Operational Parameters, Evaluation Methods, And Fundamental

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Operational Parameters, Evaluation Methods, And Fundamental Mechanisms: Aspects of Nonaqueous Extraction of Bitumen from Oil Sands Xingang Li,†,‡ Lin He,† Guozhong Wu,† Wenjun Sun,† Hong Li,†,‡ and Hong Sui†,‡,* †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Centre for Distillation Technology, Tianjin 300072, China

‡

ABSTRACT: Nonaqueous solvent extraction is a promising technology for bitumen recovery from oil sands. In this study, influences of temperature, contact time, stirring rate, and solvent-to-oil sands ratio (V/M) on bitumen recovery, using a mixture solvent of n-heptane and toluene (V/V, 3:1), were investigated by L9 (34) orthogonal design. Under the orthogonal experiment conditions, the overall impacts of factors were ranked: V/M > stirring rate > contact time > temperature. Profiles of bitumen fractions (saturates, aromatics, resins, and asphaltenes) in the dissolved bitumen, suspended particles, and residual bitumen were investigated in single factor experiments. Asphaltenes have higher temperature sensitivity than other fractions. About 3−7 wt % bitumen particles coexisted with clay minerals (50−70 wt % of the suspended particles) suspended in the solution, most of which were composed of asphaltenes. Approximately 75−90 wt % of SAR fractions (saturates, aromatics, and resins) were dissolved in the composite solvent under the experimental conditions. The amount of residual fractions varied with conditions, and multistage extraction enhanced the bitumen recovery by up to 99%. The evaluation method for bitumen recovery based on dissolved fraction outperformed the methods based on the sum of dissolved and suspended particles. The conceptualization of the solvent extraction process in this study would improve the knowledge base for bitumen recovery mechanism and serve for future work on the engineering applications.

1. INTRODUCTION The worldwide distributed oil sands (or tar sands1), naturally occurring mixtures of sand grains, water, clay materials, and a dense and extremely viscous form of petroleum,2−4 are an unconventional energy source with huge reserves (about 650 and 170 billion barrels technically recoverable5 all over the world and in Canada, respectively6,7). It is reported that about 1.5 million barrels crude bitumen from oil sands processing are produced per day in Alberta,8 and the National Energy Board (NEB) forecasted that the production of oil sands bitumen will increase up to 2.8 million barrels per day in 2020.9 Many schemes for the separation of minable oil sands have been proposed over the past decades, including hot water extraction, hydrocarbon solvent extraction, pyrolysis, and solvent extraction enhanced by microwave, ionic liquid, ultrasonication, and supercritical fluid.1,3,7,10−18 The most common commercial technology for minable oil sands is hot water extraction because of its convenient operation. However, the issues of water pollution and extraordinary high-energy consumption need to be addressed.19,20 To circumvent these drawbacks, solvent extraction has been proposed as an alternative method because (i) little or no water is required, which avoids the subsequent treatment of tailing ponds, (ii) bitumen recovery may theoretically approach 100%, (iii) it works at ambient pressure and temperature, and (iv) it is a relatively clean process with tailings containing little residual solvent and chemical additives.17,21 In addition, the problem of bitumen transportation could be solved due to the addition of solvent during the extraction operation, which reduces the viscosity of the bitumen.21,22 However, the nonaqueous solvent extraction has not been used commercially in industry due to © 2012 American Chemical Society

