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Vol. 18, No. -- 2
Adsorption Studies on the Decolorization of Mineral Oils’ By T. H. Rogers, F. V. Grimm, and N. E. Lemmon STANDARD OIL COMPANY (INDIANA), WHITING, TND.
T
HE use of various porous materials, particularly fuller’s
standard oil, for it has been found that use of a single oil earth, for the decolorization of mineral oil products has for the whole range of color, involving the measurement of been a common refinery practice for many years. A1- different kinds of oil, will give only approximate color values. though the action of such materials has been ascribed to Unless the oil being measured and the standard match exchemical reaction by Lob,2 various authors3 have concluded actly when brought to equivalent depths, the ratio of depths that the action is wholly mechanical. Day4 has pointed out found will vary according to the depth taken. That is, with that, in addition to decolorization, fuller’s earth effects marked two solutions of cylinder stock, apparently quite the same in changes in the composition of crude oils, which he regards as hue, but differing appreciably in “grayness,” it ‘was found due to the selective action of capillary diffusion. Parsons5 that the oils matched a t depths of 6.32 and 1.1 mm., respechas suggested that the decolorization of oils by fuller’s earth tively, while using a deeper range the depths when the oils -matched-were 12.75 and 2.0 is due-to adsorption, and mm., r e s p e c t i v e l y . The subsequent work, especially ratio between the two oils is that of Gurwitsch,6 has led Although the decolorization of mineral oils by porous thus not constant, being to a general acceptance of materials is generally considered to be due to adsorp5.75 in the first case and this point of view. Howtion, i t has not previously been shown whether the 6.37 in the second. It was ever, except for the recent laws of adsorption are applicable. Decolorization exdemonstrated that this difarticle of Dunstan, Thole, periments, using a number of kinds of oils and several ference was not caused by a n d R e m f r ~ but , ~ little adsorbents, show that Freundlich’s equation applies i n a c c u r a c y d u e to the quantitative data have been accurately, when amount of color removed is used as greater “grayness” of one of published and nokffort has a measure of amount of material adsorbed. the oils, as substantially the been made to analyze data Color measurements are expressed on a true color s a m e r e s u l t was obtained on the d e c o l o r i z a t i o n of scale and details of the technic of color measurement with or without a compenmineral oils from t h e standare described. The slopes of the color adsorption sating shadiiig (use of a point of the general laws of curves vary widely for different oils; those for kerosene f r o s t e d g l a s s under the adsorption. differ essentially from the well-established characterlighter oil). Such discrep It is the purpose of this istics of typical adsorption. The significance of such ancies are still more marked article to describe a convenabnormalities is discussed. It is shown that color if d i f f e r e n t kinds of oil, ient and quantitative formation of cracked kerosene in the presence of clay which have roughly but not method of studying the deis due to oxidation. exactly the same color, are colorization of mineral oils, used for the standard and to analyze such data in comu n k n o w n oil-as for inparison with typical cases of adsorption, and to discuss color formation in the presence of stance, the use of a cylinder stock solution as standard in measuring the color of a red oil. clay, especially in cracked distillates. To obviate these difficulties different standard oils were Methods used, choosing as a standard for each series of oils an oil identical with those under investigation, of such a color that COLOR MEAsuRmmNT-It is necessary to have an accurate it could be matched a t a convenient depth with the Lovimeans of measuring color and a rational scale for expressing bond 42 glass. Approximately this same depth of standard the determinations. Such requirements are largely satis- was thereafter used in comparing it with the unknown oil. fied by the method developed by Parsons and Wilson,* in Since it was demonstrated that Beer’s law applies accurately which the color of the oil is compared with that of a standard when two oils that match perfectly are being compared, this oil in a Duboscq colorimeter. The color of the standard oil procedure eliminates the inaccuracies referred to above. is determined by comparison with a Lovibond color glass, and For the purpose of orientation reference may be made to color values are expressed in terms of this primary standard, Figure 1,9 which shows the relation of the true color values being inversely proportional to the depth of the oil. Accord- to the various color systems in common use. This was obing to Beer’s law, these color units are therefore proportional tained by making up solutions of filtered cylinder stock in to the concentration of t h e coloring matter and may be taken naphtha to match each point of the A. S. T. M. color system. as a measure thereof. The color of these oils was then measured on the Tag-RobinConsiderable care must be exercised in the choice of the son colorimeter and also by the true color method, to obtain the relation between the three systems. The relation of the 1 Presented before the Division of Petroleum Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, Saybolt system was obtained by measuring the true color of 1925. Received September 30, 1925. a Saybolt color disk. Only a portion of the Saybolt range is 2 Chem. Rei!. F e l t - H a r s - I d . , 15, 80 (1908). shown on the chart; the remainder may be readily calculated 8 Engler and Albrecht, 2. angew. Chem., 36, 889 (1901); Grafe, Perroby using the value 0.0256 for the true color of a 25 color oil. Zcum Z., 3, 292 (1907). 4 Proc. A m . Phil. Soc , 36, 112 (1897); THIS JOURNAL, 1, 449 (1909). Owing to differences in the light absorptio? of the color glasses 6 J . Am. Chem. SOL.,29, 598 (1907). used in the four different color systems, an oil made up to 0 “Wissenschaftliche Grundlagen der Erdolverarbeitung,” 2nd ed., 1924, p. 337. Julius Springer, Berlin. 7 J. SOL.Chcm. Ind., 43, 179T (1924). THISJOURNAL, 14, 269 (1922).
