Article pubs.acs.org/EF
Thermal Conversion Regimes for Oilsands Bitumen Ashley Zachariah†,‡ and Arno de Klerk*,† †
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada Praxair Canada Inc., 9020 24 Street NW, Edmonton, Alberta T6P 1X8, Canada
‡
S Supporting Information *
ABSTRACT: Thermal conversion of oilsands bitumen at 400 °C was investigated to gain a better understanding of temporal changes in liquid properties. The work approximated a mild thermal cracking (visbreaking) process with run-lengths extending into the coking region. Reaction progress could be divided into three main regimes: (I) stable visbreaking, (II) coking visbreaking, and (III) coking. In addition to observations anticipated from the literature, the work revealed aspects of the reaction progression that was not fully appreciated before. Stable visbreaking with minimal formation of coke had a “productive” period during which viscosity decreased, while asphaltenes content and gas yield were unchanged, followed by an “unproductive” period during which the viscosity, asphaltenes content, and gas yield all increased. After the onset of precipitation of solids, the solids (“coke”) yield increased and the asphaltenes content in the liquid decreased, but the viscosity increased. The origin of increased viscosity was not due to increased asphaltenes content but due mainly to free radical addition products that remained soluble in the bulk liquid. The insolubility of molecules in the liquid and meso-phase that ultimately led to precipitation as solids and coke formation at higher temperatures, appeared to be caused by addition reactions more than as result of asphaltenes and hydrogendepletion. The investigation does not prove this conclusively, but if this interpretation is correct, the coking limit of stable visbreaking operation at 400 °C is not as result of a solubility constraint caused by hydrogen-depletion; instead, it is the result of free radical addition that manifests as a solubility constraint leading to hydrogen-depletion. In the reaction sequence, the importance of hydrogen disproportionation of alkyl cycloalkane functionality in the bitumen was highlighted to explain the major changes, including the onset of increased gas yield and increased viscosity that was attributed to free radical addition.
1. INTRODUCTION Poor fluidity is a major obstacle to the recovery and transport of Canadian oilsands-derived bitumen.1 Viscosity values are typically 10 to 100 Pa·s (104 to 105 cP) at 25 °C for bitumens recovered from the Athabasca and Cold Lake regions.2 There are two strategies for making the bitumen fluid enough so that it can be transported by pipeline, namely, dilution with light hydrocarbons and upgrading to convert the bitumen into a more fluid oil. Mild thermal cracking, or visbreaking, is a technology that was originally developed to decrease the viscosity of heavy oil fractions to meet fuel oil specifications. Visbreaking is a thermal conversion technology and is one of few thermal technologies that is still in widespread use. Many other thermal technologies have since been displaced by catalytic technologies. The visbreaking process entails passing the oil through a furnace and optionally a soaker vessel, before the hot oil is flash quenched and fractionated (Figure 1).3 The soaker is an
adiabatic vessel downstream from the furnace to provide additional residence time for the hot oil. Conversion is determined by the combination of furnace coil outlet temperature and residence time. For the same level of conversion, residence time must be doubled for every 15 °C decrease in temperature.4 Conversion in visbreaking context refers to the decrease in material above a specific boiling point temperature, typically conversion of vacuum residue (525 °C and higher boiling material) to products, irrespective of the nature of the products. There is a large body of literature describing visbreaking kinetics in terms of this definition of conversion.5 Visbreaking technology can handle low amounts of coke formed during the process, but the time on stream until a shutdown is necessary is normally determined by the coke buildup in the unit.3−5 Maximum conversion in a visbreaker unit is limited by the onset of coke formation during cracking and ways to estimate the onset of coking have been reported.6 The conversion and extent of viscosity reduction that can practically be achieved for a given feedstock is therefore dependent on the coking propensity of the feedstock. When visbreaking is applied as a partial upgrading technology to improve the fluidity of bitumen for pipeline transport, achieving good viscosity reduction is more important than achieving high conversion. However, the decrease in viscosity during visbreaking is often seen as synonymous with Received: October 10, 2015 Revised: December 13, 2015
Figure 1. Flow diagram of a generic visbreaker unit. © XXXX American Chemical Society
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DOI: 10.1021/acs.energyfuels.5b02383 Energy Fuels XXXX, XXX, XXX−XXX
