Thermogravimetric Monitoring of Crude Oil and Its Cuts in an Oil

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Thermogravimetric monitoring of crude oil and its cuts in an oil refinery Agustin Garcia Barneto, Jose Ariza Carmona, and Antonio Barrón Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 17 Mar 2015 Downloaded from http://pubs.acs.org on March 18, 2015

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Thermogravimetric monitoring of crude oil and its cuts in an oil refinery Agustín García Barneto,1* José Ariza Carmona,1 Antonio Barrón2 1

Department of Chemical Engineering, Physical Chemistry and Organic Chemistry,

University of Huelva (Campus de Excelencia Internacional Agroalimentario, ceiA3), Spain. 2

CEPSA, La Rábida Refinery, Huelva, Spain.

Abstract The thermal degradation profile for any type of oil-based sample under thermogravimetric analysis (TGA) conditions exhibits three distinct stages, namely: vaporization from room temperature to 340–350 ºC, cracking from 340–350 ºC to 480– 500 ºC and char oxidation from 500 ºC to 570 ºC. The former two stages occur in both inert (nitrogen) and oxidative (air) environments, whereas the latter only occurs in the presence of oxygen. Deconvoluting thermogravimetric data allows one to estimate the composition of oil derivatives with a view to expeditiously obtaining useful information from a refining process. To this end, thermal degradation of crude oil and its main refining cuts were modelled here by using the smallest possible number of representative pseudocomponents. In order to ensure accurate fitting of thermogravimetric results, mass losses were interpreted in terms of autocatalytic kinetics. Fitting to an nth-order kinetics was useful below 350 ºC (vaporization), but not above this temperature owing to cracking with fast mass losses in the vicinity of certain temperatures. This modelling scheme for thermogravimetric results afforded the following conclusions: (a) atmospheric gas-oil typically contains 10% residual kerosene fraction; (b) atmospheric residue still contains 35–45% distillable compounds; (c) the main component of visbreaking feed (nearly 66%) degrades at a similar temperature as asphaltenes; (d) visbreaking residue is similar to feed at high temperatures but contains light components similar to naphtha or gas-oil which vaporize at low temperatures; (e) simulating crude oil allowed us to estimate the potential production of distillates 1 ACS Paragon Plus Environment

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(62.93% from heavy crudes and 79.61% from light crudes). Such useful information can be used by process engineers to assess the performance of equipment such as distillation columns or visbreaking units, and also to estimate the quality of some streams such as atmospheric gas-oil or visbreaking feed. Keywords: oil refining, autocatalysis, thermogravimetry, pyrolysis, visbreaking. * Corresponding author. Tel.: +34 959 219982; fax: +34 959 219983. Universidad de Huelva, Facultad de Ciencias Experimentales, Campus El Carmen, 21071 Huelva, Spain. E-mail address: [email protected].

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1. Introduction There is wide consensus that thermal degradation of crude oil occurs mainly through evaporation of light compounds at low temperatures but involves complex reactions at high temperatures (over 350 ºC) —particularly in the air, where the process includes exothermic oxidation reactions of carbonaceous compounds.1 Apparently, the oxidation of oil in the air involves the following steps: (a) distillation by heating from ambient temperature to approximately 320 ºC; (b) low-temperature oxidation (LTO) over the range 320–370 ºC, where heterogeneous cracking reactions on side chains of heavy molecules yield oxygenated compounds; (c) medium-temperature oxidation (MTO) from 370 to 470 ºC, where previously formed pyrolysis products are oxidized in the gas phase; and (d) high-temperature oxidation (HTO) from 470 to 570 ºC, where fuel formed previously formed from MTO is oxidized.1–3 Isothermal and non-isothermal TGA have often been used to calculate kinetic parameters with a view to characterizing the thermal degradation of oil in terms of firstorder kinetics —particularly with the approaches of Arrhenius and Coats, and Redfern2,4–7. These approaches have revealed differences in activation energy between chemical reactions involved in the thermal degradation of oil. Typically, the activation energy falls below 100 kJ/mol at low temperatures but increases markedly with increasing temperature —to levels above 150 kJ/mol in some cases.8 The degradation of such complex systems as oil and biomass can be accurately characterized by using the distributed activation energy model (DAEM), which assumes the simultaneous occurrence of many irreversible first-order reactions. Based on this model, nonisothermal oxidation of heavy oil involves three stages,9 namely: (1) low-temperature oxidation from 200 to 320 ºC with an apparent activation energy E = 100 kJ/mol; (2) medium-temperature oxidation from 320 to 350 ºC; and (3) high-temperature oxidation above 350 ºC with E = 190–230 kJ/mol. On the other hand, model-free methods such as those of Friedman and Ozawa–Flynn–Wall10 have enabled the characterization of the thermal degradation of four oil crudes.11 Thus, the activation energy for heavy oil degradation was calculated to range from 52.57 to 244.52 kJ/mol at reaction 3 ACS Paragon Plus Environment

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conversions from 0.1 to 0.9; also, the pre-exponential factor was found to increase with increasing conversion. In this work, thermogravimetric data were interpreted according to alternative criteria. Thus, the description of the thermal degradation process was expanded with autocatalytic kinetics. This approach was previously used to explain biomass pyrolysis and combustion.12 As with crude oil, the thermal degradation of biomass at high temperatures —and also high heating rates— cannot be explained in terms of nth-order kinetics. In fact, autocatalytic kinetics is required to fit the results for any acceleratory stage resulting in very high mass loss rates —in an atmospheric (oxidative) environment mainly. In this work, however, the complex chemical blend of compounds typically present in crude oil, and its main refining streams, were modelled in terms of pseudocomponents (viz., fractions with a characteristic thermal degradation behaviour).13 This approach assumes crude oil degradation to be a combination of the individual degradation of several pseudo-components. The number of pseudo-components used here (seven) was the smallest allowing the thermal behaviour of the main oil cuts (naphtha, gas-oil) to be accurately simulated. The proposed approach was also used to interpret thermogravimetric data from various crude oil fractions with typical thermal degradation profiles, namely: atmospheric gasoil (AGO), which is largely vaporized during the process; visbreaking feed (VF), which is mostly cracked; and atmospheric residue (AR) and visbreaking residue (VR), which undergo vaporization and cracking. Fitting the TGA results allowed potentially useful information for process engineers to be extracted. Such information includes atmospheric distillate production from crude oil, the thermal characteristics of AGO and AR, and the pseudo-components of VF and VR, which can in turn be used for purposes such as estimating the yield of an atmospheric distillation unit or that of a visbreaking unit.

