WO2018122717A1

WO2018122717A1 - Compact plant and optimized process for the production of an aqueous urea solution

Info

Publication number: WO2018122717A1
Application number: PCT/IB2017/058356
Authority: WO
Inventors: Riccardo ANNIBALE; Claudio Mascialino
Original assignee: Ama S.P.A.
Priority date: 2016-12-28
Filing date: 2017-12-22
Publication date: 2018-07-05
Also published as: IT201600131605A1

Abstract

The present invention refers to a plant (1) for the preparation of an aqueous urea solution (AUS) suitable to be used in an SCR process for removing nitrogen oxides from a flow of exhaust gas comprising a mixing tank (10), an inlet circuit (20) equipped with first pumping means (P1) to charge demineralized water into said tank (10), loading means (30) to charge urea in solid form into the tank (10) and mixing means to obtain said solution (AUS). In particular, the mixing means comprise a recirculating and mixing circuit (100) extending outside said tank (10) and comprising second pumping means (P2) suitable to collect in a batch mode a plurality of tangential nozzles (101) arranged and configured so as to maintain a turbulent swirling agitation in the liquid contained therein. The present invention also concerns a process for the preparation of an aqueous urea solution.

Description

COMPACT PLANT AND OPTIMIZED PROCESS FOR THE PRODUCTION

OF AN AQUEOUS UREA SOLUTION

DESCRIPTION

TECHNICAL FIELD OF INVENTION

[001]. The present invention relates to a compact plant and a process for the production of an aqueous urea solution suitable to be used in processes of selective catalytic reduction, commonly referred to as SCR, for the removal of nitrogen oxides from a gaseous stream.

PRIOR ART

[002]. Selective catalytic reduction (SCR) is a known process for the removal of nitrogen oxides from a gaseous stream, such as for example an exhaust gas generated by the combustion of fossil fuels, both inside industrial or commercial plants, and in internal combustion engines for motor vehicles.

[003]. In particular, the SCR systems are commonly used in the treatment of exhaust gases generated by vehicles powered by large diesel engines, on land travel as well as in maritime or river navigation.

[004]. In this specific instance, the SCR system converts nitrogen oxides NOx, such as in particular NO and NO2, into inert components such as gaseous nitrogen N2 and water vapor. The reaction involves the addition of a chemical reducing agent in a liquid or gaseous state, in general ammonia NH3, to exhaust gases in the presence of a catalyst and at a temperature generally between 180 °C and 350 °C.

[005]. The ammonia used in the SCR systems can be metered directly or obtained from the decomposition in situ of urea provided in an aqueous solution. The latter case is preferable since it makes it possible to avoid the problems connected with the storage and transportation of ammonia in a pure state or in an aqueous solution; in fact, advantageously, aqueous urea solutions are neither toxic nor flammable, and are not considered hazardous to handle.

[006]. For this purpose, generally an aqueous solution is used at a concentration of 30 - 40% in weight of urea, preferably about 32.5%, for example, known in the European market with the trade name AdBlue®; the quality of this product is regulated by international standards of reference, such as in particular the ISO 22241 -5 Standard.

[007]. An aqueous solution of urea can be obtained by dissolving solid urea, for example in granular form, in demineralized water, or it can be synthetically produced directly from the ammonia/urea production process.

[008]. Generally, the synthetic production is preferred, as carried out in a controlled manner and in large quantities in complex plants of considerable size; in this manner, the purity of the product and the absence of contaminating substances is guaranteed. Afterwards, the aqueous solution obtained is carried to suitable distributors all over the world, taking care to avoid the contamination of the product with any foreign substances.

[009]. However, one shortcoming lies in the fact that the transportation of the product obtained represents a considerable cost both from the economic and environmental point of view, also considering that most of the transported solution consists essentially of water.

[0010]. Moreover, if the product is to be used in the naval field, it is necessary to provide a large tank to contain a suitable quantity of product; in this case, the space requirement is not of the essence, due to the fact that the product consists mostly of water, which is a plentiful resource during navigation, considering also that usually there is already a water purification and demineralization plant on board ships for the treatment of seawater used for other purposes.

[0011]. It would instead be desirable, and in fact it is the main objective of the present invention, to provide a compact plant, and a relative optimized process for its operation, that is capable of producing in situ an aqueous solution of urea by dissolution and of meeting the required standard parameters of purity/quality, so as to dodge the network of transportation and distribution and the times of delivery.

