US20250198695A1

US20250198695A1 - Method for liquefying a methane-rich feed gas, and corresponding facility

Info

Publication number: US20250198695A1
Application number: US18/849,192
Authority: US
Inventors: Laurent Benoit; Gabrielle MENARD
Original assignee: Engie SA
Current assignee: Engie SA
Priority date: 2022-03-23
Filing date: 2023-03-22
Publication date: 2025-06-19
Also published as: WO2023180391A1; FR3133908A1; EP4496969A1; CA3245465A1

Abstract

The invention relates to a method for liquefying methane-rich feed gas (12), the method comprising:

- mixing the feed gas with recycled gas (18), compressing the gas to a pressure of between 19 and 70 bar absolute, and purifying the gas,
- a first pre-cooling of the gas to a temperature of between −40° C. and −15° C., and second pre-cooling of the gas by heat exchange with the recycled gas,
- liquefying the gas by heat exchange only with a liquefaction refrigeration cycle (46),
- sub-cooling the gas by heat exchange with at least the recycled gas, and
- expanding the sub-cooled liquid (52) to obtain a liquefied gas (14) and the recycled gas representing a mole fraction of less than 35%.

Description

The present invention relates to a method for liquefying a methane-rich feed gas, comprising a purification of the feed gas to obtain a purified gas, a pre-cooling of the purified gas to obtain a pre-cooled gas, a liquefaction of the pre-cooled gas to obtain a stream of liquid, a sub-cooling of the stream of liquid to obtain a stream of sub-cooled liquid, and expansion of the stream of sub-cooled liquid to obtain a liquefied gas.
The invention further relates to a facility for using such a method.
The gas-to-be-treated is e.g. biogas (resulting from the fermentation of organic matter). The relevant market is, e.g., retail LNG (liquefied natural gas), with final storage of the LNG produced at a pressure below 3 bar (105 Pa) absolute. Such market requires relatively low liquefied gas production capacities, typically less than 20 tons of gas to be liquefied per day, or a mechanical power consumption of less than 1 MW.
The liquefaction of the feed gas consists in cooling the gas to a sufficiently cold temperature to remain in liquid form at thermodynamic equilibrium for pressures that make the gas transportable by prior art storage techniques, i.e. at pressures less than 20 bar absolute and more often less than 3 bar absolute. At such pressures, the thermodynamic equilibrium of a methane-rich gas is reached at temperatures below −100° C. and more often below −140° C.
To cool the feed gas to such temperatures, same is usually pretreated to remove any compounds that may crystallize at said cryogenic temperatures. The feed gas can then also be compressed to a pressure higher than the initial pressure thereof. Then, cooling is generally carried out in three main steps summarized hereinafter.
The gas is first pre-cooled, but not sufficiently to condense to the pressure under consideration. Such pressure is, to within head losses, the pressure at the outlet of the pretreatment and is generally greater than the final storage pressure of the liquefied gas.
The gas then undergoes a condensation or a proper liquefaction stage, during which same effectively liquefies and remains, within head losses, at the cooling inlet pressure.
Finally, the gas undergoes a sub-cooling step which continues the cooling of the liquid and, after a final expansion to reach the storage pressure, the remaining liquid is collected and stored.
The last two steps are carried out at cryogenic temperatures, typically below −80° C., and are very expensive. Indeed, on the one hand, the extracted heat, in other words the cooling supplied, at such low temperatures requires a lot of energy and, on the other hand, the equipment suitable for said cryogenic temperatures is much more specific and expensive than the equipment designed for lower temperatures, such as the pre-cooling temperature.
It would be possible to increase the temperature of the liquid at the outlet of the sub-cooling exchanger, just before the final expansion. Unfortunately, when the liquid is less sub-cooled, the expansion produces a large quantity of final vapor (end-flash gas). All or part of the end-flash gas is usually reinjected at the inlet of the liquefaction process, but thereof requires the vapor to be recompressed. Thereby, raising the temperature of the sub-cooled liquid significantly increases the flow rate of gas to be liquefied and the compression requirements of the method.
For facilities with a large capacity of more than 20 tons per day, most of the time the feed gas is already at high pressure, more than 40 bar absolute, because the gas comes either directly from a geological reservoir or from a network. Generally, the sub-cooling is continued down to very low temperatures, of about −150° C., in order to generate less than 10% by volume of vapor during the final expansion.
For facilities with a smaller capacity of less than 20 tons per day, e.g. biogas-biomethane and re-liquefaction of stored LNG vapors or BOG (Boil-Off Gas), it is known to achieve a compression of the feed gas at higher pressure, beyond 80, even 120, absolute bar, combined with a more moderate cooling that stops at −80° C., or even −50° C. The advantage of raising the final sub-cooling temperature is to limit the cost of liquefaction and sub-cooling equipment, but at the cost of producing a large quantity of vapor during the final expansion (more than 45% by volume) and, on the other hand, initial compression of the feed gas that requires expensive compressors and requires equipment specifically suitable for very high pressures.
Furthermore, in current methods, the cold of any vapor produced is generally recovered and used to cool the feed gas over the entire cooling range, i.e. during pre-cooling, liquefaction and sub-cooling, in order to reduce the overall energy consumption of the refrigeration cooling cycle(s). However, thereof does not reduce the cost of the most expensive equipment which have a significant contribution in small capacity facilities.
Thus, the cost of liquefaction of methane-rich gas remains high, in particular on a small scale.
A goal of the invention is thus to propose a liquefaction method which makes it possible to reduce the overall production cost, more particularly for capacities of less than 20 tons per day.
To this end, the subject matter of the invention relates to a method for the production of liquefied feed gas comprising at least 40% by volume of methane, the method comprising the following steps:

