Method for liquefying a natural gas, including a phase change

Abstract

Process for liquefying natural gas in a cryogenic heat exchanger by flowing in indirect contact with refrigerant fluid entering heat exchanger at a first inlet at temperature T0 and pressure P1, and flowing through the exchanger as co-current with the natural gas stream, leaving the heat exchanger in the liquid state, then being expanded at the cold end of the exchanger to return to gaseous state at a pressure P1 P1 and temperature T1 T0, before leaving the hot end of exchanger by outlet orifice in gaseous state T0. The fluid is then reliquefied to the inlet of the exchanger via compression followed by partial condensation and phase separation, a first liquid phase taken to the first inlet, a first gaseous portion compressed by a second compressor and cooled in desuperheater by contact with portion of the first liquid phase, prior to condensing in a second condenser.

Claims

1. A process for liquefying natural gas comprising a majority of methane, and other components, the other components essentially comprising nitrogen and C-2 to C-4 alkanes, in which said natural gas is liquefied by causing a gaseous stream of said natural gas at a pressure P0 greater than or equal to atmospheric pressure, to flow in at least one first cryogenic heat exchanger in indirect contact with at least one first stream of a first refrigerant fluid comprising a first mixture of compounds circulating in at least one first closed circuit loop with change of phase, said at least one first stream of first refrigerant fluid entering a first one of the at least one first cryogenic heat exchangers via a first inlet at a hot end of said first one of the at least one first cryogenic heat exchangers at a pressure P1 greater than P0 and at a temperature substantially equal to a temperature T0 which is the inlet temperature of the natural gas entering said first one of the at least one first cryogenic heat exchangers, said at least one first stream of first refrigerant fluid passing through said first one of the at least one first cryogenic heat exchangers as a co-current stream with said gaseous stream of natural gas and leaving said first one of the at least one cryogenic heat exchangers via a cold end in the liquid state, said first stream of first refrigerant fluid in the liquid state being expanded by a first expander at a cold end of said first one of the at least one cryogenic heat exchangers in order to return to the gaseous state at a pressure P1 less than P1 and at a temperature T1 less than T0 inside said first one of the at least one cryogenic heat exchangers at said cold end, then leaving said first one of the at least one cryogenic heat exchangers via an outlet orifice at said hot end in the gaseous state and substantially at the temperature T0, said at least one first stream of first refrigerant fluid in the gaseous state then being reliquefied at least in part and taken to the first inlet at said hot end of said first one of the at least one cryogenic heat exchangers to constitute a feed of said at least one first stream of first refrigerant fluid in the liquid state thus circulating in a closed circuit, the liquefaction of said at least one first stream of first refrigerant fluid in the gaseous state comprising first compression in a first compressor followed by first partial condensation in a first condenser, and phase separation in a first separator tank separating a first liquid phase of first refrigerant fluid and a first gaseous phase of first refrigerant fluid, said first liquid phase of first refrigerant fluid at a low outlet from said first separator tank being taken by a pump substantially at the pressure P1 at least in part to said first inlet at said hot end of said first one of the at least one cryogenic heat exchangers in order to constitute said at least one first stream of first refrigerant fluid in the liquid state, said first gaseous phase of said first refrigerant fluid at a high outlet from said first separator tank being compressed substantially to the pressure P1 by a second compressor and then condensed at least in part in a second condenser, wherein said first gaseous phase of said first refrigerant fluid at the outlet from said second compressor is cooled in a desuperheater by coming into contact with a portion of said first liquid phase of first refrigerant fluid at the outlet from said first separator tank, said portion of first liquid phase of the first refrigerant fluid being micronized and vaporized within said desuperheater, prior to said condensation in said second condenser.

2. The process according to claim 1, wherein said portion of first liquid phase of first refrigerant fluid represents less than 10% by weight of the total flow of a total of said first liquid phase of first refrigerant fluid, so as to be vaporized entirely within said desuperheater, and so that after the first liquid phase of first refrigerant fluid has been so-vaporized, the first refrigerant fluid at the outlet from said desuperheater is entirely in the gaseous phase prior to being at least partially condensed in said second condenser, the flow of said first liquid phase portion of first refrigerant fluid being adjusted by at least one control valve.

3. The process according to claim 1, wherein said first gaseous phase of said first refrigerant fluid after being cooled in said desuperheater and condensed in a second condenser, is taken at least in part to a second inlet at the hot end of said first one of the at least one first cryogenic heat exchangers to form a second stream of first refrigerant fluid passing through said first cryogenic heat exchangers as a co-current stream with said stream of natural gas and remaining in the gaseous state and being expanded by a second expander at the cold end of said first one of the at least one first cryogenic heat exchangers to return to the gaseous state at a pressure P1 less than P1 and at a temperature T1 less than T0 inside said first one of the at least one first cryogenic heat exchangers beside the cold end and then being mixed with said first stream of first refrigerant fluid expanded by said first expander in the gaseous state inside said first one of the at least one first cryogenic heat exchangers, the mixture of said first and second streams in the gaseous state leaving said outlet orifice at the hot end said first one of the at least one first cryogenic heat exchangers substantially at the temperature T0, to be taken subsequently to said first compressor with said first stream of first refrigerant fluid in the gaseous state at the outlet from the hot end of said first one of the at least one first cryogenic heat exchangers, to form said first stream of first refrigerant fluid in the gaseous state to be liquefied by a new cycle of said liquefaction, said new cycle comprising first compression in said first compressor.

