INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
172
LITERATURE CITED
(1) Baker, W.O.,IND.ENG.CHEM., 37,246 (1945). (2) Battista, 0.A.,Ibid., 42,502 (1950). (3) Brenner, F. C.,Frilette, V. J., and Mark, H., J . Am. Chem. Sor., 70, 877 (1948). (4) Conrad, C. C., and Scroggie, A. G., IND.ENG.CHEM.,37, 592 (1945). (5) Conrad, C.M.,Tripp, V. W., and Mares, T., paper presented a t the 117th Meeting of the AMERICAN CHEMICAL SOCIETY, Detroit. Mich. ( 6 ) Davidion,-G. F., J . Teztile Insl., 32, T132 (1941).
(7) Ibid., 34, T87 (1943). (8) . , Frilette. V. J., Hade. J.. and Mark, H., J. Am. Chem. SOC.,7 0 , 1107 (1948). (9) Hermans, P. H., and Weidinger, A., J. Polymer Sci., 4, 135 (1949). 10) Ibid., 4, 317 (1949). 11) Heyn, A. N. J., Testile Research J . , 19, 163 (1949). 12) Howsmon, J. A.,Ibid., 19, 152 (1949). 13) Ingersoll, H.G., J . Applied Phys., 17, 924 (1946). 14) Jorgensen, L.,Acta Chem. Scund., 4, 185 (1950).
or
ressures
0
J
Vol. 44, No. 1
(15) Mitchell, R. L.,IND.ENQ.CHEM., 43, 1786 (1951). (16) Morehead, F. F.,Teztile Research J., 20, 549 (1950). ENG.CHEM.,33, 1022 (1941); 34, 86 (17) Niskeraon, R. F., IND. (1942): 34.1480 (1942). (18) Niokerson, R: F., and Haberle, J. A., Ibid., 37, 115 (1945);38, 299 11946). --, (19) Ibid.i39, 1507 (1947). (20) Philipp, H.J., Nelson, M. L., and Ziifle, H. M., Tertile Research J., 17, 585 (1947). (21)Reeves, R.E., Schwartz, W. M., and Giddens. J. E., J. Am. Chem. SOC.,68, 1383 (1946). (22) Roseveare, W. E., Ibid., 53, 1651 (1931). (23) Roseveare, W.E.,Waller, R. C., and Wilson, J. N., Tertile Research J . , 18,114 (1948). (24) Sisson, W. W., and Saner, W. R., J. P h y s . Chem., 45,717(1941) (25) Staudinger, H., and Sorkin, M., Bw., 70B, 1565 (1937). (26) Waller, R. C., Bass, K. C., and Roseveare, W. E., IND.ENG. CHEM.,40,138 (1948). (27) Ward, K.,Jr., Textile Research J., 20,363 (1950). (28) Windeck-Schultze, K., and Peiper, I., Melliand Textilber., 29 (1948).
.--
RECEIVBD October 18, 1950.
istillation Some
J TRQY A. SCOTT, JR., DUlPilCAN MACMILLAN, AND EUGENE W. MELVIN Northern Regional Research Laboratory, Peoriu, Ill.
T
HE usefulness of distillation methods as applied to the Cia
fatty acid methyl esters has been principally in the preparation of samples for analysis (10). Although it is not difficult to separate saturated acid esters from one another, only a slight enrichment can be brought about by the fractional distillation of mixtures of the CISunsaturated esters. With Bonhorst, Althouse, and Triebold (3) having reported decomposition a t temperatures higher than 200" C., any hope of separation by distillation would be a t low temperatures and pressures. Thus, it was decided to determine the requirements for a still that was capable of fractionating the C1sunsaturated acid esters, and to extend the range of vapor pressure measurements to as low as 0.1 mm. of merCWY.
