Enhancement of Mixing and Mass Transfer Performance with a

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Enhancement of Mixing and Mass Transfer Performance with a Microstructure Minireactor for Controllable Preparation of CaCO3 Nanoparticles K. Wang, Y. J. Wang, G. G. Chen, G. S. Luo,* and J. D. Wang The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China

CO2/Ca(OH)2 precipitation reaction was used to prepare CaCO3 nanoparticles in this work. As a fast reaction system, nice mixing and a fast mass transfer rate of CO2 are required to enhance supersaturation. To increase the mixing performance, a microstructure reactor, a membrane dispersion minireactor, which has microfiltration membranes as the dispersion media, was introduced and CaCO3 nanoparticles with average diameters ranging from 34.3 to 110 nm were prepared. Several operating conditions were investigated for the purpose of controlling the particle size. In addition, a deep analysis on the mass transfer flux density of CO2 was carried out and the particle size decreased with the increasing of the mass transfer flux density during the reaction. Compared with a membrane-free reactor, it was found that the mixing performance was significantly enhanced by the effect of the micropore membrane, and nanoparticles cannot be prepared without it. Introduction Nanoparticles have received wide attention over the past decade, due to their special characteristics, such as surface effect and quantum effect. Among them, nanosize calcium carbonate is of great interest, since it can be used as a well pigment or functional filler in plastics, rubber, paper, paints, etc.1,2 Several methods have been developed to prepare nanosize calcium carbonate, including emulsion liquid membrane technology3 and gas-liquid carbonation method.4 Considering the demand from industrial preparation, carbonation is one of the best processes, since calcium carbonate can be easily precipitated from the mixing of carbon dioxide and calcium hydroxide. However, this process is not easily controlled and the study of dispersion, mixing, and mass transfer problems in the synthesis of nanoparticles is still insufficient. According to the theories about crystallization kinetics, the control of the nucleation and growth process by supersaturation of the solution is the key to controlling the particle size.5,6 To obtain nanoscale particles, the nucleation process should be enhanced and the growth process must be restricted, since nanoparticles are formed from small nuclei and enlarged during the growth process.7 Compared with the calcium carbonate growth process, the nucleation process is more sensitive to supersaturation, and high supersaturation is required for synthesizing nanoparticles.8,9 In the process of calcium carbonate precipitation, the supersaturation ratio can be defined by eq 1.

[Ca2+][CO32-] s) Ksp

Figure 1. Membrane dispersion minireactor: 1, continuous phase vessel; 2, measuring pump; 3, membrane dispersion minireactor; 4, valve; 5, flow meter; 6, pressure gauges; 7, source of the mixed gas; 8, pH indicator; 9, temperature control bath.

The mechanisms involved in this process have been well understood as shown in the following schemes.10

CO2 + OH- f HCO3(k1 ) 8 × 103 L mol-1 s-1 at 25 °C) (I) HCO3- + OH- f CO32- + H2O (k2 ) 6 × 109 L mol-1 s-1 at 25 °C) (II)

(1)

where [Ca2+] is the concentration of calcium ion (mol/m3), [CO32-] is the concentration of carbonate ion (mol/m3), and Ksp is the solubility product of calcium carbonate. Since high supersaturation is required to produce nanoparticles, high concentrations of calcium ion and carbonate ion are needed. * To whom correspondence should be addressed. Tel.: +86 10 62783870. Fax: +86 10 62783870. E-mail: [email protected].

Ca(OH)2 T Ca2+ + 2OH-

(III)

From the view of kinetics the bicarbonate ion can be formed rapidly by the direct reaction of carbon dioxide and the hydroxyl ion, and then it will be transformed to carbonate ion quickly, too. In general, the calcium hydroxide dissociates strongly in aqueous solution, so the calcium ion can be assumed to be sufficient.10 Although these processes are ultrafast, the mass transfer rate of carbon dioxide from the gas phase to the liquid phase is a slow process, and the concentration of carbonate ion

10.1021/ie061502+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007

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Figure 2. Visualization system: 1, computer; 2, high-speed CCD video camera; 3, microscope; 4, membrane dispersion reactor with wide exit; 5, mixing room; 6, valve; 7, flow meter; 8, source of the mixed gas; 9, water (25 °C); 10, measuring pump; 11, pressure gauges.

