1198
INDUSTRIAL AND ENGINEERING CHEMISTRY
new process and the old process. The proportion tells us, however, that we must compare the small-scale run by the new process (n) with the small-scale run by the old process (o), to see how much difference is made by the change in process. Then we compare the small-scale run by the old process (o), with the factory run by the old process (0), to see how much difference is made by the change from smallscale to factory-scale. Here, we have only one factor varying in each comparison; hence, we can draw more reliable conclusions. A second common error is to attempt to design an entirely new commercial process with only small-scale runs as a basis, without any large-scale tests or “semiworks” runs. The dangers of doing this have often been described.2 An attempt to set up the “Small Scale to Factory Proportion” brings us to the same conclusion. In developing an entirely new commercial process we have the smallscale runs but no factory-scale results whatever. We have two terms of the proportion, but not three, and we know that three terms are necessary for the solution of a proportion. Many people think that the small-scale products ought to be the same as the factory products and, as this is seldom the case, that little can be learned by small-scale runs. The proportion n:o = N : O clearly shows us, however, that n need not equal N and that o need not equal 0. In fact, they seldom do so. 14, 2
Whiting, 8th Intern. Cong. A g p l . Chem., Section Xa, p. 204.
Vol. 18, No. 11
Limitations
Now for the limitations of this proportion. First, it must be emphasized that it gives only relative terms and that results must be expressed as “more” or “less.” Results are not strictly quantitative and the proportion is not a rigid mathematical equation, though it has been written as such for the sake of brevity and simplicity. One side is only approximately equal to the other, so that, strictly speaking, our proportion should be written n:o = approximately N :0. Another precaution to be borne in mind is that the smallscale method must involve all the essential factors of the factory method. The more nearly the small-scale formula, times, temperatures, and methods of handling approximate the large-scale factory conditions, the more reliable will this proportion be. Experience in comparing small-scale results with factory results in any given series of experiments will indicate how closely this “Small Scale to Factory Proportion” can be followed. Wherever small-scale runs are made it is necessary to get an idea of this, just as it is necessary to form an idea of the size of the experimental error in any experimentation. If small-scale conditions differ so much from factory conditions that results on a small scale vary without relation to results on a large scale, this fact should be known as soon as possible. 8uch small-scale runs are evidently actually misleading and not worth making a t all, being like work in which the experimental error is greater than the differences to be observed.
The Chemistry of Gasolines’ Particularly with Respect to Gum Formation and Discoloration By Benjamin T. Brooks 40
EAST4 1 s ~S T , N s w YORK,N. Y.
The first step in the formation of gum in cracked gasolines is the formation of organic peroxides. These break up in a complex manner, with the formation of aldehydes, ketones, including formaldehyde, water, carbon dioxide, and further oxidation to organic acids. The fluid gum observed in gasoline consists mostly of organic peroxides and fluid aldehydes and ketones; old fully oxidized resin, after expelling volatile products, is resin acids, not aldehyde condensation products. Small proportions of easily oxidizable hydrocarbons, such as diolefins, promote the formation of large proportions of gum. Oxidation of ordinary olefins with “gum” formation also occurs, but at a very much slower rate. . . .. .. .
The most common cause of the discoloration of gasolines and kerosenes is the development of a trace of acidity. Sulfur dioxide and its oxidation to sulfuric acid is the most common cause of this acidity. The known oxidation of mercaptans and alkyl disulfides to sulfonic acids may also cause discoloration. Gasolines which have not been acid-treated may develop acidity and discoloration. The function of steam in redistilling acid-treated cracked gasolines is to take up the sulfur dioxide formed during distillation. Alkalies or oil-soluble bases prevent discoloration by developed acidity.
I
of the practical value of the motor fuel, pronounced color of a gasoline, when not artificially colored, may be, and generally is, an indication of poor refining, and the complaint that color has no significance usually comes from refiners who have difficulty in producing gasolines up to the usual commercial standards. Like the question of color, gum formation in gasoline claimed public notice when cracked gasoline began to be widely marketed. The first to mention a gum test or specification was apparently E. W. Dean,2 who states that it was devised by F. C. Robinson and his associates in the Atlantic Refining Company, as a test to be applied to aviation gasoline. Dean suggests that amounts of gum up to 0.03 per cent
N T H E present paper no attempt is made to correlate
the color or gum-forming properties of gasolines with their value as motor fuel. Generally, these and other tests have been self-imposed by refiners themselves, probably to insure that their products are above any possible criticism. Practically all refiners manufacture gasoline far better with respect to color than was required by U. S. Government specifications, but since color is one of the few qualities which the ordinary consumer can easily note, this is still s, standard which most distributors prefer to maintain. While it is generally admitted that color, of itself, is not a good criterion 1 Presented under the title “The Chemistry of Gasoline, with Respect to Color, Stability, and Gum Formation” before the Division of Petroleum Chemistry a t the 71st Meeting of the American Chemical Society, Tulsa, Okla., April 6 to 9, 1926.
