PROCESS AND APPARATUS FOR RELIQUEFACTION AND RECYCLING OF BOG INTO AN LNG TANK

Abstract

Method and apparatus for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank, the method including: Withdrawing BOG (F2) from the headspace of an LNG tank; compressing the BOG in a first compression stage to a first pressure p.sub.1 between 8 and 18 bara and tapping of a first portion of this gas; further compressing a second portion of the gas from step in a final compression stage to a second pressure p.sub.2?120 bara; cooling at least part of the further compressed gas to a first temperature T.sub.1 between ?20? C. and ?100? C.; expanding the gas from step to a third pressure p.sub.3 between 8 and 20 bara; and separating the gas from step into a liquid phase and a gaseous phase to combine the gaseous phase with the tapped first portion of the gas from the first compression stage and to return the liquid phase into the LNG tank.

Claims

1. A method for reliquefying and returning boil-off (BOG) to a liquefied natural gas (LNG) tank, comprising the steps of: a) withdrawing BOG from the headspace of an LNG tank; b) compressing the BOG in a first compression stage to a first pressure p.sub.1 between 8 and 18 bara and tapping of a first portion of this gas; c) further compressing a second portion of the gas from step b) in a final compression stage to a second pressure p.sub.2?120 bara; d) cooling at least part of the further compressed gas from step c) to a first temperature T.sub.1 between ?20? C. and ?100? C.; e) expanding the gas from step d) to a third pressure p.sub.3 between 8 and 20 bara; f) separating the gas from step e) into a liquid phase and a gaseous phase to f.sub.1) combine the gaseous phase with the tapped first portion of the gas from step c); and f.sub.2) return the liquid phase to the LNG tank.

2. The method according to claim 1, wherein in step f.sub.2) the liquid phase before being returned to the LNG tank is cooled to a temperature T.sub.2 between ?140 and ?161? C.

3. The method according to claim 1, wherein the cooling in step d) is carried out at least partly by heat exchange with cooling BOG from the headspace of the LNG tank.

4. The method according to claim 1, wherein the cooling in step d) is carried out at least partly by heat exchange with the gaseous phase from step f).

5. The method according to claim 1, wherein in step d) a portion of the further compressed gas from step c) is fed to a supply line for a high-pressure gas injection engine.

6. The method according to claim 1, wherein in step f) the pressure p.sub.3 is monitored and controlled so that it has a value within a predetermined range.

7. The method according to claim 1, wherein in step f) a volume of the liquid phase is monitored in order to regulate the return quantity into the LNG tank as a function of the value.

8. An apparatus for reliquefying and returning boil-off gas (BOG) into a liquefied natural gas (LNG) tank comprising a first heat exchanger comprising a line for passing through cooling fluid and a line for passing through compressed gas to be cooled; a multi-stage compressor comprising at least a first compression stage and a final compression stage, the first compression stage being configured to compress BOG from the LNG tank to a first pressure p.sub.1 between 8 and 18 bara and wherein the final compression stage is configured to compress pre-compressed BOG to a second pressure p.sub.2?120 bara; a branch line which is arranged downstream of the first compression stage in a fluid-conducting manner and which opens further downstream into at least one of a supply line for a low-pressure gas injection engine and a gas combustion unit; a return line; a first expansion unit configured to expand compressed gas from a second pressure p.sub.2 to a third pressure p.sub.3, wherein p.sub.3 is between 8 and 20 bara; a gas-liquid separator configured to separate a liquefied gas portion for feeding back into the LNG tank at a pressure p.sub.3 and to feed a gaseous portion into a bypass line, the bypass line opening into the branch line; wherein the multi-stage compressor is connected upstream in a fluid-carrying manner to the headspace of the LNG tank, and wherein the multi-stage compressor is connected downstream in a fluid-carrying manner via the return line to the line of the first heat exchanger for passing through compressed gas to be cooled, is connected further downstream to the first expansion unit, and is connected still further downstream to the gas-liquid separator.

9. The apparatus according to claim 8, further comprising a second heat exchanger, having a line for passing through cooling fluid, and a line for passing through compressed gas to be cooled; wherein in the second heat exchanger the line for passing through compressed gas to be cooled is arranged in a fluid-conducting manner between the liquid outlet of the gas-liquid separator and the LNG tank.

