Thread Content
1 Overview Since its introduction at the beginning of the 20th century as a high-concentration nitrogen fertilizer, urea has seen rapid development; to date, the total global production of urea has reached around 80 Mt per year. Since the 1960s, our country has developed and introduced urea production technology, building more than 30 large-scale units and over 40 medium-scale units. However, the production of urea inevitably generates ammonia-containing wastewater. Through years of effort, Yunnan Yuntianhua Co., Ltd. has managed to recycle and treat all the wastewater generated during normal production, ensuring that it fully meets **discharge standards**. However, the issue of generating large amounts of ammonia wastewater during the shutdown of the equipment remains a serious problem. Currently, major fertilizer manufacturers in China typically treat the ammonia wastewater and then release the ammonia into the atmosphere. This practice not only results in the loss of a certain amount of ammonia but also causes air pollution, posing a threat to the health of local residents and employees. With technological advancements and increasingly stringent environmental regulations, such treatment methods urgently need to be improved. Yunnan Yuntianhua Co., Ltd. has improved its existing facilities and shutdown emission procedures, thereby achieving \"zero\" emissions during the shutdown of its urea production units. This paper provides a brief analysis and review of the process to achieve this goal. 2 Liquids Generated During the Shutdown of the Urea Plant and the Original Treatment Methods 2.1 Main Sources of Liquids That Need to Be Treated During Plant Shutdown The urea plant operated by Yunnan Yuntianhua Co., Ltd. uses the carbon dioxide stripping process developed by Stamicarbon of the Netherlands, with a design capacity of 1620 t/year. After several technical improvements, by 2003 the production capacity of the urea plant had been increased to 2310 t/year. Like other large urea plants, the plant must be emptied during shutdown for maintenance or when the shutdown lasts more than 12 hours. A large amount of liquid is generated in the system during shutdown and draining; the liquid that the shutdown system needs to handle mainly comes from the synthesis system and the raw ammonia system. 1) Synthesis system. The synthesis system mainly consists of 5 units: the 2 stripping towers are falling-film heat exchangers. During normal operation, the liquid level is kept at a low level, so the volume of these 2 stripping towers can be disregarded. The high-pressure ammonium methoxide condenser is also a falling film heat exchanger, and can be ignored as well. The lower part of the high-pressure ammonium washer is an immersion condenser, which is one of the liquids that need to be processed in the high-pressure system. During normal operation, the urea synthesis tower maintains a high liquid level; the process medium essentially fills the entire space, and it is the main equipment in the high-pressure system responsible for producing the liquid to be processed. The effective volume of the urea synthesis tower is 183 m3. Generally, the components of the synthesis tower are: about 30% urea, 30%–35% ammonia, 20%–25% carbon dioxide, and 15%–20% water. According to the material balance table in the process manual, the specific gravity of the materials in the synthesis tower is 0.99; therefore, the amount of liquid that needs to be processed is approximately 180 tons, of which about 55 tons is ammonia and about 54 tons is urea. 2) Raw ammonia system. From the synthesis workshop to the inlet of the high-pressure system, the entire material system is filled with liquid ammonia; this includes liquid ammonia buffer tanks, ammonia preheaters, ammonia heaters, ammonia pumps, and centrifugal ammonia pumps, all of which are filled with liquid ammonia. Additionally, approximately 200 meters of pipelines are also filled with liquid ammonia. The liquid ammonia buffer tank contains about 7 tons of liquid ammonia, and together with other equipment and pipelines, the ammonia system has approximately 10 tons of liquid ammonia. There is also some liquid that needs to be treated in the other systems at 0.7 MPa, as well as in the circulation system and hydrolysis system. Based on this calculation, approximately 70 tons of ammonia-containing liquid need to be processed after parking. 