Process for the production of urea by steps with differentiated yields, and relevant implementations also on existing plants

Abstract: A process is described for the industrial synthesis of urea, making the ammonia (NH3) and the carbon dioxide (CO2) react, in at least one reaction space, at high pressures and temperatures and recycling at least in part the unreacted products obtained in a recycle section, characterized by: a) a synthesis reaction between reactants of high purity; and b) a synthesis reaction between less pure reactants, substantially recycled by the so-called recycle section. The corresponding new plant includes a reactor (R1) of high yield ("once through"), a reactor (R2) of lower yield and a section of recovery and recycle. The application of the process to preexisting plants requires the simple addition of a reactor of high yield and of pumping devices.

Description: The present invention concerns a process for the industrial synthesis of urea, making ammonia and carbon dioxide react in at least one reaction space, at high pressure and temperature, and recycling at least partially the unreacted products. The invention concerns also the applications of this process to conventional systems in order to improve the urea yield and reduce energy consumptions. New or modified plants obtained by the implementations of the above mentioned process and its applications to pre-existing systems, are within the scope of the invention. Known are several types of processes, systems and plants for the industrial synthesis of urea from NH₃ (in excess) and CO₂, with recycling after stripping. In order to have immediately clear ideas referring to emblematic cases, one among the most stated processes is the one based on the technology (Snamprogetti) of isobaric stripping with the synthesis reactor (stripping with NH₃, selfstripping), (fig. 1). In the stripper (S) a great part of the carbamate included in the urea solution coming out of the reactor (R) and part of the free NH₃ present are stripped and recycled into the reactor, whilst a urea solution (SU) coming from the stripper (S) having a relatively low content of residual CO₂ (5÷7 % weight) and obtaining a relatively high content of NH₃ (22÷25% weight). This solution (SU) is treated in a middle pressure phase (SMP) where it is distilled at 18÷20 bar and the obtained vapours are sent to a rectification column (CR) enabling to obtain, first, NH₃ at high purity (NEP) and then carbamate solution (SC). The NH₃ at high purity, after condensation, joins the NH₃ feed (NA) and is pumped (pump P) into the reactor (R), and the same thing happens with the carbamate solution (pump P'). The main technical characteristics of the isobaric stripping process (Snamprogetti) can be resumed as follows : synthesis pressure : approx. 150 bar NH₃/CO₂ mol in the reactor : approx. 3.2÷3.4 H₂O/CO₂ mol in the reactor : approx. 0.6÷0.7 temperature of the reactor : 190 °C yield : approx. 62÷63% vapour consumption : approx. 900 kg/MT urea The above indicated values are rather consolidated and drastic improvements of the process do not seem to be possible. Among the other processes with separate recycle of NH₃ widely used in the past can be mentioned the ones of Toyo Engineering Ltd. indicated with the abbreviation TRC - TRC-I (Total Recycle C, TRC Improved), characterized by the fact that the ratio NH₃/CO₂ in the reactor is equal to approx. 4÷4.5 mol and the separation of NH₃ at 18÷20 bar is more impressive than the previous process (Snamprogetti), also for the lack of a isobaric stripping. Another generation of processes has been recently developed for increasing the yield in the reactor of the so called stripping processes (Snamprogetti NH₃ selfstripping and Stamicarbon CO₂ stripping), of the conventional values 55÷65% up to 70÷75%, providing at the same time, the above mentioned high yield reactors with isobaric loops. Reference is made to the new and more recent processes as "IDR" (Isobaric Double Recycle) of Montedison and "ACES" of Toyo Engineering Ltd. In the above mentioned recent processes "IDR" and "ACES", still operating with high ratios NH₃/CO₂ in the high yield reactor, all the unaltered reactants, including the high excess of NH₃, are recycled to the reactor under the form of carbamate solution (the separate recycling of NH₃ does not exist). Without doubt much better yields are obtained (compared with the very first processes), but these advantages are coupled with a high complexity and complications in the construction of the equipment. Furthermore these processes have the drawback of having to strip the unreacted products under high temperture conditions with real risks of decomposition of the (laboriously) developed urea. The first aim of the present invention is to provide a process without the above mentioned drawbacks, which can be operated easily, with total high yields and low consumptions. Another aim of the invention is the applicability of this process to the conventional systems already