Effects of spherical adsorbent fluidization and self-rotation on removal of VOCs in a cyclonic fluidized bed

Journal Pre-proof Effects of spherical adsorbent fluidization and self-rotation on removal of VOCs in a cyclonic fluidized bed Liang Ma, Guolin Xiang, Yuan Huang, Mengya He, Jianping Li, Pengbo Fu

PII:

S1226-086X(20)30059-9

DOI:

https://doi.org/10.1016/j.jiec.2020.01.039

Reference:

JIEC 4959

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

12 August 2019

Revised Date:

21 December 2019

Accepted Date:

31 January 2020

Please cite this article as: Ma L, Xiang G, Huang Y, He M, Li J, Fu P, Effects of spherical adsorbent fluidization and self-rotation on removal of VOCs in a cyclonic fluidized bed, Journal of Industrial and Engineering Chemistry (2020), doi: https://doi.org/10.1016/j.jiec.2020.01.039

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Effects of spherical adsorbent fluidization and self-rotation on removal of VOCs in a cyclonic fluidized bed

Liang Ma1, Guolin Xiang1, Yuan Huang2*, Mengya He1, Jianping Li1, Pengbo Fu1

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School of Mechanical and Power Engineering, East China University of Science and Technology,

2School

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Shanghai, 200237, P. R. China. of Chemistry and Molecular Engineering, East China University of Science and Technology,

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Shanghai, 200237, P. R. China.

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Graphical abstract

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Highlights 

A novel cyclonic fluidized bed for VOCs adsorption was proposed.



Adsorbents fluidization in the cyclonic fluidized bed was quantitatively analyzed.



Self-rotation of spherical activated carbon adsorbent was measured.



Effects of particle fluidization and self-rotation on VOCs adsorption were tested.

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Abstract

A fluidized bed has the advantages of treating large flows, intensifying mass and heat transfer, and lowering costs. This study proposed a cyclonic fluidized bed packed with

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spherical activated carbon adsorbents for volatile organic compounds adsorption. The

fluidization and self-rotation of the AC particles in a 25 mm cyclonic fluidized bed were

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studied with high-speed camera testing technology. The effects of the particle movement on the adsorption efficiency of toluene were also tested. The results show that most of the

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particles at the inlet side of the cyclonic fluidized bed were moving up when the inlet airflow rate was greater than 2.0 m3/h. The particles began to move in clusters when the relative

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packing height increased to a critical value of 0.57. Increasing the gas flow rate and the diameter and height of the core column will increase the self-rotation speed of the total particles. The maximum self-rotation speed of spherical adsorbents reached 1700 rad/s at the

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inlet flow rate of 2.5 m3/h. In the case of the same axial velocity of the gas phase in the upper space of the core column, increasing the particle self-rotation speed can slightly improve the

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adsorption efficiency. The maximum adsorption efficiency reached 99% when the inlet flow rate is 1.0 m3/h with relative packing height 0.65.

Keywords: Cyclonic fluidized bed; VOC adsorption; Particle self-rotation; Activated carbon Nomenclature Symbols 2

Inlet height of the cyclonic fluidized bed, mm

A

Projected area of the test zone in the cyclonic fluidized bed, mm2

Ai

Projected area of the particles in the test zone, mm2

b

Inlet width of the cyclonic fluidized bed, mm

d

Diameter of the core column, mm

D1

Inner diameter of the outlet of the cyclonic fluidized bed, mm

D2

Inner diameter of the cyclonic fluidized bed, mm

f

Frame rate of high-speed camera, fps

h

Height of the core column, mm

h1

Height of the cylinder section of the cyclonic fluidized bed, mm

h2

Height of the conical section at the bottom of the cyclonic fluidized bed,

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a

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mm

Total height of the cyclonic fluidized bed, mm

H1

Height of the dead zone near the inlet side of the cyclonic fluidized bed,

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h3

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mm H2

Height of the dead zone on the opposite of the inlet of the cyclonic fluidized bed, mm