The boiling points of the methyl esters of some of the fatty acids, a t pressures down to 1 or 2 mm. of mercury, have been determined by Bonhorst, Althouse, and Triebold (Q),Norris and Terry (ii), and Althouse and Triebold (1). Bonhorst et al. (8) measured vapqr pressure by observing the pressure a t which the level of a liquid in a capillary fell. Norris and Terry (li), using a simple type of ebulliometer, reported boiling points to 0.5" C. Althouse and Triebold (i), using the dynamic method of Ramsay and Young, reported boiling pointa to 1O C. The three papers disagree on boiling points at certain pressures by as much as 4 or 5 degrees. All three papers show straight-line relationships between log p and the reciprocal of the absolute temperature.
ture a t which the plateau is found is taken as the correct boiling point. This procedure was followed in the work reported in this paper. Figure 1 represents the modification of the Hickman tensimeter used in this work. During the evolution of this instrument, both external and internal heaters were used. It was found that the combination of an internal heater of Nichrome wire and the large surface resulting from the widely flared bottom gave the smoothest boiling and the longest plateau in the plot of boiling temperature us. wattage. About 40 ml. of liquid are required to operate the tensimeter. The iron-constantan thermocouple used was calibrated a t 122 C. with the National Bureau of Standards benzoic acid apparatus (id),and a t the ice point. A copper-constantan thermocouple was not satisfactory because of heat loss through the copper wire. The electromotive force of the thermocouple was measured with a Leeds and Northrup potentiometer No. 8662. A wattmeter was used to measure the input to the tensimeter (8). Pressures were measured with a McLeod gage and an oil manometer; these were protected from possible contamination by a trap surrounded by a slush of ethanol and solid carbon dioxide. Methyl esters of fatty acids are not easy materials to work with and Hickman's apparatus was adopted after trying without success another type of ebulliometer, the gas saturation method, and the dew point method. Hickman's original design was better suited to pressures less than 0.1 mm. of mercury and the present authors' modification was more satisfactory a t higher pressures.
APPARATUS
I n the present study a tensimeter similar to one described by
Hickman, Hecker, and Embree (8) was used. These authors operated their tensimeter by applying an increasing wattage to the heater and making a plot of boiling temperature us. wattage. I n this plot, a plateau occurs which represents constant boiling temperature over a fairly wide range of wattage. The tempera-
PREPARATION OF MATERIAL
The methyl esters of the even-numbered fatty acids from caprylic through palmitic were obtained by careful fractionation of the esters made from coconut oil in a Podbielniak column with a 4-foot length of continuous helical packing. In the fractional distillation the products were collected in small portions; transition
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
173
The freezing points of the saturated esters, the calculated freezing points of the Repure esters, and the calculated purity are Refractive fraetive Index shown in Table I. In the case of methyl Index, ny (a), n%? stearate, high viscosity near the freezing 1.4148 point prevented determining a smooth 1.4237 1,4298 freezing curve, and the freezing point of 1.4350 1.4351 400 c 1.4330 (do0 c.) pure methyl stearate could not be calcu1.4363 t40° c:] 1.4362 (40' c.1 l a k d with certainty. Although the freering point is 0.9" C. lower than the literature value, a capillary determination agrees well with the latter value. The freezing Doint of 18.35"C. for methyl myristate, asfound by Garner and Rushbrooke (6),is difficult to reconcile with the value of 18.0" C., which the present authors calculated to be the freezing point of pure methyl myristate and which is believed to be accurate within 0.1" C. No freezing curve waa run on methyl oleate because the amount of sample was insufficient. Since debromination produces mixtures of cis and trans isomers, freezing curves were of no value in determining the purity of the unsaturated esters.
TABLE I. PHYSICAL CONSTANTS AND PURITY OF METHYL ESTERS
"-
Cala?. Calcd. Freeaing Methyl Freezing Pt. of Pure Ester Pt., e C. Ester, O C. &0.58/, Caprylate -37.27 -36.5 98 0 .. 5 5 Caproate -13.34 -13.0 9 Laurate 4.80 4.9 99.5 99.0 M 'state 17.86 Pagtate 29.27 29.5 18'0 98.5 Btearate a0 .go 38 95b 0 Capillary melting point, 37.4' to 38.0' C. b & 4%.
&fv
Literature Freezing Pt. --41 1 s to -40 5
56:89;37.78
(0)
34/45
VAPOR PRESSURE
For-each of the esters, a plot of log p us. 1/T gave a smooth, slightly curved line, showing that the simple Ciawius-Clapeyron relation, log p = A
4mm. O.D.
+ B-,T would not adequately describe the data.
LEADS TO LNTERNAL HEATER
wm. 8.5cm.
