Method and apparatus for reliquefaction of bog

Abstract

The invention relates to a method for reliquefying BOG, said BOG containing a volatile fraction, said method having the following steps: a) compressing a BOG stream, wherein said BOG stream is discharged from a final compression stage as a final compressed BOG stream having a final pressure; b) condensing said final compressed BOG stream in order to obtain an at least partially condensed final compressed fluid stream; c) providing a fluid receiving vessel having a fluid stream inlet and a fluid stream outlet, wherein the position of said fluid stream outlet is chosen such that it is located above a preset fluid receiving volume; d) introducing the fluid stream from step b) into said fluid receiving vessel through said fluid stream inlet; e) determining a desired liquid level for said fluid receiving vessel such that said desired liquid level is at the height of or exceeds by a preset measure the upper edge of said fluid stream outlet; f) determining an upper limit final pressure for said final compression stage; g) measuring the liquid level in said fluid receiving vessel; h) measuring said final pressure; i) conducting a fluid stream from the fluid receiving vessel through said fluid stream outlet; j) cooling the fluid stream from step i) to a temperature corresponding to the saturation temperature of said fluid stream at a pressure below said final pressure in order to condense the gaseous fraction of said fluid stream; k) diverting said cooled fluid stream if the measured liquid level is at least equal to said desired liquid level and / or if the measured final pressure is equal to said limit final pressure. The invention also relates to a device for carrying out said method.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for re-liquefying BOG (Boil Off Gas) which contains, inter alia, a volatile fraction, for example ethane, and to an apparatus for carrying out the method. BACKGROUND

[0002] BOG is boil-off liquefied gas. Liquefied gas and thus BOG is usually a mixture of substances having components whose evaporation temperatures are usually different from one another. BOG arises as a result of unavoidable heat input into the liquefied gas tank (hereinafter referred to as tank) in which the liquefied gas is either stored on land or transported, for example on a ship, or is carried as fuel for its own consumption and / or into the pipeline route in which the liquefied gas is flowing. In recent times, an increased fraction of volatile components has been determined more and more frequently in liquefied gas. As a result, LPG tanks (for example commercial propane) having an increased ethane content in the range from 5% mol in the tank liquid to 8% mol in the tank liquid have become widespread. This leads to the fact that the re-liquefaction methods in transport which have been known for a long time are no longer able to condense the volatile components of the increased fraction. A so-called non-condensable fraction arises.

[0003] For environmental protection and economic reasons, in particular in order to reduce the emission of hydrocarbon gases in normal operation and the loss of the load, the re-liquefaction of BOG is desirable.

[0004] The problem of the non-condensable fraction in the load vapor during the re-liquefaction process operation has been known for a long time. Various solutions have been established on the market:

[0005] - When the amount of non-condensable substances is small (for example, residual nitrogen gas from tank flushing), venting to the atmosphere at the first operating time is an effective remedy. If the non-condensable components are regular components of the load so that the amount of gas to be vented becomes too large, it cannot be used.

[0006] - The residual gas condenser is a heat exchanger which is supplied with the non-condensable gas fraction out of the condenser via an automatically or manually operated vent valve at the condenser. This heat exchanger cools the gas mixture almost at the final pressure of the compressor to a temperature level close to the saturation temperature of the tank. Thereby most of the high-boiling hydrocarbons are condensed and the non-condensable gases in the resulting small amount of BOG are more and more. This is an effective means for reducing the cargo loss. Since as cooling medium in this process usually liquefied condensate at storage pressure is used, the method significantly reduces the available cooling performance for cooling the tank. The method can be applied when the concentration of the volatile cargo components is low and can be used during product changeover to liquefy the cargo.

[0007] - Cascade cooling is also used. Many ships, especially semi-cooled ships, are designed so that they can transport ethane / ethylene as pure cargo. This takes place in a cooling cascade with a refrigerant system which provides a temperature level of up to -40°C for condensation. This also applies to handling any type of commercial LPG at a pressure level that can be achieved by two-stage compression. The disadvantage of this method is the high equipment and machinery effort for the additional cascade facility.

[0008] - In recent years, so-called "Vent Coolers" have been developed, as known, for example, from WO 2012 / 143699 A1. There, liquefaction takes place in the cargo condenser with the help of a two-stage liquefaction process with seawater as coolant. The development described in WO 2012 / 143699 A1 is similar in principle to the residual gas condenser. The technical advantage is that the temperature level is related to the intermediate pressure between the two compression stages and not to the tank pressure. This is usually sufficient for the condensation of the cargo. At the same time, the cooling performance of the facility is less strongly reduced compared to the residual gas condenser.

