FR2836059A1 - Continuous recovery process, e.g. for volatile organics, involves condensing or solidifying fraction from gaseous flow - Google Patents

Abstract

Recovery of fraction involves condensing it as a liquid film or solidifying it as a solid deposit on the surface of inert particles (3) which have been pre-cooled and maintained at a temperature below that of the mixture. The inert particles form a fluidized bed under the action of the gas flow, part of which is evacuated from the top part of the fluidization reactor (10) and then recirculated. The speed of gas flow is at least equal to the minimum fluidization speed for the particles loaded with the condensed or solidified fraction. The fluidized bed is a turbulent recirculating bed or a circulating bed. The part of the particles which is recirculated undergoes a stage of separating the liquid film or solid deposit from the particles by heating until the molecules forming the film or deposit are vaporized. The particles are then re-cooled and reintroduced into the fluidized bed.

Description

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 The present invention relates to the field of methods for reducing the emissions of volatile organic compounds and more particularly to a process for the continuous recovery of a fraction of a gaseous mixture, by condensation, and its use in the depollution of gaseous effluent charged with volatile organic compounds.

 The reduction of emissions of Volatile Organic Compounds (VOCs) from industrial activities (for example activities using paints, inks ...) which use large quantities of organic solvents has been the subject up to now many studies.

 When solutions making it possible to reduce these emissions at source are not possible or insufficient, techniques are used to treat the polluted air before it is released into the atmosphere. These techniques fall into two main categories: on the one hand, the destructive techniques of these molecules, such as biological treatment, thermal or catalytic oxidation of gaseous discharges, on the other hand, so-called recovery techniques, such as adsorption. , absorption, separation on membranes or condensation.

 The treatment by condensation of a gaseous effluent containing C.O.V. consists in transforming the C.O.V present in the gaseous state into liquid, then in recovering this liquid with a view to its possible re-use.

 Among the methods currently used, a number use a technique of cooling by indirect contact with a cooling fluid.

In condensers, contact between the effluent to be treated and the cold source is made by means of an exchanger. This type of condenser has the main disadvantages of not ensuring a uniform temperature within said condenser, and of causing the formation of frost which leads to a significant drop in the efficiency of the heat exchange and to the creation of preferential passages. effluent, and which can go as far as clogging. It is therefore essential to provide a period of periodic heating of the exchange surface to remove this frost, which considerably increases the energy cost of the treatment.

 Recent methods use direct contact of the gaseous effluent to be treated, against the current with solid beads having a temperature lower than that of said effluent.

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 Among these, the method of the Messer Griesheim Company (EP-B-0 537 473) makes it possible to pass the VOCs present in the effluent in the gaseous state, directly to the solid state by crystallization on beads. steel, arranged in a heap and pre-cooled by liquid nitrogen to-150 C. The balls then fall by gravity, against the flow of the gas to be treated, carrying the ice and the crystallized solvents to their surface, towards the bottom of the condenser which is at room temperature. The balls are heated there, and allow the water and the C.O.V. to drip, in the form of a liquid condensate, before being cooled again and recycled at the top of the condenser. This process requires the injection of a continuous flow of liquid nitrogen as a refrigerant. It does not allow good ball / gas contact, therefore the heat transfer, and therefore the deposition of condensate, is insufficient.

 FR-A-2 731 364 describes a method for depolluting atmospheres polluted by accretion around balls of ice maintained in lift in an enclosure, which, when weighed down by the polluting elements captured on their surface, fall at the bottom of the enclosure . These ice balls are then removed from the enclosure and undergo a partial melting so as to recover by decantation or by phase separation the polluting elements. This document does not provide for any other contribution of cold than that of the ice balls; however contact with the warmer polluted atmosphere also melts the surface of these beads. If one wishes to avoid this phenomenon, it is necessary to provide for a significant renewal of the latter, and therefore a very significant consumption of beads.

 In addition, this process is based on the separation of so-called heavy beads, from other still light beads, in the same enclosure where freezing takes place. This segregation has the drawback of being very difficult to achieve when the speed of the effluent is high and keeps these balls in suspension (case of Figures 1 and 2 of this document).

 The aim of the present invention is to overcome the aforementioned drawbacks by proposing a process for condensing, and / or solidifying on cold solid particles, the condensable or solidifiable fraction of a gaseous mixture having a high flow rate, so as to treat, continuously, a large volume of gas.

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 Another object of the invention is to use non-consumable solid particles, that is to say particles which can be quickly reused, therefore recyclable in the gas mixture treatment circuit.

 Another object is to increase the heat exchange between the gas to be treated and the particles to promote the condensation of the condensable fraction of said gas with a high yield.

 Another important object of the invention is to reduce the icing of the exchange surfaces, in particular of the walls or conduits of the condenser.

