Rate constant.
k , cc s2I3
(A1 2D)
For a flow rate of 400 cc. (STP) per minute, P equals 3.07 X 10-10 mole/p.p.m. sec., and using a coke density of 1.6 grams per cc. (8), the slope becomes s =
5.78 X 106 ( z u , ) ” ~ (!?>”:‘
t
=
ffH
= mole fraction carbon converted = mole fraction hydrogen converted
P Pc
reaction time, sec.
dimensional conversion factor, moles/p.p.m. sec. = coke density, g./cc. =
(02)3n12
Literature Cited Acknowledgment
T h e author gratefully acknowledges the technical dexterity of J. Tabacek and W. Faust, who carried out the experimental portion of the investigation.
Nomenclature
(Cot
+ CO) == function gas concentration, p.p.m. of given in Equation 10 CYH
fD(ffH) fd(ffC)
F
Z kc
= = = = = = = = = = = = = = = = = = = = =
kD
k,
ki k2 MC m, m, n (02)
rc TH
ro S
S S O
W.3
wo
function of a , given in Equation 2 gas flow rate. cc. STP/min. intercept of Equation 3, p.p.m.l/z carbon surface rate constant, mole/sq. cm. sec. atm. rate constant for oxygen diffusion, set.-' constant in Equation 2, set.-' constant in Equation 6, moles/cm. sec. constant in Equation 9, set.-' molecular weight of carbon, g./mole carbon remaining, moles total carbon reacted, moles order of rate with respect to 0 2 oxygen partial pressure, atm. coke particle radius, cm. radius a t coke-carbon interface, cm. initial particle radius, cm. slope of Equation 3, p.p.m.1/2/sec. carbon particle surface area, sq. cm. initial coke surface area, sq. cm. initial weight of carbon, g. initial weight of coke, g.
(1) Appleby, W. G., Gibson, J. W., Good, G. M., Division of Petroleum Chemistry, ACS Preprint 5, No. 4, B-71 (September 1960). (2) Barrer, R. M., Phil. M a g . 35, 802 (1944). (3) Biscoe, J.,Warren, B. E., J. Appl. P l y . 13, 364 (1942). (4) Dart, J. C., Savage, R. T., Kirkbridge, C. G., Chem. Eng. Progr. 45, No. 2, 102 (1949). (5) Eberly, P. E., Jr., Kimberlin, C. N., Jr., Miller, W. H., Drushel, H. V., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 5,193 (1966). (6) Effron, E., Hoelscher, H. E., A.I.CI2.E. J . 10, 388 (1964). (7) Essenhigh, R. H., Froberg, R., Howard, J. B., Ind. Eng. Chem. 57, No. 9, 32 (1965). (8) Haldeman, R. G., Botty, M. C., J. Phys. Chem. 63, 489 (1959). (9) Haldeman, R. G., Botty, M. C., private communication. (10) Hall, J. W., Rase, H. F., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 2, 25 (1963). (11) Hoynant, G., Comfit. Rend. 259, 2827 (1964). (12) Laine, PI’. R., Vastola, F. J., Walker, P. L., Jr., J. Phys. Chem. 67, 2030 (1963). (13) Massoth, F. E., Hensel, W. E., Jr., Zbid., 63, 697 (1959). (14) Mische, R. A., Smith, J. M., Ind. Eng. Chem. Fundamentals 1, 288 (1962). (15) Shiraski, T., Ozaki, A,, Shokubai ( T o k y o ) 4, 88 (1962). (16) Snow, C. W., Division of Rubber Chemistry, 148th Meeting ACS, Chicago, September 1964. (17) Snow, C. W., Wallace, D. R., Lyon, L. L., Crocker, G. R., Proceedings of Third Conference on Carbon,” p. 279, Pergamon Press, New York, 1959. (18) Walker, P. L., Jr., Rusinko, F., Jr., Austin, L. G., Aduan. Catalysis 11, 134 (1959). (19) Ibid., p. 140. (20) Zbid., p. 155. (21) Weisz, P. B., Goodwin, R. D., J . Catalysis 2, 397 (1963).
