Preparation of CaCO3 nanoparticles in a surface-aerated tank stirred by a long-short blades agitator

Powder Technology 333 (2018) 339–346

Contents lists available at ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

Preparation of CaCO3 nanoparticles in a surface-aerated tank stirred by a long-short blades agitator Li Ding, Bin Wu, Peicheng Luo ⁎ School of Chemistry & Chemical Engineering, Southeast University, 211189 Nanjing, China

a r t i c l e

i n f o

Article history: Received 24 January 2018 Received in revised form 19 April 2018 Accepted 20 April 2018 Available online 22 April 2018 Keywords: Calcium carbonate Stirred tank Carbonation Long-short blades (LSB) Mass transfer Nano-particle

a b s t r a c t In this work, a novel long-short blades agitator was used to intensify the mass transfer process in the Ca(OH)2H2O-CO2 system for the preparation of CaCO3 nanoparticles. CO2 was entrained into the liquid through surface aeration in a 10 L stirred tank. Very uniform CaCO3 nanoparticles with the mean size of 24–110 nm were obtained. The prepared particles are calcite crystals. The mass transfer of the precipitation process has also been analyzed. The gas-liquid interfacial area was measured by using the transport-reaction equations. The influences of the rotating speed of the agitator, the reaction temperature, the volume fraction of CO2 in the gas phase, and the initial concentration of Ca(OH)2, on the mean size, the particle size distribution, and the reaction time were discussed. Compared with other carbonation methods, this work provides an easy-operation and high-efficiency method for preparing uniform CaCO3 nanoparticles with low risk of equipment blockage. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Calcium carbonate (CaCO3), in particular, at the nanoscale, has been used as a well pigment or functional filler in plastic, rubber, paper, paints, etc. [1,2]. In these fields, the morphology and size distribution of the particles are of great importance for practical application. Thus, a considerable amount of effort has been devoted to control these characteristics [3–7]. Moreover, it is one of the most popular materials for precipitation studies by heterogeneous carbonation reactions. In the process of calcium carbonate precipitation by heterogeneous reactions of CO2-H2O-Ca(OH)2, high supersaturation level is desirable for preparing nano-sized CaCO3 particles at high nucleation rates [8–10], i.e. high concentrations of calcium ion and carbonate ion are required. In the slurry of the slaked lime, calcium hydroxide dissolves very fast and dissociates strongly in aqueous solution, so the concentration of calcium ion can be assumed to be sufficient [11]. Thus, increasing the concentration of carbonate ion is of crucial importance to prepare nano-sized calcium carbonate. It has been well understood that the carbonate ion is formed rapidly by the reaction of dissolved CO2 with the hydroxyl ion, followed by the quick transformation of bicarbonate ion to carbonate ion. Compared with these ultrafast reactions, the mass transfer rate of carbon dioxide from the gas phase to the liquid phase is relatively slow [11]. Therefore, the supersaturation level in the carbonation route is directly determined by the gas-liquid mass transfer process. ⁎ Corresponding author. E-mail address: [email protected]. (P. Luo).

https://doi.org/10.1016/j.powtec.2018.04.057 0032-5910/© 2017 Elsevier B.V. All rights reserved.

In recent years, to enhance the mixing and mass transfer performance, many reactors such as high-gravity reactor [3,12], microstructure reactor [4], membrane reactor [5–7], etc. have been employed to prepare CaCO3 nanoparticles. However, there still exists numerous margin for the improvement of the process. In addition, blockage of the equipment, e.g. the orifices on the gas-sparger of the stirred tanks, the micro-channel of the microreactors, still creates big obstacles to the practical applications. Thus, developing a low-cost carbonation process for easy operation control is still the demands of industry for the reduction of carbon emission. Recently, our group has proposed and designed a new impeller, the long-short blades agitator [13]. The previous studies show that the gas can be entrained into the liquid and excellent gas-liquid mass transfer performance can be achieved when the LSB agitator is used for surface aeration [14]. The mechanisms of surface air entrainment [15] and the turbulent flow characteristics [16] in the vessel have been investigated experimentally and numerically. Compared with the conventional agitators, e.g. the classical Rushton turbine agitator, the LSB agitator can achieve high mixing performance and much better homogeneity in the distribution of the turbulent kinetic energy [14]. Therefore, the LSB impeller has potential applications in the preparation of CaCO3 nanoparticles by carbonation method. The objective of this work is to carry out an experimental study of carbon dioxide absorption into the slaked lime slurry for the preparation of CaCO3 nanoparticles in the vessel equipped with the LSB agitator, as well as the theoretical discussion of the mass transfer process. The operating conditions, e.g. the rotating speed of the LSB agitator, the volume fraction of CO2 in the gas phase, the reaction temperature,

