Hydrodesulfurization of Gasoline Fractions with Tungsten-Nickel Sulfide Catalyst ROBERT M. COLE'
.
.
A process has been
developed for the desulfurization of catalytically and thermally cracked gasoline fractions by controlled hydrogenation over a commercial tungstennickel sulfide catalyst. The catalyst has excellent activity and life and can be successfully regenerated. The process has been used on a large scale to augment supplies of aviation gasoline. This paper presents a brief description of the pilot plant and data obtained in the laboratory and pilot plant during the development of the process.
D
URING the war it became urgently necessary to inareitst' the rich-mixture rating of aviation gasolines by the incorporation of considerable percentages of aromatics. Obvious sources of aromatics were catalytically cracked gasolines and some thermally cracked gasolines. However, many of these stocks were unsuitable because of high sulfur and olefin contents, and some form of additional treatment was necessary. Conventional refining with sulfuric acid gave very high treating losses, and therefore hydrodesulfurization was investigated as an alternative. Using a commercial tungsten-nicbel sulfide catalyst manufactured by Shell Oil Company, Inc., a process was developed for the controlled hydrogenation of these stocks to give quantitative yields of products meeting aviation gasoline specifications. The catalyst proved to be both rugged and sulfur-resistant and the process simple and economical to operate. Therefore, commercial plants were constructed at the Wood River, Ill., and Dominguez, Calif., refineries of Shell Oil Company, Inc., greatly augmenting their supplies of aviation gasoline. DESCRIPTION OF PROCESS The process can best be visualized by reference to Figure 1, which is a simplified flow diagram of the pilot plant. a
AND
D. D. DAVIDSON
Shell Development Company, Wilmington, Calif.
1
Present addreas, Shell Development Company, Emeryville, Calif.
Z
P HYDROGEN
,
9A n
RECYCLE COMPRESSOR
SODA SCRUBBER
The liquid feed and hydrogen were preheated in a gas-fired furnace and passed through the adiabatic reaction chamber. The products were condensed and the phases separated. The gas phase rincipally hydrogen, was scrubbed t o remove hydrogen suffifle, compressed, and returned to the reheater along with the make-up hydrogen. The liquid was a i 0 scrubbed t o remove hydrogen sulfide and was then generally doctor sweet and needed no further treatment. The ilot plant reactor consisted of a 12-foot by 4.25-inch inside 8ameter tube of 4 to 6% chrome steel. T o prevent heat losses, the reactor was electrically compensated. The bottom 6 inahes contained ceramic balls 0.5 inch in diameter which acted as a catalyst support. Upflow was used through the 6foot catalyst bed containing 0.57 cubic foot of 0.25-inch catalyst pellets, the remainder of the tube being empty. The catalyst had a bulk density of about 130 pounds per cubic foot and a surface area of about 10 square meters per gram. The catalyst contained approximately 40% tungsten and 25% nickel by weight. Three smaller units used f o r laboratory studies were similar in flow, except that the hydrogen was not recycled. The reactors consisted of 0.5-inch iron-pipe-size, stainless-steel tubes, each containing 25 to 50 ml. of 6/arinch pellets. Although these small reactors were not adiabatic, it was possible to obtain excellent correspondence between the Iaboratory and pilot plant results. Typical operating conditions for the pilot plant were: Pressure, lb. per sq. inch LMoleretio, hydrogen t o feed Liquid hourly space velocity Liquid feed r a p gallons per hour Temperature, b.
720 7 to 1 10 42 650
The liquid yield was slightly over 100 volume yobecause of the decrease in density on hydrogenating sulfur compounds, olefins, and aromatics. The sulfur compounds in the feed showed up a5 pentanes, hexanes, etc., in the liquid product. Because the pilot plant was designed to process about 25 barrels per day, considerable quantities of feed stock were required for evaluation of process variables. Laboratory data indicate that blends of thermally cracked gasoline and low-sulfur straight-run gasoline were suitable for this purpose. Therefore, processing studies were carried out with blends having an average sulfur content of 0.25% by weight (about 80% thiophenic) and with an average bromine number of 25. All these blends boiled in the aviation gasoline range. The data thus obtained were supplemented by runs on fractions from catalytically cracked gasolines. Some results with specific feed stocks are given herewith.
PROCESS VARIABLES L LIQUID
FEED
ADIABATIC
AS
F FROM STORAGE
(U
In the following discussion, the extent of reaction is generally measured in terms of per cent sulfur retention, which is defined as 100 times the pounds of sulfur in the liquid product divided by the pounds of sulfur in the feed. Similar ratios are used to define olefin retention and aromatic retention. The catalyst age is measured in barrels of product per barrel of catalyst.
