FR3123345A1 - Process for the treatment of water by adsorption on activated carbon in micro-grains integrating regeneration by microwaves of the activated carbon. - Google Patents

Abstract

Process for treating water with a view to reducing the content of dissolved organic pollution and micropollutants comprising bringing water into contact with activated carbon in a single contactor, extracting treated water from the contactor, extraction, dehumidification, drying of used activated carbon and desorption by microwaves of the pollution adsorbed on the activated carbon without stopping the bringing into contact of water with the activated carbon and the recycling of activated carbon thus regenerated in the contactor, the entire process being implemented in the same installation, characterized in that: the activated carbon is activated carbon in self-draining micro-grains having an iodine value of between 900 and 1000 mg/g , a particle size of between 300 μm and 1000 μm, an average particle size (D50) of between 500 μm and 600 μm and a proportion strictly less than 5% by weight of grains having a size of less than 400 μm; the extraction, dehumidification, drying, desorption and recycling of the activated carbon into micro-grains are implemented without equipment causing the active carbon to be stirred extensively, the dehumidification comprising a simple draining of the activated carbon; the drying is carried out by microwaves, the drying and the desorption being carried out in the same microwave equipment operating according to a drying cycle in order to obtain a dried activated carbon having a residual moisture content of less than 5% by weight followed by a desorption cycle; microwave drying and microwave desorption are carried out under a controlled atmosphere; and in that it includes the destruction of volatile organic compounds emitted during microwave desorption. Figure for the abstract: [Fig. 1]

Description

Process for the treatment of water by adsorption on activated carbon in micro-grains integrating regeneration by microwaves of the activated carbon.

The present invention relates to the field of water treatment.
More specifically, the invention relates to a process for treating water with a view to reducing the content of organic matter, in particular, where appropriate, the content of micropollutants (pesticides and their metabolites, endocrine disruptors, drug residues, residues of industrial products, etc.).
The method according to the invention falls within the scope of water treatment methods using a granular material allowing the adsorption of organic matter and micropollutants that it contains.
The method according to the invention finds its application in particular in the field of making water drinkable, in the field of tertiary treatment of waste water and in the field of the treatment of industrial water with a view to their discharge into the natural environment or their reuse.

Prior art

Various water treatment processes are known in the prior art using adsorbent materials to eliminate the organic matter and, where appropriate, the micropollutants that it contains.

Among these adsorbent materials, activated carbon is a preferred material due to its very high specific surface area proportional to its adsorption capacity.

It is thus known to use powdered activated carbon (PAC) in the context of water treatment. According to those skilled in the art of water treatment, powdered activated carbon consists of particles having an average size of between 5 μm and 50 μm, preferably between 15 μm and 25 μm.

These techniques use reactors containing PAC in which the water to be treated is brought into contact with the PAC for a sufficiently long time to allow good adsorption on the PAC of the materials to be eliminated which it contains. This bringing into contact can be carried out on a fluidized bed of CAP or by injection of CAP into a reactor. In the latter case, the mixture of water and PAC is, after the contacting step, the subject of a separation step leading on the one hand to obtaining a powdered activated carbon charged in adsorbed materials and on the other hand clarified water. This separation step can be carried out in different ways, mainly by settling or by membrane or mechanical filtration, or even by fluidization (hydraulic speed change).
Although it can be recycled a certain number of times, PAC nevertheless loses its adsorbent power fairly quickly and it is necessary to regularly replace part of the PAC implemented within the reactor with new PAC. Quantities of new PAC must therefore be injected regularly into the reactor to compensate for the loss of adsorption of the used PAC.

Although this type of process allows the replacement of part of the used PAC with new PAC without having to shut down the installations that implement it, it also has other drawbacks.
In particular, used PAC cannot be regenerated in the sense that there is no known economically effective treatment making it possible to restore PAC to its original adsorbent power or close to it. This results in the production of PAC sludge which must be removed from the plant, dewatered before being transported, which increases the costs associated with its landfilling or incineration.

PAC being an expensive material, its use in the context of water treatment therefore comes up against economic imperatives, the techniques implementing it having the disadvantage of involving, as a result, high operating costs.

Other processes use activated carbon not in the form of powder but in the form of grains. According to those skilled in the art of water treatment, granular activated carbon (GAC) consists of carbon grains having a size much larger than that of powdered activated carbon particles and in practice generally between 1000 µm and 3000 µm. In recent years, a new type of activated carbon has also been used for water treatment. This activated carbon corresponds neither to the classic definition of a powdered activated carbon (PAC) nor to the classic definition of a granular activated carbon (GAC). It comes in the form of micro-grains with a particle size lower than that of GAC and much larger than that of CAP generally between 300 μm and 1000 μm. They also have specific surfaces of the same order as those of CAP. This activated carbon in micro-grains (“CAμG”) allows excellent adsorption of organic matter and micropollutants.

