I
F, W. KIRSCH, HEINZ HEINEMANN, and D. H. STEVENSON Houdry Laboratories, Houdry Process Corp., Marcus Hook, Pa.
Selective Hydrodesulfurization of Cracked Gasolines Without loss of octane number, gasolines of high sulfur content can be hydrodesulfurized sufficiently for blending into refinery pools
INCREASING
USE of high-sulfur content crude oils has focused attention on methods of desulfurizing cracked gasolines obtained from them. The Houdresid process (3, 7) for cracking whole crudes or residua, has made it possible to crack high-sulfur feeds because the catalyst is unaffected by such content of the charge stock. However, if the sulfur content of the residuum is high, some
0.4
will appear in the product gasoline; thus, to permit blending in the refinery gasoline pool, desulfurization is necessary. Although the process described here was developed primarily for use with the Houdresid process it is equally applicable to other catalytically cracked gasolines. Ring-type sulfur compounds are the most difficult to remove from petroleum
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fractions; yet they constitute a large percentage of all sulfur compounds in catalytically cracked distillates. Hydrogenation with conversion to hydrogen sulfide is the most effective method known for all types-the product gasoline shows an improved lead response, less corrosiveness, and better stability. For catalytically cracked gasolines, which are usually rich in olefins and aromatics, both high octane number hydrocarbon types, hydrodesulfurization must be selective to minimize hydrogenation of olefins and aromatics, and preserve the octane number. Improved lead response resulting from desulfurization, however, can compensate and often outweigh a minor loss of clear octane rating. Casagrande and others ( 7 ) have described a selective desulfurization process, using a tungsten-nickel sulfide catalyst, which permits about 60y0 desulfurization without loss of octane number. However, a higher degree is frequently necessary. 'Without loss of octane number, 807, or more desulfurization can be obtained by hydrodesulfurizing, under carefully chosen operating conditions, only selected gasoline fractions and subsequently reblending treated and untreated fractions. Selectivity can also be improved by using naturally occurring inhibitors to prevent olefin saturation. Exper imenta I
U
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200 MID-POINT Figure 1.
OF FRACTIONS,
OF.
Sulfur distribution in Houdresid gasolines
0 Mixed crude source A Wofro
646
400
300
INDUSTRIAL AND ENGINEERING CHEMISTRY
All hydrodesulfurization runs were made in isothermal stainless steel reactors assembled in a typical flow system for moderate pressure operation ( 4 ) . Liquid products were stabilized at a vapor temperature of 75' F. and then washed with cadmium chloride solution to remove hydrogen sulfide. Weight balances showed substantially no cracking
H Y D R O G E N IN THE PETROLEUM I N D U S T R Y
96
and only traces of low-boiling products were found. Hydrocarbon-type analyses were done by mass spectrometer, and based primarily on the method of Lumpkin and Thomas (5). A heated inlet system vaporized all the sample. Houdresid and catalytically cracked gasolines were studied; synthetic stocks were blended from commercial-grade isoheptenes, n-heptane reference fuel, and thiophene. Hydrogen was purified over platinum catalyst a t 800' F. and dried before use. Houdry Process Corp.'s Series A cobalt-molybdena-alumina catalyst was used for all runs.
-
. a
260'- BTM
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LL
F-l t 2CC TEL
w
z 92 a
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6 0
Composition of Catalytically Cracked Gasolines
High-boiling fractions contain most of the sulfur present in Houdresid and other catalytically cracked gasolines (a), and are poorest in olefins. Cole a n d Davidson (2) observed that highboiling olefins hydrogenate less readily than lower boiling olefins, while the hydrogenation rate of sulfur compounds is not similarly sensitive to boiling range. Selective sulfur removal by hydrogenating the high-boiling fractions therefore appeared likely. When the fractions processed are blended with untreated light cuts, over-all olefin hydrogenation should be small, with little if any loss in octane number. I n Figure 1, one of the gasolines is a commercial Houdresid gasoline from the Sun Oil Co. in Sarnia, derived from a mixed base crude and containing 0.0770 sulfur over the 110' to 420' F. boiling range. The other, a pilot plant product from Houdresid cracking of a Wafra crude, contains 0.5701, sulfur over
40 80 DESULFURIZATION, % Figure 3. Hydrodesulfurization of catalytically cracked Arabian gasoline. Temperature, 650" to 700" F. at 300 pounds per square inch gage; LHSV, 3 to 24; hydrogen-oil molar ratio, 3 0 Processing whole gasoline A Processing 360' F.-to-bottoms fraction
its 123' to 404' F. boiling range. I n both cases, a much lower average sulfur content prevails below about 325' F. than in the total gasoline. Catalytically cracked gasolines from gas oil give similar pictures. A commercial catalytically cracked gasoline containing 0.3170 sulfur from a n Arabian crude was cut in three fractions (Figure 2). For the Arabian and
ARABIAN
WAFRA
160-r-l
-
120-
804
m -
- 0
g
-
08-
Z o- -4_ _;- - 13
LL -I
I _ 260
SULFUR ---, 260 -360'
0
rl
1 7 tli3 04-
----
360'- ETM.
