Determination of Aromatics in Light Petroleum Distillates By Use of

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Determination of Aromatics in Light Petroleum Distillates By Use of Specific Dispersions between Sodium H. M. THORNE, WALTER MURPHY,

AND

D

Line and Mercury g Line

JOHN S. BALL

Petroleum and Oil-Shale Experiment Station, Bureau of Mines, Laramie, w y o .

The aromatic content of a hydrocarbon mixture in the gasoline boiling range containing no glefins or diolefins may be determined from the refractive indexes for the mercury g and the sodium D lines, and from the density, all determined at POa C. The specific dispersion of the mixture is calculated by the equation:

The per cent aromatics in the mixture may then be obtained from the eauation: .

W =

- 192.4 sa - 122.4 x

S.

100

+c

where W is the weight per cent of aromatics in the sample, S. is the specific dispersion of the sample, So is the specific dispersion of the aromatic being determined, 122.4 is the average specific dispersion of paraffins and naphthenes, and C is the correction obtained from Figure 3. The values of specific dispersion for the several aromatics are: benzene, 248.4) toluene, 241.4) ethylbenzene, 228.1 o-xylene, 234.8) m-xylene, 237.1 / p-xylene, 238.2) and isopropylbenzene, 21 5.8.

The per cent of aromatics in a sample may then be determined by the following equation: (3) where W S, S,, S, C

The correction, C, is necessary because the relationship between specific dispersion and concentration is not exactly linear. Equation 3 may be applied to mixtures containing more than one aromatic by the use of an average value for their specific dispersions, but, unless the values for the aromatics are virtually the same, an error will be introduced. The presence of unsaturated hydrocarbons (olefins and diolefins) makes necessary a modification of the method in which diolefins are removed and a correction for olefins is calculated from the bromine number as suggested by Grosse ( 6 ) . Although oxygen, nitrogen, sulfur, and halogen compounds would interfere, they are usually present in too small amounts in petroleum products to affect the results.

A

METHOD for the determination of aromatics in hydrocarbon mixtures of the gasoline boiling range which has been widely used is that of Grosse and Wackher (6). This method is based upon the determination of the specific dispersion between the F and C lines of hydrogen. The recent availability of sodium and mercury light sources of increased convenience over the hydrogen lignt sources has made possible a modification of the method which is susceptible of greater accuracy than the original method. Refractometers of either an Abbe type with compensating prisms, or a Pulfrich type, were used in Grosse and Wackher's method but, since its publication, precision refractometers of an improved Abbe type using monochromatic light have come into general use. These refractometers, which are supplied with mercury and sodium light sources, are capable of accuracies approaching those of the Pulfrich instrument and are available from domestic manufacturers. DETERMINATION OF AROMATIC HYDROCARBONS

The specific dispersion of a substance is defined as the difference between the refractive indexes for two specified wave lengths divided by the density, each of the properties being determined a t the same temperature. A factor of lo4 is usually included, so that the specific dispersion results in a convenient number. This definition may be expressed in mathematical terms as:

where S is specific dispersion, no and nb are refractive indexes measured for wave lengths a and b, and d is the density. A temperature of 20" C. is usually selected for the measurements. The method of G r o w and Wackher is based u on the principle that the specific dispersion of mixtures of naphtfenes and paraffins using the hydrogen F (4861 A.) and C (6563A.) lines is essentially constant while aromatics have substantially higher specific dispersions. For these lines, the specific dispersion may be calculated: n:o np S = x 10' (2) dO :

weight per cent aromatics in sample specific dispersion of sample = specific dispersion of naphthenes and paraffins = specific dispersion of aromatic being determined = correction = =

