Method and apparatus for re-liquefying boil-off gas stream obtained from liquefied gas in gas reservoir of floating storage unit

Abstract

The invention relates to a method of re-liquefying a boil-off gas stream obtained from liquefied gas in a gas reservoir (12) of a floating storage unit (1), comprising the steps of directing boil-off gas from the gas reservoir (12) to a gas compressor (14) and increasing the pressure of the boil-off gas, providing pressurized boil-off gas, the pressurized boil-off gas is directed to a first condenser (16), and at least a portion of the pressurized boil-off gas is reliquefied to form a first reliquefied and pressurized boil-off gas portion and a remainder of the pressurized boil-off gas, at least a portion of the remainder of the pressurized boil-off gas is reliquefied in a second condenser (18) to form a second reliquefied and pressurized boil-off gas portion. The second condenser (18) comprises a thermoelectric cooler (18.1, 18.2, 18.3), the first condenser (18) comprising a first re-liquefied and pressurized gas portion, thereby forming a second re-liquefied and pressurized gas portion, expanding a majority of the first re-liquefied and pressurized gas portion and the second re-liquefied and pressurized gas portion to a lower pressure, and introducing the re-liquefied gas portion into the gas reservoir (12), in which the second condenser (18) comprises a thermoelectric cooler (18.1, 18.2, 18.3), and the condensation power of the thermoelectric coolers (18.1, 18.2, 18.3) is controlled by controlling the electrical power input to the thermoelectric coolers (18.1, 18.2, 18.3).

Description

Technical Field

[0001] The invention relates to a method for reliquefying a boil-off gas stream obtained from liquefied gas in a gas storage tank of a floating storage unit according to the preamble of claim 1 .

[0002] The invention relates to an apparatus for reliquefying a boil-off gas stream obtained from liquefied gas in a gas storage tank of a floating storage unit according to the preamble of a further independent claim. Background Art

[0003] It is generally known to store gases in a liquefied state. Compositions that are gaseous under normal atmospheric conditions can be liquefied by lowering their temperature and / or increasing their pressure. Examples of liquefied gases include liquid air, liquefied ammonia, liquefied natural gas, and liquefied petroleum gas, but many other gases can also be liquefied separately. Liquefied gas storage can be land-based facilities or floating storage and regasification units.

[0004] During tank loading and storage of liquefied gases such as LPG, the liquefied gas can evaporate. So-called boil-off gas (BOG) is produced by the evaporation of gas from the liquid portion of the gas in the tank. The formation of BOG can excessively increase the pressure in the tank, necessitating pressure relief. While it is possible to vent the gas directly to the atmosphere or through expansion, this is not a sustainable approach to addressing this issue.

[0005] Therefore, there's a clear need to reliquefy the portion of boil-off gas that won't be used in gas-consuming devices. To prevent or minimize gas loss due to boil-off gas accumulation in the tank, a system can be provided to reliquefy the boil-off gas so that it can be returned to the storage tank in a liquefied state. This can be achieved by compressing and cooling the boil-off gas. In many systems, the compressed boil-off gas is cooled and reliquefied, utilizing seawater as a heat sink.

[0006] Document EP2702311 A1 discloses a method for cooling a boil-off gas stream from a liquefied cargo, in which the cargo itself serves as the working medium of the cooling system. The method comprises at least the following steps: compressing the boil-off gas stream from the liquefied cargo in two or more compression stages to provide a compressed boil-off gas exhaust stream; cooling the compressed boil-off gas exhaust stream to provide a cooled exhaust stream and a cooled compressed boil-off gas stream; optionally expanding a portion of the cooled compressed boil-off gas stream after further cooling to provide an expanded cooled boil-off gas stream; and heat exchanging the expanded cooled boil-off gas stream with the cooled exhaust stream to provide a further cooled exhaust stream. In this method, the cooling capacity depends on the pressure available after compression of the boil-off gas stream, so that increased capacity requires increased compressor power consumption, and / or on the cooling capacity of the heat exchange process, which typically relies on seawater. The capacity requirements of the reliquefaction system also vary for different gas compositions, so that increasing proportions of volatile components (e.g., light hydrocarbons) require higher pressure levels for condensation. Consequently, the capacity of water-based cooling can be limited in certain circumstances.

[0007] It is an object of the present invention to provide a method for reliquefying boil-off gas from a liquefied gas storage, wherein the liquefaction capacity is significantly improved compared to prior art solutions. Summary of the Invention

[0008] The objects of the invention may be substantially met as disclosed in the independent claim and the other claims describing more details of different embodiments of the invention.

[0009] According to the present invention, a boil-off gas stream obtained from liquefied gas in a gas storage tank of a floating storage unit is reliquefied, comprising the following steps:

[0010] directing boil-off gas from a gas storage device to a gas compressor and increasing the pressure of the boil-off gas to provide pressurized boil-off gas,

[0011] directing the pressurized boil-off gas to a first condenser and reliquefying at least a portion of the pressurized boil-off gas to form a first reliquefied and pressurized gas portion and a remaining portion of the pressurized boil-off gas,

[0012] reliquefying at least a portion of the remaining portion of the pressurized boil-off gas in a second condenser forming a second reliquefied and pressurized gas portion,

[0013] expanding the first and second reliquefied and pressurized gas portions to a lower pressure and directing a majority of the reliquefied gas portions back into the gas storage,

[0014] In the method, the second condenser includes a thermoelectric cooler. The thermoelectric cooler includes a first heat exchanger and a second heat exchanger, the remaining portion of the pressurized boil-off gas stream flows through the first heat exchanger and is at least partially reliquefied therein, the heat transfer medium flows through the second heat exchanger, and condensation of the thermoelectric cooler is controlled by controlling the electrical power input to the thermoelectric cooler.

[0015] This condensing mode of the device is less dependent on the available temperature of the heat transfer medium, as the thermoelectric cooler provides a lower operating temperature to the first heat exchanger in a controllable manner. As a result, excess boil-off gas is efficiently reliquefied and returned to the gas storage.

