Pressure control of gas liquefaction system after shutdown

#This application is a Section 371 National Stage Application of International Application PCT/EP2009/058318 filed Jul. 2, 2009 and published as WO2011/000424 in English.

#The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.

#During the manufacture of liquefied gas, for example LNG, often use is made of a liquefaction process using an evaporating refrigerant. During shut down of the liquefaction process (for example when the process plant is subject to repairs or servicing) heat ingress from the environment will lead to evaporation of part of the liquid refrigerant contained inside the refrigerant circuit with concurrent potentially problematic pressure increase. On the other hand, when the liquefaction process is started up after such a period of standstill a fast cooling down of the system and in particular of its main cryogenic heat exchanger (MCHE) sometimes may lead to thermal stresses potentially causing leaks.

#Hereinafter the invention will be elucidated while referring to the drawing, in which:

#FIG. 1 schematically shows a first embodiment of the invention, and

#FIG. 2 schematically shows a second embodiment of the invention.

#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 The gas is supplied by a feed line 1 and is withdrawn as liquefied gas by a discharge line 2.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.

#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.

#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.