Method for reducing coke and oligomer formation in a furnace
A method for reducing the formation of coke and oligomers in the convection section of a hydrocarbon pyrolysis furnace, the method comprising reducing the partial pressure of the hydrocarbon feed to the furnace before that feed is mixed with dilution steam.
1 . In a method for thermally cracking an essentially liquid hydrocarbonaceous feed in a pyrolysis furnace having a convection section, said feed initially having a first partial pressure as it is fed into said furnace, said feed being pre-heated in said convection section to vaporize at least a portion thereof, after pre-heating of said feed said pre-heated feed being mixed in said convection section with a total operating weight of dilution steam for said cracking process, the resulting mixture of feed and dilution steam being further heated in said convection section, the improvement comprising, before said feed is mixed with said dilution steam reducing said first partial pressure of said feed to a lower second partial pressure, said second partial pressure being effective to cause increased vaporization of said feed before it is mixed with said dilution steam.
2 . The method of claim 1 wherein said first partial pressure is reduced in an amount effective to vaporize at least about 75 wt % of said feed based on the total weight of said feed at least by the time said feed is mixed with said dilution steam.
3 . The method of claim 1 wherein said first partial pressure is reduced to said second partial pressure by adding steam to said feed before said feed is mixed with said total operating weight of dilution steam, said additional steam being in addition to said total operating weight of dilution steam that is later mixed with said pre-heated feed.
4 . The method of claim 3 wherein said additional steam is added as said feed enters said furnace.
5 . The method of claim 3 wherein said first partial pressure is reduced to said second partial pressure by adding a portion of said total operating weight of said dilution steam to said feed before the remainder of said total operating weight of said dilution stream is added to said feed.
6 . The method of claim 5 wherein no steam in addition to said apportioned total weight of dilution steam is employed.
7 . The method of claim 5 wherein said apportioned total weight of dilution steam is added as said feed enters said furnace.
8 . The method of claim 5 wherein said portion of dilution steam is from about 10 to about 30 wt % of said total weight of said total operating dilution steam employed in said cracking process.
9 . The method of claim 1 wherein said feed is first pre-heated by itself in said convection section and then mixed with said total operating weight of dilution steam, said resulting mixture of feed and dilution steam being further heated in said convection section in a series of coils that are in fluid communication, and said second partial pressure of said feed is sufficiently lowered to essentially vaporize at least about 75 wt % of said feed based on the total weight of said feed before said dilution steam is mixed with said feed.
10 . The method of claim 9 wherein said lowering of said first partial pressure of said feed is accomplished by adding additional steam to said feed before the addition of said total operating weight of dilution steam to said feed.
11 . The method of claim 9 wherein said first partial pressure is reduced to said second partial pressure by adding a portion of said total operating weight of said dilution steam employed in said cracking process before said feed is mixed with the remainder of said total operating weight of dilution steam.
12 . The method of claim 11 wherein said portion of dilution steam is from about 10 to about 30 wt % of said total operating weight of dilution steam employed in said cracking process.