ADAPTATION TO 8 AND D LINES

A few authors (3, 16) have referred to the use of the sodium D (5893 A.) and the mercury g (4358A.) lines in analyzing hydrocarbon mixtures by dispersion methods, but none of the data presented appeared to be sufficiently accurate for adaptation of Grosse's method. The values necessary for application of Equation 3 to g-D data are: (1) the g-D specific dispersion of naphthenes and paraffins over the boiling range desired, (2) the g-D specific dispersions for the various aromatics that may be present in this boiling range, and (3) the evaluation of corrections necessary because the data .deviate slightly from the basic equation. The effect of olefins or diolefins was not considered because most of the bureau's work has been done on straight-run gasolines or condensates, which are usually free from olefinic compounds. The best method for obtaining specific dispersion values for the various groups of hydrocarbons would be from measurements on highly purified compounds. However, this meth6d was impractical for the naphthenes and paraffins. Therefore, it was necessary to make measurements on mixtures derived from petroleum distillates in which no aromatic was present. The values thus obtained were checked by measurements on two purified compounds. The specific dispersions for the aromatics were obtained from measurements on purified compounds. As a considerable amount of F-C data is available, and it has been stated (17) that these may be converted to other wave lengths by means of the Cauchy equation, such conversions were made as a check upon the results. The results of these conversions will be discussed later. SPECIFIC DISPERSIONS OF PETROLEUM FRACTIONS

Compilations of data such as those of Ward and Kurtz (17) indicate that the F-C specific dispersions of paraffins and naphthenes are approximately constant over the gasoline boiling range. The assumption waa made that the g-D specific dispersion would also be a constant, and experiments were made t o determine its value.

-

481

INDUSTRIAL A N D ENGINEERING CHEMISTRY

482 Table

I. Specific Dispersion Valuer of Fractions Boiling Outside the Aromatic Rangea for 48 Naphthas Boiling Range,

F. Preceding benzene Following benzene Following toluene

Table

100-140 185-203 243-258

Specific Dispersion Limits Average 120.0-125.1 122.8 119.6-123.8 122.3 121 .0-125.6 123.3

II. Specific Dispersion Values of Dearometized Mixtures of Paraffins and Naphthener Fractionated from Naphthas

Boiling Range, F. 100-140 140-185 185-203 203-243 243-258 Above 258

Hydroformer Charging Stock No. Av. of B ecific frac- &pertions sion

...

i

8 3 2

122:1 122.3 122.4 122.1

Macedonia, Ark., Condensate No. Av. of specific frac- dispertions aion 123.8 122.3 122.6 122.3 122.6 122.7

Lance Creek, WYO., Naphtha No. Av. of specific fracdispertions sion 5 123.3 23 122.4 15 122.4 32 122,5 24 122.7 58 122.3

I n early analytical work by the bureau, the average specific dispersion of a number of fractions, recovered before and after the boiling ranges of the aromatics involved, was used as the specific dispersion of paraffins and naphthenes. The average specific dispersion values for those ranges obtained by analyzing forty-eight naphthas are given in Table I. The wide limits of results indicated either that relatively large differences existed among naphthas from various sources, or that the determination of specific dispersion of p a r f i n s and naphthenes under these conditions was less accurate than had been thought. The latter assumption is believed to be correct for two reasons: (1)it is exceedingly difficult to be certain by refractometric methods that aromatics are absent from a given fraction even though their boiling points are remote from that of the fraction; (2)the precision refractometer in routine service does not give results of high accuracy unless very close control of conditions and use is maintained. The accuracy is discussed below. Furthermore, even if sufficient accuracy in the measurement could be attained, the values which should be used are those for the aromatic range rather than those for fractions just outside that range. For these reasons specific dispersions were determined for the paraffin-naphthene mixtures remaining .after dearomatization of the naphthas. Three types of naphthas were used, the first two of which were a hydroformer charging stock of high naphthepe content, and a condensate from the Macedonia, Ark., field which had a low n a p h t h e n e c o n t e n t . These naphthas were separated into 5% fractions by distillation and the aromatics were removed from each fraction by filtration through silica gel (9). The specific dispersions of the dearomatized fractions were then determined. The third naphtha was from Lance Creek, Wyo., crude oil and also had a low naphthene content. This naphtha was dearomatized by treatment with silica then separated into 0.5$$?f?~~~ tions in a 95-plate fractionating column. Subsequent analysis of the dearomatized naphtha by an ultraviolet s p e c t r o p h o t o me t e r showed less than 0.02% aromatics. The data for these three naphthas are plotted in Figure 1, and some averages are presented in Table 11.

Table

111.

