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Hydroconversion Behavior of Asphaltenes under Liquid-phase Hydrogenation Conditions Nan Jin, Gang Wang, Shuang Han, Yiming Meng, Chunming Xu, and Jinsen Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02765 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 24, 2016
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Hydroconversion Behavior of Asphaltenes under Liquid-phase
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Hydrogenation Conditions
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Nan Jin, Gang Wang*, Shuang Han, Yiming Meng, Chunming Xu, Jinsen Gao
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State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China
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University of Petroleum, Beijing 102249, China
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ABSTRACT
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To fully utilize deoiled end-cut (DOE) from selective asphaltene extraction process,
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Venezuela n-pentane DOE was subjected to hydroconversion in an autoclave reactor
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using tetralin as hydrogen donor. Venezuela DOE and its hydroconversion products were
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separated into gas, n-heptane maltenes (HM), n-heptane asphaltenes (HAs), and coke.
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The effects of reaction conditions including reaction temperature, solvent-to-feedstock
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(S/F) weight ratio, and reaction time on product distribution were investigated. High
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temperature, large S/F ratio, and long time benefited the generation of gas, HM, and coke
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to some extent. Under optimal conditions, over 50 wt% HAs in DOE was converted into
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HM fraction, with less than 3 wt% coke yield. The elemental compositions and molecular
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weights of HAs and HM, along with reaction time, were also analyzed. The
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hydrogen-to-carbon (H/C) ratio of HAs declined from 1.115 to 0.871, indicating that HAs
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underwent dehydrogenation and dealkylation reactions. However, the H/C ratio of HM
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initially increased from 1.405 to 1.548, showing that hydrogenation reaction occurred,
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and then decreased to 1.374 because of the cracking of HAs into HM and/or the
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secondary cracking of HM. The average molecular-weight decrease both for HAs and
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HM confirmed disaggregation and cracking reactions. The molecular composition and 1 ACS Paragon Plus Environment
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transformation of nitrogen and sulfur compounds before and after hydroconversion were
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determined by negative- and positive-ion electrospray ionization Fourier transform
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ion-cyclotron-resonance mass spectrometry, respectively. N1, S1, and O2 classes were
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dominant in the feedstock. After hydroconversion, N1 and S2 compounds decreased in
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HAs, indicating that they were reactive species. N1 compounds mainly cracked into small
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N1 compounds and also condensed into N2 compounds, while S2 compounds generally
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decomposed into S1 compounds.
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1. INTRODUCTION
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A confluence of different factors, including increasing oil demand and depleting
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conventional oil sources, has led to the consideration of inferior heavy oils as important
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feedstock in the energy basket. However, inferior heavy oil contains many heteroatoms,
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such as S, N, O, V, and Ni, which are difficult to be processed in the factory1.
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At present, inferior heavy oils are usually processed by initial conversion into
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synthetic crude oils, which could then be used to produce light oil, such as gasoline and
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diesel, as well as industrial chemicals, such as ethylene and propylene. Aside from coking
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process for inferior heavy oils in Venezuela2, 3, and the integration technology of coking
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and hydrotreating for oil sands in Canada2, 4, selective asphaltene extraction (SELEX-Asp)
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process, which was developed by State Key Laboratory of Heavy Oil Processing at the
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China University of Petroleum (Beijing), is another important pretreatment method5. The
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SELEX-Asp technology separates inferior heavy oils into several deasphalted oil (DAO)
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fractions and one deoiled end-cut (DOE). DAOs can be processed by fluid catalytic
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cracking and/or hydrotreating easily6–8, however, the DOE, accounting for a substantial
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amount (20 wt%–30 wt%)9 of feedstock, is not well utilized so far. 2 ACS Paragon Plus Environment
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As the heaviest fraction in the SELEX-Asp process, DOE concentrates the most
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polar and complex material present in inferior heavy oil. Currently, the DOE utilization
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involves producing road asphalt10, light oil11, syngas12, and carbon material13. Among
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these methods, producing light oil is considered as the most efficient way to maximize
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the value of energy, especially in the circumstance of coal-to-liquids. However, ultrahigh
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amount of asphaltenes in DOE seriously restrain it from efficient conversion. As is well
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known, asphaltenes are coke precursors14–16 during petroleum refining and catalyst
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poisons17–19 for catalytic processing. Therefore, asphaltenes are significant for efficient
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utilization of DOE.
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Several researchers have been making efforts to improve the efficient conversion of
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asphaltenes. Better than thermal cracking, which is usually accompanied with over 50 wt%
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coke yield at a temperature of 400 °C and time of 1 h20, 21, hydrogenation has become an
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effective way to convert asphaltenes with low coke yield. Studies on hydrogenation of
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asphaltenes can be classfied into two sections based on hydrogen source: (1) gas phase
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hydrogenation by hydrogen and (2) liquid phase hydrogenation by hydrogen donor
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solvents, such as tetralin. Gas phase hydrogenation allows asphaltenes from different
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sources to yield 15 wt%–50 wt% lighter fractions with 5 wt% coke yields at 400 °C22.