- mixing the feed gas with a first stream of recycled gas to obtain a gas-to-be-treated, and compressing the gas-to-be-treated to a treatment pressure comprised between 19 and 70 bar absolute,
- purification of the gas-to-be-treated to obtain a purified gas,
- first pre-cooling of the purified gas to obtain a first pre-cooled gas having a temperature of less than or equal to −15° C. and greater than or equal to −40° C.,
- second pre-cooling of the first pre-cooled gas by heat exchange with at least one second stream of recycled gas to obtain a second pre-cooled gas and the first stream of recycled gas,
- liquefaction of the second pre-cooled gas to obtain a stream of liquid, by heat exchange only with a liquefaction refrigeration cycle,
- sub-cooling of the stream of liquid by heat exchange with at least one third stream of recycled gas to obtain a stream of sub-cooled liquid and the second stream of recycled gas, the stream of sub-cooled liquid being at a sub-cooling temperature, and
- expansion of the stream of sub-cooled liquid to obtain a liquefied gas and the third stream of recycled gas, said expansion and the sub-cooling temperature being such that the third stream of recycled gas represents a mole fraction, relative to the stream of sub-cooled liquid, of less than 35%.

According to particular embodiments, the method has one or a plurality of the following features, taken individually or according to all technically possible combinations:

- the sub-cooling of the stream of liquid comprises: a first sub-cooling of the stream of liquid by heat exchange with the third stream of recycled gas, to obtain an intermediate stream of sub-cooled liquid and the second stream of recycled gas; and a second sub-cooling of the intermediate stream of sub-cooled liquid to obtain the stream of sub-cooled liquid;
- the second sub-cooling of the intermediate stream of sub-cooled liquid is achieved by exchange with a stream of liquid nitrogen, the second sub-cooling producing the stream of sub-cooled liquid a stream of vaporized nitrogen, the second pre-cooling of the first pre-cooled gas taking place by heat exchange the stream of vaporized nitrogen;
- the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by heat exchange with a refrigerant fluid to form the first pre-cooled gas without heat exchange with the first stream of recycled gas, the refrigerant being produced by the pre-cooling refrigeration cycle;
- the pre-cooling refrigeration cycle and the liquefaction refrigeration cycle are disjoint;
- the treatment pressure is less than 45 bar absolute;
- the stream of liquid at the outlet of the liquefaction unit has a temperature between −113° C. and −90° C.;
- the second pre-cooled gas is liquefied by the liquefaction unit with a sub-cooling less than or equal to 5° C.;
- the liquefaction refrigeration cycle is a Stirling cycle or a reversed Brayton cycle; and
- the expansion of the stream of sub-cooled liquid is carried out in at least one Joule-Thomson valve or by an expansion turbine.

