Preseparation – What is the process of predistillation process? What are the effects of implementing this process?

[Recommend an article by Qian Lao] Fully understand and develop the potential of increasing production and saving energy in aqueous solution full cycle urea equipment Summary by Qian Jingqing, former chief engineer of Xiangjiang Nitrogen Fertilizer Plant: The article introduces the 40-year technical development experience of the aqueous solution full cycle process urea unit in China, and details the production increase and energy-saving processes and equipment used in the process of technological progress. On this basis, the author proposes a new aqueous solution full cycle process that can efficiently recover heat energy with a steam unit consumption comparable to that of the stripping process unit. Preface The author has devoted his life to the production technology and technology development of aqueous solution full cycle urea plant. Since 1958, it has been implemented in the urea pilot plant of Nanjing Fertilizer Company. * and study abroad * The urea production process has been around for 48 years, of which the first 28 years were spent in medium-sized urea plants, engaged in production command and technology development. ； Since 1986, he has been engaged in the guidance of production and technology development of small urea plants. Since small urea plants have always had an objective need to continuously increase production, this gives the author an excellent opportunity to study and explore the potential of small urea plants to increase production and save energy. Thanks to the cooperation of many small urea plants, the author combined the practical experience in the mid-urea plant and constantly summarized and improved the production practices of each small urea plant. The 40 years of technological development and development of the aqueous solution full cycle process in China is no longer the backward process with high energy consumption in the 1960s. In practice, we have a full understanding of the two advantages of the aqueous solution full cycle urea device. The author expects that there will be 192 sets of aqueous solution full cycle urea process units in my country, and there will be about 150 sets of original small urea units. Among them, a large proportion of the single unit production capacity will be developed from the original 60kt/a to 180-200kt/a. ； The original 110kt/a urine device has been developed to 260-300kt/a ； The newly designed 200kt/a device of Yuanhua Fourth Hospital can reach 300kt/a, which is a consensus among factory colleagues: (1) Only the aqueous solution full cycle process device can expand the production capacity by 1-2 times through a small number of technical transformation measures and low investment in technical transformation. ； (2) While increasing production, the existing heat energy recovery technology is used to increase the heat energy recovery rate for generating methylammonium, and the steam consumption has been reduced to 1105-1110kg. The author's newly developed technology can reduce steam consumption to less than 900kg without the need for high-pressure circulation loop modifications. It is expected that this technology can be shared and used by many factories with full-circulation aqueous solution process equipment. The investment in technical transformation only requires 2-3 million yuan (each urea factory can * * Independent technological transformation) can achieve high benefits of increasing production and saving energy. In this article, two advantages of a full-circulation urea plant for aqueous solutions will be explained step by step. 1. It is hoped that many colleagues engaged in urea production will be fully aware of the technical advantages that exist in the design of small urea plants. 1.1 Adopt pre-distillation process The technology introduced in my country from the Dutch Stamicarbon Company in 1966 is the pre-separation process. Before the development of the small urea unit, design and research units had been debating whether the first stage of the decomposition system should adopt a pre-separation process or a pre-distillation process. They believe that since the pre-distillation process cannot save steam, it is better to engage in pre-separation. Therefore, in addition to the second version of Zhongnuo's design (about 1973) that used the pre-distillation process developed on the urea pilot plant of Nanhua Company, the subsequent third and fourth versions of the design still used the pre-separation process. However, medium-sized urea plants have realized the superiority of the pre-distillation process in practice, and most plants use pre-distillation towers instead of pre-separators. When it comes to small urea plants, engineering designers engaged in the design of small nitrogen fertilizers have already affirmed the advantages of this process. Compared with the pre-separation process,: 1.1.1 Since the temperature of the pre-distilled gas drops to 125°C, compared with the original one-point gas of 160°C, the amount of water vapor entering the first-stage recovery system is reduced, which is beneficial to the water balance of the system. Therefore, the water consumption in the first- and second-stage absorption can be increased, improving the stability of the first- and second-stage absorption operations. 1.1.2 The total process water circulation volume is reduced, the H2O/CO2 molecular ratio of the material entering the synthesis tower can be controlled at 0.65, and the conversion rate of the urine tower can reach 67%. 1.1.3 Due to the increase in water absorbed in the second stage, the CO2 component in the diuretic liquid can be reduced to 16-18%, and the gas-liquid balance pressure of the dimethyl liquid is reduced. Therefore, the operating pressure of the second stage can be reduced, and the temperature of the second stage can be reduced to 135°C for operation. In this second stage, the added value of the diuretic shrinkage is almost zero, which increases the rate of high-quality finished products. 