Enthalpies of vaporization

Enthalpy of Vaporization The enthalpy (heat) of vaporization AH is the difference between the molar enthalpies of the saturated 

Experimental heats of vaporization can be effectively correlated with 

A simple method for obtaining AH at one temperature from a known value at a reference temperature, say, at the normal boiling point, is to truncate Eq. (2-34) after the B term, set B = 0.38, and take a ratio of the AH values at the two conditions to give the Watson [Thek, R. E., and L. I. Stiel, AlChE J, 12 (1966) 599 13(1967) 626] correlation 

If an accurate correlation for F and accurate values for Zg and Zl are available, Eq. (2-33) is the preferred method for obtaining enthalpies of vaporization. Otherwise, the CS methods shown below should be used. 

Expected uncertainty Varies significantly with temperature and with the quality and temperature range of the vapor pressure data used in the correlation. 

Equation (51) permits evaluation of B + ClVm.) if Af/y,Pb(T and Hare measured or are known with requisite accuracy. For example, Douslin and co-workers derived values of B for hexafluorobenzene round the normal boiling temperature, on the assumption that C and higher virial coefficients have a negligible effect when Ps 1 bar. 

The operation costs of distillation processes are mainly influenced by the enthalpy of vaporization and, to a smaller extent, by the specific isobaric heat capacity of liquids. Therefore, its importance can easily be understood. 

For non-associating substances, enthalpies of vaporization can be very well correlated with the extended Watson equation 

The order of magnitude of the average deviation should be 0.5%. For temperatures near the critical point, the values for the enthalpy of vaporization are small and the slopes are large therefore, higher deviations can be accepted, and it is recommended to leave this region out of the database for regression. The parameters C, D, and E should only be fitted if many data with a high quality are available, otherwise, they should be set to 0. In that case, B is often close to B = 0.38. 

users of a simulation program could use enthalpies of vaporization for supercritical components, which is physically unrealistic but unavoidable in practical applications. For that case, it is useful to define an extrapolation function to avoid a mathematical error. 

There is a peculiarity for the estimation of enthalpies of vaporization the Clausius-Clapeyron equation 

Applicability Organic compounds for which group contributions have been regressed. 

Description An expression for All can be obtained from Eq. (2-33) by using an appropriate vapor pressure correlation. If one differentiates the Riedel vapor pressure correlation, Eq. (2-22), in accordance with Eq. (2-33), one obtains the heat of vaporization as 

The calculated value differs from the DIPPR 801 recommended value of 

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The enthalpies of vaporization for the pure components are in excellent agreement with experiment, as is the composition of the azeotrope. The enthalpy of the saturated vapor is also in... 

Table 1 indicates that the enthalpy of mixing in the liquid phase is not important when calculating enthalpies of vaporization, even though for this system, the enthalpy of mixing is large (Brown, 1964) when compared to other enthalpies of mixing for typical mixtures of nonelectrolytes. 

T,K Enthalpy of Saturated Liquid, kj/mole Integral Enthalpy of Vaporization, kj/mole ... 

This chapter presents quantitative methods for calculation of enthalpies of vapor-phase and liquid-phase mixtures. These methods rely primarily on pure-component data, in particular ideal-vapor heat capacities and vapor-pressure data, both as functions of temperature. Vapor-phase corrections for nonideality are usually relatively small. Liquid-phase excess enthalpies are also usually not important. As indicated in Chapter 4, for mixtures containing noncondensable components, we restrict attention to liquid solutions which are dilute with respect to all noncondensable components. 

Taking into account the mass enthalpy of vaporization at 25°C, 2443 kS/k, /-rjJ... 

For hydrocarbons, this gain remains moderate since their mass enthalpy of vaporization, between 300 and 500 kJ/kg, usually represents only 0.8% to 1% of the NHV. It is thus not extremely useful to know in this case whether the motor fuel is introduced as a vapor or a iiquid. The same is not true for... 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

Majer V and Svoboda V 1985 (lUPAC Chemical Data Series No 32) Enthalpies of Vaporization of Organic Compounds (Oxford Blackwell)... 

