Gas Flow Characteristics in a Rotating Packed Bed by Particle Image

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Gas flow characteristics in a rotating packed bed by Particle Image Velocimetry (PIV) measurement Xueying Gao, Guang-wen Chu, Yi Ouyang, Haikui Zou, Yong Luo, Yang Xiang, and Jian-Feng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03286 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Industrial & Engineering Chemistry Research

Gas flow characteristics in a rotating packed bed by Particle Image Velocimetry (PIV) measurement Xue-Ying Gao 1,2, Guang-Wen Chu1,2, Yi Ouyang1,2, Hai-Kui Zou2, Yong Luo1,2, Yang Xiang1,2*, Jian-Feng Chen1,2* 1. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, P.R. China, 100029 2. Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, P.R. China, 100029

∗Corresponding

authors:

Associate Prof. Y. Xiang ([email protected]), Tel: +86 10 64453979, Fax: +86 10 64434784, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing P.R. China, 100029. Prof. J. F. Chen ([email protected]), Tel: +86 10 64446466, Fax: +86 10 64434784, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing P.R. China, 100029.

1

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Abstract Rotating packed bed (RPB) has drawn wide attention owing to its outstanding advantage in process intensification. In present study, gas flow characteristics in RPB were investigated by particle image velocimetry (PIV) measurement. The velocity and turbulent kinetic energy (TKE) in packing zone were obtained and the effects of various parameters were analyzed. During rotation, gas rapidly acquired tangential velocity, which was dominant in resultant velocity. Mean gas velocity increased with the increase of rotational speed, but was almost independent on gas flow rate and packing size at lower rotational speed. Moreover, mean radial and tangential slip velocities were overall positively affected by gas flow rate and rotational speed, respectively. Higher TKE near the inner packing than those in the bulk zone revealed the existence of gas end-effect zone in RPB. The results could provide theoretical basis for further study on gas-solid catalytic reaction or gas-liquid mass transfer process in RPB. Keywords: Rotating packed bed, Gas flow characteristics, Particle image velocimetry, Gas end-effect zone

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1. Introduction Process intensification1 is the key technique to improve the production efficiency at lower economic costs by greatly reducing the equipment size, energy consumption per unit and environmental pollution. In recent years, high-gravity technology, typically with rotating packed bed (RPB)2-3 and rotating fluidized bed4-6, has drawn wide attention for process intensification. Particularly, RPB has been considered as the promising device due to its outstanding performance in the processes of micromixing7,8, multiphase mass transfer9 and reaction10. Therefore, RPB has been applied to various fields with gas-liquid, liquid-liquid and liquid-solid systems, such as absorption (removal of CO2, H2S and NOx)11,12, desorption13, distillation14, production of nanoparticles15 and polymerization reaction16. Very few studies have been involved in gas-solid system. Chen et al.10 firstly applied RPB to Fischer-Tropsch Synthesis (FTS) reaction experimentally, and found product distribution could be adjusted by high gravity level, indicating that RPB was recommended a suitable option for gas-solid catalytic reaction. In previous study, many researchers focused on liquid-side flow characteristics. Sun and Sang et al.17,18 observed liquid flow pattern and mean droplets size at external cavity by means of high-speed camera, but couldn’t get detailed information inside packing due to the blocking of wire mesh. Yang et al.19 introduced X-ray CT technique to obtain the liquid distribution in the packing zone of RPB, but this technique was inapplicable to gas flow behavior. To our knowledge, few works have been reported on gas-side flow behavior in RPB. Zheng and Sandilya et al.20,21 found gas experienced high frictional drag by packing surface, and acquired the tangential velocity of packing near the entering region. Chandra 3



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and coworkers22-24 proposed a novel split-packing RPB to increase tangential slip velocity and enhance mass transfer rate. However, these studies haven’t analyzed the detailed gasside flow filed in RPB, which is fundamental for gas-liquid mass transfer or gas-solid catalytic reaction processes and needs new technique for further exploration. Up to now, visualization techniques have become powerful method for flow measurement, which undergone the following stage developments: pitot tube, hot-wire anemometer, laser doppler velocimetry (LDV), laser induced fluorescence (LIF), and particle image velocimetry (PIV) reviewed by Mavros25. Among these techniques, PT and HWA are intrusive to flow pattern and can only measure one-point velocity; LDV is first realized nonintrusive measurement, but still limited to one-point measurement; LIF is enough applicative for the flow measurement throughout entire targeted domain, but requires special photochromic dyes to analyze flow pattern, resulting in limited measurement application. Compared with above methods, PIV is the comprehensive technique by virtue of its advantageous features, such as: 1) nonintrusive; 2) the whole target-region (2-D or 3-D) flow pattern obtained; 3) accurate and quantitative analysis of velocity and other parameters, like turbulent kinetic energy (TKE). Thus, PIV method has been widely used to study flow characteristics of various types of reactors: randomly packed porous bed26, stirred tank27, confined impinging jet reactor28, micro-channels reactor29, gas-solid vortex reactor30, etc. For gas flow visualization, Yu et al.31 analyzed air flow behavior in a rotating disk with tracer particles via PIV, and Lam32 investigated vortex-shedding flow behavior behind a rotating cylinder at various initial velocities and rotational speeds by PIV, recommending PIV method to be applied for measurement of gas flow characteristics in a rotating device. 4


