US20190310014A1

US20190310014A1 - Device and method for liquefying a natural gas and ship comprising such a device

Info

Publication number: US20190310014A1
Application number: US16/470,218
Authority: US
Inventors: Hicham Guedacha
Original assignee: Engie SA
Current assignee: Engie SA
Priority date: 2016-12-22
Filing date: 2017-12-15
Publication date: 2019-10-10
Also published as: FR3061277B1; EP3559572A1; CN110537064A; FR3061277A1; WO2018115659A1; BR112019012890A2

Abstract

The device (100) for liquefying a natural gas comprises:

- a compressor (105) for a first vaporized coolant chemical mixture,
- a means (110) for fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first heat exchange body (115) for heat exchange between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second heat exchange body (120) for heat exchange between the light fraction of the first mixture and the cooled natural gas in the first exchange body in order to liquefy the natural gas, and
- a return pipe (125) for return of the first vaporized coolant mixture in the heat exchange body to the compressor (105),
- upstream from an inlet (116) for the natural gas in the first exchange body (115) or downstream from an outlet (121) of liquefied natural gas from the second exchange body (120), a third heat exchange body (130, 135) for heat exchange between the natural gas and a second coolant chemical compound, and
- a means (140, 145) for compressing the second vaporized compound.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a device for liquefying a natural gas, a method for liquefying a natural gas, and a ship comprising such a device. It applies, in particular, to the offshore or onshore liquefaction of natural gas.

STATE OF THE ART

Liquefying the gas allows natural gas to be transported in a smaller volume compared to transporting non-liquefied natural gas.
Over the last few decades, liquefaction technologies have focused on large gas capacities for reasons of economy of scale.
Implementation of the technologies thus used requires very substantial investments and has very high transportation costs (marine liquefaction and reception facilities). As a result, firstly the trend for liquefaction capacities has been to increase the volume of natural gas transported in order to obtain economies of scale and to make these projects economically more attractive. Secondly, the investments made to implement these technologies focused on this sizing, and the construction of liquefaction methods had to be the most efficient possible in order to minimize the subsequent operating costs.
Today, the number of large-scale projects has decreased significantly, and there is renewed interest in the production of a small capacity of liquefied natural gas from natural gas or biogas.
In effect, the upcycling of small gas sources, by-product gases and biogas are new opportunities encouraged in particular by a growing environmental awareness among populations and governments, or a wish to reach isolated consumers in areas with no gas transportation and/or distribution infrastructure. However, these opportunities are too small to justify the use of technologies intended for large-scale production (the transposition of conventional technologies is not appropriate, as they are too complex and cannot be used to support the economic viability of these technologies), hence the need to propose new technologies that can meet the two main challenges relating to liquefaction on a small scale:

- reducing investment costs as much as possible while keeping efficiency as high as possible in order to minimize operating costs; and
- increasing the efficiency of the method so as to minimize product loss: the gas volumes to be upcycled are small, which makes every molecule important.

Today, offshore and near-shore gas resources are booming, leading to technical solutions adapted to the marine environment, known as “FLNG” (for “Floating Liquefied Natural Gas”), being used.
In particular, several types of liquefaction cycles are known:

- cascade cycles;
- mixed coolant cycles; and
- expansion cycles.

The liquefaction cycles are based on these cycles or on a combination of these cycles. This is notably the case of the “Integral Incorporated Cascade” process, aka CII (short for “cascade intégrale incorporée”).
In the CII system:

- a mixture of coolants is compressed and then fractionated into a heavy fraction and a light fraction;
- the heavy fraction is utilized in a first plate heat exchanger to exchange heat between this heavy fraction and the natural gas in order to cool it;
- the light fraction is compressed;
- the light fraction is utilized in a second plate heat exchanger to exchange heat between this heavy fraction and the cooled natural gas in order to liquefy this natural gas; and
- the two reheated fractions of the mixture are then collected and supplied once more compressed.

This system has several drawbacks:

- plate heat exchangers are very sensitive to the distribution of fluids, which poses a problem of marine adaptation for the marine applications;
- the coolant mixture has a significant number of components, especially heavy compounds, and these compounds crystallize in the heat exchangers under particular pressure and temperature conditions whose arrival is difficult to forecast; and
- the method has limited flexibility, especially in terms of operating flow rate, and a limited production capacity per compression means.

In the CII method, the coolant mixture is a mixture of nitrogen and hydrocarbons (methane, ethane, ibutane, nbutane, ipentane and npentane). The partial vaporization of this mixture at low pressure makes it possible to cool and liquefy the natural gas, and super-cool the LNG produced:

- vaporization at low pressure of the heavy fraction of the coolant mixture (gas at the top of the fractionating column) makes it possible to contribute the negative heat necessary for cooling at least the natural gas; and
- the light fraction will make it possible to liquefy the natural gas and super-cool the LNG.

On output from the exchange line, the coolant mixture is completely vaporized.
For the CII system the main drawbacks are that:

- the CII method had been developed for large-scale LNG production on land;
- the method is not very flexible: efficiency drops significantly with any deviation from the operating/design point;
- the coolant mixture contains too many constituents (hydrocarbons), making the logistics and operational aspect more complex;
- storage increases the weight of the facilities, critical for offshore facilities;
- the method has difficulties concerning the ethane supply, which poses significant difficulties for offshore facilities;
- the method presents risks of alteration to the equipment (exchangers) due to the risk of crystallization of the pentane (iC5 and nC5) contained in the coolant mixture;
- the efficiency of the method is limited by the dimensions of the exchanger and by manufacturing constraints; and
- the method presents problems concerning the installation of equipment at sea (significant drop in the performance of the plate exchanger if the distribution is not right).

