Self-sufficient system for evaporation of LNG

DESCRIPTION

TECHNICAL FIELD

[0001] The subject-matter disclosed herein relates to a system for evaporating liquefied natural gas (=LNG) which is at least self-sufficient, i.e. which does not require any electrical supply. According to some advantageous embodiments, subject-matter disclosed herein relates to a system for evaporating liquefied natural gas (=LNG) which does not require any electrical supply and which generates electrical power.

BACKGROUND ART

[0002] In general, natural gas is stored as liquefied natural gas (=LNG) around boiling point at ambient pressure (which is around minus 162 °C at atmospheric pressure of 1 bar) or slightly higher pressure and is then pumped in pipelines for gas transporting and delivering after having been pressurized and evaporated. Typically, the LNG, which is stored in suitable vessels, is sent to a LNG pump in order to increase the LNG pressure and, after LNG pressurization, it is sent to at least one heat exchanger to heat it up and perform LNG evaporation; typically, it is used sea water or ambient air (that is, in general, around 15 - 25 °C) to rise up the temperature of LNG and performing LNG evaporation. Finally, the natural gas is sent into the pipeline.

[0003] However, even if sea water or ambient air are easy to use as they can be easily exploited, the heat exchange process between them and LNG is inefficient due to the high difference of temperature in the heat exchanger and therefore the overall process of the LNG evaporation station is not optimized. Moreover, in order to improve the efficiency of the heat exchange process, it would be necessary to split the process using a plurality of heat exchangers, increasing the costs and the footprint of the station. Finally, ambient air or sea water have limited maximum temperature which does not allow to reach superheating of natural gas at the outlet of the heat exchanger, therefore limiting the work which can be extracted through the expander. Furthermore, the marine environment could be adversely affected from the local temperature change.

[0004] From document JPS55148907A it is known a system for vaporizing liquefied natural gas which uses a gas turbine-Rankine combined cycle. The system according to this solution uses the heat provided by the Rankine cycle to heat the liquified natural gas, so that it partly evaporates. At the same time, the gas turbine cycle provides heat to the Rankine cycle so to heat the low- boiling point fluid used into the Rankine cycle. The gas turbine cycle burns compressed air and natural gas as a fuel to produce burned gases to be expanded into a turbine so to generate electricity and exhaust gases to be used to provide heat both to the Rankine cycle and to the liquified natural gas. It is to be noted that the system according to this document is a complex and expending solution which requires external electrical energy to drive the pumps of the system and also generates burned gas containing pollutant.

[0005] Therefore, it would be desirable to have a LNG evaporating station which has higher heat exchange process efficiency and which is at least self- sufficient, i.e. which does not require any electrical supply, for example to pump LNG before evaporation. In other words, it would be desirable to have a LNG evaporating station which has higher overall efficiency.

SUMMARY

[0006] According to an aspect, the subject-matter disclosed herein relates to a self-sufficient system for evaporation of liquefied natural gas (=LNG) which comprises an evaporation station configured to receive LNG at first pressure and first temperature and to provide natural gas (=NG) at second pressure and second temperature which are higher than first pressure and first temperature, a heat pumping station being configured to implement a closed-loop refrigeration cycle using a mixed refrigerant as working fluid and a main heat exchanger fluidly coupled to the evaporation station and the heat pumping station and configured to transfer heat from the heat pumping station to the evaporation station in order to evaporate liquefied natural gas (LNG) and supply natural gas (NG) downstream the main heat exchanger.

The evaporation station comprises a first pump and a first expander which are mechanically coupled to each other so that the first expander drives the first pump. The heat pumping station comprises a second pump and a second expander which are coupled to each other so that the second expander drives the second pump.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. 1 shows a general schematic drawing of a first embodiment of an innovative self-sufficient system for evaporation of liquefied natural gas (LNG),

Fig. 2 shows a general schematic drawing of a second embodiment of an innovative self-sufficient system for evaporation of liquefied natural gas (LNG) with electrical power generation, and Fig. 3 shows a general schematic drawing of a third embodiment of an innovative self-sufficient system for evaporation of liquefied natural gas (LNG) with electrical power generation.

