Method And Plant For Storing And Transporting A Liquefied Combustible Gas

Abstract

For the purpose of storing and transporting a liquefied combustible gas, in a methane tanker for example, together with a virtually inert gas such as nitrogen in contact with said combustible gas, use is made of additional energy either, as the case may be, in order to liquefy the inert gas -- utilizing the evaporation of the combustible gas to that end -- or in order to reliquefy the evaporated combustible gas -- utilizing the evaporation of the liquid inert gas to that end.

Description

This invention covers a method and a plant for storing and transporting a liquefied combustible gas, such as the plant aboard a methane tanker, in which use is made of a virtually inert gas such as nitrogen which may be in contact with the combustible gas and exist likewise in liquid form in the plant.

In certain cryogenic plants it is occasionally required to alternately liquefy one of two gases, depending on the phases of operation of the plant.

A case in point arises with ships used to transport liquid hydrocarbon gases, usually methane tankers. In such vessels several possible operating modes or phases exist, with variants in each case.

In one mode of operation, when the tanker is travelling empty towards its loading port, an inert gas, usually nitrogen, is required in order to place and maintain the liquefied natural gas tanks in a neutral atmosphere until the port is reached. For this reason such vessels are equipped with liquid nitrogen tanks or with inert-gas generators.

In a second mode, corresponding to the tanker loading phase, a similar requirement for liquid nitrogen exists during these operations in order to maintain the atmosphere neutral before the methane gas fills all the spaces not occupied by the fuel.

In a third mode, when the loaded tanker leaves the port on its way to the terminal port, the ship consumes in its propulsion machinery the natural gas that evaporates from the tanks, or approximately 0.1 to 0.3 percent of the transported quantity daily. More often than not a good state of equilibrium exists between evaporation on the one hand and consumption by the ship's boilers or engines on the other, but if the engines must be slowed or stopped there is a surplus of evaporated gas and the problems of eliminating it then arise and have heretofore been unsatisfactorily resolved.

The fourth mode corresponds to the phase of approach to the terminal port and the time during which the ship is laid up with its engines stopped. In this case it is impossible to burn the gas in the engines since ship propulsion requirements are minimal or nil, so that elimination of the evaporated gas poses problems heretofore unresolved.

A fifth mode corresponds to the unloading phase. In this case it is usually possible to discharge the evaporated gas to land through pipes provided for the purpose.

A sixth mode corresponds to the degassing phase, namely the discharging into the atmosphere of the gas contained in the empty tanks, in conjunction with the process of filling with nitrogen and the subsequent entry of atmospheric air. This operation requires a certain amount of nitrogen which is produced by an inert-gas generator or a store of liquid nitrogen, and in order to accomplish all the above-mentioned operations a very large stock of liquid nitrogen may be carried on board, namely approximately one ton of liquid nitrogen for every thousand cubic metres of natural gas tankage.

Thus the operation of methane tankers involves two requirements which it has heretofore been the custom to meet separately. One is the production and/or storage of liquid nitrogen, the other is the elimination of evaporated natural gas when consumption of the latter is slowed or arrested -- by design or not -- when the vessel's propulsion system is slowed or stopped on reaching port.

As stated precedingly, this second requirement has been met unsatisfactorily or not at all heretofore.

The present invention rests on the discovery that it is possible to meet both these requirements together under satisfactory conditions.

To that end and in accordance with the subject method of this invention, which may be used for storing and transporting a liquefied combustible gas with a virtually inert gas in contact therewith, the inert gas is liquefied by making use of additional energy derived from the evaporation of the combustible gas, or else the evaporated combustible gas is reliquefied by making use of the evaporation of the liquid inert gas, as the case may be.

The associated plant or equipment, most notably for a methane tanker, includes a machine capable of receiving surplus energy for assuring the two herein-specified operating modes in alternation, some at least of the component parts of said machine being used for both operating modes.

The description which follows of a possible embodiment of the invention, given with reference to the accompanying non-limitative exemplary drawings, will provide a clear understanding of how the invention can be carried into practice.

In the drawings :

FIG. 1 is a schematic diagram of the overall arrangement of a plant according to this invention, as used aboard a methane tanker ;

FIG. 2 shows the same plant adapted for producing liquid nitrogen ; and

FIG. 3 shows the same plant adapted for reliquefying natural gas.

In FIGS. 2 and 3, the piping circuits which are in service are shown in bold lines, while the others, which are inoperative through closure of appropriate cocks and valves, are shown in fine lines.

