US 3305480 A
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Feb. 21, 1967 l. W. MILLS PREPARATION OF OILS HAVING IMPROVED OXIDATION STABILITY Filed Nov. 20, 1964 300 40o OXIDATION TIME, HRS.
9 co Q) INVENTORS |VO R w. MILLS BY JOHN J. MELCHIORE cum 8.
ATTORNEY United States Patent Office 3,305,480 Patented Feb. 21, 1957 3,305,480 PREPARATION F ()lLS HAVING IMPROVED UXHDATION STABILITY Ivor W. Miils, Glenolden, and John J. Melchiore, Wallingford, lla., assignors' to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Filed Nov. 20,1964, Ser. No. 412,678
4 Claims. (Cl. 208-273) 1 This invention relates to the preparation of hydrocarbon oils having improved oxidation stability and particularly improved resistance to oxidation in the presence of copper and iron. 7 The invention is applicable generally to the preparation of improved hydrocarbon distillate oils boiling above 500 F. but is especially useful for improving oils intended for use in electrical systems, such as transformers and capacitors, and also refrigerator oils.
It is well known that the presence of copper in petroleum distillates can have an adverse effect on oxidation stability of the hydrocarbons during use. Certain oils, such as transformer oils, cable oils, refrigerator oils and the like, are normally used in systems in which copper is present, and the copper can have a pronounced effect in accelerating deterioration of the oil through oxidation. Other petroleum distillate oils, while not necessarily being intended for use in copper-containing systems, often contain deleterious amounts of copper due to contact with copper-containing equipment during processing and subsequent handling. Thus copper can be present in suflicient amount to promote oxidation and result in gum or sludge formation in such distillates as furnace oils, diesel oils and lubricating oils. The accelerated oxidation of these products may give rise to sludging in storage tanks, clogging of filters or nozzles, corrosion of equipment, loss of desirable product qualities such as dielectric strength, and
other undesirable effects.
The present invention provides a novel method of producing from petroleum distillate oils boiling above 500 F. refined oil products which have exceptionally good oxidation stability in the presence of copper and also iron. While the method has general applicability to the treatment of distillate oils boiling in the gas oil and lubricating ranges, it is especially useful for refining oils which have viscosities in the range of 50-250' SUS at 100 F. These include transformer oils which generally have viscosities in the range of 50-65 SUS at 100 F., capacitor oils of 90-110 SUS at 100 F. and refrigerator oils whose viscosities usually fall in the range of 140-220 SUS at'100 F.
Charge oils used in practicing the present invention have aromatic contents of at least 10% by weight and usually have a considerably higher content of aromatic hydrocarbons. Untreated distillate oils from naphthenic crudes generally have aromatic contents in the range of 30-50% by weight, while those from paraffinic crudes after being dewaxed usually contain 20-30% aromatics. These distillate oils can the solvent extracted prior to treatment according to the present invention and this is often preferable. Extraction with a solvent such as furfural using conventional extraction conditions usually will reduce the aromatic contents of the napht-henic distillates to 20-35% and of the dewaxed parafiinic distillates to 10-20%. These oils, whether solvent extracted or not, invariably contain an appreciable amount of sulfur'and nitrogen compounds.
The method according to the present invention involves first treating a charge oil as described above with a sulfonating agent and utilizing severe treating conditions such that the aromatic content of the treated oil is less than by weight, more preferably less than 2% by weight. In other words this severe sulfonation treatment produces a refined oil of the type generally referred to as a technical grade white oil, which not only has a low aromatic content but also only trace or negligible amounts of constituents containing sulfur and nitrogen. After separating sulfonated material from the treated oil, the latter is then catalytically dehydrogenated under conditions such that the dehydrogenated product boiling above 500 F. will have an aromatic content in the range of 15-40% by weight. This product will have outstanding stability characteristics and resistance to oxidation in the presence of metals such as copper and iron.
