METHOD AND APPARATUS FOR RELIQUEFACTION AND RECYCLING OF BOIL-OFF GAS INTO AN LNG TANK

Abstract

A method for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank, having the steps of: compressing a BOG taken from the headspace of an LNG tank to a pressure p.sub.high, cooling the compressed gas to a temperature T.sub.1, such as by water cooling; expanding at least part of the gas from step c) to a pressure p.sub.expand; cooling the gas expanded in step d), such as by a heat exchanger in counterflow with cooling BOG (F2) from the head space of the LNG tank 3 to a temperature T.sub.4; returning the gas from step e) to the LNG tank; wherein the pressure p.sub.high is at least 200 bar, and wherein the pressure p.sub.expand is 80 to 180 bar.

Claims

1-14. (canceled)

15. A method for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank, comprising the steps: a) removal of BOG from a headspace of an LNG tank; b) compressing the BOG to a pressure p.sub.high; c) cooling the compressed gas to a temperature T.sub.1; d) expanding at least part of the gas from step c) to a pressure p.sub.expand; e) cooling the gas expanded in step d) to a temperature T.sub.4; f) returning the gas from step e) to the LNG tank; wherein the pressure p.sub.high is at least 200 bar, and wherein the pressure p.sub.expand is 80 to 180 bar.

16. The method according to claim 15, wherein in step d) at least a first portion of the gas from step c) is supplied to a high-pressure gas injection engine via an outlet to an extent of a fuel requirement of the high-pressure gas injection engine, and at least a second portion of the gas from step c) is expanded to the pressure p.sub.expand.

17. The method according to claim 15, wherein step b) is preceded by step 0), and step 0) is a selection between an operating mode i) compressing the BOG to a pressure p.sub.high comprising steps b) to d); and an operating mode ii) compressing the BOG to a pressure p.sub.low, comprising the steps of b1) compressing the BOG to a pressure p.sub.low of 100 to 199 bar; and d1) expansion of at least part of the gas from step c) to a pressure p.sub.expand, if p.sub.low>p.sub.expand; wherein, in the case of operating mode ii), steps b1) and d1) replace steps b) and d).

18. The method according to claim 17, wherein in step 0) in operating mode i) a setpoint value p.sub.high in a range from 200 to 300 bar is selected, or in operating mode ii) a setpoint value p.sub.low in a range from 100 to 199 bar is selected.

19. A device for reliquefying and returning boil-off gas (BOG) into a liquefied natural gas (LNG) tank, comprising a heat exchanger, comprising a line for a passage of cooling fluid, and a line for a passage of compressed gas to be cooled; a multi-stage compressor arrangement which is set up to compress BOG from the LNG tank to a pressure p.sub.high; a first cooler; a return line; and a first expansion unit, which is set up to expand compressed gas from a pressure p.sub.high to a pressure p.sub.expand; wherein the multi-stage compressor arrangement is fluidly connected upstream to a withdrawal line opening into a head space of the LNG tank, and wherein the multi-stage compressor arrangement is fluidly connected downstream to the first cooler, further downstream via the return line to the first expansion unit, and still further downstream to the line of the heat exchanger for passing through compressed gas to be cooled, in order to return the compressed and cooled gas to the LNG tank; wherein the pressure p.sub.high is at least 200 bar, and wherein the pressure p.sub.expand is 80 to 180 bar.

20. The device according to claim 19, wherein the device is part of a fuel gas supply system for supplying a high-pressure gas injection engine with gas stored in an LNG tank, additionally comprising an outlet which is arranged downstream of the multi-stage compressor arrangement in a fluid-conducting manner and opens further downstream into a supply line for a high-pressure gas injection engine, wherein the compressed gas, insofar as its quantity exceeds a fuel requirement of the high-pressure gas injection engine, can be fed to the return line.

21. The device according to claim 19, further comprising a second expansion unit, which is set up to expand compressed gas from a pressure p.sub.expand to a pressure p.sub.knock-out-drum, a gas-liquid separator which is fluidly connected downstream of the second expansion unit and is set up to feed a liquefied gas portion back into the LNG tank at a pressure p.sub.knock-out-drum and to feed a gaseous portion into the withdrawal line; wherein the second expansion unit and the gas-liquid separator are arranged in a fluid-conducting manner between the line of the heat exchanger for the passage of compressed gas to be cooled and the LNG tank.

