KR20220135206A - Method and device for reliquefaction of bog - Google Patents

Abstract

Proposed is a method for reliquefication of BOG (boil gas) containing volatile components. In the case, a BOG stream (11) is compressed in at least two compression stages (9, 12) and the BOG stream (11) from a final compression stage (12) exits as a final compressed BOG stream (11) with a final pressure. The final compressed BOG stream (11) is condensed to obtain an at least partially condensed final compressed fluid stream (11a), and the partially condensed fluid stream (11a) is introduced into a fluid container (4). In order to condense the gaseous component of the fluid stream (11a), the partially condensed fluid stream (11a) discharged from the fluid container (4) is cooled to a temperature corresponding to the saturation temperature of the fluid stream (11a) at a pressure lower than the final pressure. In the case, the liquid stream of the fluid container (4) is used as a coolant and an intermediate pressure between the at least two compression stages (9, 12) is used to depressurize and evaporate the coolant. After cooling, the coolant stream enters the BOG stream (11) between the first compression stage (9) and the final compression stage (12). When the coolant stream still contains the residual amounts of the liquid coolant, the residual amounts are evaporated before entering a subsequent compression stage. A cooled fluid stream (11b) is delivered for further use when the measured liquid level in the fluid container (4) is at least equal to a target liquid level (25) and/or when the measured final pressure is equal to a limit final pressure. The invention also relates to a device for carrying out a method. The BOG with a high content of volatile components can be re-liquefied at a relatively low cost by means according to the present invention.

Description

METHOD AND DEVICE FOR RELIQUEFACTION OF BOG

The present invention relates to a method and apparatus for carrying out the method for the reliquefaction of BOG (Boil Off Gas) comprising (in particular) volatile components such as, for example, ethane.

BOG is, for example, vaporized liquefied gas such as vaporized LNG (Liquefied Natural Gas) or vaporized LPG (Liquefied Petroleum Gas). Liquefied gases and thus BOG are, in general, a mixture of substances comprising components with substantially different evaporation temperatures. BOG is produced by the inevitability of heat entering into liquefied gas tanks (hereinafter simply referred to as tanks) where the liquefied gas is stored on land, transported by ship, or carried as fuel for its own consumption and/or into pipelines through which the liquefied gas flows. Occurs. In recent years, an increase in the content of volatile components in liquefied gas has been observed more and more frequently. Thus, LPG tank loads (eg commercial propane) with increased ethane content ranging from 5% mol in tank liquid to 8% mol in tank liquid have become common. As a result, the long-disclosed reliquefaction method can no longer condense the increased content of volatile components during transport. A so-called non-condensable fraction occurs.

For reasons of environmental protection and economics, reliquefaction of BOG is desirable, especially during normal operation, to reduce emissions of hydrocarbon gases and to reduce tank load losses.

The problem of non-condensable fractions in the load vapor during operation of the reliquefaction process has been known for a long time. There are several established solutions in the market:

- Venting to atmosphere during the first operating hours is an effective remedy in cases where the amount of non-condensable material is small (eg residual nitrogen from tank purge). If the non-condensable component is a constant constituent of the load, so that the amount of gas to be exhausted becomes too large, the exhaust cannot be used.

- Residual gas condenser is a heat exchanger that receives a non-condensable gas fraction from the condenser through an exhaust valve operated automatically or manually in the condenser. This heat exchanger cools the gas mixture at approximately the final pressure of the compressor to a temperature level close to the saturation temperature of the tank. This causes most of the heavily boiling hydrocarbons to condense and the remaining small amount of BOG obtained is concentrated to a non-condensable gas. This is an effective means to reduce load loss. Since the condensate of reliquefaction at storage pressure is generally used as a cooling medium in this process, this method greatly reduces the cooling capacity available for cooling the tank. This method is used when the concentration of volatile load components is low and may be used to recover the load during the product conversion process.

- Cascade cooling is also used. Many vessels (especially semi-cooled) are designed to transport ethane/ethylene as a net load. This is done in a cooling cascade with a coolant system providing temperature levels up to -40°C for condensation. It is also suitable for processing all types of commercial LPG at pressure levels achievable by two-stage compression. Disadvantages of this method are the high equipment and mechanical costs for the additional cascade system.

- Recently, a so-called "Vent Cooler" has been developed, for example as disclosed in WO 2012/143699 A1. There, reliquefaction is accomplished by a two-stage reliquefaction process using seawater as a coolant in a cargo-condenser. The development described in WO 2012/143699 A1 is in principle similar to residual gas condensers. The technical advantage is that the temperature level relates not to the tank pressure but to the intermediate pressure between the two compression stages. This is usually sufficient to condense the load. At the same time, the cooling capacity of the system is reduced much less compared to the residual gas condenser.

- In DE 10 2013 101 414 A1 it is proposed to supply tank liquid to an economizer. In this approach, the fact that the tank liquid has a much lower concentration of volatile components than vapor (BOG) is exploited. Feeding the economizer with tank liquid instead of condensate from the liquefier reduces the concentration of volatile components in the BOG stream from the economizer towards the second compression stage, thus enabling operation of the two-stage compressor system even in hot water conditions. A disadvantage is that the tank liquid for injection has to be pressurized, which makes the process more complicated and requires additional equipment.

However, this published two-step reliquefaction process also shows difficulties in handling loads with increased content of volatile components, such as ethane. Ethane is a volatile component, and its concentration in BOG is much higher than that of the liquid phase of liquefied gas, hereinafter also referred to as bulk fluid.

- The use of a three-stage compressor is also disclosed: by using a three-stage compressor, the condensing temperature at the discharge pressure can be raised to a level that is easily accessible under world trade conditions and even in warm sea water. The downside of this obvious solution is the high investment in more demanding equipment.

Ethane contents of 2.5%, 5% and 8% in the bulk liquid are standard load specifications and the LPG system is designed for this. For large LPG systems IMO-Model A tanks with tank operating pressures of 0 to 0.4 bar g are generally suitable. The BOG composition obtained from the specified ethane content requires increasing condensing pressure at a given temperature level. As mentioned above, the use of a two-stage piston compressor and seawater in LPG reliquefaction is prior art. Seawater at 32°C is considered for worldwide use. However, somewhat warmer conditions prevail in many ports and major trading areas.

