FR2966055B1

FR2966055B1 - PROCESS AND DEVICE FOR TREATING CHLORINATED POLLUTANTS BY PHOTOCATALYSIS

Info

Publication number: FR2966055B1
Application number: FR1058390A
Authority: FR
Inventors: Fabien Gerardin; Jean-Claude Andre; Anaelle Cloteaux; Marie Faure
Original assignee: Institut National De Rech Et De Securite Pour La Prevention Des Accidents Du Travail Et Des Maladies; Centre National de la Recherche Scientifique CNRS; Institut National de Recherche et de Securite INRS
Current assignee: Institut National De Rech Et De Securite Pour La Prevention Des Accidents Du Travail Et Des Maladies; Centre National de la Recherche Scientifique CNRS; Institut National de Recherche et de Securite INRS
Priority date: 2010-10-14
Filing date: 2010-10-14
Publication date: 2019-06-21
Anticipated expiration: 2030-10-14
Also published as: FR2966055A1

Abstract

Ce procédé de conversion de polluants chlorés, contenus dans un effluent à traiter, en composés chlorés oxydants, comprend : a) une étape de strippage des substances chlorées volatiles contenues dans l'effluent liquide, et b) une étape de photocatalyse desdits polluants chlorés. Ce procédé peut être utilisé en particulier pour décomposer le trichlorure d'azote et le chloroforme.This process for converting chlorinated pollutants, contained in an effluent to be treated, into oxidizing chlorine compounds comprises: a) a step of stripping the volatile chlorine substances contained in the liquid effluent, and b) a step of photocatalysis of said chlorinated pollutants. This process can be used in particular to decompose nitrogen trichloride and chloroform.

Description

Method and device for treating chlorinated pollutants by photocatalysis

The present invention relates to a process for converting chlorinated pollutants, contained in an effluent to be treated, into oxidizing chlorine compounds.

In particular, it relates to the conversion of chlorinated pollutants present in aquatic establishments and in the agro-food industry.

The present invention also relates to a device implementing such a conversion method.

Chlorine is widely used because of its low cost, its good bactericidal properties and its simplicity of implementation. It can be used in various forms, for example gaseous or liquid (bleach, ...) and is, to date, the disinfectant most commonly used in aquatic facilities, such as swimming pools.

Chlorine is a particularly reactive product, especially in contact with nitrogenous and / or carbonaceous substances. Chlorine solution, which is predominantly in the form of hypochlorite ion or hypochlorous acid, participates in chemical reactions with nitrogenous materials generated by human activity such as sweat, saliva or urine, or with vegetable or animal waste. These reactions lead, inter alia, to the formation of by-products, such as trihalomethanes (THMs), haloacetic acids, halo-ketones and chloramines whose most halogenated form, trichloride of nitrogen or trichloramine , of formula NCI3, is very volatile and irritating to the respiratory tract and to the eyes.

Nitrogen trichloride generates severe eye and respiratory pollution, particularly with monitoring personnel stationed near aquatic facility basins. Nitrogen trichloride has been the subject of toxicological and epidemiological studies. This work led to the proposal of an average exposure value of 0.5 mg.m-3 in the air, a value below which the exposed persons do not feel a constraining discomfort.

Similarly, intoxication by ingestion or inhalation of trichloromethane or chloroform, CHCI3 formula, can cause irritation of the skin, eyes or, in large doses, a coma. Chloroform has been classified as a probable carcinogen by the International Agency for Research on Cancer (IARC).

In addition, a limit value of occupational exposure in the air of workplaces has been established in France for chloroform. The Labor Code, in accordance with Article R. 4412-149, determined this value at 10 mg.m-3 for an eight-hour reference period.

There is therefore a need to provide prevention solutions for reducing exposures to these compounds. One of the technological solutions for mitigating nitrogen trichloride exposures is based on the principle of gas / liquid extraction, commonly known as stripping. This technique involves contacting the liquid effluent to be treated with air and leads to a significant, industrially validated attenuation of the nitrogen trichloride exposure. It also allows the stripping of volatile copollutants such as chloroform.

However, this solution is not satisfactory from an environmental point of view since the extracted pollutants are released into the environment without having undergone previous chemical or physicochemical treatment.

There already exist, in the state of the art, solutions for treating this type of pollutants.

EP 0 614 682 discloses a process for eliminating pollutants, especially chlorinated organic compounds, using titanium dioxide or a mixture of titanium dioxide and activated carbon.