RECEIVED for review May 16, 1966 ACCEPTED October 21, 1966 Division of Petroleum Chemistry, 151st Meeting, ACS, Pittsburgh, Pa., March 1966.
UPGRADING OF RESIDUES BY COMBINATIONS
OF RESIDUE DESULFURIZATION AND VIS BREAK1NG B. K. S C H M I D AND HAROLD B E U T H E R
Gulf Research €3 Development Co., Pittsburgh, Pa. 75230
ROWING
emphasis on air pollution from sulfur oxides has
G led to restrictions on the sulfur content of residual fuels in some areas, and future trends may mean more such limitations ( 3 ) . Since much of the present residual fuel will not meet probable future expectations of limits on sulfur content, it will become necessary to remove sulfur from these fuels. A recent survey of various methods of removing sulfur indicates that catalytic hydrodesulfurization has the most promise ( 4 ) . Much of the current research in hydrodesulfurization of residual fuels has been directed toward decreasing process costs. One potential method for decreasing the over-
all costs or improving economics of residual fuel desulfurization is to increase the production of higher-value distillate fuels as by-products. Thus, process schemes which include a residual hydrodesulfurization step and a mild thermal cracking (visbreaking) step have been investigated. Residue hydrodesulfurization, as exemplified by the Gulf HDS process, upgrades residues by decreasing sulfur content, viscosity, and pour point, and increasing API gravity ( 2 ) . T h e extent of the decrease in viscosity and pour point is a function of the sulfur removed. I n the processing of many vacuum residues, the sulfur content must be decreased to a very VOL. 6
NO. 2
APRIL 1967
207
~
The combination of visbreaking and hydrodesulfurization has been investigated as a method of upgrading residues to produce distillates and low-sulfur residual fuels. Visbreaking may be used before or after the residue hydrodesulfurization step. Either combination provides the flexibility required to vary the viscosity and sulfur levels of the residual fuel independently. In production of low-sulfur residual fuels, mild desulfurization prior to visbreaking enhances the results obtained in the visbreaking step and appears advantageous for production of low-sulfur fuel oils. Visbreaking prior to desulfurization decreases coke deposits in severe hydrodesulfurization, and has no detrimental effect on the rate of hydrocracking. The latter combination shows promise for producing substantial quantities of distillate fuels from residues.
low value (usually 1% or lower) to produce a residual fuel meeting the viscosity specifications for No. 6 fuel oil. For example, desulfurization of Kuwait vacuum residue to a level of 1% sulfur is required to produce a typical No. 6 fuel oil without the use of outside cutter oil (7). I t is anticipated, however, that many applications will exist for desulfurization to sulfur values of 1.5 to 3.0%; the viscosity of the treated residual fuel oil will often be too high for direct use as a No. 6 fuel oil, and will require the addition of substantial volumes of distillate oils to meet viscosity specifications. Under certain circumstances this will involve the use of valuable straight-run distillates when sufficient cracked distillates are not available. T o decrease the sulfur level below that actually required only for viscosity reduction would be costly and would increase hydrogen consumption and reactor volume. To eliminate the need for outside cutter stocks and to produce a low-sulfur residual fuel oil, a combination of residue desulfurization and visbreaking appears promising. For most residual stocks visbreaking alone cannot eliminate the need for outside cutter oil, for the severity of visbreaking is limited by furnace coking and the stability of the fuel oil produced. In addition, visbreaking does not effect significant desulfurization of the residue. By combining visbreaking with the Gulf HDS process, it is possible to use the visbreaking step either before or after hydrodesulfurization processing. There are advantages to using the catalytic and thermal steps in either sequence. Discussion
Residue Desulfurization Followed by Visbreaking. Residue desulfurization followed by visbreaking was investigated for t\vo Kuwait residues, an 800' F.+ residue (atmospheric tower bottoms) and a 1030" F.+ residue (vacuum toiver bottoms) (Table I). The residue was catalytically hydrodesulfurized to a moderate sulfur level where about 60% of the sulfur was removed. T h e desulfurization product was then distilled to remove the gasoline and furnace oil: 16% for the 800' F.+ residue and 11% for the 1030' F.+ residue. The furnace oil-free, desulfurized residues from these two Kuwait stocks were then subjected to visbreaking a t several severities. This resulted in substantial reduction of viscosities and pour points (Table 11). T h e Saybolt Furol viscosity a t 122' F. Table 1.
