EP3550238B1 - Method for liquefaction of gaseous methane by vaporisation of nitrogen, installation for the liquefaction of gaseous methane implementing the method - Google Patents

Description

The subject of the present invention is a process for the liquefaction of gaseous methane by vaporization of nitrogen. It also relates to an installation for the liquefaction of gaseous menthane implementing the process.

More specifically, the present invention relates to a process and an installation for the liquefaction of a flow of gaseous methane, containing at least 80% molar methane from a source of methane, to produce a flow of liquid methane, by an open cycle using a flow of liquid nitrogen from a cold source of liquid nitrogen.

The source of methane can be, for example, a natural gas transport network, or a bio-methane production site equipped with methanizers, such as certain purification stations or certain waste landfill centres.

The term bio-methane refers to the methane composing a fraction of the gases produced by a biological process of degradation of organic matter in an anaerobic environment.

The emergence of many methane production sites, as well as their remoteness from the natural gas transport networks, make it necessary to develop liquefaction processes for small gaseous methane flow rates, whose investment costs are low, and which are easy to operate.

The liquefaction of a flow of methane using the enthalpy of vaporization of a flow of liquid nitrogen, that is to say a method of liquefying gaseous methane by vaporization of nitrogen is a well-known method. However, it involves the continuous consumption of liquid nitrogen, which is stored in one or more tanks, which limits the size of these installations to methane flow rates of around 10 tonnes per day.

Indeed, beyond this value, it is necessary to frequently refill the tanks, which increases the operating costs of the installation. For flow rates of methane to be liquefied greater than 10 tons per day, there are installations of the prior art, the investment costs of which are very high due to the presence of rotating machines such as pumps, compressors, or expansion turbines which participate in the circulation of the Refrigerant.

Consequently, a first problem which the invention proposes to solve is that of developing a process and an installation for the liquefaction of methane by vaporization of liquid nitrogen, the investment costs of which are low, and the operation of which is very easy to use. In particular, the objective is to develop an installation which is devoid as much as possible of rotating machines, advantageously which does not contain any.

To solve these problems, the document US 6,598,423 B1 describes a process for the liquefaction of a gas flow by vaporization of liquid nitrogen. The gas flow circulates through a single high-pressure exchanger, in which it is liquefied on contact with a flow of liquid nitrogen, which vaporizes by recovering the calories from the gas flow. This process makes it possible to produce liquefied gas on the one hand, and to fill cylinders with gaseous nitrogen on the other.

However, this unit for liquefying a flow of methane by vaporization of liquid nitrogen presents several difficulties.

Thus, the flow of methane arriving from the gaseous methane source has, in practice, a pressure greater than 1 bar absolute. Under these conditions, its minimum liquefaction temperature is 11 1.5K. However, liquid nitrogen as sold by suppliers is available at a single temperature of 77K, which is lower than the temperature of the triple point of methane, which is 90.7K.

By condensing, within an exchanger, said flow of methane at a temperature greater than 111.5K with a flow of liquid nitrogen having a temperature of 77K coming directly from the source of liquid nitrogen, the methane risks being cooled by below its triple point temperature. As a result, there would be production of solid methane within the exchanger, clogging the pipes of the exchanger and thus causing a stoppage of the liquefaction unit by clogging of the pipes.

On the other hand, under these conditions, the temperature difference between the two flow rates is at least 34.5 K, a value which is all the higher as the pressure of the methane flow rate is high. This significant thermal difference induces strong mechanical stresses on the parts of the exchanger, with a risk of embrittlement of the latter and breakage.

Consequently, a second problem which the invention proposes to solve is that of developing a process and an installation for the liquefaction of methane by vaporization of liquid nitrogen, which does not lead to the obstruction of the pipes and whose difference temperature between the two inlet and outlet terminals of the exchangers is reduced so as to limit the mechanical stresses.

