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Kinetic study of pre-oxidized asphaltene hydroprocessing in aqueous phase Parsa Haghighat, Lante A Carbognani Ortega, and Pedro Rafael Pereira-Almao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00898 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016
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Kinetic study of pre-oxidized asphaltene hydroprocessing in aqueous phase Parsa Haghighat*, Lante Carbognani Ortega, Pedro Pereira-Almao Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada Abstract The solubilized asphaltene in water (SAW) was prepared by low temperature oxidation in aqueous phase and used as a feedstock of hydroprocessing reaction. Hydroprocessing experiments were carried out in a 100 ml batch reactor within the temperature range of 280320oC in the presence of presulfided NiMo/γ-Al2O3 catalyst. A lumped kinetic model with four components including water soluble fractions, water insoluble fractions, liquid hydrocarbons and gas products was proposed, which accurately predicted the experimental results. The activation energy of global reaction was calculated to be 83 kJ/mol. At 320oC, the liquid hydrocarbons yield increased around 8% by prolonging the residence time from 1 hour to 6 hours. For 3 hours residence time, by increasing the reaction temperature from 280oC to 320oC, the liquid yield was increased 5% and the conversion was enhanced by 8%. Increasing the reaction temperature affected the quality of products, i.e., liquid hydrocarbons with lower boiling point distribution were obtained at higher reaction temperatures. At 320oC, phenol derivative products disappeared indicating the progress of deoxygenation at higher reaction temperatures. FTIR analyses confirmed that disappearance of carboxylate functional groups through decarboxylation or protonation was the main reason for the production of water insoluble fractions after the hydroprocessing. Extended Henry’s law with γ-ϕ approach was implemented to predict the thermodynamics status of the system at reaction conditions. The occurrence of reaction in the
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liquid phase was confirmed, where at 320oC more than 80 wt. % of water remained in liquid phase and the liquid level in the reactor elevated 25%. Keywords: lumped kinetic model, hydroprocessing in aqueous phase, decarboxylation and solubilized asphaltene in water (SAW) Introduction Reaction of heavy oil fractions in the solvent media attracted wide attention in the twenty first century as a feasible technology to replace traditional upgrading methods. In several studies the role of solvent in reaction media was discussed, where some authors described the effect of solvent as a physical role that enhances the solubility of hydrocarbons and improves the mass transfer limitation, while others proved the chemical effect of solvent as a hydrogen donor.1-4 Environmental benefits of using water as solvent coupled with the special properties of this compound at high temperatures close to its critical point (374oC, 22.1 MPa), made it the most studied solvent in the heavy oil and biomass conversions. Among these studies, Morimoto et al. examined the effect of three different reaction media including water, toluene and high pressure nitrogen on Canadian bitumen upgrading, where the produced coke in supercritical water (SCW) was found to be more aromatic with lower H/C ratio compared to the other media.3 Vilcáez et al. used a column flow reactor for conversion of bitumen-water mixture at 300oC and 3-6 MPa where the improvement of upgrading and suppression of coke formation were simultaneously observed.5 Dutta et al. studied the effect of water on thermal cracking of Athabasca bitumen in an autoclave reactor in presence of D2O, where the coke formation was suppressed and H/C ratio decreased for the liquid products in the water media.2 The same result of coke reduction was observed by Moriya et al. in polyethylene thermal cracking in supercritical water.6 Some commercial processes were also introduced for heavy oil upgrading in SCW. In the US patent 2 ACS Paragon Plus Environment
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described by Banerjee, the slurry mixture of catalyst/oil is combined with supercritical water in an up flow reactor to upgrade the oil to transportation specifications without use of external gases.7 In another work, Choi proposed a two-step patented process which starts with premixing the water and heavy oil by ultrasonic wave generator and finishes with hydrothermal upgrading of heavy oil/water mixture in hot pressurized water.8 In parallel with heavy oil upgrading in aqueous phase, behavior of other hydrocarbon solvents as a reaction medium was also explored. Viet et al. studied vacuum residue hydrocracking in aromatic and paraffinic solvents with activated carbon catalyst at 400oC, where higher naphtha yield was obtained in aromatic solvents and more middle distillate was produced in alkanes media.9
In another study, catalytic
