Refining of corrosive insulating oils by selective liquid-liquid extraction process

Abstract

In recent years copper sulphide formation in transformer windings has been recognized as one significant risk factor for operation of power transformers, due to number of failures which have occurred as a consequence of copper sulphide formation. Since sulphur compounds present in the insulating oil are recognized as the cause for copper sulphide formation, and in order to lower the risk of transformers failures, one of the possible solutions is removal corrosive sulphur from the oil, i.e. corrosive oils refining. In this paper the results obtained using selective liquid-liquid extraction process to remove the potentially corrosive and corrosive sulfur compounds from the oil are presented. Liquid -liquid extraction processes were widely used in petroleum industry in virgin base oil refining and waste oil re-refining to decrease aromatic content of the vacuum gas oil fractions and/or to remove other impurities from selected refinery streams. Selective liquid-liquid extraction (SLLE) process was developed and optimized for corrosive sulfur compounds extraction in insulating oils. Besides extraction of potentially corrosive sulfur species this process is highly efficient in used and waste oil re-refining. Amounts of co-solvent used in SLLE were finely tuned to increase selectivity for extraction of sulfur compounds in balance with application oil properties (electrical properties and oxidation stability). Solvent mass ratios were balanced according to polarity figures and surface tension properties. Other important process parameters: operating temperatures, number of cycles, solvents dosages were all optimized to achieve highest performance characteristics. Optimization of the process parameters in terms of extraction yield was performed in order to balance the oxidation stability of re-refined oils.