Mechanism of Occurrence of Metals in Petroleum Distillates ROBERT A. WOODLE AND WILLIAM B. CHANDLER, JR. The Texas Co., Port Arthur, Tex.
A study is described which was undertaken to investireduced crude contained 0.51 have discussed various gate the mechanism whereby vanadium and other metals p.p.m. vanadium and was contained in the parent crude oil enter petroleum disconsidered t o be essentially aspects of the problem of metals in charge stocks for free of metals. tillates. Several experimental approaches to this problem were employed. They included vacuum steam distillaEquipment and Procedure. catalytic cracking. Mills ( 3 ) presented data which demtions and molecular distillations of metal-containing Vacuum steam distiIIations onstrate t h a t minute crudes, and color determinations on blends of distillates were performed in the 10with residua. The data obtained support the conclusion gallon capacity distillation amounts of heavy metalsthat the presence of metals in vacuum distillates is caused iron, nickel, vanadium, unit shown in Figure 1. The by actual volatilization of metal compounds indigenous still pot of this unit is a n 18copper-in distillate cracking charge stocks cause inch length of 16-inch standto the crude oil. Experimental evidence suggests that greatly increased gas and vanadium may exist in crude oil in a family of compounds ard steel pipe, closed on the covering a broad molecular weight range but that only ends by welded plates of l/4coke yields and reduced gas+ line production. Wrightthose compounds of relatively low molecular weight i n c h s t e e l . An i n t e r n a l contribute to the metal content of vacuum distillates. knockout drum consists of a son (9)observed that as the p e r c e n t a g e overhead ob12-in length of 4-inch standtained by vacuum distillaard steel pipe with welded ends of '/l-inch steel plate and with the two 1 x 4 inch slots on tion of a metal-containing reduced crude is increased, the concentration of metal in the distillate increases in a regular top which serve as vapor passages. Steam is admitted t o the manner with the yield of distillate. still pot through a n inclined l/Z-inch line perforated with 3/82Skinner ( 7 ) recently reported that all the vanadium in Santa inch holes. The still pot is directly fired by means of a gas Maria Valley crude exists as a vanadium porphyrin complex burner. Vacuum is provided by a three-stage steam jet. In every vacuum steam distillation, a routine procedure was which is stable up t o 230" F. but breaks down or changes its form followed: at some temperature between 230" and 840" F. There are a t present two schools of thought regarding the A weighed amount of oil (about 17,000 grams) was charged mechanism of occurrence of metals in distillates. One (8, 3 ) after the pot had been evacuated and preheated Over a low holds that mechanical entrainment of residue is responsible, while fire to 300" F. The temperature and vacuum were maintained the other ( 4 ) concludes that the metals are Present in t h e form during the charging operation. The fire was then adjusted t o of volatile metal-organic compounds. Heretofore, no convincing bring the sample UP t o distillation temperature, and quart distiIlate cuts were taken a t a evidence has been presented rate of about 100 t o 150 ml. in support of either mechper minute through the backanism. trap line at the bottom of the This paper reports the repacked tower. Steam was PACKED TOWER admitted t o the still when the sults of an experimental instill pot temperature reached vestigation which had as its 600" F. or prior to that if the object the determination of product rate was observed t o t h e mechanism whereby decrease rapidly. The distillation was concluded when vanadium and other metals the still pot temperature enter petroleum distillates. reached 700" F. ThroughThis study was comprised of out this work, distillation vacuum steam distillations pressures were maintained at about 10 mm. of mercury. and molecular distillations of The molecular distillations metal-containing crudes and were conducted by t h e color determinations on Beacon Laboratories of The blends of distillates with Texas Go., employing a 14inch centrifugal cyclic batch residua. molecular still. This unit was charged with about 5 gallons EXPERIMENTAL of oil. By successively passStocks. Inspections on the ing the undistilled portion of the charge over the 14-inch four reduced crudes selected diameter rotor (1725 r.p.m.) for study are shown in Table I. and increasing the rotor temThree of these reduced crudes perature after each pass, were regarded as relatively quart d i s t i l l a t e f r a c t i o n s were obtained. rich in metals and ranged from 235 p.p.m. (very high) Figure 1. Schematic Diagram of Vacuum Steam Analysis. The methods t o 14 p.p.m. (low) vanadium concentration. The fourth Distillation Unit used in analyzing for vana-
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EVERAL recent papers
November 1952
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Table I .
