Viscosity Changes during Mild Oxidation of Oilsands-Derived Bitumen

DOI: 10.1021/ef501341h. Publication Date (Web): September 23, 2014 ... Low-temperature oxidative asphaltenes liquefaction for petrochemicals: fact or ...
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Viscosity Changes during Mild Oxidation of Oilsands-Derived Bitumen: Solvent Effects and Selectivity José L. García Zapata and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada S Supporting Information *

ABSTRACT: The low-temperature oxidation of oilsands bitumen was investigated to determine how viscosity was affected by oxidation and whether oxidative hardening could be suppressed by solvent dilution. This work was performed to support the development of an oxidative desulfurization process, but it could also find application in processes for bitumen recovery by in situ low-temperature oxidation. The experimental investigation was conducted at 140−200 °C and near atmospheric pressure with oilsands-derived bitumen, air, and mesitylene as the solvent. Solvent dilution decreased the bitumen viscosity through diluent action, but it did not suppress oxidative hardening of the bitumen. In fact, with the presence of a solvent, the bitumen viscosity increased more than by oxidation of bitumen on its own. This could be explained in terms of easier hydrogen abstraction from bitumen relative to the solvent, which increased the probability of addition reactions. The increase in viscosity with oxidation extent was also investigated, and apparently conflicting reports in the literature were reconciled. At a constant temperature, different periods of near constant viscosity increase with the increase in the oxygen consumption were identified. It was also found that, during the first free radical chain-propagation-dominated oxidation period, the extent of viscosity increase was different at different oxidation temperatures for the same level of O2 consumption. Oxidative hardening is not just related to oxidation extent, but it is also affected by changes in oxidation selectivity because of the conditions at which the oxidation was performed. oxidation (LTO)7 and the use of oxidative conversion as pretreatment for in situ bitumen conversion by methanogenesis.8 The relevance of understanding the solvent effect during low-temperature oxidation of bitumen to multiple applications formed the justification for the present investigation. The impact of solvent dilution and oxidation severity on viscosity changes during mild oxidation of bitumen is reported. It was possible to decouple the contribution of the solvent as a diluent to reduce the viscosity of bitumen from its role as a medium modifier during oxidation. This led to some additional insights about the selectivity of the oxidation process, and seemingly conflicting reports in the literature on the topic could be reconciled.

1. INTRODUCTION Oxidative desulfurization (ODS) is an alternative processing pathway to hydrodesulfurization (HDS) for the removal of sulfur from petroleum. The two processes are complementary, because ODS is particularly effective in oxidizing thiophenic sulfur, which is more difficult to remove by HDS.1 Many investigations in the field of ODS of refinery streams in the distillate and vacuum gas oil boiling ranges can be found in the literature.2,3 Canadian oilsands-derived bitumen has a sulfur content in the range of 4.6 ± 0.5 wt % S.4 The possibility of removing sulfur from bitumen by an oxidative process rather than consuming large amounts of H2 during HDS was therefore of interest. In a proof-of-concept study, it was found that air oxidation in the temperature range of 145−175 °C followed by water washing resulted in 46−47 wt % sulfur removal from bitumen, which is equivalent to around 20 kg of S removed/ton of bitumen.5 No emulsion formation was found. However, it was found that the viscosity of the bitumen increased by an order of magnitude, which rendered the process impractical. Hardening of bitumen by air oxidation typically at 230−325 °C is widely practiced as a method to modify bitumen for road paving.6 The extent of the viscosity increase at the milder operating conditions was not anticipated. It was found that performing the autoxidation reaction in a hydrocarbon solvent at 145 °C resulted in a lower viscosity, but by doing so, the desulfurization extent was decreased to 18 wt %.5 The use of a solvent caused a trade-off between desulfurization and hardening that was not understood and that was not desirable. The observed behavior also has bearing on the development of in situ bitumen recovery processes that employ low-temperature © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. The experimental work was performed with Canadian Cold Lake bitumen provided by Imperial Oil. The Cold Lake bitumen was characterized (Table 1). Mesitylene or 1,3,5trimethylbenzene (98%) provided by Sigma-Aldrich was employed as the solvent. Mesitylene was selected as the solvent because it is completely miscible with bitumen, does not result in asphaltene precipitation, and is a liquid below its boiling point (Tb = 164.6 °C)9 at the oxidation conditions. The oxidation reactions were performed with air as the oxidant, which was supplied as compressed extra dry air by Praxair. 2.2. Equipment and Procedure. The apparatus employed for all oxidation reactions is shown in Figure 1. It consisted of a 250 mL Received: June 15, 2014 Revised: September 6, 2014 Published: September 23, 2014 6242