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Based on above analysis, a 2-D PIV technique was introduced for the first time to measure gas flow characteristics of RPB in this work. Preliminary exploration was carried out to observe the distributions of velocity and TKE. Also the effects of operating conditions and packing size on gas flow behaviors were analyzed accordingly. Finally, gas end-effect zone was revealed experimentally based on the analysis of TKE values within the operating range. 2. Experimental method 2.1 Experimental setup and process Figure 1 illustrates a schematic representation of visualization measurement for gas flow behaviors in RPB. The entire system mainly consists of two sections: process flow system (black framed) and PIV system (orange framed) as shown in Figure 1b. Air was allowed to enter rotating platform via gas chamber and smoke facility at specific flow rate, and finally outflowed from the external shell of RPB. In PIV system, laser was emitted to illuminate tracer particles on the test plane, a Charge-Coupled Device (CCD) camera captured the images of tracer particles movements, and then all data was stored and post-processed by Davis software system.

(a) 5



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(b) Figure 1. Experimental setup: (a) photo and (b) schematic diagram: 1. Gas chamber, 2. Hand valve, 3. Pressure gauge, 4. Rotameter, 5. Smoke facility, 6. Rotating platform, 7. RPB, 8. Laser device, 9. CCD camera, 10. Camera controller, 11. Davis system (synchronizer inside), 12. Optoelectronic switch, 13. Silver paper, 14. Digital tachometer. 2.2 RPB equipment For the gas-solid reaction system or gas-liquid cocurrent system, the gas flow pattern in RPB was different from that of conventional gas-liquid countercurrent system33. Gas flowed from internal cavity, moved outward to packing zone and discharged through external cavity of RPB. This flow mode could reduce pressure drop owing to obtained kinetic energy from centrifugal force. Considering the feasibility of PIV experiment, the structure of RPB was specially designed by us, which is depicted in Figure 2. The detailed dimensional features of RPB are listed in Table 1. 6


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(a)

(b)

Figure 2. Geometry structure of RPB: (a) radial plane and (b) axial plane (test plane marked-up). Table 1. RPB geometry size Parameters

Symbols

Values

Units

Gas inlet diameter

di

20

mm

Internal cavity diameter

dic

40

mm

Rotor inner diameter

dir

50

mm

Rotor outer diameter

dor

206

mm

Rotor height

h

50

mm

External cavity diameter

doc

300

mm

Gas outlet diameter

do

40

mm

Pore diameter at rotor wall

dpo

6

mm

Perforating ratio at rotor inner wall

ζi

16

%

7



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Perforating ratio at rotor outer wall

ζo

21

%

Packing (glass bead) diameter

dpb

25, 20, 16

mm

Packing porosity

ε

0.517, 0.496, 0.494

-

For the rotor and external shell, polymethylmethacrylate (PMMA) was selected as construction material, which is characterized with higher strength and toughness as well as better light transmission to meet mechanical and optical requirements of PIV measurement. In order to obtain more clear images on the test plane, the glass beads were arranged in strict accordance with the point to point contact between two layers. Considering laser dissipation and arrangement difficulty, three sizes of glass beads with diameters of 25mm, 20mm and 16mm were selected. The corresponding aspect ratio (the rotor width to bead diameter) was 3.12, 3.9, and 4.875, respectively. They were within reasonable range of aspect ratio for PIV measurement26,34,35. Additionally, four cross walls were set to confirm that the beads stayed relatively static when rotor was rotating and five pores were made in the cross walls to ensure gas flowed through the packing. Before conducting PIV measurement, matted black paint was sprayed onto glass beads, inner walls, cross walls of the rotor and cover plate to reduce the error from reflection during emission of laser. 2.3 PIV system and its features The flow visualization measurement was performed using a LaVision PIV system made in Germany, comprising of four main components: Double Pulsed Laser (Nd-YAG, 135mJ, 532nm, 15Hz), a CCD Camera (Imager Pro X 4M, 2048×2048 pixels), Synchronizer (Programmable Timing Unit) and Davis 8.3.0 software. Figure 3 displays the working principle of 2-D PIV system. Firstly, tracer particles were seeded in the test section. After 8


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triggered by rotation signal, lasers were emitted based on the time difference between doublelaser pulses (dt). Simultaneously, the movements of tracer particles were captured by CCD camera, and the recorded data was subjected to Davis system for post-processing to obtain detailed flow information.

