What are the consequences of not performing a thermal purge of the conversion catalyst before long-term parking?

In principle, this can easily reduce the mechanical strength of the catalyst, as a large amount of sulfur trioxide remains in the capillaries within the unheated catalyst, allowing it to absorb moisture from the air and thus causing the catalyst to become fragmented. Of course, this effect may not be apparent after just one or two occurrences, but if this practice continues, it will likely be necessary to prepare for replacing the catalyst.

The lack of hot air blowing certainly causes significant damage to the system’s equipment. After the system cools down, sulfur trioxide and water combine to form sulfuric acid; the longer this process lasts, the greater the harm caused by this dilute acid. It particularly damages catalysts, heat exchangers, economizer pipes, and valves. Sulfuric acid causes the catalyst carriers to disintegrate, reducing the catalyst’s efficiency and increasing the system’s resistance； Corrosion of heat exchangers, economizers, pipes, and valves intensifies, shortening the service life of the equipment and affecting its stable operation. This kind of damage does not become apparent immediately; it shows up during subsequent operations. I think your company should consider emergency situations such as power outages and take measures to address them, in order to minimize the occurrence of such issues.

Generally, to convert long-term parking, high-temperature gas is used to purge sulfur trioxide from the converter catalyst in order to prevent corrosion of the catalyst. The basic operation steps are as follows: Turn on the electric heating furnace 6 hours before shutdown, and after shutdown maintain the outlet temperature of the electric heating furnace above 440 degrees to carry out catalyst purging. During this process, it is necessary to keep the temperature of each section of the catalyst above 400 degrees. The purging process usually takes about two shifts, but the decision should still be based on actual measurements of sulfur trioxide in the exhaust gases. Shutdown can be carried out once the sulfur trioxide level in the exhaust gases is found to be less than 0.03%. Note: During the blowing process, the dry absorption section must be activated. Additionally, when measuring sulfur dioxide in the exhaust gases, in a process that involves two rotations and two absorptions, sulfur trioxide should be measured at each absorption inlet (i.e., the outlet of the first rotation) and the second absorption inlet (i.e., the outlet of the second rotation); it cannot be measured at the absorption outlet. When converting to parking mode, it is generally necessary to close the inlet and outlet valves as well as the bypass valves of the conversion unit, in order to prevent the catalyst temperature from dropping too rapidly. If sulfur trioxide is not completely blown out, and sulfur trioxide is introduced again once the temperatures of the catalysts in various sections of the converter drop to room temperature, it is no longer practical, as at room temperature sulfur trioxide has already condensed into acidic droplets that cause corrosion.

This is an interesting question. I think the damage is indeed severe, but there are still remedies: Step 1 is to turn on the circulation pumps of the three towers to facilitate absorption; Step 2 is to keep a low airflow rate after starting the main fan (operating with dry air at less than 1/10 of the normal level) in order to blow out as much SO3 as possible and have it absorbed by the absorption towers. It’s crucial not to circulate the water in the acid cooler – instead, try to raise the temperature of the acid. Of course, during the purging process, the wind speed outside should be as high as possible, as the instantaneous SO2 emission concentration remains high. If there are electric furnaces in the system, try to increase the temperature of the flue gas during purging; a temperature above 50 degrees can help to remove all the liquid SO3 from the catalyst. If the temperature drops below 18 degrees for an extended period after shutdown, make sure that the temperature of the dry air used for purging reaches 100 degrees. Only in this way can we hope to remove all the SO3 from the catalyst in the converter. If such conditions aren’t available, then purging with dry air for an extended period (more than 12 hours) will still provide some effect. Then increase the temperature, and the effect will be better. The damage is mainly manifested in the easy pulverization of diatomite, and the system pressure increases over time. Fortunately, however, the system valves were closed in time, so there is almost no possibility of water entering the flue system – which is a blessing in disguise!

