IT202000028508A1 - METHOD AND PLANT FOR THE RECOVERY OF AMMONIA NITROGEN FROM GAS CURRENT PRODUCED BY HYDROTHERMAL TREATMENTS - Google Patents

Description

DESCRIPTION

Attached to a patent application for INDUSTRIAL INVENTION having the title

?Method and plant for the recovery of ammoniacal nitrogen from gas streams produced by hydrothermal treatments?

FIELD OF THE INVENTION

The present invention relates to a method and related plant for the continuous recovery of ammonia nitrogen from the gaseous stream directly produced by the hydrothermal treatment (thermal hydrolysis or hydrothermal carbonization) of wastewater containing nitrogen, said ammonia nitrogen being recovered in the form of a ammonium salt.

STATE OF ART

Hydrothermal treatments, such as for example thermal hydrolysis or hydrothermal carbonization, are thermochemical processes used for the conversion of biomass at relatively low temperature and pressure conditions, in the presence of liquid water under subcritical conditions, and allow material to be decomposed organic by converting it into a carbonaceous solid, biocoal (or hydrochar ? HC), and into a liquid component.

One of the major problems present in the current state of the art concerns the possible excessive concentration of nitrogen, in the form of ammoniacal nitrogen, present in said liquid component in the case of the treatment of wastewater with a high nitrogen content such as, for example, sludge treatment plants or livestock and agro-industrial waste. Generally, if the nitrogen concentration in this liquid component is too high (?4 g/L), ? subsequent treatment is necessary in order to avoid and/or reduce environmental problems that could arise if the ammoniacal nitrogen were not effectively removed and/or recovered.

For example, if said liquid component were recirculated within an anaerobic digestion plant, the high nitrogen content would be toxic to the microorganisms present therein, thus inhibiting the biological processes.

If, on the other hand, said liquid component were used as such by spreading it, for example, on agricultural land, it would cause an excessive presence of nutrients (over-fertilisation) in the soil and contamination of surface waters and/or of groundwater due to runoff and/or infiltration into the subsoil. This would have negative effects on the entire ecosystem by altering the natural balance, not to mention the harmful effects on human health.

For these reasons, more and more efforts have been made to provide effective systems for the removal and/or recovery of ammoniacal nitrogen following hydrothermal treatments of wastewater with a high nitrogen content.

Among the various processes currently employed, there is the biological treatment of nitrification/denitrification which however requires high energy consumption for oxygenation (during nitrification) and an external source of carbon for denitrification due to the high nitrogen content present.

As an alternative to nitrification/denitrification, in recent years, innovative biological processes have been developed such as the Anammox process, i.e. a metabolic process, characteristic of some bacterial species belonging to the phylum Planctomycetes, which involves the oxidation of ammonia in anoxic conditions and which allows you to reduce energy consumption. The growth of said bacteria for the Anammox process? however slow (about 11 days) and long times are needed for the start of the process as well as? plant engineering solutions that require high retention times of the treated wastewater.

Another category of interesting processes for the reduction and/or recovery of nitrogen, following hydrothermal treatments of wastewater with a high nitrogen content, are the processes based on chemical-physical treatments carried out on the liquid component deriving from the hydrothermal treatment such as , for example, column stripping.

During such a treatment, the liquid component deriving from the hydrothermal treatment, and previously separated from the solid component, is brought to high pH and temperatures and is passed inside a desorption column with filling bodies, which have the function of increasing the contact surface between the liquid (in which nitrogen is contained in the form of hydrated ammonia NH4<.>OH) and the gas (generally air). The two fluids generally enter the column in counter-current: the air from below and the liquid from above. According to this scheme, ammonia nitrogen is passed from liquid (in the form of hydrated ammonia NH4<.>OH) to gas (in the form of ammonia, NH3). Such a stripping technique, however, requires considerable complexity. plant engineering and therefore? also high investment costs.

Furthermore, the stripping involves the use of a considerable quantity? of chemicals for regulating the pH of the liquid component and a significant amount? of air to facilitate the removal of ammonia.

