Determination of Total Per Cent Aromatics in Heavy Petroleum DistMates Modified Method Using Silica Gel Adsorption A. T. WATSOK, Humble Oil and Re5ning Co., Baytown, Tex.
The work was undertaken to develop a method for the determination of total aromatics in heavy petroleum distillates, which would be relatively rapid and suitable for routine application. The analysis utilizes the strong affinity of silica gel for aromatics, and is considered accurate to within z!=l.O%. While the procedure is based upon the same principles as those used by Liplcin et al., it has been simplified by eliminating the necessity for refractive index measurements during the determination and by reducing the sample size requirements, resulting in a signi6cant reduction in time requirement, and making it possible for one operator to carry out several analyses simultaneously. The method is particularly useful for studying the effects of process variables in such operations as solvent extractions of gas oils and lubricating oils.
A
METHOD for determining the total aromatic content of heavy petroleum distillates has been developed, which is relatively rapid and suitable for routine application. This method is based upon the principles described by Lipkin et al. (1) and is very similar to their procedure. However, certain modifications make it much more applicable to routine determinations and enable one individual to complete several determinations a day. Among the more important simplifications have been the elimination of refractive index measurements, reduction of sample requirement (from 100 ml. to 5 nil.) with a resulting greater ease of handling and simultaneous reduction of solvent requirement (about 200 ml. of n-heptane as compared to the 2 liters of pentane used by Lipkin), elimination of necessity for the use of other solvents, and simplification of the solvent-removal step by the substitution of a vacuum oven solvent stripping operation for the less convenient distillation operation described in Lipkin's procedure. Mills ( 3 ) has described a similar procedure and incorporated many of the above changes. Mills employed 10 ml. of sample and indicated that refractive index should be measured during the determination, in order to establish the break between the saturate and aromatic fractions. Such control by means of optical instruments has not been found necessary in the present study of petroleum distillates, which included stocks varying in aromatic content from about 25 to 97%. For an aromatic determination by the procedure described below, a column approximately l inch in inside diameter containing 200 cc. of 28- to 200-mesh silica gel is used. The gel is prewet with n-heptane and approximately 5 grams of sample are weighed accurately, dissolved in 15 ml. of n-heptane, and added to the column. The gel is then washed with n-heptane until the filtrate issuing from the bottom of the column is pure n-heptane. At this point, based on ultraviolet measurements, only nonaromatics have been stripped from the gel. The filtrate is caught in a tared container and then placed in a vacuum oven in which the n-heptane is stripped from the nonaromatics. From the weight of the nonaromatics remaining in the container the weight per cent of saturates can be calculated. The remainder of the material, which includes nonhydrocarbons and n-heptane-insolubles, is assumed to be aromatics.
This method, like the one described by Lipkin et al., clmsifies all compounds that contain a single aromatic ring as aromatics. This includes aromatics with long paraffin side chains, aromatics with naphthene rings, and all aromatic olefins. It is believed to be applicable to the determination of total aromatic contents of all virgin or cracked distillate stocks which have initial boiling points above 500" F.; it is not considered applicable in simple form to residua, asphalt, or bright stocks. The method has a man-hour requirement of less than 1 hour, an elapsed time requirement of about 5 hours, and an accuracy within about A0.5 to &l%. 0
IMATERIALS AND EQUIPMENT
The gel column consists of a glass tube approximately 1 inch in inside diameter and 3 fect long, which is drawn to a capillary of approximately 1 mm. a t the lower end. Silica gel (2% to 200mesh) supplied by the Davison Chemical Corp. is used. Highpurity n-heptane is employed for washing the nonaromatics from the silica gel. A vacuum oven for use in stripping solvent from the desorbed nonaromatics is highly desirable; however, the stripping operation can be carried out by distillation if necessary. Anticreep beakers are used to hold the silica gel filtrate for the stripping operation. An analytical balance is necessary for weighing the sample and desorbed nonaromatics. PROCEDURE
