The Development of the Urea Production Process Using the Fully Circulated Aqueous Solution Method in China – Commemorating 50 Years of Industrial Urea Production in China

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Preface: China’s pilot urea production plant (3,000 t/a) was built and 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 to this year. The experiments began with the aqueous solution semi-circulation method, where the ammonia content in the ton of urine-derived exhaust gas was around 650 kg; later, a more efficient semi-circulation system was developed, which improved the degree of ammonia recovery from the decomposition gas, yet 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-cycle 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 for Chemical Engineering 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 facility at the Hunan Xiangjiang Nitrogen Fertilizer Plant came online. This can be regarded as the first set of process units in China that used a full aqueous solution circulation method; 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 also China’s first urea production tower. The rectification section of this tower was of the floating valve type, making it the first of its kind as well. 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 sample 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 nitrogen fertilizer plants underwent technical upgrades to switch to producing urea, which brought about a fundamental 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 starting up successfully from the very first attempt, offering promising prospects for those small nitrogen fertilizer plants, which at that time accounted for 50% of China’s synthetic ammonia production, to shift from producing ammonium carbonate to urea. During the Seventh and Eighth Five-Year Plans, **150 small nitrogen fertilizer plants with favorable conditions were selected to switch to producing urea. In total, more than 120 such plants were involved, of which 18 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 tons per year, with some even having a capacity of 150–200 tons per year.** The rapid development of small urea production facilities is in line with China’s national conditions; it makes full use of the advantages of existing small nitrogen fertilizer plants, with many of these plants using local coal. Provinces with abundant hydro and electrical resources have an advantage, as this results in lower production costs and greater market competitiveness. Through the development during the \"Seventh Five-Year Plan\", \"Eighth Five-Year Plan\" and \"Ninth Five-Year Plan\" periods, 16 more medium-sized urea production units were added, also known as the latter 16 units; thus, the total number of such units came to 54. The latter 16 sets were mostly built in the former small nitrogen fertilizer plants, while for the second set built in the former medium-sized plants, ammonia stripping and CO2 stripping were commonly used. 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 serve as a model in China for high-output, low-consumption process installations using the aqueous solution full-circulation method. 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 unit at Luxi Group’s Plant 8 is designed for a capacity of 400 kt/a, and the actual production volume has recently reached 500 kt/a. If improved using the author’s techniques, fuel consumption is expected to drop to 1,000 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. Although the full-circulation aqueous solution process is a technology from the 1960s that has been phased out abroad, due to China’s focus on research and development in urea production using this process, extensive experience has been accumulated in terms of process design, equipment manufacturing, operating techniques, and production management. In China, this process underwent 42 years of production and technological upgrades from 1966 to 2008, achieving a steam consumption level of 1100 kg. 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. 1 Problems in the initial stage of the intermediate urine process and improvement measures: In the early stage, the focus was on identifying and addressing the issues present in the urea plants designed in China; as a result, there were four versions of the design for the intermediate urine process, with the fourth version being the most refined design. 1.1 Major issues in the design: (1) Frame height. The design of the first set at Xiangjiang Nitrogen Fertilizer Plant was influenced by the \"technological revolution,\" resulting in a frame height of only 18.5 m. As a consequence, the height difference between the equipment in the circulation system was insufficient, which led to problems such as ammonia not being able to flow into the absorption tower and the primary pump not functioning properly; this 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 was able to operate stably over long periods of time. (2) Large design margin: Due to the substantial excess in the original design, especially for the first steam heater and the second steam 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 the Dicyandiamide Content in the Final Product during the Initial Operation of the Medium-Urea Plant (1) Li Yushu, chief engineer at Qilu No.1 Chemical Plant (who was transferred from Lutianhua to this plant as the head of the urea production unit), compared the area per ton of urea produced in the first steam addition stage and the second steam addition stage of the newly introduced plant, and concluded that the area designated in the original design 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 vaporization stage should not be large in order to reduce the residence time. The second-stage heater is the key component for the increase in biuret; since the concentration reaches 