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Effects of ultrasound on oily sludge deoiling
Journal of Hazardous Materials 171 (2009) 914–917
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Effects of ultrasound on oily sludge deoiling Xu Ning a , Wang Wenxiang a , Han Pingfang b,∗ , Lu Xiaoping c a
College of Environment, Nanjing University of Technology, Nanjing, China College of Life Science and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing, China c College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing, China b
a r t i c l e
i n f o
Article history: Received 31 March 2009 Received in revised form 15 June 2009 Accepted 15 June 2009 Available online 24 June 2009 Keywords: Oily sludge Oil content Ultrasound Deoiling
a b s t r a c t Oily sludge with an initial oil content of 0.130 g g−1 (dry basis) was mixed with water and treated in an ultrasound cleaning tank. The oil was then separated from the oily sludge by air floatation. Experiments were carried out with and without 28 kHz ultrasonic irradiation at different temperatures. The results show that the minimum oil content, 0.055 g g−1 (dry basis), was obtained at 40 ◦ C after ultrasound irradiation, which was 55.6% less than without ultrasonic irradiation. In addition, this work clearly establishes that 28 kHz ultrasound is superior to 40 kHz ultrasound. The ultrasonic acoustic pressure amplitude with the 28 kHz ultrasound was 0.085 MPa; the 28 kHz ultrasound also exhibited lower oil content than the 40 kHz ultrasound, which yielded 0.120 MPa acoustic pressure amplitude. It can also be concluded that sodium silicate obstructs ultrasound oily sludge deoiling. © 2009 Published by Elsevier B.V.
1. Introduction Oily sludge, which has an annual output of about 1 million ton, is produced in crude oil production, transportation, storage, and refining. Because of its high oil content, oily sludge is listed in the China’s Dangerous Waste List. Current technologies for oily sludge treatment, such as solidification, chemical heat cleaning, and extraction, are either costly or ineffective. Therefore, oily sludge is not being disposed of properly. Ultrasonic oily sludge treatment, which is fast and effective, has recently garnered much attention. Tar sand ultrasonic heat cleaning was developed abroad prior to 1980 [1]. In Canada, there is a 1976 patent describing bitumen separation from tar sands of oil shale by treatment with ultrasound and surfactants [2]; the surfactants included benzyl peroxide and sodium silicate [3]. This kind of technology is probably applicable to cleaning up oily sludge. In China, a new practical patent describes a type of equipment in which crude oil is separated from oily sludge via cyclone. In this instrument, ultrasound and demulsification are used to further the separation [4]. Later, a two-stage oily sludge washing method using ultrasound and surfactants was developed [5]. A process for oily sludge deoiling using only ultrasound was also developed recently [6]. This technology is not only cheap and easy to apply, but it also produces no pollution by-products.
∗ Corresponding author. Tel.: +86 2583588072; fax: +86 2583588072. E-mail address: [email protected] (P. Han). 0304-3894/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2009.06.091
The objective of this work is to investigate the process of oily sludge washing by ultrasound and air flotation for the recovery of crude oil. These experiments examined the oil contents of oily sludge after washing and determined the effects of ultrasound irradiation, ultrasound frequency, and the addition of sodium silicate. 2. Materials and methods 2.1. Materials Oily sludge samples were taken from the Bingyi water treatment plant in the Shengli oil field in China. After pressure filtration, each sample was a massive and black power. The oily sludge sample breakdown is as follows: water content 38.9%. Oil content 7.9%. Oil content 0.130 g g−1 (dry basis). 2.2. Ultrasonic cleaning tank A dual-frequency ultrasonic cleaning tank (structure shown in Fig. 1) was utilized for this research. 2.3. Procedure In this research, 40 g oily sludge and 640 mL water were added to a beaker and subjected to high-speed stirring. The beaker was immersed in an ultrasound cleaning tank full of water and then preheated. After preheating, the ultrasound cleaning tank shown in Fig. 1 was turned on while the mixture was ventilated. Then crude oil was floated and skimmed from the liquid’s surface. The mixture
N. Xu et al. / Journal of Hazardous Materials 171 (2009) 914–917
915
Fig. 1. Schematic diagram of ultrasonic cleaning tank: (1) 28 kHz ultrasonic generator; (2) 40 kHz ultrasonic generator; (3) 28 kHz transducer; (4) 40 kHz transducer; (5) cleaning tank (60 cm × 30 cm × 35 cm).
