Dioxide Extraction of ComDounds and Aromatics EFFECT OF ACID PROMOTERS R. C. ARNOLD
AND
A. P. LIEN
Standard Oil Co. (Indiana), Whiting, Ind.
N e w solvents consisting of liquid sulfur dioxide promoted by strong Lewis acids are suitable for extracting both sulfur compounds and aromatics; particularly effective as promoters are boron trifluoride, aluminum chloride, and sulfur trioxide. The effects of variables have been demonstrated with petroleum fractions; comparisons with kerosine show that promoted sulfur dioxide effects two to four times more desulfurization than unpromoted sulfur dioxide or other polar solvents. Studies with individual sulfur compounds provide an insight into the mechanism of promoted sulfur dioxide extraction. O f many promoters studied, boron trifluoride has the greatest commercial promise, because it has the greatest selectivity for extraction of sulfur compounds and i s the easiest to recover.
T
HE increasing sulfur and aromatics content of petroleum
stocks in recent years has created a problem from the standpoint of the burning quality, stability, and odor of distillate fuels. Extraction studies in our laboratories on high sulfur petroleum stocks have led to some general comparisons of solvents. Nonacidic solvents, such as sulfur dioxide, furfural, and dimethylformamide, have their greatest utility in extracting aromatics. Although they effect some desulfurization, the selectivities for sulfur compounds are too lo~vto make them attractive for desulfurization alone. Acidic solvents, such as hydrogen fluoride and sulfuric acid, show almost the opposite effect; they are highly selective for sulfur compounds but remove aromatics to only a slight extent (7-9). -4 solvent having intermediate properties would be useful where both desulfurization and aromatic removal are desired. However, in the past, no such solvent has been found. The present work has shown that combinations of nonacidic solvents-notably liquid sulfur dioxide-with small amounts of Le-xis acids, such as boron fluoride and aluminum chloride, provide versatile solvents suitable for extracting both sulfur compounds and aromatirs (3-6). By adjusting the relative proportions of oil, sulfur dioxide, and acid promoter, the character of the combination solvents may be varied from that of
liquid sulfur dioxide alone t o that approaching an acidic solvent, such as hydrogen fluoride. Such solvent systems have been applied t o three petroleum stocks to show the effect of various promoters. Individual compounds have been subjected to extraction studies, as well aa vapor pressure measurements, to elicit the mechanism of extraction by promoted sulfur dioxide systems. The practical aspects of promoted sulfur dioxide extraction have been shown by comparisons with other solvents and by a simulated process application for removal of aromatics and sulfur compounds a8 separate extract streams. Experimental Procedure
Desulfurization experiments were carried out on three high sulfur petroleum stocks derived from West Texas crudes. Refraotive Index, n 1.4586 1.5003 1.4825
Sulfur,
Stock Virgin kerosine
Catalytic cycle oil Virgin gae oil
ASTM Boiling Range,
sQ
Wt. % 0.73 1,45 1.60
c.
166-284 217-279 222-343
100
80
z 0
4"
60
N a (L
40 u) W
I
8
1
AlBra
2o O
I
0
L
L
,
1.0
I
I
I
I
20
40
60
BO
A ' DESULFURIZATION
2.0
3.0
4.0
5.0
6.0
7.0
MOLES B F 3 PER ATOM OF SULFUR IN F E E 0
ID0
Figure 1. Effect of promoters on desulfurization of kerosine
234
t
Figure 2.
Desulfurization of kerosine with sulfur dioxideboron trifluoride
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47,No. 2
HYDROCARBON SEPARATIONS
0
0
MOLECULAR WEIGHT
MOLECULAR WEIOHT
A.
Figure 3.
C.
