MXPA98007629A - Process for the combined production of ammonia and u - Google Patents

Abstract

The present invention relates to a process for the combined production of ammonia and urea of the type comprising an ammonia synthesis reactor (2), a urea synthesis reactor (5) and a urea recovery section (21), which is characterized by subjecting at least part of a flow comprising carbamate in aqueous solution, coming from section (21) of recovery of urea, to a treatment of partial decomposition to obtain a flow comprising ammonia and dioxide vapor phase carbon and a flow comprising carbamate diluted in aqueous solution, which is fed together with a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably obtained by reforming as hydrocarbon vapor, and a flow comprising ammonia from the reactor (2) of ammonia synthesis to a carbamate synthesis section (3), where ammonia and carbon dioxide are reacted to obtain a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen. The flow comprising carbamate in aqueous solution is then sent to the reactor (5) of synthesis of urea, while the gas flow comprising hydrogen and nitrogen is sent to the reactor (2) of synthesis of ammonia

Description

PROCESS FOR THE COMBINED PRODUCTION OF AMMONIA AND UREA FIELD OF THE INVENTION The present invention relates to a process for the combined production of ammonia and urea in a plant comprising an ammonia synthesis reactor, a urea synthesis reactor and a urea recovery section. In the following part of the description and in the subsequent claims, the term "process for the combined production of ammonia and urea" is intended to refer to a single process that integrates the ammonia production process with the urea production process . In other words, in accordance with this technology, urea is produced - at least in part - by causing the ammonia obtained in a synthesis reactor to react with carbon dioxide contained in a crude gas flow for synthesis, which comprises between other things, hydrogen and nitrogen from, for example, a reformation section. The flow of raw gas for synthesis, without carbon dioxide, is then sent to the ammonia synthesis reactor. The processes of this type allow to eliminate or, in any case, reduce to a remarkable extent, the P1541 / 98 X decarbonation section of the raw gas flow for synthesis, the separation section of the ammonia produced in the corresponding synthesis reactor and the compression section of carbon dioxide. In addition, energy consumption and investment costs resulting from a single integrated system can be substantially lower than those resulting from two separate processes for ammonia and urea. The need to provide an integrated process is particularly felt in all those cases where all or most of the ammonia is converted to urea causing it to react with the carbon dioxide obtained as a by-product during the preparation of the synthesis gas. In the following part of the description and in the subsequent claims, the term "urea recovery section" is intended to refer to the part of the plant downstream of the urea synthesis reactor, which generally comprises one or two urea decomposers. medium pressure carbamate (approximately 18 bar a), respectively at medium and low pressure (approximately 4 bar a) and the related carbamate condensers, whose function is to separate the urea produced from the reaction mixture from the corresponding synthesis reactor and allow this P1541 / 98MX way to get a concentrated urea solution from 60% to 75%. The invention also relates to a plant for implementing the aforementioned process, as well as to a method for the simultaneous modernization of an ammonia production plant and a urea production plant. In the following part of the description and in the subsequent claims, the term "simultaneous modernization" is intended to refer to a modernization that has to do - at the same time - both with an existing plant for ammonia synthesis and with an existing plant for the synthesis of urea, with the purpose of its integration. The integration between the production process of ammonia and urea, where the carbon dioxide contained in the crude synthesis gas and the synthesis ammonia are reacted, producing an aqueous solution of carbamate that will be sent to the reactor of urea synthesis , on the one hand, involves a simplification of the plant, with special reference to the ammonia decarbonation and separation sections and the compression section of C02, and, on the other, a marked overload of the sections correlated with the urea production, essentially due to the lack of heat of formation and P1541 / 98MX to the excessive molar ratio H20 / C02 in the urea synthesis reactor with the consequent low conversion efficiency and high energy consumption. As a consequence, in the field of the combined production of ammonia and urea, the need to provide processes that increase the urea conversion efficiency in a simple way with low operating and investment costs is increasingly felt.

BACKGROUND OF THE TECHNIQUE In order to comply with the above requirement, in the field various processes have been proposed for the combined production of ammonia and urea. For example, in US-A-3 303 215 and in US-A-3 310 376, a process for the combined production according to the prior art is presented, wherein liquid ammonia purified in a suitable manner is fed to a reactor of urea synthesis, where the ammonia is reacted with the carbon dioxide comprised in a crude synthesis gas that also includes hydrogen and nitrogen. In the urea synthesis reactor, ammonia and carbon dioxide react forming ammonium carbamate which, in turn, is transforming into urea through P1541 / 98MX dehydration. A first drawback of this process lies in the fact that the high generation of heat produced during the production of carbamate and the presence of inert gases (hydrogen and nitrogen), which reduce the partial pressure of ammonia and carbon dioxide, they make it necessary to operate - in the urea synthesis reactor - at high pressures to keep the reactants in liquid phase, with the consequent high energy consumption and high operating costs. In addition, due to the introduction into the reactor of urea synthesis of a high amount of water - for example, in the form of carbamate in aqueous solution - to facilitate the absorption of carbon dioxide in the ammonia solution and the subsequent reaction in carbamate , the molar ratio H20 / C02 in this synthesis reactor is relatively high and the conversion efficiency is unsatisfactory. A further disadvantage lies in the structural and operational complexity of the urea synthesis reactor necessary to implement the process described above, which must include a special unit for the separation of inert gases (hydrogen and nitrogen) from carbon dioxide and ammonia in vapor phase. In accordance with this process of technique P1541 / 98MX above, also provides a step of condensation and separation of the ammonia produced from the unreacted gases, typical of the processes of ammonia production, which is rather demanding from the standpoints of economic consumption and energy. In US-A-3 349 1226, US-A-4 012 443, US-A-4 013 718 and US-A-4 320 103, there is disclosed another type of process according to the prior art, comprising a separate section for the absorption of carbon dioxide and the synthesis of carbamate. In accordance with this process, the ammonia from the corresponding synthesis reactor is separated from the unreacted gases - usually by absorption with water in a special absorption section - and sent to the carbamate synthesis section, where it reacts with the carbon dioxide contained in the raw gas flow for synthesis, which comes from a reforming section, forming ammonia carbamate which is sent to the urea synthesis reactor. Also in this case, the absorption of the carbon dioxide and the subsequent reaction in carbamate occurs in a water-rich environment, which is sent together with the carbamate to the urea synthesis reactor. In addition, the heat of formation of the carbamate that is released during the absorption of carbon dioxide with P1541 / 98MX the ammonia solution causes a strong evaporation of the latter, which involves the need for an additional recovery of ammonia at the outlet of the carbamate synthesis section, with the consequent problems of excessive dilution of the carbamate. At the same time, since the urea conversion reactor lacks the heat of carbamate formation, the operating conditions of the reactor become more difficult. Stated another way, in accordance with this prior art process, the production of carbamate outside the urea synthesis reactor includes not only the heat loss of related formation, but also requires the addition of water, which is in contrast to the subsequent dehydration to give urea and, therefore, does not allow satisfactory conversion yields. In conclusion, the processes for the combined production of ammonia and urea in accordance with the prior art, also require very complex plants for their implementation and involve high investment and operational costs, as well as a high energy consumption, and do not allow no case obtain a high conversion performance of urea due to the excessive molar ratio H20 / C02 present in the corresponding synthesis reactor. Due to these drawbacks, the processes P1541 / 98MX above have not found a specific application to date, although in the field there is an increasing requirement.