Figure 3. Sketch Map of a 2-D PIV system. This PIV system is characterized with five features, which are described below: 1) Appropriate tracer particles: Based on the working principle of PIV technique, the fluid velocity is indirectly measured by particle velocity. Therefore, tracking performance of tracer particles has been examined in order to avoid significant discrepancy between fluid and particle motion. Referring to the PIV experiment conducted by Yu et al.31, smoke particles generated by ignited wormwood were selected to track gas flow behavior. Also the tracking property based on STOKES’ drag law at low Reynolds number were investigated. The velocity in gravitational field is given as36: u

gd p2   p   f



18

(1)

where g is gravitational acceleration, dp is diameter of particle, μ is viscosity of fluid, ρp and ρf are densities of particle and fluid, respectively. 9



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However, in this work the centrifugal force field plays a major role in gas flow behavior and gravity can be neglected. Therefore, ‘g’ can be replaced with ‘ω2r’ in eq. (1) for settling velocity in centrifugal force field which is defined as: u

 2 rd p2   p   f  18

(2)

where ω is rotational speed (rad/s), r is radial coordinate on the test plane (m). Here dp is 2~5μm measured by Yu et.al31, ρp is 14kg/m3 (assumed ten times the density of burning incense smoke particles37), air density ρf is 1.225 kg/m3, and air viscosity μ is 1.79e05kg/(m·s). According to the maximum particle diameter of 5 μm, the centrifugal settling velocities under 100~300rpm were calculated as 8.14e-06 ~ 7.33e-05m/s. The calculated tangential velocity was 0.78~2.35m/s accordingly and the actual gas velocity should be higher than these values. It shows that the centrifugal settling velocity of tracer particle is much smaller than the actual gas velocity, indicating that smoke by ignited wormwood has good tracing property and can be used as tracer particles for gas flow in RPB of this experiment. 2) Trigger signal: For static flow field, the signal of laser emission was normally triggered by internal programming of PIV, and CCD camera simultaneously captured tracer particle images controlled by synchronizer. However, the key point in this experiment was how to synchronize the rotational signal with laser trigger signal and CCD capture signal. The detailed method is illustrated as follows: During rotation, the ‘ON’ signal of optoelectronic switch (12 in Figure 1b) was triggered by the silver paper (13 in Figure 1b) adhesion on the shaft and transmitted to the digital tachometer (14 in Figure 1b). Meanwhile, the digital tachometer converted rotation signal into voltage signal and 10


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output to the PIV system. Then, the synchronizer would synchronize the rotation signal with laser trigger and camera capture instructions (red dot-line in Figure 3). Normally, a complete cycle of rotation triggers laser emission and CCD camera capture once to match synchronized record for steady flow behavior. 3) Test plane: In order to meet the requirement of 2-D PIV measurement, glass beads were regularly stacked to form two layers in the packed bed. In this way, the middle cross section of bead contact between two layers was specified as test plane (see Figure 2b). Thus, the tracer particles on this plane could be illuminated by laser clearly. 4) Reasonable dt: Being a key parameter, the value of dt has a significant effect on the velocity vector, it is calculated by Eq. 338. dt 

ds px  f v  n px

(3)

where dspx is pixel shift which was kept 5 based on empirical value39, f is field of view whose value was 80mm, v is velocity in test section, npx is number of Kpixel with the value of 2.048. The calculated mean velocity among operating conditions was ranged from 0.69 to 2.76m/s, based on which, dt was specified in the range 70~282μs for current experiment. 5) Post-processing method: To analyze the flow field on the test plane, cross-correlation method with 32×32 pixels window was employed and the resolution of velocity vector was set 0.5mm. Then all the velocity vectors in a total field of view could be obtained by post-processing. To minimize the uncertainty of velocity, time-averaged velocity, vav, was applied to flow characteristic measurement (Eq. 4). In this work, a series of images of 50, 100, 200 and 11



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300 were captured under the same operating conditions. The deviation of vav between 200 and 300 images was less than 2% in main flow field. Thus, the time-averaged velocity with 200 images was considered as criterion to analyze the gas flow behavior at each position in RPB.

1 n  vj n j 1

vav 

(4)

Where n is image number, v and vav are local velocity on each image and time-averaged velocity of all images, respectively. Owing to rotational motion and complex flow channel by bead-packing structure, there was turbulent flow of gas inside RPB packing. The averaged turbulent kinetic energy (TKE), k, was proposed to analyze flow behavior and defined as:

k '



1 '2 '2 '2 v x  v y  vz 2



(5)

'

' where vx , v y , v z are fluctuating velocity (calculated by v '  v j  vav ) for x, y, z direction,

respectively. Considering the velocity (z-direction) perpendicular to the test plane (x, y-directions) cannot be measured directly by 2-D PIV measurement, it was assumed that TKE value in z-direction contained a similar contribution as those in x- and y-directions27,28,38. Therefore, Eq.5 can be reformed to Eq.6 as the calculation of TKE values in RPB. k



3 '2 vx  v'y2 4



(6)

2.4 Operating conditions The PIV experiment was conducted at room temperature (15±1°C) and atmospheric pressure. Considering the mechanical strength of PMMA material, the rotational speed was 12


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varied from 100~300 rpm, and the gas flow rate was ranged from 1m3/h to 4m3/h based on the limitation of PIV measurement. After the RPB system reached the steady state when rotated for 3~5min, laser was emitted and tracer particle movements were captured by CCD camera. For each condition, three runs were conducted with the maximum deviation ±5% to ensure good reproducibility of experimental data and its accuracy. 3. Results and Discussion 3.1 PIV data validation PIV image of tracer particles on the test plane inside bead-25mm packed bed is given in Figure 4. Based on PIV post-processing algorithm, velocity vectors of each interrogation window were obtained via particle displacement divided by time interval between two consecutive laser pulses.

