Process for gasifying LNG and for generating power at low temperature

Abstract

The present invention describes a process for gasifying liquefied natural gas (LNG) and for generating power, which operates efficiently at low temperatures.

Description

Technical field of the invention

[0001] The present invention finds application in the sector of the regasification of liquefied natural gas (LNG) and in energy recovery.Background artORGANIC RANKINE CYCLE

[0002] Organic fluid Rankine cycles (ORC) are widely used in the geothermal field and for applications employing biomass and waste heat recovery from industrial processes, with the possibility of selecting the working fluid among dozens of candidate fluids, allowing to achieve efficient thermodynamic cycles, even for low source temperatures and small heat availability.

[0003] The choice of a low boiling fluid also allows to reach temperatures lower than the room temperature, with the possibility of achieving a condensing cycle at cryogenic temperatures, without incurring issues of freezing or too high vacuum degrees.

[0004] Many patents concerning ORCs adapted for LNG regasification were presented in the years from 1965 to 2013 by ORMAT, R.G. JACKSON, Reickichi Nozawa, Fluor, Exxon Mobil.

[0005] Recently, also Saipem published its own patent for an ORC cycle which works with a single fluid produced starting from LNG and LPG (WO 2020 / 075112A1).

[0006] Based on a high boiling working fluid (referred to as IMR), it requires the use of a high-temperature heat source and is designed to maximize the mechanical power which may be extracted from said source by using the waste heat to vaporize an LNG stream (cooling fluid and, at the same time, cold source).

[0007] Such an idea has the advantage of not having a direct thermal impact on the environment and, generally employing a fuel, it ensures a continuous generation of power independent of environmental conditions; furthermore, the formulation of the IMR is designed so as to fully exploit the frigories contained in the vaporizing LNG stream while using a simple cycle, i.e., consisting of a single machine for the expansion (expander) of said working fluid.

[0008] However, the use of a fuel has the defect of employing a non-renewable source and involving the emission of carbon dioxide into the atmosphere, unless the ORC cycle is combined with an expensive CO 2 sequestration system.

[0009] The ever-increasing sensitivity towards environmental pollution linked to greenhouse gas emissions is orienting the market towards solutions which do not involve the use of fuels.

[0010] Inversed Mixed Refrigerant (IMR), a fluid which is easy and cheap to obtain, and with thermodynamic features similar to those of LNG, condenses completely, at LNG storage temperature, only at relatively high pressures; however, it may not be completely vaporized at room temperature except at pressures slightly above the condensation pressure.

[0011] Comprised between these two constraints, the expansion ratio available to generate power is considerably reduced to the point of making it poorly adapted for use in the presence of only roomtemperature sources.

[0012] The prior art document US 3.479.832 describes a process for vaporizing liquefied natural gas, with the separation of liquid and gaseous singlecomponent-rich fractions, and for producing energy, by means of the use of a working fluid with a variable composition.Summary of the invention

[0013] The inventors of the present patent application have developed a process for gasifying liquefied natural gas (LNG) and for producing power, which uses a working fluid obtained by mixing liquefied natural gas (LNG) and liquefied petroleum gasses (LPG), especially adapted to operate at low temperatures.

[0014] Subject of the invention is a process for gasifying liquefied natural gas (LNG) and for generating power according to claim 1.Object of the invention

[0015] In a first object, the present invention describes a process for gasifying liquefied natural gas (LNG) and for producing power.Brief description of the drawings

[0016] Figure 1 shows the diagram of a process for gasifying liquefied natural gas (LNG) and for producing power as described in the prior art document WO 2020 / 075112A1. Figure 2 shows the diagram of a process according to a first embodiment of the present invention. Figure 3 shows the diagram of the process according to a second embodiment of the present invention. Detailed description of the inventionIMR - Inversed Mixed Refrigerant

[0017] Said IMR working fluid is a liquid mixture.

[0018] In particular, such fluid is obtained by mixing commercial liquefied petroleum gas (LPG) and commercial liquefied natural gas (LNG).

