SU865870A1 - Method of control of distillation gas absorption process in urea production - Google Patents

Description

(54) METHOD OF MANAGING THE PROCESS OF ABSORPTION OF DISTILLATION GASES IN UREA PRODUCTION

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The invention relates to methods for controlling technological processes and can be used in chemical industry in automating the process of obtaining urea from ammonia and carbon dioxide.

There is a known method for controlling the process of absorption of high pressure distillation gases in the production of urea by adjusting the temperature. the absorption process by changing the liquid ammonia and water absorbent in the absorption zone fl}.

There is also known a method for controlling the process of absorption of distillation gases in the production of urea by controlling the temperatures in the upper and lower parts of the absorber by changing. supply of ammonia to the upper and lower parts of the absorber 21.

A disadvantage of the known methods is that in order to control the temperature conditions of the absorber, large amounts of water are required, which

leads to significant energy costs for its evaporation. The purpose of the invention is to reduce energy costs.

This goal is achieved by additionally controlling the temperature in the middle part of the absorber by varying the supply of a solution of low-ammonium carbonate salts to the high-pressure distillation zone.

It has been established that the need to supply a stream of condensate juice to irrigate the upper part of the wash column while fluctuating the temperature mode of the column can be eliminated without changing the irrigation rate of the upper part of the absorber. This is achieved by appropriately distributing the low ammonium salt (UAS) solution of pressure between the first stage distillation zone and the absorber (wash column). In the event of an increase in temperature in this 3 column due to the lack of water for carbon dioxide absorption, the supply of a low pressure UAS solution to the first stage distillation zone is reduced and, accordingly, the supply of this solution to the washing column is increased. Of course, both of these mode changes occur simultaneously. The first change leads to a decrease in the load of the first stage distillation zone, a decrease in the amount of carbon dioxide supplied with the distillation gases to the washing column, which ultimately smoothes the disproportion between the amount of carbon dioxide absorbed and the amount of ab . Absorbing water in the column. The second change increases the amount of water absorbent in the production column, which significantly enhances the effect of normalizing the temperature regime provided by the first change. When implementing this method, the minimum required supply of low pressure UAS solution to the washing column is maintained at its maximum allowable flow to the high pressure distillation zone, which allows to minimize the recycling of water to the urea synthesis reactor and reduces the energy consumption for the distillation of ammonia and carbon dioxide, not converted to urea. On. The drawing shows an example of the implementation of this control method. Stream 1 Gshawa synthesis of urea with temperature and pressure of 200 kgf / s throttling is up to pressure of 18 kts / cm to the top of column 2 of the first stage distillation. In heater 3, the temperature is maintained. The melt is withdrawn from the first stage distillation unit by two streams. The first of them, stream 4, is fed to the second stage distillation unit containing column 5 and heater 6. In apparatus 6, the temperature is maintained at 135-140 ° C at a pressure of 2 kgf / cm. The second stream 7 from column 2 is sent to a low pressure absorber condenser B. The melt from the second stage distillation unit - stream 9 is withdrawn for further processing. A stream 10 of second-stage distillation gases is introduced into apparatus 8, where absorption is carried out at condensation. The resulting low pressure UAS solution is withdrawn from the absorber-condenser 8 by two currents ONE of them along line 11, equipped with a valve 12, which is controlled by a regulator 13 connected to the temperature sensor 14, is passed to the distillation column 2. lines 15 are sent to the apparatus 16 - the first stage distillation gas absorber (washing column X. From the apparatus 8, gaseous stream 17 is withdrawn for further processing, stream 17. The first stage distillation gas stream 18 is sent to a remote cooler-bubbler 19, where via line 20 drain liquid The flow from the wash column 16. The UAS solution formed in the bubbler 19 is recycled through line 21 to the synthesis reactor, and the uncondensed gas stream 22 is transferred to the wash column 16. To reflux the column 16, use a urea-containing water absorbent of the condensate of the evaporation unit, the solution from the circulating tank of the plant, urea dust collection, from the dissolution tank of a non-standard product, part of the solution after the low pressure distillation step) - stream 23. The last use liquid ammonia or use ammonia gas. In addition, the upper part of the column 16 is irrigated with liquid ammonia supplied via a line 24, equipped with a valve 25, which is controlled by a regulator 26 connected to a temperature sensor 27. Liquid ammonia stream 28 is also supplied to the bottom of the wash column 16. Line 28 is equipped with a valve 29 which is controlled by a controller 30 connected to a temperature sensor (thermocouple) 31. Return ammonia vapors purified in column 16 from carbon dioxide and water & line 32 directs ngcondensation. The method is carried out as follows. In the case of increasing the load of the urea synthesis unit, the amount will increase and the composition of the first stage distillation gases will change (stream 18 and the composition of the second stage UAS solution will change (streams 11 and 15), which will cause an increase in temperature, measured by thermocouple 31, above the specified level In accordance with the signal from the temperature sensor 31, the regulator 30 will change the control action on the valve 29, which will increase the flow of liquid ammonia through line 28. If this does not lead to the normalization of the temperature in the lower part of the wash column and start the temperature in the middle part of the column is removed, according to a signal from the temperature sensor 14, the regulator 13 will reduce the opening of valve 12 and the supply of UAS solution to the distillation column 2 through line 11 will decrease. Accordingly, the supply of UAS solution through line 15 to column 16 will increase, until the temperature condition is normalized. If in this situation the temperature violation also occurs, the temperature in the upper part of the wash column 16 will increase, according to a signal from the sensor 27, the controller 26 will increase the opening degree of the valve 25 and the flow rate idkogo ammonia through line 24 will increase as long as the temperature at the top of the scrub column is reduced to the predetermined level.