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When solvent was employed, the solvent was removed from the reaction mixture at the conclusion of the experiment. Solvent was removed using a Heidolph Hei-VAP Advantage ML/G3 rotary evaporator set at 150 °C and 10 kPa absolute for 1 h. 2.3. Analyses and Calibration. The viscosities of the bitumen and all products were measured at 60 °C. Prior to measurements, samples were allowed to deaerate for at least 24 h, but no further sample conditioning was performed. Because of equipment availability, measurements were performed with different equipment, namely, a Brookfield DVIII ultra rheometer using a probe 14 and an Anton Paar RheolabQC using a CC17 cylinder in cup. The use of each is indicated with the results. The instruments were calibrated using Brookfield 5000 cP (certified at 4.82 Pa s) and 30 000 cP (certified at 31.04 Pa s) commercial silicone fluid standards (Table 2). Both rheometers were

Table 1. Properties of Cold Lake Bitumen property viscosity at 60 °C (Pa s) density at 22 °C (kg m−3) density (deg API) elemental analysis (wt %) C H N S O (by difference) SARA analysis (wt %) saturates aromatics resins asphaltenes

bitumena 11.0 ± 0.15 1001 9.8 82.8 10.0 0.6 4.5 2.1 16.8 46.1 17.7 18.7

± ± ± ±

2.4 2.7 1.2 0.55

Table 2. Calibration of Rheometers viscosity at 25 °C (Pa s)

a

Reproducibility was indicated as one sample standard deviation of analyses in triplicate.

description

5000 cP oila

30 000 cP oilb

certified standard Brookfield DVIII ultra Anton Paar RheolabQC

4.82 4.88 4.72

31.04 31.03 30.82

a Evaluated at a shear rate of 20 s−1. bEvaluated at a shear rate of 15.2 s−1.

accurate to within 2% of the certified standards. The viscosity versus shear rate behavior of the bitumen was determined (see Figure S1 of the Supporting Information). For measurements in triplicate, the viscosity values over the shear rate range of 2−100 s−1 differed by 8% or less, with the highest viscosity values observed at a shear rate around 10 s−1. Elemental analyses of the bitumen feed (Table 1) and the product samples were performed by an external laboratory. The equipment used was a Carlo Erba EA1108 elemental analyzer, which determined the carbon, hydrogen, nitrogen, and sulfur contents of the samples. The oxygen content was determined by the difference. Density was determined by volume displacement in a liquid (water) of known density and is only accurate to within 10 kg m−3. The saturates, aromatics, resins, and asphaltenes (SARA) fraction of bitumen was performed according to the ASTM D2007 standard method10 but with the modification that aromatics were desorbed using a 50:50 solvent mixture of pentane and toluene before refluxing with toluene.

Figure 1. Experimental setup employed for oxidation studies. three-neck round-bottom flask, which was connected to an air supply line and a reflux condenser. The reflux condenser was maintained at 10 °C by chilled water. The temperature was controlled through an oil bath set on top of a Heidolph MR Hei-Standard hot plate equipped with a Heidolph EKT Hei-Con thermometer to control the oil bath temperature. The reaction mixture was magnetically stirred using a 25 mm stirrer bar at 500 rpm. The bitumen load was 80 g, and when solvent was used, 80 g was used as well. The actual mass used in each experiment was measured using a Mettler Toledo ML 3002 (3200 g capacity and 0.01 g readability). The mixture was preheated to the reaction temperature, and only after the temperature was reached, the supply of air flow was started. A typical oxidation experiment was conducted at 140 °C with an air flow rate of 75 mL h−1 g−1 of bitumen. The operating conditions are indicated for each experiment. Flow control of air was performed with a certified Riteflow rotameter connected on the inlet line. Outlet gas was analyzed online by an Alpha Omega Instruments Series 9600 CO2 and O2 analyzer to quantify the oxygen consumption and CO2 production. Prolonged oxidation (12 h) with multiple viscosity measurements was conducted as two experiments. The first experiment took 5 g samples from the 80 g bitumen originally loaded at hourly intervals from 3 to 6 h. The second experiment took 5 g samples from the 80 g bitumen originally loaded at hourly intervals from 7 to 12 h. The air flow rate was kept constant based on the original amount of bitumen loaded, but the calculation of oxygen consumption took the change in bitumen mass into account.

