Patent Application
20250035375
The present invention relates to a method for liquefying a gas to be processed comprising at least 50% by volume of methane, the method comprising a purification of the gas to be processed to obtain a purified gas, a precooling of the purified gas to obtain a precooled gas, a liquefaction of the precooled gas to obtain a liquid stream, a subcooling of the liquid stream to obtain a subcooled liquid stream, and an expansion of the subcooled liquid stream to obtain a liquefied gas.
The invention also relates to a facility able to implement such a method.
The gas to be processed is, for example, a biogas (derived from the fermentation of organic matter). The market concerned is, for example, that of retail Liquefied Natural Gas, (LNG) with final storage of the LNG produced at a pressure of less than 3 bars absolute. This market requires relatively low liquefied gas production capacities, typically less than 2,000 Nm3/h of gas to be liquefied.
Existing liquefaction methods can be grouped into four categories.
A first category comprises phase-change cycles, often referred to as mixed refrigerant (MR) cycles. Cooling is provided by evaporating a refrigerant adapted to the required cooling temperature level. In order to vaporize the refrigerant over a wide temperature range, while minimizing the number of pieces of equipment, the refrigerant is composed of a mixture that evaporates progressively over the targeted temperature range. Once fully vaporized, the refrigerant is condensed in stages by means of several compression stages on the one hand and precooling on the other.
However, phase-change cycles, being technically complex, are therefore costly. To perform suitable cooling at each stage, the phase-change cycles require a large succession of compressors and a refrigerant mixture sometimes including explosive compounds, which necessitates specific compressor technologies. Thus, although effective, this type of cycle proves too costly at the small production scales targeted.
A second category comprises reverse Brayton cycles. Cold is produced by expanding, by means of a turbine, a pre-compressed refrigerant. The major difference with the phase-change cycle is that, in the reverse Brayton cycle, the refrigerant always remains gaseous. The gas to be processed is cooled by the temperature difference between the gas to be processed and the expanded refrigerant.
Nevertheless, reverse Brayton cycles remain relatively complex. Like phase-change cycles, they require a significant succession of compressors, and even higher capacities and power ratings than phase-change cycles, due to their relative lack of energy efficiency. The result is a significant cost at the production scales targeted. In addition, the key machine, the cryogenic expansion turbine, is expensive and not available “off-the-shelf” at all scales, which increases its cost at the relevant production scales.
A third category comprises Stirling cycles. The Stirling cycle resembles the reverse Brayton cycle in that it does not involve the evaporation of a refrigerant. However, its operating points are different, since the expansion stages for the refrigerant and the cooling stages of the gas to be processed, take place at the same time, and the refrigerant compression and external cooling stages also take place at the same time. For this thermodynamic reason, but also because of the technical evolution of existing Stirling solutions, the overall system generally fits into a single, relatively integrated and compact device, unlike other methods in which each stage is carried out by dedicated equipment.
However, Stirling cycles are difficult to implement for high capacities, notably greater than 50 Nm3/h of methane-rich gas to be processed. They are therefore very costly. Stirling cycles include isothermal compression and expansion, and therefore require a compact exchange line the capacity of which cannot easily be increased. Thus, the core of the cycle is often duplicated to increase capacity. As a result, the specific cost does not really benefit from the increase in capacity (no economy of scale).
In addition, Stirling cycles lack energy efficiency. The energy required to cool, liquefy, then subcool the gas to be processed, from ambient to final storage temperature, is close to that of the inverted Brayton cycle, resulting in the need for a large number of Stirling cryogenerators and high operating costs.
A fourth category comprises open cycles. Instead of using mechanical energy to create cold, by compressing, then expanding a recycled fluid, a cold carrier is used, in other words, a consumable that provides the cold. In the case of biogas liquefaction, a suitable carrier is liquid nitrogen. Once vaporized, the liquid nitrogen is released into the atmosphere, and is therefore lost, the cycle being open.
Unfortunately, open cycles present a lack of energy efficiency and high operational costs. Consumption of the cold carrier, for example, liquid nitrogen, to achieve the cooling required to liquefy the gas to be processed requires a liquid nitrogen mass flow rate greater than twice that of the gas to be processed.
One aim of the invention is therefore to propose a liquefaction method allowing the overall production costs to be reduced, particularly for capacities below 2000 Nm3/h.