the technical difficulty of solvent recovery from tailing sands.20,23 Some research work have been devoted to the influence of operational parameters (i.e., stirring rate, contact time, solventto-solid ratio) on bitumen recovery and the mass transfer coefficient of bitumen during solvent extraction.17,24 However, information on the significance level of each operational parameter on bitumen recovery is lacking, which makes decision making difficult for industrial applications. For example, temperature was observed to influence the hot water extraction efficiency by reducing bitumen viscosity, however, few studies have been carried out so far to investigate its influence in a nonaqueous solvent extraction process. Additionally, it is unclear to what extent the temperature would influence the precipitation of asphaltenes in solvent, which limits our understanding of the mass transport of bitumen during solvent extraction.17,25−27 Current studies often focus on bitumen recovery, but the effects of operational parameters on bitumen quality are rarely reported. Generally, bitumen could be divided into four fractions as saturates, aromatics, resins, and asphaltenes (SARA),28,29 which vary in polarity, solubility, density, molecular weight, and elemental composition.28−31 The SAR (saturates, aromatics, resins) fractions could be easily dissolved in paraffins (i.e., heptane) and upgraded downstream, while asphaltenes (the heaviest fraction in the bitumen) may cause hindrance to the transportation pipes, upgrading facilities and Received: February 27, 2012 Revised: May 8, 2012 Published: May 9, 2012 3553

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downstream upgrading by flocculation, deposition, coking, and poisoning catalyst.32 Therefore, the content of asphaltenes in the bitumen for upgrading should be as low as possible. Previous studies associated with solvent extraction mechanisms considered the complex bitumen as one pseudomaterial,16,17,22,33 while information on the fate and transport of each fraction is inadequate. For example, it is unclear about the kinetic factor limiting the strip of bitumen from the sand grains and the profile of bitumen fractions dissolved into the solvent. To the best of our knowledge, there are few standards available for the evaluation of bitumen recovery from oil sands. Normally, the bitumen recovery was determined by the dissolved bitumen in the solvent or the residual bitumen in the treated sands. For the latter, the suspended bitumen (particles with clays attached20) is included. However, studies are needed to quantify the composition because the extracted bitumen contains quantities of asphaltenes that are poor products with low upgradability and high transportation cost due to high viscosity. An evaluation method based on the bitumen fractionation would help to understand the composition of the products and the bitumen quality. The environmental impacts of solvent extraction process concerns the residual bitumen in the treated sands. Chemicals in the bitumen may be converted into hazardous materials such as polymer aromatics hydrocarbons (PAHs)19,33 and naphthenic acids (NAs).34−38 PAHs are toxic, mutagenic, and carcinogenic.33 NAs, widely distributed in each bitumen fraction,34 are harmful for the aquatic species when exposed in the environment.37 To date, few studies have been done to assess the environmental impacts of the oil sands after solvent extraction. There is a lack of assessment criteria, diagnostic methods, and frameworks for the recognition and minimization of environmental issues. This study aimed to (i) determine the significance level of different operation factors (i.e., solvent-to-oil sands ratio, contact time, stirring rate, and temperature) by multifactor orthogonal experiments; (ii) identify the influences of operational parameters on the recovery of bitumen fractions and the mechanism of solvent extraction; (iii) compare different methods for the evaluation of bitumen recovery.

Table 2. L9 (34) Orthogonal Matrix and Statistical Data impact factors

expt. 1 2 3 4 5 6 7 8 9 K1j K2j K3j kja K1j/kj K2j/kj K3j/kj Rj Sjb f jc Vjd F-ratios Fe optimal level optimal organization

expt. 1 2 3 4

Level 2

3

unit

temp. stirring rate contact time solvent-to-oil sands ratio (V/M)

A B C D

25 200 2 1

40 400 10 2

55 600 30 10

°C rpm min mL·g−1

D (V/M)

2 10 30 10 30 2 30 2 10 197.0 198.2 218.4 3 65.7 66.1 72.8 7.15 96.5 2 48.3 20.6 19.0 3

