Abstract:
Turbulent mixing enhancement of twisted tape for propane cracking process with detailed radial kinetics was described. Meso- and micro-scale mixing performance were analyzed to reveal the close relationship between turbulent mixing and cracking reaction. Computational fluid dynamics was used to conduct contrast simulations between bare coil and coil with twisted tape in order to present variations of transfer and cracking reaction characteristics. The simulation results show that by paying 5.31% more pressure drop than bare coil, the turbulent kinetic energy at the ratio of axial distance to total length
Z/
L=0.228 in the downstream of twisted tape is increased by 39.6%, while the overall averaged characteristic micro-mixing time is reduced to 1.72% with respect to bare coil, indicating an effective meso- and micro-mixing enhancement. The intensified turbulent mixing promotes the uniformity of field profiles to improve synergy among temperature, species concentration and velocity distributions, resulting in enhanced heat and mass transfer performance. The
Nu and
Sh of cracking coil installed twisted tape are increased by 7.50% and 2.34% compared to bare coil, respectively. The overall olefin yield is improved by 5.88% and the averaged coil skin temperature is reduced by 24.01 K. The coking suppression effect can be attributed to the strong swirling flow induced by twisted tape, generating a radial velocity component to accelerate fluid in the wall vicinity. Consequently, the residence time of fluid in the near wall region at the ratio of radial distance to cracking coil radius
r/
R=0.993 can be reduced by 53.01%. Furthermore, the enhanced turbulent mixing results in better homogenous radial field profiles, enlarging the molecular scale uniform mixed region by 5.29% to extend the reactive zone, which is beneficial for olefin yield improvement.