Energy & Fuels 2006, 20, 1959-1964
1959
Effect of Ultrasonic Treatment on the Properties of Petroleum Coke Oil Slurry Zhiqi Wang,† Hongfen Wang,‡ and Qingjie Guo*,† College of Chemical Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, Shangdong, China, and Department of Material Science and Engineering, Ocean UniVersity of China, Qingdao 266002, Shandong, China ReceiVed April 4, 2006. ReVised Manuscript ReceiVed July 9, 2006
Two types of petroleum coke and paraffine oil slurries were prepared without additives at 50 °C, with the effect of ultrasonic treatment on apparent viscosity and static stability being investigated. Experimental results indicated that ultrasonic treatment decreased the apparent viscosity of both slurries but improved their static stability markedly with little effect on the rheological behavior. The effectiveness of ultrasonic irradiation on the apparent viscosity and on the static stability was more remarkable with an increased petroleum coke concentration. Furthermore, it was found that the apparent viscosity was decreased and static stability was improved as the duration of treatment increased. Under the experimental conditions, a duration of 4 min of ultrasonic treatment was an optimum value in decreasing apparent viscosity and improving static stability.
Introduction China has rich coal resources but lacks petroleum. At the same time, about 30 million tons of fuel oil is burn out, especially in oil field and petrorefining plants. On the other hand, about 4.5 million tons of petroleum coke is produced in petrorefining plants every year in China, most of which is fuelgrade coke. Petroleum coke is mixed with coal to burn in the existing boilers because of its low volatility,1 whereas an oil boiler is mostly employed to supply steam in petrorefining plants in China. How to use the coke is an important problem for petrorefining plants in China. The slurry fuels can be pumped as oil by pipeline to burn in oil boilers. Petroleum coke oil slurry is expected to resolve the problem of handling petroleum coke and as a partial substitution of oil, which can be burned directly in existing oil boilers with slight modifications. The technology of coal slurry fuels was developed in industrialized countries in the 1970s-1980s. Its development was due to high oil prices during the economic crisis period. Coal slurry fuels are prepared by adding heavy oil to coal termed as COM (coal oil mixture); CWS (coal water slurry), a mixture of water and coal, developes. CWS has been successfully used in industry in many countries. Later, oil prices fell considerably, and, following an economic depression in the industrialized countries of Asia, they dropped to a low level. Because of the sharp increase in the price of oil, the replacement of oil fuels by slurry fuels or other ways is once again attracting much attention.2-5 * Corresponding author. Tel: +86-532-84022506. Fax: +86-53284022757. E-mail:
[email protected]. † Qingdao University of Science and Technology. ‡ Ocean University of China. (1) Wang, J.; Anthony, E. J.; Abanades, J. C. Fuel 2004, 83, 13411348. (2) Cho, H.; Klima, M. S. Energy Fuels 1996, 10, 1220-1226. (3) Vitolo, S.; Belli, R.; Mazzanti, M.; Quattroni, G. Fuel 1996, 75, 259261. (4) Burdukov, A. P.; Popov, V. I.; Tomilov, V. G.; Fedosenko, V. D. Fuel 2002, 81, 927-933. (5) Boylu, F.; Ates¸ ok, G.; Dinc¸ er, H. Fuel 2005, 84, 315-319.
