Energy & Fuels 1988,2, 653-656 to be lower than for lime because limestone can be transported in open trucks. Also, limestone is readily available in the Athabasca region of Alberta. In Figure 3 a comparison of the efficiency of the desulfurization process has been made for the two cokes investigated. These results were obtained under identical experimental conditions at a combustion temperature of 1000 "C. From the plots, sulfur capture is more efficient for Syncrude coke than for Suncor coke at higher calcium to sulfur ratios. thus, at a calcium to sulfur mole ratio of about 1:1,94% sulfur retention was observed for Syncrude coke compared with a value of 80% for Suncor coke. The reason for this difference is not understood. According to the findings of Schneider and George13the presence of calcium has a beneficial effect on the acid leaching of nickel and vanadium from coke ash. Hence, coagglomeration of coke with calcium compounds also will have the added advantage that the ash from the burnt ~~
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(13) Schneider, L. G.; George, 2.M. Extr. Metall. '81, Pap. Symp., 1981 1981,413-420.
653
agglomerates will be more suitable for heavy-metal recovery as a byproduct of the combustion process. In conclusion, it can be stated that waste coke produced in the upgrading of Athabasca bitumen can be successfully coagglomeratedwith sulfur dioxide capture agents during the recovery of bitumen or heavy oil from aqueous waste streams produced during oil sands processing. The resulting agglomerates have improved combustion properties and lower sulfur emissions compared with those for the original coke. The decrease in sulfur dioxide emitted on combustion was found to depend on the calcium to sulfur mole ratio, combustion temperature, and the type of coke. Better results were obtained for Syncrude fluid coke than for Suncor delayed coke. Acknowledgment. We wish to thank P. Maxwell, Martin Lepage, M. R. Miedema, and F. W. Meadus for technical assistance and R. D. Coleman and Roger Lafleur of EMR for kindly providing access to a Leco sulfur analyzer. Registry No. SOz, 7446-09-5; Ca(OH),, 1305-62-0.
Surface Tension of Bitumen from Contact Angle Measurements on Films of Bitumen E. I. Vargha-Butler,t T. K. Zubovits, C. J. Budziak, and A. W. Neumann* Department of Mechanical Engineering, University of Toronto, Toronto, Ontario M5S l A 4 , Canada
Z. M. Potoczny Coal Research Laboratory, C A N M E T , EMR, Devon, Alberta T6G 2C2, Canada Received December 28, 1987. Revised Manuscript Received April 25, 1988
Advancing contact angles,, ,e of water and glycerol were measured on films of bitumen. The bitumen was obtained by extracting different raw oil sands with various organic solvents. The films were prepared by film casting the bitumen from solutions (using the same solvent that was used for the extraction). The contact angles of both liquids were determined on bitumen surfaces by means of the sessile drop method at room temperature. The surface tensions, ysV,of the quasi-solid bitumen films were calculated from the contact angles by means of an equation of state approach. It was found that contact angles measured with water and glycerol depended on the solvent used for the bitumen extraction. The surface tension, ysV,of the quasi-solid bitumen films calculated from the contact angles agree with the surface tensions determined at elevated temperatures by means of the Wilhelmy plate techinque. Thus, the contact angle method provides an independent technique for the surface characterization of bitumen at ambient temperatures.
Introduction
In preceding papers1i2we reported the surface tension of bitumen determined by direct measurements using a modified Wilhelmy plate te~hnique.~The surface tension of bitumen was measured by temperature scanning from 40 to 90 OC, since at lower temperatures the high viscosity of bitument does not allow conventional surface tension determination. The bitumen samples were obtained from various raw oil sands by extraction with several solvents Present address: College of Pharmacy, Dalhousie University, Halifax, Nova Scotia B3H 355, Canada.
0887-0624/88/2502-0653$01.50/0
of different polarity and by centrifugal separation, i.e., without solvents. It was found that the surface tension of bitumen extracted with organic solvents depends on the solvent used for the extraction and that it is lower than that obtained by centrifugal separation. The differences (1) Potoczny, 2.M.; Vargha-Butler, E. I.; Zubovits, T. K.; Neumann, A. W. AOSTRA J.Res. 1984,1, 107-115. (2) Potoczny, 2.M.; Vargha-Butler, E. I.; Zubovits, T. K.; Neumann, A. W. AOSTRA J.Res. 1984 1, 117-120. (3) Neumann, A. W.; Good, R. J. In Surface and Colloid Science; Good, R. J., Stromberg, R. R. Eds., Plenum: New York, 1979; Vol. 11, pp 31-91.