1 2 10 10 1 2 2 10 1 190.4 197.4 225.9 3 63.5 65.8 75.3 11.8 235.7 2 117.9 50.4 19.0 3

bitumen recovery percentage, % 55.6 62.4 85.3 67.6 66.6 68.6 66.5 72.9 68.2

temp., °C 25, 55, 95 55 55 55

contact time, min

stirring rate, rpm

solvent-to-oil sands ratio, mL·g−1

10

600

10

2, 10, 60 10 10

600 200, 600, 800 600

10 10 2, 10, 15

mesh sieves to remove plant remains and clastic rocks before physicochemical characterization. 2.2. Optimization of Solvent Extraction Conditions. The solvent extraction conditions were optimized using multifactor orthogonal design followed by single factor experiments.39 Four parameters, including solvent-to-oil sands ratio (V/M), temperature, stirring rate, and contact time, were selected at three levels using L9(34) orthogonal array (Table 1). Generally, the number of full factorial experiments required to run is 34 = 81, which was reduced to 9 by orthogonal design, offering a great advantage in terms of experimental time and cost. Three statistical coefficients (K, R, and Fratio) were used to evaluate the orthogonal data. K is the sum of the percentages of oil sand relative bitumen recovery for each impact factor at each level, which was used to assess the optimal level of each factor so as to determine the optimum combination of the experimental conditions. The higher the K value, the higher solvent extraction efficiency of the factor considered is. The extreme difference R is a parameter representing the fluctuation degree of the extraction efficiency in accordance with the variety level of impact factor. A larger R value indicates more significant influence of the corresponding factor on the results. F-ratio was applied to evaluate whether the impact

Table 1. Parameters and Levels of the Orthogonal Experiments 1

25 200 25 400 25 600 40 200 40 400 40 600 55 200 55 400 55 600 203.3 189.6 202.7 202.0 207.6 221.9 3 3 67.8 63.2 67.6 67.3 69.2 74.0 1.6 10.8 4.7 177.0 2 2 2.3 88.5 1 37.8 19.0 19.0 3 3 D3 B3 C3A3

C (contact time)

Table 3. Single Factor Experiment Conditions

2.1. Chemicals and Samples. Toluene, n-heptane, trichloroethylene, methanol, and neutral alumina (100−200 mesh) were

symbol

B (stirring rate)

a The number of the appearance for a specific level. bSum of deviation square, Sj = kj∑(K2j/kj − y*)2; y* = ∑yi/9. cDegree of freedom, f j = the number of appearance of a specific level − 1. dVariance, Vj = Sj/f j. e Critical F value, confidence level: 95%.

2. EXPERIMENTAL SECTION

impact factor

A (temp.)

purchased from Tianjin Jiangtian Technology Co. Ltd., China. The composite solvent was made up using n-heptane and toluene at a volume ratio of 3:1. This ratio was selected because the ratio of asphaltenes (insoluble in n-heptane but soluble in toluene) to the remaining fractions (soluble in n-heptane) in the bitumen was approximately 1:3. All the solvents used were analytical grade. Athabasca oil sands samples (∼10.9 wt % bitumen) were obtained from Alberta, Canada, and were homogenized by screening through 22 3554

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Figure 1. Influence of (a) solvent to oil sands ratio, (b) stirring rate, (c) contact time, and (d) temperature on bitumen recovery.

Table 4. Bitumen Distribution and the Percentage of Solids in the Dissolved Bitumen and Suspended Particles bitumen distribution, wt % factors temp., °C