To this end, an object of the invention is a method for liquefying a gas to be processed comprising at least 50% by volume of methane, the method comprising the following steps:
According to particular embodiments, the method comprises one or more of the following features, taken alone or in any technically possible combinations:
Another object of the invention is a facility able to implement a method as described above, comprising:
The invention will be better understood on reading the following description, given by way of example only and made with reference to the appended drawing, in which
In all that follows, the same references will be used to designate a stream flowing in a pipe and the pipe carrying it. The terms “upstream” and “downstream” generally refer to the normal direction of fluid flow.
1 Nm3/h in the present document means one cubic meter per hour at a pressure of 101,325 Pa and a temperature of 0° C.
With reference to
The gas to be processed 12 is, for example, a biogas, a network gas, a synthetic gas or, more generally, a methane-rich gas.
In the case of a biogas, the gas to be processed 12 may contain up to 45% by volume of CO2 and therefore at best 50 to 55% by volume of methane.
If the gas to be processed 12 is a fossil gas, its methane content before purification is generally higher than 70% by volume.
In the example shown, the facility 10 comprises a compressor 16, followed by a cooler 18, and a purification unit 20 for purifying the gas to be processed 12 to obtain, in a manner known per se, a purified gas 22 which is liquefiable.
According to alternatives, not shown, if the gas to be processed 12 is at a sufficient pressure, the facility 10 is advantageously without the compressor 16 and the cooler 18.
The facility 10 comprises a precooling unit 24 to precool the purified gas 22 and obtain a precooled gas 26 at a temperature less than or equal to −15° C., and advantageously greater than −50° C. According to one particular embodiment, the temperature of the precooled gas 26 is between −45° C. and −15° C.
In the example, the facility 10 also comprises an expansion unit 28 for expanding the precooled gas 26.
The facility 10 comprises a liquefaction unit 30 able to liquefy the precooled, and optionally expanded, gas 26 and to obtain a liquid stream 32, with a subcooling of the liquid stream less than or equal to 5° C., for example about 3° C., at the outlet of the liquefaction unit 30.
The facility 10 comprises a subcooling unit 34 for subcooling the liquid stream 32 to obtain a subcooled liquid stream 36, and an expansion unit 38 for expanding the subcooled liquid stream 36 to obtain the liquefied gas 14.
In the example, the facility 10 comprises a single precooling refrigeration cycle 40 to perform, in the precooling unit 24, a heat exchange with the purified gas 22, and to perform, in the liquefaction unit 30, a precooling of a first refrigerant 42 implemented by the liquefaction unit 30.
By “refrigeration cycle” is meant a set of pipes and elements (not shown), such as compressors or turbines, able to cause a fluid to undergo a series of transformations with the aim of generating cold at a point in the cycle, in a manner known per se (cf. the preamble to the present document).
According to an alternative, not shown, the facility 10 includes two precooling refrigeration cycles distinct from each other (the fluids in which do not mix), one being able to supply cold to the precooling unit 24, and the other to the liquefaction unit 30.
In the example, the expansion unit 38 also being able to produce a methane-rich vapor 44, known as “flash gas”, the facility 10 further comprises a mixer 46 able to mix the gas to be processed 12 with the vapor 44 to obtain a mixture 48. In simpler terms, the vapor is recycled in the gas to be processed upstream of the compression.
The compressor 16, followed by the ambient-temperature cooler 18, allows both the gas to be processed 12 and the vapor 44 to be compressed, for example to between 19 and 40 bar absolute.
In the absence of the compressor 16, for example, because the gas to be processed 12 would already be at a sufficient pressure, only the vapor 44 is compressed by a dedicated compressor (not shown) before being mixed with the gas to be processed 12.
The purification unit 20 makes the compressed gas liquefiable at cryogenic temperatures, typically below −80° C. The purification unit 20 is conventionally able to remove volatile compounds and heavy hydrocarbons (so-called “C6+”) from the gas to be processed 12, for example using activated carbon (not shown and known per se). To reduce the water content of the gas to be processed 12, within the mixture 48, down to a few thousand ppmv (parts per million, by volume), the purification unit 20 comprises, for example a condensation system (not shown). To lower the CO2 level to less than 2.5% mol, a membrane system (not shown) is, for example, used. To lower the CO2 content to less than 50 ppmv and the water content to less than 2 ppmv, molecular sieves (not shown) can be used.
The purified gas 22 includes at least 90% by volume of methane if it is derived from a fossil gas, or 99% by volume or more if it is derived from a biogas.
