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Effects of particle type on the particle distribution in a two-pass circulating fluidized bed evaporator with baffle
Powder Technology 366 (2020) 1–11
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effects of particle type on the particle distribution in a two-pass circulating fluidized bed evaporator with baffle Na Li, Yuqing Zhang, Feng Jiang ⁎, Hongyu Wang, Xiulun Li School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2019 Received in revised form 2 December 2019 Accepted 13 February 2020 Available online 15 February 2020 Keywords: Particle types Particle distribution Circulating fluidized bed evaporator Two-pass Baffle Fouling prevention
a b s t r a c t To enhance heat transfer, as well as prevent and remove fouling, a two-pass circulating fluidized bed evaporator with baffle has been designed and built. The uniformity of particle distribution significantly influences the performance of evaporator. This paper mainly researches the influences of particle types on the particle distribution with a Charge Couple Device image measurement and acquisition system. Consequently, for four kinds of particles, particle distributions in the up-flow bed are relatively uniform; those in the down-flow bed become more uniform with the increase of circulation flow rate and decline of particle amount and baffle height. Particle maldistribution becomes obvious with the increase of particle density. Furthermore, particles with greater density are more susceptible to the operating parameters, and the increase of circulation flow rate can weaken the influences of particle amount and baffle height. Finally, phase diagrams are established to show the optimal range of the operating parameters. © 2020 Elsevier B.V. All rights reserved.
Nomenclature
1. Introduction
di ds hi H M n ns Q Res u us ρ ρs μ ε εs εsi εs
Particulate fouling on the heat exchanger surface is a major problem in heat transfer processes such as condensation of traditional Chinese medicine, wastewater disposal in the pharmaceutical manufacture, and evaporation of the salt solution. It was found that a powdery fouling layer of a few millimeters thick can lead to a reduction in heat transfer coefficient of 25% [1]. Fouling can usually be formed by suspended particles that stick to the wall or by crystalline materials that crystallize on the wall due to the local super-saturation. Eventually, the fluidized bed technology is considered to be a green and energy-saving alternative. Fluidized bed system has been widely used in industrial processes, such as food processing [2], paper making [3], water treatment [4], desalination [5], and boiler operation [6], among others [7–9], because of the heat transfer enhancement and fouling prevention at the heat transfer surfaces. Many studies on this system have conducted experimentations and simulations in the past few decades [10–17]. Based on the combination of flow boiling heat transfer with multiphase fluidization, the performance of the circulating fluidized bed is affected by several key factors, including the selection of particles, the fluidization and distribution of particles, the separation of liquid and solid phase, the pressure drop, and so on. Particle distribution in tube bundle has a significant effect on the performance of heat transfer and fouling prevention and removal in the heat exchanger. Qi et al. [18] adopted the Eulerian multiphase fluid model to simulate the particle distribution in a fluidized bed heat exchanger incorporating a tube bundle arranged in parallel and
tube internal diameter, mm solid particle diameter, mm tube height, mm baffle height, m non-uniform degree of distribution, [−] tube number, # particle number circulation flow rate, m3/h the Reynolds number of particle, [−] velocity of fluid, m/s sedimentation velocity of particle, m/s density of liquid, kg/m3 density of particle, kg/m3 viscosity of liquid, Pa ⋅ s particle amount, [−] solid holdup in the tube, [−] solid holdup in the #i tube, [−] average solid holdup, [−]
⁎ Corresponding author. E-mail address: [email protected] (F. Jiang).
https://doi.org/10.1016/j.powtec.2020.02.038 0032-5910/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the two-pass circulating fluidized bed evaporator with a baffle.
compared the simulation results with the experimental measurements. They concluded that particle distribution became uniform at a high velocity. Particle maldistribution occurred as the density or diameter of particles increased. Particle distribution became uniform as the volume fraction of the solid phase increased. Qi and Jiang [19] numerically and experimentally investigated the particle distribution in a liquid– solid fluidized bed heat exchanger with a horizontal tube bundle. They found that more particles were present in the lower tubes than in other areas of the horizontal tube bundle at a low velocity. Uniform particle distribution can be obtained at a high velocity. Wang et al. [20] studied the solid particle distribution in the heating tube bundle of a transparent multi-tube vapor–liquid–solid circulating fluidized bed evaporator by using the charge-coupled device (CCD) measuring system. They found that the heterogeneity of particle distribution decreased with increasing circulating flow rate, heat flux, and amount of particles added but increased with increasing settling velocity of particles. Zhang et al. [21] explored the particle distribution in a horizontal tube bundle after reforming the front header of a liquid–solid horizontal circulating fluidized bed evaporator by using two inlets and adding the baffle plate. They found that the angle of the baffle plate and particle diameter affected the particle distribution. Jiang et al. [22] experimentally investigated the effects of the operating parameters, such as circulation flow rate, amount of added solid particles, diameter and density of particles on the fluidization and distribution of solid particles in the horizontal tube bundle. They concluded that the non-uniform degree decreased with the increasing of the circulation flow rate and the amount of added particles, and the particles with smaller diameter and density were fluidized better and well distributed in the horizontal tube bundle. The vertical two-pass circulation evaporator with baffle has been widely used in some industrial applications, such as the sodium sulfide production and lithium industry, but has serious fouling problem. A lithium hydroxide evaporator with an annual output of 10,000 tons needs to be stopped and cleaned after continuous operation for 16–24 h. To solve the fouling problem, a two-pass circulating fluidized bed evaporator with baffle has been designed and built in the previous studies [23,24], and the particle distribution was investigated by using the Charge Coupled Device (CCD) image measurement and acquisition system. The particle used in the previous study is only polyoxymethylene,
while experiment shows that the particle type greatly influences the particle distribution. The main purpose of this paper is to study the influences of particle type on the fluidization and distribution of solid particles under a variety of operating parameters and provide the basis for particle selection. The study results may serve as a reference for the application of two-pass circulating fluidized bed evaporator with baffle to industries. 2. Experiments 2.1. Experimental setup and procedure The experimental setup is illustrated in Fig. 1. The setup consists of a heating chamber, an evaporation chamber, and a particle collector.
Fig. 2. Tube bundle distribution and target tubes.
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Table 1 Physical characteristics of the particle. Number
Particle
Size (mm)
Equivalent diameter (mm)
Density (kg·m−3)
Sedimentation velocitya (m·s−1)
1 2 3 4
Nylon 66 (PA66) (cylindrical) Resin (cylindrical) Polyoxymethylene (POM) (spherical) Thermal Conductive Nylon6 (PA6) (cylindrical)
Ф2.6 × 3 Ф2.5 × 3 Ф3.15 Ф2 × 3
3.12 3.04 3.15 2.62
1020 1270 1390 1902
0.089 0.153 0.207 0.305
a
The sedimentation velocity is estimated in water at 20 °C at atmospheric pressure.
