MXPA06013105A - Process for urea production and related plant. - Google Patents

Abstract

A process for urea production from ammonia and carbon dioxide, which are reacted in a suitable synthesis reactor (12) at a predetermined high pressure, wherein an aqueous solution comprising urea, ammonium carbamate and ammonia is obtained, also recovering carbamate and ammonia from the aqueous solution by decomposing carbamate and thermo-exhausting ammonia and carbon dioxide in a respective stripping unit (14), subsequently recondensing the same into carbamate in a respective condenser (16),and recirculating carbamate to the synthesis reactor (12), said stages along with the urea synthesis reaction being substantially performed at the same high pressure; the process also comprises the following steps: - Introducing passivation oxygen into the synthesis reactor (12); -recovering a gas effluent from said reactor (12) which comprises unreacted carbon dioxide and ammonia, as well as passivation oxygen; - feeding the gas effluent resulting from the synthesis reactor (12) to the stripping base (14b) for performing the passivation of at least a part of the content located at the inner portion of the stripping unit (14).

Description

PROCESS FOR THE PRODUCTION OF UREA AND RELATED PLANT DESCRIPTION Scope The present invention relates, in its most general aspect, to a process for the production of urea from ammonia and carbon dioxide, by reacting them at a predetermined high pressure in a suitable synthesis reactor.

In particular, the invention relates to a process of the aforementioned type, in which the product of the ammonia / carbon dioxide reaction, which essentially consists of an aqueous solution comprising urea, ammonium carbamate and ammonia, is subjected to a stage of recovery of ammonium carbamate and ammonia, which are recycled to the synthesis reactor, while the aqueous solution of urea is sent to the purification treatments to obtain urea that is free, as much as possible, of possible residues of ammonia and carbon dioxide.

More specifically, the present invention concerns a process of the type considered, in which the aforementioned recovery of carbamate and ammonia comprises the decomposition steps of the carbamate, with a thermal stripping of ammonia and carbon dioxide thus produced in a respective scrubber, its subsequent recondensation, in a respective condenser, to carbamate, which is recycled to the synthesis reactor and in which said steps, together with the urea synthesis reaction, are all carried out substantially at the same elevated pressure , constituting a cycle "hereafter loop" known in the technical field as "Loop of high pressure" (HP Loop, from English "High Pressure Loop").

The invention also relates to a plant for carrying out the aforementioned process.

Previous art It is well known to produce urea with processes of the type specified above in which for the recovery step of carbamate and ammonia, the so-called HP Loop comprises, as well as the synthesis reactor, a scrubber and a condenser which, in most Of the cases, it consists essentially of tubular beam heat exchangers. The considerable corrosive capacity of the fluids present in the processes considered here and the chemical aggressiveness against the apparatuses with which they come into contact are also known.

For this reason, and also taking into account the relatively severe operating conditions adopted in the HP Loop (pressure: 135-175 bars, reaction temperatures in the reactor: 180-200 ° C, temperatures in the scrubber: 170-210 ° C) C), the development and technology of the production of urea are greatly dependent on the availability of special steels, metals such as titanium and zirconium and other similar materials, with high resistance to corrosion.

In order to better protect against corrosion the apparatuses made with such materials, it has been advantageously suggested to carry out an effective passivation of their external and / or internal walls, in contact with the operating fluids in the HP Loop.

In particular, the prior art teaches introducing oxygen into the processes of urea synthesis, for example through controlled amounts of air (possibly enriched with oxygen), which carries out the desired passivation of the metal walls.

In the prior art, as far as the synthesis reactor is concerned, the oxygen necessary for passivation is generally provided by introduction of a controlled amount of air inside the carbon dioxide feed at the base of the synthesis reactor: from here the oxygen rises in the reactor, carrying out the desired passivation of the metal surfaces with which it comes into contact.

Most of the oxygen introduced into the synthesis reactor is thus fed into the raised part (head) of the scrubber, above the tubular bundle, together with the urea solution. This oxygen, however, is not able to ensure an adequate passivation of the scrubber, since it is separated from the urea solution and leaves the scrubber, together with the vapors of the scrubber, directly in said elevated part of the scrubber. It is therefore necessary to introduce an additional controlled amount of air into the base of the scrubber to also ensure the passivation of the remaining parts of such an apparatus: from here the oxygen rises in the scrubber, through the tubular bundle, carrying out the desired passivation.

Despite being advantageous from different points of view, a process for the production of urea of the aforementioned type has recognized drawbacks, linked to the introduction of additional air mentioned above in the base of the scrubber.

