Routine Determination of Aromatic Hydrocarbon Types in Catalytically Cracked Gas Oils by Linear Elution Adsorption Chromatography L. R. SNYDER Union Oil Company o f California, Union Research Cenfer, Brea, Calif. ,The five major aromatic hydrocarbon (plus sulfur compound) classes in catalytically cracked distillates that boil over 400" F. can be routinely determined by a combination of linear elution adsorption chromatography and ultraviolet adsorption analysis. Sample monoaromatics, diaromatics, triaromatics, pyrenes, and other tetraaromatics are reported with a relative error of about *lo% in 1 to 2 man-hours per sample. Alkyl 2,3-benzfluorenes have been found as a major constituent of the tetraaromatic hydrocarbon class in catalytically cracked samples.
M
of heavy catalytically cracked distillates (400" to 1000" F.) has been investigated by a number of workers (1-5, 7-9, 20). I n addition to saturated hydrocarbons, nonaromatic olefins, and oxygen- and nitrogen-containing "hetero" compounds, these cracked gas oils have been variously found to contain alkyl benzenes ( 4 , 5, 8, 20), styrenes (9), naphthalenes (3-5, 8, LO), fluorenes ( 2 ) phenanthrenes (2-5, 7 , 8),anthracenes (2, 4 . 5 , Y,8),and pyrenes ( S , 4 , 7 ) . Higher aromatic types are present (3, 4, Y), and certain individual compound classes have been reportede.g., chrysenes, benzanthracenes, triphenylenes, benzfluorenes, etc.-but these latter assignments appear somewhat uncertain due to the complexity of the fractions Most of these studies have used chromatographic separation over alumina and ultraviolet absorption analysis of separated fractions, supplemented or replaced in some cases by mass spectrometric analysis. These investigations, in combination with techniques developed for the analysis of related, straight-run sample types, permit the analysis of a n y cracked gas oil for aromatic hydrocarbon type. With supplemental determination of saturated hydrocarbons @ I ) , nonaromatic olefins ( I S ) , and nitrogen type (19), aromatic-type analysis essentially completes the analysis of the total sample. Two routine procedures (3, 4 ) have 774
OLECULAR COMPOSITION
a
ANALYTICAL CHEMISTRY
been described for the quantitative analysis of aromatic type in cracked gas oils by a combination of elution adsorption chromatography and ultraviolet analysis. Unfortunately, both methods suffer from excessive analysis time and cost, indeterminate accuracy, insufficient method description for straightforward application, and the need for interpretation of final data by the analyst. Most of the limitations of these routine procedures can be traced to the use of nonlinear isotherm separation conditions, as contrasted with separation by linear elution adsorption chromatography (LEAC) (IO-IS,19). This paper describes the development of a completely routine LEXC procedure for the accurate, rapid determination of aromatic type in a n y catalytically cracked gas oil. Additional information on the nature of some of these aromatic types is also presented. EXPERIMENTAL
Nonroutine Large-Scale Separations. Isolation of various compoundtype concentrates from different catalytically cracked samples was carried out in a conventional manner by elution of t h e sample from alumina of varied water content (0 t o 4y0). Detection and collection of individual compound class bands was facilitated by the ultraviolet absorption of the column eluate at key wavelengths. Details of these procedures will be given elsewhere. Routine LEAC Determination of Aromatic Type. REAGENTS.Chromatographically standardized 0.4% HzOA1203 (equivalent linear retention volume equals 23 ml. per gram for elution of naphthalene by pentane) a n d 4.0y0 H20-A1203 (1.4 ml. per gram) a r e prepared as described elsewhere (14). Phillips 99% npentane is purified by passage over activated silica gel to give material with a n absorbance against air of less than 0.10 at 225 m p (I-cm. cells). Reagent grade methylene chloride, CC14, isopropanol, and ethyl ether are used to make up various solutions. PROCEDURE. Approximately 0.5 gram of sample is weighed into a 5-ml. volumetric flask and brought to mark with pentane. If the sample does not dissolve under these conditions, ap-
