Solubility of Refined Paraffin Waxes in Petroleum Fractions A. BERNE-ALLEN, JR.,' AND LINCOLN T. WORK trically about the melting point HE purpose of this intube so as to form an annular vestiaation is to present a Columbia University, New York, N. Y. dead air space of 1/4 inch (6.4mm.) -L quantitative treaiment of about the molten wax. The determinations were carried on in a the solubility of paraffin waxes room subject to little draft where the temperature was mainin petroleum distillates. As will be developed, the varitained at 24" to 26" C. ables of importance in the solubility relation are melting point By means of a hot water bath the wax was heated to about of wax, boiling point of solvent, and equilibrium temperature 10 O C. above its melting point and then allowed to cool. During of solution. It is hoped that the relations evolved will serve in the cooling period the temperature was recorded at half-minute intervals. The melting point was regarded as t h a t temperature further study of paraffin wax purification by recrystallization. which held constant within 0.1 C. for 5 minutes or more.
T
O
Selection and Identification of Waxes
Table I presents independently determined results on duplicate samples of the nominal 130-grade wax. For the sake of brevity only data for minute intervals are given here. While the melting point is rounded off to the nearest tenth of a degree, the duplicates presented agree within 0.03' C.
Although commercially refined paraffin wax must meet definite minimum oil content and color requirements, it is sold primarily on the basis of melting point specifications. The following grades were selected for investigation : 118/20, 125, 130, 140, 148. These numerical designations, employed by the manufacturer, indicate the melting point as determined by the Saybolt method (11). For the purpose of the present investigation a modification of the A. S. T. M . procedure (1) for the determination of paraffin wax melting point was adopted. The melting points of the waxes were found to be, respectively, 49.9, 52.8, 55.6, 60.3, 64.4' C. to a precision of better than 0.05' C. MELTING POINT DETERMINATIONS. The melting point method was as follows : A &inch (12.7-cm.) depth of molten wax, measured t o a precision of 1 / 8 inch (3.18 mm.), was placed in a 1 X 8 inch (2.5 X 2 0 . 3 cm.) t e s t tube. A standardized t h e r mometer, ranging from 0 " to 100" C. and g r a d u a t e d in tenths of a ,degree, was inserted so that the 12" or 13 graduation was a t the surface of the wax. It was held in place by means of a rubber stopper and c a r e f u l l y adjusted t o run vertically down through the center of the test tube. Thus arranged, the thermometer bulb came about or 3/4 inch (12.5 o r 19 mm.) from the MOLECULAR WT. OF WAX bottom. A second tube was FIGURE1. MOLECULAR WEIGHT OF placed concenPARAFFIN WAXus. MELTING POINT
TABLEI. TIME-TEMPERATURE DATA FOR DETERMINATION OF MELTIXG POINT OF NOMINAL130-GRADE PARAFFIN WAX Elapsed Elapsed ''2.Time, -.-Temp., Time, -Temp., Min. Trial 1 Trial 1 Trial 2 Min. 62.8 10 55.60" 66.3 0 11 55.60a 59.15 1 61.9 12 55,580 57.1 2 58.5 13 55.62 56.25 3 56.9 14 55.50 55.9 56.1 4 15 55.48 55.70 5 55.85 55.40 16 55.62" 55.70 6 17 55.35 55.60" 55,635 7 55.60" 55.63" 8 Av. m. p. 55.6 55.60a 55.63a 9 a Readings used for melting point determinations. T A B L E 11.
C.Trial 2 55.565 55.52a 55.50 55.45 56.40 55.31 55.23 55.10 55.6
MOLECULAR WEIGHT O F PARAFFIN WAX us. MELTING POINT
Estd. Mol. Detd., Mol. Weight Weight M . P.,' C. (Fig. 1) (Menries Method) 49.9 333 323 52.8 346 341 55 6 356 353 56.9" 364a ... 59.1Q 374a i4o 60 3 380 376 148 64 4 408 408 0 Special blends of the other refined waxes as follows: 56.9' C. melting point sample was made by blending equal parts by weight of the original five waxes: 59.1" C.melting. point sample was prepared by blendlng equal parts by weight of 52.8O and 64.4O C. melting point waxes. Nominal M. P., F. 118 125 130
MOLECULAR WEIGHT. After a survey of the available methods for measuring molecular weight, the Menzies improved ebullioscopic method ( 5 ) was selected to determine the average molecular weight of the several waxes. The results given in Table I1 are in close agreement with values from the literature for paraffin hydrocarbons; the dots and triangles of Figure 1 indicate data from different sources. 1 Present address, E. I. du Pont de Nemours & Company, Inc., Waynesboro. Va.