Inspections on Charge Stocks T'enezuelan
Reduced Crude Vol. % basis total crude Gravity O A.P.I. ASTM distillation, 0 F. IBP 5% 10 20 30 ~. 40 50 60 70 80 90
-4 66.7 21.0 520 554 574 614 632 670 682 694 706 720 Cracked
-
B 61.2 23.3 543 572 590 620 652 680 % 7 716 726 742 760
West Texas 45.0 17.1 506 578 628 662 683 697 709 719 728 732 Cracked
-
Louisiana 40.8 25.1
The iron analysis employs the reduction of ferric iron with hydrovylamine and reaction of ferrous iron with o-phenanthroline in a buffered solution t o give a stable color. A 1-cm. cell, 510millimicron wave length, and a blue-sensitive photocell were used. Nickel is determined by the formation of nickel (IV) dimethylglyoxime, throughtheaddition of dimethylglyoxime t o an ammoniacal solution of the metal. The optical density wa.s measured using a 1-cm. cell, 520-millimicron nave length, and a blue-sensitive photocell.
724 -
730 Crackcd
RESULTS Distillations of Metal-Containing Crudes. In order t o investigate the nature and extent of vanadium concentration which could be expected in distillates, three representative metalcontaining reduced crudes were subjected t o vacuum steam distillations. The following runs were made: Charge Stock
dium, nickel, and iron were the colorimetric methods dencribed by Sandell (6). For distillate samples, the quantity of oil analyzed usually ranged from 300 to 400 grams, while for residual samples smaller amounts were used, based on the estimated metal contents. In each case, a weighed amount of oil was carefully ashed, a t a slow rate, in a platinum dish heated by a gas flame (maximum temperature about 1200" F.). The ash was taken into solution either with acid or by fusion, depending on the metal being analyzed for, and reagents were added t o produce a colored solution by reaction with the metal. The intensity of the color was determined by means of a spectrophotometer. The vanadium analysis is based on the formation of soluble yellow phosphotungstovanadic acid by the addition of phosphoric acid and sodium tungstate to an acid vanadate solution. Optical density of the solution was measured using a I-cm. cell, 400 millimicron wave length, and a blue-sensitive photocell.
Table 11. Vacuum Steam Distillations-Vanadium Analyses
Charge Stock Reduced Venezuelan crude
-4
Cumulative Kinematic Vol. % Over Color, Viscosity Basis Crude R u n C u t a t 210' F., a t Mid- Vanadium, I.a>,ibond No. So. Cs. Point of C u t P.P.M. */s-Inch Cell 1 9 7.6 64.0 0.1 ... 10 10 6 67.0 0.2 ... 11 16.1 71.0 1.0 , . . 74.5 2.8 ... 12 25.6 77.0 20 , . . 13 42.6 55.3 0.02 ... 2 6 3.6 59.2 0.05 , . . 7 4.5 63.3 0.1 ... 7.1 8 67.2 0.3 ... 9 10.7 71.3 0.8 ... 10 16.5 75.2 3.3 ... 11 27.4 78.2 23.7 ... 12 53.2 6.1 61.5 0.2 3 8 8.5 65,O 0.2 9 13.1 69.0 0.7 10 19.8 72.5 1.6 11 12 75.0 6.8 28.6 ... Rrsidue 1006 4 7 58.5 0.02 4.7 62.0 0.2 8 6.4 9 9.2 66.0 0 5 13.5 69.5 0.6 10 ... 11 73.0 2.1 20.7 ... 76.0 9.2 12 34.6 ... 78.5 44 13 60.8 5 11 10.8 67.5 0.3 195 0.8 310 12 14.8 70.0 3.6 640 13 22.8 73.5 10.7 lfi46 38.8 76.5 14 6 13 12.2 80.6 0.2 500 1.0 700 14 18.6 84.0 15 32.4 87.2 4.2 1750 16 71.6 90.4 25.3 4415 Residue ... ,. . 946 ... 7 13 12.1 83.2 0.4 ... 14 20.2 86.7 2.0 ... 15 39.3 90.1 7.3 1710 Residue ... ... 795 ... 0 2 560 80 9 8 10 22 8 0 8 1120 11 34 1 83 5 1600 86 2 2 1 12 51 I
...