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 degree of polycondensation increases very rapidly. At that time, most plants blocked part of the tube bundles to make the tonnage of urine per unit area equivalent to that of the imported equipment, thereby reducing biuret to the acceptable standard of 1.5% at that time. For example, the Xiangjiang Fertilizer Plant is designed to produce 240 tons per day, with a secondary evaporation area of 7 m2; to achieve first-class product quality, it is necessary to produce 310 tons per day. Second-generation design of the small urine treatment unit (production capacity: 180 t/day); the area for secondary evaporation is 5.6 m2. At a designed production load of 180 t/day, the concentration of biuret is around 1.5%, and it is only when the production capacity reaches 220 t/day that first-class product can be obtained. When production 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 developed. 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 tank 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 the urine contains high levels of free ammonia, the urine pump will stop pumping fluid, which prevents feeding for evaporation 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 feed to the urine flash tank remains stable, the vacuum in the first stage of evaporation 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 solved the problem 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 upon the structural defects of the pumps made by Changsha Pump Factory, resulting in a more optimized 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. In the pipeline from the outlet of the melting pump to the top of the granulation tower, the right-angled elbows are removed; instead, the pipeline runs at an angle into the tower, and the nozzles at the top of the tower also enter at an angle. This helps to prevent crystallization-related blockages and reduces the distance the fluid has to travel, as the formation of biuret is related to the time the urine stays in the pipeline. 1.3 Improving domestic operating equipment and high-pressure valves through practical production efforts. At the end of the 1960s, Class A pumps, ammonia pumps, and CO2 compressors were all manufactured by Shanghai Dalong Factory. Later, the factory separated its compressor production operations, becoming the Shanghai Compressor Factory. When developing high-pressure plunger pumps, process requirements dictate that, in order to prevent the liquid from vaporizing inside the cylinder and thereby affecting the pump’s efficiency, the plunger speed of ammonia pumps should not exceed 90 r/min, while that of methanol 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 liquid pumped; the pump’s volumetric efficiency was taken into account during the design phase. 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 operational steps should be taken based on the different conditions under which the pump is stopped in operation, with the aim of preventing the cylinder body from cooling down and thus extending the service life of the sealing ring. Due to the aging and breaking 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 block is made of corrosion-resistant carbon steel, and its service life was less than one year. Due to the presence of methylammonium in liquid ammonia, to deal with cylinder corrosion, the former Xiangjiang Nitrogen Fertilizer Factory would bore the cylinders larger, weld stainless steel strips onto them, and then bore them 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 high pressure. (2) Initially, the plunger packing was made using asbestos rope filled with graphite, compressed in custom-made molds. Its service life varied depending on the manufacturing techniques and installation methods of different factories; good-quality ones could last for half a year, while those of lower quality lasted 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 material mounted inside a steel ring; however, this guide sleeve is prone to corrosion by the medium, which can cause it to fall off 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 pump also adopted graphite guide sleeves. (3) The material of the stuffing 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 Solution A is between 90–95°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 the Xiangjiang Nitrogen Fertilizer 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 resulted in an appropriate level of 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 inlet pipe due to leakage from the valve during short interruptions in the 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 itself, 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, and the liquid column distance from the outlet of one absorption tower to the inlet of Pump 1A was only 5.5 m. Air entrainment in Pump 1A was particularly severe; when liquid containing air is compressed, its instantaneous pressure can be 2 to 3 times higher than the original pressure. Under alternating load, stress corrosion is more severe; the cylinder block cracks quickly, with the shortest service life being just over half a month. Xiangjiang Nitrogen Fertilizer Factory used many different types of steel to manufacture the cylinder bodies, yet was still unable to resolve the issue of cracking in these cylinders. They tried combining two half-cylinders together, which gave better results, but this did not solve the problem 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 Xiangjiang Nitrogen Fertilizer Factory. Additionally, the height of the frame was increased to 23.5 m, and the liquid column height 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 compressors were manufactured by copying the compressors from Xiangdan Plant. Imported compressors have not encountered any problems during use, but domestically manufactured CO2 compressors, whether