Patent Publication Number: US-6658891-B2

Title: Offshore plant for liquefying natural gas

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a plant for liquefying natural gas. 
     FIELD OF THE INVENTION 
     A plant for liquefying natural gas comprises a main heat exchanger in which the natural gas is liquefied by means of indirect heat exchange with evaporating refrigerant, and a refrigerant circuit in which evaporated refrigerant is compressed and liquefied to produce liquid refrigerant that is used in the main heat exchanger. The refrigerant circuit includes a compressor train consisting of at least one compressor. The at least one compressor is driven by means of a gas turbine that is directly connected to the shaft of the compressor. Such a plant is disclosed in U.S. Pat. No. 5,689,141. Because a gas turbine has only a limited operating window, the gas turbine is first selected and the liquefaction plant is so designed that the gas turbine operates in its limited operating window. In addition the gas turbine and the compressor are directly connected to each other, so that they form a single unit. The single unit occupies a considerable surface area. 
     There is a tendency to look for ways of reducing the surface area of such a liquefaction plant. This does not only apply to on-shore plants, but also to floating liquefaction plants. 
     Such floating liquefaction plants are used in the development of off-shore gas fields, where the gas is liquefied near the production location. Thereto the liquefaction plant is installed on a barge that serves as a floating storage of liquefied natural gas. The barge is furthermore provided with an off-loading system to transfer the liquefied natural gas into a tanker, and with a gas loading system that is connected by means of a swivel to the upper end of a riser pipe, wherein the lower end of the riser pipe is connected to a well producing natural gas. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a plant for liquefying natural gas that is flexible and that occupies a small surface area, so that, for example a barge can accommodate the liquefaction plant. 
     To this end, the plant for liquefying natural gas according to the present invention comprises a main heat exchanger in which natural gas is liquefied by means of indirect heat exchange with evaporating refrigerant, and a refrigerant circuit in which evaporated refrigerant is compressed and liquefied to produce liquid refrigerant that is used in the main heat exchanger, wherein the refrigerant circuit includes a compressor train consisting of at least one compressor driven by an electric motor. 
     It will be understood that there should be provided an electric power plant to provide electric energy to drive the electric motors. The electric power plant will include one or more gas or steam turbines each driving an electric generator. With the liquefaction plant according to the present invention, the gas or steam turbine(s) can be put everywhere where for reasons of lay-out planning or for reasons of safety they are best located. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention will now be described by way of example with reference to the accompanying drawings, wherein 
     FIG. 1 shows schematically a first embodiment of the invention; and 
     FIG. 2 shows schematically a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is now made to FIG.  1 . The plant  1  for liquefying natural gas supplied through conduit  5  comprises a main heat exchanger  10 , having a shell  11  enclosing a shell side  12  in which three heat exchanger tubes  13 ,  14  and  15  are arranged. In the main heat exchanger  10  the natural gas is liquefied by means of indirect heat exchange with refrigerant evaporating in the shell side  12 . 
     The plant  1  also comprises a refrigerant circuit  20 . The refrigerant circuit  20  comprises the shell side  12  of the main heat exchanger  10 , conduit  22 , a first and a second compressor train  23   a  and  23   b  arranged in parallel, a gas-liquid separator  25 , a pre-cooler heat exchanger  27 , a main gas-liquid separator  28  and the second and the third heat exchanger tubes  14  and  15  in the main heat exchanger  10 . 
     Before discussing the compressor trains  23   a  and  23   b  in more detail, the remainder of the refrigerant circuit  20  is discussed. The pre-cooler heat exchanger  27  has a shell  35  enclosing a shell side  36  in which two heat exchanger tubes  37  and  38  are arranged, which pertain to the refrigerant circuit  20 . The inlet end of heat exchanger tube  37  is connected by means of conduit  39  to the outlet for gas of the gas-liquid separator  25 , and the inlet end of heat exchanger tube  38  is connected by means of conduit  40  to the outlet for liquid of the gas-liquid separator  25 . The discharge end of the heat exchanger tube  38  is connected to a nozzle  42  arranged in the shell side  36  by means of a conduit  43  provided with an expansion device  44 . The discharge end of the heat exchanger tube  37  is connected by means of conduit  46  to the inlet of the main gas-liquid separator  28 . The outlet for gas of the main gas-liquid separator  28  is connected by means of conduit  48  to the inlet of the heat exchanger tube  14 , and the outlet for liquid is connected by means of conduit  50  to the heat exchanger tube  15  in the main heat exchanger  10 . The discharge end of the heat exchanger tube  14  is connected to a nozzle  52  arranged in the shell side  12  by means of a conduit  53  provided with an expansion device  54 , and the discharge end of the heat exchanger tube  15  is connected to a nozzle  58  arranged in the shell side  12  by means of a conduit  59  provided with an expansion device  60 . 