[0019] The term "commercial liquefied petroleum gas (LPG)" means a fuel whose features are well defined, for the common uses in civil and industrial fields, with the following properties: vapor pressure at 37.77°C (100°F); minimum temperature at which, in atmospheric pressure, 95% by volume of a hypothetical sample is vaporized, possibly by heating according to a precise method; the molar percentage content of molecules having a number of carbon atoms greater than 4; for the purposes of the present invention, in fact, it also contains hydrocarbons with 7 or > 7 carbon atoms.

[0020] It is known that liquefied petroleum gasses (LPG) are blended in crude oil and that they are separated therefrom by refining in a topping column.

[0021] Various refinery processes produce liquefied petroleum gasses (LPG); for example, cracking produces liquefied petroleum gas (LPG) as a byproduct.

[0022] For the purposes of the present invention, liquefied petroleum gas (LPG) is preferably defined as a combustible fluid the features of which fall within the limits defined in the following table: Properties Minimum value Maximum value Testing method Vapor pressure at 37.77°C (100°F) 482633 Pa (70 psia)1.434e+6 Pa (208 psia)ASTM D1267-0295% vol.@ atm (min T) (1)< n.a.2.22°C (36°F)ASTM D1837-64Max %C5+ (2)< n.a.2ASTM D2163-77(1) maximum temperature at which, in atmospheric pressure, the evaporation of 95% of the volume of the sample under examination is obtained. (2) content of molecules with at least 5 carbon atoms.

[0023] The term "commercial liquefied natural gas (LNG)" refers to a hydrocarbon fluid mainly in the liquid phase obtained by condensing natural gas at a temperature sufficiently low to keep it liquid even at atmospheric pressure.

[0024] It is known that natural gas is mainly composed of methane and light hydrocarbons which rarely have a number of carbon atoms > 5; it may also contain nitrogen in variable proportions.

[0025] For the purposes of the present invention, "IMR" is defined as any mixture of liquefied natural gas (LNG) and liquefied petroleum gas (LPG) obtained by mixing 1 volume of liquefied natural gas (LNG) with a quantity of liquefied petroleum gas (LPG) between 0.25 and 1.2 volumes of liquefied natural gas (LNG).

[0026] The process for preparing IMR working fluid is described in detail in patent application WO 2020 / 075112 A1 (Saipem S.p.A.).Liquefied Natural Gas (LNG) Gasification Line

[0027] For the purposes of the present invention, a liquefied natural gas (LNG) gasification line originates from a storage tank of liquefied natural gas (LNG in Figures 2 and 3), which, once gasified, is fed into the network (NG in Figure 2).

[0028] In particular, the gasification is obtained with several heating steps compatibly with the increase in system complexity due to increased process efficiency.

[0029] According to a preferred embodiment of the invention, three liquefied natural gas heating steps are provided, in which a flow of liquefied natural gas exchanges heat by acquiring it from one or more flows.

[0030] More specifically, according to what is shown in Figures 2 and 3, a flow of liquefied natural gas 40 exiting a dedicated tank (LNG) is subjected to a first heating step, thus obtaining a partially regasified flow 41, which during a second step is further heated, thus obtaining a further regasified flow 42, which, following a third heating step, gives rise to a completely regasified flow 43.LIMR Cycle - Low Temperature Inversed Mixed Refrigerant

[0031] The LIMR cycle is implemented by employing a first working fluid (1MF or LIMR) and comprises an expander (EX in Figure 2), one or more low-temperature heat sources (H1, H2 in Figure 2) and tanks (V1, V2 and V3 in Figure 2) each containing a second or third working fluid.

[0032] In particular, a first tank V1 comprises a quantity of a second working fluid (2MF), which, in a preferred aspect of the present invention, is Inversed Mixed Refrigerant (IMR).

[0033] A second tank V2 and a third tank V3 comprise a quantity of a third working fluid, which, in a preferred aspect of the present invention, is liquefied natural gas (LNG).

[0034] With regard to the expander (EX), this is preferably a turbine for generating electric power.