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was supplied as a cylinder gas by Praxair. Methylene chloride (99.9%) was employed as solvent to remove products from reactors and was supplied by Fisher Scientific. Asphaltenes content was determined by precipitation with n-pentane (98%) also supplied by Fisher Scientific. 2.2. Equipment and Procedure. All experiments were performed in microbatch reactors. The microbatch reactors were constructed using Swagelok SS 316 tubing and fittings. The reactors were 0.1 m long, with an internal diameter of 0.01 m. Reactors were heated by submerging them in a fluidized sand bath heater that was preheated. A schematic of the microbatch reactor is provided in the Supporting Information (Figure S1). Heat-up and cool-down time was previously determined,11 and it took 6 min to reach the correct internal temperature and about the same time to cool down. In a typical experiment, the reactor was loaded with around 8 g of bitumen, purged with N2, and then pressurized with N2 to 4 MPa gauge pressure. During heating to the reaction temperature, the pressure increased to 6 MPa and remained fairly constant during reaction. After cooling, the final pressure was 4.1 MPa. The pressure at which the reactions were performed was higher than typically found industrially, which is around 1 MPa.4 The N2 pressure had an effect on the vapor−liquid equilibrium during reaction that may have delayed to onset of coking reactions,13 but was otherwise inert. The main reason for employing N2 at pressure was to leak-test the microbatch reactors and confirm that no leaks developed during the experiment. The reaction time was measured from the time that the correct internal temperature was reached until the time that the reactor was cooled down (i.e., reaction time excluded the 6 min heat-up time). After cooling down the reactor, the reactor was depressurized, and products were collected by washing the reactor with methylene chloride. The solid products were separated from the liquid products by filtration through a 0.22 μm filter. Subsequently, the solvent was removed, first by rotary evaporation and then by leaving the product in a fume hood until constant mass was obtained. With each step, the mass of the reactor and products were determined using a Mettler Toledo ML 3002 balance, which had a 3200 g capacity with 0.01 g readability. Material balances were between 99 and 104 wt % of feed. 2.3. Analyses. (a) Liquid viscosity. Viscosity analyses of bitumen feed and visbroken products were performed using an Anton Paar RheolabQC viscometer. A cylinder in cup (C−CC17/QCLTC) kept at constant temperature of 60 °C was employed for the measurements. The instrument was calibrated with a Newtonian viscosity standard. An average of 4 g was used during analysis. The analyses were performed at constant shear rate of 10 s−1. (b) The asphaltenes content was determined by n-pentane precipitation, which is a modification of the standard test method ASTM D 6560.14 A ratio of 40:1 of n-pentane to oil was mixed at ambient conditions, covered to limit evaporation, and left to be stirred on a magnetic stirrer for 24 h. The mixture was then vacuum filtered through a preweighed 0.22 μm filter to collect the asphaltenes. Precipitated asphaltenes were dried overnight in a vacuum oven at 75 °C before the mass of asphaltenes was determined. (c) The microcarbon residue values were determined by thermogravimetric analysis, which is a modified method derived from the standard test method ASTM D 4530.15 The analyses were performed using a Mettler Toledo TGA/DSC1 LF FRS2MX5. The microcarbon residue value was the mass percentage of material that remained after heating a sample in an alumina crucible at a rate of 10 °C·min−1 from 25 to 600 °C under nitrogen atmosphere. (d) Elemental analysis of liquids to determine the hydrogen-tocarbon ratio was performed using a Thermo Scientific Flash 2000 CHNS-O organic elemental analyzer. (e) Proton nuclear magnetic resonance (1H NMR) spectra were obtained using a Nanalysis 60 MHz NMReady-60 spectrometer. The oil analyses were performed using 0.2 g of sample dissolved in 1 mL of deuterated chloroform. The sample
the increase in conversion. Relationships between molecular mass, density, and viscosity for crude oils were established,7 which indicate that viscosity and conversion are related. In the visbreaking literature, experimental data can also be found of the relationship between viscosity and conversion, or the decrease in vacuum residue content (e.g., refs 8,9). In aggregating materials, such as oilsands-derived bitumen, the viscosity of the oil is also affected by aggregation. Roscoe’s Law (eq 1) describes the relationship between the measured viscosity (μ) of the aggregating oil, the viscosity of the nonaggregated oil (μoil) and the effective volume fraction (Φeff) of aggregated material in the oil.10 μ = μoil ·(1 − Φeff )−2.5
(1)
By implication, any conversion leading to deaggregation would cause a decrease in measured viscosity without necessarily affecting the viscosity of the unaggregated oil. This principle was used to explain the disproportionate decrease in viscosity relative to conversion after mild visbreaking of oilsands bitumen over the temperature range 340 to 400 °C.11 What was intriguing in that work,11 was the nonmonotonous decrease in viscosity with reaction time. At all temperatures studied, a local viscosity minimum and viscosity maximum were observed, with temperature affecting only the time period that was required to reach these local minima and maxima. A similar viscosity−time relationship was subsequently reported at 300 °C,12 and again temperature affected only the time period required to reach the local viscosity minimum and viscosity maximum. Neither study fully explained the observed changes in viscosity with time or how it related to the changes in the other physical and chemical properties of the visbroken products. The purpose of this study was to gain a better understanding of the transformations taking place over time during the thermal conversion of oilsands-derived bitumen. A temperature of 400 °C was selected because it was high enough for thermal cracking to proceed at a reasonable rate, yet, also low enough so that there was sufficient time resolution to determine the order of physical and chemical changes. Due to the impact and importance of viscosity and coke formation on visbreaking, particular attention was paid to these properties and how these properties changed over time.
