Questions regarding the operation of the urea synthesis tower

Precautions: (1) During the preheating process of the synthesis tower, there should be no liquid accumulation inside the tower, which must be drained promptly. (2) Once the preheating of the synthesis tower is complete, if it is not possible to increase the ammonia pressure immediately, the amount of steam fed into the tower and the amount of condensate discharged can be reduced while maintaining a constant temperature at all points within the tower. (3) If it is necessary to stop the steam supply during the preheating process, close the steam inlet valve to the tower and open the discharge valve to allow the interior of the tower to be connected to the atmosphere, thereby preventing a negative pressure from forming inside the tower and causing damage to the lining. (4) During the heating process, the heating rate must be strictly controlled at 6–8°C/h, with a maximum of 10°C/h; the rate of sudden temperature increases should not exceed 30°C/h either. The steam inlet valve should not be opened too wide each time. (5) Regularly check whether the condensate discharge valve at the bottom of the tower is blocked. (6) Pay attention to changes in vapor pressure, as well as the relationship between pressure and temperature. (7) Record the temperature and the opening degree of the steam valve every half hour, and plot the temperature curve. (8) During the heating process, the temperature difference between the inner and outer walls of the tower should be strictly controlled to be less than 50°C. Regarding the corrosion of the synthesis tower: I believe that by properly controlling temperature, pressure, oxygen content, sulfur content, and the quality of the steam used for leak detection during daily operations, and by following procedures strictly, it is possible to prevent excessive corrosion of the synthesis tower.

There are two globe valves in front of the P4 valve at the outlet of our synthesis tower, and there is no short pipe between these two valves. Will this increase corrosion? Also, is it appropriate to replace all the valves in the outlet pipeline of the synthesis tower with A4 steel? Hope an expert can answer. Thank you!

As a supplementary note, the heating rate below 100 degrees Celsius is 8-12 degrees. Above 100 degrees Celsius and less than 30 degrees (carbon dioxide stripping process). Air supply must not be interrupted during normal production. Ensure that the oxygen content remains within the specified range for the process.

Let me talk about “how to maintain the synthesis tower at the lowest possible corrosion rate’. 1. Strictly control the oxygen addition rate, especially during startup and shutdown. 2. Strictly control the temperature of the synthesis tower; overheating is strictly prohibited. 3. Strictly prevent Cl- from entering the synthesis tower. 4. Maintain as high a pressure as possible during short shutdowns to retain pressure. 5. Operate at the lowest possible water-to-carbon ratio. 6. When upgrading the capacity of a synthesis tower, erosion and corrosion caused by the materials must be taken into account. This post was last edited by lxq700918 on 2009-2-13 21:29.]

(1) During the preheating process of the synthesis tower, there should be no liquid accumulation inside the tower, which must be drained promptly. (2) Once the preheating of the synthesis tower is complete, if it is not possible to increase the ammonia pressure immediately, the amount of steam fed into the tower and the amount of condensate discharged can be reduced while maintaining a constant temperature at all points within the tower. (3) If it is necessary to stop the steam supply during the preheating process, close the steam inlet valve to the tower and open the discharge valve to allow the interior of the tower to be connected to the atmosphere, thereby preventing a negative pressure from forming inside the tower and causing damage to the lining. (4) During the heating process, the heating rate must be strictly controlled at 6–8°C/h, with a maximum of 10°C/h; the rate of sudden temperature increases should not exceed 30°C/h either. The steam inlet valve should not be opened too wide each time. (5) Regularly check whether the condensate discharge valve at the bottom of the tower is blocked. (6) Pay attention to changes in vapor pressure, as well as the relationship between pressure and temperature. (7) Record the temperature and the opening degree of the steam valve every half hour, and plot the temperature curve. (8) During the heating process, the temperature difference between the inner and outer walls of the tower should be strictly controlled to be less than 50°C. Regarding the corrosion of the synthesis tower: I believe that by properly controlling temperature, pressure, oxygen content, sulfur content, and the quality of the steam used for leak detection during daily operations, and by following procedures strictly, it is possible to prevent excessive corrosion of the synthesis tower.

