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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Hydrodynamic Characteristics of a Pilot-Scale Dual Fluidized Bed with Continuous Feeding and Discharging of Solids: Experiment and 3D Simulation Zheng Zou,† Zhan Du,† Guoqiang Shao,† Qi Liu,†,‡ Zhaohui Xie,† Hongzhong Li,*,†,‡ and Qingshan Zhu†,‡
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†
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100190, PR China ‡ University of Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *
ABSTRACT: The hydrodynamic properties of a dual fluidized bed (DFB) with continuous feeding and discharging of solids were investigated on a pilot-scale plant in cold mode and simulated coupled with the structure-based drag model. We present a deep analysis that focuses directly on the problem of predicting the fluid dynamics behavior of this type of system for which empirical data is limited or unavailable. The fluidization of a DFB shows complex hydrodynamic characteristics because of the intricate interactions between different compartments experimentally. We prove that the simulation based on the structure-based drag model involving different fluidized structures is able to give an accurate prediction of pilot-scale DFB fluidization and capture the correct flow behaviors under different conditions. This work is expected to give thorough analysis and further exploration of the overall fluidization dynamics for DFB optimization and scale-up.
1. INTRODUCTION Since the maturity of fluidization technology, fluidized bed reactors have been widely applied in numerous industrial processes because of their ability to efficiently utilize low-grade resources, maintain their capability for flexible operation, and reduce production consumption.1 Furthermore, with increased complexity and the higher requirements of production technology, more and more complicated fluidized reaction systems, represented by the dual fluidized bed (DFB), have been improved and utilized in various large-scale energy chemical fields, such as in biomass gasification, fluid catalytic cracking (FCC), chemical looping combustion and reforming (CLC/CLR), mineral roasting, and other two-step reaction processes.2−9 Generally, the DFB system is made up of a low-velocity bubbling fluidized bed (BFB) and a high-velocity circulated fluidized bed (CFB), which are interconnected by the solids transport valve. The DFB has been considered an advanced and cost-effective multiphase reactor because it can carry out diverse types of reactions simultaneously and adjust the circulation rates of particles flexibly between the two fluidized beds. Until now, extensive experimental and theoretical research has been conducted to investigate the complex reactive flows of a DFB. Hofbauer et al. systematically studied the effects of process parameters like fluidization rate, materials © XXXX American Chemical Society
size, recirculation rate, and solids inventory on the DFB steam gasifier;2−4 Pröll et al. explored the fluid dynamics characteristics of the solids circulation rate, flow regime, bed inventory, and solids distribution between the two reactors for large-scale CLC applications;5−7 and Masten and Kunii reviewed the successive improvements and modifications of the DFB FCC technology of Exxon, UOP, Texaco, and other companies.8,9 Obviously, all the above studies merely focus on the type of DFB reactor whose terminal product is the outlet gas phase (shown in Figure 1a), whereas the loop particles just act as the catalyst, oxidant−reducer, heat carrier, or other active mediums circulated in the system constantly. On the other hand, the DFB system mainly used to process the solids phase (shown in Figure 1b) has been also widely applied in metal and nonmetal mineral roasting, direct reduction of iron (DRI), and other chemical−metallurgical processes, but surprisingly, few publications concerning the fluidization characteristics of this aspect have been available so far.10−14 It is recognized that distinguishable from a plant just containing a single fluidized bed, the interactions among the BFB, CFB, cyclone separator, Received: Revised: Accepted: Published: A
April 16, 2019 June 20, 2019 June 25, 2019 June 25, 2019 DOI: 10.1021/acs.iecr.9b02062 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Two types of DFB processes: (a) process with solids circulated in the system constantly and (b) process with continuous feeding and discharging of solids.
Figure 2. Illustration of DFB equipment with dimension labels.