Figure 4. PIV image of bead-25mm packed bed. Due to complicated flow characteristics in rotating bead-packed bed, PIV data was validated by theoretical values within the operating range. In RPB, there existed two directions of velocity: radial velocity vr and tangential velocity vt calculated by Eq. 7 and Eq. 8, respectively: 13



vr 

V 2 rh

(7)

where V is gas inlet flow rate (m3/h), r is radial coordinate of packing (m), h is packing height (m), and ε is packing porosity;

2 N r 60

vt   r 

(8)

where ω and N are rotational speed with units of rad/s and rpm, respectively. These calculations were based on two assumptions: (i) packing porosity at each r was equal to overall packing porosity; (ii) gas was synchronized with rotating packing along tangential direction. Then the calculated local velocity was expressed as: v

vr 2  vt 2

(9)

In this way, the theoretical values obtained via above calculations were compared with PIV data in the operating range in Figure 5. A noteworthy degree of agreement between them was achieved within deviation of ±15%, demonstrating that gas velocity measured by PIV was credible. 3.5 3.0 2.5

Calculated, v (m/s)

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+15% 2.0 1.5

-15% 1.0 0.5 0.5

1.0

1.5

2.0

2.5

3.0

3.5

Experimental, v (m/s)

Figure 5. PIV experimental data validation with calculated values. 14


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3.2 Flow visualization 3.2.1 Distribution of velocity field Figure 6a and 6b exemplify velocity vectors in RPB operated with gas flow rate of 1m3/h at N=0 and N=200rpm, respectively. Typical zones were selected along radial (Zone 1~4) and tangential direction (Zone 3, 5, 6) for detailed analysis. Under the static condition of RPB (N=0), gas flowing into bead-packed bed mainly depended on initial inertia force and all the velocity magnitude inside bead-packing was relatively low. Additionally, blocking effect of the beads caused the flow channel to be tortuous, which resulted in gas velocity maldistribution through the whole packed bed. As shown in Figure 6b, velocity vector profile at fixed rotational speed (N=200rpm) is obviously different from that in Figure 6a. Gas from internal cavity rapidly acquired tangential velocity after it entered the inner edge of bed (Zone 1) owing to centrifugal force and Coriolis force40. In the bulk zone (Zone 2~6), velocity magnitude was gradually increased along the radial direction, but it was found to be roughly the same along tangential direction. Therefore, the quantitative analysis in following section was focused on the flow field distributions along radial direction.

(a) 15



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(b) Figure 6. Velocity field of bead-25mm packed bed at different operating conditions: (a) N=0, V=1m3/h and (b) N=200rpm, V=1m3/h. 3.2.2 Distribution of TKE field Figure 7a and 7b present the distributions of TKE field with gas flow rate of 1m3/h at N=0 and N=200rpm, respectively. The TKE values in static RPB were found relatively lower than those in rotating bed. In both cases, the values of TKE near the inner packing were higher than those in the bulk zone due to occurrence of the violent collision between gas and packing. Figure 7c shows mean TKE value in each zone, km, along radial direction for Figure 7a and 7b. The TKE values in rotating bed were obviously higher than those in static bed, especially near the inner packing due to larger tangential slip velocity. This indicated the rotation was highly and positively influential for TKE, which could enhance gas-side mass transfer efficient or reduce gas external diffusion resistance on gas-solid catalytic reaction.

16


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(a)

(b) 0.06 N=0; V=1m3/h N=200rpm; V=1m3/h

0.05 0.04

km (m2/s2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0020 0.0015 0.0010 0.0005 0.0000

0.03 0.02

70

75

80

85

0.01 0.00 30

40

50

60

70

80

90

r (mm)

(c) 17


100


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Figure 7. TKE field of bead-25mm packed bed at different operating conditions: (a) N=0, V=1m3/h, (b) N=200rpm, V=1m3/h and (c) Mean TKE value along radial direction for (a) and (b). 3.3 Effects of various parameters on gas flow behavior 3.3.1 Rotational speed In order to quantitatively analyze the effects of various parameters on gas flow behavior, typical zones (Zone1~4) and mean values (vm, vsrm, vstm and km) of each zone were considered in following discussion. Fig. 8a-c present mean gas velocity, radial and tangential slip velocity at different rotational speeds, respectively. Mean gas velocity almost linearly increased with increasing radial coordinate or rotational speed, demonstrating its dependence on tangential velocity. The similar results of liquid velocity in RPB were also obtained in experimental study of Yang et al.41 and CFD study of Shi et al.42. Furthermore, mean radial slip velocity (vsrm) was not linearly related with radial coordinate owing to complex interaction between centrifugal force and resistance of bead-packing, and it was not obviously affected by rotational speed. However, mean tangential slip velocity (vstm) overall decreased along radial direction, indicating gas gradually synchronized with rotating packing. Thus, rotational speed had obviously positive effect on vstm near the inner packing but little effect in bulk packing zone. In Figure 8d, higher TKE near inner packing was obtained at the higher rotational speed. This happened because of the higher velocity fluctuation resulted by larger tangential slip velocity of RPB. However, in the bulk region of bed, the rotational speed was seen with smaller effect on TKE because of lower degree of disturbance and fluctuation in this zone caused by the smaller radial and tangential slip velocity between gas and rotating 18


