OIL-IMPREGNATED PAPER Sorption of Organic Acids
and Its Relation to
The extent to which six organic acids were sorbed from oil solutions by dry paper was determined for a set of conditions simulating those existing in deteriorated high-voltage underground cables. Acetic acid was almost completely sorbed, but with acids of increasingly high molecular weight the degree of sorption dropped off rapidly at first, then more gradually until a constant value of slightly over 1 per cent was reached for acids containing six or more carbon atoms. Comparison of these data with those previously reported indicates that the value of the 60-cycle power factor of oil-impregnated paper which contains the acid as an impurity is markedly affected by the amount of acid sorbed by the paper.
Power Factor JOHN D. PIPER,N. A. KERSTEIN, AND A. 0. FLEIGER The Detroit Edison Company, Detroit, Mioh.
I
N A PREVIOUS paper of this series (4),a study was made of the increase in the 60-cycle power factor of oil-impregnated paper caused by the common types of oil-soluble organic acids that may be found by oxidation of hydrocarbons. Those with low molecular weights have a marked influence on the 60-cycle power factor of oil-impregnated paper, whereas those with medium to fairly high molecular weights have a practically negligible influence. For the reasons stated (4, it was believed that acids of low molecular weight are strongly sorbed from oil by paper. No information seemed to be available, however, concerning the probable extent to which acids of different molecular weights are sorbed from oil by paper, since any published work relative to the sorption of organic liquids by cellulose products was carried out for a different purpose. The experiments outlined here were therefore performed to obtain data on the extent to which several acids are
sorbed and to establish a possible relation between the extent of sorption and dielectric behavior of such impregnated paper.
Sorption Experiments The sorption experiments consisted of assembling the apparatus and materials, drying the paper, impregnating the paper with solutions of anhydrous acids in liquid paraffin, and determining the extent to which the acid was removed from the oil by the paper. The liquid paraffin and the paper were the same kinds used in the earlier studies (3, 4). The volatile acids were desiccated over anhydrous copper sulfate in a closed system which permitted distilling them under reduced pressure to fresh portions of the desiccant. When practically no discoloration of fresh desiccant developed in contact with the acids, they were distilled into an attached apparatus,l fractionally crystallized a t least three times from their own mother liquors, and finally introduced into sample tubes of appropriate sizes. One of the less volatile acids used, lauric acid, was treated in a manner similar to that described for stearic acid ( 3 ) . The other, mixed naphthenic acid, was part of the same lot used before ( 3 ) . The latter had been stored about a year * in a dark place in a sealed evacuated tube. The melting points of all except the mixed naphthenic acids were determined and found to agree well with their respective values reported in the literature. The apparatus in which the paper was dried and impregnated diffkxed with the kind of acid used. For the more volatile acids the apparatus shown in Figure 1was employed. Except for the iron cores of the magnetic hammers and the platinum filtering cone, the entire apparatus was made of glass. As illustrated the apparatus and its contents are ready for the drying process. T o attain the conditions shown, the follom-ing steps were taken: FIGURE1. APPARATUSUSED WITH VOLATILEACIDS
1
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This apparatus will be described in a separate communication.
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Paper sample a was rolled tight to form a solid-centered paratus and admitting dry nitrogen through tube p , the techcylinder and sealed into paper compartment b containing filtering nic described to this point was the same in both cases. cone c. In practice a number of paper samples of nearly uniThe solution of oil and unsorbed acid was separated from form size were prepared in a batch and sealed into similar compartments after the weight of each sample had been determined. the paper in the following manner: The moisture content of the batch was determined on a separate piece so that the weight of "dry" paper in each sample was Compartment 6 from either apparatus was removed from the known. These paper compartbath at the end of the impregnation period, inverted, and centriments were attached, one at a fuged in a manner similar to that described previously (2). The time, to the remainder of the tube was then cracked open, the oil which had been centrifuged apparatus at d as needed. A from the paper was washed out with a neutralized solution of known weight of oil was introbenzene and alcohol, and the amount of acid in the oil was deduced into oil compartment e termined by titration with standard aqueous alkali using phenolthrough the open end of tube f, phthalein as an indicator. In a similar manner, the amount of whereupon the latter was sealed acid in the oil in compartment e and connecting parts was also off close to m as shown. The determined. Various parts of the apparatus were weighed at acid sample, in a sealed receptacle appropriate times in order that data of the type shown in Table g having a thin capillary tip, I could be obtained for each determination. For some of these was next inserted, tip foremost, measurements it was necessary to guard against loss of broken into tube h whose end was then pieces of glass. closed. By sealing hammers i and j into their respective tubes The concentrations of acid in the oil were selected to cover as shown, assembly of the appathe range between 0 . 