Method for the regasification and distribution of natural gas

Abstract

Plant (1 ) and method for the regasification of a natural gas under cryogenic conditions, comprising a cryogenic tank (10) configured for storage of the natural gas under cryogenic conditions and a capacitive tank (20). The plant (1) further comprises a feeding circuit (30), arranged for the connection between the cryogenic tank (10) and the capacitive tank (20) and comprising a pump (31 ), configured to cause an increase in pressure and temperature of the natural gas. The plant (1 ) includes a motor body (40), configured for the production of mechanical energy and comprising at least one working chamber (41 ) having an inlet gap (42), arranged in fluid communication with the capacitive tank (20), and a discharge gap (43) connected to an exhausted natural gas discharge circuit (60), and a recirculation circuit (70) adapted to convey a portion of the exhausted natural gas into the capacitive tank (20). The plant (1 ) comprises a delivery pipe, connected to the discharge circuit (60) and to a network for the transport and distribution of regasified natural gas.

Description

[0001] Method for the regasification and distribution of natural gas Technical field

[0002] The present invention relates to a method for the regasification and distribution of natural gas. The term "natural gas" is understood to primarily mean methane.

[0003] Liquefied methane (LNG) is known to be transported by LNG carriers for the purpose of feeding a distribution network, such as a national network. In particular, LNG carriers transport liquefied natural gas from a production area thereof to a predetermined delivery point. Once the delivery point is reached, the liquefied natural gas is returned again to the gaseous state through regasification processes and subsequently introduced into the national transport network.

[0004] It is known to regasify liquefied natural gas using a hot source. For example, plants exploiting heat obtained from seawater are known. Plants using the heat obtained from the combustion of a portion of the same regasified natural gas are also known.

[0005] The Applicant has noted that, even in their most modern implementations, the aforesaid transport methods by LNG carrier, with particular attention to the regasification phase, are poorly energy efficient. In fact, the energy used for liquefying natural gas is in no way recovered during the regasification process, but rather, in the case of combustion of a portion of the gas itself, the overall efficiency of the process would be further penalised. As a result, liquefied natural gas is generally more expensive than natural gas transported by pipeline.

[0006] Object of the invention

[0007] In this context, the technical task underlying the present invention is to propose a method for the regasification and distribution of natural gas that overcomes the drawbacks of the prior art mentioned above. In particular, aim of the present invention is to make available a method for the regasification and distribution of natural gas that achieves an improvement in terms of energy efficiency of the entire process and, therefore, a reduction in the costs of selling the natural gas itself.

[0008] The specified technical task and the specified aim are substantially achieved by a method for the regasification and distribution of natural gas in accordance with the appended claim 1 and / or one or more of the claims dependent thereon.

[0009] Brief description of the drawings

[0010] Further characteristics and advantages of the present invention will become more apparent from the indicative and thus non-limiting description of a preferred but non-exclusive embodiment of a method for the regasification and distribution of natural gas, as illustrated in the attached drawings, in which:

[0011] - Figure 1 schematically shows a preferred embodiment of a natural gas regasification plant for implementing a method for the regasification and distribution of natural gas in accordance with the present invention;

[0012] - Figures 2A-2C illustrate respective views of a component of the plant of Figure 1 ;

[0013] - Figures 3A-3F illustrate respective views of the component of Figures 2A-2C in different operating configurations;

[0014] - Figure 4 illustrates a Mollier diagram of the open work cycle of the plant of Figure 1 .

[0015] Detailed description of preferred embodiments of the invention

[0016] With reference to the accompanying figures, a plant for regasification of natural gas has been indicated with the numerical reference "1 ".

[0017] The term "natural gas" is understood to mean preferably methane. However, the present invention finds application for generic cryogenic liquids, such as for example nitrogen, oxygen, ammonia as well as generic fluids that have their own critical temperature well below room temperature.

[0018] With reference to Figure 1 , the plant 1 comprises a cryogenic tank 10, a capacitive tank 20, a feeding circuit 30, which connects the cryogenic tank 10 to the capacitive tank 20 and which comprises a pump 31 , a motor body 40, a discharge circuit 60 and a recirculation circuit 70. The plant also comprises a delivery pipe, connected to the discharge circuit 60 and configured to introduce the regasified natural gas into a network for the transport and distribution of the regasified natural gas.

[0019] The cryogenic tank 10 is configured for storage of natural gas under cryogenic conditions.

[0020] The term "cryogenic conditions" is understood to mean that the natural gas is in a state of low temperature, and in particular at a temperature lower than the respective critical point temperature, and of low pressure, substantially equal to the atmospheric one. Preferably, the term "cryogenic conditions" is understood to mean a liquid state of the natural gas at the aforesaid temperature and pressure conditions.

[0021] Under normal operating conditions, almost all of the natural gas inside the cryogenic tank 10 is in the liquid state. However, as will be seen in the continuation of the present disclosure, it can be provided for a relatively small percentage of natural gas stored within the cryogenic tank 10 in the gaseous state or, if necessary, the natural gas can be brought to the solid state.

[0022] Advantageously, since the natural gas is stored in the cryogenic tank 10 at a pressure substantially equal to the ambient pressure, the problems concerning the pressure tanks are solved.

[0023] In terms of sizing, the sizes of the cryogenic tank 10 can be established "ad hoc" as a function of the use of the plant and as a function of the space and autonomy needs required.

[0024] Advantageously, since almost all of the natural gas is substantially stored in a liquid state, a large quantity can be accumulated. With the same volume, in fact, natural gas in the liquid state has a mass even hundreds of times greater than the same natural gas in the gaseous state.