[0012]. In the scope of the present objective, one purpose of the present invention consists of providing a compact plant for the production of an aqueous solution of urea, such as can be housed inside a container of standardized dimensions, and an optimized process for its operation that makes it possible to produce about 20 m3 of product a day.

[0013]. Another purpose of the present invention consists of providing a plant that is absolutely safe and reliable, and simple to operate by the user.

[0014]. A further objective of the present invention is to implement a plant that is at the same time sturdy and requires little maintenance.

[0015]. A further objective of the present invention is to provide an optimized process for the production of an aqueous solution of urea having a good efficiency against a limited consumption of external resources, and a plant that can be completely supplied with electric power.

[0016]. Another purpose of the present invention is to achieve a plant made up of easily- available low-cost components and provided with all the controls required to guarantee absolute quality of the delivered product.

[0017]. The above tasks and purposes, and others that will become more evident below, are achieved with a plant as defined in claim 1 , and with a process as provided for in claim 7; further advantageous characteristics are defined in the dependent claims.

BRIEF DESCRIPTION OF THE FIGURES

[0018]. Advantages and characteristics of the invention will become evident from the description which follows, by way of example and without limitations, with reference to the enclosed figures, wherein:

- figure 1 schematically illustrates a plant for the production in batch mode of an aqueous urea solution according to the present invention;

- figure 2 schematically illustrates a first phase of a batch mode process for the production of an aqueous urea solution according to the present invention, in which the tank is filled with demineralized water;

- figure 3 schematically illustrates an optional phase of the batch process for the production of an aqueous urea solution according to the present invention, in which the loaded demineralized water is heated;

- figure 4 schematically illustrates a second phase of the batch process for the production of an aqueous urea solution according to the present invention, in which solid urea is loaded into the tank and the suspension obtained is mixed to obtain the complete dissolution of the urea;

- figure 5 schematically illustrates a third phase of the batch process for the production of an aqueous urea solution according to the present invention, in which the solution obtained in the previous phase is discharged;

- figure 6 illustrates, in a cutaway view, a mixing tank suitable to be integrated in a plant according to the present invention;

- figure 7 schematically illustrates a plant for the batch production of an aqueous urea solution according to the present invention, comprising an additional osmotic filtration section for the production of demineralized water.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

[0019]. With particular reference to figure 1 is illustrated a plant according to the present invention for the discontinuous (or batch) production and by dissolution of an aqueous urea solution AUS in a preset concentration, in particular suitable to be used in an SCR process for the removal of nitrogen oxides from a stream of flue gas.

[0020]. In particular, the preferred urea concentrations are in the order of 32.5% in weight, suitable to produce the aqueous solution referred to as AUS32 used especially in the automobile field, and of 40% in weight, to obtain the aqueous solution referred to as AUS40 that is more suitable for the naval field.

[0021]. Advantageously, a plant 1 according to the present invention is of compact size, being scaled so as to be substantially housed inside a container C of standardized size. To facilitate the work of the operators, and avoid metering errors, the plant 1 is also advantageously designed to operate in an intermittent (or batch) manner, using as basic unit individual bags of solid urea, preferably in granular form, having standard weights of 1000 kg, with which granular urea is generally distributed on the market.

[0022]. Further, as explained later in greater detail, the present invention concerns an optimized process for the production of an aqueous urea solution that makes it possible to achieve an acceptable compromise between a daily production of AUS and the consumption of energy resources.

[0023]. Said plant 1 comprises essentially a mixing tank 10 into which is loaded, preferably through an inlet circuit 20, a preset quantity Q of a solvent liquid, such as in particular demineralized water, advantageously with a high degree of purity, that is, having an electrical conductivity lower than 10-6 Siemens; moreover, in the tank 10 is loaded, through loading means 30, a corresponding quantity G of solid urea, preferably in granular or powder form, to obtain the desired aqueous solution AUS.

[0024]. As shown in particular in figure 6, said tank 10 preferably comprises a vertical axis essentially cylindrical body portion 10A, inferiorly connected to an essentially conical bottom portion 10B to smoothen the flow of the liquid within it and the outflow of the AUS solution at the end of the production process; the top opening is preferably closed by a substantially flat top wall 10C, provided with a loading hatch 12 to allow the top loading of the solid urea. Said tank 10 is preferably made of metallic material, advantageously steel, in particular AISI 316 L steel, and advantageously can be externally insulated to maintain the temperature of the liquid contained in it.

