Title of Invention

"PROCESS FOR UREA PRODUCTION AND RELATED PLANT"

Abstract

A process for urea production from ammonia and carbon dioxide, made to react at a predetermined high pressure in an appropriate synthesis reactor (12), obtaining an aqueous 5 solution comprising urea, ,ammonium carbamate and ammonia, recovery of carbamate and ammonia from the aqueous solution through steps of decomposition of the carbamate and thermal stripping of ammonia and carbon dioxide thus produced, in a respective stripper (14), their subsequent recondensation 10 into carbamate, in a respective condenser (16), recycle of the carbamate to the synthesis reactor (12), said steps, together with the urea synthesis reaction, all being carried out substantially at a same high pressure; the process comprises the further steps of: 15 -introducing passivation oxygen into the synthesis reactor (12) ; -recovering from said reactor (12) a gaseous effluent comprising unreacted carbon dioxide and ammonia and passlvatlon oxygen; 20 -feeding the gaseous effluent from the synthesis reactor (12) to the base (14b) of the stripper (14), for the passivation of at least part of the inside of the stripper (l4) .

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10; rule 13) TITLE PROCESS FOR UREA PRODUCTION AND RELATED PLANT APPLICANT UREA CASALE S.A. Via Giulio Pocobelli, 6 CH-6900 Lugano-Besso Switzerland Nationality : a Swiss company The following specification particularly describes the nature of this invention and the manner in which it is to be performed - 1 - Title: “Process for urea production and related plant” DESCRIPTION Field of application The present invention refers, in its most general aspect, to a process for urea production from ammonia and carbon dioxide, made to react at a predetermined high pressure in an appropriate synthesis reactor. In particular the invention refers to a process of the aforementioned type in which the product of the ammonia/carbon dioxide reaction, essentially consisting of an aqueous solution comprising urea, ammonium carbamate and ammonia, is subjected to a recovery step of the ammonium carbamate and of the ammonia, which are recycled to the synthesis reactor, whereas the aqueous urea solution is sent to purification treatments to obtain urea that is free as much as possible from 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 steps of decomposition of the carbamate with thermal stripping of the ammonia and of the carbon dioxide thus produced, in a respective stripper, their subsequent recondensation, in a respective condenser, into carbamate that is recycled to the synthesis reactor and in which said steps, together with the urea synthesis reaction, are all carried out substantially at a same high pressure, constituting a loop known, in the technical field, as “High Pressure Loop” (K.P. Loop). - 2 - The invention also refers to a plant for carrying out the aforementioned process. Prior 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 H.P. Loop comprises, as well as the synthesis reactor, a stripper and a condenser that, in most cases, essentially consist of tube bundle heat exchangers. The considerable corrosive capability of the fluids present in the processes considered here and their chemical aggressiveness against the apparatuses with which they come into contact are equally known. For this reason, and also taking into account the relatively severe operating conditions adopted in the H.P. Loop (pressure: 135-175 bars; synthesis temperatures in the reactor: 180-200°C; temperatures in the stripper: 170-210°C), the development and the technology of urea production are greatly dependent upon the availability of special steels, metals likes titanium and zirconium and other similar materials, with high resistance to the corrosion. In order to better protect the apparatuses made with such materials against the corrosion, it has advantageously been suggested to carry out an effective passivation of their outer and/or inner walls, in contact with the operating fluids in the H.P. Loop. In particular the prior art teaches to introduce oxygen into the urea synthesis processes, for example through controlled amounts of air (possibly enriched with oxygen), - 3 - 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 the passivation is generally provided by introducing a controlled amount of air into the carbon dioxide fed 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 high part (head) of the stripper, above the tube bundle, together with the urea solution. This oxygen, however, is not able to ensure an adequate passivation of the stripper since it separates from the urea solution and comes out from the stripper, together with the stripping vapors, directly at said high part of the stripper. It is thus necessary to introduce a further controlled amount of air at the base of the stripper to also ensure the passivation of the remaining parts of such an apparatus: from here the oxygen rises in the stripper, through the tube bundle, carrying out the desired passivation. Although advantageous for different points of view, a process for urea production of the aforementioned type has recognized drawbacks linked to the aforementioned introduction of further air at the base of the stripper. Indeed, since the operating pressure of the stripper is high, for example 140 bars, the air has to be compressed to be able to introduce it into