The heating chamber contains a tube bundle and a shell cover. The tube bundle consists of 40 glass tubes that are arranged in a triangular fashion on both sides of the baffle, as shown in Fig. 2. The tube has a diameter of 25 mm × 2.5 mm and a height of 600 mm. The entire set of equipment, excluding the tube bundle and the evaporation chamber, is made of stainless steel. The evaporation chamber is a polymethyl methacrylate barrel with a diameter of 300 mm × 10 mm and a height of 600 mm. Baffles with various heights can be placed in the middle of the evaporation chamber. The particle collector is used to collect the existing particles in the system when the particles need to be changed. The centrifugal pump is utilized to forcibly circulate fluid, whose circulation flow rate can be adjusted through frequency conversion. The CCD image measurement and acquisition system, which consists of a CCD camera and the digital video recording software StreamPix-5-S-STD, is employed to acquire the fluidization and distribution of solid particles in the system. The experiment is initiated by adding a certain amount of working fluid and solid particles in the system. Then, the frequency of the circulation pump is adjusted to the desired circulation flow rate. The fluid with particles flows from the up-flow bed to the evaporation chamber, and then flows to the down-flow bed. Particle distribution images are acquired after the steady state is confirmed under each condition. In the experiment, the CCD camera is placed at three locations: upflow bed, down-flow bed and evaporation chamber. Because the tube bundle is arranged in a triangular fashion, as shown in Fig. 2, only the outermost layer can be photographed with the CCD camera. Given that the arrangement is symmetrical, the five tubes in the up-flow bed (named #1 to #5) and five tubes in the down-flow bed (named #11 to #15) are investigated in this research. The particle distribution in
Fig. 3. Typical CCD images of the effect of particle type on the particle distributions in the up-flow bed (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%).
the inner tube bundle is analyzed and inferred by the fluidization and distribution of the particles in the evaporation chamber. In the study, by comparing the distribution of particles in the inner and outer tubes, it is found that the distribution of particles in the outer tubes could largely reflect the distribution of particles in the inner tube bundle. The distribution images of the particles can be taken within several seconds at one position. In this way, hundreds of continuous pictures can be acquired. After taking the pictures at one place and at different circulation flow rates, the location of the camera can be changed, and the above processes are repeated. 2.2. Materials Tap water and inert solid particles are used as the liquid and solid phases. The properties of the particle are shown in Table 1. In order to investigate the effect of particle density, four kinds of particles with comparable equivalent diameter but large density difference were selected in this paper. Meanwhile, in consideration of the corrosion resistance, abrasion resistance and heat transfer enhancement performance and fouling preventing and removing performance of particles, the nylon 66 (PA66), resin, polyoxymethylene (POM), and thermal conductive nylon6 (PA6) particles with equivalent diameter of 2.62–3.15 mm are chosen to investigate the effects of the particle types. The Reynolds number for a particle in a fluid is defined as Res ¼
ds uρ μ
ð1Þ
where the ds is the equivalent diameter of particles, ρ is the density of liquid, μ is the viscosity of liquid. At low Reynolds number (10−4 b Res b 1), the sedimentation velocity of particle (us) can be calculated by Eq. (2)
Fig. 4. Particle distributions in the up-flow bed at different circulation flow rates.
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Fig. 5. Typical CCD images of the effect of particle type on the particle distributions in the down-flow bed (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%).
2.3. Experiment conditions and methods
2
us ¼
ds ðρs −ρÞg 18μ
ð2Þ
where ρs represent the density of particles. For 1 b Res b 103, the sedimentation velocity of particle (us) can be calculated by Eq. (3) us ¼ 0:154
!1=1:4 1:6 g•ds •ðρs −ρÞ ρ0:4 •μ 0:6
ð3Þ
At high Reynolds number (103 b Res b 2 × 105), the sedimentation velocity of particles (us)can be calculated by Eq. (4) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ds ðρs −ρÞg us ¼ 1:75 ρ
Fig. 7. Typical CCD images of the effect of particle type on the particle fluidizations and distributions in the evaporation chamber (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%): (a) PA6; (b) POM; (c) Resin; (d) PA66.
The adjustable operating parameters used in the experiments include the circulation flow rate, particle amount and baffle height. The target parameters are the uniformity of particle distributions in the up-flow bed and down-flow bed. The circulation flow rate is set at 10.66, 11.95, 14.38, 16.71, and 19.14 m3/h. If the circulation flow rate is further increased, more air bubbles are drawn into tubes. This phenomenon greatly affects the observation. The circulation flow rate fluctuates within a certain scope at a certain frequency, so it is computed as the average of the maximum and minimum values within 2 min. The particle amount is represented by the solid holdup ε, which denotes the ratio of the bulk volume of the solid particles to the total
ð4Þ
Fig. 6. Effects of particle type on the non-uniform degrees in the down-flow bed at different circulation flow rates.
Fig. 8. Typical CCD images of the effect of particle type on the particle fluidizations and distributions in the evaporation chamber (Q = 16.71 m3/h, H = 0.1 m, ε = 0.5%): (a) PA6; (b) POM; (c) Resin; (d) PA66.
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Fig. 9. Typical CCD images of the effect of particle type on the particle distributions in the down-flow bed (Q = 16.71 m3/h, H = 0.1 m, ε = 0.5%).
volume of the liquid working medium. ε is set to 0.5%, 1.0%, and 1.5% in the various experiments conducted under different parameters. The baffle height is set to 0, 0.073, 0.1 and 0.15 m. The model of the CCD camera is GT1920, which has a frame frequency of 40.7 fps and a resolution of 1936 × 1456. The image processing software StreamPix-5-S-STD is used to continuously record pictures. The solid holdup εs can be used to investigate the particle distribution in each tube. The entire tube can be photographed using GT1920. The particles do not overlap under the present experimental conditions, so the number of particles can be determined accurately from the 2D image. ns is the average particle number of the whole tube within 2 min in steady state. εs can be calculated using Eq. (5) as follows: 1 1 3 3 3 πds ns ds ns ds n s 6 % εs ¼ 6 100% ¼ 100% ¼ 1 2 1 3600 πdi h 202 600 4 4
5
Fig. 11. Typical CCD images of the effect of particle type on the particle distributions in the up-flow bed (Q = 16.71 m3/h, H = 0.073 m, ε = 0.5%).
(7). A large M indicates a highly uneven distribution.
M¼
εs ¼
" #0:5 1 n εsi −εs 2 ∑ n i¼1 εs n 1X ε si n i¼1
ð6Þ
ð7Þ
3. Results and discussions 3.1. Effects of particle type at different circulation flow rates
ð5Þ
The non-uniform degree M reflects the difference among particle distribution in different tubes. M can be calculated using Eqs. (6) and
3.1.1. Particle distribution in the up-flow bed Fig. 3 shows the CCD images of particle distributions in the up-flow bed. The five tubes in these images are tubes #5 to #1 from left to right. The four types of particles selected in this paper have comparable equivalent diameters but different densities, so they have different
Fig. 10. Effect of particle type on the solid holdup distributions in the down-flow bed at different circulation flow rates.
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Fig. 12. Particle distributions in the up-flow bed at different baffle heights.
