Inhibitors in Cracked Gasoline I. Relation of Structure t o Inhibiting Effectiveness GUSTAVEGLOFF, J. C. RIORRELL,C. D. LOWRY, JR., AKD C. G. DRYER Universal Oil Products Company, Chicago, Ill. The testing of a large number of substances by than one hydroxyl or amine group; a n aromatic a n accelerated oxidation test has demonsfrated that hydroxyl and amine group: a single aromatic hymany will increase !he induction period qf gasoline. droxyl or amine group with one or more alkyl or Sforage tests indicate that substances doing this will other modifying groups; a n aromatic secondary also check gasoline deterioraiion f o r a considerable amine grouping. ,Wodifying groups in amines and period. DeJinite relationship has been demonstrated phenols were most effective in the ortho or para between structure and inhibiting effectiveness. position, but exerted some benejcial influence when The substances found to hare pronounced in- placed meta to the inhibiting group. hibiting properties were those containing phenolic Some compounds of other types-nitro comor aromatic amine groupings. Howerer, a single pounds, halogen compounds, ethers, etc.-were efhydroxyl or amine grouping conferred but slight fective when they contained a n aromatic amine or inhibiting action. Most of the effective compounds hydroxyl group. Inhibitors were found to reduce contained one of the following groupings: more the amount of g u m deposited in the copper dish test.
T
H E increased demand in recent years for antiknock motor fuel has made the problem of gasoline stability, as to color, gum, and knock rating, of great importance. Some cracked gasolines of high quality, which are satisfactory for use when produced, deteriorate in storage, so that they become dark in color, high in gum, and of lessened knock rating. The present studies are directed toward finding substances which will prevent depreciation of gasoline quality. I n commercial practice gasolines must often be kept in refineries or bulk stations for 6 months or longer. Many motor cars, moreover, have long periods of disuse. It is obvious that tendency to change during such storage is harmful. Loss of antiknock value would seriously lower the market value of a gasoline. Gum in gasoline is likely to deposit in the induction system of the motor, interfere with the operation of the carburetor and intake valves, and even prevent car operation. Drastic chemical treatment, as a rule, will stabilize cracked gasoline. But chemical treatment is costly, by reason of the losses it entails, particularly in gasolines of high knock rating, and by its destruction or degrading of components having valuable antiknock properties. Chemical treatment is, therefore, undesirable and should be minimized or avoided altogether. Among the many factors which influence the rate of deterioration of stored gasoline, the most important is oxygen. Gasoline out of contact with air changes but slowly. Data on samples of a Pennsylvania cracked gasoline, acid-treated in the laboratory, stored in contact with air, and under nitrogen, are as follows:
Further evidence of the part played by oxidation in gasoline deterioration is afforded by the change which occurs in gasoline when exposed to accelerated oxidation. The gasoline referred to in Table I, during exposure for 4 hours to oxygen a t 100’ C. and 100 pounds per square inch (7 kg. per sq. cm.) pressure, deteriorated in color, gum, and octane number to the following values: Octane number Color, Saybolt Gum (air jet), mg./100 cc.
47
-3
636
I n commercial gasoline storage, contact with oxygen is guarded against in many cases, but air is usually not entirely absent even where special precautions are taken. Gasoline can be effectively protected against deterioration due to oxidation by the addition of antioxidants or inhibitors. This work presents a comparison of the inhibiting effectiveness of varied type compounds, and a correlation of chemical structure with inhibiting action.
PREVIOUS LITERATURE While much has been published on the use of oxidation inhibitors in other fields, the literature on their application to gasoline is scant. Egloff, Morrell, and Faragher (4) studied a number of inhibitors to protect gasoline color and prevent increase in gum and drop in knock rating. Flood, Hladky, and Edgar (5) showed the effect of an inhibitor on the oxidation of pure hydrocarbons. Mardles and Moss (9) checked gum formation by the use of phenol, thymol, and a few other substances. Wagner and Hyman (17) reported gum inhibition by hydroquinone. Gum difficulties are also encountered in benzene refining. TABLEI. EFFECTOF AIR ON GASOLINE DETEHIORATIONHoffert and Claxton (6) found that tricresol (their preferred UNDERAIR UNDERNITROQEN inhibitor), a number of other phenols, amines, nitro comORIOINAL 0 M o m m 6 months 12 months pounds, and pyridine acted as inhibitors of gum in motor Octane number 71 40 05 67 benzene, whereas sulfur dioxide, thiophenol, and alkyl sulfates Color Saybolt 30 2 28 28 Gum ‘mg./100 cc.: accelerated resinification. Cipper dish 33 002 165 138 Air j e t
8
028
10
12
In contact with air, deterioration was extensive but was markedly less under nitrogen.
