Effects of particle size on the particle distribution of a two-pass circulating fluidized bed evaporator with a baffle

    Effects of particle size on the particle distribution of a two-pass circulating fluidized bed evaporator with a baffle Na Li, Yuqing Zhang, Feng Jiang, Guopeng Qi, Xiaoyu Han, Yawei Bian, Xiulun Li PII: DOI: Reference:

S0032-5910(17)30119-5 doi:10.1016/j.powtec.2017.02.006 PTEC 12344

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

17 October 2016 30 January 2017 2 February 2017

Please cite this article as: Na Li, Yuqing Zhang, Feng Jiang, Guopeng Qi, Xiaoyu Han, Yawei Bian, Xiulun Li, Effects of particle size on the particle distribution of a two-pass circulating fluidized bed evaporator with a baffle, Powder Technology (2017), doi:10.1016/j.powtec.2017.02.006

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Effects of particle size on the particle distribution of

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a two-pass circulating fluidized bed evaporator with

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a baffle

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Na Li a, Yuqing Zhang a, Feng Jiang a,*, Guopeng Qib, Xiaoyu Hana, Yawei Biana and Xiulun Lia School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR

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School of Biological and Environmental Engineering, Tianjin Vocational Institute, Tianjin

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300410, PR China

*Corresponding Author

Tel: (+86) 22-8740-1722.

Email address: [email protected].

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ACCEPTED MANUSCRIPT ABSTRACT

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A two-pass circulating fluidized bed evaporator with a baffle is designed and built to enhance heat transfer, as well as prevent and remove fouling. The effects of particle size on particle

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distribution under various operating parameters are systematically investigated using the CCD

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image measurement and acquisition system. The particles used in the experiment are

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polyoxymethylene particles with diameters of 2.0, 3.15, 4.0, and 5.5 mm. The height of the baffle is 0.1 m. In this study, the particle distribution in the up-flow bed is uniform, whereas that

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in the down-flow bed is uneven, especially for large particles. In the down-flow bed, the particle distribution becomes uniform, and large particles have a fast reduction in non-uniform degree as

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the circulation flow rate increases. When the circulation flow rate is low, the distribution of large

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particles becomes uneven as the amount of particles added is increased, whereas that of small

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particles remains uniform. However, when the circulation flow rate is high, the amount of particles added and particle size only slightly affect particle distribution. Finally, four phase diagrams are established to show the optimal range of the operating parameters. Keywords：Particle distribution; Particle size; Circulating fluidized bed evaporator; Two-pass; Baffle; Visualization

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ACCEPTED MANUSCRIPT 1. Introduction

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Circulating fluidized bed heat exchangers have been widely used in industrial processes, such as food processing [1], paper making [2], water treatment [3], desalination [4], and boiler [5],

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among others [6–8], because of their heat transfer enhancement and fouling prevention at the

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heat transfer surfaces. Many studies on this system have conducted experimentations and

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simulations in the past few decades [9–16].

On the basis of the combination of flow boiling heat transfer with multiphase fluidization, the

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performance of a circulating fluidized bed heat exchanger is affected by several key factors, including particles election, particle fluidization and distribution, liquid and solid phase

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separation, and pressure drop.

Particle distribution in a tube bundle significantly influences the performance of heat transfer

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as well as the prevention and removal of fouling in a heat exchanger. Qi et al. [17] adopted

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Eulerian multiphase fluid model to simulate the particle distribution in a fluidized bed heat exchanger incorporating a tube bundle arranged in parallel and compared the simulation results with the experimental measurements. They concluded that particle distribution becomes uniform at a high velocity. Particle maldistribution occurs as the density or diameter of particles increases. Particle distribution becomes uniform as the volume fraction of the solid phase increases. Qi and Jiang [18] 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 are 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. [19] studied the solid particle distribution in the heating tube bundle of a transparent multi-tube vapor–liquid–solid

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ACCEPTED MANUSCRIPT circulating fluidized bed evaporator by using the charge-coupled device (CCD) measuring system. They found that the heterogeneity of particle distribution decreases with increasing

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circulating flow rate, heat flux, and amount of particles added but increases with increasing settling velocity of particles. Zhang et al. [20] explored the particle distribution in a horizontal

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tube bundle after reforming the front header of a liquid–solid horizontal circulating fluidized bed

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evaporator by using two inlets and adding the baffle plate. They found that the angle of the baffle

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plate and particle diameter affect the particle distribution.

