MXPA97005053A - Process and installation for the production of ureacon high performance of conversion and low consumode ener - Google Patents

Abstract

The present invention relates to a process for the production of urea of the type comprising the steps of: carrying out a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture containing urea, carbamate and free ammonia in aqueous solution, subjected to a treatment of partial decomposition of the carbamate and partial separation of said free ammonia in aqueous solution to obtain a first flow containing ammonia and carbon dioxide in the vapor phase and a flow containing urea and residual carbamate in solution aqueous, subject said first flow containing ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqueous solution, recycle said first portion of carbamate to said reaction space, feed said flow containing urea and residual carbamate in aqueous solution to a urea recovery section; The urea in said recovery section said residual carbamate to obtain a second portion of carbamate in aqueous solution, characterized in that it comprises the additional steps of: subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a partial decomposition treatment to obtain a second flow containing ammonia and carbon dioxide in vapor phase and a flow containing residual carbamate in aqueous solution, subjecting said second flow containing ammonia and carbon dioxide in vapor phase to at least a partial condensation to obtain a third portion of carbamate in aqueous solution; recycle said third portion of carbamate to said reaction space;

Description

PROCESS AND INSTALLATION FOR THE PRODUCTION OF UREA WITH HIGH PERFORMANCE OF CONVERSION AND LOW ENERGY CONSUMPTION Technical Field In its general aspect the present invention relates to a process for the production of urea. The present invention relates specifically to a process for producing urea of the type containing the steps of: performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture containing urea, carbamate and free ammonia in aqueous solution; subjecting said mixture to a treatment of partial decomposition of the carbamate and partial separation of said free ammonia in aqueous solution to obtain a first flow containing ammonia and carbon dioxide in vapor phase and a flow containing residual urea and carbamate in aqueous solution; subjecting said first flow containing ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of the carbamate in aqueous solution; recycling said first carbamate portion in said reaction space; feeding said flow containing urea and residual carbamate in aqueous solution to a urea recovery section; separating said residual carbamate from the urea in said recovery section to obtain a second portion of carbamate in aqueous solution. The present invention also relates to an installation for carrying out the aforementioned process and to a method for modernizing an existing urea installation to obtain an installation in accordance with the present invention. As is known, in the field of urea production there is always a greater growth in the need for facilities that have greater capacity and flexibility of operation on the one hand, and on the other hand, that always require lower investments and operating costs, in particular in terms of energy. BACKGROUND OF THE INVENTION To this end, a series of urea production processes essentially based on the performance of a conversion reaction in a reaction space fed with ammonia (NH3) and carbon dioxide have been proposed and implemented in the art. C02) and in which the unreacted substances contained in the urea solution leaving the reaction space, in particular ammonia, carbon dioxide and carbamate in aqueous solution, are recycled. A process of this type is shown in FIG. 1, and comprises downstream, a reaction space, a carbamate decomposition unit and a urea recovery section to separate the substances that did not react to be recycled from the urea solution. If, on the one hand, this recycling allows almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand it also involves sending large amounts of water (H20) to the reactor, which are mostly detrimental. of the conversion efficiency of carbon dioxide to urea, the yield in general being between 59% and 63% SUMMARY OF THE INVENTION. The fundamental technical problem of the present invention is therefore to conceive and make available a process for the production of urea that achieves a high conversion efficiency that can be technically simple to be implemented and can comprise low operating and investment costs. According to the present invention, this problem is solved by a process of the above-mentioned type which is characterized in that it comprises the additional steps of: subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a partial decomposition treatment to obtain a second flow containing ammonia and carbon dioxide in vapor phase and a flow containing residual carbamate in aqueous solution; subjecting said second flow containing ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution; recycling said third portion of carbamate to said reaction space. According to this invention, at least part of the carbamate in aqueous solution leaving the urea recovery section is advantageously subjected to a partial decomposition treatment that separates the unreacted ammonia and carbon dioxide from a water-rich solution. containing residual carbamate. By doing so, the non-reacted substances that are recycled in the reaction space have a very low water content, and thus, it is possible to substantially limit the water supply to the reaction space, allowing a high conversion efficiency. In order to obtain a high degree of decomposition of at least part of said second carbamate portion in aqueous solution, it is preferably subjected to a decomposition treatment at a pressure substantially corresponding to the pressure in the reaction space. To improve and assist