SU1109384A1 - Process for producing urea - Google Patents

Abstract

A method for producing urea, including its synthesis from ammonia and carbon dioxide at elevated temperature and pressure, is obtained From the melt of the synthesis of unreacted NHj and COg by step distillation followed by their absorption-condensation at two pressure stages, the pressure at the first stage being 60 -120 kgf / cm, introduction of a portion of fresh CG2 into the site of supply and absorption of NHj and CO of the first stage, characterized in that, in order to reduce energy costs, the reaction mixture after the first stage of distillation subjected to Additional separation is carried out at a pressure of 30–60 kgf / cm, and the gases emitted during this are directed to contact with a stream of fresh carbon dioxide having a pressure of 200–250 kgf / cm and intended for introduction into the gaseous phase and OO) sorption-condensation NH - and CO. first from the stage.

Description

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TJU The invention relates to a process for the production of urea from ammonia and carbon dioxide. A method of producing urea with a complete liquid recycling of non-converted NH, and COj is known and has the most widespread use. The essence of the method is that the synthesis of urea from NH and CO is carried out at high temperatures (160-230 ° C) and pressures (160400 kgf / cm), as well as with a significant excess of ammonia (molar ratio NH,: COj is 3.6-6.0), the melt of urea synthesis is distilled in several steps at a successively decreasing pressure, the distillation gases are condensed in the presence of an aqueous absorbent to form carbon ammonium salts (AAS) 1, which are recycled in solution 1. One of the drawbacks of the method with liquid recycling is a high flow rate -energy vehicles. The closest in technical essence and the achieved effect to the invention is a method for producing urea, including its synthesis from ammonia and carbon dioxide, isolation of unreacted NHj and COj from the melt synthesis by unreacted distillation followed by absorption by condensation, at least on two steps pressure, the pressure at the first stage is 60120 kgf / cm, the introduction of part of fresh CO, into the site of separation and absorption of condensation NHj and COg of the first stage. According to this method, before distillation Excessive ammonia separation of the melt at a pressure of 60-120 kgf / cm is carried out in the first stage distillation in a stream COj, the first stage distillation gases are condensed in two zones with an excess of COj in the gas phase in the first of these zones, separated by excess NH, is fed to the second the condensation zone, and the ammonium salt solution is mixed with liquid ammonia before recycling. This method provides a significant reduction in energy consumption f2. However, the known method, as well as method 1, is characterized by a fundamental flaw, concluding with a limited degree of distillation of unconverted NHj and the first distillation step. Since the heat of condensation-absorption of distillation gases at the stages of medium pressure (about 12 kgf / cmO and low pressure (close to atmospheric)) is almost impossible to utilize, at these stages, on the contrary, it is necessary to consume additional energy in the form of reverse cooling The aim of the invention is to reduce energy consumption by increasing the amount of heat utilized and reducing the flow of cooling water. The goal is achieved by the method of producing urea, including its synthesis from miak and carbon dioxide, at elevated temperature and pressure, separation of Henjio-reacted NHj and COj from the synthesis synthesis by stepwise distillation followed by their absorption-condensation at least in two pressure stages, the pressure in the first stage being 60-120 kgf / cm introduction of fresh C02 to the site of NH separation and absorption and condensation of the first stage, the reaction mixture after the first stage of distillation is subjected to additional separation at a pressure of 3060 kgf / cm, and the gases emitted during this with a stream of fresh carbon dioxide, having a pressure of 200-250 kgf / cm, and preliminarily to the introduction of the first stage of COj into the stage of absorption and absorption-condensation. Figures 1 and 2 are diagrams of plants for the production of urea. For contacting with a stream of fresh carbon dioxide having a pressure of 200-250 kgf / cm, it is envisaged to direct the gas stream separated by additional separation at a pressure of 30-60 kgf / cm in full or in part. In the latter case, the remaining part of the gas stream with a pressure of 30-60 kgf / cm is processed by known methods in the distillation units of medium or low pressure. Due to the additional separation of the fusion synthesis (without heating or with heating) at a pressure of 3060 kgf / cm and transfer of the released gases to the first stage of distillation and absorption-condensation of W and C02, the amount of distillation gases at medium and low pressure stages. Accordingly, the consumption of recycled water for the condensation of these gases is reduced. Separated by separation at a pressure of 30–60 kgf / cm, gases mixed with a stream of fresh carbon dioxide are introduced into the absorption zone and condensation of the first stage distillation gases under a pressure of 60–120 kgf / cm directly, or through a zone of distillation of the melt under this pressure. additional supply of gases, separated by separation under pressure of 30-60 kgf / cm, to the zone of absorption-condensation of gases under pressure of 60120 kgf / cm increases. the amount of heat utilized in this zone. This reduces steam consumption in the process as a whole.

By reducing the amount of distillation gases in the medium and low pressure stages, the water consumption for their absorption is reduced. Ultimately, this serves as a prerequisite for reducing the recycle of water to the synthesis zone, increasing the degree of raw material conversion and additional energy savings at the stages of isolation and recovery of unreacted Shu and CO.

