METHOD AND INSTALLATION FOR THE PRODUCTION OF A LIQUEFIED CRYOGENIC FLUID

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French patent application no. FR2410657, filed Oct. 3, 2024, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a method for producing a liquefied cryogenic fluid and to an installation for producing a liquefied cryogenic fluid.

Eliminating heavy hydrocarbons, such as hydrocarbons containing 6 or more carbon atoms, from a gaseous stream of gas that is to be cooled is a commonplace operation performed notably in natural gas liquefaction plants. Such elimination is necessary at least to prevent these hydrocarbons from freezing in the heat exchanger in which the gas that is to be cooled is liquefied, or for recovering these hydrocarbons as by-product.

One known method consists in performing an expansion operation upstream of the liquefier. Such an expansion operation is disadvantageous because it causes an excessive reduction in the pressure of the gaseous stream of gas that is to be cooled. Moreover, such a method does not generally achieve sufficient elimination of hydrocarbons containing 6 or more carbon atoms.

Another known solution is the use of a distillation column. Such a solution is relatively complex and better suited to large-sized installations.

SUMMARY

The present invention seeks to effectively overcome these disadvantages by proposing a method for producing a liquefied cryogenic fluid such as liquefied natural gas from a gaseous stream of gas that is to be cooled containing at least methane and heavy hydrocarbons containing 6 or more carbon atoms, comprising the steps of:

• • mixing the gaseous stream of gas that is to be cooled with part of the liquefied cryogenic fluid withdrawn from downstream of a liquefier, to form a two-phase stream;
  • in a tank such as a phase separator, separating the two-phase stream into a gaseous first fraction and a liquid second fraction, the liquid second fraction containing the majority of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled;
  • introducing the gaseous first fraction into the liquefier to produce the liquefied cryogenic fluid.

Such a method enables simple and effective elimination of the heavy hydrocarbons from a gaseous stream of gas that is to be cooled. This makes it possible to avoid such heavy hydrocarbons freezing in the heat exchanger in which the gas that is to be cooled is liquefied.

According to one embodiment, the majority of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled represents at least more than 50%, notably at least 70%, of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled.

According to one embodiment, the gaseous stream of gas that is to be cooled is at ambient temperature, notably at a temperature of between −30° C. and +50° C.

According to one embodiment, the step of mixing the gaseous stream of gas that is to be cooled with the part of the liquefied cryogenic fluid is performed upstream of the tank before the two-phase stream is introduced into the tank, notably by means of a connection in piping for fluidically connecting the gaseous stream of gas that is to be cooled with the downstream side of the liquefier.

According to one embodiment, the step of mixing the gaseous stream of gas that is to be cooled with the part of the liquefied cryogenic fluid is performed by injecting, notably spraying, the part of the liquefied cryogenic fluid into the tank, for example into the upper part thereof, notably in combination with an injection of the gaseous stream of gas that is to be cooled into the tank, for example into the lower part thereof.

According to one embodiment, the method comprises a step of pre-cooling the stream of gas that is to be cooled before it is mixed with the part of the liquefied cryogenic fluid, notably to bring its temperature to between 0° C. and −50° C.

According to one embodiment, the method comprises a step of raising the pressure of the part of the liquefied cryogenic fluid before it is mixed with the gaseous stream of gas that is to be cooled, notably by means of a hydrostatic head and/or of a compression member such as a pump.

According to one embodiment, the hydrostatic head is created between a withdrawal point at which the part of the liquefied cryogenic fluid is withdrawn and a mixing point at which the withdrawn part of the liquefied cryogenic fluid is mixed with the gaseous stream of gas that is to be cooled, with the withdrawal point being at a greater height than the mixing point so that the pressure of the liquid column between the withdrawal point and the mixing point compensates for pressure losses between the mixing point and the withdrawal point, through the tank and the liquefier.

According to one embodiment, the method comprises a step of expanding the gaseous stream of gas that is to be cooled, notably by means of an expansion member comprising, for example, at least a valve and/or an ejector, the expansion member notably being positioned upstream of the tank.

According to one embodiment, the expansion member is positioned in the circuit for the gas that is to be cooled.

According to one embodiment, the ejector is designed to draw up the part of the liquefied cryogenic fluid.