In fact, since the operating pressure of the scrubber is high, for example 140 bars, the air must be compressed to be able to enter it inside the scrubber: there is therefore the need to install a complex apparatus such as an air compressor, which presents disadvantages such as, for example, significant costs and energy consumption for reliable preventive maintenance.

Moreover, another drawback is linked to the fact that a reliable device must be provided to control the air flow introduced into the reactor and into the scrubber, in order to avoid introducing excess amounts of air that can lead to dangerous operating conditions. is, can lead to the formation of explosive mixtures.

A further drawback is linked to the fact that additional air introduced into the scrubber, in addition to that introduced into the reactor, involves, in parallel, the introduction of a considerable amount of inert substances to be separated in the downstream sections of the HP Loop , with the need for complex units of separation of inert substances. In particular, since the air introduced into the reactor and inside the scrubber also carries with it reagents such as ammonia and carbon dioxide, it is necessary to provide a complex and effective ammonia and carbon dioxide recovery unit, to avoid a loss of reagents , with the consequent diminution of the global yield of the HP Loop, as well as to avoid important ecological problems by possible releases to the environment.

Summary of the invention The underlying technical problem of the present invention is that of conceiving and providing a process for the production of urea of the type considered above, in which a passivation of the devices of an effective, homogeneous and cost-effective HP Loop is achieved and simultaneously able to overcome the limitations and / or drawbacks referred to with reference to prior art, in an equally effective and cost-effective manner.

This problem is solved, according to the present invention, by a process for the production of urea from ammonia and carbon dioxide, which are reacted at a predetermined high pressure in an appropriate synthesis reactor, obtaining an aqueous solution which comprises urea, ammonium carbamate and ammonia, recovery of carbamate and ammonia from said aqueous solution through decomposition steps of the carbamate and thermal purification of ammonia and carbon dioxide thus produced, in a respective scrubber, its subsequent recondensation to carbamate in a respective condenser, recycled from said carbamate to the synthesis reactor, all said steps being carried out, together with the synthesis reaction of urea, substantially the same elevated pressure, characterized in that said process comprises the additional steps of: - introducing passivating oxygen into the synthesis reactor; - recovering from said reactor a gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen; - feeding said gaseous effluent from the synthesis reactor to the base of the scrubber, for the passivation of at least part of the interior of said scrubber.

Thanks to the aforementioned process, a considerable reduction in the introduction of air in the HP Loop is advantageously obtained and, therefore, a significant increase in the overall performance of the HP Loop, while at the same time ensuring a good passivation of the internal walls. of the scrubber subjected to corrosive fluids.

The additional features and advantages of the urea production process according to the present invention will become clear from the following description of a preferred embodiment thereof, made for indicative and non-limiting purposes, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically represents a urea production plant carrying out the process of the present invention.

Figure 2 schematically represents a detail, on an enlarged scale, of the plant of Figure 1.

Detailed description of a preferred embodiment With reference to the figures, there is shown a urea production plant, globally indicated with 10, which carries out the process according to the present invention.

According to the aforementioned process for the production of urea, ammonia and carbon dioxide are reacted at a predetermined elevated pressure in an appropriate synthesis reactor 12.

From reactor 12 an aqueous solution is obtained comprising urea, ammonium carbamate and ammonia.

From said aqueous solution, carbamate and ammonia are recovered through decomposition steps of the carbamate and thermal purification of ammonia and carbon dioxide thus produced, in a respective scrubber 14, their subsequent recondensation to carbamate in a respective condenser 16, and recycling said carbamate to the synthesis reactor 12. The aforementioned steps, together with the urea synthesis reaction, are all carried out at substantially the same elevated pressure, constituting the high pressure Loop (HP Loop).

Preferably, the aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the synthesis reactor 12, is fed to the head 14a of the scrubber 14.

In accordance with one aspect of the present invention, such a process comprises the additional steps of: - introducing passivating oxygen into the synthesis reactor 12; - recovering from said reactor 2 a gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen; - feeding said gaseous effluent from the synthesis reactor 12 to the base 14b of the scrubber 14, for the passivation of the internal metal walls of the scrubber 14, or for the passivation of at least part of the interior of said scrubber 14.

Preferably, the step of introducing passivating oxygen is carried out by the addition of air to the carbon dioxide, which is fed to the synthesis reactor 12, that is, feeding the air and relative oxygen directly to the synthesis reactor 12. The passivation oxygen addition is carried out in a controlled manner, in order to avoid introducing quantities of oxygen that can lead to dangerous conditions, that is, that can lead to the formation of explosive mixtures.

