COMPARISON BETWEEN ASPHALTENES (SUB)FRACTIONS EXTRACTED FROM TWO DIFFERENT ASPHALTIC RESIDUES: CHEMICAL CHARACTERIZATION AND PHASE BEHAVIOR

http://dx.doi.org/10.5935/0100-4042.20150172 Quim. Nova, Vol. 39, No. 1, 26-31, 2016 Artigo COMPARISON BETWEEN ASPHALTENES (SUB)FRACTIONS EXTRACTED FROM TWO DIFFERENT ASPHALTIC RESIDUES: CHEMICAL CHARACTERIZATION AND PHASE BEHAVIOR Silas R. Ferreiraa, Fabio R. Barreiraa, Luciana S. Spinellia, Katia Z. Lealb, Peter Seidlc and Elizabete F. Lucasa,d,* a Instituto de Macromoléculas, Universidade Federal do Rio de Janeiro, Av. Horácio Macedo, 2030, 21941-598 Rio de Janeiro – RJ, Brasil b Instituto de Química, Universidade Federal Fluminense, Outeiro São João Batista, s/n, 24020-150 Niterói – RJ, Brasil c Escola de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 149, 21941-909 Rio de Janeiro – RJ, Brasil d Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa em Engenharia, Universidade Federal do Rio de Janeiro, Av. Horácio Macedo, 2030, Bloco F, 21941-598 Rio de Janeiro – RJ, Brasil Recebido em 02/06/2015; aceito em 14/09/2015; publicado na web em 11/11/2015 Asphaltenes are blamed for various problems in the petroleum industry, especially formation of solid deposits and stabilization of water-in-oil emulsions. Many studies have been conducted to characterize chemical structures of asphaltenes and assess their phase behavior in crude oil or in model-systems of asphaltenes extracted from oil or asphaltic residues from refineries. However, due to the diversity and complexity of these structures, there is still much to be investigated. In this study, asphaltene (sub)fractions were extracted from an asphaltic residue (AR02), characterized by NMR, elemental analysis, X-ray fluorescence and MS-TOF, and compared to asphaltene subfractions obtained from another asphaltic residue (AR01) described in a previous article. The (sub)fractions obtained from the two residues were used to prepare model-systems containing 1 wt% of asphaltenes in toluene and their phase behavior was evaluated by measuring asphaltene precipitation onset using optical microscopy. The results obtained indicated minor differences between the asphaltene fractions obtained from the asphaltic residues of distinct origins, with respect to aromaticity, elemental composition (CHN), presence and content of heteroelements and average molar mass. Regarding stability, minor differences in molecule polarity appear to promote major differences in the phase behavior of each of the asphaltene fractions isolated. Keywords: asphaltene extraction; asphaltene characterization; asphaltene phase behavior; asphaltene precipitation. INTRODUCTION Asphaltenes, together with paraffins with high molar mass, are among the main compounds that cause organic deposits in the oil industry. Unlike paraffins, asphaltenes do not melt, and they can form deposits in rock formation pores, valves, pumps, storage tanks and refinery lines.1-5 Asphaltenes along with naphthenic acid salts are also blamed for stabilizing water-in-oil emulsions, making the demulsification process more difficult.6-12 In general, their complex structures consist in polynuclear aromatic rings with different alkyl branches containing acid and basic groups and some elements such as sulfur, oxygen, nitrogen, vanadium and nickel.4 Asphaltenes are constituted by a family of molecules with structures that have similar characteristics of molar mass and polarity, when compared with other petroleum components. Besides that, the content and the chemical structure of asphaltenic fractions depend on the source of the crude oil. These molecules tend to autoaggregate in a lamellar way because present interconnected aromatic rings. These interactions generally occur by hydrogen bonds, donor-acceptor electrons in complexes with transition metals and relocation of π electrons in condensed aromatic rings.9 Asphaltenes can remain soluble in the oil or can precipitate if the petroleum equilibrium is shifted by changing pressure or oil composition. Asphaltenes stability depends basically on their content and light fractions one in the oil. For instance, aromatic fractions act as solvents for asphaltenes, and, on the other hand, hydrocarbons do not, i.e. any adverse disturbance in the balance between the hydrocarbon and aromatic fractions can lead to the precipitation of asphaltenes. *e-mail: Because of these problems, many studies have been performed to elucidate various aspects of asphaltenes, such as their chemical composition, chemical structure, phase behavior in crude oil and model systems, and solubility parameters.4,13-19 However, due to the diversity and complexity of these structures, there is still much to be investigated. The aim of this study was to extract asphaltene (sub)fractions from an asphaltic residue (AR02), characterize the (sub)fractions by nuclear magnetic resonance (NMR), elemental analysis, X-ray fluorescence and mass spectrometry (MS-TOF), and to compare the results with those for the same types of (sub)fractions extracted from an asphaltic residue (AR01), described in a previous article,13 in order verify differences in asphaltenes fraction in function of asphaltic residue source. We also evaluated the phase behavior of all these (sub)fractions in model systems containing 1 wt% of asphaltenes in toluene, by monitoring the precipitation onset through optical microscopy, in order to compared phase behaviors and try to correlate that with characterization data. EXPERIMENTAL Materials n-Decane P.A., 99.5% n-heptane, 99.0% n-nonane, n-octane P.A. and 99.0% n-pentane were acquired from Vetec Química Fina (Xerém, RJ, Brazil) and used as received. Commercial toluene obtained from the same supplier was used after distillation and drying in alumina. Deuterated chloroform, from Cambridge Isotopic Laboratory (Tewksbury), was also used as received. The asphaltic residue from unit 1790 of the Duque de Caxias Refinery (REDUC) Vol. 39, No. 1 Comparison between asphaltenes (sub)fractions extracted from two different asphaltic residues (Duque de Caxias, RJ, Brazil), sampled on August 18, 2010, was donated by the Petrobras Research Center (CENPES), here called AR02. Asphaltene (sub)fractions,13 obtained from another asphaltic residue from the same unit and refinery, sampled on February 1, 2008 and called AR01, were used for comparison. Methods Extraction of asphaltene fractions from asphaltic residue AR02 Two fractions were separated from asphaltic residue AR02 by extraction with n-pentane: resins C5S and asphaltenes C5I. For this purpose, about 30 g of asphaltic residue was ground and left under the action of 1 L of a paraffinic solvent (n-pentane) to precipitate for 24 hours under stirring. Then the material was filtered through filter paper at room temperature (25 °C), to obtain two fractions: the resins soluble in the paraffinic solvent and the precipitated asphaltenes caught in the filter paper. This precipitate was placed in a Whatman cartridge (internal diameter: 94 mm) and submitted to extraction in a Soxhlet extractor to separate and (...truncated)