in the sulfonation step of the present process the charge oil containing in excess of 10% aromatics is treated with either fuming sulfuric acid (oleum) or sulfur trioxide employing treating conditions which are well known in the art for producing white oils. With either oleum or sulfur trioxide as the sulfonating agent, the treatment conventionally is carried out in a series of steps in each of which a portion of the total sulfonating agent is contacted with the oil. After each contact treatment the reaction mixture is settled. The resulting upper oil layer and lower acid sludge layer are separated from each other, and the oil layer is then treated with another portion of the sulfonating agent. After sufficient'successive treats have been carried out to reach the desired white oil quality, the acidic oil is extracted with aqueous alcohol and then neutralized with caustic soda to remove all remaining sulfonated material from the oil.
Sulfonating procedures for producing white oils are described in Ind. and Eng. Chem, vol. 49, pages 31-38, January 1957, and in Lipkin et al. US. Patent No. 2,680,- 716 and need not be described in further detail herein.
The treated oil free of sulfonatedmaterial next is catalytically dehydrogenated under conditions to effect substantial conversion of naphthcnic components to aromatics. Conditions of conversions are such as to avoid hydrocracking while effecting dehydrogenation of naphthenes to an extent such that the dehydrogenated product boiling above 500 F. will have an aromatic content of at least 15% by weight. The aromatic content of the dehydrogenated product may range up to 40% depending upon the naphthenic character of the original charge stock and the degree of dehydrogenation effected. Generally a small amount of product boiling below 500 R, will be produced via hydrocracking reactions but the degree of hydrocracking can be minimized by appropriate selection of reaction conditions.
The dehydrogenation reaction is carried out by contacting the treated oil in a hydrogen atmosphere with a dehydrogenation catalyst. Temperature and pressure conditions that should be used will depend upon the particular catalyst selected but in general will be in the ranges, re-
spectively, of 650-825" F. and -750 p.s.i.g. The specific temperature and pressure conditions should be correlated with regard to the activity of the particular dehydrogenation catalyst selected so as to minimize cracking while effecting dehydrogenation to a degree whereby the product which boils above 500 F. will have between 15% and 40% aromatic constituents. As a general rule the specific temperature selected within the above-specified range of 650-825 F. should vary inversely with the activity of the catalyst employed.
A preferred catalyst for use in the dehydrogenation step is platinum-on-alumina in which no halogen component has been incorporated. This catalyst is highly active for dehydrogenation of naphthenes and generally can be used at lower temperature than can other conventional dehydrogenation catalysts. While platinum catalysts normally are sensitive to sulfur, this presents no problem in the present process since the treated oil from the sulfonation step will have a negligible sulfur content.
Typical conditions for using a platinum-on-alumina catalyst include a temperature of 650-775 F., a pressure of 100-400 p.s.i.g., a hydrogen recycle ratio of 2000-10,000 s.c.f./bbl. of charge oil and an LHSV of 0.5-2.
Other dehydrogenation catalysts that can be used include nickel sulfide-molybdenum sulfide-on-alumina and nickel sulfide-tungsten sulfide-on-alumina. Typical conditions for using either of these catalysts include a temperature of 750-800 F., a pressure of 400-500 p.s.i.g., a hydrogen recycle ratio of 4000-10,000 s.c.f./bbl. of oil and an LHSV of 0.5-2.0.
Another commercially available catalyst that can be used for the present purpose is cobalt oxide-molybdenum oxide-on-alumina, often referred to as cobalt molybdate catalyst. This catalyst is somewhat less active than those mentioned above. Typical conditions for its use include a temperature of 775-810 F., a pressure of 350-450 p.s.i.g., a hydrogen recycle ratio of 5000- .000 s.c.f./bbl. of oil and an LHSV of 0.25-1.25.
