V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1 In order to provide a basis for comparison between the oil content of refinery streams determined by the semimicromethod and those obtained by the evaporation procedure, a large number of samples were run by both methods; some of the results are tabulated in Table IV and plotted in Figure 6. The results by the evaporation method in the region from 100 to 200 p.p.m. are about 60 to 707, of the results by the semimicromethod. The relatively lower results for the evaporation method in the lower range (only 25% of the semimicromethod values in the 10 to 25 p.p.m. range) are to be expected in view of the proportionately higher losses shown for the smaller samples in Figure 1. The reduction to 1 hour in the elapsed time required pel d e t e r m i n a t i o n 4 5 minutes if the extraction and distillation steps are overlapped-was achieved principally by the substitution of the rapid distillation procedure for evaporation from dishes in the removal of solvent from the extract. Additional saving accrued from this substitution because it eliminated the need for dehydration of extract by filtration, as the water is carried over with the distilling solvent. The method is readily adaptable to
1379 multiple simultaneous determinations, in which case an average time per determination of less than a half hour is possible. Recent tests, in which the individual losses from the two distillation steps of the present method were determined separately, showed the semimicrostill (sapable of recovering components boiling above 225" F. This indicates that the present 300" F. limitation is determined by the performance of the preliminary still. A modification of the unpacked column now used which will provide reflux is under way and should extend recovery toward the 225" F. limit set by the present miniature still. Further recovery of light ends will require the use of a lower boiling solvent and this modification of the present procedure will also be investigated. LITERATURE CITED
( 1 ) American Petroleum Institute, "Manual on Disposal of Refinery Wastes," 3rd ed., Section I. Appendix 5, 1941. (2) Kirschman, H. D., and Pomeroy, R., ANAL. C H E i i . , 21, 793 (1949). RECEIVED April 20, 1951.
Determination of Total Alkyl Benzenes in Selected Crude Fractions A n Ultraviolet Method JOHN F. KINDER, Sinclair Research Laboratories, Inc., Harvey, 111.
Solvent properties of petroleum fractions, interest in petrochemicals, and expanded output of heavier fuels have demanded additional information as to the aromaticity of crude fractions boiling above 300' F. A n application of ultraviolet analytical methods for a rapid estimate of total aromatics in petroleum crude cuts which have a boiling range of 200' to 395' F. is described. The basis of the test is the use of an average aromatic absorptivity calculated at a wave length of 215 mp. Typical analyses,
T
HE classification of aromatic compounds contained in petro-
leum crudes is based commonly upon the type of nuclear structure. Within boiling ranges characterized by naphthas, gasoline, and distillate fuels, most identified aromatir compounds consist of alkyl derivatives of benzene, Tetralin, naphthalene, biphenyl, phenanthrene, and/or anthracene. Gasoline is the most important of the crude products. An ultraviolet spectrometric niethod has been devised for the detection and analysis of specific aromatics contained in such petroleum stocks ( 5 ) . Solvent properties of petroleum fractions, interest in petiochemicals, and expanded output of heavier fuels have demanded additional information as to the aromaticity of crude fractions boiling above 300' F. This knowledge, to a certain extent, may be made available by application of the ilSTM method ( 1 ) . Ultraviolet inspection of closely fractionated crude cuts which boil between 200" and 395' F. shows that aromatic content can be rapidly estimated. The results are not separated widely from those obtained by the ASThl method.