3. RESULTS 3.1. Impact of Solvent on Oxidative Viscosity Change. The first objective of the work was to quantify the difference in the bitumen viscosity after mild oxidation in the presence and absence of a solvent. The bitumen was oxidized under constant airflow with and without a solvent. After the solvent was removed from the samples that were oxidized with a solvent, the viscosities of the oxidized bitumen samples were determined (Table 3). Table 3. Viscosity of Bitumen after Autoxidation at 140 °C for 3 h at near Atmospheric Pressure and the Airflow Conditions Indicated viscosity at 60 °C (Pa s)a,b description −1

−1

oxidation with 75 mL h air g of bitumen oxidation without forced airflow

without solvent

with solventc

14.6 ± 0.42 12.9 ± 0.04

7.0 ± 0.85 4.5 ± 0.38

a Average and one sample standard deviation were reported. bViscosity was measured using a Brookfield rheometer at a shear rate of 20 s−1. c Measured after solvent removal at 150 °C and 10 kPa absolute.

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One variable that could not be manipulated and that was affected by the solvent was the bubble size of the air bubbles. The bitumen diluted with solvent had a lower viscosity and lower surface tension. Because the maximum stable bubble size is proportional to the square root of the surface tension,11 the reduced surface tension resulted in smaller air bubbles. Smaller air bubbles implied a larger gas−liquid interface area and potentially a higher transport flux of oxygen from the air to the liquid phase for oxidation of the bitumen. The reduced viscosity improved the oxygen diffusion rate in the liquid phase, because the binary diffusion coefficient is inversely proportional to viscosity.12 A second series of experiments were therefore performed without any forced airflow, while all other experimental conditions were kept the same. Diffusion of oxygen from air to the liquid was limited to the stagnant air− bitumen interface. In this way, the gas−liquid interface area of the bitumen and bitumen diluted with solvent remained constant during oxidation, because the liquid surface areas during exposure to air were of comparable size. As before, the viscosity of the oxidized bitumens was determined (Table 3). On the basis of the data (Table 3), some general observations could be made: (a) It was observed that the bitumen oxidized without solvent had a higher viscosity than that of bitumen oxidized in a solvent. (b) Forced airflow increased the viscosity of oxidized bitumen relative the bitumen oxidized without forced airflow. (c) The viscosity of the bitumen that was oxidized in the absence of a solvent was higher than the viscosity of the bitumen feed, which was 11.0 ± 0.15 Pa s (Table 1). Conversely, the viscosity of the bitumen oxidized in the presence of a solvent was lower than that of the bitumen feed. (d) The variability of viscosity data obtained without forced airflow was less that that obtained with forced airflow. (e) It was noteworthy that the relative variability in the viscosity data of bitumen oxidized without a solvent (2.9 and 0.3%) was much less than the variability in the viscosity data of bitumen oxidized with a solvent (12.1 and 8.4%). Upon closer inspection, it turned out that there was another variable affecting the outcome of the viscosity measurements. Even though the solvent was removed after oxidation at a temperature that was more than 50 °C above its boiling point at a reduced pressure (see Figure S2 and Table S1 of the Supporting Information), solvent remained in the bitumen, the bitumen contained around 5 wt % solvent. Because it was not possible to remove all of the solvent from the oxidized bitumen, it was decided to perform the viscosity comparison on a solvent-containing basis. The measurements without solvent were repeated, and the solvent was added after the oxidation took place. The solvent that was added was then removed in a similar way as in the oxidation experiments that were conducted in the presence of a solvent. The addition and removal of solvent was also performed with the Cold Lake bitumen feed, so that the viscosity of the unconverted solvent containing bitumen could be measured. A similar amount of solvent remained in all of the products. The viscosity of the resulting products was measured and compared (Table 4). The impact of solvent on the viscosity of oilsands-derived bitumen was correlated, and it was additive with respect to the mole fraction of material.13 It was reasoned that, if the solvent concentration was the same in all samples, differences in the measured viscosity would be due to the viscosity of the bitumen only. At 95% confidence, the viscosity of the bitumen that was oxidized in the presence of a solvent was higher than the viscosity of the bitumen oxidized in the absence of a solvent.