Slurry fuels such as CWS with maximum coal loading should be relatively stable at static state and during transportation, exhibiting good rheological behavior. As a non-Newtonian fluid, CWS is characterized as pseudoplastic with shear thinning, which has a higher apparent viscosity at static, preventing coal sedimentation, but has a lower apparent viscosity at the pumping process. However, some coals forming slurry with water show dilatant fluids that exhibit shear-thickening behavior and poor static stability. To meet the requirement of its transportation, storage, and combustion in industrial application, expensive additives including dispersants and stabilizers have usually been used to improve the rheological behavior and the static stability of CWS (only the stabilizer is needed for COM). Furthermore, ideal CWS usually has low ash content, but sometimes a coal de-ashing process is required. Therefore, the cost of CWS is a key factor that hinders the large-scale industrial application of CWS technology, especially in petroleum plants. Petroleum coke oil slurry has more advantages of burning in the existing boilers than CWS. Low ash content and high heating value of petroleum coke are expected to burn petroleum coke oil slurry directly in existing oil boilers without any or with minimum modifications. Petroleum coke is the solid residue from the residual oil coking process, which benefits the properties of petroleum coke oil slurry such as stability and rheology without additives. Moreover, low contents of sulfur and nitrogen in petroleum coke oil slurry favor pollution control. Ultrasonics is a very useful approach in facilitating a reaction in both homogeneous liquid systems and heterogeneous liquidsolid systems. It is well-known that ultrasound is capable of producing extraordinarily high transient temperature and pressure in a localized spot by the occurrence and collapse of acoustic cavitation.6-8 Based on the features of ultrasound, ultrasound technology has been applied in a variety of applications: extraction of coal, desulfurization and liquefaction of coal, (6) Suslick, K. S. Science 1990, 247, 1439-1340. (7) Lauterborn, W.; Hentschel, W. Ultrasonics 1986, 24 (2), 59-65. (8) Suslick, K. S.; Casadonte, D. J.; Green, M. L. H.; Thompson, M. E.Ultrasonics 1987, 25 (1), 56-59.
10.1021/ef060144k CCC: $33.50 © 2006 American Chemical Society Published on Web 08/30/2006
1960 Energy & Fuels, Vol. 20, No. 5, 2006
Wang et al. Table 1. Analysis of Petroleum Coke Properties
proximate analysis (wt %)
ultimate analysis (wt %)
contact angle
phenolic hydroxyl
carboxyl
density
sample
moisture
ash
volatile
C
H
N
O
S
dega
mg‚g-1 a
mg‚g-1 a
g‚cm-3
Shengli Jinzhou
0.67 0.56
0.13 0.10
11.53 10.31
90.47 91.30
1.41 1.30
0.41 0.22
7.23 6.86
0.47 0.32
106.6 117.9
0.36 0.71
0.41 0.37
1.31 1.29
a
Contact angle to distilled water. b Phenolic hydroxyl group and carboxyl group contents on the coke particle surface.
Figure 1. Schematic diagram of ultrasonic treatment apparatus.
preparation of coal-water slurry, preparation of nanoparticle materials, and upgradation of heavy gas oil.9-16 Fan et al.11 have used ultrasound technology to load calcium on coal for enhanced SO2 captured. The result showed that, as compared to the mechanical stirring, the ultrasonic treatment significantly improved the SO2 removal. Li and Li16 have studied the ultrasonic irradiation on CWS. The results showed that ultrasonic irradiation markedly improved static stability of CWS with 1% of the anionic additive of NSF and changed the rheological behavior of CWS from dilatant flow to pseudoplastic one but slightly increased apparent viscosity of CWS containing 1% additive. They considered that the changes of coal particle size distribution and the dissolved ion in CWS were related to the improvement of CWS’s properties. Considering the significant influence of ultrasonic treatment on the liquid-solid dispersed system, the effect of ultrasonic treatment on the properties of petroleum coke oil slurry without additive such as rheological behavior, static stability, and apparent viscosity were studied in this paper. Experimental Section Samples. Two types of Chinese petroleum coke from a Shengli petrorefining plant and a Jinzhou petrorefining plant and paraffin oil (Luoyang petrorefining plant, Zhongyuan oilfield) were used in this study. The petroleum coke was ground to pass through 120 mesh, 80% of which was less than 200 mesh. The properties of the cokes are summarized in Table 1. The viscosity of paraffin oil is 28 mPa‚s at 50 °C, and the density of paraffin oil is 0.833 g‚cm-3, which was measured using a bottle densimeter at 50 °C. Ultrasonic Treatment Apparatus. The ultrasound generator schematic diagram is shown in Figure 1. The frequency of the ultrasonic generator was 20 kHz; the ultrasonic intensity was 50 W‚cm-2. The signal generator provided the continuous electrical signal with an adjustable frequency. The power amplifier and the matching circuit resonated this electrical signal to the piezoelectric transducer in which ultrasonic waves were produced. Theses (9) Zaidi, S. A. H. Fuel Process. Technol. 1993, 33 (2), 95-100. (10) Bjorndalen, N.; Islam, M. R. J. Pet. Sci. Eng. 2004, 43, 139-150. (11) Fan, H.; Matsuoka, K.; Wang, J.; Tomita, A. Fuel 2003, 82, 481486. (12) Krzesinska, M. Fuel 1996, 75, 1267-1270. (13) Suslick, K. S.; Doktycz, S. J. J. Am. Chem. Soc. 1989, 111, 23422344. (14) Gopinath, R.; Dalai, A. K.; Adjaye, J. Energy Fuels 2006, 20, 271277. (15) Okitsu, K.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2005, 109, 20673-20675. (16) Li, Y. X.; Li, B. Q. Fuel 2000, 79, 235-241.