2. Material and methods 2.1. Samples 4 ACS Paragon Plus Environment

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Crude oil (HC and LC), atmospheric gas-oil (AGO), atmospheric residue (AR), visbreaking feed (VF) and visbreaking residue (VR) were supplied by La RábidaCEPSA Refinery (Huelva, Spain). Sampling was programmed to ensure that all refined samples (AGO, AR, VF and VR) would come from a specific crude. After two months of monitoring, two representative series of samples were collected for analysis. One was obtained from heavy Iraqi crude (HC) on the grounds of its high viscosity (6.23 cSt), and its also high contents in sulphur (2.1 wt%) and asphaltenes (2.4 wt%). The other was obtained from a light West African crude (LC) on the grounds of its lower viscosity (5.60 cSt), and also lower contents in sulphur (0.25 wt%) and asphaltenes (0.65 wt%).

2.2. Chemical analysis Physico–chemical analyses of the samples were performed according to the following standards: ISO-12185 and ASTM 4052 for density; ISO 3104 and ASTM -D445 for viscosity; ISO 10370 and ASTM D-4530 for carbon residue; ISO 8754 and ASTM D4294 for sulphur; IP 143 and ASTM D6560 for asphaltenes; and ASTM D-2887, ASTM D-3710, ASTM D-7096, ASTM D-7169 and ASTM D 7213 for simulated distillation.

2.3. Thermogravimetric analysis All

thermogravimetric

analyses

were

performed

on

a

Mettler

Toledo

TGA/SDTA851e/LF1600 balance, using a sample mass of ca. 5 mg under nitrogen for pyrolysis runs, and a 4:1 mixture of nitrogen and oxygen for combustion runs. The temperature was raised from 25 to 900 ºC at three different rates (5, 10 and 20 ºC/min).

3. Results and discussion 3.1. Thermal degradation path for crude oil and its main cuts in an inert environment As can be seen from fig. 1, the thermogravimetric analysis revealed significant differences in thermal degradation between crude oil samples. For example, light crude (LC) exhibited increased mass loss rates at low temperatures (below 350 ºC), and so did heavy crude (HC) at high temperatures (above 350 ºC) (fig. 1b). Also, the overall mass 5 ACS Paragon Plus Environment

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loss at 600 ºC in a nitrogen environment (pyrolysis) was greater for LC than it was for HC. In other words, HC produced more char (coke) than did LC during pyrolysis. Figure 2 shows the thermal degradation profiles in a nitrogen environment (pyrolysis) for selected oil cuts. As can be seen from fig. 2a, the main difference between AGO and the other samples was the absence of residue after heating in the inert environment —the total mass loss was 100%. As a rule, oil derivatives undergoing thermal degradation below 340–350 ºC (viz., AGO) leave no carbonaceous residue upon pyrolysis. AGO gave a band from 100 to 320 ºC peaking at ca. 310 ºC (fig. 2b). The peak can vary slightly in shape and position with the nature of crude being refined but usually exhibits a small shoulder close to 235 ºC. On the other hand, those cuts that are entirely or partially degraded above 340 ºC undergo pyrolysis with variable char production; as a result, their overall mass loss never reaches 100% at the end of pyrolysis (ca. 500–510 ºC). The greatest amount of char at 600 ºC was produced by VR (19.1%), followed by VF (16.2%), AR (8.5%) and oil crude (5.5%) (see fig 2a). As can be seen from the DTG curves for AR and crude oil in fig. 2, atmospheric residue (AR) exhibited lower mass loss rates than oil crude at low temperatures but the opposite was true at high temperatures (above 340 ºC). This was obviously a result of the nature of AR. Thus, AR is obtained from crude oil after removal of distillates during atmospheric distillation; as depicts fig. 2b, however, the residue still retained substantial amounts of compounds vaporizing within the temperature range for naphtha and gas-oil (100–340 ºC). On the other hand, the DTG curve for AR at high temperatures (above 340 ºC) was similarly shaped to that for crude oil even though the former exhibited higher mass loss rates as a result of its increased proportion of heavy compounds. In addition, AR contained greater amounts of heavy molecules, and exhibited higher mass loss rates at high temperatures, than the crude. The thermogravimetric behaviour of the visbreaking streams (feed and residue) was very interesting. Visbreaking facilitates smooth thermal cracking in the liquid phase. As a result, large molecules produce volatiles and light and medium-weight distillates that are separated on a stripper column. The residue, which is supplied with a 6 ACS Paragon Plus Environment

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small amount of distillates to facilitate handling, is much less viscous than visbreaking feed (VF). VF was the most refractory sample, which was thermally degraded at very high temperatures (250–520 ºC) leading a smooth, well-defined mass loss band peaking at ca. 455 ºC (see fig 2b). This result was to be expected from the nature of VF; in fact, the visbreaking unit is fed with the heaviest refinery streams (vacuum residue, mainly). As depicts in fig. 2b, VF underwent insubstantial thermal degradation below 340 ºC; therefore, degradation of this extra-heavy component was the result of reactions at high temperatures. Visbreaking residue (VR) exhibited a peak in the same position as VF (fig. 2b) and suggesting that their main components must be similar. VR additionally contained other light components not present in VF, however. Fig. 1 Fig. 2

3.2. Thermal degradation path for crude oil and the main oil cuts in an oxidative environment Figures 1 and 3 show that the thermal degradation profiles for AGO, AR, VF, VR and crude oil in an oxidative environment were much more complex than those in an inert environment, particularly at high temperatures. Thus, as can be seen from figs 1b and 3b, below 340 ºC the samples vaporized similarly, and exhibited smooth thermal degradation profiles, in the air and in nitrogen. Near 340 ºC, however, the presence of oxygen caused a sharp decrease in mass loss rate (see figs 1b and 3b) and resulted in marked differences in thermal degradation at low and high temperatures —no similar differences were observed in the nitrogen environment, however. Above 340 ºC, the mass loss profiles exhibited several (usually three) peaks for all samples. This result is very important since all samples, whichever their nature (AR, VF, VR or crude oil), exhibited three peaks —or one plus an overlapped signal— from 340 to 500 ºC in the presence of oxygen. Consequently, oxygen has two prominent effects during thermal degradation at a high temperature, namely: (a) increasing char production —and reducing volatile production as a result—; and (b) causing mass losses to cluster and 7 ACS Paragon Plus Environment

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sharp peaks to appear around specific temperatures. Also, as expected, char was oxidized and gave a characteristic peak above 500 ºC.

