[mookxi]

Why is steam added into the cracker in thermal cracking?

My guess is that it is added to reduce carbon build-up during the process, is that the reason?

Thanks

[mbeychok]

Mookxi:

Thermal crackers to produce cracked gasoline from heavy gas oils in petroleum refineries have largely gone out of existence and been replaced by fluid catalytic crackers and hydrocrackers.  However, when they were used back in the 1930's and 1940's, steam was always injected into the process tubes of the fired heaters that raised the temperature of the gas oils to the point where they cracked into smaller molecules ... and the purpose of the steam was to inhibit the rate of coking in the heater's process tubes so that the heaters could stay online continuously for some months.

Today's delayed coking units in petroleum refineries are designed to produce and lay down saleable coke in very large vertical drums.  In those delayed cokers, steam is again used in the feed heaters to inhibit and delay the coking from occuring until the heated, vaporized feed reaches the large vertical coke drums.

[Donaldson Tan]

... and the purpose of the steam was to inhibit the rate of coking in the heater's process tubes so that the heaters could stay online continuously for some months

Coking = Carbon Build Up

You don't want carbon to cover the fired heaters because the built-up carbon reduces the overall heat transfer coefficient in the long run. Steam increases temperature of the process stream, thus promoting cleaner cracking.

[vijayppt]

As mbeychok has written,Steam(more often condensate water) is injected in furnace tubes so as to increase the velocity inside tubes to turbulent zones which will avoid any coke settling down in furnace tube inside.