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A batch reactor study of the effect of aromatic diluents to reduce sediment formation during hydrotreating of heavy oil Alexis Tirado, and Jorge Ancheyta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02452 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017
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Energy & Fuels
A batch reactor study of the effect of aromatic diluents to reduce sediment formation during hydrotreating of heavy oil Alexis Tirado1, Jorge Ancheyta2 1
Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del instituto
Politécnico Nacional, Legaria 694. Colonia Irrigación, Mexico City, 11500, México 2
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, San Bartolo Atepehuacan, Mexico City, 07730, México
ABSTRACT The effect of different aromatic diluents on sediment formation during the catalytic hydrotreating of heavy crude oil was studied. The experiments were carried out using a heavy crude oil with 11.69 °API and three aromatic streams from an industrial Fluid Cracking Catalytic plant: light cycle oil, heavy cycle oil and decanted oil. All tests were performed in a batch reactor at reaction time of 2 to 4 h, reaction temperature of 380 to 420 °C and addition of aromatic diluents in 5 and 10 wt. % with respect to the crude oil. The reaction pressure and stirring rate were kept constant at 100 kg/cm2 and 1000 rpm, respectively. The results of the hydrotreating tests showed that the amount of sediment increases as the time and reaction temperature do, being the temperature the variable that has the greatest effect on sediment formation. Light cycle oil provides a greater suppression of sediments. This effect was attributed to the greater amount of total aromatic compounds (mainly di-aromatics) able to keep asphaltenes solvated. Keywords: aromatic diluent, sediment, hydrotreating, heavy oil.
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1. INTRODUCTION The worldwide tendency in the petroleum industry to accomplish the growing demand and stricter specifications of fuels is to convert the heavy crude oils into lighter crude oils. Catalytic hydrotreating (HDT) is one of the most important upgrading processes for achieving such a purpose, whereby the heavy crude oil is subject to a series of chemical reactions carried out in the presence of a catalyst in a hydrogen-rich atmosphere, thus removing the greatest amount of impurities, such as sulfur, nitrogen, metals and asphaltenes1. Due to the presence of high amounts of asphaltenes in heavy crude oils the conversion of the residue fraction is limited due to sediment formation. It is generally accepted that residue conversion higher than 40-50% causes high formation of sediments during catalytic hydrotreating, provoking operational problems such as blocking of lines, heat exchangers, reactors as well as enhancing catalyst deactivation which eventually ends up in the shutdown of HDT commercial units. Sediment formation is the result of an imbalance between asphaltenes and those compounds that kept them in solution during HDT, in other words, resins loss their peptization properties causing precipitation of asphaltenes. Moreover, asphaltenes become more aromatic because they undergo dealkylation reactions. It is known experimentally that when the conversion of residue exceeds certain limit, sediment formation increases drastically due the incompatibility of polymerized and condensed asphaltene components in the product, as shown in Figure 12. Due to this, refineries limit sediment content in the hydrotreated product to a maximum value of 0.8-1.0 wt. %, to ensure continuous operation of the HDT unit. For this reason, the maximum allowable residue conversion is determined by sediment formation. 2 ACS Paragon Plus Environment
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Mochida et al.3 studied the molecular structure and some properties of sediments generated during hydrotreating of an atmospheric residue. The reaction was carried out at different temperatures: 395, 405 and 418 °C, which were considered conditions of no sediment formation, beginning of sediment formation and some formation of sediments, respectively. The structural analysis of the hexane insoluble fraction resulted to be rather polar due to a considerable amount of heteroatoms, with shorter alkyl groups. Similar results were reported by Stanislaus and Hauser4 for the characterization of sediments collected from the hydrocracking of Kuwait residues by means of nuclear magnetic resonance, elemental analysis and molecular weight. Mochida et al.5 also reported that the addition of an aromatic compound (1methylnaphtalene) during the hydrocracking of heavy crude oil suppresses the amount and size of sediments generated when operating under severe conditions. Marafi et al.6 studied the effect of aromatic compounds on the control of sediment formation during hydrotreating and investigated the influence of adding 10% of aromaticrich diluents to the original feed. Although no appreciable improvement in the removal of sulfur, metals, nitrogen and other impurities was observed, sediment formation was substantially reduced when heavy cyclic oil and light cyclic oil were employed, as shown in Figure 2. Ortega-García et al.7 evaluated the effect of injecting a decanted oil from fluid catalytic cracking unit (FCC) together with a vaccum residue to an ebullated-bed hydrocracking pilot plant. They stated that when decanted oil is added to the feedstock, its effect on sediment formation is marginal because the aromatic hydrocarbons of the diluent were saturated