[0009] - In DE 10 2013 101 414 A1 it is proposed to feed the tank liquid into the fuel economizer. In this method approach the fact is utilized that the tank liquid has a significantly lower concentration of volatile components than the vapor (BOG). Supplying the fuel economizer with tank liquid instead of condensate from the condenser reduces the concentration of volatile components in the BOG flow from the fuel economizer to the second compressor so that operation of the two-stage compressor system is achieved even under hot water conditions. The disadvantage is that the tank liquid has to be pressurized to be filled in, which makes the process more complex and requires additional modifications.

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

[0011] The use of a three-stage compressor is also known: by using a three-stage compressor, it is possible to increase the condensation temperature at the outlet pressure to a level which can easily be reached under world trade conditions and even in warm sea water. The disadvantage of this obvious approach is the high investment cost of the high demands for retrofitting.

[0012] Ethane contents of 2.5%, 5% and 8% in the bulk liquid are standard cargo specifications for designing LPG systems. For larger LPG systems, IMO type A tanks are usually associated with tank operating pressures between 0 and 0.4 bar g. The BOG composition resulting from the given ethane contents requires an increased condensation pressure at the temperature level provided. As already mentioned above, in LPG reliquefaction, the use of a two-stage piston compressor and sea water is state of the art. For worldwide use, a sea water temperature of 32°C is considered. However, warmer conditions prevail in many harbors and important commercial areas.

[0013] Obviously, for standard compressor configurations, ethane concentrations in the bulk liquid above 3.5% cannot be used under all ambient conditions. In this case, the standard approach consists in installing a vent valve on the LPG condenser, which vents the part of the gas which cannot be condensed at the available pressure / temperature combination.

[0014] A typical case is described in the following numerical example:

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

[0016] At a condensation temperature of 40°C (due to warm sea water or contaminated heat exchangers), a share of about 3% (mol) of the BOG remains in the vapor phase. This amount is very sensitive to slight fluctuations of the composition of the tank liquid. Thus, for example, a very small increase of the ethane content to 5.5% increases the share to 14%.

[0017] In normal operation, this gas is either vented to the atmosphere, which means not only an undesired greenhouse gas release, but also a loss of cargo, or the gas is led back into the tank as vapor, thereby significantly reducing the available cooling capacity of the refrigeration plant. SUMMARY

[0018] In contrast, the object of the present application is to propose a method and a device by means of which BOG having a high proportion of volatile components can be reliquefied and which are economical.

[0019] According to the application, this object is achieved by the method according to claim 1 and the device according to claim 11.

[0020] By means of the measures according to the application, BOG having a high proportion of volatile components can be reliquefied with relatively little outlay.

[0021] The application is based on the insight that if a desired level is preset for a fluid receiving vessel for receiving partially condensed fluid and a maximum final pressure, the limit final pressure, is preset for the final pressure, and the discharge from a cooling device connected downstream of the fluid receiving vessel and for cooling the fluid at a temperature preset in accordance with the final pressure is only released in accordance with the desired level being reached or exceeded or before the limit final pressure is reached, the proportion of the liquid phase in the partially condensed fluid can be increased in a simple manner.

[0022] Since a maximum limit final pressure is preset for the final compression stage, it has to be accepted that the BOG can not be compressed to a final pressure which is necessary for the complete condensation of all volatile components in the subsequent condenser if the BOG contains a proportion of volatiles, for example a high proportion of ethane, or at a high condensation temperature, for example due to hot water or a contaminated condenser, and the final compression pressure must therefore be increased further. This results in an increase in the proportion of the gaseous phase in the partially condensed fluid and thus a drop in the level in the fluid receiving vessel. Since the desired level is fixed and the upper edge of the fluid outlet of the fluid receiving vessel is at the height of the desired level or below it by a preset measure, only liquid flows to the cooling device until the level drops below the upper edge of the fluid outlet. Furthermore, since the measurement of the level is also only up to the desired level not being exceeded so that the actuator diverts the flow of the cooled fluid, it is ensured that only liquid is diverted by the actuator until the limit final pressure is reached.

[0023] At the closing of the actuator, the fluid flow is reversed and the fluid in the cooling device is further cooled. Due to the reversal, the level in the fluid receiving vessel rises again. The final pressure also increases as the share of the gaseous phase in the BOG flow continues to increase. With the increase in the final pressure, the condensation temperature also increases, i.e. condensation by means of a less cold coolant is achieved in the negative temperature range. Thus, with the reaching of a suitably preset limit final pressure, it is possible for the gaseous phase of the fluid, at least the most volatile constituents of the fluid, to be completely condensed in the cooling device by means of a relatively "warm" coolant. Thus, at the reaching of the limit final pressure, the actuator is opened again, even if the desired level in the fluid receiving vessel has not yet been reached, and, due to the complete or at least as complete as possible condensation, a completely or as completely as possible liquid fluid flow is discharged from the cooling device.

