Ind. Eng. Chem. Res. 2009, 48, 4507–4513
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Preparation of Uniform Microcapsules Containing 1-Octanol for Caprolactam Extraction Xingchu Gong, Yangcheng Lu, Zhixi Qian, and Guangsheng Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China
This paper presents a novel method to prepare uniform polysulfone (PSF) microcapsules that contain 1-octanol to prevent emulsification when pure 1-octanol is used as the solvent. In this method, a coaxial microchannel device was designed and N2 gas was introduced to control the drop size of polymer solution. Uniform microcapsules were successfully prepared and the diameter decreased linearly with the increase of the N2 flow rate. The pore volume and 1-octanol loading ratio could be controlled by changing the PSF content in polymer solution. The maximum loading of 1-octanol was 6.98 g/g PSF when the pore volume was 8.28 mL/g. The extraction capacity of the prepared microcapsules was determined, and it was found that the uptake of caprolactam was mainly influenced by the 1-octanol mass fraction in microcapsules, because the PSF shell did not adsorb much caprolactam. A maximum uptake of 58.8 mg caprolactam/g microcapsule was reached. The extraction kinetics of the prepared microcapsules also was determined. It was determined that the masstransfer rate was determined by inner diffusion. The apparent diffusion coefficient of 9.1 × 10-11 m2/s was obtained. 1. Introduction The production of caprolactam, which is the monomer of nylon 6, discharges a large quantity of wastewater that contains a low concentration of caprolactam.1 Biomethods are widely applied to treat the wastewater, but they cannot recover caprolactam.2 Solvent extraction is an effective way to recover caprolactam massively, but selection of the extractant is difficult. Benzene is the most widely used extractant; however, it is toxic and its extraction capacity is low. When dealing with caprolactam in wastewater, the distribution coefficient is usually 0.9 and much lower toxicity.5 However, because of its relatively high viscosity and low interfacial tension between water, 1-octanol is easy to form an emulsion when directly applied in liquid-liquid extraction.6 The immobilization of extractants is an effective way to solve the problems in phase mixing and phase separation.7-11 Microcapsules or resins that contain extractants have been successfully applied to recover metal ions and organic compounds.12-18 In previous work, 1-octanol was encapsulated in polysulfone (PSF) microspheres, using a two-step solvent extraction method.19 High 1-octanol loading was achieved and caprolactam could be extracted without the formation of emulsion. However, because of the large diameter of the prepared microcapsules (∼2 mm), the extraction kinetics of 1-octanol microcapsules is relatively slow. More than 60 min was required to reach the equilibrium state. Recently, a new technique of microdispersion process has been applied to produce microcapsules.20-23 A polymer was dissolved in a good solvent to form a polymer solution. The polymer solution then was dispersed in a microchannel with a cross-flow rupture by another liquid, which was used as the continuous phase. Uniform polymer solution drops with smaller diameter were then produced. After solidification, monodispersed microcapsules were prepared successfully. Here, the good * To whom correspondence should be addressed. Tel.: +86-01062783870. Fax: +86-010-62783870. E-mail:
[email protected].
solvent and the continuous phase should be carefully selected. The solubility of the good solvent in the continuous phase should be small. Otherwise, polymers will solidify in the microchannel, which results in an obstruction or microcapsules with bad appearances.22 After dispersion, because polymer solution drops were surrounded by the continuous phase, the solidification process can be time costly on some occasions.21 Gas, such as N2, is considered as an insoluble fluid with conventional liquids. Compared to the liquid continuous phase, the gas continuous phase can overcome the solubility problem, which can enlarge the selection range of good solvents for polymers. In addition to this, the polymer drops can be easily separated from the gas continuous phase after dispersion, which indicates the possibility of using more-efficient solidification methods. However, the shearing force of gas is weak, because of its low viscosity. Therefore, it is difficult to use a similar microdispersion apparatus reported by other researchers.24 A special designed microchannel device is highly required when a gas continuous phase is applied. A porosity shell can be obtained when N,N-dimethyl formamide (DMF) or N-methyl-2-pyrrolidone (NMP) is used as the good solvent for PSF.19 However, most liquids are soluble in DMF or NMP. Therefore, a gas continuous phase is considered. In this work, a coaxial microchannel device was designed, and a gas-liquid microdispersion process in the microchannel device was employed for controllable preparation of 1-octanol microcapsules. The influence of preparation conditions on the diameter and loading ratio of the microcapsules were determined. The extraction performance of the prepared microcapsules to caprolactam was measured and analyzed. 2. Experimental Section 2.1. Chemicals. Caprolactam (>99.9%) was kindly provided by Shijiazhuang Chemical Fiber Plant, SINOPEC (China). PSF (intrinsic viscosity: 0.56) was purchased from Beijing Trihigh Membrane Technology Co., Ltd. (China). Ethanol (>99.7%), 1-octanol (>99.8%), and NMP (>99.0%) were purchased from
10.1021/ie900046u CCC: $40.75 2009 American Chemical Society Published on Web 03/27/2009
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Figure 1. Schematic of the experimental setup. Table 1. Preparation Conditions and Characteristics of Microcapsules No.