Reduced T'enezuelan crude B
Reduced West Texas crude
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Itetluced Venezuelan crude -4. Reduced Venezuelan crude B Reduced K e s t Texas crude
V in Total Crude,
P.P.M. 175 75 6
Run No. 1, 2, 3:4, 5 6, 8
Product analyses are shown in Table 11. In Figure 2, the vanadium concentration of each distillate is plotted versus its depth in the total crude. In every case, the vanadium concentration increased sharply with increasing depth in the crude, but no relationship was apparent between the relative positions of the curves and the vanadium 'contents of the total crudes. Inspections on the distillates shoxved that all possessed characterization factors (8) within the narrow range from 11.7 to 12.0. Therefore, viscosity was chosen as an independent correlating variable indicative of average molecular weight and boiling point. Figure 3 demonstrates that for the three crudes under investigation, log vanadium concentration is a linear function of log kinematic viscositr (210" F.). hloreover, the lines are parallel within the precision of the data, and are in an orderly arrangement according to the vanadium contents of the total crudes. 40
IO
En
g
1.0
U
z U >
0.1
0.02 50
60 70 80 90 CUMULATIVE V O L . % OVER BASIS CRUDE AT MID-POINT OF CUT
Figure 2. Vacuum Steam Distillations of Metal-Containing Crudes, Vanadium-Yield Correlation
INDUSTRIAL AND EN GINEERING CHEMISTRY
Vol. 44, No. 11
PETROLEUM-COMPOSITION These data did not, of themselves, establish the mechanism of entry of metals into distillates. However, visual inspections of the distillates did not reveal any abnormal coloring of the fractions which wouId indicate entrainment of residues, and the sharp rate of increase of vanadium concentration with depth in the crude suggested a volatilization mechanism.
4 0 -
-
10
fa
for that crude-Le., entrainment was negligible. Comparison of the curves for Venezuelan crude A and vanadium pentoxidecontaining Louisiana crude leads t o the conclusion t h a t the presence of vanadium in t h e Venezuelan crude distillates was necessarily the result of actual volatilization of vanadium compounds contained in the crude oil. Color Blends. Figure 2 shows that the amount of vanadium contained in the distillates produced from any of the crudes evaluated was only a low percentage (about 20% or less) of t h a t which was contained in the parent reduced crude. Analyses confirmed that the vacuum residues were highly enriched in vanadium as well as in black, asphaltic color bodies. Using representative stocks, it was possible t o prepare blends of a vacuum residue in distillates t o determine whether a n entrainment mechanism was consistent with the observed colors of vanadiumcontaining distillates. Several distillates and a refiidue (590 p.p.m. vanadium) from vacuum steam distillation of reduced Venezuelan crude Awere selected for this work. Colors of the distillates were determined before and
i
2 1.0 I s U
40
IO
H
VENEZUELAN CRUDE 0
a
P
z-
0 002
2
2 1.0 e
6 IO 20 40 60 100 DISTILLATE VISCOSITY, KIN. AT 210'F: 4
d
z
Figure 3. Vacuum Steam Distillations of Metal-Containing Crudes, Vanadium-Viscosity Correlation
4
At this point it was desirable t o determine the extent of bottoms entrainment which could reasonably be expected in vacuum steam distillations of t h e type employed. Charge stock for this work was a reduced Louisiana crude which contained 0.5 p.p.m. vanadium. This stock wm divided into two portions; t o one portion was added sufficient vanadium pentoxide, in finely powdered form, to raise the vanadium concentration to about the same value (209 p.p.m.) as t h a t of the reduced Venezuelan crude A (235 p.p.m.). Both portions were subsequently vacuum steam distilled and t h e distillates analyzed for vanadium. D a t a for these runs are presented in Table I11 and are compared in Figure 4 with data for Venezuelan crude A. Analyses of distillates produced from the Louisiana crude, with and without added vanadium pentoxide, showed t h a t t h e presence of vanadium pentoxide had no effect on t h e vanadium-viscosity correlation
Table 111. Vacuum Steam Distillations of Louisiana Crude-Vanadium Analyses
Charge Stock ReducedLouisiana crude
Run No.
9
Cut No. Charge 13 14 15 10 17
ReduoedLouisia n a crude f VZ06
November 1952
10
Charge 15 16 17
Kinematic Viscosity
Color, Lovibond '/e-Inoh Cell
at 210' F., cs.