[0024] If the share of the volatile in the BOG decreases again, the share of the gaseous phase in the partially condensed fluid also decreases and the level in the fluid receiving vessel continues to increase and the final compression pressure decreases again. If the final compression pressure falls below the limit final pressure, the actuator is closed and remains closed until the level again reaches the desired level. In this way, the transfer of fluid with a significant share of the gaseous phase from the cooling device is prevented. The actuator is only reopened when the level reaches or exceeds the desired level. Here, the opening can take place in a continuous control loop.

[0025] Thus, the measures according to the application make it possible to reliquefy BOG with volatile constituents at small outlay.

[0026] Preferably, the actuator is a valve. By means of the valve, the transfer of the fluid flow cooled in the cooling device can be controlled at low cost. Here, the valve can be part of the cooling device and be arranged directly at the fluid flow outlet thereof. However, the valve can also be arranged in a fluid flow discharge line which is in flow connection with the fluid flow outlet of the heat exchanger. Furthermore, it is conceivable that the valve is part of a liquefied gas tank or of a consumer into which the cooled fluid flow is to be introduced.

[0027] It is also conceivable that the actuator is a metered volume delivery device, for example a turbine, which is then for example speed-controlled and interrupts, i.e. stops, the flow of the cooled fluid at a speed of "zero".

[0028] Preferably, the cooling in step j) takes place by means of a coolant circuit, wherein a coolant is passed through the heat exchanger, wherein the fluid flow out of step i) is introduced into the heat exchanger and a cooled fluid flow is conducted out of the heat exchanger. The fluid flow can be cooled in this way at low cost.

[0029] Here, the liquid coolant advantageously flows through the heat exchanger, and the coolant is stored in a coolant reservoir, wherein the coolant is in its liquid phase in a lower region of the coolant reservoir and in its gaseous phase in an upper region. The liquid coolant ensures good heat transfer, while the coolant reservoir ensures that the heat exchanger is always sufficiently supplied with coolant.

[0030] The coolant reservoir can be structurally separated from the heat exchanger, thereby achieving a high degree of flexibility in the spatial arrangement and facilitating maintenance and repair work.

[0031] Alternatively, it is also conceivable to integrate the coolant reservoir into the heat exchanger. In this way, a compact, space-saving construction is achieved. Furthermore, no connection pipe lines need to be laid, which reduces costs and, in addition, avoids heat input via these connection pipe lines.

[0032] In a preferred embodiment of the application, the method has the features of claim 6 and the apparatus has the features of claim 17. In this case, the BOG is compressed in a two-stage process, and the reliquefied BOG is used as coolant. By connecting the cooling device on the one hand to the BOG flow between the first compression stage and the final compression stage and on the other hand providing a feed-in valve in the feed line which is only opened for the feed of the reliquefied BOG into the coolant circuit, the pressure level present in the coolant circuit corresponds to the intermediate pressure level at the connection point between the first compression stage and the final compression stage. The reliquefied BOG entering the coolant circuit from the fluid receiving vessel is thus depressurized and cooled upon entry. If the final compression pressure reaches the limit final pressure, the gaseous fluid entering the heat exchanger from the fluid receiving vessel is on the one hand at an especially high pressure and on the other hand the pressure drop and thus the temperature drop for the reliquefied BOG entering the coolant circuit is especially large, so that the gaseous BOG in the heat exchanger is completely or at least almost completely condensed.

[0033] It is particularly advantageous here to take the liquid flow for introduction into the coolant circuit as coolant from the coolant circuit at the bottom of the fluid receiving vessel. Thereby, it is ensured in a simple manner that no gaseous fluid enters the coolant circuit.

[0034] In an advantageous refinement of the measures according to claim 8 or 19, the outlet of the liquid coolant from the coolant reservoir is located above the inlet of the coolant into the heat exchanger. Thereby, sufficient coolant delivery to the heat exchanger is ensured by gravity alone.

[0035] Preferably, the cooling device for cooling the fluid flow exiting from the fluid receiving vessel is a thermosyphon cooling device. Thereby, the technical outlay for cooling remains relatively low.

[0036] Preferably, the finally compressed BOG stream is condensed in a condenser by means of sea water, since this is particularly cost-effective. BRIEF DESCRIPTION OF DRAWINGS

[0037] In the following, the application is explained in more detail by way of example with reference to the drawings. The drawings show:

[0038] Figure 1 is a flow chart of a first embodiment of the device according to the application, and

[0039] Figure 2 is a flow chart of a second embodiment of the device according to the application. DETAILED DESCRIPTION

[0040] Figure 1 The embodiment of the device according to the application shown in Fig. 1 has a compressor 2, a condenser 3, a fluid receiving vessel 4, a cooling device 5 and an actuator 6, which is configured as a valve and is arranged in a fluid outflow line 7.

