LIQUID DIELECTRICS Sixty-Cycle Power Factors, Conductivities, and Polar Contents of Systems Containing Sulfur or Nitrogen Compounds in Liquid Paraffin JOHN D. PIPER, A. G. FLEIGER, C. C. SMITH, AND N. A. KERSTEIN The Detroit Edison Company, Detroit, Mich.
Measurements have been made of the 60-cycle power factors, conductivities, and polar contents of systems of liquid paraffin with one or more highly purified sulfur or nitrogen compounds selected to represent types that may be found in new insulating oils or formed in them as the result of deterioration in service. Homogeneous-appearing solutions of lauryl sulfonic acid and of some of the products resulting from reactions between bases and acids in liquid paraffin had high power factors and conductivities. In the cases of the reaction products, low concentrations of base and acid caused power factors, conductivities, and indicated polarities (15) equal to the average values produced by equimolecular proportions of base and acid alone. In higher eoncentrations the resulting values were markedly greater than these averages. The concentrations of lauryl sulfonic acid and
NSULATING oils often deteriorate under service conditions, with resultant increase in power factor. Although the general causes of the deterioration-oxidation, corona discharge, radiant energy, and thermal cracking-are fairly well known, the physicochemical nature of the deterioration products that actually cause significant power factor increases are understood only incompletely. This is partially because such a variety of products is formed that identification of the effect of a given type is practically impossible. To minimize the number of variables, the authors prepared numerous systems consisting of liquid paraffin and one or more highly purified compounds selected to represent types of products that might be found in or formed in insulating oils during deterioration, and investigated the power factors and related properties of these systems. The systems investigated were prepared to simulate deteriorated oils containing oilsoluble oxidation products (12, 13), sparingly soluble oxidation products (11), copper and lead soaps such as are formed by oxidation of oils in contact with these metals (8), and products resulting from corona discharge on insulating oils (16). For concentrations of contaminants that could reasonably be expected to be formed in service, significantly high power factors have been found only when one component of the contaminant is by itself incompletely soluble in the oil. Often this insoluble component is stabilized by other components to form suspensions that resemble true solutions. The present paper describes an extension of the work to include sulfur and nitrogen compounds. This work was undertaken because commercial insulating oils usually contain traces of these compounds. For example, in eight such oils analyzed by one of the authors in 1931, from 0.09 to 0.4 per
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1
For the first four papers in this series, see literature oitations 8,11, 18,
and 13.
of the reaction products that resulted in high power factors were higher than the concentrations of asphalt or asphaltlike deterioration products that will cause power factors of similar magnitude in insulating oils. It seems improbable that soluble bases or their soluble salts, in the small concentrations that could be formed from the nitrogen compounds occurring in insulating oils, contribute to the increasing power factors that result from deterioratiori. Soluble sulfonic acids formed from the sulfur compounds of insulating oils may, however, be one of the minor causes of increasing power factor. Homogeneous solutions of the remaining compounds, like those of oxygen compounds previously described (12), had low power factors and conductivities even when the compounds were present in much higher concentrations than the equivalent concentrations of sulfur and nitrogen found in insulating oils.
'
cent by weight of sulfur and 0.006 to 0.05 per cent of nitrogen were found.
