Oil Recovery from Oil Sludge through Combined Ultrasound and

Ind. Eng. Chem. ... Publication Date (Web): June 20, 2012 .... Then, the oil layer was kept in the filter paper for further analysis. ... (12) The the...
0 downloads 0 Views 550KB Size
Download PDF
Article pubs.acs.org/IECR

Oil Recovery from Oil Sludge through Combined Ultrasound and Thermochemical Cleaning Treatment Yuqi Jin,*,† Xiaoyuan Zheng,† Xiaoliang Chu,‡ Yong Chi,† Jianhua Yan,† and Kefa Cen† †

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China ‡ Yantai Longyuan Power Technology Co., Ltd., Yantai 264006, China ABSTRACT: Oil sludge from an oil storage tank was treated using a combined ultrasound and thermochemical cleaning method. The effects of cleaning temperature, cleaning time, ultrasound frequency, ultrasound power, and other factors on the oil recovery were investigated. Experiments on the screening and remixing of reagents were conducted and indicated that the best constituents of detergent solution were sodium silicate, sodium dodecyl benzene sulfonate, and fatty alcohol ethoxylates in a ratio of 1:1:1. The optimum conditions of the combined treatment system were also determined. The results showed that, under the optimum conditions, the oil content of oil sludge dropped from 43.13% to 1.01%, 0.53% of solids remained in the separated oil layer, and 99.32% oil recovery could be achieved when the concentration of detergent solution was to 2 g/L. Compared with traditional thermochemical cleaning, the oil recovery was 17.65% higher for the combined treatment system.

1. INTRODUCTION In the process of crude oil production, transportation, storage, and refining, oil sludge is one of the main solid wastes, which is a complex water-in-oil emulsion with suspended solids, typically containing 30−50% oil, 30−50% water, and 10−12% solids by mass.1 Because of its high concentration of petroleum hydrocarbons, it is regarded as a hazardous waste and poses a threat to human health and the environment if improperly discarded. Currently, the effective disposal of oil sludge is drawing increasing attention worldwide. A number of approaches have been suggested in the literature. The disposal of oil sludge by pyrolysis2 requires complex operations and rigorous reaction conditions. This method has existed at the laboratory scale to date. The method of incineration3 is restricted by rigorous environmental standards, as a result of its potential to cause pollution. The process of biological treatment4 might involve prohibitive operating costs. Second, the efficacy of this method in degrading heavy hydrocarbons is questionable, as in a number of cases, degradation remains incomplete. Because of the requirements of a high ratio of solvent to sludge, solvent extraction5 is restricted as well. Thermochemistry6 can recover oil and reduce the volume of sludge with low cost and high efficiency. Therefore, it is considered as a promising and feasible treatment that works by destroying the stable emulsion system and changing the properties of oil sludge, such as high degree of emulsion and viscosity.7 Jing et al.6 determined the optimum conditions for washing oil sludge with different surfactants (AEO-9, Peregal O, Triton X-100, sodium dodecylbenzene sulfonate, and Na2SiO3·9H2O). Under the optimum conditions, the washing effect of Na2SiO3·9H2O was best, with a residual oil rate of 1.6%. Li et al.7 reported that, under optimum conditions (liquid-to-solid ratio of 5, agitation temperature of 80 °C, agitation intensity of 2000 rpm, agitation time of 40 min, pH of 10), 0.28% oil remained in the dry oil sludge when the concentration of surface modification reagents, © 2012 American Chemical Society