Table 4. Viscosity of Bitumen Containing 5 wt % Mesitylene after Autoxidation at 140 °C for 3 h at near Atmospheric Pressure and the Airflow Conditions Indicated viscosity at 60 °C (Pa s)a,b description Cold Lake bitumen feed (no oxidation) oxidation with 75 mL h−1 air g−1 of bitumen oxidation without forced airflow

without solventc with solventd 2.7 ± 0.09 3.7 ± 0.57

7.0 ± 0.85

3.0 ± 0.29

4.5 ± 0.38

a

Average and one sample standard deviation were reported. bViscosity was measured using a Brookfield rheometer at a shear rate of 20 s−1. c Solvent was added and removed at 150 °C and 10 kPa absolute before viscosity was measured. dRepeated from Table 2 for ease of comparison.

It was also important to determine whether there was any systematic bias because of oxidation extent and/or the solvent addition and removal procedure. Rather than reporting just the average and standard deviation, the results of individual experiments are provided for the next two tests, because it shows how variability in the oxidation extent, solvent removal, and viscosity measurements affected the results. The oxidation of bitumen without dilution was repeated, and the oxygen consumption was recorded over time. At the end of the reaction, the viscosity and amount of solvent that remained in the oxidized bitumen samples were determined (Table 5). The induction time for bitumen oxidation was around 5 min. The overall oxygen consumption after 3 h of oxidation was 1.0−1.1 g of O2 kg−1 of bitumen, and the oxidation rate was around 0.3 g h−1 O2 kg−1 of bitumen. No systematic bias because of correlated errors was found. However, there seemed to be a meaningful difference between sets of experiments, e.g., the viscosity results in Table 4 (3.7 ± 0.6 Pa s) and Table 5 (4.5 ± 0.2 Pa s). In an analogous way, the oxidation of bitumen with solvent dilution was repeated, but in this case, the reaction time was controlled on the basis of the oxygen consumption (Table 6). The induction time for oxidation of the bitumen with solvent was around 5 min. The final oxidation rate was around 0.3 g h−1 O2 kg−1 of the bitumen−solvent mixture. 3.2. Extent of Solvent Oxidation. The solvent that was employed to dilute bitumen was mesitylene. As is the case with most hydrocarbons, it has been known for a long time that mesitylene is not resistant to oxidation in the vapor and in the liquid phase.14,15 Although most of the investigations reporting mesitylene oxidation were conducted in the presence of a catalyst, autoxidation without a catalyst was also reported. The oxidation of mesitylene could affect the bitumen oxidation rate by either reducing the availability of dissolved oxygen (i.e., reducing the bitumen oxidation rate) or forming hydroperoxides as oxidation intermediates to propagate oxidation (i.e., increasing the bitumen oxidation rate). Hence, the extent of solvent oxidation was determined (Figure 2). An induction period of around 10 min was observed, which was followed by continuous oxidation as measured by consumption of oxygen from the forced airflow. There was some variability in repeat experiments, but on average, the rate of oxygen consumption after 3 h of reaction reached a steadystate value of around 0.3 g h−1 O2 kg−1 of mesitylene or 1.2 × 10−3 mol h−1 O2 mol−1 of mesitylene. The overall oxygen consumption after 3 h of oxidation was 0.6−0.7 g of O2 kg−1 of mesitylene, which reflected the lower initial oxygen con6244

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Table 5. Experimental Variability in the Autoxidation of Bitumen at 140 °C for 3 h with 75 mL h−1 Forced Air Flow description

experiment 1

experiment 2

experiment 3

average

standard deviation

O2 consumption (g kg−1)a final oxidation rate (g h−1 kg−1)a solvent concentration (wt %)b viscosity at 60 °C (Pa s)c

1.1 0.4 4.4 4.7

1.0 0.3 4.8 4.5

1.1 0.3 3.9 4.4

1.1 0.3 4.4 4.5

0.06 0.04 0.46 0.15

Expressed on a per kilogram of bitumen basis. bSolvent was added and removed at 150 °C and 10 kPa absolute before viscosity was measured. Viscosity was measured using an Anton Paar RheolabQC viscometer at a shear rate of 10 s−1.

