Abstract:
The invention relates to a plant for regasification of liquefied natural gas (GNL), comprising a liquefied gas storage reservoir ( 10 ) and a regasification device ( 12 ) for the GNL through which the natural gas and a heat transfer medium flow. According to the invention, the plant comprises a loop circuit ( 16 ) in which the heat transfer medium circulates in the form of a low-viscosity organic liquid with a low crystallisation point and the regasification device ( 12 ) comprises at least two exchangers ( 60, 62 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a liquefied natural gas regasification plant and to a method used in same. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally, when natural gas has to be transported from a production site to an exploitation site that are close together, this transport occurs through onshore or underwater pipelines. In this case, the natural gas is transported in gaseous form and it can be used as such at its point of destination. 
         [0003]    However, when the two sites are too far away from one another or if the field configuration does not allow pipe laying, the gas is transported in liquefied form by land vehicle or by boat (generally LNG carriers) between the production site and the exploitation site. The natural gas is therefore liquefied in the vicinity of the production site during compression and cooling operations to a temperature of −160° C. The liquefied natural gas (LNG) is thereafter stored in suitable tanks, then transferred in liquid form to tanks for ground transportation or shipping to the exploitation site. Once on this site, this liquefied gas is unloaded into LNG storage tanks from which this gas can be regasified on demand and used either directly on the exploitation site or transported in gaseous form through pipelines to other exploitation sites. 
         [0004]    Usually, in the case of LNG shipping, the liquefied gas is stored, then transported in the vicinity of the shore terminal in isothermal tanks of the LNG carrier. This liquefied gas is either regasified from the LNG carrier tanks, then transported in gaseous form through pipelines to exploitation sites, or sent in liquid form to tanks of the shore terminal where they are stored and regasified on demand. 
         [0005]    Currently, to carry out the regasification operation, the gas in liquid form is pumped from the tank, then it flows through a set of heat exchangers acting as vaporizers or regasifiers. In order to provide heat exchange, seawater, possibly heated, is sent through this set of heat exchangers so that the calories present in this water are transmitted to the gas. This calories transmission allows the gas to be heated as it progresses through the set of exchangers, it progressively changes state and leaves the set of exchangers in gaseous form. 
         [0006]    Such layouts however involve quite considerable drawbacks, as regards nature preservation as well as integrity of the exchangers. 
         [0007]    In fact, the seawater that has passed through the heat exchangers is discharged at sea at a very low temperature, which causes degradation of the submarine flora and fauna. Besides, seawater is a corrosive agent towards all the metallic parts of the exchangers and it therefore requires closer maintenance of these exchangers. Furthermore, considering the fact that the LNG circulates in the exchangers at a very low temperature, the seawater has to flow through the exchangers with a high flow rate so as to prevent crystal formation, which requires large-size and expensive pumping facilities. 
         [0008]    The present invention aims to overcome the aforementioned drawbacks by means of a regasification plant using a heat carrier allowing to respect the environment and that can be used far from shore terminals. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention thus relates to a liquefied natural gas regasification plant comprising a tank for storing the gas in liquefied form and an LNG regasification device through which a heat carrier and the natural gas flow, characterized in that the plant comprises a loop circuit in which the heat carrier circulates in form of a low-viscosity and low-crystallization point organic fluid, and in that the regasification device comprises at least two exchangers. 
         [0010]    The plant can comprise a heat carrier heating unit. 
         [0011]    Advantageously, air can flow through the heating unit. 
         [0012]    The heat carrier can have a crystallization temperature ranging between −90° C. and −150° C. 
         [0013]    Preferably, the heat carrier can be an alcohol such as methanol, ethanol or propanol. 
         [0014]    One of the exchangers can be co-current between the LNG and the heat carrier and the other exchanger can be counter-current. 
         [0015]    The counter-current exchanger can be in two parts between which a phase separator is interposed. 
         [0016]    At least the counter-current exchanger can be of brazed plate-fin exchanger type. 
         [0017]    The heat carrier circulation circuit can comprise an additional heating exchanger. 
         [0018]    The plant can comprise means for liquefying a hydrocarbon by calorific exchange with the heat carrier. 
         [0019]    The hydrocarbon can be in gaseous form after being used for driving a turbine. 
         [0020]    Advantageously, the hydrocarbon can be propane. 
         [0021]    The plant can also comprise means for CO 2  trapping by the heat carrier. 