In the opposite case, if there will be a decrease in temperature at points 31 and 14 below a predetermined level, the flow rate of liquid ammonia through line 28 will decrease, as will the flow rate of UAS solution along line I5 to the washing column 16 (and accordingly, the flow rate of UAS solution will increase distillation column 2). The flow rate of liquid through line 24 is kept at a minimum level at which the desired temperature at point 27 is observed.

Thus, to control the temperature in the wash column 16. no change in the flow rate of the top-absorbing colony absorbent is required (stream 23J, which eliminates the need for additional supply of condensate juice steam.

Efficiency and flexibility of temperature control by changing the distribution of the 7th dividing of the ASA solution from the degree of low pressure between the washing column

and the distillation unit of the first stage is due to the fact that simultaneously with an increase in the supply of water (and the composition of the UAS solution) to the washing column, the amount of distillation gases entering it decreases. A reduction in the distillation gas flow rate into the wash column is achieved without reducing the load on the urea production unit (accompanied by a decrease in the production of a commercial product) and without decreasing the temperature in the first stage distillation zone (leading to an increase in the load of other processing stages

melting of urea synthesis, increased consumption of energy resources and an increase in ammonia losses). The elimination of the need for additional supply of juice vapor to the washing column of the co-condensate of the juice vapor prevents the additional water from entering the node of the cooker and, accordingly, reduces the energy consumption for its mixing in the amount of 0.05-0.1 tonnes. steam (per 1 ton of urea).

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Claims (2)

-. .-. Invention Formula A method for controlling the process of absorption of distillation gases in urea production, by controlling the temperatures in the upper and lower parts of the absorber by varying the supply of liquid ammonia to the upper and lower parts of the absorber, characterized in that, in order to reduce energy costs, the temperature in the middle part of the inlet liquid is additionally regulated. sorber by changing the supply of a solution of low ammonium ammonium salts to the high pressure distillation zone. Sources of information taken into account during the examination 1. Kucherv V.I. and others. Synthesis and use of carbamide L., Chemistry, 1979, p. 196.  2. Synthesis of KN and others. Production of carbamides., Himi, 1970, p.113.