What are the advantages of using two-stage evaporation in urea plants?

The concentration of urine after flash evaporation is around 71% (by weight), with a boiling point of 95°C. To evaporate the urine to 99.7% (by weight) through single-stage evaporation, it is necessary to raise the temperature to 140°C (above the melting point of 132.7°C). The partial pressure of water vapor in the gas phase should not exceed 0.0034 MPa; under such conditions, the urine will crystallize when it reaches a certain concentration due to the crystallization temperature being lower at that concentration, which prevents normal production. At the same time, evaporating a large amount of water under these conditions is also uneconomical, as condensing the secondary steam at such low pressures is difficult, and significant energy is required to remove the inert gas; therefore, it is advisable to carry out the evaporation in two stages.

At a certain pressure, the boiling point of the urea solution increases as the concentration of urea rises. At a certain temperature, the lower the operating pressure for urine evaporation, the higher the corresponding saturated urine concentration. Therefore, if the operating pressure is reduced, the evaporation of the urea solution can take place at lower temperatures, which reduces the hydrolysis and condensation reactions of urea; from this perspective, using vacuum evaporation is advantageous. However, at lower pressures, such as below 26.66 kPa, urea solutions exhibit a double-boiling point property; once the temperature of the urine reaches this point, it automatically separates into solid urea and water vapor, making it difficult for the evaporation process to continue. Therefore, in industry, it is common to first carry out the first stage of evaporation in a vacuum environment where no crystallization occurs, in order to concentrate the urine to a higher concentration, and then to proceed with the second stage of evaporation at a lower pressure. Since 86% of the water in the urine has already been evaporated during the first stage of evaporation, during the second stage it is possible to maintain the temperature of the system within the K1–K2 temperature range above K2. This allows for measures to be taken to quickly remove the water vapor from the solid-gas mixture, thereby obtaining urine with a high concentration of 99.7% (by mass). The evaporation and concentration process of urea solutions employs two-stage vacuum evaporation.

Avoid the crystallized area of urine to reduce equipment investment. Reduce costs

　　Based on the basic theory of the urea-water binary phase diagram, as well as the characteristic that urine has two boiling points during evaporation at low pressures, it is impossible to evaporate urine with a concentration of around 70% and a temperature of around 90°C to 99.7% purity using very low pressures in just one step. This is because, before reaching the second boiling point of 131°C, the urine enters the crystallization zone, and the crystals formed will block the pipes, preventing production from continuing. 　　Therefore, in actual production, two-stage evaporation is necessary. Of course, unless some type of large-particle granulation technique is used, the second stage of evaporation can be omitted as long as the urine concentration reaches 95%. At this point, a period of evaporation can be used.

This answer really taught me a lot, including the responses from those above. I used to think that when using evaporation to raise the urine concentration to 99.7%, the amount of steam required and the size of the evaporation heater would be enormous; I also didn’t know that urine has two boiling points at low pressures. It seems there’s still so much to learn! Thank you again to everyone who replied above! ! ! !

Regarding the 6th floor: Of course, unless some type of large-particle granulation technology is used, since only a urine concentration of 95% is required, it is possible to do without the second stage of evaporation. At this point, evaporation can be used; unless some type of large-particle granulation technique is employed. What is this technique? Is the technology mature?

When using Japan’s Toyo large-particle granulation technology, since the urine concentration only needs to reach around 95%, a single stage of evaporation is sufficient.

Using two-stage evaporation is based on considerations of both process and quality: Can 99.5% concentration be achieved with single-stage evaporation? Sure it can. As long as the control pressure is above 26.6 kPaABS (as mentioned on floor 4). To prevent crystallization from occurring in the two-phase region as the urea concentration keeps changing during the evaporation process, thereby blocking the pipes, the urea concentration will reach the level you need. The prerequisite is that the temperature of the urine must be increased further. Because temperature and pressure are in a balanced relationship; the higher the pressure, the higher the saturation temperature. This will result in a large amount of biuret. Using a section of evaporation with a pressure below 26.6 kPaabs is not feasible

The concentration of urine has the following characteristic: when evaporated at a lower pressure, as the concentration of the urine increases during evaporation, crystals will form, resulting in a solid-liquid mixture. If evaporation occurs at an absolute atmospheric pressure of 0.1, when the urine concentration reaches 70% and the boiling point is 59°C, the urine becomes a saturated solution; further evaporation will result in the formation of crystals within the urine. We refer to this temperature at which a saturated solution is achieved as the first boiling point of the urine. If urine containing precipitated crystals is further heated and evaporated, at a certain higher temperature and concentration the crystals in the urine disappear; this phenomenon is known as the second boiling point of urine. The table below lists the first and second boiling points of urine at different pressures, along with their corresponding concentrations. To prevent the formation of crystals in the urine, the evaporation process should be carried out at a pressure slightly higher than 0.273 absolute pressure (200 mmHg). A large amount of water is first evaporated to raise the concentration of the urine to 95%; the saturation temperature for 95% concentrated urine is 120°C, with actual operation taking place at 128–132°C. If the evaporation pressure in Stage 1 is too high, too little water will evaporate in that stage, which will overload Stage 2 evaporation. The second stage involves evaporating the urine to achieve a high concentration, of 99.7%. To reach this concentration, the evaporation pressure should be as low as possible, and the temperature must be above the melting point of urea (132.7°C); the normal operating range is 138–142°C, in order to prevent the formation of large amounts of biuret. When the urine concentration is 99.7% and the temperature is 140°C, the saturated vapor pressure of the solution is 25 mmHg absolute pressure. Therefore, the operating pressure for the two-stage evaporation process should not exceed 25 mmHg (absolute).

To obtain a product with a low water content, urine must be concentrated to 99.7% before being used in the production process. To achieve this, the water vapor partial pressure in the urine must not exceed 25 mmHg; if the pressure is reduced too much, the urine will evaporate at temperatures below its crystallization point, resulting in crystal formation that can cause blockages and prevent proper evaporation. Therefore, two-stage evaporation is employed, with concentration carried out in two steps to avoid crystal precipitation and enable the evaporation to proceed normally. This post was last edited by lxq700918 on 2009-3-26 21:41.]