(184b) Computational Fluid Dynamic Design of Steam Cracking Reactors: Simulation of Dynamic Coke Growth
- Conference: AIChE Spring Meeting and Global Congress on Process Safety
- Year: 2016
- Proceeding: 2016 AIChE Spring Meeting and 12th Global Congress on Process Safety
- Group: Innovations in Process Research and Development
- Time: Wednesday, April 13, 2016 - 2:00pm-2:30pm
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.
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.
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