(184b) Computational Fluid Dynamic Design of Steam Cracking Reactors: Simulation of Dynamic Coke Growth

Steam cracking of hydrocarbons is the predominant commercial process for producing many platform chemicals such as light olefins (i.e. ethene, propene, and butadiene) and aromatics (i.e. benzene, toluene, and xylenes). A major factor for the process efficiency is the formation of a coke layer on the inner surface of the tubular cracking reactors. Due to this insulating carbonaceous layer, a less efficient transfer of heat to the process gas is obtained, leading to excessively high tube metal temperatures. Additionally, the cross sectional area for flow is reduced and the reactor pressure drop increases, resulting in a loss of olefin selectivity. Decoking of industrial reactors is thus inevitable. In consideration of this energetic and economic drawback, many efforts have been made towards the development of technologies to reduce coke formation. Three-dimensional coil geometries are often introduced to enhance radial mixing resulting in lower coking rates and longer run lengths.

Our group has successfully applied computational fluid dynamics (CFD) for the evaluation of the effect of 3D reactor geometries on pressure drop, coking rates and product yields¹. These studies however focused on start-of-run performance, whereas the most attractive characteristic of the enhanced reactor designs is the potential extension of the run length. Determining the full economic potential of a coil hence involves tracking its performance throughout the run. In the case of enhanced tubular geometries or reactors with a strongly non-uniform heat flux profile (e.g. due to shadow effects), the growth of the coke layer will generally not be uniform. Because of this, the reactor geometry will change in time, which will in turn influence the fluid dynamics, product yields and successive coke formation. To take this into account, the coke layer growth needs to be incorporated in the CFD simulations. An algorithm was therefore developed for simulating a run length of a steam cracking reactor and tracking the geometry deformation caused by the growing coke layer. In this algorithm, the reactor mesh is updated on a regular basis as coke deposits on the reactor wall until an end-of-run constraint is met, indicating that decoking is required.

An OpenFOAM post-processing utility has been developed, which is based on mesh generation rather than mesh motion. Using this utility, structured meshes of high quality on a ribbed tube surface can be automatically generated. When calling the utility, it reads the non-uniform temperature and concentration fields at the moving gas/cokes interface and calculates the corresponding coking rate. Based on this information and the prescribed duration over which the coking rate is assumed constant, it then creates a new mesh with the same amount of cells but taking into account the presence of the coke layer, after which calculation is resumed. Because of the simplicity of this routine, the procedure can be repeated multiple times for different stages of the coking process. As a proof-of-concept, a Millisecond propane cracker was simulated over the first days of its run length for different reactor configurations.

References

1.         Schietekat, C. M.; Van Cauwenberge, D. J.; Van Geem, K. M.; Marin, G. B., Computational fluid dynamics-based design of finned steam cracking reactors. AIChE Journal 2014, 60, (2), 794-808.