produced 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 compression coefficient of the gas; pure CO2 experiences a greater reduction in volume at pressures above 3.0 MPa, resulting in significant differences from the compression 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 boosting the volume of cooling water circulation, thereby reducing the gas inlet temperature to 35 °C (it was originally 40 °C, a value based on a 100% purity level; with 4% air present now, reducing the cooling temperature to 35 °C prevents liquefaction). This approach takes advantage of the design principle of four-stage compressors, which aims to avoid the problem of CO2 gas 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, and after cooling this vapor turns into acidic water that causes corrosion. In some plants, accidents have occurred due to the failure of oil-water separators as a result of corrosion, leading to 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 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: When the urea production plant in Hunan is put into operation, the high-pressure valves are manufactured by Hunan Yueyang Sihua Construction Factory. These valves do not leak when the temperature of the fluid in the pipelines changes; those manufactured by Sichuan Chemical Industry’s construction factory also do not leak. However, other valve manufacturers use the same structure as that of the high-pressure valves from the original ammonia synthesis system – the valve seat and the valve body are not integrated but are assembled separately. For use in the urea production system, only the material used for these valves 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 operates 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 ； After a short shutdown of the synthesis tower, the material pipeline before it enters the tower cools down, and leakage will occur at the valves or flanges when the tower is restarted. After the synthesis tower and medium-pressure system are shut down, the system temperature drops. At the beginning of operation of the urea plant, when restarting after 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, as well as the condenser, are integrated with the separator, which results in a reduced efficiency of gas-liquid separation, severe ammonia loss from the system, and increased ammonia consumption. 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 larger 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 tower clogging was particularly severe. 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, accidents occur in which the grating plates are damaged on many occasions ； 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 Technological development of the mid-urea plant in terms of process engineering 2.1 Understanding the objective laws governing exhaust gas explosions Initially, the urea synthesis tower in the mid-urea 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 occurs in the medium-pressure section, and explosions take place when the bypass valves of the pressure control valves in that section are suddenly opened. Under such conditions, due to the high condensation efficiency of the ammonia condenser and the effective washing performance of the medium-pressure scrubber, explosive gases 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 to 98% as much as possible (in the propylene decarboxylation 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°C for the urea plant in the cooling cycle.) Therefore, the circulation water volume must be controlled in the three 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 caused by the need to activate a bypass line due to system overpressure ； The ammonia condenser, as well as the flanges of the equipment and pipelines that follow 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. Currently, in 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, making them promising for further adoption. 2.2 The emergence of the pre-distillation process: The pre-distillation process was developed in the pilot plant of Nanjing Fertilizer Plant in the mid-1960s. When Shijiazhuang Fertilizer Plant, the first factory for urine processing in China, began operations in 1966, it already used a pre-distillation process. At that time, the workshop supervisor, Mr. Zhu Junbiao, had been transferred from the Nanhua pilot plant. In 1973, the design review for the second version of the medium-pressure urine 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 in the third and fourth revision designs. However, the manufacturers recognized the advantages of this process in practice, and most units were converted to a pre-distillation process. Therefore, the pre-distillation process is the first technical achievement of China in improving the urea process using the aqueous solution full-circulation method. 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 and thus demonstrating great viability. 2.2.1 Advantages of the pre-distillation process flow: A pre-separation section is located at the upper part of the pre-distillation tower, a pre-distillation section is in the middle, and a gas-liquid separation section for the first stage of decomposition liquid is at the lower part. 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 of 120 kt/a scale has a steam consumption of 1380 kg. 2.2.2 The system’s water balance conditions were improved: the vaporization temperature was reduced from 160 ℃ to 125 ℃, which led to a corresponding decrease in the water vapor pressure. As a result, the amount of water introduced into the first absorption tower decreased, by 85–90 kg per ton of urea produced. This allowed for more water to be used in the second absorption stage, and it also increased the amount of water returned to the first absorption stage, thereby **improving the stability of operations in the first absorption tower**. By entering the rectification section of the absorber, the ammonia concentration on the tray 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 avoiding issues such as 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 entering the urine tower to be reduced to 0.65, and the conversion efficiency of the urine tower increases to 67%, thereby reducing the steam consumption in the first stage. 