WO2013057387A1 - Method for capturing a compound, contained in a gas, by means of adsorption on a vertical bed - Google Patents

Abstract

The gas, that is to be treated and arrives via the pipe GE, is separated into different streams, GE1 and GE2, that are injected into parallelepiped columns. The column comprises two vertical beds, L1 and L2, of adsorbent material. The gas GE1 is sent into the space E1 separating both beds L1 and L2. Preferably, both beds L1 and L2 are symmetrically located relative to the system for injecting the gas. The gas passes through each bed in an essentially horizontal direction, perpendicular to the largest section of the bed.

Description

 METHOD FOR CAPTURING A COMPOUND CONTAINED IN A GAS BY

 ADSORPTION ON VERTICAL BED

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process for the capture of a compound contained in a gas, for example the capture of CO ₂ in combustion fumes by adsorption.

CONTEXT

Much of the energy is produced by burning fossil fuels that rejects C0 ₂ in the atmosphere. These emissions increase the concentration of C0 ₂ in the atmosphere and cause global warming. To limit the extent of this warming, it is necessary to reduce C0 ₂ emissions related to energy production. One solution is to capture and store the C0 ₂ emitted by combustion. The main stationary sources of C0 ₂ emissions are: thermal power plants, iron and steel industry, cement plants, refineries, etc. A C0 ₂ capture process separates C0 ₂ from the fumes produced by the combustion of fossil fuels and thus makes it possible to recover a concentrated C0 ₂ flux (> 90%, preferentially> 96%), which can be sequestered or used in the combustion process. chemical industry, agribusiness, etc.

However, the installation of a CO ₂ capture process requires considerable investment and the process itself consumes energy which can cause secondary emissions of C0 ₂ . In a power plant, the installation of a capture process reduces energy efficiency in the plant - part of the energy produced by burning fossil fuels is consumed by the capture unit - and increases the cost of energy. electricity through investment costs, operating costs and reduced energy efficiency. The aim is to minimize the investment costs as well as the energy consumption of the capture process.

STATE OF THE PRIOR ART

A reference capture process is to date the absorption of C0 ₂ by a basic solution, for example by an aqueous solution of monoethanolamine, as described in document FR 2 958 454. This process has several drawbacks: the energy consumption for regeneration of the amine is important and requires taking from the turbine cycle of the thermal power plant good quality steam that could have been used to provide electricity, the degradation of the amine which causes problems corrosion and emissions of volatile organic compounds requiring additional treatment. There are several alternatives to the solvent absorption process, for example, the separation of C0 ₂ by membrane or the separation of C0 ₂ by adsorption described in document FR 2 944 217. The adsorption on a solid avoids corrosion problems and can potentially reduce the energy cost of regeneration.

An adsorption process is a cyclic process in which a CO ₂ adsorption step is carried out alternately with a regeneration step of the adsorbent. There are two possibilities for regenerating the adsorbent: by modulating the temperature commonly called "TSA" or "Temperature Swing Adsorption" or by modulating the pressure (total or partial) commonly called "PSA" or "Pressure Swing Adsorption".

Whatever the choice of the regeneration mode (TSA or PSA), the C0 ₂ adsorption methods in a fixed bed are confronted with the problems of pressure drop and the size of the equipment (bed volume, footprint ...). The flow rate of fumes emitted by a thermal power station is enormous (about 1, 8 Nm ³ / h for a 600 MW power plant). Generally the gas flows vertically in the bed of adsorbent material arranged in a column.

 To reduce the pressure drop in adsorption processes, it is possible to use either very large diameter columns and low heights, or a very large number of columns, which in both cases is not optimum in term congestion and construction costs.

This is because the gas velocity in the adsorbent bed is calculated by dividing the flow rate by the total section of the adsorption mode columns. To minimize the total section of the columns, it is necessary to increase the speed of the gas. However, an increase in the gas velocity also increases the pressure drop in the column. To compensate for this loss of charge it is necessary to compress the fumes upstream of the adsorption column. This compression has a high energy cost which is not desirable. It is therefore essential to limit the pressure drop, which imposes a constraint on the height of the column. This constraint can affect the quality of the separation, that is to say the purity and the yield of the C0 ₂ recovered, the yield being defined as the fraction of C0 ₂ in the fumes which is eliminated by the capture process. .