To solve this problem, the document EP 1 030 135 A1 describes a method for cooling a flow of hot fluid. The hot fluid flow loses its calories within an exchanger, which are transferred to an intermediate fluid flow. The intermediate fluid flow forms a closed loop, and is cooled by passing through an exchanger, in which, by losing the calories acquired in contact with the hot fluid flow, the intermediate fluid evaporates a liquid nitrogen flow. The nature of the intermediate fluid flow is chosen so that its vaporization temperature is lower than the cooling or liquefaction temperature of the hot fluid flow to be cooled, and so that its condensation temperature is higher than the evaporation temperature of the 'liquid nitrogen. Therefore, by using an appropriate intermediate fluid, this method thus avoids freezing of the flow of hot fluid, the latter never being in direct contact with the flow of liquid nitrogen.

Nevertheless, this method does not make it possible to solve the problem posed by the high temperature difference at the terminals of the heat exchangers. In addition, the installation requires the presence of a pump making it possible to circulate the flow of hot fluid.

In the document EP 3 026 379 A1 , the gaseous methane flow is passed through a pre-cooler until it reaches its dew point, then through an exchanger in which it will be liquefied. The exchanger and the pre-cooler are supplied by the same open-loop gaseous nitrogen circuit, allowing the gaseous methane flow to be cooled and then liquefied by two gas/gas exchanges. This process therefore offers a solution to avoid the solidification of the stream to be liquefied, by controlling the condensation temperature of said stream by adding liquid nitrogen. However, it has the disadvantage of requiring the use of at least one rotating machine, in this case a cryogenic pump or a cryogenic compressor. This equipment is very expensive, and has reduced maintenance intervals. Thus, they meet neither the requirement of simplicity of operation, nor that of having low investment and operating costs for the installation. On the other hand, the energy transmitted to the rotating machine is entirely transferred to the flow of gaseous nitrogen, which has the effect of reducing the efficiency of the process, because an additional consumption of cold is required to balance this contribution of energy, by decreasing the temperature of said flow. Finally, the use of gaseous nitrogen as refrigerant for the liquefaction of the fluid implies a particularly high exchange surface within the heat exchange elements that are the pre-cooler, the cooler and the exchanger, because the coefficients exchange coefficients in the gaseous phase are lower than the exchange coefficients for fluids in the liquid phase or two-phase. This therefore implies the use of large exchangers, increasing the total cost of the installation.

A method for liquefying methane according to the preamble of claim 1 is known from document DE-A-10 2010 044869 .

The object of the present invention is to overcome the aforementioned drawbacks.

To do this, the applicant has developed a process for liquefying a gas flow that does not require a rotating machine, and comprising two successive exchangers, combined with a phase separator balloon, minimizing the temperature differences at the terminals of these exchangers. , thus reducing the risk of mechanical breakage; and on the other hand avoiding the risk of solidification of the gas flow to be liquefied, by ensuring that the temperature of the nitrogen circulating in the exchangers is always higher than the temperature of the triple point of the gas flow to be liquefied.

• on fournit une source de méthane gazeux, produisant un débit de méthane gazeux à une température T1 et à une pression P1,
• on fournit une source d'azote liquide, produisant un débit d'azote liquide, de pression P2 et dont la température T2 est inférieure à la température du point triple du méthane,
• au sein d'un premier échangeur, on refroidit, à contre-courant, le débit de méthane gazeux de température T1 jusqu'à une température T3 correspondant à celle de son point de rosée par échange de chaleur avec un débit d'azote gazeux de température T4 provenant d'un ballon séparateur de phase,
• on transfère le débit de méthane gazeux de température T3 dans un second échangeur à contre-courant,
• on transfère le débit d'azote liquide de température T2 depuis la source d'azote liquide jusqu'au ballon séparateur de phase,
• on applique dans le ballon séparateur de phases une température T4 supérieure à la température du point triple du méthane et une pression P4 de telle sorte que soient présentes au sein dudit ballon une phase liquide de température T4 produisant un débit d'azote liquide, et une phase vapeur de température T4 produisant un débit d'azote gazeux,
• dans le second échangeur, on liquéfie le débit de méthane gazeux de température T3 en produisant un débit de méthane liquide de température T5, par vaporisation du débit d'azote liquide de température T4 en produisant un débit d'azote vaporisé de température T4,
• on transfère le débit d'azote vaporisé de température T4 dans le ballon séparateur.