gas flow rate (L/h)
polymer solution flow rate (µL/min)
PSF content in polymer solution (wt %)
mean diameter (mm)
pore volume (mL/g)
loading ratio (g 1-octanol/g PSF)
1 2 3 4 5 6 7 8 9 10
100 125 150 175 200 150 150 150 150 150
300 300 300 300 300 100 500 300 300 300
8 8 8 8 8 8 8 6 10 12
1.67 1.44 1.20 1.00 0.86 1.19 1.20 1.09 1.22 1.25
7.24 7.14 7.25 7.20 7.37 7.06 7.19 8.28 6.17 5.01
5.87 5.88 6.07 5.95 6.04 5.71 5.88 6.98 5.05 4.18
VAS Chemical Co., Ltd. (China). All materials were used as received without any further purification. 2.2. Preparation of Microcapsules. The experimental setup for preparing PSF microspheres is shown in Figure 1. PSF was dissolved in NMP to form a polymer solution. The PSF solution, as the dispersed phase, then was pumped into the microdevice by a syringe pump (TS2-60, Lange). The outside diameter of the dispersed phase channel is 0.7 mm. The inner diameter of the main channel for gas flow is 2.6 mm. The two phases cocurrently flowed in the coaxial microdevice. To enhance the shearing force, the microdevice was fixed vertically so that the gravity force was used as an assistant force to disperse the polymer solution. Under the co-action of the rupture of N2 and the gravity force, droplets of PSF solution were formed and solidified when they dropped in a solidification solution. The solidification solution was a mixture of 500 mL of ethanol and
1500 mL of deionized water. PSF microspheres were kept in solidification solution for 1 h before filtered out and washed with deionized water five times. After that, microspheres were dried at 110 °C for 5 h. PSF microspheres then were immersed in 1-octanol and subjected to ultrasonic treatment for 3 h. The power of the ultrasonic cleaner (SB3200DTD, Ningbo Scientz Biotechnology Co., Ltd.) was 180 W. The prepared PSF microcapsules then were kept in 1-octanol for 24 h. Finally, the microcapsules that contained 1-octanol were filtered out and kept in deionized water. The preparation conditions are listed in Table 1. 2.3. Characterization of Microcapsules. The structures of the microspheres were characterized by scanning electron microscopy (SEM) (Model JSM-7401F, JEOL) micrographs. The diameters of the microspheres were measured from digital photos. The pore volume of the PSF microspheres was determined using the mercury intrusion method (Autopore II 9220, Micromeritics). The loading ratio of microcapsules was determined using a weighing method. The extraction process with the microcapsules was performed by contacting caprolactam aqueous solutions. When determining the maximum uptake
Figure 2. Schematic of the apparatus of suspended microextraction.
Figure 3. Digital photo of Microcapsule No. 3.
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rotated at 850 rpm and formed a steady rotating flow field to reduce the mass-transfer resistance in the outer aqueous phase. After extraction, microcapsules were taken out and contacted with 1 mL of ethanol for 10 h to ensure that all the caprolactam and the 1-octanol in the microcapsules were dissolved in ethanol. The caprolactam concentration in ethanol then was determined with gas chromatography. All the experiments were repeated at least three times. 3. Results and Discussions
Figure 4. Cross-section of Microcapsule No. 3.
