DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a method and related plant for the continuous recovery of ammoniacal nitrogen from the gas stream directly produced by hydrothermal treatments (thermal hydrolysis or hydrothermal carbonization) of nitrogen-containing wastes, said ammoniacal nitrogen being recovered in the form of an ammonium salt.

PRIOR ART

Hydrothermal treatments, such as 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 water in the liquid state under subcritical conditions, and allow organic material to be decomposed by converting it into a carbonaceous solid, biocarbon (or hydrochar - HC), and 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 waste with a high nitrogen content such as, for example, sludge or manure and agro-industrial waste. Generally, if the concentration of nitrogen in such a liquid component is too high (>4 g/L), a subsequent treatment is necessary in order to avoid and/or reduce environmental problems which could arise if the ammoniacal nitrogen is not removed and/or recovered effectively.

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

If, on the other hand, said liquid component is used as such by spreading it, for example, on agricultural land, it would cause an excessive presence of nutrients (over-fertilization) in the soils and contamination of the surface water and/or groundwater due to leakage and/or infiltration into the subsoil. This would have negative effects on the entire ecosystem, altering natural balances, without considering the harmful effects on human health.

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

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

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

Another category of interesting processes for the reduction and/or recovery of nitrogen, following hydrothermal treatments of waste 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 resulting from the hydrothermal treatment, and previously separated from the solid component, is brought to high pH and temperatures and is passed into a desorption column with filling bodies, which have the function of increasing the contact surface between the liquid (in which the nitrogen is contained in the form of hydrated ammonia NH4OH) and the gas (generally air). The two fluids generally enter the column in countercurrent: the air from below and the liquid from above. According to this scheme, the ammoniacal nitrogen is passed from the liquid state (in the form of hydrated ammonia NH4OH) to the gas state (in the form of ammonia, NH3). However, such a stripping technique requires considerable plant complexity and therefore also high investment costs.

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

A similar process is 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 thus separated by means of stripping the ammonia in a special reactor.

WO201 7055131 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 involve, 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 is therefore clear how the need remains in the sector to find a system which allows the efficient recovery of ammoniacal nitrogen directly from the treated waste and continuously with the hydrothermal treatment process thereof, without carrying out the solid-liquid separation steps.

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

SUMMARY OF THE INVENTION

The present invention relates to a method for the continuous recovery of ammoniacal nitrogen from a gas stream directly produced by thermal hydrolysis or hydrothermal carbonization of nitrogen-containing wastes, said method comprising the steps of:.

(a) in a closed reactor, subjecting nitrogen-containing wastes to a thermal hydrolysis or hydrothermal carbonization treatment, said treatment comprising the step of heating said waste to a process temperature comprised between 120 and 250 °C for a period of time comprised between 0.5 and 8 hours, until registering an increase in the vapour pressure inside said reactor due to the formation of a gaseous phase comprising water vapour and gaseous Nhb, said vapour pressure being comprised between 2 and 50 bar as a function of said process temperature;

(b) continuously collecting 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 comprised between 0.2 and 2 bar above the aforesaid vapour pressure;

(c) condensing the water vapour comprised within said gaseous phase at a temperature comprised between 5 and 90 °C, until obtaining the separation of a liquid aqueous phase and a gaseous phase comprising gaseous Nhb;

(d) recovering 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 steps (c) and/or (d) are steps operating continuously with steps (a) and (b).

For the purposes of the present invention, said thermal hydrolysis or hydrothermal carbonization is to be intended as performed in a single step (single step thermal hydrolysis or hydrothermal carbonization), i.e. step (a) described above.

According to a preferred embodiment of the invention, said step (a) comprises the continuous loading of said nitrogen-containing wastes within said closed reactor, preferably by means of a pumping system. In other words, said loading takes place continuously and simultaneously with the thermal hydrolysis or hydrothermal carbonization treatment. Preferably, said nitrogen-containing wastes are continuously drawn from a storage tank in which they are preferably maintained at room temperature and atmospheric pressure.

