Correlation of Predicted and Observed Storage Stability of Cracked Gasoline E. L. WALTERS, D. L. YABROFF, AND H. B. MINOR Shell Development Company, Emeryville, Cali,f.
.
Predicted storage stability lives have been determined for a number of cracked gasolines by the extrapolated gum time method. In general, the extrapolations have been found to correlate with actual storage within precision of measurement. The gasolines tested represented all types of thermally cracked products in various stages of refinement, and for these particular fuels the extrapolated gum time procedure provided a more reliable measure of stability than did the induction period or copper dish gum methods.
gen depletion effects from influencing the rate of degradation until gum formation was excessive. This type of storage should check with the gum time predicted from the accelerated method, but not necessarily with commercial storage wherein oxygen availability may govern degradation rates. In developing a laboratory storage procedure, no consideration was given to vented storage (S), because of the uncertainty which might arise from changing composition. The detailed laboratory storage procedure is as follows: The gasoline under test was cooled to 5" * 2' C., and a 500-ml. portion was introduced into each of forty-five separate half-gallon bottles through a funnel, the stem of which extended to within 0.5 inch of the bottom. In this manner displacement of air from the bottles by gasoline vapors was minimized. After each bottle was filled, it was stoppered tightly a t once with a cork which was cut off flush with the bottle top. A Bakelite cap was then screwed on, and a heavy coating of black lacquer was applied around the bottom edge of the Bakelite cap. This particular method of sealing was adopted since it provided a virtually perfect closure and prevented loss of air from the bottles when they were placed in the warmer constant temperature rooms. Some twenty to twenty-five such bottles containing a given gasoline were placed in storage in ea:h of the two dark rooms which were maintained at 90' and 100 * 1 F., respectively. At given intervals, depending on the length of the predicted stability time, bottles were withdrawn from the constant temperature rooms, cooled in ice, opened, and the contents analyzed. Each bottle thus constituted a separate storage test. The complete curve for a given gasoline, relating deterioration with storage time, can thus be considered as an average curve in which the effect of possible erratic points has been minimized. The amount of oxygen present in the half-gallon bottles (air/ gasoline ratio = 2.8) was insufficient in some cases to allow the formation of deterioration products to proceed normally for the entire period of storage. Where this occurred, some of the bottles were withdrawn from storage, cooled, and opened, and a fresh air supply was introduced by passing air over the gasoline surface. The bottles were then resealed in the usual way and replaced in * storage.
T
H E gum stability of a given gasoline blend is governed to a greater or lesser extent by such factors as composition, treatment, and inhibition. A reliable method for measuring gasoline stability is required, therefore, to further the study of problems of this nature. Existing gasoline stability tests, such as the induction period (6)and copper dish gum ( 4 ) ,are empirical and may lead to erroneous conclusions regarding stability, especially when widely varying types of gasolines are subjected to test. In storage stability the pertinent factor is the rate a t which gum forms during storage, or, in other words, the length of time before the gasoline forms objectionable amounts of gum and related deterioration products. It would thus appear that the rate of gum formation should form the logical basis of a stability test. A stability test of this nature has been developed during the past several years. The rate of formation of gum is measured a t several accelerated conditions, and the results are then extrapolated to given conditions of storage to furnish a prediction of the actual storage life. The experimental methods have already been outlined (9). The methods employed in the present studies were modified somewhat, however, based on more recent findings. This paper shows the application of the extrapolated gum time method for predicting storage stability and its correlation with actual storage. The equipment and experimental procedure are identical with those already presented by Yabroff and Walters (9). The gum time, which is used consistently throughout the present paper, refers to the time required to reach a given A.S.T.M. (D381-36) gum content-for example, 5 mg.-under the specified conditions of aging. Other measurements of gum, such as the I.P. 38/44 (T) method (8)are satisfactory as well. Peroxide determinations were carried out in accordance with the Yule and Wilson method (11). Copper dish gums were determined on an Atlantic Refining Company type bath, employing a 150-minute evaporation time for full range products. Induction periods are reported by A.S.T.M. method D52542T, although actually determined in Universal Oil Products bombs, the results being converted by means of established relations (10).
