Determination of the effective interfacial area in rotating packed bed

Determination of the effective interfacial area in rotating packed bed

Chemical Engineering Journal 168 (2011) 1377–1382 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsev...

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Chemical Engineering Journal 168 (2011) 1377–1382

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Short communication

Determination of the effective interfacial area in rotating packed bed Kuang Yang a , Guangwen Chu b,∗ , Haikui Zou b , Baochang Sun b , Lei Shao b , Jian-Feng Chen a a

Key Lab for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China Research Center of the Ministry of Education for High Gravity Engineering and Technology, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China b

a r t i c l e

i n f o

Article history: Received 2 April 2010 Received in revised form 25 January 2011 Accepted 29 January 2011 Keywords: Effective gas–liquid interfacial area Rotating packed bed Cavity zone End effect Mass transfer

a b s t r a c t A sampling tube was installed closely to the rotor in a rotating packed bed (RPB) to collect the liquid immediately flowing out of the packing so as to measure the real effective gas–liquid interfacial area in the packing of the RPB (i.e. excluding the cavity zone). It was calculated that the contribution of the cavity zone to mass transfer accounted for about 13–25% of the overall mass transfer in the whole RPB. The existence of the end effect in the packing was confirmed by an investigation on the effective gas–liquid interfacial area in the packings with different radii. Effects of radial thickness of the packing, rotation speed, liquid and gas volumetric flow rate on the effective interfacial area were also studied. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The effective interfacial area is a fundamental parameter in the design of absorption equipment on the industrial scale because the knowledge of this parameter is necessary for calculating individual mass-transfer coefficient. This value depends on the type of the gas–liquid contact device, operational conditions and the properties of the gas–liquid system. The methods of determining effective interfacial area are classified as physical methods and chemical methods. The physical methods are based on some physical properties of the gas–liquid system [1–4], whereas the chemical methods that are based on the study of a reaction with a well-know kinetics, in which the absorption rate is a function of the interfacial area, have been widely employed in the determination of the effective interfacial area in various kinds of reactors, i.e. packed columns [5], microchannel reactors [6], airlift reactors [7], bubble columns [8]. A rotating packed bed (i.e. RPB or a Higee device), which creates a high gravity environment up to several hundred times larger than the gravitational acceleration of the earth, was first introduced as a novel gas/liquid contactor to enhance mass transfer in 1981 [9]. In the high gravity environment, thin liquid films and tiny liquid droplets can be generated in the RPB, thus decreasing mass transfer resistance and, meanwhile, increasing gas–liquid interfacial area. As a result of a larger mass-transfer area and a higher liquid mass-transfer coefficient (kL ) compared to conventional apparatuses, the efficiency of mass transfer in the RPB can be significantly

∗ Corresponding author. Tel.: +86 10 64443134; fax: +86 10 64434784. E-mail address: [email protected] (G. Chu). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.01.100

enhanced. Nowadays, the RPBs have been successfully applied to the gas–liquid processes such as desorption, absorption, distillation, ozone oxidation, and nano-materials preparation [10–16]. Because the liquid is highly dispersed in the RPB, the increase of the effective gas–liquid mass-transfer area is crucial to the intensification of mass transfer. However, studies on the effective interfacial area in the RPB are rare. Munjal et al. [17] reported a set of experimental measurements, based on chemical absorption of CO2 in NaOH solution, of the gas–liquid interfacial areas in a rotating bed as a function of the rotation speed and the liquid flow rate. The results showed that the gas–liquid effective interfacial area of the RPB with glass beads and commercial packing is in the range of 222–1109 m−1 under different operational conditions. Using the same chemical adsorption method, Chen et al. [18] obtained the interfacial area of a multi-stage centrifugal atomization RPB. He also performed some experiments to measure the effective interfacial area of a corrugated disk RPB and found that the effective interfacial area of the corrugated disk RPB is between 700 and 1300 m−1 [19]. Mass transfer in the RPB is performed in three zones [20]: end effect zone (the part of the packing close to the inner edge of the rotor, about 10 mm in radial thickness), bulk packing zone (the rest of the packing) and cavity zone (the zone between the packing and the casing of the RPB). Zhang’s study [21] showed that the contribution of the cavity zone to the overall mass transfer in the RPB cannot be ignored. But the effects of the cavity zone on mass transfer were not eliminated when measuring the effective interfacial area in the packing in the literature, which led to an exaggeration of the effective interfacial area of the packing. In this study, a sampling tube was installed closely to the rotor in a RPB to collect the

