Environ. Sci. Technol. 2008, 42, 2150–2154
Supercritical CO2 Desorption of Activated Carbon Loaded with 2,2,3,3-Tetrafluoro-1-Propanol in a Rotating Packed Bed CHUNG-SUNG TAN* AND PEI-LUN LEE Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013 ROC
Received August 29, 2007. Revised manuscript received October 26, 2007. Accepted December 17, 2007.
Desorption of activated carbon loaded with 2,2,3,3-tetrafluoro1-propanol (TFP) by supercritical carbon dioxide in a rotating packed bed was investigated in this study. The experimental data show that the time required to achieve complete desorption of TFP from activated carbon in a rotating packed bed was much lower than that in a static packed bed. The reduction of desorption time is attributed to the presence of centrifugal force. The supercritical CO2 desorption efficiency in a rotating packed bed was observed to increase with increasing rotation speed, pressure, and CO2 flow rate. To enhance desorption efficiency, a smaller activated carbon particle size was suggested. At low operating pressures such as 8.96 and 11.72 MPa, a better desorption efficiency was found to occur at lower temperatures in a temperature range of 305-335 K. However, at high operating pressures such as 15.86 MPa, a temperature of 315 K was found to be more appropriate for desorption, as compared to other temperatures. Due to a reduction of packed bed volume and an increase in desorption efficiency, supercritical CO2 desorption in a rotating packed bed is suggested for recovering TFP from the exhaust gases.
Introduction 2,2,3,3-Tetrafluoro-1-propanol (TFP) has been used extensively as a solvent in the DVD production process because of the ease with which it dissolves polycarbonate and its high volatility. TFP is expensive and causes environmental problems (1, 2); it is therefore essential to recover it from the exhaust gases of DVD plants. There are several existing techniques to remove volatile organic compounds from gases such as condensation, adsorption, catalytic oxidation, and thermal oxidation (3). However, to deal with a ppm concentration level of TFP in a huge gas flow rate and to recover TFP for further use, adsorption onto mesoporous adsorbent is believed to be the most appropriate approach. One of the keys to the success of the adsorption process is dependent on the quality of the adsorbent; it should possess a porous structure and a high adsorptive capacity. Among the adsorbents, activated carbon is commonly used because of its high surface area and low cost. When the adsorption equilibrium of activated carbon is approached, desorption is required if activated carbon is desired for further use. The desorption of activated carbon loaded with organic adsorbate such as benzene, toluene, * Corresponding author email:
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and ethyl acetate by supercritical carbon dioxide has proved to be effective (4–8). There are many advantages to using supercritical CO2 as the desorbent over the conventional steam desorption technique. These include a better subsequent adsorption efficiency using the same adsorption and desorption temperature, the absence of the condensed water in activated carbon pores, and safer operation due to the low desorption temperature and the inert environment (6). In addition, there is, in general, no steam in the display and semiconductor industries in which TFP is commonly used, a supercritical fluid can then be considered as an alternative desorbent. In conventional adsorption processes, a packed bed with large volume is commonly used. But for the gas containing TFP that is exhausted from a clean room in DVD plants, the adsorption of TFP and the desorption of activated carbon can only be carried out in a limited room area due to the compacted space in DVD plants. Recently, operation in a rotating packed bed (RPB) has been shown to be effective not only in gas–liquid systems, including distillation (9), absorption (10, 11), and stripping (12) resulting from a significant increase in contact areas and mass transfer rates (13), but also to liquid adsorption (14, 15) resulting from an increase in diffusion and mass transfer rates. Operation in a RPB is called higee, and it was first proposed by Ramshaw and Mallinson (16). Because of the presence of centrifugal force, depending on the rotation speed and radius of the bed, 5-250 G can be generated in operation. The density of a supercritical fluid is relatively close to that of a liquid, and as such, increased efficiency in the presence of centrifugal force is therefore expected in supercritical fluid desorption as well. As a result, smaller equipment requirements and less energy for desorption of activated carbon in higee are expected as compared to the conventional packed bed operation. The objective of this study is to evaluate a novel process for desorption of activated carbon loaded with TFP. Recovery of TFP from a DVD plant not only meets environmental regulations but also reduces operation cost. In the study, the adsorption of TFP by activated carbon in a static packed bed was first performed and then the supercritical CO2 desorption was performed in the same packed bed at a high rotation speed. A positive effect of centrifugal force was observed in the experiments. The effects of rotation speed, temperature, pressure, supercritical CO2 flow rate, and activated carbon particle size on supercritical CO2 desorption efficiency were studied. From the obtained results on desorption efficiency, the most appropriate operating conditions could then be determined.
