Intensification of Mass Transfer in Hollow Fiber Modules by Adding

Aug 6, 2009 - A method of improving the absorption performance in hollow fiber contactor by adding a third solid phase into the shell side absorbent i...
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Ind. Eng. Chem. Res. 2009, 48, 8655–8662

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Intensification of Mass Transfer in Hollow Fiber Modules by Adding Solid Particles Weidong Zhang,* Geng Chen, Jiang Li, and Junteng Liu State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Thecnology, Beijing 100029, China

A method of improving the absorption performance in hollow fiber contactor by adding a third solid phase into the shell side absorbent is proposed. Powdered kieselgur, graphite, and BaSO4 are chosen as the additives to intensify the absorption process of the CO2/NaOH(aq) system. Ultrasound is used in this work as an approach to make the solid particles suspend in the liquid absorbent. The mass-transfer rate is enhanced about 40% by adding solid particles into the absorbent liquid in the presence of ultrasound. As for different types of particles, the smaller the density difference between the absorbent and the solid, the higher enhancement factor obtained. The enhancement factor is a function of the solids loading as well as the liquid velocity in the shell side of the module. The mass-transfer coefficient and enhancement factor remain almost the same with increasing pH from 7 to 11; as pH further increases, both the mass-transfer coefficient and the enhancement factor are increasing dramatically. The results also indicate that the enhancement factors increase with an increase of the packing density. The residence time distribution (RTD) curves are measured to observe the flow status in the shell side; the results demonstrate that addition of solid particles can improve the flow conditions in the shell side. A mathematical model for the intensification process based on surface renewal theory is developed; the calculated results have a good agreement with the experimental results under the present experimental conditions. 1. Introduction Emission of the greenhouse gas CO2 is the most important reason for global warming. Carbon dioxide capture and storage (CCS) is gaining attention because it is an effective way to control CO2 emission. There are several techniques for separation of CO2 from gas mixtures, such as chemical and physical absorption, adsorption, conversion, cryogenic separation, membrane separation, and membrane gas absorption.1 Among these technologies, membrane gas absorption is a promising alternative to the conventional technologies in CO2 capture applications.2,3 The membrane gas absorption technique has the advantage of operational flexibility and a large masstransfer area in a given volume, without the limitation of flooding, foaming, and entrainment, which are often encountered in the conventional apparatus.4 The attractive characteristics of the membrane contactors have driven considerable research in this area. The absorption of CO2 by sodium hydroxide in a membrane hollow fiber module was initially studied by Qi and Cussler.5,6 Since then, many researchers have worked on membrane gas absorption technology. However, some investigators have found that the nonideal flow in the shell side, such as channeling in the hollow fiber bundle and uneven flow distribution in the shell side, led to a reduction in the mass-transfer coefficient.7–17 Several researchers have made efforts to enhance the mass-transfer coefficient of hollow fiber modules. Cussler18 suggested the use of regular packed fibers into a fabric to obtain more uniform spacing. Wang7 and Wichramasinghe et al.8 used modules employing a fabric to strip oxygen or toluene from water into nitrogen. Bhaumik et al.9 used hollow fiber fabric to absorb CO2 into water. All of them obtained mass-transfer coefficients that were * To whom correspondence should be addressed. P.O. Box 1, No. 15, North Third Ring Road East, Beijing 100029, People’s Republic of China. Phone: +86-10-6442-3628. Fax: +86-10-6443-6781. E-mail: [email protected].