SUMMARY OF THE INVENTION The problem underlying the present invention is to conceive a process for the combined production of ammonia and urea, in such a way that it allows, on the one hand, to obtain a high yield of urea conversion and that, on the other hand, of simple implementation with low operating and investment costs and, also with a low energy consumption. The above problem is solved, according to the invention, by a process of the aforementioned type comprising the steps of: subjecting at least part of a flow comprising carbamate, in aqueous solution from the recovery section of urea, to a partial decomposition treatment, to obtain a flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising carbamate diluted in aqueous solution; - feeding the flow comprising ammonia and carbon dioxide in vapor phase to the urea synthesis reactor; - feed the flow comprising carbamate P1541 / 98MX diluted in aqueous solution resulting from the treatment step, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably obtained by reforming with hydrocarbons and a flow comprising ammonia from the ammonia synthesis reactor, to a carbamate synthesis section; - reacting the ammonia with the carbon dioxide in the carbamate synthesis section to obtain a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen; feeding the flow comprising carbamate in aqueous solution to the urea synthesis reactor; feeding the gas flow comprising hydrogen and nitrogen to the ammonia synthesis reactor. Advantageously, thanks to the process according to the present invention and, in particular, to the step of the partial decomposition of the carbamate from the urea recovery section, it is possible to send a water-rich solution to the carbamate synthesis section. and sending at the same time to the urea synthesis reactor a flow comprising ammonia and substantially anhydrous carbon dioxide, which allows reducing the H20 / C02 molar ratio in the reactor, thereby increasing the conversion yield of urea.

P1541 / 98MX In this way, in addition to maintaining a low molar ratio of H20 / C02 in the urea synthesis reactor, it is also possible to advantageously exploit at least part of the water contained in the carbamate in aqueous solution from the section of recovery of urea, recycling it in a simple and economic way to the carbamate synthesis section, in order to facilitate the absorption of carbon dioxide and to keep the carbamate produced in the form of an aqueous solution, thus avoiding the undesirable crystallization of the same. An additional advantage that results from the present process resides in the fact that when sending to the urea synthesis reaction a flow of gas comprising ammonia and carbon dioxide, it is possible to supply at least part of the heat of reaction necessary for the synthesis. of urea directly from the heat generated by the reaction between the ammonia and the carbon dioxide in the urea synthesis reactor (heat of formation of the carbamate). By doing this, it is possible to eliminate the problem of the heat balance in the urea synthesis reactor, even in those cases when substantially all the carbon dioxide comprised in the crude synthesis gas is transformed into carbamate in the specific synthesis section .

