I
PAUL H. JOHNSON, C. R. EBERLINE, and R. V. DENTON Research Division, Phillips Petroleum Co., Bartlesville, Okla. P
Catalytic Cracking of Petroleum Residuum With increased demand for iet and gas turbine fuels, cycle oils-an upgraded product of residuum crack.ing become increasingly important
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T R m m in petroleum refining are toward reduction in quantity of residual fuels and increase in distillate fuels because of increased demand for gasoline and heating oils and growing requirements for jet and gas turbine fuels. Much work has been done on obtaining lighter fractions from residua. This article reports a bench-scale study of fluid catalytic cracking of residua and compares it with conventional gas-oil cracking. The study of residuum cracking was a natural outgrowth of earlier work (4) aimed a t the recovery of the maximum amount of the high-grade cracking stocks from residuum. Because of poor recovery techniques, these stocks, in most cases, are left to be burned as fuel. Commercial cracking of residual stocks has been described by Dart, Mills, Oblad, and Peavy (2). Equipment Fluidized confined-bed cracking units, developed in this laboratory, for use in bench-scale catalyst testing and feed stock evaluation have been described (4,5). Except for greater quantities of steam diluent, essentially the same procedure was used in residuum cracking studies. *_
R
Typical Operating Conditions Temperature, O B. Pressure, lb./sq. inch gage Space rate, Ib. oil/lb. catalyst/hr. Steam diluent, lb./barrel feed Process period, minutes
900 10 0.7-3 30-160 5-25
In a given series of tests, all conditions were held constant except space rate, which was varied to obtain a range of conversions. The effects of variations in cracking temperatures were not investigated, but variations in quantity of steam and contamination of catalyst by metals received some attention. Equilibrium natural clay (montmorillonite) catalyst from a commercial unit was used in most of this study. The Phillips activity index ( 5 ) was 107, corresponding to approximately 32.5 activity by the Jersey D L method (7). Heavy metal contaminants on the catalyst amounted to approximately 200 p.p.m. expressed as nickel oxide plus vanadium pentoxide.
+
Exploratory Studies
After application of the best recovery technique to crudes, there remains residual product which contains relatively large quantities of materials regarded as undesirable in catalytic cracking feed stocks. The heavy resinous and asphal- ' tic constituents of the residua produce high carbon yields and contain heavy metals which contaminate catalysts. Accumulation of these metals increases carbon and decreases gasoline yield. High carbon deposition is not necessarily uneconomic, if the additional yields of liquid products justify suitable process equipment. Western Kansas residuum of 140° F. softening point was separated into oil, resin, and asphaltene fractions for cracking studies. The asphaltenes were precipitated with pentane; the nonasDhaltene Dortion was fractionated with isobutane into an oil and resin fraction (Table I). The product distribution from the oil fraction shows that even the heaviest oils, when relatively free of asphaltenes . and resins, do not Droduce large amounts of carbon: Gasofine yields i r e similar to those from lighter stocks. The gasoline yield from the resin fraction is as high as 25 volume %. Because portions of the most highly reduced crude residuum can produce substantial yields of high quality gasoline, whereas other portions produce abnormally high yields of carbon when cracked, investigation of the carbon-forming process of these heavy materials seemed warranted. Carbon Deposition
The relationship of catalyst deposit to processing period, conversion, and
Table I.
feed rate differs significantly between gas oils and residua. Voorhies (9) has shown for gas oils that weight per cent of carbon on catalyst is a function of process period but independent of oil rate or conversion, with other conditions constant. This has been confirmed for clean (low carbon residue) gas oils in this laboratory. The lower, horizontal line of Figure 1 demonstrates the constancy of carbon deposition with changing feed rate at a constant process period for a Gulf Coast-West Texas gas oil. Residuum cracking data, on the other hand, show that carbon on catalyst increases with increasing feed rate at a constant processing period. The sloping line on Figure 1 represents data for a Gulf Coast-West Texas residuum. This straight line may be interpreted to mean that an additional increment of carbon is superimposed upon the carbon curve predicted by the Voorhies relationship. This increment is a constant fraction of the weight of feed. Expressed algebraically, for a constant processing period
Cc =
a
+- bF
where Cc = carbon, weight per cent of catalyst and F = weight of feed per unit weight of catalyst. Slope b represents the constant proportion of feed which appears as the added increment of carbon on catalyst. With the gas oil, b is essentially zero, so that Cc = a-i.e., carbon on catalyst is constant, in accordance with the Voorhies relationship. With residua, however, b has a finite positive value. When processing period is changed, the value as Dredicted bv the Voorof a changes " hies relationship, but the value of b remains essentially constant and is ap-
Characteristics and Cracking Data of Fractions of Western Kansas 140' F. Softening Point Residuum
Characteristics Vol. % of residuum Gravity, API c6 insoluble, wt. yo Carbon residue, Ramsbottom, wt. % Cracking yields at 50% conversion, % of feed Carbon, wt. Gasoline, vol. Cycle oil, vol.
Residuum
011 Fraction
Resin Fraction
100.0
61.5
14.5
8.4 27 19
18.9
3.8
6.6
...
