MXPA97004694A - Process and plant for the production of urea with reaction spaces that have performance difference - Google Patents

Abstract

A process for the production of urea is made to react substantially pure ammonia and carbon dioxide in a main reaction space (R-1) from which a reaction mixture that undergoes purification is obtained to obtain a partially purified mixture which is send to a recovery section of urea (3). From this section (3) a diluted carbamate solution is obtained which is recycled to an auxiliary reaction space (R-2), wherein the residual carbamate is converted to urea. This process achieves a high average conversion performance with a reduced energy consumption.

Description

PROCESS AND PLANT FOR THE PRODUCTION OF UREA WITH REACTION SPACES OUE HAVE DIFFERENTIATED PERFORMANCES TECHNICAL FIELD In its general aspect, the present invention relates to a method for modernizing a urea production plant of the type comprising: a urea synthesis reactor; - a scrubbing device for subjecting a first reaction mixture leaving the reactor to a treatment of partial decomposition of the carbamate and to a partial separation of the free ammonia in aqueous solution, present in the first mixture, - - a means for condensing, at least partially, the vapors leaving the purification equipment and having a first carbamate solution recycled to the reactor; - a recovery section for separating the urea produced in the reactor from a second aqueous carbamate solution. The present invention also relates to a plant for producing a urea obtainable by the method of modernization of the present invention. As is known, in the field of urea production, there is an increasing need for plants that have 11. '! 7 / M; greater operational capacity and flexibility, on the one hand, and requiring lower investment and operating costs, particularly in terms of energy, on the other hand.

BACKGROUND OF THE ART For this purpose, a series of urea production processes have been proposed and implemented in the art which are essentially based on the conversion reactions with differentiated yields, in the reaction spaces set in parallel, as described by example in the European patent application EP-A-0 479 103. In these processes, the total production of urea is distributed between a main reaction space, designed to cover most of the required production capacity (generally 60). % to 80% thereof) and which operates under conditions of low yield, and an auxiliary reaction space, also called "one-pass", which operates under high performance conditions and is designed to bring production capacity to the final amount required. Within the framework of the aforementioned process, the reaction mixture coming from the main reaction space is subjected to a preliminary purification treatment in order to obtain a concentrated solution of recycled carbamate to the reaction space and a solution of urea which is further processing and purification, together with the reaction mixture leaving the auxiliary reaction space, in a separation and recovery section. From this last section a dilute carbamate solution and a substantially pure urea solution can be obtained. According to the constant teachings of the prior art and in order to increase as much as possible the conversion efficiency in the auxiliary reaction space, this space is exclusively fed with substantially pure carbon dioxide and ammonia, while the carbamate solution diluted is recycled exclusively to the main reaction space, which, consequently, is delegated the task of converting into urea all the carbamate obtained in the purification sections located downstream thereof. While essentially meeting the aforementioned need, these processes exhibit both a weighted average yield limited by the rather poor performance in the main reactor and a series of plant limitations linked to the need to send high flow rates of recirculation to the reactor. main and provide a large auxiliary reactor // • '' MX high cost. The large amount of water in the recycling solution to the main reaction space also imposes (notwithstanding the high conversion efficiency of the auxiliary reaction space) an upper limit to the average conversion efficiency that can be achieved by the plant, which in turn limits the energy savings that can be achieved in terms of reducing the consumption of high pressure steam. As stated in the patent applications EP 0 544 056 and EP 0 624 571, processes for the industrial synthesis of urea which are carried out by entirely new plants are also known in the art, where the high purity ammonia and the carbon dioxide are reacted in a first reaction space, while a solution of the recycled carbamate, which comes from a recovery section of urea, is sent to a second reaction space, where they react with an unreacted portion of the ammonia and carbon dioxide. None of the documents EP 0 544 056 nor the EP 0 624 571, addresses, however, the problem of the modernization of existing urea production plants, including a purification equipment to treat the reaction mixture containing urea in order to highlight the production capacity while the same i i ••. / 'wrr: time we have a high average weighted performance.