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If the following inequality is satisfied,



Nomenclature A a ci c1 , c0 CCO

3

2−

total area of the gas–liquid interface in the packing, m2 effective interfacial area, m−1 concentration of CO2 at the gas–liquid interface, mol/L Na2 CO3 concentration at the outer edge of the packing and the liquid outlet, respectively, mol/L concentration of CO3 2− ion in sample solution,

mol/L normal concentration of hydrochloric acid, mol/L concentration of OH− in solution, mol/L diffusivity of CO2 in solution, m2 /s solubility coefficient of CO2 in the electrolyte solution, kmol/m3 MPa H0 solubility coefficient of CO2 in purified water, kmol/m3 MPa h constant related to h+ , h− , hG h+ , h− , hG constant of positive ion, the negative ion and vapor solute, respectively I ionic strength, kmol/m3 kL liquid side mass-transfer coefficient, m/s k1 pseudo-first-order rate constant, s−1 k2 pseudo-second-order rate constant between CO2 and OH− , m3 /kmol s k2∞ rate constant for reaction in infinitely dilute NaOH solution, m3 /kmol absorption rate of CO2 , kmol/m2 s Ni PCO2 partial pressure of CO2 in gas phase, MPa pressure of gas phase, MPa Ptotal Pi partial pressure of CO2 in equilibrium at interface of gas and liquid, MPa y mole ratio of CO2 in gas phase i inlet mole ratio of CO2 in gas phase yCO 2 o yCO outlet mole ratio of CO2 in gas phase cHCl cOH D H

2

V V1 V2 V0 Zi 

volume of the packing, m3 volume of hydrochloric acid consumed at the first titration end-point, ml volume of hydrochloric acid consumed at the second titration end-point, ml volume of sample, ml valence of ion contribution of cavity zone to mass transfer

1/2 ≤1+

kL2

cOH 2ci

(2)

Reaction (1) can be treated as a pseudo-first-order reaction. According to Danckwerts’ mass-transfer model [22], the reaction rate can be expressed as:



Dk1 + kL2 Aci

Ni =

(3)

The mass-transfer process of the absorption of CO2 is controlled by the liquid film, according to Henry’s law: ci = HPi = HPCO2

(4)

Substituting Eq. (4) into Eq. (3), the solution is:



Ni HPCO2

2

 = A2 Dk1 + A2 kL2 = A2 Dk1

1+

1 2

 (5)

When the inequality  ≥ 3 [7] is satisfied, rapid chemical reaction is not sensitive to kL , the above equation can be expressed as:



Ni HPCO2

2

= A2 Dk1

(6)

Ni can be obtained from the experiments; the absorption rate of CO2 can be expressed by the formation rate of Na2 CO3 , and the concentration of Na2 CO3 in the solution can be obtained by potentiometric titration. k1 is the rate constant of the pseudo-first-order reaction, and its relationship with temperature and electrolyte concentration is: k1 = k2 cOH−

(7)

k2 k2∞

(8)



lg



= 0.1987I − 0.012I 2 2.382 T

lg(k2∞ ) = 11.895 −

(9)

The partial pressure of CO2 can be written as: PCO2 =

o i − yCO yCO 2

2

o ) i ln(yCO /yCO 2

× Ptotal

(10)

2

The solubility coefficient of CO2 in the electrolyte solution can be estimated as follows: lg

liquid immediately flowing out of the packing so as to obtain the real effective interfacial area of the packing to the exclusion of the influence of the cavity zone. The mass-transfer characteristics of stainless steel wire mesh packings with different radial thickness were also investigated.