higher than those achieved with random packing fibers. Luo et al.19 studied the mass-transfer performance of a membrane module with a single helically wound hollow fiber in the membrane extraction process. It was found that the helically wound hollow fiber membrane provided a much better masstransfer performance. Although these modules are of good masstransfer performance, they are not easy to manufacture. Wang and Cussler7 investigated the mass-transfer performance in a fully baffled cylindrical module, and the results indicated the mass-transfer coefficients were enhanced by the improvement of the flow condition. However, this kind of module would cause a large pressure drop through the membrane modules, resulting in high energy consumption. Zhang et al.20 studied mass-transfer characteristics in hollow fiber contained liquid membrane and improved the mass-transfer coefficients of the process with the air agitating in the shell side of the module. It is an effective way to intensify the mass transfer of the membrane extraction process with the advantage of easy operation. Liu et al.21 applied an ultrasonic technique to the deoxygenation process of immersed hollow fiber membrane modules. They found that the mass transfer was improved by an ultrasonic wave in the membrane-based deoxygenation process. The enhancement effects of fine particles addition in the gas absorption process with and without reaction have been studied in the mechanically agitated reactor by several authors.22–28 Sharma and Mashelkar23 were first to qualitatively demonstrate the increase in the mass-transfer rate in a bubble column by small particles. Dagaonkar et al.24 investigated the physical absorption of pure CO2 in various liquids containing micrometersized particles in a stirred cell. As for the chemical absorption process in a stirred cell, Saha et al.28 undertook experimental investigations on the absorption of CO2 in alkanolamines in the presence of fine activated carbon particles. One of the main mechanisms for the gas-liquid mass-transfer enhancement by adding solid particles is boundary layer mixing. The collisions of solid particles can reduce the effective liquid boundary layer

10.1021/ie9004964 CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

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thickness, and the turbulence caused by the solid particles can yield a good mixing degree of the boundary layer and the liquid bulk. In the membrane gas absorption process, it is generally considered that there is nonideal flow in the module shell side; for this purpose, the third solid particles can be introduced into the membrane gas absorption process to improve the flow condition and intensify the mass transfer. On the basis of the previous research,7–28 a method to improve the mass-transfer coefficient of the membrane gas absorption process by adding the solid particles is proposed. In this process, physicochemical stable particles are introduced into the absorbent and ultrasound is applied to make the particles suspending in the absorbent. The effects of solid concentration, type of particles, pH value of the liquid absorbent, liquid phase velocity, and module packing density on the enhancement factor are investigated. Also, the effects of ultrasound and particles without ultrasound are studied.

Under the condition with the presence of solid particles in the absorbent, the turbulence of the particles would increase the mass-transfer coefficient. As discussed above, the solid particles added into the absorbent improve the mixing degree of the boundary layer and the liquid bulk. This phenomenon can be described by the surface renewal theory. Therefore, based on the surface renewal theory, the shell side mass-transfer coefficient, kL,R, can be obtained as follows32–34 kL,R ) RD1/2sD1/2 ) RD1/2

1 1 1 1 ) + + KL kG km mEchemkL

(1)

where KL is the overall mass-transfer coefficient based on the liquid phase and kG, km, and kL are individual mass-transfer coefficients of the gas side, the membrane, and the liquid side, respectively. Since pure CO2 is used and the membrane is hydrophobic, the mass-transfer resistances in the gas side and membrane phase can be neglected.30,31 The variable m represents the distribution coefficient of CO2 between the gas and liquid phases. As for the experimental condition without adding particles into the absorbent, the liquid side mass-transfer coefficient, kL, could be obtained by the correlation developed by Prasad and Sirkar14

( )

Sh ) β

dH (1 - φ)Re0.6Sc0.33 L

(2)

where β is a constant, φ is the packing density of the module, L is the effective length of the module, Re is the Reynolds number of the shell side, and Sc is the Schmidt number, defined as Sc )

µ FD

1/4

) RD1/2Re1/4Φ-1/4 (6)

where R is a constant, which can be obtained by model fitting from experimental data, sD is the surface renewal rate, and Φ is the hold-up of the dispersed phase. Echem is the enhancement factor due to chemical reaction,35–37 which is given by the following

2. Theory The overall mass-transfer coefficient can be expressed in a resistance in series model29 as eq 1

( Fgu µΦ )

Echem

(Ha*)2 + )2(E∞* - 1)



E∞*(Ha*)2 (Ha*)2 +1 + 4(E∞* - 1)2 E∞* - 1 (7)