More specifically, the subject of the invention is a process for the liquefaction of a gaseous flow, in particular a gaseous methane flow, in which:

• a source of gaseous methane is provided, producing a flow of gaseous methane at a temperature T1 and at a pressure P1,
• a source of liquid nitrogen is supplied, producing a flow of liquid nitrogen, of pressure P2 and whose temperature T2 is lower than the temperature of the triple point of methane,
• within a first exchanger, the flow of methane gas at temperature T1 is cooled, counter-current, to a temperature T3 corresponding to that of its dew point by heat exchange with a flow of nitrogen gas of temperature T4 from a phase separator flask,
• the flow of gaseous methane at temperature T3 is transferred into a second counter-current exchanger,
• the flow of liquid nitrogen at temperature T2 is transferred from the source of liquid nitrogen to the phase separator drum,
• a temperature T4 higher than the temperature of the triple point of methane and a pressure P4 are applied in the phase separator flask such that a liquid phase of temperature T4 is present within said flask producing a flow of liquid nitrogen, and a vapor phase at temperature T4 producing a flow of gaseous nitrogen,
• in the second exchanger, the flow of gaseous methane at temperature T3 is liquefied by producing a flow of liquid methane at temperature T5, by vaporization of the flow of liquid nitrogen at temperature T4 by producing a flow of vaporized nitrogen at temperature T4,
• the flow of vaporized nitrogen at temperature T4 is transferred to the separator flask.

According to the invention, the flow of liquid methane at temperature T5 is transferred and then stored in a storage tank, containing a phase of liquid methane and a phase of gaseous methane; said gaseous methane phase escaping from said storage tank forming a methane leak flow.

• la pression P1 est d'au moins 1 bar absolu, avantageusement égale à 1,5 bar absolu,
• la température T1 est comprise entre 270K et 320K, avantageusement égale à 300K,
• la pression P2 est d'au moins 6 bar absolu, avantageusement égale à 10 bar absolu,
• la température T2 est comprise entre 75K et 85K, avantageusement égale à 77K,
• la température T3 est comprise entre 110K et 130K, avantageusement égale à 116.6K,
• la pression P4 est comprise entre 4 et 6 bars absolus, avantageusement égale à 5 bar absolu,
• la température T4 est comprise entre une valeur strictement supérieure à 90,7 K et 100K, avantageusement égale à 94K,
• la température T5 est comprise entre 110K et 120K, avantageusement égale à 115K.

In a preferred embodiment:

• the pressure P1 is at least 1 bar absolute, advantageously equal to 1.5 bar absolute,
• the temperature T1 is between 270K and 320K, advantageously equal to 300K,
• the pressure P2 is at least 6 bar absolute, advantageously equal to 10 bar absolute,
• the temperature T2 is between 75K and 85K, advantageously equal to 77K,
• the temperature T3 is between 110K and 130K, advantageously equal to 116.6K,
• the pressure P4 is between 4 and 6 bar absolute, advantageously equal to 5 bar absolute,
• the temperature T4 is between a value strictly greater than 90.7 K and 100K, advantageously equal to 94K,
• the temperature T5 is between 110K and 120K, advantageously equal to 115K.

The Applicant has demonstrated that the use of a phase separator drum advantageously made it possible to increase the temperature of the liquid nitrogen flow, from T2 to T4, making it possible to avoid freezing of the liquid methane flow and clogging of the second exchanger and to avoid significant thermal differences at the terminals of the first and second exchangers.

Indeed, the method never involves direct exchange between the flow of liquid nitrogen at the temperature T2 and the flow of gaseous methane. Therefore, the first and second exchangers are subject to less significant thermal gradients, which has the effect of limiting the mechanical stresses to which they are exposed, and of increasing the life of these elements.

Furthermore, the phase separator flask also makes it possible to create a circulation of nitrogen through the various members, of the thermosiphon type, thus avoiding the use of a rotating machine.

As said previously, the pressure P4 and the temperature T4 present in the phase separator flask allow the coexistence of nitrogen within this flask in the form of two phases, the liquid phase and the vapor phase, said flask making it possible to separate by seriousness of these two phases. The liquid phase present in the phase separator flask is at the origin of the flow of liquid nitrogen at temperature T4. This flow exits the separator balloon by gravity.

