Gas-side Mass Transfer in a Rotating Packed Bed with Structured

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Gas-side Mass Transfer in a Rotating Packed Bed with Structured Nickel Foam Packing Meng-Jun Su, Yong Luo, Guang-Wen Chu, Wei Liu, Xiao-Hua Zheng, and Jian-Feng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00269 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Industrial & Engineering Chemistry Research

Gas-side Mass Transfer in a Rotating Packed Bed with Structured Nickel Foam Packing

Meng-Jun Su1,2, Yong Luo1,2,*, Guang-Wen Chu1,2,*, Wei Liu1,2, Xiao-Hua Zheng1,2, Jian-Feng Chen1,2

1

State Key Laboratory of Organic-Inorganic Composites and 2Research Center of the

Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China

* Corresponding author. Tel: +86 10 64446466; Fax: +86 10 64434784. E-mail address: [email protected]; [email protected]

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ABSTRACT In this work, the gas-side mass transfer was investigated in a rotating packed bed (RPB) with structured nickel foam packings, with different surface properties, either a non-modified nickel foam packing (NNP) or a hydrophobic surface-modified nickel foam packing (SNP). Effects on gas-side volumetric mass transfer coefficient (kGae) of rotational speed, liquid flow rate, and gas flow rate were experimentally studied in the RPB with NNP and SNP. It can be found that the RPB loaded with SNP has a higher mass transfer efficiency, compared with that loaded with NNP. Based on the experimental data, correlation of kGae in a RPB with structured nickel foam packing was proposed, which can hence provide a fast calculation of mass transfer coefficients for the RPB design with structured nickel foam packing.

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1. Introduction

A rotating packed bed (RPB), which uses the centrifugal field to simulate the high gravity environment, is a typical example of the process intensification separators1,2 or reactors3,4 for the chemical industry. In a RPB, liquids are broken up into numerous tiny liquid elements, such as droplets, films, and ligaments, by the rotating packing, which can considerably enhance the efficiency of micromixing and mass transfer processes.5,6 Mass transfer parameters such as the liquid-side volumetric mass transfer coefficient (kLae), gas-liquid effective interfacial area (ae), and gas-side volumetric mass transfer coefficient (kGae), are of great importance to describe the mass transfer behaviors for different packings. The liquid-side mass transfer controlled process has been studied widely, and correlations of kLae and ae were developed from conceptual model or fitted from experiential values with various packings.7-9 But gas-side mass transfer controlled processes in a RPB are studied much less. Kelleher and Fair10 researched kGae for the distillation process in a RPB with the metal spongelike packing and a correlation of kGae was proposed. Rao et al.11 investigated the gas-side mass-transfer coefficient in a two-motor RPB with split Ni-Cr metal foam packing. They measured kGae based on the SO2-NaOH system. The two-motor RPB used has two rotating disks driven by two motors which can rotate in the co-rotation or counter-rotation direction. Results of kGae were fitted for co-rotation and counter-rotation operating conditions, respectively. Chen12 calculated kGae using the two-film theory and an empirical equation for kGae was therefore proposed. Benefiting from the study of bionics, the micro surface morphology and chemical composites of materials can influence the liquid flow pattern on its surface.13 It inspired our curiosity that whether the chemical processes can be affected by

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surface modification of packing in a RPB. In our recent work, we have studied the micromixng and liquid-side mass transfer property of non-modified structure nickel foam packing (NNP) and surface-modified nickel foam packing (SNP) in a RPB. Results indicated that the SNP can enhance the efficiency of micromixing and liquid-side controlled mass transfer process.14,15 The studies of the gas-side mass transfer process in a RPB packed with NNP and SNP was rare. In this work, an experimental study on the gas-side mass transfer in a RPB with NNP and SNP using a SO2-NaOH system was presented. Effects of rotational speed, liquid flow rate, and gas flow rate on kGae were examined. In addition, in order to estimate the efficiency for different processes, comparison of the gas-side mass transfer controlled process and the liquid-side mass transfer controlled process for NNP and SNP were also made. Correlations of mass transfer coefficients with structured nickel foam packing were proposed for a RPB, which is essential for the RPB design applying for mass transfer processes.