Disulflder
Extraction of individual sulfur compounds
All reagents and individual compounds used were commercial grade. Extraction of Petroleum Distillates. The treating procedure, when boron trifluoride was employed as a promoter, consisted of single-stage batch extractions in a closed reactor equipped with a stirrer and with inlet and drawoff valves (11). The extraction temperature was controlled by circulating cold acetone through a jacket surrounding the reactor. Oil, sulfur dioxide, and boron trifluoride were added successively through the inlet valve. When stirring was started, the pressure in the reactor fell rapidly. Stirring was continued for 15 minutes, after which the oil and solvent were allowed to settle for an additional 15 minutes. The extract and raffinate phases were removed separately through the drawoff valve; on warming to room temperature, most of the sulfur dioxide and part of the boron trifluoride were liberated. The two phases were washed separately with 10% aqueous sodium hydroxide and water and finally were dried. Percentage desulfurization was calculated from the change in sulfur content between feed and raffinate divided by sulfur content of feed; sulfur determinations were made by the lamp method (1)except in the case of cycle oil and gas oil where sulfur was determined by the bomb method ( 2 ) . Dearomatization was estimated from the change in refractive index by using, as a base line, the index of completely dearomatized and desulfurized oil obtained by adsorption on silica gel. No correction was made for the effect of sulfur compounds on refractive index. Sulfur selectivity was calculated by dividing the percentage desulfurization by the extract yield in volume per cent. With promoters other than boron trifluoride, glass separatory funnels cooled by immersion in an acetone bath, were used in place of the steel reactor, The funnels were withdrawn from the cooling bath only to permit periodic agitation and final phase separation. In the mechanism studies with individual compounds, enough of each was dissolved in n-heptane to give a solution having a sulfur content of about 1.5%. Each blend was extracted at -20' C. with 50 volume yo of sulfur dioxide promoted by 1 mole of aluminum chloride per gram-atom of sulfur in the blend. Vapor Pressure Measurements. Vapor pressure experiments at -20' C. were carried out in the reactor used for the boron trifluoride extraction experiments. Boron trifluoride was introduced from a storage vessel with a pressure gage calibrated to measure the weight of delivered boron trifluoride by pressure difference. The total pressure in the reactor after each boron trifluoride addition was observed on a mercury manometer. I n carrying out an experiment, a known weight and volume of sulfur compound or aromatic was added to a known volume of solvent in the reactor. The mixture was stirred about 30 February 1955
MOLECULAR WEIGHT
B. terf-Thiols
n-Thiols
minutes to allow it to cool to -20' C. A measured amount of boron trifluoride was then introduced, and stirring was continued until equilibrium had been established as indicated by the manometer. More boron trifluoride was added in small measured increments until a pressure of about 1500 mm. H g was reached. The boron trifluoride vapor pressure was calculated by subtracting the partial pressures of sulfur dioxide and residual air from the total reactor pressure. The moles of boron trifluoride in the liquid phase were calculated from the difference between the total added and that in the free space. The boron trifluoride was assumed to behave as a perfect gas. Extraction of Petroleum Distillates
The effect of acid promoters on a number of solvents was first investigated. From the standpoint of versatility and commercial promise] sulfur dioxide appeared most attractive and was selected for further study. Various promoters in liquid sulfur dioxide were compared on the basis of results obtained from the extraction of kerosine. The effect of variables has been investigated by studies with boron trifluoride as the promoter. Comparison of Promoters. Table I presents data obtained with various promoters. The relative desulfurizations obtained in the presence of 50 volume 70sulfur dioxide are shown graphically in Figure 1. These comparisons show that promoted sulfur dioxide gives twice as much desulfurization as sulfur dioxide alone. The strong Lewis acid, aluminum chloride, increased desulfuriza-
Table 1.
Comparison of Promoters
(Feed stock, kerosine, weight % sulfur = 0.73, volume % aromatics = 20; promoter concentration, 2 moles/gram-atom sulfur in feed; temperature, -200
son4 Promoter None BFr AlClr
sol
Vol.'
%
25
96
100 25 50 100 25
86 94 90 85 90
100 26 50 100 50 50 50 50
82 90 87
50 ..
50
FeCla TiClr AlBra HgCle Based on kerosine.
C.)
Raffinate Properties, % Yield, DesulDearovol. furisation matisation 91 ~~
88
INDUSTRIAL AND ENGINEERING CHEMISTRY
25
26 42
58 79 83 87 85 86 94 62
57
43 ~.