[0035] According to a first object of the present invention, a process for gasifying liquefied natural gas (LNG) and for generating power is described, which comprises the steps of: 1) subjecting a flow of a first working fluid 1MF in the gaseous phase 1 to a first cooling step, thus obtaining a cooled flow 2 of said first working fluid 1MF, 2) sending said cooled flow 2 of said first working fluid 1MF to a first tank V1 containing a quantity of a second working fluid 2MF, 3) separating a first condensed fraction 3 and a first gaseous portion 6, 4) subjecting said first gaseous portion 6 to a second cooling step, thus obtaining a first condensed portion 7, 5) sending said first condensed portion 7 to a second tank V2 containing a quantity of a third working fluid 3MF, 6) separating a second condensed fraction 8 and a second gaseous portion 12, 7) subjecting said second gaseous portion 12 to a third cooling step, thus obtaining a condensed final portion 13, 8) sending said condensed final portion 13 to a third tank V3 containing a quantity of said third working 3MF, thus obtaining a third condensed fraction 14, 9) subjecting said first condensed fraction 3 to a pumping step, thus obtaining a first pumped condensed fraction 4, and to a heat exchange step, thus obtaining a first portion for mixing said working fluid 5, 10) subjecting said second condensed fraction 8 to a pumping step, thus obtaining a second pumped condensed fraction 9, to a first heat exchange step, thus obtaining a second pumped and higher-temperature condensed fraction 10, and to a second heat exchange step, thus obtaining a second mixing portion for said working fluid 11, 11) subjecting said third condensed fraction 14 to pumping, thus obtaining a third pumped condensed fraction 15, to a first heat exchange, thus obtaining a third pumped and higher-temperature condensed fraction 16, and to a second heat exchange, thus obtaining a third mixing portion for said working fluid 17, 12) mixing said first 5, second 11 and third 17 mixing portions for said first working fluid in a mixer M, thus obtaining an initial flow of said first working fluid 18, 13) heating said initial flow of the first working fluid 18 by means of a second low-temperature heat source H2 thus obtaining a heated flow of said first working fluid 19, 14) expanding said heated flow of the first working fluid 19 into a power-producing expander EX, thus obtaining a flow of the first expanded and cooled working fluid 20, 15) heating said flow of the first expanded and cooled working fluid 20 by means of a first low-temperature heat source H1, thus obtaining the flow of a first working fluid in the gaseous phase 1 of step 1).

[0036] According to an aspect of the present invention, during step 1), said flow of a first working fluid 1 is mainly in the gaseous phase.

[0037] According to a particular aspect of the present invention, during step 3), the first condensed fraction 3 is obtained by mixing the liquid fraction of the cooled flow 2 with said second working fluid 2MF contained in the first tank V1.

[0038] According to an aspect of the present invention, during step 4) said first condensed portion 7 obtained is not completely condensed but only partially condensed.

[0039] According to another aspect of the present invention, during step 6), the second condensed fraction 8 is obtained by mixing the liquid fraction of said first partially condensed portion 7 with said third working fluid 3MF contained in the second tank V2.

[0040] According to an aspect of the present invention, during step 11), said third pumped and higher-temperature condensed fraction 16 may not be condensed.

[0041] For the purposes of the present invention, steps 13) and 15) are carried out using a low-temperature heat source.

[0042] In particular, low-temperature heat sources may be: ambient air, sea water, low-temperature solar thermal, exhausted heat of a low-temperature thermodynamic cycle, heat recovery from processes and / or low-temperature machinery, where "sea water" refers not only to pumped sea water suitably treated to remove sediments, but, more generally, to environmental water, obtained from rivers, canals, wells, natural basins such as lakes, etc. and artificial basins.

[0043] In general, a low-temperature source is a source which has a temperature approximately between 0-55°C.

[0044] When reference is made to a flow having a "higher" temperature or a "higher" pressure, it means that such flow has been subjected to a heating or pumping step which has increased the temperature, or the pressure thereof compared to before.