[0016] According to an embodiment of the invention, the first condenser comprises a heat exchanger which extracts heat from the pressurized boil-off gas and transfers it directly or indirectly to the seawater, thereby reliquefying at least a portion of the pressurized boil-off gas.

[0017] Since the method is applied in a floating storage unit, it is advantageous and cost effective to use seawater as a heat sink for the initial or first condensation stage to receive excess heat. In this way, a significant portion of the gas can be simply reliquefied using seawater.

[0018] According to an embodiment of the present invention, the second heat exchanger of the thermoelectric cooler transfers heat directly or indirectly to the seawater.

[0019] Using seawater to receive heat from a thermoelectric cooler is advantageous and cost-effective.

[0020] According to an embodiment of the present invention, the first condenser includes a heat exchanger that extracts heat from the pressurized boil-off gas and transfers it directly or indirectly to the seawater, thereby reliquefying at least a portion of the pressurized boil-off gas, and the second heat exchanger of the thermoelectric cooler transfers the heat directly or indirectly to the seawater.

[0021] Since the method is applied in a floating storage unit, using seawater as the heat sink for both the primary condensation stage and the secondary condenser is an advantageous and cost-effective approach, as a common piping system can be utilized. This makes the system simpler, saving materials and space in the floating storage.

[0022] According to an embodiment of the present invention, the method includes the following steps: separating the first reliquefied and pressurized gas portion from the remaining portion of the pressurized boil-off gas in a first gas-liquid separator downstream or in the first condenser, and directing the remaining portion of the pressurized boil-off gas separated from the first reliquefied and pressurized gas portion to a second condenser, and reliquefying at least a portion of the remaining portion of the pressurized boil-off gas to form the second reliquefied and pressurized gas portion, and expanding the first reliquefied and pressurized gas portion and the second reliquefied and pressurized gas portion separately, and after expansion to a lower common pressure, combining most of the first reliquefied gas portion and the second reliquefied gas portion, and directing the combined flow back to the gas storage.

[0023] In this embodiment, the results from the first condenser, i.e., the first reliquefied and pressurized gas portions from the remaining portion of the pressurized boil-off gas, are separated from one another, and only the remaining portion of the pressurized boil-off gas (i.e., the gas portion of the results from the first condenser) is directed to the second condenser. Thus, the capacity of the second condenser is not required for further cooling of the first reliquefied and pressurized gas portion and / or for arranging the flow of the already reliquefied portion through the second condenser, thereby avoiding unnecessary compression work, but the capacity of the second condenser is only used to condense the remaining portion of the pressurized boil-off gas.

[0024] According to an embodiment of the present invention, the step of increasing the pressure of the boil-off gas comprises two or more stages, and part of the pressurized boil-off gas is cooled between two compression stages in a liquid pool heat exchanger, and wherein at least one of the first reliquefied and pressurized gas part and the second reliquefied and pressurized part is cooled in the liquid pool heat exchanger before being expanded to a lower pressure, wherein a liquid-gas pool is formed at a bottom portion of the liquid pool heat exchanger, which receives heat from the cooled reliquefied and pressurized gas part.

[0025] This provides an efficient way of improving the compression process and reliquefaction of gas by exploiting the cooling potential of the reliquefaction and expansion of the pressurized gas.

[0026] According to an embodiment of the present invention, the first reliquefied and pressurized gas portion includes those components of the boil-off gas that can be reliquefied at the temperature of seawater under the prevailing pressure, and the thermoelectric cooler reliquefies the remaining components of the boil-off gas. This provides the effect of achieving partial liquefaction of the gas using only the cooling capacity of seawater.

[0027] According to an embodiment of the present invention, the second condenser is controlled by controlling the thermoelectric cooler, and the electric power input to the thermoelectric cooler is controlled by measuring the pressure of the gas upstream of the thermoelectric cooler, setting a target value as the pressure, comparing the measured value with the target value, and controlling the electric power input to the thermoelectric cooler based on a signal obtained using the measured value and the target pressure value.

[0028] The gas pressure upstream of the second condenser provides good feedback for controlling the second condenser, as gas condensation effectively affects the gas pressure. For example, the density of liquid propane is approximately 270 times that of gaseous propane at atmospheric pressure and temperature. Preferably, when operating, the compressor, preferably a piston compressor, operates at a constant flow rate, and the pressure of the compressed and reliquefied gas is controlled by controlling gas condensation in the second condenser.

[0029] The device according to the invention for reliquefying a boil-off gas stream obtained from liquefied gas in a gas storage of a floating storage unit comprises:

[0030] A gas compressor for increasing the pressure of the boil-off gas to provide pressurized boil-off gas,

[0031] a first condenser for reliquefying at least a portion of the pressurized boil-off gas,

[0032] a second condenser for reliquefying at least a portion of the remaining portion of the pressurized boil-off gas,

[0033] at least one valve for partially expanding the reliquefied and pressurized gas to a lower pressure,

[0034] The second condenser comprises a thermoelectric cooler comprising a first heat exchanger in which a remaining portion of the pressurized boil-off gas flow is arranged to be reliquefied and a second heat exchanger in which a heat receiving transfer medium is arranged to flow, and a power management system for supplying power to the thermoelectric elements of the thermoelectric cooler in a controllable manner.

[0035] With this arrangement, the condensing capacity is less dependent on the available temperature of the heat transfer medium, since the thermoelectric cooler provides a lower operating temperature to the first heat exchanger in a controllable manner. Thus, excess boil-off gas is efficiently reliquefied and returned to the tank.

[0036] According to an embodiment of the invention, the first condenser comprises a heat exchanger arranged to extract heat from the pressurized boil-off gas and transfer the heat directly or indirectly to the seawater, thereby reliquefying at least a portion of the pressurized boil-off gas.

[0037] Using seawater as a heat sink for the initial or first condensation stage to absorb excess heat is an advantageous and cost-effective means of allowing a substantial portion of the gas to be reliquefied simply using seawater.

[0038] According to an embodiment of the invention, the second heat exchanger of the thermoelectric cooler is arranged to transfer heat directly or indirectly to seawater.Using seawater to receive heat from the thermoelectric cooler is advantageous and cost effective.