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However, all of the evaporators mentioned above are single-pass evaporators. The mechanism by which particles are distributed in a multi-pass circulation fluidized bed evaporator with a

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baffle remains unclear. A two-pass evaporator with a baffle has been used in some industrial

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applications, such as sodium sulfide production, but this device suffers from serious fouling. To solve the fouling problem, a transparent multi-tube two-pass forced circulation evaporator with a

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baffle was designed and built in this study. Applying the circulating fluidized bed technology to the evaporator, we investigated the particle distribution in different tubes by using the CCD image measurement and acquisition system. This study aims to investigate the influence of particle size under various operating parameters, including circulation flow rate and amount of particles added, on the fluidization and distribution of solid particles. The results of this study may serve as a reference for industries that apply fluidized bed technology in this type of evaporator.

2. Experiments 2.1. Experimental setup and procedure

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ACCEPTED MANUSCRIPT The experimental setup is illustrated in Fig. 1. The setup consists of a heating chamber, an evaporation chamber, and a particle collector. The heating chamber contains a tube bundle and a

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shell cover. The tube bundle consists of 40 glass tubes that are arranged in a triangular fashion on

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both sides of the baffle, as shown in Fig. 2. The tube has a diameter of 25 mm × 2.5 mm and a

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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

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with a diameter of 300 mm × 10 mm and a height of 600 mm. A baffle with a height of 0.1 m is placed in the middle of the evaporation chamber. The particle collector is used to collect the

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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

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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

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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 flows from the up-flow bed to the evaporation chamber and then to the down-flow bed. Particle distribution images are acquired after the steady state is confirmed under each condition. During the experiment, the CCD camera is placed at three locations: the up-flow bed, the down-flow bed and the 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. Meanwhile the arrangement is symmetrical, so the five tubes of the outmost layer in the up-flow

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ACCEPTED MANUSCRIPT bed are chosen and named #1 to #5, while the five tubes of the outmost layer in the down-flow bed are chosen and named #11 to #15. With the software streamPix-5-S-STD, the distribution

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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

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circulation flow rates, the location of the camera can be changed, and the above processes are

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repeated.

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2.2. Materials

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Tap water and inert solid particles are used as the liquid and solid phases. The properties of the

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particles are shown in Table 1.

Table 1 Physical characteristics of the particles. Density

Terminal velocity a

(mm)

(kg·m−3)

(m·s−1)

Voidage

Polyoxymethylene (POM-2.0)

2.0 b

1390

0.123

0.292

Polyoxymethylene (POM-3.15)

3.15

1390

0.207

0.38

Polyoxymethylene (POM-4.0)

4.0

1390

0.216

0.396

Polyoxymethylene (POM-5.5)

5.5

1390

0.253

0.402

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Diameter

Particle name

a: the terminal velocity is measured in water at 20 °C at atmospheric pressure. b: the equivalent diameter of the flat globose particle.

2.3. Experiment conditions and methods The adjustable operating variables used in the experiment include the circulation flow rate and the amount of particles added. The target parameters are the distribution of particles with different sizes in the down-flow and up-flow beds.

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ACCEPTED MANUSCRIPT The amount of particles added is represented by the solid holdup ε, which denotes the ratio of the bulk volume of the solid particles to the total volume of the liquid working medium. ε is set

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to 0.5%, 1.0%, 1.5%, and 2.0% in the various experiments conducted under different parameters.

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The circulation flow rate is set at 10.66, 11.95, 14.38, 16.71, and 19.14 m3/h. More bubbles are

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drawn into the tubes when the circulation flow rate is further increased. This phenomenon affects the observation. The circulation flow rate also fluctuates within a certain scope at a certain

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frequency. The circulation flow rate is computed as the average of the maximum and minimum

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values of the circulation flow rate within 2 min.