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The specific hydrogenation results depend on factors such as operating conditions22, type
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of catalyst23, and properties of feedstock22 employed. However, gas phase hydrogenation
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is limited by the contact efficiency between hydrogen and asphaltenes because of their
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different phases and complicated asphaltene aggregates24. By contrast, hydrogen donor
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solvents can not only provide hydrogen to asphaltene radicals efficiently, but also
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dissolve coke precursors to prevent phase separation, which is followed by coke
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formation. The extensive studies on conversion of residues25–27, asphaltenes28–30 and
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coals31,
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presence of tetralin, 28 wt% heptane-maltenes can be obtained for California asphaltene
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without coke formation30. Researchers also pointed out that the solvents stabilize the
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decomposition products, which would tend to polymerize to insoluble coke at higher
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temperatures29, 33.
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in hydrogen donor solvents have confirmed the above advantages. In the
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Previous studies have provided significant amount of information on the
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contribution of hydrogen donor solvent to increasing the conversion efficiency of
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asphaltenes. However, only few studies have focused on the conversion products. The
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structural properties of the conversion products are worth of further exploration and
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research, which will be of great importance to further utilization of the conversion
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products effectively. The current study aims at maximizing the hydroconversion of
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asphaltenes with little coke formation in the presence of tetralin. Moreover, we made the
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attempt to explain how asphaltenes convert into desired products under favorable
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hydrogenation conditions through analyzing feedstock and product comprehensively and
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deeply.
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2. EXPERIMENTAL SECTION
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2.1. Hydroconversion Experiments
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Hydroconversion experiments were conducted with Venezuela DOE, which was
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obtained from the industrial demonstration unit of SELEX-Asp process with n-pentane of
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Venezuela vacuum residues (VR). This sample was used as received without further
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treatment. The main properties of the feedstock are listed in Table 1.
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The hydroconversion experiments were carried out in a 250 mL autoclave stainless
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steel reactor, equipped with mechanical stirring and automatic temperature control. A
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schematic of the experimental setup is presented in Figure 1. A typical experiment was
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performed as follows: DOE and tetralin were introduced into the reactor at a ratio of 1:3–
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1:9. The reactor was pressurized with hydrogen to 3.0 MPa; hence the reaction pressure
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under the reaction temperature was 7.0–8.0 MPa. The temperature was increased at a rate
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of about 7 °C/min and kept at 380–420 °C for 1–7 h while stirring at 300 rpm. The
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reaction was terminated by pumping cooling water into the coil located inside the reactor,
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as depicted in blue in Figure 1.
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2.2. Product Separation
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After hydroconversion, gas, liquid, and solid products were separated, respectively,
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as displayed in Figure 2. The gases were collected by displacement method, and then
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released into a 1 L capacity gas bag for further analysis.
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The liquid product was divided into HM and HAs fractions after determining the
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HAs conversion. HAs content was determined according to Chinese Standard Analytical
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Method for Petroleum and Natural Gas Industry SH/T0509-2010. The conversion of HAs
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was estimated by HAs content in DOE and its hydroconversion products. To obtain HAs
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and HM, the liquid product was mixed at a ratio of 40:1 (volume/weight) n-heptane /
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liquid sample and stirred at 60 °C for 3 h. After 12 h, the mixture was filtered under
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vacuum using a quantitative filter paper, and then rinsed with n-heptane. The HAs
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recovered in the filter paper were dried in an oven at 80 °C for 24 h. The HAs-free
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sample was concentrated by a micro distillation apparatus to obtain a solvent-free sample
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(HM). The distillation conditions were respectively 100 °C under atmospheric pressure 5 ACS Paragon Plus Environment
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for n-heptane and 75 °C under 200 Pa for tetralin and its products.
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Coke content in the liquid product was almost 0, which was determined by a method
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similar to HAs content determination. Thus, all the coke was staying on the bottom of the
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reactor. The coke was washed out of the reactor by toluene, filtered using quantitative
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filter paper, and refluxed to colorless with toluene. The coke recovered in the filter paper
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were dried in an oven at 100 °C for 24 h. Coke yields were calculated from the weight
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increase of the filter paper. Through above method, entrained HM could be separated
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from coke and dissolved in the toluene. This part of HM was in trace amounts, and
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thereby ignored during the material balance.
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All tests in this article were repeated twice. Yields of gas, HM, HAs, and coke were
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calculated based on the HAs amount in the DOE and defined as follows:
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For gas, HAs, and coke, Yield (%)=
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where m is the mass of the sample and ω is the mass fraction of HAs in the sample.
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During the distillation process, some light components can be easily co-distilled with the
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solvents; hence, the yield of HM was calculated by subtraction method.