Experimental Section Virgin activated carbon (Calgon, WSIV) with an average pore size of 3.81 nm and a BET surface area of 932 m2/g was ground and sieved to obtain particles with an average diameter of 1.0 mm. The activated carbon particles were then boiled in deionized water to remove fines and dried in an oven at 393 K. About 1.0 g of activated carbon mixed with glass beads of the same particle size was packed in a rotating packed bed (RPB) with an inner diameter of 2.6 cm and a height of 3 cm. The experimental apparatus for adsorption and supercritical CO2 desorption is illustrated in Figure 1. In the adsorption operation, 160 cm3 of TFP with a purity of 99.7% (Seedchem) was first loaded into a saturator. A nitrogen gas stream containing 84 ppm of TFP (Boclh, a purity of 99.99%) 10.1021/es702162y CCC: $40.75
2008 American Chemical Society
Published on Web 02/15/2008
FIGURE 3. Experimental data for adsorption and supercritical desorption at 1000 rpm, 315 K, and 11.72 MPa in reproducibility tests.
FIGURE 1. Experimental apparatus for adsorption and supercritical desorption.
a purity of 99.5% (Boclh) was first compressed by a pump (Milton Roy, MD93) and then stored in a surge tank. The CO2 stream with the desired temperature and flow rate flowed into the packed bed with a preset rotation speed via the central axis. The pressure was found to be controlled within (1.0% of the desired value. The effluent supercritical CO2 stream was expanded via a metering valve. The total volume of the expanded CO2 was measured by a wet gas meter (Ritter, TG05/6). The TFP in the expanded CO2 was collected in a cold trap from which samples were taken out frequently and sent to a FID gas chromatograph for analysis.
Results and Discussion
FIGURE 2. Configuration of rotating packed bed. was prepared by passing the nitrogen stream through the saturator containing a fixed level of TFP with a flow rate of 36 cm3/min. The gas bypassed the RPB until the concentration of TFP became stable, and then it flowed into the RPB through the central axis, which had three evenly distributed holes with a diameter of 1.0 mm in each on the side and a total 12 holes, as shown in Figure 2, by switching a three-way valve. The adsorption was performed at a temperature of 315 K. Because there was no rotation of the packed bed, the operation could be regarded as conventional adsorption in a packed bed. The TFP in the effluent stream was condensed and collected in a cold trap filled with 10 mL of methanol. The temperature of the cold trap was maintained at a temperature of 268 K. The TFP collected in the cold trap was analyzed with a FID gas chromatograph (Shimadzu, GC14A) using styrene as the internal standard. The flow rate of the effluent gas was determined by a wet test meter (Ritter, TG05/6). A breakthrough curve was then obtained from the measured TFP collected in the cold trap and gas flow rate. After a certain period of operation, the adsorption was stopped and a subsequent supercritical CO2 desorption was performed. For the supercritical CO2 desorption, CO2 with
The amount of TFP adsorbed onto the activated carbon at 315 K and 180 min was found to be roughly 0.38 g of TFP per gram of activated carbon. The outlet concentration of TFP was of about 80% of the inlet concentration at 180 min, read from the measured breakthrough curves, showing that the equilibrium adsorption capacity of the activated carbon was higher than the presently measured adsorption amount, and thus that activated carbon was appropriate for removal of TFP from a nitrogen gas stream. The reproducibility tests for both adsorption and desorption were carried out at different operating conditions. Some typical results are illustrated in Figure 3; the average deviation of the measured data was found to be less than 4%, demonstrating the reliability of the apparatus for adsorption and desorption. It should be noted here that though only two sets of data at 315 K, 11.72 MPa, and 1.57 cm3/min are shown in Figure 3, the reproducibility tests at these operating conditions were performed frequently during the operation, nearly the same desorption data were observed. Figure 4 shows desorption efficiencies at different rotation speeds for a temperature of 315 K, a pressure of 11.72 MPa, and a supercritical CO2 flow rate of 1.57 cm3/min. In the studied range of rotation speeds, 5-25 gravitational forces were generated. The desorption efficiency was defined as the amount of TFP collected in the cold trap divided by the total amount adsorbed. A desorption efficiency of 100% represents complete desorption. When the rotation speed was set at zero, desorption by supercritical CO2 could be regarded as to occur in a static packed bed. It can be seen from Figure 2 that only 55% of the adsorbed TFP was desorbed by supercritical CO2 in a 30 min operation. However, when the packed bed was rotated, more effective desorption over a static packed bed was observed. The desorption efficiency was found to increase with increasing rotation speed. When the rotation speed was 1600 rpm, complete desorption of activated carbon loaded with TFP could be achieved in a 30 VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Effect of rotation speed on desorption efficiency at 315 K, 11.72 MPa, and a CO2 flow rate of 1.57 cm3/min.
FIGURE 6. Effect of flow rate on desorption efficiency at 315 K, 11.72 MPa, and 1000 rpm.
FIGURE 5. Effect of pressure on desorption efficiency at 315 K, 1000 rpm, and a CO2 flow rate of 1.57 cm3/min.