the condensation and separation steps of the substances that did not react in the urea recovery section, the residual carbamate-containing flow in aqueous solution resulting from the partial decomposition treatment of the second carbamate portion is advantageously fed to said recovery section of urea. According to another embodiment of the present invention, the process comprises the steps of: feeding the reaction mixture containing urea, carbamate and free ammonia in aqueous solution to a decomposition unit; feeding at least part of said second portion of carbamate in aqueous solution to said decomposition unit, wherein the treatment of the partial decomposition of the reaction mixture and the second carbamate portion is advantageously carried out in the same unit of decomposition to obtain said first and second streams containing ammonia and carbon dioxide in vapor phase and a flow containing urea and residual carbamate in aqueous solution According to this embodiment, the implementation of the urea production process is very simple technically that no relevant additional equipment is required compared to prior art processes, and this involves low investment costs. It is particularly satisfactory to have obtained by subjecting at least 50% and preferably at least 65% of said second carbamate portion in aqueous solution to partial decomposition treatment. According to another aspect of the present invention, the technical problem set forth above is solved by designing an installation for implementing the aforementioned urea production process comprising: a urea synthesis reactor; a first separation unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and a partial separation of the free ammonia in aqueous solution present in said mixture; means for condensing at least in part the vapors leaving said first separation unit and for recycling a first portion of carbamate in aqueous solution in said reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said first separation unit for the separation of urea produced in the reactor from a second portion of carbamate in aqueous solution; whose installation is characterized in that it comprises: a second separation unit for subjecting at least part of said second carbamate portion in aqueous solution to a partial decomposition treatment; means for at least partially condensing the vapors leaving said second separation unit and for recycling a third portion of carbamate in aqueous solution in said reactor. According to still another embodiment of the invention, the plant for the production of urea comprises: a urea synthesis reactor; a separation unit for subjecting a reaction mixture leaving said first reactor to a treatment partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said unit and for recycling a first portion of carbamate in aqueous solution for said first reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution whose installation is characterized in that it comprises: means for feeding at least part of said second carbamate portion in aqueous solution to the separation unit. In accordance with the present invention, the facilities designated to carry out the urea production process can be provided, either new or for modified pre-existing facilities, in order to obtain an expansion of production capacity and at the same time improve the performance from the point of view of energy consumption. According to another aspect, the present invention therefore makes available a method for modernizing a urea production facility of the type comprising: a urea synthesis reactor; a first separation unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said first separation unit and for recycling a first portion of the carbamate in aqueous solution in said reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said first separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution; which method is characterized in that it comprises the steps of: providing a second separation unit for subjecting at least part of said second portion of the carbamate in aqueous solution to a partial decomposition treatment; providing means for at least partially condensing the vapors leaving said second separation unit and for recycling a third portion of carbamate in aqueous solution in said reactor. In an alternative embodiment, the method of modernization of the present invention comprises the steps of: providing a second separation unit for subjecting at least part of said second carbamate portion in aqueous solution to a partial decomposition treatment; providing means for feeding the vapors leaving said second separation unit to said means for condensing vapors left by said first separation unit. According to a further embodiment, the modernization method of the present invention comprises the step of: providing means for feeding at least part of said second carbamate portion in aqueous solution to the separation unit. Additional features and advantages of the present invention are set forth in the detailed description of some preferred embodiments thereof given below by way of non-limiting example with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 shows a block diagram of a urea production process according to the prior art; Figure 2 shows a block diagram of a first embodiment of the urea production process according to the present invention; and Figure 3 shows a block diagram of a second embodiment of the urea production process according to the present invention. DETAILED DESCRIPTION OF A PREFERRED MODE Figure 1 shows a block diagram illustrating the steps of a urea production process according to the prior art. Block 1 indicates a high pressure reaction space for the synthesis of urea which is fed by gas flows 21 and 22 which contain substantially pure ammonia and carbon dioxide, respectively. Typical operating conditions in the reaction space are: • NH3 / C02 molar ratio at the inlet: 2.9 to 3.4; • H20 / C02 molar ratio