The CII method is based on a single compression line using centrifugal compressors.
A centrifugal compressor is used to compress a gas and as a result increase its pressure. Centrifugal compressors are fitted with wheels rotating around a shaft driven by a turbine or by an electric motor. These rotating wheels allow the kinetic energy contained in the gas to be transformed into potential energy in order to increase its pressure.
As the possible pressure increase that can be achieved by a wheel is limited, it is necessary to increase their number in order to achieve the desired discharge pressure.
The set of wheels is contained in a body called the “casing”. A casing can contain from eight to a maximum of ten wheels; the greater the number, the more the compressor is likely to present stability problems.
Compression is central to a liquefaction method. In effect, each additional point of efficiency gained at the compressor allows the production of liquefied natural gas to be increased.
In addition, the compression train is the capital-intensive component in a liquefaction unit.
Increasing the efficiency of a centrifugal compressor leads to an increased investment. Conversely, reducing the investment leads to solutions that are less efficient and/or significantly less flexible.
The financial value of a compressor is directly linked to the number of casings. In effect, the higher the number of casings, the higher the investment to be made but the greater the operational flexibility. Conversely, a reduction in the number of casings results in a loss of operational flexibility, sometimes accompanied by a loss of efficiency.
The challenge for the present invention therefore consists of providing a better compromise between efficiency and investment so as to keep satisfactory performance levels in as broad an operating range as possible.
The compression trains of the CII method comprise a low- and medium-pressure section and a high-pressure compression section. The compression steps are grouped together in one, two or three casings.
The low- and medium-pressure section makes it possible to compress the coolant mixture at low pressure on output from the cryogenic exchange line.
The high-pressure section makes it possible to compress the light fraction of the coolant mixture, which will make it possible to contribute the negative heat necessary for liquefaction and the super-cooling of the liquefied natural gas.
The actuation of low- and high-pressure sections in the compression train by the same shaft results in a single speed of rotation between the turbines of these sections. This speed of rotation can be differentiated by using multiplication mechanisms between the speed of rotation of the turbines relative to the single shaft. However, with or without a multiplication mechanism, the rotation speeds must be proportional or identical, depending on the case, which makes the compression train inflexible when the flow rates entering each section are not identical or proportional. This can pose problems of mechanical stability.
In addition, a large drop in efficiency is observed in the high-pressure section, even more significant when one wishes to include both sections in a single casing.
Lastly, this configuration is not very flexible, which limits the field of opportunities that can be addressed: a drop in efficiency, which can be significant, can be observed if one deviates from the operating point for which the equipment was sized (natural gas, and precise production conditions) and mechanical stability problems can occur leading to more frequent maintenance.
The current CII systems present the following drawbacks:

- a variation in the flow rate between the low-pressure and high-pressure sections of the compression train leading to an imbalance between these two sections, which can lead to mechanical instability problems during the shutdown and start-up phases;
- a significant drop in efficiency is observed between the low-pressure section and high-pressure section; and
- limited flexibility for the compression train in terms of flow-rate range and composition.

Systems are known, for example, such as those described in document WO 2011/039279. In such systems, a hydrocarbon flow is separated in a first fractionating device to produce an upper first hydrocarbon flow and a lower first hydrocarbon flow. The top flow is liquefied and at least one portion is then cooled by a coolant fluid to provide a cooled liquefied hydrocarbon flow and a hot coolant flow. The cooled liquefied hydrocarbon flow is then expanded and stored.
This document thus describes a liquefaction method using three different pure coolant compounds circulating in separate cooling circuits. This description is difficult to implement, especially from an operational, supply/logistics point of view, and requires a large area for implementation.
The description in the document “New trends in LNG process design”, by E. Flesh et al., is also known. This description has several limitations:

- to obtain efficiency of the cryogenic exchange line limited by construction constraints, this technology requires a surface area ratio to be observed between the lower portion and the upper portion, limiting the operational flexibility of this technology;
- the presence of heavy compounds in the coolant mixture (pentanes), which can crystallize and create equipment damage, if the operational envelope specified during the design of this method is deviated from.

SUBJECT OF THE INVENTION

The present invention aims to remedy all or part of these drawbacks.
To this end, according to a first aspect, the present invention relates to a device for liquefying a natural gas, comprising:

- a compressor for a first vaporized coolant chemical mixture,
- a means for fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first body for exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second body for exchanging heat between the light fraction of the first mixture and the natural gas cooled in the first exchange body in order to liquefy the natural gas, and
- a conduit for returning the first coolant mixture vaporized in the heat exchange body to the compressor,
  which comprises:
- upstream from an inlet for the natural gas in the first exchange body or downstream from an outlet for liquefied natural gas from the second exchange body, a third body for exchanging heat between the natural gas and a second coolant chemical compound, and
- a means for compressing the second vaporized compound.

When the third exchange body is positioned upstream from the inlet for the natural gas in the first exchange body, the device makes it possible to perform a pre-cooling of the natural gas before the cooling performed by the first exchange body.
When the third exchange body is positioned downstream from the outlet for liquefied natural gas from the second exchange body, the capacity of the device is increased.
This configuration increases the advantages of the conventional CII configuration by reducing its drawbacks, in particular by:

- reducing the flow rate of the coolant mixture and the impact on the device's power consumption;
- reducing the weight of installations: reducing the sizes of the compressors for the coolant mixture, simplifying the logistics, and reducing on-site storage capacities;
- reducing the device's dimensions; and
- maximizing production and efficiency by minimizing the quantity of equipment.

Therefore, the present invention uses a single initial cooling mixture which, thanks to one or two fractionations, is decomposed into two or three mixtures circulating in a single closed circuit. The quantity of equipment used is substantially reduced and control is simplified.
In some embodiments, the third exchange body is positioned upstream from the inlet for the natural gas in the first exchange body, the device comprising:

- a means for cooling the second compressed compound and
- a conduit for transferring the second cooled compound to the third exchange body.