Properties of Kuwait Residues Atmospheric Vacuum Residue Residue
...
210' F.
Sulfur, wt. 70 Carbon residue, Conradwn
4.57
7100 5.2
Wt
5% 208
for the desulfurized vacuum tower residue was reduced from 4800 to about 430 by moderate severity visbreaking; for the desulfurized atmospheric tower bottoms, from 353 to the range 64 to 101. Pour points were also reduced significantly. I n all but one very mild visbreaking run, it was possible to produce specification No. 6 fuel oil from the combination of the two processes. Table I11 gives yields and inspection data of the
Table 11.
Visbreaking of Hydrodesulfurized Kuwait Residue Charge to VisbrGking (670OF.S Product from Hydrodesulfuriza- - Visbreaking Severity tion) Mild Medium Severe
VACUUM RESIDUE Yields, vol. of charge to visbreaking Gasoline (CS-~OOO F.) ... 5.4 6.5 8.4 Residue (300" F.+) ... 94.6 93.5 91.6 Inspections Gasoline ( C S - ~ O O ~F.) Gravity, API 65.0 66.0 65.3 Sulfur.' &t. % 0.597 0.287 0.246 Aromatics, Vol. % ... 7.7 7.3 5.9 Residue (30,0° F.+) Gravity, API 11.4 10.7 11.5 11.5 Viscosity, Saybolt Universal sec. 100' F. 191,733 20,270 12,293 10,938 210' F. 1.532 390 301 267 Viscosity, Saybolt Furol sec. 122' F. 4,800 675 428 381 Viscosity, Redwood No. 1, sec. 100' F. 170,259 18,000 10,916 9,713 Sulfur, wt. 70 2.26 2.58 2.22 2.03 Pour point, ' F. 80 35 30 35 O
ATMOSPHERIC RESIDUE Yields, vol. yo of charge to visbreaking Gasoline (Cj-3OOD F.) .,. 3.7 Residue (300 O F. . ,) ... 96.3 Inspections Gasoline (CS-~OO' F.) Gravity, API ... 64.9 Sulfur, wt. % ... 0.422 Aromatics, vol. yo ... 6.4 Residue (30,O" F.+) Gravity, API 14.1 13.8 Viscosity, Saybolt Universal sec. 100' F. 2,437 14,080 210' F. 255 122 Viscosity, Saybolt Furol sec. 122' F. 353 101.3 Viscosity, Redwood No. 1, sec. 100" F. 12,503 2,164 Sulfur, wt. 70 2.58 2.14 Pour point, F. 30 45
+
O
I&EC PROCESS DESIGN A N D DEVELOPMENT
8.5 91.5 64.3 0.218 8.3 14.8 1,382 101 64.1 1,227 2.10 40
Table 111. Production of No. 6 Fuel Oil by Hydrosulfurization Processing Followed by Visbreaking of Kuwait Residues Atmospheric Residue Vacuum Residue
Visbreaking severity
Mild
Yield, vol. 70of original charge Gaso1ine(C~-40OoF.) 9 . 5 Excess furnace oil (400-670' F.) 15.1 No. 6 fuel oil 76.9 Inspections of No. 6 fuel oil
Gravity, ' API Viscosity, Saybolt Furol see., 122" F. Pour point, F. Sulfur, wt. yo
12.6
Severe Mild
Table IV. Effect of Hydrodesulfurization Processing on Sulfur Removal during Visbreaking Stage
Charge.