1
Bur. Mines, Tech. P a g e 914.
November, 1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
lene, by allowing it to stand in air in diffused daylight and removing the unchanged amylene by a vacuum, they obtained a sirupy residue containing 31.9 per cent oxygen, of which about 45 per cent was in the form of peroxide oxygen. Gum Formation Trimethyl ethylene gave a similar product containing 29 In 1918 I. W. Humphrey and the writer3 suggested that to 30 per cent oxygen. A sample of hexylene showed 7.2 “diolefins are probably the cause of the resinification which per cent oxygen after exposure to oxygen for 48 hours. That has been observed when highly cracked gasolines are per- the conjugated dienes absorb oxygen and resinify with extramitted to stand for several months,” and refining with dilute ordinary rapidity was shown by Thiele” in the case of cyclosulfuric acid of 85 to 90 per cent concentration was recom- pentadiene and the fulvenes, and cases have been noted in mended for such gasolines. In a patent applied for in 1913 which the spontaneous oxidation of such hydrocarbons was and issued in 1919, Ellis4 suggested the use of even more so rapid that the oil ignited. The fluid resin, or “gum,” which forms in cracked gasoline dilute acid for this purpose, the object being to polymerize those constituents mainly responsible for gum formation on exposure to air is normally pale honey-yellow in color, without alteration or loss of the other unsaturated hydro- if formed without the development of free mineral acid acidcarbons, a result only fully appreciated since the antiknock ity. In the presence of traces of sulfuric acid esters, or of value of the unsaturated hydrocarbons has become known. sulfur compounds capable of oxidation, the resin is dark In a later paper6 considerable evidence was brought out brown to black and shows traces of sulfuric or sulfonic acids. indicating that the volatile yellow coloring matter of freshly The peroxides present in the fluid, pale yellow, freshly distilled, highly cracked gasoline, particularly that made formed resin, even when evaporated on a steam bath, may by vapor-phase cracking a t atmospheric pressure, was due decompose suddenly and violently with the liberation of to diolefins of the conjugated type. This is indicated by the considerable heat and blackening of the resin. This is not ease of polymerizing these highly unsaturated, yellow hydro- observed when evaporating fresh distillates. After the carbon constituents, by dilute acid, by heat alone, by fuller’s decomposition of the peroxides, particularly if the reaction earth, metallic sodium, etc., and by their rapid oxidation is violent with marked rise in temperature, the resulting resin is no longer fluid but quite firm to the touch, after cooling. to form large proportions of so-called gum. Although Engler and Staudinger6 stated that the dienes Similarly, Stobbe12 found that the dimeride of cyclopentawere formed by cracking oil a t high temperatures and diene forms a diperoxide, which on warming ‘polymerized Armstrong and Miller’ isolated derivatives of butadiene or condensed’’ to a hard resin. The composition of the fluid gummy material as it separates from the light liquid condensate from oil gas, the responsibility of the dienes for gum formation in gasoline has never from cracked gasoline is quite different from the resin obyet been strictly proved. As a matter of fact, the formation tained in the usual way by evaporating until substantially of so-called gum is B general property of all olefinic hydro- no further loss in weight occurs. ALSO the composition of carbons, though, as shown in the present paper, at very the resinous deposit changes on long standing a t ordinary different rates. The literature indicates clearly the general temperatures. Thus, when rapidly produced by oxidation in direct sunlight, with free access of air, the resinous deposit character of the process. In 1922* the writer stated: contains large proportions of organic peroxides, but after The behavior of turpentine or pinene on air oxidation is, in general, typical of the behavior of the olefins, including un- long standing the organic peroxides decompose or react with saturated petroleum oils. With all such substances air oxida- other substances. Also the percentage of organic acids intion is accompanied by the formation of organic peroxides, water, creases with the period of oxidation; old samples give, on carbon dioxide, simple organic acids, resinous substances, and evaporation, a resin which is completely soluble in dilute other oxidation products among which alcohols, aldehydes, aqueous alkali. One sample of pale yellow resin, about and ketones have frequently been noted. 6 months old, contained 30 per cent of material freely soluble All these types of products can easily be detected in samples in water, about 30 per cent of neutral oil consisting chiefly of cracked gasoline which have become slightly oxidized. of aldehydes and ketones, and about 40 per cent of resin acids Smith and Cookea showed that “gum” formed by the oxida- soluble in 3 per cent aqueous alkali. The water-soluble tion of cracked gasoline by air was an oxidation product material consists mostly of the simpler aldehydes, ketones, containing as much as 18 per cent oxygen, and also obtained and organic acids. These aldehydes, ketones, and organic qualitative tests for aldehydes, though they were inclined peroxides are responsible for the fluidity of the original to regard the resins, or “gum,” as condensation products “gum,” and the former volatile products are lost in the usual of aldehydes, probably formaldehyde, but showed that