Article pubs.acs.org/EF
Sodium Conversion of Oilsands Bitumen-Derived Asphaltenes Yipei Styles and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, 9211 116th Street, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: Alkali metals are strong electron donors and can form electron donor−acceptor ion pairs with multinuclear aromatics. Single electron donation converts the aromatic into a radial ion, and two electron donations convert the aromatic into a dianion. The anionic aromatic species are more susceptible to reductive addition reactions, such as hydrogen transfer from more hydrogen-rich molecules, such as employed in Birch reduction. The conversion of heavy oils in the presence of alkali metals is claimed to be capable of bulk desulfurization with little hydrogen consumption. In this work, the conversion of oilsands bitumen asphaltenes with sodium was investigated over the temperature range of 60−250 °C under an inert atmosphere to limit the contribution of thermal conversion. Control experiments conducted with asphaltenes without sodium revealed that the asphaltenes were reactive on their own, and the nature of products was affected by the phase behavior of the asphaltenes that gradually changed from a solid to liquid between 124 and 142 °C. When fluidity was limited, intermolecular hydrogen transfer was restricted; intramolecular hydrogen disproportionation and “in cage” intermolecular reactions resulted in the formation of more condensed and potentially more aromatic products. Sodium markedly affected the natural hydrogen transfer and disproportionation. Conversion of the asphaltenes with sodium did not result in an increased maltene yield, but the maltenes had a higher hydrogen/carbon ratio, less sulfur, and less nitrogen. Model compound reactions were employed to study the influence of the reaction atmosphere (N2 or H2) and hydrogen donor properties of the organic matrix. In the presence of H2, the formation and reaction of NaH appeared to influence selectivity and the reaction network. In the absence of H2, desulfurization of thiophenic compounds by sodium proceeded by hydrogenolysis and hydrogenation pathways, with the hydrogenolysis pathway being favored by the lack of hydrogen donor molecules. It was also found that hydrogenolysis of the carbon−carbon bond in the thiophenic ring of dibenzothiophene is reversible.
1. INTRODUCTION Alkali metal conversion of petroleum was originally proposed for the bulk desulfurization of lighter boiling fractions containing refractory sulfur-containing compounds, i.e., thiophenic sulfur.1 Sodium and potassium were preferred alkali metals for this purpose. The application of alkali metal conversion to heavy asphaltene-rich oils, was proposed as a second conversion step after conventional hydrotreating to achieve better desulfurization.2 The objective of alkali metal conversion was desulfurization of multinuclear aromatics. The general concept was later extended to include basic salts as well as alkaline earth metals.3−5 One specific challenge in the use of alkali metals for upgrading of heavy oil that was identified was recycling of the alkali metal. A method to perform desulfurization of heavy oil with sodium in the presence of H2, so that the sodium can be recovered as NaSH, instead of Na2S, was developed.6 More recently, alkali metal conversion was evaluated for conversion of various sulfur-rich heavy oils, bitumens, and shale oils.7 Lithium and sodium were employed in this study, and it corroborated previous claims of desulfurization, denitrogenation, and demetalation. As a result of the high H2 consumption associated with desulfurization of sulfur-rich oils, alkali metal conversion was of particular interest for the upgrading of Canadian oilsands-derived bitumen that has a sulfur content of around 5 wt %.8 All of the aforementioned processes were developed for application of alkali metal conversion above 250 °C but mainly at temperatures in excess of 340 °C. This was in stark contrast to most of the fundamental studies in base catalysis of alkali metals, which were conducted at much milder conditions.9−12 © XXXX American Chemical Society
Alkali metals facilitate conversion of multinuclear aromatic compounds by forming an electron donor−acceptor ion pair with the aromatic compound. The alkali metal is a very strong electron donor, and the conversion can be illustrated by the half reaction of sodium (eq 1) and the half reaction of the aromatic (eq 2). Na → Na + + e−
(1)
aromatic + e− → [aromatic•]−
(2)
Sodium is the electron donor, and the multinuclear aromatic is the electron acceptor. A single electron donation to form the Na+[aromatic•]− ion pair and two electron donations to form the (Na+)2[aromatic]2− ion pair are both possible. The former can also be described as a radical anion, because the single electron that was donated is unpaired. The electron(s) donated to the aromatic molecule are delocalized in the π-electron system.10 These ion pairs can be isolated, and crystal structures have been determined for various combinations of alkali metals and aromatics.13 The solvent environment determines to what extent disproportionation between the free radical anion and the dianion takes place (eq 3), with the dianion being favored by poorly polarizable solvents.14 Special Issue: 65th Canadian Chemical Engineering Conference Received: January 15, 2016 Revised: March 1, 2016
A
DOI: 10.1021/acs.energyfuels.6b00106 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
