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Registry No. PEG 400, 25322-68-3; benzoic acid, 65-85-0.
Literature Cited Braun, R.; Parrott, E. J . Pharm. Sci. 1972, 67, 592. Chien, Y. W. “Novel Drug Dellvery Systems”; Marcel Dekker: New York. !MJ . 1972, 78, 958.
Holmes, J. T.; Wllke, C. R.; Olander, D. R. J . h y s . Chem. 1903, 6 7 , 1469. Kaufmann, T. 0.;Leonard, E. F. A I C E J . 1988, 74, 110. Keshary, P. R.; Chlen, Y. W. Bug Dev. Ind. Pharm. 1984, 70, 883. Smith, K. A.; Colton, C. K.; Menill, E. W.; Evans, L. B. Chem. €ng. hog., Symp. Ser. 1908, 64, 45. ToJo, K. J . Soc. Powder Technd., Jpn. 1984, 27, 490. Tojo, K.; Fan, L. T. Math. Blosci. 1981, 57, 279. Tojo, K.; Miyanaml, K. Chem. €ng. Commun. 1982, 76, 159. ToJo, K.; Ghannam, M.; Sun, Y.; Chlen, Y. W. J . ContrdledReleese 1985. 7 , 197. Wllke, C. R. Chem. Eng. Rog. 1949, 45, 218.
Solvent Extraction of Mined Athabasca Oil Sands Helen Leung and Colin R. Philllps’ Department of Chemical Engineerlng and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 7A4, Canada
A semiempirical analysis of the solvent extraction process for Athabasca oil sands was carried out in order to arrive at process design criteria. A generalized extraction equation of the form (1 - t)”’ = 1 - T‘ where [ = extraction efficiency and T’ = dimensionless contact time is derived. The analysis is applied to experimental data on kerosene in a rotating contactor together with previously published results for several solvents in stirred vessels. The effects of the oil sand to solvent ratio and stirrer speed (or rotational speed for the rotating contactor) are examined.
Introduction Extraction schemes proposed for recovery of bitumen from oil sands include the Clark hot water process (Clark, 1944), cold water processes (Grant et al., 1980; Djinghenzian, 1951), and anhydrous solvent extraction processes (Cottrell, 1963; Kenchington et al., 1981; Cormack et al., 1977; Fu and Phillips, 1979; Funk, 1979). A solvent extraction process appears to promise both high extraction efficiency at a reasonable cost and reduction of the environmental problems that arise in the commercially adopted hot water process. A detailed technical and economic analysis has been made on a process for the production of 1900 m3/day of bitumen from 136000 kg/h of Athabasca oil sands (Kenchington and Phillips, 1981). The process design consisted of the following major steps: (i) solvent extraction, (ii) draining and washing, (iii) solvent recovery from spent sands, and (iv) solvent recovery from bitumen solution. The present work constitutes a detailed investigation of solvent extraction in rotating contactors. Preliminary work in stirred tanks at laboratory scale has already been reported (Cormack et al., 1977). In the present work, a semiempirical analysis of the extraction process is applied in order to arrive at process design criteria. The analysis is supplemented by experimental work in large laboratory scale rotating contactors. Comparison is made between the stirred tanks and the rotating contactor. Model The process of dissolution of bitumen from oil sands aggregates depends on the solvent to oil sands ratio, solvent type, and degree of agitation. The process may involve the following stages: (1)transfer of solvent from the main fluid through the fluid film to the surface of the solid particle, (2) diffusion of solvent into the aggregates, (3) breakup of the aggregate due to the “softening” effect of the solvent together with the effect of the agitation, (4) dissolution of bitumen from the aggregate surface, and (5) 0196-4313l85/1024-0373$01.50l0
transfer of bitumen from the surface of the aggregate through the fluid film into the main body of the fluid. In the case of a dilute system (that is, a high solvent to oil sands ratio) and a solvent having a high solubilizing power, stages 1-3 may not be important and the extraction becomes a simple convective diffusion process. Oil sands aggregates consist of sand particles fairly uniformly embedded in a bitumen film, the whole aggregate being roughly spherical in shape (Figure 1). A mass balance on a spherical oil sands aggregate results in the equation
in terms of k , the mass transfer coefficient. Here, CA is the mass concentration of bitumen in the main solution, CAOis the equilibrium mass concentration of bitumen in the solution at the interface between the oil sands particle and the fluid, ,C is the density of the bitumen, and el and e2 are, respectively, the fraction of the exposed aggregate surface which is bitumen and the volume fraction of bitumen in the aggregate. In the case of a large particle and a high Reynolds number (1 C Re C 450, Sc < 250) (Hughmark, 1969) and for larger particles (>1500 pm) (Nienow, 1975)
k
a
U:I2/R1l2
Ut
R112
(2) (3)
Here
k = &-‘I4 (4) where a is a dimensional proportionality constant. Substitution of eq 4 into eq 1 yields the equation
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Table I. Equilibrium Contact Time for Various Solvents in Stirred Vessel' ~ , reciprocal b of equil contact time, equil contact solvent min-' time, min benzene 0.166 6.02 toluene 0.154 6.49 Gulfaol 2329 0.147 6.80 Gulfaol 3139 0.132 7.58 kerosene 0.122 8.20 av 0.144 av 7.00
Water
Bitumen f I I In-
Radius of the aggregate
'Interpreted results from Cormack et al. (1977). *From Figure
Figure 1. Schematic diagram of oil sands aggregate.