was dewatered by pressure filtration. Finally, the oil content of the sludge after deoiling was measured. 2.4. Oil content determination According to previous studies [1,3], the oil in sludge can be extracted from petroleum ether (boiling ranger, 30–60 ◦ C) under ultrasound irradiation. In this case, after extraction, the petroleum ether was recovered from the extract liquid at 65 ◦ C, and the weight of the oil was measured. The weight of the sludge residue from the extraction was measured after drying at 105 ◦ C. The weight ratio of oil to dry residue is the oil content (dry basis). 2.5. Ultrasonic acoustic pressure amplitude determination The hydrophone was immersed in the ultrasonic field. The maximum value on the oscilloscope was noted. P=
U M
(1)
where U is the maximum value on the oscilloscope, measured in V, and M is the sensitivity of the hydrophone, measured in V MPa−1 . The sensitivity of the hydrophone in a 28 kHz ultrasonic field is 25.8 V MPa−1 , while that of a 40 kHz ultrasonic field is 28.2 V MPa−1 . 3. Results 3.1. Effect of ultrasonic irradiation According to Bougrier et al. [6], ultrasound irradiation can separate crude oil from solid particles. In this study, the oil content of the sludge after deoiling with and without ultrasound irradiation was determined experimentally. The results are given in Fig. 2. It can be seen from Fig. 2 that there was little change in oil content, and the final content was greater than 0.11 g g−1 without ultrasound irradiation. The measured oil contents with ultrasound irradiation are less than those without ultrasound. At 40 ◦ C, the oil content was 0.055 g g−1 , which is the minimum value. This is 55.6% less than that observed without ultrasonic irradiation. When the temperature is above 40 ◦ C, oil content increases. This is because when the temperature is too high, the intensity of the ultrasonic cavitation is weakened, which causes the process of separation of crude oil from solid particles to be retarded [7]. The cavitation effect is maximum at low temperatures. But high temperature is good to molecular movement and increasing number of cavitation nucleus. Thus oil separation from sludge particle. Under these two factors, the optimum temperature is 40 ◦ C.
Fig. 2. Effects of ultrasound on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; acoustic pressure amplitude 0.10 MPa; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
measured oil contents of the oily sludge after deoiling by ultrasonic generators are shown in Fig. 3. It can be seen from the results that 28 kHz ultrasound is superior to 40 kHz. The potential reasons for this are as follows. First, frequency is an important factor in ultrasonic cleaning, and 28 kHz is more suitable for washing solid particles of this size. Because the 28 kHz ultrasonic cavitation threshold is lower than that at 40 kHz, cavitation intensity is higher at 28 kHz. The separation efficiency at 28 kHz is higher as a result. Second, Kotyusov’s research showed that optimum frequency for particle coagulation was 21–25 kHz. For frequencies above 25 kHz, the lower the frequency, the higher the coagulation efficiency is [8]. Cavitation number increases when ultrasound frequency increases. Thus high frequency ultrasound generates more intense blasts shook. However, cavitation is more difficult to happen under high frequency ultrasound than that under low frequency ultrasound. Consequently, 28 kHz was more suitable for crude oil particle coagulation than 40 kHz. Finally, the 28 kHz ultrasound propagates vertically upward and has a levitation effect [9]. As a result, the oily sludge was distributed evenly in the water at 28 kHz, making this setting was more beneficial for washing than 40 kHz. It is clear from Fig. 3 that oil content is lowest at 0.10 MPa. Oil content increased when the ultrasonic acoustic pressure amplitude was above 0.10 MPa, since ultrasonic cavitation was too high at larger
3.2. Effect of ultrasonic frequency The deoiling efficiency of two kinds of ultrasonic generators, one at 28 kHz and the other at 40 kHz, was tested. The ultrasonic acoustic pressure amplitude varied with ultrasound frequency. The
Fig. 3. Effects of ultrasonic frequency on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
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N. Xu et al. / Journal of Hazardous Materials 171 (2009) 914–917
Table 1 Oil content of sludge after bifrequency ultrasonic deoiling. Ultrasonic acoustic pressure amplitude (MPa) 28 kHz