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packing. 3.5 N=100rpm, V=4m3/h N=200rpm, V=4m3/h N=300rpm, V=4m3/h

3.0

vm (m/s)

2.5 2.0 1.5 1.0 0.5 30

40

50

60

70

80

90

100

r (mm)

(a) 0.40 N=100rpm, V=4m3/h N=200rpm, V=4m3/h N=300rpm, V=4m3/h

0.35 0.30

vsrm (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.25 0.20 0.15 0.10 0.05 0.00

30

40

50

60

70

80

90

r (mm)

(b)

19


100


0.40 N=100rpm, V=4m3/h N=200rpm, V=4m3/h N=300rpm, V=4m3/h

0.35

vstm (m/s)

0.30 0.25 0.20 0.15 0.10 0.05 0.00 30

40

50

60

70

80

90

100

r (mm)

(c)

0.16

N=100rpm, V=4m3/h N=200rpm, V=4m3/h N=300rpm, V=4m3/h

0.14 0.12

km (m2/s2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.020 0.015 0.010 0.005 0.000

0.10 0.08 0.06

70

75

80

85

0.04 0.02 0.00 30

40

50

60

70

80

90

100

r (mm)

(d) Figure 8. (a) Mean gas velocity, (b) Mean radial slip velocity, (c) Mean tangential slip velocity and (d) TKE of bead-25mm packed bed at various rotational speeds. 3.3.2 Gas flow rate Fig. 9a-c show mean gas velocity, radial and tangential slip velocity at different gas flow rates, respectively. Gas flow rate had little influence on mean gas velocity and tangential slip velocity in all zones, but had positive influence on mean radial slip velocity. Moreover, gas flow rate was observed to affect the TKE near inner packing positively (Figure 9d), similar 20


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with the effect of rotational speed on TKE. It might be engendered by higher gas flow rate leading to the greater fluctuation velocity of radial direction and higher averaged turbulent kinetic energy near the inner packing of RPB. 2.5 N=200rpm, V=1m3/h N=200rpm, V=2m3/h N=200rpm, V=4m3/h

2.0

vm (m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

1.0

0.5 30

40

50

60

70

80

90

100

r (mm)

(a) 0.40 N=200rpm, V=1m3/h N=200rpm, V=2m3/h N=200rpm, V=4m3/h

0.35 0.30

vsrm (m/s)

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0.25 0.20 0.15 0.10 0.05 0.00 30

40

50

60

70

80

90

r (mm)

(b)

21


100


0.40 N=200rpm, V=1m3/h N=200rpm, V=2m3/h N=200rpm, V=4m3/h

0.35

vstm (m/s)

0.30 0.25 0.20 0.15 0.10 0.05 0.00 30

40

50

60

70

80

90

100

r (mm)

(c) 0.14 N=200rpm, V=1m3/h N=200rpm, V=2m3/h N=200rpm, V=4m3/h

0.12 0.10

km (m2/s2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.08

0.012 0.008

0.06

0.004 0.000

0.04

70

75

80

85

0.02 0.00 30

40

50

60

70

80

90

100

r (mm)

(d) Figure 9. (a) Mean gas velocity, (b) Mean radial slip velocity, (c) Mean tangential slip velocity and (d) TKE of bead-25mm packed bed at various gas flow rates. 3.3.3 Packing size Figure 10a shows more detailed flow information of bead-16mm packed bed compared to that of bead-25mm packed bed (Figure 4). As shown in Figure 10b, the velocity magnitude increased along radial direction and remained almost the same along tangential direction. Also the velocity direction was close to tangential direction, manifesting the dominance of tangential velocity in resultant velocity. Figure 10c shows higher TKE near inner packing 22


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than those of bulk zone. The similar trend was found in bead-25mm packed bed as given in Figure 6b and 7b.

(a)

(b)

23



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(c) Figure 10. (a) PIV image, (b) velocity field, and (c) TKE field of bead-16mm packed bed (N=200rpm, V=1m3/h). Quantitative analysis of mean gas velocity and TKE in three different size packing is depicted in Figure 11. The gas velocities of bead-16mm and bead-20mm packed bed are similar with those of bead-25mm packed bed at lower rotational speed. That means packing size has no significant effect on gas velocity in this case. The TKE values of packed bed increased with the decrease of bead size, probably because more bead surfaces with smallersize packing could provide more tortuous channel and more disturbance to the gas flow pattern. Therefore, larger fluctuating velocity would arise and lead to higher turbulent kinetic energy.