[0025]. It was also found that a tank having a capacity of about 6000 liters makes it possible to achieve an output of about 20m3 a day of AUS solution, while being at the same time sufficiently compact to be easily housed inside the container C, together with all the other means and devices making up the plant 1 according to the present invention.

[0026]. Preferably, inside said tank 10 are provided temperature sensing devices ST, preferably consisting of at least one pair of sensors positioned at different heights, suitable to sense the temperature of the liquid loaded in it.

[0027]. Turning again to figure 1 , said inlet circuit 20 includes essentially an inlet duct 21 , connected to a first check valve EV1 , such as a valve with preferably electro-pneumatic operation, and to first pumping means P1 , such as for example a centrifugal pump, through which it is possible to draw said quantity Q of demineralized water, for example from a storage tank 50, imparting on the same pump the driving power necessary to reach said tank 10.

[0028]. If necessary, as shown in figure 7, the demineralized water used in the plant 1 of the present invention can be produced in a contiguous filtration plant 70, of known type, for example using membranes for reverse osmosis, itself also advantageously lodgeable inside the same container C that houses said plant 1 .

[0029]. Advantageously, said inlet circuit 20 also includes a first flow control means CL, such as an in-line flowmeter, for example a liter-counter, suitable to sense the flow of demineralized water in the inlet duct 21 , transmitting the sensed data to suitable well- known of command and control means 1 1 , such as for example a PLC unit; when the preset quantity Q is reached, the pump P1 is disengaged and said first electromagnetic valve EV1 is closed, so as to prevent any further inflow of demineralized water into the circuit 20 and eventually into the tank 10.

[0030]. If necessary, said inlet circuit 20 can also include heating means H, advantageously in-line and preferably electrical, such as for example an electric heater with a power of 55kW, suitable to raise the temperature of the demineralized water flowing in said inlet duct 21 ; in fact, advantageously, the higher the temperature of the demineralized water the shorter will be the mixing time necessary to obtain the desired AUS solution. It must also be considered that, according to experimental tests carried out, the dissolution of the urea in the demineralized water causes a lowering of about 15 °C in the temperature of the liquid, compared to the initial temperature.

[0031]. According to an advantageous characteristic, the preferable mixing temperature tM, that is, the temperature of the demineralized water inside the tank before adding the urea, is preferably higher than 32 °C, and is advantageously included between 38 °C and 40 °C. In fact, this temperature is a compromise between the power consumption tied to supplying the plant, the stability of the solution obtained, and the rate of dissolution of the urea, which reflects on the mixing time, thus making it possible to obtain a suitable daily output of aqueous solution.

[0032]. In this case, advantageously, said first pumping means P1 consist of a centrifugal pump with a low flow rate, preferably about 90 - 100 l/min, allowing in this manner a controlled flow of the stream of demineralized water through the electric heater H to reach the desired temperature in a single passage during the loading of the tank 10.

[0033]. The demineralized water, drawn in the preset quantity Q and possibly heated, is then fed into said tank 10, advantageously through a plurality of nozzles 101 , advantageously tangential to the wall of the same tank and spaced substantially along the vertical extension of the latter, preferably in positions corresponding with said body portion 10A and possibly also in said bottom portion 10B. A non-return valve V3 is advantageously arranged along said inlet duct 21 upstream of said tangential nozzles 101 .

[0034]. If necessary, if a single passage through the heater H is not sufficient to raise the temperature of the demineralized water to the desired mixing temperature tM, said plant 1 can be equipped with an extraction duct 102, connected at one end to the tank 10, preferably at the bottom portion 10B through a suitable second retaining means V1 , such as a valve, and connected at the opposite end to a first recirculating duct 22, the access to which is controlled by a suitable first valving unit formed, for example, by a pair of electromagnetic valves EV6, EV7, through which a heating flow rate Q1 of demineralized water can be recirculated in a continuous stream in the inlet circuit 20, upstream of the first pump P1 and of said heater H, to flow again through said heater until the temperature of the demineralized water in the tank 10, measured at different heights in the tank thanks to said pair of sensors ST, reaches the preset value for the mixing temperature tM.

[0035]. Preferably, during the heating phase, a first by-pass duct 23 and a relative second valving unit, comprising for example a pair of valves with electropneumatic operation EV2, EV3 suitably arranged, can be provided to by-pass the flowmeter CL.