Dk2 cOH

H H0

=−



hI

(11)

h can be determined by the constant of the positive/negative ions in the electrolyte solution, and the vapor solute: h = h+ + h− + hG

(12)

h+ , h− , hG is the constant of the positive ion, the negative ion and the vapor solute, respectively. The solute constant of Na+ , OH− and CO3 2− is 0.091, 0.066 and 0.021, respectively. The constant of the vapor solute hG is −0.019. The formula for the ionic strength is as follows:

2. Measurement system and liquid phase analysis method 2.1. Measurement system



In this study, NaOH–CO2 –N2 system with a pseudo-first-order reaction was used to measure the effective interfacial area because the kinetics of this system is well-known and the chemicals are easy to handle. When the NaOH solution is in excess, the reaction can be expressed as:

ci can be obtained by the potentiometric titration, and Zi is the valence of the ion. The formula for the diffusion coefficient of CO2 in the solution is as follows [23]:

2NaOH + CO2 = Na2 CO3 + H2 O

D = 1.97 × 10−9 (1 − 0.129cOH− − 0.261cCO

(1)

I=

1 2

ci Zi2

(13)

3

2−

)

(14)

K. Yang et al. / Chemical Engineering Journal 168 (2011) 1377–1382

1600

Table 1 Specifications of the RPB used in this study.

V2

1400

d / mV

1200 1000

V1

800

1379

Item

Value

Inner radius of the rotor, m Axial height of the rotor, m Outer radius of the packing, m

0.08 0.054 0.2225, 0.2055, 0.1885, 0.1715, 0.1545, 0.1375, 0.1205, 0.088 0.317 Stainless steel wire mesh 499.7 97%

Inner radius of the casing, m Packing type Specific area of the packing, m2 /m3 Voidage of the packing

600 400 200 0 0

2

4

6

VHCl /mL

8

10

Fig. 1. Curve of potentiometric titration.

The effective interfacial area in the packing can be expressed as: ˛=

A V

(15)

A can be obtained from Eq. (6), V represents the overall volume of the packing. 2.2. Potentiometric titration method The concentration of the liquid components after absorption and reaction was measured by an automatic potentiometric titrator (ZDJ-2D, Beijing Xianqu Weifeng Technology Development Co., China). 0.565 M HCl solution, which was calibrated by a certain concentration of anhydrous Na2 CO3 solution, was used for titration analysis to determine the concentration of Na2 CO3 in the collected sample. The electric potential curve measured by the automatic potentiometric titrator is plotted in Fig. 1. V1 and V2 represent HCl volume consumed at the first titration end-point and the second titration end-point, respectively. The concentration of Na2 CO3 with a sample volume of V0 can be expressed as: cNa2 CO3 =

V2 − V1 × cHCl V0

(16)

3. Experimental 3.1. Experimental setup The experimental set-up is shown in Fig. 2. The specifications of the RPB employed in this work are given in Table 1. The packing con-

sists of stainless steel wire mesh with a wire diameter of 0.22 mm. In order to eliminate the effects of cavity zone on mass transfer, an L-shaped tube was installed between the rotor and the casing. The vertical section of the tube, with an opening of 20 mm in length to collect the liquid immediately flowing out of the packing, was configured near the outer edge of the rotor and in parallel with the axis, while the horizontal section of the tube was fixed at the casing via a thread connection. To investigate the influence of the packing thickness on the mass-transfer efficiency packings with eight different outer radii were tested, and eight L-shaped tubes with different horizontal section lengths were employed accordingly to match the packings. There are two kinds of covers at the lower and the upper ends of the rotor: the lower cover and the upper cover. The lower cover has a certain dimension and was fixed at the axis, whereas there are eight upper covers with different radii. When a certain packing was tested, a corresponding upper cover was chosen to match the packing’s radius. Then the packing was held between these two covers by screws going through the rim of the upper cover and connecting the upper cover with the lower cover. 3.2. Experimental procedure NaOH solution (1 mol/L) at 20 ◦ C was pumped from the stock tank into the inner edge of the packing via a liquid distributor. The liquid distributor consists of two pipes configured in parallel with the axis, and each pipe has four Ø 3 mm holes aligned along the length direction with an interval of 15 mm between two adjacent holes. Two absorbent streams jetted from these two pipes, respectively, and reached the opposite points of the inner edge of the packing before entering the packing. The aqueous absorbent moved outward and left from the outer edge of the packing under the action of the centrifugal force. The mixed gas stream of CO2 and N2 with a CO2 /N2 molar ratio of 1/9 was introduced into the RPB tangentially from a gas inlet and contacted the absorbent liquid