[0039] According to an embodiment of the present invention, the first condenser includes a heat exchanger arranged to extract heat from the pressurized boil-off gas and transfer the heat directly or indirectly to seawater, thereby reliquefying at least a portion of the pressurized boil-off gas, and the second heat exchanger of the thermoelectric cooler is arranged to transfer the heat directly or indirectly to the seawater. Using seawater as a heat sink for both the initial condensation stage and the second condenser is an advantageous and cost-effective approach, as a common piping system can be utilized. This results in a simpler system, saving materials and space in the floating storage tank.

[0040] According to an embodiment of the invention, the apparatus comprises a first gas-liquid separator after the first condenser and a second gas-liquid separator after the second condenser, the first gas-liquid separator being arranged to separate the first reliquefied and pressurized gas portion from the remaining portion of the pressurized boil-off gas.

[0041] In this embodiment, the results from the first condenser, i.e., the first reliquefied and pressurized gas portion from the remaining portion of the pressurized boil-off gas, can be separated from each other, and only the remaining portion of the pressurized boil-off gas, i.e., the gas portion of the results from the first condenser, is directed to the second condenser. In this way, the capacity of the second condenser is not required for further cooling of the first reliquefied and pressurized gas portion and / or for arranging the flow of the already reliquefied portion through the second condenser, thereby avoiding unnecessary compression work, but the capacity of the second condenser is only used for condensing the remaining portion of the pressurized boil-off gas.

[0042] According to an embodiment of the present invention, the gas compressor includes two or more stages, and a gas cooler between two compression stages, and the gas cooler includes a liquid pool heat exchanger.

[0043] This provides an efficient way of improving the compression process and reliquefaction of gas by exploiting the cooling potential of the reliquefaction and expansion of the pressurized gas.

[0044] According to an embodiment of the invention, the pressure probe is arranged upstream of a thermoelectric cooler, which is arranged in data transmission communication with a power management system of the thermoelectric cooler.

[0045] A thermoelectric cooler is a solid-state semiconductor device that converts DC power into a temperature difference. When voltage is applied to a thermoelectric cooler, an electric current is generated. This current induces the Peltier effect, which causes heat to move from the cold side to the hot side.

[0046] In this context, the word "gas" may refer to both gaseous and liquid gases, and the state of the gas is referred to by describing the phase of the gas when necessary. It should also be understood that the word "gas" refers to a mixture of various components that may change over the reliquefaction process.

[0047] The exemplary embodiments of the invention presented in this patent application should not be interpreted as limiting the applicability of the appended claims. The verb "comprise" is used in this patent application as an open limitation that does not exclude the presence of features not yet recited. Unless explicitly stated otherwise, the features described in the dependent claims are freely combinable with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Hereinafter, the present invention will be described with reference to the accompanying exemplary schematic drawings, in which

[0049] Figure 1 shows an apparatus for reliquefying a boil-off gas stream according to an embodiment of the present invention,

[0050] Figure 2 FIG. 2 shows an apparatus for reliquefying a boil-off gas stream according to another embodiment of the present invention.

[0051] Figure 3 shows an apparatus for reliquefying a boil-off gas stream according to another embodiment of the present invention, and

[0052] Figure 4 An apparatus for reliquefying a boil-off gas stream according to another embodiment of the present invention is shown. DETAILED DESCRIPTION

[0053] Figure 1 A schematic diagram illustrates an apparatus 10 for reliquefying a boil-off gas stream according to an embodiment of the present invention. As is known in the art, boil-off gas is generated from liquefied gas stored in a gas reservoir, which may also be referred to as a tank 12. The figures depict an open-cycle reliquefaction system. The apparatus 10 is connected to the tank 12 such that a gas supply line 102, arranged to supply boil-off gas to the apparatus 10 and remove boil-off gas from the tank 12, leads to the upper portion of the tank 12, the so-called dead space, and a liquefied gas return line 210 is arranged to supply reliquefied gas from the apparatus 10 back to the tank 12.

[0054] The apparatus 10 includes a gas compressor 14 having at least two stages 14.1 and 14.2. The compressor is preferably a reciprocating piston compressor. Compressor 14 can be, for example, a piston compressor. Advantageously, a gas cooler 22 is disposed between compressor stages 14.1 and 14.2. The outlet of the first compressor stage 14.1 is connected to the gas cooler 22 via a first gas line 104, and the gas cooler 22 is connected to the inlet of the second compressor stage 14.2 via a second gas line 106. The apparatus includes a first condenser 16, which is connected to the outlet of the second compressor stage 14.2 via a third gas line 108. The first condenser 16 is a heat exchanger in which heat from the pressurized boil-off gas is preferably transferred directly or indirectly to seawater. The first condenser 16 can be referred to as a passive heat exchanger because heat transfer is based on convection on the surface and conduction through the heat exchanger material. Also disposed within the apparatus 10 is a second condenser 18, which is connected to the outlet of the first condenser 16 via a first condensed gas line 202. The second condenser 18 is connected to the gas-liquid separator 20 via a second condensed gas pipeline 204. Figure 1 As can be clearly seen in the figure, the compressor 14, the first condenser 16, and the second condenser 18 are sequentially connected upstream of the gas-liquid separator 20 along the flow direction of the gas. The gas-liquid separator 20 includes two outlets, one of which is connected to the exhaust gas pipeline 110 and the other is connected to the third condensate pipeline 206. The exhaust gas pipeline 110 is provided with a valve 32, which reduces the pressure of the exhaust gas before it is directed to the desired further processing. For clarity, the first condenser 16, the second condenser 18, and the gas-liquid separator 20 are shown as separate devices, but they can also be integrated into a polymerization device with corresponding functions.

[0055] Second condenser 18 includes thermoelectric coolers 18.1, 18.2, and 18.3. These operate according to the well-known Peltier effect. A first heat exchanger 18.1 removes heat from the gas stream and transfers it to thermoelectric element 18.2, while a second heat exchanger 18.3 removes heat from thermoelectric element 18.2 and transfers it to a heat transfer medium arranged to flow through second heat exchanger 18.3. Second heat exchanger 18.3 preferably transfers heat directly or indirectly to seawater.