reactive characteristics.22,23 Zhou et al. performed full loop simulation to study the gas−solid flow behaviors and interactions among the riser, BFB, and port-seal for the FCC application.24 Obviously, few computations have been carried out on a DFB with continuous feeding and discharging of solids as well, which hampers the exploration of hydrodynamics modeling for DFB design and scale-up. Meanwhile, it should be admitted that CFD is still at the verification and validation stage for modeling the multiphase flow system.25,26 As a crucial parameter for determining the accuracy of hydrodynamics predictions, more improvements regarding the interphase drag coefficient are required for the conventional models that simply assume the homogeneous fluidized conditions and generally overestimate the drag forces among different phases.27−29 Among the various modified drag models, the prominent structure-based drag correlation proposed by Li et al.30−34 takes into account of the influence of mesoscale structure on the momentum transfer within a fluidized bed; it has thus exhibited more accurate results with wide applications and has been considered a valid way to compute nonuniform flow by the other researchers.35−38 In order to explore the hydrodynamic properties and loopseal characteristics of the DFB with continuous feeding and discharging of solids, experiments were first carried out on a pilot-scale DFB in cold mode, and then 3D CFD computations coupled with the modified structure-based drag models were conducted and validated through comparisons to experimental
loop-seal, feed and outlet valves within the DFB are more complicated. Furthermore, longer start-up time, pressure drop fluctuations, unsteady solids circulation, emergency shutdown, and all the other risks of large-scale projects make comprehensive understanding and deep analysis of the hydrodynamic properties required for the rational design and optimal operation of a DFB with continuous feeding and discharging of solids. With the rapid development of computational ability, computational fluid dynamics (CFD) has substituted the empirical numerical model and become a valid and effective tool for characterizing the properties of fluidization. With regard to the two main simulation approaches, for high accuracy consumption of a huge computing resource, the application of the discrete element method (DEM) in largescale fluidized bed equipment is still challenging, yet more and more successful applications of the Eulerian approach have been reported because of its advantages of flexibility and efficiency; thus, this approach was adopted in this study as well.15−19 Numerous studies have been also carried out to simulate the fluidization of a DFB, similar to the status of the experimental investigation of a DFB with constantly circulating solids; Liu et al. conducted a series of numerical studies to explore the impacts of operating parameters on the solids circulation rate for the DFB gasification system.20,21 Lu et al. simulated the complete CLC/CLR system by using different drag coefficient models to predict gas−solid flow behaviors and B
DOI: 10.1021/acs.iecr.9b02062 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
controlling the solids transport rate and avoiding gas leakage within the standpipe, which is achieved by only injecting the aeration gas into the recycle chamber; therefore, a U-valve is adopted as the feed and outlet valves and loop-seal in the experimental apparatus.41−45 From the studies of hydrodynamics analyzed before, the functioning of the loop-seal is mainly dependent on the aeration rates, the pressure balance between the BFB and CFB, and other operational parameters.46−49 Considering the combined effects of gas velocity on the solids flow rate and gas leakage, the recycle chamber aeration velocity for the outlet valve of the CFB (ug,rc‑CFB,o) is set at 0.1 m/s (10umf); all the other aeration velocities (ug,rc or ug,l) for the feed and outlet valves of the BFB and the loop-seals of the BFB and CFB were 0.025 m/s (2.5umf).50 The detailed operating conditions for the experiments are summarized in Table 2.
data. This work is expected to give a thorough analysis and further exploration of the overall flow behavior for DFB design and optimization.
2. EXPERIMENTS 2.1. Experimental Setup. Experiments were carried out in cold-mode DFB equipment (6.8 m width, 2.5 m depth, 12 m height) that was assembled in combined Plexiglas and steel devices for removal of the static electricity generated in significant amounts in the large-scale apparatus. A laboratory photograph and schematic illustration of the DFB equipment are displayed in Figure S1 in the Supporting Information and in Figure 2, respectively. The whole system consists of two root blowers, which provide the fluidizing air for the DFB; valves and several loop-seals; the bucket elevator, which lifts the solids materials to the top feed bin; the conveyer, which adjusts the feed rate of solids to the DFB; the bag filter, which purifies the off gas; and the DFB equipment. For a better understanding of the system core part, the DFB is divided into five parts (BFB, CFB, BFB feed and outlet valves, and CFB outlet valve), with the dimensions and height labeled in the right part of Figure 2. It should be noted that the sizes of all valves and loop-seals are identical to each other. 2.2. Materials. The bed materials used in this study were silica sand particles (dp = 100 μm) which generally exhibited better fluidized properties. The minimum fluidization velocity (umf) was determined from the model of Wen and Yu,39 and the terminal velocity (ut) was determined from the model of Schiller and Naumann.40 Air was used as the fluidizing gas, the flow rate was regulated by simply adjusting the blower frequency and measured accurately with a flowmeter, covering the range of 0−1200 m3/h. The physical properties of the particles are given in Table 1.