By contact with the flow of gaseous methane at temperature T3, the flow of liquid nitrogen at temperature T4 receives calories from the flow of gaseous methane, producing a partial vaporization of this flow of liquid nitrogen to form the flow of vaporized nitrogen. By partially changing state, the flow of vaporized nitrogen maintains the temperature of the flow of nitrogen at temperature T4.

The second exchanger and the phase separator tank being in level balance, the flow of vaporized nitrogen will naturally join the phase separator tank, and thus create a thermosiphon-type circulation allowing the mixing of the volume of nitrogen present in the tank. phase separator. This ensures satisfactory mixing of the flow of liquid nitrogen at temperature T2 introduced into the balloon, with the nitrogen in liquid and vapor form already present in said balloon, at temperature T4.

The nitrogen of temperature T2 newly introduced into the ball instantaneously goes to the temperature T4 due to the mixing, and the constant supply of heat by the flow of vaporized nitrogen coming from the second exchanger. There is therefore no longer any risk of sending too cold liquid nitrogen into the second exchanger, thus eliminating the risk of freezing of the gas flow to be liquefied.

Finally, the vapor phase contained in the phase separator drum, fed by the fraction of gaseous nitrogen present in the flow of vaporized nitrogen, is continuously drained in the form of a flow of gaseous nitrogen at temperature T4, in order to to maintain a constant pressure P4 in said balloon. This gaseous nitrogen flow is then directed into the first exchanger, where it captures calories from the gaseous methane flow of temperature T1, so that the latter reaches its dew point of temperature T3.

In a particular embodiment, the temperature T4 is controlled by regulating the pressure P4 prevailing within the phase separator flask. In practice, a PIC-type regulator and a valve limiting the gaseous nitrogen flow are used together.

According to another feature to maintain the level balance between the second exchanger and the phase separator drum, the volume of the liquid nitrogen phase at temperature T4 contained in the phase separator drum is maintained by regulating the nitrogen flow liquid of temperature T2 and pressure P2 entering said balloon.

According to the invention, the flow of liquid methane at temperature T5 is transferred and then stored in a storage tank, containing a phase of liquid methane and a phase of gaseous methane; said gaseous methane phase escaping from said storage tank forming a methane leak flow.

Still according to the invention, the leak rate of methane at temperature T5 is directed into the first exchanger, countercurrent to the flow rate of gaseous methane at temperature T1, and co-current to the flow rate of gaseous nitrogen at temperature T4, thus helping to cool said flow of methane gas.

In the event that the methane to be liquefied is not sufficiently pure, the content of impurities in the flow of gaseous methane between the source of gaseous methane and the first exchanger is reduced.

To do this, the gaseous methane is purified by passing said methane through at least one adsorber, the adsorber preferably being regenerated by means of the gaseous nitrogen flow coming from the first exchanger.

The invention also relates to an installation according to claim 6 implementing the method described above.

The invention and the resulting advantages will clearly emerge from the following example, based on the appended figure.

There figure 1 is a schematic representation of an installation according to the invention.

There is a flow of gaseous methane (1), coming from a source of gaseous methane, at a pressure P1 equal to 1.5 bar absolute, and at a temperature T1 equal to 300K.

There is also a source of liquid nitrogen stored in a reservoir (22) supplying a flow of liquid nitrogen (10, 12) with a pressure P2 equal to 10 bars absolute, and a temperature T2 equal to 77K.

The flow of gaseous methane (1) at temperature T1 is introduced into a first exchanger (2), where it is cooled against the current of a flow of gaseous nitrogen (16) at temperature T4, until it reaches a temperature T3, corresponding to its dew point temperature.

In this example, the temperature T3 is equal to 116.6K, and the temperature T4 is equal to 94K, corresponding to a pressure P4 of 5 bar absolute.

The flow of gaseous methane (3) leaves this first exchanger (2) at its dew temperature T3, and is directed into a second exchanger (4). In this second exchanger (4), the flow of gaseous methane (3) of temperature T3 is mainly liquefied by producing a flow of liquid methane (5) of temperature T5, by vaporization of a flow of liquid nitrogen (14) of temperature T4 coming from a separating drum (13) by producing a flow of vaporized nitrogen (15) of temperature T4.

