Coke formation on metal foils from heavy oils

Abstract

The coke deposition on nickel and steel foils during pyrolysis of heavy oils has been studied. The amount of coke deposited on the foils was found to be dependent on foil type, reactor history, feed type and reaction conditions. On nickel, an asphaltene-containing feed gave more coke than an asphaltene-free feed. The coking on the metal foils increased in the order SS-302 < Inconel 718, Inconel X < Nickel. Introducing a rougher surface by rubbing the foils increased the coking. Scanning electron micrographs of the coked foils are presented and reveal details of two types of deposit. Type i is a catalytically formed deposit found only on nickel. The surface is covered in grains, crystalline in appearance. The grains are either irregular or needle-shaped, depending on the feed. The structure of the grains is not affected by the reaction conditions. Type ii is a deposited layer of coke, often containing droplets or mesophase spheres. The thickness and structure of the layer depends on the feed and the reaction conditions. This coke is formed in the liquid phase and deposited onto the surface. Surface roughness is important in determining the amount of coke deposited. A mechanism explaining these phenomena is suggested.