Those upstairs have mentioned a lot regarding the control during the heating process, so I won’t repeat that here. I would like to share my three points of view briefly: 1. After stopping the heating, it is important to ensure a continuous supply of air, so that the pressure and temperature meet the basic requirements for heating; meanwhile, insulation and pressure maintenance should be maintained for more than 4 hours； 2. When feeding the material in, it is essential to ensure that the oxygen content in CO2 remains within the specified range. This is not a concern for systems that use hydrogen peroxide; however, even in those cases, the amount of hydrogen peroxide added must also be within the specified range, or it can be added in slightly larger quantities ； 3. During normal production, it is essential to keep the pressure and temperature in the synthesis tower within the specified ranges; exceeding these limits can pose a serious threat of corrosion to the equipment in the synthesis tower.

Once the preheating of the synthesis tower is complete, a constant temperature can be maintained by reducing the amount of steam fed into the tower and the amount of condensate discharged, thus keeping the temperature at all points within the tower constant. If it is necessary to stop the steam supply during the preheating process, special care must be taken to close the steam inlet valve of the tower, and to open the bottom drain valve and the top sampling valve, so as to allow the interior of the tower to be in communication with the atmosphere and prevent a negative pressure from forming inside the tower, which could damage the lining. Nitrogen can be filled at this time for corrosion prevention. During normal production, it is essential to ensure that the hydrogen sulfide content in the carbon dioxide gas fed into the synthesis tower remains below 10 ppm. Additionally, the pressure and temperature in the synthesis tower must be kept within specified ranges; overheating and overpressure must be avoided. By controlling the H/C and N/C ratios appropriately, the synthesis conversion rate can be improved.

Our factory uses a full-circulation process for aqueous solutions. In the full-circulation aqueous solution process unit, the synthesis tower is not a critical component for the operational process; by adjusting the ratio of materials fed into the tower, the temperature and pressure inside it reach equilibrium, which in turn ensures a high conversion rate of CO2. The focus is rather on the maintenance of the synthesis tower, in order to reduce the degree of corrosion of its lining, with an annual corrosion rate of less than 0.2 mm. During normal operation, 1) it is necessary to maintain constant values for the NH3/CO2 molecular ratio and the H2O/CO2 molecular ratio; the amount of ammonia fed into the tower as well as the amount of monomethylamine should not be adjusted arbitrarily. This is done to prevent overheating, ensuring that the temperature of the 316L stainless steel lining does not exceed 188°C. When the temperature reaches 190°C, the corrosion rate increases significantly ； Maintain a low H2O/CO2 ratio to reduce the corrosivity of the reaction materials on the lining, thereby minimizing effects such as urea hydrolysis. 2. Control the O2% and H2S content in the CO2 feed gas. During normal operation, when there is a trace amount of H2S, 0.4% oxygen addition is sufficient. When H2S is less than 10 mg/m3, increase it to 0.5%. Keeping the oxygen content in the solution above the minimum level, generally between 20 and 50×10-6, can prevent metal corrosion. In actual production, there are significant differences in the dissolved oxygen levels in the reaction solutions at different locations. Depending on the oxygen partial pressure in the gas phase, as well as the liquid phase temperature and the degree of gas-liquid contact, the equilibrium level of oxygen dissolution varies, which results in more severe corrosion in certain areas. 3. The pressure control valve at the outlet of the synthesis tower (PV204) should not be opened suddenly, as an increase in the pressure difference before and after the valve can easily cause the valve stem to bend or break, or it may result in the valve getting stuck and being unable to move up or down freely. 4. Check once a day whether the leak detection steam contains ammonia. With long-term use, the lining of the synthesis tower may leak due to corrosion, cracking, and other factors. The material used for the tower shell is not resistant to corrosion; under the attack of the synthetic fluid, it suffers severe corrosion, which can even lead to serious accidents. Therefore, regularly checking for leaks in the lining is an important task in maintaining the synthesis tower. The leak detection steam can be obtained from the steam expansion tank, with a pressure of around 0.5 MPa; its temperature is similar to that of the synthesis liquid inside the tower. If the high-pressure steam pressure is too high, it will cause excessive erosion at the leak detection groove. The ammonia ion concentration in the leak detection steam is below 0.5×10-6; otherwise, the accumulation of chloride ions in the leak detection tank causes stress corrosion cracking on the back side of the lining. 5. The three material pipes at the bottom of the tower must not vibrate, otherwise the welds at the junctions between the tower’s material pipes and the lining are prone to cracking. This post was last edited by Boating on the Five Lakes on 2009-3-14 20:41]