For each experiment, the reactor was filled with 3−4 g of industrial asphaltenes. The weight ratio of asphaltene/sodium was 4:1. The sodium metal was cut while submerged in mineral oil to minimize exposure to air, and then only the cut sodium pieces were transferred to the reactor preloaded with asphaltenes. The freshly cut sodium was exposed to air for only a short period during the transfer. The size of the sodium pieces was not measured or controlled, because at most of the reaction conditions, sodium was a liquid. The loaded reactors were leak-tested with nitrogen at an absolute pressure of 4.1 MPa. For the reactions, the reactors were purged and then pressurized with nitrogen to an initial pressure of 3.5 MPa. The run length was 1 h measured from the time that the reactor internal temperature reached the set point; the temperature range investigated was 60−250 °C. Heating time was around 5 min. At the end of the experiment, the reactor was cooled, and once cooled, the reactor was depressurized. The gas released from the reactor was collected for analysis. All other products were removed from the reactor by washing with n-pentane. n-Pentane was also used for deasphalting at the same time. The remaining unconverted sodium was deactivated by reaction with isopropanol, methanol, and water sequentially. The mixture was then vacuum-filtered using a Millipore membrane filter of 0.22 μm pore size and 47 mm diameter to separate the liquid and solids. The solids were dried and weighed. The liquid was a two-phase mixture containing an organic phase (maltenes in n-pentane solution) and an aqueous phase (mainly NaOH in water solution). The aqueous solution was separated from the maltene solution in a separating funnel. The maltene solution and aqueous solution were individually dried on a heating plate at 45 and 110 °C, respectively. After evaporation of all of the water from the aqueous solution, the product became a light yellow- and white-colored mixed powder. For the control experiments, the same procedure was followed but no sodium was added; thus, no deactivation of sodium was necessary. However, to keep the workup procedure consistent, alcohols and water were added and phase-separated from the product. All materials were weighed on a Mettler Toledo ML3002E balance with 3200 g capacity and 0.01 g readability. 2.2.2. Experiments with Anthracene. The same equipment and procedure that were used for the asphaltenes were employed for the experiments with anthracene but with a few modifications, as indicated. For each experiment, the reactor was filled with around 1 g of anthracene. Na was cut in mineral oil and added so that the molar ratio of anthracene/sodium was 1:2.2. Decalin was used as the solvent, and the molar ratio of anthracene/decalin was 1:16.2. The reactors were purged and leak-tested in the same way, but the initial pressure after purging was 0.86 MPa of either nitrogen or hydrogen. The reactor was kept at 150 °C for 22 h before it was cooled, and the products were collected for analysis. Reaction products were analyzed using gas chromatography. 2.2.3. Experiments with Sulfur Compounds. For a typical experiment, about 2 g of targeted reactant (dibenzothiophene or diphenyl sulfide) was added to a 250 mL flask. Then, 25 mL of solvent (decalin or 1-methylnaphthalene) was added to the same flask. Lastly 5.3 g of sodium was added. The flask was connected to a nitrogen cylinder, and nitrogen flow was maintained through the flask for at least 10 min before heating the reaction mixture to 150 °C in an oil bath on a hot plate. The mixture was stirred at 500 rpm, and the nitrogen flow was maintained for the whole experiment, so that the reaction was conducted in an oxygen-free atmosphere. The reaction mixture was kept at 150 °C for 22 h before it was cooled, and the product was analyzed by gas chromatography. The reaction of dibenzothiophene under a hydrogen atmosphere was performed in the same way as the experiments with anthracene. 2.3. Analyses. CHNS elemental analysis was performed by the Analytical and Instrumentation Laboratory of the Department of Chemistry at the University of Alberta, using a Thermo Scientific Flash 2000 CHNS and oxygen combustion analyzer. Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA/DSC 1. The microbalance had a maximum capacity of 5 g and 0.1 μg readability. Analyses were conducted under a constant flow of 45 mL/min N2. The temperature program used varied on the
2Na +[aromatic•]− ⇌ (Na +)2 [aromatic]2 − + aromatic (3)
These alkali metal and aromatic ion pairs are active for hydrogen exchange reactions, and the aromatic ion is also susceptible for partial hydrogenation. The Birch reduction is the best known form of the partial hydrogenation reaction. Birch reduction is the “hydrogenation” that takes place with an alkali metal in liquid ammonia with an alcohol as the combined co-solvent and proton donor.15 If reduction temperatures higher than −33 °C are required, organic amines can be used instead of ammonia. Although the upgrading of heavy oil by alkali metal conversion was demonstrated and sodium conversion in particular,6,7 it was not clear what the mechanistic advantage of using sodium was. If the primary benefit is the formation of a sodium−aromatic ion pair, then there is no need for hightemperature conversion. In fact, keeping the reaction temperature below 250 °C would avoid the need for a furnace for preheating, which can be performed with steam instead, and potentially reduce both capital and operating costs. The sodium conversion of asphaltenes was studied over the temperature range of 60−250 °C. This temperature range was selected specifically to ensure that sodium is in the liquid phase for most of the experiments and to limit the impact of thermal conversion, which may otherwise obscure the origin of conversion that is observed. A second objective was to explore the potential practical benefit of operating at temperatures below that claimed in the literature on heavy oil upgrading with alkali metals2−7 but well within the temperature range where fundamental studies demonstrated the reactivity of sodium.9−12 To better interpret the observations, the applied work was supplemented with sodium-mediated conversion of model compounds to evaluate the performance of sodium in aromatic hydrogenation reactions and desulfurization reactions.