     Now the parallel compressor trains will be discussed in more detail. Each of the compressor trains  23   a  and  23   b  consists of three interconnected compressors, a low pressure compressor  65   a ,  65   b , an intermediate pressure compressor  66   a ,  66   b  and a high pressure compressor  67   a ,  67   b . Conduit  22  is connected to the inlets of the low pressure compressors  65   a  and  65   b  by means of conduits  22   a  and  22   b . The outlets of the low pressure compressors  65   a ,  65   b  are connected to the inlets of the intermediate pressure compressors  66   a ,  66   b  by means of conduits  70   a  and  70   b , provided with an air cooler  71 . The outlets of the intermediate pressure compressors  66   a ,  66   b  are connected to the inlets of the high pressure compressors  67   a ,  67   b  by means of conduits  72   a  and  72   b , provided with an air cooler  73 . The outlets of the high pressure compressors  67   a ,  67   b  are connected to the inlet of the gas-liquid separator  25  by means of conduits  74 ,  74   a  and  74   b , provided with an air cooler  75 . 
     The shell side  36  of the pre-cooler heat exchanger  27  is connected to the inlets of the intermediate pressure compressors  66   a ,  66   b  by means of conduit  80 . 
     The compressors of each compressor train  23   a  or  23   b  are arranged on the same shaft  82   a  or  82   b  driven only by an electric motor  83   a  or  83   b . The electric motors  83   a  and  83   b  are connected to an electric generator (not shown) by means of electric conduits  84   a  and  84   b.    
     During normal operation natural gas supplied through conduit  5  is passed through heat exchanger tube  13  arranged in the shell side  12  of the main heat exchanger  10 , and liquefied natural gas is removed from the discharge end of the heat exchanger tube  13 . Evaporated refrigerant is removed from the shell side  12 , and it is passed through conduits  22 ,  22   a ,  22   b  to the inlets of the low pressure compressors  65   a ,  65   b  of the parallel compressor trains  23   a  and  23   b , in such a way that substantially equal amounts of refrigerant are supplied to the compressor trains  23   a  and  23   b . In the compressors  65   a ,  65   b ,  66   a ,  66   b ,  67   a ,  67   b  the refrigerant is compressed from a low pressure in stages to a high pressure, and in between the heat of compression is removed in the air coolers  71  and  73 . 
     At the high pressure the refrigerant is supplied to the air cooler  75  in which it is partly liquefied. The partly liquefied stream of refrigerant is separated into a gaseous stream and a liquid stream in the gas-liquid separator  25 . 
     The liquid stream is used for autorefrigeration and for partly liquefying the gaseous refrigerant stream. To this end the liquid stream is passed at high pressure through heat exchanger tube  38  and expanded in expansion device  44 . In expanded form the liquid stream is introduced in the shell side  36  through nozzle  42 . The gaseous stream is partly liquefied in the heat exchanger tube  37 , and passed to the main gas-liquid separator  28 . 
     In the main gas-liquid separator  28 , this stream is separated into a gaseous stream and a liquid stream, which are both used for autorefrigeration and for liquefying the natural gas stream in the main heat exchanger  10 . 
     To this end the liquid stream is passed at high pressure through heat exchanger tube  15  and expanded in expansion device  60 . In expanded form the liquid stream is introduced through nozzle  58  in the shell side  12 , where it is allowed to evaporate at low pressure. The gaseous stream is passed at high pressure through heat exchanger tube  14 , wherein it is partly liquefied, and this partly liquefied stream is subsequently expanded in expansion device  54  and introduced in the shell side  12  through nozzle  52 , where it is allowed to evaporate at low pressure. 