4. The process according to claim 3, wherein said gaseous phase of first refrigerant fluid cooled in said desuperheater is totally condensed in said second condenser, and is then taken in the liquid state substantially at said pressure P1 and at said temperature T0 to said hot end of said first one of the at least one cryogenic heat exchangers to pass therethrough as a co-current stream with said stream of natural gas mixed with said at least one first stream of first refrigerant fluid in the liquid state, to form the second stream of first refrigerant fluid in the liquid state passing through said first one of the at least one cryogenic heat exchangers as a co-current stream with said natural gas stream and leaving said first one of the at least one cryogenic heat exchangers in the liquid state and being expanded by the second expander at the cold end of said first one of the at least one cryogenic heat exchangers in order to return to the gaseous state at a pressure P1 less than P1 and at the temperature T1 less than T0 inside said first one of the at least one cryogenic heat exchangers next to the cold end of said first one of the at least one cryogenic heat exchangers, and then leaving said first one of the at least one cryogenic heat exchangers via the outlet orifice at the hot end of said first one of the at least one cryogenic heat exchangers in the gaseous state and substantially at the temperature T0 in order to be taken to said first compressor with said first stream of first refrigerant fluid in the gaseous state at the outlet from the hot end of said first one of the at least one cryogenic heat exchangers.

5. The process according to claim 1, wherein said natural gas leaving said cold end of said first one of the at least one cryogenic heat exchangers at a temperature substantially equal to T1 is cooled and at least partially liquefied in a second cryogenic heat exchanger having a hot end and a cold end, in which said natural gas for liquefying is liquefied by causing the stream of said natural gas to flow in indirect contact with at least one first stream of a second refrigerant fluid comprising a second mixture of compounds flowing in at least one second closed circuit loop with phase change, said first stream of second refrigerant fluid entering into said second cryogenic heat exchanger at a first inlet at the hot end of said second cryogenic heat exchanger at a temperature substantially equal to T1 and at a pressure P2, passing through said second cryogenic heat exchanger as a co-current stream with said stream of natural gas, and leaving said second cryogenic heat exchanger in the liquid state at the cold end of said second cryogenic heat exchanger, said first stream of second refrigerant fluid in the liquid state being expanded by a third expander at the cold end of said second cryogenic heat exchanger in order to return to the gaseous state at a pressure P2 less than P2 and at a temperature T2 less than T1 within said second cryogenic heat exchanger at the cold end of said second cryogenic heat exchanger, and then leaving via an outlet orifice at the hot end of said second cryogenic heat exchanger in the gaseous state substantially at the temperature T1, said first stream of second fluid in the gaseous state then being partially reliquefied and taken to the inlet at the hot end of said second cryogenic heat exchanger in order to constitute the feed of said first stream of second cooling fluid in the liquid state thus circulating in a closed loop, the liquefaction of said first stream of the second refrigerant fluid in the gaseous state comprising compression to the pressure P2 by a third compressor and then cooling substantially to T0 in a cooling heat exchanger, with said first stream of second cooling fluid in the gaseous state then being taken to an inlet at the hot end of said first one of the at least one first cryogenic heat exchangers and being output via the cold end of said first one of the at least one first cryogenic heat exchanger in the partially liquefied state substantially at the temperature T1, and then being subjected to phase separation in a third separator tank having a low outlet and a high outlet, said third separator tank separating a liquid phase of the second refrigerant fluid from a gaseous phase of the second refrigerant fluid, the liquid phase of the second refrigerant fluid at the low outlet from said third separator tank being taken substantially at the temperature T1 and the pressure P2 to said first inlet at the hot end of said second cryogenic heat exchanger in order to form said first stream of second refrigerant fluid in the liquid state, said gaseous phase of said second refrigerant fluid at the high outlet from said third separator tank being taken to a second inlet at the hot end of said second cryogenic heat exchanger substantially at the temperature T1 and at the pressure P2 in order to form a second stream of second refrigerant fluid passing through said second cryogenic heat exchanger in the gaseous state and leaving at the cold end of said second cryogenic heat exchanger prior to leaving from an outlet orifice at the hot end of said second cryogenic heat exchanger in order to be taken to said third compressor with said first stream of second fluid in the gaseous state.

6. The process according to claim 5, wherein said natural gas leaving the cold end of said second cryogenic heat exchanger is cooled and fully liquefied at a temperature T3 lower than T2 in a third cryogenic heat exchanger having a hot end and a cold end, in which said natural gas flows in indirect contact as a co-current stream with at least one third stream of second refrigerant fluid fed by said second stream of second refrigerant fluid in the gaseous state leaving the cold end of said second cryogenic heat exchanger substantially at the temperature T2 and at the pressure P2, said third stream of second refrigerant fluid passing in the gaseous state through said third cryogenic heat exchanger as a co-current stream with said stream of liquefied natural gas and leaving said third cryogenic heat exchanger substantially in the gaseous state and being expanded by a fourth expander at the cold end of said third cryogenic heat exchanger to return to the gaseous state at a pressure P2 less than P2 and at a temperature T3 less than T2 within said third cryogenic heat exchanger proximate the cold end of the third cryogenic heat exchanger, and then leaving said third cryogenic heat exchanger via an orifice at the hot end of said third cryogenic heat exchanger in the gaseous state and substantially at the temperature T2 in order subsequently to be taken to an orifice at the cold end of said second cryogenic heat exchanger in order to leave said second cryogenic heat exchanger via an orifice at the hot end of said second heat exchanger in order to be taken to said third compressor together with said first stream of second fluid in the gaseous state.

7. The process according to claim 1, wherein said first expander comprises a valve with an opening percentage that is suitable for being controlled in real time.

8. The process according to claim 1, wherein the compounds of the natural gas and of the first refrigerant fluid are selected from methane, nitrogen, ethane, ethylene, propane, butane, and pentane.

9. The process according to claim 1, wherein the composition of the natural gas lies within the following ranges for a total of 100% of the following compounds: methane 80% to 100%; nitrogen 0% to 20%; ethane 0% to 20%; propane 0% to 20%; and butane 0% to 20%.

10. The process according to claim 1, wherein the composition of the first refrigerant fluid lies within the following ranges for a total of 100% of the following compounds: methane 2% to 50%; nitrogen 0% to 10%; ethane and/or ethylene 20% to 75%; propane 5% to 20%; butane 0% to 30%; and pentate 0% to 10%.