10. The apparatus according to claim 8, further comprising a third heat exchanger, the cooling line of which is part of the bypass line and the line to be cooled is part of the return line.

11. The apparatus according to claim 8, further comprising a second expansion unit configured to expand compressed gas from the third pressure p.sub.3 to atmospheric pressure, wherein the second expansion unit is configured to conduct fluid between the liquid outlet of the gas-liquid separator and the LNG tank.

12. The apparatus according to claim 8, wherein the apparatus is part of a fuel gas supply system for supplying a high-pressure gas injection engine with gas stored in the LNG tank, additionally comprising an outlet which is arranged downstream of the second compression stage of the multi-stage compressor in a fluid-conducting manner and opens further downstream into a supply line for a high-pressure gas injection engine, whereby the compressed gas, insofar as the quantity exceeds the fuel requirement of the high-pressure gas injection engine, can be fed to the return line.

13. The apparatus according to claim 8, wherein the gas-liquid separator comprises a pressure sensor to measure the pressure in the gas-liquid separator, and a controller to actuate a valve arranged between a gas outlet of the gas-liquid separator and the bypass line as a function of the measured pressure.

14. The apparatus according to claim 8, wherein the gas-liquid separator comprises a level sensor and a controller to actuate a valve arranged between a liquid outlet of the gas-liquid separator and the LNG tank as a function of the measured level.

15. (canceled)

16. The method according to claim 2, wherein cooling of the liquid phase is carried out by heat exchange with cooling BOG from the headspace of the LNG tank.

17. The apparatus according to claim 8, wherein the cooling fluid is BOG from an LNG tank.

18. The apparatus according to claim 8, wherein the first heat exchanger is configured for heat exchange between the line for passing through cooling fluid and the line for passing through compressed gas to be cooled in counterflow.

19. The apparatus according to claim 8, wherein the multi-stage compressor is connected upstream in a fluid-carrying manner to the headspace of the LNG tank via the line of the heat exchanger for passing through cooling BOG.

20. The apparatus according to claim 9, wherein the line for passing through cooling fluid is arranged in a fluid-conducting manner between the head space of the LNG tank and the line of the first heat exchanger for passing through cooling fluid.

21. The apparatus according to claim 11, wherein the second expansion unit is configured to conduct fluid between the line of the second heat exchanger for passing through compressed gas to be cooled and the LNG tank.

Description

[0049] The invention is further explained by means of figures. The figures are for illustrative purposes and are not to be understood as limiting.

[0050] It shows:

[0051] FIG. 1 Schematic representation of an apparatus according to the present invention;

[0052] FIG. 2 Schematic Mollier diagram illustrating a method according to the present invention.

[0053] FIG. 1 is a schematic representation of an apparatus for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank. The BOG F2 is removed from the LNG tank 3 at approx. ?161? C. and initially fed to the second heat exchanger 21, where it is passed as a cooling fluid through line 5 in counterflow to the reliquefied gas to be cooled. Further downstream, the BOG is fed to a first heat exchanger 20, namely a line for passing cooling fluid, in counterflow to compressed gas to be cooled. The BOG thus heated to temperatures of approximately 30? C. is then fed to a multi-stage compressor 10 and compressed in a first compression stage 70a. Preferably, the first compression stage has one or two piston compressors 71, 72 connected in parallel or in series, each with subsequent water cooling. The first compression stage 70a is set up to compress the BOG to a first pressure p.sub.1 of, for example, 12 bara. After the first compression stage 70a, a branch line 6 is arranged downstream in a fluid-conducting manner, which opens further downstream into a supply line for a low-pressure gas injection engine 4. The gas can be depressurized to the pressure required by the gas injection engine 4, for example 6 bara, by a valve arranged on the branch line 6. The figure shows that the multistage compressor arrangement 10 has a second compression stage, which is also the final compression stage 70b. The final compression stage 70b is configured to compress pre-compressed BOG to a second pressure p.sub.2 of approx. 300 bara. This is achieved by means of three piston compressors 73, 74, 75, each with subsequent water cooling. However, a different number or type of compressors can also be used and they can be connected in parallel or in series.