phe- evaporation test. One sample of fluid gum, separated as such from the gasoline, gave only 18 per cent of resin dry nols were not present. Although all of the probable chemical reactions in this to the touch? after evaporating on a steam bath. The resin series of chemical changes have not been worked out, the or “gum” as usually obtained is only a small portion of the principal. effects have been clearly established. Practically, original fluid material which separates from the gasoline, the most important reaction in the series is the formation Also the gasoline layer of oxidized samples contains aldehydes of organic peroxides, which for some reason or other has been and ketones, including formaldehyde, acetaldehyde, propyl generally overlooked in this connection. Working with aldehyde, butyric aldehyde, and undoubtedly a very large pure hydrocarbons, Engler and Weissberg,lo in their work number of similar products. The odor of these aldehydes on “autoxidation,” isolated the peroxides of commercial and ketones is very pronounced in cracked gasolines that have amylene, trimethyl ethylene, and a hexylene. From amy- been oxidized by air, and this can readily be shown by extraction with concentrated aqueous sodium bisulfite. One * J. Am. Chem. Soc.. 40, 852 (1918). ‘U.S. Patent 1,318,061. sample of cracked gasoline, 900 cc., which was exposed to 6 Brooks and Parker, THIS JOURNAL, 16, 587 (1924). the air for two weeks in a glass flask, was thus extracted and German Patent 265.172 (1912). mixture became heated so strongly that it was necessary ’ J . Chem. SOC.(London), 49, 74 (1886);cf. also Noyes, Blinks, and the to cool it under the tap. #Onadding a slight excess of sodium Mory, J. Am. Chcm. Soc., 16, 688 (1894). 8 “The Non-Benzenoid Hydrocarbons,” New York, 1911, p. 427. carbonate to the bisulfite solution and distilling with steam, *c. A . , 16, 4332 (1922). 11 Ber., 88, 866 (1900). are permissible. The sales organizations of some companies impose on their own refining departments an upper limit of 15 mg. per 100 cc. (evaporated in a copper dish).
I
I
1199
lo B e . .
88, 1094 (1900).
‘2
Ibid., 61, 1436 (1919).
INDUSTRIAL AND ENGI NEERING CHEMISTRY
1200
18 grams of oily aldehydes and ketones were obtained. In order to follow quantitatively the series of reactions which take place in such oxidations, it is obvious that pure hydrocarbons must be studied. A small sample, 100 cc., of commercial amylene was dissolved in 200 cc. of pharmaceutical “paraffin oil” and exposed to air oxidation over a layer of water, in an open glass flask. Although most of the amylene was fost by evaporation, 4 cc. of butyl aldehyde and a small amount, not quantitatively determined, of acetone were isolated. . Formaldehyde is copiously evolved when cracked gasoline, recently exposed to air oxidation, is distilled. Also, as shown by Smith and C ~ o k eit, ~may be detected in such gasolines by extraction with water. This formaldehyde is probably formed by the decomposition of peroxides of olefins having the >C=CHZ group, as has frequently been noted in the oxidation of hydrocarbons in the terpene series. >C=CHg
+ Os->C-CHz->C=O I
0-0
+ HCHO
1
It has frequently been suggested that the resins formed in gasoline are condensation products of formaldehyde or other aldehydes, possibly with olefins. But, as shown in the present paper, formaldehyde and other aldehydes are formed in abundance and are to be regarded as one of the normal type reaction products of the oxidation of the olefins by air, and are not themselves responsible for resin formation, although the condensation of formaldehyde, and possibly other aldehydes, with olefins can be brought about under suitable conditions. Thus, Prinsl3 showed that in acetic acid solution with the addition of a small proportion of concentrated sulfuric acid, reaction occurs with formaldehyde as follows: RCH=CHR1+ HCHO-RCH-CHRi Hs0
1
RCH-CHRi
I
I
CHzOH OH
-
I
+
HzC-0 RCH=CHRi
1
-
CHzOH
In this reaction resinous reaction products are not formed unless larger proportions of sulfuric acid are employed, leading to the ill-defined products of variable composition known as the “formolite” resins of Nastjukoff. Ormandy and Craven14 also showed that on prolonged refluxing of cracked gasolines with hydrochloric acid resins are formed. (m-Xylene also resinXes fairly rapidly under these conditions.) The present writer finds, however, that the addition of formaldehyde to cracked gasoline followed by evaporation in the usual manner on a steam bath does not appreciably increase the amount of gum obtained. That the resins, misnamed ‘‘gums,” formed by the air oxidation of cracked gasolines, are not condensation products of aldehydes and other substances is shown by the following considerations : (1) The amount of resin formed, in carrying out the usual evaporation test, is not increased by the addition of formaldehyde or other simple aldehydes and ketones. (2) Energetic condensation reagents such as sulfuric acid (Nastjukoff) or hydrochloric acid (Ormandy and Craven) are necessary to produce resinous substances from formaldehyde and olefins, and even small proportions of these yield simple condensation products, such as the glycols and unsaturated alcohols, noted by Prins. (3) The initial product, or gum, formed in gasoline is a mixture of peroxides of the olefins; diolefins oxidize much more rapidly than simple olefins. The peroxides yield resin and other decomposition products, including aldehydes and ketones. (4) The g u m or resin produced by the air oxidation of turpentine contains a resin acid similar to abietic acid, in other words, not a formaldehyde condensation product.16 Further-
.
