NL8900152A - Method for preparing urea - Google Patents

Abstract

Method for preparing urea from NH3 and CO2, the bulk of the urea synthesis solution produced being passed to a high-pressure decomposition stage and being subjected to a stripping treatment using CO2 and then being passed to a low-pressure decomposition stage. The remaining portion of the urea synthesis solution is passed from the synthesis zone to a treatment stage operating at 15-25 bar and then to the low-pressure decomposition stage. A corresponding portion of the NH3 and CO2 prepared in the synthesis is fed to the treatment stage operating at 15- 25 bar.

Description

METHOD FOR THE PREPARATION OF UREA

The invention relates to a process for the preparation of urea from ammonia and carbon dioxide.

When ammonia and carbon dioxide are introduced into a synthesis zone under a suitable pressure (for example 125-250 atm) and at a suitable temperature (for example 170-220hC), ammonium carbamate is first formed, according to the reaction:

Urea is subsequently formed from the ammonium carbamate formed by dehydration according to the reaction:

The extent to which the conversion to urea proceeds depends, among other things, on the temperature and the excess of ammonia used. As a reaction product, a solution consisting mainly of urea, water, ammonium carbamate and unbound ammonia is obtained. The ammonium carbamate and the ammonia should be removed from the solution and in most cases returned to the synthesis zone.

A widely used embodiment for the preparation of urea, known as "Stamicarbon carbon dioxide stripping process", is described, for example, in European Chemical News, Urea Supplement of January 17, 1969 pages 17-20 and in Nitrogen no. 143 of May-June 1983 pages. 32-38. In this process, in a high pressure part, the urea synthesis solution formed in the reaction zone at high pressure and temperature is subjected to a stripping treatment at synthesis pressure by contacting the solution in countercurrent with gaseous carbon dioxide under the application of heat, so that most of the the carbamate solution present is decomposed into ammonia and carbon dioxide. These decomposition products are expelled from the solution in gaseous form and are removed together with a small amount of water vapor and the carbon dioxide used for stripping. The gas mixture obtained during the stripping treatment is condensed for the most part in a condensation zone operated at synthesis pressure and absorbed in an aqueous solution from the further treatment of the urea-containing solution, using the heat of condensation released to form steam which is again used in the process, or for heating process flows. In order to create an optimum NH 4 / CO 2 ratio for the condensation zone, the majority or all of the fresh ammonia required in the process is supplied, so that the condensation can take place at the highest possible temperature, which results in the highest possible pressure of the recovered low-pressure steam. From the condensation zone, both the aqueous carbamate solution formed and the non-condensed gas mixture are fed to the reaction zone for urea formation. The heat required for the conversion of carbamate to urea is obtained here by further condensation of the gas mixture.

The stripped urea synthesis solution is then fed to a low-pressure work-up section, relaxed to a pressure of, for example, 2-6 bar, and heated by steam to remove ammonia and carbon dioxide still partially present in the stripped urea solution as carbamate. The gas mixture obtained in these operations is condensed and absorbed in an aqueous solution in a low pressure condensation zone and the dilute carbamate solution formed thereby is pumped back to the high pressure portion of the urea synthesis and finally passed into the reaction zone. The residual urea-containing solution is further depressurized and worked up into a urea solution or melt which is optionally further processed into solid urea. For this purpose the aqueous urea solution is usually evaporated in two evaporation steps under vacuum and the urea melt obtained thereby is processed into granules.

Urea plants are designed for a certain capacity. Increasing the capacity of an existing urea preparation installation by increasing the quantities of starting material and the throughput of process flows is generally only possible to a limited extent. If it is desired to expand the capacity of an installation by increasing the throughput flows, it is necessary to ensure that the increased process flows achieve good returns in the respective process steps, otherwise no good overall synthesis efficiency is obtained. This is especially true for the process steps that are performed in the high pressure part of the urea synthesis. In particular, bottlenecks in this section may include stripping of the urea synthesis solution in the stripping zone and condensation in the condensation zone of the gases obtained in the stripping operation. After all, if the liquid load of the stripping zone is increased too high, the stripping action is largely lost due to "flooding". In the condensation zone at the condensation of the gas mixture obtained in the stripping operation, it must remain possible that with the existing heat exchanging surface, even with the increased supply of the quantity of gas mixture, the released heat can be converted into usable steam that can be used in the urea process. . The aforementioned bottlenecks are largely the reason that expansion of the capacity of an existing installation, without having to adjust or replace expensive high-pressure equipment, is only possible to a limited extent.