EXTRAPOLATED GUM TIME TESTS
DESCRIPTION OF TESTMETHODS.The formation of gum during all but a small final portion of the induction period generally proceeds a t a simple exponential rate under fixed conditions of temperature and oxygen pressure; thus a plot of the logarithm of the gum content as a function of time of aging (or oxidation) yields a straight line up to about 85% of the induction period. Beyond this point the rate of gum formation becomes faster than the exponential rate. From such a gum curve the time required to reach a gum content of 5 or 10 mg. per 100 ml. can be determined, and such a gum time serves as a measure of the stability of the gasoline a t that specific temperature and oxygen pressure. Temperature and oxygen pressure, the test accelerants, function independently of each other, a.s indicated by Figure 1 which shows the effect of temperature to be similar at different oxygen pressures and, conversely, the effect of oxygen pressure to be similar at different temperatures. Therefore the susceptibility of the test gasoline to temperhure can be determined at any convenient oxygen pressure, and also the susceptibility to oxygen pressure can be determined a t any temperature. On deter-
LABORATORY STORAGE TESTS
Laboratory storage tests were conducted in some 900 sealed glass containers, using a reasonably high (2.8/1) air/gasoline volume ratio. This was found necessary on the basis of preliminary experiments which showed that given amounts of air would form fixed amounts of gum in a given gasoline, and that a ratio of this magnitude (or greater) was required to prevent oxy-
423
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
424
constant for all gasolines but may vary grestly in certain cases. More recently, unpublished observations a t the Shell Oil Wood River Research Laboratories indicated that the effect of oxygen pressure on the gum time (or induction period) is a log-log function which may be expressed by the following equation:
20
&
Vol. 40, No. 3
15
$10
log tg = C
. I
k8 3 6 Y
where t g
+ D log P (at constant temperature)
(2)
= z-mg. gum time, hours = oxygen pressure, pounds absolute
P C, D = constants which depend on thr gasoline
.5: $ 4
3 2
2.70
2.74
2.78
1 / T (" K.) X 10'
1.8 1.9 2.0 2.1 Log 0:Press., Lb./Sq. Inch Abs.
Figure 1, Independent Effects of Temperature and Oxygen Pressure on Gasoline Stability Measurements
B X 10'
Figure 2.
D
Range of Effects of Temperature and Oxygen Different Gasolines
mining the individual effects, these are combined to give an extrapolation of gum times to specified storage conditions. It has been shown previously (9,9,IO), and found valid in the present studies, that the effect of temperature on the gum time (or induction period) may be expressed by the following equation:
log tg = A where tg
+ B / T (at constant oxygen pressure)
(1)
The value of C, the intercept constant, depends on both the particular temperature employed and the gum level chosen for the gum time; D, the slope constant, is independent of these factors. The magnitude of extrapolations involved in temperature and oxygen pressure measurements is shown in Figure 2, which shows separate plots of the effect of temperature ( B value) and oxygen pressure (D value) a t high and low temperatures and a t high and low oxygen pressures, respectively. The ranges of B and D values shown are those encountered in the present studies. In applying the extrapolated gum time method to a given gasoline, the effect of temperature (at a fixed oxygen pressure) was generally determined by measuring gum times a t four fixed temperatures, usually a t 5 " C. intervals. The effect of oxygen pressure (at a fixed temperature) was usually determined a t two or more elevated oxygen pressures, The range of accelerating conditions varied for different gasolines because of differences in stability levels. Likewise, the gum value (5 mg., 10 mg., etc.) varied because of differences in initial gum contents, rates of gum formation, etc. The 5-mg. and 10-mg. gum times were used whenever feasible. Examples are presented showing typical graphical measurements involved in applying these methods to one of the test gasolines, sample 4. Figure 3 shows both a coordinate and semilogarithmic plot of the rate of gum formation Pressure for plotted against the time of aging a t several elevated temperatures, with an oxygen pressure of 100 pounds per square inch gage. The 5- or 10mg. times can be read directly from these plots. Table I summarizes the complete gum stability measurements for sample 4. Figure 4 presents separate plots showing the effect of temperature and oxygen pressure on both the induction period and gum times for this gasoline. The slopes of the lines in Figure 4 give the values of B (for the temperature effect) and the value
= 2-mg. gum time, hourso
T = absolute temperature, K. A , B = constants which depend on the gasoline
The value of A , the intercept constant, depends on the particular oxygen pressure employed and the gum level chosen for the gum time; B, the slope constant, is independent of these factors, but the B value for induction periods may be somewhat different from the value for gum times, possibly because of uncertainty in induction period measure ments a t temperatures lower than 100' C. Previous investigators ( I , 6, 9) have reported the effect of oxygen pressure on the gum time or induction period to be small and to be expressible as a linear or semilogarithmic function of the stability time US. the oxygen pressure. Indications by Rogers (7) and some of the present authors (9) showed, however, that the oxygen pressure effect is not a
0
Figure 3.