According to a particularly preferred embodiment, the method according to the present invention is characterized in that it does not comprise solid-liquid separation steps. Preferably, the method according to the present invention is characterized in that said nitrogen-containing wastes are not subjected to solid-liquid separation steps, more preferably to pre- and/or post-treatments comprising solid-liquid separation steps and/or addition of chemical reagents.

In other words, according to a particularly preferred embodiment of the invention, said nitrogen-containing wastes, initially contained within said storage tank are continuously loaded within said reactor without being subjected to solid-liquid separation steps, preferably to pre- and/or post-treatments comprising solid-liquid separation steps and/or addition of chemical reagents.

According to a preferred embodiment, said step (b) comprises continuously taking said gaseous phase from the reactor of step (a) and conveying it to a condensation and separation step, and, simultaneously, continuously discharging from the reactor of step (a) the reaction sludge resulting from said treatment of thermal hydrolysis or hydrothermal carbonization of said nitrogen-containing wastes, maintaining the pressure inside said reactor at a value between 0.2 and 2.0 bar above said vapour pressure. A further object of the present invention is a plant, preferably operating according to the method described above. Said plant comprises:

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

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

- a line (1a) exiting from said reactor (1) and entering into said condenser (3);

- a line (3a) exiting from said condenser (3) and entering into said two-phase separator (4);

- a tank (6) comprising an acid;

- a line (4b) exiting from said two-phase separator (4) and entering into 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 the line (4b), wherein said regulating device, on the basis of the pressure value detected inside said reactor, varies the degree of opening thereof.

Preferably, said regulating device (5) is a regulating valve. According to a preferred embodiment, the plant of the present invention comprises a second regulating device (5') placed on a line (1a') exiting said reactor (1) and configured to control the pressure inside said reactor (1), wherein said regulating device, based on the pressure value detected inside said reactor, varies its degree of opening so as to favour, continuously, the exit of the reaction sludge resulting from thermal hydrolysis or hydrothermal carbonization of said nitrogen-containing wastes in said reactor (1) and maintain a pressure inside said reactor at a value between 0.2 and 2.0 bar above said vapour pressure; said regulating device being preferably a regulating valve (5a').

Preferably, the plant according to any of the above-described embodiments is characterized in that it does not comprise solid-liquid separators and/or systems for adding chemical reagents to said effluent.

Preferably, said nitrogen-containing wastes are selected in the group consisting of: sludge, manure, leached agro-industrial waste, organic fractions of municipal solid waste, digestates and a combination thereof. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a block diagram depicting the components of the plant according to an embodiment of the present invention in which the first regulating device is a regulating valve, in particular a regulating valve (5b) placed on the line (4b) and the second regulating device (5') is also a regulating valve (5a') placed on the line (1a'). Figure 2 shows a block diagram depicting the components of the plant according to an alternative embodiment of the present invention in which the first regulating device is a regulating valve, in particular a regulating valve (5a) placed on the line (3a) and the second regulating device (5') is also a regulating valve (5a') placed on the line (1a').

DETAILED DESCRIPTION OF THE INVENTION For the purposes of the present invention, the term “hydrothermal treatment” generally refers to the thermal hydrolysis treatment or the hydrothermal carbonization treatment.

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

For the purposes of the present invention, "room temperature" means a temperature value, measured under atmospheric pressure conditions, comprised between 5 and 40 °C, preferably between 15 and 30 °C, more preferably between 20 and 25 °C.

For the purposes of the present invention, "atmospheric pressure" or "ambient pressure" means a pressure value within ± 0.05 atm of the normal or standard atmospheric pressure measured at a latitude of 45°, at sea level and at a temperature of 0 °C on a surface unit of 1 cm ² , which corresponds to a mercury column pressure of 760 mm, or corresponding to 1 atm.

For the purposes of the present invention, “vapour pressure” means the pressure exerted by the vapour of a substance on the condensed phase (solid or liquid) of the same substance when such phases are under thermodynamic equilibrium conditions with each other within a closed system, i.e., under saturated vapour conditions.

For the purposes of the present invention, the terms “aqueous ammonium sulphate solution” and “ammonium sulphate in aqueous solution” are used in a perfectly interchangeable manner.

