Novel Cracking Coil Design Based On Flowing Field Regulation And Its Synergetic Reinforcement Mechanism

Thermal cracking is a vital process to provide basic raw materials for the petrochemical industry.In view of the huge energy consumption of cracking furnace,improving its energy efficiency by developing heat and mass transfer enhanced cracking coil can bring promising social and economic benefits.Among all the intensification methods of cracking coil,novel insert enhancing strategy has drawn more and more attention for its lower manufacturing cost and good performance.This research has developed novel cracking coil based on the concept of positive constructing of synergetic flowing field.The novel hollow cross disk(HCD)internal was employed to disturb the local flowing pattern and achieve a more synergetic field profiles which is more suitable for cracking process.Computational fluid dynamics(CFD)method combining with the experiments were applied in this study to reveal the distinct transfer and reaction characteristics of the designed novel cracking coil.The research results can provide a new developing idea and structure for the design of novel cracking coil as well as some new criterials for the simulation of cracking process.The main works are summarized as following:(1)A 3D mathematical model for the cracking coil was established to describe the pyrolysis process.Validation and verification tests were conducted to demonstrate the reliability and numerical precision of the established model.Furthermore,the mesh number estimation formula for mass transfer controlled turbulent reactive process simulation was derived based on isotropic turbulence theory as well as the physical meaning of eddy dissipation concept model(EDC)with the dimensionless Da number,The cracking simulation case of bare coil was employed to validate this formula.The results showed that when averaged characteristic mesh scale approaching 1.80mm with the inlet Reynolds number of 7.27 ×104 for the 11m millisecond cracking coil,the relative error of C2H4 yield between simulation and industrial value decreased to 1.18%,indicating applicability of the derived mesh number estimation formula.(2)Compressible flowing characteristic of the cracking coil and its effect on heat transfer was studied.Compressibility was caused by extremely high heat flux along the coil skin and can result in abnormal variations of density,temperature and velocity profiles near the entrance and exit of the U-shape bend in cracking coil.The simulation results showed that with the developing of fluid in cracking coil,vorticity induced by density variations of compressible flow accounted for 18.40%of the total amount,indicating an obvious promoting effect for vortex generation.The volume expansion of compressible flow absorbed part of heat and resulted in a 51.21k lower temperature in centerline comparing with the case neglecting compressive effect.As the induced disturbence in compressible flow can weaken towards upstream and downstream with local speed of sound,perturbance caused by the curved bend can maintain the distance about 25 to 30 times of coil diameter at upstream while the downstream effect zone ranged from about 26 to 50 times of the coil diameter.Strong perturbance disturbed and reconstructed a different flowing field than that in normal pipe flow.In addition,the Dean vortex couples induced by the curved bend can improve Nusselt number Nu maximumly by 3.87 times comparing with that at the entrance of the bend,showing an effective heat transfer enhancement.(3)Contrastive thermal cracking and heat exchange experiments were conducted between the novel coil with HCD insert and bare coil.Cracking experiment results showed that the overall coking material weight of the four sampling parts at novel coil is 4.67%less than that at the bare coil.While the heat exchange results implied that the Nu of designed novel coil can achieve a maximum increase of 5.89%,validating the practical heat transfer intensification effect of HCD insert.Further CFD simulation for industrial scale cracking coil with HCD was conducted to analyze its multiscale mixing enhancement character and revealed the close relationship between turbulent mixing and cracking reaction.The numerical results disclosed that HCD can induce radial velocity component at the cost of pressure loss to improve turbulence in the near wall region.The averaged kinetic energy at z/L=0.597 of HCD was increased by 12.13%,while the overall averaged characteristic micro-mixing time was only 1.96%of the bare coil,showing an effective meso-and micro-mixing enhancement.The improved multiscale mixing performance resulted in the Eddy dissipation concept cell volume fraction increased by 0.58%,which meaned more region inside the cracking coil had mixed uniformly at molecular scale,resulting a larger reactive region.Consequently,the ethylene and propylene yield were improved by 0.36%and 7.96%respectively comparing with bare coil.Furthermore,the exchanging flow between the near wall region and center of coil decreased its averaged coil skin temperature by 21.76k.Residence time in the near wall region was sharply reduced to prevent excessive cracking.Comparative simulations between HCD and twisted slice presented their distinct near wall region and central enhancing character respectively.HCD insert can achieve obvious strengthening performance with fewer pressure cost and had no risk of central blocking by coking material.Generally,HCD had an advantage over twisted slice for near wall region enhancement.(4)A combination of the novel HCD insert and twisted slice was employed to reconstruct the local flowing field and improve synergy between temperature,velocity and raw material concentration profiles.Featured segmentation of cracking coil was conducted to analyze strengthening requirement of different section.Net reaction rate of C3H8 and attenuation distance of induced vortex were employed to quantitatively determine the best installing position for HCD and twisted slice.The formula of mass transfer synergy angle considered source item for reaction process was deduced and employed to explain its reinforcement mechanism of HCD and twisted slice combination strategy.The results implied that the cosine value of heat and mass transfer synergy angle can achieve a maximum increase of 114.5 and 111.2 times at the downstream of both inserts comparing with bare coil.The "Synergy Index"criterial(SI)was put out to evaluate the matching attribution of reconstructed field profiles.Results showed that SI value at downstream of both HCD and twisted slice can maximumly increase 63.73%and 94.41%.The synergy improvement of reconstructed flowing field resulted in an increase of 3.86%for the overall olefin yield and a reduced coil skin temperature of 28.45k comparing with bare coil,indicating an effective intensification performance in product distribution optimization and coking depression.