and comparisons with the ASTI11 D 875-46T acid absorption procedurc, are giben for fractions from three crudes. The ultraviolet procedure is not designed for crude fractions containing benzene and/or naphthalene; both of these compounds are removed readily by distillation. To date, the analytical data have been obtained from petroleum stocks which have low bromine numbers. For such materials, the ultraviolet method should be useful as an analysis to confirm o r supplement the ASTM test. ment of calibration data, and subsequent analysis of test samples. .4t a scanning speed of 0.4 mg per second, absorption curves were recorded directly in terms of absorbance. Iso-octane (2,2,4trimethylpentane), treated with silica gel for removal of aromatics, was employed as a solvent. Calibration data were obtained from aromatics supplied by the American Petroleum Institute. All had a purity higher than 99%, as tested and certified by the Sational Bureau of Standards. DEFINITIONS AND SYMBOLS
Absorption relationships in the ultraviolet region are defined by the Bouguer-Beer law ( 3 ) . log,,
whrre
A I c
O I = '1
=
alc
sample absorbance path length of light in absorbing medium sample concentration ZO = energy incident on sample Z = energy transmitted by sample a = absorptivity, A/lc JV-ave lengths are expressed in millimicrons. = = =
APPARATUS AND MATERIALS
THEORY AND DEVELOPMENT OF METHOD
The Cary ultraviolet automatic recording spectrophotometer was used for preliminary exploration of crude fractions, develop-
The strongest ultraviolet absorption band for each type of aromatic nuclei occurs at a wave length dependent upon the
ANALYTICAL CHEMISTRY
1380 fundamental ring structure. With respect to the most important classification of aromatics, these wave lengths are: Ring Structure Mononuclear (benzene) Condensed dinuclear (naphthalene) Condensed trinuclear (anthracene)
Band Wave Length, ml.r 183 221 252
Alkylation of the parent molecule produces a shift towards loriger Jmve lengths in the location of this strong band. Alkyl benzenes, for example, show a niasimum between 183 and 210 n i p for derivatives in the 200" to 395" F. boiling range of petroleum ( 4 ) . Within this temperature interval, monoalkyl, di:tlk>~l,trialkyl, and tetraalkyl benzenes are possible components. This primary band, which occurs a t a wave length dependent upori the number of attached alkyl groups, has an absorptivity of api)rosimately the same order of magnitude for many alkyl ben~ ( ~ I I C S .Present-day ultraviolet instruments, however, do not Iwrrnit observation of absorption ut wave lengths much shorter t1un 210 mp. Both the manner in which spectral data are obtxined and the method used to measure band intensities must be eh:inged to Ivork in the 183 to 210 nip rcgion. Such instrumentation is not available commerciall>-.
WAVELENGTH (mmul
Figure 1. A. B.
Ultraviolet Absorption Spectra of Various Alkylbenzenes
Toluene m-Xylene
C. 1,3-Dimethyl-5-ethylbenzene D . 1,2,3,5-Tetramethylbenzene
Further study of the primary absorption band reveals a shoulder on the long wave-length side. This shoulder, located between 200 and 230 mp, is somewhat flat and possesses the same propertj as the primary band; its absorptivity is approximately the same for the various alkyl benzenes. -4pplication of absorption data in this particular wave-length region is the basis of total aromatic analysis. It would be desirable to use an interval over which there is a very small change in absorptivity with a large variation in wave length. With a lower working limit of 210 mp, this requirement is fulfilled only for the tri- and tetrasubstituted benzenes. To select one point a t which measurements are to be obtained, 215 mp is chosen as the optimum wave length which is attainable, and satisfies, in pal t at least, the criterion of location on a flat portion of the curve. The curve shape for mono- and dialkyl derivatives has a pronounced slope a t 215 mp. However, repeated measurements show good reproducibility for the biggest offender, toluene. -4bsorption curves for typical alkyl benzene classes are shown in Figure 1. The use of pure aromatic conlpounds which have boiling points within or near the range of 200' to 395" F. yields absorption spectra which are recorded over an interval of approximately 240 to 213 mp. Individual absorptivities then are calculated at a wave length of 215 mp to give the data in Table I. There are several noticeable differences in absorptivities as listed. Benzene, with no groups on the ring, has a spectrum which is not shifted to longer wave lengths. Measured absorption
-_
Table I.
Measured Absorptivities of Pure Aromatics Boiling Point,
Absorptivity at 215 rnr
176.2 231,l 277.1 282.4 281.0 292.0 306.3 329.3
1.40 43.75 48.29 71.93 69.31 69.03 38.52 6L.50 70.44 44 11 72.24 59.57 62.31 65,69 54.28 71,12 61,18 60.45
F.
