November. 1930
I,VDUSTRIAL AND ENGINEERING CHEMISTRY
Bohnson, J . Phys. Chem., 25, 19 (1921). Bohnson and Robertson, J . .4m.Chem. Soc., 45, 2493, 2512 ( 19231. Bray and Livingstone, Ibid.,45, 1255 (1923). Hrode, Z. p h y s i k . Chem., 37, 237 (1901). Elissafoff, Z . Illektrochem., 21, 352 (1915). (8) Fenton and Jackson, J . Chem. Soc., 75, 1 (1899). (9) Fenton and Jones, Ibid., 77, 69 (1900). (10) Kastle and Loevenhart, A m . Chem. J . , 29, 397 (1903). (11) Kiss and Lederer, Rfc. Irav. chim., 46, 453 (1927). (12) Lemoine, Combt. r e n d . , 161, 47 (1915). (3) (4) (5) (6) (7)
(13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
1237
Maas and Hatcher, J . A m . Chem. SOC.,42, 2368 (1920) Mellor, “Modern Inorganic Chemistry,” p. 543. Osborne, Am. J . Sci., 32, 334 (1886). Rice, J . .4m. Chem. Soc., 43, 2099 (1926). Robertson, Ibid., 47, 1299 (1925). Spitalsky and Petin, Z . physik. Chem., 113, 161 (1924). Taylor, J . Phys. Chem., 27, 322 (1923). Thenard, A n n . chim. phys., 9, 441 (1818) Walton, Z . physik. Cizem., 47, 185 (1904). Zotier, B i d . SOC. chim., 21, 241 (1917).
Cracking Value of Straight-Run and Cycle Gas Oil’ H. Sydnor and A. C. Patterson TECHSICAL SERVICED I V I S I O N , STAXDARDOIL COMPANY OF
T HAS been appreciated for some time, in the oil industry, that the gas oils produced as by-products of the cracking process are less valuable as cracking stock than the straight-run gas oils originally fed to the process. The term “straight-run gas oil” has been used to apply t o gas oils that have not been subjected to cracking conditions except in so far as incipient cracking may occur upon crude distillation. The gas oils produced as by-products of the cracking process are known as “cycle gas oils.” The cycle gas oils may be produced as by-products of the cracking of either straightrun gas oils or rrude residuums. The Cracking Coil Cycle
I
SEW
JERSEY,ELIZABETH, h-.J.
coming straight-run gas oil decreases. Furthermore, since the unit is operating on a feed stock containing a higher percentage of straight-run gas oil in the total feed, the quantity of gasoline produced per unit of time increases as the quantity of cycle gas oil withdrawn is increased. I t has also been noted that the yield of fuel oil of a given gravity increases in proportion to the yield of gasoline as the quantity of cycle gas oil withdrawn from the system is diminished. I n order more accurately to evaluate these factors, a study was made of the cracking value of the cycle gas oil produced by successive passes through a miniature cracking coil. The results of four successive passes through the equipment in which no recycling was inrolved were combined and compared with an operation on the same equipment when currently recycling to produce the same yield of gasoline on the straight-run gas oil as was produced by the combined successive operations. I n the successive “once-through” operations, the cycle gas oil produced in each pass served as the feed stock for the succeeding pass. The history of the straight-run gas oil used in this work is
Most modern cracking processes recycle to some extentthat is, they return a portion or all of the cycle gas oil produced to the system as feed stock along with the incoming straight-run gas oil. I n such a system the total feed rate (straight-run plus cycle oils) is maintained constant and the amount of straight-run gas oil added per unit of time must be the equivalent of the gasoline, fuel oil, cycle gas oils, gas, and coke removed from the system. Cvcle gas oils may be re” moved from the system in several ways. They may be removed directly as cuts from the fractionating equipment. They may be removed by increasing the A. P. I. gravity of the fuel oil produced, in which case they are designated as fuel oil unless subjected t o a redistillation process and recovered as overhead products. I n most low-pressure (atmospheric to 350 pounds) cracking equipment they are taken out to some extent along with the gasoline composing the product designated as “distillate.” After removal of the gasoline, the remaining cycle gas oil is either returned to the cracking process or sold to the trade. Modern high-pressure (750 to 1000 pounds) cracking equipment is built with fractionating equipment that will permit all of the cycle gas Fi‘ O& oil to be retained within the system until completely converted to 400” F. end point Figure 1-Flow D i a g r a m of L a b o r a t o r y E q u i p m e n t specification gasoline, fixed gases, and fuel oil of the gravity desired d6n-n to about 5” A. P. I. not definitely known, but it is believed t o be a wide gas oil The percentage of incoming straight-run gas oil in the total cut from Midcontinent crude. feed mill, of course, increase with increasing withdrawal of Description of Equipment cycle gas oil from the system. It follows that, since decomposition of the cycle gas oil is necessary to obtain the This work was carried out in laboratory equipment ultimate yield of gasoline, the yield of gasoline based on in- is a miniature tube and tank cracking in all essentials, Figure 1 is a flow diagram of the apparatus. Figure 2 is 1 Received October 16, 1930.
