Boil-off gas reliquefaction method and apparatus

Abstract

A method for reliquefaction of BOG containing a volatile fraction is proposed: a BOG stream is compressed in at least two compression stages and the BOG stream out of the last compression stage is a last compressed BOG stream. The last compressed BOG stream is condensed and introduced into a fluid containing vessel. The introduced fluid stream is cooled to the saturation temperature of the fluid stream at a pressure lower than the final pressure, wherein a liquid stream out of the fluid containing vessel is used as a coolant and the intermediate pressure between at least two compressors is used. Thereafter, the coolant stream is introduced into the BOG stream between the first compression stage and the last compression stage. If a residual amount of liquid coolant is still contained in the coolant stream, this residual amount is evaporated before entering the subsequent compression stage. The cooled fluid stream is forwarded when the liquid level measured in the fluid containing vessel is at least equal to a desired liquid level and / or when the measured final pressure is equal to a limit final pressure. The invention also relates to an apparatus for performing the method.

Description

Technical Field

[0001] The present invention relates to a method for reliquefying BOG (evaporated gas), particularly a volatile fraction such as ethane, and apparatus for performing said method. Background Technology

[0002] BOG is evaporated liquid gas, such as evaporated LNG (liquefied natural gas) or evaporated LPG (liquefied petroleum gas). Liquid gas, and thus BOG, is typically a mixture of substances having different evaporation temperatures. BOG is caused by the unavoidable heat input into liquid gas tanks (hereinafter referred to as tanks), where the liquid gas is either stored on land, transported, for example, on ships, or carried as fuel for its own consumption and / or enters pipelines in which the liquid gas flows. Recently, the proportion of volatile components in liquid gas has been increasingly determined. Therefore, LPG tank loading (e.g., commercial propane) with increased ethane content, ranging from 5% molar to 8% molar in the tank liquid, has become common. This causes long-known reliquefaction methods during transport to no longer be able to condense the increased proportion of volatile components, resulting in so-called non-condensable fractions.

[0003] For environmental and economic reasons, especially to reduce hydrocarbon emissions during normal operation and to minimize loading losses, BOG reliquefaction is desirable.

[0004] During the reliquefaction process, when the load vapor... The problem of non-condensable fractions in distillates has long been known. Several solutions have been developed on the market:

[0005] When the amount of non-condensable substances is low (e.g., residual nitrogen from tank flushing), venting to the atmosphere during the first operating time is an effective remedy. This is not applicable when the non-condensable components are a regular part of the load, resulting in an excessive amount of gas to be vented.

[0006] The residual gas condenser is a heat exchanger that carries non-condensable gas fractions from the condenser via an automatically or manually operated vent valve. This heat exchanger cools the gas mixture, which is almost at the final pressure of the compressor, to a temperature level close to the tank's saturation temperature. This condenses most of the high-boiling hydrocarbons and enriches the remaining small amount of BOG and non-condensable gases. This is an effective means of reducing load loss. Because the condensate recondensed at storage pressure is typically used as a cooling medium in this process, this method significantly reduces the refrigeration power available for cooling the tank. The method can be applied when the concentration of volatile load components is low and can also be used to recover loads during product changes.

[0007] - Additionally, cascade cooling is used. Many vessels—especially semi-cooled vessels—are designed to transport ethane / ethylene as a pure load. This occurs in a cooling cascade with a refrigerant system that provides temperature levels down to -40°C for condensation. This also applies to handling any type of commercial LPG at pressure levels achievable through two-stage compression. The disadvantage of this method is the high equipment and machinery costs associated with the additional cascade facilities.

[0008] - In recent years, so-called "exhaust gas coolers" have been developed, such as those known from WO 2012 / 143699 A1. Here, reliquefaction is carried out by means of a two-stage reliquefaction process using seawater as the coolant in the cargo condenser. In principle, the development described in WO 2012 / 143699 A1 is similar to a residual gas condenser. The technological advantage lies in the fact that the temperature level is related to the intermediate pressure between the two compression stages, rather than to the tank pressure, which is typically sufficient to condense the cargo. Simultaneously, the refrigeration capacity of the facility is less drastically reduced compared to a residual gas condenser.

[0009] - A method proposed in DE 10 2013 101 414A1 involves feeding tank liquid into the fuel economizer. This approach utilizes the fact that tank liquid has a significantly lower concentration of volatile components compared to boil-off gas (BOG). Feeding the fuel economizer with tank liquid instead of condensate from the condenser reduces the concentration of volatile components in the BOG stream flowing from the fuel economizer to the second compression stage, thus enabling the two-stage compressor system to operate even under hot water conditions. A disadvantage is that the tank liquid must be pressurized for filling, which complicates the process and requires additional equipment.

[0010] However, these known two-stage reliquefaction processes also present difficulties when handling loads with an increased proportion of volatile components such as ethane. Ethane is a more volatile component and its concentration in the BOG is much higher than its concentration in the liquid phase of the liquid gas, hereinafter also referred to as bulk liquid.

[0011] The use of three-stage compressors is also known: by using three-stage compressors, it is possible to raise the condensing temperature at the outlet pressure to a level easily achievable under world trade conditions and even for warm seawater. The obvious drawback of this approach is the high investment cost of investing in more demanding equipment.

[0012] Ethane contents of 2.5%, 5%, and 8% in bulk liquids are standard loading specifications designed for LPG systems. For larger LPG systems, IMO Type A tanks with tank operating pressures between 0 and 0.4 bar g are typically important. The BOG composition produced from the given ethane contents requires increased condensation pressures at the provided temperature levels. As already mentioned above, the use of a two-stage reciprocating compressor and seawater in LPG reliquefaction is current technology. For global use, seawater at 32°C is considered. However, considerably warmer conditions prevail in many ports and important trading areas.

[0013] Clearly, standard compressor configurations cannot handle ethane concentrations above approximately 3.5% in bulk liquids under all environmental conditions. In these cases, the standard approach involves installing a vent valve on the LPG condenser that can purge the portion of the gas that cannot be condensed at the available pressure / temperature combination.

[0014] The following examples illustrate a typical case:

[0015] In a fully cooled tank condition (1 bar a), the BOG concentration of ethane for a 5% ethane load is approximately 26%. At 36°C, this mixture can be processed without problems by a two-stage compressor with a maximum delivery pressure of 21 bar a.