contact time, min

stirring rate, rpm

solvent-to-oil sand ratio, mL·g−1

percentage of solids in products, wt %

levels

dissolved

suspended

residual

suspended particles

dissolved bitumen

25 55 95 2 10 60 200 600 800 2 10 15

69.0 72.2 87.0 64.9 72.2 80.1 65.8 72.2 72.7 63.2 72.2 73.0

4.3 3.7 5.1 5.5 3.7 3.7 3.4 3.7 4.6 7.7 3.7 4.4

26.7 24.1 7. 9 29. 6 24.1 16.2 30.7 24.1 24.6 29.2 24.1 22.5

68.1 66.1 67.1 48.0 66.1 75.9 67.4 66.1 60.8 60.6 66.1 58.8

11.1 8.7 10.1 6.7 8.7 12.2 9.2 8.7 8.4 14.3 8.7 7.5

factors were statistically significant or not, which was calculated using the method previously described by Gonder et al.40 and then compared with the critical F value, which could be found in most of the statistics and experimental design books.41 Briefly, oil sands samples (3.0 g) and composite solvent (30 mL) were weighted into a centrifuge tube (50 mL), which was agitated in a magnetic blender (DF-101S, Corey Gongyi Instrument Co., Ltd.; temperature fluctuation: ± 0.1 °C) under the designed conditions (Table 2). The mixture was centrifuged at 5000 rpm for 5 min, and the supernatant was transferred into a flask. The bitumen and composite solvent were separated by distillation (distillation speed: 5 mL·min−1). The distillation residue was vacuum-dried at 80 ± 1 °C and 20 ± 0.5 kPa (absolute pressure) until insignificant mass loss was observed. To gain insights into the influence of factors on the recovery of each bitumen fraction, the optimal operation parameters determined by the orthogonal experiments were used in the subsequent single factor experiments. 2.3. Single Factor Experiments. Composite solvent with the designed volume (Table 3) was added into a flask containing oil sands (5.0 g). After agitation in the magnetic blender, the mixture was retained for about 1 min, when three products including (1) solution, (2) suspended particles, and (3) residual sands were obtained. To determine the concentration of total bitumen and each bitumen fraction in the products, analysis samples were prepared as follows: The solution (product 1) and suspended particles (product 2) were transferred to a centrifugation tube. After centrifugation at 5000 rpm for 5 min, the supernatant was transferred into a weighted flask. The residue was ultrasonically extracted using toluene (15 mL) in triplicate, which was followed by distillation and vacuum-drying at 80 ± 1 °C and 20 ± 0.5 kPa (absolute pressure) until insignificant mass loss was

observed. The same procedures, including centrifugation, distillation, and vacuum-drying, were performed to the supernatant. The residual sands (product 3) were ultrasonically extracted using toluene (15 mL) in triplicate, which was followed by distillation and vacuum-drying at 80 ± 1 °C and 20 ± 0.5 kPa (absolute pressure) until insignificant mass loss was observed. The mineral solid in the suspended particles were separated by ultrasonic extraction as follows: the suspended particles obtained by centrifugation were extracted by toluene under the condition of ultrasonic extraction (10 min once for three times at ambient temperature) in the centrifuge tube. Then, the mixture was centrifuged at 5000 rpm for 5 min. The procedures including ultrasonic extraction and centrifugation were repeated in triplicate. The residual solid was oven-dried at 110 °C until insignificant mass loss was observed. With this method, the bitumen content and mineral solid content in the suspended particles and the extracted bitumen were measured. 2.4. Analytical Method. The concentration of total bitumen in the oil sands was determined by the mass difference between the initial oil sands and those after solvent extraction. Fractionation of bitumen into four fractions (i.e., saturates, (naphthene) aromatics, resins (or polar aromatics), and asphaltenes) was carried out using ASTM D4124.42 Briefly, bitumen was dissolved in n-heptane and separated into soluble and insoluble fractions (asphaltene). The soluble fraction was loaded on the top of neutral alumina column. The saturate fraction was eluted with 100 mL heptanes followed by the aromatic fraction with 100 mL toluene. The resin fraction was eluted with 100 mL mixture of methanol and toluene (V/V, 1:1) followed by 100 mL trichloroethylene. 3555

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Figure 2. Influence of (a) temperature, (b) contact time, (c) stirring rate, and (d) ratio of solvent-to-solid on the profile of SARA in the dissolved bitumen (lines, saturates; light gray, aromatics; dark gray, resins; black, asphaltenes; ▲, dissolved bitumen).