which was made up of Sx4056, petroleum sulfonate, and sodium silicate in a ratio of 1:4:10, was up to 3.5 g/L. To effectively remove oil from solid particles in oil sludge and decrease the stability of the water-in-oil emulsion,8 ultrasound irradiation has been employed. In recent years, ultrasound has been confirmed to be an effective treatment method because of the combination of mechanical vibration and resulting cavitation effects. Zhang et al.9 investigated oil recovery from refinery oil sludge through a combined ultrasound and freeze/thaw treatment and further examined the effects of ultrasound power, ultrasound treatment duration, sludge/water ratio in the slurry, and biosurfactant and salt concentrations on the combined process. They reported an oil recovery of up to 80.0%.9 Xu et al.10 investigated the process of oil sludge washing by ultrasound and air flotation for the recovery of crude oil and determined the effects of ultrasound irradiation, ultrasound frequency, and sodium silicate addition. It was found that the minimum oil content (dry basis, initial oil content of the oil sludge was 0.130 g g−1), 0.055 g g−1, could be achieved. Hence, the combined ultrasound and thermochemical cleaning treatment can enhance the oil recovery and is an effective and feasible method for oil sludge treatment. However, few studies have focused on this combination system for oil recovery. The purpose of this study was to investigate the oil recovery obtained through the combination of thermochemical cleaning and ultrasound irradiation. The effects of cleaning temperature, cleaning time, ultrasound frequency, ultrasound power, and other factors on the degree of oil recovery were evaluated. Received: Revised: Accepted: Published: 9213

May 1, 2012 June 10, 2012 June 20, 2012 June 20, 2012 dx.doi.org/10.1021/ie301130c | Ind. Eng. Chem. Res. 2012, 51, 9213−9217

Industrial & Engineering Chemistry Research

Article

a duration of 10 min, a temperature of 60 °C, a power of 500 W, and a frequency of 28 kHz. 3.1.1. Screening of Reagents. Figure 2 shows that the oil recovery increased with increasing concentration of the

2. MATERIALS AND METHODS 2.1. Materials. The oil sludge used in these experiments was taken from an oil storage tank in Zhoushan District, China. The sludge was black and sticky. Its characteristics were as follows: water content, 46.72%; oil content, 43.13%; solids content, 10.15%. The water content was determined according to Chinese standard method GB/T 11146-1999,11 the oil content was determined by Soxhlet extraction, and the solids content was obtained by difference. The chemicals, namely, sodium carbonate, sodium silicate, sodium dodecyl benzene sulfonate (LAS), and fatty alcohol ethoxylate (AEO-9) (AR grade), were purchased from Sinopharm Chemical Reagent Co. Ltd. (SCRC, Beijing, China). 2.2. Experimental Design. 2.2.1. Procedure. Oil sludge (10 g) and appropriate amounts of detergent solution were placed in a 150 mL beaker, and the value of the pH was adjusted. The samples were then blended in a water bath with an automatic stirrer (JJ-4C) at a set temperature, agitation intensity, and agitation duration. After this process, the samples were transferred to an ultrasound cleaning tank and were treated at a certain ultrasound frequency, ultrasound power, ultrasound temperature, and ultrasound duration. Figure 1

Figure 2. Effect of reagent dosage on the degree of oil recovery.

reagents LAS, AEO-9, Na2CO3, and Na2SiO3. It can be seen that the order of oil recovery was Na2CO3 > Na2SiO3 >AEO-9 > LAS. AEO-9 is usually used as a nonionic surfactant, and LAS is usually used as an anionic surfactant. Recently, LAS and AEO-9 have been used together to maximize cleaning efficiency in the fields of industry and research. Thus, LAS and AEO-9 were chosen in this study. Figure 2 also shows that Na2CO3 was slightly better than Na2SiO3. However, considering the costs of the reagents, Na2SiO3 was chosen. 3.1.2. Remixing of Reagents. Figure 3 shows that the oil recovery increased with increasing dosage of detergent solution

Figure 1. Schematic diagram of the combined ultrasound and thermochemical cleaning treatment system.

illustrates the combined ultrasound and thermochemical cleaning treatment system. When the ultrasound treatment was finished, the oil layer was collected and weighed. Then, the oil layer was kept in the filter paper for further analysis. 2.2.2. Definition of Oil Recovery. Oil recovery is defined as the ratio of the mass of recovered oil to the mass of oil in the original oil sludge was calculated as ωm R (%) = 1 1 × 100% ω2 m 2 (1) where R is the oil recovery (%); ω1 and ω2 (%) are the oil contents of the recovered oil layer and the original oil sludge, respectively; and m1 and m2 (g) are the masses of the recovered oil layer and the original oil sludge, respectively.

