Binary mixtures Boiling-point calculation

An additional program took the energy parameters of the binary systems making up ternary mixtures and calculated the boiling point of the ternary and the equilibrium composition of the vapor phase. Comparison of the measured boiling point with the predicted boiling point for the same composition and pressure was used as a criterion of successful performance of the NRTL equation. 

In Section A.l, the general laws of thermodynamics are stated. The results of statistical mechanics of ideal gases are summarized in Section A.2. Chemical equilibrium conditions for phase transitions and for reactions in gases (real and ideal) and in condensed phases (real and ideal) are derived in Section A.3, where methods for computing equilibrium compositions are indicated. In Section A.4 heats of reaction are defined, methods for obtaining heats of reaction are outlined, and adiabatic flame-temperature calculations are discussed. In the final section (Section A.5), which is concerned with condensed phases, the phase rule is derived, dependences of the vapor pressure and of the boiling point on composition in binary mixtures are analyzed, and properties related to osmotic pressure are discussed. 

Figure 1. Boiling-point diagram for the binary system -octane and n-dodecane at P = 20 kPa. Open and filled symbols are used for the TraPPE and SKS force fields, respectively. The dashed line represents the experimental data. The calculated boiling points for the pure substances are shown as circles. Simulation results for binary mixtures are depicted as diamonds, upward-pointing triangles, squares, and downward-pointing triangles for simulations containing total mole fractions of -octane of 0.1,0.25,0.5, and 0.75, respectively.

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

Binary Mixtures.—Since a mixture of constant boiling point behaves like a single substance on distillation, it is possible, if we know the composition of the mixture distilled, to calculate that of the binary mixture. 

In the calculation of the composition from the specific gravity of the redistilled binary mixture, the necessary correction has been introduced for the contraction that occurs on mixing the components. In many cases the boiling point of the binary mixture is too near that of one of the components to allow of a determination of composition being made with that component in excess, but if the boiling point is greatly depressed one may frequently determine the composition even when the more volatile of the two original components is in excess. 

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