The dehydrogenated product formed under conditions as described above generally will contain a minor amount of material boiling below 500 F. as a result of some amount of hydrocracking occurring during the reaction. Usually this lower boiling material will amount to between 1% and 10% by volume of the total product, although under the more severe reaction conditions the amount may range up to 20%. The product obtained upon removal of this lower boiling material will contain between and 40% by weight of aromatic hydrocarbons. It is usually preferable to correlate and adjust the reaction conditions in the dehydrogenation step so that the product boiling above 500 P. will have an aromatic content in the range of -35%. This product will have exceptionally good oxidation stability in the presence of copper and/or iron. If desired, the product can be distilled into fractions of any selected boiling ranges each of which fractions likewise will have outstandingly good oxidation stability in the presence of such metals.
For the purpose of illustrating the invention more specificaly, the invention will be described with reference to the prepartion of transformer oils. Such oils generally boil in the range of 500-775 F. and have viscosities in the range of 50-65, more preferably 55-60, SUS at 100 F. A list of typical characteristics of commercial transformer oils is given in the text by F. M. Clark entitled Insulating Materials for Design and Engineering Practice (1962), page 135. The present description is presented in conjunction with the accompanying sheet of drawings in which curves are shown which represent, respectively, a conventional commercial transformer oil, a white oil made from a transformer stock and a dehydrogenated oil prepared from such white oil in accordance with the invention, all as more fully described hereinafter.
Commercial transformer oils customarily are tested by the Doble Oxidation Test for electrical insulating oils developed by the Doble Engineering Company of Belmont, Massachusetts. This procedure has been described in ASTM Standards on Electrical Insulating Liquids and Gases, pages 307-313, December 1959, under the title Suggested Method of Test for Oxidation Characteristics of Mineral Transformer Oil. It involves bubbling air through a known amount of the oil held at a temperature of 95 C. in the presence of copper and iron and making two types of tests daily on small samples of the oil. One type of test is an acidity measurement. The other is a precipitation test in which one volume of the oil is diluted with five volumes of pentane, the mixture is allowed to stand at least eight hours and the presence or absence of a sludge precipitate is noted. The endpoint of the Doble test is taken as the number of days of oxidation either before the acidity of the oil reaches 0.25 mg. of KOH per gram or before a positive precipitation test for sludge is obtained. Commercial transformer oils heretofore available usually have a life of only about threedays under Doble test conditions. Longer life values could be obtained by adding an oxidation inhibitor to the oil but the use of such additive traditionally has been considered unacceptable by transformer manufacturers and users.
An amplification of the Doble test recommended by the Doble Engineering Company is the so-called Power Factor Valued Oxidation (PFVO). This involves operating in the manner described above but also determining the power factor of the oil at two hour intervals throughout the oxidation period. A curve is obtained by plotting the power factor against the oxidation time. The three curves shown in the accompanying drawing are curves obtained in this manner.
Curve A is a typical PFVO curve representing commercial transformer oil prepared by conventional treatment of a transformer oil stock with sulfuric acid and clay. It can be seen that a sharp hump in the PFVO curve occurs in the earlier stages of oxidation. Thereafter the power factor begins to rise, and after an oxidation period of hours a rapid increase in power factor is exhibited. However, usually before 100 hours has been reached, the oil has failed in the Doble test due to sludging. Failure generally occurs at about three days (72 hours).
The preparation of the oils corresponding to curves B and C of the drawings is described in the following example which constitutes a specific embodiment of the invention.
EXAMPLE A paraffin distillate stoc-k boiling in the approximate range of 570-740 F. and having a viscosity of 57 SUS at 100 F. was dewaxed to form a charge oil having a pour point of about 15 R, an aromatic content of about 25% by weight, a nitrogen content of 25 p.p.m. and a sulfur content of 0.2%. This charge oil was treated at about 80 F. for ten successive times using 20% oleum each time in amount of 20 lbs./bbl. of charge oil. After each treat the mixture was settled and the acid sludge layer was separated from the oil. After the tenth treat with oleum the acidic oil was extracted with an equal volume of 30% aqueous isopropanol, and then was Water washed followed by a caustic soda wash. Finally the treated oil was dried by contacting it mm 2 lbs./bbl. of adsorptive clay.