The object of the invention is to provide a process for the preparation of urea, wherein it is possible to increase the capacity of a urea preparation plant without having to make adjustments in the high-pressure part of the synthesis. This objective can be achieved by passing the surplus production of urea synthesis solution via a bypass to an additional treatment step in which this amount of urea synthesis solution is contacted with an additional amount of carbon dioxide at a pressure of 15-25 bar. The required additional amount of ammonia is also supplied at this pressure.

The invention therefore relates to a process for the preparation of urea in which a carbamate and free ammonia-containing urea synthesis solution is formed from carbon dioxide and excess ammonia in a reaction zone at a temperature of at least 175 ** and the associated pressure of at least 125 bar, a high-pressure decomposition stage at synthesis pressure or Lower pressure by a stripping treatment with carbon dioxide under heating, decomposes a part of the carbamate and at least partly condenses the gas mixture obtained thereby and returns the condensate and any non-condensed part of the gas mixture to the synthesis zone, - in a low-pressure decomposition stage at a pressure of 2-6 bar the carbamate still present at least largely decomposes and the gas mixture formed separates, and the residual urea-containing solution is further processed by evaporation to a concentrated urea solution and, if desired, solid urea, characterized in that part of the urea synthesis solution carries from from the reaction zone to a treatment stage operating at a pressure of 15-25 bar, add to this treatment stage an amount of ammonia and carbon dioxide corresponding to the amount required for the preparation of the supplied portion of the urea synthesis solution and the solution discharged from the treatment stage further processed in the low-pressure decomposition stage.

They may suitably contact the urea synthesis solution in the treatment step, with or without the addition of heat, with the amount of carbon dioxide to be supplied and add the amount of liquid ammonia to the resulting carbamate decomposition gas. In this way the molar N / C ratio of the mixture can be adjusted to any desired value. The resulting liquid phase is fed to the low pressure decomposition stage. When the mixture of carbamate decomposition gases and liquid ammonia is completely condensed in heat exchange with the produced about 70-75% by weight urea solution, the heat of condensation released is generally sufficient to concentrate the urea solution in a pre-evaporation rate to about 80%. -85% by weight urea solution. Such an already partially concentrated urea solution, despite the larger amount of urea solution due to the overproduction, can be concentrated without problems in the existing two-stage evaporation system to a urea content of at least 99% by weight.

without having to increase the evaporation capacity.

It is noted that it is known from EP-A-213,669 to subject a part of a urea synthesis solution produced at synthesis pressure to a thermal stripping treatment with carbon dioxide and to contact the remaining part under adiabatic conditions at synthesis pressure with carbon dioxide. In this way it is achieved that in the decomposition of carbamate still present in the urea synthesis solution, a gas mixture is formed in a low-pressure stage with a favorable molar N / C ratio which can be condensed without requiring extremely large amounts of water. According to the present invention, a part of the urea synthesis solution, with or without heat supply, is contacted with carbon dioxide at a pressure of 15-25 bar and liquid ammonia is added to the resulting gas mixture.

The method according to the invention achieves the great advantage that the capacity of an existing urea preparation installation can be expanded to approximately 125 to 140% of the original capacity, without it being necessary to expand or adapt the high-pressure equipment. Such an increase in capacity does require the installation of an extra treatment stage and a pre-evaporation stage with associated equipment, but the equipment required for this is relatively cheap compared to the costs of high-pressure equipment. The method according to the invention achieves that the above-mentioned bottlenecks, namely the occurrence of "flooding" in the stripping zone and the danger of too low pressure of the steam generated in the condensation zone, are avoided. Consumption of high-pressure steam also decreases per ton of urea produced.

The invention will be elucidated on the basis of the figure and the examples, without however being limited thereto.

