October, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
this sirup contained all of the glucose formed by the hydrolysis of the wood. Table VI shows that glucose is converted almost quantitatively into lactic acid by these bacteria. Commercial Applications
From the data in this paper it appears that the production
1165
of lactic and acetic acid from spent wood liquors is a feasible process, although many technical details would have t o be worked out before the process could be put on an industrial scale. If these laboratory experiments can be duplicated on a commercial scale, the yields of products per ton of wood as given in Table VI11 might be expected.
Evaluation of Transformer Oils'9' By J. G. Ford WESTINGHOUSE F$LEC!TRIC
& &fANUFACTURING
N EVALUATING oils for transformer use, consideration must be given to flash and fire points, viscosity, specific gravity, pour point, dielectric strength, presence of inorganic acids, alkalies and salts, sulfur content, and finally to the tendency to form deposits, known as sludge. It is a simple matter to choose an oil having the desired physical characteristics, but it is more difficult to select an oil that is desirable from the standpoint of sludge formation. Service tests in the apparatus in which the oils are to be used are the most reliable criterion of this property. Transformer oil has two principal functions-( 1) to act as an insulating medium, and ( 2 ) t o carry the heat generated in the windings and core of the transformer to the cooling surf aces. Sludge is usually itself a good insulator and therefore does not interfere primarily with t'he insulating qualities of the oil. The rate of sludge formation varies with temperature but, not necessarily directly with the temperature. This sludge formation is due t o chemical changes the mechanism of which is not well understood. The precipitation of this sludge in the ventilating ducts and on the cooling surfaces results in increased temperature of the transformers and this increased temperature increases the rate of sludge formation. It is therefore necessary to select an oil that will have a minimum of sludge formed under normal operating conditions and to recondition the oil when the amount of sludge formed becomes sufficient to increase operating temperatures. Recondit'ioning should be necessary only occasionally with a properly designed transformer and a properly selected oil. It is well known that sludging is caused by oxygen being in continual contact with the oil. This oxidation takes place not only at the surface but throughout the body of the oil, since it readily dissolves oxygen. I n fact, new oil before being placed in a transformer cont'ains considerable amounts of oxygen in solution. I t is also known that heat, light, and the mat'erials used in transformer construction accelerate this oxidation. All mineral oils in the presence of oxygen oxidize to a certain extent under operating conditions of a transformer. Some oils, chiefly the water-whit e type. form little or no sludge but produce on oxidation considerable amounts of a large number of organic acids, the lower boiling members of which are mostly vo1at)ilized a t operating temperatures. Mineral oils slowly oxidize, even a t room temperature, in the presence of light, moisture, and oxygen. The steps in the development of oxidation products have already been described by other investigators.3-6
'
Received June 16. 1927. Presented before the Division of Petroleum Chemistry a t the 74th Meeting of the American Chemical Society, Detroit, Mich., September 6 t o 10, 1927. * Scienlil:c Pa$er 245 from the Westinghouse Electric & Manufacturing Company. Sligh, PYOC. A m . SOL.Testing M'nieviais, 24, 964 (1924). 4 Staeger, THISJ o n R x A L , 17, 1272 (1925). Haslam and Frolich, I b i d . , 19, 292 (1927). 6 Musatti and Pichetto, Ann. chim. agplicala, 15, 238 1:1925).
COMPANY,
EASTPITTSBURGH, P A .