1’C. A . , 14, 1662 (1920). J . Ins:. Petroleum Tech., 10, 99 (1924).
14
Wienhaus and Schumm, Ann., 439, 20 (1924).
Vol. 18,No.11
more, the “gum” or resin formed in gasoline, after long standing, consists chiefly of resins soluble in aqueous alkali. (5) The gum deposits formed by carbureted water gas, and by the light-oil drips from such gas, consist chiefly of oxidation products and polymers of styrene and indene, as shown by Brown.16
The styrene and indene isolated from carbureted water gas “drips” by Brown are formed by cracking a t very high temperatures (1350’ to 1450’ F.) and have not been reported in cracked gasolines. Their rapid oxidation and resinification is well known and parallels the behavior of aliphatic diolefins in these respects. Although the dienes are very readily polymerized under certain conditions, as by 85 per cent sulfuric acid3*”and by fuller’s earth, the hydrocarbon polymers are readily soluble in gasoline and are quite different in composition and character from the sparingly soluble oxidized products normally deposited from gasoline on air oxidation. Note-Styrene and 85 per cent sulfuric acid gives a brittle resinous polymer. Even a trace of oxygen induces the polymerization of styrene. The aliphatic dienes react vigorously with 85 per cent acid forming black tars. Indene is polymerized by 60 per cent sulfuric acid to diindene and a solid resin melting a t about 320’.
Advantage is taken of the much greater ease with which they are polymerized, as contrasted with simple olefins, in refining with dilute sulfuric acid, and by fuller’s earth, in the liquid phase as used by Leslie and Barbre, or in the vapor phase as carried out in the Gray process.’* A very marked positive test for peroxides can be obtained in a refined commercial cracked gasoline after exposure to air and sunlight (which accelerates the oxidation) for half an hour. These peroxides are very liable to form in samples kept in glass bottles and such oxidized samples will then show a so-called “doctor” reaction with alkaline plumbite solution, which is not due to mercaptans but which very closely resembles the true mercaptan test.’g One result of air oxidation is the oxidation of mercaptans, if present, so that exposure of small samples of ordinary “sour” gasoline, containing mercaptans, to air and sunlight for about an hour is sufficient to oxidize the mercaptans to give “sweet” gasoline, negative to alkaline plumbite; further oxidation gives the lead peroxide reaction noted above. The experimental results given in Table I show that a small proportion of a readily oxidizable hydrocarbon in a cracked gasoline causes the resinscation of a relatively very large proportion of other unsaturated hydrocarbons, as carried out in the usual evaporation test. Saturated hydrocarbons, as represented by straight-run gasolines, are affected only slightly, if a t all, by the oxidation of small proportions of the dienes. This is shown by the following evaporation tests using a refined cracked gasoline containing about 40 per cent of olefins, the sample having lost about 4.5 per cent on acid treatment and redistillation, and which showed after refining only 6 to 7 mg. of gum. Table I-In0uence Expt. 1 2 3 4
5
of Easily Oxidized Hydrocarbons o n G u m
Formation Mg./100 Gum Gasoline 46 Straight-run gasoline 2 per cent limonene 634 Refined cracked gasoline 2 per cent limonene Refined cracked gasoline alone 7 5 per cent isoprene and exposed to Straight-run gasoline light 48 hours 52 5 per cent isoprene and exposed Refined cracked gasoline 521 to light 48 hours
CC.
