Title of Invention	A PROCESS AND A PLANT FOR UREA PRODUCTION
Abstract	In a process for the production of urea, substantially pure anmonia and carbon dioxide are reacted in a reaction space (1) from which conies out a reaction mixture subjected to stripping (2) to obtain a partially purified mixture sent to a urea recovery section (3,4,7,8). From the recovery section (3,4,7,8) it is obtained a dilute carbomate solution, which is subjected to stripping (9) with recycling of vapours to the reaction space (1) after condensation (6). This process achieves high conversion yield with reduced energy consumption and low implementation costs. PRICE: THIRTY RUPEES

Title of Invention

A PROCESS AND A PLANT FOR UREA PRODUCTION

Abstract

In a process for the production of urea, substantially pure anmonia and carbon dioxide are reacted in a reaction space (1) from which conies out a reaction mixture subjected to stripping (2) to obtain a partially purified mixture sent to a urea recovery section (3,4,7,8). From the recovery section (3,4,7,8) it is obtained a dilute carbomate solution, which is subjected to stripping (9) with recycling of vapours to the reaction space (1) after condensation (6). This process achieves high conversion yield with reduced energy consumption and low implementation costs. PRICE: THIRTY RUPEES

Full Text	Title: "Process and plant for the production of urea with high conversion yield and low energy consumption" DESCRIPTION Field o£ application In general, the present invention relates to a process for urea production. Specifically, the present invention relates to a process for urea production of the type comprising the steps of: - performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture including urea, carbamate and free ammonia in aqueous solution; subjecting the mixture to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution to obtain a flow including ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution; - subjecting the flow including ammonia and carbon dioxide in vapour phase to at least partial condensation to obtain a first portion of carbamate in aqueous solution; - recycling the first portion of carbamate to the first reaction space; - feeding the flow including urea and residual carbamate in aqueous solution to a urea recovery section; and - separating in the recovery section the residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution. The present invention also relates to a plant for implementation of the above mentioned process and to a method for modernization of an existing urea plant to obtain a plant in accordance with the present invention. As known, as concerns urea production, the requirement to have on the one hand plants of every greater capacity and management flexibility and on the other hand ever smaller investment and operating costs, in particular in energy terms, is increasingly felt. Prior Art The best established urea production processes in the industry are based substantially on the performance of a conversion reaction in a reaction space fed with ammonia and carbon dioxide and to which are recycled the unreacted substances contained in the urea solution at the outlet of the reaction space, in particular ammonia, carbon dioxide and carbamate in aqueous solution. This recycling, if on the one hand it allows almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand also involves the sending to the reactor of large quantities of water which penalize the overall yield of conversion of the carbon dioxide to urea with the yield being generally between 59% and 63%. For the purpose of increasing the conversion yield there have been proposed successively and implemented in the industry a series of processes for urea production based fundamentally on the performance of conversion reactions with differentiated yields in reaction spaces placed in parallel with each other, as described for example in European Patent Application EP-A-0 479 103. Although giving a high weighed average yield the above mentioned processes exhibit a series of drawbacks linked to the complexity of the plant facilities and the high costs of construction of the plants assigned to their implementation. This is substantially due to the presence of two separate reaction spaces. Summary of the invention The technical problem underlying the present invention is therefore to conceive and make available a process for urea production achieving high conversion yield which would be technically simple to implement and would involve low investment and operating costs. In accordance with the present invention this problem is solved by a process of the above mentioned type characterized in that it comprises the further steps of: subjecting at least part of the second portion of carbamate in aqueous solution obtained in the recovery section to a partial decomposition treatment to obtain a flow including ammonia and carbon dioxide in vapour phase and a flow comprising residual carbamate in aqueous solution; - subjecting the flow including ammonia and carbon dioxide in vapour phase to condensation to obtain carbamate in aqueous solution; - recycling the carbamate to the reaction space; and - feeding the flow comprising residual carbamate in aqueous solution to the urea recovery section. ["hanks to the process in accordance with the present Invention • a large part of the water contained in the reaction mixture is recirculated in_ the urea recovery section and no longer in the reaction space. In this manner it is possible to keep the water content in the reaction space within optimal values permitting high conversion yield. Advantageously the decomposition treatment of at least part of the second portion of carbamate in aqueous solution is performed in the same decomposition stage of the reaction mixture coming from the reaction space. In accordance with another aspect of the present invention the technical problem indicated above is solved by a plant designed to implement the above mentioned process for urea production and including: - a urea synthesis reactor; - a first stripping unit for subjecting a reaction mixture leaving the reactor to a partial decomposition treatment of the carbamate and partial separation of the free ammonia in aqueous solution present in the mixtures-means of condensing at least partially the vapours leaving the first stripping unit and of recycling a first carbamate solution to the reactor; - a recovery space for separating the urea produced in the reactor from a second aqueous carbamate solution; - a second stripping unit for subjecting at least a part of the second carbamate solution to a partial decomposition treatment; - means of feeding at least part of the second carbamate solution obtained in the recovery section to the second stripping unit; - means of feeding the vapours leaving the second stripping unit to the condensation means; and - means of feeding the aqueous carbamate solution obtained from the second stripping unit to the recovery section. In accordance with the present invention the plants assigned to implementation of the process for urea production can be provided either completely new or by modifying existing plants to obtain enlargement of the productive capacity and at the same time improved performance in terms of energy consumption. In accordance with another aspect the present invention it makes available a modernization method for a urea production plant of the type comprising: - a urea synthesis reactor; - a first stripping unit for subjecting a reaction mixture leaving the reactor to a partial decomposition treatment of the carbamate and partial separation of the free ammonia in aqueous solution present in the above mentioned mixture; - means of at least partial condensation of the vapours leaving the first stripping unit and of recycling of a first carbamate solution to the first reactor; - a recovery section for separation of the urea produced in the reactor from a second aqueous carbamate solution; with the method being characterized in that it comprises the steps of: - providing a second stripping unit for subjecting at least part of the aqueous carbamate solution to a partial decomposition treatment of the carbamate; - connecting the second stripping unit to the condensation means for at least partial condensation of the vapours leaving the second stripping unit; and providing means of recycling the aqueous carbamate solution produced in the second stripping unit to the reaction section. Further characteristics and advantages of the present invention are set forth in the description of some embodiments thereof given below by way of non-limiting example with reference to the plants illustrated in the annexed drawings. Brief description Q£ the drawings In the drawings: - FIG. 1 shows a block diagram representing a process for urea production in accordance with the prior art; FIG. 2 shows a block diagram representing a first embodiment of the process in accordance with the present invention, and FIG. 3 shows a block diagram representing a second embodiment of. the process in accordance with the present invention. Detailed description of a preferred embodiment ol ths. present invention FIG. 1 (state of the art) shows a block diagram in which appear the different steps of a process implemented in accordance with a modern urea production plant of the total recycling type. With reference signs 1, 2 and 3, it is indicated a high pressure synthesis loop comprising a synthesis reactor 1, a stripping section 2 and a condensation section 6. The reactor 1, is fed with pure reagents and an aqueous carbamate solution from recycling. The average operating conditions of the reactor are: • molar ratio NH3/CO2 at input 2.9 to 3.4; • molar ratio H20/CO2 at input 0.4 to 0.7; • conversion yield of the COj into urea 59% to 63%; • pressure 150 bar; • temperature 185°C to 19Q°C. The urea solution SU leaving the reactor 1 fed with ammonia NH3 A and carbon dioxide COj C passes into the stripping section 2 (isobaric with the high pressure reactor HP) where a large part of the unreacted substances, notably NH3 and carbamate, are stripped and recycled to the reactor 1 after condensation in section 6. As stripping agent there can be used the fed NH3 or the fed CO2 C as in the case of FIG. 1, or self-stripping operation can be applied in which the evaporated NH3 facilitates decomposition of the carbamate. With reference signs 3, 4, 7 and 8, it is indicated a urea recovery section comprising stripping sections 3 and 4 at medium respectively low pressure, and condensation sections 7 and 8 at medium respectively low pressure. From the section 2 the partially distilled urea solution SUl passes into the following stripping sections 3 at medium pressure and 4 at low pressure (18 and 4 bar respectively). If the medium pressure sections 3 and 7 (18 bar) are missing, the recovery section will include only the low pressure sections 4 and 8 (4 bar) for the final distillation. The vapours VI coming from the section 2 are condensed in the section 6 and recycled to the synthesis section 1 (stream L). There is obtained from 3 a 70%-72% urea solution SU2 still containing small quantities of NH3 and CO2. This solution is concentrated up to 99.7% of urea in the section 4 (stream U) and is prilled or granulated in the section 5 to obtain the final product P. The vapours V3 coming from the section 4 are condensed and subjected to further distillation treatments in the section 8 for condensation and treatment of the water. The formation water purified of all traces of NH3 and urea is discharged from the plant (stream H) , while the remaining part containing the residues of NH3 and CO2 present in it is recycled (stream L2) as a solution of carbamate to the condensation section 7. In section 7, the vapours V2 coming from the corresponding stripping section 3 are condensed and the carbamate solution (stream LI) is sent to the condensation section 6 at high pressure HP, where it promotes condensation of the vapours at high pressure HP coming from 2. The carbamate solution obtained in the section 6 is then recycled directly to the reactor 1 (stream L). In some cases the, 18 bar condensation section 7 also contains a separation column for the NH3 to obtain liquid NH3 with high purity which is sent to the reactor together with the fed NH3 (stream Al). In the general block diagram of FIG. 1 the recycled water