Patent Application FR 2 794 033 discloses a process for purifying gaseous effluents by photocatalytic reaction according to which, under ultraviolet radiation, the gaseous effluent is brought into contact with a licking movement with one or more supports, then it is crosses a second support, the supports being coated with at least one photocatalyst agent.

Furthermore, US Pat. No. 5,832,361 proposes a device for decomposing nitrogen trichloride gas by photolysis.

However, the known techniques do not make it possible to reduce the concentration of pollutants in the effluent to be treated sufficiently or do not respect the environment.

The object of the invention is therefore to propose a treatment of gaseous effluents making it possible to further reduce the concentration of pollutants in the liquid effluent.

It therefore relates to a process for converting chlorinated pollutants, contained in a liquid effluent to be treated, into oxidizing chlorine compounds, comprising: a) a stripping step of volatile chlorinated substances contained in the liquid effluent, and b) a step photocatalysis of said chlorinated pollutants.

This combination has proved particularly suitable for the conversion of chlorinated pollutants into chlorinated oxidizing compounds. It allows in particular the formation of hypochlorous acid, whose bactericidal properties are recognized, and allowing a partial recycling of chlorine.

For the purposes of the invention, the term "oxidizing chlorine compounds" means compounds containing chlorine at an oxidation degree of 0 or +1.

Such a method makes it possible, on the one hand, to decompose nitrogen trichloride and chloroform and, on the other hand, to form compounds that are non-toxic to humans and their environment, and that even have bactericidal properties. These compounds, with recognized bactericidal properties, can then be introduced, in a beneficial manner, into the effluent treatment circuit, in particular the waters of the basins.

Preferably, the chlorinated pollutants are nitrogen trichloride and / or chloroform.

Advantageously, the method according to the invention further comprises a photolysis step before the photocatalysis step.

Indeed, the photocatalytic process makes it possible to increase the decomposition of nitrogen trichloride relative to photolysis alone.

Preferably, the chlorine oxidizing compounds are chosen from chlorine gas (Cl 2), hypochlorous acid (Cl 2) and mixtures thereof.

Moreover, the photocatalytic degradation of chloroform leads mainly to the formation of chlorides, without the concentration of tetrachloromethane (CCI4) exceeding 1% of the average value of exposure.

In a particular embodiment, the photocatalyst step according to the invention employs, as a catalyst, titanium dioxide. Preferably, titanium dioxide is used in its anatase crystalline form. The photocatalysis step is carried out by UV radiation of wavelength less than 388 nm, in particular between 200 and 388 nm, and preferably between 250 and 388 nm.

The conversion process may also comprise at least one step of washing the catalyst with ultrapure water for laboratory or weakly mineralized water, subsequent to the photocatalysis step. It allows the regeneration of the catalyst. The subject of the invention is also, according to a second aspect, a processing device for implementing the method as defined above.

This installation comprises a reactor intended to be supplied with effluent to be treated and means for treating the effluent by photocatalysis.

According to another characteristic of this installation, the reactor is furthermore provided with a catalyst placed on a support.

It can still be equipped with photolysis means. The installation may further comprise means for washing the catalyst. Other aims, features, and advantages of the invention will become apparent on reading the following description, made with reference to the accompanying drawings, in which: FIG. 1 schematically illustrates a device for treating chlorinated pollutants according to FIG. invention, laboratory scale; FIG. 2 is a diagrammatic front view of a first configuration of a device according to the invention; FIG. 3 is a side view of the device of FIG. 2; - Figure 4 is a side view of the device according to another embodiment; and FIG. 5 schematically illustrates the treatment of the effluent according to the invention.

FIG. 1 illustrates a device for treating chlorinated pollutants according to the invention. Such a device can be used for the treatment of NCI3 nitrogen trichloride and chloroform CHCl3 present in aquatic basins and, more particularly to convert these pollutants into non-toxic chlorine oxidizing compounds, in this case chlorine gas (CI2) and or hypochlorous acid (HC10), which can be subsequently used for their bactericidal properties.

More particularly, a device of this type is intended to be installed at buffer tanks of an aquatic installation, to treat the air leaving the buffer tank, loaded in particular with nitrogen trichloride and chloroform.

But it will be noted that the device shown in FIG. 1 is intended to be implemented on a laboratory scale and is therefore suitable for the treatment of a few liters per minute of liquid effluent. In actual situation, the flow is then several hundred m3 / h. However, an industrial device is based on structural and functional principles identical to those of the device of FIG.