2. Experimental 2.1 Equipment The main structure of the RPB includes a motor, a casing, a rotor, a liquid distributor, a liquid inlet/outlet, a gas inlet/out, and a structured nickel foam packing. The static casing has an outer diameter of 150 mm and its height is 40 mm. The inner and outer diameter of the structured nickel foam packing are 35 and 80 mm, and the height is 18 mm. The packing (40 ppi) has a specific surface area of 2916 m2/m3 and porosity of 0.96. The hydrophobic modification was conducted according to the method developed by Li et al.16 Water contact angle on the surface of SNP was measured by a DSA-30 optical contact angle meter (Krüss Company Ltd., Germany)

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and the average value was about 126.1° 2.2 Experimental procedure and calculation The system of SO2 absorbed by an aqueous solution of NaOH was used to determine the gas-side mass transfer coefficient in a RPB.17,18 Figure 1 shows the schematics of the experimental setup for the mass transfer measurements by using a SO2-NaOH system. The SO2 concentration of SO2-N2 mixed gas was about 300 ppm and the concentration of NaOH solution was about 0.1 mol/L. The liquid was introduced into the RPB by a liquid distributor with four holes (1 mm diameter) and the liquid flows outward through the packing by the centrifugal force. Then liquid droplets were sprayed onto the stationary housing and leaving the RPB at the liquid outlet. The gas was fed into the RPB at the gas inlet and flowing across the packing from the outer edge to inner edge of the rotor. As a result, counter-current gas-liquid contacting was achieved inside the RPB. Two SO2 analyzers (KM940, Kane Quintox Co., UK) were employed to measure the SO2 concentration at the gas inlet and outlet, respectively. Its measurement accuracy is 1 ppm. The liquid samples were collected at the liquid inlet and outlet, respectively. The total reaction of SO2-NaOH system can be expressed as:

2 NaOH + SO2 → Na 2SO3 + H 2O

(1)

Then, the gas-side volumetric mass transfer coefficient can be obtained by the following formula: k G ae =

 C G in QG ln  2 π ( r − r1 ) h  C G out 2 2

  

(2)

3. Results and discussion 3.1 Gas-side volumetric mass transfer coefficient (kGae) 5



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Figure 2 illustrates the effects of rotational speed, liquid flow rate, and gas flow rate on kGae of the RPB with NNP and SNP. It can be seen that the values of kGae increased with the increase of the rotational speed and liquid flow rate as presented in Figures 2(a) and (b). The values of kGae for SNP maximally increased by 10.44% and 10.70% compared to NNP when the rotational speed at 2000 r/min and the liquid flow rate at 8.4 L/h (the liquid velocities at the inner periphery is about 0.74 m/s). The previous work of imaging study of the liquid flow demonstrated that more gas-liquid interfacial surface area might be generated, due to the violent collisions between liquid and the mirco-nano surface of SNP.15 When the gas flow rate increased from 1000 L/h to 1800 L/h (the gas velocities at the outer periphery ranging from 0.06-0.11 m/s), the values of kGae for SNP maximally increased by 11.73% compared to NNP in Figure 2(c). These results may be attributed to the violent turbulence of the gas phase in the complex and porous nickel foam structure. 3.2 Comparisons of kGae, kLae, and ae From our previous studies of the mass transfer experiments with liquid-side controlled system, the hydrophobic surface-modified structured nickel foam packing can promote the liquid-side volumetric mass transfer coefficients.15 It is necessary to illustrate the effect on the two mass transfer controlled processes with the two kinds of structured nickel foam packing. Table 1 shows the experimental data under the conditions of different rotational speeds, liquid flow rates, and gas flow rates for kGae, kLae, and ae. It can be calculated that the values of kGae maximally increased by 11.73% for the RPB with SNP, compared to that with NNP under different operation conditions. Nonetheless, the maximum enhancement rate of kLae was 94.43%.15 It indicates that the surface-modified hydrophobic structured nickel foam packing can significantly enhance the liquid-side mass transfer controlled process, but the