Initial height of the particle bed before operation, mm

k

Relative packing height of the activated carbon adsorbents

Nf

Number of frames recording the sphere self-rotation Number of tracer particle self-rotations Particle fluidization factor, %

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q

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N

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H3

Q

Airflow rate, m3/h

t

Movement time of the tracer self-rotation particle, s

E

Adsorption efficiency, %

Greek letters 3

ω

Particle self-rotation speed, rad/s

Acronyms Activated carbon

CFBs

Circulation fluidized beds

DIA

Digital image analysis

PIV

Particle image velocimetry

VOCs

Volatile organic compounds

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AC

1 Introduction

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Volatile organic compounds VOCs include various olefins and alkanes found in fatty and aromatic groups and are mainly derived from industrial production, storage, and

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transportation processes, which are involve in the petrochemical industry, rubber and plastic processing industry, biopharmaceutical industry, etc. Statistics reveals that VOCs emissions

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from Chinese industries increased from 1.15×106 tons in 1980 to 13.35×106 tons in 2010, representing an increase of nearly 12 times [1]. Some of these substances are very toxic and

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can invade the human body through the respiratory system and skin contact, causing great damage to humans [2]. Therefore, the control and management of VOCs pollution is an

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essential endeavor. Studies on VOCs management have been carried out earlier in western countries. In 1990, the US Clean Air Act was amended to control organic emissions.

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However, China's pollution control work on VOCs started late. In 2012, the Ministry of Environmental Protection issued the “12th Five-Year Plan for Air Pollution Prevention and Control in Key Areas”, which first proposed VOCs gas emission reduction targets and managed key industries. Adsorption is a method that is low cost and highly efficient; as such, adsorption has received great attention from many researchers for removal of heavy metal [3, 4], dyes [5], organic matters [6] and VOCs [7, 8] in aqueous or gas. The principle of adsorption is 4

intercepting VOCs by using activated carbon (AC) particles, carbon fibers, zeolites, and other adsorbents with multi-level structures. Carbon adsorbents have a large surface area, highly porous structure, and a strong adsorption ability; thus, these materials are widely used in gas purification, especially in the treatment and recovery of VOCs [9]. Activated carbon adsorption has been extensively applied to the adsorption of benzene-based compounds and halogenated hydrocarbons [7, 10]. Many researchers have conducted in-depth studies on the modification, application, and production of AC materials. Müller [11] investigated the effect

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of the particle size and surface area on the adsorption of albumin-bonded bilirubin on AC. The results demonstrated that the adsorption rate of bilirubin increases with decreasing particle sizes and with increasing mesopore volumes.

Commonly used adsorbers include fixed beds, zeolite rotors and fluidized beds.

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Fluidized beds have high mass transfer rates that can improve adsorption and desorption.

Other expected advantages are continuous processing, the ability to treat large flows under a

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moderate pressure drop, the possibility to handle dusty gases or slurries, a good fluid side mass and less formation of localized hotspots (avoiding the spontaneous combustion of the

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adsorbent [12, 13]). Guo et al. [14] used a fluidized bed to adsorb toluene gas and AC as the adsorbent. Their results showed that the fluidized bed effectively increased the adsorption rate

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of toluene on AC. Clarke et al. [15] removed VOCs from a gas stream using bioreactors containing fluidized and fixed beds and found that the operating mode of the former is better

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than that of the latter. The maximum elimination capacity of ethanol in the fluidized bed mode and packed bed mode were 1520 g/(m3h) and 530 g/(m3h), respectively. As the gas velocity

