V O L U M E 21, N O . 11, N O V E M B E R 1 9 4 9 Table V.
Reproducibility of Fluorometric Method for Determination of 2-Naphthol
Number of samples, N 2-Naphthol present in each sample Average fluorometric reading P Average deviation of tluorom’etric readings Standard deviation of fluorometric readings
48
14,OO micrograms 8 7 . 1 units 1 . 2 0 units 1 . 3 5 units
1377 1-naphthol is not greater than that of %naphthol. If the amount of the 1-naphthol is equal to that of %naphthol, the error is approximately 5% positive. If the ratio of 1-naphthol to 2naphthol is 5 t o 1, the error is approximately 15% positive. LITERATURE CITED
Assoc. Offic.d g i . Chemists, “Official and Tentarlve Methods of
produced by distilled water and therefore the latter may conveniently be substituted for the zero adjustment of the fluorescence meter. Table IT illustrates the reproducibility of the fluorescence intensity readings for 48 samples. Each sample consisted of 14.00 micrograms of 2-naphthol. The fluorometric method may be used for the determination of 2-naphthol in the presence of I-naphthol, if the concentration of
hnalysis,” 6th ed., pp. 395-6, 1946. Barr, C. G., Plant Phvsiol., 23, 443-54 ( 1 9 4 8 , . Dey, B.. Rao, R., and Ssnkaranarayan, Y., J . Indian Chem. SOC., 9,7177 ( 1 9 3 2 ) . (4) Eegriwe, E., Z . anal. Chem., 89, 121 ( 1 9 3 2 ) .
(5) Peckman, H., and Welsh, W., Ber., 17, 1651 (18541. RECEIVEDJanuary 19, 1949. Presented before the southeastern Regional SOCIETY, Oak Ridge, Tenn., J u n e 10, Meeting of the . 4 M E R I C A N CHEMICAL 1949. -4bstracted from a portion of a thesis submitted by Sidney Katz in partial fulfillment of the requirements for t h e P h . D . degree.
HYDROGARBON TYPE ANALYSIS Estimation of Six-Membered Ring Naphthenes H. C. RAMPTON, Anglo-Zranian Oil Co., L t d . , Sunbury-on-Thumes, England A method of hydrocarbon type analysis of petroleum naphthas is described, based on examination of cuts of specified boiling ranges, prepared by fractionation of original and dearomatized samples. The cut points selected ensure the segregation of particular groups of hydrocarbons and contents of aromatics, paraffins, and total naphthenes are obtained in detail throughout the boiling range initial boiling point to 225” C. The “reactable” six-membered ring naphthene contents are determined by a dehydrogenation procedure and do not embrace gemin i l cyclohexane derivatives. The latter are included in the “unreactable” naphthene content, together with the cyclopentane derivatives.
F
R O N the commencement of the petroleum industry many
investigations have been carried out with the object of correlating the physical properties and chemical constitution of naturally occurring hydrocarbon mixtures, but, until recent years, little progress had’ been made in the development of reliable analytical methods for the accurate assessment of hydrocarbon type composition. The analytical difficulties encountered in such work are extremely great and are due to the involved and complex chemical nature of the raw materials. Nevertheless, considerable effort has been directed toward the ultimate goal of analytical petroleum chemistry-i.e., the resolution of petroleum into individual hydrocarbons and other constituents-and much progress has been accomplished. The hydrocarbons present in petroleum may be conventionally and broadly classified as paraffins, naphthenes, and aromatics, a classification that is sharp for the gasoline boiling range but becomes ambiguous for compounds of high molecular weight, which may contain the characteristic structure of all three type. combined in the one molecule. For the purposes of the present paper, the simple classification is tenable, since the investigation has been restricted to materials boiling within the gasoline 1 ange. The paraffins represent the well-defined saturated open-chain structure hydrocarbons. The aromatics may be specified as cyclic hydrocarbons, as a predominant feature of their structure is the benzene ring with its characteristic resonating double bond unsaturation. The naphthenes have fully saturated carbon ring structures. In the absence of evidence t o the contrary, straightrun petroleum fractions are considered to contain five- and sixmembered ring naphthenes only and it is with the estimation of