METHODS OF GUMDETERMINATION Before discussing the experimental work, it is desirable to consider what is meant by “gum” in gasoline and the sig-
1375
1376
INDUSTRIAL AND ENGINEERING CHEMISTRY
nificance of current tests. Gum may signify either nonvolatile organic material in gasoline, which is more precisely termed “preformed” or “actual” gum, or it may signify sensitive hydrocarbons, initially volatile, which, during the execution of a test or in storage, change in character and become non-
FIGURE1.
Vol. 24, No. 12
the deposit in the inlet passage . . . ” The gum that collected on the intake system in their tests amounted to 8 to 22 per cent of the total preformed gum in the fuel. Hunn, Fischer, and Blackwood (‘7) also state that the preformed gum is the only criterion of the amount of deposit a
STAINLESS-STEEL 2.25-INCH BOMB
volatile. This is termed “potential” gum. However, as there are no universally accepted tests differentiating these two types of gum, investigators do not always agree as to just how much of the residue obtained from a gasoline sample represents actual and how much potential gum. I n the copper dish method of gum determination, catalysis by the copper occurs, converting potential into actual gum, so that the amount of gum found is larger than in methods where catalytic action is unlikely. Moreover, the copper dish method is influenced by the rate of evaporation and may give low results with gasolines of low end point. The amount of gum found by this method also depends on the degree of exposure of the evaporating sample to air. To eliminate catalysis and contact with air, Cooke (3) and Voorhees and Eisinger (16) determined preformed gum in gasoline by evaporation in glass in an atmosphere of steam. This steam oven test, however, is somewhat time-consuming, and Mardles and Moss (9) believe results obtained by i t are not always trustworthy, since the gum is not inert to steam. Evaporation in a glass dish in the open usually gives somewhat higher values than use of the steam oven, owing to conversion of potential to actual gum during the considerable period required for evaporation. A figure not greatly above the steam oven value is usually obtained if the time of evaporation is reduced by impinging a jet of air on the surface of the gasoline (Littlejohn, Thomas, and Thompson, 8). A method of determining preformed gum by instantaneous evaporation has been proposed by Norris and Thole (14) in which the gasoline is dropped into a heated spiral. It is generally agreed that only the preformed gum, and not all of that, will deposit in the induction system of a motor. The potential gum is of little significance with reference to the motor use of the gasoline a t the time of test, although it indicates to some degree the stability of the gasoline in storage. Voorhees and Eisinger (16)reported that a gasoline showing 712 mg. of gum per 100 cc. by copper dish, or 8 mg. by steam oven, gave no gum deposit in a motor. They state that “an evident relation exists between actual, or preformed gum, and
gasoline will form in the intake system of a motor, and that potential gum is not transformed in an appreciable degree into actual gum during passage through the manifold. They found that a gasoline containing 10 mg. of gum per 100 cc. by the air-jet evaporation method was satisfactory in a Buick engine, while a sample showing 25 mg. caused loss of power after 3000 miles (4828 km.)and in this running deposited 88 mg. of resinous material per gallon (23 mg. per liter) of gasoline in the induction system of the motor. Hoffert and Claxton (6) make similar statements with regard to gum. They found the deposit in the induction system of a motor proportional to the dissolved gum (determined by glass dish evaporation). “Little if any resinification of the volatile unsaturates would take place under normal conditions in an engine.” KO test for preformed gum gives any indication of the behavior of gasoline in storage or the amount of gum it would deposit if used a t a time after the test is made. As an indication of storage stability, resistance of gasoline to accelerated oxidation appears the best available test. The time before measurable absorption of oxygen begins in the oxygen bomb test is assumed to be an approximate measure of the length of time the gasoline will be stable in storage. No definite relationship between the bomb test and storage stability has been demonstrated as yet, and further work is required to clarify this point. The most commonly used oxidation test wa8 devised by Hunn, Fischer, and Blackwood (7). The effects of variables in the test were studied by Ramsey (16),and attempts to correlate test results with storage stability were made by Aldrich and Robie (2).