[0056] In other words, the thermoelectric cooler can be considered an active heat exchanger, a controllable solid-state device, that transfers heat from the cooler's first heat exchanger 18.1 to the cooler's second heat exchanger 18.3. The thermoelectric cooler is provided with a power management system 34 for supplying power to the thermoelectric elements of the thermoelectric cooler in a controlled manner. By controlling the power input to the thermoelectric cooler, the heat transfer of the thermoelectric cooler can be controlled, and the condensation power of the gas condensed in the second condenser 18 can be controlled.

[0057] The gas stream is at least partially reliquefied in the first heat exchanger of the thermoelectric cooler 18, preferably a majority of the gas is liquefied, and the cooling and / or condensation of the first heat exchangers of the thermoelectric coolers 18.1, 18.2, 18.3 is controlled by controlling the power input to the thermoelectric coolers. Due to its controllable condensing power, the second condenser 18 actually completes the reliquefaction of the boil-off gas, although some small amount of residual gas may remain uncondensed.

[0058] The gas cooler 22 is described in more detail below. The gas cooler comprises a liquid pool heat exchanger, wherein a liquid-gas pool is formed at the bottom of the liquid pool heat exchanger. This pool receives heat from the first reliquefied and pressurized gas portion. The gas cooler 22 is configured to cool the partially pressurized boil-off gas between compressor stages 14.1 and 14.2. Another function of the gas cooler 22 is to cool the reliquefied gas before it is fed back to the tank 12. A heat transfer device 24, such as a coil or tube bundle, is provided in the lower portion of the gas cooler 22. A third condensation line 206 is connected to the heat transfer device 24 at its input, through which the reliquefied and pressurized gas is directed. The heat transfer device 24 is connected at its outlet to a liquefied gas return line 210, through which the reliquefied gas can be returned to the tank 12. The liquefied gas return line is provided with a valve 30 for expanding the reliquefied and pressurized gas portion to a lower pressure before returning to the tank 12.

[0059] The gas cooler 22 is a container having a space inside. The third condensation line 206 is provided with a branch line 208 which opens at its upper portion into the space inside the gas cooler 22. A valve 28 is arranged to the branch line 208, which causes the condensed, i.e., reliquefied, gas to expand before entering the space in the gas cooler 22. Due to the expansion to a lower pressure, the gas in the liquid phase at least partially evaporates and cools. Figure 1As can be clearly seen in the diagram, the pressure in the branch line needs to be reduced to approximately the pressure prevailing at the outlet of first compressor stage 14.1. The expanded reliquefied gas mixes in the container with the pressurized boil-off gas fed from first compressor stage 14.1 to the gas cooler, so the gas leaving gas cooler 22 to second compressor stage 14.2 is cooler than the gas from first compressor stage 14.1. The mass flow of gas fed to gas cooler 22 via branch line 208 is small compared to the mass flow returned to tank 12 via liquefied gas return line 210, and therefore, a large portion (i.e., greater than 85%) of the first and second reliquefied and pressurized gas portions is introduced back into gas storage 12. Due to the gas flow to the gas cooler via branch line 208, the gas flow through second compressor stage 14.2 is greater than the gas flow through first compressor stage 14.1.

[0060] Heat transfer device 24 is arranged to lower a portion of gas cooler 22 so that, in use, the heat transfer device is immersed in the liquefied gas. The liquefied gas in gas cooler 22 is at a lower temperature than the pressurized boil-off gas from liquefied gas return line 210 and gas-liquid separator 20 because it has been discharged to a lower pressure through valve 28 in branch line 208 and cooled to a lower pressure by expansion.

[0061] according to Figure 1 The apparatus for reliquefying the boil-off gas flow obtained from tank 12 in an embodiment operates as follows. According to the present invention, excess boil-off gas is efficiently reliquefied and returned to the tank. The apparatus according to the present invention is activated or its power is controlled, for example, based on the measured pressure in the tank. When the pressure in the tank exceeds a set limit or range, the apparatus is activated or its reliquefaction power is increased, allowing a portion of the boil-off gas to be reliquefied and returned to the tank in liquid form. During operation of the apparatus, boil-off gas is directed from gas storage tank 12 to gas compressor 14, where its pressure is increased, thereby providing pressurized boil-off gas. In this context, the term "pressurized" means that the pressure is increased from the level prevailing in tank 12. Although only two stages are shown, the compressor preferably includes two or more stages 14.1 and 14.2, wherein the boil-off gas pressure is increased in each stage. Furthermore, the boil-off gas is cooled between the compression stages by gas coolers 22, which may be referred to as intercooling.

[0062] Intercooling occurs in the open space of the gas cooler 22 so that the boil-off gas is allowed to mix with the expanded pressurized boil-off gas from the liquefied gas return line 210 and the gas-liquid separator 20, which cools the gas in the open space of the gas cooler 22. The expanded reliquefied gas mixes in the vessel with the pressurized boil-off gas fed into the gas cooler from the first compressor stage 14.1, so the gas leaving the gas cooler 22 to the second compressor stage 14.2 is cooler than the gas from the first compressor stage 14.1.

[0063] The pressurized boil-off gas is next directed from the compressor 14 to a first condenser 16 where the gas is at least partially condensed by utilizing, directly or indirectly, cooling energy obtained from the sea water. The portion of the condensed gas depends on the composition of the gas, the condensation temperature of the gas available in the first condenser 16 and the actual pressure of the boil-off gas. The condensation temperature is influenced by the temperature of the water in which the floating storage unit 1 floats and it is generally not possible to obtain a complete liquefaction of the gas using the available sea temperature levels. Since only a portion of the pressurized boil-off gas can be liquefied in the first condenser 16, the output of the first condenser comprises a first reliquefied and pressurized gas portion, which is in liquid form and the remaining portion of the pressurized boil-off gas is a mixture. Figure 1 In the illustrated embodiment, the mixture of the first reliquefied and pressurized gas portion and the remaining pressurized boil-off gas portion is directed to a second condenser 18, where a major portion of the remaining pressurized boil-off gas portion is condensed, thereby forming a second reliquefied and pressurized gas portion. In the second condenser 18, the remaining portion of the pressurized boil-off gas stream flows through a first heat exchanger 18.1 of the thermoelectric cooler, where the majority of the gas stream is reliquefied. A suitable heat transfer medium flows through a second heat exchanger 18.3, receiving heat from the pressurized boil-off gas stream. The condensing capacity of the thermoelectric cooler is controlled by controlling the electrical power input to the thermoelectric cooler. When electrical power is supplied to the thermoelectric cooler, the condensing capacity is significantly increased relative to the capacity available only with seawater as a heat sink. This offers advantages, particularly when the temperature of the water surrounding the unit 1 increases. This can be particularly problematic when operating the device 10 for extended periods of time in confined areas, such as ports.