P1S41 / 98 X Therefore, the process according to the present invention allows to obtain in an extremely simple and effective way a combined production of ammonia and urea at low investment and operating costs and, also with a low energy consumption and a high urea conversion efficiency. Unlike the processes according to the prior art, the present process advantageously allows to eliminate also cumbersome step of the separation of the ammonia - by condensation or by absorption - of the unreacted gases. Actually, according to the present invention, ammonia and carbon dioxide are separated from the respective flows at the same time and are directly reacted in a single carbamate synthesis section when exploiting or taking advantage of their high chemical reaction capacity, obtaining a carbamate solution that will be sent to the urea synthesis reactor. Preferably, the flow coming from the ammonia synthesis reactor comprises vapor phase ammonia, so that the synthesis of ammonia carbamate can occur at least partially in the gas phase, with an extremely rapid reaction between ammonia and carbon dioxide , which means that it does not require a previous absorption of carbon dioxide in the flow that P1541 / 98MX comprises ammonia. If a certain amount of water greater than the amount included in the diluted flow comprising the carbamate in aqueous solution resulting from the treatment step is required in the carbamate synthesis section, the process according to the present invention advantageously comprises the additional step of feeding a flow comprising water from a urea concentration section to the carbamate synthesis section. In this way, when recycling the water obtained in one of the sections downstream of the urea synthesis reactor, it is no longer necessary to send to the carbamate synthesis section a flow comprising the water coming from outside the process, and obtain from This way a savings in operating costs. In order to advantageously increase the conversion performance of urea, the process according to the present invention further comprises the steps of: subjecting at least part of the flow comprising carbamate in aqueous solution obtained in the synthesis section of carbamate to a partial decomposition treatment, obtain a flow comprising ammonia and carbon dioxide in vapor phase and a flow that P1541 / 98MX comprises carbamate diluted in aqueous solution; feeding the flow comprising ammonia and carbon dioxide in vapor phase to the urea synthesis reactor; - feeding the flow comprising carbamate diluted in aqueous solution resulting from the treatment step towards the carbamate synthesis section. In fact, by doing this, it is possible to send to the urea synthesis section a substantially anhydrous flow comprising ammonia and carbon dioxide which allows to further reduce the H20 / C02 molar ratio with the consequent increase in conversion efficiency, recycle in advantageous to the carbamate synthesis section is the water present in the carbamate flow from said section. In order to control the temperature inside the urea synthesis reactor and to ensure optimum operating conditions for the conversion of the urea, the process according to the present invention also comprises the steps of: preheating a flow comprising ammonia recycled from the urea synthesis section; and feeding the preheated flow comprising ammonia to the urea synthesis reactor. In accordance with an alternative modality of P1541 / 98MX process according to the present invention, the temperature inside the urea synthesis reactor is controlled thanks to the fact that it additionally comprises the steps of: - cooling a flow comprising ammonia and carbon dioxide in vapor phase which results from the treatment of partial carbamate decomposition; feeding the cooling flow in this way to the urea synthesis reactor. The two previous alternatives allow to exert a control of the direct and effective temperature in the urea synthesis reactor, which allows to supply exactly the amount of heat necessary for a high conversion efficiency. In the first case, the urea synthesis reactor is fed with a preheated flow in a suitable manner comprising recycled ammonia, while in the second case, a flow comprising ammonia and vapor dioxide carbon is adequately cooled before being fed to the urea synthesis reactor. For the implementation of the aforementioned process, the invention advantageously provides a plant for the combined production of ammonia and urea, comprising: an ammonia synthesis reactor, a P1S41 / 98 X carbamate synthesis section, a urea synthesis reactor, a urea recovery section and a carbamate decomposition section; - means for feeding at least part of a flow comprising carbamate in aqueous solution from the recovery section of urea to the decomposition section; means for feeding a flow comprising ammonia and vapor dioxide carbon obtained in the decomposition section to the urea synthesis reactor; a respective medium for feeding a flow comprising carbamate diluted in aqueous solution obtained in the decomposition section, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably coming from a steam reforming section of hydrocarbons and a flow that comprises ammonia from the ammonia synthesis reactor to the carbamate synthesis section; - means for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the urea synthesis reactor; means for feeding a gas flow comprising hydrogen and nitrogen, obtained in the section of P1541 / 98 X carbamate synthesis, towards the ammonia synthesis reactor. In accordance with a further aspect of the invention, a method is also provided for the simultaneous modernization of an ammonia synthesis plant and a urea synthesis plant, respectively comprising an ammonia synthesis reactor and a synthesis reactor. of urea and a urea recovery section, characterized in that it comprises the steps of: providing a carbamate synthesis section and a carbamate decomposition section; - providing a means for feeding at least part of a flow comprising carbamate in aqueous solution, coming from the recovery section of urea to the decomposition section; - providing a means for feeding a flow comprising ammonia and vapor dioxide carbon obtained in the decomposition section to the urea synthesis reactor; - providing a respective means for feeding a flow comprising carbamate diluted in aqueous solution obtained in the decomposition section, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably coming from a P1541 / 98MX steam reforming section of hydrocarbons and a flow comprising ammonia from the ammonia synthesis reactor to the carbamate synthesis section; - providing a means for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the urea synthesis reactor; providing a means for feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section to the ammonia synthesis reactor. Thanks to the aforementioned modernization method that combines an existing ammonia plant and an existing urea plant, it is possible to obtain - in a simple and economical way - a high urea conversion efficiency and, at the same time, significant reductions in costs operative and in the consumption of energy. The features and advantages of the inventions are set forth in the description of a modality thereof provided hereinafter by way of non-limiting example with reference to the accompanying drawing.

P1541 / 98MX BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows schematically a plant for the combined production of urea and ammonia, in accordance with the invention, carried out either ex-novo or by modernizing an existing plant for the production of ammonia. and of a plant exist for the production of urea of the conventional type.

DETAILED DESCRIPTION OF A PREFERRED MODALITY With the sole purpose of simplifying the description of the present invention, reference will be made to the connection ducts of the different parts of the plant, as well as to the same parts of the plant described in what is described above. follows and represented in Figure 1, conventional by themselves, only when strictly necessary. With reference to Figure 1, indicated generally by the number 1, there is a plant for the combined production of ammonia and urea according to the invention. Advantageously, the plant 1 comprises an ammonia synthesis reactor 2, a carbamate synthesis section 3, a urea synthesis section 4, a urea recovery section 21 and a carbamate decomposition section 23.

P1541 / 98MX The urea synthesis section 4 comprises, arranged or arranged in series with respect to each other, a urea synthesis reactor 5 and a high pressure scrubber 6 (approximately 180 bar a), for partial decomposition of the carbamate and the separation of the free ammonia in aqueous solution present in the reaction mixture coming from the reactor 5. As will be seen later, the process for the combined production of ammonia and urea according to the present invention, allows obtaining in the synthesis reactor 5 of urea a performance comparable to the yield obtainable with the urea production plants according to the prior art, that is, a yield between 62% and 70%. An example of the operating conditions of the urea synthesis reactor 5 obtainable with the present invention is: molar ratio NH3 / C02 of 3.8, molar ratio of H20 / C02 of 0.8, 64% conversion efficiency, pressure of 180 bar to , temperature of 190 ° C In the example of Figure 1, the part of the plant for the production of urea is of the total recycling type, that is, with the recycling of the reactants to the synthesis reactor 5. However, the present invention is not limited to a particular type of urea synthesis process but, advantageously, it can be P1541 / 98MX also implemented in plants that operate with a urea synthesis process, for example, of a partial recycling type or of the "one pass" type, without recycling of reactants. By numbers 7, 8, 9a and 9b, ducts are indicated for feeding a gas flow comprising hydrogen, nitrogen and carbon dioxide, a flow comprising ammonia, a flow comprising water and a flow comprising carbamate diluted in water. aqueous solution, respectively, to section 3 of carbamate synthesis. The gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably coming from a hydrocarbon steam reforming section, comprising a primary reformer unit and a secondary reformer unit, not shown in Figure 1, since it is of the conventional type and, therefore, known to those skilled in the art. In the following part of the description and in the subsequent claims, by the term "hydrocarbons", it is intended to refer generically to a raw material which is the source of the hydrogen and carbon, such as for example, methane or a mixture of liquid hydrocarbons and / or gaseous, such as natural gas and naphtha.