It is clear that for standard compressor configurations ethane concentrations greater than about 3.5% in the bulk liquid cannot be handled at all ambient conditions. The standard approach in this case is to install a vent valve on the LPG-condenser that vents some of the gases that cannot be condensed under the available pressure/temperature combination.

A typical situation is given in the following numerical example:

The BOG concentration of ethane for a 5%-ethane load in fully cooled tank conditions (1 bar a) is about 26%. Such mixing at 36° C. can be achieved without problems with a maximum conveying pressure of 21 bar a.

At a condensation temperature of 40 °C (due to warm seawater or contaminated heat exchangers) a BOG content of about 3% (mol) remains in the vapor phase. These quantities are very sensitive to slight fluctuations in the tank liquid composition. Thus, for example, a very small increase in the ethane content to 5.5% increases the content to 14%.

During normal operation this gas is exhausted to the atmosphere, which not only means an undesirable release of greenhouse gases, but also a loss of load, or returned to the tank as vapor, greatly reducing the available cooling capacity of the refrigeration unit.

On the other hand, it is an object of the present invention to propose a method and apparatus in which BOG having a higher content of volatile components can be reliquefied and at the same time particularly economical.

According to the invention, the above object is solved by a method according to claim 1 and an apparatus according to claim 12 . The invention also includes a ship equipped with the device according to the invention.

By means of the measures according to the present invention, BOG having a high content of volatile components can be reliquefied at a relatively low cost.

The present invention provides that a maximum ultimate pressure, ie, a limit final pressure, is predetermined for a target liquid level and a final pressure for a fluid container for receiving a partially condensed fluid, connected downstream of the fluid container, and the fluid depends on the final pressure The fact that the liquid content in the partially condensed fluid can be increased in a simple way if the discharge of the fluid from a cooling device cooled at a predetermined temperature only opens depending on whether the target liquid level is reached or exceeded or a limiting final pressure is reached. based on

A maximum limiting final pressure is predetermined for the final compression stage, so that when the BOG contains volatile components (eg high ethane content) or the condensation temperature is high (eg due to warm water or a contaminated condenser) and thus the compression final pressure is forced to rise further) in the subsequent condenser it may not be compressed to the final pressure required for complete condensation of all volatile components. This causes the gaseous content in the partially condensed fluid to increase, thereby reducing the liquid level in the fluid container. Because a target level is defined and the upper edge of the fluid flow outlet of the fluid container is at or less than this target level by a predetermined limit, only liquid fluid flows into the cooling device until the level falls below the upper edge of the fluid flow outlet . Also, based on the measurement of the liquid level, since the actuator delivers the cooled fluid flow only until the target liquid level is reached, it is ensured that only liquid is delivered by the actuator until a limiting final pressure is reached.

Upon closure of the actuator, the fluid flow is stopped and the fluid in the cooling device is further cooled. The stagnation causes the liquid level in the fluid container to rise again. As the gaseous content of the BOG stream continues to increase, the final pressure also increases. As the final pressure continues to increase, the condensation temperature also rises, ie in the negative temperature range, condensation is achieved with a less cold coolant. It is thus possible to completely condense the gas phase of the fluid, at least most of the volatile components of the fluid, into a relatively "warm" coolant in the cooling device once a suitably predetermined limiting ultimate pressure is reached. Thus, upon reaching the final limit pressure, the actuator reopens, even if the target liquid level in the fluid container has not yet been reached again, and the complete or at least extensive condensation drains completely or most of the liquid fluid flow from the cooling device.

As the volatile components in the BOG decrease again, the gaseous content in the partially condensed fluid also decreases and the liquid level in the fluid container continues to rise, again lowering the compression final pressure. When the compression final pressure falls below the limit final pressure, the actuator closes and remains closed until the liquid level again reaches the target liquid level. In this way, a fluid with a significant gaseous content is prevented from being transferred from the cooling device. Only when the liquid level reaches or exceeds the target liquid level, the actuator is reopened. In this case the opening can be made in a continuous control loop.

According to the invention, the BOG is in this case compressed in at least a two-step process and the reliquefied BOG in the fluid container is used as coolant for the partially condensed fluid flow. The cooling device comprises a heat exchanger, wherein a coolant outlet of the heat exchanger is fluidly connected with a BOG flow between a first compression stage and a final compression stage, and a coolant inlet of the heat exchanger is fluidly connected through a throttle to the fluid container; , the pressure level in the heat exchanger corresponds to the intermediate pressure level at the point of connection for the BOG flow between the first and final compression stages. Accordingly, the reliquefied BOG flowing into the heat exchanger from the fluid container in which the final compression pressure prevails is decompressed and cooled while flowing through the throttle. When the compression final pressure reaches the limiting final pressure, on the one hand the fluid in gaseous form entering the heat exchanger from the fluid container is subjected to a particularly high pressure and on the other hand the pressure drop and thus the temperature of the reliquefied BOG entering the heat exchanger. The drop is particularly large, so that the gaseous fluid in the heat exchanger is completely or at least almost completely condensed and the depressurized coolant is evaporated.

The measure according to the invention makes it possible that the entire amount of coolant supplied from the fluid container, which is required to cool the fluid flow, is guided through the heat exchanger. This has several positive effects:

The inlet temperature of the coolant is always the flash point temperature of the coolant.

The entire amount of coolant is involved in heat exchange.

Countercurrent flow in the heat exchanger may be selected. Because the coolant has a significant temperature difference between its dew point and boiling point depending on the material mixture, the outlet temperature of the supercooled fluid stream is lower compared to co-current. As a result, the cooling capacity of the whole process is increased.

The measures according to the invention thus also make it possible to reliquefy BOG comprising volatile components at low cost.

The actuator is preferably a valve. The delivery of the cooled fluid flow in the cooling device can be inexpensively controlled by means of a valve. The valve in this case may be part of the cooling device and may be arranged directly at the fluid flow outlet of the device. However, the valve may also be disposed in a fluid flow outlet line fluidly connected to the fluid flow outlet of the heat exchanger. It is also conceivable that the valve is part of a liquefied gas tank or consumer into which the cooled fluid flow enters.