71
77 85 84 74 74
28 43 58
43
50 65 46 54 65 46 45 48 41
Sulfur Seleotivity 6.3 4.8
4.1
13.0 8.3 5.8 8.5 5.2 6.2 5.5 4.5
235
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT tion from 43 to 86%. Approximately the same desulfurization is obtained with ferric chloride, titanium tetrachloride, and boron trifluoride. Other promoters, such as sulfur trioxide, aluminum bromide, and mercuric chloride, give slightly less desulfurization. Although data are not shown for weaker Lewis acids, such as aluminum fluoride, antimony chloride, stannic chloride, and zinc chloride, these compounds exhibited only a slight promotional effect. When kerosine was contacted with promoters in the absence of sulfur dioxide, desulfurization seldom exceeded 10%. Therefore, a synergistic effect occurs between sulfur dioxide and the promoter. The ability of sulfur dioxide to extract aromatics is not enhanced by boron trifluoride. On the other hand, aluminum chloride and sulfur trioxide a t low sulfur dioxide concentrations result in about 50% greater dearomatization than sulfur dioxide alone. The effect of other promoters is less clear but aluminum bromide appears to enhance extraction of aromatics whereas mercuric chloride seems t o have little effect. A comparison of sulfur selectivities is shown in the last column of Table I. Whereas sulfur trioxide increases desulfurization, it shows little improvement over sulfur dioxide alone in selectivity for sulfur compounds. Both boron trifluoride and aluminum chloride increase the sulfur selectivity of sulfur dioxide, especially a t low sulfur dioxide concentrations. The most effective promoted solvent, sulfur dioxide-boron trifluoride, is twice as selective as sulfur dioxide alone. Effect of Variables. The effects of solvent and promoter concentrations and extraction temperatures have been studied with the system sulfur dioxide-boron trifluoride. The data are shown in Table I1 and Figure 2. At a constant amount of sulfur dioxide, desulfurization increases rapidly with increasing boron trifluoride to about 2 moles per gram-atom of sulfur in the feed. Additional amounts of boron trifluoride increase desulfurization only slightly. Desulfurization a t constant boron trifluoride concentration increases as the ratio of sulfur dioxide to oil increases, but to a lesser extent when a promoter is present than when no promoter is used. Sulfur selectivity increases with increased boron trifluoride concentration and decreased sulfur dioxide concentration. Thus, a small amount of sulfur dioxide with a high concentration of promoter results in a system similar to acidic solvents, whereas a large amount of sulfur dioxide with a low concentration of promoter results in a system approaching sulfur dioxide alone.
Table 11.
Extraction of Kerosine with Boron KaifluoridsPromoted Sulfur Dioxide Rttffinate Properties, %
SO%, Yol.
DesulDemofurisation matisation 96 25 20 25 26 25 0.5 - 20 94 41 27 94 25 1.7 - 20 74 28 - 20 94 28 25 3.8 79 91 42 50 0.0 - 20 43 50 0.78 - 20 89 63 44 50 1.7 - 20 80 88 43 - 20 90 44 50 2.2 83 88 50 3.8 85 - 20 43 50 6.7 45 88 87 - 20 50 2.0 90 43 85 -35 0 90 50 2.0 34 78 - 20 57 100 0.0 86 58 100 0.63 - 20 60 85 70 100 2.0 - 20 58 a5 87 100 4.8 20 90 62 85 0 Moles boron triauoride per gram-atom feed sulfui
%
Temp.,
BF'a/S4 0.0
-
c.
Yield, vol.
-
Sulfur Selectivity 6.3 6.8 12.0 13.0
4.8 5.7 6.7 8.3 7.1 7.2 8.5 7.8 4.1 4.7 5.8 6.0
The data in Table I1 show t h a t aromatic extraction is independent of boron trifluoride concentration; a t constant temperature, dearomatization depends only on sulfur dioxide/oil ratio. Lowering the temperature from -20' to -35' C. had little effect on the extraction. However, a temperature increase to 0' C. reduced both sulfur and aromatic selectivities. The effectiveness of promoters varies inversely with the molecular weight of the material being extracted. Desulfurization of virgin naphthas is greater than that of kerosine. However, the sulfur dioxide-boron trifluoride system is less effective for desulfurization of heavier stocks, such as cycle oil from catalytic cracking or vigin gas oil. Data obtained with these stocks are shown in Table 111.
Studies with Individual Compounds Further insight into the chemistry of extraction with promoted sulfur dioxide was obtained by extracting synthetic blends of sulfur compounds in n-heptane with sulfur dioxide-aluminum chloride mixture. In addition, equilibrium vapor pressures were obtained for boron trifluoride over mixtures of sulfur dioxide and various individual compounds. Data from these experiments serve as a basis for a proposed mechanism of promoted solvent extraction.