7.4
9.7
11.5
19.3 69.0
0.0
96.6a
0.3 92.7
1.4 89.2
12.4
12.6
12.0
11.8
200b + 40 55 25 25 30 2.66 2.56 2.40 2.10 1 . 9 4 a 1 . 3 aol. yo 300-400° F. naphtha ( a l l ) plus 1.S% outside furnace oil required to meet uiscosity specijication f o r N o . 6 fuel oil. b 200 SFS at 122' F. taken as typical No. 6 fuel oil. A S T M maximum specgcation +
O
300 SFS at 122' I;.
No. 6 fuel oils, together with yields of excess furnace oil and gasoline produced by this combination of processes. I n one case, processing of the lighter Kuwait residue, as much as 3470 of gasoline plus furnace oil was produced in addition to the specification No. 6 fuel oil. While it would be expected that any residue could be reduced in viscosity by a proper thermal treatment, these results indicate that a desulfurized residue is more susceptible to visbreaking than a nonhydrogenated stock. A comparison has been made between visbreaking of a straight-run and a catalytically desulfurized residue (Table IV). Visbreaking of a 1030' F. vacuum-reduced Kuwait crude a t a typical severity showed a decrease in cutter oil requirement to produce No. 6 fuel oil of 42Y0; after desulfurization, visbreaking under the same operating conditions, showed a decrease in cutter oil requirement of 76.8y0. (Because of different viscosity levels of the two residues, an absolute viscosity comparison cannot be made.) T h e above comparison indicates that hydrogenation increases the susceptibility of residues to thermal cracking. I t is believed this increased ease of thermal cracking is the result of saturation of several rings of condensed aromatic ring structures, making them more susceptible to thermal cracking. Thermal cracking of a completely condensed ring structure can lead only to its degradation by activation and condensation of the aromatics to a larger molecule. This same principle is used commercially in hydrogenation of heavy catalytic gas oils before recycling them in catalytic cracking, to allow the cracking of condensed aromatics to useful products rather than coke. A second advantage of following residue desulfurization with visbreaking is the large reduction in pour point of the finished fuel oil. IYhile any residual oil can be reduced in pour point by reducing viscosity, a t some levels waxy constituents limit the value of further reductions in viscosity. I n catalytic desulfurization, waxy constituents find it difficult to compete for the catalyst surface with highly condensed aromatics which are adsorbed much more strongly. Thus, they are not preferentially converted. In thermal cracking, on the other hand, there is no adsorptivity factor and the relative decomposition of the molecules depends largely on their structures. Since paraffins crack thermally more readily than aromatics, they are selectively converted. A combination of catalytic and thermal processing thus allows a maximum reduction in pour point. Catalytic hydrogenation provides
70 of
crude)
HDS Prior
Moder- Severe ate
14.4
Kuwait vacuum residue (20 vol. Visbreaking Alone 300' F.+ Feed to vis- product breaking f r o m visstage breaking
Gravity, API 6.7 Sulfur, yo 5.2 Visc-sity, Saybolt Universa1 sec.;21Oo F. 7100 Pour point, F. 100 Cutter oil required to make No. 6 fuel oil (200 SFS vis. at 122' F.), vol. % 38.7 Decrease in cutter oil requirement by visbreaking, 75 ...
5.9 5.5
O
950 60
to
Visbreaking 3000
Feed to visbreaking stage
F.+ product from uzsbreaking
11.4 2.26
11.5 2.03
1532 80
267 35
22.5
25.0
5.8
41.8
...
