NL8900055A - Method for reducing the amount of dissolved ammonia while a urea solution is being concentrated - Google Patents

Abstract

Method for reducing the amount of dissolved ammonia in a urea solution obtained in urea synthesis from ammonia and carbon dioxide while the solution is being concentrated, the solution initially formed, which contains about 70 wt% of urea, being passed to a buffer zone and then being concentrated by evaporation of water under vacuum. The reduction in the ammonia content is achieved if the urea content of the solution to be concentrated is increased, before the solution is stored in the buffer zone, by about 2-10% by weight by water being evaporated from the solution.

Description

METHOD FOR REDUCING THE AMOUNT OF AMMONIA DISSOLVED DURING CONCENTRATION OF A UREA SOLUTION

The invention relates to a method for reducing the amount of dissolved ammonia while concentrating a urea solution by evaporating water from the urea solution.

In the preparation of urea from carbon dioxide and an excess of ammonia, a carbamate and free ammonia-containing urea synthesis solution is first formed in the synthesis section at a pressure of 125-350 bar and the associated temperature of 170-250 ° C. The carbamate is decomposed into ammonia and carbon dioxide and the decomposition products are removed from the solution in several steps together with part of the ammonia and water contained in the solution. In the end, an urea solution in water is obtained which contains approximately 70% by weight of urea and in which ammonia and carbon dioxide are still present. This solution is not suitable for fertilization purposes and for use in the resin production. As a rule, it must first be processed into solid urea. The approx. 30% by weight of water and the ammonia and carbon dioxide still present are at least largely removed by evaporation, or urea is allowed to crystallize from the solution and the crystals are melted, after which the urea melt obtained is converted into granular form. Since when urea solutions are exposed to high temperatures, urea decomposition and biuret formation occur, which biuret is undesirable both when using urea for fertilization purposes and for resin manufacture, as a rule, the evaporation of urea solutions is carried out under reduced pressure to avoid excessively high evaporation temperatures. avoid. Moreover, for economic reasons, the evaporation process is usually carried out in two or more stages, whereby in the first stage (s), at a moderately reduced pressure, the majority of the water present is removed, after which the evaporation in the last stage is carried out under a significantly more reduced pressure. continued until a practically anhydrous urea melt is obtained which contains less than 0.5 weight percent water.

Evaporation processes for urea solutions in more than one step have been described in the literature. For example, a two-stage evaporation process for urea solutions is known from U.S. Pat. No. 3,171,770, in which the solutions are concentrated in the first stage at a pressure above 200 mm Hg (0.267 bar) to 87-96% by weight of urea, after which in the second stage at a pressure below 100 mm Hg (0.133 bar), preferably between 50 and 80 mm Hg (0.067-0.107 bar), further evaporation takes place to a water content below 0.5% by weight at temperatures below 145hC. In current practice, lower pressures are usually used in the second evaporation stage, for example about 0.03 bar. Urea solutions with a water content of less than 0.5% by weight can suitably be pelleted by prilling in a pelletizer. If other granulation techniques are used for granulating, for example fluid bed granulation, it is often not necessary to adjust the water content. to be reduced to 0.5 weight percent or less In such cases, they can usually be based on urea solutions with a urea concentration of about 90-96% by weight, which can be obtained in the first evaporation step. however, under the moderately reduced pressure used in that step, a still relatively considerable amount of dissolved ammonia is in the order of 1500-2000 ppm. When these urea solutions containing ammonia are processed into granules and a large proportion of these granules are stored, the ammonia originally dissolved in the urea solution, except that this causes a considerable If ammonia loss occurs, this also means that the environment is not acceptable.

In practice, the solution containing approximately 70% by weight of urea, which is obtained from the last decomposition stage of the urea synthesis under a pressure of, for example, 3-5 bar, is subjected to a pressure drop, after which the solution is passed to a buffer zone and the save time. The ammonia content of the solution is now on the order of about 5000 ppm. The solution is then passed from the buffer zone to the heating zone of the first evaporation stage, in which a pressure of, for example, 0.35 bar is maintained. The conditions in the first evaporation stage are so mild that urea hydrolysis and biuret formation, and the accompanying formation of ammonia, only take place to a small extent. The concentrated urea solution with a concentration of, for example, 90-96% by weight of urea, which now still contains 1500-2000 ppm of dissolved ammonia, is removed from this heating zone.

Surprisingly, it has now been found that the amount of dissolved ammonia in concentrated urea solutions can be significantly reduced by increasing the urea content of the solution drained from the urea synthesis, containing about 70% by weight urea, by evaporating water before storing it in the buffer zone. . The increase in the urea content can then be 2-10% by weight, for example about 6% by weight. The water vapor formed during this evaporation is entrained with a considerable part of the original dissolved amount of ammonia, so that the remaining amount of dissolved ammonia decreases sharply.