This flow of liquid methane (5) is then directed and introduced into the storage tank (6). The evaporations of this storage tank (6), otherwise called "Boil Off Gas”, as well as the residual gaseous fraction present in the flow of liquid methane (5), escape from the storage tank (6) forming a leak flow of methane (8).

This methane leak flow (8) is directed and introduced against the current into the exchanger (2), in order to recover the cold enthalpy of this fluid to cool the gaseous methane flow (3), concomitantly with the flow nitrogen gas (16).

The temperature T5 is advantageously 115K.

The flow of liquid nitrogen (10, 12) of temperature T2 and pressure P2 from the source of liquid nitrogen is introduced into the phase separator drum (13) within which the temperature T4 and the pressure P4 prevail. The temperature T4 directly depends on the pressure P4. These pressure and temperature conditions induce the presence of two phases within said separating drum (13): a liquid phase producing the flow of liquid nitrogen (14) at temperature T4, and a vapor phase producing the flow of gaseous nitrogen (16) temperature T4.

The volume of the liquid phase of the separator drum (13) is kept constant by acting on a valve (11) limiting the flow of liquid nitrogen (10) to form the flow of liquid nitrogen (12), according to the instruction sent by an LIC level regulator (20) installed on said separator tank (13).

The flow of liquid nitrogen (12) of temperature T2 mixes in the separating drum (13) with the flow of vaporized nitrogen (15), and by enthalpy balance, the mixing temperature obtained is 94K.

The flow of liquid nitrogen (14) at temperature T4 leaves the separator tank (13) by gravity and is introduced into the second exchanger (4), in which it absorbs heat from the flow of gaseous methane at temperature T3.

In doing so, it partially vaporizes and loses density forming the vaporized nitrogen flow (15), composed of liquid nitrogen and partially vaporized nitrogen, at the same temperature T4, which allows it to escape of the exchanger. The flow of vaporized nitrogen (15) is then introduced into the separator drum (13). The fraction of nitrogen gas contained in the flow of vaporized nitrogen (15) feeds the vapor phase contained in the separator drum (13) while maintaining its temperature T4.

In order to maintain a constant pressure P4 in the separator drum (13), the gaseous nitrogen flow (16) of temperature T4 is evacuated from the separator drum (13), to be introduced into the exchanger (2), where it will exchange sensible heat with the flow of methane gas (1).

The gaseous methane flow (17) leaving the exchanger (2) is used to regulate the pressure within the separator tank (13) by means of a PIC pressure regulator (21) acting on a valve (18) placed on the pipe in which said flow of gaseous methane (17) circulates.

The invention and the advantages resulting therefrom clearly emerge from the foregoing description. We note in particular the absence of rotating machines which makes the installation a low-cost installation. In addition, the presence of two successive exchangers, combined with a phase separator tank, makes it possible to minimize the temperature differences at the terminals of these exchangers, thus reducing the risk of mechanical breakage. On the other hand, there is no longer any risk of solidification of the gas flow to be liquefied, because the temperature of the nitrogen circulating in the exchangers is always higher than the temperature of the triple point of the gas flow to be liquefied. .

Claims (6)

1. A method for liquefying gaseous methane, in which:

   - a source of gaseous methane is provided, producing a flow of gaseous methane 5 at a temperature T1 and at a pressure P1,

   - a source of liquid nitrogen is provided, producing a flow of liquid nitrogen at a pressure P2 and at a temperature T2 which is lower than the temperature of the triple point of methane,

   - in a first exchanger, the flow of gaseous methane 10 at temperature T1 is cooled, as a counter-current, to a temperature T3 corresponding to that of its dew point by the exchange of heat with a flow of gaseous nitrogen at temperature T4 originating from a phase separator drum,

   - the flow of gaseous methane at temperature T3 is transferred into a second counter-current exchanger,

   - the flow of liquid nitrogen at temperature T2 is transferred from the source of liquid nitrogen to the phase separator drum,