In addition, to limit the pressure drop, the speed of the gas in the columns is reduced, which implies that the total section of the columns increases. The ground occupation of the system increases accordingly. In addition, the gas distribution becomes very difficult when the section of a column is large relative to its height. There are technological constraints that limit the size of the column diameters to values close to 8 m to 10 m. An increase in section therefore corresponds to an increase in the number of columns. "Lively et al., Ind. Eng Chem Res 2010, 49, 7550" discusses the problem of pressure drop in a fixed bed. It describes a particular shaping adsorbents. The adsorbents are mixed with polymers and shaped into woven hollow fibers. The authors claim that the pressure drop in an adsorber, which consists of a set of these hollow fibers, is lower than in a conventional fixed bed. The use of these hollow fibers is also indicated in the documents US 2008/0282888 A1, US 2008/0282885 A1 and US 2008/0314244 A1, but, if the three documents highlight their ability to improve the transfer of material and heat transfer, they do not fulfill their effect on the pressure drop.

The formatting proposed in the documents cited above is complex and expensive. The manufacture of very long fibers, necessary for the size of the adsorbers used for the capture of C0 ₂ , is not controlled to date.

The present invention proposes to circulate the gas in a substantially horizontal direction in a bed of adsorbent material arranged vertically to limit the pressure drop in a fixed bed adsorber, while limiting the ground occupation of the adsorption system and maintaining the required quality of separation, in terms of C0 ₂ purity and efficiency. SUMMARY OF THE INVENTION

 In general, the present invention provides a method for capturing a compound contained in a gas in which said gas is brought into contact with a material that adsorb the compound, said material being disposed in a first bed. The method is characterized in that the gas is circulated in the first bed in a horizontal direction and in that the passage section of the gas in the first bed is perpendicular to said horizontal direction.

According to the invention, the passage section of the gas in the first bed may be a rectangle having a height of between 2 and 30 meters and having a length at least equal to 5 meters.

 The length may be greater than the height.

 The depth of the first bed traversed by the gas may be between 0.2 and 5 meters.

The material adsorbing the compound may be disposed in the first bed and in a second bed separated by a space of the first bed, the passage section of the gas in the second bed being perpendicular to said horizontal direction and in that the gas is injected. in said space. The gas can be injected into said space by a gas distributor to homogenize the distribution of the gas in the first and second bed.

 The gas can be brought into contact with the adsorbent material at a temperature of between 20 ° C. and 250 ° C. and at a pressure of between 1.1 and 5 bars absolute.

The length, the height and the depth can be chosen to circulate, through the first bed, a gas flow rate greater than 1000 Nm ³ / h at a speed of between 0.05 m / s and 1.5 m / s. .

 The adsorbent material can be regenerated by contacting the material with steam at a temperature of between 50 ° C. and 90 ° C. and at a pressure of between 0.1 and 0.7 bar absolute.

According to the invention, the gas may consist of combustion fumes and the compound may be CO ₂ .

DETAILED DESCRIPTION OF THE INVENTION

 Other features and advantages of the invention will be better understood and will become clear from reading the description given below with reference to the drawings among which:

 FIG. 1 schematizes a deacidification process,

 FIG. 2 schematizes the adsorption and desorption steps; FIG. 3 shows a sequencing of adsorbers,

 FIGS. 4A and 4B show embodiments of a vertical bed according to the invention,

 FIGS. 5, 6 and 7 represent implementations of the vertical beds according to the invention,

 FIG. 8 schematizes the flow of the gas in the device of FIG. 5,

FIG. 9 schematizes a gas distributor to be implemented in the devices of FIGS. 5 to 7;

 FIG. 10 schematizes the flow of the gas through an adsorbent bed with a gas distributor of FIG. 9.

The gas to be treated by the process according to the invention may be a combustion smoke comprising CO ₂ , for example in a content of between 5% and 30% by volume. Combustion fumes can be produced by a thermal power plant for the generation of electricity. The capture process according to the invention can also be applied to all combustion fumes, produced for example in a refinery, in a cement plant, in a steel plant. In general, the method according to the invention can be applied to the capture of all types of compounds, for example SOx, NOx, contained in a gas.