Vol. 17, No. 8

Referring to Table I1 and Figure 1, it will be noted that the regions of largest deviation from constancy are those outside the aromatic boiling ranges, which are 140" to 185' F. for benzene and 203" to 243" F. for toluene. The first fraction which boils below benzene has a comparatively high specific dispersion. This may be the result of inaccuracies in determination, as the results on low-boiling fractions are often erratic, but data on pure hydrocarbons (17) in this region suggest that slightly higher dispersions may be expected. The material recovered between the toluene and ethylbenzene boiling ranges (243"to 258' F.)also has a high specific dispersion, probably caused by the presence of trans-l,land trans-l,4dimethylcyclohexaneswhich have high values. The best value for the specific dispersion for those ranges where aromatics actually are present is 122.4 and the small deviations between ranges suggests that this value is substantially constant. Extensive data on pure compounds for the F-C specific dispersion (17) indicate considerable variations, but large differences between fractions are not obtained because: (1)the fractionation employed usually yields mixtures rather than pure compounds, and (2) the compounds with values that deviate most widely from the average either are absent, or are present only in small amounts. Within the limits of accuracy of determination of the specific dispersions, these facts will correlate the results obtained here with the variations in F-C data. SPECIFIC DISPERSIONS O F PURE C O M P O U N D S

Table 111 gives the g-D specific dispersions of n-heptane and methylcyclohexane as determined upon specially purified products. Several physical properties of these materials are included to indicate their purity by comparison with literature values. Previous data for g-D specific dispersions of paraffins and naphthenes are also given. The previous data for specific dispersions are not in agreement among themselves and are therefore unsuitable for the present purpose. The values for the two compounds determined in this laboratory check those obtained for petroleum fractions. In the study of avaiation gasolines, the aromatic hydrocarbons boiling below n-propylbenzene have been of greatest interest. As the published data for g-D dispersion on these compounds were inadequate, the seven hydrocarbons in the group were purified and the properties determined. A comparison of the properties of the purified compounds with published values in Table IV indicates that each of the compounds, with the exception of oxylene, is of high purity. A freezing point determination of impurity in the benzene made by Huffman and Knowlton ( 7 )

Comparison of Properties of Purified Compounds with Literature Values

Compound

Boiling Point, 760 Mm.

' C. n-Heptane

Methyloyclohexane

213.8

100.80

100.8 Literature data on other Fraffins and naphthenes n- entane n-Hexane n-Decane 2,7-Dimethyloctane Cyclohexane

.... ....

68.9

0.68380 0,68375 0.68378

1.38758 1.38774 1.38775

,39596 1.38926

122.6

0,76940 0.7692 0.76944 0 ,7707

1.42297 1.4230 1.42310 1.4243

,43239 1,42486

124.1 122.4

~

... ... ...

159.9 81.0

ny

Refractive Indexesn l O l l n',"

e

Disper- Refersion, g-D ence

F.

98.5 209.3 98.4 98.40 101.0 100.3

Density, dZo

...

...

..... .....

.4343

1.4256

.....

.....

0.6602

1.3762

1.3848

1.3772

0.7240

1.4092

1.4185

1.4104

0.7781

1.4263

.....

1.4362

.....

1.4276

1.4387 1.4765

1 ,4489 1.4876

1.4400 1.4779

.....

Dimethylcyclohexane .... 1-Methyl-4-isopropylcy168.4 clohexane ... 0.7950 192.4 Decahydronaphthalene . . . 0.8895 a Refractive index for mercury e line at 5461 A.

,....

.....

..... .....

.....

130 125.6 125 124.8 130 125.6 128 122 127

i25.5 128 125

(4)

(do)

(3)

(4)

(20) (16)

(3)

ANALYTICAL EDITION

August, 1945

em

(2:

483

125

124

W

% 0 0

$

I23 122A I22 121

Id

R VI

120

150

100

200

250

MID-BOILING POINT Figure 1.

