Elsevier

Powder Technology

Volume 295, July 2016, Pages 47-58
Powder Technology

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

https://doi.org/10.1016/j.powtec.2016.03.024Get rights and content

Highlights

  • The particle distribution in the up-flow bed is uniform.

  • The down-flow bed shows an uneven particle distribution.

  • A high flow rate enhances the uniformity of the particle distribution.

  • The optimal range of the operating parameters is established.

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

Abstract

A liquid–solid circulating fluidized bed evaporator is designed and built to enhance the heat transfer and prevent and remove fouling in a two-pass evaporator. The effects of the operating parameters, such as circulation flow rate, amount of added particles, and height of the baffles, on the fluidization and distribution of solid particles in the two-pass circulating fluidized bed evaporator are systematically investigated using the charge-coupled device image measurement and acquisition system. Polyoxymethylene particle is used in the experiment. The circulation flow rate ranges from 10.66 m3/h to 19.14 m3/h, the amount of added particles changes from 0.5% to 2.0%, and the height of the baffles are 0.073, 0.1, 0.15, and 0.2 m. The particle distribution in the up-flow bed is uniform, whereas that in the down-flow bed is uneven. This study primarily investigates the particle distribution in the down-flow bed, which becomes increasingly uniform when the circulation flow rate is increased or when the height of the baffle is reduced, except when the baffle height is 0.2 m. When the circulation flow rate is low, the distribution becomes increasingly uneven as more particles are added. However, when the circulation flow rate is high, the amount of added particles has little influence on the particle distribution. Finally, two phase diagrams are established to show the optimal range of the operating parameters. These findings may serve as a valuable reference for the industrial application of the fluidized bed heat transfer enhancement and fouling prevention technology in multi-pass evaporators with baffles.

Introduction

The circulating fluidized bed heat exchangers have been developed by adding solid particles for enhancing heat transfer and preventing fouling at the heat transfer surfaces. It has been used for a wide range of industrial processes, such as aluminum production [1], pharmaceuticals [2], paper making [3], water treatment [4], desalination [5], and so on [6], [7], [8]. Wen et al. [9] reported that the heat transfer coefficient for vapor–liquid two-phase flow boiling falls significantly without addition of polytetrafluoroethylene particles. While the vapor–liquid–solid three-phase flow boiling can maintain a gently higher heat transfer coefficient for more than 1500h because the solid particles shear the heat transfer wall and keep it clean.

Due to the potential applications and advantages, many studies on circulating fluidized beds system have been made from the point of view of both experiment and simulation in the past few decades. These studies relate to the heat transfer characteristics [10], [11], the frequency and force of particle–wall collisions [12], the effect of particle size [13], and so on [14], [15], [16].

Based on the combination of flow boiling heat transfer with multiphase fluidization, the performance of the circulating fluidized bed heat exchanger is affected by several key factors, including the selection of particles, the distribution and fluidization of particles, the separation of liquid and solid, the pressure drop, and so on.

Particle collision on the wall has a significant effect on the performance of heat transfer and fouling prevention and removal in the heat exchanger. So the research on particle distribution in tube bundle is of great importance. There have been a limited number of studies on particle distribution in the tube bundle. Qi et al. [17] adopted the Eulerian multiphase fluid model to simulate the particle distribution in the fluidized bed heat exchanger incorporating tube bundle arranged in parallel and compared the simulation results with the experimental measurements. They drew the conclusion that particle distribution became more uniform in the high velocity. With the increase in density or diameter of particle, particle maldistribution took place. With the increasing of volume fraction of solid phase, particle distribution became slightly more uniform. Jiang et al. [18] numerically and experimentally investigate the particle distribution in liquid–solid circulating fluidized bed heat exchanger with the horizontal tube bundle. They found that more particles occurred in the lower tubes of the horizontal tube bundle in low velocity. The uniformity distribution of particles was obtained in high velocity. Wang et al. [19] researched the solid particle distribution in the heating tube bundle of a transparent multi-tube vapor–liquid–solid circulating fluidized bed evaporator using the charge-coupled device (CCD) measuring system. The results show that the nonhomogeneity of particle distribution decreases with the increase of circulating flow rate, heat flux and amount of added particles; the nonhomogeneity increases with the increase of settling velocity of particles. Zhang et al. [20] studied the particle distribution in the 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 angel of the baffle plate and particle diameter have the effect on the performance of the particle distribution.