[0045] With particular reference to the diagram shown in Figure 2, a flow 1 of a first working fluid is cooled during a heat exchange step.

[0046] The cooling determines the condensation of the heavy fraction 3 of said first working fluid, which is separated inside the first tank V1, in which the second working fluid 2MF is contained.

[0047] Heavy fraction means the liquid fraction with a chemical composition similar to IMR, whereas "similar" means a molecular weight variation of approximately more or less 40%.

[0048] Said heavy fraction 3 is pumped by a first pump P1, thus obtaining a pumped heavy fraction 4, which, following a heating step, forms a first mixing portion for the first working fluid 5.

[0049] The same cooling also determines the separation of a first gaseous portion 6, which is subjected to a further heat exchange step, from which, in particular, a further cooled first gaseous portion 7 is obtained.

[0050] The cooling determines the further condensation of the heavy fraction 8, which is separated inside the second tank V2, containing liquefied natural gas LNG as the third working fluid 3MF.

[0051] Said heavy fraction 8 is pumped by a second pump P2, thus obtaining a pumped heavy fraction 9, which is subjected to a first heating, providing a preheated flow 10, and to a second heating, forming a second mixing portion for the first working fluid 11.

[0052] The same cooling also determines the separation of a second gaseous portion 12, which is subjected to an even further cooling step, thus obtaining a final condensed portion 13.

[0053] Said final condensed portion 13 is sent to a third tank V3 containing liquefied natural gas (LNG) as a third working fluid 3MF.

[0054] The outflow from the third tank V3 is then subjected to a first heat exchange step and a subsequent second heat exchange step, thus obtaining a third mixing portion for the first working fluid 17.

[0055] With regard to the third mixing portion for the first working fluid 17, this is then mixed inside a mixer (M in Figure 2) with the above-mentioned first and second mixing portions (corresponding to the flows 5 and 11, respectively, in Figure 2).

[0056] The working fluid obtained from the mixing inside the mixer M 18 (LIMR) is then subjected to a heating step using a second low-temperature heat source (H2 in Figure 2), thus obtaining a heated flow 19, which is subsequently expanded in an expander (EX in Figure 2) with production of energy.

[0057] Following the expansion, the expanded flow 20 is subjected to a heating step using a first low-temperature heat source (H1 in Figure 2), thus obtaining an expanded and heated flow 1.

[0058] Such expanded and heated flow 1 may therefore re-enter the cycle described above as the first working fluid 1MF.

[0059] For the purposes of the present invention, the flow of liquefied natural gas 40 is gasified in thermal exchanges with one or more of the flows described above.

[0060] More specifically, the flow of liquefied natural gas 40 is subjected to: a first heat exchange corresponding to step 7) indicated above, thus obtaining a flow of partially regasified liquefied natural gas 41; a second heat exchange corresponding to step 4) and to steps 10) and 11) described above, thus obtaining a flow of further regasified liquefied natural gas 42; a third heat exchange corresponding to steps 1), 9), 10) and 11) described above, thus obtaining a flow of completely regasified natural gas 43.

[0061] According to an embodiment of the present invention, shown, for example, in Figure 3, during the first heat exchange step a third low-temperature heat source H3 may be further involved, which, by means of an intermediate carrier fluid, heats: - the further heated flow of natural gas 42 (after a second heating step), the first pumped condensed fraction 4, the second pumped and higher-temperature condensed fraction 10, the third pumped and higher-temperature condensed fraction 16.

[0062] As shown in the diagram of Figure 3, in fact, the flow of a carrier fluid 50 is pumped by a fourth pump P4, thus obtaining a flow at greater pressure 51.

[0063] Such higher-pressure flow 51 releases calories during the first heat exchange step, thus obtaining a cooled flow of carrier fluid 52.

[0064] From the above description, the advantages offered by the process of the present invention are immediately apparent to the person skilled in the art.

[0065] In particular, such process exploits low-temperature heat sources, thereby meaning sources at room temperature, such as, for example, sea water or natural or artificial reservoirs.