of caprolactam, microcapsules were contacted with 25-mL caprolactam aqueous solutions with different concentrations under stirring for 24 h to reach the extraction equilibrium state. The caprolactam aqueous solution was saturated with 1-octanol before experiments. After extraction, the microcapsules were taken out and contacted with 2 mL of ethanol for 10 h, to ensure that all the caprolactam and 1-octanol in microcapsules were dissolved in ethanol. The caprolactam content in ethanol and aqueous phase then were determined with gas chromatography (Agilent 6890, Agilent). A method of suspended microextraction was applied to determine the extraction kinetics, as shown in Figure 2. To control the temperature, the microextraction apparatus was placed in a dry oven (CS101-2A, Chongqing Yinhe Experimental Instrument Co., Ltd.). Several microcapsules and 5 mL of caprolactam aqueous solution saturated with 1-octanol were placed in a sample bottle at 30 °C. The bottle
3.1. Surface and Internal Morphology of Microcapsules. PSF microcapsules with uniform size are shown in Figure 3. Figure 4 is an SEM image showing the cross-section of a microcapsule, which shows that the prepared microcapsule has a large cavity and large pore volume. Figure 5 shows SEM photos of microcapsule Nos. 3, 9, and 10. Figure 5a shows that there are small pores on the surface of microcapsules. The pores will be helpful to improve the performances of mass-transfer kinetics. When compared to Figure 5b and Figure 5c, it can be concluded that the greater the amount of PSF in the polymer solution, the more compact the surface that is formed. 3.2. Diameter. In our experiments, the mean diameter of microcapsules was between 1.67 mm and 0.86 mm, under different preparation conditions, as listed in Table 1. Compared with the microcapsules reported in our previous work, microcapsules prepared with a microdispersion process is much smaller.19 The main reason is that the shearing force of N2 can result in more-frequent rupture of the polymer solution. Smaller size will lead to a larger specific surface area and shorter masstransfer distance, which help to reach extraction equilibrium
Figure 5. Scanning electron microscopy (SEM) images showing the surfaces of microcapsules: (a) microcapsule No. 3, (b) microcapsule No. 9, and (c) microcapsule No. 10.
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Figure 6. Microcapsule diameter changes with the gas flow rate.
Figure 8. Microsphere pore volume and loading ratio change with the PSF content in the polymer solution.
Figure 9. Uptakes of microcapsules to caprolactam. Figure 7. Pore size distribution of Microcapsule No. 1.
state more quickly. The polymer solution flow rate apparently did not affect microcapsule diameter. With the increase of PSF content in the polymer solution, the diameter of the microcapsule slightly increased. However, the diameter of the microcapsule decreased linearly when the gas flow rate increased, as shown in Figure 6. 3.3. Pores and Loading Ratio. To investigate the pore size distribution of microspheres, Microsphere No. 1 was cut off in liquid nitrogen. The result of mercury intrusion experiment is plotted in Figure 7. The pore size distribution ranges from several nanometers to several hundred micrometers. The pore volume of the PSF microspheres is listed in Table 1. The gas flow rate and the polymer solution flow rate had little effect on the microsphere pore volume. However, the microsphere pore volume decreased as the PSF content in the polymer solution increased. The maximum pore volume, 8.28 mL/g, was achieved when the PSF content was 6% in the polymer solution. Note that the pores of the PSF microspheres were full of 1-octanol after an impregnation process, which has been described in previous work.19 Therefore, the influence of preparation conditions on the loading ratio of microcapsules follows a similar rule with microsphere pore volume. In our experiments, the increase of the PSF content in the polymer solution was determined to lead to a linear decrease of microsphere pore volume and 1-octanol loading, as shown in Figure 8. This result indicates that the loading ratio of microcapsules can be controlled by varying the PSF content in the polymer solution.
3.4. Microcapsules’ Extraction Performance. Maximum Uptake of Microcapsules. The uptakes of Microcapsule Nos. 3, 8, 9, and 10 to caprolactam under different concentrations were investigated at 30 °C. The results are plotted in Figure 9. The uptake of microcapsules increased as the caprolactam concentration in the aqueous phase increased. The maximum uptake is 58.8 mg/g. It is also observed that the maximum uptake of caprolactam linearly increases as the mass fraction of 1-octanol in the microcapsules increases, as shown in Figure 10. To verify this result, the adsorption ability of the PSF shell to caprolactam dissolved in 1-octanol was measured, as listed in Table 2. The PSF shell obviously adsorbs little caprolactam in the 1-octanol phase. It can be concluded that 1-octanol provides the extraction ability. This result also can explain the relationship shown in Figure 10. A prediction of the distribution coefficient was also tested. The Lyngby modified UNIFAC model was used to calculate the liquid-liquid equilibrium of the ternary system caprolactam + water + 1-octanol. All the binary interaction parameters were taken from the literature.5,25 The prediction results were plotted in Figure 10. All the prediction results are ∼15% lower than the experimental results; this is due to the calculation deviation in liquid-liquid equilibrium. Because of low caprolactam content in equilibrium system, the Lyngby modified UNIFAC model with existing interaction parameters cannot describe the caprolactam distribution accurately. Extraction Kinetics. The extraction kinetics of microcapsules was determined, and the results are plotted in Figure 11. In Figure 11a and b, it is obvious that microcapsules with a larger diameter took a longer time to reach extraction equilibrium at different caprolactam concentrations. In Figure
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d(CLm) ) )
d(NLm) NLm - NLm+1 ) VLm VLm NLm - NLm+1 (4/3)πRm3 - (4/3)πRm+13
(3)
In layer nmax, the caprolactam concentration differs in dt as d(CLnmax) )
NLnmax VLnmax
)
NLnmax (4/3)πh3
(4)
At time t, the sum of caprolactam in all of the layers is the total caprolactam content in a microcapsule. The average Figure 10. Extraction distribution coefficient changes with 1-octanol mass fraction in microcapsules ((9) calibration results and (0) prediction results). Table 2. Adsorption Ability of PSF Shell to Caprolactam in 1-Octanol Phase Mixture of Caprolactam and 1-Octanol
No.