340

L. Ding et al. / Powder Technology 333 (2018) 339–346

and the initial concentration of calcium hydroxide, are considered to study their effect on the performance of the prepared CaCO3 nanoparticles, including the crystal form, the morphology, the mean particle size and the particle size distribution (PSD). The gas-liquid interfacial area is also predicted to evaluate why the carbonation process can be intensified by the LSB agitator.

2. Experimental 2.1. Materials An analytical grade of calcium oxide (98% purity, Xilong Science Company), CO2 gas (99.9% purity, Nanjing Shangyuan Industrial Gas Company), N2 gas (99.9% purity, Nanjing Shangyuan industrial gas Company) and distilled water were used in the preparation of CaCO3 nanoparticles. To prepare calcium hydroxide slurry, CaO was first slaked in distilled water for 4 h at 70 °C with continuous stirring, the slurry was then aged for another 24 h without stirring.

2.2. Apparatus and methods The process flow diagram is shown in Fig. 1a. A cylindrical stirred tank with an elliptical bottom is used for the carbonation reactions. The inner diameter and the height of the tank are 210 mm and 245 mm, respectively. Constant-temperature circulating water is pumped through the jacket of the tank to control the reaction temperature. Four baffles of 19 mm in width and 200 mm in length are mounted at 90° intervals around the internal wall of the tank. The gas phase is entrained into the liquid through surface aeration by the LSB agitator, as shown in Fig. 1b, which consists of six short blades (SBs), two connection rings, three long blades (LBs) and one fixed bracket. Six SBs (width × height = 30 mm × 25 mm) are fixed in 60° under the connector rings. Three LBs are fixed on the outer connector ring of 10 mm in width and 100 mm in diameter in 120°. The characteristic size of LSB agitator, D, is defined as the diameter of the sweeping circle of the LBs and it is fixed at D = 100 mm. The pressure is regulated by a constant pressure valve.

Approximately 8 L calcium hydroxide slurry was initially poured into the stirred tank. When the temperature of the slurry reached a certain value, N2 was firstly induced into the tank to reach a certain pressure. Then the valve of N2 stream was closed and the valve of CO2 stream was opened. The total pressure of the gas phase was fixed at 400 kPa by a constant pressure valve in this work. The CO2 valve kept open during the whole process to supply CO2 consumed by the reactions in the liquid. Thus, CO2 partial pressure in the gas phase was a constant value. The pH values and the conductivity of the liquid were monitored during the whole reaction process. When the pH reached 7, the process was terminated. The suspension was then sampled for further filtration and drying. 2.3. Characterization of CaCO3 nanoparticles The crystal structure and the purity of the synthesized nanoparticles were measured by a multi-function levels X-ray powder diffraction (XRD, Ultima IV) using Cu Ka radiation (40 kV, 40 mA). Transmission electron microscope (TEM, JEM-2100) was used to characterize the morphology and measure the particle size distribution (PSD) of the prepared CaCO3 nanoparticles. Above 300 particles from N5 TEM pictures are counted to ensure the accuracy of the PSD results.

3. Theory The mass transfer process of CO2 absorption into Ca(OH)2 slurry is illustrated in Fig. 2. From the view of two-film theory, firstly, carbon dioxide diffuses from the bulk of the gas phase to the gas-liquid interface. When the Ca(OH)2 slurry is used, calcium hydroxide dissolves from the solid phase to the liquid quickly, and then dissociates strongly in aqueous solution. A broad consensus opinion is that the processes of dissolution and dissociation are fast enough that they are not the controlled steps [4], thus, the calcium and hydroxide ions can be assumed to be constant almost all the time. However, the mass transfer of CO2 from the gas bulk to the liquid is a relatively slow process. Consequently, it is considered as the dominant step in the building up of the supersaturation.

