EP3116626A1

EP3116626A1 - Device for separating gas components contained in a gas mixture, and use thereof for separating methane and carbon dioxide from a biogas

Info

Publication number: EP3116626A1
Application number: EP15715171.3A
Authority: EP
Inventors: Gilles Hebrard; David BENIZRI; Pierre LABEYRIE; Nicolas Dietrich
Original assignee: Aria-Energie Developpement Environnement; Centre National de la Recherche Scientifique CNRS; Institut National des Sciences Appliquees de Toulouse; Institut National de la Recherche Agronomique INRA
Current assignee: Aria-Energie Developpement Environnement; Centre National de la Recherche Scientifique CNRS; Institut National des Sciences Appliquees de Toulouse; Institut National de la Recherche Agronomique INRA
Priority date: 2014-03-10
Filing date: 2015-03-10
Publication date: 2017-01-18
Also published as: FR3018205A1; FR3018205B1; WO2015135921A1

Abstract

The invention relates to a device for separating a first gas component and a second gas component which are contained in a gas mixture, comprising: a liquid/gas absorption separation column (10), comprising a lower tank (12) and an upper separation area (11); means for placing the gas mixture in contact, with a liquid solution capable of selectively absorbing the second gas component, via counter-flow circulation in the separation area (11); and means (19) for recovering the first gas component, separate from the second gas component. The tank (12) stores a volume (16) of absorbing solution. The transverse cross-sectional surface of the tank (12) of the absorption column (10) is no lower than twice the transverse cross-sectional surface of the separation area (11) of the absorption column (10).

Description

DEVICE FOR SEPARATING GASEOUS CONSTITUENTS FROM GASEOUS MIXTURE, AND USE THEREOF FOR THE SEPARATION OF

METHANE AND CARBON DIOXIDE OF A BIOGAZ

The present invention is in the field of the separation of gaseous constituents from a gaseous mixture comprising them. More particularly, it relates to a device for the separation of a first gaseous component and a second gaseous component contained in a gaseous mixture, as well as the use of such a device for such separation.

An application for which the device according to the invention is particularly advantageous, and which will be described in detail in the following description, is the separation of methane and carbon dioxide contained in a biogas. The device according to the invention however also applies, in a similar manner, to the separation of gaseous constituents contained in any other type of gaseous mixture, for example to the separation of carbon dioxide and other constituents of a gaseous gas. synthesis, a mixture of hydrocarbons, etc. In the current context of energy transition, the development of environmentally friendly techniques to produce energy is of great importance. Anaerobic digestion is one of these techniques. It is a natural process of anaerobic degradation of carbon-source biomass to produce a combustible gas, called biogas, composed mainly of methane (about 54%) and carbon dioxide (about 43%). %), as well as a small percentage of nitrogen and water in the form of steam and, if necessary, small amounts of sulfur gas. In particular, anaerobic digestion systems are installed more and more frequently on farms, for the recovery of organic waste of plant or animal origin that is produced there.

Numerous solutions have been proposed by the prior art for upgrading the raw biogas produced by anaerobic digestion systems, in particular to purify it to obtain a gas with a high methane content. Such purification involves the separation of methane and carbon dioxide contained in the biogas. Among the techniques proposed by the prior art for this purpose, a particular technique consists in washing the biogas under high pressure with a liquid aqueous solution, called an absorbent solution, capable of specifically absorbing the carbon dioxide contained in the biogas. This technique takes advantage of the differences in dissolution properties in water of methane and carbon dioxide, the dissolution constant in water of carbon dioxide being much greater than that of methane. This characteristic is described by thermodynamics using Henry's constant, describing the equilibrium between a liquid phase and a gas phase at given temperature and pressure conditions.

An example of a device making it possible to carry out such an absorption separation is described in particular in patent FR-A-2 972 643. Such a device comprises a column filled with packing, in which a high pressure is maintained, and in which the biogas product is produced. by anaerobic digestion is forced to flow countercurrently from a specifically absorbing solution of carbon dioxide. This absorbent solution, usually water, flows through the column from top to bottom, dripping. Due to their different properties, carbon dioxide is transferred to water, which is the liquid phase, and the methane remains in the gas phase. It is recovered at the top of the column, to be valued. Carbon dioxide is extracted from water, and can also be used in agriculture or the chemical industry, for example. The present invention aims to improve such a device, so as to improve the efficiency of separation of methane and carbon dioxide from a gaseous effluent containing them, in particular from a biogas produced by the fermentation of biomass, and to obtain methane with a degree of purity higher than that obtained by means of the devices of the prior art, which can in particular be injected into the natural gas distribution network. The invention aims in particular at the fact that such a device makes it possible to obtain a separation efficiency such as methane on the one hand, and carbon dioxide on the other hand, can be recovered with respective purities of greater than or equal to 96%. Such high purity levels make it possible, in particular, to recover the methane thus purified as natural gas, as automobile fuel gas, for conventional cogeneration engines, etc., and to recover the carbon dioxide recovered for industrial purposes. for example in agriculture, algae production or chemical industry.

More generally, the present invention aims to provide a device for separating, with a high separation efficiency, a first gaseous component and a second gaseous component contained in a gaseous mixture.

