ARTICLE pubs.acs.org/EF
Influence of Thermomechanical History on Chemical and Rheological Behavior of Bitumen M. Mouazen,†,‡ A. Poulesquen,*,† and B. Vergnes‡ † ‡
Atomic Energy Commission Marcoule, Nuclear Energy Direction, DTCD/SPDE/L2ED, BP17171, 30207, Bagnols Sur Ceze, France Mines Paristech, Cemef, UMR 7635, BP 207, 06904, Sophia Antipolis, France ABSTRACT: It is well-known that asphaltene content plays an important role in determining the high viscosity of bitumen. This paper presents an experimental study of the specific effects of extrusion operating conditions on the physical and chemical properties of bitumen. Five bitumen samples were prepared by twin screw extrusion with different operating conditions (feed rate Q and screw speed N). Physical properties were studied by rheological measurements. Viscosity values were measured by steady state flow tests. Chemical changes in the bitumen structure were followed in the infrared region with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy by measuring the evolution of the bands areas at 1700 cm1 (CdO), 1030 cm1 (SdO), and 1600 cm1 (aromatic CdC) and the bands between 900 and 730 cm1 attributed to aromatic C—H. An increase in feed rate Q induces a decrease in the Newtonian viscosity, as a result of a decrease in the asphaltene volume fraction. The characterization by ATR confirms that the decrease in feed rate entails the creation of CdO functional groups and the increase in sulfoxide (SdO) functional groups and CdC bonds, accompanied by a decrease in the C—H aromatic bonds. These results indicate a structure that is more oxidized and more aggregated at low feed rate, certainly as the result of an increase in the residence time into the extruder. The increase in screw speed also induces decreases in the viscosity and the volume fraction of asphaltenes, until a point after which the situation reverses. This change may be explained by the appearance of new peaks between 1200 and 1050 cm1, attributed to CdS bonds, and between 640 and 540 cm1, for S—S bonds. A competition between shear rate and residence time takes place. The thermomechanical history has, thus, a great influence on the chemical and rheological behavior of pure bitumen, and the chemical changes observed show that the asphaltene volume fraction is not the unique parameter that explains the variations in viscosity.
1. INTRODUCTION Bitumens differ in their physical and chemical properties as a result of the nature of their crude oil source and the operations involved in their production by fractional distillation.1 They are usually characterized by a large number of empirical standards tests, which include penetration index, softening point, asphaltene content, and viscosity. Bitumen material can be considered as a colloidal suspension, in which the asphaltene particles (28 nm) are dispersed in a maltene matrix.1 The effect of asphaltene content on the heavy oil viscosity has been studied for a long time. To estimate the effect of asphaltene content on viscosity, Dealy2 prepared a sample by adding 5 wt % of asphaltenes to a bitumen with an initial asphaltene content of 16%. He observed that the viscosity was more than three times higher compared to that of the initial bitumen. In the same way, Henaut et al.3 measured the rheological behavior of a heavy crude oil, which clearly shows an increase in viscosity with asphaltene content. The viscosity of reconstituted oil with 18% asphaltenes was 50 times higher than that of the maltenes (0% asphaltenes) at 20 °C. Two domains can also be distinguished by plotting the Newtonian viscosity of bitumen versus the inverse of absolute temperature, according to an Arrhenius law. The calculation of the activation energy Ea confirms these two regions. Mouazen et al.4 and Henaut et al.3 reported a critical temperature around 50 °C, above which a significant difference in activation energy was found. The increase in Ea for temperatures lower than 50 °C means that the structure of the heavy oil becomes more rigid. It might be attributed to the appearance of interactions between r 2011 American Chemical Society