these two classes of compound that this investigation is concerned. ESTI&IATION OF HYDROCARBON T Y P E S
Fractional Distillation. The primary step in the estimation of hydrocarbon type is that of fractional distillation, and progress in recent years in this connection has been stupendous. Various forms of ultraefficient column packing have been designed and used with great success in the analytical laboratory, notably the glass and stainless steel helices developed by Fenske et al. (3,4,10, 11), the spiral packings due to Podbielniak (9).and the spiral screen packing described by Lecky and Ewe11 (1I . Using columns packed with such materials it has become normal practice to fractionate a gasoline sample into close boiling cuts, with subsequent analysis of these for aromatic, naphthene, and paraffin contents (6, 8). By these means a hydrocarbon type analysis throughout the hoiling range can be obtained. I t has been realized that the removal of aromatic hydrocarbons, prior to the fractionation, facilitated estimation of naphthene and paraffin hydrocarbons. This fact is due partly to abnormal vapor pressure relationshipi existing hetween certain of these compounds and the aromatic hydrocarbons, and also to the reduction in the number of individual compounds present in the distillat,ion charge. The removal of aromatics, without affecting the other hydrocarbons present, is accomplished by the convenient process of selective adsorption. The sample is percolated through a column packed with silica gel adsorbent (each 100 grams of gel absorb 8 grams of aromatics) and the aromatic-free percolate of constant refractive index is segregated, equivalent t,o the naphthene-
ANALYTICAL CHEMISTRY
1378
paraffin component, which can then be fractionated and analyzed. The accuracy and fullness of the resultant naphthene-paraffin analysis depend on the time expenditure permitted for the necessary fractionation. For ordinary routine analysis purposes, the distillation time may be as short as 40 hours, whereas for more exacting research programs greater precision in fractionation may be necessary. For the program of work comprising the subject matter of this paper, the fractionations have been those of the &st or routine kind. Two types of fractionating column have been employed, both equivalent to 40 theoretical plates in efficiency. The first comprised a &foot (150-em.) length of 20-mm. inside diameter tubing packed with 0.125-inch Fenske glass helices, capable of handling distillation charges of 1 to 3 liters. The second consisted of an 18-inch (45-em.) length of 16-mm. inside diameter tubing packed with l/&nch stainless steel gauze cylinders, suitable for distilling smaller charges of 200 to 500 ml. The reflux ratio employed normally in such distillations was 40 to 1. Estimation of Aromatics. The method of analysis for aromatic content aims a t the determination, as far as possible, of individual aromatic hydrocarbons, and employs fractionation and ultraviolet spectroscopy. A sample of the original gasoline is fractionated (40 theoretical plates, 40 to 1reflux ratio, not less than 20 to 1 charge-hold-up ratio) into cuts of specific boiling ranges: I 2 3
1.B.P.- 60' 60-117O 117-1200 120-126O 4 126-140O 5 140-145O 6 145-150O 7 150-175" 8 175-200' 9 10 200-225°
C. C. c. C. C. C. C. C. C. C.
+ toluene) (ethylbenzene) (ethylbenzene + isomeric xylenes) (xylenes, mainly m- and o-) (benzene
A quantity of the gasoline sample is dearomatized by percolation through silica gel and the aromatic-free percolate is distilled through a fractionating column (40 theoretical plates, 40 to 1 rBflux ratio, not less than 20 to 1 charge-holdup ratio). Boiling point data on a weight basis are recorded and specific cuts are segregated, of boiling ranges: 1 1.B.P.- 15' C. 2 15- 32.5O C. 3 32.5- 45' C.
4
4.i- fifio
6 7 8 9 10 11 12 13 14
75- 8 5 O 85- 950 95-109' 109-1200 120-130° 130-150° 150-175' 175-200° 200-225O
i
66-
i s 0
c
E:
C. c.
C.
c.
C. C. C. C. C.
These cut points were chosen with the deliberate intention of facilitating and simplifying the subsequent analysis for naphthene and paraffin hydrocarbons, and were based on experience gained by analyses carried out in greater detail. The advantage of the specific cut points may be best assessed by reference to Table I, wherein are listed the predominant hydrocarbons considered to be present.