DETAILED DESCRIPTION OF EMBODIMENTS

[0008] According to an aspect, the subject-matter disclosed herein relates to a self-sufficient LNG evaporation system for evaporating liquefied natural gas (which is previously stored in liquid phase, i.e. in conditions below the boiling point) in order to send evaporated natural gas to distribution pipelines. As previously stated, the term “self-sufficient” means that the embodiments of the system described herein do not require any electrical supply to, for example, pump LNG before evaporation. The system comprises an evaporation station which receives LNG from the storage, raises the pressure of the LNG with a pump and then increases the temperature of the LNG through a main heat exchanger in order to supply evaporated NG to an expander which can extract mechanical work from NG expansion and finally expanded NG is provided to distribution pipelines. The pump and the expander of the evaporation station are mechanically coupled with a shaft so that the mechanical work extracted by the expander can drive the pump, avoiding then the need for an external electrical supply to the evaporation station, in particular to the pump.

[0009] As mentioned above, the main heat exchanger provides the heat needed to evaporate the LNG; in particular, the main heat exchanger is configured to transfer heat from a mixed refrigerant to the LNG, thereby cooling the mixed refrigerant and heating the LNG. The mixed refrigerant is the working fluid of a closed-loop cycle implemented by a heat pumping station which is fluidly coupled to the main heat exchanger: the main heat exchanger condensates the mixed refrigerant which is then heated in a secondary heat exchanger using an external heat source, for example ambient air or sea water or process waste heat, such as boil-off gas (=BOG, i.e. part of LNG which evaporates into gas phase during LNG storage process). The heat pumping station comprises also an expander, which expand the mixed refrigerant and extract work from the mixed refrigerant expansion, and a pump, which raises the pressure of the mixed refrigerant. The pump and the expander of the heat pumping station are coupled so that the expander can drive the pump (directly, for example through a common shaft, or indirectly, for example by supplying electrical energy through a power generator, as it will better explained in the following).

[0010] Reference now will be made in detail to embodiments of the disclosure, examples of which are illustrated in the drawings. The examples and drawing figures are provided by way of explanation of the disclosure and should not be construed as a limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. In the following description, similar reference numerals are used for the illustration of figures of the embodiments to indicate elements performing the same or similar functions. Moreover, for clarity of illustration, some references may be not repeated in all the figures.

[0011] Fig. 1 shows a schematic drawing of a first embodiment of an innovative self-sufficient system for evaporation of liquefied natural gas (LNG) 1000, referred in the following as “innovative self-sufficient system for LNG evaporation” or simply “LNG evaporating system”. A second and a third embodiment 2000 and 3000 of an LNG evaporating system will be described in the following with the aid of Figs. 2 and 3. It is to be noted that elements in Figs. 2 and 3 which have the same reference number of the elements in first embodiment 1000 shown in Fig. 1 may be identical or similar to the elements in Fig. 1 and perform the same or similar functions.

[0012] With non-limiting reference to Fig. 1, the LNG evaporating system 1000 includes an evaporation station 100, which is configured to receive liquefied natural gas (=LNG) at first pressure Pl and first temperature T1 and to provide natural gas (=NG) at second pressure P2 and second temperature T2. For example, the liquefied natural gas may be provided by a LNG storage 12, e.g. one or more appropriate vessel in which LNG is stored, typically slightly below the boiling point, which for LNG is typically around - 162° C at 1.013 bar (ambient pressure); in other words, for example, first pressure Pl may be around 1 bar and first temperature T1 may be around - 162° C. LNG evaporating system 1000 may be configured to provide NG to an external system 28, e.g. distribution pipelines, for example in a range of 30 - 200 bar and minus 15 - plus 20 °C, depending on external system 28 requirements.

[0013] The LNG evaporating system further comprises a heat pumping station 200 which is configured to implement a closed-loop refrigeration cycle using a mixed refrigerant as working fluid. Advantageously, the mixed refrigerant comprises one or more of methane, ethane, ethylene and propane. It is to be noted that, depending on the available heat sources of the system, variations and combinations of all the mentioned species are possible. According to one possibility, the mixed refrigerant composition may be:

- Methane 0,35 %

- Ethane/Ethylene 0,50 %

-- Propane 0,15 %

According to another possibility, in particular in embodiments where the mixed refrigerant is superheated, as it will better explained in the following, the mixed refrigerant composition may be:

Propane 0,50 %

- i -Butane 0,40 %

- i -Pentane 0,10 % Advantageously, C4/C5 species like butane or isopentane may be added to the mixed refrigerant compositions. As will be more clear from the following, the heat pumping station 200 is configured to provide heat to the evaporation station 100 in order to perform LNG evaporation.