[0016] At a condensation temperature of 40°C (due to warm seawater or a contaminated heat exchanger), approximately 3% (molar) of the BOG remains in the vapor phase. This amount is highly sensitive to slight fluctuations in the composition of the tank liquid. Therefore, an increase in the ethane content, for example, by only a very low 5.5%, would raise this share to 14%.

[0017] During normal operation, the gas is either vented into the atmosphere, which not only means the undesirable release of greenhouse gases but also the loss of the cargo, or it is returned to the tank as vapor, thus significantly reducing the available cooling capacity of the refrigeration system. Summary of the Invention

[0018] In contrast, the object of the present invention is to provide a method and an apparatus by which BOG with a high proportion of volatile components can also be reliquefied, and the method and apparatus are particularly economical in this regard.

[0019] According to the invention, this objective is achieved by the method according to the invention and the apparatus according to the invention. The invention also includes ships equipped with the apparatus according to the invention.

[0020] According to the method of the present invention, BOG with a high content of volatile components can be reliquefied with relatively little consumption.

[0021] This invention is based on the knowledge that if a desired liquid level is preset for a fluid container used to contain partially condensed fluid and a maximum final pressure, i.e., an ultimate final pressure, is preset for the final pressure, then the liquid phase fraction in the partially condensed fluid can be increased in a simple manner, and the fluid is released from a cooling device connected downstream of the fluid container only in relation to reaching or exceeding the desired liquid level or reaching the ultimate final pressure, and the fluid is cooled in the cooling device at a temperature preset in relation to the final pressure.

[0022] By pre-setting a maximum ultimate final pressure for the final compression stage, it is possible to tolerate BOG—if it contains a volatile component (e.g., a high ethane content) or if the final compression pressure must be further increased due to high condensation temperatures (e.g., due to warm water or a contaminated condenser)—not being compressed to the final pressure necessary for complete condensation of all volatile components in the subsequent condenser. This results in an increase in the gas phase fraction of the partially condensed fluid, causing a drop in the liquid level in the fluid containment vessel. Because a desired liquid level is determined, and the upper edge of the fluid outlet of the fluid containment vessel is at or below this desired liquid level by a predetermined amount, only liquid fluid flows to the cooling device until the liquid level drops below the upper edge of the fluid outlet. Furthermore, because the actuator, based on the measurement of the liquid level, also only transfers the cooling fluid flow until below the desired liquid level, it is ensured that only liquid is transferred by the actuator until the ultimate final pressure is reached.

[0023] When the actuator is off, the fluid flow accumulates and flows back, and the fluid in the cooling device is further cooled. Due to the accumulation and backflow, the liquid level in the fluid container rises again. As the gas phase fraction in the BOG flow continues to increase, the final pressure also increases. With the increase in final pressure, the condensation temperature also increases, that is, condensation is achieved in the negative temperature range by means of a less cold coolant. Upon reaching the appropriately preset ultimate final pressure, it is therefore feasible for the gas phase of the fluid to be completely condensed by the relatively "warm" coolant in the cooling device, at least completely condensing most of the volatile components of the fluid. Therefore, the actuator is turned on again when the ultimate ultimate pressure is reached, even if the desired liquid level in the fluid container has not been reached again, and due to complete or at least significant condensation, a completely or largely liquid fluid flow exits from the cooling device.

[0024] As the volatile fraction in the BOG decreases again, the vapor fraction in the partially condensed fluid also decreases, and the liquid level in the fluid container continues to rise while the final compression pressure decreases again. If the final compression pressure is below the ultimate final pressure, the actuator shuts off and remains shut off until the liquid level reaches the desired level again. This prevents fluid with a significant vapor fraction from being transferred from the coolant. The actuator only reopens when the liquid level reaches or exceeds the desired level. This reopening can be performed in a continuous control loop.

[0025] According to the invention, BOG is compressed in at least two stages and used as a coolant for the partially condensed fluid flow of reliquefied BOG from the fluid reservoir. The cooling device has a heat exchanger in which the pressure level corresponds to the intermediate pressure level at the junction with the BOG flow between the first and final compression stages. The coolant outlet of the heat exchanger is flowably connected to the BOG flow between the first and final compression stages, and the coolant inlet of the heat exchanger is flowably connected to the fluid reservoir via a throttle valve. Therefore, the reliquefied BOG entering the heat exchanger from the fluid reservoir at the final compression pressure is depressurized and cooled at its cross-flow throttle valve. If the final compression pressure reaches the ultimate final pressure, the gaseous fluid entering the heat exchanger from the fluid reservoir is under particularly high pressure, while the pressure drop and temperature drop of the reliquefied BOG entering the heat exchanger are particularly large, causing the gaseous fluid to completely or at least nearly completely condense in the heat exchanger, and the depressurized coolant to evaporate.

[0026] The measure according to the invention allows the entire dose of refrigeration required for the cooling fluid flow delivered from the fluid containment vessel to be directed through the heat exchanger. This has several positive effects:

[0027] The refrigerant's inlet temperature is always its flash temperature.

[0028] All of the refrigerant is involved in heat exchange.

[0029] Counter-current flow can be selected in the heat exchanger. Because the refrigerant and the substance mixture have a significant temperature difference between the dew point and the boiling point, this counter-current flow results in a lower outlet temperature compared to a fluid flow that causes subcooling. Therefore, the cooling efficiency of the entire process is increased.

[0030] Therefore, the method according to the invention also achieves the reliquefaction of BOG containing volatile components with low consumption.

[0031] Preferably, the actuator is a valve. The transfer of the fluid flow for cooling in the cooling device can be controlled at low cost by the valve. Here, the valve can be part of the cooling device and can be directly installed at its fluid outlet. However, the valve can also be installed in a fluid outlet line that is flowably connected to the fluid outlet of the heat exchanger. Furthermore, it is conceivable that the valve is part of a liquid gas tank or consumer into which the cooling fluid flow should be introduced.

[0032] Another consideration is that the actuator is a volumetric delivery device, such as a turbine, which is then, for example, speed-controlled and stops the flow of cooling fluid at “zero” speed.