2.2 Handling methods for shutting down the original unit When the unit needs to be shut down for major maintenance or when the high-pressure system needs to be drained, the conventional method used was as shown in Figure 1. The high-pressure systems of the carbon dioxide stripping unit are equipped with discharge lines; through these specially designed discharge lines, the fluid is routed away from the stripping tower and into the circulation system. Once inside the circulation system, ammonia and urea are separated using a distillation tower. Urea enters the urine storage tank through pre-evaporation. The ammonia coming out of the distillation tower will exist in gaseous form and be vented in large quantities through flues; the unvented portion will be condensed by a low-pressure ammonium methanate condenser before being sent to the ammonia water storage tank. The ammonia from the ammonia system is directly discharged into the ammonia water storage tank; part of it exists in gaseous form and is vented through a chimney, while the other part is sent into the ammonia water storage tank. The liquid in the urine storage tank is easy to handle; it is usually either reused to restart the evaporation and granulation system to produce finished urea, or stored after being diluted. In any case, the treatment of urine does not cause pollution or losses. The ammonia in the ammonia storage tank presents particular challenges. The effective volume of the ammonia storage tank in the existing plant is 500 m3; if used as a buffer tank during normal operation, filling it up after shutdown would create pressure when the plant restarts. Therefore, it is usually emptied to maintain a low liquid level before the plant is restarted, and thus it is not generally used for storage purposes. And with increasingly strict environmental regulations, this ammonia solution cannot be discharged into sewers. Therefore, to ensure compliance with emission standards, when the plant is shut down for emissions control, the gases are processed through a hydrolysis unit to separate ammonia from water; the ammonia is released through a chimney while the water is discharged via sewers. After the high-pressure system is drained each time the plant is shut down, it takes 12 hours to deal with the dilute ammonia in the ammonia tank; this increases shutdown costs significantly and pollutes the atmospheric environment. The raw ammonia system is shown in Figure 2; after shutdown, the valves on the liquid ammonia pipelines in the synthesis workshop and those leading into the synthesis system have been closed. When discharging ammonia from the replacement system, it is sent to the ammonia storage tank through the pump outlet discharge pipeline. Due to ammonia’s high volatility, a large amount of ammonia is released into the chimney with each discharge. 3 Problems Existing in the Failure to Achieve “Zero” Emissions During the Shutdown of the Original Plant and Improvement Methods 3.1 Existing Problems Upon analyzing the wastewater and gases generated during shutdown, it is found that ammonia in the gas phase cannot be retained; moreover, since there are not enough buffer tanks available, dilute ammonia water lacks buffering capacity. In order to ensure that the wastewater meets discharge standards, it must be treated through a hydrolysis system before being released. Therefore, it is necessary to add an ammonia water storage tank to enhance the buffering capacity and to improve the operating methods so that ammonia is not released into the atmosphere. All 70 tons of ammonia can be converted into dilute ammonia water for storage, and then recovered through hydrolysis when normal production resumes. This not only saves costs associated with starting up and shutting down the system but also eliminates air pollution. After the plant is shut down, the synthesis system usually has its tower sealed, and then the contents are discharged into the circulation system through the discharge pipeline; therefore, no ammonia is lost from the synthesis system during this discharge process. During the discharge process of the synthesis system, the circulation system releases a large amount of gaseous ammonia; during the shutdown discharge process, the discharge rate can be reduced without opening the vent valve, allowing the ammonia to condense in large quantities within the circulation system and be discharged into the buffer tank. The existing discharge pipeline in the circulation system that leads to the ammonia water storage tank is a DN50 pipeline; generally, a flow rate of 3–4 m/s is sufficient to achieve a discharge volume of 21–28 t/h. As can be seen from the material balance diagram, at 70% load, the amount of ammonium methylate condensed in the existing circulation system is around 25 t/h; therefore, the current pipelines are sufficient to meet the discharge requirements. The ammonia concentration in the ammonia water condensed in the circulation system will be around 30%, with a temperature of around 70°C. To store ammonia solution at atmospheric pressure without it evaporating, it must be cooled. Since the medium in the circulation system is not just ammonia but also contains about 30% carbon dioxide, which reacts at low temperatures to form ammonium bicarbonate and thus crystalline substances, it is necessary to consider diluting the concentration while lowering the temperature. Therefore, to store dilute ammonia solution, it is also necessary to consider the cooling and dilution of the ammonia solution. It was found during the parking emission process that approximately 30 t/h of the refined liquid from the hydrolysis system is discharged on-site; cooling this liquid and recovering it for use in cooling and diluting ammonia would represent an ideal source of water. During the shutdown of the plant, the hydrolysis system is used to treat the ammonia in the ammonia