Ind. Eng. Chem. Res. 2007, 46, 2231-2234
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Efficient Reduction of Gasoline Volatility through Ultrasonic Atomization Kazuo Matsuura,† Susumu Nii,*,‡ Tetsuo Fukazu,† and Katsumi Tsuchiya§ Ultrasound Brewery Co., Ltd., 19 Yanaginomoto, Ikenotani, Oasa-cho, Naruto, Tokushima 779-0303, Japan, Department of Chemical Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Chemical Engineering and Materials Science, Doshisha UniVersity, 1-3 Miyakodani, Tatara, Kyotanabe, Kyoto 610-0321, Japan
Being relatively volatile components of a commercial gasoline, hydrocarbons whose carbon number ranges from four to six, C4 to C6, were effectively removed by ultrasonic atomization. The ultrasonic frequency was 2.4 MHz, and the voltage applied to the oscillating unit was 24 V. More than 60% of C4 and C5 hydrocarbons were removed during 900 s of operation. In regard to the energy requirement for the separation, ultrasonic atomization needs only about half the energy required for evaporation. This finding is interpreted as a result of the phase change of liquids; the input energy was effectively utilized for the removal of light hydrocarbons. Operation at ambient temperature is advantageous for making use of a difference in vapor pressure among hydrocarbons. Introduction Loomis1
Since Wood and discovered the phenomena, ultrasonic atomization has been recognized as a technique to produce fine liquid particles.2-5 The average diameter of less than 10 µm is easily attained, and the drop size distribution is much narrower than that for conventional spraying. Because of the simplicity of the device and the ease of operation, ultrasonic atomization is playing important roles in analytical chemistry, materials preparation,6,7 and medical treatments using fine liquid particles. The applications are found in liquid atomizers for atomic absorption spectroscopy, humidifiers for controlling humidity, and a delivery device for inhaling medicine. Only a little attention has been paid to the composition of atomized liquid from solutions. Matsuura and his co-workers first reported that ethanol is highly enriched in the resulting “vapor phase” through ultrasonic atomization of mixtures of ethanol and water.8 The vapor phase is a mixture of fine liquid particles, vapor of volatile components, and a carrier gas. Highly concentrated ethanol can be recovered through cooling of the gaseous mixture. Furthermore, the energy requirement for producing the vapor phase enriched with the volatile components was much smaller than the latent heat for evaporation. The finding potentially leads to a development of an innovative separation technique which outperforms conventional distillation which is the most common process in the petroleum industry. Another aspect of the technique is significant enlargement of surface area. Attempts have been made to separate nonvolatile components such as surfactants9 and amino acids10 from the aqueous solutions. Here, we report on the application of ultrasonic atomization to remove relatively volatile components from gasoline. A unique separation characteristic has been presented, and the energy requirement for the separation has been discussed. The reduction of volatile components in automobile gasoline has been promoted worldwide. The government has introduced standards to control the emission of hydrocarbons of lower molecular weight. In the United States, the Environmental Protection Agency (EPA)11 has been regulating the vapor * To whom correspondence should be addressed. Tel.: +81-52-7893390. Fax: +81-52-789-3269. E-mail:
[email protected]. † Ultrasound Brewery Co., Ltd. ‡ Nagoya University. § Doshisha University.
pressure of gasoline since 1989 and the use of reformulated gasoline has been strongly recommended in some areas since 1994.12 In Japan, the vapor pressure allowance for automobile gasoline has been reduced lately by 10%. Estimation shows that the renovation of existing distillation plants needs more than 40 billion Japanese Yen, JPY(23 billion JPY for fixed costs and 17 billion JPY for operating costs)13 in Japan. Distillation, which is a widely applied separation technique, is a combination of heating liquids to evaporate and cooling of vapor to recover volatile substances. Thus, in principle, the method requires a large amount of heating energy. Development of a highly efficient separation method is a key to cope with the regulation and also to reduce the environmental impact of gasoline. When the supplied heat is exclusively spent to vaporize those volatile species, not to heat up the whole liquid, the separation should require less energy than distillation. Ultrasonic atomization is a method that enhances the vaporization and also separation of highly volatility species from a liquid mixture. Gasoline components with a relatively high vapor pressure are lower molecular weight hydrocarbons, and they contribute to raising the vapor pressure. Figure 1 represents the vapor pressure of typical gasoline components at different temperatures against the carbon number of hydrocarbons. Among the components of an ordinary gasoline,14,15 species were selected from alkane, olefin, and naphtene. In this plot, the vapor pressure of the hydrocarbons linearly decreases with increasing carbon number. Interestingly, the slope for the lower temperature is steeper than that for the higher temperature. Since the separation of a mixture is based on differences of one or more properties among the components, the condition should be selected to maximize these differences. The condition which provides the larger slope would be favorable for the separation. Therefore, we chose the ambient temperature for the atomization experiments. Experimental Two kinds of liquids are applied to ultrasonic atomization experiments. One is the binary mixture of benzene and toluene, and the other is the commercial gasoline. Reagent-grade benzene and toluene were used to prepare the mixture, and a regulargrade gasoline was purchased for the experiments. The schematic diagram of the experimental setup is shown in Figure 2. A glass vessel for atomization was placed in the
10.1021/ie0611305 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
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Figure 1. Vapor pressure of gasoline components.