[0033] In an advantageous embodiment of the invention, the compressed BOG sub-stream is separated from the BOG stream between the first condenser and the condenser connected to the final compression stage, and mixed with the coolant stream leaving the heat exchanger, causing any residual liquid coolant that may still be present in the coolant stream to evaporate. This ensures that no liquid enters one or more compressors or compressors and damages them.

[0034] Therefore, a droplet separator is preferably provided, having a coolant inlet flowably connected to the coolant outlet of the heat exchanger and a BOG stream inlet flowably connected to a BOG sub-stream of the BOG stream flowing between a first condenser and a condenser connected to the final compression stage. The droplet separator also has a mixing outlet from which a mixture of evaporated coolant and the BOG sub-stream flows, and this mixing outlet is flowably connected to a connection point of the BOG stream between the first and final compression stages. Superheated, compressed gas is guided through the droplet separator. Heat is introduced in this way, causing the residual amount to evaporate. These measures achieve an efficient and extremely low-cost evaporation device for the possible residual amount of liquid in the coolant stream leaving the heat exchanger.

[0035] Particularly preferably, a droplet separator level sensor is provided in the droplet separator for measuring the liquid level in the droplet separator, and a throttle valve is configured to reduce the coolant volume flow when the measured liquid level exceeds a preset maximum liquid level and increase the coolant volume flow when the measured liquid level is below the preset maximum liquid level. This ensures that, on the one hand, the residual amount of liquid in the coolant flow leaving the heat exchanger does not become excessive, while on the other hand, sufficient coolant is delivered to the heat exchanger to ensure adequate cooling of the fluid flow. This creates the possibility of stable regulation: a low fill level or liquid level can be adjusted within a constant evaporation rate generated by the heat input in the droplet separator. Therefore, the refrigerant injected into the heat exchanger is slightly more than the refrigerant required to cool the fluid flow.

[0036] The magnitude of the coolant volume flow entering the heat exchanger can also be controlled by temperature measurement and the creation of a selected temperature difference:

[0037] Based on the first possibility

[0038] 1.1 Measure the outlet temperature of the coolant stream leaving the heat exchanger;

[0039] 2.1 Measure the temperature of the coolant before depressurization;

[0040] and

[0041] If the outlet temperature from 1.1 is lower than the coolant temperature from 2.1 by a preset amount, then increase the amount of coolant entering the heat exchanger; and

[0042] If the outlet temperature from 1.1 is lower than the coolant temperature from 2.1 by a greater than preset amount, then reduce the amount of coolant entering the heat exchanger.

[0043] Preferably, the temperature of the liquid from step 2 is measured in the fluid container. A temperature sensor can be installed there at a relatively low cost, since a level sensor for measuring the liquid level in the fluid container is always provided there.

[0044] According to the second possibility 2.1 (as described above in 1.1) for regulating the size of the coolant volume flow entering the heat exchanger, the outlet temperature of the coolant flow leaving the heat exchanger is measured;

[0045] 2.2 Measure the temperature of the coolant entering the heat exchanger after its pressure decreases to an intermediate pressure; and

[0046] If the outlet temperature from 2.1 exceeds the refrigerant temperature from 2.2 by a predetermined amount, then reduce the amount of refrigerant entering the heat exchanger; and

[0047] If the outlet temperature from 2.1 exceeds the coolant temperature from 2.2 by a preset amount, then increase the amount of coolant entering the heat exchanger.

[0048] According to the third possibility 3.1, measure the outlet temperature of the cooled fluid flow leaving the heat exchanger at its final pressure;

[0049] 3.2 (as described in 2.2 above) Measure the temperature of the coolant entering the heat exchanger after its pressure has decreased to an intermediate pressure;

[0050] and

[0051] If the outlet temperature from 3.1 exceeds the coolant temperature from 3.2 by a less than preset amount, then reduce the amount of coolant entering the heat exchanger; and

[0052] If the outlet temperature from 3.1 exceeds the coolant temperature from 3.2 by a preset amount, then increase the amount of coolant entering the heat exchanger.

[0053] If the heat exchanger is equipped with a corresponding excess capacity, the evaporated refrigerant can travel through any of the three possibilities described above under superheated conditions sufficient for complete evaporation. This allows for a minimized refrigerant charge when injected into the heat exchanger, and eliminates the need to address any liquid transfer into the droplet separator during normal operation.

[0054] In other words, by measuring the temperature, excessive refrigerant can be avoided from being injected into the heat exchanger during normal controlled operation. Evaporated refrigerant can become severely overheated. This reduces the amount of refrigerant required in the heat exchanger and reduces the resulting intermediate pressure in the facility or equipment. Consequently, this leads to increased cooling capacity.

[0055] When the droplet separator is combined with the regulation of the coolant volume flow into the heat exchanger by means of temperature measurement or temperature difference based on at least one of the three possibilities mentioned above, a preset value for regulating the temperature difference can be selected such that it causes liquid to perforate into the droplet separator. Liquid perforation can then be automatically corrected, for example, simply via cascaded regulation or override regulation. If this method is further combined with guiding a sub-volume of hot gas from the compressed, uncondensed BOG stream through the droplet separator, then the system automatically adapts to different product and load conditions.

[0056] Advantageously, the throttle valve is configured to increase the coolant volume flow when a preset limit temperature for the final compressed BOG flow, i.e., before it enters the condenser, is reached or exceeded. This ensures that the final compressed BOG flow can reach its preset ultimate pressure, and that the final compression pressure is not limited by the preset limit temperature.

[0057] Accordingly, the final or limiting temperature of the ultimately compressed BOG stream can also be correlated with the desired value of refrigerant superheat or fluid flow subcooling, for example, through cascaded or override control. This ensures that more refrigerant can be injected into the heat exchanger and the final temperature reduced if needed.

[0058] A particularly advantageous aspect is that the liquid flow is drawn from the fluid reservoir at the bottom of the reservoir and introduced into the heat exchanger as a coolant. This ensures, in a simple way, that no gaseous fluid enters the coolant circuit.

[0059] Preferably, the final compressed BOG stream is condensed in a condenser using seawater, as this is particularly low-cost. Attached Figure Description

[0060] The invention will now be described in more detail by way of example with reference to the single accompanying drawing.