the stripper: there is thus the need to install a complex apparatus as an air - 4 - compressor, which has drawbacks such as, for example, significant energy consumption and costs for a reliable preventive maintenance. Moreover, another drawback is linked to the fact that a reliable device has to be provided that controls the air flow rate introduced into the reactor and into the stripper, in order to avoid introducing excess amounts of air that can lead to dangerous operating conditions, i.e. can lead to the formation of explosive mixtures. A further drawback is linked to the fact that the further air introduced into the stripper, 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 sections downstream of the H. P. Loop, with the need of complex units of separation of the inert substances. In particular, since the air introduced into the reactor and into the stripper also carries with it reaccants such as ammonia and carbon dioxide, it is necessary to provide a complex and effective recovery unit of ammonia and carbon dioxide, to avoid a loss of reactants with consequent lowering of the overall yield of the H.P. Loop, as well as to avoid important ecological problems for possible releases in the environment. Summary of the invention The technical problem underlying the present invention is that of devising and providing a process for urea production of the type considered above, in which an effective, homogeneous and cost-effective passivation of the apparatuses of a H.P. Loop can be achieved and that is simultaneously able to overcome the limitations and/or the - 5 - drawbacks quoted with reference to the prior art, in an equally efficient and cost-effective manner. This problem is solved, according to the present invention, by a process for urea production from ammonia and carbon dioxide, made to react at a predetermined high pressure in an appropriate synthesis reactor, obtaining an aqueous solution comprising urea, ammonium carbamate and ammonia, recovery of carbamate and ammonia from said aqueous solution through steps of decomposition of the carbamate and thermal stripping of ammonia and carbon dioxide thus produced, in a respective stripper, their subsequent recondensation into carbamate, in a respective condenser, recycle of said carbamate to the synthesis reactor, said steps, together with the urea synthesis reaction, all being carried out substantially at a same high pressure, characterized in that it comprises the further steps of: - introducing passivation oxygen into the synthesis reactor; recovering from said reactor a gaseous effluent comprising unreacted carbon dioxide and ammonia and passivation oxygen; - feeding said gaseous effluent from the synthesis reactor to the base of the stripper, for the passivation of at least part of the inside of said stripper. Thanks to the aforementioned process, a considerable reduction in the air introduced into the H. P. Loop and therefore a significant increase in the overall yield of the H. P. Loop is advantageously obtained, a good passivation of the inner walls of the stripper subjected to the corrosive fluids at the same time being ensured. - 6 - Further characteristics and the advantages of the process for urea production according to the present invention shall become clear from the following description of a preferred embodiment thereof, made for indicating and not limiting purposes, with reference to the attached drawings. Brief description of the drawings Figure 1 schematically represents a urea production plant that carries 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, a urea production plant is shown, globally indicated with 10, which carries out the process according to the present invention. According to the aforementioned process for urea production, ammonia and carbon dioxide are made to react at a predetermined high pressure in an appropriate synthesis reactor 12. From the reactor 12 an aqueous solution is obtained comprising urea, ammonium carbamate and ammonia. From said aqueous solution, carbamate and ammonia are recovered through steps of decomposition of the carbamate and thermal stripping of ammonia and carbon dioxide thus produced, in a respective stripper 14, their subsequent recondensation into carbamate, in a respective condenser 16, and recycle of said carbamate to the synthesis reactor 12. The aforementioned steps, together with the urea synthesis reaction, are all carried out substantially at a - 7 - same high pressure, constituting the High Pressure Loop (H.P. 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 stripper 14. In accordance with an aspect of the present invention, such a process comprises the further steps of: - introducing passivation oxygen into the synthesis reactor 12; - recovering from said reactor 12 a gaseous effluent comprising unreacted carbon dioxide and ammonia and passivation oxygen; - feeding said gaseous effluent from the synthesis reactor 12 to the base 14b of the stripper 14, for the passivation of inner metal walls of the stripper 14, or for the passivation of at least part of the inside of said stripper 14. Preferably, the step of introducing passivation oxygen is carried out by adding air to the carbon dioxide, which is fed to the synthesis reactor 12, i.e. by feeding the air and the relative oxygen directly to the synthesis reactor 12. The addition of passivation oxygen is carried out in a controlled manner, in