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increased, the removal efficiency decreased while the elimination capacity increased. Song et al. [16] used a batch operation circulating fluidized bed to adsorb toluene from an air stream with a polymeric adsorbent, and efficient toluene abatement was achieved. At present, studies on the AC adsorption method mainly focus on the relationship between AC materials and VOCs properties. Studies on particle movement in fluidized beds are limited and mainly describe theoretical analyses and numerical simulations. Many scholars have studied the distribution, mechanical characteristics and movement behavior of a 5

single particle or several particles in fluidized beds by different methods, and most of them have investigated the rise and settlement process of large particles in the fluidized bed [17, 18]. For example, Hernández-Jiménez et al. [19] conducted experimental research on bubble fluidized beds using particle image velocimetry (PIV) and digital image analysis (DIA) technology and, compared to experimental results with numerical simulations, found that the two sets of results are only qualitatively consistent in two dimensions. Chen et al. [20] studied the orientation distribution of cylindrical and spherical particles during co-fluidization using

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an X-ray particle tracking speedometer based on an X-ray stereoscopic imaging system. Xu et al. [21] studied the movement of particles in fluidized beds using the positron emission and radioactive particle tracking techniques. However, the physical properties of the tracer

particles sometimes used in this method may differ from those of the particles to be tested.

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The distribution of particles in fluidized beds has aroused concern from many scholars.

Particle clusters are an important fluidization phenomenon and a strong factor influencing

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uneven fluid dynamics in fluidized beds. Wang et al. [22] used optical and electrical measurements and imaging methods to monitor the particle-clustering phenomenon occurring

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in circulating fluidized beds (CFBs) and showed that particles were found to cluster in highflux CFBs. Particle rotation plays an important role in several aspects of gas-solid two-phase

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flow. Wu et al. [23, 24] studied the self-rotation of particles in a CFB and the effect of selfrotation on the particle distribution. Hagemeier et al. [25] used half-page blackened tracer

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particles to estimate the particle velocities and particle rotation simultaneously within a 2Dfluidized bed. Liu et al. [26] used a pseudo two-dimensional discrete particle model to

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investigate the influence of the Magnus lift force in a fluidized bed. In this paper, we proposed a cyclonic fluidized bed to study how the fluidization and

self-rotation of spherical AC adsorbent influences VOCs adsorption. Commercial spherical AC with a painted marker on its surface was used as the tracer particles, whose motion processes were captured by a high-speed camera under different working conditions. The effects of the amount of particles, inlet gas velocity, and core column structure on the motion of AC particles were measured. The relationship between the motion characteristics and 6

adsorption efficiency of the AC particles was studied via an experiment performed on the ACbased adsorption of toluene.

2 Experiments 2.1 Adsorbents and tracer particles The adsorbents used in the test were pitch-based AC microspheres produced by the East

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China University of Science and technology. The microspheres have a diameter of 0.6~0.8 mm, a stacking density of 0.642 g/ml and a specific surface area of 1,200 m2/g. The large specific surface area makes the adsorbent have high adsorption performance.

To observe the self-rotation of the microspheres in the cyclonic fluidized bed, some of

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the microspheres (tracer particle) were marked by spraying white paint on half of the surface. The minimum detection angle or measurement accuracy is π. Normal temperature air was

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used in the experiment.

2.2 Experimental apparatus and procedure

A Perspex cyclonic fluidized bed reactor with an inner diameter of 25 mm was used, and

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its structure parameters are shown in Fig. 1(a) and Table 1. To study the effect of the core column structure on the motion of the particles in the cyclonic fluidized bed, five core

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columns with different diameters and heights were designed (

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Table 2). The structure of the core columns was recorded as d-h, where d and h represent the diameter and height of the core column, respectively, in units of millimeters. Thus, these core columns are labeled 11-22, 11-33, 11-44, 13-22 and 16-22. The schematic diagram of the test system were shown in Fig. 1(b) and (c). the test system consisted of four parts: VOCs generating device, gas generating device, VOCs adsorption device and high-speed imaging device. Two kinds of experiments were conducted with combining some of the devices. 2.2.1

Detection of adsorbents fluidization and self-rotation

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The test system for detecting adsorbents fluidization and self-rotation were combined by the gas generating device, VOCs adsorption device and high-speed imaging device. Before

the experiment, a certain weight of the particles, including tracer particles, was added to the

reactor. Then, air was blown into the tangential inlet of the cyclonic fluidized bed reactor. The

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airflow rate and inlet pressure were measured by a hotwire anemometer and pressure gauge. The airflow rate was limited to a certain range to prevent the adsorbent particles from being

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blown out of the reactor.