Scope and Technique This paper deals primarily with the effect that sulfur and nitrogen contained in insulating oils would have on the power factors of these oils if the sulfur and nitrogen were present as chemical compounds of certain types. The investigation has thus far been conducted only with low-molecular compounds for the same reason that the investigations with oxygen compounds were similarly limited; i. e., only beginnings such as those described by Fenske and co-workers (6) have been made in elucidating the structures of the high-molecular condensation and polymerization products. The procedure for the study consisted of: (a) careful purification of the compounds, (6) addition of each compound to liquid paraffin having a very low power factor, (c) compsrison between power factors, conductivities, and polar contents of the solutions with those of the liquid paraffin at selected temperatures. Power factor measurements were made a t 60 cycles and 1970 volts per mm. (50 volts per mil) with the apparatus described previously (11, 1.2). Conductivity measurements were made immediately after the power factor measurements had been made and a t the same temperature, using a stress of 98.5 volts per mm. (2.5 volts per mil) and taking the reading after one minute of voltage application in accordance with A. S. T. M. Designation D257-33. Determinations of the polar contents of the systems were computed from the differences between the specific polarizations and the specific refractions according to the suggestion of Sommerman (16). Dielectric constants used in the computations were calculated from the data obtained for deter1505
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INDUSTRIAL AND ENGINEERING CHEMISTRY
mining the power factors a t 60 cycles and 60"C. Refractive indices a t 60" C. were determined with an Abbe refractometer, and the densities were determined by standard methods. It was thought that the data might be useful in attempting to identify the types of sulfur and nitrogen compounds that exist in deteriorated insulating oils, and that changes in the character of the systems with increasing concentrations of compounds might be revealed by the shapes of curves on which are plotted the calculated polar contents against the respective concentrations of compounds.
0.001 0.000
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CONCENTRATION OF SULFUR COMPOUND I N SOLUTION, PER C E N T BY W E I G H T
FIGURE 1. EFFECTON POWER FACTOR O F ADDING
T O LIQUID
PARAFFIN
tions of the oxygen-containing compounds previously studied (12, IS). This indicates that in proportions eauivalent to the sulfur &tent of insulating oils, none of these types of sulfur compounds, not even the mercaptans, cause high power factors when in homogeneous solution in insulating oils. Except for the solutions of thianthrene, all the sohtions indicated by Figure 1 were homogeneous a t the lowest temperature, 20" C., a t which dielectric measure. ments were taken. Power factor values a t the lower temperatures were progressively lower than the respective values a t 80" C. The solution containing 2.7 per cent thianthrenecrystallized upon cooling to 20" C., but the resulting power factor values did not materially change as the system b e c a m e heterogeneous. The fact that the systems containing thiophenol remained homogeneous and had low power factors at the
THE
SULFURCOMPOUNDS OF TYPESTHAT HAVEBEENFOUND IN PETROLEUM
The liquid paraffin had a viscosity of 285 Saybolt Universal seconds a t 40" C. and 50.5 seconds a t 100" C. Details concerning this material and the techniques employed were described previously (12).
Sulfur Compounds The sulfur compounds mere selected to be as nearly identical as possible with those isolated from petroleum as described by Challenger (5) and by Borgstrom and Reid (4). Highly volatile compounds had to be avoided because they would have been partially removed from solution during the filling of the cell under vacuum. The compounds selected and the purification treatments to which each was subjected are listed in Table I. Except for the lauryl sulfonic acid, which was obtained from the Chemistry Department of Stanford University, all of the compounds were originally of the best quality commercially obtainable from Eastman Kodak Company. The lauryl sulfonic acid was used to represent, not sulfur compounds that might be present in new insulating oils, but sulfonic acids that might be formed from sulfur compounds upon oxidation. Figure 1 gives the power factors of the liquid paraffin solutions containing the sulfur compounds used to represent sulfides, disulfides, heterocyclic sulfur compounds, and both aryl and alkyl mercaptans. The power factors of all of these solutions m-ere low, as were those of the homogeneous solu-
Vol. 34, No. 12 I # /
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0.00 0 0.5 1.0 CONCENTRATION OF L A U R Y L SULFONIC
ACID, %BY WT.
FIGURE 2,
POWER FkCSOLUTIONS CovTAINING VARIOUSCONCEUTRATIOXS OF LAURYL SULFONIC ACID IN LIQUID PARAFFIN TORS OF
Dotted line gives power factors at 60' C of a mixture of asphalt In transformer oil in the con~ centrations ~ ~ ~ ~ indicated
$ ~ t ~ ~ ~ in contrast with the phenolcontaining systems previously studied ( I I ) , which upon cooling formed emulsions that had high power factors. Figure 2 shows the power factors of the lauryl sulfonic acid solutions. These solutions had the highest power factors of any of the homogeneous systems studied in this series of investigations. The data show that, if the sulfur contained n insulating oils were converted into soluble sulfonic acids by Oxidation, the resulting power factors would be high. If, for TABLE I. TREATMENT GIVENT H E n-Heptyl sulfide n-Amyl disulfide Thianthrene Thiophene
SULFUR COMPOUNDS
Distd. in vacuum; middle fraction used Dried over anhydrous CaSOa; distd. in vacuum 3 times; middle fraction used Untreated except for drying in vacuum a t 1000 c.
