Numerical simulation of hydrogen reduction of U3O8 in fluidized bed reactors using CPFD method

doi:10.11949/0438-1157.20240358

Abstract

Abstract:

Gas-solid fluidized bed has been used in many links of natural uranium conversion process due to its advantages such as high gas-solid contact efficiency and fast interphase mass and heat transfer. However, the current understanding of fluidization reaction performance of high-density particle systems remains insufficient, complicating precise design and control efforts. This work employs the computational particle fluid dynamics (CPFD) method to conduct a comprehensive three-dimensional numerical simulation of an industrial-scale continuous U₃O₈ reduction fluidized bed system, focusing on the statistical analysis and comparison of key parameters such as macroscopic gas-solid flow, heat transfer, and reaction characteristics across different particle size distributions within the fluidized bed reduction system. The results indicate that under conditions of 80% excess hydrogen, the fluidization state of particles with three different sizes performs poorly, with most particles in a non-fluidized state and low bed expansion ratios observed. Analysis of the product distribution at the gas and solid outlets reveals that smaller particle sizes correspond to higher system temperatures and product conversion rates. However, due to the generally poor fluidization state, even with smaller particle sizes, the overall conversion rate remains low. This result suggests that further optimization of operational conditions and structural configuration of the fluidized bed may be necessary in practice to enhance fluidization effects and reaction conversion rates. Through this study, we aim to offer new perspectives and methodologies for a deeper understanding of the flow and reaction characteristics of high-density particles in fluidized beds, providing robust support for technological advancements in nuclear chemical engineering and related fields.

Key words: gas-solid fluidized bed, hydrogen reduction, high-density particles, CPFD, numerical simulation, natural uranium conversion, nuclear chemical engineering

摘要：

气固流化床因具有气固接触效率高、相间传质传热快等优点已用于天然铀转化工艺的多个环节，但目前对这类高密度颗粒系统的流化反应性能认识不足，难以对其精确设计和操控。采用计算颗粒流体力学（computational particle fluid dynamics， CPFD）方法对工业尺度连续U₃O₈还原流化床进行全三维数值模拟，对不同颗粒粒径分布的流化床还原系统中宏观气固流动、传热、反应特性等重要参数进行统计分析。结果显示，在氢气过量80%的条件下，3种不同粒径的颗粒在流化床中的流化状态均表现不佳，大部分区域颗粒处于非流化状态，床层膨胀率低。分析出口处产物分布发现，颗粒粒径越小产物的转化率越高，然而由于流化状态普遍较差，即便是在粒径较小的条件下转化率的总体水平仍然偏低。以上结果表明，在实际操作中可能需要进一步优化流化床的操作条件和结构形式，以提高流化效果和反应转化率。本研究期望能够为高密度颗粒在流化床中的流动与反应特性的深入认识提供新的视角和方法，为核化工及相关领域的技术进步提供有力支持。

关键词: 气固流化床, 氢还原, 高密度颗粒, 计算颗粒流体力学, 数值模拟, 天然铀转化, 核化工

CLC Number:

TQ 051.3

Shuyue LI, Huan WANG, Shaoqiang ZHOU, Zhihong MAO, Yongmin ZHANG, Junwu WANG, Xiuhua WU. Numerical simulation of hydrogen reduction of U₃O₈ in fluidized bed reactors using CPFD method[J]. CIESC Journal, 2024, 75(9): 3133-3151.

李舒月, 王欢, 周少强, 毛志宏, 张永民, 王军武, 吴秀花. 基于CPFD方法的U₃O₈氢还原流化床反应器数值模拟[J]. 化工学报, 2024, 75(9): 3133-3151.