On the basis of Leveque model, the asymptotic infinite enhance37 ment factor, E*, ∞ is defined as

(

E∞* ) 1 +

)( )

DCO2 COH-,0DOHνOH-mCCO2DCO2 DOH-

1/3

(8)

where DCO2 and DOH- are the diffusion coefficients of CO2 and hydroxyl in aqueous solution, respectively. COH-,0 is the hydroxyl concentration at the inlet of the module. VOH-, the stoichiometric coefficient of OH-, is 2 and m, the partial reaction order with respect to CO2, is 1, which are obtained from the chemical reaction between CO2 and OH-. The Hatta number in eq 7 can be described based on the conditions at the module inlet Ha* )

√krDCO ,LCOH ,0 2

kL

-

(9)

where kr is the chemical reaction rate constant, which is obtained from ref 29. The overall mass-transfer coefficient with and without the presence of the solid particles can be obtained by solving eqs 1-9.

(3) 3. Experimental Section

Sh is the Sherwood number, defined as Sh )

kLdH D

(4)

where dH is the hydraulic diameter of the shell side, defined as the cross-sectional flowing area divided by the wetted perimeter dH )

ds2 - nd2o ds + ndo

(5)

In the present experimental conditions, the ultrasound has little effect on mass transfer, as discussed later. Thus, eq 2 can be used to calculate the mass-transfer coefficient in a particle-free liquid with and without the presence of the ultrasound.

3.1. Experimental Setup and Procedures. Absorption experiments are carried out in a hollow fiber membrane contactor, with gas phase flowing inside the fiber. Figure 1 schematically shows the experimental setup. The pure CO2 gas, through a gas flow meter, is fed into the lumen side. The gas flow rate is held constant for all runs while the absorbent flow rate is varied. The liquid absorbent is introduced by a peristaltic pump from the solution tank, through a liquid-flow meter, to the fiber shell side. The gas and absorbent flow are in a countercurrent mode. The liquid-phase pressure is slightly larger than that at the gas phase in order to prevent bubble formation; however, the transmembrane pressure drop is less than 0.015 MPa, which is far lower than the breakthrough pressure. In order to prevent fiber wetting, air is used to make the outer surface of the fibers drying prior to each experiment

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Figure 1. Schematic flow diagram for membrane gas absorption. Table 1. Properties of Solid Particlesa particles

FS (kg · m-3)

kieselguhr graphite BaSO4

2.4 × 10 2.6 × 103 4.2 × 103 3

D10 (µm) 16.00 10.78 4.16

Table 2. Module Characteristics

D50 (µm 0 24.93 22.20 8.12

D90 (µm) 41.20 43.05 17.53

DAV (µm) 27.10 25.07 9.69

a

Note: D10, D50, and D90 are the diameters at percentage points 10%, 50%, and 90%, respectively. DAV is the average diameter.

and the operation time of each experiment (in the presence and absence of ultrasound) is kept below 4 h. The membrane module is immersed into a stainless steel ultrasound bath (53 length × 32 width × 15 height cm3, with a frequency of 40 kHz and a power output of 500 W). Here, ultrasound is used to make the particles suspending in the liquid absorbent when the absorbent with solid particles flow through the shell side of the module. In addition, the use of ultrasound could reduce the fouling of the membrane in the membrane gas absorption process. The water in the ultrasound bath is circulated and cooled outside of the bath to eliminate the thermal effect caused by the ultrasound; the temperature of the absorbent remains constant. The liquid absorbent at the inlet of the module maintains fresh, so the CO2 loading is zero. CO2 gas diffuses through the membrane pores into the liquid in the shell side and is absorbed by the absorbent. Steady state is indicated by a constant gas flow rate at the outlet of the module, which is measured by a soap film flow meter. Two or three duplicate measurements are taken for each operating condition and averaged. 3.2. Materials and Modules. Pure carbon dioxide (Beijing Praxair Industrial Gas Co., Ltd.) of more than 99.9% purity is used as the feed gas. The solvent used in the study is of analytical grade. Deionized water is used to prepare the aqueous sodium hydroxide solutions. The absorbent is 0.1 mol/L NaOH aqueous solution. Kieselgur (Beijing Yili Refined Chemical Co., Ltd.), graphite, and BaSO4 (Tianjin Fuchen Chemical Reagent Plant) are used as the third solid phase adding into the absorbent liquid. The properties of the used solid particles are given in Table 1. The solid particles’ size is determined by a laser particle size analyzer (Rise 2002, Jinan Runzi Technology Co., Ltd.). The hollow fiber membrane contactors used in the experiments are made of glass; therefore, the shell side condition can be observed clearly. The material of the fibers is polypropylene, supplied by Hangzhou Qiushi Membrane Technology Ltd. The ends of the fibers are potted with an epoxy. Modules with different packing densities are made. Details of the modules used in this study are listed in Table 2. The fiber distribution in