1200
01
loo0
E I
800
s D
lL
0
600 3 K
% W
(L K
400
-I
5
t;
d
200
0
-
0
0
1.0
M O L RATIO B F ~ :n - BUTYL SULFIDE
I.O 2.0 MOL R A T I O B F g : ETHYL DISULFIDE
0
B
A
Figure 4.
236
I .o
MOL RATIO BF=' n - BUTYL MERCAPTAN
C
Vapor pressure studies showing complex formation
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
Vol. 47,No. 2
HYDROCARBON SEPARATIONS Table 111. Extraction of Heavier Distillates with Boron Trifluoride Sulfur Dioxide Raffinate Properties
SO*, VOl.
%
Temp.,
BFa/SQ
..
0.0 1.9
50 50
Yield, vol.
%
C.
..
Sulfur, %
Desulfuriaation,
%
Catalytic Cyole Oil
..
-20 -20
1.5003 1.4695 1.4705
65 65
..
1.45 0.59 0.46
59
1.50 1.30 1.05
12 30
68
Virgin Gas Oil
..
1.4825 io0 0.b -io 94 1.4760 50 1.0 +10 89 1.4737 a Moles BFI per gram-atom feed sulfur.
..
on the other hand, extracts only 14% of 5,6-dithiadecane (nbutyl disulfide). Extraction of a variety of other sulfur compounds with sulfur dioxide and sulfur dioxide-aluminum chloride is shown in Table IV. The addition of aluminum chloride to sulfur dioxide increases the extractions as follows: thiophene, from 65 to 87%; diphenylthiamethane (phenyl sulfide), from 68 t o 93 %; 7-thiatridecane (hexyl sulfide), from 9 to 86%; and 1,3-phenyl-2, thiapropane (benzyl sulfide), from 91 t o 97%. COMPARISON WITH HYDROFLUORIC ACID. The results of previous studies with hydrofluoric acid were explained on the basis of a n acid-base reaction where the sulfur compound acts as a Lewis base by donating a pair of electrons for coordination with the acid (7-9). The effect of structure on relative basicity has been summarized as follows:
MI Extraction Studies. The effect of structure on complexing tendencies of sulfur compounds has been demonstrated by extraction from heptane solutions with anhydrous hydrofluoric acid (8, 9). Similar studies with sulfur dioxide, both alone and promoted by aluminum chloride, were carried out to show the effect of promoter and t o provide a comparison with hydrofluoric acid EFFECTOF PROMOTER. Aluminum chloride promotes the extraction of every type of sulfur compound tested. However, the amount of increased desulfurization over sulfur dioxide alone depends on the type and molecular weight of the sulfur compound being removed. The mthiols of low molecular weight are much more amenable to extraction with sulfur dioxide-aluminum chloride than with sulfur dioxide alone, as shown in Figure 3A. As the molecular weight increases to about 200, the curves converge until neither sulfur dioxide nor sulfur dioxide-aluminum chloride extracts n-thiols. tert-Thiols are extracted t o about the same extent as n-thiols by sulfur dioxide alone. On the other hand, when aluminum chloride is added, tert-thiols are more easily extracted than n-thiols. For example, Figure 3B shows that tert-thiol having a molecular weight of 150 would be 69% removed, whereas a n-thiol of the same molecular weight would be only 13%removed. Disulfides having molecular weights in the range of 100 t o 180 are almost completely extracted by sulfur dioxide-aluminum chloride, as shown by Figure 3C. Unpromoted sulfur dioxide,
Nature of Substituent
Sulfur Type
Phenvl Desulfide Sulfide
The results obtained with sulfur dioxide-aluminum chloride are consistent with this explanation. Thus, sulfides and disulfides are more readily extracted than thiols, and tert-thiols are more readily removed than n-thiols. As in the case of hydrofluoric acid, desulfurization with sulfur dioxide-aluminum chloride decreases with increasing molecular weight for a given sulfur type: However, the sulfur dioxide-aluminum chloride system shows some interesting differences when compared with hydrofluoric acid. Phenyl sulfide appears in the sulfur dioxide-aluminum chloride system to be a much stronger base than hexyl sulfide. This apparent reversal of-basicity may be partially explained in that sulfur dioxide alone extracts phenyl sulfide because of its aromatic character. The small additional desulfurization obtained with sulfur dioxide-aluminum chloride over that with sulfur dioxide alone (Table IV) is probably due to weak complexing between aluminum chloride and the aromatic nucleus. Whereas disulfides are weaker bases than sulfides in the hydrofluoric acid system, results obtained with sulfur dioxide-aluminum chloride would indicate the reverse for promoted sulfur dioxide systems. An explanation for this apparent anomaly is not evident on the basis of present experimental results.