1.1.4 Due to the heat and mass transfer in the pre-distillation tower, the steam consumption per minute is reduced. During the pre-separation process, since the conversion rate is 63%, the steam consumption per minute is 900kg/tur. ； During the pre-distillation process, since the conversion rate reaches 67%, the steam consumption per minute is 540 kg/tur. The measured steam consumption of the pre-separation process of the medium-sized Lunan Fertilizer Plant in 1981 was 1500kg (the conversion rate of the urine tower was 65%). The measured steam consumption of the pre-distillation process of the Sichuan Fertilizer Plant in 1983 was 1330kg. ； When the Fujian Yongan small urea plant operates at 120kt/a, the steam consumption value is 1380kg. 1.2 Adding an external suction cooler to the first absorption tower. Adding an external suction cooler absorbs the technology of Japan's improved "C" method, which shifts the absorption load of the first absorption tower outward. A ton of urea requires an absorption volume of 0.4m3, so the space between the tubes of the external cooler becomes the volume for absorbing CO2, thereby increasing the production capacity of the first absorption system, reducing the absorption load in the first absorption tower, and reducing the amount of reflux ammonia. The medium-sized device does not have an external cooler. During the pre-separation process, the amount of reflux ammonia at the top and bottom of the first absorption tower and urea per ton is 2.0 m3 ； There is an external suction cooler in a small device, so bottom reflux is not needed. The amount of ammonia used for refluxing urea per ton depends on how much heat is taken out by the external suction cooler, which is between 0.6-0.8 m3. After the production load of the entire absorption system is increased, there is no need to increase the area of ​​the ammonia condenser, because the role of a 1 m2 suction external cooler is equivalent to 5 m2 of an ammonia condenser. Make good use of the first-suction external cooler. When the production capacity of the device is increased, the circulation volume of desalted water must be increased. Simply increasing the area of ​​the first-suction external cooler is not enough. And twist-shaped aluminum strips should be added to each cooling tube to increase the turbulent velocity of the water, thereby improving the heat transfer system and removing the heat generated by methylammonium. The temperature of the desalted water is controlled at 90-95°C so that there is no risk of crystallization of concentrated methylammonium in an external suction cooler. The upper limit is to adjust the temperature of water to 95°C. If it is circulated at 95-100°C, the circulation volume of the pump will decrease due to vaporization, and the temperature at the bottom of the absorption tower will rise. Setting up or adding an external suction cooler is an effective measure to improve the production capacity of the urea plant absorption system, and at the same time, the heat energy recovery rate is also increased accordingly. The desalted water volume of the cooling cycle should be able to remove the corresponding heat load. In this desalted water circulation system, a cold desalted water volume of 2.5 m3/tur can be supplied, the water supply pressure is not less than 0.4MPa, and the desalted water circulating volume should be 20-25m3/tur to configure a circulating water pump of corresponding capacity. The external suction cooler can not only reduce the amount of absorption absorbed by the bubbling section of the first suction tower, but also improve the operating conditions of the fine washing section of the first suction tower, reducing the amount of CO2 absorbed by the ammonia water in the upper part of the tower, preventing methane ammonium from being saturated and entering the solution stratification zone, causing the methane ammonium to crystallize and precipitate, thus improving the operational stability of the first suction tower. Another area where the small urea device can improve production capacity is the heat energy utilization section of the evaporation heater. The design unit only calculated the heat energy utilization, and did not calculate the heat of methylammonium generation by absorbing CO2, and did not calculate the absorption capacity of this section. Therefore, without any modification, the original 60kt/a small urea unit can reach 80kt/a. By adding a suction external cooler of the same area, the output can reach 100kt/a. 1.3 Process improvement of dimethyl liquid in the first-stage absorption system In the pre-distillation process, the CO2 content in dimethyl liquid can be reduced to 16-18%. It is first sent to the heat utilization section of a steam heater, then to an external suction cooler, and then to the bubbling section of a suction tower. This can make full use of the space between the tubes of the first steam heating utilization section and an external suction cooler to absorb NH3 and CO2, and remove the heat generated by methylammonium. They are two parts of the pre-distillation process unit that can both increase production and improve the heat energy recovery rate. At present, a few factories have not controlled the operation of reducing the dimethyl liquid component, and the control is between 18-20%. After the medium-sized equipment was changed to the pre-distillation process, the dimethyl liquid component was not reduced from 20%. This is an inappropriate operating process and fails to achieve the benefits of increasing production and saving energy. 1.4 One-point heater The one-point heater is a rising film tube heater with an orifice added at the bottom of the tube. When this technology was hosted by the Nitrogen Division of the former Chemical Fertilizer Division and Stamicarbon Company conducted technical exchanges, the company provided this technology that saves energy and reduces the added value of biuret in one minute. The orifice technology allows urine to enter each row of tubes evenly at the bottom sealing head. The urine is already in a boiling state. After being introduced into the tubes through the orifice, the flow rate is increased, causing the fluid to form a liquid film to rise, and the liquid side heat transfer coefficient is increased. This can save steam, reduce the residence time of the material in the tubes, and avoid back-mixing of urine in the original tube heater, thereby reducing the value-added amount of biuret. If there is no orifice, the urine rises greatly from the middle tube. When it reaches the upper end of the tube, due to the temperature difference between the urine, a large amount of low-temperature urine can flow back into the tubes around the heater, forming a back-mixing of the circulating flow. The combination of the one-point heater and