2.2.3 The heat recovery rate in the first vapor heating section has been 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 gas stream 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 return amount per ton of urine to 0.6–0.8 m3. 2.2.4 It is possible to 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 vapor-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 reduced to 135 ℃ ； As a result, the energy consumption for binary decomposition 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-grade 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 absorption units were equipped with external coolers; the pre-separated gas entered directly into the bubbling section of the absorption tower. Following the restoration of the pre-separator are Lanzhou Fertilizer Plant and Zhejiang Quzhou Chemical Plant. The purpose of reinstating 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 (it is designed to be able to separate 50% of the total vapor volume), the amount of material entering the pre-distillation tower can be reduced, thereby lowering the vapor consumption in that tower. Additionally, with a pre-separation temperature of 115 °C, not only is the amount of water carried into the first absorption tower reduced, but the thermal load on that tower is also decreased. 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 in that stage also decreased not significantly. Furthermore, 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 Technical development of small urea plants since 1986: This year is used as a dividing point because three small urea pilot plants 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 carried out after three months, and the results showed that the device met all the design-related technical and economic requirements; 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, together with leaders from various ministries, held a field meeting in Hui County, Henan Province, 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**, funding for technical upgrades was provided for 150 such units in total. After 1986, due to the construction of numerous small urea plants, the output of these small plants accounted for 50% of the country’s total urea production at that time (25% from large urea plants and 25% from medium-sized urea plants). Due to the continuous technological development at the former small urea plant, its production capacity was increased to over 100 kt/a; many plants reached capacities of 150–200 kt/a, and new medium-scale plants with a capacity of 200 kt/a were also built at the former small nitrogen plant. After technical upgrades, its capacity reached 300 kt/a. The 400 kt/a urea plant using the aqueous solution full-circulation process owned by Luxi Group has now reached a production capacity of 500 kt/a, reaching the scale of large-scale urea plants. This shows that, through technological development in China, the advantages of the aqueous solution full-circulation urea process have been recognized by many enterprises operating such plants. Therefore, many of the newly built 200 kt/a, 300 kt/a, and 400 kt/a plants still employ the urea process with a complete 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-urea products. 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 plant in Hui County, as well as the insights gained from the successful operation of its third experimental 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 pre-distillation process is used for medium-pressure decomposition; this process improves the conditions of horizontal balance in the urea production system based on the aqueous solution full-circulation method. Maintaining system water balance is a fundamental condition for ensuring a healthy circulation of aqueous solutions, and the improved favorable operating conditions resulting from proper system water balance are manifested in the following aspects. (1) As the amount of water carried from the first stage decomposition tower to the absorption tower decreases, it is possible to increase the amount of liquid from the second stage circulation that can return to the first stage, thereby improving the operational stability of the absorption tower. (2) Due to the reduction in the total amount 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 conventional tray tower to 67%. This allows for a reduction in the volume of circulating liquid in stage 1, which in turn reduces the steam consumption for decomposition in that stage as well as the energy required to pump the circulating liquid. (3) Improvement of the two-stage absorption condition. 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 decreases 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 column becomes 0.25 MPa, allowing the separation temperature to be reduced from 150 ℃ to 135 ℃. This facilitates an increase in the decomposition rate in the second stage and reduces 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 aqueous ammonia solution also 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 gains in terms of increased production 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 weaker concentration of dimethyl ether in these two heat exchange units allows it to absorb more CO2 from the gas stream; as a result, the heat recovery rate in that section increases, which 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 circulating ammonia 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 section decreases, and issues such as the formation of ammonium methylate crystals or slight excess levels of certain substances on the ammonia water plates in the purification section 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 these tubes, thereby enhancing the heat transfer coefficient and cooling efficiency. 