Simulation of Coking in Convection Section of Steam Cracker

Abstract

This work presents CFD simulations of coke formation in the tubes of the mixture over-heater in the convection section of a steam cracker. An actual industrial feed (Gas condensate) composition is used in the simulations. The spray flow (liquid droplets in the feed) entering the tubes is simulated based on Eulerian-Lagrangian approach including a species model and a multicomponent evaporation of the droplets, using ANSYS FLUENT 13.0. The tubes of the mixture over-heater makes 3 horizontal passes (11.3 m long and 0.077 m in diameter) with U-bends connecting the passes. Droplet-wall interaction like splashing, limited splashing, rebound and stick are considered. Due to these interactions some liquid is deposited on the tube wall. The liquid deposited on the tube wall undergo partial evaporation and thermal degradation to form coke based on phase separation model of Wiehe (1993). Finally, the spatial variation in coke formation in the over-heater tube is presented as a function of various outer tube wall temperatures. For outer tube wall temperatures below the boiling point of the highest boiling compound in the feed, coke formation (~ 1 mm thick in 1 month) is observed mainly at the bends. For outer tube wall temperatures above this boiling point no coke is formed in the tubes except at the bends.