Fig. 3

3.3. Interpretation of the thermal degradation process Clearly, oil and its derivatives initially undergo vaporization. The sequence in which oil components change from liquid to gas is governed by their vaporization enthalpies as measured at normal boiling temperature, which increase with increasing boiling point and are only slightly influenced by molecular structure. Among linear alkanes, the vaporization enthalpy is 48.16 kJ/mol at 253.6 ºC for tetradecane (C14H30); 51.84 kJ/mol at 286.9 ºC for hexadecane (C16H34); 55.23 kJ/mol at 316 ºC for octadecane (C18H38) and 58.49 kJ/mol at 343 ºC for eicosane (C20H42). Among polyaromatic hydrocarbons (PAHs), the enthalpy is 43.18 kJ/mol at 218 ºC for naphthalene (C10H8) and 52.35 kJ/mol at 340 ºC for anthracene (C14H10).14,15 As can be seen, the vaporization enthalpy for hydrocarbons boiling near 340 ºC is close to 60 kJ/mol and increases with increasing boiling point. For example, the enthalpy for fluoranthene (C16H10) is 66.52 kJ/mol at 383 ºC, that for chrysene (C18H12) 69.54 kJ/mol at 441 ºC and that for picene (C22H14) 75.66 kJ/mol at 519 ºC. During vaporization, mass losses are essentially independent of the particular environment in thermogravimetric runs. As can be seen from fig. 1, for a given crude (HC or LC), the mass loss rate (TG line in fig. 1a) and its derivative (DTG line in fig. 1b) nearly overlapped up to 350 ºC in both types of environment (nitrogen and air). In addition, however, oil vaporization was accompanied by slight oxidation in the atmospheric environment. For example, heating AGO produced no coke under nitrogen but gave 2–3% coke at 340 ºC in the air (see figs 3a and 3b). Vaporization was followed by a much more complex stage above ca. 340 ºC that introduced cracking reactions. Therefore, accurately interpreting the thermogravimetric results required considering the mechanism for volatile and coke formation at high


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temperatures. In short, volatiles resulted from cracking, whereas coke (char) came from cracking and asphaltene aggregation. Volatile formation was the result of thermal cracking, a process by which larger hydrocarbon molecules give smaller hydrocarbon molecules via carbon–carbon bond breaking reactions. Thermal cracking involves free-radical chain reactions16 including an initiation stage by which hydrocarbon molecules give two radicals, an H-transfer reaction and a radical decomposition reaction (β-scission, mainly)17. The amount of energy need to trigger this process is much greater than that required to vaporize hydrocarbons (e.g., ca. 400 kJ/mol is needed to break a C–C or C–H bond). Understanding spontaneity in cracking reactions therefore entails examining entropy changes. Since radical formation substantially increases entropy, the entropic factor (TΔS) offsets the enthalpic factor (ΔH) above 340 ºC. Also, the rate of cracking reactions increases with increasing number of carbon atoms in the hydrocarbon molecules. At specific temperatures, large molecules undergo faster cracking than small molecules.18 However, the most salient feature of the cracking stage is the influence of oxygen on the mass loss of sample. In contrast to the previous vaporization stage, cracking is strongly influenced by the presence of oxygen. As can be seen from fig 1a, from 350 ºC (the start of the cracking stage) to 490 ºC (the end of cracking and start of char oxidation), the mass loss (TG line) in nitrogen always fell above that in the air. Cracking led to an increased char mass in the presence of oxygen. Based on the experimental results, oxygen inhibited volatile formation and promoted char formation throughout the temperature range. Also, mass losses clustered around certain temperatures in the presence of oxygen. As a result, the DTG line was smooth in the nitrogen environment but sawtoothed (several peaks) in the air (see fig. 1b). The occurrence of large mass losses near some temperatures requires further research; based on available information, however, we can hypothesize that such temperatures must coincide with the breaking of specific structures present in hydrocarbon molecules contained in all crudes. Consequently, the first peak might correspond to breaking of the weaker C–C bonds present in all samples, namely: Calkyl–Calkyl (alC–alC) bonds. 9 ACS Paragon Plus Environment

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Similarly, the second peak might correspond to breaking of Calkyl–Caryl (alC–arC) bonds, and the third to Caryl – Caryl (arC – arC) bonds. According to Blanksby and Ellison (2003),19 the bond dissociation energies for these structures are approximately 75 kcal/mol for alC–alC bonds, 100 kcal/mol for alC–arC bonds and 120 kcal/mol for arC– arC bonds. Coke formation during thermal degradation of crude oil and its derivatives is usually explained in the light of the model of Wiehe (1993)20. According to this author, coke formation during bitumen pyrolysis results from phase separation. Thus, in open reactors coke forms from a mesophase enriched with modified asphaltenes. As pyrolysis progresses, asphaltene molecules are cracked, thereby losing side chains and gaining aromaticity. The resulting asphaltene cores are less soluble in the oil matrix and more reactive to free radicals. As a consequence, addition reactions lead to coke formation through aggregation of asphaltene cores. When the concentration of asphaltene aggregates exceeds their solubility, coke starts to deposit. There is much evidence that coke formation involves free radicals. For example, the presence of a proton-donor as solvent reduces coke yield in bitumen pyrolysis by effect of free-radical addition reactions competing with hydrogen abstraction.21 The fact that a thermobalance operates like an open reactor in which volatile products are continuously removed by a nitrogen or air flow allows thermogravimetric results to be explained in the light of the Wiehe model. Thus, asphaltene aggregation in a nitrogen environment must coexist with moderate coke formation because the inert environment will promote the formation of volatiles. When oxygen is present, however, coke from asphaltenes is very probably accompanied by substantial amounts of coke from cracking reactions because oxidative cracking promotes coke formation. Increased coke formation by effect of the presence of oxygen during TGA has been observed in other contexts. According to Cinar et al. (2011),22 the nature and amount of coke obtained by heating heavy crude oil depends on the presence or absence of oxygen. Thus, they found heating heavy oil containing 10% asphaltenes at 673 K under atmospheric pressure to exhibit an induction period of 45 min where no coke was 10 ACS Paragon Plus Environment