Activated carbons in grains or micro-grains have the advantage of having excellent adsorption qualities.

On the other hand, they have the disadvantage of being relatively quickly saturated by the materials that are adsorbed on them. When they are saturated, the objectives of treatment and elimination of organic matter and micropollutants can no longer be met. In addition, when the granular or micro-grain activated carbon is saturated, a release of the organic substances adsorbed into the water during treatment can occur. Such a release can have negative results on the final quality of the treated water. To avoid such a release, the renewal of the activated carbon in grains or in micro-grains must intervene frequently.

The disadvantage of granular activated carbon and micro-granular activated carbon consisting of the fact that they saturate relatively quickly is compensated by their ability to be regenerated. Regeneration, also called reactivation, consists of subjecting these materials to an appropriate treatment to restore their original adsorption capacities or adsorption capacities close to them. To do this, the process most commonly implemented consists of thermal regeneration, carried out in an oven inside which a high temperature prevails (around 800°C) allowing the adsorbed molecules to be desorbed and/or destroyed by heat. Water vapor can be used to perfect this regeneration, the activated carbon in grains or in micro-grains then being able to find a structure very close to its initial structure free of any pollutant. The regeneration of activated carbon in grains or in micro-grains can also be carried out by acid or basic washing which, however, does not generally make it possible to recover all of its initial performance.

Activated carbon regeneration operations are generally not carried out on site. The granular activated carbon extracted from the installations is in practice stored and, when the quantity stored is sufficient, loaded, transported and unloaded in a factory where it can undergo a regeneration process. These different storage and transport operations considerably lengthen and complicate the time and logistics required to obtain regenerated activated carbon. Moreover, regeneration is only implemented on large quantities of used activated carbon, which does not respond well to the frequent needs for small quantities of regenerated activated carbon.

A problem subsequent to these operations of storage and transport, then of regeneration, hitherto little documented is constituted by the fact that they cause an increase in the proportion of fine particles in the regenerated activated carbon compared to the new activated carbon. . In practice, all granular or micro-grain activated carbons available on the market include a small proportion of fine particles. The main drawback related to the presence of these particles is that, due to their fineness, they can be found in the treated water and thus degrade the quality of the treatment. New good quality granular or micro-granular activated carbons have a reduced rate of such fine particles. However, when they return from the regeneration channels, granular or micro-grain activated carbons have higher levels of such fine particles. The abrasion of granular or micro-grain activated carbons, a source of loss of adsorbent materials, comes largely from the multiple manipulations (loading, unloading, transfer, shoveling, etc.) that the activated carbon undergoes during its transport to these dies and during its treatment therein, in particular when this treatment is a heat treatment carried out in rotary kilns. The structure of these furnaces and their operation mean that the activated carbon treated there undergoes significant mixing. This mixing causes an abrasion of the activated carbon grains and therefore the appearance of fine particles in it.

It will also be noted that, in many installations of the state of the art, the renewal of the activated carbon in grains or micro-grains which they implement, that is to say the replacement of the activated carbon saturated by new activated carbon or regenerated activated carbon, cannot be done without stopping their operation. All the disadvantages associated with such an interruption ensue, and mainly the loss of productivity of the installations.

To remedy this problem in particular, water treatment installations have been proposed in the prior art including a contactor in which activated carbon in grains or in micro-grains is brought into contact with water to be treated and provided with means by which used activated carbon can be gradually extracted from the contactor, be roughly cleaned, for example by hydrocycloning, then returned to the contactor via a recycling pipe. A purge makes it possible to regularly evacuate the saturated or partially saturated activated carbon and means are provided to bring new or regenerated activated carbon into the contactor in order to compensate for the purged activated carbon. It is thus possible to extend the residence time of the activated carbon in the installation and to proceed gradually to its renewal in the latter without necessarily having to interrupt its operation. Such installations are in particular marketed under the names FILTRAFLO®CARB and OPACARB®FL.

A disadvantage of cleaning activated carbon with a view to its recycling is that such cleaning produces sludge which constitutes an effluent which itself must be treated to reduce the content of pollutants. In addition, the cleaning of the activated carbon within the installation does not constitute regeneration of the activated carbon, in the sense that it only restores a small proportion of the adsorption capacities of the activated carbon used.