is determined by the mixing process. Therefore, the supersaturation is mainly affected by the mass transfer process of carbon dioxide. According to this, the enhancement of mixing of the two phases and fast mass transfer rate of carbon dioxide are strongly called for to synthesize calcium carbonate nanoparticles, and the particle size can be controlled by controlling the mass transfer process. In recent years microstructure reactors have triggered an explosion of scientific and industrial interest.11-13 These reactors exhibit numerous practical advantages in reactor design and production process, including efficient enhancement of mixing and the mass transfer process.14,15 In our laboratory a microstructure reactor, a membrane dispersion minireactor with microfiltration membranes as the dispersion media, has been developed and used to prepare nanoparticles in homogeneous mixing systems.7,16,17 With the help of a microporous filtration membrane, nice mixing behavior and a fast mass transfer rate were obtained in this minireactor with low equipment cost.14,18 In this work, calcium hydroxide suspension and carbon dioxide/nitrogen mixed gas (29.8% volume fraction CO2) were selected as the working system to prepare calcium carbonate nanoparticles in this minireactor. As a heterogeneous gasliquid-solid reaction system, gas bubbles with average diameters ranging from 0.9 to 2 mm were produced by the strong shear flow of the continuous phase of calcium hydroxide suspension, and a fast mass transfer rate of carbon dioxide was obtained between the two phases. Effective mixing was achieved by the effect of strong turbulence caused by the high flow rate in this minireactor. Additionally, the section of mixing chamber in this minireactor where the two phases contact each other was only 4 mm × 2 mm, and large amounts of minibubbles were produced in this confined space under the experimental conditions. Because of the short space time (just 20-40 ms), the coalescence of the bubbles could be neglected and the fluid of continuous phase was efficiently segmented to thin films by the bubbles, which enhanced the mass transfer of carbon dioxide in the liquid phase with a short mass transfer distance. Calcium carbonate nanoparticles with average diameters ranging from 34.3 to 110 nm were prepared at different operating conditions. For further analysis, the bubbles produced in this minireactor were recorded with a high-speed camera using the mixed gas and water system. The mass transfer flux density of carbon dioxide was calculated, and the effect of the mass transfer rate of carbon dioxide on the particle size was investigated in this article. A membrane-free minireactor was introduced in this research, too, and a bad mixing performance

Figure 3. XRD results at different operating conditions. (A) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (B) FC ) 159 mL min-1, FD ) 199 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (C) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.37, 5 µm membrane, 25 °C; (D) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 30 µm membrane, 25 °C.

was obtained without the membrane. Compared with the particles prepared in the membrane dispersion minireactor and the membrane-free reactor, it was found that the supersaturation was efficiently enhanced by the dispersion effect of the micropore membrane. Experimental Section Materials. The chemicals mainly used in this work included calcium hydroxide (analysis pure, Beijing Modern Eastern Fine Chemical Co. Ltd.) and mixed gas (29.8% volume fraction CO2, Beijing Haiwen Gas Co. Ltd.). Experiments. Figure 1 shows the membrane dispersion minireactor (equipment 3) and reaction circuit in our experiments. A stainless steel microfiltration membrane was used in the minireactor as the dispersion medium. The active membrane area was 16 mm2, and the volume of the mixing chamber was 221 mm3. During the reaction process the mixed gas, stored in equipment 7, was pressed though the membrane and transformed to small bubbles by the cross-flow drag force. A 250 mL volume of calcium hydroxide suspension was used as the continuous phase and stored in a vessel with a stirrer to help the dissolution of calcium hydroxide. The mixed gas was pressed though the membrane at the beginning of each experiment, and then calcium hydroxide suspension was pumped directly into the mixing chamber. The pressure of the mixed gas was fixed at 0.2 MPa, and the flow rate was adjusted by the valve behind

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Figure 4. TEM images of CaCO3 nanoparticles. (A) FC ) 56 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (B) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.19, 5 µm membrane, 25 °C; (C) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 10 µm membrane, 25 °C; (D) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 40 °C.

Figure 5. Size distribution of CaCO3 nanoparticles (A) FC ) 365 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (B) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C.

Figure 7. Effect of disperse phase flow rate on particle size.

Figure 6. Effect of continuous phase flow rate on particle size.