a c

Table 6. Experimental Variability in the Autoxidation of Bitumen Diluted in a Solvent at 140 °C with 75 mL h−1 Forced Air Flow description

experiment 1

experiment 2

experiment 3

average

standard deviation

reaction time (min) O2 consumption (g kg−1)a final oxidation rate (g h−1 kg−1)a solvent concentration (wt %)b viscosity at 60 °C (Pa s)c

88 0.9 0.6 4.4 3.5

77 0.9 0.6 3.2 3.9

91 0.9 0.6 4.6 3.3

85 0.9 0.6 4.1 3.6

7.6 0.01 0.01 0.75 0.31

Expressed on a per kilogram of bitumen basis. bSolvent was added and removed at 150 °C and 10 kPa absolute before viscosity was measured. Viscosity was measured using an Anton Paar RheolabQC viscometer at a shear rate of 10 s−1.

a c

sulfur compounds before the sulfur content of the oxidized bitumen was determined (Table 7). No emulsion formation was observed. Analysis of the water phase revealed that little sulfur was removed by water washing. This is may not be surprising considering the low water solubility of dibenzothiophene sulfone and its analogues; e.g., the solubility of dibenzothiophene sulfone in water ranges from 1.8 × 10−4 to 2.2 × 10−4 g g−1 at 30−70 °C.16 Although there were numerical differences in the sulfur content determined for the different oxidized bitumen samples, these differences were not statistically meaningful. The results confirmed that, at the conditions employed, there was little (5− 11%) oxidative desulfurization. 3.4. Impact of the Oxidation Extent on Oxidative Viscosity Change. A prolonged oxidation experiment was conducted with bitumen. This experiment was conducted without any solvent addition during or after the experiment. The objective was to determine how the viscosity changed in relation to the oxidation extent. This was important to understand the sensitivity of viscosity changes to the extent of oxidation. The oxidation was performed at 140 °C with a constant air flow rate of 75 mL h−1 g−1 of bitumen initially charged (Figure 3). The oxygen consumption calculation accounted for the change in bitumen mass as a result of sampling. Viscosity measurements were performed using an Anton Paar RheolabQC at a shear rate of 10 s−1. Little CO2 was observed in the product gas. Viscosity increased with increasing oxygen consumption (Figure 3), and the results suggested that there were two regions. In the first region, the increase in viscosity was around 1 Pa s per 1000 mg of O kg−1 of bitumen oxygen consumption.

Figure 2. Oxygen consumption during oxidation of mesitylene at 140 °C with an air flow rate of 75 mL h−1 g−1 of bitumen. The results for three experiments (■, ●, and ◆) are indicated.

sumption. Under the assumption that oxidation was equimolar and that no secondary oxidation of products took place, the maximum conversion of mesitylene at 140 °C over a 3 h period was 0.3%. 3.3. Extent of Oxidative Desulfurization. The oxidation conditions were mild, 140 °C for 3 h, with little or no forced airflow. Little oxidative desulfurization of the bitumen was anticipated. This was deliberate, because it was important to limit conversion; higher conversion could lead to precipitation that would affect the viscosity. The extent of oxidative desulfurization was also an indirect indication of the overall oxidation extent and potentially the observed increase in viscosity. To make the measurements comparable to previous work,5 the same product workup procedure was followed. The oxidized bitumen was water-washed to remove water-soluble

Table 7. Sulfur Content of Autoxidized and Water-Washed Bitumen sulfur content (wt %)a

a

desulfurization (%)

