Method of preheating hydrocarbons for thermal cracking

In a method of operating a cracking furnace suitable for the cracking of different hydrocarbon feedstocks in the presence of steam to produce ethylene, said cracking furnace having a radiant heating zone provided with fuel-fired burners and through which a preheated mixture of said hydrocarbon feedstock and said steam is passed, combustion gases from said radiant heating zone passing into a convection zone being provided with heat exchanger means for preheating said feedstock and said steam, the improvement which comprises selecting different operation states for the cracking furnace by the steps of:

• subdividing the heat exchanger means in said convection zone into a plurality of functionally separated heat exchanger bundles;
• passing said hydrocarbon feedstock selectively through a first bundle or a first series of bundles of the heat exchanger means;
• passing said steam separately from said hydrocarbon feedstock selectively through a second bundle or a second series of bundles of the heat exchanger means;
• mixing said preheated hydrocarbon feedstock and said preheated steam before conducting the mixture into said radiant zone of said cracking furnace;
• and
• selectively choosing the bundles for preheating of said hydrocarbon feedstock and said steam in accordance with the composition of said hydrocarbon feedstock.