- "E

Specific Dispersion (g-D) of Naphtha Fractions

of the principles of azeotropic distillation and its applications has been given in Bureau of Standards publications ( 1 , 10, 13, Boiling Point, Density Refractive, n:o Indexes n p o ~ (isper- Refer16, 18, 81) and some examples of Compound 760 Mm. di0 ny sion p D ence azeotrppes used in various cases a C. F. are given. Among the azeoBen~ene 80.2 176.4 0.87900 1.50108 1.52291 1.50518 248.4 tropes suggested were ethanol 80.102 0.87896 1.50123 (6) for benzene, methyl cyanide for 80.26 0.8807 1.5014 1.5237 1.5049 253 (16) 248.6 (3) toluene, and acetic acid for the group of aromatics boiling within Toluene 110.7 231.3 0.86688 1.49682 1.51775 1.50077 241.4 110.88 0.86697 1.49685 (6) the range 130" to 175' C. These 110.8 0.8677 1.4966 1.5180 1.5000 247 (16) were used for the purification 238 (3) work described in this section, alEthylbenzene 136.1 277.0 0.86713 1.49577 1.51555 1.49953 228.1 though a later communication 136.15 0.86690 1.49587 (6) 135.8 0.8682 1.4955 1.5155 1.4987 230 (16) from Rossini (14) suggests more c o n v e n i e n t azeotropic a g e n t s 1.49575 1.51626 1.49959 238.2 pXylene 138.4 281.1 0.86104 would be methanol for benzene, 138.40 0.86100 1.49615 (6) ethanol for toluene and Cellom-X y 1ene 139.2 282.6 1.49712 1.51761 1.50094 237.1 0.86410 solve or methyl C'ellosolve for 139.30 0.86410 1.49741 (6) 139.8 0.8661 1.4975 1.5184 1.5009 241 (16) the eight- and nine-carbon-atom u-Xylene 144.3 291.7 0.8791 1.50449 1.52513 1.50839 234.8 aromatics. 144.05 0.88011 1.50547 (6) Chemical Reactions. The treatIsopropylbenzene 152.4 306.3 0.88186 1.49121 '1.50981 1.49471 215.8 ment of aromatics with sulfuric 152.53 0.8620 1 ,4922 (6) acid is one method commonly 1.3,5-Trimethylbenused for purification. I n the zene 164.0 0.8828 1.4962 1.5163 1.4995 233 (16) treatment of benzene and toluene 217 (3) the purpose is to remove the Refractive index for mercury a line at 5461 A. more reactive compounds such as unsaturates and sulfur-containing compounds (thiophene), and the reaction with the aromatics is negligible. However, with m-xylene the purpose is to form ma t the Bartlesville, Okla., Bureau of Mines laboratory indicates xylenesulfonic acid and then t o hydrolyze that material. The the liquid soluble, solid insoluble impurity t o be 0.01 * 0.003 separation of the xylenes and ethylbenzene is discussed in a nummole yo. The data for o-xylene which w&s not highly purified are ber of publications (8, 11, 18, 19). Sulfonation appears.to be the included, not only because they provide the only value for g-D best means of separation. When the sulfonation IS carried out at low temperatures (0' C.) p - lene is not sulfonated. The sulfonic specific dispersion available, but also because the error in this acid of m-xylene is h y d r o i y z s a t temperatures of 130" to 135" C., property is probably very small, &s the most likely impurities while those of ethylbenzene and o-xylene require higher temperaare the other xylenes which have very similar sptcific dispersions. tures (155' to 160' C.), I t haa also been stated that m-xylene is METHODSOF PURIFICATION. The methods used for purificamuch easier to sulfonate than other C8-aromatics. tion may be grouped under four headings: adsorption, crystallization, distillation, and chemical reactions. The use of these methods is summarized in Table V Table

IV.

Comparison

of Properties of Purified Aromatic Compounds with Literature Values

...

ii

Adsorption. The removal of aromatic or unsaturated im urities from saturated compounds is readily accomplished by Htering the material through silica gel (9). Crystallization. Although crystallization is one of the best methods for purification, it waa used in only two instances (benzene and p-xylene), because the other compounds require very low temperatures for crystallization for which suitable equipment was not available. Distillation. Two types of distillation were used in these puritications: distillation a t atmos heric pressure, and azeotropic distillation. Columns having higi efiiciencies were used for both. In general, the distillation at atmospheric pressure served only for rough separation, and was followed by some further purification. Azeotropic distillation ,of close-boiling fractions extends their boiling range allowing separation by types of compounds with paraffins, naphthenes, and aromatics distilling in that order. In purifying aromatics by azeotropic distillation it was necessary only t o distill the azeotropic mixture until all possible impurities (other than aromatics) were removed, A discussion