It may be contemplated that the actuator is, for example, a volumetric conveyor such as a turbine which is speed controlled and stops, ie stops, the flow of cooled fluid at speed “0”.

In a preferred refinement of the invention, the compressed BOG partial stream branches in the BOG stream between the first condenser and the condenser connected to the final compression stage and is mixed with the refrigerant stream exiting the heat exchanger so that it can still be present in the refrigerant stream. The remaining liquid coolant evaporates. Thereby, no liquid enters the compressor or into the compressors and the compressor is not damaged.

A droplet separator is preferably provided for this purpose, said separator having a coolant inlet fluidly connected to the coolant outlet of the heat exchanger, flowing between the first condenser and the condenser connected to the final compression stage, the BOG stream of BOG a mixed flow outlet having a BOG flow inlet fluidly connected to the partial flow, wherein a mixture of the evaporated coolant and the BOG partial flow is discharged and fluidly connected to a connection point towards the BOG flow between the first and final compression stages to provide The superheated compressed gas is guided through the droplet separator. Heat is introduced in this way, which causes the residual amount to evaporate. This measure provides an evaporative device which is effective and very inexpensive for the amount of liquid remaining in the coolant stream exiting the heat exchanger.

It is particularly preferred that a droplet separator level sensor for measuring a liquid level in the droplet separator is arranged in the droplet separator, the throttle being configured to reduce the coolant volume flow rate if the measured liquid level exceeds a predetermined maximum liquid level and if the measured liquid level is It is set to increase the coolant volume flow rate when a predetermined maximum liquid level is not reached. This ensures that, on the one hand, the residual amount of liquid in the coolant stream exiting the heat exchanger does not become excessively large, but on the other hand, sufficient coolant is supplied to the heat exchanger to ensure proper cooling of the fluid flow. Thus, a stable control possibility is given. A low state of charge or liquid level can be adjusted if the rate of evaporation produced by the introduction of heat in the droplet separator is constant. As a result, slightly more coolant is injected into the heat exchanger than is needed to cool the fluid stream.

The magnitude of the coolant volume flow entering the heat exchanger can also be controlled by measuring the temperature and forming a selected temperature difference:

According to the first possibility,

1.1 The outlet temperature of the coolant stream exiting the heat exchanger is measured;

2.1 The coolant temperature is measured before depressurizing the coolant;

and

if the outlet temperature from 1.1 falls below the coolant temperature from 2.1 by less than a predetermined amount, the amount of coolant entering the heat exchanger increases;

If the outlet temperature from 1.1 is below the coolant temperature from 2.1 by more than a predetermined amount, the amount of coolant entering the heat exchanger is reduced.

Preferably the temperature of the liquid of 2 in the fluid container is measured. Since a level sensor for measuring the liquid level in the fluid container is arranged there anyway, the corresponding temperature sensor can be arranged there at relatively low cost.

According to a second possibility for regulating the magnitude of the coolant volume flow entering the heat exchanger,

2.1 The outlet temperature of the coolant stream exiting the heat exchanger (as in 1.1 above) is measured;

2.2 the temperature of the coolant entering the heat exchanger is measured after depressurization of the coolant to an intermediate pressure;

and

if the outlet temperature from 2.1 exceeds the coolant temperature from 2.2 by less than a predetermined amount, the amount of coolant entering the heat exchanger is reduced;

When the outlet temperature from 2.1 exceeds the coolant temperature from 2.2 by more than a predetermined amount, the amount of coolant entering the heat exchanger increases.

According to a third possibility,

3.1 The outlet temperature of the fluid stream discharged from the heat exchanger, cooled and subjected to final pressure is measured;

3.2 (as in 2.2 above) the temperature of the coolant entering the heat exchanger is measured after depressurization of the coolant to an intermediate pressure;

and

if the outlet temperature from 3.1 exceeds the coolant temperature from 3.2 by less than a predetermined amount, the amount of coolant entering the heat exchanger is reduced;

If the outlet temperature from 3.1 exceeds the coolant temperature from 3.2 by more than a predetermined amount, the amount of coolant entering the heat exchanger increases.

If the heat exchanger is equipped with a suitable overcapacity, the evaporating coolant can be reliably operated with sufficient superheat for complete evaporation by each of the three possibilities described above. In this way, it is possible to achieve a minimized amount of coolant when injected into the heat exchanger and to ensure that no carryover of liquid into the separator is carried out during regulating operation.

Thus, by measuring the temperature in normal regulating operation, it is possible to prevent excessive coolant injection into the heat exchanger. Evaporated coolant can overheat significantly. This reduces the amount of coolant required in the heat exchanger and consequently reduces the intermediate pressure in the system or device. As a result, this increases the cooling capacity.

When the droplet separator is combined with the regulation of the coolant volume flow entering the heat exchanger using temperature measurement or temperature difference according to at least one of the three possibilities described above, the setpoint for adjusting the temperature difference is: may be chosen to leak. The liquid leak can then be corrected automatically, simply by means of, for example, a cascade control or an override control. When this method is further combined with the perfusion of a partial amount of hot gas of a compressed yet uncondensed BOG stream through a droplet separator, the system automatically adapts to different products and loading conditions.

Preferably the throttle is set to increase the coolant volume flow for the final compressed BOG stream, ie if a predetermined threshold temperature is achieved or exceeded prior to entry of said stream into the condenser. This ensures that the final compressed BOG stream can reach a predetermined limiting final pressure of said flow and that the compressed final pressure is not limited by a predetermined limiting temperature.

Correspondingly, the final or limit temperature of the final compressed BOG stream may be linked to a setpoint of coolant superheat or fluid flow subcooling, for example in cascade regulation or override regulation. In this way, it is ensured that more coolant is injected into the heat exchanger if necessary and the final temperature decreases.

In this case, it is particularly advantageous if the liquid stream is removed from the heat exchanger for introduction as coolant into the heat exchanger at the bottom of the fluid container. This simply ensures that no gaseous fluid enters the coolant circuit.