Figures/Tables 18

Fig.1 Schematic diagram of unreacted shrinking core model for U3O8 reduction reaction

Fig.2 (a) Schematic diagram of the U3O8 reduction fluidized bed system structure; (b) cross-sectional mesh for three-dimensional model

Table 1 Main operating parameters of U3O8 reduction fluidized bed reactor

操作参数	数值
U₃O₈进料
进料量	$600 k g / h \times \frac{M_{U O_{8 / 3}}}{M_{U}} = 707.56 k g / h$
进料温度	866 K
密度	6690 kg/m³
气体进料
进气量（H₂过量80%）	氢气量：3025.21 mol/h 氮气量：1008 mol/h 折算气速：0.064 m/s
固体产物出口
温度	866 K
压力	400 kPa
出料量	707.56 kg/h
UO₂密度	6870 kg/m³
壁面条件	绝热壁面

Table 1 Main operating parameters of U3O8 reduction fluidized bed reactor

操作参数	数值
U₃O₈进料
进料量	$600 k g / h \times \frac{M_{U O_{8 / 3}}}{M_{U}} = 707.56 k g / h$
进料温度	866 K
密度	6690 kg/m³
气体进料
进气量（H₂过量80%）	氢气量：3025.21 mol/h 氮气量：1008 mol/h 折算气速：0.064 m/s
固体产物出口
温度	866 K
压力	400 kPa
出料量	707.56 kg/h
UO₂密度	6870 kg/m³
壁面条件	绝热壁面

Table 2 Simulation parameters and initial boundary conditions

参数	数值
颗粒堆积体积分数	0.56
最大碰撞动量再定向系数/%	40
切向颗粒-壁面碰撞恢复系数	0.99
法向颗粒-壁面碰撞恢复系数	0.3
回弹系数	0
颗粒应力模型的压力常数p_s/Pa	10
颗粒应力模型的无量纲常数β	3
计算颗粒代表的实际颗粒数	125
颗粒应力模型的无量纲常数 α	10^-7
湍流模型	LES

Fig.3 Variation of bed pressure drop with fluidization velocity (a) and UO2 conversion with different grid number (b)

Fig4 Schematic diagram of experimental apparatus for fluidized bed pressure measurement

Fig.5 (a) Three-dimensional model of a bubbling fluidized bed constructed based on laboratory apparatus; (b) particle size distribution of iron oxide particles

Fig.6 (a) Characteristics of bed pressure drop under different fluidization velocities; (b) comparison between experimental and simulation results for maximum bed pressure drop and predicted values of minimum fluidization velocity

Fig.7 Thermal U3O8 reduction reactor with different particle sizes: (a) bed pressure varying with fluidization velocity and flow pattern classification; (b) comparison of bed pressure drop under the fully fluidized state and minimum fluidization velocity

Fig.8 Particle distribution in thermal U3O8 reduction reactor under different fluidization velocities: (a) 50—100μm; (b) 100—150 μm; (c) 150—200 μm

Fig.9 Transient particles distribution with 80% excess hydrogen (fluidization velocity of 0.064 m/s, coloring based on particle axial velocity)

Fig.10 (a) Time-averaged particle distribution in steady flow state; (b) time-averaged distribution of particle concentration at different heights in reactor

Fig.11 Time evolution of pressure drop in beds with different particle sizes (a) and power as MSA (b)

Fig.12 Evolution of instantaneous distribution of fresh feeding particles with different sizes inside reactor (coloring based on particle residence time）

Fig.13 Time variation with mass flux (a) and mole fraction (b) of H2O at the gas outlet with different particle sizes

Fig14 H2O distribution in U3O8 reduction reactor: (a) 50—100 μm; (b) 100—150 μm; (c) 150—200 μm

Fig.15 Time variation with UO2 mass fraction of different particle sizes at solid outlet: (a) particle mixture; (b) initial packing particles; (c) fresh feeding particles

Fig.16 Gas temperature distribution in U3O8 reduction reactors with different particle sizes: (a) 50—100 μm; (b) 100—150 μm; (c) 150—200 μm

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Numerical simulation of hydrogen reduction of U₃O₈ in fluidized bed reactors using CPFD method

基于CPFD方法的U₃O₈氢还原流化床反应器数值模拟

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