Fig. 1. (a) Experimental setup for the preparation of nano-CaCO3. (1) CO2 cylinder; (2) N2 cylinder; (3, 4) valves; (5) rotating speed controller; (6) constant pressure valve; (7) electric motor; (8) stirred tank; (9) LSB impeller; (10) discharge control valve; (11) pH sensor; (12) temperature sensor. (b) Schematic diagram of the LSB agitator configuration. (13) Fixer; (14) Long blades; (15) Outer and inner connector rings; (16) Short blades.

L. Ding et al. / Powder Technology 333 (2018) 339–346

341

Fig. 2. Schematic diagram of the mass transfer process of CO2 absorption into Ca(OH)2 slurry.

It has been well understood that carbon dioxide and the hydroxyl ion react in the liquid film, forming the bicarbonate ion rapidly, which is then transformed to carbonate ion quickly, as shown in the following schemes. CO2 ðaqÞ þ OH− ðaqÞ → HCO− 3 ðaqÞ − 2− HCO− 3 ðaqÞ þ OH ðaqÞ → CO3 ðaqÞ þ H2 OðlÞ

ðiÞ ðiiÞ

Reactions (i) and (ii) are all second-order reactions, and the latter is far quicker than the former [17]. Thus, the apparent reaction rate of the two consecutive reactions, rA, can be written as, r A ¼ k2 C b C A

ð1Þ

applied to solve Eq. (3). The apparent liquid-side volumetric absorption rate of CO2, NA, can be calculated as, NA ¼ kL aC Ai γ= tanhγ ≈ kL aC Ai γ ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k2 C b DAL aC Ai

ð5Þ

where, a is the effective gas-liquid interfacial area. When the gas phase is pure carbon dioxide, the gas-side mass transfer resistance is zero. The gas-liquid interfacial area, a, at different rotating speed of the LSB agitator, can be measured by Eq. (5). The equilibrium concentration at the gas-liquid interface, CAi, is calculated as, C Ai ¼ HPyAi ¼ HPyA

ð6Þ

where, H is solubility coefficient, P is total pressure of gas. NA can be calculated by the consumption of CO2 during the total reaction time of t, i.e.

where, Cb is the concentration of hydroxyl ion, CA is the concentration of dissolved carbon dioxide, and k2 is the rate constant of Reaction (i), which can be calculated by Eq. (2), [18].

NA ¼


 6666 k2 ¼ 4:2  1013 exp − T þ 273

where m0 is the total mass of calcium oxide added to the system, M0 is the molecular mass of calcium oxide, and V is the liquid volume of the slurry.

ð2Þ

When the reaction occurs in the liquid film, the reaction-diffusion equation can be constructed by applying the conservation equation of mass, i.e. 2

d CA 2

dx

dx ¼ k2 C A C b dx

ð3Þ

where, DAL is the diffusion coefficient of CO2 in the liquid, When the absorption process is accompanied by fast chemical reactions, the reaction plane can be evaluated by the Hatta number, γ, which is defined as the ratio of the reaction rate in a liquid film to the rate of diffusion through the film. For a second order reaction, the maximum rate of reaction assumes that the liquid film is saturated with gas at the interfacial concentration, thus, γ2 ¼

k2 C Ai C b δL k2 C b DAL ¼ 2 DAL kL C Ai δL

ð4Þ

where, δL is the thickness of the liquid film, kL is the liquid-side mass transfer coefficient. For the absorption of CO2 into aqueous solution of Ca(OH)2, γN N 1, the boundary conditions of x = 0, CA = CAi, and x = δL, CA = 0, are

m0 M0 Vt

ð7Þ

4. Results and discussion 4.1. Crystal form, morphology and size distribution XRD measurement and TEM observation are first carried out to characterize the CaCO3 particles prepared by the method in this work at different operating conditions (the reaction temperature, the rotating speed of the LSB agitator, the initial concentration of Ca(OH)2, the volume fraction of CO2). Fig. 3 exhibits XRD patterns of the prepared CaCO3 particles. We can see that the sharp fluctuations of lines all occur between 0° and 80°, indicating that the prepared particles are of calcite structure despite different operating conditions. In addition, no extra peak is observed, which implies that the prepared CaCO3 is very pure. TEM images of CaCO3 particles at different operating conditions are shown in Fig. 4. It is seen that the size of the CaCO3 particles is tens of nanometer with very narrow size distribution. 4.2. Mass transfer of the carbonation process and the reaction time The pH value and the conductivity level are two indicators for the monitoring of the carbonation process. Fig. 5 shows the variations of pH value and conductivity level during a selected carbonation process (T = 25 °C, N = 300 rpm, CB0 = 4 wt%, yA = 0.25). We can see that

