Article pubs.acs.org/IECR
Removal of Sulfur Compounds from Mineral Insulating Oils by Extractive Refining with N-Methyl-2-pyrrolidone Jelena M. Lukić,† Draginja Nikolić,† Valentina Mandić,† Sandra B. Glisić,‡ Dušan Antonović,‡ and Aleksandar M. Orlović*,‡ †
Electrical Engineering Institute Nikola Tesla, Koste Glavinica 8A, 11000 Belgrade, Serbia, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia
‡
ABSTRACT: Copper sulfide deposits in the insulation systems of power transformers decrease the dielectric properties of the insulation and therefore constitute a significant risk for the operation of power transformers. Disulfides, oxidized sulfur compounds, thiols, and elemental sulfur, which could be present in insulating oil, have been recognized as the sources of reactive sulfur responsible for copper sulfide formation. Selective liquid−liquid extraction using N-methyl-2-pyrrolidone solvent was investigated for the purification of mineral insulating oils through the removal of compounds and precursors responsible for copper sulfide formation. The efficiency of the extraction process was evaluated using corrosive sulfur test IEC 62535, SEM/EDX measurements of paper before and after the IEC 62535 test, and measurements of the dibenzyl disulfide concentration in the oil using GC-ECD method. The oxidation stability of refined oils was evaluated using the IEC 61125 method. Precursors of copper sulfide deposits were completely removed from different mineral oils as a result of purification by extraction with N-methyl-2pyrrolidone and 1.0 wt % water as a cosolvent.
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INTRODUCTION The application of mineral insulating oils in electrical equipment requires excellent long-term thermal stability and insulating characteristics in the presence of electrical stresses and catalytic effects of different metals, such as copper, iron, zinc, and silver. In recent years, problems related to the use of mineral insulating oils have emerged in the form of conductive copper sulfide deposition in transformer windings (copper windings insulated with layers of cellulose paper). Copper sulfide is a highly conductive compound, and when deposited in the transformer windings, it significantly decreases the paper/oil insulation dielectric properties. Coupled with external electrical stresses, copper sulfide deposits can trigger electrical breakdown through solid insulation in highly loaded transformers operating at high temperatures.1 Certain sulfur compounds present in mineral insulating oils have been recognized as precursors responsible for copper sulfide formation.1,2 Mineral insulating oils are refined crude oil vacuum light distillates, containing hydrocarbons as major constituents and trace amounts of hetero compounds (sulfurand nitrogen-containing compounds). The concentration levels of naturally occurring sulfur compounds present in insulating oils can be correlated to the crude oil origin, applied refining technology, and/or degree of base oil refining.3−5 Sulfur compounds are normally present in dicyclic and polycyclic aromatic fractions,6,7 in the form of mono- and disulfides and various types of thiophene derivates. The presence of dibenzyl disulfide (DBDS) was confirmed in most corrosive oils (as determined by test method IEC 62697 CDV), which led to the conclusion that DBDS could be responsible for conductive copper sulfide formation.8 Other disulfides, thiols, and their oxygenated derivates, as well as elemental sulfur, can also lead to the formation of Cu2S.9 Thermal decomposition of disulfides (including DBDS) and thiols, under high oxygen concen© 2012 American Chemical Society
trations, can result in the formation of their oxidized derivates and elemental sulfur.9,13−17 Disulfides are readily oxidized to form sulfoxides, sulfones, and carbonyl compounds, whereas thiols, which can be formed by disulfide degradation, can oxidize to form disulfides again.14−18 Elemental sulfur, as a possible degradation product of various disulfides,15 and oxygenated derivates of disulfides and thiols (thiolate radicals and thiolate anions) can react with CuO generated through decomposition of copper peroxides,19 resulting in the formation of copper sulfide under certain conditions.9 A risk-mitigation strategy for “in-service” power transformers