Process for urea production with differentiated yield reaction spaces

Abstract: Process for the industrial synthesis of urea, by reacting ammonia (NH3) and carbon dioxide (CO2) in at least one reaction space, at high pressures and temperatures and by recirculating at least part of the non-reacted products obtained in a recovery section, characterized by the fact that the following takes place : a synthesis reaction A) between highly pure reagents, and a synthesis reaction B) between less pure reagents, substantially recycled from the said recovery section, the reaction A) being either of adiabatic type A1) or with partial reaction heat removal A2).

Description: This invention concerns a process for the industrial production of urea by reacting ammonia (NH 3 ) and carbon dioxide (CO 2 ) in a synthesis section consisting of many differentiated yield reaction spaces, in one of these spaces a majority part of the total conversion occurs by feeding it with highly pure reagents, and in the other of these spaces the remaining minority part of the conversion occurs by feeding it with less pure reagents that have been substantially recirculated by a recovery section. A method for the synthesis of urea of the type set forth in the introduction is described in European patent application EP 0 479 103 corresponding to Swiss patent application No. 03216/90-1 deposited on October 3rd, 1990, by the same Applicant. The plant for the carrying out of this process comprises: a first high-yield reactor fed with CO 2 and fresh NH 3 from the outside and with very pure recovery NH 3 ; a second reactor, parallel with the first, with a yield lower than the first and substantially fed with reagents from the recovery mixture; and a system or recovery section for the recovery of reaction mixtures obtained from said first and second reactors. In European patent application EP 0 497 215, which is relevant only as to the novelty of the present invention, the Applicant has described an embodiment of the process according to said first patent application, which lends to a particularly efficient and advantageous result, because of the small investment and minimum consumption of energy it requires. Characteristically, the majority synthesis reaction stage (A) with high-yield, between highly pure reagents, operating at a higher pressure (Pmax), for instance, above 300 bar abs and preferably around 400 bar abs, is followed by a flash stage F1 operating at pressures lower than approximately 200 bar abs, the gaseous effluent GF1 of the above mentioned flash stage F1 being fed to a minority synthesis reaction stage B with less pure reagents operating at a pressure lower than 200 bar abs, while the liquid effluent EL1 of the above mentioned flash stage, together with the effluent EB from minority reaction stage B operating in parallel to the majority reaction stage A, are fed to a recovery section RE consisting of two decomposition stages D-1 and D-2 operating in series: the first D-1 being lower than 100 bar abs preferably at 50 bar abs; the second D-2 working at a pressure lower than 50 bar abs preferably at 20 bar abs. Each decomposition stage consists of a decomposer D-1 and D-2 respectively (heat exchangers for the distillation of reagents not transformed into urea), whose gas effluents, consisting of NH 3 +CO 2 +H 2 O, feed a condensation system with direct heat recovery from the process, where the partial condensation of said effluents is carried out and then completed in a fractionating column with a head condenser. Additionally, a process for the synthesis of urea wherein NH 3 and CO 2 are reacted in a synthesis space implying A) a majority synthesis reaction between highly pure reagents and B) a minority synthesis reaction between less pure reagents and a decomposition stage between A) and B) has also been disclosed by DE-A-1 643 092. In the uninterrupted research and experimentation in this important technical field, the Applicant had succeeded, not without surprise, to perfect an articulated and flexible process that will cope, with a particularly efficient and advantageous embodiment, with the most varied and frequent requirements by minimizing each time the investment costs and/or energy consumptions, and by maximizing the yields by adopting the most appropriate operating conditions according to the capacity of the plant. These objectives are reached by a process according to claim 1 attached hereunder. The various aspects and advantages of the invention will be better illustrated by the following description of one of the possible embodiments with heat removal, (preferred, but not limitative), shown in the attached drawing, in which Figure 1 is a process flow diagram. Differentiated Yield HEC (High Efficiency Combined) Process This new process has the advantage of requiring low energy and inexpensive recycle units. Moreover, the type of reactor for the reaction A) (a "once-through" type) has a high efficiency, great reliability, is substantially corrosion free, and requires low residence time i.e. small dimensions. The plant for carrying out the process consists of the following sections: a) synthesis section with two reactors (R-1, R-2) in parallel; b) a medium pressure recycling section with one decomposition stage surmounted by an ammonia fractionating system to produce a purified urea solution, a carbamate solution and pure ammonia streams that are sent respectively to the secondary converter (R-2) (carbamate solution), and to the main converter (NH 3 ); c) concentration section of the solution and for finishing. Heat Removal Process (Figure 1) The main characteristic of the process is that the heat developed by the reaction is utilized to bring the reagents to the reaction temperature. The majority reaction A) is carried out in the following conditions: NH 3 /CO 2 molar ratio : 3.5 - 4 H 2 O/CO 2 molar ratio : 0 CO 2 conversion : 80% NH 3 inlet