ultrasonic waves were transmitted into the slurry sample by the probe, whose diameter was 25 mm. The oscilloscope was used to detect the electrical signal of the power amplifier. About 200 mL of petroleum coke oil slurry was irradiated by ultrasound with immersing the probe to the middle of the samples. Warm water was circulated through the water bath by a pump to maintain petroleum coke oil slurry at 50 °C. Slurry Preparation and Measurement of Slurry Properties. The desired quantity of pulverized petroleum coke was added into the oil with total mass of mixture as 1000 g and then stirred vigorously with a homomixer. Agitation lasted 15 min to mix uniformly. For comparison, the slurry sample was divided two parts. One was irradiated using an ultrasonic generator for several minutes; the other was as a control sample. The frequency and intensity of the ultrasonic generator used were 20 kHz and 50W‚cm-2, respectively. The samples were poured into the sample cylinders stored statically in a thermostatic chamber at 50 °C. The volumes of the stored slurries were 100 mL. The sample cylinder was made of quartz glass with graduation, so the separated clear paraffin oil could be recorded every day. The volume of separated clear oil was used to evaluate the static stability of the slurry. The larger volume of clear oil within the same storage time indicated that petroleum coke particles had a high sedimentation rate, whose slurry had a poor static stability. An NXS-11 rotary viscometer equipped with a circulating water bath could keep the temperature within 0.5 °C was adopted for investigating the rheological properties of the slurry. Samples were placed in the temperature-controlled measurement container (stainless steel; volume is 125 mL) and allowed to equilibrate to the required temperature for 10 min before the measurements were made. The shear-dependent rheological properties of the samples were determined by changing the shear rate from 3.178 to 113.5 s-1 and then acquiring the apparent viscosity as a function of shear rate. For some of the samples, the apparent viscosity was measured at a constant shear rate of 28.38 s-1 by taking the average of measurements acquired at this shear rate over a 2-min period.