         [0022]    Preferably, the heat carrier can be used as solvent of the CO 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]    Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein: 
           [0024]      FIG. 1  diagrammatically shows the LNG regasification plant according to the invention, 
           [0025]      FIG. 2  is a partial sectional view of the heater used in the plant according to the invention, 
           [0026]      FIG. 3  is a diagrammatic sectional view of the regasifier used in this plant, 
           [0027]      FIG. 4  is a first variant of the regasification plant according to the invention, 
           [0028]      FIG. 5  is another variant of the regasification plant according to the invention, 
           [0029]      FIG. 6  shows an example of a particular use of the plant according to the invention, and 
           [0030]      FIG. 7  shows another example of a use of the plant according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  diagrammatically shows a liquefied natural gas (LNG) regasification plant comprising a storage tank  10  for storing the LNG at atmospheric pressure and at a temperature close to −160° C., a regasification device with a heat exchanger unit, or regasifier  12 , through which a heat carrier and the LNG from the tank flow, and a heat carrier heating unit  14 . 
         [0032]    The heat carrier is an organic fluid whose crystallization point is close to that of the LNG and it has a viscosity that is low enough to be led to readily circulate in these pipes even at very low temperatures. Furthermore, this heat carrier remains in the liquid state under conditions of use at atmospheric pressure and at ambient temperature. Preferably, this heat carrier can be an alcohol or a hydrocarbon or one of their compounds. In the description hereafter, the organic fluid considered by way of example is methanol whose crystallization point is approximately −98° C., but it is also possible to use other alcohols such as ethanol (crystallization point: −114° C.) or propanol (crystallization point: −126° C.). 
         [0033]    This plant comprises a heat carrier circulation loop  16  that, in the example shown, is a closed loop with a warm part and a cold part. This loop comprises a circulation pump  18 , a line  20  for circulation of the heat carrier between the pump and regasifier  12 , a circulation line  22  between the regasifier and heating unit  14 , a return line  24  between this heating unit and the circulation pump, a heat carrier tank  26  being arranged on this return line. The plant also comprises an LNG suction pump  28  generally immersed in tank  10 , a LNG circulation line  30  between this pump and a circulation pump  32 , a line  34  bringing the LNG from this circulation pump to regasifier  12 , and an outlet line  36  intended to carry the gas in gaseous form from the regasifier to any suitable means. A heating fluid  38  that is, in the example illustrated, outside air at ambient temperature also flows through the heating unit comprising a line  40  for discharge of the condensates from this air. Of course, this heating air can also come from any equipment present in the exploitation site, such as the fumes discharged by a gas turbine. 
         [0034]    To achieve regasification, the LNG is pumped from tank  10  by pumps  28  and  32 , then it circulates in lines  30  and  34  to be sent to regasifier  12 . This gas circulates in the regasifier through which the methanol used as heat carrier also flows. The methanol present in tank  26  is therefore pumped by pump  18  and it is sent through line  20  to regasifier  12 . In this regasifier, the calories present in the methanol are transmitted to the LNG and heat it so that the liquid phase of the LNG is changed into a gas phase by vaporization then, if necessary, it is overheated to reach a temperature close to the ambient temperature. 
         [0035]    The temperature of the methanol at the inlet of regasifier  12  is about 20° C. and the temperature of the LNG circulating in line  34  is about −160° C. At the outlet of this regasifier, the natural gas is at a temperature close to 5° C. whereas the methanol reaches a temperature of about −70° C. at the outlet of this regasifier in line  22 . 
         [0036]    During exchange in the regasifier, the methanol is cooled to a temperature above its crystallization point, i.e. −70° C. for the example considered. The cold methanol is sent through line  22  to heating unit  14  so that the air circulating in this unit, whose temperature is higher than that of the cold methanol, exchanges its calories with this methanol to obtain a heated methanol in line  24  and, consequently, in tank  26 . 
         [0037]    The temperature of the methanol at the heating unit inlet is of the order of −70° C., whereas the air is fed into this heater at a temperature close to 30° C. After calorific exchange in this unit, the methanol is discharged at the unit outlet at a temperature close to 0° C., whereas the air leaves the unit at a temperature close to 5° C. 
         [0038]    Thus, the warm part of loop  16  is made up of line  24 , tank  26 , pump  18  and line  20 , whereas the cold part of this loop comprises line  22 . 