40 kHz
0.129 0.129 0.129 0.085 0.085 0.085 0.062 0.062 0.062
0.120 0.085 0.071 0.120 0.085 0.071 0.120 0.085 0.071
Oil content (g g−1 )
0.062 0.069 0.080 0.067 0.070 0.076 0.072 0.073 0.073
acoustic pressure amplitudes [10]. This resulted in very small solid particle sizes, which meant that the specific surface area was also large; the surface adsorption capacity increased as well. 3.3. Dual-frequency ultrasonic irradiation The 28 and 40 kHz ultrasonic generators were also used together; the oily sludge washing was investigated, and the results are shown in Table 1. The experimental parameters were as follows: sludge water ratio, 1:16; ultrasonic acoustic pressure amplitude, 0.10 MPa; ultrasonic irradiation time, 20 min; air liquid volume ratio, 0.27. The results indicate that when the ultrasonic acoustic pressure amplitude at 28 kHz was 0.085 MPa and that at 40 kHz was 0.120 MPa, oil content was minimized. 3.4. Effect of sodium silicate Chemical heat cleaning always attracts much attention. Surfactant types were studied, and it was determined that sodium silicate was fit for oily sludge washing [11]. In this study, sludge oil contents after deoiling with or without ultrasonic irradiation were investigated. Sodium silicate dosage was determined as the ratio of sodium silicate mass to dry oily sludge. The results are provided in Fig. 4. In addition to the ultrasonic acoustic pressure amplitude, sludge oil contents after deoiling with or without sodium silicate addition were investigated.
Fig. 4. Effects of sodium silicate dosage on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; ultrasonic acoustic pressure amplitude 0.10 MPa; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
Fig. 5. Effects of sodium silicate dosage on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
Fig. 4 shows that ultrasonic irradiation clearly promoted oily sludge deoiling. In addition, washing oily sludge with sodium silicate independently encouraged oily sludge deoiling. Sodium silicate reduced the surface tension of the oil. As a result, it was beneficial to crude oil separation from the surface of the solid particles. Oil content reached near 0.09 g g−1 with 1% sodium silicate dosage. However, sodium silicate addition before ultrasonic irradiation increased oil content. The more sodium silicate added, the higher the oil content after deoiling was. Fig. 5 indicates that the oil content with sodium silicate addition is higher than without sodium silicate. Both Figs. 4 and 5 show that sodium silicate addition inhibits deoiling when ultrasound is utilized. 4. Discussion After ultrasonic irradiation, the structure of the oily sludge was altered. The crude oil was stripped from the surface of the solid particles. The ultrasonic cavitation and mechanical vibrating reduced the crude oil content on the surface of the solid particles. The process is dependent mainly on the ultrasonic cavitation, especially the cavitation happening at the interphase boundary of the oil and solid. When cavitation occurs, powerful high-density micro jets, which can be as fast as 400 km h−1 , are produced. The micro jets impact the surface of the solid particles. This causes the corrosion phenomenon to occur on the solid, and the crude oil splits off. When cavitation bubbles collapse, the gas pressure is as high as 3 × l08 N m−2 , and the temperature is 103 K [12]. These conditions are beneficial for oil–solid separation. The cavitation effect is maximum at low temperatures. But high temperature is good to molecular movement and increasing number of cavitation nucleus. Cavitation number increases when ultrasound frequency increases. Thus high frequency ultrasound generates more intense blasts shook. However, cavitation is more difficult to happen under high frequency ultrasound than under low frequency ultrasound. The higher the ultrasonic intensity is, the higher the cavitation intensity. Thus, high ultrasonic intensity is more advantageous for oil–solid separation. On the other hand, the crude oil drops in water aggregate because of the vibration and impact in the ultrasonic field. The crude oil drops vibrate from the ultrasound, which causes the impact of the crude oil drops. The tension of the crude oil drops is enough to cause the observed aggregation. In this process, if the ultrasonic intensity is too high, the crude oil drop aggregation will be suppressed. After being expelled from the surface of the solid particles, oil drops condense and float on the surface of water. However,