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For HM,
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2.3. Analytical Methods
mi，after reaction − mi，before reaction × 100 (i =gas, HAs, coke) mDOE × ωHAs, DOE
YieldHM (%)=100 − YieldGas − YieldHAs − YieldCoke
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Gas chromatograph (GC). The gas products were analyzed by GC 9790 II gas
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chromatograph, equipped with a flame ionization detector and a thermal conductivity
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detector, using argon as the carrier gas. The temperature program consisted of an
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isothermal step at 60 °C for 15 min, followed by a ramp of 5 °C/min to reach 180 °C, and
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then moving on to the final isothermal step at 180 °C for 17 min. After measuring the 6 ACS Paragon Plus Environment
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volume percentage of C1−C6 hydrocarbons, CO and CO2, the equation of the state for an
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ideal gas was used to convert the data to mass percentages.
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Size exclusion chromatography (SEC). The molecular weight of HM and HAs
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were estimated by SEC. SEC experiments were conducted on a Waters 515-2410 System
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with a molecular-weight distribution capacity of 500–8 500 000 g/mol, using
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tetrahydrofuran as a mobile phase. All the injected samples had a concentration of 2–3
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mg/mL. The flow rate was fixed at 1 mL/min at 30 °C. The eluent was analyzed with a
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2410 refractive index detector from the Waters Company.
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Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).
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The HM and HAs samples were analyzed by Bruker apex-ultra FT-ICR MS equipped
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with a 9.4 T superconducting magnet.
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For characterization of nitrogen compounds, samples were dissolved in a blended
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toluene and methanol (1:3, v/v) solvent at a concentration of 0.1 mg/mL, which was an
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ultralow concentration, indicating that the effect of aggregation should be ruled out.
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Approximately 15 µL of aqueous solution of ammonium hydroxide (28 wt%−35 wt %)
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and 10 µL of formic acid were added for negative-ion electrospray ionization (ESI) to
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enhance molecular ionization. The typical conditions for negative-ion formation were 4.0
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kV emitter voltage, 4.5 kV capillary column introduced voltage, and −320 V capillary
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column end voltage.
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For characterization of sulfur compounds, samples are needed methylation before
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FT-ICR MS analysis. About 100 mg samples were diluted with 5 mL of dichloromethane.
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Then, 300 mg of silver tetrafluoroborate and 0.3 mL of methyl iodide were added to the
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sample solution. After stirred in darkness for 48 h, the reaction mixture was centrifuged
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to remove the yellow silver idodide precipitate. The samples derived from methylation
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were diluted with acetonitrile to a concentration of 0.005 mg/mL, and then analyzed by
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positive-ion FT-ICR MS. The typical conditions for positive-ion formation were -2.5 kV
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emitter voltage, -3.0 kV capillary column introduced voltage, and 320 V capillary column
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end voltage.
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ICR was operated at 200−800 Da mass range and 4 M acquired data size. FT-ICR
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MS was internally calibrated with the N1 class homologous series. Mass spectrum peaks
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with a relative abundance five times greater than the standard deviation of the baseline
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noise were exported to a spreadsheet. Data analysis was performed by a custom software.
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Compositional assignment of each peak was performed within 0.0015 Kendrick mass
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defect tolerances. The detail of data processing can be found elsewhere6, 34.
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3. RESULTS AND DISCUSSION
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3.1. Hydroconversion of Venezuela DOE
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Suitable operating parameters are essential to maximize the HM yield with little
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coke formation during hydroconversion. Therefore, the effects of reaction conditions,
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such as temperature, solvent-to-feedstock (S/F) weight ratio, and reaction time on the
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product distribution were investigated.
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3.1.1. Effect of Reaction Temperature
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Maintaining the S/F ratio and reaction time at 9 and 1 h, respectively, the effect of
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reaction temperature on the product distribution was investigated in the range of 380–
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420 °C and displayed in Figure 3(a).
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As the reaction temperature increased, the gas yield almost increased linearly from
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1.20 wt% to 4.45 wt%, while the coke yield increased substantially from almost 0 to 7.23
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wt% when the temperature increased from 400 °C to 420 °C. This result may be
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attributed to the fact that higher temperature is favorable to the cracking of HAs
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molecules, which leads to the formation of macromolecular radicals. These
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macromolecular radicals can combine with hydrogen radicals abstracted from tetralin,
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and finally become HM fraction. However, at higher temperature, massive
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macromolecular radicals are generated instantly, which will mutually collide into larger
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radicals and form coke before encountering the hydrogen radicals. As expected, the yield
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of HM experienced a slight drop from 34.55 wt% at 400 °C to 32.99 wt% at 420 °C
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because of the mutual polymerization within macromolecular radicals. In addition,
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secondary reactions of HM strengthened as the reaction temperature increased. Therefore,
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the reaction temperature should be lower than 420 °C to guarantee lower coke yield and
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higher yield of the desired product.
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3.1.2. Effect of Solvent-to-feedstock Weight Ratio
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The effect of S/F ratio was investigated in the range of 3–9 at a constant reaction
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temperature of 400 °C and time of 1 h, as shown in Figure 3(b). The S/F ratio is the
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amount of hydrogen donor contacted by unit weight of feedstock, indicating the average
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hydrogen donation ability of the hydrogen donor solvent to the feedstock.
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Figure 3(b) showed that the coke yields were particularly low (