X 0 U.
9
I
z n
U.
m 0 LL
-
MOL FRACTION OF BF3 IN SO2
MOL FRACTION OF BF3 IN SO2 I .o
MOL
RATIO
BF3:THIOPHENE
A
0
1.0
-
0
MOL RATIO BF3: PHENYL SULFIDE
1.0
MOL RATIO BF3 : MESITYLENE
B
Figure 5. February 1955
U
C
Vapor pressure studies showing no complex formation
INDUSTRIAL AND ENGINEERING CHEMISTRY
237
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
A.
0
20 */e
40 60 DESULFURIZATION
100
1
SOz-AIC13
0
80
ever, the complex must be relatively unstable because the boron trifluoride curve does not reach the theoretical curve for a 2: 1 complex even a t a boron trifluoride partial pressure of 1000 mm. Hg. A possible explanation is that the addition of the first boron trifluoride molecule decreases the electron density around the second sulfur atom. Consequently, the attraction for a second boron trifluoride molecule is reduced. Boron trifluoride shows little tendency to complex with thiophene in the presence of sulfur dioxide. The straight line curve shown in Figure 5A, although possessing a somewhat smaller slope than the curve for sulfur dioxide-boron trifluoride alone, extrapolates to the same point. Diphenylthiamethane (phenyl sulfide) also fails t o form a complex with boron trifiuoride, as shown in Figure 5B. The relatively basic aromatic, mesitylene, does not complex with boron trifluoride in the presence of sulfur dioxide, as shown in Figure 5C. By way of contrast, this particular aromatic has been shown to form a complex with hydrofluoric acid-boron trifluoride ( I O ) .
20
40
'/e
Mechanism of Extraction
60
DE AR OMAT I2 AT IO N
80
100
Both the extraction experiments with pure compounds and vapor pressure measurements provide strong evidence for complex formation between sulfur compounds and Lewis acids in the presence of sulfur dioxide. Desulfurization by promoted solvent extraction then probably involves complex formation between the electron-deficient promoter molecule and the electron-rich sulfur atom of the sulfur compound and physical extraction of the resulting complex by the solvent.
L.
RAFFINATE
0
3
6 9 SULFUR SELECTIVITY
12
t
IS
t
95 VOLS. 74% DESULFURIZATION 16% DEAROMATIZATION
83 VOLS. $8 % DESULFURIZATION 59% OEAROMATIZATION
100 VOLS.
Figure 6. Comparison of promoted sulfur dioxide with other solvents
Vapor Pressure Measurements. The straight line relationship in Figure 4 shows that boron trifluoride in sulfur dioxide obeys Henry's law. The curve for sulfur dioxide-boron trifluoride alone crosses the abscissa a t a boron trifluoride mole fraction of about 0,004, instead of a t 0 as expected. This is probably due to small amounts of impurities in the sulfur dioxide-for example, traces of moisture would form a roinplex with boron trifluoride. If such a complex were formed, the partial pressure of boron trifluoride would remain close to zero to mole fraction of about 0.5 instead of rising rapidly after only 0.004 mole fraction of boron trifluoride was added. The compound 5-thianonaiic (n-butyl sulfide) forms a 1: 1 molar complex with boron trifluoride. The vapor pressure curve is shown in Figure 4A. If the complex were completely stable, the vapor-pressure curve would remain a t zero until the mole fraction of boron trifluoride in sulfur dioxide reached point S , which corresponds to 1 mole of boron trifluoride per mole of sulfide in sulfur dioxide. Thereafter, the boron trifluoride partial pressure would be expected to increase along the line AB. Actually, the vapor pressure curve approaches line AB asymptotically; the complex therefore attains complete stability only under pressure of excess boron trifluoride. A complex similar to t h a t with 5-thianonane is formed with 1-butanethiol (n-butyl mercaptan), as shown by Figure 4B. Boron trifluoride reacts with disulfides in a somewhat different manner. Figure 4C shows that the complex probably contains two boron trifluoride n~oleculesper disulfide molecule, How-
238
so 2
4
BF3
I
2
c--2
100 VOLS. FEE0
e+
0.73% S
T
2 8 VOLS.