The invention therefore relates to a method for reducing the amount of dissolved ammonia of a urea solution obtained in the urea synthesis from ammonia and carbon dioxide during the concentration of the solution, wherein the initially formed solution containing about 70% by weight of urea is introduced into a buffer zone and then concentrated by evaporation of water under vacuum. The process according to the invention is characterized in that the urea content of the solution to be concentrated, before it is stored in the buffer zone, is increased by about 2-10% by weight by evaporation of water from the solution. Preferably, the concentration of the urea solution will be increased from about 70% by weight to about 75-80% by weight, because the temperature in the buffer zone can then be kept below about 90 ° C, so that biuret formation during the storage period is negligibly low.

The method according to the invention has the advantage that ammonia losses during the granulation process are considerably reduced and the environment is less burdened.

The invention will now be elucidated with reference to the figure and the exemplary embodiment, without, however, being limited thereto.

In the figure, A represents a urea preparation plant in which an approximately 70 wt% urea-containing solution is produced. B is a pre-evaporation stage, C a buffer zone, D and E are a first and a second evaporation stage, respectively. In the pre-evaporation stage B, a flash zone F is provided in which the urea solution to be concentrated is expanded via 1 and expansion valve 2 to a pressure between 1.5 and 0.5 bar, for example 1 bar. The gas mixture of ammonia, carbon dioxide and water vapor formed during the expansion is discharged via 3 and processed elsewhere in the urea preparation installation. From the flash zone F, the solution is supplied via 4 to the heating zone of the pre-evaporation stage B at a pressure between 0.8 and 0.2 bar, for example 0.3 bar. From the pre-evaporation stage B, the concentration increased, for example about 75% by weight urea solution, is passed via 5 to the buffer zone C. The water vapor formed during evaporation is discharged from the vapor space of the pre-evaporation stage B via 6. The urea solution is then, by means of a pump 7 through 8, from the buffer zone C to a two-stage evaporation system, comprising the first evaporation stage D, in which the solution is concentrated to about 90-96 at a pressure of, for example, about 0.3 bar. % by weight, after which the already partially concentrated urea solution is passed via 9 to the second evaporation stage, in which the solution is further concentrated to, for example, more than 99% by weight at a pressure of, for example, about 0.03 bar. The concentrated urea solution is removed via 11. The water vapor formed in both evaporation stages is removed via 10 and 12 respectively.

Example

In the preparation of urea in an installation with a capacity of 1000 tons per day, the method according to the figure was used for concentration. The amounts are indicated in kg per hour.

From the urea preparation plant, 61820 kg of solution under a pressure of 4.1 bar and a temperature of 135ος was obtained, which contains 42237 kg of urea, 225 kg of biuret, 650 kg of CO2, 1322 kg of NH3 = 2.1% by weight) and 17386 kg water and which was supplied via 1 to the flash zone F at a pressure of 1 bar. The temperature of the solution dropped to 101πς and 3020 kg of gas mixture was formed containing 591 kg of CO2, 877 kg of NH3 and 1552 kg of water vapor, which gas mixture was discharged through 3. The remaining 58800 kg of solution containing 32237 kg of urea, 225 kg of biuret 59 kg COi, 445 kg NH 3 <= 0.8 wt%) and 15834 kg water vapor, were discharged from the flash zone via 4 and passed through the heating zone of the pre-vaporized rap B at a pressure of 0.3 bar. 5, 54449 kg of solution containing 42203 kg of urea, 225 kg of biuret, 0.1 kg of CO2, 43.4 kg of NH3 (= 797 ppm) and 11978 kg of water was discharged. This solution was stored for 2 hours in buffer zone C at a pressure of 1 bar and a temperature of 86βς and then pumped via 8 to the first evaporation stage. The solution was concentrated at a pressure of 0.3 bar to about 95% by weight solution. 42221 kg solution of 130πς was obtained, which contained 41859 kg urea, 262 kg biuret, 22 kg NH3 (= 500 ppm) and 2078 kg water. This solution was passed via 9 to the second evaporation stage and further concentrated under a pressure of 0.03 bar. Via 11, 41773 kg of urea melt was obtained, which contained 41276 kg of urea, 351 kg of biuret, 20.9 kg of NH3 (= 500 ppm) and 125 kg of water.

If the concentration process was carried out without using the pre-evaporation step, the NH3 content of the solution discharged from the evaporation steps was 1500-2000 ppm.

Claims (4)

A method for reducing the amount of dissolved ammonia from a urea solution obtained in the urea synthesis from ammonia and carbon dioxide during the concentration of the solution, wherein the initially formed about 70 wt% urea-containing solution is placed in a buffer zone and it is then concentrated by evaporation of water under vacuum, characterized in that the urea content of the solution to be concentrated, before being stored in the buffer zone, is increased by about 2-10% by weight by evaporation of water from the solution. 2. Process according to claim 1, characterized in that the urea content of the solution to be concentrated is increased by about 6% by weight. 3. A method for reducing the amount of dissolved ammonia in a concentrated urea solution as described and explained. Concentrated urea solutions with a reduced amount of dissolved ammonia obtained by the method of claim 1 or 2.