Table 2. Operating Parameters of Each Experimental Run parameter gas flow rate of fluidizing air for DFB, Gg gas velocity of BFB, ug,BFB gas velocity of CFB, ug,CFB solids feed rate, Gs,BFB,f ug,rc for feed and outlet valves of BFB, ug,rc‑BFB,f/o ug,l for loop-seal of BFB and CFB, ug,l‑BFB and ug,l‑CFB ug,rc for outlet valve of CFB, ug,rc‑CFB,o
value
units
mean particle diameter, dp particle density, ρp gas density, ρg minimum fluidization velocity, umf terminal velocity, ut
100 2620 1.29 0.01 0.59
μm kg/m3 kg/m3 m/s m/s
Run 2
Run 3
unit
540
900
900
m3/h
0.3 7.5 0.516
0.5 12.5 0.516 0.025
0.5 12.5 0.258
m/s m/s kg/s m/s
0.025
m/s
0.1
m/s
2.4. Experimental Procedures. For the start-up procedure of the DFB system, all three feed and outlet valves, two loop-seals and corresponding standpipes were filled with silica sand first; and then air was introduced into the CFB gas inlet and released from the outlet of the BFB second cyclone; solids were then charged into the feed valve of the BFB with the aeration gas injected into its recycle chamber; the BFB outlet valve began to run when the solids overflowed from the BFB; and the CFB outlet valve started up, discharging the particles; the loop-seals of the BFB and CFB began to work as soon as the solids flowed into the corresponding bed. The DFB experiments were carried out to measure the pressure distribution of the whole fluidized system and the solids discharge rate from the outlet valve of the CFB to determine whether the system reached steady state.
Table 1. Physical Properties of Experimental Materials property
Run 1
2.3. Operating Conditions. Generally, metal or nonmetal mineral roasting, fossil fuel conversion, and other endothermic or exothermic fluidized processes with high reaction temperatures and huge heat exchanges are always accomplished on the basis of the massive gaseous reactant, which improves the fluidized quality of the large-scale reactor through simultaneously raising the gas velocity. Therefore, in order to obtain comprehensive hydrodynamic characteristics of a DFB reactor, the superficial gas velocities in the BFB (ug,BFB) and the riser of CFB (ug,CFB) were 0.3 and 0.5 m/s (30−50umf) and 7.5 and 12.5 m/s (13−21ut), respectively. Furthermore, the experiments were investigated under two solids feed rates (Gs,BFB,f = 0.258 and 0.516 kg/s) to explore the effect of the solids flow rate on the hydrodynamics of the DFB. As is known, steady operation of the solids transport valve is critical to the operational safety and running efficiency of a DFB system. Among the different types of nonmechanical solids transport valves (U-, L-, V-, and J-valves), the U-valve (with solids flows in the supply and recycle chambers being in moving and fluidized bed modes, respectively) is best for
3. CFD MODEL The CFD model was solved by using ANSYS Fluent (ANSYS Inc.). The two-fluid model with the simplified kinetic theory of granular flow (KTGF) for particulate phase stresses was used.51 The complete model equations are summarized in Table S2 in the Supporting Information. As is known, the drag force between the gas−solid phases dominates the flow behavior, and an accurate definition of drag force is crucial to the correct simulation of fluidized hydrodynamic properties. Furthermore, because most gas−solids systems exhibit significant local and overall heterogeneity, the drag coefficient (CD) should depend on the formation of a heterogeneous structure.52 Therefore, in this study, the homogeneous Gidaspow drag law is revised by the heterogeneous structure-based model, which takes into account of the influence of mesoscale structure on the momentum transfer between the gas and solid phases. As illustrated in Figure 3, the C
DOI: 10.1021/acs.iecr.9b02062 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
heterogeneous flow structure is formulated by uniformly using the following equation: βs =
FDεg (ug − u p)
=
ρg 3 (1 − εg)C̅ D |ug − u p|εg 3 4 dp
(1)
The equations for the averaged drag coefficients, CD, for the BFB (CD,BFB) and CFB (CD,CFB) are C̅ D,BFB = C De(1 − fb )
Figure 3. Separation of the heterogeneous flow structure by using the multiscale method.