Such a process? described for example in KR2018025326, in which after the hydrothermal carbonization treatment, a solid-liquid separation is carried out and the ammoniacal nitrogen is recovered by treating the liquid phase so? separated by stripping the ammonia in a special reactor.

WO2017055131 instead describes the recovery of nitrogen by means of a vacuum distillation of the liquid component deriving from the hydrothermal carbonization process and previously separated from the solid component.

However, all these systems necessarily provide, following the hydrothermal treatment, at least one solid-liquid separation step to isolate the liquid component which is then treated to remove and/or recover the nitrogen present therein.

It therefore appears clear how the need remains in the sector? to find a system that allows the efficient recovery of ammonia nitrogen directly from the treated wastewater and in continuous with the hydrothermal treatment process of the same, without carrying out solid-liquid separation steps.

The present invention solves the problems of the prior art, in particular the problems relating to an excessive use of chemical substances potentially harmful to the environment and to the operators and the excessive plant complications required for example by traditional stripping systems or vacuum distillation systems , providing a method and a related plant that allow the effective recovery of ammonia nitrogen directly from the thermal hydrolysis or hydrothermal carbonization reactor, without carrying out solid-liquid separation steps, and in continuous with the treatment of the wastewater inside of said reactor. The method and related plant according to the present invention therefore make it possible to recover ammoniacal nitrogen from wastewater with a high nitrogen content quickly, without the need to of complex equipment and with low process costs. Furthermore, the method and the relative plant according to the present invention make it possible to recover the ammoniacal nitrogen, not in a further waste product to be disposed of, burned or further processed, but in the form of an ammonium salt which is directly usable for commercial purposes in various technological sectors, such as for example the inorganic fertilizer sector.

SUMMARY OF THE INVENTION

The present invention relates to a method for the continuous recovery of ammonia nitrogen from a gaseous stream directly produced by thermal hydrolysis or hydrothermal carbonization of wastewater containing nitrogen, said method comprising the steps of:

(a) in a closed reactor, subjecting wastewater containing nitrogen to a treatment of thermal hydrolysis or hydrothermal carbonization, said treatment comprising the step of heating said wastewater to a process temperature ranging from 120 to 250°C for a period of time ranging between 0.5 and 8 hours, until an increase in the vapor pressure is recorded inside said reactor due to the development of a gaseous phase comprising water vapor and gaseous NH3, said vapor pressure being between 2 and 50 bar in operation of said process temperature;

(b) continuously withdrawing said gaseous phase from the reactor of step (a) and conveying it to a condensation and separation phase, maintaining the pressure inside said reactor at a value between 0.2 and 2 bar above the said vapor pressure;

(c) condensing the water vapor contained within said gaseous phase at a temperature ranging from 5 to 90°C, until obtaining the separation of a liquid aqueous phase and a gaseous phase comprising gaseous NH3;

(d) recovering the ammoniacal nitrogen in the form of an ammonium salt by reacting the gaseous NH3 separated in step (c) with an acid.

Preferably, the method according to the present invention provides that phases (c) and/or (d) are phases operating continuously with phases (a) and (b).

A further object of the present invention is a plant preferably operating according to the method described above. This plant includes:

- a reactor (1) of thermal hydrolysis or hydrothermal carbonization, said reactor being a closed reactor;

- a capacitor (3) coupled with a two-phase separator (4);

- a line (1a) leaving said reactor (1) and entering said condenser (3);

- a line (3a) leaving said capacitor (3) and entering said two-phase separator (4);

- a tank (6) comprising an acid;

- a line (4b) leaving said two-phase separator (4) and entering said tank (6);

- a regulating device (5) configured to control the pressure inside said reactor (1) said regulating device being a regulating device (5a) placed on the line (3a), or a regulating device (5b) placed on line (4b), in which said regulating device, on the basis of the pressure value detected inside said reactor, varies its degree of opening.

Preferably, said adjustment device (5) ? a regulating valve.