APPARATUS AND METHODS IN PRESENT WORK I n the present work, gum was determined by copper dish and by evaporation in porcelain in a current of air (air jet method). The copper dish method is widely used in the oil industry and is a fair measure of total gum, both potential and preformed. The air jet determination shows approximately the preformed gum, and the values obtained are con-
December, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
sidered by those who have published data on t h k subject to be related to the amount of residue the gasoline would deposit in a n engine if used a t the time of the test. The test was carried out according to the directions of Hunn, Fischer, and Blackwood with due regard to the effects of variables in the test found by Bridgernan and Aldrich ( 2 ) . The results obtained in this way, according to the work of RIolitor ( I I ) , are somewhat higher than would be obtained in steam oven evaporation. I n the accelerated oxidation test, apparatus built in this laboratory similar to that of Hunn, Fischer, and Blackwood was used. The bombs differ in some respects from theirs and from other published designs, and as they are well adapted to rapid manipulation, a brief description is given, with diagrams (Figures l and 2) and a photograph (Figure 3). The bomb is of the flange t e with ground joint between head and body of the bomb. was adopted because it could be used without a gasket and was thought less likelv to leak than a bomb having a screw cap. It ii large enough to accommodate an 8-ounce oil sample bottle, which allows use of sufFjcient gasoline for analysis to be made after, ex osure to oxygen. The bomb is machined from 41/&ch 51.4-cm.) stainless steel. (Some have been made from cold-rolled steel and chromium plated.) On the bottom of each bomb is a half-inch (1.3-cm.) projection extending across the face. This projection fits into a slotted bar, also of stainless steel, withm the copper water bath in which the bomb is mounted, arid holds it in place. This eliminates the necessity of removing the bomb from the bath when the head is removed. In bombs of this type, introduction of oxygen and ronnection to the pressure gage are usually made through the head of the bomb. It is preferable to make connections through the flange. A hole is drilled part way through the lower portion of the flange, tapped to take a quarter-inch (0.6-cm.) brass nip le, and continued to the interior of the bomb witi a diameter of 3/32 inch (0.24 cm.). In this way loosening of connections during the use of the bomb is avoided. In the present set-up, four bombs are placed in line in each water bath. Connection to each bomb is made by a brass nipple, from which a brass pipe runs to the reducing valve on the oxygen tank. Steel-stem brass needle valves are used in the oxygen line, as they have been found superior to brass needle valves in eliminating leaks, The reducing valve bears a gas release set a t 150 pounds per square inch (10.5 kg. per sq. cm.). Connection to the fourpoint recording pressure gage is made with highpressure cop er tubing. Use of the gage avoids the need of watchng the bombs and taking readings during the tests, and provides an accurate permanent record. In making a test, 200 cc. of gasoline in an open 8ounce bottle are placed in the bomb, the head put in position, and the bolts tightened. Oxygen is introduced to 100 pounds per square inch (7 kg. per sq. cm.) pressure and released to clear the system of nitrogen originally present. This purging is repeated, and oxygenintroduced to 110 pounds (7.7 kg.). When the bomb and contents are at the temperature of the cold water bath (about 18" C.) the pressure is reduced to exactly 1OOpounds(7kg.). While Aldrich and Robie (1) use an ice bath this appears disadvanta eous, as it delays heating the bomb. &teamis then turned on to feat the bath to 100' C . As the temperature increases, the pressure rises, reaches a maximum, and continues near this maximum for a shorter or longer period of time, de ending upon the stability of the gasoline. The test is continued 4 hours or until a break in the curve is noted. At the end, the steam is shut off and the bomb cooled slowly, running a small stream of cold water into the bath. Rapid cooling of the bombs, as suggested by Zublin ( I @ , is not favored, as this is likely to cause gasoline to distil out of the bottle into the bomb. When the bombs have cooled to room temperature, the samples are removed. The period from slightly before attainment of maximum pressure (approximately 15 minutes from the beginning of heating) until more than a slight drop in pressure takes place (usually a sharp break in the pressure curve occurs) is recorded as the mduotion period. Aldrich and Robie (1) have developed a means of
TE
1377
calculating a correction for the time in which a bomb is acquiring maximum temperature. The bombs used in the present investigation heat more rapidly than theirs, however, and it is believed that the subtraction of about 15 minutes from the period of test is a sufficientlyaccurate allowance for general use.