Compound Benzene Toluene Ethylbenzene m-Xylene p-Xylene o-Xylene Is0 ropylbenzene 1-&!ethyl-P-ethylbenzene 1,Z,A-Trimethylbenzene Isobutylbenzene 1,2.3-Trimethylbenzene 1-hlethyl-4-isopropylbenzene 1,a-Diethylbenzene 1,3-Dimethy1-5-ethylbenzene 1-hlethyl-4-tert-butylbenzene 1,2,3,5-Tetramethylbenzene 1,2,3,4-Tetrahydronaphthalene Average absorptivity (excluding benzene)
336.8 343.0 348.9
350.8 358. 0 362.8 374.4
388,3 405,3
a t 215 nip will be low for benzene; for this reason, berizeiie is eseluded from the analysis and a temperature range which includes toluene as the lowest boiling aromatic is selected. Monoalkyl derivatives, such as toluene and isopropylbenzene, have a lower than average absorptivity, because absorption is increasing towards shorter wave lengths and a nieasureinent a t 215 mp is of less magnitude than one obtained on the true shoulder. The diand trialkj-1 aromatic absorptivities are calculated from nieasurements made on or very near to the shoulder and are similar in value but of higher magnitude than those of the monoalkyl compounds. To obtain calibration data for application to crude fractions which contain several classes of alkyl benzenes, a number of synthetic mixtures are prepared. These contain aromatic compounds selected from the set in Table I and are formed to be representative of a variety of alkyl benzene types. The composition, boiling range covered by the blend, and computed absorptivities a t 215 mp are contained in Table 11. Wide variations in boiling range and aromatic type are represented by this series of synthetics. Furthermore, the absorptivities are similar and can be used as a guide for the selection of the most serviceable average aromatic absorptivity. Blends 4 and 5, particularly 4, are more typical if compared to probable distributions of aromatics contained within specified petroleum crude boiling ranges. The absorptivity of blend 4 or 5 , then, is considered applicable both to 200' t o 395" F. range stocks and to closely cut fractions boiling between 250" and 395" F.
Table 11. Absorptivities at 215 m p of Several Aromatic Blends Containing Mixed Alkylbenzene Types Alkylbenzene Type, Weight %--
c
Blend So.
Boiling Range,
F.
1 2 3
281-374 337-405 231-292
4
231-405 329-405
5
Monoalkyl
...
...
44
24.5
9.8
Dialkyl 100
Trialkyl
...
..,
alkyl
...
62
20
18.8 32.9
6.2 10.9
...
56 45.2 37.1
Tetra-
...
Tetralin
... ...
18
Absorptivity 63.3 68.0
61.4
5.3
62.0 63.8
9.3
The majority of analyses by this method have been carried out on fractions boiling between 300' and 400" F. For these tests, an average absorptivity of 63.8 was used and applied likewise to 200" to 395" F. full range stocks. iVith this factor, which is the absorptivity of a mixture composed of 100% mixed alkyl benzenes, the total aromatic content of each blend is calculated from such a formula as: Total aromatics (weight %)
=
blend absorptivi2 63.8
x
100
Results for the preceding five synthetic mixtures are tabulated in Table 111.
1381
V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1 The largest deviation ( +6.47,j occurs for the blend which is free OS monoalkyl benzenes. This type of sample would not exist iiniong wide boiling range petroleum cuts. Predominant amounts of trialkyl and/or tetraalkyl benzenes in a close boiling crude cut would cause similar high calculated aromatic content. The aromatics contained in blend 4 represent a boiling range of 231' t0405' F. and include both toluene and ethylbenzene as monoalkyl derivatives. Analysis of this mixture as 97.1 weight Yo aromatic is indicative of the accuracy antiripated for a 200' to 395' F. (.rude fraction.
tion of 0.015 gram per liter used for 100% aromatic stochs, a factor of 10, or a concentration of 0.150 gram per liter, is necessary for 10% aromatic content. rl spectral interval-240 to 212 nip, for example-is scanned and permits visual inspection for the detection of interfering components. The absorbance value a t 215 m p is read and the total alkyl benzene content then is calculated from the following formula: Total alkyl benzene content (weight
70)
whfxre A = sample absorbancc a t 215 nig and c centration, grams per liter.