The flow rates of gas, for example fumes, to be treated by the process according to the invention can be significant, of the order of 10 ⁵ Nm ³ / h for a cement plant, 3.10 ⁵ Nm ³ / h for a steel plant, and of the order of 10 ⁶ to 3.10 ⁶ Nm ³ / h for a thermal power station.

Combustion fumes contain nitrogen and CO ₂ , oxygen and water vapor, and traces of NOx and SOx. The volume concentration of CO ₂ is typically in the range of 3 - 30%, the volume concentration of H ₂ O can be in the range of 1 - 15%, and the cumulative volume concentration of nitrogen and oxygen can be between 50 and 95%. The pressure of the combustion fumes can be close to atmospheric pressure and at a temperature of between 100 and 250 ° C.

Before capturing C0 ₂ by the fixed bed adsorption process, the fumes can be prepared by carrying out one or more pretreatment steps. Preprocessing steps may include:

 (i) cooling the charge to a temperature between 20 and 100 ° C,

 (ii) a step of removing the oxides of nitrogen and the oxides of sulfur,

(iii) the compression of the fumes at the adsorption pressure of between 1.1 bar and 5 bar absolute and preferably less than 2 bar,

(iv) dehydration of the fumes.

 Cooling of the load and dehydration of fumes are optional steps. They make it possible to adapt the temperature and the humidity rate of the fumes to the optimal conditions for the operation of the adsorbent.

 One embodiment of the process according to the invention is illustrated in FIG. 1 in which the gas to be treated arrives via line 1. Fumes 1 can be cooled in heat exchanger E1 to a temperature of between 20 and 100 ° C, preferably between 25 and 50 ° C. The water that can be condensed during the cooling in E1 is separated from the flue gases in the separator tank B. The condensates are discharged from the flask B via the duct 2, the cooled flue gases are discharged through the duct 3.

 The cooled fumes 3 are introduced into the U units for the removal of nitrogen oxides and sulfur oxides. These units are commonly named deSOx and deNOx.

The purified fumes 4 are sent into a compressor or a fan K, to be compressed to a pad pressure of between 1.1 bar absolute and 5 bar absolute, preferably between 1.1 bar absolute and 2 bar absolute. Then the fumes can be introduced into the dehydration unit DH. For example, the dehydration process is that described in document FR 2 740 468. The level of dehydration is chosen as a function of the adsorbent. Some adsorbents tolerate moisture and do not require dehydration of fumes, others are very sensitive to water and require extensive dehydration of fumes. The dehydrated gas is discharged from DH via line 5.

 In other embodiments, the order of the operations of the pretreatment of the fumes can be modified. It is possible, for example, to place a deNOx unit, operating at high temperature, upstream of the exchanger-separator E1-B, and a deSOx unit downstream of the exchanger-separator E1-B, or to carry out the deNOx and deSOx operations at high temperature, that is to say, upstream of the exchanger-separator E1-B.

After undergoing pretreatment, the fumes flowing in the pipe 5 are at a temperature of between 20 and 250 ° C, preferably between 30 and 200 ° C. The partial pressure of C0 ₂ in the flue gases flowing in duct 5 is between 0.03 * Pads and 0.3 ^* Pads absolute bar, the Pads pressure being the pressure during contact of the gas to be treated with the adsorbent material in the unit. C described below.

After pretreatment, the flue gases are introduced into the CO ₂ capture unit C. In this unit, the fumes are brought into contact with one or more adsorbent materials arranged in fixed bed adsorbers. Unit C makes it possible to separate the stream 5 from the flue gases into a stream 7 rich in C0 ₂ and a stream 6 constituted by fumes depleted in C0 ₂ .

The adsorbent material is selective for the adsorption of CO ₂ with respect to nitrogen and oxygen, that is to say, in contact with a mixture of gases containing oxygen, nitrogen and of C0 ₂ , it adsorbs mainly C0 ₂ . Other favorable characteristics of the adsorbent are: (i) a low affinity for water, (ii) a moderate CO ₂ adsorption heat, (iii) a low resistance to material transfer. The present invention is not directed to the adsorbent and is applicable to any adsorbent. Non-limiting examples are zeolites, carbon molecular sieves, MOFs, amines immobilized on a support or basic oxides. The adsorbent material is in the form of a bed composed of particles in the form of beads or extrudates.