v)
3
''0
20
F-l CLEAR
91.7
r
v,
€0
40 92.1
80
88.0
100
m - 0 0
926
92 I
934
93.4
87.3
GASOLINE FRACTIONS, VOL. %
Figure 2. Composition of Houdresid Wafra and catalytically cracked Arabian gasolines; temperature, " F.
Wafra gasoline, 82 and 42%, respectively, of the total sulfur is in the 360" F. +fraction. Also, the fraction with the highest sulfur content has the lowest bromine number and octane number (Figure 2). Table I shows that only 8 to 11% of the total olefins present are in the highest boiling fractions. Therefore, complete hydrodesulfurization of these fractions would cause only relatively minor reduction of the total olefin content and a minor loss of octane number. Hydrodesulfurization. The catalytically-cracked Arabian gasoline (Figure 2, Table I) was selectively desulfurized over commercial cobalt-molybdenaalumina catalyst. Processing of the whole gasoline and two fractions shows the advantage of hydrotreating the highboiling range material and then blending with untreated light gasoline (Figure 3). Severity was varied by changing space rate between 3 and 24 volumes per volume per hour and maintaining other operating conditions constant a t 700 ' F., 300 pounds per square inch gage, and a hydrogen-hydrocarbon molar ratio of 3. When the whole gasoline was hydrotreated, octane number loss began after 20Y0 desulfurization (Figure 3). Treating the 260' F.-to-bottoms fraction (66.ly0 by volume) and reblending with the untreated fraction, resulted in a smaller octane number loss at 45 to 75% desulfurization. Removing 80% of the sulfur by either method, however, produced a loss of 5 octane numbers. VOL. 49, NO. 4
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APRIL 1957
647
DESULFURIZATION, % Figure 4. Hydrodesulfurization of whole Houdresid Wafra and catalytically cracked Arabian gasolines. Temperature, 650" to 700' F. at 300 pounds per square inch gage; LHSV, 6 to 25; hydrogen-oil molar ratio, 3
0
A
Arabian Wafra
The gasoline then contained about 0.0670 sulfur. A gain of about 0.6 octane number occurred by treating the 360' F.-tobottoms fracIion (31.7'30 by volume) and blending with untreated material to obtain 0.067c sulfur in the finished gasoline. Table I1 gives a breakdown for one case of treating whole gasoline and two of its fractions. Table 111 shows the results of hydrocarbon analyses. The aromatic content of the 360' F.-to-bottoms fraction after processing equaled that before treatment; the whole gasoline obtained by blending with this fraction is therefore also unchanged in its aromatic content of 42yG. Hydrogenating 14y0 of the total aromatics accompanied the 5-octane number loss obtained by hydrotreating either the whole gasoline or the 260' F.-to-bottoms fraction. Since the 260' to 360' F. fraction contains 69y0 of all the Ca, 75% of the Cg, and O in the total 25Y6 of the C ~ aromatics gasoline, the C Sand Cg aromatics appear to be particularly susceptible to hydrogenation a t these conditions, and must be preserved. When the total Arabian gasoline is hydrotreated to 8070 desulfurization, a n olefin hydrogenation of 51% occurs. If only the heaviest fraction is treated and blended with the untreated fraction, olefin hydrogenation is only 25% a t 80% total desulfurization. Treatment of fractions of the Wafra gasoline confirmed these results for the Arabian gasoline; however, when the whole Wafra gasoline is treated, octane loss was smaller. At 80% desulfurization (0.114y0 sulfur in the product) the F-1 clear octane number dropped from 93 to 91; at 90% desulfurization (0.06% sulfur in the product) it was 90. Comparing extent of olefin saturation a t various desulfurization levels when
648
Table I.