In the embodiment according to the figure, 1 denotes a reaction zone, 2 represents a high-pressure decomposition stage, for example a stripping zone, 3 represents a first condensation zone and 4 represents a washing zone. These devices belong to the high-pressure part of the urea synthesis, which can be operated at a pressure of 125-250 bar, for example 140 bar. Expansion valves are indicated with 5 and 6 and 15. An expansion zone is indicated at 7A, a heating zone provided with a vapor separator at 7B and an adsorption zone at 7C. Together with the supply and discharge pipes, these form the extra treatment stage that is operated at a pressure of 15-25 bar. Sis shows a Low-pressure decomposition stage, with 9 a second condensation zone operated at Low-pressure, 10 is a pre-evaporation stage, 11 is a storage vessel for the urea solution to be evaporated, and 12 is a storage vessel for the carbamate solution obtained during the heat exchange in the pre-evaporation stage.

Most of the liquid ammonia is fed via 18 and an ejector 17, together with the carbamate solution supplied from the washing column 4 via 19, via 20 into the high-pressure condensation zone 3.

The gas mixture from strip zone 2 is also fed to this via 21. This gas mixture is obtained by counter-flowing the majority, for example 60-75 wt.%, Of the urea synthesis solution discharged from synthesis zone 1 via 22 with heat input in the stripping zone 2 with a proportional part of the total amount of carbon dioxide required via 27 is entered in this zone. This high-pressure condensation zone can for instance be designed as a vertical pipe heat exchanger. The heat released in this zone by the formation of ammonium carbamate is dissipated using boiler feed water, which is converted into low-pressure steam of 4-5 bar. The stripping zone can also be designed as a vertical pipe heat exchanger. The heat required for stripping is supplied in the form of high-pressure steam of, for example, 15-30 bar. The carbamate solution formed in the high-pressure condensation zone 3 and the non-condensed gas are supplied via 23 to the synthesis zone 1. In this zone sufficient heat is generated by the further condensation of ammonia and carbon dioxide to carbamate to cover the heat requirement of the endothermic conversion of carbamate to urea. Via 24, the non-condensed gas mixture in the synthesis zone containing the inert gases, which are introduced into the process with the fresh ammonia and carbon dioxide and optionally as passivation air or oxygen, is fed to washing column 4, where ammonia and carbon dioxide present in the gas are washed out with carbamate solution supplied via 26. The inert gases are vented through 25.

The stripped urea synthesis solution is withdrawn from stripping zone 2 and passed through the expansion valve 5, in which the pressure is reduced to 2-6 bar, for example about 4 bar, to the low-pressure decomposition stage 8.

The remainder of the urea synthesis solution produced in reaction zone 1, for example 25-40%, is expanded via 14 and expansion valve 15 to a pressure of 15-25 bar in expansion zone 7A, the remaining liquid phase is fed to heating zone 7B, in which heat and then to the absorption zone 7C in which the relaxed urea synthesis solution is brought into direct countercurrent contact with carbon dioxide supplied via 16. The amount of carbon dioxide is adapted to the amount of urea synthesis solution and is, for example, also 25-40% of the total amount required in the process. The gas mixtures obtained in the zones of the treatment step are combined and the resulting gas mixture is discharged via 30. A proportional part of the amount of ammonia required in the process, for example 25-40%, is supplied via 31 and the mixture condensed with the aid of a carbamate solution supplied via 33, in the heating space of the pre-evaporation stage 10. The carbamate-containing stage remaining in the medium pressure decomposition stage urea synthesis solution is passed via the 29 and expansion valve 6, in which the pressure is reduced to 2-6 bar, for example 5 bar, into the low-pressure decomposition stage 8. The solution remaining after the stripping treatment in stripping zone 2 is also passed to this via 28, after the pressure in expansion valve 5 has been reduced. The majority of carbamate still present is decomposed in this step by the supply of heat and the formed gases are passed via 32 to the low-pressure condensation zone 9 and condensed there using an aqueous solution, for example process condensate, via 35. Via 34 70-75 Z-urea solution, which still contains small amounts of ammonia and carbon dioxide, was rapidly fed to the pre-evaporation.

In the pre-evaporation step 10, the solution is concentrated to a solution containing about 80-85% by weight of urea. The heat required for the concentration is obtained by condensation in the heating space of the mixture obtained by combining the streams 30 and 31 using the carbamate solution supplied via 33. The remaining carbamate solution is brought under synthesis pressure and is partly fed from storage vessel 12 to the washing column 4 to wash out ammonia and carbon dioxide from the inert gases and partly directly into the condensation zone 3.