There are a t present numerous testing methods employing elevated temperatures and various accelerating means which are supposed to classify oils according t o their value for transformer use. Considerable controversy exists among different investigators concerning the temperature of test, measurement of end products, and the use of catalysts such as copper and iron. I n general, the results of these different tests show little agreement among themselves. An oil which may be indicated excellent when tested by one method may give poor results when tested by another. The author believes that none of these tests can be intelligently criticized until more definite knowledge is obtained concerning the action of oxygen on oils of different chemical constitution, what products of oxidation are det)rimental t o the transformer, and the effect of temperature and catalysts on the rate of oxidation and formation of end products. However, two criticisms of the majority of tests are that the time consumed in making them is disadvantageous and it is difficult to reproduce results. According to Staeger4mineral oils may contain the following groups of hydrocarbons: I-Unsaturated hydrocarbons ( A ) Cycl1c (a) Aromatic hydrocarbons (benzene, naphthalene) ( b ) Alicyclic hydrocarbons (terpenes, polyterpenes, dihydroxy, tetrahydroxy combinations) ( B ) Aliphatic (olefins, polyolefins) 11-Saturated hydrocarbons ( A ) Cyclic (naphthenes, polynaphthenes, condensed naphthenes) ( B ) Aliphatic (paraffins)
Experience has shown that under the same operating condition some oils fail in one or two years while others function properly after ten years. This difference in the behavior of oils is undoubtedly connected with their chemical constitution. Since oils may be composed of any or all of the above groups of hydrocarbons in various proportions and these groups probably act differently towards oxygen, it is not astonishing that oils behave differently in transformers. It seems reasonable to suppose that if we knew the approximate composition of a transformer oil and how the various hydrocarbons therein acted towards oxygen, we could predict just how this oil vould perform and therefore choose the most suitable oil for a given set of operating conditions. To isolate and test the individual hydrocarbons occurring in petroleum would be almost out of the question on account of the difficulty and labor involved in obtaining the pure hydrocarbons. On the other hand, to simplify matters a5 much as possible, some valuable information might be gained by a proximate analysis of a number of oils taken from various geographical locations; determining, if possible, the percentage of the different groups of hydrocarbons present. These oils may then be subjected to oxidation tests in an
1166
INDUSTRIAL AND ENGINEERING CHEMISTRY
attempt to correlate constitution and behavior. Before any such program can be attempted the necessity of having reliable methods of analysis becomes apparent. The author found that some water-white oils, which had been freed from unsaturated compounds by oleum treatment, were very susceptible to oxidation, whereas the addition of a small amount of unsaturated hydrocarbons to these oils retarded the oxidation considerably. Other investigators have also found this to be trues4,' The following experiment demonstrates that the presence of these hydrocarbons in an oil has a marked effect on the course of oxidation:
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this work. Not only were the unsaturated compounds not completely removed, but results varied with temperature of test, method and time of agitation of the acid and oil mixture, and the relative amounts of acid and oil used. The method is intended to indicate the degree of refinement of a n oil and not to serve as a quantitative determination of unsaturated hydrocarbons in transformer oil. After considerable experimental work, however, a revision of t h e met'hod was developed which eliminated the major variables
The oxygen held in chemical combination by two different oils was measured under the same conditions. One oil was practically free from unsaturated hydrocarbons while the other contained 10.5 per cent. The unsaturate-free oil consumed about three hundred times as much oxygen as the sample containing the unsaturates. In the absence of unsaturated compounds large quantities of alcohols, aldehydes, esters, acids, water, and carbon dioxide were formed. Other oils free from unsaturated hydrocarbons did not generally form sludge, while those containing these compounds, after continuous heating, threw down a deposit of asphaltic nature.
It can therefore be concluded that unsaturated compounds play a n important role in the oxidation of mineral oils both as to extent and character of products. Oxidation T e s t s
I n order to eliminate as many variables as possible, it was decided to make oxidation tests in the absence of all catalysts except those that might already be present in the oil. Five hundred cubic centimeter samples of oil were filtered into Pyrex beakers and placed into ovens designed to give uniform heat treatment. Air was passed freely into the ovens to maintain normal oxygen concentration. The air-bath temperature was regulated within * 1" C., and naturally the actual temperature variation of the samples was even less than this. Tests were made at 80, 100, 110, and 120" C., respectively, for long periods of time. Neutralization values and the percentage sludge were determined periodically. D e t e r m i n a t i o n of U n s a t u r a t e d Hydrocarbons
To make a correct comparison between sludge formation and the unsaturated hydrocarbon content, a dependable method for the determination of these hydrocarbons is imperative. Determinations of bromine, iodine, and formolite numbers were not made because these tests are known to give inconsistent results. Furthermore, Musatti and Pichetto6 have shown that no relation exists between theue tests and the oxidation of oils. On the contrary, they did find some correlation between the amount of unsaturated hydrocarbons in the oil which are soluble in liquid sulfur dioxide and the quantity of sludge formed in a definite time. Whereas the bromine and iodine numbers are thought to be a measure of the olefin hydrocarbons, the formolite number and liquid sulfur dioxide absorption are thought to indicate the aromatic content of an oil. According to Egloff and Morrell,s olefin hydrocarbons are absorbed and polymerized by 80 per cent (by weight) sulfuric acid while aromatic hydrocarbons are not attacked. On treating a number of transformer oils with sulfuric acid of this strength little or no absorption took place, indicating that the unsaturated hydrocarbons present were mostly aromatic compounds. The United States Government method9 for unsaturated hydrocarbon content, as printed, proved unsatisfactory for Waters, Bur. Standards, Bull. 7, 305 (1911); Circ. 49 (1920). THISJOURNAL, 18, 354 (1926). 9 Bur. M i n e s , Tech. Paper 323A, p. 89.