76.8
cracking of high viscosity asphaltic molecules which give a high pour point, while thermal cracking destroys the wax. The combination of residue desulfurization and visbreaking has another advantage-desulfurization. T h e visbreaking of straight-run stocks generally does not effect any desulfurization of the fuel oil (after 300" F. end point gasoline is removed). I n fact, the sulfur content is often higher (Table IV). T h e visbreaking of a desulfurized stock, on the other hand, does show a small reduction in sulfur content of the gasoline-free fuel oil-from 2.26 to 2.O3yO sulfur. I t is postulated that sulfur compounds containing aromatic rings are partially saturated to allow thermal cracking of the ring and the formation of an easily decomposed sulfur compound. Visbreaking Followed b y Residue Desulfurization. An alternative to visbreaking the desulfurized residue is to visbreak first and then desulfurize. This sequence produces a hydrogenated fuel oil which should be more stable in long-time storage. Since some gas and gasoline are made in visbreaking, the more expensive desulfurization unit can be reduced in size by as much as 15%. The visbreaker oil is also less susceptible to coke formation during desulfurization. This second sequence has been investigated over a range of visbreaking and desulfurization severities using a mixed Kuwait and Eastern Venezuela vacuum-reduced crude (Table V). T h e reduced crude was first processed by visbreaking a t several severities. After the removal of a 400" F. end point gasoline, the visbreaker residues were desulfurized a t several levels of desulfurization. T h e visbroken stocks are designated according to severity by the volume per cent gasoline removed-Le., 3% gasoline-free visbreaker residue and 11.8% gasoline-free visbreaker residue. T h e results of these experiments are given in Table VI, together with corresponding data from desulfurization of the straight-run residue. T h e properties of the hydrodesulfurized Properties of Kuwait-Eastern Venezuela Vacuum Residue 1,030f Boiling range, F.
Table V.
Gravity, O API Viscosity, Saybolt Universal sec. 100' F. 210° F. Sulfur, wt. yG Carbon residue, wt. yo VOL. 6
6.8
915,625 8,621 4.20 19.7
NO. 2 A P R I L 1 9 6 7
209
Hydrodesulfurization of Visbreaker Tars
Table VI.
Charge to hydrodesulfurization Gasoline yield in visbreaking, vol. %. HDS severity Charge Yield. vol. % of HDS charge Gasoline (C,-400' F.) " ... Furnace oil (403-670' F.) 8.2 Residue (670' F.+) 91.8 Hydrogen consumption, scf./bbl. .. Inspections of residues dravitv. ' API 7.0 Viscoshy, Saybolt Universal sec. 100' F. 320, 522 210'
Table VII.
37,091 405
7225 146
29,3854 1,060
...
1,435
520
15,818 182
7,580
1,046
246
4,610
1,740
450
32,950 50 4.61 1.56 400°F.++--670'
6,420
173,000
284.500
...
80
i.ii
F.+--+
% of Straight-Run Charge
VisNo breaking processing only0 11.8
10.6
Visbreaking plus HES 25.0~
HDS only
0
0
26.5
23.0
44.1
22.2
0
0
144.1 110.4 66.4 Totalgasoline yield from both visbreaking and HDS.
56.6
furnace oil-free residue (670' F.+) are substantially improved in comparison with the charge in each case. At the higher residues are almost desulfurization severity, the 670' F. equivalent to the No. 6 fuel oil viscosity specifications without cutter oil. An increase in severity in either the visbreaking or the desulfurization stage increases the yield of distillate products and decreases the viscosity and yield of 670' F.+ residue. T h e yield data are summarized in Table VI1 on the basis of blending to No. G fuel oil for the operation using the highest severity in each step. T h e furnace oil yield is the excess over that required for blending to No. G fuel oil or cutter oil requirement if outside furnace oil must be used. This combination produced a total distillate yield of 48y0 (gasoline plus furnace oil) and a specification No. G fuel oil. These total distillate yields are substantially higher than for either process alone a t the same conditions. T h e severity of the desulfurization step in this case is substantially higher than the mild hydrodesulfurization step prior to visbreaking discussed above. I n general, it is anticipated that this combination (visbreaking followed by desulfurization) would be used where high yields of distillate products are desired. An unexpected effect in the combination of visbreaking followed by residue desulfurization is the low catalyst coke deposit. When desulfurizing a straight-run residue, the carbon on catalyst increases with a n increase in temperature,
+
l&EC PROCESS DESIGN A N D DEVELOPMENT
Charge
Medium
...