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influence on the gas-side mass transfer controlled process is relatively weak. This can be explained by the fact that the special hydrophobicity micro-nano structure of SNP has a significant influence on the liquid flow pattern in the micro channels inside the packing with the centrifugal force and then promotes the gas-liquid phase mixing efficiency, but which has little influence on the macro gas flow. The intensification of the gas-side mass transfer controlled process is largely caused by the increase of ae15 and the enhancement of the gas-side mass transfer coefficient (kG) is not obvious. Therefore, we can further promote liquid-side mass transfer efficiency through micro-nano modification on the surface of packing. For the gas-side mass transfer process, changing the macro structure of the packing to promote the turbulence of gas flow is suggested. 3.3 Correlations Some correlations of kGae in a RPB with different packings have been proposed,10-12 which could provide a reference for the RPB design. Recent studies have found that the liquid flow behavior and surface property of packings can also influence the mass transfer processes.7,8 Therefore, a modified correlation for kGae, which includes the influence of liquid flow, gas flow, and packing’s surface property, needs to be proposed. The surface tension (σ) of 0.1 mol/L NaOH solution is about 72 mN/m at 25°C.8 Regarding the packing’s surface property of hydrophobicity, the critical surface tension (σc) was taken into consideration for the correlation of kGae. The value of σc of nickel is about 75 mN/m, and the surface-modified nickel is about 28 mN/m.16,19,20 Based on the above experimental results, the fitted correlation of gas-side volumetric mass transfer coefficient for the RPB with structured nickel foam packing has the form as Eq. 3.

7



kG ae d p DG a t

−1

= 4.56 × 10 Re

0.59 L

0.63 G

Re We

0.07 L

−0.06 L

Fr

σ     σc 

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0.10

(3)

The ranges of the dimensionless groups in Eq. 3 are 2.31×10-2 < (kGaedp)/(DGat) < 4.52×10-2, 2.59×10-2 < ReL < 6.04×10-2, 0.56 < ReG < 1.01, 1.10×10-7 < WeL < 5.99×10-7, 1.17×10-6 < FrL < 2.93×10-5, and 1.57 < σ/σc < 3.13. It can be seen that the liquid and gas Reynolds numbers have relative large exponents, resulting a positive effect to the gas-side controlled mass transfer process that is consistent with experimental data. Weber number expresses the ratio of inertial force to surface tension force, which also has a positive influence on the mass transfer. Froude number expresses the ratio of inertial force to centrifugal force, having a negative influence on the mass transfer. Notably the influence of packing’ surface tension cannot be ignored, because the packing’s surface tension influences the liquid behavior which passes through the packing’ surface and then influences the liquid distribution inside the packing to affect the mass transfer process. All the values of kGae predicted by the fitted correlation are within ±20% of the experimental data, as shown in Figure 3, indicating the applicability of this correlation for the RPB with structured nickel foam packing.

4. Conclusions In this work, a gas-side mass transfer controlled absorption system SO2-NaOH was employed to measure kGae in a RPB with structured nickel foam packing. Experimental results show that the kGae with the SNP maximally increased by 10.44%, 10.70%, and 11.73% under various rotational speeds, liquid flow rates, and gas flow rates, respectively, compared to the NNP. Comparison with the enhancement rate of kGae and kLae for the NNP and SNP in the RPB reveals that the hydrophobic 8


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surface-modified nickel foam packing has better applications for the chemical process that was controlled by the liquid-side mass transfer, and the enhancement on the gas-side mass transfer controlled process is relatively weak. The predicted kGae by the correlation can fit well with the experimental data. The results of this study provide a better understanding of the mass transfer for structured nickel foam packing in RPBs, and can consequently help to improve its design and operation.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21676009, 21725601, and 21436001).