[0041] The compressor 2 has an inlet 8 for a BOG stream 9. This inlet 8 can be, for example, in flow connection with the gas phase region of a liquefied gas tank.

[0042] The BOG stream 9 is compressed in the compressor 2 in a final compression stage 10 to a final pressure. The final pressure is related to the composition of the substance mixture making up the BOG stream 9 and increases with the share of volatile constituent parts in the substance mixture or the BOG stream 9.

[0043] In the embodiment shown in Fig. 1, a two-stage or multi-stage compressor 2 is shown, but the compressor 2 can also be configured in a single stage in this embodiment. In this case, the only compression stage is also the final compression stage 10. Figure 1 The limit final pressure is determined as the maximum final pressure which decisively influences the operation of the actuator 6, i.e. the valve.

[0044] The final compression stage 10 has an outlet 11 for the finally compressed BOG stream 9, which is in flow connection 13 with a BOG stream inlet 12 of the condenser 3. In the condenser 3, the finally compressed BOG stream 9 is cooled at a temperature which is predetermined independently of the final pressure. The condenser 3 can therefore be, for example, sea water cooled.

[0045] It is therefore possible in the BOG stream 9 with volatile constituent parts that the determined limit final pressure can not be sufficient to condense all the volatile constituent parts of the BOG stream at the existing condenser temperature, so that the BOG stream is only partially condensed.

[0046]

[0047] ​In the following, the BOG flow leaving the condenser 3 is generally referred to as fluid flow 9a, since it can contain liquid and / or gaseous components. Accordingly, the associated outlet is referred to as fluid flow outlet 14.

[0048] The fluid flow outlet 14 of the condenser 3 is in flow connection 16 with the fluid flow inlet 15 of the fluid receiving vessel 4.

[0049] The fluid receiving vessel 4 has a fluid flow outlet 17, which is located above a predetermined fluid receiving volume 18 of the fluid receiving vessel and is in flow connection 20 with a fluid flow inlet 19 of the cooling device 5.

[0050] In the fluid receiving vessel 4, the gaseous and liquid phases of the fluid are separated into a lower liquid phase region 21 and an upper gaseous phase region 22. For the fluid receiving vessel 4, it is desirable that the liquid level 23 is fixed in the height of the upper edge of the fluid flow outlet 17 or exceeds the outlet by a predetermined distance.

[0051] In the fluid receiving vessel 4, there is also provided a liquid level sensor 24 for measuring the liquid level. The measurement signal is transmitted to the valve control device 6a, by means of which the valve 6 in the fluid flow discharge line downstream of the cooling device can be brought into the open or closed position.

[0052] The cooling device 5 has the already mentioned fluid flow inlet 19 and a fluid flow outlet 25, which is in flow connection 26 with the fluid flow discharge line 7. In the cooling device 5, the fluid flow 9a is cooled to a temperature which corresponds to the saturation temperature of the fluid flow 9b at a pressure which is less than the final pressure.

[0053] From the point at which the BOG flow 9, also referred to as fluid flow 9a from leaving the condenser 3, leaves the final compression stage 10 of the compressor 2, it is at the final pressure. This final pressure is measured by means of a pressure sensor 27, which is arranged at any point in the region extending from the outlet of the final compression stage 10 of the compressor 2 to the valve 6 in the fluid flow discharge line 7 downstream of the cooling device 5 and at the final pressure. For example, this pressure sensor 27 can be arranged in the fluid receiving vessel 4. The measurement signal is transmitted to the valve control device 6a, by means of which the valve 6 in the fluid flow discharge line 7 can be brought into the open position or the closed position, wherein in the open position the reliquefied BOG is delivered to a further use, for example introduced into a tank for liquefied gas.

[0054] That is to say, the actuator position or valve position is controlled by means of the measurement signals of the liquid level sensor 24 and the pressure sensor 27, more precisely as follows:

[0055] A) Open position

[0056] The actuator or valve 6 in the fluid stream discharge line 7 opens in the following case:

[0057] a) the liquid level corresponds at least to the desired liquid level 23

[0058] and / or

[0059] b) the end pressure reaches the limit end pressure.

[0060] Case a)

[0061] Since the upper edge of the fluid stream outlet 17 of the fluid receiving vessel 4 is in the height of the desired liquid level 23 or below said height by a preset measure, only the liquid phase 21 of the fluid, i.e. only the reliquefied BOG, flows into the cooling device 5 and further into the fluid stream discharge line 7 upon reaching the desired liquid level 23.

