Chromatographic Analysis of Gas Oils for Hydrocarbon Types Examination of Techniques R. J. CLERC, C. B. KINCANNON, AND T. P. WIER, JR. Research Laboratory, Shell Oil Company, Houston, Tex. In analyzing mineral oils, gas oils, etc., in terms of groups of molecules of the same hydrocarbon type, it is desirable to separate the sample into fractions, each of which contains a single hydrocarbon type. Of the separation processes useful for this purpose, chromatography is outstanding. Three chromatographic methods of analysis were examined in a study of the hydrocarbon type composition of a
I
straight-run gas oil and a catalytically cracked gas oil. The use of a weak developer to displace a saturate band and to elute several aromatic bands allows the desired separations into saturates, monocyclic aromatics, dicyclic aromatics, and tricyclic aromatics. Moreover, the fractions so recovered are auitable for further characterization by measurement of spectral and other physical properties,
N THE study of petroleum refining processes it is frequently
Work by Mair provided an adsorption method for determining
necessary to determine compositions of liquid hydrocarbons boiling in the gas oil and higher ranges. The approach to the analysis of these petroleum fractions has resolved itself into three lines of attack during recent years-Le., analysis in terms of individual hydrocarbons, number of carbon atoms in rings and in alkyl chains, and molecular types. The isolation of individual hydrocarbons by exhaustive distillation, crystallization, solvent extraction, etc., is exemplified by the work of A.P.I. Project 6 at the National Bureau of Standards (1.9). Such work must be confined to the isolation of as many pure hydrocarbons as possible from a relatively few samples.
the total aromatic content of gasolines (9),which has now been extended to higher boiling fractions ( 11, IS). Chromatographic techniques have also been reported by Lipkin et al. (6, 7) for measuring the paraffin, monocyclic naphthene, and dicyclic contents, etc., and the total aromatic content of the higher boiling oils. The main interest of the chromatographic work described in this paper was the determination of the amounts of monocyclic, dicyclic, and tricyclic aromatics, in addition to the separation of the saturate hydrocarbons from the aromatic hydrocarbons. Although the present paper deals only with the analysis of gas oil samples, the techniques described also apply, with some limitations, to materials of higher boiling range.
Table I. Property A.S.T.M. distillation Initial boiling oint, C. 10% recoverexat 0 C. 50% recovered a t a C. 9 0 9 recovered a t C. point, 0 C. Recovery, % v. Lose,.% y. Refractive index, n$? Density. d?O Bromine $0. Sulfur % W. Molechar weight
E~J
Properties of Samples West Texas SR Gas Oil 240 260 277 302 329 97.5 2.5
1.478
0.858 6 0.96 224
Pilot Plant Catalytically Cracked Gas Oil 240 254 279 321 338 91.0 9.0 1.515 0.896 12 0.99 208
The so-called ring analysis-Le., an analysis in which the percentage of the total carbon atoms present in aromatic and naphthenic rings is determined-is exemplified by the methods of Deanesly and Carleton (l),Lipkin et al. (6, 8),and others. For the most part, these methods are based on simple physical properties which allow application of the methods to a large number of samples without too great an expenditure of work on each. These methods yield figures for the average composition of a sample in terms of the different kinds of atom bondings, but it is usually impossible to interpret the results in terms of the quantitative distribution of molecular types present in the sample except by assumption of the types present. More recently, effort has been directed toward separation of groups of molecules of the same hydrocarbon type, such aa paraffins, naphthenes, aromatics, etc., which allows analysis of complex petroleum fractions without assumptions of the hydrocarbon types present. Of the separation processes available today, one of the most efficient and useful is chromatography.