Additional objectives of the present invention are that this device is inexpensive to manufacture, simple to use and maintenance, it can be installed on the very sites where the biogas systems are located, and that it is suitable for continuously treat a stream of gas mixture from such systems.

For this purpose, it is proposed by the present invention a device for separating a first gaseous component and a second gaseous component contained in a gaseous mixture, in particular methane and carbon dioxide contained in a gaseous mixture, such as 'a biogas, of which they are the main components. By major components is meant here that the gas mixture has no other constituent in an amount greater than or equal to 10%. The device according to the invention comprises:

a liquid-gas absorption separation column, called an absorption column, comprising a lower part, called a reservoir, and an upper part, called the separation zone,

contacting means in the separation zone of the absorption column, by counter-current circulation, and preferentially at high pressure, the gaseous mixture and a liquid solution capable of selectively absorbing the second gaseous constituent, called the absorbent solution,

the reservoir of the column being intended to receive and store a volume of absorbent solution in which the second gaseous component is dissolved, and comprising an outlet orifice, out of the reservoir, of the absorbent solution in which the second gaseous constituent is dissolved,

and means for recovering the first gaseous component separated from the second gaseous component, in particular in the upper part of the separation zone. By selectively absorbing, it is meant in the present description that the absorbent solution absorbs more the second gaseous component than the other components of the gaseous mixture. Thus, the concentration of the second gaseous component in the gaseous mixture gradually decreases, by passing this second gaseous component in the absorbent solution, as the gaseous mixture rises in the separation zone of the absorption column. .

The reservoir of the absorption column is further dimensioned so that its cross-sectional area is large enough so that, when the reservoir contains a volume of absorbent solution, the falling speed of the absorbent solution, in this volume of solution absorbent, is lower than the rate of rise of the gaseous mixture, more particularly of bubbles of the gaseous mixture, in this same volume of absorbing solution. Specifically, the reservoir of the absorption column is dimensioned such that its cross-sectional area is greater than or equal to twice the cross-sectional area of the separation zone of the absorption column.

It has been found by the present inventors that such a dimensioning of the reservoir, with respect to the separation zone of the absorption column, whose dimensioning is dictated in particular by the characteristics of the gaseous mixture whose constituents must be separated, in particular its nature and its flow rate to be treated, makes it possible, whatever these characteristics of the gaseous mixture, to ensure that, when the reservoir contains a volume of absorbing solution, the descending speed of the absorbing solution coming from the separation, in this volume of absorbent solution, is less than the rate of rise of bubbles of the gaseous mixture in the same volume of absorbing solution.

The reservoir of the absorption column has the main function, in a conventional manner in itself, of receiving and storing the absorbent solution which has been charged with the second gaseous component. This absorbent solution contained therein will be referred to for convenience in the remainder of the present description by the terms "liquid stopper". Under normal conditions of operation of the device, its volume is fixed by calculations of the skilled man, so as to be sufficient to direct the gas mixture introduced into the absorption column towards the separation zone , and thus to limit leakage of gaseous mixture at the outlet orifice, out of the tank, of the absorbent solution loaded with the second gaseous component.

The present inventors have recognized that regardless of the volume of the liquid plug, in prior art devices, always occurred in which the cross-sectional area of the tank is typically substantially equal to the cross-sectional area of the tank. separation zone of the absorption column, a gaseous mixture leak, containing not only the second gaseous component, but also the first gaseous component, out of the tank, and that this gas leak was prohibitive for obtaining efficiencies of very high separation.

In a very advantageous way, they have been able to take advantage of this observation, and determine that this residual gas mixture leak could be significantly limited, or even completely eliminated, by an increase in the cross-sectional area of the tank of the column. absorption, relative to the columns proposed by the prior art, for a cross-sectional area of the separation zone of the column constant absorption, so as to obtain a cross-sectional area of the tank, in which the liquid plug is intended to be, at least twice as large as that of the separation zone of the absorption column, which ensures that the absorbent solution charged as the second gaseous component, flowing from the separation zone of the absorption column towards the reservoir, circulates in the liquid plug contained in the latter at a speed lower than the rate of rise in this plug liquid gaseous mixture bubbles entrained in the tank by the absorbent solution flowing therein. It is thus advantageously avoided that the absorbent solution charged with a second gaseous component carries with it part of the gaseous mixture introduced into the column, in the form of bubbles, into the liquid stopper and up to the outlet orifice of the solution. absorbent, out of the tank. It advantageously results in a better separation efficiency of the first gaseous component and the second gaseous component contained in the gaseous mixture, all of which is then directed into the separation zone of the column.

By such a reduction, or even elimination, leakage of gaseous mixture through the liquid stopper contained in the reservoir of the absorption column, the device according to the invention allows a complete and varied recovery of the constituents of the gas mixture, which can recovered with a high degree of purity, including the main constituents of biogas that are methane and carbon dioxide.

The cross-sectional area of the separation zone of the absorption column, and more generally its dimensioning, are dictated, in a conventional manner in itself, by considerations relating to the field of chemical engineering, and determinable by calculations of the the skilled person, taking into account in particular the capture kinetics of the second gaseous component and the cyclic capacity of the absorbent solution, to ensure the best absorption efficiency of the second gaseous component by the absorbent solution.