         [0039]    To achieve heating of the methanol at the regasifier outlet, and as illustrated in  FIG. 2 , heating unit  14  comprises a heat exchanger including a vertical shell  42  with an air inlet  44  and an air outlet  46  arranged at each end of this shell. This shell houses a set of vertical tubes  48  connected, at one end thereof, by an intake manifold  50  to an inlet  52  for the cold methanol coming from the regasifier and, at the other end thereof, by a discharge manifold  54  to an outlet  56  linked to line  24  leading to methanol tank  26 . In this heat exchanger, the methanol flows in through inlet  52 , enters intake manifold  50 , circulates in all the vertical tubes  48  and ends in discharge manifold  54  prior to being discharged through outlet  56 . Simultaneously, air, either at ambient temperature or heated by any known means, is fed into shell  42  through inlet  44 , then it scavenges all the tubes and the manifolds. During scavenging, the calories contained in this air are transmitted to the methanol so as to heat it and to obtain a warm methanol at outlet  56 . During this exchange, the water droplets contained in the air are condensed, then they fall through gravity at the bottom of shell  42  prior to being discharged in form of condensates through line  40 . Tubes  48  can be coated with a hydrophobic material film (water shedding film) of polymethylsiloxane type to facilitate separation of the water droplets. 
         [0040]    In connection with  FIG. 3 , the regasifier comprises a vertical shell  58  that contains at least two exchangers in which the gas and the methanol circulate, an upper exchanger  60  arranged in the upper part of the shell and a lower exchanger  62  arranged in the lower part of this shell. Preferably, these exchangers are in form of brazed plate-fin exchangers, advantageously made of aluminium. The upper exchanger, referred to as counter-current exchanger because the natural gas and the methanol circulate in opposite directions whereas the lower exchanger is referred to as co-current exchanger, the fluids circulating in the same direction. Thus, the lower exchanger comprises, on one of its sides and in the lower part thereof, a methanol inlet  64  connected to line  20  and an outlet  66  on one side of the exchanger. This lower exchanger also comprises an inlet  68 , connected to LNG line  34 , which is located in the lower part and on the side opposite the methanol inlet, and an outlet  70  located in the upper part of the exchanger. Thus, in lower exchanger  62 , the methanol and LNG streams circulate in the same direction, i.e. from the bottom to the top of this exchanger. The skin temperature within this exchanger therefore remains above −100° C. and the exchange surfaces can be minimized. Methanol outlet  66  is connected by a line  72  to an inlet  74  of the upper exchanger that is located in the upper part and on one side of this exchanger. Similarly, natural gas outlet  70  is connected by a line  76  to a gas inlet  78  located in the lower part of this exchanger. The gas in vapour form is discharged through an outlet  80  located in the upper part of this exchanger whereas methanol outlet  82  is located in the lower part of this exchanger and connected to line  22  leading to the heating unit. This exchanger is thus referred to as counter-current exchanger because the gas and methanol streams circulate in opposite directions, the gas from the bottom to the top of the exchanger and the methanol from the top to the bottom of the exchanger. 
         [0041]    In the variant shown by way of example in  FIG. 4 , regasifier  12  is divided into two distinct parts. Thus, co-current exchanger  62  comes in form of a shell-and-tube exchanger, and it comprises inlets  64 ,  68  and outlets  66 ,  70  for methanol and LNG. Outlets  66  and  70  are connected by lines  72 ,  76  to counter-current exchanger  60  that is a brazed plate-fin exchanger, advantageously made of aluminium, comprising inlets  74 ,  78  and outlets  82 ,  80  for methanol and natural gas. 
         [0042]    Preferably, the shell-and-tube exchanger comprises a mechanical expansion joint  83  that absorbs all the dimensional variations of this exchanger as the LNG and the methanol flow therethrough. 
         [0043]    In this variant, operation of the plant is the same as described in connection with  FIGS. 1 to 3 . 
         [0044]      FIG. 5  shows a variant of the regasification plant illustrated in  FIG. 4 , which therefore comprises the same reference numbers for the common elements. 
         [0045]    This variant differs in that regasification is carried out in several stages. Furthermore, counter-current exchanger  60  is in two parts  60 A,  60 B and a phase separator  84  is provided between these two parts of the exchanger. 
