Gas Pressure Drop and Mass Transfer Characteristics in a Cross-flow

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Ind. Eng. Chem. Res. 2010, 49, 3732–3740

Gas Pressure Drop and Mass Transfer Characteristics in a Cross-flow Rotating Packed Bed with Porous Plate Packing Wei Zhou Jiao, You Zhi Liu,* and Gui Sheng Qi Research Center of Shanxi ProVince for High GraVity Chemical Engineering and Technology, North UniVersity of China, Taiyuan 030051, People’s Republic of China

Structured packing were proposed and designed according to gas-liquid mass transfer and kinetic balance characteristics in a cross-flow rotating packed bed (RPB). Two types of novel porous plate packing were investigated respectively in a cross-flow RPB to examine their gas pressure drop and mass transfer characteristics with CO2-NaOH solution system. Compared with the literature, the pressure drop of the cross-flow rotating packed bed with new structured packing was slower than that of wire-gauze packing and was no more than a tenth that of the countercurrent rotating packed bed in the same operational condition. The volume mass transfer coefficient of RPB with new structured packing was higher than that with wire-gauze packing, which was higher than that of countercurrent RPB and was 1-2 orders of magnitude higher than that of the conventional packed tower. Moreover, the correlative expressions of gas pressure drop and volume mass transfer coefficients of RPB were obtained using the MATLAB program and in good agreement with the experimental results. 1. Introduction Process intensification is the term which describes the strategy of making dramatically smaller process plant systems in order to reduce their capital cost, hazardous inventory, and environmental cost.1–3 Ramshaw4 invented rotating packed bed (RPB), a novel gas-liquid reactor that utilizes centrifugal acceleration to intensify mixing and mass transfer. Thus, 1-2 orders of enhancement in mass transfer can be achieved in a rotating packed bed. Consequently, the size and the capital of the processing system would be extremely reduced. The high gravity rotating packed bed (HIGEE) provides a means for replacing the gravitational acceleration, g, by a centrifugal acceleration of 100-1000g. The high centrifugal force permits the use of packing with a large surface area in the range of 1000-4000 m2/m3, which is 5-10 times higher compared to the packing used in conventional columns, which would lead to be 5-10 times higher mass transfer rate. Since the liquid flows as thin films under the high centrifugal acceleration, there is an enhancement in the liquid-side mass-transfer coefficient. On the basis of these advantages, the feasibility of the RPB has been extensively studied in many systems such as distillation,5,6 absorption,7–10 stripping,11 reactive precipitation,12–14 and ozone oxidation.15 To reduce the pressure drop of the countercurrent-flow RPB used in high gas flow rates for the gas absorption process, the cross-flow RPB with stainless steel packing was developed for some gas absorption processes, such as NH3 absorption by water and SO2 absorption by ammonium sulfite solution.10,16 On the basis of these studies, the cross-flow RPB would be more applicable for treating pollutants in gaseous streams having higher flow rates, whereas flooding may occur in the countercurrent-flow RPB. Moreover, investigating the gas pressure drop and mass transfer performance of the cross-flow RPB is necessary for designing the cross-flow RPB accurately and economically. Kumar et al.17 reported pressure drop and mass transfer data of a countercurrent-flow rotating packed bed with wiremesh sheets. Singh et al.8 published the pressure drop and area of a transfer unit data on a rotating metal sponge bed of voidage of 0.95 and specific area of 2500 1/m and on wire gauze of voidage * To whom correspondence should be addressed. Tel.: 86-3513921986. Fax: 86-351-3921497. E-mail: [email protected].

