Pressure control of gas liquefaction system after shutdown

Abstract

A method is provided for operating a system for the liquefaction of gas of the type comprising a main heat exchange vessel, a bundle for the gas to be liquefied extending through said MCHE and a refrigerant compression circuit of which a first end leads evaporated refrigerant from the vessel towards a compressor and a second end supplies the compressed and cooled refrigerant from the compressor towards the MCHE. For avoiding problems during heat up or during start up of the heat exchanger the pressure within the liquefaction system is controlled by regulating the amount of evaporated refrigerant in the liquefaction circuit.

Claims

1. A method for operating a liquefaction system for the liquefaction of gas comprising a main heat exchanger or vessel (MCHE), a bundle for the gas to be liquefied extending through said MCHE, comprising: providing a refrigerant compression circuit of which a low pressure part leads evaporated refrigerant from the MCHE towards a compressor and a condenser for providing a compressed and cooled refrigerant that is provided to a high pressure part having compressed and cooled refrigerant at a pressure higher than said evaporated refrigerant in the low pressure part, the high pressure part supplying the compressed and cooled refrigerant from the compressor towards the MCHE; providing a drum having a quantity of said refrigerant partly in the form of a condensed refrigerant and partly in the form of said evaporated refrigerant and heat transfer members having surfaces configured to heat said condensed refrigerant or cool the evaporated refrigerant, connecting the drum to the low pressure part by a balance line, controlling pressure within the liquefaction system by operating the heat transfer members for regulating a quantity of said evaporated refrigerant in the drum and, through an exchange of said evaporated refrigerant between the drum and low pressure part through the balance line only, in the low pressure part by heating said condensed refrigerant via the heat transfer members in a first operating, state for supplying said evaporated refrigerant from the drum to the low pressure part through the balance line, and by cooling said evaporated refrigerant via the heat transfer members in a second operating state for withdrawing said evaporated refrigerant from the low pressure part towards the drum through the balance line.

2. The method according to claim 1, wherein operating the heat transfer members comprises operating the heat transfer members in the second operating state to withdraw heat from the refrigerant in the drum during heat up of the heat exchanger so as to withdraw refrigerant from the low pressure part of the liquefaction system.

3. The method according to claim 1, wherein operating the heat transfer members comprises operating the heat transfer members in the first operating state to supply heat to the refrigerant in the drum during start up of the heat exchanger so as to supply the refrigerant to the low pressure part.

4. The method according to claim 3, and further comprising injecting liquid refrigerant directly into the MCHE.

5. The method according to claim 1, wherein the refrigerant is a mixed refrigerant, comprising a mixture of propane, ethane, methane and nitrogen.

6. The method according to claim 1, wherein operating the heat transfer members comprises operating the heat transfer members in the second operating state to withdraw heat from the refrigerant in the vapor/liquid separator during heat up of the heat exchanger so as to withdraw refrigerant from the low pressure part of the liquefaction system.

7. The method according to claim 1, wherein the heat transfer members comprise a heat transfer coil.

8. The method according to claim 7, wherein the heat transfer coil circulates a secondary refrigerant or a heating medium.

9. The method according to claim 8, wherein the secondary refrigerant is LNG or liquid nitrogen.

10. The method according to claim 1, and further comprising injecting refrigerant directly into the MCHE.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter the invention will be elucidated while referring to the drawing, in which:

(2) FIG. 1 schematically shows a first embodiment of the invention, and

(3) FIG. 2 schematically shows a second embodiment of the invention.

DETAILED DESCRIPTION

(4) Firstly referring to FIG. 1 a first embodiment of a cryogenic heat exchanger for the liquefaction of gas is illustrated fit for carrying out the method according to an aspect of the invention. The gas is supplied by a feed line 1 and is withdrawn as liquefied gas by a discharge line 2. The heat exchanger illustrated schematically is of the type comprising a main cryogenic heat exchanger or vessel (MCHE) 3, a bundle 4 for the gas to be liquefied extending through said MCHE 3 between the feed and discharge lines 1 and 2, respectively, and a refrigerant circuit 5-5′ of which a first end is the low pressure part 5′ of the liquefaction system that leads evaporated refrigerant, coming from the pressure letdown valve 10 through the distributor 11 in top of vessel 3, via line 6 towards a compressor 7 and of which a second end is the high pressure part 5 of the liquefaction system that leads the compressed refrigerant from compressor 7 via a condenser 17 towards the MCHE 3.

(5) The refrigerant entering the MCHE 3 by means of line 8 of the compression circuit 5′ flows upward through a bundle 9 and (after passing pressure letdown valve 10 not further elucidated here) is discharged by distributor 11 and falls down by gravity while evaporating. The evaporated refrigerant is collected by line 6 of the compression circuit at the bottom of the MCHE.

(6) The refrigerant passing through the MCHE 3 is in a heat exchange relation with respect to the gas passing through the MCHE (bundle 4) in a manner known per se which, therefore, needs no further explanation.

(7) As refrigerant for use in such a cryogenic heat exchanger optionally a so-called mixed refrigerant may be used, comprising a mixture of, for example, propane, ethane, methane and nitrogen.

(8) FIG. 1 shows an embodiment of the invention. In this embodiment a balance line 12 connects the MCHE 3 to a refrigerant drum 13 which contains refrigerant and which is provided with heat transfer means 14 and 16. In the illustrated embodiment the heat transfer means 14 comprise a heat transfer coil above the liquid level through which a secondary refrigerant may be circulated, such as for example LNG (which has a lower boiling point than the mixed refrigerant). The heat transfer means 16 comprises a heat transfer coil below the liquid level through which a heating medium may be circulated, such as for example steam, water or electricity.

(9) By means of the refrigerant drum 13 and balance line 12 the pressure within the MCHE 3 may be controlled by regulating the quantity of evaporated refrigerant. For example, during heat up of the MCHE 3 (this may occur when the heat exchanger is not operative for reasons of servicing, repairs or otherwise of the process plant) the heat exchange means 14 withdraw heat from the refrigerant within the drum 13, and part of the evaporated refrigerant within the drum condenses which will lead to a corresponding flow and withdrawal of evaporated refrigerant from the MCHE 3 through the balance line 12.

(10) During start up of the heat exchanger (for example after a period of standstill) evaporated refrigerant is supplied to the MCHE 3. This is achieved by supplying heat to the refrigerant in the drum 13 by circulating a heating medium through the heat transfer means 16, which results in a corresponding evaporation of part of the refrigerant in the drum 13 and a flow thereof through the balance line 12 into the MCHE 3.

(11) As an alternative liquid refrigerant may be injected directly into the MCHE 3 as illustrated in FIGS. 1 and 2 by supply line 19 and injector 20.

(12) FIG. 2 shows an alternative embodiment of the invention. In this embodiment the additional drum 13 is omitted and the high pressure part 5 of the liquefaction system is provided with heat transfer means 14 and 16 which are operated for withdrawing heat from the refrigerant in the compression circuit and for supplying heat thereto (during heat up or start up, respectively).

(13) In this embodiment the compression circuit 5 comprises a vapor/liquid separator 15 which is provided with said heat transfer means 14 and 16. The separator 15 is connected to the MCHE by a vapor line 8′ and a liquid line 8″. Basically the operation is as explained with respect to the embodiment according to FIG. 1, but now the vapor line 8′ operates as balance line.

(14) It is noted that the high pressure part of the liquefaction system 5 also may be provided with other components which, in a corresponding manner, are provided with heat exchange means 14 and 16 for withdrawing/supplying heat.

(15) The invention is not limited to the embodiments described before which may be varied in many ways within the scope of the invention as defined by the appending claims.