[0062] Case b)

[0063] With the limit end pressure being reached, the fluid is at a relatively high pressure, so that cooling to a temperature below the saturation temperature of the fluid stream at the limit end pressure, i.e. even cooling by only a small amount (e.g. 1 °K), brings about a high further condensation of the gaseous component of the fluid stream 9a, and the fluid stream 9b leaving the cooling device 5 is almost completely or even only liquid.

[0064] B) closed position

[0065] The actuator or valve 6 in the fluid stream discharge line 7 enters its closed position in the following case:

[0066] the liquid level falls below the desired liquid level 23

[0067] and

[0068] the end pressure is below the limit end pressure.

[0069] If the share of uncondensed BOG increases (e.g. because the share of volatile components in the BOG has increased, or because the sea water 28 has been warmed in the sea water cooled condenser 3), the share of the gaseous phase 22 in the fluid increases (and thus the share of the liquid phase 21 decreases) and the end pressure increases.

[0070] If the liquid level falls below the desired liquid level 23 and subsequently continues to fall below the upper edge of the fluid stream outlet 17 of the fluid receiving vessel 4, the boundary between the gaseous phase 22 and the liquid phase 21 of the fluid is first located in the region of the fluid stream outlet 17 of the fluid receiving vessel 4. In this case, a mixture consisting of gas and liquid leaves the fluid receiving vessel 4 and enters the cooling device 5.

[0071] If the liquid level drops so that the fluid flow outlet 17 is completely located in the gas phase region 22 of the fluid receiving vessel 4, only gaseous BOG is discharged.

[0072] Because the actuator or the valve 6 in the fluid flow discharge line 7 downstream of the cooling device 5 is closed, the fluid flows back, whereby the liquid level in the fluid receiving vessel 4 rises again. Because the partially condensed BOG stream 9a leaving the condenser 3 and entering the fluid receiving vessel 4 still contains a liquid phase fraction, although it contains an increased gas phase fraction.

[0073] As already mentioned above, on the one hand the final pressure increases with the increasing fraction of non-condensed components in the BOG, while on the other hand the liquid level in the fluid receiving vessel 4 increases, so that over time at least one of the two states described under A)a) and A)b) above is reached again and the actuator or the valve 6 opens again.

[0074] In Figure 1 one embodiment according to the application, a device 1 with an external cooling device 5 is shown.

[0075] The cooling device 5 has a heat exchanger 29 with an inlet 30 and an outlet 31 for a coolant 32 and an inlet 33 and an outlet 34 for the fluid stream 9a or 9b.

[0076] The heat exchanger 29 is part of an external coolant circuit.

[0077] The fluid stream inlet 33 of the heat exchanger 29 is in flow connection 20 with the fluid stream outlet 17 of the fluid receiving vessel 4, and the fluid stream outlet 34 of the heat exchanger 29 is connected to the fluid flow discharge line 7 of the cooling device.

[0078] In an embodiment according to Figure 2 the liquefied BOG is used as coolant 32.

[0079] Here, the cooling device 5 also has a heat exchanger 29 with an inlet 30 and an outlet 31 for a coolant 32 and an inlet 33 and an outlet 34 for the fluid stream 9a or 9b. The heat exchanger 29 is part of a coolant circuit 35.

[0080] As in the embodiment shown in Figure 1 the fluid stream inlet 33 of the heat exchanger 29 is in flow connection 20 with the fluid stream outlet 17 of the fluid receiving vessel 4, and the fluid stream outlet 34 of the heat exchanger 29 is connected to the fluid flow discharge line 7 of the cooling device.

[0081] In Figure 2In the embodiment shown, the fluid receiving vessel 4 additionally has a bottom outlet 36, which thus forms a second outlet of the fluid receiving vessel, more precisely only for reliquefied BOG, i.e. only for liquid flow.

[0082] The bottom outlet 36 is connected via a feed line 37 with a coolant inlet 38 of a coolant reservoir 39. In the embodiment shown, this coolant inlet 38 is arranged in the bottom of the coolant reservoir 39.

[0083] The coolant 32, i.e. the reliquefied BOG used for this, is partially re-evaporated in the coolant circuit 35, in particular in the heat exchanger 29, so that the coolant 32 exists in the lower portion 40 of the coolant reservoir 39 in the liquid phase and in the upper portion 41 thereof in the gaseous phase.

[0084] In the liquid phase region 40, the coolant reservoir 39 has a coolant outlet 42, which is above the coolant inlet 30 of the heat exchanger 29 and is in flow connection 43 therewith.

[0085] In the coolant reservoir 39, a coolant level sensor 44 is arranged for measuring the filling state 45, i.e. the level, of the liquid phase of the coolant 32.