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obtained, followed by another where substantial amounts of coke were formed. Coke formation at that stage was governed by phase separation of asphaltenes. As suggested by Wiehe (1993),20 the solubility of asphaltenes in maltenes governs coke formation in a nitrogen environment. In the air, the results are very different, however. Thus, coke formation is subject to no induction period; rather, coke forms from the beginning via a mechanism other than separation of asphaltenes. Cinar et al. (2011)22 hypothesized that maltenes are less effective solvents for asphaltenes because they tend to be oxidized by oxygen. For that reason, the distillation step (vaporization) in the process of oil heating in the presence of oxygen presence is followed by oxygen addition in the liquid phase. This process governs coke formation under these conditions. Also, coke obtained in the presence of oxygen is more reactive than coke forming in a nitrogen environment. Seemingly, oxygen causes the formation of carboxyl and hydroxyl groups on the surface of coke. Fig. 4 Figure 4 illustrates the above-described ideas, which can be summarized as follows: •

The thermal degradation profile for any type of oil under TGA conditions exhibits three distinct stages, namely: (1) vaporization from room temperature to 340–350 ºC, (2) cracking from 340–350 ºC to 480–500 ºC and (3) char oxidation from 480–500 ºC to 570 ºC. The former two stages occur in both inert (nitrogen) and oxidative (air) environments, whereas the latter only occurs in the presence of oxygen (see fig. 1b).

•

Vaporization. Crude oil and its derivatives vaporize (i.e., volatile compounds successively pass from the liquid phase to the vapour phase similarly as in atmospheric distillation of crude during refining) upon heating from room temperature to 340–350 ºC.

•

Cracking and asphaltene aggregation. At atmospheric pressure, the boundary between vaporization and thermal cracking is 340–350 ºC. At higher temperatures, cracking gains increasing importance while vaporization loses it. 11 ACS Paragon Plus Environment

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During the cracking stage, large molecules are broken into smaller ones (volatiles) and char. This process is very complex and involves the formation of new compounds that can undergo further cracking. For thermogravimetric purposes, however, we are only concerned with the overall production of volatiles and char. At high temperatures, char (coke) formation occurs via a different mechanism: asphaltene aggregation. Thus, as the temperature is raised, the resins solubilizing asphaltenes are thermally degraded (vaporized or cracked). Based on the model of Wiehe (1993),20 these conditions facilitate aggregation of asphaltenes and hence char (coke) formation. Therefore, char can form via two completely different mechanisms. On the one hand, cracking reactions leave a carbonaceous residue resulting from the breaking of heavy molecules; on the other, aggregation of asphaltenes results in coke formation via a physical mechanism involving mechanical aggregation without production of volatiles. •

Char oxidation. The third stage of oil thermal degradation occurs approximately from 490 to 570 ºC and involves the oxidation of char previously formed during cracking in an atmospheric environment.

3.4. Using autocatalytic kinetics to fit the thermal degradation profile for crude oil and its cuts Based on the foregoing, the pyrolysis of oil and its cuts under thermogravimetric conditions can be modelled as follows: Vaporization: Si Vi1 Cracking: Si Vi2 + Char The pyrolysis model for thermal degradation in an atmospheric environment must be expanded with char oxidation: Char oxidation: Char + O2 Vi3


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In the previous schemes, Si are pseudo-components in the samples, Char is the carbonaceous residue (coke), and Vi1, Vi2 and Vi3 are volatiles produced by vaporization, cracking and char oxidation, respectively. In order to ensure accurate fitting of the thermogravimetric results, the previous results were modelled in terms of autocatalytic kinetics. For an nth-order kinetics, the reaction rate can be represented by equation (1), where α is the reaction conversion (fraction reacted), n the reaction order and k the kinetic constant, which is usually expressed according to the Arrhenius law (equation 2) with k0 as the pre-exponential factor and E as the activation energy. dα / dt = k (1 − α ) n

(1)

k = k 0 exp(− E / RT ) (2) With n =1, eq. (1) reflects a first-order kinetics, which is typically used to represent oil thermal degradation and assumes the reaction rate to be proportional to the amount of unreacted fraction (1 – α). Although this simple model is useful in many cases, it cannot be applied to thermal degradation processes involving an acceleratory stage (e.g., kerogen pyrolysis),23,24 conditions under which using an nth-order kinetics to fit thermogravimetric data results in a compensation effect. Thus, changes in activation energy are offset by changes in pre-exponential factor and unusually large k0 – E values are obtained as a result. This is a well-known fact in biomass pyrolysis.25 As an alternative to an nth-order kinetics, Burnham (2000)24 suggests fitting pyrolysis results to the extended Prout–Tompkins equation (3)26,27 when acceleratory phases are involved. In this approach, the reaction rate depends on both the unreacted fraction (α) and the reacted fraction (1 – α). Also, the reaction order, n, has the same meaning as in nth-order kinetics, but the nucleation order, m, appears as an exponent measuring the degree of autocatalysis and s is a constant usually taken to be 0.01 that ensures accurate values for the reaction rate at extreme reaction conversions (close to 0 or 1). If autocatalysis is not significant, then m = 0 and eq. (3) simplifies to an nth-order equation.

dα / dt = k (1 − α ) n ( s + α

m

)

