A preliminary study on a new method for temperature-induced reduction of medium-temperature catalysts

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Source: China Nitrogen Fertilizer and Methanol Technology Network. Our plant’s two ammonia synthesis systems produce 180 kt of alcohol-ammonia per year. For the two φ3800 mm medium-temperature shift converters, the heating and reduction of new catalysts in the past have mostly employed the semi-water gas cycle method. On May 15, 2008, a single-system shutdown for maintenance was carried out; Unit 1 continued to operate normally, while the catalyst in Unit 2 was replaced, from the original B117 type to the B116-2 type (60 t). For this heating and reduction process, the traditional semi-water gas cycle was not used; instead, a new method involving shift gas + semi-water gas cycle was employed. This method is easy to operate, results in stable temperature increases, prevents overheating, and saves time – the total reduction time was 41.5 hours (2.5 hours longer than the planned time due to the relatively humid insulation layer inside the furnace). 1 Introduction to the process flow: Our factory has two production systems. For the convenience of production operations, series valves are installed on the inlet and outlet pipes of the conversion system; the process flow is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/081016920317038.jpg Figure 1: Schematic diagram of the series connection process of the two conversion systems. Figure 2: Heating and reduction scheme (Table 1). Table 1: Heating and reduction scheme for the medium-temperature converter. http://www.nmtech.com.cn/jishuwang/upload1/081016921202339.jpg 3. Heating and reduction operations: (1) Before heating and reduction, CO2 is used for purging to ensure that the oxygen content in the gas exiting the conversion water separator is less than 0.5%. (2) After the system replacement is successful, slightly open change gas series valve-1 and series valve-2 (as shown in Figure 1) to supply additional gas. When the pressure is raised to 0.1–0.2 MPa, one MH compressor is started first, and the two-way valve is opened to circulate the reforming gas and raise its temperature. (3) After raising the temperature from room temperature to 120 °C, start another MH compressor, add an appropriate amount of semi-water gas, and increase the gas circulation volume as much as possible. When the temperature rises to 180°C, the series valve is closed, and the temperature is maintained at an appropriate level to enable better dehydration of the catalyst layer and reduce the temperature difference across the bed. (4) The reduction reaction is significant when the catalyst layer temperature rises to 200°C, and it becomes active at 250°C. At this time, to restore the main phase and ensure complete reduction of the catalyst, the hot water pump can be started for hot water circulation only when the temperature at all points in the catalyst layer is above 150°C. During the reduction process, when the levels of CO and H2 in the recycle gas are low, it is appropriate to vent some of the gas and introduce semi-water gas instead. (5) When the temperature of the upper catalyst layer reaches 450°C, reduce the lower catalyst ； When the temperature at the lowest point reaches 350°C and the CO content at the analysis outlet is ≤3.0%, maintaining this temperature for another 4 hours completes the heating reduction process. (6) During the heating process, it is preferable to have as small a temperature difference as possible throughout the furnace; an axial temperature difference within the same section should be ≤30°C. (7) During the heating and reduction process, the inlet valve of the compressor must be fully open, and the addition of fresh gas can be controlled by venting the recycled gas. (8) During the heating and reduction process, when increasing the CO content in the recycle gas, the venting must be done slowly; the CO content in the recycle gas should not be increased too rapidly, otherwise the large amount of heat released during the reaction can cause the catalyst to overheat. (9) Be careful when adding steam when the temperature at the lowest point of the catalyst layer reaches 180°C, as this can easily lead to overheating; the hot water pump should be started promptly after adding steam. 4 Production operation status: (1) During the direct temperature-raising reduction process using transformed gas + semi-water gas, we followed the plan strictly; the temperature remained stable, with no sudden increases or overheating occurring. (2) The catalyst exhibits good low-temperature activity after reduction, reducing the CO content in the shift gas at the outlet of the medium-shift reactor from 12%–13% to 7%–8%, thereby alleviating the load under low-shift conditions. 5 Conclusion Using a transformed gas + semi-water gas cycle to raise the temperature for the reduction of the transformation catalyst is convenient to operate and safe and reliable. The temperature of the catalyst layer throughout the process is easy to control, which not only saves time but also reduces the workload.