Preferably the final compressed BOG stream is condensed using seawater in a condenser, especially since it is inexpensive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is now described in more detail by way of example with reference to a single figure.

1 is a flow chart showing an embodiment of an apparatus according to the invention;

The embodiment shown in FIG. 1 of a device 1 according to the invention comprises a compressor 2 , a condenser 3 , a fluid container 4 , a cooling device 5 , an evaporation device 6a and an actuator 7 . wherein the actuator is configured as a valve and is arranged in the fluid flow outlet line (8).

In the embodiment according to FIG. 1 , the compressor 2 is designed in two stages. The first compression stage 9 has an inlet 10 for the BOG stream 11 to be compressed. This inlet 10 can be fluidly connected, for example, with the gas phase region of a liquefied 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 intermediate compressed BOG stream 11 to a final pressure.

The final pressure depends on the composition of the substance mixture constituting the BOG stream 11 and increases with the content of volatile components in the substance mixture or BOG stream 11 .

As the maximum critical final pressure for the actuation of the actuator 7 , ie the valve, a limit final pressure is determined.

The final compression stage 12 has an outlet 13 for the final compressed BOG stream 11 , which outlet is fluidly connected 15 to the BOG stream inlet 14 of the condenser 3 . In the condenser 3 , the final compressed BOG stream 11 is cooled to a predetermined temperature irrespective of the final pressure. The condenser 3 can thus be cooled, for example, with seawater.

Thus, in the case of BOG stream 11 with volatile components, the defined limiting final pressure is not sufficient to condense all volatile components of BOG stream 11 to the existing condenser temperature, so that BOG stream 11 is only partially condensed. do.

Hereinafter, the BOG stream exiting the condenser 3 is generally referred to as the fluid stream 11a, since it may contain components in liquid and/or gaseous form. The associated outlet is therefore referred to as the fluid flow outlet 16 .

The fluid flow outlet 16 of the condenser 3 is fluidly connected 18 with the fluid flow inlet 17 of the fluid container 4 .

The fluid container 4 is above a predetermined fluid container volume 20 of the fluid container 4 and is in fluid connection 22 with the fluid flow inlet 21 of the cooling device 5 , the fluid flow outlet 19 . ) has

In the fluid container 4 , the gas phase and liquid phase of the fluid are separated into a lower liquid phase region 23 and an upper gas phase region 24 . In the case of the fluid container 4 , the target liquid level 25 is determined at predetermined intervals at or above the level of the upper edge of the fluid flow outlet 19 .

A level sensor 26 for measuring the liquid level is also arranged in the fluid container 4 . A measurement signal is transmitted to the actuator or valve control device 7a, by means of which the actuator or valve 7 in the fluid flow discharge line 8 downstream of the cooling device 5 moves to the open or closed position. can get

The cooling device (5) has a heat exchanger (27) which has an inlet (28) and an outlet (29) for the coolant (30) and an inlet (31) for the fluid stream (11a) and a cooled fluid flow It has an outlet (32) for (11b).

The fluid flow inlet 31 of the heat exchanger 27 is fluidly connected 22 with the fluid flow outlet 19 of the M fluid container 4 , and the fluid flow outlet 32 of the heat exchanger 27 comprises: It is fluidly connected with the fluid flow discharge line (8).

In the heat exchanger 27 , the fluid stream 11a is cooled to a temperature corresponding to the saturation temperature of the fluid stream 11a at a pressure lower than the final pressure.

The reliquefied BOG is used as coolant 30 . The fluid container 4 in the embodiment shown in FIG. 1 additionally has a bottom outlet 33 , which outlet thus provides a second outlet of the fluid container 4 only for, in particular, the reliquefied BOG, ie the liquid flow. to form

The bottom outlet 33 is connected to the coolant inlet 28 of the heat exchanger 27 via a supply line 34 . A supply valve 35 is arranged in the supply line 34 , wherein the magnitude of the coolant volume flow entering the heat exchanger 27 can be changed by means of the supply valve so that the supply valve acts as an adjustable throttle. do.

The difference in the measured temperature can be used as a control variable for the magnitude of the coolant volume flow rate.

The device thus comprises a number of temperature sensors:

A first temperature sensor (36) for measuring the outlet temperature (T1) of the coolant stream exiting the heat exchanger (27).

A second temperature sensor (37) for measuring the temperature (T2) of the coolant (30) prior to the depressurization of the coolant (30).

A third temperature sensor (38) for measuring the temperature (T3) of the coolant (30) entering the heat exchanger (27) after depressurization of the coolant to an intermediate pressure.

A fourth temperature sensor (39) for measuring the outlet temperature (T4) of the fluid stream (11b) under the final pressure discharged from the heat exchanger (27) and cooled.

The following possibilities are provided for regulating the magnitude of the coolant volume flow entering the heat exchanger 27 using the throttle, for example:

First possibility:

When the outlet temperature T1 falls below the coolant temperature T2 by less than a predetermined amount, the coolant volume flow rate increases; and

When the outlet temperature T1 falls below the coolant temperature T2 by more than a predetermined amount, the coolant volume flow rate decreases.

Second possibility:

When the outlet temperature T1 exceeds the coolant temperature T3 by less than a predetermined amount, the coolant volume flow rate decreases; and

When the outlet temperature T1 exceeds the coolant temperature T3 by more than a predetermined amount, the coolant volume flow rate increases.

Third possibility:

When the outlet temperature T4 exceeds the coolant temperature T3 by less than a predetermined amount, the coolant volume flow rate decreases; and

When the outlet temperature T4 exceeds the coolant temperature T3 by more than a predetermined amount, the cooling volume flow rate increases.

Each of the three possibilities can be used alone or a combination of one or two other possibilities for the regulation of the coolant volume flow.

The coolant volume flow rate should be adjusted so that the coolant flow completely evaporates when it leaves the heat exchanger 27 once the fluid flow in the heat exchanger has cooled sufficiently.

All three possibilities can also be combined with further regulation of the coolant volume flow based on additional temperature measurements.