having recycling sections for ammonia (or adapted with a new recycling section of the ammonia), reaching global yields of urea transformation in the synthesis sections equal or higher to the ones obtained with the modern processes "IDR", "ACES", etc. A further aim of the invention is represented by the simple and effective plants (new or modified) resulting from the implementation of the said process. These and other aims are achieved, according to the invention, by a process as defined in attached claim 1. Further characteristics of the process are deducible from the following claims 2 to 7. The application of this process to conventional systems with recycling of ammonia is characterized by the simple addition of one high yield reactor as well as the devices for the additional pumping of highly pure reactants to a further reaction space. The plant for the implementation (particularly ex-novo) of the process according to the invention comprises: a first high-yield urea synthesis reactor; means for feeding high purity ammonia and carbon dioxide to said first reactor; a second urea synthesis reactor of yield lower than said first reactor in parallel with said first reactor; a recovery section for separating urea produced in the first and the second urea synthesis reactors from an aqueous solution of unreacted products; conduit means for connecting said first and second urea synthesis reactors with the recovery section; conduit means for recycling the aqueous solution of unreacted products from the recovery section to said second urea synthesis reactor; conduit means for recycling high-purity ammonia recovered in said section to the first urea synthesis reactor. The different aspects and advantages of the invention are seen better in the description of its preferred embodiment illustrated in scheme of fig. 2. In fig. 2 is represented a form of simple and effective and therefore advantageous embodiment of the process according to the invention in its more significant conceptual features. These derive from the fact that the process can be represented schematically as essentially constituting of three sections, two of reaction S1 and S2 of differentiated yield and the last one of recycling SE-RI (S3). S1 includes the reactor R1 fed, characteristically, only (or substantially only) with pure reactants CO₂ (line 1) and NH₃ (line 2). Receiving only reactants of high purity, R1 is to be considered immediately as reactor of high yield (HY). The mixture M1 of the reaction products in R1 (HY), made up substantially of urea U1, of the carbamate solution SC1 and the unreacted compounds UN1 (principally NH₃ fed in excess from line 2'), is brought from line 3 to recycling section SE-RI, to which is made flow into, through line 4, the same mixture M2 of the products from the reaction in the second reactor R2, mixture made up of urea U2, of carbamate solution SC2 and of unreacted compounds UN2 (principally NH₃ fed in excess). From the said recycling section SE-RI, outflow : 1) on line 5, all the produced urea (U) resulting from the partial ones (U1) and (U2) obtained in the two respective reactors R1 and R2; 2) on line 6, ammonia of high purity (NEP) which, according to a feature of the invention, is recycled again only into reactor R1 together with NH₃ fed from the outside (line 2'), having NEP (6) purity substantially compatible with the last one (NA); c) on line 7, the aqueous effluent SC = Eff. (H₂O+NH₃+CO₂), i.e. the mixture consisting in the carbamate aqueous solution and NH₃ in excess. Characteristically, the last aqueous effluent (SC) rich in NH₃ of low purity grade and containing CO₂, is recycled, through line 7, only into reactor R2 which, receiving reactants less pure than the ones fed into R1, has an average yield (MY), i.e. lower than that of R1 (HY). R2 received fresh CO₂ from feeding 1 on line 1' and eventually additional quantity of fresh NH₃ on line 2". The applications of the process, according to the invention, to the conventional systems with separate recycling of NH₃ in order to improve substantially the performances are of particular importance. As a not limitative example, we describe in fig. 3 the application to the selfstripping process NH₃ of Snamprogetti. Such a process, as most of the urea processes used actually on industrial scale, is of total recycling type, which means that unreacted NH₃ and CO₂ coming out of the reactor (or out of isobaric stripper) are recycled to the reactor in form of aqueous solution. The presence of recycled H₂O in the reactor reduces the yield of carbamate conversion into urea penalizing the capacity and the consumption of the plant. On the other side, it is known that feeding a synthesis reactor with only pure reactants (NH₃ and CO₂) we can obtain very high yields. For example in the process known as "Vulcan" (now abandoned) the following results are reached : NH₃/CO₂ in the reactor : 4.5 H₂O/CO₂ in the reactor : 0 P : 400 bar T : 215 °C yield : 80% Volume of reactor : 0.05 m³/td urea To be