Figure 3. Effect of different ratios of detergent solution on the degree of oil recovery.

3. RESULTS AND DISCUSSION 3.1. Screening and Remixing of Reagents. LAS and AEO-9 were chosen as surfactants, and Na2CO3 and Na2SiO3 were used as dispersants. The surfactants played the role of wetting hydrophobic substances and lowering the interface tension. They also could change the size of the contact angle between phases. As a result, the rate of oil separated from oil sludge could be accelerated. The dispersants used in this experiment caused a high level of dispersion of sand particles, which hindered these particles from settling and achieving cohesion.12 The thermochemical cleaning conditions were a temperature of 50 °C, a duration of 40 min, an agitation intensity of 250 rpm, a pH of 10, and an oil sludge/detergent solution (S/D) ratio of 1:8, and the ultrasound conditions were

prepared with different constituents. In terms of oil recovery, the sequence was Na2SiO3 + LAS + AEO-9 (1:1:1) > Na2SiO3 + AEO-9 (1:2) > Na2SiO3 + AEO-9 (1:1) > Na2SiO3 + LAS (1:1). No further significant increase in oil recovery was observed when the concentration of detergent solution was greater than 2 g/L. The main reason was that LAS and AEO-9 could form mixed micelles that had a solubilization effect such that the oil separation rate was accelerated.13 Thus, the experiments showed that the optimal ratio of sodium silicate, sodium dodecyl benzene sulfonate (LAS), and fatty alcohol ethoxylate (AEO-9) was 1:1:1, and the optimal concentration of detergent solution was 2 g/L. 3.2. Effects of Parameters on the Degree of Oil Recovery. To further understand the process of combined ultrasound and thermochemical cleaning treatment and obtain 9214

dx.doi.org/10.1021/ie301130c | Ind. Eng. Chem. Res. 2012, 51, 9213−9217

Industrial & Engineering Chemistry Research

Article

more information about this method, the individual influences of different parameters on the oil recovery in the process were further investigated. With a given set of conditions (ultrasound duration of 10 min, ultrasound power of 500 W, ultrasound frequency of 28 kHz, and ultrasound temperature of 60 °C), thermochemical cleaning experiments were conducted at different levels of cleaning temperature, cleaning duration, agitation intensity, pH, and S/D ratio. Based on the optimal conditions determined in this step, ultrasound experiments were conducted at different levels of ultrasound frequency, ultrasound power, ultrasound duration, and ultrasound temperature. 3.2.1. Effects of Thermochemical Cleaning Parameters. Figure 4a shows that the oil recovery varied as a function of cleaning temperature. The results indicate that the oil recovery increased with increasing cleaning temperature and remained constant when the temperature was higher than 55 °C. This is mainly due to the fact that, as the temperature increased, the viscosity decreased, facilitating oil liberation, and the electrostatic repulsion between oil and sand also increased, so the oil− sand adhesion became weaker, especially when the pH was higher than 8.14 However, a higher temperature means more energy consumption, so the optimal temperature of 55 °C was chosen. From Figure 4b, it can be seen that the oil recovery increased with increasing cleaning time and became stable for cleaning durations of more than 30 min. Before the point of 30 min, the oil did not have enough time to react with sodium silicate to produce natural surfactant and contact with detergent solution. When the cleaning time was too long, more energy was required, and an oil-in-water emulsion formed easily. Thus, a 30-min cleaning duration was considered appropriate. According to Figure 4c, the oil recovery increased with pH approximately in the form of a sinusoid. This can be attributed to the strong adsorption of HSiO3− groups onto the polar surfaces of the sands with rising pH. In sodium silicate solutions, OH−, SiO32−, and HSiO3− are formed. At reduced pH, the main species is H2SiO3. When the pH is above 8−9, HSiO3− is the main species of sodium silicate to act as a dispersant.12 These groups occupy the adsorption sites of hydrocarbons on the sand surfaces. Hence, both oil and sand surfaces become more negatively charged, and the repulsion between them becomes stronger.14,15 At the same time, when the pH increases, the oil−water interfacial films formed by asphaltenes become progressively weaker.16 This benefits demulsification to enhance oil recovery. In contrast, the difficulty of treating effluent increases when the pH is too high. On the basis of the experimental results, a pH of 9 was determined to be suitable. Figure 4d shows that the oil recovery increased markedly as the agitation intensity increased. When the agitation intensity was 200 rpm, the oil recovery achieved its maximum value of 99.26%. The components of oil sludge can be separated effectively by stirring mechanically. Although a high agitation intensity has the potential to accelerate oil separation from oil sludge, an oil-in-water emulsion can be formed. At the same time, the energy consumption of the equipment increases. Thus, 200 rpm was considered to be appropriate. Figure 4e presents the oil recovery at different S/D ratios. It can be observed that oil recovery increased slightly from 99.20% to 99.24% as the S/D ratio increased from 1:8 to 1:6 and then decreased to 74.82% for an S/D ratio of 1:1. More oil could be recovered at increasing S/D ratio, but when the S/D