342

L. Ding et al. / Powder Technology 333 (2018) 339–346

Fig. 3. XRD patterns of CaCO3 prepared under different operating conditions. (a) T = 20 °C, N = 300 rpm, yA = 0.25, CB0 = 4 wt%; (b) T = 20 °C, N = 180 rpm, yA = 0.25, CB0 = 4 wt%; (c) T = 20 °C, N = 300 rpm, yA = 0.25, CB0 = 6 wt%; (d) T = 20 °C, N = 300 rpm, yA = 0.5, CB0 = 4 wt%.

the pH values of the liquid phase do not change much during almost all the time, which gives a further proof that the dissolution of solid Ca(OH) 2 is fast enough that it is not the controlled step. Whereas an obvious decrease and recovery in the conductivity level is observed at this stage, which is caused by the increase and recovery of emulsion viscosity [19]. When the solid Ca(OH)2 dissolves completely, calcium ion and hydroxyl ion cannot be replenished in time, leading to a sharp decrease in the pH value and the conductivity level. When the calcium ion in the solution has been consumed completely, the pH comes to 7 and the

Fig. 5. Variation of pH value and conductivity level during the carbonation process. P = 400 kPa, T = 20 °C, N = 300 rpm, yA = 0.25, CB0 = 4 wt%.

conductivity reaches a minimum value. Then, the pH value changes very slowly since the reaction is just the dissolution of CO2 into the water. In this work, the carbonation process was terminated when the pH value reached 7. In addition, the concentration of hydroxyl ion was assumed as a constant value for the theoretical study of the process because the period of pH decrease was very short compared to the whole carbonation process. In the published literatures, several reactors, e.g. high gravity reactor [12], microstructure reactor [20], membrane reactor [7], have been proposed to intensify the mass transfer process of the carbonation process. Table 1 lists the reaction time and the size of the prepared CaCO3, by

Fig. 4. TEM photographs of CaCO3 prepared under different operating conditions. (a) T = 20 °C, N = 300 rpm, yA = 0.25, CB0 = 4 wt%; (b) T = 20 °C, N = 300 rpm,yA = 0.5, CB0 = 4 wt%; (c) T = 20 °C, N = 240 rpm,yA = 0.25, CB0 = 4 wt%; (d) T = 10 °C, N = 300 rpm, yA = 0.25, CB0 = 4 wt%.

L. Ding et al. / Powder Technology 333 (2018) 339–346 Table 1 Comparison of the reaction time and the mean size of prepared CaCO3 particles by using different reactors. Reactor

High gravity Microstructure reactor [12] reactor [4]

Membrane reactor [6]

Gas-sparged stirred tank [28]

This work

yA t (min) d (nm)

0.9999 5–21 17–36

0.488 ~165 330–750

0.20 32 6120

0.25–1.0 0.7–11 24–110

0.298 50–380 65–80

343

longer. Additionally, in the traditional stirred tank equipped with a gas sparger, it is hard to produce nano-sized particles. For the carbonation method intensified by the LSB agitator in this work, we can see that the mean size of the particles is the same order of magnitude as that by the high-gravity method [12]. Meanwhile, super short reaction time of the carbonation process of this work indicates that it is a highefficiency method for producing CaCO3 nanoparticles in the stirred tank. In addition, no gas-sparger is used, which reduce the risk of possible equipment blockage, e.g. the blockage of the gas-sparger or the microchannel of the microreactor. 4.3. Effect of the operating conditions

Fig. 6. Effect of the rotating speed on the mean size of CaCO3 nanoparticles and the reaction time. P = 400 kPa, T = 20 °C, yA = 0.5, CB0 = 4 wt%.

