Petroleum Control Testing by X-Ray Absorption RALPH C. VOLLMAR, EARL E. PETTERSON,
AND
PAUL A. PETRUZZELLI
Standard Oil Company of California, Richmond, Calif. X-ray absorption techniques have been successfull? applied to quality control of petroleum products. Procedures are described for rapid, direct, and nondestructive analytical determinations of sulfur, tetraeth>llead, and metallic additives. Jleasurements were made on an x-ra? photometer using petroleum derivathes containing small amounts of elements substantially higher in atomic number than carbon. .ipplication to petroleum products includes the useful range, method of calibrations, and standardization of test conditions. Accuracy is compared with gravimetric or other established procedures. Limitations, including nonspecifirit?, are covered.
T
HE economic production of petroleum products has resulted
in a great variety of testing to ensure uniformity of product as well as to meet complex specifications. Much of the testing is tedious and complex, and unfortunat,ely does not always give a direct answer. It would be desirable to replace any chemical test with a simple instrument measurement, which might be performed by technicians with a minimum of training. Aborn and Brown ( 1 ) first demonstrated the practical importance of the absorption characteristics of tetraethyllead exposed t o x-ray radiation. Their work made use of the greater absorption of increasing masses of material a t a constant lmve length. They pointed out that accurate control of operating conditions was extremely important. Since then improvements in electrical and electronic controls have resulted in greater utilization of x-rays as an analytical tool. I t was found that testing could be greatly siniplified and standardized by application of photometric methods (3, 4 ) . Liebhafsky, Smith, Tanis, and Winslow ( 7 ) developed a technique for measuring the x-ray absorption photometrically. ii combination of phosphor and multiplier phototube made possiblc a reasonably precise measurement of x-ray absorption. By recognizing and controlling all the variables in the system it was possible to show that even small amounts of heavier elements were detectable in mixtures. As in any photometric method, ronstancy of absorpt,ion for a given mass of material depends on the nonvarying frequency and intensity of the beam of radiations. Michel and Rich (8) have reported on a highly useful instrument for measuring relative x-ray absorption. The authors have applied this instrument to a wide variety of control testing in a routine petroleum inspection laboratory where a rapid and reasonably accurate evaluation of a blending or additive compound is desired. The procedure involves a differentiation between two samples, which means a comparison between reference and blended stock. The only other requirement is that the additive have one or more elements of relatively higher atomic nunibrr than the hnse stock and t,hat measui,able amounts he involved. X-RAY PHOTOVETER
All measurements were made on a Model 632830Gl x-rav photometer manufactured by the General Electric Company. It is drsigned to measure the relative x-ray absorption of two saniples in terms of the amount of aluminum necessary to add to one path of a dividrd beam to compcnsate for a higher absorption in the sample in the other path. The instrument operates on a null method in which the unknown is balanced 1Tith a known, bv introducing a variable amount of aluminum in the form of a tapered disk controlled by a calibrated dial. The aluminum needed for balance, compared to that required foi knoivn blends as determined by a previous calibration, gives a measure of the desired component in the unknown. The x-ray photometer, shown in Figure 1 in its simplest form, includes a tungsten-target x-ray head vith suitable water-cooling facilities and control devices. An applied voltage of about 40 kv.
produces a spcbctruni of x-rays with effective wave length of 0.4 A. nnitted as pulses from the x-ray tube on the positive peak of the BO-cycle alternating current power input. h two-bladed synchronous motor-driven chopper is placed so that it alternately interrupts and divides the beams of x-rays, one half passing through the sample, S , and one half through the standard, X . The x-rays which pass through the cell impinge alternately on the left and right half of the fluorescent screen where the x-radiation is converted to light. The light from either half passes through a light tunnel, or collector, to a multiplier-type phototube. The electrical output of the phototube then passes through an amplifier to a phase detector. The phase detector is also fed by the output of a small 30-cycle generator which is carried on the same ohaft of the synchronous motor driving the two-bladcd chopper. The inputs from the phototube and generator are converted by the phase detector into a direct current whose magnitude and direction depend upon the relative intensities of x-rays transmitted through the sample and the standard. This direct current is read on a balance indicator consisting of a zero-centered microammetcr. PHOTO Tb9E
LIGHT COLLECTOR
SCREEN
CALIBRATED ATTENUPTOR MOTOR UNIT
Figure 1.