[0064] Preferably, the electrical power input to the thermoelectric cooler is controlled by measuring the gas pressure upstream of the thermoelectric cooler using a pressure probe 26. A target value for the pressure upstream of the thermoelectric cooler is set, and the measured value is compared with the target value. The electrical power input to the thermoelectric cooler is controlled based on a signal obtained using the measured pressure value and the target pressure value.

[0065] Optionally, a second pressure probe 26' is arranged in the device 10, which is located adjacent to the inlet of the last compressor stage in the case of a compressor comprising several stages, or adjacent to the inlet of a single compressor in the case of a compressor comprising only one compressor stage. When the device is provided with pressure probes 26', 26 on both sides of the compressor, it is operated so that a pressure difference is measured across the last (or only) compressor stage 14.2. The measured difference is compared with a target value, and the electrical power input to the thermoelectric cooler is controlled based on a signal obtained using the measured pressure difference and the target pressure difference value. When the pressure difference reaches a predetermined value or above, power is supplied to the thermoelectric cooler, thereby controlling the compressor at the predetermined pressure difference. This feature is useful on existing ships with compressors that have pressure ratio restrictions compared to new compressors.

[0066] exist Figure 1 In this case, the first reliquefied and pressurized gas portion obtained from the first condenser and the second reliquefied and pressurized gas portion obtained from the second condenser 18 are mixed with each other in the second condensed gas line 204. The gas stream exiting the first heat exchanger 18.1 is directed to the gas-liquid separator 20, in which any uncondensed gas components in the reliquefied gas are separated from the reliquefied gas. Finally, the first and second reliquefied and pressurized gas portions are expanded to a lower pressure before being fed back to the tank 12, after which the reliquefied gas portions are introduced into the gas storage tank 12. The expansion is achieved by directing the reliquefied gas through the valve 30.

[0067] The first reliquefied and pressurized gas portion comprises those components of the boil-off gas which are capable of being reliquefied at the temperature of seawater under the prevailing pressure, and the thermoelectric cooler reliquefies a majority of the remaining components of the boil-off gas.

[0068] Figure 2 A device 10 for reliquefying a boil-off gas stream according to another embodiment of the present invention is schematically depicted.

[0069] and Figure 1 , the apparatus 10 is connected to the tank 12 such that the gas feed line 102 arranged for feeding boil-off gas to the apparatus 10 opens into an upper portion above the level of the liquefied gas in the tank 12, and such that the liquefied gas return line 210 is arranged for feeding reliquefied gas from the apparatus 10 back to the tank 12. Typically, Figure 2 Examples and Figure 1 The embodiment corresponds to the embodiment of FIG. 1 , but here, the first condenser 16 and the second condenser 18 are both provided with gas-liquid separators 20 and 20' at their downstream, and the reflux route of the liquefied gas is the same as that of FIG. Figure 1 The embodiments are different.

[0070] The apparatus 10 comprises a gas compressor 14 having at least two stages 14.1, 14.2. Figure 1 In the embodiment of FIG. 1 , a gas cooler 22 is arranged between the compressor stages 14.1 and 14.2. The outlet of the first compressor stage 14.1 is connected to the gas cooler 22 via a first gas line 104, and the gas cooler 22 is connected to the inlet of the second compressor stage 14.2 via a second gas line 106. The gas cooler 22 is similar to the one in the embodiment of FIG. Figure 1 The disclosed gas cooler, which also operates in a corresponding manner, is therefore not disclosed in more detail here and is combined with Figure 1 The disclosure of the gas cooler 22 itself is applicable to Figure 2 .

[0071] The apparatus 10 includes a first condenser 16, which is connected to the outlet of the second compressor stage 14.2 via a third gas line 108. The first condenser 16 is a passive heat exchanger, in which heat from the pressurized boil-off gas is preferably transferred directly or indirectly to the seawater. Following the first condenser 16 is a first gas-liquid separator 20'. The first gas-liquid separator 20' is arranged to separate the first reliquefied and pressurized gas portion from the remaining pressurized boil-off gas portion. The first gas-liquid separator 20' includes two outlets, one of which is connected to the fourth gas line 112 and one of which is connected to the third condensate line 206.

[0072] There is also a second condenser 18 arranged in the apparatus 10, which is connected to the outlet of the first gas-liquid separator 20' via a fourth gas line 112. The gas separated in the first gas-liquid separator 20' is introduced into the second condenser 18. The second condenser 18 is connected to the second gas-liquid separator 20' via a second condensed gas line 204. The second condenser 18 is similar to the one in the combined Figure 1 The disclosed condenser also operates in a corresponding manner, so it is not combined with Figure 2 Due to its controllable condensing power, the second condenser 18 practically completes the reliquefaction of the boil-off gas, although some insignificant amount of residual gas may remain uncondensed.

