Ind. Eng. Chem. Res. 2001, 40, 3843-3847
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RESEARCH NOTES Enhancement of Distillation Efficiency by Application of an Electric Field Costas Tsouris,* Kevin D. Blankenship, Junhang Dong, and David W. DePaoli Oak Ridge National Laboratory,† Chemical Technology Division, Oak Ridge, Tennessee 37831-6224
The effects of an applied electric field on distillation were investigated using a laboratory-scale two-stage column. The electric field was applied in the liquid region between a distillation plate and a rod-shaped electrode immersed in the liquid and placed parallel to the plate. It was found that the distillation column performance, i.e., the plate efficiency and distillate flow rate, can be increased by application of a direct-current (DC) electric field to the liquid phase of the plate. The cause for such an increase in the distillation efficiency appears to be the enhancement in mass- and heat-transfer rates under the applied electric field. For a binary system of water and 2-propanol, a 2.2% increase in the plate efficiency of the second stage and a 4-fold increase in the distillate flow rate were obtained by application of a voltage of 14 kV. A 20% increase in the heat energy input was observed under this voltage. These enhancements were not observed in the distillation of a nonconductive binary system containing 2-butanone and toluene with applied voltages of up to 30 kV. Introduction Distillation is the most common separation process in several industries. It consumes approximately 3% of the total energy in the United States and often holds a large portion of the total equipment investment in a chemical plant. After having been studied for almost a century, distillation technologies still attract great interest for further development because even a small improvement in the distillation efficiency can lead to significant energy savings. It has been reported in the literature that the application of an external electric field can modify vaporliquid equilibria and enhance mass- and heat-transfer rates at a fluid interface.1-5 A decrease in the temperature of the boiling point for pure alcohols under applied electric fields has also been reported.6,7 O’Neal1 studied the electrified distillation of polar-nonpolar organic mixtures by applying a DC high voltage on a glass condenser. He found an increase in the concentration of the polar component of liquid samples collected at the bottom of the negatively charged central electrode. Maximuc et al.3 observed up to a 70% increase in the mass-transfer rate of a binary mixture distilled under an electric field. Austin et al.8 reported a 10-fold increase in the mass-transfer coefficient by application of a voltage of 2 kV across a plane interface of ethyl acetate/ water. Using electric fields, Yabe et al.4,5 showed a 3-fold increase in the boiling heat-transfer rates of a nonazeotropic mixture of HCFC-132 and HFC-134a and en* Author to whom correspondence may be addressed. E-mail address:
[email protected]. † Managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.
hancements in heat transfer by factors of 5-100 for several processes such as boiling, condensation, and defrosting. Experiments have recently been conducted in our laboratory to investigate the effects of electric fields on binary vapor-liquid equilibria, distillation, and liquidliquid interface behavior.9-11 Changes in the separation factor as high as 10% have been observed for some binary systems containing water and 2-propanol under an electric-field strength of a few kilovolts per centimeter.9,10 Experimental observations and modeling results suggest that the enhancement in the separation efficiency under electric fields might be attributed to charge accumulation at the vapor-liquid interface, which can change the local concentration of certain components. In experiments of liquid-liquid mixing, application of an electric field across the liquid-liquid interface was found to cause Maragoni flow in the vicinity of the interface, which accelerated interface renewal.8 When the applied electric field was further increased, liquid column formation and droplet formation could be observed. These phenomena induced surface renewal and droplet jetting that caused enhancements in mass and heat transfer. In another recent study, we found that gases could be sprayed into a liquid phase through application of an electric field12,13 so that they formed small bubbles, thus increasing the gas-liquid contact area for enhanced mass and heat transfer. It is expected from the above experimental results and discussions that the application of electric fields can improve distillation efficiency in three ways: (1) by enhancing mass-transfer rates and plate efficiency, (2) by enhancing heat-transfer rates and preventing temperature drop and temperature nonuniformity, and (3) by modifying vapor-liquid equilibria. This work focuses
10.1021/ie000869q CCC: $20.00 © 2001 American Chemical Society Published on Web 07/28/2001
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Figure 1. Schematic diagram of the distillation column used in this study.
