The issue of declining sulfur-based acid production
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Due to high system resistance, especially that of the catalyst (in the two-stage conversion and two-stage absorption process), the temperature at the inlet of Stage 1 is now 430 degrees. The standard process parameter required is 425 degrees, with a tolerance of plus or minus 2 degrees. All the valves on the high-temperature bypass line, namely those used for the boiler’s bypass, are closed (due to internal leaks; they need to be replaced during maintenance). As a result, the temperature has to be increased repeatedly until it reached 430 degrees at present; going any higher would affect the catalyst’s activity and damage it. The amount of sulfuric acid produced is also being reduced repeatedly – from 11.5 tons originally to just over 9 tons now. This means that less than ten tons of gas are produced per hour at medium pressure, resulting in a loss of over 200 tons per day. I would like to ask those with expertise in sulfuric acid production: what should be done? Is it possible to lower the temperature at the inlet of Stage 1 by opening the cold air valves, thereby increasing the amount of sulfuric acid produced? What are the advantages and disadvantages of this approach?
Based on your operating conditions, the fan should already be running at full capacity with no excess capacity left; in such a situation, it’s pointless to open the cold air valve, as the total air volume remains the same. There is always a trade-off: to increase production, one must find ways to reduce system resistance. Generally speaking, the section of catalyst where the system resistance increases the most is likely to be that part; if so, it can be handled in the following manner. There is an opportunity for parking and maintenance; a short-circuit pipe is arranged from one inlet to one outlet, with a diameter of about one-third that of a section of the flue. This can reduce the resistance in that section by about 30–40%, and it is expected that, as long as emissions remain within limits, the sulfide level can be brought back to 10.5. However, this is a short-term approach that comes at the cost of conversion rates. Converting the imported 430 isn’t a problem; the key is that the outlet pressure must not exceed 630. Based on your production volume of 300,000 tons of sulfuric acid, it’s likely that water-tube boilers with a pressure of 3.82 MPa are being used. What is the outlet temperature of the boiler? If it exceeds 370, then there is a problem with the boiler as well – the area isn’t sufficient. I’ve encountered this situation before. Previously, the plant had a unit with a capacity of 200,000 tons; the boiler’s outlet pressure was 420 at later stages. Due to internal leakage in the high-temperature valves, the pressure inside the conversion unit reached 445. During a major repair, I added a set of evaporation tubes, which reduced the pressure to 340.
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