proximately 2 grams of sample are weighed into a 5-ml. flask and brought to mark with CC14. The sample solution is now separated by one of two LEAC separation procedures, depending upon the sample end point, with the column arrangement of Figure 1 and auxiliary facilities described previously (11). One to eight samples can be analyzed simultaneously. For samples with 95% boiling point under 600" F., separation mode A is used, while for samples with 95% boiling point over 600" F., separation mode B is used. Mode -4 (Samples with 95% boiling point under 600" F.). Fifty microliters of sample solution (in pentane) are charged to the column inlet system (Figure 1) in the usual manner (dry column) (11). Elution is begun with n-pentane, with the center stopcock set so as to pass eluent from the bottom column, I, to the top column, 11. A 35-ml. fraction (A) of pentane eluate is collected a t the top of column I1 in a 50-ml. flask to which I5 ml. of pentane had been added initially. The center stopcock is changed to collect eluate at that point, the pentane eluent is replaced with 4% volume methylene chloride-pentane, and a 50-ml. fraction (B) is collected. The eluent is changed to 507, volume isopropanol-pentane, elution continued, and a 25-ml. fraction collected at the center stopcock and discarded. Eluent is changed to pentane, and another 25-ml. fraction collected at the center stopcock and discarded. The center stopcock setting is changed to permit passage of eluent from column I onto column 11, the eluent is changed t o 10% volume ethyl ether-pentane, and 50 ml. of eluate are collected at the tou of column I1 as fraction D. Mode B (samples with 95% boiling point over 600" F.). Fiftv microliters tin pentane) or 10 pl. (in CClr) of sample solution are charged to the same column as above. EIution is begun with n-pentane and a 50-ml. fraction (A) is collected a t the top of column 11. The center stopcock setting is changed to permit collection a t that point, the eluent changed to 4% volume methylene chloride-pentane, and a 50-ml. fraction (B) collected. Eluent is changed to 12% volume methylene chloride-pentane, and a 50-ml. fraction (C) collected at the center stopcock. Eluent is changed successively to 50y0 volume isopropanol-pentane
+ + 8mm.OD /IZ/S
boiling range, as shown in Table I. W refers to the weight in milligrams of sample charged to the LEAC separation, and A 1 5 , A?30,etc., refer to the various absorbance measurements on the LEAC fractions.
CLASS JOINTS
LEAC SEPARATION OF CRACKED G A S OILS OVER ALUMINA
The LEAC separability of Petroleum aromatic hydrocarbons over alumina has been discussed in detail in the initial
0.4 %
Hz0 -AI203 COLUUN II
1 I
I I85mm.
2-mm 3 - w N STOPCOCMS
I
Table 1.
Ultraviolet Coefficients for Various Sample Boiling Point Ranges"
cm FBT IBT 450 475 500 525 550 575 600 625 650 675 700 750 800 900
500 525 550
575 600
625 650
675 700
750 800
900
1000
110 113
115 117
139 i46 154 166 178 188 199 211 223 234 246 271 299
148
125 129 131 136 137 142 148
121 126 132 138 145 151 157 163 169
127 131
118 121 126
117 122 127 132 138 143 149
450 475 500 525 550 575 600 625 650 675 700 750 800 900
24.9 20.8 18.6 21.5 18.5 17.0 16.5 15.6 14.8
400 500 550 600 650 700 800 900
15.2
107
iio iii iii
iio i23
I19
i24
130 136 142 137 152 157
123 i29 135 141 148 154 161 167 174 180
i33 i%
140 147 155 162 170 177 185 192 200
145 153 162 170 179 188 197 206 215 234
is7
167 181 195 208 222 237 253 268 284 317 354 444
ELUENT MANIFOLD
Figure 1. Apparatcls used in LEAC separation of aromatic types
and pentane, and 25-ml. fractions of each eluent are collected a t the center and discarded. The center stopcock setting is changed to permit elution from column I onto 11, eluent changed to 10% volume ethyl ether-pentane, and a 50-ml. fraction ( D ) collected at the top of column 11. Ultraviolet Analysis. T h e absorptivities in 1-cm. celh of each of the previous fractions are obtained a t the following wavelengths: fraction A, 215 mp; fraction B, 248 and 255 mp (if pyrenes are to be determined separately, 334, 336, 33(3, and 340 mp); fraction C, 265 nip; and fraction D, 230 mk. These absorbance readings are norIhe solvent commally measured posing the fraction. For maximum accuracy, a blank run should be made and each fraction measured us. the corresponding blank fraction. This precaution was used throughout the present studies, but does not appear necessary as long as the eluting solvent and reference solvent are from the same batch. The various aromatic types are determined a s follows: wt. 70monoaromatics = C, A215/W corrected for any ccl4 charged A215 (CCl,) = O.O28/fil. wt.% diaromatics = C d .42,0/W ~ t . triaromatics 7~ plus 3yrenes = tis.