A further subject matter of the invention is a facility for using a method such as described hereinabove, comprising:

- a mixer for mixing the feed gas with the first stream of recycled gas and obtaining the gas-to-be-treated, and at least one compressor suitable for compressing the gas-to-be-treated to the treatment pressure,
- a purification unit suitable for purifying the gas-to-be-treated and for obtaining the purified gas,
- a first pre-cooling unit suitable for pre-cooling the purified gas and for obtaining the first pre-cooled gas,
- a second pre-cooling unit suitable for pre-cooling the first pre-cooled gas by heat exchange with at least the second stream of recycled gas and for obtaining the second pre-cooled gas and the first stream of recycled gas,
- a liquefaction unit for liquefying the second pre-cooled gas and for obtaining the stream of liquid, the liquefaction unit including the liquefaction refrigeration cycle,
- a sub-cooling unit suitable for sub-cooling the stream of liquid to the sub-cooling temperature by heat exchange with at least the third stream of recycled gas and to obtain the stream of sub-cooled liquid and the second stream of recycled gas, and
- an expansion unit for expanding the stream of sub-cooled liquid to obtain the liquefied gas and the third stream of recycled gas, the sub-cooling unit and the expansion unit being configured so that the third stream of recycled gas represents a mole fraction, relative to the stream of sub-cooled liquid, of less than 35%.

The invention will be better understood upon reading the following description, given only as an example and making reference to the enclosed drawing, wherein:

FIG. 1 is a schematic view of a facility according to the invention, suitable for using the method according to the invention; and

FIG. 2 is a schematic view of a facility according to the invention constituting a variant of the facility shown in FIG. 1 .