acts as carrier for conveyance of the unreacted substances to the reactor. The penalization due to the recycling is severe and involves negatively the main process steps, i.e.: - less reactor conversion; - less performance in the distillers, for instance in the high pressure stripper HP (section 2) and in the medium-and low-pressure sections 3 and 4; and - more water to be treated. On the other hand the recycled water is essential for condensation and recovery of the unreacted substances which are freed in the distillation treatments (sections 2, 3 and 4), and thus it cannot be excluded. In accordance with a first embodiment of the process according to the present invention shown in FIG. 2, the recycled carbamate solution L1 coming from section 7 is distilled in an additional section 9 opera'ting at the same pressure of the synthesis loop(high pressure HP) to obtain a stream of vapours V4 rich in NH3 and CO2 and poor in H2O (only a few percentage points) which is sent directly to the section 6 for condensation at high pressure. From the section 9 it is also obtained a stream of solution L3 very rich in water which. The solution L3 is further distilled in the stage 10, obtaining water-poor vapours V5 recirculated to the unit 7, and a liquid stream L4 which is sent to section 8 and again recycled to the section 7 (stream L2) . The stream L2 consists practically of water with small quantities of NH3 and CO2 There is thus obtained a separate closed circulation loop of process water which serves for the essential function of promoting condensation of NH3 and CO2 vapours (streams V2 and V3) in the stages 7 and 8, witheut interfering negatively in the main process streams feeding the reactor. Operating in this way, the reactor is fed with pure reagents and a highly concentrated solution of carbamate L, poor in H2O, and operating consequently at high conversion yield conditions typical of once-through reactors (e.g. conversion yield of 70% to 72%) . It is noted that the stripper 2 of the synthesis loop and the urea recovery section (distillation sections 3 and 4), are promoted by the smaller flow rates to be treated, because of the greater yield and the absence (or nearly) of recycled water. The process according to the present invention can be advantageously implemented not only in newly built plants, but also to increase the capacity of existing plants thanks to a considerable balancing of the main units making up the plant. In accordance with a second embodiment of this process it may be profitable to use the existing stripping section 2 to strip not only the urea solution coming from the reactor 1 but also the recycled carbamate solution L1 (see FIG. 3). In this case the carbamate solution, stream L1, coming from section 7 directly feeds the stripping section 2. The plant designed to implement the process for urea production in accordance with the present invention can be provided either entirely new or by modernizing an existing plant. This invention relates to process and plant for the production of urea with high conversion yield and low energy consumption. In its general aspect the present invention relates to a process for urea production. The present invention relates specifically to a process for urea production of the type comprising the steps of: - performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising 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 comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution; - subjecting said first flow comprising 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 portion of carbamate to said reaction space; - feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section; separating in said recovery section said residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution. The present invention also relates to a plant for carrying out the above mentioned process and to a method for modernizing an existing urea plant; to obtain a plant according to the present invention. As known, in the field of urea production the need is ever more growing of plants having greater capacity and operating flexibility on the one hand, on the other hand, requiring ever smaller investment and operating costs, in particular in energy terms. Prior Art To this end, there have been proposed and implemented in the art 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 (CO2) and to which are recycled the unreacted substances contained in the urea solution leaving the reaction space, in particular ammonia, carbon dloxide and carbamate in aqueous solution. A process of this type is shown in FIG. 1, and comprises downstream to a reaction space, a cambamate decomposition unit and a urea recovery section for separating from the urea solution the unreacted subtil,ances to be recycled. If, on the one hand, this recycle allowr. almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand it also involves the sending to the reactor of large quantities of water (H2O) which are detrimental to the overall yield of conversion of the carbon dioxide to urea, with the yield being generally between 59% and 63%. Summary of the invention The technical problem underlying the present invention is accordingly to conccivo and make avall.ililc a [Jioconr, for urea production achieving high couveruion yield which would be technically simple to implement .twd would involve low investment and operating costs. In accordance with the present i nven: hii, this problem is solved by a process of the above mc, n, , ; .ned type which is characterized in that it comprif;^-; thi ■ , lilitionai .nLcps of: - subjecting at least part of. sa i c) /;econd portion of carbamate in aqueous solution obLa.i II'M1 in said recovery section to a treatment of partial deconiiiooition to obtain a second flow comprising, ammonia and cdihoii dioxide in vapor phase and a flow comprising re; idual carbamate in aqueous solution; - subjecting said second flow comprising 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, al ]i>ist (lait of the carbamate in aqueous solution leaving the urea j (.'covery section is advantageously subjected to a treatment of partial decomposition separating unreacted ammonia and carbon dioxide from a solution rich in water comprising residual carbamate. So doing, the unreacted .sub.sl aricn.s wliich are rocyclod to thi' reaction space liave a vei y I nw watci content , a\)d I hu;; it i.s possible to ;>ubfjt ant ia IJ y limit I lie w.itci i cci to tlic reaction space, permitting a high conversion yield. In order to obtain a high degree of decomposition of said at least part of the second portion of carbamate in aqueous solution, the same is preferably subjected to a ft decomposition treatment at a pressure substantially corresponding to the pressure in l:lie reaction space. To improve and assist the condensation and separation steps of the unreacted substances in the urea recovery section, the flow comprising residual carbamate in aqueous solution resulting from the treatment of partial decomposition of the second portion of carbamate is advantageously fed to said urea recovery section. According to another embodiment ol: tlie present invention, the process comprises the steps of s - feeding the reaction mixture compr:irjing urea, carbamate and free ammonia in aqueous solui:ion to a decomposition unit; feeding said at least part df tin. oecond portion of carbamate in aqueous solution to naid fie -iJinposition unit, wherein the treatment of partial ijri; : rnposition of the reaction mixture and of the secoini pc i i: hm of carbamate is carried out advantageously in th.- saiix decomposition unit to obtain said first and second flow ( omprising ammonia and carbon dioxide in vapor phase and a I I ov/ comprising urea and residual carbamate in aqueoun Bolul.ioii. In accordance with this embodiment, the implementation of the urea production process is technically very simple, since no relevant additional equipment is required compared to the prior art processes, and it involves low investment costs. Particulaily satisfactory results have been obtained by subjecting at least 50% and preferably at least 65% of said second portion of carbamate in aqu According to another aspect of the i - a urea synthesis reactor; - a first stripping unit for yul i jecti iui a reaction mixture leaving said reactor to .1 treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; means for condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor; - a recovery section of a flow comprising urea and residual carbamate in aqueous solution leciving said first stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution; which plant is characterized in that it comprises: - a second stripping unit for subjecting at least part of said second portion of carbamate} in aqueous solution to a treatment of partial decomposition; means for condensing at le.ist partially the vapors leaving said second stripping unit and o(: recycling a third portion of carbamate in aqueou;; .stilut ion to r.aid reartor. In accordance with a still further embodiment of the invention, the plant for urea production comprises: - a urea synthesis reactor; a stripping unit for subject; inc) ,1 reaction mixture leaving said first reactor to a I leatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueou solution present in said mixture-means for condensing at least partially the vapors leaving said stripping unit and of recycling a first portion of carbamate in aqueous solution to said first reactor; - a recovery section of a flow comprining urea and residual carbamate in aqueous solution leaving said stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueouu solution; which plant is characterized in that it comprises: - means for feeding at least part of said second portion of carbamate in aqueous solution to the stripping unit. In accordance with the present. inveuition the plants delegated to carry out the urea production process may be provided either new or by modifying pre-existing plants so as to obtain a production capacity expansion and at the same time improved performance from the energy consumption viewpoint. According to another aspect, the present invention accordingly makes available a method for modernizing a urea production plant of the type comprising: - a urea synthesis reactor; - a first stripping unit for subgectarg a reaction mixture leaving said reactor to of partial decomposition of the carbamate and plant separation of the free ammonia in aqueous solution present in said mixture; means for condensing at leant: partially the vapors leaving said first stripping unit and of recycling a first portion of carbar^ate in aqueous solution to said reactor; - a recovery section of a flow comprising] urea and residual carbamate in aqueous aolutiou leaving naid first stripping unit for separating 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 stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial deomposition; - providing means for condensing al least partially the vapors leaving said second striping unit and of recycling a third portion of carbamate in aqueous solution to said reactor. In an alternative embodiment, the modeerization method of the present invention comprises tlie HI: 'in of : - providing a second stripping unit bjectihg at least part of said second portion of c.imate in aqueous solution to a treatment of parti.ii! dt :in\|iosition; providing means for feeding the vapors leaving said second stripping unit to saiti ineaiin fur condensing the vapors leaving said first stripping 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 portion of carbamate in aqueous solution to the stripping unit . Accordingly the present invention provides a process for urea production comprising the steps of: (l)performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution; (2)subjecting said mixture to a treatment of partial decomposition of the carbamate and partial separation of said free anunonia in aqueous solution to obtain a first flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution; (3) subjecting said first flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqeuous solution; (4)recycling said first portion of carbamate to said reaction space; (5)feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section; (6) separating in said recovery section said residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution; (7)subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a treatment of partial decomposition to obtain a second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising residual carbamate in aqueous solution; (8) subjecting said second flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution; (9) recycling said third portion of carbamate to said reaction space and recovering urea from the urea recovery section in a known manner. The invention also provides a plant for urea production comprising: - a urea synthesis reactor; - a first stripping 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 condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor; - a recovery section of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution; - a second stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition and - means for condensing at least partially the vapors leaving said second stripping unit and of recycling a third portion of carbamate in aqueous solution to said reaction . Further characteristics 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: - FIG. 1 shows a block diagram of a urea production process according the prior art ; - FIG. 2 shows a block diagram of a first embodiment of the urea production process according to present invention; and - FIG. 3 shows a block diagram of a second embodiment of the urea production process according to the present invention. Detailed description of a preferred embodiment FIG. 