P1541 / 98MX A gas flow comprising hydrocarbons and water vapor is fed through the pipeline 11 to the primary reformer unit of section 10, where the first decomposition with hydrocarbon vapor occurs resulting in the formation of hydrogen, monoxide of carbon and carbon dioxide. Then, decomposition is continued in the secondary reforming stage, where a gas flow comprising nitrogen (usually air) is also added. Numbers 12, 13 and 14 indicate, respectively, a flow desulfurization section comprising hydrocarbons, a high temperature conversion section and a low temperature conversion section for the conversion of carbon monoxide to carbon dioxide. Sections 12, 13 and 14 are of a conventional type and, therefore, will not be described in greater detail in the next part of the description. For the purposes of the present invention, the gas flow comprising hydrogen, nitrogen and carbon dioxide fed via pipeline 7 into the carbamate synthesis section 3, can be produced by any other known technique, as an alternative to reforming with hydrocarbon vapor. The flow that comprises ammonia comes from P1541 / 98 X Ammonia synthesis reactor 2 and is fed to section 3 through line 8. According to the example of Figure 1, it is worth noting that line 8 connects directly to reactor 2 of ammonia synthesis with section 3 of carbamate synthesis. In this way, it is possible to feed to this last section a flow comprising free ammonia in vapor phase which reacts instantaneously with the carbon dioxide present in section 3, facilitating the synthesis of carbamate. Advantageously, at least part of the water fed to the carbamate synthesis section 3 to accelerate the absorption of the carbon dioxide and its immediate reaction with the ammonia, is contained in the flow comprising carbamate diluted in aqueous solution coming through medium. of the duct 9b - of a carbamate decomposition section 23, which will be described in more detail later. In the example of Figure 1, a prefixed amount of water is also fed to the carbamate synthesis section by means of the duct 9a. This water - or part of it - may come from a source external to the plant 1 or, advantageously, from a section 22 of urea concentration. However, feeding a flow that P1541 / 98MX comprises water through the duct 9a is totally optional and, mainly, serves to increase the water content inside the carbamate synthesis section 3. Actually, it is provided - but it is not represented - an embodiment of the invention wherein all the water is fed to section 3 through line 9b and comes from section 23 of carbamate decomposition. According to a further embodiment of the present invention - not shown - the carbamate synthesis section 3 is fed only by means of the duct 9b, with additional amounts of water coming from the external forces to the plant 1 or to the section 22 of Urea concentration. Preferably, about 30 to 40% of the total amount of water fed into the carbamate synthesis section 3 is fed through the duct 9a and, about 60 to 70% (e.g., 65%) is fed through. of the duct 9b. From the carbamate synthesis section 3, the ducts 15 and 16 are derived to feed a flow comprising carbamate in aqueous solution to the urea synthesis reactor 5 and a flow comprising hydrogen and nitrogen to the ammonia synthesis reactor 2 , respectively.

P1541 / 98MX Before being fed to the ammonia synthesis reactor 2, the flow of gas comprising hydrogen and nitrogen is passed through the pipe 16 to a meta-section section 17 and to a drying section 18 of type conventional, where the gas flow is purified properly. In particular, in section 17 of methanation, the possible traces of carbon monoxide and / or carbon dioxide in suitable form are transformed into methane. On the other hand, in the drying section 18, the gas flow comprising hydrogen and nitrogen is dehydrated by washing with liquid ammonia to eliminate possible traces of water. In this regard, a flow comprising liquid ammonia is introduced into the duct 16 through the duct 19 and is then fed, along with the flow of gas comprising hydrogen and nitrogen, to the drying section 18 which generally includes a separator. of gas / fluid. In the separator, the water present in the gas flow is absorbed by ammonia, so that an aqueous solution of ammonia is obtained, which is advantageously recycled to the urea synthesis section 4, by means of the ducts 20 and 25; at the same time, the flow of water-free gas comprising hydrogen and nitrogen is fed to the ammonia synthesis reactor 2 through the P1541 / 98MX duct 16. The operating conditions of pressure and temperature inside the ammonia synthesis reactor 2 are those typical of a conventional ammonia synthesis plant, well known to those skilled in the art. In addition to the synthesis section 4, the part of the plant for the production of the urea also comprises a urea recovery section 21, a urea concentration section 22 and, advantageously, a carbamate decomposition section 23. In the example of Figure 1, the urea recovery section 21 is of the type comprising a carbamate disintegrator 24 at a medium pressure (approximately 18 bar a), a carbamate disintegrator at low pressure (approximately 4 bar a). ) and a distillation column 26 of ammonia. The urea concentration section 26 comprises in turn a pair of vacuum stills 27 and 28, respectively and a vacuum unit 29, shown by a dashed line in Figure 1. The urea synthesis reactor 25 is connected - in the part or lower end thereof and by means of ducts 15 and 30 - with the carbamate synthesis section 3 and with the duct 19 that feeds a flow that P1541 / 98MX comprises ammonia, respectively. Between the carbamate synthesis section 3 and the urea synthesis reactor 5, there is provided a separator for extracting from the flow comprising carbamate in aqueous solution - through the duct 31 - possible entrainments of hydrogen and nitrogen. The reactor 5 is also connected - always at its lower end and through the duct 33 - with the scrubber 6, from which comes a vapor phase (which includes ammonia, carbon dioxide and water vapor) which is recycled to the reactor 5 through the duct 34 and a liquid phase (including a partially purified urea solution) which is fed to the carbamate disintegrator 24 of the urea recovery section 21 through the pipe 35. The pipe 35 passes through sections 21 and 22 of concentration and recovery of urea, so that at the outlet of the still 28 a flow of purified urea is obtained which is sent - always by means of the pipeline 35 - to conventional terminating devices and, therefore, are not represented. The number 36 indicates the ducts for feeding vapors comprising ammonia to the vacuum unit 29, where these vapors are condensed according to a well-known method.