METHODS OF DETERMINATION OF PROPERTIES. B d i n Points. The boiling points of the compounds were determinecf a t 760 mm. of mercury pressure in a Cottrell-type boiling point apparatus equipped with a pressure-regulating device with which the pressure can be regulated to * 1 mm. of mercury. Temperatures were measured by means of Anschutz thermometers (graduated to 0.2" C.), and corrections as determined by the Bureau of Standards were applied. Refractive Indexes. The refractive indexes of the compounds were determined with a Bausch & Lomb precision refractometer, using as light sources a sodium arc for the D line a t 5893 A. and a mercury arc with a blue filter for the g line a t 4358 A., and with a green filter for the e line a t 5461 A. The temperature variations are thought to be =+=0.03" C. and the temperature a t which the determinations were made is probably 19.96" C. A temperature correction has been applied to all refractive indexes to cor'rect them to 20" C. The averages of the individual daterrninations are believed to be accurate to *0.00003.

INDUSTRIAL AND ENGINEERING CHEMISTRY

484 Table

V.

Compound

Purification of Compounds Properties after Treatment di0 ;'n

Purification Method

n-Heptane

None Filtration through silica gel Fractionation in 60-plate column None Filtration through silica gel Fractionation in 60-plate column None Treatment with sulfuric acid Fractionation in 60-plate column Areotropic distillation with ethanol Crystallization

Methylcyclohexane Benzene

0.6835 0.6836 0.6838 0.7689 0.7687 0.7694 0.8767 0.8770 0,8775

1.38767 1.38767 1,38758 1.42308 1.42311 1.42297 1.49996 1.50008 1,50054

0.8782 0.8790

1.50093 1.50108

Vol. 17, No. 8

Densities. The densities of the compounds were determined with pycnometers of the type described by Robertson ( I d ) . These pycnometers were of slightly more than 23-ml. ca acity, and were calibrated using freshly boiled distilled water. $wo to six determinations were made on each compound and the maximum deviation from the average was O.oooO5. The bath showed a variation of *0.03" C. from the average temperature of 19.96' C., and volume readings on the pycnometer were taken over this temperature range to obtain maximum and minimum values. The average of these readings was taken as the volume corresponding to the average of the temp.erature extremes. The densities were checked using other types of pycnometers. The densities a t the averaged temperature were corrected to 20.00" C. CALCULATION OF g-D SPECIFIC DISPERSIONS

Many different equations have been suggested for the calculation of refractive indexes with respect to other wave lengths, having given the values for refractive indexes a t two or three wave lengths. Ethylbenzene

None Fractionation. in 60-plate column AzeotroDic distillation with acetic acid Fractionation in 60-plate column None Treatment with sulfuric acid Azeotropic distillation with acetic acid and redistillation

o-Xylene

m-Xylene

None Sulfpnation, hydrolysis, and redistillation None Fractionation in 60-plate column Crystallization None Fractionation in 60-plate column Azeotropic distillation with acetic acid Fractionation in 60-plate column

pXylene Isopropylbenzene

I

I.

e

I.

I

I

I

0.8662 1.49570 0.8671 1.49531

O:&il 0.8779

....

1:49577 1.50372 1,60417

o.8791

1,60449

0.8634 0.8641 0.8599 0.8610 0,8610 0.8569 0.8616

1.49674 1.49712 1.49545 1.49569 1.495% 1.48842 1.49057

0:8&9

1:49i2l

I

Perhaps the earliest equation is that of Cauchy:

B

n = A + s + $

C

(4)

where n is the refractive index for light of wave length A, and A , B, and C are constants for each compound. This equation is often simplified by omitting the third term:

n = A + -

B

(5)

A2

Sellmeier's equation may be written:

.*

A

= 1

A2 '.O

I.

I TOLUENE

.

a W n

I

1.2

I

I

.

I

@I

I

I

I

I

I

/.I,

I

I

I

I

I

0 - X Y L ENE

1

I

€THY L BENZENE

1 1 a

: a

.4

a .2 0 V

;1

.e

I

210

I

310 410

I

@I

510

610

I

1

.

710

810

910

I

I

I

.8

ISOPROPYLBENZENE

I

Figure 2.

30

I

I.

