Process and system for reliquefying boil-off gas (BOG)

Abstract

A reliquefaction system and process for innovative reliquefaction of LNG boil-off gas (BOG), where the reliquefaction is propelled by LNG gas fuel. The reliquefaction system is preferably installed on shipboard including LNG carrier or harbor tug, where the LNG carrier and harbor tug use a gas fuel engine.

Claims

1. A boil-off gas (BOG) reliquefaction system comprising: an in-tank fuel pump 1; a LNG storage tank 2; a heat exchanger 3; a multistage compressor 4; a compressor after cooler 5; an expansion valve 6; and a LNG flash drum 7; wherein the in-tank fuel pump 1 is disposed inside the LNG storage tank 2 for drawing LNG from the LNG storage tank 2; wherein the heat exchanger 3 is fluidly coupled with the in-tank fuel pump 1 for receiving the LNG from the in-tank pump 1 and coupled with the storage tank 2 to receive BOG from the storage tank 2; wherein the LNG is vaporized, and the vaporized LNG and the BOG provide cold sources, resulting in cold energy recovered BOG; wherein the inlet of the multistage compressor 4 is coupled to the heat exchanger 3 to receive the cold energy recovered BOG, and the outlet of the multistage compressor 4 to the inlet of the compressor after cooler 5; wherein the cold energy recovered BOG is compressed; and wherein the compressor after cooler 5 removes heat from the compressed BOG; wherein the outlet of the compressor after cooler 5 is coupled with the heat exchanger 3; and wherein the compressor after cooler 5 discharges the compressed and after cooled BOG to the heat exchanger 3 at a temperature ranging from 20 C. to 45 C.; wherein inside the heat exchanger, the compressed and after cooled BOG is cooled down further by the cold sources from the vaporized LNG and BOG; wherein the inlet of the expansion valve 6 is coupled with the heat exchanger 3 to receive the cryogenically cooled compressed BOG; wherein the outlet of the expansion valve 6 is coupled with the flash drum 7; wherein the cold compressed BOG is expanded via the expansion valve 6, resulting in the expanded BOG that is close to atmospheric pressure; and wherein the flash drum 7 receives the expanded BOG, and returns flash gas and LNG recovered to the LNG storage tank 2.

2. The BOG reliquefaction system of claim 1, wherein the BOG is compressed in the multistage compressor 4 to a pressure ranges from 30 to 300 barg.

3. The BOG reliquefaction system of claim 1, wherein the resultant cold compressed BOG leaves the heat exchanger at a temperature ranges from 130 C. to 155 C., preferably around 150 C.

4. The BOG reliquefaction system of claim 1, further comprising a LNG booster pump 18, wherein the LNG booster pump 18 is disposed between the LNG storage tank 2 and the heat exchanger 3, and increases the pressure of the LNG to supply high-pressure fuel gas.

5. The BOG reliquefaction system of claim 1, further comprising an additional vaporizer 20, wherein the vaporizer 20 is disposed downstream of the heat exchanger 3.

6. The BOG reliquefaction system of claim 5, wherein the discharge cooling medium from the compressor after cooler 5 is used to heat the vaporizer 20.

7. The BOG reliquefaction system of claim 1, wherein the expansion valve 6 is a Joule Thomson (JT) valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments according to the present invention will now be described with reference to the Figures, in which like reference numerals denote like elements.

(2) FIG. 1 is a schematic configuration of the BOG reliquefaction system in accordance with one embodiment of the present invention.

(3) FIG. 2 is a schematic configuration of the BOG reliquefaction system in accordance with another embodiment of the present invention.

(4) FIG. 3 is a schematic configuration of the BOG reliquefaction system in accordance with another embodiment of the present invention.

(5) FIG. 4 is a schematic configuration showing the details of the integration of compressor after cooling water with fuel gas trim heater in accordance with another embodiment of the present invention.

(6) FIG. 5 is a flow chart showing the BOG reliquefaction process in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention may be understood more readily by reference to the following detailed description of certain embodiments of the invention.

(8) Throughout this application, where publications are referenced, the disclosures of these publications are hereby incorporated by reference, in their entireties, into this application in order to more fully describe the state of art to which this invention pertains.

