Posted By Flic If we use the Contact process for the manufacture of sulphuric acid as an example, the overall process is that sulphur is burned in excess air to form sulphur dioxide, which is then oxidised to sulphur trioxide. This is then reacted with sulphuric acid to form oleum and finally diluted back down to form sulphuric acid. There are three intermediates - sulphur dioxide, sulphur trioxide and oleum. If we take the process from beginning to end all the intermediates fall into category (a). All of them stay in the reaction vessels except, perhaps for sampling. If we were to remove one of them and store it for later (e.g. if we had a shutdown and there was some gas in the system), it would fall into category (b). I am uncertain whether this example can be turned into category (c), since these intermediates are products in their own rights. I suspect that, within the definition, we were to transport oleum to another site, solely for dilution to sulphuric acid, it would fall into category (c), but am unsure. Flic