asphaltenes, which become stronger below 50 °C (the structure of bitumen becomes more rigid), whereas, above this temperature, the Brownian motion is predominant and asphaltenes may move “more freely” in the maltene matrix.4 These two domains have also been identified by X-ray scattering, but most of the X-ray studies have been performed after the dispersion of the asphaltene molecules within an organic solvent, which is quite different from the case for the real system (crude oil). However, Barre et al.5 and Pierre et al.6 suggested the presence of two different domains corresponding to q-values larger and smaller than 2.5 103 A1. On the other hand, the amount of heteroatoms (sulfur, S; oxygen, O; and nitrogen, N) and polar molecules in the crude oil plays an important role in the association of the asphaltene molecules. Moreover, the heteroatoms and their amounts seem also to have a strong influence on the chemical and physical properties of bitumen. Michalica et al.7 supposed that a higher content of heteroatoms leads to a higher degree of self-assembling and association of molecules, which will result in an increase in the binder stiffness. On the other hand, lower heteroatom content could be responsible for insufficient intermolecular associations, resulting in a weak viscosity. Several authors found that oxidative aging was responsible for a further increase in the content of heteroatoms,7,8 mainly oxygen, which, again, can result in an increase in the binder viscosity.9 Generally, the oxidation of bitumen Received: July 8, 2011 Revised: August 30, 2011 Published: August 30, 2011 4614
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Table 1. Extrusion Conditions (Barrel Temperature Varies 140180 °C along the Extruder) screw speed, N
feed rate,
N/Q ratio
residence
sample
(rpm)
Q (g/h)
(rpg)
time (s)
A B
70 70
1150 840
3.65 5
162 202
C
70
395
10.6
368
D
150
360
25
370
E
300
372
48.4
347
results in an increase in the asphaltene content and a decrease in the saturates and aromatics contributions. Moschopedis and Speight, by exposing the bitumen to oxygen, have found that the oxygen acts as a catalyst in the production of additional asphaltenes. Bitumen aging, thermo-oxidation process, UV radiation,10 and storage mode conditions also lead to significant variations in bitumen properties. Two types of mechanisms are generally involved; the first one is irreversible and is characterized by chemical changes, including oxidation and the loss of volatile components. The second mechanism is a reversible process called physical hardening; it may be attributed to a molecular structuring to approach an optimum thermodynamic state.11 The principal condition for the first mechanism is the presence of oxygen and its insertion into the carbon chains, leading to the formation of oxygen-containing groups. The aromatization (increase in the carbon-to-hydrogen ratio and formation of CdC bonds) and, finally, the cross-linking process produced by the formation of intermolecular and intramolecular bonds also take place. At room temperature, the oxidation process is only possible at the bitumen surface because the oxygen diffusion is limited.8 However, the increase in temperature leads to the acceleration of the O2 diffusion process, and an oxidation process is then favored. Several methods have been used to replicate the effect of aging. Three major tests are used: TFOT (Thin Film Oven Test), RTFOT (Rolling Thin Film Oven Test),9,11 and PAV (Pressure Aging Vessel).12 Recently, a new method was developed through thermo-oxidation in a rheo-reactor.13 The mixing process at high temperature also has significant effects on the bitumen properties. An increase in the viscosity was observed with the increase in processing time.14 These modifications are related in the literature to several phenomena, such as asphaltene oxidation15 and structuring process of molecules and clusters.16,17 Moreover, these modifications cannot be attributed to the loss of volatile components because these components do not significantly contribute to an increase in the viscosity value. In the present work, we investigate the influence of extrusion conditions (screw speed N and feed rate Q) on the pure bitumen behavior. Indeed, in the nuclear industry, twin screw extrusion is used to embed radioactive elements into a bitumen matrix. This thermomechanical history has a great influence on the chemical, physical, and rheological behavior of pure bitumen. The chemical changes, mainly due to oxidation and mechanical stresses, will be assessed by infrared spectroscopic techniques. On the other hand, the physical modifications will be analyzed by rheological characterizations.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Extruded Bitumen. Pure bitumen (Azalt 70/100, provided by Total) was introduced at a temperature of