The cyclonic fluidized bed was installed vertically, and the origin of the coordinate

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system was fixed at the bottom center of the reactor. The particle motions were recorded by a high-speed digital camera (FastCam SA X2, Photron, Japan), and images with a 512×896

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pixel resolution were obtained at a camera frame rate of f=16000 fps from the x-direction, as shown in Fig. 1(c). The detection zone covered the whole cylindrical section. Two 100 W

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white light-emitting diodes (LED) illuminated the detection zone. When the airflow rate was stable, the camera would start to record the particle motions. The typical self-rotation of a

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tracer particle is shown in Fig. 1(d). To study the effect of the inlet flow, amount of particles, and core column structure on

the particle fluidization and self-rotation, twenty-five different operating conditions were designed with four inlet flow rates, five particle packing amounts and five core column structures, as shown in Table 2.

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Fig. 1 Experiment setup. (a) Structure of the cyclonic fluidized bed; (b) Schematic of the test system. 1blower, 2,3-gas flow meter, 4-toluene cylinder, 5-Pressure gauge; (c) High-speed camera imaging system;

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2.2.2

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(d) Processing image sequences of particle self-rotation.

Image analysis

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(1) Calculation of the particle self-rotation speed ω

Based on the typical sample images of the tracer microsphere self-rotation, as shown in Fig. 1(c). The sequence images obtained by the high-speed camera were analyzed by a digital

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software. The white paint on the sphere surface enabled us to track the self-rotation of the particles. The microsphere self-rotation speed is shown by Eq. (1),

n f N f 1

(1)

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

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where nπ is the self-rotation angle observed from Fig. 1(c) (n=1, 2, 3…), f is the frame rate of high-speed camera, fps; Nf is the number of frames recording the sphere self-rotation. According to Eq. (1), the self-rotation speed of microsphere shown in Fig. 1(c) was 2218 rad/s with inlet flow rate of 2 m3/h, adsorbents packing amount of 1.25 g and the core column 13-22. In this paper, at least ten samples were used for each case to calculate the mean particle self-rotation speed. (2) Relative packing height k 9

When the gas flow rate is constant, the average axial velocity of the gas in the reactor is determined, so the packing height of the adsorbent determines the adsorption time, which directly affects the final adsorption efficiency. The total height of the cylinder of the cyclonic fluidized bed is h1 = 88 mm, and the relative packing height k is calculated by Eq. (2),

k

H3 h1

(2)

where H3 is the initial packing height of the particles in the cyclonic fluidized bed, mm.

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(3) Adsorbents fluidization factor q The spatial distribution rate of the AC particles in the cyclonic fluidized bed is an

important criterion to qualify particle fluidization. However, it is difficult to measure the

particle’s 3D distribution. Therefore, in this paper, particle fluidization is characterized by the

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ratio of the projected area of the particles to that of the cyclonic fluidized bed in the two-

defined by Eq. (3):

1 10 Ai  100%, (i  1, 2,3 10) 10 i 1 A

(3)

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q

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dimensional images obtained by the high-speed camera. Particle fluidization factor q is

where A is the projected area of the test zone (see Fig. 1(c)) in the cyclonic fluidized bed,

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mm2; Ai is the projected area of the particles in the test zone, mm2; and i represents the sample number. According to the particle motions of the different conditions, a rectangular area with

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a height from 25 mm to 65 mm was selected from the two-dimensional images of the reactor to calculate particle fluidization. Since the fluidization of some of the working conditions

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shows a certain periodicity, the concentration of the fluidized particles in the same position changes remarkably over time. Therefore, q represents the mean value of 10 different time points in one cycle.

