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Thermogravimetric determination and pyrolysis thermodynamic parameters of heavy oils and asphaltenes Alexandra Boytsova, Natalia Kondrasheva, and Jorge Ancheyta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01584 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017
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Thermogravimetric determination and pyrolysis thermodynamic parameters of heavy oils and asphaltenes Alexandra Boytsovaa,*, Natalia Kondrashevaa, Jorge Ancheytab a
Saint Petersburg Mining University, 21 line of Vasilievsky island 2, 199106 Saint Petersburg, Russia, Email:
[email protected] b
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, México City 07730, México Abstract
Thermal analyses of Yarega heavy crude oil, its atmospheric residue, vacuum residue and asphaltenes were carried out for better understanding of the pyrolysis of high-molecular weight hydrocarbons. The degree of influence of asphaltenes on the pyrolysis was determined. Kinetic and thermodynamic parameters of the pyrolysis were also investigated. Activation energy and pre-exponential factor were calculated. It was found that the degree of conversion depends on the average molecular weight of the liquid oil systems. The higher the molecular weight, the lower the final degree of conversion. It was determined that the activation energy of pyrolysis of the liquid oil systems is higher than that of the asphaltenes obtained from these systems. This process occurs due to deasphalting of the leaching of a solvate layer as a result of the existence of two phases (α-phase and β-phase). β-phase is not soluble in the low-molecular weight hydrocarbons, but partially broken and converted into asphaltenes in the vacuum distillation. Images obtained by scanning electron microscopy showed that the asphaltenes size decreases with increase in density. Asphaltenes from heavy crude oil and atmospheric residue were found to have the highest strength and bond orders and asphaltenes from vacuum residue have highest strength of structure.
Keywords: Thermogravimetric analysis, Scanning electron microscope, asphaltenes, heavy oil, atmospheric residue, vacuum residue, pyrolysis, kinetic parameter, thermodynamic parameter 1
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1. Introduction Petroleum is a complex fluid containing many components ranging from low to high molecular weight. Most of them are kept soluble in the fluid under reservoir conditions but may precipitate as solid materials such as asphaltenes at some stage in upstream, transportation, and downstream processes of crude oil. Heavy oil contains a significant amount of asphaltenes [1]. It is known that they are macromolecular compounds consisting of sheets of condensed polynuclear and heterocyclic aromatic systems with alkyl side chains and naphthenic rings with sulfur, nitrogen, and metals (V and Ni) compounds [2]. Mostly, asphaltenes are responsible for coke formation and catalyst deactivation in hydroprocessing during refining of petroleum [3-5]. To optimize the usage and conversion of heavy oil, it is necessary to understand the structure of the asphaltene and its processability. In recent years, studies on the structure and physical properties of asphaltenes were made with significant progress. For instance Moschopedis et al. [6] determined the molecular weight of the asphaltenes with vapor-pressure in osmometry equipment and the effect of solvent nature on temperature was determined. Storm et al. [7] compared the macrostructures of the asphaltenes in toluene and vacuum residue. Speight et al. [8] concluded that asphaltenes had several types of structures rather than a definitive molecular structure. Trejo et al. [9] characterized asphaltenes using SEC, LDMS, MALDI, NMR and XRD. Aguiar and Mansur [10] determined the interaction enthalpies between resins and asphaltenes by microcalorimetry. Rogel et al. [11] compared asphaltene deposits obtained during cleaning of a submersible pump and asphaltenes extracted from the crude oil with heptane, and found that asphaltenes present in the deposits are less soluble and more aromatic than those coming from the original crude oil. Moreover, not only the structure of asphaltenes, but also the kinetics of asphaltenes decomposition have gained considerable importance in recent years. Marafi et al. [12] presented a literature review on kinetics and modeling of petroleum residues hydroprocessing and summarized values of kinetic order and activation energy of asphaltenes conversion during 2
