IN D U S T R I A L A N D . E N G IN E E R IN G C H E M 1 S T R Y
May, 1946
oxygen in 50-50 acetone-ethanol solutions are from 2 to 470 lower than the ones calculated from the data for the individual components; the reverse is true for the 50-50 solution of iso-octane and ethanol. For many practical purposes such a n error may not be significant, and the solubility of air in these solutions may therefore be calculated from the data given. Figures 2 and 3 are plots of the logarithm (base 10) of the mole fractions of nitrogen and oxygen against 1f T when the partial pressure of the gas is one atmosphere. From the slopes of these curves the heats of solution may be calculated from the equation:
TABLE 111. HEATSOF SOLUTION AT 25 C. Nitrogen, cal./mole 110 310 120 220 390 560 190 510 190
Solvent Abs. ethanol 95% ethanol Methanol Isopropanol n-Butanol Acetone Iso-octane Acetone ethanol Iso-octane ethanol
++
TABLEIV.
Solvent Methanol Ethanol Acetone
Oxygen, cal./mole 290 140 240 -250 - 290 120 -230 80 -270
-
COMPARISONOF OSTWALDCOEFFICIENTSWITH THOSE OF OTHERINVESTIGATORS Gas
N2
N2 N2 N2
N2
0 2
Temp.,
C. 25 25
-25 0
25 -25
01
0
02
25
-0stwald Present investigation 0.1645 * 0.1489
0.1340 0.1554 0.1816 0.2390 0.2570 0.2794
Coefficient Just 0.1415 0.1432
....
....
0.1460
,...
.... ....
so9
-
Horiuti
.... ... I
0,1336 0.1553 0.1795 0.2357 0.2550 0.2791
d In N / d T = AH/RT2
The values for A H , the heat of solution per mole a t 25 a C. (Table 111),range from +560 calories for nitrogen in acetone t o -290 for oxygen in ethanol, an indication that heat may either be absorbed or evolved in this process. Except for isolated values only a few data are available with which the present values may be compared. The comparison in Table IV shows good agreement with those of Horiuti (4) while those of Just (6) are too low. Presence of water might have been responsible for his low results. ACKNOWLEDGMENT
individual measurements from the mean was 0.3%; maximum deviation amounted to 0.9%. This agrees well with the a priori estimate of the probable error of 0.5%. Table I1 shows the experimentally determined data as well as the calculated values for the solubility of air. I n this case the aolubility of argon was assumed to be equal to that of oxygen since the work of Lannung (7) showed that such an assumption will introduce only a slight error. The solubility of air in all the alcohols as well as in acetone is considerably less than in iso-octane. The decided reduction of &hesolubility of air in 95% ethanol compared with pure ethanol checks Just’s measurements (8) of the solubility of nitrogen in methanol-water solutions. He found that practically all the drop occurred between 0 and 50% water. The experimentally determined values of 7 for nitrogen and
?he authors wish to thank Lyle C. Woods for assistance in making these measurements. LITERATURE CITED 1) Brunel, Crenshaw, and Tobin, J.A m . Chem. SOC., 43, 561 (1921). 2) Clarke, Robinson, and Smith, J . Chem. SOC.,1927,2647. 13) Fieser, “Experiments in Organic Chemistry”, p. 363, New York, D. C. Heath and Co., 1941. (4) Horiuti, Juro, Sci. Papers Inst. Phys. Chem. Research (Tokyo), 17, No. 341,125-256 (1931). (5) International Critical Tables, Vol. 111,pp. 33, 218 (1928). (6) Just, Gerhard, 2.physik. Chem., 37,’342 (1901). (7) Lannung, Axel, J . A m . Chern. SOC.,52, 68 (1930). (8) Lund and Bjerrum, Ber., 6 4 , 2 1 0 (1931). (9) Osborne, Natl. Bur. Standards, Bull. 9, 327 (1913). (10) Walden, Ulich, and Laufl, 2.physik. Chem., 114, 275 (1925). (11) Wojciechowski, J . Research Natl. Bur. Standards, 17, 721 (1936), [ f
Purification of Commercial Benzene bv AzeotroDic Distillation I J JOHN GRISWOLD AND R. H. BOWDEN’ The University of Texas, Austin, Texas Nonaromatic hydrocarbon impurities were separated from an “industrial pure” coke-oven benzene by azeotropic distillation with acetone. From a fractionation analysis, the impurities were found to consist chiefly of cyclohexane, C, paraffins, and naphtherres, in which n-heptane and dimethylcyclopentanes predominate. The approximate purity of the original benzene was 98.5%; it is shown that benzene of approximately 99.7% or higher purity may be readily prepared by azeotropic distillation of the commercial material with acetone.
