CORRESPONDENCE Thermodynamic Properties of Light Hydrocarbons SIR: Since the data on this subject were published [ I s n . ENG. CHEM.,34, 590 (1942)],,the enthalpy of hydrocarbon vapors a t zero pressure as a function of temperature has been found to be in error, because of errors in the latent heats of the individual hydrocarbons at 32" F. and zero pressure absolute. A revised chart giving correct values of the enthalpy of hydrocarbon vapors a t 32; F. and zero pressure above liquid a t 32" and zero pressure is presented here. The latent heats of vaporization a t 32" F. and zero pressure absolute used were computed from the latent heats of the pure compounds in the following manner: The latent heat of vaporization a t 32" F. and the bubble point pressure were determined for each compound except methane. The isothermal change in enthalpy of the vapor at 32" F. between bubblezpoint and zero pressures was added to the latent heat at 32 and the bubble-point pressure t o give the vapor enthalpy at 32" and zero pressure above liquid a t 32' and the bubble-point pressure for each compound. The liquid enthalpy for each compound was then vorrected to zero pressure from the bubble-point pressure at 32" F. The difference between the vapor enthalpy at 32" and zero pressure and the liquid enthalpy a t 32' and zero pressure is the latent heat a t 32" and zero pressure as used in constructing the figure. The latent heat of methane used here is the same as that in Figure 8 of the original paper. The specific heats used in calculating the vap2r enthalpies at zero pressure and temperatures other than 32 F. are substantially the same as those previously presented. The latent heats at 32" F. and zero pressure were calculated in the manner just described in preference to the method previously used involving the use of the equation,
-dlnK =__ AH
dT
where
RT2
K = equilibrium vaporization constant T = temperature, F. 4G0 R = gas constant, 1.987 B.t.u.jlb. mole/" R. AH = enthalpy of hydrocarbon in vapor phase minus enthalpy in liquid phase at same temperature and pressure O
+
because of the inaccuracy of calculating heats of solution from equilibrium constants which are not exact a t low temperatures and pressures. The use of the accompanying figure for paraffin hydrocarbon mixtures a t zero pressure involves the assumption that such mixtures are ideal solutions, because the latent heats incorporated in the figure are based on pure compounds. The errors introduced by this assumption for paraffin hydrocarbon mixtures are probably no greater than the errors in reading the enthalpy charts. This figure, when used with charts previously presented, offers a convenient means for determining the enthalpy of pure paraffins or paraffin mixtures as a function-of temperature and pressure. The application of these data to commercial operations gives heat balances which are within the accuracy of measuring temperatures, pressures, flow quantities, and compositions for such operations.
DYSART E. HOLCOMB