7
8
0 TIME O F AGITATION IN MINUTES
Figure 1-Absorption of Unsaturated Hydrocarbons by 95 Per Cent Sulfuric Acid a t Room Temperature
and proved to be satisfactory for the determination of those unsaturated hydrocarbons which are soluble in concentrated sulfuric acid-namely, the olefins and part of the aromatic compounds. Briefly, the revised method is as follows: Ten cubic centimeters of concentrated sulfuric acid (sp. gr. 1.84) are poured into a 25-cc. glass-stoppered tube which has been calibrated in 0.25-cc. divisions at room temperature. Fifteen cubic centimeters of the oil t o be tested are gently poured into the tube from a pipet, causing as little agitation as possible a t the oil and acid interface. The volume of the oil is then accurately read while the tube is held in a vertical position. The tube is mounted on a suitable shaking device and agitated. The tubes are alternately shaken for 30 minutes and centrifuged for 5 to 10 minutes until no change in the oil volume is detected. The time required for complete absorption varies with the amount of unsaturated hydrocarbons originally present. With oils containing up to 12 per cent about 1.5 t o 2 hours' shaking is required, while oils containing from 12 t o 25 per cent may be agitated as long as 3 hours before absorption is complete. The final volume of residual oil is read and the calculated volume loss is taken as the percentage of unsaturated hydrocarbons by concentrated sulfuric acid.
Results by this method are easily reproducible and are subject to only slight variations when conditions of the test are considerably changed. As shown in Figure 1, oils that are tested by the United States Government method
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30 40 50 60 70 TIME O F HEATING IN DAYS
Figure 2-Sludging
80
Tests on Oils a t 120' C.
90
October, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
and show very little difference in absorption give entirely different results by the revised method. For instance, oils 1and 3 showed only a slight difference in absorption when they were shaken 1.25 minutes, as required by the government test, while these same oils show a difference of 11.5 per cent when the absorption values become constant. It will be shown later that there is also considerable difference in the sludging tendencies of these oils. Under the conditions of the test, naphthene and paraffin hydrocarbons are but slightly attacked. A naphthene-base oil, after removal of unsaturated hydrocarbons, was treated with fresh acid. After 2 hours of continuous agitation no decrease in volume could be detected, although the acid was darkened considerably. Treating oil with concentrated sulfuric acid until no further absorption can be detected absorbs and polymerizes the olefins but apparently does not completely remove the aromatic hydrocarbons. Judging from the rate a t which the residual aromatic compounds are sulfonated, it would require lengthy treatment for their removal. For instance, a sample of oil which imparted only a slight coloration to the acid after 2 hours' treatment gave 9.7 per cent aromatic hydrocarbons by the nitration method of Egloff and hIorrelIb for like compounds in gasoline. To obtain the total percentage of unsaturated compounds, the percentage as obtained by nitration should be added to the sulfuric acid absorption value.
1167
are very slowly sulfonated by 95 per cent sulfuric acid on sludge formation. Nevertheless, results from a number of heat tests indicate that when the total unsaturated hydrocarbons are considered, there is some relationship between these values and the quantities of sludge formed. Since oils vary in percentage of the different hydrocarbons, especially naphthenic and paraffinic, one would think that sludging may depend, not only upon the unsaturated hydrocarbons, but also upon the basic compounds of the oil. Tests
I
1
,
PARAFFIN
OIL
I
,833
R e l a t i o n of U n s a t u r a t e d H y d r o c a r b o n s to Q u a n t i t y of Sludge
.
Having developed a method for the determination of unsaturates by which reproducible results could be obtained, and having shown that oils differ in their percentage of unsaturated compounds, the preliminary work for correlating I05
i n
I 2 3 4 TREATMENTS WITH 15% OLEUM Figure 3-Rise i n Aniline Point of a Highly Refined Paraffin Oil with Successive Treatments of 15 Per Cent Oleum
, I
I
I
2
3
TREATMENTS WITH 15% OLEUM
I
4
Figure 4-Change in Physical Characteristics of Insulating Oil when Treated w i t h 15 Per Cent Oleum
were made on both naphthene- and paraffin-base oils having approximately the same sulfuric acid absorption values. The results indicated that when sufficient unsaturated compounds are present to prevent relatively high acid formation the quantity of sludge formed is practically independent of the base of the oil. Although naphthene-base oils seem to be slightly better solvents for the asphaltic oxidation products and do not precipitate this material quite so readily as the paraffin oils, the quantity of sludge formed by each is approximately the same when tests are continued for a long period and equal amounts of unsaturated compounds are in the two types of oil. D e t e r m i n a t i o n of N a p h t h e n e and Paraffin H y d r o c a r b o n s
sludge tests with unsaturated hydrocarbon content was completed. Figure 2 shows the development of sludge in a number of oils heated at 120" C. for approximately 1400 hours. The curves show that increased sulfuric acid absorption results in an increase in the sludge value. As an apparent exceptiori i t will be noticed that, although oil 3 contains only 1.6 per cent unsaturated hydrocarbons (by sulfuric acid), it forms more sludge than oil 8 which gives an absorption value of 9.5 per cent. This number 3 oil also developed high organic acidity. Further reference m-ill be made to this oil later. Oxidation tests on the same series of oils a t 100" and 110' C. show approximately the same relationship between sulfuric acid absorption and quantity of sludge formed as the 120' C. tests. Although tests have been in progress a t 80" C. for about 13 months, sufficient sludge has not formed in most of the samples to enable one to draw any definite conclusions. The question arises whether the sulfuric acid absorption alone, or this value plus the nitration value, should be used for comparison with sludge tests. The author has not yet done sufficient experimental work to draw definite conclusions concerning the influence of the aromatic compounds which
Egloff and Morrells developed a method for the determination of naphthene and paraffin compounds occurring in gasoline based on the lowering of the temperature of complete miscibility of aniline and the paraffin hydrocarbons by the presence of the naphthene compounds. I n brief, the test is made as follows: Ten cubic centimeters of freshly distilled aniline and a n equal volume of oil, which has been freed from unsaturated hydrocarbons, are placed in a test tube t h a t is jacketed by a larger test tube. A thermometer (calibrated in 0.1 C.) and a stirring rod are placed in the smaller test tube. The tube is heated until the solution is just above the cloud point and then allowed to cool until the cloudiness reappears. The temperature a t which the cloud reappears is read and taken as the aniline point.
