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This post was last edited by mkp369 on 2018-6-27 at 17:21. A layer of catalyst is usually formed as a scab on the top. In recent years, there have been several cases where the catalyst in the middle and lower layers of a conversion unit changed color or developed scars; sometimes the ceramic balls at the bottom of those layers would even crack, leaving users very confused. After several years of monitoring, it was found that some factories proceed with cold blowing without first performing hot blowing, while other factories use a short duration for hot blowing. During cold blowing, the process gas containing a small amount of SO2 is heated above its ignition temperature as it passes through an upper catalyst layer, resulting in the formation of SO3 (with some of this SO3 remaining within the pores of the catalyst). Subsequently, as the cold gas passes through the middle and lower layers of the bed, SO3 combines with the moisture present in the process gas to form condensed acid. When there is a high amount of acid on the mesh belt of the upstream drying tower, under normal operation the acid carried along is decomposed into SO3 and water vapor before it reaches the converter. However, during cold blowing, the acid carried along is not completely decomposed, which can also cause scarring on the catalyst in the middle and lower sections.