1. Approximately 5 grams of sample are weighed accurately and diluted with 15 ml. of n-heptane. 2. A small amount of glass R'OO~ is tamped into the bottom of the column, 200 cc. of fresh 28- to 200-mesh silica gel are placed in the column, and the column is tapped in order t o pack the gel tightly. Twenty milliliters of n-heptane are used to prewet the gel. 3. The sample-n-heptane blend is introduced into the column after the prewet n-heptane has just disappeared into the gel. After all the blend has entered the gel the beaker is rinsed with approximately 10 ml. of n-heptane. This n-heptane is poured into the column. 4. When the n-heptane used for rinsing the beaker has entered the gel, 180 ml. of n-heptane are added to the column. 5. The effluent is caught in an accurately weighed anticreep beaker until the effluent ceases to flow from the bottom of the column. 6. The anticreep beaker containing the effluent is then placed in a vacuum oven held a t 125' F. and 10-mm. pressure and kept there for 4 hours. This length of time has been found adequate to assure evaporation to a constant weight. 7 . After the anticreep beaker has cooled, it is weighed. From the weight of the saturates remaining in the beaker the weight per cent of nonaromatics is calculated. The weight per cent aromatics is determined by subtracting this value from 100. If volume per cent is desired, the specific gravities of the original sample and of the nonaromatic fractions are determined and volume per cents are calculated utilizing these values. ACCURACY AND REPRODUCIBILITY
In Table I are given the results of three determinations of total aromatics on a sample of heavy West Texas virgin gas oil. The volume of n-heptane used was varied in order to illustrate that the results are not affected seriously by the quantity of n-heptane wash. In Table I1 are given the results of two determinations on catalytic cycle stock. The results given in these two tables indicate that the method is reproducible to better than =t0.5% based on total sample. I t is also considered accurate for total saturates
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ANALYTICAL CHEMISTRY
Table I. Rim So. 1 2
Aromatic Determinations on West Texas Heaby Virgin Gas Oil n-C7Used, Weight yo Calcd. Weight 7' Aromatics
3
M1. 175 200 180
Nonaromatlcs 51.9 51.7 51.8
and Nonhydrocarbons 48.1 48.3 48.2
Table 11. Aromatic Determinations on 650" to 950" F. Cut of Catal) tic Cycle Oil Run No.
?
Weight % Nonaromatics 59.4 59.7
Calcd. Weight % Aromatics and Nonhydrocarbons 40.6 40.3
solvent extraction of heavy stocks. Table V gives results of the measurement of total aromatics in the raffinate and extract from a series of phenol extraction studies on a light motor oil, in which one of the conditions was systematically varied. Table V illustrates not only the utility of the procedure for studying the effect of such variables in extraction studies but also its accuracy, In the five sets of samples t h e sum of the aromatics in the extract and raffinate varied only from 37.4 to 38.8%. As the nonaromatics in the extracts and raffinates were similar, the sum of the aromatics shown in Table V closely approximates the weight per cent of aromatics in the charge. RECOVERY OF AROMATIC FRACTION
Table 111. Ultraviolet Absorption of Materials Measured as Saturates in Aromatic Determinations Ultraviolet Coefficients, Liter/G. Cm. 260 mp 235 mp 0.018 0.055 0.002 0.006
Source of Saturates Process gas oil Catalytic cyoie oil
t o within &0.5%. Evidence that the materials measured as
saturates contain almost no aromatics is shown by the results of ultraviolet absorptioo,measurements on the saturates obtained from two samples, one of which was a virgin gas oil and one a cracked stock, using the silica gel procedure described (Table 111). As the Rrocedure is so designed that all materials insoluble in nheptane and all nonhydrocarbons are calculated as aromatics, the accuracy of the method for the determination of aromatics per s e is dependent upon the concentration of these nonhydrocarbon materials in the sample, but it is believed to be generally within ~ t 0 . to 5 *l.O%. These percentages refer to the nonhydrocarbon elements present; with stocks of high molecular weight the mole fraction of such compounds may be considerably higher. A more detailed discussion of the behavior of these compound types on percolation has been reported by Wanless, Eby, and Rehner (3), who used a silica gel chromatographic method which employed a series of solvents, rather than a single one, in an effort to differentiate among nonaromatics, aromatics, and nonhydrocarbon ring compounds. That the quantity of silica gel used is adequate for samples containing up to 100% of aromatics is evidenced by the fact that similar results have been obtained for ultraviolet coefficientson the nonaromatics obtained from solvent extract samples of catalytic cycle stocks which contained as much as 97 weight % aromatics. Table 1V. Estimated Time Requirement for Modified Silica Gel Determination of Per Cent Aromatics and Nonhydrocarbons in Heavy Petroleum Distillates Working Time, Minutes