[0061] Figure 1 An embodiment of the device according to the present invention is illustrated as a flowchart. Detailed Implementation

[0062] According to the device 1 of the present invention Figure 1 The embodiment shown includes a compressor 2, a condenser 3, a fluid container 4, a cooling device 5, an evaporator 6a, and an actuator 7, which is configured as a valve and is disposed in a fluid outflow line 8.

[0063] According to Figure 1 In this embodiment, the compressor 2 is configured in two stages. The first compression stage 9 has an inlet 10 for the BOG stream 11 to be compressed. This inlet 10 can, for example, be flowably connected to the gas phase region of a liquid gas tank. The first compression stage 9 compresses the BOG stream 11 to an intermediate pressure. The second compression stage is the final compression stage 12 and compresses the intermediately compressed BOG stream 11 to the final pressure.

[0064] The final pressure is related to the composition of the mixture of substances constituting BOG stream 11 and increases with the proportion of volatile components in the mixture of substances or BOG stream 11.

[0065] The ultimate pressure is determined as the maximum and decisive final pressure for operating actuator 7, i.e., the valve.

[0066] The final compression stage 12 has an outlet 13 for the final compressed BOG stream 11, which is in flow connection to the BOG stream inlet 14 of the condenser 3. In the condenser 3, the final compressed BOG stream 11 is cooled at a preset temperature independent of the final pressure. Therefore, the condenser 3 can be, for example, seawater cooled.

[0067] Therefore, in BOG stream 11 with volatile components, it is feasible that the determined ultimate pressure is insufficient to condense all the volatile components of BOG stream 11 at the existing condenser temperature, so that BOG stream 11 is only partially condensed.

[0068] Hereinafter, the BOG flow exiting the condenser 3 will generally be referred to as fluid flow 11a, because the BOG flow may contain liquid and / or gaseous components. Therefore, the associated outlet is referred to as fluid flow outlet 16.

[0069] The fluid outlet 16 of the condenser 3 is in fluid connection 18 with the fluid inlet 17 of the fluid container 4.

[0070] The fluid container 4 has a fluid outlet 19, which is located above a preset fluid holding volume 20 of the fluid container 4 and is in flow connection 22 with the fluid inlet 21 of the cooling device 5.

[0071] In the fluid container 4, the gas and liquid phases of the fluid are divided into a lower liquid phase region 23 and an upper gas phase region 24. The desired liquid level 25 is fixed at or above the upper edge of the fluid outlet 19 at a predetermined distance for the fluid container 4.

[0072] A level sensor 26 for measuring the liquid level is also provided in the fluid container 4. The measurement signal is forwarded to the actuator or valve control device 7a, by means of which the actuator 7 or valve can be put into an open position or a closed position in the fluid outflow line 8 downstream of the cooling device 5.

[0073] The cooling device 5 has a heat exchanger 27 having an inlet 28 and an outlet 29 for the coolant 30, an inlet 31 for the fluid flow 11a, and an outlet 32 ​​for the cooled fluid flow 11b.

[0074] The fluid inlet 31 of the heat exchanger 27 is fluidly connected 22 to the fluid outlet 19 of the fluid container 4, and the fluid outlet 32 ​​of the heat exchanger 27 is fluidly connected to the fluid outflow pipe 8. The fluid flow 11a is cooled to a lower temperature in the heat exchanger 27, the temperature corresponding to the saturation temperature of the fluid flow 11a at a pressure below the final pressure.

[0075] The reliquefied BOG is used as coolant 30. Fluid container 4 is in... Figure 1 The embodiment shown additionally has a bottom outlet 33, which thereby forms a second outlet of the fluid containment container 4, more precisely for BOG only for reliquefaction, i.e. only for liquid flow.

[0076] The bottom outlet 33 is connected to the coolant inlet 28 of the heat exchanger 27 via a delivery pipe 34. A delivery valve 35 is provided in the delivery pipe 34, which can be used to change the volumetric flow rate of the coolant entering the heat exchanger 27, thus acting as an adjustable throttle valve.

[0077] The measured temperature difference can be used as an adjustment variable for the magnitude of the coolant volume flow.

[0078] Therefore, the device has multiple temperature sensors:

[0079] The first temperature sensor 36 is used to measure the outflow temperature T1 of the coolant stream leaving the heat exchanger 27.

[0080] The second temperature sensor 37 is used to measure the temperature T2 of the coolant 30 before its pressure is reduced. The third temperature sensor 38 is used to measure the temperature T3 of the coolant 30 entering the heat exchanger 27 after its pressure is reduced to an intermediate pressure.

[0081] The fourth temperature sensor 39 is used to measure the outlet temperature T4 of the cooled fluid flow 11b that leaves the heat exchanger 27 and is under final pressure.

[0082] The possibility of regulating the volumetric flow of coolant entering heat exchanger 27 by means of a throttle valve thus provides, for example, the following:

[0083] First possibility:

[0084] When the outlet temperature T1 is lower than the coolant temperature T2 by a preset amount, increase the coolant volume flow; and

[0085] When the outlet temperature T1 is lower than the coolant temperature T2 by a predetermined amount, the coolant volume flow is reduced.

[0086] Second possibility:

[0087] When the outlet temperature T1 exceeds the coolant temperature T3 by a preset amount, reduce the coolant volume flow; and

[0088] When the outlet temperature T1 exceeds the coolant temperature T3 by a preset amount, the coolant volume flow is increased.

[0089] Third possibility:

[0090] When the outlet temperature T4 exceeds the coolant temperature T3 by a preset amount, reduce the coolant volume flow; and

[0091] When the outlet temperature T4 exceeds the coolant temperature T3 by a preset amount, the coolant volume flow is increased.

[0092] Each of the three possibilities can be used alone to regulate the coolant volume flow or in combination with one or two other possibilities.

[0093] The coolant volume flow should be regulated such that the coolant flow is fully evaporated when leaving the heat exchanger 27 while adequately cooling the fluid flow in the heat exchanger.

[0094] All three possibilities can also be linked to further regulation of the coolant volume flow, which is based on further temperature measurements.

[0095] The device 1 also includes a fifth temperature sensor 40, which measures the temperature T5 of the final compressed BOG stream 11, i.e., before the final compressed BOG stream enters the condenser 3, wherein the throttle valve is configured to increase the coolant volume flow when the temperature T5 reaches or exceeds a preset limit temperature for the final compressed BOG stream 11.