no.

shell i.d. (cm)

fiber o.d. (µm)

fiber thickness (µm)

fiber length (cm)

no. of fibers

contact area (cm2)

packing fraction (%)

1 2 3 4 5

1.0 1.0 1.0 1.0 1.0

450 450 450 450 450

50 50 50 50 50

30 30 30 30 30

20 50 100 150 180

84.82 212.06 424.12 636.17 763.41

4 10 20 30 36

the shell is not uniform, which is attributed to the random packing of fibers in the module and polydispersity of fiber diameters. 3.3. Data Analysis and Enhancement Factors. In order to eliminate the gas side mass-transfer resistance, experiments are carried out using pure CO2 in the tube side. The total masstransfer rate coefficient based on the liquid side is expressed as KL )

N Am∆Cm

(10)

where N is the gas absorption rate, obtained by measuring the difference of the gas flow rate between the inlet and the outlet of the module since pure CO2 was used, Am is the mass-transfer area based on the surface area of gas-liquid contact, and ∆Cm is the logarithmic mean concentration difference of CO2 in the liquid phase. In all experiments, the fresh absorbent is used, so the carbon dioxide concentration at the inlet of the liquid phase, CLin, is zero and ∆Cm can be calculated as follows ∆Cm )

(

ln

Cout L HCG HCG - Cout L

)

(11)

where H is the Henry’ law coefficient, CG is the carbon dioxide concentration in the gas phase, and CLout is the carbon dioxide concentration at the outlet of liquid phase. The liquid samplers are measured by the chemical titration method. The mass-transfer enhancement factor, E, is calculated from E)

KL (KL)0

(12)

where KL is the overall mass-transfer coefficient based on liquid with suspended particles in the presence of ultrasound and (KL)0 is the coefficient in the particle-free liquid in the presence of ultrasound.

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Figure 2. Effect of particles loading on the mass-transfer coefficient at different liquid velocities for the kieselgur/NaOH(aq) system (CNaOH ) 0.1 mol/L, module 3).

3.4. Residence Time Distribution. RTD (residence time distribution) analyses are conducted to demonstrate how the packing density of modules and the addition of particles affect the flow conditions. A pulse tracer method using saturated KCl solution as tracer is applied to examine the flow status in hollow fiber modules. Data collected from each tracer evaluation were transformed into RTD curves for normalized comparison. The normalized RTD curves are obtained as follows E(t) )

C(t)

∫

∞

0

(13)

