WO2007003778A1 - Method for processing gaseous wastes containing organic compounds by capturing or condensation - Google Patents

Abstract

The invention relates to a method for processing and depolluting gaseous wastes containing volatile organic compounds (COV) and/or polycyclic aromatic hydrocarbons (HAP) by adsorption and/or condensation by powder mineral particles, consisting (c) in introducing powder alumna particles in a gaseous waste flow, wherein at least one part of the COV and / or HAP are deposited on the particles, (d) in separating the powder alumna particles loaded with the COV and / or HAP from the processed gaseous wastes. The inventive method is characterised in that (1), prior to the stage (c) for introducing the powder alumna particles, at least one stage for pre-cooling the treatable wastes in inserted, and (2) at the stage (c) for introducing the powder alumna particles, the used particles are cooled prior to the introduction thereof.

Description

 Process for the treatment of gaseous effluents containing organic compounds by capture or condensation

Field of the invention

The invention relates to a method for treating gaseous effluents containing as pollutants, organic compounds of very diverse molecular masses, from heavy to light, which are captured, adsorbed by and / or condensed on a powdery mineral support. This adsorption and / or condensation depollution process can treat effluents from carbonaceous product production units, and more particularly furnaces for producing pre-cooked anodes.

State of the art:

Furnaces for cooking carbon products, such as anode baking ovens used in primary aluminum production plants, emit fumes loaded with Volatile Organic Compounds (VOCs). These VOCs come from the raw materials used (typically pitches and cokes) and may include molecules with a fairly wide molecular weight distribution. They contain molecules of light compounds and heavy compounds (including soot) that can condense in the form of "tars". The VOCs emitted by the baking furnaces of carbonaceous products comprise compounds whose vapor pressures (typically greater than 0.13 Pa at 0 ^° C., ie greater than 10 Pa at 25 ° C.) and boiling temperatures (typically less than about 15O ⁰ C at 200 ⁰ C) promote their presence mainly in gaseous form. VOCs emitted by furnaces for cooking carbon products also include Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are compounds that all have one or more aromatic rings (or rings) that are mostly classified as carcinogens. They are part of so-called "semi-volatile" compounds since their boiling temperatures (typically between 120 ° C and 35O ⁰ C) allows their presence (under "normal" conditions of temperature and pressure) in two forms: gaseous and condensed. They will be particularly sensitive to the evolution of the temperature which has a direct influence on the ratio between the gaseous fraction and the condensed fraction of each of these compounds. Among the PAHs, sixteen molecules (collectively referred to as OSPARCOM HAP ₁₆ ) are particularly targeted by the regulations on environmental protection and occupational health:

Three Cycle Molecules: Phenanthrene, Anthracene Four Cycle Molecules: Fluoranthene, Pyrene, Benzo (e) Fluorene,

Benzo (b) fluorene, Benzo (a) anthracene, Chrysene Five-ring molecules: Benzo (b) fluoranthene, Benzo (j) fluoranthene,

Benzo (k) fluoranthene, Benzo (e) pyrene, Benzo (a) pyrene, Dibenzo (a, h) anthracene, six-ring molecules: Indeno (1,2,3, c, d) pyrene, Benzo (g, h , i) perylene,

Dibenzo (a, e) pyrene

The fumes emitted by the furnaces to cook carbonaceous products thus constitute a nuisance and a risk for the health and must be cleaned up (purified) in order to reduce the rate of VOC or PAH residual in the gas which will be finally rejected in the atmosphere.

Processes in furnaces for cooking carbon products are usually cyclic. A typical duration of a cycle is of the order of 24 to 30 hours. Cooking does not necessarily proceed at a constant temperature and involves chemical reactions in the carbonaceous mass to be cooked. Therefore, neither the temperature nor the quantity nor the composition of the VOCs or PAHs emitted are constant during a cooking cycle. The present invention is designed to depollute fumes entering the pollution control device with a maximum temperature Ti not exceeding 200 to 22O ⁰ C. During certain phases of the oven cycle, the inlet temperature T ₁ in said depollution devices can be significantly lower, up to 80 ^° C.