140 °C in the feeding zone of a laboratory scale corotating twin screw extruder (Werner ZSK25WLE; length, L = 1000 mm; screw diameter, D = 25 mm; L/D = 40). The temperatures of the various barrel elements were set between 140 °C (bitumen introduction) and 180 °C (extruder exit). The used bitumen mainly consists of carbon (typically 84.7 wt %) and hydrogen atoms (10.2 wt %). In addition, heteroatoms such as sulfur (4.3 wt %), nitrogen (1.3 wt %), and oxygen (0.5 wt %) are generally present. Traces of metals are also found, with the most numerous being typically vanadium (116 ppm) and nickel (37 ppm). These elements were quantified by specific infrared detector with precision measurement around (0.30%. The ratio C/H for the used bitumen is nearly 0.69. All elementary measurements were performed at the Service Central d0 AnalyzeSolaize/France (USR-59/CNRS). Five extruded samples have been prepared using three different screw speeds, N (rpm), and three different feed rates, Q (g/h). Extrusion conditions are given in Table 1. 2.2. Rheological Study. The rheological properties were analyzed using a controlled stress rheometer (MCR 301, Anton Paar) in parallel plate geometry, with disks of 25 mm diameter and 1 mm gap. The rheometer has been used in strain mode. For dynamic tests, the strain sweeps were conducted with a frequency of 100 rad/s by varying the strain from 106 to 100 to determine the linear viscoelastic region of the samples. The frequency sweeps were then performed from 100 to 0.01 rad/s at a strain value within the linear viscoelastic domain. The flow curves were obtained by starting from 106 and going up to 100 Pa (five stress levels per decade), measuring the corresponding viscosity once steady state flow was observed. 2.3. Asphaltene/Maltene Separation. The experimental procedure for the separation of asphaltenes from the bitumen consists of mixing 1 g of bitumen with 40 mL of n-pentane, which was used as a precipitant. The mixture was agitated for 4 h and then filtered through 3 μm filter paper. The precipitated asphaltenes were rinsed with npentane until they had a light brown color. The precipitated and the remaining maltenes were slowly dried under the fume for 12 h and then for 12 h in an oven at 50 °C. Nonextruded bitumen contains around 23.6% asphaltene mass fraction. 2.4. Infrared Spectra. Chemical changes in the bitumen structure were followed by Fourier transform infrared (ATR-FTIR) spectroscopy (Thermo Scientific) by measuring the evolution of the band areas at 1700 cm1 (CdO), 1030 cm1 (SdO), and 1600 cm1 (CdC double bonds) and the three bands included between 900 and 730 cm1 attributed to the C—H aromatics. Several indices were calculated from the areas of the IR18 bands shown in Figure 1. They are characteristic of oxidation (A1700/∑A); aromaticity (A1600/∑A);18,19 aliphaticity ((A1452 + A1373)/∑A); ramification (A1373/(A1452 + A1373));19 presence of sulfoxide group (A1030/∑A); presence of aromatic H ((A874 + A809 + A747)/ ∑A), 747 cm1 (4 or 5 adjacent C—H), 809 cm1 (2 adjacent C—H), and 874 cm1 (one isolated C—H);18,19 and long chains A720/(A1452 + A1373).19 The sum of the areas is calculated from
∑A ¼ A2950 þ A2860 þ A1700 þ A1600 þ A1452 þ A1373 þ A1030 þ A874 þ A809 þ A747 þ A720
3. RESULTS 3.1. Rheological Characterizations. 3.1.1. Effect of Feed Rate. Figure 2 presents the evolution of the dynamic storage
modulus G0 with strain amplitude, at 100 rad/s and 50 °C for the three samples A, B, and C. It is observed that the storage modulus clearly decreases with the feed rate. At the same time, the critical strain γc slightly increases, from 0.16 to 0.25. This change in critical strain may be related to structural changes.14 Similar results were observed for the loss modulus (G00 ) and the complex viscosity (η*). Figure 3 shows the evolution of the shear viscosity 4615
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Figure 1. Typical ATR-FTIR transmission spectrum of bitumen.
Figure 2. Storage modulus G0 vs strain for bitumen prepared at constant speed (70 rpm) and different feed rates (100 rad/s, 50 °C).