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 comprising substantially pure ammonia and carbon dioxide respectively. Typical operating conditions in the reaction space are: • molar ratio NH3/CO2 at input: 2.9 to 3.4; • molar ratio H2O/CO2 at input: 0.4 to 0.7; • conversion yield of the CO2 into urea: 59% to 63%; • pressure: 150 bar a; • temperature: 185°C to 190°C. Blocks 2, 5 and 6 indicate respect1veiy a high pressure decomposition unit, a urea granulation or prilling section and a high pressure condensation unit The decomposition and condensation unit 2 and 6 generally operates at the same pressure conditions i >uis as the reaction space 1. A urea recovery section is generlly ludicated by blocks 3, 4, 7 and 8. In particular, blocks 3 and 4 indicate a stripping or distillation unit and blocks 7 and 8 indicate a condensation unit. Block 4 also indicates a urea finishing unit, wherein a solution with an urea content of up to 99.7% is obtained. Block 8 also indicates a waste water treatment unit for purification of the water to be discharged from the urea production process. Typically, blocks 3 and 7 operate at medium pressure (about 18 bar), while blocks 4 and 8 operate at low pressure (about 4 bar). Flow line 23 represents a liquid flow of a reaction mixture coming from block 1 and comprising urea and unreached substances, notably carbamate and free ammonia in aqueous solution. The liquid flow 23 is fed to block 2, where it is subjected to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia. The decomposition unit indicated by block 2 generally comprises a stripper apparatus which operates with a flow 30 of carbon dioxide as stripping agent coming from the flow line 22. At: the outlet of block 2, flow line 24 and 25 are shown which represent respectively a gas flow comprising ammonia and carbon dioxide in vapor phase and a liquid flow comprising urea and residual carbamate in aqueous solution. Flow line 24 traverses the condensation unit represented by block 6, where the ammonia and carbon dioxide in vapor phase are condensed obtaining a flow of carbamate in aqueous solution which is recycled to the. reaction space 1. The flow comprising urea and residual carbamate in aqueous solution indicated by flow line 25 traverses the distillation units of the urea recovery section indicated by blocks 3 and 4, where the residual carbamate is decomposed and separated from the urea solution. Generally, the urea content in the liquid flow 25 is between 70%-72% after block 3 and about 99% after block 4. Flow lines 26 and 27 represent. a gaw flow comprising ammonia and carbon dioxide in gaseing phase obtained in blocks 3 and 4 respectively. The flow 27 traverses the condensation unit represented by block 8, where the ammonia and carbon dioxide in vapor phase are condensed obtaining a of carbamate in aqueous solution, and is fed toirhc Miidensation unit 7, where it promotes condensation o) ihc q flow 26. Analogously, the flow 26 travortu.is In- condensation unit represented by block 7, when the ammonia and carbon dioxide in vapor phase are condensed obtaining a flow of carbamate in aqueous solution, and is fed Lo the condensation unit 6, where it promotes condensation of the gas flow 24. Part of the water contained in the aqueous solution obtained in the condensation unit of block 8 is further treated and purified of almost aill ktces of ammonia and urea in the treatment unit also i.tidi ,H =:l by block 8. From block 8 departs a flow line 28 ci a water] flow to be discharged from the urea producn: j on . Finally, the urea solution flow 25 i comining from block 4 traverses the granulation or prillini oec-tion indicated by block 5, where it is transformed t;o a final product leaving the urea production process by I Jew line 29. In accordance with 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 the reaction space 1 in addition to the flow 21, as indicated by flow line 31 in FIG. 1. As shown in FIG. 1, in the urea production process according to the prior art, all the carbamate containing aqueous solution separated from t lie uvea is recycled to the reaction space 1, with the large? amount of water therein contained being used for condensation and conveyance of the unreacted substances. In FIGS. 2 and 3, it is indicated a block diagram of a first respectively second embodiment of the urea production process according to the present Invention. In said figures the detaj hi of the block diagram structurally and functionally ecjuivalent to those shown in FIG. 1 are indicated by the same reference numbers and not further described. In FIG. 2, block 9 indicates a high pressure decomposition unit operating at the same pressure conditions as the reaction space 1. The liquid flow 26 containing carbamale in aqueous solution and having a high water content in fed to block 9, where it is advantageously subjected to a treatment of partial decomposition of the carbamate. At the outlet of block 9, f]ow lincin ^2 and 23 aro fshown which rcproaoiiL rcupectively flow compriuiing amuiunia and carbon dioxide in vapor hase and a liquid flow comprising urea and residual carbamate in aqueous solution. The gas flow 32, which is very rich in ammonia and carbon dioxide and poor in water (only a fevi percentages points) , traverses the condensation unit represented by block 6, where ammonia and carbon dioxide are cuudeatied obtuiuiuy a flow of carbamate in aqueous solution, and is recycled to the reaction space 1 through flow line 24. In the example of FIG. 2, all the carbamate in aqueous solution separated from the urea in tlie recovery section is subjected to the decomposition treatment in block 9. However, satisfactory results have licn obtained also by feeding to block 9 only a part of Milamate leaving the urea recovery section. Preferably, »l. least 50% of this carbamate may be sent to block 9 According to the process of the invention, a reaction between ammonia and caibcm cl HJ-:i cte is performed in reaction space 1 obtaining a reaction mixture comprising urea, carbamate and free ammonia i ti a moiis solution, which is subjected in decomposition unit ,i' to a treatment of partial decomposition of the cailfimate and partial separation of said free ammonia In a.:.\|U(ous solution. From the decomposition unit 2 departn first flow 24 comprising ammonia and carbon dioxide in vapor phase and a flow 25 comprising urea and residual carbamate in aqueous solution. The flow 24 is then subjected to at least partial condensation in block 6 to obtain a first portion of carbamate in aqueous solution which is recycled to the reaction space 1. The flow 25 is on the contrary fed to a urea recovery section (blocks 3, 4, 7 and 8) where the urea is separated from a second portion o) arbamate in aqueous solution indicated by flow lino :!(;. Advantageously, in accordanrc wich procp;;;; stepn of the present invention, at lea:U pai t: of the flow 26 is further subjected to a treatment: of pruicial decomposition in block 9 to obtain a second J:low I2 comprising ammonia and carbon dioxide in vapor phasK; and a flow 3 3 comprising residual carbamate in aqueous solution. The flow 3 2 is then at least partially condensed in block 6 to obtain a third portion of carbamate in aqueous solution recycled to the reaction space via flow line 24. By operating in this manner it is possible to obtain a high conversion yield in the reaction space since a highly- concentrated solution of carbamate which is very poor in water is recycled to the same. According to the present urea produ.'Lion process, it is poaoible to acliieve a conversion y.l.elil ol. carbon dioxide to urea of about 70% to 75%, which is notably greater than that obtainable with the prior art processes. Moreover, this high conversion yield ancJ the substantially absence of water to be recycled to the reaction space 1 also result in a smaller flow of nubotancoo to bo scparated from the urea ooluLion, and ihiwi it reaction in .in incroP in the performance of the reaction unit 2 an(i of the distillation units 3 and 4 of the recovery section. In the example of FIG. 2, the liquid flow 33, which is very rich in water, is advantageously recycled to the urea recovery section in order to promote condensation and recover of the unreacted substance.g which are freed in the distillation units 3 and 4. Preferably, flow line 33 traverses * distillation unit indicated by block 10, where the reaction carbamate is further subjected to decomposition on order to obtain a solution very rich in water which is ted to block 8. From block 10 also departs a flow line 34 of a wafer poor vapor flow comprising residual ammonia and carbon dioxide which is fed to the condensation unit indicated by block 7 of the recovery section. There is thus obtained a separate circulation loop ol process water which advantageously promotes condensation of ammonia and carbon dioxide vapors in units 7 and 8 without being recycled to the reaction apace 1 and thus without negatively affecting the reaction between ammonia and carbon dioxide. In the alternative embodiment of the process according to the present invention disclosed in FlG). 3, the flow line 26 coming from the urea recoveiy Beet ion, namely the condensation unit 7, instead of l:)eing recycled directly to the reaction space 1 through block 6 as in the prior art process, it is advantageously fed to the decomposition unit indicated by block 2, where a f low of ammonia and carbon dioxide in vapor phase is obtained whicih is recycled via flow line 24 to the reaction space 1 after condensation in the block 6. So doing, the treatment of partial decomposition of the second portion of carbamate in aqueous solution (flow 26) is carried out in the same deeompsition unit of the reaction mixture (flow 23), thi:;rel)y permitting an implementation of the process accouding to the present invention which does not requiie the.' use of relevant additional equipment if compared lo the prior art. As stripping agents for the decompsition i.( i on unit 2 it can also be used a part of the ammonia flow fed to the reaction space. Alternatively, the block 2 can be operated in a Rcl f - St ripping niodo, wherein the ovaporat od ammonia promotes the decomposition of the carbamate. Moreover, the urea recovery section may only comprises the low pressure units 4 and 8. In this case, the flow comprising urea and residual carbamate in aqueous solution coming from the decomposition iniit 2 is fed directly to block 4 for final separation of t he (itrsi solution from the unreacted substances. Reference is now made to a )>:ii;int: j or urea production specifically designed to carry DUI; ti :: p' ocess according to the present invention. i The urea production plant advantage uri'ly comprises a urea synthesis reactor indicated by bio I': 1 , a first and a second stripping unit indicated li/ blockn 2 and 9 respectively, a urea recovery on indicated by blocks 3, 4, 7 and 8, and respect ivi means for condensing and recycling to the reactor the vapors leaving the first and the second stripping unit. With reference to the embodiment of FIG. 2, the means for condensing the vapors leaving the second stripping unit 9 preferably comprises the means for condensing the vapors leaving the first stripping unit 2 and is indicated by block 6. A distiller unit indicated by block 10 is also disposed between the second stripping unit and the recovery section. Witli reference to the embodiment ot VIG. 1, the urea production plant comprises feeding means indicated by flow line 26 between the recovery section and the stripping unit 2. In this case the stripping unit 9 and the distiller "unit 10 are noL needed. The plant designed to implemcnl; t lie i)rocess for urea production in accordance with tin pmuiiil: invention may be a brand new plant or a plant obtained Ir/ modernizing a pre¬existing plant such as the plant:. i ('suiting from the implementation of the proce.'