According to the state of the art, the effluents or fumes loaded with VOCs or PAHs from the furnaces to be cooked anodes are typically treated by direct injection of alumina, the alumina having the dual function of condensation nucleus and adsorbent: the alumina grains condense by cooling the light (volatile) fractions of the VOCs or PAHs, and fix the heavy fractions by adsorption. After this depollution action, the treated gaseous effluents are separated into two solid / gas fractions: the solids (aluminas already loaded with VOCs or condensed PAHs and soot) are reintroduced in part (with fresh alumina) in the gaseous effluents to be treated to capture the organic compounds present, and another part is evacuated. The treated gases are discharged into the atmosphere via a chimney. As a separating means, for example, a bag filter is used, the separated alumina being collected in a hopper.

However, the state of the art, based on the principle of condensation of organic compounds (volatile at the flue gas temperature) condensable and on the principle of the adsorption of organic condensates by a suitable inorganic powder carrier (alumina), is not fully satisfactory because the treated gaseous effluents discharged into the atmosphere are not sufficiently cleaned up to meet, in particular, the standards of environmental protection. This deficiency relates more particularly to the most volatile compounds.

The patent application FR 2 836 059 (National School of Industrial Engineering and Mines of Nantes) describes a method of removal by condensation on an inert powder carrier (such as alumina), condensable gaseous pollutants, for example VOCs, present in hot gaseous effluents to be purified continuously. This process consists in creating a deposit (liquid or solid) of condensable pollutants on the surface of the pre-cooled, fluidized inert particles in the stream of gaseous effluents to be cleaned.

EP 0228 373 B1 (A. Ahlstrom Corporation) describes a method for purifying gases containing condensable pollutants in a fluidized bed zone provided with exchange (cooling) surfaces, this process consisting in reintroducing into the flow of gaseous effluents to be treated with cooled solid particles from the gas / solid separation of the fluidized bed. The reintroduction of cooled particles in the gaseous effluents to be treated takes place in the fluidization zone upstream of the cooling surfaces, in such a way that the condensation of the condensable effluents is effected before the cooling surface zone.

US Patent 5,307,638 (Messer Griesheim) discloses a device and a method for recovering solvents present in hot gases to be purified, comprising contacting countercurrent in a treatment device, hot gases polluted with objects. circulants (such as steel spheres) which are previously strongly cooled by liquid nitrogen. The recovered solvents are extracted from the treatment device, while the exhaust gases are evacuated by passing through the cooling zone and the circulating objects leave the device, are cooled again and reintroduced into said treatment device.

US Patent 3,977,846 (Aluminum Company of America) discloses a method of cleaning gaseous effluents from an electrode baking furnace comprising a step of contact with a fluidized bed of cooled alumina particles.

The patent application CA 2035212 (Klaus Jungk and Ulrich Huwel) describes a process for the depollution of gaseous effluents from anode baking furnaces, in which the gaseous effluents to be cleaned up and containing tars and other organic compounds to be removed, are introduced into an adsorption chamber, in which they are mixed with particles of alumina on which are deposited (by condensation) the pollutants present in the gaseous effluents. The alumina particles loaded with pollutants extracted from the gaseous effluents are then introduced into a combustion chamber in which the pollutants are eliminated by combustion (between 700 ^° C. and

900 ° C).

Patent GB 1 448 369 (Aluminum Company of America) describes a process for recovering organic compounds present in a gas to be cleaned, which consists in treating in a fluidized bed the gaseous effluents polluted with solid particles (alumina) kept in suspension in the said effluents and maintained at a temperature at most equal to the condensation temperature of the organic compounds by introduction of a cooling liquid (water) in the fluidized bed, the liquid having the property of covering at least momentarily each particle of a thin film.

US Pat. No. 4,966,611 (Custom Engineered Materials) describes a device and method for adsorbing VOCs by adsorbent materials, and a process for regenerating these adsorbent materials after adsorption and combustion of post-desorption VOCs. In a first cycle, the gas stream containing VOCs is contacted with adsorbent materials until saturation of said materials. During this adsorption cycle are regenerated adsorbent materials already saturated in a previous cycle by means of a hot gas stream generated in the combustion chamber burning the desorbed VOC adsorbents materials. The temperature of the gas flow implemented in the regeneration being high, it is lowered by spraying water before this gaseous flow brought to the proper temperature desorbs the adsorbed VOCs to burn them.