sedimentation velocities, as shown in Table 1. It can be seen in Fig. 3, different types of particles are evenly distributed in the up-flow bed. Fig. 4 reflects the influences of particle type on the non-uniform degrees of particle distributions in the up-flow bed at different circulation flow rates. As shown in Fig. 4, the non-uniform degrees of particle distributions in the up-flow bed are low and fluctuate with the circulation flow rate for four kinds of particles, which is conducive to preventing, removing fouling and enhancing heat transfer. This is because, when the system is running, the fluidized solid particles together with the fluid leave the centrifugal pump and enter the shell cover in the up-flow bed side. In the shell cover, the particles are particulate fluidization status in the turbulent fluid, so that the particles can be evenly distributed in the shell cover. Therefore, when fluid carrying particles enters the up-flow bed from the shell cover, the solid holdup in each tube of the up-flow bed is relatively nearly and the particles are evenly distributed. That is to say, the uniform mixing and distribution of particles caused by the particulate fluidization in the shell cover is the fundamental reason for the uniform distribution of particles in the up-flow bed. 3.1.2. Particle distribution in the down-flow bed Fig. 5 is the CCD images of distributions of different types of particles in the down-flow bed. The five tubes in these images are tubes #11 to #15 from left to right. The difference in particle sedimentation velocity affects the fluidization and distribution of particles. As shown in Fig. 5, particles with high sedimentation velocity easily accumulate in tubes close to baffle and unevenly distribute in tube bundle of the downflow bed. Fig. 6 reflects the effect of particle type on the non-uniform degrees of particle distributions in the down-flow bed as the circulation flow rate increases. It can be seen in Fig. 6 that the non-uniform degree of PA6 is the highest, indicating that it is the most uneven distributed in the down-flow bed. Then followed by POM and Resin, and PA66 is the most evenly distributed in the down-flow bed. The non-uniform degrees significantly decline as the circulation flow rate increases regardless of the particle type. Furthermore, with the increase of circulation flow rate, the difference of non-uniform degrees of different types of particles in the down-flow bed decreases gradually, indicating that the increase of circulation flow rate can weaken the influences of particle type on the particle distributions in the down-flow bed. Fig. 7 shows the fluidizations and distributions of different types of particles in the evaporation chamber at low circulation flow rate.
Fig. 13. Typical CCD images of the effect of baffle height on the particle distributions in the down-flow bed (Q = 11.95 m3/h, ε = 1.5%): (a) H = 0.073 m (b) H = 0.15 m.
As shown in Fig. 7, from PA6 to PA66, with the decrease of particle sedimentation velocity, on the one hand, particles become easy to be fluidized, so the difference of solid holdups between the two sides of baffle in the evaporation chamber decreases. On the other hand, after crossing the baffle and reaching the down-flow bed, the decrease in sedimentation velocity of the particles increases the time of the particles moving in the evaporation chamber vertically and horizontally. The particles can move horizontally to a position far away from the baffle in the evaporation chamber, so they are evenly distributed in the evaporation chamber and the down-flow bed. As the circulation flow rate increases, the particles are much easier to be fluidized and have more kinetic energy to reach the down-flow bed across the baffle and horizontally move for longer distances in the evaporation chamber. The difference of solid holdups between the two sides
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Fig. 14. Effects of particle type on the non-uniform degrees in the down-flow bed at different baffle heights and circulation flow rates.
of baffle in the evaporation chamber decreases, as shown in Fig. 8. In this way, the influences of particle sedimentation velocity on particle distributions in the down-flow bed are weakened, and the distributions of different types of particles in the down-flow bed are more uniform, as shown in Fig. 9. Fig. 10 quantitatively describes the effect of particle type on solid holdup distributions in each tube of down-flow bed. When the circulation flow rate is low, the particles tend to be distributed in the tubes near baffle (like #11 and #12), especially for the particles with high sedimentation velocity. The difference of solid holdups is very obvious between the tubes near and far away from the baffle. PA6 has a high sedimentation velocity, which makes it difficult to cross the baffle and reach the down-flow bed at low circulation flow rate. Therefore, compared with other types of particles, PA6 has the lowest solid holdup in each tube of the down-flow bed. With the increase of circulation flow rate, it can be obviously found that the distributions of all types of particles in each tube become more uniform, and the average solid holdup in each tube also increases. The increase of circulation flow rate can reduce the difference of particle velocity and solid holdup in each tube.
Fig. 15. Particle distributions in the up-flow bed at different particle amounts.
It also can be found in Fig. 10 that the average solid holdup of PA66 in the down-flow bed is the largest under both low and high circulation flow rates. This is also because the density and sedimentation velocity of PA66 are small, the velocity and solid holdup of PA66 in each area of the evaporator are more nearly than other particles.
3.2. Effects of particle type at different baffle heights 3.2.1. Particle distribution in the up-flow bed Fig. 11 shows the CCD images of particle distributions in the tube bundle of up-flow bed. As shown in Fig. 11, particle distributions in the up-flow bed are uniform. Fig. 12 shows the influences of baffle height on the distributions of four kinds of particles in the up-flow bed. As shown in Fig. 12, the non-uniform degrees are low in the upflow bed and slightly fluctuate as the baffle height increases regardless of the particle type. Given that particle distributions in the up-flow bed depend on the particle mixture in the shell cover, baffle height has little influence on the particle mixture in the up-flow bed.
Fig. 16. Effects of particle type on the particle distributions in the down-flow bed at different particle amounts (H = 0.1 m, Q = 10.66 m3/h).
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Fig. 17. Typical CCD images of the effect of particle amount on the particle distributions in the down-flow bed (Q = 10.66 m3/h, H = 0.1 m): (a) ε = 0.5% (b) ε = 1.5%.
3.2.2. Particle distribution in the down-flow bed Fig. 13 describes the influences of baffle height on the distributions of four types of particles in the tube bundle of down-flow bed. As shown in Fig. 13, under difference baffle heights, particle distribution of PA6 is the most uniform and that of PA66 is the most uneven. With the increase of baffle height, particle distributions become uneven and differences of particle distributions for four types of particles become more obvious. Fig. 14 shows the influences of baffle height on the non-uniform degrees of four types of particle distributions in the down-flow bed. As shown in Fig. 12, the non-uniform degree of PA6 still is the highest, and then followed by POM and Resin, PA66 is the most evenly distributed in the down-flow bed. When the circulation flow rate is low, the non-uniform degrees of four kinds of particles increase along with the baffle height. Furthermore, for different kinds of particles, the increments of the nonuniform degrees are different. When the baffle height increases from 0 to 0.15 m, the non-uniform degrees of PA66, Resin, POM and PA6 are increased by 0.084, 0.30, 0.61, and 0.63 respectively. It is deduced that particles with higher sedimentation velocity are more susceptible to the baffle height at low circulation flow rate. When the circulation flow rate is high, the non-uniform degrees slightly increase along with the baffle height. It can be concluded that the increase of circulation flow rate weakens the influences of particle type and baffle height on the particle distributions in the downflow bed. As the baffle height increases, the flow resistance increases, so particles especially particles with higher sedimentation velocity are hard to climb over the baffle from left to right. When the particles leave the tube bundle of the up-flow bed and enter the evaporation chamber, due to the expanding of sectional area, the velocity and kinetic energy of particles decrease gradually, particles are difficult to climb over the baffle from the up-flow bed to the down-flow bed. At the same time, after climbing over the baffle, the particles tend to fall in place close to baffle in the evaporation chamber due to the low horizontal velocity and kinetic energy. Therefore, the distributions of particles in the tube bundle of the down-flow bed are more uneven. From PA66 to PA6, with the increase of particle sedimentation velocity, on the one hand, particles become harder to be fluidized, so the consumed kinetic energy increases to climb over the baffle from left to right. On the other hand, after crossing the baffle and reaching the down-flow bed, the increase in particle sedimentation velocity decreases the move time of particles in the evaporation chamber. The particles with high sedimentation velocity only can horizontally move a short distance in the evaporation chamber, so they are unevenly distributed in the evaporation chamber and the down-flow bed. The nonuniform degree of particle distribution increases along with the particle sedimentation velocity.
Fig. 18. Effects of particle amount on the solid holdup distributions in the down-flow bed at different particle amounts.
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Fig. 19. Effects of particle type on the particle distributions in the down-flow bed at different particle amounts (H = 0.1 m, Q = 16.71 m3/h).