Preferably, said nitrogen-containing wastewaters are selected from the group consisting of: purification sludge, zootechnical wastewater, percolated agro-industrial wastewater, organic fractions of municipal solid waste, digestates and a combination thereof.

BRIEF DESCRIPTION OF THE FIGURES

In Figure 1 ? shown is a block diagram depicting the components of the system according to an embodiment of the present invention in which the regulating device is a regulating valve, in particular a regulating valve (5b) placed on the line (4b).

In Figure 2 ? shown is a block diagram depicting the components of the system according to an alternative embodiment of the present invention in which the regulating device is a regulating valve, in particular a regulating valve (5a) placed on the line (3a).

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the term ?hydrothermal treatment? generally refers to thermal hydrolysis treatment or hydrothermal carbonization treatment.

For purposes of the present invention, the terms ?linked? and ?in communication? or ?connected? and ?connected?, are used as perfectly interchangeable synonyms.

For the purposes of the present invention, by "ambient temperature" is meant a temperature value, measured under atmospheric pressure conditions, between 5 and 40 ?C, preferably between 15 and 30 ?C, plus preferably between 20 and 25 ?C.

For ?atmospheric pressure? or ?ambient pressure?, for the purposes of the present invention, is meant a pressure value around ? 0.05 atm of normal or standard atmospheric pressure measured at latitude 45?, at sea level and at a temperature of 0 ?C over a unit area of 1 cm<2>, which corresponds to the pressure of a column of mercury of 760 mm, or corresponding to 1 atm.

For purposes of the present invention, for ?vapour pressure? it means the pressure exerted by the vapor of a substance on the condensed phase (solid or liquid) of the same substance when these phases are in conditions of thermodynamic equilibrium between them within a closed system, the cio? under saturated steam conditions.

For the purposes of the present invention, the expressions ?aqueous solution of ammonium sulfate? and ?ammonium sulphate in aqueous solution? are used perfectly interchangeably.

For the purposes of the present invention, the terms ?liquid stream? and ?liquid phase? what? such as the terms ?gas stream? and ?gas phase" are used, unless otherwise specified, as perfectly interchangeable synonyms. For the purposes of the present invention, the expression ?ammoniacal nitrogen? refers both to nitrogen in the form of ammonium ion NH4<+ >(when in solution , ?N-NH4<+>?) and in the form of gaseous ammonia (NH3 gas).

The present invention relates to a method for the continuous recovery of ammonia nitrogen from the gas stream directly produced by the hydrothermal treatment (thermal hydrolysis or hydrothermal carbonization) of wastewater with a high nitrogen content.

The method according to the present invention comprises the steps of:

(a) in a closed reactor, subjecting wastewater containing nitrogen to a treatment of thermal hydrolysis or hydrothermal carbonization, said treatment comprising the step of heating said wastewater to a process temperature ranging from 120 to 250°C for a period of time ranging between 0.5 and 8 hours, until an increase in the vapor pressure is recorded inside said reactor due to the development of a gaseous phase comprising water vapor and gaseous NH3, said vapor pressure being between 2 and 50 bar in operation of said process temperature;

(b) continuously withdrawing said gaseous phase from the reactor of step (a) and conveying it to a condensation and separation phase, maintaining the pressure inside said reactor at a value between 0.2 and 2 bar above the said vapor pressure;

(c) condensing the water vapor contained within said gaseous phase at a temperature ranging from 5 to 90°C, until obtaining the separation of a liquid aqueous phase and a gaseous phase comprising gaseous NH3;

(d) recovering the ammoniacal nitrogen in the form of an ammonium salt by reacting the gaseous NH3 separated in step (c) with an acid.

According to an embodiment of the invention, step (a) comprises the step of heating said wastewater to a process temperature preferably ranging from 180 to 190°C and for a period of time preferably ranging from 0.5 to 2 hours until registering an increase in the vapor pressure inside said reactor due to the development of a gaseous phase comprising water vapor and gaseous NH3, said vapor pressure being between 10 and 14 bar, as a function of said process temperature.