SULFUR EXTRACT 7 VOLS * 6.9 WT.%S
d
Y
AROMATIC EXTRACT IO VOLS. 1.1 WT,%
s
Figure 7. Two-step promoted sulfur dioxide extraction
The synergistic effect of promoted solvent may be explained on the basis that the Lewis acid forms a highly polar but relatively unstable complex with sulfur compounds alone. The presence of sulfur dioxide with its high dielectric constant stabilizes the complex and shifts the equilibrium in its favor. Once formed, the complex is selectively extracted from the hydrocarbon by sulfur dioxide. Although strong Lewis acids form stabilized complexes with sulfur compounds in mlfur dioxide, they are much less reactive
INDUSTRIAL AND ENGINEERING CHEMISTRY
Yol. 47, No. 2
HYDROCARBON SEPARATIONS Table IV.
Extraction of Miscellaneous Sulfur Compounds with Sulfur Dioxide-Aluminum Chloride
(Temperature, -2OO C.; feed, solution of sulfur compound in n-heptane: promoter concentration, 2 moles/grarn-atom sulfur in feed) Sulfur Compound Hexyl sulfide Thiophene Phenyl sulfide Benzyl sulfide
Desulfurization, Yo SOI-AlCl;
Molecular Weight
so2
202 84 186 214
9 65 68 91
86 87 93 97
toward aromatics. I n the case of sulfur dioxide-boron trifluoride no appreciable reaction is obtained with the aromatic-as disclosed in the vapor pressure studies with mesitylene. The increased sulfur selectivity of sulfur dioxide-boron trifluoride, as compared with sulfur dioxide alone, is explained on this basis. The somewhat stronger acid, aluminum chloride, apparently complexes with aromatics to some extent; the system sulfur dioxidealuminum chloride therefore exhibits greater aromatic extraction and lower sulfur selectivity than sulfur dioxide-boron trauoride. Sulfur trioxide, a strong promoter, may form complexes with sulfur compounds. However, it also acts as a strong oxidizing agent for sulfur compounds and a strong sulfonating agent for aromatics. All of these effects may contribute to the observed effects with the sulfur dioxide-sulfur trioxide system.
Practical Aspects I n order to orient the results obtained with promoted sulfur dioxide extraction, comparative experiments have been conducted with conventional polar solvents, such as furfural and dimethylformamide, and with the acid solvent hydrogen fluoride. The versatility of promoted sulfur dioxide and a unique application have been illustrated by a simulated countercurrent extraction sequence. Comparison with Other Solvents. An insight into the nature and potentialities of promoted solvent extraction is provided by comparisons with other well-known solvents. Table V shows results obtained on kerosine with hydrofluoric acid and with the organic solvents, furfural and dimethylformamide. Graphic comparisons are provided by the bar charts shown in Figure 6. The comparisons are made by interpolation or extrapolation of the data in Tables I and V on the basis of 30 volume yoof solvent, based on kerosine.
Table V.
Extraction of Kerosine with Miscellaneous Solvents (Extrartion temperature, 25' C.)
Solvent
HF
Furfural Dimethylformamide
Solvent, Raffinate Properties, % Vol. Yield, DesulDearo% vol. furization matization 30 95 77 12
25 50 100 50
100
96 91 84 87 81
14 21 36
20 32
16 27 41 29 45
Sulfur Selectivity 15
4.0 3.0 2.6 1.2 1.2
Figure 6.4 shows that promoted sulfur dioxides gives 3 to 5 times as much desulfurization as typical organic solvents. The solvents sulfur dioxide-boron trifluoride and sulfur dioxidealuminum chloride are more effective than hydrofluoric acid. Although sulfur dioxide promoted by sulfur trioxide gives less desulfurization than hydrofluoric acid, it is still several times as effective as the o'rganic solvents. Whereas boron trifluoride does not increase the ability of sulfur dioxide t o extract aromatics, sulfur dioxide promoted with aluminum chloride or sulfur trioxide effects from 2 t o 4 times February 1955
greater extraction of aromatics than is obtained with hydrofluoric acid or the organic solvents. These results are shown graphically in Figure 6B. The selectivity of extraction of sulfur compounds as compared with aromatics is provided by Figure 6C. The most effective promoted solvent, sulfur dioxide-boron trifluoride is twice as selective as sulfur dioxide alone and 3 to 6 times as selective as the organic solvents. The selectivity factor of 12 for sulfur dioxide-boron trifluoride is almost as high as that for hydrofluoric acid. Process Application. The most promising promoter from a commercial standpoint is boron trifluoride, not only because it is selective, but also because it can be easily recovered and re-used. I n addition, the difference between the sulfur dioxide and the sulfur dioxide-boron trifluoride systems in selectivity for sulfur compounds and aromatics may be employed to remove concentrates of these materials in separate extract phases. A twostep extraction process shown in Figure 7, was simulated by carrying out successive single-stage batch extractions. When the process is operated continuously, fresh feed would be contacted in the first step with boron trifluoride and solvent, which is a mixture of sulfur dioxide and aromatics produced as extract phase in the second extraction step. I n the second step, raffinate from the first step would be extracted with fresh sulfur dioxide.