The model of the CCD camera is GT1920, which has a frame frequency of 40.7 fps and a

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continuously record pictures.

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resolution of 1936 × 1456. The image processing software StreamPix-5-S-STD is used to

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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. 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. 1 as follows:

1 1 3 d s 3 ns d s ns 3 d n 6 s  6  100%   100%  s s % 1 1 3600 d i 2 h  20 2  600 4 4

(1)

The non-uniform degree M reflects the difference between particle distributions in different tubes. M can be calculated using Eqs.2 and 3. A large M indicates a highly uneven distribution.

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ACCEPTED MANUSCRIPT  1 n     2  s   M     si  n i1   s  

(2)

1 n   si n i1

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s 

0.5

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3. RESULTS AND DISCUSSION

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3.1. Effects of circulation flow rate on the down-flow bed

Previous studies [21] show that the baffle mainly influences the particle distribution in the

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down-flow bed. Thus, the situation in the down-flow bed is discussed first. Fig.3 shows the CCD images of the particle distribution in the evaporation chamber at a

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circulation flow rate of 10.66 m3/h. In these images, the up-flow bed is located on the left, the down-flow bed is on the right, and the baffle is in the middle. The particles climb over the baffle

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from left to right and then flow down to the tubes. Fig.4 shows the influence of particle size on the particle distribution as the circulation flow rate increases. As shown in Fig.4, the particle distribution is significantly affected by the circulation flow rate. For each circulation flow rate, particles with different sizes show a similar trend in the solid holdup curve. When the circulation flow rate is low, the solid holdups in Tubes #11 and #12 are significantly higher than those in the other tubes. Furthermore, large particles have a great difference in the solid holdup in different tubes. This phenomenon occurs because the liquid and particle velocities along the horizontal direction are low when the circulation flow rate is low, thereby restricting these particles to move farther and enter Tubes #13, #14, and #15. Given that the terminal velocity of large particles is high, the time these particles take to drop in the evaporation chamber is short, so the large particles move a shorter distance, as shown in

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ACCEPTED MANUSCRIPT Fig.3. As the circulation flow rate increases, the particle distribution becomes increasingly uniform, and the curve becomes flat. When the circulation flow rate reaches 19.14 m3/h, the

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particles are well distributed regardless of the particle size. Because, when the circulation flow rate is high, more particles, especially large particles, can climb over the baffle and move across

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long distances along the horizontal direction and then enter Tubes #13, #14, and #15. Fig. 5 shows the effect of particle size on the non-uniform degree in the down-flow bed as the

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circulation flow rate increases. As shown in Fig. 5, the larger the particle size, the higher the

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non-uniform degree. The non-uniform degree significantly declines as the circulation flow rate increases regardless of the particle size. Moreover, the non-uniform degree declines to different

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extents in the case of different particle sizes. A larger particle has a faster reduction in the non-

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uniform degree. When the circulation flow rate increases from 10.66 m3/h to 19.14 m3/h, the non-uniform degrees of POM-5.5, POM-4.0, POM-3.15, and POM-2.0 are reduced by 1.12, 1.03,

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1.02, and 0.76, respectively. In other words, large particles are more susceptible to the circulation flow rate than small particles. In consideration of the high terminal velocity of large particles, the particle distribution is heterogeneous and the non-uniform degree is high when the circulation flow rate is low. When the circulation flow rate is high, the velocity of particles along the horizontal direction increases, and the turbulence degree increases at the same time, which makes the particles mix evenly in radial direction, so the particles are well distributed, and the non-uniform degree is low, regardless of the particle size. 3.2. Effects of circulation flow rate on the up-flow bed Fig. 6 shows that the solid particles are relatively well distributed in the up-flow bed. This phenomenon occurs because the particles form particulate fluidization in the up-flow bed. Fig. 7

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ACCEPTED MANUSCRIPT quantitatively describes the particle distribution further. The distribution of POM-2.0 is the most uniform at each circulation flow rate, whereas the solid holdup of POM-2.0 is the lowest. This

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result may be attributed to the fact that POM-2.0 is easier to accumulate in the evaporation chamber after climbing over the baffle because of its lower terminal velocity compared with the

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other particles. It also can be seen in Fig. 7 that the distribution tendencies of POM-3.15, POM-

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4.0, and POM-5.5 are similar.