Preferably said nitrogen-containing wastewaters are selected from the group consisting of: purification sludges, livestock wastewaters, agro-industrial wastewaters, leachates, organic fraction of municipal solid wastes, digestates or a combination thereof.

Preferably said wastes are liquid wastes comprising a quantity of total water greater than 70% w/w, preferably greater than 80% w/w, preferably greater than 85% w/w, more? preferably between 70 and 90% p/p, even more? preferably between 85 and 90%w/w. Preferably, the nitrogen content (in the form of ammonium ion N-NH4<+ >or organic nitrogen) in said wastewater ? between 0.5 and 4 g/l, preferably between 1 and 2 g/l.

Preferably, said gaseous phase developed, in phase (a), following the thermal hydrolysis or hydrothermal carbonization reactions inside the reactor, can include, in addition to water vapor and gaseous NH3, also other types of gas, such as CO2, depending on the type of wastewater treated.

Preferably, said thermal hydrolysis or hydrothermal carbonization treatment takes place in the presence of water, said water being already? included within the wastewater containing nitrogen (liquid waste comprising a total amount of water preferably greater than 70% w/w preferably greater than 80% w/w, preferably greater than 85% w/w, more preferably included between 70 and 90% w/w, still more preferably between 85 and 90% w/w) or being added to them in the event that said wastewaters have a water content lower than 70% w/w, preferably less than? 80% p / p, more? preferably lower than 85% w/w until reaching a total water percentage preferably between 85 and 90% w/w.

According to one embodiment of the invention, step (b) is a step of continuously withdrawing said gaseous phase from the reactor of step (a) and conveying it to a condensation and separation step, maintaining the pressure inside said reactor at a value preferably between 0.5 and 1.0 bar per above the above vapor pressure. Without wishing to be bound to a specific theory, the Applicant has found that, thanks to the specific temperature conditions in which said wastewater is treated inside said closed reactor, to the corresponding increase in vapor pressure, and to the maintenance of an overpressure between 0.2 and 2.0 bar, preferably between 0.5 and 1.0 bar above said vapor pressure, ? possible to optimize not only the hydrothermal treatment of wastewater but, simultaneously and continuously, also carry out an effective recovery of ammonia nitrogen directly from said wastewater without interrupting the hydrothermal treatment and/or subjecting said wastewater to solid-liquid separation phases and/or add chemical reagents. The method according to the present invention therefore advantageously allows ammonia nitrogen to be recovered continuously directly from the thermal hydrolysis or hydrothermal carbonization reactor (or directly from the wastewater), quickly, without the need to of complex equipment and with low process costs. In the case of traditional systems, in fact, the need? to carry out solid-liquid separation steps and/or to add chemical reagents directly to the wastewater contained in the reactor or to the separated liquid component, greatly increases the complexity? of the operations as well as the cost/environmental impact in terms of quantity? use of additional chemical reagents.

According to a preferred embodiment, step (c) ? a step of condensing the water vapor contained within said gaseous phase at a temperature comprised between 5 and 90°C, preferably between 20 and 60°C until obtaining the separation of a liquid aqueous phase and gaseous NH3. Preferably, said phase (c) ? effected by means of a capacitor coupled with a two-phase separator.

Pi? preferably, during said phase (c), the vapor pressure inside said condenser and said two-phase separator ? between 2 and 50 bar, preferably between 10 and 14 bar.

According to an embodiment of the present invention, the acid used in step (d) is selected from the group consisting of: sulfuric acid, hydrochloric acid or nitric acid.

According to a preferred embodiment, step (d) of the method according to the present invention ? a step of recovering the ammoniacal nitrogen in the form of an ammonium salt by reacting the gaseous NH3 separated in step (c) with an aqueous solution of said acid. Preferably, said aqueous solution of an acid ? therefore selected from the group consisting of: an aqueous solution of sulfuric acid, an aqueous solution of hydrochloric acid or an aqueous solution of nitric acid.