.:;:; IL1U;M ;, led in, the block diagram of FIG. 1. According to a first embodiment, th 1.:: nuadernization takes place by means for the steps of: providing a second stripping unit: (block 9) for subjecting at least part of the carbamate in aqueous solution leaving the recovery oection (flow line 26) to a treatment of partial decomposition; - providing means for condensing at least partially the vapors leaving said second stripping unit and of recycling the so obtained high concentrated carbamate solution to the, reactor (block 1). In accordance with another embodiment of the invention, the modernizing method preferably coniprim's the step of: - providing means for feeding the vapors leaving the second stripping unit (block 9) directly to the means for condensing the vapoi's leaving tin; I i r.'U stripping unit 2, represented by block 6. Advantageously, the method for modernixing a pre-existing plant additionally comprise tlie step of : providing means for feedinri (flow line 33) a flow comprising residual carbamate in aqueouf? solution from the second stripping unit (block 9) to the urea recovery section. In a particular and advanLageouj; bi reaction of the present invention, the modernizing methoiJ c:onij )r :i :.i::i3 the step of: - providing means for feeding (f1civ n 26) at least part of the carbamate solution comiiig fiiiiii the urea recovery-section to the stripping unit indicatiil by block 2. Thanks to the modernizing mothod of I h reaction invontion, not only the conversion yield of the pre-existing urea synthesis reactor can be drastically increased but also its capacity. In fact, since only a very small amount of water is recycled to the reactor 1, a bigger flow of ammonia and carbon dioxide may be fed to the same without causing a capacity overcharge in the reactor itself as well as in the decomposition unit 2 and in the distillation units 3 and 4 of the recovery section. In the next examples there are compared by way of merely indicative and non limiting example the conversion yields obtainable by a plant implementing the process according to the proaent invontion or moderni/.c^d by the mcLliod of .the present invention and by a planl. implemtMiting the process according to the prior art. EXAMPLE 1 A pre-existing plant operating according to the prior art process described with reference to Fig. 1 is modernized in ordrr to operalo accorcing to the reaction with reference to FIG. 2. The pre-existing plant is based on the so-called ammonia self-stripping process, wherein no ammonia or carbon dioxide stripping agent is fed 1;o t lie decompostion unit indicated by block 2. Therefore, in tills case flow line 30 is missing. The operation conditions of the urea synthesis reactor before the plant modernization are the followings: • molar ratio NH3/CO2 at input: i.2; • molar ratio H2O/CO2 at input; 0.6; • conversion yield of the CO2 into ured: 61%; • pressure: about 150 bar a; • temperature: 190°C; • capacity: 1800 MTD urea After modernizing the pre-eXISTIng plant by providing a second stripping unit 9 fed with 77% of the carbamate solution coming from the urea recovery section, and by feeding the vapors leaving said second stripping unit to the reactor space 1 via the condenser unit 6, as described with reference to FIG. 2, the new operating conditions of the reactor are the followings: • molar ratio NH3/CO2 at input; - . 2 ,; • molar ratio H2O/CO2 at input: .3 19- • conversion yield of the CO2 into ured; 70%; • pressure: about 150.bar a; • temperature: 190°C; • capacity: 2500 MTD urea Thanks to the present invention, it is possible to increase the conversion yield of 9 points percentage and to increase the capacity of 700 MTD urea, i.e. 39% more than the original capacity. Such a relevant increase in the conreion yield and the much lower amount of water recycled to the reactor allow to obtain the new increased capacity with only minor modifications to the pre-existing plant and with low investment costs. Moreover, this high conversion yield also results in a reduction in energy consumption of the modernized plant. EXAMPLE 2 A pre-existing plant operating according to the prior art process described with reference to FIG. 1 is modernized in order to operate according to the process described with reference to FIG. 2. The pre-existing plant is based on the carbon dioxide stripping process, wherein a flow of carbon dioxide as stripping agent is fed to the decompostion unit indicated by block 2 (flow line 30) . In this case, the ammonia separation unit comprised in block 7 and flow line 31 are missing. Also units 3 and 7 are missing, and the urea recovery section only comprises the low pressure units 4 and 8 . The operation conditions of the urea synthesis reactor before the plant modernization are the followings: • molar ratio NH3/CO2 at input: 3.0; • molar ratio H2O/CO2 at input: 0.5; • conversion yield of the CO2 into urea: about GO"..; • pressure: about 14 5 bar a; • temperature: 185°C; • capacity: 1900 MTD urea After modernizing the pre-existing plant by providing a second stripping unit 9 fed with70% of the carbamate solution coming from the urea recovery y section, and by feeding the vapors leaving said Become stripping unit to the reactor space 1 via the condeesel unit 6, as described with reference to FIG. 2, the new operating conditions of the reactor are the followings: • molar ratio NH3/CO2 at input: 3.0; • molar ratio H2O/CO2 at input: 0.25; • conversion yield of the CO2 into urea: 66%; • pressure: about 150 bar a; • temperature: 'l90°C; • capacity: 2500 MTD urea Thanks to the present invention, it is possible to increase the conversion yield of 6 points percentage and to increase the capacity of 600 MTD urea, i.e. 32% more than the original capacity. Also in this case, only minor modifications to the pre¬ existing plant and low investment costs are required to obtain a relevant increase in the capacity and the conversion yield. EXAMPLE 3 In this example, the convertion yield of a reactor operating in a brand new plant implementing the process according to the present invention as described in FIG. 2 has been simulated. As for the above example 2, the plant is based on the carbon dioxide stripping process, wherein all the carbon dioxide to be fed to the rea The operation conditions of the reactor in the urea production plant according to the present invention are the followings: • molar ratio NH3/CO2 at input: 3.2; • molar ratio H2O/CO2 at input: 0.1; • pressure: about 150 bar a; • temperature: 190oC; • capacity: 400 MTD urea The conversion yield of the CO2 into urea obtained by the present reactor is very high: 72% . Moreover, the content of water in the carbamate solution recyoled to the reactor is particularly low. Since low amounts of water and unreacted substances are contained in the reaction mixture sent to the decomposition unit and further to the recovery section, it results that the duties of the process equipment are lower if compared to the conventional plants and consequently also the energy consumption and the investment coste. The results given in the above examples have been obtained by means for well known calculation a l gor Ithms. WE CLAIM: 1. A process for urea production comprising the steps of: (1) performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution; (2) 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 comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution; (3) subjecting said first flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqeuous solution; (4) recycling said first portion of carbamate to said reaction space; (5) feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section; (6) separating in said recovery section said residual carbamate fix>m the urea to obtain a second portion of carbamate in aqueous solution; (7) subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a treatment of partial decomposition to obtain a second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising residual carbamate in aqueous solution; (8) subjecting said second flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution; (9) recycling said third portion of carbamate to said reaction space and recovering urea from the urea recovery section in a known manner. 2. The process as claimed in claim 1, wherein the treatment of partial decomposition of the said at least part of the second portion of carbamate in aqueous solution is carried out at a pressure substantially corresponding to the pressure in the reaction space. 3. The process as claimed in claim 1, wherein it further comprises the step of: feeding the flow comprising residual carbamate in aqueous solution resulting from the treatment of partial decomposition of the second portion of carbamate to said urea recovery section. 4. The process as claimed in claim 1, wherein it comprises the steps of: feeding the reaction mixture comprising urea, carbamate and free ammonia in aqueous solution in step (1) to a decomposition unit. - feeding said at least part of the second portion of carbamate in aqueous solution to said decomposition unit, - wherein the treatment of partial decomposition of the reaction mixture and of the second portion of carbamate is carried out in the same decomposition unit to obtain said first and second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution. 5. The process as claimed in claim 1, wherein at least 50% of said second portion of carbamate in aqueous solution is subjected to the treatment of partial decomposition. 6. The process as claimed in claim 5, wherein at least 65% of said second portion of carbamate in aqueous solution is subjected to the treatment of partial decomposition. 7. A plant for urea production by the process claimed in claim 1 comprising: - a urea synthesis reactor (1); - a first stripping unit (2) for subjecting a reaction mixture leaving said reactor (1) to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture; - means (6) for condensing at least partially the vapors leaving said first stripping unit (2) and of recycling (24) a first portion of carbamate in aqueous solution to said reactor (1); - a recovery section (3,4,7,8) of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit (2) for separating the urea produced in the reactor (1) from a second portion of carbamate in aqueous solution; - a second stripping unit (9) for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition and - means for condensing at least partially the vapors leaving said second stripping unit (9) and of recycling a third portion of carbamate in aqueous solution to said reaction (1). 8. The plant as claimed in claim 7, wherein said means for condensing the vapors leaving said second stripping unit (9) comprises said means (6) for condensing the vapors leaving said first stripping unit (2). 9. The plant as claimed in claim 7,wherein means (33) for feeding a flow comprising residual carbamate in aqueous solution from said second stripping unit (9) to said recovery section (3,4,7,8) is provided. 10. The plant for urea production as claimed in claim 7, wherein means for feeding (26) at least part of the second portion of carbamate in aqueous solution is provided. 11. A process for urea production substantially as hereinabove described and illustrated with reference to figures 2 and 3 of the accompanying drawings. 12. A plant for urea production substantially as hereinabove described and illustrated with reference to figures 2 and 3 of the accompanying drawings.