806
JULY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
This paper comprises a study of the solubility relations of commercially refined paraffin waxes in petroleum distillates. The wax samples used as solutes and the petroleum distillates employed as solvents were carefully identified by thorough laboratory inspections. I t was demonstrated that the average boiling points of petroleum fractions and the melting points of the paraffin waxes are directly related to the molecular weights of the solvents and solutes, respectively. Quantitative relations between paraffin wax solubilities in the petroleum fractions and the following variables-paraffin wax melting point, average boiling point of solvent, and solution equilibrium ternperature-were intensively investigated ; an empirical equation was established to express paraffin wax solubility in petroleum fractions in terms of these relations. The effect of changes in the molecular structure of hydrocarbon solvents was also introduced, and the concept of a coefficient of solubility is discussed.
80
807
120
160
200
MOLECULAR W E I G H T FIGURE 2. MOLECULAR WEIGHTOF ALIPH.4TIC HYDROCARBON US. BOILING POINT
that most closely represents the average physical characteristics of the petroleum fraction in question. Table IV presents the molal boiling. points of the petroleum fractions. These were derived by the method given by Watson and Nelson (9). MOLECULAR WEIGHT. The boiling points of normal paraffin hydrocarbons were plotted against their molecular weights as shown in Figure 2. The various average boiling points and the molal boiling points were applied to the graph to estimate the molecular weights of the petroleum solvents. Those estimated from molal boiling points agreed most closely with determined values for solvents 1, 2, and 3. Consequently, molecular weights estimated by this means were considered as acceptable for petroleum fractions A through F. The determined molecuIar weights were obtained by the cryoscopic method. Benzene was used as the freezing solvent into which the petroleum fractions were introduced by means of a weighing pipet. The precision of these results is estimated to be within 2 per cent. I n Table IV the esti-
Selection and Identification of Solvents
Three samples of low-boiling straight-run petroleum distillates from high-grade mid-continent crudes were obtained through the courtesy of the Standard Oil Company of New Jersey. They were used directly as solvents (as received) and hereafter will be referred to as solvents 1, 2, and 3, respectively, in the order of their increasing average boiling points, The solvents were selected with the point in view of obtaining a wide spread over all the lighter fractions from petroleum. Carefully fractionated TABLE111. A. S. T. M. DISTILLATION AND AVERAGEBOILING cuts were prepared from solvents 1 and 3 to proPOINTS OF PETROLEUM FRACTIONS vide additional solvents for experimentation. Fraction Fractions A, B, C, from solvent 1, and D, E, F, 1 2 3 h B C D E B Distillation Temperature, F. from solvent 3 were lettered in order of increasing 70distilboiling point. All the solvents were carefully late: 296 100 140 218 244 355 Initial 113 208 451 identified by methods to be described later. How330 110 268 .. . 367 5 143 230 472 ever, owing to the limited quantity of the lettered 114 ibi 238 369 338 274 10 163 234 473 121 170 243 355 1 37 278 181 20 243 477 fractions the test for unsaturates and the Morgan.. . .. . 128 177 250 283 30 ... 374 479 ... 136 183 256 .. . 2 87 40 ... 376 482 Soule paraffin-naphthene ratio were of necessity 390 144 191 263 291 50 226 379 269 486 omitted on these fractions. .. . 