(9) The present invention provides a reliquefaction system and process for innovative reliquefaction of LNG boil-off gas (BOG), where the reliquefaction is propelled by LNG gas fuel. The reliquefaction system is preferably installed on shipboard including LNG carrier or harbor tug, where the LNG carrier and harbor tug use a gas fuel engine. The reliquefaction system and process of the present invention have many advantages including lower capital cost, smaller footprint, less equipment and lower weight, least complexity and lowest electrical consumption comparing to the reliquefaction systems available in the market.

(10) Referring now to FIG. 1, there is provided a BOG reliquefaction system in accordance with one embodiment of the present invention. It is preferable to use the BOG reliquefaction system onboard a LNG fuel ship comprising a low pressure gas fuel engine. As shown in FIG. 1, the reliquefaction system comprises: an in-tank fuel pump 1, a LNG storage tank 2, a heat exchanger 3, a multistage compressor 4, a compressor after cooler 5, an expansion valve 6, and a LNG flash drum 7.

(11) The in-tank fuel pump 1 is disposed inside the LNG storage tank 2. In operation, in-tank fuel pump 1 draws LNG from the LNG storage tank 2.

(12) The heat exchanger 3 is fluidly coupled with the in-tank fuel pump 1. The IN LNG stream 8 represents the LNG from the in-tank pump 1 to the heat exchanger 3, where the LNG is at close to atmospheric pressure and 160 C. Inside the heat exchanger 3, the LNG is fully vaporized and transfers its cold, and becomes superheated up to close to room temperature at the outlet of the heat exchanger 3, represented by the OUT LNG stream 9. In one embodiment, the heat exchanger 3 is a diffusion bonded heat exchanger. The source of heat comes from the compressed BOG, which will be described in more details hereinbelow.

(13) The heat exchanger 3 is also fluidly coupled with the LNG storage tank 2 to receive BOG from the LNG storage tank 2, where the BOG is represented by the IN BOG stream 10. The IN BOG stream 10 is close to atmospheric pressure and at 160 C. when it is drawn from the LNG storage tank 2 into the heat exchanger 3. Inside the heat exchanger 3, the BOG transfers its cold, and becomes superheated up to close to room temperature at the outlet of the heat exchanger 3, represented by the OUT BOG stream 11.

(14) The inlet of the multistage compressor 4 is coupled to the heat exchanger 3 to receive the cold energy recovered OUT BOG stream 11, and the outlet of the multistage compressor 4 to the inlet of the compressor after cooler 5. The outlet of the compressor after cooler 5 is coupled with the heat exchanger 3. When the low-pressure OUT BOG stream 11 is transported through the multistage compressor 4, the BOG is compressed in the multistage compressor 4 to a pressure ranges from 30 to 100 barg, preferably close to 50 barg for optimal efficiency and cost effectiveness in material and equipment selection. The compressed BOG is represented by the compressed BOG stream 12 and discharged with temperature of 100 to 150 C. to the compressor after cooler 5. The compressor after cooler 5 cools down the compressed BOG stream 12 and discharges the cool compressed BOG stream 13 to the heat exchanger 3. In certain embodiments, the temperature of the BOG stream 13 ranges from 20 C. to 45 C. depending upon the cooling medium such as cooling water, air cooler, etc. Inside the heat exchanger, the cool compressed BOG stream 13 is cooled down further by the cold sources from the IN LNG stream 8 and IN BOG stream 10, resulting in the cryogenically cooled compressed BOG stream 14. The resultant cryogenically cooled compressed BOG stream 14 leaves the heat exchanger at a temperature ranges from 130 C. to 155 C., preferably around 150 C.

(15) The inlet of the expansion valve 6 is coupled with the heat exchanger to receive the cold compressed BOG stream 14. The outlet of the valve 6 is coupled with the flash drum 7. The cold compressed BOG stream 14 is expanded via the expansion valve 6, resulting in the expanded stream 15. The pressure of the expanded stream 15 is close to atmospheric pressure. In one embodiment, the expansion valve 6 is a Joule Thomson valve. The flash drum 7 receives the expanded stream 15. Inside the flash drum 7, some flash gas is formed and returned to the LNG storage tank 2 via the flash stream 16 with a temperature around 160 C. and near atmospheric pressure. The LNG recovered is returned to the LNG storage tank 2 via the RELIQUEFIED stream 17 with a temperature around 160 C. and near atmospheric pressure.