EXHIBITION OF THE INVENTION Consequently, the technical problem with the present invention is that of trying to conceive and elaborate a method available for the modernization of a urea production plant, which allows to solve the aforementioned problems of the prior art. According to a first embodiment of the present invention, this problem is solved by a method of the aforementioned type, characterized in that it comprises the additional steps of: providing a second urea synthesis reactor upstream of the purification equipment; - connect the second reactor with the purification equipment; - providing a means for recycling the second carbamate solution obtained in the recovery section to the second reactor. In accordance with the present invention, it has surprisingly been found that it is possible to further reduce the steam consumption and further simplify the plant designed to carry out the aforementioned process, this by the production of urea under high performance conditions in the space of main reaction (the main part in terms of production) and the low performance in the auxiliary reactor (the least important part in terms of production). According to the present invention, the aforementioned high-performance conversion conditions in the main reaction space can be achieved by feeding the pure reactants and only the concentrated carbamate solution coming from the partial purification operations (decomposition). partial of the carbamate, partial separation of the free ammonia and condensation) of the reaction mixture leaving the main reaction space. In extreme contrast to the constant teachings of the prior art, the diluted carbamate solution that comes from the urea separation and recovery section located downstream, both from the main reaction space and the auxiliary reaction space, is recycled alone and exclusively towards the auxiliary reaction space. The latter will be able to work both in conditions of low yield, due to the high quality of the water in the recycling solution, and in terms of the low production. In an alternative embodiment, the problem identified above is solved by a method of the aforementioned type, which is characterized by comprising the additional steps of: providing a second urea synthesis reactor upstream of the purification equipment; - providing a means for recycling the second carbamate solution, obtained in the recovery section, to the second reactor; - connecting the second reactor with distillation equipment, to subject a second reaction mixture, which leaves the second reactor, to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in the aqueous solution present in the second mixture; - provide a means to recycle the vapors leaving the distillation equipment to the second reactor, - connect the distillation equipment to the recovery section of urea.

WAYS TO CARRY OUT THE INVENTION According to a first embodiment of the present invention, the synthesis reaction in the main reaction space is carried out according to the following process parameters: NH3 / C02 mol .- 2.8-3.4, preferably 3.0 H20 / C02 mol: 0.1-0.25, preferably 0.18 > ! 'i "17MX.

Reaction temperature: 180-195 ° C, preferably 190 ° C Pressure: 140-155 bar, preferably 145 bar Conversion efficiency of C02: 69-71% Advantageously, when operating with a molar ratio of ammonia / carbon dioxide of less than 4 and preferably of about 3, a reduction of the volume of the high-performance reaction space and of the requirement can be achieved. of the heat to pre-heat the reactants up to the reaction temperature. The synthesis reaction in the auxiliary reaction space is carried out according to the following process parameters: NH3 / C02 mol: 4.2-4.6, preferably 4.5 H20 / C02 mol: 1.2-1.6, preferably 1.5 Reaction temperature: 180-192 ° C, preferably 190 ° C Pressure: 140-155 bar, preferably 145 bar Conversion efficiency of C02: 56-60% According to this embodiment of the present invention, the reaction mixture leaving the auxiliary reaction space is subjected to a partial purification treatment (partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution) together with the reaction mixture that leaves the main reaction space. Advantageously, the purification treatment of the reaction mixture is carried out at a temperature between 180 and 192 ° C, at the inlet, and between 165-170 ° C, at the outlet of the treatment, and at a pressure essentially equal to which exists in the main reaction space (140-155 bar) using a carbon dioxide feed as the scrubbing agent. Preferably, only the partial condensation of the flow including ammonia and carbon dioxide in the vapor phase, obtained from the purification operations with carbon dioxide, is provided. In this form, part of the flow of residual ammonia and vapor dioxide carbon is fed into the auxiliary reaction space, in order to control the ammonia / carbon dioxide molar ratio within the aforementioned range and to close the global thermal balance of the reaction space. The molar ratio of ammonia / carbon dioxide in the auxiliary reaction space can also be reacted, if necessary, and maintained at an optimum value by feeding essentially pure ammonia to this space, preferably the ammonia is taken from the feeding of ammonia.

In a preferred embodiment and once again, in order to control the molar ratio of ammonia / carbon dioxide within the aforementioned range and to close the overall thermal balance, the aforementioned uncondensed waste stream, formed of ammonia and dioxide of carbon, can also be partially fed into the main reaction space. In another embodiment of the present invention, the reaction mixture leaving the auxiliary reaction space is subjected to distillation before being transported to the purification treatment. In this case, an advantageous reduction of the liquid flow rates sent to the treatment of purification, which are going to undergo partial purification, and together with it the decrease of the heating requirements to implement this treatment can be achieved. By distilling this reaction mixture, a vapor phase flow of ammonia and carbon dioxide is obtained, which is recycled to the auxiliary reaction space in order to control the molar ratio of ammonia / carbon dioxide and achieve a thermal balance Of the same. Preferably, the step of distilling the reaction mixture leaving the auxiliary reaction space is carried out at a pressure essentially equal to that existing in the auxiliary reaction space, by conventional, well-known methods. In accordance with the present invention, the conversion reaction of the carbamate in urea, in the auxiliary reaction space, can be carried out either at a pressure practically equal to that existing in the main reaction space or at a pressure that is 4 to 8 bar lower. In the first case, the ammonia, carbon dioxide and water present in the vapor phase that is in the vapor flow that comes out at the top of the two reaction spaces, are absorbed by the diluted carbamate solution that it leaves the urea recovery section and is recycled in liquid phase to the auxiliary reaction space. In the latter case, the vapors leaving the upper part of the main reaction space can be fed into the auxiliary reaction space, in order to contribute to the control of the molar ratio of ammonia / carbon dioxide, within the latter. Advantageously and additionally, a certain simplification of the plant can be achieved, because the need to raise the auxiliary reaction space with respect to the main reaction space is eliminated.