The patent EP 0 668 343 (Foster Wheeler Energy Corp.) describes a process for purifying and cooling hot gaseous effluents containing gaseous pollutants (sulfur compounds and corrosive gases such as HCl, CO, NH3). The polluted gas stream is treated in a circulating fluidized bed reactor, comprising solid particles adsorbing gaseous pollutants, suspended in the polluted gas stream. At the outlet of the treatment reactor, a gas / solid separation is practiced. The solid fraction (adsorbent solid particles) is cooled in a heat exchanger and reintroduced into the gas stream to be treated in the reactor.

Patent EP 0 368 861 B1 (A. Ahlstrom Corporation) describes a process for treating industrial gases containing gaseous pollutants consisting in placing in a treatment chamber, the industrial gases polluted in contact with solid particles in a fluidized bed, to be cooled. the fluidized medium (polluted industrial gases and solid particles) by means of heat exchange present in the treatment chamber, then to carry out a gas / solid separation by reintroducing into the treatment chamber the cooled particles and a part of the cooled exhaust gases, such that the temperature of the recycled solid particles and recycled waste gases is lower than the temperature of the polluted industrial gases to be treated. The gases to be treated enter the device with a very high temperature, of the order of 1000 ^° C. to 1300 ° C.

Processes are also known for cleaning gaseous effluents from aluminum-producing cells by igneous electrolysis using pulverulent alumina intended to at least partially capture the HF contained in these effluents. The patent application FR 2,848,875 (Aluminum Pechiney) describes a process involving a step of cooling the effluents by vaporization of droplets of a cooling fluid. Similar processes are described in French patent application FR 2,259,164 (Vereinigte Aluminum Werke AG) and in US Pat. No. 4,065,271 (Metallgesellschaft). More generally, patent application EP 0 736 321 A1 (Danieli & C. Officine Meccaniche) describes a process for capturing halogenated molecules using activated carbon. Each of these documents describes a means of cooling the medium containing the gaseous effluents to be cleaned. However, there is an important demand to further reduce the residual VOC content of the gases resulting from such a process of depollution, this request being justified by the requirement of respect for the environment and compliance with national and international regulations in the field. material.

OBJECT OF THE INVENTION

Therefore, the objective of the invention is to improve the efficiency of the depollution of gaseous effluents containing VOCs, and more particularly PAHs ₅ and in particular the depollution of gaseous effluents produced in the baking furnaces of carbon products. It is desirable that this new process can be implemented without technological disruption of current flue gas treatment facilities.

Indeed, given the size of effluent purification devices in an industrial environment, it is desirable that this improvement can be obtained in an existing device that requires as little modification as possible. More particularly, it is desirable to have a method that can be implemented in a device already installed according to the state of the art which will be slightly modified ("retro-fit"), or in a new device that will be a variant of an existing device to be able to reuse a maximum of existing components and to minimize the modifications to be made to the plans of the set. As such, it should be noted that in most existing installations in igneous electrolysis aluminum production plants, the temperature of the gaseous effluents emitted by the anode baking furnaces enter the treatment device with a temperature T ₁ between 80 ⁰ C and 22O ⁰ C.

This is the reason why it was chosen to start from a process in which this depollution is carried out by means of mineral particles introduced into the flow of gaseous effluents, by adsorption and / or condensation of the VOCs or PAHs present in the effluents. gaseous directly by and on the particles; it has also been chosen to recirculate the pulverulent particles of alumina, charged, in the gaseous effluents to be cleaned.

Thus, to achieve these objectives, it has become apparent that the lowering of the temperature must not be done by the implementation of a single cooling means but must be done by the combination of several means arranged between them in such a way that this lowering of the temperature of the gaseous effluents containing organic compounds (volatile or liquid at the temperature of said effluents) is carried out in successive stages, in cascade, to reach the temperature of approximately 80 ^° C. at the time of treatment with the powdery mineral particles.

Objects of the invention

All these objectives are achieved by the method and device according to the present invention.

A first object of the present invention is a process for the treatment of pollution control of gaseous effluents containing volatile organic compounds (VOCs), and in particular Polycyclic Aromatic Hydrocarbons (PAHs), adsorption and / or condensation by pulverulent particles of alumina which combines at least two cooling means: a cooling zone by spraying a liquid, and a cooling zone by injection of a powdered solid cooled in a fluidized bed. Third cooling means, which is optional, is provided by the injection of a gaseous fluid (dilution) prior to the injection of a solid powdery solid.