 Each adsorber of unit C alternates cyclically between at least one adsorption stage and a desorption stage. The steps implemented in unit C are described with reference to FIGS. 2A and 2B, which represent an adsorbent bed C1. a) Adsorption step:

With reference to FIG. 2A, the combustion fumes arriving via the pipe 5 are brought into contact with the adsorbent material in the bed C1. The contacting in C1 can be carried out at a total pressure Pads of between 1 and 5 bar absolute, preferably between 1.1 and 2 bar absolute, and more preferably between 1.1 and 1.3 bar absolute. The optimum temperature level depends on the type of adsorbent chosen. A part, and preferably at least 50% by volume, or even at least 70% by volume or at least 90% by volume, of the CO ₂ contained in the combustion fumes is adsorbed by the adsorbent material C1. The C0 ₂ depleted fumes are removed from C1 by line 6. When the adsorbent material in C1 is saturated with C0 ₂ , the adsorption step is stopped. b) Desorption stage:

 Three variants for carrying out the desorption step are described with reference to FIG. 2B.

In the first variant, the adsorbent C1 is brought into contact with a purge gas, preferably water vapor, which arrives via the conduit 8. The vapor causes the desorption of CO ₂ either by an increase in the temperature of the adsorbent C1 either by a dilution effect or by a combination of these two effects. The mixture of CO ₂ and water vapor is evacuated via line 7.

Some adsorbents are sensitive to water. Regeneration by water vapor is therefore not feasible. In this case, the desorption of CO ₂ is carried out under vacuum, without purge gas. The material is placed in the adsorber C1 under vacuum at a pressure Pdes of between 0.01 and 0.20 bara (absolute bar), preferably between 0.05 and 0.10 bara. The evacuation is performed by closing the adsorber at one end by the valve V2 and connecting a vacuum pump P at the other end. The evacuation of the column C1 makes it possible to release the C0 ₂ absorbed by the adsorbent material. The stream rich in C0 ₂ is pumped from column C1 and discharged through line 7.

In a third variant, the action of a purge gas can be combined with the evacuation of the column. If the purge gas is water vapor at low pressure and low temperature, evacuation of the column avoids condensation of water in the column. This last case is particularly interesting in the case of industrial sites such as refineries or waste incineration sites where low-temperature steam is available. Indeed, in these cases, the energy of this steam at low temperature (generally less than 90 ° C or even lower than 80 ° C) is considered as not usable, it is lost and discharged in heat exchangers air-cooling type. In the present case, this vapor can be recovered and recovered at the regeneration stage by contacting the adsorbent material with water vapor under partial vacuum (for example at a temperature of between 50 ° C. and 90 ° C. preferably between 50 ° C and 80 ° C, and at a pressure between 0.1 and 0.7 bar absolute, preferably between 0.1 and 0.6 bar absolute), avoiding ai if a consumption of energy necessary for the production of steam.

 In cases where the desorption is carried out under vacuum, the desorption step must be followed by a repressurization step. One end of the adsorber is closed by a valve and the other end of the adsorber is brought into contact with the fumes. When the pressure in the adsorber has reached the Pads value, the adsorber is ready to go through a new adsorption and desorption cycle.

In a preferred embodiment, several adsorbers can be simultaneously treating the fumes. The number of adsorbers N _ads , which are simultaneously in adsorption mode, can be between 1 and 8, preferably between 2 and 6. The total number N of the adsorbers can be between 3 and 24, preferably between 4 and 4. and 12. the sequencing ensures that there is always at least _a N d _S of the N adsorbers in the adsorption mode while the remaining adsorbers are in desorption mode or repressurization.

Figure 3 shows an example of sequencing of C1 to C10 adsorbers. The cycle comprises the ADS adsorption, DES desorption and PP repressurization steps. Repressurization time t _rp is the cycle time t _cyc i _e divided by the number of adsorber N, in this case at 10. The beginning of the cycle of an adsorber is shifted by t with respect to _rp

 4

the previous adsorber. In our example, the adsorption time is _cycle .

