Determining Trace Metals in Petroleum Distillates An Acid-Extraction Technique J.
E. BARNEY I I
Research Department, Standard O i l Co. (Indiana), Whiting, Ind.
Catalytic refining of petroleuni deniands a knowledge of minute traces of metal compounds t h a t occur in petroleuni fractions. 3letals anal>-ses t h a t involve combustion of t h e oil may- :-ield low results because of bolatilization and entrainment. . i n acid-extraction technique has been developed for separating these metals from the oil, after which the:- are determined bjspectrographic a n a l y i s . The oil is extracted first with sulfuric acid and then with a mixture of hydrochloric acid, acetone. and water. After concentration, t h e extract is transferred to graphite electrodes for spectrographic anal>-sis i n a direct-current arc. If spectrographic equipment is not available, sensitive chemical methods can be used. The coefficient of variation of t h e method is 1 1 7 ~ . About 6 man-hours and 72 hours of elapsed time are required per anal>-sis. Six methods for recovering copper, iron, lead, nickel, and vanadium from a gas oil have been compared with t h e acid-extraction technique: simple ashing, partial sulfated ashing, total sulfated ashing, extraction w-ith iodine, extraction with a mixture of hydriodic and acetic acids, and extraction with a mixture of hydrobroniic and acetic acids. Total sulfated ashing, extraction with hydrobromic and acetic acids. and acid extraction recovered t h e five metals quantitati\-el:-. The other four methods were iinsatisfactory. Partial sulfated ashing gave no better rcsiilts t h a n simple ashing.
T
HI,: illcreased use of catalytic pi'ocesyes in t h e refining of petroleum has intensified t h e study of trace metals occurring i n p ~ t r o l e u mfractions. These metals may have been present i n the original petroleum or introduced during prior processing. Only it fen. tenths of a part per million of such metals as copper, iron. lead, nickel, and vanadium in t h e feed m a y decrease catalJ-*t activity. Knowledge of the concentrations of these metals is thus important t o the refiner. 11:Lny schemes have been devised for the determination of nietnls in petroleuni fractions. Some cover only concentrations almve 1 p,p,ni.: others extend the detectable limit t o 0.1 p.p.rn. Copper, iron, nickel, and vanadium have been determined in crude oils by niicropolarographic and colorimetric techniques in concentrations as low as 0.1 p.p.m. (10). Colorimetry hae d a o heen applied t o iron, nickel! a n d vanadium in petroleiini fraction:: 139) and vanadium in fuel-oil ash (23, 14). The emission spectrograph offers several advantages over these terhiiicjiies. If t h e metals content is high enough, t h e oil can be nnnl!~aed directly Ivithout prior ashing. Seireral spectrographers have dirertly tlt~terminedmetals in lubricating oils ( 3 , 4 ,7 , IO, 2 2 ) . A cathode-layer technique ( 1 5 ) has been used in the direct analysis of heavy petroleum stocks. Key and Hoggan (16) used a nitrogen atmwphere t o spark residual fuels. These procedures are a:itiPfitrtory for rrude oils, residual fuels, and other fractions cont:iining m:iiiy parts per million of metals, hut they are not sensitive enough for gas oils and naphthas. Concentration of the m c t d s Iic>fore measuring t h e :iniounts present, is therefore necessary. Ilven \vhe11 direct analysis of t h r nil i - not feasible, spectrographic, niethotls can be carried out upon ashed residues in less time than cheniical methods. S o sepmitions are required, and the nitxtals :LW determined siniiiltarico~isl?.. A semiquantitative
method for determining metals in ashed residues has been devised by Murray and Plagge (20). Three quantitative methods have recently been proposed ( 1 , 5 , 1 4 ) . In all four methods, ashing t h e oil may lead t o loss of metals by volatilization and entrainment (16, 29). Chemical oxidation of t h e oil would eliminate tlieqe difficulties, but it requires 1:trgc :iniount.s of mineral acids and is slow. -4 new extraction technique has been developed that eliminates ashing of t h e oil and does not reVYCOR FLASK qiiire large amounts of mineral acids. -4s l i t t l e a s 0.01 p.p.ni. of some nictals can be deFigure 1. Extraction apparatus termined in o n l ~ . 50 grams of oil. T h r oil is extracted first with concentrated sulfuric arid and then with a mixture of ronrentrated hydrochloric acid, acetone, and watrr. T h e estrart is evaporated to dryness, t h e carbonweoils material is removed by ignition, and the inorganic residue is dissolved in hydrochloric arid. T h e acid soluticn is evaporated t o ahout 0.2 nil. a n d transferred t o grstphite electrodes for spectrographic in a direct-current arc. 31etals in the oil are determined from working curves based on standardized aqueous solutions. If spectrographic equipment is not available, t h e acid ;;olutioIis may he analyzed by any sensitive chemical procedure. EQUIPMEST
For extraction of the oil, a 300-ml. two-necked flat-bottomed flask, especially constructed of Vycor brand glass (96% silica,). and a 30-em. reflux condenser are used. T h e extraction flask is shown in Figure 1. T h e stirring bar is covered with VJ-cor arid inserted in a Teflon sleeve. Spectrographic equipment consists of a Bausch and Lomb large Littrow spectrograph, a Xultisource power supply, an A.R.T,.Dietert comparator-densitometer, and other conventional equipment. Table I summarizes the excitation conditions and other pertinent spectrographic information. ;1 two-step adjustable sector is inserted in the optical path to provide coppcr lines of
Table I.