[0014] The LNG evaporating system further comprises a main heat exchanger 300 which is fluidly coupled to the evaporation station 100 and the heat pumping station 200 and which is configured to allow the transfer of heat from the heat pumping station 200 to the evaporation station 100 in order to evaporate (and possibly also superheat) liquefied natural gas and supply natural gas downstream the main heat exchanger 300. In particular, the main heat exchanger 300 is configured to generate NG at fourth pressure P4 and fourth temperature T4. For example, the NG just downstream the main heat exchanger 300 may be at around 30 °C and 100 bar; in other words, for example, fourth pressure P4 may be around 100 bar and fourth temperature T4 may be at around 30 °C. In general, NG just downstream the main heat exchanger 300 may be in a range of 0 - 60 °C and 30 - 200 bar.

[0015] As it will be better disclosed in the following, since the main heat exchanger 300 features the heat exchange of more streams of fluids in the same equipment, the using of a Brazed Aluminum Heat Exchanger (=BAHX) is preferred. According to another possibility, other types of heat exchangers may be used, for example Wound Coil Heat Exchanger (=WCHE) or Diffusion Bonded Heat Exchanger (=DBHE).

[0016] It is to be noted that the main heat exchanger 300 is a critical component of the system and it has technical limitations in the maximum design conditions, such as maximum operating pressure and maximum temperature difference between hot and cold streams in the main heat exchanger 300. According to an embodiment, if a BAHX is used, the maximum temperature difference between the LNG and the mixed refrigerant is about 50°C; in particular, a proper mixed refrigerant has to be used to ensure this requirement.

[0017] With non-limiting reference to Fig. 1, the evaporating station 100 comprises a first pump 10 which has a fluid inlet 11 configured to receive LNG; in particular, the fluid inlet 11 may be selectively fluidly coupled to the LNG storage 12 in order to supply LNG to the first pump 10. The first pump 10 is configured to pump the LNG, so to rise up LNG pressure (and possibly increase slightly LNG temperature) and provide LNG at third pressure P3 and third temperature T3 at a fluid outlet 19, which is fluidly coupled to the main heat exchanger 300. For example, LNG at the fluid outlet 19 may be around minus 155 °C and 100 bar; in other words, for example, third pressure P3 may be around 100 bar and third temperature T3 may be around minus 155 °C. In general, LNG at the fluid outlet 19 may be in a range of minus 160 - minus 150 °C and 50 - 200 bar.

[0018] As shown in Fig. 1, the evaporating station 100 comprises further a first expander 20 mechanically coupled to the first pump 10 so that the first expander 20 drives the first pump 10. In particular, the first expander 20 and the first pump 10 have a common shaft which couples them and allows the transmission of the mechanical work to drive the first pump 10. It is to be noted that the evaporating station 100 does not need any electrical driver as the first expander 20 may provide sufficient power to drive the first pump 10. In order to drive the first pump 10, the first expander 20 is fluidly coupled to the main heat exchanger 300 and receives NG from the main heat exchanger 300 to perform NG expansion. In particular, the first expander 20 has a fluid inlet 21 which is configured to receive NG from the main heat exchanger 300 at fourth pressure P4 and fourth temperature T4 and a fluid outlet 29 which is configured to provide NG to the external system 28. It is to be noted that at the fluid outlet 29 the NG is at second pressure P2 and second temperature T2 due to NG expansion to extract work from it. For example, NG at the fluid outlet 39 may be around 5 °C and 70 bar; in other words, second pressure P2 may be around 70 bar and the second temperature T2 may be around 5 °C.

[0019] As explained, the main heat exchanger 300 is configured to transfer the heat for LNG evaporation from the heat pumping station 200 to the evaporation station 100, the heat pumping station 200 being configured to implement a closed-loop refrigeration cycle using a mixed refrigerant as fluid. As shown in Fig. 1, the heat pumping station 200 comprises a second pump 30, a second expander 40 and advantageously a first secondary heat exchanger 35. As it will be better explained in the following, the second pump 30 and the second expander 40 are coupled to each other so that the second expander 40 can drive the second pump 30.

[0020] According to a preferred solution, in particular to ensure that no other electrical inputs are required from the system (i.e. the system is self-sufficient), the first pump 10 and the second pump 30 have a minimum efficiency equal to or greater than 60% and the first expander 20 and the second expander 40 have isentropic efficiency preferably at least 90%. For example, if the efficiency of the machines is lower, in particular the efficiency of the first pump 10, the first pump 10 may have to raise the LNG pressure in the main heat exchanger 300 to a higher level, risking exceeding the maximum design pressure possible with BAHX; it is to be noted that the risk may be overcome by using DBHE instead of BAHX, which is however associated with increased cost and/or size limitations of the system. The same may be valid for the first expander 20 and its efficiency. According to another possibility, if the efficiency of the machines is lower, there may be the need of external heat sources and higher heating temperatures of the evaporating mixed refrigerant, for example even beyond the ambient temperature.