In the embodiment shown, this device essentially comprises a reactor 1, comprising a cylinder 2 made of a material transparent to light radiation, for example pyrex®, and a set of fluorescent tube-type light sources 3, here six in number. , regularly arranged around the reactor. They are for example placed on a support 4 coaxial with the reactor.

It further comprises a catalyst 5 placed on a support 6.

Titanium dioxide, preferably in its crystalline anatase form, which is illuminated by one or more light sources 3, all or part of which emits radiation below a wavelength of 388 nm, can be used as catalyst 5. .

It is possible to use titanium dioxide, pure or associated with a mineral substance, allowing a greater absorption in the UV / visible range. Such compounds are well known in themselves.

This reactor 1, when it is supplied with effluent to be treated, implements, on the one hand, a photocatalysis by the action of the catalyst 5 and under the action of the tubes 3, and on the other hand, a photolysis under the action tubes 3.

Of course, when a reactor of this type is installed in an aquatic installation, it is then provided with all the connection means (not shown) allowing its installation within the basin and the continuous treatment of the air to be treated by maintaining a continuous flow of air to be treated in the reactor.

In the application targeted by the embodiment of FIG. 1, the photocatalysis of nitrogen trichloride and of chloroform is carried out in reactor 1, according to the following operating conditions:

The flow rate of the effluent to be treated is here equal to 1.35 × 10 4 m -1.

With regard to the concentration of the chlorinated pollutants at the inlet of the reactor 1, for the treatment of NCI3, this concentration is from 10 -4 to 5 × 10 -3 mol.m-3, preferably of the order of 2.10 -4 mol. For CHCl 3, the concentration is 10 -4 to 5.10 -3 mol-3, preferably of the order of 10 -3 mol-3. The average residence time is 34 s.

The temperature in the reactor is from 1 to 100 ° C., preferably equal to 50 ° C.

Finally, the relative humidity is 10 to 90%, preferably 70%.

In order to control the conversion efficiency, the wall of the piston-type reactor 1 is provided with four taps 7, for example separated by 0.15 m from each other, and which are intended for monitoring the longitudinal evolution of the concentrations. different species. The effluent to be treated is taken at the inlet and outlet of the reactor 1, as well as at each nozzle located along the reactor.

Fluorescent tubes 3 emit mainly in the UV-A range, with a photon flux of 1.1 × 10 -5 mol of photons.s-1.

Any other light source may be used to deliver radiation all or part of which is less than a wavelength of 388 nm.

Of course, the optical properties (reflection and transmission) of all the constituent elements of the reactor 1, in particular the stainless steel body of the reactor, the Pyrex cylinder 2, the supports 6 are adapted to the emission spectrum of the lamps 3 .

The photocatalytic degradation of nitrogen trichloride and chloroform can be implemented according to two different configurations: through flow and leaching flow.

FIG. 2 illustrates the flow through configuration of reactor 1. This configuration corresponds to the introduction of three catalyst disks 5, 0.08 m in diameter. These supports 6 are placed perpendicular to the flow and spaced 0.15 m apart. The surface area and the total catalyst mass are 0.15 m 2 and 0.27 g, respectively.

FIG. 3 illustrates another view of the flow-through configuration of reactor 1 in which the photocatalysis step occurs.

FIG. 4 illustrates the liquefying flow configuration of the reactor 1. This configuration corresponds to the placement of the support 6 along two axial planes of the reactor 1 and placed perpendicularly with respect to each other. The area and total catalyst mass is 0.22 m 2 and the total catalyst mass is 4 g.

But it should be noted that the principle of flow through or licking is also applicable to industrial size reactors adapted to the treatment of a stream from a buffer tank of a pool.

FIG. 5 illustrates the treatment of the effluent according to the invention. In the exemplary embodiment shown, the water basin 8 comprises chlorinated pollutants such as, for example, nitrogen trichloride and chloroform. This effluent to be treated then flows through the first conduit 9 to be conveyed to the buffer tank 10, where the stripping step of the volatile chlorinated substances contained in this liquid effluent is carried out. The second conduit 11 then allows the passage of the buffer tank 10 to the treatment device 12, these volatile compounds, including nitrogen trichloride and chloroform gas. The treatment device 12 allows the treatment of said volatile compounds in oxidizing chlorine compounds, by the photolysis and / or photocatalysis steps. Said oxidizing chlorine compounds are then reintroduced into the buffer tank 10, through the third conduit 13. The buffer tank 10 then allows the absorption of said oxidizing chlorine compounds. This water enriched in oxidizing chlorine compounds, and especially in hypochlorous acid whose bactericidal properties are recognized, is then reintroduced into the aquatic basin 8, through the fourth conduit 14.