The present invention provides an implementation of the contacting between the gas and the adsorbent material. According to the present invention, it is a question of producing a bed of vertical adsorbent traversed horizontally by the gas, which makes it possible to have passage section dimensions which may be much greater than those possible in conventional circular columns.

According to the invention, the bed of adsorbent material is vertical, that is to say that the passage section of the gas through the bed is perpendicular to a horizontal direction. In addition, the gas passes through the bed in a horizontal direction. The passage section of the gas corresponds to the surface of the bed through which the gas enters the bed. The passage section may be defined by projecting the bed surface on a vertical plane which is perpendicular to the horizontal direction of passage of the gas. What is commonly referred to as bed height corresponds, in the present invention, to the depth. Figures 4A and 4B show different embodiments of a vertical bed according to the invention. In FIG. 4A, the bed is a rectangular parallelepiped of depth P, of height H and of length L. The gas flows in the horizontal direction of the arrow G1 to cross the bed in its depth P. In this case, the section of gas passage is the dimension rectangle H and L, the surface of the rectangle being perpendicular to the horizontal direction G1. With reference to FIG. 4B, the bed is formed of an assembly of several rectangular parallelepipeds of height H, of depth P and of length L1, L2, L3, L4 and L5. The passage section of the gas is formed by a rectangle of height H and length L perpendicular to the direction G2 gas passage. The gas flows in the horizontal direction of the arrow G2 to cross the bed in its depth P. The input and output surfaces referenced A and B in FIGS. 4A and 4B are produced by means of grids which have the property of containing the adsorbent material and to be gas permeable. For example, Johnson grids can be used. The hatched surfaces in FIGS. 4A and 4B are solid and impervious to gas. Thus, the adsorbent material is disposed between the grids A and B and is contained by the hatched surfaces. Preferably, the material is in the form of particles, for example beads or extrudates, so that it can be easily introduced and removed from the space between the grids A and B.

The shape of the vertical beds makes it possible to use a large quantity of adsorbent material whose height H and / or length L is clearly greater than the depth P of the bed. According to the invention, the height H can be between 2 and 30 m and preferably between 5 and 10 m, the bed depth P of which is between 0.2 and 5 m and preferably between 0.5 and 3 m and whose length, L, can be very large, preferably greater than 5 m, or even greater than 10 m or even greater than 20 m. Since there is little construction stress along the length of the vertical bed, according to the invention, the L / H ratio can be large, preferably greater than 1 or 2, or even greater than 5. The dimensions of the bed, that is to say the length L, the height H and the depth P, may be chosen so as to circulate, through the first bed, a large flue gas flow, greater than 1000 Nm ³ / h, preferably greater than 1500 Nm ³ / h or even 2000 Nm ³ / h, and at a speed between 0.05 m / s and 1, 5 m / s.

 An implementation according to the invention of a vertical bed is shown schematically in Figures 5 and 6. The unit R is composed of two vertical beds L1 and L2 arranged in a chamber E, for example rectangular parallelepiped shape height Ha, of width La and depth Pa. In the enclosure E, the beds L1 and L2 are separated by a space E1, the entrance surfaces in the beds L1 and L2 communicating with the space E1. In addition, in the enclosure E, the exit surface of the bed L1 communicates with the space E2 and the exit surface of the bed L2 communicates with the space E3.

The adsorption unit R shown in FIG. 5 operates in adsorption mode. The gas to be treated arriving via the conduit GE is separated into different streams, GE1 and GE2, which are injected into parallelepiped-shaped columns. The column comprises two vertical beds L1 and L2 of adsorbent material. The gas GE1 is sent into the space E1 separating the two beds L1 and L2. Preferably, the two beds L1 and L2 are located symmetrically by compared to the gas injection system. The gas passes through each of the beds in a substantially horizontal direction perpendicular to the larger section of the bed. After passing through the bed L1, the gas opens into the chamber E2. After passing through the bed L2, the gas opens into the chamber E3. In the enclosures E2 and E3, the gas is respectively collected in outlet tubes GS1a and GS1b located on either side of the crossed beds and collected on a larger scale from the evacuation pipes GS.

