The development of the fully recirculating urea process in aqueous solutions in China

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By chance, I came across an article written by Professor Qian Jingqing, a veteran expert in China’s urea industry. It describes the remarkable 50-year development of urea production in our country, and I found it very interesting. I am sharing it here with all those who are dedicated to working hard in the urea industry. Please ask the moderator to remove it later. Commemorating the 50th Anniversary of the Birth of China’s Urea Industry – The Development of the Fully Recirculating Aqueous Solution Urea Process in China. Author/Source: Qian Jingqing. Date: 9-18-2008. China’s pilot urea production facility with a capacity of 3,000 tons per year was put into operation at the Nanjing Fertilizer Plant in 1958; thus, it has been 50 years since the industrial production of urea began in China. The experiments started with the aqueous solution semi-circulation method, where the amount of ammonia in the ton of urine-derived exhaust gas was around 650 kg; later, the high-efficiency semi-circulation method was adopted, which improved the degree of ammonia recovery from the decomposition gas, but 176 kg of ammonia remained in the exhaust gas. In 1960, China introduced a 10 kt/a non-circulating urea plant from the Soviet Union, which was put into operation at the Taiyuan Fertilizer Plant. In 1962, the semi-circulation system at Nanjing Yonglining Plant passed **technical appraisal. Two sets of 40 kt/a semi-cyclic plants designed by the Fourth Research Institute of Chemistry using this process were put into operation successively at the Wujing Chemical Plant in Shanghai and the Quzhou Chemical Plant in Zhejiang. In 1966, two 80 kt/a aqueous solution full-circulation process units from the Dutch company Stamicarbon were put into operation at the Luzhou Natural Gas Chemical Plant in China. Meanwhile, the Fourth Research Institute of Chemistry utilized the process technology data related to the aqueous solution full-circulation method obtained from the Shanghai Institute of Chemical Technology, and by drawing on the design of the imported units, it developed its own 80 kt/a and 110 kt/a urea production facilities (with all equipment for these facilities also manufactured domestically); these facilities began operations at the Shijiazhuang Fertilizer Plant in November 1966. Due to **interference, the plant stopped operating after the test run. By January 1970, the newly built 45 kt/a ammonia synthesis unit coupled with an 80 kt/a urea production plant at the Hunan Xiangjiang Nitrogen Fertilizer Factory came online. This can be regarded as the first set of process equipment in China that used a full aqueous solution circulation system; a pre-separation process was employed, and the lined synthesis tower (with an inner diameter of 1.4 m) was developed by the Shanghai Boiler Factory – this was the first urea production tower made in China. The rectification section of this tower was a floating valve tower as well, making it the first of its kind. The author organized and supervised the commissioning of China’s first domestically produced urea plant, and continuously improved and refined it to ensure its normal operation. From the 1970s to the early 1980s, China built 32 plants equipped with medium-scale urea production units of 80–110 kt/a, totaling 38 sets (including two imported sets), which are referred to as the first 38 sets. During this period, the design of the mid-urine collection system was continuously modified, resulting in a total of four versions. The fourth edition is a more improved version, and it is used in projects such as those at Quhua in Zhejiang and ammonia plants in Jiangxi. In 1986, a new phase of development emerged in China’s urea industry; numerous small ammonia plants underwent technical upgrades to produce urea, which brought about a qualitative change in these plants. First, three pilot plants capable of producing 40 kt/a of urea using the aqueous solution full-circulation process were put into operation in Zoucheng and Pingdu in Shandong Province, as well as in Hui County in Henan Province. In particular, the plant in Hui County, which was operated under the leadership of the author, succeeded in its first trial run, offering promising prospects for converting those small ammonia plants – which at that time accounted for 50% of China’s synthetic ammonia production – from producing ammonium carbonate to producing urea. During the Seventh and Eighth Five-Year Plans, **150 small nitrogen plants with favorable conditions were selected to switch to producing urea. In total, there were over 120 such plants; 18 of them had two production units each. Shandong’s Luxi region and Shanxi’s Linyi region each had three such units. Approximately 70% of these production units were upgraded to a capacity of over 100 kt/a, with some even having a capacity of 150–200 kt/a.** During the \"Seventh Five-Year Plan\" to \"Ninth Five-Year Plan\" periods, 16 more medium-sized urea plants were added, also known as the latter 16 plants; thus, the total number of such plants reached 54. The latter 16 sets were built in the original small nitrogen plants; in the original medium-sized plants, ammonia stripping and CO2 stripping were used for the second set built there. Since the late 1990s, small urea plants have intensified their technological upgrades, resulting in a significant increase in plant capacity and a substantial reduction in steam consumption; they have thus become models of high productivity and low consumption for processes using the full-circulation aqueous solution method in China. Through simple upgrades aimed at increasing production efficiency and reducing energy consumption, the steam consumption can be reduced to 1100 kg. Many new urea production plants built from former small nitrogen fertilizer facilities still use this process; the investment required for such plants is less than half that of plants using the CO2 stripping process. Plants with capacities of 8–13, 12–20, and 18–30 have been constructed successively. The aqueous solution full-circulation process plant at LuXi Group’s Eighth Factory is designed for a capacity of 400 kt/a, and its actual production volume has recently reached 500 kt/a. If improved using the author’s techniques, the fuel consumption is expected to drop to 1000 kg. This is the largest aqueous solution full-circulation process unit in our country at present, and in terms of the level of steam consumption reduction, it can compete with large-scale CO2 stripping and ammonia stripping processes. The following introduces the development of the fully closed-loop urea technology in aqueous solutions in our country. ?1 Problems in the initial stage of operation of the urine treatment system and improvement measures 1.1 Major design issues (1) Frame height: The design of the first set at Xiangjiang Nitrogen Fertilizer Plant was influenced by the “technological revolution”; as a result, the frame height was only 18.5 m. This led to insufficient height difference between the equipment in the circulation system, causing problems such as ammonia from backflow not being able to enter the absorption tower and the first-stage pump not functioning properly, which made it difficult for the plant to operate normally. It was changed to 23.5 m at the design of the second version, and to 26.5 m at the design of the fourth version. In 1986, the elevation of the three small urea test units was 28.0 m; by the time the design for the 40 kt/a small urea production unit was finalized, this elevation had risen to 30 m. As a result, once the small urea production unit went into operation, it could operate stably over long periods of time. ? (2) Large design margin: Due to the substantial excess in the original design, especially for the first vapor heater and the second vapor heater, the area per ton of urine is much larger than that of the imported units. The increase in biuret in each section was not taken into account during the design; the allowable biuret value for each process is known from the installation specifications. At that time, all urea production facilities were new plants, and their ammonia synthesis capacity had not yet reached the designed level; as a result, the urea plants operated at low load, causing the content of biuret in the final product to be 3%–4%. This situation also occurred when the small urea plant was put into operation, due to the inability to increase the ammonia synthesis capacity. 