Microwave-assisted ultraviolet digestion of petroleum coke for the simultaneous determination of nickel, vanadium and sulfur by ICP-OES

doi:10.1016/j.talanta.2015.07.060

Talanta

Volume 144, 1 November 2015, Pages 1052-1058

https://doi.org/10.1016/j.talanta.2015.07.060 Get rights and content

Highlights

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Microwave and UV radiation were combined for the first time for digestion of petroleum coke.
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High sample masses (up to 500 mg) were digested using the MW-UV method.
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MW-UV method allowed the use of diluted acids for digestion of petroleum coke.
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Simultaneous determination of Ni, V and S in digests was possible to be performed by ICP-OES.

Abstract

A method for the simultaneous determination of Ni, V and S in petroleum coke by inductively coupled plasma optical emission spectrometry (ICP-OES) after microwave-assisted ultraviolet digestion (MW-UV) in closed vessels was proposed. Digestion was performed using electrodeless discharge lamps positioned inside quartz vessels and turned on by microwave radiation. The following parameters were evaluated: HNO₃ concentration (15 mL of 1, 4, 7, 10 or 14.4 mol L⁻¹), volume of H₂O₂ (30%, 1 or 3 mL), sample mass (100, 250 or 500 mg) and heating time (40 or 60 min) with or without the use of UV lamps. Digestion efficiency was evaluated by the determination of the residual carbon content (RCC) in digests. Using UV lamps lower RCC was obtained and the combination of 4 mol L⁻¹ HNO₃ with 3 mL of H₂O₂ and 60 min of heating allowed a suitable digestion of up to 500 mg of petroleum coke (RCC< 21%). The agreement with the reference values for Ni, V and S (obtained by digestion of petroleum coke by microwave-induced combustion) and with a certified reference material of petroleum coke was between 96 and 101%. The proposed method was considered as advantageous when compared to American Society for Testing and Materials method because it allowed the simultaneous determination of Ni, V and S with lower limit of detection (0.22, 0.12 and 8.7 µg g⁻¹ for Ni, V and S, respectively) avoiding the use of concentrated nitric acid and providing digests suitable for routine analysis by ICP-OES.

Graphical abstract

Introduction

Petroleum coke is a solid residue obtained from thermal processing of heavy petroleum fractions with high carbon content [1], [2], [3]. The quality of petroleum coke is dependent on the concentration of some elements (mainly Ni, V and S), which cause poisoning of catalysts used in refining processes [1], [4] and also serious problems during its burning, such as abrasion and fouling in the combustion chamber [5], [6]. For petroleum coke used in aluminum production, Ni and V are undesirable because they catalyze oxidation reactions leading to higher consumption of carbon in the electrolysis cell [7]. Sulfur compounds are also undesirable, because they are responsible for the SO_x emission when coke is burned, causing serious environmental problems [1], [4]. Therefore, the content of Ni, V and S in petroleum coke is a decisive factor for its quality evaluation [1].

The determination of Ni, V and S in petroleum coke has been performed by analysis of solid samples, according to method recommended by American Society for Testing and Materials (ASTM, D 6376-10 method) using wavelength dispersive X-ray fluorescence spectrometry [8]. However, some interferences caused by changes in sample composition could be observed and calibration with certified reference materials (CRM) is usually required [1]. The determination of Ni and V can also be performed according to ASTM D 5056-04 (Reapproved 2010) and ASTM D 5600-14 methods by flame atomic absorption spectrometry (FAAS) [9] and inductively coupled plasma optical emission spectrometry (ICP-OES) [10], respectively. These official methods describe a preliminary step of digestion in order to obtain a solution suitable for introduction in FAAS and ICP-OES equipment. In both methods, sample preparation is carried out by dry ashing with a subsequent fusion step. Initially, petroleum coke is ignited at temperatures up to 700 °C and residual ash is further fused with Li₂B₄O₇ at temperatures up to 1000 °C [9], [10]. Despite the high efficiency of digestion, these official methods present some disadvantages, such as high digestion time and problems related to contamination and losses of analytes by volatilization [11].

Microwave-assisted wet digestion (MW-AD) in closed vessels has been also applied for petroleum coke. Usually, concentrated inorganic acids (e.g., HNO₃) and high temperature (up to 280 °C) and pressure (80 bar or more) are used, allowing a decrease of digestion time and reagent consumption [12], [13], [14]. Using MW-AD some advantages in comparison to dry ashing are observed, such as the minimization of risks of contamination and losses of volatile species as well as a reduction of digestion time [15]. However, even using concentrated acids, few applications of MW-AD for petroleum coke were proposed and reliable digestion was only observed for lower sample masses (50 mg) [16].

