Tetraethyllead Susceptibilities Of
Gasoline
Doctor Treatment
VS.
Caustic Washing
L. M. HENDERSON,
hydroxide solutions than the higher boiling ones, the odor is e l i m i n a t e d i n larger proportion than are the reThe Pure Oil Company, spective mercaptan compounds. At the same time, Chicago, 111. sulfur compounds having a harmful effect on the tetraethyllead susceptibility of the gasoline are removed. Various investigators ( I , 6, 6) reported that the addition of certain sulfur compounds to gasoline lowered the octane number and reduced the tetraethyllead susceptibility. Mercaptans decreased the tetraethyllead susceptibility, and disulfides decreased it even more than mercaptans. These observations are significant in any comparison of doctor sweetening with sodium hydroxide washing, for the sodium plumbite plus sulfur treatment (doctor sweetening) converts mercaptans into disulfides which remain in the gasoline and depreciate the tetraethyllead susceptibility even more than the equivalent mercaptans. When the gasoline stock is washed with sodium hydroxide, mercaptans are dissolved to a greater or less extent in the sodium hydroxide solution, are thereby removed from the gasoline, and are not converted into other gasoline-soluble compounds which have an objectionable effect on the tetraethyllead response of the gasoline.
The experimental evidence presented shows that, when gasolines are treated with sodium plumbite and sulfur, they require more tetraethyllead to produce a given octane value than do those treated with sodium hydroxide alone and, similarly, more than do those which are thoroughly scrubbed with caustic and subsequently treated with sodium plumbite and sulfur. The differences in quantity of tetraethyllead required are related qualitatively to the amount of mercaptan sulfur removed by the caustic washing. The decreased requirements of tetraethyllead accomplished by efficient scrubbing of gasoline w i t h sodium hydroxide solutions would result in large economic savings to the petroleum industry.
w. B. ROSS, AND c. M. RIDGWAY
L
ARGE quantities of tetraethyllead are being used daily by the petroleum industry for the purpose of increasing the antiknock value of gasoline. Inasmuch as different gasoline stocks require different amounts of tetraethyllead to produce specific antiknock ratings, an investigation of the factors which affect the tetraethyllead susceptibility of gasolines is of economic importance. One of the factors requiring serious consideration is the influence of refining processes. Many observers have noted that the tetraethyllead susceptiRelative Influence of Caustic Washing and bility of gasoline stocks decreases to a greater or less extent on Doctor Sweetening treatment with the conventional doctor solution (sodium The present investigation was initiated for the purpose of plumbite) and sulfur (1,6,6).This treatment is applied quite comparing the tetraethyllead susceptibilites of caustic-washed generally at present for the purpose of eliminating the odor due gasolines with those treated with sodium plumbite and sulfur. to certain mercaptans normally found in gasolines in relatively Accordingly, gasoline stocks were thoroughly scrubbed with small amounts. This treatment converts substantially all sodium hydroxide solutions under controlled conditions so mercaptans in the gasoline, malodorous as well as those withdesigned as to limit the cause of any observable change in out objectionable odor, into disulfides which remain in solution octane value and tetraethyllead susceptibility to the caustic in the gasoline. A gasoline thus treated will pass the so-called washing only. Corresponding samples of the gasolines were doctor test. There is, however, a possibility that the doctor test is much more severe than is justified on a basis of intrinsic qualities of a gasoline. A gasoline containing, for example, 0.0007 per cent mercaptan sulfur will normally pass the doctor test, while another containing but slightly 0.0012 per more mercaptan sulfur-namely, cent-will generally not pass the test even though the former sample may contain much more total sulfur than the latter. The unfortunate feature of the so-called doctor test when written into gasoline specifications lies in the fact that it necessitates processing a product to meet an arbitrary test that apparently is not adequately related to the merits or intrinsic qualities of the product. The lower boiling mercaptans possess objectionable odors, whereas the higher boiling mercaptans have progressively less objectionable odors. Thorough washing of a mercaptan-con0 PUMP taining gasoline stock with sodium hydroxide MIXER solution removes the mercaptans to a greater or ENLARGED SKETCH less degree (&S,4). And inasmuch as the lower OF MIXERS boiling mercaptans are more soluble in sodium FIGURE 1. SIX-STAGE COUNTERCURRENT CONTINUOUS TREATER 27
INDUSTRIAL AND ENGINEERING CHEMIsTRY
28