Figure 1. Tensimeter Used to Determine Boiling Points of Methyl Esters cuts were discarded and the esters used for further study were portions composited on the basis of constant refractive index. Methyl palmitate was crystallized from methyl alcohol before use. Refractive indexes reported in Table I are in good agreement with those reported by Althouse, Hunter, and Triebold (9). The metbyl stearate was prepared from technical grade methyl etearate by 12 crystallizations from methanol. Methyl oleate, linoleate, and linolenate were prepared from the bromides. This method of preparation gave a mixture of isomers but no attempt a t further pyrification was made because it is unlikely that the presence of isomers would have had much influence on the vapor pressure of tbe esters, as even in lower molecular weight isomers the difference is small (4). Methyl oleate had 8n iodine value of 85.3 (theoretical value, 85.6); methyl linoleate, 171.8 (theory, 172.4); and methyl linolenate, 259.8 (theory, 260.4). The sample of methyl stearate had an iodine valueof 0.1. FREEZING CURVES
To determine the purity of the esters, freezing curves were determined according to the m.ethod of Glasgow, Streiff, and Rossini (6),by using a platinum Pesistance thermometer calibrated by the National Burwu of Standards and a Leeds and Northrup Mueller bridge, Model 8069. A cryoscopic constant of 3.4"per mole WM obtained by meaaurjng the freezing point of methyl w r i s t a t e to which aumene had been added as solute. This constant was used for all cakulations because members of a homologous series have nearly idSn&d pryoswpjc constants.
20 9
I1
13
15
17
19
NUMBER OF CARBON ATOMS Figure 2. Boiling Points of Methyl Ester us. Number of Carbon Atoms in the Esters After the pressure range had been covered for each ester and a plot made of log p vs. 1/T, another boiling point determination was made a t an intermediate pressure. This final determination when plotted always fell in place on the curve, indicating no decomposition over the range of pressure and temperature covered. The vapor pressure data for each of the esters were fitted to an
INDUSTRIAL AND ENGINEERING CHEMISTRY
174
Vol. 44, No. 1
TABLE 11. EXPERIMENTAL AND CALCULATED BOILINGPOINTS AT VAE~OUELPRESSURES Methyl Caprylate Pressure,
texp..
' c.
Mm. H g 0.Y47 1.140 2.136 2.772 3.536 5.505 7.82 10.56
L__
0.2098 0.2182 0.618 0.672 0.968 1.132 2.302 3.039 4.92
33.69 39.84 49.26 53.41 57.45 64.94 71.16 76.76
tcalod.,
c.
33.76 39.78 49.27 53.42 57.40 64.94 71.21 76.79
Methyl Myristate 91.25 91.18 91.67 91.70 106.82 106.77 108.15 108.08 113.90 113.97 116.42 116.59 129 _ 08 ~ .~ . 129.30 134.87 134.66 144.63 144.54
Methyl Caprate texp. tcalod.,
c.
-0.20
Antoine equation: log p = A
+ t +B C' ~
-4.576 [E
c.