These embodiments make it possible to produce a pre-cooling loop for the natural gas.
In some embodiments, the second chemical compound is a pure substance composed of nitrogen, propane and/or ammonia or a mixture of nitrogen and propane.
The use of such a composition to form the second compound allows the natural gas to be cooled before this natural gas enters into the exchange line formed of the first and second exchange body. This pre-cooling makes it possible to simplify/limit the number of constituents in the first cooling mixture used, which also makes it possible to reduce the dimensions of the exchange surfaces between the natural gas and the first cooling mixture.
In some embodiments, the first cooling mixture comprises nitrogen and methane and at least one compound amongst:

- ethylene;
- ethane;
- propane; and/or
- butane.

Use of such a mixture makes it possible to minimize the energy supply to the system for liquefying the natural gas.
In effect, in the CII system as currently implemented, heavy compounds are used in the first coolant mixture, these compounds having the advantage of ensuring the vaporization of the first mixture before it enters the first compressor.
However, these heavy compounds have the drawback of crystallizing in the coldest portion of the exchanger as a function of the content of these compounds and of the specified operating conditions possibly being exceeded temporarily. To date, no clear universal limit is known that makes it possible to determine when the crystallization occurs, which leads to uncertainty and risks of damage. However, the person skilled in the art, who currently favors the gaseous state on input to the first compressor, uses this type of compounds despite this drawback.
In some embodiments, the third exchange body is positioned upstream from the inlet for the natural gas in the first exchange body, the device comprising, downstream from the outlet for liquefied natural gas from the second exchange body:

- a fourth body for exchanging heat between the liquefied natural gas and the nitrogen;
- a means for compressing the vaporized nitrogen;
- a means for cooling the compressed nitrogen; and
- a line for transporting the cooled nitrogen to the fourth exchange body.

These embodiments make it possible to obtain the combination of a pre-cooling of the natural gas and a subsequent cooling of the liquefied natural gas.
In some embodiments, the device that is the subject of the present invention comprises, between an outlet for the second compound of the means for cooling the second compound and the third exchange body, a circuit for cooling the second compound by the heavy fraction of the first mixture inside the first exchange body.
These embodiments make it possible to cool the second compound in the cascade of exchangers formed of the first and second exchangers.
In some embodiments, one portion of the cooling circuit is configured to cool the second compound by exchanging heat with the light fraction of the first mixture in the second exchange body.
These embodiments make it possible to cool the second compound in the cascade of exchangers formed of the first and second exchangers.
In some embodiments, the first exchange body and/or the second exchange body is a coil exchanger.
In some embodiments, the means for cooling the second compound is an exchanger of heat between the second compound and water.
According to a second aspect, the present invention relates to a ship, which comprises a device for liquefying a natural gas that is the subject of the present invention.
According to a third aspect, the present invention relates to a method for liquefying a natural gas, comprising:

- a step of compressing a first vaporized coolant chemical mixture,
- a step of fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first step of exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second step of exchanging heat between the light fraction of the first mixture and the natural gas cooled during the first exchange step in order to liquefy the natural gas, and
- a step of returning the first coolant mixture vaporized in the heat exchange bodies to the compressor step,
  which comprises:
- before an input of natural gas of the first exchange step or after an output of liquefied natural gas from the second exchange step, a third step of exchanging heat between the natural gas and a second coolant chemical compound, and
- a step of compressing the second vaporized compound.

As the particular aims, advantages and features of the ship and the method that are the subjects of the present invention are similar to those of the device that is the subject of the present invention, they are not repeated here.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages, aims and particular features of the invention will become apparent from the non-limiting description that follows of at least one particular embodiment of the device, ship and method that are the subjects of the present invention, with reference to drawings included in an appendix, wherein:

FIG. 1 represents, schematically, a first particular embodiment of the device that is the subject of the present invention;

FIG. 2 represents, schematically, a particular embodiment of the ship that is the subject of the present invention;

FIG. 3 represents, schematically and in the form of a logical diagram, a first particular series of steps of the method that is the subject of the present invention;

FIG. 4 represents, schematically, a second particular embodiment of the device that is the subject of the present invention;

FIG. 5 represents, schematically and in the form of a logical diagram, a second particular series of steps of the method that is the subject of the present invention;

FIG. 6 represents, schematically, a third particular embodiment of the device that is the subject of the present invention;

FIG. 7 represents, schematically, a particular embodiment of the compressors of the device that is the subject of the present invention; and

FIG. 8 represents, schematically and in the form of a logical diagram, a third particular series of steps of the method that is the subject of the present invention.

DESCRIPTION OF EXAMPLES OF REALIZATION OF THE INVENTION

The present description is given in a non-limiting way, each characteristic of an embodiment being able to be combined with any other characteristic of any other embodiment in an advantageous way.
It is now noted that the figures are not to scale.
FIG. 1, which is not to scale, shows a schematic view of an embodiment of the device 100 that is the subject of the present invention. This device 100 for liquefying a natural gas, comprising:

- a compressor 105 for a first vaporized coolant chemical mixture,
- a means 110 for fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first body 115 for exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second body 120 for exchanging heat between the light fraction of the first mixture and the natural gas cooled in the first exchange body in order to liquefy the natural gas, and
- a conduit 125 for returning the first coolant mixture vaporized in the heat exchange bodies to the compressor 105,
  which comprises:
- upstream from an inlet 116 for the natural gas in the first exchange body 115 or downstream from an outlet 121 of liquefied natural gas from the second exchange body 120, a third body, 130 or 135, for exchanging heat between the natural gas and a second coolant chemical compound, and
- a means, 140 or 145, for compressing the second vaporized compound.