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IIVDUSTRIAL A X D ENGINEERING CHEMISTRY
Vol. 22, No, 11
Figure 2-Tube
a flow diagram of a modern tube and tank unit. The oil feed is forced under cracking pressure through a spiral preheater coil immersed in an electrically heated lead bath, which corresponds to the heating coil in the refinery units. The coil discharges into the bottom of a soaking drum placed in an electric furnace. Three soaking-drum temperatures are continuously recorded and are controlled by rheostats. The pressure on the coil and soaking drum is controlled by releasing the cracked material into a packed fractionating column from which distillate of any desired final boiling point, fuel oil of any desired gravity down to 6" A . P. I., and a cycle gas oil stream are withdrawn. It is necessary t o apply heat to the laboratory soaking drum and fractionating equipment because of the small quantities of heat involved a t the slow feed rates, and the relatively large radiation losses. The gas produced is separated from the distillate a t atmospheric pressure. I n once-through operations the feed pump is connected directly to the fresh feed storage blowcase, and all cycle gas oil is removed from the system. I n recycling runs the cycle gas oil is mixed with fresh feed in the accumulator and the mixture is fed back into the unit. Procedure
I n this series of runs, the soaking drum temperature was held between 855' and 860" F. and the release pressure was held a t 760 pounds per square inch gage. The feed rate bore the same relation t o the soaking drum volume that 15,000 gallons per hour would bear to the volume (1100 cubic feet) of the soaking drums on the 750-pound refinery units. An effort was made to cut 400" F. E. P. distillate and 10" 4. P. I. fuel oil from the tower. Because the quantities of fuel oil produced were small, it was not possible to control the gravity exactly. The distillates cut in runs 1 and 2 contained a small amount of material boiling above 400" F. They were rerun in true boiling equipment ( I ) to determine the yield of gasoline, and the bottoms from the rerunning operation were reported as cycle stock. I n the single run in which the cycle gas oil was currently recycled, similar cracking conditions were employed, and a
and Tank Unit
12.9" A. P. I. fuel oil and a distillate containing 92.3 per cent of gasoline were withdrawn from the tower. The distillations and fuel-oil inspections were carried out in accordance with the specifications of the A. S. T. M. The octane numbers of the gasolines were determined on a standard Ethyl Gasoline Corporation knock engine. The aniline point is the temperature a t which equal volumes of dry aniline and dry sample are completely miscible. Nofe-The A. P. I. gravity is simply a modification of the Baume scale adopted by the American Petroleum Institute. T o convert A. P. I . gravities to specific gravities the following equation is used: Specific gravity (60' F./60° F.) =