(3) 13

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Equation (3) is useful to represent sigmoidal α – t curves when the thermal degradation of a solid involves the formation and growth of active sites called “nuclei”. The reaction is initially slow because it is governed by the formation of nuclei at different sites in the solid but becomes increasingly faster as the nuclei grow until reactant spheres come into contact and the reaction stops. Under this kinetic approach, the position of peaks in the DTG curve obtained by plotting the reaction rate against time or temperature depends mainly on the pre-exponential factor and activation energy. On the other hand, their shape depends on the reaction order and nucleation order. Basically, peaks become sharper with decreasing n or increasing m. The influence of the nucleation order on peak shape allows sharp peaks to be fitted while avoiding a compensation effect. Since a thermobalance allows one to measure the amount of volatiles V released by a sample at a given time, the reaction conversion must be expressed as α = V / V∞ , where V∞ is the amount of volatiles released at infinite time. In this way, eq. (3) can be rewritten as eq. (4). Consequently, the optimization process should allow us to determine five different parameters for each pseudo-component, namely: preexponential factor, activation energy, reaction order, nucleation order and mass of volatiles at infinite time. To this end, the kinetic equations were integrated and optimized by using the Runge–Kutta and Gauss–Newton methods with the objective function (OF) shown in eq. (5), where dmexp/dt and dmcal/dt are the experimental and calculated mass loss rate, respectively, for the n points in each experiment. The model was validated in terms of the coefficient of variation (CV) of eq. (6), where N and P are the number of data points and parameters fitted, respectively, and dm exp / dt is the average experimental mass loss rate. dV / dt = kV∞ (1 − V / V∞ ) n [s + (V / V∞ ) m ] n

(4)

2

OF = ∑ (dm exp / dt − dm cal / dt )

(5)

i =1

CV (%) = 100 OF /( N − P ) /( dmexp / dt )

(6)

3.5. Simulation of thermogravimetric data 14 ACS Paragon Plus Environment

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In order to improve the significance of the results, the fitting of thermogravimetric data was subjected to various restrictions, namely: •

The number of pseudo-components used to simulate the thermal degradation of samples was minimized. Such a number was the smallest needed to ensure accurate fitting of the thermal degradation profiles. Making pseudo-components comparable between samples required considering their relative positions in the vaporization or cracking zones. Pseudo-components were designated as follows in sequence of increasing temperature: PsA, PsB, PsC and PsD for vaporization, and PsE, PsF and PsG for cracking.

•

The vaporization of a pure substance is described by a zero-order kinetics.28 However, oil-related samples are mixtures of many pure substances, so their vaporization does not fit it and an actual sample is simulated by using a number of pseudo-components vaporizing according to a zero-order kinetics or a few vaporizing according to a low-order (usually 0.40–0.60) kinetics. The latter choice is to be preferred since it involves easier kinetic calculations and allows samples to be described in terms of fewer components —which leads to better fitting.

•

The pre-exponential factor (k0) is chosen in accordance with the nature of the process to be fitted. During fitting, the pre-exponential factor may vary slightly around the initial values where it was previously optimized. Thus, Ln (k0) was close to 7 for vaporization; 12 for cracking and 19 for char oxidation.

•

During fitting, the reaction order for the cracking process may vary slightly around 1.

3.5.1. Atmospheric gas-oil (AGO) TGA detected differences between two atmospheric gas-oil samples (AGO1 and AGO2) obtained from two different crude oils (heavy crude and light crude, respectively) (see fig. 5a). As can be seen, thermal degradation of AGO2 ended at a higher temperature; also, its mass loss rate peaked at a higher temperature (314 ºC versus 309 ºC in AGO2). 15 ACS Paragon Plus Environment

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In order to explain these differences, the DTG curves were deconvoluted in accordance with the above-described criteria. As depicts in fig. 5b, the thermal degradation of AGO was simulated by using only two pseudo-components degrading according to the kinetic parameters of table 1. Both pseudo-components exhibited a low reaction order (0.4–0.6) and a near-zero nucleation order (m = 10–4). Therefore, the vaporization of AGO can be described by an nth-order kinetics. Under these conditions, the major component of AGO (ca. 90 wt%) was a pseudo-component PsD vaporizing with an activation energy close to 60 kJ/mol, which is the expected vaporization enthalpy for hydrocarbons vaporizing close to 340 ºC.28 The less abundant pseudocomponent, PsC, vaporized with a slightly lower activation energy (ca. 50 kJ/mol). This is consistent with the expected outcome for the residual kerosene fraction (ca. 10 wt%) remaining in AGO. Consequently, based on its thermal degradation profile, AGO was a blend of 90% gas-oil and 10% kerosene. Fig. 5 Table 1

3.5.2. Atmospheric residue Thermogravimetric analysis revealed that the atmospheric residue still contained substantial amounts of light compounds distilling below 350 ºC (see fig. 6). In fact, based on the results, AR contained 35–45% distillable compounds, the proportion depending on its origin. AR2, which was obtained from light crude, contained greater amounts of light compounds than did AR1, which was obtained by atmospheric distillation of heavy crude (see fig. 6a). Fig. 6 Deconvolution of the thermogravimetric curves allowed quantitative information about the thermal degradation of AR to be obtained (see table 2 and fig. 6b). Fitting the mass loss rate–temperature curves for AR in the nitrogen environment required using six pseudo-components. The first three (PsB, PsC and PsD) vaporized similarly to AGO, and exhibited a near-zero nucleation order, a reaction order of 0.6 and an 16 ACS Paragon Plus Environment

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activation energy of 52.01–63.19 kJ/mol. These components accounted for 2.61%, 6.32% and 29.09%, respectively, of the total mass of atmospheric residue. Therefore, 38.02% of the initial mass of AR1, which was obtained from heavy crude, vaporized. The vaporization stage was followed by a cracking stage the mass losses in which were simulated by using another three pseudo-components: PsE, PsF and PsG. In this case, thermal degradation failed to fit an nth-order kinetic and required considering nucleation kinetics via the PT equation. For these pseudo-components (PsE-PsG), the nucleation order was 0.37–0.40 but reaction order and activation energies remained largely unchanged; thus, the reaction order was near-unity and activation energies 122.45 kJ/mol (PsD), 128.87 kJ/mol (PsE) and 137.49 kJ/mol (PsF). The total mass released as volatiles during cracking in the nitrogen environment (53.35%) was spread between these pseudo-components (11.72%, 8.30% and 33.33%, respectively). The last pseudo-component, PsG, was associated to asphaltene cracking. Table 2 The most salient feature of the thermal degradation of AR was the influence of oxygen on the production of volatiles and coke during the cracking stage. Its significance can be easily understood by comparing mass losses during TGA in the nitrogen and atmospheric environments. After the vaporization stage, where losses were almost identical in both environments (viz., 41.97% in the air and 38.02% under nitrogen), volatile formation during the cracking stage was greatly affected by the presence of oxygen (viz., 32.52% in the air versus 53.35% under nitrogen). Obviously, coke formation was affected in the opposite direction: 25.08% in the air and 8.65% under nitrogen (i.e., a difference of 16.43%). This is a general phenomenon for oil derivatives whose thermal degradation includes cracking. As revealed by the TGA results, oxygen induced coke formation —at least above 340 ºC. Also, coke was produced not only by asphaltenes (PsG) but also by all pseudo-components undergoing some cracking (PsE and PsF). In summary, TGA showed coke formation in a nitrogen environment to be largely associated to asphaltene aggregation. In the inert environment, cracking produced large amounts of volatiles but little coke. In the 17 ACS Paragon Plus Environment

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presence of oxygen, however, a different cracking mechanism led to increased coke formation throughout the temperature range. Under those conditions, coke had a twofold origin, namely: cracking and asphaltene aggregation.