tank. To reduce ammonia losses, after shutdown, the feed liquid for the hydrolysis process can be replaced with condensate from the condensate pump; this allows the hydrolysis system to be cleaned out simultaneously while the high-pressure system is drained and replaced. Due to the significant reduction in the load on the reflux condenser, the refined liquid at the pump outlet of the hydrolysis unit is sent via newly added pipelines to the top of the reflux condenser, where it is cooled before being discharged into a newly added ammonia storage tank for the cooling and dilution of ammonia. The discharge pipe of the reflux tank is a DN40 pipeline; considering that the discharge capacity may not be sufficient to meet the requirements for cooling and diluting ammonia water, an additional DN40 pipeline has been added at the outlet of the reflux liquid pump leading to the newly installed ammonia water tank. This allows the cooled purified liquid to be sent directly to that tank, where it can be used in the cooling and dilution cycle for the ammonia water being discharged. 3.2 Improved emission process for the high-pressure system: After the improvements, the process during shutdown emissions is shown in Figure 3. During emissions from the high-pressure system, the flow proceeds through the high-pressure discharge pipe, with the discharge rate controlled by LV-203, and the fluid is sent to the circulation system. The circulation system (original) remains under control based on the process parameters used during normal production; after urine and ammonia are separated in the distillation tower, the urine is still sent to a urine storage tank for storage or further treatment. After being absorbed by the circulation system, ammonia is discharged through the discharge line of the absorber level tank to the newly added ammonia water storage tank. After the system is shut down, the hydrolysis system immediately stops the supply of ammonia from the ammonia storage tank and switches to using steam condensate instead. The hydrolysis purification liquid is sent to the top of the reflux condenser; the water that has been cooled in this reflux condenser is then discharged into the newly added ammonia storage tank. This allows the highly concentrated ammonia discharged by the circulation system to be diluted and cooled for storage. In this way, not only is the cooling and dilution of ammonia water in the circulation system resolved, but the ammonia remaining from hydrolysis is also treated and recovered. 3.3 Improvements in the displacement emission process for ammonia systems: To reduce ammonia losses and minimize environmental pollution in the discharge process of ammonia systems, it is necessary to consider converting gaseous ammonia into liquid form. An attempt was made to pump ammonia through a centrifugal ammonia pump and seal the ammonia recovery pipeline to send it to the synthesis workshop, but this approach had to be abandoned due to slow discharge caused by the long length of the pipeline when the pressure in the ammonia system dropped. Therefore, the treatment of the ammonia system must take into account the conversion of gaseous ammonia into ammonium hydroxide, as shown in Figure 4. The liquid ammonia buffer tank is equipped with a pipeline that leads to the atmospheric pressure absorption system for venting inert gases; through this pipeline, the ammonia is sent to the atmospheric pressure absorption tower, where it is absorbed to form ammonia water, which is then sent to an ammonia water tank for storage. For safety, a 2mm flow-limiting orifice plate is installed on this pipeline; to increase the discharge rate, the orifice plate is removed once the pressure in the ammonia system drops below 0.1 MPa, and a shut-off valve is used to control the discharge volume in order to boost the discharge speed. 4 Operating Requirements and Precautions 4.1 Key Points for Parking The standard operating procedure for parking requires that the vent valves of the 0.7MPa absorption system, the vent valve of the circulation system, and the vent valve at the top of the reflux condenser be fully opened. Meanwhile, the circulation coolers of the 0.7MPa system’s absorption tower, those of the circulation system’s absorption tower, as well as the circulating water system of the reflux condenser should be turned off so that they do not function and ammonia is not condensed and released. To reduce ammonia losses and achieve \"zero\" emissions, the vent valves of the 0.7 MPa absorption system, the vent valves of the circulation system, and the vent valve at the top of the reflux condenser are kept closed during shutdown ； Maintain the normal opening of the coolers in the circulation system and the return condenser’s circulating water to ensure their proper operation, thereby reducing ammonia loss. 4.2 Key control points for discharging from the high-pressure system 1) The discharge rate is controlled using a stripper level control valve when discharging from the high-pressure system. 