LIQUID PRODUCT TO STORAGE
PREHEATER
Figure 1. Simplified Flow Diagram of Hydrodesulfurization Pilot Plant
PRESSURE
The effect of increasing total reaction pressure is beneficial, Lower pressures not only reduce conversion but also cause more rapid deactivation of m shown in Figure 2, for fresh catalyst.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2712
the catalyst. This is shown in Figure 3 for operation a t 800" F. maximum catalyst-bed t e m p e r a t u r e . This curve gives the increase in sulfur retention as the catalyst age increases from 2000 to 3000 barrels of feed per barrel of catalyst at various pressures. At lower temperatures the catalyst life is very greatly extended, but it is still considered desirable to maintain a total pressure of at least 650 pounds per square inch. The preferred range is 700 to 1000pounds per square inch.
REACTION PRESSURE
, P5I.G.
Figure 2. Effect of Total Pressure on Sulfur Retention at 800" F. 5 liquid hourly space velocity 7 to 1
mole ratio of hydrogen t o liquid feed
SPACE VELOCITY
At a given liquid hourly space velocity the conversion is a function of both catalyst age and temperature. Some typical results with an aged catalyst are shown in Figure 4. In coinmercial practice it is considered desirable to design for moderate liquid hourly space velocities of the order of 5 to 15, even though sat,isfact,ory desulfurization may be obtained a t much higher throughputs. This conservative design allows a considerably longer catalyst life before the activity declines to a point vc-here the desired degree of desulfurization cannot be obtained. For a plant of a given capacity the use of low space velocities does not increase eit,her the heat or recycle gas requirements but merely requires a somewhat larger adiabatic reactor. Thus, the operating cost is not increased and the capital cost is only marginally greater. TEMPERATURE
With a fresh catalyst it is generally necessary to operate at low temperatures or a t very high space velocities to control the extent of react,ion. Such control may be necessary t o prevent the loss of valuable aromatics or olefins or simply to minimize hydrogen consumption. Because operation at constant and moderate space ve1ocit;- is 100 60.
'
40
1
I
I
Vol. 41, No. 12
generally preferable, it is common to initiate the reaction a t a low temperature. As the catalyst activity declincs, the temperature is gradually raised to maintain the desiied amount of desulfurization. The practical range is about 450" to 800" F. Above 800" F., unless the sulfur content of the feed is low, the rate of decline in catalyst activity is greatly accelerated, and regeneration soon becomes necessary. The extent of reaction has been found to correlate best with the maximum bcd temperature, and therefore the temperature iise in the adiabatic reactor must be taken into account. This is approximately 4 " F. for each mole per cent of olefins in the feed hydrogenated (at 7 to 1 mole ratio of hydrogen t o liquid feed). If the reaction is allowed t o proceed so far that aromatics are hydrogenated, the temperature rise is, of course, much greater. In practice the reaction is controlled by varying the preheat temperature. Figure 5 gives some pilot plant data on the variation of sulfur retention with temperature for a fresh catalyst. With an aged catalyst the curve would be shifted to the right. HYDROGEN RATIO AND CONSUMPTION
Variations in the mole ratio of hydrogen to liquid feed above 2 to 1 have very little effect on the sulfur retention. To maintain the catalyst activity, however, it has been found desirable to recycle about GO00 cubic feet of gas per barrel of charge. When the liquid feed is in the aviation gasoline range, this corresponds to a mole ratio of about 7 to 1. This is adequate to'niaintain the catalyst activity if the hydrogen concentration in the recycle gas does not fall below about 80Vc. The hydrogen consumption may be calculated almost quantitatively from the changes in sulfur, olefin, and aromatic content of the feed. I n addition, it is, of course, necessary t o allow for leakage and a small loss in the liquid product. Because very little fixed gas is produced in the process, there is little tendency for inerts to build up in the recycle stream if reasonably pure hydrogen is used for make-up, particularly as the liquid product will absorb and remove a certain amount of contaminants. If impure hydrogen is used, however, a portion of the recycle gas must be discarded to maintain the desired hydrogen concentration, or else the impurities (principally light hydrocarbons) must be removed by conventional means. FEED COMPOSITION
For a given set of operating conditions the sulfur retention is independent of the sulfur content of the feed. Hydrogen sulfide in the recycle gas stream also has little influence on the catalyst activity, but the catalyst is mechanically more rugged if
I 40
\ I
20 \
10 8
6
I
\
I
0 0.4 300
400
500 6
600
700
800 1
LlQUlD HOURLY SPACE VELOCITY
PRESSURE, !FIG.