153 199 271 .. . 297 382 60 ... 490 .. . 303 .. . 166 206 280 388 70 ... 495 ENGLER DISTILLATIONS. Engler distillations 447 179 220 291 311 395 80 276 288 502 205 241 308 324 477 404 90 311 302 511 were made of all nine fractions. The results are 498 . . . 240 339 415 95 343 324 525 presented in Table 111. They were made in ac535 240 264 331 340 Find 379 379 418 532 Final 70 cordance with the A. S. T. M. method (serial .. . recovered. . . .. . 95 96 99 96 98 98 5 Residue, D86-21T). From these data the average boiling . . . 2 1 2 0.5 2 1.5 00. .. . .. , points of the various petroleum fractions were ... 0 2 4 2 0.5 .. . 0 LOSS, % .. . estimated by three different means as indicated Average Boiling Point, ' F. (" C.) 407.5 at the bottom of Table 111. 159.5 202 273 299 386.5 492 268 10,90% 237 (131.1) (208.6) (70.9) (94.4) (133.9) (148.3) (197) (255.6) (113.9) MOLALBOILINGPOINT.The molal boiling 296.3 384 401.7 151 198.3 268.3 10, 50, 233.3 269.7 490 point is an empirically corrected value of the five90% (111.8) (131.3) (205.4) (66.1) (92.4) (132.1) (146.8) (195.6) (254.4) 10, 30, point average A. S. T. M. boiling point which, 151.4 382.8 488.8 295 195.6 267.8 268 399 50, 70, 231.7 (146.1) (194.9) (253.8) according to Watson and Nelson (9), gives the (131.1) (203.9) (66.3) (90.9) (131) 90% (111) bdiling point of the individual paraffin hydrocarbon 7
~
INDUSTRIAL AND ENG.INEERING CHEMISTRY
808
OF PETROLEUM FRACTIONS TABLEIV. PROPERTIES
cut
No. 1 2 3
A B
C
D E F
Molal B. P.O 'F. 'C. 221 105 263 128 389 198 62 88 128 144 193 252
-Mol. WeightEstd. from DeterSp. Gr. molal b. p. mined (60/60°F.)
K
Unsaturates
% 104 116 157 83 95 116 124 154 198
,105 114 150
..
.. . .. . .. . .. .
0.722 0,669 0.704 0.743 0.749 0.785 0.762 0.783 0.812
12.2 12.6 12.3 12.1 12.0 12.111.9 12.112.1
2.5 6.0 6.5
...
.. . ... .. . ...
...
Corrections which are subtracted from the average boi!ing point of the A. 6. T. M. Engler distillation were read from the chart given by Watson and Nelson (9). 0
,paper were estimated. lowing relation :
VOL. 30, NO. 7
These were calculated from the fol-
%naphthenes = - (100 - U) N - P where U = unsaturates, 7S = sp. gr. of petroleum fraction a t 20/4' C. N , P = sp. gr. at 20/4 O C. of pure paraffins and pure naphthenes, respectively, as read from Figure 3 for temp. equal to av. boiling point of petroleum fraction Table V summarizes the results of the investigation of paraf fin-naphthene ratio. RATIOSFOR SOLVENTS TABLEV. PARAFFIN-NAPHTHENE
mated values are compared with the determined molecular weights and are shown to agree within 1 to 4 per cent. According to Watson and NelCHARACTERISTIC FACTOR. son (9) the characteristic factor or paraffinicity constant is a definite criterion of the nature of a petroleum fraction. The characteristic factor K may be defined as the cube root of the temperature of the molal boiling point in degrees Rankine divided by the specific gravity of the fraction a t 60/60° F. (15.6/15.6" C.). The authors show that the constant for pure paraffins falls between 12.5 and 12.8; for benzene it is 9.8. Table IV gives the specific gravities and characteristic factors for the nine solvents under investigation. The specific gravities were determined by the plummet immersion method and by means of A. P. I. hydrometers. TOTALUNSATURATES.The total unsaturates present in solvents 1, 2, and 3 were determined by the standard inspection method (10) ; 66' BB. sulfuric acid was used. These data are also included in Table IV.
Paraffin-Naphthene Ratio The following method was proposed by Morgan and Soule (6) for the determination of paraffin-naphthene ratio in petro-
leum distillates.