22. The device according to claim 19, wherein the first cooler is a water cooler.

23. The device according to claim 19, wherein the multi-stage compressor arrangement comprises a first compression stage, wherein the first compression stage is arranged to compress BOG from the LNG tank to a first pressure p1 between 6 and 18 bar, and wherein the first compression stage comprises at least one labyrinth-sealed piston compressor.

24. The device according to claim 19, wherein the multi-stage compressor arrangement comprises a middle and a last compression stage, which are arranged to compress pre-compressed gas from a first pressure p1 to the pressure p.sub.high, wherein the last compression stage has a bypass with a controllable valve in order to control the return flow and thus the delivery pressure after the last compression stage, wherein gas can be fed back via the bypass in such a way that the gas at the outlet has the predetermined target pressure value p.sub.high.

25. The device according to claim 24, wherein the bypass with controllable valve be regulated in such a way that any setpoint pressure between 100 and 300 bar can be set.

26. The device according to claim 24, wherein the last compression stage comprises at least one piston compressor sealed with a piston ring.

27. The device according to claim 23, further comprising: a branch line which is arranged downstream of the first compression stage in a fluid-conducting manner, and which opens further downstream into a supply line for a low-pressure gas injection engine and/or a gas combustion unit.

28. A method of use of a device according to claim 19 on a vessel, comprising the steps of providing said device on the vessel, and reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank, wherein reliquefying and returning comprises the steps: a) removal of BOG from a headspace of the LNG tank; b) compressing the BOG to a pressure p.sub.high, wherein the pressure p.sub.high is at least 200 bar; c) cooling the compressed gas to a temperature T.sub.1; d) expanding at least part of the gas from step c) to a pressure p.sub.expand, wherein wherein the pressure p.sub.expand is 80 to 180 bar; e) cooling the gas expanded in step d) to a temperature T.sub.4; f) returning the gas from step e) to the LNG tank.

Description

[0053] It shows:

[0054] FIG. 1 a schematic representation of a device according to the present invention;

[0055] FIG. 2 a schematic Mollier diagram illustrating a method according to the present invention.

[0056] FIG. 1 schematically shows a fuel gas supply system according to the present invention with a device for reliquefying and returning boil-off gas (BOG) to a liquefied natural gas (LNG) tank 3. The BOG (F2) accumulating in the head space of LNG tank 3 is fed via extraction line 5 to a heat exchanger 20 to be used as a cooling fluid in indirect heat exchange and then compressed by a multi-stage compressor arrangement 10 to a high pressure p.sub.high of typically approx. 300 bar.

[0057] In the embodiment shown, the multi-stage compressor arrangement 10 comprises a first compression stage with piston compressors 71 and 72 and associated coolers, a middle compression stage with piston compressor 73 and associated cooler, and a final compression stage with piston compressors 74 and 75 and associated coolers. The first compression stage 71, 72 is set up to compress the BOG to a pressure p1 of typically 7 bar. Part of the BOG compressed in this way, preferably without lubricant, can be fed to the low-pressure gas injection engine 4 via branch line 6, if required. The first compression stage can be pressure-controlled by means of a bypass with a pressure control valve (not shown).

[0058] The multi-stage compressor arrangement 10 is fluidly connected downstream to a first water cooler 50, wherein the compressed gas is cooled to typically 40 C. The outlet pressure of the highest compression stage, here consisting of piston compressors 75 and 74 and associated water coolers, is regulated via a bypass 19 with pressure control valve 9. After leaving the first water cooler 50, the compressed BOG can either be fed to a high-pressure gas injection engine 2 as fuel via the outlet 7 or fed to a first expansion unit 60, for example an expansion valve or an expander, via the return line 8. Typically, excess BOG that exceeds the fuel requirement of the engine 2 is fed to the return line 8. The gas is expanded in the first expansion unit 60 to a pressure p.sub.expand of approx. 150 bar. Due to the isenthalpic pressure reduction, the compressed natural gas undergoes renewed cooling and can therefore be further cooled from approx. 20 C. in indirect heat exchange with the BOG (F2) from the LNG tank 3 in the heat exchanger 20.