Results and Disscusion Effect of Ultrasonic Treatment on the Rheological Properties of the Slurry. Both the stability and the fluidity of slurry fuel are very important for slurry handling and storage. So it is important to characterize the rheological characteristics of the slurry. The slurry fuel is characterized by a non-Newtonian fluid. The desired fluidity should be as pseudoplastic flow or a nearBingham fluid not a dilatant one. The fluidity characteristic is dependent on the coal kind, particle size distribution, concentration (loading), additive, and flow state. The paraffin oil shows Newtonian behavior as clean fluids at 50 °C. But for the slurries, as seen from Figure 2, the Newtonian behavior shifted to the non-Newtonian one. The whole shear rate region was pseudoplastic with shear thinning: the apparent viscosity of the slurries decreased with increasing shear rate before and after ultrasonic treatment. The viscosity of the slurries was very sensitive to shear rate over the range 3-60 s-1 (Figure 2). Also, it was observed that ultrasonic irradiation had little effectiveness on the rheological behavior. However, ultrasonic irradiation could decrease the apparent viscosity of both slurries. Gunal and
Ultrasonic Treatment of Coke Oil Slurry
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Figure 2. Rheological behavior of Shengli and Jinzhou slurry before and after ultrasonic treatment for 4 min. Table 2. Changes of Apparent Viscosity at 28.38 s-1of the Slurries before and after Ultrasonic Treatment for 4 min concentration
viscosity before η1
viscosity after η2
∆η ) (η1-η2)
∆η/η1
wt %
mPa‚s
mPa‚s
mPa‚s
%
58 60 62 64 66
451.7 636.1 946.5 1296 2163
390.9 532.3 788.9 1060 1756
60.8 103.8 157.6 236 407
13.5 16.3 16.7 18.2 18.8
Islam17 have studied the alteration of asphaltic crude rheology with ultrasonic irradiation. They also found that ultrasonic treatments do not alter crude oil rheology. Effect of Ultrasonic Treatment on Viscosity of the Slurry. The effect of ultrasonic treatment on the viscosity of the slurries with different petroleum coke concentration is shown in Table 2. From the data shown in the table, one can see that higher petroleum coke loading slurry decreases the apparent viscosity more than the lower loading one after being treated by ultrasound. An increasing percentage of decrease in viscosities is found to occur when going up from 58 to 66 wt % petroleum coke loading in the slurry. The value of decrease in viscosity for a solid loading of 58 wt % is 13.5%; however, the value for a solid loading of 66 wt % is up to 18.8%. Figure 3 shows the change of apparent viscosity (at shear rate of 23.38 s-1) with ultrasonic treatment time. From Figure 3, one can find that the apparent viscosity of the slurry decreases with increasing ultrasonic treatment period. At the experimental condition (20 kHz and 50 W‚cm-2), the apparent viscosity reduces sharply from 0 to 4 min and then tends to reach a constant. Therefore, 4 min is a sufficient period for ultrasonic treatment on the slurry. Effect of Ultrasonic Irradiation on the Static Stability of the Slurry. Slurry fuels are a heterogeneous mixture. To enable the powder to disperse in the dispersed phase homogeneously and hinder the sedimentation rate of the powder, many surfactants are added as stabilizer of the slurry. According to the study of slurry fuels (COM and CWS), it has been demonstrated that additives were absorbed on the surface of the coal particle to control the interparticle force for hindering settling rate or forming a loosely dispersed flocculated network. The adsorption of additives on the coal particle surface was significantly affected by coal surface properties such as surface area, porosity, polarity, ash content, and oxidizability. Generally, the coal particle, which has more surface and porosity, can sorb a lot of additives, so the slurry fuels will represent better stability.18 (17) Gunal, O. G.; Islam, M. R. J. Pet. Sci. Eng. 2000, 26, 263-272.
Figure 3. Effect of ultrasonic treatment time on the apparent viscosity at 28.38 s-1 of the slurry.
In this study, petroleum coke is the solid residue from the residual oil coking process, so there is affinity between petroleum coke and oil. They represent mutual solubility to a certain degree. It is reasonable that petroleum coke oil slurry maybe have a kind static stability even when no additive is introduced. Without ultrasonic treatment, Shengli petroleum coke oil slurry displayed a relative good stability, whose volume of clear oil separate from slurry was much less than that of Jinzhou as shown in Figure 4a-f. The separation between coke particle and oil was mainly induced by gravity, so the difference between densities of dispersed solid particles and suspending liquid had an important influence on the stability of suspension. Coke densities of Shenli and Jinzhou are 1.34 and 1.32 g‚cm-3, respectively (Table 1); therefore, the different stability of two coke slurries cannot attribute to the different particle density. Whereas, Shengli coke particle surface properties such as phenolic hydroxyl content, carboxyl content, and contact angle to distilled water represented more hydrophobic slurries than Jinzhou particle, this might be the reason that Shengli coke slurry had a better stability than Jinzhou coke slurry. Figure 4 also shows the effect of ultrasonic treatment for 4 min on the static stability of the different concentration slurries. Ultrasonic treatment can also improve the static stability of petroleum coke oil slurry, but the promotion varied with the different coke loading slurries. The final clear oil volume (FV) was defined as one separated from the slurry without ultrasonic treatment after being stored for 40 days. As shown in Figure 4a-d, the FV of different (18) Kaushal, K. T.; Sibendra, K. B.; Kumaresh, C. B.; Somnath, B.; Kamlesh, K. M. Fuel Process. Technol. 2004, 85, 31-42.
