GB796217A - Process for the preparation of electrical insulating oils - Google Patents

Abstract

A process for the preparation of electrical insulating oils starting from a mineral oil having a viscosity of 1-9 DEG E. at 20 DEG C., and aromatic and sulphur contents of at least 35 per cent and 1 per cent by weight respectively comprises preparing an oil A by catalytically hydrofining the starting oil to a sulphur content of 0.05 to 0.6 per cent wt. and then treating it with concentrated sulphuric acid or oleum, separating the acid sludge, neutralizing the oil and treating it with a solid adsorbent; preparing an oil B by first extracting the starting oil with a selective solvent for the aromatic components to obtain a raffinate with a maximum aromatic content of 25 per cent wt. and then treating it with acid as for oil A; and mixing 80 to 40 parts by weight of oil A with 20 to 60 parts by weight of oil B. The starting oil for oils A and B may be the same oil or different oils within the prescribed Specifications. In the preparation of oil A the oil may be hydrogenated in the liquid or gas phase at 300-400 DEG C., 10-200 kg./sq. cm., a gas discharge rate of 50-5000 litres/kg. of oil and an oil flow rate of 0.3-3 kg./litre of catalyst/hour. Suitable catalysts are oxides and sulphides of metals of groups VI and VIII of the Periodic Table such as cobalt oxide, molybdenum oxide, tungsten sulphide and nickel sulphide which may be applied to carriers such as active carbon, fuller's earth, kieselguhr, silica or alumina, e.g. in the form of bauxite, pumice or burnt clay. Hydrogen sulphide formed may be removed by washing the oil with caustic alkali solution or by blowing an inert gas, e.g. nitrogen, through the oil. If this treatment lowers the flash point overmuch it may be restored by distilling off the low boiling components of the oil under reduced pressure, preferably before the acid treatment. Examples of the selective solvent used to obtain oil B are liquid sulphur dioxide and furfural. The acid treatment is the same for both oils and may be carried out with 80-100 per cent concentrated sulphuric acid or oleum (up to 30 per cent free SO3) in amounts of 5-30 per cent wt. at 0-75 DEG C. for 5 sec. to 1 hour. After removal of the acid sludge the oil is neutralized with alkali, e.g. aqueous or alcoholic caustic soda, the used alkaline material separated and the oil washed with water and/or alcohol. Finally the oil is treated with a solid adsorbent, e.g. fuller's earth, bauxite or clay, whether or not activated with acid, to which small quantities of alkaline material such as lime may be added. The oil may be heated and an inert gas, e.g. nitrogen, passed through during the treatment. Optional ingredients mentioned are antioxidants such as alkyl phenols, e.g. 2,6-di-t-butyl-4-methyl phenol. In examples a distillate of viscosity 5.5 DEG E./20 DEG C., flash point 156 DEG C., sulphur content 1.76 per cent wt. and aromatic content 50 per cent wt. obtained from a naphthenic crude oil by distilling and redistilling over lime is used as the starting oil from which 3 oils A are obtained by hydrofining with cobalt and molybdenum oxides supported on bauxite as catalyst under the following conditions: temperature 370 DEG , 375 DEG , 375 DEG C.; hydrogen pressure 50, 50, 50 kg./sq. cm.; oil flow rate 1.1, 0.5, 2.0 kg./litre of catalyst/hour; gas discharge rate 225, 260, 125 litre/kg. oil. These oils are acid treated with 10 per cent wt. oleum (176 per cent SO3), and a fourth oil A is obtained by the same treatment as the first except the 25 per cent wt. of 96 per cent H2SO4 is used instead of oleum. These are blended with two oils B, extracted with liquid sulphur dioxide to 9 per cent wt. and 19 per cent wt. aromatic content and treated with 10 per cent wt. and 17 per cent wt. of oleum respectively. One example incorporates 0.3 per cent wt. of 2,6-di-t-butyl-4-methyl phenol. Specifications 589,149, 589,150, 657,521, 665,575 and 699,455 are referred to.