11. The process according to claim 6, wherein the temperatures have the following values: T0: 10 C. to 60 C.; T1: 30 C. to 70 C.; T2: 100 C. to 140 C.; and T3: 160 C. to 170 C.

12. The process according to claim 6, wherein the pressures have the following values: P0: 0.5 MPa to 10 MPa; P1: 1.5 MPa to 10 MPa; and P2: 2.5 MPa to 10 MPa.

13. An installation on board a floating support for performing a process for liquefying natural gas, wherein the installation comprises: at least one first cryogenic heat exchanger having an enclosure, a cold end, a hot end, a first inlet and a second inlet, and comprising: a first duct having an inlet and a cold outlet, the first duct passing through said at least one first cryogenic heat exchanger and suitable for causing a first stream of first refrigerant fluid in the liquid state to flow therethrough; a second duct having an inlet and a cold outlet, the cold duct passing through said at least one first cryogenic heat exchanger and suitable for causing a second stream of first refrigerant fluid in the gaseous or liquid state to flow therethrough; and a third duct having a cold outlet, the third duct passing through said at least one first cryogenic heat exchanger and suitable for causing said natural gas to flow therethrough; a first expander between the cold outlet of said first duct and the first inlet at the cold end of the enclosure of said at least one first cryogenic heat exchanger; a second expander between the cold outlet of said second duct and the second inlet at the cold end of an enclosure of said at least one first cryogenic heat exchanger; a first compressor having an inlet, an outlet, and a connection pipe, the connection pipe located between an outlet at the hot end of the enclosure of said at least one first cryogenic heat exchanger and the inlet of said first compressor; a first condenser having an inlet, an outlet, and a connection pipe, the connection pipe located between the outlet of said first compressor and the inlet of said first condenser; a first separator tank having an inlet, an outlet, and a connection pipe, the connection pipe located between the outlet of said first condenser and said first separator tank; a second compressor having an inlet, an outlet, and a connection pipe, the connection pipe located between the outlet of said first separator tank and the inlet of said second compressor; a desuperheater having an inlet, an outlet, and a connection pipe, the connection pipe located between an outlet of said second compressor and an inlet of the desuperheater for admitting gas into said desuperheater; a second condenser having an inlet, an outlet, and a connection pipe, the connection pipe located between an outlet of said desuperheater and an inlet of said second condenser; a pump having an outlet and an inlet and a first connection pipe the first connection pipe located between the outlet of said first separator tank and said pump, and a second connection pipe fitted with a first valve between the outlet from said pump and the inlet of the desuperheater for admitting liquid into said desuperheater; at least one return pipe located between the outlet of said pump and the inlet of said first duct for carrying first refrigerant fluid; and at least one circulation pipe located between the outlet of said second condenser and the inlet of said second duct for carrying first refrigerant fluid.

14. The installation according to claim 13, further comprising: a second separator tank having an inlet, a top outlet, a bottom outlet, and a connection pipe, the connection pipe located between the outlet of said second condenser and said second separator tank; a connection pipe between the top outlet from said second separator tank and the inlet of said second duct for first refrigerant fluid; a connection pipe between the bottom outlet of said second separator tank and the inlet of said first duct for carrying first refrigerant fluid; and a connection pipe fitted with a second valve and located between the outlet from said pump upstream from said first valve, and a junction with said connection pipe between the bottom outlet of said second separator tank and the inlet of said first duct for carrying first refrigerant fluid.

15. The installation according to claim 13, further comprising: a fourth duct having an inlet and a cold outlet, the fourth duct having an hot end and passing through said at least one first cryogenic heat exchanger and suitable for causing a first stream of second refrigerant fluid in the gaseous or liquid state to flow; a second cryogenic heat exchanger having an enclosure, a cold end, a hot end, a first inlet and a second inlet and comprising: a fifth duct having an inlet and a cold outlet, the fifth duct of the second cryogenic heat exchanger passing through said second cryogenic heat exchanger and suitable for causing a first stream of second refrigerant fluid in the liquid state to flow therethrough; a sixth duct having an inlet, and a cold outlet, the sixth duct of the second cryogenic heat exchanger passing through said second cryogenic heat exchanger and suitable for causing said first stream of second refrigerant fluid in the gaseous state to flow continuously therethrough; and a seventh duct having an inlet and a cold outlet, the sixth duct of the second cryogenic heat exchanger passing through said second cryogenic heat exchanger and suitable for causing said natural gas to flow continuously through said seventh duct passing through said at least one first cryogenic heat exchanger; a third cryogenic heat exchanger having an enclosure, a cold end, a hot end, a first inlet and a second inlet, and comprising: a first duct having an inlet and a cold outlet, the first duct of the third cryogenic heat exchanger passing through said third cryogenic heat exchanger and suitable for causing a second stream of first refrigerant fluid in the gaseous state to flow continuously from said second duct passing through said second cryogenic heat exchanger; and a second duct having an inlet and a cold outlet, the second duct of the third cryogenic heat exchanger passing through said third cryogenic heat exchanger suitable for causing said natural gas to flow continuously from said third duct passing through said second cryogenic heat exchanger; a third separator tank having an inlet, a top outlet, and a bottom outlet; a connection pipe between the cold end of said fourth duct of said first cryogenic heat exchanger and said third separator tank; a connection pipe between the bottom outlet of said third separator tank and an inlet orifice at the hot end of said second cryogenic heat exchanger; a connection pipe between a top outlet from said third separator tank and a hot end of said second duct of said second cryogenic heat exchanger; a third expander between the cold outlet of said first duct of said second cryogenic heat exchanger and an inlet at the cold end of the enclosure of said second cryogenic heat exchanger; a third compressor having an inlet, and an outlet, and a connection pipe, the connection pipe located between an outlet at the hot end of the enclosure of said second cryogenic heat exchanger and the inlet of said third compressor; a gas cooling heat exchanger having an inlet and an outlet, and connection pipe located between the outlet of said third compressor and the inlet of said gas cooling heat exchanger; a connection pipe located between the outlet of said gas cooling heat exchanger and the inlet at the hot end of said fourth duct of said first cryogenic heat exchanger; a fourth expander between the cold outlet of said first duct of said third cryogenic heat exchanger and an inlet at the cold end of the enclosure of said third cryogenic heat exchanger; and a connection pipe located between an outlet at the hot end of the enclosure of said third cryogenic heat exchanger and a second inlet at the cold end of the enclosure of said second cryogenic heat exchanger.