0 1988 American Chemical Society
654 Energy & Fuels, Vol. 2, No. 5, 1988
Vargha-Butler et al.
between the surface tensions of the solvent-extracted and centrifuged bitumen samples were attributed to interactions between the solvent and the bitumen. As direct surface tension measurements with the Wilhelmy plate technique can be performed only at elevated temperatures (because of the visocosity of bitumen), we wished to explore the possibility of characterizing the surface properties of films of bitumen at ambient temperatures by means of contact angle measurements. The contact angle of a liquid drop on a solid surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions: solid/vapor, ysv, solid/liquid, ysL, and liquid/vapor, yLv. The equilibrium relation is known as Young's equation YSV - YSL
= YLV
COS @e
(1)
where de is the equi.librium contact angle. This equation yields a single, unique contact angle, assuming ideal conditions. In practice, however, the contact angle made by an advancing liquid (6,) and that made by a receding liquid (OR) are not identical, almost all of the solid surfaces exhibit contact angle hysteresis. The theory of contact angle hysteresis was discussed by several authors,"1° and various models have been proposed to explain the effects of surface heterogeneity as well as surface roughness. A thermodynamic analysis1'J2 for the hysteresis on heterogeneous and on rough solid surfaces assumes that, on an energetically heterogeneous surface, there may be patches of different compositions at different sites. It was concluded that the advancing contact angle (0,) is a measure of the wettability of the low energy portions of the solid surface, while the receding angle (dd is a measure of the higher energy sites. Conventionally, the advancing contact angle is measured and taken as being indicative of the surface properties of low-energy solids, such as polymers13or, in our case, bitumen films. An equation of state relation14J5is used to calculate the surface tension of the solid, ysv, from the contact angles obtained with different liquids. A thin layer of bitumen, however, is a far more complex, heterogeneous material than a polymer. The liquids used for contact angle measurements may interact physically or chemically with some of the components of the bitumen. Thus, it is of general interest to determine whether contact angles on bitumen films will provide useful information. The purpose of this paper is 2-fold: First, the effect of the extracting solvent on the contact angles measured on films of bitumen will be studied. Second, we shall compare the surface tensions of bitumen calculated from contact angles with the corresponding ones determined previously by direct surface tension measurements at elevated tem(4)Wenzel, R. N. Ind. Eng. Chem. 1936,28,988-994. (5)Cassie, A. B. D.; Baxter, S. Nature (London) 1945,155, 21-22. (6)Good, R.J. J. Am. Chem. SOC.1952,74, 5041-5042. (7)Johnson, R. E.,Jr.; Dettre, R. H. Adu. Chem. Ser. 1964,No. 43, 112-136. (8) Johnson, R. E., Jr.; Dettre, R. H. J. Phys. Chem. 1964, 68, 1744-1750. (9)Neumann, A. W.; Renzow, D.; Reumuth, H.; Richter, I. E. Fortschrittsber. Kolloide Polym. 1971,55,49-54. (10)Herzberg, W. J.; Marian, J. E.; Vermeulen, T. J. Colloid Interface S C ~1970, . 33, 164-171. (11)Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972,38, 341-358. (12)Eick, 3. D.; Good, R. J.; Neumann, A. W. J.Colloid Interface Sci. 1975,53,235-248. (13)Neumann, A. W. Adu. Colloid Interface Sci. 1974,4 , 105-191. (14)Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, H. J. Colloid Interface Sci. 1974,49, 291-304. (15)Neumann, A. W.; Absolom, D. R.; Francis, D. W.; van Oss, C. J. Sep. Purif. Methods 1980,9, 63-163.