[0036]. Advantageously, since the heating of the demineralized water necessary to reach the desired mixing temperature tM takes place outside the tank, the fact of providing a first pump P1 with a low flow rate, preferably in the order of 90 - 100 l/min, makes it possible to lend to the liquid an optimized flow rate through the heater H to obtain an appreciable rise in its temperature with every passage, thus shortening the time necessary to heat the quantity of demineralized water Q charged into the tank 10.

[0037]. The quantity G of solid, for example granular, urea determined on the basis of the desired AUS solution, is loaded into the tank 10, preferably after the tank has been filled with the quantity Q of demineralized water, through loading means 30 positioned on top of the tank 10 and formed essentially by a loading auger 31 , substantially horizontal, suitable to feed the product inside the tank 10 by pouring it through the loading hatch 12 provided on the top wall 10C of the tank; if necessary, a loading hopper 32 can be provided at the head of said auger 31 to smoothen the emptying of the bags of urea.

[0038]. Said auger 31 can be of fixed type, and housed advantageously inside the container that holds the plant 1 or, alternatively, it can be movable, and thus equipped with handling means to be positioned outside the container when in use or put away later.

[0039]. The use of said feeding auger 31 is advantageous since it makes it possible to charge the urea in a controlled, and if necessary a batch mode, in the demineralized water charged into the tank 10, and at the same time to crumble it to enlarge as much as possible the contact surface between the two components so as to favor its dissolution.

[0040]. Said plant 1 further comprises mixing means 100, suitable to mix the two components together inside said tank 10, initially forming essentially a suspension, to obtain a stable aqueous solution that meets the qualitative requirements imposed by the standard of reference.

[0041]. According to an advantageous characteristic of the present invention, said mixing means 100 include a recirculating and mixing circuit extending outside the tank 10 and comprising pumping means suitable to draw in a continuous mode a mixing flow rate Q2 of liquid from the previously filled tank 10 and to redirect it back into the same tank through said plurality of tangential nozzles 101 arranged and configured so as to maintain the suspension contained therein in turbulent swirling agitation to facilitate the close contact between the two components, maximizing the rate of dissolution of the urea in the demineralized water.

[0042]. If necessary, said second pumping means can coincide with said first centrifugal pump P1 ; in this case, the recirculating and mixing circuit 100 connects substantially with said inlet circuit 20, through said first recirculating duct 22, preferably by-passing the flowmeter CL and the heater H through a relative valving unit.

[0043]. Alternatively, preferably said recirculating and mixing circuit 100 comprises a second recirculating duct 103, connected to the bottom portion 10B of said tank 10 through said extraction duct 102 the access to which is controlled by said valve V1 , and through which said mixing flow rate Q2 is recirculated in a continuous mode and introduced back into the tank through said plurality of fluxing nozzles 101 .

[0044]. Advantageously, said second recirculating duct 103 is operatively connected to a second pumping means P2, different and separate from said first pump P1 , and appropriately configured to impose on the mixture a driving force that allows it to enter into the tank 10 at such a speed that, thanks also to the configuration of the nozzles 101 , it is able to create a swirling vortex and thus generate a forced mixing centrifugal swirling action.

[0045]. Preferably, said second pumping means P2 are formed by a second centrifugal pump advantageously characterized by a greater flow rate than the flow rate of said first centrifugal pump P1 , for example in the order of about 550 l/min, so as to inject the suspension into the tank, through said nozzles 101 , with a high tangential speed, so as to impose an energetic mixing to the suspension.

[0046]. Preferably, said recirculating and mixing circuit 1 00 further comprises second control means R suitable to verify the quality of the suspension/solution flowing inside the duct 100, and to determine when suitable quality requirements are achieved. For example, said second control means R may include a refractometer suitable to detect the opacity of the suspension flowing in the second recirculating duct 103, comparing it with the opacity set down by the standard of reference for the production of an aqueous urea solution suitable to be used in an SCR process.

[0047]. When this standard is met, by operating on a third valving unit, for example formed by a pair of electromagnetic valves EV8, EV10 suitably arranged, the AUS solution is deflected inside an outflow circuit 40 comprising a discharge circuit 41 branching off from said second recirculating duct 103, preferably downstream of said second pumping means P2, to be discharged into a storage container or tank 60, preferably located outside the container C that houses said plant 1 .