at entry: 0.4 to 0.7; • conversion performance from C02 to urea: 59% to 63%; • pressure: 150 bar a; • Temperature: 185 ° C to 190 ° C. Blocks 2, 5 and 6 respectively indicate a high pressure decomposition unit, a urea granulation or granulation section and a high pressure condensing unit. The decomposition and condensation units 2 and 6, generally operate at the same pressure conditions as in the reaction space 1. A urea recovery section is generally indicated by blocks 3, 4, 7 and 8. In particular, blocks 3 and 4 they indicate a separation or distillation unit and blocks 7 and 8 indicate a condensing unit. Block 4 also indicates a urea finishing unit, in which a solution with a urea content greater than 99.7% is obtained. Block 8 also indicates a wastewater treatment unit for the purification of the water to be discharged from the urea production processes. Blocks 3 and 7 typically operate at medium pressure (around 18 bars), while blocks 4 and 7 8 operate at low pressure (around 4 bars). The flow line 23 represents a liquid flow of a reaction mixture coming from block 1 and containing urea and substances that did not react, especially carbamate and free ammonia in aqueous solution. The flow of liquid 23 is fed to block 2, in which it is subjected to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia. The decomposition unit indicated by the block 2 generally comprises a separating apparatus, which operates with a flow of carbon dioxide as a separating agent from the flow line 22. At the output of block 2, the lines of flow 24 and 25 which respectively represent a flow of gas containing ammonia and carbon dioxide in the vapor phase and a liquid fluid containing urea and residual carbamate in aqueous solution. The flow line 24 passes through the condensation unit represented by the block 6, in which the ammonia and the carbon dioxide in the vapor phase are condensed, obtaining a flow of carbamate in aqueous solution which is recycled to the reaction space 1. The flow containing urea and residual carbamate in aqueous solution indicated by flow line 25 passes through the distillation units of the urea recovery section indicated by blocks 3 and 4, in which the residual carbamate decomposes and is separated from it. the urea solution. In general, the urea contained in the liquid stream 25 is between 70 and 72% after block 3 and about 99% after block 4.

The flow lines 26 and 27 represent a gas flow containing gaseous ammonia and carbon dioxide obtained in blocks 3 and 4 respectively. The flow 27 passes through the condensation unit represented by the block 8, in which the ammonia and the carbon dioxide in the vapor phase are condensed obtaining a flow of carbamate in aqueous solution, and are fed to the condensing unit 7, in where it promotes the condensation of the gas flow 26. Analogously, the flow 26 passes through the condensation unit represented by the block 7, in which the ammonia and the carbon dioxide in the vapor phase are condensed obtaining a flow of carbamate in aqueous solution, and it is fed to the condensation unit 6, wherein it promotes the condensation of the gas flow 24. Part of the water contained in the aqueous solution obtained in the condensation unit of block 8 is treated and further purified from almost all indications of ammonia and urea in the treatment unit also indicated by middle of block 8. From block 8 there is a flow line 28 of a wastewater flow to be discharged from the urea production process. Finally, the flow of the urea solution coming from block 4 goes through the granulation or granulation section indicated by block 5, in which it is transformed to a final product that leaves the urea production process through the flow line 29. According to an alternative embodiment of the prior art process, block 7 also indicates an ammonia separation column to obtain a substantially pure liquid ammonia which is sent to reaction space 1 in addition to flow 21, as indicated by the flow line 31 in Figure 1. As shown in Figure 1, in the urea production process according to the prior art, all the carbamate contained in the separated aqueous solution from the urea is recycled to the reaction space 1, with the large amount of water contained in it, being used for the condensation and transportation of substances that did not react. In figures 2 and 3, a block diagram of a first and second modalities respectively of the urea production process according to the present invention is indicated. In said figures the details of the block diagram structurally and functionally equivalent to those shown in Figure 1 are indicated by the same reference numbers and are not described later. In Figure 2, block 9 indicates a high pressure decomposition unit operating at the same pressure conditions as in reaction space 1. Liquid flow 26 containing carbamate in aqueous solution and having a high water content is it feeds the block 9, in which it is advantageously subjected to a partial decomposition treatment of the carbamate. At the outlet of block 9, the flow lines 32 and 33 are shown, which respectively represent a flow of gas containing ammonia and carbon dioxide in the vapor phase and a liquid flow containing urea and residual carbamate in aqueous solution. The flow of gas 32, which is very rich in ammonia and carbon dioxide and poor in water (only a few percentage points), passes through the condensation unit represented by block 6, in which the ammonia and carbon dioxide condensate obtaining a carbamate flow in aqueous solution, and it is recycled to the reaction space 1 through the flow line 24. In the example of figure 2, all the carbamate in aqueous solution separated from the urea in the recovery section it is subjected to the decomposition treatment in block 9. However, it is satisfactory to have also obtained by feeding to block 9 only a part of the carbamate leaving the urea recovery section. Preferably, at least 50% of this carbamate can be sent to block 9.