By downward velocity of the absorbent solution in the volume of Absorbent solution contained in the tank means in the present description the average downward speed over the entire cross-sectional area of the tank. This downward speed depends, in addition to the cross-sectional area of the tank, the flow of absorbent solution injected at the inlet of the column and the physical and physicochemical characteristics of the absorbent solution and the reservoir, such as density, viscosity and tension. surface of the absorbent solution, and the roughness of the tank wall.

When the absorbent solution is water, this descending speed of the absorbent solution, V | _iq , can be approximated relatively accurately by the equation:

V | _iq = QL / _R S where QL is the flow rate of absorbent solution introduced into the absorption column, and SR is the cross-sectional area of the reservoir. By ascending rate of bubbles of the gaseous mixture in the volume of absorbent solution contained in the reservoir is meant the ascending terminal velocity of the bubbles of the gaseous mixture in this volume of liquid at rest. This end-up rate, which depends on the diameter of the gas mixture bubbles entrained by the absorbent solution in its descent into the reservoir, and the physical and physico-chemical characteristics of the absorbent solution, such as its density, its viscosity and its tension superficial, can also be determined by calculations within the skill of the art, or empirically.

Whatever the composition of the gaseous mixture, and the diameter of the bubbles that this mixture is likely to form in the liquid stopper, and whatever the characteristics of the absorbent solution, for conventional operating conditions, especially in the field of the treatment of biogas, in the device according to the invention, the rate of rise of the gas mixture bubbles may be entrained in the volume of absorbent solution contained in the reservoir is preferably always greater than the descending speed of the absorbent solution in this same volume, so that the gas mixture leaks out of the tank, through the liquid stopper, are greatly reduced.

In particular, when the gaseous mixture is composed mainly of methane and carbon dioxide, as is the case for biogas from anaerobic digestion, and the operating conditions used provide that low gas mixture flow rates are introduced. in the absorption column, for example of the order of 40 Nm ³ / h, corresponding to the flow rates of biogas produced by common methanization systems of small or medium size, with a flow rate adapted accordingly absorbing solution, typically of the water, of about 10 Nm ³ / h, in the device according to the invention the separation zone is dimensioned, conventionally in itself, to have a cross-sectional area of about 0.07 m ² , and the tank has a cross-sectional area greater than or equal to 0.14 m ² . This makes it possible to ensure that all the bubbles that can be entrained in the reservoir by the absorbent solution flowing from the separation zone of the absorption column, go up towards this separation zone more rapidly than the absorbing solution. does not flow into the tank.

This can easily be verified by observation, by means of a camera, of the fluid escaping at the outlet orifice out of the reservoir, this fluid conveying no gas bubble.

Otherwise, it is within the skill of those skilled in the process engineering profession to verify it theoretically, for each given gaseous mixture - absorbent solution pair, on the basis of his general knowledge, illustrated in particular in chapter 7 of R's work. Clift, JR Grace and ME Weber, Bubbles drops and particles, Dover Publications, Inc. Mineola, New York, 1978.

In particular, the diameter of the gas bubbles for which the rate of rise in the liquid stopper contained in the reservoir is equal to the descending speed of the absorbent solution in the same liquid stopper can be approximated by the following equation, derived from the Stokes equation: (QL / SR) ^{/ 2} x 18 χ μ

where d is the diameter of the gas bubbles (m), QL is the flow of absorbent solution introduced into the absorption column (m ³ / s), S _R is the cross-sectional area of the reservoir (m ² ), pi _iq is the density of the liquid (kg / m ³ ), p _gas is the density of gas (kg / m ³ ), g is the gravitational acceleration (m / s ² ), and μ is the viscosity of the solution absorbent (Pa.s).

For all the larger diameter bubbles, in the liquid stopper the rate of rise is higher than the falling speed of the absorbent solution. It is then easy for a person skilled in the art to verify that this theoretical diameter is well below the diameter of the bubbles likely to form in reality in the liquid plug.

For the particular case described above, wherein the gaseous mixture consists mainly of methane and carbon dioxide, and the absorbent solution is water, the reservoir having a cross-sectional area equal to twice the surface area. cross section of the separation zone, that is to say equal to 0.14 m ² , we deduce from this equation a bubble diameter of 134 μηι. This diameter corresponds to the diameter of the gas mixture bubbles for which the rate of rise is equal to the descending speed of the absorbing solution, and are therefore likely to be carried with it. This theoretical diameter is smaller than the diameter of the gaseous mixture bubbles actually capable of being entrained in the reservoir by the absorbent solution flowing from the separation zone of the absorption column. An observation under real experimental conditions has enabled the present inventors to determine that this average real diameter is about 500 μηι, and the diameter of the smallest bubbles observed being about 150 μηι. This clearly demonstrates that in this case, the dimensioning of the tank with respect to the separation zone of the absorption column, as recommended by the present invention, advantageously makes it possible to avoid any leakage of gas through the liquid plug during the implementation of the device for separating the constituents of the gas mixture.

Preferably, the cross-sectional area of the reservoir of the absorption column is chosen so as to obtain that the descending speed of the absorbent solution, in the volume of absorbent solution contained in the reservoir, is at least two times lower than the speed ascension of the gaseous mixture in this same volume of absorbing solution. It is the responsibility of the skilled person to determine, taking into account the explanations above, the cross-sectional area of the tank required for this purpose.