hydrotreating of asphaltene in supercritical toluene was explored at 380oC and 5 MPa hydrogen pressure, where NiMo/MgO catalyst exhibited the best performance, increasing the reaction conversion by 16%. It was reported that toluene medium played an effective role for asphaltene conversion even in absence of catalyst.10 Despite vast research on heavy oil conversion in solvent media, kinetics study of these reactions is rarely addressed. The few conducted researches are limited to the use of traditional kinetic models for direct reaction without presence of solvent, which neglects the probable effect of water on reaction pathways.11,12 On the contrary, in the field of biocrudes conversion abundant research can be found on biomass degradation, their kinetics and reaction pathways in the solvent medium. 13-21 Zhang et al. studied the hydrothermal reaction of 5 different lignin samples in water with a simple kinetic model containing two phases. In the first phase, lignin is converted to water soluble low molecular fragments (monomers) and water-acetone soluble larger fractions (oligomers); in the second phase the insoluble polymer and char were produced by condensation of soluble components.13 Yong and Matsumura developed a reaction pathway containing a series
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of first order reactions for guaiacol and lignin conversion in water phase. Their proposed kinetic model comprised several real components including phenolic derivatives such as phenol, catechol and cresol plus lumped components such as water soluble liquid products, aromatic compounds, char and gas.17-19 The same kinetic model was adopted by Forchheim et al. to describe lignin depolymerization in water.15 Wahyudiono et al. compared the kinetic parameters of thermal decomposition of guaiacol in sub and supercritical water by applying a single first order reaction with respect to guaiacol conversion where the pre-exponential factor increased one order of magnitude in supercritical water and activation energy stayed relatively constant around 35 kJ/mol.21 In addition to kinetic modeling of non-catalytic reaction in aqueous phase, kinetic study of catalytic reactions in water was also carried out. Forchheim et al. described the catalytic degradation of lignin in presence of Raney nickel catalyst in aqueous phase by a consecutive kinetic model with two first order reactions. Based on this model, lignin first converted to guaiacol and then to product where the apparent activation energy of guaiacol conversion was found to be around 170 kJ/mol.14 Due to the complex structure of biomass derivatives and production of different compounds through their degradation, proposing a kinetic model with the ability to predict the formation rate of all specific products is very difficult. To propose such a model, extensive characterization is required to identify each single product with accurate quantitative analyses, which makes the study time-consuming and impractical. For instance, Gasson et al. attempted to develop a kinetic model for lignin decomposition in ethanol to predict the formation of several monomeric phenol compounds. Although the reaction elements for kinetic study did not exceed 11 components, the mass balances in some experiments reached lower that 85% which strongly affected the accuracy of calculated kinetic parameters.16 For this reason, in the kinetic study of complex feedstocks such as lignin and bitumen, lumped models
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are mostly applied where each lumped fraction represents the range of compounds with similar properties. In the majority of lignin decomposition kinetic studies, all the gas products are reported as one single gas component, all precipitated materials after reaction are lumped as one solid component and all the aromatic hydrocarbons pooled into a single lumped fraction.17,18 Also in kinetic studies of heavy oil fractions, the crude oil is divided into several fractions with different boiling point ranges and the reaction is described based on the change of lumped components such as Naptha, Distillates, VGO, VR and Coke.22-24 In our previous article, the possibility of solubilizing asphaltene in aqueous phase via low temperature oxidation and formation of value added products by further hydroprocessing of those molecules was discussed.25 The advantages of asphaltene oxidation in water prior to hydroprocessing were described before. During the oxidation step, the asphaltene molecules break into smaller components through oxy-cracking reaction. Thus, the solubilized asphaltenes in water, i.e., the feedstock of second reaction, were partially upgraded before hydroprocessing. Furthermore, through the oxidation, the oxidized asphaltene became solubilized in water resulting in homogenous dispersion of these compounds in liquid water which significantly improves mass transfer limitations at hydroprocessing step.25 In this paper, a new kinetic model for oxidized asphaltene hydroprocessing in water was proposed, which is the first attempt on kinetic study of aqueous phase hydroprocessing of oxygenated hydrocarbons derived from heavy oil fractions. Also, controlling the system at reaction conditions by chemical reaction and negligibility of catalytic mass transfer limitation was justified and with the help of FTIR and GCMS analyses, a possible reaction pathway for aqueous phase hydroprocessing was proposed. In addition to the kinetic modeling of reaction in aqueous phase, the phase behavior of hydrogenwater system at reaction conditions was studied. Estimating the hydrogen solubility in the
