基于CPFD方法的U3O8氢还原流化床反应器数值模拟

doi:10.11949/0438-1157.20240358

摘要/Abstract

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

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

中图分类号:

TQ 051.3

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.

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.

图/表 18

图1 U3O8还原反应的未反应缩核模型示意图

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

图2 （a） U3O8还原流化床反应器结构示意图；（b）三维模型网格划分剖面图

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

表1 U3O8还原流化床反应器主要运行工况参数

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

表1 U3O8还原流化床反应器主要运行工况参数

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

表2 模拟参数及初始边界条件

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

图3 不同网格数模拟的床层压降随流化风速的变化情况(a)和出口处UO2转化率随时间的变化情况(b)

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

图4 流化床压力测量实验装置示意图

Fig4 Schematic diagram of experimental apparatus for fluidized bed pressure measurement

图5 (a) 基于实验室装置建立的三维鼓泡流化床模型；(b) 氧化铁颗粒粒径分布

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

图6 （a）不同流化风速下的床层压降变化特性；（b）实验与数值模拟最大床层压降及最小流化速度预测值的对比

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

图7 不同颗粒粒径的热态U3O8还原反应器中床层压力随流化风速的变化曲线及流型划分（a）以及完全流化状态下的床层压降统计值和最小流化速度预测值（b）

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

图8 不同流化风速下热态U3O8还原反应器床层颗粒分布：（a） 50~100 μm；（b）100~150 μm；（c）150~200 μm

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

图9 氢气过剩80%(流化风速为0.064 m/s)情况下床层颗粒的瞬态分布（通过颗粒轴向速度着色）

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

图10 (a)稳定流动状态下床层颗粒浓度时均分布云图；(b)不同高度反应器中床层颗粒浓度时均分布曲线

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

图11 不同颗粒粒径床层压降随时间演变（a）和压降频谱图（b）

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

图12 不同粒径的新鲜入料颗粒在反应器内瞬时分布的演变过程（通过颗粒停留时间着色）

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

图13 不同颗粒粒径床层中反应器气体出口处H2O质量流率（a）和摩尔分数（b）随时间的变化

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

图14 U3O8还原反应器中产物H2O分布云图：（a） 50~100 μm；（b）100~150 μm；（c）150~200 μm

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

图15 不同颗粒粒径床层中固相出口处UO2组分质量分数随时间的变化：（a）混合颗粒；（b）初始堆料颗粒；（c）新鲜入料颗粒

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

图16 不同颗粒粒径U3O8还原反应器中气体温度分布云图：（a）50~100 μm；（b）100~150 μm；（c）150~200 μm

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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基于CPFD方法的U₃O₈氢还原流化床反应器数值模拟

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

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