SU1118637A1 - Method of obtaining urea - Google Patents

Abstract

A method for producing urea, including its synthesis from ammonia and carbon dioxide at 180g230 C and a pressure of 150-220 kgf / cm, the isolation of unreacted NHj and CO- from the synthesis process, followed by their absorption-condensation and recycling to the synthesis unit of the resulting solution of carbon ammonium salts, moreover, the gases separated from the synthesis fusion are recycled into the process after contact, with a stream of substances intended to be introduced into the synthesis unit, which is distinguished by the fact that, in order to reduce energy costs, the separation from the synthesis fusion gas is spond NHj and COj Provo (d A at a pressure of 100-220 kgf / cm, as marked in the gases fed to contacting the stream with a solution ugleammoniynyh salts having The pressure of 170-300 kgf / cm.

Description

00 O) 00

The invention relates to an improved production method, urea from ammonia and carbon dioxide.

A known method for producing urea with a complete liquid recycling of non-converted NHj and CO .. The essence of it is that the synthesis of urea from X and COj is carried out at a high temperature (160–23 C and pressure (160–400 kgf / cm), and also with a significant excess of ammonia, the molar ratio NH.: CO is 3.6-6.0); The melt of urea synthesis is distilled in Nes. how many steps at consistently reduced pressure; The distillation gases are condensed in the presence of an aqueous absorbent to form a recycled solution of ammonium carbon salts (UAS) d 3.

The main disadvantage of the liquid recycling process is the high consumption of energy resources required for the stepwise distillation and subsequent compression of the recirculated gases.

The stripping process for urea production is known. This method is characterized by the following features: the distillation of the first stage is carried out in a current of COg at a pressure of 130-140 kgf / s with the utilization of the heat of condensation of the distillation gases; the pressure in the synthesis zone is maintained at the same level as in the distillation of the first stage 2.

The disadvantage of the stripping process is that it is impossible to maintain optimal parameters in the synthesis zone, which reduces the specific productivity of the synthesis reactor and causes an increase in its dimensions, metal consumption and manufacturing complexity.

In another known modification of the method with liquid recycling, I stage distillation is carried out at a pressure of 60-120 kgf / cm and 25-50% fresh CO is fed into the condensation zone of distillation gases at the same pressure with heat recovery. From this stripping process, this method compares favorably with the possibilities for the effective implementation of the synthesis process for C3.

However, this method is inferior to the stripping process in terms of distillation efficiency and has certain limitations on the level of temperature at which the heat of condensation of the distillation gases is utilized, and on the amount of this heat.

There is also known a method for producing urea, which includes its synthesis from ammonia and carbon dioxide, the isolation from the synthesis of unreacted NHj and CO2 and their subsequent absorption by condensation, at least in two stages of pressure, the pressure in the first stage being 60-120 kgf (cm), the introduction of a portion of fresh C02 into the site of release and absorption-condensation of NHj and CO of the first stage. According to this method, by distillation, excess ammonia is separated from the melt synthesis by melt separation at a pressure of 60-120 kgf / cm,

The first stage distillation is carried out in a CO-stream; 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, the separated excess NH is fed to the second condensation zone, and the solution of ammonium salts before recycling is mixed with liquid ammonia 3.

Unlike the other listed Bbmie, this method provides a significant reduction in energy consumption. However, it is also characterized by a fundamental flaw, concluding with a limited degree of distillation of unconverted NHj and COj at the first distillation stage. Since the heat of condensation-absorption of distillation gases at the stages of medium pressure (about 12 kgf / cm) and low pressure (close to atmospheric): it is almost impossible to utilize, at these steps, on the contrary, it is necessary to consume additional energy in the form of a reverse cooling water.