Figure 4. Effects of thermochemical cleaning parameters on oil recovery: (a) cleaning temperature, (b) cleaning time, (c) pH, (d) agitation intensity, and (e) S/D ratio.

ratio was increased above 1:6, the viscosity of the slurry increased. This could inhibit the formation and collapse of cavitation bubbles and weaken the effect of ultrasound, resulting in decreased oil recovery.9 Thus, the S/D ratio of 1:6 was determined to be the best choice by considering the oil recovery. 9215

dx.doi.org/10.1021/ie301130c | Ind. Eng. Chem. Res. 2012, 51, 9213−9217

Industrial & Engineering Chemistry Research

Article

3.2.2. Effects of Ultrasound Parameters. The results presented in Figure 5a indicate that the oil recovery reached

Figure 5b shows that the maximum oil recovery was 98.82% when the ultrasound frequency was 28 kHz. The same phenomenon was reported by Xu et al.,10 who used 28 and 40 kHz ultrasound to recover oil from oil sludge and found that the 28 kHz ultrasound was superior to 40 kHz. It can be considered that 28 kHz is suitable for oil desorption from sand surfaces. According to the research of Kotyusov and Nemtsov,17 a frequency in the range of 21−25 kHz is most suitable for particle coagulation. For frequencies above 25 kHz, the higher the frequency, the lower the coagulation efficiency. Moreover, the lower the frequency was, the more easily cavitation was generated. Spontaneously, under compression and sparse effect of liquids, the cavitation bubbles had enough time to increase to a larger size before collapsing, which caused a high cavitation intensity. Consequently, 28 kHz could be regarded as the best choice. Figure 5c presents the effect of ultrasound power on the oil recovery. It was found that the oil recovery increased from 81.50% to 99.27% when the ultrasound power was increased from 0 to 400 W. When the ultrasound was increased further, the oil recovery underwent no further increase. It was recognized that the phenomenon of ultrasound cavitation was responsible for the enhanced desorption of adsorbed molecules and that the effect of cavitation was influenced by the size of the bubbles because the larger bubbles could store more energy. For a low frequency of ultrasound, the radius of the bubbles generated was a few micrometers, and the bubbles were too unstable and collapsed easily. This resulted in a few larger bubbles with more energy to produce shock waves and microjets.9,18 Accordingly, there was no further increase in oil recovery when the ultrasound power was above 400 W. As a result, the ultrasound power of 400 W was considered to be appropriate. The effect of ultrasound temperature on the oil recovery is shown in Figure 5d. It can be found that the oil recovery increased from 89.50% at room temperature to 99.27% at 60 °C. The oil recovery increased slightly when the temperature was higher than 60 °C. A potential reason is that there is an inverse relationship between the temperature and viscosity of oil sludge. The viscosity and adhesion stress of oil between oil and sands decreased when the temperature increased.13 On the other hand, there was a positive correlation between the number of cavitation cores and the ultrasound temperature. The higher the temperature, the larger the number of cavitation cores. However, when the temperature was too high, the inner pressure of the cavitation bubbles increased, which caused the cavitation intensity to reduce.19 Consequently, an ultrasound temperature of 55 °C was selected. 3.3. Comparison with Traditional Thermochemical Cleaning. The traditional thermochemical cleaning experiment was conducted under the following conditions: cleaning time of 30 min, agitation intensity of 200 rpm, cleaning temperature of 55 °C, pH of 9, S/D ratio of 1:6, and detergent solution concentration of 2 g/L. The experimental results of traditional thermochemical cleaning are summarized in Figure 6. It is shown that the combined ultrasound and thermochemical cleaning treatment had an oil recovery of 99.32%, which was 17.65% higher than that achieved with traditional thermochemical cleaning. Therefore, the combined treatment system worked more effectively. It was recognized that traditional thermochemical cleaning destroyed the stable system and changed the properties of oil sludge through the combined effects of mechanical action,