[0073] In this embodiment, the first condenser 16 condenses the gas at least partially by directly or indirectly utilizing cooling energy obtained from seawater. The portion of the gas condensed depends on the composition of the gas, the condensation temperature of the gas available in the first condenser 16, and the actual pressure of the boil-off gas. The condensation temperature is affected by the temperature of the water buoying the floating storage unit 1, and it is generally not possible to achieve complete liquefaction of the gas using available sea temperature levels. Since only a portion of the pressurized boil-off gas can be liquefied in the first condenser 16, the output of the first condenser includes a first reliquefied and pressurized gas portion, which is in liquid form and represents the remainder of the pressurized boil-off gas. The first reliquefied and pressurized gas portion is separated in the first gas-liquid separator 20' and passed to the third condensate line 206. The remainder of the pressurized boil-off gas is then directed to the second condenser 18 via the fourth gas line 112. In the second condenser 18, the majority of the remaining portion of the pressurized boil-off gas is condensed, forming a second reliquefied and pressurized gas portion. In the second condenser 18, the remaining portion of the pressurized boil-off gas stream supplied from the fourth gas line 112 flows through the thermoelectric cooler's first heat exchanger 18.1, where the majority of the gas stream is reliquefied. A suitable heat transfer medium, preferably in heat transfer communication with the seawater, flows through the second heat exchanger 18.3 and receives heat from the pressurized boil-off gas stream. The condensing power of the thermoelectric cooler 18 is controlled by controlling the power input to the thermoelectric cooler. When power is supplied to the thermoelectric cooler, the condensing capacity is significantly increased compared to the capacity available by utilizing only seawater as a heat sink. Furthermore, because the second condenser 18 receives and processes only the remaining portion of the pressurized boil-off gas from the first gas-liquid separator 20', its capacity is more efficiently utilized at lower flow rates.

[0074] The second condenser 18 is connected to a second gas-liquid separator 20 ″ via a second condensed gas line 204 . The second gas-liquid separator 20 ″ comprises two outlets, one of which is coupled to the exhaust gas line 110 and one of which is coupled to the fourth condensed gas line 212 . The exhaust gas line 110 is provided with a valve 32 which reduces the pressure of the exhaust gas before directing it to the desired further processing. During normal operation, the valve 32 is closed.

[0075] about Figure 2 The gas cooler 22 of the embodiment is connected with Figure 1In contrast to the embodiment of FIG. 1 , the heat transfer device 24 immersed in the liquefied gas at the lower portion of the gas cooler 22 is configured here to cool only the reliquefied gas condensed in the first condenser 16, which is directed from the first gas-liquid separator 20 to the heat transfer device 24. The heat transfer device 24 in the gas cooler 22 is connected from its outlet to a liquefied gas return line 210, through which the reliquefied gas can be returned to the tank 12. The third condensation line 206 is provided with a valve 30 on the downstream side of the heat transfer device 24 for expanding the reliquefied and pressurized gas portion to a lower pressure before returning to the tank 12. The fourth condensate line 212 is also connected to the gas return line 210, through which the reliquefied gas from the second gas-liquid separator 20" is returned to the tank 12. The fourth condensate line 212 is also provided with a valve 30', by which the second reliquefied and pressurized gas portion is expanded to a lower pressure before being combined with the flow from the third condensate line 206. The liquefied gas in the gas cooler 22 is at a lower temperature than the pressurized boil-off gas in the liquefied gas return line 210 and the first gas-liquid separator 20', because it has been discharged to a lower pressure through the valve 28 in the branch line 208.

[0076] according to Figure 2 An embodiment of the apparatus for reliquefying the boil-off gas stream obtained from tank 12 operates as follows. According to the present invention, excess boil-off gas is efficiently reliquefied and returned to the tank. The apparatus according to the present invention is activated, or its capacity is controlled, for example, based on the measured pressure in the tank. When the pressure in the tank exceeds a set limit or range, the apparatus is activated or its reliquefaction capacity is increased, allowing a portion of the boil-off gas to be reliquefied and returned to the tank in liquid form. During operation of the apparatus, boil-off gas is directed from gas storage tank 12 to gas compressor 14, where the pressure of the boil-off gas is increased, thereby providing pressurized boil-off gas. In this context, the term "pressurized" means that the pressure is increased from the level prevailing in tank 12. The compressor includes two or more stages 14.1 and 14.2, wherein the pressure of the boil-off gas is increased in each stage. Furthermore, the boil-off gas is cooled between the compression stages by gas coolers 22, which may be referred to as intercooling. Intercooling occurs in the open space of the gas cooler so that the boil-off gas is allowed to mix with the expanded pressurized boil-off gas from the liquefied gas return line 210 and the gas-liquid separator 20, which cools the gas in the open space of the gas cooler 22. The expanded reliquefied gas mixes in the vessel with the pressurized boil-off gas fed into the gas cooler from the first compressor stage 14.1, so the gas leaving the gas cooler 22 to the second compressor stage 14.2 is cooler than the gas from the first compressor stage 14.1.

[0077] The pressurized boil-off gas is next directed from the compressor 14 to a first condenser 16, wherein the gas is at least partially condensed by utilizing cooling energy obtained directly or indirectly from the seawater. The portion of the condensed gas depends on the composition of the gas, the condensation temperature of the gas available in the first condenser 16 and the actual pressure of the boil-off gas. The condensation temperature is influenced by the temperature of the water in which the floating storage unit 1 floats and it is generally not possible to obtain a complete liquefaction of the gas using the available sea temperature levels. Not all of the pressurized boil-off gas can be liquefied in the first condenser 16, the output of the first condenser comprising a first reliquefaction and pressurized gas portion, which is in liquid form and the remainder of the pressurized boil-off gas. Figure 2 In the illustrated embodiment, a mixture of the first reliquefied and pressurized gas portion and the remaining portion of the pressurized boil-off gas is directed to a first gas-liquid separator 20', where the first reliquefied and pressurized gas portion is separated from the remaining portion of the pressurized boil-off gas. The first reliquefied and pressurized gas portion comprises those components of the boil-off gas that are capable of reliquefying at seawater temperatures under the prevailing pressure, and a thermoelectric cooler reliquefies a majority of the remaining components of the boil-off gas. The first gas-liquid separator 20' can also be integrated with the first condenser, with the first reliquefied and pressurized gas portion separated from the remaining portion of the pressurized boil-off gas in the first gas-liquid separation chamber of the first condenser 16.