the pre-distillation process device can reduce the area of one-point addition, that is, one-point addition can increase production capacity under the same area. In the second version of the design of the small urea device, the one-point addition area is increased to 94m2, which is larger than that of the medium urea pre-distillation device. The area is larger than the tons of urea in the process device. Moreover, the pre-distillation process is adopted and the bottom end of the tube is equipped with a throttling hole. Therefore, 94m2 can pass a 360t/d output. If a pre-separator is added, the output can be 420t/d. The requirements for good use of One Point Plus are that the secondary anti-corrosion air added at the bottom of One Point Plus must remove oil mist, otherwise oil scale and carbon deposits will be produced on the surface of the tubes, which will affect the heat transfer effect. However, in the current small urea unit, the one-point addition tube is scaled and difficult to clean, so the throttle hole is removed and the heat transfer effect caused by the scaling is poor. When increasing production, the area of ​​the one-point addition is increased, and no orifice technology is used. As a result, the steam consumption increases while increasing production. All heaters in the urea plant adopt the rising film type, which has its unique advantages. When the output increases, due to the increase in the rising flow rate of the material, the heat transfer coefficient increases. In the same area, the output can be doubled, and high-quality urea can be obtained in the evaporation system. As for the corrosion of evaporator heater tubes, most of them have now adopted titanium or titanium alloy materials that are resistant to erosion and corrosion. 1.5 The desorption tower is equipped with a reflux condenser. The top of the desorption tower is only filled with ammonium bicarbonate liquid without heat exchange (called cold flow), and the temperature at the top of the tower is controlled by adjusting the cold flow rate, as well as other design factors. The adaptability is poor and the operation flexibility is small. The desorption system of the small urea plant is equipped with a reflux condenser, and the reflux condensate is used as the reflux liquid in the 1-6# distillation section at the top of the desorption tower. Therefore, it is more convenient to control the temperature after desorption and condensation at 112°C. The water vapor content in the desorbed gas is relatively stable, which is very beneficial to the stable operation of two circulations and one cooling. The cooling water of the reflux condenser is 95-100°C hot water from the first suction external cooler, and the outlet water temperature is 105°C. Since the reflux condenser is at an altitude of 24m, the hot water flows automatically to the boiler room according to the level difference. A ￠700mm desorption tower, 1 A 6m2 reflux condenser can desorb 7-8m3 of ammonium bicarbonate liquid per hour. The waste liquid discharge indicator does not exceed the design value. Many factories also send the dilute ammonia water of the scrubbing liquid of the refined gas of the synthesis system to the desorption tower for desorption, which can desorb 10m3 per hour. 1.6 The inert scrubber is equipped with an explosion-proof space in the separation space above the inert gas scrubber. The separated gas contains a large amount of hydrogen (about 30-35%) and about 10% oxygen, which is an explosive gas. As long as there is a trace detonation source, a chemical explosion will occur. About 1/4 of medium-sized urea plants have experienced explosions at different parts in the early stages, and the probability of explosions is greater in the separator part of the inert scrubber, so small urea devices are commonly used It is designed to install an explosion-proof plate at the separator of the inert scrubber. The space above the plate is an explosion-proof space. The gas from the ammonia condenser A (the gas contains 80% ammonia) passes through the explosion-proof space and reaches the ammonia condenser B. The gas with high ammonia partial pressure is used as the explosion-extinguishing gas source. Once an explosion occurs, the explosion-proof plate ruptures and the explosive gas enters the explosion-proof space. Due to the dilution of a large amount of ammonia, the explosion stops to avoid a crushing explosion at the separation part of the inert scrubber. Due to design reasons, the explosion-proof panel will break if there is a slight physical pressure difference, affecting its use. Sichuan Chuanhua Group Co., Ltd. has improved the installation method of explosion-proof panels to avoid rupture of explosion-proof panels and truly play an explosion-proof role. 2. Development progress of small urea plant technology 2.1 Development of pre-separation-pre-distillation process technology In 1983, the original pre-separator was restored in front of the pre-distillation tower in the secondary urea plant of Qilu No. 1 Chemical Industry Co., Ltd. At that time, the secondary urea plant did not have an external suction cooler, and the pre-separated gas directly entered the bubbling section of the absorption tower for absorption. Looking back now, this process did not have a major impact at that time, and only Lanhua and Juhua plants were improved. The reason is that the original pre-separator has a small volume (￠800×2600, V=1.3 m3), and the pressure in the pre-separator rises during operation, and the pre-separation effect at the design pressure of 1.7MPa cannot be achieved. ； Secondly, the pre-separated gas directly enters the first suction tower without the function of an external suction cooler. In terms of operation technology, although the medium-sized plants at that time changed to the pre-distillation process, they did not realize the superiority of the pre-distillation process and adjust the process indicators. If the CO2 component of the dimethyl liquid remains at 20%, it will still enter the first absorption tower directly. The dimethyl liquid component of the small urea plant drops to 16-18%. This dimethyl liquid is first sent to the first steam heating and utilization section, and merges with the pre-distilled gas. The partial pressure of CO2 gas rises to 18.6%, resulting in more methylammonium reactions. Then it enters the first suction external cooler and absorbs 92% of the ammonia in the pre-distilled gas. Because