Numerical simulation of hollow catalyst with pores in gas-solid reaction system

doi:10.12034/j.issn.1009-606X.220229

Abstract

Abstract:

In the gas-solid flow, fluid and particle usually aggregate to form dense-phase and dilute-phase respectively, resulting in unbalanced mass transfer and reaction rates between the dense- and dilute-phase. The unbalance of dilute- and dense-phase reduces the overall efficiency of the reactor. To solve this problem, a hollow catalyst particle with pores structure is designed. It is aimed to enhance the overall efficiency of fluidized reactors by improving the mass transfer rate between the dense- and dilute-phase. Dense- and dilute-phase are widely distributed during fluidization processes, which can be respectively described by the cluster system and single particle system. In these two phases, the flow, reaction process and mass transfer process of hollow porous particles are studied by numerical simulation, and compared with the solid spherical particle system at the same fluidization condition. Then, the frequency of particle clusters formation and breakage is studied under various fluidization conditions. The time scale is analyzed to measure the possibility of gas transport in the fluidization process. It is found that the time-scales of the mass transfer, the reaction, and the movements of particles between the dilute- and dense-phase could be at the same order for some fluidization conditions. Thus, hollow porous catalyst particle can store the reacting material in the dilute-phase efficiently. When moving to the dense phase, the particle would release the reacting material to provide additional material for the dense-phase reaction. When the reaction is faster than the mass transfer, the overall reaction rate of the hollow porous catalyst system is 26.92%~29.55% higher than that of the solid spherical catalyst system at the studied conditions. It could be predicted that this hollow porous catalyst would be capable to improve the overall reaction efficiency of large gas-solid fluidized bed reactors due to wider distributions of the dilute- and dense-phase.

Key words: hollow porous catalyst, gas-solid flow, mass transfer rate, reaction rate, numerical simulation

摘要：

气固流态化过程中流体和颗粒分别聚集，形成稀密两相，严重限制其传质效率和反应速率的提高。针对此问题，本工作设计了一种中空多孔结构的催化剂颗粒，通过模拟方法研究该颗粒对稀密两相气相传质与反应的影响，及其在稀密相间转换的时间尺度。结果表明，一定的流动强度时，在颗粒稀密相转换的时间尺度内，中空多孔结构的颗粒能够有效地在稀相存储反应气体，并在密相释放，为密相提供额外的反应气体，增强体系的整体反应效率。当催化反应速率高于传质速率时，在所研究的流动条件下中空多孔颗粒体系的反应效率比实心球形颗粒体系高出26.92%~29.55%。可以预见在稀密相分布更广的大型气固流化床反应器中，中空多孔结构的催化剂颗粒能够更为有效地提高反应器的整体效率。

关键词: 中空多孔催化剂, 气固流态化, 传质速率, 反应速率, 数值模拟

CLC Number:

TQ021

Qiuying WU, Lingkai KONG, Ji XU, Wei GE, Shaojun YUAN. Numerical simulation of hollow catalyst with pores in gas-solid reaction system[J]. The Chinese Journal of Process Engineering, 2021, 21(7): 774-785.

吴秋莹, 孔令凯, 徐骥, 葛蔚, 袁绍军. 气固两相流内中空多孔催化剂性能的数值模拟[J]. 过程工程学报, 2021, 21(7): 774-785.

Figures/Tables 21

Fig.1 Catalyst structures and the computing meshes

Table 1 Parameters of numerical simulation

Parameter	Value
Particle diameter/m	5.0×10^-3
Grid size/m	5.0×10^-4~2.0×10^-3
Time step/s	2.0×10^-5~5.0×10^-5
Inlet mass fraction of A	1.0
Inlet mass fraction of B	0.0
Inlet mass fraction of I	0.0

Fig.2 Single particle simulation system

Fig.3 Mass fraction distributions of A component

Table 2 Particle Sherwood number (Shs) of the single solid spherical catalyst

Re_s	Frössling	Ranz-Marshall	Lu et al^[27]	Simulation
10	3.75	3.90	3.68	3.80
20	4.47	4.68	4.51	4.79
40	5.49	5.79	5.67	6.14
60	6.28	6.65	6.56	7.16
100	7.52	8.00	7.94	8.51
200	9.81	10.49	10.46	11.31

Table 3 Parameters of numerical simulation in dilute phase

Parameter	Value
Particle diameter/m	1.0×10^-4
Grid size/m	1.0×10^-5~5.0×10^-5
Time step/s	1.0×10^-7
Inlet mass fraction of A	0.9
Inlet mass fraction of B	0.0
Inlet mass fraction of I	0.1

Fig.4 The velocity distributions (Res=10): (a) solid spherical particle; (b) hollow porous particle

Fig.5 The mass fractions of product B (Res=10): (a) solid spherical particle; (b) hollow porous particle

Fig.6 The mass fractions of reactant A in hollow porous particle systems (Res=10)

Fig.7 The storage of the gas reactant A and its growth rate in hollow porous particles (Res=10)

Fig.8 The process of storing gas reactant A in hollow porous particles

Fig.9 Particle cluster simulation system

Table 4 Parameter of numerical simulation in dense phase

Parameter	Value
Particle diameter/m	1.0×10^-4
Grid size/m	1.0×10^-5~5.0×10^-5
Time step/s	1.0×10^-7
Inlet mass fraction of A	0.1
Inlet mass fraction of B	0.0
Inlet mass fraction of I	0.9

Fig.10 The average mass fraction of the reactant A remaining in the cavity of hollow porous catalyst

Fig.11 Velocity distributions in particle cluster system (Res=10): (a) solid spherical particle; (b) hollow porous particle

Fig.12 The mass fractions of product B in particle cluster system (Res=10): (a) solid spherical particle; (b) hollow porous particle

Fig.13 The mass fractions of reactant A in particle cluster system (Res=10): (a) solid spherical particle; (b) hollow porous particle

Fig.14 Configuration of the simulation system

Fig.15 Porosity distributions of particle clusters (Res=10)

Fig.16 Analysis of particle aggregation and dispersion (Res=2)

Fig.17 Analysis of particle aggregation and dispersion (Res=10)

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