Dried over anhydrous CaSOd; distd. in vacuum; crystallized in vacuum 3 times from mother liquor in closed system (3) Dried in contact with small amount of P2Ofi; Thiophenol fractionally distd. in vacuum; middle fraction crystallized 3 times from mother liquor in vacuum n-Heptyl mercaptan Dried over anhydrous CaSO4; distd. in vacuum; crystallized 3 times from mother liquor in vacuum; redistd. in closed system at 60' C. Lauryl sulfonic acid Dried by evacuation at 60' C. and 0.1 mm. Hg; distd in all-glass molecular still (IO); crystallized 4 times from purified hexane in closed system (IO) in atmosphere of hexane; hexane removed from crystals at room temperature and 0.5 mm. Hg
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INDUSTRIAL AND ENGINEERING CHEMISTRY
example, an insulating oil containing approximately 0.1 per cent sulfur could be oxidized selectively by 0.15 per cent of oxygen to convert the sulfur compounds to oil-soluble sulfonic acids that have average molecular weights equivalent to that of lauryl sulfonic acid (corresponding to 0.78 per cent lauryl sulfonic acid), a power factor (at 60 cycles and 80" C.) of approximately 0.10 would result, provided the oil had the same viscosity as that of liquid paraffin. 2.45
2000
KEY
- OONDUOTIVITV EQUIVALENT A.C. CA CULATED FROM POWER FACTOR
CONDUCTIVITV AFTER ONE MINUTE OF VOLTAOC APPLIOATION.
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became heterogeneous on cooling. Figure 3 shows that both the equivalent alternating- and direct-current conductivity values decreased in the normal manner as the solution cooled to the point of heterogeneity, and then increased sharply as the mixture was further cooled. The dielectric constant also increased sharply a t the transition, as shown. The phase that separated appeared to be definitely crystalline. Benzene sulfonic acid was used in an attempt to prepare systems containing other sulfonic acids. This acid was found to be practically insoluble in liquid paraffin. The power factor of liquid paraffin in contact a t 80" C. with a globule of molten benzene sulfonic acid was identical with that of the liquid paraffin. Dispersions having higher power factors were made but they were very unstable. Figure 4 is a plot of the concentrations of polar materials in the solutions, as measured by the differences between the specific polarizations and the specific refractions, against the respective molecular proportions of the sulfur compounds actually present in the solutions. Within the experimental error the indicated concentrations of polar materials were proportional to the respective concentrations of compounds added. The order of decreasing polarity for given molecular concentrations of the compounds in liquid paraffin was: lauryl sulfonic acid, amyl disulfide, thianthrene, heptyl mercaptan, thiophenol, and thiophene. The indicated negative polarity of liquid paraffin was probably caused by a small systematic error either in the determination of the third decimal of the dielectric constants or in the calibration of the refractometer.
Nitrogen Compounds
20
40 BO TEMPERATURE. DEGREES C.
FIGURE3. DIELEICTRIC PROPERTIES DURING THE COOLINGOF A SYSTEM CONTAININQ 0.72 PERCENTBY WEIGHT OF LAURYL SULFONIC ACIDIN LIQUID PARAFFIN In regions to left of half vane the systems were cloudy and were clear to the right.