[0078] exist Figure 2 In the illustrated embodiment, the remaining portion of the pressurized boil-off gas is directed to a second condenser 18, where a substantial portion of the remaining portion of the pressurized boil-off gas is condensed to form a second reliquefied and pressurized gas portion. In the second condenser 18, the remaining portion of the pressurized boil-off gas flow is fed through a first heat exchanger 18.1 of a thermoelectric cooler, where a substantial portion of the gas flow is reliquefied. In the thermoelectric cooler, a substantial portion of the remaining portion of the pressurized boil-off gas is condensed, thereby forming a second reliquefied and pressurized gas portion.

[0079] A suitable heat transfer medium, preferably in heat transfer communication with the seawater, flows through the second heat exchanger 18.3 and receives heat from the pressurized boil-off gas stream. The condensing capacity of the thermoelectric cooler 18 is operated by controlling the electrical power input to the thermoelectric cooler. When electrical power is supplied to the thermoelectric cooler, the condensing capacity is significantly increased relative to the capacity available using only seawater as a heat sink.

[0080] Preferably, the electric power input to the thermoelectric cooler is controlled by measuring the gas pressure near the upstream side of the thermoelectric cooler using a pressure probe 26. A target value for the pressure upstream of the thermoelectric cooler is set, and the measured value is compared with the target value. The electric power input to the thermoelectric cooler is controlled based on a signal obtained using the measured pressure value and the target pressure value.

[0081] The gas containing condensate and gaseous gas is directed from the outlet of the second condenser to the second gas-liquid separator 20" via the second condensed gas pipeline 204. The second gas-liquid separator 20" includes two outlets, one of which is connected to the exhaust gas pipeline 110 and one of which is connected to the fourth condensation pipeline 212. The exhaust gas pipeline 110 is provided with a valve 32, which reduces the pressure of the exhaust gas before being directed to the desired further processing.

[0082] exist Figure 2 In the case of a condensed gas phase, the first reliquefied and pressurized gas portion (in the third condensed gas line 206) and the second reliquefied and pressurized gas portion (in the fourth condensed gas line 212) are preferably mixed with each other before being fed back into the tank. The first reliquefied and pressurized gas portion is directed to the heat transfer device 24 in the gas cooler 22 before being returned to the tank 12 via the liquefied gas return line 210. The first reliquefied and pressurized gas portion in the third condensed gas line 206 is expanded to a lower pressure on the downstream side of the heat transfer device 24 through the valve 30 before being returned to the tank 12.

[0083] The second reliquefied and pressurized gas portion is discharged from the second gas-liquid separator 20" via the fourth condensed gas line 212. The second reliquefied and pressurized gas portion is expanded to a lower pressure in the valve 30' and then returned to the tank 12. The fourth condensed gas line 212 is also connected to the gas return line 210, through which the reliquefied gas can be returned to the tank 12. After the streams are separately expanded in the valves 30, 30', the third condensed gas line 206 and the fourth condensed gas line 210 are joined to each other into the liquefied gas return line 210.

[0084] Figure 3 An embodiment of the present invention is disclosed which is similar in all respects to the embodiment of the present invention except that the compressor 14 includes only one stage. Figure 2 Therefore, no need Figure 2 The gas cooler 22 shown in FIG is omitted. The outlet of the compressor 14 is directly connected to the first condenser 16 via the third gas line 108. The third condensate line 206 and the fourth condensate line are connected to the liquefied gas return line 210 downstream of the valves 30', 30 provided in the condensate lines.

[0085] Figure 4 An optional variation of the invention is disclosed which is otherwise similar to the invention except that the heat exchanger 24 is connected to the fourth condensation line 212 instead of the third condensation line 206. Figure 2 In practice, this may not be as effective as Figure 2The configuration in is less attractive because the first condenser 16 is able to condense most of the pressurized boil-off gas. In some practical applications, both the fourth condensate line 212 and the third condensate line 206 can be alternatively arranged to be connectable to the branch line 208.

[0086] Although the present invention has been described herein by way of example in conjunction with what are currently considered to be the most preferred embodiments, it will be apparent to those skilled in the art that, as technology advances, the basic concept of the invention can be implemented in many different ways. Therefore, the present invention and its embodiments are not limited to the examples and samples described above, but rather may be varied within the scope of the patent claims and their legal equivalents. Details mentioned in conjunction with any of the above embodiments may be used in conjunction with another embodiment when such a combination is technically feasible.

[0087] Parts List

[0088] Floating Storage Unit 1

[0089] Apparatus 10 for reliquefying a boil-off gas stream

[0090] Gas storage tank 12

[0091] Gas compressor 14

[0092] Compressor stages 14.1, 14,2

[0093] First condenser 16

[0094] Second condenser 18

[0095] Thermoelectric coolers 18.1, 18.2, 18.3

[0096] First heat exchanger 18.1

[0097] Thermoelectric element 18.2

[0098] Second heat exchanger 18.3 gas-liquid separator 20

[0099] First gas-liquid separator 20'

[0100] Second gas-liquid separator 20" gas cooler 22

[0101] Heat transfer device 24

[0102] Pressure probe 26

[0103] Valve 28

[0104] Valve 30

[0105] Valve 30'

[0106] Valve 32

[0107] Power management system 34 Gas supply line 102 First gas pipeline 104 Second gas pipeline 106 The third gas pipeline 108 Exhaust gas line 110 Fourth gas pipeline 112 Liquefied gas reflux pipeline 201 First condensed gas pipeline 202 Second condensed gas pipeline 204 The third condensation line 206 Branch line 208 Liquefied gas reflux line 210 Fourth condensation line 212

Claims

1. A method for reliquefying a boil-off gas stream obtained from liquefied gas in a gas storage tank (12) of a floating storage unit (1), said method comprising the following steps directing boil-off gas from the gas reservoir (12) to a gas compressor (14) and increasing the pressure of the boil-off gas to provide pressurized boil-off gas, directing the pressurized boil-off gas to a first condenser (16) and reliquefying at least a portion of the pressurized boil-off gas to form a first reliquefied and pressurized gas portion and a remaining portion of the pressurized boil-off gas, reliquefying at least a portion of said remaining portion of said pressurized boil-off gas in a second condenser (18) to form a second reliquefied and pressurized gas portion, expanding the first reliquefied and pressurized gas portion and the second reliquefied and pressurized gas portion to a lower pressure and directing a majority of the reliquefied gas portion back to the gas storage (12), It is characterized in that The second condenser (18) comprises a thermoelectric cooler (18.1, 18.2, 18.3), the thermoelectric cooler comprising a first heat exchanger (18.1) and a second heat exchanger (18.3), the remaining part of the pressurized boil-off gas flow flowing through the first heat exchanger and at least partially reliquefied in the first heat exchanger, the heat transfer medium flowing through the second heat exchanger, and the condensing power of the thermoelectric cooler (18.1, 18.2, 18.3) being controlled by controlling the electrical power input to the thermoelectric cooler (18.1, 18.2, 18.3).