[0025] According to one aspect of the present invention, the cryogenic tank 10 may comprise a vacuum suction pump 11 configured to extract from the cryogenic tank 10 a portion of natural gas in the gaseous state so as to obtain inside the cryogenic tank 10 a pressure lower than atmospheric pressure.

[0026] In particular, said vacuum pump 11 may be operatively arranged in an upper portion of the cryogenic tank 10, so as to draw from the overlying gaseous portion of the natural gas the portion of natural gas in the liquid state.

[0027] According to a preferred use of such a vacuum pump 11 , it can be used to create within the cryogenic tank 10 pressure and temperature conditions such as to cause the triple point thermodynamic state of the natural gas. Even more preferably, the vacuum pump 11 can be used so that in the cryogenic tank 10 a pressure and a temperature lower than the pressure and the temperature determining the triple point thermodynamic state are reached.

[0028] This characteristic can be advantageously used, by way of non-limiting example, in naval applications where it is necessary to at least partially solidify the natural gas stored inside the cryogenic tank 10, in such a way as to limit if not eliminate the resonance phenomena preventing the ship from overturning.

[0029] This condition can be regulated.

[0030] Downstream of the cryogenic tank 10, the feeding circuit 30 which connects the cryogenic tank 10 with the capacitive tank 20 is operatively arranged.

[0031] Generally, the feeding circuit 30 is configured to change the thermodynamic conditions of the natural gas. In particular, the feeding circuit 30 is configured to increase the temperature and pressure of the natural gas so as to make it advantageously exploitable from an energy point of view. In accordance with an aspect of the present invention, the feeding circuit 30 is configured to increase the temperature and the pressure of the natural gas so as to promote a passage of state (from liquid to gaseous), at least partially, of the natural gas, as will be clear in the continuation of the present invention.

[0032] The feeding circuit 30 comprises the aforesaid pump 31 , arranged inside the cryogenic tank 10 or in fluid communication with the cryogenic tank 10 by means of a conduit. Specifically, the pump 31 is operatively arranged in such a way that it can draw the natural gas in the liquid state from the cryogenic tank 10.

[0033] A non-return valve 33 may also be provided interposed between the cryogenic tank 10 and the pump 31 . Advantageously, this non-return valve 33 allows the use of the pump 31 intermittently without producing "regurgitations" towards the cryogenic tank 10 and therefore pressure rises in the cryogenic tank 10 due to the return of natural gas from the feeding circuit 30 to the cryogenic tank 10. This allows the cryogenic tank 10 to be optimally sized and to cope with thermal insulation optimally.

[0034] In accordance with an aspect of the present invention, the pump 31 can be controlled and regulated according to the rate of the motor body 40.

[0035] At a functional level, the pump 31 is configured to cause an increase in pressure and temperature of the natural gas. Preferably, the pump 31 is configured to promote a passage of the natural gas to a supercritical state. The term "supercritical state" is understood to mean that natural gas is brought to a thermodynamic condition such that the temperature of the natural gas is higher than its critical temperature while the pressure is higher than its critical pressure. In this thermodynamic condition, the properties of the substance are partly similar to those of a liquid and partly similar to those of a gas.

[0036] The work carried out on the natural gas by the pump 31 can be separated into two contributions, identified in Figure 4 with the segment "A" -"B" and with the segment "B"-"C" on the Mollier diagram.

[0037] Firstly, the pump 31 causes an increase in pressure of the natural gas, so as to obtain a natural gas in the liquid state and at high pressure with a moderate increase in temperature. Preferably, the natural gas is brought to a hypercritical pressure value. This transformation is illustrated in Figure 4 on the Mollier diagram with segment

[0038] Advantageously, by operating on a liquid that is substantially incompressible under the aforesaid cryogenic conditions, the pump 31 requires a negligible operating energy expenditure with respect to the mechanical (and therefore electrical) energy produced by the plant 1 as a whole.

[0039] Secondly, in a section 32 of the feeding circuit 30 interposed between the pump 31 and the capacitive tank 20, an increasing quantity of natural gas is accumulated under the aforesaid hypercritical conditions: the pump 31 continuously introduces further natural gas so that in this section of conduit 32 the pressure of the natural gas increases further. At the same time, there is a significant increase in the temperature of natural gas. In other words, the pump drive work increases the pressure and the temperature of the natural gas. As a result of this drive work, the natural gas is in a hypercritical state under conditions of high pressure and high temperature. Such transformation can be considered substantially adiabatic. This second contribution is illustrated in Figure 4 on the Mollier diagram with segment "B"-"C".

[0040] Preferably, downstream of the pump 31 there is arranged a non-return valve 34, which is operatively interposed between the pump 31 and the aforesaid capacitive tank 20.

[0041] To sum up, the natural gas stored in the cryogenic tank 10 is in cryogenic conditions, i.e. very low temperatures (below the melting temperature of the respective natural gas) and at a pressure substantially equal to the atmospheric one. In other words, the natural gas is in such conditions that it cannot be exploited in an energy advantageous way, identified with the reference "A" on the Mollier diagram of Figure 4.

[0042] The feeding circuit 30 switches the thermodynamic condition of the natural gas from the initial cryogenic conditions, to a thermodynamic condition of high temperature and high pressure, indicated by the reference "C" on the Mollier diagram of Figure 4. Preferably, said thermodynamic condition "C" corresponds to a hypercritical state of the natural gas. This condition will now be identified in the continuation of the present disclosure as a thermodynamic condition of "ex liquid".