order to avoid introducing amounts of oxygen that can lead to dangerous operating conditions, i.e. can lead to the formation of explosive mixtures. In a preferred embodiment, the process comprises further steps of: - 8 - recovering a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, at the head 14a of the stripper 14, and feeding said effluent to said condenser 16, to obtain said recondensed carbamate and for the passivation of inner metal walls of the condenser 16, or for the passivation of at least part of the inside of said condenser 16. With greater . reference to the figures, the structural characteristics of the plant 10 for the production of synthesis urea from ammonia and carbon dioxide shall now be specified. The plant 10 comprises the synthesis reactor 12, the stripper 14 and the condenser 16, in fluid communication with each other to constitute the High Pressure Loop. More specifically, the reactor 12 is fed with an ammonia feeding duct 2 and with a carbon dioxide feeding duct 3. In the plant 10 a passivation oxygen inlet duct 5 into said synthesis reactor 12 is provided and a fluid communication 6, i.e. a duct, is provided between the head 12a of the reactor 12 and the base 14b of the stripper 14. Preferably, the passivation oxygen inlet duct 5 is connected to the carbon dioxide feeding duct 3. Again preferably, the passivation oxygen is contained in air introduced into said inlet duct 5. An aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the reactor 12, is fed to the head 14a of the stripper 14 with a duct 4. More specifically, the stripper 14 comprises a substantially vertical tube bundle heat exchange unit 24, - 9 - the aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the reactor 12, being introduced at an upper end of said tube bundle 24, or through an inlet opening 33 arranged at head 14a of the stripper 14 above said tube bundle 24, and going down from top to bottom inside said tube bundle 24. The gaseous effluent comprising unreacted carbon dioxide and ammonia and passivation oxygen coining from the head 12a of the reactor 12, being introduced at a lower end of said tube bundle 24, or through an inlet opening 34 arranged in the base 14b of the stripper 14 under said tube bundle 24. In figure 2 a tube of said tube bundle 24 is schematically represented with 2 4a, whilst respective outlet openings of a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen and of a urea solution with residues of ammonia and carbon dioxide are represented with 35 and 36. In a preferred embodiment, the plant 10 provides a fluid communication 7, i.e. a duct, between the head 14a of the stripper 14 and the condenser 16. The operation of the plant 10 according to the present invention is specified hereafter. The aqueous solution comprising urea, ammonium carbamate and ammonia, coming from the reactor 12, flows as a film from top to bottom inside the tubes 24a of the tube bundle 24; a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, coming from the head 12a of the reactor 12, runs through the duct 6 and rises inside the tubes 24a of the tube bundle 24 uniformly contacting all of the inner parts of the stripper 14 subjected to corrosion (inner walls of the base 14b of the stripper, inner surface - 10 - of the tubes 24a and inner walls of the head 14a of the stripper). It should be noted that the operating pressure of the synthesis reactor 12 is greater than that present in the stripper 14 for better, it is slightly greater than that present in the stripper 14, given that the H.P. Loop operates substantially at a same pressure) , so that the gaseous effluent enters into the stripper 14 without needing any compression device. The heat necessary for the stripping is provided through the use of condensing steam, for example at a pressure of about 2 5 bar, which is fed to the stripper 14 at the tube bundle 24 and flows outside of the tubes 24a of the tube bundle 24. In figures, respective feeding and recovery ducts of said condensing steam are indicated with 36 and 37, whereas the corresponding inlet and outlet openings of the condensing steam formed in said stripper 14 are respectively indicated with 38 and 39. The gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, coming from the head 14a of the stripper 14, runs through the duct 7 and goes into the condenser 16, to obtain the recondensed carbamate and for the passivation of the inner walls of the condenser 16. In the condenser 16 (which comprises a tube bundle heat exchanger) the condensation (in the case illustrated in the example of figure 1 the condensation is substantially total) of the gaseous effluent coming from the head 14a of the stripper 14 takes place: with the heat developed by the aforementioned condensation, low pressure steam (for example 3-5 bar) is advantageously produced, used in the sections downstream of the H.P. Loop for the purification of the urea. - 11 - The level of the urea solution at the head 12a of the reactor 12 is suitably controlled with means 4a, associated with said duct 4, per se known and therefore not represented in detail, to allow a correct separation of the aqueous solution comprising urea, ammonium carbamate and ammonia, sent to the head 14a of the stripper 14, from the gaseous effluent comprising unreacted carbon dioxide and ammonia and passivation oxygen, sent to the base 14a of the stripper 14. In the gaseous effluent there are also