   - a temperature T4 which is higher than the temperature of the triple point of methane and a pressure P4 are applied in the phase separator drum in such a manner that a liquid phase at temperature T4 producing a flow of liquid nitrogen and a vapour phase at temperature T4 producing a flow of gaseous nitrogen are present inside said drum,

   - in the second exchanger, the flow of gaseous methane at temperature T3 is liquefied, thereby producing a flow of liquid methane at temperature T5 by vaporization of the flow of liquid nitrogen at temperature T4, thereby producing a flow of vaporized nitrogen at temperature T4,

   - the flow of vaporized nitrogen at temperature T4 is transferred into the separator drum,

   - the flow of liquid methane at temperature T5 is transferred to and then stored in a storage tank containing a liquid methane phase and a gaseous methane phase; said gaseous methane phase escaping from said storage tank, thereby forming a methane leakage flow, characterized in that

   - the methane leakage flow at temperature T5 is directed into the first exchanger as a counter-current to the flow of gaseous methane at temperature T1, and as a co-current with the flow of gaseous nitrogen at temperature T4, thereby contributing to cooling said flow of gaseous methane.
2. The method as claimed in claim 1, characterized in that the temperature T4 is maintained inside the phase separator drum by regulating the pressure P4 inside said drum.
3. The method as claimed in claim 1 or claim 2, characterized in that the volume of the liquid nitrogen phase at temperature T4 is maintained inside the phase separator drum by regulating the flow of liquid nitrogen at temperature T2 and pressure P2 entering said drum.
4. The method as claimed in one of claims 1 to 3, characterized in that the impurities content of the flow of gaseous methane between the source of gaseous methane and the first exchanger is reduced.
5. The method as claimed in one of claims 1 to 4, characterized in that:

   - the pressure P1 is at least 1 bar absolute, advantageously equal to 1.5 bar absolute,

   - the temperature T1 is in the range 270K to 320K, advantageously equal to 300K,

   - the pressure P2 is at least 6 bar absolute, advantageously equal to 10 bar absolute,

   - the temperature T2 is in the range 75K to 85K, advantageously equal to 77K,

   - the temperature T3 is in the range 110K to 130K, advantageously equal to 116.6K,

   - the pressure P4 is in the range 4 to 6 bar absolute, advantageously equal to 5 bar absolute,

   - the temperature T4 is in the range from a value strictly greater than 90.7K to 100K, advantageously equal to 94K,

   - the temperature T5 is in the range 110K to 120K, advantageously equal to 115K.
6. A facility implementing the method as claimed in claims 1 to 5 for the production of liquid methane and comprising:

   - a source of gaseous methane at temperature T1 and pressure P1;

   - a source of liquid nitrogen (22) at a temperature T2 and pressure P2;

   - a phase separator drum (13) connected to the source of liquid nitrogen (22) at temperature T2 and pressure P2;

   - a storage tank (6) connected to the liquid methane outlet of a second exchanger (4) and containing a liquid methane phase and a gaseous methane phase; said gaseous methane phase escaping from said storage tank, thereby forming a methane leakage flow;

   - a first exchanger (2) in which the flow of gaseous methane (1) at temperature T1 and at pressure P1 circulate as a counter-current to a flow of gaseous nitrogen (16) at temperature T4 originating from the separator drum (13) and as a counter-current to the methane leakage flow at temperature T5;

   - the second counter-current exchanger (4) into which the flow of gaseous methane (3) enters at temperature T3 and into which the flow of liquid nitrogen (14) at temperature T4 originating from the separator drum (13) enters, and from which the flow of liquid methane (5) leaves at temperature T5 and the flow of vaporized nitrogen (15) leaves at temperature T4;

   - a valve (18) positioned downstream of the first exchanger (2) in the direction of circulation of the flow of gaseous nitrogen (16, 17) in a manner such as to regulate the pressure P4 inside the phase separator drum (13);

   - a valve (11) positioned downstream of the source of liquid nitrogen (22) in the direction of circulation of the flow of liquid nitrogen (10, 12) at temperature T2, in a manner such as to regulate the volume of the liquid phase inside the phase separator drum (13).

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