The process according to the invention is a process for the treatment of the depollution of gaseous effluents containing volatile organic compounds (VOCs) and / or polycyclic aromatic hydrocarbons (PAHs), by adsorption and / or condensation by powdery mineral particles. It comprises successively:

(c) a step of introducing pulverulent mineral particles of alumina into the effluent gas streams, during which said step, at least a portion of said VOC and / or PAH are deposited on said particles;

(d) a step of separating powdery mineral particles loaded with VOCs and / or PAHs and treated off-gases; and said process is characterized in that (1) before the step of introducing the pulverulent mineral particles of alumina, (c) is inserted a step (a) of pre-cooling the effluents to be treated, by injection of a liquid in said gaseous effluents; optionally followed by a step (b) of diluting the gaseous effluents by the introduction of a gaseous fluid; (2) in the step of introducing pulverulent particles of alumina (c), the particles used are cooled before being introduced;

(3) in the step of separating the cleaned off-gases and the pulverulent particles of alumina (d), the off-gases are either removed, for example discharged to the atmosphere, or can, if the dilution step (b) ) is present, partly to be recirculated in said dilution step (b).

A second object of the present invention is a device for the removal of gaseous effluents containing volatile organic compounds (VOCs) or polycyclic aromatic hydrocarbons (PAHs) comprising

(i) at least one cooling zone of said gaseous effluents by injection of a liquid (called "liquid injection zone"); (ii) at least one cooling zone of said effluents by injection of a gaseous fluid (called "gas injection zone"); (iii) at least one cooling zone (called "conditioning zone") of pulverulent particles of alumina;

(iv) at least one zone for cooling and decontaminating said effluents by contact with pulverulent particles of alumina (called "pollution control zone"), (v) at least one separation zone of the pulverulent particles of alumina and effluents gaseous treated (so-called "separation zone").

This device must allow, if necessary, the simultaneous use of cooling by injection of a liquid and by gas injection.

A third object of the present invention is the use of the method or the device according to the invention for the depollution of cooking furnace effluents of carbonaceous products, and especially of anode baking furnace. This use can be advantageously done in an igneous electrolysis aluminum production plant, so as to recycle at least a portion of the alumina charged with VOC and / or PAH from the separation zone or the separation step. (d) in the igneous electrolysis process.

Brief description of the figures Figures 1 and 2 show an embodiment of the device according to the invention.

1 Entry of effluents

2 Liquid injection area

21 Injector (spray) of water

22 Water inlet

23 Air inlet (for spraying)

24 Purge

3 Gas injection area

31 Air inlet (for dilution)

32 Injection dilution gas flow regulator

33 Exit to the depollution zone

34 Exit to the depollution zone in parallel

4 Pollution control area

42 Fresh alumina inlet

43 Cooled alumina inlet

5 Separation area

51 VOC alumina recovery zone

52 Means of separation (eg bag filter)

53 Evacuation of separated alumina for recycling

54 Evacuation of alumina charged with VOCs or PAHs

55 Outflow of the effluents removed

6 Conditioning area

62 Fresh alumina inlet (alternative to 42)

65 Packing hopper

650 Cooling liquid outlet

651 Refrigerant inlet

652 Fluidizing air inlet

653 Evacuation of cooled alumina

654 Cooling coil

655, 656, 657 1 ^st, 2 ^nd, 3 ^rd cooling stage

658 Spilled alumina

659 Fluidizing fabric

7 Fan

8 Exit of cleaned effluent (eg chimney)

Figure 1 gives an overview of a device according to the invention. Figure 2 shows an embodiment of the conditioning hopper (65) with three cooling stages.

Figure 3 shows the results of a numerical simulation of an embodiment of the method according to the invention. The abscissa represents the temperature of the alumina before reinjection in the depollution zone. The temperature of 70 ^° C. (marked by a vertical bar) corresponds typically to the state of the art (no cooling). The ordinate represents the content of treated effluents (in mg / Nm ³ ) in naphthalene (left scale) and phenanthrene, anthracene or fluoranthene (right scale). Detailed description of the invention

The process according to the invention applies to any pollutant that can be condensed under process temperature conditions, from the heaviest to the lightest, but it is particularly targeted at VOCs and more particularly at PAHs. H is based on a fine control of the temperature and its evolution during the various stages, from the entry of effluents into the treatment plant to the outlet of the purified effluents.