[0043] By using the feeding circuit 30, action is taken increasing the pressure of the natural gas by means of pump 31 and, subsequently, thanks to the aforesaid drive work, raising the temperature until a natural gas is obtained in the condition of "ex liquid".

[0044] In accordance with an aspect of the present invention, the feeding circuit 30 may comprise a dosing tank 73, a valve 72 configured to isolate the feeding circuit 30 and a valve 73 that is interposed between the dosing tank 71 and the capacitive tank 20.

[0045] The capacitive tank 20 is configured to collect and mix a certain quantity of "ex liquid" natural gas coming from the feeding circuit 30 with a relative quantity of natural gas of recirculation recovered from the motor body 40 through the recirculation circuit 70, in order to optimally feed the motor body 40.

[0046] In other words, said capacitive tank 20 is suitably sized in such a way as to perform a function of mixing the "ex liquid" natural gas and the natural gas of recirculation in order to obtain a certain quantity of natural gas defined as "feed natural gas". The natural gas of recirculation is in a gaseous state at a lower pressure than the "ex liquid" natural gas, as will be explained in the continuation of the present disclosure. Likewise, inside the capacitive tank 20, the "ex liquid" natural gas coming from the feeding circuit 30 reaches a complete evaporation caused in particular by a partial expansion of the same.

[0047] The capacitive tank 20 is suitably sized in such a way as to perform a function of dosing the feed natural gas with which to feed the motor body 40.

[0048] This natural gas defined as "feed natural gas" has pressure and temperature conditions averaged with respect to the pressure and temperature conditions of the "ex liquid" natural gas and the natural gas of recirculation. In this "feeding" thermodynamic condition, the natural gas is in a gaseous state. This "feeding" condition is indicated in Figure 4 with the reference "E".

[0049] The characteristics of the recirculation circuit 70 as well as the dosage ratio between the "ex liquid" natural gas and the natural gas of recirculation will be illustrated in detail in the continuation of the present disclosure.

[0050] The "recirculation" thermodynamic condition is instead indicated in Figure 4 with the reference "D".

[0051] The motor body 40 is configured for the production of mechanical energy and comprises at least one working chamber 41 having an inlet gap 42, arranged in fluid communication with the capacitive tank 20, from which it is fed with the feed natural gas and a discharge gap 43, connected to the discharge circuit 60 of natural gas in exhausted conditions, indicated in Figure 4 with the reference "F".

[0052] The expansion of the "ex liquid" natural gas is indicated in Figure 4 with the segment "E"-"F".

[0053] Said working chamber 41 is configured to transform into mechanical work, by means of at least one movable wall 44 the expansion and / or the movement of the feed natural gas.

[0054] Preferably, the movable wall 44 is constrained to make a translation between an upper dead point and a lower dead point. Alternatively, the movable wall 44 may be constrained to rotate about an axis.

[0055] The term "exhausted natural gas" is understood to mean natural gas in conditions subsequent to such transformation, in which the natural gas has a low enthalpy content and temperature and pressure conditions that are suitable for the introduction into a transmission and distribution network.

[0056] The motor body 40 can be made according to any type as long as it is suitable for the required purpose.

[0057] According to a preferred embodiment, the motor body 40 is of the reciprocating motion type.

[0058] In particular, in a manner known per se, the motor body 40 comprises at least one cylinder 45 defining the working chamber 41 having the inlet gap 42, associated with a feeding valve 46, and the discharge gap 43, associated with a discharge valve 47. A piston 48, integral with the respective movable wall 44, and a connecting rod 49, which is constrained to the piston 48 are constrained slidingly inside the cylinder 45. Finally, the connecting rod 49 is constrained to a motor shaft 50.

[0059] Functionally, the motor body 40 is configured such that the transformation work of the motor body 40 on the feed natural gas can be substantially divided into two distinct operating phases.

[0060] In the first operating phase, with the feeding valve 46 open, a transfer of feed natural gas at high pressure, coming from the capacitive tank 20, to the working chamber 41 of the motor body 40 takes place, which causes a first movement of the movable wall 44 and therefore a first movement of the motor shaft 50.

[0061] Since this is a mechanical phenomenon of mass transport, in this first operating phase the pressure, temperature and enthalpy of the feed natural gas can be considered substantially constant.

[0062] In other words, mechanical energy is generated as a result of the transfer of mass of the feed natural gas within the working chamber 41 .

[0063] Still, in the first operating phase, the feed natural gas does not undergo thermodynamic transformations but maintains pressure and enthalpy substantially constant.

[0064] Once the first operating phase is finished, a second operating phase begins. This second operating phase consists of a transformation similar to a polytropic transformation that exchanges mechanical work with the movable wall 44 of the working chamber 41 .

[0065] In particular, in the second operating phase part of the enthalpy of the feed natural gas is transformed into mechanical energy.

[0066] In particular, temperature and pressure of the feed natural gas are reduced and the natural gas can be considered as exhausted natural gas.

[0067] In the second operating phase, since the transfer of mass of the feed natural gas from the capacitive tank 20 to the working chamber 41 is finished, the mass of the natural gas inside the working chamber can be considered as constant.

[0068] The mechanical energy obtained in this second expansion operating phase is negligible with respect to the mechanical energy obtained in the first transfer operating phase.

[0069] In the following description, a movement cycle of the motor body 40 is described as a function of the angle assumed by the motor shaft 50 during its rotation, which occurs clockwise.

[0070] In particular, the position of the motor shaft 50 whereby the movable wall 44 is in the upper dead centre is assumed as a 0-degree angle.

[0071] In particular, in the first operating phase the motor shaft 50 is moved from 12 degrees to 50 degrees while in the second operating phase the motor shaft 50 is moved from 50 degrees to 180 degrees.