The unit R shown in FIG. 6 operates in desorption mode. The purge gas arriving via the conduit GP is subdivided into several flows, the flows arriving through the conduits GP and GP1b which supply the unit R respectively in the speakers E2 and E3. For example, to inject the gas into the unit R, the same introduction systems can be used as those used for the evacuation of the purified fumes at the adsorption stage. The purge gas passes through the beds L1 and L2 and is collected in the space E1 located between the beds L1 and L2. The gas is evacuated from the space E1 of the unit R via the exhaust duct GD1, preferably distinct from the supply system of the adsorption phase. The fact of having different evacuation systems either purified fumes or purge gas loaded with C0 ₂ depending on whether one is in the adsorption or desorption phase, makes it possible to avoid back-mixing phenomena. in the pipes and in the empty zones of the reactors and thus makes it possible to avoid degrading the separation performance of the process (capture rate in the adsorption phase, C0 ₂ purity in the desorption phase).

 FIG. 7 represents the adsorption unit of FIG. 5 in which two beds whose shape is shown in FIG. 4B have been installed.

Preferably, a gas injection and collection system is used downstream and upstream of the beds in order to ensure the most homogeneous distribution possible. Indeed, if no injection system is used, a flow as shown in FIG. 8 is obtained. In FIG. 8, the space E2 between the beds L1 and L2 is fed by the conduit GE1 as proposed. FIG. 8 represents the gas flow lines in the adsorption phase. It is observed that if, at the bottom of the space E1, the current lines feed the bed homogeneously, it is not the same at the entrance of the space E Indeed, we observe on both sides two recirculation zones, often called dead zones, which will accumulate fluid during the adsorption phase, and which is always present during the desorption phase, which can strongly impede the purity C0 ₂ in the purge gas. It is therefore preferable to use gas distribution systems for example as shown in FIG. 9. It is a system composed of a supply pipe T which feeds a multitude of identical pipes T1, they -Same equipped with slots or O holes for distribution (injection of fumes or collection of purge gas loaded C0 ₂ respectively for the adsorption phases and desorption). The present invention can be used with a wide variety of such systems well known to those skilled in the art. The distribution system is installed in the space E2 between the beds L1 and L2.

 Figure 0 illustrates the flow of the gas dispensed by the system described with reference to Figure 9, through a bed of adsorbent material L1. The perforated tube system T1 effectively distributes homogeneously both to the injection on the left of FIG. 10 and to the collection on the right of FIG. 10.

The examples of digital applications presented below make it possible to illustrate the operation and the advantages of the method according to the invention.

Example 1

The adsorbent used for this example is a silica impregnated with the amine tetraethylenepentamine (see GD Pirngruber, S. Cassiano-Gaspar, S. Louret, A. Chaumonnot, B. Delfort, Energy Procedia 1 (2009) 1335). Its 348 K isotherm is represented by a Langmuir equation q = q ^■ ^ or Q = Q

l + bp ~ ^mt l + Bc

where p is the CO ₂ partial pressure (in bar), where c is the CO 2 concentration in mole / m 3, the adsorbed amount of CO 2 in moles / kg, and the CO 2 concentration in the adsorbed phase in moles / m 3 adsorbent. The values of the parameters b, q _its t, B and Q _sat are:

b = 116 bar ^"1 or B = 3.356 m ³ / mol

q _sat = 2.1 mol / kg or Q _sat = 2300 mol / m ³

The C0 ₂ diffusion coefficient in the particles of the silica is estimated D _c = 5 * 10 ^"1 m ² / s. In the type of silica used, the diffusion in the particles dominate over the other resistance to transfer. The relationship between the diffusion constant and the material transfer constant k is therefore, according to Gluckauf (Faraday Trans Soc 51, 1955, 1540).

