427669 Numerical Study on Enhancement of Crack Reaction Employing a Novel Coil

Wednesday, November 11, 2015: 4:06 PM
250B (Salt Palace Convention Center)
Dehong Bai1, Yuan Zong1 and Ling Zhao2, (1)State key laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China, (2)State key laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, China

Numerical Study on Enhancement of Crack Reaction Employing a Novel Coil

Dehong Bai, Yuan Zong, Ling Zhao

State Key laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai, China, 200237

Olefins are important feedstock for chemical industry which are produced mainly by thermal cracking nowadays. Thermal pyrolysis technology is reliable and easy to realize while its drawback of huge energy consumption is apparent. One of the factors for process energy efficiency is the large heat resistance caused by the turbulent boundary layer. A novel internal - hollow cross disk (HCD), which involves a series of protruding crinkle structures, is expected to generate vortical flow to disrupt the boundary flow. As a result, the enhanced heat transfer can impair coke formation and the improved flow field may affect the reaction rate. However, the latter is rarely studied. The research here focus on the effect of HCD on propane cracking process by performing a comprehensive CFD study.

Based on the pilot cracking experiment by Van Damme[1], the present work has coupled turbulent model, energy equation and the improved 9-reactions propane pyrolysis kinetics[2,3] to quantitatively evaluate the thermal performance of the enhanced coil with HCD and the characteristic of reactive flow in it. Specially, The EDC model has been introduced to present the interaction between process of mixing and intrinsic reaction, which has been proven more applicable for kinetics with more than 2 reactions compared with the commonly used FR/EDS model[4].

By comparison with smooth tube, the introduction of HCD induces distinct flow pattern, as shown in Figure 1. The strengthened mixing between the vortex near the wall and main flow results to increase of global Nu by 14.73% while that for friction factor only improved by 4.52% compared with smooth tube. Subsequently, the concentration of C2H2 and C3H6 are nonuniformly distributed in the coil. Figure 2 presents the distribution of these product downstream from the HCD. The results show the reaction has close relationship with that of the flow field and it also proves the necessity to perform 3D numerical investigation for the cracking process instead of the traditional 1D simulation.

Figure 1. Velocity distribution downstream from HCD

(a) C2H4                      (b) C3H6

Figure 2. Distribution of product downstream from the HCD

To sum up, the total calculated mass fraction of desired products, including C2H4 and C3H6, has a rise of 13.15% over that of smooth tube. The conversion is also found to increase by 1.14% and the rise for olefin selectivity is 7.12%. Additionally, the mass fraction of C6+ and C2H2 have decreased dramatically in the enhanced coil which is beneficial for coke restriction. Taking both heat transfer and reaction analysis into consideration, it clearly proves that intense mixing and reduced radial temperature gradients generated by the HCD are beneficial to the promotion of reaction rate as well as olefin selectivity.

[1]P.S. Van Damme, S.Narayanan, G.F. Froment. Thermal cracking of propane and propane-propylene mixtures: pilot plant versus industrial data [J], AIChE journal, 1975, 21(6): 1065-1073.

[2]K.M. Sundaram, G.F. Froment. Modeling of thermal cracking kinetics-I: Thermal cracking of propane and their mixtures[J], Chemical Engineering Science, 1977, 32(6), 601-608.

[3]Pramod Kumar, Deepak Kunzru. Modeling of naphtha pyrolysis [J], Industrial and Engineering Chemistry Process, 1985, 24: 774-782.

[4]Guihua Hu, Honggang Wang, Feng Qian, Kevin M.Van Geem, Carl M. Schietekat, Guy B. Martin. Coupled simulation of an industrial naphtha cracking furnace equipped with long-flame and radiation burners [J], Computers and Chemical Engineering, 2012, 38: 24-34.


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