2. EXPERIMENTAL SECTION 2.1. Materials. The investigation was performed using Canadian oilsands-derived bitumen. The density and elemental analysis of the bitumen is listed in Table 1. Other properties are reported together with the experimental results for the visbreaking studies to facilitate comparison. Nitrogen (99.999%) was used for purging reactors and
Table 1. Oilsands Derived Bitumen Characterization description
experimental dataa
density at 30 °C (kg·m−3) elemental analysis (wt %) carbon hydrogen nitrogen sulfur oxygenb
1013.2 ± 1.0 82.9 ± 0.1 10.1 ± 0.01 0.6 ± 0.002 4.9 ± 0.03 1.5
a Analyses in triplicate; average ± standard deviation. bDetermined by difference.
B
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Energy & Fuels mixtures were placed in 5 mm diameter NMR tubes and analyzed using the following parameters: spectral width 14 ppm; digital resolution: 0.03 H; number of scans per sample: 16; active scan time: 4.7 s. The average scan time was 14.7 s, and 4000 points were recorded per scan. (f) Refractive index of liquid samples was measured relative to air using 589 nm (sodium D-line). All analyses were performed at constant temperature of 30 °C using an Anton Paar Abbemat 200 refractometer. (g) Stereomicroscopy was performed using a Carl Zeiss SteREO Discovery V20. (h) The particle size distribution of solids was determined by laser diffraction using a Malvern Mastersizer 3000. The solids were dispersed in isobutanol for analysis. The analysis used a Mie scattering model, which approximates the particle size based on laser light scattering, which assumes the particles to be spherical in shape.
The composition of the gas phase was not rigorously monitored. The gas composition after 60 min of reaction contained 30 ± 4 mol % methane, 59 ± 3 mol % C2−C5 hydrocarbons, and the remainder being COx. 3.2. Liquid Viscosity. The viscosity of the bitumen feed was 9.6 ± 0.3 Pa·s at 60 °C. This measurement was performed on the raw bitumen, and the bitumen contained 1.3 wt % solids. The viscosity of the reaction products was measured after product workup (Figure 2) and did not include solids, which were removed during the workup procedure. Numerical values can be found in the Supporting Information (Table S1).
3. RESULTS 3.1. Product Yield. The product yields were determined for each experiment (Table 2) as the average and sample standard deviation of experiments performed in triplicate. Table 2. Product Yields on Bitumen Feed Basis after Reaction at 400 °C for the Reaction Times Indicated solids (wt %)
a
liquids (wt %)a
Figure 2. Product viscosity after reaction at 400 °C for the reaction times indicated.
a
gases (wt %)
reaction time (minutes)
x
s
x
s
x
s
bitumen feed 0 30 60 80 90 180 240 270 280 300 330 360
1.3 1.4 1.8 1.8 1.5 5.0 8.0 11.0 11.7 14.9 14.5 23.6 24.1
0.05 0.03 0.4 0.5 0.4 2.1 0.3 1.0 0.3 2.8 2.2 2.2 1.9
98.7 97.5 97.5 94.1 93.0 88.8 84.2 82.1 81.4 77.6 75.0 68.8 67.1
0.5 0.4 0.4 0.3 0.5 1.7 1.5 1.0 2.0 8.0 4.7 2.6 1.6
0.3 0.7 4.1 5.5 6.2 7.8 8.5 8.7 10.9 10.6 11.2 11.9
0.2 0.6 0.2 1.5 0.5 1.7 1.5 2.0 4.0 4.0 2.5 2.1
The viscosity decreased with increase in reaction time, reaching a minimum viscosity value of 0.15 ± 0.03 Pa·s after 30 min reaction. With increasing reaction time, a slight increase in viscosity was observed, although it became meaningfully higher only after 80 min reaction. The viscosity increased to reach a local maximum of 1.2 ± 0.1 Pa·s at 90 min, which is an order of magnitude increase over the value after 30 min. With a further increase in reaction time beyond 90 min, the viscosity decreased by an order of magnitude again to reach a local minimum of 0.18 ± 0.03 Pa·s after 180 min reaction. When the reaction time was prolonged, an increase in viscosity was observed. The viscosity passed through another local maximum at 300 min and local minimum at 330 min. All of these changes in viscosity represented statistically meaningful differences. 3.3. Asphaltenes Content. The n-pentane precipitated asphaltenes content of the bitumen feed was 18.8 ± 1.0 wt % on a total feed basis. This value had to be corrected for solids present in the liquid (Table 2), because the raw feed was not subjected to a product workup procedure that removed solids, as was the case with the reaction products. The asphaltenes content of the solids-free reaction products was determined by precipitation with n-pentane and expressed as a mass percentage of the bitumen fraction, which was 98.7 wt % of the total feed mass (Figure 3). The asphaltenes content appeared to follow the same reaction-time-dependent pattern as was observed for viscosity (Figure 2). However, this was strictly speaking true only for the latter stages of the reaction. Even though the patterns in Figures 2 and 3 were similar, there were some important differences. During the initial stages of the reaction, the asphaltenes content remained fairly constant, but the viscosity decreased by roughly 2 orders of magnitude. Furthermore, the asphaltenes content reached a local maximum after 80 min reaction time, whereas the local viscosity maximum was reached after 90 min, after there had been a significant decrease in the asphaltenes content.