temperature : about 100°C CO 2 inlet temperature : about 150°C Reaction temperature : 195°C Pressure : 240 bar abs Reactor conversion : 75% Reactor lining : AISI 316 L.U.G. The urea solution from reactor R-1 undergoes flash from 240 to 150 bar in separator D-1. The vapours produced in flash in D-1 (substantially ammonia) are sent to R-2, while the urea solution in D-1 is mixed with the solution from reactor R-2 and feeds the medium pressure decomposer E-1 downstream of D-2. The operating conditions of the secondary reactor (R-2) are: NH 3 /CO 2 molar ratio : 4.5 H 2 O/CO 2 molar ratio : 1.3 CO 2 conversion yield : 60% Pressure : 150 bar abs Temperature : 190°C 75% of the urea production is obtained in R-1, whereas the remaining 25% is obtained in R-2. The weighed average efficiency of the two reactors is close to 71.5%, that is very high in comparison to the efficiency of very recent processes. Characteristically, the CO 2 and fresh ammonia are fed by a medium pressure decomposer-prereactor E-1 tube side, in which ammonium carbamate and urea are formed. E-1 is part of the majority reaction stage A2) with R-1. A part of the reaction heat is removed from the urea solution that circulates outside said tubes. Advantageously, this heat has a high thermal level (about 170°C) that is exploited for the distillation of the same urea solution in decomposer E-1 operating at 18 bar that can be of the falling-film type or up-flow type ("process-to-process heat exchange"). The carbamate/urea solution in the pre-reactor E-1 is sent to reactor R-1, in which the majority part of the carbamate is dehydrated to urea. Characteristically, the control of the heat developed in E-1 is carried out regulating the temperature of liquid ammonia through E-9, while the temperature of R-1 is maintained by-passing, if necessary, the CO 2 to E-1. The auxiliary reactor R-2 is fed with flash vapours and with the recycled carbamate solution, as well as with the vapours coming from the high pressure decomposer E-2, preferably of falling-film type, wherein the heat is given by means of medium pressure steam with the aim of performing the thermal balance of reactor R-2 and to distill the solution coming from R-2. The urea solution leaving D-2 is then distilled in decomposer E-6 operating at 3.5 bar. The urea solution then passes in the vacuum section where it is concentrated up to 96% w in the first evaporator E-3 that operates at 0.35 bar, and then up to 99.7% w in the second evaporator E-4 operating at 0.05 bar. The vapours obtained in D-2 are partially condensed in the first part of the first evaporator E-3A, in which a part of the process heat is recovered (with a double-effect system), and are sent to the fractionating column C-1. In the latter pratically all the CO 2 and H 2 O vapours are condensed as carbamate solution and sent to reactor R-2. Pure NH 3 at the head of column C-1 is condensed in E-7 and the liquid ammonia obtained is used, together with the fresh NH 3 feed, as reflux for column C-1 and feeds pre-reactor E-1. The quantity of NH 3 reflux is determined by the thermal balance of column C-1. Consumptions The specific consumptions, referring to 1000 kg of urea, are: Liquid NH 3 at 32°C, 18 bar (kg) : 568 CO 2 (kg) : 734 Steam at 25 bar (kg) : 400 Electric power (kWh) : 115 Example Described hereunder, with a demonstrative aim, but not limitative, is the application of the invention to modernize existing Weatherly type plants. These plants, developed in the U.S.A. in the years 1960 - 1970, are characterized by the fact that the synthesis reactor is fed with pure reagents (NH 3 and CO 2 ) without recycle water, which has enabled these plants to obtain very high conversion yields of CO 2 in urea (75%). The operating data of these Weatherly reactors are: - NH 3 /CO 2 molar ratio : 4 - H 2 O/CO 2 molar ratio : 0 - yield : 75% - pressure : 260 bar abs - temperature : 195°C The process of this invention also provides a simple, inexpensive, efficient and convenient extension of its European application EP 0 479 103 to the existing urea synthesis plant of the "once through" type, mainly Weatherly processes. Particularly, the Weatherly synthesis section of these plants represents the majority portion described herein. In fact, it has been found that it is surprisingly possible to revamp, in a simple and safe manner, using this invention's processes as described and claimed, existing urea production plants of the "once through" type. These "once through" plants present the following main disadvantages : partial utilization of the CO 2 fed to the reactor, connected to the yield of the reactor itself (in the Weatherly reactor, with a yield equal to 75%, only 75% of the fed CO 2 is converted into urea), whereas in total recycle plants the utilization CO 2 yield is equal to approximately 100%; very high energy consumptions in the case of residual CO 2 recovery through the use of an absorbing solution as the MEA type; very little operating flexibility, the urea plant's run being bound to the utilization of gaseous effluents (vapours rich in NH 3 ) in other plants. Among the main advantages of the process according to the invention, the following are mentioned: 1) High urea yield in the synthesis section with consequent recycle sections downstream simple and inexpensive. 2) Elimination of all critical equipment subject to corrosion, used in modern stripping processes, such as strippers, high pressure carbamate condensers, scrubbers, etc., resulting in a longer life of the plant. 3) Very low carbamate recycle to the low pressure converter (with respect to conventional high pressure total recycle processes) and very small carbamate pumps. 4) Very low utility and energy consumptions. 5) Absence of recovery steam generation in the plant with consequently minor heat transfer surface requirements, and absence of recovery steam to be used elsewhere. 6) Reduced investment costs.