To this end, the device 1 additionally has a fifth temperature sensor 40 which measures the temperature T5 of the final compressed BOG stream 11 , ie before entering the condenser 3 , in which case the throttle is the temperature When (T5) reaches or exceeds a predetermined threshold temperature for the final compressed BOG stream (11), it is set to increase the coolant volumetric flow rate.

The measurement signals of the temperature sensors T1-T5 are transmitted to the supply valve control device 35a, and in order to set the respective predetermined magnitude of the coolant volume flow entering the heat exchanger 27, the supply valve 35 can be moved to each predetermined throttle position by the control device.

The coolant outlet 29 of the heat exchanger 27 is fluidly connected 42 with the coolant inlet 41 of the evaporation device 6a, which in the illustrated embodiment is designed as a droplet separator 6 .

The droplet separator 6 also has a BOG flow inlet 43 , which in the illustrated embodiment fluidly connects with the BOG flow outlet 44 of the first compression stage 9 and through which intermediate compression A partial stream 46 of the hot BOG stream 11 is introduced.

The coolant flow exiting the heat exchanger 27 is thus mixed with the hot gas of the first compression stage 9 at medium pressure in the droplet separator 6 . If there is still a residual amount of liquid coolant in the coolant stream exiting the heat exchanger 27, it is evaporated by the hot gas.

For example, if the coolant volume flow rate is too high to temporarily complete evaporation, the liquid coolant is collected in the droplet separator 6, forming a liquid phase region 47 in the lower region of the droplet separator 6 and vapor phase in the upper region A region 48 is formed.

All of the above-mentioned possibilities of adjusting the coolant volume flow rate based on the measured temperatures T1 to T5 may also be combined with a further control of the coolant volume flow rate based on liquid level or fill level measurements of the droplet separator 6 .

Accordingly, a droplet separator level sensor 49 for measuring the liquid level in the droplet separator 6 is arranged in the droplet separator 6 . The throttle, ie the supply valve 35, is

- if the measured liquid level exceeds a predetermined maximum liquid level, to reduce the coolant volume flow; and

- it is set to increase the coolant volume flow rate when the measured liquid level falls below a predetermined maximum liquid level;

The measurement signal of the droplet separator level sensor 49 is transmitted to the supply valve control device 35a, in order to set the respective predetermined magnitude of the coolant volume flow entering the heat exchanger 27, the supply valve 35 is It can be moved to each predetermined throttle position by the control device.

The gas phase region 48 of the droplet separator 6 has a mixed flow outlet 50 from which a mixture of evaporated coolant and a hot BOG partial stream is discharged, which outlet is connected to the first compression stage 9 and the final It is fluidly connected 51 to the BOG flow 11 between the compression stages 12 , and in the illustrated embodiment to the BOG flow inlet 52 of the second or final compression stage 12 . Thus, on the one hand, evaporated coolant, ie BOG in gaseous form, is 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 prevailing at the connection or introduction point 53 between the first compression stage 9 and the final compression stage 12 also prevails in the droplet separator 6 and thus the heat exchanger 27 . The boundary between the final pressure and the intermediate pressure in the supply line 34 is the throttle, ie the supply valve 35 . The intermediate pressure in the illustrated case of the two-stage compressors 2 , 9 , 12 corresponds to the compression pressure of the first compression stage 9 .

In the illustrated embodiment the actuator 7 is a valve arranged in the fluid flow outlet line 8 connected to the fluid flow outlet 32 of the heat exchanger 27 . Hereinafter this valve is referred to as the discharge valve 7 . The valve can be moved to an open and closed position and is actuated as follows.

The BOG stream 11 is at a final pressure state from the discharge from the final compression stage 12 of the compressor 2 (referred to as the fluid stream 11a from the discharge from the condenser 3 ). This final pressure extends from the outlet of the BOG stream 11 of the final compression stage 12 of the compressor 2 to the discharge valve 7 in the fluid flow discharge line 8 downstream of the heat exchanger 27 and reaches the final pressure state. measured by a pressure sensor 54 placed at any point within the area of . For example, this pressure sensor 54 may be disposed between the final compression stage 12 and the condenser 3 or within the fluid container 4 . The measurement signal is transmitted to the discharge valve control device 7a, by means of which the discharge valve 7 in the fluid flow discharge line 8 can be moved to the open position or the closed position, in which case it is reloaded from the open position. Liquefied BOG is supplied for further use, for example introduced into a liquefied gas tank.

The fluid stream 11a , 11b or the final compressed BOG stream 11 upstream of the discharge valve 7 is thus the final pressure state, ie the maximum at a predetermined limit final pressure state.

When the coolant 30 at its final pressure or limit final pressure flows through the supply valve 35 towards the heat exchanger 27, the coolant 30 is reduced to an intermediate pressure and thus cooled.

Due to the high pressure condition of the fluid stream 11a flowing from the fluid container 4 to the heat exchanger 27 at the limiting final pressure and the relatively low temperature level in the heat exchanger 27, the fluid stream 11a is not fluid at the intermediate pressure. Cooling to a temperature close to the saturation temperature of stream 11a, the gaseous content of fluid stream 11a will condense in this state, and open discharge valve (at limit final pressure) in fluid flow discharge line 8 ( Only BOG reliquefied via 7) or only BOG almost reliquefied is discharged to the tank, for example.

As soon as the final pressure falls below the limit final pressure, this discharge valve 7 again closes until the target liquid level 25 of the fluid container 4 is reached and the discharge valve 7 opens again.

The actuator or discharge valve position is controlled by the measurement signals of the level sensor 26 and the pressure sensor 54 as follows:

A) open position

The discharge valve (7) or actuator in the fluid flow discharge line (8),

a) if the level corresponds at least to the target level 25 , and/or

b) if the final pressure reaches the limit final pressure,

moved to the open position.

case a)

Since the upper edge of the fluid flow outlet 19 of the fluid container 4 is at or below the target liquid level 25 by a predetermined limit, when the target liquid level 25 is reached, the Only the fluid, ie the reliquefied BOG flows into the heat exchanger 27 and further into the fluid flow outlet line 8 .

case b)

When the final pressure limit is reached, the fluid is at a relatively high pressure, so even if the cooling (eg 1°K) is weak, the cooling from the limit final pressure to a temperature below the saturation temperature of the fluid flow is the gas in the fluid stream 11a. The fluid stream 11a exiting the heat exchanger 27 resulting in a high level of further condensation of the components is almost completely or even entirely liquid.

