Issues with purging during long-term shutdowns of sulfuric acid systems

After a long shutdown of the sulfuric acid system, it is necessary to purge with sulfur trioxide to prevent it from condensing into acid and corroding the catalyst and equipment. However, it is difficult to determine and calculate the duration required for introducing sulfur trioxide, as this depends on the amount of sulfur trioxide present in the catalyst as well as the air flow rate of the fan. When the amount of sulfur trioxide is low, less time is needed; the higher the air flow rate of the fan, the shorter the time required. Therefore, it is hard to pinpoint an exact value. Generally, the process can be considered complete when the sulfur trioxide concentration at the inlet of the first absorption tower and the second absorption tower is less than 0.03%. However, during the process of injecting sulfur trioxide, it is important to remember that the flue gas temperature must be kept above 400 degrees for this injection. Additionally, the measurement of sulfur trioxide should be taken at the inlet of the first absorption tower and the inlet of the second absorption tower, not at the outlet of the first absorption tower or the outlet of the second absorption tower.

Under normal circumstances, by maintaining the conversion temperature above 400 degrees and using the normal air flow rate during production, sulfur trioxide levels can be kept below 0.03% after 4 to 5 hours

Reply to 1# natasa_ln: When the sulfuric acid system is shut down for an extended period, it is necessary to use hot air at a temperature of over 400 degrees Celsius to clean it; approximately 6–8 hours is sufficient for this purpose: SO3 at the conversion outlet

Regarding the issue of the conversion purge temperature, is it the temperature of the gas entering the conversion section that is required, or the temperature of the gas entering each conversion layer? However, it is somewhat difficult to ensure that the inlet temperature at each stage remains above 400 degrees. Specifically, the inlet temperatures at stages 2, 3, and 5 are acceptable during the first few hours, but this cannot be guaranteed later on. I wonder what about everyone else? Additionally, after purging for several hours while ensuring that the intake temperature remained above 400 degrees, it was found that there was still a temperature difference of 10-20 degrees between the inlet and outlet of each section (with the outlet having a higher temperature). Is this normal? Is there still a reaction?

Once the sulfur burning furnace stops spraying sulfur, the conversion unit immediately reduces the air flow to 1–2 KPa, opens the exhaust valve, closes the inlet valve of the fan, and closes the inlet air valve of the sulfur burning furnace (if there is a leak in the air valve, then the exhaust gas valve of the boiler is closed) to allow it to cool down naturally. Open the cold air inlet valve fully to supply air directly to the converter, which then cools down according to a cooling curve (at about 20°C per hour). When the converter temperature drops below 200°C, increase the air flow rate to accelerate cooling. When the converter temperature drops below 100, open the sampling ports everywhere to exhaust the furnace gas from the dead corners. After 20 hours of cold blowing, the analyst was instructed to take samples at the outlets of stages 3 and 4 respectively, until the sum of the SO2 levels at these two locations was below 0.015%. Otherwise, take samples every two hours until the standard is met. When the temperatures in stages 2, 3, and 4 of the converter are all below 55°C, stop the blower and inform the supervisor to begin maintenance work