The closest to the technical essence and the achieved effect is a method for producing urea, including its synthesis from ammonia and carbon dioxide at 180-230 ° C and a pressure of 150-220 kgf / cm, isolation of unreacted NHg and CO2 from the synthesis and subsequent absorption. -condensation and recirculation of the ammonium salts in the synthesis zone of the resulting solution, and the gases separated from the melt synthesis, at a pressure of 135 kgf / cm, are in contact with the working (energy-carrying) gaseous CO or liquid NHj in the ejector to a pressure not lower than d aleni synthesis 5. This method provides a significant reduction in energy consumption. However, when the streams are contacted, there is almost no condensation and dissolution of NH and CO from the gas phase of the separation unit in the working stream (respectively, in gaseous COj or liquid NHj), which leads to significant energy consumption for adiabatic compression of the non-condensing part of the gas stream. This circumstance requires the provision of energy-carrying (working) pressure, a flow of hundreds of atmospheres above the synthesis pressure. The purpose of the invention is to reduce energy consumption at all subsequent stages of the process by increasing the distillation of unreacted NHj and CO from the urea synthesis water at the stage. high pressure. The goal is achieved by the fact that according to the method of obtaining urea, including its synthesis from ammonia and carbon dioxide at 180-230 ° C and a pressure of 150-220 CGS / CM2, the release of unreacted NH- and CO from the synthesis and subsequent absorption by condensation and recirculation after synthesis of the ammonium salts of the resulting solution, the gases separated from the synthesis fluid are recycled to the process after contact with the flow of substances intended to be introduced into the synthesis assembly, the release of unreacted NHj and CO2 pro one at a pressure of 100–220 kgf / cm and the gases emitted at the same time are directed to contact with a stream of ammonium carbon solution having a pressure of 170–300 kgf / cmH In the proposed method (as in the prototype method) and recycling the produced gases to the synthesis zone with the help of an ejector. The main difference of the proposed method is that a solution of ammonium carbon salts (AAS) is used as the working medium of the ejector, whereas in the method-prototype, gaseous CO2 or liquid is used for this purpose. In practical use of the urea process, it is necessary to maintain certain ratios between the reactants. As a result, the resources of the mentioned streams: solution UAS, gaseous CO and liquid NH. Are fairly strictly regulated. In particular, the amount of recycled ASA solution is about 3 times more than the amount of CO and NH. Supplied to the urea synthesis reactor. From the theory of jet devices, it is known that the efficiency of the ejectors is proportional to the mass flow rate of the working environment C63. In this connection, the use of a UAS solution as a working stream gives an undoubted advantage over the use of gaseous COj or liquid NHj. Moreover, when using a UAS solution in an ejector used, the efficiency of the latter additionally increases due to the high absorption capacity of the solution with respect to separated gases. A significant portion of these gases, when contacted with the solution of AAS, condenses and dissolves and, as a result, the cost of working on their adiabatic compression is eliminated. Gas different CO, and liquid NHj do not possess this ability. In the prototype method, NH should be considered as the main variant of the working environment of the ejector as the most economical variant of the known method. This is due to the fact that with the same mass flow rate, energy consumption for the compression of gaseous CO. significantly higher than the compression of liquid NH. In connection with this, as well as teaching that the present method involves using a liquid medium in the ejector (as a working stream), the use of gaseous COj in the ejector is not considered further. The description of the prototype method (including in the examples) does not indicate the pressure of the workflow before eke-. torus The calculation carried out according to the well-known Cg method showed that in order to implement the prototype method, gaseous CO2 would have to be compressed to 800 kgf / cm, and liquid NH to approximately 5000 kgf / cm. Because of these parameters, the use of the prototype method, of course, can not be considered appropriate. $ 11 When comparing the efficiency of using the two compared methods under specific conditions of the urea synthesis process, it was found that, at the pressure of the working medium (UAS solution) in the proposed method 170 kgf / cm, the ejector recirculates from the separator to the synthesis zone 8923 kg / h of the vapor-gas mixture. If, however, liquid NHj is used as the active stream as in the prototype, then with identical energy consumption for compressing the working medium, the amount of the injected stream is only 6745 kgf / h. In this case, liquid NHj must be compressed to a pressure of 570 kgf / cm. Special equipment is necessary for this. If the pressure of liquid NH supplied as a working medium to the ejector is assumed to be equal to 170 kgf / cm, then the ejector will not actually work (the injected flow will be only 22.3 kg / h). Thus, the advantages of using the UAS solution as the working medium of the ejector compared to the flow of liquid Shz are obvious. The pressure ranges adopted in the additional separation zone (100-220 kgf / cm) and for the UAS flow (170-300 kgf / cm) are due to the following considerations. With a separation pressure below uOqc / cm, it becomes difficult to introduce a significant part of the separated gas flow into the synthesis unit without additional energy consumption. The pressure of more than 220 kgf / cm in the additional separation unit cannot be ensured, since the technological process in the synthesis unit is carried out at a pressure of not less than 220 kgf / cm .. When the pressure of the UAS flow is lower than 170 kgf / cm, the introduction of UAS and the gas phase from the separator into the synthesis column. The pressure of the same flow above 300 kgf / cm is not rational to create, since this will require significant additional energy costs. The drawing shows a diagram of the implementation of the proposed method. Example 1. According to the drawing, in column 1 of urea synthesis, in which a temperature of 195С and a pressure of 220 kgf / cm is maintained, 35417 liquid ammonia (stream 2) with a temperature and pressure of 220 kgf / cm is fed (stream) 3 - 34,000 gaseous CO and 355 inerts with a temperature and pressure of 220 kgf / cm and a stream of 4 - 128478 recycled solution (66683 ammonia, 41700 carbon dioxide, including 11,833 fresh carbon dioxide, 19,900 water and 195 inerts). The synthesis melt 5 discharged from column 1 (62500 urea, 66683 ammonia, 29867 carbon dioxide, 38650 water and 550 inerts) is throttled to 100 kg / cm and fed to the separator 6. From apparatus 6, where the temperature is maintained, the liquid phase 7 is removed ( 62500 urea, 57948 ammonia, 28752 carbon dioxide, 36720 water and 300 inerts) and gases. A part of the gases (stream 8) in the amount of 1920 ammonia, 245 carbon dioxide, 425 water and 55 inerts are sent to the absorption-condensation unit 9. Another part of the gases - stream 10 (6815 ammonia, 870 carbon dioxide, 1505 water and 195 inerts) - is introduced into contact with a stream 11119093 of recirculated UAS solution (59868 ammonia, 40830 carbon dioxide and 18395. water) with a temperature of 130 ° С and pressure 300 kgf / cm in the ejector .12. The mixed stream 4 after the ejector 12 is fed to the synthesis column 1. The liquid phase 7 is throttled and sent to the unreacted NH and CO unit 13, where the process is carried out successively at three pressure levels of -70, 12 and 3.5 kgf / cm and respectively at temperatures of 160, 155 and 125 ° C. A solution 14 (62500 urea, 1800 ammonia, 1100 vooxide, carbon, and 27300 KINDS) with appropriate temperature and pressure, as well as gases 15 (56148 ammonia, 27652 carbon dioxide, 9420 water and 300 inerts) are removed from node 13. The gases 15 are sent to the absorption-condensation unit 9, where a stream 8 of gases (the composition indicated earlier) from the separator 6, a stream 16-11833 of fresh carbon dioxide and a stream 17 of a solution of low pressure carbonate salts (1800 ammonia, 1100 carbon dioxide and 8550 waters). From the absorption-condensation unit 9, the output from the ejector 12 is the flow of 11 ammonium carbon salts (indicated earlier) and a stream 18 (355 nerts). Flow 14 is directed to the solution concentration unit 19, where the process is carried out in two stages with the following parameters: first stage - temperature 130 ° C, pressure 0.35 kgf / cm and second stage temperature LAOc and pressure 0.04 kgf / cm. In node 19, a stream of urea melt 20 (62500 urea, 100 ammonia, 50 carbon dioxide and 2100 water) with a temperature and pressure of 0.04 kgf / cm is obtained, which is then sent to granulation. Vapor 21 (25200 water, 1700 ammonia and 1050 carbon dioxide) is isolated, which is sent to absorption-condensation and then returned to the process. The parameters, the composition of the streams in examples 2, 3, 4 are given in the table.