24


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N=100rpm, V=4m3/h Bead-25mm Bead-20mm Bead-16mm

1.4

vm (m/s)

1.2 1.0 0.8 0.6 0.4 30

40

50

60

70

80

90

100

r (mm)

(a)

N=100rpm, V=4m3/h Bead-25mm Bead-20mm Bead-16mm

0.5 0.4 km (m2/s2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2 0.1 0.0 30

40

50

60

70

80

90

100

r (mm)

(b)

Figure 11. (a) Velocity and (b) TKE along radial coordinate with different packing sizes. 3.4 TKE analysis In previous studies, Dudukovic et al.43 observed a liquid-side end-effect zone near the inner bead-packing in RPB. Guo et al.9 employed oxygen-water system to measure gas-liquid mass-transfer coefficient and found the mass-transfer contribution of the liquid-side endeffect zone was much higher than those in other regions. However, very few studies have 25



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been reported on gas-side end-effect zone. Liu et al.44 conducted 3D simulation of a RPB with wire-mesh packing by CFD method, and found gas-side end-effect zone near the outer packing of RPB. Reddy et al.23 found that a novel-structured RPB with split-packing could artificially create multiple zones significantly increasing the gas-side mass transfer coefficient. But to our knowledge, the phenomenon of gas end-effect zone in RPB with beadpacking has not been experimental validated. As already mentioned, the TKE values near the inner packing were higher than those in bulk packing zone. In order to analyze the TKE values in all operating conditions, two dimensionless Reynolds numbers — ReG and Reω were proposed based on the competition between initial inertia force and rotation-shear force (mainly centrifugal force and Coriolis force). The formulas were expressed as: ReG 

d pbU 0 

(10)



where ReG is related with gas inlet flow rate and bead-packing size, dpb is bead diameter (m), U 0 is

the superficial velocity given by U 0 

V (m/s); 2 Ri h

and

Re 

R2 

(11)

where Reω is characterized by the magnitude of rotation-shear force, ω is rotational speed (rad/s), R is equivalent length of packed bed given by R 

Ri2  Ro2 (m). The calculated 2

ReG and Reω for bead-25mm packed bed in the operating range are listed in Table 2. Table 2. The values of ReG and Reω of bead-25mm packed bed Case 1 2 3

V(m3/h) 1 2 4

N(rpm) 100 100 100

ReG 61 121 242

26


Reω 4025 4025 4025

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4 5 6 7 8 9

1 2 4 1 2 4

200 200 200 300 300 300

61 121 242 61 121 242

8050 8050 8050 12074 12074 12074

Figure 12a illustrates the values of TKE in all zones of RPB at various ReG and Reω. Generally, for all cases, the TKE values in Zone 1 are obviously higher than those in other zones. This fact revealed the existence of a gas-side end-effect zone near the inner packing of RPB, which probably affected the gas-side mass transfer coefficient or the reaction rate. The reason was speculated that in this zone, the maximum slip velocity between gas and rotating bed resulted in highest fluctuating velocity and turbulent intensity near the inner packing of RPB. This could reduce the thickness of boundary layer on bead surface and gasside mass transfer resistance so as to enhance gas-side mass transfer coefficient. Moreover, this gas end-effect could give good explanation for the intensification of gas-side mass transfer performance of split-packing RPB and provide basis for design of more novel-type RPB. In addition, the TKE in Zone 1 at various operating conditions was compared and it was found that the higher values of TKE were obtained under the condition of ReG > 121 and Reω > 8050 (Figure 12b). The increase in ReG and Reω could lead to more turbulent flow pattern and larger thickness of gas end-effect zone of RPB, which is worthy of further investigation in the future work. It was noted that the similar TKE behavior showed in bead20mm and bead-16mm packed beds.

27



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(a)

(b)

Figure 12. TKE values in (a) each zone and (b) Zone 1 of bead-25mm packed bed in the operating range. 4. Conclusions In this study, gas flow characteristics in a RPB with glass-bead packing were investigated by PIV measurement for the first time. The distributions of velocity and TKE in packing zone were obtained. It was observed that as a result of RPB rotation, gas rapidly acquired tangential velocity, which was dominant in resultant velocity; mean gas velocity gradually 28


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increased along the radial direction but remained almost the same along tangential direction. The rotation of bed was observed to be a vital parameter regarding the TKE, which resulted a significant rise in TKE as compared with static bed. The effects of various parameters including rotational speed, gas flow rate and beadpacking size on flow behaviors were quantitatively analyzed. The velocity increased with the increase of rotational speed. Gas flow rate was found an insignificant parameter since the velocity had negligible dependence on it. Under the condition of lower rotational speed and higher gas flow rate, gas velocity was not obviously affected by packing size. Moreover, mean tangential and radial slip velocities were overall positively affected by rotational speed and gas flow rate, respectively. The TKE was descried to be positively dependent on rotational speed and gas flow rate but negatively dependent on packing size near the inner packing of RPB. Two dimensionless Reynolds numbers were proposed to analyze TKE within operating range. The result showed TKE values near the inner packing were higher than those in the bulk zone at the given operating conditions. This fact revealed the existence of gas end-effect zone in RPB, which could probably affect gas-side mass transfer coefficient or the reaction rate in gas-solid catalytic reaction. Despite the limitation of PIV experiment with respect to the operating range and packing size, the present results are reliable enough to extend this study for detailed flow information inside the entire bead-packing bed. Also based on this, a corresponding optimization strategy for flow characteristics at various operating conditions can be proposed in further study.