Figure 3.
Effect of Total Reaction Pressure on Loss of Catalyst Activity
Figure 4,
Effect of Liquid Hourly Space Velocity on Desulfurization
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949
4
0
20
r
1
I-
o 2
8 20
x a
W
t; E
2713
IO 8
8
5
6
15
m
4z
4
2
'0
8
2
a
m 5
I
450
500
550
600
MAX. CATALYST
a
700
650
BE0 TEMP.
750
excessive amounts of hydrogen sulfide are not recirculated. Therefore, it is standard practice to treat the recycle stream for removal of hydrogen sulfide unless the sulfur content of the liquid feed is fairly low. Sulfur compounds are hydrogenated more readily than olefins and to a certain extent inhibit their reaction. This is illustrated in Figure 6 for a case where the sulfur content of the charge was increased by adding a concentrate of substituted thiophenes extracted from catalytically cracked gasoline. Olefins, in turn, inhibit the hydrogenation of aromatics, and as long as the bromine number of the product does not drop below about 3, the loss of aromatics is negligible. I n producing aviation blending stocks, i t is generally desirable to desulfurize as completely as possible without loss of aromatics. This can be conveniently accomplished by running for a bromine number of about 5 to 10 in the product. This allows sufficient latitude for control purposes and comfortably meets all aviation gasoline requirements. Under these conditions, diolefins will usually be completely hydrogenated. As the average boiling point of the feed stock increases, i t is necessary to increase the average reaction temperature somewhat in order to maintain the same sulfur retention. Some representative values are as follows:
150 200 250 300
F
Catalyst Bed Inlet Temperature,
08
06
12
IO
Figure 6.
Effect of Sulfur in Feed on Hydrogenation of Olefins Bromine n u m b e r s of feeds 35 to 47
100
80
z
2
60
Iz w
40
I-
W
x
z
LL
w 20 -1
0 I-
z w o
10
E 8 Q
6
2
I
4
6
20
8 1 0
40
60
80
PER CENT SULFUR RETENTION
Figure 7.
Effect of Boiling Range of Feed on Relation of Olefin Retention to Sulfur Retention
Three fractions of California catalytically cracked gasoline Fraction
I 11 I11
Boiling Range,
F. 160-255 255-320 320-340
Sulfur, Wt.
%
Bromine No. 106
33 12
F.
455 515 580 640
^*
c
04
WEIGHT PER CENT SULFUR IN FEED
Figure 5. Effect of Maximum Catalyst Bed Temperature on Desulfurization at 10 Liquid Hourly Space Velocity
A.S.T.M. 50% Point of Feed,
02
0
OF:
The rate of hydrogenation of olefins decreases more rapidly with boiling point than the hydrogenation of sulfur compounds. This is illustrated in Figure 7 , which shows the olefin retention versus sulfur retention for three fractions froin a Los Angeles Basin catalytically cracked gasoline. The narrow spread between fractions I1 and I11 is probably caused by the higher sulfur content of fraction 11. CATALYST LIFE
The catalyst may lose its activity rapidly if it is overheated or if the hydrogen recirculation fails. Normally the loss in activity is gradual and is caused by the accumulation of tarry deposits, u hich also increase the pressure drop across the bed. The rate a t which these deposits form is a function of the cleanliness of the feed, and it is therefore desirable to use freshly distilled charge stocks which are held in gas-blanketed tanks to prevent contact with air. Catalyst deposits can also be minimized by the use of a small removable guard section of old catalyst at the inlet to the catalyst bed.
It has been found that the pressure drop may be reduced and the activity restored by recirculating hydrogcn without feed for several hours at a temperature of 900" to 1000" F. But after two or three such treatments the carbon deposits on the catalyst build up to a point where hydrogen recirculation is no longer effective, and the catalyst must be regenerated with steam and air. It is possible to process 15,000 t o 25,000 barrels of feed per barrel of catalyst before regeneration is necessary. The ultimate catalyst life is seldom determined b y loss in activity but by the slow mechanical disintegration of some of the catalyst pellets. After two or three steam-air regenerations it is generally necessary to drop the catalyst from the reactor and to screen it to remove fines. The unbroken pellets are recharged to the reactor. The only catalyst actually consumed is that necessary t o replace the fines and to permit occasional renewal of the guard section. Therefore, the ultimate catalyst life is extremely long and becomes a negligible factor in the cost of operating the process. REGENERATION OF THE CATALYST The act,ivity of the spent catalyst can be completely restored to that of fresh catalyst by oxidizing with air in the presence of
Vol. 41, No. 12
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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-
Table I.