on the effect of an electric field on the mass transfer and separation efficiency in distillation processes. Experimental Section Apparatus and Methods. Experiments were conducted in a distillation column consisting of a boiler flask, an additional stage, and a condenser, as schematically shown in Figure 1. Liquid mixtures were boiled in a 2000-mL round-bottom flask (Ace Glass) heated by an electric heating jacket. The vapor flowed upward through 0.25-mm-i.d. orifices constructed from 1.6-mm-o.d. stainless steel tubing (total of 20) in a Teflon distillation tray (8-cm-diameter) into the second stage of the column, which was an 8-cm-diameter Pyrex pipe. The stainless steel orifices were interconnected with copper wires and linked to high voltage (see Figure 1). The vapor flowed into a vertical water-cooled glass condenser and then was refluxed as liquid back into the second stage of the column. A Teflon overflow pipe (1.3cm o.d., 0.6-cm i.d.) in the second stage returned liquid to the first stage of the column through Teflon tubing extended down into the reaction flask. This overflow maintained the level of the liquid in the second stage at a fixed height of approximately 7.0 cm above the tray. The large spacing between the tray and the total condenser made it difficult to cause flooding of the tray. High voltage was applied to the distillation tray, and an electrically grounded stainless steel rod of 1/4-in. diameter was placed completely beneath the liquid surface above the tray (Figure 1). The rod was mounted horizontally through the wall of the distillation column, parallel to the tray, with a Teflon seal and chemically resistant O-ring. The shortest distance between the rod
and the tray was 6.0 cm. An electric field was therefore formed in the liquid region between the tray and the rod. The effect of the electric field on the size of the vapor bubbles rising through the liquid was observed by CCD camera (SVC-09, Xybion Electronic Systems). Furthermore, the effect of the electric field on mass transfer between the vapor and liquid was investigated by measuring the concentration of the final distillate with and without an electric field applied in the secondstage region of the distillation column. The samples taken from the final stage were analyzed by a HewlettPackard 5890 Series II gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and a Porapak Q 1/8-in., 6-ft column. Details of the GC analysis are given elsewhere.9 The experiments were carried out at hood pressure, which was slightly lower than the atmospheric pressure. The pressure in the still and at the top of the condenser was measured with an accuracy of (0.002 psig using pressure sensors (SC Series, model TJE/0713, Sensotec, Columbus, OH). The vapor temperature was measured using a thermistor connected to a temperature recorder (1560 Black Stack, Hart Scientific, American Fork, UT) with an accuracy of (0.01 °C. High voltage was provided by a direct current (DC) power supply (Glassman Series EQ, Whithouse Station, NJ) with a maximum voltage of 30 kV and a current limit of 40 mA. The current in the system was read from a multimeter (Fluke 29 Series 2, John Fluke Manufacturing, Everett, WA) attached to the remote control of the high-voltage power supply. The mixture was heated at total reflux until a steady vapor temperature was reached prior to any measurements. A binary system containing water and 2-propanol (>99.9%, J. T. Baker, Phillipsburg, NJ) was used as the
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Figure 2. Bubble size reduction in a distillation stage with increasing applied voltage. Average bubble diameters at different voltage levels: (a) 0 V, 4.4 mm; (b) 10 kV, 3.0 mm; (c) 20 kV, 1.6 mm; and (d) 30 kV, 0.8 mm.
feed stream in the distillation experiments. The molar composition of the mixture was 57.2% 2-propanol and 42.8% water, and the conductivity of the solution was 0.75 µmho/cm. Results and Discussion The average diameter of the bubbles in the liquid on the second stage was measured by a video technique.14 Changes in the total number of bubbles in the liquid phase of the second stage were estimated from the average bubble size and the rate of vapor flow through the plate, which was determined by measuring the distillate flow rate under total condensation conditions. The bubble size was measured with and without applied voltages. Figure 2 shows images of the bubbles flowing upward through the liquid under different applied voltages. The average diameter of the bubbles decreased from 4.4 mm at 0 V to 0.8 mm at 30 kV. Because a total condensation and reflux was employed, the composition of the final distillate was the same as that of the vapor at the second stage. Therefore, the plate efficiency of the second stage could be calculated from the 2-propanol concentration in the final distillate, which was sampled from the condenser. Figure 3 shows the relationship between the concentration of 2-propanol in the final distillate and the applied voltage. As shown, the 2-propanol concentration of the final distillate increased with an increase in the applied voltage. This increase appears to have been caused mainly by the increased plate efficiency due to the enhancement of mass transfer between the vapor
Figure 3. Effect of applied voltage on plate efficiency. Dashed lines indicate 95% confidence interval for 2-propanol mole fraction with no electric field applied.