of the reaction of CO2, 80% of the total CO2 in one gas is absorbed in front of the first absorption tower (calculated based on the heat energy recovery rate). The increase in heat energy recovery rate also reduces the heat load of the first absorption tower. The amount of urea reflux ammonia in one absorption tower is reduced from 2.0M3 in the pre-separation process to 0.6M3. Therefore, after the pre-separator was restored in the intermediate urine unit at that time, the advantage of recovering the heat generated by methylammonium in this process was not fully demonstrated. However, due to the splitting effect of the pre-distilled gas and the pre-separator in the first-stage decomposition system, the overall pressure of the first-stage system is reduced, which is beneficial to the first-stage decomposition rate. At that time, the intermediate urine device did not restore the pre-separator to improve production capacity. It just believed that the gas phase materials separated by the pre-separator did not need to be heated to save steam consumption, and the water vapor partial pressure in the pre-separated gas phase was low and the water absorbed by the first stage could be reduced. The two small urea plants in Shandong Mingshui are small urea plants that use pre-separators with a volume of 3.5M3 as recommended by the author. It is introduced that the installation height difference of the pre-separator in Qilu Yihua is 18 meters higher than that of the pre-distillation tower. These data are raised to question the installation height difference of the original design and research unit which is only 5 meters or 7 meters, so that the installation height difference of this plant is 12 meters. It is also the first factory to put the pre-separated gas into the first suction external cooler and then into the first suction tower. (The original design of a certain hospital has a pre-separator and a suction external cooler, but the pre-separated air still enters a suction tower). After the pre-separator was added to the two sets of devices, the steam consumption was significantly reduced, which was reflected in the decrease in the steam side pressure in the first heater. There is still a pressure balance valve installed on the pre-separated gas phase pipe. When the production load of the device is low, the two sets of devices can save steam. When the load of the two sets of devices was increased, the position difference was less than 15 meters, and the pre-separated gas phase tube was still ￠139 tube without enlargement. Therefore, the pressure balance valve of the old system is fully closed, while the pressure balance valve of the new system can only be opened a little, thus losing the function of the pre-separator. The author analyzes this situation and believes that it is not a problem with the pre-separation-pre-distillation process, but the result of not installing a pre-separator properly. The author believes that the installation of the pre-separator must meet two process requirements. One is to ensure that during the production process, the pressure inside the pre-separator must be close to the design pressure 1.7MPa in order to be close to the design value of the pre-separation efficiency. For this reason, the volume of the pre-separator and the diameter of the pre-separated gas phase pipe must match the corresponding production load. Unnecessary resistance losses should be eliminated during the process, such as canceling the pressure balance valve on the pipeline to reduce the actual pressure in the pre-separator. Second, the pre-separator and the pre-distillation tower must have a sufficient head difference when installed. In the absence of a pressure balance valve on the pre-separation gas phase pipe, the pre-separated liquid can flow smoothly into the pre-distillation tower. Therefore, in the second plant, under the production load at that time, the pre-separator still used 3.5M3, the installation height difference was 15 meters, and the pre-separator gas phase pipe diameter was ￠159, which is the same as the pre-distillation gas phase pipe diameter. There was no pressure balance valve installed on the pre-separator gas phase pipe. The system logistics was normal, and the raw steam consumption was 1380kg. After installing the pre-separator, the steam consumption was 1280kg. To implement supporting measures for this process, it is necessary to adjust the area of ​​the first suction external cooler and the temperature-regulated desalted water circulation volume of the first suction external cooler, because the amount of CO2 absorbed increases. If the temperature of the gas-liquid mixture rises and the ability to absorb CO2 decreases, the amount entering the first absorption tower increases, making the operation of the first absorption tower difficult. 2.2 After adding a pre-separator, why does the steam consumption decrease? The design pressure of the pre-separator is 53% under the design pressure of 1.7MPa. The decomposition rate of methylammonium is 15%. The partial pressure of ammonia in the pre-separated gas phase accounts for 92%, CO2 accounts for 3.5%, and H2O4.5%. Therefore, the pre-distilled gas The NH3/CO2 ratio in the medium is reduced from the original 8.03 to 3.94. The pre-distilled gas with a reduced NH3/CO2 ratio is condensed in the first steam heating and utilization section. The NH3/CO2 ratio in the condensate also decreases, causing the boiling point of the solution to rise. As a result, the shell temperature of the heat exchange section is increased and the heat exchange amount is increased. ； At the same time, the partial pressure of CO2 in the pre-distilled gas increased from 10.4% to 18.7%, which increased the driving force for the liquid to absorb CO2. The author used the concept of water balance in the operation process to adjust the amount of water added in the second circulation and one cooling, so that the CO2 content in the dimethyl liquid dropped to 14-15%. The amount of CO2 absorbed in the first steam heating and utilization section increased, and the heat exchange rate also increased. Since the pre-separated gas containing 92% ammonia is introduced into the first suction external cooler, more residual CO2 in the first gas from the first steam heating and utilization section can be absorbed, and the amount of CO2 entering the first absorption tower is reduced. This process can reduce the amount of reflux ammonia