4

I

. I

I

I

I I P-XYLENE

8 -.2@ I O

PERCENT (BY WE1G H T ) ARO M A T I C S

20

20

30

40

50

80

70

80

PERCENT (BY WEIGHT) A R O M A T I C S

Determination of Correction C u n w for Aromatic Contents by Specific Dispersion

90

ANALYTICAL EDITION

August, 1945

where A and XO are constant for each compound. A still more elaborate equation is that of Hartmann:

(7) where TL,, c, Xo, and a are constants for each compound. A simplified form of this equation is: ?

where no, c, and

l

=

n

f

i

+

(8)

L

X-xo

are constants.

while Table VI11 gives converjions of Bureau of Mines data to

F-C specific dispersions. It will be noted that the two sets of data approach the same basis as the more complicated equations are used. Of the five equations, the best agreement is reached by the use of the simplified Hartmann equation in converting from g-D to F-C specific dispersion. There were not sufficient data available to use the more complicated form of this equation. Although the calculations did not show agreement within experimental error, the fault seems to lie with the equations, for the differences between the experimental results obtained for the g-D specific dispersions and the literature values of F-C specific dispersions show consistent changes between hydrocarbons.

Equation 5 is the most conPARAFFINS AND NAPHTHENES. venient equation to use, as it becomes a straight-line function when used t o convert one dispersion to another-for example: no -

?ZD

= 124.89

(np

- nc)

( 9)

The results of the use of several of the equations on various sets of data are given in Table VI. From these values, Equation 9 appears t o give results which correlate best with the value of 122.4 which was the average value found for paraffin and naphthene mixtures from petroleum fractions. I t will be notcd that this value, 122.4, converts to 98.0 for the F-C specific dispersion, This average value agrees with the figures suggested for the accurate analysis of gasoline by Grosse and Wackher.

Table

VI.

Compound n-Heptane Methylcyrlohexane

Calculation of Specific Dispersions of Paraffins and Naphthenes Calculated g-D Specific Diapersion Data Equa- Equa- Equa- Equa- Equa- ExperirnenSource tion 9 tion 5 tion 4 tion 6 tion 8 tal Value ($0)

(6)

($1

123.1 122.6 122.2 121,7

123.7 120.8 123.3 122.7 122.9 120.3 122.6 118.6

125.8 124.5 124.9 123.6

122.6 124.3 122.2 120.3

122.6

.... .. , ,

122.4

Calculated F-C Specific Dispersion .-Heptane

This

Xlethylcyclohexane

This

work

98.1

97.5

98.3

95.9

98.7

98.1(6)

work

98.0

97.5

99.2

96.2

98.6

97.5(6)

A n equation similar to 9 was tested on the data of Wibaut et al. (20) by converting F-C dispersions to G'-D dispersions. The G' line is the hydrogen line a t 4340 A. The average deviation in calculating G'-D dispersion of twenty-four paraffin hydrocarbons was 0.4 dispersion unit and for six naphthene hydrocarbons it was 0.5 dispersion unit. As the GI-D dispersion had a spread very close to the g-D dispersion, it was assumed that about the same errors which are uTithin the experimental variation would hold for the latter. The conversion from g-D to F-C specific dispersion is more accurate than the reverse calculation, inasmuch sts it involves interpolation rather than extrapolation. The more complicated equations give variable results, and show no advantages over the Cauchy equations for calculations involving naphthenes and paraffins. No results are given for the Hartmann equation, because the difficulty of solution makes it impractical for use. AROMATICS. The calculation of g-D specific dispersions from F-C values is less satisfactory and more difficult for the aromatic hydrocarbons than for the paraffin and naphthene compounds. The simplified Cauchy equation which is satisfactory for the latter compounds does not give satisfactory results in the case of the aromatics and i t is necessary to resort to the more complicated equations. Tables VI1 and VI11 show the results obtained by the use of the various equations in converting specific dispersions of aromatics from one set of wave lengths to another. Table VI1 shows Grosse and Wackher's data converted to g-D specific dispersions

485

CORRECTIONS FOR DETERMINATION OF AROMATICS

As stated previously, the per cent aromatics (by weight) does not follow exactly a straight-line relationship with specific dispersion. This deviation is corrected for by a term, C, in the equation: (10)

This correction, C, was evaluated by preparing solutions containing known amounts of an aromatic compound in an aromaticfree base. The aromatic-free base was in most cases the appropriate boiling range fraction from the dearomatized Lance Creek naphtha described above. The results of specific dispersion determinations on the blends are shown in Figure 2. There are considerable variations in the points shown because the magnitude of the correction is only slightly greater than the accuracy of tne determination. However, sufficient points were obtained for most aromatics to determine the general shape of the curve and the magnitude of the correction.

