FR3072886A1 - EXTRACTION OF VOLATILE COMPOUNDS FROM A LIQUID CONTAINING DISSOLVED SALT - Google Patents

Abstract

The present invention relates to a process for extracting a volatile compound from a liquid containing it and further containing at least one dissolved inorganic or organic salt, characterized in that it comprises the following steps: a) supply of a liquid containing a volatile compound and at least one dissolved inorganic or organic salt; b) providing a carrier gas for extracting the volatile compound; c) optionally providing a reagent to facilitate the extraction of the volatile compound and / or the precipitation or crystallization of the at least one inorganic or organic salt; d) extracting the volatile compound and simultaneously precipitating or crystallizing the at least one inorganic or organic salt by bringing the liquid of step a) into contact with the gas of step b) in the presence of a vacuum stirred reactor reagent of step c); e) recovering the carrier gas containing at least a part of the volatile compound; f) recovering the liquid having a reduced content of volatile compound and said at least one dissolved inorganic or organic salt mixed with particles of said at least one inorganic or organic precipitated or crystallized salt.

Description

The present invention relates to the extraction of volatile dissolved organic or inorganic compounds using a gas, also known as stripping.

The elimination of dissolved organic or inorganic compounds is a process commonly encountered in liquid effluent treatment chains.

Although many solutions for extracting gases or organic compounds dissolved in a liquid exist, most suffer from certain disadvantages limiting their performance. Thus, fouling is often encountered in conventional packed columns (fouling of the media constituting the packing) or membrane contactors, especially if effluents or sludges rich in suspended matter or in salts which can precipitate are used. This can even make the use of membrane contactors susceptible to clogging unthinkable.

Extraction processes using specific solvents (adsorption of the organic or inorganic compound in a solvent) are expensive and require complex separation systems, limited in size.

Bubble columns under vacuum or atmospheric are limited by gas mixing.

Vacuum stripping has been described in the past and applied in fields such as anaerobic digestion for the stripping of ammonia as described in US20060006055. This type of extraction makes it possible to increase the exchange surfaces between the gas bubbles transferred into the liquid to be purified and consequently to increase the transfers and the purifying yields within the reactor. The use of vacuum also makes it possible to limit the sensible heat sometimes required to displace the organic or inorganic compound in the carrier gas. Energy saving is also possible on the sometimes necessary heating of the carrier gas insofar as the quantity of carrier gas required is reduced compared to existing conventional solutions. However, fouling problems have not been resolved.

The mineral precipitation is also carried out in mixing reactors or crystallization reactors (with mechanical stirring and guide flow) such as flocculation reactors.

However, no prior art suggests combining these two techniques.

The inventors surprisingly realized that it was possible to combine the stripping technique under vacuum and precipitation / crystallization within the same reactor and that it was therefore possible to simultaneously perform stripping (stripping) ) and precipitation / crystallization.

Thanks to this process, stripping can be carried out without risk of fouling when effluents rich in suspended matter or in salts which can precipitate are treated. Thanks to the use of vacuum, the requirements for carrier gases are reduced, which consequently limits the size of the structures (columns) for desorption of the target gas to be eliminated and which reduces the amount of energy sometimes required to heat the carrier gas. .

The present invention therefore relates to a process for extracting a volatile compound from a liquid containing it and also containing at least one dissolved inorganic or organic salt, characterized in that it comprises the following steps:

a) supply of a liquid containing a volatile compound and at least one dissolved inorganic or organic salt;

b) supply of a carrier gas intended to extract the volatile compound;

c) optional supply of a reagent to facilitate the extraction of the volatile compound and / or the precipitation or crystallization of the at least one inorganic or organic salt;

d) extraction of the volatile compound and simultaneous precipitation or crystallization of the at least one inorganic or organic salt by contacting in a stirred reactor under vacuum the liquid of step a) with the gas of step b) in the possible presence of reagent from step c);

e) recovery of the carrier gas containing at least part of the volatile compound;

f) recovery of the liquid having a reduced content of volatile compound and of said at least one dissolved inorganic or organic salt mixed with particles of said at least one precipitated or crystallized inorganic or organic salt.