EXPERIMENTAL
The successful application of chromatography to petroleum fractions depends largely on the use of suitable developers-Le., liquids added to the column to increase resolution of the adsorbed sample components. Liquids which serve this purpose are divisible into two groups, depending upon their strength of adsorption in relation to the material being separated. Those developers which displace adsorbed material from the column by virtue of their greater strength of adsorption are termed displacents. In such operations the displaced material is caused to move down the column ahead of and a t the same rate g,a the displacent. The less strongly adsorbed developers are termed eluents, because, in effect, they are used to move the adsorption bands by a washing process (ez here: to wash out). When a sample consists of material of a wide range of adsorption affinities, a developer may be both an eluent and a displacent. There are important differences in results, depending upon whether a developer of high adsorption affinity or of low affinity is used and upon the manner of use. Three techniques are compared in this paper: (1)use of a strong initial developer, (2) use of a weak initial developer to displace saturates only, and (3) use of a weak initial developer to displace a saturate band and to elute several aromatic bands. Use of Strong Initial Developer (TotalDisplacement Development). In the adsorption technique of Mair et oil. (9,11, 13) the sample is added to a column of adsorbent, and alcohol (a strong developer) is added immediately thereafter to displace the entire sample down the column ahead of the alcohol. This technique thus falls hi the category of displacement development. The displacement technique was examined for the separation of gaa oil components. For example, sample of catalytically
V O L U M E 2 2 , NO. 7, JULY 1 9 5 0
865 after the saturates have been removed, other bands will be obtained. The material in these bands is eluted by the developer rather than displaced.
L
1.42 0
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16 20 24 28 32 36 40 44 EFFLUENT VOLUME, mL
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52
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Figure 1. Displacement Development with a Strong Initial Developer, Ethyl Alcohol 50 m l . of catalytically cracked ga0 oil charged t o 149 grams of Davi00n silica gel, 28- t o %OO-mesh, as received, i n jacketed three-sectioned column. Upper, 1.9 om. (diameter) X 60 cm.; middle, 1.2 X 60 cm.; lower, 0.6 X 55 c m . Temperature 50' C. Flow rate, approximately 0.5 m l . per minute
In carrying out the elution technique, ordinary Corning borosilicate glass pipe sections are frequently used as adsorption columns. In use they are partially filled with 28- to 2Wmesh Davison silica gel, the upper, vacant portion of the pipe serving aa a reaervoir. (The long, jacketed columns needed for displacement developments experimeats were found to be unnecessary when a weak developer was used.) In the present experiments, developer w a s first poured through the gel to dissipate the heat of wetting. then the sample was added. The proper size of gaa oil ssmpie was predicted from the aromatic adsorption ca acity of the el- for optimum efficiency only a slight excess of geyis recommen8ed After the straight-run gas oil had been charged into the gel; developer was added continuous1 When the initial developer had ceased to elute chromatograp& bands, a developer of slightly higher adsorption affinity-viz., a solution of 10% toluene in methylcyclohexane-was added to obtain another band. Toluene and finally acetone were added to displace the remainin material from the adsorbent. The refractive index of the eflfuent waa obtained on single drops of effluent a t intervals of volume, the results of which are shown in Figure 3. After a particular adso tion band was collected, the oil was recovered b removing lowboiling developer by atmospheric distillation hlowed by dis-
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 cracked gas oil (Table I ) was passed over Davison 28to 200- mesh silica gel and developed with ethyl alcohol. The refractive index of the effluent, plotted in Figure 1 as a function of the volume of effluent, reveals the overlapping between the various major hydrocarbon types. Although work here and elsewhere has shown that undoubtedly a much sharper separation can be obtained by the use of a higher temperature, a longer column, a more viscous displacent, and finer silica gel. However, I L I some overlapping is an inherent characteristic of this L4It method. L Use of a Weak Initial Developer to Displace Saturates 1.391 Only (Selective Displacement Development). The use of a weak developer (pentane) to displace saturates, followed 0 IO 20 30 40 50 60 70 80 90 100 110 I20 I 3 0 I40 IS0 by