It has also been proposed in the prior art water treatment plants by adsorption on activated carbon in which the activated carbon is regenerated within the installation itself without therefore the need for this to do so. extract. On this subject, we can cite CN1260138C which describes a technique in which a contactor containing activated carbon is sometimes subjected to adsorption cycles during which water to be treated passes through the contactor and sometimes subjected to activated carbon regeneration cycles. by microwaves during which no fluid circulates in the contactor. Such a contactor must therefore be equipped with means making it possible to produce microwaves within it and be made of materials allowing the action of microwaves. Moreover, the adsorption and regeneration steps are, according to this technique, linked by the same contactor geometry. Finally, the water treatment must be interrupted to proceed with the regeneration of the activated carbon. In practice, the use of such a technique is therefore confined to the laboratory scale and is not economically feasible on an industrial scale.

Also known from US7303684B2 is an installation combining two contactors mounted in parallel allowing the treatment of water by adsorption on activated carbon thanks to which the passage of fluid to be treated can be interrupted alternately in each of the two contactors to carry out regeneration by microwaves of the activated carbon it contains, the adsorption being continued in the other contactor. In addition to the disadvantages of the technique mentioned in the previous paragraph, such an installation also has the disadvantage of having to implement two contactors and not just one.

Objectives of the invention

An object of the present invention is to provide a water treatment process to reduce the content of dissolved organic pollution and micropollutants in this water by adsorption on an activated carbon, in which the activated carbon used can be regenerated, and which makes it possible to minimize the release, by the regenerated activated carbon, of polluting substances and fine particles of carbon into the treated water.

Another objective of the present invention is to describe such a process which very significantly reduces the needs for loading, unloading and transporting the activated carbon in order to carry out its regeneration.

Yet another object of the present invention is to disclose such a process which allows, in at least some of its embodiments, to increase the quality of the regenerated activated carbon, by maximizing the restoration of its original adsorption properties. .

Yet another objective of the present invention is to describe such a method which, in at least some of its embodiments, leads to minimizing the quantities of fresh activated carbon that must be implemented.

Another object of the present invention is to provide such a method which makes it possible, in at least some of its embodiments, to regenerate small quantities of activated carbon and thus to adapt continuously and in a synchronized manner to the operation of the water treatment facility implementing such a process under economically acceptable conditions.

Finally, an objective of the present invention is to disclose such a process which can operate without the use of chemicals and without the production of sludge or other solid residues.

These various objectives, or at least some of them, are achieved by virtue of the invention which relates to a process for treating water with a view to reducing the content of dissolved organic pollution and micropollutants comprising a step of bringing said water into contact with activated carbon in a single contactor, a step for extracting treated water from said contactor, a step for extracting used activated carbon from said contactor, a step for dehumidifying said used activated carbon extracted from said contactor , a step of drying the dehumidified used activated carbon, a step of desorption by microwaves of the pollution adsorbed on said dehumidified and dried activated carbon, and a step of recycling activated carbon thus regenerated in said contactor, said extraction steps , dehumidification, drying and desorption of the activated carbon being implemented without stopping said step of bringing said water into contact with said activated carbon and the assembly said process being implemented in the same installation

characterized in that:

said activated carbon is self-draining micro-grain activated carbon (CAμG) having an iodine value of between 900 and 1000 mg/g, a particle size of between 300 μm and 1000 μm, an average particle size (D50) of between 500 μm and 600 μm and a proportion strictly less than 5% by weight of grains having a size of less than 400 μm,

said steps of extracting, dehumidifying, drying, desorbing and recycling the activated carbon into micro-grains are implemented without equipment causing the active carbon to be stirred extensively, said dehumidifying step comprising a simple draining of the extracted activated carbon of the contactor;

said drying step is a microwave drying step, said microwave drying step and said microwave desorption step being carried out in the same microwave emitting equipment operating according to a drying cycle in order to obtaining a dried activated carbon having a residual moisture content of less than 5% by weight followed by a desorption cycle;

said microwave drying step and said microwave desorption step are carried out under a controlled atmosphere;

and in that it comprises a step for destroying the volatile organic compounds emitted during said microwave desorption step.

The process according to the invention makes it possible to minimize the release by the activated carbon in regenerated micro-grains of polluting substances and fine particles of carbon into the treated water.

Such a process has the advantage of using a regenerated activated carbon whose particle size and adsorption characteristics are essentially the same as those of the original activated carbon. Thus, the method of the invention can make it possible to greatly minimize the quantities of new activated carbon that have to be added during its implementation, or even to completely do away with it over relatively long periods of time that can go up to several months.