In another embodiment of the present invention, the purification treatment of the reaction mixture is carried out at a pressure essentially equal to that which exists in the main reaction space (preferably 150 bar) under conditions termed isobaric or "self-purging". The synthesis reaction in the main reaction space is carried out according to the following process parameters: NH3 / C02 mol: 3.0-3.4, preferably 3.2 H20 / C02 mol: 0.08-0.2, preferably 0.1 Reaction temperature: 185-195 ° C, preferably 190 ° C Pressure: 140-155 bar, preferably 150 bar Conversion efficiency of C02: 70-73% In this case, once again operating with a molar ratio of ammonia / carbon dioxide of less than 4, and preferably of about 3.2, there is an advantageous reduction of both the volume of the high performance reaction space and the heat required to heat the reactants at the reaction temperature. The autodepuration treatment is carried out at a temperature between 190 and 210 ° C.

The reaction synthesis in the auxiliary reaction space is carried out according to the following process parameters: NH / C02 mol: 4.2-4.6, preferably 4.5 H20 / C02 mol: 1.0-1.5, preferably 1.3 Reaction temperature: 185-195 ° C, preferably 190 ° C Pressure: 145-155 bar, preferably 150 bar Conversion efficiency of C02: 58-62% Again, in this embodiment of the present invention, the reaction mixture leaving the auxiliary reaction space is subjected to a partial purification treatment (partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution) together with the mixture of reaction that leaves the main reaction space. Preferably, the process of the present invention also provides, in this case, a partial condensation of the flow that includes ammonia and carbon dioxide in vapor phase, obtained by the isobaric purification operation. In this way, the residual flow of ammonia and vapor dioxide carbon is fed into the space '/ > M 'of auxiliary reaction, in order to control the molar ratio of ammonia / carbon dioxide within the aforementioned range and to reduce the thermal balance of the synthesis reaction. In this embodiment, the flow (which includes ammonia, carbon dioxide and water in the vapor phase) leaving the top of the main reaction space, is further subjected to the isobaric purification treatment together with the reaction mixture, while the flow leaving the top of the auxiliary reaction space is fed directly to the urea recovery section. Similarly, to the above described embodiment, the conversion reaction of the carbamate to urea can be carried out in the auxiliary reaction space, either at a pressure essentially equal to that existing in the main reaction space, or at a pressure that is 4 to 8 bar lower. Preferably, the distillation step of the reaction mixture leaving the auxiliary reaction space is carried out at a pressure of between 135 and 155 bars, preferably 150 bars, and at a temperature of between 190 and 210 ° C, with procedures and equipment known, totally conventional. Again, in this case, an advantageous reduction of the liquid flow rate sent to the •1. "//" MX purification treatment, which is going to undergo partial purification, and together with this, the heating requirements to implement this treatment In accordance with another aspect of the invention, the technical problem established above is solved with a plant that can be obtained by the aforementioned modernization method, comprising: a first urea synthesis reactor, a purification equipment for subjecting a first reaction mixture, leaving the first reactor, to a partial decomposition treatment. of the carbamate and partial separation of the free ammonia in aqueous solution, present in the first mixture, - - a means for partially condensing the vapors of the purification equipment and recycling a first carbamate solution to the first reactor; - a second reactor for the synthesis of urea, in parallel with the first reactor; - a recovery section for separating the urea produced in the first and in the second reactors from a second aqueous solution of carbamate; - means for recycling the second carbamate solution obtained in the reaction space into the second reactor, - means for feeding a second reaction mixture leaving the second reactor to the purification equipment. According to a still further embodiment of the invention, the aforementioned urea production plant comprises: a first urea synthesis reactor; - a purification equipment for subjecting a first reaction mixture leaving the first reactor, to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution, present in the first mixture, - - a means for condensing by at least partially the vapors of the purification equipment and recycle a first carbamate solution to the first reactor, - a second urea synthesis reactor, in parallel with the first reactor, - - a recovery section to separate the urea produced in the first and in the second reactors from a second aqueous solution of carbamate; - means for recycling the second carbamate solution obtained in the recovery section to the second reactor; - a distillation equipment for subjecting a second reaction mixture, leaving the second reactor, to a partial decomposition treatment of carbamate and to /. ! f | V a partial separation of the free ammonia in aqueous solution, present in the second mixture; - a means to recycle the vapors leaving the distillation equipment to the second reactor. According to the present invention, the plants that carry out the urea production