[0096] The measurement signals from temperature sensors T1-T5 are forwarded to the delivery valve control device 35a, which allows the delivery valve 35 to enter the desired throttling position in order to set the desired variable of the coolant volume flow into the heat exchanger 27.

[0097] The coolant outlet 29 of the heat exchanger 27 is in a flow connection 42 with the coolant inlet 41 of the evaporator 6a, which in the illustrated embodiment is configured as a droplet separator 6.

[0098] The droplet separator 6 also has a BOG flow inlet 43, which in the illustrated embodiment is in flow connection 45 with the first compression stage 9 and through which a sub-flow 46 of the intermediate compressed, hot BOG flow 11 is introduced into the droplet separator 6.

[0099] Thus, in the droplet separator 6, the coolant stream from the heat exchanger 27 mixes with the hot gas from the first compression stage 9 at an intermediate pressure. If a residual amount of liquid coolant is required in the coolant stream from the heat exchanger 27, this residual amount evaporates through the hot gas.

[0100] If complete evaporation is temporarily not feasible due to, for example, excessive coolant volume flow, then liquid coolant is collected in droplet separator 6, such that a liquid phase region 47 is formed in the lower region of droplet separator 6 and a gas phase region 48 is formed in the upper region.

[0101] All of the above possibilities for regulating the coolant volume flow based on the measured temperatures T1 to T5 can also be associated with further regulation of the coolant volume flow based on measurements of the liquid level or fill level in the droplet separator 6.

[0102] Therefore, a droplet separator level sensor 49 for measuring the liquid level in the droplet separator 6 is provided in the droplet separator 6. Therefore, the throttle valve, i.e., the delivery valve 35, is configured for...

[0103] - When the measured liquid level exceeds the preset maximum liquid level, reduce the coolant volume flow; and - when the measured liquid level is below the preset maximum liquid level, increase the coolant volume flow.

[0104] The measurement signal from the droplet separator level sensor 49 is forwarded to the delivery valve control device 35a, which allows the delivery valve 35 to enter a desired throttling position in order to set the desired volumetric flow of coolant into the heat exchanger 27.

[0105] The gas phase region 48 of the droplet separator 6 has a mixing outlet 50 from which a mixture consisting of evaporated coolant and hot BOG sub-stream exits and is in flow connection 51 with the BOG stream 11 between the first compression stage 9 and the final compression stage 12, or in the illustrated embodiment, with the BOG stream inlet 52 of the second or final compression stage 12. Therefore, on the one hand, evaporated coolant, i.e., gaseous BOG, can be introduced from the droplet separator 6 into the BOG stream 11 between the first compression stage 9 and the final compression stage 12. On the other hand, the intermediate pressure present at the connection or inlet 53 between the first compression stage 9 and the final compression stage 12 also dominates in the droplet separator 6 and thus in the heat exchanger 27. The boundary between the final pressure and the intermediate pressure in the delivery line 34 is a throttling valve, i.e., a delivery valve 35. In the illustrated case of a two-stage compressor, the intermediate pressure corresponds to the compression pressure of the first compression stage 9.

[0106] In the illustrated embodiment, actuator 7 is a valve disposed in a fluid outflow line 8, which is connected to the fluid outlet 32 ​​of heat exchanger 27. Hereinafter, this valve will be referred to as the outlet valve. The valve can be moved to an open position and a closed position and is operated as follows:

[0107] From the outlet of the final compression stage 12 of compressor 2, the BOG flow 11—referred to as fluid flow 11a from the outlet of condenser 3—is at its final pressure. This final pressure is measured by means of a pressure sensor 54, which is located at any point in a region extending from the outlet of the final compression stage 12 of compressor 2 to the outlet valve in the fluid outflow line 8 downstream of heat exchanger 27 and is at its final pressure. The pressure sensor 54 can be located, for example, between the final compression stage 12 and condenser 3 or in the fluid container 4. The measurement signal is forwarded to an outlet valve control device 7a, by means of which the outlet valve in the fluid outflow line 8 can be moved to an open or closed position, wherein in the open position, the reliquefied BOG is delivered to a further application, such as being introduced into a liquid gas tank.

[0108] In other words, upstream of the outlet valve, the fluid flows 11a, 11b or the finally compressed BOG flow 11 are under the final pressure, that is, at most under the preset ultimate final pressure.

[0109] Thus, when the coolant 30, which is at its final pressure or ultimate final pressure, passes through the delivery valve 35 to the heat exchanger 27, the coolant 30 is depressurized to an intermediate pressure and cooled accordingly.

[0110] Because the fluid flow 11a from the fluid container 4 to the heat exchanger 27 is under high pressure at the ultimate pressure and the temperature level in the heat exchanger 27 is relatively low, the fluid flow 11a is cooled to a temperature close to the saturation temperature of the fluid flow 11a at an intermediate pressure, causing the gas phase fraction of the fluid flow 11a to be condensed in this state and continue to be transferred only or approximately only to the reliquefied BOG, for example, by opening the outlet valve in the fluid outflow line 8 (at the ultimate pressure), for example, by discharging it into a tank.

[0111] Once the final pressure drops below the ultimate final pressure, the outlet valve closes again until the desired liquid level 25 is reached again in the fluid containment container 4 and the outlet valve reopens.

[0112] The position of the actuator or outlet valve is controlled by means of measurement signals from the level sensor 26 and the pressure sensor 54, more specifically, in the following manner:

[0113] A) Open location

[0114] a) When the liquid level corresponds to at least 25% of the desired liquid level

[0115] and / or

[0116] b) When the final pressure reaches the ultimate final pressure.

[0117] The actuator or outlet valve in the fluid outflow line 8 is in the open position.

[0118] Case a)

[0119] Because the upper edge of the fluid outlet 19 of the fluid container 4 is at or below the desired liquid level 25 by a preset amount, when the desired liquid level 25 is reached, only the fluid from its liquid phase, i.e. only the reliquefied BOG, flows into the heat exchanger 27 and continues to flow into the fluid outflow pipe 8.