In particular embodiments of the invention, the cross-sectional area of the reservoir of the absorption column is between 2 and 4 times the cross-sectional area of the separation zone of the absorption column, and preferentially still between 2 and 3 times the cross-sectional area of this separation zone. In particular, such upper bounds of these value ranges advantageously facilitate the manufacture and operation of the device according to the invention.

The absorbent solution is chosen as a function of the gaseous mixture to be treated, and in particular of the first gaseous component and the second gaseous component that it contains. In the case of particular application of the device according to the invention in which the gaseous mixture is a biogas, it is preferably an aqueous solution, such as an aqueous solution of sodium hydroxide, amine (s) or lime, and more generally a basic solution. Preferably, the absorbent solution is water. According to particular embodiments, the invention also fulfills the following characteristics, implemented separately or in each of their technically operating combinations.

In particular embodiments of the invention, the outlet orifice, out of the reservoir of the absorption column, of absorbent solution in which the second gaseous constituent is dissolved, is provided in a side wall of the tank, preferably near a bottom wall of the latter. By such a characteristic, the device according to the invention differs from the devices proposed by the prior art, which provide on the contrary an outlet of the liquid through the bottom wall of the tank. This characteristic advantageously makes it possible to further limit, if it is necessary, the gas mixture leaks at the exit orifice of the liquid solution. It indeed decreases the possibilities of moving fluid, in the form of a vortex, at the level of the liquid plug, and consequently the risk that a "piercing" effect of the liquid plug which could be induced by such a Put into motion.

Such an advantageous result is all the better achieved when, according to a particular characteristic of the invention, at least one deflection plate, preferably a plurality of deflection plates, is / are arranged in the tank of the absorption column, so as to extend in the direction of flow of the absorbent solution. This or these plate (s) of deflection allow in particular to further limit the risk of generation of a vortex in the liquid stopper, and thus to avoid drilling of the latter.

Each deflection plate is preferably disposed in a peripheral zone of the reservoir, so as to extend substantially from the lateral wall of the latter, towards its center. Each plate is also preferably disposed in a lower part of the tank.

In the case of a plurality of deflection plates, the latter are preferably arranged at regular intervals from each other. In particular embodiments of the invention, the separation zone of the absorption column contains means for facilitating the absorption of the second gaseous component in the absorbent solution, such as trays or packing, in particular a bulk packing, for increasing the contact area between the liquid phase and the gas phase flowing countercurrently in the separation zone. The lining may for example be a plastic type lining. It can in particular consist of Raschig rings

According to a particularly advantageous characteristic of the invention, the means for facilitating the absorption then preferably extend in the upper part of the reservoir of the absorption column, so as to ensure that it does not exist, or very little free space between these means of facilitating absorption and the upper surface of the liquid plug which fills the reservoir, in operation of the device. In the devices of the prior art, a large gap exists between the lining of the absorption column and the liquid plug, which promotes the acceleration of drops of absorbent solution flowing from the lining, and consequently a strong impact of these drops on the surface of the liquid stopper, and the entrainment of microbubbles of gaseous mixture in the liquid stopper.

On the contrary, in the device according to the invention, this source of gaseous mixture leakage in the liquid plug is eliminated, which advantageously contributes to improving the separation efficiency of the device.

In particularly preferred embodiments of the invention, the absorption column comprises, between the reservoir and the separation zone, an intermediate portion, which comprises a narrowing of its surface in cross section, so as to fit, on one side, at the cross-sectional area of the separation zone, and on the other side, at the cross-sectional area of the reservoir of the absorption column. The absorption facilitation means then occupy the entire internal volume of this intermediate portion, until coming flush with the surface of the liquid stopper. The absorption column is then preferably configured so that the gaseous mixture to be purified is introduced at this intermediate portion, or in the lower part of the separation zone.

In particular embodiments of the invention, the device comprises means for separating the absorbent solution and the second gaseous component dissolved therein. These separation means are fluidly connected to the outlet orifice, out of the reservoir of the absorption column, of the absorbing solution in which the second gaseous component. Thus, the present invention is also expressed in terms of a device for recovering the second gaseous component from the gaseous mixture containing it in a mixture with the first gaseous component.

The means for separating the absorbent solution and the second gaseous component may be of any conventional type in itself.

Preferably, these separation means comprise:

a duct connected fluidically to the reservoir of the absorption column at a first end, and opening into a degassing tank at an opposite second end, and a mixer disposed in this duct.

Such separation means are described in particular in the aforementioned FR-A-2,972,643 patent.

According to a particularly advantageous characteristic of the invention, the device comprises means for circulating, in the separation zone of the absorption column, the absorbent solution separated from the second gaseous component by the separation means that the device comprises. thus ensuring a recycling of the absorbent solution which, once freed of the second gaseous component, is then again used to wash the gaseous mixture in the separation zone of the absorption column.

Since the device according to the present invention makes it possible to limit gaseous mixture leakage very strongly through the liquid plug, the absorbent solution which is thus regenerated is almost entirely free of a first gaseous component. The loss of first gaseous component is thus avoided.

The device may further comprise means for pressurizing the gaseous mixture before it is introduced into the absorption column, and means for pressurizing the absorbent solution before it is introduced into the absorption column, conventional in themselves. same. It may also include automated control means for its various components, including valves, compressors, circulation and injection pumps, etc.