     In the main heat exchanger  10 , the natural gas stream  5  is liquefied and sub-cooled while passing through the heat exchanger tube  13  by indirect heat exchange with the expanded streams that are introduced into the shell side  12  through nozzles  52  and  58 . 
     Preferably, natural gas is pre-cooled, and to this end, it is supplied via conduit  85  to the inlet end of a heat exchanger tube  86  in the pre-cooler heat exchanger  27 . The outlet end of the heat exchanger tube  86  is connected to conduit  5 . 
     Reference is now made to FIG. 2, showing schematically an alternative embodiment of the invention. Parts that are similar to parts discussed with reference to FIG. 1 have been referred to with the same reference numerals. The plant  2  of FIG. 2 differs from the plant  1  shown in FIG. 1 in that the refrigerant circuit  20  includes auxiliary heat exchangers  90  and  91 . In auxiliary heat exchangers  90  and  91  the refrigerant is partly liquefied by indirect heat exchange with auxiliary refrigerant. The auxiliary heat exchangers  90  and  91  also form part of the auxiliary refrigerant circuit  100 . The auxiliary heat exchangers  90  and  91  take the place of the air cooler  75  and the pre-cooler heat exchanger  27  as shown in FIG.  1 . In addition each of the first and the second compressor trains  23   a  and  23   b  consists of a single compressor  65   a  and  65   b.    
     Now the auxiliary refrigerant circuit  100  of the plant  2  will be discussed. The auxiliary refrigerant circuit  100  comprises shell side  101  of the auxiliary heat exchanger  91 , conduit  102 , a first and a second auxiliary compressor train  103   a  and  103   b  arranged in parallel, a heat exchanger tube  104  arranged in the auxiliary heat exchanger  90 , and a heat exchanger tube  106  in the auxiliary heat exchanger  91 . 
     The auxiliary compressor trains  103   a  and  103   b  consist of two-stage compressors  110   a  and  110   b , which are arranged to receive two streams of evaporated auxiliary refrigerant from the shell side  101  of the auxiliary heat exchanger  91  through conduits  102 ,  102   a ,  102   b , and from shell side  112  of the auxiliary heat exchanger  90  through conduits  105 ,  105   a  and  105   b . The compressors  110   a  and  110   b  are driven only by an auxiliary electric motor  113   a  or  113   b . The auxiliary electric motors  113   a  and  113   b  are connected to an electric generator (not shown) by means of electric conduits  114   a ,  114   b.    
     The outlets of the two-stage compressors  110   a  and  110   b  are connected to the inlet of the heat exchanger tube  104  of the auxiliary heat exchanger  90  by means of conduits  116   a ,  116   b ,  116 , provided with air cooler  117 . The discharge end of the heat exchanger tube  104  is connected to a nozzle  120  arranged in the shell side  112  by means of a conduit  125  provided with an expansion device  126  to supply during normal operation part of the auxiliary refrigerant to the shell side  112 . The remainder is passed through conduit  130 , which is connected to the inlet end of the heat exchanger tube  106  in the auxiliary heat exchanger  91 . The discharge end of the heat exchanger tube  106  is connected to a nozzle  135  arranged in the shell side  101  by means of a conduit  140  provided with an expansion device  144 . 
     During normal operation natural gas supplied through conduit  5  is passed through heat exchanger tube  13  arranged in the shell side  12  of the main heat exchanger  10 , and liquefied natural gas is removed from the discharge end of the heat exchanger tube  13 . 
     Evaporated refrigerant is removed from the shell side  12 , and it is passed through conduits  22 ,  22   a ,  22   b  to the inlets of the parallel compressor trains  23   a  and  23   b , in such a way that substantially equal amounts of refrigerant are supplied to the compressor trains  23   a  and  23   b . The heat of compression is removed in the air coolers  71   a  and  71   b . The refrigerant is passed on through the conduit  74  to heat exchanger tube  150  in the auxiliary heat exchanger  90  and subsequently to heat exchanger tube  155  in the auxiliary heat exchanger  91 , and during this passage the refrigerant is partly liquefied by indirect heat exchange with evaporating auxiliary refrigerant. 