The expansion unit 28 comprises, for example, a Joule-Thomson valve or a gas expansion turbine.
The liquefaction unit 30 includes a Stirling refrigeration cycle 50 distinct from the precooling refrigeration cycle 40 and implementing the first refrigerant 42. The liquefaction unit 30 is adapted so that the liquid stream 32 has a temperature of between −115° C. and −90° C. at the outlet of the liquefaction unit 30.
By “Stirling refrigeration cycle” 50 here is meant a refrigeration cycle implemented by a Stirling machine 51 known per se to the person skilled in the art.
The Stirling machine 51 is a thermomechanical device configured to perform a single-phase, closed, alternating, regenerative thermodynamic cycle. The cycle comprises successive phases of compression and expansion of the working fluid, in this case the first refrigerant 42, at different temperature levels.
The cycle is said to be “regenerated”, as the first refrigerant 42 passes through a regenerator (not shown), the purpose of which is to cool or heat the first refrigerant 42 according to its direction of transit.
The cycle is said to be “closed”, as the movement of the first refrigerant 42 is entirely controlled by the variation in the internal volumes of the Stirling machine 51, without recourse to devices for isolating the various parts occupied by the first refrigerant 42, in other words, in particular without recourse to valves.
The cycle is said to be “alternating” as, at each point of the internal volumes occupied by the first refrigerant 42, at least one thermodynamic characteristic of the fluid, such as temperature or pressure, is not stationary during the cycle.
The cycle is said to be “single-phase”, in the sense that the first refrigerant 42 remains single-phase during the cycle.
Conversely, a refrigeration cycle is “distinct from a Stirling cycle” if it is not implemented by a Stirling machine or does not possess one of the properties listed above.
The Stirling refrigeration cycle 50 is further able to perform precooling of the first refrigerant 42 by heat exchange with a second refrigerant 52 implemented by the precooling refrigeration cycle 40, the second refrigerant 52 heating up, for example, by less than 5° C. during this heat exchange.
The Stirling refrigeration cycle 50 is advantageously able to precool the first refrigerant 42 to a temperature less than or equal to −15° C.
The precooling refrigeration cycle 40 used is not a Stirling cycle. The precooling refrigeration cycle 40 is, for example, a glycol-water cycle, a CO2 cycle, an ammonia cycle, a freon cycle or a propane cycle, which are known per se and will not be described in detail.
In the example, the precooling refrigeration cycle 40 comprises a cooling module 54 for producing a stream of cooled second refrigerant 56, and a flow divider 57 for dividing the stream of cooled second refrigerant 56 into two flows 58, 60 used respectively to perform the precooling of the first refrigerant 42 and the precooling of the purified gas. The precooling refrigeration cycle 40 comprises, for example, a mixer 62 able to mix the two flows 58, 60 after their respective passages through the liquefaction unit 30 and the precooling unit 24, and to reconstitute a second refrigerant stream 64 directed toward the cooling module 54.
The cooling module 54 is able to cool the second refrigerant stream 64 and produce the cooled second refrigerant stream 56.
The subcooling unit 34 is able, for example, to cool the liquid stream 32 by heat exchange with an open cycle 66 implementing liquid nitrogen.
Alternatively, or additionally, not shown, the subcooling unit 34 is able to cool the liquid stream 32 by heat exchange with the vapor 44 produced by the expansion unit 38.
The expansion unit 38 advantageously comprises an expansion module 68 able to expand the subcooled liquid stream 36 and to obtain an expanded subcooled stream 70 at a pressure of less than 3 bar absolute, for example a pressure of 2 bar absolute. The expansion unit 38 comprises, for example, a flash drum 72 for separating the expanded subcooled stream 70 into the liquefied gas 14, for example received in a storage 74, and the vapor 44 recycled upstream of the compressor 16.
A method according to the invention, implemented by the facility 10, will now be briefly described.
In the example, the gas to be processed 12 and the vapor 44 from the flash drum are mixed by the mixer 46 to form the mixture 48.
The gas to be processed 12, within the mixture 48, is compressed in the compressor 16, then cooled to approximately ambient temperature, for example 20° C., in the cooler 18. Then, the gas to be processed 12, in the example within the mixture 48, is purified in the purification unit 20 to form the purified gas 22.
The purified gas 22 is precooled in the precooling unit 24 to a temperature less than −15° C., by heat exchange with the second refrigerant 56, in the example with the stream 60.