100 .I 63.8 c
=
~-
=
s~rmplrcon-
Table 111. Calculation of Total Aromatics in Synthetic Blends Blend Yo.
c/a .4romaties (Synthetic)
% Aromatics (Calculated)
100.0 100,o 100.0 100.0 100.0
The Ion absorptivity of benzene a t 215 mp, together. with a realization of the volume per cent of crude boiling below 200" F., invalidates the application of the ultraviolet method t o IHP200" F. crude fractions. Naphthalene and 1-methylnaphthalene have absorptivities of 541 and 301, respectively, a t 215 my. -in upper temperature limit of 395" F., which does not permit inclusion of more than 0.1% naphthalene or 0.2% total naphthalenes, is chosen. These lovi percentages of naphthalenes ill not distort the absorption curve between 240 and 213 my. Objectionable amounts, however, can be detected readily by scanning the sample spectrum between 310 and 330 my, a t considerab1)- increased s:tmple concentrations. APPLICATION TO-ANALYSES OF PETROLEUM CRUDES
By the use of recording ultraviolet instrumentation, total alkyl benzenes are determined rapidly and reproducibly. The procedure requires only that a sample be weighed and made up to an appropriate concentration in iso-octane. With a conceiitra-
'I'able I \ .
Total Aromatics Determined by Lltraiinlet and ASTM Methods Boiling Range, 0 F.
% S
Criide 1
0.29 Crude 2
Crude 3
284-286 3119-299 317-320 330-331 339-340 343-346 362-364 375-375 377--378 384-387 412-412 415-4 16 4 18-4 18 182--398
ASTM D875-46T 20.0 22.0 24.0 25.0 19.0 15 0 17.0 17.7 9.0 2.0 26.0 18.0
15.0 8.0 21.0 16.0 12.0 22.0 13.0 9.0 10.0 13.5
Ultraviolet
17 21 22 21 18 13
3 I
0 0 1
I5 1 18 7
-, -,
0.8 '7.3 10.3 18.3 9.2 22 0 15.7 11 0 23.6 12.i
;.
14 2
"
O
7
1
-
/oi,A\,,I, ,I, j1, 4
210
230
210
230
210
230
,
210
2 30
WAVELENGTH (mmU1
Figure 2. Ultraviolet Absorption Spectra of Representative Petroleum Fractions A. B.
Boiling range, 176-300 338-368
F. C. 390-398 D.
176-398
Many petrolewn,crude cuts have been analyzed by this methoti in the Sinclair Laboratories. Actual tests on fractions from threc, crudes are presented and compared with the ASTM procedure i i i Table IT.', which gives data for a number of fractions having narrow, intermediate, and wide boiling ranges. Typical rccorded spectra for these ranges are reproduced in Figure 2.
S o . 1 is a high-sulfur crude and the aromatics (ASTM) in the first seven fractions were calculated on t,he basis that total acid absorption was due to alkyl benzenes only. No corrections for bromine number were made. The ASTSI analysis for the coniposite full range sample, l i 6 " to 398" F., was corrected for a bromine number of 3.5. Yltraviolet determinations for the sevcu fractions are lower than the corresponding ASTM analyses, uncorrected for bromine number. This is evidence that sulfur compounds and/or unidentified materials present in the crude, but reacting with bromine, do not interfere with the ultraviolet test. Crudes 2 and 3 have negligible bromine numbers and no corrections on any of the ASTM analyses are necessary. Several alkyl benzene contents are given for fractions boiling above 400" F. In each of these tests naphthalenes were absent and the aromatics are identified as total alkyl benzenes and Tetrnlins. DISCUSSION OF R E S U L T S AND hIETHOD
Deviations between the -4STLI and ultraviolet methods for low-sulfur crudes are random. This can be interpreted by a consideration of the ultraviolet calibration dataand an inspection of the absorptivities (Table I ) . Low values for the alkyl benzene content, bj- the ultraviolet method, indicate a concentration of monoalkyl derivatives, whereas high values point to a prepouderance of trialkyl and/or tetraalkyl benzenes. I n investigation of crudes from widely separated fields, n o interference with the test method has been observed. Many classes of compounds, i n addition t o alkyl benzenes, absorb ultraviolet light a t 215 m p . \Tithin a boiling range of 200" to 395' F., these are mainly conjugated diolefins and sulfur compounds. The first does not exist in crude stocks; the second is not likely to be present in an amount sufficient to produce appreciable absorption a t 215 my for the sample concentrations commonly used In the case of a fraction which contains 2070 alkyl benzene, the probable error due to 1% of alkyl sulfides, disulfides, or metcaptans would be less than 0.2% t,otal aromatic value. The same