[0066] With respect to the process and the power cycle described in patent application WO 2020 / 075112, the power cycle of the present invention has the same efficacy and the same system simplicity.

[0067] Advantageously, the cycle of the present invention may also operate at low temperatures.

[0068] Since it does not require a fuel, the process described above does not depend on a non-renewable energy source and does not involve the production of carbon dioxide.

Claims

1. A process for gasifying liquefied natural gas (LNG) and for generating power that comprises the steps of: 1) subjecting a flow of a first working fluid 1MF in the gaseous phase (1) to a first cooling step, thus obtaining a cooled flow (2) of said first working fluid 1MF, 2) sending said cooled flow (2) of said first working fluid 1MF to a first tank V1 containing a quantity of a second working fluid 2MF, 3) separating from said first tank V1 a first condensed fraction (3) and a first gaseous portion (6), 4) subjecting said first gaseous portion (6) to a second cooling step, thus obtaining a first condensed portion (7), 5) sending said first condensed portion (7) to a second tank V2 containing a quantity of a third working fluid 3MF, 6) separating from said second tank V2 a second condensed fraction (8) and a second gaseous portion (12), 7) subjecting said second gaseous portion (12) to a third cooling step, thus obtaining a condensed final portion (13), 8) sending said condensed final portion (13) to a third tank V3 containing a quantity of said third working 3MF, thus obtaining a third condensed fraction (14), 9) subjecting said first condensed fraction (3) to a pumping step, thus obtaining a first pumped condensed fraction (4), and to a heat exchange step, thus obtaining a first portion for mixing said working fluid (5), 10) subjecting said second condensed fraction (8) to a pumping step, thus obtaining a second pumped condensed fraction (9), to a first heat exchange step, thus obtaining a second high-pressure and higher-temperature condensed fraction (10), and to a second heat exchange step, thus obtaining a second portion for mixing said working fluid (11), 11) subjecting said third condensed fraction (14) to pumping, thus obtaining a third pumped condensed fraction (15), to a first heat exchange, thus obtaining a third pumped and higher-temperature condensed fraction (16), and to a second heat exchange, thus obtaining a third portion for mixing said working fluid (17), 12) mixing said first (5), second (11) and third (17) portions for mixing said first working fluid in a mixer M, thus obtaining an initial flow of said first working fluid (18), 13) heating said initial flow of the first working fluid (18) by means of a second low-temperature heat source H2 thus obtaining a heated flow of said first working fluid (19), 14) expanding said heated flow of the first working fluid (19) into a power-producing expander EX, thus obtaining a flow of the first expanded and cooled working fluid (20), 15) heating said flow of the first expanded and cooled working fluid (20) by means of a first low-temperature heat source H1, thus obtaining the flow of a first working fluid in the gaseous phase (1) of step 1), wherein: - during step 7) a heat exchange is carried out with a flow of liquefied natural gas (40), thus obtaining a flow of partially regasified natural gas (41), - during steps 4), 10) and 11) a heat exchange b) is carried out with said flow of partially regasified natural gas (41), thus obtaining a flow of further regasified natural gas (42), - during steps 1), 9), 10) and 11) a third heat exchange is carried out with said flow of further regasified natural gas (42), thus obtaining a completely regasified flow (43), and wherein: said second working fluid 2MF is a fluid obtained by adding a quantity of liquefied petroleum gas (LPG) to a quantity of liquefied natural gas (LNG) in a quantity such that 0.25 up to 1.2 volumes of liquefied petroleum gas (LPG) is added to said 1 volume of liquefied natural gas, wherein said third working fluid 3MF is liquefied natural gas (LNG).

2. A process according to the preceding claim, wherein low-temperature heat sources are employed during steps 13) and 15).

3. A process according to claim 1 or 2, wherein during said heat exchange step of step 1) a third low-temperature heat source (H3) is further employed.

4. A process according to the preceding claim, wherein the heat exchange with said third low-temperature heat source (H3) is indirect by means of a carrier fluid.

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