3.
Based on the analysis of Cormack et al. (1977), eq 5 may be solved for the condition CA toluene > Gulfsol 2329 > Gulfsol 3139 > kerosene. The equilibrium contact time for kerosene is 1.4 times that for toluene. On the basis of the time for complete rather than 85% dissolution, Cormack et al. (1977) reported that kerosene required 3.5 times the contact time needed by toluene. However, the time for complete dissolution depends on successive sampling and analysis, and the final extraction efficiency for many of their experiments does not differ much from 85%. Since it is difficult to determine accurately the time for complete extraction, the 85% criterion is used here. Correlations of the relative mass transfer coefficient (normalized with respect to kerosene) with the aromatic content and the boiling temperature of the solvent are examined in Figures 5 and 6. The results indicate that aromatic compounds are better solvents for bitumen, as suggested by Mitchell and Speicht (1973), and that the relative mass transfer coefficient decreases with increasing boiling temperature of the solvent. It should be noted, however, that these correlations are not necessarily independent of one another, since aromaticity and boiling temperature may be interdependent.
The extraction curve shows a break at about 5 min (Figure 3). The amount of bitumen left in the oil sand at the breakpoint depends on the type of solvent. The amount of bitumen left at the breakpoint is plotted against the aromaticity and the diffusivity of the solvent in Figure 7 . As before, it should be noted that the aromaticity and the diffusivity may be interdependent. The diffusivities of the solvents in Athabasca bitumen are taken from the work of Fu and Phillips (1979). Because Gulfsol 2329 and Gulfsol 3139 are mixed solvents, no diffusivity data are reported for them. The results suggest that diffusion of solvent into the aggregate may become important in the late stages of extraction since diffusion is a slow process, and the degree of extraction beyond the breakpoint increases very slowly with time. B. Stirrer Speed. The speed of agitation is important in the mass transfer process. Terminal velocity theory suggests that the transfer coefficient depends on the square root of stirrer speed. Harriott (1962) reports that the transfer coefficient is proportional to the power of the turbine speed for particles larger than 100 pm. It is therefore appropriate to plot the relative mass transfer coefficient against the square root of stirrer speed. Figure 8 shows the variation of ko (normalizedwith respect to the same solvent, at a stirrer speed of 125 rpm) with the stirrer speed. (The ratio ko/ko,lzsrpm for a given solvent at the same oil sand to solvent ratio and the same initial size distribution is equal to the ratio PlP,,, rpm.) In Figure 8 each square represents an average of five stirred tank experiments, one for each solvent. The circles represent single experiments for benzene. The results show that the mass transfer coefficient is a linear function of the square root of the stirring rate. For benzene and the average result of the five solvents, the mass transfer coefficient approaches zero at stirring speeds of 31 and 10 rpm, respectively. These stirring speeds correspond approximately to the stirrer speed required for complete suspension (minimum stirrer speed). Estimates may be made through Zweitering's equation (1958) to calculate the minimum stirrer speed. The minimum stirrer speed is estimated to be 36 rpm for all solvents if the minimum
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28
24
z
20
N 0
r \
’6
0
2
12
Re
Figure 10. General results for rotating contactors as a function of the Reynolds and Froude numbers (Deiber and Cerro, 1976). The solid rectangular line shows the mapping of operating conditions used in this work.
(Stirrer speed )”‘,
( RPM)”‘
Figure 8. Effect of stirrer speed on relative mass transfer coefficient (normalized with respect to the same solvent at 125 rpm): ( 0 ) benzene, (w) average for five solvents (data from Cormack et al. (1977)). Correlation coefficient for benzene = 1.0; average = 0.95.
0
Table 11. Correlation of Relative Mass Transfer Coefficient, ko/ko,l:l, with Oil Sand to Kerosene Ratio (rpm = 30, X = 2) oil sand to kerosene ratio, w/w re1 mass transfer coeff ko/ko,l:. 1:l 1 1:3 0.969 1:4.6 0.805
of the flow (Figure 10). Three regions were identified: an oscillating boundary layer region, a falling film region, and a discontinuous solutions region. The discontinuous solutions region can be classified into two types, transition type I (dripping or cascading) and transition type I1 (viscous lifting). The flow field of a rotating contactor with lifters is slightly different due to the enhancement of the inertial force provided by the lifters. The system can first be treated in the absence of lifters. In this case, the system used falls into the discontinuous solutions region shown by the rectangular solid lines in Figure 10. The Reynolds number is defined as
Aromaticity, %
Figure 9. Change in mass transfer coefficient with agitation speed as a function of aromaticity of the solvent (normalized with respect benzene, (w) toluene, (0) to the same solvent at S = 125 rpm): (0) Gulfsol 2329, ( 0 )Gulfsol 3139, (A)kerosene (all solvents in stirred vessel (Cormack et al., 1977)).