Figure 3. Viscosity vs shear rate for bitumen prepared at constant speed (70 rpm) and different feed rates (100 rad/s, 50 °C).
obtained from steady state flow tests for the three samples. The behavior is globally Newtonian, with a yield stress appearing at
Figure 4. Storage modulus G0 vs pulsation for bitumen prepared at constant speed (70 rpm) and different feed rates (100 rad/s, 50 °C).
very low shear rate (less than 104 s1). The decrease above 10 s1 is due to sample fracturing.4 We observe a decrease in the Newtonian viscosity η when increasing the feed rate Q. The values of η are 540, 640, and 950 Pa 3 s for the three samples A, B, and C, respectively. To further investigate the microstructure, dynamic frequency sweep tests have been performed. Figure 4 presents the plot of storage modulus G0 versus angular frequency ω in the linear region (γ = 0.001) for the three samples, measured at 50 °C. At high pulsation, we observe an increase in G0 when Q decreases, which is consistent with the previous results obtained in strain sweeps. On the other hand, at low pulsation, a plateau appears, which is characteristic of a solidlike behavior. This effect is more pronounced for the lowest feed rates with a larger plateau. On the other hand, the loss modulus G00 (not represented here) scales as ω at low pulsations. These results globally indicate an increase in the interactions between asphaltene particles when the feed rate is decreased. In the literature, this phenomenon is often related to a secondary relaxation process, related to the existence of a structure resulting 4616
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Table 2. Asphaltene Mass and Volume Fractions and Newtonian Viscosity at 50 °C for Each Sample screw speed, feed rate, sample N(rpm) Q (g/h) A B C D E F
0
Figure 5. Storage modulus G vs strain for bitumen prepared at constant feed rate (≈ 375 g/h) and different screw speeds (100 rad/s, 50 °C).
70 70 70 150 300
1150 840 395 360 372
asphaltene content (% wt)
asphaltene content (% vol)
viscosity (Pa 3 s)
17.6 18.75 24 19.5 21.9 0
15.5 16.5 21.1 17.1 19.2 0
540 640 980 295 430 14
where Fbitumen = 1.02 g/cm3 is the density of the bitumen, Fashaltene = 1.165 g/cm3 is the density of the asphaltenes, and wt % is the weight percentage of asphaltenes. Density measurements were performed at 20 °C. The Newtonian viscosity of the bitumen with zero asphaltene content (i.e., maltenes) was also measured. Its value is 14 Pa 3 s at 50 °C. All the data resulting from these analyses are listed in Table 2.
4. DISCUSSION
Figure 6. Storage modulus G0 vs pulsation for bitumen prepared at constant feed rate (≈ 375 g/h) and different screw speeds (100 rad/s, 50 °C).
from the interactions between the asphaltene aggregates.4,20 In other words, at low angular frequency, the particles of asphaltene have the time to restructure and to approach an optimum thermodynamic state under a specific set of conditions (formation of aggregates). 3.1.2. Effect of Screw Speed. Figure 5 presents the evolution of the dynamic storage modulus G0 with strain amplitude, measured at 50 °C for the three samples C, D, and E, produced at different screw speeds, N. We observe first that the changes of G0 values with N are more pronounced than with Q. The increase in N from 70 to 150 rpm leads to a decrease in the storage modulus G0 (from 1.66 104 to 2.6 103 Pa) and an increase in the critical strain (from 0.16 to 0.25), but the situation reverses for the sample E extruded at 300 rpm (G0 = 5.2 103 Pa, γc = 0.20). In Figure 6 we present the frequency sweep tests for these three samples. In the whole pulsation range, the same ranking as in strain sweep tests is obtained. Once again, we observe the development of a plateau for G0 at low pulsations. 3.2. Asphaltene/Maltene Separation. To explain the differences in the rheological behavior, the asphaltene content for the five samples has been evaluated by separating asphaltenes and maltenes, using n-pentane as solvent. The volume fraction is calculated from F ð1Þ ϕ ¼ bitumen wt % Fasphaltene