hydrotreating the total gasolines (Figure 4) shows that the Arabian gasoline is more sensitive to hydrogenation and therefore, loses octane rating more easily. This is true even though Wafra contains more total olefins than the Arabian (bromine numbers 92 and 47, respectively) as well as more olefins in the lower boiling range where the rate of olefin hydrogenation is Faster (2). Better preservation of octane number by Wafra is the combined effect of relatively large amounts of cyclic olefins (characceristic of Houdresid gasoline) in the gasoline (Table I) ; larger amounts of light paraffins; and smaller amounts of aromatics (33 us. 42YG) that make it less sensitive to aromatics hydrogenation. Factors Affecting Catalyst Selectivity. Meerbott and Hinds (6) reported that olefins inhibit desulfurization. This was confirmed in the present work by measuring desulfurization and olefin hydrogenation with mixtures of thiophene, heptane, heptenes and C7 cycloolefins.
Hydrocarbon-Type Composition of Two Catalytically Cracked Gasolines (T'ol.
Yo of whole gasoline)
Wafra Init. 250°
Paraffins Naphthenes Monoolefins Cycloolefins Aromatics
Table II.
16.1 3.3 17.3 6.7 4.5
Arabian Init.
250-360' 360°-btms. Total 4.1 2.5
5.1 6.6 17.6
2.3 0 2.0 1.3 10.6
22.5 5.8 24.4 14.6 32.7
19.2 4.7 4.6 3.0 2.4
After treatment at 700' F., 300 p.s.i.g. LHSV, vol./vol. Sulfur, wt. % BrP no. F-1,clear Gasoline, after blending treated and untreated fractions Sulfur, wt. Olefins, vol. % F-1,clear
4.4 3.8 1.3 0.4 21.8
28.7 13.6 10.4 5.6 41.7
Gasoline
Bottoms
Bottoms
100 0.312 46.7 89.7
66.1 0.412 31.5 90.7
37.1 0.741 20.6 88.0
6 0.061 22.0 85.0
6 0.070 12.0 84.7
0.026 4.9 85.0
0.061
0.060 14 85.0
0.060 16 90.3
7 85.0
3
Hydrocarbon-Type Composition of Treated Arabian Gasoline after 80% Desulfurization Wt. %
F-1, Clear
Paraffins, Vol. %
Processed fraction Whole gasoline 260' F.4- ; blended with Init. 260' F. 360' F. ; blended with Init. 360' F. Gasoline before processing
0.062
85.0
0.063
+
Xaphthenes Olefins,
+
Vol. %
Aromatics, Vol. o/o
31
33
36
85.0
36
28
36
0.060
90.3
30
28
42
0.312
89.7
29
29
42
S,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
5.1 5.1 4.5 2.2 17.5
Hydrotreatment of Arabian Gasoline over Cobalt-MolybdenaAlumina Whole 260" F.360°1?'.-
Gasoline, vol. % Fraction Sulfur, wt. Yo Br2 no. F-1, clear
Table 111.