5.9
Yields, Val.
210
Severe
11.3
(Charge. Typical vacuum residue from Kuwait-E. Venezuela mixed crude)
a
Medium
11.6
Production of No. 6 Fuel Oil by Visbreaking Followed by Hydrodesulfurization
Gasoline (64-400' F.) Furnace bil (400-670' F.) Excess after.cutting to No. 6 fuel oil Outside cutter bil required Total Nb. 6 fuel oil (200 Saybolt Furol Sec. at 122' F.)
Charge
13.8 30.0 60.1 970
Viscosity, Saybolt Furol sec. 122' F. Viscositv. Redwood No. 1. sec. 100' F.
Medium Severe
Straight-Run Vacuum Residue
2.4 19.6 80.1 740
F.
Pour point, ' F. Sulfur, wt. % Boiling range a At 730'F.
F.+ Visbreaker Product 1- 1- .8
400" 3
5.5 10.4 85.9 650
10.6 29.5 63.4 800
14.2
12.7
915,6250 8,621
35,099 492
10,006 184
179,200
1,127
338
9.9 90.1
...
...
4.39 400' F.+
9.5
,. .
70 1.37 +--67O0
9.7
14,300 70
1.20
F.+.-+
Severe
6.8
*..
32,100 8,800 60 90 1.32 0.93 +--670° F.+--+
. .. .. 4.2 400' F.+
other conditions remaining the same. With the visbroken tars, however, coke deposits on the catalyst d o not increase with a n increase in processing temperature (Figure 1). I n spite of the variation in yield of distillate products, the coke yield is approximately constant throughout the normal temperature range for desulfurization. As a result of this effect, the coke yield from desulfurization of the visbreaker tar is considerably less than for processing of the straight-run residue a t the higher temperature. Thus, despite the fact that thermal treatment generally degrades some of the heavy hydrocarbons and actually makes asphaltenes, the use of moderate severity visbreaking apparently decreases coke formation in the subsequent HDS stage. Perhaps many of the thermally unstable residue components are either cracked to lighter products or condensed to a stable asphaltene during the thermal treatment. I n any case, a residue which has already been thermally processed appears to be less susceptible to additional thermal cracking during catalytic processing; and this mild thermal treatment of residues prior to desulfurization offers a possible method for reducing coke formation and increasing cycle length in the HDS stage. c'
3
rr. . v)
/
32r
/'i 1
28
M I X E D CRUDE VACUUM RESIDUE
0 0
a
3
Y
n
T E M P E R ATU R E
F.
Figure 1. Effect of visbreaking on catalyst deposits in residue hydrodesulfurization
Kinetics of HDS Processing of Visbroken Residues,
T h e rate of desulfurization of residues can be generally expressed by a simple second-order kinetic equation (7) : C --
- k-
1-C
1 LHSV
T h e reaction rate constant was also calculated for hydrocracking of the 1000° F. residue portion of the charge in all cases, assuming second-order kinetics. T h e reaction rate constant is shown as a function of reciprocal temperature in Figure 4. I n this case, there is surprisingly little difference in activation energy or in the reaction rate constant for the
where
c
= 1 -
k
=
% %
sulfur in product, wt. sulfur i n charge, wt.