noted that above mentioned values have been reached (and still can be reached) for years on industrial scale in synthesis reactor lined with zirconium, against corrosion. This system has been abandoned because the recovery of residual reactants not transformed in the reactor was complex and uneconomical, by means of a selective chemical separation in order to obtain NH₃ of high purity to recycle into the reactor. According to the present invention (see fig. 3), the excess of separated ammonia (NEP) in the recycling sections of the Snamprogetti process (or in processes Toyo TRC and TRC-I), together with fresh feeding ammonia (NA), is fed (NA + NEP) into the reactor (R1) of very high yield "once through" for example of the "Vulcan" type operating with pressures above 300 bar and temperature above 190°C, where it is transformed into urea together with part of the fresh CO₂ (CA) fed in the stoichiometric quantity requested for reaching the desired ratio NH₃/CO₂ higher than 4, having now the conventional reactor (R2) foreseen in the Snamprogetti scheme (and operating with pressure lower than 300 bar and at temperature lower or equal to 190 °C), the function of transforming into urea the only carbamate recycle (SC) together with the remaining part of fresh CO₂ (CA') fed into said reactor in such quantity as to maintain in it ratios NH₃/CO₂ lower than 4. In a preferred embodiment form, the new plant for obtaining the transformation of reactants into urea with very high yields includes two reactors in parallel, the first of which works as "one through" (without recycles) at high pressure (from 250 to 450 bar) and temperatures between 200 - 230 °C, fed substantially with ammonia and pure CO₂, and the second reactor working at lower pressure, for example 130 - 200 bar and lower temperatures, for example 180 - 200 °C (better if lower than 190 °C) coupled with a total recycle system and consequently functioning with feeding of part of the fresh reactants not sent into the first reactor and recuperating all the recycle of the total recycle system. In particular, the solution of urea (SU) coming out of the reactor (R1) is preferably expanded in the separator (SEP) operating at reactor pressure (R2) and the freed flash vapours (VF) are sent into the reactor (R2), whilst the solution of flash urea (SUF) is sent into the section at medium pressure (SMP) for the recovery of the unreacted substances (NH₃ and CO₂). With this system, in the above mentioned reactors (R1 respectively R2) yields of transformation into urea are reached, in the first one superior from 75 to 85% and in the second one from 60 to 70%; therefore the yield of combined transformation of the synthesis system is very high depending from the optimal distribution of load between the two reactors. As already anticipated, the advantage of the process according to the invention is that it can be conveniently used for the realization of new production plants of high yields and low energy consumptions, well as for the improvement of urea processes with separate recycle of NH₃ for example of Snamprogetti type (fig. 3) or Toyo Engineering Ltd. As already mentioned, a plant modernized by the application of the invention to conventional systems is characterized by the addition of one reactor of high yield R1 and of the devices for bringing the reactants to the higher operative conditions of R1 compared with the old reactor R2. A new plant is obtained simply by the implementation of the process scheme according to fig. 2. EXAMPLE 1 An example referred to the Snamprogetti process (see scheme of fig. 3) is given hereunder: the recycle NH₃ of high purity (NEP) which is pumped into the reactor is 0.4* , times the stoichiometry feeding NH₃ (NA). Feeding with total NH₃ (NA + NEP) the parallel reactor R1 (high efficiency parallel converter HY) type "once through" proposed by the present invention, we have : available NH₃ : ≈1.4x570 = 798 kg/t urea = 46.94 kmol/t urea NH₃/CO₂ : 4.5 mol necessary CO₂ : 46.94 / 4.5 = 10.43 Kmol/t = 459 kg/t H₂O/CO₂ : 0 With an obtainable yield of 80%, the CO₂ transformed into urea is : 459x0.8 = 367.2 kg/t = 367.2 60 44 = ≈ 500 kg urea/t urea It is therefore possible to produce approx. 50% of the urea in reactor R1, i.e. in high yield conditions. absolutely not limitative of the invention, an average value of 0,4 has been presumed. A Snamprogetti plant with reactor in parallel of high yield (HY) will therefore operate with an average yield equal to approx. 63 + 80 / 2 = 71.5% and the vapour consumptions will consequently be notably reduced (550-600 kg/t urea against the conventional value of 900 kg/t). It can be worth mentioning that in the US patent No. 4,670,588 a process with two reaction zones or reactors in series is described, the philosophy of which is however completely different from the one of the present invention. Also in the "IDR" it is possible to utilize reactors with two isobaric zones in series functioning with different molar ratios of the reactants.