Throughout hereinafter, the same references will identify stream flowing through a pipe and the pipe which carries the stream. The terms “upstream” and “downstream” generally extend with respect to the normal direction of flow of a fluid.
1 Nm³/h means, in the present document, one cubic meter per hour at a pressure of 101,325 Pa and a temperature of 0° C.
The facility 10 according to the invention is described with reference to FIG. 1 . The facility is suitable for liquefying a feed gas 12 comprising at least 40% by volume of methane and obtaining a liquefied gas 14 (i.e. a liquid), e.g. for marketing on the retail LNG (liquefied natural gas) market.
The feed gas 12 is e.g. at low pressure, close to atmospheric pressure. The feed gas 12 is at a temperature close to room temperature, i.e. much hotter than the bubble temperature thereof at atmospheric pressure (101,325 Pa).
The feed gas 12 is e.g. a biogas.
The liquefied gas 14 is advantageously stored at a pressure of less than 3 bar absolute (300 kPa).
In the example, the facility 10 comprises a mixer 16 for mixing the feed gas 12 with a first stream of recycled gas 18 and obtaining a gas-to-be-treated 20. The facility 10 comprises at least one compressor 22 for compressing the gas-to-be-treated 20, e.g. followed by a cooler 24 and a purification unit 26 suitable for purifying the gas-to-be-treated 20 and for obtaining a purified gas 28.
The facility 10 comprises a first pre-cooling unit 30 suitable for pre-cooling the purified gas 28 and obtaining a first pre-cooled gas 32, the first pre-cooling unit including in the example, a pre-cooling refrigeration cycle 34.
“Refrigeration cycle” refers to a set of pipes and elements (not shown), such as compressors or turbines, suitable for subjecting a fluid to a series of transformations with the aim of generating a cold at a place of the cycle, in a manner known per se.
The facility 10 comprises a second pre-cooling unit 36 suitable for pre-cooling the first pre-cooled gas 32 by heat exchange with a second stream of recycled gas 38 and to obtain a second pre-cooled gas 40 and the first stream of recycled gas 18.
The facility 10 comprises a liquefaction unit 42 for liquefying the second pre-cooled gas 40 and for obtaining a stream of liquid 44, the liquefaction unit including a liquefaction refrigeration cycle 46.
The facility 10 comprises a sub-cooling unit 48 suitable for sub-cooling the stream of liquid 44 to a sub-cooling temperature by heat exchange with at least a third stream of recycled gas 50 and for obtaining a stream of sub-cooled liquid 52 and the second stream of recycled gas 38.
The facility 10 comprises an expansion unit 54 for expanding the stream of sub-cooled liquid 52 and obtaining the liquefied gas 14, e.g. received in a storage 56, and the third stream of recycled gas 50.
The compressor 22 is suitable for compressing the gas-to-be-treated 20 to a treatment pressure comprised between 19 and 70 bar absolute, which makes the gas-to-be-treated 20, after purification, liquefiable at cryogenic temperatures which stay nevertheless higher than −113° C.
The treatment pressure is advantageously less than 45 bar.
The purification unit 26 is suitable for removing from the gas-to-be-treated 20 the compounds which can crystallize downstream. The purification unit 26 is conventionally suitable for removing volatile compounds and heavy hydrocarbons (called “C6+”), e.g. by means of activated carbons (not shown and known per se). To reduce the water content down to a few thousand ppmv (parts per million, by volume), the purification unit 26 comprises e.g. a condensation system (not shown). To reduce the CO2 content to less than 2.5 mol. %, e.g. a membrane system (not shown) is used. To lower the CO2 level below 50 ppmv and the water level below 2 ppmv, molecular sieves can be used (not shown).
The purified gas 28 includes at least 90%, or even 99%, of methane by volume.
The first pre-cooling unit 30 comprises e.g. a heat exchanger 58 suitable for making heat exchange between the purified gas 28 and a refrigerating fluid 60 produced by the pre-cooling refrigerating cycle 34, without any heat exchange with the first stream of recycled gas 18.
In the present example, the pre-cooling refrigeration cycle 34 is disjoint from the liquefaction refrigeration cycle 46. “Disjoint” means that the two refrigerating cycles do not share a refrigerating fluid that would be common to both.
The pre-cooling refrigeration cycle 34 used is e.g. a glycol water cycle, a CO2 cycle, an ammonia cycle, a freon cycle or a propane cycle, which are known per se and which will not be described in detail.
The temperature of the first pre-cooled gas 32 is comprised between −40° C. and −15° C.
The second pre-cooling unit 36 comprises e.g., a heat exchanger 62 for making the heat exchange with the second stream of recycled gas 38.
The liquefaction unit 42 comprises e.g. a heat exchanger 64 suitable for making a heat exchange between the second pre-cooled gas 40 and a refrigerating fluid 66 produced by the liquefaction refrigerating cycle 46, without any heat exchange with the second stream of recycled gas 38.
In the example, the liquefaction refrigeration cycle 46 is suitable for providing all the necessary cold to the liquefaction unit 42.
The liquefaction refrigeration cycle 46 is e.g. a Stirling cycle.
“Stirling cycle” refers herein to a refrigeration cycle implemented by a Stirling machine known per se to a person skilled in the art.
In a variant, the liquefaction refrigeration cycle 46 is e.g. a reversed Brayton cycle, also known per se to a person skilled in the art.
The sub-cooling unit 48 and the expansion unit 54 are configured so that the third stream of recycled gas 50 represents a mole fraction, relative to the stream of sub-cooled liquid 52, of less than 35%, and preferably comprised between 10% and 30%. Thereof is possible in particular by sufficiently lowering the temperature of the stream of sub-cooled liquid 52.
Still in the example shown in FIG. 1 , the sub-cooling unit 48 comprises a first heat exchanger 68, a second heat exchanger 70 and a sub-cooling refrigeration cycle 72.
The first heat exchanger 68 is suitable for performing a first sub-cooling of the stream of liquid 44 by heat exchange with the third stream of recycled gas 50, and for obtaining an intermediate stream of sub-cooled liquid 74 and the second stream of recycled gas 38.
The second heat exchanger 70 is suitable for performing a second sub-cooling of the intermediate stream of sub-cooled liquid 74 for obtaining the stream of sub-cooled liquid 52, by heat exchange with a refrigerating fluid 76 produced by the sub-cooling refrigerating cycle 72, without any heat exchange with the third stream of recycled gas 50.
The expansion unit 54 advantageously comprises an expansion member 78 for expanding the stream of sub-cooled liquid 52 and obtaining an expanded sub-cooled stream 80, e.g. to a pressure of less than 3 bar absolute. The expansion unit 54 comprises e.g. a flash drum 82 for separating the expanded sub-cooled stream 80 into the liquefied gas 14 and a vapor forming the third stream of recycled gas 50.
