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Design of the purification section in an ammonia synthesis plant Author/Source: Wang Zhiwu, Li Liang, Liu Yitang (Shandong Mingshui Chemical Co., Ltd., Zhangqiu 250200) Date: 2008-10-201 Review In June 2005, Shandong Mingshui Chemical Co., Ltd. established a successful partnership with Shanxi Jinmei Group. Taking advantage of this opportunity, we began work on a technical upgrade project for urea production with a capacity of 520 kt/a in January 2006. This project was successfully commissioned in March 2007, enabling the company to gain a certain advantage in terms of production scale and further reducing production costs. The entire technical renovation project makes use of DCS centralized control, and is divided into three operational units: gas production, purification and synthesis, and urea production, which helps to save labor costs and space. The process uses a fixed-bed gas furnace for intermittent gas production ； tannin-based desulfurization ； Medium-Low-Low transformation ； Level 7 compression ； Carbopropyl method decarburization ； Alcohol alkylation for refining and co-production of methanol ； 22.0 MPa synthetic ammonia ； Production of urea by full-circulation aqueous solution method ； Large-particle granulation technology. The gas generation, purification, and utility systems in this plant were designed by our own team, incorporating the advanced and mature process equipment technologies developed by medium and small nitrogen fertilizer manufacturers over the past few years. The author was responsible for the design of the purification section and participated in the initial commissioning of the system. The design concepts for the purification section are briefly outlined below for discussion and exchange. 2 Design of the process scheme: The purification section can be further divided into several sub-sections such as semi-dehydrogenation, shift reaction, hydrodehydrogenation, decarburization, and advanced desulfurization. 2.1 Semi-deactivation section: Based on years of practical experience in our plant, we continue to use the well-established and reliable tannin method. This method produces large sulfur particles that are easy to separate; the suspended sulfur content in the weak solution is low, which reduces the risk of tower blockage. It requires less investment, has a simple process, and there is no issue of excessive ammonia nitrogen in the discharged liquid, thus avoiding environmental pollution. (1) The production capacity of the desulfurization unit is designed to be 20×104 t/year of ammonia alcohol; the diameter of the desulfurization tower is 6000, and a packing flushing device is installed on the tower. The regeneration tank has been enlarged to φ10000, with a regeneration time of 16–18 minutes. A heater is installed in the regeneration tank to ensure proper regeneration of the solution during winter. (2) A safety water seal is installed at the outlet of the Roots blower, and the system’s circulating air is controlled by electric valves, enabling timely and effective adjustment of the air volume and proper control of the gas tank level to ensure safety. (3) Two electrostatic defoggers are connected in parallel, and the inlet and outlet pipes with a diameter of φ1600 use U-shaped water seals instead of valves. (4) A continuous sulfur melting reactor without a high-level tank is used, along with a solution sedimentation tank. At a gas flow rate of 7.6φ104 Nm3/h, the system pressure is 38 kPa, the system resistance is 0.72 kPa, and the regeneration temperature is 48 ℃. The total alkalinity of the desulfurization solution is currently 24.2 g/L, the tannin content is 1.56 g/L, and the suspended sulfur content is 0.23 g/L. The H2S level was around 1.1 g/m3 before half-desorption, and around 0.016 g/m3 after half-desorption. The main equipment for semi-disassembly is shown in Table 1. 2.2 Transformation Section The transformation system is an important process in ammonia synthesis production, and it plays a key role in ensuring the stable operation of ammonia synthesis as well as in determining energy consumption. Considering the actual conditions of our factory, we adopt a mature and reliable medium-low-temperature process. (1) Use high-efficiency medium- and low-temperature shift catalysts. (2) The medium-temperature furnace is divided into 3 sections, each equipped with **, steam, and gas cooling to regulate the temperature of the catalyst layer; it features automatic control and is easy to operate ； The low-temperature converter is divided into two sections, and a temperature-regulating water heater is installed at the inlet of each section to control the temperature of the gas entering the converter. (3) The saturated hot water tower uses a structured packing tower, which reduces system resistance and improves the efficiency of mass and heat transfer in the system. The system is equipped with multiple high-efficiency, low-resistance specialized thin-tube heat exchangers to fully recover waste heat. (4) In view of the corrosive conditions under such transformations, the first heat exchanger is made entirely of stainless steel, and the outlet pipes of the saturation tower are also made of stainless steel. At a gas flow rate of 7.6×104 Nm3/h, the system pressure is 0.78 MPa and the system resistance is 0.06 MPa ； The temperature of the semi-water gas at the exit of the saturation tower is 112 ℃, and the temperature of the shifted gas at the exit of the hot water tower is 68 ℃ ； Medium-pressure outlet temperature: 328 ℃, low-pressure outlet temperature: 181 ℃ ； The CO output at medium pressure conversion is 9.3%, while it is 3.1% at low pressure conversion; steam consumption is below 180 kg/t of ammonia. The main equipment for transformation is shown in Table 2. 