This device is particularly suitable for the treatment of small inflow gas mixture flows, in particular of the order of 40 Nm ³ / h. It has in particular been found by the present inventors that in operation at such flow rates, there is no engorgement of the absorption column, constant inflow can be maintained for both the gaseous mixture and the absorbent solution, and it can be recovered, at the outlet at the top of the absorption column, a gas whose concentration by volume of methane is greater than 96%, and, at the outlet of the gas-liquid separation means, a gas whose volume concentration in Carbon dioxide is also greater than 96%.

In addition to high separation performance, the device according to the invention has great compactness, energy saving, simplicity of operation and maintenance, as well as a reasonable manufacturing cost. For the purification of biogas, it also requires the implementation of no chemicals.

Another aspect of the invention is the use of the device corresponding to one or more of the above characteristics for the separation of a first gaseous component and a second gaseous component contained in a gaseous mixture.

In particular, the device according to the invention can be used for the separation of methane, as the first gaseous component, and of carbon dioxide, as the second gaseous component, contained in a biogas.

The device according to the invention can then belong to a more global installation, comprising an anaerobic digestion system, for the production of biogas, at the output of which the device according to the invention is fluidly connected, so as to allow direct and continuous recovery. , from the biogas produces, on the one hand methane, and on the other hand carbon dioxide, at high levels of purity.

The device according to the invention will now be more precisely described in the context of preferred embodiments, which are in no way limiting, represented in FIGS. 1 to 6, in which:

- Figure 1 schematically shows the absorption column of a device according to a particular embodiment of the invention;

- Figure 2 shows a cross-sectional view along the plane A-A of the column of Figure 1;

- Figure 3 shows a longitudinal sectional view of a portion of the absorption column of Figure 1, including its reservoir;

FIG. 4 schematically represents a device according to a particular embodiment of the invention, comprising the absorption column of FIG. 1;

FIG. 5 shows photographs of the fluid escaping from the outlet of the reservoir, (a) for a separation device according to the prior art, in which the reservoir has a diameter equal to the diameter of the zone of separation, (b) for a device according to the invention, wherein the reservoir has a diameter equal to 2 times the diameter of the separation zone;

and FIG. 6 shows a graph representing the theoretical diameter of the bubbles retained in the liquid stopper of a device for separating gaseous components from a gaseous mixture, as a function of the ratio (diameter of the reservoir / diameter of the separation zone). .

The exemplary device according to the invention described below is with reference to a preferred field of application of the invention which is the separation, from biogas, of methane (first gaseous component) and carbon dioxide. carbon (second gaseous component), with a view to recovery for their valuation. Such a field of application is however in no way limiting of the present invention.

The absorption column 10 of this device according to the invention is illustrated in FIG. Part of the internal components of this column, normally not visible, are shown in this figure for reasons of better understanding of its description.

The absorption column 10 comprises three successive parts: an upper part 1 1, called the separation zone, a lower part 12, called a reservoir, and connecting the two, an intermediate portion 13. The reservoir 12 and the separation zone 1 1 can have any shape. Preferably, both have a substantially cylindrical shape. According to the present invention, the internal diameter of the reservoir 12 of the absorption column shown in this FIG. 1 is at least two times greater than the internal diameter of the separation zone 11, more precisely 2 times greater.

The intermediate portion 13 has a portion in decreasing cross section, so as to have, at a lower end 131, an inner diameter substantially equal to the internal diameter of the reservoir 12, and at an opposite upper end 132, an inner diameter substantially equal to the internal diameter of the separation zone 1 1.

The intermediate portion 13 is fixed, fluid-tight, on the one hand to the separation zone 1 1, by means of a connecting flange 133, and on the other hand to the tank 12, by means of a flange junction 134.

The separation zone 1 1 is filled with a lining 1 1 1, represented by hatched transparency in FIG. 1. This lining is for example formed of Raschig rings made of plastic material. It extends in the intermediate portion 13, and beyond, in the upper portion 126 of the reservoir 12.

Lining support grids 1 12, 1 13 are respectively disposed in the separation zone 1 1 and in the tank 12, in the upper part 126 of the latter. The absorption column 10 comprises a first pipe 14 for its hydraulic connection with a source of biogas under pressure. This first pipe 14 can lead into the lower part of the separation zone 11, or, as shown in FIG. 1, into the intermediate portion 13, in which packing 11 1 1 is preferentially located.

The absorption column 10 also comprises a second pipe 15 for its hydraulic connection with a source of absorbent carbon dioxide solution, for example water, under pressure. This second pipe 15 opens into the upper part of the separation zone 1 1, above the lining 1 1 1. It is preferably associated with spraying means 151 distributed in the separation zone January 1 so that they allow uniform spraying of the absorbent solution on the lining 1 1 1.

The reservoir 12 is associated with two sensors 127, 128 of liquid level inside, arranged one above the other at the upper part 126 of the tank 12. These sensors are intended to regulate the level of liquid which is contained in the reservoir 12 when the absorption column 10 is in operation. In Figure 1, the column 10 is shown with a volume of absorbent solution 1 6 contained in the tank 12. This volume 1 6 is referred to in the remainder of this description by the terms "liquid stopper" or "foot of water. The device is driven from the information recorded by the level sensors 127, 128, so as to ensure that the upper surface 1 61 of the liquid cap 1 6 is always located between these level sensors 127, 128 during operation of the device. according to the invention.