Table VI.
Two-step Promoted Sulfur Dioxide Extraction of Kerosine
Yield vol. % Sulfui, wt. % Refractive index, Desulfurization, Yo Dearomatization, % '
Step 1 Raffinate Extraqt'
95
.. ..
74 16
7 6.9 1.5188
.. ..
Step_____ 2 Raffinate Extract 83 10 1.1
.. ..
88 59
1.5134
..
..
Extraction of aromatics in the first step is suppressed because the solvent ratio is low and because the sulfur dioxide is already saturated with aromatics. The presence of a promoter, such as boron trifluoride, increases the affinity of sulfur dioxide for sulfur compounds. Consequently, sulfur dioxide removes sulfur compounds from the fresh feed and liberates aromatics to the desulfurized feed. The desulfurized feed from the first step is extracted in a second zone where the aromatics are removed by unpromoted sulfur dioxide. The extract from zone 2 is divided, and a portion is used as solvent for the first extraction step. The remaining extract obtained from unpromoted sulfur dioxide is a low sulfur aromatic concentrate; the over-all desulfurization and dearomatization is slightly greater than obtained by one-step sulfur dioxide-boron trifluoride extraction. Results obtained from treating kerosine by this simulated twostep process are also shown in Table VI. The unpromoted extraction step used 100 volume yo sulfur dioxide. The promoted step employed 28 volume %, based on feed, of extract phase produced in the second step plus 2 moles of boron trifluoride per gram-atom of feed sulfur. The advantage of this type of operation over one-step extraction is that the yield of low grade high sulfur extract is reduced by about 50%. The aromatic extract of low sulfur content is a valuable by-product.
Conclusion These studies have revealed a new class of selective solvents intermediate between typical physical solvents on the one hand and highly acidic solvents on the other. Such solvent systems have possible application where it is desirable to obtain a high degree of desulfurization and a t the same time to lower substantially the aromatic content of treated stocks. Thus, the solvents are particularly useful in treatment of high sulfur, highly
INDUSTRIAL AND ENGINEERING CHEMISTRY
239
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT aromatic distillate fuels where the sulfur compounds impart a n offensive odor, as well as poor color and poor stability,, and where aromatics impart adverse burning characteristics. Acknowledgment
The authors thank J. F. Deters for obtaining the data on furfural and D. A. McCaulay and Harold Shalit for supplying the data on hydrogen fluoride.
Method D 129-52 (1953). (3) Arnold, R. C., U. S. Patent 2,673,830 (March 30, 1954). (4) Arnold, R. C., and Lien, -4. P., Ibid., 2,646,390(July 21, 1953). (5) Ibid., 2,671,046(March 2, 1954) (6) Ibid., 2,671,047 (March 2 , 1954). (7) Lien, A. P., and Evering, B. L., IND.ENO. CHEM.,44, 874 (2) Ibid., p. 78,
(1952). (8) Lien, A. P., McCaulay, D. A., and Evering, B. L., I b i d . , 41,2698 (1949).
(9) Lien, A. P., MeCaulay, D. A, and Evering, B. L.,Proc. Srd World Petroleum Congr., Sec. 111, 145 (1951). McCaulay, D. A., and Lien, a.P., J . Am. Chem. Xoc., 73, 2013
(10)
Literature Cited (1) Am. SOC.Testing
Materials, “A.S.T.M. Standards on Petroleum Products and Lubricants,” p. 22, Method D 90-50T (1953).