Of course, the reactor 1 may be further equipped with many additional members that have not been shown in the figure for reasons of simplification.

The stream of nitrogen trichloride sampled is collected on a support successively composed of: a tube of silica gel impregnated with sulfamic acid, capable of retaining chlorinated compounds such as hypochlorous acid, mono- and dichloramines, and a cassette containing two quartz fiber filters impregnated with a solution of arsenic trioxide and sodium carbonate on which the nitrogen trichloride is collected. The tubes are analyzed by potentiometry and that of the impregnated filters either by ion chromatography with or without a suppressor column, or by capillary electrophoresis. The analysis of the concentration of chloroform is, for its part, carried out sequentially by gas chromatography and by mass spectrometry.

These processes, photolytic and photocatalytic, provide access to global quantum efficiencies, which are defined as the ratio of the overall degradation rate of a compound to the light intensity absorbed by the pollutant in the case of photolysis and by the catalyst 5 in the case of photocatalysis.

These global quantum efficiencies are thus determined for photolysis and for photocatalysis, respectively, as follows:

(A) in which φι corresponds to the overall photolysis quantum yield, r corresponds to the overall degradation rate of the compound (mol.m ^ .s1), V corresponds to the reactor volume (m3) and Ia corresponds to the luminous intensity absorbed by the pollutant (mol of photons.s'1),

(B) in which <p2 corresponds to the global quantum yield of photocatalysis, r corresponds to the overall degradation rate of the compound (mol.g'1 .s'1), m corresponds to the catalyst mass (g) and Ia

corresponds to the light intensity absorbed by the catalyst 5 (mol of photons.s'1).

It will be noted that for NCI3 and for CHCI3, the yield values φ1 and φ2 are of the order of: Pi Pi is about 10, as regards NCI3; φ2 is about 0.1-0.2, with respect to NCI3 and φ2 is about 0.02-0.05, with respect to CHCl3.

The calculation of the global quantum efficiencies is determined, in particular, by the measurement of the absorbed luminous intensity. This estimate, at each point of the reactor space 1, was carried out by simulation according to the Monte-Carlo method.

Monte Carlo simulation is a probabilistic technique that consists of calculating a numerical value using random methods.

This simulation consists in producing a large number of photon shots from the light sources 3 and following their trajectory in the space of the reactor 1. The simulation algorithm has been designed taking into account the geometry and the optical properties of the light sources. different elements of the reactor 1, as well as the law and the emission spectrum of the six light sources 3 emitting UV radiation.

The programming of the algorithm was carried out with Matlab® software, marketed by Mathworks Inc.

According to one embodiment, during the decomposition of nitrogen trichloride, a photolysis step is coupled to the step of photocatalysis of nitrogen trichloride. The combined action of these two steps makes it possible to convert nitrogen trichloride more efficiently.

According to this embodiment, it is a question of converting the nitrogen trichloride, contained in an effluent to be treated, into oxidizing chlorine compounds, according to the following successive stages: a step of stripping said effluent containing nitrogen trichloride; a step of photolysis of nitrogen trichloride, then - a step of photocatalysis of nitrogen trichloride.

These steps, nor their succession in time, not being limiting of the invention, other steps can be implemented before, during and after these steps.

According to another preferred embodiment, the photocatalytic step is followed by a step of washing the catalyst with ultrapure water for laboratory or with weakly mineralized water, intended to eliminate the inorganic chloride ions deposited at the surface of the photocatalyst. This washing step is preferably carried out for the photocatalytic conversion of chloroform.

By "ultra-pure water" is meant a water characterized by a conductivity of about 0.054 mS.cm1 at 25 ° C.

This washing with ultra-pure water makes it possible to regenerate the photooxidative properties of the catalyst 5, titanium dioxide, preferably in its crystalline anatase form, by dilution of the mineral compounds likely to be derived from the contact between the treated pollutant and the catalyst, preventing photocatalytic degradation.

This catalyst can then be reused in a new photocatalysis step.

According to one embodiment, the process according to the invention comprises one or more catalyst regeneration steps 5.

According to another embodiment, the process according to the invention comprises a step of reintroducing the oxidizing chlorine compounds, resulting from the treatment of chlorinated pollutants, in particular nitrogen trichloride and chloroform, by the photolysis steps and or photocatalysis, in the effluent to be treated, and more particularly in a buffer tank connected to an aquatic basin.