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during the reaction. However, when decanted oil is added to the product, sediment formation was significantly reduced (> 40%). It is clearly seen that sediment formation is a critical process parameter during hydrotreating and hydrocracking of heavy oils. Nevertheless, it has been shown that the use of streams produced in an FCC unit can be a relevant process solution due to their high content of aromatic compounds and relative low cost. However, which type and properties of the diluent generate a higher performance in the reduction of sediments is not fully understood. This is the main motivation for studying the effect of different aromatic diluents on sediment formation during catalytic hydrotreating of crude oils. 2. EXPERIMENTAL 2.1 Materials A heavy crude oil and three aromatic streams recovered from an FCC commercial unit were used: light cycle oil (LCO), heavy cycle oil (HCO) and decanted oil (DO), which properties are presented in Table 1. Blends of 5 and 10 wt.% of diluent plus heavy crude oil were prepared. The properties of the blends are reported in Table 2. 2.2 Hydrotreating experiments Hydrotreating tests were carried out in a Parr batch reactor model 4575 of 500 mL of capacity, which is equipped with control for stirring rate and temperature. The hydrotreating catalyst used in all experiments was a commercial NiMo/γ-Al2O3 sample with the following main properties: 0.58 % Ni, 2.2 % Mo, 197 m2/g specific surface area, 0.85 mL/g mean pore volume and 173 Å mean pore diameter. In each experiment 1 g of fresh catalyst and 100 g of feedstock were loaded to the batch reactor. The catalyst was
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sieved with a mesh size of 20 (0.84 mm) in order to avoid the presence of fine catalyst particles which could be erroneously quantified as sediments. Prior to the hydrotreating tests the catalyst was sulfided ex situ in a fixed-bed glass reactor. It was first dried under a flow rate of nitrogen of 30 mL/min of N2 at 120 °C for 2 h, and then activated with 40 mL/min of a H2/CS2 mixture at 400 °C during 3 h. After finishing the presulfiding step, the flow was switched to nitrogen to keep the catalyst in inert atmosphere. The sulfided catalyst was transferred into a basket at room temperature and introduced in the batch reactor together with the feedstock, so that contact which air is avoided. 2.3 Reaction conditions Four variables were studied to evaluate their effect on sediment formation: reaction time, reaction temperature, type of diluent and dilution percentage. Pressure and stirring rate were kept constant at 100 kg/cm2 and 1000 rpm, respectively. Firstly, experiments were performed with the undiluted heavy crude oil at 400 °C in order to determine a reaction time such that the amount of sediments is sufficiently high to magnify the difference when using the diluents. Reaction time was varied between 2 and 4 h. Once the reaction time was defined, the effect of reaction temperature was evaluated in the range of 380 to 420 °C with the undiluted crude oil. A series of experiments were then carried out to evaluate the effect of the addition of aromatic diluents. Dilution was studied at 5 and 10 wt. % of aromatic stream. 2.4 Characterization of feedstock and products
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The physical and chemical properties of all the streams were determined by the ASTM methods reported in Table 1. The distribution of aromatic compounds of the diluents was analyzed by chromatography of supercritical fluids, in which tri-aromatic includes aromatics with three and more rings. 1
H and
13
C NMR spectra of the different streams were determined in accordance with
ASTM D5292 method using deuterated chloroform (CDCL3) as a solvent at resonance frequency of 300 and 75 MHz, respectively. The average molecular parameters were calculated through a series of rules based on the results obtained from elemental analysis, molecular mass and nuclear magnetic resonance spectra. The results of these parameters are presented in Table 3. 3. RESULTS AND DISCUSSION The hydrotreating experiments focused on the study of the effect of different process variables on sediment formation. The results of modifying the reaction time, reaction temperature, the addition of different aromatic diluents and dilution percentage are presented below. Each experiment was performed by changing one variable at a time. 3.1 Effect of reaction time Reaction time was considered the variable to be firstly defined to find content of sediment in the hydrotreated product in which conversion can affect it in considerable manner. It was varied from 2 to 4 h. The effect of reaction time on sediment content and on API gravity of the upgraded oil is shown in Figure 3. These experiments were done only with the heavy crude oil. It was confirmed that the longer the reaction time the higher the sediment content as well as the higher the API gravity, because of the conversion of heavy molecules into lighter hydrocarbons. The changes in API gravity due to hydrocracking and hydrogenation 6 ACS Paragon Plus Environment
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reactions lead to the alteration of the equilibrium between aliphatic and aromatic phases, thus causing a problem of solubility that produces asphaltenes precipitation and subsequent formation of sediments8. The results show that at 3 h of reaction a high amount of sediments (>0.7 wt. %) is generated. While at 4 h sediment formation is excessive (>1.5 wt. %) and at 2 h it is low (