Figure 3. Separation of benzene-toluene mixture through ultrasonic atomization: initial liquid temperature 298 K, air inlet temperature 293 K, air flow rate 2.2 × 10-4 m3/s.
Figure 2. Experimental setup.
bath whose temperature was set constant at 303 K. The shape of the atomization vessel was cylindrical, the inner diameter was 11.5 cm, and the height was 29.5 cm. An ultrasonic oscillator (HM-2412, Honda electronics Co. Ltd.) was mounted on the bottom of the center of the vessel. The testing liquid was charged to a level 3.5 cm above the oscillator to obtain a better atomization condition. The oscillator was driven by applying 24 V, and the ultrasonic frequency was 2.4 MHz. The ultrasonic irradiation from the bottom of the liquid to the surface causes the sound pressure to lift up the liquid above the oscillator, and the fountain liquid jet appears. The air at ambient temperature was supplied to the vessel at flow rates of 2 × 10-4 to 3 × 10-4 m3/s. The airflow was directed from the bottom of the fountain jet to the top to carry the mist as well as the vapor to the outside of the vessel. The inlet air temperature was around 300 K, and the temperature was set constant for each experiment. The outlet gas temperature was monitored with a thermocouple. The operation was started by applying the voltage to the oscillator just after introducing air to the vessel. The duration of the operation was 900 s. The change of liquid weight and density was measured to obtain the amount of liquid atomized. The liquid density was measured with a density meter (DA-505, Kyoto Electronics Manufacturing Co., Ltd.) The gasoline composition was analyzed with a method using gas chromatography in accordance with an industrial standard from Japan, JIS K-2536-2. Results and Discussion The formation of ultrasonic mist started right after starting ultrasonic irradiation. When the hydrocarbon mixture was applied, very fine liquid droplets which are unobservable with the naked eye were produced from the side surface of the fountain jet. For the ultrasonic atomization of water, the size of the liquid particles is bigger than that for the hydrocarbons and the mist formation can be observed around the side surface
Figure 4. Change of gasoline composition through ultrasonic atomization: (black bar) before sonication, (white bar) after sonication.
of the fountain. Although the diameter of hydrocarbon droplets is not directly observed yet, there should be very fine particles. Yano et al.16 suggested the formation of nanometer size droplets of a mixture of ethanol and water under the irradiation of ultrasound using the same device. In addition, it takes about a few seconds to start the mist formation. The convenience of this operation is highly advantageous in contrast to the operation of distillation columns which requires a much longer time for startup. Figure 3 represents the result of the separation of binary mixtures of benzene and toluene. The vertical axis is the calculated values of benzene mole fractions in the vapor phase on the basis of mass balance. The value of the benzene mole fraction in the gas, yB, is calculated from the change of liquid weight and composition during the operation.