 15

 k = -

R _p is the radius of the particle. To evaluate the performance of the adsorbent in a PSA process we have used a very simplified model of a PSA, proposed by Hirose (J Chem Eng Japan, 20 (1987) p 339). This model considers a PSA process in two stages: (i) a adsorption step at the pressure p _ads , a gas velocity that was empty of u _ads and a duration of t _ads , (ii) a desorption stage at the pressure p _of a purge gas velocity u of _the empty drum and a duration of t _of . The concentration of C0 ₂ in the feed is c ₀ . The analytical model of Hirose calculates, based on u _ads Udes reports and t _ads / es t _d, which is the number of transfer units required to achieve a certain average concentration of C02 in the outlet of the adsorber, d. The number of NTL transfer units ⁾ is defined as NTU = ^kl - ^£) - ^P - ^q: (1)

^U ads ^' 0

 k = material transfer constant (s-1)

 ε = porosity of the bed

 P = depth of the bed

 qO * = amount adsorbed at equilibrium (mol / kg)

NTU increases as the adsorption capacity of the adsorbent increases and when the material transfer constant increases. NTU is also proportional to the residence time in the adsorber (the ratio P / u _ads ). For a given adsorbent and a given temperature, k and q ₀ * are set and the main parameter for varying NTU is the residence time.

In the present case, the concentration of CO ₂ in the fumes is 15 mol%, the remainder is considered as an inert gas (N 2 or O 2). The total flue gas flow is 1.76 ^* 10 ⁶ Nm ³ / h. The adsorption and desorption are considered isothermal at 348 K. The desorption is carried out under vacuum at 0.3 bar. The porosity of the adsorber is ε = 0.37.

The capture efficiency of C0 ₂ is 1 - c- | / c ₀ . It is set at 90%.

Purge gas is water vapor. The speed was empty in desorption is equal to the speed was empty in adsorption (u _ads / u _d es = 1).

The ratio t _of / t _ad s is fixed at 1, 5. Therefore, there are two columns in desorption mode for a column in adsorption mode.

By setting _t / t _d s, the model calculates the value of Hirose NTU necessary to achieve the desired performance (90%).

Then the speed u _ads , the pressure drop and the particle diameter are calculated as a function of the depth of the bed, while respecting equation (1) and the additional fluidization stress of the adsorbent particles (the speed u _ads is limited to 80% of the fluidization rate).

The exercise is first conducted for a classical geometry of an adsorption column. The typical dimensions of an adsorption column are: height = 8 m, diameter = 8 m. Calculations show that we need 15 columns of this size to treat fumes. This figure greatly exceeds the number of columns typically found in current PSA processes. The total mass of adsorbent is approximately 31,500 liters. The pressure drop is 0.56 bar. According to conventional reasoning, the performance of the adsorption capture process depends on the residence time in the adsorber, that is to say, the ratio P / u _to d _S , which is equal to the ratio (total adsorber volume) / (smoke flow rate). Following this logic, the total volume of the adsorbers as well as the mass of adsorbent must remain constant to maintain the performance.

 But the calculations show that in a system that is constrained by the fluidization velocity, the mass of adsorbent decreases as the depth of the bed decreases. Table 1 indicates that the adsorption mass decreases from 31500 t to 19200 t when the depth of the bed is reduced from 8 to 3 m. The pressure drop also decreases which implies that the mechanical energy consumption for the compression of fumes upstream of the adsorption process decreases.

 Nevertheless, the number of columns necessary to treat the fumes becomes unreasonable (187 instead of 115) and the land use increases sharply.

 If the adsorbent is dispensed in an adsorber according to the invention, the number of adsorbers may remain below 17. The required area is generated by adjusting the horizontal dimension of the adsorbers (30 m). Land use is significantly lower than with vertical columns.

 When the depth of the bed is reduced to 2 m, the mass of adsorbent, the occupation of the soil and the loss of charge decrease again.

Table 1: bed depth (P), gas u u _ads velocity, particle diameter d _p , adsorption adsorption number Nads, adsorber dimensions (diameter for conventional geometry, vertical length x horizontal length for geometry according to the invention ), land use, total adsorbent mass, pressure drop (Δρ).

Example 2

The exercise of Example 1 is repeated for an adsorbent optimized in terms of capacity, regenerability and material transfer. b = 40 bar-1, q _sat = 3 mol / kg (r = 0.13), D = 6 ^* 10 ^-9 m ² s-1

Thanks to the very favorable isotherm of this optimized adsorbent, the adsorption process can be operated with a ratio Wt _to ds very low, that is to say, t _of / t _a ds = 1 -0.