3. RESULTS AND DISCUSSION 3.1. Multifactor Orthogonal Experiments. Results of orthogonal experiments are shown in Table 2. The extreme difference analysis showed that the highest K value for each impact factor was at the third level. Therefore, the optimal operating conditions were determined as solvent extraction for 30 min at 600 rpm and 55 °C with the V/M of 10 mL·g−1. The R values of parameters were ranked: V/M > stirring rate > contact time > temperature (Table 2). The most significant factor influencing solvent extraction efficiency was V/M (F = 50.4), followed by stirring rate (F = 37.8) and contact time (F = 20.6). Temperature had an insignificant influence on extraction efficiency as the F-ratio (1.0) was much lower than the critical F-value (19.0). The main effects of factors on the overall bitumen recovery are shown in Figure 1. As expected, the bitumen recovery increased with the increase of V/M and stirring rate (Figure 1, parts a and b). The bitumen recovery did not change in the first 10 min, but increased by 7.2% after 30 min (Figure 1c). Figure 1d further evidenced the insignificant influence of temperature. A previous study24 demonstrated the mass transfer resistance in solvent extraction was predominantly at the bitumen layer when n-pentane was used; however, the solvent−bitumen interface was the main compartment limiting mass transfer when toluene was used. Since the composite solvents used here

were made up with n-heptane and toluene (V/V, 3:1), we deduced that the mass transfer resistance may be equivalently significant at the bitumen layer and solvent−bitumen interface during the bitumen extraction. 3.2. Single Factor Experiments. Results indicated that higher values of the four factors were favorable to the bitumen recovery (Table 4). Particularly, the suspended particles comprised of undissolved bitumen (30−50 wt %) and some mineral solids (50−70 wt % of the suspended particles or 7−12 wt % of the extract), which are clay materials.20,43,44 The mineral solids may be attached with the suspended bitumen fractions (i.e., asphaltenes) through strong interactions.45 Results also indicated that higher temperature and stronger stirring rate were beneficial for the separation of clays from bitumen, leading to more bitumen soluble in the solution. 3.2.1. Profile of Four Fractions in the Extracted Bitumen. Figure 2, parts a and b, indicated that asphaltene was more sensitive than other fractions to temperature and contact time, because its relative content in the bitumen increased by 11.8% as temperature increased from 25 to 95 °C and increased by 7.3% when contact time increased from 2 to 60 min. The high temperature sensitivity may be evidenced by the facts that (i) a higher temperature promoted the dissolution of monomer asphaltene46 and (ii) the relative content of SAR fractions decreased by 11.8% (Figure 6), although their absolute 3556

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Figure 3. Influence of (a) temperature, (b) contact time, (c) stirring rate, and (d) ratio of solvent-to-solid on the profile of asphaltenes in the suspended bitumen particles.

extraction under the conditions optimized by orthogonal experiments. 3.2.2. Asphaltenes in the Suspended Bitumen Particles. Results indicated that the relative content of asphaltenes in the suspended bitumen particles decreased by 35.5% as temperature increased from 25 to 95 °C (Figure 3a), which was opposite to the trend observed in the extracted bitumen (Figure 2a). This finding may be attributed to the activity sites of asphaltenes, which are functional groups that can be used to link with similar molecules.46,47 A high temperature broke down the large asphaltenes particles into small molecules, which provided more active sites available for the adsorption of small bitumen fractions (i.e., resins and aromatics).46 Therefore, the relative content of asphaltenes decreased, while that of other fractions increased in the suspended bitumen. As expected, the proportion of asphaltenes in the bitumen particles increased significantly with the increase of contact time and V/M (Figure 3, parts b and d). It is expected that a high stirring rate would facilitate desorption of light fractions from the asphaltenes, resulting in the increase in the proportion of asphaltenes in the suspended bitumen particles (Figure 3c). However, the results demonstrated that the stirring rate had relatively insignificant influence on the relative content of