The urea solution from the pre-evaporation step 10 is fed via 36 to the storage vessel 11. From storage vessel 11, the solution is passed via 37 to the concentration steps (not shown) and concentrated there, for example, to a practically aqueous melt. The water vapor formed in the pre-evaporation step, which contains small amounts of ammonia and carbon dioxide, is discharged through 38 and, after condensation, is sent to a process condensate upgrading plant (not shown).

Example 1

In an installation for the preparation of urea which had an original and capacity of 1740 tons per day, an installation as shown in the figure was obtained by applying an extra treatment step (7A, 7B, 7C) and an extra pre-evaporation step (10). By increasing the supply flows of ammonia and carbon dioxide, the capacity was increased to 2175 tons per day.

The pressure in the high-pressure section was 140 bar, in the additional treatment stage 15 bar, in the low-pressure decomposition stage 5 bar and in the pre-evaporation zone 0.3 bar. The temperature in the reaction zone was 184 ° C. The stripping zone was heated with steam of 22.5 bar. The same stripping efficiency was achieved as before the increase in capacity. In the condensation zone, low-pressure steam of 4.4 bar was obtained. The amounts are indicated in kg per hour.

51.519 kg NH3 and 27.66678 kg CO2 were supplied to the installation via 18. Of the urea synthesis solution produced in the reaction zone 1, 260,400 kg (81,132 kg urea, 260 kg biuret, 48,432 kg CO2, 77,271 kg NH3, residual water) were added via 22 to the stripping zone 2 and 61,232 kg (19,078 Ig urea, 61 kg biuret, 11,389 kg CO2 / 18,170 kg NH3, residual water) are supplied via 14 to the additional treatment step. In the stripping zone 2, the amount of urea synthesis solution supplied was stripped with 57,320 kg of gas mixture containing 55,900 kg of CO2 and the remainder consisted of inert, mainly air. The gas mixture discharged from the stripping zone containing 97,745 kg CO2, 72,750 kg NH3, 10,733 kg H2O and 1,389 kg inert was fed to the condensation zone, the carbamate solutions supplied via 19 and 13 totaling 115,332 kg containing 47,228 kg C.O2, 42,295 kg NH3, 25,701 kg H2O and 108 kg urea and part of the liquid NH3 supplied via 18, 48,645 kg which contained 146 kg H2O.

Via 14, 61,232 kg of urea synthesis solution (19,078 kg of urea, 61 kg of biuret, 11,389 kg of CO2, 18,170 kg of NH3, residual water) was fed via expansion zone 7A, heating zone 7B to the absorption zone 7C and brought into contact here with a gas mixture supplied via 16 containing 10,778 kg of CO2. The gas mixture discharged via 30 containing 66,460 kg CO2, 13,927 kg NH3 and 2,210 kg H2O was mixed with 3,028 liquid NH3 containing 9 / kg H2O and the mixture condensed in the heating zone of the pre-evaporation step 10, feeding 33 of 56,645 kg carbamate solution containing 108 kg urea, 18,819 kg CO2, 15,055 kg NH3 and 22,662 kg H2O.

The liquid phase discharged from contact zone 7C via 29, containing 19,078 kg urea, 61 kg biuret, 3,659 kg CO2, 4,243 kg NH3 and 10,747 kg H2O, was fed to the low-pressure decomposition stage 8. The solution discharged from the stripping zone was also fed via 28. containing 72,886 kg urea, 364 kg biuret, 12,540 kg CO2, 9,146 kg NH3, 40,136 kg H2O and 30 kg inert ingredients. 132.839 kg of urea solution was discharged via 34, which, in addition to 91.908 kg of urea and 472 kg of biuret, also contained 269 kg of CO2, 740 kg of NH3 and the remainder water. A gas mixture containing 14,638 kg CO2, 9,705 kg NH 3, 4,577 kg H 2 O and 259 kg inert was discharged via 32, which was condensed in the low-pressure condensation zone 9. The solution obtained via 34 was concentrated in the heating zone of the pre-evaporation stage 10 using of the heat released during the condensation of the gas mixture formed in the additional treatment step. 46.455 kg and 26 47.589 kg of the condensate formed here were fed to condensation zone 3 and washing zone 4, respectively. From the pre-evaporation step 10, 107,299 kg of urea solution was obtained via 36, which contained 91,685 kg of urea (urea content 85.4 X), 472 kg of biuret, 269 kg of CO2 / 738 kg of NH3 and 14,134 kg of water, which was further concentrated in a evaporation system.