process can be provided with either new plants or with pre-existing plants that are modified in order to obtain an expansion of the production capacity and at the same time a performance improved from the point of view of energy consumption. Other features and advantages of the present invention are set forth in the detailed description of some of the preferred embodiments thereof, which is given by way of non-limiting example in relation to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically a plant for the production of urea according to the prior art and according to the process called purification of C0; Figure 2 schematically shows a first embodiment of a plant according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of Figure i; Figure 3 shows schematically a second embodiment of a plant, according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of the Figure Figure 4 shows schematically a third embodiment of a plant, according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of the Figure 1; Figure 5 shows schematically a plant for the production of urea according to the previous prior art and according to the so-called self-cleaning process or isobaric process, - Figure 6 shows schematically a fourth embodiment of a plant, according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of Figure 5; Figure 7 shows schematically a fifth embodiment of a plant, according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of Figure 5; Figure 8 schematically shows a sixth embodiment of a plant, according to the present invention, either as a totally new plant or as one obtained by the modernization of the plant of Figure 5.

DESCRIPTION OF THE PREFERRED MODALITY For the sole purpose of simplifying the disclosure of the present invention, specific reference is made to the connecting ducts of the different parts of the plant described below and shown in the Figures, in a conventional manner, only when it is strictly necessary. With reference to Figure 1, the reference number 1 indicates, as a whole, a urea production process of the so-called purification type with C0 / according to the prior art. The plant 1 comprises a synthesis section 2 and a section 3 for the purification and recovery of the urea produced, and in turn includes a number of pieces of equipment that operate either at low pressure or under vacuum, as will be described in more detail then. The synthesis section 2 comprises, in series, a reactor Rl for the synthesis of urea (in which a respective reaction space is defined), a high pressure scrubber 4 for the partial decomposition of the carbamate and the partial separation of the free ammonia in aqueous solution, present in the reaction mixture leaving Rl, and a carbamate 5 condenser for the absorption of the purification vapors with the ammonia feed and with a carbamate solution leaving section 3. The Rl reactor it is connected, by its lower end and by means of a duct 6, to the scrubber 4, from which a vapor phase (which includes ammonia, carbon dioxide and steam) is sent to the carbamate 5 condenser, through a duct 7, and a liquid phase (which includes a partially purified urea solution) which is sent to the recovery and purification section 3, through a duct 14. The reactor Rl is also connected, on the opposite upper part and by means of a duct 31, with a purifier 8, wherein the vapors leaving the reactor Rl (which essentially include ammonia, carbon dioxide and steam) are absorbed by means of a diluted solution of recycled carbamate, which comes from section 3 via pipeline 9. rence numbers 10 and 11 indicate a centrifugal compressor and a high pressure pump for sending carbon dioxide to the scrubber 4 and the ammonia feed to the carbamate condenser 5, via the ducts 30a, 30b, respectively. An ejector 12 sends to the carbamate condenser 5, either a carbamate recycling solution coming from the purifier 8, through a duct 9a, or an aqueous carbamate solution taken from the R-1 reactor, near the end of the reactor. background thereof. A duct 13, which extends between the carbamate condenser 5 and the reactor R-1, allows the recycling thereto of an aqueous solution including the recycling of carbamate as well as a feed of ammonia and carbon dioxide. As already mentioned, the duct 14 allows a partially purified urea solution leaving the synthesis reactor 2 to be sent to the recovery section 3. The recovery section 3 comprises, in turn, a low pressure distiller 15 (which works at about 3-4 bar) connected to a pair of conventional vacuum distillers 16, 17, in series, with the interposition of a urea solution collection tank 18. Rence number 19 indicates a duct for sending purified urea which leaves section 3 towards the conventional finishing equipment not shown. The rence numerals 20 and 21 indicate the supply ducts of the ammonia vapors to a vacuum section 22, where these vapors are condensed in a known manner. The condensates obtained and containing a certain amount of residual ammonia are sent to a section of water treatments through the pipeline 23. The vapors leaving the distiller 15 are combined with a diluted solution of recovered ammonia coming from section 22 and they condense at 24 and are sent (in the form of a diluted carbamate solution) to the purifier 8, through a high pressure pump 25 and through the aforementioned duct 9. With regard to the plant 1 described above, some embodiments of the plant according to the present invention, shown in Figures 2 to 4, will be