Figure 8. Effect of Ca(OH)2 volume fraction on particle size.

the flow meter. The two phases contacted each other in the mixing chamber and then circulated back to the bottle and separated from each other. The pH values in the bottle (inlet of the reactor) and the outlet of the minireactor were captured to monitor the reaction process. To control the temperature, a water bath was used, and the fluctuation of the temperature was less than 2 °C. Visualization and Analysis. For the purpose of capturing the bubbles produced in this minireactor, a transparent window

in the mixing chamber was introduced in this work as shown in Figure 2. The mixed gas and water at 25 °C were chosen as the system to get clear pictures. A high-speed CCD video camera with a microscope (equipment 3) was used to record the images at the outside of the mixing chamber with a frequency of 100 images per second. The average bubble size (d32G) was determined by measuring the sizes of at least 100 minibubbles from the images using custom-made image-analysis software.

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Figure 9. Effect of membrane pore size on particle size.

Figure 12. Mixed gas bubbles produced in membrane dispersion minireactor at different operating conditions: (A) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (B) FC ) 262 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (C) FC ) 365 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (D) FC ) 159 mL min-1, FD ) 65 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C.

Figure 10. Effect of temperature on particle size.

Figure 13. Change of pH values with reaction time at the inlet and the outlet of the reactor. FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C. Figure 11. Concentration profiles for gas/liquid absorption with chemical reaction.23

After reaction the particles were separated with a centrifugal separator (LD5-2A, Beijing Medical Centrifugal Separator Factory). Then they were washed three times with deionized water and once more with absolute ethyl alcohol. In the end the particles were dried in a drying cabinet (80 °C) for 12 h. The crystal form of the nanoparticles was characterized by X-ray diffraction analysis (Rigaku Corporation D/max-RB), and the morphology was recorded by transmission electron microscopy (TEM; JEOL-2010 120 kV) images. The average diameter of the particles (d32) was determined by measuring at least 200 particles in the images. Results and Discussion Crystal Form, Morphology, and Size Distribution. The results given by the X-ray diffraction analysis are shown in Figure 3, which indicates that all of the CaCO3 particles are of calcite structure19 despite different preparation conditions. The TEM images are shown in Figure 4, and the particle size distributions are shown in Figure 5, where n refers to the number

Figure 14. Mass transfer flux densities of CO2 and particle sizes at different continuous phase flow rates.

fraction. It is clear that the sizes of the particles are in the nanometer range, and their distribution is narrow. Effects of Operating Conditions on the Particle Size. For the controllable synthesis of CaCO3 nanoparticles, the effects of operating conditions on the particle size were investigated in this work. The average diameters of CaCO3 particles prepared at different continuous phase flow rates are given in Figure 6. Increasing the continuous phase flow rate can provide a strong cross-flow drag force, which reduces the geometric scale of the disperse phase, shortens the mass transfer distance, and provides

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Figure 15. Mass transfer flux densities of CO2 and particle sizes at different disperse phase flow rates.

a large mass transfer area.20 In addition, the disturbance in the reactor is enhanced with the increasing continuous phase flow rate, which intensifies the mixing in the reactor, too. Therefore, supersaturation was increased with the increase of the continuous phase and the particle size was reduced. Figure 7 shows the influence of the disperse phase flow rate on average sizes of CaCO3 particles. Increasing the flow rate of the disperse phase not only increases reactant CO2, but also accelerates the mass transfer rate by increasing the concentration driving force between the two phases. The results show that the particle size decreased with the increasing of the disperse phase flow rate under the experimental conditions. Since mixing can be affected by solid particles in solution, the average size of CaCO3 particles will be affected by the content of Ca(OH)2 in the suspension of the continuous phase.21 Figure 8 shows the influence of the volume faction of Ca(OH)2 on the average diameter of CaCO3 particles. From the result it can be seen that the mixing was enhanced with the increasing of the volume faction at low Ca(OH)2 content, and the particle size of CaCO3 was reduced. However, at the high content of Ca(OH)2 a large amount of CaCO3 primary nuclei were produced; thus the coalescence of CaCO3 primary nuclei was intensified during the reaction. Also, the particle size increased quickly with the increasing of the Ca(OH)2 volume fraction. The effect of the pore size of the membrane on the particle size is shown in Figure 9. It can be seen that the average particle sizes increase with the increasing of membrane pore size in the experimental conditions. According to the results of membrane emulsification process, the size of the dispersed phase is mainly determined by the pore diameter of microfiltration membranes.22 Therefore, with an increase of the membrane pore size, the bubbles of the mixed gas were enlarged and the mixing in the reactor was weakened in this minireactor, and thus the size of the particles increased. It is well-known that the temperature is one of the most important factors in the reaction process. Several properties will be changed with the change of the reaction temperature, especially the diffusion of CO2 and the solubility of Ca(OH)2 in this process. On one hand, the diffusion is increased with the increase of the temperature, which increases the supersaturation. On the other hand, the solubility of Ca(OH)2 is decreased as the temperature becomes higher, which reduces the supersaturation for the low concentration of Ca2+. The experiment results showed that the particle size was decreased with the increase of temperature below 40 °C but changed to increase above this degree, as shown in Figure 10. CO2 Mass Transfer in the Membrane Dispersion Minireactor and Its Influence on the Particle Size. In the last section, it can be seen that CaCO3 nanoparticles with average diameters ranging from 34.3 to 110 nm were prepared under different operating conditions. Considering the reaction mechanisms, the