 These aims, as well as others, are achieved by the process of the present invention, of continuous recovery of a condensable or solidifiable fraction of a mixture in the form of a gas stream, by condensation into a liquid film or solidification. in a solid deposit on the surface of inert particles, precooled and maintained at a temperature lower than that of said mixture, characterized in that the inert particles form, under the action of the gas stream, a fluidized bed of which at least a part is discharged into the upper portion of the fluidization reactor and then recirculated.

 The creation of a fluidized bed of particles has the advantage of considerably increasing the heat transfers between the exchanger proper and the particles, and of favoring the intense mixing of the gas and the cold particles to allow a high yield of condensation and / or solidification of the condensable and / or solidifiable fraction (for example crystallizable) over the entire population of particles.

 The presence of particles in constant motion within the condenser further limits the formation of frost due to the possible presence of water vapor in the gas mixture. In fact, the attritive capacities of these particles in a fluidized bed are used here to prevent the frigory exchange walls from frosting or becoming covered with undesirable deposits. The process of the present invention therefore does not require a step of defrosting the condenser.

 By inert particles is meant here particles of chemical nature different from that of the constituents of the gas mixture, in contrast to known crystallization processes, such as aluminum chloride, where particles of aluminum chloride serve as seeds of crystallization.

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In the process according to the invention, the particles have at least a triple function: they constitute a cold source within the reactor itself, they serve to support deposits of liquid and / or solid condensed and / or solidified after transformation of state of the condensable or solidifiable molecules of the gas mixture, and they allow the transport of this condensed and / or solidified fraction outside the reactor.

These particles can also be porous particles. In this case, the exchange surface is not only the peripheral surface of said particle but also the internal surface accessible to the molecules to be retained.

Advantageously, the speed of the gas stream is at least equal to the minimum fluidization speed of the particles loaded with condensed and / or solidified fraction, preferably at least equal to twice this minimum fluidization speed, for example greater than or equal to 2, 5 m / s approximately, even close to 3 mis.

This high speed limits the residence time of each particle inside the reactor, and therefore avoids the condensation or solidification of a large quantity of liquid, respectively solid, on each of the particles. Thus the thickness of the liquid film or of the solid deposit on the surface of the particles is reduced, consequently reducing the bonding phenomena of said particles, that is to say of adhesion between them.

A film or deposit of small thickness makes it possible to have no weighted particles, for example saturated with condensed liquid, which would cause stratification within the fluidized bed and would unbalance its operation. With a fluidization speed at least equal for example to twice the minimum fluidization speed, the bed remains homogeneous, the particles covered with the liquid film or the solid deposit do not fall to the bottom of the reactor (unlike FR-A-2 731 364), but are discharged in the upper part of the reactor, preferably continuously, within the flow of the outgoing gas mixture.

The speed of the gas stream should not however be too high, to ensure a contact time between the particles and the gaseous effluent sufficient to cause condensation or solidification on the surface of said particles. A maximum value of the order of 8 to 10 m / s is acceptable.

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 The fluidized bed used in the fluidization reactor can be a turbulent recirculating fluidized bed or a circulating bed. In the first case, the recirculation of the particles is partial, in the second case, this recirculation is total.

 In a subsequent step, the part of the recirculated particles is subjected to a step of separation of the liquid film or of the solid deposit of said particles.

 This separation can be carried out by reheating the latter until the molecules forming the film and / or the deposit have vaporized.

 After separation, the particles can be cooled again and then reintroduced into the fluidized bed, thus allowing them to be recycled and used over a long period of operation.

 In the recycling loop, i.e. in the particle circuit outside the main fluidization reactor (which is a condensation and / or freezing reactor), the particles are freed from the condensed and / or solidified fraction which is deposited there by a heating of said particles which can also be used in a second fluidization reactor, provided with heat exchangers.

 Likewise, the step of pre-cooling the particles before their reintroduction at the level of the lower portion of the main fluidization reactor, can also be in the form of a third fluidized bed.

 Advantageously, the solid particles have an average harmonic diameter of less than approximately 100 μm, preferably close to 60 μm. The mean harmonic diameter is the diameter of the sphere having the same area per unit of volume.

 The present invention also relates to the use of the process described above for the depollution of a gaseous effluent loaded with volatile, condensable or solidifiable compounds, in particular volatile organic compounds (VOCs), such as solvents, in particular for the purpose of their recycling.

 The invention will be better understood in the light of the illustrative example below and the appended figures in which:

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 Figure 1 is a diagram of a complete installation for carrying out the method according to the invention: Figures 2 and 3 are diagrams of simplified installations for carrying out the method according to the invention.

 According to Figure 1, the complete installation comprises a fluidization reactor, here a condenser (10) having the form of a vertical column provided at its base with an inlet (1) for the gaseous effluent (E) charged with pollutants (P), and a refrigeration zone (F) provided with exchangers (2) in which a refrigerant circulates.