of 0.934 and specific area of 2067 1/m. Liu et al.9 reported the pressure drop and mass transfer measured across rotating packed beds with plastic of cubic and elliptic cylindrical grains. Munjal et al.18,19 proposed that the gas-liquid mass transfer could be enhanced by a countercurrent-flow RPB with wire commercial packing under CO2 absorption by an aqueous NaOH solution. The result showed that both the gas-liquid interefacial area (a) and the liquid-phase mass-transfer coefficient (kLa) increased with rotational speed. Chen et al.20 investigated the removal of VOCs from the air into an aqueous phase by a countercurrent-flow RPB with random packing. The packing used in this study were acrylic beads, the specific surface area and porosity of which were 1200 m2/m3 and 0.4, respectively. The experimental results showed that the overall volumetric gasphase mass-transfer coefficient (KGa) increased as a function of the gas Grashof number (GrG) to the power of 0.18. Lin et al.16 investigated the feasibility of a cross-flow rotating packed bed (RPB) with stainless steel wire mesh on the removal of carbon dioxide (CO2) from gaseous streams. Tan et al.21 presented more results concerning CO2 absorption by different absorbents including piperazine (PZ) and its mixtures with MEA, AMP, and ethlydiethanolamine (MDEA) in a countercurrent-flow RPB with the stainless wire-mesh packing. Chen et al.22 examined the mass transfer efficiency of a countercurrent-flow RPB with different radii of the packed bed using stainless steel wire-mesh packing. Experimental results showed that kLa increased with decreasing volume of the packed bed, which may contribute to the significant end effects as the volume of the packed bed is reduced. Lin et al.7 investigated the removal of CO2 from gaseous steams containing 1-10% CO2 in a countercurrent-flow RPB with the stainless wire mesh using NaOH solution. Zheng et al.,23 working with the foam metal packing with specific surface areas of 1000 m-1 and porosities of 94%, reported the pressure drop data on the different rotor. Liu et al.9 proposed that the mass transfer (stripping of ethanol) and pressure drop (water-air) of a rotating packed bed were investigated under 13-273 times gravitational force with two random packings. The first packing is rectangular, with a size of 5 × 5 × 2.8 mm3, specific surface area of 524 m2/m3, and voidage of 0.533. The second one is an elliptical cylindrical packing with a size of 3 × 2.6 × 3 mm3, specific surface area

10.1021/ie9009777  2010 American Chemical Society Published on Web 03/24/2010

Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010

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Table 1. Structure and Properties of Packing packing type

A

B

material packing plate number packing thickness/mm porosity specific surface area a/1/m packed density/kg/m3 plate distance/mm opening ratio hole diameter/mm hole distance/mm

polypropylene 40 1 0.827 671.8 164.2 2 0.55 5 7

1Cr18Ni9Ti 30 0.4 0.911 574 706.7 2.67 0.32 3 5

Table 2. Details of Rotor and Casing rotor material outer diameter inner diameter axial width of packing casing material casing geometry diameter of casing height of casing

stainless steel 150 mm 40 mm 80 mm stainless steel column box 320 mm 120 mm

of 1027 m2/m3, and voidage of 0.389. The results indicated that the mass-transfer coefficient (KGa) increased with the gas rate, the liquid rate, and the rotor speed. Moreover, different shapes of packing gave different mass-transfer performances in the rotating packed bed. As to pressure drop, the gas rate seems to be a more important factor than the liquid rate because of less liquid holdup under centrifugal force. According to the above literature, pressure drop and mass transfer characteristics in RPB have been investigated using random packing such as glass beads, acrylic beads, and wire mesh packing. But, few studies on structured packing have been reported. However, the above random packing are difficult to arrange correctly according to the demand of uniformity and symmetry in the RPB. The asymmetry of packing will lead to vibration of the RPB in operation. The vibration will worsen the symmetry of packing. The worsening asymmetry of packing will enhance libration of the RPB. The vicious circle will influence the life of the RPB. According to the literature,24 some inner supports were added in the RPB to prevent asymmetry of the wire-gauze packing, but these had no obvious effect. Therefore, the two novel types of structured packing, a rippled porous plate made of stainless steel and a porous plate made of plastic, have been designed successfully according to gas-liquid mass transfer characteristics in the cross-flow RPB. Therefore, investigating the hydraulic and mass transfer performance of an RPB equipped with the structured packing is necessary for designing a cross-flow RPB accurately and economically. The aim of this work is to evaluate the gas pressure drop and liquid-phase mass transfer performance of the cross-flow RPB under various of operating conditions using CO2 absorption. Results in this work provide further insight into the application of the cross-flow RPB in the absorption processes. 2. Experiments In this paper, experiments were carried out in the cross-flow RPB with the structured packing. Employing the CO2-NaOH solution system, the gas pressure drop and mass transfer across the unit are measured with a manometer under different gas and liquid flow rates at ambient conditions. The experimental data and are used for 0 < G e 100 m3/h, 0 < L e 250 L/h. Table 1 shows the properties and geometric parameters of the two structured packing (parameters of the RPB units are given in Table 2). One is a plastic porous plate (packing A)

Figure 1. Appearance of plastic porous plate packing.

Figure 2. Appearance of rippled porous plate packing.