a
Average (x) and sample standard deviation (s) of experiments performed in triplicate.
The Long Lake bitumen feed contained 1.3 wt % solids. After reaction times of 80 min and less, the mass of solids increased only slightly. The mass increase was between 0.1 and 0.5 wt %, and in most cases, the sample standard deviation was around 0.3 wt %, so that there was no meaningful statistical difference between these values. A significant change in the solids content was observed only after a reaction time of 90 min. At 90 and 280 min reaction times, the sample standard deviations were higher than other experiments. In both cases it was accompanied by a step change in solids content with respect to 80 and 270 min reaction times, respectively. The higher observed standard deviations were not just due to experimental error, but also due to the changes caused by reaction. The products before and after these reaction times were different in nature. The liquid yield decreased with increasing reaction time. Taking the sample standard deviation into consideration, the decrease in liquid yield with increase in reaction time can be described as a monotonous decrease. C
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Figure 3. Asphaltenes content by n-pentane precipitation of the solidsfree liquids after reaction at 400 °C for the reaction times indicated.
Figure 5. Hydrogen-to-carbon molar ratio of liquid products after reaction at 400 °C for the reaction times indicated.
3.4. Microcarbon Residue. The microcarbon residue (MCR) is a measure of the coking propensity of a liquid. It is an alternative method to the standard test method for determining the Conradson Carbon Residue (CCR) and it was found that MCR = CCR.16 The raw bitumen and the liquid reaction products were analyzed (Figure 4). As with the asphaltenes content of the raw bitumen, the measured MCR value of 10.2 ± 0.8 wt % had to be corrected for the presence of solids.
Table 3. 1H NMR Characterization of Selected Liquid Products after Reaction at 400 °C for the reaction times indicated hydrogen distribution (%) reaction time (minutes)
aromatic
aliphatic
aliphatic methyl
bitumen feed 60 180 280 300
9 ± 0.8 11 ± 1.7 16 ± 4.2 20 ± 0.4 20 ± 2.4
91 89 84 80 80
± ± ± ± ±
16 ± 0.5 3 ± 0.4 5 ± 0.7 11 ± 0.6 11 ± 2.7
0.8 1.7 4.2 0.4 2.4
the values are therefore only indicative of the ratio of aromatic to aliphatic hydrogen. Nevertheless, the results qualitatively indicated that there was an increase in aromatic hydrogen with an increase in reaction time. The amount of aliphatic hydrogen that was in methyl groups was determined by considering the area from 0.5 to 1.0 ppm as the methyl range. This is only indicative of the methyl content, but a clear change in this part of the 1H NMR spectrum was seen with increasing reaction time. Initially there was a sharp drop in the aliphatic hydrogen present in methyl groups, but over time this value gradually increased again. 3.6. Refractive Index. The refractive index of oil is easily measured and can provide information about the chemical changes taking place during reaction. It was found that the refractive index of the liquid products increased with an increase in reaction time (Figure 6). The numerical values are also reported as Table S2 in the Supporting Information. It is noteworthy that there was no statistically meaningful difference between the refractive index of the bitumen feed, 1.5788 ± 0.0032, and that of the reaction product after heating to 400 °C and then immediately cooling down, 1.5801 ±
Figure 4. Microcarbon residue content of the solids-free liquids after reaction at 400 °C for the reaction times indicated.