P1541 / 98MX The condensates obtained, which contain part of residual ammonia in aqueous solution, are sent to the water treatment section (not represented by means of the pipeline 37. According to a particular invention, the pipeline 9a to feed a flow that it comprises water to the carbamate synthesis section 3, is in fluid communication with the pipeline 37 through the pipeline 38 (represented by a dashed line in Figure 1) .Thereby, it is possible to feed the section 3 of carbamate synthesis a flow comprising water from the section 22 of urea concentration, thus advantageously recycling part of the water already present in the plant. The vapors comprising water, ammonia and carbon dioxide, obtained in the carbamate disintegrators 24 and 25, are sent - after being at least partially condensed - to the ammonia distillation column 26 which substantially separates pure ammonia from a solution aqueous carbamate. The ammonia resulting from the distillation is condensed and recycled at least partially to the urea synthesis reactor 5 through the ducts 19 and 30 and to the drying section 18 through the ducts 19 and 16 , respectively.

P1541 / 98MX In accordance with a particularly advantageous feature of the present invention, the flow comprising carbamate in aqueous solution from the bottom of the ammonia distillation column 26 is sent - through the pipeline 39 - to the section 23 of carbamate decomposition. In this way, a substantially anhydrous flow comprising ammonia and carbon dioxide in vapor phase is obtained, which is recycled to the urea synthesis section 5 through ducts 40 and 34 and a very dilute flow of carbamate in aqueous solution which is advantageously recycled to the carbamate synthesis section 3 through the duct 9b. Advantageously, in accordance with the process for the combined production of ammonia and urea of this invention, at least part of a flow comprising carbamate in aqueous solution coming from (duct 39) of the recovery section of urea is subjected to a partial decomposition treatment resulting in the production of a flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising carbamate diluted in aqueous solution. The flow comprising ammonia and carbon dioxide in vapor phase is fed (ducts 40, 34) to the urea synthesis reactor 5, while the flow comprising carbamate diluted in P1541 / 98MX aqueous solution together with a gas flow comprising hydrogen, nitrogen and carbon dioxide and a flow comprising ammonia from an ammonia synthesis reactor 25 is fed (ducts 9b, 7 and 8) to a synthesis section of carbamate, wherein the ammonia and carbon dioxide are reacted to obtain a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen. The flow comprising carbamate in aqueous solution is then fed (duct '15) to a urea synthesis reactor 5, while the flow of gas comprising hydrogen and nitrogen, is fed (duct 16) to the synthesis reactor 2 ammonia. Thanks to the present invention, it is possible to control - and maintain at low levels - the amount of water sent to the urea synthesis reactor 5, which is advantageously recycled to the carbamate synthesis section 3, which allows this mode - in a simple and effective way - obtain high yields of urea conversion. In other words, the step of the partial decomposition of the carbamate allows a high flexibility of the process, since it allows one to operate with large amounts of water in section 3 of urea synthesis, without adversely affecting the molar ratio of H20 / C02 in the urea synthesis reactor 5 and, for P1541 / 98MX therefore, the conversion performance. According to a particularly advantageous embodiment of the present invention, it is possible to further reduce the molar ratio of H20 / C02 in the urea synthesis reactor 5, consequently increasing the conversion efficiency, by feeding at least part of the flow comprising carbamate in aqueous solution from section 3 to section 23 of carbamate decomposition through duct 41 (represented by the dotted line in Figure 1), and obtain a substantially anhydrous flow comprising ammonia and carbon dioxide in vapor phase , which is sent to the urea synthesis reactor 5 through the ducts 40 and 34, and a very dilute flow of carbamate in aqueous solution which is advantageously recycled to the carbamate synthesis section 3 through the duct 9b. As a consequence, the process for the combined production of ammonia and urea is further characterized in that at least part of the flow comprising carbamate in aqueous solution coming from (duct 15) of the carbamate synthesis section 3 is advantageously subjected to a treatment of partial decomposition to obtain a flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising carbamate diluted in aqueous solution that is sent (ducts 40 and 34) P1541 / 98MX to the urea synthesis reactor 5, and to the carbamate synthesis section 3 (duct 9b), respectively. Advantageously, depending on the water content of the flows comprising carbamate in aqueous solution from sections 3 and 21, more or less large parts of said flows are sent to the carbamate decomposition section 23, to obtain a recycle of water to the carbamate synthesis section 3 and to send the substantially urea-reactive urea synthesis reactor 5. The partial decomposition of the carbamate comprised in the flow coming from the carbamate synthesis section 3 and in the flow coming from the urea recovery section 21, respectively, can occur in two separate decomposition units or -as shown in the Figure 1 - in a single decomposition unit forming the section 23. Preferably, the carbamate decomposition section 23 operates at the same pressure and temperature conditions of the scrubber 6, which are also the same for the urea synthesis reactor 5 . According to a particularly advantageous point of the present invention, the carbamate synthesis section 3 comprises three chambers 42, 43 and 44 separated by two film absorbers 45 and 46.

P1541 / 98 X In the example of Figure 1, the chambers 42-44 and the film absorbers 45 and 46 are comprised within a single substantially vertical tubular device. The first chamber 42 is located at the lower end section 3 and is in fluid communication with the duct 7 which feeds the gas flow comprising hydrogen, nitrogen and carbon dioxide towards the carbamate synthesis section 3, respectively with the duct 15 which feeds the flow comprising carbamate in aqueous solution obtained in section 3 of carbamate synthesis to the urea synthesis reactor. The second chamber 45 is located in the central part of section 3 and is in fluid communication with the duct 8 which feeds a flow comprising ammonia from the ammonia synthesis reactor 2 to the carbamate synthesis section 3. The third chamber 44 is located at the upper end of the section 3 and is in fluid communication with the ducts 9a and 9b which feed a flow comprising water, respectively, carbamate diluted in aqueous solution, to the carbamate synthesis section 3 and the duct 16 feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section 3 to the ammonia synthesis reactor 2.

