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I. TEC ACES Process This method combines an aqueous solution circulation process with carbon dioxide stripping technology. It is characterized by a high ammonia/carbon dioxide ratio and a high conversion rate ； Plate trays are installed in the upper part of the stripping tower, while a liquid film heat exchanger is located in the lower part, resulting in high stripping efficiency ； The heat from the high-pressure ammonium methoxide condenser is used to recover by-product steam, resulting in a high heat utilization rate. The process flow is described as follows. 1. Synthesis Liquid ammonia supplied from outside is pressurized by a high-pressure ammonia pump and then fed into the urea synthesis tower via an ammonia preheater. Carbon dioxide supplied from outside is compressed to 18.2 MPa by a carbon dioxide compressor and then sent to the air lift tower; air for corrosion prevention is added at the middle section of the carbon dioxide compressor. The recycled ammonium methoxide solution from the recycling section is pumped by a high-pressure ammonium methoxide pump to Ammonium Methoxide Condenser No. 2 and the high-pressure washer. The operating pressure of the synthesis tower is 18.04 MPa, the operating temperature is 190°C, the molar ratio of ammonia to carbon dioxide is 4, the conversion rate of carbon dioxide is X%, and the products of the synthesis reaction overflow from the central tube and are discharged to the stripping tower at the bottom of the tower. At the upper part of the stripping tower, the synthetic urea solution from the synthesis tower comes into contact with carbon dioxide gas from the lower part, enabling effective carbon dioxide stripping. At the lower part of the stripping tower, ammonium carbamate and excess ammonia contained in the synthetic urea solution are decomposed and separated through carbon dioxide stripping and steam in a falling film heat exchanger. The operating pressure of the stripping tower is 18.04 MPa, and the temperature is 177°C; the gas at the top of the tower is sent to Ammonium Methane Condensers No. 1 and No. 2. In the ammonium methanate condenser, the gas from the top of the stripping tower is condensed and absorbed by the recycled ammonium methanate solution. The heat of condensation and the heat of absorption are used to generate steam at 0.59 MPa in the No. 1 ammonium methanate condenser, as well as to heat the urea solution at the outlet of the stripping tower in the No. 2 ammonium methanate condenser. Both the gas and the solution at the bottom of the methylammonium condenser are sent to the synthesis tower. The gas coming out of the top of the synthesis tower contains a small amount of ammonia and carbon dioxide, which is sent to a high-pressure scrubber for recovery. In the scrubber, ammonia and carbon dioxide are recovered using a circulating ammonium methoxide solution, which is then sent to Ammonium Methoxide Condenser No. 1 as an absorbent. The gas emerging from the top of the scrubber is sent to a high-pressure decomposer to further recover ammonia and carbon dioxide. 2. Purification The urea solution coming out of the bottom of the stripping tower is first preheated to 155°C in Condenser No. 2 using ammonia methanol, and then sent to the high-pressure decomposer. There, it is further heated by the steam condensate from the internal heat exchanger, which causes ammonium carbamate to decompose into gaseous ammonia and carbon dioxide. The resulting gases are then sent to the high-pressure absorption tower. After most of the ammonium carbamate is separated in the high-pressure decomposer, the urea solution is sent to the low-pressure decomposer at a pressure of 0.35 MPa, where it is further purified to achieve residual ammonia and carbon dioxide levels of 0.5% and 0.4%, respectively. The gas separated by the low-pressure decomposer is sent to the low-pressure absorption tower, while the urea solution is sent to the flash separator for the final stage of purification; residual ammonia and carbon dioxide are further separated through vacuum flashing. The urea solution at the outlet of the urea solution tank contains about 70% urea and about 0.4% ammonia; this urea solution is pumped by a urine pump to the concentration process. 3. Concentration and granulation The urea solution is first sent to a vacuum concentrator, where it is concentrated to about 84% urea. The urea solution is heated to 132°C by low-pressure steam under a vacuum of 0.02 MPa, resulting in a urea concentration of 95.5% in the vacuum concentrator. The concentrated urea solution is sent to the final concentrator, where it is heated to 138°C by low-pressure steam. In the final separator, under a vacuum of 0.3 MPa, the solution is concentrated to contain 99.8% urea, and then it is pumped by a molten urea pump to the top of the granulation tower. Granulation is sprayed into the tower through granulation nozzles; the urea that falls to the bottom of the tower is sent to a warehouse via conveyor belt for storage or packaging. 