[0021] As it can be easily understood for the person skilled in the art, the closed-loop refrigeration cycle implemented by the heat pumping station 200 includes at least a step of heating the mixed refrigerant, in particular performed by the secondary heat exchanger 35, advantageously totally evaporating the mixed refrigerant; a step of expanding the mixed refrigerant, in particular performed by the second expander 40, advantageously extracting mechanical work from the mixed refrigerant expansion; a step of cooling the mixed refrigerant, in particular performed by the main heat exchanger 300, advantageously condensing the mixed refrigerant; a step of pumping the mixed refrigerant, in particular performed by the second pump 30, advantageously rising up the mixed refrigerant pressure; and a step of heating the mixed refrigerant, in particular performed by the main heat exchanger 300. In particular, the secondary heat exchanger 35 is arranged upstream the second expander 40, in particular the secondary heat exchanger 35 is directly coupled to the second expander 40 inlet. In particular, the second expander 40 is directly coupled to the main heat exchanger 300, so that the mixed refrigerant exiting from the second expander 40 is directly supplied to the main heat exchanger 300.

[0022] As it is apparent from Fig. 1, the step of cooling the mixed refrigerant in order to condensate it and heating the mixed refrigerant at a higher pressure level, i.e. after being pumped by the second pump 30, may both be performed by the main heat exchanger 300, in particular by two different sections of the main heat exchanger 300 (see sections 301 and 302 respectively in Fig. 1). Advantageously, part of the heat that is removed from the mixed refrigerant (so that mixed refrigerant is condensed) in section 301 of the main heat exchanger is partially transferred to the LNG in order to evaporate it (see section 303 in Fig. 1) and partially transferred to the mixed refrigerant itself at a higher pressure level (see section 302 in Fig. 1).

[0023] It is to be noted that the mixed refrigerant exiting from section 302 may be only partly evaporated, as the available heat in the main heat exchanger 300 may not allow for complete evaporation. Advantageously, complete evaporation of the mixed refrigerant may be reached, for example by staggered the cold side outlet streams so that they don’t have the same temperature and/or by further heating the mixed refrigerant exiting from section 302 of the main heat exchanger 300, before being expanded by the second expander 40. Advantageously, the mixed refrigerant can be fully evaporated performing heat transfer from ambient air (which can have a temperature in the range 15-45 °C) to the mixed refrigerant, for example by exposing to the ambient air the coils in which the mixed refrigerant can flow. More advantageously, the mixed refrigerant can be used to cool a gas turbine air inlet, therefore performing heat transfer from gas turbine air inlet to the mixed refrigerant, in particular in a first secondary heat exchanger 35.

[0024] Even more advantageously, in order to reach higher temperatures at the second expander 40 inlet (hence, increasing the work that can be extracted by the second expander 40), the mixed refrigerant can be fully evaporated and possibly superheated by exploiting an external heat source 31 (see for example Figs. 1-3): the first secondary heat exchanger 35 may transfer heat from the external heat source 31 to the mixed refrigerant in order to supply mixed refrigerant in the form of gas downstream the first secondary heat exchanger 35. Advantageously, the external heat source 31 may be ambient air or sea water or process waste heat. In particular, process waste heat may be heat deriving from Boil-Off-Gas (=BOG) compression systems, which are typically required in LNG storage installations. The BOG compression system handles the part of LNG that evaporates into the gas phase during LNG storage process. The gas is compressed in the BOG compressor, providing a flow of evaporated NG which has a higher temperature with respect to the mixed refrigerant (for example in a range of 40 - 80 °C, preferably higher than 60 °C) and which therefore can be used to provide heat to the mixed refrigerant. [0025] As already described, the second pump 30 and the second expander 40 are coupled to each other. According to some embodiments, for example the embodiment shown in Fig. 1, the second expander 40 and the second pump 30 may have a common shaft which couples them and allows the transmission of the mechanical work generated by the second expander 40 to drive the second pump 30. It is to be noted that the heat pumping station 200 does not need any electrical driver as the second expander 40 may provide sufficient power to drive the second pump 30. According to other embodiments, for example the embodiment shown in Fig. 2, the second expander 40 may be mechanically coupled to a first power generator 62 which is configured to generate electrical power and provide the electrical power required to drive the second pump 30. In particular, the second pump 30 is typically driven by an electric motor (not shown in any figures) which converts electrical power into mechanical power, in order to drive the second pump 30; advantageously, the first power generator 62 may be electrically coupled to the electric motor, in order to provide the electrical power to the electric motor, so that there is no need of an external electric source. According to still other embodiments, for example the embodiment shown in Fig. 3, the second expander 40 and the second pump 30 may have a common shaft which couples them, so that the second expander 40 mechanically drives the second pump 30, and the second expander 40 is further coupled to a first power generator 62 configured to generate electrical energy, which can be supplied for example outside the self-sufficient system 3000. It is to be noted that the embodiment of Fig. 3 can be applied in particular when the mixed refrigerant is superheated upstream the second expander 40 inlet, extracting thus surplus power from the mixed refrigerant expansion and drive other rotating equipment or convert it into electrical energy.