The increase of circulation flow rate increases the kinetic energy of particles. So the particles become easier to be fluidized and horizontally move for longer distances in the evaporation chamber after climbing over the baffle. In this way, the influences of particle sedimentation velocity and baffle height on particle distributions in the down-flow bed are weakened, and the distributions of different types of particles in the down-flow bed are more uniform. 3.3. Effects of particle type at different particle amounts 3.3.1. Particle distribution in the up-flow bed Fig. 15 shows the influences of particle amount on the distributions of different kinds of particles in the up-flow bed. As shown in Fig. 15, particle distributions in the up-flow bed are uniform, and the nonuniform degrees slightly fluctuate as the particle amount increases regardless of the particle type. Given that particle distributions in the up-flow bed depends on the particle disturbance and mixture in the shell cover, particle amount has little influence on the particle disturbance and mixture, which is consistent with the conclusion above. 3.3.2. Particle distribution in the down-flow bed Fig. 16 reflects the influences of particle amount on the non-uniform degrees of particle distributions in the down-flow bed at low circulation flow rate. As shown in Fig. 16, when the circulation flow rate is low, the non-uniform degrees of four kinds of particles increase along with the particle amount. But the increments of the non-uniform degrees are different. When the particle amount increases from 0.5% to 1.5%, the nonuniform degrees of PA66, Resin, POM and PA6 are increased by 0.055, 0.052, 0.30 and 0.29 respectively. That is to say, particles with higher sedimentation velocity are more susceptible to the particle amount at low circulation flow rate. These phenomena can be explained with the CCD images and solid holdup distributions in the tube bundle of the down-flow bed at different particle amounts as shown in Figs. 17 and 18. When the circulation flow rate is low, as the particle amount increases, the solid holdups of PA66 and Resin increase in tubes close to baffle as well as in tubes far away from baffle, whereas those of POM and PA6 increase only in tubes close to baffle. These all attribute to the sedimentation velocity. The sedimentation velocities of PA66 and Resin are low, so the movement times of particles after climbing over the baffle are long, particles can go into each tube
Fig. 20. Typical CCD images of the effect of particle amount on the particle distributions in the down-flow bed (Q = 16.71 m3/h, H = 0.1 m): (a) ε = 0.5% (b) ε = 1.5%.
even though the circulation flow rate is low. While the sedimentation velocities of POM and PA6 are high, so the movement times are short, particles only can go into tubes close to baffle regardless of the particle amount. So, as the particle amount increases, the non-uniform degrees of PA66 and Resin slightly increase whereas those of PA6 and POM obviously increase when the circulation flow rate is low. When the circulation flow rate is high, as shown in Fig. 19, the nonuniform degrees of PA6 and POM slightly increase along with the particle amount, and those of PA66 and Resin fluctuate in a certain range as particle amount increases. When the particle amount increases from 0.5% to 1.5%, the non-uniform degrees of POM and PA6 are increased by 0.12 and 0.06. That is to say, the increase of circulation flow rate can weaken the influences of particle amount on particle distribution. This phenomenon can be explained with the CCD images of particle distributions in the tube bundle of the down-flow bed at different particle amounts as shown in Fig. 20. When the circulation flow rate is
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Fig. 21. Three-dimensional phase diagram of the influences of particle parameters and operating parameters on the non-uniform degrees in the down-flow bed.
high, as the particle amount increases, the solid holdups of POM and PA6 increase in tubes close to baffle as well as in tubes far away from baffle, whereas those of PA66 and Resin increase in each tube evenly. As is mentioned above, the increase of circulation flow rate increases the kinetic energy of particles to move horizontally further and distribute evenly in the evaporation chamber after climbing over the baffle. In this way, the influences of particle sedimentation velocity and particle amount on particle distributions in the down-flow bed are weakened, and particle distributions in the down-flow bed are more uniform. 3.4. Optimal range of the operating parameters On the basis of the above mentioned results, the sedimentation velocity of particles is the main factor affecting their fluidizations and distributions in the down-flow bed. Five phase diagrams about sedimentation velocity and operating parameters are established to provide the basis for particle selection. The particle distributions in the up-flow bed are uniform, whereas those in the down-flow bed depend on the operating parameters. According to the experimental research and industrial application, the particle distribution can meet the demand of fouling prevention and heat transfer enhancement when the nonuniform degree is no more than 0.4. The optimal ranges of the particle parameters and operating parameters under the present experimental conditions are shown in Fig. 21.
For four kinds of particles, particle distributions in the down-flow bed become more uniform with the increasing of circulation flow rate and decline of particle amount and baffle height. Great density of particle results in high sedimentation velocity and difficult to be fluidized, thus particle maldistribution becomes obvious with the increase of particle density. Also, particles with greater density are more susceptible to the circulation flow rate, particle amount and baffle height; the increase of circulation flow rate can weaken the influences of particle density, particle amount and baffle height on particle distribution. In general, particle sedimentation velocity is the main influence factor on particle fluidizations and distributions in the down-flow bed. Finally, phase diagrams are established to show the basis for particle selection and the optimal range of the operating parameters under the present experimental conditions.
Acknowledgment This work is supported by the open foundation of State Key Laboratory of Chemical Engineering (SKL-ChE-18B03), the Municipal Science and Technology Commission of Tianjin, China under the Contract No. 2009ZCKFGX01900, National Natural Science Foundation of China (No. 21676180, 21076143), and by the Key Technologies Research and Development Program of Tianjin (15ZCZDSF00160). References
4. Conclusions In this paper, influences of particle type on particle distribution are investigated systematically in a system of two-pass circulating fluidized bed. The main results are summarized as follows. The particle distributions of four kinds of particles in the up-flow bed are relatively uniform and are slightly affected by the circulation flow rate, particle amount and baffle height. This may because the particles are mixed evenly with the flow of the fluid in the shell cover before they go into tubes.
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N. Li et al. / Powder Technology 366 (2020) 1–11 [7] M.A.O. Arumemi-Ikhide, Boiling Enhancement Characteristics of an Antifouling three-Phase (Vapour–Liquid–Solid) Circulating Fluidised Bed Heat Exchanger, pH.D. Thesis University of Edinburgh, 2006. [8] D.G. Klaren, Self-cleaning heat exchangers: principle, industrial applications and operating installations, Industrial Heat Transfer Conference, Dubai, Saudi Arabia, Sept 24-26, 2000. [9] D.G. Klaren, E.F. De Boer, D.W. Sullivan, Cost savings of 'zero-fouling' crude oil preheaters, Hydrocarb. Process. 84 (2005) 84–89. [10] L.B. Zhang, X.L. Li, A study on boiling heat transfer in three-phase circulating fluidized bed, Chem. Eng. J. 78 (2) (2000) 217–223. [11] P. Pronk, C.A. Infante Ferreria, G.J. Witkamp, Prevention of fouling and scaling in stationary and circulating liquid–solid fluidized bed heat exchangers: particle impact measurements and analysis, Int. J. Heat Mass Transf. 52 (2009) 3857–3868. [12] B.-Å. Andersson, Effects of bed particle size on heat transfer in circulating fluidized bed boilers, Powder Technol. 87 (1996) 239–248. [13] M.Y. Liu, Y. Yang, X.L. Li, Concentration of gengnian'an extract with a vapor–liquid– solid evaporator, AICHE J. 51 (3) (2005) 759–765. [14] J.P. Wen, H. Zhou, X.L. Li, Performance of a new vapor–liquid–solid three-phase circulating fluidized bed evaporator, Chem. Eng. Process. 43 (2004) 49–56. [15] T. Zhao, K. Liu, H. Murata, et al., Experimental and numerical investigation of particle distribution behaviors in a rolling circulating fluidized bed, Powder Technol. 258 (2014) 38–48.