Figure 3. Processing mold of the ripple porous plate packing.

whose sketch is seen from Figure 1. Figure 2 is the simplified sketch of the rippled porous plate made of stainless steel packing with a protruded ripple plate (packing B), the concentric ring waved from disk plate, which has been desiged and produced. As the radial channel of the plate is orderly, it causes smaller resistance to the gas flow, and as the roughed plates can intensify liquid film separation, the gas-liquid interfacical area can be greatly improved. The mold of the stainless steel packing with the protruded ripple plate is shown in Figure 3. Figure 4 illustrates the experimental setup for mass transfer with the CO2-NaOH system. The compressed air flows axially from the bottom of the packing owing to the pressure drop. In

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Figure 6. Distribution of high gravity factor in radial packing.

3. Results and Discussions Figure 4. Schematic diagram of the experimental setup: (1) liquid tank; (2) liquid pump; (3, 12) rotameter; (4) liquid inlet; (5) gas outlet; (6) liquid distributor; (7) liquid outlet; (8) gas inlet; (9) packing; (10) rotating packed bed; (11) variable speed motor; (13) CO2 cylinder; (14) roots blower; (15) air; (16) manometer.

The high gravity factor (β) is a ratio of the average high gravitational acceleration in the RPB and the gravitational acceleration g. On the basis of formula 1, the high gravity factor is changed by adjusting the rotor speed or the radius, that is to say, the high gravity fields are changed freely. The high gravity factor (β) increases linearly with the augment of the rotor radius when the rotor speed is not changed (see Figure 6). β)

ω2r g

(1)

Generally, the average high gravity factor describes the magnitude of the high gravity fields ranging from the inner radius to outer radius. In fact, the high gravity field has the characteristics of the cubic distribution, which is seen as a plane distribution when the packing is axially well-distributed. Therefore, the average high gravity factor β′ is described as formula 2. Conveniently, the mean high gravity factor β′ is replaced by the high gravity factor β. The high gravity factor β varied from 0 to 271.8 by the variable speed motor.

∫ β·2πr dr 2ω (r + r r + r ) 3(r + r )g ∫ 2πr dr r2

β′ )

2

2

r1

1

r2

1 2

1

2 2

)

(2)

2

r1

Figure 5. Schematic diagram of the cross-flow rotating packed bed: (1) motor; (2) shaft seal; (3) liquid distributor; (4) gas outlet; (5) top clapboard of packing; (6) liquid outlet; (7) liquid inlet; (8) gas inlet; (9) packing; (10) nether clapboard of packing; (11) casing; (12) rotor; (13) rotor shaft.

the meantime, the liquid is introduced from the tank into the inner edge of the packing through a liquid distributor. The liquid distributor had two tubes, which were provided with four equidistant holes on the side facing the inner periphery. The holes size is 1 mm in diameter. The liquid travels radially in the packing due to the centrifugal force and leaves the packing from the outer edge. This process make the CO2 and NaOH solution contacted in the cross-flow mode within the cross-flow RPB. The exiting air stream is discharged finally from the top of cross-flow RPB, while the liquid is expelled from the bottom of the cross-flow RPB. The gas-liquid flow rate was measured by the rotameters. The rotor speed was measured using a speed controller. The CO2 content in the outlet liquid stream was determined by the standard chemical absorption method. A schematic diagram of the cross-flow rotating packed bed is shown in Figure 5. Gas enters the casing via the gas inlet passing the spraying area and then leaves it. Liquid enters the pipe axially and irrigates packing through liquid distributor. Liquid flows along radial direction of the rotating packing due to centrifugal force, is collected at the wall, and leaves the casing via the liquid outlet. It can use centrifugal force to enhance the gas-liquid mass transfer process with a large gas flow rate and small gas pressure drop.

3.1. Gas Pressure Drop. 3.1.1. Centrifugal Pressure Drop (∆Pc). ∆Pc )

∫

r2

r1

Fg

υ02 dr ) Fg r

∫

r2

r1

rω2 dr )

Fg 2 2 ω (r2 - r12) 2

(3) The pressure drop across the rotor is mearsured without the flow of gas and liquid at different rotor speed. These values correspond to centrifugal pressure drop (∆Pc), which can be estimated from formula 3. The centrifugal pressure drop (∆Pc) can be found as shown below. Let the angular velocity of the rotor be ω. The average tangential velocity of the gas, Vθ, at radius, r, could be less than rω. To express this deviation, a mean slip factor, Ks, is used such that Vθ ) Ksrω. So, the centrifugal pressure drop (∆Pc) can be written such as the formula 4. ∆Pc )

Fg (K ω)2(r22 - r12) 2 s

(4)

Figure 7 displays the effect of high gravity factor (β) ranging from 4.25 to 271.9 on the centrifugal pressure drop (∆Pc). As expected, increasing the high gravity factor increases the centrifugal pressure drop (∆Pc). It is still shown from Figure 6 that the ∆Pc of the packing A is smaller than that of packing B, which indicates that the slip factor (Ks) of packing A is lower


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Figure 7. Effect of high gravity factor on centrifugal pressure drop.