2.3 Shift Conversion Section: Since coal contains not only inorganic sulfur but also some organic sulfur, this organic sulfur is converted into inorganic sulfur during the high-temperature conversion process. To ensure the normal operation of subsequent processes over extended periods, desulfurization equipment is installed after the conversion stage for secondary desulfurization. We adopted the tannin method for desulfurization. Currently, the H2S level before desulfurization is around 150 mg/m3, while it is around 6 mg/m3 after desulfurization. The main equipment for transformation and stripping is shown in Table 3. 2.4 Decarburization Section: Through comparison and taking into account factors such as energy consumption, purification level, and investment, and in line with the ammonia synthesis process of this project, the propylene carbonate method for decarburization is adopted. (1) The decarburization is designed as 2 systems, with both decarburization towers having a diameter of 3000 and being arranged in parallel. The two sets of systems operate as independent units, offering considerable flexibility in production; they can run individually at low load levels. (2) Five decarburization pumps are installed, of which 1 set can be shared by 2 systems. The decarbonization pump is equipped with a turbine, resulting in significant energy savings. The entire system employs three-stage flashing and staged recovery, featuring a simple and rational process flow. At an gas flow rate of 7.6×104 Nm3/h, the system pressure is 2.6 MPa, while the flashing pressure is 0.46 MPa ； In the normal exhaust gas, CO2 accounts for 98.4% and O2 accounts for 0.5%, while in the purified gas, CO2 is 0.2% ； The temperature of propylene carbonate is 27 ℃. 2.5 Precision Desulfurization Section: Our plant’s ammonia synthesis system employs an alcohol-alkane alkylation purification process; trace amounts of sulfur can easily poison the alcohol-alkane catalyst, which is why a precision desulfurization section is installed. Since the decarboxylation solvent, propylene carbonate, is also an excellent desulfurization agent, most of the organic sulfur in the gas, along with a small amount of inorganic sulfur remaining after decarboxylation, is removed as a byproduct of this process. The remaining H2S and organic sulfur are then removed using a advanced desulfurization unit, reducing the total sulfur content to below 0.05×10‑6. Desulfurization tanks are arranged in parallel, with a bypass line provided. T102 is installed on the top of each slot, and T104 is installed at the bottom. The main equipment for decarburization and desulfurization is shown in Table 4. 2.6 Refining Section For the refining of raw gas, the alcoholification process developed by Hunan Anchun Company is employed. This process combines the alcohol synthesis process with the hydrocarbonation and purification processes, allowing the harmful components in the ammonia synthesis feed gas such as CO and CO2, which are present at the outlets of the shift and decarburization systems, to react with hydrogen to produce methanol of high value. The levels of CO and CO2 in the gas fed to the hydrocarbonation catalyst are in the range of (50–200)×10‑6, while there is virtually no increase in CH4 levels in the ammonia synthesis feed gas. The alcohol-to-hydrocarbon refining process uses methanol synthesis and hydrocarbonation at 13.5 MPa, as well as ammonia synthesis at 22.0 MPa. A new set consists of two φ1600 methanol towers connected in series and parallel; the annual production capacity of methanol as a by-product can reach (3–4)×104 tons. Following these towers is a φ1400 hydrocarbonation tower, with the same pressure rating of 13.5 MPa. The refining section and the ammonia synthesis section are built within one framework, occupying a small area with a simple layout. They are controlled using DCS, which saves labor costs and also provides greater production flexibility, allowing for the adjustment of methanol and urea production levels according to market conditions. Methanol is produced in the first tower, while the second tower is used for purification; the hydrocarbonation tower controls trace amounts. 3 Conclusions The system has been in operation for over 8 months; the coal consumption for raw materials is around 1.2 tons, and the urea/ammonia consumption is around 0.6 tons. Production control is stable, and the expected results have been achieved. In summary, our company’s 520 kt/a urea technology upgrade project was successful; it reduced the energy consumption in ammonia synthesis, increased the company’s economic benefits, and created a certain advantage in terms of production scale.