Vanadium,
14.0 18.0 24.6 40.0 74.2
loo-
s a
0
0'
0
t
I
I
I
I
0
IO
20
30
40
101
DISTILLATE VISCOSITY, KIN. AT 210" F:
Figure 7.
Color-Viscosity Correlations, Vacuum Steam and Molecular Distillates
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 11
PETROLEUM-COMPOSITION Figure 8 shows the vanadium-viscosity curves for both t h e virgin and the deasphalted reduced crudes. Despite the fact that less than 5 % of the vanadium remained, the deasphalted crude yielded distillates which contained 60% as much vanadium as did distillates derived from the virgin crude. These data help to elucidate the nature of the distribution of vanadium compounds in the virgin crude. As stated, propane deasphalting effects separation according t o molecular weight. The vanadium compounds retained in the deasphalted crude were those of lowest molecular weight; although they constituted a very small percentage of the total, those vanadium compounds accounted for over half of the distillate vanadium content. The great bulk of the vanadium was rejected in the higher molecular weight asphaltic fraction. It does not seem unreasonable t o suggest that vanadium may exist in crude oil in a family of compounds covering a broad molecular weight
40
-
10
za i
g 1.0
-
U
z
3
0.1
40
-
IO f 1 , I I 0.02 I 2 4 6 IO 20 40 '60 I00 DISTILLATE VISCOSITY, KIN. AT 210' I? Figure 8. Vacuum Steam Distillation of Deasphalted Reduced Venezuelan Crude A, Vanadium-Viscosity Correlation
z 0.
1 1.0
c w of the aame viscosity. This would appear t o indicate entrainment during the vacuum steam distillations until it is realized that the vacuum steam distillates were exposed t o sufficiently higher temperatures during their preparation t o account for their higher colors. Distillation of Deasphalted Crude. Propane deasphalting is very similar to distillation, in that both processes effect separations according to molecular weight. By propane deasphalting reduced Venezuelan crude A, a t 100" F. using an 800 volume % dosage, a propane-soluble fraction was prepared which represented 84 volume % of the parent reduced crude but which contained less than 5 % as much vanadium (10 p.p.m.). The fraction so prepared, termed the deasphalted reduced crude, was vacuum steam distilled and the distillates were analyzed for vanadium. Data are presented in Table VII.
Table VI.
Molecular Distillations of Venezuelan Crude A-Vanadium Analyses
4
1
7
2
8 9 5 6 7
11.6 16.8 26.4 8.2 11 0 16.2
69.1 72.5 76.0 65.9 69.1 72.2
70
0.1 0.8 3.2 0.02 0.2 1.2
160
560 45 65
...
Table VII. Vacuum Steam Distillation of Deasphalted Venezuelan Crude A-Vanadium Analyses cut
No.
Kinematic Viscosity a t 210° F., Cs.
Charge 12 13 14
19.5 30.0 45.5
November 1952
...
Color
Vanadium,
P.P.M.
Lovibohd l/s-Inoh Cell
10 0.86 3.5 8.5
520 1200 2080
...
H
0.1
I
, .I I . I 0.02 2 4 6 IO 20 40 60 100 DISTILLATE VISCOSITY, KIN. AT 2lO'F. Figure 9. Vacuum Steam Distillation of Reduced Venezuelan Crude A, Vanadium-, Nickel-, and Iron-Viscosity Correlations range. It appears, however, that only those metal compounds of relatively low molecular weight enter vacuum distillates. Nickel and Iron. Mills (3) has shown that vanadium, nickel, and iron are three of the most offensive metals in so far as catalytic cracking is concerned. To investgiate the nickel and iron contents of distillates, a vacuum steam distillation was conducted on reduced Venezuelan crude A. The data are presented in Table VIII, and the metal-viscosity curves for all three metals are compared in Figure 9. It is interesting that the nickel-viscosity curve parallels t h e vanadium-viscosity curve. Moreover, for a given viscosity, the ratio of vanadium t o nickel in the distillate is essentially constant and is the same as the ratio of those metals in the reduced crude-i.e., about lo/,. The iron results are unique in two respects: T h e concentrations of iron in the distillates are extremely high relative t o t h e concentration in the parent reduced crude (13.6 p.p.m.), when compared with vanadium and nickel; and the slope of the iron-
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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Table VIII. Vacuum Steam Distillation of Reduced Venezuelan Crude A-Nickel and Iron Analyses Cut No.