Figure 9. Effect of gas flow rate on pressure drop for dry bed.

Figure 8. Effect of high gravity factor on pressure drop for dry bed.

Figure 10. Effect of gas and liquid flow rate on pressure drop for wet bed.

than that of packing B. This phenomenon may be due to the minor packing material difference between the gas tangential velocity and rotor speed. 3.1.2. Gas Pressure Drop of Dry Bed (∆Pd). Figure 8 presents the pressure drop as a function of the high gravity factor (β) ranging from 26.5 to 271.9 at the different gas flow rate. It indicates that the gas pressure drop values (∆Pd) increases along with the augment of high gravity factor (β). In fact, there are two main reasons for this trend. One is that the centrifugal pressure drop, as the part of the pressure drop of dry bed, increases with the increasing of the high gravity factor. Another is that the augment of high gravity factor (β) intensifies the collision between gas and packing, which lead to the loss of airflow kinetic energy. On the basis of above, the packing B has larger gas pressure drop than that of packing A, which may attribute to more loss of airflow kinetic energy for packing B in the same operational condition. Figure 9 shows the gas pressure drop as the function of gas flow rate ranging from 40 to 100 m3/h for the dry bed. The gas pressure drop (∆Pd) increases with the increment of gas flow rate (G), which is in agreement with conventional towers. Depending on the Fanning equation, the skin drag and form drag enhance when the increasing gas overpasses the bending packing tunnel in high gravity fields, which finally brings on the increase of the gas pressure drop for dry bed. Moreover, the difference between the pressure drop of packing A and B is more pronounced when the gas flow rate is more than 80 m3/h. 3.1.3. Gas Pressure Drop of Wet Bed (∆Pw). Figure 10 shows the effect of the liquid flow rate ranging from 50 to 250 L/h on the pressure drop for a wet bed at the different gas flow rates. Comparing Figures 9 and 10, the ∆Pd

is smaller than the ∆Pw at the same high gravity factor β ) 106.2. Practically, the ∆Pw not only includes the ∆Pd, but also includes the pressure drop contributed by the flow of the thin films, fogdrop, and liquid hold-up in the cross-flow RPB. Moreover, the liquid in the RPB not only is sheared forcibly into very thin films and fogdrop, but also is rapidly renewed to form new thin films and fogdrop in the high gravity fields. It is also made sure that the ∆Pw is about 10-100 Pa higher than the ∆Pd. Meanwhile, as is shown in the Figure 10, the ∆Pw is slightly effected by liquid flow rate in this experimental condition. This is because that the cross-flow RPB could offer less liquid holdup. Therefore, the pressure drop is not obviously influenced by the liquid flow rate. An important thing is that the phenomenon of entrainment is not found in the operational condition. So, it is possible to allow higher gas and liquid flow rates in the RPB than that in a conventional packed column. Figure 11 shows the relation between pressure drop and high gravity factor at constant liquid flow rate (L ) 150 L/h) for packing A and B. The results indicate that the ∆Pw was enhanced with the increase of both the high gravity factor and the gas flow rate. Especially, the pressure drop of packing B is higher than that of packing A in the same operational condition. This implies that packing B would hold higher liquid and loss of airflow kinetic energy than that of packing A in the experimental condition. 3.2. Mass Transfer. 3.2.1. Theoretical Analysis of Liquid-Side Volumetric Mass-Transfer Coefficient kLae. The basic reaction between

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NaOH solution and CO2 is presented by the following overall reaction: CO2 + OH- f HCO3-

(5)

HCO3- + OH- f CO32- + H2O

(6)

Since eq 6 can be taken as instantaneous, all HCO3- are supposed to react with excess NaOH solution with perfect mixing. Otherwise, reaction 5 is much slower than the neutralization reaction 6, which is the controlling step. Therefore, (-rCO2) ) NCO2 ) EkLC*CO2

G × 2πr dr dy ) EkLaeC*CO22πr dr dz π(r12 - r22)

(10)

C*CO2 ) HdPCO2 ) HdPTy

(11)

where

According to the above equations, we obtain the following relation G × dy ) EkLaeHdPTy dz π(r1 - r22) 2