The MCR increased with increasing reaction time. Although the increase from data point to data point was not statistically meaningful, the trend is clear, and the difference between short and long reaction times were meaningful. The change in average MCR value with increasing reaction time appeared gradual, with the exception of two inflections that were observed around 80 to 90 min and around 270 to 280 min. 3.5. Hydrogen Content. The increase in microcarbon residue with increase in reaction time (Figure 4) suggested that the liquid product gradually became more hydrogen depleted and aromatic in nature. This was confirmed by elemental analysis. The hydrogen-to-carbon ratio of the liquid product decreased with an increase in reaction time (Figure 5). A large decrease in hydrogen-to-carbon ratio occurred between 80 and 90 min reaction time, with the hydrogen-tocarbon molar ratio of the liquid decreasing from 1.32 ± 0.003 to 1.21 ± 0.001. An inflection in the hydrogen-to-carbon molar ratio trend was observed around 270 to 280 min reaction time. The nature of the hydrogen in the liquid products was determined by analyzing selected samples with 1H NMR (Table 3). The total aliphatic hydrogen content was determined by integrating the area over the 0.5 to 4.5 ppm range, the remainder being considered aromatic. Olefins produced during thermal cracking would cause overlap and
Figure 6. Refractive index of liquid products after reaction at 400 °C for the reaction times indicated. D
DOI: 10.1021/acs.energyfuels.5b02383 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels 0.0032. With increasing reaction time, the refractive index of the liquid products increased. The shape of the relationship of reaction time with refractive index (Figure 6) closely resembled that of the microcarbon residue (Figure 4) and horizontal mirror image of the hydrocarbon-to-carbon molar ratio (Figure 5). 3.7. Characterization of Solids. It was observed during the workup of the reaction products that the solids had very different filtration characteristics. After filtration and drying of the solids, the solids were visually inspected using stereomicroscopy. It was found that the solids changed in color and shape with increasing reaction time (Figure 7).
Figure 8. Particle size distribution of solids after 240 min reaction at 400 °C.
Figure 7 that the mineral matter is present as platelets, which is a morphology that is poorly described by an equivalent spherical particle diameter. Some observations can be made on the basis of Figure 8. The particle size distribution appears to be bimodal. Most particle volume (∼75%) is found in a normal distribution with mean at a particle size of 50 μm. The rest of the particle volume (∼25%) is found in a chi-square-like distribution that has a maximum at a particle size of 15 μm. The median particle size for the total distribution is 45 μm based on the particle volume distribution with size.
4. DISCUSSION The progress of thermal conversion with reaction time proceeded through different regimes, or time-periods, which were dominated by specific events. It is not suggested that these regimes have clear boundaries. In some cases, the regimes were separated by an easily measurable change but not in all cases. The discussion will be organized on the basis of the identified regimes. It is suspected that the discussion of the different visbreaking regimes can be generalized, because similar observations were made at 380, 360, 340, and 300 °C.11,12 However, the present discussion is based only on the experimental evidence for thermal conversion at 400 °C, and no specific proof is provided that the discussion can be generalized. 4.1. Stable Visbreaking during Dynamic Preheating (Regime Ia). The preheating period is a period characterized by dynamic heating. It is considered separately, because there are changes taking place during the heat-up period. The most apparent of these were the increase in asphaltenes content (Figure 3) and the increase in microcarbon residue (Figure 4). Yet, at the same time there was little change in the refractive index of the liquid (Figure 6), which is an indicator of composition. Whatever happened, it affected the solubility and subsequent behavior of some material, without affecting the bulk composition much. A small amount of gas was released (Table 2). An accurate quantification of the gas yield from small-scale experiments is usually challenging, and it was tempting to disregard the small gas yield as a value within experimental error. However, it was reported that both trapped gas and gas produced by reaction were evolved from oilsands materials at temperatures in the range 25 to 210 °C.17,18 These temperatures are well below temperatures typically associated with thermal cracking and the dominant products were light hydrocarbons. In a different study with mined oilsands, the formation of percentage level gaseous products were also observed at temperatures below 400
Figure 7. Microscope images of the solids fraction of the products after reaction at 400 °C for (a) 0 min, (b) 60 min, (c) 240 min, and (d) 300 min.
