Separate Acid Circuits for IPAT, DT, and FAT

It is to be noted that SO2 is soluble in the acid to a certain extent. Since the conversion of SO2 to SO3 is only about 92-95% complete by the time the gases exit the third catalyst pass, the concentration of SO2 in these gases is about 0.7-1.1% depending on the plant conditions. 

A considerable amount of SO2 therefore dissolves in the circulating acid in IPAT. The acid pump sends the same acid to the FAT. 

The gases at the inlet of the FAT contain only about 0.05-0.12% SO2 and hence stripping ofS02 can occur from the acid coming down in the FAT. 

The exit gases from the FAT can therefore contain a higher concentration ofS02 (0.20-0.25%) which is unacceptable due to the stringent air pollution control laws in all countries. This situation is taken care of by providing separate acid circulation tanks for the FAT (and for DT+IPAT) with appropriate instrumentation for addition of dilution water and level control in both tanks. 

These were made to improve the working of the plant and were based on the experience gained over many years by plant engineers and designers. 

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