Morawetz E 1972 Enthalpies of vaporization of/i-alkanes from C, 2to C2q J. Chem. Thermodyn. 4 139... 

References D. D. Wagman, et ah, The NBS Tables of Chemical Thermodynamic Properties, in J. Phys. Chem. Ref. Data, 11 2,1982 M. W. Chase, et ah, JANAF Thermochemical Tables, 3rd ed., American Chemical Society and the American Institute of Physics, 1986 (supplements to JANAF appear in J. Phys. Chem. Ref. Data) Thermodynamic Research Center, TRC Thermodynamic Tables, Texas A M University, College Station, Texas I. Barin and O. Knacke, Thermochemical Properties of Inorganic Substances, Springer-Verlag, Berlin, 1973 J. B. Pedley, R. D. Naylor, and S. P. Kirby, Thermochemical Data of Organic Compounds, 2nd ed.. Chapman and Hall, London, 1986 V. Majer and V. Svoboda, Enthalpies of Vaporization of Organic Compounds, International Union of Pure and Applied Chemistry, Chemical Data Series No. 32, Blackwell, Oxford, 1985. 

Enthalpy of Vaporization (or Sublimation) When the pressure of the vapor in equilibrium with a liquid reaches 1 atm, the liquid boils and is completely converted to vapor on absorption of the enthalpy of vaporization ISHv at the normal boiling point T. A rough empirical relationship between the normal boiling point and the enthalpy of vaporization (Trouton s rule) is ... 

Selected physical properties are given in Table 1 and some thermodynamic properties in Table 2. Vapor pressure (P) and enthalpy of vaporization (H) over the temperature range 178.45 to 508.2 K can be calculated with an error of less than 3% from the following equations wherein the units are P, kPa Pi, mj/ mol T, K and = reduced temperature, T/ T (1) ... 

Extensive tables and equations are given in ref. 1 for enthalpy of vaporization and heat capacity at constant pressure. 

Fluoroacetic acid [144-49-OJ, FCH2COOH, is noted for its high, toxicity to animals, including humans. It is sold in the form of its sodium salt as a rodenticide and general mammalian pest control agent. The acid has mp, 33°C bp, 165°C heat of combustion, —715.8 kJ/mol( —171.08 kcal/mol) (1) enthalpy of vaporization, 83.89 kJ /mol (20.05 kcal/mol) (2). Some thermodynamic and transport properties of its aqueous solutions have been pubHshed (3), as has the molecular stmcture of the acid as deterrnined by microwave spectroscopy (4). Although first prepared in 1896 (5), its unusual toxicity was not pubhshed until 50 years later (6). The acid is the toxic constituent of a South African plant Dichapetalum i mosum better known as gifirlaar (7). At least 24 other poisonous plant species are known to contain it (8). 

Correlations for Enthalpy of Vaporization. Enthalpy or heat of vaporization, which is an important engineering parameter for Hquids, can be predicted by a variety of methods which focus on either prediction of the heat of vaporization at the normal boiling point, or estimation of the heat of vaporization at any temperature from a known value at a reference temperature (5). 

Enthalpies of vaporization, at the normal boiling point, T, can be estimated roughly with the simple rule of Pictet and Trouton (19) which... 

Correlation Methods Vapor pressure is correlated as a function of temperature by numerous methods mainly derived from the Clapeyron equation discussed in the section on enthalpy of vaporization. The classic simple equation used for correlation of low to moderate vapor pressures is the Antoine S equation (2-27). 

Enthalpy of Vaporization The enthalpy (heat) of vaporization AHv is defined as the difference of the enthalpies of a unit mole or mass of a saturated vapor and saturated liqmd of a pure component i.e., at a temperature (below the critical temperature) anci corresponding vapor pressure. AHy is related to vapor pressure by the thermodynamically exact Clausius-Clapeyron equation ... 

Tbe enthalpy of vaporization decreases with temperature and is zero at tbe critical point. If tbe value of an enthalpy of vaporization AH is known at temperature T, this temperature dependency can be represented by tbe Watson relation to calculate another enthalpy of vaporization AH at any other temperature To ... 

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