Claims: A process of producing urea comprising the steps of reacting high purity ammonia and carbon dioxide in a first reaction space (S1) at high temperature and pressure to produce a first reaction mixture (M1), withdrawing said first reaction mixture (M1) and feeding it into a urea recovery section (SE-RI), characterized in that it further comprises the steps of: a) recovering urea (U) and a mixture (SC) including an aqueous carbamate solution from said urea recovery section (SE-RI); b) introducing said mixture (SC) into a second reaction space (52) and reacting it therein to produce a second reaction mixture (M2); c) withdrawing said second reaction mixture (M2) from the second reaction space (S2) and recycling it to the urea recovery section (SE-RI); d) recovering high purity ammonia (NEP) from the recovery section (SE-RI) and recycling it to said first (S1) reaction space. A process according to claim 1, characterized in that the step of urea synthesis in the first reaction space (S1) accounts for 80 to 20% of the total urea produced. A process according to claim 1, characterized in that it further comprises the step of feeding high purity carbon dioxide and optionally high purity ammonia to said second reaction space (S2). A process according to claim 1, characterized in that the step of urea synthesis in the second reaction space (S2) accounts for 20 to 80% of the total urea produced. A process according to claim 1, characterized in that urea synthesis in the first reaction space (S1) takes place in substantial absence of recycle water. A process according to anyone of the preceding claims, characterized in that urea synthesis in the first reaction space (S1) is carried out at pressures of from 250 to 450 bar and at temperatures of from 200 to 230°C. A process according to anyone of the preceding claims, characterized in that urea synthesis in the second reaction space (S2) is carried out at pressures of from 130 to 200 bar and at temperatures of from 180 to 200°C. A urea production plant, comprising: a first high-yield urea synthesis reactor (R1); means for feeding high purity ammonia and carbon dioxide to said first reactor (R1); a second urea synthesis reactor (R2) of yield lower than said first reactor (R1) in parallel with said first reactor (R1); a recovery section (SE-RI) for separating urea produced in the first (R1) and the second (R2) urea synthesis reactors from an aqueous solution (SC) of unreacted products; conduit means (3, 4) for connecting said first (R1) and second (R2) urea synthesis reactors with the recovery section (SE-RI); conduit means (7) for recycling the aqueous solution (SC) of unreacted products from the recovery section (SE-RI) to said second urea synthesis reactor (R2); conduit means (6) for recycling high-purity ammonia (NEP) recovered in said section (SE-RI) to the first urea synthesis reactor (R1). A plant according to claim 8, characterized in that it further comprises conduit means (2'') for feeding high purity carbon dioxide to said second reactor (R2). A plant according to claim 8, characterized in that said first urea synthesis reactor (R1) is of the "once through" high-yield type. A method of retrofitting a pre-existing plant for urea production including a reactor (R2) for urea synthesis in fluid communication with a recovery section (SE-RI) for separating urea from an aqueous solution (SC) of unreacted products leaving the reactor (R2), comprising the steps of: a) providing a further reactor (R1) for urea synthesis upstream of said recovery section (SE-RI); b) connecting said further reactor (R1) with said recovery section (SE-RI) and with means for feeding high purity ammonia and carbon dioxide; c) providing conduit means (7) between said recovery section (SE-RI) and the reactor (R2) for feeding thereto said solution (SC) of unreacted products; d) providing conduit means (6) between said recovery section (SE-RI) and said further reactor (R1) for recycling thereto high-purity recovered ammonia (NEP).

Inventors: PAGANI GIORGIO, PAGANI, GIORGIO
Applicants: UREA CASALE SA, UREA CASALE S.A.
Application year: 1991
Main classification: C07C273
IPC codes: C07C273