Physical and Chemical Properties of Petroleum Fractions I. Behavior in Dilute Benzene Solutions HARRY T. RALL AND HAROLD M. SMITH Petroleum Experiment Station, Bartlesville, Okla.
A modified Beckmann apparatus and method for the cryoscopic determination of molecular weight are described. Results obtained with this apparatus, using benzene as the solvent, are given for oils having molecular weights of approximately 200, 300,500, and 700. Comparative cryoscopic data from eleven cooperating laboratories for the same oils in benzene solution are given and discussed. Data are presented showing that impurities (aside from moisture) and moisture exert adverse effects on molecular weight determinations. The presence of impurities probably at least partly explains the discrepancies universally found i n cryoscopic determinations of the molecular weights of oils.
tories cooperated in this work. Most of these laboratories were connected with oil companies, but educational and industrial institutions were also included. A sample of each of the four oils used as solutes, and described below, was sent to each of the laboratories with the request that they determine the molecular weight in their accustomed manner. A questionnaire covering the method, procedure, and solvents used was also enclosed, the data from which are summarized in this report.
Bureau Methods and Apparatus CRYOSCOPIC METHOD. The cryoscopic apparatus which probably provides the most accurate results is that which employs two cells, as described by Adams ( I ) and developed further by Kraus and Vingee (IS). In this apparatus one cell contains an equilibrium mixture of pure liquid and solid solvent; the other cell contains the solution. Temperature differentials are determined by multiple thermocouples and a potentiometer. The authors' equipment did not permit the use of this more exact method, and the apparatus described below is essentially that of Beckmann, but with several modifications.
P
RESENT studies dealing with petroleum chemistry now being carried out by the Petroleum Division of the Bureau of Mines included the determination, under comparable conditions, of physical and chemical properties of narrow-cut fractions from selected typical crude oils, coupled with the study of the relationship of these properties to each other, and where possible to similar properties of known compounds. One property that is of importance not only in itself but because of its relation to other properties is molecular weight. This is usually evaluated with data obtained from dilute solutions of the oil in certain solvents. The investigation of dilute solutions of hydrocarbons is interesting from the research standpoint because of the occurrence of many deviations from the theoretical. It seems possible that after suitable study relationships may be found between these deviations and structural differences in the oils and also among certain of the more common characteristic properties, thus giving results of practical value. Considerable work dealing with the cryoscopic determination of molecular weights has been reported in the literature. Normann (16), in 1907, obtained some interesting molecular weight-concentration curves. Since then Wilson and Wylde (go), Devine (4),Steed (19), Gullick (11),Epperson and Dunlap (B), Fenske et al. (7), and FitzSimons and Thiele (9) have contributed to various phases of the problem. However, each investigator has assumed that his method is correct and has made little attempt to determine why his results do not agree with others, Further, in many cases the oils investigated have not been identified sufficiently to render the data of further use. The present paper deals with attempts to obtain concordant molecular weights from cryoscopic measurements in dilute solutions of benzene and reports results obtained by the bureau and by cooperative investigators. Sixteen labora-
Referring to the cryoscopic apparatus, Figure 1, the bath container, A , is a double-walled papier-mach6 tub such as is used with calorimeters. This is covered at the top by a closely fitting wooden cover, B. Through this top, reading from left to right, are the cold-water inlet, C, the propeller shaft, D,the bath thermometer, E (graduated in 0.1' C., 0.02' C. estimated), the mercury thermoregulator, F, the outside jacket, G, of the freezingtube assembly, and the overflow tube, H . Referring now to the freezing-tube assembly, the outside jacket, G, is 48 mm. in inside diameter and the freezing tube proper, I,is 35 mm. in inside diameter. These tubes are held in position relative to each other by a flanged wooden ring, J . A close-fitting Bakelite plug, K , closes the top of the tube, I , and through this plug pass the brass propeller shaft, L, and the Beckmann thermometer, M . The propeller proper, N , IS made from a circular brass disk 19 mm. in diameter, with four radial slots at 90" to each other. The leaves or blades thus formed are slightly tilted out of horizontal t o produce pitch. This propeller runs at a speed of approximately 900 r. p. m. To decrease heat conduction from the exterior to the solution, the metal shaft is made in two pieces joined by a glass tube, 0, and cemented with kaolin and water glass. There is a small air s ace separating the ends of the shaft within the glass tube. eT !l shaft is mounted in hardwood bearings (not shown), one at each end of the Bakelite plug, Between these bearings the plug is hollowed slightly t o accommodate a few strands of oil-saturated cotton, which maintains a film of oil on the shaft without danger of the oil flooding the lower bearing and dropping into the solution below. The Bakelite plug also has a 3-mm. hole (not shown) for the introduction of pellets of solid solutes and for ['seeding." Auxiliary apparatus not shown are an insulated tank for ice water with a magnetically operated valve activated through a relay by the mercury thermoregulator; magnifying glasses for reading the Beckmann and bath thermometers; and a seeding rod consisting of a lon glass rod with a 3-strand (NO.30 B & S) nichrome wire coil se$ed into one end. The solute, if solid, is compressed into pellets in a stainless steel press; if li uid, it is weighed and delivered from a Nicol tube with the deyivery tip bent down. These tubes are filled by suction and the liquid is forced out by air pressure from a small aspirator bulb. The Bakelite plug must be removed momentarily for introduction of liquid solutes. 324