According to Tizard and Marshal1,IO the paraffin hydrocarbons are completely miscible with aniline a t 70" C. and the cloud point is depressed 0.3" C. for each 1 per cent of naphthene hydrocarbons present. An attempt was made to use this test in the analysis of transformer oils, but difficulties were encountered. To 10
J . SOC.Chcm. Ind., 40, 20T (1921).
INDUSTRIAL AND ENGINEERING CHEMISTRY
1168
begin with, the aniline point of a Pennsylvania paraffinbase oil which had been refined with oleum was 102.3' C. instead of 70" C., or less if the sample contained naphthene compounds. It a t once became apparent that the aniline point varied not only with the percentage of naphthene hydrocarbons present but also with the molecular weight of
" I
~
50
60 70 80 90 100 110 VISCOSITY SAYBOLT AT 100°F: IN
120
130
146
SECONDS
i n Aniline Point with Viscosity of "Pure" P a r a a n Oil
Figure &Variation
the hydrocarbons in question. Under these circumstances it was necessary to obtain as a standard with which to compare other oils an oil that was essentially composed of paraffin hydrocarbons. It was also necessary to find the variation in aniline point with molecular weight. That naphthene and paraffin hydrocarbons are both attacked by fuming sulfuric acid is well known. It occurred to the author that if these two series of hydrocarbons were treated with fuming sulfuric acid the irrelative rates of reaction might be such that one group would be removed faster than the other. I n this manner the percentage of naphthene or paraffin hydrocarbons in the residual oil might be increased. Since the aniline point changes with the concentration of naphthene compounds, this should be a good indication as to whether the acid treatment sulfonates one group more readily than the other. Accordingly, the Pennsylvania oil mentioned above was given a number of successive treatments with 15 per cent oleum. Aniline points were determined after each sulfonation. Figure 3 shows that the aniline point rises, indicating the probable
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20 30 40 50 60 70 80 90 100 110 190 130 HEATING AT IZO'C. IN DAYS
TINE OF Figure 6-Oxidation
of Insulating Oils at 120' C.
removal of naphthene hydrocarbons. Upon repeated treatment of the oil with acid the aniline point reached a value that was practically constant, although if fresh acid was added there was further action, due probably to sulfonation of the remaining paraffins. It was recognized that the rise in aniline point might be
Vol. 19, No. 10
due to the removal of either naphthene hydrocarbons or some of the lighter hydrocarbons, resulting in a mixture of higher average molecular weight. T o test this possibility, viscosity measurements were made. Since viscosity is a function of molecular weight, removal of the lighter hydrocarbons should give increased viscosity to the oil. The viscosity of the oil remained practically constant throughout, indicating that the rise in aniline point was due to the removal of naphthene compounds. -4nother sample of oil, prepared by compounding a naphthene-base oil with the above paraffin oil, was treated similarly with oleum. Specific gravity, aniline point, and refractive index were determined after each treatment. The results are shown in Figure 4. Instead of an increase in density, there was a decrease as shown by the curve, indicating conclusively a removal of naphthenes rather than of lighter paraffins. The decrease in refractive index together with the rise in aniline point give further confirmation of the conclusion that naphthenes were being removed. A Texas oil, very high in naphthene and freed from unsaturated compounds, was also treated with 15 per cent oleum, with the following results: BEFORETREATMENT AFTER TREATMENT Viscosity, Saybolt at 100' F. (38' C . )
Specific gravity (26' C . ) Refractive index Pour test Aniline point
55 seconds 0.868 1.4752 -55' F. (-48.3' 80.6' C.