reaction rate constant LHSV = liquid hourly space velocity, volume of liquid feed per hour per volume of catalyst
-
3 .O-
-
.u
I2
4
2.0-
c v)
T h e reaction rate constant as defined above was calculated for desulfurization of gasoline-free residues from visbreaking a t various severities. Figure 2 shows the reaction rate constant as a function of visbreaking severity using moderate hydrodesulfurization conditions. There appears to be a slight increase in desulfurization rate as a result of mild visbreaking, but this beneficial effect is lost as the visbreaking severity is further increased. This effect is believed to be real, because the rate of desulfurization is generally greater for lower molecular weight feedstocks. After visbreaking, the molecular weight of the residue, even the gasoline-free residue, would be less than that of the straight-run residue. At higher visbreaking severities, however, this effect is probably overwhelmed by the increased conversion of sulfur-containing compounds to different, more difficult-to-desulfurize compounds. Figure 3 is a plot of reaction rate constant us. the reciprocal of the absolute temperature for the desulfurization of two visbroken stocks and the original straight-run residue. While the rate constant is only slightly affected by visbreaking a t moderate hydrodesulfurization temperatures, there are significant differences a t higher temperatures-Le., their activation energies are low in comparison with the straight-run residue. T h e activation energies for desulfurization of these stocks are given in Table V I I I . Here, again, the use of higher temperatures has less effect on stocks which have already been thermally cracked. Apparently the prior thermal processing converts some of the sulfur compounds to forms which are less readily available for desulfurization.
c1
.* w
I-
t
4 Iv)
z 0 0
.L
0 W
c 4
I
.o-
K
-
0.8-
-
2
!!
0.6 -
w
0.4 -
c 0 a a
-I
I
I
0.76
I 0.84
I
0.80
0.82
x
QT ( O R )
I
0.86
103
Figure 3. Reaction rate constants for desulfurization as a function of temperature I
-
I
I
I
I
I
2.0-
.u
-
I2
4
c
g
1.0-
0
0
;0.7 a K
0.5-
i i . e n GASOLINE-FREE VISBREAKER RESIDUE
4e! u 4
Y K
0.3
I
0.78
2.0
400°F+
-
2 0
I
I
I
0 .8 0 I/T
I
I
0.84
0.82 (*R)
x io3
Figure 4. Reaction rate constants for hydrocracking as a function of temperature
1
I
I
I
4
8
12
16
VISBREAKING SEVERITY E.P. G A S O L I N E Y I E L D : % B Y
I 20
I
VOLUME
Figure 2. Effect of visbreaking severity on reaction rate constant for desulfurization
Table VIII. Activation Energies for Hydrodesulfurization and Hydrocracking in Processing of Straight-Run and Visbroken Residues Activation Energy, Kcal.-G. Mole DesulfurizaHydrotion cracking
Straight-run residue Visbroken residue 3 7 , gasoline removed 12yo gasoline removed
39.5
58.2
22.2
60.2
8.8
54.0
Mild desulfurization conditions
VOL. 6
NO. 2
APRIL 1 9 6 7
211
three stocks. In other words, the visbroken residue, even though it has been thermally cracked, can still be hydrocracked about as readily as the straight-run residue. In view of these facts, the decrease in activation energy for desulfurization after visbreaking is somewhat surprising. I t appears that the sulfur compounds are changed during thermal cracking to make them less susceptible to desulfurization. This effect is opposite to that observed for hydrodesulfurization prior to visbreaking.
difficult in the following dehydrosulfurization step. O n the other hand, if hydrocracking is the major consideration, it may be advantageous to use visbreaking prior to dehydrosulfurization processing to decrease coke formation by residue thermal cracking during the dehydrosulfurization step. The prior visbreaking step has no apparent detrimental effect on the rate of hydrocracking as it does on the rate of desulfurization. Either combination can result in substantially more upgrading of the residue than is normally obtainable by one process alone.
Conclusions
T h e combination of visbreaking and hydrodesulfurization processing is capable of substantially decreasing the viscosities, the pour points, and sulfur contents of residues from typical crudes, so that little or no cutter oil is required for blending to residual fuels. In addition, it provides the flexibility required to vary the viscosity and sulfur levels of the residual fuel independently. I t is probably advantageous to use hydrodesulfurization followed by visbreaking when low-sulfur residual fuels are to be produced. Generally, prior desulfurization enhances the results obtained in the visbreaking step, while visbreaking first makes desulfurizaton somewhat more
literature Cited
(1) Beuther, H., Flinn, R. A., Schmid, B. K., Oil Gas J . 58, No. 13 130 (1960). ( 2 ) Beuther, H., Schmid, B. K., Proceedings of Sixth World Petroleum Congress, Section 111, Paper 20-PD7, Frankfurt, Germany, June 19-26, 1963. ( 3 ) Carpenter, H. C., Cottingham, P. L., U. S. Dept. Interior, Bur. Mines, IC-8156 (1963). (4) Chem. Week 97, No. 4, 20 (July 24, 1965). RECEIVED for review June 1, 1966 ACCEPTEDOctober 21, 1966 Division of Petroleum Chemistry, 151st Meeting, ACS, Pittsburgh, Pa., March 1966.