The compressor 105 is, for example, a centrifugal compressor fitted with a wheel rotating around a shaft driven by a turbine or by an electric motor. This rotating wheel allows the kinetic energy contained in the gas to be transformed into potential energy in order to increase the pressure of said gas. In order to increase the compression carried out, the number of wheels is increased in order to achieve a defined discharge pressure.
The pressure of the compressor 105 on input is, for example, a minimum of around 2 bars absolute. The compression ratio produced in the compressor 105 is, for example, between 2 and 6.
This compressor 105 is, for example, configured to compress a first coolant mixture that comprises nitrogen and methane and at least one compound amongst:

- ethylene;
- ethane;
- propane; and/or
- butane.

The composition of the first compound is adjusted as a function of the composition of the natural gas to be liquefied in the device. This adjustment is carried out as a function of the vapor characteristics curve, i.e. the pressure/temperature balance, of the composition of the gas along the exchange line formed of the first exchange body 112 and the second exchange body 120.
The use of propane is aimed at balancing out volatility differences between the heavy compounds and light compounds of the first mixture.
This compressor 105 comprises an inlet (unnumbered) for vaporized coolant mixture and an outlet (unnumbered) for compressed coolant mixture.
The compressed coolant mixture is preferably cooled in a fifth heat exchanger 106. This heat exchanger 106 is, for example, a tubular exchanger in which the cold source is air or water. The colder the source, the more efficient the method. Preferably, the maximum cooling temperature is equal to the temperature of the air or water plus fifteen degrees Celsius.
The coolant mixture, preferably cooled in the fifth exchanger 106, is supplied to the fractionating means 110. This fractionating means 110 is, for example, a fractionating column.
The flow input to the fractionating column is diphasic, one portion being gaseous and one portion being liquid. The gaseous fraction flows in the column to emerge from the top and the liquid fraction from the bottom.
This fractionating means 110 comprises:

- an inlet (unnumbered) for compressed coolant mixture;
- an outlet (unnumbered) for a light fraction of the coolant mixture positioned in a bottom portion of the fractionating means 110; and
- an outlet (unnumbered) for a heavy fraction of the coolant mixture in a top portion, relative to the outlet for the light fraction, of the fractionating means 110.