1382
ANALYTICAL CHEMISTRY
amount of substituted thiophenes would be calculated as approximately 0.6% alkyl benzenes. SIGNIFICANCE O F ULTRAVIOLET METHOD
This analytical procedure was developed from a combination of the need for a test confirming ASTM D 875-16T results and a study of the absorption intensit,y of alkyl benzenes in the short wave-length region. Examination of many spectra was made possible by the use of automatic rec-oi,dinp u1trnviolr.t instrumeritation. The ultraviolet test proves that thc ALYl'.\;Iprocedure, a t least in the absence of high broinine number, is very reliable for straight-run petroleum stocks in the range of 1 to 95% alkylbenzene content. As a supplementary test for aromatics i t has proved its merit in cases of disagreement between the POSA ( 2 ) and ASTM methods. The aromaticity of crude fractions boiling between 250" and 395' F. may be estimated in an average elapsed time of 10 to 20 minutes with an average reproducibility of analysis of i O . 5 weight % alkyl benzenes. Calibration data 6 months old are still in use. The test can be utilized as an aid to existing standard procedures. Its main advantage is the facility of rapid determinatioii of total alkyl benzene content in crude fractions boiling between 300" and 395" F. A need for ultraviolet equipment for use in the region between 183 aud 210 m p is indicated. Absorption data within this interval could be of considerable value in the evnluntion of petroleurn stocks. LIMITATIONS OF THE METHOD
A knowledge of the sample absorption characteristics a t as many wave lengths as possible is an asset to any spectrometric. analysib. For routine analyses this becomes possible only with the aid of automatic recording equipment. Absorbance measurements a t 215 mp should be accompanied by additional measurenirnts a t several other wave lengths to cnsure the absence of inter-
fering components. If done by a manual procedure, the method becomes less effective. Measurements at 215 mp are very close to the lowest working limit of many ultraviolet instruments. This, together with criteria of cell matching and reproducibility of absorbance values obtained from fiteep curves, imposes restrictions in regard to the method of analysis. Reproducibility of analyses and calibration data, together with the fact that unmatched cells can be compensated on the Cary instrument, has justified application of the procedure. The alkyl benzene absorptivities tabulated in Tables I and I1 w e significant for the instrument used for development, of calibration data. In view of the probable variations between instruments, some of the values should be rechecked and a correction i:ictor applied if necessary. .411 data have been developed from straight-run stocks. Preliminary results on cracked products indicate a substantial discrepancy in aromatic content as determined by the ASTM and ultraviolet methods. Further investigations t o determine the applicability of the ultraviolet method to surh petroleum fractions are in progress. ACKNOWLEDGMENT
The author is indebted to many members of the department for their cooperation and assistance in the development of analytical data. Appreciation t o Sinclair Research Laboratories, Inc., is vxpressed for permission to publish this paper. LITERATURE CITED
(1) Am. Soc. Testing Materials, B.S.T.M. Standards on Petroleum Products and Lubricants. pp. 323-30, D 575-461T, Xovember
1950. (2) Grosse, A. V., and Wackher, R. C., IND.EXG.CHEM.,Ah-ar.. ED.,
11, 614~-24(1939). ( 3 ) )fellon, hl. G., "Analytical Absorption Spectroscopy," p. 93, Yew T o r k , John JViley 8: Sons, 1950. (4) Platt, J. R., and Klevens, H. B., Cheni. Revs.. 41, 301 -10 (1947).
(5) Tunnicliff, D. D.. Brattain, R. R., and Zumwalt. L. R., (!HEM.. 21, 590-94 (1949).
.\K.