2.2.3

Adsorption of VOCs The test system of adsorbing VOCs were combined by the VOCs generating device, gas

generating device and VOCs adsorption device. According to the preliminary factory survey, 10

air with a toluene concentration of 8 ppm was used as the simulated VOCs gas to test and verify the adsorption behavior of the cyclonic fluidized bed. The inlet and outlet toluene concentrations were measured by a VOCs detector (PGM7300 MiniRAE Lite, RAE Systems Inc., USA). For VOCs adsorption process, the VOCs removal efficiency in the cyclonic fluidic bed reactor packing with AC adsorbents is defined as follows:

ci  co 100% ci

(4)

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E

where ci and co are the concentrations of VOCs in the inlet and outlet airflows respectively,

ppm. To evaluate the VOCs removal efficiency of the cyclonic fluidized bed packed with AC adsorbents, the E values were calculated by Eq. (4) with corresponding concentrations of

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VOCs in airflow at various values of the operation variables including inlet flow rate (Q),

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3 Results and Discussion

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adsorbents self-rotation speed (ω) and relative packing height (k).

3.1 Fluidization of the adsorbent particles in the cyclonic fluidized bed

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Since the cyclonic fluidized bed has only one tangential inlet, the dead zone of the granular bed on the opposite side of the inlet inevitably appears. To reduce the dead zone, the

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effects of the inlet airflow rate and packing height of the AC particles were investigated with different core columns.

Effect of the gas flow rate on particle fluidization

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3.1.1

When the airflow rate Q was low, only a small portion of the particles on the surface of

the bed were fluidized. However, when the airflow rate Q was large enough, most of the particles near the inlet side were blown to the upper space and circulated in the cyclonic fluidized bed in the form of particle groups (see supplementary movie 1-Fluidization of the AC particles). Fig. 2 shows the fluidization of the AC particles in the cyclonic fluidized bed with the core column 16-22 at different airflow rates. The relative packing height k = 0.29 11

remained unchanged. It can be seen that the airflow rate has a significant effect on particle fluidization. The height H1 is significantly reduced as the airflow rate increases. In particular, when Q = 2.5 m3/h, H1 = 0 mm. However, the height H2 remains essentially unchanged for heights slightly more than 24 mm. These results indicate that multiple inlets are needed to

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significantly improve particle fluidization at the reactor bottom.

Fig. 2 Images of particle fluidization in the cyclonic fluidized bed with different airflow rates. The core

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column is 16-22, the amount of packed adsorbents is 6.250 g，corresponding to relative packing height

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k=0.29.

Effect of the relative packing height on particle fluidization

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The particle fluidization process that occurs in the cyclonic fluidized bed with core

column 13-22 and different relative packing heights k is shown in Fig. 3. The airflow rate was Q= 2.0 m3/h. The size of the dead zone significantly increased as the amount of particles increased. The amounts of the particles in Fig. 3(a)-(c) were low. The size of the dead zone increased steadily as the amount of the packing particles increased. In Fig. 3(c), H1 is 10.67 mm, which is close to the inlet height (a=11 mm). The H1 values in Fig. 3(d) and (e) are significantly smaller than those shown in the other figures. The proportion of the fluidized 12

particles in Fig. 3(d) and (e) is more than that in Fig. 3(a)-(c), and the expansion heights of the fluidized particles exhibit irregular fluctuations. This shows that as the packing height increases, the airflow path changes and tends to prefer a horizontal tangential motion. In addition, the particles began to move in clusters when the relative packing height increased to

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a critical value of 0.57.

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Fig. 3 Images of particle fluidization in the cyclonic fluidized bed with different relative packing heights. The core column is 13-22, and the airflow rate is Q= 2.0 m3/h.