For the purposes of the present invention, the terms “liquid stream” and “liquid phase” as well as the terms “gas stream” and “gaseous phase” are used, unless otherwise specified, as perfectly interchangeable synonyms. For the purposes of the present invention, the term “ammoniacal nitrogen” refers both to nitrogen in the form of ammonium ion NFl4 ⁺ (when in solution, “N-NFl4 ⁺ ”) and in the form of gaseous ammonia (gaseous NFb).

For the purposes of the present invention, "room temperature" means a temperature between 20 and 25 °C considered at atmospheric pressure.

The present invention relates to a method for the continuous recovery of ammoniacal nitrogen from a gas stream directly produced by the hydrothermal treatment (thermal hydrolysis or hydrothermal carbonization) of waste with a high nitrogen content. The method according to the present invention comprises the steps of:

(a) in a closed reactor, subjecting nitrogen-containing wastes to a thermal hydrolysis or hydrothermal carbonization treatment, said treatment comprising the step of heating said waste to a process temperature comprised between 120 and 250 °C for a period of time comprised between 0.5 and 8 hours, until registering an increase in the vapour pressure inside said reactor due to the formation of a gaseous phase comprising water vapour and gaseous Nhb, said vapour pressure being comprised between 2 and 50 bar as a function of said process temperature;

(b) continuously collecting 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 comprised between 0.2 and 2 bar above the aforesaid vapour pressure;

(c) condensing the water vapour comprised within said gaseous phase at a temperature comprised between 5 and 90 °C, until obtaining the separation of a liquid aqueous phase and a gaseous phase comprising gaseous Nhb;

(d) recovering 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 waste at a process temperature preferably comprised between 180 and 190 °C and for a period of time preferably comprised between 0.5 and 2 hours until registering an increase in the vapour pressure inside said reactor due to the formation of a gaseous phase comprising water vapour and gaseous NH3, said vapour pressure being comprised between 10 and 14 bar, as a function of said process temperature.

Preferably, said nitrogen-containing wastes are selected in the group consisting of: sludge, manure, leached agro-industrial waste, organic fractions of municipal solid waste, digestates and a combination thereof.

Preferably said waste is liquid waste comprising a total water amount greater than 70% w/w, preferably greater than 80% w/w, preferably greater than 85% w/w, more preferably comprised between 70 and 90% w/w, still more preferably comprised between 85 and 90%w/w. Preferably, the nitrogen content (in the form of ammonium ion N-NH4 ⁺ or organic nitrogen) in said waste is comprised between 0.5 and 4 g/l, preferably between 1 and 2 g/l.

Preferably, said gaseous phase formed, in step (a), following the thermal hydrolysis or hydrothermal carbonization reactions inside the reactor, may comprise, in addition to water vapour and gaseous NH3 also other types of gases, such as CO2, depending on the type of waste treated.

Preferably, said thermal hydrolysis or hydrothermal carbonization treatment occurs in the presence of water, said water being already comprised within the nitrogen- containing wastes (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 comprised between 70 and 90% w/w, even more preferably comprised between 85 and 90%w/w) or being added thereto if said waste has a water content of less than 70% w/w, preferably less than 80% w/w, more preferably less than 85% w/w until reaching a total water percentage preferably comprised between 85 and 90% w/w.

According to an embodiment of the invention, step (b) is a step of continuously collecting 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 comprised between 0.5 and 1.0 bar above the aforesaid vapour pressure.

According to a particularly preferred embodiment of the invention, step (b) is a step of continuously drawing said gaseous phase from the reactor of step (a) by conveying it to a condensation and separation step, and, simultaneously, continuously discharging from the reactor of step (a) the reaction sludge resulting from said treatment of thermal hydrolysis or hydrothermal carbonization of said nitrogen- containing wastes, maintaining the pressure inside said reactor at a value between 0.2 and 2 bar, preferably between 0.5 and 1.0 bar above said vapour pressure. According to a preferred embodiment of the invention, said step (a) provides for the continuous loading of said nitrogen-containing wastes inside said closed reactor, preferably by means of a pumping system. In other words, said loading takes place continuously and simultaneously with the thermal hydrolysis or hydrothermal carbonization treatment. Preferably, said nitrogen-containing wastes is continuously drawing from a storage tank in which it is preferably maintained at room temperature and atmospheric pressure. In other words, said withdrawal takes place continuously and simultaneously with the loading of the effluent into the reactor and the thermal hydrolysis or hydrothermal carbonization treatment.