3.1.3 Measures to improve the system’s water balance: A distillation section is installed at the top of each process unit, with the aim of reducing the amount of water vapor carried away by 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; in a split tower, a distillation section is provided both in the form of trays and packing. 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, the ammonia solution enters plate 3#; the recycled ammonia is added at plate 1#. The ammonia concentration at plate 3# is 89%, so 2 more 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 there 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, which 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, thereby raising the heat transfer coefficient. This not only helps to save steam usage but also reduces the amount of biuret formed in that section. 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 coil tubes, where scaling occurs. After switching to the pre-distillation process, the area utilization rate of the first distillation unit increases; with the same area, its production capacity can be increased 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 exchanger in the evaporation process, with a refrigeration energy consumption of 209–251 MJ per ton of urine produced. 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 has been adopted 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 second surface cooler before reaching the first surface cooler, thereby increasing the flow rate of water in both coolers. Even without a booster, 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 saving energy in small urea plants. Since the small urine treatment unit is equipped with an external cooler, 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 Principles of energy savings in the pre-separation–pre-distillation process (1) When properly installed, the pre-separator 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) It altered the composition of the vapor from the pre-distillation stage; the NH3/CO2 ratio decreased from 8.03 to 3.94, while the CO2 content increased from 10.4% to 18.6% ； As the gas-phase NH3/CO2 levels decrease, the NH3/CO2 levels in the liquid phase of the heat utilization section also drop. 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 CO2 in the heat utilization section rises, resulting in an increase in the condensation heat generated from methammonium. In the small urea plant, the CO2 content in the dimethyl liquid is 16%–18%. On the units with a high yield increase, 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, while the steam consumption for that process decreased. (3) The pre-separated gas containing 92% gaseous ammonia is sent for external cooling; there it reacts with the pre-distilled gas from the vapor heating section, thereby fully absorbing 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 systems; the cold water is employed to cool the secondary surface coolers in urea production units 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, which significantly reduces the amount of CO2 entering 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 Effects 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 evaporation stage decreases. Moreover, the heat recovery rate in the heating section of the first evaporation stage and in the external cooler for absorption increases, leading 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 drops from 0.6 m3 to 0.35 m3. (2) The author believes that making good use of the evaporation heating section and the external absorption cooler will play an important role in increasing production while saving energy. Therefore, in technological upgrades aimed at achieving significant production increases, it is necessary to expand the area of the evaporation heating section and the external absorption cooler; this is a measure that yields three benefits: it increases the volume available for CO2 absorption as well as the evaporation capacity, while also raising the heat recovery rate, thereby reducing the steam consumption per ton of urine to 1180 kg and the amount of ammonia returned per ton of urine in the first absorption tower to 0.25 m3. (3) With this process in the capacity-increasing technical upgrade, 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 prior to 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: Pre-separators are also used in some processes aimed at increasing production and saving energy, but the energy-saving effects do not meet expectations; in some cases, the steam consumption is even higher than that of the original pre-distillation process. The difference between the author and these others who also use the pre-separator process is that, in the author’s approach, the pre-separator is installed in such a way that its efficiency is brought as close as possible to the designed value. When operating at a design pressure of 1.7 MPa, the pre-separator is able to remove 53% of the excess ammonia, which leads to significant changes in the composition of the vapor stream prior to distillation, thereby enabling it to function effectively within the respective heat exchange systems. 