REFINING OF CORROSIVE INSULATING OILS BY SELECTIVE LIQUID - LIQUID EXTRACTION PROCESS

摘要

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. Efficiency of SLLE process in corrosive oils refining was evaluated using CIGRE A2.32. CCD, SEM/ EDX measurements of paper before and after CCD in comparison with EDX of new unused paper DBDS concentration in the oil before and measurement of DBDS concentration in the oil before and after treatment using GC - AED method. Results of corrosive oil refining process performed on thirteen corrosive oils using SLLE are presented. The following oil samples have been investigated: corrosive oils from the CIGRE A2.32.TF.01 RRT, two new corrosive oils widely used in electrical equipment and four aged corrosive oils. Efficiency of SLLE process was determined also for re-refining of used corrosive oils and artificially aged corrosive oils. This investigation was performed in order to get better insight in development of oil corrosiveness along with oil ageing, to correlate these findings to transformer service life and applicability of mitigation techniques. SLLE process was efficient in removing all of the metal passivator present as well as 2.6-di-tert-butyl-paracresol (DBPC) from the oil, confirmed by HPLC (CIGRE A2.32.TF02 method) and IR measurements (IEC 60666). Re-inhibiting after corrosive oils re-refining is necessary in order to achieve oxidation stability limits according to IEC 61125. Oils containing DBDS and corrosive oils without DBDS were successfully refined and purified. Aged corrosive oils from service also showed very good response to SLLE treatment in terms of non-corrosive behavior and major improvement of all oil characteristics, meeting the IEC 60296 requirements.