5
10 1.5 0 5 Time of Oxidation, Hr.
10
15
Formation of Gum at Elevated Temperatures and 100 Pounds Oxygen Pressure for Sample 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1948
425
position and treatment. I n general, they were fu!l-range cracked gasolines or commercial. gasoline blends repre15 sentative of those likely to be encountered in actual practice. The gasolines were tested immediately upon 10 receipt, if possible, or storedsat 0" C. A 8 in the event of necessary delay. 8 6 ACCELERATED STABILITY TESTB. g 5 Complete accelerated stability data, were obtained for all gasolines, similar 4 to the example shown in Figures 3 3 and 4 for sample 4. A summary of the complete data is given in Table 2 111, which includes as well the extrapolated gum times, in months, a t 1 storage temperatures of 90 O and 2.72 2.76 2.8 1.6 1.7 1.8 1.9 2.0 2.68 100" F. Log 0%Press., Lb./Sq. Inch. Abs. 1 / T (" K.) X 108 For different gasolines the staFigure 4. Effect of Temperature and Oxygen Pressure on Stability Times for bilitv levels. the ratio of gum time Sample 4 to iAductiod period, and the effects of temperature and oxygen pressure on gum time varied appreciably. With minor exceptions, the of D (for the oxygen pressure effect). In practice the slopes were B values (temperature effect) and D values (oxygen prescalculated from a least-squares solution of the experimental sure effect) fell within the ranges of 5000 to 6000 and 0 data rather than from a direct graphic plot. to -0.50, respectively. This represents, as shown in Table PROPERTIES OF GASOLINE SAMPLES.Twenty-two gasolines I11 and Figure 2, a marked difference in the relative stability were investigated; their general properties are listed in Table of the gasolines a t high and low temperatures and oxygen 11. The gasolines were derived from different crudes (Calipressures. fornia, mid-continent, and Texas) and differed widely in comACCURACY OF EXPERIMENTAL AND EXTRAPOLATED MEASUREMENTS. The accuracy of the experimental and extrapolated (predicted) stability measurements can be estimated with some degree of certainty. Errors in experimental measurement can TABLE I. TYPICAL SUMMARY OF GUMFORMATION UNDER arise from uncertainty in gum determinations, test temperatures, ACCELERATED CONDITIONS (SAMPLE 4) and pressures. Temperature measurements are especially sus(Initial A.S.T. M. gum: observed, 0.6, 0.8 mg.; extrapolatedo, 0.7 mg./100 ceptible to error, since an error of 0.2" C. represents an experiml') Gum Time b Av. Oxygen mental error of 201, (g). It is estimated that the experimental 10-mg. 5-mg. InducPressure, % of induc% of induction Lb./ error in accelerated gum time determinations is on the order of +" tin" ".-.. Period, Temp., Sq. In. =2.5 t %. Hr. period Hr. Hr. C. Gage period The extrapolations involving temperature and oxygen pressure 3.08 68.4 2.28 50.7 4.50 100 100 5.17 68.9 3.82 50.9 7.50 95 100 are both exponential; hence small experimental errors, particu8.67 67.1 6.41 49.6 12.92 90 100 14.58 67.6 10.78 50.0 21.58 85 100 larly in temperature measurements, may lead to large uncer68.4 50.7 3.08 4.50 2.28 100 100 tainties in the final extrapolation result. In the usual case, 4.87 ... ... 100 75 5.33 2.76 5i:s 3.73 70:O 100 50 extrapolated gum time measurements were made a t four tempera6.08 3.24 53.3 4.38 72.0 100 25 tures (loo', 95O, 90",and 85' C.) and three oxygen pressures 20
.