N. Xu et al. / Journal of Hazardous Materials 171 (2009) 914–917
the oil drops will be adsorbed on the surface of the solid particles again if ultrasound stops. Fig. 3 suggests that both too high and too low ultrasonic acoustic pressure amplitudes inhibit sludge deoiling, because high-acoustic pressure amplitudes prevent merging and low acoustic pressure amplitudes make it difficult to separate crude oil from solid particles. Fig. 4 shows that 0.10 MPa was the optimum ultrasonic acoustic pressure amplitude. This paper investigated the effects of ultrasonic acoustic pressure amplitude, temperature, and frequency on ultrasonic washing. Fig. 2 shows that 40 ◦ C was the optimum washing temperature for ultrasonic cavitation [7]. Because the cavitation threshold was enhanced with increasing frequency [9], 28 kHz was more suitable for oily sludge deoiling than 40 kHz. Sodium silicate reduced the surface tension of the oil. As a result, it was beneficial to crude oil separation from the surface of the solid particles. However, the oil was easily emulsified with water and did not float upward. 5. Conclusion (1) The oil contents of oily sludge after deoiling by ultrasonic generator were tested for different ultrasonic acoustic pressure amplitudes. After ultrasound irradiation, the minimum oil content (dry basis), 0.055 g g−1 , was at 40 ◦ C. This is a reduction of 55.6% compared to oil content without ultrasonic irradiation. Thus, 40 ◦ C was the optimum ultrasonic washing temperature. (2) The results showed that 28 kHz was superior to 40 kHz ultrasound, and 0.10 MPa was determined to be the optimum ultrasonic acoustic pressure amplitude. (3) The 28 kHz ultrasonic generator was combined with the 40 kHz ultrasonic generator to determine their effect on oily sludge washing. The results indicated that oil content was minimized at ultrasonic acoustic pressure amplitudes of 0.085 MPa at 28 kHz and 0.120 MPa at 40 kHz.
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(4) The oil contents of sludge after deoiling with or without sodium silicate were also investigated. The addition of sodium silicate inhibited deoiling. Acknowledgements The authors wish to express their thanks to the National High Technology Research and Development Program of China (863 Program) (no. 2006AA02Z244) and Jiangsu University Science Research Project (no. 06KJB530041). References [1] Baswick, D. Sandy, Ultrasonic Separation of Bitumen From Tar Sands, CA 996485, Canada, 1976. [2] J.F. Kuo, K. Sadeghi, L.K. Jang, et al., Enhancement of bitumen separation from tar sand by radicals in ultrasonic irradiation, Applied Physics Communication 6 (2–3) (1986) 205–212. [3] V.C. George, M.S. Kazem, M.A. Sadeghi, et al., Development of a new method for the extraction of bitumen from tar sands using sonication and sodium silicate, Chemistry and Technology of Fuels and Oils 25 (1) (1989) 3–11. [4] N. Xu, X.P. Lu, Y.R. Wang, Study on ultrasonic degradation of pentachlorophenol solution, Chemical and Biochemical Engineering Quarterly 20 (3) (2006) 343–347. [5] V.S. Moholkar, M.M.C.G. Warmoeskerken, Mechanistic aspects and optimization of ultrasonic washing, Acta Review 2 (2) (2002) 34–37. [6] C. Bougrier, H. Carrere, J.P. Delgenes, Solubilisation of waste-activated sludge by ultrasonic treatment, Chemical Engineering Journal 106 (2) (2005) 163–169. [7] S. Canoglu, B.C. Gultekin, S.M. Yukseloglu, Effect of ultrasonic energy in washing of medical surgery gowns, Ultrasonics 42 (1–9) (2004) 113–119. [8] A.N. Kotyusov, B.E. Nemtsov, Induced coagulation of small particles under the action of sound, Acustica 82 (5) (1996) 459–463. [9] C. Pettier, A. Francony, Ultrasonic waste-water treatment: incidence of ultrasonic frequency on the rate of phenol and carbon tetrachloride degradation, Ultrasonics Sonochemistry 4 (4) (1997) 295–300. [10] H. Behrend, Schubert, Influence of hydrostatic pressure and gas content on continuous ultrasound emulsification, Ultrasonics Sonochemistry 8 (3) (2001) 271–276. [11] P. Mokrejs, M. Mladek, F. Langmaier, et al., Washing of MgO from chrome sludge in an acidic environment, Research Journal of Chemistry and Environment 11 (1) (2007) 79–85. [12] N. Feng, H.M. Li, Sonochemics and Its Application, Anhui Scientific Publishing Company, Hefei, China, 1992.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Effects of ultrasound on oily sludge deoiling Xu Ning a , Wang Wenxiang a , Han Pingfang b,∗ , Lu Xiaoping c a