peratures, treating the bitumen as a liquid and not as a quasi-solid, as in the contact angle measurement. Experimental S e c t i o n Materials. For the contact angle measurements two low grade (Peace River and Pelican Lake) and two medium grade (No. 25 and No. 86) oil sands were selected from samples studied previously.1*2Sample No. 25 was obtained from the SUNCOR site and No. 86 from the GCOS site of the Fort McMurray deposit. The oil sands together with their analytical data were provided by the Oil Sands Sample Bank of the Alberta Research Council, Edmonton, Canada. The following six organic solvents were used for the bitumen extraction: n-hexanol; benzene; tetrahydrofuran; xylene; toluene; cyclohexanone. The solvents were laboratory grade, Fisher Scientific or Eastman products. The analytical data for the oil sands and the physical properties (boiling points, dipole moments, and surface tensions) of the solvents are given in ref 1. Methods. The bitumen was extracted a t the boiling points of the solvents (hot extraction) from the oil sand samples in a conventional Soxhlet extraction apparatus. For more details of the extraction, see ref 1. 1. Preparation of the Bitumen Films by Film Casting. The bitumen test surfaces were prepared by film casting in a manner similar to the one used by Davidson and Lei16 for the preparation of polymer films. A concentration of 0.5 g of bitumen/2 mL of the same solvent used for the hot extraction was found to give satisfactory results. Film casting involves placing a few drops of the solution on a carefully cleaned glass slide. The glass slide is then set to spin on the rotor of a modified centrifuge a t a relatively low speed (at 3900 rpm) under slightly evacuated conditions until all the solvent has evaporated, leaving a thin and smooth film of bitumen on the glass slide. These specimens were kept in a n oven a t 90 "C for at least 2 h to provide a thermal history similar to that of the measurements performed with the Wilhelmy plate techinque. The bitumen slides were stored in an evacuated desiccator for at least 48 h to remove any pcmible traces of the solvent remaining in the bitumen film. Two slides were prepared from each of the bitumen samples. 2. Contact Angle Measurements. The sessile drop technique is one of the most convenient methods for routine measurements of contact a n g l e ~ . ~The J ~ experimental setup for the sessile drop method consists of a horizontal stage, a source of illumination from behind the drop with a light filter (to minimize heating by the light source) and a telescope equipped with a goniometer eyepiece. The telescope is mounted so that it can be moved right or left relative to the stage on which the specimen (glass slide with the bitumen film) is mounted. A drop of the liquid was placed on the surface of the film of bitumen; it was formed by using a 2.0-mL micrometer syringe (Gilmont Instrument Inc.) with a 30-cm stainless-steel chromatographic needle. Contact angles with water and glycerol were measured on each bitumen surface a t room temperature (23 "C). For each liquid, a different syringe an dneedle were used. Both were cleaned carefully prior to the measurements. The glass barrel was cleaned with chromic acid; the needles were repeatedly rinsed with the measuring liquid. The advancing contact angles, BA, were measured by taking readings on both sides of the droplets to check for drop symmetry. For convenience, the volume of the drops was chosen in the range 0.02-0.05 mL. The reading of the contact angle was taken immediately after the formation of the drop, to minimize the effect of liquid penetration and any chemical interaction on the contact angle. T o obtain a statistically significant value of BA, a t least 6-10 drops were measured on both sides, providing 12-20 advancing contact angle values for each surface. The resulting data were then averaged. The standard deviation and the population mean were calculated from the sample mean assuming a t distribution. The errors given are the 95% confidence limits. From these data, the surface tension, ysv, of the quasi-solid bitumen was calculated by means of a n equation of state a p p r ~ a c h . ' ~ * ' ~ (16)Davidson, E.B.; Lei, G. J. Polym. Sci. 1971,9, 569-572.
Energy & Fuels, Vol. 2, No. 5, 1988 655
Surface Tension of Bitumen
Table I. Contact Angles and Surface Tensions of Bitumen Films at 23 "C for Sample No. 25" surface tension (ysv), mJ/m2 contact angle (e), deg extracting solvent water glycerol water glycerol mean value 92.5 f 0.6 83.4 f 1.0 27.0 f 0.4 26.4 f 0.6 26.7 n-hexanol 86.5 f 1.2 24.7 f 1.1 24.7 f 0.8 24.7 tetrahydrofuran 96.5 f 1.8 toluene 93.1 f 1.2 84.6 f 0.5 26.9 f 0.8 25.7 f 0.3 26.3 benzene 93.6 f 1.0 85.4 f 1.0 26.5 f 0.6 25.3 f 0.6 25.9 xylene 92.7 f 0.8 83.2 f 0.6 27.1 f 0.5 26.5 f 0.4 26.8 cyclohexanone 97.1 f 1.4 84.7 f 1.2 24.3 f 0.9 25.7 f 0.8 25.0 The errors are 95% confidence limits.
Table 11. Contact Andes and Surface Tensions of Bitumen Films at 23 O C for Samole No. 86" contact angle (e), deg surface tension (ysv), mJ/m2extracting solvent water elvcerol water elvcerol mean value n- hexanol 92.1 f 1.0 82.8 f 0.8 27.5 f 0.6 26.7 f 0.5 27.1 84.1 f 1.0 tetrahydrofuran 94.2 f 1.2 25.9 f 0.8 26.0 f 0.6 26.0 toluene 93.6 f 0.6 84.5 f 1.0 26.2 f 0.4 25.8 f 0.6 26.0 82.5 f 1.4 26.6 f 0.6 26.9 f 0.9 26.8 benzene 93.5 f 1.0 96.6 f 0.5 87.7 f 0.8 24.7 i 0.3 24.1 f 0.5 24.4 xylene 93.5 f 0.8 83.6 f 1.0 26.6 f 0.5 26.3 f 0.6 26.5 cyclohexanone a
The errors are 95% confidence limits.