Kinematic Viscosity at 210’ F., CS.
Nickel,
P.P.hI.
Iron, P.P.M.
Color Lovibohd ‘/z-Inch Cell
viscosity curve is appreciably less than the slopes of the vanadium and nickel-viscosity curves. Both of these points suggest that’ the mechanism of iron entry may not have been volatilization; but in the absence of additional data, one can only speculate as t o what that mechanism might have been, It must be kept iri mind that both the preliminary reduction of the total crude and the subsequent vacuum steam distillation of the reduced crude were performed in iron equipment; whereas the only source of vanadium and nickel was the crude itself, some iron may have been introduced into the reduced crude or into the distillates by chemical reaction with the equipment. The data presented here, therefore, do not provide positive evidence of the existence of volatile iron compounds in the crude oil. DISCUSSION
Skinner ( 7 ) recently described an investigation which was directed toward determining the physical and chemical state of vanadium in Santa Maria Valley crude and the properties of the vanadium bearer. The vanadium was shown t o be present as a vanadium porphyrin complex which is dissolved or dispersed i n the crude oil, and which possesses a high degree of stability. Skinner and others ( 1 ) have pointed out, the relatively high concentrations of some of the less common metals in crude oils suggest that the metals are of biological origin and that they were introduced through the process of formation of the oil. Skinner reported that, the vanadium-bearing molecule was concentrated in the high molecular weight, asphaltic fraction of the crude and that it became thermally unstable a t some temperature between 230” and 840” F. The experimental results reported here demonstrate that (1) the vanadium and nickelbearing molecules are thermally stable u p t o 700” F., and (2) at temperatures approaching 700” F., these molecules exert very small but significant vapor pressures. Although this study was theoretical in nature, the results are of considerable practical interest t o t,he petroleum refiner. In
any plant vacuum crude still, entrainment probably occurs t o some extent, depending on the tower design and the throughput a t which it is operated, and may contribute heavily to distillate metal contents when metal-containing stocks are being charged. Although elimination of entrainment will effect some reduction of distillate metal concentration, the work reported here s h o m that for a given crude there is a metal concentration below which it is not possible t,o descend solely by eliminating entrainment. Further, Figure 2 demonstrates that it may be economical, in aggravated cases, t o decrease slightly the yield of vacuum distillate and thereby preferentially reject the portion of the distillate containing the highest metal concentration. CONCLUSION
An experimental investigation has been conducted to determine the mechanism whereby vanadium and other metals enter distillates derived from metal-containing crude oil. Data are presented which demonstrate that metals enter vacuum distillates by actual volatilization of metal compounds indigenous t o the crude oil. Experimental evidence suggests that vanadium may.exist in crude oil in a family of compounds covering a broad molecular weight range, but that only those compounds of relatively low molecular weight contribute t o the metal content of vacuum distillates. These results are of practical interest to the petroleum refiner. ACKNOWLEDGMENT
The authors are indebted t o The Texas Co. for permission to publish this work. The assistance of the Beacon Laboratories of The Texas Co. and the helpful comments of N. B. Haskell arc gratefully acknowledged. LITERATURE CITED Dunstan, “The Science of Petroleum,” Vol. 11, p. 1053, New York, Oxford Univ. Press, 1938. Johnson, P. H., and Mills, K. L., IND.ENG.CHEM.,44, 1624-9 (1952). Mills, G. A., Ibid., 42, 182-7 (1950). Nelson, W. L., Oil Gas J . , 50, No. 15, 160 (Aug. 16, 1951). Sachanen, “The Chemical Constituents of Petroleum,” pp. 397-9, New York, Reinhold Publishing Co., 1945. Sandell, “Colorimetric Determination of Traces of Metals,” 2nd ed., pp. 378, 476, 607, New York, Interscience Publishers, Inc., 1950. Skinner, Davis A., IND.ENG.CHEDI.,44, 1159-65 (1952). Watson, K. M., Nelson, E. F., and Murphy, G. B., Ibid., 27, 1460 (1935). Wrightson, Frances M., -4naZ. Ckem., 21, 1543-5 (1949). RECEITED for reviev August 2 5 , 1952.
ACCBPTED September 16, 1952.
Catalytic Cracker a t Sundown
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