Indeed, the various characteristics of the process according to the invention contribute on the one hand to limit the abrasion of the activated carbon in micro-grains during its extraction from the contactor and the stages allowing its regeneration and on the other hand to allow the obtaining an activated carbon in regenerated micro-grains having regained most of these adsorption capacities and freed from all the polluting matter which had been adsorbed therein.

Firstly, the micro-grain activated carbon used is very specific and was chosen after numerous tests for its combined ability to abrade little and to present excellent qualities of adsorption of organic matter and in particular of the micropollutants contained in the water to be treated.

Secondly, and although the recommended micro-grain activated carbon already has a very high resistance to abrasion, any significant mixing of the carbon during its extraction from the contactor, its dehumidification, its drying and the desorption of the polluting materials which are adsorbed, is according to the present invention prohibited. To the knowledge of the inventors, it had never been envisaged that such significant mixing could contribute to the appearance of the technical problem resulting from the abrasion of the activated carbon, namely the appearance of fine particles in the regenerated activated carbon. liable to pass into the treated water and degrade the quality of the treatment. It is emphasized that the prior art recommended for these steps indifferently equipment causing such significant mixing, such as in particular pipes provided with pumps to extract the activated carbon from the contactors and fluidized bed dryers or rotary kilns to dry it and desorb polluting materials, that other materials do not cause. It clearly appears from this prior art that the problem posed by the significant mixing of the activated carbon during its extraction and during the various stages leading to its regeneration had hitherto, in practice, been totally neglected. Contrary to the a priori that such heavy mixing had no effect on the quality of the granular activated carbon, the inventors were able to demonstrate the contrary.

Consequently, according to the invention, the dehumidification of the activated carbon in micro-grains extracted from the contactor is carried out simply by draining the latter. Such draining does not cause any significant mixing of the material. Such a mode of dehumidification is only possible because the micro-grains of the activated carbon recommended in the context of the present invention are self-draining. In practice, self-draining means the fact that, placed on a sieve whose mesh retains the grains while allowing the water to pass, part of the water with which they have been mixed can flow out by simple gravity. . Thus, a simple draining makes it possible to dehumidify it. This is a property possessed only by a few granular activated carbons available on the market.

Third, the steps of drying and desorption of the micrograin activated carbon are both carried out in the same equipment emitting microwaves. Thus, no transfer operation, and therefore no significant mixing of the material, occurs between these two steps.

Fourthly, the step of desorption of the pollutants adsorbed on the activated carbon in micro-grains being carried out thanks to the action of microwaves, taking care to destroy the volatile compounds thus desorbed, the method according to the invention allows to restore the adsorption capacities of the activated carbon in micro-grains and to subsequently avoid any release of pollutants into the water treated by this activated carbon thus regenerated.

It will be noted that the use of microwaves has already been envisaged in the prior art for the purpose of regenerating activated carbon used in the context of water treatment processes. Microwaves have the advantage of being able to uniformly heat this type of material and to desorb the materials that have adsorbed on it. The present invention proposes to implement these microwaves under a controlled atmosphere, that is to say low in oxygen, in order to avoid any combustion of the activated carbon and to complete this implementation by a destruction of the volatile compounds resulting of microwave application. Thus, it is possible to regenerate the activated carbon in micro-grains and avoid any subsequent release of pollutants into the treated water.

According to a preferred variant of the process, a temperature of between 90° C. and 150° C. is uniformly maintained within the micro-grain activated carbon for a period of between 2 min and 30 min thanks to said microwave emitting equipment during said drying cycle.

Also preferably, a temperature of between 600° C. and 900° C. is uniformly maintained within the micro-grain activated carbon for a period of between 1 min and 20 min thanks to said microwave emitting equipment during said step of desorption.

Advantageously, said step of extracting used micro-grain activated carbon from said contactor consists in transferring said activated micro-grain carbon from said contactor directly into a filtering structure.

Also advantageously, the method comprises a step of filtering the treated water coming from said step of extracting treated water from said contactor.

According to a variant, during said step of bringing the water to be treated into contact with said micro-grain activated carbon in said contactor, said micro-grain activated carbon is in the form of a fixed bed.

According to another variant, during said step of bringing the water to be treated into contact with said micrograin activated carbon in said contactor, said micrograin activated carbon is in the form of a fluidized or semi-fluidized bed. fluidized.

Also according to a variant, said step of extracting used micro-grain activated carbon from said contactor is implemented in an essentially continuous manner.