The effluents to be decontaminated enter the device through an inlet (1) at a temperature Ti. In a first step (a), the temperature of the gaseous effluents to be decolled advantageously is lowered to a value T ₂ of between approximately 90 ° C. and approximately 95 ^° C., typically by water injection, and typically in the form of droplets. . The liquid, typically water, is injected, advantageously by at least one spray nozzle (21) into a cooling tower. It is preferable that the amount of water injected be adjusted so that the vaporization of the water is complete. Indeed, it is preferred to avoid the condensation of water at any point in the pollution control system downstream of the water injection point (21), since this may promote the corrosion of the components of the device, for example sheaths, and this is especially so since the water is capable of dissolving gases such as SO ₂ , HCl, CO ₂ or possibly NH 3 which form corrosive aqueous solutions. For the same reason, it is preferred that the temperature T ₂ of the aqueous effluents at the end of this first step does not fall below 9O ⁰ C. It is preferable that this temperature remains constant, in order to avoid as much as possible the condensation of water or corrosive media in cold points of the installation downstream of the liquid injection zone (2), and in particular in the depollution zone (4). Advantageously, the vaporization rate of the droplets is controlled by means of a detector located in the liquid injection zone (2) or downstream thereof.

The amount of water injected must be adjusted according to the temperature T ₁ and the amount of fumes to be treated; this temperature T ₁ and this quantity vary according to the cycle of the furnace which generates said fumes. It may happen occasionally that the fumes are already at a relatively low temperature, around T ₁ = 95 ° C, which may require the temporary stop of the injection of water. It is possible, in the context of the present invention, to control and regulate the temperature of the injected water. However, in practice, it will only be in exceptional cases that the additional investment and exploitation costs that this entails will be considered justified.

The liquid injection zone (2) may or may comprise a cooling tower of known type. For example, it is possible to circulate the effluents to be treated in a venturi and all or part of the fluid droplets are injected into the venturi or upstream of the venturi. This makes it possible to accelerate the vaporization of the droplets in contact with the hot effluents. It is possible to inject some of the droplets downstream of the venturi.

In a second step (b), the gaseous effluents from the liquid injection zone (2) are diluted in a gas injection zone (3) by the introduction of a gaseous flow. The gaseous dilution fluid may be air, or gaseous effluents released from the separation zone and which are reinjected into the circuit, or a mixture of the two. In practice, the volume of gas flow injected should not unacceptably increase the volumes of gas to be treated by the separation zone, as this requires an oversizing of the capacity of the separation means. In a typical embodiment of the process according to the invention, the gaseous volume injected does not exceed about 10% by volume of the volume of effluents to be treated.

This dilution has the effect of cooling the effluents. In a typical embodiment, the temperature of the effluents decreases by about 5 to 10 ^° C. in the gas injection zone, reaching at the outlet of the gas injection zone a temperature T ₃ of at least 80 ° C. C. Advantageously, T ₃ is between 80 and 90 ^° C. The installation, and in particular the separation zone (5), will be dimensioned so as to accommodate this additional flow rate.

Simultaneous use of steps (a) and (b) is not always necessary, but step (a) is necessary, and if both steps (a) and (b) are used, step (a) is ) must precede step (b). The need to use step (b) in addition to step (a) depends on the temperature T ₂ of the effluents leaving the liquid injection zone (2). When the temperature T ₂ is very low, step (b) can be omitted.

In practice, it is found that the temperature of the effluents generated by the furnace varies according to the thermal cycle of the furnace. The temperature T ₁ is therefore subject to strong variations, especially at the beginning and at the end of the thermal cycle. It typically varies between 80 ⁰ C and 22O ⁰ C. Advantageously, the use of precooling means is adjusted according to the thermal cycle of the furnace that generates the effluents to be treated; they can be used permanently or intermittently. In a preferred embodiment of the method according to the invention, step (a) is used permanently, and adds, if necessary, step (b). Paradoxically, it is often particularly practical to add the injection of a gaseous fluid when T ₂ is not constant and low enough. The inventors have indeed found that the temperature T ₂ is more difficult to stabilize when the difference T ₂ - T ₁ is low. However, it is important that the thermal conditions in the depollution zone and in the separation zone are constant.