described below. In the following description and in the figures, the parts of the floor 1 that are structurally or functionally equivalent to those discussed above in relation to the Figure 1, will be indicated with the same rence numbers and will no longer be described. In addition, according to a particularity of the present invention, the following embodiments of the plant 1 represent either a totally new plant, or a plant obtained by the modernization of a previously existing plant, as will be explained in more detail ''. '//' / MX below. In relation to Figure 2, the synthesis section 2 of the plant 1 according to the present invention, further comprises a second reactor of synthesis of urea R-2, in parallel with the reactor R-1. According to a particular feature of the present invention, the Rl reactor will act as the main reactor, which is the most important in terms of both yield and urea production, while the R-2 reactor will act as the auxiliary reactor, with lower importance both in terms of yield and in terms of urea production, since it is dedicated to converting the diluted carbamate solution that comes from the recovery section 3 into urea. For this purpose, plant 1 includes a suitable medium, in this specific case a pipeline 26, to make recycle the dilute carbamate solution leaving the purifier 8 to the auxiliary reactor R-2. According to another feature of the present invention, the auxiliary reactor R-2, (in which a respective reaction space is defined) is connected through a duct 27 to the scrubber 4, where a treatment is carried out. of partial decomposition of the carbamate and partial separation of the free ammonia present in the reaction mixtures, which come out of the two reactors Rl and R-2. Advantageously, the duct 27 extends between the auxiliary reactor (R-2) and a branch 6a of the duct 6, to allow considerable simplification of the plant. In order to control the molar ratio N__3 / C0 and close the thermal balance of the reactions in the main reactor Rl and in the auxiliary reactor R-2, the branches 28a, 28b of a duct 28 allow to send towards the same part of the ammonia and carbon oxide vapors that were not condensed in the carbamate condenser 5. Advantageously, the molar ratio NH3 / C02 and the thermal balance in the auxiliary reactor R-2 can be further controlled by removal of part of the ammonia feed through a duct 29, which it extends between an ammonia feed pipe 30b and pipe 26. According to a further feature of the present invention, the bottom of the auxiliary reactor R-2 is placed at a higher level, relative to the ground, than the bottom of the main reactor R1, in order to facilitate feeding to the reactor R-2 the non-condensed vapors rich in ammonia that come from the ammonia condenser 5, through the pipeline 28b. The reference numeral 32 indicates a duct for sending the vapors leaving the top of the auxiliary reactor R-2 towards the purifier 8. As already mentioned, the plant 1 of Figure 2 can be either a totally new plant or a plant obtained by the modernization of a previously existing plant, as shown in Figure 1. Preferably, the modernization of the plant of Figure 1 is carried out by means of the steps of: - providing the auxiliary reactor R-2 in upstream of the scrubber 4 and in parallel with the existing reactor Rl; - connecting the reactor R-2 with the scrubber 4 by means of the duct 27; - providing duct 26 for recycling to the R-2 reactor the diluted carbamate solution, obtained in the recovery section 3 and leaving the purifier 8. According to the embodiment shown in Figure 2, the auxiliary reactor R-2 preferably placed so that its bottom is provided at a higher level with respect to the ground than the bottom of the reactor Rl. Preferably, the modernization method of the present invention also comprises the steps of providing the duct 28b to make the R-2 reactor recycle the ammonia-rich vapors, not condensed in the carbamate 5 condenser, and to provide the duct 32 for sending to the purifier 8, the vapors coming out of the upper part of the reactor R-2. According to another embodiment of the plant 1, according to the present invention and as shown in Figure 3, the synthesis section 2 comprises a distiller 35 wherein the reaction mixture leaving the reactor R-2, via the pipe 27, is distilled in order to generate a flow of ammonia-rich vapors, which is recycled to reactor R-2 through another pipeline 36. Distiller 35 in turn is connected to scrubber 4, via pipeline 37 , designed to feed the scrubber with a partially purified reaction mixture by distillation. Furthermore, in this embodiment of the plant 1, the non-condensed ammonia-rich vapors coming from the carbamate condenser 5 are fed, via the branch 28b of the duct 28, to the distiller 35 and not to the reactor R-2, in order to facilitate the distillation operations. Advantageously, the distiller 35 makes it possible to reduce the liquid flow rate (reaction mixture produced in R-2) sent to the scrubber 4, with a reduction in the thermal load of that equipment. This feature is particularly advantageous when it is necessary to modernize a previously existing plant, where the scrubber is already operating at full capacity. In another embodiment of the plant 1, according to the present invention and as shown in Figure 4, the bottom of the auxiliary reactor R-2 is at the same level as the bottom of the main reactor Rl and, in any case, practically at level of earth, thus avoiding the necessary work to raise the reactor