Example 1. According to FIG. 1, kg / h: 35-458 of liquid ammonia (stream 2) with a temperature of 124 ° C and a pressure of 220 kgf / cm, 33952 gaseous are fed to the urea synthesis column 1, in which the temperature and pressure are maintained at 220 kgf / cm. COj (stream 3) with a temperature of 160 ° C and a pressure of 220 kgf / cm and 128792 recirculated solution of ASA (stream 4), 66584 ammonia, 41726 carbon dioxide and 19881 water. The synthesis melt 5 withdrawn from column 1 (62565 urea, 66589 ammonia, 29798 carbon dioxide, 39250 water is throttled to 70 kgf / cm and stage 6 is fed to the distillation unit 1, where the process is carried out at the same Lenin temperature and temperature. Next jtnae urea synthesis (stream 7), 62565 urea, 22542 ammonia, 7263 carbon dioxide and 32807 water — are withdrawn from node 6, throttled to a pressure of 30 kgf / cm and fed to a separator 8. From apparatus 8, where the temperature is maintained 130-135 s , liquid phase (stream 9) 62565 urea, 21284 ammonia, 6856

carbon dioxide and 32724 of water — are further directed to subsequent stages of distillation and for processing by known methods into commodity forms, and the gas stream 10 (1258 ammonia, 407 carbon dioxide and 83 water) is brought into contact with the stream

11 fresh gaseous carbon dioxide (11930 CO) with a temperature of 160 ° C and a pressure of 220 kgf / cm in the ejector 12.

Mixed stream 13 after the apparatus

12 (1258 ammonia, 12337 carbon dioxide and 83 water) are served together with the gas phase (stream 14) from the

5 6 (44047 ammonia, 22535 carbon dioxide, 6443 water) at the stage of condensation - absorption of the first stage distillation gases. In the zone 15 of the condensation-absorbsto served and the solution

0 UAS from the low pressure condensation unit — stream 16 (21453 ammonia 6854 carbon dioxide, 13296 water). The uncondensed gas phase (stream 17) —174 ammonia is directed

5 on the absorption, and the solution UAS (stream 4) return to the column) 1 synthesis.

Example 2. (Fig 1). It differs from Example 1 in that the melt of urea synthesis (stream 7) is withdrawn from node 6, throttled to a pressure of 45 kgf / cm and fed to a separator 8, where the temperature is maintained. The liquid phase (stream 9) contains 62565 urea, 18122 ammonia, 5851 carbon dioxide and 32499 water, and the gas phase (stream 10) contains 4420 ammonia, 1412 carbon dioxide and 308 water. The mixed stream 13 after the apparatus 12, containing 4420 ammonia, 13342

0 carbon dioxide and 308 water, served together with gas stream 14 from unit 6 (composition of the stream is the same as in example 1) to the stage of condensation-absorption of the first stage distillation gases

5 15. In this example, in the zone 15 of condensation-absorption serves solution AAS (stream 16) of the following composition: ammonia 18291, carbon dioxide 5849, water, 13130.

0

Example 3. According to FIG. 2, stream 2 of liquid ammonia (30599 Shz), stream 3 (32199) are supplied to a synthesis column 1 in which the temperature and pressure are maintained at 250 kgf / cm.

55 gaseous carbon dioxide) and stream 4 is a solution of UAS (68573 ammonia, 41637 carbon dioxide, 20819 water) from node 5 of condensation-absorption of distillation gases. The melt (stream 6) leaving the 1 {62606 urea column, 63381 ammonia, 28105 carbon dioxide and 39735 water) is throttled to 90 kgf / cm and fed to the separator 7 The synthesis melt (stream 8) from the separator 7 (62606 urea, 35389 ammonia, 19503 carbon dioxide and 39636 water are fed to the first stage distillation unit 9, where the process is carried out at the same pressure and temperature. The liquid phase (stream 10) from node 9 (62606 urea, 10445 ammonia 10306 carbon dioxide, 33209 water ) are throttled to a pressure of 60 kgf / cm and sent to the separator 11 where the temperature is maintained. From the apparatus 11 the liquid phase is The stream 12 (62606 urea, 8016 ammonia, 8760 COj and 32768 water) is fed further to the subsequent distillation and processing into commodity forms by known methods, and the gases 13 released during this (2429 ammonia, 1546 carbon dioxide and 441 water) are introduced into the apparatus 14 on contacting with the flow

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W 15 - fresh gaseous carbon dioxide with a pressure of 250 kgf / cm (13800 COj). The mixed stream 16 (2429 ammonia, 15346 carbon dioxide and 441 water) after the apparatus 14 is fed to the distillation column 9. Gas stream 17 from separator 7 (27955 ammonia, 8587 carbon dioxide) and gas stream 18 from distillation column 9 (27373 ammonia, 24543 carbon dioxide, 6868 water) are sent to condensation-absorption unit 5. A UAS solution — stream 19 (13272 ammonia, 8507 carbon dioxide, 13951 water) from a low pressure condensation unit is fed to this node. The uncondensed gas phase — stream 20 (27 ammonia) is sent to absorption, and the UAC solution (stream 4) is returned to synthesis column 1. The proposed method allows to reduce the specific (per 1 ton of commercial urea) steam consumption by about 0.05-0.1 t and cooling water consumption by 4-8 m.

Claims (1)

METHOD FOR PRODUCING UREA, including its synthesis from ammonia and carbon dioxide at elevated temperature and pressure, separation of unreacted NHj and СО ₂ from the melt of synthesis by step distillation followed by their absorption-condensation - at two pressure stages, the pressure at the first stage being 60-120 kgf / cm ² , the introduction of a portion of fresh CO? in the unit of incidence and absorption-condensation узел3 and CO2 of the first stage, characterized in that, in order to reduce energy costs, the reaction mixture after the first stage of distillation is subjected to additional separation at a pressure of 30-60 kgf / cm ² , and the gases liberated in this case are directed for contacting with a stream of fresh carbon dioxide having a pressure of 200-250 kgf / cm ² and intended to be introduced at the stage of incrustation and absorption-condensation ΝΗ3 and С0 _{2 of the} first stage. <pvtf 1 1109384