1 and 0.6 millimole of acid per gram of ratus and its contents was cpmpleted. oil, which was approximately the same range as that used in The paper was dried, and the the previous work (4, 5). The ratio of the weight of the imoil was dried and degassed by pregnating solution to the weight of dry paper was, however, immersing the apparatus up to different. In the previous work an excess of oil was necessary evacuation tube k in a bath m a i n t a i n e d at 100" C. and to ensure complete contact of the oil-impregnated paper with evacuating the system between 1 the electrodes of the measuring cell. Since this excess was and 0.1 mm. of mercury for 4 unnecessary in the present work, the ratio was reduced to 0.6 hours. The time beyond the 1(about * 5 per cent) in order to approximate more closely the hour period used in the previous work (4) to obtain a practically conditions obtaining in the insulation of high-voltage underconstant power factor was conground cables. sidered adequate for a condition Before computing the extent of sorption, it was necessary of e uilibrium to be reached durFIGURE2. A P P A R A T U S to differentiate between the acid sorbed by the paper and that ing &e drying process, the extent USED WITH NONVOLATILE of which could not readily be left in solution in the impregnating oil. I n the process deACIDS checked in the present study. scribed it was impossible to remove all the oil from the paper. The small amount of moisture In practice, about two-thirds was removed, this proportion beremaining in cable paper after similar treatment was discussed by ing removed without subjecting the glass tubes to stresses Greenfield (1). During the drying process receptacle g was kept reasonably cool to prevent premature and violent release of the which might break them. The concentration of acid in the acid sample. At the conclusion of the drying period the vacuum remaining oil in the paper could be estimated only indirectly. connection was sealed off at 1, the apparatus removed from the It was assumed that a t the end of the impregnation process bath, and the oil compartment isolated from the paper compartequilibrium between the concentration of acid in the oil and ment by collapsingthe walls of connecting line m. The apparatus was then taken from the bath and allowed to cool to about 25" C. that in the paper had been reached and that the concentration In reparing the impregnating solution, hammer i was raised, of acid remaining in solution was uniform throughout. Since and tEe apparatus inclined and tapped to bring the tip of sample the case is somewhat analogous to the partition of a solute bereceptacle g under the hammer which was then released to crush tween two immiscible solvents, it was further believed that, if the tip. The sample receptacle and its T-shaped containing tube were heated, thus driving the acid into compartment e part of the solution were removed, the point of equilibrium whereupon the T-shaped tube was removed by sealing it off at should not be materially shifted. If this were true, the concenn. After the seal had cooled, the acid and oil were thoroughly tration of acid in successive parts removed should be equal. mixed to form a homogeneous solution. The apparatus, which was tilted in various positions during the mixing, was then placed To test this hypothesis, three experiments were performed in i? an upright position and window o was broken with hammer which several portions of each solution were centrifuged into j . A known volume of solution, measured by marks on the oil separate vials, and the concentration of acid in each portion compartment, was transferred from that compartment to the was determined. I n these experiments, which involved acetic paper compartment. Tube p was connected by rubber tubing to a three-way stopcock which gave access to a vacuum pump acid solutions, it was not possible to keep the sample adeand a source of dry nitrogen. After the connecting lines were quately protected from moist air while the oil was being reflushed with dry nitrogen and evacuated, tip q was broken within moved, because the additional length of compartment b (Ffgthe rubber tube and dry nitrogen was gradually admitted until ure 1 or 2) necessary to collect multiple samples could not be the system reached atmospheric pressure, when tube p was again sealed. Compartment b was sealed off at T and placed accommodated in the centrifuge. Nevertheless, the results of in an upright position in a bath at 80" C . after the seal had the three experiments indicated that the concentration of acid cooled. The bath was allowed to cool slowly to approximately in successive portions removed was essentially the same. Al25" C . Before proceeding to the next step, compartment b mas though the use of a solvent as a general means for removing left standing in the bath at least 16 hours to allow equilibrium to be established. That this length of time is sufficient for the oil was objectionable because a solvent might remove some equilibrium to be attained is indicated by the fact that the of the acid which had been sorbed by the paper, it was believed power factor of paper impregnated with similar solutions bethat information obtained by the use of a solvent might be usecomes constant in much less than 16 hours. ful in conjunction with that obtained by the centrifugal The apparatus (Figure 2) used in the cases involving the method. less volatile acids was simpler than that with the more volaAccordingly, two experiments were performed in which the tile acids. Corresponding parts of each apparatus are indisolution was removed by extracting it with hexane in a closed cated by the same letters. Except that the acid and oil were system. The values found for the sorption of acetic acid fell mixed before being placed in the apparatus, and that transfer along the same curve as those obtained by centrifuging the of part of the impregnating liquid from compartment e to solution from the paper. From the results of these tests it compartment b was accomplished simply by inverting the apwas concluded that throughout the range of concentrations