C(t) dt

where E(t) is the residence time distribution function and C(t) is the concentration of KCl in outlet liquid phase at time t. 4. Results and Discussion 4.1. Influence of Solids Concentration. The turbulence caused by particles in the absorbent could improve the flow condition in the shell side, leading to a high mass-transfer coefficient. The key point of applying the technique to the membrane gas absorption is to keep the fine particles suspending in the absorbent during the absorption runs. In this work, ultrasound is applied to make the particles suspending in the absorbent. Nevertheless, the ultrasound has less effect on the gas absorption process, and the enhancement effect is small in the membrane gas absorption process when ultrasound is used alone. Here, ultrasound is just an approach to suspend the particles in the liquid; the power and frequency of the ultrasound are kept constant. The absorption processes are carried out under several conditions, such as in particle-free liquid with and without the presence of ultrasound, with the addition of solid particles in the presence and absence of ultrasound, for comparison. It can be seen in Figure 2 that the ultrasound has less effect on mass transfer under the present experimental conditions, which may be caused by the frequency of the ultrasound and position of the module placed in the ultrasound bath in this work. The solid particles have a smaller influence on the mass-transfer enhancement in the absence of ultrasound, since the particles cannot suspend in the liquid absorbent under the condition without ultrasound; the turbulence caused by the particles may not occur in such circumstances. The following experiments are all carried out under the presence of ultrasound. Experiments were also carried out at different particles concentrations of kieselgur in the range of 0-2.2 kg/m3 with 0.1 mol/L sodium hydroxide solution as absorbent in the presence of ultrasound. The dependence of mass-transfer

Figure 3. Effect of particles loading on the enhancement factor at different liquid velocities for the kieselgur/NaOH(aq) system (CNaOH ) 0.1 mol/L, module 3).

coefficients and enhancement factors on the solid loading at four different liquid velocities (1.06, 2.13, 3.19, and 4.26 cm/ s) are presented in Figures 2 and 3, respectively. The results indicate that the mass-transfer coefficients are enhanced by adding the solid particles into the absorbent. With different particles loading, the mass-transfer coefficients increase quickly and remain flat after S ) 1.0 kg/m3. The higher mass-transfer coefficients and enhancement factors are observed at particles concentration between 1.0 and 1.5 kg/m3. However, above 1.5 kg/m3, the mass-transfer coefficient and enhancement factor gradually decrease as particles concentration increases. The enhancement factor is strongly affected by the particles loading. Dagaonkar et al.24 investigated the absorption of pure CO2 in water containing TiO2 particles in a stirred cell at different solid concentrations. The results indicated that the enhancement factors increased with an increase of the solid loading and then appeared to level off. In the experiment carried out by Ozkan et al.,26 the enhancement efficiency was increased at low solids volume fraction and reduced as the solids volume fraction further increased. In this work, at low solids concentration, with the increasing of solid loading, the effective number of the particles intensifying the turbulence in the shell side liquid increase and the flow condition in the shell side is improved. Consequently, the mass transfer in the shell side is enhanced. With the solids loading continuously increasing, the solid particles added into the absorbent may play a passive role in the mass-transfer process; for instance, the existence of the particles could increase the apparent viscosity of the fluid in the shell side, which could increase the mass-transfer resistance. Therefore, the mass-transfer coefficients decrease slightly at higher solid loading. 4.2. Influence of the Type of the Solid Particles. Figure 4a and 4b show the enhancement of the mass-transfer coefficient versus the particles loading at different liquid velocities. The results show that the enhancement factors for kieselgur and graphite are similar, while the results of BaSO4 are relatively smaller. Ozkan et al.26 investigated the effect of inert solid particles on mass transfer in mechanically agitated reactors; they found the solid particles could increase the mass-transfer coefficient in the O2/water system, while the enhancement efficiency was different for various particles. The different performance of various kinds of solid particles could be attributed to the nature of the particles. It can be seen from Table 1 that kieselgur and graphite have similar average diameters and densities, so these two kinds of particles have similar performance, while the density of powdered BaSO4 is relatively larger than others. The mechanism of the enhancement by adding solid particles is the enhanced

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Figure 4. (a) Comparison of the mass-transfer enhancement factor by different solid particles, kieselgur, graphite, and BaSO4 (liquid velocity, uL ) 1.06 cm/s, CNaOH ) 0.1 mol/L, module 3). (b) Comparison of the masstransfer enhancement factor by different solid particles, kieselgur, graphite, and BaSO4 (liquid velocity, uL ) 3.19 cm/s, CNaOH ) 0.1 mol/ L, module 3).

