(586d) Numerical Study on Enhancement of Crack Reaction Employing a Novel Coil

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.