In a preferred embodiment, the process comprises the additional steps of: - recovering a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, in the head 14a of the scrubber 14, and - feeding said effluent to said condenser 16, to obtain said recondensed carbamate and for passivation of the internal metal walls of the condenser 16, or for the passivation of at least part of the interior of said condenser 16.

With further reference to the figures, the structural characteristics of the plant 10 for the production of synthesis urea from ammonia and carbon dioxide will be specified below.

The plant 10 comprises the synthesis reactor 12, the scrubber 14 and the condenser 16, in fluid communication with one another to form a high pressure Loop. More specifically, the reactor 12 is fed with an ammonia feed line 2 and a carbon dioxide feed line 3.

In the plant 10 there is provided a passivation oxygen inlet conduit 5 into said synthesis reactor 12, and a fluid communication 6, that is, a conduit, which is provided between the head 12a of the reactor 12 and the base 14b of the scrubber 14. Preferably, the passivation oxygen inlet conduit 5 is connected to the carbon dioxide feed conduit 3. Again preferably, the passivation oxygen is contained in the air introduced into said inlet conduit 5.

An aqueous solution comprising urea, ammonium carbamate and ammonia is fed from the reactor 12 to the head 14a of the scrubber 14 with a conduit 4.

More specifically, the scrubber 14 comprises a substantially vertical tubular bundle heat exchange unit 24, the aqueous solution comprising urea, ammonium carbamate and ammonia, coming from reactor 12, which are introduced into an upper end of said tubular bundle. 24, or through an inlet opening 33 accommodated in the head 14a of the scrubber 14 above said tubular bundle 24, and descending from the upper part to the lower part inside the tubular bundle 24. The gaseous effluent comprising dioxide carbon and unreacted ammonia and passivation oxygen coming from the head 12a of the reactor 12, is introduced into the lower end of said tubular bundle 24, or through an inlet opening 34 accommodated in the base 14b of the scrubber 14 under said bundle tubular 24.

In figure 2, a tube 24 of said tubular bundle 24 is represented schematically with 24a, while 35 and 36 respective gaseous effluent outlet openings are represented, comprising carbon dioxide, ammonia and passivating oxygen and a urea solution with residues. of ammonia and carbon dioxide.

In a preferred embodiment, the plant 10 provides a fluid communication 7, that is, a conduit, between the head 14a of the scrubber 14 and the condenser 16.

The operation of the plant 10 according to the present invention is specified hereinafter.

The aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the reactor 12, flows as a film from the top to the bottom inside the tubes 24a of the tubular bundle 24; a gaseous effluent comprising carbon dioxide, ammonia and passivating oxygen, coming from the head 2a of the reactor 12, passes through the conduit 6 and ascends inside the tubes 24a of the tubular bundle 24 uniformly contacting all the internal parts of the purifier 14 subjected to corrosion (the internal walls of the base 14b of the scrubber, the internal surface of the tubes 24a and the internal walls of the head 14a of the scrubber). It should be noted that the operating pressure of the synthesis reactor 12 is higher than that present in the scrubber 14 (or better, it is slightly higher than that present in the scrubber 14, since the HP Loop operates at substantially the same pressure), so that the gaseous effluent enters the scrubber 14 without the need for any compression device.

The heat required for the scrubbing is provided through the use of condensing steam, for example at the pressure of about 25 bar, which is fed to the scrubber 14 in the tubular bundle 24 and flows out of the tubes 24a of the tubular bundle 24 In the figures, 36 and 37 respective conduits for feeding and recovering said condensation vapor are indicated, while the corresponding inlet and outlet openings for the condensation vapor formed in said scrubber 14 are indicated respectively with 38 and 39.

The gaseous effluent comprising carbon dioxide, ammonia and passivating oxygen, coming from the head 14a of the scrubber 14, passes through the conduit 7 and enters the condenser 16, to obtain the recondensed carbamate and for the passivation of the internal walls of the capacitor 16.

In the condenser 16 (which comprises a tubular beam heat exchanger) the condensation of the gaseous effluent coming from the head 14a of the scrubber 14 takes place (in the case illustrated in the example of FIG. 1, the condensation is substantially total ): with the heat developed by the aforesaid condensation, low pressure steam (for example 3-5 bar) is advantageously produced, used in the downstream sections of the HP Loop for the purification of urea.

The level of the urea solution in the head 12a of the reactor 12 is suitably controlled by means 4a, associated with said duct 4, in a manner known per se and therefore not shown in detail, to allow a correct separation of the solution Aqueous comprising urea, ammonium carbamate and ammonia, sent to the head 14a of the scrubber 14, from the gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen, sent to the base 14a of the scrubber 14.