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solvent at reaction conditions is a very important aspect mostly ignored in previous studies of hydroprocessing in solvent media. In the literature, studying of gas-water binary systems and prediction of gas solubility in aqueous phase is limited to a few number of gases such as light alkanes, CO2 and H2S due to their presence in reservoirs with water and their application in enhance oil recovery methods such as SAGD, ES-SAGD and N-Solve. In general, thermodynamics modeling of light gases in water is divided to symmetric (ϕ -ϕ) and asymmetric (γ-ϕ) approaches. In ϕ –ϕ method, fugacity of both phases are predicted by equation of state. The application of this method for prediction of gas solubility in the liquid solvents can be found in several studies.26-30 In (γ-ϕ) approach, the fugacity of liquid phase is calculated by activity coefficient while the gas phase fugacity is determined by equation of state. In the work of Valtz et al. performance of both methods was evaluated to predict the solubility of CO2 in water, where
both approaches accurately matched the experimental results.31 The activity model is capable of predicting aqueous phase containing salt and electrolytes and it is more favored when thermodynamics study on non-pure water and gases is desired.32 In this study, the thermodynamic status of the system at reaction conditions was predicted using the γ-ϕ equilibrium approach by applying the extended Henry law for hydrogen in water. Experimental approach Feed preparation The oxidized asphaltene soluble in aqueous phase, identified as SAW (solubilized asphaltene in water) is the reactant for hydroprocessing experiments. To prepare the required amount of SAW, wet oxidation experiments were conducted and repeated in a 550 ml model 4575B HP/HT Parr batch reactor. The reaction was carried out at 240oC, 2 h and initial oxygen pressure of 1000 psig. The carbon content of aqueous phase after oxidation reached 10300 mg/l. 6 ACS Paragon Plus Environment
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The results of oxidation reaction are provided in Table S1 of the supporting information. For these experiments, asphaltene was precipitated by normal pentane (>99%, Sigma-Aldrich) from Athabasca vacuum residue. The elemental composition of virgin asphaltene was as follows: C: 82.5 wt. %, H: 8 wt. %, N: 0.8 wt. % and S: 6.5 wt. %. Carbon and Hydrogen contents were determined with a LECO 628 analyzer and Nitrogen and Sulphur measurement were carried out with an Antek 9000 analyzer. The detailed description of asphaltene solubilization in aqueous phase by low temperature oxidation was presented in our previous work.25 Hydroprocessing experiment A 100 ml Micro Parr batch reactor model 4598 was used for hydroprocessing of solubilized asphaltene in water. 25 grams of SAW solution produced through the previously mentioned low temperature oxidation plus 0.5 gram of presulfided NiMo/Al2O3 catalyst were added to reactor. The preparation and activation procedure of the in-house prepared NiMo/Al2O3 catalyst was described previously.25 The leak test was conducted by nitrogen at 2000 psig, then the vessel was pressurized with hydrogen (99.999% ultra-high purity, Praxair) and stirring was started. Finally the reactor was heated up to the set point temperature. After experiment completion, the reactor was quenched to ambient temperature and the gas phase was collected for GC analysis. The gas analysis was carried out with SRI 8610C Multiple Gas #3 gas chromatograph with TCD detector for carbon containing components such as CO2, CO and CH4 and HID detector for Hydrogen analysis. The liquid phase after reaction was mixed with 25 ml of CS2 (Sigma-Aldrich) for 60 minutes, with a stirring rate of 1000 rpm to extract the liquid products by CS2 from the water phase. The aqueous phase inside the reactor was filtered and the solid was dried and characterized with infrared analysis. For all infrared analyses, dried samples were mixed with 7 ACS Paragon Plus Environment
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KBr powder and analyzed with a Nicolet 6700 FTIR spectrometer (Thermo Electron Corporation) provided with smart diffuse reflectance cell (DRIFTS). Boiling point distribution analysis of the CS2 phase which contains extracted liquid products was performed with an Agilent 6890N gas chromatograph using the ASTM D7169 method, following the technique explained by Carbognani et al.33 To quantify the amount of liquid hydrocarbons in the CS2 phase, a calibration curve with 4 levels of hydrocarbon concentrations in CS2 solution was prepared. The total carbon (TC), inorganic carbon (IC) and organic carbon (TOC) in the water phase were measured with a Shimadzu TOC analyzer (TOC-V CPH/CPN) for water soluble fractions before and after hydroprocessing (after extraction of the liquid products from the aqueous phase). The carbon based conversion was calculated by comparing the amount of soluble carbon in water before and after reaction. The product yields including liquid, gas and solids (water insoluble fraction) were all determined by dividing the carbon content of each fraction by the initial content of carbon in SAW before reaction. The liquid products extracted with CS2 were also analyzed with a Shimadzu GCMS analyzer. The QP5000 mass spectrometer with scan range of 35-300 m/z, scan interval of 0.56 second and interface temperature of 310oC, was coupled directly to the GC-17A gas chromatograph. The procedure of product separation and fractions characterization before and after hydroprocessing reaction is summarized in Figure 1.