,I"_
6 Gum value for zero oxidation time as indicated by ourve of Figure 3. b An a-mg. gum time is defined a8 the time required to reach a-mg. A.S.T.M. gum under the specified conditions of aging.
TABLE 11. GENERAL PROPERTIES OF TEST FUELS A.P.I. GravSample No. 60' F. 1 62.3 59.7 2 59.7 3 65.6 4 62.1 5 62.3 6 60.5 7 63.7 8 48.3 9 47.4 10 65.0 11 58.4 12 61.5 13 50.6 14 61.3 15 46.5 16 62.9 17 60.5 18 62.9 19 62.9 20 62.1 21 62.1 22 a
B~o-
mine No., G.Brz/ 100 G.
A.S.T.M. Gum, Mg./ 100 MI. 0,0.2 0.8,l.O 0.4,0.6 0.6,O.S 0.8,O.S 4.4,4.4 0.2,o .2 0.4,0.4 3.4.3.4 0.109 1 . 0 , 1.o 0.115 2.4,2.8 0.028 4.0,4.2 0.69 6.0,6 . 8 0 077b 0:2566," 7.0,S.O 1.0,1.2 0.130 0.79 0.6,O.S 0.055 0.6,l.O 0.064 0.8,l.O 1.0,l.O 0.055 0.055 0.8,l.O 0.4,0.4 0.182 0.238 0.4,0.4 Total Sulfur, Wt. % 0.236 0.140 0.140 0.113 0.194 0.135 0.033 0.030
Copper Dish Gum, Mg./ 100 M1. 5,6 19,26 9,9 8,lO 10,11 84,99 24,25 11,13 127,132 596,628 70,76 138,161 295,306 86,95 36,38
. .10,lO ...
8,lO 10,lO 10,lO 73 5,6
Peroxide
No.
(If) 0.14 0.10 0.04 0.15 0.23 0.89 0.15 0.07 0.45 0.19 0.40 0.14 0.44 0.47 0.46 0.22 0.07 0.15 0.07 0.07 0.19 0.09
A.S.T.M. Octane TEL NO., Content, A.8.T M. Distillation, F., of Added Induction Initial Final RecovInhibitor Motor M1./ Period, Method Gal, Hr. b.p. b.p.. 10% 50% 90% ery, % Type" Unknown 9.08 99 385 142 235 331 9 8 . 5 71.5 0.31 9 7 . 5 Aminophenol 91 133 252 360 72.0 0.22 10.08 104 396 140 253 358 9 7 . 0 Catechol 15.50 72.0 0.19 97.5 Aminophenol 90 380 126 210 319 4.50 74.5 0.76 96.0 Aminophenol 95 383 135 246 327 5.25 71.5 0.32 Aminophenol 91 399 127 219 354 97.0 3.67 71.6 0.37 Unknown 104 396 149 248 358 96.6 5.75 66.5 95 390 138 235 351 95.5 6.33 2:58 80.0 122 433 194 307 401 98.0 4.00 69.0 98.5 Unknown 135 439 189 291 396 3.92 70.0 0.33,O. 42 66.0 80 370 133 239 345 9 3 . 5 Unknown .. 9 5 . 5 Unknown 95 403 147 271 376 63.0 >24 91 401 133 237 363 9 7 . 0 Unknown 68.0 2.50 Unknown 65.0 1.25 97 390 131 241 356 96.2 5.10 73.9 i:is 102 401 142 234 352 9 8 . 8 Aminophenol 333 374 9 8 . 0 ...... 6.33 270 406 304 60.5 99 345 135 243 306. 9 7 . 5 ,...... 1.08 71.0 .. 250 316 9 7 . 5 102 356 147 ....... 69.0 1.50 97.5 99 345 135 243 306 ........ 71.0 3.92 99 345 135 hminophenol 243 306 9 7 . 5 3.75 71 .O 379 1 3 6 , 241 340 9 6 . 5 Aminophenol 4.33 0 : is 100 74 .O Unknown 406 131 235 381 9 7 . 0 13.42 75.0 1.05 95
...