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[16] X.P. Tang, M.Y. Liu, F. Jiang, Studies on the hydrodynamic and heat transfer in a vapor-liquid-solid flow boiling system with a CCD measuring technique, Chem. Eng. Sci. 59 (4) (2004) 889–899. [17] H. Wang, M.Y. Liu, R.T. Lin, Visual investigations on radial solid holdup in vapor– liquid–solid fluidized bed evaporator with a CCD measuring system, Chem. Eng. Sci. 61 (2) (2006) 802–813. [18] G.P. Qi, F. Jiang, Parametric study of particle distribution in tube bundle heat exchanger, Powder Technol. 271 (2015) 210–220. [19] G.P. Qi, F. Jiang, Numerical investigation on prevention of fouling in the horizontal tube heat exchanger: particle distribution and pressure drop, Desalination 367 (2015) 112–125. [20] B.B. Wang, F. Jiang, G.P. Qi, et al., Solid particle distribution in vapor–liquid–solid multi-pipe circulating fluidized bed evaporator, J. Tianjin Univ. (Sci. Technol.) 46 (2) (2013) 133–137(in Chinese). [21] S.F. Zhang, G.D. Gong, Y. Liu, Experimental study of particles distribution in liquid–solid horizontal circulating fluidized bed, Chem. Eng. 36 (4) (2008) 24–2640. (in Chinese). [22] F. Jiang, Y.W. Bian, G.P. Qi, et al., Study on the particle distribution of a horizontal multi-tube circulating fluidized bed, Powder Technol. 295 (2016) 272–283. [23] N. Li, F. Jiang, X.Y. Han, et al., Study on the particle distribution of a two-pass circulating fluidized bed evaporator with baffle, Powder Technol. 295 (2016) 47–58. [24] N. Li, Y.Q. Zhang, F. Jiang, et al., Effects of particle size on the particle distribution of a two-pass circulating fluidized bed evaporator with a baffle, Powder Technol. 311 (2017) 456–464.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effects of particle type on the particle distribution in a two-pass circulating fluidized bed evaporator with baffle Na Li, Yuqing Zhang, Feng Jiang ⁎, Hongyu Wang, Xiulun Li School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2019 Received in revised form 2 December 2019 Accepted 13 February 2020 Available online 15 February 2020 Keywords: Particle types Particle distribution Circulating fluidized bed evaporator Two-pass Baffle Fouling prevention
a b s t r a c t To enhance heat transfer, as well as prevent and remove fouling, a two-pass circulating fluidized bed evaporator with baffle has been designed and built. The uniformity of particle distribution significantly influences the performance of evaporator. This paper mainly researches the influences of particle types on the particle distribution with a Charge Couple Device image measurement and acquisition system. Consequently, for four kinds of particles, particle distributions in the up-flow bed are relatively uniform; those in the down-flow bed become more uniform with the increase of circulation flow rate and decline of particle amount and baffle height. Particle maldistribution becomes obvious with the increase of particle density. Furthermore, particles with greater density are more susceptible to the operating parameters, and the increase of circulation flow rate can weaken the influences of particle amount and baffle height. Finally, phase diagrams are established to show the optimal range of the operating parameters. © 2020 Elsevier B.V. All rights reserved.
Nomenclature
1. Introduction
di ds hi H M n ns Q Res u us ρ ρs μ ε εs εsi εs
Particulate fouling on the heat exchanger surface is a major problem in heat transfer processes such as condensation of traditional Chinese medicine, wastewater disposal in the pharmaceutical manufacture, and evaporation of the salt solution. It was found that a powdery fouling layer of a few millimeters thick can lead to a reduction in heat transfer coefficient of 25% [1]. Fouling can usually be formed by suspended particles that stick to the wall or by crystalline materials that crystallize on the wall due to the local super-saturation. Eventually, the fluidized bed technology is considered to be a green and energy-saving alternative. Fluidized bed system has been widely used in industrial processes, such as food processing [2], paper making [3], water treatment [4], desalination [5], and boiler operation [6], among others [7–9], because of the heat transfer enhancement and fouling prevention at the heat transfer surfaces. Many studies on this system have conducted experimentations and simulations in the past few decades [10–17]. Based on the combination of flow boiling heat transfer with multiphase fluidization, the performance of the circulating fluidized bed is affected by several key factors, including the selection of particles, the fluidization and distribution of particles, the separation of liquid and solid phase, the pressure drop, and so on. Particle distribution in tube bundle has a significant effect on the performance of heat transfer and fouling prevention and removal in the heat exchanger. Qi et al. [18] adopted the Eulerian multiphase fluid model to simulate the particle distribution in a fluidized bed heat exchanger incorporating a tube bundle arranged in parallel and
tube internal diameter, mm solid particle diameter, mm tube height, mm baffle height, m non-uniform degree of distribution, [−] tube number, # particle number circulation flow rate, m3/h the Reynolds number of particle, [−] velocity of fluid, m/s sedimentation velocity of particle, m/s density of liquid, kg/m3 density of particle, kg/m3 viscosity of liquid, Pa ⋅ s particle amount, [−] solid holdup in the tube, [−] solid holdup in the #i tube, [−] average solid holdup, [−]
⁎ Corresponding author. E-mail address: [email protected] (F. Jiang).
https://doi.org/10.1016/j.powtec.2020.02.038 0032-5910/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the two-pass circulating fluidized bed evaporator with a baffle.
compared the simulation results with the experimental measurements. They concluded that particle distribution became uniform at a high velocity. Particle maldistribution occurred as the density or diameter of particles increased. Particle distribution became uniform as the volume fraction of the solid phase increased. Qi and Jiang [19] numerically and experimentally investigated the particle distribution in a liquid– solid fluidized bed heat exchanger with a horizontal tube bundle. They found that more particles were present in the lower tubes than in other areas of the horizontal tube bundle at a low velocity. Uniform particle distribution can be obtained at a high velocity. Wang et al. [20] studied the solid particle distribution in the heating tube bundle of a transparent multi-tube vapor–liquid–solid circulating fluidized bed evaporator by using the charge-coupled device (CCD) measuring system. They found that the heterogeneity of particle distribution decreased with increasing circulating flow rate, heat flux, and amount of particles added but increased with increasing settling velocity of particles. Zhang et al. [21] explored the particle distribution in a horizontal tube bundle after reforming the front header of a liquid–solid horizontal circulating fluidized bed evaporator by using two inlets and adding the baffle plate. They found that the angle of the baffle plate and particle diameter affected the particle distribution. Jiang et al. [22] experimentally investigated the effects of the operating parameters, such as circulation flow rate, amount of added solid particles, diameter and density of particles on the fluidization and distribution of solid particles in the horizontal tube bundle. They concluded that the non-uniform degree decreased with the increasing of the circulation flow rate and the amount of added particles, and the particles with smaller diameter and density were fluidized better and well distributed in the horizontal tube bundle. The vertical two-pass circulation evaporator with baffle has been widely used in some industrial applications, such as the sodium sulfide production and lithium industry, but has serious fouling problem. A lithium hydroxide evaporator with an annual output of 10,000 tons needs to be stopped and cleaned after continuous operation for 16–24 h. To solve the fouling problem, a two-pass circulating fluidized bed evaporator with baffle has been designed and built in the previous studies [23,24], and the particle distribution was investigated by using the Charge Coupled Device (CCD) image measurement and acquisition system. The particle used in the previous study is only polyoxymethylene,
while experiment shows that the particle type greatly influences the particle distribution. The main purpose of this paper is to study the influences of particle type on the fluidization and distribution of solid particles under a variety of operating parameters and provide the basis for particle selection. The study results may serve as a reference for the application of two-pass circulating fluidized bed evaporator with baffle to industries. 2. Experiments 2.1. Experimental setup and procedure The experimental setup is illustrated in Fig. 1. The setup consists of a heating chamber, an evaporation chamber, and a particle collector.
