After several weeks of operation, more and more of the coke produced as a by-product during the process has deposited in the pipes in which the cracking process is taking place. These must be "decoked" every 40 to 90 days. For this purpose, the pipes are normally regenerated by introducing a preheated air-vapour mixture into them. In the process, the coke is burned off or "gassed-off" revealing the metallic surface of the tubes, which is also catalytically active for the cracking process.
This process is accelerated by the use of oxygen-enriched air and results in a much more thorough removal of the coke than when using non-enriched air. The use of oxygen in de-coking has the following advantages compared to pure air operation:
High quality decoking and associated energy savings
Greater flexibility in operation