It is based on the fact that the densities
vs. boiling points of the residual oils, after the removal of u n s a t u r a t e s , offer a method of approximating the p r o p o r t i o n s of paraffins and naphthenes. Approximately 300 cc. of oil were treated with con120 180 240 300 centrated sulfuric BOILING POINT, 'C. acid a n d oleum FIGURE3. SPECIFICGRAVITYvs. until all unsatuBOILINGPOINTFOR DETERMINATION r a t e s were reOF PARAFFIN-NAPHTHENE RATIO moved. The oil was then washed with water and neutralized with 5 per cent caustic solution. It was filtered dry and carefully measured into a distilling flask. Upon distillation, a record was kept and the distillate was divided into four fractions. The gravity of each cut was determined, and the average boiling points were calculated from the distillation data. Figure 3, where the specific gravities a t 20/4O C. of the pure paraffins and pure naphthenes are plotted against their corresponding boiling points, was constructed from data taken from the literature. By means of it the paraffin-naphthene ratios of the petroleum fractions employed as solvents in this
Solvent No. Av. sp; gr. (20/4' C.) B. Q., C . Av. sp. gr. (20/4" C . ) : Paraffins Naphthenes Paraffins, yo Naphthenes, %
1 0.7214 122.1
2 0.7375 134.7;
3 0.7742 207.8
0.7012 0.7615 64.9 32.6
0.7090 0.7670 47.7 46.3
0.7455 0.7977 42.1 51.4
Variables Affecting Solubility A review of theoretical concepts shows that three prime variables affect solubility-namely] equilibrium temperature of solution, molecular weight of solvent, and molecular weight of solute. The effect of changes in pressure upon solubility of solids in liquids are negligible and, where ideal solutions a r e concerned, the chemical nature of the solvent may be disregarded. Figures 1 and 2 show that there are functional relations between molecular weights and melting points of waxes and between molecular weights and average boiling points of solvents. Therefore] these quantities were substituted as variables in place of the respective molecular weights, and a relation was developed in terms of them and of the solution equilibrium to express paraffin wax solubility. Pressure as a variable was neglected] and the chemical nature of the solvent was held essentially constant by careful selection of the petroleum fractions. The cloud point method was employed to determine the. paraffin wax solubilities in the various petroleum fractions. This method has been used on a number of occasions by other, investigators (3,8)and has been accepted as reasonably accurate. The method consists essentially in dissolving a definitely known quantity of wax in an accurately determined volume of solvent and then slowly chilling the solution to the point where finely dispersed wax crystals first begin to appear. as a cloud in the clear solution. This temperature is taken as the equilibrium point or solubility temperature for the wax solution. In order to express the solubilities in comparative. terms, all solubilities were converted to grams of wax dissolved per 100 cc. of solvent.
Solubility Determinations The solubility determinations were made in 1 by 8 inch tubes,. securely sealed by a No. 5 one-hole rubber stopper, through which a calibrated thermometer, ranging from 0 t o 100 C. and graduated in tenths of a degree, was inserted. The range of wax solubility was studied between 5 and 40 C.. Accurately weighed samples of wax were prepared, in most cases approximating 1, 3, 7, and 15 grams in weight. The solvents were carefully measured into the test tubes containing the wax by means of a 25-cc. and 50-cc. calibrated pipet. After addition of the solvent, the test tubes were stoppered, the wax was dissolved by being warmed in a water bath, and the cloud point was determined. The precipitated wax was then just barely redissolved by a careful application of heat and the cloud point again determined. This procedure was repeated in orderO
O
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JULY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
100 80
100
60
60
40
40
20
20
10
10
6
6
4
4
2
2
809
ao
a
a
1.0
1.0
0.0
0.8
0.6
0.6 ',A' 0.4 /'
/ 1 /
'
0.4
0
20
20
10
10
10
8
8
8
6
6
6
ci 20
,
,ly /I
/
10
,
SOLVENT 1
20
30
3:
40
c, 0
2
10 8
6 4
2 1.0 0.0
/SOLVENT 0
10
20
30
40
0
10 20 3 0 40 TEMPERATURE," C.
FIGURE4. SOLUBILITIES OF PARAFFIN WAXES IN VARIOUSSOLVBNTS
F
INDUSTRIAL AND ENGINEERING CHEMISTRY
810
a
-.I
g
VOL. 30, NO. 7
-40
&
J 20
TEMPERATURE,' C.