Figure 5. Effects of ultrasound parameters on oil recovery: (a) duration, (b) frequency, (c) power, and (d) temperature.

99.28% within 15 min of ultrasound treatment after thermochemical cleaning and then remained stable when ultrasound irradiation was extended to 30 min. The oil recovery was 99.29% when the treatment duration was 30 min, which is close to that after 15 min of treatment. Initially, the oil was desorbed from the sand surface under ultrasound, and oil− water emulsions were formed at relatively low concentration. At the beginning, because of the high rate of desorption, the oil recovery increased, but when the concentration of oil−water emulsions increased, the readsorption of oil onto the removed sand surfaces increased at the same time. When the rate of desorption is equal to that of readsorption, a further increase in ultrasound duration is not associated with an enhancement of oil recovery.15 Therefore, the optimal ultrasound duration was determined to be 15 min. 9216

dx.doi.org/10.1021/ie301130c | Ind. Eng. Chem. Res. 2012, 51, 9213−9217

Industrial & Engineering Chemistry Research

■

REFERENCES

(1) Ramaswamy, B.; Kar, D. D.; De, S. A study on recovery of oil from sludge containing oil using froth flotation. J. Environ. Manage. 2007, 85, 150. (2) Liu, J. G.; Jiang, X. M.; Zhou, L. S.; Han, X. X.; Cui, Z. G. Pyrolysis treatment of oil sludge and model-free kinetics analysis. J. Hazard. Mater. 2009, 161, 1208. (3) Shie, J. L.; Lin, J. P.; Chang, C. Y.; Wu, C. H.; Lee, D. J.; Chang, C. F.; Chen, Y. H. Oxidative thermal treatment of oil sludge at low heating rates. Energy Fuels 2004, 18, 1272. (4) Liu, W. X.; Luo, Y. M.; Teng, Y.; Li, Z. G.; Christie, P. Prepared bed bioremediation oily sludge in an oilfield in northern China. J. Hazard. Mater. 2009, 161, 479. (5) Taiwo, E. A.; Otolorin, J. A. Oil recovery from petroleum sludge by solvent extraction. Pet. Sci. Technol. 2009, 27, 836. (6) Jing, G.; Chen, T.; Luan, M. Studying oily sludge treatment by thermo chemistry. Arabian J. Chem., published online Jun 15, 2011, http://dx.doi.org/10.1016/j.arabjc.2011.06.007. (7) Li, X. B.; Liu, J. T.; Xiao, Y. Q.; Xiao, X. Modification technology for separation of oily sludge. J. Cent. South Univ. Technol. (Engl. Ed.) 2011, 18, 367. (8) Ye, G. X.; Lu, X. P.; Han, P. E.; Peng, F.; Wang, Y. R.; Shen, X. Application of ultrasound on crude oil pretreatment. Chem. Eng. Process. 2008, 47, 2346. (9) Zhang, J.; Li, J. B.; Thring, R. W.; Hu, X.; Song, X. Y. Oil recovery from refinery oily sludge via ultrasound and freeze/thaw. J. Hazard. Mater. 2012, 203−204, 195. (10) Xu, N.; Wang, W. X.; Han, P. F.; Lu, X. P. Effects of ultrasound on oily sludge deoiling. J. Hazard. Mater. 