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being investigated no significant error could be created by assuming that, if all the oil were removed, it would contain the same concentration of acid as the two-thirds usually removed in practice. Percentage of acid concentration sorbed was therefore calculated as shown in Table I. TABLEI. DATAFOR
A
TYPICAL DETERMINATION
Weight of paper, grams Moisture, per cent Weight of dry paper, grams Weight of oil (total), grams Weight of propionic acid, gram Weight of soln. used t o impregnate paper, grams Oil-paper ratio Weight of soln. not used for impregnation,,gpms Concn. of acid (by titer) in original s o h , millimole/gram of oil Weight of s o h . removed from paper, grams Concn. of acid (by titer) of s o h . removed from paper, millimole/gram of oil Percentage of acid ooncn. sorbed by paper, 0.351 0.222 loo = 0.351
-
27.0 6 8 26.2 27 2 0 729 15 0 0.59 12 9
0.351 10.3 0 222
36.8
VOL. 29, NO. 9
cyclic acid. Cycloparaffinic acids with molecular weights lower than that of cyclohexanecarboxylic acid were not available. Cyclohexanecarboxylic acid was sorbed to about the same small extent as lauric acid a t similar concentrations. This is shown in Figure 3 where the curves for the two acids nearly coincide. The extent of the sorption of the mixed naphthenic acids was also small where the concentration was between 0.1 and 0.3 millimole of acid per gram of oil. An insufficient amount of purified acid was available for determining the sorption of the acid at higher concentrations. Only a small difference was found in the extent of the sorption of a given acid over the range of concentrations between 0.1 and 0.6 millimole of acid per gram of oil. This difference in the case of acetic, propionic, and butyric acids was only slightly greater than the average deviation from the means of E
03
$ '
I
Relation of Sorption Results t o Power Factor The results of the sorption experiments are shown in Figure 3. Acetic acid was almost completely sorbed (95 to 98 per cent) from the oil by the paper under the conditions of the experiments. This acid has the lowest molecular weight of the fatty acids, which are completelymiscible withliquid paraffin a t the Concentrations and temperatures employed. The double-circled points on the curve represent the data obtained by the hexane extraction method. The decrease in the extent of sorption of the fatty acids with increasing molecular weight was marked. Propionic acid was sorbed from about 31 to 36 per cent; butyric acid, 10 to 13 per cent; and lauric acid, 1to 4 per cent.
?i5 B
3 o.2
XANECARBOXYLIC
0
z
2
0.2 0.4 06 EQUILIBRIUM CONCENTRATION, MILLIMOLES OF ACID PER GRAM OF
OIL
FIGURE 4. EQUILIBRIUM SORPTION ISOTHERMS
MILLIMOLES OF ACID PER GRAM OF OIL
,
FIGURE 3. SORPTIOK OF ACIDS FROM OIL PAPER
BY
Oil-paper ratio, 0.6
I n order that the work be related as nearly as possible to the conditions existing when oil in oil-impregnated paper is oxidized in service, two naphthenic acids were included in the group of acids examined. Cyclic acids are generally believed t o be among the chief constituents of naphthenic acids, although this belief is not supported by the data of Williams and Richter ( 7 ) . Mixed naphthenic acid was included because it is formed by the actual oxidation of hydrocarbons and because it was used in the previous work (4). Cyclohexanecarboxylic acid was selected as representative of a low-molecular-weight
the values determined a t given concentrations. Curves representing the sorption, nevertheless, seem best drawn as straight lines with the small positive slopes shown. On the other hand, curves representing the sorption of lauric and cyclohexanecarboxylic acids have definitely negative slopes. The equilibrium conditions existing in the several samples are illustrated in Figure 4,where the amounts of acid sorbed per unit weight of dry paper are plotted against the equilibrium concentrations of acid in the oil. As the figure indicates, acetic acid was sorbed so strongly that only slightly more was left in the oil when the paper contained the larger concentrations of acid than when it contained the smaller concentrations. I n the cases of propionic and butyric acids, on the other hand, considerably more acid was left in the oil when in contact with paper containing the larger concentrations of acid than when in contact with paper containing the smaller concentrations. Although the extent of sorption of the acids of higher molecular weight was very small, the curves representing these acids also have positive slopes. Except in the case of the mixed naphthenic acids no attempt was made to measure the extent of sorption of acids from solutions containing less than about 0.1 millimole of acid per gram of oil. In the case of this acid determinations were made a t 0.018 and 0.016 millimole per gram. At these concentrations the percentage sorption was much higher than a t 0.1 millimole per gram. Judging from the work of Trillat and Vaille ( 6 ) ,who reported that the concentration of oleic acid in paraffin oil was decreased from 1 part in 3,000 to 1part in 20,000 by passing oil through filter paper, it is possible that if all the curves in Figure 3 were extended towards zero concentration all would be upturned as zero concentration was ap-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
proached. The lower concentrations were not investigated, however, because none of the acids studied, when present in very low concentrations, caused significant increases in the power factor of impregnated paper (4).