Summary of Spectrographic \.ariables
-1.r. innut. volts D.C. 0Utpr;t. volts Capacitance, pfd. Inductance, ph. Resistance, ohms Exposure, seconds Slit width, microns Wedge. riini. .inalytical gap. m m . Upper electrode5 Louer electrodes Internal standard Emulsion Developer
Dei-elopment. min. Acid stop, scc. Fix, min. JYash. min. Drying, min.
230 ~~~
300 60 480 18
60 20 J
9 Undercrit center-pojt Flat chamfered Cobalt S .A .-2 D-19 3 20 10 3 ~~~
1283
~
~
ANALYTICAL CHEMISTRY
1284 suitable intensity. T h e photographic emulsion is calibrated by standard methods ( 1 2 ) .
Table 11.
Spectral Lines Used for Anal? sis Concn. Range,
Line,
0.02-0.5 0 . 5 -5.0 0.05-1.0 0 . 5 -5.0 0.02-0 2 0.2 -5.0 0.02-0.5 0.2 - 5 0 0.02-0.5 0.05-2.0 1.0 -5.0 1 . 0 -5.0 0.02-0 5 0 . 2 -5,o 0 2 -5.0 01-50
3274. On 2618.4 2973.2 2912.2 2833.1 2603.4 2801.1 2925.6 3003.6 2992.5 2821.3 5895.9 3056.3 3198.0 2914.9 3345.0 3044.0
REAGENTS
All mineral acids used in t,he extraction are of microanalytical grade. Other chemicals are reagent grade. All reagents and distilled water were checked for impurities b y spectrographic analysis, and those showing the least impurities were selected. The reference solution contains copper, iron, lead, manganese, nickel, sodium, vanadium, and zinc in concentrations of 0.100 mg. per ml. It is prepared from stock solutions of the metal chlorides or nitrates, standardized by any convenient procedure. A solution containing 0.02 mg. of cobalt per ml. supplies the internal-standard element. It is prepared b y diluting the correct amount of 6% Hexogen cobalt s-ith a highly refined mineral oil.
Copper Iron Lead lfanganese Nickel Sodium Vanadium
PROCEDURE
A representative 50-gram portion of the oil, warmed if necessary, is weighed into the extraction flask. Fifty milliliters of nheptane and 1.00 ml. of the internal-standard solution are added, and stirring is begun. Five milliliters of concentrated sulfuric acid is then added, and the mixture is refluxed 1 hour at, about 100" C., as governed by n-heptane reflux. The oil layer is decanted, and the acid layer is transferred t o a 250-ml. T'ycor Erlenmeyer flask. T h e oil layer is returned t o the extraction flask and refluxed 1 hour a t 100" C. with 50 ml. of a mixture containing 2 parts of concentrated hydrochloric acid, 1 part of acet,one, a d 1-part of 'distilled xater. T h e oil-acid mixture is transferred to a separatory funnel and shaken thoroughly. After the aqueous layer settles, i t is removed and added t o the Erlenmeyer flask. T h e remaining oil is m-ashed n-ith 20 ml. of a 50 to 50 mixture of acetone and distilled r a t e r , and the washings are added t,o the Erlenmeyer flask. Any asphaltic residues remaining in the extraction flask are removed b y several rinses with 10-ml. portions of acetone and added to the acid extracts. T o the combined extracts is added 2 ml. of concentrated sulfuric acid. T h e mixture is heated progressively until all the acetone, water, hydrochloric acid, and sulfuric acid have been removed. Care should be exercised to avoid superheating. T h e residue is ignited a t 550" C. until all carbonaceous material has been removed. The remaining inorganic material is taken up in a minimum amount of hydrochloric acid and concentrated to about . flat chamfered 1/4-inch graphite electrode is sealed by 0.2 ml. % allowing it t o soak u p one drop of mineral oil under an infrared lamp. The hydrochloric acid concentrate is evaporated in increments on the electrode under the infrared lamp. T h e mineral oil is removed b y heating the electrodes in ashing coils ( 2 ) . T h e deposits are subjected to spectrographic excitation using a directcurrent arc. T o allov correction for metal impurities in the reagents and by carrying out the extraction distilled witer, a blank is prepared . . procedure without oil. To prepare calibration standards, 1 ml. of the cobalt solution