Preferably the acid used in step (d) is sulfuric acid, pi? preferably an aqueous solution of sulfuric acid saturated with ammonium sulphate.

According to a particularly preferred embodiment, step (d) of the method according to the present invention is carried out by bubbling the gaseous NH3 separated in step (c) into said aqueous solution of said acid. Preferably, according to this embodiment, said aqueous solution of said acid ? maintained at a temperature comprised between 10 and 30°C, preferably between 20 and 25°C in order to avoid any excessive overheating due to the exothermic neutralization reactions between the ammonia and said aqueous solution of said acid.

According to the embodiments described above, the ammonia nitrogen ? therefore recovered in the form of an ammonium salt, preferably selected from the group consisting of: ammonium sulphate, ammonium chloride or ammonium nitrate.

Preferably, the ammoniacal nitrogen ? recovered in the form of a crystalline ammonium salt, preferably in the form of a monocrystalline ammonium salt; said ammonium salt being selected from the group consisting of: ammonium sulphate, ammonium chloride or ammonium nitrate. According to a preferred embodiment of the invention, when the acid used in step (d) is an aqueous solution of sulfuric acid saturated with ammonium sulphate, ammoniacal nitrogen ? recovered in the form of a monocrystalline ammonium sulfate salt.

Preferably l? ammoniacal nitrogen ? recovered in the form of an ammonium salt solution, preferably selected from the group consisting of: an ammonium sulphate solution, an ammonium chloride solution or an ammonium nitrate solution. Said ammonium salt or said solution of an ammonium salt are therefore obtained following the reaction between the gaseous NH3 separated in step (c) and said acid or said aqueous solution of an acid.

The method according to the present invention therefore allows ammoniacal nitrogen to be recovered directly from wastewater with a high nitrogen content, obtaining, not a further waste product to be disposed of, burned or further treated, but an ammonium salt (or a solution of an ammonium salt) that ? directly usable for commercial purposes in various technological sectors, such as for example the inorganic fertilizer sector.

According to an embodiment, the method according to the present invention comprises a step (c?) of controlling the pH of the liquid aqueous phase separated in step (c) and, in case said pH is > 8, heating said liquid aqueous phase up to a temperature between 60 and 90°C obtaining a further separation of gaseous NH3 possibly dissolved in said separated liquid aqueous phase.

According to an alternative embodiment, the method according to the present invention comprises a step (c1) (alternative to step (c?)) of maintaining the liquid aqueous phase separated in step (c) at a temperature between 60 and 90°C and adding a base until a pH of said liquid aqueous phase is greater than 8, preferably between 8 and 12, obtaining a further separation of gaseous NH3 possibly dissolved in said separated liquid aqueous phase.

Preferably, the liquid aqueous phase obtained following step (c), or following step (c)+(c?) or (c)+(c1), comprises a nitrogen content between 0.2 and 7, 0 g/l, preferably less than 0.5 g/l.

According to a preferred embodiment, the method according to the present invention ? characterized in that steps (c), (c?), (c1) and/or (d) as described above are steps operating continuously with steps (a) and (b). This advantageously allows the recovery of ammoniacal nitrogen to be carried out continuously with the treatment of thermal hydrolysis or hydrothermal carbonization without interrupting such a wastewater treatment process. In other words, the method according to the present invention allows ammonia nitrogen to be recovered directly from the wastewater to be treated without any additional treatment such as, for example, a pH adjustment of said wastewater before, during or after the hydrothermal treatment or a solid separation - liquid after hydrothermal treatment.

A further object of the present invention is a plant for the continuous recovery of ammoniacal nitrogen from a directly gaseous stream produced by thermal hydrolysis or hydrothermal carbonization of wastewater containing nitrogen, preferably operating according to the method described above.