[0086] A feed-in valve 46 is arranged in the feed line 37. The measurement signal of the coolant level sensor 44 is transmitted to a valve control device 46a, by means of which the feed-in valve 46 can be brought into an open position or a closed position, wherein in the open position reliquefied BOG is fed into the coolant reservoir 39 as coolant 32. The filling state 45 of the liquid phase of the coolant 32 in the coolant reservoir 39 is regulated by opening and closing the feed-in valve 46, so that the coolant outlet 42 of the coolant reservoir 39 is always located in the liquid phase region 40. It is thus ensured that sufficient liquid coolant 32 is always delivered to the heat exchanger 29.

[0087] The coolant outlet 31 of the heat exchanger 29 is in flow connection 47 with the feed line 37 downstream of the feed-in valve 46. In this way, the coolant circuit 35 is formed, in which the liquid coolant 32 successively flows through the coolant reservoir 39 and the heat exchanger 29. The coolant circuit 35 works like a thermosyphon cooling device.

[0088] In the embodiment according to Figure 2 the compressor 2 is formed in two stages. The first compression stage 48 has an inlet 8 for the BOG flow 9 to be compressed and compresses the BOG flow 9 to an intermediate pressure, which is below the final pressure. The second compression stage is the final compression stage 10 and compresses the intermediate- compressed BOG flow to the final pressure and has an outlet 11 for the final compressed BOG flow.

[0089] The gaseous phase region 41 of the coolant reservoir 39 has an outlet 49 which is in flow connection 50 with the BOG stream between the first compression stage 48 and the final compression stage 10. Thus, on the one hand, evaporated coolant, i.e. gaseous BOG, can be introduced from the coolant reservoir 39 into the BOG stream between the first compression stage 48 and the final compression stage 10. On the other hand, the intermediate pressure prevailing at the introduction site 51 between the first compression stage 10 and the final compression stage 10 is also prevailing in the coolant reservoir 39 and thus in the entire coolant circuit 35. The boundary between the final pressure and the intermediate pressure in the feed line 37 is the feed-in valve 46.

[0090] Upstream of the feed-in valve 46, the fluid stream 9a or the BOG stream 9 after final compression is at the final pressure, i.e. at most at the preset limit final pressure.

[0091] When the coolant 32 at the final pressure or limit final pressure enters the coolant circuit 35 through the feed-in valve 46, the coolant 32 is thus depressurized to the intermediate pressure and is correspondingly cooled here.

[0092] Due to the high pressure at which the fluid stream 9a flows from the fluid receiving vessel 4 to the heat exchanger 29 at the limit final pressure and the relatively low temperature level in the coolant circuit 35, the fluid stream 9a is cooled to a temperature close to the saturation temperature of the fluid stream 9a at the intermediate pressure, so that the gaseous fraction of the fluid stream 9a will condense in this state and continue to be guided further only or almost exclusively with re- liquefied BOG, for example to be discharged into a tank, through the open valve 6 in the fluid stream discharge line 7 at the limit final pressure.

[0093] As soon as the final pressure falls below the limit final pressure, the valve 6 is closed again until the desired level 23 in the fluid receiving vessel 4 is reached again and the valve 6 is opened again.

[0094] The measure for re-liquefying BOG according to the present application is further illustrated below according to the example with numerical values shown in Figure 2 Table 1. In said example, the BOG to be re- liquefied is derived from a liquefied gas tank for propane and the condenser 3 is cooled by sea water. The liquid and gaseous components as well as the pressure and temperature ratios given below are based on flash calculation with NIST (National Institute of Standards and Technology) data in the individual method steps / equipment elements:

[0095] a) in the liquefied gas tank, BOG is removed from the liquefied gas tank for re-liquefaction,

[0096] Liquid: propane

[0097] Ethane content 5% mol

[0098] BOG: ethane content of about 26% mol

[0099] Pressure: 1 bar a

[0100] b) in a two-stage compressor 2

[0101] BOG stream: ethane content of about 26% mol

[0102] Intermediate pressure: 5 bar a

[0103] Final pressure: 21 bar a

[0104] For the final compressed BOG stream 9 discharged from the compressor 2, at an ethane content of about 26% mol and a pressure of 21 bar a, the temperature of complete condensation is about 25°C.

[0105] c) in a condenser 3

[0106] On the coolant side:

[0107] Seawater 28 with a water temperature of 32°C,

[0108] Resulting from the heat input in the condenser 3 at a condensation temperature of about 40°C.

[0109] The BOG stream 9 is thus only partially condensed.