is oxidized with 1 ml. of concentrated sulfuric acid, ignited t o remove the organic residue, and taken up in 1 ml. of concentrated hydrochloric acid. The proper amount of the reference solution is added t o give a series of standards representing 0.02 t o 5.0 p.p,m. of the various elements in terms of a 50-gram sample of oil, These solutions are treated in the same manner as the hydrochloric acid solutions of the ashed residues. hnalvtical working curves are prepared by plotting the parts per million of the elements represented bv each standard against the corresponding ratio of the intensity of the spectral lines listed in Table I1 t o the intensity of the cobalt internal-standard line. If a 50-gram sample of oil is being analyzed, concentration in parts per million can be read directly from these curves. DISCUSSIOY
Completeness of extraction was demonstrated b y carrying out semiquantitative analyses ( 1 1 ) of both t h e extract and the extracted oil for several different oils. Photometering of suitable spectral lines for the metals present shon-ed t h a t more than 05% of the copper, iron, lead, manganese, nickel, and vanadium were extracted from the oil. Sodium and zinc viere not present in t h e oils in significant amounts; hoxvever, these two elements are easily removed from petroleum fractions. T h e precision of the procedure was established b v triplicate analyses of two different oils. T h e results are shown in Table 111. When applied t o the determination of copper, iron. lead, nickel, and vanadium, the procedure s h o w R coefficient of variation of 11%.
a
Zinc Cobalt b Sectored line.
A.
P.P.hI.
Element
,... b
Internal-standard line.
Table 111. Triplicate i n a l l ses of T w o Petroleum Fractions (Parts per million) Cu Fe Mid-continentgas oil
Av. Boscan gas oil
Ax. a
0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0
22 18 15 18
68 78 85 77
51 35 44
43
03 92 95 97
Pb 73 68 66 69 48
0 0 0 0 0 0 0 0
44
52 48
V
Si
0 0 0 0
61 72 6-1 66
1 75 1 23 1 65 1 .54
"
0 0 0 0
a a
21 18 18 19
Too low for accurate analysis.
Table IV.
,inalysis of an Enriched Gas Oil
Element
Added
Copper
2 45
Iron
2.50
Lead
2.50
T'anadium
2.50
Parts per hfillion Found
2 20,2.05 2.26,1.85 2 . 3 4 , 2 74 2 19,2.74 2 91,2.64 2.66,2.01 2 43,2.63 2.68,2.58
Accuracy of the procedure was estimated by analyzing a n oil containing known added amounts of four elements. A mid-continent gas oil was enriched with mineral-oil solutions of copper, iron, lead, and vanadium naphthenates t h a t added 2.5 p.p.m. of each element. Both t h e original and the enriched gas oil v e r e analyzed by t h e extraction procedure, and the difference between the two values was calculated. Results are shown in Table IT'. On the average, a single analysis is \Tithin 1275 of t h e true value. T h e sensitivity of the procedure is governed by the ratio of the concentration of a given element in t h e oil t o that in t h e blank. Hence, t h e sensitivity for any element m a y he increased by analyzing a larger sample. Fifty grams was chosen as a compromise between ease of handling and sensitivity. K i t h this sample size, the sensitivity for eight metals t h a t often must be determined in gas oils is: Element Limit, p.p.m.
Cu 0 02
Fe 0 05
Pb 0 02
Mn 0 01
pi1
0 01
Na 1 0
V 0 01
Zn 0 1
Other metals may be determined if they are included in t h e standards. Silicon and magnesium cannot be determined because they always are found in large amounts in the blank.
concxmox The extraction procedure is particularly vel1 suited to t h e analysis of petroleum fractions containing less than 0.5 p.p.m. of metals hhout 6 man-hours and '72 hcurs of elapsed time are required per analysis. l l t h o u g h the procedure is long, i t is shorter than other procedures giving comparable results. Sensitive methods other than spectrographic may also be used for the analysis of the acid extracts.