Preferably, the lining 1 1 1 extends to the height of the upper level sensor 127, or between the two level sensors 127, 128, so as to ensure that the free space between the upper surface 1 61 of the liquid cap 1 6 and lining 1 1 1 is as small as possible, or substantially nonexistent.

The reservoir 12 is delimited peripherally by a side wall 121, and it has a bottom wall 122. Near this bottom wall

122, its side wall 121 is pierced with an orifice 17 for the liquid outlet out of the tank 12, more precisely absorbent solution loaded with carbon dioxide extracted from the biogas, via a third pipe 18. At the upper end 1 14 of the separation zone 1 1, the absorption column 10 comprises a fourth pipe 19, for the exit, outside the separation zone 11, of the gas having passed through the zone of separation 11, that is to say say a gas highly enriched in methane, and substantially free of carbon dioxide. The absorption column 10 further comprises, in the tank 12, deflection plates 124. In the particular embodiment illustrated in FIG. 2, these plates 124 are 4 in number, arranged at regular intervals one on the other hand, in a peripheral zone 123 of the reservoir 12. These plates 124 extend, for example, for their width, from the side wall 121 of the reservoir 12, towards its center, and as illustrated in FIG. for their height, from the bottom wall 122 of the tank 12, in the flow direction 125, in the tank, liquid from the separation zone 11 of the absorption column 10. The deflection plates 124 s preferably extend over more than half the height of the tank 12, preferably substantially over the entire height of the latter. By way of example, they may each have a height of 800 mm, a width of 50 mm and a thickness of 10 mm.

An example of a device according to the invention in a complete configuration for its operation for separation and recovery, from biogas, methane and carbon dioxide, is shown in FIG. 4.

This device comprises the absorption column 10 described above with reference to FIGS. 1 to 3.

In addition to the absorption column 10, it mainly comprises the following components. On the first pipe 14, intended to supply the absorption column 10 with biogas to be treated, are successively mounted a biogas desulfurization member 21, based for example on iron filings, and a compressor 22 for pressurizing the gas. gas before its injection into the absorption column 10. The compressor 22 is controlled by automatic control means, to obtain the desired pressure and the flow rate of biogas at the inlet of the absorption column 10. The biogas reaches the first line 14 from the plant in which it was produced, in the direction indicated at 41 in Figure 4, to be injected into the intermediate portion 13 of the absorption column 10.

In the absorption column 10, the biogas naturally rises, for the most part, towards the upper end 114 of the separation zone 11, as indicated at 42 in FIG.

On the second line 15, which brings the absorbent solution, for example water, into the separation zone 1 1, is mounted a pump 23, which, like all the other members of the device, is controlled by control means automatically, so as to feed the absorption column 10 with the desired flow of absorbent solution, at the desired pressure. These automatic control means are conventional in themselves and are not shown in FIG. 4.

The absorbent solution, injected towards the lining 1 1 1 in the direction indicated at 43 in FIG. 4, and flowing through this lining, then comes into contact with the biogas, which amounts, for its part, against current, in the separation zone 1 1. There is a selective dissolution in the absorbent solution of the carbon dioxide contained in the biogas, on a large contact surface. The biogas thus sees its concentration of carbon dioxide decrease throughout its ascent of the separation zone 1 1, while the absorbent solution is in turn responsible for this carbon dioxide throughout its descent. When it reaches the upper end 1 14 of the separation zone

1 1, the purified gas of carbon dioxide, and very rich in methane, borrows the fourth pipe 19, which has a mechanical regulation of its internal pressure through a discharger 24 mounted on this pipe 19. This discharger 24 opens only proportionally to the incoming flow, and when the desired pressure is reached. The purified gas has three circulation possibilities: a methane recovery route for its recovery, in the direction indicated at 44 in FIG. 4, or, where appropriate, recirculation channels in the absorption column 10 or emptying 26, for shutting down the device.

The absorbent solution loaded with carbon dioxide flows in drop form from the lining 1 1 1 to the tank 12, in the direction indicated at 125 in FIG. 4, to form the liquid plug 1 therein. 6. As explained above, the level of absorbent solution in the liquid plug 1 6 is regulated so that its upper surface 1 61 is permanently located between the level sensors 127, 128. This regulation is provided by a control system comprising automatic control means, according to the information recorded by the level sensors 127, 128, the opening or closing of a valve 27 mounted on the third pipe 18, absorbent solution outlet , charged with carbon dioxide, out of the tank 12. The control of the valve 27 makes it possible in particular to control the flow of liquid flowing in the third pipe 18.

When it flows into the lining 1 1 1, the absorbent solution is likely to lead with them microbubbles of biogas, towards the reservoir 12 and in the liquid stopper 1 6. It has been determined by the present inventors that such microbubbles have an average diameter of 0.5 mm. For this, the inventors have implemented a separation device of the prior art, as described in the document FR-A-2 972 643, in which the reservoir has the same diameter as the separation zone of the column. absorption (ie 0.3 m), with a biogas inflow of 40 Nm ³ / h and an inflow of absorbent solution (water) of 10 Nm ³ / h. An observation of the solution exit pipeline absorbent, loaded with carbon dioxide, out of the reservoir of the device, was carried out. The observed image is shown in Figure 5 (a). In the liquid there is the presence of numerous gaseous bubbles, in the form of white spots, the average diameter of which is measured at 0.5 mm. The diameter of the smallest bubbles observed is about 250 μηι.