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The curves in Fig. 8 show that the tendency of the non-uniform degree varies as the circulation

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flow rate increases. Given that the particles can be evenly mixed with the flow of the fluid in the shell cover, the particle distribution is uniform in the up-flow bed, and the non-uniform degrees

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are slightly affected by the circulation flow rate regardless of the particle size.

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3.3. Effects of the amount of particles added on the down-flow bed.

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Fig. 9 shows that the trend of the solid holdup curves is similar when the circulation flow rate is low regardless of the particle amount and size. The solid holdup initially decreases from Tubes #11 to #13 and then almost remains constant from Tubes #13 to #15. As is mentioned above, when the circulation flow rate is low, the particle velocity along the horizontal direction is low, which restricts these particles to move farther and enter Tubes #13, #14, and #15. For large particles, only the solid holdups of Tubes #11 and #12 significantly increase along with the particle amount. Given their low horizontal velocity and high terminal velocity, large particles can only move a short horizontal distance after climbing over the baffle. A large amount of particles added indicates low particle distribution uniformity when the circulation flow rate is low. Fig. 10 describes the problem further. The non-uniform degrees of POM-3.15, POM-4.0, and POM-5.5 increase along with the amount of particles added, whereas that of POM-2.0

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ACCEPTED MANUSCRIPT slightly declines. Given the uniform distribution of POM-2.0, a large amount of particles added can reduce the tendency of the particle distribution to fluctuate randomly, thereby enhancing the

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uniformity of particle distribution.

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When the circulation flow rate is high, the particles are well distributed regardless of the

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particle size (Fig. 11). The trend of the distribution curve is only slightly affected by the particle size and the amount of particles added. A comparison of Figs. 9 and 11 shows that the solid

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holdups of Tubes #13, #14, and #15 also significantly increase at a high circulation flow rate.

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These results may be attributed to the fact that a high circulation flow rate denotes a fast horizontal velocity and a long movement distance of the particles.

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When the circulation flow rate is high, the non-uniform degree is relatively low in the down-

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flow bed and is slightly affected by the amount of particles added (Fig. 12).

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3.4. Effects of the amount of particles added on the up-flow bed For the four types of particles, the solid holdup increases along with the amount of particles added (Fig. 13). The particle distribution in the up-flow bed is uniform and the tendency of particle distribution is similar regardless of the particle size and the amount of particles added. As shown in Fig. 14, the non-uniform degree is relatively low, and is slightly affected by the amount of particles added regardless of the particle size in the up-flow bed. 3.5. Optimal range of the operating parameters On the basis of the above mentioned results, four phase diagrams (Fig. 15) are established to determine the optimal range of the operating parameters. The particle distribution in the up-flow

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ACCEPTED MANUSCRIPT bed is uniform, whereas that in the down-flow bed depends on the operating parameters. According to the experimental research and industrial application, the particle distribution can

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meet the demand of fouling prevention and heat transfer enhancement when the non-uniform degree is less than 0.4. The optimal range of the operating parameters under the present

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experimental conditions is shown in Fig. 15.

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4. Conclusions

The effects of particle size on the particle distribution of a two-pass circulating fluidized bed

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evaporator with a baffle are systematically investigated with a CCD measuring technique. The main results are summarized as follows. The distribution of POM-2.0 is better than that of the

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other particles, but the solid holdup of particle POM-2.0 is still the lowest. The solid particles in

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the up-flow bed have a relatively uniform distribution. The non-uniform degrees in the up-flow

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bed are slightly affected by the circulation flow rate and the amount of particles added regardless of the particle size.