a strong developer (alcohol) to displace total aromatics, EFFLUENT a U M E , ml is illustrated by the work of Mair and Forziati (IO) and Figure 2. Displacement Development of S a t u r a t e Band Only Lipkin et al. (6). This principle was examined, as illustrated with Methylcyclohexane in Figure 2, using a sample of straight-run gas oil (Table I ) 50 ml. of Went Texas straight-run gas oil charged to 147 gram# of Davison silica gel (not previoudy wet with developer), followed by 100 ml. of methylwith methylcyclohexane (MCH) as the weak developer cyclohexane, and finally d i a p l a d with ethyl aloohol. Same column, temperature, and flow rate as in Figure 1 and ethvl alcohol as the strong- developer. The results show that this procedure eliminates mixing between saturates and aromatics, as reported in the literature (6, IO). HowTable 11. Properties of West Texas Straight-Run Gas Oil Chromatographic ever, the aromatic fraction remains a Bands complex mixture which cannot be readily Chromatographic Band characterized in terms of mono-, di-, and B C D E F A tricyclic aromatic content. Proceas Elut. Elut. Displ. Displ. Displ. Although a weak developer (methyl10% tpluToluene Acetone Developer MCH MCH m e In cyclohexane) was used, the saturates MCH ._ ~Oil recovered were displaced rather than eluted, inasmuch aa they were the first components to issue from the column, which had not oeen previously wet with developer. 1.043 1.054 ... 1.077 Use of Weak Initial Developer to DisS ecific dispersion, SF,C 97 147 ... 230 ... ... place Saturates and Elute Aromatic olecular weight C 238 215 188 ... Bands (Elution Development). In using 0.0 1.0 2:i 4.8 9.7 i:o Sulfur, yo W. the procedure described immediately Hydrocarbon type analysis Total % v. above, when the refractive index of the 72 ... ... ... ... 72 Saturatea effluent (Figure 2) drops to that of the Aromatics Monocyclic ... 11 2 ... ... ... 13 weak developer (methylcyclohexane), Dicyclic ... ... 9 ... 9 ... ... ... ... ... 0 Tricyclio the completion of the saturate band is ... ... ... 2 ' . 4 6 Residue indicated. If, instead of introducing 100 --a Methylcyclohexane. ethyl alcohol to displace the aromatics, b Material recovered waa semisolid black residue. the weak initial developer is allowed 0 Molecular weight obtained from mid-boiling point and density (14). t o continue flowing throigh the column
...
...
s
...
...
... ...
-
ANALYTICAL CHEMISTRY
866
aromatic rings, regardless of whether they are condensed or noncondensed and regardless of the number of naphthenic rings which may be present in side chains. The compositions are in error-i.e., too high-by the amount of sulfur compounds present in each fraction.
Table 111. Properties of Catalytically Cracked Gas Oil Chromatographic Bands Prwess
Developer
A 4 -. 1.447 0.805
Ref&tive indsr, n%? Density, d i 0 h t n a t i v i t r intemept. (n
- %)1.044
...
8 i50 dispersiond SF,C d%oular weight sulfur, % w.
210
0.0
Hydroaarbon type analysis, % v. Bsturatea
... ...
Monocyclic Dicyclic Trioyclio Itmidue
7
9
7b
1.560 0.981
1.581 0.986
1.645 1.071
...
1.079
1.088
1.109
215 178 1.9
248 170 2.9
318
i:i
...
... ... ...
i''
23
...
... ...
...
9
...
...
Methylcyclohexane. Material recovered WM semisolid black residue. Molecular weight obtained from mid-boiling point and density ( 1 4 ) .
1.431
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oil movered Yield % V.
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ChromatogFaphic Band B Cand D E Elut. Elut. Elut. MCH MCH 10% tolu-
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2000
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EFFLUENT VOLUME,
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4000
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5000
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Figure 3. Use of Weak Initial Developer (Methylcyclohexane) to Displace Saturate Band and Elute Aromatic Bands West Texas straight-run gas oil 51 m l . of oil charged to 272 grams of Davison silica gel, 2% to 200-mesh (praviously wet with developer) i n straight column 2.85 c m . (diameter) X 61 cm. Average percolation rate, 3.7 ml. per minute at room temperature