16. The process according to claim 3, wherein said gaseous phase of first refrigerant fluid cooled at the outlet from said desuperheater is condensed in part in said second condenser, and then a second phase separation is performed in a second separator tank having a low outlet, the second separator tank separating a second liquid phase of first refrigerant fluid from a second gaseous phase of first refrigerant fluid, said second liquid phase of first refrigerant fluid at the low outlet from said second separator tank being mixed with a remainder of said first liquid phase of first refrigerant fluid and taken to said first inlet at the hot end of said first one of the at least one cryogenic heat exchangers to form said at least one first stream of first refrigerant fluid in the liquid state substantially at the temperature T0 and substantially at the pressure P1, and said second gaseous phase at a high outlet from the second separator tank being taken at said pressure P1 and substantially at the temperature T0 to the second inlet at the hot end of said first one of the at least one cryogenic heat exchangers to form a second stream of first refrigerant fluid passing through said first one of at least one cryogenic heat exchangers in the gaseous state as a co-current stream with said stream of natural gas, and leaving said first one of the at least one cryogenic heat exchangers in the gaseous state and being expanded by the second expander at the cold end of said first one of the at least one cryogenic heat exchangers to return to the gaseous state at a pressure P1 less than P1 and at a temperature T1 less than T0 inside said first one of the at least one cryogenic heat exchangers next to said cold end, and then leaving via said outlet orifice at said hot end of said first one of the at least one cryogenic heat exchangers in the gaseous state and substantially at the temperature T0, to be taken subsequently to said first compressor with said at least one first stream of first refrigerant fluid in the gaseous state at the outlet from said hot end of said first one of the at least one cryogenic heat exchangers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear in the light of the following detailed description of various embodiments given with reference to the following figures:

(2) FIG. 1A is a diagram of a standard two-loop liquefaction process with change of phase, making use of coil cryogenic heat exchangers;

(3) FIG. 1B shows a variant of FIG. 1A in which the second and third cryogenic heat exchangers C2 and C3 are in continuity and of the so-called cold box type (made of brazed aluminum plates);

(4) FIG. 2 is a diagram of a liquefaction process of the invention including a circuit in the primary refrigeration loop for recycling a portion of the refrigerant fluid in the liquid state to the portion of the refrigerant fluid in the gaseous state, in a desuperheater situated upstream from a refrigerant fluid condenser;

(5) FIG. 2A is a cutaway side view showing a detail of the desuperheater of FIG. 2; and

(6) FIG. 3 is a diagram of a liquefaction process in a preferred version of the invention including a liquid phase and gas phase separator tank in the primary refrigeration loop downstream from the FIG. 2 condenser itself situated downstream from a desuperheater.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

(7) FIG. 1A is a process flow diagram (PFD), i.e. a diagram showing the streams in a standard dual-loop liquefaction process with change of phase known as a dual mixed refrigerant (DMR) process that uses as its refrigerant gases mixtures of gases that are each specific to a respective one of said two loops and that are referred to as the first refrigerant fluid and as the second refrigerant fluid, respectively, the two loops being totally independent of each other.

(8) Natural gas flows in ducts of coil shape Sg passing successively through three cryogenic heat exchangers in series EC1, EC2, and EC3. Natural gas enters at AA into the first cryogenic heat exchanger EC1 at a temperature T0, greater than or substantially equal to ambient temperature and at a pressure P0 lying in the range 20 bar to 50 bar (2 megapascals (MPa) to 5 MPa). The natural gas leaves at BB at T1=50 C. approximately. In this heat exchanger EC1, the natural gas is cooled but it remains in the gaseous state. Thereafter, it passes at CC into a second cryogenic heat exchanger EC2 of temperature lying in the range T1=50 C. approximately at its hot end CC to T2=120 C. approximately at its cold end DD. In this second heat exchanger EC2, all of the natural gas becomes liquefied as LNG at a temperature T2=120 C. approximately. Thereafter, the LNG passes at EE into a third cryogenic heat exchanger EC3. In this third heat exchanger EC3, the LNG is cooled to the temperature T3=165 C., thereby enabling the LNG to be discharged in the bottom portion at FF, and then enabling it to be depressurized at GG so as to be able finally to store it in liquid form at ambient atmospheric pressure, i.e. at an absolute pressure of about 1 bar (i.e. about 0.1 MPa). Throughout that passage of the natural gas along the circuit Sg through the various heat exchangers, the natural gas is cooled, delivering heat to the refrigerant fluid, which in turn become heated by vaporizing as described below and needs to be subjected continuously to complete thermodynamic cycles with change of phase in order to be able to extract heat continuously from the natural gas entering at AA.

(9) Thus, the passage of the natural gas is shown on the left of the PFD where said natural gas flows downwards along the circuit Sg, its temperature decreasing on moving downwards, from a temperature T0 that is substantially ambient at the top at AA, to a temperature T3 of about 165 C. at the bottom at FF; the pressure being substantially equal to P0 down to the level FF of the cold outlet from the cryogenic heat exchanger EC3.