4.1. Effect of Feed Rate. According to Table 2, it is observed that, at a constant screw speed (70 rpm), the decrease in feed rate from 1150 to 395 g/h leads to an increase in asphaltene volume fraction from 17.6 to 24%, accompanied with an increase in viscosity (from 540 to 980 Pa 3 s at 50 °C), which is fully consistent with the literature.14 An asphaltene content variation of 5.5 wt % leads to a viscosity two times higher. The loss and storage moduli are also higher, and at low pulsation, the formation of asphaltene aggregates is more pronounced when the volume fraction of asphaltenes increases. To more deeply interpret these results from a microstructural point of view, a series of infrared measurements has been conducted. Figure 7 shows the changes in the infrared spectra for the three samples A, B, and C. The decrease in the feed rate induces an increase in the carbonyl (CdO) absorption band at 1700 cm1 at a temperature between 140 and 180 °C. Other absorption bands, such as C—O single bond stretching (≈9101300 cm1) and sulfoxide SdO (1030 cm1), are also increased with the decrease in feed rate. The area values for each peak, calculated by OMNIC infrared software (Thermo Scientific), are listed in Table 3. In addition, Figure 7 also shows that the CdC aromatic bonds at 1600 cm1 grow and the area of the three bands between 900 and 730 cm1, characterizing the C—H aromatics bonds, decreases with the decrease in feed rate. This indicates that the ratio C/H increases, which means that the aromatic rings fused to produce some sheets of asphaltenes. Consequently, the structure becomes more compact. Therefore, an increase in the asphaltene content is observed from both macroscopic and microscopic points of view. Furthermore, the long chains located at 720 cm1 and the aliphaticity index ((A1452 + A1373)/∑A) decrease with the decrease in feed rate (Table 3), which confirms that the structure becomes more compact. The residence time of the bitumen in the extruder could be the main cause of this variation. Indeed, in a twin screw extruder, the residence time increases when the feed rate decreases.21 In our conditions, we can estimate the average residence time at 162, 202, and 368 s for the feed rates 1150, 840, and 395 g/h, respectively. Consequently, a long residence time facilitates the 4617
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Figure 7. ATR-FTIR spectra of the three samples at different feed rates: (a) in the range 40002000 cm1 and (b) in the range 2000400 cm1.
Table 3. Relative Area Values for Some Peaks of Samples A, B, and Ca samples
a
A(CdO)/
A(CdC)/
A(SdO)/
A(C—H)/
aliphaticity
A(1700 cm1)
A(1600 cm1)
A(1030 cm1)
A(900730 cm1)
(A1452 + A1373)/A)
long chains
A
0
0.032
0.012
0.060
0.278
0.081
B
0.011
0.074
0.016
0.057
0.270
0.068
C
0.016
0.078
0.022
0.050
0.250
0.050
A is the total area defined in section 2.4.
Figure 8. ATR-FTIR spectra of the three samples at different screw speeds: (a) in the range 40002000 cm1 and (b) in the range 2000400 cm1.
diffusion of oxygen, leading to a more oxidized matrix. The high temperature of the extruder barrels (140180 °C) also facilitates oxygen diffusion, leading to an acceleration of the aging process and an increase in the asphaltene content and size.13 4.2. Effect of Screw Speed. At constant feed rate, according to Table 2, an increase in screw speed leads to a decrease in viscosity and asphaltene volume fraction until it reaches a value above which these two parameters increase again. An observation of the microstructure by infrared spectroscopy clarifies the situation and explains this change. Figure 8 shows the effect of the screw speed on infrared spectra for the three samples C, D, and E. By comparing samples C and D, we see that an increase in screw speed leads to a decrease in the area of the bands CdC, CdO, and SdO and an increase in C—H bands, aliphaticity index, and long chains. All the values are listed in Table 4. Thus,
for the sample at 150 rpm, it seems that the oxidation is reduced but it cannot be explained by the residence time, which is similar: 368 s at 70 rpm and 370 s at 150 rpm. However, the infrared spectrum for sample E at 300 rpm is very different. Indeed, a high asphaltene content is observed, concomitant with a decrease in oxidation peaks (CdO), indicating a lower oxidation. It does not seem very consistent with the previous results, where an increase in asphaltene content was accompanied by an increase in the oxidation peaks. However, some new peaks appear in the region 1200500 cm1; an intense CdS band at 12001050 cm1 and a S—S band around 640540 cm1 are clearly identified. These two sets of peaks can be the principal cause of the increase in viscosity and asphaltene content without oxidation phenomenon. Unlike the other operating conditions, where the residence time in the extruder dominates, for the high screw speed, the 4618
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Table 4. Relative Area Values for Some Peaks of Samples C, D, and Ea samples
a
A(CdO)/
A(CdC)/
A(SdO)/
A(C—H)/
aliphaticity
A(1700 cm1)
A(1600 cm1)
A(1030 cm1)
A(900730 cm1)
(A1452 + A1373)/A)
long chains
C
0.016
0.078
0.022
0.050
0.250
0.050
D
0.006
0.037
0.011
0.066
0.330
0.102
E
0
0.058
0.005
0.055
0.276
0.078
A is the total area defined in section 2.4.