using different methods. In the work of Chen et al. [12], a high gravity reactor was used for the carbonation process, and the mean size could be controlled in the range of 17–36 nm, with a short reaction time of 5–21 min. Although the microstructure reactor and the membrane reactor can produce nano-scaled CaCO3 particles, the reaction time is much

4.3.1. Rotating speed of the LSB agitator The LSB agitator used in this work can entrain the gas into the liquid by the interaction between the long blades and the liquid surface. In our previous study, the gas-liquid mass transfer coefficient, kLa, is directly influenced by the rotating speed of the agitator, N [14]. For the reaction system of Ca(OH)2-CO2-H2O, the process is mainly determined by the gas-liquid mass transfer. Thus, we fixed other operating conditions and varied the rotating speed to study its effect on the carbonation process. It is noted that the Reynolds number for the LSB agitator, which is defined based on the characteristic size, D (Re = ρND2 / μ), ranges from 13,032 to 30,409, indicating that the flow is in the turbulent regime in this work. Fig. 6 shows the effect of rotating speed on the mean size of CaCO3 nanoparticles, d32, and the reaction time, t. It is seen that increasing the rotating speed leads to a sharp decrease in d32 and t when N is b300 rpm. When N N 300 rpm, the trend of decrease becomes slow. From Eq. (5) we can see that the apparent liquid-side volumetric absorption rate of CO2 is proportional to the effective gas-liquid interfacial area, a. Thus, increasing N will accelerate the absorption of CO2, leading to an increase in the concentration of carbonate ion. Therefore, the supersaturation level is increased, resulting in a high nucleation rate, which corresponds to smaller particles. Moreover, increasing N will improve the mixing efficiency and the turbulent energy level, which is beneficial to preparing uniform particles. The PSD at different N shown in Fig. 7 has proven this point, i.e. the PSD becomes narrower when N increase from 180 rpm to 420 rpm.

Fig. 7. Histogram of particle size distribution at different rotating speed of the LSB agitator. P = 400 kPa, T = 20 °C, yA = 0.5, CB0 = 4 wt%.

344

L. Ding et al. / Powder Technology 333 (2018) 339–346

4.3.2. Reaction temperature Temperature is one of the most important factors that affect the reaction process. The properties of the reaction system, in particular, the diffusion of CO2 and the solubility of Ca(OH)2, will be changed with the variation of the temperature [7]. It is known that the diffusion of CO2 is increased by increasing the temperature, whereas the solubilities of Ca(OH)2 and CO2 are all decreased, which leads to the reduction of the concentrations of hydroxyl ion and carbonate ion. Thus, the supersaturation level depends on which has a prevailing effect. Fig. 8 shows the effect of the temperature on the reaction time and the mean size of particles, d32. In this work, the temperature is varied from 10 °C to 40 °C. It is seen that d32 increases when the temperature is increased, whereas the reaction time decreases by increasing the temperature. This implies that the temperature has a prevailing effect on the solubilities of Ca(OH)2 and CO2, i.e. the supersaturation is decreased by increasing the temperature below 40 °C, leading to producing big particles at lower nucleation rates. The results are similar to those reported by Zhou et al. [7], in which CaCO3 nanoparticles were prepared in a membrane reactor. However, in the work reported by Wang et al. [4], d32 decreases by increasing the temperature below 40 °C, but changes to increase above 40 °C in a microstructure minireactor. 4.3.3. Volume fraction of CO2 in the gas phase On one hand, the volume fraction of CO2 in the gas phase, yA, directly determines the equilibrium concentration at the gas-liquid interface. On the other hand, gas-side mass transfer resistance will be increased when yA is decreased. Fig. 9 shows the effect of yA on the mean size of the particles, d32, and the reaction time. It is seen that d32 decreases slightly when yA is varied from 0.25 to 0.4, and then change to increase again yA when yA N 0.4. This indicates that the gas-side mass transfer resistance makes a significant contribute to the process when yA b 0.4. In this case, increasing yA will improve the mass transfer rate of CO2, thus accelerating the nucleation process and producing smaller particles. When yA N 0.4, it is seen that d32 has a sharp increase by increasing yA. It is known that the isoelectric point of calcite is pH = 8.2 [20], i.e. Ca2+ ions are excessive at pH higher than 8.2, whereas an excess of CO2 will be anticipated at pH lower than 8.2. In this work, high initial concentration of Ca(OH)2 is used, leading to pH higher than 8.2 and an excess of Ca2+ ions during nearly the whole reaction process. In this case, Jung et al. [21] have found that the mean size will increase with an increase of molar gas flow rate of CO2 in a stirred tank with a gas

Fig. 8. Effect of the reaction temperature on the mean size of CaCO3 nanoparticles and the reaction time. P = 400 kPa, N = 300 rpm, yA = 0.25, CB0 = 4 wt%.