Full Text

Title: "Process and plant for the production of urea with high conversion yield and low energy consumption"
DESCRIPTION
Field o£ application
In general, the present invention relates to a process for urea production.
Specifically, the present invention relates to a process for urea production of the type comprising the steps of:
- performing a reaction between ammonia and carbon dioxide
in a reaction space to obtain a reaction mixture including
urea, carbamate and free ammonia in aqueous solution;
subjecting the mixture to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution to obtain a flow including ammonia and carbon dioxide in vapour phase and a flow comprising urea and residual carbamate in aqueous solution;
- subjecting the flow including ammonia and carbon dioxide in vapour phase to at least partial condensation to obtain a first portion of carbamate in aqueous solution;
- recycling the first portion of carbamate to the first reaction space;
- feeding the flow including urea and residual carbamate in aqueous solution to a urea recovery section; and
- separating in the recovery section the residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution.

The present invention also relates to a plant for implementation of the above mentioned process and to a method for modernization of an existing urea plant to obtain a plant in accordance with the present invention.
As known, as concerns urea production, the requirement to have on the one hand plants of every greater capacity and management flexibility and on the other hand ever smaller investment and operating costs, in particular in energy terms, is increasingly felt.
Prior Art
The best established urea production processes in the industry are based substantially on the performance of a conversion reaction in a reaction space fed with ammonia and carbon dioxide and to which are recycled the unreacted substances contained in the urea solution at the outlet of the reaction space, in particular ammonia, carbon dioxide and carbamate in aqueous solution.
This recycling, if on the one hand it allows almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand also involves the sending to the reactor of large quantities of water which penalize the overall yield of conversion of the carbon dioxide to urea with the yield being generally between 59% and 63%.
For the purpose of increasing the conversion yield there have been proposed successively and implemented in the industry a series of processes for urea production based fundamentally on the performance of conversion reactions with differentiated yields in reaction spaces placed in parallel with each other, as described for example in European Patent Application EP-A-0 479 103.

Although giving a high weighed average yield the above mentioned processes exhibit a series of drawbacks linked to the complexity of the plant facilities and the high costs of construction of the plants assigned to their implementation. This is substantially due to the presence of two separate reaction spaces.
Summary of the invention
The technical problem underlying the present invention is therefore to conceive and make available a process for urea production achieving high conversion yield which would be technically simple to implement and would involve low investment and operating costs.
In accordance with the present invention this problem is solved by a process of the above mentioned type characterized in that it comprises the further steps of:
subjecting at least part of the second portion of carbamate in aqueous solution obtained in the recovery section to a partial decomposition treatment to obtain a flow including ammonia and carbon dioxide in vapour phase and a flow comprising residual carbamate in aqueous solution;
- subjecting the flow including ammonia and carbon dioxide in vapour phase to condensation to obtain carbamate in aqueous solution;
- recycling the carbamate to the reaction space; and
- feeding the flow comprising residual carbamate in aqueous solution to the urea recovery section.
["hanks to the process in accordance with the present Invention • a large part of the water contained in the reaction mixture is recirculated in_ the urea recovery

section and no longer in the reaction space. In this manner it is possible to keep the water content in the reaction space within optimal values permitting high conversion yield.
Advantageously the decomposition treatment of at least part of the second portion of carbamate in aqueous solution is performed in the same decomposition stage of the reaction mixture coming from the reaction space.
In accordance with another aspect of the present invention the technical problem indicated above is solved by a plant designed to implement the above mentioned process for urea production and including:
- a urea synthesis reactor;
- a first stripping unit for subjecting a reaction mixture leaving the reactor to a partial decomposition treatment of the carbamate and partial separation of the free ammonia in aqueous solution present in the mixtures-means of condensing at least partially the vapours
leaving the first stripping unit and of recycling a first carbamate solution to the reactor;
- a recovery space for separating the urea produced in the reactor from a second aqueous carbamate solution;
- a second stripping unit for subjecting at least a part of the second carbamate solution to a partial decomposition treatment;
- means of feeding at least part of the second carbamate solution obtained in the recovery section to the second stripping unit;
- means of feeding the vapours leaving the second stripping

unit to the condensation means; and
- means of feeding the aqueous carbamate solution obtained
from the second stripping unit to the recovery section.
In accordance with the present invention the plants assigned to implementation of the process for urea production can be provided either completely new or by modifying existing plants to obtain enlargement of the productive capacity and at the same time improved performance in terms of energy consumption.
In accordance with another aspect the present invention it makes available a modernization method for a urea production plant of the type comprising:
- a urea synthesis reactor;
- a first stripping unit for subjecting a reaction mixture leaving the reactor to a partial decomposition treatment of the carbamate and partial separation of the free ammonia in aqueous solution present in the above mentioned mixture;
- means of at least partial condensation of the vapours leaving the first stripping unit and of recycling of a first carbamate solution to the first reactor;
- a recovery section for separation of the urea produced in the reactor from a second aqueous carbamate solution;
with the method being characterized in that it comprises the steps of:
- providing a second stripping unit for subjecting at least part of the aqueous carbamate solution to a partial decomposition treatment of the carbamate;
- connecting the second stripping unit to the condensation means for at least partial condensation of the vapours

leaving the second stripping unit; and
providing means of recycling the aqueous carbamate solution produced in the second stripping unit to the reaction section.
Further characteristics and advantages of the present invention are set forth in the description of some embodiments thereof given below by way of non-limiting example with reference to the plants illustrated in the annexed drawings.
Brief description Q£ the drawings
In the drawings:
- FIG. 1 shows a block diagram representing a process for urea production in accordance with the prior art;
FIG. 2 shows a block diagram representing a first embodiment of the process in accordance with the present invention, and
FIG. 3 shows a block diagram representing a second embodiment of. the process in accordance with the present invention.
Detailed description of a preferred embodiment ol ths.
present invention
FIG. 1 (state of the art) shows a block diagram in which appear the different steps of a process implemented in accordance with a modern urea production plant of the total recycling type.
With reference signs 1, 2 and 3, it is indicated a high pressure synthesis loop comprising a synthesis reactor 1, a stripping section 2 and a condensation section 6.

The reactor 1, is fed with pure reagents and an aqueous carbamate solution from recycling.
The average operating conditions of the reactor are:
• molar ratio NH3/CO2 at input 2.9 to 3.4;
• molar ratio H20/CO2 at input 0.4 to 0.7;
• conversion yield of the COj into urea 59% to 63%;
• pressure 150 bar;
• temperature 185°C to 19Q°C.
The urea solution SU leaving the reactor 1 fed with ammonia NH3 A and carbon dioxide COj C passes into the stripping section 2 (isobaric with the high pressure reactor HP) where a large part of the unreacted substances, notably NH3 and carbamate, are stripped and recycled to the reactor 1 after condensation in section 6.
As stripping agent there can be used the fed NH3 or the fed CO2 C as in the case of FIG. 1, or self-stripping operation can be applied in which the evaporated NH3 facilitates decomposition of the carbamate.
With reference signs 3, 4, 7 and 8, it is indicated a urea recovery section comprising stripping sections 3 and 4 at medium respectively low pressure, and condensation sections 7 and 8 at medium respectively low pressure.
From the section 2 the partially distilled urea solution SUl passes into the following stripping sections 3 at medium pressure and 4 at low pressure (18 and 4 bar respectively).
If the medium pressure sections 3 and 7 (18 bar) are missing, the recovery section will include only the low

pressure sections 4 and 8 (4 bar) for the final distillation.
The vapours VI coming from the section 2 are condensed in the section 6 and recycled to the synthesis section 1 (stream L).
There is obtained from 3 a 70%-72% urea solution SU2 still containing small quantities of NH3 and CO2. This solution is concentrated up to 99.7% of urea in the section 4 (stream U) and is prilled or granulated in the section 5 to obtain the final product P.
The vapours V3 coming from the section 4 are condensed and subjected to further distillation treatments in the section 8 for condensation and treatment of the water. The formation water purified of all traces of NH3 and urea is discharged from the plant (stream H) , while the remaining part containing the residues of NH3 and CO2 present in it is recycled (stream L2) as a solution of carbamate to the condensation section 7.
In section 7, the vapours V2 coming from the corresponding stripping section 3 are condensed and the carbamate solution (stream LI) is sent to the condensation section 6 at high pressure HP, where it promotes condensation of the vapours at high pressure HP coming from 2.
The carbamate solution obtained in the section 6 is then recycled directly to the reactor 1 (stream L).
In some cases the, 18 bar condensation section 7 also contains a separation column for the NH3 to obtain liquid NH3 with high purity which is sent to the reactor together with the fed NH3 (stream Al).
In the general block diagram of FIG. 1 the recycled water acts as carrier for conveyance of the unreacted substances

to the reactor.
The penalization due to the recycling is severe and involves negatively the main process steps, i.e.:
- less reactor conversion;
- less performance in the distillers, for instance in the high pressure stripper HP (section 2) and in the medium-and low-pressure sections 3 and 4; and
- more water to be treated.
On the other hand the recycled water is essential for condensation and recovery of the unreacted substances which are freed in the distillation treatments (sections 2, 3 and 4), and thus it cannot be excluded.
In accordance with a first embodiment of the process according to the present invention shown in FIG. 2, the recycled carbamate solution L1 coming from section 7 is distilled in an additional section 9 opera'ting at the same pressure of the synthesis loop(high pressure HP) to obtain a stream of vapours V4 rich in NH3 and CO2 and poor in H2O (only a few percentage points) which is sent directly to the section 6 for condensation at high pressure. From the section 9 it is also obtained a stream of solution L3 very rich in water which. The solution L3 is further distilled in the stage 10, obtaining water-poor vapours V5 recirculated to the unit 7, and a liquid stream L4 which is sent to section 8 and again recycled to the section 7 (stream L2) . The stream L2 consists practically of water with small quantities of NH3 and CO2
There is thus obtained a separate closed circulation loop of process water which serves for the essential function of promoting condensation of NH3 and CO2 vapours (streams V2 and V3) in the stages 7 and 8, witheut interfering

negatively in the main process streams feeding the reactor.
Operating in this way, the reactor is fed with pure reagents and a highly concentrated solution of carbamate L, poor in H2O, and operating consequently at high conversion yield conditions typical of once-through reactors (e.g. conversion yield of 70% to 72%) .
It is noted that the stripper 2 of the synthesis loop and the urea recovery section (distillation sections 3 and 4), are promoted by the smaller flow rates to be treated, because of the greater yield and the absence (or nearly) of recycled water.
The process according to the present invention can be advantageously implemented not only in newly built plants, but also to increase the capacity of existing plants thanks to a considerable balancing of the main units making up the plant.
In accordance with a second embodiment of this process it may be profitable to use the existing stripping section 2 to strip not only the urea solution coming from the reactor 1 but also the recycled carbamate solution L1 (see FIG. 3). In this case the carbamate solution, stream L1, coming from section 7 directly feeds the stripping section 2.
The plant designed to implement the process for urea production in accordance with the present invention can be provided either entirely new or by modernizing an existing plant.

