Apparatus and method for reliquefaction of boil-off gas of vessel

1. An apparatus for reliquefaction of boil-off gas (BOG) generated in a liquefied gas storage tank provided to a vessel, the apparatus comprising:

a compressor configured to compress BOG discharged from the storage tank to provide compressed BOG;

a heat exchanger configured to cool the compressed BOG using additional BOG discharged from the storage tank, the compressed BOG from the heat exchanger being divided into at least two flows comprising a first flow and a second flow;

a first expansion unit configured to expand the first flow to provide an expanded first flow;

a first intermediate cooler configured to cool the second flow using the expanded first flow to provide a cooled second flow;

a receiver configured to receive the cooled second flow for holding BOG in the receiver;

a level control line configured to discharge BOG from the receiver for regulating a BOG level in the receiver and further configured to divide the BOG discharged from the receiver into at least two flows comprising a third flow and a fourth flow;

a second expansion unit configured to expand the third flow to provide an expanded third flow;

a second intermediate cooler configured to cool the fourth flow by heat-exchanging with the expanded third flow to provide a cooled fourth flow and a heat-exchanged, expanded third flow; and

a third expansion unit configured to expand the cooled fourth flow to provide an expanded, cooled fourth flow,

wherein the apparatus is configured to return the expanded, cooled fourth flow to the liquefied gas storage tank and further configured to send the heat-exchanged, expanded third flow to the compressor.

2. The apparatus according to claim 1 , further comprising:

a pressure control line configured to discharge a fluid from the receiver for regulating a pressure inside the receiver,

wherein the fluid discharged through the pressure control line is configured to be returned to the liquefied gas storage tank.

3. The apparatus according to claim 1 , wherein the compressor is configured to compress the BOG discharged from the liquified gas storage tank to a pressure in the range of 40 to 100 bara.

4. The apparatus according to claim 1 , wherein the BOG compressed by the compressor has a temperature of 80° C. to 130° C.

5. The apparatus according to claim 1 , further comprising:

an after-cooler disposed downstream of the compressor and configured to cool the BOG compressed by the compressor,

wherein the BOG cooled by the after-cooler has a temperature of 12° C. to 45° C.

6. The apparatus according to claim 1 , wherein the BOG expanded by the first expansion unit has a pressure of 4 to 15 bara.

7. The apparatus according to claim 1 , wherein the BOG expanded by the second expansion unit has a pressure of 2 to 5 bara.

8. The apparatus according to claim 1 , wherein the compressor is a multistage compressor comprising multiple compressors, and each of the first flow having passed through the first intermediate cooler and the third flow having passed through the second intermediate cooler is supplied downstream of any one of the multiple compressors.

9. A method for reliquefying boil-off gas (BOG) generated in a liquefied gas storage tank provided to a vessel, the method comprising:

compressing, by a compressor, BOG generated from the liquefied gas;

cooling the compressed BOG using additional BOG generated from the liquefied gas;

dividing the cooled BOG into a first flow and a second flow;

expanding the first flow to provide an expanded first flow;

cooling the second flow using the expanded first flow to provide a cooled second flow;

supplying the cooled second flow to a receiver;

controlling a pressure downstream of the compressor by controlling a pressure of the receiver;

discharging BOG from the receiver for returning to the liquefied gas storage tank and controlling discharging of the BOG from the receiver to maintain an inner pressure of the receiver or the pressure of BOG downstream of the compressor at a preset pressure,

dividing the BOG discharged from the receiver into at least two flows comprising a third flow and a fourth flow;

expanding the third flow to provide an expanded third flow;

cooling the fourth flow with the expanded third flow to provide a cooled forth flow; and

supplying the cooled fourth flow to the liquified gas storage tank.

10. The method according to claim 9 , wherein the pressure downstream of the compressor is set in the range of 40 to 100 bara.

11. The method according to claim 9 , wherein the cooled fourth flow is expanded and supplied to the liquified gas storage tank and a level of the receiver is measured to regulate a degree of expansion of the cooled fourth flow.

12. The method according to claim 9 , wherein the first flow is expanded to a pressure of 4 to 15 bara,

the third flow is expanded to a pressure of 2 to 5 bara, and

the expanded first flow and the expanded third flow are supplied to the compressor after cooling the second flow and the fourth flow,

the third flow being supplied farther downstream of the compressor than the first flow.

13. The method according to claim 9 , wherein the BOG compressed by the compressor is cooled to 12° C. to 45° C. before heat exchange with the BOG generated from the liquefied gas.