29



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Acknowledgement This work was supported by the National Natural Science Foundation of China (No.21620102007, U1462127) and National Key Research and Development Plan (No.2016YFB0301501).

Nomenclature di = Gas inlet diameter (mm) dic = Internal cavity diameter (mm) dir = Rotor inner diameter (mm) do = Gas outlet diameter (mm) doc = External cavity diameter (mm) dor = Rotor outer diameter (mm) dpb = Packing (glass bead) diameter (mm) dpo = Pore diameter at rotor wall (mm) dp= Tracer particle diameter (μm) dspx = Pixel shift dt = Time difference between double-laser pulses (μs) f = Field of view (mm) h = Packing height (mm) k = Value of turbulent kinetic energy (m2/s2) km = Mean Value of turbulent kinetic energy in each zone (m2/s2) N = Rotational speed (rpm) 30


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n = Image number npx = Number of Kpixel P= Operating pressure (kPa) r= Radial coordinate of RPB (m) R = Equivalent radius of RPB (m)

Ri= Rotor inner radius (m) Ro= Rotor outer radius (m) ReG= Reynolds number of gas flow ( ReG  d pbU 0  f  ) Re= Reynolds number of rotating packing ( Re   R 2  f /  ) T= Operating temperature (°C) u = settling velocity of tracer particles (m/s) U0 = Gas superficial velocity (m/s) V = Gas inlet flow rate (m3/h) v = Local velocity (m/s) v' = Fluctuating velocity (m/s) vav = Time-averaged velocity of multiple image (m/s) vj = Local velocity of each image (m/s) vm = Mean velocity in each zone (m/s) vr = Gas radial velocity (m/s) vsrm = Mean radial slip velocity between gas and packing (m/s) vstm = Mean tangential slip velocity between gas and packing (m/s) vt= Gas tangential velocity (m/s) 31



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Greek letters

 = Packing porosity ζi = Perforating ratio at rotor inner wall (%) ζo = Perforating ratio at rotor outer wall (%)

 = Angular velocity of RPB (rad/s)  = Density, kg/m3 μ = Viscosity of fluid, kg/(m·s) Subscript c = Cavity of RPB f = Fluid in bed i = Inlet or inner of RPB j= Each image number o= Outlet or outer of RPB p= Tracer particle References (1) Green, A. Process Intensification : The Key to Survival in Global Markets. Chem. Ind. 1998, 5, 168. (2) Ramshaw C., Mallinson R. H. Mass transfer apparatus and process. U.S. Patent 4400275. 1983. (3) Rao, D. P.; Bhowal, A.; Goswami, P. S. Process Intensification in Rotating Packed Beds (HIGEE): An Appraisal. Ind. Eng. Chem. Res. 2004, 43, 1150. 32


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(4) Ahmadzadeh,

A.; Arastoopour, H.; Teymour, F. Numerical Simulation of Gas and

Particle Flow in a Rotating Fluidized Bed. Ind. Eng. Chem. Res. 2003, 42, 2627 (5) Trujillo, W. R.; De Wilde, J. Influence of Solids Outlets and the Gas Inlet Design on the Generation of a Gas-Solids Rotating Fluidized Bed in a Vortex Chamber for Different Types of Particles. Chem. Eng. Sci. 2017, 173, 74. (6) Verma, V.; Li, T.; De Wilde, J. Coarse-Grained Discrete Particle Simulations of Particle Segregation in Rotating Fluidized Beds in Vortex Chambers. Powder Technol. 2017, 318, 282. (7) Jiao, W. Z.; Liu, Y. Z.; Qi, G. S. Micromixing Efficiency of Viscous Media in Novel Impinging Stream-Rotating Packed Bed Reactor. Ind. Eng. Chem. Res. 2012, 51, 7113. (8) Guo, T. Y.; Shi, X.; Chu, G. W.; Xiang, Y.; Wen, L. X.; Chen, J. F. Computational Fluid Dynamics Analysis of the Micromixing Efficiency in a Rotating-Packed-Bed Reactor. Ind. Eng. Chem. Res. 2016, 55, 4856. (9) Guo, K. A Study on Liquid Flowing Inside the Higee Rotor. Ph.D. Dissertation, Beijing University of Chemical Technology, 1996. (10) Chen, J. F.; Liu, Y.; Zhang, Y. Control of Product Distribution of Fischer−Tropsch Synthesis with a Novel Rotating Packed-Bed Reactor: From Diesel to Light Olefins. Ind. Eng. Chem. Res. 2012, 51, 8700. (11) Qian, Z.; Xu, L. B.; Li, Z. H.; Li, H.; Guo, K. Selective Absorption of H2S from a Gas Mixture with CO2 by Aqueous N-Methyldiethanolamine in a Rotating Packed Bed. Ind. Eng. Chem. Res. 2010, 49, 6196. (12) Sun, B. C.; Sheng, M. P.; Gao, W. L.; Zhang, L. L.; Arowo, M.; Liang, Y.; Shao, L.; 33