The down-flow bed shows an uneven particle distribution, and large particles have a more uneven distribution than small particles. The solid holdup distribution becomes increasingly uniform as the circulation flow rate increases, especially for large particles. When the circulation flow rate is low, the non-uniform degrees of POM-3.15, POM-4.0, and POM-5.5 increase along with the amount of particles added, whereas that of POM-2.0 slightly declines. When the circulation flow rate is high, the non-uniform degree is relatively low and is slightly affected by the amount of particles added. Finally, phase diagrams are established to show the optimal range of the operating parameters under the present experimental conditions.

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ACCEPTED MANUSCRIPT Nomenclature tube internal diameter, mm

ds

solid particle diameter, mm

h

tube height, mm

M

non-uniform degree of distribution, [–]

n

tube number, #

ns

particle number

Q

circulation flow rate, m3/h



amount of particles added, [–]

s

solid holdup in the tube, [–]

 si

solid holdup in the #i tube, [–]

s

average solid holdup, [–]

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ACCEPTED MANUSCRIPT ACKNOWLEDGMENT The authors are grateful to the Municipal Science and Technology Commission of Tianjin,

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China for financially supporting under the contract no. 2009ZCKFGX01900.

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ACCEPTED MANUSCRIPT REFERENCES [1] R. Rautenbach, T. Katz, Survey of long time behavior and costs of industrial fluidized bed

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heat exchangers, Desalination. 108 (1996) 335–344.

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[2] D. G. Klaren, Self cleaning heat transfer, Hydrocarb. Eng. 6 (2001) 83–92. [3] R. Rautenbach, C. Erdmann, J.S. Kolbach, The fluidized bed technique in the evaporation

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ACCEPTED MANUSCRIPT [18] 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.

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[19] B.B. Wang, F. Jiang, G.P. Qi, et al. Solid particle distribution in vapor–liquid–solid multi-

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fluidized bed evaporator with baffle, Powder Technol. 295 (2016) 47–58.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1 Schematic of the two-pass circulating fluidized bed evaporator with a baffle.

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Fig. 2 Tube bundle distribution and target tubes.

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Fig. 3 Particle distribution in the evaporation chamber (ε= 0.5%, Q = 10.66 m3/h): (a) POM-2.0,

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(b) POM-3.15, (c) POM-4.0, and (d) POM-5.5.

Fig. 4 Distribution of particles with different sizes in the down-flow bed (ε= 0.5%).

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Fig. 5 Effect of particle size on the non-uniform degree in the down-flow bed as the circulation flow rate increases (ε=0.5%).

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Fig. 6 CCD image of particle distribution in the up-flow bed (ε = 0.5%, Q = 10.66 m3/h). Fig. 7 Distribution of particles with different sizes in the up-flow bed (ε = 0.5%).

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Fig. 8 Effect of particle size on the non-uniform degree in the up-flow bed as the circulation flow rate increases (ε = 0.5%).

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Fig. 9 Distribution of particles with different sizes in the down-flow bed (Q= 10.66 m3/h). Fig. 10 Effect of particle size on the non-uniform degree in the down-flow bed as the amount of particles added increases (Q=10.66 m3/h). Fig. 11 Distribution of particles with different sizes in the down-flow bed (Q = 19.14 m3/h). Fig. 12 Effect of particle size on the non-uniform degree in the down-flow bed as the particle amount increases (Q=19.14 m3/h). Fig. 13 Distribution of particles with different sizes in the up-flow bed (Q= 11.95 m3/h). Fig. 14 Effect of particle size on the non-uniform degree in the up-flow bed as the amount of particles added increases (Q=11.95 m3/h). Fig. 15 Three-dimensional phase diagram of the influence of operating parameters on the nonuniform

degree

in

the

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down-flow

bed.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights The particle distribution in the up-flow bed is uniform.

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The particles unevenly distribute in the down-flow bed especially for the big ones.

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A high flow rate greatly enhances the uniformity of the big-particle distribution.

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The distribution of particle POM-2.0 is the best among the four sizes of particles.

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Results can serve as references for the industrial application of such equipment.

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