In the device according to the invention as described above, the liquid plug 1 6, due to the cross-sectional area of the reservoir 12, which has been advantageously chosen for this purpose, the microbubbles have an ascending speed, in the direction of the separation zone 1 1 overhanging the reservoir 12, which is much greater than the descending speed of the absorbent solution, loaded with carbon dioxide, in the liquid plug 1 6. The microbubbles of biogas thus quickly rise to the surface 1 61 the liquid plug 1 6, and only the absorbent solution in which is dissolved carbon dioxide, but devoid of methane, reaches the bottom of the tank 12, the outlet port 17.

The observation of the fluid flowing in the pipe 18 extending from this outlet orifice 17, was carried out in an experiment carried out according to the operating conditions described above with reference to FIG. 5 (a). The reservoir has a diameter twice that of the separation zone of the absorption column (0.6 m and 0.3 m respectively), the inflow of biogas is 40 Nm ³ / h and the inflow of solution absorbent (water) is 10 Nm ³ / h. The observed image is shown in Figure 5 (b). The presence of any gas bubble is not observed in the liquid, which demonstrates the efficiency of the sizing of the reservoir recommended by the present invention. The very low occurrence of biogas leakage through the outlet orifice 17 outside the tank 12 is all the more accentuated by the absence of free space between the lining 1 1 1 and the upper surface 1 61 of the liquid plug 16, by the presence in the tank of the deflection plates 124 and in that the outlet orifice 17 is located on the side wall 121 of the tank 12, and not in the bottom thereof. The absorbent solution charged with carbon dioxide flows from this outlet orifice 17 into the third pipe 18, in the direction indicated at 45 in FIG. 4, at a rate regulated by the valve 27. This valve 27 also constitutes a means of relaxation, to bring the absorbent solution to atmospheric pressure.

The third line 18 brings the absorbent solution to a system for separating the absorbent solution and carbon dioxide. This system comprises a static mixer 28, which creates a great mechanical and passive agitation. This agitation allows degassing of carbon dioxide which, at atmospheric pressure, is less soluble in the absorbent solution.

The assembly then circulates in a long-dimension pipe 29, which brings it into a large volume degassing tank, in which it is stored. There is a separation of the absorbent solution and carbon dioxide, the latter rising to the surface in the form of microbubbles, to escape in the direction indicated at 46 in Figure 4. The device may comprise means recovery of this carbon dioxide (not shown in the figure), for its revaluation.

The second pipe 15 is fluidically connected, at an end opposite the absorbent solution injection end in the separation zone 1 1 of the absorption column I O, to this degassing tank 30. The pump 23 mounted on this second line 15 then ensures the circulation of regenerated absorbent solution, that is to say freed of carbon dioxide, in the direction indicated 47 in Figure 4, in this pipe 15, for its reinjection into the column d Such a step of recycling the absorbent solution advantageously makes it possible to reduce the operating cost of the device.

As indicated above, the operation of the device according to the invention can be controlled in a completely automated manner, in particular with regard to the circulating flows of gas and liquid, the pressure inside the column and the level of the plug. liquid 1 6 in the tank 12.

The various calculation and control steps are preferably performed by a computer-programmed system comprising at least one microprocessor and storage means (magnetic hard disk, flash memory, optical disk, etc.) in which a product is stored. computer program, in the form of a set of program code instructions to be executed to implement the various calculation steps of the control method of the device according to the invention.

By way of example, it is realized the following experiment of implementation of the device according to the invention described above.

The sizing of the absorption column 10 is as follows: total height: 2.10 m; internal diameter of the separation zone 1 1: 0.25 m; internal diameter of the tank 12: 0.6 m; height of the liquid plug 1 6: 0.8 m. The pressure inside the absorption column 10 is set at 7 bar. The gas mixture flow rate (biogas) entering the column 10 is 40 Nm ³ / h, and the absorbent solution flow (water) entering the column 10 is 10 Nm ³ / h. The temperature is 20 ° C.

For such conditions, it is determined that the diameter of the biogas bubbles that may be entrained by the trickling water from the separation zone 1 1 of the column 10 into the liquid plug 1 6 is equal to 0.5 mm. We deduce an ascending terminal velocity of these bubbles in the liquid stopper 1 6 at rest close to 0.06 m / s. Furthermore, the downward velocity of the water in the liquid cap 1 6 is determined at 0.01 m / s. This speed is very much lower than the rate of rise of the gas mixture, as recommended by the present invention.

During operation of the device under such conditions, for 4 hours, there is no clogging of the absorption column 10. is recovered at the top of the column, a gas with a methane volume concentration greater than 96 %, and, at the outlet of the gas-liquid separation means, a gas whose volume concentration of carbon dioxide carbon is also greater than 96%.

The relevance of choosing a cross-sectional area ratio of the reservoir, relative to the cross-sectional area of the separation zone of the absorption column, greater than or equal to 2, has been theoretically confirmed and experimental way.