tiilation at reducled pressure. The portion of the charge recovered in each band and the physical properties of these fractions are shown in Table 11. Similarly, the above chromatographic procedure was applied to a catalytically cracked gas oil (Table I). The refractive indexvolume of effluent profile is shown in Figure 4. Again several bands were eluted with the initial developer. In this case, however, the second developer, 10% toluene in methylcyclohexane, eluted a band (E) rather than displaced it. This is shown by the short eeotion of the 10% toluene plateau which was observed b e fore band E issued from the column. The properties of the oil contained in each band are shown in Table 111. The discontinuities in the flow of sample components from the gel-Le., the distinct bands-strongly indicated major differences in hydrocarbon types between successive bands. In these experimenta, molecular weight played little part. To demonstrate this, bands of constant or closely similar adsorption a f i i t y such as those above were subjected to precision distillation and spectroecopic examination of the cuts. Each eluted band proved to be wentially one hydrocarbon type. On the basis of such background data, the material in the chromatographic bands waa =signed the hydrocarbon type indicated by physical properties, a shown in the lower portion of Tables I1 and 111. The types of ammatica, mono-, di-, and tricyclic, refer to the number of
... DISCUSSION
Although development by the total displacement technique does not yield hydrocarbon types which are comTotal pletely separated, the technique can be 54 used for analysis in casea where there 0 is a satisfactory method of characteriz30 9 .. ing each portion of the intermediate 7 7 volume-i.e., the portion of the effluent 100 in which different major hydrocarbon types exist together. For example, Gooding and Hopkins ( 4 ) have used refractivity intercept to characterize the intermediate portions of kerosenes. The validity of such proportioning depends on the assumptions that at most only two hydrocarbon types are present in each fraction and the specific average values chosen for the physical properties of the two types are correct. Because of the spread in physical property values (Table IV and Figure 1) and because synthetic products sometimes do not contain all hydrocarbon types, the use of "total displacement" development can frequently lead to such errors as the quantitatively reported presence of a component which is actually absent-for example, the absence of monocyclic aromatics is noted in the catalytically cracked gas oil, The use of an elution development technique employing a weak initial developer presents disadvantages in the form of the larger volumes of material to be handled, the introduction of a distillation step, and the longer total time required compared to the use of a strong initial developer. However, these disadvantages are usually more than offset by the facts that suitable fractions for further characterization are in ,hand and that the analyses obtained on the hasis of the distinct bands are not so dependent on amumptions. Thus, it is not necessary to assume which major groups are present in any fraction nor to assume a distribution of the individual compounds comprising each major group in order to arrive at an average value of a physical p r o p erty for calculation purposes.
...
... ...
1.431
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1.429
-
1.427
-
1.420
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EFFLUENT VOLUME, ml.
Figure 4. Use of Weak Initial Developer (Methylcyclohexane) to Displace Saturate Band and Elute Aromatic Bands Catalytically cracked gas oil 85 m l . of oil charged to 269 y a m s of Davi6on silica gel 28- to 200mesh (Baturated with deve oper) i n itraight columh 2.85 cm. (diameter) X 56 c m . Average flow rate, 2.9 ml. per minute a t
room temperature
V O L U M E 2 2 , NO. 7, J U L Y 1 9 5 0
867 M. P., “Physical Constants of the Principal Hydrocarbons,” 4th ed., New York, Texas Co., 1943. (3) Egloff,G., “Phyeical Properties of Hydrocarbone,” Vols. 3 and 4, New York, Reinhold Publishing Corp., 1946-47. (4) Gooding, R. M., and Hopkins, R. L., paper presented before Division of Petroleum Chemistry, 110th Meeting, AY. CHEM.Soc., Chicago, Ill., 1946. (5) Lipkin, M. R., et al., ANAL.CHEM.,20, 130-4 (1948). (6) Lipkin, M. R., and Martin C. C., Ibid., 19, 183-9 (1947). (7) Lipkin, M. R., Martin, C. C., and Hoffecker, W. A., paper presented before Division of Petroleum Chemistry, 113th Meeting, Aaa. CHEM.SOC., Chicago, Ill., 1948. (8) Lipkin, M. R., Martin, C. C., and Kurtz, 8. S., Im. ENQ. CHEM.,ANAL.ED.,18,376-80 (1946). (9) Mair, B. J., J . Research Natl. Bur. Standarde., 34, 436-61 (2) Do=,
Table IV.
Range of Physical Properties of Gas Oil Aromatic Typesa
Monocyclic Dicyclic Tricyclic a From data of Doss b Estimated.
n %o 1.48-1.54 1.53-1.60 1.59-166b (2)and Egloff ( 8 ) .
8F.C 125-160 160-260 300-,550
-
3
(n 1.05-1.07 1.09-1.11 >1.1
The monocyclic aromatics of the straight-run oil were divided into two distinct bands. The same was true of the dicyclic aromatics of the catalytically cracked material; this indicated that further type distinctions may derive from the application of this approach. ACKNOWLEDGMENT
The authors wish to acknowledge the advice and cooperation of K. E. Train, R. G. Appleby, hl. 0. Baker, and G. P. Hinds, Jr., in connection with this work. LITERATURE CITED
( I ) Deanesly, R. M., and Carleton, L. T.. IND.ENG.CHEM., ANAL.