Table 111. Contact Angles and Surface Tensions of Bitumen Films at 23 OC for the Peace River Sample' contact angle (O), deg surface tension (ysv), mJ/m2 extracting solvent water glycerol water glycerol mean value 82.4 f 0.6 n-hexanol 95.7 f 0.8 25.2 f 0.5 27.6 f 0.4 26.4 tetrahydrofuran 91.7 f 0.9 78.3 f 1.0 27.7 f 0.6 29.9 f 0.4 28.8 toluene 92.9 f 1.1 79.9 f 0.5 27.0 f 0.8 29.0 f 0.5 28.0 80.8 0.8 26.9 f 0.4 28.5 f 0.2 27.7 benzene 93.0 f 0.5 85.6 f 1.2 31.6 f 0.8 29.4 f 0.5 30.5 79.1 0.6 xylene cyclohexanone 93.3 f 0.6 79.9 f 1.2 26.7 f 0.4 29.0 f 0.2 27.9
* *
OThe errors are 95% confidence limits.
Table IV. Contact Angles and Surface Tensions of Bitumen Films at 23 OC for the Pelican Lake Sample" contact angle (e), deg surface tension (ysv), mJ/m2 extracting solvent water glycerol water glycerol mean value n-hexanol 90.9 f 1.0 76.9 f 0.8 28.3 f 0.6 30.6 f 0.5 29.5 82.2 f 0.6 27.4 f 0.9 27.7 f 0.4 27.6 tetrahydrofuran 91.9 f 1.4 96.3 f 0.6 81.7 f 1.4 24.7 f 0.4 28.0 f 0.9 26.4 toluene 24.7 f 0.4 25.9 f 0.5 25.3 benzene 96.3 f 0.5 85.5 f 0.8 98.0 f 0.7 85.9 f 0.8 23.8 f 0.5 25.8 f 0.5 24.8 xylene 79.8 f 1.2 cyclohexanone 91.6 f 1.4 27.9 f 0.9 29.0 f 0.8 28.5 "The errors are 95% confidence limits.
Results and Discussion The contact angles, OA, measured with water and glycerol on bitumen films obtained from the four oil sands after extractions with different solvents are given in Tables I-IV. These contact angles represent the averaged values of 12-20 readings. In the same tables, the calculated surface tensions of the bitumen films are also summarized. The errors given are the 95% confidence limits. In the last column of each table the mean values of the surface tensions obtained from Owater and Oglycerol for each bitumen sample are tabulated. It has been ~hown'~J'that surface tensions, ysv, of homogeneous solids calculated by means of an equation of state14J6relation from contact angles of different liquids on well-prepared solid surfaces of low energy are virturally independent of the liquid. The results obtained for films of bitumen extracted from medium grade oil sands No. 25 and No. 86 (cf. Tables I and 11) show that the surface tensions calculated from Owater and for the same sample differ from each other by on y approximately 1
O F
(17) Omenyi, S. N.; Neumann, A. W.; van Oss, C. J. J. Appl. Phys.
1981,52, 189-195.
mJ/m2. These differences, in most of the cases, are within the 95 % confidence limits given for the surface tensions of the two medium grade bitumen samples. Such minor discrepancies between surface tensions, calculated from water and glycerol contact angles, have been observed previously even for contact angles measured on very smooth and impuirity-free polymer surfaces, with extremely pure liquids. These minor differences were sometimes attributed to small but finite values of the equilibrium spreading pressures.lg In cases of bitumen extracted from the low-grade oil sands (samples from the Peace River and Pelican Lake deposits, cf. Tables I11 and IV); however, the differences in surface tensions calculated from the contact angles of water and those of glycerol, with few exceptions, exceed 2 mJ/m2. These differences are slightly outside the error limits given in Tables I11 and IV for the surface tensions of the bitumen. Bitumen from low-grade oil sands contains more residual solids (cf. ref 1) than the medium-grade samples. According to microscopic observations and digital (18) Spelt, J. K. Ph.D. Thesis, University of Toronto, Toronto, Can-
ada, 1985.
656 Energy & Fuels, Vol. 2, No. 5, 1988
Vargha-Butler et al.