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residue hydroprocessing under different reaction conditions. For thermal cracking of asphaltene, Wiehe [13] obtained a first-order reaction rate coefficient of 0.026 min−1 for conversion of Cold Lake asphaltenes at 400°C. Zhao et al. [14] conducted a kinetic study and reported an activation energy of 176 kJ/mol for cracking of Athabasca asphaltenes in the temperature range from 350 to 430 °C. Zhao and Ying [15] compared the reactivities of asphaltenes in thermal cracking and catalytic hydrocracking at 430°C and determined reaction kinetics based on experimental data. Nowadays thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have important role to study asphaltene behavior at different conditions. TGA is a good analytical technique to determine the thermal behavior of high-molecular oil fractions at high temperature. The first attempt to use TGA data to understand the kinetics of oil combustion was performed by Tadema [16] in 1959. After that, more and more efforts have been made to apply thermal analysis methods to oil oxidation and pyrolysis. Trejo et al. [17] compared thermal analyses of asphaltenes, resins and sediments and found that sediments obtained from hydrotreated crude have higher tendency to form coke than resins and asphaltenes. They studied the kinetics of asphaltene cracking and found values of activation energy from 122.2 to 217.6 kJ/mol. Kok et al. [18] also compared thermogravimetric analysis of two oils and their SARA fractions at three different heating rates (5, 10, 15°C/min) under air atmosphere. They identified two reaction regions and observed that the activation energy changed with the degree of conversion and was higher for heavier constituents of crude oils. Kok et al. [19] also carried out experiments with Bati Raman and Garzan crude oils and their fractions by TGA. It was concluded that pyrolysis mechanism depends on the chemical nature of the constituents, and each fraction in the whole crude oil follows a reaction pathway independent of the other fractions. In another work, Kok et al. [20] observed that the cracking activation energy depends on asphaltenes content and increases as crude oils become heavier and they studied the oxidation of two heavy oils using thermogravimetry (TGA), derivative 3
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thermogravimetry (DTG) and FTIR spectroscopy techniques in the temperature range from 25 to 900oC [21]. Based on the obtained results these authors found four different reaction regions: low temperature oxidation (LTO), fuel deposition (FD), high temperature oxidation (HTO) and decomposition of limestone. Activation energy values varied between 6.9–10.6 kJ/mol in low temperature oxidation and 91.8–181.9 kJ/mol in high temperature oxidation regions. Varfolomeev [22] found two different reaction regions in combustion of Siberian and Tatarstan regions crude oils using DSC at different heating rates. Reaction intervals and peak temperatures are increased as the heating rate is increased. They also noticed that the heavier the oil the higher the activation energy and the higher peak temperature. Khulbe et al. [23] studied pyrolysis of asphaltene fraction from Cold-Lake bitumen in a nitrogen environment in the temperature range of 20-845°C using TGA. Activation energy for the decomposition of the asphaltenes was 56.5 kcal/mol. However there is no information in the literature about entropy, enthalpy and free energy for pyrolysis of high-molecular hydrocarbon systems (heavy oil and its residues). This information could be used to determine the structural changes during high-temperature processes and structure differences between asphaltenes obtained from heavy crude oil, atmospheric residue and vacuum residue. The aim of this research is to study the thermal decomposition of heavy crude oil, its atmospheric and vacuum residues and the corresponding asphaltenes. The influence of asphaltene on coke formation is also studied. Activation energies, pre-exponential factors, entropy, enthalpy and free energy of hydrocarbon cracking reaction were obtained for better understanding of the differences between cracking processes in different high-molecular oil structures. 2. Experimental The experimental methodology applied in this research is depicted in Figure 1. 2.1. Atmospheric and vacuum distillation 4
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Heavy crude oil (HCO) from Yarega oil field (Russia) was used to produce atmospheric residue (AR 330°C+) and vacuum residues (VR). Atmospheric residue was obtained according to the ASTM D7345 method. Two vacuum residues were obtained by ASTM D1160 method in an automatic vacuum distillation unit “Herzog HDV 632” at pressures of 1 and 13 torr (boiling point of 530°C and 500°C respectively). It means that vacuum residue obtained at 1 torr (VR1) has higher molecular weight than vacuum residue obtained under pressure of 13 torr (VR13). The main properties of crude oil and its residues are shown in Table 1. 2.2. Precipitation of asphaltenes All oil samples were used to precipitate asphaltenes by solvent extraction with n-heptane using a crude-to-solvent ratio of 1:30 (w/v) in accordance with ASTM 6560. The mixture of crude oil and solvent was settled for 16 hours in dark to allow asphaltenes to precipitate. It was then filtered with filter paper on a funnel and finally dried. Precipitated asphaltenes were washed with 150 mL of n-heptane. Asphaltenes were washed in Soxhlet equipment for 6 hours under nheptane reflux in order to obtain resins free from asphaltenes. A colored liquid mixture was obtained from Soxhlet equipment washing which was composed by n-heptane and adsorbed resins. The density of asphaltenes was calculated according to equation (1) [24] and the results are summarized in Table 2.