24.7 28.2 50.0
VOL. 49, NO. 8
8.5
9.9
38.9 50.0
AUGUST 1957
24.0 25.1 50.0
1255
6
5
4
Figure 1. Effect of feed rate on carbon deposition at constant processing period
3
2
I 0.1
0.2
0.3
0.4
WEIGHT OF FEED
0.5
proximately equal to the Ramsbottom carbon residue of the residuum. I n the example shown in Figure 1, the value of 6 is 9.3, whereas the Ramsbottom carbon residue of the residuum is 8.8 weight %. Such correspondence of Ramsbottom carbon residue with slope 6 has been noted not only for residua, but also for heavy gas oils. Both carbon residue and the increment of carbon yield represented by b are assumed to result from components of the feed stock which do not crack to produce volatile fragments, or are easily converted to high boiling materials and subsequently into carbon. In view of
"40
60
50
CONVERSION, VOL%
Figure 2. Carbon yields from fluid catalytic cracking of Gulf Coast-West Texas residuum and gas oil Table II.
1256
0.8
the different conditions prevailing in catalytic cracking and in the carbon residue test, quantitative equivalence of the two values must be regarded as fortuitous. Actually, variation in cracking conditions, such as quantity of steam diluent, affects the value of 6 .
Residuum vs. Gas Oil Cracking The foregoing indicates that catalytic cracking of residua would be expected to yield more carbon and less liquid product than gas oil cracking. Cracking data at 50y0 conversion for several gas oils and residua, produced by vacuum flashing of topped crudes, ar'e compared in Table 11. The most significant difference between residua and gas oils is the much greater proportion of the residuum deposited as carbon on the catalyst during cracking. Residua yielded more than four times as much carbon as the same weight of the corresponding gas oils. This is the result of the high carbon residue of the residuum, which produces a constant increment of carbon yield regardless of conversion. In Figure 2 carbon yield is plotted as a function of conversion for the residuum
Figure 3. Gasoline yields from fluid catalytic cracking of Gulf Coast-West Texas residuum and gas oil
Comparative Fluid Catalytic Cracking Data for Residua and Gas Oils
Crude source Cracking feed yo crude Feed properties Gravity, API 5-957, boiling range, O F. 50% distillation temp., ' F. Carbon residue, Ramsbottom, wt. CSinsoluble, wt. % Sulfur, wt. % Yields from catalytic cracking, % of feed Gas (Ce and lighter), cu. ft./bbl. CB Ca olefins, vol. CS-40O0 F. end point, gasoline, vol. Cycle oil, vol. Carbon, wt. Cracked products properties CS-40O0 F. end point gasoline Research octane No. (3 ml. TEL) Bromine No. Sulfur, wt. % 400' F. and heavier cycle oil Gravity, API Correlation index Sulfur, wt. %
+
0.7
0.6
PER UNIT WEIGHT OF CATALYST
and gas oil from Gulf Coast-West Texas crudes. Thecurves are essentially parallel. The large carbon yields must be reflected in smaller yields of gasoline and light olefins. While the data show this to be true, the differences in volume yields of gasoline from gas oils and residua are not large. Only in the case of the residuum of highest carbon residue is the volumetric gasoline yield more than 5yoof feed below that of the corresponding gas oil. Figure 3 shows gasoline yields as a function of conversion for the residuum and gas oil from Gulf Coast-West Texas crudes. The two curves are almost identical. If carbon residue is defined as a portion of the residuum incapable of yielding gasoline, the gasoline yield based upon the balance of the residuum compares favorably with the gasoline yield from gas oil, and the displacement of the curve shown in Figure 3 would be expected. The gasolines produced from residua tend to be higher in sulfur content and bromine number and lower in antiknock quality than gasolines from the corresponding gas oils. The octane number difference shown in Table I1 between gasolines from residua and gas oils is 3 to 4 units, showing that gasolines from residuum cracking are high quality
Gulf Coast-West Texas Residuum Gas oil 6.1
...
15.1
.. ...
8.8 1.8 0.46
280 6.5 38.2 50.0 11.5
26.7 685-975 790 0.08
West Texas-Panhandle Residuum -4 Residuum B Gas oil 10.1 7.6
...
14.9
... 6 . .
...
13.0
180 6.8 43.0 50.0 3.0
280 5.8 35.2 50.0 15.3
0.3
16.7
...
0.89
9.6 2.7 0.74 250 7.2 36.0
29.8 550-1 000 750 0.3 0.51 140 9.4
13.4
39.9 50.0 3.2
50.0
Western Kansas Residuum Gas oil 10.4 .
8.4
*..
...
19 27 1.4 320 5.4 28.2 50.0
24.7
...
0.86 185 6.4 42.0 50.0 4.5
96.8 78 0.025
93 95 0.12
93 95 0.12
96.6 77 0.05
90 98 0.17
94 70
21.5 61 0.4
25.5 49 0.2
24 53 0.6
23 54
28.5 44 0.4
22 56 1 .o
25
INDUSTRIAL AND ENGINEERING CHEMISTRY
.
24.2 600-1 100 910 0.05
93.5 92 0.08
...
I
...
52
0
.%
Figure 4. E f f e c t o f Z L 500 steam diluent in catalytic ~2 400 300 cracking of West Texas- ZJ nm 200 Panhandle 10.1% residuum at so?& conver100 sion 0
3
,
3 41
c3
40 5 39
y 40 w .L z
4
-W Wlr.
3%35
db38
2s 37 0 . -I