In the embodiment shown in the drawings, the plant includes a turbo-compressor set consisting of an alternating or rotary compressor 1 coupled either directly or through gearing to a reciprocating or rotary expansion engine 2 and to an extra-power motor 3, which motor may be an electric motor or a heat or steam engine, or be formed of a plurality of motors. Preferably, it is powered with natural gas taken from the natural gas transported in the ship.

The plant includes a plurality of heat-exchangers, the drawings most clearly showing the cooling exchanger 4 which utilizes water supplied by the ship's cooling network 5, and the cooling exchanger 6 which in this case involves four separate fluid flow-paths 6a, 6b, 6c, 6d.

The plant further comprises :

a liquid nitrogen separator 7 and a natural gas liquefier 8 ;

liquid methane tanks as represented by the tank 9, and liquid nitrogen tanks as represented by the tank 10 ;

a natural-gas-fired boiler 11, the combustion smokes from which are discharged through the stack 12 ;

and associated pipes, valves (see below) and other gear.

When it is required to produce nitrogen, the gaseous nitrogen generator 13 is activated and extracts the carbon dioxide from the stack gases (CO.sub.2 + N.sub.2) and retains the nitrogen.

The cycle generated by the turbine engine 1-2 can be designed to work at atmospheric pressure during suction of compressor 1. Alternatively, a two-pressure cycle could be used, in which the lower pressure would be higher than atmospheric pressure. In this latter event, as well as in certain operating modes, the gaseous nitrogen produced by generator 13 is compressed by a booster 14 driven by an appropriate motor 15.

Manually-operated or automatic isolating valves allow of establishing different circuits for performance of the plant's different functions, these valves being designated from left to right by reference numerals 16, 17 . . . . 33, 34 (only the valves relevant to the present description being shown on the drawing).

When valve 26 is open it allows boiler 11 to be fed directly with evaporated gas without passing through the heat exchanger 6 when the production of nitrogen is stopped.

When valve 27 is open, the ship's ancillary systems and services are supplied with nitrogen in the gaseous state.

As indicated precedingly, the plant is capable of operating in at least two different ways :

in a first operating mode liquid nitrogen is produced, and for exemplary purposes the case will be taken of a refrigerating system the lower pressure of which is higher than atmospheric pressure.

As shown in FIG. 2, this is accomplished by opening the valves 19, 20, 22, 24, 25, 28, 29, 31, 32, 33 and closing the valves 16, 17, 18, 21, 23, 26, 30, 34.

The shaft lines 1, 2, 3 and 14, 15 are set in rotation, and the evaporated gas is caused to pass through open valve 24 and heat exchanger circuit 6d, where the evaporated gas at around - 161.4.degree. C relinquishes its `frigories` and issues at around 40.degree. C in order to feed the boiler 11 through open valve 25.

The operating conditions of a nitrogen-cycle refrigerating machine are accordingly established this way, the nitrogen being delivered under pressure and cooled in separator 7, whence the liquid nitrogen is discharged into tank 10, while the cold gaseous nitrogen under pressure is directed to the inlet side of expansion turbine 2.

The plant achieves steady-state operating conditions when it liquefies a quantity of nitrogen equal to the nitrogen produced by the generator. Production of liquid nitrogen is facilitated by the extra cold which is produced by the methane evaporated in tanks 9 and which is given up in heat exchanger 6.

Alternatively, the plant may be designed to produce liquid nitrogen without extra cold from methane evaporation in the tanks 9 (e.g., when the latter are waiting to be filled with, or else no longer contain any, liquefied methane), in which case the extra power supplied by motors 3 and 15 will attain its highest level, these motors being accordingly designed to be capable of continuous operation under these conditions without being overloaded.

In a second operating mode, the plant is devised in such manner as to reliquefy the evaporated natural gas from the tanks 9.

As shown in FIG. 3, this is accimplished by opening the valves 16, 17, 18, 21, 23, 34 and closing the valves 19, 20, 22, 24, 25, 26, 28, 30, 32, 33.

Boiler 11 and generator 13 are shut off and the shaft lines 1, 2, 3 and 14, 15 are set rotating. Liquid nitrogen is conveyed from tank 10 (assumed to already contain liquid nitrogen from a previous operation or an external source) to cooling heat-exchanger 6b and issues therefrom under gravity, pressure or pumping. This additional cooling effect enables the nitrogen refrigeration cycle to be initiated in a closed circuit.

The plant will achieve steady operating conditions when the extra `frigories` provided by the liquid nitrogen and the extra energy provided by the motor 3 allow all the evaporated gas from the tanks to be liquefied in the condenser 8, the liquid procuded in this way returning to tank 9 through open valve 17.

