Patent Publication Number: US-10330381-B2

Title: Plant for the liquefaction of nitrogen using the recovery of cold energy deriving from the evaporation of liquefied natural gas

Description:
This application is the national stage of PCT/IB2016/051368, filed Mar. 10, 2016, which claims priority from Italian Application No. BG2015A000018, filed Mar. 17, 2015. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a plant and to a process for the liquefaction of nitrogen using the recovery of cold energy deriving, from the evaporation of liquefied natural gas. 
     To be able to transport the maximum amount of natural gas, natural gas is transported in liquid form, maintaining it at cryogenic temperatures. 
     To return to gaseous form, the natural gas must be vaporized and heated, and therefore must transfer its cold energy to another fluid. 
     The patent EP1469265, by the same applicant, describes a process of this kind. 
     The object of the present invention is to recover cold energy deriving from the evaporation of liquefied natural gas to use it in the liquefaction of nitrogen. 
     Another object is to reduce electricity consumption in the liquid nitrogen liquefaction process, exploiting the cold energy obtained from evaporation of liquid natural gas. 
     A further object is to recover cold energy deriving from the evaporation of liquefied natural gas to use it in the liquefaction of nitrogen that is more advantageous than the processes currently adopted. 
     In accordance with the present invention, these objects and yet others are achieved by a method for the liquefaction of nitrogen using the recovery of cold energy deriving from the evaporation of liquefied natural gas comprising the steps of: sending a flow of nitrogen to be liquefied to a precooler; sending a flow of nitrogen gas exiting said precooler to a heat exchanger of the high pressure recirculation compressor; sending a flow of nitrogen exiting said heat exchanger to a high pressure recirculation compressor; sending a flow of nitrogen exiting said compressor to a liquefaction heat exchanger; sending to said liquefaction heat exchanger a flow of natural gas, countercurrent to the flow exiting said compressor; sending a flow of nitrogen exiting said liquefaction heat exchanger to said heat exchanger countercurrent to said flow of nitrogen gas and to said flow of nitrogen; sending a flow of nitrogen exiting said heat exchanger to said precooler countercurrent to said flow of nitrogen to be liquefied; sending the flow of nitrogen exiting said liquefaction heat exchanger to an expander; sending the flow of nitrogen exiting said expander to a medium pressure separator that delivers an exiting flow of nitrogen. 
     These objects are also achieved by a plant for the liquefaction of nitrogen using the recovery of cold energy deriving from the evaporation of liquefied natural gas comprising sending the nitrogen to be liquefied to the following elements positioned in series: a precooler; a heat exchanger of the high pressure recirculation compressor; a high pressure recirculation compressor; a liquefaction heat exchanger that also receives a countercurrent flow of natural gas and supplies a flow of nitrogen; an expander; a medium pressure separator that delivers a flow of nitrogen; and said flow of nitrogen passes through said heat exchanger and said precooler. 
     Further features of the invention are described in the dependent claims. 
     The advantages of this solution with respect to solutions known in the art are various. 
     The present plant has a specific consumption of less than a 0.1 kW/Nm3 of LIN for a liquefier with a capacity of 400 TPD, and therefore a reduction of the specific consumption for liquefaction of nitrogen of around 80% is obtained with respect to the classic liquefaction cycle that does not use the recovery of cold energy from LNG, which typically has a specific consumption of 0.52 kW/Nm3 LIN. 
     A considerable reduction in electricity consumption is also obtained with respect to the aforesaid patent EP1469265, in fact, the present solution uses one less compressor, as the nitrogen that is liquefied (processed by the high pressure recirculation compressor) acts both as coolant and as liquefying product. As a result of this, the high pressure recirculator of the previous patent is integrated in the compressor that takes the nitrogen to the liquefaction pressure. 
     Moreover, this synergy leaves intact the condition that the liquefied natural gas is never used directly to cool the gas processed by the compressors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The features and the advantages of the present invention will be apparent from the following detailed description of a practical embodiment thereof, illustrated by way of non-limiting example in the accompanying drawing, wherein: 
         FIG. 1  shows a plant for the liquefaction of nitrogen using the recovery of cold energy deriving from the evaporation of liquefied natural gas in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the accompanying FIGURE a plant for the liquefaction of nitrogen using the recovery of cold energy deriving from the evaporation of liquefied natural gas in accordance with the present invention receives the gaseous nitrogen to be liquefied at a pressure of 10 bar and at the ambient temperature of 15° C., while the natural gas is at the temperature of −156° C. 
     The flow of nitrogen  100  to be liquefied is supplied to a precooler  101 . 
     The flow  102  of precooled nitrogen is combined with the flow  103  of recirculating gas coming from the turbine  104  and with the flow  105  of cold gas recovered from the low pressure recirculation that are combined in the cold nitrogen collector  106  at a pressure of 10 bar and at a temperature of −110° C. 
     The flow  107  exiting the collector  106  is sent to the heat exchanger  108  of the high pressure recirculation compressor to be further cooled to −145° C. 