2009, 171, 914. (11) Crude oilsDetermination of waterKarl Fischer reagent method; Chinese Standard Method GB/T 11146; Standardization Administration of China: Beijing, 1999 (in Chinese). (12) Li, H.; Zhou, Z. A.; Xu, Z.; Masliyah, J. H. Role of acidified sodium silicate in low temperature bitumen extraction from poorprocessing oil sand ores. Ind. Eng. Chem. Res. 2005, 44, 4753. (13) Yang, J. S.; Xu, H. Experimental research on crude oil removal from oily sludge and sands by ultrasound. Acta Pet. Sin. 2010, 26, 300 (in Chinese). (14) Qi, D.; Keng, H. C. Bitumen−sand interaction in oil sand processing. Fuel 1995, 74, 1858. (15) Feng, D.; Aldrich, C. Sonochemical treatment of simulated soil contaminated with diesel. Adv. Environ. Res. 2000, 4, 103. (16) John, R. F. Petroleum Engineering Handbook; Society of Petroleum Engineers: Richardson, TX, 2006; Vol. 1: General Engineering. (17) Kotyusov, A. N.; Nemtsov, B. E. Induced coagulation of small particles under the action of sound. Acustica 1996, 82, 459. (18) Breitbach, M.; Bathen, D.; Schmidt-Traub, H. Effect of ultrasound on adsorption and desorption processes. Ind. Eng. Chem. Res. 2003, 42, 5635. (19) Feng, N.; Li, H. M. Sonochemistry and Its Applications; Anhui Science & Technology Publishing House: Hefei, China, 1992 (in Chinese).

Figure 6. Comparison of the two methods.

heating, and chemical additives, which are mainly macroscopic effects, whereas ultrasound enhanced the effects in the form of the shock waves and microjets, which are mainly microscopic effects. Therefore, the latter saves time and provides a higher efficiency.

4. CONCLUSIONS (1) Oil recovery from oil sludge was investigated in this study under different conditions. The combined ultrasound and thermochemical cleaning treatment was identified as an effective method in comparison with traditional thermochemical cleaning. (2) Based on the experimental results, Na2SiO3, LAS, and AEO-9 at an optimal ratio of 1:1:1 and a concentration of 2 g/L were selected. (3) The results of examining the individual impacts of different factors on the treatment process indicated an effective oil sludge/detergent solution (S/D) ratio of 1:6, a cleaning time of 30 min, a cleaning temperature of 55 °C, an agitation intensity of 200 rpm, a pH of 9, an ultrasound duration of 15 min, an ultrasound temperature of 55 °C, an ultrasound power of 400 W, and an ultrasound frequency of 28 kHz. (4) Under the specified experimental conditions, an oil recovery of up to 99.32% was observed, and the oil content of oil sludge decreased from 43.13% to 1.01%. The solids content in the oil layer separated from the oil sludge was 0.53%. (5) Compared with traditional thermochemical cleaning, the oil recovery of the combined ultrasound and thermochemical cleaning treatment was 17.65% higher. Therefore, the combined treatment system worked more effectively.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87952037. Fax: +86-571-87952438. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the National Key Technology Research and Development Program of China (No. 2012BAB09B00), National High Technology Research and Development Program of China (No. 2009AA064704), National Basic Research Program of China (2011CB201500), Research Project of Environmental Protection Commonweal Industry (201209023-4), and Important Project on Science and Technology of Zhejiang Province of China (2008C13024-1). 9217

dx.doi.org/10.1021/ie301130c | Ind. Eng. Chem. Res. 2012, 51, 9213−9217