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The corresponding value of the dead zone heights H1 and H2, which is affected by the relative packing height k, is shown in Fig. 4. The height of the rectangular inlet a is 11 mm,

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and the value of H1 cannot exceed a. As the relative packing height k of the particles increases, H1 increases first and then decreases. When the relative packing height is less than

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0.2, some of the particles near the inlet are carried to the opposite side, which results in the value of H2 being greater than the initial packing height H3. However, more airflow tends to rotate tangentially as the relative packing height increases. This indicates that increasing the particles’ packing height would prevent airflow from escaping directly. In addition, particle fluidization q is proportional to the relative packing height in the test conditions, which means that the higher the packing height, the better the particle fluidization that occurs in the cyclonic fluidized bed. 13

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Fig. 4 Effect of relative packing height on the dead zone height H1 and H2. The airflow rate is Q= 2.0 m3/h, the core column is 13-22.

Effect of the core column size on particle fluidization

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3.1.3

The core column size is an important parameter affecting fluidization. As shown in Fig.

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5(a), the height H1 of the dead zone with different core column diameters is significantly reduced as the airflow rate increases. In contrast, H2 is less affected by the airflow rate.

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However, H2 increases with increasing the diameter of the core column, whereas H1 is unaffected. The reason is that the left side of the dead zone is close to the tangential inlet, and

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the fluidized particles are mostly located in the left half of the dead zone. In addition, the QH2 curves are all under the initial packing height line because just some of the adsorbents are fluidized.

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The effects of the core column diameter on particle fluidization q that occurs in the cyclonic fluidized bed are shown in Fig. 5(b). All the core columns have the same heights, 22

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mm. As shown in Fig. 3, particle fluidization increases significantly with increasing the airflow rate Q. Particle fluidization increases as the core column diameter increases. The reason is that with the increase in the core column diameter, the volume of the annulus around the core column decreases. Therefore, the gas velocity in the annulus increases, and the amount of fluidized particles increases accordingly.

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Fig. 5 Effect of the core-column diameter on adsorbents fluidization. (a) Effect on the dead zone heights H1 and H2. The dotted lines indicate the initial packing height. (b) Effect on the fluidization factor.

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The particle fluidization that occurs in the cyclonic fluidized bed with different core-

column heights but the same diameter is shown in Fig. 6. The effect of the airflow rate on the

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dead zone heights is similar as that observed in Fig. 5(a). The height of the core column does not significantly affect H1 and H2. In particular, the H2 values of the fluidized beds with

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different core-column heights are very close to each other. This indicates that the crosssectional area of the cyclonic fluidized bed has a greater effect on particle fluidization.

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The effect of the core column height on particle fluidization q in the cyclonic fluidized bed with the same diameters of 11 mm was evaluated. Particle fluidization increases with increasing the core column height. By comparing Fig. 7(a) and Fig. 5(b), we can see that the

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diameter of the core column has a greater influence on particle fluidization than the height of the core column. The gas velocity in the annulus changes significantly when the diameter of

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the core column is changed, so particle fluidization changes significantly. However, the gas velocity in the annulus remains nearly unchanged at different core column heights, which only slightly affects the gas velocity above the annulus.

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Fig. 6 Effect of the core-column height on adsorbents fluidization. (a) Effect on the dead zone heights H1

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and H2. The dotted lines indicate the initial packing height. (b) Effect on the fluidization factor

3.2 Particle self-rotation

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Many factors, such as shear flow and particle-particle collision, lead to the particles selfrotating in the cyclonic fluidized bed. The effect of the core column diameter and height on

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the particle self-rotation speed are shown in Fig. 7(a) and (b), respectively. The particle selfrotation speed steadily increases as the airflow rate increases, which is consistent with

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previous studies[27, 28]. From Fig. 7, it can be seen that the maximum self-rotation speed of adsorbents reaches 1700 rad/s. The self-rotation speed of microspheres detected in this study

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is lower than that detected by Wu et al. [23] in a CFB，but in the same order of magnitude. This may own to low concentration of particles in the CFB.