In a third step (c), powdery mineral particles are introduced into the flow of pre-cooled effluents. Advantageously, the pulverulent mineral particles injected are alumina. According to the invention, their temperature at the time of injection must not exceed that of the gases flowing in the depollution zone; advantageously, these particles are cooler than said gases. However, the temperature of said particles must not be less than 0 ^° C. in order to avoid the risk of condensing the ambient humidity and forming frost. By way of example, a temperature of approximately 0 ^° C. of the particles before their injection leads to a lowering of the temperature of approximately 20 ^° C. This facilitates the condensation of the heavy fractions, and in particular PAHs, including a drop in a factor of 3 to 4 can be observed with the process according to the invention compared with a method according to the state of the art.

The quantity of mineral particles, their particle size and the contact conditions are chosen so as to optimize the depollution, and in such a way that the temperature T ₄ at the separation zone (55) does not fall below a value which is in practice between about 70 ^° C. and 90 ^° C., and preferably between 75 ° C. and 85 ^° C. The high value is limited by the depollution efficiency, since the adsorption and the condensation of the VOCs or condensable PAHs on the mineral particles decrease as the temperature increases. The low value is limited by the desire to avoid as much as possible the condensation of corrosive products in the cold spots of the installation. In practice, the limiting parameter is often the SO ₂ dew point in contact with moisture. A temperature of 80 ^° C. generally gives good results. In an advantageous embodiment of the invention, the velocity of the gases at the injection point of the cooled alumina can vary from about 2 m / s to 35 m / s, and is preferably between 8 m / s and 20. m / s. The low value is limited by the size and the mass of the powdery mineral particles, the high value is limited by the dimensioning of the device.

Any alumina of the type and particle size of those used for the Hall-Héroult electrolysis process may be suitable. This alumina is known as Smelter Grade Alumina (SGA). In an advantageous embodiment of the invention, the quantity of alumina injected into the gases at the level of the reactor is approximately 100 to 350 g / Nm ³ of gas, and the average particle size of the grains of fresh alumina injected into the gases. is about 70 to 100 μm. By way of example, here is the particle size distribution of the fresh alumina measured during a test on a pilot plant (all the percentages refer to the mass):> 150 μm = ^> 5,13%

> 106 μm => 40.46%

> 75 μm => 80.82%

> 53μm => 92.69%

> 45μm => 95.66% <45μm => 4.34%

<20u.m => N / A

In a fourth step (d), the powdery mineral particles loaded with VOCs or PAHs are separated from the gaseous effluents treated. Any suitable means of separation may be suitable, such as a bag filter, known as such, associated with a collection hopper. If a bag filter is used, the filtration rate is advantageously between 1 and 2 cm / s.

In the interest of a good depollution efficiency, the contact time between the particles and the effluents must not be too short. Thus, the average residence time of the gases in the depollution zone (4) and in the separation zone (5) is of the order of 0.5 to 3 seconds, and preferably between 1.5 and 2.5. seconds. The depollution can be done in a single central decontamination zone or in several parallel depollution zones; in the latter case, there is provided at the end of the gas injection zone (3) an outlet (34) towards the zone or zones of depollution placed in parallel.

In order to maintain stationary conditions in the separation zone (5), it is necessary to replace the quantity of mineral particles extracted (separated). This is advantageously done by introducing fresh particles. For example, we use to

10 to 15 t of alumina circulating in the depollution zone and about 1.5 t / h of fresh alumina. "Fresh" particles are understood here to mean particles with little or no

VOCs or PAHs condensed or adsorbed, and in particular particles that have not yet been used in the pollution control area (4). The extracted alumina particles can be used in an igneous electrolysis process for the production of aluminum. During this process, the tars condensed on the particles are destroyed.

As indicated above, the powdery mineral particles are alumina, and must have a lower temperature than the effluents with which they come into contact. Advantageously, they are cooled to a temperature between 0 ^° C. and

50 ⁰ C, and preferably between ⁰ C and 30 ⁰ C. This cooling takes place in a zone called "conditioning zone" (6) which comprises at least one cooling means.

