CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102019000006002 filed on April 17, 2019, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a system and a method for feeding a urea synthesis reactor.

In particular, the invention relates to a urea synthesis reactor provided with a feeding system for feeding the reactor with ammonia, carbon dioxide and non-converted reactants recovered downstream of the reactor; and a method for feeding a urea synthesis reactor with ammonia, carbon dioxide and non-converted reactants recovered downstream of the reactor.

STATE OF THE PRIOR ART

As is known, urea is produced on an industrial scale through processes based on the reaction, under conditions of high temperature and high pressure, between carbon dioxide and ammonia forming ammonium carbamate (intermediate), and on the subsequent decomposition reaction of ammonium carbamate forming urea and water.

The urea synthesis reaction is carried out in a reactor from which an aqueous urea solution is obtained, which is then progressively concentrated, with recovery and recycling of the non-converted reactants, and solidified in a finishing section (for example, a granulator or a prilling tower) .

The urea solution leaving the reactor contains non- converted reactants, which are recovered in different ways, depending also on the plant scheme and on the process adopted, for being then fed back to the reactor.

A typical urea synthesis reactor of a urea plant is therefore supplied with an essentially gas flow of carbon dioxide and an essentially liquid flow of ammonia and ammonium carbamate in solution.

The carbamate is normally recovered in aqueous solution at a pressure which is not sufficient for being fed to the bottom of the reactor, which is typically found at higher pressures.

For the recycling of the carbamate to the reactor, in particular, as is known, an ejector (carbamate ejector) is used .

Thanks to the geometry of the ejector, the carbamate to be recycled to the reactor is sucked up and recycled to the reactor exploiting the loss of kinetic energy of the ammonia at high pressure which acts as driving fluid and is also fed to the reactor.

The ejector is an apparatus which, although extremely simple and reliable being free of moving mechanical parts, is characterized by a low efficiency: in order to be able to recycle the carbamate to the reactor, even where the pressure drop is relatively small, the driving fluid (ammonia) pressure demand at the ejector inlet necessary for the entrainment of the carbamate is very high, with a pressure drop between inlet and outlet being also significant.

Especially for large plants, the use of an ejector can therefore be highly limiting in terms of efficiency and energy consumption, as well as availability on the market.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome the drawbacks of the prior art highlighted above; in particular, it is an object of the invention to provide a system and a method for feeding a urea synthesis reactor, and consequently a plant and a method for producing urea, capable of having improved efficiency with respect to the prior art, allowing in particular a reduction in energy consumption .

The present invention therefore relates to a urea synthesis reactor provided with a feeding system for feeding the reactor with ammonia, carbon dioxide and non-converted reactants recovered downstream of the reactor, and a method for feeding a urea synthesis reactor with ammonia, carbon dioxide and non-converted reactants recovered downstream of the reactor, as defined in essential terms in the appended Claims 1 and 8, respectively.

The invention also relates to a urea plant as defined by Claim 7; as well as a process for the synthesis of urea by direct biphasic reaction of ammonia and carbon dioxide at high temperature and high pressure, as defined in Claim 14.

Preferred additional features of the invention are indicated in the dependent claims.

The invention provides a plant and a method for producing urea which are particularly efficient in terms of energy consumption, in particular in the feeding of the urea synthesis reactor.

Instead of an ejector or other poorly efficient apparatus, in accordance with the invention, a turbocharger is used for the recycling of non-converted reactants (carbamate) to the reactor: as in an ejector, also in the turbocharger, the ammonia is exploited as driving fluid, but in this case the ammonia transfers part of its kinetic energy to the carbamate, supplying the energy necessary for pumping the carbamate into the reactor, acting on a first impeller connected to a second impeller which increases the pressure of the carbamate.

The use of a turbocharger allows to reduce the pressure required for the ammonia and the relative pressure drop compared to plant configurations with an ejector, reducing energy consumption and thus improving efficiency. The turbocharger does not present any sealing problems, since the shaft that connects the two impellers of the turbocharger is not externally connected to any engine, and does not require a forced separation of the two flows in transit, which can partly mix without substantial reductions in efficiency.

In addition to reducing energy consumption, the solution of the invention also allows to keep the flows of ammonia and carbamate fed into the reactor separate.

In this way, unlike with an ejector, the ammonia and carbamate streams downstream of the turbocharger remain separate and can thus be fed to the reactor in optimal positions .

In particular, this makes it possible to feed ammonia not only into the bottom of the reactor but also along the vertical extension of the same, in an optimal position to facilitate the conversion.

Carbamate can also be fed at an optimal reactor height. Furthermore, while in known plant configurations the carbamate is sent in full to the carbamate condensers, it is possible to send part of the carbamate directly from the high-pressure pumps to the reactor, thus allowing for greater operational flexibility.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become clear from the description of the following non-limiting embodiment examples, with reference to the figures of the attached drawings, wherein:

- Figure 1 is a schematic view of a urea production plant (urea plant) according to the invention;

- Figure 2 is a schematic view in greater detail of a portion of the urea plant of Figure 1;

- Figure 3 is a schematic view of an apparatus (in particular, a turbocharger) used in the urea plant of Figures 1 and 2 ;

- Figure 4 shows variants of the urea plant of the invention .

