Application of presulfided sulfur-resistant shift catalysts in coal chemical industry production

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Application of Presulfided Sulfur-Resistant Shift Catalysts in Coal Chemical Industry Production Abstract: Following the successful development of presulfided sulfur-resistant shift catalysts, their application in coal chemical industry production is discussed. Keywords: presulfiding; sulfur-resistant shift catalyst; catalytic activity I. Introduction Cobalt-molybdenum-based sulfur-resistant low-temperature shift catalysts are widely used in medium and small nitrogen fertilizer plants; prior to use, these catalysts must be sulfided at high temperatures. The common methods for sulfidation involve using CS2 or H2S as sulfiding agents in situ. However, during on-site vulcanization, improper operations often lead to overheating of the catalyst, which damages its activity; in severe cases, the entire catalyst batch may be classified as flammable. Furthermore, CS2 and H2S are harmful to living organisms, especially CS2, which poses a great risk to operators ； Furthermore, in-situ vulcanization not only consumes time, gas, electricity, vulcanizing agents, etc., but it also typically takes 2–3 days before normal production can resume. Therefore, it is very necessary to develop a new generation of sulfur-resistant, low-shrinkage catalysts. Our plant produces 400,000 tons per year of synthetic ammonia and 700,000 tons per year of urea, using coal as raw material. This project uses coal as a raw material and employs water-coal slurry pressurized gasification technology to produce crude syngas. The crude syngas from the gasification system enters the shift unit, where it is completely shifted to produce synthetic ammonia shift gas. The fully transformed crude syngas is purified through low-temperature methanol washing and refined via liquid nitrogen washing to produce syngas that meets the requirements for ammonia synthesis. This syngas is then used in an ammonia synthesis unit to yield liquid ammonia, which is subsequently fed into a urea production unit to produce the final product, urea. The crude synthesis gas for ammonia synthesis, produced from raw materials such as coal and residue oil, contains large amounts of CO (12–49%). Carbon monoxide is not only not a direct raw material required for ammonia synthesis, but it is also toxic to the catalysts used in ammonia production. Therefore, carbon monoxide must be removed from the crude syngas before it is sent to the ammonia synthesis unit. Our company’s gasification unit employs a GEGP water-coal slurry quenching process, and to match this, the shift unit uses a sulfur-resistant shift process. Previously, at the end of March 2014, our plant replaced the catalyst in the first shift converter; the catalyst used after the replacement was still a sulfur-resistant shift catalyst. Sulfurization of the catalyst was carried out using gaseified feed gas. To reduce the sulfurization time, sulfur was added to the gaseified coal slurry tank, and the sulfurization process took 43 hours ； 15 T of sulfur is consumed (estimated at 10 tons) ； At the start of vulcanization, the bed temperature was 250°C; during vulcanization, the temperature remained within the range of 250–300°C for most of the time, with the highest temperature reaching 320°C under conditions of 0.4 MPa℃ ； During the sulfidation stage, it was possible to maintain the inlet temperature of R1502 within the range of 130–150°C, thereby effectively protecting the catalyst in R1502. II. Introduction to Presulfided Sulfur-Resistant Shift Catalysts: The ex-vessel presulfiding technique is a process by which fresh cobalt-molybdenum catalysts in their oxidized state are pre-sulfided before being installed in industrial plants. It involves using a special industrial treatment process to combine a sulfiding agent, in the form of a certain sulfide, with the active components cobalt and molybdenum, thereby converting the oxidized catalysts into sulfided catalysts. The presulfided catalyst is a cobalt-molybdenum-based CO sulfur-tolerant shift catalyst that uses magnesium-aluminum spinel as a carrier; it is suitable for the conversion of sulfur-containing gases produced from heavy oil, residue, asphalt, cinder, and coal in gas production processes, and it can operate over a wide range of temperatures, sulfur concentrations, and water vapor ratios. This catalyst exhibits high activity and stability as well as high strength stability. Thanks to the pre-sulfidization technique, the active components cobalt and molybdenum in the catalyst exist in the form of cobalt sulfide and molybdenum sulfide; during use, only nitrogen is required to raise the temperature, and gas can be passed once the desired operating temperature is reached. The presulfided sulfur-resistant shift catalyst contains no alkali metals, nor any substances that are harmful to equipment or human health. During the heating process, only a small amount of water is generated, which is discharged along with the process gas and poses no risk to the equipment. Its technical features are: 1. High mechanical strength and strength stability. 2. High activity and strong resistance to hydration. 3. Good selectivity and stability of activity. 4. Strong adaptability to high space velocities and wide water vapor ratios, with great operational flexibility. III. Application of presulfided sulfur-resistant shift catalysts in actual production: Our plant replaced the catalyst in the second shift furnace in January 2015, and the catalyst used this time was a presulfided sulfur-resistant shift catalyst. (1) Heating: It took 12 hours to raise the temperature of the second converter to 90°C this time, after which it was maintained at that temperature for 3 hours. It took 9 hours to raise the temperature further to 120°C, after which the temperature was maintained at that level for 3 hours. Finally, it took 14 hours to raise the temperature to 220°C. Maintain at a constant temperature for 10 hours. (II) Gas introduction: On January 21st, at 19:20, the pipes were warmed up; the gas introduction was completed at 3:48. The second converter used in this process employed a presulfided sulfur-resistant conversion catalyst, so sulfuration was not necessary. For this gas introduction, process gas was mixed with N3; the initial flow rate of nitrogen was set at 17,000 Nm3/h, and the system pressure was maintained at 0.25 Mpa during the gas introduction process. After completion of this process, the maximum temperature of the bed reached 310°C. The entire gas guiding process took 8 hours and 28 minutes. Compared to the 43 hours required for sulfidation and gas introduction in the first converter, 35 hours and 30 minutes were saved. During gas introduction, set the converter vent to manual mode; as the gas flow increases, slowly open the vent valve of the converter system to maintain a system pressure of around 0.25 MPa. If the bed temperature rises too rapidly, the heat can be shifted backward by increasing the amount of N3, increasing the vent volume, and raising the gas space velocity in order to maintain the bed temperature. At the same time, the gas introduction rate should be adjusted appropriately based on the bed temperature, with the process gas flow being increased gradually until the vaporization vent is closed. After the shift gas conversion was completed, during normal operation, the temperature rise in the bed of the second shift reactor was concentrated in the upper part; the overall temperature rise of the shift reactor was 20°C, and the CO content at the outlet was around 0.7%. The CO level in the process gas leaving the shift system was within the specified range, indicating that the catalyst had good activity. Data before replacing the catalyst in the second shift converter:
Time: 12.22, 12.23, 12.24, 12.25, 12.26, 12.27, 12.28
CO content: 1.41, 1.46, 1.48, 1.34, 1.22, 1.34, 1.63