[0059] The BOG compressed by the compressor arrangement and cooled by water cooling 50, expansion 60 and heat exchange 20 is expanded again in a second expansion unit 30, now to a pressure p.sub.knock-out-drum of typically 1 bar, and finally separated into a liquid component and a gaseous component by a gas/liquid separator 40. The liquid component separated by the gas/liquid separator 40 is fed back into the LNG tank, and the gaseous component separated by the gas/liquid separator is combined in the withdrawal line 5 with the BOG emerging from the LNG tank and then fed to the heat exchanger 20 to be used as a cooling fluid.

[0060] In the BOG reliquefaction system as shown in FIG. 1, reliquefaction of natural gas is carried out using BOG taken from the storage tank as a refrigerant without the need for a separate cycle to reliquefy BOG. It will be understood that the present invention is not limited thereto, and a separate refrigeration cycle may be established to ensure reliquefaction of all BOG, if required. Such a separate circuit can ensure the reliquefaction of the BOG, but separate equipment or an additional energy source is required.

[0061] FIG. 2 shows the compression and cooling cycle in the schematic Mollier diagram with dashed lines. In its initial state, the BOG is located to the right of the dew line at an atmospheric pressure of 1 bar and approx. 160 C. When removed from the LNG tank a), especially when used as a coolant in indirect heat exchange, the BOG heats up to ambient temperature or higher, approx. T1.

[0062] In step b), compression to p.sub.high takes place, for example in accordance with the compressor arrangement shown in FIG. 1 using five piston compressors. These can be arranged as first compression stage 101, middle compression stage 102 and last compression stage 103, each with subsequent water cooling to the temperature T.sub.1. Overall, the natural gas can thus be compressed to a pressure of typically 300 bar. After the final cooling to T.sub.1 by water cooling in step c), at least part of the gas is isenthalpically expanded in step d) to a pressure p.sub.expand of typically 150 bar. Due to the Joule-Thomson effect, the temperature of the gas is reduced to T.sub.2, typically to approx. 20 C. In step e), the gas is cooled further to a temperature T.sub.4 of approx. 75 C. In the device shown in FIG. 1, this takes place in indirect heat exchange 20 with cooling BOG from the headspace of the LNG tank. Step f) is the return of the gas to the LNG tank, which typically involves further isenthalpic expansion and the separation of liquid and gaseous parts in the gas/liquid separator.

[0063] FIG. 2 shows the advantage of compressing the gas to the pressure p.sub.high with subsequent cooling c) and expansion d): Compared to compression to just p.sub.expand followed by water cooling, an enthalpy difference 104 can be obtained in the form of a lower temperature T.sub.2 of the gas. If compression is only to p.sub.expand, the gas is at a higher temperature T.sub.1 at the same pressure p.sub.expand and the cooling capacity of the coolant available in the heat exchanger must be used to cool warmer gas (dotted line). As the coolant is not available indefinitely in the case of BOG, the less compressed gas in the heat exchanger can often only be cooled to the temperature T.sub.3 instead of T.sub.4, and the subsequently expanded gas is only reliquefied to a lesser extent, corresponding to arrow 105.

[0064] Not shown in FIG. 2 is operating mode ii) as described above, in which the BOG is compressed to a pressure p.sub.low and expanded to p.sub.expand in the case of p.sub.low>p.sub.expand. This operating mode requires that the middle and last compression stages are set up to compress pre-compressed gas from a first pressure pi optionally to a pressure p.sub.high of, for example, 300 bar, or to a pressure p.sub.low of, for example, 150 bar. The pressure can, for example, be regulated by a bypass with a controllable valve, which is arranged in the last compression stage 103. If the controllable valve can be regulated in such a way that any target pressure between p.sub.low and p.sub.high can be set, further pressures in the return line 8 of FIG. 1 or in the diagram of FIG. 2 parallel below the dotted line c) are conceivable.