FIGURE 5 . SOLUBILITIES O F PARdFFIN WAXES A AND G IN SOLVENT 1
MELTING POINT OF WAX,'C. WAXSOLUBILITIES AT FIGURE 6. PARAFFIN 25" C. us. MELTINGPOIKTS
2i
..?m
2 3 0.1
i
i
32 0 340 360 100,000X R E C I P R O C A L ABS.TEMP.
t o obtain several readings. To FIGURE7. MOLECULAR SOLUbe acceptable, three cloud point BILITY us. RECIPROCAL ABSOLUTE values were required t o agree TEMPERATURE FOR SOLVENT1 within 0.1" C. of their mean cloud point temperature. From four to eight solubility determinations were made with each of the solvents 1, 2, 3, B, and C, with the five grades of wax; and in the case of the additional solvents A, D, E, and F, two points were determined with each wax. The data for the wax solubility determinations are presented graphically in Figure 4. Figure 5 represents the results of solubility determinations on mixed waxes A and G in solvent 1. It indicates that the characteristics of the temperature-solubility functions as determined b y cloud point a r e not altered appreciably by the width of the wax fraction involved. Wax A was prepared by blending equal portions by weight of the five original samples; G was made up of equal proportions by weight of the 125" and 148" F. nominal grades (Table 11). It is interesting to note that the densities of the various waxes under investigation are approximately the same. Therefore, blends can be made either by weight or volume and remain equivalent.
W a x Solubility WAX SOLUBILITY A T 2 5 O C . FIGURE8. PARAFFINWAX SOLUBILITIES AT 25" C. us. AVERAGE BOILING POINTOF SOLVENT
0 4 WAY SOLUBILITY,
8
12
G./ 100 C C. SOLVENT
FIGURE9. SOLUBILITY OF WAX OF 55.6" C. MELTINGPOINTos. AVERAGEBOILINGPOINT OF SOLVENT
Application of the theory of least mean squares (4) to the data indicates that the slope, M, of the solubility lines in Figures 4 and 5 is essentially constant. Thus, a mathematical analysis supports t h e graphical demonstration that the temperature-solubility functions are parallel. The maximum variation of the values of M is from 0.0523 to 0.0602. The mean value was estimated as 0.0556 * 0.001. It is worthy of comment that there was no systematic variation of M. Of the forty-seven values treated, only thirteen were more than 2.5 per cent from the mean and only three were more than 5 per cent from the mean; the maximum variation was 8.3 per cent. If the true slope of the semilogarithmic temperature-solubility functions is 0.0556, as has been estimated, the solubility of paraffin wax in straight-run petroleum fractions doubles with each 5.4" C. (9.7" F.), increase in temperature. The effect of wax melting point on solubility is shown in Figure 6. The various lines are lettered to indicate the solvent used. Treatment of these data by least mean squares shows the slope of the functions to be 0.0560, or essentially equal numerically to the slope of the temperature-solubility functions but opposite in sign. The negative value indicates a n inverse function. From these findings, it can be shown that solubility doubles with each 5.4" C. decrease in melting point of the wax. The experimental data obtained from the solubility of the various waxes in solvent 1 were converted to molecular solubilities and plotted against the reciprocal of the equilibrium absolute temperatures as presented in Figure 7. Linear functions as expected from theoretical consideration result.
JULY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
811
and m, 45" to 70" C. The calculated solubility values agree well with the estimated solubilities interpolated from the experimentally established data. The average deviation of calculated values from experimental determinations may be regarded as about 5.0 per cent. The maximum variation of forty random values shown by these calculations is 15 per cent; fourteen values are within 3 per cent. This equation is graphically presented for more convenient application as Figure 10, the use of which is as follows: From the point on the left-hand ordinate which represents the average boiling point of the solvent, move horizontally to the right until the proper paraffin wax melting point line is intersected. These lines have a negative slope. Pass a vertical line through the point of intersection and follow it up or down until the proper solution equilibrium temperature line is intersected. These lines have a positive slope. Then pass horizontally to the right-hand ordinate where the paraffin wax solubility in grams of wax per 100 cc. of solvent is given.