eleven . 1118 Thus, in the proposed method, due to the additional separation of the melt synthesis (without heating or with heating) at a pressure of 100-220 kgf / cm and transfer of the released gases to the synthesis node and the first distillation stage is 1.051.15 times compared to the prototype the amount of distillation gases is reduced at subsequent pressure levels, respectively, the size and intensity of equipment of these stages is reduced, and the flow of circulating water for the condensation of these gases is reduced accordingly. Due to the additional supply of gases, separated by separation under a pressure of 100-220 kgf / cm, the number of uchi, 12, increases in contact with the UAS stream supplied to the synthesis column.

lysed in this heat node. This reduces steam consumption in the process as a whole.

Due to the additional compared with the prototype, reducing the amount of distillation gases at the high, medium and low pressure stages, water consumption for their absorption is reduced, which is a prerequisite for reducing water recycling to the synthesis zone, increasing the degree of raw material conversion and additional saving of energy resources at stages division and recovery of unreacted NHj and CO.

The proposed method makes it possible to reduce the specific (per 1 t of commercial urea) steam consumption by about 0.03-0.06 tons and cooling water consumption by 2-4 m.

Claims (1)

METHOD FOR PRODUCING UREA, including its synthesis from ammonia and carbon dioxide at 180g230 ° C and a pressure of 150-220 kgf / cm ² , isolation of unreacted NH ₃ and СО2 from a melt of synthesis, followed by their absorption-condensation and recirculation to the synthesis unit of the resulting solution of carbon ammonium salts moreover, the gases separated from the synthesis melt are recycled to the process after contacting with a stream of substances intended to be introduced into the synthesis unit, characterized in that, in order to reduce energy costs, the separation of unreacted from the melt synthesis vshih NH ₃ and C0 ₂ is carried out at a pressure of 100-220 kgf / cm ^2, and wherein the selected gas is directed to contacting with a stream of solution ugleammoniynyh salts having ^ pressure of 170-300 kgf / cm ^2. 1 1118637