ρa =
ωa 1
ρo
−
1 − ωa
ρm
(1)
where ρa is the density of asphaltenes, kg/m3; ωa the weight fraction of asphaltenes; ρo the density of oil, kg/m3; and ρm the density of maltenes (deasphalted oil), kg/m3.
2.3. Thermogravimetric analysis 5 mg of sample was used for TGA experiments in SDT Q-600 analyzer. Hydrocarbon decomposition was analyzed under nitrogen gas atmosphere for determination of the coke
5
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formation behavior with flow rate of 100 mL/min. Heating rate was 20⁰C/min. The temperature ranged from 50 up to 1000⁰C.
2.4 SEM analysis To evaluate differences in the structure of the asphaltenes obtained from HCO, AR, VR13 and VR1 SEM analyses were performed in Tescan VEGA 3 SBH instrument, which has energydispersive unit for elemental microanalysis and evacuating of samples (up to 9·10-3 Pa).
3. Kinetic and thermodynamic analysis of hydrocarbon cracking 3.1 Kinetics Thermal decomposition of asphaltenes can be written as follows [17]: k
A → aC + (1 − a)V
(2)
where A is the reactant, C is coke, V the volatile fraction, and a the stoichiometric coefficient. According to this reaction, the following kinetic expression for volatile products could be derived [17]: 1 dV V = k0 e − Ea / RT (1 − ) V0 dt V0
x=
V V0
(3)
(4)
Where V0 is the total amount of volatilized material (wt.%), x is the degree of conversion, t is the time (seconds), k0 the pre-exponential factor (1/s), Ea the activation energy (J/mol), and R the universal gas constant (8.314 J/mol·K). After transformation of eq. 3: ln(
1 dV V E ) = ln[k0 (1 − )] − a V0 dt V0 RT
(5)
Substituting eq. 4 into 5:
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ln(
E 1 dx ) = ln[k0 (1 − x)] − a ⋅ dt R T
(6)
For a constant degree of conversion a plot of ln(dx/dt) vs 1/T can be constructed. The slope of this straight-line plot is then used to calculate activation energy and then pre-exponential factor.
3.2 Thermodynamics The entropy (∆S), enthalpy (∆H) and free energy (∆G) were calculated using the following equations [25]: ∆S = 2.303R·log(k0h/kT)
(7)
∆H = Ea – RT
(8)
∆G = ∆H – T∆S
(9)
where k is Boltzmann constant (1.38·10-23 J/K) and h is Planck constant (6.63·10-33 J·s). According to the theory of absolute rates and free volume model [26], each molecule in the fluid is considered to be localized in a potential well of energy (in the region of minimum of potential energy), which is determined by the molecule interaction with its neighbors. This interaction leads to the establishment of short-range, while long-range order is almost negligible. According to the Eyring theory, liquid flow is carried by hopping of individual molecules in free adjacent. These hoppings always occur in the fluid, but in the absence of the flow, it happens only under the influence of heat fluctuation. Frequency of hopping molecules is determined by the following equation:
J=
1
τ
(10)
τ = τ 0e − E
а
/ RT
(11)
where τ is the relaxation time (Frenkel equation); Temperature increasing leads to evaporation of low-molecular hydrocarbons, and transition of high-molecular hydrocarbons from molecular state in structure. This results into a change in 7
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overall system power. Thermodynamic calculation helps determine the possibility of individual reaction stages and the possible depth of the process under these conditions. To achieve this, it is necessary to develop proper thermodynamic and kinetic calculations for effective usage of heavy oil and its high-molecular residues.