(7) where

According to the Danckwerts mass-transfer model: E)

kL′ √D(S + k1) ) 1 + Dk /k 2 ) √1 + Ha2 ) √ 1 L kL √DS

E ) √1 + Ha2,

(8)

Assumptions. (1) The plug-flow condition is applied to both gas and liquid phases. (2) The gas pressure drop is uniformly distributed in both axial and radial directions. (3) Liquid-side volumetric mass-transfer coefficient and gas-liquid flow rate is considered to be constant. (4) Isothermal absorption takes place in the RPB. As is seen from Figure 12, the mass balance equation for the microelement ring for the case of absorption NCO2

(12)

L -G × 2πr dr dy ) dz dx × 2aeπr dr dz ) 2 2 Z π(r2 - r1 ) (9)

combining rate eq 7, we have

Ha2 )

k2DCO2COHkL2

(13)

Substituting eq 13 into eq 12 and integrating over the axial equation from z ) 0 to z ) Z with the CO2 conditions y ) y1 and y ) y2, respectively.

∫

Z

0

kLae dz)

G π(r1 - r22) 2

∫

y2

y1

1 dy EHdPTy

(14)

Therefore, kLae is written as kLae )

πZ(r1

2

G ln(y2 /y1) - r22)EHdPT

(15)

For the kLae tests, the inlet hydroxide concentration, CNaOH, between 0.03 mol/L (0.03 N) and 0.05 mol/L (0.05 N) is used. This leads to lower Hatta numbers (0.7 < Ha < 1.6). In this range of Hatta numbers (Ha), equations 15 is sensitive to kLae and therefore can be used for the estimation of kLae. 3.2.2. Volumetric Mass-Transfer Coefficient kLae. Figure 13 shows the dependence of kLae on the high gravity factor β for various of liquid flow rates with the two packing. It is clear that the mass transfer conefficients increase with increasing high gravity factors. This is probably due to the fact that the centrifugal acceleration decreases the film resistance in the mass transfer process. In a centrifugal field, thin liquid films and tiny liquid droplets are generated, which result in a decrease in mass transfer resistance and an increase in the volumetric mass transfer coefficient. The kLae of packing B is higher than that of packing A in the same operational conditions. This

Figure 11. Effect of high gravity factor on pressure drop for a wet bed.

Figure 12. Schematic diagram of mass balance.

Figure 13. Effect of high gravity factor on volumetric mass transfer coefficient.


Figure 14. Effect of liquid flow rate on volumetric mass transfer coefficient.

implied that packing B would hold better wetting properties and structure than packing A in the experimental conditions. Figure 14 presents the kLae as a function of liquid flow rate ranging from 50 to 250 L/h at a fixed high gravity factor. The liquid flow rate has a significant influence on the kLae value; that is, the increase of the liquid flow rate lead to the increase of the kLae values. The possible reason for this phenomenon was that a higher liquid flow rate provided a greater liquid-side mass transfer, which was caused by more liquid spreading over the surface of the structured packing. Moreover, this phenomenon was attributed to the fact that more NaOH is used to absorb CO2 under a given NaOH concentration. Also, Figure 14 indicates the effect of the gas flow rate from 40 to 100 m3/h on the kLae at the high gravity factor of 106.2. The kLae values did not change significantly when the gas flow rate was increased from 40 to 100 m3/h, implying that the liquid side was the controlling resistance to mass transfer. These behaviors were also observed at a liquid flow rate of 150 L/h, implying that that the kLae values remained relatively unchanged as the gas flow rate was increased. As shown in Figures 13 and 14, the kLae of packing A is slower than that of packing B though specific surface area of packing A is larger than that of packing B at the same operational condition. The reason for this phenomenon is due to the different geometric structure and wetting performance of both packings. The experimental results showed that the kLae of the stainless steel protruded ripple plate packing (packing B) is bigger than that of the polyproplene protruded plate packing (packing A). By its periodic changeable ripple area, the stainless steel protruded ripple plate packing (packing B) could increase its effective gas-liquid interfacial area, because the periodic changeable ripple area not only makes full use of its interfacial area, but also its intensified gas-liquid contact area. Therefore, the packing B is superior to packing A in enhancing gas-liquid turbulence and mass transfer efficiency. The minimum wetting rate of the packing can be described by the following formula:25 MWR ) (4.8 × 10-9g/υε)/[(1 + cos θ/2]

(16)

The contact angle θ beween water and stainless steel packing is 79°, and that beween the water and polyproplene packing is 102°. Substituting the contact angle into formula 3, the minimum wetting rate of packings A and B are 0.08672 and 0.14360 m3/ (m · h), respectively. It is found that the minimum wetting rate with stainless steel packing was 1.65 times higher than that of polyproplene packing. The experimental results showed that the kLae of packing B was 1.4-2 times higher than that of packing A. According to the above analysis, the crucial factors on the

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Figure 15. Effect of gas-liquid flow rate on the comprehensive performance.