College of Environment, Nanjing University of Technology, Nanjing, China College of Life Science and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing, China c College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing, China b
a r t i c l e
i n f o
Article history: Received 31 March 2009 Received in revised form 15 June 2009 Accepted 15 June 2009 Available online 24 June 2009 Keywords: Oily sludge Oil content Ultrasound Deoiling
a b s t r a c t Oily sludge with an initial oil content of 0.130 g g−1 (dry basis) was mixed with water and treated in an ultrasound cleaning tank. The oil was then separated from the oily sludge by air floatation. Experiments were carried out with and without 28 kHz ultrasonic irradiation at different temperatures. The results show that the minimum oil content, 0.055 g g−1 (dry basis), was obtained at 40 ◦ C after ultrasound irradiation, which was 55.6% less than without ultrasonic irradiation. In addition, this work clearly establishes that 28 kHz ultrasound is superior to 40 kHz ultrasound. The ultrasonic acoustic pressure amplitude with the 28 kHz ultrasound was 0.085 MPa; the 28 kHz ultrasound also exhibited lower oil content than the 40 kHz ultrasound, which yielded 0.120 MPa acoustic pressure amplitude. It can also be concluded that sodium silicate obstructs ultrasound oily sludge deoiling. © 2009 Published by Elsevier B.V.
1. Introduction Oily sludge, which has an annual output of about 1 million ton, is produced in crude oil production, transportation, storage, and refining. Because of its high oil content, oily sludge is listed in the China’s Dangerous Waste List. Current technologies for oily sludge treatment, such as solidification, chemical heat cleaning, and extraction, are either costly or ineffective. Therefore, oily sludge is not being disposed of properly. Ultrasonic oily sludge treatment, which is fast and effective, has recently garnered much attention. Tar sand ultrasonic heat cleaning was developed abroad prior to 1980 [1]. In Canada, there is a 1976 patent describing bitumen separation from tar sands of oil shale by treatment with ultrasound and surfactants [2]; the surfactants included benzyl peroxide and sodium silicate [3]. This kind of technology is probably applicable to cleaning up oily sludge. In China, a new practical patent describes a type of equipment in which crude oil is separated from oily sludge via cyclone. In this instrument, ultrasound and demulsification are used to further the separation [4]. Later, a two-stage oily sludge washing method using ultrasound and surfactants was developed [5]. A process for oily sludge deoiling using only ultrasound was also developed recently [6]. This technology is not only cheap and easy to apply, but it also produces no pollution by-products.
∗ Corresponding author. Tel.: +86 2583588072; fax: +86 2583588072. E-mail address: [email protected] (P. Han). 0304-3894/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2009.06.091
The objective of this work is to investigate the process of oily sludge washing by ultrasound and air flotation for the recovery of crude oil. These experiments examined the oil contents of oily sludge after washing and determined the effects of ultrasound irradiation, ultrasound frequency, and the addition of sodium silicate. 2. Materials and methods 2.1. Materials Oily sludge samples were taken from the Bingyi water treatment plant in the Shengli oil field in China. After pressure filtration, each sample was a massive and black power. The oily sludge sample breakdown is as follows: water content 38.9%. Oil content 7.9%. Oil content 0.130 g g−1 (dry basis). 2.2. Ultrasonic cleaning tank A dual-frequency ultrasonic cleaning tank (structure shown in Fig. 1) was utilized for this research. 2.3. Procedure In this research, 40 g oily sludge and 640 mL water were added to a beaker and subjected to high-speed stirring. The beaker was immersed in an ultrasound cleaning tank full of water and then preheated. After preheating, the ultrasound cleaning tank shown in Fig. 1 was turned on while the mixture was ventilated. Then crude oil was floated and skimmed from the liquid’s surface. The mixture
N. Xu et al. / Journal of Hazardous Materials 171 (2009) 914–917
915
Fig. 1. Schematic diagram of ultrasonic cleaning tank: (1) 28 kHz ultrasonic generator; (2) 40 kHz ultrasonic generator; (3) 28 kHz transducer; (4) 40 kHz transducer; (5) cleaning tank (60 cm × 30 cm × 35 cm).
