3 replies to this topic

[gvdlans]

1. Residence time in the cracker heater is very short, around 0.05 s. There are a number of reactions involved, for example:
ethane <=> ethylene + H2 <=> acethylene + H2, but also addition reactions that ultimately lead to coke.
So the longer the residence time, the more ethylene is converted into acethylene and coke. If you would wait long enough, you would have only coke and H2 left.

2. Pressures are relatively low (a few barg). Further reducing the partial pressure is possible but not very attractive: the equipment must be large... The steam dilution is used to avoid addition reactions which would ultimately lead to coke.

3. One of the objectives of the selected process conditions and equipment is to prevent coke formation as much as possible. Still. a heater needs to be decoked about once a year. This is often done on-line by burning off the coke by sending in a steam/air mixture. Some operating companies remove the coke mechanically, but this means that flanges must be opened etc. So depending on the decoking method, there is either no or a limited amount of coke produced. I would say the coke formed is anyway not of a high quality, e.g. when compared with coke produced in the delayed coking process in refineries.

If you want to know more, maybe you can find a copy of the book "Ethylene - a Keystone to the Petrochemical Industry", by Ludwig Kniel e.a.