*
9 This chart was described in a paper presented by T.H. Rogers before the Petroleum Division of the 68th Meeting of the American Chemical Society, Ithaca, N. y.,September 8 to 13, 1924.
.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
match exactly a color glass of one of the systems will be rroffll shade when compared even a t the proper depth with a color glass of each of the other three systems. Therefore, in view of what has been said above, it is apparent that a somewhat different relation between the systems will be obtained depending on the oil used for an intercomparison. The relations shown in Figure 1 are fairly exact and are reproducible when dealing with cylinder stock solutions such as was used in making the comparison; however, for general use the relations are only approximate and cannot be taken as absolute. DECOLORIZATION EXPERIMENTS-The so-called contact method of decolorization was used in all the experiments reported in this article. The oil was agitated with the adsorbent in a mechanical shaker, then filtered through a Buchner funnel and the color of the filtrate determined. The experiments were carried out at room temperature with the exception of those with wax and red oil, which required a temperature of 66” C. (150’ F.). The time of contact required to reach equilibrium between the adsorbent and oil varied between 10 and 60 minutes and was determined for each series of experiments. The adsorbents, which were finely divided materials of about 200 mesh, were dried by heating to suitable temperatures. Application of Freundlich’s Adsorption Equation
I n the development of a suitable method for comparing various adsorbents for decolorization of mineral oils, it was found that the application of Freundlich’s equationlo offered many advantages over the more common methods in general use. The equation is where z = the amount adsorbed by m grams of the adsorbent, C = equilibrium concentration, and n and K are characteristic constants. Applying this to decolorization results, z = units of color removed, ni = grams of adsorbent per 100 grams of oil, and c = equilibrium color. This gives arbitrary values depending on the units chosen, but the value of n will not 10
“Kapillarchemie,” 3rd ed , 1933, p. 232.
Leipzig.
Figure 1-Relation
165
vary provided a color scale in which the values are additive is used. Expressing this equation as log
5
rn
= log K
1 +; log c
and plotting the values of log x/m against log C, a straight line having a slope of l / n is obtained. The straight-line curves plotted in Figure 2 show that Freundlich’s adsorption equation is an accurate expression of the experimental results tabulated in Table I. Particular care was exercised in the color determinations in order to test accurately the applicability of Freundlich’s equation. A slight error in the color determination of the original oil causes a curvature in the upper portion of the curve but does not affect appreciably the lower portion. Table I-Decolorization
of Cylinder Stock Solution w i t h Clay
Grams adsorbent Equ;y;m 5 values per 100 grams solution m Adsorbent A with 260 Color Solution 5.0 204 11.2 10.0 154 10.6 20.0 84 8.8 60 8.0 25.0 33.3 37 6.7 50.0 13.6 4.93 Adsorbent D u’ith 263 Color Solution 8.7 5.0 219 7.4 10.0 189 6.6 20.0 131 22.8 6.26 120 33.3 5.3 86.5 56.5 4.4 46.8 4.19 53.3 50.0 26.2 80.0 2.96
Comparison of Various Adsorbents for Decolorization
Three widely different petroleum products were used for the comparison of the different adsorbents-namely, a straight-run kerosene, paraffin wax, and a naphtha solution of cylinder stock. The adsorbents A , B , C, and D are clays obtained from different sources, E and F are activated carbons, and G is an inorganic gel. The results are shown in Figures 3, 4, and 5 . The convenience of this method of plotting results may be emphasized, since with a minimum of tests the amount of adsorbent required for a given degree of decolorization may be calculated.
