Vaporization, enthalpies

Similarly, the vapor enthalpy can be expressed in terms of humid heat and latent heat in relation to a base condition ... 

The integration can be carried out graphically or numerically using a computer. For illustrative purposes the graphical procedure is shown in Figure 5. In this plot of vapor enthalpy or FQ vs Hquid temperature (T or T, the curved line is the equiHbtium curve for the system. For the air—water system, it is the 100% saturation line taken direcdy from the humidity diagram (see Fig. 3). 

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). 

Calculate liquid densities, molar tray and condenser-reflux drum holdups, ana hquor and vapor enthalpies. Determine holdup and enthalpy derivatives with respect to time by forward difference approximations. 

When 0 °C dry air is chosen as the zero point of dry air enthalpy, and 0 °C watet as the zero point of water vapor enthalpy, the enthalpies of dty air and water vapor can be calculated from the equations... 

Figure 5-8. Hydrocarbon vapor enthalpies at various Watson K factors.

II2 = specific enthalpy at the evaporator outlet (saturated vapor enthalpy at the evaporator pressure (J kg-1, kJkg ) H4 = specific enthalpy at the condenser... 

Calculation of the power requirements for the three refrigerants requires the flowrate to be calculated for a duty of 3 MW. This can be calculated from the enthalpy difference across the evaporator (H2 — H1). The enthalpy difference across the evaporator is assumed to be the difference between the saturated vapor enthalpy at the evaporator pressure and the saturated liquid enthalpy at the condenser pressure. This assumes no subcooling of the refrigerant. 

The terms enthalpy of fusion, enthalpy of vaporization, enthalpy of combustion, and many more cause some students to believe that there are many different kinds of enthalpies. There are not. These names merely identify the processes with which the enthalpy term is associated. Thus, there are processes called fusion (melting), vaporization, sublimation, combustion, and so forth. The corresponding enthalpy changes are called by names that include these descriptions. 

Piacente, V., Pompili, T., Scardala, P., Ferro, D. (1991) Temperature dependence of the vaporization enthalpies of n-alkanes from vapour-pressure measurements. J. Chem. Thermodyn. 23, 379-396. 

Verevkin, S.P., Wandschneider, D., Heintz, A. (2000) Determination of vaporization enthalpies of selected linear and branched C7, C8, C9, Cu and Cu monoolefin hydrocarbons from transpiration and correlation gas-chromatography methods. J. Chem. Eng. Data 45, 618-625. 

What about measurements of enthalpies of combustion of condensed phase species 49 and 50 and accompanying enthalpies of vaporization Enthalpies of formation of the gaseous hydrocarbons can be directly obtained from these studies as well. There are two recent studies that provide us with useful information. The first42 results in the values of 104.6 0.6 and 104.8 0.6 kJmol-1 respectively. The second accompanies the earlier cited cyclic bisallene (and polycyclic monoolefin) study, in which the authors20... 

The saturated liquid and vapor enthalpies can be expressed as a linear function of compositions. 

One of the critical issues in vapor pressure methods is the choice of the procedure to calculate the vaporization enthalpy. For instance, consider the vapor pressures of ethanol at several temperatures in the range 309-343 K, obtained with a differential ebulliometer [40]. The simplest way of deriving an enthalpy of vaporization from the curve shown in figure 2.4 is by fitting those data with the integrated form of the Clausius-Clapeyron equation [1] ... 

In both cases, the variation of the vaporization enthalpy with temperature is implicit. AVap// is now calculated, at any temperature, from equation 2.40, and the correction to the standard states, if required, is made as already described. 

Equations 2.39 and 2.40 lead to Avap//°(C2l I5OH) = 42.4 0.5 kJ mol-1 [40], which agrees with the mean of the calorimetric results for the same liquid, 42.30 0.04 kJ mol-1 [39]. Note that the less sophisticated approach (equation 2.33) apparently underestimates the vaporization enthalpy by 0.6 kJ mol-1. However, this is not true because AvapH = 41.8 kJ mol-1 refers to the mean temperature, 326 K. A temperature correction is possible in this case, because the molar heat capacities of liquid and gaseous ethanol are available as a function of T [40]. That correction can be obtained as ... 

Calculate liquid and vapor enthalpies, using subroutine ENTH. 

Either the temperatures or flows could be adjusted first. The common choice is to correct the temperature. Correction of temperatures is usually done through either bubble-point or dew-point determinations on the calculated stage compositions. After correcting the stage temperatures, the liquid and vapor enthalpies may be obtained from the calculated compositions, and the flows corrected by solution of the now linear heat balance equations of Table I. 

Using PCA, Cramer found that more than 95% of the variances in six physical properties (activity coefficient, partition coefficient, boiling point, molar refractivity, molar volume, and molar vaporization enthalpy) of 114 pure liquids can be explained in terms of only two parameters which are characteristic of the solvent molecule (Cramer 111, 1980). These two factors are correlated to the molecular bulk and cohesiveness of the individual solvent molecules, the interaction of which depends mainly upon nonspecific, weak intermolecular forces. 

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