The liquid containing a volatile compound and at least one inorganic or organic salt dissolved in step a) according to the present invention can be any effluent such as industrial, municipal waste water and / or industrial drilling water, ground water or an industrial solvent. It can also be a liquid, ie pumpable, sludge, for example coming from a digester.

The volatile compound to be removed from the liquid can be an organic or inorganic compound. It is advantageously chosen from the group consisting of ammonia (NH ₃ ), methane (CH ₄ ), carbon dioxide (CO ₂ ), hydrogen sulfide (H ₂ S), hydrocyanic acid (HCN ) BTEX (benzene, toluene and xylene), PAHs (polycyclic aromatic hydrocarbons), VOCs (volatile organic compounds) and pesticides, more advantageously the volatile compound is ammonia.

The inorganic salt dissolved in the liquid can be chosen from the group consisting of struvite, calcium phosphates, calcium carbonate, magnesium phosphate, magnesium carbonate, calcium sulfate, calcium fluorides, magnesium hydroxide, a metal hydroxide and their mixtures, advantageously it is chosen from the group consisting of calcium carbonate, struvite and their mixtures, in particular it is calcium carbonate and struvite.

The organic salt dissolved in the liquid can be chosen from the group consisting of dimethyl sulfide, dimethyl disulfide, calcium citrate, calcium oxalate, EDTA, a cyanide and their mixtures. Advantageously, the organic salt dissolved in the liquid can be chosen from the group consisting of dimethyl sulfide, dimethyl disulfide and their mixtures when the volatile compound is hydrogen sulfide (H ₂ S). In another advantageous embodiment, the organic salt dissolved in the liquid can be chosen from the group consisting of calcium citrate, calcium oxalate, EDTA and their mixtures when the volatile compound is ammonia or carbon dioxide. In yet another advantageous embodiment, the organic salt dissolved in the liquid can be a cyanide, which can for example precipitate in the form of an iron cyanide, when the volatile compound is hydrocyanic acid.

The carrier gas which can be used in step b) of the process according to the present invention can be chosen from the group consisting of air, nitrogen, steam and their mixture, advantageously from the group consisting of air, steam and their mixture. In particular, in the case where the volatile compound is ammonia, the carrier gas is chosen from the group consisting of air, steam and their mixture, more advantageously by air and an air / steam mixture , even more advantageously by air.

The reagent which can be used in any step c) of the process according to the present invention can be any reagent which allows precipitation / crystallization (for example by modification of the pH of the effluent) of the at least one dissolved inorganic or organic salt . It is advantageously chosen from the group consisting of sodium hydroxide, lime, ash, magnesium chloride, magnesium oxide, ferric chloride, hydrochloric acid, sulfuric acid and phosphoric acid, preferably it it is soda or lime. In an advantageous embodiment, the reagent of step c) is lime or soda, in particular lime when the volatile compound is ammonia. In another advantageous embodiment, the reagent of step c) is chosen from the group consisting of hydrochloric acid, sulfuric acid and phosphoric acid when the volatile compound is CO2, hhS or l HCN.

In a particular embodiment, step d) of the method according to the invention comprises the following simultaneous or successive steps, advantageously simultaneous:

- dl) bringing the carrier gas from step b) into contact with the liquid from step a)

- d2) entry of the mixture obtained in step dl) into the reactor stirred under vacuum, possible contact with the reagent of step c), and extraction of the volatile compound and simultaneous precipitation or crystallization of the at least one inorganic salt or organic.

In fact, the injection of the carrier gas can occur at several points in the system: in the supply circuit of the reactor enclosure (eg injection upstream by pressurization, or direct in the supply pump, or in a static mixer), in the very enclosure of the reactor by microbubbling, in the recirculation loops. The injection into the reactor enclosure can be done for example by bubbling by means of a pierced injection cane, a gas-liquid diffuser (membranes, ejectors, etc.) or direct diffusion via the tree of agitation.