[0120] Scenario b)

[0121] As the ultimate pressure is reached, the fluid is under relatively high pressure, causing the temperature to cool below the saturation temperature of the fluid flow at the ultimate pressure, even if the cooling is only slight—for example, 1°K—resulting in a significant further condensation of the gaseous components of fluid flow 11a and the fluid flow 11a leaving the heat exchanger 27 being nearly entirely liquid or even just liquid.

[0122] B) Closed position

[0123] When the liquid level drops to 25 below the desired level and the final pressure is below the ultimate final pressure...

[0124] The actuator or outlet valve in the fluid outflow line 8 enters its closed position.

[0125] If the proportion of uncondensed BOG increases (e.g., because of increased volatile components in the BOG or because the seawater 55 has become hotter in the case of a seawater-cooled condenser 3), then the gas phase proportion in the fluid increases (and thus the liquid phase proportion decreases) and eventually the pressure increases.

[0126] If the liquid level is lower than the desired level 25 and subsequently continues to drop below the upper edge of the fluid outlet 19 of the fluid container 4, then the boundary between the gas phase region 24 and the liquid phase region 23 of the fluid is initially located in the region of the fluid outlet 19 of the fluid container 4. In this case, the mixture of gas and liquid leaves the fluid container 4 and enters the heat exchanger 27.

[0127] If the liquid level drops to such that the fluid outlet 19 is completely within the gas phase region 24 of the fluid container 4, then only the gaseous BOG leaves.

[0128] Because the actuator or outlet valve is closed downstream of the heat exchanger 27 in the fluid outflow line 8, the volumetric pressure of the fluid flows backward, causing the liquid level in the fluid container 4 to rise again. Although the partially condensed BOG flow leaving the condenser 3 and entering the fluid container 4 contains an increased gas phase fraction, it always contains a liquid phase fraction.

[0129] As already mentioned above, on the one hand, the pressure eventually rises as the proportion of uncondensed components in the BOG increases, and on the other hand, the liquid level in the fluid container 4 rises, causing it to reach at least one of the two states described above as A)a) and A)b) again over time, and the actuator or outlet valve opens again.

[0130] The measures for reliquefying BOG according to the present invention are as follows: Figure 1 The embodiments shown are further illustrated with reference to several examples. In these examples, the BOG to be reliquefied is drawn from a liquid gas tank for propane and condenser 3 is cooled by seawater. The liquid and gas compositions, as well as the pressure and temperature ratios given in the various method steps / equipment components below, are based on flash memory calculations using NIST (National Institute of Standards and Technology) data:

[0131] a) In the liquid gas tank for extracting BOG for reliquefaction.

[0132] Liquid: Propane

[0133] Ethane content 5% molar BOG: Ethane content approximately 26% molar pressure: 1 bar a

[0134] b) In the two-stage compressor 2, the BOG stream contains approximately 26% mol of ethane.

[0135] Intermediate pressure: 5 bar

[0136] Final pressure: 21 bar

[0137] For the final compressed BOG stream 11 leaving compressor 2, with an ethane content of approximately 26% mol and a pressure of 21 bar a, the temperature at which complete condensation occurs is approximately 25°C.

[0138] c) In condenser 3

[0139] On the coolant side:

[0140] The seawater at a temperature of 32°C 55 condenses at a temperature of approximately 40°C due to the heat input in condenser 3.

[0141] BOG stream 11 was therefore only partially condensed.

[0142] On the gas / condensate side:

[0143] Pressure (final pressure): 21 bar

[0144] The incoming BOG stream 11 contains approximately 26% molar ethane.

[0145] The condensed fluid flow 11a that leaves the area:

[0146] (Larger, approximately 97% molar BOG) liquid fraction (condensate):

[0147] Ethane content approximately 25% molar (smaller, approximately 3% molar BOG) of uncondensed gas fraction:

[0148] Ethane content approximately 45% molar d) Liquid fraction (condensate): Ethane content approximately 25% molar Gas fraction: Ethane content approximately 45% molar

[0149] Pressure (final pressure): 21 bar ae) in heat exchanger 27

[0150] On the coolant side:

[0151] In the delivery line 34, upstream of the delivery valve 35, the condensate has an ethane content of approximately 25% and a final molar pressure of 21 bar.

[0152] In the delivery line 34 downstream of the delivery valve 35, i.e. in the heat exchanger 27, the pressure (intermediate pressure) is 5 bar a (pressure decreasing from the final pressure to the intermediate pressure).

[0153] Condensate: Temperature approximately -6.5℃

[0154] The ethane content is approximately 8% molar (the values ​​for the stated temperature and ethane content occur because a portion of the ethane evaporates due to the pressure drop, thus increasing the propane content in the condensate).

[0155] On the fluid side:

[0156] In the gaseous fraction: ethane content is approximately 45% molar.

[0157] Pressure (final pressure): 21 bar a (gas fraction completely liquefied)

[0158] With an ethane content of approximately 45 mol% in the gas fraction on the fluid side and a coolant side temperature of approximately -6.5°C, the saturation pressure of the gas fraction on the fluid side is approximately 10 bar a. Since the final pressure on the fluid side is 21 bar a, the gas fraction in the fluid is completely liquefied.