yB ) (xB,1w1 - xB,2w2)/(w1 - w2)
(1)
where xB refers to the benzene mole fraction in liquid and w refers to the liquid weight. The subscript 1 represents the initial and 2 the final value. Within the concentration range examined, benzene is preferentially separated into the gas phase in the form of fine liquid droplets as well as the vapor. The more volatile components are transferred to the gas phase with the help of the atomization and the airflow. Figure 4 shows how ultrasound changed the gasoline composition before and after atomization. Carbon number distribution is compared between before and after atomization. Substantial decrease was observed for species whose carbon number ranges from four to five, C4 to C5. After the ultrasonic atomization, the main component’s carbon number shifted to a higher value. Table 1 represents experimental data of the composition of the raw gasoline and also the calculated values of the mist (mixture of vapor and fine liquid droplets) composi-
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2233 Table 1. Composition of Tested Gasoline before Sonication and Produced Mist through Atomization gasoline mist (calculated value) [vol %]
tested gasoline [vol %] carbon number
paraffin
3 4 5 6 7 8 9 10 11 12 13 Total
0.1 3.7 15.1 13.7 8.1 5 1.8 1.5 0.9 0.4
olefin 1.8 4.4 2.7 3 1.7 0.8 0.5 0.4 0.1
0.3 1.1 1.7 1.3 0.9 0.2 0.1
50.3
15.4
5.6
sulfur content
naphtene
73.0 ppm
aromatic
0.3 8.7 3.5 10.1 4.5 1.4 0.2 28.7
paraffin
olefin
naphtene
0.3 11.6 36.3 17.6 4.8 1.2 0.2 0.4 0.2
5.5 9.6 2.9 1.7 0.4 0.1 0.2
0.5 1.1 0.8 0.2 0.2
72.6
20.4
2.8
aromatic
0.3 3.1 0.4 0.3 0.1
4.2
19.7 ppm
Table 2. Comparison between with and without Ultrasonic Atomizationa
final gasoline density [kg/m3] final gasoline temperature [K] air temperature at inlet [K] gas temperature at outlet [K] gasoline reduction rate [kg/s]
without sonication
with ultrasonic atomization
753.8 296 299 299 8.3 × 10-5
768.8 295 301 293 16.6 × 10-5
Initial gasoline density 736.8 kg/m3, air flow rate 3.7 × 10-4 m3/s, operation time 900 s. a
tion. Light paraffin and olefin are transferred into the mist from the liquid. Naphtene and aromatic tend to remain in the liquid. In addition, components containing sulfur are also separated into mist. The separation tendency is similar to the distillation. The result clearly suggests that the lowering of vapor pressure is successfully attained through ultrasonic atomization. Sato et al. reported in their former work8 that the energy required for converting the mixture of ethanol and water into fine liquid particles was about 1/3 of the latent heat for evaporation. The reason for the low energy requirement is the partial evaporation, in other words, partial phase change. Unlike in distillation, the liquid is not boiled in ultrasonic atomization. Most of the energy applied to the oscillator is used to produce liquid mist, and a remarkably large interface is provided for evaporating species. Table 2 compares the experimental results between with and without ultrasonic atomization for the gasoline tested. The air was supplied at the same flow rate for with and without ultrasound. Thus, the comparison is between evaporation and atomization. The rate of the liquid reduction for ultrasonic atomization is about twice as large as that for without atomization. The fact is due to the formation of fine droplets. The liquid gasoline temperature dropped from the initial temperature of 303 K because of the evaporation. The heat transferring from the bath to the atomization vessel does not compensate for the heat leaving from the liquid to the gas. The final temperature reaches around 295 K for both conditions. The reasons for the similar temperature drop of the different liquid reduction rate are as follows. For ultrasonic atomization, a part of ultrasonic energy irradiated to the liquid turns to heat. Due to the heat supply, the temperature drop was reduced for ultrasonic atomization as compared to that for without atomization. Also, the change of liquid into fine droplets requires less energy than the phase change. The gas outlet temperature for ultrasonic atomization decreased from the inlet temperature also due to the evaporation from liquid droplets.
Figure 5. Heat inputs to the vessel: (a) without atomization, (b) with ultrasonic atomization.
Between the two operations, energy input per unit weight of liquid reduction was compared to discuss the energy requirement of these operations. As is schematically shown in Figure 5, energy inputs to the vessel for with and without ultrasonic atomization are the heat of supplied air, EG or EG′, heat from the bath, EB, and energy input from the ultrasonic unit, EU, for sonication. The total energy inputs of both operations during the operation time of 900 s are EG + EB for without atomization and EG′ + EB + EU for ultrasonic atomization. The values of EG and EG′ are calculated from the data given in Table 2 and the specific heat of the air. They are 96 kJ and 103 kJ, respectively. The value of EB is obtained from a heating experiment of the liquid gasoline from 293 K using the same experimental setup for atomization. After reaching the water temperature in the bath at 303 K, the vessel filled with the liquid gasoline of 293 K was soaked in the