O’zbekiston respublikasi oliy ta’lim vazirligi toshkent kimyo texnologiya instituti

CHAPTER 3 FUNDAMENTALS OF ENERGY BALANCES

(reproduced from Sterbacek et al. (1979), with permission)

3.8 ENTHALPY OF MIXTURES 87

3.8 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.
C_pðmixtureÞ¼x_aC_pa þx_bC_pb þx_cC_pc þ… (3.20)
where x_a, x_b, x_c, etc., are the mole fractions of the components a, b, c (or weight fractions if the specific heat per unit mass is used).
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:
Hmixture,t ¼xaHa,t þxbHb,t þΔHm,t (3.21)
where H_a,t and H_b,t are the specific enthalpies of the components a and b and ΔH_m,t is the heat of mixing when 1 mol of solution is formed, at temperature t.

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.
For hydrocarbon mixtures, the heat of mixing is usually small compared with the other heat quantities, and can usually be neglected when carrying out a heat balance to determine the process heating or cooling requirements. The heats of solution of organic and inorganic compounds in water can be large, particularly for the strong mineral acids and alkalis.

3.8.1 INTEGRAL HEATS OF SOLUTION

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.
Tables of the integral heat of solution over a range of concentration, and plots of the integral heat of solution as a function of concentration, are given in the handbooks for many of the materials for which the heat of solution is likely to be significant in process design calculations.
The integral heat of solution can be used to calculate the heating or cooling required in the preparation of solutions, as illustrated in Example 3.5.