Although the data indicate that the presence of soluble sulfonic acids can cause high power factors in insulating oils, they do not indicate that a hypothesis of formation of soluble sulfonic acids is adequate to explain the high power factors that insulating oils sometimes acquire in service. Continuing the example, if the oil used could be selectively oxidized to convert the 0.1 per cent of sulfur to sulfonic acids, the sulfonic acids formed would be equivalent to an acid number of 1.7 mg. potassium hydroxide per gram of oil. This value is much higher than is found for the total acidity of deteriorated insulating oils, even those having power factors greater than 0.11 at 80" C. No solutes of known structure have yet been found that, for equal concentrations by weight, will cause such high power factors in insulating oils as will asphalt or certain asphaltlike bodies that are formed in the course of service degradation. The dotted curve on Figure 2 shows that the power factors a t 60" C. of mixtures of asphalt in transformer oil were much higher than were the power factors of the solutions consisting of equal concentrations by weight of lauryl sulfonic acid in liquid paraffin. Quantitative comparison of the effects of the two additives cannot be made, however, because the transformer oil had a lower viscosity than had liquid paraffin. Like several of the systems studied previously, the solutions containing the higher concentrations of lauryl sulfonic acid
According to Poth, Armstrong, Cogburn, and Bailey ( I d ) , most of the nitrogen in petroleum crudes is nonbasic, but these nonbasic compounds appear to be converted to bases upon distillation. Although nearly all the bases are presumably removed during refining, it seemed possible that small concentrations of bases might be left in insulating oils or be formed there from inert nitrogen compounds during deterioration of the oils. Accordingly, bases were selected for test. Nonbasic nitrogen compounds were not investigated because the structures of such compounds from petroleum have not been described.
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FIGURE 4. DIFFERENCES BETWEEN SPECIFIC POLARIZATIONS AT 60 CYCLES AND SPECIFIC REFRACTIONS FOR VARIOUS SOLUTIONS OF SULFUR COMPOUNDS IN LIQUIDPARAFFIN AT 60" C
Maybery and Wesson (7) pedicted and Bailey and coworkers (1) showed, that many of the nitrogen bases of petroleum (California distillates) are quinoline derivatives. One of these that had actually been isolated by Biggs and Bailey ($)-namely, 2,4-dimethylquinoline-was available and therefore was selected for test. To exemplify naphthenic bases, which Bailey et al. (2) have shown also to be in petrol-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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eum, piperidine was used; naphthenic bases that have actually been isolated were not available. A tertiary aliphatic amine, tri-n-butylamine was also included. The three bases were obtained from Eastman Kodak Company, dried in a closed evacuated system in contact with fused potassium hydroxide, and vacuum-distilled. Several distillations were required in order to obtain colorless 2,4dimethylquinoline which had to be used immediately because color soon developed upon exposure even to subdued daylight in sealed, evacuated glass tubes.
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CONCENTRATION OF NITROGEN COMPOUNDS PER CENT B Y WEIGHT
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FIGURE 5. POWER FACTORS OF VARIOUS CONCENTRATIONS O F NITROGEN COMPOUNDS DISSOLVED IN LIQUID PARAFFIN
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Figure 5 shows that the power factors of liquid paraffin solutions containing 2,4-dimethylquinoline, present in concentrations far greater than the equivalent concentrations of nitrogen ever found in insulating oils, are low. Figure 5 also shows that the solutions containing tri-n-butylamine, selected to represent aliphatic bases, and piperidine, selected to represent the strongest organic bases known, also had low power factors. Azobenzene was used more to represent colored oil-soluble compounds in general than a type of nitrogen compound. Experiments with the azobenzene solutions confirmed the observation made by studying solutions containing colored hydrocarbons (Ifi)-namely, that oils owing their color to the fact that they contain compounds having chromophores do not necessarily have high power factors. This observation does not exclude the possibility that substances producing color in oils (as a result of their state of colloidal dispersion) may also cause these oils t o have high power factors. ' Organic Salts of h'itrogen Bases I n order to determine whether the power factors of an insulating oil would be increased markedly by the presence of organic salts formed by interaction of nitrogen bases and organic acids resulting from oxidation, several systems were prepared by adding equimolecular proportions of base and acid to liquid paraffin and the resulting power factors were determined. Figure 6 compares the power factors of the solutions containing both bases and acids and those containing equimolecular proportions of either. When the bases and acids were present together in low concentrations, their effects appeared to be additive. When they were present in larger concentrations, howevAr, the resulting power factors were much higher than the power factors of solutions containing an equimolecular proportion of either. Similarly, Figure 7 shows that the contents of polar materials, as measured by the differences between the specific polarizations and the specific refractions, were additive for low concentra-
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PIPERIDINE LAURATE
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M I L L I M O L E S OF AODEO M A T E R I A L PER LITER OF SOLUTION
FIGUR 6 .~ POWER FACTORS OF SOLUTIONS OF ORGANIC SALTS COMPARED WITH POWER FACTORS OF EQUIVALENT CONCENTRATIONS OF THEIRPARENT BASESAND ACIDS Concentration of each s a l t is plotted a s t h e Bum of the concentration8 of t h e base a n d acid from which each was
formed.