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Nomenclatures ae = gas-liquid effective interfacial area (m2/m3) at = specific surface area of the packing (m2/m3) ac = centrifugal acceleration (m/s2) CGin, CGout = concentration of solute at the inlet and outlet gas stream, respectively (mol/L) dp = effective diameter of the packing = 6(1-ε)/at (m) DG = diffusion coefficient of gas phase (m2/s) h = axial height of the packing (m) kG = gas-side mass transfer coefficient (m/s) kGae = gas-side volumetric mass transfer coefficient (1/s) kLae = liquid-side volumetric mass transfer coefficient (1/s) QL, QG = volumetric liquid and gas flow rate, respectively (m3/s) r1, r2 = inner and outer radius of the packing, respectively (m)

r = average radius of packing =

r12 + r22 (m) 2

Greek Letters ρL, ρG = density of liquid and gas, respectively (kg/m3) µL, µG = viscosity of liquid and gas, respectively (kg/(m·s)) σc = critical surface tension of material (mN/m) σ = surface tension of water (mN/m)

Dimensionless Groups

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ReL = liquid Reynolds number =

ReG = gas Reynolds number =

ρ LQLd p (2π rh ) µL

ρGQG d p (2π rh) µG Q L2 ρ L d p

WeL = liquid Weber number =

(2π rh ) 2 σ

FrL = liquid Froude number =

Q L2 a c2 (2π rh ) 2 d p

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References (1) Lin, C. C.; Liu, H. S. Adsorption in a centrifugal field: basic dye adsorption by activated carbon. Ind. Eng. Chem. Res. 2000, 39, 161. (2) Gudena, K.; Rangaiah, G. P.; Lakshminarayanan, S. Optimal design of a rotating packed bed for VOC stripping from contaminated groundwater. Ind. Eng. Chem. Res.

2014, 51, 835. (3) Chen, J. F.; Wang, Y. H.; Guo, F.; Wang, X. M.; Zheng, C. Synthesis of nanoparticles with novel technology: high-gravity reactive precipitation. Ind. Eng. Chem. Res. 2000, 39, 948. (4) Sun, B. C.; Wang, X. M.; Chen, J. M.; Chu, G. W.; Chen, J. F.; Shao, L. Synthesis of nano-CaCO3 by simultaneous absorption of CO2 and NH3 into CaCl2 solution in a rotating packed bed. Chem. Eng. J. 2011, 168, 731. (5) Yang, H. J.; Chu, G. W.; Zhang, J. W.; Shen, Z. G.; Chen, J. F. Micromixing efficiency in a rotating packed bed: experiments and simulation. Ind. Eng. Chem. Res.

2005, 44, 7730. (6) Luo, Y.; Chu, G. W.; Zou, H. K.; Wang, F.; Xiang, Y.; Shao, L.; Chen, J. F. Mass transfer studies in a rotating packed bed with novel rotors: chemisorption of CO2. Ind. Eng. Chem. Res. 2012, 51, 9164. (7) Chen, Y. S.; Lin, F. Y.; Lin, C. C.; Tai, Y. D.; Liu, H. S. Packing characteristics for mass transfer in a rotating packed bed. Ind. Eng. Chem. Res. 2006, 45, 6846. (8) Chen, Q. Y.; Chu, G. W.; Luo, Y.; Sang, L.; Zhang, L. L.; Zou, H. K.; Chen, J. F. Polytetrafluoroethylene wire mesh packing in a rotating packed bed: mass transfer studies. Ind. Eng. Chem. Res. 2016, 55, 11606. (9) Luo, Y.; Luo, J. Z.; Chu, G. W.; Zhao, Z. Q.; Arowo, M.; Chen, J. F. Investigation of effective interfacial area in a rotating packed bed with structured stainless steel