Claims

1. A method for reliquefying BOG, wherein the BOG comprises a volatile component, the method comprising the following steps: a) Compressing the BOG stream (11) in at least two compression stages, wherein the BOG stream (11) exits from the final compression stage (12) as a final compressed BOG stream (11) with final pressure; b) The final compressed BOG stream (11) is condensed to obtain a final compressed fluid stream (11a) that is at least partially condensed; c) Provide a fluid container (4) having a fluid inlet (17) and a fluid outlet (19), wherein the position of the fluid outlet (19) is selected such that the fluid outlet is higher than a preset fluid holding volume (20); d) The fluid flow (11a) from step b) is introduced into the fluid container (4) through the fluid inlet (17); e) Determine the desired liquid level (25) of the fluid container (4) such that the desired liquid level (25) is at the height of the upper edge of the fluid outlet (19) or above it by a preset measure. f) Determine the final pressure limit for the final compression stage (12); g) Measure the liquid level in the fluid container (4); h) Measure the final pressure; i) A fluid flow (11a) is drawn out from the fluid container (4) through the fluid outlet (19); j) Cooling the fluid flow (11a) from step i) to a temperature corresponding to the saturation temperature of the fluid flow (11a) at a pressure less than the final pressure, in order to condense the gaseous portion of the fluid flow (11a), wherein the cooling includes the following steps: j1) provides a heat exchanger (27); j2) The fluid flow (11a) from step i) is introduced into the heat exchanger (27); j3) The liquid flow from the fluid container (4) is introduced into the heat exchanger (27) as coolant (30) via the throttle valve (35); j4) A coolant flow is drawn from the coolant outlet (29) of the heat exchanger (27); j5) A flow connection is established between the coolant outlet (29) of the heat exchanger (27) and the BOG stream (11) located between the first compression stage (9) and the final compression stage (12) to set a pressure in the heat exchanger (27) corresponding to an intermediate pressure present at the connection point (53) to the BOG stream (11) that is lower than the final pressure, so that the coolant (30) entering the heat exchanger (27) is reduced from the final pressure to the intermediate pressure to evaporate at least partially; j6) The coolant stream from step j4) is introduced into the BOG stream (11) located between the first compression stage (9) and the final compression stage (12); j7) If the coolant stream from step j4) still contains a residual amount of liquid coolant, then the residual amount is evaporated before entering the subsequent compression stage; k) When the measured liquid level is at least equal to the desired liquid level (25) and / or when the measured final pressure is equal to the ultimate final pressure, the cooled fluid flow is transferred from the heat exchanger (27).

2. The method according to claim 1, Its features are, In step j7), a compressed BOG sub-stream (46) is separated from the BOG stream (11) located between the first compression stage (9) and the condenser (3) connected to the final compression stage (12) and mixed with the coolant stream from step j4), such that any possible residual amount of liquid coolant is evaporated.

3. The method according to claim 2, Its features are, Step j7) also includes the following steps: j7.1) Provides a droplet separator (6); j7.2) The coolant stream from step j4) is introduced into the droplet separator (6); j7.3) The BOG sub-stream from claim 2 is introduced into the droplet separator (6); j7.4) A mixture consisting of BOG sub-stream and evaporated coolant is drawn out from the droplet separator (6); and Step j6) includes introducing the mixture from step j7.4) into the BOG stream (11) located between the first compression stage (9) and the final compression stage (12).

4. The method according to claim 3, Its features are, Step j7) also includes the following steps: j7.5) Determine the maximum liquid level for the droplet separator (6); j7.6) Measure the liquid level in the droplet separator (6); j7.7) When the measured liquid level reaches or exceeds the maximum liquid level, the amount of the introduced coolant (30) is reduced in step j3).

5. The method according to any one of claims 1 to 4, Its features are, Step j3) also includes the following steps: j3.1a) Measure the outlet temperature T1 of the coolant flow from step j4); j3.2a) Measure the temperature T2 of the coolant (30) before its pressure decreases; j3.3a) When the outflow temperature T1 from step j3.1a) is lower than the coolant temperature T2 from step j3.2a) by a preset amount, increase the amount of coolant (30) entering the heat exchanger (27); j3.4a) When the outflow temperature T1 from step j3.1a) is lower than the coolant temperature T2 from step j3.2a) by a preset amount, reduce the amount of coolant (30) entering the heat exchanger (27).

6. The method according to claim 5, Its features are, In step j3.2a), the temperature of the liquid in the fluid container (4) is measured.

7. The method according to any one of claims 1 to 4, Its features are, Step j3) also includes the following steps: j3.1b) Measure the outlet temperature T1 of the coolant flow from step j4); j3.2b) Measure the temperature T2 of the coolant (30) entering the heat exchanger (27) after its pressure reaches the intermediate pressure; j3.3b) When the outflow temperature T1 from step j3.1b) exceeds the coolant temperature T2 from step j3.2b) by a preset amount, reduce the amount of coolant (30) entering the heat exchanger (27). j3.4b) When the outflow temperature T1 from step j3.1b) exceeds the coolant temperature T3 from step j3.2b) by a preset amount, the amount of coolant (30) entering the heat exchanger (27) is increased.

8. The method according to any one of claims 1 to 4, Its features are, Step j3) also includes the following steps: j3.1c) Measure the outlet temperature T4 of the cooled fluid flow leaving the heat exchanger (27) at final pressure from step k); j3.2c) Measure the temperature T3 of the coolant (30) entering the heat exchanger (27) after its pressure reaches the intermediate pressure; j3.3c) When the outflow temperature T4 from step j3.1c) exceeds the coolant temperature T3 from step j3.2c) by a less than preset amount, reduce the amount of coolant (30) entering the heat exchanger (27). j3.4c) When the outflow temperature T4 from step j3.1c) exceeds the coolant temperature T3 from step j3.2c) by a preset amount, the amount of coolant (30) entering the heat exchanger (27) is increased.

9. The method according to any one of claims 1 to 4, Its features are, Step j3) also includes the following steps: j3.1d) Determine the limiting temperature of the BOG stream (11) used for final compression; j3.2d) Measure the temperature T5 of the finally compressed BOG stream (11); j3.3d) When the temperature T5 measured in step j3.2d) reaches or exceeds the limit temperature from step j3.1d), the amount of coolant (30) entering the heat exchanger (27) is increased.

10. The method according to any one of claims 1 to 4, Its features are, In step j3), the liquid flow is extracted from the fluid container at the bottom of the fluid container (4).

11. The method according to any one of claims 1 to 4, Its features are, In step b), condensation is achieved by means of seawater (55).