Preferably, the light fraction leaving the fractionating means 110 enters the first exchange body 115 and is cooled by the heavy fraction traversing the first exchange body 115. Depending on the operating conditions, this light fraction can also act as cold source in the heat exchange occurring with the natural gas entering by the inlet 116 of the first exchange body 115.
In some preferred variants, the fractionating means 110 also comprises an inlet for the reflux of a portion of the light fraction and this portion of the light fraction is collected, for example, in a reflux drum 111.
The fractionating means 110 therefore preferably comprises packing, making it possible to improve the mass transfer between the gaseous flow and the liquid fraction coming from the reflux drum 111, which absorbs the heaviest compounds of the gaseous fraction making it possible to obtain a flow rich in nitrogen and methane at the head.
The fractionating means 110 is preferably fitted with meshing to limit the carry-over of droplets in the gaseous fraction.
This reflux drum 111 is connected to the outlet for the light fraction of the fractionating means 110, with or without intermediate exchange in the first body 115, and operates in a similar way in separating the light fraction from heavy fraction residues transported unexpectedly by the light fraction away from the fractionating means 110.
The drum 111 is preferably fitted with meshing to limit the carry-over of droplets in the gaseous fraction.
The light fraction leaving the fractionating means 110, or the reflux drum 111 when such a drum 111 is present, is preferably compressed by a second compressor 112.
The second compressor 112 is, for example, a centrifugal compressor. This centrifugal compressor is preferably actuated by the turbine utilized at the location of the compressor 105 when this compressor 105 is a centrifugal compressor.
The pressure on output from the second compressor 112 is, for example, around 40 bars absolute and the compression ratio is preferably between 2 and 4.
The light fraction, with or without compression in the second compressor 112, is preferably cooled in a sixth heat exchanger 113.
This heat exchanger 113 is, for example, a tubular exchanger in which the cold source is air or water. The colder the source, the more efficient the cooling. Preferably, the maximum cooling temperature is equal to the temperature of the air or water plus fifteen degrees Celsius. The resulting flow contributes the cold or negative heat necessary for cooling the natural gas.
The light fraction, compressed or not in the second compressor 112, cooled or not in the sixth exchanger 113, is transmitted to the first heat exchanger 115.
The first heat exchanger 115 is, for example, a coil exchanger in which the light fraction acts as cold source and the natural gas acts as hot source. Preferably, the first exchanger 115 and the second exchanger 120 are formed from a single coil exchanger.
The natural gas enters the first exchanger 115 by the inlet 116.
The light fraction vaporized during the exchange with the natural gas in the first exchange body 115 is preferably directed towards a drum 114 configured to separate the light fraction into two portions, one being heavier than the other.
The device 100 preferably comprises a valve 136 upstream of the drum 114. This valve creates, for example, an expansion of the gaseous portion of the first mixture of around 20 to 25 bars.
The two portions of the light fraction are transmitted to the second exchange body 120, the light fraction acting as cold source in the heat exchange carried out with the natural gas cooled beforehand in the first exchange body 115.
When the device 100 utilizes a drum 114, the heavy portion of the light fraction, after traversing the second exchange body 120, is expanded in an expander 118, then is transmitted to the compressor 105 via the return conduit 125.
This expander 118 is positioned in place of a valve 123 or in parallel to this valve 123.
In some variants, between the expander 118 and the compressor 105, the heavy portion of the light fraction, compressed, is reinjected into the second exchange body 120.
The light portion of the light fraction is transmitted to the compressor 105 via the return conduit 125.
In some variants, the light portion of the light fraction, upon leaving the second exchange body 120, is expanded in a valve 122, and is reinjected into this second exchange body 120 before being directed to the compressor 105.
The valve 122 creates an expansion to reach a pressure of about 4 to 5 bars as a function of the pressure drop of the downstream circuit, for example.
The heavy fraction of the coolant mixture leaving the fractionating means 110 is transmitted to the first exchange body 115 and acts as cold source in the exchange occurring with the natural gas.
In some variants, the device 100 comprises an expander 127 in parallel to the valve 122.
Preferably, the device 100 comprises the expander 127 and does not comprise a valve 122.
In some variants, the heavy fraction leaves the first exchange body 115 and is reinjected into this first exchange body 115, after being expanded in a regulator 119, before being directed to the compressor 105.
The regulator 119 creates an expansion to reach a pressure of about 4 to 5 bars as a function of the pressure drop of the first exchange body 115, for example.
In some variants, the return conduit 125 comprises a drum 126 between the first exchange body 115 and the compressor 105.
This drum 126 makes it possible to ensure that, on input to the first compressor 105, the first coolant mixture is exclusively gaseous.
The drum 126 is preferably fitted with meshing to limit the carry-over of droplets in the gaseous fraction.
Preferably, the device 100 comprises a conduit connecting one portion of the drum 126, intended to receive the liquid portion of the first mixture, to the fractionating means 110. Preferably, this conduit is fitted with a pump. Preferably, this pump is actuated as a function of a level of liquid captured, by a sensor, in the portion of the drum 126 intended to receive the liquid portion of the first mixture.
In this way, as can be understood, the natural gas is liquefied thanks to two successive cooling steps. The first step takes place in the first exchange body 115 and the second step takes place in the second exchange body 120.
The natural gas circulates, in the first body 115 and in the second body 120, preferably in counter-current to the first coolant mixture.
The cooled natural gas preferably leaves the first body 115 at a temperature of about −30° C. This cooled natural gas is then preferably directed to a fractionation section (not shown) to separate any condensates from the gaseous fraction. The gaseous fraction is transmitted to the second body 120 to be liquefied.
In addition to these two steps, the present invention proposes adding a third cooling step positioned either before or after the first two steps.
In the first case, a third exchange body 130 is positioned upstream from the inlet 116 for the natural gas in the first exchange body 115. This third exchange body 130 is, for example, a tubular exchanger using, as cold source, a second coolant compound, and as hot source the natural gas entering the device 100 so as to be liquefied.
The second chemical compound is, for example, a pure substance composed of nitrogen, propane and/or ammonia or a mixture of nitrogen and propane.
Preferably, when ammonia is used, this ammonia is used alone.
In the second case, a third exchange body 135 is positioned downstream from the outlet 121 for liquefied natural gas from the second exchange body 120. This third exchange body 135 is, for example, a tubular exchanger using, as cold source, a second coolant compound, and as hot source the liquefied natural gas exiting from the device 100 to be stored or used. The natural gas liquefied in this way can be expanded to atmospheric pressure by a regulator (not shown) before storage. The evaporation gas, called “BOG” (for “Boil-off gas”), collected in the storage of the liquefied natural gas can be reinjected into the device 100 at the location of the gaseous fraction leaving the fractionation section between the first exchange body 115 and the second exchange body 120.
The second coolant compound is here, for example, liquid nitrogen.
Downstream from this third body, 130 or 135, the device 100 comprises a means, 140 or 145, for compressing the second compound.
This compressor, 140 or 145, is for example a centrifugal compressor.
In some preferred embodiments, the device 100 comprises both the upstream cooling step and the downstream cooling step.
In these embodiments, the third exchange body 130 is designated the exchange body positioned upstream from the first exchange body 115, and the fourth exchange body 135 is designated the exchange body positioned downstream from the second exchange body 120. The second coolant compound is designated the coolant mixture utilized in the third body 130, and the third coolant mixture is designated the coolant mixture utilized in the fourth exchange body.
In some embodiments, the device 100 comprises:

- a means 150 for cooling the second compound compressed in the compression means 140; and
- a conduit 155 for transferring the second compressed compound to the third exchange body 130.

The cooling means 150 is, for example, an exchanger of heat between the second compound vaporized during the heat exchange with the natural gas in the third exchange body 130 and the air or water.
In some embodiments, such as that shown in FIG. 1, the device 100 comprises, between an outlet 131 for the second compound of the means 150 for cooling the second compound and the third exchange body 130, a circuit 170 for cooling the second compound with the heavy fraction of the first mixture inside the first exchange body 115.
This cooling circuit 170 is achieved, for example, by inputting the second cooled compound into the first exchange body 115, the second cooled compound acting as hot source relative to the heavy fraction and any light fraction traversing this first exchange body 115. This second vaporized compound can simultaneously act as cold source relative to the natural gas input into the first body 115 by the inlet 116 for gas natural.
In some variants, the second vaporized compound leaves the first body 115, is expanded in a regulator 124 then reinjected into the first body 115 or into the second body 120.
The second compound is expanded, for example, to a pressure between 3 and 4 bars as a function of the pressure drop in the upstream conduits.
The purpose of this cooling circuit 170 is to facilitate the cooling occurring in the cooling means 150.
In some embodiments, such as that shown in FIG. 1, one portion of the cooling circuit 170 is configured to cool the second compound by exchanging heat with the light fraction of the first mixture in the second exchange body 120.
In these embodiments, the second compound, cooled by heat exchange in the first exchange body 115, is injected into the second exchange body 120. The second vaporized compound then acts as hot source relative to the light fraction of the first coolant mixture traversing this second exchange body 120. At the same time, this second vaporized compound can act as cold source relative to the natural gas input into the second exchange body 120.
In some embodiments, the device 100 comprises:

- a means 160 for cooling the compressed nitrogen; and
- a line 165 for transporting the cooled nitrogen to the fourth exchange body 135.