57.5 seconds 0,844
C.)
1.4639 - 3 5 ° F . (-37.2'C.) 94.6' C.
These changes in physical characteristics also indicate the removal of naphthene hydrocarbons. Although sufficient evidence has not been presented to permit the statement that the oleum-treated Pennsylvania oil is composed solely of paraffin hydrocarbons, there is no doubt that it contains a very high percentage of these compounds, and therefore satisfactorily serves as a standard for comparing other oils. For analytical purposes it will be assumed that this oil is 100 per cent paraffin hydrocarbons. As stated before, the aniline point varies with the average molecular weight as indirectly indicated by viscosity. To compare oils by the aniline point method it mas necessary to obtain a curve showing the change of aniline point with molecular weight. Consequently, a Pennsylvania oil was treated with oleum until a constant aniline point was obtained. A one-liter sample of this oil was vacuum-distilled into ten fractions of 100 cc. each. The fractions were again treated n-ith oleum, after which viscosity and aniline points were determined on each. The results are shown in Figure 5 . The composite acid-treated oil and mixtures of any of the fractions in various proportions all fall on the curve. By the aid of this curve the percentage naphthene hydrocarbons in a given sample of oil is easily calculated as f O l l O N S : Before making the aniline point determination the unsaturated compounds must be removed from the sample. These hydrocarbons also depress the aniline point and would therefore give erroneous results. T o remove the unsaturated hydrocarbons, a 400-cc. oil sample is treated with 350 cc. of 95 t o 98 per cent sulfuric acid (sp. gr. 1.84) for approximately 3 hours. The acid and oil are thoroughly agitated by passing a current of air through the reacting mixture. The oil is then decanted off, washed with water and weak alkali, and dried with calcium chloride or anhydrous sodium carbonate. As stated before, the aromatic compounds are not completely removed by the sulfuric acid treatment, so some other means must be applied to eliminate them before making the aniline point determination. Attempts were made to remove these compounds by nitration and then recovering the oil, but owing to nitration products, soluble in the oil, which seemed to be powerful emulsifying agents, recovery of the oil was extremely difficult. After trying a number of different schemes, freshly activated silica-gel was found t o remove these aromatic hydrocarbons almost completely. After agitating 50 grams of the moisture-free oil withoan equal weight of pulverized silica-gel for 15 minutes a t 120 C. the mass i s
October, 1927
I S D U S T R I A L d S D E S G I S E E R I S G CHEXISTRY
1169
filtered through a n ordinary filter paper. Ten cubic centimeters of the filtered oil are used for the aniline point determination, according to the method previously described. The above silica-gel treatment has the disadvantage that the aromatic hydrocarbons are not entirely removed and introduce a small error due to depression of the aniline point by these hydrocarbons. The error is practically the same for most oils and amounts t u about 0.5" C. The viscosity of the remaining acidtreated oil is determined and the percentage of naphthene hydrocarbons calculated.
From tests made there appears to be some difference in the behavior of low and high molecular weight naphthene compounds toward oxygen. The oxidation of mixtures of paraffin and naphthene hydrocarbons ranging in viscosity from 50 to 60 seconds Saybolt a t 100" F. (38" C.) results in the formation of sludge and excessive quantities of acid. Samples from the same crude oil with a viscosity of 80 to 90 seconds a t the same temperature are quite resistant to oxidation, forming only a trace of sludge and very little acid. I n fact, these oils prove to be much superior to any oils that have been tested. The superiority of these mixtures over the ordinary transformer oils is shown in Figure 6. The first appearance of sludge is indicated by the first point on the respective curve of each oil. Oil 1-D is a water-white oil, composed chiefly of paraffin hydrocarbons. It forms no sludge under the conditions of the test, but develops excessive quantities of organic acids, a large part of which are volatilized. Oil 15-D is what might be classed a poor grade of insulating oil, since sludge appears in 3 days and develops rapidly thereafter. However, the quantity of acid formed is small. Sample 2-D represents a good insulating oil when graded aCcording to American specifications, since 15 days are required for sludging. On continuing the test considerable sludge and a small amount of acid are formed. Oil 0 2.0 40 bO 80 100 120 140 160 I80 T I M E OF HEATING IN DAYS AT l l O % 23-D is composed of paraffin hydrocarbons compounded with Figure 7-Effect of Acid o n Precipitation of Sludge a t l l O o C. high molecular rveight naphthene hydrocarbons. The Relation of Quantity of Sludge Formed to T i m e of Heating and Percentage of Unsaturation unsaturated hydrocarbons were removed from both components. Sludge does not appear until after 45 days of heating. Example. Assume the viscosity of a n oil to be 30 seconds Say- The small quantity which forms remains practically conbolt a t 100" F. (38" C.) and the aniline point, 7 5 " C. By referstant for an additional 55 days of heating. The quantity ence t o the curves given in Figure 5 it is found t h a t a mixture of paraffins of the same viscosity has a n aniline point of 98" C. of acid that develops during this time is practically negligible. Applying the correction for residual aromatic compounds, the de- This oil would undoubtedly be a serviceable transformer oil, pression resulting from naphthene compounds is 12.5' C. Since, since the desirable characteristics are a low sludge value according to Tizard and Marshall, the aniline point is depressed 0 . 