The invention further relates to an installation for producing a liquefied cryogenic fluid such as liquefied natural gas from a gaseous stream of gas that is to be cooled containing at least methane and heavy hydrocarbons containing 6 or more carbon atoms, and comprising:

• • a circuit for gas that is to be cooled and having an upstream end intended to be connected to the gaseous stream of gas that is to be cooled and a downstream end for delivering the liquefied cryogenic fluid;
  • a tank, in the circuit for gas that is to be cooled, the installation being configured so that the tank receives the gaseous stream of gas that is to be cooled and part of the liquefied cryogenic fluid so as to form a two-phase stream and/or the installation being configured so that the tank receives a two-phase stream derived from the mixing-together of the gaseous stream of gas that is to be cooled and the part of the liquefied cryogenic fluid, the tank being configured to allow the two-phase stream to be separated into a gaseous first fraction and a liquid second fraction containing the majority of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled;
  • a liquefier configured to cool the gaseous first fraction and produce the liquefied cryogenic fluid.

According to one embodiment, the installation comprises a compression member, such as a pump, the compression member being notably positioned in a loop of the circuit for gas that is to be cooled, and configured to be able to raise the pressure of the part of the liquefied cryogenic fluid before it is mixed with the gaseous stream of gas that is to be cooled.

According to one embodiment, the installation is configured to create a hydrostatic head between a withdrawal point at which the part of the liquefied cryogenic fluid is withdrawn and a mixing point at which the withdrawn part of the liquefied cryogenic fluid is mixed with the gaseous stream of gas that is to be cooled.

According to one embodiment, the withdrawal point is at a greater height than the mixing point so that the pressure of the liquid column between the withdrawal point and the mixing point compensates for pressure losses between the mixing point and the withdrawal point, through the tank and the liquefier.

According to one embodiment, the installation comprises an expansion member comprising for example at least a valve and/or an ejector, the expansion member being configured to expand the gaseous stream of gas that is to be cooled and being notably positioned upstream of the tank.

According to one embodiment, the expansion member is positioned in the circuit for the gas that is to be cooled.

According to one embodiment, the ejector is designed to draw up the part of the liquefied cryogenic fluid.

According to one embodiment, the installation does not have a distillation column for separating the heavy hydrocarbons containing 6 carbon atoms from the gas that is to be cooled.

The invention may also relate to any alternative device or method comprising any combination of the features given above or below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more clearly from reading the following description and from studying the accompanying figures. These figures are given only by way of illustration and do not in any way limit the invention.

FIG. 1 is a schematic depiction of an installation according to the invention; and

FIG. 2 is a schematic depiction of a detail of a variant of the installation according to FIG. 1.

FIG. 3 is a schematic depiction illustrating the primary steps of the method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is made to FIG. 1 which depicts an installation 1 for producing a liquefied cryogenic fluid 7 such as liquefied natural gas from a gaseous stream of gas that is to be cooled 6 containing at least methane and heavy hydrocarbons containing 6 or more carbon atoms.

The installation 1 comprises a circuit for gas that is to be cooled and having an upstream end intended to be connected to the gaseous stream of gas that is to be cooled 6 and a downstream end for delivering the liquefied cryogenic fluid 7.

The installation 1 also comprises a tank 3 in the circuit for gas that is to be cooled.

The installation 1 further comprises a liquefier 2, notably in the circuit for gas that is to be cooled.

In the example depicted in FIG. 1, the installation 1 is configured so that the tank 3 receives the gaseous stream of gas that is to be cooled 6 and part 11 of the liquefied cryogenic fluid 7, so as to form a two-phase stream 8. In the example depicted in FIG. 1, the two-phase stream 8 is formed inside the tank 3.

In the example depicted in FIG. 2, the installation is configured so that the tank 3 receives a two-phase stream 8 derived from the mixing-together of the gaseous stream of gas that is to be cooled 6 and the part 11 of the liquefied cryogenic fluid 7.

In the examples depicted in FIGS. 1 and 2, the tank 3 is configured to enable the two-phase stream 8 to be separated into a gaseous first fraction 9 and a liquid second fraction 10 containing the majority of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled.

The tank 3 may comprise a phase separator for ensuring effective separation of the two-phase stream 8 into the gaseous first fraction 9 and the liquid second fraction 10.

As shown in FIG. 1 or 2, the liquefier 2 is configured to cool the gaseous first fraction 9 and produce the liquefied cryogenic fluid 7.

The installation 1 comprises a compression member 4, such as a pump, the compression member 4 being notably positioned in a loop of the circuit for gas that is to be cooled, and configured to be able to raise the pressure of the part 11 of the liquefied cryogenic fluid 7 before it is mixed with the gaseous stream of gas that is to be cooled 6.