The cooling means 160 is, for example, an exchanger of heat between the compressed third mixture and the air or water.
The natural gas can undergo pre-treatment prior to the third exchange body 130.
The compressors and compression means, 105, 112 and 140, utilized in this embodiment can be replaced by the compressors, 605, 610 and 620, described with reference to FIG. 6 and whose functions are similar.
FIG. 2, which is not to scale, shows a schematic view of an embodiment of the ship 200 that is the subject of the present invention. This ship 200 comprises:

- a device 100 for liquefying a natural gas as described with regard to FIG. 1;
- a device 400 for liquefying a natural gas as described with regard to FIG. 4; or
- a device 600 for liquefying a natural gas as described with regard to FIG. 6.

FIG. 3 shows, schematically and in the form of a logical diagram, a particular series of steps of the method 300 that is the subject of the present invention. This method 300 for liquefying a natural gas, comprising:

- a step 305 of compressing a first vaporized coolant chemical mixture,
- a step 310 of fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first step 315 of exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second step 320 of exchanging heat between the light fraction of the first mixture and the natural gas cooled during the first exchange step in order to liquefy the natural gas, and
- a step 325 of returning the first coolant mixture vaporized in the heat exchange bodies to the compressor step,
  which comprises:
- before an input of the natural gas of the first exchange step or after an output of liquefied natural gas from the second exchange step, a third step 330 of exchanging heat between the natural gas and a second coolant chemical compound, and
- a step 335 of compressing the second vaporized compound.

This method 300 is performed, for example, by utilizing the device 100 as described with regard to FIG. 1. As can be understood, all the variants, all the examples and all the embodiments of the device 100 can also be transposed as steps within method 300.
FIG. 4, which is not to scale, shows a schematic view of an embodiment of the device 400 that is the subject of the present invention. This device 400 for liquefying a natural gas, comprising:

- a compressor 105 for a first vaporized coolant chemical mixture,
- a means 110 for fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first body 115 for exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second body 120 for exchanging heat between the light fraction of the first mixture and the natural gas cooled in the first exchange body in order to liquefy the natural gas, and
- a conduit 125 for returning the first coolant mixture vaporized in the heat exchange bodies to the compressor,
  comprises:
- a regulator 405 of the liquefied natural gas,
- a collector 410 of evaporation gas produced during the expansion of the gas in the regulator 405, and
- a conduit 415 for injecting the evaporation gas on input to the second exchange body.

In this FIG. 4, the various means relative to the first coolant mixture are identical to the means described with reference to FIG. 1 or 6, including the particular variants and embodiments described with reference to these FIGS. 1 and 6. These means are:

- the compressor 105,
- the exchanger 106,
- the fractionating means 110,
- the reflux drum 111,
- the compressor 112,
- the exchanger 113,
- the drum 114,
- the first exchange body 115,
- the inlet 116 for natural gas,
- the expander 118,
- the regulator 119,
- the second exchange body 120,
- the outlet 121 for liquefied natural gas from the second exchange body 120,
- the regulator 122,
- the expander 126,
- the return conduit 125,
- the drum 126, and
- the valve 136.

In this way, as can be understood, the natural gas is liquefied thanks to two successive cooling steps. The first step takes place in the first exchange body 115 and the second step takes place in the second exchange body 120.
Between these two steps, i.e. between an outlet (unnumbered) for cooled natural gas from the first body 115 and an inlet (unnumbered) for cooled natural gas into the second body 120, the device 400 preferably comprises a fractionation section configured for removing condensates from the gas flow.
The liquefied natural gas leaving the second body 120 by the outlet 121 traverses a regulator 405 configured to expand the liquefied natural gas to atmospheric pressure.
The regulator 405 is, for example, a valve utilizing the Joule-Thomson effect.
This expansion leads to the appearance of evaporation gas, aka BOG.
The BOG generated in this way is collected in a collector 410 and injected, via a conduit 415, on input to the second exchange body 120. This injection can take place upstream from, in or downstream from the fractionation section, if such a section is present.
The collector is, for example, a gas/liquid separator drum 410 fitted with meshing to limit the carry-over of droplets in the gaseous fraction.
Preferably, the conduit 415 is equipped with a compressor 416 compressing the gaseous fraction leaving the collector 410.
In some embodiments, such as that shown in FIG. 4, a third exchange body 420 is positioned upstream from the inlet 116 for the natural gas in the first exchange body 115. This third exchange body 420 is, for example, a tubular exchanger using, as cold source, a second coolant compound, and as hot source the natural gas entering the device 400 so as to be liquefied.
The device 400 therefore comprises a compressor 425 of the second vaporized compound downstream from the third exchange body 420. This compressor 425 is, for example, a centrifugal compressor.
The second chemical compound is, for example, a pure substance composed of nitrogen, propane and/or ammonia or a mixture of nitrogen and propane.
In some embodiments, the device 400 comprises:

- a means 430 for cooling the second compressed compound; and
- a conduit 435 for transferring the second cooled compound to the third exchange body 420.