dissolution amount increased (Table 4). The significant time sensitivity might be attributed to the fact that the continuous stirring resulted in the release of the monomer asphaltenes that were initially aggregated to form large asphaltene particles by intermolecular force.47 However, the increase in the relative content of asphaltenes in the extracted bitumen would reduce the bitumen quality,32 which is adverse for the downstream upgrading process. Results also demonstrated the insignificant influences of V/ M and stirring rate on the relative content of four fractions in the extracted bitumen (Figure 2, parts c and d). Despite of the different treatments, the proportions of saturates, aromatics, resins, and asphaltenes were 22.5%, 35.3%, 28.7%, and 13.5%, respectively. The results suggested that the recovery of total bitumen could be promoted by increasing the value of factors (i.e., temperature, contact time, V/M, and stirring rate). However, it is not recommended to carry out nonaqueous solvent extraction at high temperature (i.e., 95 °C) and long contact time (i.e., 60 min) because of the relative high content of asphaltenes in the bitumen, which would reduce the recovered bitumen quality. Thus, it is recommended to operate the 3557

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Figure 4. Influence of (a) temperature, (b) contact time, (c) stirring rate, and (d) ratio of solvent-to-solid on the profile of SARA in the residual bitumen (lines, saturates; light gray, aromatics; dark gray, resins; black, asphaltenes; Δ, residual bitumen).

asphaltenes in the bitumen particles. This implied the shearing force provided by stirring rate (≤800 rpm) was inadequate to break the bonding between asphaltenes and light fractions (especially resins). It is known that light fractions (i.e., saturates, aromatics and resins) are upgradable, while it seems difficult to upgrade asphaltenes into lighter fractions in the downstream. Because the suspended bitumen particles are mainly asphaltenes, which are adverse for transportation and catalysis,32 the light fractions adsorbed on the asphaltenes need to be separated from suspended particles. For this purpose, treatments such as filtration are required before transportation and upgrading. 3.2.3. Profile of Four Fractions in the Residual Bitumen. Figure 4 showed the influences of operational conditions on the relative contents of SARA fractions in the residual bitumen. The relative content of asphaltenes decreased from 41.4 to 23.1 wt % as temperature increasing while that of resins and aromatics varied slightly (Figure 4a). When temperature increased to 95 °C, insignificant difference was found between the relative contents of the four fractions. This suggests that most bitumen components have stripped from the surface of sand grains and redistributed homogeneously around the sand grains. One possible reason was that the high temperature weakened the bond force between fractions, which resulted in the dissolution (Figure 2a) of SARA fractions in the solvent.

Results showed that the concentration of bitumen in the residual oil sands decreased by 45.2% when contact time increased from 2 to 60 min. The proportion of asphaltenes in the residual bitumen decreased as time increased (Figure 4b), which appears inconsistent with previous studies24 where a long extraction time enhanced the recovery of soluble fractions and therefore increased the proportion of insoluble fractions in the residual bitumen in oil sands. In this study, because the surface of the sand grains is normally hydrophilic or covered by an aqueous film,48 it is thermodynamically easier for the sand to absorb the polar substances than low-polar materials when they were dipped into the hydrophobic hydrocarbon solvents (i.e., nheptane and toluene). With the assistance of continuous stirring, sand grains would be surrounded by the bitumen fractions whose polarities were higher than that of solvent. On the other hand, large asphaltenes particles would form by the association of monomer asphaltene,44,49,50 which were expected to strip from the surface of sands and resulted in the rearrangement of fractions around the sands surface. Consequently, the relative content of asphaltenes in the residual bitumen decreased, while that of the other fractions increased. Figure 4c showed that bitumen remaining in the oil sands decreased by 19.9% when the stirring rate increased from 200 to 800 rpm. Moreover, a high stirring rate resulted in a high proportion of asphaltene fraction in the residual bitumen in oil sands, which suggested that the effects of stripping and 3558

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Figure 5. Bitumen recovery determined by three evaluation methods.