2. EXPERIMENTAL SECTION 2.1. Materials. Industrially produced asphaltenes was used as feed material. The asphaltenes feed was obtained from the n-pentane-based solvent deasphalting unit at the Long Lake Upgrader of Nexen Energy ULC. The Long Lake Upgrader is situated in the Athabasca oilsands region, and it upgrades bitumen recovered from a subsurface oilsands deposit by steam-assisted gravity drainage (SAGD). Characterization of the asphaltenes feed is presented in section 3.1 Sodium sticks (99%, in mineral oil) were purchased from Alfa Aesar. n-Pentane (99.7%) used as washing solvent and solvent for reprecipitation of asphaltenes from the industrial asphaltene feed was purchased from Fisher Scientific. Nitrogen (99.998%) and hydrogen (99.95%) were obtained as cylinder gases from Praxair Canada. The model compounds used were anthracene (98%), dibenzothiophene (98%), diphenyl sulfide (98%), 1-methylnaphthalene (96%), and decalin (cis- and trans-decahydronaphthalene mixture, 97%) purchased from Sigma-Aldrich. 2.2. Equipment and Procedure. 2.2.1. Experiments with Asphaltenes. The experiments for this investigation were conducted in microbatch reactors. The microbatch reactors were constructed from Swagelok 316 stainless-steel 0.5 in. tubing (12.7 mm outside diameter and 10.2 mm inside diameter), approximately 80 mm in length. The temperature inside the reactor was monitored using a thermocouple in a 1/16 in. diameter (1.6 mm outside diameter) stainless-steel sleeve. The microbatch reactors were placed in a preheated fluidized sand bath heater, Omega fluidized bath, to heat the reactors to the reaction temperature. These microbatch reactors were not equipped with any form of agitation and were not suitable for kinetic measurements. B
DOI: 10.1021/acs.energyfuels.6b00106 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels basis of the nature of the analysis, but in all analyses, a heating rate of 10 °C/min was employed. The sample mass used for analysis was around 5 mg. Proton nuclear magnetic resonance (1H NMR) spectroscopy was performed with a Nanalysis 60 MHz NMReady-60 spectrometer. Samples were dissolved in deuterated chloroform to obtain a concentration of 0.2 g/mL. Hydrogen with shift values below 6.3 ppm was considered aliphatic, and hydrogen with shift values above 6.3 ppm was considered aromatic. The main underlying assumption for this classification is that the products are substantially free of olefinic compounds. Differential scanning calorimetry (DSC) combined with microscopy was conducted using a Mettler FP84HT thermal analysis microscopy cell and Olympus BX51 microscope. For this analysis, a sample of the solid asphaltenes (around 5 mg) was placed in a glass crucible. The sample was heated at 10 °C/min, and temperature ranges for study were selected to cover both melting and onset of thermal decomposition. A camera mounted on the microscope recorded the change in visual appearance with the temperature, which could be matched to the energy flow that was simultaneously recorded. The analysis was performed under a static air atmosphere. Scanning electron microscopy with energy-dispersive X-ray (SEM− EDX) microanalysis was performed by the Scanning Electron Microscopy Laboratory at the Earth Sciences Building of the University of Alberta using a Zeiss EVO MA 15 SEM with Bruker Quantax 200 EDX. Gas chromatography coupled with mass spectrometry (GC−MS) was performed using an Agilent 7820 with 5977E mass spectrometer. Separation was performed on a HP-5 column (30 m × 0.25 mm × 0.25 μm) using helium as the carrier gas. The temperature program started at 90 °C, with a hold of time of 0.5 min, after which the temperature was increased by 5 °C/min up to 325 °C and then held for 5 min. Analysis by GC−MS was used only for product identification. The products were quantified using an Agilent 7890A gas chromatograph with a flame ionization detector (GC−FID). The GC−FID was used with a DB-5 MS column (30 m × 0.25 mm × 0.25 μm), which is similar to the HP-5, and the same temperature program was employed. The FID response factor for dibenzothiophene was experimentally determined to be 1.12.
Good repeatability was obtained during CHNS elemental analysis. Material balance over the analyses of the fractions compared to that of the feed indicated a difference of