En.,14. 220-6 (1942).
(1945). (10) Mair, B.J., and Forriati, A. F., Ibid., 32, 165-83 (1944). (11) Mair, B. J., Gaboriault, A. L., and Rossini, F. D., Znd. Eng. Chem., 39,1672-81 (1947). (12) . , Mair. B. J.. and Rossini. F. D.. A.P.I. Project 6. “ReDort on Fractionation Analysis and ‘Isolation of Hydroca&ns in Petroleum,” Mareh 31, 1947. (13) Mair, B. J., Sweetman, A. J., and Rossini, F. D., Znd. Eng. Chem., 41, 2224-30 (1949). (14) Mills, I. W., Hirschler, A. E., and Kurtz, S. S.. Ibid., 38, 442-50 (1946). RECEIVED March 10, 1950. Presented before the Division of Petroleum Chemistry, Symposium on Adsorption, at the 116th Meeting of the AMmBICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Separation of Nitrogen Compounds by Adsorption from Shale Oil J. R. SMITH, C. R. SMITH, JR., AND G. U. DINNEEN Petroleum and Oil-Shale Experiment Station, Bureau of Mines, Luramie, Wyo. One of the complicating factors in elucidating the composition of distillates from Colorado shale oil is the presence of large quantities of nitrogen compounds. The content of these compounds may be as much as 50 weight % on some fractions. A procedure is described for separating shale-oil distillates into two fractions-one containing primarily hy-
M
OSTofthedistillateobtainedfromColoradoshatleoilisin the boiling range above 400’ F. Consequently, knowledge of the composition of this higher boiling fraction is of paramount importance in developing processes for producing engine and other distillate fuels from shale oil. The majority of the methods ( 2 , 6 , 1 1 ) for the analysis of petroleum distillates in the higher boiling ranges estimate the carbon atoms as paraffinic, naphthenic, or aromatic. These methods are based on correlations that usually are not valid for samples containing olefinic unsaturation and nonhydrocarbon material. If the method utilizes optical properties, it is difficult, if not actually impossible to make the required determinations on dark samples. Consequeiitly, higher boiling shale-oil distillates, which always contain large quantities of olefins and of nitrogen compounds and are dark, cannot be analyzed satisfactorily by direct application of available methods. Separation and recovery of the nitrogen compounds, therefore, would aid materially the analysis of the residual hydrocarbons as well as analysis of the nitrogen compounds themselves. A large part of the nitrogen compounds in shale-oil naphthas and kerosene8 can be removed by treatment with dilute aqueous acid (I ), However, this procedure is ineffective for distillates having a 50% boiling point above approximately 525” F., al-
drocarbons and the other primarily nitrogen and other heterocyclic compounds. This separation is based on adsorption and employs Florisil, a synthetic magnesium silicate, as the adsorbent. Factors investigated in selecting optimum conditions for the process are discussed. Reaults obtained on several shale-oil distillates are presented.
though such distillates may contain 30 to 50 weight 70 nitrogen compounds. Adsorption had proved effective in analyzing shaleoil naphtha (8), so an attempt was made to apply the technique to the separation of nitrogen compounds from higher boiling fractions. After preliminary tests on a number of adsorbents, Florisil was selected the best available for the desired application. Conditions were established for a procedure that would separate a shale-oil distillate into two fractions-a lightrcolored one containing essentially hydrocarbons and a dark one containing principally nitrogen compounds. Results obtained by application of the procedure to several samples are presented. APPARATUS
Adsorbent. Florisil, a synthetic magnesium silicate manufactured by the Floridin Company. Air-dried material, 30- to 60mesh,wiw used. Column. A glass tube, similar to that described by Mair and having a fritted disk or other device near the lower end White (8), for supporting the adsorbent end having a reservoir of approximately 200-ml. ca acity on the top. If necessary for some applications, the corumn may be equipped with a heating or cooling jacket and a connection for a plying pressure by means of an inert gas. In the work reporteam this paper Florisil was em-