(1951). (11)
McCaulay, D. A, Shoemaker, B. H., and Lien, A. P., IND. ENQ. CHEM.,42, 2103 (1950).
RECEIVED for review September 29, 1954.
ACCEPTEDDecember 14, 1954.
Sulfur Compounds in Kerosine Range of Middle East Crudes S. F. BIRCH, T. V. CULLUM, R. A. DEAN, AND R. L. DENYER Research Sfation, The British Petroleum Co., ttd., Sunbury-on-Thames, Middlesex, England
The mixture of sulfur compounds extracted from an Iranian kerosine by sulfuric acid and released on dilution has been investigated. Separation of individual compounds was effected b y fractional distillation, followed b y successive partial extractions of the fractions with aqueous mercuric acetate solution, and two-stage recovery from the extracts. Compounds were identified by physical properties and b y the hydrocarbons formed on desulfurization with Raney nickel. The main constituents identified were monocyclic and bicyclic sulfides (including bridged-ring compounds) and thiophenes; dialkyl sulfides contributed only 5 to 10%. No evidence of the presence of other types of sulfur compounds was obtained.
T
H E examination of the sulfurous oil (tar oil), which separates when sludge acid from the refining of light petroleum naphthas is diluted with water, has yielded much useful information concerning the sulfur bodies present in these distillates (14, 15, 17, 81, 38). This oil, provided the distillates treated are straight run products free from unsaturated hydrocarbons, consists mainly of unchanged sulfur compounds. Present in lesser amount are aromatic hydrocarbons (6) and traces of ketones (22) of unknown origin, the solubility of these substances in the acid being enhanced by the solubilizing effect of the sulfur compounds and sulfonic acids derived from the aromatics. The investigation of the sulfur bodies of petroleum distillates through the medium of a spent refining agent such as sludge acid is, however, open to several objections, not the least of which is that i t is difficult to establish any reliable quantitative relationship between the composition of the tar oil and that of the original distillate. Thus the solvent action of the acid is almost certainly selective for certain types of sulfur compounds and will also vary with molecular weight; extraction is often incomp1et.e and is affected by acid concentration, acid-hydrocarbon ratio, and temperature while the products of chemical reaction of the acid-e.g., aromatic sulfonic acids, water from oxidation of mercaptans, and sulfonation-have a marked effect on its solvent action. Nevertheless, tar oil undoubtedly provides a convenient source of those neutral compounds which are soluble in, but unaffected by, concentrated sulfuric acid. Sludge acid, being a waste material, is available in quantity. Separation of the tar oil is simple and, if desired, can be carried out on a very considerable scale. Alternative methods for concentrating the neutral sulfur bodies, which are dependent on the formation of metal salt or nonmetal-
240
lic halide complexes-e.g , mercuric salts, aluminum chloride ( I S ) . boron trifluoride, or on some form of adsorption-do not readily lend themselves to large scale operations nor are they as easy to conduct. With large quantities of material available, separation can be attempted on a scale adequate to provide samples of end products for physical and chemical examination. Certain of the compounds isolated in the present worlr were found t o be difficult to synthesize, and it proved easier to separate them directly from the tar oil, in sufficient quantity and of the required purity for determination of physical properties. Further. iarger scale working Lends to favor detection of substances present only in a small amount. Another advantage is that identification can sometimes be effected through desulfurization products, when isolation of individual sulfur compounds proves impossible The first investigation in these laboratories of the sulfur compounds recovered from acid sludge was concerned with material derived from the treatment of the sulfur dioxide extract of a 100” to 160” C. naphtha of mixed Persian origin ( 1 7 ) . Only alkane and simple cyclic sulfides could be identified; disulfides, if present, were not there in sufficient quantity to enable separation to be effected by the methods employed, mainly fractionation and adsorption on silica gel. The investigation haa now been extended to the kerosine boiling range, tar oil from the treatment of a refinery tractor fuel blend of similar origin being used. Since the boiling ranges of the naphtha and the tractor fuel blend overlapped, some of the compounds identified were found in both. Unlike the tar oil from the naphtha, that from the higher boiling kerosine contained only 5 to 10yoof alkane sulfides; the bulk consisted of cyclic sulfides and thiophenes. Of the
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
Vol. 47, No. 2