mass of PSF microspheres (g)
A1 A2 A3 A4 A5 A6
0.136 0.147 0.137 0.153 0.176 0.22
weight (g)
caprolactam content (wt %)
caprolactam content after adsorption (wt %)
2.802 2.750 2.753 2.707 2.826 2.806
1.01 1.88 2.80 3.90 6.05 7.18
1.01 1.89 2.81 3.87 6.12 7.18
11c, microcapsules with smaller 1-octanol loading extracted less caprolactam, when contacting the aqueous phase for the same length of time. Up to Microcapsule No. 5, the caprolactam content in microcapsules increased slowly after 25 min. Here, a simple model is derived to describe the extraction kinetics. In this model, the diffusion in the microcapsules is assumed to be the rate-controlling step. The model also assumes that the microcapsule is uniform inside. Because the mass of outer aqueous phase was much greater than that of the microcapsules, the caprolactam content in the aqueous phase is considered to be invariable. In the calculation, a numerical method was applied, to calibrate the apparent diffusion coefficient, and a microcapsule was divided into a series of layers with a same thickness, as shown in Figure 12. Caprolactam distributes evenly in each layer, but the caprolactam concentration varies in different layers. In a very short time (dt), Fick’s law is used to determine the diffusion flux of caprolactam in each layer. To layer 1, the diffusion of caprolactam into this layer during a time period dt is described as NL1 ) DAA1
(DCW - CL1) dC dt ) DA(4πR2) dt dr h
(1)
To layer n (n > 1), the diffusion of caprolactam into this layer during a time period dt is described as NLn ) DAAn
(CLn-1 - CLn) dC dt ) DA(4πRn2) dt dr h
(2)
Therefore, in layer m (m < nmax), the caprolactam concentration increases during time period dt as
Figure 11. (a) Microcapsules with different diameters. (Conditions: caprolactam solution concentration, 34.23 g/L; lines refer to calculated results, and data points refer to experimental results.) (b) Caprolactam solution with different concentrations. (Details: (b) Microcapsule No. 3, caprolactam solution concentration ) 53.97 g/L; (O) Microcapsule No. 3, caprolactam solution concentration ) 34.23 g/L; (2) Microcapsule No. 9 in ref 19, caprolactam solution concentration ) 41.26 g/L; (4) Microcapsule No. 9 in ref 19, caprolactam solution concentration ) 10.54 g/L. Lines refer to calculated results.) (c) Microcapsules with different 1-octanol loading. (Conditions: caprolactam solution concentration ) 34.23 g/L; lines refer to calculated results, and data points refer to experimental results.)
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used as the continuous phase to disperse the polymer solution drops uniformly. After solidification, monodispersed polysulfone (PSF) microspheres can be prepared. By controlling the flow rate of N2 in the microchannel, the diameter of microcapsules changes linearly. The minimum diameter was 0.86 mm. Microsphere pore volume increases with the decrease of PSF content in polymer solution. Accordingly, the 1-octanol loading ratio can be controlled by varying the PSF content in the polymer solution. The maximum loading of 1-octanol is 6.98 g/g PSF when the microsphere pore volume is 8.28 mL/g. When applied to address caprolactam in the aqueous phase, the extraction capacity and kinetics of microcapsules were also investigated. The extraction capacity increased as the caprolactam concentration in the aqueous phase increased. The PSF shell adsorbs little caprolactam that has been dissolved in 1-octanol. Therefore, the maximum uptake of caprolactam increased as the weight content of 1-octanol in a microcapsule increased. The maximum uptake was 58.8 mg/g. Microcapsules with smaller diameter can reach the equilibrium state more quickly. When Microcapsule No. 5 was used to extract caprolactam, the equilibrium state can be reached within ∼25 min. A model was derived, based on the assumption of largest diffusion resistance occurred in microcapsules. The calculation results agreed well with the experimental results, and the relative deviation was