turbulence caused by the particles in the module shell side, therefore improving the flow condition of the shell side. Thus, the density difference between solids and liquid absorbent may have a significant effect on the intensity caused by the vibration of fine particles. On one hand, it is easier to vibrate for particles with a small density difference than those with a relatively larger density difference in the same condition. On the other hand, with the larger density difference between particles and liquid, the depositing motion of the particles in the liquid bulk plays a dominant role in the movement of particles. Under this condition, the degree of mass-transfer intensification is small. The density difference between powdered BaSO4 and absorbent is larger than that of other two types of particles, so the masstransfer enhancement of BaSO4 particles is lower than that of other particles. Besides the density of the particles, some other characteristics, such as the affinity, degree of sphericity, chemical property, etc., may have an influence on the intensification process. It is necessary to choose more appropriate types of solid particles to investigate the effect of particles characteristics on the masstransfer intensification process in further studies. In the following experiments, kieselgur is chosen as the addition particles. 4.3. Influence of pH. Experiments are carried out at different pH in the range from 7 to 13 to investigate the effect of absorbent pH on the mass-transfer coefficient and mass-transfer enhancement efficiency. The absorbent pH is controlled by adjusting the concentrations of NaOH. Powdered kieselgur is added to the absorbent as the third solid phase to enhance the mass transfer. Figure 5a and 5b plot the mass-transfer coefficient and enhancement factor versus absorbent pH values. As can be seen, the mass-transfer coefficient remains almost the same with increasing pH from 7 to 11; enhancement factors in this condition reamin around 1.46 and 1.38 at s liquid velocity of

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Figure 5. (a) Effect of absorbent pH on the mass-transfer coefficient and enhancement factor (liquid velocity, uL ) 1.06 cm/s, solid concentration of kieselgur, S ) 1.0 kg/m3, module 3). (b) Effect of absorbent pH on the mass-transfer coefficient and enhancement factor (liquid velocity, uL ) 3.19 cm/s, solid concentration of kieselgur, S ) 1.0 kg/m3, module 3).

Figure 6. Mass-transfer coefficient and enhancement factor as a function of liquid velocity in the range from 1.06 to 5.23 cm/s (solid concentration of kieselgur, S ) 1.0 kg/m3, CNaOH ) 0.1 mol/L, module 3).

1.06 and 3.19 cm/s, respectively. However, as pH further increases, both the mass-transfer coefficient and the enhancement factor increase dramatically. With pH ranges from 7 to 11, the concentrations of NaOH in the absorbent are so small that the enhancement effect of chemical reaction on the mass transfer can be neglected. As the concentration of hydroxyl further increases in the liquid absorbent, especially when the pH is above 12, the absorption rate changes to diffusion controlled since the reaction rate of CO2 with NaOH is fast. As mentioned above, pure CO2 gas is used as feed gas; the overall mass-transfer resistance is equal to the mass-transfer resistance in the liquid phase. Thus, the overall mass-transfer resistance in the membrane absorption process is controlled by diffusion of CO2 in the liquid absorbent. With the presence of solid particles in liquid absorbent, the turbulence caused by the particles can improve the flow condition in the module shell side, which affects the diffusion behavior of the solute in the

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Figure 7. Effect of module packing density on the mass-transfer enhancement efficiency (solid concentration of kieselgur S ) 1.0 kg/m3, CNaOH ) 0.1 mol/L).

