Simulation of scale-up effect of particle residence time distribution characteristics in continuously operated dense-phase fluidized beds

doi:10.11949/0438-1157.20201004

Abstract

Abstract:

Scale-up of fluidized bed is a great challenge in chemical engineering. In order to explore the particle flow behavior and residence time distribution characteristics of the continuously operated polydisperse fluidized bed during the scale-up process, a large-scale parallel GPU-based coarse-grained CFD-DEM method coupled with a polydisperse and non-spherical particle drag model was used. Long-term particle residence time simulations were carried out in a continuously operated three-dimensional fluidized bed. Through the simulation of fluidized beds of different sizes (lengths), it is found that the mean residence time (MRT) of particles of different sizes has a linear relationship with the length of the fluidized bed. This relationship can be used to predict the particle MRT of larger fluidized beds. As the length of the fluidized bed increases, the difference in MRT of particles with different sizes becomes larger, indicating that the increase in the length of the fluidized bed has a certain ability to regulate the residence time of particles with different sizes.

Key words: gas-solid flow, polydisperse particles, residence time distribution, discrete simulation, scale-up

摘要：

采用基于GPU（graphics processing unit）大规模并行的粗粒化CFD-DEM（computational fluid dynamics-discrete element method）方法，耦合多分散、非球形颗粒曳力模型，对连续操作的三维流化床进行了长时间颗粒停留时间模拟。通过对不同尺寸（长度）流化床的模拟发现不同粒径颗粒平均停留时间（mean residence time，MRT）与流化床长度呈线性关系，该关系可以用来预测更大尺寸流化床内的颗粒停留时间。随着流化床长度的增加，不同粒径颗粒MRT的差异变大，说明流化床长度的增加对不同尺寸颗粒的停留时间具有一定的调控能力。

关键词: 气固流, 多分散颗粒, 停留时间分布, 离散模拟, 放大

CLC Number:

TQ 021.1

LAN Bin, XU Ji, LIU Zhicheng, WANG Junwu. Simulation of scale-up effect of particle residence time distribution characteristics in continuously operated dense-phase fluidized beds[J]. CIESC Journal, 2021, 72(1): 521-533.

兰斌, 徐骥, 刘志成, 王军武. 连续操作密相流化床颗粒停留时间分布特性模拟放大研究[J]. 化工学报, 2021, 72(1): 521-533.

Figures/Tables 12

Fig.1 Simulation schematic diagram of 3D continuously operated fluidized beds

Fig.2 Particle size distribution used in experiment

Table 1 Representative particle sizes and their mass fractions used in simulations

Diameter/μm

Mass fraction/%

251

418

640

38.3

53.2

8.5

Table 2 Simulation parameters for fluidized bed and properties of gas and particles

Parameter	Value
particle
density/(kg/m³)	3135
Sauter mean particle diameter/μm	341
minimum fluidization velocity/(m/s)	0.095
voidage at the minimum fluidization condition	0.48
sphericity	0.65
mass flow rate of solids at the solid inlet/(g/s)	5.3
Young’s modulus/Pa	1×10⁷
Poisson’s ratio	0.3
characteristic velocity/(m/s)	1
coefficient of restitution	0.95
coefficient of friction	0.3
coefficient of rolling friction/mm	0.05
coarse-graining ratio	5
time step/s	1×10^-5
gas
density/(kg/m³)	1.2
viscosity/(Pa·s)	1.8×10^-5
gas grid size/mm	5
superficial gas velocity/(m/s)	0.4
time step/s	1×10^-4
operating pressure/Pa	101325

Fig.3 Axial pressure distribution of fluidized beds with different lengths

Table 3 Total pressure drop of fluidized beds with different sizes

Bed	Average	Total pressure	Total pressure
length/m	holdups/kg	drop/kPa	drop (exp.)/kPa
0.07	1.03	2.303	—
0.15	2.16	2.231	2.390
0.31	4.71	2.373	—
0.63	9.61	2.382	—

Fig.4 Contour of solid volume fraction at t=1000 s in central vertical plane for fluidized beds with different scale

Fig.5 Radial distribution of solid volume fraction in fluidized beds with different scale

Fig.6 Radial distribution of solid velocity in fluidized beds with different scale

Fig.7 Particle RTD of fluidized beds with different scale

Table 4 Particle MRT of fluidized beds with different scale

Bed length/m	Particle diameter/μm	Particle recovery rate/%	MRT of each particle/s	Total MRT/s	Total MRT(exp.)/s	$σ θ 2$
0.07	251	99.6	167.2	198.1	—	0.64
0.07	418	99.2	191.1
0.07	640	97.7	221.0
0.15	251	99.7	365.9	405.4	387.7	0.76
0.15	418	99.2	405.7
0.15	640	99.2	422.7
0.31	251	99.5	732.6	847.2	—	0.77
0.31	418	98.1	882.8
0.31	640	96.7	998.7
0.63	251	99.5	1458.9	1708.5	—	0.80
0.63	418	97.8	1822.2
0.63	640	97.0	2061.6

Table 4 Particle MRT of fluidized beds with different scale

Bed length/m	Particle diameter/μm	Particle recovery rate/%	MRT of each particle/s	Total MRT/s	Total MRT(exp.)/s	$σ θ 2$
0.07	251	99.6	167.2	198.1	—	0.64
0.07	418	99.2	191.1
0.07	640	97.7	221.0
0.15	251	99.7	365.9	405.4	387.7	0.76
0.15	418	99.2	405.7
0.15	640	99.2	422.7
0.31	251	99.5	732.6	847.2	—	0.77
0.31	418	98.1	882.8
0.31	640	96.7	998.7
0.63	251	99.5	1458.9	1708.5	—	0.80
0.63	418	97.8	1822.2
0.63	640	97.0	2061.6

Fig.8 Relationship between MRT of particles with different sizes and bed length

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