141.5
131 5
+
A. P. I.
Discussion of Data
Table I shows the results as obtained by the initial cracking of the straight-run gas oil and the cracking in successive passes of the cycle gas oil produced in the previous pass. The built-up yields from the four passes are compared with the results obtained when currently recycling. Table I-Results of Successive Passes a t 750 Pounds Pressure, 15,000 Gallons per Hour t o 1100 Cubic Feet Soaking D r u m Average soaking-drum temperature, 855-860' F. YIELDSox RUN 1 STRAIGHTSTRAIGEITRUN 4 RUN RUN MID- RUN 2 RUN 3 CYCLE GASOIL COFTI- CYCLE CYCLE GAS BUILT- WHEN NENT GAS OIL GAS OIL OIL UP CURRENTLY G.4s OIL PROM 1 FROM 2 PROM 3 YIELDSRECYCLING ~~
~~~~~~
% Yields on fresh feed gasoline, 400' F. End pointa. . . . . 2 4 . 3 Cycle gas oil&.. . 6 3 . 5 4.4 Fuel o i l a . . . . . . . Gash ..... . . . . . . 5.5 Cokeb., , . . . . , . 0.5 Material balanceb 9 6 , 3 Ratio fuel-oil yield to gasoline yield 0.18 C o n v e r s i o n per passo.. . . . , , , . , 2 4 . 3 Fuel oil gravity, 0 . 4 . P . I. . . . . . . . 1 0 . 0 0 Per cent by volume. b Per cent by weight.
"
%
19.3 66.2 7.3 4.2 0.3 96.4 0.38
% 16.7 67.5 8.6
4.5 0.7 97.4 0.51
% 14.9 67.1 11.0 5.5 1.5
99.0 0.74
% 47.8 19.1 15.7 11.7 1.2 92.6 0.33
% 48.2 22.3 19.1 12.7 1.7 100.2
0.40
19.3
16.7
14.9
21.3
12.0
9.0
8.5
12.9
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1930
1239
8 It
B
*6
5 B
h
8 i!
PASS
Figure 3-Effect
/vUMDSi?
of Successive Passes on Yields
I n Table I1 the results of the individual passes have been Figure 4-Effect of Successive Passes on Gasoline Yield Aniline corrected to a common fuel-oil basis (12" A. P. I.). This Point of Cycle Gas Oil a n d Ratio of Fuel Oil Yield (12O'A. P. I.) on Fresh Feed Gasolink Yield correction was made by calculating the quantity of cycle gas oil required to elevate the fuel oil to 12" -4.P. I. and this quantity was subtracted from the cycle-gas-oil yield and amounts of coke. Furthermore, conversion to gasoline added to the fuel-oil yield. I n actual plant-scale operations would be obtained only at the expense of the yield of cycle the cycle gas oil that would be included in the fuel oil would gas oil. It will also be noted that both the fuel-oil and coke be of somewhat lower A. P. I. gravity than the average gravity yields are increasing in absolute percentage as well as in of the cycle gas oil used. However, the fractionation be- relation to the gasoline yield. The fuel-oil yield increased tween fuel oil and cycle gas oil is poor in most cracking equip- from 4.4 per cent of 10' A. P. I. for the virgin feed to 11.0 ment and it is probable that no great error is introduced. I n per cent of 8.5"-4.P. I. for the fourth pass. The ratio of the any case the correction amounts to not more than 2 per cent fuel-oil yield t o the gasoline yield increased from a value of the fresh feed. I n Table 1112the built-up yields, based on the straight-run gas oil, are shown after each pass. The yields are corrected to a 12" 8. P. I. fuel-oil basis. The built-up yields were obtained by basing the yields for each pass back on the original virgin gas oil and combining the yields of any one pass with those from the preceding passes. Table IV and Figures 5 and 6 present the inspections of the feed stocks used and of the products obtained on the successive passes. Figures 3 and 4 pre%4POR BMPEm4c(rURE A T CUT P O f m - S - *F sent the results of Tables I, 11, and I11 Figure 5-Effect of Successive Passes on Gravity of 10 Per Cent Cuts on Gas-Oil Fractions in graphical form. I n Table I and Figure 4 it will be seen that the conversion to gasoline decreases with each successive pass through the equipment from a value of 24.3 per cent for the virgin gas oil to 14.9 per cent for the fourth pass. I n all cases the temperature, pressure, and feed rate were maintained approximately constant. While the results do not represent the maximum conversion to gasoline that could be obtained from different gas oils and conditions of operations, they are comparable among themselves and are representative of average operation on this type of stock when not producing excessive Work along this line has also been carried by A. N. SdChanOV and M. D. Tilitscheyew a n d is reported in a translation t h a t will appear in the near future, Out