Fig. 2. Tube bundle distribution and target tubes.
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Table 1 Physical characteristics of the particle. Number
Particle
Size (mm)
Equivalent diameter (mm)
Density (kg·m−3)
Sedimentation velocitya (m·s−1)
1 2 3 4
Nylon 66 (PA66) (cylindrical) Resin (cylindrical) Polyoxymethylene (POM) (spherical) Thermal Conductive Nylon6 (PA6) (cylindrical)
Ф2.6 × 3 Ф2.5 × 3 Ф3.15 Ф2 × 3
3.12 3.04 3.15 2.62
1020 1270 1390 1902
0.089 0.153 0.207 0.305
a
The sedimentation velocity is estimated in water at 20 °C at atmospheric pressure.
The heating chamber contains a tube bundle and a shell cover. The tube bundle consists of 40 glass tubes that are arranged in a triangular fashion on both sides of the baffle, as shown in Fig. 2. The tube has a diameter of 25 mm × 2.5 mm and a height of 600 mm. The entire set of equipment, excluding the tube bundle and the evaporation chamber, is made of stainless steel. The evaporation chamber is a polymethyl methacrylate barrel with a diameter of 300 mm × 10 mm and a height of 600 mm. Baffles with various heights can be placed in the middle of the evaporation chamber. The particle collector is used to collect the existing particles in the system when the particles need to be changed. The centrifugal pump is utilized to forcibly circulate fluid, whose circulation flow rate can be adjusted through frequency conversion. The CCD image measurement and acquisition system, which consists of a CCD camera and the digital video recording software StreamPix-5-S-STD, is employed to acquire the fluidization and distribution of solid particles in the system. The experiment is initiated by adding a certain amount of working fluid and solid particles in the system. Then, the frequency of the circulation pump is adjusted to the desired circulation flow rate. The fluid with particles flows from the up-flow bed to the evaporation chamber, and then flows to the down-flow bed. Particle distribution images are acquired after the steady state is confirmed under each condition. In the experiment, the CCD camera is placed at three locations: upflow bed, down-flow bed and evaporation chamber. Because the tube bundle is arranged in a triangular fashion, as shown in Fig. 2, only the outermost layer can be photographed with the CCD camera. Given that the arrangement is symmetrical, the five tubes in the up-flow bed (named #1 to #5) and five tubes in the down-flow bed (named #11 to #15) are investigated in this research. The particle distribution in
Fig. 3. Typical CCD images of the effect of particle type on the particle distributions in the up-flow bed (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%).
the inner tube bundle is analyzed and inferred by the fluidization and distribution of the particles in the evaporation chamber. In the study, by comparing the distribution of particles in the inner and outer tubes, it is found that the distribution of particles in the outer tubes could largely reflect the distribution of particles in the inner tube bundle. The distribution images of the particles can be taken within several seconds at one position. In this way, hundreds of continuous pictures can be acquired. After taking the pictures at one place and at different circulation flow rates, the location of the camera can be changed, and the above processes are repeated. 2.2. Materials Tap water and inert solid particles are used as the liquid and solid phases. The properties of the particle are shown in Table 1. In order to investigate the effect of particle density, four kinds of particles with comparable equivalent diameter but large density difference were selected in this paper. Meanwhile, in consideration of the corrosion resistance, abrasion resistance and heat transfer enhancement performance and fouling preventing and removing performance of particles, the nylon 66 (PA66), resin, polyoxymethylene (POM), and thermal conductive nylon6 (PA6) particles with equivalent diameter of 2.62–3.15 mm are chosen to investigate the effects of the particle types. The Reynolds number for a particle in a fluid is defined as Res ¼
ds uρ μ
ð1Þ
where the ds is the equivalent diameter of particles, ρ is the density of liquid, μ is the viscosity of liquid. At low Reynolds number (10−4 b Res b 1), the sedimentation velocity of particle (us) can be calculated by Eq. (2)
Fig. 4. Particle distributions in the up-flow bed at different circulation flow rates.
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Fig. 5. Typical CCD images of the effect of particle type on the particle distributions in the down-flow bed (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%).
2.3. Experiment conditions and methods
2
us ¼
ds ðρs −ρÞg 18μ
ð2Þ
where ρs represent the density of particles. For 1 b Res b 103, the sedimentation velocity of particle (us) can be calculated by Eq. (3) us ¼ 0:154
!1=1:4 1:6 g•ds •ðρs −ρÞ ρ0:4 •μ 0:6
ð3Þ
At high Reynolds number (103 b Res b 2 × 105), the sedimentation velocity of particles (us)can be calculated by Eq. (4) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ds ðρs −ρÞg us ¼ 1:75 ρ
Fig. 7. Typical CCD images of the effect of particle type on the particle fluidizations and distributions in the evaporation chamber (Q = 10.66 m3/h, H = 0.1 m, ε = 0.5%): (a) PA6; (b) POM; (c) Resin; (d) PA66.
The adjustable operating parameters used in the experiments include the circulation flow rate, particle amount and baffle height. The target parameters are the uniformity of particle distributions in the up-flow bed and down-flow bed. The circulation flow rate is set at 10.66, 11.95, 14.38, 16.71, and 19.14 m3/h. If the circulation flow rate is further increased, more air bubbles are drawn into tubes. This phenomenon greatly affects the observation. The circulation flow rate fluctuates within a certain scope at a certain frequency, so it is computed as the average of the maximum and minimum values within 2 min. The particle amount is represented by the solid holdup ε, which denotes the ratio of the bulk volume of the solid particles to the total
ð4Þ
Fig. 6. Effects of particle type on the non-uniform degrees in the down-flow bed at different circulation flow rates.
Fig. 8. Typical CCD images of the effect of particle type on the particle fluidizations and distributions in the evaporation chamber (Q = 16.71 m3/h, H = 0.1 m, ε = 0.5%): (a) PA6; (b) POM; (c) Resin; (d) PA66.
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Fig. 9. Typical CCD images of the effect of particle type on the particle distributions in the down-flow bed (Q = 16.71 m3/h, H = 0.1 m, ε = 0.5%).
volume of the liquid working medium. ε is set to 0.5%, 1.0%, and 1.5% in the various experiments conducted under different parameters. The baffle height is set to 0, 0.073, 0.1 and 0.15 m. The model of the CCD camera is GT1920, which has a frame frequency of 40.7 fps and a resolution of 1936 × 1456. The image processing software StreamPix-5-S-STD is used to continuously record pictures. The solid holdup εs can be used to investigate the particle distribution in each tube. The entire tube can be photographed using GT1920. The particles do not overlap under the present experimental conditions, so the number of particles can be determined accurately from the 2D image. ns is the average particle number of the whole tube within 2 min in steady state. εs can be calculated using Eq. (5) as follows: 1 1 3 3 3 πds ns ds ns ds n s 6 % εs ¼ 6 100% ¼ 100% ¼ 1 2 1 3600 πdi h 202 600 4 4
5
Fig. 11. Typical CCD images of the effect of particle type on the particle distributions in the up-flow bed (Q = 16.71 m3/h, H = 0.073 m, ε = 0.5%).
(7). A large M indicates a highly uneven distribution.