+ +
+
+
Gas Age-Record, 50, 571 (1922). Stobbe and Farber, Be?., 67, 1838 (1924). 18 Treatment of liquid gasoline with fuller’s earth greatly increases the gum obtained in the evaporation test owing to the more prolonged oxidation of the high-boiling oily polymers. Cf. the patents of Leslie and Barbre, U. S. Patent 1,337,523 (1920); Hall, C. A . , 14, 994 (1918);Gray, U. S. Patent 1,340,889(1920). 1) Brooks, THISJOURNAL, 16, 588 (1924). 16
17
iovember, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The following series of results, obtained by evaporating 100 cc. of the freshly redistilled oils on a steam bath in a copper dish in the usual manner, indicates the approximate proportion between the rate of oxidation, or gum formation, and the number of double bonds in the molecule, these hydrocarbons being selected on account of their boiling points *beingvery close together and their chemical structure known, with exception of the light hydrocarbon distillate obtained by the destructive distillation of rubber. The fraction of the latter distilled a t 124' to 340' F. (51' to 171' C.). between G u m Formation a n d Number of Olefin Groups Boiling point N o of olefin Gum C. bonds Mg /lo0 cc. Hydrocarbon Pinene 156 to 157 1 940 Limonene 2 1960 175 to 176 Rubber distillate 51 to 171 2660 Mvrcene" 3960 166 t o 169 These gums were very soft. The evaporation test for gums, as is well known, gives very inaccurate results when large amounts of gum are dormed. T a b l e 11-Relation
i
The addition of various alcohols, aldehydes, and ketones ,does not appreciably affect the amount of gum formed in the evaporation test. Formaldehyde, acetone, mesitylene, butyl aldehyde, and amyl alcohol were tried, with negative results or with differences which were too small to be significant. 2o It is believed that if peroxides were the initial product and t h a t the gum or resin was formed directly from the peroxides the addition of water, which is known to hydrolyze organic peroxides, should show a marked reduction in the amount of gum obtained, as is confirmed by the following: Gum Mg.
Untreated cracked gasoline 'Untreated cracked gasoline evaporated with 25 cc. of water Refined cracked gasoline after 6 hours' exposure to light and air Refined cracked gasoline exposed with water
463 120 1002 625
The amounts of peroxide oxygen in two samples of refined cracked gasoline after 6 hours' standing in sunlight and exposed to air in Erlenmeyer flasks, partially filled, one with water added to cover the bottom and the other dry, were determined by ascertaining the amount of iodine liberated from dilute hydriodic acid: Dry gasoline after 6 hours' exposure Moist gasoline after 6 hours' exposure
Peroxide oxygen (:rams/liter 1.856 0.384
The pronounced effect of a relatively small proportion of easily oxidized hydrocarbon in promoting gum formation in cracked gasolines may have several explanations. First, it is well known that organic peroxides induce polymerization of unsaturated hydrocarbons, particularly of such sensitive hydrocarbons as styrene and the dienes. From 0.5 to 1.0 gram benzoyl peroxide is said to be sufficient to induce the polymerization of 1 kg. of vinyl chloroacetate. The formation of higher boiling polymers, if not removed, naturally gives higher gum results in the evaporation test, since the less volatile polymers are subjected to much more prolonged oxidation. Also, the peroxides themselves may react with olefins, in cold neutral solvents, as has been shown by Prileshajew,*l to form addition products which may be readily decomposed to give alkylene oxides. Such oxides are readily converted by moisture and a trace of acid to aldehydes and ketones, which may be the mechanism of the formation of the aldehydes and ketones found in the oxidized oils. The literature is so definite with regard to the principal 2 0 The common requirement for aldehyde condensations is the presence of an alkali. Unsaturated ketones may be resinified by alkalies, as the dibenzylidene acetone, CsHs. CH=CH. CO. CH=CH. CsHb, derivatives studied by Herzog and Kreidl, Z. angew. Chcm. 81, 465 (1922). 11 B n . , 49, 4812 (1909);48, 609 (1911); 44, 613 (1912).
