High-precision diffusion measurement of ethane and propane over SAPO-34 zeolites for methanol-to-olefin process

doi:10.1007/s11705-017-1684-5

Front. Chem. Sci. Eng.

2018, Vol. 12

Issue (1) : 77-82 https://doi.org/10.1007/s11705-017-1684-5

RESEARCH ARTICLE

High-precision diffusion measurement of ethane and propane over SAPO-34 zeolites for methanol-to-olefin process

Dali Cai, Yu Cui, Zhao Jia, Yao Wang, Fei Wei(

)

Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

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Abstract

The methanol-to-olefin (MTO) process has attracted much attention and many problems including lifetime and selectivity of light olefins have all been connected to the diffusion problems in zeolite crystals. However, a quantitative study of diffusion problems in SAPO-34 zeolites is lacking. In this paper, we performed a high-precision diffusion measurement of the diffusion behavior of ethane and propane, which represent ethylene and propylene respectively, over SAPO-34. The diffusions of ethane and propane over fresh and coked SAPO-34 zeolites with different crystal sizes were carefully studied. Ethane and propane show different diffusion behavior in SAPO-34. The diffusion of ethane is almost not influenced by the crystal size and coke percentage, whereas that of propane is strongly affected. A slower diffusion velocity was observed in bigger crystals, and the diffusion velocity decline significantly with the coke percentage increasing. The diffusion coefficient was calculated with both the internal and surface diffusion models, and the results show that the surface diffusion plays a key role in the diffusion process of both ethane and propane. We believe that this work would be helpful for understanding the diffusion of different molecules in SAPO-34 zeolites, and may lay the foundation of MTO research.

Keywords diffusion measurement methanol-to-olefin process

Corresponding Author(s): Fei Wei

Just Accepted Date: 25 September 2017 Online First Date: 09 January 2018 Issue Date: 26 February 2018

Cite this article:

Dali Cai,Yu Cui,Zhao Jia, et al. High-precision diffusion measurement of ethane and propane over SAPO-34 zeolites for methanol-to-olefin process[J]. Front. Chem. Sci. Eng., 2018, 12(1): 77-82.

URL:

https://academic.hep.com.cn/fcse/EN/10.1007/s11705-017-1684-5
https://academic.hep.com.cn/fcse/EN/Y2018/V12/I1/77

Fig.1 Characterization of SAPO-34 zeolites with different crystal diameters. SEM images of (A) d_m = 26.4 µm, (B) d_m = 11.2 µm, (C) d_m = 1.4 µm, and (D) SAPO-34 with hierarchical structures (d_eq = 0.2 µm); TEM images of (D) on a scale of (E) 0.5 µm and (F) 50 nm; and (G) the diameter distribution from (A) to (C)

Fig.2 Flow diagram and physical map of the equipment designed for the high-precision diffusion measurement. Adsorption amount is converted into pressure difference, measured with sensitive differential pressure sensor

Fig.3 Adsorption amount vs. time with the adsorbents being (A) ethane, and (B) propane on SAPO-34 zeolites

Fig.4 Diffusion velocity vs. coke percentage. The diffusion velocity is defined as the reciprocal of the time at which the diffusion reaches 99% of the equilibrium. Coke mass percentage was measured with TG-DSC

Diameter /µm	$D c$ (Propane) /(10^?¹⁵?m²·s^?¹)	$D c$ (Ethane) /(10^?¹⁵?m²·s^?¹)
26.4	11.97	299.20
11.2	6.24	92.93
1.4	0.19	2.15
0.2	0.04	0.04

Tab.1 Diffusion coefficients of propane and ethane in SAPO-34 zeolites with different diameters

Tab.2 Surface diffusion coefficient

Fig.5 Relationship between diffusion coefficient and diameter, from traditional diffusion theory (left) and from surface diffusion theory (right)

Fig.6 Several SAPO-34 molecular sieves with different Thiele modulus and efficiency. Calculation is based on propane

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