Mole Sieve Steam Deactivation

At the Dow Fort Saskatchewan hydrocarbons plant, the cracked gas dryers are critical to remove moisture from the cracked gas stream in order to meet product specification and prevent hydrate formation in the back end of the plant. The molecular sieve in the FS Cracked Gas Dryers needs replacement approximately every 3 years in service. Bed life is limited by water adsorption capacity and run length. In order to safely remove the mole sieve from the vessel, it must first be deactivated to prevent undesired pyrophoric reaction. Water flooding is considered best practice to deactivate dryer mole sieves, however, the Fort Saskatchewan dryers were not designed to be water flooded. There have been two previous process safety near misses at the Fort Saskatchewan plant due to unsuccessful mole sieve removal causing smouldering. In 2017, a new procedure was trialed using steam to deactivate the mole sieve in order to deactivate the bed without exceeding the weight capacity of the vessel. This method was documented in the mole sieve vendor literature but very little practical experience or data existed on this method. Using this new procedure, the plant has demonstrated the ability to safely deactivate the mole sieve and prevent additional process safety near misses. Additionally, the process has resulted in decreased bed removal time, reducing downtime and cost associated with the outage. This paper and presentation will focus on the process safety incidents encountered during mole sieve change-outs, the success of the new procedure, and improvement plans which will be further trialed during the next mole sieve change out.