Article pubs.acs.org/EF
Study on the Preparation of Mesophase Pitch from Modified Naphthenic Vacuum Residue by Direct Thermal Treatment Dong Liu,*,† Bin Lou,† Ming Li,† Fengjiao Qu,‡ Ran Yu,† Yuanxi Yang,† and Chongchong Wu§ †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China Exploration and Development Research Institute, Hua Bei Oil Field Company, Renqiu 062550, China § University of Calgary, Calgary, Alberta T3A2L4, Canada ‡
ABSTRACT: Two feedstocks (LPA and HPA) obtained from modification of naphthenic vacuum residues were selected to prepare mesophase pitch by direct thermal treatment. The influence of reaction temperature, soaking time, reaction pressure, and molecular structures of the feedstock on mesophase development was systematically investigated by analyzing variations in carbonization yield, carbon residue, quinoline insolubles (QI) content, density, optical textures, crystal structure, and surface morphology of derived products. It is found that the mesophase development and the properties of resultant mesophase products were closely related to the molecular structure of the original materials and preparation conditions. Compared with LPA, HPA was the preferable feedstock for thermal treatment because of its high degree of aromaticity and a large proportion of naphthenic carbon, and the resultant product obtained under the optimum conditions showed large flow domain mesophase, fewer alkyl side chains, and a high degree of molecular orientation.
1. INTRODUCTION In recent years, the carbonaceous mesophase known as nematic liquid crystal has attracted considerable attention as precursor of advanced carbon materials of excellent properties such as carbon foam,1,2 C/C composites,3 and carbon fibers4 and is commonly prepared from coal-tar pitch, petroleum pitch, and aromatic hydrocarbons.5 In past years, Korai. et al.6 have reported that the solubility properties and development of anisotropy in the resultant mesophase pitches obtained from pyrolysis of A240 pitch and coal-tar pitches, respectively, exhibited a strong relationship with the alkyl and naphthenic groups in the constituent molecules of feedstock. Also, an investigation conducted by Miyake. et al.7 showed that the mesophase content increased with a rise in hydrogen amount caused by hydrogenation for the mesophse pitch but varied with the types and amounts of the alkyl groups introduced by reductive alkylation for the mesophase pitch. Mochida. et al.8 further proved through the analysis of mesophase pitch that some naphthenic and short alkyl chains groups surviving from the feedstock during the pyrolysis are essential for the mesophase pitch to keep its low softening point and reasonable stabilization reactivity. Aromaticity of the original materials for pyrolysis is also important because of the formation of the aromatic, discotic, nematic liquid crystal phase leading to mesophase and anisotropy.9,10 Besides this, the fluidity of reaction medium necessary for the movement and orientation of the mesogens can be affected by the reaction conditions,11−14 so some other researches investigated by ́ Rodriguez-Reinoso et al.15−17 surveyed chemical and physical interactions during the pyrolysis of the feedstock, which influence mesophase growth and coalescence of mesophase within semicokes. To sum up, not only the chemical compositions and structures of the original feedstock but also the reaction conditions incorporated into the system such as heating rate, the soak time, final reaction temperature and pressure within the system are functions of the properties of © XXXX American Chemical Society
resultant mesophase products that ultimately determine their application. Hence, as for the feedstock of different properties, different preparation processes such as direct thermal polymerization,18,19 hydrogenation treatment,20,21 catalytic polymerization,22,23 and cocarbonization24 were employed to balance constituent molecular reactivity and mobility, leading to improved quality of the resultant mesophase products. Among these processes, direct thermal polymerization is the most common and practical method, which is performed by maintaining the feedstock isothermally for a certain time. Although the demand for mesophase pitch based advanced carbon materials such as carbon fiber, needle coke, and carbon foam have constantly increased, the high price of mesophase pitch is considered to be a major applicable obstacle. At present, A240 pitch and naphthalene-based pitch are universally recognized as excellent precursors for preparing mesophase pitch.8 A240 pitch can be obtained from FCC decant oil by a series of complicated refined processes, and naphthalene-based pitch is generally derived by catalytic