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Results and Discussion Thermodynamics verification of the system at reaction conditions For the hydroprocessing study in the solvent medium, conducting the reaction in the liquid phase is a primary objective, thus before starting the experiments it should be ensured that at reaction temperatures and pressures, water does not vaporize and stays in the liquid phase. For the current study, γ-ϕ approach with extended Henry law is used to predict the gas solubility and ultimately to evaluate the reaction phase. The possible electrolyte effect of soluble hydrocarbons in the aqueous phase was considered to be negligible. This was evidenced by comparing the experimental data including recorded temperature and pressure for (pure water/H2) with (SAW/H2) system where the exact same behavior was obtained. Therefore, for the thermodynamics modeling of this work, the aqueous phase was assumed to be a pure water. First, liquid-vapor equilibrium for each component (i) is written as: =
(1)
Fugacity of components in the liquid phase is given by:
( − ) =
( − ) =
(2)
(3)
where (reference pressure) is the saturation pressure at related temperature, is the partial
infinite molar volume of hydrogen and is the molar volume of water at corresponding
saturation condition. Activity coefficients are taken to be 1 by Henry law where ( → 0, → 1) and ( → 1, → 1).
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Despite the lack of data on hydrogen-water binary systems, a few correlations can be found for Henry dependency on temperature.34-36 For this study, the correlation proposed by Alvarez et al. was adopted due to the wide range validity of correlation up to the water critical
point.36 This correlation is shown in Equation 4, where " corresponds to the critical temperature of water.
#$( /&'() = −3.44134 +
2.1262 × 100 −0.46295 × 104 + /1
" − " − − 4.8138 6 7 ln ( ) "
(4)
The vapor phase fugacity is determined by Equation 5, where the Peng Robinson Equation of State (PR-EOS) was applied to calculate the fugacity coefficient. The detailed description of PR-EOS can be found elsewhere.37 = ':
(5)
VMGSim Dynamics 8.0 was implemented for flash and equilibria calculations by applying Equation 4 for Henry correlation. For the experimental part, the reactor containing 25 grams of pure water was pressurized with pure hydrogen to 600 psig at room temperature, then heated up to 320oC and finally it was quenched to initial condition while the temperature and related pressure during the transition time were continuously recorded. The system was kept for five minutes at each experimental point to ensure it reached the equilibrium condition before recording the data. The comparison of experimental data with modeling results is presented in Figure 2. The predicted line lies within the experimental points (heating up and cooling down points), which indicates the good agreement of thermodynamics model with experiment. Furthermore, it is 10 ACS Paragon Plus Environment
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observed that the experimental data for heating up and quenching the reactor are overlapping confirming that the reactor passed through the reversible thermodynamics process which justifies the presence of equilibrium state at all experimental conditions. Results of thermodynamics modeling including the variation of vapor fraction, hydrogen solubility and liquid level in the reactor by increasing the temperature are presented in Figure 3. It is observed that the hydrogen solubility in aqueous phase increased by enhancing the reaction temperature, suggesting higher accessibility of hydrogen molecules to reactants at reaction conditions, which diminishes the hydrogen mass transfer limitation in reaction media. The calculated solubility of hydrogen at 320oC supports the experimental data reported for hydrogen/water system in another published article.36 The liquid volumetric holdup in the reactor and vapor fraction of the system increased with increasing the temperature as shown in Figure 3a and b. The main reason for increasing the liquid level in the reactor is the change of water density by temperature, reaching 666 kg/m3 at reaction conditions (320oC). The properties of system at reaction condition shown in Table 1 indicate that at 320oC more than 20 grams of water remains in the liquid phase, confirming the presence of a liquid medium through hydroprocessing. Effect of initial hydrogen pressure on hydroprocessing results In hydroprocessing reaction of heavy oil fractions, excess amount of hydrogen is required to suppress the coke formation by capping the aromatic free radicals. The hydrogen addition also increases the product quality by saturating the hydrocarbon molecules. For the case of reaction in solvent media, the presence of external hydrogen facilitates the hydrogenation although hydrogen can be donated by solvent molecules as well.1,2 To ensure that hydrogen is not a limiting agent in SAW hydroprocessing, a set of experiments with different initial hydrogen 11 ACS Paragon Plus Environment
