why steam is added at thermalcracking of propane 1

[ahp]

I've read some books about thermalcracking of propane or ethane to produce ethylene, and it is written that steam is added to increase the selectivity. Can anyone tell me more about the aim of adding the steam ?
thank before.

 

[TD2K]

Coke formation in cracking increases at higher hydrocarbon pressures and is undesireable of course. Steam is added to reduce the hydrocarbon partial pressure and therefore reduce coking. It also increased the selectivity towards lighter compounds like ethylene, over pyrolysis gasolines. Velocities would also be increased which would directionally reduce skin temperatures and therefore coking tendancies though this could also be handled by just selecting a smaller ID tube in the design so I don't think this is a likely reason.

Another advantage of adding steam is if you have a major tube leak, you want to pull the HC feed out of the furnace NOW. If you had no steam, you'd also then have no flow through the tube and no cooling medium for that tube from the firebox temperatures. Very quickly you'd have a deformed (or melted) tube from overheating. With steam, you can immediately go to maximum steam flow through that tube while you pull hydrocarbon feed and start an orderly furnance shutdown. You don't just want to ESD a furnace as you still have a lot of residual heat off the firebrick (which can overheat the tubes) and a furnance ESD frequently results in coking spalling off the tubes and plugging them up in the worse case requiring tubes to be cut out and replaced (BTDT).

We used to add 0.3 lb steam/lb HC on our ethane cracking furnaces. Not sure what the ratio is for propane crackers.

 

[saidsaz]

more on dilution steam addition purpose ,,,

1- steam has a sufficient oxidising effect on iron and nickel to minimise carbon dorming reactions ...

2- used in start up of a furnace in warm up operations ..

3- used for coils decoking operations ..

ratio typical ...

ethane cracking 0.25- 0.4

propane cracking 0.3-0.50

hope this helps

 

[saidsaz]

more on dilution steam addition purpose ,,,

1- steam has a sufficient oxidising effect on iron and nickel to minimise carbon formation reactions ...

2- used in start up of a furnace in warm up operations ..

3- used for coils decoking operations ..

ratio typical ...

ethane cracking 0.25- 0.4

propane cracking 0.3-0.50

hope this helps

 

[ahp]

thank you, those are really help me.