1.2 Exploration to reduce biuret in the early operation phase of the medium-volume urine treatment plant (1) Li Yushu, chief engineer at Qilu First Chemical Industry, compared the area per ton of urine for the first vaporization stage and the second vaporization stage of the imported plant, and concluded that the originally designed area was too large. The Nitrogen Division of the former Fertilizer Department specifically held a technical exchange meeting on quality issues at this factory. At the meeting, Qilu Yihua shared its experience in reducing dicyandiamide formation; for a production capacity of one ton of urea per day, one pipe should be used ； The area of the second evaporation stage should not be large in order to reduce the residence time. The second heater is the key component for increasing the amount of biuret; since the urine concentration is already 99%, the increase in biuret levels is proportional to the square of this concentration ； Moreover, it operates at 140 °C; with a large area, the increase in biuret content occurs very rapidly. At that time, most plants blocked part of the heat exchange tubes to make the tonnage of urine per unit area equivalent to that of the imported equipment, thereby reducing the biuret level to the acceptable standard of 1.5% at that time. For example, if Xiangdan is designed to produce 240 t per day with a secondary evaporation area of 7 m2, it needs to produce 310 t per day in order to achieve first-class quality. Design for the second version of the small urine treatment unit (capacity: 180 t/day); the area required for secondary evaporation is 5.6 m2. At a design capacity of 180 t/day, the concentration of biuret is around 1.5%, and it is only when the production volume reaches 220 t/day that first-class product can be obtained. ? When production capacity increased, the tube bundles suffered severe wear, and their service life was less than half a year. Xiangdan Plant was the first to use titanium alloy for this purpose; it has been in use since 1986 and remains intact to this day. (2) Through collective exploration, the vacuum feeding operation method was created. During the introduction process, after being divided into two streams, the urine first enters the urine tank and is then continuously pumped to a section for evaporation. The urine tray must be operated at a low liquid level. Biuret increases by 0.1% in the urine tank, sometimes 0.2%. If the two-stage decomposition rate does not meet the requirements and there is a high level of free ammonia in the urine, the urine pump will stop pumping fluid, which prevents feeding and thus forces the urine to circulate ； Furthermore, since the exhaust pipe of the urine tank is connected to the exhaust stack, it also increases ammonia consumption. Thus, the vacuum feeding method was proposed; as long as the feeding to the urine flash tank is stable, the vacuum in the first-stage evaporation section will also remain stable. (3) The Xiangjiang Nitrogen Fertilizer Factory, in collaboration with the Changsha Water Pump Factory, successfully developed a melting pump equipped with a liquid level auto-regulation function. The purpose of this is to maintain the liquid level of molten urea in the pump’s inlet pipeline, preventing it from accumulating in the secondary vapor separator – a device that is crucial for ensuring that levels of biuret do not exceed acceptable limits. This technology won the First Prize for Scientific and Technological Progress. Although it only received a certificate, it made a significant contribution to the entire domestic urea production facility; it served as a reliable guarantee for producing urea of first-class quality, and it also resolved the issue of having to import melt pumps from Japan for the construction of such facilities. The initial urea plants built used the melt pumps produced by Changsha Pump Factory. Later, Jiangsu Jingjiang Stainless Steel Pump and Valve Factory and Xi’an Titanium Pump and Valve Factory improved the structural defects of the melt pumps from Changsha Pump Factory, thereby enhancing their design. Xiangjiang Nitrogen Fertilizer Factory also made improvements during use to make the operating performance of the melting pump more stable. However, later pump manufacturers did not make much effort in terms of automatic level control; instead, they installed pipe sight glasses in the pump inlet lines to manually maintain the liquid level at 7 meters. (4) The evaporation granulation system reduces the pipeline length by eliminating right-angle elbows, achieving the shortest possible pipeline configuration. The pipeline from the melt pump outlet to the top of the granulation tower runs at an angle into the tower, and the nozzles at the tower top also enter at an angle. This serves two purposes: to prevent crystallization-related blockages and to shorten the distance, as the formation of biuret is related to the residence time of urine in the pipelines. 1.3 Improving domestic operating equipment and high-pressure valves through practical production experiences: When developing high-pressure plunger pumps, process requirements were established to prevent liquid from vaporizing inside the cylinder, which could affect the pump’s efficiency; thus, the plunger speed of ammonia pumps should not exceed 90 r/min, while that of propylene pumps should not exceed 70 r/min. During later manufacturing, the piston diameter was increased to boost the pump’s liquid delivery volume. Increasing the speed further afterward is not a good approach, but the value indicated on the nameplate already represents the actual volume of fluid pumped; the pump’s volumetric efficiency was taken into account during the design. 1.3.1 Problems gradually emerging in ammonia pumps during use: (1) Check valves are used at the liquid ammonia inlet and outlet on the cylinder body, with neoprene \"O\" rings for sealing. All seals of the pump body use neoprene “O” rings. As soon as the ammonia pump stops operating, the liquid ammonia vaporizes and the temperature drops, causing the rubber to age; this leads to leaks when the pump is restarted. Later, nitrile rubber was used, which slows down the aging process, but it is not the best solution either. Appropriate measures should be taken based on the conditions during pump shutdown in order to prevent the cylinder body from cooling down, thereby extending the service life of the sealing ring. Due to the aging and rupture of the sealing ring inside the cylinder, high-pressure liquid leaks into the low-pressure side, causing erosion and corrosion within the cylinder. The cylinder body is made of corrosion-resistant carbon steel, but its service life was less than a year due to the presence of methylammonium in the liquid ammonia. Regarding the handling of cylinder body corrosion, the former Xiangjiang Nitrogen Fertilizer Factory would bore the cylinder larger, weld stainless steel filler metal onto it, and then bore it smooth again. In some factories, after enlarging the cylinder block, stainless steel sleeves are installed; all sealing surfaces of the pump are changed to flat-seal types, using 2 mm thick nylon gaskets to ensure a fit with the uneven sealing surfaces. Otherwise, there is a risk of liquid ammonia leaking under excessive pressure. (2) Initially, the plunger packing was made using asbestos rope filled with graphite, compressed in custom-made molds. Its service life varies depending on the manufacturing techniques and installation methods of different factories; good quality ones can last for half a year, while those of lower quality last less than a month. Later, manufacturers produced filling materials specifically for TFA (carbon fiber) molding, which yielded good results. The guide sleeve in the plunger packing ring consists of babbitt alloy mounted inside a steel ring; however, this guide sleeve is prone to corrosion by the medium, which can cause it to come loose and abrade the surface of the plunger, thereby reducing the service life of the packing. The manufacturer switched to graphite guide sleeves, and later the Type 1A pumps also adopted graphite guide sleeves. (3) The material of the filler box is also carbon steel; however, if sealed water circulation is switched to using a level gauge, corrosion-resistant chromium steel should be used instead. ? (4) The ammonia pumps manufactured by Shandong Weifang Shengjian completely overcome the aforementioned shortcomings: all cylinder bodies and components are made of chromium steel, combined valves are used inside the cylinders, and all seals have been replaced with flat seals. 