Comparison of Feeds and Products i n Hydrodesulfurization of Catalytically Cracked Gasolines Los Angeles Basin Naphtha -.
Crude source Catalytic cracking srook
Feed Gravity, 'A.P.1. Bromine number Maleic anhydride value6 Sulfur wt. 70 Aromktios, wt. Gum, accelerated, mg.il0U I i 1 i . C Aviation method octane number C:.F.R. F-4 performance number A.S.T.M. distillation, F.
52.2 66 12 0.065 39
+
. . i
4.6 mi. TEL
I.B.P.
5
Si1 0 001s
I1
.
4 98.8 148
PI3
110 137 207 269 288
144
10%
209 270 280
%3point 5
90
55.0
. ~ .
50%
a b
Pl~oduct
Los Angeles Basin Flashed Distillatea Feed Product
19.1 75 38 0.26
33 80
..
54.0 8.4 0 0.002
33 3 96
.
Feed
Depentanized product
56.0 68
2.3
*
~
...~
208 277 312
286
316
co.01 32 ~ 2 93.5 119 3.44 170 206 277 312
1138 170
I38 177
~
I . . .
0.10 29 l..l
230
West 'Tex,as Plashed..~~~. Distillateo Feed Ihpentanieed product
5 8 ~ 0
....
*
120
I52 188 238 287 315
Weat Texas
Flashed Distillateo _______
Very heavy gas oil produLed b y high temperature > a o u u i u f i i t n l r i n g of straight-run residue. Reflux method of Ellis and Jones (D. (1).
Figure 8. Variation of Clear Octane Number with Extent of Desulfurization Three rractions of California thermally cracked gasoline Boiling Range, Sulfur, A.S.T.M. Fraction F. Wt. % Octane No. I 0.4i 73.8 70.4 I1 0.66
I11
1.04
66.6
steam and then resulfiding with hydrogen sulfide. T o carry out such a regeneration, the pressure is dropped to atmospheric and the catalyst bed is purged with steam at 850' to 950" F. During this operation some hot zones may appear in the catalyst bed and some hydrogen is evolved. Following the purge, air is added to the inlet steam at such a rate as to maintain the catalyst hot zones in the range of 1000" to 1250" F. An oxygen concentration of about 2% is generally adequate. The hot zone temperature is controlled almost entirely by the oxygen concentration in the inlet gases and is practically independent of vapor velocity. However, as the oxygen is almost quantitatively consumed, the total time for regeneration is inversely proportional to the rate at which air is admitted. When the hoe zones have passed through the catalyst bed and unreacted oxygen builds up in the exit gases, the regeneration is considered complete. Following the oxidation step, hydrogen sulfide is admitted a t a temperature of 850' to 950" F. and atmospheric pressure. Another wave of hot zones will occur. These are again controlled to a maximum temperature of about 1250" F. by steam dilution. About 10 mole % hydrogen sulfide in the mixture is generally satisfactory. The resulfiding is considered complete when the hot zones have passed out of the bed. I n this work the moles of hydrogen sulfide required 17 ere generally about equal to those of oxygen consumid in thP oxidation step.
I
WEIGHT PER CENT SULFUR IN PRODUCT
Figure 9. Variation of Leaded Octane Number with Sulfur Content Three fractions of California thermally cracked gasoline Boiling Range, Sulfur, A.S.T.M.Octane Fraction F. Wt. 70 No., 1 M1.TEL I 1134210 0.41 76.5 I1 1%-190 0.66 73.3 I11 29C-370 1.04 69.1
I
I
2
4
6
8 1 0
I
I
20
40
I
I
50 80
PER CENT SULFUR RETENTION
Figure 10. Variation of Olefin Retention with Sulfur Retention on Hydrodesulfurization Three fractions of California thermally cracked gasoline Boiling Range, Sulfur, Bromine Wt. % No. Fraction F. I 110-210 0.41 ea 180-290 0.66 87 I1 I11 290-370 1.04 60
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1949
Table 11. Hydrodesulfurization of Straight-Run Gasoline and Severely Cracked Aromatic Distillate
(fmvity, ' A.P.J. Bromine number Sulfur nt. % Maleid anhydride value Aromatics, wt. % A\.S.T.M. octane nnrnber 1 ml. TEL I-C octane number f 4.6 ml. TEL E'-4 performance number A.S.T.M. distillation, F.