     The flow  110  exiting the heat exchanger  108  is combined, in the collector  113 , with the flow  111  of flash gas (−165° C.) coming from the medium pressure separator  112 . 
     The flow  114  exiting the collector  113  is compressed in the first stage  115  of the high pressure recirculation compressor. 
     The flow  116  exiting the first stage  115  is cooled in the heat exchanger  108  and is sent to the second stage  117  of the high pressure recirculation compressor, removing the compression heat, so that suction of the machine takes place at the lowest temperature possible. (−150° C.). In this way, the electricity consumption is reduced considerably as the volumetric flow rate to be compressed is lower. 
     The flow  120  of nitrogen exiting the second stage  117  of the high pressure recirculation compressor is at a pressure of around 40 bar so as to liquefy the nitrogen (−154° C.) as a result of the natural gas available (−156° C.). 
     The flow  120  exiting the second stage  117  is sent to the liquefaction heat exchanger  121 . 
     The flow  123  of natural gas enters, countercurrent with respect to the nitrogen, the heat exchanger  121 , from which the flow  124  exits. The natural gas gasifies (up to −125° C.) and the nitrogen liquefies at a temperature a few degrees above the temperature of the natural gas entering. 
     The flow  126  of liquid nitrogen produced is divided into two flows. 
     A first flow  127  (around 10% of the total) is sent to the heat exchanger  128  of the low pressure recirculation compressor to remove the heat of compression downstream of each compression stage. 
     The nitrogen, still cold, see the flow  157 , is immediately recirculated to the turbine  104 . 
     The second flow  129  (the remaining 90%) is divided again (approximately in half) into two flows  130  and  150 . 
     One flow  130  is further cooled by a reduction in pressure, by an expander  131 , to the value of the suction pressure of the high pressure recirculation compressor (around 10 bar), and reaches the medium pressure separator  112 . 
     The liquid phase  132  is separated from the vapour phase  111  in the medium pressure separator  112 , recovering the cold flash  111  (around 25% of the flow rate that expands at −165° C.) directly at the suction side of the first stage (−150° C.) of the high pressure recirculation compressor  115 . 
     The flow  132  of liquid nitrogen at equilibrium pressure at 10 bar is further cooled by a reduction in pressure, by an expander  133 , to the storage pressure value (the pressure downstream of the expander is ambient pressure plus of the tank loading head, causing gasification of 25% of the flow  132  at equilibrium temperature of −193° C.). 
     The flow exiting the expander  133  is sent to a low pressure separator  136  where the liquid phase  134  is separated from the vapour phase  135 . 
     The liquid phase  134  is sent to storage, while the vapour phase  135  is sent to the first stage  140  of the low pressure recirculation compressor. The flow  141  exiting the first stage  140  is cooled in the heat exchanger  128  and sent to the second stage  142  of the low pressure recirculation compressor. 
     The low pressure and the high pressure recirculation compressors have two stages and comprise the intermediate heat exchangers, respectively  128  and  108 . The heat exchanger  128  should be considered optional; in this way the electricity consumption of the low pressure recirculation compressor can be further reduced, as the volumetric flow rate to be compressed is lower. 
     The flow  143  exiting the second stage  142  of the low pressure recirculation compressor is once again sent to the heat exchanger  128 . The flow  105  exiting the heat exchanger  128  is sent to the collector  106 . 
     The other flow  150 , coming from the second flow  129  is sent to the heat exchanger  108  of the high pressure recirculation compressor. 
     The flow  151  exiting the heat exchanger  108 , is divided into the two flows  152  and  153 . 
     The flow  152  is sent to the precooler  101 , the flow  155  exiting from which is combined with the flow  153  and with the flow  157  exiting the heat exchanger  128 , related to the flow  127 . 
     The resulting flow  158  is further cooled by means of a reduction in pressure, by a turbine  104 , which expands the entering flow to the pressure of the precooled gas to be liquefied (10 bar and −110° C.). 
     The plant has been divided into blocks to facilitate understanding thereof. 
     The block  200  receives the nitrogen to be liquefied, and performs precooling. 
     The block  201  receives the natural gas and performs liquefaction of the nitrogen. 
     The block  202  is used for the production of liquid nitrogen. 
     The block  203  is used for subcooling. 
     The block  204  is used for compression and for cold energy to (temperature) recovery. 
     The block  205  is used for the work (pressure) recovery. 
     The block  203  is optional, as, if storage of liquid nitrogen at the same pressure as the flow  100  (entering nitrogen gas) is required, the low pressure separator  136 , the low pressure recirculation is compressor  140  and  142 , and the heat exchanger  128  are not installed, as subcooling is not required. 
     When entering the block  203  the liquid nitrogen is at the pressure as the flow  100  (entering nitrogen gas) and therefore the flow  132  is sent directly to storage. 
     Although maintaining the block  203  in the plant, the heat exchanger  128  of the low pressure recirculation compressor is also optional: this heat exchanger  128  is only installed if the low pressure recirculation compressor  140 ,  142  has a capacity large enough to offset the installation cost of the heat exchanger  128  with the energy gain deriving from intercooling of the compression stages. 