3 for each 1 per cent of naphthene hydrocarbons present, the c o u p l e d w i t h low sample would contain 41.6 per cent naphthene hydrocarbons. acid formation. Paraffin hydrocarbon content is taken as the difference. The higher molecular weight naphAlthough the writer realizes that this method (of oil analysis is not free from error, several conclusions may be drawn: thene c o m p o u n d s seem t o have a n 1--The results are reproducible if conditions are carefully antioxidizing effect followed. somewhat similar to 2-Considerable differences are shown in oils from different the unsaturated geographical locations. 3-Correlation exists between results obtained by the analysis h y d r o c a r b o n s, aland those obtained by oxidation tests. though they do not precipitate sludge so Table I shows the analysis of sereral oils refined for transreadily. A suspicion former use. t h a t t h e o d d beTable I-Analysis of Transformer Oils havior of the parafVISCOSITY SAYBOLT UNSATURATED X-APH-PARAFFIN fin-naphtliene h y A T 100'F. (38OC.1 .&XIHYDROCARBONS THESE HYDROOIL ACID-TREATED drocarbon mixture LIXE By95V0 By HYDROCARCARSAMPLE OIL POINT HgSOc nitration BONS BOSS was due to sulfur Secoiids C . P e r cent Pev cent P e r cent P e r cent compounds waq won Pennsylvania 57 99.1 10.5 5.2 5,9 78.4 Texas 55 80.6 13.0 11.2 26.1 dispelled when the California :2 82.2 1.8 9.7 2:;: 33.0 sulfur content was Figure 8-Effect of Acidity o n Precipitation found to be only 0.04 of Sludge a t lZOo C. Oxidation of M i x t u r e s of Paraffin a n d N a p h t h e n e Hydro(Neutralization value-D-188) carbons per cent.
Results from a large number of oxidation tests on naphthene- and paraffin-base oils containing considerable amounts (5 per cent or over) of unsaturated hydrocarbons show that,, regardless of the base of the oil, the quantity of sludge formed is dependent upon the coilcentration of the unsaturated hydrocarbons. Oxidation of mixtures of parafin and naphthene hydrocarbons alone generally resulted in the formation of sludge and large quantities of organic acids. As yet sufficient data have not been obtained to permit one to draw definite conclusions concerning the effect of naphthene hydrocarbons on sludge formation in mixtures. with paraffin hydrocarbons.
Effect of Acid F o r m a t i o n on Precipitation of Sludge
Oils, containing only a small amount of unsaturated hydrocarbons, when subjected to oxidizing conditions after heating for some time, develop high acidity and form exce:" saire quantities of sludge. As stated before, the unsaturated compounds evidently are retarding agents. The oxidation of the saturated hydrocarbons is retarded until the unsaturated are partially or completely oxidized, after which they oxidize, forming alcohols, aldehydes, esters, acids, water, and carbon dioxide. Either the acid or some intermediate product, formed in the oxidation of the saturated compounds,
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
causes rapid precipitation, possiblyby condensation and polymerization. This precipitate is usually much lighter in color than the regular asphaltic sludge. It is sometimes almost white. Figure 7 illustrates the behavior of oils of this composition. Although oil 6 has four times as great a sulfuric acid absorption value as oil 3, it forms much less sludge during the time of test. From the slope of the curves, one might predict that, if the tests were continued, oil 6 would eventually contain more sludge than oil 3, and thereby correlate the absorption values. I n the absence of excessive quantities of acid, correlation is shown between sulfuric acid absorption values and sludge a t 110" C. by oils 2 , 4 , and 6. These curves and a number of others not shown here illustrate that the oxidation of the saturated compounds,
TIME OF HEATING
IN
DAYS
Figure 9-Effect of Temperature o n Rate of Oxidation of a Transformer Oil Containing 1.6 Per Cent Unsaturates (Neutralization value-A. S. T. M. D-188)
with the formation of alcohols, aldehydes, etc., merely accelerates the precipitation of asphaltic sludge, which is formed from the oxidation of the unsaturated compounds. After this precipitation the sludging rate is very slow. The formation of sludge, a t this temperature, must surely arise from the oxidation of unsaturated hydrocarbons, since oil 1, which forms no sludge but large quantities of acid, is practically oil 3 with the unsaturated hydrocarbons removed. The effect of acid formation is further illustrated in Figure 8. The oils are mixtures of paraffin and high molecular weight naphthene hydrocarbons from which the unsaturated compounds have been removed. The curves show that the high molecular weight naphthene compounds retard the oxidation in somewhat the same way as do the unsaturated compounds. Oil 21 develops acid in 4 days' heating and soon thereafter precipitates sludge. This oil contains 11.5 per cent naphthene hydrocarbons. Oil 20, containing 26 per cent naphthene compounds, acts in the same way except that the time for acid formation was 45 days. It is interesting t o note that, with the increased naphthene hydrocarbon concentration, the quantity of sludge precipitated in the presence of acid increased. Oil 23, containing 58 per cent naphthene compounds, forms very little sludge or acid. A logical conclusion from the foregoing work is that a good, serviceable transformer oil should contain sufficient naphthene hydrocarbons to prevent acid formation. Effect of Temperature on the Rate of Oxidation