A
SUPPLY of benzene was recently needed which would be
pure enough for experimental work on fractional distillation. Commercial preparation of the specification grades of benzene from by-product coke-oven light oil includes acid treat1
Present address, Magnolis Petroleum Company, Beaumont, Texas.
ment and fractionation, which remove most of the nonaromatic impurities from the oil. However, near-boiling nonaromatic hydrocarbons form nonideal solutions and, in some cases, azeotropes with benzene (6); this behavior prevents complete elimination of such compounds by regular distillation. Nonideal hydrocarbon mixtures (including azeotropes) may be separated by various other methods, including distillation in the presence of a selective extraneous component. Azeotropic distillation is one of the easiest t o carry out, and ita principles have been discussed (9). Its mathematical aspects have also been presented (2, 3 ) . The process is applicable to the separation of cyclohexane and impurities from benzene (4), and it is the basis of two methods using methanol and methyl ethyl ketone, respectively, for the purification of toluene from petroleum fractions ( 2 , S , 8). The work reported here is an introductory study of the =eotropic purification of a commercial coke-oven benzene with ace-
INDUSTRIAL AND ENGINEERING CHEMISTRY
510 81
TO
1
1.5010
1 BENZENE-
1
W O
JY1.5000
I w 1.4990-
ANALYTICAL OF
ORIGINAL
DISTI LLATION BENZENE
118O
Vol. 38, No. 5 The third fractionator was a Podbielniak Heligrid column, 11 mm. i l l diameter. A t the vapor velocities used, thc column tested over sisty equivalent theoretical plates on heptanemethylcyclohexane at total reflux (11). Refractive indicc7s (n2: and n2i) were a determined with Bausch & Lomb prcvision oil refractornetor,. FRACTIONATIOS PROCEDURE
An analytical fractionation was made 011 the benzene, as IeI 2 1.4980 wived, in the Hcligrid I column with total re0 10 20 30 40 50 80 70 80 90 IC flux and intermittent VOLUME Yo D I S T I L L E D sample take-off. The results, plotted in Figure Figure 1 1, indicate the prcsenco of both low-boiling and 4 7 GAL. high-boiling impurities of relatively low rcfraclivc: (178 LITERS) index. The purification operations arc slioivn diaI grammatically in Figure 2. ( 4 GAL The plate column was charged with 51 gallon-: (193 liters) of benzene, and 47 gallons (178 liters) were taken overhead with a reflux ratio between 7 and 8 t o 1 and continuous product takeoff. Thcl refractive indices of overhead and bot toms were ' ACETONE 5 GAL, i ''ET0":C not significantly different from that of I Ii:, original ?NO PURE' 0 a vIATEK benzene, an indication that no substantial c1irnin:aBENZENE 2I I 51 GAL. tion of high boiling impurities had occurred. (193 LITERS P 96 ML. The column was drained and then charged with NON-AROMATIC HYDROCAR EON 5 the redistilled benzene and 5 gallons (18.95 liters) BTM S BT M S. 99.7V0 BENZENE of acetone. The aaeotropic distillation was con4 GAL. 38 GAL. (15.2 LhERS) (144 LITERS) ducted as follows: A boiling rate of 0.37 to 0.54 gallon (1.4 to 2.05 liters) per minute was maintairied under Figure 2. Summary of Purification Procedure total reflux, and a 2-quart (1.9-liter) sample of the overhead condensate was withdrawn evcry hour. The initial distillate x-m very high in acetone. The acetone contone, and includes an analysis of the separated impurities. The centration in the distillate decreased as the run proceeded, and benzene was "industrial pure" (2") grade, donated by the Kopwas substantially all distilled TI-hen about 14 gallons (63 liters) pers