         [0046]    The natural gas leaving co-current shell-and-tube exchanger  62  through outlet  70  is preheated to its boiling point corresponding to the pressure in separator  84 . This heated liquid natural gas flows through lower part  60 A of counter-current exchanger  60  to achieve a phase conversion through vaporization. This converted natural gas is sent through a line  86  to separator  84  where separation of the natural gas in gaseous form occurs in upper part  88  of this separator with a lower composition, molecular weight and calorific value, and in liquid form in lower part  90  of this separator. The natural gas in vapour form present in the separator is then sent, through a line  92 , from this separator to the inlet of part  60 B of exchanger  60  where it undergoes, by exchange with the methanol circulating therein, a temperature rise until it reaches outlet  80 . The liquid phase, whose molecular weight and calorific value are higher than that of the vapour, is extracted by a pump  94  connected to this separator by a line  96 . The liquid phase leaving pump  94  is sent through a line  98  to any storage means prior to being treated. Advantageously, it is possible to control the composition and the calorific value of the natural gas in gaseous form in line  92  before it enters exchanger  60  by injecting a predetermined amount of liquid coming from the separator through a line  98 A starting after pump  94  on line  98  and ending on line  92 . 
         [0047]    In this configuration, the temperature of the natural gas at the regasifier outlet is of the order of 0° C., the temperature of the methanol is about −70° C. 
         [0048]    In addition, it is possible to heat the methanol at the outlet of pump  18  by placing on line  20  a heat exchanger  100  for exchange between the methanol and a warm fluid that is commonly used in or close to this regasification plant, such as warm water from trickle towers. 
         [0049]    As described above, the methanol at the regasifier outlet is at low temperature, of the order of −70° C., and it has to be heated to provide conversion of the LNG to gas phase in the regasifier. It is therefore possible to take advantage of the presence on the site of an electric power plant with a combined cycle gas turbine as illustrated in  FIG. 6 . In this case, plant  102  is supplied with air through a channel  104  and with natural gas through a channel  106 ; this channel can be a bypass of line  36  described above. Combustion of the air-natural gas mixture within the turbine generates, after recovery of the calories generated (HRSG), at outlet  108 , fumes with temperatures of the order of 130° C. As shown in  FIG. 6 , these fumes are fed through an inlet  110  into a heat exchanger assembly  112 , divided into at least three parts  112 A,  112 B,  112 C, and leave through a discharge line  114  prior to being sent through a line  116  to any suitable means, such as a chimney. A phase-change fluid such as propane also flows through the heat exchanger assembly and circulates in a closed loop  118 . This loop comprises a liquid propane tank  120 , a circulation pump  122  connected to the tank by a line  124  and a propane phase separator  126  connected to the pump by a line  128 E carrying the liquid propane to part  112 A of the heat exchanger assembly and a line  128 S carrying the propane, preheated to its boiling point, into this separator. Two lines start from this separator: a line  130 , referred to as liquid line, wherein the liquid contained in the separator is brought to part  112 B of the heat exchanger assembly, then flows therethrough and flows back in gaseous form into separator  126 , and a line  132 , referred to as gas line, that carries the gas phase of the propane contained in the separator to part  112 C of the heat exchanger assembly so as to overheat this propane gas. A line  134  brings the propane in pressurized gaseous form to an expansion turbine  136  coupled in rotation to any energy producing means such as an alternator  138 . At the outlet of the expansion turbine, the propane gas is sent through a line  140  to a heat exchanger  142 , referred to as condenser, in order to cool this propane gas and thus to cause a phase change so as to obtain a liquid phase before it flows back to tank  120  through a line  144 . To cool the propane, the methanol circulating in line  22 , as described above, flows through condenser  142  and, at the outlet of this condenser, the methanol is at a higher temperature than at the inlet because it has collected the calories contained in the propane in gas phase. 
         [0050]    During operation, the propane in liquid form is pumped from tank  120  and flows through part  112 A of exchanger assembly  112 . The preheated propane in liquid form is thereafter sent to separator  126 . The liquid phase extracted from this separator flows through part  112 B of assembly  112  and flows back in nearly gaseous form into the separator for separation of the liquid phase and the gas phase of the propane. The gas phase contained in this separator is also extracted to flow through part  112 C of exchanger assembly  112  to be totally converted to gas phase and overheated if necessary. The propane in gaseous form flows through turbine  136  that it drives into rotation, said turbine driving alternator  138  into rotation. At the turbine outlet, the propane in gaseous form flows through condenser  142  where it undergoes phase change and changes to the liquid phase by exchanging its calories with the cold methanol that also circulates in this condenser. At the outlet of this condenser, the liquid propane is stored in tank  120 . 
         [0051]    The treating group as diagrammatically shown in  FIG. 7  shows a potential use of the LNG regasification plant with a methanol loop for collecting and liquefying the CO 2  contained in discharges, such as the fumes from gas turbines. 
         [0052]    This configuration involves an LNG regasification plant  146 , a CO 2  collection/separation plant  148 , a methanol heating unit  149  and a CO 2  liquefaction unit  150 . 