To obtain the best service from an oil the transformer should be operated at the lowest possible temperature, because the rate of sludging accelerates with a rise in temperature. Electrical engineers generally have accepted 90" C. as the maximum operating temperature and design the transformers accordingly. Before specifying test temperatures for evaluating transformer oil, it is necessary to know the rates of deterioration of different oils a t various temperatures. When making heat tests the operating temperature is almost out of the question on account of the time consumed in obtaining results. With the idea of making tests as short as
possible, the tendency has been to specify elevated temperatures ranging from 110' to 200°C. Since oils vary in their chemical constitution and some hydrocarbons are more sensitive to oxidation than others, it is apparent that changes of temperature will cause the oils to behave differently. The author made tests a t 80', loo", l l O o , and 120" C. and found that the relative rates of oxidation differ with certain oils. A number of oils, especially those containing over 5 per cent unsaturated compounds, appear to accelerate a t about the same rates with temperature increase, while some oils give apparently different results. Assuming the unsaturated compounds act as retarding agents in the oxidation of oils, the rate of oxidation of these compounds, or the time required to form excessive quantities of organic acid, should be an approximate measure of their rate of oxidation. Figure 9 shows curves which represent tests on a paraffin-base oil containing 1.6 per cent unsaturated hydrocarbons. The curves show the formation of acid at 80°, loo", l l O o , and 120" C. The first points on the curves indicate the time after which acid rapidly increased or, in other words, the time required to oxidize the unsaturated hydrocarbons present. Although only 190 days are shown on the curve, the 80" C. test has been carried on over a period of 300 days without acid development. It can be readily seen that the oxidation of this oil is considerably different at 80" C. than a t 100" C. or higher. The general statement has been made that the rate of oxidation approximately doubles with each 10 degrees rise in temperature. The author found that this holds true with a number of oils, especially those containing large quantities of unsaturated compounds, but the oil discussed above acts differently. Any number of oils could be chosen to illustrate this point. Figure 10 shows the effect of temperature of test on the development of acid in a paraffin oil. freed from unsaturated and most of the naphthene hydrocarbons by oleum treatment.
0
20
40
80 100 I20 140 160 180 TIME OF HEATING IN DAYS of Temperature of Test to Neutralization
60
Figure 10-Relation Values (A. S . T . M. D-188) of Water-White Transformer Oils
The curves show that the oil contains larger amounts of acid a t 80" C. than a t 100" or 120" C. This is probably due to volatilization of acid a t the higher temperatures and possibly to a change in the mechanism of oxidation resulting in polymerization and condensation of intermediate oxidation products or acids. Since, as previously shown, acids or intermediate oxidation products in oils conhining unsaturated compounds influence the rate of sludging, different temperatures will cause different sludging rates. Therefore, elevated temperature tests do not correlate the results obtained in actual service. Summary
The work herein reported, although covering several years of efforts, is still in its infancy. It has, however, been fruitful of some new ideas regarding the use and selection of transformer oils, that may be numerated as follows:
October, 1927
I S D U S T R I A L .4SD ELVGI.VEERISG CHEMISTRY
I-Unsaturated hydrocarbons play a n important role in the oxidation of transformer oils. 2-There is a definite relationship between the quantity of sludge formed and the unsaturated hydrocarbons as measured by 95 per cent sulfuric acid absorption. 3-The oxidation of saturated hydrocarbons is retarded by the presence of unsaturated compounds. 4-Small amounts of unsaturated hydrocarbons retard oxidation only temporarily and finally large quantities of sludge are precipitated. 5-With oils containing over 5 per cent unsaturated hydrocarbons the quantity of sludge formed appears t o be independent of the base of the oil. 6--illthough the method for the determination of naphthenes is not entire6 free from error, it is of value for determining the approximate source of an oil and for showing how it will act under certain conditions.