The expansion member 78 is e.g. a Joule-Thomson valve or an expansion turbine.
A first method according to the invention, used by the facility 10, will now be briefly described.
The feed gas 12 and the first stream of recycled gas 18 (i.e. the vapor coming from the flash drum 82, after successive heating in the sub-cooling unit 48 and then in the second pre-cooling unit 36) are mixed by the mixer 16 to form the gas-to-be-treated 20.
The gas-to-be-treated 20 is compressed in the compressor 22, then cooled to approximately ambient temperature, e.g. 20° C., in the cooler 24. The gas-to-be-treated 20 is then purified in the purification unit 26 to form the purified gas 28.
The purified gas 28 undergoes a first pre-cooling in the first pre-cooling unit 30, by heat exchange with the cooling fluid 60, to form the first pre-cooled gas 32.
The first pre-cooled gas 32 undergoes a second pre-cooling in the second pre-cooling unit 36, by heat exchange with the second stream of recycled gas 38, to form the second pre-cooled gas 40. The second stream of recycled gas 38 heats up and becomes the first stream of recycled gas 18.
The second pre-cooled gas 40 is liquefied in the liquefaction unit 42 and forms the stream of liquid 44.
In other words, the second pre-cooling unit 36 does not perform any liquefaction. Liquefaction is entirely performed by the liquefaction unit 42.
The second pre-cooled gas 40 is liquefied by the liquefaction unit 42, with a sub-cooling advantageously less than or equal to 5° C., e.g. of about 3° C. In other words, the temperature of the stream of liquid 44 at the outlet of the liquefaction unit 42 is e.g. 3° C. below the bubble temperature of the second pre-cooled gas 40. The temperature of the stream of liquid 44 at the outlet of the liquefaction unit 42 is preferably between −90° C. and −113° C.
The liquefaction refrigeration cycle 46 advantageously supplies all the cold for the liquefaction of the second pre-cooled gas 40.
The stream of liquid 44 is then sub-cooled in the sub-cooling unit 48 to form stream of sub-cooled liquid 52 by heat exchange with at least the third stream of recycled gas 50, i.e. the vapor coming from the flash drum 82. The third stream of recycled gas 50 heats up and becomes the second stream of recycled gas 38.
In the example, the stream of liquid 44 undergoes a first sub-cooling in the first heat exchanger 68 by heat exchange with the third stream of recycled gas 50, then a second sub-cooling in the second heat exchanger 70 by heat exchange with the refrigerant fluid 76 to form the stream of sub-cooled liquid 52.
The sub-cooling applied reduces the evaporation rate at the outlet of the flash drum 82 to a value of less than 35% in moles. Advantageously, the evaporation rate remains greater than or equal to 20% in moles.
The stream of sub-cooled liquid 52 is expanded in the expansion member 78 to form the expanded sub-cooled stream 80, which is received in the flash drum 82. The liquefied gas 14 is e.g. recovered full-length from the flash drum 82 and sent into the storage 56.
The vapor coming from the flash drum 82 is recycled into the gas-to-be-treated 20. The vapor forms the third stream of recycled gas 50, which first becomes the second stream of recycled gas 38 after passing through the sub-cooling unit 48, then becomes the first stream of recycled gas 18 after passing through the second pre-cooling unit 36.
The vapor does not pass into the liquefaction unit 42, or in any case the vapor does not yield the cold to the liquefaction unit 42.
In the example shown in FIG. 1 , the vapor does not pass into the first pre-cooling unit 30, or in any case does not yield the cold to the first pre-cooling unit 30.
According to a variant (not shown), the vapor can yield part of the cold thereof in the pre-cooling unit 30, in particular depending upon of the size of the facility 10.
For example, if the production of liquefied gas 14 is less than 20 tons per day, the recovery of cold in the first pre-cooling unit 30 from the first stream of recycled gas 18 will be prevented, as shown in FIG. 1 .
On the other hand, if the production of liquefied gas 14 is greater than or equal to 20 tons per day e.g. the recovery will be preferred. To achieve the recovery, the first stream of recycled gas 18 is passed e.g. into the first pre-cooling unit 30.
A facility 100 according to the invention, which is a variant of the facility 10, is described with reference to FIG. 2 . The facility 100 is similar to the facility 10 shown in FIG. 1 . Similar elements have the same numerical references and will not be described again. Hereinafter, only the differences will be described in detail.
In the facility 100, the first pre-cooling of the purified gas 28 is performed by heat exchange with a liquefaction refrigeration cycle 146. In other words, the liquefaction unit 42 and the first pre-cooling unit 30 share the same refrigeration cycle 146, which provides the cold used for the first pre-cooling and for the liquefaction.
The refrigerating fluid 66 yields cold to the second pre-cooled gas 40 in the heat exchanger 64 of the liquefaction unit 42, and becomes the refrigerating fluid 60. The refrigerating fluid 60 yields cold to the purified gas 28 in the heat exchanger 58 of the first pre-cooling unit 30.
As in the facility 10, the second pre-cooling unit 36 of the facility 100 does not receive cold from the refrigeration cycle 146.
In a variant or in addition, in the sub-cooling unit 48, the second heat exchanger 70 does not receive cold from a refrigeration cycle dedicated to sub-cooling, but from an open loop 172 with liquid nitrogen.
In the open loop 172, a stream of liquid nitrogen 174 (coming from a source not shown, such as a liquid nitrogen storage) yields cold to the intermediate stream of sub-cooled liquid 74 in the second heat exchanger 70 of the sub-cooling unit 48, and vaporizes to become a stream of nitrogen gas 176.
Advantageously, the stream of gaseous nitrogen 176 then yields cold to the first pre-cooled gas 32 in the heat exchanger 62, and becomes a stream of nitrogen 178.
According to a particular embodiment, an addition of nitrogen 180 is brought to the stream of gaseous nitrogen 176 before the latter enters the second pre-cooling unit 36. The addition 180 has a temperature lower than the temperature of the stream of gaseous nitrogen 176 before the addition. The addition 180 is advantageously made in liquid form.
Thereby, the second pre-cooling is performed by heat exchange with the second stream of recycled gas 38 and advantageously with the stream of gaseous nitrogen 176, possibly increased by the addition 180.
The operation of the facility 100 is identical to the operation of the facility 10.
According to yet another variant (not shown), the second heat exchanger 70 of the sub-cooling unit 48 is absent, as is the sub-cooling refrigeration cycle 72 (FIG. 1 ) or the open loop 172 (FIG. 2 ). The sub-cooling performed by the sub-cooling unit 48 then takes place only in the first exchanger 68, by heat exchange with the third stream of recycled gas 50.