HYDROCRACKING PREHYDROGENATED SHALE OIL P H I L I P L. C O T T I N G H A M AND HARRY C. CARPENTER Laramie Petroleum Research Center, U. S. Bureau of Mines, Laramie, Wyo.
Shale oils with lowered carbon residue and metals contents were prepared by prehydrogenating crude gascombustion shale oil a t 1000 pounds' pressure and 600" to 660" F. The prehydrogenated oils were oncethrough hydrocracked at 1000 or 1500 pounds with temperatures from 800" to 900" F. At a given conversion, naphtha yields were about the same a t both pressures, but sulfur and nitrogen percentages were lower at 1500 than a t 1000 pounds. At 900" F., naphthas obtained at 1000 pounds averaged below 0.1 0% nitrogen and those obtained at 1500 pounds averaged 0.024/,. These percentages were reduced to below 0.01 % when the tar bases were removed. A 66-hour recycle hydrocracking run at 1000 pounds and 907" F. produced 80.6 and 106.4 volume % of conversion as, respectively, Cg+ and C4+ naphthas.
HALE
oils retorted from the Greeen River formation of
S Colorado, Ctah, and Wyoming by conventional, aboveground retorts usually are heavy, viscous oils that contain little material boiling in the gasoline range. Unless thermally cracked during retorting, they have pour points of about 70" to 90" F., gravities in the range 16" to 20" A.P.I., carbonhydrogen weight ratios of about 7.5, sulfur content of nearly 1%, nitrogen content of over 2%, and oxygen content of about 1 to 2% ( 7 , 5, 6 ) . The high percentages of sulfur, nitrogen, and oxygen compounds-for example, over 60y0 of one shale oil (I)-render the raw oil unsuitable for refining to high quality motor fuels by conventional refining processes. Recycle hydrocracking at 3000 pounds' pressure was shown to produce a high yield of naphtha with low sulfur and nitrogen content, but the pressure greatly exceeded the capacity of ordinary refining equipment (4). Once-through hydrogenating coker distillate fractions a t 1100 and 1500 pounds' pressure (2, 3 ) produced products of poorer quality and in lower quantity than were obtained in the 3000-pound operation. 212
l&EC PROCESS DESIGN A N D DEVELOPMENT
The purpose of the present study was to investigate the hydrocracking of prehydrogenated shale oil (shale oil hydrogenated a t mild conditions prior to hydrocracking) a t pressures of 1000 and 1500 p.s.i., lvhich would be in the range of present petroleum hydrocracking equipment. Apparatus and Procedure
Prehydrogenating and hydrocracking experiments were done in the hydrogenation equipment whose simplified flow diagram is shown in Figure l . The Type 347 stainless steel reaction vessel 40 inches long and 11/2 inches in i.d. contained a thermowell 9/16 inch in diameter. A section 123/4 inches long near the bottom contained 300 cc. of 4- to 8-mesh catalyst supported in the heated zone of an electric furnace, and a n upper 19-inch section serving as the preheater contained 4-mesh Alundum grain. Temperatures were measured by six thermocouples spaced a t 2l/d-inch intervals in the catalyst. Hydrogen was measured by displacement with corrosioninhibited water from two hydrogen storage cylinders maintained a t reaction pressure. These cylinders were used alternately and were refilled from a higher-pressure source when emptied.