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7-High molecular weight naphthene hydrocarbons do not appear to be so sensitive t o oxidation as those of low molecular weight. &Mixtures of high molecular weight naphthene with paraffin hydrocarbons give an oil with high resistance to oxidation. 9-No general rule can be given concerning the effect of different temperatures on the rate of oxidation of different oils. 10-Temperatures other than operating temperatures will give false results with certain oils. 11-From the analysis and oxidation tests herein described, one may predict the approximate behavior of a n oil at a definite temperature.
Acknowledgment The author is indebted to D. R. Kellogg and his associates who were of assistance in facilitating this work.
Stainless Iron and Its Application to Chemical Plant Construction’ By Walter M. Mitchell CENTRAL ALLOYSTEEL CORPORATION, MASSILLON.OHIO
N
OT many years ago the metallurgist had recourse to the
chemist for solution of the problems encountered in the development of metallurgical processes; and now, in turn, the chemist finds it necessary to consult the metallurgist when problems concerning the selection of materials for specific purposes are to be solved. The selection of materials suitable for the construction of apparatus for the manufacture of corrosive products is one of the major difficulties experienced in many of the developments in chemical industries, and no process of chemical manufacture can be considered as satisfactorily established until a suitable material has been found in which the reactions involved can be carried out on a large scale. I n the past this subject has perhaps been overlooked, but with the newer synthetic processes involring very high temperatures or pressures, frequently both, the materials of construction become vitally important. The advent, therefore, of a class of alloys with high strength, obtainable in virtually all necessary shapes, possessing good general corrosion resistance, and high specific resistance to a definite class of compounds, has naturally been heralded with considerable enthusiasm. This class of alloys comprises the so-called “stainless” irons and steels, the characteristic properties of which are due to the presence of a considerable proportion of chromium-a white, hard, somewhat brittle metal, noted for its high melting point and its resistance to oxidation and oxidizing agents. Commercial Development
It would be interesting to trace the development of these alloys from the time of Faraday and Stodart, who in 1820 were the first to experiment with the addition of chromium to iron, to the products of the present time, but we are not so much concerned with the historical development as with the practical application of the alloys at present produced. It is sufficient to note that the earliest experiments with the addition of chromium to steel were with the idea of its substitution as a hardening element in place of the more customary carbon. We know today that chromium confers no hardening properties in itself, but merely forms a solid solution with the iron. The earlier experimenters noted that the alloys of iron rich in chromium were hard, fused with difficulty, 1 Presented before t h e meeting of t h e Indiana Section of t h e American Chemical Society, S o u t h Bend, I n d , April 20, 1927.
resisted oxidation and atmospheric attack, and in addition were not attacked by a number of chemical reagents, the most prominent of which was nitric acid. The first commercial development of these alloys, making use of their corrosion-resisting qualities, appears to have been due to Harry Brearley, of the laboratory of Thomas Firth and Sons in Sheffield, England. Brearley conducted an extensive research to determine resistance to erosion of various types of steels with reference to their use for rifles and naval guns. Certain of these steels carrying high chromium were not attacked a t all, or only very slowly, by the usual metallographic etching reagents, nor were the specimens rusted after months’ exposure to the atmosphere of the physical laboratory. The ultimate outcome of this was the marketing of the now familiar “stainless steel” containing 11to 13 per cent chromium with 0.30 to 0.35 per cent carbon, as a t present used for cutlery, surgical instruments, etc. Patents were eventually applied for, and later granted in Canada in 1915 and the following year in the United States, Specific claims covered the composition of the alloy, also the necessity for heat treatment and the preparation of a polished surface to produce full stainless qualities. At about the same time Elwood Haynes, of the United States, was granted a patent for a ferrous alloy containing high chromium, disclosing the “noble” character of the polished surfaces of wrought-metal articles of the composition specified. This is a broader and more generic patent covering stainless materials in general, while the Brearley patent may be considered as covering a specific product included in the broader classification allowed to Haynes. Probably the most interesting of the patents granted, and it may eventually develop, because the date of application precedes the above patents, that they are also the most important, are the Strauss patents, granted in 1919 and 1920 in the United States covering alloys containing nickel (0.5 to 20 per cent) in addition to chromium (6.0 to 40.0 per cent). Both these patents and the British Pasel patents emanate from the same inventor, who a t the time was a member of the staff of the Krupp gun works a t Essen. Practically all the straight iron-chromium alloys which had been produced in a commercial way up to this time contained sufficient carbon to confer hardening properties and the “stainlessness” was only developed after proper heat
