826
Vol. 35, No, 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
of orange shellac were kept in glass jars at room temperature for months; one was stored over calcium chloride, the other over wet sodium chloride which gives 75 per cent humidity. After 1000 days the condition of the shellacs was as follows: Oiange Shellac High-flow Low-flow
Over Dry CaCll
Over Wet NaCl Hardened
Still fusible
Infusible
Stlll fuslhla
An experiment was commenced with orange shellac in t n o friction-top tin cans, covered with cloth to permit passage of moisture; one contained a small jar of calcium chloride, the other of 1%-etsodium chloride. The two cans were kept in a n oven a t 40" C. until the shellacs became infusible. Over dry calcium chloride the shellac was infusible after 400 days; m e r wet sodium chloride, after 175 days. CONCLUSIONS
Shellac can best be preserved by storage in a dry atmosphere. Since, however, the fluidity and other molding properties vary with moisture content (2), the dry resin can be restored t o the desired condition by adding a measured amount of water shortly before it is t o be used.
Shellac at temperatures below 150" C. approaches the gelled state whether dry, moist, or wet. Shellac in storage should be kept dry t o retard the reactions of hardening. At higher temperatures gelation is greatly retarded by water and does not occur in water above 150" C. Gelation under water is reversible between about 150" and 165" C. Since the effect of mater at room temperature is to hasten gelation and at higher temperatures to retard it, there must be an intermediate range where water is without effect upon the rate; this temperature seems t o be between 4 0 " and 70" C. ACKEOW LEDG MENT
For criticisms of the manuscript, the writer expresses thanks to A. T. Krogh and L. R. Hill of this company, and to Paul F. Bruius of the Polytechnic Institute of Brooklyn. LITERATCRE CITED
(1) Rangaswami, M., and Aldis, R. W., Indian Lac Research I n s t . Bull. 419 (1934). (2) Townsend, R. V., and Clayton, W. R.. IND. ENG.CHEM.,A N ~ L . E D ,8. 108 (1936).
CORRESPONDENCE Calculation of Relative Volatilitv -J
SIR: A recent paper by John Griswold [IxD.ENG.CHEW,35, 247 (1943) ] shows convincingly the need for accurate relative volatility values in distillation calculations. There is, however, a slight erroi in his reference to my equation for the relative volatility of normal liquids a t atmospheric pressure [ J . Int. Petroleum Tech., 2 5 , 558 (1939)l. The constant of 11.5 xas not derived from the Clausius-Clapeyron equation and Trouton's rule, but was a mean value obt,ained from consideration of a large number of hydrocarbon mixtures, mostly wide boiling. The constant obtained in the theoretical derivation was 11.1. Converting natural to common logarithms, this is nearly the same as the modified constant proposed in Griswold's Equation 4A. Thus it appears that the "constant" varies from 11.1 for close-boiling to 11.5 or more for wide-boiling mixtures. If the approximate Clausius-Clapeyron equation is combined with Kistiakowsky's equation instead of with Trouton's rule, the following is obtained: loga =
T - TA TB log T A +
where R
= gas
T
log T B
constant, (cc.) (atm.)
82.048
TB - TA +7 log R
(" C.) - 1 (mole) -1 =
P A P B = vapor pressures of components at abs. temp. T T A T B = atm. boiling points of components, abs. scale a = relative volatility at abs. temp. T
This equation applies only to nonpolar liquids at atmospheric pressure when the gas law and Raoult's law deviations are such that the relation = P A / P Bcan be assumed. It contains no empirical constant and is likely t o be more accurate than my earlier equation. Checked against the experimental results of Griswold and a few others, it gives values of (a 1) from 4 t o 15 per cent low. (Y
-
R. EDGEWORTH-JOHSSTONE TRINIDAD LEABEHOLDB, LTD. POINT-A-PIERRE, TRINIDAD. B. W.I.
SIR: The new formula by Johnstone for calculating
CYideal
( = P A / P B )is a real contribution.
I t is probably as accurate as can be developed from theoretical considerations only without becoming complicated. For accuracy in wide-boiling hydrocarbon systems, both this formula and my Equation 6 leave something t o be desired. When the vapor pressure curve of each component fits the type form equation : log P
= n
- C/l'
as is usually the case, then : log
(aided)
= iOg ( P A / P B ) = f l / T
-
"{
P and y are constants applicable over fairly wide ranges. Two vapor pressures for each component are needed to evaluate p and y, one of which may be the normal boiling point. When a vapor pressure curve has not been determined, a synthetic curve may be drawn through the boiling point on a suitable hydrocarbon vapor pressure chart, of which several are available. This procedure is recommended as the best a t the present time for calculation of w e a l of wide-boiling hydrocarbon systems, and it is applicable to pressures other than atmospheric. THE UNIVERSITY o r TEXAS AUSTIN,TEXAS
JOHXGRISWOLD
...
SIR: I am obliged t o Griswold for his remarks on my formula for agree that with wide-boiling components for which vapor pressure curves are available or can be approximated, his method is probably better than any theoretical formula. With very closeboiling components it is possible that the formula might be more satisfactory, since atmospheric boiling points are often known with greater accuracy than vapor pressures at other temperatures. R. EDGEWORTH-JOHNSTONE a, and