.. .. .. .. ..
.. ..
Concentrations not known with certainty hence not specified. These samples contained appreciable amdunts of mercaptan sulfur: all others dootor sweet,
..... . . . .
.
I
.
.
.
.
.
. .. .
426
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 3
Figure 3. Effect of Magnification of Experimental Uncertainties on Extrapolation of Temperature
loo?
&
100
w
;
.
3
-
.B +
a d
10
(100, 50, and 25 pounds per square inch'gage). Errors in temperature or oxygen pressure measurements may be cumulative or tend to cancel out. I n the former case they n-ould be of opposite sign a t the extreme conditions. Figure 5 s h o m the maximum uncertainty in t,emperature extrapolations resulting from given percent,agc errors in accelerated measurements. These uncertainties, which are appreciable, are considered in subsequent correlation of prcdicted and observed storage stability. The storage tests, as well as accelerated tests, are subject t,o experimental uncertainty in measurement. The uncertainty in t,he measured storage lives is estimat,ed to be approximately + 5 to 770, the longer test period accounting for the larger error in comparison with that from accelerated test measurements. COMPARISON OF PREDICTED AND OBSERVED STORAGE T E S T S
1
0.1 2.6
2.8 3.0 10,00O/T( " K.)
TABLE111.
3.2
SUMMARY OF A C C E L E R S T E D AND P R E D I C T E D ST.IBILITY D A T A
Gum Time D a t a Gum time, hr. (1000
used ld
9.08
0,o.2
0.20
3 5
'
2
10.08
0.8,l.O 1.2
3
15.50
0.4,O. 6
0.60
4
4.50
0.6,0.8
0.70
5
5.25
0.8,O . 8
0.55
6
3.67
4.4,4.4 4 . 4
7
5.75
0.2,o.z 0.56
8
6.33
0.4,0.4 0.38
5
10 3 5
5 1; 10 5
10 5
10 5 10 5
RESULTS OF STORAGE L~EBSUREMEXTS. Xeasurements wcre made at both storage temperatures of the change in B.S.T.LI. gum, copper dish gum, peroxide number, and A.S.T.M. induction period. The A.S.T.11. gum measurements were obtained for correlation purposes. The other measurements were obtained primarily to determine the relative trends under storage and accelerating condit,ions. An example of the complete st'orage data for sample 4 is s h o r n in Figure 6, which indicates as well the calculated rate of formation of A.S.T.M. gum.