[0048]. Advantageously, said outflow circuit 40 is also connected to said flowmeter CL, suitably separated from the inlet circuit 20 to which it is connected thanks to a suitable valving unit formed, for example, by a pair of electromagnetic valves EV3, EV4, so as to sense and verify the exact quantity of aqueous solution AUS discharged. A third retaining means EV1 1 , such as a pneumatically controlled electromagnetic valve controls the outflow of the aqueous solution AUS toward the storage tank 60.

[0049]. If necessary, an additional valve EV9 controls the access to a second by-pass duct 104, through which it is possible to reduce the flow rate of said second pump P2.

[0050]. Clearly, a plant 1 according to the present invention can be equipped with additional known means and devices, useful for its operation, that, as they are not relevant for the purposes of this invention, have been omitted from the present description and are not illustrated in the enclosed figures. For example, suitable pneumatic control means can be provided, such as a compressor, connected to the many electromagnetic valves present in the plant. In addition, safety means can be provided, such as for example pressure gauges for controlling the pressure in the line, thermostats for controlling the temperature of the liquid, overpressure valves and draining points, to be used in particular to carry out the routine and the extraordinary maintenance of the plant.

[0051]. Said plant 1 works essentially according to the following operative phases: with reference to figure 2, is illustrated a first operative phase of the plant, in which the filling of said mixing tank 10 with a preset quantity Q of demineralized water is carried out through said inlet circuit 20.

[0052]. This first phase is activated by a command from the operator, acting for example through a relative interface connected to said command and control means 1 1 , which activate the opening of said first electromagnetic valve EV1 and actuate said first pump P1 .

[0053]. The flow of demineralized water collected is made to flow through the inside of said inlet duct 21 to be sent to the tank 10 through said plurality of fluxing nozzles 101 , advantageously passing through said in-line flowmeter CL that senses if the quantity of water matches the quantity preset by the user on the basis of the type of aqueous urea solution that is to be obtained, and, if present, through the heating device H which, if necessary, heats the demineralized water to bring it to the preset mixing temperature tM, advantageously higher than 32 °C and preferably included between 38 °C and 40 °C.

[0054]. When the preset quantity Q is reached, said command and control means 1 1 close said first electromagnetic valve EV1 and stop said first pump P1 , ending in this manner the first operative phase of the plant 1 .

[0055]. If necessary, if said temperature probes ST prearranged in the tank 10 sense that the temperature of the demineralized water is lower than the desired mixing temperature tM, it is possible to operate a phase for heating the liquid, optional, illustrated in figure 3; the first pump P1 is started again and at the same time said second retaining means V1 is opened, allowing the access of a flow rate of heating water Q1 of demineralized water in the extraction duct 102 and, at the same time, operating appropriately on the first valving unit EV6, EV7, deflected inside the recirculating duct 22.

[0056]. This last duct is connected, upstream of said first pump P1 , to said inlet duct 21 , so as to allow again the passage of the flow rate Q1 of demineralized water in the heating device H, preferably by-passing the flowmeter CL through said by-pass duct 23 controlled by said second valving unit EV2, EV3, and finally returning back inside the tank 10 through said plurality of nozzles 101 .

[0057]. The heating phase ends when both temperature probes ST sense that the temperature of the demineralized water inside the tank 10 is the same as the preset mixing temperature tM; at this point, said said first pump P1 is disabled and, by appropriately operating said second valving unit, the access to the first recirculating duct 22 is prevented.

[0058]. With reference to figure 4 is illustrated a third effective phase of the plant operating procedure, in which a suitable quantity G of solid urea is loaded and there is an intense mixing of the suspension initially created until there is the complete dissolution of the urea and thus the desired aqueous solution AUS is obtained.

[0059]. Initially, the command and control means 1 1 activate said second pump P2, which has advantageously a greater flow rate than said first pump P1 , and suitably operate said second valving unit to convey a mixing flow rate Q2 of demineralized water to the extraction duct 102 and toward the second recirculating duct 103, accessing therefore said mixing and recirculating circuit 100, which returns it, after a passage through said refractometer R, into the tank 10 through said tangential nozzles 101 .