12. An apparatus for performing the method according to any one of claims 1 to 11, the apparatus having -At least one two-stage compressor (2), - The first compression stage (9) of the compressor has an inlet (10) for the BOG flow (11), and - The final compression stage (12) of the compressor finally compresses the BOG stream (11) to the final pressure and has a BOG stream outlet (13) for the final compression of the BOG stream (11). -Condenser (3) - The condenser has a BOG flow inlet (14) that is flowably connected to the BOG flow outlet (13) of the final compression stage (12), and - The condenser is configured to at least partially condense the finally compressed BOG stream (11) into a fluid stream (11a), and -The condenser has a fluid outlet; - A fluid-containing container (4), the fluid-containing container having - Fluid inlet (17), which is in flow connection with the fluid outlet of the condenser (3), - Fluid outlet (19), which is located above a preset fluid containment volume (20); - A level sensor (26) for measuring the liquid level in the fluid container (4), - A pressure sensor (54) for measuring the final pressure; - Cooling device (5), the cooling device -Having a heat exchanger (27), the heat exchanger - It has a fluid inlet that is in flow connection with the fluid outlet (19) of the fluid container (4), and - It has a fluid outlet from which the cooled fluid flows out, and it constitutes the fluid outlet of the cooling device (5). - It has a coolant inlet (28) which is in flow connection with the outlet (33) for liquid flow of the fluid container (4) via a throttle valve (35), wherein the liquid flow forms a coolant flow, and the throttle valve (35) is configured to limit the coolant volume flow to a preset amount. - It has a coolant outlet (29) that is in flow connection with the BOG stream (11) located between the first compression stage (9) and the final compression stage (12) to introduce evaporated coolant into the BOG stream (11) and to set a pressure in the heat exchanger (27) corresponding to an intermediate pressure present at the connection point (53) that is less than the final pressure, and - The cooling device is configured to cool the fluid flow (11a) to a temperature corresponding to the saturation temperature of the fluid flow (11a) when the pressure is less than the final pressure; -Actuator (7), - The actuator is in fluid connection with the fluid outlet (32) of the cooling device (5), and - When the measured liquid level is at least equal to the desired liquid level (25) and / or when the measured final pressure is equal to the preset ultimate final pressure, the actuator can enter the open position to transfer the cooled fluid flow, and - The actuator can otherwise enter a closed position, in which the actuator interrupts the flow of the cooled fluid.

13. The device according to claim 12, Its features are, The actuator (7) is a valve.

14. The device according to claim 12 or 13, Its features are, An evaporator (6a) is inserted between the coolant outlet (29) of the heat exchanger (27) and the connection (53), the connection leading to the BOG flow flowing between the first compression stage (9) and the final compression stage (12), the evaporator being configured to evaporate any residual amount of liquid coolant that may be present in the coolant flow.

15. The device according to claim 14, Its features are, The evaporation apparatus (6a) has a droplet separator (6), which has - The coolant inlet is in flow connection with the coolant outlet (29) of the heat exchanger (27). -BOG stream-inlet, which is in flow connection with the BOG sub-stream (46) of the BOG stream (11), the BOG sub-stream flowing between the first compression stage (9) and the condenser (3) connected to the final compression stage (12), - Mixed outlet (50), from which the mixture of the evaporated coolant and the BOG sub-stream exits, and the mixed outlet is in flow connection with the connection (53) leading to the BOG stream (11) between the first compression stage (9) and the final compression stage (12).

16. The device according to claim 15, Its features are, A droplet separator level sensor (49) is provided in the droplet separator (6) to measure the liquid level, and the throttle valve (35) is configured to... - When the measured liquid level exceeds the preset maximum liquid level, reduce the volumetric flow of the coolant, and - When the measured liquid level is lower than the preset maximum liquid level, increase the volumetric flow of the coolant.

17. The device according to claim 12 or 13, Its features are, - A first temperature sensor (36) is provided, which is used to measure the outflow temperature T1 of the coolant flow leaving the heat exchanger (27).

18. The device according to claim 12 or 13, Its features are, - A second temperature sensor (37) is provided, which is used to measure the temperature T2 of the coolant (30) before its pressure decreases.

19. The device according to claim 12 or 13, Its features are, - A third temperature sensor (38) is provided, which is used to measure the temperature T3 of the coolant (30) entering the heat exchanger (27) after its pressure drops to the intermediate pressure.

20. The device according to claim 12 or 13, Its features are, - A fourth temperature sensor (39) is provided, which is used to measure the outlet temperature T4 of the cooled fluid flow leaving the heat exchanger (27) at the final pressure.

21. The device according to claim 12 or 13, Its features are, - A first temperature sensor (36) is provided, which is used to measure the outlet temperature T1 of the coolant flow leaving the heat exchanger (27). - A second temperature sensor (37) is provided, which is used to measure the temperature T2 of the coolant (30) before its pressure decreases. The throttle valve (35) is configured to be used for, - When the outlet temperature T1 is lower than the coolant temperature T2 by a preset amount, increase the coolant volume flow; - When the outflow temperature T1 is lower than the coolant temperature T2 by a preset amount, the coolant volume flow is reduced.

22. The device according to claim 12 or 13, Its features are, - A first temperature sensor (36) is provided, which is used to measure the outlet temperature T1 of the coolant flow leaving the heat exchanger (27). - A third temperature sensor (38) is provided, which is used to measure the temperature T3 of the coolant (30) entering the heat exchanger (27) after its pressure drops to the intermediate pressure. The throttle valve (35) is configured to be used for, -When the outlet temperature T1 exceeds the coolant temperature T3 by a preset amount, reduce the coolant volume flow; - When the outflow temperature T1 exceeds the coolant temperature T3 by a preset amount, the coolant volume flow is increased.

23. The device according to claim 12 or 13, Its features are, - A third temperature sensor (38) is provided, which is used to measure the temperature T3 of the coolant (30) entering the heat exchanger (27) after its pressure drops to the intermediate pressure. - A fourth temperature sensor (39) is provided, which is used to measure the outlet temperature T4 of the cooled fluid flow leaving the heat exchanger (27) at its final pressure. The throttle valve (35) is configured to be used for, -When the outlet temperature T4 exceeds the coolant temperature T3 by a preset amount, reduce the coolant volume flow; - When the outflow temperature T4 exceeds the coolant temperature T3 by a preset amount, the coolant volume flow is increased.

24. The device according to claim 12 or 13, Its features are, - A fifth temperature sensor (40) is provided, which is used to measure the temperature T5 of the finally compressed BOG stream (11). -The throttle valve (35) is configured to increase the coolant volume flow when the temperature T5 measured by the fifth temperature sensor (40) reaches or exceeds a preset limit temperature for the final compressed BOG flow (11).

25. The device according to claim 12 or 13, Its features are, The outlet (33) for liquid flow of the fluid container (4) is formed in the bottom of the fluid container (4).

26. The device according to claim 12 or 13, Its features are, The coolant in the condenser (3) is seawater (55).

27. A ship having the equipment according to any one of claims 12 to 26.

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