[0110] On the gas / condensate side:

[0111] Pressure (final pressure): 21 bar a

[0112] Input BOG stream 9: ethane content of about 26% mol

[0113] Discharged, partially condensed fluid stream 9a:

[0114] (Larger, liquid fraction (condensate) of about 97% mol BOG: ethane content of about 25% mol

[0115] (Smaller, uncondensed gas fraction of about 3% mol BOG: ethane content of about 45% mol

[0116] d) in a fluid receiving vessel 4

[0117] Liquid fraction (condensate): ethane content of about 25% mol

[0118] Gas fraction: ethane content of about 45% mol

[0119] Pressure (final pressure): 21 bar a

[0120] e) in a cooling device 5

[0121] On the coolant side:

[0122] In the feed line 37 upstream of the feed-in valve 46

[0123] Condensate: ethane content of approximately 25% mol

[0124] Pressure (final pressure): 21 bar a

[0125] In the feed line 37 downstream of the feed-in valve 46, i.e. in the coolant circuit 35

[0126] Pressure (intermediate pressure): 5 bar a (depressurization from the final pressure to the intermediate pressure)

[0127] Condensate: temperature of approximately -6.5°C

[0128] Ethane content of approximately 8% mol

[0129] (The values for the temperature and the ethane content occur because a portion of the ethane evaporates as a result of the pressure drop, so that the propane content in the condensate increases.)

[0130] On the fluid side:

[0131] In the gas fraction: ethane content of approximately 45% mol

[0132] Pressure (final pressure): 21 bar a (complete liquefaction of the gas fraction)

[0133] With an ethane content of approximately 45% mol in the gas fraction on the fluid side and a temperature of approximately -6.5°C on the coolant side, the saturation pressure of the gas fraction on the fluid side is approximately 10 bar a. Since a final pressure of 21 bar a prevails on the fluid side, the gas fraction in the fluid is completely liquefied.

Claims

1. A method for reliquefying BOG, wherein the BOG contains a volatile component, the method comprising the following steps: a) Compressing the BOG stream (9), wherein the BOG stream (9) is discharged from the final compression stage (10) as a final compressed BOG stream (9) with final pressure; b) Condensing the final compressed BOG stream (9) to obtain at least a partially condensed final compressed fluid stream (9a). c) Provide a fluid receiving container (4) having a fluid inlet (15) and a fluid outlet (17), wherein the location of the fluid outlet (17) is selected such that it is located above a preset fluid receiving volume (18). d) The fluid flow (9a) from step b) is introduced into the fluid receiving container (4) through the fluid inlet (15); e) Determine a desired liquid level (23) for the fluid receiving container (4) such that the desired liquid level (23) is at the height of the upper edge of the fluid outlet (17) or exceeds the upper edge by a preset amount; f) Determine the upper limit final pressure for the final compression stage (10); g) Measure the liquid level in the fluid receiving container (4); h) Measure the final pressure; i) The fluid flow (9a) is discharged from the fluid receiving container (4) through the fluid flow outlet (17); j) Cool the fluid flow (9a) from step i) to a temperature corresponding to the saturation temperature of the fluid flow (9a) at a pressure below the final pressure, in order to condense the gaseous portion of the fluid flow (9a). k) If the measured final pressure is equal to the ultimate final pressure, then transfer the cooled fluid flow (9b).

2. The method according to claim 1, characterized in that, Step j) comprises the following steps; j1) Provides a coolant circuit (35) in which coolant (32) flows through a heat exchanger (29); j2) The fluid flow (9a) from step i) is introduced into the heat exchanger (29); and Step k) includes removing the cooled fluid flow (9b) from the heat exchanger (29).

3. The method according to claim 2, characterized in that, Step j1) includes providing a coolant circuit (35) in which liquid coolant (32) flows through the heat exchanger (29); and Step j1) further includes storing the coolant (32) in a coolant reservoir (39), wherein the coolant (32) is in its liquid phase in the lower region (40) and in its gas phase in the upper region (41) of the coolant reservoir (39).

4. The method according to claim 3, characterized in that, Step j1) includes storing the coolant (32) in a coolant reservoir (39), which is structurally separate from the heat exchanger (29).

5. The method according to claim 3, characterized in that, Step j1) includes integrating the coolant reservoir (39) into the heat exchanger (29).

6. The method according to any one of claims 3 to 5, characterized in that, In step a), the BOG stream (9) is compressed in at least two compression stages (48, 10); and Step j1) also includes the following steps: j1.1) The liquid flow from the fluid receiving container (4) is fed into the coolant circuit (35) as coolant (32) by means of the feed line (37); j1.2) A feed valve (46) is provided in the feed line (37), and the feed valve (46) is opened to feed in and otherwise kept closed; j1.3) Establish a flow connection (50) between the gas phase region (41) of the coolant reservoir (39) and the BOG stream (9) located between the first compression stage (48) and the final compression stage (10) to introduce the evaporated coolant into the BOG stream (9) and to set a pressure in the coolant circuit (35) that corresponds to an intermediate pressure present at the inlet (51) and is therefore less than the final pressure.

7. The method according to claim 6, characterized in that, In step j1.1), the liquid flow is taken out from the fluid receiving container at the bottom of the fluid receiving container (4).