description

without solvent

with solvent

without solvent

with solvent

oxidation with 75 mL h−1 air g−1 of bitumen oxidation without forced air flow

4.1 ± 0.24 4.4 ± 0.30

4.4 ± 0.06 4.1 ± 0.27

11 5

5 11

Average and one sample standard deviation were reported. 6245

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145 °C.5 Analogous results were found during prolonged oxidation of bitumen at 130 °C, with little sign of oxidative degradation.17 The hardening could be eliminated by performing the ODS in a solvent.5 However, including a solvent as cofeed to the ODS process design increased the operating cost, and fundamentally, it was not clear whether the beneficial effect was due to suppression of oxidative hardening or just solvent dilution. The results of the present study (Table 4) indicated that the use of a solvent was not helpful to reduce oxidative hardening of bitumen. Other insights were also gathered from the work: (a) The oxidation rate of the bitumen, the solvent, and the bitumen−solvent mixture was within experimental variation the same, 0.3 g h−1 O2 kg−1 of liquid at near steady-state oxidation conditions. It is considered “near” steady state, because it is a semi-batch oxidation reaction and the composition of the liquid phase is changing slowly with time. The solvent did not accelerate or suppress O2 consumption. (b) Despite similar oxidation extents, the increase in viscosity when bitumen was oxidized in the presence of a solvent was marginally higher than when the bitumen was oxidized on its own. The implication is that the rate of O2 consumption at a constant oxidation temperature is not the only variable that affected oxidative hardening. The solvent seemed to promote reactions that led to oxidative hardening. The invariability of O2 consumption between bitumen, solvent, and bitumen−solvent mixture at near steady state can be explained in terms of liquid-phase hydrocarbon oxidation. Initiation of oxidation is slow, order of magnitude of 10−3 g h−1 of O2/kg, at 140 °C.18 Initiation of oxidation usually follows third-order kinetics for hydrocarbons, first order in O2, and second order in the hydrocarbon.19 Evidence for slow initiation can also be seen from oxidation studies using differential scanning calorimetry. The onset of low-temperature oxidation of Turkish oilsands was 300 °C when a heating rate of 5 °C/ min was employed and higher onset temperatures at higher heating rates.20 However, after initiation at near steady-state oxidation, the oxidation rate is constant at an order of magnitude of 10−1 g h−1 of O2/kg, which is consistent with free radical chain-propagation-dominated oxidation. Under these conditions, the oxidation rate is dependent upon the hydroperoxide concentration, which is at a near constant concentration. If the viscosity can be used as a measure of product selectivity, the presence of mesitylene as a solvent changed the selectivity of propagation and/or termination during bitumen oxidation to increase oxidative hardening. This makes sense if one considers the relative ease of hydrogen removal by peroxyl radicals (eq 1).

Figure 3. Viscosity in relation to oxygen consumption during bitumen oxidation at 140 °C in the absence of a solvent.

In the second region, the viscosity was more strongly affected by oxygen consumption and viscosity increased to around 7 Pa s per 1000 mg of O kg−1 of bitumen. The relationship between viscosity increase and oxygen consumption appeared linear in both regions. 3.5. Impact of the Oxidation Temperature on Oxidative Viscosity Change. Previous work conducted with bitumen oxidation indicated that, under otherwise similar oxidation conditions, the increase in viscosity during oxidation at 175 °C was considerably more than the increase in viscosity during oxidation at 145 °C.5 To confirm that the increase in viscosity with the increase in the temperature that was observed was just due to the increased oxidation extent, a series of oxidation experiments were performed at different temperatures (Table 8). The following observations were made: (a) Table 8. Viscosity of Bitumen after Oxidation for 3 h at Different Temperatures at an Air Flow Rate of 75 mL h−1 g−1 of Bitumen in the Absence of a Solvent temperature (°C)

oxygen consumption (mg of O/kg)

viscosity at 60 °C (Pa s)a

140 160 180 200

990 1030 1260 2900

12.2 14.1 16.6 17.5

a

Viscosity was measured using an Anton Paar RheolabQC viscometer at a shear rate of 10 s−1.

The oxygen consumptions within the first 3 h of oxidation at 140, 160, and 180 °C were very close to each other, and a meaningful increase in oxygen consumption was seen only upon further increasing the temperature to 200 °C. (b) The viscosity increased with an increase in the oxidation temperature from 140 to 180 °C, even though the oxygen consumption was very similar. (c) Bitumen oxidized at 140 °C at an oxygen consumption of 990 mg of O/kg (Table 8) and 1350 mg of O/kg (Figure 3) had a similar viscosity, 12.2 Pa s. This range of oxygen consumption straddles the range of oxygen consumption of the oxidation experiments performed at 160 and 180 °C, which supported the observation made in point b.