[0072] According to a further embodiment, not illustrated in the accompanying figures, the motor body 40 may be of the flow motor type.

[0073] In such an embodiment, the first operating phase and the second operating phase take place substantially simultaneously.

[0074] Once the operating phases are finished, the exhausted natural gas is at least partially conveyed into the discharge circuit 60. The discharge circuit 60 is adapted to discharge the natural gas inside the delivery pipe under the conditions indicated in the Mollier diagram of Figure 4 with the reference "G". The discharge circuit 60 may comprise an exhausted natural gas collection tank 61 and a discharge conduit adapted to at least partially expel the exhausted natural gas from the plant 1 .

[0075] The discharge circuit 60 may further comprise a discharge valve 62.

[0076] At the operating level, the discharge circuit 60 is connected to the delivery pipe, connected or connectable in turn to the regasified natural gas transmission and distribution network. The discharge circuit 60 is configured to receive part of the exhausted natural gas from the motor body 40 and to modify its thermodynamic condition so as to make it suitable for the introduction into the regasified natural gas transport and distribution network.

[0077] According to a further aspect of the present invention, the plant 1 may comprise a stop system 80 to stop the operation of the motor body 40 which is configured to interrupt the operation of the plant 1 .

[0078] Preferably, such a stop system 80 can be associated with the pump 31 so as to be able to block the extraction of natural gas from the cryogenic tank 10 and therefore the feeding of the plant 1 .

[0079] The stop system 80 may also act through the valve 74, connected to the stop system 80.

[0080] According to an aspect of the present invention, the plant 1 may comprise a replenishment circuit 90 associated with the discharge circuit and configured to replenish into the cryogenic tank 10 a portion of the exhausted natural gas passing through the discharge circuit 60, and in particular a portion of the exhausted natural gas passing through the collection tank 61 .

[0081] Alternatively, the plant 1 may comprise a replenishment circuit 90 associated with the feeding circuit and configured to replenish into the cryogenic tank 10 a portion of the natural gas in the gaseous state exiting the main heat exchanger 32.

[0082] Advantageously, thanks to the replenishment circuit 90, it is avoided that the decrease in pressure in the cryogenic tank 10, due to the natural gas in the liquid state being drawn by the pump 31 , excessively decreases the pressure inside the cryogenic tank 10 avoiding problems related, for example, to natural gas solidification.

[0083] In fact, the natural gas in the gaseous state introduced into the cryogenic tank 10 by the replenishment circuit 90, keeps the pressure inside the cryogenic tank 10 substantially constant, except for the natural gas in the liquid state extracted by the pump 31 .

[0084] Advantageously, moreover, the replenishment circuit 90 allows the pump to withdraw from the cryogenic tank 10 quantities such as to equalize the pressure reduction caused by the instantaneous consumption of natural gas in the liquid state required by the operation of the plant 1 .

[0085] In other words, as the pump 31 takes natural gas from the cryogenic tank 10, the operating pressure in the cryogenic tank 10 is restored by replacing the volume of the natural gas in the liquid state, taken by the pump 31 , with the volume of the exhausted natural gas in the gaseous state replenished.

[0086] For the regulation of the flows in the discharge circuit 60 and in the replenishment circuit 90, driven shut-off and flow regulation valves may be operatively arranged.

[0087] According to a peculiar aspect of the present invention, the recirculation circuit 70 is adapted to convey in the capacitive tank 20 a portion of the exhausted natural gas withdrawn from the working chamber 41 of the motor body 40.

[0088] Advantageously, thanks to the use of the recirculation circuit 70, the exhausted natural gas, introduced into the delivery pipe from the discharge circuit 60, has temperature and pressure conditions such as to be safe and adequate for the transport and distribution operations of the network itself. In other words, the exhausted natural gas is discharged at a pressure and at a temperature that do not damage the plant 1 and the regasified natural gas transmission and distribution network.

[0089] The recirculation circuit 70 is in fact configured such that it withdraws part of the exhausted natural gas from the working chamber 41 and introduces it into the capacitive tank 20 after a polytropic compression, indicated in the Mollier diagram in Figure 4 with the segment "F" - "D", which increases temperature and pressure. In the capacitive tank 20, the natural gas of recirculation mixes with the "ex liquid" natural gas coming from the feeding circuit 30. Within the recirculation circuit, the natural gas has thermodynamic conditions indicated in the Mollier diagram in Figure 4 with the reference "E".

[0090] In fact, the temperature of the natural gas of recirculation, following polytropic compression, is higher than the temperature of the exhausted natural gas. Likewise, the pressure of the natural gas of recirculation is also greater than the pressure of the exhausted natural gas.

[0091] The mixing of the natural gas of recirculation with the "ex liquid" natural gas arriving from the feeding circuit 30 takes place in a predetermined and controlled manner, so as to define the feed natural gas. In other words, the quantities of natural gas of recirculation and the "ex liquid" natural gas arriving from the feeding circuit 30 must respect a predetermined reciprocal relationship, as will be explained in the continuation of the present disclosure.

[0092] According to a preferred embodiment, this mass ratio between the natural gas of recirculation and the "ex liquid" natural gas is 23 to 1 .

[0093] The polytropic compression, as a function of the embodiment of the plant 1 , can be performed by means of a special compressor or performed advantageously by means of the motor body 40, taking advantage of the return stroke from the lower dead point to the upper dead point of the piston 48.

[0094] In the following, two embodiments of the plant 1 will be described in detail, with particular attention to the technical characteristics of the motor body 40 and of the recirculation circuit 70, since the characteristics of the cryogenic tank 10 and the feeding circuit 30 are substantially the same.