and liquid phases under the electric field. In an earlier study,9 it was reported that, when both electrodes are located in the liquid phase, no change in phase equilibrium occurs. The mass-transfer enhancement can be attributed to the increased vapor-liquid contact area and interface instability caused by the electric field. The power input and distillate rate were also measured, with results shown in Figure 4. In this experiment, the distillate was collected by a graduated cylinder rather than being refluxed back into the column. With an applied voltage of 14 kV, a 4-fold increase in the distillate flow rate and a 20% increase in the heat energy input (including energy input via the electric field on the second stage) were observed. This means that, by the application of an electric field, the produc-
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Figure 4. Distillation flow rate and total power input at different applied voltages.
ducted for a binary mixture of 2-butanone and toluene, which had a much lower conductivity (11 nanomho/cm) than the binary system containing water and 2-propanol. No significant change in the performance of the distillation column was observed with an applied voltage of up to 30 kV. This result indicates that the electrodistillation method is not suitable for a highly insulating system with a low dielectric constant. The reason is that the electric force in such systems is small, and therefore, the electrohydrodynamic flow is limited. Under these conditions, no spraying or pumping takes place on the distillation stage.12,15,16 Electrohydrodynamic pumping and spraying are possible for insulating fluids at much stronger electric fields; however, undesirable phenomena, such as discharge, occur more frequently at stronger fields.
Table 1. Distillation of Water/2-Propanol Binary Mixture for 0- and 14-kV Applied Voltage
Conclusions
applied voltage parameter
0 kV
14 kV
mole fraction of 2-proponal in final distillate average bubble diameter, mm vapor flow rate (or distillate rate), mL/min electric energy converted to heat, kJ/min NV)14/NV)0a ) 65.7 AV)14/AV)0b ) 11.0
0.738 4.4 ∼2.0 15
0.754 1.8 ∼9.0 19
a N V)14 and NV)0 are the total numbers of bubbles in the liquid observed under applied voltages of 0 and 14 kV, respectively. b A V)14 and AV)0 are the total vapor-liquid contacting areas in the observed sample under applied voltages of 0 and 14 kV, respectively.
tivity of a distillation column can be increased without causing a decrease in the concentration of the distillate. The increase of the distillate rate can be explained by taking into account two effects of electric fields: (1) enhancement of heat transfer,4,5 which facilitates the vaporization rate of a liquid, and (2) enhancement of the vapor transport from one stage to the next by electropumping.15 The pressure difference between the top of the condenser and the still increased from 0.172 to 0.241 psig, with a temperature increase of 0.24 °C in the liquid phase of the still, when the applied voltage was increased from 0 to 25 kV. This change in pressure difference was caused by (1) the electrostatic force on the vapor-liquid interface at the electrified tray, which reduced the upstream pressure from 14.382 to 14.374 psig, and (2) the decrease of the vaporization rate, which increased the pressure in the still from 14.554 to 14.615 psig. Meanwhile, the increase of the vapor volume fraction in the liquid layer with the applied electric field decreased the hydraulic resistance to vapor transport through the tray, which also increased the distillate rate. Some important parameters are given in Table 1 to compare the column performance with and without an electric field applied. Table 1 indicates that the vaporliquid contact area and the total flux of vapor through the liquid can be dramatically increased by the application of an electric field. It should be noted that big bubbles did not form as the vapor holdup in the liquid phase increased because the electric field inhibited bubble coalescence. These observations suggest that increases in mass and heat transfer are the main causes of the enhancement of the distillation performance. To investigate the effect of the liquid conductivity on the electrodistillation, similar experiments were con-