P1541 / 98MX The first film absorber 45 is located between the first and second chambers 42 and 43 and comprises a plurality of tubes having the opposite ends in fluid communication with respectively the first and second chambers. The second film absorber 46 is located between the second and third chambers 43 and 44 and comprises a plurality of tubes having the opposite ends in fluid communication respectively with the second and the third chamber. Thanks to the thus structured section 3 of carbamate synthesis, it is possible to obtain a rapid and effective reaction between carbamate and carbon dioxide in a small and structurally simple device that involves low costs of realization and operation. The flow comprising carbamate diluted in aqueous solution from the urea recovery section 21 is preferably fed to the third chamber 44 - through the duct 9b - close to the second film absorber 46. In addition, particularly satisfactory results have been obtained feeding the flow comprising carbamate diluted in aqueous solution to the third chamber 44, near the upper end of the section P1541 / 98MX 3 of carbamate synthesis, as shown in Figure 1 by dashed line. In the same way, the flow comprising water is fed to the third chamber 44 - through the duct 9a - close to the upper end of the carbamate synthesis section 3. Advantageously, the third chamber 44 - which operates preferably in adiabatic conditions comprises a plurality of horizontal perforated plates of conventional type, which allows increasing the absorption performance. In accordance with the particular structures of the carbamate synthesis section 3 of Figure 1, the flow comprising hydrogen, nitrogen and carbon dioxide coming from the hydrocarbon steam reforming section 10 is fed through the duct 7. to the first chamber 42. From the chamber 42, the flow between the tube side is made to the first film absorber 45, where it flows countercurrently with a luxury comprising ammonia and carbamate in aqueous solution from the second chamber. 43. In this part, most of the carbon dioxide reacts with the free ammonia - preferably either in vapor form or in liquid form - formed at the P1541 / 98MX carbamate that is collected in chamber 42. An example of the composition of the flow comprising carbamate in aqueous solution and leaving chamber 42 - obtainable with the process according to the present invention - is as follows: 37.7% by weight of ammonia, 43.7% by weight of carbon dioxide and 19.0% by weight of water. The gas flowing out of the first film absorber 45 is mixed - in chamber 43 - with the ammonia flow from the ammonium synthesis reactor 2 through the pipeline 8, and enters - on the pipe side - into the second film absorber 46, wherein most of the carbon dioxide and vapor phase ammonia are absorbed by a dilute solution of ammonia that comes from the third chamber 44. The third chamber 44, which is fed through the ducts 9a and 9b by a flow comprising water from the urea concentration section 22, respectively by a flow comprising carbamate in aqueous solution that comes from the urea recovery section 21, allows the elimination or final removal of the ammonia and dioxide of residual carbon. Thanks to the present invention, it is possible to obtain, for example, a gas flow comprising hydrogen and nitrogen coming from chamber 44 (duct P1541 / 98MX 16) having a residual ammonia content equal to 1% mol and residual carbon dioxide equal to approximately 0.05 mol%. The heat of reaction that develops in the carbamate synthesis section 3 is advantageously removed by indirect heat exchange with a cooling fluid (water, for example) which is preferably passed on the shell side in the film absorbers. 45 and 46. In this way, it is possible to preserve the temperature inside the carbamate synthesis section 3 within a range of values such as to avoid any crystallization of the carbamate in the tubes of the film absorbers 45 and 46. Optimal values of pressure and temperature inside the carbamate synthesis section 3 are included, for example, between 140 and 200 bar a (preferably 180 bar a) and between 110 and 150 ° C preferably 130 ° C), respectively. The exhaust gas rich in inert substances such as nitrogen and methane from the ammonia synthesis reactor 2 is suitably separated from the gas that reacted and washed in a washing section 47 with the flow comprising water fed to the section 3 synthesis of carbamate through line 9a. Once P1541 / 98MX was washed, the exit gas is sent to a conventional type recovery plant (not shown). By doing this, it is possible to recover - through the wash water - most of the ammonia entrained in the outlet gas, which is advantageously fed into the carbamate synthesis section 3. By means of number 50 there is represented a cooling apparatus that allows to cool the flow comprising ammonia coming from the synthesis reactor 2 to values lower than approximately 100 ° C. According to a particularly advantageous feature of the present invention, there is provided in the plant for the combined production of ammonia and urea, a means for cooling the gas flow comprising hydrogen, nitrogen and carbon dioxide by indirect heat exchange with a flow comprising urea in aqueous solution of the urea recovery section 21. In particular, as shown in Figure 1, the flow of gas comprising hydrogen, nitrogen and carbon dioxide from the reforming section 10 is passed through the duct 7 through the carbamate disintegrators 24 and 25 of section 21 of recovery of urea, where it is cooled by indirect thermal exchange with the flow of urea P1541 / 98 X partially purified. In this way, the double objective of cooling is advantageously obtained, on the one hand, to the gas flow coming from the reforming section 10 that will be fed to the carbamate synthesis section 3 and, on the other hand, to supply the necessary heat for the decomposition of the carbamate comprised in the flow of urea partially purified, without having to resort to external heat sources, which results in significant savings from the point of view of energy consumption and operating costs. In accordance with another feature of the present invention, the temperature of the urea synthesis reaction is advantageously controlled by feeding to the reactor 5 a flow comprising ammonia preheated in a suitable manner. To this end, the plant for the combined production of ammonia and urea is additionally provided with a medium (shown in Figure 1 by heat exchanger 48) to preheat a stream comprising recycled ammonia - through ducts 29 and 30 - from the recovery section 21 of urea, and a means (duct 30) for feeding the flow comprising ammonia thus heated to the reactor 5 for synthesis of urea).