Aromatic Distillate Feed Product 34.1 37.2 28 0.5 0.08 0.003 70 0.3 77 69
+
w
I.B.P.
10% 60% 90%
End point
*..., 90 ,,
192 207 230 268 293
.,
95 190
Venioe Gasoline Feed Product
__-
,.*.
....
0.61
...'
...
....,
L
62.7
68.7
. ~ . ,
,.... ,....
,.I.
183 202 225 263 322
.. .. .. ....
0,008
I
100 .,.,
....
...*
400
100
...... .... ..... 400
Q
PROPERTIES OF THE PRODUCTS
2715
increase the aromatic content. Therefore, partial desulfurization of these stocks was investigated with a view to increasing their usefulness in leaded motor fuels. Three fractions from a Los Angeles Basin thermally cracked gasoline were used. Figures 8 and 9 show the variations in clear and leaded octane number with sulfur content for each fraction. Figure 10 gives the relations between sulfur retention and olefin retention for these materials. The lower boiling fractions proved most amenable to this treatment. They gave small increases in both clear and leaded octane number with partial desulfurization. It is possible by severe treatment to produce potential aviation blending stocks by thermal cracking. One such material is an aromatic distillate produced as a by-product of butadiene synthesis by vapor-phase cracking of mixed Los Angeles Basin naphthas a t high temperature in regenerative stoves. Table I1 gives some properties of this material before and after hydrodesulfurization.
CATALYTICALLY CRACKED GASOLINES
STRAIGHT-RUN GASOLINE
The most suitable boiling range of catalytically cracked gasoline ior optimum results in hydrodesulfurization depends on the nature of the feed stock, the end use, and in some cases, limitations o f the hydrogen supply, which might make it desirable to eliminate highly olefinic fractions. In the present case, the interest was almost wholly in producing aviation blending stock, and generally the feed was depentanized and had an A.S.T.M. end point of 270' t o 320" F. However, other fractions were also investigated. Table I gives some results for catalytically cracked fractions from Los Angeles Basin and West Texas flashed distillates and Los .4ngelw Basin naphtha.
Straight-run gasolines are generally relatively low in sulfur and are also comparatively easy t o refine, because they contain no olefins. However, it was of interest to examine a high-sulfur straight-run gasoline. A sample from Venice, Calif. , crude was selected, and the results are shown in Table 11.
ACKNOWLEDGMENT The authors take pleasure in acknowledging the assistance of many members of the Wilmington and San Francisco staffs of the Shell Development Company. They are particularly indebted t o A. J. Johnson for his guidance and advice.
LITERATURE CITED
THERMALLY CRACKED GASOLINES
Conventional thermally cracked g a s o h e s are generally unsuitable for the production of high-quality aviation blending stocks unless they have been subjected to a severe treatment to
(1) Am. SOC.Testing Materials, A.S.T.M. D 873. (2) Ellis and Jones, Analyst, 61, 812 (1936). RWOBIVBD June 14,
1949.
STABILITY OF FUEL OILS IN
STORAGE Effect of Sulfur Compounds b
R. B. T H O M P S O N , L. W. DRUGE,AND J. A. CHENICEK Universal Oil Products Company, Riverside, Ill. Because high-sulfur furnace oils and doctor-sweetened oils have been reported to have poor storage stabilities, the effect of a number of sulfur compounds of the type that may be present in fuel oils has been investigated. Changes in the oils during storage were followed by determining sludge and soluble gum. The following effects were observed : Free sulfur promotes sludge formation: thiophenes, aliphatic mercaptans, and aliphatic sulfides show little effect : disulfides and polysulfides promote sludge formation : and thiophenol is particularly effective in forming sludge. The effect of di- and polysulfides suggests a reason for the adverse action of doctor sweetening. On the other hand caustic methanol extraction, by which the mercaptans are removed, gives a product of enhanced stability.
W
I T H the widespread use of cracked and virgin stocks in blends of fuel oils, some difficulties have arisen wit,hrespect to sludge formation, especially in blends containing components derived from high-sulfur crudes. The same high-sulfur crudes yield oils that are usually in need of some treatment to improve odor for sales purposes; in several cases doctor sweetening has resulted in products of decreased stability. I n certain oils sufficient sludge has been formed to permit its analysis. With two different oils consisting of blends of catalytic and virgin stocks, sludges of high sulfur content precipitated as shown in Table I. The greater sulfur content in the sludge as compared with the oil indicates that sulfur compounds play a prominent role in the formation of sludge. Further evidence, which is more suggestive of a method of investigating such a problem, was afforded by a series of experiments studying different methods of sweetening the sour compo-