Fig. 2. Schematic of the experimental setup. a: Liquid inlet; b: liquid outlet; c: liquid sample collecting device; d: gas inlet; e: gas outlet; A1: sample analysis at gas inlet; A2: sample analysis at gas outlet; A3: sample analysis at outer edge of packing; A4: sample analysis at liquid outlet.

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0.24 0.23

7000 6000

0.21 -1

0.20

a/m

3

CNaCO / mol/L

0.22

8000

3

L=3L/min, G=20m /h c0 c1

0.19 0.18 0.17

5000 4000 3000

0.16

2000

0.15

1000

0.14

600

800

1000

N / rpm

1200

3

L = 3L/min, G = 20m /h N = 600rpm N = 800rpm N = 900rpm N = 1000rpm N = 1200rpm N = 1400rpm

0

1400

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

R/m

Fig. 3. Na2 CO3 concentration vs. rotation speed. Fig. 5. Effective interfacial area vs. outer radius of packing.

countercurrently in the packing. The temperature and the pressure in the RPB were 20 ◦ C and 0.1 MPa (absolute pressure), respectively. The content of CO2 at the gas inlet “A1” and the gas outlet “A2” was measured by two infrared gas analyzers (GXH-3010F, Huayun Analytical Instrument Company, China), respectively. Liquid samples were collected at the outer edge of the packing “A3” and the liquid outlet “A4”, respectively. For convenience, Na2 CO3 concentration at the outer edge of the packing and the liquid outlet was designated as c1 and c0 , respectively. 4. Results and discussion 4.1. Contribution of cavity zone to mass transfer As can be seen from Fig. 3, the concentration of Na2 CO3 from the outer edge of the packing is lower than that from the liquid outlet. The difference is ascribed to the contribution of the cavity zone to the gas–liquid mass transfer, which can be expressed as: =

c0 − c1 × 100% c0

(17)

 = 13 − 17 % in Fig. 3. The exact value of  depends on the experiment conditions. It can be observed from Fig. 4 that the contribution of the cavity zone to the overall mass transfer in the RPB decreased from about 25% to 17% as the outer radius of the packing increased from 0.088 m to 0.2225 m. As the outer radius of packing increased, the velocity of the liquid flying out of the packing increased due to the action of a larger centrifugal force, and the travel distance and residence time

0.26

3

L=3L/min, G=20m /h, N=900rpm

0.25 0.24 0.23

φ

0.22 0.21

4.2. Effect of radial thickness of packing The existence of the end effect in the RPB was reported in previous studies [20,21]. The so-called end effect means that a great proportion of mass transfer is performed in the part of the packing close to the inner edge of the rotor. By an investigation on the interfacial area of the packings with varying outer radii, the masstransfer efficiency at different radial positions of a certain packing can be revealed. It can be seen from Fig. 5 that the effective interfacial area increased sharply with the decrease of the outer radius of the packing. The effective interfacial area was less than 500 m−1 when the outer radius was 0.1885 m, whereas the effective interfacial area reached 7500 m−1 when the outer radius reduced to 0.088 m. In the inner packing zone, the collision between the liquid and the packing is the most violent, resulting in a large effective interfacial area. When flowing outwards in the packing, the liquid gradually gains a circumferential speed under the centrifugal force, and the relative velocity between the liquid and the packing becomes smaller and smaller, which will result in a smaller shearing stress upon the liquid and consequently the decrease of the effective interfacial area. This result confirms the existence of the end effect. In the following section, the experiments were conducted with a packing outer radius of 0.2225 m unless otherwise stated. 4.3. Effect of rotation speed As can be seen from Fig. 6, the effective interfacial area increased from 190 m−1 to 350 m−1 with the increase of the rotation speed from 600 rpm to 1400 rpm. It is easy to understand that with the increase of the rotation speed, the shearing force imposed on the liquid by the packing enhanced, which resulted in a better dispersion of the liquid and an increase of the effective interfacial area. 4.4. Effect of liquid volumetric flow rate