With reference to Figure 1 and Figure 2, the plant according to the present invention comprises:

- a thermal hydrolysis or hydrothermal carbonization reactor (1) suitable for receiving said wastewater containing nitrogen and for operating at a process temperature between 120 and 250°C and at a vapor pressure, as a function of said process temperature, between between 2 and 50 bars; said reactor (1) being a closed reactor;

- a capacitor (3) coupled with a two-phase separator (4);

- a line (1a) leaving said reactor (1) and entering said condenser (3) for transporting said gaseous stream comprising water vapor and gaseous NH3;

- a line (3a) leaving said capacitor (3) and entering said two-phase separator (4);

- a tank (6) comprising an acid for the recovery of the ammoniacal nitrogen in the form of an ammonium salt;

- a line (4b) leaving said two-phase separator (4) and entering said tank (6) for transporting the gaseous NH3 separated into said two-phase separator (4);

- a regulating device (5) configured to control the pressure inside said reactor (1) said regulating device being a regulating device (5a) placed on the line (3a) (Figure 2), or a regulating device regulation (5b) placed on the line (4b) (Figure 1), in which said regulation device, on the basis of the pressure value detected inside said reactor, varies its degree of opening so? to maintain a pressure inside said reactor at a value between 0.2 and 2.0 bar above the aforementioned vapor pressure. According to a preferred embodiment of the invention, said adjustment device (5, 5a and 5b) ? a regulating valve.

According to one embodiment, the plant according to the present invention also comprises a heating system (2) connected to said reactor (1).

Preferably, said heating system ? selected from the group consisting of: a heating jacket, a steam blowing system, or a combination thereof.

Preferably said condenser (3) and said two-phase separator (4) are at a temperature between 5 and 90°C, preferably between 20 and 60°C. Preferably, the vapor pressure inside said condenser (3) and said two-phase separator (4) is? between 2 and 50 bar, preferably between 10 and 14 bar.

According to an embodiment of the invention, said regulating device, preferably said regulating valve, (5) varies its opening degree as follows: to maintain a pressure inside said reactor at a value preferably comprised between 0.5 and 1.0 bar above the aforementioned vapor pressure.

According to a particularly preferred embodiment, the plant according to the present invention comprises a storage tank (7), coupled to said two-phase separator (4) to collect the liquid aqueous phase deriving from the condensation of the water vapor separated from the gaseous NH3.

Preferably said storage tank (7) is in ambient pressure and temperature conditions.

According to an embodiment of the present invention, said two-phase separator (4) comprises a heating system (8) and said storage tank (7) comprises a system for detecting the pH of the liquid stream collected inside said storage tank , said heating system being operated and regulated to heat said two-phase separator (4) to a temperature comprised between 60 and 90°C on the basis of the increase of said pH of the liquid stream above 8. According to an alternative embodiment of the present invention, said two-phase separator (4) ? connected to a dosing system of a base and said storage tank (7) comprises a system for detecting the pH of the liquid stream collected inside said storage tank, said dosing system being operated and regulated to obtain regulation of said pH up to a value between 8 and 9. According to this embodiment, said two-phase separator (4) also comprises a heating system (8) which allows to maintain the temperature of said two-phase separator (4) at a value between between 60 and 90 ?C.

Preferably said base ? chosen from the group consisting of: NaOH and KOH. Preferably, said heating system (8), according to the previous embodiments, is selected from the group consisting of: heating jacket and steam blowing or a combination thereof.

According to a particularly preferred embodiment, said tank (6) comprises an acid selected from the group consisting of: sulfuric acid, hydrochloric acid or nitric acid.

According to a particularly preferred embodiment, said tank (6) comprises an aqueous solution of said acid.

Preferably, said tank (6) comprises sulfuric acid, plus? preferably an aqueous solution of sulfuric acid saturated in ammonium sulphate.

Preferably, said tank (6) also comprises a cooling system and a safety valve whose opening ? activated and regulated on the basis of the (possible) increase in pressure detected inside said tank (6).

Preferably said cooling system advantageously allows to keep said tank (6) at a temperature between 10 and 30 ?C, preferably between 20 and 25 ?C in order to avoid any excessive overheating due to the exothermic neutralization reactions between the NH3 gaseous and the acid, preferably the aqueous solution of an acid, included inside said tank. Similarly, said safety valve advantageously makes it possible to avoid an excessive increase in pressure inside said tank as a result of said neutralization reactions.