The effects of applied electric fields on the distillation process of a binary mixture of water and 2-propanol were studied using a laboratory-scale two-stage column. It was found that application of an electric field increased both the distillate flow rate and the distillate concentration of the volatile component, or the plate efficiency and throughput of the column. The reasons for this enhancement in the distillation performance might include (1) enhancement in mass and heat transfer between the vapor and liquid phases and (2) facilitated vapor transport through the plate and liquid by electropumping and electrospraying. However, no significant change was observed in the distillation of a 2-butanone/toluene binary system under an applied voltage of up to 30 kV. The reason for this difference is that the electrohydrodynamic effects were negligible in this system. Acknowledgment Funding for this research was provided by the Environmental Management Science Program, Office of Environmental Management, and the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC0500OR22725 with UT-Battelle, LLC. The authors are also grateful to Dr. Marsha K. Savage for editing the manuscript. Literature Cited (1) O’Neal, J. M. The Compositional Behavior of Selected Binary Mixtures in the Presence of A Non-Uniform Electric Field. Ph.D. Dissertation, University of Southern Mississippi, Hattiesburg, MI, 1973. (2) Bulanov, G. A.; Butkov, V. V. Rectification of Binary Mixtures in an Electrostatic Field. Sov. Surf. Eng. Appl. Electrochem. 1987, 2, 91. (3) Maximuc, E. P.; Bologa, M. K.; Gordeev, Y. N. The Influence of Electric Fields on the Intensity of Rectification. J. Electrost. 1993, 30, 413. (4) Yabe, A.; Taketani, T.; Maki, T.; Aono, H. Experimental Study of EHD Enhanced Evaporator for Non-Azeotropic Mixtures. Trans. ASHRAE 1992, 98, 455. (5) Yabe, A.; Mori, Y.; Hijikata, K. Active Heat Transfer Enhancement by Utilizing Electric Fields. Annu. Rev. Heat Transfer 1995, 7, 91. (6) Katti, P. K.; Chaudhri, M. M. Effect of Strong Electric Fields on the Boiling Point of Some Alcohols. Nature 1961, 192, 1285. (7) Biswas, S.; Basu, S. P. Effect of Electric Field on the Boiling Points of Liquids. Indian J. Technol. 1976, 14, 165
Ind. Eng. Chem. Res., Vol. 40, No. 17, 2001 3847 (8) Austin, L. J.; Banczyk, L.; Sawistowski, H. Effect of Electric Field on Mass Transfer Across a Plane Interface. Chem. Eng. Sci. 1971, 26, 2120. (9) Blankenship, K. D.; Shah, V. M.; Tsouris, C. Distillation Under Electric Fields. Sep. Sci. Technol. 1999a, 34, 1393. (10) Blankenship, K. D.; DePaoli, D. W.; Hylton, J. O.; Tsouris, C. Effect of Ellectrode Configurations on Phase Equilibria Under Electric Fields. Sep. Purif. Technol. 1999b, 15, 283. (11) Tsouris, C.; Dong, J. Effects of Electric Fields on Phase Inversion of Liquid-Liquid Dispersions. Chem. Eng. Sci. 2000, 55, 3571. (12) Tsouris, C.; DePaoli, D. W.; Feng, J. Q.; Scott, T. C. Experimental Investigation of Electrostatic Dispersion of Nonconductive Fluids into Conductive Fluids. Ind. Eng. Chem. Res. 1995, 34, 1394. (13) Shin, W. T.; Yiacoumi, S.; Tsouris, C. Experiments on
Electrostatic Dispersion of Air in Water. Ind. Eng. Chem. Res. 1997, 36, 3647. (14) Tsouris, C.; Lizama, H. M.; Spurrier, M. A.; Takeuchi, T. L.; Scott, T. C. Hydrodynamics of Bioreactor Systems for LiquidLiquid Contacting. Appl. Biochem. Biotechnol. 1996, 57, 581. (15) Tsouris, C.; Shin, W. T.; Yiacoumi, S. Pumping, Spraying, and Mixing of Fluids by Electric Fields. Can. J. Chem. Eng. 1998, 76, 589. (16) Raco, R. J. Electrically Supported Column of Liquid. Science 1968, 12, 311.
Received for review October 5, 2000 Revised manuscript received June 4, 2001 Accepted July 5, 2001 IE000869Q