3.5.3. Visbreaking feed and residue Pyrolysis of VF1 was simulated by using the four pseudo-components of fig. 7a. The first pseudo-component (PsD) represented vaporization of residual light compounds (2.10%) degrading below 350 ºC, whereas the other three (PsE, PsF and PsG) simulated compounds undergoing thermal cracking from 350 to 525 ºC. Based on their respective masses (2.97%, 12.28% and 66.36%), the extra-heavy pseudo-component (PsG), which degrades at a similar temperature as asphaltenes (461 ºC), was the most important. This is consistent with the substantial production of char during pyrolysis of VF1 at 600 ºC: 16.39%. Fig. 7 Table 3 testifies to the importance of nucleation during pyrolysis of VF1 at high temperatures, even in the absence of oxygen. As can be seen, the nucleation order increased in the sequence PsE < PsF < PsG. This indicates that accurately describing the cracking of heavy oil fractions requires considering nucleation kinetics —especially at high temperatures. For example, pyrolysis of the major pseudo-component (PsG) occurred with an activation energy of 138.00 kJ/mol, a near-unity reaction order (1.12) and a moderate nucleation order (0.45). On the other hand, the product of the visbreaking unit (visbreaking residue, VR1) was much more complex than its feed (VF). Figure 7b shows the seven pseudo-components required to simulate VR1 pyrolysis: four for vaporization below 350 ºC and three for cracking from 350 to 520 ºC. Table 3 Deconvolution the thermogravimetric curves was useful to understand the effect of the visbreaking unit on VF1 because it allowed its efficiency in breaking the heaviest compounds to be assessed. A comparison of the pseudo-components undergoing 18 ACS Paragon Plus Environment

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cracking in tables 3 and 4 (viz., PsE, PsF and PsG) in VF1 and VR1 revealed that the content in the component degrading at the highest temperature (PsG, asphaltene zone) was significantly reduced from 66.36% to 42.37%; in parallel, however, the content in that cracking at the lowest temperature (PsE) increased from 2.97 to 10.23%. Also, cracking of visbreaking feed yielded 17.05% of compounds in VR1 vaporizing bellow 350 ºC, namely: PsA (1.89%), PsB (2.87%), PsC (4.91%) and PsD (7.38%) (see table 4). Table 4

3.5.4. Crude oil Figure 8 shows the deconvolution of the DTG curves for the two oil crudes (HC and LC) as obtained in the nitrogen environment (pyrolysis). As can be seen, only seven pseudo-components were needed to fit the experimental data. Using the smallest possible number of pseudo-components made fitting difficult, particularly in the vaporization zone (i.e., below 350 ºC). The reason is quite simple: the fraction of vaporized oil contained a vast number of compounds each passing into the gaseous state according to a zero-order kinetics. Since our approach used the smallest possible number of pseudo-components to simulate samples, each pseudo-component represented the thermal behaviour of a large number of compounds; as a result, the thermal degradation of the pseudo-components exhibited a reaction order considerably greater than zero. This was also the case with the deconvoluted curves for atmospheric gas-oil (AGO) and visbreaking residue (VR), which required using a reaction order of 0.6 to fit the vaporization zone accurately. Crude oil, however, required increasing the reaction order of vaporizing pseudo-components to 1. As can be seen from tables 5 and 6, the pseudo-components PsA, PsB and PsC vaporized with a reaction order of 1, and PsD with one of 0.7. Based on the temperature at which the mass loss rate of these pseudo-components peaked (tables 5 and 6), PsA and PsB must be associated to light and heavy naphtha, and PsC and PsD to kerosene and gas-oil. On these grounds, TGA can be used to estimate the potential production of distillates from crude oil. For 19 ACS Paragon Plus Environment

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example, based on tables 5 and 6, heavy crude (HC) and light crude (LC) could provide 62.93% and 79.61% of distillates, respectively. Fig. 8 Table 5 Table 6 Thermogravimetric analysis is also useful to estimate coke formation from crude oil. As can be seen from fig. 8, and tables 5 and 6, PsE, PsF and PsG represented crude fractions undergoing cracking and hence potential coke precursors. As expected, these components were present in greater amounts in HC (31.24%) than they were in LC (17.88%), consistent with the fact that coke production amounted to 5.83% with HC but only 2.51% with LC.

4. Conclusions Using autocatalytic kinetics to deconvolute thermogravimetric data allows one to obtain highly useful information about the composition of the main streams and processes in an oil refinery for purposes such as monitoring oil refining. Under thermogravimetric conditions, pyrolysis of oil-based samples involves vaporization up to 350 ºC followed by cracking up to 520 ºC. In the presence of oxygen, cracking increases char production and mass losses cluster around three specific temperatures, which reflects as sharp peaks in the DTG curve. Our approach allows these thermal processes to be modelled in terms of a small number of pseudo-components representing the main oil cuts. Below 350 ºC, the oilbased samples usually vaporize with an activation energy of 40–60 kJ/mol, a less-thanunity reaction order and a near-zero nucleation order; therefore, autocatalysis has little influence on the process and thermogravimetric data can be modelled according to an nth-order kinetics. Above 350 ºC, however, cracking occurs with an activation energy of 120–140 kJ/mol, a near-unity reaction order and a nucleation order of up to 0.45 in an inert environment or 1.20 in an oxidant environment. These results underline the significance of high temperatures and the presence of oxygen to nucleation and confirm 20 ACS Paragon Plus Environment

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that an nth-order kinetics cannot be used to fit thermogravimetric data under those experimental conditions. Deconvoluting thermogravimetric data allows one to estimate the composition of oil derivatives from pseudo-components with a view to expeditiously obtaining useful information from a refining process. For example, thermogravimetric analysis of atmospheric residue allows one to calculate the proportion of distillable compounds that cannot be separated by an atmospheric column (typically 35–45% depending on the nature of the crude) or to control the composition of atmospheric gas-oil, which consists primarily of two fractions the proportions of which depend on the nature of the distilled crude. In addition, TGA allows the extent of cracking to be estimated from the visbreaking streams. Thus, the content in the major and heaviest pseudo-component of visbreaking feed is typically reduced from 66% to 42%, which allows the production of light compounds to be estimated. Finally, TGA allows the potential production of major cuts from distillable compounds in crude oil, and their tendency to yield coke, to be assessed.