GASOLIKE USED Two Pennsylvania cracked gasolines from different stocks were used in this study. One was untreated, and the other was treated in the refinery with fuller's earth in the vapor phase. Their properties were as follows: UNTREATED
Color
19
C:pper dish Air jet Octane number Induction period (oxygen bomb), min.
85 4 77 15
Gum mg./100 cc.:
TREATED WITH FULLER'S EARTH 30 163 0 73 45
A 0. T . M. 1W-CC. DISTILLATION
The gasolines were shipped in drums direct from the refinery. On arrival at the laboratory, excess pressure was re-
P L A N Y/€W
FRONT E&P!T
/ON
FIGURE2. ACCELERATED GUM TESTAPPARATUS leased, and the gas space filled with nitrogen, so that until used the distillates were out of contact with air. Samples were taken by forcing gasoline out of the drum with nitrogen. Successive drums of the treated gasoline showed almost exactly the same induction period, when tested both with and without inhibitors. The two gasolines differed considerably in their response to inhibitors, as shown by the following:
Gasoline done Inhibitors (0.01%): Catechol Hydroquinone
INDUCTION PERIOD Treated with fiiller's earth Min. 45
Untreated Min. 15 260 55
2400 ~...
86
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I N D U S T R I A L A N D E N G I N E E R I N G C H E M I ST R Y
CORRELATION OF STORAGE AKD BOMB TESTS In the selection of inhibitors the accelerated oxidation test is of value only if substances prolonging the induction period of unstable gasoline in the oxygen bomb are able to prevent deterioration of gasoline in storage. To make sure this was the case, a number of inhibitors which had been tested in the oxygen bomb were added to samples of the untreated gasoline and stored for 6 months in glass bottles, with opportunity for contact with air. The uninhibited gasoline, as shown in Table 11, deteriorated considerably during this period of storage, whereas the samples protected by inhibitors suffered little depreciation. TABLE11. STORAGE TESTS WITH INHIBITORS GUM Copper Air OCTANE IXDUCTION COLOR dish jet No. PERIOD M e . per 100 cc. Min
.
Gasoline alone (untreated): Initial properties 19 After 6 months 11 Inhibitors (0.01%): Catechol: 20 Initial 6 months 22 1,6-Dihydroxynaphthalene: Initial 17 6 months 16 Pyrogallol: Initial 18 6 months 21 p- Aminophenol: Initial 13 months Hydroquinone: Initial 21 6 months 21 2-Amino-6-hydroxytoluene: Initial 18 6 month 14 p-p’-Diaminodiphenylamine : 19 Initial Brown 6 months a-Naphthol: 19 Initial 16 6 months
‘3
’
85 842
4 538
77 65
...
14 7
2 12
77 75
Over 240
9 10
11
6
77 75
Over 240
4 7
8
0
77 75
Over 240
109
6
0 6
77 75
Over 240
2 5
0 6
77 75
55
4 8
2 4
77 75
Over 240
15
15
12
6
77 75
Over 240
14 12
6 2
77 74
Over 240
6
In general, the bomb test enables one t o distinguish substances effective from those not effective as inhibitors. It is not assumed that proportionality exists between the induction period in the bomb and the time during which an inhibitor will protect gasoline in storage. It is seen in Table I1 that hydroquinone, for example, gave a comparatively short bomb induction period, yet it protected gasoline for 6 months and might have been effective for a longer period. Some of the compounds to be tested could not be dissolved in gasoline in the desired concentration. A sparingly soluble substance was dissolved in a small amount of an organic solvent, and the solution added to the gasoline. The solvents used were absolute methanol, absolute ethyl alcohol, acetone, and benzene. Comparative tests of inhibitors with and without solvent indicated that the presence of benzene did not affect the induction period. This solvent was therefore used wherever possible. The others mentioned made no change in the induction period in many cases, but some compounds showed considerable variation in induction period in different solvents. With acetone, low induction periods were given by certain amines, and methanol appeared to be detrimental to a few polyhydroxy compounds. This variation is being studied further. Some inhibitors were tested in more than one solvent, and the longest induction period found was used in the tables, as it was believed to be the best measure of true inhibiting power of the substance. Table I11 presents a comparison of induction periods of inhibitors dissolved alone in gasoline, and inhibitors in gasoline with use of a second solvent. The gasoline used was the sample treated with fuller’s earth used throughout this study. I n each test 10 cc. of solvent was used to 200 cc. of gasoline, and the inhibitors were in the proportion of 0.01 per cent by weight.