Example 2

While maintaining a pressure of 25 bar in the additional treatment stage, using high pressure steam of 20.5 bar in the stripping zone, and under otherwise the same conditions and yield as in Example 1, urea was prepared in an installation as shown in the figure.

The composition of the streams was as follows: stream 18: 51,523 kg NH3 155 kg H20 stream 31: 5,953 kg NH3

18 kg H2O

stream 27: 55,900 kg CO2 1,420 kg inert stream 16: 10,754 kg C02 273 kg inert stream 22: 86,526 kg urea 277 kg biuret 46,935 kg C02 77,023 kg NH3 49,639 kg H20 stream 14: 12,964 kg urea 41 kg biuret 7,032 kg C02 11,540 kg NH3 7,437 kg H20 stream 21: 95,588 kg CO2 69,980 kg NH3 8,857 kg H2O 1,387 kg inert stream 28: 78,970 kg urea 388 kg biuret 14,688 kg CO2 11,274 kg NH3 38,555 kg H2O 33 kg inert stream 29: 12,964 kg urea 41 kg biuret 5,958 kg CO2 5,294 kg NH3 7,096 kg H20 2 kg inert flow 30: 11,828 kg CO2 6,245 kg NH3 762 kg H20 230 kg inert flow 32: 19,115 kg CO2 13,109 kg NH3 4,414 kg H2O 260 kg inert flow 33: 65 kg urea

23,070 kg CO2 18,456 kg NH3 17,419 kg H2O

stream 34: 91,879 kg of urea

477 kg biuret 1,668 kg COi 3,594 kg NH3 41,246 kg H2O

stream 36: 91,793 kg of urea

477 kg biuret 269 kg COi 739 kg NH3 23,587 kg H2O

stream 13: 28 kg of urea

14,934 kg C02 13,132 kg NH3 7,815 kg H2O

stream 26: 37 kg urea

19,827 kg CO2 17,435 kg NH3 10,375 kg H2O

Claims (6)

1. Process for the preparation of urea in which a carbamate and free ammonia-containing urea synthesis solution is formed in a high-pressure decomposition stage at synthesis pressure from carbon dioxide and excess ammonia in a reaction zone at a temperature of at least 175 ° C and the associated pressure of at least 125 bar. or lower pressure by a stripping treatment with carbon dioxide under heating, decomposes a part of the carbamate and at least partly condenses the gas mixture obtained hereby and returns the condensate and any non-condensed part of the gas mixture to the synthesis zone, - in a low-pressure decomposition stage at a pressure of 2-6 bar the carbamate still present at least largely decomposes and the gas mixture formed separates, and the residual urea-containing solution is further processed by evaporation into a concentrated urea solution and, if desired, solid urea, characterized in that part of the urea synthesis solution is removed from the reaction zone leads to one at a pressure of 15-25 bar operating step, supplies to this treatment step an amount of ammonia and carbon dioxide corresponding to the amount required for the preparation of the supplied part of the urea synthesis solution and the solution discharged from the treatment step further processed in the low-pressure decomposition step. 2. Process according to claim 1, characterized in that the urea synthesis solution is stripped in the treatment step operating at a pressure of 15-25 bar with the amount of carbon dioxide supplied to this decomposition step. Process according to claim 1 or 2, characterized in that the treatment with carbon dioxide is carried out under adiabatic conditions in the treatment step operating at a pressure of 15-25 bar. 4. Process according to any one of claims 1-3, characterized in that the liquid ammonia fed to the treatment step operating at a pressure of 15-25 bar is mixed with the carbamate decomposition gases formed during the stripping treatment in this step and the mixture is condensed in heat exchange with the urea solution to be concentrated. 5. Process for the preparation of urea as described and explained with reference to the figure. Concentrated urea solutions and solid urea obtained by the method according to any one of claims 1-4.