5. Acknowledgements CEPSA “La Rábida” refinery is gratefully acknowledged for kindly supplying the samples and performing the chemical analyses.

6. References (1) Kok, M.V.; Karacan, O.; Pamir, R. Energy & Fuels 1998, 12, 580–588. (2) Ambalae, A.; Mahinpey, N.; Freitag, N. Energy & Fuels 2006, 20, 560–565. (3) Mahinpey, N.; Murugan, P.; Mani, T. Energy Fuels 2010, 24, 1640–1645. (4) Kok, M.V. Fuel Processing Technology 2011, 92, 1026–1031 [5] Al-Saffar, H.B.; Hasanin, H.; Price, D.; Hughes, R. Energy & Fuels 2001, 15, 182– 188. (6) Khansari, Z.; Gates, I.D.; Mahinpey, N. Energy & Fuels 2012, 26, 1592–1597. (7) Murugan, P.; Mahinpey, N.; Mani, T.; Freitag, N. Fuel 2009, 88, 1708–1713. 21 ACS Paragon Plus Environment

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(8) Jia, H.; Zhao, J.Z.; Pu, W.F.; Zhao, J.; Kuang, X.Y. Energy & Fuels 2012, 26, 1575– 1584. (9) Fan, C.; Zan, Ch.; Zhang, Q.; Ma, D.; Chu, Y.; Jiangb, H.; Shi, L.; Wei, F. Fuel Processing Technology 2014, 119, 146–150. (10) Opfermann, J.R.; Kaisersberger, E.; Flammersheim, H.J. Thermochim Acta 2002, 391, 119–27. (11) Mothe, M.G.; Carvalho, C.; Servulo, E.; Mothe, Ch. J Therm Anal Calorim 2013, 111, 663–668 (12) Barneto, A.G.; Ariza, J.; Martín, J.E.; Sánchez, R. Bioresource Technology 2010, 101: 3220–3229 (13) Barneto, A.G.; Moltó, J.; Ariza, J.; Conesa, J.A. Journal of Analytical and Applied Pyrolysis 2014, 105, 8–13 (14) CRC Handbook of Chemistry and Physics (2015). 95th Edition. Internet Version. (Accessed December 2014) http://www.hbcpnetbase.com/ (15) White, C.M. J Chem Eng Data 1986, 31, 198–203 (16) Rice, F.O. J Am Chem Soc 1933; 55: 3035–3040. (17) Xiao, Y.; Longo, J.M.; Hieshima, G.B.; Hill, R.B. Ind Eng Chem Res 1997, 36, 4033–4040 (18) Abbot, J.; Dunstan, P. Ind Eng Chem Res 1997, 36, 76–82 (19) Blanksby, S.J.; Ellison, G.B. Acc Chem Res 2003, 36, 255–263. (20) Wiehe, I.A. Ind Eng Chem Res 1993, 32, 2447−2454. (21) Zachariah, A.; Wang, L.; Yang, S.; Prasad, V.; Klerk, A. Energy and Fuels 2013, 27, 3061−3070. (22) Cinar, M.; Castanier, L.; Kovscek, A. Energy and Fuels 2011, 25, 4438–4451 (23) Burnham, A.K.; Braun, R.L.; Coburn, T.T.; Sandvik, E.I.; Curry, D.J.; Schmidt, B.J.; Noble, R.A. Energy & Fuels 1996, 10 (1): 49–59 (24) Burnham, A.K. Journal of Thermal Analysis and Calorimetry 2000, 60, 895–908 (25) Agrawall, R.K. Thermochimica Acta 1985, 90, 347–351 (26) Prout, E.G.; Tompkins, F.C. Trans Faraday Soc 1946, 42, 468–472 22 ACS Paragon Plus Environment

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(27) Prout, E.G.; Tompkins, F.C. Trans. Faraday Soc 1944, 40, 488–498 (28) Shen, L.; Alexander, A. Thermochimica Acta 1999, 340–341, 271–278


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AGO1

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AGO2

PsC

PsD

PsC

PsD

Ln k0 (s–1)

6.89

7.01

6.80

6.90

Ea (kJ/mol)

51.03 59.97 49.59 60.38

n

0.60

0.40

0.60

0.44

m

10–4

10–4

10–4

10–4

V∞ (g/100 g) 10.17 89.60 11.23 88.76 Tmax (ºC)

224

302

216

309

Table 1. Kinetic parameters for the pyrolysis of two atmospheric gas-oil samples (AGO1 and AGO2, obtained from heavy and light crude, respectively). Each AGO was modelled as the combination of two pseudo-components (PsC and PsD). Tmax is the temperature at which the degradation rate peaked.


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AR1 PsB

PsC

PsD

PsE

PsF

PsG

Ln k0 (s–1)

7.00

7.00

7.00

18.02

18.06

18.03

Ea (kJ/mol)

52.01 57.57 63.19 122.45 128.87 137.49

n

0.60

0.60

0.60

0.99

1.00

1.10

m

10–4

10–4

10–4

0.40

0.40

0.37

V∞ (g/100 g)

2.61

6.32

29.09

11.72

8.30

33.33

Tmax (ºC)

233

283

330

381

412

453

Table 2. Kinetic parameters describing the thermal degradation of atmospheric residue in a nitrogen environment. The overall process was deconvoluted by using six pseudocomponents. The first three (PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. Tmax is the temperature at which the degradation rate peaked.