+ -'
I l l l l l l l l l l / l / l l / l l I l l l l l l l l l l l l l l l $ & I
Figure 6-Effect
J50
640
650
700
750
840
k%POR %WP€RArURh - .F of Successive Passes on Boiling Range of Gas-Oil Fractions
INDUSTRIAL AND ENGINEERING CHEMISTRY
1240
of 0.18 for the straight-run gas oil to 0.33 for the cycle gas oil fed to the fourth pass. It is interesting to note that there is not much change in the percentage of gas produced, though in general the ratio of gas to gasoline increases. T a b l e 11-Calculated
Yields for E a c h Pass W h e n C u t t i n g 12' A. P. I. F u e l Oil PASS 1 PASS2 PASS3 PASS4
% Yields on fresh feed: Gasoline, 400' F., end pointa.. . . 2 4 . 3 Cycle gas oila.. . . . . . . . . . . . . . . . 6 3 . 0 12' A. P. I. fuel o i l Q . .. . . . . . . . 4 . 9 5.5 Gasb.. . . . . . . . . . . . . . . . . . . . . . . . 0.5 Cokeb., . . . . . . . . . . . . . . . . . . . . . Material balanceb.. . . . . . . . . . . . 9 6 . 3 Ratio fuel-oil yield t o gasoline yield 0.20 0.23 Ratiogasyieldtogasolineyield . . . a
b
%
%
%
19.3 66.2 7.3 4.2 0.3 96.4 0.38 0.22
16.7 65.8 10.3 4.5 0.7 97.4 0.62 0.27
14.9 64.2 13.9 5.5 1.5 99.0 0.93 0.37
Per cent by volume. Per cent by weight.
A comparison of the built-up yields from the individual oDerations with the results of the oDeration when currently recycling shows that approximately the same results are obtained when the cycle gas oils are cracked alone and in the presence of the straight-run gas oil, provided, of course, that no change is made in the operating conditions. The material balance for the built-up yields is poor because of low recoveries on three of the individual operations, resulting in a large cumulative error.
Vol. 22, No. 11
vidual passes. The gasoline produced by the fourth pass had an octane number three points lower than the product from the virgin-gas-oil operation. This result is contrary to the idea that seems to be prevalent that cycle gas oils produce gasolines of higher antiknock value than virgin gas oils from the same crude when cracked under the same conditions. T a b l e 111-Calculated Yields on S t r a i g h t - R u n G a s Oil a f t e r E a c h P a s s W h e n C u t t i n g 1Z0 A. P. I. Fuel Oil YIELD WHEN CURP A S S 1 PASS 2 P A S S 3 P A S 5 4 RESTLY RECYCLING
% Yields on straight-run gas oil: Gasoline. 460' F. endpoint". . 24.3 Cycle gas oil5. . . . 6 3 . 0 12' A. P. I. fuel oil= 4 . 9 Gas b.. . . . . . . . . . . . 5 . 5 Cokeb.. . . . . . . . . . . 0 . 5 Material balanceb. 9 6 . 3 Ratio fuel-oil yield t o
%
%
%
36.5 41.7 9.5 8.1 0.7 94.0
43.4 27.4 13.8 10.0 1.0 92.9
47.5 17.6 17.6 11.5 1.4 92.6
R , p t ~ ' ~ & y i , ' f ~ l ~o.20 .t; gasoline y i e l d . . . ' . 0
b
0.23
o.26 0.22
0.32 0.23
0.40 0.26
Per cent by volume. Per cent by weight.
Summary and Conclusion
With successive passes through the equipment under the same operating conditions (1) the yield of gasoline on the feed stock to each pass decreases; (2) the ratio of fuel-oil
T a b l e IV-Inspection of Feed S t o c k s and P r o d u c t s PASS 1 PASS 4 STRAIGHT-RUN PASS2 PASS 3 CYCLEGAS OIL MIDCONTINEKTCYCLEGASOIL CYCLEGASOIL FROM 2 FROM 3 FROM 1 GAS OIL Feed-stock insoections: Gravity, A. P . I . . . . . . . . . . . . . . . Aniline point, F. . . ... Distillation :
0.37 0.24
%
CYCLE GAS OIL FROM 4
YIELDS O N STRAIGHT-RUN GAS OIL WHENCURRENTLY REDUCING
24.7 84 F. A . P. I. 447 34.4 464 3 2 . 5 480 3 0 . 7 496 2 8 . 9 512 27.2 530 25.6 553 23.4 583 2 1 . 4 635 1 7 . 9 712
33.7 169 F . A . P. I . 496 42.1 534 3 8 . 1 559 36.6 580 35.5 600 34.5 622 3 3 . 6 648 3 2 . 5 678 3 1 . 3 732 2 9 . 6 757
~~
33.7 169
.
