The chemical formula of urea is CO(NH2)2. The production method involves reacting ammonia with carbon dioxide under high pressure to first produce ammonium carbamate, which is then dehydrated to produce urea. The reaction equations are:

2NH3 + CO2 = NH2COONH4

NH2COONH4 = CO(NH2)2 + H2O

Based on this reaction principle, various urea production processes are developed using different pressures, temperatures, and ammonia-to-carbon ratios.

Urea preparation can be classified into coal-based urea, gas-based urea, and oil-based urea based on the raw materials used. Due to the high industrial production cost of oil-based urea, coal-based and gas-based urea are the primary methods used in actual production.

Regarding costs, a rough calculation shows that coal-based urea production costs are as follows: Coal consumption per ton of ammonia is between 1.1 and 1.5 tons, and ammonia consumption per ton of urea is between 570 kg and 620 kg. Therefore, coal consumption per ton of urea is approximately 0.6 to 0.9 tons, roughly 0.7 tons. Adding electricity costs of 800 yuan/ton, steam costs of 200 yuan/ton, and depreciation costs of 200 yuan/ton, the urea production cost calculation formula is:

Urea production cost = 0.7 * price per ton of coal + 800 + 200 + 200

For natural gas urea production, the cost calculation is: producing one ton of urea requires approximately 700 cubic meters of natural gas and 300 yuan in other costs.

Urea Production Process:

1. Liquid Ammonia Pressurization: After sampling and analysis, liquid ammonia from the boundary area is pressurized to 14 MPa by a high-pressure pump and sent to the high-pressure injector.

2. Reaction Stage: Liquid ammonia acts as the jet propulsion, carrying the ammonium carbamate liquid from the high-pressure scrubber into the high-pressure ammonium carbamate condenser. There, ammonium carbamate is generated: CO₂ + 2NH₃ = NH₄·COONH₂ (exothermic reaction). Some heat is recovered, shifting the reaction equilibrium towards the forward reaction.

The resulting gas and liquid flow directly to the bottom of the synthesis tower. In the synthesis tower (14 MPa, 183℃), urea is generated: NH₄·COONH₂ = CO(NH₂)₂ + H₂O (endothermic reaction), while the gaseous reaction CO₂ + 2NH₃ = NH₄·COONH₂ is exothermic. Therefore, the synthesis tower is in self-heating equilibrium, and multiple trays are provided to prevent backmixing.

The urea-ammonium carbamate liquid generated from the reaction is sampled and analyzed from the bottom of the synthesis tower before being sent to the top of the stripping tower. There, it comes into countercurrent contact with the high-pressure CO₂ from the lower layer. Under high temperature and CO₂... Driven by gas, the ammonium methyl ester solution is further decomposed. Urea and the ammonium methyl ester solution are sampled and analyzed from the bottom of the stripping tower and then sent to the distillation tower. In the distillation tower, they are circulated and heated to further decompose the ammonium methyl ester solution. The resulting urea solution has a concentration of approximately 68%. After sampling and analysis, it enters a flash tank (at atmospheric pressure) to flash-evaporate CO2/N3/H2O. The urea solution then flows into the urea storage tank.

3. Urea Granulation

After sampling the urea solution from the storage tank, it is sent to a first-stage evaporator heater (0.03 MPa, 130℃). The gas and moisture in the urea solution are fully decomposed and evaporated. The urea solution exiting the first-stage heater has a concentration of approximately 95%. It then flows into a second-stage evaporator heater (0.003 MPa, 140℃) for further heating and concentration. The resulting urea is in a molten state (99.7%). After sampling, it is sent to the granulation system to be directly granulated, packaged, and stored.

4. Other Instructions

Gas generated during the process are either recovered and reused, or sent to the exhaust stack. They are absorbed by the solution generated during the process and returned to the ammonia tank. Gases that cannot be absorbed are discharged into the atmosphere.