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 volume, 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 separate flow for the two gases. The pre-distilled gas still follows the original process: it passes through a heating section, and then enters an absorption tower cooler ； The pre-separated gases pass through only one external cooler, allowing them to utilize the characteristics of their respective gas phase compositions and serve different functions in various heat exchange devices. In some processes, one or two heat exchange units are inserted into the two-gas flow, 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 technology of full-circulation 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 technology was already implemented at the Yongan urea plant in Fujian with a production capacity of 190 kt/a back in 1997 ； Followed by the Fujian Sanming Intermediate Urine Plant, which reached 1,180 kg when it was upgraded from 110 kt/a to 260 kt/a in 1999. (This plant is not equipped with a flash heater.) If installed, the steam at 0.6 MPa generated by the flash evaporation of high-pressure steam condensate as a heat source can reduce steam consumption by 80 kg. ) In Yongan, Fujian, when the production capacity increased to 200 kt/a, the technical upgrades aimed at recovering heat energy were not implemented simultaneously, resulting in a rise in steam consumption 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 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 was reduced from 1380 kg to 1280 kg. (2) Increasing the area of the steam heating section raises the heat recovery rate, resulting in a steam consumption of 1180 kg under the same production load. (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. 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 heat from the ammonium methoxide reaction in the medium-pressure system. 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 capacity matches the production capacity, increasing the area of the corresponding steam heating section and the external suction cooler, and simultaneously raising the circulation volume of the desalinated water, it is possible to achieve a steam consumption of 1100 kg. Another prerequisite for this 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%, then the steam consumption will further decrease. Various types of high-efficiency tray designs with gas chambers, as well as liquid-bypass high-efficiency trays, provide strong technical support for increasing the production capacity of urea production towers, while maintaining a conversion rate of 67% or higher. This represents another advancement in the technology of domestic urea production processes using fully circulating aqueous solutions. 4 A modified aqueous solution full-circulation urea process for efficient heat recovery: Starting from the pre-distillation process developed in China, and considering the innovative pre-separation–pre-distillation process, steam consumption gradually decreases. In summary, the key to reducing steam consumption in these two process flows is to continuously decrease 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 to improve 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 retrofitting measures in the first step, only the heat recovery rate of the CO2 gas generated from the decomposition of ammonium in one stage was improved, reaching 85%, in the steam heating section and the external cooler. For further heat recovery, it is necessary to extract part of the CO2 that originally enters the synthesis tower for reaction and send it to a recovery device that requires high-level heat; this involves adding a heat recovery section to the first-stage heater in order to reduce the steam consumption there, as well as increasing the area of the steam heating section in the first stage in order to reduce the steam consumption associated with evaporation in that stage. 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 between the author and the Hunan Chemical Engineering Design Institute. ZL99233018.1 – Urea synthesis tower equipped with heat exchanger internals; ZL0227385.1 – 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 through 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 absorption water used 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 of reaction generated can 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°C, 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 ammonia in Stage 1, the H2O/CO2 ratio entering the tower rises to 0.85. Under these operating conditions, to maintain a conversion rate of at least 70%, it is necessary to design new internal components for the synthesis tower in order to ensure that the conversion rate remains above 70% under high H2O/CO2 conditions. 4.3 Guiding principles for designing new urine tower internals: The internals of an isothermal-type urine tower are designed such that 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 more than 188 °C in order to increase the conversion rate; for this purpose, the existing high-efficiency trays in the tower are utilized after minor 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 Drawing on 44 years of operational experience with full-circulation process units for aqueous solutions, and based on the continuous innovation in energy-saving technical improvements by production, research, and design organizations, the author combines his long-term experience in working with this process – starting from the pilot plant at Nanhua Factory in 1958 – to focus on the production, management, and development of urea; this work has now spanned 50 years. I was a witness and participant in the industrial production of urea in China, as well as in the development of the full-circulation process technology for its aqueous solutions. The new process developed by the author by fully tapping into the potential of this technological setup for increasing production and saving energy can be regarded as a Chinese-style new process for cost and energy savings through full circulation of aqueous solutions. Table 1 shows a comparison between the new process and the originally introduced process. Table 1 http://www.nmtech.com.cn/jishuwang/upload1/0810161702154623.jpg