[0060]. After a predefined time interval, for example about 30 seconds, during which the demineralized water contained in the tank 10 undergoes a turbulent swirling agitation thanks to the recirculating and mixing circuit 100, the granular urea is added through said loading means 30; the quantity G of urea that is to be used, defined on the basis of the desired concentration of the solution, is loaded into the loading hopper 32 and from there it is fed to the auger 31 , to be poured into the tank 10 through said hatch 12, while at the same time the liquid is continually recirculated in the circuit 100.

[0061]. Advantageously, said auger 31 works with a timed alternating mode, so as to add the urea into the tank 10 in a batch mode, guaranteeing a greater efficiency to the dissolution process.

[0062]. At the completion of the loading phase for the predefined quantity G of urea, said auger 31 is disabled, while the action of recirculating the suspension through the recirculating and mixing circuit 100 is continued until the urea is completely dissolved.

[0063]. Advantageously, thanks to said refractometer R, it is possible to determine precisely when the suspension/solution reaches the opacity required by the standard of reference to be considered suitable to be used in an SCR system.

[0064]. At this point, as shown in figure 5, the discharge phase is actuated: by operating on said third valving unit EV8, EV10, said command and control means 1 1 deflect the obtained AUS solution into said outflow circuit 40, to be conveyed through a discharge duct 41 , controlled by said third check valve EV1 1 , toward said storage tank 60, after a possible passage through said flowmeter CL.

[0065]. If necessary, by operating on said fourth valving unit which controls said second by-pass duct 104, it is advantageously possible to reduce the flow rate of said second pump P2 in the discharge phase.

[0066]. Below will be described a process according to the present invention for the discontinuous (or batch) preparation and the dissolution of an aqueous urea solution AUS in a preset concentration, suitable to be used in an SCR process for removing nitrogen oxides from a flow of exhaust gas. Said process is suitable to be carried out preferably, but not exclusively, in a plant 1 according to the present invention.

[0067]. Said process comprises essentially the following steps, preferably in the following order:

a) loading into a tank a preset quantity Q of demineralized water;

b) loading into the same tank urea in solid form in a preset quantity G, based on the concentration of the desired aqueous solution;

c) mixing the suspension formed by said two components previously loaded into the tank to obtain the dissolution of the urea in the demineralized water and the formation of an AUS solution, and finally

d) discharging the resulting solution (AUS) from the tank.

[0068]. In particular, according to an advantageous characteristic of the present invention, said steps b) and c) are carried out by keeping the demineralized water inside the tank in a turbulent swirling agitation, wherein the vortex agitation is created by continuously withdrawing a mixing flow rate Q2 of the liquid from the tank and recirculating said liquid back into the tank through a plurality of tangential nozzles. Advantageously, the swirling agitation can be generated before carrying out said step b) of loading solid urea into the tank.

[0069]. If necessary, after said step a) and before said step b), a step a') can be provided for heating the demineralized water, carried out by continually collecting a heating flow rate Q1 of demineralized water from the tank, and recirculating it back into the same tank at a higher temperature until a mixing temperature tM higher than about 32 °C, preferably between 38 °C and 40 °C, is achieved.

[0070]. Preferably, the mixing flow rate Q2 is greater than said heating flow rate Q1 .

[0071]. Further, preferably said step b) is carried out by loading the urea through the top of the tank, advantageously in a batch mode to favor its dissolution in the demineralized water.

[0072]. In conclusion, from the above it is evident how a plant 1 and a process for the production of an aqueous solution of urea suitable to be used in an SCR system according to the present invention makes it possible to achieve the initially foreseen purposes and advantages.

[0073]. In effect, a compact plant has been designed, along with a relative optimized process, capable of producing in situ, by dissolution, an aqueous urea solution that respects the standard required parameters of purity/quality, so as to sidestep the network of transportation and distribution and the times of delivery.

[0074]. Thanks to a plant and a process according to the invention it is possible to produce about 20 m3 of product a day, with a good efficiency against a limited consumption of external resources.

[0075]. Advantageously, a plant 1 according to the present invention is quite functional and compact, capable of being housed inside a container C of standardized dimensions, and it is also easy to manage by the user, being entirely dimensioned on the basis of individual bags of standard dimensions with which granular urea is commonly distributed on the market.

[0076]. As will be evident to a person skilled in the field, a plant according to the present invention is sturdy, as it is not equipped with moving mechanical parts inside the tank to obtain the mixing of the suspension, and thus requires little maintenance; furthermore, in this manner it is possible to avoid impurities that could build up and eventually lead to contaminations of the product and cause problems in the operation of the plant.