R-2, which are usually complicated and expensive. In this case, the pressure in the auxiliary reaction space is reduced by 4-8 bar, compared to that already existing in the primary reaction space. In this modality, the molar ratio NH3 / C0 and the thermal balance in the auxiliary reactor R-2 are controlled by sending to the latter the vapors leaving the upper part of the main reactor Rl, through the pipeline 33, instead of removing the condensed vapors that come from the carbamate condenser 5. In this case, the duct 28 extends only between the carbamate condenser 5 and the main reactor Rl. Due to the lower synthesis pressure in the R-2 reactor, the reaction mixture leaving it is sent to the scrubber 4 by means of a pump 34, provided on line 27 in the upstream of that equipment. With reference to Figure 5, a urea production plant of the type called isobaric depuration (self-cleaning) according to the prior art will be described below. Again, in this case, the parts of the floor 1 that are structurally or functionally equivalent to those already discussed above in relation to Figures 1-4, are indicated with the same reference numbers and will no longer be described. In the synthesis section 2 of the plant 1 of Figure 5 and in accordance with the conventional process conditions of the auto-purification plants, the carbon dioxide and ammonia feed is sent directly to the reactor Rl and respectively to a tank of collection 38 which is an integral part of the recovery section of urea 3. For this purpose, the plant 1 comprises suitable ducts 39, 40 extending between a centrifugal compressor and a high pressure pump (not shown) and the aforementioned reactor R1 and tank 38. In the reaction section 2 there is also provided, downstream of the carbamate condenser 5, a separator 41 designed to separate vapors including ammonia, carbon dioxide and steam, which did not condense in 5 a from a recycled carbamate solution to the reactor Rl, through a pipe 42 and an ejector 43. The vapors coming out of the separator 41 are sent by the pipeline 44 to an exhaust 45 medium pressure gauge (operating at approximately 18 bar) provided in the recovery section 3 immediately downstream of the scrubber 4. The recovery section 3 comprises, in this case, in addition to the above medium pressure still 45, the distiller low pressure 15 and vacuum distillers 16, 17, a distillation column 46 designed to essentially remove pure ammonia from a diluted carbamate solution, both recycled to reactor Rl. The diluted carbamate solution leaving the bottom of the distillation column 46 in particular is recycled to the carbamate 5 condenser via line 47 and pump 48, while the pure ammonia vapors leaving the top of the column 46 is condensed at 49 and is sent to tank 38 through a pipe 50. Therefore, the ammonia is recycled to Rl together with a new ammonia feed, through pumps 51, 52 and a pipe 53 in which ejector 43 is mounted. Reference number 54 indicates another pipeline that 1 . / '/ M \ extends downstream of pump 51 and which is designed to feed column 46 an adequate reflux of liquid ammonia for rectification operations. In the distillation column 46 a diluted carbamate solution obtained by condensation of the vapors leaving the upper part of the medium and low pressure distillers, 45 is subjected to rectification. , in respective capacitors indicated by 55 and 56. The recovery section 3 also comprises a tank 57 in which a diluted carbamate solution is collected, which comes from the condenser 56, and a pump 58 designed to feed this solution to the column 46. With regard to the plant 1 of Figure 5, some alternative embodiments of the plant according to the present invention, shown in Figures 6 to 8, will be described below. With reference to Figure 6, the synthesis section 2 of the plant 1 comprises a second reactor of synthesis of urea R-2, placed in parallel to the reactor R-1. Similar to the previous embodiments of the invention described above, the R1 reactor acts as a main reactor, the most important being both in terms of performance and in terms of production capacity, while the R-2 reactor acts as a reactor. an auxiliary reactor, with less importance in terms of yield of urea and production of urea, the latter has the objective of converting into urea the solution of diluted carbamate that comes from the recovery section 3. Plant 1 comprises a suitable medium, in this specific case, it is the duct 66, to recycle the diluted carbamate solution from the pump 48 and leaving the bottom of the distillation column 46 to the auxiliary reactor R-2. The auxiliary reactor R- 2 in turn is connected through a duct 59 with the scrubber 4, where a partial decomposition of the carbamate is carried out and a partial separation of the free ammonia present in the reaction mixtures, which leaves the Rl and R reactors. -2. In order to control the molar ratio NH3 / C0 and the thermal balance in the auxiliary reactor R-2, the duct 44 now feeds this reactor with non-condensed vapors rich in ammonia leaving the carbamate condenser 5. In this embodiment of the invention, the medium pressure distiller 45 it is fed with vapors leaving the upper part of the auxiliary reactor R-2 through another duct 60. Advantageously, the bottom of the auxiliary reactor R-2 is supported at a higher level, with respect to the '/ "/ 1X earth, than the bottom of the main reactor Rl, in order to facilitate the supply to R-2 of the non-condensed vapors coming from the separator 41. The