Any means for cooling the powdery mineral particles may be suitable. In an advantageous embodiment of the invention, the conditioning zone comprises at least one conditioning hopper (65). According to an advantageous embodiment, the powdery mineral particles are cooled to a temperature of between approximately -0 ^° C. and approximately 20 ^° C. It is preferred that the temperature be at least 5 ° C. in order to avoid crystallization (gel ) water vapor on the alumina and on the inner walls of the device.

If several separation elements are provided, the packaging can be done in a common packaging zone to all the separation elements, or in a central packaging unit.

In an advantageous embodiment, the conditioning hopper (65) is placed below the separating means (52) and comprises coils (654) in which a refrigerant circulates (see FIG. 2). The fluidized bottom of the hopper can be divided into several stages (655, 656, 657) in which the slurry passes successively through a spill device (658); for example, three floors give a good result. A big part or even the entire fluidization surface of this hopper is covered by the refrigeration network. The refrigerant circulates in the opposite direction of the recycled alumina. From the last stage (657) (ie the coldest stage), the recycled alumina, which can reach a temperature of the order of 0 ^° C., leaves the conditioning hopper (65) via an outlet (653). ) and is reinjected (through a conduit 43) into the effluents to be treated, upstream of the pollution control zone (4) or (as in the embodiment of Figure 1) directly in this zone.

The fresh alumina can also be conditioned in temperature. For example, it may be simply mixed (after being conveyed through a conduit (62)) to recycled alumina at the first cooling stage (655) of the conditioning hopper (65); alternatively, it may be injected independently upstream of the depollution zone, or (as in the embodiment of Figure 1) directly in this zone (through a conduit 42).

VOC or PAH-laden alumina can be used as a raw material in the igneous electrolysis process (Hall-Héroult process). If the oven to cook carbonaceous products that generates the fumes to be cleaned up is an oven to cook the anodes used in the primary aluminum production plants, it is possible to return the alumina charged with VOCs or PAHs to the electrolysis tanks. (via a transport) and reuse it, preferably mixed with alumina from other sources (for example fresh or fluorinated alumina), possibly after intermediate storage in a storage silo. The adsorbed "tars" will be destroyed (oxidized) directly in the bath of electrolyte and molten aluminum then reduced to CO ₂ and CO.

Test n ° 1

Measurements have been made on a pilot device according to the invention that processes fumes from an anode baking oven of the type used in a primary aluminum production plant. To cool the fumes, the water spray and the air dilution were combined. The results are shown in Table 1. Three different temperatures were explored.

Table 1

The thermodynamic calculations applied to PAHs indicate that the injection of recycled fresh alumina, at the rate of 250 g / Nm ³ of gas to be treated, cooled to 0 ^° C. in the filter reactor, causes an overall cooling of the gas mixture. / alumina ratio of about 2O ⁰ C. the temperature drop associated with the saturated vapor pressure PAH OSPARCOM 16, causes the condensation of a portion of the volatile phase and concentrations estimated output of the dry treatment is divided by a factor 3 or 4 compared to the same treatment without cooling of the alumina.

Test n ° 2

It is known that the effluents of an anode baking furnace consist mainly of PAHs, of which phenanthrene, anthracene and fluoranthene alone account for more than 65% by weight of the condensed tar emissions. Using the known physicochemical properties of these compounds (Tébuiiitto Tfusio _n j _n> vapor pressures at 2O ⁰ C ^and / _or 25 ° C), developed a mathematical model based on the law of Dupre:

etLθg P _steam = - A / (T + C) + B with A

and

C = 18 - 0.19 Tebuiiition if 125K <T _boiling in <400K and

C = 60 - 0.294 Tbuiiition If Tbuiiition> 400K and the laws of thermodynamics. This model makes it possible to describe, for the process according to the invention as well as for the processes according to the state of the art, the influence of a cooling by alumina on the emissions of these compounds at the end of the process of treatment. The following parameters were used for this calculation: • T ₂ (after cooling by water spray) = 105 ⁰ C Concentration of pollutant in the gases to be treated: 300 mg / Nm of naphthalene,

6 mg / Nm .3 of phenanthrene 1 mg / Nm ³ of anthracene 6 mg / Nm ³ of fluoranthene • Cooling temperature of the alumina before reinjection in the gases to be treated

(depollution zone 4) ranging from 5 ° C. to 70 ^° C.