In the embodiment which has just been described, the invention relates to the conversion of nitrogen trichloride and chloroform present in aquatic establishments, such as swimming pools. But the invention also applies to the conversion of nitrogen trichloride and chloroform, present in packaging establishments of the food industry, such as fresh fruit and vegetable packing plants.

The following examples are intended to illustrate the invention without limiting its scope. EXAMPLES

EXAMPLE 1 The evolution of the nitrogen trichloride concentration along the reactor, during the photolysis and photolysis steps coupled with photocatalysis in a through-flow configuration, is determined in Table 1 below:

Table 1

The concentrations corresponding to the distances 0 m and 0.5 8 m are the concentrations at the inlet and outlet of the reactor.

Those corresponding to the distances 0.065 m, 0.215 m, 0.365 m and 0.515 m are the concentrations at each stitching.

As for those corresponding to the distances 0.14 m, 0.29 m and 0.44 m, these are the concentrations upstream and downstream of the three catalyst disks. Since the photocatalysis step occurs only at the catalyst level, the process is considered to be solely photolytic between each catalyst disk.

Concentration differences appear between the upstream and downstream of the catalyst disks, which corresponds to the photocatalytic process alone.

The degradation yields for the first and second catalyst disc are 65% and 74%, respectively.

Finally, at the outlet of the reactor the yields are respectively 99% for the photolysis step coupled with the photocatalysis step and 76% for the photocatalysis alone.

EXAMPLE 2 The evolution of the nitrogen trichloride concentration along the reactor, during the photolysis and photolysis steps coupled with the photocatalysis stage in liquefying flow configuration, is determined in Table 2 below:

Table 2

The concentrations corresponding to the distances 0 m and 0.565 m are the concentrations at the inlet and outlet of the reactor.

In this liquefied flow configuration, the photolysis step coupled to the photocatalysis step makes it possible to achieve a pocket degradation yield of 95%, compared to 75% in the case of photolysis alone.

Claims

1. Process for converting nitrogen trichloride and / or chloroform, contained in a liquid effluent to be treated, into oxidizing chlorine compounds, characterized in that it comprises: a) a step of stripping nitrogen trichloride and / or volatile chloroform, contained in the liquid effluent, and b) a step of photocatalysis of nitrogen trichloride and / or chloroform.

2. Method according to claim 1, characterized in that the oxidizing chlorine compounds are selected from gaseous chlorine, hypochlorous acid, and mixtures thereof. 3. Method according to any one of the preceding claims, characterized in that it further comprises a photolysis step before the photocatalysis step. 4. Method according to any one of the preceding claims, characterized in that the photocatalysis step employs, as catalyst (5), titanium dioxide, preferably in its crystalline form anatase. 5. Method according to any one of the preceding claims, characterized in that the photocatalysis step is carried out by UV radiation of wavelength less than 388 nm, in particular between 200 and 388 nm, and preferably between between 250 and 388 nm, 6. Process according to any one of the preceding claims, characterized in that it comprises at least one step of washing the catalyst with ultrapure water or weakly mineralized water, subsequent to the photocatalysis step. 7. Method according to claim 6, characterized in that the step of washing the catalyst with ultrapure water or weakly mineralized water is implemented for the photocatalytic conversion of chloroform. 8. Process according to claim 7, characterized in that the catalyst is reused in a new photocatalysis step. ST Process according to any one of the preceding claims, characterized in that it comprises one or more regeneration steps of the catalyst (5).

10. Process according to any one of the preceding claims, characterized in that it comprises a step of reintroducing the oxidizing chlorine compounds, resulting from the treatment of chlorinated pollutants by the photolysis and / or photocatalysis steps, in the effluent. treat. 11. Application of the process according to one of claims 1 to 10 for the conversion of nitrogen trichloride and chloroform, present in aquatic establishments, such as swimming pools. 12. Application of the process according to one of claims 1 to 10 for the conversion of nitrogen trichloride and chloroform, present in packaging establishments of the food industry, such as fresh fruit packaging plants and fresh vegetables. 13. Processing device for implementing the method according to one of claims 1 to 12, characterized in that it comprises a reactor (1) to be supplied with effluent to be treated and means for treating the effluent by photocatalysis, said reactor (1) is further provided with a catalyst (5) placed on a support (6), and in that it comprises means for washing the catalyst (5). 14. Device according to claim 13, characterized in that the reactor (1) is provided with photolysis means.