December, 1942
INDUSTRIAL A N D ENGINEERING CHEMISTRY
z 0.03 2 c
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From consideration of the data obtained it is believed that, although the presence of sufficiently high proportions of soluble salts resulting from reaction between nitrogen bases and carboxylic acids can cause high power factors in insulating oils, the presence of such compounds in proportions equivalent to the low nitrogen content of insulating oils does not appreciably affect the power factors of the solutions. For example, an insulating oil containing 0.05 per cent by weight of nitrogen could form only approximately 0.03 mole per liter of an organic salt, a concentration that these data show to be too small to exert a significantly adverse effect when the acid involved is carboxylic. Salts of the stronger sulfonic acids have not yet been investigated.
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Conductivity Values
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The direct-current conductivity values for typical systems
W u
at 80" C. are contrasted in Table I1 with the respective
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MILLIMOLES OF ADOEO MATERIAL PER L I T E R O f SOLUTION
FIGURE 7. POLAR CONTENTSOF ORGANICSALTS COMPARED WITH THOSE OF THE PARENT BAS~S AND ACIDS
tions; but for higher concentrations the indicated contents were much greater than the average of the contents indicated for the respective base and acid. It seems probable that the difference between the dielectric behavior of the solutions containing both base and acid and the behavior of the solutions containing either the one or the other was caused by the formation of a soluble organic salt that was more highly dissociated electrolytically than were the parent acid and base. Inasmuch as all the systems with higher power factors than solutions containing equimolecular proportions of acid or base became heterogeneous upon cooling (whereupon their power factors increased), an alternative explanation seems possible. Previous work with soaps (8) and with oxidation products (11) showed that systems appearing to be homogeneous but becaming visually heterogeneous a t lower temperatures often exhibit dielectric behaviors resembling those of heterogeneous rather than of homogeneous systems.
equivalent alternating-current conductivity values computed from the power factor data2. I n general, the d. c. values, which were taken after one minute of voltage application, were less than one fourth of the corresponding a. c. values for the solutions of sulfur compounds discussed in connection with Figure 1; this indicates that the charge carriers were depleted rapidly from the solutions by the applied field. For the solutions containing lauryl sulfonic acid, the nitrogen bases, and the salts of these bases, the d. c. values approached the a. c. values more closely; thus the charge carriers that were swept away by the field were rapidly replaced by others, probably by electrolytic dissociation.
Acknowledgment The authors are grateful to J. W. McBain of Stanford University through whose cooperation the lauryl sulfonic acid was obtained.
Literature Cited
TABLE 11. COMPARISON BETWEEN DIRECTAND ALTERNATINGCURRENTCONDUCTIVITY VALUESAT 80' C. OF HOMOGENEOUS- (8) APPEARING SYSTEMS CONTAINING SULFUR AND NITROGEN COMPOUNDS IN LIQUIDPARAFFIN Conductivity Mhos/Cm. Cube 2 1014 Concn., D. C. A. C. Added Constituent &:Et (2.5 v./mil) (50 v./mil) n-Amyl disulfide 6.0 0.0 3.7 2.2
