Process and plant for urea production

Abstract: The process to modernize existing urea plants which use a stripping with carbon dioxide, and to increase urea yields and flexibility under overload conditions while at the same time reducing energy consumption, corrosion phenomena and possible risks of explosive mixtures, comprises substantially a passivation stage with the introduction of an oxidising agent and reduction of the air fed to the system; a medium pressure distillation stage of the products leaving the stripping section, and a condensation of the products of said distillation, effected in a pre-evaporation phase to concentrate at low pressure the urea solution. The modernized plant, comprising at the start at least a reactor, a scrubber, a condenser, a stripper and the evaporators, now includes also a passivation section, a medium pressure distillation section, and a distillation section with double-effect technique.

Description: This invention concerns a process and plant for urea production, and more particularly to such a process and plant which allow to increase urea yields, reduce energy consumption, inhibit corrosion phenomena and make the operation flexible under overloading conditions, especially in those plants where the product leaving the reactor for the synthesis of urea from ammonia (NH 3 ) and carbon dioxide (CO 2 ) is stripped with one of the reagents, more particularly with CO 2 , the carbamate is condensed in a condenser and the vapours are treated at high pressure in a scrubber. As is well known, in the synthesis of urea aqueous solutions of urea are produced containing unreacted compounds and more particularly ammonium carbamate and NH 3 which must be treated downstream to decompose said carbamate and recover the urea solutions as well as the unreacted ammonia and CO 2 . More particularly, in the greater part of existing urea plants at least one stripping of the effluents from the reactor is carried out using as stripping agent at least one of the two reagents (CO 2 or NH 3 ). The major characteristics of the numerous plants of the first type (CO 2 ) can be summarized as follows : the flow of CO 2 being fed to the reactor is used as agent to remove said unreacted products; synthesis pressure is relatively low (for example, 140 bar); the major part of the residual carbamate in the stripper operating at the same pressure as the synthesis reactor is removed; a single carbamate recycle stage at low pressure, for example between 3.5 and 4.5 bar; consumption of medium pressure steam for the process of the order of 900÷1000 kg/MT of urea. In order to fix immediately ideas, Fig. 1 shows a scheme indicating the most important sections of said existing plants with CO 2 as stripping agent. R indicates the reactor fed through line 1 and the condenser CO.CA with NH 3 , and through line 2 and stripper ST with CO 2 . SCRU indicates the high-pressure scrubber, DECO is the decomposer fed through line 3 from stripper ST. By way of example, the most significant operating conditions can be summarized as follows : - molar ratio NH 3 /CO 2 in the reactor 2.8 - molar ratio H 2 O/CO 2 in the reactor 0.4 - conversion yield of CO 2 into urea in the reactor 57 % - temperature of reactor effluent 183°C - pressure in the reactor 141 bar - steam consumption at 20 bar 930 kg/MT urea for the stripper; 70 kg/MT for the second urea concentrator - exported steam 4.5 bar This process has the advantage of operating at not very high synthesis pressure, of having an efficient recycle of unreacted substances directly to the reactor and of requiring a reduced number of finishing stages, thanks to the efficiency of the stripping with CO 2 . To this simplicity of the process we find, by contrast, a not indifferent heat consumption (steam) and little flexibility in operation due to the presence of a single low pressure distillation stage before the vacuum section. This stage is operated with difficulty when the performance of the stripper gets worse (for example by overloading the stripper) and thus the amount of unreacted substance (NH 3 and CO 2 ) to be sent to the LP stage (DECO) increases. Besides, in order to inhibit corrosion phenomena in the stripper, in the carbamate condenser and in the high pressure scrubber, a relatively large amount of air (above 6000 ppm with reference to the CO 2 ) is added, for example to the feed CO 2 to be able to passivate the above equipment. This anomalous amount of air (and thus inert gas) makes it compulsory to operate with low NH 3 /CO 2 ratio in the reactor, with the result of low yields and relatively high steam consumption. The above problems get more acute when the plant must operate under overloaded conditions, hence the unsuitability of the process to operate under harder conditions than those planned for in the design. In European patent 0 096 151 a method is described to eliminate corrosion in the stripping equipment of urea plants operating at high temperature and pressure and in contact with an evaporating process fluid in the liquid state, such stripping being carried out in at least one stage and with at least a stripping agent chosen from CO 2 or NH 3 at a pressure between 120 and 240 bar; the method is characterized by the fact that for the protection of the strippers' metal surfaces a passivating system is used, injecting oxygen in synergic combination with a liquid agent or in a solution chosen from the group consisting of H 2 O 2 , nitrites (alcaline, alcaline-earthy, ammonic), persulphate (alcaline or ammonic), alcaline perborate, peracetic acid and organic peroxide. In the embodiment exemplified (applied to the protection of metal walls in austenitic stainless steel) the passivating combination results from putting together: O 2 from 500 to 2000 ppm (with respect to the fed CO 2 ) and H 2 O 2 in amounts correspoding to a content of active oxygen between 1 and 25 ppm, preferably 2 and 10, and even better between 4 and 6 ppm in respect of the global flow of the process fluid. Belgian patent 625.397 also describes a method for passivating the internal surfaces of a urea synthesis reactor using oxygen as the passivating agent and suggesting it be replaced at least partially with other passivating agents such as H 2 O 2 alcaline or alcaline-earthy peroxides. This method which seemed to have solved the problem of corrosion in reactors where the process fluid is substantially in the liquid state, has proved however insufficient for the strippers where corrosion reaches a very high level because said process fluid is present as an evaporating liquid and temperatures may be higher than those in the reactor. The present state of the technique seems to suffer from a few contradictions. The second prior art reference suggested a method which, although suitable for the reactor, was inadequate for the strippers. The first prior art document certainly solved this last problem but does not seem to cover entirely the whole system. This and other drawbacks of the Known Technique mentioned may be found in the fact that previously the problem of corrosion was dealt with on its own, isolating it from the rest of the problems and leaving unexplored the possibility of studying whether it could be solved in a wider context. In fact in the two previous patents efforts are made to solve only the problem of corrosion, admitting that there are gradients of the corrosion phenomenon for example going from its values in the reactor and other parts of equipment to the higher values in the stripper. To this diversity of position other difficulties are added: in effect the amounts of oxygen (even if combined with H 2 O 2 ) have been determined keeping in mind the requirements of passivation only and omitting to study whether those optimal figures where corrosion was concerned did not introduce secondary effects unhelpful where other characteristics of the process are concerned. In other words, in the state of the art the aim has been almost exclusively the elimination of corrosion, so that there seems to be a lack of teaching regarding the opportunity of exploring whether passivation measures may be considered flexibly within the framework of a system of modernization for the entire plant, and thus be co-ordinated with measures concerning yield, consumption, flexibility etcetera. Finally, EP 0 306 614 teaches to add - in order to reduce the steam consumption of urea plants including a carbon dioxide stripping step - a medium pressure distillation stage in which the distillation heat is provided by recovery heat/steam and condensing the vapor phase thereby obtained in a first vacuum distiller. According to this reference, furthermore, a pre-distiller must be added - with or without the medium pressure distillation stage - upstream of the carbon dioxide stripping stage. The object of this invention is to eliminate the above disadvantages, and more particularly to increase yields, reduce energy consumption, corrosion phenomena and the danger that explosive mixtures may form, as well as to make the process more flexible, as required by the plans for the modernization of urea production plants of this type. It has been found, not without surprise, that it is possible to eliminate the drawbacks mentioned above (such as low conversion yield, high energy consumption, corrosion phenomena, little operational flexibility) by means of a process and plant as defined in attached claims 1 and 9. According to the invention, this object has been accomplished by adopting few simple measures, and more particularly at least those concerning : the drastic reduction of the amount of passivating air, for example to about 1/3 of the design amount, compensating for the lack of oxygen preferably in the stripper and in the carbamate condenser by means of injecting small amounts of an oxidizing agent in the liquid phase upstream of the equipment to be passivated; the feed of H 2 O 2 and the resulting reduction of air are carried out in the PASS section. In pre-existing plants the passivating air is equal to amounts of O 2 between about 6000 and 8000 ppm referred to the feed CO 2 ; the increase of excess NH 3 in the reactor of between 2.8 mol and 3.4 mol, thanks to the reduction of inerts (air) and therefore increase of the reactor conversion yield; the addition of a medium pressure (MP) distillation stage capable of reducing the heat load of the upstream stripper (ST) and of the low-pressure downstream distiller (DECO.LP), with the resulting possibility of overloading these two pieces of equipment. In a particularly advantageous and therefore preferred embodiment of the invention, the new distiller DIS.MP is connected, according to the technique of double effect, to a new vacuum pre-distiller in which vapours MP condense with the consequent partial distillation of the urea solution. This combination makes it therefore possible to use the heat transferred to the new MP distillator