Simulation Study on the Reaction-Diffusion Coupling Processes in Simple Pore Structures

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.