SU1072799A3 - Process for preparing urea - Google Patents

Abstract

In the prepn. of urea (I) from NH3 and CO2 with recycle of condensate from carbamate decomposition to the reactor with excess NH3, the synthesis gas with excess recycled NH3 is introduced into a first reaction zone, while the condensate with excess NH3 is introduced into a second zone. Both zones are sepd. by a heat-exchanger surface. Pref. the NH3:CO2 mole ratio is 2.5-5 in the first zone and 3.5-8 in the second. Both zones can operate at the same pressure, pref. 100-250 atm., or the first is at a pressure up to 200 atm. higher. Better yields and improved heat economy, even when using relatively low temp. and pressure, are obtd.

Description

This invention relates to an improved process for the preparation of urea.

A known method for producing urea by reacting ammonia and carbon dioxide under pressure. In this method, the urea synthesis reactor is subdivided into a series of sections in which the reactions take place: in adjacent zones, the carbamate, which has not been converted to urea, decomposes. The reaction heat is used to decompose the G1J carbamate.

The disadvantage of this method is the low yield (60%) of urea and the ineffective use of the heat of reaction.

The closest in technical essence to the present invention is a method for producing urea by reacting ammonia and carbon dioxide at 160-22 ° C and a pressure of 200-300 atm in two consecutive reaction zones with a molar ratio of ammonia and carbon dioxide in the first zone 3.2- 4.5: 1 to produce urea and ammonium carbamate by dehydrating in the second zone the resulting carbamate into the pot using the heat of reaction, separating the strain in the first zone, including the recycling of unreacted carbam and ammonium 2.

In the first stage, the heat of reaction is used to produce steam, and in the second stage, the heat content of the carbamate is used to dehydrate it into urea.

The disadvantage of this method is the low yield of the target product ().

The purpose of the invention is to increase the yield of the target product.

This goal is achieved by the fact that according to the method of producing urea by reacting ammonia and carbon dioxide at 160220 ° C and elevated pressure in two successive reaction zones with a molar ratio of ammonia and carbon dioxide in the first zone of 3.2-4.5: 1 to produce urea and ammonium carbamate, by dehydrating in the second zone the ammonium carbamate formed using the heat of reaction generated in the first zone, including recycling of the unreacted ammonium carbamate, the process is carried out at a pressure of 160,220 atm, oreagirovavshy ammonium carbamate recycled to the second reaction zone with simultaneous supply into it gshmiaka to maintain the molar ratio of ammonia and carbon dioxide is 5: 1, combined reaction streams exiting from both zones, and they are isolated from the unreacted ammonium carbamate.

In both reaction zones, the pressure is maintained at 180 atm.

In the first zone, the pressure is maintained at 220, and in the second, 160 atm.

The method using a highly efficient reactor allows to achieve high yields even when operating at relatively low pressures and temperatures, thereby eliminating the drawbacks associated with high power consumption, high cost due to the use of expensive materials and the technological problems mentioned. In addition to TorOj, this method allows working with a low stripping pressure, facilitating the decomposition of the carbamate

5 in the distant column, although at the same time high yields in the reactor are achieved.

The proposed method includes a step of supplying fresh ammonia and carbon dioxide (stoichiometric amounts together with part of the excess recycle ammonia to the first reaction zone separated by heat exchange surfaces from the second reaction zone located in the reactor itself, into which the condensates obtained by decomposing ammonium carbamate, along with the remaining part of the excess ammonia.

0 Two zones Can operate at the same pressure. In the range of 100–250 atm or at different pressures, and in the first reaction zone a higher pressure is maintained than in the second. The pressure in the first zone is sensitive to the excess pressure in the second zone, up to a pressure of 200 atm.

In the first reaction zone, there is a 0 C. large excess of heat compared to the amount of heat required to maintain an appropriate reaction temperature. To the second zone, the missing heat is supplied through

e separation toploobmennuyu surface in the form of excess heat released in the first zone.

In the second zone, where the carbamate and / or carbonate solution and part of the excess recycle N, Ha are supplied, a small amount of Cp2 can be supplied if necessary (no more than 10% so as to ensure an optimal ratio of NHj to CO2 and H2O / CO AND FOR

5 no heat balance.

It should be noted that the ratio of ammonia to carbon dioxide in two different zones of a highly efficient reactor must be regulated1) in a manner that is NHj / CO– 2.5–5 volts. the first zone fed with fresh reagents and part of the excess recycled ammonia, and 3.5-8 in the second zone, where