liquid phase. Consequently, the intensifying effect is much stronger in the diffusion-controlled condition, such as higher absorbent pH value. 4.4. Influence of the Liquid Flow Rate in the Shell Side. Experiments were performed with a solid loading of 1.0 kg/m3 at different absorbent velocities in the range of 1.06-5.23 cm/s. The mass-transfer coefficients and enhancement factors as a function of the liquid velocity are demonstrated in Figure 6. Since pure CO2 is used in the process, the mass transfer was controlled in the liquid side; as a result, the mass-transfer coefficients increased with the liquid velocity. A higher liquid flow rate provides a higher degree of turbulence in the shell side, leading to a higher renewal rate of the liquid near the membrane surface, which reduces the mass-transfer resistance greatly. The results also indicate that the enhancement factor decreases with increasing liquid velocity. Similar results are also observed in Figures 2-5. This may be explained by the difference of the residence time of the particles in the shell side at different liquid velocities. With increasing liquid velocity, the residence time of the particles in the shell side becomes short. In addition, since the mass-transfer coefficient is high at higher liquid velocity, the space for further improvement of the

mass-transfer coefficient by solid particles is limited at higher liquid velocity. Consequently, the mass-transfer enhancement efficiency caused by solid particles is reduced at higher liquid velocity. However, the ratio of KL/(KL)0 is still greater than 1 at higher liquid velocity, which means it still has a strengthening effect. In addition, the enhancement factor is higher at lower liquid velocity; the results indicate that the particles can better improve the mixing degree of the boundary layer and liquid bulk at lower liquid velocity, and therefore, the intensification effect becomes better. 4.5. Influence of the Fiber Packing Density. The merit of the hollow fiber contactors is that they can provide a relatively large mass-transfer area in a given volume. However, with increasing module packing density there exists nonideal flow in the shell side of the module, which is attributed to the random packing of fibers in the module, polydispersity of fiber diameters, and influence of the wall effect of the module, and so on. As a consequence, the mass-transfer coefficient decreases as packing fraction increases.11,14 In this study, the third solid particles are added into the absorbent, aiming to improve the flow condition and increase the mass-transfer performance. In order to investigate the performance of the solid particles in various modules with different packing density, the specific experiments are carried out, and the enhancement factors obtained at different liquid velocities and various packing densities are plotted in Figure 7. It can be seen that the enhancement factors increased with an increase of packing density in the range from 0.08 to 0.36. RTD curves under varying module packing density are presented in Figure 8, which demonstrates that the liquid flow in the module shell side is maldistribution and different at each packing fraction. The researches of Sirkar et al.,14 Fane et al.,16 Wu and Chen,11 and Gawronski and Wrzesinska15 showed that the packing density of hollow fiber modules had a great effect on the shell side flow condition and mass-transfer efficiency. In the predictions of Sirkar et al.,14 Fane et al.,16 as well as Gawronski and Wrzesinska,15 the mass-transfer coefficient in the shell side decreases with increasing module packing density.

Figure 8. RTD curves for impulse of KCl tracer under varying module packing densities (liquid velocity, uL ) 2.13 cm/s, packing density ) (a) 0.04, (b) 0.10, (c) 0.30, and (d) 0.36).