was dewatered by pressure filtration. Finally, the oil content of the sludge after deoiling was measured. 2.4. Oil content determination According to previous studies [1,3], the oil in sludge can be extracted from petroleum ether (boiling ranger, 30–60 ◦ C) under ultrasound irradiation. In this case, after extraction, the petroleum ether was recovered from the extract liquid at 65 ◦ C, and the weight of the oil was measured. The weight of the sludge residue from the extraction was measured after drying at 105 ◦ C. The weight ratio of oil to dry residue is the oil content (dry basis). 2.5. Ultrasonic acoustic pressure amplitude determination The hydrophone was immersed in the ultrasonic field. The maximum value on the oscilloscope was noted. P=
U M
(1)
where U is the maximum value on the oscilloscope, measured in V, and M is the sensitivity of the hydrophone, measured in V MPa−1 . The sensitivity of the hydrophone in a 28 kHz ultrasonic field is 25.8 V MPa−1 , while that of a 40 kHz ultrasonic field is 28.2 V MPa−1 . 3. Results 3.1. Effect of ultrasonic irradiation According to Bougrier et al. [6], ultrasound irradiation can separate crude oil from solid particles. In this study, the oil content of the sludge after deoiling with and without ultrasound irradiation was determined experimentally. The results are given in Fig. 2. It can be seen from Fig. 2 that there was little change in oil content, and the final content was greater than 0.11 g g−1 without ultrasound irradiation. The measured oil contents with ultrasound irradiation are less than those without ultrasound. At 40 ◦ C, the oil content was 0.055 g g−1 , which is the minimum value. This is 55.6% less than that observed without ultrasonic irradiation. When the temperature is above 40 ◦ C, oil content increases. This is because when the temperature is too high, the intensity of the ultrasonic cavitation is weakened, which causes the process of separation of crude oil from solid particles to be retarded [7]. The cavitation effect is maximum at low temperatures. But high temperature is good to molecular movement and increasing number of cavitation nucleus. Thus oil separation from sludge particle. Under these two factors, the optimum temperature is 40 ◦ C.
Fig. 2. Effects of ultrasound on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; acoustic pressure amplitude 0.10 MPa; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
measured oil contents of the oily sludge after deoiling by ultrasonic generators are shown in Fig. 3. It can be seen from the results that 28 kHz ultrasound is superior to 40 kHz. The potential reasons for this are as follows. First, frequency is an important factor in ultrasonic cleaning, and 28 kHz is more suitable for washing solid particles of this size. Because the 28 kHz ultrasonic cavitation threshold is lower than that at 40 kHz, cavitation intensity is higher at 28 kHz. The separation efficiency at 28 kHz is higher as a result. Second, Kotyusov’s research showed that optimum frequency for particle coagulation was 21–25 kHz. For frequencies above 25 kHz, the lower the frequency, the higher the coagulation efficiency is [8]. Cavitation number increases when ultrasound frequency increases. Thus high frequency ultrasound generates more intense blasts shook. However, cavitation is more difficult to happen under high frequency ultrasound than that under low frequency ultrasound. Consequently, 28 kHz was more suitable for crude oil particle coagulation than 40 kHz. Finally, the 28 kHz ultrasound propagates vertically upward and has a levitation effect [9]. As a result, the oily sludge was distributed evenly in the water at 28 kHz, making this setting was more beneficial for washing than 40 kHz. It is clear from Fig. 3 that oil content is lowest at 0.10 MPa. Oil content increased when the ultrasonic acoustic pressure amplitude was above 0.10 MPa, since ultrasonic cavitation was too high at larger
3.2. Effect of ultrasonic frequency The deoiling efficiency of two kinds of ultrasonic generators, one at 28 kHz and the other at 40 kHz, was tested. The ultrasonic acoustic pressure amplitude varied with ultrasound frequency. The
Fig. 3. Effects of ultrasonic frequency on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
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Table 1 Oil content of sludge after bifrequency ultrasonic deoiling. Ultrasonic acoustic pressure amplitude (MPa) 28 kHz
40 kHz
0.129 0.129 0.129 0.085 0.085 0.085 0.062 0.062 0.062
0.120 0.085 0.071 0.120 0.085 0.071 0.120 0.085 0.071
Oil content (g g−1 )
0.062 0.069 0.080 0.067 0.070 0.076 0.072 0.073 0.073
acoustic pressure amplitudes [10]. This resulted in very small solid particle sizes, which meant that the specific surface area was also large; the surface adsorption capacity increased as well. 3.3. Dual-frequency ultrasonic irradiation The 28 and 40 kHz ultrasonic generators were also used together; the oily sludge washing was investigated, and the results are shown in Table 1. The experimental parameters were as follows: sludge water ratio, 1:16; ultrasonic acoustic pressure amplitude, 0.10 MPa; ultrasonic irradiation time, 20 min; air liquid volume ratio, 0.27. The results indicate that when the ultrasonic acoustic pressure amplitude at 28 kHz was 0.085 MPa and that at 40 kHz was 0.120 MPa, oil content was minimized. 3.4. Effect of sodium silicate Chemical heat cleaning always attracts much attention. Surfactant types were studied, and it was determined that sodium silicate was fit for oily sludge washing [11]. In this study, sludge oil contents after deoiling with or without ultrasonic irradiation were investigated. Sodium silicate dosage was determined as the ratio of sodium silicate mass to dry oily sludge. The results are provided in Fig. 4. In addition to the ultrasonic acoustic pressure amplitude, sludge oil contents after deoiling with or without sodium silicate addition were investigated.