1.3.2 Problems encountered during the operation of the Type 1A pump: (1) The sealing surfaces of the two check valves at the inlet and outlet of the cylinder are fitted with PTFE gaskets, while the springs of these check valves are made of Mo2Ti material. During use, since the temperature of Liquid A is between 90 and 95 °C, the gasket tends to soften and get squeezed out, failing to provide a seal. The spring stiffness of the Mo2Ti material is insufficient, causing the check valve to leak fluid; in severe cases, no fluid is delivered at all. The solution adopted by Xiangdan Plant was to replace the tetrafluoro gaskets with nylon gaskets, and to use electric furnace wires from scrapped ammonia synthesis towers to wind the spring steel (Cr20Ni80); this ensured the proper stiffness and prevented leakage from the valve discs. The spring in the check valve at the bottom of the synthesis tower was later made from this material as well, which solved the problem of blockage in the material feed pipe to the tower due to leakage from the valve during short interruptions in operation of the tower. (2) The cylinder block of the No. 1 pump is cracked. Since there are two check valves at the inlet and outlet of the cylinder body, the cylinder body has four holes, and it is called a four-way cylinder body. Due to the high stress at the opening in the cylinder block where it intersects with the cylinder block, the suction pressure during operation is 1.7 MPa, while the discharge pressure is 20.0 MPa. Under the influence of alternating loads, stress corrosion at this intersection is severe. At that time, the height of the framework at the Xiangjiang Nitrogen Fertilizer Plant was only 18.5 m; the liquid column distance from the outlet of one absorption tower to the inlet of Pump 1A was only 5.5 m. Air entrapment in Pump 1A was particularly severe, and when liquid containing air is under pressure, its instantaneous pressure can be 2 to 3 times higher than the original pressure. Under alternating loads, stress corrosion is more severe; the cylinder block cracks quickly, with the shortest service life being just over half a month. Xiangdan Factory has used many different types of steel to manufacture cylinder bodies, yet the problem of cracking in these cylinders has not been resolved. The factory tried combining two half-cylinders together, which gave better results, but this did not solve the issue at its root. The final solution was to replace the one-way valves at the inlet and outlet with combined valves, and to create a cylinder body with one fewer hole – what is known as a three-way cylinder body. This approach was first proposed and put into use by Xiangdan Factory. Additionally, the height of the frame was increased to 23.5 m, and the height of the liquid column at the inlet of the No. 1 pump’s cylinder body was adjusted to 10 m, thereby resolving the problem of cracking in that pump’s cylinder body. 1.4 Overpressure issues in the fourth and fifth stages of the CO2 compressor: The CO2 compressors at Xiangjiang Nitrogen Fertilizer Plant are imported; one is an Italian four-stage compressor, while three are Japanese five-stage compressors. The domestic four-stage compressor was manufactured after mapping the compressors from the Xiangdan Plant. Imported compressors have not encountered any problems during use, but CO2 compressors manufactured domestically, whether by Shanghai Compressor Factory, Shenyang Compressor Factory, or Sichuan Jianyang Compressor Factory, all suffer from overpressure issues in stages 4 and 5. The analysis indicates that the cause lies in process factors: during design, the manufacturer intended to use pure CO2, but when air was used instead of oxygen for corrosion protection in the synthesis tower, the purity of CO2 dropped to 95%–96%. This reduced the compressibility coefficient of the gas. Pure CO2 experiences a significant reduction in volume at pressures above 3.0 MPa, resulting in a large difference from the compressive behavior of an ideal gas; the presence of air led to overpressure in stages 4 and 5. The solution is to enlarge the bore diameters of the fourth and fifth stages as the compressor’s production capacity increases. If there is no need for increased production, the issue can be resolved by increasing the area of the four-stage cooler or raising the volume of cooling water circulation, thereby reducing the gas inlet temperature to 35 °C (it was originally 40 °C, based on a CO2 purity of 10.0%; with 4% air present now, a cooling temperature of 35 °C prevents liquefaction, as the design of four-stage compressors aims to avoid the problem of CO2 liquefying easily when compressed to 8.0 MPa and then cooled). The material used for the inter-stage coolers of compressors was originally carbon steel. The designers were unaware that CO2 gas contains saturated water vapor; after cooling, this water becomes acidic and corrosive. In some plants, accidents have occurred due to the failure of oil-water separators caused by corrosion, resulting in insufficient strength and subsequent explosions during operation. For the outlet pipelines of each stage of the compressor, since the temperature of the gas after compression is high and does not reach the dew point temperature, carbon steel can be used; however, the oil-water separators used to separate the fluid after cooling in each stage’s cooler, as well as the inlet pipelines from these separators to each stage, must be made of stainless steel. The inlet and outlet pipelines of the desulfurization tower, as well as the pipelines between towers, must also be made of stainless steel. The bottom head of the desulfurization tower is lined with stainless steel plates. 1.5 Structure of high-pressure valves: In Xiangni’s urea production facility, the high-pressure valves were manufactured by Hunan Yueyang Sihua Construction Factory. These valves do not leak when the temperature of the fluid in the pipelines changes. The valves manufactured by Sichuan Chemical Construction Factory also do not leak. However, other valve manufacturers use a different structure for high-pressure valves in ammonia synthesis systems, where the valve seat and valve body are assembled separately; for use in urea production systems, only the material used is changed to Mo2Ti, while the structure remains unchanged. Due to the temperature changes in the urea system pipelines during startup and shutdown, when the temperature is high, the fasteners securing the sealing surfaces loosen, resulting in leaks when the system is started at low temperatures. For example, in the case of high-pressure liquid ammonia pipes, the liquid ammonia must be heated to 150 °C during pressure increase; once the pressure increase and ammonia circulation are complete and liquid ammonia is introduced, the temperature drops to around 40 °C, at which point leakage occurs from the valves. At the beginning of operation of the urea plant, when restarting after a shutdown, all equipment flanges, pipeline flanges, valve connection flanges, and valve stem packing caps must be tightened; otherwise, leaks will occur everywhere during startup. 1.6 The separator and the heater (or cooling condenser) are combined into a single unit. To avoid detection by patent authorities, the heater or cooler and the condenser are merged with the separator, which results in a reduced efficiency of gas-liquid separation and significant ammonia loss. It was separated during the design of the second edition. Medium-sized plants built according to a single design scheme separate them on their own. Unfortunately, the small urea plant was also designed as an integrated unit, reportedly following the model of the large urea plants; however, due to improper parameter selection, the separation efficiency was poor. As a result, various factories separated these components on their own during upgrades aimed at increasing production. 