Block Diagram of X-Ray Photometer
A zero reading on t.his balance indicator nieans that the intensities of the alternating pulses of x-rays as received on the fluorescent screen are of equal intensit,ies. .I dial on the front panel controls the tapered aluminum disk in the beam which passes through the standard in such a fashion as to insert,a known absorption. By changing the angular position of this disk, the difference in x-ra absorption between t,he standard and the sample may be bafanced, thus obtaining a direct indication of the difference in absorption of the sample and of the standard. The thickness of aluminum which must he interposed on the standard side to obtain a balance is then a measurc or' the increased absorpt,ion due to t,he additivo. By rcfcrring the dial reading to :tn appropriate calibration, the concentration of the heavier material in thr sample may be determined. USEFUL RAIYGE
The absorption of the x-rays by various materials in terms of the aluminum disk, was measured to give a comparison of the 1491
1492
ANALYTICAL CHEMISTRY Table I.
X-Ray Photometer Sensitivities Sensitivity
Element Ka hf g A1 Si P
S
c1 C8
Mn Fe
co Ni cu
Zn Mo
2 Pb
Atomic NO. 11 12 13 14 15 16 17 20 25 26 27 28 29 30 42 47 56 82
%/dial division 0.27 0.17 0.12 0.087 0.065 0.051 0.040 0,023 0.014 0.012 0.010
0.0097 0,0089 0,0083
0,0057 0.0044 0.0039 0.0033
Dial division/% 3.7 5.8 8.2 11.5 15.3 19.7 25.0 44.1 70.9 81.3 97.1 103.1 112.4 120.5 175.4 227.3 256.4 303.0
is L5eighed into the right-hand cell compartment, and the same weight of referencestock (standard) into the left. The cellis placed in optical alignment and the x-ray tube is energized. The disk is rotated to vary the thickness of aluminum in the standard side. When the balance indicator reads zero, the reading on the calibrated dial is recorded. This reading is the mils of aluminum equivalent in x-ray absorption under the experimental conditions to all the constituents present in the unknown but not in the standard. Other things equal, the contribution to this equivalent thickness of each such constituent is proportional to its mass. If the difference in absorption between standard and unknoa n exceeds the capacity of the calibrated disk, aluminum blocks of known thickness are interposed on the standard side. The instrument is calibrated for a particular element with samples containing known amounts of the desired constituent. The procedure refers to the use of a base stock which is the most reliable procedure. However, refinery practices may not always permit this because of the frequent doubt or nonavailability of the exact base stock. Refined petroleum stocks contain essentially only carbon and hydrogen in slightly varying proportions from the lightest to the heaviest fractions, and, in addition, a varying percentage of sulfur. Because the x-ray photometer makes no measurable distinction between the small changes in carbonhydrogen ratio in the range in which the authors worked, it is possible to substitute a highly refined product with a minimum sulfur content for all base stocks concerned. There is, however, an appreciable sensitivity for sulfur in hydrocarbons. SULFUR
9
% Figure 2.
SULFUR (COMBUSTION) Coniparison of Test Results
Sulfur in hydrocarbon mixtures can be measured by x-ray absorption. Kehl and Hart (6) have recently reported on a procedure using an x-ray spectrometer with a Geiger counter. Zemany, Winslow, Poellmitz, and Liebhafsky (11 ) used the x-ray photometer for evaluation of sulfur in crude oils. The increased absorption of the x-rays due to sulfur is noticeable throughout the usual range of petroleum hydrocarbons. A straight-line relationship was found between mils of aluminum (dial reading) and per cent sulfur in hydrocarbon materials. This was determined by making a series of blends and comparing the photometer readings with the known sulfur value for each blend. Comparison of test results is shown in Figure 2. The deviations have been within the accepted A.S.T.M. tolerance for accuracy. The petroleum samples used in the above work were, for the most part, mixtures of treated distillates and naturally occurring sulfur compounds present in or recovered from the refining prncess.
sensitivity of the instrument toward a number of elements. These measurements were made on liquid samples predominantly hydrocarbons but containing a measurable amount of heavier materials. Under these test conditions substantial variations in the ratio of carbon to hydrogen produce little or no change in x-ray absorption. This simplifies the problem of looking for elements of higher atomic number in petroleum-base stocks. The data presented in Table I show sensitivities. “Sensitivity” refers to the per cent by weight of the determined element which on the authors’ instrument gave 1dial division change in absorption when a 150gramsample was used. The repeatability of readings is 0.2 dial division. For sulfur 0.051 weight % in a 150-gram sample gives the same response as 0.0033 weight %of lead, or a differenceof more than tenfold. Reference to this table will be helpful in determining application of this technique to any particular analysis. It also indicates the magnitude of possible error in case of contamination of a sample. PROCEDURE
The primary voltage across the x-ray tube, usually 100 volts, and the voltage determining the level of amplification are kept constant. h sample of unknown
Figure 3.
Sulfur by X-Ray and A.S.T.M. Methods
V O L U M E 21, NO. 1 2 , D E C E M B E R 1 9 4 9
-
120
110100-
1493
CALIBRATION TEL CONTENT
OF GASOLINE5
-
90
40t 30 20
0
MOTOR
/
/
/
/
/
II)
GRADE
GASOLINE
_-__AVIATION G R A D E GASOLINE
/
I
I
I
I
2.0
3.0
40
5.0
CC TELIGAL Figure 4.