The solids initially had the shape of platelets (Figure 7a,b). The platelets were the mineral matter present in the oilsands bitumen. Deposition of organic material on these minerals was low (Table 2) and the presence of organic material did not visibly alter the shape of the minerals. Microscopy suggests that all organic material that was deposited had deposited on the minerals. The filtration of reaction products took time, but filtration did present a problem. Products after about 180 min of reaction time became extremely difficult (almost impossible) to filter, and the 0.22 μm filter was readily clogged. The reason for this is apparent from Figure 7c, which shows the formation of a fine solids fraction. The dominant solid product was no longer the mineral platelets, but fine coke particles that formed as a separate solids phase. These small coke particles hampered filtration by forming a dense filter cake. After about 270 min reaction time, the coke particles started agglomerating to form physically larger structures (Figure 7d). Filtration became easier than for any of the samples produced at shorter reaction times. The agglomerated coke formed a more porous filter cake, and the solids were easily separated from the liquid phase during filtration. In order to obtain a more quantitative description of the solids, the particle size distributions of the solids were determined. However, the mineral-rich solids obtained at short reaction time, as well as the agglomerated solids, did not maintain particle integrity during sample preparation in isobutanol. Only the coke obtained after 240 min reaction time, shown in Figure 7c, appeared to be mechanically strong enough for particle size analysis. Because it was not possible to guarantee particle integrity even for this sample, the particle size distribution (Figure 8) should be considered qualitatively, rather than quantitatively. Furthermore, it is obvious from E
DOI: 10.1021/acs.energyfuels.5b02383 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels °C, and it was further noted that the yield was little affected by heating rate over the range 0.4 to 1000 °C·s−1.19 (In the present study the average heating rate was 1 °C·s−1.) A loss of more hydrogen-rich material could be seen from the slight decrease in hydrogen-to-carbon ratio (Figure 5) of the liquids. The experimental observations were consistent; a slight decrease in hydrogen-to-carbon ratio would impose a stoichiometric hydrogen constraint during pyrolysis and cause an increase in coke yield as seen by the increase in microcarbon residue. It is possible that other changes also took place during the heat-up period that affected the bitumen. The viscosity of the bitumen halved during the heat-up period (Figure 2), but such a direct comparison should not be made. It was previously reported that when reaction products after heating were not filtered, the measured viscosity was ∼55% higher due to the presence of the solids.11 The observed difference in viscosity between the feed and the product after 0 min reaction at 400 °C might not be a real decrease in viscosity, although a decrease in viscosity during this period could not be ruled out. 4.2. Stable Visbreaking (Regime Ib). The stable visbreaking regime is the time-period during which thermal reactions take place without much production of solids at constant temperature. It can be argued that the reaction progresses in the same way as during dynamic preheating (Regime Ia) except that the reaction now proceeds isothermally. The differentiation between Regime Ia and Ib was made only to facilitate discussion of the experimental results. In the furnace coil of an industrial visbreaking unit, there is always a temperature profile. In the present study, the stable visbreaking period is represented by the data after 0, 30, 60, and 80 min of reaction. During this period, the nonmineral solids yield calculated from Table 2 remained in the range 0.1 to 0.5 wt %, with a sample standard deviation of 0.5 wt %. There was practically no coke and whatever carbonaceous deposits formed, formed on the mineral matter (Figure 7a,b). Even though the reaction progressed in a similar fashion as described in the literature on thermal cracking (e.g., refs 3,4), we would like to divide the stable visbreaking regime into a “productive” and an “unproductive” period. This distinction is a consequence of the difference in perspective between visbreaking for partial upgrading of bitumen and visbreaking as a refinery process. In a petroleum refinery, conversion is important, because the aim is to produce naphtha and distillate from the visbreaker feed, while converting the remainder of the heavier oil fraction sufficiently to meet fuel oil viscosity requirements. In a partial upgrader, the aim is to lower the viscosity of the feed material and not to obtain a high vacuum residue conversion, or to obtain a meaningful yield of lighter products specifically. Table 4 summarized the experimental observations to illustrate the difference between “productive” and “unproductive” visbreaking. Hydrogen disproportionation proceeds over time and there is no observable difference in the impact of hydrogen disproportionation on the change in microcarbon residue, hydrogen-to-carbon ratio, and refractive index between the “productive” and “unproductive” periods. The loss of hydrogen from the asphaltenes subfraction is possibly higher, considering that the hydrogen-to-carbon ratio of asphltenes fractions from different vacuum residues were reported to decrease from 1.1 to 1.0−0.9 after 30 min reaction time, with little further change after 60 min reaction time at 400 °C.20
Table 4. Changes in Properties during “Productive” (0 and 30 min Reaction) and “Unproductive” (60 and 80 min Reaction) Visbreaking at 400 °C change in Regime I description
results
“productive”
“unproductive”
gas yield viscosity asphaltenes content microcarbon residue H:C molar ratio refractive index
Table2 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6
constant decrease constant increase decrease increase
increase increase increase increase decrease increase
In both “productive” and “unproductive” periods, there were an increase in microcarbon residue, decrease in hydrogen-tocarbon ratio, and increase in refractive index. However, there was a decrease in viscosity only in the “productive” period, and it was likely related to a decrease in the effective volume fraction (Φeff) of aggregated material (eq 1). In the “unproductive” period, the viscosity deteriorates somewhat and there is an increase in asphaltenes content, as well as an increase in gas yield. Despite the lack of coke formation and seemingly steady progression of hydrogen disproportionation, why does the reaction behavior change? One possible explanation for the observations is that hydrogen disproportionation over time makes some pendant groups more susceptible to cracking, while at the same time increasing the probability of unfavorable free radical addition reactions. The chemistry leading to the cracking of alkyl chains is illustrated in Figure 9.