In one exemplary embodiment, the installation is configured to create a hydrostatic head between a withdrawal point at which the part 11 of the liquefied cryogenic fluid 7 is withdrawn and a mixing point at which the withdrawn part 11 of the liquefied cryogenic fluid 7 is mixed with the gaseous stream of gas that is to be cooled 6.

The withdrawal point is at a greater height than the mixing point so that the pressure of the liquid column between the withdrawal point and the mixing point compensates for pressure losses between the mixing point and the withdrawal point, through the tank 3 and the liquefier 2. Height or elevation is measured with respect to a determined level such as the ground at a determined location.

As depicted in FIG. 1, the mixing point is level with the inlet into the tank 3. As depicted in FIG. 2, the mixing point is level with a connection between the loop of the circuit for gas that is to be cooled and the circuit for gas that is to be cooled, upstream of the tank 3.

The installation 1 comprises an expansion member 5 comprising for example at least a valve and/or an ejector.

The expansion member 5 is positioned in the circuit for gas that is to be cooled, upstream of the tank 3, and is configured to expand the gaseous stream of gas that is to be cooled 6.

In one exemplary embodiment, the expansion member 5 comprises at least an ejector for drawing up the part 11 of the liquefied cryogenic fluid 7. Such an ejector may advantageously replace the compression member 4 which can thus be omitted.

Reference is now made to FIG. 3, which is a flowchart illustrating the primary steps of the method. The method begins at step E1, which involves mixing the gaseous stream of gas to be cooled (6) with a portion (11) of the liquefied cryogenic fluid (7) withdrawn downstream of a liquefier (2) to form a two-phase stream (8). Following the mixing step E1, the process proceeds to step E2, where the two-phase stream (8) is separated in a tank (3) into a gaseous first fraction (9) and a liquid second fraction (10). The liquid second fraction (10) contains the majority of the heavy hydrocarbons. Finally, at step E3, the gaseous first fraction (9) is introduced into the liquefier (2) to produce the liquefied cryogenic fluid (7). The method may also include other steps as described herein, such as pre-cooling, pressure raising, or expansion A method for producing a liquefied cryogenic fluid 7 such as liquefied natural gas from a gaseous stream of gas that is to be cooled 6 containing at least methane and heavy hydrocarbons containing 6 or more carbon atoms, in an installation as described in connection with FIGS. 1 and 2, is described hereinafter.

The method comprises the steps of:

• • mixing the gaseous stream of gas that is to be cooled 6 with part 11 of the liquefied cryogenic fluid 7 withdrawn from downstream of a liquefier 2 to form a two-phase stream 8;
  • in a tank 3 such as a phase separator, separating the two-phase stream 8 into a gaseous first fraction 9 and a liquid second fraction 10, the liquid second fraction 10 containing the majority of the heavy hydrocarbons, containing 6 or more carbon atoms, of the gaseous stream of gas that is to be cooled;
  • introducing the gaseous first fraction 9 into the liquefier 2 to produce the liquefied cryogenic fluid 7.

As depicted in FIG. 1, the step of mixing the gaseous stream of gas that is to be cooled 6 with the part of the liquefied cryogenic fluid is performed by injecting, notably spraying, the part 11 of the liquefied cryogenic fluid 7 into the tank 3, for example into the upper part thereof, notably in combination with an injection of the gaseous stream of gas that is to be cooled 6 into the tank 3, for example into the lower part thereof.

As a variant, as depicted in FIG. 2, the step of mixing the gaseous stream of gas that is to be cooled 6 with the part of the liquefied cryogenic fluid is performed upstream of the tank 3 before the two-phase stream 8 is introduced into the tank 3, notably by means of a connection in piping for fluidically connecting the gaseous stream of gas that is to be cooled with the downstream side of the liquefier 2.

The method comprises a step of pre-cooling the stream of gas that is to be cooled 6 before it is mixed with the part 11 of the liquefied cryogenic fluid 7, notably to bring its temperature to between 0° C. and −50° C.

The method comprises a step of raising the pressure of the part 11 of the liquefied cryogenic fluid 7 before it is mixed with the gaseous stream of gas that is to be cooled 6, notably by means of a hydrostatic head and/or of a compression member 4 such as a pump 4.

The method comprises a step of expanding the gaseous stream of gas that is to be cooled 6, notably by means of an expansion member 5 comprising for example at least a valve and/or an ejector, the expansion member notably being positioned upstream of the tank 3.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.