In the gaseous effluent there are also inert substances such as hydrogen (which is contained in the carbon dioxide feed to the reactor 12 and which comes from the urea production plant, accommodated upstream of the urea 10 production plant, and in the liquid ammonia fed to the reactor 12) and nitrogen, contained in the air introduced into the carbon dioxide fed to the reactor 12.

A conduit 8 carries the carbamate solution produced in the condenser 16 within a separator 28 provided downstream thereof. In the separator 28, most of the inert substances, which still contain a part of oxygen, are separated from the carbamate solution, said inert substances being carried, through the conduit 9a, to a final purification section of urea 32, the called urea recovery section.

The carbamate solution coming from the separator 28, practically without inert substances, is carried through a conduit 9 to an ejector 30. Through of the ejector 30, operated with the liquid ammonia fed to the reactor 12, the carbamate solution is recirculated to the reactor 12.

The final purification section of urea 32 is also fed, via a conduit 13, with the urea solution comprising ammonia residues and carbon dioxide, collected in the base 14b of the scrubber 14. The oxygen contained in the inert substances fed to section 32 through conduit 9a is useful for passivating, where necessary, portions of said section 32.

At the outlet of section 32, molten urea is obtained, suitable for example for granulation, to which it is sent with a conduit 1, and a flow of inert substances that still contain residual parts of NH3 and CO2, fed to purification devices suitable (not shown) through conduit 9b. Such residual parts of NH3 and C02 are removed in said purification devices, before they are released into the environment.

At the outlet of the urea treatment section 32, a so-called carbamate weak solution, or an aqueous solution of carbamate, produced in said section, which is recirculated through a conduit 11 and the conduit 7, is also obtained. , towards the condenser 16, as an agent for absorbing the NH3 and C02 vapors coming from the scrubber 14.

In the case of plants with a large capacity, the condenser 16 of the HP Loop is advantageously of the submerged type, known as "Full Condenser" ™, manufactured with a tubular beam heat exchanger with vertical tubes configured so as to obtain a time of residence of the carbamate solution that is optimal for the formation of urea. In this way, the aforementioned condenser 16 acts as a pre-reactor with a significant reduction in the volume of the main reactor 12.

The present invention also relates to a passivation method of a high pressure Loop of a urea production plant, comprising a synthesis reactor 12, a scrubber 14 and a condenser 16, in fluid communication with one another. The method comprises the steps of: - introducing passivating oxygen into the synthesis reactor 12; - recovering from said reactor 12 a gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen; - feeding said gaseous effluent from the synthesis reactor 12 to the base 14b of the scrubber 14, for the passivation of at least part of the interior of said scrubber 14.

In a preferred embodiment variant of the method of the invention, this comprises the additional steps of: - recovering a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, in the head 14a of the scrubber 14, and - feeding said effluent to said condenser 16, to obtain said recondensed carbamate and for the passivation of at least part of the interior of said condenser 16.

Moreover, the present invention also relates to the use of the gaseous effluent, which comprises carbon dioxide and unreacted ammonia and passivation oxygen, recovered in the synthesis reactor 12 of the urea production plant 10, to passivate at least one lower part of the interior of the scrubber 14, the passivating oxygen being introduced into the synthesis reactor 12.

From the previous description, it can be seen that the urea production process according to the invention solves the technical problem and achieves numerous advantages, the first of which lies in the fact that it is obtained an unusually homogenous passivation of the devices present in the HP Loop.

Moreover, the process is simple and reliable to carry out.

An additional advantage is that linked to the fact that, thanks to the present invention, it has surprisingly been noted that it is possible to use most of the oxygen introduced into the reactor to effectively passivate the scrubber (as well as the devices downstream thereof): the oxygen consumption is therefore drastically reduced with respect to the prior art.

Moreover, there is no longer a need for the prior art to manufacture particularly complex units for separating inert substances and recovering reagents.

Of course, one skilled in the art can bring numerous modifications and variants to the urea production plant described above, in order to meet specific and contingent requirements, all of which are covered by the scope of protection of the present invention, such as it is defined by the following claims.