Fig. 4. Effects of sodium silicate dosage on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; ultrasonic acoustic pressure amplitude 0.10 MPa; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
Fig. 5. Effects of sodium silicate dosage on oil content after oily sludge deoiling (ultrasound frequency 28 kHz; sludge water ratio 1:16; temperature 40 ◦ C; ultrasonic irradiation time 20 min; air liquid volume ratio 0.27).
Fig. 4 shows that ultrasonic irradiation clearly promoted oily sludge deoiling. In addition, washing oily sludge with sodium silicate independently encouraged oily sludge deoiling. Sodium silicate reduced the surface tension of the oil. As a result, it was beneficial to crude oil separation from the surface of the solid particles. Oil content reached near 0.09 g g−1 with 1% sodium silicate dosage. However, sodium silicate addition before ultrasonic irradiation increased oil content. The more sodium silicate added, the higher the oil content after deoiling was. Fig. 5 indicates that the oil content with sodium silicate addition is higher than without sodium silicate. Both Figs. 4 and 5 show that sodium silicate addition inhibits deoiling when ultrasound is utilized. 4. Discussion After ultrasonic irradiation, the structure of the oily sludge was altered. The crude oil was stripped from the surface of the solid particles. The ultrasonic cavitation and mechanical vibrating reduced the crude oil content on the surface of the solid particles. The process is dependent mainly on the ultrasonic cavitation, especially the cavitation happening at the interphase boundary of the oil and solid. When cavitation occurs, powerful high-density micro jets, which can be as fast as 400 km h−1 , are produced. The micro jets impact the surface of the solid particles. This causes the corrosion phenomenon to occur on the solid, and the crude oil splits off. When cavitation bubbles collapse, the gas pressure is as high as 3 × l08 N m−2 , and the temperature is 103 K [12]. These conditions are beneficial for oil–solid separation. The cavitation effect is maximum at low temperatures. But high temperature is good to molecular movement and increasing number of cavitation nucleus. Cavitation number increases when ultrasound frequency increases. Thus high frequency ultrasound generates more intense blasts shook. However, cavitation is more difficult to happen under high frequency ultrasound than under low frequency ultrasound. The higher the ultrasonic intensity is, the higher the cavitation intensity. Thus, high ultrasonic intensity is more advantageous for oil–solid separation. On the other hand, the crude oil drops in water aggregate because of the vibration and impact in the ultrasonic field. The crude oil drops vibrate from the ultrasound, which causes the impact of the crude oil drops. The tension of the crude oil drops is enough to cause the observed aggregation. In this process, if the ultrasonic intensity is too high, the crude oil drop aggregation will be suppressed. After being expelled from the surface of the solid particles, oil drops condense and float on the surface of water. However,
N. Xu et al. / Journal of Hazardous Materials 171 (2009) 914–917