inert substances such as hydrogen (which is contained in the carbon dioxide fed to the reactor 12 and coming from an ammonia production plant, arranged upstream of the urea production plant 10, 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 duct 8 carries the carbamate solution produced in the condenser 16 into a separator 2 8 provided downstream thereof. In the separator 28, most of the inert substances, still containing a part of oxygen, are separated from the carbamate solution, said inert substances being carried, through a duct 9a, to a final urea depuration section 32, the so-called urea recovery section. The carbamate solution practically without inert substances, coming from the separator 28, is carried through a duct 9 to an ejector 30. Through the ejector 30, actuated with the liquid ammonia of feed to the reactor 12, the carbamate solution is recirculated to the reactor 12. The final urea depuration section 32 is also fed, through a duct 13, with the urea solution comprising residues of - 12 - ammonia and carbon dioxide, collected at the base 14b of the stripper 14. The oxygen contained in the inert substances fed to the section 32 through the duct 9a in of use to passivate, where necessary, parts of said section 32. In output from the section 32, melted urea is obtained, for example suitable for granulation, to which it is sent with a duct 1, and a flow of inert substances that still contain residual parts of NH3 and CO2, fed to suitable depuration devices (not represented) through a duct 9b. Such residual parts of NH3 and CO2 are removed in such depuration devices, before they are released into the environment. In output from the urea depuration section 32 a so-called weak carbamate solution, or a aqueous carbamate solution, produced in such a section, is also obtained, which is recirculated, through a duct 11 and the duct 7, to the condenser 16 as absorption agent of the NH3 and CO2 vapors coming from the stripper 14. In the case of plants with a large capacity, the condenser 16 of the H. P. Loop is advantageously of the submerged type, known as “Full condenser”™, made with a tube bundle heat exchanger with vertical tubes sized in order to obtain a residence time of the carbamate solution that is optimal for urea formation. In this way, the aforementioned condenser 16 acts as pre-reactor with a significant reduction in the volume of the main reactor 12. The present invention also refers to a passivation method of a High Pressure Loop of a urea production plant 10, comprising a synthesis reactor 12, a stripper 14 and a - 13 - condenser 16, in fluid communication with each other. The method comprises the steps of: - introducing passivation oxygen into the synthesis reactor 12; recovering from said reactor 12 a gaseous effluent comprising unreacted carbon dioxide and ammonia and passivation oxygen; - feeding said gaseous effluent from the synthesis reactor 12 to the base 14b of the stripper 14, for the passivation of at least part of the inside of said stripper 14. In a preferred variant embodiment of the method of the invention, it comprises the further steps of: - recovering a gaseous effluent comprising carbon dioxide, ammonia and passivation oxygen, at the head 14a of the stripper 14, and - feeding said effluent to said condenser 16, to obtain said recondensed carbamate and for the passivation of at least part of the inside of said condenser 16. Furthermore, the present invention also refers to the use of the gaseous effluent, comprising unreacted carbon dioxide and ammonia and passivation oxygen, recovered in the synthesis reactor 12 of the urea production plant 10, to passivate at least a lower part of the inside of the stripper 14, passivation oxygen being introduced into the synthesis reactor 12. From the previous description it can clearly be seen that the process for urea production according to the invention solves the technical problem and achieves numerous - 14 - advantages the first of which lies in the fact that an unusually homogeneous passivation of the apparatuses present in the H.P. Loop is obtained. Moreover, the process is simple and reliable to carry out. A further 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 stripper (as well as the apparatuses downstream of it) : oxygen consumption is thus drastically reduced with respect to the prior art. Moreover, there is no longer the need of the prior art to have particularly complex units of separation of the inert substances and of recovery of the reactants. Of course, a man skilled in the art can bring numerous modifications and variants to the urea production plant described above, in order to satisfy specific and contingent requirements, all of which are covered by the scope of protection of the present invention, as defined by the following claims.

Documents:

362-CHE-2006 CORRESPONDENCE OTHERS 22-10-2013.pdf

362-CHE-2006 AMENDED CLAIMS 02-09-2014.pdf

362-CHE-2006 AMENDED PAGES OF SPECIFICATION 02-09-2014.pdf

362-CHE-2006 EXAMINATION REPORT REPLY RECIEVED 02-09-2014.pdf

362-CHE-2006 OTHERS 02-09-2014.pdf

362-CHE-2006 POWER OF ATTORNEY 02-09-2014.pdf

362-che-2006-abstract.pdf

362-che-2006-claims.pdf

362-che-2006-correspondence-others.pdf

362-che-2006-description(complete).pdf

362-che-2006-form 1.pdf

362-che-2006-form 26.pdf

362-che-2006-form 3.pdf

362-che-2006-form 5.pdf

Form 3.pdf

Petition for Annexure.pdf

Petition for POR.pdf