This invention relates to process and plant for the production of urea with high conversion yield and low energy consumption.
In its general aspect the present invention relates to a process for urea production.
The present invention relates specifically to a process for urea production of the type comprising the steps of:
- performing a reaction between ammonia and carbon dioxide
in a reaction space to obtain a reaction mixture comprising
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 comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution;
- subjecting said first flow comprising 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 portion of carbamate to said reaction space;
- feeding said flow comprising urea and residual carbamate
in aqueous solution to a urea recovery section;
separating in said recovery section said residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution.

The present invention also relates to a plant for carrying out the above mentioned process and to a method for modernizing an existing urea plant; to obtain a plant according to the present invention.
As known, in the field of urea production the need is ever more growing of plants having greater capacity and operating flexibility on the one hand, on the other hand, requiring ever smaller investment and operating costs, in particular in energy terms.
Prior Art
To this end, there have been proposed and implemented in the art 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 (CO2) and to which are recycled the unreacted substances contained in the urea solution leaving the reaction space, in particular ammonia, carbon dloxide and carbamate in aqueous solution.
A process of this type is shown in FIG. 1, and comprises downstream to a reaction space, a cambamate decomposition unit and a urea recovery section for separating from the urea solution the unreacted subtil,ances to be recycled.
If, on the one hand, this recycle allowr. almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand it also involves the sending to the reactor of large quantities of water (H2O) which are detrimental to the overall yield of conversion of the carbon dioxide to urea, with the yield being generally between 59% and 63%.
Summary of the invention
The technical problem underlying the present invention is

accordingly to conccivo and make avall.ililc a [Jioconr, for urea production achieving high couveruion yield which would be technically simple to implement .twd would involve low investment and operating costs.
In accordance with the present i nven: hii, this problem is solved by a process of the above mc, n, , ; .ned type which is characterized in that it comprif;^-; thi ■ , lilitionai .nLcps of:
- subjecting at least part of. sa i c) /;econd portion of
carbamate in aqueous solution obLa.i II'M1 in said recovery
section to a treatment of partial deconiiiooition to obtain a
second flow comprising, ammonia and cdihoii dioxide in vapor
phase and a flow comprising re; idual carbamate in aqueous
solution;
- subjecting said second flow comprising 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, al ]i>ist (lait of the carbamate in aqueous solution leaving the urea j (.'covery section is advantageously subjected to a treatment of partial decomposition separating unreacted ammonia and carbon dioxide from a solution rich in water comprising residual carbamate.
So doing, the unreacted .sub.sl aricn.s wliich are rocyclod to thi' reaction space liave a vei y I nw watci content , a\)d I hu;; it i.s possible to ;>ubfjt ant ia IJ y limit I lie w.itci i cci to tlic reaction space, permitting a high conversion yield.
In order to obtain a high degree of decomposition of said at least part of the second portion of carbamate in aqueous solution, the same is preferably subjected to a

ft
decomposition treatment at a pressure substantially corresponding to the pressure in l:lie reaction space.
To improve and assist the condensation and separation steps of the unreacted substances in the urea recovery section, the flow comprising residual carbamate in aqueous solution resulting from the treatment of partial decomposition of the second portion of carbamate is advantageously fed to said urea recovery section.
According to another embodiment ol: tlie present invention, the process comprises the steps of s
- feeding the reaction mixture compr:irjing urea, carbamate and free ammonia in aqueous solui:ion to a decomposition unit;
feeding said at least part df tin. oecond portion of carbamate in aqueous solution to naid fie -iJinposition unit,
wherein the treatment of partial ijri; : rnposition of the reaction mixture and of the secoini pc i i: hm of carbamate is carried out advantageously in th.- saiix decomposition unit to obtain said first and second flow ( omprising ammonia and carbon dioxide in vapor phase and a I I ov/ comprising urea and residual carbamate in aqueoun Bolul.ioii.
In accordance with this embodiment, the implementation of the urea production process is technically very simple, since no relevant additional equipment is required compared to the prior art processes, and it involves low investment costs.
Particulaily satisfactory results have been obtained by subjecting at least 50% and preferably at least 65% of said second portion of carbamate in aqu
According to another aspect of the i - a urea synthesis reactor;
- a first stripping unit for yul i jecti iui a reaction mixture leaving said reactor to .1 treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture;
means for condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor;
- a recovery section of a flow comprising urea and residual
carbamate in aqueous solution leciving said first stripping
unit for separating the urea produced in the reactor from a
second portion of carbamate in aqueous solution;
which plant is characterized in that it comprises:
- a second stripping unit for subjecting at least part of
said second portion of carbamate} in aqueous solution to a
treatment of partial decomposition;
means for condensing at le.ist partially the vapors leaving said second stripping unit and o(: recycling a third portion of carbamate in aqueou;; .stilut ion to r.aid reartor.
In accordance with a still further embodiment of the invention, the plant for urea production comprises:
- a urea synthesis reactor;
a stripping unit for subject; inc) ,1 reaction mixture leaving said first reactor to a I leatment of partial

decomposition of the carbamate and partial separation of the free ammonia in aqueou solution present in said mixture-means for condensing at least partially the vapors leaving said stripping unit and of recycling a first portion of carbamate in aqueous solution to said first reactor;
- a recovery section of a flow comprining urea and residual
carbamate in aqueous solution leaving said stripping unit
for separating the urea produced in the reactor from a
second portion of carbamate in aqueouu solution;
which plant is characterized in that it comprises:
- means for feeding at least part of said second portion of
carbamate in aqueous solution to the stripping unit.
In accordance with the present. inveuition the plants delegated to carry out the urea production process may be provided either new or by modifying pre-existing plants so as to obtain a production capacity expansion and at the same time improved performance from the energy consumption viewpoint.
According to another aspect, the present invention accordingly makes available a method for modernizing a urea production plant of the type comprising:
- a urea synthesis reactor;
- a first stripping unit for subgectarg a reaction mixture leaving said reactor to of partial decomposition of the carbamate and plant separation of the free ammonia in aqueous solution present in said mixture;

means for condensing at leant: partially the vapors leaving said first stripping unit and of recycling a first portion of carbar^ate in aqueous solution to said reactor;
- a recovery section of a flow comprising] urea and residual
carbamate in aqueous aolutiou leaving naid first stripping
unit for separating 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 stripping unit for subjecting at least
part of said second portion of carbamate in aqueous
solution to a treatment of partial deomposition;
- providing means for condensing al least partially the
vapors leaving said second striping unit and of recycling
a third portion of carbamate in aqueous solution to said
reactor.
In an alternative embodiment, the modeerization method of the present invention comprises tlie HI: 'in of :
- providing a second stripping unit bjectihg at least
part of said second portion of c.imate in aqueous
solution to a treatment of parti.ii! dt :in|iosition;
providing means for feeding the vapors leaving said second stripping unit to saiti ineaiin fur condensing the vapors leaving said first stripping 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
portion of carbamate in aqueous solution to the stripping
unit .

Accordingly the present invention provides a process for urea production comprising the steps of:
(l)performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution;
(2)subjecting said mixture to a treatment of partial decomposition of the carbamate and partial separation of said free anunonia in aqueous solution to obtain a first flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution;
(3) subjecting said first flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqeuous solution;
(4)recycling said first portion of carbamate to said reaction space;
(5)feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section;
(6) separating in said recovery section said residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution;
(7)subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a treatment of partial decomposition to obtain a second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising residual carbamate in aqueous solution;

(8) subjecting said second flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution;
(9) recycling said third portion of carbamate to said reaction space and recovering urea from the urea recovery section in a known manner.
The invention also provides a plant for urea production comprising:
- a urea synthesis reactor;
- a first stripping 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 condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor;
- a recovery section of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution;
- a second stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition and
- means for condensing at least partially the vapors leaving said second stripping unit and of recycling a third portion of carbamate in aqueous solution to said reaction .

Further characteristics 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:
- FIG. 1 shows a block diagram of a urea production process according the prior art ;
- FIG. 2 shows a block diagram of a first embodiment of the urea production process according to present invention; and
- FIG. 3 shows a block diagram of a second embodiment of the urea production process according to the present invention.
Detailed description of a preferred embodiment
FIG. 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 comprising substantially pure ammonia and carbon dioxide respectively.
Typical operating conditions in the reaction space are:
• molar ratio NH3/CO2 at input: 2.9 to 3.4;
• molar ratio H2O/CO2 at input: 0.4 to 0.7;
• conversion yield of the CO2 into urea: 59% to 63%;
• pressure: 150 bar a;
• temperature: 185°C to 190°C.

