Enthalpy of mixtures

For gases, the heats of mixing are usually negligible and the heat capacities and enthalpies can be taken as additive without introducing any significant error into design calculations as was done in Example 3.3. 

For mixtures of liquids and for solutions, the heat of mixing (heat of solution) may be significant, and so must be included when calculating the enthalpy of the mixture. 

For binary mixtures, the specific enthalpy of the mixture at temperature t is given by  

Heats of mixing and heats of solution are determined experimentally and are available in the handbooks for the more commonly used solutions. 

If no values are available, judgement must be used to decide if the heat of mixing for the system is likely to be significant. 

Heats of mixing and heats of solution are determined experimentally and are available in the handbooks for the more commonly used solutions. If no values are available, judgment must be used to decide if the heat of mixing for the system is likely to be significant. 

Heats of solution are dependent on concentration. The integral heat of solution at any given concentration is the cumulative heat released, or absorbed, in preparing the solution from pure solvent and solute. The integral heat of solution at infinite dilution is called the standard integral heat of solution. 

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In Chap. 8 we discuss the thermodynamics of polymer solutions, specifically with respect to phase separation and osmotic pressure. We shall devote considerable attention to statistical models to describe both the entropy and the enthalpy of mixtures. Of particular interest is the idea that the thermodynamic... 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

For the problem to be tractable, the enthalpies of the two phases must be known as functions of the respective phase compositions. When heats of mixing and heat capacity effects are small, the enthalpies of mixtures may be compounded of those of the pure components thus... 

According to Reid et al. (1987) the simple cubic equations of state can be used to estimate the enthalpies of mixtures of light hydrocarbons, C02, H2S, and nitrogen to within 4 kj/kg (1.7 Btu/lb). 

The enthalpy of mixtures is dependent on temperature, pressure and composition. Its differential is given by... 

In the solution of distillation problems, the only functions generally required are the enthalpies and fugacities of the pure components and the enthalpies of mixtures as well as the fugacities of the components in a mixture. Thus, only the expressions for these quantities are given in tables for the equations of state. 

Saturation Temp., F Ethanol Concentration Enthalpy of Mixture Btu/lb ... 

PREDICTION OF VAPOR-LIQUID EQUILIBRIA AND ENTHALPIES OF MIXTURES AT LOW TEMPERATURES. 

Vit] [1994Wit] Integral enthalpy of mixture in the liquid phase 1627°C, < 7 mass% C... 

The enthalpy of mixture for this system is not available in the literature. Only the phase diagram is given [14, 15]. The phase diagram presents a eutectic point at x(NdF3) = 0.69 and T = 1488 K (Figure 3.5.1). This figure shows a partial miscibility in a solid state and an azeotrope at x(NdF3) = 0.21 and T = 1688 K. 

For the material and energy balances, pure-component heat capacity and density data are needed. These are among the most widely measured data and are available on process simulators for more than a thousand substances. (See Chapter 13 for details of process simulators.) There are also reasonably accurate group-contribution techniques for use when no data are available [8]. The enthalpies of mixtures require an accurate equation of state for gases and nonionic liquids. The equations of state available on process simulators are accurate enough for these systems. However, additional heat of solution data are needed for electrolyte solutions, and these data may not be as readily available. For these systems, care should be taken to use accurate experimental data, because estimation techniques are not as well defined. 

Banos, I. Valero, J. Perez, P. Gracia, M. Gutierrez Losa, C. Excess molar enthalpies of mixtures containing 1,3-dichloropropaneX Chem. Thernwdyn. 1990,22, 67-72... 

Garcia-Lisbona, N. Garcia Vicente, I. Munoz Embid, J. Velasco, L Otin, S. Kehiaian, H. V. Thermodynamics of mixtures containing bromoalkanes. II. Excess enthalpies of mixtures of 1-bromoalkane with cyclohexane, with benzene, or with tetrachloromethane. Measurement and analysis in terms of group contributions (DISQUAC) Fluid Phase Equilib. 1989,45, 191-203... 

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