Preferably, the method comprises a step of bringing the activated carbon from said desorption step to ambient temperature.

The energy emitted by said activated carbon in micro-grains during said step of bringing it to ambient temperature is then advantageously used to implement a step of pre-drying the micro-grains of activated carbon provided just before said drying step of these micro-grains.

Preferably, said pre-drying step is implemented in said microwave transmitter equipment.

Optionally, the method also comprises a step of injecting steam or CO ₂ into the activated carbon in micro-grains carried out after said step of desorption by microwaves. This injection, which may only be intermittent, is intended to participate in the reactivation of the activated carbon in micro-grains, that is to say in the restoration of these adsorption capacities. This injection can preferably be carried out either at the level of the microwave oven, or directly at the outlet thereof by means of a contactor. It will be carried out before the step of bringing the activated carbon to room temperature when such a step is implemented.

Brief description of figures

: there schematically represents an installation allowing the implementation of the method according to the invention.

Claims (12)

Process for treating water with a view to reducing the content of dissolved organic pollution and micropollutants comprising a step of bringing said water into contact with activated carbon in a single contactor, a step of extracting treated water from said contactor, a step of extracting used activated carbon from said contactor, a step of dehumidifying said used activated carbon extracted from said contactor, a step of drying the dehumidified used activated carbon, a step of desorption by microwaves of the pollution adsorbed on said dehumidified and dried activated carbon, and a step of recycling activated carbon thus regenerated in said contactor, said steps of extracting, dehumidifying, drying and desorbing the activated carbon being implemented without stopping said step of bringing the said water with said activated carbon and all of said process being implemented in the same installation
characterized in that:
said activated carbon is activated carbon in self-draining micro-grains having an iodine value of between 900 and 1000 mg/g, a particle size of between 300 μm and 1000 μm, an average particle size (D50) of between 500 μm and 600 μm and a proportion strictly less than 5% by weight of grains having a size of less than 400 μm,
said steps of extracting, dehumidifying, drying, desorbing and recycling the activated carbon into micro-grains are implemented without equipment causing the active carbon to be stirred extensively, said dehumidifying step comprising a simple draining of the extracted activated carbon of the contactor;
said drying step is a microwave drying step, said microwave drying step and said microwave desorption step being carried out in the same microwave emitting equipment operating according to a drying cycle in order to obtaining a dried activated carbon having a residual moisture content of less than 5% by weight followed by a desorption cycle;
said microwave drying step and said microwave desorption step are carried out under a controlled atmosphere;
and in that it comprises a step for destroying the volatile organic compounds emitted during said microwave desorption step. Process according to Claim 1, characterized in that during the said drying cycle, a temperature of between 90°C and 150°C is uniformly maintained within the micro-grain activated carbon for a period of between 2 min and 30 min thanks to said microwave emitting equipment. Process according to Claim 1 or 2, characterized in that during the said desorption cycle, a temperature of between 600°C and 900°C is uniformly maintained within the micro-grain activated carbon for a period of between 1 min and 20 minutes thanks to said microwave emitting equipment. Method according to one of Claims 1 to 3, characterized in that the said step of extracting used micro-grain activated carbon from said contactor consists in transferring the said granular activated carbon originating from said contactor directly into a filtering structure. Method according to one of Claims 1 to 4, characterized in that it comprises a step of filtering the treated water coming from the said step of extracting treated water from the said contactor. Process according to one of Claims 1 to 5, characterized in that during the said step of bringing the water to be treated into contact with the said micro-grain activated carbon in the said contactor, the said micro-grain activated carbon is in the form of a fixed bed. Process according to one of Claims 1 to 5, characterized in that during the said step of bringing the water to be treated into contact with the said micro-grain activated carbon in the said contactor, the said micro-grain activated carbon is in the form of a fluidized or semi-fluidized bed. Method according to one of Claims 1 to 7, characterized in that the said step of extracting spent micro-grain activated carbon from the said contactor is carried out essentially continuously. Process according to one of Claims 1 to 8, characterized in that it comprises a step of bringing the activated carbon from the said desorption step to room temperature. Process according to Claim 9, characterized in that the energy emitted by the said activated carbon in micro-grains during the said stage of bringing it to ambient temperature is used to implement a pre-drying stage provided just before the said drying stage. . Method according to claim 10 characterized in that said pre-drying step is implemented in said microwave emitting equipment. Process according to one of Claims 1 to 11, characterized in that it comprises a step of injecting steam or CO ₂ into the activated carbon in micro-grains carried out after the said step of desorption by microwaves.