2 2 (1 − εe) ijj Use yzz (1 − εe) ρp d p ijj Usb yzz jj zz + C Db fb jj zz (1 − εg) jk Us z{ (1 − εg) ρg db jk Us z{
(2)
where
BFB and CFB fluidized structures are separated into the emulsion and bubble phases and into the cluster phase, interphase, and dispersed phase, respectively, in the multiscale method. Then, the drag coefficient in consideration of
C De = 200
(1 − εe)μg εe 3ρg d pUse
+
7 and C Db = C Dbo(1 − fb )−0.5 3εe 3
and
Table 3. Detailed Drag Models for the Gas−Solid Phase System
l ρg εgεs|ug − u p| o o 3 o o εg −2.65 o C D0 o o dp 4 o o o o o o o o ij 3 yz ρg εgεs|ug − u p| o o jj εg −2.65zzzzHd,BFB βs_BFB = o m o jjj 4 C D0 z o dp o o { o ok o o 2 o o u u ε μ ε ρ | − o s g p| s o g g o o 150 + 1.75 o 2 o o d εgd p o p n l ρg εgεs|ug − u p| o o 3 o o εg −2.65 o o 4 C D0 o dp o o o o o o o o ji 3 ρg εgεs|ug − u p| o zy o jj εg −2.65zzzzHd,CFB jj C D0 βs_CFB = o m o z j o 4 d o p o { k o o o o 2 o o εs μg εsρg |ug − u p| o o o o 150 + 1.75 o 2 o o dp d ε o g p n l o i 24 y 0.687 o o ) (Re ≤ 1000) o jjj zzz(1 + 0.15Re Re C D0 = o { k m o o o o o 0.44 (Re > 1000) n Re =
device
(εg ≥ εg_u)
(εg_u > εg > εg_l)
(εg ≤ εg_l)
(εg ≥ εg_u)
(εg_u > εg > εg_l)
(εg ≤ εg_l)
εgρg ds|ug − u p| μp
ug (m/s)
εg_l (−)
εg_u (−)
0.3
0.45
0.75
Hd,BFB = 1.22 − 5.6εg + 0.10h + 6.68εg 2 − 0.36εgh + 0.04h2
0.5
0.45
0.78
Hd,BFB = 0.89 − 4.11εg + 0.12h + 4.91εg 2 − 0.42εgh + 0.04h2
Hda
BFB
Hd,CFB = 7402εg 6 − (3.369 × 104)εg5 + (6.355 × 104)εg 4 7.5
0.525
− (6.364 × 104)εg 3 + (3.568 × 104)εg 2 − (1.063 × 104)εg
0.995
+ 1315
CFB
Hd,CFB = 8901εg 6 − (4.077 × 104)εg 5 + (7.731 × 104)εg 4 12.5
0.510
0.995
− (7.764 × 104)εg 3 + (4.357 × 104)εg 2 − (1.295 × 104)εg + 1594
BFB feed or outlet valve
0.025
0.45
0.53
Hd,BFB,f/o = 5.38 − 27.47εg + 0.05h + 36.40εg 2 − 0.13εgh + 0.01h2
CFB outlet valve
0.1
0.45
0.66
Hd,CFB = 2.39 − 11.35εg + 0.10h + 14.0εg2 − 0.28εgh + 0.02h2
a
Parameter h is the relative elevation above the corresponding air distributor, not the absolute height in the system. D
DOI: 10.1021/acs.iecr.9b02062 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. (a) Variation of Hd,BFB with gas voidage and bed height and (b) variation of Hd,CFB with gas voidage at different gas velocities for the BFB and CFB. d p yzij U yz (1 − εc) ijj jj1 − 2 zzzjjj sc zzz + C Dd(1 − fc ) j (1 − εg) k dc z{jk Us z{ 2
C̅ D,CFB = C Dc fc
d p ij Usi yz fc (1 − εd) ijj Usd yzz jj zz jj zz + C Di j z j z (1 − εg) k Us { (1 − εg) dc jk Us z{ 2
where
C Dc/i/d
variation of Hd,BFB with gas voidage and bed height at different gas velocities for the BFB. It is shown that Hd,BFB is decreased as the voidage is reduced, which implies that our model is more applicable to revising the drag force for the dense phase with an obvious heterogeneous structure. In particular, in a region with a higher gas voidage, the Hd,BFB is reduced as the bed height or gas velocity increases; this is ascribed to the fact that the bubbles grow during the rising process and enlarge with more abundant gas, which leads to a more heterogeneous flow structure. Figure 4b plots the curves of Hd,CFB versus gas voidage under different gas velocities; similar to that in the BFB, the drag force in the CFB is also reduced by Hd,CFB (