(70 ^° C. corresponding to the state of the art, that is to say without a cooling device for alumina)

The results are plotted in FIG. 3. It can be seen that the process according to the invention makes it possible to divide by three the discharges of certain pollutants (in particular those of naphthalene, phenanthrene, anthracene and fluoranthene) by the simple benefit of cooling. alumina at about 5 ° C before reinjection into effiuents to be treated.

Advantages of the invention

The temperature at which the gaseous effiuents contaminated by VOC or HAP are processed, according to known methods, for alumina, is not sufficiently low (110 ⁰ C - 115 ° C) to condense and adsorb all organic compounds hot. The method according to the invention makes it possible to lower this temperature, while avoiding a too low temperature likely to lead to the condensation of corrosive liquids. The process according to the invention improves the depollution efficiency. It can be installed on existing pollution control installations with relatively small modifications, which are compatible with the most expensive components (eg the flue gas collection system, the cooling tower, the bag filter, if one or more of these components are already installed).

Claims

claims 1. Process for the treatment of pollution control of gaseous effluents containing volatile organic compounds (VOCs) and / or polycyclic aromatic hydrocarbons (HAP), by adsorption and / or condensation by pulverulent particles of alumina, said process comprising successively: (c) a step of introducing pulverulent particles of alumina into the streams of gaseous effluents, during which said step at least a portion of said VOCs and / or PAHs are deposited on said particles; (d) a step of separating powdery mineral particles loaded with VOCs and / or PAHs and treated off-gases; said method being characterized in that (1) before the step of introducing the pulverulent particles of alumina (c), is inserted a step (a) of pre-cooling the effluents to be treated, by injecting a liquid into said gaseous effluents; optionally followed by a step (b) of diluting the gaseous effluents by the introduction of a gaseous fluid; (2) in the step of introducing pulverulent particles of alumina (c), the particles used are cooled before being introduced; (3) in the step of separating the cleaned off-gases and the pulverulent particles of alumina (d), the off-gases are either removed or can, if the dilution step (b) is present, partly be recirculated in said dilution step (b). 2. Method according to claim 1, characterized in that it comprises successively stages' (a), (b), (c) and (d). 3. Method according to any one of claims 1 or 2, characterized in that the temperature T ₄ effluents at the end of step (c) is between 70 ° C and 90 ^° C, andpreferentially between 75 ° C and 85 ° C. 4. Method according to any one of claims 2 or 3, characterized in that the temperature T ₂ of the effluents at the end of step (a) is between 9O ⁰ C and 95 ⁰ C, and the temperature T ₃ of effluents at the end of step (b) is between 80 ^° C. and 90 ^° C. 5. Method according to any one of claims 1 to 4, characterized in that the pulverulent particles of alumina are cooled before introduction at a temperature between 0 ⁰ C and 50 ⁰ C, and preferably between 0 ⁰ C and 30 ⁰ C, and even more preferably between 0 ⁰ C and 20 ⁰ C. 6. Method according to any one of claims 1 to 5, characterized in that the alumina particles loaded with VOC and / or PAHs from the separation step (d) are at least partially reintroduced in step (c). ). 7. Method according to claim 6, characterized in that said particles of alumina loaded with VOC and / or PAHs from the separation step (d) are cooled before reintroduction in step (c). 8. Device for the removal of gaseous effluents containing volatile organic compounds (VOCs) and / or polycyclic aromatic hydrocarbons. (PAHs). comprising (i) at least one cooling zone of said gaseous effluents by injection of a liquid (called "liquid injection zone"); (ii) at least one cooling zone of said effluents by injection of a gaseous fluid (called "gas injection zone"); (iii) at least one cooling zone (called "conditioning zone") of pulverulent particles of alumina; (iv) at least one zone for cooling and decontaminating said effluents by contact with pulverulent particles of alumina (called "pollution control zone"), (v) at least one separation zone of the pulverulent particles of alumina and effluents gaseous treated (so-called "separation zone"). 9. Use of the method according to any one of claims 1 to 7 or the device according to claim 8 for the decontamination of effluents from a baking furnace of carbon products, and in particular an anode baking oven. 10. Use according to claim 9 of the process according to claim 7 or the device according to claim 8 in an igneous electrolysis aluminum production plant, so as to recycle at least a portion of the VOC-laden alumina and / or PAH from the separation zone or the separation step (d) in the igneous electrolysis process.