PREFERRED EMBODIMENT OF THE INVENTION

Figure 1 shows an extremely schematic view of a urea plant 1 (i.e. a plant for the production of urea), operating for carrying out a process for the synthesis of urea by direct biphasic reaction of ammonia and carbon dioxide at high temperature and high pressure.

The general configuration of the urea plant 1 can be of various types, as well as the urea production process carried out in the plant.

For example, but not necessarily, the urea plant 1 is of the type operating essentially according to the technology known as "Snamprogetti™ Urea Technology", to which the changes described below are applied for the implementation of the present invention. However, it is understood that the invention also applies to other urea production plants/processes .

In general, the urea plant 1 comprises: a urea synthesis reactor 2 wherein a urea synthesis reaction from ammonia and carbon dioxide takes place; treatment sections 3, shown as a whole with dotted lines in Figure 1, which can have various configurations (comprising for example: a high pressure recovery section, a medium pressure recovery section, a low pressure recovery section, a vacuum concentration section connected to a process condensate treatment section) , wherein a urea solution produced in the reactor 2 is progressively concentrated with removal therefrom of unreacted ammonia and carbon dioxide and water and recirculation of the recovered components; a finishing/solidification section 4 comprising, for example, a granulator or a prilling tower.

A urea line 7 brings a urea solution produced in the reactor 2 to the treatment sections 3, where the urea is progressively concentrated and separated from the non- converted reactants, before being sent to the finishing/solidification section 4.

Here and in the following, not all the components of the various sections and of the circuits which connect them are indicated and described, but only those useful for the understanding of the present invention.

The reactor 2 is fed with the reactants required for the urea synthesis reaction, i.e. C02 and NH3, as well as with non-converted reactants recovered from the treatment sections 3 and recirculated to the reactor 2, via a feeding system 10.

In greater detail and with reference also to Figure 2, the reactor 2 has a first inlet 11, a second inlet 12 and a third inlet 13, connected to respective feeding lines 21, 22, 23 of the feeding system 10 and precisely: a C02 feeding line 21, an ammonia feeding line 22, and a carbamate feeding line 23 (here and in the following, ammonium carbamate) .

The inlets 11, 12, 13 are distinct and separate from one another and positioned in different positions on the reactor 2, preferably in a lower portion of the reactor 2 if the latter, as in the example shown, has a substantially vertical development (i.e. it extends along a vertical longitudinal axis in use) .

In particular, the inlet 11 connected to the C02 feeding line 21 and the inlet 12 connected to the ammonia feeding line 22 are, for example, positioned at a lower end of the reactor 2, through a bottom wall, for example shaped like a cap, of the reactor 2.

In the non-limiting example illustrated, the inlet 13 connected to the carbamate feeding line 23 is positioned higher in the reactor 2 (at a higher level, i.e. at a greater height along the longitudinal axis of the reactor 2) with respect to the inlet 12 connected to the ammonia feeding line 22, in particular through a side wall, for example of substantially cylindrical shape, of the reactor 2.

However, it is understood that the inlets 11, 12, 13 can be positioned differently than as shown herein by way of example only. In particular, the inlet 13 connected to the carbamate feeding line 23 does not necessarily have to be positioned higher than the inlet 12 connected to the ammonia feeding line 22, and can be positioned lower or even substantially at the same height, for example this too through the bottom wall of the reactor 2, of the inlet 12.

Furthermore, the reactor has an outlet 26, positioned at an upper end of the reactor 2 and connected to the urea line 7, from which an aqueous urea solution containing non- converted reactants is withdrawn from the reactor 2.

The urea line 7 connects the outlet 26 to the treatment sections 3 (known and not illustrated), where the urea solution produced in the reactor 2 is progressively concentrated and the non-converted reactants, in particular carbamate (ammonium carbamate) , are recovered.

The methods for recovering carbamate (and in general non-converted reactants) can be various, also depending on the urea process carried out in the plant 1, and require variously configured and connected components.

The carbamate recovered in the treatment sections 3 is then sent to the reactor 2 through the carbamate feeding line 23.

In the purely exemplary embodiment illustrated in particular in Figure 2, the carbamate (understood, here and in the following, as a solution containing carbamate) is withdrawn from the treatment sections 3 via a carbamate line 27, equipped with a high pressure pump 28 and a flow rate control valve 29 (associated with a flow controller FC) , and sent into a carbamate condenser 30 (defined in particular by a heat exchanger) and into a carbamate separator 31, arranged in series along the carbamate line 27 and connected by a section 27a of the carbamate line 27.