c.,
100 lb. oxygen) 8.23 8.90 3.47 5.17 12.58 15.0 2.28 3.08 4.35 4.53 0.18 1.18 2.78 3.67 4.04 5.03
~
Effect of Acceierants on Gum Times Temperature Oxygen Pressure Calcd. time ratio, Calcd. time ratio, Calcd. Storage Time under 90' F. (mo.) 1 atm. air 1 Atm. Air. Months B \-aIueb looo C. (hr.) D valuec 1001b./sq,in.oxygen 100' F. 90' F. 35 or 19 1.48 80 or 40; 6180 or 5680 6.520r3.28 -0,108 37 or 2ob 86 o r 43 7 1.17 3.5 s250 1.82 -0.045 11 5.4 65 130 6.56 1.61 -0,515 5160 78 155 16 37 3.1(3) 5.16 -0.312 6010 22 50 13 0.53 29 5.45 -0,116 6050 17 38 1.36 0.2 0 .5 5280 1.QO -0.086 1.4 3.0 8 3.81 16 5110 1.50 -0.369 10 21 5 10 1.68 5070 1.42 -0,140 12 6.3 0.7 0.35 1.92 4800 0.98 -0.178 2.4 1.2 1.56 1.0 0.6 4630 0.78 -0.122 1.6 0.8 0.05 0.11 I . l(3) 537.0 .2,15 -0.028 0.11 0.22 1.00 1.47 0 0.25 0.5 5100 0.65 1.3 2.35 0.4 0.75 1.19 -0.239 4940
0.36 1.25 0.87 1.30 5 0.38 2.4,2.8 2.55 0.05 11 10 0.09 0.36 10 4.0,4.2 4 . 0 12 > 24 20 0.87 10 2.50 0.27 6.0,6.8 6.0 13 ,... 15 0.60 6.4 0.94 -0.510 10 0.8 1.25 7.0,8.0 8 . 0 0.024 4770 14 0.45 0.14 20 1.62 4.32 -0.134 1.0,1.2 1.8 1.53 4.8 5 5880 5.10 15 8 10 2.55 1.67 10 5,34 -0,140 2.56 1.15 5 6040 6.33 0.6,0.8 16 15 10 3.78 1.23 0.75 1.58 5 0.77 5150 -0.060 1.08 0.6,l.O 0.6 17 0.9 10 0.92 1.23 2.07 -0,061 1.4 1.10 5320 0.8,l.O 0.9 5 1.50 18 1.6 1.25 10 1.28 8 4.08 -0.069 5 3.92 3.50 5830 1.0,l.O 0.75 19 9 10 3.92 41 6.2 5.37 -0,500 2.75 6030 3 3.75 0.8,l.O 0.9 20 51 3.45 5 1.25 1.40 -0,064 2.5 2.83 5060 5 4.33 0.4,0.4 0.5 21 3.3 10 3.67 1.19 8 1.69 -0,048 8.10 5190 0.5 5 13.25 22 0.4,0.4 10.5 10.55 10 a Gum content indicated for zero oxidation time by curves similar to Figure 3. From Equation 1, log time = A B/T. D log P. c From the equation log time = C d Two calculations &e shown for sample 1 because of uncertainties in gum time measurements arising from the low rate of gum formation. 9
4.00
3.4,3.4
3.8
10
3.92
1.0,l.O
1.25
++
10 5 10
.... 0.15 0.85
11 18 23 34
1.5
1.8 2.8 3.2 18 20 92 115 5 6.5 16 21
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1948
421
Calculation by means of Equation 1, which relates gum time (storage life) with temperature, shows that a change of 10" F. in the storage temperature should change the storage life by a factor of about two. This is confirmed by the storage data of Table V and thus establishes the validity of the time-temperature equation a t storage temperatures as well as a t accelerated temperatures. . I n general, peroxide formation closely paralleled gum formation under both accelerating and storage conditions. Thus, the observations already noted for gum formation apply generally to peroxide formation as well. The copper dish gum content generally increased in storage a t a rate approximately tenfold greater than did the A.S.T.M. gum; usually the shape of the two curves was roughly similar. Induction periods decreased continually during storage from small to appreciable amounts. In the extreme case, sample 2 ,
6
i
n 0
Figure 6.