However, all of the evaporators above are single-pass. It is still unclear how the particle distribution is in the multi-pass circulation fluidized bed evaporator with baffle. The two-pass evaporator with baffle has been used in several industrial applications, such as the sodium sulfide production, but has serious fouling problem. To solve the fouling problem, a transparent multi-tube, two-pass forced circulation evaporator with baffle is designed and built in this study. Applying the circulating fluidized bed technology to the evaporator, the particle distribution in the different tubes is investigated using the CCD image measurement and acquisition system. The main purpose of this paper is to study the influence of operating parameters, such as circulation flow rate, amount of added particles, and height of the baffles, on the fluidization and distribution of solid particles. The research findings may serve as a valuable reference for industries that apply fluidized bed technology in this kind of evaporator.

Section snippets

Experimental setup and procedure

The experimental setup is illustrated in Fig. 1. The setup consists of a heating chamber, evaporation chamber, and particle separator. The heating chamber contains a tube chamber and bundle. The tube bundle consists of 40 glass tubes. The tube has a diameter of 25 mm × 2.5 mm and a height of 600 mm. These tubes are arranged in a triangular fashion on both sides of the baffle, as shown in Fig. 2. The entire set of equipment, excluding the tube bundle and evaporation chamber, is made up of

Estimation of holdup of solid particles

Solid holdup εs can be used to investigate the distribution of the particles in each tube. With the GT1920, the whole tube can be photographed. The particle overlapping does not appear under the present experimental conditions, as shown in Fig. 3 when ε is 2.0% and the circulation flow rate is 19.14 m3/h in the up-flow bed, so the number of particles can be got accurately from the 2D image. The value of ns is the average value of the particle number of the whole tube within two minutes in

Particle distribution in the up-flow bed

Fig. 4 is the typical image of particle distribution when the circulation flow rate is 10.66 m3/h. The particles are relatively well distributed. Fig. 5 shows the particle distribution in the different tubes in the up-flow bed. The solid holdup slightly increases when the circulation flow rate increases from 10.66 m3/h to 11.95 m3/h, and a high circulation flow rate enhances the uniformity of the distribution of particles in the different tubes. So the particle distribution in the up-flow bed

Conclusions

A CCD measuring technique is used to systematically investigate the particle distribution in a system of a two-pass circulating fluidized bed evaporator with baffle. The main results are summarized as follows:

The solid holdup in the different tubes in the up-flow bed has a relatively uniform distribution, and the distribution in the up-flow bed is slightly affected by the circulation flow rate, amount of added particles and baffle height. This may because the particles are mixed evenly with the

Acknowledgments

The authors are grateful to the Municipal Science and Technology Commission of Tianjin, China for financially supporting this work under the contract no. 2009ZCKFGX01900.

References (20)

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Cited by (13)

  • Effect of distributor structure on the particle distribution in a vertical two-pass circulating fluidized bed evaporator with a baffle

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    Meanwhile, it can also be found that the semicircular distributors with appropriate structure have more stable performance and better potential in the application to this type of heat exchanger and evaporator comparing with the trapezoid distributors. The decrease in baffle height is beneficial to the particle distribution in the down-flow bed, which has been proved in the previous study [31,32]. Therefore, at low baffle height, the particle distribution in the down-flow bed is more uniform compared with that at high baffle height, as show in Fig. 11(a) and (h).

  • Effects of distributor on the particle distribution in a horizontal multi–tube liquid–solid circulating fluidized bed heat exchanger

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    The discrepancy in the fluidization of particles maybe exists in different directions of beds due to the effect of forces exerted on the particles. However, so far the investigation on the fluidization and distribution of particles mainly focus on the vertical beds [20,21,23–26], especially the up–flow bed. The horizontal circulating fluidized bed exchanger has the advantage of easy installation and good stability compared with the vertical beds.

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

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    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.

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