0.20 0.19 0.18 0.17 0.16

of the liquid in the cavity zone decreased accordingly, resulting in a decrease in the mass-transfer effect of the cavity zone.

0.10

0.12

0.14

0.16

0.18

R/m

0.20

0.22

0.24

Fig. 4. Contribution of cavity zone to mass transfer vs. outer radius of packing.

It can seen from Fig. 7 that the effective interfacial area increased significantly from 213 m−1 to 277 m−1 as the liquid volumetric flow rate increased from 3 L/min to 8 L/min, because more tiny liquid droplets, small threads and thin films were produced at a larger liquid volumetric flow rate. However, when the liquid volumetric flow rate increased further to 10 L/min, the effective interfacial area exhibited a trend to decrease (from 277 m−1 to 274 m−1 ). The liquid residence time in the packing decreased with the increase of the liq-

K. Yang et al. / Chemical Engineering Journal 168 (2011) 1377–1382

400

rate from 10 m3 /h to 40 m3 /h. The gas radial velocity at the inner periphery of the packing also increased from 0.1 m/s to 0.4 m/s and the ratio of gas to liquid volumetric flow rate increased from 55 to 220. The increase in the gas volumetric flow rate and radial velocity led to a stronger disturbance of the gas and liquid phases, a better dispersion of the liquid and an increase of the gas–liquid interface, resulting in the increase of the effective interfacial area.

3

L=3L/min, G=20m /h

a/m

-1

350 300 250

5. Conclusion In this paper, the classical chemical method of CO2 absorption into NaOH solution was employed to obtain the real effective interfacial area of mass transfer in the packing of the RPB with stainless steel wire mesh packing. Experimental results indicated that the effective interfacial area increased with the increase of the rotation speed and gas volumetric flow rate. The effect of the packing thickness on the effective interfacial area was also studied, and the existence of the end effect was verified by a revelation of the much higher effective interfacial area in the packing with smaller outer radius. Moreover, the contribution of the cavity zone to mass transfer was calculated to account for about 13–25% of the overall mass transfer in the whole RPB.

200 600

800

1000

1200

N / rpm

1400

Fig. 6. Effective interfacial area vs. rotation speed.

320

3

G = 20m /h, N = 900rpm

300

-1

280

a /m

1381

260

Acknowledgements

240

This work was supported by the National Nature Foundation of China (No. 20821004 and No. 20990221) and National Fundamental Research Program of China (973 Program) (No. 2009CB219903).

220 200

References

180 1

2

3

4

5

6

7

8

9

10

11

L / (L/min) Fig. 7. Effective interfacial area vs. liquid volumetric flow rate.

uid volumetric flow rate. This factor predominated over the effect of the increased number of the tiny liquid droplets, small threads and thin films and led to a decrease of the effective interfacial area when the liquid volumetric flow rate exceeded 8 L/min. 4.5. Effect of gas volumetric flow rate Fig. 8 shows that the effective interfacial area increased from 191 m−1 to 300 m−1 with the increase of the gas volumetric flow

320

L=3L/min, N=900rpm

300

a/m

-1

280 260 240 220 200 180 10

15

20

25

30

35

3

G / (m /h) Fig. 8. Effective interfacial area vs. gas flow rate.

40

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