M¼
εs ¼
" #0:5 1 n εsi −εs 2 ∑ n i¼1 εs n 1X ε si n i¼1
ð6Þ
ð7Þ
3. Results and discussions 3.1. Effects of particle type at different circulation flow rates
ð5Þ
The non-uniform degree M reflects the difference among particle distribution in different tubes. M can be calculated using Eqs. (6) and
3.1.1. Particle distribution in the up-flow bed Fig. 3 shows the CCD images of particle distributions in the up-flow bed. The five tubes in these images are tubes #5 to #1 from left to right. The four types of particles selected in this paper have comparable equivalent diameters but different densities, so they have different
Fig. 10. Effect of particle type on the solid holdup distributions in the down-flow bed at different circulation flow rates.
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Fig. 12. Particle distributions in the up-flow bed at different baffle heights.
sedimentation velocities, as shown in Table 1. It can be seen in Fig. 3, different types of particles are evenly distributed in the up-flow bed. Fig. 4 reflects the influences of particle type on the non-uniform degrees of particle distributions in the up-flow bed at different circulation flow rates. As shown in Fig. 4, the non-uniform degrees of particle distributions in the up-flow bed are low and fluctuate with the circulation flow rate for four kinds of particles, which is conducive to preventing, removing fouling and enhancing heat transfer. This is because, when the system is running, the fluidized solid particles together with the fluid leave the centrifugal pump and enter the shell cover in the up-flow bed side. In the shell cover, the particles are particulate fluidization status in the turbulent fluid, so that the particles can be evenly distributed in the shell cover. Therefore, when fluid carrying particles enters the up-flow bed from the shell cover, the solid holdup in each tube of the up-flow bed is relatively nearly and the particles are evenly distributed. That is to say, the uniform mixing and distribution of particles caused by the particulate fluidization in the shell cover is the fundamental reason for the uniform distribution of particles in the up-flow bed. 3.1.2. Particle distribution in the down-flow bed Fig. 5 is the CCD images of distributions of different types of particles in the down-flow bed. The five tubes in these images are tubes #11 to #15 from left to right. The difference in particle sedimentation velocity affects the fluidization and distribution of particles. As shown in Fig. 5, particles with high sedimentation velocity easily accumulate in tubes close to baffle and unevenly distribute in tube bundle of the downflow bed. Fig. 6 reflects the effect of particle type on the non-uniform degrees of particle distributions in the down-flow bed as the circulation flow rate increases. It can be seen in Fig. 6 that the non-uniform degree of PA6 is the highest, indicating that it is the most uneven distributed in the down-flow bed. Then followed by POM and Resin, and PA66 is the most evenly distributed in the down-flow bed. The non-uniform degrees significantly decline as the circulation flow rate increases regardless of the particle type. Furthermore, with the increase of circulation flow rate, the difference of non-uniform degrees of different types of particles in the down-flow bed decreases gradually, indicating that the increase of circulation flow rate can weaken the influences of particle type on the particle distributions in the down-flow bed. Fig. 7 shows the fluidizations and distributions of different types of particles in the evaporation chamber at low circulation flow rate.
Fig. 13. Typical CCD images of the effect of baffle height on the particle distributions in the down-flow bed (Q = 11.95 m3/h, ε = 1.5%): (a) H = 0.073 m (b) H = 0.15 m.
As shown in Fig. 7, from PA6 to PA66, with the decrease of particle sedimentation velocity, on the one hand, particles become easy to be fluidized, so the difference of solid holdups between the two sides of baffle in the evaporation chamber decreases. On the other hand, after crossing the baffle and reaching the down-flow bed, the decrease in sedimentation velocity of the particles increases the time of the particles moving in the evaporation chamber vertically and horizontally. The particles can move horizontally to a position far away from the baffle in the evaporation chamber, so they are evenly distributed in the evaporation chamber and the down-flow bed. As the circulation flow rate increases, the particles are much easier to be fluidized and have more kinetic energy to reach the down-flow bed across the baffle and horizontally move for longer distances in the evaporation chamber. The difference of solid holdups between the two sides
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Fig. 14. Effects of particle type on the non-uniform degrees in the down-flow bed at different baffle heights and circulation flow rates.
of baffle in the evaporation chamber decreases, as shown in Fig. 8. In this way, the influences of particle sedimentation velocity on particle distributions in the down-flow bed are weakened, and the distributions of different types of particles in the down-flow bed are more uniform, as shown in Fig. 9. Fig. 10 quantitatively describes the effect of particle type on solid holdup distributions in each tube of down-flow bed. When the circulation flow rate is low, the particles tend to be distributed in the tubes near baffle (like #11 and #12), especially for the particles with high sedimentation velocity. The difference of solid holdups is very obvious between the tubes near and far away from the baffle. PA6 has a high sedimentation velocity, which makes it difficult to cross the baffle and reach the down-flow bed at low circulation flow rate. Therefore, compared with other types of particles, PA6 has the lowest solid holdup in each tube of the down-flow bed. With the increase of circulation flow rate, it can be obviously found that the distributions of all types of particles in each tube become more uniform, and the average solid holdup in each tube also increases. The increase of circulation flow rate can reduce the difference of particle velocity and solid holdup in each tube.
Fig. 15. Particle distributions in the up-flow bed at different particle amounts.
It also can be found in Fig. 10 that the average solid holdup of PA66 in the down-flow bed is the largest under both low and high circulation flow rates. This is also because the density and sedimentation velocity of PA66 are small, the velocity and solid holdup of PA66 in each area of the evaporator are more nearly than other particles.
3.2. Effects of particle type at different baffle heights 3.2.1. Particle distribution in the up-flow bed Fig. 11 shows the CCD images of particle distributions in the tube bundle of up-flow bed. As shown in Fig. 11, particle distributions in the up-flow bed are uniform. Fig. 12 shows the influences of baffle height on the distributions of four kinds of particles in the up-flow bed. As shown in Fig. 12, the non-uniform degrees are low in the upflow bed and slightly fluctuate as the baffle height increases regardless of the particle type. Given that particle distributions in the up-flow bed depend on the particle mixture in the shell cover, baffle height has little influence on the particle mixture in the up-flow bed.
Fig. 16. Effects of particle type on the particle distributions in the down-flow bed at different particle amounts (H = 0.1 m, Q = 10.66 m3/h).
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Fig. 17. Typical CCD images of the effect of particle amount on the particle distributions in the down-flow bed (Q = 10.66 m3/h, H = 0.1 m): (a) ε = 0.5% (b) ε = 1.5%.
