Thermodynamic analysis of reaction pathways and equilibrium yields for catalytic pyrolysis of naphtha

doi:10.1007/s11705-022-2207-6

Front. Chem. Sci. Eng.

2022, Vol. 16

Issue (12) : 1700-1712 https://doi.org/10.1007/s11705-022-2207-6

RESEARCH ARTICLE

Thermodynamic analysis of reaction pathways and equilibrium yields for catalytic pyrolysis of naphtha

Dongyang Liu, Yibo Zhi, Yuen Bai, Liang Zhao(

), Jinsen Gao, Chunming Xu

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China

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Abstract

The chain length and hydrocarbon type significantly affect the production of light olefins during the catalytic pyrolysis of naphtha. Herein, for a better catalyst design and operation parameters optimization, the reaction pathways and equilibrium yields for the catalytic pyrolysis of C_5–8 n/iso/cyclo-paraffins were analyzed thermodynamically. The results revealed that the thermodynamically favorable reaction pathways for n/iso-paraffins and cyclo-paraffins were the protolytic and hydrogen transfer cracking pathways, respectively. However, the formation of light paraffin severely limits the maximum selectivity toward light olefins. The dehydrogenation cracking pathway of n/iso-paraffins and the protolytic cracking pathway of cyclo-paraffins demonstrated significantly improved selectivity for light olefins. The results are thus useful as a direction for future catalyst improvements, facilitating superior reaction pathways to enhance light olefins. In addition, the equilibrium yield of light olefins increased with increasing the chain length, and the introduction of cyclo-paraffin inhibits the formation of light olefins. High temperatures and low pressures favor the formation of ethylene, and moderate temperatures and low pressures favor the formation of propylene. n-Hexane and cyclohexane mixtures gave maximum ethylene and propylene yield of approximately 49.90% and 55.77%, respectively. This work provides theoretical guidance for the development of superior catalysts and the selection of proper operation parameters for the catalytic pyrolysis of C_5–8 n/iso/cyclo-paraffins from a thermodynamic point of view.

Keywords naphtha catalytic pyrolysis reaction pathway equilibrium yield

Corresponding Author(s): Liang Zhao

Online First Date: 09 November 2022 Issue Date: 19 December 2022

Cite this article:

Dongyang Liu,Yibo Zhi,Yuen Bai, et al. Thermodynamic analysis of reaction pathways and equilibrium yields for catalytic pyrolysis of naphtha[J]. Front. Chem. Sci. Eng., 2022, 16(12): 1700-1712.

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https://academic.hep.com.cn/fcse/EN/10.1007/s11705-022-2207-6
https://academic.hep.com.cn/fcse/EN/Y2022/V16/I12/1700

Tab.1 Feedstocks composition after simplification

Scheme1 Reaction pathways of n-paraffins and iso-paraffins.

Scheme2 Reaction pathways of cyclo-paraffins.

Fig.1 Equilibrium constants for the protolytic cracking pathway of C_5–8 n-paraffins to form (a) light olefins and (b) light paraffins.

Fig.2 Equilibrium constants for (a) dehydrogenation and (b) hydride transfer cracking pathways of C_5–8 n-paraffins.

Fig.3 Equilibrium constants for the protolytic cracking pathway of C_5–8 iso-paraffins to form (a) light olefins and (b) light paraffins.

Fig.4 Equilibrium constants for (a) dehydrogenation and (b) hydride transfer cracking pathways of C_5–8 iso-paraffins.

Fig.5 Equilibrium constants for (a) protolytic, (b) dehydrogenation and (c) hydrogen transfer cracking pathways of C_5–8 cyclo-paraffins.

Fig.6 Effects of temperature on thermodynamic equilibrium yield for (a) n-pentane, (b) n-hexane, (c) n-heptane and (d) n-octane at 0.1 MPa.

Fig.7 Equilibrated olefin groups of C_2–8 olefin systems at 0.1 MPa.

Fig.8 Effects of total hydrocarbon pressure on thermodynamic equilibrium yield for (a) n-pentane, (b) n-hexane, (c) n-heptane, and (d) n-octane at 800 K.

Fig.9 Effect of cyclo-paraffin content on pyrolysis product of (a) n-pentane, (b) n-hexane, (c) n-heptane, and (d) n-octane.

Fig.10 The joint effect of temperature and pressure on the equilibrium yields of (a) ethylene and (b) propylene as well as contour maps of (c) ethylene and (d) propylene.

$Δ f H m θ$	Standard molar formation enthalpy, kJ
T	Reaction temperature, K
C_p,m	Molar heat capacity at constant pressure, J·mol^–1·K^–1
$Δ f S m θ$	Standard molar formation entropy, J·K^–1
C_p,m,i	Contribution value of each group to the C_p,m
N_i	The number of groups
$Δ r H m θ$	Standard molar reaction enthalpy, kJ
$Δ r S m θ$	Standard molar reaction entropy, J·K^–1
$Δ r G m θ$	Standard molar reaction Gibbs free energy, kJ
$K θ$	Standard equilibrium constant
R	Molar gas constant, J·mol^–1·K^–1
G_t	Total Gibbs free energy of mixed system, kJ
n_i	Numbers of moles of species i
$μ i$	Chemical potential of species i
$λ k$	Lagrange multiplier of the kth element
$β k i$	Number of atoms of the kth element in a mole of the ith species
b_k	Total moles of the kth element, mol
$Δ G i,f θ$	Standard mole generation Gibbs free energy of species i, kJ·mol^–1
P	Total hydrocarbon pressure, MPa
$P θ$	Standard pressure, MPa

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