4 Results and discussion 4.1 Physical and chemical properties of the feedstock From Table 1 it is seen that the total content of resins and asphaltenes for all samples is around 50% (from 47 to 55 wt.%). Metals are concentrated as the oil becomes heavier, from 207 ppm Ni+V in HCO to 522 ppm Ni+V in VR1. The density of the asphaltenes varied from 1009 kg/m3 in HCO to 1027 kg/m3 in VR1. The heavier the feedstock, the higher the density of their asphaltenes.
4.2 TGA of heavy oil, atmospheric and vacuum residues and asphaltenes Figure 2 shows the TG curves for all the samples. The weight loss of the HCO sample starts at 156⁰C and pyrolysis completely at 480⁰C, which is typical of a one-step process. On the contrast residues and asphaltenes pyrolysis processes involve two main stages: LTP (light-temperature pyrolysis) and HTP (high-temperature pyrolysis), indicating that different types of reactions occur. The first stage starts at about 131-207⁰C for AR and the asphaltenes obtained from HCO and AR, while for VR13, VR1 and their asphaltenes, it starts at about 324-356⁰C. This period is characterized by sharply decreasing of weight. The first step completes and the second step starts at about 500⁰C for all samples. Initial volatilization of HCO, AR and their asphaltenes is mainly due to light alkanes distillation which continues up to 350⁰C. Alvarez et al. [24] observed that in the range of 100-320⁰C, weight loss occurs by volatilization of mono-, di-, and tri-aromatics and higher aromatic compounds. Rapid volatilization from 350⁰C to 500⁰C indicates cracking of heavy fractions such as asphaltene and resins. From 500⁰C to 1000⁰C (HTP stage) volatilization 8
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continues, but not so as in the LTP stage, which explains the polycondensation processes resulting in more solid and dense structure of the high-molecular hydrocarbon. The same behavior during second step could indicate that in this range, the reactions involving asphaltenes dominate those of other components present in the heavy hydrocarbon structure. The coke yield of each sample is shown in Table 3. Interestingly, the HCO is completely reacted at a temperature of 480⁰C, and its asphaltenes give 18.1 wt. % of coke yield. AR asphaltenes produce about 13 times more coke yield than AR, while the asphaltenes obtained from VR13 and VR1 give more coke yield at about 10 and 3 times respectively. The higher the density of the liquid oil, the lower the ratio of coke yields from asphaltenes and oil. Thus it can be stated that gelation in petroleum systems is a result of distillation (atmospheric and vacuum), which lead to the formation of more dense high-molecular structures (precursors to the formation of coke during pyrolysis). It is observed that temperatures of maximum weight loss (Tmax) for HCO, AR, VR13 and VR1 are similar to those of the corresponding asphaltenes, which may indicate that asphaltenes are responsible for the behavior and temperature range of pyrolysis process. Comparing the TGA curves for the asphaltenes obtained from HCO and AR with those for VR or its asphaltenes, it is clear that the curves of the former have a sharp decline in the temperature range at above 270-320⁰С. According to this, it could be assumed that incomplete leaching of solvate layer can be a composition of oil distillates (0), the process does not occur spontaneously. Oil systems go from dispersed condition to molecular solution. As for enthalpy (Figure 7), which is responsible for the strength of oil systems during the pyrolysis, it was revealed that ∆H>0, which means that the reactoin is endothermic for all samples (heat release). The strength of the molecular bond of asphaltenes is lower than that of liquid oil systems (∆Hasph< ∆Hliq.sys.). It could be the reason why EaAsph is lower than EaLiq.sys. It is proved by voids formed under deasphalting process by washing solvate layer after the penetration in which the system will be destroyed faster (EaAsph.