According to the process of the present invention, a reaction between ammonia and carbon dioxide is carried out in the reaction space 1 obtaining a reaction mixture containing urea, carbamate and free ammonia in aqueous solution, which is subjected to the decomposition unit 2 to a treatment of partial decomposition of the carbamate and to the partial separation of said free ammonia in aqueous solution. Coming from the decomposition unit 2, a first flow 24 containing ammonia and carbon dioxide in vapor phase and a flow containing urea and residual carbamate in aqueous solution. The flow 24 is consequently subjected to at least partial condensation in the block 6 to obtain a first portion of carbamate in aqueous solution which is recycled to the reaction space 1. The flow 25 is in feed contrary to a section of recovery of urea (blocks 3, 4, 7 and 8) in which the urea is separated from a second portion of carbamate in aqueous solution indicated by the flow line 26. Advantageously, according to other steps of the process of the In the present invention, at least part of the flow 26 is further subjected to the partial decomposition treatment in block 9 to obtain a second flow 32 containing ammonia and carbon dioxide in the vapor phase and a flow 33 containing residual carbamate in aqueous solution. Accordingly, the stream 32 is at least partially condensed in block 6 to obtain a third portion of carbamate in aqueous solution recycled to the reaction space via the flow line 24. By operating in this manner it is possible to obtain a high yield of conversion in the reaction space because a highly concentrated carbamate solution which is very poor in water is recycled thereto. According to the present urea production process, it is possible to achieve a yield of conversion of carbon dioxide to urea from approximately 70% to 75%, which is above all much greater than that which can be obtained with the processes of the prior art. In addition, this high conversion efficiency and the substantial absence of water to be recycled into the reaction space 1 also results in a smaller flow of substances to be separated from the urea solution, and thus results in an increase in the performance of the decomposition unit 2 and the distillation units 3 and 4 of the recovery section. In the example of figure 2, the liquid flow 33, which is very rich in water, is advantageously recycled to the recovery section of urea in order to promote condensation and recover the non-reactive substances which are released in the distillation units 3 and 4. Preferably, the flow line 33 passes through a distillation unit indicated by the block 10, in which the residual carbamate is further subjected to decomposition in order to obtain a solution very rich in water. which is fed to the block 8. From the block 10 also flows a flow line 34 of a water-poor vapor stream comprising residual ammonia and carbon dioxide which is fed to the condensing unit indicated by the block 7 of the recovery section. Thus, a circulation circuit separated from the process water is obtained which advantageously promotes the condensation of the ammonia and carbon dioxide vapors in the units 7 and 8 without being recycled to the reaction space 1 and without adversely affecting the reaction between ammonia and carbon dioxide. In the alternative embodiment of the process according to the present invention described in Figure 3, the flow line 26 from the urea recovery section, called the condensation unit 7, instead of being recycled directly to the reaction space. through block 6 as in the prior art process, it is advantageously fed to the decomposition unit indicated by block 2, in which a flow of ammonia and carbon dioxide is obtained in the vapor phase, which is recycled via the flow line 24 to the reaction space 1 after the condensation in block 6. By doing so, the treatment of the partial decomposition of the second portion of the carbamate in aqueous solution (flow 26) is carried out in the same decomposition unit of the reaction mixture (flow 23), thereby enabling an implementation of the process according to the present invention which does not require the use of relevant additional equipment compared to the prior art. As the separation agents for the decomposition unit 2 it can also use a part of the flow of ammonia fed into the reaction space. Alternatively, block 2 can be operated in a self-separation mode, in which the evaporated ammonia promotes the decomposition of the carbamate. In addition, the urea recovery section may comprise only the low pressure units 4 and 8. In this case, the stream containing urea and residual carbamate in aqueous solution coming from the decomposition unit 2 is fed directly to block 4 for the final separation of the urea solution from the substances that did not react. Reference is now made to an installation for the production of urea specifically designed to carry out the process according to the present invention. The urea production plant advantageously comprises a urea synthesis reactor indicated by block 1, first and second separation units indicated by blocks 2 and 9 respectively, a recovery section of urea indicated by blocks 3, 4, 7 and 8, and respective means for condensing and recycling the vapors leaving the first and