The cooling means 430 is, for example, an exchanger of heat between the second compressed compound and the water or glycol water.
In some embodiments, such as that shown in FIG. 4, the device 400 comprises, between an outlet 421 for the second compound of the means 430 for cooling the second compound and the third exchange body 420, a circuit 440 for cooling the second compound with the heavy fraction of the first mixture inside the first exchange body 115.
This cooling circuit 440 is achieved, for example, by inputting the second cooled compound into the first exchange body 115, the second cooled compound acting as hot source relative to the heavy fraction and any light fraction traversing this first exchange body 115. At the same time, this second cooled compound can act as cold source relative to the natural gas entered into the first body 115 by the inlet 116 for gas natural.
In some variants, the second vaporized compound leaves the first body 115, is expanded in a regulator 424 then reinjected into the first body 115 or into the second body 120.
The second compound is expanded, for example, to a pressure of 3 to 4 bars on output from the regulator 424.
The purpose of this cooling circuit 440 is to facilitate the cooling that occurs in the cooling means 430.
This circuit 440 can also comprise a second portion, in the second exchange body 120, as described with regard to FIG. 1.
In some particular embodiments, the first cooling mixture comprises nitrogen and methane and at least one compound amongst:

- ethylene;
- ethane;
- propane; and/or
- butane.

FIG. 5 shows, schematically, a particular embodiment of the method 500 that is the subject of the present invention. This method 500 for liquefying a natural gas, comprising:

- a step 505 of compressing a first vaporized coolant chemical mixture,
- a step 510 of fractionating the compressed mixture into a heavy fraction and a light fraction,
- a first step 515 of exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second step 520 of exchanging heat between the light fraction of the first mixture and the natural gas cooled during the first exchange step in order to liquefy the natural gas, and
- a step 525 of returning the first coolant mixture vaporized in the heat exchange bodies to the compressor step,
  comprises:
- a step 530 of expanding the liquefied natural gas;
- a step 535 of collecting evaporation gas produced during the expansion step; and
- a step 540 of injecting the evaporation gas on input to the second exchange body.

This method 500 is performed, for example, by utilizing the device 400 as described with regard to FIG. 4. As can be understood, all the variants, all the examples and all the embodiments of the device 400 can also be transposed as steps within method 500.
FIGS. 6 and 7 show, schematically, a particular embodiment of the device 600 that is the subject of the present invention. This device 600 for liquefying a natural gas, comprising:
a first centrifugal compressor 605 for a first vaporized coolant chemical mixture,

- a means 110 for fractionating the compressed mixture into a heavy fraction and a light fraction,
- a second centrifugal compressor 610 of the light fraction,
- a first body 115 for exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second body 120 for exchanging heat between the compressed light fraction of the first mixture and the natural gas cooled in the first exchange body in order to liquefy the natural gas, and
- a conduit 125 for returning the first coolant mixture vaporized in the heat exchange bodies to the first compressor,
  comprises:
- upstream from an inlet 116 for the natural gas in the first exchange body, a third body 420 for exchanging heat between the natural gas and a second coolant chemical compound;
- a third centrifugal compressor 620 for compressing the second vaporized compound, the first and third centrifugal compressor being actuated by a common single turbine 630;
- a casing 635 common to the first compressor and the third compressor;
- a means 430 for cooling the second compressed compound; and
- a conduit 435 for transferring the second cooled compound to the third exchange body 420.

The term “casing” refers to a housing that comprises at least one compressor. Each compressor comprises one or more wheels.
In this FIG. 6:

- the exchanger 106,
- the fractionating means 110,
- the reflux drum 111,
- the exchanger 113,
- the first exchange body 115,
- the inlet 116 for natural gas,
- the regulator 119,
- the second exchange body 120,
- the outlet 121 for liquefied natural gas from the second exchange body 120,
- the regulator 122,
- the expander 127,
- the return conduit 125,
- the drum 126,
- the valve 136,
- the third exchange body 420,
- the transfer conduit 435,
- the fourth exchange body 430,
- the outlet 421 for the second coolant compound,
- the cooling circuit 400,
- the regulator 424,
- the regulator 405,
- the collector 410, and
- the injection conduit 415,
  are identical to the corresponding means described with reference to FIG. 1 or 4, including the particular variants and embodiments described with reference to these FIGS. 1 and 4.

The third compressor 620 corresponds to the third compressor 140 as described with reference to FIG. 1. However, this third compressor 620 is actuated by the utilization of a single turbine shared with the turbine actuating the first compressor 605. The first compressor corresponds to the first compressor 105 as described with reference to FIG. 1.
The fourth compressor 615 is configured to increase the pressure of the light portion of the light fraction of the first coolant mixture. This fourth compressor shares a common single turbine with the second compressor 610, this second compressor 610 corresponding to the second compressor 112 as described with reference to FIG. 1.
In some preferred embodiments, such as that shown in FIGS. 6 and 7, the device 600 that is the subject of the present invention comprises:

- between the second compressor 610 and the second exchange body 120, a fourth centrifugal compressor 615 for the light fraction of the first mixture, the second and fourth centrifugal compressor being actuated by a common single turbine 640; and
- a casing 645 common to the second compressor and the fourth compressor.

In some preferred embodiments, such as that shown in FIG. 6, the device 600 that is the subject of the present invention comprises:

- a separator 650 of a gas fraction and a liquid fraction of the compressed light phase, the fourth compressor 615 compressing the separated gas fraction;
- a regulator 625 for the liquid fraction of the light phase heated in the second exchange body;
- the turbine 640 of the fourth compressor being actuated by the expansion energy.

The separator 650 is, for example, similar to the reflux drum 114 as described with regard to FIG. 1. The regulator 625 is, for example, similar to the expander 118 as described with regard to FIG. 1.
In some preferred embodiments, such as that shown in FIG. 6, the second chemical compound comprises nitrogen, propane and/or ammonia.
In some preferred embodiments, such as that shown in FIG. 6, the first cooling mixture comprises nitrogen and methane and at least one compound amongst:

- ethylene;
- ethane;
- propane; and/or
- butane.