Tetraethyllead in Gasoline
IVhen testing fuel oils the authors noticed a slightly higher absorption for a given sulfur content than their basic curve would lead them to expect. -4study of comparative x-ray and analytical determinations showed it was possible to correlate these results. 4 new curve could be established parallel to and above the basic calibration allowing for slightly increased absorption. This shifting was found to compensate for the low ash content normally present in black fuels. This method of adapting - the x-ray photometer will be useful in many instances r h e i e calibration can he made for particular stocks even though such calibrations might not be identical with curves prepared using pure compounds. Figure 3 shows data comparing sulfur by x-ray :iiid by A S T.M. Method D 1 2 9 4 4 ( 2 ) .
bration is made with arepresentative gasoline of knowii density. A correction may be applied t o results by multiplying by the ratio of the density of the sample to that of the calibrating stock. The magnitude of such a correction amounts to 0.02 ml. of tetraethyllead per galloii for 1 O A.P.I. The authors’ solution was to prepare two sets of calibration curves. For motor gasolines normally blended with motor grade tetraethyllead fluid a family of curves is prepared with sulfur content from 0 to 0.30%. A single curve is adequate for aviation gasolines blended with aviation grade fluid, as sulfur content is always low on aviation gasolines (Figure 4). A comparison of x-ray with analytical determinations by A.ST.hl. Method D 526 (a) is shown in Figure 5. COMPOUhDI>G IN OILS
The variety and number of compounding materials I added to petroleum products are legion. Such additives may improve oiliness, detergency, viscosity index, low temperature characteristics, load-carrying capacity, or other qualities. I t is desirable to determine whether the Droner amount of such comDounding - habeen added. .\]though tests such as viscosities or pours will sometimes satisfy this need, in many instances an analytical chemical determination has been felt necessary as a control on quality. Many of these additives contain one or more elements of higher atomic number than carbon and hydrogen. If only one such element is present, as in the case of a metal soap, we may calibrate the photometer directly in terms of this metal. Frequently additives are complex derivatives whose formulation inclurlec:mnre than nne elrment of hiphrr atomic. number.
. .
TETRAETHYLLE-AD IN G.4SOLINE
The determination of tetraethyllead in gasoline was recognized as a possible application of the principle of x-ray absorption technique. As far back as 1929 ( 1 ) it, waR demonstrated that it was possible to measure lead content following this bitsic approach. In 1940 Gross and Staab ( 5 ) using the improved counter tubes of Trost ( I O ) succrssfully mc3:isurerl this additive in various types of fuels. The recent work of %emany,\Vinslo\v,Poellinitz, and Liebhafsky (1 1) showed excellent pre ion in determining tetraethyllead by x-ray absor on when the exact base stock is available for comparison. The increased sensitivit?. of the Gieger counter type of radiation detector for x-ray absorption measurements has hren reported by Sullivan and Friedman (9). The refinery control laboratory performing tetraethyllead analyses is faced with certain particular conditions. For many of the samples, including those from outside sources, no base stock is available. Sulfur content, as mentioned above, will influence x-ray absorption when sulfur percentage is several times the lead concentration. For euample, a. gasoline 0.25% sulfur and 2 ml. of tetraethyllead per gallon actually contains only about 0.05% lead or about one fifth a8 much lead as sulfur. Variations in chlorine and bromine content of the tetraethyllead fluids can introduce a further slight inaccuracy. For purposes of uniformity it is desirable to weigh all samples while t,etraethyllead content is expressed on a volume basis. Cali-
Figure 5.
Tetraethyllead in Gasoline
This is beat illustrated by a fairly simple example involving a gear lubricant additive containing substantial percentages of sulfur and chlorine. Suppose we are blending 10% of this additive with 90% oil. rJrc take a series of readjnas on the base stock and several blends. Results mav be reaortdd in w r active .incent compounding, or in ternis of one or more of gredients. This procedure has been applied to lubricating 011 compounding,lvhere the actual aInount of active metals was only
a fraction of that prrsrnt in the example chosen.