1.7 The diameter of the granulation tower is 9 m. At the initial stage of construction of the medium-urea plant, the diameter of the granulation towers was 9 m, and sticking to the tower was a particularly serious problem. When the production capacity exceeds 100 kt/a, it is necessary to increase the rotation speed of the nozzles, but this in turn exacerbates the sticking problem on the tower walls and bottom. When urea sticks to the tower walls and heavy urea lumps fall downward, numerous accidents occur in which the grating plates are damaged ； When short stops require urea lumps to be applied on the grid plates at the lower part of the tower, the lumps on the tower walls fall, resulting in several injuries and deaths; this is caused by an unreasonable design. Therefore, in the early stages of operation of the urea plant, the employees also paid a heavy price for this. 2 Technical development in terms of process design 2.1 Understanding the objective laws behind exhaust gas explosions: Initially, the urea synthesis tower in the urine treatment plant was protected from corrosion using oxygen; therefore, a small air separation unit with a capacity of 50 m3/h was installed specifically for this purpose. At that time, the exhaust system of the medium-temperature urine treatment unit experienced multiple explosions; due to the high oxygen content in the exhaust gases, air was used instead for corrosion prevention in an attempt to reduce the oxygen level in those gases. However, after switching to air, due to the use of a pre-distillation process, secondary air must be supplied at the bottom of Unit 1 for corrosion prevention (2 m3 per ton of urine), and the oxygen content in the exhaust gas remains around 10%. According to the explosion phase diagram, if the oxygen content in the exhaust gas is below 4%, the exhaust gas composition is not within the explosion zone. An analysis of the factories where explosions occurred shows that they are located in the northeast, northwest, and north China regions. In winter, overpressure often occurred in the medium-pressure section, and explosions took place when the bypass valves of the pressure control valves in that section were suddenly opened. Under these conditions, due to the high condensation efficiency of the ammonia condenser and the effective cleaning performance of the medium-pressure scrubber, explosive gases can become concentrated. When overpressure occurs, a bypass line is activated, which creates a throttling effect that generates static electricity, thereby leading to an explosion. Therefore, in the subsequent operating procedures, it was specified that the purity of the feed gas CO2 must not be lower than 95%, and it should be adjusted as close as possible to 98% (in the propylene decarburization process). When it reaches 90%, the main control operator must stop the tower immediately without any prior request. The specified condensation and washing process parameters for the ammonia condenser and the scrubber are that the temperature of the condensed ammonia should be no lower than 25 °C, and the temperature of the ammonia solution being washed in the scrubber should be no lower than 35 °C, in order to control a certain ammonia partial pressure in the exhaust gases. This is the control parameter for summer, based on an inlet water temperature of 32 ℃ for the urea plant in the cooling cycle. Therefore, the circulation water volume must be controlled in the northern regions during winter. Most of the current devices have increased production, and it is not too difficult to keep these temperatures within safe operating limits. A pressure control valve must not be closed completely during startup and operation, in order to prevent an explosion that could occur if the bypass line has to be activated due to system overpressure ； The ammonia condenser, as well as the flanges of the equipment and pipelines following it, are equipped with an electrostatic grounding system. These systems are checked once a year before the thunderstorm season, and the pipelines are replaced with stainless steel to ensure a smooth inner surface, thereby reducing the generation of static electricity due to friction in the airflow. Since then, no similar explosion accidents have occurred in medium-sized urea plants or small urea plants built after 1986. The Lanzhou Institute of Chemistry once developed a technology for removing hydrogen from CO2 gas, which was tested at the Liujiaxia facility in Gansu; this technology can address the issue of exhaust gas explosions at their source. In recent years, Hubei Institute of Chemistry has developed CO2 gas dehydrogenation technology, which was tested at Sichuan Meifeng Group with excellent results. However, this technology requires the H2S content in the CO2 gas to be less than 1×10‑6, and due to factors such as the high cost of catalysts, it has not been adopted in urine treatment facilities. In current plants that use natural gas as a raw material, the H2S content in the CO2 gas is very low, so there is no need for a desulfurization step; this technology holds promise for wider adoption. 2.2 The emergence of the pre-distillation process: The pre-distillation process was developed in the mid-1960s on the South Chemical pilot plant. When Shijiazhuang Fertilizer Plant, the first factory for urine processing in China, began operations in 1966, it already used a pre-distillation process (as the plant manager at that time, Mr. Zhu Junbiao, had been transferred from a pilot plant of Nanhua). In 1973, the design review for the second version of the medium-pressure uranium plant was held in Nanjing, where a pre-distillation process was adopted. At that time, three plants were built according to the second-phase design: Hunan Zijiang Nitrogen Fertilizer Plant, Sichuan Chemical Plant, and Guangxi Hechi Nitrogen Fertilizer Plant. At that time, the designers at the Fourth Chemical Engineering Institute failed to reach a consensus on this process, and the pre-separation process was used again in the third and fourth revision designs. However, manufacturers have come to recognize the advantages of this process in practice, and most have switched to a pre-distillation process. Therefore, the pre-distillation process is the first technical achievement of China in improving the fully closed-loop urea process using aqueous solutions. In 1986, the design of the small urea plant adopted a pre-distillation process flow, and an external cooling unit was added, enabling this process to increase production while saving energy, thus demonstrating great viability. ? The advantages of the pre-distillation process lie in the following aspects. ? (1) Reduced steam consumption? A pre-separation section is located at the upper part of the pre-distillation tower, a pre-distillation section is in the middle, and the lower part serves as a gas-liquid separation section for the first stage of decomposition liquid. The 160 ℃ vapor separated in the separation section undergoes mass and heat exchange with the 115 ℃ material coming from the urine tower in the pre-distillation section. The temperature of the pre-distillation vapor is between 125 and 130 ℃, while the temperature of the pre-distillation liquid rises to 130–135 ℃; it then goes to the first-stage heater, as a result of which the steam consumption for this stage is reduced. In 1981, Lunan Fertilizer Plant, which used the original pre-separation process, measured its steam consumption at 1500 kg (with a conversion rate of 65%), whereas for the pre-distillation process used by Sichuan Chemical Plant, the actual steam consumption in 1983 was 1330 kg, which is 170 kg less than that of the pre-separation process. A well-operating small urea plant with a capacity of 120 kt/a consumes 1,380 kg of steam. (2) The system’s water balance conditions were improved: the vaporization temperature dropped from 160 ℃ to 125 ℃, resulting in a corresponding decrease in the vapor pressure; as a consequence, the amount of water introduced into the first absorption tower decreased. This allowed for a reduction of 85–90 kg of water per ton of urine processed. This in turn increased the amount of water available for use in the second stage of absorption, as well as the amount of water returned to the first absorption stage, **thereby improving the stability of operations in the first absorption tower. By entering the distillation section of the absorber, the ammonia concentration on the trays can be reduced from 95% to 89%, thereby improving the ability to purify CO2. This results in few ammonium methylate crystals forming on the trays, as well as fewer instances of excessive CO2 levels in the absorbed gas. In the first absorption tower, the amount of water brought in by the pre-distillation gas is reduced by 85–90 