Fig. 9. Effect of the volume fraction of CO2 on the mean size of CaCO3 nanoparticles and the reaction time. P = 400 kPa, T = 20 °C, N = 300 rpm, CB0 = 4 wt%.

sparger, due to a reduction in the excess Ca2+ ion concentration. From Eq. (6) we can see that the equilibrium concentration of CO2 at the gas-liquid interface is proportional to yA. Therefore, increasing yA will leads to an obvious reduction in the excess Ca2+ ion concentration, resulting in a lower level of supersaturation. Thus, bigger particles are produced by increasing yA in the case of yA N 0.4, as shown in Fig. 9. 4.3.4. Initial concentration of Ca(OH)2 in the suspension When the Ca(OH)2 slurry is used, the solution is saturated with Ca2+ due to the quick dissolution and dissociation of Ca(OH)2 from the solid phase, thus, the concentration of Ca2+, in theory, is constant when the initial concentration of Ca(OH)2, CB0, is changed. Therefore, the precipitation process will be prolonged because more Ca2+ ions need to be consumed when CB0 is increased, i.e. crystal growth occurs in a prolonged period, leading to producing bigger particles. The influence of CB0 on the reaction time and mean size of nanoparticles shown in Fig. 10 has proven this point. Meanwhile, new nuclei formation continues to occur, which leads to a wider PSD when CB0 is increased, as shown in Fig. 11.

Fig. 10. Effect of the initial concentration of Ca(OH)2 on the mean size of CaCO3 nanoparticles and the reaction time. P = 400 kPa, T = 20 °C, N = 300 rpm, yA = 0.5.

L. Ding et al. / Powder Technology 333 (2018) 339–346

345

Fig. 11. Histogram of particle size distribution at different initial concentration of Ca(OH)2. P = 400 kPa, T = 20 °C, N = 300 rpm, yA = 0.5.

In addition, solid particles have been found having a significant effect on the mass transfer rate when the solid phase formation occurs near or at the gas-liquid interface. Gomez-Diaz et al. [22] has found that the presence of the solid phase formation at the interface invokes a decrease in the effective mass transfer area. A clear declining trend of the gasliquid mass transfer area has been observed when the initial concentration of Ca(OH)2 is increased in their study. In this sense, increasing CB0 will cause an increase in the mean size of the particles, due to the reduction in the mass transfer rate.

4.4. Effective gas-liquid interfacial area To have a further understanding of the carbonation process intensified by the LSB agitator, we have measured the gas-liquid interfacial area, a, at different rotating speed of the agitator, according to the mass transfer rate represented by Eq. (5). In the experiments, pure CO2 was used for the gas phase so that the equilibrium concentration, CAi, in Eq. (5) could be calculated directly by using Henry's law. Fig. 12 shows a comparison of the measured a in the tank stirred by the LSB agitator with those through sparger aeration by using the conventional impellers [23–25]. We can see that the gas-liquid interfacial area is strongly dependent on the rotating speed of the agitator. This is consistent with the results of our previous work [14], in which increasing the rotating speed of the agitator results in an obvious increase in the gas-liquid mass transfer coefficients, kLa. In addition, the values of a are two to three times as large as those reported in the literature in the case of sparger aeration. It is noted that the gas-liquid interfacial area reported in the work of Sridharan et al. [26] has been measured by the absorption of the gas mixture of CO2 and N2 into cuprous amine solution in the stank of 100 mm in diameter, equipped with a six bladed Rushton turbine agitator. In the work reported by Dluska et al. [25], a stirred tank of 130 mm in diameter equipped with four-pitched-blades turbine was used and the values of a were measured by using the absorption of pure oxygen into NaOH solution. Despite different methods for measuring a, the LSB agitator has proven to be an excellent impeller to intensify the mass transfer process in the stirred tank, thus providing an easyoperation and high-efficiency method to produce uniform CaCO3 particles of several tens of nanometers. 5. Concluding remarks