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wire mesh packing. Chem. Eng. Sci. 2017, 170, 347. (10) Kelleher, T.; Fair, J. R. Distillation studies in a high-gravity contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (11) Reddy, K. J.; Gupta, A.; Rao, D. P.; Rama, O. P. Process intensification in a HIGEE with split packing. Ind. Eng. Chem. Res. 2006, 45, 4270. (12) Chen, Y. S. Correlations of mass transfer coefficients in a rotating packed bed. Ind. Eng. Chem. Res. 2011, 50, 1778. (13) Liu, M. J.; Wang, S. T.; Jiang, L. Nature-inspired superwettability systems. Nat. Rev. Mater. 2017, 2, 17036. (14) Chu, G. W.; Song, Y. J.; Zhang, W. J.; Luo, Y.; Zou, H. K.; Xiang, Y.; Chen, J. F. Micromixing efficiency enhancement in a rotating packed bed reactor with surface-modified nickel foam packing. Ind. Eng. Chem. Res. 2015, 54, 1697. (15) Zheng, X. H.; Chu, G. W.; Kong, D. J.; Luo, Y.; Zhang, J. P.; Zou, H. K.; Zhang, L. L.; Chen, J. F. Mass transfer intensification in a rotating packed bed with surface-modified nickel foam packing. Chem. Eng. J. 2016, 285, 236. (16) Li, M.; Xu, J.; Lu, Q. Creating superhydrophobic surfaces with flowery structures on nickel substrates through a wet-chemical-process. J. Mater. Chem. 2007, 17, 4772. (17) Sandilya, P.; Rao, D. P.; Sharma, A. Gas-phase mass transfer in a centrifugal contactor. Ind. Eng. Chem. Res. 2001, 40, 384. (18) Commenge, J. M.; Obein, T.; Framboisier, X.; Rode, S.; Pitiot, P.; Matlosz, M. Gas-phase mass-transfer measurements in a falling-film microreactor. Chem. Eng. Sci.

2011, 66, 1212. (19) Perry, R. H.; Green, D. W. Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1999.

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(20) Yan, X. C.; Luo, M. D. Surface Chemistry. Chemical Industry Press: Beijing, 2005.

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Figure Captions Figure 1. Schematics of gas-side mass transfer experimental setup. Figure 2. Effects of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on kGae in the RPB with NNP and SNP.

Figure 3. Comparison of experimental and calculated data of kGae

Table Legends Table 1. Comparison of kGae, kLae and ae in the RPB with NNP and SNP

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Figure 1. Schematics of gas-side mass transfer experimental setup. (1, SO2 cylinder; 2, N2 cylinder; 3, drain tank; 4, pump; 5, stock tank; 6, RPB; A1, sample at gas inlet; A2, sample at gas outlet; B1, sample at liquid inlet; B2, sample at liquid outlet)

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16 NNP SNP

15

L=6 L/h G=1200 L/h

kGae (1/s)

14 13 12 11 10

400

800

1200 N (r/min)

1600

2000

(a) 18

NNP SNP

17

N=1200 r/min G=1200 L/h

16

kGae (1/s)

15 14 13 12 11 10 3

4

5

6

7

8

9

L (L/h)

(b) 17

NNP SNP

16 15 14

L=1200 L/h G=3.6 L/h

13

kGae (1/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


12 11 10 9 8 7 1000

1200

1400

1600

1800

G (L/h)

(c)

Figure 2. Effects of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on kGae in the RPB with NNP and SNP. 17



18

NNP, this work SNP, this work 16

Experimental kGae (1/s)

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+20%

14

12

-20% 10

8

6 6

8

10

12

14

16

18

Calculated kGae (1/s)

Figure 3. Comparison of experimental and calculated data of kGae

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Table 1. Comparison of kGae, kLae, and ae in the RPB with NNP and SNP Variables

kGae (1/s) NNP

Variables

SNP

Rotational speed 11.17-13.50 11.17-14.91 (400-2000 r/min)

Rotational speed (400-2400 r/min)

ae (m2/m3)

kLae (1/s) NNP

SNP

NNP

SNP

0.58-0.74 0.64-0.90 1054-1139 1082-1343

Liquid flow rate (3.6-8.4 L/h)

10.80-15.33 10.99-16.97

Liquid flow rate (18-40 L/h)

0.40-0.70

0.51-1.34

905-1421

1050-1496

Gas flow rate (1000-1800 L/h)

8.61-14.07

Gas flow rate (800-1600 L/h)

0.60-0.90

0.68-0.95

940-1084

980-1100

9.62-15.51

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Table of Contents (TOC) graphic

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Gas-side Mass Transfer in a Rotating Packed Bed with Structured

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