Methyl Laurate texp. todad.,
-
0.00 -0.07 4-0.06 +0.01 -0.14 +o. 01 +0.06 -0.07 -0.08 +0.01
-~ Methyl Palmitate
___
105.24 112.60 116.28 129.71 138.78 149.37 162.14 172.05
0.1283 0.2142 0.2739 0.639 1.085 1.938 3.710 5.936
105.34 112.63 116.14 129.66 138.78 149.34 162.20 172.07
0.1147 0.1916 0.1933 0.3829 0,3842 0.712 1,071 1.343 1.982 3.305 4.856
Methyl Linoleate 118.93 118.90 126.55 126.69 126.69 126.71 137.50 137.48 137.55 137.59 148.00 147.90 155.30 155.29 159.50 159.51 166.96 167.02 177.29 177.30 185.49 185.46
All calculated boiling
points were found to be within lltO.2" C. of experimental values. A comparison of calculated and experimental boiling points is shown in Table 11. The Antoine equation for each ester and the calculated boiling points a t various pressures are shown in Table 111. At a given pressure, the boiling points of a pair of consecutive esters of even-numbered, saturated acids differ by 20" to 30" C. In contrast, the largest difference of boiling points at a given pressure for the unsaturated esters is 1.4' C.-for example, methyl linoleate vs. methyl linolenate a t 0.1 mm. Heats of vaporization (Table IV) were calculated from the equation AH =
toalod.,
51.12 09 09 64.25 69.82 69.98 77.46 82.45 92.25 92.72 97.23
0.00
-0.08
C. 51.12 59.02 64.31 69.83 69.84 77.47 82.51 92.18 92,64 97.24 O
0.3508 0.624 0.888 1,278 1.291 2,053 2.756 4.76 4.88 6.19
-0.05 -0.03
$0.07 -0.03 +0.05 +0.07 -0.07 -0.17 -0.22 +0.21 4-0.09
texp.,
Mm. H g
-0.07 3.0.06 -0.01 -0.01 +0.05
-0.22 +O. 11 +o. 12 +0.07 -0.20 -0.11 +o. 18 +O. 16 +0.19
0,2031 0,2689 0,2461 0,4420 0.814 1.115 1.846 2.452 3.060 3.806 4.709
Pressure,
+ (273.16 - C ) (log p
- A)I2
B
where A , E , and C are the Antoine constants. Figure 2 gives a plot of boiling temperatures at 1 mm. of mercury us. the number of carbon atoms in the saturated esters. The individual compounds fall on a regular curve. REQUIREMENTS OF STILL FOR SEPARATING ESTERS
From the vapor pressure-temperature data, it is possible to calculate the distillation conditions necessary to separate the
")
- log esters. The equation, nmln.= __ 1 - x ' log 01 (log __ 1 - Y gives the minimum number of plates needed in a column a t total reflux. In this equation, n is the number of plates; a,the relative volatility; y, the mole fraction of more volatile component a t top of column; and 2, the mole fraction of more volatile component in still pot. The minimum number of plates required to separate each successive pair of saturated C8 to Cla fatty acid esters is presented in Table V. In one set of calculations it is assumed that an equimolar mixture is in the still pot, and that the desired purity of the distillate is 99%. In the other set the same initial mixture is as~
+0.10 +O. 03 -0.14 -0.05
0.00 -0.03
+0.06
+o. 02
-0.03
+O. 14
+0.02 -0.02 +O. 04 -0.10 -0.01 +o. 01 +O. 06 f0.01 -0.03
C.
Pressure, Mm. Hg. 0.1326 0.3861 0.811 1.743 1.798 2.275 2.284 2.354 2.507 3.214 5.114 10.56
63.17 77.68 88.49 101.18 101.56 105.81 105.90 106.27 107.67 112.18 120.89 135.82
0.1349 0.1434 0.2289 0.2997 0.3270 0.5736 0.876 1.189 1.245 2.460 4.009 5.975
Methyl 125.61 126.68 133.62 137.91 139.40 148.45 155.80 161.30 162.13 175.43 185.32 194.16
texp.. O
0.1232 0.1580 0.2562 0.3663 0.487 0.823 1.601 3.03 4.70
C.
tosicd.,
' c.
63.16 77.58 88.66 101.16 101.69 105.79 105.86 106.40 107.52 112.04 120.90 135.87 Stearate 125.77 126.66 133.64 137.83 139.21 148.44 155.79 161.32 162.17 175.32 185.44 194.19
texp. tcalod.,
-
' c.