EXAMPLES

The present examples include one or a plurality of the following characteristics, in all possible combinations.
The treatment pressure is 40 bar absolute (pressure of the gas-to-be-treated after compression by the compressor 22).
The second pre-cooled gas 40 has a temperature of −53.5° C.
The stream of liquid 44, at the outlet of the liquefaction unit 42, has a temperature of −90° C.
The sub-cooling unit 48 and the expansion unit 54 are configured to obtain an evaporation rate comprised between 20% and 30% in moles.
The following two tables define four cases and serve to compare same with each other:

- case 1 (counter-example): no recovery of the cold contained in the flash gas (the third stream of gas 50 does not pass through the exchangers 68 and 62), nor open loop 172 with liquid nitrogen;
- case 2 (example according to the invention): with recovery of the cold contained in the flash gas (the third stream of gas 50 passes through the exchangers 68 and 62), but no open loop 172 with liquid nitrogen. Case 2 corresponds substantially to FIG. 1 ;
- case 3 (counter-example): no recovery of the cold contained in the flash gas (the third stream of gas 50 does not pass through the exchangers 68 and 62), but presence of the open loop 172 with liquid nitrogen; and
- case 4 (example according to the invention): with recovery of the cold contained in the flash gas (the third stream of gas 50 passes through the exchangers 68 and 62), and presence of the open loop 172 with liquid nitrogen. Case 4 corresponds substantially to FIG. 2 .


