SU696014A1 - Method of preparing urea - Google Patents

Description

one

This invention relates to a process for the production of urea from ammonia and carbon dioxide.

A known method for producing urea 1 from NHgH COjnpH at elevated temperatures and pressures, followed by distillation of the melt synthesis in several stages, with progressively decreasing pressure; low pressure distillation gases are absorbed by an aqueous absorbent to form a solution of ammonium salts (UAS); distillation gases of high pressure are fed into a washing column, which is irrigated with highly concentrated ammonia water and a solution of UAS from a low pressure stage; In the wash column, two zones can be conditionally distinguished: 1) fine purification of the return vapor from the ammonia from COg impurity and 2) formation of recycled UAS solution, the streams withdrawn from both zones are returned to the reactor.

The drawing shows the diagram of the wash column.

The diagram indicates recycled solution of UAS If high pressure distillation gases 2, refrigerant 3 (for example, water); a solution of AAS 4 from a stage of low pressure, ammonia water 5,

return ammonia vapors 6, fine cleaning zone 7 of the return vapor Hj, zone 8 forming a recirculated CAA solution.

In the described method, the system for recovering unreacted feedstock has a significant disadvantage in that the supply of a significant amount of water (as part of an AHC) to the reactor reduces the efficiency of the processes of synthesis and distillation, as well as entails a great waste of energy.

The aim of the invention is to reduce energy costs in the production of urea.

The goal is achieved by melting the urea obtained from NHj and COj at elevated temperatures and pressures under distilled pressure reduction, and the distillation gases recovered at higher than atmospheric pressure are processed by mixing carbonic ammonium salts with liquid flows in the recirculating solution formation zone. and in the zone of fine purification of ammonia from carbon dioxide at a pressure equal to the distillation pressure, a 5-100% solution from the purification zone is rectified without a change in pressure, the separated gases are added are combined with the distillation gases, and the remaining ammonium salt solution is recycled, diluted before recycling urea so that the molar ratio of COCNHj): C02. In the reagent mixture before the synthesis column does not exceed 0.15, and the flow containing urea, is fed to the zone of formation of the heated solution under the distillation gas injection point. A distinctive feature of the method is that 5-100% of the solution from the fine purification zone rectifies without pressure change, the separated gases are added to the distillation gases, and the ammonium salt solution is diluted with urea before its recirculation (outside the fine purification zone) so that the molar ratio of CO (NH2) .i: CO 2 in the mixture of reagents of the synthesis column does not exceed 0.15. In the framework of the proposed method, an additional method is provided, in which the urea-containing stream is supplied to the zone of formation of the recirculated solution below the injection point of the distillation gases, or to a separate zone. With this dilution of the UAS solution with urea, the recirculated flow becomes ammonia-unsaturated, thereby eliminating the danger of its transfer into the suction lines and valve systems of the pumps used to pump the OAS solution. : A dilute urea, a melt of urea from an evaporator, or may be used to dilute the recycled solution of UAS. from any distillation stage, suspension of urea in water (from crystallizers), mother liquor (after centrifuges), solution of urea in liquid ammonia, etc. The process of rectification of the solution from the fine cleaning zone envisaged in the proposed method is not necessarily carried out in a special column; For the same purpose, said stream can be fed to a distillation column. Example 1. Urea synthesis is fed to the column (all quantities in;. Kg / h) of N Hz 590, CO2 of 297 and recirculated ruemogr of UAS 481 solution (including N Hz of 175, CO 2 of 143, HgO 77, of urea 86). The process is conducted at a tempera. tour 190 ° C and a pressure of 2.10 kgf / cm. The melt of the synthesis of urea (LH3 536, CO 2 143, urea 491 and water 198) is sent to the first stage distillation column. The parameters of high-pressure distillers and 17 kgf / NH 4 488, COj 129 and 36 are distilled off from the melt. The distillation gases (1 cm stream, Fig. 2) are then introduced into the distillation column 2, and the remaining melt is distilled at a temperature of 140 s and a pressure of 2 kgf / cm. After the second distillation stage, the urea solution (water 119, urea 491) is evaporated, and then the product is obtained in crystalline or granular form. Low pressure distillation gases are absorbed with water to obtain a solution of AAS (NH, 48, CO. 14, 53). A part of this solution (NH, 26, CO 7.6, H, 0 28.7) is fed to the washing column (stream 3); the other part (Nfl 22, CO2 6.4, HjO 24.3) is rectified in column 4 (stream 5). For irrigation of the wash column 2, also water (192, HgO 48) - stream 6 is also used. The solution from the refined ammonia reflux zone (stream 7) containing NH, 92, CO2 3, 48. is removed at a temperature for rectification. Column rectification 4 is equipped with a heater (8 - heating steam) and a dephlegmator (9 - cooling water). The following parameters are maintained in column 4: pressure — 17 kgf / cm temperature in the lower part — 200 ° C, in the upper part — 100 ° C. The solution fed to the rectification is preheated to the heat of the bottom of the washing column or by cooling the high-pressure distillation gases before the washing column. Liquid flow 10 (H2O 60, the content of NH and COj is negligible) from column 4 is transferred to the wastewater node, and gas (stream 11) containing NHj 114, CO5 9.4 ,. HjO 12.3, return to the wash column. Due to the heat removal by cooling water (stream 9), the temperature in the lower part of the wash column is 95 s, in the upper part it is 45 ° C. From column 2, the output t stream 12 return vapor NH; 553. To dilute the UAS solution into a separate zone below the injection point of the distillation gases 1, CO (NH2) 2 86 is fed - stream 13. The UAS solution with the addition of urea is stream 14 (NH 175, CO2 143, CO (H2) 286, H220 , CO (NH2) 2 is recycled to the synthesis column. Using this method of urea production provides, compared to existing methods, the possibility of increasing the productivity of the main equipment of the urea production unit, which allows to reduce the specific capital costs and reduce the cost of production. effect on a pair. Only due to a decrease in steam consumption, the economic effect of using the method in one workshop

capacity of 360 thousand tons in 1 year will be about thousand rubles.

Claims (2)

1. A method of producing urea from ammonia and carbon dioxide at elevated temperatures and pressures, including the distillation of the reaction solution under stepwise pressure reduction, the processing of distillation gases BED separated at a pressure above atmospheric / by mixing liquid ammonium salts in the recurring solution formation zone and in the ammonia fine zone from carbon dioxide at a pressure equal to the distillation pressure, and the recirculation of the solution of ammonium carbonate salts, so that, in order to reduce energy costs, 5-100% of the solution from the purification zone rectifies without pressure change, attach the gases to the distillation gases, 5 and dilute the ammonium salt solution before recycling (outside the fine purification zone) urea so that the molar ratio of CO (NH2) z COjB of the mixture of reagents before the synthesis column does not exceed 0.15. 2. The method, which is also distinguished by the fact that the liquid stream containing urea is fed into the zone of formation of the recycled solution below the injection point of the distillation gases. five Information sources; taken into account in the examination 1. Sineva K.N. Production of urea, M., Khim, 1970, 58-60 (prototype),