[0095] A first embodiment is schematically illustrated in Figures 1 , 2A-2C and 3A- 3F.

[0096] In such an embodiment, the motor body is of the aforesaid reciprocating motion type, illustrated in Figures 2A-2C

[0097] In such an embodiment, the motor body 40 is configured to:

[0098] - house the feed natural gas;

[0099] - host an expansion phase of the feed natural gas;

[0100] - convert a displacement and / or an expansion of the feed natural gas into mechanical energy; and

[0101] - host a compression phase of the exhausted natural gas.

[0102] In other words, the motor body 40 is configured to perform on the feed natural gas the first and second operating phases and the polytropic compression phase.

[0103] In this embodiment, moreover, the motor body 40 is integral to the recirculation circuit 70 and to the rest and mixing tank 20.

[0104] In other words, the capacitive tank 20 and the recirculation circuit 70 are obtained inside the motor body 40 and defined by the operation and by the movement of its components.

[0105] In detail, the motor body 40 has a feeding chamber 51 and a discharge chamber 52, which are obtained in the cylinder and interposed respectively between the working chamber 41 and the inlet gap 42 and between the working chamber 41 and the discharge gap 43.

[0106] The feeding chamber 51 and the discharge chamber 52 are respectively associated with the feeding valve 46 and the discharge valve 47.

[0107] In particular, each of the valves 46, 47 is of the mushroom type and comprises a planar lower element 46a, 47a configured to occlude a bottom portion of the respective chamber 51 , 52 so as to define a sealed separation with the working chamber 41 , and a stem 46b, 47b, integral with the planar lower element 46a, 47a.

[0108] Each of the valves 46, 47 is constrained slidingly in the respective chamber 51 , 52 so as to define a translation movement having a linear trajectory.

[0109] The inlet gap 42 is obtained in the motor body 40 in an upper portion thereof and is substantially transverse to a longitudinal axis of the feeding chamber 51 .

[0110] Similarly, the discharge gap 43 is obtained in the motor body 40 in an upper portion thereof and is substantially transverse to a longitudinal axis of the discharge chamber 52.

[0111] According to a particular structural aspect of the feeding valve 46, it has a cavity 46c obtained inside the stem 46b, which defines a first containment volume "V1". The stem 46b also has a through hole 46d for such a cavity 46c, preferably obtained transversely in the stem 46b.

[0112] The valve also has a closing element 46e for the cavity 46c.

[0113] Preferably, this closing element 46e is threaded and depending on how tightened it is in the cavity 46c, it allows to adjust the size of the first containment volume "V1 ".

[0114] The feeding chamber 51 , together with the feeding valve 46, defines a second containment volume "V2". In other words, this second containment volume "V2" is defined as the volume of the feeding chamber 51 from which the encumbrance of the feeding valve 46 and the first containment volume "V1" is removed.

[0115] In this embodiment, the first containment volume "V1" and the second containment volume "V2", thus defined, define the capacitive tank 20.

[0116] According to a further aspect of the present invention, the dimensional ratio between the first containment volume "V1" and the second containment volume "V2" is 1 to 23.

[0117] The feeding valve 46 is movable inside the feeding chamber 51 so that it can assume four respective operating configurations.

[0118] In particular, the feeding valve 46 can assume a closed configuration, also defined as the first configuration, illustrated in Figure 2c, in which the through hole 46d faces the inlet gap 42 of the motor body 40 and in which the planar lower element 46a occludes the feeding chamber 51 below. Furthermore, in this closed configuration, the stem 46b is substantially adhering to the walls of the motor body 40, occludes the upper feeding chamber 51 . By lowering, the feeding valve 46 can assume a second configuration, in which the through hole 46d is not facing the inlet gap 42 which is occluded by the stem 46b and in which the planar lower element 46a occludes the feeding chamber 51 below. In such a configuration, the stem 46b still occludes the feeding chamber 51 above so that the first containment volume "V1" is not in fluid communication with the second containment volume "V2".

[0119] By further lowering, the feeding valve 46 can assume a third configuration in which the through hole 46d is not facing the inlet gap 42 which is occluded by the stem 46b and in which the planar lower element 46a occludes the feeding chamber 51 below. In this configuration, the first containment volume "V1" is in fluid communication with the second containment volume "V2".

[0120] Finally, the feeding valve 46 can assume an open configuration, also defined a fourth configuration, in which the stem 46b occludes the inlet gap 42 and in which the first containment volume "V1" and the second containment volume "V2" are in fluid communication with the working chamber 41 .

[0121] The discharge valve 47 can instead assume two operating configurations.

[0122] In particular, the discharge valve 47 can assume a closed configuration in which the discharge valve 47 occludes the feeding chamber 52 and the discharge gap 43 below, and an open configuration in which the discharge gap 43 is in fluid communication with the working chamber 41 .

[0123] Advantageously, as illustrated in the accompanying figures, according to a further structural aspect, since in the open configuration the feeding valve 46 or the discharge valve 47 could at least partially enter the working chamber 41 , on the movable wall 44 recesses are made at least partially counter-shaped to the feeding and discharge valves 46, 47 so as not to come into abutment with them.

[0124] In the following, a movement cycle of the embodiment of the motor body 40 just described will be described in detail. In the following description, a movement cycle of the motor body 40 is described as a function of the angle assumed by the motor shaft 50 during its rotation, which occurs clockwise.

[0125] In particular, the position of the motor shaft 50 whereby the movable wall 44 is in the upper dead centre is assumed as a 0-degree angle.