Figure 16. Effect of high gravity factor on the comprehensive performance.

volumetric mass transfer coefficient are material quality and structure of packing. In all, In order to improve the wetting properties of the packing, the packing surface changed by physical and chemical modification to reduce the wetting angle and intensify mass transfer efficiency. 3.3. Comprehensive Performance ∆P/kLae. To assess the packing efficiency in high gravity fields, the pressure drop and mass transfer coefficient using the absorption of CO2 by aqueous NaOH solution have been measured. But single pressure drop or mass transfer coefficient could not evaluate roundly the performance of packing. Therefore, combining the pressure drop (∆P) and mass transfer coefficient (kLae); that is, the pressure drop per unit mass transfer coefficient ∆P/kLae was regarded as the comprehensive performance of packing. Figure 15 showed the ∆P/kLae values as functions of the liquid flow rate from 50 to 250 L/h. As expected, the ∆P/kLae values reduced with the increasing liquid flow rate owing to the increasing liquid flow rate and the mass transfer coefficient having more obvious influence than gas pressure drop. In the same way, Figure 15 presents the effect of the gas flow rate from 40 to 100 m3/h on the ∆P/kLae values under the high gravity factor of 106.2. The gas flow rate influenced the ∆P/ kLae values, that is, the ∆P/kLae values increased with the gas flow rate as the liquid flow rate and high gravity factor is given. An increasing gas flow rate produced the higher effect on the gas pressure drop than the mass tranfer coefficient. Figure 16 indicates the effect of the high gravity factor ranging from 26.5 to 271.8 on the ∆P/kLae values at the gas flow rate of 60 m3/h. As expected, the increasing high gravity factor had influence on the ∆P/kLae values, which could provide

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∆Pw ) 0.08486β0.519G0.8529L0.0945

(20)

For packing B ∆Pd ) 0.1515β0.5201G0.9117

(21)

∆Pw ) 0.07956β0.4499G1.0387L0.0792

(22)

3.4.2. Liquid Phase Volumetric Mass Transfer Coefficient kLae. Apart from the high gravity factor and gas-liquid flow rate, the structure of packing and the rotating packed bed are the crucial factors on the volumetric mass transfer coefficient kLae. For the unwetting phase (gas phase), the gas phase ReG was related to gas flow rate, material quality, structure of packing and rotating packed bed, etc. Figure 17. Comparison of experimental and correlation values.

deugFg µG

ReG )

thinner liquid films and more violent collision between gas and liquid, thus leading to better mass transfer and higher pressure drop. This phenomenon was also found in that the increasing high gravity factor produced a higher effect on the gas pressure drop than the mass tranfer coefficient. As can be seen from Figure 16, the ∆P/kLae of packing B are superior slightly to that of packing A. The ∆P/kLae of packing A have a bigger range than those of packing B, that is, packing B has better stability than packing A. Moreover, packing A (polyproplene packing) has a smaller pressure drop than that of packing B. Also, the packing density of packing A is 0.42 times that of packing B, which reduces obviously the dynamics consumption of the motor. In addition, it is possible to reduce the wetting angle and intensify mass transfer effeciecy by physical and chemical modification of the packing A surface. Therefore, the packing A (polyproplene packing) has good application prospects in the future. 3.4. Correlative Expression. 3.4.1. Gas Pressure Drop. A pressure drop correlation has been proposed for the different packing structures. The dry and wet pressure drop are respectively assumed to be comprised of the following parameters: ∆Pd ) A′βaGb

(17)

∆Pw ) AβaGbLc

(18)

(23)

For the wetting phase (NaOH solution), WeL and Ga reflected the liquid flow rate, high gravity fields, surface tension, and reaction system, etc. WeL )

Ga )

uL2 FLaσ

βgl3FL2 µL2

(24)

(25)

For kLae, correlations have been proposed for the different packing structures. The term kLae was assumed to be comprised of the following dimensionless number parameters: ReG, WeL, and Ga kLae ) AReGaWeLbGac

(26)