10 20 0 10 Storage Time, Months
g
20
5
3
\
gg 20o
Storage Test Resylts for Sample 4
15
2 E; 10 In general, the curves predicted for gum formation have the same form as those actually observed in storage, excepting a t high gum levels (latter periods of storage) where effects due to oxygen depletion were noted. Aw example of this is shown in Figure 7 for sample 17. For this gasoline at the higher storage temperature the rate of gum formation was observed to diminish as oxygen depletion became evident but to resume normally on further access to air. Similar effects due to oxygen depletion were noted on other stability measurements as well. These occurred above degradation levels used for correlating predicted and observed stability lives. The predicted and observed gum times in storage for all gasolines are summarized in Table IV. The storage data a t 100' F. are more complete; the correlation of these with predicted gum times is shown in Figure 8. The 10-mg. gum times are shown whenevef possible; whenever other gum times are plotted these are differentiated by appropriate symbols. The dotted lines of Figure 8 indicate the maximum uncertainty (due to temperature) resulting from a 2.5y0 error in the accelerated gum time measurements; this uncertainty would be increased by an additional 7% if comparable errors in oxygen pressure measurements were also included. The agreement between the predicted and observed storage times is considered to be satisfactory on the whole, especially since the storage lives range from days to years. In general, the predictions agree with the observed results within the precision of measurement. The errors in samples 7, 8, and 21 are greater than for the others. The correlation of predicted and observed 5-mg. gum times for sample 8 is reasonably satisfactory; correlation at a 10-mg. gum level was rendered difficult by the deposition of insoluble lead compounds, usually considered as gum, during the latter stages of storage. The B value of sample 21 (5060) i s probably too low; an "average" B value of 5500 would bring this sample in line. Some of the samples with short (less than 2 months) storage lives also fell outside the dotted area, as shown in the enlarged section of Figure 8, although the correct order of magnitude was obtained in every case. Because of the low rate of gum formation in sample 1, the experimentally determined B value was in doubt, and two calculations were made. The observed storage life agrees fairly well with the second calculation.
$5
0
0
0.5 1 1.5 Time in Storage, Months
Figure 7. Gum Formation Storage for Sample 17
in
TABLEIV. PREDICTED AND OBSERVED STORAGE LIVES (GUM TIMES) FOR VARIOUS GASOLINE SAMPLES Sample No.
1
2 3 4 5
6 7 8 9 10 11
12 13 14
.
15
16 l7
Gum Value, Mg./ 100 M1. 3 5 5 10 3 5 5 10 5 10 5 10 5 10 5
i~. n 5 10 5 10 5 10
;:
10 15 10 20 5 10
5 10 5
in
Storage Life a t 1 Atm. Air, Months, st loOD F. 90' F. Predicted Obsvd. Predicted Obsvd. 35 or 19 80 or 40 21 > 26 >26 37 or 20 86 or 43 24.5 7 3.6 3.5 1.9 7.5 ii 5.4 3.7 65 130 >24 >24 155 78 >24 >24 24 16 37 >24 22 50 >24 >24 13 29 >20 * 11 38 17 13 >20 0.1 0.5 0.2 0.05 1.4 3.0 1 .o 16 8 3 21 10 5 5 10 6.5 6.3 11 12 0.7 0.35 0.05 2.4 1.2 0.4 0.6 1.0 0.6 0.8 1.6 1.0 0.03 0.11 0.05 0.22 0.11 0.06 0.25 0.5 0.12 1.3 0.8 0.65 0.75 0.15 0.4 1.7 0.4 0.9 0.15 0.08 0.01 0.45 0.85 0.06 11 3 5 4.8 14 8 18 6.5 23 10 8.5 ...... 34 15 12 1.5 1.7 0.75 0.8 2.0 ' 1 .o 1.8 0.9 1.9 2.8 1.4 1.1 2.1 3.2 1.6 1.4 10.5 18 8 5.5 20 12.5 6.5 9 92 > 14 41 >14 115 > 14 >14 51 5 >11 2.5 6.5 >11 ' 6.5 3.3 8 16 8 > 12 > 12 10.5 21 > 12 >12
I
428
Vol. 40, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE V. PREDICTED AND OBSERVED RATIOS OF GUMTIMES AT 90" F. TO THOSE AT 100' F. lo errors of 2.5% in occeleroled
/
Sample
20'
KO.