1201
reactions involved that no attempt has been made in the present investigation to follow this series of reactions with pure hydrocarbons and peroxides. More resin is formed from the higher boiling fractions than from the lighter ones. Since the less volatile fractions are exposed to air oxidation over a longer evaporation time, it was expected that they would show much more resin in the evaporation test. Thus, an unrefined commercial cracked gasoline made by pressure distillation showed increasing amounts of resin, from 0.005 gram per 100 cc. in the first fraction distilling to 110' C. (230" F.) to 0.365 gram in the fraction 170' to 195' C. (338-383' F.). A sample of cracked gasoline made by vapor-phase cracking showed 0.106 gram of resin in the corresponding low-boiling fraction and 1.980 gram in the corresponding high-boiling fraction. A series of these fractions (five successive fractiom) of the unrefined vapor-phase gasoline was exposed for two weeks in open Erlenmeyer flasks, in direct sunlight. The first fraction then showed no visible separation of fluid resin mixture, the second a small amount, the next a larger amount, and much the largest amount of separated resinous material in the highest boiling fraction. When it was attempted to evaporate these fractions in evaporating dishes on a steam bath, violent and almost explosive decomposition occurred with the first two fractions, vitiating the quantitative results, but also showing that in such cases the gasoline layer may also contain substantial proportions of peroxides. Relation between Gum Formation and Color of Freshly Distilled Gasolines
Reference has been made5 to the evidence that the volatile yellow coloring matter in highly unsaturated and freshly distilled gasolines is probably due to dienes. In the paper referred to it was shown that a number of well-known methods of polymerizing the dienes, as developed in the researches on synthetic rubber, also polymerize the dienes in highly cracked yellow gasolines. This may be accomplished, as in the case of isoprene and dimethylbutadiene, by moderate heat alone, under pressure. There is some indication that in the fractional distillation of cracked gasolines, under pressure, in which cases the temperatures are higher than in atmospheric pressure distillation, polymerization of these highly unsaturated yellow hydrocarbons occurs, just as in the autoclave method. An interesting confirmation of this was noted in two fractionating towers operating under pressure, on two units of the same cracking process, cracking the same oil under the same conditions. One tower was much larger than the other, carried more liquid on the plates, and required a much longer time for the vapors to work through it. Both towers were operated to give gasoline of the same end point and distillation range, but the larger tower gave gasoline of about 25 Saybolt color and the smaller tower yielded gasoline averaging about 10 Saybolt color. Redistillation in the laboratory showed that the color of the latter was not due to entrainment of higher boiling material. The evaporation tests for resin of these two gasolines, without further refining, showed 0.190 gram for the one of 25 color and 536 grams for the yellow sample. Discoloration of Petroleum Distillates
The findings noted in the present paper relate to the discoloration of gasolines and kerosenes, but the causes noted in the case of these oils may also be important factors in the discoloration of lubricating oil distillates. Air oxidation is often regarded as the cause of the discoloration of light distillates, probably on account of the frequency with which pronounced discoloration accompanies oxidation and gum formation. According to this view the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1202
discoloration is due to nonvolatile oxidation products of an asphaltic nature. That air oxidation alone is not a cause of discoloration is shown fist by the fact that pure hydrocarbons, either saturated or olefinic, may be oxidized by air to a considerable extent without appreciable discoloration. The writer has frequently observed well-rehed specimens of cracked gasoline which, on standing in the light exposed to air until a distinct layer of pale honey-yellow “gum” had separated, still showed good color, 23 to 24 on the scale. To test this point further a sample of commercially cracked gasoline was carefully refined with 3 per cent by volume of 90 per cent sulfuric acid, redistilled with steam, and washed with alkali. After this treatment the gasoline contained about 30 per cent of olefinic hydrocarbons, was better than 25 in color, and showed no color change after 3 days’ exposure to direct sunlight. The sample then gave about 5 mg. of gum in the copper dish test and showed no blackening or corrosion of the copper. [On distilling the original gasoline from the pressure distillate, in the laboratory, the cut was made at 401” F. (205’ C.).] Substantial amounts of various substances chemically typical of all the known oxidation products of unsaturated hydrocarbons were then added to as many different Counce samples of the gasoline and, together with a sample of the gasoline alone, with nothing added, were exposed to direct sunlight for 10 days, the cork stoppers being vented to the air. The results show that the addition of none of the type oxidation products caused discoloration to an extent greater than two points in the scale-i. e., to 23 color at the end of the test. Table 111-Color
Stability Tests on Refined Cracked Gasoline
Expt. Substance added 1 Nothinn 2 Nothing 3 Butyl aldehyde 4 Benzaldehyde 5 Acetone 6 Acetic acid 7 Benzoyl peroxide 8 Water
9
Ammonia, aqua
10 Soda lime
il
...
Color after 10 days Remarks 25 Sealed
4 ‘drOps
3 drops 2 cc. 4 drops 0.05 gram 2 CC.
23 23 24
2
24
CC.
0.5 0.02
11 Sulfur 12 Sulfur benzoyl peroxjde 13 Hydrochlonc acid, aqua
E:1;
14 Sulfur dioxide
0.003 gram
+
\,
Amount
CC.
I
25 12 12
.. ..
Stopper vented Water layer colorless Aqueous layer very slightly yellow
Color 15 after 3 hours Color 18 after
hour
4 These tests indicate that organic peroxides, aldehydes, and ketones have nothing to do with discoloration. The discoloration caused by sulfur was pronounced but was not increased by the addition of the peroxide. This discoloration is quite independent of oxidation, as was shown by sealing in glass a sample of the gasoline containing sulfur, expelling the air by light gasoline vapor before sealing; this sealed sample showed the same rate of discoloration as numbers 11 and 12 in Table 111. In the case of sulfur no deposit was formed. The most striking case was the sample containing sulfur dioxide (14). I n fact, the small proportion of sulfur dioxide employed in this case was arrived a t by gradually diminishing the amount of sulfur dioxide added. Noh-The amount of sulfur dioxide added was controlled by making a “standard” solution of sulfur dioxide in refined straight-run gasoline, and adding a small proportion of this to 100 cc. of the cracked gasoline. On extracting kerosene with liquid sulfur dioxide the SO*-insoluble portion may not be discolored if allowed to stand for some time without removing the sulfur dioxide; the extracted portion containing the olefins is very quickly and strongly discolored.