The foregoing sultfonation treatment produced a technical grade white oil having an aromatic content of 0.3%, a sulfur content of 25 ppm. and a negligible 1 p.p.m.) nitrogen content. In the Doble Oxidation Test this oil had a life of 5 days, with acidity build-up being the limiting criterion. The PFVO curve for this oil is curve B in the drawing. From curve B it can be seen that this oil was somewhat better than the typical commercial oil illustrated by curwe A. The white oil had good insulating characteristics as shown by low power factor but only for about hours. Thereafter a sharp rise in the power factor occurred and the oil was no longer suitable as a transformer oil.
The technical grade white oil was then dehydrogenated in a flow reactor in a hydrogen atmosphere maintained in the usual manner by recycling efiluent hydrogen while adding fresh make-up hydrogen. A platinum-on-alumina catalyst substantially free of halogen was employed. Reaction conditions used comprised a temperature of about 775 F., pressure of about 150 p.s.i.g., LHSV of 1.0 and a hydrogen recycle ratio of 10 moles per mole of white oil charged. The dehydrogenation product contained about 5% hydrocarbons boiling below 500 F. which were removed'by distillation.
The dehydrogenated oil thus obtained had an aromatic content of 32.5% by weight, which was composed of 23.8% mononuclear, 6.0% dinuclear and 2.7% trinuclear aromatics. Carbon type analysis of the oil showed 11% aromatic carbons, 20% naphthenic carbons and 69% parafiinic carbons. The oil had 16 ppm. of sulfur. and essentially no nitrogen constituents.
In the Doble Oxidation Test the dehydrogenated oil showed an inordinately long life, namely, 25 days. The PFVO curve is curve C which shows that that the dehydrogenated oil had outstanding stability as measuered by power factor. From curve C it can be seen that an upward break in the PFVO curve did not ocur until the oxidation time had exceeded 950 hours.
The foregoing example shows that the present invention can produce refined transformer oil which has outstanding oxidation stability in the presence of copper and iron. When the invention is used to refine other stocks such as capacitor oils, cable oils, refrigerator oils and the like, analogous improvements in the oil stability are effected.
1. Method of preparing an oil having improved oxidation stability in the presence of copper which consists of:
(a) treating a petroleum distillate oil boiling above 500 F, having a viscosity in the range of 5065 SUS at 100 F. and having an aromatic content of at least by weight with a sulfonating agent selected from the group consisting of fuming sulfuric acid and sulfur trioxide in amount to reduce the aromatic content of the oil to less than 5% by weight,
(b) separating sul-fonated material from the treated oil,
(0) catalytically dehydrogenating the treated oil to an extent such that the dehydrogenated product boiling above 500 F. has an aromatic content in the range of -40% by weight,
(d) and recovering the dehydrogenated product boiling above 500 F. as said oil having improved oxidation stability.
t5 2. Method according to claim 1 wherein the amount of sulfonatin g agent used is sufficient to reduce the aromatic content to less than 2% by Weight.
3. Method of preparing an oil having improved oxidation stability in the presence of copper which consists of: (a) treating a petroleum distillate oil boiling above 500 F., having a viscosity in the range of 65 SUS at 100 F. and having an aromatic content of at least 10% by weight with a sulifonating agent selected from the roup consisting of fuming sulfuric acid and sulfur trioxide in amount to reduce the aromatic content of the oil to less than 5% by weight, (b) separating sulfonated material from the treated oil, (c) dehydrogenating the treated oil by contacting the same in a hydrogen atmosphere with a dehydrogenation catalyst under dehydrogenating conditions including a temperature in the range of 650825 F. and a pressure in the range of 100750 p.s.i.g., said conditions being correlated such that the dehydrogenated product boiling above 500 F. has an aromatic content in the range of 15-40% by weight, (d) and recovering the dehydrogenated product boiling above 500 F. as said oil having improved oxidation stability. 4. Method according to claim 3 wherein the amount of sulfonating agent used is sufiicient to reduce the aromatic content to less than 2% by weight.
References Cited by the Examiner UNITED STATES PATENTS 5/1961 De Chel lis et al 208- 9/1961 Murray et al 20 8-90