Company; it tested zero bromine number. The acetone of distillate had been taken out. The bottom product was ahout was "pure" grade, purchased from the General Chemical Com99% pure, as calculated from refractive indices. pany, and was used without further purification. The first eight samples (4gallons) of distillate consisted niosbly Three fractionators were used in the course of the viork. The of acetone and came oyer below 70" C. These cuts wcre com6rst was a 6-inch-diameter twelve-plate column with a 55-gallon bine4 and fractionated in tiTo batches in the packed laboratory steam-heated still. Each plate contained two 2.5-inch Vulcan column. I n these two distillat'ions, a reflux ratio ( L I D ) of about bubble caps with a baffle between. The plate construction \yas 15 to 1 was maintained with continuous take-off. When watersimilar to that described by Huffman and Treybal ( 7 ) . The washed free of acetone, the first 110 ml. of distillate yielded 30 plate spacing was 12 inches. Reflux and product streams were ml. of nonaromatic hydrocarbons. As the distillation proceeded, measured by Rotameters. Temperatures were measured by the percentage of hydrocarbons decreased, and the distillation iron-constantan thermocouples with a low-range sensitive pomas discontinued when no hydrocarbons appeared in the distentiometer, giving an accuracy of about =t0.5" C. Subsequent tillate. The benzene left in the still was about 99.7Y0 pure by tests showed an average plate efficiencyof 65y0 on benzene-toluene calculation from its refractive index (Table 11). A total of 96 at total reflux. ml. of nonaromatic hydrocarbons was recovered by water washThe second fractionator was a laboratory column constructed ing the entire distillate from the laboratory column. This maof 1-inch standard pipe and packed with 48 inches (122 cm.) of terial was charged to the Heligrid column with 25 ml. of aniline '/s-inch (3-mm.) single-turn helices. The bottom half of the as bottoms. It was fractionated into approximately 1-ml. porpacking wm of Nichrome and the top half of glass helices. tions, using total reflux and intermittent take-off. The over-all Over the range of vapor velocities used, the column tested from reflux ratio was about 40 to 1, and a sufficiently sharp separation nineteen t o twenty-one equivalent theoretical plates on heptanewas obtained to permit analysis of the constituents. The boiling m6thylcyclohexane at total reflux. I
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May, 1946
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
511
point curve (corrected to 760 mm.) and the refractive index curve (25" C.) of the cuts are plotted on Figure 3, with values for the pure compounds indicated ( 5 ) . This figure shows no indication of the presence of benzene or of olefins. The presence of olefins w~ts not exOF IMPURITIES FROM BENZENE pected since the bromine number of the original benzene was zero, indicating complete olefin removal in the refining operations. A quantitative analysis was obtained from Figure 3, using the general method described by the National Bureau of Standards (6). The results are given in Table VOLUME *A DISTILLED I. The distillation loss probably acFigure 3 I I I I I crued entirely from the handling of the small cuts; hence, the analysis is based on the recovered volume. ferences in fracAccording to Table I, the sample contained about 60% paraffins tionating ability of OC by volume. the two columns. In the pregent COMPOSITION OF IMPURITIES sample the followA distillation on similar material obtained from nitrationing paraffin