Figure 16. Change of pH values with reaction time in the minireactor and the membrane-free reactor.

preparation of nanosize CaCO3 is a consecutive process organized by mass transfer, reaction, and precipitation, and the mass transfer of CO2 is the rate-determining step. For these reasons, it can be assumed that different mixing performances were obtained at different operating conditions, so with different sizes of particles prepared, and the better the mixing is, the smaller the particle size becomes. In our early work, the homogeneous and heterogeneous mixing performance in this minireactor was tested by Chen and Xu et al.14,18 with RTD curve measurement, Dushman reaction, and dye extraction method. Nice mixing and a fast mass transfer rate were indicated by a very small segregation index (lower than 0.002) and high extraction Murphee efficiency (nearly 100%) with a contact time barely less than 0.45 s. To learn the mixing performance and its influence on the particle size in this system, the mass transfer flux density of CO2 was calculated and its effect on the particle size was investigated in this section. According to the mass transfer film theory, the concentration profiles at the gas-liquid interface can be shown as in Figure 11, and it is clear the reaction takes place in the liquid film.23 Since Ca2+ can be assumed to be sufficient in the liquid phase, the mass transfer rate of CO2 is the main factor to affect the supersaturation and the particle size. Considering that reaction has taken place in the liquid film, the supersaturation increases with the increase of the mass transfer quantity of CO2 per time and per area. Therefore, the particle size is changed with the changing of the mass transfer flux density of CO2. From the mass transfer film theory, the mass transfer flux density can be obtained from eq 2.

NCO2 ) MCO2/A

(2)

where NCO2 is the mass transfer flux density of CO2 (mol s-1 m-2), MCO2 is the mass transfer flux of CO2 (mol s-1), and A is the interfacial area in the reactor (m2). To determine the interfacial area, the bubbles produced in the membrane dispersion minireactor were recorded by a high-speed CCD camera, which provided four pictures of the mixed gas bubbles at different operating conditions, respectively. Large numbers of bubbles were formed in this minireactor, and the average diameters (d32) were calculated by measuring the bubbles from these pictures. The interfacial area could be calculated with eq 3.

A ) Vw(6/d32G)

(3)

where V is the volume of the reactor (m3), w is the volume fraction of the gas phase (v/v), which can be obtained from the

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Figure 17. TEM images of CaCO3 particles prepared by the minireactor and the membrane-free reactor: (E) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 5 µm membrane, 25 °C; (F) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, 50 µm membrane, 25 °C; (G) FC ) 159 mL min-1, FD ) 134 mL min-1, XS ) 0.74, without membrane, 25 °C.