Optionally, the precooled gas 26 is expanded in the expansion unit 28 to obtain a precooled and expanded gas 76.
Then, the precooled and expanded gas 76 (or the precooled gas 26, in the absence of the optional expansion) is liquefied by the liquefaction unit 30, with minimal subcooling, less than or equal to 5° C., for example about 3° C. In other words, the temperature of the liquid stream 32 at the outlet of the liquefaction unit 30 is 3° C. below the bubble temperature of the precooled and expanded gas 76 (or of the precooled gas 26, in the absence of optional expansion). The temperature of the liquid stream 32 at the outlet of the liquefaction unit 30 is preferably between −115° C. and −90° C.
The liquefaction unit 30 implements the Stirling refrigeration cycle 50, which provides the cold allowing the precooled gas 26 to be liquefied.
In particular, the Stirling 50 refrigeration cycle performs the precooling of the first refrigerant 42 to a temperature less than −15° C., by heat exchange with the second refrigerant flow 58. During this heat exchange, the second refrigerant 52 heats up by less than 5° C.
The liquid stream 32 is then subcooled in the subcooling unit 34, expanded in the expansion module 68 to, for example, 2 bar absolute, and sent to the flash drum 72. The liquefied gas 14, is, for example, collected at the foot of the flash drum 72 and sent to the storage 74. In the example shown, the liquid stream 32 is cooled by heat exchange with the liquid nitrogen open cycle 66. In other words, the liquid nitrogen is vaporized and transfers its cold to the subcooling unit 34.
The vapor from the flash drum 72, as mentioned above, is recycled into the gas to be processed 12.
In the precooling refrigeration cycle 40 shown, the second refrigerant stream 64 is cooled by the cooling module 54 to produce the cooled second refrigerant stream 56, The cooled second refrigerant stream 56 is divided into the two flows 58, 60. The flow 60 passes into the precooling unit 24 to transfer its cold to the purified gas 22 and perform the precooling of the purified gas. The flow 58 passes into the liquefaction unit 30 to transfer its cold to the Stirling refrigeration cycle 50 and perform precooling of the first refrigerant 42. After their respective passages through the liquefaction unit 30 and the precooling unit 24, the two streams 58, 60 are mixed to reconstitute the second refrigerant stream 64.
In the example, the precooling of the purified gas 22 and the precooling of the first refrigerant 42 are therefore performed in parallel by the single precooling refrigeration cycle 40.
These examples comprise one or more of the following features, according to any combination.
The gas to be processed 12 is compressed to between 19 and 40 bar absolute, for example to 40 bar absolute.
The purified gas 22 is precooled between −15° C. and −50° C., for example at −35° C., by the precooling unit 24.
There is no expansion of the precooled gas 26 before it enters the liquefaction unit 30.
The temperature of the liquid stream 32 at the outlet of the liquefaction unit 30 is between −115° C. and −90° C., for example −90° C.
The liquid stream 32 is subcooled by the subcooling unit 34 to a temperature which allows to obtain a molar evaporation rate in the flash drum 72 of between 20% and 50%, preferably between 20% and 25%.
Thanks to the features described above, the method allows to reduce the overall production cost of liquefied gas 14, particularly for production capacities of less than 2000 Nm3/h.
Indeed, precooling of the purified gas 22 to a temperature between −15° C. and −50° C., and the subcooling of the liquid stream 32 are performed by dedicated, low-cost systems (separate from the liquefaction unit 30). In the liquefaction unit 30, the Stirling refrigeration cycle 50 is itself precooled to a temperature less than −15° C. by a distinct precooling refrigeration cycle 40, which is not a Stirling cycle. This improves the performance of the Stirling refrigeration cycle 50.
The subcooling of the liquid stream 32 in the liquefaction unit 30 is advantageously reduced to 5° C. or less. The actual subcooling is performed in the dedicated subcooling unit 34, and not by the Stirling refrigeration cycle 50 of the liquefaction unit 30.
All this minimizes the thermal expenditure of the liquefaction unit 30, which alone accounts for half of the overall energy expenditure in the prior art.
Liquefaction is performed by the low-complexity Stirling 50 refrigeration cycle. This cycle is well suited to targeted cooling and advantageously integrates all the elements of self-contained cooling in a single machine.
Subcooling, advantageously performed with liquid nitrogen, further enhances savings.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2113017 | Dec 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/084367 | 12/5/2022 | WO |