could be carried out by removing reactive sulfur compounds (known as “corrosive sulfur”) responsible for the formation of conductive copper sulfide deposits. Adsorptive methods, which are widely used on an industrial scale, have been investigated and applied for the purification of corrosive oils. Reported results indicate potential applicability of the developed methods but also uncertainties related to potential deposits of silver and copper sulfide.10,11 Liquid−liquid extraction processes, widely used in the petroleum industry for virgin oil refining and waste oil rerefining, is a method that could be used for the purification of mineral insulating oils containing DBDS and other copper sulfide precursors. Different solvents can be used in liquid−liquid extraction processes, but one of the most commonly used is N-methyl-2-pyrrolidone (NMP), because of its selectivity and ease of application and handling. In this study, liquid−liquid extraction using NMP was applied for the selective removal of corrosive sulfur compounds from insulating oils. To increase solvent power and selectivity for the extraction of sulfur compounds, water was used as a Received: November 11, 2011 Accepted: March 7, 2012 Published: March 7, 2012 4472
dx.doi.org/10.1021/ie300450e | Ind. Eng. Chem. Res. 2012, 51, 4472−4477
Industrial & Engineering Chemistry Research
Article
dosage, low extraction temperature to minimize oil extractive losses (refined oil yield was above 90% for all investigated oils), excellent performance of treated oils in the covered conductor deposition (CCD) test, and good oxidation stability of refined oils. A single extraction stage consisted of mixing the oil with the solvent mixture for 30 min of intense agitation, followed by separation of the upper raffinate layer (refined oil) and bottom extract layer. The latter contained solvent and the extracted oil fraction enriched with sulfur compounds, aromatics, and other heretocyclic and polar products from the oil. Contacting and layer separation were performed at the temperature of the particular extraction stage. The raffinate layer from the first extraction stage was applied to the following stage at higher temperature. After each extraction stage, solvent was recovered from the bottom extract layers and reused in the following extraction stage (Figure 1). Solvent recycling was performed by
cosolvent. The extraction process was performed in three consecutive batch extractions, using a specified amount of cosolvent, solvent/oil ratio, and temperature gradient. The process efficiency for the extractive removal of DBDS was investigated, along with the improvement of oil performance, that is, the absence of copper sulfide deposits in IEC 62535 testing of treated oils.
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EXPERIMENTAL SECTION Materials. Seven different mineral insulating oils containing DBDS were investigated in this study with the main goal of examining the efficiency of applied extraction process in the removal of corrosive sulfur compounds (DBDS in particular). Typical characteristics of unused and in-service mineral insulating oils are reported in Table 1. Table 1. Typical Properties of Unused and in-Service Mineral Insulating Oils Containing Di-tert-butyl-para-cresol (DBPC) Antioxidant and DBDS propertya
unused oil A
used oil F
TAN [(mg of KOH)/g] tgδ ρ (GΩ m) σ (mN/m) nD(20 °C) flash point (°C) DBPC (wt %) CA (wt %) total S (ppm) DBDS (ppm) corrosive sulfur test (IEC 62535) oxidation stability (IEC 61125 B; IP, h)
0.00 0.003 91.9 47 1.4801 144 0.28 9.2 130 174 corrosive 296
0.06 0.027 8.4 27 1.4840 147 0.19 12.3 330 38 corrosive 68
a TAN, total acid number in milligrams of KOHper gram; tgδ, oil dielectric dissipation factor; ρ, oil specific resistivity; nD(20 °C), oil refraction index; DBPC, di-tert-butyl-para-cresol; CA, aromatic hydrocarbon content; total S, total sulfur compounds content; DBDS, dibenzyl disulfide content; IP, induction period.
N-Methyl-2-pyrrolidone (NMP, p.a. grade) and deionized water were used as solvent and cosolvent. Kraft paper prepared according to method IEC 60554-3-1 and Cu-ETP copper plates were used to study Cu2S deposition. The paper characteristics were as follows: density, 700−850 kg/m3; thickness, 0.060− 0.100 mm; air permeability, 0.5−1 μm/(Pa s); conductivity,