Where A, a, b, and c are required coefficients. Correlative expression of the kLae in the cross-flow RPB are obtained with MATLAB program. All correlation coefficients from the regression analysis are above 0.99. Figure 18 shows the good agreement between the calculated and experimental kLae. For packing A kLae ) 0.003689ReG0.5449WeL0.5476Ga0.3428

(28)

Where A, a, b, and c are required coefficients. The correlative expression of gas pressure drop of the cross-flow RPB are obtained with the MATLAB program. All correlation coefficients from the regression analysis are above 0.99. Figure 17 shows the calculated and experimental values, respectively. It can be seen that the experimental results lie within 10% of the values calculated by these correlations. The expression can be used when the high gravity factor (β) is 0-271.8, gas flow rate (G) is 0-120 m3/h, and liquid flow rate (L) is 0-250 L/h. At the same time, the correlative expression coefficient shows that the high gravity factor for dry bed pressure drop is slightly larger than that for a wet bed, which is contrary to the gas flow rate. This indicate that the liquid reduces the gas flow channels, which obviously effect the wet bed pressure drop. For packing A ∆Pd ) 0.06920β0.6764G0.8203

(19)

Figure 18. Comparison of experimental and predicted KLae using existing correlations.


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Table 3. Pressure Drop Characteristics of Different Packing Structures gas-liquid contact

packing type

gas flow rate/m3/h

liquid flow rate/m3/h

high gravity factor

dry pressure drop/Pa

wet pressure drop/Pa

cross-flow cross-flow cross-flow countercurrent countercurrent

packing A packing B wire gauze26 random plastic9 foam metal23

40–120 40–120 40–120 0–4.2 0–160

0.05–0.25 0.05–0.25 0.05–0.20 0–0.138 0–0.6

0–271.9 0–271.9 0–271.9 10–402 0–192.6

10–120 15–180 12–240 300–3000 150–2800

10–130 17–240 25–260 100–5000 200–1000

Table 4. Volumetric Mass Transfer Coefficient Characteristics of Different Packing Structure gas-liquid contact

packing type

gas flow rate/m3/h

liquid flow rate/L/h

high gravity factor

volumetric mass transfer coefficient/1/s

cross-flow cross-flow countercurrent countercurrent countercurrent cross-flow countercurrent countercurrent

packing A packing B wire mesh17 wire gauze18 glass beads27 wire gauze26 wire gauze8 packed tower28

40–120 40–120 3–48 0–33

0.05–0.25 0.05–0.25 0–0.36 0.26–0.77 0.118–0.176 0.05–0.20 0–19.58

0–271.9 0–271.9 610–1140 50.7–150.4 70–134 0–271.9 40–200

0.10–0.99 0.28–1.33 0.1–0.15 0.5–1.14 0.33–0.66 0.21–0.35 0.14–1.08 0.0004–0.07

40–160 148–1026

For packing B kLae ) 0.003968ReG0.1399WeL0.4336Ga0.2724

(29)

3.5. Comparison with the Literature. 3.5.1. Gas Pressure Drop. Table 2 shows the pressure drops in this work and that in the literature, respectively. Compared with the literature data of cross-flow RPBs, the gas pressure drops of packings A and B are lower than that of the wire gauze in the cross-flow RPB.26 At the same time, it is found that the gas pressure drop of packing A is lower than that of packing B in the same operational conditions. But the new structured packings, packings A and B, have better kinetic balance than the wire gauze. The new structured packing is regularly arranged for its complete unit according to the demand of uniformity and symmetry in the RPB before installation. Therefore, the new structured packings, packings A and B, have better kinetic balance than wire gauze or random packing. To verify the pressure drop performance of the cross-flow RPB, the pressure drop obtained from this study is compared with that of the countercurrent-flow RPB equipped with random packing9 and foam metal packing.23 The gas pressure drop of the cross-flow RPB is less than 240 Pa in Table 3, which is slower than that of the cross-flow rotating packed bed with wiregauze packing, and was no more than a tenth that of the countercurrent rotating packed bed in the same operational condition. Thus, the cross-flow RPB would offer a low gas pressure drop characteristic, which has good application prospects. 3.5.2. Liquid Phase Volumetric Mass-Transfer Coefficient kLae. To verify the mass transfer performance of a cross-flow RPB during CO2 absorption, Table 4 shows the kLae values obtained from this work and those of a countercurrent flow RPB8 at different operational conditions. It is found that the kLae values of a cross-flow RPB are higher than that of a countercurrentflow RPB. The discrepancy may be caused by the difference of packing structure. Table 3 depicts that the kLae values of a crossflow RPB in this work are 0.95-2.1 times higher than that in the cross-flow rotating bed with wire-gauze packing, which is higher than that of the countercurrent rotating bed and is 1-2 orders of magnitude higher than the conventional gas-liquid mass transfer equipment in the same operational conditions, respectively. 4. Conclusion The pressure drop and volume mass transfer coefficient for a cross-flow rotating packed bed with new structured packing