between Various Color Systems and True Color Scale
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It is to be observed that the order of decolorizing power for the clays is the same in the three cases. On the other hand, the activated carbons are effective adsorbents for the darker colored compounds contained in the heavier oil, and have but slight affinity for the coloring matter present in kerosene. Furthermore, the choice of the most suitable adsorbent for a given oil depends in some cases upon the amount of coloring matter to be removed. For example, adsorbent G is a good adsorbent for refining kerosene to a 21 Saybolt color (0.0426
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products is, on the basis of adsorption theory, of the same order of magnitude. However, the most striking feature is the marked variation in the slopes of the curves (values of l/n). The slopes increase in a regular way as one passes from darker to lighter oils. The significance of this is much more clearly seen when these curves are plotted on ordinary coordinate paper (Figure 7). In this case for comparative purposes amounts adsorbed and equilibrium concentrations are expressed in per cent of original concentrations. Only the curves of greatest and least slope are included, and for comparison there are plotted the results of Davis" on the adsorption of iodine from toluene solution by charcoal. All known cases of true adsorption have curves similar to the latter. Freundlich'? cites over twenty cases of adsorption from solution, in all of which the values of l/n are between 0.24 and 0.55, whereas the values of l / n for the oils in Figure 6 vary from 0.53 to 2.2. The slopes of the curves for cylinder stock, red oil, technical white oil, and pale oil are larger than those from most adsorption data, but are still less than one; this according to Freundlich's definition indicates adsorption. However, the decolorization of kerosene is absolutely out of that category. It is typical of adsorption that the amount adspbed is relatively greater in lower concentrations-i. e., is proportional t o a root of C-while in this case the amount adsorbed is relatively greater the more concentrated the solution-is proportional to a power of C. The practical result of this is the extreme difficulty of completely decolorizing kerosene. As a matter of fact, this is never attempted in refinery practice, whereas it is interesting to note that in other practical applications of adsorptive materials the basis for their use is the utilization of this character-
true color), but is very poor for refining the oil to 25 Saybolt color (0.0256 true color). Although the range of color is the same in kerosene and wax, the slopes of the wax curves are, with the exception of adsorbent A , less than those for kerosene, and lie between those for kerosene and for cylinder stock solution. Although variations in the solvent may account for these differences in the decolorization curves for the three products, a possible explanation would be that the nature of the coloring matter is not the same in light and heavy .oils. An alternative explanation, with a more complete discussion of the significance of the slopes of the curves, will be given in the following section. Significance of Constant l/n
As shown above, the exponential formula applies to the results of decolorization of the same oil by different amounts of an adsorbent. Since there is considerable ground for the belief that the coloring matter in different kinds of oils consists of the same type of compounds, it is interesting to compare the results of the treatment of different types of oils over a wide range of color with the same adsorbent. Figure 6 shows the results, again plotted in terms of Freundlich's equation, of the decolorization of six oils by adsorbent D. Since the solvent, or nonadsorbable portion of the oils, varies so widely, it is not surprising that such curves fail to fall on a single straight line. The various curves, taken as a whole, do very roughly approximate a single sweep. This means in a practical way that the ease of decolorization of various
istic feature of adsorption-for example, the complete removal of very slight traces of noxious gases by active charcoal. At first glance this suggests a different mechanism for the decolorization of kerosene than for dark-colored oils. I n 11
J. Chem. Sor., 91, 1671 (1907).
12
Loc. c % L ,p. 236.
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making this comparison, however, it is wise to bear in mind the assumptions that have been made in applying the exponential formula to decolorization results. When dealing with reversible adsorption of a single known substance, as is the case for the examples of typical adsorption cited, determinations are made and results are expressed in terms of actual amounts of material adsorbed, whereas in decolorization
Figure 4-Decolorization of Cylinder Stock S O l u t i O M by Various Adsorbenrs
experiments the color is used as a measure of the concentration of colored substances-probably a complex mixture of chemical individuals in colloidal13 solution. No account IS taken of other noncolored bodies adsorbed, or of the presence of colored substances formed in contact with the adsorbent, which Gurwitsch’4 has shown to be an important feature. The ratio of the color removed by adsorption t o the total amount of material adsorbed will therefore be quite variable, depending on the quality of the oil being investigated. Since a measure of only an indefinite part of the total amount adsorbed is being obtained, it is indeed surprising that the exponential formula applies so well even under certain fixed conditions. Altogether, these considerations indicate that the abnormal values of the slopes of these decolorization curves do not necessitate the conclusion that decolorization is not due to adsorption, and a continuation of the same point of view will provide a possible explanation of the variation in the values of l / n with different oils, Assuming that the amount of material formed by contact with the adsorbent and irreversibly adsorbed is constant for each gram of adsorbent, this means that a constant value should be added to the z/m values obtained by difference in color before and after treating. This would tend to diminish the slope of the logarithmic curves and bring them in harmony with true adsorption curves. Passing from the treatment of dark-colored to light-colored oils it is reasonable to expect that the amount adsorbed, as the writers measure it, is a smaller proportion of the total amount adsorbed. Thus, the ronstant value to be added n-ill be rela’3 1‘
Rakusin, Petroleum Z Lor. ~ t l . 372. ,
, 4, 922 (1908).