The liquid nitrogen is evaporated and heats up in the circuit 6b of heat-exchanger 6 and is discharged to the stack at a temperature of approvimately 20.degree. C to 30.degree. C.

Thus a plant devised according to this invention allows of accomplishing two entirely different functions, to wit, producing liquid nitrogen and reliquefying the natural gas evaporating from the methane tanker's tanks, These functions are moreover performed by utilizing the same basic elements, namely the liquefaction line formed by compressor 1, turbine 2, motor 3, heat-exchanger 4 and heat-exchanger 6, an important feature of the invention being this dual function of the elements 1, 2, 3, 4 6.

The efficiency and economics of the plant are thus considerably increased, making the plant economically viable aboard a methane tanker. This contrasts with liquid nitrogen producing plants or methane reliquefying plants devised in conventional fashion or as individual systems, which are of debatable profitability.

It goes without saying that the invention has been described in schematic form only, but with sufficient clarity for the specialist in the art to be able to carry it into practice without difficulty. Obviously, alternatives could be introduced in the exemplary schematic form described herein, and for instance the heat-exchanger 6 could be devised as a plurality of separate elements grouped in series or in parallel. It will therefore be manifest that many changes and substitutions of parts could be made without departing from the scope of the invention as defined by the claims .

Claims

I claim:

1. In a system for handling a liquefiable combustible gas for storage and transportion, including a container for liquefiable inert gas in liquid form, a container for a liquefiable combustible gas in liquid form, a source of said inert gas in gaseous form, and a source of said combustible gas in gaseous form, an improved arrangement for optionally liquefying a selected one of said gases in gaseous form for delivery to the corresponding container comprising:

a. means for circulating said inert gas through a refrigeration cycle including an expansion stage, a compression stage, and power means for supplying to said compression stage at least a portion of the energy required thereby;

b. heat exchange means through which said circulating inert gas passes while in expanded cooled condition;

c. means for expanding one of said gases from its liquefied form in the corresponding container and passing the expanded cooled gas through said heat exchanger;

d. means for applying the cooling effect of said two expanded gases to the selected gas from its source to liquefy said gas; and

e. means for delivering the thus liquefied gas to the container therefor.

2. The system according to claim 1 including

f. means for passing surplus expanded combustion gas from said liquefied combustion gas container in heat exchange relation with said cooled inert gas to re-liquefy the same for return to said container, liquefied inert gas from the container thereof being expanded to cool the same and passed in cooled condition through said heat exchange means (b).

3. The system according to claim 2 wherein compressed inert gas from said compression stage is cooled by passage through said heat exchange means (b) and then passed in heat exchange relation with said expanded combustible gas in means (f).

4. The system according to claim 1 wherein said inert gas after compression in said compression stage is cooled to liquefy the same and a fraction of said liquefied inert gas is delivered to said inert gas container, and make-up fresh inert gas from said source thereof is added to said circulating means.

5. The system according to claim 4 wherein said compressed inert gas is cooled by passage through said heat exchange (b).

6. The system according to claim 4 including means for combusting said combustible gas and wherein said inert gas is extracted from the flue gases of said combusting means.

7. The system according to claim 6 wherein said combusting means is a boiler and including further means for treating the resulting smoke, which further means include means for eliminating the carbon dioxide from the smoke whereby to extract nitrogen inert gas therefrom, and a booster connected to said circulating means for compressing this nitrogen and injecting it into said circulating means whereby to liquefy it.

8. The system according to claim 6 wherein liquefied combustible gas from said container thereof is expanded and passed through said heat exchange means before being combusted in said combusting means.

9. In a method of handling a liquefiable combustible gas for storage and transportion, including a container for liquefiable inert gas in liquid form, a container for a liquefiable combustible gas in liquid form, a source of said inert gas in gaseous form, and a source of said combustible gas in gaseous form, an improved arrangement for optionally liquefying a selected one of said gases in gaseous form for delivery to the corresponding container comprising:

a. circulating said inert gas through a refrigeration cycle including an expansion stage, a compression stage, while supplying to said compression stage at least a portion of the power required thereby;

b. passing said circulating inert gas while in expanded cooled condition through a heat exchanging zone;

c. expanding one of said gases from its liquefied form in the corresponding container and passing the expanded cooled gas through said heat exchanging zone;

d. applying the cooling effect of said two expanded gases to the selected gas from its source to liquefy said gas; and

e. delivering the thus liquefied gas to the container therefor.