[0126] In particular, Figure 3A illustrates an initial phase in which the feeding valve 46 is in the closed configuration, or first configuration, and the discharge valve 47 in the closed configuration.

[0127] In this phase, the natural gas of recirculation is contained in the second containment volume "V2".

[0128] The first containment volume "V1" is filled with the "ex liquid" natural gas coming from the feeding circuit 30 via the inlet gap 42.

[0129] Preferably, according to a preferred use of the plant 1 , the ratio between the mass of "ex liquid" natural gas and the natural gas of recirculation is 1 to 23. Advantageously, this allows achieving very low consumption.

[0130] The movable wall 44 is in proximity of the upper dead centre.

[0131] In this phase, the motor shaft 50 is moved from the 356-degree angle to the 6-degree angle.

[0132] Figure 3B illustrates a subsequent phase of the movement cycle in which the discharge valve 47 is in the closed configuration. In this phase, the feeding valve 46 is switched firstly in the second configuration so as to occlude the inlet gap 42 and then it is switched in the third configuration so that the first containment volume "V1" is in fluid communication with the second containment volume "V2". In such a configuration, the natural gas of recirculation may mix with the "ex liquid" natural gas coming from the feeding plant 30, obtaining the feed natural gas.

[0133] This phase corresponds to the first operating phase of the motor body 40 described above.

[0134] In this phase, the movable wall 44 is still substantially in proximity of the upper dead centre and the motor shaft 50 is moved from the 6-degree angle to the 12-degree angle. Figure 3C illustrates a phase in which the feeding valve 46 is switched to the open configuration, or fourth configuration, while the discharge valve 47 is in the closed configuration.

[0135] In this phase, the first containment volume "V1" and the second containment volume "V2" are in fluid communication with the working chamber 41 so that the feed natural gas can be transferred inside the working chamber 41. This phase corresponds to the second operating phase of the motor body 40 described above. The movable wall 44 is moved downward by the thrust of the natural gas under the feeding conditions. In this phase, the motor shaft 50 is moved from the 12-degree angle to the 170-degree angle.

[0136] The 3D figure illustrates a phase of the movement cycle in which both the feeding and discharge valves 46, 47 are in the open configuration.

[0137] In this phase, a quantity of exhausted natural gas, corresponding to the quantity of natural gas coming from the feeding circuit 30, is conveyed into the discharge circuit 60 leaving the working chamber 41 . The movable wall 44 is located in proximity of the bottom dead centre.

[0138] In this phase, the motor shaft 50 is moved from the 170-degree angle to the 180-degree angle.

[0139] Figure 3E illustrates a phase of the movement cycle in which the feeding valve 46 is in the open configuration, or first configuration, while the discharge valve 47 is switched in the closed configuration. In this phase the adiabatic compression of the exhausted natural gas is carried out by means of the movable wall 44.

[0140] In this phase, the motor shaft 50 is moved to the 180-degree angle.

[0141] In this phase, moreover, the working chamber 41 contains a quantity of natural gas corresponding to the natural gas of recirculation.

[0142] Finally, Figure 3F illustrates a cycle phase of the movement cycle in which, following polytropic compression, the natural gas of recirculation is located in the capacitive tank 20.

[0143] In this phase, the motor shaft 50 is moved from the 180-degree angle to the 356-degree angle.

[0144] Advantageously, this embodiment has several advantages that make its use extremely performing.

[0145] The first concerns the structural simplicity of the motor body 40. The motor body 40 is in fact substantially structured as a generic Diesel motor. Advantageously, in other words, it is possible to convert any existing Diesel motor or Otto into such motor body 40.

[0146] In particular, it is possible to obtain the motor body 40 of the invention by modifying an existing Diesel or Otto motor. In this case, the modifications are limited to the head and to the control of the valves that can be executed mechanically or electronically.

[0147] The second advantage is related to the compactness of the plant 1 . In fact, the recirculation circuit 70 and the capacitive tank 20 are obtained inside the motor body 40.

[0148] A further embodiment of the plant 1 , not illustrated in the accompanying figures, is now described.

[0149] In such an embodiment, the recirculation circuit 70 is associated with the collection tank 61 of the discharge circuit 60 and comprises a compressor connected and moved by the motor body 60.

[0150] Essentially, the compressor is configured to perform three distinct functions, in particular:

[0151] - extract from the collection tank 61 , a portion of exhausted natural gas in the quantity calculated for recirculation, in volumetric terms, and as a function of the desired discharge temperature of the plant 1 by means of driven shut-off and flow regulation valves;

[0152] - compress the natural gas;

[0153] - transfer the compressed exhausted natural gas into the capacitive tank 20, where pressure and temperature can be measured by appropriate measuring instruments.

[0154] In addition, a non-return valve can be arranged between the compressor and the capacitive tank 20, so that the natural gas contained in the capacitive tank 20 does not return to the compressor.

[0155] According to an aspect of the present invention, the operation of the plant can rely on the rotation of the motor shaft 50 or on a control unit.

[0156] In accordance with an aspect of the present invention, the motor shaft 50 may be connected to an electrical machine that transforms the mechanical work available to the motor shaft 50 into electrical energy.

[0157] The present invention also concerns a method for the regasification of natural gas under cryogenic conditions, preferably executable by the aforesaid plant 1 .

[0158] The method provides for preliminary phases in which the cryogenic tank 10 containing a natural gas at cryogenic temperature Tcryo and at a pressure level Pcryo is provided. This state of the natural gas is indicated in the Mollier diagram in Figure 4 with the reference "A".