B) closed position

The discharge valve (7) or actuator in the fluid flow discharge line (8),

when the level falls below the target level (25), and

When the final pressure is less than the limit final pressure,

closed position.

If the content of non-condensed BOG increases (e.g. because the content of volatile components in the BOG has increased or because the seawater 55 is warmer in the seawater-cooled condenser 3), the gaseous content in the fluid increases (and thus The liquid content decreases) and the final pressure increases.

When the level drops below the target level 25 and further below the upper edge of the fluid flow outlet opening 19 of the fluid container 4, the boundary between the gaseous region 24 and the liquid phase 23 of the fluid is first in the area of the fluid flow outlet 19 of the container 4 . In this case, the mixture of gas and liquid is discharged from the fluid container 4 and introduced into the heat exchanger 27 .

When the liquid level drops to such a degree that the fluid flow outlet 19 is completely within the gas phase region 24 of the fluid container 4 , only BOG in gaseous form is discharged.

Since the actuator or discharge valve 7 in the fluid flow discharge line 8 downstream of the heat exchanger 27 is closed, the fluid stagnates, as a result of which the level in the fluid container 4 rises again. This is because the partially condensed BOG stream 11a leaving the condenser 3 and entering the fluid container 4 contains an increased gaseous content, but still a liquid content.

As described above, on the one hand, as the content of non-condensable components in the BOG increases, the final pressure increases and on the other hand, the liquid level in the fluid container 4 rises, so that over time A)a) and A ) when at least one of the two states described above in b) is reached, the actuator or discharge valve 7 is opened again.

The measures according to the invention for the reliquefaction of BOG are further described below with reference to the embodiment shown in FIG. 1 including numerical examples. In the numerical example, the BOG to be reliquefied is discharged from a liquefied gas tank for propane and the condenser 3 is seawater cooled. The pressure- and temperature conditions as well as the liquid- and gas compositions specified below of the individual method steps/device elements are based on flash calculations using National Institute of Standards and Technology (NIST) data.

a) in liquefied gas tanks from which BOG is removed for reliquefaction

Liquid: propane

Ethane content 5% mol

BOG: About 26% mol of ethane

pressure: 1 bar a

b) in the two-stage compressor (2)

BOG flow: About 26% mol of ethane

Medium pressure: 5 bar a

Final pressure: 21 bar a

For the final compressed BOG stream 11 leaving the compressor 2 the temperature for complete condensation at a pressure of 21 bar a and an ethane content of about 26% mol is about 25° C.

c) in the condenser (3)

Refrigerant side:

Seawater 55 having a water temperature of 32 ° C.

The heat introduction in the condenser 3 results in a condensation temperature of about 40° C.

Thus, the BOG stream 11 is only partially condensed.

Gas/Condensate Side:

Pressure (final pressure): 21 bar a

Inlet BOG flow (11): About 26% mol of ethane

Partially condensed discharged stream 11a:

(more, about 97% mol BOG) Liquid content (condensate): ethane content about 25% mol

(lesser, approx. 3% mol BOG) Non-condensable gas content: ethane content approx. 45% mol

d) in the fluid container (4)

Liquid content (condensate): About 25% mol of ethane

Gas content: About 45% mol of ethane

Pressure (final pressure): 21 bar a

e) in the heat exchanger (27)

Refrigerant side:

in the supply line (34) upstream of the supply valve (35)

Condensate: About 25% mol of ethane

Pressure (final pressure): 21 bar a

In the supply line 34 downstream of the supply valve 35 , ie in the heat exchanger 27 .

Pressure (medium pressure): 5 bar a (reduced from final pressure to medium pressure)

Condensate: temperature of about -6.5 ℃

ethane content of about 8% mol

(The values for temperature and ethane content are adjusted, as the pressure release causes some of the ethane to evaporate, increasing the propane content in the condensate.)

Fluid side:

Gas content: About 45% mol of ethane

Pressure (final pressure): 21 bar a (gas content fully liquefied)

If the ethane content in the gas content on the fluid side is about 45% mol and the temperature on the coolant side is about -6.5°C, the saturation pressure for the gas content on the fluid side is about 10 bar a. The final pressure on the fluid side is 21 bar a, so the gas content in the fluid is completely liquefied.

Claims (27)