Step Clean, weigh beaker and sample, and prepare blend Prepare (pack and prewet) column n-Heptane wash Tare beaker for filtrate Strip ing and cooling Weigiing stripped sample Total working time Total elapsed time
Elapsed Time Hours Minutes
I n the procedure described for the rapid determination of total aromatics no means has been provided for recovering the aromatic fraction, which is often wanted for utilization in further tests, such as spectroscopic study or measurement of other physical or chemical properties. In order to recover the aromatic fractions after completion of the n-heptane wash, approximately 50 ml. of benzene are first added to the column. After the benzene has entered the gel, 200 ml. of absolute ethyl alcohol are added to the column. The beaker in which the nonaromatics were caught is removed for stripping, as described in step 6 of the procedure, and all of the effluent issuing from the column is retained in a second tared antiCree beaker. When the effluent ceases to flow from the bottom of &e column following the addition of the ethyl alcohol, this anticreep beaker is placed in a vacuum oven a t 125' F. and 10-mm. pressure for 4 hours. After the anticreep beaker has cooled, the weight of the aromatics is determined. Recovery should be a t least 98%. The aromatics thus obtained are considered suitable for study of the physical properties of the aromatics that were present in the original oil. Table V. -4romatic Determinations on Products from Phenol Extraction Studies on Light Motor Oil R u n No. Raffinate Yield, vpl. % Aromatics. weight ?4 Extract Yield, vol. % Aromatics,. weight % Aromatics in r a 5 n a t e plus extract ~
~~~
1
2
3
4
5
67 22.9
70 22.2
71 20.9
73 20.7
77 24.0
33 30 29 27 23 7 1 . 1 77.2 80.0 8 2 . 4 87.9 3 8 . 8 38.7 3 8 . 0 37.4 3 8 . 7
~~
~~~
~~
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Table VI. Application of Modified Silica Gel Method to Aromatic Determinations on Residuum Sample Determination 1 2 3 4 Weight of sample, grams 6 . 1 0 1 5 4 . 9 1 8 4 2.2657 1.8481 Approximate el/oil ratio 40/1 40/1 90/1 110/1 Sampleinraf&ate, weight % 41.8 46.6 41.2 36.6 Ultraviolet absorption coefficients of raffinates, liters/g. om. 2.43 0.970 0.492 2.02 260 mp 0.68 0.65 0.059 0.018 360 mp
9
9
LIMITATIONS OF METHOD
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ti
The silica gel procedure has been found suitable for application to all heavy distillate streams tested thus far. However, none of these streams has included materials boiling much above 1000° F. That the method as described was not considered suitable for virgin or cracked petroleum residua is illustrated in Table VI, where the aromatics in a sample of dewaxed, deasphalted residuum were determined. Samples of different sizes were used as the charge to the silica gel and the results obtained varied widely. As can be seen from the ultraviolet coefficients, the materials measured as nonaromatics by the silica gel were not simple saturates. The use of smaller samples in the charge to the silica gel gives somewhat more reproducible results for residua, but even this improvement will not yield a nonaromat,ic fraction which is transparent to ultraviolet wave lengths in regions where it should be transparent if it were totally free of aromatic rings. Mills ( 2 ) has indicated that the use of clay in addition to silica gel
10
i
10 6 47
4
6
5
0
..
30 6
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TIME REQUIREMENTS
The total time required for a single sample analysis is about 6 hours; however, actual working time for a single sample is only about 45 minutes and one operator can analyze several samples at a time. A breakdown of the approximate time requirements is given in Table IV. TYPICAL APPLICATION
The method is particularly useful for following the effects of varying the conditions in research work on such operations as
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V O L U M E 24, NO, 3, M A R C H 1 9 5 2 in the adsorption will overcome the difficulties encountered in analyzing residua. However, no investigation of the use of clay was undertaken in the present work. CONCLUSION
The strong affinity of silica gel for aromatic compounds has been utilized to obtain an accurate, relatively rapid method of analysis for total aromatics in heavy petroleum distillates. Though the method is similar to that described some time ago by Lipkin et al., certain modifications make it suitable for routine determinations in laboratories and relatively rapid when several determinations are made simultaneously by the same operator.