The reactor according to the invention therefore allows both extraction and crystallization / precipitation. It can therefore manage a high level of solid provided that the latter is kept in suspension in the liquid by stirring. In particular, the level of solid in the reactor of step d) or d2) is maintained between 0.1 and 900 g / L of solid in suspension, advantageously between 0.1 and 350 g / L, in particular at a average concentration of 150g / L.

The reactor is therefore perfectly agitated.

The hydraulic residence time in the reactor of step d) or d2) is defined as a function of the desired exhaustion of the volatile compound to be extracted (from a few seconds to several hours), advantageously it is between 10 minutes and 6 hours , more advantageously between 30 minutes and 2 hours.

In an advantageous embodiment, the reactor of step d) or d2) does not include a lining or membrane contactors which will therefore not be fouled.

The operating point (temperature and pressure) within the reactor is defined as a function of the boiling temperature of the liquid in step a), so that it does not boil in the case where the gas vector is not steam.

In the case where the carrier gas is steam, the operating point corresponds to the temperature and the boiling pressure of the liquid in step a).

Step d) or d2) is carried out under vacuum, that is to say at a pressure below atmospheric pressure, advantageously at an absolute pressure between 500 and 99,000 Pa, in particular at an absolute pressure higher than the boiling pressure of the liquid in step a) from 1000 to 10 000 Pa at the operating temperature in the case where the carrier gas is not steam.

The reactor of step d) or d2) must therefore withstand these pressures.

Advantageously, step d) or d2) is carried out at a temperature below the boiling temperature of the liquid in step a) at the operating pressure in the case where the carrier gas is not of the steam, advantageously at a temperature below between 1 and 20 ° C from the boiling point of the liquid in step a) at the operating pressure, in particular at a temperature between -50 ° C and 180 ° C, more advantageously at 60 ° C.

The thermal energy source if necessary to obtain the desired temperature can come from the effluent itself or from another energy source (steam boiler, air heating, heat exchanger, heat pump, heating with thermal fluid). In the case where heating is required for stripping, the system can therefore be equipped with exchangers known to those skilled in the art (plate / tube exchanger, double-shell reactor, etc.).

The advantage of using a vacuum reactor is that the amounts of carrier gas to be used are reduced.

Thus, in an advantageous embodiment, step d) or d2) is implemented with a carrier gas flow rate less than 10 to 100 times that required for operation at atmospheric pressure. Thus advantageously when the volatile compound is ammonia, the gas volume flow can be between 2.5L / h and 150L / h for lL / h of liquid, more advantageously it is lOOL / h of gas for lLJh of liquid . This gas flow can therefore be adapted according to the volatile compound to be eliminated. Advantageously, in the case where the volatile compound is ammonia and where the liquid of step a) boils at a pressure close to 20,000 Pa and is heated to a temperature of about 60 ° C, the optimal operating pressure of step d) or d2) of the method according to the present invention will advantageously be between 22,000 and 25,000 Pa absolute with a carrier gas / liquid ratio of the order of 2.5 to 150 Nm ³ of air per m ³ of effluent, if the carrier gas is air.

In an advantageous embodiment, the supply of the reactor enclosure of step d) or d2) with liquid from step a) is carried out by a pump or by gravity or by means of the vacuum system.

The introduction of the liquid into the enclosure can thus be carried out in a natural or forced manner by means of a spraying system or injection nozzle.

Step e) of the method according to the present invention therefore makes it possible to recover a part of the volatile compound, advantageously at least 50% by weight relative to the total weight of the volatile compound initially present in the liquid in step a), more advantageously the major part, even more advantageously more than 70% by weight, in particular more than 75% by weight relative to the total weight of the volatile compound initially present in the liquid in step a) in the case where the volatile compound is ammonia .

Step f) of the process according to the present invention therefore makes it possible to recover the liquid having a reduced content of volatile compound and of said at least one dissolved inorganic or organic salt mixed with particles of said at least one precipitated or crystallized inorganic or organic salt. .