According to a particularly preferred embodiment of the invention, the method as described above is characterized in that it does not comprise solid-liquid separation steps. Preferably, the method according to the present invention is characterized in that said nitrogen-containing wastes are not subjected to solid-liquid separation steps, more preferably in that they are not subjected to pre- and/or post-treatment comprising solid-liquid separation steps and/or addition of chemical reagents.

In other words, according to a particularly preferred embodiment of the invention, said nitrogen-containing wastes, initially contained within said storage tank is continuously loaded within said reactor without being subjected to solid-liquid separation steps, more preferably to pre- and/or post-treatments comprising solid- liquid separation steps and/or addition of chemical reagents.

Without wishing to be bound by a specific theory, the Applicant has found that, thanks to the specific temperature conditions in which said waste is treated inside said closed reactor, to the corresponding increase in vapour pressure, and to the maintenance of an over-pressure comprised between 0.2 and 2.0 bar, preferably comprised between 0.5 and 1.0 bar above said vapour pressure, it is possible to optimize not only the hydrothermal treatment of the waste but, simultaneously and continuously, also to carry out an effective recovery of ammoniacal nitrogen directly from said waste without interrupting the hydrothermal treatment and/or subjecting said waste to solid-liquid separation steps and/or adding chemical reagents. Therefore, the method according to the present invention advantageously allows ammoniacal nitrogen to be continuously recovered, directly from the thermal hydrolysis or hydrothermal carbonization reactor (i.e., directly from the waste), quickly, without the need for complex equipment and with low process costs. In fact, in the case of traditional systems the need to perform solid-liquid separation steps and/or to add chemical reagents directly to the waste contained in the reactor or to the separate liquid component greatly increases the complexity of the operations as well as the cost/environmental impact in terms of the amount of additional chemical reagents used.

According to a preferred embodiment, step (c) is a step of condensing the water vapour comprised within said gaseous phase at a temperature comprised between 5 and 90 °C, preferably comprised between 20 and 60 °C until the separation of a liquid aqueous phase and gaseous Nhb is obtained. Preferably, said step (c) is carried out by means of a condenser coupled with a two-phase separator.

More preferably, during said step (c), the vapour pressure inside said condenser and said two-phase separator is comprised 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 in the group consisting of: sulphuric acid, hydrochloric acid or nitric acid. According to a preferred embodiment, step (d) of the method according to the present invention is a step of recovering 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 is therefore selected in the group consisting of: an aqueous solution of sulphuric acid, an aqueous solution of hydrochloric acid or an aqueous solution of nitric acid.

Preferably the acid used in step (d) is sulphuric acid, more preferably an aqueous solution of sulphuric 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) within said aqueous solution of said acid. Preferably, according to such an embodiment, said aqueous solution of said acid is 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 ammoniacal nitrogen is therefore recovered in the form of an ammonium salt, preferably selected in the group consisting of: ammonium sulphate, ammonium chloride or ammonium nitrate. Preferably, the ammoniacal nitrogen is recovered in the form of a crystalline ammonium salt, preferably in the form of a monocrystalline ammonium salt; said ammonium salt being selected in 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 sulphuric acid saturated with ammonium sulphate, the ammoniacal nitrogen is recovered in the form of a monocrystalline ammonium sulphate salt.

Preferably the ammoniacal nitrogen is recovered in the form of a solution of an ammonium salt, preferably selected in 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 as a result of the reaction between the gaseous Nhb separated in step (c) and said acid or said aqueous solution of an acid.

Therefore, the method according to the present invention allows ammoniacal nitrogen to be recovered directly from waste 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) which is directly usable for commercial purposes in various technological sectors, such as the inorganic fertilizer sector.

According to an embodiment, the method according to the present invention essentially consists of or consists of steps (a), (b), (c) and (d) described above. According to an embodiment, the method according to the present invention comprises a step (o’) of controlling the pH of the liquid aqueous phase separated in step (c) and, if said pH is > 8, heating said liquid aqueous phase up to a temperature comprised between 60 and 90 °C, obtaining a further separation of gaseous NH3 possibly dissolved in said separated liquid aqueous phase.