per ton of urea in the first absorption tower to 0.35m3. In the first stage of the decomposition system, due to the installation of the pre-separator, about half of the gas volume after decomposition of the material coming out of the synthesis tower is the pre-separated gas phase volume, that is, 53% of the excess ammonia does not enter the pre-distillation tower, so it enters the pre-distillation tower. The amount of material decreases, and after passing through the pre-distillation section, a large amount of gas phase enters the pre-distillation gas phase, and the amount of material entering the one-minute addition will decrease significantly, and the steam consumption of the one-minute addition will also decrease at any time. An example of the decrease in the amount of material entering the one-minute addition is: When the original small urea unit was expanded to more than 100kt/a, the material pipe at the bottom of the pre-distillate was replaced to ￠159. After using the pre-separator, when the pre-separation efficiency was high, serious corrosion occurred in this pipe. This corrosion condition occurred in many factories. Only by reducing the pipe diameter and using ￠108 pipes can the corrosion problem be solved. This was caused by periodic cavitation corrosion in the original ￠159 pipe when the liquid level dropped to the pipeline. At the same time, severe corrosion also occurred in the packing or trays in the pre-distillation section due to lack of oxygen. The first time the author encountered this working condition was in the Shanxi Jincheng Chemical Plant with a 200kt/a pre-distillation unit. When the unit was installed, the author suggested adding a pre-separator. Because the daily output of the plant was low, about 600t/d, steam corrosion occurred in the ￠159 pipe. ； In Shanxi Linyi Plant, in the same device, after adding a pre-separator, due to the daily production of 750 t/d, no corrosion problems occurred in the ￠159 pipe. It can be analyzed from this that the pre-distillation gas phase temperature is 115°C, which is caused by 53% excess ammonia being separated in the pre-separator. If all enters the pre-distillation tower, the pre-distillation outlet temperature is 125-130°C, then the total heat enthalpy value in one part of the gas increases, and this part of the heat is provided by one part of the added steam. Therefore, the steam consumption of one minute of addition has decreased, and this decrease should be the difference between the enthalpy value of the total amount of one minute gas and the heat enthalpy value of the pre-divided gas phase at 115°C. At the same time, due to the split flow of the two gases, the overall pressure of the first-stage system decreases, and the first-stage decomposition rate and total ammonia steaming rate increase. The first-stage decomposition rate and total ammonia steaming rate can be achieved without heating to 160°C. 2.3 The author’s technical transformation process in reducing the steam consumption value of small urea plants to 1050kg. Since small urea plants have the requirement to continuously increase production capacity, there are already many single-set units with a capacity of 180-200kt/a. Through continuous practice, the author and many factories have gradually improved the capacity of single-set units and gradually reduced the steam consumption value to 1050-1100kg. 2.3.1 Use the pre-separator well. Set up a pre-separator in front of the pre-distillation tower. It was first used in the design of the Shandong Provincial Institute in 1986. Later, the pre-separator was used in the SHS energy-saving process and the self-stripping one-stage tower process. According to various reports or field visits, the author found that the pre-separation efficiency did not reach Based on the original design value level of the Fourth Chemical Hospital, and based on the installation situation of the pre-separator after the medium-sized unit was changed to the pre-distillation process, it was analyzed that the pre-separator must first be installed so that the actual pressure of the pre-separator during operation must be close to the design value of 1.7MPa to obtain the designed pre-separation efficiency. Therefore, when setting up the pre-separator, it is necessary to have an appropriate volume and pre-separated gas phase pipe diameter. There is no heat exchange equipment in the pipe, and there is no pressure balance valve on the pre-separated gas phase pipe. The gas phase pipe diameter must match the corresponding production load, and the installation position difference must be greater than the position difference between the pre-separator and the pre-distillation tower. In the process where the pre-separator is set up by the author in the small urea plant, the pressure of the pre-distillation tower is 0.1MPa greater than the pressure of the pre-separator (actual measured value), and the theoretical head difference is 10 meters, so it is reasonable to set 15 meters during installation. Materials can circulate in the actual process. The position difference value of 15 meters has been agreed upon by many manufacturers and in the design of new energy-saving processes. 2.3.2 The supporting measures for increasing production in this process device are the area of ​​a steam heating utilization section and the area of ​​a suction external cooler that are suitable for the production load, which can play a dual role. For example, increasing the area of ​​the first steam heating and utilization section can not only increase the absorption capacity and evaporation capacity of the first section, but also increase the recovery rate of the heat energy of the ammonium methane section. When increasing the area of ​​the first steam heating and utilization section, it must be considered that the flow rate of urine in the tubes should not decrease, otherwise the K value will decrease and the heat exchange rate will decrease. The tubes of the first-suction external cooler are originally equipped with twisted aluminum rods to increase the flow rate of circulating temperature-controlled water in the tubes and increase the K value on the water side. When many factories increase the area of ​​the first-suction external cooler, they do not install twisted aluminum rods and do not increase the circulation volume of the temperature-controlled water. Simply increasing the area (usually adding one unit) will not have a big effect. When