second separation units to the reactor. With reference to the embodiment of Figure 2, the means for condensing the vapors leaving the second separation unit 9 preferably comprises the means for condensing the vapors leaving the first separation unit 2 and indicated by the block 6. A The distillation unit indicated by the block 10 is also placed between the second separation unit and the recovery section. With reference to the embodiment of Figure 3, the installation for the production of urea comprises feeding means indicated by the flow line 26 between the recovery section and the separation unit 2. In this case, the separation unit 9 and the distillation unit 10 are not needed. The installation designed to implement the process for the production of urea in accordance with the present invention can be an entirely new installation or an installation obtained by modernizing a pre-existing installation such as the installation resulting from the implementation of the process illustrated in the block diagram. of Figure 1. According to a first embodiment, this modernization takes place by means of the steps of: providing a second separation unit (block 9) to subject at least part of the carbamate in aqueous solution leaving the recovery section ( flow line 26) to a partial decomposition treatment; providing means for at least partially condensing the vapors leaving said second separation unit and for recycling the highly concentrated carbamate solution thus obtained to the reactor (block 1). According to another embodiment of the invention, the method of modernization preferably comprises the step of: providing means for feeding the vapors leaving the second separation unit (block 9) directly to the means for condensing the vapors leaving the first unit of separation 2, represented by block 6. Advantageously, the method for the modernization of a pre-existing installation further comprises the step of: providing means for feeding (flow line 33) a flow containing residual carbamate in aqueous solution originating from the second separation unit (block 9) to the recovery section of urea. In a particular and advantageous embodiment of the present invention, the method of modernization comprises the step of: providing means for feeding (flow lines 26) at least part of the carbamate solution from the urea recovery section to the unit separation indicated by block 2. Thanks to the method of modernization of the present invention, not only can the conversion performance of the preexisting urea synthesis reactor be increased drastically but also its capacity. In fact, because only a very small amount of water is recycled to reactor 1, a larger flow of ammonia and carbon dioxide can be fed thereto without causing an overload of capacity in the reactor itself as well as in the reactor unit. decomposition 2 and in distillation units 3 and 4 of the recovery section. In the following examples, only an example of the conversion performances that can be obtained by means of an installation that implements the process according to the present invention or modernized by the method of the present invention and by means of an example are compared only by way of indication and not by way of limitation. an installation that implements the process according to the prior art. EXAMPLE 1 A pre-existing installation operating in accordance with the prior art process, described with reference to Figure 1, is modernized in order to be operated in accordance with the process described with reference to Figure 2. The pre-existing installation is based on the aforementioned process of self-separation of ammonia, in which no ammonia or carbon dioxide separation agent is fed to the decomposition unit indicated by bLoque 2. Consequently, in this case the flow line 30 is misses. The operation conditions of the urea synthesis reactor before the modernization of the installation are the following: • NH3 / C02 molar ratio at the inlet: 3.2; • H20 / C02 molar ratio at entry: 0.6; • conversion performance from C02 to urea: 61%; • pressure: approximately 150 bar a; • temperature: 190 ° C. • capacity: 1,800 MTD of urea After modernizing the pre-existing installation by providing a second separation unit 9 fed with 77% of the carbamate solution from the urea recovery section, and by feeding the vapors leaving said urea second separation unit towards the reaction space 1 through the condenser unit 6, as described with reference to figure 2, the new operating conditions of the reactor are as follows: • molar proportion NH3 / C02 at the inlet : 3.2; • H20 / C02 molar ratio at the inlet: 0.19; • conversion efficiency from C02 to urea: 70%; • pressure: approximately 150 bar a; • temperature: 190 ° C; • capacity: 2500 MTD of urea Thanks to the present invention, it is possible to increase the conversion efficiency of 9 percentage points and increase the capacity of 700 MTD of urea, that is, 39% more than the original capacity. Such a relevant increase in the conversion performance and the much smaller amount of water recycled to the reactor allows the new increased capacity to be obtained with only minimal modifications to the pre-existing installation and with low investment costs. further, this high conversion efficiency also results in a reduction in