[0054] Downstream of the multi-stage compressor arrangement 10, a fluid-conducting outlet 7 is arranged on the one hand, which opens into a supply line for a high-pressure gas injection engine 2. On the other hand, a return line 8 is arranged, the contents of which are indirectly cooled further downstreamamong other thingsto a temperature of approx. ?70? C. using the first heat exchanger 20. As a result, the compressed gas can be fed to the return line 8 if the quantity exceeds the fuel requirement of the high-pressure gas injection engine 2. Further downstream of the first heat exchanger 20, a first expansion unit 30 is connected, which is set up to expand the compressed and cooled gas is enthalpically from the pressure p.sub.2 to a third pressure p.sub.3 of approx. 15 bara, whereby the temperature is further reduced to approx. ?110? C.

[0055] A gas-liquid separator 40 is connected downstream of the expansion unit 30, which is set up to separate a liquefied gas portion for feeding back into the LNG tank 3 at pressure p.sub.3 and to feed a gaseous portion into a bypass line 9, the bypass line 9 opening into the branch line 6. It can be seen from FIG. 1 that the exemplary device comprises a third heat exchanger 22, the cooling line of which is part of the bypass line 9 and the line to be cooled is part of the return line 8, the latter part corresponding to a section of the return line upstream of the first heat exchanger 20. Based on the embodiment shown, the highly compressed, water-cooled BOG from the last compression stage 70b is first cooled in indirect heat exchange with the gaseous phase from the gas-liquid separator before and then further cooled in indirect heat exchange with the BOG from the LNG tank before expansion and phase separation takes place.

[0056] The liquid phase leaves the gas-liquid separator 40 and is indirectly cooled further in the second heat exchanger 21 to a temperature T.sub.2 of only around ?155? C. using BOG, which was fed directly from the LNG tank and into a cooling fluid jacket 5. Finally, the liquid is expanded to atmospheric pressure in the expansion unit 31 and returned to the LNG tank.

[0057] The valves 80 and 31, respectively 50, serve to control the pressure and liquid level in the gas-liquid separator 40. They can be actuated as a function of a pressure and/or level measured in the gas-liquid separator. Optionally, a valve can also be arranged between the outlet of the gas-liquid separator 40 and the line of the second heat exchanger 21 for the passage of fluid to be cooled in order to control the fill level in the gas-liquid separator.

[0058] FIG. 2 shows a schematic Mollier diagram to illustrate a method according to the present invention. The x-axis shows the enthalpy of the system, the y-axis the pressure of the gas. Certain temperatures are shown as isothermal lines T.sub.w, T.sub.1 anddottedT.sub.2; likewise the boiling line and dewline. The method steps that are associated with a change in enthalpy, temperature and/or pressure are shown as dashed lines.

[0059] In step a), the BOG is withdrawn from the headspace of the LNG tank and heated to a temperature T.sub.w by ambient temperature but also by using it as a coolant in one or more heat exchangers. In step b, the BOG is compressed to a first pressure p.sub.1 of between 8 and 18 bara in a first compression stage, in this case comprising two compression operations with subsequent water cooling, and a first portion of this gas is tapped (not shown). In step c), the gas is further compressed to a high pressure p.sub.2 in a final compression stage, in this case consisting of three compression operations, each with subsequent water cooling. This is followed in step d) by the cooling of at least part of the further compressed gas from step c), initially by means of water cooling to T.sub.w and then to a first temperature T.sub.1 between ?20? C. and ?100? C. Step e) is followed by isenthalpic expansion to a third pressure p.sub.3 between 8 and 20 bara. Step f) follows with the separation of the gas into a liquid and a gaseous phase in order to combine the gaseous phase with the diverted first part of the gas from step b) (sub-step f.sub.1) and to return the liquid phase to the LNG tank 3 (sub-step f.sub.2).

[0060] FIG. 2 shows that in sub-step f.sub.2) the liquid phase is further cooled to a temperature T.sub.2 that is only slightly above the boiling point of natural gas before being returned to the LNG tank at ambient pressure, which corresponds to the cooling in the second heat exchanger 21 in FIG. 1. In sub-step f.sub.1) it can also be seen that the gaseous phase can be further expanded and/or heated again before combining with the pre-compressed BOG from the first compression stage b), for example in indirect heat exchange with compressed gas to be cooled, which corresponds to its use as a refrigerant in the heat exchanger 22 of FIG. 1.