SEPTEMBER 15, 1936
ANALYTICAL EDITION
The pure solvent is placed in the freezing tube, I, and weighed quickly to 1 mg., the Bakelite plug with thermometer and stirrer is inserted] and the solvent is cooled until crystallization occurs. It is then warmed slightly, and, in order to obtain constant freezing points, the freezin is repeated three to six times. The solvent is then supercoofed without crystallization and placed in the tube, G, and stirring begun. Temperature readings are taken at intervals. Since the solvent is below the convergence temperature, it will warm to this temperature and remain constant. For certain solvents, such as cyclohexane, it is necessary to approach the convergence temperature from above as the solvent will not supercool sufficiently without “spontaneous” crystallization. If the convergence temperature thus found is more than 0.1’ C. below the freezing point of the solvent, the bath temperature must be increased accordingly. After the bath temperature is thus regulated the solvent is allowed to cool to the convergence temperature again and is seeded by introducing a small crystal of solvent. Crystallization starts immediately and the temperature rises abruptly to the freezing point, where it remains constant for some time, the length of the plateau varying with different solvents. This freezing point is checked by again warming, supercooling, and freezing. No solute is added until this point will give two consecutive checks within O.O0lo to 0.002’ C. After this point has been determined accurately a weighed amount of solute is introduced. If the approximate freezing point cannot be calculated, a preliminary freezing of the fiolution must be made so that proper adjustment of the bath temperature can be made. The process described for the pure solvent is now repeated] except that checks usually are not made, since it has been found that if the first addition of solute is not made until the pure solvent gives check results the freezing points of the solutions generally will check. Three or more additions of solute are made. In certain runs approximately 4 grams of anhydrous magnesium perchlorate were added directly to the solvent in the freezing tube prior t o use. The bureau’s cryoscopic apparatus and procedure were developed primarily for research, and the procedure a t least cannot be recommended for routine determinations of molecular weight. The experiments have certainly been carried out with more attention to details than is usually expected in a routine laboratory determination. CALCULATIONS. Both molecular weights and constants for individual determinations have been calculated from the usual relationship between change in freezing point and concentration
325
Benzene A-4 was prepared from the same benzene as A-2. Two approximately equal portions were separated by crystallization and the crystallized portion was treated with sulfuric acid, water, sodium hydroxide, calcium chloride] and finally refluxed and distilled from sodium. The 4.67 to 90.97 per cent portion was collected and divided into approximately equal ortions by crystallization, the crystallized portion being retainedl Benzene A-5 is the mother liquor from the first crystallization of A-4. Benzenes A-2, A-4, and A-5 were each divided into two portions. One portion, termed “wet,” was used as prepared; the other portion, termed “dry,” was dried with anhydrous magnesium perchlorate. This same drying agent was used in the determination. Table I records the freezing points and cryoscopic constantsolute concentration relationships for these solvents. The first numerical term on the right of the equation is the cryoscopic constant a t infinite dilution, and the second term gives the slope of the constant-concentration curve. The data are plotted in Figure 3. Rejected data are enclosed within a dotted circle. TABLE I. DATAON SOLVENTB Freezing Point Benzene Dry Wet 0
A-1 A-2 A-4 A-5
c. ..
5:14 5.53 4.94
0
c.
5.0 5.08 5.48 4.88
Constant-Concentration Equation Dry Wet .
I
.
I..
K = 66.54 - 0 . 4 1 C K = 65.42 - 0 . 0 2 C K 66.82 - 0 . 6 6 C
K K K K
= 64.98
= 66.99 = 68.45 = 67.63
- 00 .. 31 96 CC - 0.56 C - 0.47 C
SOLUTES.Tables I1 and I11 give the properties of all solutes used. Naphthalene was used as the standardizing substance for determining the constants of the solvents. Oils 0-1, 0-2, 0-3, and 04 were employed in the cooperative work. I n the studies on the effect of impurities in the solvent only oil 0-1 was used.
a /L
where A =
depression of freezing point
m = W = M =
molecular weight of solute weight of solvent molecular weight of solvent molar cryoscopic constant
w = weight of solute
K
=
The concentration of the solute in the solvent has been expressed as grams of solute per gram of solvent
The final accepted values for both constants and molecular weights have been obtained by extrapolation of a straight line through the points to zero concentration. In the case of the constants, this line was determined by the method of least squares. For determining the molecular weight curves the method of averages was used.