FIGURE 10. CHARTFOR PARAFFIN WAX SOLUBILITY IS PETROLEUM FRACTIONS
As shown in Figure 8, the solubilities of the various paraffin waxes at 25" C. were plotted against the average boiling points of the solvents. The data for each grade of paraffin wax approximate a straight line. These lines when projected appear to converge a t one point. This imaginary point of zero wax solubility (it is not intended to suggest that this really exists) appears to lie at about 377" C. average boiling point of solvent. This point corresponds to the boiling point of the paraffin hydrocarbon docosane, C22H46. Each of the lines radiating from the imaginary point of zero solubility takes a definite position and slope which is a function of the wax melting point. Since the solubility of paraffin wax increases with rise in equilibrium temperature, it is readily seen that the solubility lines tend to spread open in a counterclockwise direction with increase of temperature, and to close towards the vertical position as temperature decreases (Figure 9). It lias been qualitatively stated in the literature (8) that wax solubility in petroleum fractions increases as wax melting point decreases and as the average boiling point of the petroleum fraction decreases. However, as a result of the developments in this paper, i t is now possible to express the solubility of any paraffin wax in any predominantly nonaromatic saturated petroleum solvent quantitatively by the equation: Y
=
-0.3367(X)1.1357(m-1)
+ 377
or X = (1120 - 2.97Y) 1.1357P-m) where X = wax dissolved in 120 cc. of solvent, grams Y = av. solvent b. p., C. (based on temps. at 10,30,50, 70, 90% poin?, A. S. T. M. distillation) m = m. p. of wax, C.
t
=
solution equilibrium temp.,
O
C.
This equation holds well within the following limits: Y , 60" to 300" C.; t, 0" to within 10°C. of the waxmelting point;
Solubility Coefficient In order to study the solubility of paraffin wax with relation to the nature of the hydrocarbon solvent, the solubility of wax with a melti n g p o i n t of 55.6" C . was determined in benzene, toluene, xylene, and in a 3: 1mixture of solvent 2 and benzene. The latter yielded a mixture of average molecular weight equal to that of solvent 1. The data for these solubilities are set forth graphically in Figure 11. At 31" C. the solubility of paraffin wax in benzene is the same as the wax solubility in toluene and xylene.
0
20
40
TEMPERATURE OC. FIGURE 11. SOLUBILITY OF PARAFFIN WAX OF 55.6' C. MELTING POINT IN VARIOUS SOLVENTS
812
INDUSTRIAL AND ENGINEERING CHEMISTRY
Toluene and xylene have almost identical solubilities for the wax throughout the entire temperature range investigated; that of benzene is less below 31" C. and greater above this temperature, than the corresponding solubility on the toluene and xylene curves. In other words, a t 31" C. these three aromatic solvents yield identical solubility for the wax in question; toluene and xylene yield the same percentage change in solubility with temperature; that for benzene is considerably greater. Figure 11B shows that, although the mixed solvent contained 25 per cent of benzene and 75 per cent of petroleum fraction 2, it took the characteristic slope of the petroleum solvent and was not altered in respect to slope by the benzene. Also, the mixed solvent showed a higher capacity to dissolve the wax than did petroleum fraction 2. This was probably due to the fact that the average molecular weight of the mixed solvent was 105 whereas that of solvent 2 was 114. The solubility coefficient or, more explicitly, the thermal logarithmic solubility coefficient, K , may be defined as the rate of change of the logarithm of solubility in relation to the rate of change of temperature. That is to say, it is the differential of the logarithm to the base 10 of solubility expressed in grams per 100 cc. of solvent in respect t o temperature in degrees Centigrade, and may be expressed as :
-
tl)
The solubility coefficient of the petroleum solvents was carefully worked out to be 0.0556 by least mean squares. For the aromatics and the solvent composed of benzene and solvent 2 in the ratio of 1 to 3, the solubility coefficients were determined graphically to be as follows: benzene, 0.0787; toluene, 0.0627; xylene, 0.0627; 3:l mixture of petroleum solvent 2 and benzene, 0.0556; and petroleum solvents 1 and 2,both 0.0545. The solubility coefficients for solvents 1 and 2 were also determined by least mean squares to be 0.0570 and 0.0539, respectively. It is apparent that the more desirable solvents for dewaxing petroleum stocks 19,000 20,000 21,000 are those with a MOL. H E A T O F FUSION,CAL./G.MOLE relatively high FIGURE 12. HEAT OF FUSION OF PARAFFINWAX os. MOLECULAR coefficient of solubilit,y. On t h k WEIGHT AND MELTINGPOINT
VOL. 30, NO. i
account benzene has been named in a number of patents on dewaxing ( 2 ) . Figure 11 shows that the addition of one methyl group to the aromatic nucleus tends to cause the resulting toluene to assume the solubility characteristic or coefficient approaching that of the straight-chain hydrocarbons. The dominating influence of the straight-chain hydrocarbons is distinctly demonstrated in Figure 11B,which shows that the addition of as much as 25 per cent by volume of benzene to solvent 2 does not alter the solubility coefficient of the latter appreciably.