dissolution resulting from enhanced shear by stirring was more significant for soluble fractions than for insoluble fractions. Similarly, the increase of V/M from 2 to 15 mL·g−1 resulted in a 27.3% increase of asphaltene, a 14.5% decrease of resin, and insignificant changes of saturates and aromatics in residual bitumen in oil sands (Figure 4d). 3.2.4. Evaluations of the Solvent Extraction Efficiency Applied in Oil Sands Processing. Figure 5 shows the bitumen recovery determined using three different evaluation methods based on (i) the dissolved bitumen in the solvent (DS); (ii) the total bitumen recovery, which is the sum of the dissolved and undissolved but suspended bitumen in the solvent (RS), and (iii) the dissolved SAR fractions in the solvent (RL), respectively. Generally, bitumen recovery assessed by RS was 3−7% higher than that assessed by DS because the suspended bitumen particles were included in the former method. It is recommended to use DS for evaluating the ungradable recovered bitumen if the suspended particles need to be taken into account because of its harmful effects, but RS would be more preferable for the assessment of total bitumen recovery. Information on bitumen recovery determined by RS and DS was inadequate to assess the mass transfer of each bitumen fraction because they consider the bitumen as a whole. Figure 5 indicated that about 75−90% SAR fractions were

extracted despite of the conditions, while a portion of the asphaltenes was extracted, also concluded from Figure 2. This finding tells us that the RL provides more information on the extracted bitumen with which we could evaluate the quality of bitumen more clearly. 3.2.5. Environmental Impacts of Solvent Extraction. Temperature had a slight effect on the amount of the residual fraction in the range from 25 to 55 °C (Figure 6a). However, the residual SAR fractions and aromatics decreased by 61.5% and 63.0%, respectively, when the temperature increased from 55 to 95 °C. On the other hand, the temperature increase resulted in a rapid decrease of asphaltenes in the amount of the residual bitumen. These results indicated that high temperature was beneficial for the removal of the residual bitumen (especially asphaltenes). The contact time caused positive effects to the reduction of the residual fractions (Figure 6b). The amount of residual asphaltenes and aromatics decreased by 81.8% and 15.9% after 60 min, respectively. The residual SAR fractions changed slightly as time increased. The increase in stirring rate facilitated the reduction of residual SAR fractions (Figure 6c). The residual SAR and aromatics decreased by 27.4% and 19.1%, respectively, when the stirring rate increased from 200 to 800 rpm. This demonstrated the possibility of removing the residual materials by increasing the stirring rate. 3559

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Figure 6. Influence of (a) temperature, (b) contact time, (c) stirring rate, and (d) ratio of solvent-to-solid on the profile of the amount of asphaltenes, SAR fractions, and aromatics in the residual materials.

4. MECHANISM OF SOLVENT EXTRACTION PROCESS

Similarly, the increase in V/M caused a significant reduction in the residual SARA fractions (Figure 6d), because the more the solvent used, the more the soluble fractions (SAR) dissolved. The amounts of the residual SAR fractions and aromatics decreased by 48.4% and 41.4%, respectively, when V/M increased from 2 to 15. On the contrary, the content of asphaltenes in the residue increased with the increase of solvent volume. These phenomena suggested that it was feasible to reduce the amount of residual SAR factions by increasing V/M. Overall, the residual bitumen in the treated sands ranged from 9510 to 36970 mg·kg−1 under the experimental conditions (Figure 6), which was much higher than the “threshold level” (when no further reduction is necessary) of 50 mg·kg−1 in the U.K. regulations.51,52 Therefore, it is inappropriate to discard the treated oil sands directly. The bitumen in the treated oil sands could be extensively removed by multiple stages solvent extraction. For purpose of comparison, we carried out multistage solvent extraction with solvent to solid ratio of 10 at 55 °C for 10 min, where the stirring rate was controlled at 600 rpm. Results indicated that the bitumen recovery was improved to 88.5% and 99.0% by duplicate and triplicate solvent extraction, respectively. Moreover, the resulted oil sand could be reused as an energy resource instead of waste.