kg; the amount of water returned from the second stage to the first stage increases. As a result, the total amount of water decreases. This allows the H2O/CO2 ratio in the urine tower to be reduced to 0.65, and the conversion rate of the urine tower increases to 67%, thereby reducing the steam consumption in the first stage. ? (3) The heat recovery rate in the first vapor heating section has increased. As more water can be used for absorption in the second stage, the CO2 content in the dimethyl ether drops from 20% to 16%–18%; as a result, more CO2 is absorbed from the vapor in the first vapor heating section, leading to an increase in the heat recovery rate. As a result, the heat load entering the absorption tower decreases, reducing the ammonia reflux per ton of urine to 0.6–0.8 m3. (4) Can it reduce the pressure and temperature in the second stage of operation? As the CO2 content in the dimethyl ether solution decreases to 16%–18%, the gas-liquid equilibrium pressure of this solution drops by 0.05 MPa. Therefore, the operating pressure in the second cycle and first cooling stage can be maintained at 0.2 MPa (gauge pressure); the pressure after the first cooling stage can be reduced from 0.2 MPa to 0.15 MPa. The pressure in the second separation column is then 0.25 MPa, allowing the decomposition temperature in the second stage to be lowered to 135 ℃ ； As a result, the energy consumption for binary addition can be reduced, and the amount of biuret hardly increases during this process, which plays a very important role in increasing the proportion of first-class finished products. 2.3 Development of the pre-separation–pre-distillation process: In 1983, Qilu No.1 Chemical Plant developed this pre-separation–pre-distillation process, which involved reinstalling the pre-separator from the original 110 kt/a plant (φ800×2600 mm, V=1.3 m3) in front of the pre-distillation tower, with an installation height difference of 18 m. At that time, none of the medium-pressure urine treatment units were equipped with external coolers; the pre-separated gas entered directly into the bubbling section of the absorption tower. Following in the footsteps of those that have restored their pre-separators are Lanzhou Fertilizer Plant and Zhejiang Quzhou Chemical Plant. ? The purpose of restoring the pre-separator at that time was to reduce steam consumption and improve the operation of the primary absorption tower. If the pre-separator performs well (and is designed to separate 50% of the total vapor volume), it is possible to reduce the amount of material entering the pre-distillation tower, thereby lowering the vapor consumption in that stage. Furthermore, with a pre-distribution temperature of 115 °C, it not only reduces the amount of water introduced into the first absorption tower but also lowers its thermal load. In fact, this process development did not have a significant impact. Based on practical experience, the reason may be that the volume of the pre-separator in the original design was too small, which led to an increase in pressure within the pre-separator and prevented it from achieving the desired pre-separation effect at the designed pressure of 1.7 MPa. As a result, the load on the pre-distillation tower decreased only slightly, and hence the steam consumption for the first stage of distillation also decreased not significantly. Additionally, both the pre-separated gas and the dimethyl liquid (with a CO2 content of 20%) go directly to the bubbling absorption section at the bottom of the first absorption tower. Unlike in smaller urea production plants where the CO2 content in the dimethyl liquid is reduced to 16%–18%, these fluids first pass through a vaporization heating section and an external cooling unit, which increases heat recovery and reduces the thermal load on the first absorption tower, thus playing an important role in ensuring its stable operation. 3 Development of small urea technology since 1986: This year is used as a dividing point because three small urea testing units were put into operation one after another in 1986. The first two sets started operating in Zoucheng, Shandong in January 1986, and in Pingdu, Shandong in May 1986; the operation of those first two sets was not smooth. Commissioned by the then Minister of Chemical Industry and the Director of the Fertilizer Department, the author participated in the technical renovation and commissioning of the plant in Huixian, Henan. The unit was successfully commissioned on December 31, 1986, and became profitable in January 1987. Systematic measurements were conducted after three months, and the results showed that the facility met all the designed technical and economic criteria; in particular, when the production rate was 140 t per day, the product quality was fully up to standard. In April 1987, the Ministry of Chemical Industry and leaders from various ministries held a field meeting in Hui County, Henan, to discuss the technical feasibility and possibilities of converting small-scale nitrogen fertilizers into urea. Following this meeting, during the **7th Five-Year Plan period, 77 small urea production units were approved for construction (of which 2 included CO2 stripping units and 3 medium-pressure combined urea production units; all 5 of these units were later converted to use a full-water-solution circulation process). During the **8th Five-Year Plan period, investment was provided for the technical upgrades of 150 such units. After 1986, due to the construction of numerous small urea plants, the output of these small urea facilities accounted for 50% of the country’s total urea production at that time. Due to the continuous technological development in the existing small urea plants, their production capacity has been increased to over 100 kt/a; many plants now have a capacity of 150–200 kt/a, and new medium-sized plants with a capacity of 200 kt/a have also been built in these facilities. After technical upgrades, the production capacity has reached 300 kt/a. This shows that the urea production process using the aqueous solution full-circulation method, thanks to technological advancements in China, has gained recognition for its advantages among various enterprises employing this method. Therefore, many of the newly built 200 kt/a, 300 kt/a, and 400 kt/a plants still employ the urea process with a full aqueous solution cycle. Urea produced by small and medium-sized urea plants that use this process still accounts for over 50% of the total national urea production. ? Since 1986, the author has mainly been engaged in the technical development of low-cost urea. The Ministry of Chemical Industry ran four small urea plants in the past. To train the technical backbone of these small urea plants and share with them the experience gained from operating medium-sized plants, the author discussed various technical improvements that led to the successful operation of the Huixian plant, as well as the insights gained from the successful operation of Huixian’s third pilot plant. In June 1986, the author attended a consultation held in Zoucheng, Shandong, organized by the Fertilizer Department, and submitted suggestions for revisions. In 1987, I was invited by the plant in Pingdu, Shandong, to guide them through the modification work. At that time, Pingdu had only one set of equipment and was in urgent need to increase its production capacity; it was I who advised the plant on how to boost its production capabilities. The work I carried out in 1986 laid the foundation for **deciding to develop small urea plants; thus, it can be said that I was the one who blazed the path for building such facilities. 3.1 Technical advantages in the process design of small urea plants: When small urea plants were first developed, they had advantages over traditional medium-scale urea plants, and these advantages paved the way for subsequent developments in technologies aimed at increasing production and reducing energy consumption. 