(32b) Simulation Study on the Reaction-Diffusion Coupling Processes in Simple Pore Structures

Yanping
Li ^a,b,c, Mingcan Zhao ^c,d, Chengxiang Li ^c*, Wei Ge ^b,c,d*
a School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China
^b Collaborative Innovation Center of
Chemical Science and Engineering (Tianjin),

Tianjin
300072, China
^c State Key Laboratory of Multiphase Complex
Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China
^d School of Chemistry and Chemical
Engineering,

University
of the Chinese Academy of Sciences, Beijing 100049, China

E-mail addresses: liyanping@ipe.ac.cn
(Yanping Li), licx@ipe.ac.cn
(Chengxiang Li), wge@ipe.ac.cn
(Wei Ge).

Keywords: molecular
simulation, HS-PPM, reaction-diffusion coupling, pore

Nanoporous
materials have attractive and wide applications and vital importance in many
chemical processes because of the controlled porosity and large surface area. Their
performance would be significantly affected by the diffusion and reaction
processes insides the pore structures. To achieve high accuracy and high
efficiency with computer simulations, a coupling method is introduced to
investigate such processes.

Molecular
dynamic simulation combing hard-sphere (HS) and pseudo-particle (PP) has been developed
[1,
2].
The diffusion in a reaction-free area can be simulated by HS without
considering the structure of the reactants. However, the HS model has poor
scalability for parallel simulation due to its event-driven algorithm. To
improve the scalability of the HS model, PP modeling (PPM), a time-driven
modeling approach is coupled with HS model to construct a scalable parallel and
high efficiency method through domain decomposition. Therefore, HS-PPM has the
potential to be applied to large-scale parallel computing of diffusion-reaction
processes inside nanoporous materials with complex structure.

 The
HS-PPM coupling method is first applied to study the effect of coke deposition
on the diffusion of methane in nanoporous zeolite ZSM-5 (MFI) [2].
The reaction-diffusion coupling leads to a complex coke distribution in
zeolite, which has a significant influence on gas diffusion process. Two
different coke distribution models are proposed to simulate the actual coke
formation process. The results show that a better compromise of reaction and
diffusion process in pores leads to a uniform distribution of coke deposition,
which has little influence on gas diffusion, especially for a low amount of
coke. On the contrary, a worse compromise will result in a concentrated
distribution of coke at some active sites, which block the diffusion of gas in
the pore completely. Therefore, the coke will have significant influence on gas
diffusion, even for a little amount of coke deposition.

Then
the HS method is applied to study the reaction-diffusion coupling processes in
several simple pore structures with the same volume, including the straight,
T-shaped and cross-shaped pores, to understand the coupling mechanism. For a first-order
isothermal reaction A¡úB, a diffusion factor D
and reaction factor R are proposed to quantitatively characterize the
reaction and diffusion performance of the pore structures. In terms of the
proposed factors, D and R, the optimal structures are
investigated through optimizing the overall productivity of these structures.
The results show that the productivity depends on the competition of D
and R, which are both determined by the size and shape of the pore
structures. The results are expected to be useful for understanding the
reaction-diffusion coupling mechanism in the pore structures.

These
simulation results demonstrate the effectiveness of the simulation approach
used for evaluating the performance of the simple pore structures for simple
reactions and the potential of its application in more complicated and
practical cases. It also suggests the effectiveness of the proposed factors, D
and R, for charactering the diffusion and reaction processes at
molecular level.

References

1.
Zhang CL, Shen GF, Li CX, Ge W, Li JH. Hard-sphere/ pseudo-particle modeling
(HS-PPM) for efficient and scalable molecular simulation of dilute gaseous flow
and transport, Mol Simulat.  2016;42:1171-1182.

2. Li YP, Zhang CL, Li CX, Liu ZC,
Ge W. Simulation of the effect of coke deposition on the diffusion of methane
in zeolite ZSM-5, Chem Eng J. 2017;320:458-467.