 The reactor exchange surfaces (here condenser) (10) can be arranged at the bottom, at the top or over the entire height of the reactor. These exchangers may consist of a combination of finned tubes (constituting at least a portion of the wall of the condenser) or of pins more or less submerged inside said condenser (10) depending on the number of frigories to be communicated to the gas-particle mixture; that is to say depending on the temperature level that it is desired to reach, the possible exchange coefficient taking into account the size of the particles and the flow rate of incoming gas as well as its inlet temperature.

 The inert solid particles (3) constitute a fluidized bed under the action of the incoming gas stream (E + P). The gas stream (E + P) is cooled in contact with the exchangers in zone (F) and inert solid particles (3) which are covered with part of the pollutants (P) which can be condensed or solidified in the condenser (10) at during their upward transport by the gas stream to be then evacuated by the outlet (4), in the upper portion of the fluidization reactor (condenser) (10). This gas / solids mixture (E + PS) is then directed to a cyclone (5) making it possible to separate the purified effluent (E) (discharged through the outlet (6)) and the particles covered with the liquid film (PS) (recovered in the lower part of said cyclone (7)).

 The solid particles (PS) are firstly heated, in a first enclosure (8) provided with an exchanger (9) in which a heat transfer fluid circulates, until the liquid film is vaporized and the pollutants discharged under vapor form (P) at outlet (11).

 The particles thus regenerated (S) are transferred into a second enclosure (12), provided with an exchanger (13) in which a cooling fluid circulates, to be there

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 pre-cooled, before being returned to the bottom of the zone (F) of the condenser (10) and thus being recycled.

 Preferably, the solid particles (3) are particles which are inert chemically with respect to the pollutants to be recovered, and which are optionally porous. They can for example be particles of silica, alumina, or ceramic.

 Figure 2 shows schematically an installation identical to that of Figure 1 with the exception of the precooling chamber (12): the particles, as soon as they are rid of the liquid film are directly returned to the base of the condenser (10) in the area (F) refrigeration.

 In the simplified installation according to Figure 3, after the cyclone (5), the solid particles (PS) are directly returned to the condenser (10). A periodic or permanent sampling (14) of these solid particles (PS) charged with liquid is carried out at the bottom of the condenser, and a continuous supply (15) of fresh solid particles compensates for this sampling. The separation of the solid (S) and the liquid pollutant (P) is carried out subsequently.

EXAMPLE:
An exemplary embodiment of the method according to the invention relates to the depollution of an air charged with acetone vapors, a solvent commonly used in industry.

 The device used is a condenser consisting of a vertical column 10 m high and of cylindrical section one meter in diameter.

 The gas entering the condenser is air at 20 C containing about 100 g / m3 of acetone. The fluidized bed consists of 500 kg of alumina particles with a mean harmonic diameter of 60 J. m. For an air + acetone flow velocity of around 3 mm, the heat transfer coefficient is at least 200 W / m2. OK in the condenser assembly. The temperature of the bed is maintained at -60 ° C. by refrigeration of the particles at the bottom of the column and cooling of the walls by means of vertical tubes with fins in which circulates a cooling fluid, preferably over the entire height of the reactor.

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 Almost 95% of the acetone is condensed, forming a liquid film with an average thickness of between 10 and 50 μm approximately, on these alumina particles with an average residence time of one hour.

 The particles are renewed either by external input (see Figure 3), or by regeneration of the alumina particles after separation of the air, using the cyclone, then evaporation of the condensed acetone film.

Claims (9)

1-Process for the continuous recovery of a condensable or solidifiable fraction of a mixture in the form of a gas stream (E + P), by condensation in a liquid film or solidification in a solid deposit on the surface of inert particles ( 3), precooled and maintained at a temperature lower than that of said mixture, characterized in that the inert particles (3) form, under the action of the gas stream, a fluidized bed at least part of which is evacuated in the portion upper part of the fluidization reactor (10) then recirculated.  2-A method according to claim 1, characterized in that the speed of the gas stream is at least equal to the minimum fluidization speed of the particles loaded with condensed or solidified fraction.  3-Method according to one of claims 1 or 2, characterized in that the fluidized bed is a turbulent recirculating fluidized bed.  4-Method according to one of claims 1 or 2, characterized in that the fluidized bed is a circulating bed.  5-A method according to any one of claims 1 to 4, characterized in that the part of the recirculated particles is subjected to a step of separation of the liquid film or of the solid deposit of said particles.  6-A method according to claim 5, characterized in that the separation of the liquid film or the solid deposit of the particles is carried out by heating the latter until the molecules forming the film and / or the deposit are vaporized.  7-A method according to claim 6, characterized in that after separation, the particles are again cooled, then reintroduced into the fluidized bed.  8-Use of the method according to any one of claims 1 to 7 in the depollution of a gaseous effluent charged with volatile compounds condensable or solidifiable.  9-Use according to claim 8 in the depollution of a gaseous effluent loaded with volatile organic compounds.