Claims: A process of producing urea in a plant including at least one synthesis space (SS) for reacting ammonia and carbon dioxide at high temperature and pressure and a recovery section for recovering unreacted reagents, comprising the steps of: (a) reacting highly pure ammonia and carbon dioxide in a first reaction stage (E-1, R-1) at a predermined pressure and temperature sufficient to carry out the reaction; (b) flash separating a product stream from the first reaction stage (E-1, R-1) into ammonia-containing vapours and a first liquid effluent including urea; (c) reacting the ammonia-containing vapours thus obtained and a carbamate solution recycled from the recovery section in a second reaction stage (R-2) at a pressure less than 200 kg/cm 2 and at a temperature sufficient to carry out the reaction so as to obtain a second liquid effluent including urea; (d) feeding the liquid effluents including urea from said first (E-1, R-1) and second (R-2) reaction stages to the recovery section, decomposing the liquid effluents in the recovery section and withdrawing a purified urea solution therefrom, wherein the highly pure ammonia and carbon dioxide are reacted in said first reaction stage (E-1, R-1) with partial removal of the reaction heat at a pressure less than 300 kg/cm 2 , at a temperature not higher than 200°C and at an ammonia/carbon dioxide ratio less than 4. A process according to claim 1, wherein the first reaction stage (E-1, R-1) operates at a pressure of 240 bar abs. A process according to claim 1, wherein the product stream from the first reaction stage (E-1, R-1) is flash-separated at a pressure equal to the synthesis pressure in the second reaction stage (R-2). A process according to claim 3, wherein said flash separation is carried out at a pressure of 150 bar. A process according to claim 1, wherein 75% of the urea is produced in the first reaction stage (E-1, R-1) and 25% of the urea is produced in the second reaction stage (R-2). A process according to claim 1, wherein the reaction between highly pure ammonia and carbon dioxide in said first reaction stage (E-1, R-1) is carried out in a decomposer-prereactor (E-1) and in a synthesis reactor (R-1) in series, the reaction heat being partially removed in said decomposer-prereactor (E-1). A process according to claim 6, wherein the liquid effluents including urea from said first (E-1, R-1) and second (R-2) reaction stages are distilled in said decomposer-prereactor (E-1) by means of the reaction heat removed therein from the urea solution that circulates outside the tubes of said decomposer-prereactor (E-1). A process according to claim 6, wherein the removal of the reaction heat in said decomposer-prereactor (E-1) is controlled by regulating the temperature of the ammonia stream recycled from the recovery section. A process according to claim 6, wherein the temperature of said first reaction stage (E-1, R-1) is maintained by by-passing the highly pure carbon dioxide to said decomposer-prereactor (E-1). A process according to claim 1, further comprising the step of recycling to the first reaction stage (E-1, R-1) an ammonia stream obtained by decomposing in the recovery section the liquid effluents from said first (E-1, R-1) and second (R-2) reaction stages. A process according to claim 1, further comprising the step of distilling the liquid effluent from the second reaction stage (R-2) in a high pressure decomposer (E-2). A process according to claim 11, wherein said distillation step is carried out by countercurrently contacting in the high pressure decomposer (E-2) the liquid effluent including urea from the second reaction stage (R-2) and the flash-separated ammonia-containing vapours from the first reaction stage (E-1, R-1).

Inventors: PAGANI GIORGIO, ZARDI UMBERTO, PAGANI, GIORGIO, ZARDI, UMBERTO
Applicants: UREA CASALE SA, UREA CASALE S.A.
Application year: 1992
Main classification: C07C273
IPC codes: C07C273