3.1.1 The medium-pressure decomposition process employs a pre-distillation technique, which improves the conditions of horizontal balance in the urea production process using an aqueous solution recirculation system. Maintaining the system's water balance is a fundamental condition for ensuring a healthy circulation of aqueous solutions. The improved operational conditions resulting from maintaining this balance are evident in the following aspects. ? (1) As the amount of water carried from the first stage into the absorption tower decreases, it is possible to increase the amount of liquid from the second stage cycle that can return to the first stage, thereby improving the stability of the operation in the first absorption tower. (2) Due to the reduction in the total volume of circulating liquid in stages 1 and 2, the H2O/CO2 ratio upon return to the urine tower is reduced to 0.65, thereby increasing the conversion efficiency of the traditional tray tower to 67%. This allows for a reduction in the amount of circulating liquid in stage 1, which in turn reduces the steam consumption associated with decomposition in that stage as well as the energy required to circulate the liquid. ? (3) Improvement of the two-stage absorption conditions. Due to the improvement in the water balance conditions, the amount of water used for absorption in the two-circulation-one-cooling and two-cooling processes can be increased, and the concentration of dimethyl liquid is reduced from 20% to 16%–18%. The CO2 content in the dimethyl liquid decreases, with its equilibrium partial pressure being 0.15 MPa; maintaining an operating pressure of 0.20 MPa during cooling is sufficient. As a result, the pressure in the secondary distillation column becomes 0.25 MPa. This allows the temperature for separation to be reduced from 150 ℃ to 135 ℃, which facilitates an increase in the decomposition rate in the second stage as well as a reduction in the steam consumption required for separation. After adding more water during the first cooling stage, the absorption capacity increases; the amount absorbed in the second cooling stage decreases, and the concentration of ammonia in the ammonia solution in that stage drops. As a result, the ammonia content in the exhaust gas from the second stage decreases, which helps to reduce ammonia consumption. 3.1.2 An external cooler for the first absorption stage was added, thereby enabling the pre-distillation process to achieve greater improvements in productivity and energy savings. As the CO2 content in the dimethyl ether solution decreases, and since this solution is not fed directly into the bubbling section of the first absorption tower as in the pre-separation process, it first passes through a heating section, then enters the external cooler for the first absorption stage, and finally reaches the bubbling section of that tower. The less concentrated dimethyl ether solution can absorb more CO2 from the gas stream in these two heat exchange devices, resulting in an increased rate of heat recovery in that section; this is also one of the factors contributing to reduced steam consumption in the pre-distillation process ； At the same time, it reduced the thermal load on the absorption tower, lowering the amount of ammonia returned per ton of urine from 2 m3 to 0.6–0.8 m3; as a result, the condensation area required for the ammonia recycling system became more sufficient. By adding an external absorption cooler, the task of removing CO2 from the gas stream in the absorption tower is taken over by this new cooler. As a result, the amount of CO2 that needs to be removed in the purification stage decreases, and issues such as the formation of ammonium methylate crystals or minor excess levels on the ammonia water plates in the purification stage do not occur. In the pre-separation process, the ammonia concentration on the ammonia plate is as high as 95%, while in the pre-distillation process it is 89%. Therefore, the addition of an external cooler for the first absorption stage improves the operational stability of this stage, making it easier for both the process and the operators to control its operation. Coiled aluminum rods are installed in the tubes of the external cooler to increase the turbulence of the cooling water within those tubes, thereby enhancing the heat transfer coefficient and cooling efficiency. 3.1.3 Measures shall be taken to improve the system level balance conditions. A distillation section is installed at the top of each process unit, with the aim of reducing the amount of water vapor entrained in the gas entering the first and second absorption stages after passing through the distillation process. The pre-distillation tower itself is a device equipped with a distillation section; both tray and packed columns in a two-stage column have such a distillation section, while the desorption tower is fitted with a desorption condenser, and the condensed reflux liquid is sent to the first tray ； At the top of the absorption tower, ammonia water enters plate 3#; the reflux ammonia is added at plate 1#. The ammonia concentration at plate 3# is 89%, so 2 distillation plates are required in the upper section. During the pre-separation process, ammonia solution enters plate 2#, with a concentration of 95%, and only one distillation plate is required on it. 3.1.4 All heaters are rising-film tube heaters. The material is brought to its boiling point before entering the heater; it then enters the rising-film tubes and rises in a film-like manner, which increases the K value and allows for a reduction in the heater area. A throttle hole is also provided at the bottom end of each heater tube; this allows the urine to be distributed evenly among the various tubes. Moreover, as the urine passes through the throttle hole, its upward flow velocity within the tubes increases, which raises the heat transfer coefficient. As a result, both steam consumption is reduced and the formation of biuret in that section is decreased. With the throttle orifice in place, if the secondary air is oil-free, the corrosion products and oil residues from the synthesis tower do not accumulate in the tubes of the first section, but rather in those of the second section. This is exactly the opposite of the situation with medium-sized units that do not have throttle holes in their finned tubes, where scaling occurs. After switching to the pre-distillation process, the area utilization rate of Unit 1 increased. At the same area, a 1% increase in production capacity can boost it by 60%. 3.1.5 No booster is installed in the evaporation system; ammonia cooling is used instead of water cooling for the secondary heat exchangers in the evaporation process, with a freezing capacity of 209–251 MJ per ton of urine. Under the summer rainy climate conditions in the south, the vacuum level can be reduced to the specified value (0.0067 MPa), and the moisture content of urea meets the standards for first-class quality. In most plants in the north, the temperature of the circulating water is low; therefore, water cooling is used in place of ammonia cooling, which allows for the elimination of one chiller (for a production capacity of 60 kt/a). The plants in the south also switched to water cooling later, with the total surface area of the secondary coolers being the same as that of the primary coolers. After making improvements to the cooling water system based on that of a medium-sized plant, the circulating cooling water first passes through the secondary surface cooler before reaching the primary surface cooler, thereby increasing the flow rate of water in both coolers. Even without a booster pump, the vacuum level in the second stage can still meet the design specifications. This cooling water system also played a significant role in increasing the production capacity of the facility. 3.2 Application of the pre-separation–pre-distillation process in small urea plants The pre-separation–pre-distillation process has played a significant role in the technological upgrades aimed at increasing production and reducing energy consumption in small urea plants. Since the small urine treatment unit is equipped with an external cooling device, during a technical upgrade at a certain factory, the steam consumption was reduced from 1380 kg to 1280 kg by using a pre-separator. 