R−O−O• + R−H → R−O−O−H + R•

(1)

It is more difficult to remove hydrogen from the methyl groups of mesitylene than it is to remove hydrogen from the cycloalkane structures in naphthenic−aromatic compounds,19 which are abundant in oilsands bitumen. It is therefore likely that, in bitumen−solvent mixtures, the rate of free radical chain propagation to create bitumen-derived radicals was higher than bitumen on its own. This was because, at similar peroxyl radical concentrations, the probability of mesitylene-derived peroxyl radicals reacting with bitumen was higher than the probability of bitumen-derived peroxyl reacting with mesitylene. Termination by the addition of two bitumen-derived radicals (eq 2) would produce a heavier product and increase the viscosity; the

4. DISCUSSION 4.1. Solvent-Mediated Bitumen Oxidation. Previous work on the development of an ODS process for oilsands bitumen found that oxidative hardening causing an increased viscosity was a problem even when oxidation was conducted at 6246

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selectivity to termination of free radical intermediates by addition reactions (eq 2) increased. Elemental analysis was too insensitive to determine the extent of oxygen incorporation, and the bitumen composition was too complex to differentiate addition products from material in the feed. The reason for the oxidation temperature dependence of the viscosity increase at a constant O2 consumption was not resolved. It should also be mentioned that, at longer periods of oxidation in the 140−160 °C temperature range, it was found that the viscosity of oxidized bitumen converged to a single value, irrespective of the oxidation temperature.17 (d) In the fourth stage, there is an increase in the magnitude of the viscosity increase with O2 consumption and time, as suggested by Babu and Cormack.25 Initially, the second derivative of viscosity change with respect to O2 consumption is positive, but the results (Figure 3) suggest that this is not a continuous change. The change in viscosity with O2 consumption settles on a higher, albeit constant, value than in the third period. The present investigation did not explore extended oxidation further. (e) There is an implied fifth stage, where the viscosity increase approaches a limit and further oxidation causes little further increase in viscosity. This is the behavior that was described by Petersen.23

probability that this addition reaction took place was increased in the bitumen−solvent mixture.

R• + ′R• → R−′R

(2)

It is suspected that different compound classes will have different propensity for free radical addition reactions. It was shown that, for coal liquids, benzyl alcohol and tetralin contributed to the formation of addition products, with benzyl alcohol causing more addition reactions.21 The free radical addition of tetralin was also confirmed under autoxidation conditions similar to that in the present investigation.22 4.2. Oxidative Bitumen Hardening. The relationship between oxidation extent and viscosity change was not clear from the literature. Petersen23 indicated that, during oxidation at 60 and 113 °C, the viscosity of asphaltic bitumen increased with increasing the low-temperature oxidation time but that the rate of viscosity increase over time decreased over time; i.e., the second derivative of viscosity with time is negative. Severin et al.24 reported that there was a linear relationship between the oxygen content incorporated in crude oil that was oxidized at 100 °C and the viscosity; i.e., the second derivative is zero. Babu and Cormack25 investigated the oxidation of oilsands bitumen over the temperature range of 47−97 °C and found that the viscosity initially increased linearly with increasing the oxidation extent, but thereafter, the rate of increase in viscosity with oxidation extent increased; i.e., the second derivative of viscosity with time is positive. These reports are not necessarily conflicting, because different bitumens, different conditions, and different time periods of oxidation were employed. The results that were presented (Figures 2 and 3) indicated that there are at least four stages of bitumen oxidation, with a fifth stage being implied but not experimentally confirmed. (a) The first stage is the induction period. Initiation of oxidation takes place, and little O2 is consumed. This period was found to last around 10 min for bitumen autoxidation at 140 °C and near atmospheric pressure; i.e., the O2 partial pressure was in the range of 20−25 kPa absolute. (b) The second stage was a transition from free radical initiation to free radical chain propagation. Some variability in the length of this transition was observed in repeat experiments (Figure 2). A likely cause of this variability is variability in the bubbling and agitation of the air− bitumen mixture, which will influence the time that is required to reach a near steady-state hydroperoxide concentration. Little oxidation takes place in this period, and at the end of this period, the bitumen viscosity is still very close to the bitumen feed viscosity. (c) The third stage is a free radical chainpropagation-dominated oxidation stage. The rate of O2 consumption with time was constant. During this period, the rate of viscosity increase with time and O2 consumption was also constant, which was coincident with the oxidation period described in the work by Severin et al.24 It was found that, for oxidation conducted at 140 °C, this period lasted until the cumulative O2 consumption was around 3000 mg of O kg−1 of bitumen or 0.3% O2 relative to the mass of bitumen. An aspect that was not fully appreciated before is that, although the increase in viscosity was linear with respect to O2 consumption and time in this period, the viscosity increase was not independently related to the O2 consumption. The viscosity increase for the same O2 consumption was different for different oxidation temperatures and increased with the temperature for the same O2 consumption (Table 8). Two possibilities were that the fraction of oxygen incorporated in bitumen relative to O2 consumption increased or that the