Advantageously, the liquid has a content reduced by at least 50% by weight of volatile compound, more advantageously by 75% by weight in the case where the volatile compound is ammonia.

In a particularly advantageous embodiment, the extraction yield of the volatile compound, in particular when the volatile compound is ammonia, is at least 50% by weight, in particular at least 60% by weight, more particularly at least 75% by weight. The yield is calculated as follows:

Yield (in% by weight) = 1- (Quantity of the final volatile compound in the liquid in step (f) / Quantity of the initial volatile compound in the liquid of step (a))

In an advantageous embodiment, the method according to the present invention comprises an additional step g) of liquid / solid separation implemented on the liquid obtained in step f) so as to recover on the one hand the particles of said at least a precipitated or crystallized inorganic or organic salt and on the other hand the liquid having a reduced content of volatile compound and of said at least one dissolved inorganic or organic salt.

The process according to the present invention can also comprise an additional step h) of separation of the volatile compound from the carrier gas obtained in step e) and an optional step of recycling of the carrier gas devoid of volatile compound in the reactor of step d) or in step dl).

The method according to the present invention can be implemented in continuous, semi-continuous or discontinuous mode.

The present invention will be better understood with reference to the description of the figures and examples which follow.

FIG. 1 represents a schematic view of the reactor usable in the process according to the invention.

FIG. 2 represents the quantity of ammonia removed from a liquid by the process according to different conditions of pressure and theoretical or real pH (examples) as a function of the carrier gas / liquid ratio at a temperature of 60 ° C.

The reactor (1) which can be used in the process according to the present invention is therefore a three-phase reactor (liquid / gas / solid) stirred and under vacuum.

It therefore includes:

- a liquid inlet means (2) for the implementation of step a), advantageously located in the middle of the reactor;

- a gas inlet means (3) for the implementation of step b), advantageously located at the bottom of the reactor;

- an input means for the reagent (4) of step c), advantageously located in the middle of the reactor;

- An outlet means (5) for the gas containing the volatile compound for the implementation of step e), advantageously located at the very top of the reactor;

- a means for possible heating (6);

- a stirring means (7)

a means allowing the reactor to be placed under vacuum,

- An outlet means (8) for the liquid for the implementation of step f), advantageously located at the very bottom of the reactor.

It is shown in Figure 1.

Example 1:

The process was carried out in batch mode in order to extract the ammonia (volatile compound according to the invention) from an aqueous effluent rich in calcium carbonate and magnesium hydroxide heated to 60 ° C.

The effluent comes from the digestion of solid waste and has a dry extract of 3300 mg / L and a pH of 8.3 and at a temperature of 20 ° C.

Its boiling pressure is close to 20,000 Pa absolute at 60 ° C.

It contains :

- 450 mg / L of NH ₄ ⁺ ;

- 75 mg / L of Ca,

- 230 mg / L of K

- 75 mg / L of Mg,

- 500 mg / L of Na

- 700 mg / L Cl

- 0.5 mg / L of Cu

-1,150 mg / L Fe

- 320 mg / L of SO ₄ -16 mg / L of NO ₃

-1550 mg / L of CO ₃ ² '

The reagent used to promote precipitation by modifying the pH is 30.5% sodium hydroxide.

At pH 9.5 the precipitated salt obtained is therefore calcium carbonate and at pH 11 calcium carbonate and Mg (OH> 2 (magnesium hydroxide).

The carrier gas is air. The carrier gas comes into contact with the effluent within the reactor by microbubbling with an injection pipe.

Different pH conditions (9.5 and 11), different pressure conditions (atmospheric pressure, 101,300; 37,000, 55,000, 27,000, 28,000, 25,000 Pa) and different carrier gas / effluent ratio were tested ( air flows: 4; 7; 10; 11; 12; 34 L / h, which corresponds to gas-liquid ratios between 4 and 76 L / L).

The level of solid in the reactor is 0.5 g / L at pH 9.5 and 0.4 g / L at pH 11.