.
" F .A . P . I . 496
42.1
. . . 534 38.1 ..
.
50 % 60% 70%
...
..
.
...
80 %
90 7% Final boiling point
z.
Gasoline inspections: Gravity, A. I . .. . . . . . . . . . . Aniline point, C. Octane number. . . . . . . . . . . . Distillation: 140° F . . . . . . . . . . . . . . . . . 158' F. . . . 212' F 284'F. . . 356' F. 374' F . . . . . . . . . . . . Final boiling point, ' F.. . . . . Fuel-oil inspections: Gravity ' A . P I Flash-Pknsky-Marten, F Furol viscosity a t 122' F., sec Water and sediment, %5 Insoluble sediment, 70
..
...
559 3 6 . 6 680 3 5 . 5 600 34.5 622 33.6 648 3 2 . 5 678 3 1 . 3 732 29.6 757
28.9 126
30.4 142
F. A . P.I .
474 38.9 500 3 6 . 8 524 35.2 544 3 3 . 8 565 32.6 590 3 1 . 4 613 3 0 . 1 642 28.4
a
F. A . P. I.
459 490 508 528 547 564 596 628 680 732
38.0 35.2 33.2 32.1 30.8 29.4 28.0 25.9 23.3
26.5 106
F. A . P. I.
453 471 488 505 525 546 572 610 659 734
36.8 34.7 32.0 30.2 28.6 26.7 24.8 22.5 19.9
a
58.8 42.0 84 .~
58.8 41.5
57.5 41.5 67
56.0 41.6 65
59.5 42.0 66
2.0 6.5 24.0 56.5 90.5 94.5 383
2.5 6.5 23.0 54.0 87.5 94.0 387
2.5 11.0 29.0 65.0 90.5 93.0 401
3 5 8.0 26.0 65.0 93.0 94.5 403
5.5 11.0 29.5 61.5 91.5 95.5 381
10 0 353 210 0 15 0 01s
12 0 168 22 0 90 0 260
9 0 245 25 0 90 0 260
8 5 147 18 1 8 0 370
12.9 169 12 1.3 0.390
70
%
Figure 3 shows the built-up yields on virgin gas oil after each pass. The cycle-gas-oil yield of necessity decreases with increasing yields of gasoline, fuel oil, and gas. I n Table IV and Figures 5 and 6 it will be observed that the boiling range of the cycle gas oil decreases with each pass through the system. Furthermore, the A. P. I. gravity of the cut corresponding to any one temperature on the distillation curve is lower for the cycle gas oil from each succeeding pass, indicating the presence of more unsaturated compounds. Additional evidence of the change in the chemical composition of the gas oils is found in the fact that the aniline point decreases from a value of 169' F. for the straight-run gas oil to 84" F. for the cycle gas oil from the fourth pass. The boiling ranges of the gasolines are somewhat erratic, but they do not appear to be widely different for the indi-
7%
E "/.
yield to gasoline yield increases; ( 3 ) the boiling range of the cycle gas oil produced is lowered; (4) the A . P. I. gravity corresponding to a given boiling range is lowered; ( 5 ) a decided change in the chemical composition of the cycle gas oils occurs, as indicated by the decrease in aniline point. It is therefore concluded that the cracking value of cycle gas oils in terms of the products that can be derived from them decreases with successive passes through the equipment. Apparently the same results are obtained either by currently recycling the fractions of cycle gas oil or by cracking them separately under the same operating conditions. Literature Cited (1) Beiswenger and Child, IND. EXG. CHBJI., Anal Ed., 2, 284 (1930)