Figure 9. Hydrogen disproportionation of alkyl naphtheno-aromatic compounds to produce alkyl aromatic compounds that facilitate thermal cracking of alkyl groups.
The transfer of hydrogen during hydrogen disproportionation does not produce hydrogen free radicals (H·), and the loss of hydrogen indicated in Figure 9 is by hydrogen disproportionation. Hydrogen disproportionation is a less-demanding reaction, because it involves the transfer of hydrogen, rather than cracking to produce new free radicals. The energy associated with hydrogen disproportionation between a methyl radical and various nonradical hydrocarbons that have transferable hydrogen is in the range 30 to 45 kJ·mol−1.21 Hydrogen disproportionation can readily take place at temperatures much lower than 400 °C. Oilsands bitumen is rich in cycloalkane groups (naphthenes).2 When alkyl groups are attached to a cycloalkane, the C−C bond dissociation energy is typical of aliphatic C−C bonds, around 365 kJ·mol−1.22 When a cycloalkane is converted F
DOI: 10.1021/acs.energyfuels.5b02383 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
tive” visbreaking, there is a continuous decrease in hydrogen-tocarbon ratio of the liquid (Figure 5). The phase behavior of different solubility fractions of raw feed materials and thermally cracked products were shown to correlate with the hydrogento-carbon ratio of the material.24 The decrease in hydrogen-tocarbon ratio affects the solubility of the hydrogen-poor material in the bulk liquid phase. This can be seen from the increase in the measured asphaltenes content of the liquid products (Figure 3). Under thermal cracking conditions, the decreased solubility leads to the formation of a second liquid phase, the meso-phase. This second liquid phase is the precursor of coke formation.25 The duration of this period before the onset of coking is influenced by the vapor−liquid equilibrium, with higher pressure suppressing the onset of coking.13 The present work was conducted at higher pressure than typically found in industrial visbreaking units, and it is anticipated that at lower pressure the duration of Regime 1b could be somewhat shorter. 4.3. Coking Visbreaking (Regime II). The end of the stable visbreaking period (Regime I) is marked by an increase in coke formation (Figure 10).
into an aromatic by hydrogen disproportionation, the C−C bond dissociation energy of the alkyl group decreases to around 320 kJ·mol−1,22 due to the resonance stabilization of the benzylic radical. This is shown in Figure 9 by the hydrogen disproportionation reaction of the alkyl naphtheno-aromatic compound (1) to produce the alkyl aromatic compound (2). Subsequent thermal cracking of the alkyl group in (2) is therefore easier than in (1). It is possible that the stepwise hydrogen disproportionation leading from (1) to (3) to (4) shown in Figure 9 provides an even easier route for dealkylation. The C−C bond scission of (4) produces a nonradical aliphatic molecule and a benzylic radical by intramolecular hydrogen disproportionation, rather than the formation of two free radical fragments. The same reaction sequence as shown in Figure 9, when applied to multinuclear compounds with multiple cycloalkane structures, can explain the formation of longer chain hydrocarbons. The scission of the C−C in compound (1) or (3) between the benzylic carbon and the tert-carbon containing the alkyl group, would produce a C7 alkyl group, as opposed to the original C4 alkyl group. It is therefore possible to also produce aliphatic chains of increased length using the same reaction sequence, as shown in Figure 9, but applied to multinuclear compounds with multiple cycloalkane structures. This may help explain the reported formation of long-chain aliphatic molecules during the thermal cracking of asphaltenes without such long alkyl groups being present in the raw material (e.g., ref 23). Further support for the reaction sequence in Figure 9 was obtained from the 1H NMR data (Table 3). A consequence of this thermal cracking pathway is that the methyl content of the reaction product should increase over time. Much of the hydrogen in methyl groups in bitumen is lost during the initial stages of thermal cracking, but over time an increase in the percentage of hydrogen in methyl groups from 3.4 ± 0.4% at 60 min reaction to 11.4 ± 2.7% at 300 min reaction was observed. Other reactions likely also contributed to this change, but directionally the 1H NMR results support the pathway shown in Figure 9. During the initial stages of Regime I, the bitumen is selfstabilizing, because it contains naphtheno-aromatics that have good hydrogen donor properties. Hydrogen disproportionation performed by compounds that are hydrogen donors, converts free radicals formed by cracking reactions into nonradical products by H· transfer but without forming new free radical species. For example, as shown in Figure 9, the alkyl naphtheno-aromatic (1) is a good hydrogen donor, and multiples of 2 H· transfer leads to the nonradical products (2) and (3). With increasing reaction time, hydrogen disproportionation depletes the hydrogen donor content and increases the cracking propensity. The free radical concentration in the reacting mixture is increased as a consequence of both aforementioned outcomes, namely, a decrease in hydrogen donor content and an increase in cracking. Because free radical addition is a bimolecular process, the probability of free radical addition is increased by the increase in free radical concentration. The increase in free radical addition products is reflected in the slight increase in viscosity (Figure 2), which characterizes the “unproductive” period in Regime I. With the increase in gas yield (Table 2) and loss of lighter and more hydrogen-rich volatile products during “unproduc-
Figure 10. Coke-yield with reaction time at 400 °C calculated from Table 2.