Blocks 2, 5 and 6 indicate respect1veiy a high pressure
decomposition unit, a urea granulation or prilling section
and a high pressure condensation unit
The decomposition and condensation unit 2 and 6 generally operates at the same pressure conditions i >uis as the reaction space 1.
A urea recovery section is generlly ludicated by blocks 3, 4, 7 and 8. In particular, blocks 3 and 4 indicate a stripping or distillation unit and blocks 7 and 8 indicate a condensation unit.
Block 4 also indicates a urea finishing unit, wherein a solution with an urea content of up to 99.7% is obtained.
Block 8 also indicates a waste water treatment unit for purification of the water to be discharged from the urea production process.
Typically, blocks 3 and 7 operate at medium pressure (about 18 bar), while blocks 4 and 8 operate at low pressure (about 4 bar).
Flow line 23 represents a liquid flow of a reaction mixture coming from block 1 and comprising urea and unreached substances, notably carbamate and free ammonia in aqueous solution.
The liquid flow 23 is fed to block 2, where it is subjected to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia.
The decomposition unit indicated by block 2 generally comprises a stripper apparatus which operates with a flow 30 of carbon dioxide as stripping agent coming from the flow line 22.

At: the outlet of block 2, flow line 24 and 25 are shown which represent respectively a gas flow comprising ammonia and carbon dioxide in vapor phase and a liquid flow comprising urea and residual carbamate in aqueous solution.
Flow line 24 traverses the condensation unit represented by block 6, where the ammonia and carbon dioxide in vapor phase are condensed obtaining a flow of carbamate in aqueous solution which is recycled to the. reaction space 1.
The flow comprising urea and residual carbamate in aqueous solution indicated by flow line 25 traverses the distillation units of the urea recovery section indicated by blocks 3 and 4, where the residual carbamate is decomposed and separated from the urea solution.
Generally, the urea content in the liquid flow 25 is between 70%-72% after block 3 and about 99% after block 4.
Flow lines 26 and 27 represent. a gaw flow comprising ammonia and carbon dioxide in gaseing phase obtained in blocks 3 and 4 respectively.
The flow 27 traverses the condensation unit represented by block 8, where the ammonia and carbon dioxide in vapor phase are condensed obtaining a of carbamate in aqueous solution, and is fed toirhc Miidensation unit 7, where it promotes condensation o) ihc q flow 26.
Analogously, the flow 26 travortu.is In- condensation unit represented by block 7, when the ammonia and carbon dioxide in vapor phase are condensed obtaining a flow of carbamate in aqueous solution, and is fed Lo the condensation unit 6, where it promotes condensation of the gas flow 24.
Part of the water contained in the aqueous solution obtained in the condensation unit of block 8 is further

treated and purified of almost aill ktces of ammonia and urea in the treatment unit also i.tidi ,H =:l by block 8. From block 8 departs a flow line 28 ci a water] flow to be discharged from the urea producn: j on .
Finally, the urea solution flow 25 i comining from block 4 traverses the granulation or prillini oec-tion indicated by block 5, where it is transformed t;o a final product leaving the urea production process by I Jew line 29.
In accordance with 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 the reaction space 1 in addition to the flow 21, as indicated by flow line 31 in FIG. 1.
As shown in FIG. 1, in the urea production process according to the prior art, all the carbamate containing aqueous solution separated from t lie uvea is recycled to the reaction space 1, with the large? amount of water therein contained being used for condensation and conveyance of the unreacted substances.
In FIGS. 2 and 3, it is indicated a block diagram of a first respectively second embodiment of the urea production process according to the present Invention.
In said figures the detaj hi of the block diagram structurally and functionally ecjuivalent to those shown in FIG. 1 are indicated by the same reference numbers and not further described.
In FIG. 2, block 9 indicates a high pressure decomposition unit operating at the same pressure conditions as the reaction space 1.
The liquid flow 26 containing carbamale in aqueous solution and having a high water content in fed to block 9, where it

is advantageously subjected to a treatment of partial decomposition of the carbamate.
At the outlet of block 9, f]ow lincin ^2 and 23 aro fshown which rcproaoiiL rcupectively flow compriuiing amuiunia and carbon dioxide in vapor hase and a liquid flow comprising urea and residual carbamate in aqueous solution.
The gas flow 32, which is very rich in ammonia and carbon dioxide and poor in water (only a fevi percentages points) , traverses the condensation unit represented by block 6, where ammonia and carbon dioxide are cuudeatied obtuiuiuy a flow of carbamate in aqueous solution, and is recycled to the reaction space 1 through flow line 24.
In the example of FIG. 2, all the carbamate in aqueous solution separated from the urea in tlie recovery section is subjected to the decomposition treatment in block 9. However, satisfactory results have licn obtained also by feeding to block 9 only a part of Milamate leaving the urea recovery section. Preferably, »l. least 50% of this carbamate may be sent to block 9
According to the process of the invention, a
reaction between ammonia and caibcm cl HJ-:i cte is performed in reaction space 1 obtaining a reaction mixture comprising urea, carbamate and free ammonia i ti a moiis solution, which is subjected in decomposition unit ,i' to a treatment of partial decomposition of the cailfimate and partial separation of said free ammonia In a.:.|U(ous solution. From the decomposition unit 2 departn first flow 24 comprising ammonia and carbon dioxide in vapor phase and a flow 25 comprising urea and residual carbamate in aqueous solution. The flow 24 is then subjected to at least partial condensation in block 6 to obtain a first portion of carbamate in aqueous solution which is recycled to the reaction space 1. The flow 25 is on the contrary fed to a

urea recovery section (blocks 3, 4, 7 and 8) where the urea is separated from a second portion o) arbamate in aqueous solution indicated by flow lino :!(;.
Advantageously, in accordanrc wich procp;;;; stepn of the present invention, at lea:U pai t: of the flow 26 is further subjected to a treatment: of pruicial decomposition in block 9 to obtain a second J:low I2 comprising ammonia and carbon dioxide in vapor phasK; and a flow 3 3 comprising residual carbamate in aqueous solution. The flow 3 2 is then at least partially condensed in block 6 to obtain a third portion of carbamate in aqueous solution recycled to the reaction space via flow line 24.
By operating in this manner it is possible to obtain a high
conversion yield in the reaction space since a highly-
concentrated solution of carbamate which is very poor in
water is recycled to the same.
According to the present urea produ.'Lion process, it is poaoible to acliieve a conversion y.l.elil ol. carbon dioxide to urea of about 70% to 75%, which is notably greater than that obtainable with the prior art processes.
Moreover, this high conversion yield ancJ the substantially absence of water to be recycled to the reaction space 1 also result in a smaller flow of nubotancoo to bo scparated from the urea ooluLion, and ihiwi it reaction in .in incroP in the performance of the reaction unit 2 an(i of the distillation units 3 and 4 of the recovery section.
In the example of FIG. 2, the liquid flow 33, which is very rich in water, is advantageously recycled to the urea recovery section in order to promote condensation and recover of the unreacted substance.g which are freed in the distillation units 3 and 4.

Preferably, flow line 33 traverses * distillation unit indicated by block 10, where the reaction carbamate is further subjected to decomposition on order to obtain a solution very rich in water which is ted to block 8.
From block 10 also departs a flow line 34 of a wafer poor vapor flow comprising residual ammonia and carbon dioxide which is fed to the condensation unit indicated by block 7 of the recovery section.
There is thus obtained a separate circulation loop ol process water which advantageously promotes condensation of ammonia and carbon dioxide vapors in units 7 and 8 without being recycled to the reaction apace 1 and thus without negatively affecting the reaction between ammonia and carbon dioxide.
In the alternative embodiment of the process according to the present invention disclosed in FlG). 3, the flow line 26 coming from the urea recoveiy Beet ion, namely the condensation unit 7, instead of l:)eing recycled directly to the reaction space 1 through block 6 as in the prior art process, it is advantageously fed to the decomposition unit indicated by block 2, where a f low of ammonia and carbon dioxide in vapor phase is obtained whicih is recycled via flow line 24 to the reaction space 1 after condensation in the block 6.
So doing, the treatment of partial decomposition of the second portion of carbamate in aqueous solution (flow 26) is carried out in the same deeompsition unit of the reaction mixture (flow 23), thi:;rel)y permitting an implementation of the process accouding to the present invention which does not requiie the.' use of relevant additional equipment if compared lo the prior art.
As stripping agents for the decompsition i.( i on unit 2 it can

also be used a part of the ammonia flow fed to the reaction space. Alternatively, the block 2 can be operated in a Rcl f - St ripping niodo, wherein the ovaporat od ammonia promotes the decomposition of the carbamate.
Moreover, the urea recovery section may only comprises the low pressure units 4 and 8. In this case, the flow comprising urea and residual carbamate in aqueous solution coming from the decomposition iniit 2 is fed directly to block 4 for final separation of t he (itrsi solution from the unreacted substances.
Reference is now made to a )>:ii;int: j or urea production
specifically designed to carry DUI; ti :: p' ocess according to
the present invention. i
The urea production plant advantage uri'ly comprises a urea synthesis reactor indicated by bio I': 1 , a first and a second stripping unit indicated li/ blockn 2 and 9 respectively, a urea recovery on indicated by blocks 3, 4, 7 and 8, and respect ivi means for condensing and recycling to the reactor the vapors leaving the first and the second stripping unit.
With reference to the embodiment of FIG. 2, the means for condensing the vapors leaving the second stripping unit 9 preferably comprises the means for condensing the vapors leaving the first stripping unit 2 and is indicated by block 6.
A distiller unit indicated by block 10 is also disposed between the second stripping unit and the recovery section.
Witli reference to the embodiment ot VIG. 1, the urea production plant comprises feeding means indicated by flow line 26 between the recovery section and the stripping unit 2. In this case the stripping unit 9 and the distiller "unit

10 are noL needed.
The plant designed to implemcnl; t lie i)rocess for urea production in accordance with tin pmuiiil: invention may be a brand new plant or a plant obtained Ir/ modernizing a pre¬existing plant such as the plant:. i ('suiting from the implementation of the proce.'.:;:; IL1U;M ;, led in, the block diagram of FIG. 1.
According to a first embodiment, th 1.:: nuadernization takes place by means for the steps of:
providing a second stripping unit: (block 9) for subjecting at least part of the carbamate in aqueous solution leaving the recovery oection (flow line 26) to a treatment of partial decomposition;
- providing means for condensing at least partially the
vapors leaving said second stripping unit and of recycling
the so obtained high concentrated carbamate solution to the,
reactor (block 1).
In accordance with another embodiment of the invention, the modernizing method preferably coniprim's the step of:
- providing means for feeding the vapors leaving the second
stripping unit (block 9) directly to the means for
condensing the vapoi's leaving tin; I i r.'U stripping unit 2,
represented by block 6.
Advantageously, the method for modernixing a pre-existing plant additionally comprise tlie step of :
providing means for feedinri (flow line 33) a flow
comprising residual carbamate in aqueouf? solution from the
second stripping unit (block 9) to the urea recovery
section.