the energy consumption of the modernized installation. EXAMPLE 2 A pre-existing installation operating according to the prior art process described with reference to Figure 1 is modernized in order to operate according to the process described with reference to Figure 2. The pre-existing installation is based on the process of carbon dioxide separation, wherein a flow of carbon dioxide as the separating agent is fed to the decomposition unit indicated by block 2 (flow line 30). In this case, the ammonia separation unit comprises a block 7 and the flow line 31 is lost. Also units 3 and 7 are lost and the urea recovery section only includes the low pressure units 4 and 8. The operating conditions of the urea synthesis reactor before the modernization of the installation are the following: • molar ratio NH3 / C02 at the entrance: 3.0; • H20 / C02 molar ratio at entry: 0.5; • conversion performance from C02 to urea: around 60%; • pressure: approximately 145 bar a; • temperature: 185 ° C. • capacity: 1900 BAT of urea After the modernization of the pre-existing installation by providing a second separation unit 9 fed with 70% of the carbamate solution that comes from the recovery section of urea, and by feeding the vapors that leave said second separation unit towards the space of the reactor 1 through the unit of the condenser 6, as described with reference to figure 2, the new operating conditions of the reactor are the following: • molar proportion NH3 / C02 at the inlet: 3.0; • H20 / C02 molar ratio at the inlet: 0.25; • conversion performance from C02 to urea: 66%; • pressure: approximately 150 bar a; • temperature: 190 ° C. • capacity: 2500 MTD of urea Thanks to the present invention, it is possible to increase the conversion efficiency by 6 percentage points and increase the capacity of 600 MTD of urea, that is, 32% more than the original capacity. Also in this case, only minor modifications to the pre-existing installation and low investment costs are required to obtain a significant increase in capacity and conversion performance.

EXAMPLE 3 In this example, the conversion performance of a reactor operating in an entirely new installation implementing the process according to the present invention has been simulated as described in Figure 2. As in Example 2 above, the installation was based on the carbon dioxide separation process, in which all the carbon dioxide to be fed to the reactor 1 is first fed as a separation agent in the decomposition unit indicated by the block 2 through the flow line 30 In this case, the flow line 22 is lost as well as the ammonia separation unit comprised in block 7 and flow line 31. The urea recovery section comprises the medium and low pressure units 3, 4, 7 and 8. The operating conditions of the reactor in the urea production plant according to the present invention are the following: • molar ratio NH3 / C02 at the inlet: 3.2; • H20 / C02 molar ratio at the inlet: 0.1; • Pressure: around 150 bar a; • temperature: 190 ° C; • capacity: 400 MTD of urea. The conversion efficiency of C02 to urea obtained by the present reactor is very high: 72%. In addition, the water content in the recycled carbamate solution in the reactor is particularly low. Because low amounts of water and unreacted substances are contained in the reaction mixture sent to the decomposition unit and in addition to the recovery section, the result is that the costs of the processing equipment are low compared to the costs of processing. conventional facilities and consequently also energy consumption and investment costs. The results given in the previous examples have been obtained by means of well-known calculation algorithms.

Claims (14)

1. NOVELTY OF THE INVENTION Having described the present invention is considered as a novelty and therefore the content of the following claims is claimed as property. A process for the production of urea of the type comprising the steps of: performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture containing urea, carbamate and free ammonia in aqueous solution; subjecting said mixture to a treatment of partial decomposition of the carbamate and partial separation of said free ammonia in aqueous solution to obtain a first flow containing ammonia and carbon dioxide in vapor phase and a flow containing residual urea and carbamate in aqueous solution; subjecting said first flow containing ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqueous solution; recycling said first carbamate portion to said reaction space; feeding said flow containing urea and residual carbamate in aqueous solution to a urea recovery section; separating said residual carbamate from the urea in said recovery section to obtain a second portion of carbamate in aqueous solution; characterized in that it comprises the additional steps of: subjecting at least part of said second carbamate portion in aqueous solution obtained in said recovery section to a partial decomposition treatment to obtain a second flow containing ammonia and carbon dioxide in vapor phase and a stream containing residual carbamate in aqueous solution; subjecting said second flow containing ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution; recycling said third portion of carbamate to said reaction space.