increasing the output of the device, such as adding an external cooler, the circulating water volume of the two external coolers must be increased. If a large-capacity pump is replaced, a larger-diameter water pipe needs to be replaced. It is best to use one pump for one cooler as a circulation system to increase the circulation volume of temperature-adjusting water. The pre-separated gas phase pipe line should not be connected in series through two external coolers and connected to the original external cooler close to a suction tower. The diameter of the connected pipe should be enlarged accordingly, and the corresponding desalted water circulation volume needs to be increased in the other coolers. During the technical transformation to increase production, some plants only installed pre-separators, but did not accurately grasp some of the specific experiences raised by the author in practice. After increasing production, steam consumption increased, and the amount of reflux ammonia in the first absorption tower increased, causing the ammonia condenser to operate overloaded, and the opening of the first-stage pressure regulating valve increased, gradually causing ammonia consumption to increase. 2.3.3 Utilization of high-pressure steam condensate waste heat and increasing urine flash heater Adding urine flash heater is a need for the evaporation system to improve the overall production capacity. In the pre-distillation process unit, the urine concentration after the second stage decomposition is 66.9%, which is one of the important signs of the system's process water balance (the pre-separation process is 70%). After the original simple flash separation, 70% of the urine is obtained (the pre-separation process is 74%). The author inserts a flash heater in the pipeline before the flash separator to reduce the resistance loss of the urine in the pipeline after bisection, so that the bisection tower can enter the flash separator at 0.25MPa. The heat source comes from the high-pressure steam condensate passing through 0.6MP * * 0.6MPa steam from the intermediate flash evaporation tank. Under a certain production load, with a suitable flash heater area, its heat supply and demand are balanced. The 0.6Mpa intermediate expansion tank can flash out 15% of the steam volume of the condensate. According to on-site observations, after a factory uses this waste heat utilization measure, the urine concentration after flash evaporation can reach more than 74%, which can improve the production level of the evaporation system and save 80kg/tur of urea steam consumption. The original steam consumption of the plant was 1380 kg/tur. After installing the pre-separator, the steam consumption dropped to 1280 kg/tur under the same production load. After adding a flash heater, the steam consumption dropped to 1200 kg/tur. 2.3.4 While increasing the production capacity of the first-stage absorption system and evaporation system, the steam consumption is further reduced to 1050 kg/tur. When improving production capacity, the author tries his best to tap the production potential of existing equipment, with the premise of adding as little equipment as possible. Based on the above practice, it is considered that as long as the area of ​​a steam heating utilization section and a suction external cooler is increased, the absorption volume of the section is increased. At the same time, the production capacity of the evaporation system is also increased, and the heat energy recovery for generating methylammonium is also improved. When the device capacity reaches 180t/a, the steam consumption per ton of urea drops from 1200 kg/tur to 1050 kg/tur. This 1050 kg/tur steam consumption also includes the overall scale benefits after the production scale is expanded. During the practice of this plant, the author got the consensus from Sanming, a medium-sized urea plant in Fujian. Sanming is the original 110 kt/a unit. It has added technological measures to increase the production of urine towers. In 1999, it adopted the technology of adding a pre-separator. At that time, flash heaters that utilized waste heat had not yet been used. In the technical summary of the plant, it was reported that the annual output of the device in 2001 was 260kt/a, and the steam consumption was 1100kg. It is recommended that the plant use a flash heater that utilizes waste heat, and the steam consumption will be reduced to 1050kg. This technical transformation is simple, does not change the original production process and production conditions, and the investment in technical transformation is very low. It can be solved by the current self-raised funds of small and medium-sized urea plants. 2.3.5 The conversion rate of the synthesis tower must reach 67%. During the process of increasing the production of the unit, the conversion rate of the urea synthesis tower cannot be less than 67%, otherwise the steam consumption per minute will increase. After increasing production, the production intensity of the urine tower increases. Generally, high-efficiency trays or new synthetic tower internals need to be used. If the production increase is higher than the production intensity of a single tower I=20, double towers must be used. When using twin towers, according to the author's investigation, when the production intensity I ≤ 14 in the twin towers, there is no need to use high-efficiency trays. Only 12 traditional trays (which can reduce the height of the non-tray section at the top of the tower) and 3 cyclone plates are used. For example, in a ф1400 urine tower, 5 cyclone plates need to be installed. The cyclone plate installation interval is preferably 400mm, and the tray spacing is preferably 1000-1500mm, otherwise the conversion rate will be affected. 