8. The method according to claim 4, characterized in that, The outlet (42) of the liquid coolant (32) leaving the coolant reservoir (39) is located above the inlet (30) of the coolant (32) entering the heat exchanger (29).

9. The method according to any one of claims 1 to 5, characterized in that, In step j), a thermosiphon cooling device is used as the cooling device.

10. The method according to any one of claims 1 to 5, characterized in that, In step b), condensation is achieved by means of seawater (28).

11. An apparatus for performing the method according to any one of claims 1 to 10, comprising: - Compressor (2), - The compressor has an inlet (8) for the BOG flow (9), and - The final compression stage (10) of the compressor finally compresses the BOG stream (9) to the final pressure and has a BOG stream outlet (11) for the finally compressed BOG stream. - Condenser (3) - The condenser has a BOG inlet (12), which is flowably connected (13) to the BOG outlet (11) of the final compression stage (10), and - The condenser is configured to at least partially condense the ultimately compressed BOG stream (9) into a fluid stream (9a); and - The condenser has a fluid outlet (14). - Fluid receiving container (4), the fluid receiving container having: - Fluid inlet (15), which is flowably connected (16) to the fluid outlet (14) of the condenser (3). - Fluid outlet (17), which is located above a preset fluid receiving volume (18); - Liquid level sensor (24), the liquid level sensor is used to measure the liquid level in the fluid receiving container (4), - Pressure sensor (27), the pressure sensor is used to measure the final pressure; - Cooling device (5), the cooling device having - A fluid inlet (19), which is flowably connected (20) to the fluid outlet (17) of the fluid receiving container (4), and - Fluid outlet (25) for the cooled fluid flow (9b), and - The cooling device is configured to cool the fluid flow (9a) to a temperature corresponding to the saturation temperature of the fluid flow (9a) at a pressure below the final pressure, in order to condense the gaseous portion of the fluid flow (9a). - Actuator (6), the actuator - It is flow-connected to the fluid outlet (25) of the cooling device (5), and - If the measured final pressure is equal to the preset ultimate final pressure, then the actuator enters the open position to divert the cooled fluid flow (9b). - In other cases, it can enter the closed position, in which the actuator interrupts the flow of cooled fluid (9b).

12. The device according to claim 11, characterized in that, The actuator is a valve (6).

13. The device according to claim 11 or 12, characterized in that, The cooling device (5) has: a coolant circuit (35) in which a coolant (32) cross-flow heat exchanger (29) has: a fluid inlet (33) which is flowably connected (20) to a fluid outlet (17) of the fluid receiving container (4); and a fluid outlet (34) which constitutes the fluid outlet (25) of the cooling device (5).

14. The device according to claim 13, characterized in that, The coolant (32) flowing through the heat exchanger (29) is liquid, and the coolant circuit (35) has a coolant reservoir (39) in which the coolant (32) is in its liquid phase in the lower region (40) and in its gas phase in the upper region (41) of the coolant reservoir (39).

15. The device according to claim 14, characterized in that, The coolant reservoir (39) and the heat exchanger (29) are structurally separate from each other.

16. The device according to claim 14, characterized in that, The coolant reservoir (39) is integrated into the heat exchanger (29).

17. The device according to any one of claims 14 to 16, characterized in that, The compressor (2) is a compressor with at least two stages, wherein the first compression stage (48) has an inlet (8) for the BOG flow (9), and The cooling device (5) also has: - Feed line (37), which connects the inlet of the coolant circuit (35) for liquid coolant (32) to the outlet (36) of the fluid receiving container (4) for liquid flow; - Feed valve (46), which is disposed in the feed line (37) and is capable of entering the open position to feed the liquid flow, and in other cases can enter the closed position; - A conduit (50) for establishing a flow connection between the vapor phase region (41) of the coolant reservoir (39) and the BOG stream (9) located between the first compression stage (48) and the final compression stage (10) to introduce evaporated coolant into the BOG stream (9) and to set a pressure in the coolant circuit (35) corresponding to an intermediate pressure present at the inlet (51) that is less than the final pressure.

18. The device according to claim 17, characterized in that, The outlet (36) for liquid flow of the fluid receiving container (4) is formed in the bottom of the fluid receiving container (4).

19. The device according to claim 15, characterized in that, The outlet (42) of the liquid coolant (32) leaving the coolant reservoir (39) is located above the inlet (30) of the coolant (32) entering the heat exchanger (29).

20. The device according to claim 11 or 12, characterized in that, The cooling device (5) is a thermosiphon cooling device.

21. The device according to claim 11 or 12, characterized in that, The coolant in the condenser (3) is seawater (28).

22. A ship having the equipment according to any one of claims 11 to 21.

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