Fig. 12. Comparison of gas-liquid interfacial area at different rotating speed with literature data. (■) this work, T = 20 °C, P = 400 kPa; (●) the results from the work of Sridharan et al. [27], T = 20 °C, the volumetric gas flow rate is 12.5 × 10−6 m3·s−1, (▲) the results from the work of Dluska et al. [26], T = 25 °C, the volumetric gas flow rate is 12.5 × 10−6 m3·s−1.

In this work, a novel surface aerator, the LSB agitator, has been employed for the carbonation process to prepare very uniform CaCO3 nanoparticles, whose mean size ranges from 24 nm to 110 nm. The prepared particles are of high-purity calcite crystals. The influences of the operation conditions on the mean size, the PSD, and the reaction

346

L. Ding et al. / Powder Technology 333 (2018) 339–346

time have been investigated. The mass transfer process accompanying by carbonation reactions was further analyzed and the model for predicting the overall absorption rate of CO2 was built up. The gasliquid interfacial area, a, was then measured by the proposed model. It is found that the values of a in this work are two to three times as large as those in the tanks stirred by the conventional impellers in the case of sparger aeration. Therefore, this work provides an easyoperation and high-efficiency carbonation method for preparing uniform CaCO3 nanoparticles in the stirred tank. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21476048), the Fundamental Research Funds for the Central University of China (Grant No. 104.205.2.5), and Project Funds by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] J.J. Yin, C.L. Qin, H. An, A. Veeraragavan, B. Feng, Influence of hydration by steam/ superheating on the CO2 capture performance and physical properties of CaObased particles, Ind. Eng. Chem. Res. 52 (2013) 18215–18224. [2] V. Materic, M. Hyland, M.I. Jones, B. Northover, High temperature carbonation of Ca (OH)2: the effect of particle surface area and pore volume, Ind. Eng. Chem. Res. 53 (2014) 2994–3000. [3] M. Wang, H.K. Zou, L. Shao, J.F. Chen, Controlling factors and mechanism of preparing needlelike CaCO3 under high-gravity environment, Powder Technol. 142 (2004) 166–174. [4] K. Wang, Y.J. Wang, G.G. Chen, G.S. Luo, J.D. Wang, Enhancement of mixing and mass transfer performance with a microstructure minireactor for controllable preparation of CaCO3 nanoparticles, Ind. Eng. Chem. Res. 46 (2007) 6092–6098. [5] Z.Q. Jia, Z.Z. Liu, F. He, Synthesis of nanosized BaSO4 and CaCO3 particles with a membrane reactor: effects of additives on particles, J. Colloid Interface Sci. 266 (2003) 322–327. [6] Z.Q. Jia, Q. Chang, A. Mamat, Preparation of nanoparticles with a semi-batch gasliquid membrane contactor, Chem. Eng. Process. 50 (2011) 810–814. [7] J. Zhou, X. Cao, X.Y. Yong, S.Y. Wang, X. Liu, Y.L. Chen, T. Zheng, P.K. Ouyang, Effects of various factors on biogas purification and nano-CaCO3 synthesis in a membrane reactor, Ind. Eng. Chem. Res. 53 (2014) 1702–1706. [8] M. Euvrard, F. Membrey, C. Filiatre, A. Foissy, Crystallization of calcium carbonate at a solid/liquid interface examined by reflection of a laser beam, J. Cryst. Growth 265 (2004) 322–330. [9] D. Chakraborty, S.K. Bhatia, Formation and aggregation of polymorphs in continuous precipitation .2. Kinetics of CaCO3 precipitation, Ind. Eng. Chem. Res. 35 (1996) 1995–2006.