3.2.2. Particle distribution in the down-flow bed Fig. 13 describes the influences of baffle height on the distributions of four types of particles in the tube bundle of down-flow bed. As shown in Fig. 13, under difference baffle heights, particle distribution of PA6 is the most uniform and that of PA66 is the most uneven. With the increase of baffle height, particle distributions become uneven and differences of particle distributions for four types of particles become more obvious. Fig. 14 shows the influences of baffle height on the non-uniform degrees of four types of particle distributions in the down-flow bed. As shown in Fig. 12, the non-uniform degree of PA6 still is the highest, and then followed by POM and Resin, PA66 is the most evenly distributed in the down-flow bed. When the circulation flow rate is low, the non-uniform degrees of four kinds of particles increase along with the baffle height. Furthermore, for different kinds of particles, the increments of the nonuniform degrees are different. When the baffle height increases from 0 to 0.15 m, the non-uniform degrees of PA66, Resin, POM and PA6 are increased by 0.084, 0.30, 0.61, and 0.63 respectively. It is deduced that particles with higher sedimentation velocity are more susceptible to the baffle height at low circulation flow rate. When the circulation flow rate is high, the non-uniform degrees slightly increase along with the baffle height. It can be concluded that the increase of circulation flow rate weakens the influences of particle type and baffle height on the particle distributions in the downflow bed. As the baffle height increases, the flow resistance increases, so particles especially particles with higher sedimentation velocity are hard to climb over the baffle from left to right. When the particles leave the tube bundle of the up-flow bed and enter the evaporation chamber, due to the expanding of sectional area, the velocity and kinetic energy of particles decrease gradually, particles are difficult to climb over the baffle from the up-flow bed to the down-flow bed. At the same time, after climbing over the baffle, the particles tend to fall in place close to baffle in the evaporation chamber due to the low horizontal velocity and kinetic energy. Therefore, the distributions of particles in the tube bundle of the down-flow bed are more uneven. From PA66 to PA6, with the increase of particle sedimentation velocity, on the one hand, particles become harder to be fluidized, so the consumed kinetic energy increases to climb over the baffle from left to right. On the other hand, after crossing the baffle and reaching the down-flow bed, the increase in particle sedimentation velocity decreases the move time of particles in the evaporation chamber. The particles with high sedimentation velocity only can horizontally move a short distance in the evaporation chamber, so they are unevenly distributed in the evaporation chamber and the down-flow bed. The nonuniform degree of particle distribution increases along with the particle sedimentation velocity.
Fig. 18. Effects of particle amount on the solid holdup distributions in the down-flow bed at different particle amounts.
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Fig. 19. Effects of particle type on the particle distributions in the down-flow bed at different particle amounts (H = 0.1 m, Q = 16.71 m3/h).
The increase of circulation flow rate increases the kinetic energy of particles. So the particles become easier to be fluidized and horizontally move for longer distances in the evaporation chamber after climbing over the baffle. In this way, the influences of particle sedimentation velocity and baffle height on particle distributions in the down-flow bed are weakened, and the distributions of different types of particles in the down-flow bed are more uniform. 3.3. Effects of particle type at different particle amounts 3.3.1. Particle distribution in the up-flow bed Fig. 15 shows the influences of particle amount on the distributions of different kinds of particles in the up-flow bed. As shown in Fig. 15, particle distributions in the up-flow bed are uniform, and the nonuniform degrees slightly fluctuate as the particle amount increases regardless of the particle type. Given that particle distributions in the up-flow bed depends on the particle disturbance and mixture in the shell cover, particle amount has little influence on the particle disturbance and mixture, which is consistent with the conclusion above. 3.3.2. Particle distribution in the down-flow bed Fig. 16 reflects the influences of particle amount on the non-uniform degrees of particle distributions in the down-flow bed at low circulation flow rate. As shown in Fig. 16, when the circulation flow rate is low, the non-uniform degrees of four kinds of particles increase along with the particle amount. But the increments of the non-uniform degrees are different. When the particle amount increases from 0.5% to 1.5%, the nonuniform degrees of PA66, Resin, POM and PA6 are increased by 0.055, 0.052, 0.30 and 0.29 respectively. That is to say, particles with higher sedimentation velocity are more susceptible to the particle amount at low circulation flow rate. These phenomena can be explained with the CCD images and solid holdup distributions in the tube bundle of the down-flow bed at different particle amounts as shown in Figs. 17 and 18. When the circulation flow rate is low, as the particle amount increases, the solid holdups of PA66 and Resin increase in tubes close to baffle as well as in tubes far away from baffle, whereas those of POM and PA6 increase only in tubes close to baffle. These all attribute to the sedimentation velocity. The sedimentation velocities of PA66 and Resin are low, so the movement times of particles after climbing over the baffle are long, particles can go into each tube
Fig. 20. Typical CCD images of the effect of particle amount on the particle distributions in the down-flow bed (Q = 16.71 m3/h, H = 0.1 m): (a) ε = 0.5% (b) ε = 1.5%.
even though the circulation flow rate is low. While the sedimentation velocities of POM and PA6 are high, so the movement times are short, particles only can go into tubes close to baffle regardless of the particle amount. So, as the particle amount increases, the non-uniform degrees of PA66 and Resin slightly increase whereas those of PA6 and POM obviously increase when the circulation flow rate is low. When the circulation flow rate is high, as shown in Fig. 19, the nonuniform degrees of PA6 and POM slightly increase along with the particle amount, and those of PA66 and Resin fluctuate in a certain range as particle amount increases. When the particle amount increases from 0.5% to 1.5%, the non-uniform degrees of POM and PA6 are increased by 0.12 and 0.06. That is to say, the increase of circulation flow rate can weaken the influences of particle amount on particle distribution. This phenomenon can be explained with the CCD images of particle distributions in the tube bundle of the down-flow bed at different particle amounts as shown in Fig. 20. When the circulation flow rate is
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N. Li et al. / Powder Technology 366 (2020) 1–11
Fig. 21. Three-dimensional phase diagram of the influences of particle parameters and operating parameters on the non-uniform degrees in the down-flow bed.
high, as the particle amount increases, the solid holdups of POM and PA6 increase in tubes close to baffle as well as in tubes far away from baffle, whereas those of PA66 and Resin increase in each tube evenly. As is mentioned above, the increase of circulation flow rate increases the kinetic energy of particles to move horizontally further and distribute evenly in the evaporation chamber after climbing over the baffle. In this way, the influences of particle sedimentation velocity and particle amount on particle distributions in the down-flow bed are weakened, and particle distributions in the down-flow bed are more uniform. 3.4. Optimal range of the operating parameters On the basis of the above mentioned results, the sedimentation velocity of particles is the main factor affecting their fluidizations and distributions in the down-flow bed. Five phase diagrams about sedimentation velocity and operating parameters are established to provide the basis for particle selection. The particle distributions in the up-flow bed are uniform, whereas those in the down-flow bed depend on the operating parameters. According to the experimental research and industrial application, the particle distribution can meet the demand of fouling prevention and heat transfer enhancement when the nonuniform degree is no more than 0.4. The optimal ranges of the particle parameters and operating parameters under the present experimental conditions are shown in Fig. 21.
For four kinds of particles, particle distributions in the down-flow bed become more uniform with the increasing of circulation flow rate and decline of particle amount and baffle height. Great density of particle results in high sedimentation velocity and difficult to be fluidized, thus particle maldistribution becomes obvious with the increase of particle density. Also, particles with greater density are more susceptible to the circulation flow rate, particle amount and baffle height; the increase of circulation flow rate can weaken the influences of particle density, particle amount and baffle height on particle distribution. In general, particle sedimentation velocity is the main influence factor on particle fluidizations and distributions in the down-flow bed. Finally, phase diagrams are established to show the basis for particle selection and the optimal range of the operating parameters under the present experimental conditions.
Acknowledgment This work is supported by the open foundation of State Key Laboratory of Chemical Engineering (SKL-ChE-18B03), the Municipal Science and Technology Commission of Tianjin, China under the Contract No. 2009ZCKFGX01900, National Natural Science Foundation of China (No. 21676180, 21076143), and by the Key Technologies Research and Development Program of Tianjin (15ZCZDSF00160). References
4. Conclusions In this paper, influences of particle type on particle distribution are investigated systematically in a system of two-pass circulating fluidized bed. The main results are summarized as follows. The particle distributions of four kinds of particles in the up-flow bed are relatively uniform and are slightly affected by the circulation flow rate, particle amount and baffle height. This may because the particles are mixed evenly with the flow of the fluid in the shell cover before they go into tubes.
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