[0077]. Finally, a plant according to the present invention is made up of easily-available low-cost components and provided with all the required controls to guarantee the absolute quality of the delivered product.

[0078]. It is pointed out that in the above description, the directional terms such as "above, below, vertical, horizontal, lower and upper", like any other similar terms, should be interpreted with reference to the means and devices of a plant according to the present invention when in actual use, as shown in the enclosed figures.

[0079]. Naturally, the present invention is susceptible to many applications, modifications or variants without thereby departing from the scope of patent protection, as defined by the enclosed claims.

[0080]. Moreover, the materials and equipment used to implement the present invention, as well as the shapes and dimensions of the individual components, can be the most suitable according to the specific requirements.

Claims

1. Plant (1 ) for the production of an aqueous solution of urea (AUS) suitable for use in a SCR process for nitrogen oxides removal in a gaseous stream, said plant (1 ) comprising a mixing tank (10), an inlet circuit (20) provided with first pumping means (P1 ) for filling said tank with demineralized water, loading means (30) for loading urea in solid form into said tank (10) and mixing means for obtaining said solution (AUS) characterized in that said mixing means comprise a recirculating and mixing circuit (100) extending outside said tank (10) and comprising second pumping means (P2) adapted to continuously withdraw a mixing flow rate (Q2) of a liquid from the tank (10) and to recirculate said flow rate back into said tank through a plurality of tangential nozzles (101 ) configured and arranged so as to generate a turbulent swirling action in said tank (10).

2. Plant (1 ) according to claim 1 , wherein said inlet circuit (20) comprises a first inlet duct (21 ) associated with said first pumping means (P1 ) and connected to said tank (10) through said plurality of nozzles (101 ).

3. Plant (1 ) according to claim 2, wherein said inlet circuit (20) further comprises inline heating means (H), said plant (1 ) being equipped with an extraction duct (102) connectable to said inlet duct (21 ) upstream with respect to said heating means (H) whereby a heating flow rate (Q1 ) of demineralized water is continuously withdrawn from the filled tank (10) by means of said first pumping means (P1 ) and recirculated back to the tank (10) at a higher temperature.

4. Plant (1 ) according to any one of claims 1 to 3, wherein said tank (10) comprises a vertical axis cylindrical body portion (10A), inferiorly associated with an essentially conical bottom portion (10B), and superiorly closed with a top wall (10C) provided with a loading hatch (12).

5. Plant (1 ) according to any one of claims 1 to 4, wherein said first pumping means (P1 ) and said second pumping means (P2) comprise respectively a first centrifugal pump and a second centrifugal pump, said second centrifugal pump (P2) having a greater flow rate than the flow rate of said first centrifugal pump (P1 ).

6. Plant (1 ) according to any one of claims 1 to 5, wherein said loading means (30) comprise an auger (31 ) and a loading hopper (32).

7. Process for the production of an aqueous solution of urea (AUS) suitable for use in a SCR process for nitrogen oxides removal in a gaseous stream, comprising in sequence the following steps: a) filling a tank with a preset quantity (Q) of demineralized water, b) loading a preset amount (G) of urea in solid form into said tank, c) mixing the obtained suspension until complete dissolution of the urea so as to obtain said solution (AUS),

characterized in that

said steps b) and c) are performed while maintaining the liquid in the tank in turbulent swirling agitation, wherein the vortex agitation is created by continuously withdrawing a mixing flow rate (Q2) of the liquid from the tank and recirculating said liquid back into the tank through a plurality of tangential nozzles.

8. Process according to claim 7, wherein said turbulent swirling agitation is triggered before introducing the urea into the tank as in step b).

9. Process according to claim 7 or 8, wherein after said step a) and before said step b), a step a') of heating said demineralized water is provided, said step a') being performed by continuously withdrawing a heating flow rate (Q1 ) of demineralized water from the tank and recirculating it back at a higher temperature until the demineralized water in the tank reaches a mixing temperature (tivi) greater than about 32 °C.

10. Process according to claim 9, wherein said mixing temperature (tivi) is between 38 °C and 40 °C.

11. Process according to claim 9 or 10, wherein said mixing flow rate (Q2) is greater than said heating flow rate (Q1 ).

12. Process according to any one of claim 7 to 1 1 , wherein said step b) is discontinuously performed from the top of said tank so as to facilitate the dissolution of the urea in the demineralized water.