As the correction curves are to equal zero a t 0 and 100% aromatics, an equation of the following form is indicated:

c = k(W - 0.01W2) where C is the correction, W is the apparent weight per cent aromatics, and k is a constant. Evaluation of k by the method of least squares gives the following values for each aromatic: benzene, 0.0833; toluene, 0.0404; ethylbenzene, 0.0391 ; . o-xylene, 0.0489; m-xylene, 0.0448; p-xylene, 0.0266; and is0 ropylbenzene, 0.0274. Similar constants from Grosse's data (67 usin the F and C lines are: benzene, 0.0744; toluene, 0.0504; a n i xylenes, 0.0188. The curves as developed in Figure 2, are combined in Figure 3 to simplify their use. Based on the determina-

Table

VII. Calculation of g-D Specific Dir errionr of Aromatic

Compound Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Isopropylbenzene

Hydrocarbons from Grosse Lata Calculated g-D Specific Dispersion B~~~~~of Equa- Equa- Equa- Equa- EquaMined tion 9 tion 5 tion 4 tion 6 tion 8 Value 237.5 230.9 218.2 224.4 226.4 227.3

...

238.9 232.2 219.6 225.8 227.8 228.6

...

254.8 237.3 234.8 235.6 246.4 247.2

...

246.0 236.1 225.4 231.9 234.1 234.9

...

260.3 241.3 240.6 240.2 252.7 253.6

...

248.4 241.4 228.1 234.8 237.1 238.2 215.8

Calculation of F-C Specific Dispersions of Aromatic Hydrocarbons from Bureau of Mines Data Calculated F-C Specific Dispersion ~ i Equa- Equa- Equa- Equa- Equature Compound tion 9 tion 5 tion 4 tion 6 tion 8 Value (6) Benzene 198.9 197.7 188.6 188.3 189.3 190.2 193.3 192.2 184.3 Toluene 186.9 184.6 184,9 Ethylbenzene 182.6 181.5 175.1 176.0 175.5 174.7 188.0 186.9 179.2 o-Xylene 181.9 179.8 179.7 189.8 188.8 178.9 m-Xylene 183.5 179.5 181.3 190.7 189.5 180.5 p-Xylene 184.4 181.1 182,O 172.8 171.8 Ieopropylbenzene 164.5 167.5 164.6 Table

VIII.

~

~

~

INDUSTRIAL A N D ENGINEERING CHEMISTRY

486

tions shown in Figure 2, the probable error of the correction is *0.2% aromatics. ACCURACY OF DETERMINATION OF AROMATICS

The fundamental factor involved in determining the accuracy of aromatic determination by this method is the accuracy with which the refractive index may be determined. With the precision refractometer (Bausch & Lomb), the procedure for reading involves estimation of half units on a vernier or one twentieth of a scale division. This is equivalent over most of the range to 0.00003 refractive index unit. However, in actual practice, attainment of this accuracy is difficult and requires great care. Experience has indicated that the precision is generally *0.00003.

Vol. 17, No. 8

the probable error of the aromatic content becomes 0.7under the following assumptions: Probable error of S, Probable error of S. Probable error in value 122.4 Probable error in C Fraction term of equation Denominator of fraction

= = = = = =

0.6 0.6 0.3 0.2 0.50 (50% aromatics) 119.0 (for toluene)

The value 122.4 * 0.3 is given the lower probable error by analysis of the results presented in Figure 1. The sum of the squares of the deviations for 160 fractions waa 30.39. The probable error then was

0.6745

4%

= 0.28

This value is the probable error of one determination rather than of the mean, inasmuch as the value, 122.4,is robably constant only within the limits of the experimental &termination. Thus evaluation of the various factors involved in the determination 6y the specific dispersion method of a single aromatic in a single naphtha fraction shows the probable error t o be from 0.6 to 1.0,depending upon the quantity of aromatics, and upon the aromatic compound. When the content of an aromatic in a gasoline or naphtha is determined by analyzing the boiling-range fraction containing that aromatic which is usually 20% or less of the original naphtha, the reported result on the naphtha will have a probable error of not more than =+=0.2%, Experimental results of analyses of a number of synthetic naphthas (dearomatized Lance Creek naphtha plus known amounts of purified aromatics), all differed less than *0.2% from the true aromatic content. LITERATURE CITED

Bruun, J. H., and Hicks-Brunn, M. M., J . Research Natl. Bur. Standards, 5, 933 (1930). Daniels, F., Mathews, J. H., and Williams, J. W., “Experimental Physical Chemistry”, 3rd ed., pp. 449-60, New York, McGraw-Hill Book CO., 1941. Dixmier, G., Chimie et Industrie, Special No., 3 8 - 4 0 (Sept PERCENT

Figure

[BY WEIGHT)

1926).