In a particular and advanLageouj; bi reaction of the present invention, the modernizing methoiJ c:onij )r :i :.i::i3 the step of:
- providing means for feeding (f1civ n 26) at least part of the carbamate solution comiiig fiiiiii the urea recovery-section to the stripping unit indicatiil by block 2.
Thanks to the modernizing mothod of I h reaction invontion, not only the conversion yield of the pre-existing urea synthesis reactor can be drastically increased but also its capacity.
In fact, since only a very small amount of water is recycled to the reactor 1, a bigger flow of ammonia and carbon dioxide may be fed to the same without causing a capacity overcharge in the reactor itself as well as in the decomposition unit 2 and in the distillation units 3 and 4 of the recovery section.
In the next examples there are compared by way of merely indicative and non limiting example the conversion yields obtainable by a plant implementing the process according to the proaent invontion or moderni/.c^d by the mcLliod of .the present invention and by a planl. implemtMiting the process according to the prior art.
EXAMPLE 1
A pre-existing plant operating according to the prior art process described with reference to Fig. 1 is modernized in ordrr to operalo accorcing to the reaction with reference to FIG. 2.
The pre-existing plant is based on the so-called ammonia self-stripping process, wherein no ammonia or carbon dioxide stripping agent is fed 1;o t lie decompostion unit indicated by block 2. Therefore, in tills case flow line 30 is missing.

The operation conditions of the urea synthesis reactor before the plant modernization are the followings:
• molar ratio NH3/CO2 at input: i.2;
• molar ratio H2O/CO2 at input; 0.6;
• conversion yield of the CO2 into ured: 61%;
• pressure: about 150 bar a;
• temperature: 190°C;
• capacity: 1800 MTD urea
After modernizing the pre-eXISTIng plant by providing a second stripping unit 9 fed with 77% of the carbamate solution coming from the urea recovery section, and by feeding the vapors leaving said second stripping unit to the reactor space 1 via the condenser unit 6, as described with reference to FIG. 2, the new operating conditions of the reactor are the followings:
• molar ratio NH3/CO2 at input; - . 2 ,;
• molar ratio H2O/CO2 at input: .3 19-
• conversion yield of the CO2 into ured; 70%;
• pressure: about 150.bar a;
• temperature: 190°C;
• capacity: 2500 MTD urea
Thanks to the present invention, it is possible to increase the conversion yield of 9 points percentage and to increase the capacity of 700 MTD urea, i.e. 39% more than the original capacity.

Such a relevant increase in the conreion yield and the much lower amount of water recycled to the reactor allow to obtain the new increased capacity with only minor modifications to the pre-existing plant and with low investment costs. Moreover, this high conversion yield also results in a reduction in energy consumption of the modernized plant.
EXAMPLE 2
A pre-existing plant operating according to the prior art process described with reference to FIG. 1 is modernized in order to operate according to the process described with reference to FIG. 2.
The pre-existing plant is based on the carbon dioxide stripping process, wherein a flow of carbon dioxide as stripping agent is fed to the decompostion unit indicated by block 2 (flow line 30) . In this case, the ammonia separation unit comprised in block 7 and flow line 31 are missing. Also units 3 and 7 are missing, and the urea recovery section only comprises the low pressure units 4 and 8 .
The operation conditions of the urea synthesis reactor before the plant modernization are the followings:
• molar ratio NH3/CO2 at input: 3.0;
• molar ratio H2O/CO2 at input: 0.5;
• conversion yield of the CO2 into urea: about GO"..;
• pressure: about 14 5 bar a;
• temperature: 185°C;
• capacity: 1900 MTD urea

After modernizing the pre-existing plant by providing a second stripping unit 9 fed with70% of the carbamate solution coming from the urea recovery y section, and by feeding the vapors leaving said Become stripping unit to the reactor space 1 via the condeesel unit 6, as described with reference to FIG. 2, the new operating conditions of the reactor are the followings:
• molar ratio NH3/CO2 at input: 3.0;
• molar ratio H2O/CO2 at input: 0.25;
• conversion yield of the CO2 into urea: 66%;
• pressure: about 150 bar a;
• temperature: 'l90°C;
• capacity: 2500 MTD urea
Thanks to the present invention, it is possible to increase the conversion yield of 6 points percentage and to increase the capacity of 600 MTD urea, i.e. 32% more than the original capacity.
Also in this case, only minor modifications to the pre¬
existing plant and low investment costs are required to
obtain a relevant increase in the capacity and the
conversion yield.
EXAMPLE 3
In this example, the convertion yield of a reactor operating in a brand new plant implementing the process according to the present invention as described in FIG. 2 has been simulated.
As for the above example 2, the plant is based on the carbon dioxide stripping process, wherein all the carbon

dioxide to be fed to the rea The operation conditions of the reactor in the urea production plant according to the present invention are the followings:
• molar ratio NH3/CO2 at input: 3.2;
• molar ratio H2O/CO2 at input: 0.1;
• pressure: about 150 bar a;
• temperature: 190oC;
• capacity: 400 MTD urea
The conversion yield of the CO2 into urea obtained by the present reactor is very high: 72% . Moreover, the content of water in the carbamate solution recyoled to the reactor is particularly low.
Since low amounts of water and unreacted substances are contained in the reaction mixture sent to the decomposition unit and further to the recovery section, it results that the duties of the process equipment are lower if compared to the conventional plants and consequently also the energy consumption and the investment coste.
The results given in the above examples have been obtained by means for well known calculation a l gor Ithms.

WE CLAIM:
1. A process for urea production comprising the steps of:
(1) performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution;
(2) 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 comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution;
(3) subjecting said first flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqeuous solution;
(4) recycling said first portion of carbamate to said reaction space;
(5) feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section;
(6) separating in said recovery section said residual carbamate fix>m the urea to obtain a second portion of carbamate in aqueous solution;
(7) subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a treatment of partial decomposition to obtain a second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising residual carbamate in aqueous solution;

(8) subjecting said second flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution;
(9) recycling said third portion of carbamate to said reaction space and recovering urea from the urea recovery section in a known manner.

2. The process as claimed in claim 1, wherein the treatment of partial decomposition of the said at least part of the second portion of carbamate in aqueous solution is carried out at a pressure substantially corresponding to the pressure in the reaction space.
3. The process as claimed in claim 1, wherein it further comprises the step of:
feeding the flow comprising residual carbamate in aqueous solution resulting from the treatment of partial decomposition of the second portion of carbamate to said urea recovery section.
4. The process as claimed in claim 1, wherein it comprises the steps of:
feeding the reaction mixture comprising urea, carbamate and free ammonia in aqueous solution in step (1) to a decomposition unit.

- feeding said at least part of the second portion of carbamate in aqueous solution to said decomposition unit,
- wherein the treatment of partial decomposition of the reaction mixture and of the second portion of carbamate is carried out in the same decomposition unit to obtain said first and second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution.

5. The process as claimed in claim 1, wherein at least 50% of said second portion of carbamate in aqueous solution is subjected to the treatment of partial decomposition.
6. The process as claimed in claim 5, wherein at least 65% of said second portion of carbamate in aqueous solution is subjected to the treatment of partial decomposition.
7. A plant for urea production by the process claimed in claim 1 comprising:
- a urea synthesis reactor (1);

- a first stripping unit (2) for subjecting a reaction mixture leaving said reactor (1) to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture;
- means (6) for condensing at least partially the vapors leaving said first stripping unit (2) and of recycling (24) a first portion of carbamate in aqueous solution to said reactor (1);
- a recovery section (3,4,7,8) of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit (2) for separating the urea produced in the reactor (1) from a second portion of carbamate in aqueous solution;
- a second stripping unit (9) for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition and
- means for condensing at least partially the vapors leaving said second stripping unit (9) and of recycling a third portion of carbamate in aqueous solution to said reaction (1).
8. The plant as claimed in claim 7, wherein said means for condensing the vapors leaving said second stripping unit (9) comprises said means (6) for condensing the vapors leaving said first stripping unit (2).

9. The plant as claimed in claim 7,wherein means (33) for feeding a flow comprising residual carbamate in aqueous solution from said second stripping unit (9) to said recovery section (3,4,7,8) is provided.
10. The plant for urea production as claimed in claim 7, wherein means for feeding (26) at least part of the second portion of carbamate in aqueous solution is provided.
11. A process for urea production substantially as hereinabove described and illustrated with reference to figures 2 and 3 of the accompanying drawings.
12. A plant for urea production substantially as hereinabove described and illustrated with reference to figures 2 and 3 of the accompanying drawings.

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Patent Number

191952

Indian Patent Application Number

981/MAS/1995

PG Journal Number

30/2009

Publication Date

24-Jul-2009

Grant Date

19-Aug-2004

Date of Filing

01-Aug-1995

Name of Patentee

M/S. UREA CASALE S.A

Applicant Address

VIA SORENGO 7, CH-6900 LUGANO-BESSO.

Inventors:

#	Inventor's Name	Inventor's Address
1	GIORGIO PAGANI,	VIALE FAENZA 26/6, I-20142 MILANO.
2	UMBERTO ZARDI,	VIA LUCINO 57; CH-6932 BREGANZONA,

PCT International Classification Number

C07C126/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA