METHOD IN THE UTILIZATION OF BOIL-OFF FROM LIQUID GAS AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract: In a plant for the production of fuel gas from boil-off and from liquefied gas with the aid of a combined heat exchanger (9), a compressor (4) is placed upstream of the heat exchanger in the gas line carrying boil-off. Compressed boil-off and liquefied gas pass in heat exchange in a combined heat exchanger (9) which has separate flow banks (7, 8) for liquefied gas and boil-off, respectively. From the combined heat exchanger (9), the two volumes of gas pass in a mixture to a consumer through a pipeline (5).

Description: METHOD IN THE UTILIZATION OF BOIL OFF FROM LIQUID GAS AND APPARATUS FOR CARRYING OUT THE METHOD The invention relates to a method of utilising boil-off from liquefied gas as fuel gas in combination with gas produced by vaporising the liquefied gas. The invention also relates to a plant for carrying out said method, wherein boil-off and vaporised liquefied gas are transported together to a consumer, comprising a storage tank for liquefied gas, a draw-offline for boil-off from the storage tank, a pump with attached draw-off line for liquefied gas from the storage tank, and a combined heat exchanger having two separate flow banks in a common surrounding shell, where one bank is connected to the draw-offline for liquefied gas for vaporisation thereof and the other bank is connected to the draw-off line for boil-off, which banks open into a common mixing chamber, which is connected to a flowline leading to the consumer. NO-PS 176 454 makes known a method for utilising boil-off from liquefied gas as fuel gas in combination with gas produced by vaporising the liquefied gas, wherein boil-off and vaporised gas are brought together and compressed, and where the boil-off and the liquefied gas prior to compression are superheated and vaporised, respectively, each in its respective section in a combined heat exchanger, and brought together in a common mixing chamber. The same publication also makes known a plant wherein boil-off and vaporised gas are brought together and compressed. The plant comprises a storage tank for liquefied gas, draw-off lines for boil-off and liquefied gas, respectively, from the storage tank, and a combined heat exchanger having two separate flow banks in a common surrounding shell. One of the flow banks is connected to the liquefied gas draw-offline, whilst the other flow pipe bank is connected to the boil-off draw-off line. The flow banks open into a common mixing chamber, which is connected to a flowline leading to a consumer. One advantage obtained with the above-mentioned known art is that both the method and the plant are simplified, it being possible to dispense with an otherwise customarily used separator, because the suction temperature of the compressor will be considerably higher, and nor does the compressor need to be cryogenic. A disadvantage of this prior art is that the compressor must necessarily carry the total volume of gas, i.e., both the natural boil-off and vaporised gas. The objective of the present invention is to provide a method and a plant as mentioned by way of introduction, wherein the advantages of the prior art from NO-PS 175 654 are retained, whilst simplifying the plant even further. This is achieved according to the invention by a method as mentioned above, wherein the boil-off is compressed and passed to heat exchange with the liquefied gas, and is subsequently mixed with the liquefied gas vaporised during the heat exchange. By virtue of the fact that the compression takes place prior to the heat exchange and that only the boil-off is compressed, the advantage is obtained that the compression applies only to the volume of boil-off and not the total volume of gas. This results in reduced power requirements and simpler capacity control. According to the invention, a plant is also proposed as mentioned above, which plant is characterised in that a compressor is provided in the boil-off draw-off line before the combined heat exchanger. By placing the compressor in the boil-offline upstream of the heat exchanger, the compressor only needs to carry the boil-off volume and not the total volume of gas. The compressor will have to be constructed to work with gas at temperatures as low as - 140°C when LNG is involved, but this disadvantage will be more than compensated for by the advantages obtained, namely that the compressor in terms of size will be more compact, have reduced power requirements, a smaller working area and simpler capacity control. An additional advantage is that the combined heat exchanger will be capable of being dimensioned for a higher pressure drop, which will result in reduced shell dimensions and lower production costs. It is thus only the choice of material for the compressor which tends to be unfavourable, but as mentioned this disadvantage is cancelled out as a result of the compressor being made more compact. The invention will now be explained in more detail with reference to the drawing which is a schematic illustration of a plant according to the invention. On the figure the reference numeral 1 denotes a storage tank for liquefied gas, for example, an LNG tank on board a LNG ship. As a result of heat penetration from the surroundings, so-called boil-off will be produced. This accumulates in the dome 2 of the storage tank 1 and can be withdrawn through a boil-off draw-off line 3. This draw- off line 3 runs to a compressor 4. In the compressor 4 the boil-off is compressed, with attendant rise in temperature. From the compressor, the compressed boil-off passes to a combined heat exchanger 9. The boil-off is used as fuel gas in the propulsion engine of the ship. The plant is such that if the boil-off is not sufficient, i.e., larger amounts of fuel gas are required than the boil-off can supply, liquefied natural gas is withdrawn with the aid of a pump 11, and led through a draw-offline 10 to the combined heat exchanger 9. The combined heat exchanger 9 has separate flow banks 7 and 8 for liquefied gas and boil-off, respectively. The liquefied gas and the boil-off are heat exchanged with an external hot flow, indicated by mean of the arrow in the figure. This flow, which in most cases will be vapour, is conducted from the shell side of the heat exchanger. The two gas flows are accumulated in the mixing chamber 6 and thence pass through a flowline 5 to a non-illustrated consumer, e.g., to a burner.

Claims: P a t e n t c l a i m s l. A method for utilising boil-off from liquefied gas as fuel gas in combination with gas produced by vaporising the liquefied gas, characterised in that the boil-off is compressed and passed to heat exchange with the liquefied gas, and is subsequently mixed with the liquefied gas vaporised during the heat exchange. 2. A plant for carrying out the method according to claim 1 , wherein boil-off and vaporised liquefied gas are transported together to a consumer, comprising a storage tank (1) for liquefied gas, a draw-offline (3) for boil-off from the storage tank (1), a pump (11) with draw-offline (10) attached thereto for liquefied gas from the storage tank (1), and a combined heat exchanger (9) having two separate flow banks (7, 8) in a common surrounding shell, where one flow bank (7) is connected to the liquefied gas draw-offline (10) and the second flow-through unit (8) is connected to the boil-off draw-off line (3), which flow banks open into a common mixing chamber (6) which is connected to a flowline (5) to the consumer, characterised in that a compressor (4) is provided in the boil-off draw-off line (3) before the combined heat exchanger (9).

Inventors: CHRISTIANSEN PER, CHRISTIANSEN, PER
Applicants: CHRISTIANSEN PER, KVAERNER MARITIME AS, CHRISTIANSEN, PER, KVAERNER MARITIME A.S
Application year: 1997
Main classification: F17C9
IPC codes: B63B25, B63H21, F17C9