Optionally, on the carbamate line 27 a vapor line 32 is inserted, upstream of the carbamate condenser 30, which adds a gas flow containing process vapors, also recovered from the treatment sections 3, to the carbamate solution circulating in the carbamate line 27.

The carbamate separator 31 is configured for carrying out the separation of a liquid phase containing carbamate from a gas phase, which is removed from a separator head outlet through a gas discharge line 33.

The liquid phase exits from a bottom outlet of the carbamate separator 31 and constitutes an essentially liquid stream containing carbamate (carbamate stream) which is sent to a further section 27b of the carbamate line 27 to be fed to the reactor 2.

Indicatively, the carbamate stream leaving the carbamate separator 31 and circulating in the section 27b has a pressure comprised between about 140 bar (g) and about 150 bar (g) .

An ammonia line 34, equipped with one or more high pressure pumps 35, brings ammonia, precisely an essentially liquid stream containing ammonia (ammonia stream) , to the ammonia feeding line 22.

Advantageously, the ammonia line 34 is provided with a ramification 36 controlled by a flow rate control valve 37 (associated with a flow controller FC) and connected via an auxiliary line 38 to the treatment sections 3, so as to split the ammonia between a first stream, which feeds the reactor 2, and a second stream, which is sent to the treatment sections 3.

Indicatively, the ammonia circulating in the ammonia line 34 downstream of the high pressure pump 35 has a pressure comprised between about 190 bar (g) and about 210 bar ( g) .

Furthermore, the line 34 is optionally provided with a flow rate control valve 39 (associated with a flow controller FC) positioned downstream of the pump 35 to possibly reduce the ammonia pressure in a final section 34b of the line 34.

According to the invention, the feeding system 10 comprises a turbocharger 40, which is used for feeding ammonia and carbamate to the reactor 2.

The turbocharger 40 is an apparatus (precisely, a turbomachine) , schematically represented in Figure 3, essentially comprised of two impellers 41a, 41b integrally connected to each other by a rotating shaft 42. The impellers 41a, 41b are rotatably housed in respective separate housings

43a, 43b, equipped with respective inlets 44a, 44b, and respective outlets 45a, 45b.

In accordance with the invention, the inlets 44a, 44b of the turbocharger 40 are connected to the ammonia line 34 (precisely, to its final section 34b) and to the carbamate line 27 (precisely, to its section 27b) , respectively; the outputs 45a, 45b are connected to the ammonia feeding line

22 and to the carbamate feeding line 23, respectively. Therefore, the turbocharger 40 has an ammonia-side impeller 41a and a carbamate-side impeller 41b.

The ammonia-side impeller 41a is rotationally driven by the passage of the ammonia stream fed to the turbocharger 40 through the ammonia line 34 and transmits the rotation, via the shaft 42, to the carbamate-side impeller 41b, which acts on the carbamate stream supplied through the carbamate line 27 and increases the pressure thereof.

The ammonia acts as driving fluid in the turbocharger 40, transferring part of its kinetic energy to the carbamate and thus supplying the energy necessary for pumping the carbamate into the reactor 2 at the required pressure.

Indicatively, but not necessarily, the turbocharger 40 operates with an ammonia stream having an inlet pressure comprised between about 160 bar (g) and about 190 bar (g) , and an outlet pressure comprised between about 150 bar (g) and about 160 bar (g) ; and a carbamate stream having an inlet pressure comprised between about 140 bar (g) and about 150 bar (g) , and an outlet pressure comprised between about 150 bar (g) and about 160 bar (g) .

The ammonia stream, after passing through the turbocharger 40, where it transfers part of its kinetic energy, is fed into the reactor 2 through the ammonia feeding line 22, optionally equipped with a control valve 46.

The carbamate stream, after passing through the turbocharger 40 and being brought to the required pressure, is fed into the reactor 2, separately from the ammonia stream, through the carbamate feeding line 23, in a different position in the reactor 2, for example (but not necessarily) at a higher height with respect to ammonia. As already highlighted, the carbamate stream can be fed into the reactor 2 at a higher, lower or even substantially equal height with respect to ammonia.

In the variant shown in Figure 4, the ammonia feeding line 22 divides, downstream of the turbocharger 40, in two (or more) ammonia feeding branches 47, equipped with respective control valves 46 and connected to respective inlets 12 of the reactor 2.

In this way, the ammonia can be fed into the reactor 2 in different positions, not only on the bottom of the reactor 2, but also at different heights along the reactor 2.

According to a further variant, also illustrated in Figure 4, the carbamate line 27 has, downstream of the high pressure pump 28, a branch 48 connected to an auxiliary carbamate line 49. The auxiliary carbamate line 49, equipped with a control valve 50, is connected directly to the reactor 2, in particular to a supplementary inlet 51 of the reactor 2.

In this way, a carbamate portion is sent directly to the reactor 2 by the high pressure pump 28.

Clearly, the variants just described can also be used separately from each other.

Finally, it is understood that further modifications and variations can be made to the plant and method described and illustrated herein, that do not fall outside the scope of the attached claims.