(10) In FIGS. 1 to 3, to clarify explanation, the cold ends of the heat exchangers are physically closer to the bottom ends of said heat exchangers, and vice versa the hot ends of the heat exchangers are at their top ends. Likewise, to clarify explanation, the various phases of the refrigerant fluids are represented as follows: liquid phases are represented by bold lines; gaseous phases are represented by dashed lines; and two-phase phases are represented using ordinary lines.

(11) In the right-hand portion of the PFD, there are shown the thermodynamic cycles to which the refrigerant fluids are subjected in the two loops, as described below.

(12) In conventional manner, the cryogenic heat exchangers EC1, EC2, and EC3 are constituted by at least two fluid circuits that are juxtaposed but that do not communicate fluids between each other, the fluids flowing in said circuits exchanging heat all along their passage through the said heat exchanger. Numerous types of heat exchanger have been developed for various industries, and in the context of cryogenic heat exchangers, two main types are known: firstly coil heat exchangers and secondly heat exchangers using brazed aluminum plates, and commonly referred to as cold boxes.

(13) The description of the invention with reference to FIGS. 1A, 2, and 3 makes reference to heat exchangers EC1, EC2, and EC3 of the coil type. Coil heat exchangers of this type are known to the person skilled in the art and sold by the suppliers Linde (Germany) or Five Cryognie (France). Such heat exchangers comprise a leaktight and lagged enclosure 6, and the natural gas and the refrigerant fluids flow therein in pipes of coiled shapes Sg, S1, and S2, said coils being arranged in said enclosure that is leaktight and lagged relative to the outside in such a manner that heat is exchanged between the inside volume of the enclosure and the various coils with a minimum of heat losses to the outside, i.e. to the ambient medium. In addition, gases and liquids may be respectively expanded or vaporized directly within the enclosure rather than in a duct inside the enclosure and as described below.

(14) FIG. 1B shows a variant of FIG. 1A in which the cryogenic heat exchangers are of the plate heat exchanger type: all of the circuits are in thermal contact with one another in order to exchange heat, but the leaktight and lagged enclosure 6 seeks merely to thermally insulate the various ducts it contains, with no fluid being introduced therein directly, all of the fluids that flow therein thus being prevented from mixing. Heat exchangers of this cold box type are known to the person skilled in the art and they are sold by the supplier Chart (USA).

(15) The process has a first loop referred to as a primary loop or a primary mixed refrigerant (PMR) loop that is made up as follows. A flow d1 of a first stream of the first refrigerant fluid enters the first cryogenic heat exchanger EC1 at its cold end AA at a point AA1 where its temperature is substantially equal to T0 and at a pressure P1, where P1 lies for example in the range 1.5 MPa to 10 MPa. Said first refrigerant fluid passes in the liquid state into the first heat exchanger EC1 in a first pipe of coil shape S1. The first stream of refrigerant fluid leaves the heat exchanger EC1 at BB at a temperature T1 of 50 C. approximately, prior to being directed to a first expander D1 that is constituted by a servo-controlled valve, said valve being in communication at BB1 with the inside of the enclosure 6 of the first heat exchanger EC1 beside the cold end of the heat exchanger EC1. Because of its expansion to a pressure P1 less than P1, where P1 lies in particular in the range 2 MPa to 5 MPa, the liquid of the first refrigerant fluid vaporizes, absorbing heat from the natural gas circuit Sg and heat from the other circuits of the first loop within the first heat exchanger as described below, and also, where appropriate, heat from the duct forming part of the second loop as described below, or indeed other loops when using multiple loop circuits referred to as multiple mixed refrigerant (MMR) circuits.

(16) The first refrigerant fluid in the gaseous state at BB1 passes through the enclosure as a countercurrent and leaves the enclosure of the first heat exchanger EC1 at AA3 at its hot end AA, while still in the gaseous state and substantially at a temperature T0. Said first stream of refrigerant fluid in the gaseous state is then reliquefied and taken to the hot inlet AA1 of said first heat exchanger EC1 in order to constitute the feed of a said first stream of first refrigerant fluid in the liquid state to the inside of the duct S1, thus circulating around a closed circuit.

(17) For this purpose, the stream of the first refrigerant fluid leaving the cold end of the enclosure of the first heat exchanger EC1 at AA3 while in the gaseous state is initially compressed from P1 to P1, where P1 lies in the range P1 to P1, in a first compressor C1, and is then condensed in part in a first condenser H0. The two-phase mixture of the first refrigerant fluid leaving the first condenser H0 is subjected to phase separation in a first separator tank R1. A first liquid phase of the first refrigerant fluid is extracted from the bottom of the first separator tank R1 and redirected as a flow d1a and at a pressure substantially equal to P1 by means of a pump PP to the inlet of a second condenser H1. A gas phase of the first refrigerant fluid is extracted from the top end of the separator tank R1 and is compressed substantially to the pressure P1 as a flow d1b by a second compressor C1A, the temperature at the outlet from said compressor being about 80 C. to 90 C. To facilitate condensation of this gaseous phase d1b, it is mixed with the liquid phase d1a prior to introducing the two-phase mixture d1 that is obtained into the second condenser H1.

(18) In the prior art embodiment shown in FIGS. 1A and 1B, the condensation of the gaseous phase at the outlet from the second condenser H1 is not total and the fluid leaving it may still be a two-phase fluid. The gas that it contains gives rise to a rise in the pressure of the refrigerant fluid. However since the pipes are designed to operate at some given maximum pressure, a safety valve is generally inserted that is rated at a pressure slightly below the limit pressure that can be tolerated by the pipes, said valve (not shown) being connected to a flare 5, serving to eliminate the discharged gas by combustion, given that the quantities involved are small compared with the mass of refrigerant fluid in the loop. This gives rise to a problem because the fraction of gas that is sent to the flare is richer in the lighter components of the mixture constituting the first refrigerant fluid, thereby having the consequence of modifying the composition of the refrigerant mixture and thus of modifying the lowest temperature T1 that is reached on vaporizing the liquid refrigerant fluid in the first expander D1 within the enclosure of the first heat exchanger EC1.