[10] C.Y. Tai, C.K. Chen, Particle morphology, habit, and size control of CaCO3 using reverse microemulsion technique, Chem. Eng. Sci. 63 (2008) 3632–3642. [11] J.R. Burns, J.J. Jachuck, Monitoring of CaCO3 production on a spinning disc reactor using conductivity measurements, AICHE J. 51 (2005) 1497–1507. [12] J.F. Chen, Y.H. Wang, F. Guo, X.M. Wang, C. Zheng, Synthesis of nanoparticles with novel technology: high-gravity reactive precipitation, Ind. Eng. Chem. Res. 39 (2000) 948–954. [13] P.C. Luo, H. Wu, J. Wu, Y.C. Tai, X.Y. Luo, Inventors; Southeast University, Assignee. Long-Short Blades AgitatorPCT/CN2014084156 2014. [14] P.C. Luo, J. Wu, X. Pan, Y.Q. Zhang, H. Wu, Gas-liquid mass transfer behavior in a surface-aerated vessel stirred by a novel long-short blades agitator, AICHE J. 62 (2016) 1322–1330. [15] Y.Q. Zhang, X. Pan, Y.H. Wang, P.C. Luo, H. Wu, Numerical and experimental investigation on surface air entrainment mechanisms of a novel long-short blades agitator, AICHE J. 64 (2018) 316–325. [16] X. Pan, L. Ding, P.C. Luo, H. Wu, Z. Zhou, Z.B. Zhang, LES and PIV investigation of turbulent characteristics in a vessel stirred by a novel long-short blades agitator, Chem. Eng. Sci. 176 (2018) 343–355. [17] P. Huber, Kinetics of CO2 stripping and its effect on the saturation state of CaCO3 upon aeration of a CaCO3-CO2-H2O system: application to scaling in the papermaking process, Ind. Eng. Chem. Res. 50 (2011) 13655–13661. [18] H.S. Yu, Z.C. Tan, J. The, X.S. Feng, E. Croiset, W.A. Anderson, Kinetics of the absorption of carbon dioxide into aqueous ammonia solutions, AICHE J. 62 (2016) 3673–3684. [19] H. Wei, Q. Shen, Y. Zhao, D.J. Wang, D.F. Xu, Influence of polyvinylpyrrolidone on the precipitation of calcium carbonate and on the transformation of vaterite to calcite, J. Cryst. Growth 250 (2003) 516–524. [20] J. Burger, V. Papaioannou, S. Gopinath, G. Jackson, A. Galindo, C.S. Adjiman, A hierarchical method to integrated solvent and process design of physical CO2 absorption using the Saft-γ Mie approach, AICHE J. 61 (2015) 3249–3269. [21] P. Somasundaran, G.E. Agar, Zero point of charge of calcite, J. Colloid Interface Sci. 24 (1967) 433–440. [22] W.M. Jung, S.H. Kang, K.S. Kim, W.S. Kim, C.K. Choi, Precipitation of calcium carbonate particles by gas-liquid reaction: morphology and size distribution of particles in Couette-Taylor and stirred tank reactors, J. Cryst. Growth 312 (2010) 3331–3339. [23] D. Gomez-Diaz, J.M. Navaza, B. Sanjurjo, Analysis of mass transfer in the precipitation process of calcium carbonate using a gas/liquid reaction, Chem. Eng. J. 116 (2006) 203–209. [24] K. Sridharan, M.M. Sharma, New systems and methods for the measurement of effective interfacial area and mass transfer coefficients in gas—liquid contactors, Chem. Eng. Sci. 31 (1976) 767–774. [25] J.G. Lu, Z.Y. Lu, S. Cao, L. Gao, X. Gao, J.T. Wang, Y.Q. Tang, Y. Chen, Synthesis, characterization, physical properties, and CO2 absorption performance of monoethanolamine glycinate ionic liquid, J. Solut. Chem. 44 (2015) 1997–2007. [26] E. Dłuska, S. Wroński, T. Ryszczuk, Interfacial area in gas–liquid Couette–Taylor flow reactor, Exp. Thermal Fluid Sci. 28 (2004) 467–472. [27] K. Sridharan, M.M. Sharma, New systems and methods for the measurement of effective interfacial area and mass transfer coefficients in gas-liquid contactors, Chem. Eng. Sci. 31 (1976) 767–774. [28] J.G. Carmona, J.G. Morales, R. Rodriguez-Clemente, Rhombohedral-scalenohedral calcite transition produced by adjusting the solution electrical conductivity in the system Ca(OH)2-CO2-H2O, J. Colloid Interface Sci. 261 (2003) 434–440.