According to a preferred embodiment, the ammonia nitrogen ? then recovered, by means of the plant of the present invention, in the form of an ammonium salt, preferably selected from the group consisting of: ammonium sulphate, ammonium chloride or ammonium nitrate. Preferably l? ammoniacal nitrogen ? recovered in the form of an ammonium salt solution, preferably selected from the group consisting of: an ammonium sulphate solution, an ammonium chloride solution or an ammonium nitrate solution.

Preferably, the concentration of ammonium ions in said solution of an ammonium salt is between 2 and 15 g/l, preferably between 6 and 10 g/l. Advantageously, the plant according to the present invention therefore makes it possible to recover ammoniacal nitrogen directly from wastewater with a high nitrogen content, obtaining, not a further waste product to be disposed of, burned or further processed, but an ammonium salt (or a solution of an ammonium salt) that ? directly usable for commercial purposes in various technological sectors, such as for example the inorganic fertilizer sector.

EXAMPLES

Example 1

The test ? carried out in a plant that continuously treats about one ton/hour of wastewater with a high nitrogen content (sewage sludge) with an ammonia nitrogen concentration of about 0.9 g/l. Carbonization test? was carried out using a reactor with a volume of one cubic meter heated by means of a diathermic oil jacket. Proof ? was carried out at a temperature of about 190 ?C, one hour of residence time at a pressure of about 13 bar. During the test? The relief pressure of the regulating valve has been set at approximately 13.5 bar, i.e. approximately 1 bar above the vapor pressure of water at that temperature. An excess pressure higher than the pressure set in the regulating valve allows the vapors formed in the reactor, first in the exchanger 3 and then in the separator 4, to escape, both maintained at a pressure of about 13-13.5 bar. The separator 4 ? equipped with a heating system to keep the condensed fraction at about 45 ?C. The gaseous fraction containing the ammonia flows into the tank 6 containing the acid agent, maintained at a temperature of about 20°C and at a pressure of 1 bar. Under these conditions, by making about 3000 liters of waste water flow, about 30 liters of aqueous solution containing 3.9 g/l of ammonia nitrogen and about 14.6 kg of microcrystalline ammonium sulphate were recovered continuously, obtained by bubbling the gas in a aqueous solution of sulfuric acid saturated with ammonium sulphate.

Example 2

A second test conducted using the plant and process conditions of the first example, with a sludge with approximately 0.9 g/l of ammoniacal nitrogen. The vapors evacuated from the continuous process were cooled in the exchanger 3 and then partially in the separator 4 which? been maintained at a temperature of 90 ?C. In this case, the concentration of ammoniacal nitrogen in the liquid fraction? was 0.46 g/l, instead? 3.90 g/l obtained at 45 ?C. The vapors were then made to flow inside a tank containing 100 liters of an aqueous solution of sulfuric acid 1/10 vol/vol, obtaining an ammonium sulphate solution at a concentration of 150 g/l.

Claims (10)