generated shear rate (which is proportional to the rotation speed) probably becomes the dominant parameter. Consequently, the specific mechanical energy supplied by the extruder is sufficient to modify the microstructure by the creation of intermolecular or intramolecular bonds, leading to an increase in asphaltene content. In fact, the high content of heteroatoms, especially sulfur (4.35%), facilitates the achievement of self-assembling and association of molecules (CdS and S—S), evidenced by FTIR, which will result in a higher stiffness of the bitumen. 4.3. Discussion. A paradoxical result is obtained for the two samples B and D. Indeed, they have approximately the same asphaltene volume fraction (16.5 for B and 17.1 for D), but the Newtonian viscosity of B is more than two times higher than the one of D (640 versus 295 Pa 3 s). If we compare the FTIR data for samples B and D, we notice that the carbonyl peak (CdO) is higher for D, which is completely consistent with its longer residence time in the extruder: 202 s for B compared to 370 s for D. On the other hand, the aromatic (CdC) and sulfoxide (SdO) peaks are higher for sample B, which is consistent with the viscosity evolution. Furthermore, we observe that the aromatic C—H index, the aliphaticity index, and the long chains ratio are higher for sample D, which can confirm the lowest value of viscosity obtained (295 Pa 3 s). Consequently, we can assume that the increase in screw speed from 70 to 150 rpm induces a separation of heteroatoms or polar molecules, which leads to smaller asphaltene aggregates (i.e., the number of sheets of asphaltene into the aggregates decreases). Therefore, a competition between the screw speed (shear rate, strain, and specific energy) and the feed rate (residence time) takes place, and the viscosity of bitumen is given by a compromise between the asphaltene content and the quantity of heteroatoms existing and/ or inserted (oxidation process) into the bitumen. In twin screw extrusion, the variations of screw speed N and feed rate Q are currently accounted for by considering the ratio N/Q (expressed in rpg, revolution per gram), to which the specific energy is proportional. To better interpret the previous results, we have plotted in Figure 9 the different area ratios determined by ATR-FTIR as a function of N/Q. It clearly evidences the existence of two domains defining the evolution of the bitumen structure and composition. For N/Q values smaller than approximately 18 rpg (low speed region), we observe an increase in oxidation and simultaneously a decrease in C—H bonds to the benefit of CdC bonds. Moreover, we note a minimum of CH2 long chains, which confirms the higher viscosity and asphaltene ratio obtained in the case of sample C. Above 18 rpg (high speed region), oxidation decreases and C—H bonds increase to the detriment of CdC bonds. Furthermore, for sample D, the weak CdC contribution and the maximum obtained on CH2 chains confirm the lowest value obtained. The viscosity of the bitumen is directly affected by these structural evolutions.
Figure 9. Area ratios obtained by ATR-FTIR as a function of the N/Q ratio: (a) for SdO and CO bonds (oxidation) and (b) for CdC and CH2 bonds (aromatization and long chains).
5. COMPARISON BETWEEN EXPERIMENTAL RESULTS AND RHEOLOGICAL MODELS In the literature, there are a lot of theoretical models and empirical correlations for predicting the crude oil viscosity. However, most of these models are based on a simplified system where asphaltenes are dispersed in an organic solvent, because of the rather complex structure and composition of the crude oil.5,6,22 According to the literature, bitumen is a colloidal suspension in which the asphaltene particles (28 nm) are dispersed in a maltene matrix.1 It is well-known that the relative viscosity of a colloidal dispersion (ratio of the viscosity η of the dispersion to the viscosity of the dispersing liquid η0) can be expressed by a KriegerDougherty type equation:23 !q ϕef f η ηr ¼ ¼ 1 ð2Þ η0 ϕmax where q is a factor depending on the dispersed particles, ϕmax is the maximum packing volume fraction, and ϕeff is the effective volume fraction of the dispersed phase. The value q is usually expressed as q = [η]ϕmax, where [η] is the intrinsic viscosity. In a 4619
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Figure 10. Relative viscosity of the bitumen samples vs the asphaltene volume fraction at 50 °C, experimental points (symbols) and theoretical fit by KriegerDougherty equations (full lines).