and grade (1") benzene by fractional crystallization and silica eel adnaphthene hydrosorption was recently reported by Anderson and Engelder (1). carbons boiling beWhile they did not attempt a quantitative analysis of the nontween 70' and 100O MOL FRACTION BENZENE IN LQUID aromatic impurities, their boiling point and refractive index C. are probably absent: trimethylFigure 4. Relative Volatility, a curves are in fair agreement with Figure 3, allowing for the difof Acetone to Benzene b u t a n e , 2,2-dimethylpentane, 2, P-dimethylpentane, 3,3-dimethylpentane, methylcyclopentane, TABLE I. COMPOSITION OF'NONAROMATIC IMPURITIES SEPARATED and 1,l-dimethylcyclopentane. The presence of a small amount of FROM BENZENE BY AZEOTROPIC DISTILLATION WITH ACETONE 3-ethylpentane is uncertain and is indicated only by the slope of Normal B.P., O C. % by Vol. the boiling point curve. Not more than 1% of cis-1,2-dimethylHexanes 5848.7 2 cyclopentane is present. Cyclohexane 80.74 14 2,3-Dimethylpentane 89.8 5 A considerable proportion of the impurities probably remained ZMetLvlhexane 90.05 fra~a-l,3-Dimethyloyclopentane 90.8 39 in the acetone-water wash and were lost t o the final material. ttane-1 2-Dimethylcyclopentane 91.9 The last traces of benzene were probably eliminated from the 3-Met&lhewne 91.96 4 93.47 3 (max.) impurities in the same step. The solvent action of the acetone 98.43 21 99.24 10 tends to remove more naphthenes than paraffins; hence the analytical figure of 60% paraffins in the impurities is probably 100.9 2 somewhat higher than the true value for the impurities in the benzene. TABLE 11. REFRACTIVE INDICES AND CALCULATED PURITIES OF MATERIALS
.
f
Purit vol. 1.49800 1.5012' 100 1.39521 1.3976b 0 1.49647 1.49966 98.5 1.4970 1.5002b 99.0 1.49773 i . 6 o o ~ a 99 7
ng
Pure benzene Nonaromatic impurities Original sample of "ladustrial pure" benzene Bottom product, plate column Bottom product, lab. column a Average of several modern determinations. Calculated from valuea a t 25' C.
*
ny.
&
PURITY OF PRODUCTS
The purities calculated by assuming refractive index linear with volume fractions are summarized in Table 11. The vapor-liquid equilibrium of acetone-benzene (IO) wm calculated over to relative volatility and is given in Figure 4, It wm determined from these data that 18.5 theoretical plates a t total reflux are required t o separate a benzene mixture con-
512
INDUSTRIAL AND ENGINEERING CHEMISTRY
taining 10 mole yGacetone into acetone containing 0.1 mole yo benzene. This explains why pure benzene mas not obtained in the twelve-plate column, which contained seven to eight theoretical plates. The charge to the packed laboratory column (tlventg equiva,lent theoretical plates) consisted of nonaromatic impurities, benzene, and a high percentage of acetone. No benzene wa,s found in the distillate, of which the initial portion consisted of 27.3 volume yo or approximately 58 mole yo nonaromatic hydruocarbons. The foregoing considerations shoTy that benzene of high purity may be made by azeotropic distillation of a commercial material with acetone in a column of twenty or more equivalent throretical plates. Also, 10% of acetone in the charge will eliminate 27, of nonaromatic impurities in a twenty-theoretical-plate column when using an adequate reflux ratio. The same produrt should
Vol. 38, No. 5
be obtainable with a somewhat lower reflux ratio by charging a higher proportion of acetone. LITERATURE CITED
Anderson and Engelder, IXD.ENG.CHEM.,37, 541 (1945). Benedict, Johnson, Solomon, and Rubin, Trans. Am. Inst Chem. Enors., 41, 371 (1945).