In theory, in the embodiment described above of a reservoir and a separation zone of cylindrical shape, the diameter of the bubbles retained in the liquid plug has been calculated, for different ratios DR / D _z , where DR represents the diameter of the reservoir and D _z represents the diameter of the separation zone, using the equations below. The following values were determined: D _z = 0.3 m and QL = 10 m ³ / h, with methane as a gas and water as an absorbent solution.

= d bubble

and

4 * QL

^ ⁷ ulle

Ti * T ± Γ) j where _R ² dbuiie is the diameter of retained gas bubbles (m), QL is the flow rate of absorbent solution introduced into the absorption tower (m ³ / s), D _R is the diameter of reservoir (m), puquide is the density of the liquid (kg / m ³ ), pgaz is the density of gas (kg / m ³ ), g is the gravitational acceleration (m / s ² ), and μ is the viscosity of the absorbent solution (Pa.s).

The results shown in Table 1 below are obtained.

Table 1 - Diameter of the bubbles retained in the liquid cap as a function of the ratio (diameter of the tank D _R / diameter of the separation zone D _z ) The graph shown in Figure 6 has been drawn from these values. This graph shows that beyond a ratio (diameter of the reservoir / diameter of the separation zone) equal to 2, the size of the bubbles retained varies little, and is much less than the actual size of the bubbles that can be observed experimentally. When this ratio is less than 2, the size of the retained bubbles varies significantly depending on the ratio of diameters.

These theoretical results were supplemented by real experiments carried out as described above with reference to FIGS. 5 (a) and 5 (b). As explained above, an observation of the liquid at the outlet of the tank has shown, for a ratio D _R / D _z equal to 1, the presence of numerous bubbles. For a ratio D _R / D _z equal to 1, 8 (observations not shown in the figures), it has always been observed the presence in the liquid of gas bubbles, of small diameter, in a lesser amount, however. For a ratio D _R / D _z equal to 2 (observation shown in FIG. 5 (b)), in accordance with the present invention, no gas bubble was observed in the liquid.

Claims

Apparatus for separating a first gaseous component and a second gaseous component contained in a gaseous mixture, comprising:

a liquid-gas absorption separation column, called an absorption column (10), comprising a lower part, called a reservoir (12), and an upper part, called separation zone (1 1),

countercurrent circulation contacting means, in the separation zone (1 1) of said absorption column (10), of said gaseous mixture and of a liquid solution capable of selectively absorbing said second gaseous constituent , said absorbent solution, - the reservoir (12) of the column being intended to receive a volume

(1 6) of absorbent solution in which the second gaseous component is dissolved, and having an outlet orifice (17) out of said reservoir (12) of said absorbent solution in which said second gaseous component is dissolved, and means (19) ) recovering said first gaseous component separated from said second gaseous component, said device being characterized in that the cross-sectional area of the reservoir (12) of the absorption column (10) is greater than or equal to twice the area in section cross-section of the separation zone (1 1) of the absorption column (10).

2. Device according to claim 1, wherein the cross-sectional area of the reservoir (12) of the absorption column (10) is between 2 and 4 times the cross-sectional area of the separation zone (1 1). of the absorption column (10).

3. Device according to any one of claims 1 to 2, in wherein the outlet orifice (17), out of the reservoir (12) of the absorption column (10), of absorbing solution in which the second gaseous component is dissolved, is formed in a side wall (121) of said reservoir ( 12), preferably near a bottom wall (122) of said tank (12).

4. Device according to any one of claims 1 to 3, wherein at least one deflection plate (124), preferably a plurality of deflection plates (124), are arranged in the reservoir (12) of the column d absorption (10) so as to extend in the flow direction (125) of the absorbent solution, preferably in a peripheral zone (123) of said reservoir (10).

5. Device according to any one of claims 1 to 4, wherein the separation zone (1 1) of the absorption column (10) contains means (1 1 1) for facilitating the absorption of the second component. gaseous in the absorbent solution, such as trays or packing, and wherein said absorption facilitation means (1 1 1) extends into the upper portion (126) of the reservoir (12) of the column of absorption (10).

6. Device according to any one of claims 1 to 5, comprising separation means (28, 29, 30) of the absorbent solution and the second gaseous component dissolved therein, fluidly connected to the orifice (17) of discharging, out of the reservoir (12) of the absorption column (10), said absorbing solution in which said second gaseous component is dissolved.

7. Device according to claim 6, wherein the means (28, 29, 30) for separating the absorbent solution and the second gaseous component dissolved therein comprise:

a conduit (29) fluidly connected to the reservoir (12) of the absorption column (10) at a first end and opening into a degassing tank (30) at an opposite second end,

- and a mixer (28) disposed in said duct (29).

8. Device according to claim 7, comprising means (15,23) for circulating, in the separation zone (1 1) of the absorption column (10), the absorbent solution separated from the second gaseous component by said separation means (28, 29, 30).

9. Use of a device according to any one of claims 1 to 8 for the separation of a first gaseous component and a second gaseous component contained in a gaseous mixture.

10. Use according to claim 9, wherein the gaseous mixture is a biogas, the first gaseous component is methane and the second gaseous component is carbon dioxide.