ACKNOWLEDGMENT
Permission of the management of the Humble Oil and Refining Co. to release the material presented herein is gratefully acknowledged. An expression of thanks is also due V. H. Rushing, whose assist,ance throughout the work was invaluable. LITERATURE CITED
(1)
Lipkin, M. R., Hoffeoker, W. A., Martin, C. C., and Ledley, R. E.,
ANAL. CHEM.,20, 130 (1948). (2) Mills, I. W., Proc. Am. Petroleum Inst., 29M (III), 50 (1949). (3) Wanless. G. G... Ebv. . L. T.. and Rehner. John, Jr., . ~ N A L .CHEM.,
23, 563 (1951). RECEIVED for review April 12, 1951. Accepted January 14, 1952.
Rapid Potentiometric Determination of Chloride at low Concentrations tl
t n Solutions of High lonic Strength W. J. BLAEDEL, W. B. LEWIS', AND J. W. THOMAS2 University of Wisconsin, Madison, Wis. st stand rd methods of chloride determination possess severe shortcomings when applied to the rapid and routine determination of low concentrations of chloride in solutions of high ionic strength. A potentiometric method is described which allows detection of a limiting chloride concentration of 2 X lo-' M (0.7 microgram per 10 ml.) in solutions containing about 1 M sulfuric acid and/or sodium sulfate. The relative error is about 270 at chloride concentrations from 3 X 10-4 to 0.05 M . The method
T
HERE exist many methods for the analysis of chloride, but only a few are applicable to the rapid, routine determination of low chloride concentrations in solutions of high ionic strength. Such determinations are involved in the analysis of chemicals for traces of chloride, of air for chlorine-containing gases after absorption and hydrolysis in absorbing solutions, and of organic compounds after Eschka or Parr bomb fusion. Several analytical methods were investigated with a view toward &ding one that was capable of detecting a limiting concentration of chloride of loa M or less in solutions containing acids and/or salts a t moderately high concentrations-Le., equivalent to about 1 M sulfuric acid or sodium sulfate. As the objective was to apply the procedure to routine determinations on large numbers of samples, speed and simplicity were important requirements. For concentrations of chloride well above the limiting concentration, a relative error of 1 to 3% waR considered tolerable for most of the kinds of analyses mentioned above. A great deal of preliminary experimental work indicated that most accepted methods of chloride analysis did not satisfy these requisites. The high ionic strength rendered turbidimetric and nephelometric methods very unreliable. Direct titration with standard silver nitrate, using adsorption indicators like dichlorofluorescein or diphenylamine blue, was unsuccessful because of the high ionic strength and- low chloride concentration. The Mohr method, using chromate as an indicator, was not sensitive enough. Indirect argentimetric methods, such as those of Volhard, McPresent address, Los Alamos Soientific Laboratory, Los Alamos, N. M. 'Present address, Bureau of Dairy Industry, U. S. Department of Agriculture, Beltsville, Md.
involves measuring the voltage difference between two silver-silver chloride electrodes in a concentration cell, one arm of which contains the unknown solution and the other a standard solution. The chloride content of the unknown is found by the aid of a nomogram. Application to the determination of phosgene or cyanogen chloride in air after absorption in alcoholic sodium hydroxide is described. A pair of workers may prepare and analyze over 200 samples per day.
Lean and Van Slyke, and Liebig, were satisfactory from the standpoint of sensitivity and accuracy; but the manipulation and care required made them poor for routine use. The precision of such indirect methods, Khen used routinely, falls much farther below the nonroutine precision by which the method is gaged than does the precision of direct methods. The mercurimetric method, involving direct titration of the chloride with standard mercuric nitrate, was not sensitive enough and required large and variable corrections for low chloride concentrations. Potentiometric titration methods, both direct and differential, possessed sufficient sensitivity and precision, but required too much time. The common potentiometric methods consist of measuring the voltage of a concentration cell, one half of which is an appropriate electrode immersed in the unknorn solution, and the other half is a standard half-cell. Most of these involve troublesome and time-consuming corrections for liquid junction potentials and ionic strength effects. Furman and Low (8), however, have devised a simple differential method which eliminates error due to these two effects. The method was conceived primarily for measuring low chloride concentrations in solutions of high ionic strengths, and may be easily adapted to routine analysis. THEORY O F DIFFERENTIAL POTENTIOMETRIC METHOD
Briefly, the method consists of measuring the voltage of a silver-silver chloride electrode immersed in the unknown solution against an identical electrode immersed in a standard 0.00100 M sodium chloride solution. Both solutions must have practically
