Mechanism of Catalytic Coke Formation and Some Means to Limit it in Refinery Processes

Abstract

The formation of carbon filaments which occurs at carbon activities ac > 1 in a range of temperatures 450-700°C is a major problem in many chemical, petrochemical and refinery processes where hydrocarbons or other strongly carburizing atmospheres are involved. An excessive carbon deposition causes deterioration of the furnace alloys, such as an important migration of carbon into the alloys. In order to better control and limit this deterioration, this work has been performed to on one side get a more accurate understanding of the mechanisms of formation of catalytic coke and on the other side to find remedies as the injection of selected additives in the feed. Thermogravimetric analyses (TGA) were performed on iron samples in simulated conditions of isobutane dehydrogenation. X ray diffraction (XRD) and scanning electron microscopy examinations were used to identify the different steps during the formation of the catalytic coke. The selection of appropriate remedies to reduce the catalytic coke deposition, requires accurate understanding on both mechanisms of the catalytic particles formation and of the growth of the graphite filaments. We have studied the first steps of the catalytic coke formation on high purity iron that has been previously reduced or oxidised. The comparison of the catalytic coke deposition kinetics indicates that the mass gain is much faster on a pre oxidised state than on a reduced one. In refinery and petrochemical processes, several methods can be selected in order to limit the deposition phenomena of catalytic coke: selection of an appropriate metallurgy, protection of the surfaces by application of coatings, injection of additives with the feed. Steric inhibitors (that block the adsorption sites and slow down the germination and diffusion steps) such as sulfur additives are currently industrially used but special care has to be taken in order to prevent consequential secondary effects such as, for catalytic refinery process, the deactivation of catalysts. Based on TGA experiments, the accurate amount of inhibitor to be injected has been selected regarding the oxidising state of the iron surface.