(16) Referring now to FIG. 2, there is provided a BOG reliquefaction system in accordance with another embodiment of the present invention. It is preferable to use the BOG reliquefaction system onboard a LNG fuel ship comprising a high pressure gas fuel engine. The high pressure gas fuel engine can be a MEGI engine. As shown in FIG. 2, the reliquefaction system is similar to the one shown in FIG. 1 as described above, except that it further comprises a LNG booster pump 18, where the LNG booster pump 18 is disposed between the LNG storage tank 2 and the heat exchanger 3. When the in-tank fuel pump 1 draws the LNG from the storage tank 2, the IN LNG stream 8 is close to atmospheric pressure and 160 C. The LNG booster pump 18 increases the pressure of the LNG, resulting in the pressured LNG stream 19. At the discharge of the LNG booster pump 18, the pressured LNG stream 19 carries LNG at a pressure of 300 barg into the heater exchanger 3. The fully vaporized LNG in the OUT LNG stream 9 will supply the required high-pressure fuel gas to the MEGI engine. The other streams and equipment in FIG. 2 are to operate in the same conditions and manners as described in FIG. 1.

(17) Referring now to FIG. 3, there is provided a BOG reliquefaction system including trim heater/vaporizer to produce gas fuel for high demand scenarios in accordance with another embodiment of the present invention. In this embodiment, the reliquefaction system acts as the main LNG fuel supply source, in parallel as a reliquefaction system. As shown in FIG. 3, the reliquefaction system is similar to the one shown in FIG. 1 as described above, except that it further comprises an additional vaporizer 20, where the vaporizer 20 is disposed downstream of the heat exchanger 3. In addition, the LNG booster pump 18 as shown in FIG. 2 can also be included if there is a need to supply high pressure fuel gas to an MEGI engine. The reliquefaction system as shown in FIG. 3 operates in the same conditions and with the same process flow as in FIG. 1 and FIG. 2 except that it has the capability of vaporizing LNG and superheating the LNG fuel stream 9 to the required temperature at around 50 C., represented by the stream 21. The vaporizer 20 can have hot water or steam as a heating medium.

(18) Referring now to FIG. 4, it shows the details of the integration of compressor after cooler with fuel gas trim heater in accordance with another embodiment of the present invention. The reliquefaction system can have enhanced energy and utility supply efficiency by using the discharge cooling medium from the compressor after cooler 5 for the vaporizer 20. As shown in FIG. 4, stream 22 is the hot medium at the discharge of the compressor after cooler 5, entering the vaporizer 20 as heating medium. The other streams and equipment in FIG. 4 are to operate in the same conditions and manner as described in their identical streams and equipment in FIGS. 1-3.

(19) Referring now to FIG. 5, there is provided a process of reliquefaction of LNG boil-off gas (BOG) in accordance with one embodiment of the present invention. The process 500 comprises:

(20) providing 510 cold BOG from a LNG storage tank, wherein the cold BOG is at close to atmospheric pressure and 160 C.;

(21) supplying 520 cold sources by passing cold LNG and the cold BOG through a heat exchanger, where the cold LNG is at close to atmospheric pressure and 160 C. and from the LNG storage tank, and where the cold BOG is heated in the heat exchanger to room temperature, and the LNG is vaporized in the process;

(22) compressing 530 the heated BOG from the heat exchanger to a pressure ranges from 30 to 300 barg, preferably close to 50 barg for optimal efficiency and cost effectiveness in material and equipment selection, where the compressed BOG is discharged with temperature of 100 to 150 C.;

(23) cooling 540 the compressed BOG to remove heat from the compressed BOG, resulting in a cooled compressed BOG at a temperature ranging from 20 C. to 45 C.;

(24) further cooling 550 the cooled compressed BOG to a temperature ranging from 130 C. to 155 C., preferably around 150 C.;

(25) expanding 560 the further cooled compressed BOG into flash gas and LNG close to atmospheric pressure and 160 C.; and

(26) returning 570 the flash gas and LNG to the storage tank.

(27) In the reliquefaction process of the present invention, there is no external refrigerant such as nitrogen to generate cold energy utilizing close loop refrigeration cycle. Also, there is no refrigerant compressors, expanders or pulse tube refrigerators utilized in the process of the present invention. Essentially this is the most compact, least complex, lowest energy consumption and low cost solution, which integrates two separate systems; fuel gas supply system and reliquefaction system into one module. Total electrical consumption for the present invention is less than 50% of conventional reliquefaction systems.

(28) While preferred embodiments of the present subject matter have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.