stirrer speed for benzene is taken at 31 rpm. The effect of agitation rate for each solvent is shown in Figure 9. From Harriott’s data (1962), dko/k0,260pm/d[(S/280) r ~ m ] l is/ ~calculated to be 1.37. Compared to 1.37, the value of dkO/k0,126,/d[(S/125) r ~ m ] lfor / ~ the solvents presented here is t i e same order of magnitude. The speed effect is less for kerosene than for toluene due mainly to the lower aromaticity of kerosene. At a given rpm, toluene dissolves bitumen more rapidly. 11. qotating Contactor. A rotating contactor fitted with lifters was used with kerosene as the solvent. The data from the rotating contactor are analyzed by plotting (1-’t)1/2 against the contact time. Each extraction curve is normalized by multiplying the contact time by 8, the reciprocal of the equilibrium contact time. Typical data are plotted in Figure 4. The results from both the rotating contactor and the stirred vessel fall on the same straight line. The hydrodynamics of the rotating drum are very different from those of the stirred vessel. Deiber and Cerro (1976) characterized the flow behavior of rotating contactors by the Reynolds number and the Froude number
R e = ub2 - - - inertial force v
viscous force
in which b is the mean film thickness of the liquid when uniformly distributed around the inside periphery of the cylindrical contactor. The Froude number is defined as
Fr
inertial force gravitational force
The presence of lifting devices results in an increase in the inertial force. Both Re and Fr increase. The increases in Re and Fr tend to shift the rectangular solid region in Figure 10 to the right and upward toward the transition type I region. A. Oil Sand to Solvent Ratio. Extraction performance in terms of the relative mass transfer coefficient (equal to /3/P,, w/w-l) is shown as a function of the oil sand to solvent ratio in Table 11. The rotational speed is constant at 30 rpm. An increase in the relative mass transfer coefficient is experienced as the oil sand to kerosene ratio increases. This increase is due in part to the particle-particle interaction which is enhanced by the tumbling action of the contactor. Particle-particle interaction may cause bitumen removal in addition to the convective diffusion processes. Softening effects caused by the solvent in the bitumen may have a significant effect on the process. For the stirred tank experiment, the effect
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Table 111. Correlation of Relative Mass Transfer Coefficient, kO/k0,30 rpm, with the Rotational Speed (A = 2) at Various Oil Sand to Kerosene Ratios re1 mass transfer coeff __I_.--- rum wlw = 1:l w / w = 1.46 30 56 76 41
i
1
0.94‘3
1.058 0.359 0.651
0.6’76 1 63’7
of oil sand to solvent ratio is not significant a t times less than 5-10 min since very dilute solutions were used, ranging from 1:6 to 1:16.2 w/w for kerosene. In such dilute solutions little or no particle-particle interaction is likely. For the rotating contactor, the average p (0.0237 min-’1 for the three oil sand to kerosene ratios (ranging from 0.22 to 1:O w/w) is lower than that for kerosene in the stirred tank experiment (0.122 min-’ in Table I). However, the operating conditions are different. For the rotating contactor, the rotational speed is low (30 rpm), whereas for the stirrer in the stirred tank, it is high (300 rpm). Another reason for the low mass transfer coefficient in the rotating contactor is the fact that a t 30 rpm the particles may not be fully suspended. Strictly, comparisons between the two systems should be at equal power consumption rather than a t equal rpm. However, due to lack of data on power consumption, comparisons had to be made on the basis of rpm. B. Rotational Speed. Both the Reynolds number and Froude number increase with the rotational speed. Since the Reynolds number and the Froude number determine the flow characteristics of the rotating contactor, the rotational speed directly affects the flow condition. Table I11 shows the dependence of the relative mass transfer coefficient on the rotational speed. The relative mass transfer coefficient is not proportional to the (rotational speed)li2as is the case for stirred tanks. A minimum mass transfer efficiency occurs at a rotational speed of about 70 rpm, which corresponds roughly to the change of the flow regime from transition type I1 to transition type I. If the rotational speed continues to increase, the “rimming” speed is reached, corresponding to the upper limit of the rate of rotational speed. The “rimming” speed is estimated to be 250 rpm in the present situation. Similarity in the limiting stirrer speeds (or rotational speed) exists between the flow conditions generated in the rotating contactor and those in the stirred tank. For the latter, a minimum stirrer speed exists at which the particles are just completely suspended; the upper limit of the stirrer speed corresponds to the onset of aeration. Conclusions The convective diffusion mass transfer process is the most important mechanism involved in the early stages of solvent extraction of bitumen from Athabasca oil sands in dilute systems (