For long-term parking, if the residual SO2 and SO3 in the converter catalyst are not removed using high-temperature hot blowing, the catalytic activity will gradually decline over time, the conversion temperature difference will decrease, and the conversion rate will drop significantly. When performing hot blowing, one can observe the exhaust stack to ensure that no white smoke is emitted; generally, this process takes about 6 hours per session.

A power outage lasting over 20 days, along with the resulting natural cooling, must have an impact on the catalyst. If the moisture content of the flue gas at the outlet of the drying tower is within the specified limits, the impact is likely to be minimal. However, the requirements outlined in the catalyst-related manuals are not absolute; we have encountered extreme situations on many occasions, such as natural insulation for 40 hours with a bottom temperature below 150 degrees, or even introducing large amounts of low-concentration flue gas before the conversion temperature was reached (of course, this was not an idea of us who work in the sulfuric acid industry). During subsequent maintenance, it was found that white sulfates had precipitated on the surface of the catalyst, causing a crust to form there. This post was last edited by in the blink of an eye on 2008-3-1 07:52]

The power went out, and it wasn’t restored until over 20 days later; as a result, the acid circulation system and fans could not be operated, so cooling had to occur naturally, and damage was inevitable.

Reply to floor 5: You might not have understood it – natural cooling does cause damage, but it’s not inevitable; in other words, the damage can be minimized. Please note that I am referring to remedies after the call. Young people should consider more ways to address unexpected situations; there are many solutions. As the original poster mentioned, by reflecting on such situations we can come up with good methods, and that is experience gained, which is truly invaluable! I hope this can also become a small asset for you!

I have encountered this situation before: the shaft of a motor used in one of my systems broke during production, and since there was no spare high-voltage motor available and heat treatment was not carried out either, it took over 20 days to send it to the motor manufacturer for repair. Before starting the vehicle after the motor was returned, I was shocked when I checked the catalyst; its color had turned black. I thought the catalyst had been poisoned, so I took a sample and sent it to Xiangfan Jingxin Technology Center for testing and activity assessment. It turned out that the black color was due to sulfur trioxide condensing as a result of the catalyst not having been properly heated, but the vanadium content was still within acceptable limits. After driving, the conversion efficiency remains the same as before, but the catalyst lifespan is greatly affected.

It’s a rare disaster; with our capabilities, we have almost no emergency response measures. We lack the water, electricity, and steam needed for cleanup, so all we can do is watch helplessly. If the weather forecast is fairly accurate, it’s best to stop the device and avoid it. Yet most of them still rely on luck.

If there is no hot blowing and the equipment is restarted after 20 days of shutdown, white smoke will definitely come from the chimney, and acid mist droplets may also be emitted. As a remedial measure, during the startup heating process, strive to raise the inlet temperature of each section to the specified range so as to reach the ignition point of the catalyst; maintain this temperature for five to six hours. This approach helps to remove a large amount of sulfur trioxide from the catalyst’s capillaries at the time of startup. Secondly, maintaining a high temperature for several hours can significantly restore the catalyst’s activity; thirdly, it facilitates smooth gas introduction.

There is indeed an impact: in a system that has not been purged, large amounts of sulfur trioxide and moisture will form condensed acid, which coats the surface of the catalyst. This not only leads to the catalyst becoming fragmented and losing its activity, but it also causes the catalyst particles on the surface to agglomerate, increasing the resistance within the system.

Due to the freezing disaster at the beginning of the year, our two sulfuric acid production units lost power suddenly due to issues with the power supply grid; as a result, it was not possible to heat or cool the conversion catalysts, so they were left to cool down naturally. Power was restored more than 20 days later. Before resuming production, we checked the catalysts by exposing them to air, and they turned black. This change in color had no significant impact on the conversion rate after operations resumed, but I believe it definitely affected the lifespan of the catalysts! This violates the operating procedures! I’ve been working with sulfuric acid for over 20 years, and this is the first time I’ve encountered such an irresistible natural disaster!