The equipment used is shown in Figure 1 and is as follows:

- 0.5L glass reactor fitted with an agitator containing the effluent

- Hotplate

- Temperature sensor 0-300 ° C

- Manometer 0-1 bar abs equipped with mechanical regulation valve for pressure control

- Max vacuum pump 50 mbar abs

The injection of reagents is done by micropipette.

The NH ₃ measurement is implemented by HACH LANGE spectrophotometry.

The hydraulic residence time in the reactor is between 0 and 130 minutes.

The results are shown in Figure 2.

A theoretical computational approach was also carried out in order to confirm the results and help the optimal conduct of the process.

The results obtained on this ammonia stripping show that the ammonia elimination yields at a fixed temperature increase significantly with the rise in pH. It is therefore more advantageous to work at high pH if there is no economic or chemical constraint on the addition of an alkaline compound (lime or soda). It has been shown, for example, that the choice of working pH particularly influences the nature of the precipitated salts and that it was possible to find compromises between the nature of the desired precipitates (depending on the quality of the liquid to be treated) and the pressure / gas-liquid ratio couple to maintain an ammonia reduction greater than 50% by weight.

The influence of temperature and pH on the stripping of ammonia being well known on the performance of ammonia elimination, attention focused on the pressure / gas-liquid ratio couple.

It has been shown in particular that the reduction in pressure alone does not allow the elimination of ammonia from the liquid unless this pressure advantageously approaches the boiling point of the water-ammonia mixture (case of stripping). steam).

To obtain advantageous performance in eliminating ammonia with air as carrier gas, the results have shown that a good compromise was to maintain a pressure close enough to the boiling pressure of the liquid (i.e. operating pressure of 25,000 Pa and therefore with a delta of 5000 Pa corresponding to a temperature difference of 5 ° C on the boiling of water (-5 ° C compared to boiling)) and to increase the gas-liquid volume ratio. A very low supply of carrier gas allows in most tests to obtain elimination yields of 50% by weight (at 60 ° C., gas-liquid volume ratio (GL) of the order of 20 at high pH of 11 and a GL ratio of around 60 at a pH close to 9.5)

The tests show by extrapolation that even at a relatively low pH for conventional ammonia stripping (pH 9.5), it is possible to obtain NH3 removal yields greater than 70% by weight with gas- liquid of the order of 80, these ratios being 10 times lower compared to the conventional ratios used at atmospheric pressure for similar yields.

Thanks to the numerical model designed, an extrapolation is possible to obtain higher yields by increasing the Gas-Liquid ratio.

In addition to the fact that the stripping yields of the dissolved volatile compound are comparable between vacuum technologies and technologies at atmospheric pressure, the possibility of being able to simultaneously strike and precipitate one or more salts selectively in the reactor is an advantage. In the tests, this precipitation in the reactor was carried out by controlling the pH (addition of sodium hydroxide as reagent for the precipitation of CaCO ₃ or Mg (OH) ₂ ). This controlled precipitation cannot be envisaged in conventional stripping technologies with lining or trays. In other applications, the addition of other reagents to the reactor in order to recover other dissolved salts is therefore possible.

Claims (17)