[0026] Advantageously, if higher amounts of surplus heat are available, for example from the external heat source 31 or a different external source (see for example second external source 41 in Fig. 3), the mixed refrigerant exiting from the first secondary heat exchanger 35 may be superheated in a second secondary heat exchanger 45 before being supplied to the second expander 40; in other words, the second secondary heat exchanger 45 is arranged upstream the second expander 40, in particular the second secondary heat exchanger 45 is directly coupled to the second expander 40 inlet. The second secondary heat exchanger 45 is configured to transfer heat from the second external heat source 41 to the mixed refrigerant to supply mixed refrigerant in the form of superheated gas downstream the second secondary heat exchanger 45. Advantageously, the second external heat source 41 is process waste heat; according to one possibility, process waste heat may be heat deriving from Boil-Off-Gas (=BOG) compression systems.

[0027] Advantageously, if higher amounts of surplus heat are available, for example from the external heat source 31 or a different external source (see for example third external source 51 in Fig. 3), the natural gas (=NG) exiting from the main heat exchanger 300 may be superheated in a third secondary heat exchanger 25 before entering the first expander 20; in other words, the third secondary heat exchanger 25 is arranged upstream the first expander 20. The third secondary heat exchanger 25 is configured to transfer heat from the third external heat source 51 to the NG to supply NG in the form of superheated gas downstream the third secondary heat exchanger 25. Advantageously, the third external heat source 51 is process waste heat; according to one possibility, process waste heat may be heat deriving from Boil-Off-Gas (=BOG) compression systems.

[0028] Advantageously, if the natural gas (=NG) is superheated by the third secondary heat exchanger 25 before entering the first expander 20, the work that can be extracted by the first expander 20 may be higher than the one needed to drive the first pump 10. Hence, the evaporation station 100 may further comprise a second power generator 63 configured to generate electrical energy, which can be used for example outside the self-sufficient system 3000. As already described, when the natural gas (=NG) is superheated upstream the first expander 20 inlet, surplus power may be extracted from the natural gas expansion and drive other rotating equipment or convert it into electrical energy.

[0029] Advantageously, if the system is designed properly, the start-up of the system may be possible with only the first pump 10 and the first expander 20 featuring a helper motor (i.e. the evaporation station 100 is temporarily coupled to an helper motor). Advantageously, the heat pumping station 200 may be at high pressure with only little to even no mixed refrigerant in the closed-loop refrigeration cycle; more advantageously, the heat pumping station 200 comprises isolation valves (not shown in any Figures), in particular the valves may be located at the mixed refrigerant cold side outlet (i.e. the outlet of section 301) of the main heat exchanger 300 and at the outlet of the second expander 40. When the evaporation station 100 is started, the main heat exchanger 300 will cool down. Advantageously, during start-up of the system, the mixed refrigerant is isolated in the portion of the heat pumping station 200 comprised between the two isolation valves (i.e. between the outlet of the second expander 40 and the outlet of section 301). The heat provided by the main heat exchanger 300 to the evaporation station 100 not only will evaporate LNG but also will start to condense the isolated mixed refrigerant; said condensation will cause a drop of the pressure in the isolated portion of the heat pumping station 200. Once the pressure in the isolated portion dropped sufficiently (and a sufficient liquid level is present in the second pump 30), the isolation valve at the outlet of the second expander 40 will be opened and the heat pumping station 200 will be started. Advantageously, shortly after the starting of the heat pumping station 200, also the isolation valve at the mixed refrigerant cold side outlet is opened.