P1541 / 98MX If no significant amounts of ammonia and carbon dioxide are sent to the urea synthesis reactor 5, the control of the reaction temperature is no longer carried out (only) by preheating a flow comprising ammonia but by cooling in a suitable manner to the flow which comprises ammonia and carbon dioxide. In fact, in this case, the heat necessary for the urea synthesis reaction is supplied at least partially by the heat of formation of the carbamate produced within the reactor 5. As a consequence, depending on the amount of ammonia and carbon dioxide fed to the reactor 5, it may be necessary to supply additional heat through the heat exchanger 48 or, reduce the heat inside the reactor 5 by removing the excess heat. This latter possibility generally occurs when, in addition to the scrubber 6 of the urea synthesis section 4, the carbamate decomposition section 23 is also provided. In this case, the plant according to the present invention advantageously includes a means (represented in Figure 1 by the heat exchanger 49) for cooling a flow comprising ammonia and vapor dioxide carbon dioxide from a carbamate decomposition section (reference numerals 6 and 23) P1541 / 98MX and a medium (ducts 30 and 34) to feed the flow cooled in this way to the urea synthesis reactor 5. Preferably, a water flow is used as the cooling fluid, to produce steam recovered at a high thermal level, for example, 5 bar a). The plant of Figure 1 can be a new plant or it can be done by modernizing an existing plant for the production of ammonia and an existing plant for the production of urea. According to the present invention, a method for the simultaneous modernization of an ammonia synthesis plant and a urea synthesis plant comprising respectively an ammonia synthesis reactor (2), a urea synthesis reactor (5) and a urea recovery section (21), which advantageously comprises the steps of providing, respectively, a carbamate synthesis section (3) and a carbamate decomposition section (23), a means for feeding at least part of a flow comprising carbamate in aqueous solution from the section (21) of recovery of urea to the decomposition section, a means (40, 34) to feed a flow comprising ammonia and vapor dioxide carbon obtained in the decomposition section to the urea synthesis reactor (5), P1541 / 98MX respective means (9b, 7 and 8) for feeding a flow comprising carbamate diluted in aqueous solution obtained in the decomposition section, a flow comprising hydrogen, nitrogen and carbon dioxide, which preferably comes from a reforming section with hydrocarbon vapor and a flow comprising ammonia from the ammonia synthesis reactor (2) to the carbamate synthesis section (3), a means (15) for feeding a flow comprising carbamate in aqueous solution obtained in the section (3) synthesis of carbamate to the reactor (5) of synthesis of urea and a means (16) to feed a gas flow comprising hydrogen and nitrogen obtained in the section (3) of synthesis of carbamate to the reactor (2) ) of ammonia synthesis. In addition, in accordance with additional embodiments of the modernization method according to the present invention, additional sections or means are advantageously provided, as described in the dependent claims 18 to 22 appended to the present invention. In the present description and in the subsequent claims, with the term "a means for feeding", it is generally intended to refer to the various parts of a plant, such as for example ducts, pumps and compressors, which serve to transport a fluid liquid or gaseous from a part of P1541 / 98MX the plant to another. From the preceding description clearly emerge the numerous advantages achieved by the present invention; in particular, a process is obtained for the combined production of ammonia and urea at high yield, of simple implementation, with low investment and operation costs and with low energy consumption.

P1541 / 98MX

Claims (23)