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Under the same working conditions, the self-rotation speed of the particles increases with increasing the diameter of the core column (Fig. 7(a)). This is due to the increase in the gas velocity in the space around the annulus, which causes the increase in the gas velocity (especially the tangential velocity) in the whole cylinder. Also under the same working conditions, the particle self-rotation speed increases with increasing core column heights (Fig. 7(b)). The gas velocity around the annulus is greater than that above the annulus. In addition, the height of the core column increases, so the average gas velocity in the entire cylinder 16

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space increases.

Fig. 7 Relationship between the particle self-rotation speed and gas flow rate. (a) the core column has

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same height but different diameter; (b) the core column has same diameter but different height

The particle self-rotation velocities at different heights in the cyclonic fluidized bed with

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core column 11-33 are shown in Fig. 8. The self-rotation speed of the particles in the annulus (hp <33 mm) is significantly faster than that of the particles in the upper space (hp >33 mm).

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This is mainly due to the greater tangential velocity of the gas stream in the annulus, resulting in a more intense interaction between the particles or with the wall. From Fig. 7 and Fig. 8, it

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could be seen the self-rotation speed of particles fluctuate greatly, which indicates that the

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high self-rotation speed is induced mainly by the collision of particle-particle or particle-wall.

Fig. 8 Self-rotation of the particles at different heights. The core column is 11-33 with a relative packing 17

height of 0.29.

3.3 Effect of particle motion on VOCs adsorption The adsorption efficiencies of the AC adsorbents in the cyclonic fluidized bed with different core columns are shown in Fig. 9. In this paper, the concentration of the toluene in the inlet air flow was set to 8 ppm, and the amount of the AC adsorbents was 6.25 g for each experiment. It can be seen from Fig. 9 that the adsorption efficiency of the cyclonic fluidized

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bed with different core columns decreases rapidly as the air flow rate increases. This result is same as other adsorbers, such as fluidized bed peat bioreactor [15], rotating packed bed [29] and bubbling fluidized bed [30]. Therefore, the adsorption efficiency is mainly affected by the

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residence time of the airflow contacting the adsorbent particles.

Fig. 9 Adsorption efficiency of the cyclonic fluidized bed reactor. (a) the diameter of core column on

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adsorption efficiency; (b) the height of core column on adsorption efficiency. The packing amount of adsorbents is 6.25 g.

Fig. 9(a) shows that E16-22 > E13-22 > E11-22, which indicates that the adsorption

efficiency of the cyclonic fluidized bed is improved as the diameter of the core column increases under the same airflow rate. Compared to Fig. 7(a), it can be seen when the core column has the same height, improving the self-rotation speed can slightly increase the adsorption efficiency. Fig. 9(b) shows that E11-22 > E11-33 > E11-44, which indicates that the 18

adsorption efficiency increases when the height of the core column decreases. It can be concluded that reducing the annulus volume is beneficial to improving the adsorption efficiency. This could be explained from two aspects. First, reducing the annulus volume leads to some of the adsorbents being transferred to the upper space of the annulus. Due to the axial velocity of the airflow in the upper space is much lower than that in the annulus, VOCs and the adsorbent particles have a longer contact time in the upper space. Second, the large tangential velocity allows for the good fluidization and self-rotation speed of the adsorbent

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particles. The relationship between the adsorption efficiency E and the relative packing height k of the adsorbents in the cyclonic fluidized bed with core column 13-22 is shown in Fig. 10. The adsorption efficiency E gradually increases with increasing the relative packing height k. The

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inlet air flow rate is 1.0 m3/h, which is the lowest of all the conditions and corresponds to a superficial velocity of 0.566 m/s in the cyclonic fluidized bed. The value of superficial

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velocity is similar to that in the bubbling bed for ethanol adsorption with high-wear-resistant asphalt-based spherical-particle activated carbon [30].The maximum adsorption efficiency

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can reach more than 99% when the relative packing height is k=0.65.