plant 1 of Figure 6 can be a totally new or a plant obtained by the modernization of a previously existing plant, such as the one shown in Figure 5, by means of a series of steps equivalent to those already mentioned in relation to the plant of Figure 2. In another modality of the plant 1, according to the present invention and as shown in Figure 7, the auxiliary reactor R-2 is installed at a level lower or equal to that of the main reactor Rl, while the pressure in the space of the auxiliary reaction is reduced by 4-8 bar with respect to the pressure already existing in the primary reaction space.As the synthesis pressure is lower in the R-2 reactor, the reaction mixture leaving it is sent to the scrubber 4 by means of a pump 61, pr ovista in pipeline 59 upstream of this equipment. Again, in this case, when the auxiliary reactor R-2 operates at a pressure lower than that of the main reactor R1, it is possible to achieve the simplification of the plant and a network in the costs of manufacturing (or modernization) of the plant. plant 1 In another embodiment of the plant 1, according to the present invention and as shown in Figure 8, the synthesis section 2 comprises a distiller 62 in which the reaction mixture leaving the reactor R- 2, through a duct 63, is distilled in order to generate a flow of ammonia-rich vapors, recycled to the reactor R-2 through another duct 64. The distiller 62 is in turn connected with the distiller 45 of medium pressure, through a duct 65 designed to feed the latter the distilled reaction mixture. In this embodiment, the duct 65 is also used in this terminal section to transport to the medium pressure distiller 45 the reaction mixture that comes from the scrubber 4 through the duct 14. Advantageously, the distiller 62 allows reducing the flow rate of flow of liquid (reaction mixture produced in R-2) that is sent to the scrubber 4, with a reduction of the thermal load of this equipment. This feature is particularly advantageous when it is desired to modernize a previously existing plant, where the scrubber is already working at full capacity.

H '/' Í7MX

Claims (14)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A method for modernizing a plant for the production of urea, of the type comprising : a urea synthesis reactor; a purification equipment for subjecting a first reaction mixture leaving the reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution, present in the first mixture, - a means for at least partially condensing the vapors leaving the purification equipment and having a first carbamate solution recycled to the reactor; a recovery section for separating the urea produced in the reactor from a second aqueous carbamate solution, characterized in that it comprises the steps of: providing a second urea synthesis reactor in the upstream of the scrubbing equipment; connect the second reactor with the purification equipment; providing a means for recycling the second carbamate solution obtained in the recovery section to the second reactor. The method according to claim 1, characterized in that the second reactor is placed with its bottom higher than the bottom of the first rector. The method according to claim 1, characterized in that it also comprises the step of providing a means for recycling the non-condensed vapors that come from the purification equipment to the second reactor. The method according to claim 1, characterized in that it further comprises the step of providing a means for condensing vapors leaving the top of the first and / or second reactors and having the solution obtained in this way recycled to the second reactor. The method according to claim 1, characterized in that it further comprises the step of providing a duct for making the vapors coming out of the upper part of the first reactor recycle towards the second reactor. The method according to claim 1, characterized in that it further comprises the step of providing a duct for feeding the vapors leaving the top of the second reactor to the urea recovery section. The method according to claim 1, characterized in that it also comprises the steps of: connecting the second reactor with a distillation equipment to subject a second reaction mixture leaving the second reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution, present in the second mixture, - providing a means for recycling the vapors leaving the distillation equipment to the reactor; connect the distillation equipment with the purification equipment. The method according to claim 7, characterized in that it also comprises the step of providing a means for recycling the non-condensed vapors that come from the purification equipment to the distillation equipment. The method according to claim 1, characterized in that it further comprises the step of connecting the upper part of the second reactor with a medium pressure distiller, provided in the urea recovery section. 