The term “coke” is rather loosely used in the thermal cracking literature, and “coke” generally refers to the solid carbonaceous deposits formed during thermal conversion, irrespective of the degree of pyrolysis. In this study, the maximum temperature was 400 °C, and the coke was not fully pyrolyzed, but the material was insoluble in toluene. Hence, solid carbonaceous deposits met the solubility definition for coke, but it was unlikely that the material was as carbon-rich as the coke produced during higher temperature thermal conversion. The solids nucleated as small particles in the liquid to produce coke with a small particle diameter (Figures 7 and 8), making separation by filtration very difficult. The mineral matter present in the oil has only a finite capacity for coke growth, and additional coke particles are nucleated within the liquid phase, which resulted in the observed bimodal particle size distribution (Figure 8). The formation of organic solids is a consequence of both insolubility and hydrogen depletion. As thermal conversion progresses, some of the molecules become more hydrogen depleted. As explained before, this causes the solubility of the molecules in the bulk oil to decrease to a point where a second liquid phase is formed. As the molecules become even more hydrogen depleted, they become insoluble and precipitate as a solid phase, i.e. coke. Wiehe26 provides a particularly lucid description of this process and how it leads to coke formation. G
DOI: 10.1021/acs.energyfuels.5b02383 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
association that does not require bond formation at 400 °C. As a side-line comment, it is interesting to note that many of these observations also apply to radiation-induced cracking of bitumen at low temperature (e.g., ref 30). The viscosity of the liquid passes through a local minimum during the coking visbreaking period. At least some of the heavier products formed by free radical addition were either cracked to lighter products, or became insoluble and were precipitated as solids. The local viscosity minimum was likely the result of the competing processes of heavier product formation and heavier product removal from the liquid. 4.4. Coking (Regime III). The transition between coking visbreaking (Regime II) and coking (Regime III) took place between 270 and 280 min reaction time at 400 °C. The transition was evident from the increase in the rate of coke formation (Figure 10), as well as in the nature of the coke (Figure 7). The coke started to aggregate and form a solid mass, which could be separated more easily from the liquid by filtration. There was also acceleration in the rate of decrease in the hydrogen-to-carbon ratio of the liquid (Figure 5), with a concomitant increase in microcarbon residue (Figure 4) and refractive index (Figure 6). Coking taking place in both Regimes II and III produced a coke that is different from the coke observed in higher temperature processes, such as delayed or fluidized bed coking. Although the coke was not studied in detail, the changes in the liquid product suggested that at 400 °C the coke was not appreciably pyrolyzed once formed. The coke appeared to be a stable precipitation product rather than a hydrogen-depleted product. There was not much additional gas evolved during the coking period (Table 2), but the hydrogen-to-carbon ratio of the liquid kept on decreasing. The liquid product did not become lighter boiling either. The results indicated that the changes in the liquid product were mainly due to hydrogen disproportionation and free radical addition reactions. The liquid viscosity increased (Figure 2), and it approached a viscosity close to that of the bitumen feed at the maximum reaction time studied. The increase in viscosity was not monotonous, but was affected by the partitioning of molecules between the liquid and solid phases. For example, there was a local minimum in viscosity observed at 330 min reaction time, which occurred at the same time as an increase in coke yield from 14.5 ± 2.2 to 23.6 ± 2.2 wt % to remove heavy material from the liquid phase. Continued reaction of the bitumen at 400 °C in the coking period had no observable product upgrading benefit. 4.5. Implications for Oilsands Bitumen Upgrading. Visbreaking of oilsands bitumen at 400 °C was able to decrease the viscosity of the bitumen measured at 60 °C from 9.6 ± 0.3 to 0.15 ± 0.03 Pa·s after 30 min reaction time. At the same time, there was