Table V. Comparison of the Surface Tensions of Bitumen at 23 "C Calculated from Contact Angles and Obtained from the Extrapolated Values of Liquid Bitumen Measured Directly by Means of the Wilhelmy Plate Technique for No. 25 and No. 86 Oil Sands" surface tension of bitumen at 23 "C, mJ/m2 from contact from direct angles measurements extracting solvent No. 25 No. 86 No. 25 No. 86 25.8 27.5 26.7 27.1 n-hexanol 24.7 26.0 25.6 28.6 tetrahydrofuran 27.0 25.3 26.3 26.0 toluene 26.8 26.9 27.5 25.9 benzene 24.4 25.5 24.9 26.8 xylene 26.5 24.3 27.8 25.0 cyclohexanone
Table VI. Comparison of the Surface Tensions of Bitumen at 23 "C Calculated from Contact Angles and Obtained from the Extrapolated Values of Liquid Bitumen Measured Directly by Means of the Wilhelmy Plate Technique for Peace River and Pelican Lake Oil Sands" surface tension of bitumen at 23 "C.mJ/m2 from direct from contact aneles measurements extracting Peace Pelican Peace Pelican solvent River Lake River Lake 29.6 28.4 n-hexanol 26.4 29.5 31.7 29.0 tetrahydrofuran 28.8 27.6 toluene 28.0 26.4 24.4 30.2 26.2 26.2 benzene 27.7 25.3 25.6 26.9 xylene 30.5 24.8 27.5 33.8 cyclohexanone 27.9 28.5
a Bitumen was extracted with different solvents from mediumgrade oil sands.
Bitumen was extracted with different solvents from low-grade oil sands.
image analysis results, a large quantity of clay particles is distributed in the bitumen from the low-grade oil sands.lg The quantity of clays present in the solventextracted bitumen depends also on the solvent used for the extraction. The surface tension, yLv, of bitumen measured by means of the Wilhelmy plate technique (between 40 and 90 "C) was Yound not to be influenced by the presence of clays.20~2' In the case of thin films of bitumen, however, the different surface tensions obtained from contact angles with water and glycerol may be due to heterogeneity and also roughness of the thin, quasi-solid bitumen films caused by the embedded (clay) particles. After establishing the surface tensions of quasi-solid bitumen in films obtained via contact angle measurements a t ambient temperatures, one may compare these results with the results obtained directly1 for liquid bitumen at elevated temperatures by means of the Wilhelmy plate technique. Since the contact angles were measured at 23 "C, for the purpose of this comparison, the surface tensions of the liquid bitumen, measured between 40 and 90 "C, were extrapolated to 23 "C. In these measurements' the surface tension of bitumen was found to be a linear function of the temperature in the range from 40 to 90 "C. These straight lines were extrapolated to 23 "C. This procedure was further validated by a measurement down to 25 "C in one particular case.2o The surface tension results, from contact angles and the extrapolated values of the liquid bitumen given a t 23 OC, are summarized in Tables V and VI for bitumen obtained from medium- and (19) Moy, E. M.A.Sc. Thesis, University of Toronto, Toronto, Canada,
low-grade oil sands, respectively. Error limits are not available for the surface tensions of the solvent-extracted liquid bitumen. The error for the surface tensions of liquid bitumen separated mechanically, by centrifuge, from various oil sands was found to be as small as 0.5 mJ/m2.' From the present work, the error in the surface tenion of the quasi-solid bitumen films varied from 0.2 to 1.1 mJ/m2. Comparing the corresponding surface tensions obtained by the two approaches for each bitumen sample, we found that the surface tensions of medium-grade bitumens (No. 25 and 86) obtained from contact angles and direct measurements differ from each other, with one exception, by only 0.4-1.3 mJ/m2 (cf. Table V). The differences in the relevant surface tensions of bitumen from low-grade oil sands (Peace River, Pelican Lake) obtained by direct and indirect methods, however, varied from 0.9 to 5.3 mJ/m2, indicating most likely an effect of the clay particles on the contact angles. Overall, we reached the following conclusions: 1. The surface tensions calculated from 0 (indirect method) and those determined by means of the Wilhelmy plate technique (direct method) are in agreement of bitumen obtained by solvent extraction from different oil sands. The agreement is good for bitumen from mediumgrade oil sand and fair for bitumen from low-grade oil sand. 2. The surface tensions, ysv, calculated from contact angles show a slight solvent dependence similar to that found with the direct Wilhelmy plate measurements. 3. The contact angle method is a good approach for the determination of the surface tension of bitumen at ambient temperatures.
(20)Budziak, C. J. M.A.Sc. Thesis, University of Toronto, Toronto, Canada, 1985. (21) Budziak, C. J.; Vargha-Butler,E. I.; Hancock, R. G. U.; Neumann, A. W.Fuel, in press.
Acknowledgment. This work was supported by the Alberta Oil Sands Technology and Research Authority through Contract No. 4300.235, Project No. 201.
1984.
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