(19) In that primary loop, the composition of the refrigerant mixture is generally determined in terms of alkane components C1, C2, C3, and C4 in the manner described below in order to reach a lowest temperature T1 of about 50 C. However, once a lighter portion of the components has been eliminated, the composition of the mixture changes and its lowest temperature T1 then becomes 40 C. or 45 C., or even 35 C. This results in a drop in the efficiency of the primary loop and thus in a drop in the overall efficiency of the liquefaction process.

(20) In an improved variant of FIGS. 1A and 1B, an additional accumulator tank R1 (not shown) is included downstream from the condenser H1 with the function of receiving a liquid phase, and where appropriate a multiphase phase so that the gas contained in the multiphase phase collects in the top portion of said accumulator tank, where it is trapped, the liquid phase contained in R1 being taken from the bottom of said accumulator tank and being directed to EC1. If the quantity of gas in R1 increases, the pressure within R1 increases and said gas condenses and mixes with the liquid phase before being discharged to the cryogenic heat exchanger EC1. When the pressure of the gas reaches a limit value, a valve opens and releases a portion of the gas to the flare 5 so that its pressure drops back to an acceptable level, thereby preventing the gas from reaching the low point from which liquid phase is taken from said accumulator tank, where it would produce a two-phase mixture with said liquid phase, and where expansion of that mixture in the expander D1 presents a difficult problem. However, under all circumstances, the liquid phase leaving R1 and recycled through S1 presents a composition having a content of lighter components that is either unchanged or else that is decreased.

(21) The adaptations to the primary loop of the present invention as described below with reference to FIGS. 2 and 3 make it possible to overcome the problem of instability and of deterioration in the overall efficiency of the above-described liquefaction process that results therefrom.

(22) The embodiments of FIGS. 1 to 3 include a second loop of a refrigerant fluid that co-operates with all three cryogenic heat exchangers EC1, EC2, and EC3, as described below.

(23) At the cold outlet BB from the cryogenic heat exchanger EC1, the natural gas at temperature T1 is partially liquefied and then passes into the second cryogenic heat exchanger EC2, which it leaves at the temperature T2 while partially liquefied, prior to being cooled and liquefied completely at a temperature T3 in the third cryogenic heat exchanger EC3. A second mixture of refrigerant fluid flows in a second closed circuit loop with phase change as follows. The second refrigerant fluid reaches the hot end CC of EC2 at CC1 while in the liquid state at the temperature T1 and at the pressure P2, where P2 lies for example in the range 2.5 MPa to 10 MPa. The second refrigerant fluid in the liquid state passes through the second heat exchanger EC1 in a coil-shaped duct S2 as a countercurrent to the natural gas fluid in Sg. This first stream of second refrigerant fluid in the liquid state as a flow d2a is then expanded in an expander D2 at the cold end DD of the second heat exchanger EC2 at a point DD1 to a pressure P2 less than P2 and at a temperature T2 less than T1, inside the enclosure of the second heat exchanger EC2. Thereafter, this first stream of second refrigerant fluid leaves the second enclosure via an orifice CC3 at the hot end of the second heat exchanger EC2, while in the gaseous state and substantially at a pressure P2 and a temperature T1. This stream of second refrigerant fluid in the gaseous state is then compressed from P2 to P2 in a compressor C2 that it leaves at a temperature lying in the range 80 C. to 100 C., approximately, prior to being cooled in a temperature cooling heat exchanger H2 that it leaves while still in the gaseous state and at a temperature substantially equal to T0 (20 C. to 30 C.) This second refrigerant fluid gas is then taken at AA4 to the hot end AA of the first cryogenic heat exchanger EC1 in order to be cooled on passing through it in a coil-pipe type S1B that it leaves at BB3 at the cold end BB of the first heat exchanger EC1 at a temperature T1=50 C. approximately and in a multiphase state, i.e. a partially-liquefied state, as a flow d2 in order to be separated in a second separator tank R2, where it is separated into a liquid phase and a vapor phase. The liquid phase is sent as a flow d2a via CC3 to the hot end CC of the second heat exchanger EC2 in order to constitute the feed of said first stream of the second refrigerant fluid in the liquid state within the coil S2 for the purpose of performing a new cycle as described above. The vapor phase flow d2b leaving the second separator tank R2 is likewise taken to the hot end CC of the second heat exchanger EC2 at substantially T1 and substantially P2 in order to feed via CC2 another coil-shaped duct S2A within the second heat exchanger EC2. The gaseous stream d2b of the second refrigerant fluid leaves via DD3 in the vapor state at a pressure substantially equal to P2 and at a temperature T2=120 C. approximately in order to be taken to the hot end EE of the third cryogenic heat exchanger EC3, still at T2=120 C. approximately, within which heat exchanger it is cooled in a coil-shaped duct S3. The refrigerant fluid leaves the duct S3 at FF while still in the gaseous state at a pressure of substantially P2 and at a temperature T3=165 C. approximately prior to being expanded to P2 less than P2 in an expander D3 directly within the enclosure EC3 at a cold end via FF1 in order to leave it at its hot end via EE1 at approximately a pressure P2 and a temperature T2=120 C. and being taken to the cold end of the second enclosure EC2 via DD2. This second stream d2b of second refrigerant fluid in the gaseous state is then in a mixture with the first stream d2a of the second refrigerant fluid vaporized to the gaseous state on expanding in the expander D2 at DD1, the mixture of the two gases leaving the second heat exchanger EC2 as a flow d2=d2a+d2b via CC3 in order to perform a new cycle through the compressor C2 and the cooler E2, as described above.