Table I. Boiling Range,
c.
I.B.P.-15 15-32.5 32.5- 45 45- 66 66- 75 75- 85 85- 95 95-109
Predominant Hydrocarbons in Specific Cut. Para5ns Butanes Isopentane n-Pentane Isohexanes n-Hexane Dimethylpentanes Methylhexanes n-Heptane
Saphthenee
... ...
Cyclopentane Cyclopentane Methylcyclopentane Cyclohexane Dimethylc yclopentanea Methylcyclohexane, ethylcyoloyantane, and trimethylcyclopentanes Trimethylcyclopentanes and dimethylIso-octanes cyclohexanes CaHla, 5. and 6-membered ring n-Octane CeHls, 5- and 6-membered ring Iso- and n-nonanes ClaHto 5- end 6-membered ring 180- and n-decanes Iso- and n-undecanes CnHw: 5- and 6-membered ring C ~ H P 5, , and 6-membered ring 150- and n-dodecanes
Each of the above cuts is examined by ultraviolet spectroscopic methods (6) for individual aromatic hydrocarbons likely to be 109-120 present from a consideration of the boiling point-via., cut 2, 120-130 benzene and toluene, cut 5, ethylbenzene, 0-, m-, and p-xylene, 130-150 150-175 etc. Cut 3 is low in aromatic content and serves as a buffer cut, 175-200 avoiding the estimation of ethylbenzene in high concentration of 200-225 toluene. In the case of the higher boiling cuts, 150" to 175' C., 175" to 200" C., and 200" to 225" C., the number of iso03Sas I 1 I I I I I I I I I mers does not permit at the I present juncture complete 0-0 estimation of individuals and a total aromatic content only is obtained. Experience has 3475 shown that aromatics are absent from the cut of initial boiling point to 60" C. and i t s examination can be omitted. A knowledge of the weight per cent yields of the cuts, together with their determined aromatic contents, enables the total aromatic content of the sample to be calculated, including a detailed distribution throughout the whole boiling range. Estimation of Naphthenes and Paraffins. Various methods for estimation of naphthene and paraffin contents are available, the majority 0.3300 based on relationships between physical properties. The particular method chosen here employs specific refracFigure 1. Mean Specific Refraction Values for Paraffins and Naphthenes tion measurements.
,
t
pP5
V O L U M E 21, NO. 11, N O V E M B E R 1 9 4 9
1379
H Y D R O G E N INLET BUBBLER
-OIL J A C K E T FEED Y E S E L
PREHEATER
3x2'
180 WATTS)
(WINDIUG
T U E R MOCOUPLE
A5BE3T05
I'
REACTOR
IO'X I
ID
B D
6"
AATALYST
(;)---*
,DOUBLE
SURPACE
CONDENSER
U------SRODUCT RECEIVER
Figure 2.
All-Glass Dehydrogenation Apparatus
The density a t 20' C. in grams per milliliter (accuracy +=0.0001 gram per ml.) and the refractive index a t 20" C., sodium D line (accuracy *O.OOOl), of each cut are determined and specific refractions are calculated according to the formula: n s 1 n2+2';i where d = density a t 20" C., gram per ml. (by pycnometer, 2) and n = refractive index a t 20" C. (sodium D line) (by Abbe or Pulfrich instrument). By linear interpolation, using mean values for the specific refractions of the naphthenes and paraffins assumed to be present, the naphthene and paraffin contents of each cut are determined and expressed as weight percentages of the sample. A plot of the mean specific refraction values employed against mid-boiling point of cut is given in Figure 1, and has been conatructed from the values listed in Table IX. DIFFERENTIATION AND ESTIMATION O F NAPHTHENE TYPES
The scheme of analysis outlined above enables a hydrocarbon type composition of any straight-run gasoline to be calculated in terms of aromatics, paraffins, and naphthenes throughout the boiling range. It is of interest, for various reasons, to distinguish between, and, as far as possible, estimate the two main types of naphthene hydrocarbons-namely, the five-membered ring or cyclopentane derivatives and the six-membered ring or cyclohexane derivatives. For the boiling ranges below 95' C., this can be accomplished by reference to boiling point data and the total naphthene contents of the specified cuts, as the chosen cut points separate the following hydrocarbons: cyclopentane (boiling point 49.2 O C.), methylcyclopentane (boiling point 71.9' C.), cyclohexane (boiling point 80.8" C.), and the dimethylcyclopentanes of boiling point 85' to 95' C. Above 95' C., however, substituted cyclopentane and cyclohexane derivatives occur together and recourse must be made to more elaborate methods. I t was decided to investigate the reaction discovered by Zelinsky (12) and his co-workers (19)-namely, the dehydrogenation of cyclohexane derivatives to the corresponding benzene hydrocarbons in the presence of platinum or palladium catalysts:
CBHnR. +CsH6R
+ 3Ha
Subsequent estimation of the increase in aromaticity on dehydrogenation can furnish an analytical measure of the content of six-membered ring naphthenes. Zelinsky showed that, under certain conditions, the above reaction was quantitative and selective, and proceeded in one stage without formation of intermediate partially dehydrogenated cyclohexanes-Le., cyclohexenes-and paraffins, and cyclopentane derivatives were unaffected. Equipment. The apparatus used in the present investigation consisted of an all-glass reactor and preheater connected to a combined feed vessel and electrolytic pump, with receiver trap and condenser, the latter operated mith methanol cooling a t -30" C. For maintaining the slow feed rate of 6 ml. per hour, the electrolytic pump functioned in a most satisfactory manner. A diagrammatic sketch is given in Figure 2. The catalyst space measured 70 ml. and the dead space of 50 nil. a t the top of the reactor was filled with Gooch asbestos. Catalyst Preparation. The platinum catalyst was prepared from analytical grade platinum chloride, hydrochloroplatinie ,icid, HzPtCl8.6HzO. A quantity of 30 grams of granular active charcoal was added to an aqueous solution containing 13.6 grams of platinum chloride, the whole was evaporated on a water bath, and the mixture was stirred until dry. The resulting solid was introduced into the catalyst space of the reactor and flow of hydrogen commenced. After flushing, generation of the active catalyst was achieved by raising the temperature a t a rate to reach 300' C. a t the end of 10 hours. The catalyst was then ready for use, and thereafter an atmosphere of hydrogen was maintained in the apparatus. The composition of the finished catalyst corresponded to 15% (weight) platinum, 85% (weight) charcoal. Procedure. To render the apparatus free from leaks, heav) dudco grease No. 657 was used on all ground-glass joints save those of the feed vessel and pump, where the light grade, Yo. 356, proved satisfactory. Freedom from leaks was tested by checking the relative rates of entry and exit of hydrogen gas when no sample was passing. The feed was introduced through the filling funnel and three-way stopcock into the electrolytic pump and the preheater temperature l-ias adjusted to 200' C. Preliminary Experiments. As an initial test of the catalyst, methylcyclohexane was used as feed in a few preliminary experiments prior to the completion of the electrolytic pump, using gravity feed from a small buret into the preheater. The physical constants of the methylcyclohexane, density a t 20" C., gram per ml., 0.7681, and refractive index n z o i c ,1.4225, compared with literature values for the pure hydrocarbon of 0.7694 and 1.4231 and correspond to a purity of approximately 9 9 7 . The refractive indexes of the first few drops of product approached that of toluene-namely, 1.4969-but rapidly declined for subsequent yields to that of the feed. The reason for this rapid loss of activity was not, a t first, obvious and such variables as catalyst bed temperature and hydrogen rate were investigated without success. I t was then realized that, because platinumcontaining catalysts are notoriously sulfur-sensitive, the rubber connection between the temporary feed buret and the preheater might be responsible. Steps were taken, therefore, to ensure that all connections in the final design of apparatus were of glass. Aromatic-free materials, produced by exhaustive extraction of aromatics by concentrated sulfuric acid, when dehydrogenation was attempted, exhibited the same phenomenom-Le., rapid losa of catalyst activity. This was probably due to the presence of some sulfur-containing compounds formed during the acid treatment and not removable by the customary alkali wash, The technique of dearomatization by percolation through silica gel obviates this trouble, as the sulfur compounds present are selectively absorbed and one obtains a naphthene plus paraffin component, the sulfur content of which is