According to an embodiment, the method according to the present invention essentially consists of or consists of steps (a), (b), (c), (o’) and (d) described above.

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

According to an embodiment, the method according to the present invention essentially consists of or consists of steps (a), (b), (c), (d) and (d) described above. Preferably, the liquid aqueous phase obtained following step (c), or following step (c)+(c’) or (c)+(c1), comprises a nitrogen content comprised 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 is characterized in that steps (c), (o’), (d) and/or (d) as described above are steps operating continuously with steps (a) and (b).

This advantageously allows to carry out the recovery of ammoniacal nitrogen continuously with the treatment of thermal hydrolysis or hydrothermal carbonization without interrupting such a waste treatment process. In other words, the method according to the present invention allows ammoniacal nitrogen to be recovered directly from the waste to be treated without any additional treatment such as, for example, a pH adjustment of said waste before, during or after the hydrothermal treatment or a solid-liquid separation before and/or after the hydrothermal treatment.

A further object of the present invention is a plant for the continuous recovery of ammoniacal nitrogen from a gas stream directly produced by thermal hydrolysis or hydrothermal carbonization of nitrogen-containing wastes, 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) adapted to receive said nitrogen-containing wastes and to operate at a process temperature comprised between 120 and 250 °C and at a vapour pressure, as a function of said process temperature, comprised between 2 and 50 bar; said reactor (1) being a closed reactor;

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

- a line (1a) exiting from said reactor (1) and entering into said condenser (3) for the conveyance of said gas stream comprising water vapour and gaseous NH3; - a line (3a) exiting from said condenser (3) and entering into 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) exiting from said two-phase separator (4) and entering into said tank (6) for the conveyance of the gaseous NFb separated in said two-phase separator (4);

- a first 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 (5b) placed on the line (4b) (Figure 1), wherein said regulating device, on the basis of the pressure value detected inside said reactor, varies the degree of opening thereof so as to promote, continuously, the venting of the gaseous current from the reactor (1) and to maintain a pressure inside said reactor at a value comprised between 0.2 and 2.0 bar above the aforesaid vapour pressure.

According to a preferred embodiment of the invention, said regulating device (5, 5a and 5b) is a regulating valve.

According to an embodiment, said plant comprises a storage tank (0) of said wastes preferably maintained at room temperature and atmospheric pressure.

In other words, said storage tank (0) of said wastes is a storage tank preferably maintained at room temperature and atmospheric pressure.

Preferably, said reactor (1) is a closed reactor, wherein said nitrogen-containing wastes are loaded within said reactor (1) continuously by means of a pumping system (V) connected to said wastes storage tank (0).

In other words, said pumping system (V) allows the drawing of said wastes from said storage tank continuously with their loading inside the reactor (1) and with the reaction of thermal hydrolysis or hydrothermal carbonization taking place inside said reactor.

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

Preferably, said heating system is selected in the group consisting of: a heating jacket, a water vapour blowing system, or a combination thereof. Preferably said condenser (3) and said two-phase separator (4) are at a temperature comprised between 5 and 90 °C, preferably comprised between 20 and 60 °C. Preferably, the vapour pressure inside said condenser (3) and said two-phase separator (4) is comprised between 2 and 50 bar, preferably between 10 and 14 bar. According to an embodiment of the invention, said first regulating device, preferably said regulating valve, (5) varies the degree of opening thereof so as to promote, continuously, the venting of the gaseous current from the reactor (1) and to maintain a pressure inside said reactor at a value preferably comprised between 0.5 and 1.0 bar above the aforesaid vapour 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 vapour separated from the gaseous Nhb.

Preferably, said storage tank (7) is under 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 based on 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) is connected to a base dosing system 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 activated and regulated so as to obtain the adjustment of said pH up to a value comprised between 8 and 9. According to such an embodiment, said two-phase separator (4) also comprises a heating system (8) which allows the temperature of said two-phase separator (4) to be maintained at a value comprised between 60 and 90 °C.

