1. Field of the Invention
This invention pertains to the field of processing transformer oils.
This invention relates to mineral oils used as electrical insultating oils as in transformer switches and the like and generally called transformer oils. More particularly, it relates to the use of a particular processing sequence beginning with the preoxidation of the mineral oil before further processing steps.
Oils used as electrical insulating oils in transformers or switches must be capable of resisting current conduction at voltage levels much higher than the voltages at which a transformer switch is normally operated since severe surges of voltage can occur in transformers and switches exposed to systemic disturbances such as lightning. This property of an insulating oil is termed its impulse strength. In addition, these oils must have an inherent resistance to oxidative processes which break down such oils and make then unfit for their intended purpose. With additive oxidation inhibitors they should show a substantial increase in oxidative resistance over their inherent oxidative resistance.
Heretofore insulating oils with acceptable properties have been produced by various methods which usually included treating with large doses of sulfuric acid. However, large doses of sulfuric acid are not preferred since it produces large amounts of sludge which must be disposed of. Environmental considerations demand that processes be developed which reduce or eliminate this sludge problem. In the present invention, a preoxidation step using a small amount of sulfuric acid as catalyst is used which when used in combination with the other steps produces an oil having superior properties necessary for insulating oils and resulting in substantially less sludge. U.S. Pat. No. 3,725,253 discloses a process for the purification of lubricating oils which comprises first reacting the mineral oil with an oxygen containing gas catalytically at temperatures ranging from 226.degree. F to 536.degree. F in the presence of 2.5% sulfuric acid. This severe process results in the destruction of a large percentage of the incoming charge stock resulting in a very large amount of sludge. Thus, vacuum distillation is required to remove the compounds produced in the oxidation step. The process of the patent is completely different from the process of the present invention since the preoxidation step in the present invention is carried out at a much lower temperature resulting in almost no impurity generation. As a consequence, vacuum distillation is not required at the end of the oxidation step in the present process. Thus, it is clear that the patent is directed to a completely different process which has as its aim a completely different objective and achieves different results than this invention.
U.S. Pat. No. 3,105,812 describes a process for removing nitrogen-containing compounds from cracking and hydrocracking feed stocks by catalytic oxidation followed by hydrogenation. The oxidation is catalyzed by phosphorous oxide or a phosphorous oxide and vanadium oxide mixture. As the patent points out, the vanadium oxide catalyst, which is a relatively well known oxidation catalyst, is not very effective used alone. Although the claims of the patent include a temperature between 100.degree. and 600.degree. F for the oxidation step, the examples given in the patent were carried out at from 300.degree. to 400.degree. F. It has been found in using the process of our invention that oxidation of transformer oil stocks can be carried out at a much lower temperature rountinely. This is surprising in view of the data in U.S. Pat. No. 3,105,812. At column 10, lines 51-59 the patent teaches that a charge stock boiling in the range of a typical transformer oil distillate (550.degree.-750.degree. F) is best hydrogenated at 800-1600 psi. Using the process of our invention, the hydrogenation pressure is much lower.
The invention to be disclosed below uses a unique catalyst system for preoxidizing a transformer oil feed stock at very mild conditions. The fact that this can be done is surprising in view of the prior art which teaches oxidation of hydrocarbon oil feed stocks at much more severe conditions. The mild conditions to be delineated below have very real advantages in fuel savings, required metallurgy, and capital investment as well as other considerations.