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Chu, G. W.; Zou, H. K.; Chen, J. F. Absorption of Nitrogen Oxides into Sodium Hydroxide Solution in a Rotating Packed Bed with Preoxidation by Ozone. Energy Fuels 2017, 31, 11019. (13) Tan, C. S.; Lee, P. L. Supercritical CO2 Desorption of Toluene from Activated Carbon in Rotating Packed Bed. J. Supercrit. Fluids. 2008, 46, 99. (14) Lin, C. C. Distillation in a Rotating Packed Bed. J.Chem.Eng.Jpn. 2002, 35, 1298. (15) Lin, C. C.; Lin, C. C. Feasibility of Using a Rotating Packed Bed with Blade Packings to Produce ZnO Nanoparticles. Powder Technol. 2017, 313, 60. (16) Chen, J. F.; Gao, H.; Zou, H. K.; Chu, G. W.; Zhang, L.; Shao, L.; Xiang, Y. Cationic Polymerization in Rotating Packed Bed Reactor : Experimental and Modeling. AIChE J. 2010, 56, 1053. (17) Sun, R. L.; Xiang, Y.; Yang, Y. C.; Zou, H. K.; Chu, G. W.; Shao, L.; Chen, J. F. Visualization Study of Fluid Flow in a Rotating Packed Bed. J. Chem. Eng. Chin. Univ. 2013, 3, 411. (18) Sang, L.; Luo, Y.; Chu, G. W.; Zhang, J. P.; Xiang, Y.; Chen, J. F. Liquid Flow Pattern Transition, Droplet Diameter and Size Distribution in the Cavity Zone of a Rotating Packed Bed: A Visual Study. Chem. Eng. Sci. 2017, 158, 429. (19) Yang, Y. C.; Xiang, Y.; Chu, G. W.; Zou, H. K.; Luo, Y.; Arowo, M.; Chen, J. F. A Noninvasive X-Ray Technique for Determination of Liquid Holdup in a Rotating Packed Bed. Chem. Eng. Sci. 2015, 138, 244. (20) Zheng, C.; Guo, K.; Feng, Y. D.; Yang, C.; Gardner, N. C. Pressure Drop of Centripetal Gas Flow through Rotating Beds. Ind. Eng. Chem. Res. 2000, 39, 829. 34


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and Mass Transfer Between Two Coaxially Rotating Disks. Numer. Heat Tr. 2001, 39, 285. (41) Yang K. Study on Micromixing and Gas–Liquid Mass Transfer Characteristic in Rotating Packed Bed. Ph.D. Dissertation, Beijing University of Chemical Engineering, 2010. (42) Shi, X.; Xiang, Y.; Wen, L. X.; Chen, J. F. CFD Analysis of Liquid Phase Flow in a Rotating Packed Bed Reactor. Chem. Eng. J. 2013, 228, 1040. (43) Munjal, S.; Duduković, M. P.; Ramachandran, P. Mass-Transfer in Rotating Packed beds—II. Experimental Results and Comparison with Theory and Gravity Flow. Chem. Eng. Sci. 1989, 44, 2257. (44) Liu, Y.; Luo, Y.; Chu, G. W.; Luo, J. Z.; Arowo, M.; Chen, J. F. 3D Numerical Simulation of a Rotating Packed Bed with Structured Stainless Steel Wire Mesh Packing. Chem. Eng. Sci. 2017, 170, 365. Figure captions Figure 1. Experimental setup: (a) photo and (b) schematic diagram Figure 2. Geometry structure of RPB: (a) radial plane and (b) axial plane (test plane markedup). Figure 3. Sketch Map of a 2-D PIV system. Figure 4. PIV image of bead-25mm packed bed. Figure 5. PIV experimental data validation with calculated values. Figure 6. Velocity field of bead-25mm packed bed at different operating conditions: (a) N=0, V=1m3/h and (b) N=200rpm, V=1m3/h. Figure 7. TKE field of bead-25mm packed bed at different operating conditions: (a) N=0, V=1m3/h, (b) N=200rpm, V=1m3/h and (c) Mean TKE value along radial direction for (a) and 37



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(b). Figure 8. (a) Mean gas velocity, (b) Mean radial slip velocity, (c) Mean tangential slip velocity and (d) TKE of bead-25mm packed bed at various rotational speeds. Figure 9. (a) Mean gas velocity, (b) Mean radial slip velocity, (c) Mean tangential slip velocity and (d) TKE of bead-25mm packed bed at various gas flow rates. Figure 10. (a) PIV image, (b) velocity field, and (c) TKE field of bead-16mm packed bed (N=200rpm, V=1m3/h). Figure 11. (a) Velocity and (b) TKE along radial coordinate with different packing sizes. Figure 12. TKE values in (a) each zone and (b) Zone 1 of bead-25mm packed bed in the operating range. Table legends Table 1. RPB geometry size. Table 2. The values of ReG and Reω of bead-25mm packed bed.

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For Table of Contents Only

39


Gas Flow Characteristics in a Rotating Packed Bed by Particle Image

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