10. A method to modernize a plant for t 1. '< // "" MX production of urea, of the type comprising: a urea synthesis reactor, - a purification equipment for subjecting a first reaction mixture leaving the second reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution, present in the first mixture, a means for condensing, at least partially, the vapors leaving the purification equipment and having a first carbamate solution recycled to the reactor, - a recovery section for separating the urea produced in the reactor of a second aqueous carbamate solution, characterized in that it comprises the steps of: providing a second urea synthesis reactor upstream of the purification equipment; provide a means for recycling the second carbamate solution obtained in the recovery section to the second reactor, - connecting the second reactor with distillation equipment to subject a second reaction mixture, which leaves the second reactor, to a decomposition treatment of the carbamate and partial separation of the free ammonia in aqueous solution present in the : I .. "" '/' 'MX reaction mixture; provide a means to recycle the values leaving the distillation equipment to the reactor; connect the distillation equipment with the urea recovery section. The method according to claim 10, characterized in that it also comprises the step of providing a means for recycling the vapors leaving the purification equipment to the second reactor. The method according to claim 10, characterized in that it further comprises the step of providing a duct for feeding the upper vapors leaving the reactor to the urea recovery section. 13. A plant for the production of urea comprising: a first urea synthesis reactor, - a purification equipment for subjecting a first reaction mixture leaving the first reactor to a treatment of partial decomposition of the carbamate and partial separation of the ammonia free in aqueous solution, present in the first mixture, - a means to condense, at least partially, the vapors leaving the i. '//' 'MX debugging and having a first carbamate solution recycled to the first reactor, - a second urea synthesis reactor in parallel with the first reactor, - a recovery section to separate the urea produced in the first reactor and the second reactors from a second aqueous carbamate solution, a means for recycling the second carbamate solution obtained in the recovery section to the second reactor, a means for feeding a second reaction mixture leaving the second reactor to the second reactor. 14. The plant according to claim 13, characterized in that it also comprises a means for feeding a flow of carbon dioxide fed to the purification equipment 15. The plant according to claim 13, characterized in that the bottom of the second reactor is more elevated than the bottom of the first reactor 16. The plant according to claim 13, characterized in that it also comprises a It gave to recycle non-condensed vapors that come from the purification equipment to the first reactor and / or the second reactor. 17. The plant according to claim 13, / '' MX characterized in that it also comprises a means for condensing the vapors leaving the upper part of the first and / or second reactors and having the solution obtained in this way recycled to the second reactor. The plant according to claim 13, characterized in that it also comprises at least one duct for making the vapors coming out of the upper part of the first reactor recycle towards the second reactor. The plant according to claim 13, characterized in that it also comprises at least one duct for feeding the vapors leaving the upper part of the second reactor towards the recovery section of urea. 20. A plant for the production of urea comprising: a first urea synthesis reactor, - a purification equipment for subjecting a first reaction mixture leaving the first reactor to a treatment of partial decomposition of the carbamate and partial separation of the ammonia free in aqueous solution, present in the first mixture; means for condensing, at least partially, the vapors leaving the purification equipment and having a first carbamate solution recycled to the first reactor, - 11. A second urea synthesis reactor placed in parallel with the first reactor, a recovery section for separating the urea produced in the first reactor and in the second reactor from a second aqueous carbamate solution, a means for recycling the second carbamate solution obtained in the recovery section towards the second reactor, - a device for subjecting a second reaction mixture leaving the second reactor to distillation to obtain a flow that includes ammonia and carbon dioxide in vapor phase and a partially purified reaction mixture, - a means to make recycle the flow that includes ammonia and carbon dioxide in vapor phase, to the second reactor. 14. A plant according to claim 20, characterized in that it further comprises a means for feeding the partially purified reaction mixture leaving the distillation equipment to the urea recovery section. The plant according to claim 20, characterized in that it further comprises a means for feeding the partially purified reaction mixture which I l_ '' / > 'M \ leaves the distillation equipment to the purification equipment. 23. The plant according to claim 20, characterized in that it also comprises a means for recycling the non-condensed vapors leaving the purification equipment to the second reactor. The plant according to claim 20, characterized in that it also comprises a means for recycling the non-condensed vapors leaving the purification equipment to the distillation equipment. 25. The plant according to claim 20, characterized in that it further comprises a duct for feeding the upper vapors leaving the second reactor to the urea recovery section. 26. The plant according to claim 20, characterized in that it further comprises means for condensing the vapors leaving the upper part of the first reactor and / or the second reactor and having the solution obtained in this way recycled to the second reactor. í 1_4 '/' 7MX