+0.01 +0.10 -0.17 +o. 02 -0.13 +o, 02 +0.04 -0.13 +O. 15 + O . 14 -0.01 -0.05
__
-0.16 10.02 -0.02 +0.08 +o. 19
+0.01
+0.01 -0.02 -0.04 +O.ll -0.12 -0.03
-0.13 +0.02 f O . 08
f0.01 -0.05 + O . 13 -0.08 $0.02 -0.01
sumed, but the desired purity of distillate is taken as 90%. The saturated fatty acid esters can be separated readily by a column having a small number of plates. However, this calculation gives only the minimum number of plates under total reflux and the actual number required is much greater. A fair approximation of the requirements under operating conditions can be obtained by multiplying the minimum number of plates by two. Although the separation of methyl esters of saturated fatty acids offers little difficulty, this is not true in the case of the C18 group of saturated and unsaturated acid esters. Methyl stearate can be separated from the unsaturated acids but the separation by distillation of the unsaturated esters seems to be nearly hopeless. Table VI shows the minimum numbers of plates required to separate each of the binary mixtures. The first set of values is based on the assumption that a 50 mole % mixture is in the still pot and that 99% pure distillate is required. The second set is for a 90% pure distillate from a 50 mole yo mixture, and the third is for a twofold enrichment of a mixture containing 40 mole yoof the desired material. Since the minimum number of plates must be doubled a t least, it is quite obvious that present-day stills are not capable of producing 99% pure material. Inspection of Table VI shows that while the number of plates required for separation of most pairs increases with increasing temperature, one pair, oleatelinolenate, passes through a minimum, and another pair, linoleateoleate, passes through a maximum. This is due to lack of parallelism of the vapor, pressure-temperature curves. Since the minimum number of plates for one pair occurs a t approximately the maximum for the other pair, fractionation of the ternary mixture of oleate, linoleate, and linolenate is still further complicated. DISCUSSlON
In the past it has been customary to use iodine values, thiocyanogen values, and neutralization equivalents as criteria of purity of methyl esters of fatty acids. These chemical methods are useful for characterizing materials but, because in most cases equal amounts of adjacent impurities could balance out to give analyses for pure materials, these methods cannot be regarded as
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1952
BOILINGPOINTS ( ' C.) OF THE METHYLESTERSOF FATTY TABLE 111. CALCULATED ACIDSAT VARIOUSPRESSURES 0 1
0 2
0.4
0.6 30.7
10 37 9
2.0 48.2 51.5'
4.0 59.5 62.5'3
6.0 66.4 68.7"
Methyl caprate 1704 8 logp 6.9858 - -
35 5
43.8
52 9
58 5
66 0
77.0 78.7"
89.1 90.8''
96.6 98.1"
Methyl laurate
59 6
68 5
78.1
84.0
92 0
103.5 102.8"
116.1 124.1 116.0a 124.4-
81.7
90 5
100 2
106 3
114 5
126.7 127.l a 125b
140.2 148.8 141.3a 150.3"
150.0 148.5" 1496
163.7 163.2"
155 5b
171.2 169.4" 170b
185.4 194.3 184.6- 193.6"
Pressure, mm. mercury Methyl caprylate log p = 7 9678
2102 6
- t-+ 226
3
t
log p = 7.2253
+ 178
7 - t1921 + 174
Methyl myristate log p = 5.9342
- +
1350.1 t 113
Methyl paImitate log p
-
7.5253
101 8
-
Methyl oleate log D
6.8805
Methyl linoleate log p = 7 8693
122 1
128 7
- +
121 5
131 6
142 5
149 2
- +
158 2
172.3 172.6"
118 2
128 0
138 8
145.5
154 4 152 5b
167.4 166.5b
181.7
190.8
117 0
127 2
138 2
145 0
154 0 149.5b
167.1 163,
181.3
190.2
118 4
128 5
139.3
146.1
155.0
168.0
182.2
191.1
1983 5 - ml 2.589 4 -t + 175
137.3 136b
2313 6 t 152
Methyl linolenate log p = 7 4392
111 6
2285 8 t 166
Methyl stearate log p = 7.4593
114b
2298 8 -t + 154
a Bonhorst Althouse. and Triebold ( 8 ) . b Norris add Terry ( 1 1 ).
TABLE IV.
absolute assurances of purity. Refractive index is also used extensively but it is open to the same criticism and, as usually employed, is not particularly sensitive. In this work, it was desired to determine the purity of esters by an independent physical method-estimating composition by freezing curves. It was found applicable to the saturated esters and the results show that, in general, compounds were being dealt with that had relatively high degrees of purity. It is unfortunate that the method was not applicable to poly-unsaturated esters because they are mixtures of cis-trans isomers. For the linoleate and the linoienate only the conventional data can be presented and it is emphasized that the vapor pressures given are for synthetic, and not naturally occurring acids. The vapor pressures of methyl oleate and methyl linoleate increase with increasing unsaturation; however, methyl linolenate, with three double bonds, has the lowest vapor pressure of the unsaturated CIS group. A possibility exists that, if pure isomers were measured the vapor pressures and the degree of unsaturation would form an orderly series.