[0159] The method may comprise a previous phase in which the natural gas under cryogenic conditions is transported via a special means of transport, preferably an LNG carrier, and is transferred into the cryogenic tank 10, which is preferably stationary and part of a fixed plant.

[0160] Alternatively, the method may be performed by transferring (e.g. on an LNG carrier) the cryogenic tank, after filling it with gas under cryogenic conditions.

[0161] The method envisages phases operated cyclically.

[0162] In particular, the method provides for a phase in which, by means of the feeding circuit 30 exiting the cryogenic tank, a predetermined flow rate of the natural gas under cryogenic conditions is extracted. The pressure of the natural gas is raised, in the feeding circuit 30, from the level Pcryo to the level Pproc, where Pproc is greater than Pcryo and greater than Prec and the temperature of the natural gas is raised from Tcryo to a process temperature Tproc, where Tproc is greater than Tcryo. This condition is indicated in the Mollier diagram of Figure 4 with the reference "C".

[0163] These phases take place in the section 32 of the feeding circuit 30 as a result of the aforesaid work carried out by the pump 31 . Through these phases, the natural gas reaches the thermodynamic condition called the "ex liquid" condition. In particular, the natural gas is brought to the hypercritical state, at high temperature and at high pressure, in particular at a point in the Mollier diagram arranged above the critical isotherm.

[0164] The method then provides for a phase in which the capacitive tank 20 is fed with a mass M1 of natural gas at the temperature Tproc and at the pressure level Pproc.

[0165] The mass of recirculation M2 of the natural gas is also fed to the capacitive tank 20 at a recirculation temperature Tree and at the pressure level Prev. This mass M2 of natural gas is in the recirculation conditions, indicated in the Mollier diagram of Figure 4 with the reference "D".

[0166] Preferably, the mass M2 of the natural gas comes from the recirculation circuit 70 while the mass M1 of the natural gas comes from the feeding circuit 30.

[0167] At this point, the method provides for a phase in which the masses M1 and M2 of the natural gas, respectively "ex liquid" and recirculation one, are mixed, obtaining a mass M1 +M2 of the natural gas at the feed temperature Tfeed and at the pressure level Pfeed.

[0168] The values of pressure Pfeed and temperature Tfeed are a weighted average of the respective pressure values Pproc and Prec and of the respective temperature values Tproc and Tree.

[0169] It should be remembered that preferably the pressure Prec of the natural gas of recirculation is lower than the pressure Pproc of the natural gas coming from the feeding circuit 30. Likewise, the temperature Tree of the natural gas of recirculation is preferably lower than the temperature Tfeed of the natural gas coming from the feeding circuit 30.

[0170] Therefore, the value of the pressure Pfeed is lower than Pproc and higher than Prec. Likewise, the temperature value Tfeed is lower than Tproc and higher than Tree.

[0171] According to an alternative embodiment, the pressure Prec of the natural gas of recirculation is lower than the pressure Pproc of the natural gas coming from the feeding circuit 30 but the temperature Tree of the natural gas of recirculation is higher than the temperature Tfeed of the natural gas coming from the feeding circuit 30.

[0172] This mass M1 +M2 is under the above-mentioned natural gas feeding conditions. This condition is indicated in the Mollier diagram of Figure 4 with the reference "E".

[0173] Once the mass M1 +M2 of the natural gas is obtained, it is fed from the capacitive tank 20 to the motor body 40 at the pressure level Pfeed and at the feeding temperature Tfeed.

[0174] The method then provides for a phase in which to expand in the motor body 40 the mass M1+M2 of natural gas, so as to lower the pressure from the Pfeed level to the Pex level, in which Pex is lower than Pfeed, and so as to lower the temperature from Tfeed to Tex, wherein Tex is lower than Tfeed, producing mechanical energy.

[0175] This phase is indicated in the Mollier diagram of Figure 4 with the segment "E"-"F". The natural gas expansion end condition is indicated in the Mollier diagram in Figure 4 with the reference "F".

[0176] Finally, the method provides for a phase of discharging a mass MT (corresponding to the initial mass M1 ) of natural gas into the delivery pipe connected to the regasified natural gas transmission and distribution network. This phase is carried out firstly by means of a discharge circuit 60 and subsequently in the delivery pipe and then introduced into the regasified natural gas transmission and distribution network connected thereto. The discharge conditions are indicated in the Mollier diagram of Figure 4 with the reference "G".

[0177] The method also comprises a phase in which the remaining mass of natural gas is compressed, corresponding to the mass of recirculation "M2", so as to raise the pressure from the Pex level to the Prec level and so as to raise the temperature from Tex to Tree and feed the mass M2 at the pressure level Prec and at the feeding temperature Tree of the capacitive tank 20. This phase is indicated in the Mollier diagram of Figure 4 with the reference "F"-"D".

[0178] Preferably, the phase of compressing the mass M2 of natural gas so as to raise the pressure from the Pex level to the Prec level and so as to raise the temperature from Tex to Tree and of feeding with the mass M2 at the pressure level Prec and at the feeding temperature Tree of the capacitive tank 20 is performed by means of the motor body 40, in particular by means of a return stroke opposite to the previous expansion, and delivers the compressed natural gas in the recirculation circuit 70.