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

According to a particularly preferred embodiment, said tank (6) comprises an acid selected in the group consisting of: sulphuric 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 sulphuric acid, more preferably an aqueous solution of sulphuric acid saturated with ammonium sulphate.

Preferably, said tank (6) also comprises a cooling system and a safety valve whose opening is operated and regulated based on the (possible) pressure increase detected inside said tank (6).

Preferably, said cooling system advantageously allows said tank (6) to be 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 gaseous Nhb and the acid, preferably the aqueous solution of an acid, comprised inside said tank.

Similarly, said safety valve advantageously allows to avoid an excessive increase in pressure inside said tank as a result of said neutralization reactions.

According to a preferred embodiment, the ammoniacal nitrogen is thus recovered, by means of the plant of the present invention, in the form of an ammonium salt, preferably selected in the group consisting of: ammonium sulphate, ammonium chloride or ammonium nitrate.

Preferably the ammoniacal nitrogen is recovered in the form of a solution of an ammonium salt, preferably selected in 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 comprised between 2 and 15 g/l, preferably between 6 and 10 g/l.

According to a particularly preferred embodiment, the plant according to the present invention comprises a second regulating device (5') placed on a line (1a') exiting from said reactor (1) and configured for controlling the pressure inside said reactor (1), wherein said regulating device, based on the pressure value detected inside said reactor, varies its degree of opening so as to favour, continuously, the exit of the reaction sludge resulting from thermal hydrolysis or hydrothermal carbonization of said nitrogen-containing wastes in said reactor (1) and maintain a pressure inside said reactor at a value between 0.2 and 2.0 bar above said vapour pressure; said regulating device being preferably a regulating valve (5a').

Without wishing to be bound by a specific theory, the Applicant has found that it is particularly advantageous for the purposes of continuous recovery of ammoniacal nitrogen, to regulate the pressure inside the reactor (1) by the joint action of the two regulating devices (5) and (5') described above.

According to a particularly preferred embodiment, the plant of the present invention is characterized in that it does not comprise solid-liquid separators and/or systems for adding chemical reagents to said effluent.

Advantageously, the plant according to the present invention therefore allows ammoniacal nitrogen to be recovered directly from waste 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) which is directly usable for commercial purposes in various technological sectors, such as the inorganic fertilizer sector.

According to an embodiment, the plant according to the present invention consists essentially of or consists of the elements described above.

EXAMPLES

The test was conducted in a plant which continuously treats about one tonne/hour of waste with a high nitrogen content (sludge) with an ammoniacal nitrogen concentration of about 0.9 g/l. The carbonization test was conducted using a reactor with a volume of one cubic metre heated by means of diathermic oil jacket. The test was conducted at a temperature of about 190 °C, one hour residence time at a pressure of about 13 bar. During the test, the vent pressure of the control valve was set to about 13.5 bar, then about 1 bar above the water vapour pressure at that temperature. An excess pressure higher than the pressure set in the control valve allows the vapours formed in the reactor to escape first into the exchanger 3 and then into the separator 4, both kept at a pressure of about 13-13.5 bar. The separator 4 is provided with a heating system to keep the condensed fraction at about 45 °C. The gaseous fraction containing ammonia flows in the tank 6 containing the acid agent, maintained at a temperature of about 20 °C and a pressure of 1 bar. Under these conditions, about 3,000 litres of waste were continuously recovered by flushing about 30 litres of aqueous solution containing 3.9 g/l of ammoniacal nitrogen and about 14.6 kg of microcrystalline ammonium sulphate obtained by bubbling the gas in an aqueous solution of sulphuric acid saturated with ammonium sulphate.

Example 2

A second test was conducted using the plant and process conditions of the first example, with a sludge with about 0.9 g/l of ammoniacal nitrogen. The vapours evacuated from the continuous process were cooled in the exchanger 3 and then partly in the separator 4 which was 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 of 3.90 g/l obtained at 45 °C. The vapours were then flowed into a tank containing 100 litres of an aqueous solution of 1/10 vol/vol sulphuric acid, obtaining a solution of ammonium sulphate at a concentration of 150 g/l.