				Temperature
				of the
			Temperature	stream of		Temperature
	Flow rate	Treatment	of the	liquid 44	Heat load	of the
Flow-rate of	of the	pressure	second pre-	(at the	of the	stream of
the feed gas	gas-to-be-	(compressor	cooled gas	liquefaction	liquefaction	sub-cooled
12	treated 20	22)	40	outlet)	unit 42	liquid 52
kg/h	kg/h	bar absolute	° C.	° C.	kW	° C.

Case

1	100	222.3	40	−35	−90	25.55	−90
Case 2	100	179.0		−53	−90	18.02	−99
Case 3	100	141.6		−35	−90	16.48	−115
Case 4	100	141.6		−53.5	−90	14.36	−115


				Mole
	Pressure	Mass ratio		fraction of
Thermal	of the	of the stream		the flash		Saving on
load of the	stream of	of liquid		gas (third	Energy	liquid nitrogen
sub-	liquid	nitrogen	174/		stream of	saving	consumption
cooling unit	nitrogen	liquefied	Flash drum	recycled gas	compared	compared to
48	174	gas 14	pressure 82	50)	to case 1	case 3
kW	bar absolute	—	bar absolute	—	%	%

Case

1	0		0	2	0.548	0%
Case 2	2.67		0		0.439	−29%
Case 3	4.83	6	0.66		0.294	−35%	0%
Case 4	4.83	6	0.46		0.294	−44%	−30%

Case 1 represents a simple method, namely just liquefaction without sub-cooling and without any recovery of cold on the flash. Attempt is then made to evaluate the energy saving resulting from the gradual addition of cold recovery systems on the flash gas and of a liquid nitrogen sub-cooling system.
To evaluate the saving, in the first approach, only the thermal load of the liquefaction unit 42 is considered, because said part is the most expensive part of the method. In addition, the thermal load is used rather than the mechanical power consumed by the liquefaction cycle, because the mechanical power depends on the type of cycle used (reversed Brayton, MR, Stirling, etc.), yet the focus is on the energy saving regardless of the type of liquefaction cycle.
In conclusion, case 4 reduces the size of the liquefaction unit by 44%, resulting in a significant reduction in the overall cost of liquefied gas production.
By means of the characteristics described hereinabove, the method makes it possible to reduce the overall production cost of the liquefied gas 14, more particularly for production capacities of less than 20 tons per day.
Indeed, the treatment pressure, comprised between 19 and 70 absolute bar, is sufficiently high for the liquefaction temperature not to be too low, i.e. preferably greater than −90° C. Thereby, the equipment used is less specific and less expensive. Since the investment burden is significant for small capacities, thereof has a favorable impact on the unit cost of production. In addition, the energy spent to provide cold is also lower when the temperature of the fluid to be cooled is lower.
However, the treatment pressure remains relatively low and permits a sufficiently low sub-cooling temperature, which keeps the volume fraction of flash gas recycled upstream of the compressor in a reasonable proportion, which reduces the energy spent on compressing the gas-to-be-treated 20. In addition, the lower pressure also leads to savings on the equipment which does not have to withstand very high pressures.
In addition, the cold present in the recycled flash gas (third stream of recycled gas 50) is used specifically to amplify the pre-cooling of the gas-to-be-treated and the sub-cooling thereof. Said cold is not used in the liquefaction unit 42. Thereof reduces the cooling range of the liquefaction unit 42, and reduces the size of the liquefaction refrigeration cycles 46, 146. In addition, thereof avoids having to modify the liquefaction refrigeration cycles as such, in order to integrate a stream of recycled gas.
The sub-cooling is advantageously performed with liquid nitrogen, the cold of which is e.g. also used to amplify the pre-cooling.
The use of the cold of the flash gas, and, if appropriate, of the liquid nitrogen, is not “spread out” throughout the cooling range as in some prior art solutions, but is rather concentrated on the pre-cooling and the sub-cooling, which specifically reduces the most expensive parts of the cooling process, such as the liquefaction unit 42.