The results allow the conceptualization of the solvent extraction process as follows (Figures 7 and 8): initially, oil sands were in contact with and submerged in the composite solvent, followed by mass transfer between the solvent and the oil sands bitumen.

Figure 7. Formation of asphaltene particles in solvent extraction process. 3560

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Figure 8. Mass transport of bitumen components during the solvent extraction process: (a) solvent molecules diffuse into bitumen layer; (b) soluble materials dissolve in solvent and diffuse back to the solution; (c) formation of porous network constructed by insoluble asphaltenes; (c2) formation of porous network constructed by insoluble asphaltenes and small molecules attached to the surface of asphaltenes.

(polar fractions) tend to stay on the surface of the sand grains forming a porous network structure.24 The porous network served as a compartment sequestrating the soluble fractions inside until reaching equilibrium with the liquid phase. The absorption of lighter fractions (or low polar materials) to the asphaltenes or asphaltene particles may increase the amount of soluble materials in the network at equilibrium. The diffusion of the dissolved fractions into the solvent layer was facilitated by stirring. Meanwhile, the porous network would be broken into small particles by shearing force, stripped

Because the viscosity of the solvent is much lower than that of the bitumen on the surface of the sand grains,53 the solvent molecules tend to diffuse into the bitumen layer, which resulted in the viscosity decrease of soluble bitumen fractions. The bitumen molecules dissolved in the solvent would diffuse back into the solvent layer until there was no concentration difference between solvent phase and bitumen layer. Additionally, the polarity of bitumen was higher than that of the solvent, but lower than that of the aqueous film covered on the surface of sand grains,48 therefore, the insoluble bitumen fractions 3561

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from the bitumen layer and suspended in the solvent phase. The formation of these particles increased the interfacial area between bitumen and solvent and thereby facilitated mass transfer. Consequently, most soluble fractions would be quickly dissolved, leaving the bitumen particles composed mostly by asphaltenes that are insoluble in the solvent. After separation from the sand grains, the suspended particles would reform new particles due to collision and precipitation, because asphaltenes tend to self-associate with the active sites and form colloidal aggregates or flocks46,47,49,54 (Figure 7). Unfortunately, the existence of sand grains and clays made it difficult at current stage to characterize the collision and size changing processes by instruments, which warrants future work focus on the micro-mechanism of the collision and particle size changing in solvent extraction of oil sands.

5. CONCLUSION This study clearly demonstrated the efficiency of bitumen recovery using composite solvent consisting of n-heptane and toluene (V/V, 3:1). The V/M and stirring rate were the most significant factors influencing bitumen recovery, while the impacts of contact time and temperature were insignificant under the conditions of orthogonal experiments. The recovery of asphaltenes was more sensitive to temperature and contact time than other fractions (i.e., saturates, aromatics, and resins). A large portion of SAR fractions (up to 30% of the total) was remaining in the residual bitumen despite of the treatments, suggesting that it was infeasible to remove all the bitumen from oil sands once. However, multistage extraction was demonstrated as an effective way to extensively remove the residual bitumen. Approximately 50−70% of the suspended particles are clay minerals, which should be removed by subsequent treatment in case of harmful impacts on the downstream processes and facilities. Results highlighted the use of the dissolved fractions for the evaluation of bitumen recovery other than using the sum of dissolved and suspended materials (mainly asphaltenes), which may overestimate the solvent extraction efficiency by 3−7 wt %.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding author E-mail: [email protected]; Phone: +86 022 27404701; Fax: +86 022 27404705. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Basic Research Program (No. 2009CB219905), National Hi-Technology Research & Development Program of China (2009AA063102), Program for Changjiang Scholars and Innovative Research Team in University “PCSIRT” (IRT0936), and Municipal Natural Science Foundation of Tianjin (Nos. 11JCYBJC05400 and 12JCQNJC05300).

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