4. Recovery The overhead gas from the low-pressure decomposer is sent to the low-pressure absorption tower. The ammonium methyl solution formed in the high-pressure absorption tower is pumped by an ammonium methyl pump; part of it is circulated to the No. 2 ammonium methyl condenser, while the other part is circulated to the No. 1 ammonium methyl condenser via the wash tower in the synthesis section. 5. Treatment of process condensate The condensate from the surface condenser of the final concentrator is collected in a process condensate tank and then pumped via an absorption pump to the scrubber tower, where it is used to wash the off-gases from the high-pressure absorption tower. The process condensate from the first and second surface condensers is stored in a process condensate tank, and then pumped to a process condensate stripping tower via a process condensate pump. Ammonia and carbon dioxide are stripped from the condensate through steam stripping, and the gas at the top of the tower is sent to a low-pressure decomposer for recovery. The stripping condensate from the intermediate section of the process condensate stripper is pumped to the urea hydrolyzer, where the urea is completely hydrolyzed into ammonia and carbon dioxide. The process condensate from the urea hydrolyzer is sent back to the lower part of the process condensate stripping tower, where ammonia and carbon dioxide are stripped off. In the treated process condensate, the concentrations of urea and ammonia are both less than 1×10—6, allowing it to be used as boiler feed water after being sent out of the restricted area. II. TEC’s full-cycle improved Method C process: Its feature is that urine is first crystallized and then melted and granulated to reduce dicyandiamide formation. The process flow is briefly described as follows: 1. Urea synthesis Liquid ammonia from the outside is pressurized to 26 megapascals using a high-pressure liquid ammonia pump and then fed into the synthesis tower. Carbon dioxide gas from outside is pressurized to 26 MPa by a carbon dioxide compressor and sent into the synthesis tower. Ammonia and carbon dioxide react within the tower; the operating pressure of the synthesis tower is 25 MPa, the temperature at the top is 200°C, the ammonia/carbon dioxide ratio is 4, the water/carbon dioxide ratio is 0.37, and the conversion rate of carbon dioxide is 71.7%. 2. Decomposition and crystallization separation of urine (1) High-pressure decomposition: The reactants coming out of the top of the synthesis tower are reduced in pressure to 1.7 megapascals via a pressure reducing valve, and then sent into the high-pressure decomposition tower; heat is supplied by the reboiler in this tower. (2) Low-pressure decomposition: The solution coming out of the bottom of the high-pressure decomposition tower is reduced in pressure to 0.25 megapascals through a level control valve, and then fed into the upper part of the low-pressure decomposition tower. There, sensible heat is used to vaporize some of the ammonia and carbon dioxide; these gases then enter the heat exchanger and reboiler simultaneously before returning to the packing section at the lower part of the low-pressure decomposition tower, where they come into countercurrent contact with the rising carbon dioxide gas, thereby facilitating further decomposition of methylammonium into ammonia and carbon dioxide. (3) Gas separation: The urea solution containing a small amount of ammonia and carbon dioxide, coming out from the bottom of the low-pressure decomposition tower, is further depressurized to 0.03 MPa and fed into the upper part of the gas separation tower. Ammonia and carbon dioxide separation is achieved using sensible heat. After flashing, it overflows to the lower part of the tower ； At the lower part of the tower, there is a packing section and a U-tube heater. The urine comes into countercurrent contact with air supplied by an exhaust gas recirculation blower via the packing, thereby enabling the separation of ammonia and carbon dioxide. Subsequently, the urine is heated using the U-tube heater, at which point its concentration reaches 74%. (4) Urine crystallization: The urea solution with a concentration of 74% coming from the bottom of the gas separation tower is pumped by a urine pump into the crystallizer. Vacuum crystallization is employed in the upper part of the crystallizer; through vacuum evaporation in that upper section, the concentration of urine reaches 82%–86%. In the lower part of the crystallizer, crystalline urea is formed, along with concentrated urine containing this crystalline urea. This mixture is pumped to a centrifuge for separation. The crystalline urea obtained after centrifugation contains less than 2.4% water and 0.1% biuret. The reduction in the biuret content is achieved by adding biuret-containing wash water, which carries the biuret back into the mother liquor. The pseudo-mother liquor coming out of the centrifuge is collected in the mother liquor storage tank; a portion of it is pumped by the mother liquor pump into the low-pressure absorption tower and eventually returns to the synthesis tower, while the rest returns to the crystallizer for further concentration and crystallization. 