Claims

1. A method for liquefying a feed gas comprising at least 40% by volume of methane, the method comprising the following steps:

mixing the feed gas with a first stream of recycled gas to obtain a gas-to-be-treated, and compressing the gas-to-be-treated to a treatment pressure comprised between 19 and 70 bar absolute,

purification of the gas-to-be-treated to obtain a purified gas,

first pre-cooling of the purified gas to obtain a first pre-cooled gas having a temperature of less than or equal to −15° C. and greater than or equal to −40° C.,

second pre-cooling of the first pre-cooled gas by heat exchange with at least one second stream of recycled gas to obtain a second pre-cooled gas and the first stream of recycled gas,

liquefaction of the second pre-cooled gas to obtain a stream of liquid, by heat exchange only with a liquefaction refrigeration cycle,

sub-cooling of the stream of liquid by heat exchange with at least one third stream of recycled gas to obtain a stream of sub-cooled liquid and the second stream of recycled gas, the stream of sub-cooled liquid-being at a sub-cooling temperature, and

expansion of the stream of sub-cooled liquid to obtain a liquefied gas and the third stream of recycled gas, said expansion and the sub-cooling temperature being such that the third stream of recycled gas-represents a mole fraction, relative the stream of sub-cooled liquid, of less than 35%.

2. The method according to claim 1, wherein the sub-cooling of the stream of liquid comprises:

a first sub-cooling of the stream of liquid by heat exchange with the third stream of recycled gas to obtain an intermediate stream of sub-cooled liquid and the second stream of recycled gas, and

a second sub-cooling of the intermediate stream of sub-cooled liquid to obtain the stream of sub-cooled liquid.

3. The method according to claim 2, wherein the second sub-cooling of the intermediate stream of sub-cooled liquid is achieved by exchange with a stream of liquid nitrogen, the second sub-cooling producing the stream of sub-cooled liquid and a stream of vaporized nitrogen, the second pre-cooling of the first pre-cooled gas being performed by heat exchange with the stream of vaporized nitrogen.

4. The method according to claim 1, wherein the purified gas undergoes the first pre-cooling in a first pre-cooling unit including a pre-cooling refrigeration cycle, by exchanging heat with a refrigerant to form the first pre-cooled gas without exchanging heat with the first stream of recycled gas, the refrigerant being produced by the pre-cooled refrigerant cycle, the pre-cooling refrigeration cycle and the liquefaction refrigeration cycle being disjoint.

5. The method according to claim 1, wherein the treatment pressure is lower than 45 bar absolute.

6. The method according to claim 1, wherein the stream of liquid at the outlet of the liquefaction unit has a temperature comprised between −113° C. and −90° C.

7. The method according to claim 1, wherein the second pre-cooled gas is liquefied by the liquefaction unit with a sub-cooling of less than or equal to 5° C.

8. The method according to claim 1, wherein the liquefaction refrigeration cycle is a Stirling cycle or a reversed Brayton cycle.

9. The method according to claim 1, wherein said expansion of the stream of sub-cooled liquid is performed in at least one Joule-Thomson valve or using an expansion turbine.

10. A facility suitable for implementing a method according to claim 1, comprising:

a mixer for mixing the feed gas with the first stream of recycled gas and obtaining the gas-to-be-treated, and at least one compressor suitable for compressing the gas-to-be-treated to the treatment pressure,

a purification unit suitable for purifying the gas-to-be-treated and for obtaining the purified gas,

a first pre-cooling unit-suitable for pre-cooling the purified gas and for obtaining the first pre-cooled gas,

a second pre-cooling unit suitable for pre-cooling the first pre-cooled gas by heat exchange with at least the second stream of recycled gas and for obtaining the second pre-cooled gas and the first stream of recycled gas,

a liquefaction unit for liquefying the second pre-cooled gas and for obtaining the stream of liquid, the liquefaction unit including the liquefaction refrigeration cycle,

a sub-cooling unit suitable for sub-cooling the stream of liquid to the sub-cooling temperature by heat exchange with at least the third stream of recycled gas and for obtaining the stream of sub-cooled liquid and the second stream of recycled gas,

an expansion unit for expanding the stream of sub-cooled liquid to obtain the liquefied gas and the third stream of recycled gas, the sub-cooling unit and the expansion unit being configured so that the third stream of recycled gas represents a mole fraction, with respect to the stream of sub-cooled liquid, of less than 35%.