C ! P(A248
+
A255)/W
wt.yc tetraaromatics = CtA266/W (wt.% pyrenes (if deterinined) = Cp (A334 A336 113,s As40 Ztp(Az55)/W The various constants-C,, Cd, C,, C f , Cp, and Ifp-vary with sample
+
+
+
C d
17.5 16.3 15.2 14.7 14.6
17.2 16.2 15.3 15.0 15.3 15.7
16.2 15.0 14.3 14.1 14.3 15.4 16.5
16.5 15.3 14.7 14.7 15.1 16.4 17.7 19.3
16.7 16.9 15.6 15.9 1 5 . 1 15.4 1 5 . 1 15.4 15.6 16.1 17.0 17.6 18.5 19.1 20.1 20.8 21.8 22.6 23.8
17.2 17.5 16.2 16.5 15.8 16.1 15.9 1 6 . 3 16.6 1 7 . 1 1 8 . 3 18.9 20.0 20.8 22.0 23.0 23.9 25.2 25.5 27.0 27.0 28.8 31.6
13.3 13.3 13.3 13.2 13.0
13.4 13.4 13.4 13.3 13.2 13.4
18.0 17.0 16.7 17.0 17.9 20.0 22.1 24.7 27.3 29.6 31.9 35.1 38.0 ~
18.4 17.5 17.2 17.6 18.7 20.9 23.2 26.2 29.2 31.8 34.5 38.4 41.7 47.0
Ca 15.2 15.2
44.5 14.5 14.4
13.8 13.8 13.8 13.6
13.6 13.9 13.6 13.9 13.6 13.9 13.6 13.9 13.5 13.8 13.8 14.2 15.9 18.0 22.0
Ct FBT
700
750
800
900
1000
IBT 400 500 600 700 800 900
51.7 51.7 51.7
41.0 41 .O 41.0 40 7
35.8 35 8 35.8 35.5
31.1 31.1 31.1 31.1 38.8
30.0 30.0 30.0 30.0 28.8 28.8
ZlP
400 500 600 700 750 800 900 400 500 600 700 750 800 900 a IBP, FBP = 5,
0.05 .. ._ 0.05 0.06
10.5 10.5 10.5
0.07 .0.07 0.07 0.08
0.07
0.07 0.07 0.09 0.11
0.OR .. ._ 0.08 0.08 0.11 0.13 0.17
0.09 .. ..
0.09 0.09 0.12 0.14 0.18 0.30
11.3 11.3 11.3 11.4 17.0
95T0 Engler distillation temperature, "F.
VOL. 36, NO. 4, APRIL 1964
775
no
-7
(17.3) v)
0
+ a I 7.5
a
4
18.2
a W
I-
(18.0) 111
13.3
0
14.0
$ -
(21.3)
0
(20)
5 I
a
14.3
I
(14.0)
@ -
5
E
15.5
' N ' d
776
2 1.6
ANALYTICAL CHEMISTRY
24.2
As shown in Table 11, the various compound types are generally grouped according to the number of aromatic rings. Major exceptions are fluorene and pyrene which separate with the triaromatics, and benzfluorenewhich separates with the tetraaromatics. The pentaaromatic hydrocarbons generally overlap the elution of certain oxygen and nitrogen compounds previously observed (1, 19) in cracked gas oils. Use of a stronger eluent and adsorbent tends to improve the latter separation, but previous studies (19) have shown incomplete separation even under such conditions. The separation between adjacent and diaromatics-is seldom complete, and it frequently becomes necessary t o estimate the contamination of one chromatographic band by another. This is normally a difficult task because the diversity of chemical types t h a t fall under a particular band greatly complicates the quantitative analysis of a fraction for the relative amounts of different bands. However, the elution band tail of a chromatographically homogeneous group of compounds falls off exponentially, so that a logarithmic plot of band concentration vs. eluate volume is linear. This relationship has been cited previously in the elution of total-nonaromatic monosulfides (12) and total saturated hydrocarbons (21) from alumina. Its applicability to the chromatographic classes of Table I1
row boiling distillation fractions from a catalytically cracked gas oil (elution conditions for these various separations correspond t o those used in routine separation procedures). Band concentration is followed in e:tch case by eluate absorptivity a t a selected wavelength in the ultraviolet. I n Figure 2, a-d, the boiling point ran5e of each of these fractions precludes the presence of significant amounts of the next eluted chromatographic clas j -e.g., essentially no diaromatics in thch sample of Figure 2a, no triaromatics in Figure 2b, etc.a n d the linear nature of each plot in the band tail region is evident. I n Figure 2, e and f , for some higher boiling samples where interfwence by the next eluting class does occur, the various elution curves are linear for a short distance past the band maximum, but, because of interference, become nonlinear as the next elu ed class makes its appearance. Correction of the band tail absorption measL rements for interference by the followmg class generally restores the linear “orm of the first ehited band within the accuracy of such correction?. M‘ith cr nithout interference. use of plots such as those of Figure 2 , combined with spectral correction for interfering elution bands where necessary, permits the extent of elution of a band to be calculated, and the contamination of each fraction by adjacent elution bands to be estimated within narrow limits. The separation procedures described above, which use the divided column of Figure 1, assume the initial elution of sample monoaromatics and diaromatics paqt the column midpoint (center stopcock). Saturates and olefins do not interfere with the separation or analysis of aromatic iypes, are not determined in the preseqt procedure, and will not he considered further. Following the elution of mono- and diaromatics from column I, elution is changed so as to collect eluent at the center vent. The hig,her aromatic hydrocarbon classes (tri-, tetraaromatics) are eluted directly from column I as separated fractions i nd collected for iiltraviolet analysis. The remaining, more strongly adsorbing compounds on column I (pentaaromzkics, oxygen, and nitrogen compounds) i r e stripped from t h e column through the center vent and discarded. The conditions of separation a r e such that the monoaromatics leave column I1 at the time t h a t the diaromatics leave column I , so t h a t the final required step is the stripping of the diaromatics from column 11. Table I11 summarizes these separation steps. Figure 3 summarizes the elution characteristics of the mono-, di-, and triaromatic classes in feparation modes A and LI. Recoveries of each aromatic type (in its fraction) and interference (relative per cent) from adjacent frac-
DI
MON 0 -AROMATIC S
> *6
l 90 80
o
o
- AR 0MAT ICS
n ‘--A
i” ‘ii W
a:
70
70
.\“ 60
60
400
600
800
1m ; TRI-AROMATICS
60
400
800
600
400
600
800
OF
O F
Figure 3. Recovery of and interference to various aromatic fractions in routine LEAC separation vs. sample boiling point, narrow boiling catalytically cracked fractions
tions are plotted us. sample average boiling point. The tetraaromatic class shows an approximately constant recovery (98’%), with negligible interference from more strongly adsorbing compound types. The data of Figure 3 were obtained from the study of elution curves (as in Figure 2) for a number of routine separations involving narrow boiling (25’ F.) fraction from a catalytically cracked gas oil described below. The need for two separation modes (A for light samples, B for heavy) is illustrated in the data of Figure 3 for the monoaroniatic class. Mode 13 gives excessive interference of diaromatics to the nionoaromatic fraction with very light samples, while niode A gives a very low recovery of monoaromatics in the monoaromatic fraction for heavy samples. For the LEAC separation of structurally similar sample types---Le., cracked samples-the recovery curves of Figure 3 can be assumed constant.
Table 111.
Step No. 1
2
Interference by one class to another in the routine separation procedure is generally less than 5’%, and can therefore be ignored. The one exception, interference by tetraaromatics t o triaromatics, will be considered below. Lack of interference of one compound class to another is important in the present analytical procedure, because t h e accurate detection by ultraviolet analysis of contaminating compound classes in these various fractions appears generally unfeasible. ULTRAVIOLET ANALYSIS OF VARIOUS AROMATIC FRACTIONS
Ultraviolet absorption spectra of various fractions obtained in the routine LEAC separation of cracked gas oil samples are generally similar to those reported by Charlet, Lanneau, and Johnson (3). Some examples are shown in Figure 4, a-f. A11 of the monoaromatic and diaromatic fractions (Figure 4,
Elution of Various Aromatic Classes in Routine Separation Procedure
Monoaromatics Fraction A, top vent
aromatics Column I1
Triaromatics Column I
Tetraaromatics Column I
Pentaaromatics, N and 0 Compounds Column I
Column I1
Fraction B, center vent
Column I
Column I
Fraction C, center vent
Column I
Di-
3
Column I1
4
Column I1
Rejected at center vent
Fraction D,
top vent
VOL. 36, NO. 4, APRIL 1964
777
4 Figure 4. Ultraviolet absorption spectra of various chromatographic fractions Monoaromatics from routine LEAC separation of 525-550' F. fraction ( b ) Diaromatics from routine LEAC separation of 600-625' F. fraction (E) Vinyl naphthalene concentrate from large-scale separation of 500525'F. fraction (d) Triaromatics from routine LEAC separation of 600-625' F. fraction (e) Triaromatics from routine LEAC separation of 750-775' F. fraction (f) Tetraaromatics from routine LEAC separation of 750-775' F. fraction (a)
L
......
WAVE LENGTH
mp
u and b) show the characteristic spectra of the alkylbenzenes and naphthalenes, respectively. Any alkylthiophenes in the original sample should concentrate in the monoaromatic fraction, but a previous study (12) has shown the concentration of alkylthiophenes in catalytically cracked gas oils to be quite small (