5. CONCLUSION The low-temperature oxidation of oilsands bitumen was investigated to determine how viscosity changed with oxidation extent and whether it could be suppressed by solvent dilution. It was found that solvent dilution decreased viscosity only as a diluent, but it did not suppress oxidative hardening of the bitumen. In fact, the presence of a solvent increased oxidative hardening of the bitumen relative to the oxidative hardening of bitumen in the absence of a solvent. This could be explained in terms of the relative ease of hydrogen abstraction during the free radical propagation step of oxidation. Generally speaking, hydrogen abstraction from the C−H bond in cycloalkane (naphthene) rings that are attached to aromatics is easier than hydrogen abstraction from C−H bonds in most other functional groups. In the presence of a solvent, the oxidation rate per unit mass of the bitumen−solvent mixture was the same as that for bitumen and the solvent individually, but the selectivity to hydrogen abstraction from bitumen compared to the solvent in the mixture was higher. The increase in viscosity with oxidation extent was also investigated. It was possible to identify different periods of oxidation and viscosity increase, which reconciled apparently conflicting reports in the literature. It was also found that, during the first free radical chain-propagation-dominated oxidation period, the increase in bitumen viscosity was affected by not only oxidation extent, but also the temperature for a constant oxidation extent. At the same level of O2 consumption, the oxidative hardening of bitumen increased with increasing the oxidation temperature. The reason for the oxidation temperature dependence of the viscosity increase at a constant O2 consumption was not resolved, although hypotheses to explain this experimental observation were formulated.

■

ASSOCIATED CONTENT

S Supporting Information *

Viscosity versus shear rate curve for bitumen (Figure S1), vapor pressure curve of the solvent (mesitylene) and the operating point for solvent removal (Figure S2), and solvent content 6247

dx.doi.org/10.1021/ef501341h | Energy Fuels 2014, 28, 6242−6248

Energy & Fuels

Article

(17) Siddiquee, M. N.; de Klerk, A. Continuous and prolonged oxidation of bitumen for upgrading by microbial digestion. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 2013, 58 (2), 649−651. (18) Emanuel, N. M.; Denisov, E. T.; Maizus, Z. K. Liquid-Phase Oxidation of Hydrocarbons; Plenum Press: New York, 1967. (19) Emanuel, N. M.; Zaikov, G. E.; Maizus, Z. K. Oxidation of Organic Compounds. Medium Effects in Radical Reactions; Pergamon Press: Oxford, U.K., 1984. (20) Senguler, I.; Kok, M. V. The characterization of a tar sand sample using differential scanning calorimeters. Energy Sources, Part A 2013, 35, 77−82. (21) Wang, Z.; Bai, Z.; Li, W.; Chen, H.; Li, B. Quantitative study on cross-linking reactions of oxygen groups during liquefaction of lignite by a new model system. Fuel Process. Technol. 2010, 91, 410−413. (22) Siddiquee, M. N.; de Klerk, A. Oxidation of naphthenic− aromatic compounds in bitumen. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 2014, 59 (2), 572−574. (23) Petersen, J. C. Asphalt oxidationAn overview including a new model for oxidation proposing that physicochemical factors dominate the oxidation kinetics. Fuel Sci. Technol. Int. 1993, 11, 57−87. (24) Severin, D.; Glinzer, O.; Killesreiter, H.; Neumann, H.-J. The quiet oxidation of a crude oil and the effect on its viscosity. Erdoel Kohle, Erdgas, Petrochem. 1983, 36 (3), 127−130. (25) Babu, D. R.; Cormack, D. E. Effect of oxidation on the viscosity of Athabasca bitumen. Can. J. Chem. Eng. 1984, 62, 562−564.

remaining in the bitumen (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-780-248-1903. Fax: +1-780-492-2881. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Project COSI2010-07 of the Institute of Oilsands Innovation at the University of Alberta, and permission to publish the results is appreciated. The assistance of Muhammad N. Siddiquee in the bitumen characterization is appreciated.

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REFERENCES

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dx.doi.org/10.1021/ef501341h | Energy Fuels 2014, 28, 6242−6248