 By increasing the diffusion characteristic constant, the gas velocity Uads can also increase compared to the previous example. However, when the bed depth is 8m, the pressure drop becomes greater than 2 bar, which is difficult to accept or at least generates a cost of compression too large.

 The pressure drop becomes reasonable when the depth of the bed is less than 4 m. Table 2 shows the calculations made for a bed depth of 3 m. With conventional adsorber columns, with a diameter of 8 m, which also poses problems of gas distribution, it takes 22 columns to treat the fumes. The land occupation is 1105 m2. 4 adsorbers of a geometry according to the invention, with a thickness of the equivalent bed (3 m) can treat fumes with a significantly lower land occupation. If the thickness of the bed is reduced to 2 or 1 m (which is difficult to envisage in a conventional geometry and makes the implementation according to the present invention very interesting) the mass of adsorbent and the land use decreases further. .

A second advantage of the new geometry is the fact that the constraint of the fluidization of the particles falls in particular by the possibility of maintaining the bed by grids type Johnson grid on either side of the bed. It is thus possible to reduce the particle size to improve the performance of the adsorber. The last line of Table 2 shows that a decrease in particle size below the fluidization limit reduces the adsorbent mass and the soil occupation, while the pressure drop remains acceptable.

Table 2: Bed depth (P), particle diameter dp, number of adsorbers in adsorption mode H _ads , total number of adsorbers N, dimensions of the adsorbers (diameter for conventional geometry, vertical length x horizontal length for geometry according to the invention ), land use, total mass of adsorbent, pressure drop

(Δρ).

 P uads dp Nads N Dimension Occ mass Δρ m m / s mm total soil column bar m2 t

art ant 3 1.00 2.5 10 22 o 8 1 105 1580 0.46

 2 0.85 2.3 13 28 o 8 1410 1240 0.24 invention 3 1.00 2.5 4 10 8 x 7.8 470 1800 0.46

 2 0.85 2.3 4 10 10 x 8.1 320 1480 0.25

1 0.62 1 .9 4 10 10 x 10.9 240 1 150 0.08

1 0.88 1 .6 4 10 10 x 8.1 160 770 0.20

Claims

1. A method for capturing a compound contained in a gas in which the gas is brought into contact with a material adsorbing the compound, said material being disposed in a first bed, characterized in that the gas is circulated in the first bed in a horizontal direction and in that the passage section of the gas in the first bed is perpendicular to said horizontal direction. 2. Method according to one of claims 1 and 2, characterized in that the passage section of the gas in the first bed is a rectangle having a height of between 2 and 30 meters and having a length at least equal to 5 meters. 3. Method according to claim 2, characterized in that the length is greater than the height. 4. Method according to one of the preceding claims, characterized in that the depth of the first bed traversed by the gas is between 0.2 and 5 meters. 5. Method according to one of claims 1 to 6, characterized in that the material adsorbing the compound is disposed in the first bed and in a second bed separated by a space of the first bed, the passage section of the gas in the second bed being perpendicular to said horizontal direction and in that the gas is injected into said space. 6. Method according to claim 6, characterized in that the gas is injected into said space by a gas distributor to homogenize the distribution of gas in the first and second bed. 7. Method according to one of the preceding claims, characterized in that the gas is brought into contact with the material adsorbing the compound at a temperature between 20 ° C and 250 ° C and at a pressure between 1.1 and 5 absolute bars. 8. Method according to one of the preceding claims, characterized in that one chooses the length, the height and the depth to circulate, through the first bed, a gas flow rate greater than 1000 Nm ³ / h, to a speed between 0.05 m / s and 1.5 m / s. 9. Method according to one of the preceding claims, ^{'characterized} in that regenerates the adsorbent material by bringing the material into contact with steam at a temperature between 50 ° C and 90 ° C and a pressure between 0.1 and 0.7 bar absolute. 10. Method according to one of the preceding claims, characterized in that the gas consists of combustion fumes and the compound is C0 ₂ .