twice, with an appreciable reduction of energy consumption. It can therefore be seen that by operating according to the invention it is possible to debottleneck in a surprisingly simple manner the key synthesis sections and those of distillation and recycle of the stripping process with CO 2 , making it technically workable and financially feasible to modernize this type of plant. Fig. 2 shows the flow sheet for the process and for the plant according to the invention. The top high pressure (H.P.) part, comprising the reactor (R), stripper (ST), carbamate condenser (CO.CA) and H.P. scrubber (SCRU) remain unchanged. According to a first aspect of the invention however their operating conditions change, for example as follows : the flow of passivating air, which in pre-existing plants varies between 6000 and 8000 ppm of O 2 with respect to the CO 2 , is now drastically reduced, for example to 1/3 of the design value (line 12); the NH 3 /CO 2 ratio in the reactor is therefore increased to for example 3.4 mol; the ratio H 2 O/CO 2 in the reactor is 0.5 mol; conversion yield of CO 2 into urea in the reactor is 72%; the stripper (ST), carbamate condenser (CO.CA) and H.P. scrubber (SCRU) are passivated for example with H 2 O 2 through lines 7, 8, 9. According to another aspect of the invention, a medium pressure distillation stage (DIS.MP) is introduced which is fed through line 3' from stripper (ST). Given the presence of this new distiller (DIS.MP), operating for example at about 18÷22 bar, the urea solution leaving the stripper (ST) will have a greater amount of carbamate and NH 3 than the design value. Thanks to this fact, together with the increased yield in the reactor, the stripper's (ST) (and the carbamate condenser's CO.CA) heat load can be reduced appreciably with the possibility, therefore, of overloading them as compared to the design load. The urea solution (SU) leaving the stripper (ST) is distilled in the new MP distiller (DIS.MP) up to a residual amount of carbamate and NH 3 lower than the design amount. This in turn reduces the heat load of the subsequent stage (DECO.LP) of distillation L.P., thus allowing an increased load in respect of the design value. According to a further advantage of the invention the vapours leaving the new distiller (DIS.MP) (consisting of NH 3 , CO 2 , H 2 O) are condensed, according to the double-effect technique, in a new pre-evaporator (Pr-EV) situated in series with the existing evaporator (EV1) of the 1st vacuum stage, operating at for example 0.35 bar a. In this way the condensation heat of the vapours mentioned (Vu) is used to concentrate the urea solution (SU) permitting a considerable heat recovery, while at the same time reducing the heat load in the existing evaporator (EV1). This last fact makes it therefore possible not only to open up the space for a possible overloading of the existing evaporator EV1 compared to design conditions, but also to "force" the degree of distillation of the solution so that the heat load of the second existing vacuum evaporator (EV2) can be made lighter thus allowing it to be overloaded in respect of its design value. The condensate obtained in the pre-evaporator (Pr-EV) is then recycled to the H.P. scrubber (SCRU) by means of lines 5 and 6. The importance of the invention in connection with stripping plants with CO 2 becomes therefore evident, especially where the increase in capacity is concerned. As regards the passivation stage or system it is worth noting that it can be effected according to the known technique, and more particularly according to the method described in European patent 0096151. In the overall view according to the invention it has been ascertained, for example, that if as the determining parameter for the purpose of an optimal treatment the residence time of the process products in contact with the surfaces of the equipment is assumed, passivation can be effected in the most selective and efficient manner. For example, where said residence times are below about 30 seconds as in the stripper, the amount of H 2 O 2 must be kept in the high area whereas the O 2 must be kept in the low area of the preferred respective intervals: in this way with reference to preferred intervals according to the European patent above-mentioned, i.e. for the H 2 O 2 between 4 and 6 ppm and O 2 between 500 and 2000, we have seen that for residence times for example shorter than 30 seconds the H 2 O 2 may be kept at 4 ppm and the O 2 may actually be lower than the bottom limit of 500 ppm. On the other hand, where residence times are greater, as for example at 30 seconds, H 2 O 2 may be made lower than the lowest value and be kept for example between 0 and 2 ppm and O 2 is above 2000. At the limit, the sinergic mixture H 2 O 2 +O 2 (air) may be introduced where there is permanence and in that case O 2 will not be, preferably, above 500 ppm. Where, on the contrary, the amount of H 2 O 2 is lowered (or even eliminated) a certain amount of O 2 may be introduced above 2000, or even critically equal to 2500 ppm. These indications apply above all to those plants where stripping is effected with synthesis CO 2 , i.e. feeding air to the reactor with the compressor for the synthesis CO 2 , first through the stripper and then to the reactor. If self-stripping is carried out, i.e. the feed CO 2 does not run through the stripper but goes directly to the reactor, the amounts of O 2 and/or H 2 O 2 may be selected differently.