     The block  200  is also optional, as if the nitrogen is not precooled we lose the coldest refrigeration duty at the inlet of the high pressure recirculation heat exchanger  108  with a consequent increase in the specific consumption due to the increase in recirculating flow rate that the heat exchanger must cool and the decrease in the efficiency of the turbine  104 , as the volumetric flow rate at the suction side of the turbine is lower. 
     The nitrogen gas produced by the medium pressure separator  112  is reintegrated directly on the suction side of the first stage of the high pressure recirculation compressor  115 . 
     Moreover, there is the option of recovering this gas directly in the collector  106  (together with the precooled nitrogen  102  and with recovery  105  of the nitrogen from the low pressure recirculation compressor  140 ,  142 ) before the nitrogen gas enters the high pressure recirculation heat exchanger  128 . Any recovery in the collector  106  (and not on the suction side of the machine) only affects the efficiency of the cycle, due to a slight increase in the specific consumption of the high pressure recirculation compressor. 
     The axes of the machines  104 ,  117 ,  115 ,  140 ,  142 , all or partially, can be mechanically connectable so as to be able to further reduce electricity consumptions. In particular, for small plants they can all be separate, whereas in larger plants it is advantageous to connect them. 
     In accordance with the present invention, it has been attempted to use large amounts of natural gas available in the regasification area to maintain the compression temperature at the lowest possible point to allow the compression of large amounts of gaseous nitrogen with low energy consumption. 
     Moreover, using an expander  104 , it is possible to expand the liquid nitrogen gasified by the heat exchanger  108  and heated  152 ,  155  in the expander  104  to produce a large amount of mechanical or electrical energy, which can be used by the compressor  117  and/or  115  to compress the recirculating nitrogen  107  again. 
     The function of the precooler  101  is that of lowering the operating temperature (hot side) of the heat exchanger  108  to cryogenic temperatures to allow improved specific power consumption of the high pressure recirculation compressor  115 / 117 . 
     The flow  114  of nitrogen exiting the heat exchanger  108  is sent to the high pressure recirculation compressor  115 ,  117 , at the lowest possible temperature using liquid nitrogen coming from the heat exchanger  121 , further improving energy efficiency. 
     The flow  126  exiting the liquefaction heat exchanger  121  is liquid nitrogen to be able to cool the elements downstream to the lowest possible temperature. In this way, the use of liquid nitrogen, therefore at a temperature below −155° C., allows a further reduction in the power consumption of the plant. 
     The flow  151 ,  152  of nitrogen exiting the heat exchanger  108  is sent to the precooler  101  countercurrent to the flow of nitrogen  100  to be liquefied, to heat as much as possible the nitrogen gasified in  108  to be expanded in the turbine  104  with higher mechanical/electrical energy recovery so as to reduce the energy consumption of the plant. 
     The use of an expander  131  to produce liquid nitrogen  132 , and the separator  112  that separates the nitrogen coming from the expander  131  allows a flow of cold nitrogen gas  111  to be obtained, which is not sent to a heat exchanger such as  128 , but directly to the suction side of the high pressure recirculation compressor  115 / 117  so as to lower the compression temperature for improved specific power consumption. 
     The use of a recirculation compressor  115 / 117  not only processes the nitrogen to be liquefied  132 / 134 , as end product deriving from the flow of nitrogen  100  to be liquefied, but also treats a much higher flow rate  107 / 110 / 114 / 120  so as to collect more cold energy from the liquid methane  123  so as to transfer it via the liquid nitrogen  150  to the heat exchanger  128  to obtain improved interstage cooling of the compressor  115 / 117  (in energy efficiency). This new arrangement of the compressor  115 / 117  with respect to the heat exchangers  128  and  121  makes it possible to obtain a specific consumption of the production of liquid nitrogen &lt;&lt;0.1 kW/Nm3, electricity consumption that is otherwise not possible. 
     In alternative embodiments of the present plant, perhaps with lower performance, but equally functional, the following can be implemented. 
     The nitrogen to be liquefied is sent to the following elements positioned in series: the heat exchanger  108  of the high pressure recirculation compressor; the high pressure recirculation compressor  115 ,  117 ; the liquefaction heat exchanger  121  that also receives the countercurrent flow  123  of natural gas; the expander  131 ; the medium pressure separator  112  that delivers the flow  132  of nitrogen. 
     In particular, the compressor  115 ,  117  comprises in series the first stage  115  of the high pressure recirculation compressor; the heat exchanger  108  and the second stage  117  of the high pressure recirculation compressor. 
     The precooler  101  can also be added at the inlet of the plant described above, before sending the flow  102  to the collector  106 . 
     The expander  133  where the flow is further cooled by a reduction in pressure and the other low pressure separator  136  where the liquid phase  134  is separated from the vapour phase  135  can also be added at the outlet. 
     The block  205  can therefore be added. 
     The block  203  can also be added. 
     The plants thus conceived are susceptible to numerous modifications and variants known to the those skilled in the art after the present description has come to their knowledge, all falling within the scope of the present inventive concept: moreover, all the elements used can be replaced by technically equivalent elements.