treated with sodium plumbite under conditions which eliminated loss of volatile components. Other treatments were made involving caustic washing followed by plumbite and sulfur treatment. PROCEDTJRE. The scrubbing of the gasoline was carried out in an experimental six-stage countercurrent continuous treater as shown in Figure 1. Fresh caustic soJution (approximately 25 per cent of the volume of gasoline) was fed into tower 6 and thence passed successively through five succeeding towers and drained continuously from the bottom of tower 1. The gasoline was charged countercurrently to the caustic solution, entering tower 1 and passing through the remaining five caustic towers and then to tower 7 in which it was scrubbed with water to remove traces of caustic from the treated gasoline. All samples (of untreated gasoline charged to the treater and of treated gasoline from the treater) were taken by water displacement to eliminate loss of the more volatile components of the gasoline. . That such loss was avoided is confirmed by tests showing that the Reid vapor pressure of the gasoline going to and taken from the treater was substantially unchanged in value. It was necessary that loss of volatile material be avoided in order to limit any changes in octane values of the gasoline to the treatment with caustic solutions only. On each tower a caustic recirculating pump was installed to take suction from the bottom and force the caustic along with gasoline through a mixer where intimate contact with the gasoline was secured. From the mixer the liquids were carried back into the tower where the caustic solution settled from the gasoline to the bottom of the tower while the gasoline rose to the top and passed on into the mixer of the next tower. Corresponding samples of gasoline were treated with sodium plumbite (doctor) solution and sulfur in the following manner: Precautions were taken a t all stages to avoid loss of volatile hydrocarbons. Two-gallon cans were cleaned thoroughly and washed with caustic solutions and with water. Air in the can was displaced by nitrogen. Successive portions of gasoline were introduced into the can and removed until addition of a fresh portion of asoline showed no increase in pressure in the can on shaking. $his saturated the gas space in the can and eliminated loss of light ends of gasoline to the vapor space. The sodium plumbite solution (one and a half times the theoretically required amount) was conducted into the can, followed by the gasoline charge which was introduced by water displacement and was chilled by being passed through a cold condenser before entering the can. The cans were thus filled to within one inch of the top. The calculated theoretical amount of powdered sulfur was added and the can sealed tightly. The theoretical amounts of sulfur necessary were calculated on the basis of the known mercaptan
VOL. 31, NO. 1
sulfur content of the gasoline and the conventional reactions involved in the well-known sodium plumbite and sulfur treatment. The cans were then shaken by machine for 30 minutes; then they were cooled in an ice bath for 45 minutes and water-washed in a closed system, and a sample was withdrawn by water displacement for vapor pressure tests. If the vapor pressure tests showed no loss of volatile components, octane values on the treated gasoline were determined, both by the A. 5.T. M.-C. F. R. Motor method and by the Research method. Both methods of octane rating were employed for the purpose of ascertaining whether the treatment influenced the so-called antiknock sensitivity of the gasoline as well as the conventional A. S. T. M. octane rating of the sample. The octane determinations were made by comparing samples of the differently treated products of a specific gasoline stock against each other and against a reference fuel. This gave reliable differences in octane values.
Experimental Data Table I shows gravities, A. S. T. M. distillations, and total sulfur values of the gasoline stocks used in this investigation. The octane values and tetraethyllead (TEL) response obtained on each of these gasolines, before and after treatment by the various procedures described above, are shown in Table I1 and Figure 2. In order t o simplify the table the samples treated by different procedures are designated by letters; the legend is shown a t the bottom of Table 11. TABLE I.
DATAON G.4SOLINE STOCKS
INSPECTION
Sample No. Gravity, A. P. I. A. S., T. M. distillati on, Initial b. p. 5% over
' F.:
40
%!i
Recovery % Residue, Loss, %
393 61.9
394 62.5
131 65.5
175 65.3
437 62.2
80 109 126 160 188 214 237 260 282 312 347 380 407
81 107 116 140 168 200 236 266 293 325 356 378 390
82 109 120 144 169 193 216 238 270 303 343
90 108 118 140 164 188 212 238 266 304 338
388
386
84 114 131 160 184 208 228 248 268 292 328 360 379
97.5 1.1 1.4
k
Total sulfur, %
0.035
97.5 1.1 1.4 0.037
...
...
98.0 1.2 0.8
96.5 1.0 2.5
98.0 1.0 1.0
0.058
0.080
0.030
TABLE 11. CAUSTICAND DOCTOR TREATMENTS OF CHPa AND HPa CROSSGASOLINES FROM MICHIGAN, ILLINois, AND TEXASCRUDES Tetraethyllead, Cc./gal.
-A.
8. T. M. Octane No.Bb C D
-Research B
Octane No.-
C
Tetraethyllead, Cc./gal.
D
64.1 70.4 73.8 76.0 77.9 80.3
63.6 70.3 73.4 75.8 77.6 80.2
63.8 70.0 73.2 75.3 77.0 79.5
66.4 72.0 75.9 78.8 79.6 81.7
66.5 71.8 75.2 78.2 79.4 82.0
-A.