     From the discharge end of the heat exchanger tube  155  partly liquefied refrigerant is passed through conduit  46  to the main gas-liquid separator  28 . In the main gas-liquid separator  28 , this is separated into a gaseous stream and a liquid stream, which are both used for autorefrigeration and for liquefying the natural gas stream in the main heat exchanger  10 . 
     To this end the liquid stream is passed at high pressure through heat exchanger tube  15  and expanded in expansion device  60 . In expanded form the liquid stream is introduced in the shell side  12  through nozzle  58 . The gaseous stream is passed at high pressure through heat exchanger tube  14 , wherein it is partly liquefied, and this partly liquefied stream is subsequently expanded in expansion device  54  and introduced in the shell side  12  through nozzle  52 . 
     As stated before, in order to partly liquefy the refrigerant, auxiliary refrigerant is passed through the auxiliary refrigerant circuit  100  in the following way. 
     Evaporated auxiliary refrigerant is removed from the shell side  101  of the auxiliary heat exchanger  91 , and it is passed through conduits  102 ,  102   a ,  102   b  to the inlets of the parallel auxiliary compressors  110   a  and  110   b , in such a way that during normal operation substantially equal amounts of auxiliary refrigerant are supplied to the compressors  110   a  and  110   b . In the compressors  110   a  and  110   b  the auxiliary refrigerant is compressed to high pressure. Heat of compression is removed from the compressed auxiliary refrigerant by means of air cooler  117 . 
     Auxiliary refrigerant at high pressure is passed through the heat exchanger tube  104  in the auxiliary heat exchanger  90 , and part of the cooled auxiliary refrigerant is passed through expansion device  126  to the shell side  112  where it is allowed to evaporate at an intermediate pressure. Thus cooling the auxiliary refrigerant by autorefrigeration and cooling the refrigerant passing through heat exchanger tube  150 . The remainder is supplied at high pressure to the heat exchanger tube  106  in the auxiliary heat exchanger  91 . Cooled auxiliary refrigerant leaving the heat exchanger tube  106  is passed through expansion device  144  to the shell side  101  of the auxiliary heat exchanger  91 , where it is allowed to evaporate at a low pressure. 
     Auxiliary refrigerant at the intermediate pressure is removed from the shell side  112  of the auxiliary heat exchanger  90  via conduits  105 ,  105   a  and  105   b  to the inlets of the second stage of the two-stage compressors  110   a  and  110   b , whereas auxiliary refrigerant at the low pressure is removed from the shell side  101  of the auxiliary heat exchanger  91  via conduits  102 ,  102   a  and  102   b  to the inlets of the first stage of the two-stage compressors  110   a  and  110   b.    
     Preferably, natural gas is pre-cooled, and to this end, it is supplied via conduit  158  to the inlet end of a heat exchanger tube  160  in the auxiliary heat exchanger  91 . The outlet end of the heat exchanger tube  160  is connected to conduit  5 . 
     The operating conditions of the liquefaction plants as described with reference to the Figures and the compositions of the refrigerants are well known, and will not be discussed here. 
     An advantage of the plant as discussed with reference to FIG. 2 is that the power supplied to the electric motors  83   a  and  83   b  and the electric motors  113   a  and  113   b  can be selected to match the cooling requirements in the refrigeration circuits  20  and  100 . 
     The parallel arrangement of the compressor trains is preferred because in the event of a failure in or maintenance of one compressor train the other one can continue to operate, so that the plant can continue to liquefy natural gas. 
     Each of the three separate compressors of the compressor trains  23   a  and  23   b  can be replaced by a single three-stage compressor. 
     It will be understood that air coolers can be replaced by water coolers. 
     The electric generators providing the electric power driving the electric motors  83   a ,  83   b ,  113   a  and  113   b  and the required drivers (steam or gas turbines) can be arranged at the most suitable location. They not be arranged in-line with the compressors, and therefore the present invention provides a plant for liquefying natural gas that is flexible and that occupies only a relatively small surface area, so that, for example a barge can accommodate the liquefaction plant.