2. The method according to claim 1, characterized in that The first condenser (16) includes a heat exchanger that extracts heat from the pressurized boil-off gas and transfers the heat directly or indirectly to seawater, thereby reliquefying at least a portion of the pressurized boil-off gas.

3. The method according to claim 1 or 2, characterized in that The second heat exchanger (18.3) of the thermoelectric cooler (18.1, 18.2, 18.3) transfers heat to seawater directly or indirectly.

4. The method according to claim 1, wherein The method includes separating the first reliquefied and pressurized gas portion from the remaining portion of the pressurized boil-off gas in a first gas-liquid separator (20) located downstream of or in the first condenser (16), and directing the remaining portion of the pressurized boil-off gas separated from the first reliquefied and pressurized gas portion to a second condenser (18), and reliquefying at least a portion of the remaining portion of the pressurized boil-off gas to form the second reliquefied and pressurized gas portion, and separately expanding the first reliquefied and pressurized gas portion and the second reliquefied and pressurized gas portion and, after expansion to a lower common pressure, combining at least most of the first reliquefied gas portion and the second reliquefied gas portion, and directing the combined stream back to the gas storage (12).

5. The method according to claim 4, characterized in that The step of increasing the pressure of the boil-off gas comprises two compression stages, and cooling a portion of the pressurized boil-off gas between the two compression stages in a liquid pool heat exchanger, and wherein at least one of the first reliquefied and pressurized gas portion and the second reliquefied and pressurized gas portion is cooled in the liquid pool heat exchanger before expansion to a lower pressure, wherein a liquid-gas pool is formed at a bottom portion of the liquid pool heat exchanger, the liquid-gas pool receiving heat from the cooled reliquefied and pressurized gas portion.

6. The method according to claim 2, characterized in that The first reliquefied and pressurized gas portion comprises those components of the boil-off gas that are reliquefiable at the temperature of the seawater under the prevailing pressure, and the thermoelectric cooler (18.1, 18.2, 18.3) reliquefies the remaining components of the boil-off gas.

7. The method according to claim 1, characterized in that The electrical power input to the thermoelectric coolers (18.1, 18.2, 18.3) is controlled by - measuring the gas pressure upstream of said thermoelectric cooler (18.1, 18.2, 18.3), - setting a target value for the pressure, comparing a measured value with the target value, and controlling the electric power input to the thermoelectric cooler (18.1, 18.2, 18.3) based on a signal obtained using the target pressure value and the measured value.

8. The method according to claim 9, characterized in that The electrical power input to the thermoelectric coolers (18.1, 18.2, 18.3) is controlled by - measuring the gas pressure upstream of the last compressor stage and defining the pressure difference between the gas pressure upstream of the last compressor stage and the gas pressure upstream of said thermoelectric cooler (18.1, 18.2, 18.3), - setting a target value for the pressure difference, comparing a measured value with the target value, and controlling the electric power input to the thermoelectric cooler (18.1, 18.2, 18.3) based on a signal obtained using the target pressure difference and the measured value.

9. An apparatus for reliquefying a boil-off gas stream (10) obtained from liquefied gas in a gas storage tank (12) of a floating storage unit (1), the apparatus comprising a gas compressor (14, 14.1, 14.2) for increasing the pressure of the boil-off gas to provide pressurized boil-off gas, a first condenser (16)(16) for reliquefying at least a portion of the pressurized boil-off gas, a second condenser (18) (18) for reliquefying at least a portion of the remaining portion of the pressurized boil-off gas, at least one valve (30, 30') for partially expanding the reliquefied and pressurized gas to a lower pressure, It is characterized in that The second condenser (18) (18) comprises a thermoelectric cooler (18.1, 18.2, 18.3) and a power management system (34), wherein the thermoelectric cooler comprises a first heat exchanger (18.1) and a second heat exchanger (18.3), the remaining part of the pressurized boil-off gas flow being arranged to be reliquefied in the first heat exchanger and the heat receiving transfer medium being arranged to flow in the second heat exchanger, and the power management system is used to supply electric power to the thermoelectric element (18.2) of the thermoelectric cooler (18.1, 18.2, 18.3) in a controllable manner.

10. The device method according to claim 8, characterized in that: The first condenser (16) comprises a heat exchanger arranged to extract heat from the pressurized boil-off gas and transfer the heat directly or indirectly to seawater, thereby reliquefying at least a portion of the pressurized boil-off gas.

11. The device according to claim 8 or 9, characterized in that The second heat exchanger (18.3) of the thermoelectric cooler (18.1, 18.2, 18.3) is used to transfer heat to seawater directly or indirectly.

12. The device according to claim 8, characterized in that The apparatus comprises a first gas-liquid separator (20') after the first condenser (16) and a second gas-liquid separator (20") after the second condenser (18), the first gas-liquid separator being arranged to separate the first reliquefied and pressurized gas portion from the remaining portion of the pressurized boil-off gas.

13. The device according to claim 8, characterized in that The gas compressor (14) includes two or more stages and a gas cooler (22) located between the two compression stages, and the gas cooler (22) between the two stages includes a liquid pool heat exchanger.

14. The device according to claim 8, characterized in that A pressure probe is arranged upstream of the thermoelectric cooler (18.1, 18.2, 18.3), and the pressure probe is arranged for data transmission communication with the power management system (34).

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