1. Method for the continuous recovery of ammoniacal nitrogen from a gaseous stream directly produced by thermal hydrolysis or hydrothermal carbonization of wastewater containing nitrogen, said method comprising the steps of: (a) in a closed reactor, subjecting wastewater containing nitrogen to a treatment of thermal hydrolysis or hydrothermal carbonization, said treatment comprising the step of heating said wastewater to a process temperature ranging from 120 to 250°C for a period of time ranging between 0.5 and 8 hours, until an increase in the vapor pressure is recorded inside said reactor due to the development of a gaseous phase comprising water vapor and gaseous NH3, said vapor pressure being between 2 and 50 bar in operation of said process temperature; (b) continuously withdrawing said gaseous phase from the reactor of step (a) and conveying it to a condensation and separation phase, maintaining the pressure inside said reactor at a value between 0.2 and 2.0 bar above above the above vapor pressure; (c) condensing the water vapor contained within said gaseous phase at a temperature ranging from 5 to 90°C, until obtaining the separation of a liquid aqueous phase and a gaseous phase comprising gaseous NH3; (d) recovering the ammoniacal nitrogen in the form of an ammonium salt by reacting the gaseous NH3 separated in step (c) with an acid. The method according to claim 1 comprising a step (c?) of controlling the pH of the liquid aqueous phase separated in step (c) and, in the case wherein said pH is > 8, heating said liquid aqueous phase up to a temperature between between 60 and 90°C obtaining a further separation of gaseous NH3 possibly dissolved in said separated liquid aqueous phase. The method according to claim 1, comprising a step (c1) of maintaining the liquid aqueous phase separated in step (c) at a temperature ranging from 60 to 90°C and adding a base until a pH of said liquid aqueous phase is registered greater than 8, preferably between 8 and 12, obtaining a further separation of gaseous NH3 possibly dissolved in said separated liquid aqueous phase. 4. Method according to any one of the preceding claims, wherein said phases (c), (c2), (c1) and/or (d) are phases operating continuously with phases (a) and (b). 5. Method according to any one of the preceding claims wherein said nitrogen-containing wastewaters are selected from the group consisting of: sewage sludge, livestock wastewater, agro-industrial wastewater, leachates, organic fractions of municipal solid waste, digestates and a combination thereof. 6. Plant for the continuous recovery of ammoniacal nitrogen directly from a gaseous stream produced by thermal hydrolysis or hydrothermal carbonization of wastewater containing nitrogen, said plant comprising: - a thermal hydrolysis or hydrothermal carbonization reactor (1) suitable for receiving said wastewater containing nitrogen and for operating at a process temperature between 120 and 250°C and at a vapor pressure, as a function of said process temperature, between between 2 and 50 bars; said reactor (1) being a closed reactor; - a capacitor (3) coupled with a two-phase separator (4); - a line (1a) leaving said reactor (1) and entering said condenser (3) for transporting said gaseous stream comprising water vapor and gaseous NH3; - a line (3a) leaving said capacitor (3) and entering said two-phase separator (4); - a tank (6) comprising an acid for the recovery of the ammoniacal nitrogen in the form of an ammonium salt; - a line (4b) leaving said two-phase separator (4) and entering said tank (6) for transporting the gaseous NH3 separated into said two-phase separator (4); - a regulating device (5) configured to control the pressure inside said reactor (1) said regulating device being a regulating device (5a) placed on the line (3a), or a regulating device (5b) placed on line (4b), in which said regulating device, on the basis of the pressure value detected inside said reactor, varies its degree of opening so? to maintain a pressure inside said reactor at a value comprised between 0.2 and 2.0 bar above said vapor pressure; said regulating device being preferably a regulating valve. 7. Plant according to claim 6 wherein said tank (6) comprises a cooling system and a safety valve whose opening ? operated and regulated on the basis of the pressure increase detected inside said tank (6). 8. Plant according to claim 6 or 7 comprising a storage tank (7), coupled to said two-phase separator (4) for collecting the water vapor separated from the gaseous NH3 and condensed in a liquid aqueous phase. 9. Plant according to claim 8 wherein said two-phase separator (4) comprises a heating system (8) connected thereto and said storage tank (7) comprises a system for detecting the pH of the liquid stream collected inside said storage tank, said heating system being operated and regulated to heat said two-phase separator (4) to a temperature between 60 and 90°C based on the increase of said pH of the liquid stream above 8. 10. Plant according to claim 8 wherein said two-phase separator (4) is connected to a dosing system of a base and said storage tank (7) comprises a system for detecting the pH of the liquid stream collected inside said storage tank, said dosing system being operated and regulated to obtain regulation of said pH up to a value greater than 8, preferably between 8 and 12, said two-phase separator (4) comprising a heating system (8) connected thereto; said heating system being operated and regulated to heat said two-phase separator (4) and keep it at a temperature between 60 and 90°C.