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VF1 PsD

PsE

PsF

PsG

Ln k0 (s–1)

7.00

18.00

18.00

17.99

Ea (kJ/mol)

64.04 123.91 130.87 138.00

n

0.60

1.00

1.00

1.12

m

10–4

0.10

0.22

0.45

V∞ (g/100 g)

2.10

2.97

12.28

66.36

Tmax (ºC)

329

375

414

461

Table 3. Kinetic parameters describing the thermal degradation in a nitrogen atmosphere of visbreaking feed obtained from heavy crude oil. The overall process was deconvoluted by using four pseudo-components. One (PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. Tmax is the temperature at which the degradation rate peaked.


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VR1 PsA

PsB

PsC

PsD

PsE

PsF

PsG

Ln k0 (s–1)

7.00

7.00

7.00

7.00

18.00

18.00

18.00

Ea (kJ/mol)

43.15 48.91 56.10 62.12 122.44 129.66 138.80

n

0.60

0.60

0.60

0.60

1.00

1.00

1.11

m

10–4

10–4

10–4

10–4

0.09

0.31

0.34

V∞ (g/100 g)

1.89

2.87

4.91

7.38

10.23

11.15

42.07

Tmax (ºC)

151

211

267

320

365

412

457

Table 4. Kinetic parameters describing the thermal degradation in a nitrogen atmosphere of visbreaking residue obtained from heavy crude oil. The overall process was deconvoluted by using seven pseudo-components. The first four (PsA, PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. Tmax is the temperature at which the degradation rate peaked.


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HC PsA

PsB

PsC

PsD

PsE

PsF

PsG

Ln k0 (s–1)

7.00

7.00

7.00

7.00

18.00

18.00

18.04

Ea (kJ/mol)

41.45 46.11 53.23 60.94 122.35 129.19 137.18

n

1.00

1.00

1.00

0.72

1.00

1.00

1.09

m

10–4

10–4

10–4

10–4

0.08

0.30

0.35

V∞ (g/100 g)

8.77

13.45 20.91 19.80

9.90

5.87

15.47

Tmax (ºC)

132

367

409

450

181

239

309

Table 5. Kinetic parameters describing the thermal degradation of heavy crude oil under a nitrogen atmosphere. The overall process was deconvoluted by using seven pseudocomponents. The first four (PsA, PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. Tmax is the temperature at which the degradation rate peaked.


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LC PsA

PsB

PsC

PsD

PsE

PsF

PsG

Ln k0 (s–1)

7.01

7.00

7.00

7.00

18.00

17.99

18.00

Ea (kJ/mol)

41.42 46.11 53.07 60.43 122.13 129.42 135.90

n

1.00

1.00

1.00

0.70

1.00

1.00

1.09

m

10–4

10–4

10–4

10–4

0.04

0.31

0.36

V∞ (g/100 g) 15.21 19.10 23.53 21.77

7.45

4.71

5.72

Tmax (ºC)

366

410

446

133

182

236

305

Table 6. Kinetic parameters describing the thermal degradation of light oil crude under a nitrogen atmosphere. The overall process was deconvoluted by using seven pseudocomponents. The first four (PsA, PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. Tmax is the temperature at which degradation rate peaked.


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Figure 1. Thermal degradation profiles of two representative types of oil: heavy oil (HC) and light oil (LC). a) Mass loss. b) Mass loss rate. Runs were performed in an atmospheric and a nitrogen environment, using a heating rate of 10 ºC/min. Mass loss rates were normalized to the initial mass of sample. 210x297mm (300 x 300 DPI)


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Figure 2. Thermal degradation profiles of crude, atmospheric gas-oil (AGO), atmospheric residue (AR), visbreaking feed (VF) and visbreaking residue (VR). Runs were performed at a scan rate of 10 ºC/min in a nitrogen environment. Mass loss rates were normalized to the initial mass of sample 210x297mm (200 x 200 DPI)


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Figure 3. Thermal degradation profiles of crude, atmospheric gas-oil (AGO), atmospheric residue (AR), visbreaking feed (VF) and visbreaking residue (VR). Runs were performed at 10 ºC/min in an atmospheric environment. Mass loss rates were normalized to the initial mass of sample. 210x297mm (200 x 200 DPI)


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Figure 4. Summary of changes during the thermal degradation of crude oil and its cuts under thermogravimetric conditions. * The cracking stage was considerably affected by the presence of oxygen. ** Char oxidation only occurred in the presence of oxygen 338x190mm (96 x 96 DPI)


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Figure 5. a) Comparison of the thermal degradation of AGO obtained from heavy crude (AGO1) and light crude (AGO2). b) Deconvolution of the AGO1 pyrolysis with two pseudo-components. All thermogravimetric runs were performed at 10 ºC/min under a nitrogen atmosphere. 210x297mm (200 x 200 DPI)


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Figure 6. a) Comparison of the pyrolysis of atmospheric residue obtained from heavy crude (AR1) and light crude (AR2). b) Deconvolution of AR1 pyrolysis with six pseudo-components. The first three (PsB, PsC and PsD) described vaporization of light compounds from 125 to 350 ºC and the others (PsE, PsF and PsG) thermal cracking from 350 to 480 ºC. All thermogravimetric runs were performed at 10 ºC/min. 210x297mm (200 x 200 DPI)


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Figure 7. a) Deconvolution of VF1 pyrolysis with four pseudo-components. PsD described vaporization of light compounds present in this heavy cut, and PsE, PsF and PsG thermal cracking from 350 to 525 ºC. b) Deconvolution of VR1 pyrolysis with seven pseudo-components. The first four (PsA, PsB, PsC and PsD) described vaporization of light compounds present in this cut as result of thermal cracking and the other three (PsE, PsF and PsG) thermal cracking of the remaining heavy compounds from 350 to 525 ºC. All thermogravimetric runs were performed at 10 ºC/min. 210x297mm (200 x 200 DPI)


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Energy & Fuels

Figure 8. a) Deconvolution of heavy oil crude (HC) pyrolysis with seven pseudo-components. The first four (PsA, PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. b) Deconvolution of light oil crude (LC) pyrolysis with seven pseudo-components. The first four (PsA, PsB, PsC and PsD) underwent vaporization and the other three (PsE, PsF and PsG) thermal cracking. All thermogravimetric runs were performed at 10 ºC/min. 210x297mm (200 x 200 DPI)


Thermogravimetric Monitoring of Crude Oil and Its Cuts in an Oil

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