Technical description of desulfurization and dehydrogenation for urea production by carbon dioxide stripping method

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Technical description of desulfurization and dehydrogenation of urea produced by carbon dioxide stripping method. The urea production method adopts carbon dioxide stripping method. Now the carbon dioxide stripping method technology and dehydrogenation technology will be explained in detail.: one: Carbon dioxide stripping process technology In 1965, the Dutch company Stamicarbon developed the carbon dioxide stripping urea process. In the late 1970s, the process developed into an improved carbon dioxide stripping process. In the 1990s, the Dutch company Stamicarbon successively developed a pool condenser and a pool reactor, a new process of urea 2000+TM. In the mid-1970s, my country introduced 11 large-scale carbon dioxide stripping urea units, and then China Wuhuan Chemical Engineering Company built a 240,000-ton/year domestic carbon dioxide stripping process unit at Shanghai Wujing Chemical Plant. In the 1980s, improved carbon dioxide stripping technology was introduced. Based on localization, Ningxia Chemical Plant, Zhenhai Petrochemical Plant and Guangzhou Petrochemical Company built three sets of urea plants. In the late 1980s and 1990s, China Wuhuan Chemical Engineering Company designed its own 120,000-ton/year urea plant at Sanming Chemical Plant, 160,000-ton/year Wujing Chemical Plant, 130,000-ton/year urea plant at Hebei Hejian Fertilizer Plant, and 180,000-ton/year urea plants at Meifeng Chemical Co., Ltd., Luxi Group Dong'e Fertilizer Plant, and Hubei Yihua Chemical Co., Ltd., all of which were successfully put into operation. In the late 1990s, the Tarim Oilfield Branch of China National Petroleum Corporation's Tarim Oilfield Branch's Southwest Fertilizer Plant was the first in China to adopt Stamicarbon's improved and latest pool condenser process technology. The urea unit was put into operation in November 2001. The 230,000 tons/year urea plant of Anhui Linquan Chemical Co., Ltd. was put into operation in October 2002. The carbon dioxide gas stripping urea unit consists of the following processes: The high-pressure loop includes urea synthesis tower, gas stripping tower, methane condenser, high-pressure scrubber and high-pressure ejector ； Only a low-pressure decomposition and absorption system is installed in the post-process. ； The vacuum evaporation system includes a two-stage vacuum evaporation and condensation system, and is equipped with a process condensate treatment process. The urine after vacuum evaporation is sent to the final granulation process. Its main process feature is to minimize the synthesis pressure under the optimal ammonia-to-carbon ratio. At the same time, carbon dioxide gas is used to strip the methylammonium liquid under the synthesis pressure. The decomposed ammonia and carbon dioxide are condensed under the synthesis pressure. The condensation heat is used to by-produce steam for the second-stage decomposition and first-stage evaporation as heating steam. It is also used as power steam for the steam ejector and for system insulation. Due to the use of carbon dioxide stripping, this process has lower stripping pressure and higher stripping efficiency than the ammonia stripping urea process. Therefore, the process only requires low-pressure decomposition and can meet the production requirements of the urea unit without medium-pressure decomposition. After the improvement of the process technology, the dehydrogenation technology of the raw material CO2 gas was adopted, which fundamentally eliminated the risk of explosion during the process. A 4bar absorption tower was installed after the high-pressure scrubber to absorb ammonia in the non-condensable gas, reducing the consumption of the urea unit. The process flow is short, the equipment is small, the production is stable, and the consumption is low. It has built many factories in my country and has accumulated rich experience in design, equipment manufacturing and production. In recent years, most of the newly built urea plants and the renovation of large-scale urea plants in my country have adopted new carbon dioxide stripping process technology. 2. Dehydrogenation principle and dehydrogenation reaction equation 1. Dehydrogenation principle: The CO2 feed gas dehydrogenation process includes CO2 feed gas fine desulfurization, oil removal, heating, dehydrogenation and other parts. Dehydrogenation of CO2 feed gas actually causes H2, CO, CH4 and other flammable gases in the CO2 feed gas to react with O2 to generate H2O and CO2 under the action of a dehydrogenation catalyst. In order to achieve the purpose of removing explosive gases. 2. Dehydrogenation reaction equation Catalyst 2H2 + 1/2O2 ===== 2H2O 180℃ Catalyst CO + 1/2O2 ===== CO2 180℃ Catalyst CH4 + 2O2 ===== CO2 + 2H2O 180℃ Three: CO2 dehydrogenation process The concentration of hydrogen in the five-stage outlet gas of the carbon dioxide compressor is less than 0.96%---0.01% (volume percentage). The carbon dioxide gas first enters the high-pressure CO2 heater and is heated to about 180°C. It then enters the dehydrogenation reactor equipped with a catalyst to react to burn the flammable gas components in the carbon dioxide gas to make the H2 in the CO2 gas ≤ 50ppm. The gas is then cooled to 120°C by high-pressure CO2 cooling gas and sent to the stripping tower. 4. Catalysts and main equipment 1. Dehydrogenation catalyst TH-1 type 0.4 tons (produced by Hubei Institute of Chemistry) 2. High-pressure carbon dioxide heater DN500 one set 3. High-pressure carbon dioxide cooler DN500 one set 4. Dehydrogenation reactor DN800 one set 5. CO2 feed gas desulfurization CO2 dehydrogenation catalyst is a catalyst based on precious metals such as platinum-palladium, and the price is much higher than that of ordinary catalysts. This catalyst is very sensitive to poisons such as sulfide. A small amount of sulfide can cause the catalyst activity to decrease or even become deactivated. Therefore, desulfurization must be performed before dehydrogenation. The CO2 gas from the outlet of the second stage of the CO2 compressor enters the activated carbon desulfurization tank. The sulfide in the gas interacts with the desulfurizer and is removed from 10 mg/m3 (standard) to less than 1 mg/m3 (standard). The purified gas leaving the desulfurization tank is heated by the CO2 heater and enters the Enter the COS hydrolysis tower to convert the organic sulfur in the gas into inorganic sulfur. After the gas is cooled by CO2 cooling gas, it enters the fine desulfurization tank to remove the total sulfur in the gas to less than 0.1 mg/m3 (standard). The desulfurized CO2 gas goes to the third-stage inlet of the CO2 compressor. When the sulfur content in the desulfurization agent reaches about 15% (weight), the backup desulfurization tank is activated and the desulfurization agent in the original desulfurization tank is replaced. Our company's urea production uses a refined desulfurization process to ensure the dehydrogenation effect. 6. Conclusion This process technology adopts the dehydrogenation technology of raw material CO2 gas, which fundamentally eliminates the risk of explosion during the process.