Mechanisms of fouling in steam crackers

This work investigates the complex mechanisms of fouling in steam cracking. Each section of a steam cracker, i.e. the convection, radiant, and transfer line heat exchanger (TLE), is subject to distinct fouling mechanisms due to intrinsic differences in process conditions and gas-phase compositions. In the convection section, fouling arises from incomplete evaporation of heavy hydrocarbons, leading to the formation of droplets that impinge onto the hot tube walls and act as fouling precursors. However, the precise chemical reactions that converts these impinged hydrocarbon droplets into coke remain incompletely understood. Novel techniques, such as ultra-high-resolution mass spectrometry, may provide new insights into this phenomenon. Fouling in the radiant section results from an interplay between heterogeneous catalytic and non-catalytic mechanisms. Initially, the large number of available catalytic sites leads to exponential growth of coke deposits. Over time, the growth of the coke matrix encapsulates the metal particles, reducing the extent of the heterogeneous catalytic mechanism. Once this mechanism is interrupted, coking rates decrease to a steady value, entering an asymptotic coking regime. TLE fouling is primarily attributed to the condensation of high-boiling-point compounds, particularly polycyclic aromatic hydrocarbons (PAH). Although experimental evidence is incomplete, it appears that the formation of polyaromatics might be inhibited by limiting the amount of aromatic compounds with nine or more carbons in the feedstock.