Claims: A process for producing urea comprising the steps of: reacting ammonia and carbon dioxide within a synthesis reactor (R); stripping the urea effluent leaving the reactor (R) with carbon dioxide in a stripper (ST), thereby removing from said urea effluent residual carbamate and unreacted products; condensing in a condenser (CO.CA) carbamate out of an ammonia feed stream for the reactor (R); treating vapors leaving the reactor (R) in a scrubber (SCRU); adding air to a carbon dioxide feed to the reactor (R) for passivating said stripper (ST), scrubber (SCRU) and condenser (CO.CA); characterized in that it comprises the steps of: introducing an oxidizing agent in a liquid phase upstream of the stripper (ST) and condenser (CO.CA); increasing the molar ratio of ammonia to carbon dioxide within the reactor (R) while reducing the amount of said passivating air. A process according to claim 1, characterized in that the amount of passivating air is reduced in the stripper (ST), scrubber (SCRU) and condenser (CO.CA). A process according to claim 1, characterized in that the amount of passivating air is reduced by about 1/3. A process according to claim 1, characterized in that the oxidizing agent in liquid phase comprises hydrogen peroxide. A process according to claim 1, characterized in that the molar ratio of ammonia to carbon dioxide is increased within the reactor (R) to produce an ammonia excess of between 2.8 to 3.4 mol in the reactor (R). A process according to claim 1, characterized in that it further comprises the steps of: distilling the urea solution leaving the stripper (ST) in a medium pressure distiller (DIS.MP); condensing the vapours leaving the mean pressure distiller (DIS.MP) in a pre-evaporator (Pr.EV) for the concentration of the urea solution at low pressure. A process according to claim 6, characterized in that the urea solution leaving the stripper (ST) is distilled at a pressure of from 6 to 50 bar. A process according to claim 6, characterized in that the vapors leaving the mean pressure distiller (DIS.MP) are condensed in the pre-evaporator (Pr-EV) at 0.35 bar. A plant for the preparation of urea, comprising: a) a high-pressure loop including: i) a reactor (R) for synthesizing urea from ammonia and carbon dioxide; ii) a stripper (ST) for stripping product leaving the reactor (R) with a reagent; iii) a carbamate condenser (CO.CA); and iv) a scrubber (SCRU) for treating vapors leaving the reactor (R); b) a low-pressure loop, comprising; i) a decomposer (DECO.LP) for decomposing products of the stripper (ST); and ii) at least one evaporator (EV1, EV2); c) a medium pressure distillation means (DIS.MP) located between the stripper (ST) and the decomposer (DECO.LP) for distilling an urea solution leaving the stripper (ST) and thus reducing heat load in the stripper (ST) and decomposer (DECO.LP); d) a pre-evaporator (Pr-EV) located between the medium pressure distillation means (DIS.MP) and said at least one evaporator (EV1, EV2) for concentrating the urea solution using the condensation heat of vapors from the medium pressure distillation means (DIS.MP); characterized in that it further comprises means (PASS.) for introducing an oxidizing agent into the plant at a location in the plant upstream of the stripper (ST) and carbamate condenser (CO.CA).

Inventors: PAGANI GIORGIO, ZARDI UMBERTO, PAGANI, GIORGIO, ZARDI, UMBERTO
Applicants: UREA CASALE SA, UREA CASALE S.A.
Application year: 1990
Main classification: C07C273
IPC codes: C07C273