2
m
5-
5
10
6
9
Gum Value, h'Ig./lOO M1. 5
7 8 9 10
11 13 12
10 5 10 5
5 10 6
10 5 10
10 20
10
14 15
17 18 19
5
0
10 15 Predicted Storage Life, RIonths
20
1.86 2.02
1.99 1.88 2.23 2.00
5
2.05
10 5 10
1.7 (2.0) (1.5) 1.7 1.8 (2,0)
(2.33 (2.1) 2.0 (1.0) (1.4) (1.5)s (1.77 2.0 2.2 2.1 2 , 01 1.7 1 .a 1.8
I
1.90
5
10
1.8
2 19
1.B
Calculated from B value obtained for gum time. Values in parenthesis are less certain than other values since observed gum times were quite short in these cases and were given with a smaller number of significant figures,
2 s100 2 0
10
0
$
5-
p.i
0 r
8
3
2
10 20 5 10
1.99 1.90
a
Figure 8. Comparison' of Predicted and Observed Storage Lives at 100' F. and 1 Atmosphere Air
6
1.5
Ratio of Gum Times, 90° F./lOOO F. Calcd. from Obsvd. in . accelerated data4 storageb 2.03 1.9 2.0 2.08 (2.0) 2.1 2.00 1.7
5
50
CI
13
3 E L2
u 0
conditionb. If gasoline temperature measqremennts am available, however, ah effective storage temperature caq be calculated, and a prediction made of the storage - life to be expected under the severest condition-full accesa to air. Although the validity of the extrapolated gum tims procedure has been established by the present studies, the meth'od in its indicated form is not suitable for routine and research studies because of the excessive requirements of precision, manpon er, and equipment. Ac adaptation of this method, based on the same principles but more suitable for general use, is yet to be presented,
0 0
5
10 15 20 0 5 10 15 Observed Gum Time in Storage, Months
20
LITERATURE CITED
Figure 9. Correlation of Induction Period and Copper Dish Gum with Gum Stability in Storage at 100" F.
the induction period decreased from 16 to 1.5 hours while the X.S.T.X. gum remained a t about 2 mg. per 100 mi. during 2-year storage. For this particular and widely vaiying group of gasolines, neither the induction period nor the copper dish gum methods gave reliable indications of stability, as shown in Figure 9. It is conceivable that these methods, particularly the induction period, nould provide more reliable measures of stability when applied routinely or t o gasolines of closer comparability. Gasoline end point, degree of refinement, inhibitor type and concentration, and initial gum content may all be pertinent features in these respects. SIGNIFICANCE OF PREDICTED AND ACTUALSTORAGE LIVES. Predicted (extrapolated) storage lives refer to storage under specified conditions, and, as such, provide a sound basis for comparing the stabiIity,of different types of gasolines and niethods of treatment or inhibition. Conditions prevailing in commercial storage are such, however, that the predicted storage lives may not correlate with actual storage. Commercial storage often is carried out in such a way that oxygen availability is a controlling stability factor. Commercial storage temperatures are cyclic, varying both with dailv and seasonal atmospheric
(1) Aldrich, E. if7.,and Roble, N. P., S.A.E. Journal, 30, 198-205 (1932). (2) Am. SOC.for Testing Materials, JIethod D525-42T.
(3) Dryer, C. G., LOWIY,C. D., Jr., Morrell, J. C., and Egloff, G., IKD.EKG.CHEM.,26, 885-8 (1934). (4) Egloff, G., Morrell, J. C., Wirth, J., 111, and Murphy, G. B., Proc. World Petroleum Congr., 11, 85-93 (1933). (5) Flood, D. T., Hladky, J. W., and Edgar, G., IRD.ERG.C m x . 23, 1132-4 (1931). (6) Ramsay, J. Vi'., Ibid., 24,539-42 (1932). (7) Rogers, T. H., private communication (1944). (8) Walters, E. L., and Yabroff, D. L., A S T M Bull., 135,40-3 (Aug, 1945). (9) Yabroff, D. L., and Walters, E. L., IXD. ENG.CEEM.,32,83-1 (1940). (10) Yabroff, D. L., and M'alters, E. L., IND.ENG.CHEM.,AXAL.ED., 13, 353-5 (1941). (11) Yule, J. A. C., and Wilson, C. P., Jr., IXD. ENG.CHEM., 23, 12.549 (1931). RECEIVED September 30, 1946.