Bureau Solvents and Solutes Benzene is the only solvent discussed in this report, SOLVENTB. but several supplies of benzene having different degrees of purity were prepared and used. Benzene A-1 was a reagent-quality thiophene-free benzene and was used without any purification. Benzene A-2 was prepared from commercially pure benzene by treatment with concentrated sulfuric acid, sodium hydroxide, water, calcium chloride, and distillation from sodium.
\
I l l l I W
I
fK / J
i
INDUSTRIAL AND ENGINEERIBG CHEMISTRY
326
TABLE11. Number
Name
VOL. 8, NO. 5
PROPERTIES O F SOLUTES U S E D
Source and Purification
Molecular Weight, Theory
Saybolt Unjvereal Specific Viscosity Gravity, 100° F. 130' F. 60' F.
Approximate Flqsh Pour Point Point O
Naphthalene Triphenylmethane 8-Methyl naphthalene Oil 0-1
Pure: treated melted Na; vacuum distilled through Ni on pumice Pure: twice recrystallized from absolute alcohol, dried a t 70' C. under reduced pressure Practical Acetoneextract, cuts 91,92,and 93 Cabin Creek lubrication stock (18) heated h vacuum, filtered hot Same as 4. exceot cuts 61 and 62
F.
%
128.1
...
..
...
... ...
... ... ...
...
244.13 142.08
... ...
.. ..
... ...
..
.... .... ....
1347 259 105.4
550 131 65.6
0.879 0.863 0.882
30 45
550 420
0.60
...
0.888
..
...
..
....
40
..
LEGEND
340
320
300
CONCENTRATION, CX I O 0
F.
Carbon Residue
CONCENTRATION, C X I O O
FIGURE 2. MOLECULAR WEIGHT-CONCENTRATION DATA
...
0.35
..
ANALYTICAL EDITION
SEPTEMBER 15, 1936
321
TABLE111. A. S.T. M. DISTILLATION OF SOLUTE 0-4 Per Cent Over
Temperature
68 2
F. D.
464 478 484
67 8
c.
10
20
xn ..
487
40 50 60 70
67 4
tZ
so
Q 670
90
E. P
ACCURACY.Table IV gives an indication of the accuracy that may be expected from the apparatus described above. In most instances the deviation of the data from the best curve through the data points is of the order of 1 per cent and is almost always within 2 per cent. While the precision obtained is virtually the same for oils over a wide range of molecular weights, there is no criterion by which the accuracy of the molecular weight determination of such oils may be judged.
t;; 5
666
0
2 662 Q
0
$ 658 0
5
Cooperative Solvents, Solutes, and Apparatus
654
65 0
64 6
I
I
1
I
02
04
06
1
I
I
1
I
16
I
I8
1
I
20
2'2
E 4
FIGURE3. CRYOSCOPIC COYSTANT-CONCENTRATION DATAFOR SEVERAL DIFFERENT BENZENES Tables V and VI attempt to outline the more significant information concerning the-solvents, solutes (besides oils), and apparatus Benzene is favored in the cryoscopic method. All those used by the cooperating laboratories by the cryoscopic laboratories using the cryoscopic method separated the freezand ebullioscopic methods. Table VI1 shows the number of ing solution from the bath by an air jacket similar to that laboratories that used the two methods and the solvents shown in Figure 1. One laboratory reported using for this employed. purpose a Dewar flask from which the vacuum had been' released. A bath temperature of approximately 0' C., TABLE) IV. MOLECULAR WEIGHTSOF PURESUBSTANCES IN maintained by the addition of ice t o water, seemed to be the BENZENE favorite with half of the laboratories reporting; none emMolecular Weight Solute Experimental Theoretical Error Solvent ployed a bath temperature higher than 3.0" to 3.5" C. % Stirring of the solution was done universally by means of a Phenyl ether 171 6 170.1 +O.Q A-1 &Methyl naphthalenea wire or glass loop moved up and down in the solution either 139.5 142.1 -1.8 A-2 Triphenylmethane 242.6 244.1 -0 6 A-1 by hand or mechanically. The replies were not always definite a Not purified. TABLEV.
COOPERATIVE DATAON SOLVENTS AND SOLUTES Solvent
Lab. No.
Name
1 3
Benzene Benzene
4
Benzene
7
Benzene Benzene
8
Boiling point
Purified
Reagent 1' boiling range Unknown
No Crystallization
5.3s
... ...
H ~ S O I caustic , H z 0 , dried, fr