TABLE rrr.
Vol. 24, No. 1%
SOLVEXTS IN INHIBITOR
TESTS
INDVCTION PERIOD--
No inhibitor Inhibitor (0.010/,): or-Naphthol Thymol Catechol Isoeugenol Pyrogallol 0- Aminophenol p-Phenylenediamine 4-Chloro-2-aminophenol
No Ethyl solvent Benzene Acetone Methanol alcohol Min. Min. Min. Man. Min. 45 45 45 45 45 2310 300 2250 255
.. .. .. ..
2310 315 2475 285
..... . ..
230
..
2080 1320 435 955
2250 265 1500 270 1005 735 900
795
.. .. .. zii5 1330 960 410
EFFECTOF COXEXTRATION There is a possibility that change in concentration of an inhibitor will not always produce a proportional change in its inhibiting action, and that wide variation in concentration might even change a substance from an inhibitor to an accelerator of oxidation. Moureu and Dufraisse ( I d ) state: “On account of the close relationship which exists between the two opposite types of catalytic action an inversion of the sense of the catalysis is frequently the case.” An inhibitor was added to untreated gasoline in different proportions. I n this preliminary work, increasing the amount of inhibitor used caused an increase in induction period nearly proportional to the inhibitor concentration. Further work is in progress on this question. The effect of inhibitor concentration is as follows: INDUCTION PERIOD Min. 15
No inhibitor (untreated gasoline) Catechol:
: ::2
100
160 260 350 715
:::;Ti 0.05%
EFFECTOF IKHIBITOR ON GUMTEST It was found that certain inhibitors reduced the amount of gum deposited by gasoline when tested by the copper dish method. A similar observation was made in an earlier paper from this laboratory (4). The inhibitors arrest the conversion of potential gum into actual gum, so that the value found when an inhibitor is present approximates that given by tests for preformed gum, Data on two gasolines are as follows:
Gasoline alone With inhibitors ( 0 01%): Catechol Hydroquinone a-Naphthol p-Ammophenol Air-jet gum
GUM(COPPERDISH) Untreated Gasoline treated gasoline with fuller’s earth Milligrams per 100 cc. 85 163 14 2 14 6 4
2 3 4
0 0
After a large amount of actual gum has formed in gasoline during storage, this inhibiting effect may not be observed. RESULTSOF OXYGEN BOMBTESTS Comparative tests on inhibitors were made in a uniform concentration of 0.01 per cent in the Pennsylvania cracked gasoline treated with fuller’s earth. When a compound in this concentration produced a slight change in induction period, too small for inhibitive action to be assigned to it with certainty, the test was repeated in 0.05 per cent concentration. The compounds tested were in most cases of the highest quality furnished by the Eastman Kodak Company. I n a few cases recrystallized or redistilled technical products were used. Generally the substances did not affect the color of the gasoline. Where there was a drop in color, the Saybolt color of the gasoline containing the inhibitor before oxidation is given in the tables.
The induction periods cuuld be read to the nearest 5 minutes. Repeated tests on the same compound aould usually check within a few per cent of tlie induction period. The conclusions drawn with respect to the inhibiting value of the compounds tested as a functiuii of strnrturo are hased upon tlie results obtained in aecelcratcd mygen bomb tests and do not necessarily apply to storape coiiilitions. (The relation Iictu~een induction period and the time an inhibitor will p r o t e c t gasoline in storage is bcing investigated.) I t should be ernphasiet:d that wliilc the results s h o u l d be generally applicable to cracked gasolines, they were obtained, as s t a t e d above, with gasoline from a I'ennsylvania stock. PHENOLS The data on comDorinds of a r h ~ o l i c natnre are given in Table IV. Phenol itself has moderate inhibiting action. The inhibitineI .nrunertv . " of the h\&