260-360' 360°-btnis. Total
260'
H Y D R O G E N IN T H E PETROLEUM I N D U S T R Y T h e addition of 20% heptenes to a heptane-thiophene mixture (0.5y0 sulfur) reduced desulfurization from 79 to 65y0 over cobalt-molybdena-alumina at 700" F., 300 pounds per square inch gage, and 1.1-second contact time (liquid hourly space velocity, 48). When 4% of the heptenes in this mixture was replaced by C7 cycloolefins, desulfurization was further reduced to 44% at LHSV of 48. Selectivity of desulfurization over olefin hydrogenation as a function of temperature and pressure has been studied for nickel-tungsten sulfide catalyst by Cole and others ( 2 ) and Casagrande and others (7). They have shown that rate of olefin hydrogenation increases faster with pressure than that of desulfurization. I n the present study, effect of pressure a t constant contact time for a pure-compound mixture (heptaneheptene-thiophene) with a cobalt- molybdena-alumina catalyst was investigated. Rate of hydrogenation for olefins is independent of pressure, and is a function of contact time only (Figure 5 ) . This behavior indicates a first-order reaction. Hydrodesulfurization, on the other hand, improves as pressure is decreased at constant contact time, and its order of reaction must be lower than first. Therefore, the most selective desulfurization will occur at the lowest pressure compatible with good catalyst life. Catalyst Life. I n view of the good results obtained by desulfurizing gasoline fractions over cobalt-molybdena-alumina at 300 pounds per square inch gage, testing catalyst stability seemed neces-
sary. A life test, carried out with a commercial high-boiling fraction of the same Arabian catalytically cracked gasoline previously discussed, is characteristic (Table IV). This fraction, distinct from that described in Table 11, was hydrotreated at 300 pounds per square inch gage, a liquid hourly space rate of 10 volumes per volume per hour, and a hydrogennaphtha molar ratio of 3. Temperature was adjusted during the run to maintain the F-1 clear octane number of the product within one number of the feed. Beginning a t 650' F., the temperature had been raised 50" after treating 14 barrels per pound of catalyst. Extrapolation of the deactivation rate indicates a life of a t least 40 to 50 barrels per pound prior to catalyst regeneration. Since only a fraction of the total gasoline is passed over the catalyst, life based on total production of desulfurized gasoline is not less than 100 to 120 barrels per pound. During this run, 84% desulfurization was obtained, bringing the sulfur content of the treated fraction to O.O6?%. Olefins were reduced by about 40%. Since this fraction, however, contains only about 3501, of total olefins in the gasoline, their total loss, after blending treated and untreated fractions, is about 14y0. T h e finished gasoline, after blending of such fractions, had a sulfur content of O.O?% (77% reduction); its F-1 clear octane number was 91.2, compared to 90.5 for the untreated total 3 cc. tetraethyllead gasoline, and F-1 was 97.2, compared with 95.6.
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PSlG
Table IV.
Catalytic Gasoline Inspections
(High-boiling Arabian) Gravity, API Compn. Paraffins, vol. yo Olefins, vol. yo Naphthenes, vol. % Aromatics, vol. yo Octanes F-1 clear F-1 3 CC. TEL Sulfur, wt. yo Brz no.
+
ASTM dist., vol. yo Int., F. 10 20 30 40 50 60
70 80 90 95 D.P. E.P. Recovery,
70
36.6
17 29 8 46 90.2 94.2 0.424 31.5 290 312 322 332 345 357 367 378 390 406 416 430 440 98.5
Conelusion
Cracked gasolines of relatively high sulfur content can be selectively hydrodesulfurized without loss of octane number to a sulfur level which permits their blending into the refinery gasoline pool. This is achieved by hydrogenation of a high-boiling, sulfur-rich, and olefin-poor fraction of the gasoline over cobalt-molybdena-alumina catalyst a t 300 pounds per square inch gage. Hydrogenation of C S and Cg aromatic hydrocarbons must be avoided. Catalyst life, prior to regeneration, is estimated at 100 to 120 barrels of total low-sulfur gasoline per pound of catalyst. literature Cited
(1) Casagrande, R. M., Meerbott, W. K., Santor, A. F., Trainer, R . P., IND. ENG.CHEM.47, 744 (1955). (2) Cole, R. M., Davidson, D. D., Zbid., 41. 2711 (1949). Dart: J. C..'Mills: C.,'Mills; G. A.. A., Oblad. Oblad, A. G.. G., Peavy, C. C . , Petroleum Rej&er PLavy, Rejiner 34; 34, No. 6 , 153 (1955). Heinemann, H., Mills, G. A., Hattman, J. B., Kirsch, F. W., IND. ENG. CHEM. CHEM.45,-130 45, 130 (1953). LumDkin. H. E..Thomas. B. W., Anal. C h i n . 23, 1738 (1951). ' ( 6 ) Meerbott, W. K., Hinds, G. P., Jr., IND.END.CHEM.47, 749 (1955). (7) Mills, G. A., Stevenson, D. H., Smith, R. K., Heinemann, H., Erdil u. Kohle 8 , 782 (1955). ( 8 ) Sterba, M. J., IND. ENC. CHEM.41, 2680 (1949). ~
/
CONTACT TIME, SECONDS
Figure 5. Selective desulfurization of a pure-compound mixture containing 80% heptane, 20% heptenes, and 0.570 sulfur. Temperature, 650" to 700" F.; LHSV, 12 to 48; hydrogen-oil molar ratio, 3
RECEIVED for review July 27, 1956 ACCEPTED December 7, 1956 VOL. 49, NO. 4
APRIL 1957
649