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Figure 4. Effect of ultrasonic treatment for 4 min on the static stability of the different concentration slurries. Panels a-d: Jinzhou coke, 60, 62, 64, 66 wt %. Panels e-f: Shengli coke, 64, 66 wt %.
slurries containing 60, 62, 64, and 66 wt % were 27.5, 22.0, 16.0, and13.5 mL, respectively. However, the clear oil volumes separated from different slurries with ultrasonic treatment after being stored for 40 days were 27.5, 21.0, 10.0, and 6.5 mL, respectively. The less clear oil volume within the same stored days showed that coke particles had a low sedimentation rate and that the slurry exhibited a better static stability. Under the conditions studied, the static stability of the slurry with higher coke concentration was improved more than that with lower coke concentration after ultrasonic treatment, which is similar to the decrease in apparent viscosity of the slurry as shown in Figure 2. The changes of clear oil volume of Jinzhou slurry under different duration of ultrasonic treatment are shown in Figure 5. The margin of the clear oil volume between the parent slurry and the ultrasonic treatment slurry was quite different for various coke concentrations, especially in the storage prophase. There was a visible difference of clear oil volume
separated from the slurry with different ultrasonic treatment times before being stored 12 days for higher coke loading slurry (64 wt %, Figure 5, panel 2), whereas the difference was not notable for the lower coke loading one (60 wt %, Figure 5, panel 1). According to Figure 5, it was an optimal duration that the slurry was being treated by ultrasonic for 4 min under the experimental conditions, which could improve the slurry’s stability. It is generally considered that either a bimodal or a broad size distribution is required to produce highly loaded coalwater slurry with minimum viscosity. For a high concentration of slurry fuels, pulverized coal or coke sizes should preferably be distributed over a wide range rather than sharply distributed.19 It is also known that an increase (to a certain extent) of the finer fraction when its concentration remains constant in the (19) Veytsman, B.; Morrison, J.; Scaroni, A.; Painter, P. Energy Fuels 1998, 12, 1031-1039
Ultrasonic Treatment of Coke Oil Slurry
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Figure 5. Effect of ultrasonic treatment duration on the static stability of Jinzhou coke slurry: panel 1, 60 wt %; panel 2, 64 wt %.
slurry causes a decrease in the viscosity of the slurry. The optimum ratio between the fractions and their optimum dispersity are such that smaller particles can fill the voids between larger ones. However, the incorporation of smaller particles into aggregates constituted by larger particles increases the number of contacts in aggregates. According to the structural-rheological model of slurries, such a process should cause an increase in the viscosity of coal-water slurries rather than the decrease observed in it.20 There are two effects of cavitation formed by ultrasound near the extended liquid-solid interfaces in the liquid-solid system: microjet impact and shock wave damage. The deformation of the cavity during its collapse sends a fast-moving stream of liquid through the cavity at the surface with velocities greater than 100 m‚s-1. These shock waves induce high velocity collisions among solid particles suspended in liquids. These collisions result in changes of the particle’s size, porosity, and surface. From the curve of particle size distribution shown in Figure 6, one also can see that ultrasonic treatment on the slurry results in a higher proportion of fine coke particle in coke oil slurry as compared with the absence of ultrasonic treatment. This observation was supported by other workers also.16 However, the change of the particle distribution was not very distinctive. Therefore, ultrasonic treatment on the petroleum slurry causing the decrease of viscosity just may partially be due to the change of the particle size distribution. On the other hand, the mean particle diameter in the slurry fuel is about 20 µm. There is a strong tendency for the powder