(24) In FIG. 1B, the cryogenic heat exchangers are cold box heat exchangers as described above and the gases from the fluid vaporized by the expanders D1, D2, and D3 are channeled via coil-shaped ducts S1C, S2B, and S2C respectively within the first heat exchanger EC1, the second heat exchanger EC2, and the third heat exchanger EC3 in order to leave at the hot end of the first heat exchanger EC1 via AA3 and at the hot end of the second heat exchanger EC2 at CC3.

(25) In FIG. 1B, the second and third heat exchangers EC2 and EC3 together with said pipes S2A and S3 are in continuity from the hot end CC of the second heat exchanger EC2 to the cold end FF of the third heat exchanger EC3. The return of the gaseous phase from the expander D3 via FF1 to the cold end of the third heat exchanger via the outlet CC3 at the hot end of the second heat exchanger EC2 takes place in a coil-shaped duct S2C. Likewise, the return of the gaseous phase from the expander D2 via DD1 at the cold end of the second heat exchanger in DD1 going to CC3 at the hot end of the second heat exchanger takes place in a coil-shaped pipe S2B.

(26) In FIGS. 2 and 3, there are shown two variant implementations of the process of the invention. The modifications relative to the prior art process shown in FIGS. 1A and 1B lie in the first loop of the first refrigerant fluid.

(27) In FIG. 2, the liquid phase of the first refrigerant fluid at the pressure P1 and as a flow d1a leaving the first separator tank R1 is split into two streams or flows d1c and d1b=d1, with only the liquid portion of the flow d1 being sent directly to the hot end AA of the first heat exchanger EC1 in order to constitute the feed of the first stream of liquid first refrigerant fluid in the duct S1. A portion of the flow d1c representing a mass ratio lying in the range 2% to 5% relative to the initial flow d1a is sent into a desuperheater DS, the gaseous phase d1b leaving the second compressor C1A also going to the inlet of the desuperheater DS that operates as described below. The liquid fraction of the flow d1c sent to the desuperheater DS is adjusted by the combined action of the servo-control valve V1 and of the first expander D1 as described below. This fraction d1c represents 2% to 10%, preferably 3% to 5% of the flow d1a from the pump PP.

(28) FIG. 2A is a cutaway side view of the desuperheater DS which serves to cool the gaseous phase d1b before it enters the condenser H1. The desuperheater DS is constituted in conventional manner by a gas inlet pipe 1 connected to an internal strip 3 in the form of a perforated tube having a plurality of small-section orifices 4 distributed along and at the periphery of said strip. A pipe 2 bringing in liquid from the pump PP delivering a flow d1c that is controlled by the servo-control valve V1 serves to feed the strip 3 with liquid so as to create a mist of fine liquid droplets leaving the orifices 4 because of the pressure causing the liquid to be spread through said strip 3. The fine droplets of liquid then present a large specific surface area for exchange with the gaseous phase arriving via the feed pipe 1. The latent heat of evaporation of the liquid phase then has the effect of cooling the incoming gaseous phase. Said gaseous phase presents a temperature at the inlet to the desuperheater DS of about 80 C. to 90 C., and its temperature at the outlet from the desuperheater is no more than 55 C. to 65 C. because of the heat absorbed by vaporizing the liquid fluid d1c. The quantity of liquid d1c injected into the desuperheater DS is adjusted accurately so that all of the stream leaving the desuperheater DS is in the gaseous state and thus presents a homogeneous composition of gases.

(29) A desuperheater DS of this type is sold by the supplier Fisher-Emerson (France).

(30) In FIG. 2, the first refrigerant fluid leaving the desuperheater DS is thus entirely in the gaseous state at a temperature of about +55 C. to +65 C. prior to being fully condensed in a said second condenser H1, which in this example is a total condenser. At the outlet from the second condenser H1, the first refrigerant fluid is entirely in the liquid state and represents a flow d1 that is taken at the temperature T0 and substantially at the pressure P1 to the hot inlet AA2 of the first heat exchanger EC1 through which it passes within a coil-shaped duct S1A as a co-current with the fluid passing through the coil-shaped pipes Sg and S1 and S1B, prior to being taken to a second expander D1A likewise constituted by a servo-control valve, the second expander D1A being in communication with the inside of the heat exchanger EC1 via its cold end at BB2. At this level, the second stream of the first refrigerant fluid in the liquid state vaporizes, thereby absorbing heat from the natural gas duct Sg and also absorbing heat from the streams of the duct S1, of the duct S1A, and of the duct S1B.

(31) In FIG. 2, the first stream or flow d1 and the second stream or flow d1 of the first refrigerant fluid as vaporized at BB1 and at BB2 by the first expander D1 and by the second expander D1A respectively at the cold end and inside the first enclosure EC1 mix together inside said enclosure of the heat exchanger EC1. This mixture leaves its hot end via AA3 to form the stream or flow d1=d1+d1 of gas of the first refrigerant fluid that is then compressed in the first compressor C1 from P1 to P1 in order to be subjected to a new cycle, as described above.

(32) This implementation of FIG. 2 is advantageous since during the pre-cooling of the first gas stream in the desuperheater DS, the light gas coming from the tank R1 becomes mixed with vapor coming from a heavy liquid phase d1c, and the resulting mixture is then heavier than the incoming gas phase on its own, thereby facilitating condensation in H1 and enabling condensation to be total and more efficient.

(33) The fact that the first stream or flow d1 and the second stream or flow d1 of the first refrigerant fluid in the liquid state respectively leaving the second condenser H1 and the pump PP as described above are not mixed together before passing through the first heat exchanger EC1, but rather pass through the first heat exchanger EC1 in two separate ducts S1 and S1A is also advantageous, since the two streams present different compositions of the first refrigerant fluid, and they are also at different pressures. Thus mixing them would lead to instabilities that are more problematic than those in the prior art. Nevertheless, it is possible to control the mixing of said two liquid streams using appropriate regulation systems, e.g. control valves, but that would go against the simplicity and the reliability desired in an installation of this type.