3. Drying and granulation The powdered urea coming out of the centrifuge enters an air-flow dryer. It is pumped to the top of the tower by an air blower and an extractor fan in the air-flow dryer; from there, it is conveyed through air-conveyance pipes to the top of the tower, where it enters a cyclone separator to separate the urea. The separated urea is then sent to a melter, where it is melted before being fed to the nozzles in the granulation tower for granulation. Granular urea is fed into the fluidized bed cooler, where it is cooled; afterward, it overflows from the fluidized bed and is transported via belt conveyors to the warehouse or packaged. 4. Recovery The ammonia and carbon dioxide separated in each decomposition tower are recovered separately and then returned to the synthesis tower. three ； Casale HEC urea process The HEC urea process is a patented technology of Casale Company, and its features are: (1) it includes two urea synthesis towers. The first tower is the main synthesis tower, with an operating pressure of 22–24 MPa, a temperature of 195°C, an ammonia/carbon dioxide ratio of 3.6, a water/carbon dioxide ratio of 0, and a carbon dioxide conversion rate of 75%. The second tower is an auxiliary synthesis tower, also known as a secondary tower; its operating pressure is 16 MPa, the temperature is 190°C, the ammonia/carbon dioxide ratio is 4.5, the water/carbon dioxide ratio is 1.3, and the carbon dioxide conversion rate is 61.0%. The average conversion rate of the two towers is 71%, which is higher than that of other urea production processes. (2) The equipment for medium-pressure and low-pressure recovery systems is small. (3) Lower material and energy consumption. Applying this method to the technical renovation of the full-cycle urea plant can reduce ammonia and steam consumption, resulting in good economic benefits. The evaporation system is similar to other processes; the process flows for the high-pressure section and the recovery section are described as follows: Carbon dioxide from outside the system is pressurized to 25 megapascals by a carbon dioxide compressor, and then mixed with liquid ammonia that has been pressurized by a high-pressure liquid ammonia pump. This mixture then enters the ammonium methanate condenser, where the reaction between ammonia and carbon dioxide generates heat used to produce low-pressure steam. After exiting the ammonium methanate condenser, the reaction mixture proceeds to the first synthesis tower. From the top of this tower, it goes to the separator in the high-pressure decomposer, which uses steam at 2.45 megapascals to facilitate gas stripping. The solution coming out of the upper part of the separator enters the second synthesis tower. The ammonium methoxide solution recovered from the medium-pressure absorber is pumped into the second synthesis tower using a high-pressure ammonium methoxide pump. The urea solution exiting the second synthesis tower is also sent to the separator in the upper part of the high-pressure decomposer. The urea solutions from tower 1 and tower 2 emerge at the bottom of the high-pressure decomposer and then enter the medium-pressure decomposer; from there they go on to the low-pressure decomposer. The urea solution coming out of the low-pressure decomposer is sent to an evaporator, where water is removed under vacuum until the urea concentration reaches 4%. Afterwards, it is pumped to a granulation tower using a molten urea pump for granulation. The gas separated from the upper part of the low-pressure separator is sent to the low-pressure condenser; the ammonium carbonate solution emerging from the low-pressure condenser passes through a gas-liquid separator before being pumped by a low-pressure ammonium carbonate pump to the bottom of the medium-pressure absorption tower. The gas emerging from the top of the second synthesis tower is sent to the separator in the upper part of the medium-pressure decomposer. The gas coming out of the upper separator first has its heat recovered by the lower heater of a vaporizer, and then it is sent to the medium-pressure absorption tower where it comes into contact with liquid ammonia sprayed from the upper part of the tower; this results in the formation of ammonium methanate solution, which is then pumped into the second synthesis tower using a high-pressure ammonium methanate pump. The ammonia gas exiting from the top of the medium-pressure absorption tower is sent to the ammonia condenser, where it is condensed into liquid ammonia that flows into the liquid ammonia storage tank for reuse.