2.4 There are several innovative technologies to improve the technical level of the aqueous solution full cycle urea unit. 2.4.1 Development of high-efficiency synthesis trays. my country's small and medium-sized urea towers all use traditional porous sieve plates. With the need to increase the production capacity of small urea units, new trays have been developed. Bubble bubble caps are added to the trays to increase the probability of gas-liquid phase contact reactions and accelerate the diffusion rate of reaction heat, thus improving the completion of the first reaction. In addition, it is necessary to increase the number of installed trays with a plate spacing of 1m to increase the reaction section in the tower. Therefore, under high production intensity (I≥20), the conversion rate is not less than 65%. Based on the new understanding of the reaction mechanism of urea synthesis, a variety of new trays and new internal parts have been developed that can improve the conversion rate of urea synthesis, aiming to improve the production intensity of the synthesis tower and reduce the energy consumption of the device. In recent years, the technology of synthesis tower internals has progressed rapidly, playing a positive role in reducing the energy consumption of traditional process equipment. The emergence of high-efficiency trays in the synthesis tower has deepened our understanding of factors affecting conversion rate. The diameter of the small urine tower is ф1200mm, and the volume of 17 m3 is equipped with 6 plates (3 cyclone plates, 3 porous plates), 20 m3 is equipped with 7 plates (3 cyclone plates, 4 porous plates), 24 m3 is equipped with 8 plates (3 cyclone plates, 5 porous plates), and 26 m3 is equipped with 8 plates (4 cyclone plates, 4 porous plates). The increase in reaction volume only increases the height, but does not increase the number of trays. Therefore, for a urine tower above 20 m3, about 2/5 of the space in the upper part of the tower is empty. The empty tower section has a great impact on the conversion rate. Under the designed production intensity (generally I=7-8), the conversion rate reaches 67% with a longer conversion time. As the production intensity increases, the conversion rate decreases. ； When the production intensity reaches 11, the conversion rate is lower than 60% ； When I reaches 14, the conversion rate rises to 64% instead, which shows that the rising speed of the materials in the tower increases and reduces the degree of backmixing in the empty tower section in the upper 2/5 of the tower. When I90%, ​​the average particle size is 1.95mm, the blanking temperature can meet the relevant packaging requirements. ② For daily output less than 500 tons, the tower diameter is above Φ12mm, the particles of 1.6-2.8mm are >80%, the average particle size is 1.85mm, and the temperature can meet the packaging requirements in summer. From the above report, it can be confirmed that the Φ12m tower device can use LP-type nozzles, which are the best nozzles for small and medium-sized urea devices to solve the granulation problem. 3. A new aqueous solution full cycle process that efficiently recovers heat energy. After the author improved the traditional process device with the pre-separation-pre-distillation process, the steam consumption per ton of urea has been reduced to 1050-1100 kg. This new process is based on this and is further developed to further recover the reaction heat of methylammonium in the medium-pressure circulation system, which can reduce the steam consumption per ton of urea to less than 900 kilograms. This new process uses two utility model patents that the author participated in: ZL99233018.1 Urea synthesis tower with heat exchanger internals, ZL0227385.1 New process for recovering heat energy in urea production. This process can increase the output of traditional process equipment by 1-1.5 times, and the steam consumption is comparable to that of steam stripping process equipment. Since it does not use high-pressure circulation loop modification, the investment in technical modification is low, and it is suitable for the technical modification of current small and medium-sized urea plants. Using the mature pre-separation-pre-distillation process to further develop it, the technology is simple, easy to implement, safe and reliable, and there is no need to bear the initial risks associated with using other technical modification processes. 3.1 Technical basis for new process development The author has analyzed and confirmed through practice that the traditional aqueous solution full cycle urea plant has the advantages of increasing production and saving energy after adopting the pre-separation-pre-distillation process. During the development, the first thing I thought about was that the operating pressure of the pre-separator should be close to the design value, so that improving the pre-separation efficiency is the first priority. The installation distance difference of medium-sized devices is set at more than 15m. According to the author's actual measurement on a certain device, the pressure difference between the pre-separator and the pre-distillation tower (according to the author's process flow) is 0.1MPa, so there must be a certain margin during specific implementation. The author takes 15m, and this empirical value has been confirmed on many devices. The pre-separated gas and pre-distilled gas must be separated, which is not only required to improve the production capacity of the sub-system, but also to give full play to the advantages of their different components. Special effects can be produced in the respective heat exchange equipment. Therefore, the installation of the pre-separator must pay attention to achieving the highest pre-separation efficiency. Without pre-separation efficiency, the respective advantages of the two gases cannot be exerted. Increasing the steaming area and adding a flash heater that utilizes waste heat are important elements in the author's overall technical transformation ideas. This measure can not only improve the production capacity of the device, but also reduce energy consumption. Increasing area and increasing production capacity must match each other to achieve better results. 