(34) FIG. 3 shows a preferred variant implementation of the invention, in which the second condenser H1 is not a total condenser, with only a portion of the gas stream leaving the desuperheater DS being condensed in the second condenser H1. The two-phase fluid leaving the second condenser H1 at a flow die is subjected to phase separation in a second separator tank R1A within which a second liquid phase and a second gaseous phase of the first refrigerant fluid are separated.

(35) In FIG. 3, the second liquid phase of refrigerant fluid from the low outlet of R1A is taken to the duct S1 and represents a flow d1f. The flow d1a at the outlet from the pump PP is separated into two flows, respectively d1c to the desuperheater DS, which flow is adjusted by the first control valve V1, and a residue did that is adjusted by a second control valve V1A, said two control valves being controlled closely in combination with each other; said residue did is then mixed with the liquid flow d1f and taken to the pipe S1 at the hot end of the cryogenic heat exchanger EC1, substantially at the pressure P1.

(36) In FIG. 3, the second gaseous phase of the first refrigerant fluid leaving the high outlet of the second separator tank R1A represents a flow d1. It is taken at the temperature T0 and substantially at the pressure P1 to the inlet AA2 at the hot end AA of the first heat exchanger EC1 in order to pass through it in the duct S1A while in the gaseous state and not in the liquid state as in the implementation of FIG. 2. At the cold end of the duct S1A at BB2, the second expander D1A expands the gas of the second gaseous phase of the first refrigerant fluid to a pressure P1 less than P1. This expansion of the gas at BB2 from S1A by D1A then absorbs heat from Sg, S1, S1A, and S1B, thereby cooling them, and where appropriate absorbs heat from other loops if there are multiple loop circuits (referred to as MMR as mentioned above). The fluid in the liquid state leaving the second expander D1A via BB2 mixes with the first portion of the first refrigerant fluid vaporized at BB1 in order to leave via AA3 as a flow d1 and in order to be compressed by the first compressor C1 from P1 to P1, where P1 lies in the range P1 to P1. Thereafter, it leaves the first compressor C1 in the form of a two-phase mixture having a liquid phase as a flow d1a that is compressed substantially to P1 by the pump PP, and a gaseous phase as a flow d1b that is compressed at P1 by the second compressor C1A, and then cooled within the desuperheater DS, and then partially or totally condensed within the condenser H1, and finally separated once more within the separator R1A, as described above, for a new cycle, as described above.

(37) In the variant implementation of FIG. 3, the expander D1 is a liquid-to-gas expander, whereas the expander D1A is a gas-to-gas expander.

(38) The implementation of FIG. 3 is preferred since firstly the control valve VIA associated with the control valve V1 and the expander D1 enables two liquid phases to be mixed together and enables them to be vaporized under good conditions of stability, and secondly it does not require the use of a total condenser, thereby increasing the overall stability of the process and thus its industrial reliability. In this preferred variant, the liquid stream d1 represents about 95% by weight of the stream of the first refrigerant gas, while the gaseous stream d1 represents the complement, i.e. about 5%.

(39) The condensers H0 and H1 and the cooler H2 may be constituted by water heat exchangers, e.g. exchanging heat with sea or river water, or cold air heat exchangers of the cooling tower type, known to the person skilled in the art.

(40) The compositions of the first and second refrigerant fluids are associated with the technologies used in terms of cryogenic heat exchangers and condensers, and manufacturers and suppliers all recommend their own compositions. However these compositions are also closely associated with the composition of the natural gas that is to be liquefied, and the components of the refrigerant fluids are advantageously adjusted over time whenever the characteristics of the natural gas change in significant manner.

(41) By way of example, the first refrigerant fluid operating in a loop in the heat exchanger EC1, and thus at ordinary temperature T0 (20 C. to 30 C.) down to a lowest temperature T1 of about 50 C., is constituted by the following mixture: C1 (methane)2.5% C2 (ethane/ethylene)60% C3 (propane)15% C4 (butane)20% C5 (pentane)2.5%

(42) Likewise, the second refrigerant fluid operating in a loop in the heat exchangers EC1, EC2, and EC3, and thus from T1=50 C. approximately, down to a lowest temperature of T3=165 C. approximately, is constituted by the following mixture: N2 (nitrogen)5% C1 (methane)45% C2 (ethane/ethylene)37% C3 (propane)13%

(43) The mechanical power consumed for an annular production of 2.5 mega tonnes per year (Mt/y) in the installation as a whole is of the order of 85 megawatts (MW): 50 MW being injected via the compressor C2, generally by means of a first gas turbine (not shown); and 35 MW being injected via the compressors C1 and C1A, generally by means of a second gas turbine, with C1 absorbing substantially of the power and C1A the remaining third.

(44) These powers involved by the processes of the invention are of the same order and have substantially the same distribution as the powers involved in the prior art. In contrast, said processes of the invention are much more stable and reliable, and as a result provide an optimized industrial technique.

(45) The invention is described above in the context of two-loop processes, comprising a hot first loop corresponding to the circuits S1-S1A-S1B operating in the heat exchanger EC1 (50 C.), and a cold second loop corresponding to the circuits S2-S2A-S3 operating in the heat exchangers EC2 (50 C.=>120 C.) and EC3 (120 C.=>165 C.). However, similar processes exist in which the hot loop is identical, but the cold loop is replaced by two independent loops each having its own refrigerant fluid, in general a second loop operating in the heat exchanger EC2, i.e. from 50 C. to 120 C., while the third loop operates in the heat exchanger EC3, i.e. from 120 C. to 165 C. In all of these processes, and regardless of the type of cryogenic heat exchanger, the hot loop corresponding to the heat exchanger EC1 remains substantially the same as that described with reference to FIG. 1A. Thus the invention applies to practically all processes for liquefying natural gas using multiple independent loops and changes of phase.