A method for reliquefaction of BOG comprising volatile components, comprising the steps of:
a) compressing the BOG stream (11) in at least two compression stages (9, 12); wherein the BOG stream 11 exits from the final compression stage 12 as a final compressed BOG stream 11 having a final pressure;
b) condensing the final compressed BOG stream (11) to obtain an at least partially condensed final compressed fluid stream (11a);
c) providing a fluid container (4) having a fluid flow inlet (17) and a fluid flow outlet (19), the position of the fluid flow outlet (19) being selected to lie above a predetermined fluid receiving volume phosphorus, step;
d) introducing the fluid flow 11a from step b) into the fluid container 4 through the fluid flow inlet 17 ;
e) determining the target level 25 for the fluid container 4 in such a way that the target level 25 lies at or above the level of the upper edge of the fluid flow outlet 19 by a predetermined limit;
f) setting an upper final pressure for the final compression stage (12);
g) measuring the liquid level in the fluid storage tank (4);
h) measuring the final pressure;
i) discharging the fluid stream (11a) from the fluid container (4) through the fluid flow outlet (19);
j) cooling the fluid stream (11a) from step i) to a temperature corresponding to the saturation temperature of the fluid stream (11a) at a pressure lower than the final pressure, in order to condense the gaseous component of the fluid stream (11a); Steps to follow:
j1) providing a heat exchanger (27);
j2) introducing the fluid stream 11a from step i) into the heat exchanger 27;
j3) introducing a flow of liquid from the fluid container (4) as a coolant (30) through the throttle (35) into the heat exchanger (27);
j4) discharging the coolant flow from the coolant outlet (29) of the heat exchanger (27);
j5) the intermediate pressure present at the connection point 53 to the BOG stream 11 such that the coolant 30 entering the heat exchanger 27 is reduced from the final pressure to the intermediate pressure and thereby at least partially evaporated. of the BOG stream 11 between the first compression stage 9 and the final compression stage 12 and the heat exchanger 27 to establish in the heat exchanger 27 a pressure corresponding to forming a fluid connection (45) between the coolant outlets (29);
j6) introducing the coolant flow from step j4) into the BOG flow 11 between the first compression stage 9 and the final compression stage 12;
j7) if the coolant flow from step j4) still contains a residual amount of liquid coolant, evaporating said residual amount before entering into a subsequent compression stage;
which comprises, cooling;
k) delivering the cooled fluid stream 11b from the heat exchanger 27 when the measured liquid level is at least equal to the target liquid level 25 and/or when the measured final pressure is equal to the limit final pressure
A method comprising: According to claim 1,
In step j7) the compressed BOG partial stream 46 is branched from the BOG stream 11 between the condenser 3 connected to the first compression stage 9 and the final compression stage 12, and in step j4 ) mixed with the coolant flow from ), thus evaporating a possible residual amount of liquid coolant. 3. The method of claim 2,
Step j7) is followed by the steps:
j7.1) providing a droplet separator (6);
j7.2) introducing a flow of coolant from step j4) into the droplet separator (6);
j7.3) introducing the BOG partial flow of claim 2 into the droplet separator (6);
j7.4) discharging the mixture of the BOG partial stream and the evaporated coolant from the droplet separator (6);
further comprising; and
Method according to claim 6, characterized in that step j6) comprises introducing the mixture of step j7.4) into the BOG stream (11) between the first compression stage (9) and the final compression stage (12). 4. The method of claim 3,
Step j7) is followed by the steps:
j7.5) determining the maximum liquid level for the droplet separator (6);
j7.6) measuring the liquid level in the droplet separator (6);
j7.7) reducing the amount of coolant 30 flowing from step j3) when the measured level reaches or exceeds the maximum level
Method, characterized in that it further comprises. 5. The method according to any one of claims 1 to 4,
Step j3) is followed by the steps:
j3.1a) measuring the outlet temperature (T1) of the coolant flow from step j4);
j3.2a) measuring the temperature (T2) of the coolant (30) prior to the depressurization of the coolant (30);
j3.3a) the coolant 30 flowing into the heat exchanger 27 when the outlet temperature T1 from step j3.1a) falls below the coolant temperature T2 from step j3.2a) by less than a predetermined amount ) increasing the amount;
j3.4a) when the outlet temperature T1 from step j3.1a) falls below the coolant temperature T2 from step j3.2a) by more than a predetermined amount, the coolant flowing into the heat exchanger 27 ( 30) reducing the amount of
Method, characterized in that it further comprises. 6. The method of claim 5,
A method, characterized in that 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 6,
Step j3) is followed by the steps:
j3.1b) measuring the outlet temperature T1 of the coolant flow from step j4);
j3.2b) measuring the temperature (T3) of the coolant (30) flowing into the heat exchanger (27) after the pressure of the coolant is reduced to the intermediate pressure;
j3.3b) the coolant 30 entering the heat exchanger 27 when the outlet temperature T1 from step j3.1b) exceeds the coolant temperature T3 from step j3.2b) by less than a predetermined amount ) reducing the amount of;
j3.4b) when the outlet temperature T1 from step j3.1b) exceeds the coolant temperature T3 from step j3.2b) by more than a predetermined amount, the coolant flowing into the heat exchanger 27 ( 30) increasing the amount.
Method, characterized in that it further comprises. 8. The method according to any one of claims 1 to 7,
Step j3) is followed by the steps:
j3.1c) measuring the outlet temperature T4 of the fluid stream 11b at the final pressure from step k), discharged from the heat exchanger 27 and cooled;
j3.2c) measuring the temperature (T3) of the coolant (30) flowing into the heat exchanger (27) after the pressure of the coolant is reduced to an intermediate pressure;
j3.3c) the coolant 30 entering the heat exchanger 27 when the outlet temperature T4 from step j3.1c) exceeds the coolant temperature T3 from step j3.2c) by less than a predetermined amount ) reducing the amount of;
j3.4c) when the outlet temperature T4 from step j3.1c) exceeds the coolant temperature T3 from step j3.2c) by more than a predetermined amount, the coolant flowing into the heat exchanger 27 ( 30) increasing the amount of
Method, characterized in that it further comprises. 9. The method according to any one of claims 1 to 8,
Step j3) is also followed by the following steps:
j3.1d) determining a threshold temperature for the final compressed BOG stream (11);
j3.2d) measuring the temperature (T5) of the final compressed BOG stream (11);
j3.3d) if the temperature T5 measured in step j3.2d) reaches or exceeds the limit temperature from step j3.1d), increase the amount of coolant 30 flowing into heat exchanger 27 step to let
Method characterized in that it further comprises 10. The method according to any one of claims 1 to 9,
A method, characterized in that in step j3), the flow of liquid at the bottom of the fluid container (4) is removed from the fluid container. 11. The method according to any one of claims 1 to 10,
Method characterized in that it is condensed using seawater (55) in step b). An apparatus for carrying out the method according to any one of claims 1 to 11, comprising:
- at least a two-stage compressor (2),
- the first compression stage (9) of the compressor has an inlet (10) for the BOG stream (11), and
- at least two stages, wherein the final compression stage ( 12 ) of the compressor finally compresses the BOG stream ( 11 ) to a final pressure and has a BOG stream outlet ( 13 ) for the final compressed BOG stream ( 11 ) of compressor (2);
- as a condenser (3),
- having a BOG flow inlet (14) in fluid connection (15) with a BOG flow outlet (13) of the final compression stage (12);
- configured to at least partially condense the final compressed BOG stream (11) to form a fluid stream (11a),
- a condenser (3), which has a fluid flow outlet (16);
- a fluid container (4), comprising:
- a fluid flow inlet (17), fluidly connected (18) with the fluid flow outlet (16) of the condenser (3);
- a fluid flow outlet (17) overlying a predetermined fluid receiving volume (18);
- a fluid container (4) comprising 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),
- as a heat exchanger (27),
- a fluid flow inlet (31) in fluid communication with the fluid flow outlet (19) of the fluid container (4); and
- a fluid flow outlet (32) from which the cooled fluid stream (11b) is discharged and forms a fluid flow outlet of the cooling device (5),
- a coolant inlet (28) fluidly connected with an outlet (33) of a fluid container (4) for a liquid flow through a throttle (35), said liquid flow forming a coolant flow and the throttle (35) is a coolant volume a coolant inlet (28) configured to limit the flow rate to a predetermined limit;
- for introducing the evaporated coolant into the stream (11) as said BOG and for setting in the heat exchanger (27) a pressure corresponding to the intermediate pressure present at the connection point (53) and thus lower than the final pressure; 1 having a heat exchanger (27) with a coolant outlet (29) in fluid connection (42, 51) with the BOG stream (11) between the compression stage (9) and the final compression stage (12); ,
- a cooling device (5), configured to cool the fluid stream (11a) at a pressure lower than the final pressure to a temperature corresponding to the saturation temperature of the fluid stream (11a);
- as an actuator (7),
- fluidly connected (26) with the fluid flow outlet (32) of the cooling device (5);
- when the measured liquid level is at least equal to the target liquid level (25) and/or when the measured final pressure is equal to a predetermined limit final pressure, can be moved to the open position to deliver the cooled fluid flow (11b) there is, and
- actuator (7), which can otherwise be moved to a closed position blocking the cooled fluid flow (11b)
A device comprising a. 13. The method of claim 12,
Device, characterized in that the actuator is a valve (7). 14. The method of claim 12 or 13,
Between the coolant outlet 29 of the heat exchanger 27 and the connection point 53 for the BOG stream 11 flowing between the first compression stage 9 and the final compression stage 12, an evaporation device 6a and wherein the evaporating device is configured to evaporate any residual amount of liquid coolant that may be present in the coolant flow. 15. The method of claim 14,
The evaporation device (6a) is provided with a droplet separator (6),
The droplet separator is
- a coolant inlet 41 fluidly connected with the coolant outlet 29 of the heat exchanger 27;
- a BOG flow inlet, fluidly connected (45) with the BOG partial stream (46) of the BOG stream (11) flowing between the first compression stage (9) and the condenser (3) connected to the final compression stage (12); 43),
- a mixture consisting of the evaporated coolant and the BOG partial stream is discharged therefrom and is fluidly connected to a connection point 53 for the BOG stream 11 between the first compression stage 9 and the final compression stage 12 ( 51), mixed flow outlet (50)
A device comprising a. 16. The method of claim 15,
A droplet separator level sensor (49) for measuring a liquid level in the droplet separator (6) is arranged in the droplet separator (6), and the throttle (35) comprises:
- to reduce the coolant flow rate if the measured level exceeds a predetermined maximum level, and
- A device, characterized in that it is configured to increase the coolant volume flow rate if the measured liquid level falls below a predetermined maximum liquid level. 17. The method according to any one of claims 12 to 16,
- device characterized by a first temperature sensor (36) for measuring the outlet temperature (T1) of the coolant flow exiting the heat exchanger (27). 18. The method according to any one of claims 12 to 17,
- a device characterized by a second temperature sensor (37) for measuring the temperature (T2) of the coolant (30) prior to the depressurization of the coolant (30). 19. The method according to any one of claims 12 to 18,
- a device characterized by a third temperature sensor (38) for measuring the temperature (T3) of the coolant (30) entering the heat exchanger (27) after the pressure of the coolant has been reduced to an intermediate pressure. 20. The method according to any one of claims 12 to 19,
- a device characterized by a fourth temperature sensor (39) for measuring the outlet temperature (T4) of the cooled fluid stream (11b) discharged from the heat exchanger (27) and brought to the final pressure state. 19. The method of claim 17 and 18,
The throttle 35 is
- increase the coolant volume flow rate when the outlet temperature T1 from claim 17 is below the coolant temperature T2 from claim 18 by less than a predetermined amount;
- device, characterized in that it is set to reduce the coolant volume flow rate when the outlet temperature (T1) from claim 17 is below the coolant temperature (T2) from claim 18 by more than a predetermined amount. 20. The method of claim 17 and 19,
The throttle 35 is
- to reduce the coolant volume flow rate if the outlet temperature (T1) from claim 17 exceeds the coolant temperature (T3) from claim 19 by less than a predetermined amount;
- Device, characterized in that it is set to increase the coolant volume flow rate when the outlet temperature (T1) from claim 17 exceeds the coolant temperature (T3) from claim 19 by more than a predetermined amount. 21. The method of claim 19 and 20,
The throttle 35 is
- to reduce the coolant volume flow rate if the outlet temperature (T4) from claim 20 exceeds the coolant temperature (T3) from claim 19 by less than a predetermined amount;
- Device, characterized in that it is set to increase the coolant volume flow rate when the outlet temperature (T4) from claim 20 exceeds the coolant temperature (T3) from claim 19 by more than a predetermined amount. 24. The method according to any one of claims 12 to 23,
- a fifth temperature sensor (40) for measuring the temperature (T5) of the final compressed BOG stream (11);
- the throttle 35 is the coolant volume flow rate when the temperature T5 measured by the fifth temperature sensor 40 reaches or exceeds a predetermined limit temperature for the final compressed BOG stream 11 . Device characterized in that it is set to increase. 25. The method according to any one of claims 12 to 24,
Device, characterized in that the outlet (33) of the fluid container (4) for the liquid flow (30) is formed in the bottom of the fluid container (4). 26. The method according to any one of claims 12 to 25,
The device, characterized in that the coolant of the condenser (3) is seawater (55). As a ship,
27. A ship equipped with the device according to any one of claims 12 to 26.

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