Heat of Fusion of Paraffin Wax Sakhanov and Vassiliev (7) found that heats of fusion of paraffin waxes calculated from solubility curves checked accurately with experimentally determined calorimetric values. Sakhanov and Vassiliev state that when they used a wax of average molecular weight 400, they obtained a molecular heat of fusion of approximately 21,000 calories, which agrees well with the calculated values determined from the present work. The slopes of the molecular solubility functions were estimated from the data presented in Figure 9. These slopes have been shown to be equal to L/(2.303 X 1.99), where L is the molecular latent heat of fusion. Values computed by means of this relation were plotted against their respective molecular weights and melting points as shown in Figure 12. I n B the data approximate a linear function. Only one point 4 and B (that for wax of melting point 55.6" C.) of Figure 12 ' deviates appreciably from the smooth curves presented.
Molecular Solubility of Paraffin Wax When solubility is expressed as mole fraction, the solubility of a given wax in all the petroleum solvents employed here is the same. Moreover, the solubility of all the waxes at a given temperature below the melting point is the same. These statements are borne out by Table VI, columns 3 and 4, respectively. This phenomenon explains the (m - t ) exponent which occurs in the empirical expression for wax solubility. TABLE VI. PARAFFIN WAX SOLUBILITIES AT 39.4" C. BELOW MELTINGPOINTOF THE VARIOUSWAXES,IX TERMSOF MOLECULAR RATIO
THE
Mol. Ratio for Av. Soly. of Mol. Weight Nine Solvents (m t i Temp., of Wax At 25' C." At(m t)"C. C. 0,0235 10.5 323 0.1470 49.9 0,0203 13.4 52.8 341 0,0892 0,0201 16.2 55.6 353 0.0622 0,0186 20.9 60.3 376 0.0314 0.01so 25.0 64.4 408 0.0180 For example, 0.0892 was an a The figures in this column are averages. average of the wax solubilities for each of the nine solvents. The minimum figure was 0.0766,and the maximum 0.0962. The average deviation was *5.84 per cent. While there was no absolute trend, i t appears in most cases that the value decreased slightly with increase i n boiling point of the solvent. Wax M. P.,
-
c.
-
Literature Cited Am. SOC. Testing Materials, Standards, Part 11, p. 836, Philadelphia, 1933. Glover (to Indian Refining Co.), French Patent 682,330 (1929). Henderson and Ferris, IKD.ENG.CHEM.,19, 262 (!,927). Lipka, "Graphical and Mechanical Computation, pp. 124-7, New York, John Wiley & Sons, 1918. Menzies and Wright, S. A m . Chem. SOC.,43,2314 (1921). Morgan and Soule, Chem. & Met. Eng., 26, 977 (1922). Sakhanov and Vassiliev, Neftyanoe Slantsevoe Khoz., 6,820(1924). Sullivan, McGill, and French, IND. ENG.CHEM.,19, 1042 (1927). Watson and Nelson, Ibid., 25, 880 (1933). Wilhelm, New and Revised Tag Manual for Inspectors of Petroleum, 18th ed., p. 91, Brooklyn, C. J. Tagliabue Mfg. Co., 1923. Ibid., p. 104. RECBIVH~D September 9,1937.