Claims (3)

CLAIMS A process for the production of urea from ammonia and carbon dioxide, which are reacted at a predetermined high pressure in an appropriate synthesis reactor (12), obtaining an aqueous solution comprising urea, ammonium carbamate and ammonia, recovery of carbamate and ammonia from said aqueous solution through stages of decomposition of the carbamate and thermal purification of ammonia and carbon dioxide thus produced, in a respective scrubber (14), its subsequent recondensation to carbamate, in a condenser (16) respectively, recycling said carbamate to the synthesis reactor (12), all of said steps being carried out, together with the urea synthesis reaction, substantially at the same elevated pressure, characterized in that it comprises the additional steps of: - introducing passivation oxygen into the synthesis reactor (12); - recovering from said reactor (12) a gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen; - feeding said gaseous effluent from the synthesis reactor (12) to the base (14b) of the scrubber (14), for the passivation of at least part of the interior of said scrubber (14). The process according to claim 1, characterized in that said step of introducing oxygen passivation is carried out by adding air to the carbon dioxide fed to the synthesis reactor (12). The process according to claim 1, characterized in that said aqueous solution comprising urea, ammonium carbamate and ammonia, is fed to the head (14a) of the scrubber (14). The process according to claim 1, characterized in that a gaseous effluent comprising carbon dioxide, ammonia and passivating oxygen, is recovered in the head (14a) of the scrubber (14), and said effluent is fed to said condenser ( 16), to obtain said recondensed carbamate and for the passivation of at least part of the interior of said condenser (16). A plant (10) for the production of synthesis urea from ammonia and carbon dioxide, comprising a synthesis reactor (12), a scrubber (14) and a condenser (16), in fluid communication with one another to constitute a high-pressure loop, characterized in that an inlet duct (5) of passivating oxygen is provided in said synthesis reactor (12) and a fluid communication (6) is provided between the head (12a) of the reactor (12) and the base (14b) of the scrubber (14). The plant (10) according to claim 5, characterized in that said passivation oxygen inlet conduit (5) is connected to the carbon dioxide feed to the synthesis reactor (12). The plant (10) according to claim 6, characterized in that said passivating oxygen is contained in the air introduced into said inlet conduit (5). The plant (10) according to claim 5, characterized in that an aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the reactor (12), is fed to the head (14a) of the scrubber (14). The plant (10) according to claim 5, characterized in that a fluid communication (7) is provided between the head (14a) of the scrubber (14) and said condenser (16). 0. The plant (10) according to claim 5, characterized in that said scrubber (14) comprises a substantially vertical tubular beam heat exchange unit (24), an aqueous solution comprising urea, ammonium carbamate and ammonia, which they come from the reactor (12), introduced at an upper end of said tubular bundle (24), and that descend from the upper part to the lower part inside said tubular bundle (24), said fluid communication (6) being between the head (12a) of the reactor (12) and the base (14b) of the scrubber (14), connected to said tubular bundle (24) at a lower end thereof.

1. A passivation method of a cycle or high-pressure loop of a urea production plant (10), comprising a synthesis reactor (12), a scrubber (14) and a condenser (16), in fluid communication one with another, reacting ammonia and carbon dioxide in said synthesis reactor (12) at a predetermined elevated pressure, obtaining an aqueous solution comprising urea, ammonium carbamate and ammonia, which is subjected in said scrubber (14) to decomposition of the carbamate and thermal purification of ammonia and carbon dioxide thus produced, carrying out a subsequent recondensation thereof to carbamate in said condenser (16), said carbamate being recycled to the synthesis reactor (12), characterized in that it comprises the steps of : - introducing passivating oxygen into the synthesis reactor (12); - recovering from said reactor (12) a gaseous effluent comprising carbon dioxide and unreacted ammonia and passivating oxygen; - feeding said gaseous effluent from the synthesis reactor (12) to the base (14b) of the scrubber (14), for the passivation of at least part of the interior of said scrubber (14).

2. The method according to claim 11, characterized in that it comprises the additional steps of: - recovering a gaseous effluent comprising carbon dioxide, ammonia and passivating oxygen, in the head (14a) of the scrubber (14), and - feeding said effluent to said condenser (16), to obtain said recondensed carbamate and for the passivation of at least part of the interior of said condenser (16).

3. Use of a gaseous effluent, comprising carbon dioxide and unreacted ammonia and passivating oxygen, recovered in a synthesis reactor (12) of a urea production plant (10), to passivate at least a lower part of the interior of a scrubber (14), said plant (10) of the type comprising a high pressure Loop including said synthesis reactor (12), said scrubber (14) and a condenser (16), in fluid communication with one another , wherein ammonia and carbon dioxide are reacted in said synthesis reactor (12) at a predetermined elevated pressure, obtaining an aqueous solution comprising urea, ammonium carbamate and ammonia, the passivation oxygen being introduced into the synthesis reactor (12) said aqueous solution being subjected in said scrubber (14) to decomposition of the carbamate and thermal purification of the ammonia and carbon dioxide thus produced, a subsequent condensation being carried out. ion of the same to carbamate in a condenser, said carbamate being recycled to the synthesis reactor (12).