The combination of MW-AD with ultraviolet radiation (UV) in closed systems has been an alternative method to perform digestions with higher sample masses [17]. Contrarily to conventional digestion in open systems using concentrated inorganic acids, closed systems also allow the use of diluted acid solutions with suitable digestion efficiency. The use of diluted solutions is an important feature because high acid concentration of digests can cause interferences in ICP-based analytical techniques [18]. In this way, microwave-assisted ultraviolet digestion (MW-UV) has been applied to reduce the organic carbon from natural water [19] and for digestion of skimmed milk [20], Antarctic seaweeds [21] and crude oil samples [17] using diluted acids. This method is based on the use of an electrodeless discharge lamp, which is activated by microwave radiation in a closed system that enables an efficient digestion at high temperature and pressure [17], [19], [20], [21]. Despite the advantages of the combination of microwaves with ultraviolet radiation (e.g., use of diluted reagents, high efficiency of organic matter digestion, compatibility of digests with atomic spectrometric techniques, etc.) [17], [20], [22], its application for other matrices considered as hard to digest, such as petroleum coke was not yet performed.

In this sense, a MW-UV method is proposed in the present study for the digestion of petroleum coke for subsequent simultaneous determination of Ni, V and S by ICP-OES. Experimental parameters such as HNO₃ concentration, H₂O₂ volume, sample mass, heating time and also the effect of UV radiation on digestion efficiency were evaluated. Results obtained with the proposed method were compared with those obtained using microwave-induced combustion (MIC) method [23], [24] for digestion and Ni, V and S determination by ICP-OES. The proposed MW-UV method was also compared with ASTM D 5600-14 method. Accuracy of proposed method was evaluated by digestion of a CRM of petroleum coke (NIST 2718).

Section snippets

Reagents, standards and samples

Solutions used in this study were prepared using Milli-Q water with resistivity of 18.2 MΩ cm. Concentrated nitric acid (Merck, Darmstadt, Germany) and hydrochloric acid (Merck) were distilled in a sub-boiling system (Model DuoPur, Milestone, Sorisole, Italy). Hydrogen peroxide (30%, Merck) was used as auxiliary reagent for petroleum coke digestion by MW-UV. A solution of 6 mol L⁻¹ NH₄NO₃ (Merck) was used as ignition aid for MIC digestion. Lithium tetraborate (Merck) was used for the digestion of

Results and discussion

Initially, petroleum coke sample “A” was digested by MIC in order to obtain reference values for Ni, V and S. The accuracy of this reference method was evaluated by the digestion of CRM of petroleum coke (NIST 2718) and the results presented agreement between 96 and 102% with certified values for all elements (Table 2). The values obtained by MIC for sample “A” were 147.5±6.9, 195.8±7.8 and 7470±310 μg g⁻¹ for Ni, V and S, respectively, with RCC values lower than 1% in digests. This petroleum

Conclusion

The proposed MW-UV method was suitable for the simultaneous determination of Ni, V and S by ICP-OES in petroleum coke. The use of UV lamp improved the efficiency of matrix digestion compared with digestion without this device, especially when combined with H₂O₂. When it is compared to the recommended digestion method by ASTM, the MW-UV method presented advantages due to the use of diluted solutions avoiding the use of Li₂B₄O₇ that can cause problems on the determination of analytes by ICP-OES.

Acknowledgments

The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for supporting this study and also to Centro de Pesquisa e Desenvolvimento Leopoldo Américo Miguel de Mello (CENPES-PETROBRAS) for support and by providing the petroleum coke samples.

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Microwave-assisted ultraviolet digestion of petroleum coke for the simultaneous determination of nickel, vanadium and sulfur by ICP-OES

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents, standards and samples

Results and discussion

Conclusion

Acknowledgments

Fuel Process. Technol.

Fuel Process. Technol.

Talanta

Anal. Chim. Acta

Spectrochim. Acta B

Anal. Chim. Acta

Spectrochim. Acta B

Handbook of Petroleum Product Analysis

Desulfurization of petroleum coke: a review

Ind. Eng. Chem. Res.

Petroleum Refining-Technology and Economics

Study on physical-chemical properties of petroleum cokes

Rom. J. Phys.