In some preferred embodiments, such as that shown in FIG. 6, the device 600 comprises:

- a regulator 405 of the liquefied natural gas,
- a collector 410 of evaporation gas produced during the expansion of the gas in the regulator, and
- a conduit 415 for injecting the evaporation gas on input to the second exchange body.

In some preferred embodiments, such as that shown in FIG. 6, the device 600 comprises, between an outlet 421 for the second compound of the cooling means 430 and the third exchange body 420, a circuit 440 for cooling the second compound by the heavy fraction of the first mixture inside the first exchange body 115.
In some preferred embodiments, such as that shown in FIG. 6, the first exchange body 115 and/or the second exchange body 120 is a coil exchanger.
In some preferred embodiments, such as that shown in FIG. 6, the means 430 for cooling the second compound is an exchanger of heat between the second compound and water.
FIG. 8 shows, schematically, a particular embodiment of the method 700 that is the subject of the present invention. This method 700 for liquefying a natural gas, comprising:

- a step 705 of centrifugal compression of a first vaporized coolant chemical mixture,
- a step 710 of fractionating the compressed mixture into a heavy fraction and a light fraction,
- a second step 715 of centrifugal compression of the light fraction,
- a first step 720 of exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,
- a second step 725 of exchanging heat between the light fraction of the first mixture and the cooled natural gas in the first exchange body in order to liquefy the natural gas, and
- a step 730 of returning the first coolant mixture vaporized during the heat exchange steps to the first compression step,
  comprises:
- upstream from a step 735 of inputting the natural gas into the first exchange step, a third step 740 of exchanging heat between the natural gas and a second coolant chemical compound;
- a third step 745 of centrifugal compression of the second vaporized compound, the first and third centrifugal compression step being actuated by a common single turbine;
- the first and third compression step being carried out in a common casing;
- a step 750 of cooling the second compound compressed during the third exchange step; and
- a step 755 of transferring the second cooled compound to the third exchange step.

This method 700 is performed, for example, by utilizing the device 600 as described with regard to FIGS. 6 and 7. As can be understood, all the variants, all the examples and all the embodiments of the device 600 can also be transposed as steps within method 700.

Claims

1. Device (100) for liquefying a natural gas, comprising:

a compressor (105) for a first vaporized coolant chemical mixture,

a means (110) for fractionating the compressed mixture into a heavy fraction and a light fraction,

a first heat exchange body (115) for heat exchange between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,

a second heat exchange body (120) for heat exchange between the light fraction of the first mixture and the cooled natural gas in the first exchange body in order to liquefy the natural gas, and

a return conduit (125) for return of the first vaporized coolant mixture in the heat exchange body to the compressor (105),

characterized in that it comprises:

upstream from an inlet (116) for the natural gas in the first exchange body (115) or downstream from an outlet (121) of liquefied natural gas from the second exchange body (120), a third heat exchange body (130, 135) for heat exchange between the natural gas and a second coolant chemical compound, and

a means (140, 145) for compressing the second vaporized compound.

2. Device (100) according to claim 1, wherein the third exchange body (130) is positioned upstream from the inlet (116) for the natural gas in the first exchange body (115), the device (100) comprising:

a means (150) for cooling the second compressed compound; and

a conduit (155) for transferring the second cooled compound to the third exchange body (130).

3. Device (100) according to claim 2, wherein the second compound is a pure substance composed of nitrogen, propane and/or ammonia or a mixture of nitrogen and propane.

4. Device (100) according to one of claims 1 to 3, wherein the first cooling mixture comprises nitrogen and methane and at least one compound amongst:

ethylene;

ethane;

propane; and/or

butane.

5. Device (100) according to one of claims 1 to 4, wherein the third exchange body (130) is positioned upstream from the inlet (116) for the natural gas in the first exchange body (115), the device (100) comprising, downstream from the outlet (121) for liquefied natural gas from the second exchange body (120):

a fourth body (135) for exchanging heat between the liquefied natural gas and the nitrogen;

a means (145) for compressing the vaporized nitrogen;

a means (160) for cooling the compressed nitrogen; and

a conduit (165) for transporting the cooled nitrogen to the fourth exchange body.

6. Device (100) according to claim 5, wherein the means (150) for cooling the second compound is an exchanger of heat between the second compound and water.

7. Device (100) according to one of claims 1 to 6, which comprises, between an outlet (131) for the second compound of the means (150) for cooling the second compound and the third exchange body (130), a circuit (170) for cooling the second compound with the heavy fraction of the first mixture inside the first exchange body (115).

8. Device (100) according to claim 7, wherein one portion of the cooling circuit (170) is configured to cool the second compound by exchanging heat with the light fraction of the first mixture in the second exchange body (120).

9. Device (100) according to one of claims 1 to 8, wherein the first exchange body (115) and/or the second exchange body (120) is a coil exchanger.

10. Ship (200), characterized in that it comprises a device (100) for liquefying a natural gas according to one of claims 1 to 9.

11. Method (300) for liquefying a natural gas, comprising:

a step (305) of compressing a first vaporized coolant chemical mixture,

a step (310) of fractionating the compressed mixture into a heavy fraction and a light fraction,

a first step (315) of exchanging heat between the heavy fraction of the first mixture and the natural gas in order to cool at least the natural gas,

a second step (320) of exchanging heat between the light fraction of the first mixture and the natural gas cooled during the first exchange step in order to liquefy the natural gas, and

a step (325) of returning the first coolant mixture vaporized in the heat exchange bodies to the compressor step,

characterized in that it comprises:

before an input of the natural gas of the first exchange step or after an output of liquefied natural gas from the second exchange step, a third step (330) of exchanging heat between the natural gas and a second coolant chemical compound, and

a step (335) of compressing the second vaporized compound.