2) The pressure control in the circulation system shall not exceed 0.3 MPa, and the valve position of the vent valve from the circulation system to the atmospheric pressure absorption system shall be kept below 50%. The temperature of the low-pressure heat regulation water and the temperature of the absorption tower circulation cooler in the circulation system are controlled at the lower limit values to improve the absorption efficiency, while the water volume in the circulation system is increased to the upper limit value to reduce the concentration. If the pressure in the circulation system and the opening degree of the vent valve do not meet the requirements, the discharge rate can be appropriately reduced. 3) Increase the shell-side steam pressure of the circulating heater to raise the outlet liquid temperature of the circulating heater to above 130°C, thereby ensuring effective distillation and preventing large amounts of ammonia from being carried away with the urine and thus reducing ammonia losses. If the required conditions cannot still be achieved through the shell-side steam pressure and temperature, the discharge rate can be appropriately reduced. 4) To achieve cooling and dilution effects with the ammonia water discharged from the circulation system, the feed to the hydrolysis system must maintain a condensate flow rate of 30 t/h to keep the hydrolysis system operating; all the hydrolysis purified liquid is sent to the top of the reflux condenser, thereby maintaining the dilution water flow rate at 35–40 t/h. Once the high-pressure system has completed discharging, the hydrolysis system can be shut down. 4.3 Key points for purging the high-pressure system and circulation system: After the liquid phase in the high-pressure system has been drained, there is usually still a pressure of around 3.0 MPa, at which point a large amount of ammonia remains in the gas phase of the high-pressure system. Therefore, the gas released from the high-pressure system is still discharged into the circulation system; it is prohibited to open the vapor vent valve of the synthesis tower to release pressure into the atmosphere. The exhaust gas vent valve of the synthesis system should be kept at a low opening degree, and the vapor vent valve should only be opened to connect with the atmosphere once the pressure in the high-pressure system has dropped to a lower level. The circulation system continues to operate, recovering and storing the gas phase from the high-pressure system. After the high-pressure system is depressurized, all the flushing fluid from the relevant pipelines is discharged into the circulation system. During the water replacement process in high-pressure systems, the ammonia concentration at the bottom is usually high when discharge occurs. At the start of discharge, the liquid should be sent to the circulation system; after half an hour of discharge, a sample should be taken for analysis to ensure that it meets environmental discharge standards before discharge can take place on-site. All the water displaced from the 0.7MPa absorption system and circulation system is discharged into the ammonia storage tank until the discharge standards are met, after which it is discharged on-site. 4.4 Key points for handling the ammonia system: When discharging the ammonia system, all valves except the discharge valve and the cut-off valve leading to the high-pressure system should be in the open position, to ensure thorough displacement of the ammonia from the system. During ammonia system discharge, the atmospheric pressure absorption system must remain in normal operation. After the ammonia system has been drained, nitrogen can be introduced at the ammonia isolation valve in the urea synthesis unit to purge the system; the discharge point remains the atmospheric pressure absorption tower. Wait until the ammonia concentration drops to meet the emission standards before turning on the on-site drain for discharge. 4.5 Treatment of ammonia storage tanks: If the ammonia storage tank requires maintenance or if the liquid level is too high, the ammonia can be transferred to a new storage tank using a transfer pump. If ammonia loss is kept low through proper control during each shutdown, and the concentration is maintained at around 10%, then there will be approximately 600 tons of ammonia in the new ammonia storage tank. This amount of ammonia can be slowly recovered and reused through the hydrolysis system once the plant is operating normally. 5 Implementation Results On October 17, 2006, the ammonia synthesis and urea production units had to be shut down for maintenance due to faults. During this shutdown, approximately 600 m3 of dilute ammonia water was recovered from the emissions; sampling and analysis showed that the NH3 content was 11.2% and the CO2 content was 2.88%. After the unit started operating normally, hydrolysis was increased to full capacity at 16:00 on October 19 to recover dilute ammonia water, and all of the dilute ammonia water was recovered by 17:00 on October 24. A total of 67 t of ammonia was recovered during this shutdown, which is fairly close to the estimated 70 t; this indicates that there was minimal loss of ammonia during the shutdown, thus achieving the goal of zero emissions during such periods.