polymerization of pure naphthalene. Compared with the commercial vacuum residues, the above two tailor-made pitches are costly. As a result, lowering the cost of precursor and using a simple synthesis process to prepare the mesophase pitch are the key points to contribute to application of mesophase pitch based materials on a large scale. Paraffin-, intermediate-, or naphthene-based vacuum residues are all rich in polycyclic aromatic compounds, but their constituent compounds are very different in molecular structure and carbonized behavior.25 By far, for industrial use, the vacuum residues are too reactive chemically, their yields of coke are too low, and their cokes are of too poor quality, without significant crystallinity and appropriate physical properties such as electrical Received: February 18, 2016 Revised: May 16, 2016
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DOI: 10.1021/acs.energyfuels.6b00392 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels conductance and coefficients of thermal expansion.26 In this paper, the feedstocks HPA and LPA are both China CNOOC company’s commercial products, which are obtained by a simple modified treatment for naphthenic vacuum residue in which constituent molecules generally possess a larger proportion of aromatic structure and naphthenic structure attached to aromatic rings than those in paraffin- or intermediate-based vacuum residue.27 Thus, the carbonized reactivity of LPA and HPA decreases, and the carbonized yield also has a certain degree of improvement because of the oxidative cross-linking modification. LPA or HPA possesses potential to prepare high-quality mesophase pitch in large scales, but the pyrolysis behavior of this kind of raw material has not been investigated systemically thus far. In this study, the above two modified naphthenic vacuum residues were selected to prepare the mesophase pitch by direct thermal polymerization process in order to study the effect of the chemical compositions and structures of feedstock as well as process conditions on the pyrolysis behaviors and properties of resultant products so as to have an overall understanding of their relationships.
Figure 1. Experimental reactor used for pyrolysis. valve when the pressure in the reactor exceeded the set pressure by 0.1 MPa. Because the excellent performance of advanced carbon materials derived from the mesophase pitch such as carbon fiber or needle coke has a very deep relationship with optical anisotropic structure, in this study, the selection of optimal experimental conditions was mainly based on the optical texture in the resultant mesophase products. 2.3. Characterization of Samples. The elemental compositions were determined on a PE-2400 Series HCSN elementary analyzer. The oxygen content was obtained by difference. Fourier transform infrared spectra (FTIR) of samples were recorded by using a Nicolet NEXUS 470 FTIR spectrometer equipped with mercury−cadmium telluride detectors operating at 4 cm−1 by averaging 128 scans. The orthosubstitution index that can reflect the relative size of the aromatic molecules was defined as Ios = Abs750/(Abs750 + Abs815+ Abs880), and the aromaticity index was calculated according to the formula Iar = Abs3050/ (Abs3050 + Abs2920).28 X-ray diffraction (XRD) tests were performed on an X’Pert Pro MPD X-ray diffractometer with Cu Kα radiation. 1H nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker Avance DMX500 spectrometer, using deuterated chloroform as solvent and tetramethylsilane (TMS) as internal standard. The optical textures of the carbonized residues were observed by using a XP-4030 polarized microscope (Shanghai milite Precise Instrument Co., Ltd., China) equipped with an adjusted ocular (10×), an oil immersion objective (50×), and a 1-λ retarder plate. The density, carbon residue, and quinoline insolubles (QI) content of the resultant mesophase products were measured according to GB/T 8928−2008, GB8727−88, and GB/T 2293−2008, respectively.
2. EXPERIMENTAL SECTION 2.1. Feedstock. In this paper, the two modified naphthenic vacuum residues, namely, LPA and HPA provided by CNOOC Company, were selected as the feedstock of the direct thermal polymerization. The properties of both feedstocks are listed in Table 1.
Table 1. Properties of LPA and HPA items
LPA
Elementary Analysis (wt %) 87.07 10.15 0.98 0.96 0.84 1.40 Six Component Analysis (wt %) saturates 13.56 monocyclic aromatics 3.54 dicyclic aromatics 1.31 polycyclic aromatics 4.32 resins 71.13 asphaltenes 6.14 softening point (°C) 59 carbon residue (wt %) 20.9 QI (wt %) 3.9 density (g·cm−3) 1.0139 ash content (wt %) 0 C H N S O H/C
HPA 86.76 9.94 1.00 0.91 1.39 1.37 12.68 2.83 17.30 2.09 31.23 33.87 117 23.5 5.7 1.0717
