power ranging from about 12 MW to about 40 MW and more particularly ranging from about 14 MW to about 30 MW. 
     
     
         17 . The system of  claim 1 , wherein the integrally-geared turbo-compressor comprises at least four stages, each stage comprising at least one impeller. 
     
     
         18 . A method of liquefying natural gas, comprising:
 providing a pre-cooling loop comprising: an integrally-geared turbo-compressor having a plurality of compressor stages each one being provided with movable inlet guide vanes, at least one condenser, at least one expansion element, and at least one heat exchanger;
 driving said integrally-geared turbo-compressor with a prime mover; 
 circulating a first refrigerant through the integrally-geared turbo-compressor; 
 condensing the first refrigerant delivered by the integrally-geared turbo-compressor in the condenser; 
 dividing the first refrigerant in a plurality of partial flows; 
 expanding the condensed first refrigerant in the expansion element; 
 circulating the expanded refrigerant through the heat exchanger to remove heat from the natural gas, to pre-cool the natural gas; 
 controlling independently movable inlet guide vanes to regulate the partial flows at the suction side of the compressor stages; 
 providing at least one cooling loop; 
 circulating a second refrigerant in said at least one cooling loop; 
 remove heat from the pre-cooled natural gas by heat exchange against the second refrigerant. 
   
     
     
         19 . The method of  claim 18 , wherein said prime mover comprises an electric motor. 
     
     
         20 . (canceled) 
     
     
         21 . The method of  claim 18 , wherein the movable inlet guide vanes are controlled as a function of flow conditions of the partial flows. 
     
     
         22 . The method of  claim 18 , further comprising the steps of:
 providing a plurality of sequentially arranged first expansion elements in said pre-cooling loop;   expanding condensed first refrigerant through said first expansion elements at a plurality of decreasing pressure levels;   circulating portions of the expanded first refrigerant from each said first expansion elements through a plurality of first heat exchangers, to remove heat from the natural gas;   returning through respective return paths said portions of expanded first refrigerant from said first heat exchangers to respective ones of said plurality of compressor stages.   
     
     
         23 . (canceled)