d
+
6
2
0 U
FIGURE 5. INTERPOLATION CHART Percentage sorption from solutions containing 0.35 millimole of acid per gram of oil
Concerning the relation of the sorption experiments to the previous work mentioned, the results of the sorption experiments confirm the hypothesis that the oil-soluble organic acids of low molecular weight, that caused marked increases in the power factor of impregnated paper, are strongly sorbed by the paper. The results further indicate that acids of fairly high molecular weight which had been shown to have little influence on the 60-cycle power factor of impregnated paper are only weakly sorbed. Since different oil-paper ratios were used in the two sets of experiments, the results cannot be compared quantitatively, but apparently the amount of acid sorbed by the paper is an important function in determining the increase in the power factor of oil-impregnated paper caused by that acid. The number of members of the fatty acid series which may cause significant increases in the power factor of impregnated paper by reason of their being sorbed by the paper may be roughly predicted from the extent to which they are sorbed as determined by interpolation of the data taken from Figure 3. If the logarithms of the values of the percentage sorption of acetic, propionic, and butyric acids, as taken from the curves a t a given concentration, are plotted against the number of carbon atoms per niolecule of the respective acids, an essen-
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tially straight line may be drawn through the three points (Figure 5 ) . The relation expressed by the line does not include the point for lauric acid which was sorbed to a greater extent than that indicated by the line. Since the related cyclohexanecarboxylic acid containing seven carbon atoms was sorbed to about the same extent as the lauric acid containing twelve, the interpolated values for the sorption of the acids above butyric were obtained by assuming that the degree of sorption decreases as a semilogarithmic function of the number of carbon atoms until a constant value is reached. The values obtained in this manner, which are probably as accurate as could be obtained experimentally, indicate that fatty acids containing six or more carbon atoms are not sorbed sufficiently to cause sisnificant power factor changes at commercial frequencies. Valeric and butyric acids would probably cause the power factor of impregnated paper to be increased to an extent approaching practical significance if these acids were present at the higher concentrations investigated. Since there seems to be no evidence that these acids are present in deteriorated insulation, the objectionable task of making electrical measurements upon prepared samples containing them has not been attempted. The present work and the previous study (4) seem to establish an adequate experimental proof that the formation of the so-called volatile acids and their sorption by paper are a major cause of increased dielectric losses in oil-impregnated paper. It is to be expected that in the presence of other oxidation products, including water, a given concentration of acid may have a different effect than if it were alone. It is also to be expected that a given concentration of acid may have a different effect from that of an equimolecular concentration of one containing the same number of carbon atoms but having more than one functional group. Since in either case not only the extent of sorption but also solubility of the acids in oil could be markedly different from those in the conditions prevailing in these experiments, it is unsafe to predict the resulting effect upon the power factor of impregnated paper caused by the presence of such materials. A limited amount of investigation in these fields is contemplated. For the present this will be confined to a study of the effect of a few acids of the type treated here when present with known small amounts of water.
Literature Cited (1) Greenfield, E. W., J. F r a n k l i n I n s t . , 222, 345 (1936). (2) Piper, J. D., IND. ENG.CHEix., Anal. Ed., 6,380 (1934). (3) Piper, J. D., Thomas, D. E F., and Smith, C. C . , IND.ENG. CHEX, 28, 309 (1936) (4) Ihid., 28,843 (1936). (5) Ihid., 29, 979 (1937). (6) Trillat, J., and Vaille, R., Compt. rend., 203, 159 (1936). (7) Williams, M., and Richter, R., J.Am. Chem. SOC., 57,1686 (1935). RECEIVED June 1, 1937. This woik forms part of a n investigation on t h e deterioration of high-voltage underground cable being conducted by T h e Detroit Edison Company under the direction of C. F. Hirshfeld, chief of reseaich.