66.2 71.5 75.5 78.0 79.0 81.0
A 68.3 73.1 75.7 77.6 78.9 80.6
0.0 0.5 1.0 1.5 2.0 3.0
-
B 68.1 73.5 76.2 78.1 79.1 80.9
D . 68.1 72.1 74.6 76.9 78.0 79.7
A 70.4 74.7 77.6 79.4 81.1 82.7
B 70.4 74.5 78.2 80.3 81.5 83.0
C H P Gasoline from Texas Crudes (Sample 393)C -----A. 8. T. M. Octane No. I3 C D A 64.1 64.3 64.3 64.9 70.0 70.8 70.8 71.0 73.4 74.5 74.5 74.7 76.8 7 5.5 76.8 76.5 77.1 78.3 78.4 77.5 80.2 79.1 80.4 79.4
D 70.0 73.8 77.3 79.7 80.9 82.4
C
D
Octane No.C D
-Research B
E
0.0 0.5
1.0
1.5 2.0 3.0
.
62.8 68.6 72.5 74.6 76.5 78.8
A
9. T. M. Octane No.-
B
D
F
-Research Octane A B D 61.8 61.7 60.5 67.5 67.0 63.8 71.4 70.7 68.0 73.5 74.1 71.0 75.7 76.4 73.9 78.0 78.8 76.0
No.F 61.8 67.0 71.0 74.3 75.9 79.0
C H P Gasoline from Michigan Crude (Sample 175)
A 62.4 67.5 69.9 71.9 72.9 75.5
B C 62.6 60.4 67.4 67.0 70.2 69.7 71.6 70.8 72.7 71.9 76.3 75.7
D 60.0 65.5 68.0 69.6 71.2 73.2
A 62.8 88.5 71.4 73 3 74:7 78.0
B C D 63.5 61.9 61.6 67.5 67.0 67.2 70.7 70.1 70.4 73.3 74:2 73:6 74.6 78.0 77.3 77.3
C H P Gasoline from Illinois Crude (Sample 437)
A B C D A B C D 69.0 68.7 68.3 67.6 67.0 67.2 67.2 66.0 . , 0.5 73.5 73.9 73.6 71.6 .. .. .. 1.0 75.3 75.9 75.5 74.8 .. . . 1.5 77.8 78.2 77.8 76.9 2.0 78.9 79.4 7 9 . 1 78.0 8214 8 2 : s 82:7 82:2 3.0 81.1 8 1 . 6 $1.4 8 0 . 4 doctor-treated only with 100% excess sulfur but not corrosive t o oopper strip test; F = three-stage caustic washed.. c These data represent independent treatlng and testing. 0.0
-
a CHP gasoline from combination high-pressure refinery unit: H P gasoline from high- ressure refinery unit. b A P untreatec?sample; B six-stage caustic washed. C = six-stage oaustio washed plus doctor treatment: D = doctor-treaied only: E =
-
S. T. M. Ootane No.-
0.0 0.5 1.0 1.5 2.0 3.0
H P Cross Gasoline from Texas Crudes (Sample 394) 0.0 0.5 1.0 1.5 2.0 3.0
Bb
C H P Gasoline from Michigan Crude (Sample 131)
C H P Gasoline from Texas Crudes (Sample 393)C 0.0 0.6 1.0 1.5 2.0 3.0
-A.
.. ..
.
.. .
. ...
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
29 ~~
t
IAMPLL
176 7574-
8
73727170-
; z
5 0
f
69-
68 -
rj I31
1)
{
67 66
175
~
STAGE CAUSTIC-TREATED A-DOCTOR-TREATED (MINIMUM SULFUR)
X-0-
65 -
J RAW STOCK WAS MICHIGAN COMBINATION HIGH-PRESSURE CRACKED GASOLINE SAMPLE NO. 131.
IN CAUSTIC TREAT THE RATIO OF CAUSTIC VOLUME (12%N A O W T O GASOLINE VOLUME WAS 1:4
C.C. TETRAETHYL LEAD PER GALLON
FIGURE 2. TETRAETHYLLEAD us. OCTANENUMBER
Evidence that no volatile components of the gasoline were lost in the respective treatments is afforded by the fact that vapor pressures of the samples remained substantially unchanged : VAPORPRESSURES-
-REID Sample No.
A
B
C
D
E
F
11.3 10.6 8.8
...
11.3
...
. ..
.. ..
437
{
'
DID NOT REACH 77 OCTANE WITH 3 C C . TEL.
I
*\\\\\\\\\\\\\\\?k
C C . T E L TO 70 A S I M . k 0 . , o,8 4
OCTANE
2,0
c . c . T E L To
0
0,4
0;8
77 A S I M . O C T A N E I? I$ $0
FIGURE3. EFFECTOF TREATMENT ON TETRAETHYLLEAD SUSCEPTIBILITY OF GASOLINES
mental sulfur to meet the doctor test, the tetraethyllead requirement was materially less than that required when the gasoline was sweetened only with plumbite and sulfur. Unless otherwise stated, theoretical minimum amounts of elemental sulfur were used in the sodium plumbitesulfur treatment. This theoretical amount of sulfur was calculated on the basis of the mercaptan sulfur content of the gasoline and the following equations which are generally accepted as broadly indicating the reactions involved in the doctor treatment: PbO
+ 2 RSH = Pb