the oil drops will be adsorbed on the surface of the solid particles again if ultrasound stops. Fig. 3 suggests that both too high and too low ultrasonic acoustic pressure amplitudes inhibit sludge deoiling, because high-acoustic pressure amplitudes prevent merging and low acoustic pressure amplitudes make it difficult to separate crude oil from solid particles. Fig. 4 shows that 0.10 MPa was the optimum ultrasonic acoustic pressure amplitude. This paper investigated the effects of ultrasonic acoustic pressure amplitude, temperature, and frequency on ultrasonic washing. Fig. 2 shows that 40 ◦ C was the optimum washing temperature for ultrasonic cavitation [7]. Because the cavitation threshold was enhanced with increasing frequency [9], 28 kHz was more suitable for oily sludge deoiling than 40 kHz. Sodium silicate reduced the surface tension of the oil. As a result, it was beneficial to crude oil separation from the surface of the solid particles. However, the oil was easily emulsified with water and did not float upward. 5. Conclusion (1) The oil contents of oily sludge after deoiling by ultrasonic generator were tested for different ultrasonic acoustic pressure amplitudes. After ultrasound irradiation, the minimum oil content (dry basis), 0.055 g g−1 , was at 40 ◦ C. This is a reduction of 55.6% compared to oil content without ultrasonic irradiation. Thus, 40 ◦ C was the optimum ultrasonic washing temperature. (2) The results showed that 28 kHz was superior to 40 kHz ultrasound, and 0.10 MPa was determined to be the optimum ultrasonic acoustic pressure amplitude. (3) The 28 kHz ultrasonic generator was combined with the 40 kHz ultrasonic generator to determine their effect on oily sludge washing. The results indicated that oil content was minimized at ultrasonic acoustic pressure amplitudes of 0.085 MPa at 28 kHz and 0.120 MPa at 40 kHz.
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(4) The oil contents of sludge after deoiling with or without sodium silicate were also investigated. The addition of sodium silicate inhibited deoiling. Acknowledgements The authors wish to express their thanks to the National High Technology Research and Development Program of China (863 Program) (no. 2006AA02Z244) and Jiangsu University Science Research Project (no. 06KJB530041). References [1] Baswick, D. Sandy, Ultrasonic Separation of Bitumen From Tar Sands, CA 996485, Canada, 1976. [2] J.F. Kuo, K. Sadeghi, L.K. Jang, et al., Enhancement of bitumen separation from tar sand by radicals in ultrasonic irradiation, Applied Physics Communication 6 (2–3) (1986) 205–212. [3] V.C. George, M.S. Kazem, M.A. Sadeghi, et al., Development of a new method for the extraction of bitumen from tar sands using sonication and sodium silicate, Chemistry and Technology of Fuels and Oils 25 (1) (1989) 3–11. [4] N. Xu, X.P. Lu, Y.R. Wang, Study on ultrasonic degradation of pentachlorophenol solution, Chemical and Biochemical Engineering Quarterly 20 (3) (2006) 343–347. [5] V.S. Moholkar, M.M.C.G. Warmoeskerken, Mechanistic aspects and optimization of ultrasonic washing, Acta Review 2 (2) (2002) 34–37. [6] C. Bougrier, H. Carrere, J.P. Delgenes, Solubilisation of waste-activated sludge by ultrasonic treatment, Chemical Engineering Journal 106 (2) (2005) 163–169. [7] S. Canoglu, B.C. Gultekin, S.M. Yukseloglu, Effect of ultrasonic energy in washing of medical surgery gowns, Ultrasonics 42 (1–9) (2004) 113–119. [8] A.N. Kotyusov, B.E. Nemtsov, Induced coagulation of small particles under the action of sound, Acustica 82 (5) (1996) 459–463. [9] C. Pettier, A. Francony, Ultrasonic waste-water treatment: incidence of ultrasonic frequency on the rate of phenol and carbon tetrachloride degradation, Ultrasonics Sonochemistry 4 (4) (1997) 295–300. [10] H. Behrend, Schubert, Influence of hydrostatic pressure and gas content on continuous ultrasound emulsification, Ultrasonics Sonochemistry 8 (3) (2001) 271–276. [11] P. Mokrejs, M. Mladek, F. Langmaier, et al., Washing of MgO from chrome sludge in an acidic environment, Research Journal of Chemistry and Environment 11 (1) (2007) 79–85. [12] N. Feng, H.M. Li, Sonochemics and Its Application, Anhui Scientific Publishing Company, Hefei, China, 1992.