         [0053]    Regasification plant  146 , as already described in connection with the previous figures, comprises a regasifier  12  through which flow warm methanol circulating in a loop  152  and LNG coming from line  34 . 
         [0054]    CO 2  collection/separation unit  148  comprises an absorption column  154  containing transfer elements  156  with an inlet  158  for the methanol from the regasifier, an inlet  160  for a CO 2 — containing gaseous fluid, an outlet  162  for a CO 2 -freed gaseous fluid and an outlet  164  for a mixture of ethanol and CO 2 . This CO 2  collection/separation unit also comprises a flash drum  166  with an inlet for the methanol-CO 2  mixture, an outlet  168  for the CO 2  in gaseous form and an outlet  170  for the methanol freed of a very large part of the CO 2 . 
         [0055]    Heating unit  149  comprises elements identical to those already described in connection with  FIGS. 1 and 2 , i.e. a heater through which flow the methanol coming, in the example illustrated in  FIG. 7 , from outlet  170  of drum  166  and a heating fluid  38  that can be outside air at ambient temperature. This exchanger also comprises a discharge line  40  for the condensates from this outside air. This unit finally comprises a heat exchanger  174  allowing to heat the methanol after its passage through the heater through an outlet  172  and a flash drum  175  allowing to separate the methanol in liquid form, which is then sent through a line  176  to the methanol loop, and the CO 2  in gaseous form that joins, through a line  178 , a line  180  also connecting CO 2  line  168  of flash drum  166 . 
         [0056]    Liquefaction unit  150  comprises a condenser  181  whose specific feature is to use an intermediate fluid, such as ethane, to take part in the liquefaction of the CO 2  and in heating the natural gas in vapour form. 
         [0057]    This condenser comprises an enclosure  182  that contains at least two condenser parts  184  and  186 , each one counter-current and preferably in form of brazed aluminium plates and fins, in which circulate the CO 2  in vapour form and the ethane for the first one and the LNG and the ethane for the second. Lower condenser  184  is arranged in the lower part of the enclosure and comprises, on one side thereof and in the upper part of this condenser, a CO 2  inlet  188  connected to line  180  and a liquid CO 2  outlet  190  on the lower part of the condenser. Upper condenser  186  comprises an LNG inlet  192 , connected to LNG line  34 , that is arranged in the lower part of this condenser and an outlet  194  arranged in the upper part of the exchanger. A closed ethane loop  196  allows the ethane to circulate between the two exchangers. More precisely, the vapour ethane is fed into upper ethane condenser  186  through an inlet  198  located in the upper part of the condenser, flows through this condenser and ends at a liquid ethane outlet  200  arranged in the lower part of this condenser, is brought through a line  202  to a liquid ethane inlet  204  located in the lower part of the lower CO 2  condenser, flows through the lower condenser and reaches an outlet  206  in the upper part of this condenser, and eventually reaches inlet  198  through a line  208 . 
         [0058]    During operation of the treating group described above, the LNG substantially follows the same regime as described in connection with  FIG. 1 , except that a bypass of LNG line  34  reaches inlet  192  of CO 2  liquefaction unit  150  and runs through upper condenser  186 , then through outlet  194  to join line  36 . 
         [0059]    At the regasifier outlet, the methanol is sent through inlet  158  to column  156  that also receives a fluid containing a substantial part of CO 2 , of the order of 12%, through inlet  160 . After treatment in this column, the CO 2  is collected by the methanol, and a mixture of methanol and of dissolved CO 2  is discharged through outlet  164 . The CO 2 -freed fluid is discharged through outlet  162  to any suitable means. The mixture of CO 2  and of methanol undergoes separation in flash drum  166  from which the CO 2  in vapour phase is discharged through outlet  168  to line  180  and the methanol in liquid phase from outlet  170  is heated in the heating unit by passing successively through the heater and exchanger  174 . At the outlet of exchanger  174 , the residual CO 2  contained in the methanol is again separated from this methanol in flash drum  175 . During this separation, the CO 2  is discharged through outlet  178  to join line  180  connected to outlet  168  and the CO 2 -freed methanol joins, through outlet  176 , pump  18  of the methanol loop. The CO 2  in vapour phase is liquefied in lower condenser  184  where it exchanges its calories with the ethane that circulates in a loop between the two condensers. After this exchange, the CO 2  is in liquid form at outlet  190  and it can be sent to a storage tank from where it can be removed to be possibly sequestered in underground reservoirs. 
         [0060]    The present invention is not limited to the embodiment examples described and it encompasses any variant and equivalent.