Another significant fact in connection with sulfur dioxide was noted-i. e., freshly distilled, unoxidized samples of cracked gasoline could be treated with sulfur dioxide with only slight discoloration if kept out of contact with the air,
Vol. 18, NO. I
but portions of the same samples quickly discolored on exposure to air and after a few hours showed a tarry deposit. If a trace of sulfur dioxide is added to a slightly oxidized cracked gasoline containing the organic peroxides always formed by air oxidation, the discoloration is immediate. The sulfur dioxide is probably oxidized to sulfuric acid under these conditions, the discoloration and tar deposits then resulting according to the well-known effect of this reagent. In the paper with Humphrey,3 it was shown that on treating cracked gasoline with concentrated sulfuric acid, small RO >SOz, proportions of neutral dialkyl sulfates of the type R1O are formed, which are not removed by washing with aqueous alkali and which decompose on standing, causing discoloration and the formation of a tarry deposit containing sulfuridacid. The necessity for redistilling cracked gasolines that have been treated with concentrated sulfuric acid in order to produce color-stable gasoline is well known, Sulfuric acid of about 60 per cent strength does not form such neutral, oil-soluble dialkyl sulfates, which accounts for the greater color stability of cracked gasoline which has been so treated. The distillation of acid-treated cracked gasoline with an alkali also improves the initial color and color stability of the gasoline distillatelZ2but the almost universal practice in redistilling such gasoline is with steam and fire, or more rarely, with steam alone. When such gasolines are redistilled by fire alone, without steam, discolored distillates are obtained, particularly in the heavier ends. The prevailing opinion is that the function of the steam is to lower the distillation temperature, which of course it does, and that this is the reason for the better color of the distillates so produced. The use of steam in distilling lubricating oils certainly reduces the cracking of such heavy oils, by lowering the still temperatures and promoting vigorous circulation of the oil in the still, but gasoline hydrocarbons are not appreciably cracked a t the temperatures required for their -distillation and, if pure, can be drydistilled without any discoloration or subsequent color instability. In the opinion of the writer the cause of the color in the distillate is solely the sulfur dioxide formed during the distillation. The evolution of sulfur dioxide during the distillation begins to be quite noticeable at about 107” (225O F.), and the decomposition of the dialkyl sulfates is not prevented by the introduction of the steam. The function of the steam appears to be merely a means of carrying off the sulfur dioxide with the condensation water. Accordingly, the use of a water spray in the vapor line leaving the still or in the top of the condenser should serve the same purpose, and experimental trial confirms this. On a small scale in the laboratory a water spray could not be used on account of the small amount of water required, but parallel experiments on portions of cracked pressure distillate treated with sulfuric acid, 4 pounds of acid per barrel, and washed in the usual manner, were carried out as follows: the first portion was dry-distilled to 210’ C. (410” F.), collecting 60 per cent as gasoline distillate; the second portion was distilled with fire and steam until 60 per cent gasoline distillate was obtained, the steam being passed through the gasoline, the condensed water being about 20 per cent of the volume of the oil distillate; the third portion was fire-distilled and the steam admitted into the top of the condenser, using the same amount of steam as in the second portion. The first distillate was about 22 color as distilled and quickly darkened and after 24 hours had formed a distinct tarry deposit, after the usual behavior of such distillates; but the second and third distillates were 35+ color as produced and after standing exposed to light m a south window for 3 months both
c.
29
Brooks, U. S. Patent 1,563,012 (1925).
