Fouling of Convection Section Tubes Due to Heavy Feeds in a Steam Cracker

Enhanced fouling rates of convection section tubes of a steam cracker is being increasingly observed owing to the tendency of the industrial crackers to crack heavier feeds. CFD simulations are performed to predict the extent of fouling i.e. coke formation in the over-heater tubes of the convection section of a steam cracker. Several orientations of tube geometries of an actual steam cracker were simulated each having 3 passes and each pass being 11 m long of 0.0775 m diameter ¹. Operating conditions that were simulated include inlet droplet diameter in the range 1 to 100 mm and outer wall temperature above and below the boiling point of the highest boiling component in the feed.

The spray flow was simulated using the discrete phase model (Lagrangian approach). Eleven representative chemical species were selected to represent the entire feed composition in terms of its physico-chemical properties. Evaporation of species from the multicomponent droplets while flowing through the tube was also included in the simulations. The droplet interactions with the wall (splash, limited splash, rebound and stick) were simulated using the splashing regime map of Bai et.al. (2002)²,adapted to the present conditions where needed. Movement of the liquid deposited on the wall was simulated using the Eulerian wall film model. The latter also took into of account evaporation of liquid, and generation of droplets due to shearing of the liquid wall film of already deposited droplets. The dry and wet patches on the tube wall, owing to boiling in the liquid film, is also accounted for, along with changes in the heat transfer characteristics of the wall. The quantity and the composition of the liquid on the wall film is determined using a coking rate model based on the coking model of Wiehe (1993)³.

It was seen that the coking rates are dominantly controlled by the outer wall temperature in the convection section. Higher coking rates are observed for outer wall temperatures lower than the boiling point of the feed. For outer wall temperature higher than the boiling point of the deposited liquid, coking was seen to occur only in the cold zones (inlet bends) of the tubes. The variation in inlet droplet diameters was seen to have a significant effect on the extent of liquid deposition on the wall. The effect of variation in inlet droplet diameters, the shell side outer wall temperature and the tube geometry on the amount of liquid deposited on tube wall and coking rates are discussed. The trend in coke formation rates and its effect on a change in heat transfer and pressure drop characteristics of the tubes is studied for over an operating period of 1 year.

Fraction of incoming liquid deposited at the inlet bend for different inlet droplet diameters

References

1.      S. De Schepper, G. J. Heynderickx, G. B. Marin, (2010), Modeling the coke formation in the convection section tubes of a steam cracker, Ind. Eng. Chem. Res., 49, 5752-5764.

2.      C.X. Bai, H. Rusche & A.D. Gosman, (2002),  Modeling of gasoline spray impingement, Atomization and Sprays, 12, 1-27

3.      I. Wiehe, A phase separation kinetic model for coke formation. Ind. Eng. Chem. Res. 32 (1993) 2447-2454