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With the lower packing density, as in modules 1 and 2, the shell side flow is close to ideal plug flow. There are few dead zones and channeling effects in the loosely packed module due to the fibers being widely spaced. The improvement of the shell side flow condition caused by adding particles in the absorbent is not remarkable. Consequently, the enhancement factors obtained in the loosely packed modules are relatively smaller. With the packing density increasing, there are dead zones between the bundles and also channeling effects in the shell side as a result of maldistribution and bypassing of the liquid flow in the shell side.11,13 The dead zones and channeling effects lead to the tailing of the RTD curves (see Figure 8c and 8d). Addition of solid particles reduces the degree of the tailing of RTD curves. Herein, the turbulence caused by solid particles enhances the radial mixing of the liquid flow in the shell side, which improves the flow condition in the shell side, reduces the dead zones, and eliminates the channeling effect. This can make the flow distribution in the shell side become more uniform and accordingly reduce the mass-transfer resistance. The results of membrane absorption experiments and RTD curves indicate that the intensifying effect of the third solid particles is mainly caused by improving the flow condition in the module shell side. 5. Conclusion A method by introducing the third phase solid particles into the absorbent to intensify the mass transfer of the hollow fiber membrane contactor is proposed in this work. Powdered kieselgur, graphite, and BaSO4 are chosen as additives to the absorbent to intensify the absorption of the CO2/NaOH(aq) system. Ultrasound is used to make the solid particles suspending in the liquid absorbent. The solid particles have a smaller influence on the mass-transfer efficiency in the absence of ultrasound. Under the condition in the presence of ultrasound, experiments are carried out in order to evaluate the effects of the particles concentration in the liquid, the type of solid particles, pH of the liquid absorbent, liquid velocity of the shell side, and packing density on the enhancement efficiency. It is evident that the mass transfer is enhanced with the addition of the solid particles for about 40%, which is attributed to the particles intensifying the turbulence and the radial mixing of the liquid flow. Higher enhancement factors are obtained with a smaller density difference between liquid and solid particles under the present experimental conditions. The enhancement factor is dependent on the particles concentration, and the highest enhancement factors are obtained at particles concentrations of 1.0-1.5 kg/m3. The pH of the liquid absorbent plays an important role in the intensification process; a high enhancement factor is obtained at high pH. The enhancement efficiency is also dependent on the liquid velocity of the shell side, and the enhancement factor at low liquid velocity is higher than that at high liquid velocity. The enhancement factor increased with an increase of the packing density. RTD analyses are conducted to demonstrate how the packing density of modules and addition of particles affect the flow conditions. The results show that addition of solid particles can improve the flow conditions in the shell side and reduce the tailing of the RTD curves. On the basis of the surface renewal theory, a mathematical model for the intensification process was developed. The calculated masstransfer coefficients and enhancement factors are in good agreement with experimental results.

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Acknowledgment This work was supported by the National High Technology Research and Development Program of China (863 Program; 2008AA062301) and the Program for New Century Excellent Talents in University (NCET-05-0122). Nomenclature Am ) contact area based on the fiber outside diameter (m2) C ) concentration of the solute (mol/m3) ∆Cm ) logarithmic mean concentration difference of carbon dioxide (mol/m3) dH ) hydraulic diameter of the shell side (m) do ) outside diameter of hollow fiber (m) ds ) shell side diameter of module (m) D ) diffusion coefficient (m2/s) E ) enhancement factor (-) Echem ) enhancement factor due to chemical reaction (-) E*∞ ) asymptotic infinite enhancement factor (-) E(t) ) residence time distribution function g ) gravitational acceleration (m/s2) Ha* ) Hatta number (-) k ) individual mass-transfer coefficient (m/s) kr ) reaction rate constant (m3/(mol · s)) KL ) mass-transfer coefficient of the liquid phase (m/s) m ) physical solubility (-) n ) number of hollow fibers in the module N ) carbon dioxide absorption rate (mol/s) Re ) Reynolds number sD ) surface renewal rate S ) solid concentration in the liquid phase (kg/m3) Sc ) Schmidt number Sh ) Sherwood number t ) time (s) uL ) velocity of absorbent (cm/s) Greek Letters R ) constant in eq 6 β ) constant in eq 2 Φ ) dispersed phase hold up φ ) packing density of hollow fiber module µ ) viscosity (Pa · s) ν ) stoichiometric coefficient (-) F ) density (kg/m3) Superscripts in ) inlet of liquid phase out ) outlet of liquid phase Subscripts G ) gas phase L ) liquid phase m ) membrane phase R ) based on the surface renewal theory

Literature Cited (1) Yang, H. Q.; Xu, Z. H.; Fan, M. H.; Gupta, R.; Slimane, R. B.; Bland, A. E.; Wright, I. Progress in carbon dioxide separation and capture: A review. J. EnViron. Sci. 2008, 20, 14. (2) Li, J. L.; Chen, B. H. Review of CO2 absorption using chemical solvents in hollow fiber membrane contactors. Sep. Purif. Technol. 2005, 41, 109.

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ReceiVed for reView March 26, 2009 ReVised manuscript receiVed June 20, 2009 Accepted July 7, 2009 IE9004964