[0179] According to a preferred embodiment of the method, the natural gas is methane. In this embodiment, the following pressure and temperature values are obtained:

[0180] - the pressure level Patm is equal to about the atmospheric pressure; and

[0181] - the pressure level Pproc has a value between about 200 bar and about 220 bar;

[0182] - the pressure level Pfeed has a value between about 150 bar and about 200 bar;

[0183] - the pressure level Pex has a value between about 2 bar and about 4 bar;

[0184] - the temperature Tcryo is between about -130°C and about -90°C;

[0185] - the temperature Tproc is between about +300°C and about +500°C;

[0186] - the temperature Tree is between about +250°C and about +350°C;

[0187] - the temperature Tfeed is between about +280°C and about +340°C; and

[0188] - the temperature Tex is between about -20°C and about +20°C.

[0189] Advantageously, the present invention overcomes the drawbacks complained of in the prior art.

[0190] In particular, one objective achieved is to make available a plant 1 and a method for the regasification of a natural gas under cryogenic conditions that allow a high process reliability and, simultaneously, a considerable improvement in terms of energy efficiency of the entire process.

[0191] This result is achieved thanks to the possibility of exploiting the regasification of natural gas to generate mechanical and / or electrical energy, at least partially recovering the energy spent on the previous liquefaction of natural gas in order to be able to transport it more advantageously.

[0192] Advantageously, moreover, the plant 1 and the method are reliable from a process and environmental point of view since they do not require combustion and / or pollute the environment.

Claims

CLAIMS1. Method for the regasification and distribution of natural gas, comprising the phases of:- feeding a cryogenic tank (10) with a mass MO of natural gas at a cryogenic temperature Tcryo and at a pressure level Pcryo;- extracting from said cryogenic tank a mass M1 of natural gas, where M1 is lower than MO, and raising the pressure of the mass M1 from the pressure level Pcryo to the pressure level Pproc and raising the temperature of the mass M1 from Tcryo to a process temperature Tproc, by means of a feeding circuit connected downstream of said cryogenic tank;- feeding a capacitive tank (20) with the mass M1 of natural gas at the temperature Tproc and at the pressure level Pproc by means of said feeding circuit and with a mass of recirculation M2 of natural gas at a temperature Tree, in particular lower than the raised temperature Tprocl , and at a pressure level Prec, in particular lower than the raised pressure Pproc, by means of a recirculation circuit (70);- mixing the masses M1 and M2 of natural gas inside the capacitive tank (20), obtaining a mass M1 +M2 at a feeding temperature Tfeed and at a pressure level Pfeed;- feeding a motor body (40) connected downstream of said capacitive tank (20), with the mass M1 +M2 of natural gas at the pressure level Pfeed and at the feeding temperature Tfeed;- expanding the mass M1 +M2 of natural gas in the motor body (40), so as to lower the pressure from the pressure level Pfeed to a pressure level Pex and to lower the temperature from Tfeed to Tex, producing mechanical energy;- discharging a portion MT of the mass M1 +M2 of natural gas at said temperature Tex and at said pressure level Pex from the motor body (40) to a network for the transport and / or distribution of natural gas, connected downstream of said motor body (40);- compressing inside said motor body (40) the remaining mass of natural gas, corresponding to said mass of recirculation M2, so as to raise the pressure of the mass of recirculation M2 from the pressure level Pex to the pressure level Prec and so as to raise the temperature of the mass of recirculation from the temperature Tex to the temperature Tree;- feeding said new mass M2 at the pressure level Prec and at the feeding temperature Tree to said capacitive tank (20) by means of said recirculation circuit (70), interposed between said motor body (40) and said capacitive tank (20).

2. Method according to claim 1 , wherein said temperature Tex and said pressure level Pex are suitable for the introduction of natural gas into a national transmission network and / or into a natural gas distribution network; preferably said temperature Tex is between -20°C and 40°C and wherein said pressure level Pex is between 0.5 bar and 12 bar.

3. Method according to claim 1 or 2, wherein said temperature Tproc is between 300°C and 500°C and wherein said pressure level Pproc is between 200 bar and 220 bar; preferably said natural gas being in a hypercritical state.

4. Method according to any one of the preceding claims, wherein said temperature Tree of the mass of recirculation M2 is between +250°C and +350°C.

5. Method according to claim 4 when dependent on 3, wherein said mass M1 and said mass M2 are in a mass ratio between 1 :15 and 1 :30, preferably having a value of 1 :23.

6. Method according to any one of the preceding claims, wherein said temperature Tfeed is between 280°C and 340°C and wherein saidpressure level Pfeed is between 150 bar and 200 bar, preferably said phase of mixing the masses M1 and M2 of natural gas inside the capacitive tank causing an expansion of the mass M1 , in particular passing from a hypercritical state to a gaseous state.

7. Method according to any one of the preceding claims, wherein said phase of raising the pressure of the natural gas from the level Pcryo to the level Pproc and raising the temperature of the natural gas from Tcryo to a process temperature Tproc is carried out by means of a pump operatively arranged in said feeding circuit.

8. Method according to any one of the preceding claims, comprising a phase of transporting the cryogenic tank from a point of liquefaction of the natural gas to a point of delivery of the natural gas, via a means of transport, in particular by means of an LNG carrier, said transport phase being interposed between said phase of feeding the cryogenic tank and said phase of extracting a mass M1 from the cryogenic tank.

9. Method according to any one of the preceding claims from 1 to 7, comprising a phase of transporting said mass M0 at said cryogenic temperature Tcryo and at said pressure level Pcryo from a point of liquefaction of the natural gas to a point of delivery of the natural gas via a means of transport, in particular by means of an LNG carrier, said transport phase being prior to said phase of feeding the cryogenic tank.

10. Method according to any one of the preceding claims, wherein said cryogenic temperature Tcryo is between 180°C and -140°C, preferably equal to -160°C, and wherein said pressure level Pcryo is substantially equal to the atmospheric pressure.

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