HEATSOF VAPORIZATION (CALORIES PER MOLE) OF METHYL ESTERS OF FATTY ACIDSAT 1 MM. OF MERCURY
LITERATURE CITED
Methyl Caprylate
Methyl Caprate
Methyl Laurate
Methyl Myristate
Methyl Palmitate
Methyl Stearate
Methyl Oleate
Methyl Linoleate
Methyl Linolenate
13,370
15,070
16,570
17,940
19,160
20,470
20,170
19,970
20,200
TABLE V.
115
MINIMUMNUMBEROF PLATES REQUIREDTO SEPARATE METHYLESTERS OF SATURATED ACIDS
(1) Althousb, P. M., and Trie-
bold, H. O., IND.ENQ. CHEM.,ANAL.ED., 16, 605
(1944). (2) Althouse, P. M., Hunter, G. W and Triebold, H. O., J . Am. Oil Chemists' SOC.,24, 257 (1947).
.,
(3) Bonhorst, C. W., Althouse, P. M., and Triebold, H. O., IND. ENQ.CHEM.,40,2379 (1948). (4) Egloff, Gustav, "Physical Constants of Hydrocarbons," Vol. I, pp. 176, 186, 188, 200, 204,205, 215, New York, Reinhold PubCsprylate 1.32 2.78 Caprate lishing Corp., 1939. 1000 1000 Caprate (5) Garner, W. E., and Rushbrooke, J. E., J . Chem. SOC.,1927, 3.11 1.49 Laurate 1351. 1200 1200 Laura'te (6) Glasgow, A. R., Jr., Streiff, A. G., and Rossini, F. D., J . Re1.74 3.64 Myristate search Natl. Bur. Standards, 35,355 (1945). 140' 140° Myristate (7) Haller, A., and Youssoufian, Compt. r e d . , 143,803 (1906). 1.79 3.75 Palmitate ( 8 ) Hickman, K. C. D., Hecker, J. ,C., and Embree, N. D., 160' 160' Palmitate 1.99 4.16 Stearate IND.ENQ. CHEM.,ANAL. ED.,9,264 (1937). (9) King, A. M., and Garner, W. TABLE VI. MINIMUMNUMBEROF PLATES REQUIRED TO SEPARATE METHYLESTERS OF E., J . Chem. SOC., 1936, Cts ACIDS 1372. Pair of Minimum Number of Plates Minimum Number of Platen Re- Minimum Number of Plates Re- Required to Produce 80% Pure (10) Longenecker, H. E., Oil & Methyl Esters quired to Produce 99% Pure Lower quired to Produce 90% Pure Lower Boiling Component from Soap, 17,53 (1940). Boiling Com onent from 50 Mole Lower Boiling Component from Mixture Containing 40 Mole to Be Separated Mixture 50 Mole ToMixture 7% of T h a t Component (11) Norris, F. A., and Terry, D. E., Ibid., 22,41 (1945). Temperature, ' C. Temperature, O C. Temperature, ' C. 180" 120' 140° 160' 180' 120° 140' 160' 180' 120° 140° l6Oo (12) Schwab, F. W., and Wichers, E. J., J. Research Natl. BUT. Oleate 19.11 19.96 22.27 26.16 9.14 9.64 10.65 12.51 7.45 7.86 8.68 10.20 Stearate Standa~ds,34, 333 (1945). Plates Required to Separate Lower. Boiling Component. from 50 Mole 70 Mixture 99% pure product 90% pure produot 800 800
pair of Methyl Esters to Be Separated
8
Linoleate 14.43 17.66 Stearate Linolenate 20.81 23.69 Stearate Oleate Linolenate 231.89 133.57 Linoleate Olzate Linoleate Linolenate
58.99 46 86
20.81
23.59
6.90
8.44
9.95 11.28
5.63
6.89
26.80
30.25
9.95
11.33
12.82 14.47
8.12
9.24
10.45 11.80
133.57
193.74
110.88
63.87
63.87 92.64 90.42 52.08
52.08 75 54
141.53 308.42 244.24 69.85
94.17 106.71
28.21
67.67
22.40 83.40
8.12
8.78
147.48 116.79 23.00 65.19 120.26 95.24 45.03 51.02
18.27 27.24
36.72 41.61
RECEIVED September 5, 1950.