3.2 Technical content for further development 3.2.1 In the first step of energy-saving technical transformation, the heat energy recovery rate of the CO2 gas generated in the first stage of the decomposition system is only increased in the first steam heating and utilization section and the first suction external cooler. Therefore, to develop a new heat energy recovery work, it is necessary to extract part of the CO2 that originally entered the synthesis tower reaction and send it to a section of heat energy recovery equipment that requires high energy. That is, a heat energy utilization section is added to the first section to reduce the steam consumption of the first section and increase the area of the original first steam heating and utilization section to reduce the steam consumption of the first section of evaporation. Through calculation, the heat energy recovered in these two parts can reduce steam consumption by 300kg, reducing steam consumption per ton of urea from the current 1050-1100kg to 800-900kg. 3.2.2 Improving the conversion rate of the urea synthesis tower is one of the important factors in reducing steam consumption in the new process. Due to the increased amount of methylammonium in the new process, the following two process conditions must be used when calculating process materials.: (1) The H2O/CO2 ratio entering the tower should not exceed 0.85. Therefore, the absorption water in the first and second stages can be used multiple times, and the CO2 added by the medium-pressure system can be absorbed. (2) The composition of the methane ammonium liquid should be kept consistent with the traditional process, so as not to increase the equilibrium pressure of a section of the system, so as to avoid causing changes in system operating conditions and causing adverse consequences. The temperature of the ammonium methane liquid entering the tower is still 95°C, which will not lead to increased corrosion of the methane ammonium pump cylinder and its internal parts. Therefore, the H2O/CO2 ratio entering the tower needs to be maintained at 0.85. At present, the highest conversion rate of various domestic urine tower internals is 68% when the H2O/CO2 ratio is 0.65. In this new process, the H2O/CO2 ratio entering the tower increases to 0.85 due to the increase in the methylammonium circulation amount in one stage. Under this working condition, in order to ensure that the conversion rate is not less than 70%, new synthesis tower internals must be designed to ensure that the conversion rate is 70% or above under a high H2O/CO2 ratio. The design guiding principle of the new urine tower internals is an isothermal synthetic tower internals, which keeps the temperature at the top and bottom of the tower basically consistent to improve the conversion rate. The three materials enter the tubular reactor set up in the tower with NH3/CO2 less than 3. This reactor is also a heat exchanger. This equipment is made of high-temperature resistant 25-22-2 material and is fixed under the large cover at the top of the tower. After the reactants are moved from the central tube to the lower part of the tower, liquid ammonia is added until the NH3/CO2 is 4.2-4.3 to balance the reaction heat and improve the conversion rate. For this purpose, it is still necessary to use the existing high-efficiency trays in the tower and use them after slight modification. If the original medium-sized device does not need to increase its production capacity, and this process is used for transformation, only the amount of CO2 entering the tower is adjusted, and part of the CO2 is fed into each heat exchange equipment at medium pressure, the steam consumption value can also be reduced to the design value. The internal parts of this urine tower are placed at the upper part of the urine tower. The three materials enter the tubular reactor. The first reaction is basically completed in this internal part. ； After the reaction, the material is guided from the central tube to the bottom of the tower, and the reaction heat is transferred to the rising molten material in the tower. The rising process of the material is also the process of the second reaction formula. This structure creates a good high-temperature environment for the transformation reaction. Part of the liquid ammonia needs to be added at the bottom of the tower to bring the NH3/CO2 to 4.2 to maintain a balance between the temperature at the bottom of the tower and the temperature at the top of the tower. The temperature at the top of the tower is 186-188°C. High-efficiency trays are also arranged in the tower from bottom to top, so it can be ensured that the conversion rate is not less than 70% (guaranteed value) under the condition of H2O/CO2 0.85. 3.3 Advantages of using new energy-saving technology to transform the existing traditional urea plant. The urea plant that has been transformed using pre-separation and pre-distillation can continue to be transformed according to the new process. The process changes are small, the addition of equipment is small, and the technical transformation project is easy to implement. The early technical transformation project can continue to play a role, but the investment in technical transformation is not big. Therefore, the new energy-saving process is simple and easy to master, is a low-input, high-output solution, and is suitable for the transformation of medium and small urea plants. 4． Conclusion In this article, the author introduced the 40-year technological development experience of the aqueous solution full cycle process urea unit in China, so that the steam energy consumption is the same as that of the stripping process unit. If we evaluate the advancement of technology in terms of steam consumption, we can say that the Chinese-style aqueous solution full cycle urea process is no longer a backward process. If all the domestically developed new technologies introduced in this article are used in the new design or the technical renovation of the old equipment, the ammonia consumption and quality of urea as well as the safe production of the equipment can be ranked among the world's advanced levels. The author believes that the various existing energy-saving processes in China generally require high investment in technological transformation, and some of the effects are not as good as the new cost-saving processes developed by the author. Moreover, the technological transformation task can be completed by the technical strength of our factory. Therefore, I hope that many urea manufacturers will reach a consensus with the author to transform the backward appearance of the original process equipment.

The pre-separation and pre-distillation process flow is: After the urine from the urine outlet tower is decompressed by the pressure reducing valve, it enters the pre-separator on the sixth floor of the main factory building. The urine from the pre-separator goes to the pre-distillation tower. The gas from the pre-separator merges with the primary gas and dimethyl liquid at the entrance of the primary suction cooler and enters the primary suction cooler. The effect of implementing this process is: Reduce the load on the medium-pressure system, stabilize the operation of the medium-pressure system, and reduce steam consumption.