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November, 1926
I N D U S T R I A L AND ENGINEERING CHEMIBTRY
1203
had discolored to the same degree, showing about 21 color. ammonia, though preserving the color, causes the precipitaThe use of a solution of caustic soda in place of the water tion of a slight cloud which after 3 or 4 days settles out as spray would perhaps be a further improvement. Ammonia a noticeable deposit. On examination, these deposits proved has been used in this wav with the obiect of reducing the acid to be ammonium salts. corrosion of condensers,”and beneficial results in the quality Discoloration without the Development of Acidity of the gasoline have also been noted. Although the liberation of sulfur dioxide during the reThat free sulfur will cause discoloration of gasolines without distillation of acid-treated cracked gasolines has been gen- oxidation or the development of acidity has already been erally noted, the small proportion of sulfur dioxide which mentioned. Heat or direct sunlight accelerates this change. can cause pronounced discoloration of cracked gasolines has However, in the number of cases which have come to the not been appreciated, nor has any relation been shown be- writer’s attention this may account for perhaps 5 per cent tween discoloration caused in this way and other cases of of the total. Although it may be recalled that some pure discoloration of finished distillates. substances are rapidly discolored by air oxidation, in the The examination of over thirty samples of gasolines which absence of any impurities which could produce highly acid became appreciably discolored after exposure to sunlight products, such as furfural, some of the nitrogen bases and the for 2 days then showed distinct acidity to a few drops of dilute phenols, their presence in finished refined gasoline is prscmethyl orange. These samples included both straight-run tically impossible. The writer has isolated phenols and niand cracked gasolines, refined in a variety of ways. One trogen bases from several cracked gasolines including gasosample of straight-run gasoline which has been treated lines made from mid-continent oils, and the presence of these only with alkaline plumbite solution, without the addition impurities in unrefined cracked gasolines in very small proof sulfur, changed from 25 color to about 20 color after portions is much more general than is commonly supposed. 3 days’ exposure to sunlight and then showed a distinct The nearly spent alkali used in treating cracked gasoline acidity to methyl orange. It is a common observation .with alkaline plumbite can usually be shown to contain that very “sour” gasolines hold their color better than after phenols, and where dilute acid is employed on cracked oils, treating with alkaline plumbite. It is general experience nitrogen bases can usually be recovered from them. HOWthat the addition of an excess of sulfur, when treating with ever, these impurities are easily removed substantially alkaline plumbite causes color instability and, as shown above, completely in the normal refining processes. free sulfur alone causes discoloration in the absence of oxidation. However, developed acidity appears to be a much more common cause of discoloration. The more rapid development of discoloration and acidity in cases where no sulfur is added, and the gasoline is free from elementary sulfur, is probably to be explained by the fact that the conversion of the mercaptans to disulfides, as suggested by Wendt and By G. S. Hiers D i g g ~ , is * ~an oxidation and may be considered as being the first step in the oxidations which finally result in acidity, UNIVER3ITY OF ILLLINOIS, U R B A N A , ILL. probably in the form of sulfonic acids. In oxidation by alkaline hvmchlorite solutions. as shown bv Birch and N o r r i ~ . ~ ~ mercaptans are oxidized t; sulfonic acids, if sufficient hypoN THE course of an inchlorite is employed; the first result in both cases is the oxidavestigation requiring a tion of mercaptans to disulfides. That both mercaptans and sealed mechanical stirrer in --D alkyl disulfides are readily oxidized by air to sulfonic acids a small set-up where comhas long been known.25 It is frequentIy noted that the pactness was demanded, the discoloration of gasolines and kerosenes is preceded by the arrangement indicated in formation of a slight cloud in the oil, as though due to a the accompanying figure trace of moisture, and in large samples a very small aqueous suggested itself. The usual layer may actually separate. Though water is one of the type of stirrer demands at products of oxidation by air (autoxidation at ordinary temperatures), this separation of moisture preceding dis- least three points of support coloration is undoubtedly due to the fact that such oils are -i. e., a t the flask I, at a steam-distilled or washed and are saturated with moisture point such as G, and a t the and, the solubility of water in such oils being extremely usual bearing which is insmall, the development of a slight acidity, sulfonic acids, serted just below the pulley, sulfur dioxide, or sulfuric acid, takes up this trace of dis- B. The modification suggested uses a cork or rubsolved water to form the observed cloud. I n cases where discoloration is due to dewloped acidity ber stopper which fits snugly the addition of a base, preferably an oil-soluble base, should into the glass tube G and prevent discoloration. This expectation was borne out in which c a r r i e s a glass or a large number of cases in which the finished gasoline was metal tube, E, of the proper treated with dry ammonia gas. Although very small propor- size to make a good beartions of ammonia are usually effective, sometimes as little ing. In this way only two as 0.1 gram per gallon, the odor of ammonia is noticeable in supports are required to the gasoline and the gas is gradually lost under the usual insure rigidity-at I and G. conditions of storage. I n cases where the discoloration and Such a device may be used A-Stirrer shaft; B-Pulley C- Washer the developed acidity were very pronounced, stabilizing by in set-ups where the flask is &--Cork or rubber stoppers E-Glass or metal tubes for bear28 THIS JOURNAL, 16, 113 (1924). as large as 2 liters. ings
AFModified Liquid Sealed Mechanical Stirrer’
k I
’
J . chcm. SOC. (tondon), 117, 1934 (1925). Houben-Weyl, ”Die Methoden der Organishen Chemie,” 2nd ed., 1914, v ~ i 11, . p. 162. 24
Y
1
Received August 2, 1926.
P G-Tubing
k-Mercury liquid;
of proper size or other sealing
2-Flask