have been presented. The gas pressure drop of two packings augmented with both the increasing of gas flow rate and highgravity factor and was almost independent of liquid flow rate. The gas pressure drop values of the cross-flow RPB for two packings were obviously lower than that of countercurrent RPB in the same operational condition. Moreover, the gas pressure drop of packing B was higher than that of packing A. The volumetric mass transfer coefficient increased with the augment of high gravity factor and gas-liquid flow rate, and the liquid flow rate is the main contribution. The comprehensive performance of packing B (porous corrugated plate packing with stainless steel) is superior slightly to that of packing A (plastics corrugated plate packing). Compared with the literature, the gas pressure drop of the cross-flow RPB with structured packing was slower than that of the cross-flow RPB with wire-gauze packing and was no more than a tenth of that of the countercurrent RPB at the same operational conditions. The volumetric mass transfer coefficient of porous corrugated plate packing was 0.95-2.1 times higher than that in the cross-flow rotating bed with wire-gauze packing, which was higher than that of the countercurrent rotating bed and was 1-2 orders of magnitude higher than the conventional gas-liquid mass transfer equipment at the same operational conditions. Correlative expressions of the gas pressure drop and volumetric mass transfer coefficient were obtained with the MATLAB program. The correlative expressions were in agreement with the experimental results, and their mean deviation was less than 10%. Acknowledgment This work was supported by the National Natural Science Foundation of China, Youth Science and Technology Foundation of Province Shanxi of China and the Graduate Innovation Foundation of Province Shanxi of China. Nomenclature A, a, b, c ) constants a ) specific surface area of the packing (m2/m3) ae ) effective gas-liquid interfacial area per unit volume of packed bed (m2/m3) C*CO2 ) equilibrium concentration associate with the liquid concentration (kmol/m3) COH- ) concentration of OH- ion (kmol/m3) DCO2 ) diffusivity of CO2 in sodium hydroxide solution (cm2/s) de ) hydraulic diameter (m) E ) enhancement factor of the gas-liquid flux due to the chemical reaction

3740


G ) gas flow rate (m3/h) g ) gravitational acceleration (m/s2) Hd ) solubility coefficient (kmol/kPa · m3) Ha ) Hatta numbers defined as Ha ) (k2DCO2COH-)1/2/kL Ks ) slip factor k2 ) reaction rate constant of CO2 (m3/kmol · s) kLae ) overall volumetric liquid phase mass transfer coefficient (1/s) kL ) liquid phase mass transfer coefficient (m/s) L ) liquid flow rate (L/h) l ) characteristic size of packing (m) NCO2 ) absorption rate of CO2 (mol/m2 · s) PT ) total pressure (Pa) PCO2 ) gas pressure of CO2 (Pa) ∆Pc ) centrifugal pressure drop (Pa) ∆Pd ) pressure drop in dry bed (Pa) ∆Pw ) pressure drop in wet bed (Pa) r ) radius of rotor (m) r1 ) inner radius of rotor (m) r2 ) outer radius of rotor (m) uL) liquid velocity (kg/m2 · s) ug ) gas velocity (kg/m2 · s) Vθ ) average angular velocity of gas (m/s) V ) dynamic liquid viscosity (m2/s) y1 ) mole fraction of CO2 in gas y2 ) mole fraction of CO2 in outlet gas Z ) axial height (m) Greek Symbols β ) high gravity factor β′ ) mean high gravity factor ω ) angular velocity (rad/min) θ ) contact angle (deg) ε ) porosity of the packing Fg ) gas density (kg/m3) FL ) liquid density (kg/m3) σ ) surface tension (N/m) µG ) kinematic viscosity of gas (m2/s) Dimensionless Groups ReG ) gas phase reynolds number ) (deugFg)/µG WeL ) liquid phase Weber number ) uL2/FLaσ Ga ) Galileo number ) (βgl3FL2)/µL2

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ReceiVed for reView November 29, 2008 ReVised manuscript receiVed February 7, 2010 Accepted March 5, 2010 IE9009777

Gas Pressure Drop and Mass Transfer Characteristics in a Cross-flow

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