Simulation of the coking phenomenon in the superheater of a steam cracker

Abstract

Coke formation in the convection section of a steam cracker occurs when heavy feeds are cracked. This work presents CFD simulations of coke formation in the mixture superheater tubes in the convection section of a steam cracker. The hydrocarbon feed used for the simulations is a gas condensate. Eleven representative chemical species are selected, based on their boiling points, to mimic the entire range of feed components. The liquid–vapor spray flow in the mixture superheater tube is simulated based on an Eulerian–Lagrangian approach using ANSYS FLUENT 13.0. Evaporation of multicomponent droplets suspended in the vapor phase or deposited on a tube wall is considered. The mixture superheater tubes make three horizontal passes (11.3m long and 0.077m diameter) through the convection section. The droplet–wall interaction model considers ‘Splash’, ‘Rebound induced breakup’, ‘Rebound’ and ‘Stick’. The amount of liquid deposited on the mixture superheater tube wall is obtained by simulating the spray flow. The amount of coke formed from the liquid deposited on a wall is based on the phase separation model of (Wiehe, 1993). Industrial & Engineering Chemistry Research 32, 2447–2454. Spatial variations of the coke layer formed in the mixture superheater tubes as a function of outer tube wall temperatures and initial droplet diameter are presented. For outer tube wall temperatures lower than the boiling point of the highest-boiling species in the feed a 1mm thick coke layer is formed over a period of 1 month. For outer tube wall temperatures higher than the boiling point of the highest boiling component in the feed no coke is formed in the mixture superheater tubes. This work provides guidelines to minimize the extent of coke formation in the steam cracker convection section when a heavy feed is cracked. It also provides possible remedies to completely eliminate the coking problem when cracking heavy feeds.