5, the recycled carbamate and / or carbonade is supplied together with the remaining portion of the excess recycled ammonia. A urea solution flowing from the first and second zones of the reactor operating at the same pressure in the zones or with a certain pressure gradient between the two zones (0200 Kr / CM-f, with a higher pressure being maintained in the first zone) enters the downstream processing section recirculating non-converting reagents. A reactor with two reaction zones operating at different pressures is particularly advantageous in one stage of the stripping, when stripping the solution of recurring carbamate leaving the reactor At the same time, a high yield is achieved in the reactor while at the same time ensuring a high degree of decomposition of the carbamate in the distant column. Fig. 1 shows a diagram of the method for producing urea by stripping urea solution, which is melting from the reactor; 2 is the same when using a reactor with two reaction zones operating under the same pressure; Fig. 3 is a variant of the scheme. Fresh reactants (MH, CO), shown by arrow 1 (Fig. 2). 1), entering the reaction zone 2, operating at a higher pressure. The recycle carbamate solution 3 coming out of the carbamate condenser 4, together with about 5 hours of fresh NH, is fed to the reaction zone 6, operating under the same pressure as the isobaric circuit, including the carbamate decomposition column, or the distant column 7, and 4 carbamate condenser. A solution of urea 8 flowing out of the reaction zone 6 and containing carbamate and residual NH, and a solution of urea 9 flowing out of zone 2 {high pressure, are fed to the Stand Column 7 operating under the same pressure as the reaction zone 6. Solution 10 urea, which is actually carbamate-free and contains a small residual amount of ammonia, flowing from the bottom of the distant column 7, goes further to the treatment section. The overhead columns 7 of the pair 11 NH-CC are directed to the carbamate condenser 4. The heat required by the column 7 for the decomposition of the carbamate and the evaporation of NHj c. H2O is replenished by the supply of steam 12. For ZE51 separation in apparatus 4, the heat of condensation of the NlTj – CO –HjO vapor and decomposition of the carbamate is used to produce vapor 13. The reaction zone 6 and apparatus 4 and 7 operate under the same pressure (about 160 kg / cm 1. The reaction zone 2 operates at a higher pressure, for example 220 kg / cm. Excess heat released in zone 2 is supplied to zone 6, which consumes heat, through the heat exchange surface 14. When using a reactor with two reaction zones operating under the same pressure (Fig. 2), fresh reagent s 1 NHj + COj through nozzle 15 is fed to the first reaction zone 6. A solution of 3 carbamate and part 5 of fresh ammonia is fed through a nozzle 16 to the second reaction zone 2 o Excess heat from the first reactive zone 6 is transferred to the second reaction zone 2, consumed through heat exchanging surfaces 14. According to the invention, a method is provided for providing optimal molar NH to CO ratios in the two reaction zones. In order to achieve an improved result, it is possible to work under such conditions that a part of the ammonia ammonium is separated at the outlet of the first reaction zone and is recycled to this zone. To maintain the thermal balance of the reactor, heat can be removed from the first reaction zone to produce steam or with a refrigerant. When implementing the method according to the scheme shown in FIG. 3, the urea solution 9 is sent to the separator 17, where it is divided into the upper part of the excess ammonia, which is recirculated towards line 1, and the excess heat from the reaction zone 6 is removed by the cooling coil 18. Example. The output in the reactor is 90%. In accordance with figure 1, we have two reaction zones. 1- reaction zone {flow rate at u, kg: NHj 1274, CO 733. Molar ratio: N / C {ammonia: € 02) 4.5, H / C (water: CO2) 0. Reaction conditions: 190 ° C , pressure 220 atm. Yield 83%. The output of the first reaction zone, kg: NHj 804, CO 124, urea 830, 249. The stream flowing from the second reaction zone 6, kg: MI, 247, C02 180, 8Sr. Stream 5: 95 kg. ORedin streams 5 and 3, kg: NHj 350, CO 180, 2 89. N / C 5 ratio, IL / C ratio 1.2. Reaction conditions: Dagle 160 atm. yield 69%. Output from the second reaction zone b, kg:, / CHNz 254, CO 56, urea 170, 140. Amount) streams 8 and 9, flowing from the two reaction zones, we get the following costs, kg: .NK CO 180, urea 1000, 389. The ratio N / C is 4.6, the ratio H / C is 0.24,. The output is 80%. Example 2. The reactor exit 72%. In accordance with FIG. 2 We have two reaction zones. . 1- reaction zone (feed through nozzle 15, kg: HH3 907, CO2 753. / H / C e 3.2. N / C 0.. Reaction conditions:, pressure 180 atm. Output 74%. Exit from the first reaction zone b, kg: NH, 487, CO 191, urea 740, 222. 2 — reaction zone 2 (streams 3 and 5, nozzle 16, stream 3 — recirculation. carbamate), kg: NH3 277, CO 276, ngO 135. Flow 5: NHj 156 kg. Combined flow 3 and 5, kg: NH, 533, C02 276, 136. N / C 57 N / C 1. Reaction conditions: 185 ° C, pressure 180 atm. 69% yield. Output from the second reaction zone 2, kg: NH7 386 / CO, 85, urea 260, H / jO 213. The stream 9 obtained by combining the streams results from two reaction zones, and The following composition is present in kg: NHj 873, CO 276, urine wine 1000, 435. N / c 3.7, ..H / C 0.33, 72% yield.

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Claims (3)

1. METHOD НЫ by the interaction of ammonia and carbon dioxide at 160-220 ° С and high pressure in two consecutive reaction zones with a molar ratio of ammonia and carbon dioxide in the first zone of 3.2-4.5: 1 to obtain urea and carbamate. ammonium by dehydration in the second eon of the formed carbamate. published. 1974. 2250923, published, 1970 URINE PRODUCTION using ammonia using the reaction heat generated in the first zone, including recycling unreacted ammonium carbamate, characterized in that, in order to increase the yield of the target product, * the process is conducted at a pressure of 160-220 atm, unreacted ammonium carbamate is recycled to the second reaction a zone with simultaneous supply of ammonia into it to maintain a molar ratio of ammonia to carbon dioxide of 5: 1, the reaction flows from both zones are combined, and unreacted ammonium carbamate is isolated from them . · 2. The method according to p, 1, <distinguish g, and with the fact that in both reaction zones _ support pressure; ^- 180 atm. > 3. The method according to π. 1, due to the fact that in the first reaction zone the pressure is maintained at 220, and in the second, 160 atm. < m KZ ^ 1