ANALYTICAL CHEMISTRY
1494 METAL ANALYSES
Am. SOC.Testing Materials, ”Stmdards on Petroleum Produrts and Lubricants,” A.S.T.M. Committee D-2. Clark, G . L., “Applied X-Rays,” 3rd ed., New York, McGrawHill Book Co., 1940. Conigton, A. H., and Allison, S. K., “X-Rays in Theory and Experiment,” 2nd ed., New York, D. Van Nostrand Co., 1943. Gross, W., and Staab, F., German Aeronautical Research, U. S. Dept. Commerce, Research R e p t . 1476 (1941). Kehl. W.L., and H a r t , J. C., paper presented at 28th Annual A.P.I. Meeting, Chicago, November 8, 1948. Liebhafsky, H . A., Smith, H. M., Tanis, H . E . , and Winslow, E. R., ANAL.CHEM.,19, 861 (1947). Michel, P. C., and Rich, T. A , , Gen. Eke. Rea., 50, 45 (February 1947). Sullivan, M. V., and Friedman, Herbert, IND.EXG.CHEM., AXAL.ED.,18, 304 (1946). Trost, A , , Phys. J . , 115, 456 (1940). Zemany, P. D., Winslow, E. H., Poellmita, G. S., and Liebhafsky, H. A., ANAL.CHEM.,21,493 (1949).
The manufacture of metal-organic derivatives requires a great deal of inorganic analytical testing to arrive at the desired metal content. All the metals involved are of considerably higher atomic number than carbon and are frequently present in comparatively large amounts. I t is possible to prepare standards of varying metal content and calibrate to a sensitivity as close as the usual routine analytical accuracy. The proper choice of sample weight is an important factor on samples of high metal content, for beyond a certain mass the x-rays are so completely absorbed that insufficient impulse remains to activate the amplifier. 4CKNOWLEDGMENT
The authors wish to express appreciation for the helpful criticisms and suggestions received from John Y. Beach of the California Research Corporation. LITERATURE CITED
(1) Aborn, R. H.. and Brown, R. H., IND. ESG.CHEM.,AN*L. E.D , 1 , 2 6 (1929)
RECEIVEDApril 19, 1949. Presented before the Division of Petroleum CHEMICAL SOCIETI-, San Chemistry at the 115th Meeting of the AMERICAN Francisco, Calif.
Preparation of Sugar liquors and Sirups for Color Determinations P. F. ME.4DS
AND T.
K. GILLETT
California and Hawaiian Sugar Rejining Corporation, Ltd., Crockett, Calif. Definite procedures for clarification and pH and density adjustments, essential for accurate determination of colors of sugar solutions, have been investigated in some detail in connection with a program of replacing visual color methods with photoelectric determinations. The results of this investigation led to the selection of an optimum method of preparing sugar liquors and sirups for routine color determinations. The sugar products to be tested usually require adjustment in density, to permit filtration and to give a color reading that will fall within the effective range of the color instrument. A schedule of standard dilutions has
T
H E accurate measurement of the colora of sugar liquors and sirups is of considerable importance in the sugar industry for controlling operations and maintaining product quality. This measurement depends primarily upon two factors: an accurate and reliable instrument for the determination of color, and a standardized procedure for the preparation of solutions for color determination. The first factor has been investigated in considerable detail in this laboratory and several photoelectric instruments have been developed for mearmring colors of process liquors (8), refined white sugars ( 7 ) , and soft sugars ( 6 ) . The development of these instruments has resulted in the replacement of all the visual color methods formerly used in this laboratory with photoelectric methods which are murh more accurate and reliable. The second problem of preparing solutions properly for color determinations arises principally in connection with the routine analysis of sugar sirups and liquors. The colors of a wide variety of refinery products are measured in the photoelectric colorimeter developed for this purpose (8). These products vary from the lightrcolored washed raw sugar liquor, on the one hand, to the dark-colored affination green sirup on the other hand. There is
been established for determining the color of liquor and sirup samples normally encountered in refinery practice. Based on experimental work, a standard filtration procedure has been established to provide a clear sample and j e t avoid removal of color by the diatomaceous filter medium employed. istandard pH of 7.0 has been selected and all samples are adjusted to this pH value in order to avoid the effect of variations in color due to the original acidit? or alkalinity of the sample. Color readings are determined for solutions prepared in the prescribed manner and calculated to a standard reference basis (100q~solids). This calculation is brieflj discussed.
considerable difference in the densities, pH values, and clarities of the products. Obviously, a standardized method for preparing these materials for color determination is essential if comparable results are to be obtained. The present paper describes the results of an investigation which was carried on to determine the optimum procedure. PREVIOUS WORK IN T H I S FIELD
A considerable body of literature has been developed relative to the proper preparation of sugar solutions for color determinations. The most detailed of these procedures have generally involved rather technical and complex techniques which are difficult to apply t o routine laboratory analytical work. Most previous investigators agree that color determinations on sugar solutions should be made a t the highest possible density, preferably 55” to 65” Brix (3, 9, 14). In diluting sugar solutions below these high densities, colloidal material is apt to be precipitated, which, in some cases, can be removed only with great difficulty. Consequently, the sugar solutions must be maintained a t the higher densities to avoid these problems. How-