2. 2. A process according to claim 1, characterized in that the treatment of partial decomposition of said at least part of the second carbamate portion in aqueous solution is carried out at a pressure substantially corresponding to the pressure in the reaction space.
3. 3. A process according to claim 1, characterized in that it further comprises the step of: feeding the residual carbamate-containing stream in aqueous solution resulting from the treatment of the partial decomposition of the second carbamate portion to said urea recovery section.
4. A process according to claim 1, characterized in that it comprises the steps of: feeding the reaction mixture containing urea, carbamate and free ammonia in aqueous solution to a decomposition unit; feeding at least said portion of the second portion of the carbamate in aqueous solution to said decomposition unit, wherein the treatment of the partial decomposition of the reaction mixture and the second portion of the carbamate is carried out in the same decomposition unit to obtain said first and second flows containing ammonia and carbon dioxide in vapor phase and a flow containing residual urea and carbamate in aqueous solution.
5. 5. A process according to claim 1, characterized in that at least 50% of said second portion of the carbamate in aqueous solution is subjected to a partial decomposition treatment.
6. 6. A process according to claim 5, characterized in that at least 65% of said second carbamate portion in aqueous solution is subjected to the partial decomposition treatment.
7. 7. An installation for the production of urea comprising: a urea synthesis reactor; a first separation unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said first separation unit and for recycling a first portion of carbamate in aqueous solution towards said reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said first separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution; characterized in that it comprises: a second separation unit for subjecting at least part of said second carbamate portion in aqueous solution to a partial decomposition treatment; means for at least partially condensing the vapors left by said second separation unit and for recycling a third portion of carbamate in aqueous solution to said reactor.
8. An installation according to claim 7, characterized in that said means for condensing the vapors leaving said second separation unit comprise said means for condensing the vapors that said first separation unit leaves.
9. 9. An installation according to claim 7, characterized in that it further comprises: means for fng a stream containing residual carbamate in aqueous solution from said second separation unit to said recovery section.
10. 10. An installation for the production of urea comprising: a urea synthesis reactor; a separation unit for subjecting a reaction mixture leaving said first reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said separation unit and for recycling a first portion of the carbamate in aqueous solution to said first reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution; characterized in that it comprises: means for feeding at least part of said second carbamate portion in aqueous solution to the separation unit.
11. 11. A method for modernizing an installation for the production of urea of the type comprising: a urea synthesis reactor; a first separation unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said first separation unit and for recycling a first portion of the carbamate in aqueous solution to said reactor; an un-flow recovery section containing residual urea and carbamate in aqueous solution leaving said first separation unit, for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution; characterized in that it comprises the steps of: providing a second separation unit for subjecting at least part of said second carbamate portion in aqueous solution to a partial decomposition treatment; providing means for at least partially condensing the vapors leaving said second separation unit and for recycling a third portion of carbamate in aqueous solution to said reactor.
12. 12. A method for modernizing an installation for the production of urea of the type comprising: a urea synthesis reactor; a first separation unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors leaving said first separation unit and for recycling a first portion of carbamate in aqueous solution to said reactor; a recovery section of a flow containing urea and residual carbamate in aqueous solution leaving said first separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution; characterized in that it comprises the steps of: providing a second separation unit for subjecting at least part of said second carbamate portion in aqueous solution to a partial decomposition treatment; providing means for feeding the vapors leaving said second separation unit to said means for condensing the vapors leaving said first separation unit.
13. 13. A method according to claim 11 or 12, characterized in that it further comprises the step of: providing means for feeding a stream containing residual carbamate in aqueous solution from said second separation unit to said recovery section.
14. 14 A method for modernizing an installation for the production of urea of the type comprising: a urea synthesis reactor; a separation unit for subjecting a reaction mixture leaving said first reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for at least partially condensing the vapors left by said separation unit and for recycling a first portion of carbamate in aqueous solution to said first reactor; a section of recovery of a flow comprising urea and residual carbamate in aqueous solution leaving said separation unit to separate the urea produced in the reactor from a second portion of carbamate in aqueous solution; characterized in that it comprises the step of: providing means for feeding at least part of said second carbamate portion in aqueous solution towards the separation unit.