colloidal dispersion, ϕeff can also be expressed as a function of a solvation constant K, which is temperature dependent24 and the real volume fraction ϕ1,24 ϕef f ¼ Kϕ
ð3Þ
The q factor of the asphaltene particles is variable. The value 2.5 is often proposed for the intrinsic viscosity of rigid spherical particles, but for ellipsoidal and disklike forms, it is more than 2.5. Luo and Gu24 have found that the asphaltene particles have a nonspherical shape ([η] in the range 56) and probably different sizes, that is, a polydisperse size distribution. The maximum packing volume fraction of asphaltenes in the crude oil is also very difficult to apprehend because it is structure and origin dependent. Storm and Shen25 found that the maximum volume fraction ϕmax of the asphaltene particles was close to 0.35. Luo and Gu24 estimated this value to be in the range 0.60.7. Figure 10 presents the evolution of the relative viscosity of extruded samples as a function of the volume fraction of asphaltenes. First of all, we see that we do not obtain a master curve, which indicates that the volume fraction is not the unique parameter that controls the viscosity. As shown previously, it seems that we can select two families of data, each one being more or less described by a KriegerDougherty relationship (eqs 2 and 3): the samples made in low speed conditions (70 rpm) provide a maximum packing fraction ϕmax of 0.37 and a q factor of 5.5, and the samples made at high speed (150 and 300 rpm) give a ϕmax of 0.36 and a q of 4.5. These values are close to those reported in the literature, but this distinction as a function of screw speed would remain to be confirmed. These results are related to a modification of the composition or the structure of asphaltene particles. More specifically, the oxidation process favored at low speed, by creating new peaks observed by IR spectroscopy (CdO and SdO) and the appearance of new contributions at 300 rpm, could explain these differences, by a change in the amount or the proportion of heteroatoms.
6. CONCLUSIONS Extrusion operating conditions significantly influence bitumen chemistry and rheology, by aging or mechanical processes. Aging produces fundamental modifications in the asphaltene content of bitumen and in their chemical properties. Chemical changes include the formation of carbonyl and sulfoxide bonds and an increase in CdC bands. The chemical and rheological changes are generally consistent. Actually, the aging effect is strongly
dependent on extrusion operating conditions. The decrease in feed rate leads to an increase in sample viscosity as a result of an increase in asphaltene volume fraction. This is certainly due to an increase in the residence time in the extruder, where the structure becomes more oxidized and more compact as the result of an improved oxygen diffusion process. Similar results were obtained with the increase in screw speed, until a point where the situation reverses. This change has been related by ATR-FTIR to the apparition of news peaks (CdS and S—S) and is due to the high shear rates supplied by the extruder. A competition between shear rate and residence time then takes place. The thermomechanical history has, thus, a great influence on the chemical and rheological behavior of pure bitumen, and the chemical changes observed show that the asphaltene volume fraction is not the unique parameter that explains the viscosity variations. Besides, the heteroatoms and their content also play a role in the chemical and physical properties of bitumen. A higher content of heteroatoms leads to a higher degree of self-assembling (intramolecular bonds) and association of molecules (intermolecular bonds), which results in a higher viscosity. The use of a KriegerDougherty equation to describe the relationship between the viscosity of the crude oil and the asphaltene content is not evident, as a result of the complexity of the crude oil and its sensitivity to the oxidation process. A simple variation of the amount of heteroatoms leads to a major modification of the structure and of the shape of the asphaltene particles. To summarize, we can conclude that, at low screw speed, oxidation is the major mechanism which controls the increase in asphaltene content and the viscosity. At high screw speed, oxidation is balanced by the shear induced reduction of asphaltene aggregates.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +33 466 791 801. Fax: +33 466 397 871. E-mail:
[email protected].
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