feed ratio of the two phases, and d32G is the average diameter of the mixed gas bubbles. Since it is hard to test the concentration of CO2 at the outside of the minireactor, the mass transfer flux of CO2 could be calculated from the consumption of Ca(OH)2. Figure 13 gives the pH values of the continuous phase at the inlet and the outlet of the reactor during the reaction. The operating conditions in this experiment are given in the caption below the figure. Since the pH of the inlet was measured from the bottle, where most of the Ca(OH)2 was stored (see Figure 1), it could be used to monitor the whole reaction process. Figure 13 shows that the pH did not change much at the beginning of the reaction for the dissolution of solid Ca(OH)2. However, with the reaction process carried out, when Ca(OH)2 was almost consumed, the pH dropped quickly. After that, the pH changed slowly since the reaction was slow and CO2 was just dissolved into the water. In this work we assumed that the reaction was complete at pH 10, because the Ca(OH)2 remaining in the solution was less than 0.1% of the initial quantity. As an equimolar reaction between Ca(OH)2 and CO2, the mass transfer flux of CO2 could be calculated by dividing the initial molar quantity of Ca(OH)2 fed into the reaction system by the reaction time, as shown in eq 4.

MCO2 ) (m/0.074)/t

(4)

where m is the mass of Ca(OH)2 fed into the reaction system (kg), 0.074 is the molar mass of Ca(OH)2 (kg/mol), and t is the reaction time (s). According to eqs 2-4 the mass transfer flux densities of CO2 were obtained and are shown in Figures 14 and 15 together with the particle sizes at different continuous phase flow rates and disperse phase flow rates. It is clear that the mass transfer rate of CO2 is enhanced by the high flow rate of continuous phase and disperse phase. Also, the particle size decreases withthe increase of the mass transfer flux density of CO2 at any experimental conditions in these figures. Comparison with the Mixing Performance and the Particles Prepared in the Minireactor and the Membrane-Free Reactor. Since the mixing process can be efficiently enhanced by the microstructure, the fast mass transfer process of CO2 was obtained owing to the dispersion effect of the micropore membrane in the minireactor. Figure 16 gives the pH of Ca(OH)2 suspension in the bottle during the reaction, and it is clear that when the membrane was not used the whole process required nearly twice the reaction time. Because the operating conditions in these three experiments are the same except for the choice of membrane, nice mixing and a fast mass transfer rate of CO2 will be achieved in the experiment with a short reaction time. Therefore the particles prepared with the 5 µm membrane have the smallest average

diameter. Figure 17 shows TEM images of the CaCO3 particles prepared in the different reactors. It is clear that particles cannot be prepared without a membrane in this minireactor. Conclusion In this article, efficient mixing and a fast mass transfer rate were achieved in the membrane dispersion minireactor, which provides high supersaturation to prepare calcium carbonate nanoparticles. Different mass transfer rates of carbon dioxide were obtained at different operating conditions, and different sizes of calcium carbonate nanoparticles, whose average diameters ranged from 34.3 to 110 nm with narrow size distributions, were prepared. The effects of operation conditions on the particle size were investigated, and it was found that the particle size could be controlled by controlling the continuous phase flow rate, disperse phase flow rate, volume fraction of calcium hydroxide, etc. For further analysis, the mass transfer film theory was introduced in this work and the mass transfer flux density of carbon dioxide was investigated. Since the mass transfer of carbon dioxide was the key to affecting the particle size, it was proved that the particle size decreased with increasing the mass transfer flux density of carbon dioxide. A membrane-free reactor was also used in this work for a comparison, and it was found that a nice mixing performance was obtained owing to the effective dispersion on the micropore membranes. Acknowledgment We wish to acknowledge the support of the National Natural Science Foundation of China (20476050, 20490200, 20525622) and SRFDP (20040003032) for this work. Notation A ) interfacial area in the reactor (m2) d ) diameter of CaCO3 particles (nm) d32 ) average diameter of CaCO3 particles (nm) d32G ) average diameter of bubbles (mm) DP ) membrane pore size (µm) FC ) flow rate of continuous phase (mL min-1) FD ) flow rate of disperse phase (mL min-1) Ksp ) solubility product of calcium carbonate MCO2 ) mass transfer flux of CO2 (mol s-1) m ) mass of Ca(OH)2 fed into the reaction system (kg) NCO2 ) mass transfer flux density of CO2 (mol s-1 m-2) n ) number fraction of CaCO3 particle size s ) supersaturation ratio T ) reaction temperature (°C) t ) reaction time (s) V ) volume of reactor (m3)

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ReceiVed for reView November 24, 2006 ReVised manuscript receiVed March 2, 2007 Accepted March 10, 2007 IE061502+