to agglomerate together, so even violent mechanical agitation cannot disperse the powder as well as the ultrasonic treatment. However, the violent impact by ultrasound can destroy the aggregates formed by finer particle, which disperse petroleum coke particle in the oil very well. The slurries from the bottom of the sample cylinders after being stored for 5 days are sampled and magnified 700 diameters by an optical microscope as shown in Figure 7. Panel a is the slurry with ultrasonic treatment; panel b is the slurry without ultrasonic treatment. The particles dispersed fairly uniformly in the slurry after ultrasonic treatment, but the particle blocks were observed in the parent slurry. It is very clear that ultrasonic treatment results in marked reduction of the larger agglomerations in the slurry. Hence, it is reasonable to expect that the latter is the dominating reason for the decrease in the viscosity of the slurry. At lower coke concentrations, because the coke surface coverage by oil was low, more oil was free and the particleparticle interactions were less than that of higher coke concentrations. So higher coke loading slurries exhibited better stability whether ultrasonic treatment was done or not. One can suppose that there were more agglomerates in high coke concentration slurry, that ultrasound irradiation broke all the possible agglomerates, and that the increasing coke surface coverage by oil reduced the “free oil”. Therefore, the ultrasonic treatment improved the stability of higher concentration slurries much better than the lower one. On the basis of the above discussion, it was more difficult for higher coke loading slurry to disperse well than lower coke loading slurry by a common mechanical agitation, but ultrasound possessing a unique feature could disperse the higher concentration slurry easily. Therefore, the effectiveness of ultrasonic treatment on the properties of the slurry was more remarkable with increasing petroleum coke loading. A significant improvement in static stability of the slurry treated by ultrasound may also be attributed to the mentioned two factors: (1) Ultrasonic treatment on a high solids content slurry might produce an optimum particle size distribution as generating additional fine particles, which perhaps benefits the stability. (2) In the ultrasonic treatment process, the agglomerates formed by powder were crashed and immediately wetted by the oil before they get together again. The later was the dominating factor since the particle size distribution was not very notable. In addition, ultrasound produced extraordinarily high transient temperature and pressure in a localized spot on the particle surface, which could lead to effective local softening and dramatic increases in the adsorb rates of solid-liquid,21 so
(20) Redkina, N. I.; Khodakov, G. S. Theor. Found. Chem. Eng. 2003, 37 (3), 263-267.
(21) Prozorov, T.; Prozorov, R.; Suslick, K. S. J. Am. Chem. Soc. 2004, 126, 13890-13891.
Figure 6. Changes of particle size distribution before and after irradiation. Jinzhou coke slurry.
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Figure 7. Optical microscope picture of the bottom slurry stored for 5 days before and after ultrasonic treatment, Jinzhou coke: panel a, ultrasonic treatment; panel b, parent.
oil could adhere to the impact point of coke particles. That would be favorable for the affinity and mutual solubility of coke particle and oil. This was a potential reason ultrasonic treatment could improve static stability of the slurry. Conclusion Two petroleum cokes were mixed with paraffin oil to prepare slurry fuel, and then the effect of ultrasonic treatment on the slurry’s properties was investigated. The experimental results indicated that ultrasonic treatment on the petroleum coke oil slurry not only decreased the apparent viscosity of both slurries but also promoted their static stability markedly with little effect on the rheological behavior. The improvement of ultrasonic treatment on the apparent viscosity and static stability was more remarkable with increasing the petroleum coke content. The apparent viscosity was decreased, and the static stability of the
slurries was improved in a different degree as the duration of ultrasonic treatment increased. Ultrasonic treatment for 4 min under experimental conditions (20 kHz and 50 W‚cm-2) could reach the optimum in decreasing apparent viscosity and improving static stability. Ultrasound could destroy the agglomerates in the slurry effectively, especially for higher coke loading slurry. The economic efficiency of slurry fuels increases with increasing content of their solid components, with decreasing viscosity, and with increasing static and dynamic stability. Therefore, ultrasound exhibits a valuable technology for solidliquid suspension as slurry fuels. Acknowledgment. We acknowledge the Taishan Scholar Construction Project of Shandong Province (JS200510036) for financial support. EF060144K