FIGURE 7. Effect of temperature on desorption efficiency at 8.96 MPa, 1000 rpm, and a CO2 flow rate of 1.57 cm3/min.
min operation, suggesting that centrifugal force aids the desorption process. Centrifugal force is believed to increase mass transfer rates including diffusion in bulk fluid, interphase mass transfer, and possibly intraparticle diffusion (14). It should be noted here that the enhancement of desorption was mainly caused by centrifugal force, not by agitation. This was verified by an operation in which desorption was carried out by feeding supercritical CO2 through a port on the outer surface of the RPB instead of through the central axis of the basket onto the activated carbon loaded with toluene. The measured desorption data showed that the desorption efficiency for this setup was nearly the same as that in a static packed bed, but was much lower than that in the presently proposed operation with the same rotation speed (17). It is therefore expected that the same behavior occurs for the activated carbon loaded TFP as well. When the desorption temperature, rotation speed, and CO2 flow rate were maintained at 315 K, 1000 rpm, and 1.57 cm3/min, respectively, an increase in desorption efficiency with increasing pressure was found, as shown in Figure 5. This was an expected result because more adsorbed TFP can be dissolved in supercritical CO2 at higher pressures, since solubility in a supercritical fluid is generally increased with increasing pressure (18–20). The observed dependence of desorption efficiency on pressure in a RPB is the same as that for supercritical desorption in a conventional packed bed (4–6). Because of the reduced time required to achieve higher desorption efficiency at higher pressures and higher rotation speeds, synergistic effects caused by supercritical
desorption and centrifugal force were exhibited in the proposed process. It has been shown that the desorption efficiency of supercritical CO2 in a packed bed is a function of supercritical CO2 flow rate (4–6). It was therefore of interest to see if the supercritical CO2 flow rate affects desorption efficiency in a rotating packed bed, especially at high rotation speed. Figure 6 illustrates that the desorption efficiency was also enhanced by increasing the supercritical CO2 flow rate at a temperature of 315 K, a pressure of 11.72 MPa, and a particle size of 1.0 mm, indicating that interphase mass transfer resistance played a role in the desorption even at rotation speeds as high as of 1000 rpm. It can also be seen from Figure 6 that 100% desorption could be achieved with a supercritical CO2 flow rate as low as 0.5 cm3/min at 60 min. This can not happen in a static packed bed, as seen from Figure 4, in which operation at 1.57 cm3/min resulted in 90% desorption in 60 min. The observation verified the positive role played by centrifugal force. Figures 7 and 8 show that the desorption efficiency of the activated carbon was increased with a decrease in temperature when the pressure was fixed at 8.96 and 11.72 MPa, respectively. It is known that the solubility of an organic compound in supercritical CO2 is increased with increasing supercritical CO2 density (19–21), which is beneficial to desorption, and diffusion resistance is increased with increasing viscosity (22, 23), which has an adverse effect on the removal of the adsorbed compound from the adsorbent. A decrease in desorption efficiency with increasing temperature
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therefore taken to be the same as pure CO2. However, when the pressure was increased to 15.86 MPa, 315 K was found to be an appropriate temperature, as shown in Figure 9. In this situation, both the viscosity effect and the density effect combined to play an important role in desorption. Figure 10 shows the effect of particle size on desorption efficiency for operations at a temperature of 315 K, a pressure of 11.72 MPa, a rotation speed of 1000 rpm, and a supercritical CO2 flow rate of 1.57 cm3/min. It can be seen that smaller particle sizes resulted in higher desorption efficiencies, indicating that an intraparticle diffusion resistance was present in large particles. Thus, a smaller particle size should be selected to shorten desorption time for subsequent use if the resulting pressure drop is tolerable.
Acknowledgments FIGURE 8. Effect of temperature on desorption efficiency at 11.72 MPa, 1000 rpm, and a CO2 flow rate of 1.57 cm3/min.
Financial support from National Science Council of ROC, grant number NSC- 95-2623-7-007-018-ET, is gratefully acknowledged.
Literature Cited
FIGURE 9. Effect of temperature on desorption efficiency at 15.86 MPa, 1000 rpm, and a CO2 flow rate of 1.57 cm3/min.
FIGURE 10. Effect of particle size on desorption efficiency at 315 K, 11.72 MPa, 1000 rpm, and a CO2 flow rate of 1.57 cm3/min. was therefore attributed mainly to the density effect. The density and viscosity of supercritical CO2 at different operating temperatures and pressures can be found elsewhere (24). Because the equilibrium concentrations of organic compounds in supercritical CO2 are generally not very high (18–21) and the concentrations in the desorption are much lower than the equilibrium ones, which can be seen from the total amounts of the adsorbed TFP and CO2, the density and viscosity of the supercritical mixture containing TFP were
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