Benedict and Rubin, Ibid., 41, 352 (1945). Field, Edmund, U. S.Patent 2,302,608 (Nov. 17, 1942). Forsiati, Willingham, Jlair, and Rossini, Proc. Am. Petroleum Inst., 24, 111, 34 (1943) ; J . Research Natl. BUT.Standnrds. 32. 11 (1944). Griswold and Ludwig, ISD. E N G .CHEM.,35, 117 (1943). Huffman and Treybal, IND.E N G .CHEM.,AXAL.ED., 12, 74& (1940). Lake, G. R., Trans. A m . Inst. Chem. Engrs., 41, 327 (1945). Mair, Clasgow, and Rossini, J . Research NatZ. Bur. Standards. 27, 39 (1941). Othmer, D. F., IXD.ESG. CHEM.,35, 614 (1943). Podbielniak, R. J., IND.EXC.CHEM., A X L LED., . 13, 63') (1941)-
Paper Capacitors Containing Chlorinated Impregnants MECHANISM OF STABILIZATION1 L. EGERTON -4ND D. A. AICLEAN Bell Telephone Laboratories, Inc., M u r r a y Hill, Y. J . T h e stabilization of paper capacitors containing chlorinated aromatic impregnants with small quantities of organic additives is well established commercially. Although for practical reasons anthraquinone was chosen for initial commercial application, other quinones are also effective, as are the nitroaromatics, maleic anhydride, and sulfur. Evidence is given that the mechanism of stabilization consists of the formation of barrier films on the electrodes. These barrier films, which in certain cases maj cover only the active points on the electrode surface, reduce the catalytic decomposition of the chlorinated impregnant by the electrode metal, prevent attack of the, electrodes by liberated hj-drogen chloride, and hinder electrolytic action. It appears likely that the film-forming properties of the stabilizers are dependent upon their oxidizing power. A secondary effect of stabilizers may be the formation of complexes w-ith aluminum chloride to diminish the activity of the latter or change the nature of the reactions which it induces. Conductivity measurements in HC1-saturated chlorinated diphenyl containing soluble additives are in line with known hydrogen-bonding tendencies of the additives. Compounds which are stronr organic bases do not stabilize capacitors.
T
HE paper capacitor is probably unique with respect t u tht severity of operating conditions imposed on a,n organic dielectric. For example, a common type of one-microfarad capacitor has about 6 square feet of electrode area and a dielectric only 0.0009 inch (23 microns) thick. This large area of very thin dielectric must withstand voltages of 300-600 volts €or long periods of time a t operating temperatures up to about 90" C. This performance must characterize 100% of the area; a minute failure a t any point over the 6-square-foot area short-circuits the t>woelectrodes and thus renders the capacitor worthless. 1
The fimt paper in this series appeared in January, 194.5 (91.
In one popular type of paper capacitor the dielect,ric is paper impregnated with chlorinated naphthalene or chlorinated diphenyl, occasionally in admixture with other chlorinated arornat'ics, and the electrodes are thin aluminum foils. Because it has someiThat greater stability at high temperatures and because, being a liquid, it is not subject to formation of fissures at low temperatures, chlorinated diphenyl is preferred for thc. niore expensive hermetically sealed types which operate over a wide temperahre range. The solid chlorinated naphthalene is more adaptable to potted or other types of nonhermetically sealed capacitors which, in general, operate over a more restricted t'emperature range. The chlorinated impregnants are used bec'ause of their high dielectric constants which result i n high capacity, their nonflammability, and other desirable propert,ies. As has been pointed out, such capacitors are subject to a spccia? type of degradation on direct-current potentials ( I O ) , especially itt elevated temperatures. This degradation is characterized by a leakage current which rises with time on d.c. test, an inordinately short life, visible localized decomposition of the dielectric, and corrosion of the electrodes; none of these effects are appreciable on alternating-current potentials. This indicates that the deterioration has electrochemical aspects, which can also be deduced from the facts that the anodes are preferentially atracked, and that the paper layer next, to the cathode shows earliest and most severe decomposition. The deterioration under d.c. potentials has been explained as consisting of the following steps:
1. Under the conditions existing in the capacitor, small quansities of hydrogen chloride are split off from the impregnating compound. A principal factor stimulating this deterioration is the large ratio of electrode metal surface t o dielectric volume. The amount of hydrogen chloride formed by this primary reaction is small and probably would not of itself be serious in the absence of the following secondary reactions. 2. The hydrogen chloride is electrolyzed in the d.c. field to Form aluminum chloride at the anode.