Fig. 10 Effect of the relative packing height on the adsorption efficiency. The core column is 13-22, the air flow rate is 1.0 m3/h.

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4 Conclusion A novel cyclonic fluidized bed packed with spherical AC adsorbents is proposed to adsorb VOCs. The fluidization of the adsorbent particles as well as their self-rotation were studied with a high-speed imaging system. The adsorption results show that the cyclonic fluidized bed exhibits a good performance for removing VOCs. From the afore-mentioned results and discussion, the following conclusions may be drawn: (1) The gas flow rate poses the greatest influence on particle fluidization. More packing

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particles are blown to the upper space with an increase in the inlet airflow rate; when Q= 2.5 m3/h, H1 = 0 mm, that means all the adsorbents near the inlet are blown to the upper space. In addition, multiple inlets are needed to significantly improve particle fluidization at the bottom of the cyclonic fluidized bed. The relative packing height must be higher than 0.2, that the

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airflow path changes at the cyclonic fluidized bed inlet and tends to increase horizontal

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tangential motion. The diameter of the core column has a greater influence on particle fluidization than the height of the core column.

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(2) The spherical AC adsorbents self-rotate rapidly in the cyclonic fluidized bed, especially when the inlet airflow rate is increased. The maximum self-rotation speed of adsorbents can reach 1700 rad/s in the experiments. Increasing the core column diameter and

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height will both improve the tangential velocity, which increases the particle-particle and gasparticle interactions. As a result, the particles rotate faster.

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(3) The adsorption efficiency is mainly affected by the residence time of the airflow contacting the adsorbent particles. Reducing the annulus volume is beneficial to improving

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the adsorption efficiency. The maximum adsorption efficiency is higher than 99% when the relative packing height is k=0.65 at inlet flow rate 1.0 m3/h. This paper has preliminarily studied particle fluidization, self-rotation, and the

adsorption performance of VOCs in a cyclonic fluidized bed, and more studies on the particle fluidization characteristics related to adsorption and desorption will be conducted in the future.

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Acknowledgment We would like to express our thanks for the sponsorship of the National Key Research and Development Program of China (2016YFC0204500), National Natural Science Foundation of China (21878099，51608203), Shanghai Sailing Program (19YF1412000) and

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the Fundamental Research Funds for the Central Universities.

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Table 1 Structural parameters of the cyclonic fluidized bed, mm

D2

h1

h2

h3

a

b

13

25

88

126

151

11

8

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D1

24

Table 2 Test operating conditions. The error of inlet airflow rate is within ±0.1 m3/h, and the error of amount of particles is within ±0.001 g.

Amount Flow

Amount Core

Flow

of Number rate (m3/h)

Core of

column

Number rate

particles

column

(m3/h)

structure (g)

particles structure (g)

1.0

6.250

11-22

14

1.5

6.250

13-22

2

1.5

6.250

11-22

15

2.0

6.250

13-22

3

2.0

6.250

11-22

16

2.5

4

2.5

6.250

11-22

17

1.0

5

1.0

6.250

11-33

18

1.5

6

1.5

6.250

11-33

19

2.0

7

2.0

6.250

11-33

20

8

2.5

6.250

11-33

21

9

1.0

6.250

11-44

10

1.5

6.250

11

2.0

6.250

12

2.5

6.250

13

1.0

13-22

6.250

16-22

6.250

16-22

6.250

16-22

2.5

6.250

16-22

2.0

1.250

13-22

22

2.0

3.750

13-22

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re

-p

6.250

23

2.0

6.250

13-22

11-44

24

2.0

8.750

13-22

11-44

25

2.0

11.250

13-22

na

11-44

13-22

Jo

ur

6.250

ro of

1

25