Data after replacing the catalyst in the second shift converter:
Time: 1.25, 1.26, 1.27, 1.28, 1.29, 1.30, 1.31
CO content: 0.77, 0.76, 0.71, 0.79, 0.76, 0.78, 0.79

Data after running the converter for some time after catalyst replacement:
Time: 3.28, 3.29, 3.30, 3.31, 4.1, 4.2, 4.3
CO content: 0.86, 0.88, 0.88, 0.89, 0.85, 0.84, 0.78

As can be seen from the table above, before the catalyst was replaced, the CO content in the outlet gas of the second shift converter was around 1.4%. After the converter started operating, the CO content in the outlet gas dropped to 0.7–0.8%. After two months of operation, the CO content in the outlet gas was around 0.85%. Fluctuations in the water vapor ratio of the process gas supplied by the gasification unit cause occasional changes in the CO content in the process gas at the shift outlet. For this replacement, our plant opted for a pre-sulfided sulfur-resistant shift catalyst in order to shorten the sulfiding time during startup. The advantage of not requiring sulfiding is that it simplifies the gas introduction process during shifting, saving nearly 35 hours in total. It also reduces pollution, facilitates operation, and lowers safety risks. Based on the performance in subsequent operations, its catalytic activity fully meets the requirements of production. IV. Conclusions 1. It is feasible to use pre-sulfided catalysts in coal chemical production. 2. Pre-sulfided sulfur-resistant shift catalysts are easy to put into operation, reducing the startup time by 2/3 (thereby saving on sulfuration costs). 3. They help reduce pollution, facilitate operation, minimize safety hazards, and make the entire installation safer and more reliable to operate