bath and the temperature change of the gasoline was monitored for 900 s. From the temperature difference, EB was calculated to be 122 kJ. The energy given from the ultrasonic unit to the vessel was derived from the electric power applied to the oscillator. The value was 14.7 kJ. Therefore, the total energy input for without atomization is 218 kJ and for with atomization is 240 kJ. When the values were divided by the weight of liquid reduction for the operation time, the energy to separate a unit weight of volatile or atomizable components would be obtained. The values are 2.9 × 103 kJ/kg for without atomization and 1.6 × 103 kJ/kg for ultrasonic atomization. The energy input for ultrasonic atomization is about 55% of that for simple evaporation. The result suggests that ultrasonic atomization has a lower energy requirement for converting the volatile components of gasoline into the fine droplets as well as vapor than the ordinary phase change of the liquid. From the view point of processing, additional energy will be required for the recovery of the mist. Matsuura et al.17 proposed the incorporation of adsorption units for the mist collection to the ultrasonic atomization unit for separating ethanol from its aqueous solution. For the VOC removal process from gasolines, appropriate units for the mist recovery should be combined with the atomization unit. Though further study is required for estimating the overall energy requirement for the ultrasonic
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separation, the presented results suggest that the method has a high potential to separate volatile or atomizable components efficiently from a liquid mixture. Conclusion Removal of C4-C6 compounds from a commercial gasoline is successfully attained by ultrasonic atomization. More than 60% of C4 and C5 hydrocarbons were separated out in 900 s of operation. The input energy for the ultrasonic atomization to bring a unit weight of the liquid into the gas phase was about a half of that for evaporation. The fact indicates that the partial phase change of liquid particles produced by ultrasonic atomization, and the energy, is effectively used to separate the light hydrocarbons. The ultrasonic atomization process is operated at ambient temperature unlike conventional distillation that requires heating up the whole liquid. Furthermore, operation at ambient temperature is advantageous for separating volatile components for making use of the difference in vapor pressure among hydrocarbons. Ultrasonic atomization is highly expected to be an efficient separation technique to produce environmentally friendly gasoline. Literature Cited (1) Wood, R. W.; Loomis, A. L. The physical and biological effects of high frequency sound waves of great intensity. Philos. Mag. 1927, 4, 417. (2) Lang, R. Ultrasonic atomization of liquids. J. Acoust. Soc. Am. 1962, 34, 6. (3) Bouguslavskii, Y. Y.; Eknadiosyants, O. K. Physical mechanism of the acoustic atomization of a liquid. SoV. Phys. Acoust. 1969, 15, 14. (4) Mason, T. AdVances in Sonochemistry; JAI press Ltd.: London, 1993; Vol. 3, pp 145-164. (5) Rajan, R.; Pandit, A. B. Correlations to predict droplet size in ultrasonic atomization. Ultrasonics 2001, 39, 232.
(6) Skrabalak, S. E.; Suslick, K. Porous MoS2 synthesized by ultrasonic spray pyrolysis, J. Am. Chem. Soc. 2005, 127, 9990. (7) Taniguchi, I. Physical and electrochemical properties of spherical nanostructured LiCrxMn2-xO4 particles synthesized by ultrasonic spray pyrolysis. Ind. Eng. Chem. Res. 2005, 44, 6560. (8) Sato, M.; Matsuura, K.; Fujii, T. Ethanol separation from ethanolwater solution by ultrasonic atomization and its proposed mechanism based on parametric decay instability of capillary wave. J. Chem. Phys. 2001, 114, 2382. (9) Takaya, H.; Nii, S.; Kawaizumi, F.; Takahashi, K. Enrichment of surfactant from its aqueous solution using ultrasonic atomization. Ultrason. Sonochem. 2005, 12 (6), 483. (10) Suzuki, A.; Maruyama, H.; Seki, H.; Matsukawa, Y.; Inoue, N. Enrichment of amino acids by ultrasonic atomization. Ind. Eng. Chem. Res. 2006, 45, 830. (11) American Environmental Protection Agency. http://www.epa.gov/ otaq/rfg/information.htm. (Accessed Feb 2007). (12) Chevron homepage. http://www.chevron.com/prodserv/fuels/bulletin/fed%2Drefm/. (Accessed Feb 2007). (13) Technical report of Japan Clean Air Program (in Japanese), Petroleum energy center of Japan, Tokyo, 2002; http://www.pecj.or.jp/ japanese/jcap/pdf/jcap09_13_08.pdf. (Accessed Feb 2007). (Report no. PEC2001JC-08). (14) Dunstan, A. E.; et al. The Science of Petroleum; Oxford University Press: London, 1938; Vol. V, Part 1, pp 57. (15) Nelson, W. L. Petroleum Refinery Engineering, fourth ed.; McGrawHill: New York, 1958; pp 15. (16) Yano, Y. F.; Douguchi, J.; Kumagai, A.; Iijima, T.; Tomida, Y.; Miyamoto, T.; Matsuura, K. In situ X-ray diffraction measurements of the capillary fountain jet produced via ultrasonic atomization. J. Chem. Phys. 2006, 125, 174705. (17) Matsuura, K.; Fukazu, T.; Abe, F.; Sekimoto, T.; Tomishige, T. Efficient separation coupled with ultrasonic atomization using a molecular sieve. AIChE J. 2007, 53, in press.
ReceiVed for reView August 28, 2006 ReVised manuscript receiVed January 19, 2007 Accepted February 5, 2007 IE0611305