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pressures was carried out at 320oC, 3 h, 500 rpm and 0.5 gram of catalyst. The experimental results shown in Figure 4 indicate that reaction conversion increased 4 % from 400 psig initial hydrogen pressure to 550 psig and stayed unaffected above 550 psig. A similar trend is observed for product yields where the liquid products increased by elevating the initial hydrogen pressure up to 550 psig and remained constant beyond this point, indicating that above 550 psig the reaction progressed under excess amount of hydrogen. Also it was observed that solid formation slightly diminished by increasing the hydrogen pressure up to 550 psig. The same promotional effect of hydrogen pressure on catalytic hydrodeoxygenation of biomass in solvent media is reported in other published articles.38,39 However, some authors claimed that very high ratio of hydrogen to feed inhibits the catalytic reaction since the hydrogen molecules occupy the entire catalyst sites and make them unavailable for the reactant molecules.40,41 Kinetic model for hydroprocessing of solubilized asphaltene in aqueous phase To study the kinetics of reaction, series of experiments were conducted at different residence times and temperatures following the experimental plan shown in Table 2. All the experiments were carried out at initial hydrogen pressure of 600 psig after confirming presence of excess amount of hydrogen in reactions, occurring above 550 psig. Three experiments including Exp #3, Exp #6 and Exp #12 were performed twice to verify the reproducibility of results. Experiments with mass balances lower than 90% after reaction were repeated and the average of mass balances for the reported experiments is above 95%. To avoid catalyst internal mass transfer limitation, the catalyst particles were finely grounded and sieved to the range of 0.15-0.25 µm. To justify the negligibility of catalytic mass transfer, Exp #11 was repeated under the same conditions with larger catalyst size between 1.8-3.0µm (one order of magnitude larger particles). The experimental results showed that the hydroprocessing reaction was not affected
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by variation of catalyst’s size, indicating that particles range is small enough to make the rate of internal mass transfer faster than the chemical reaction rate. Furthermore, to ensure the negligibility of external mass transfer, Exp # 12 was repeated under the same condition with a mixing rate of 1500 rpm (three times higher than initial rate of 500 rpm) where the reaction result remained unchanged, confirming that the process is controlled by chemical reaction. In the direct thermal or catalytic hydrocracking of crudes, the primary theory behind all the kinetic models is the simultaneous conversion of larger molecules to smaller ones, which is predicted by the series of first order reactions yielding to different boiling point range fractions. However, for hydroprocessing of SAW, several reactions occur at the same time such as hydrogenation, hydrodeoxygenation, decarboxylation, cracking and WGS reaction. Therefore, since cracking is not a dominant reaction in this case, classifying the products based on their boiling point range is not a correct assumption. For this research, after characterizing each component, a new lumping approach is proposed containing 4 elements, as shown in Figure 5. The first element represents the soluble materials in aqueous phase before or after reaction. The second component is the solid formed after reaction which is insoluble in water and characterized in detail with FTIR spectroscopy. The liquid products are the third lumped fraction of the kinetic model, which are the value added hydrocarbons extracted with CS2 after reaction, characterized via HTSD and GCMS. The last element is the gas product including all the carbon containing components which is primary CO2 with low content of CO and CH4. The average selectivity to CO2 in carbon containing gas products was found to be 95% for all reaction conditions. The 6 first order reactions were applied to predict the conversion of components while the order of reaction respect to hydrogen was considered to be zero. Comparison of gas composition before and after hydroprocessing experiments also evidenced that hydrogen
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consumption was negligible compared to the initial hydrogen amount; thus, pseudo first order reaction assumption was deemed reasonable. Equations 6-9 showed the kinetic reactions for each component based on the lumped model proposed in Figure 5.
SAW:
Solid:
Liquid:
Gas:
; = −(?@ + ? + ?0 )A
(6)
; B = ?@ A − (?C + ?D )A