167
tively much larger than in the case of dark-colored oils, which accounts for the very steep slope of the kerosene curves. In line with this explanation is the fact that the curves for the decolorization of wax and technical white oil, products which are more highly refined and would be expected to contain less reactive material, are much flatter than those for oils in the same color range.
Figure %Decolorization of Paraffin Wax by Various Adsorbents
Another interesting fact brought out in Figure 6 is that the colored bodies are not homogeneously adsorbed. The curve for “special pale oil” represents the results of the decolorization of a partially decolorized pale oil. Although this “special pale oil” was obtained simply by a treatment with clay of the same pale oil whose decolorization values are shown in the curve immediately above, its x / m values do not form a continuation of the original curve, but fall much below. The partial decolorization of the original oil has rendered the oil more difficult of further decolorization, indicating a selective action of the adsorbent. Such behavior is typical of all the oils studied. Catalytic Action of Adsorbents on Color Formation in Cracked Distillates
While determining the decolorizing action of adsorbent D on a cracked kerosene, it was observed that with a short time of contact the oil was decolorized to about the same extent a5 the straight-run kerosene reported above. Howeyer, if the oil was left in contact with the clay, color was rapidly developed and finally exceeded the color of the original oil. X similar catalytic effect of bauxite was reported by Dunstan, Thole, and R e r n f r ~ . ~ Although color formation in cracked distillates i s usually considered to be due to polymerization of unsaturated compounds, no definite experimental evidence has been reported on this question. I n order t o determine whether oxygen is essential t o color formation, several comparative experi-ments were carried out with and without the presence of oxygen. This was accomplished by carefully replacing the.
‘)i
1 . I
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air in the clay and oil with an inert gas and maintaining an atmosphere of this gas during the time of contact. I n these experiments, 10 grams of clay and 150 grams of cracked kerosene ( 5 Saybolt color) were used. Four gases were investigated for their effect on color formation-air, oxygen, nitrogen, and carbon dioxide. I n each case the oil was shaken for two different periods of time, 10 minutes and 65 hours. of Various Gases o n Color Formation in Cracked Kerosene COLOROF OIL IN SAYBOLT UNITS TIME OF CONTACT Air Oxygen Nitrogen COP 10 minutes 19 1s 21 21 65 hours 12 0 21 20
Table 11-Effect
-
When an atmosphere of carbon dioxide or nitrogen is used the adsorbent improves the color from 5 to 21 Saybolt in 10 minutes and no change results after 65 hours. I n the presence
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are not comparable with those on the original oil, even if the amount of adsorbent is expressed correctly in relation to the amount of original oil. Also, it has already been s h a m that decolorization of a partially decolorized oil gives results widely different from the original oil. I n fact, two oils of the same kind, produced by the same refinery operations but differing considerably in color, will give different adsorption curves, the oil of higher original color giving greater x / m values for the same value of C. Other factors, particularly the age of the oil, which has its effect chiefly in the development of oxidized substances, also affect adsorption values. Dunstan, Thole, and Remfry have applied Freundlich’s law to decolorizing naphtha by bauxite in an entirely different manner. Naphthas of widely differing colors were prepared by treatment of the same original oil with varying amounts of bauxite. These samples were then treated with the same amount of bauxite and the amount of eolor removal determined, The x / m values were then plotted against the original colors. Although an approximately straight line is obtained on logarithmic paper, the interpretation-of this fact is meaningless, for there is no apparent justification for the unusual definition of C in Freundlich’s equation. As a matter of fact, a straight line of the same slope is obtained if one re-plots their results using equilibrium instead of original color values. However, as pointed out ab&e, decolorization tests of naphthas of different degrees of dekolorixation cannot be grouped together and, neglecting t h g possibility of differences in behavior due to differences i n t h e base oil, it seems probable that tests of tpe various naphthas, each with several ratios of adsorbent, would give rise to separate adsorption
a
Figure 6-Comparison
of Decolorization Curves for Various Petroleum Oils
of air and oxygen some color is formed in the first 10 minutes and a t the end of 65 hours the color formation is very noticeable. Although it is certain that under these conditions color formation is due to oxidation, it does not follow that polymerization does not also occur. Cnder widely different conditions, such as the vapor phase treatment with clay, it is quite probable that the reaction is entirely due to polymerization. Numerous experiments have shown that the catalytic action of clay is much more pronounced on an oil which has previously been partially decolorized. For example, the oil used in the above experiment had developed a color of from 21 to 5 Saybolt units on aging in the presence of air for about 3 months. If this oil is first decolorized to 21 color and then treated with a fresh portion of clay, the color is increased t o 6 Saybolt units in 3 hours, as compared with 12 color in 65 hours for the original oil. It is also to be noted that the adsorbents differ greatly in their catalytic action on color formation and.that the action is not proportional to their decolorizing power. Adsorbent D possesses the greatest catalytic activity, whereas adsorbents C and G are practically noncatalytic.
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