I have read somewhere that; "Steam is injected in the furnace to prevent premature coking".
How is that possible?
Answers
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10/05/2018
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A:
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The coke is a product of thermal cracking. The extent the oil is cracked is influenced by time and temperature. The steam (or water which becomes steam) reduces the time that the oil is exposed to the temperature, limiting the extent of cracking.
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10/05/2018
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
Though your question is not so clear, I presume that you mean injecting steam into furnace tubes or coil.
When high viscous oil is handled in the furnace like Reduced crude or long residue, vacuum residue in DCU furnaces or visbreaker unit furnaces, steam is introduced into the tubes as velocity steam to increase the velocity of the flow and to disrupt the boundary layer flow.
When steam is introduced it creates turbulence inside the tube and this changes the boundary layer particle to get mixed in the main stream. So the residence time for the boundary layer is reduced and thereby exposure to more heat for long duration is avoided. Otherwise the boundary layer will subject to mild to severe thermal cracking leads to coke deposition. The formed coke acts as refractory and demands more energy to attain the required process temperature and the skin temperature of the furnace tubes will reach its limiting temperature much earlier. In some furnaces boiler feed water also being used instead of steam.
Introduction of steam helps to avoid premature coking.
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