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. Process for the combined production of ammonia and urea in a plant of the type comprising an ammonia synthesis reactor, a urea synthesis reactor and a urea recovery section, the process comprises the steps of: subjecting at least part of a flow comprising carbamate in aqueous solution, coming from the recovery section of urea, to a partial decomposition treatment, to obtain a flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising carbamate diluted in aqueous solution; feeding the flow comprising ammonia and carbon dioxide in vapor phase to the urea synthesis reactor; feeding the flow comprising carbamate diluted in aqueous solution resulting from the treatment step, a gas flow comprising hydrogen, nitrogen and carbon dioxide, preferably obtained by reforming with hydrocarbons and a flow comprising ammonia from the reactor of P1S41 / 98 X synthesis of ammonia towards a carbamate synthesis section; reacting the ammonia with the carbon dioxide in the carbamate synthesis section to obtain a flow comprising carbamate in aqueous solution and a gas flow comprising hydrogen and nitrogen; feeding the flow comprising carbamate in aqueous solution to the urea synthesis reactor; feeding the gas flow comprising hydrogen and nitrogen to the ammonia synthesis reactor.
2. 2. The process according to claim 1, characterized in that the flow coming from the ammonia synthesis reactor comprises vapor phase ammonia. The process according to claim 1, characterized in that it further comprises the preliminary step of cooling the gas flow comprising hydrogen, nitrogen and carbon dioxide by indirect heat exchange by a flow comprising urea in aqueous solution in the recovery section of urea. The process according to claim 1, characterized in that it also comprises the steps of: - subjecting at least part of the flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to a partial decomposition treatment, to obtain a flow which comprises P1541 / 98MX ammonia and vapor dioxide carbon dioxide and a flow comprising carbamate diluted in aqueous solution; feeding the flow comprising ammonia and carbon dioxide in vapor phase to the urea synthesis reactor; feeding the flow comprising carbamate diluted in aqueous solution resulting from the treatment step to the carbamate synthesis section. The process according to claim 1, characterized in that it further comprises the steps of: preheating a flow comprising recycled ammonia from the urea synthesis section; Y - feeding the preheated flow comprising ammonia to the urea synthesis reactor. The process according to claim 1 or 4, characterized in that it also comprises the steps of: - cooling a flow comprising ammonia and carbon dioxide in vapor phase resulting from the treatment of partial decomposition of carbamate; - feeding the flow cooled in this way to the urea synthesis reactor. The process according to claim 1, characterized in that it further comprises the step of feeding the flow comprising water coming from the urea concentration section to the synthesis section of P1S41 / 98MX carbamate. Plant for the combined production of ammonia and urea comprising: an ammonia synthesis reactor, a urea synthesis section, a urea synthesis reactor, a urea recovery section and a carbamate decomposition section; - means for feeding at least part of a flow comprising carbamate in aqueous solution from the recovery section of urea to the decomposition section; means for feeding a flow comprising ammonia and vapor dioxide carbon dioxide obtained in the decomposition section to the urea synthesis reactor; - a respective means for feeding a stream comprising carbamate diluted in aqueous solution obtained in the decomposition section, a gas stream comprising hydrogen, nitrogen and carbon dioxide, preferably coming from a reforming section with hydrocarbon vapor, and a flow comprising ammonia from the ammonia synthesis reactor to the carbamate synthesis section; means for feeding a flow comprising carbamate in aqueous solution obtained in the P1541 / 98MX carbamate synthesis section _ to the urea synthesis reactor; means for feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section to the ammonia synthesis reactor. Plant according to claim 8, characterized in that it further comprises a means for feeding a flow comprising water to the carbamate synthesis section, in fluid communication with a urea concentration section. Plant according to claim 8, characterized in that the carbamate synthesis section comprises: - a first chamber in fluid communication with the medium for feeding a gas flow comprising hydrogen, nitrogen and carbon dioxide towards the synthesis section of carbamate and with the medium for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the urea synthesis reactor, respectively; - a second chamber in fluid communication with the medium for feeding a flow comprising ammonia from the ammonia synthesis reactor to the carbamate synthesis section; P1541 / 98MX - a third chamber in fluid communication with the medium for feeding a flow comprising carbamate diluted in aqueous solution to the synthesis section of carbamate and with the medium for feeding a gas flow comprising hydrogen and nitrogen obtained in the section of synthesis of carbamate to the ammonia synthesis reactor, respectively; a first film absorber located between the first and second chamber and comprising a plurality of tubes having opposite ends in fluid communication with the first and second chamber respectively; a second film absorber located between the second and third chamber and comprising a plurality of tubes having opposite ends in fluid communication with the second and third chamber, respectively. Plant according to claim 10, characterized in that the third chamber is in fluid communication with a means for feeding a flow comprising water coming from a section of urea concentration. 12. Plant according to claim 10, characterized in that the chambers and the absorbers are comprised in a single, substantially vertical tubular device, the first chamber and the third chamber. P1541 / 98MX camera are located in a lower part with respect to an upper end of the device. Plant according to claim 8, characterized in that the means for feeding the flow comprising ammonia connects directly to the ammonia synthesis reactor with the carbamate synthesis section. Plant according to claim 8, characterized in that it further comprises a means for cooling the flow comprising hydrogen, nitrogen and carbon dioxide by indirect heat exchange with a flow comprising urea in aqueous solution in the urea recovery section. Plant according to claim 8, characterized in that it further comprises: means for feeding at least part of the flow comprising carbamate in aqueous solution from the synthesis section of carbamate to the decomposition section of carbamate. Plant according to claim 8, characterized in that it further comprises: means for preheating a flow comprising recycled ammonia from a urea recovery section; and - a means for feeding the heated flow P1541 / 98MX comprising ammonia to the urea synthesis reactor. Plant according to claim 8 or 15, characterized in that it further comprises: - a means for cooling the flow comprising ammonia and carbon dioxide in vapor phase coming from the decomposition section of carbamate; means for feeding the cooled flow to the urea synthesis reactor. 18. Method for the simultaneous modernization of a plant for the synthesis of ammonia and a plant for the synthesis of urea, comprising respectively an ammonia synthesis reactor and a urea synthesis reactor and a urea recovery section, characterized in that comprises the steps of: - providing a carbamate synthesis section and a carbamate decomposition section; - providing a means for feeding at least part of a flow comprising carbamate in aqueous solution from the recovery section of urea to the decomposition section; providing a means for feeding a flow comprising ammonia and vapor dioxide carbon dioxide obtained in the decomposition section to the urea synthesis reactor; - provide a respective means for P1541 / 98MX feed a flow comprising carbamate diluted in aqueous solution obtained in the decomposition section, a flow of gas comprising hydrogen, nitrogen and carbon dioxide, coming preferentially from a hydrocarbon vapor preforming section, and a flow that comprises ammonia from the ammonia synthesis reactor to the carbamate synthesis section; providing a means for feeding a flow comprising carbamate in aqueous solution obtained in the carbamate synthesis section to the urea synthesis reactor, providing a means for feeding a gas flow comprising hydrogen and nitrogen obtained in the synthesis section of carbamate to the ammonia synthesis reactor. Method of medernization according to claim 8, characterized in that the carbamate synthesis section comprises: - a first chamber in fluid communication with the means for feeding a gas flow comprising hydrogen, nitrogen and carbon dioxide towards the section of synthesis of carbamate, and with a medium for feeding a flow comprising carbamate in aqueous solution obtained in the synthesis section of carbamate P1541 / 98MX to the urea synthesis reactor, respectively; - a second chamber in fluid communication with the medium for feeding a flow comprising ammonia from the ammonia synthesis reactor to the carbamate synthesis section; a third chamber in fluid communication with the medium for feeding a flow comprising carbamate diluted in aqueous solution to the carbamate synthesis section and with a means for feeding a gas flow comprising hydrogen and nitrogen obtained in the carbamate synthesis section to the ammonia synthesis reactor, respectively; a first film absorber located between the first and second chamber and comprising a plurality of tubes having opposite ends in fluid communication with the first and second chamber respectively; a second film absorber located between the second and third chamber and comprising a plurality of tubes having opposite ends in fluid communication with the second and third chamber, respectively. The method of modernization according to claim 18, characterized in that it also comprises the step of providing a means for cooling to the flow that P1541 / 98MX comprises hydrogen, nitrogen and carbon dioxide by indirect heat exchange with a flow comprising urea in aqueous solution in the urea recovery section. 21. Method of modernization according to claim 18, characterized in that it further comprises the step of: providing a means for feeding at least part of the flow comprising carbamate in aqueous solution from the carbamate synthesis section to the decomposition section of carbamate . The method of modernization according to claim 18, characterized in that it further comprises the steps of: - providing a means for preheating a flow comprising recycled ammonia from a urea recovery section; and - providing a means for feeding the heated flow comprising ammonia to the urea synthesis reactor. Method of modernization according to claim 18, characterized in that it further comprises the steps of: providing a means for cooling the flow comprising ammonia and carbon dioxide in vapor phase P1541 / 98MX from the carbamate decomposition section, provide a means to feed the cooled flow to the urea synthesis reactor. P1541 / 98MX