1. Method for extracting a volatile compound from a liquid containing it and also containing at least one dissolved inorganic or organic salt, characterized in that it comprises the following steps: a) supply of a liquid containing a volatile compound and at least one dissolved inorganic or organic salt; b) supply of a carrier gas intended to extract the volatile compound; c) optional supply of a reagent to facilitate the extraction of the volatile compound and / or the precipitation or crystallization of the at least one inorganic or organic salt; d) extraction of the volatile compound and simultaneous precipitation or crystallization of the at least one inorganic or organic salt by contacting in a stirred reactor under vacuum the liquid of step a) with the gas of step b) in the possible presence of reagent from step c); e) recovery of the carrier gas containing at least part of the volatile compound; f) recovery of the liquid having a reduced content of volatile compound and of said at least one dissolved inorganic or organic salt mixed with particles of said at least one precipitated or crystallized inorganic or organic salt. 2. Method according to claim 1, characterized in that the volatile compound is chosen from the group consisting of ammonia, methane, carbon dioxide, hydrogen sulfide, hydrocyanic acid, BTEX, PAHs, VOCs and pesticides, advantageously the volatile compound is ammonia. 3. Method according to any one of claims 1 or 2, characterized in that said organic salt is chosen from the group consisting of dimethyl sulfide, dimethyl disulfide, calcium citrate, calcium oxalate, EDTA, a cyanide and their mixtures. 4. Method according to any one of claims 1 or 2, characterized in that said inorganic salt is chosen from the group consisting of magnesium phosphate, struvite, calcium phosphates, calcium carbonate, magnesium carbonate , calcium sulphate, calcium fluorides, magnesium hydroxide, a metal hydroxide and their mixtures, advantageously it is calcium carbonate and struvite. 5. Method according to any one of claims 1 to 4, characterized in that the liquid of step a) is industrial, municipal waste water and / or industrial drilling water, ground water, an industrial solvent or liquid mud from a digester. 6. Method according to any one of claims 1 to 5, characterized in that the carrier gas of step b) is chosen from the group consisting of air, nitrogen, steam and their mixture, advantageously chosen from the group consisting of air and steam. 7. Method according to any one of claims 1 to 6, characterized in that the reagent of step c) is chosen from the group consisting of sodium hydroxide, lime, ash, magnesium chloride, oxide magnesium, ferric chloride, hydrochloric acid, sulfuric acid and phosphoric acid, preferably it is soda or lime, in particular lime when the volatile compound is ammonia. 8. Method according to any one of claims 1 to 7, characterized in that step d) comprises the simultaneous or successive steps, advantageously simultaneous, as follows: - dl) bringing the carrier gas from step b) into contact with the liquid from step a) - d2) entry of the mixture obtained in step dl) into the reactor stirred under vacuum, possible contact with the reagent of step c), and extraction of the volatile compound and simultaneous precipitation or crystallization of the at least one inorganic salt or organic. 9. Method according to any one of claims 1 to 8, characterized in that the level of solid in the reactor of step d) or d2) is maintained between 0.1 and 900 g / L of solid in suspension, advantageously between 0.1 and 350 g / L. 10. Method according to any one of claims 1 to 9, characterized in that the hydraulic residence time in the reactor of step d) or d2) is between 10 minutes and 6 hours advantageously between 30 minutes and 2 hours . 11. Method according to any one of claims 1 to 10, characterized in that the reactor of step d) or d2) does not include a lining or membrane contactors. 12. Method according to any one of claims 1 to 11, characterized in that step d) or d2) is carried out at a temperature below the boiling point of the liquid in step a) at the operating pressure in the case where the carrier gas is not steam, advantageously at a temperature below between 1 and 20 ° C. from the boiling point of the liquid in step a) at the operating pressure, in particular at a temperature between -50 ° C and 180 ° C. 13. Method according to any one of claims 1 to 12, characterized in that step d) or d2) is implemented at a pressure below atmospheric pressure, advantageously at an absolute pressure between 500 and 99,000 Pa, in particular at an absolute pressure higher than the boiling pressure of the liquid in step a) from 1000 to 10,000 Pa at the operating temperature in the case where the carrier gas is not steam. 14. Method according to any one of claims 1 to 13, characterized in that step d) or d2) is carried out with a carrier gas flow rate less than 10 to 100 times that necessary for operation at atmospheric pressure . 15. Method according to any one of claims 1 to 14, characterized in that it comprises an additional step g) of liquid / solid separation implemented on the liquid obtained in step f) so as to recover from on the one hand the particles of said at least one precipitated or crystallized inorganic or organic salt and on the other hand the liquid having a reduced content of volatile compound and in said at least one dissolved inorganic or organic salt. 16. Method according to any one of claims 1 to 15, characterized in that it comprises an additional step h) of separation of the volatile compound from the carrier gas obtained in step e) and an optional step of recycling of the carrier gas free of volatile compound in the reactor of step d) or in step dl). 17. Method according to any one of claims 1 to 16, characterized in that it is implemented in continuous, semi-continuous or discontinuous mode.