3.2.1 Principle of energy savings in the pre-separation–pre-distillation process: (1) When the pre-separator is installed properly, it can separate 53% of the excess ammonia (about 50% of the total amount of gas separated), which then flows to a separate external cooler. This reduces the pressure in the first decomposition stage, thereby facilitating the decomposition process. As a result, the amount of liquid passing through the first addition unit is also reduced by half, thus saving steam consumption in that unit. (2) The composition of the vapor phase after pre-distillation was altered; the NH3/CO2 ratio decreased from 8.03 to 3.94, while the CO2 content increased from 10.4% to 18.6% ； As the NH3/CO2 concentration in the gas phase decreases, the NH3/CO2 concentration in the liquid phase of the heat utilization section also drops. This leads to an increase in the boiling point of the solution, thereby increasing the temperature difference in the heat exchange section and boosting the heat transfer rate ； Furthermore, as the CO2 partial pressure increases, the driving force for absorbing CO2 in the heat utilization section rises, resulting in an increase in the condensation heat generated by methylammonium. In the small urea plant, the CO2 content in the dimethyl liquid is 16%–18%. On the units with a high increase in production capacity, the CO2 content in the dimethyl liquid was reduced further to 14%–15%, which enhanced the absorption capacity of the absorbent solution. As a result, the heat recovery rate in the single-vapor heating section increased, and the steam consumption in that section decreased even further. (3) The pre-separated gas containing 92% gaseous ammonia is sent to an external cooler, where it reacts with the pre-distilled gas that has passed through the vapor heating section; this allows for the complete absorption of any residual CO2 in the pre-distilled gas. The heat generated by this reaction is removed by the circulating desalinated water in the external cooler, and this recovered heat is utilized outside the process boundary, for example, by feeding it to a 2.5 MPa boiler to increase its steam generation capacity ； Some are reused in the copper melt heater, reducing steam consumption ； Some plants use it in lithium bromide refrigeration equipment; the cold water is used to cool the secondary surface coolers in the urea production unit as well as the cooling coils in the ammonia synthesis system, thereby preventing a reduction in production in the ammonia synthesis system during summer. After installing the pre-separator, following the principle of separating the gas into pre-separated gas and pre-distilled gas, the advantages of their respective gas compositions are fully utilized in their respective heat exchangers; as a result, 80%–85% of the CO2 contained in the pre-separated gas is absorbed by the two absorption units at the front end of the first absorption tower, thereby significantly reducing the amount of CO2 that enters this tower. With the development of this process, the amount of ammonia recycled per ton of urine in the first absorption tower has been reduced to 0.25 m3. 3.2.2 What is the effect of this process? (1) Under the same production load, the addition of a pre-separator results in a decrease in the amount of liquid phase after pre-separation; as a result, the steam consumption in the first distillation stage decreases. Moreover, the heat recovery rate in the heating section of the first distillation stage and in the external cooler for the first absorption stage increases, which leads to a reduction in the steam consumption per ton of urine from 1380 kg to 1280 kg. At the same time, the amount of ammonia returned per ton of urine in the first absorption tower decreases from 0.6 m3 to 0.35 m3. ? (2) If the steam heating section and the external cooler for absorption are utilized effectively, they will play an important role in increasing production while saving energy. Therefore, for technological upgrades that result in a significant increase in production, I increase the area of the steam heating section as well as that of the external cooler for absorption; this is a measure that yields three benefits: it increases the volume available for CO2 absorption and the evaporation capacity, while also raising the heat recovery rate, thereby reducing the steam consumption per ton of urine to 1180 kg, and reducing the amount of ammonia returned per ton of urine in the absorption tower to 0.25 m3. (3) With this process in the capacity-increasing technical renovation, no modifications are required for the first absorption tower or the ammonia condenser, as 85% of the additional CO2 is absorbed in the two outer absorption sections before the first absorption tower. (4) Taking the one-stage heater as an example, after adopting the pre-distillation process, the original one-stage heater with an area of 94 m2 had a design capacity of 180 t/d; this area was calculated based on the pre-separation process. However, the actual production capacity can be increased by 60%, resulting in a production volume of 320 t/d. By switching to the pre-separation–pre-distillation process, a production rate of 350 t per day can be achieved; if an external cooler with the same area (71 m2) is added to the system, the daily output can exceed 400 t. 3.2.3 The key to the effectiveness of this process is to properly install the pre-separator, so as to maximize its efficiency to the design value. If the pre-separator operates at a design pressure of 1.7 MPa, it can separate 53% of the excess ammonia, resulting in a significant change in the composition of the vapor stream from pre-distillation, thereby enabling it to function in its respective heat exchange units. In actual installations, the pressure is often greater than 1.7 MPa; that is, after passing through a primary external cooler, it enters the primary absorber where the pressure is 1.75 MPa. When designing the pre-separator for this purpose, its volume should be appropriate to the production capacity; meanwhile, the diameter of the gas phase pipe should be the same as that of the pre-distillation gas phase pipe, and also suitable for the production capacity, to ensure that the pressure on the pre-separator does not increase during operation. If there is a head difference of more than 15 m with the pre-distillation tower during installation, it is not necessary to install a pressure control valve on the gas line; moreover, the pre-separated gas contains no liquid, allowing the pre-separated liquid to flow smoothly into the pre-distillation tower. The second is the principle of separating the two gases. The pre-distilled gas follows the original process: it passes through a heating section, and then enters a suction tower cooler ； The pre-separated gas passes through only one external cooler, allowing its characteristics based on gas phase composition to be utilized, so that it can serve different functions in various heat exchange devices. In some processes, one or two heat exchange units are inserted into the two-gas flow sequence, which **increases the pressure in the first-stage decomposition system; as a result, pre-separation is not as effective as it should be, and the decomposition efficiency of the first stage is also significantly affected. 3.2.4 The author’s contribution here? The author pioneered the application of a pre-separation–pre-distillation process in small urea plants, bringing the full-circulation process technology for aqueous solutions to a leading level in China; in many large-scale plants, the steam consumption has been reduced to 1100 kg. It can be said that this was already achieved at the urea plant in Yongan, Fujian, with a capacity of 190 kt/a in 1997. The fuel consumption values obtained by the author were achieved through step-by-step modifications on the plant’s equipment. ? (1) After the pre-separator was used successfully for the first time in that plant, on the same 120 kt/a unit, the steam consumption decreased from 1380 kg to 1280 kg. (2) Increasing the area of the steam heating section improves the heat recovery rate; under the same load, the steam consumption is reduced to 1180 kg. (3) The addition of a flash heater results in a three-stage evaporation mode, raising the temperature of the urine after flashing from 95 °C to 105 °C; this reduces the steam consumption of the primary evaporator, lowering the steam consumption per ton of urine to 1100 kg. The heat source for flash evaporation makes use of the waste heat from the condensed high-pressure steam; this steam is first directed to a newly installed intermediate expansion tank. The steam at 0.6 MPa generated by flash evaporation is used for this process, and the amount of steam produced must be balanced with the heat required for flash evaporation. When the plant’s production capacity was increased to 200 kt/a, the technical upgrades aimed at recovering heat energy were not implemented simultaneously, resulting in steam consumption rising back to 1200 kg; it is believed that this value can be reduced to 1100 kg