AROMATICS

Egloff, G., “Physical Constants of Hydrocarbons”, Vols. I and 11, New York, Reinhold Publishing Corp., 1939, 1940. Egloff, G., and Grosse, A. V., U. 0. P. Booklet 217, Chicago, Universal Oil Products Go.. 1938. Grosse, A. V., and Wackher, R. C., IND.ENG.CHEW,ANAL.ED..

3. Correction of Aromatic Contents as Determined by Specific Dispersion Method

The attainment of this precision requiree close attention to di. rectiona provided with the instrument. In particular, close temperature control is necessary, as the refractive indexes of hydrocarbons in the gasoline range change about 0.00005 for each 0.1”C. change in temperature. I t has also been found necessary in attaining temperature equilibrium to allow an appreciable time after introduction of a sample before taking a reading. As pointed out in the instruction manual, it is necessary to have the space between the prisms full of liquid before taking a reading. With low-boiling fractions, sufficient evaporation may occur from the prisms to give a false result. Care should be taken to check the refractometer with accurately calibrated liquids of similar characteristics, or with the test piece if these are not available, before and after each series of observations, as variations may occur for no perceptible reason.

If the probable error of the refractive index determination is taken as O.oooO3,and the error in determination of density is assumed negligible, the probable error of the specific dispersion, when the density is 0.7500,is found to be

.\i

+

= o.6

0.00003* 0.000038 0.7500

by the usual methods for determination of propagation of error (8). Applying the same methods to Equation 4, a 122.4 c, W =S Sa- 122.4

-

+

11, 614-24 (1939).

Huffman, H. M., and Knowlton, J. W., unpublished investigstion. Kizhner, N., and Yerulelshtein, G., Rum. Phya. Chem. SOC.. Chem. Part, 57, 1-12 (1928). Mair, B. J., and Forsiati, A. F., J . Research Natl. Bur. Standards, 32, 165-83 (1944). Mair, B. J., Glasgow, A . R., and Rossini, F. D., Ibid., 21, 167-84 (1938).

Nakatsuchi, A,, J . Soc. C h a . I d . , Japan, 32, Suppl. binding, 333-6 (1929); 33, Suppl. binding, 85-6B (1930). Robertson, G. R., IND.ENG.CHEM.,ANAL.E D . , 11, 464 (1939). Rose, F. W., and White, J. D.. J . Research Natl. Bur. Standards, 21. ~.167-84 (1938). ~.

Rossini, F. D., private communication. Rossini, F. D., Mair, B . J., and Glasgow, A. R., Oil Gas J . , 39, NO.2 7 , 1 5 8 - 9 , 2 1 9 (1940). (16) Ward, A. L., and Fulweiler, W. H., IND. E N G . CBEM.,ANAL. ED., 6, 398-400 (1934).

Rard, A. L., and Kurtz, S. S., Ibid., 10, 559-76 (1938). White, J. D., and Rose, F. W., J . Resear& N&. Bur. Standards, 10, 6 3 9 4 5 (1933); 9, 711-20 (1932); 21, 151-65 (1938). (19) Whitmore, F. C., “Organic Chemistry”, p, 711, New York, D. Van Nostrand Co., 1937. (20) Wibaut, J. P., et aZ., Rec. trap. chim, 58, 328-77 (1939). (21) Wojciechowski, M . , J , Rpaeurch Nail. Bur. Standards. 19,

(17) (18)

347-52 (1937). PRESEXTFTED before she Division of Petroleum Chemistry at the 108th MeetCHEMICAL SOCIETY, New York, N. Y. Published by ing of the AMERICAN permission of the Director. U.S. Bureau of Mines, Washington, D. C .