once the technical upgrades for heat recovery are completed. The author was able to develop this energy-saving technology by fully recognizing the potential of this device to increase production and save energy, and by making full use of the ability of the medium-pressure system to recover the reaction heat of ammonium methoxide. Installing a proper pre-separator is of primary importance; without good pre-separation efficiency, there is no effective heat recovery, and thus no reduction in steam consumption. Whether in large or medium-sized urea plants, by using a pre-separator whose volume matches the production capacity, increasing the area of the corresponding steam heating section and the external suction cooler, and simultaneously raising the circulation rate of the desalinated water, it is possible to achieve a steam consumption of 1100 kg. Another prerequisite for a steam consumption of 1100 kg is that the conversion rate in the urea tower must be 67%. Therefore, to prevent a decrease in this conversion rate after increasing production, it is necessary to modify the internal components of the synthesis tower or add new urea towers; if the conversion rate after such modifications remains above 67%, the steam consumption will further decrease. Various types of high-efficiency tray designs with gas chambers, as well as liquid-baffled high-efficiency trays, provide strong technical support for improving the production capacity of urea production towers, enabling the conversion rate to remain at 67% or higher. This represents another advancement in the technology of domestic urea production processes using a fully closed-loop aqueous solution system. ? 4 A improved aqueous solution full-circulation urea process for efficient heat recovery: With the development of pre-distillation processes in China, as well as the innovation of pre-separation–pre-distillation processes, steam consumption has been decreasing gradually. In summary, the key to reducing steam consumption in these two process flows lies in continuously lowering the steam consumption in the first stage of decomposition (in the pre-separation process, the steam consumption in this stage accounts for 60% of the total steam consumption) and improving the heat recovery rate in the first stage of absorption; this has therefore become the guiding principle behind the author’s development of new processes. 4.1 Key areas for heat recovery In the energy-saving upgrades carried out in the first step, only the heat recovery rate of the CO2 gas generated from the decomposition of ammonium in one of the heating sections and in the external cooling unit was improved, reaching 85%. For further heat recovery, it is necessary to extract part of the CO2 that originally enters the reactor and send it to a recovery device that requires high-level heat; in other words, a heat recovery section must be added to the heating unit in question, in order to reduce the steam consumption there. Additionally, the area of the heating section used for vaporization needs to be increased, so as to reduce the steam consumption during that process. Through calculation, the heat recovered from these two parts can reduce steam consumption by 300 kg, lowering the steam consumption per ton of urine from the current 1100 kg to 800–900 kg. This new process makes use of two utility model patents developed in collaboration with the Hunan Chemical Engineering Design Institute: ZL99233018.1, which relates to a urea synthesis tower equipped with heat exchanger components; and ZL0227385.1, which describes a new process for recovering thermal energy in urea production. 4.2 Increasing the conversion rate of the urea synthesis tower is one of the key factors in reducing steam consumption under this new process. Due to the increased amount of methylammonium in this new process, it is necessary to take into account the following two process conditions when performing material balance calculations. ? (1) The H2O/CO2 ratio entering the tower should not exceed 0.85; therefore, the water used for absorption in the first and second stages must be reused multiple times, and the CO2 introduced into the medium-pressure system must be absorbed. The heat generated by this reaction should be recovered in the heat exchange equipment of the medium-pressure stage in order to reduce steam consumption. ? (2) The composition of the methylammonium solution remains the same as that in the traditional process, preventing an increase in the equilibrium pressure of the first-stage system and thus avoiding adverse consequences resulting from changes in the system’s operating conditions. The temperature of the ammonium methoxide liquid entering the tower remains at 95 ℃, which prevents further corrosion of the ammonium methoxide pump cylinder and its internal components. Therefore, maintaining H2O/CO2 at 0.85 upon entering the tower is a key parameter of this process. At present, for various internal components of urine treatment towers in China, under the operating condition of H2O/CO2 at 0.65, the highest conversion rate recorded is 68% (with a production intensity of over 15). In this process, due to the increased circulation rate of ammonium methoxide in the first stage, the H2O/CO2 ratio entering the tower rises to 0.85. Under these operating conditions, to maintain a conversion rate of at least 70%, new internal components for the reactor must be designed to ensure that the conversion rate remains above 70% even under high H2O/CO2 conditions. 4.3 Guiding principles for designing new urine tower internals: For the design of isothermal-type urine tower internals, the three reactants enter the tubular reactor located within the tower with a NH3/CO2 ratio of less than 3; this reactor also functions as a heat exchanger. It is mounted on the top cover of the tower. After the reactants are transferred from the central pipe to the lower part of the tower, liquid ammonia is added at the bottom of the tower to maintain an NH3/CO2 ratio of 4.2–4.3, thereby balancing the reaction heat. The temperature at the bottom of the tower is adjusted to be no higher than 188 °C in order to increase the conversion rate; for this purpose, the existing high-efficiency trays in the tower are still used after undergoing slight modifications. The structure of the internal components of this tower reduces the temperature difference between the top and bottom, making it a truly isothermal urine treatment tower. 5 Conclusion Although the full-circulation aqueous solution process is a technology from the 1960s that has been phased out abroad, since China’s research and development in urea production have primarily relied on this process, extensive experience has been accumulated in terms of process design, equipment manufacturing, operation, and production management. Plants with a steam consumption of 1100 kg already exist. Processes that further recover thermal energy to reduce steam consumption to 900 kg will also emerge. Therefore, a new Chinese-style modified aqueous solution full-circulation urea production process is about to be developed; it **differs from the traditional pre-separation process used in the 1960s. It can be said to be a new Chinese-style process for full circulation of solvent solutions aimed at saving costs, energy, and water. A comparison between the new process and the originally introduced process is shown in Table 1. Table 1 Comparison between the new process and the originally introduced process
Serial Number | New Process | Originally Introduced Process
1 | Pre-separation – pre-distillation process | Pre-separation process
2 | Isothermal urea tower internals, conversion rate of 70%; traditional tray type, with 12 trays in the intermediate urea unit, conversion rate of 65% | —
3 | Presence of a heating section | None
4 | Presence of an external cooling coil for absorption | None
5 | CO2 content in dimethyl liquid reduced to 14%–15%; it first enters a heating section | None
6 | Presence of a flash heater that makes use of waste heat, enabling a three-stage evaporation mode | None
7 | Steam consumption: 900 kg per unit; actual measurement in 1981 showed 1500 kg | —
8 | Introduction of domestically produced deep hydrolysis units, resulting in ammonia consumption at a level meeting advanced national standards | None
9 | Use of granulation nozzles with a particle size of 2 mm accounting for 70%; original nozzles had small particles with a size of 1 mm accounting for 67% | —