Enthalpy ideal-gas

The thermal properties of an ideal gas, enthalpy, entropy and specific heat, can be estimated using the method published by Rihani and Doraiswamy in 1965 ... 

The specific enthalpy of a gas is calculated using the principle of corresponding states. The enthalpy of a gas mixture is equal to the sum of the ideal gas enthalpy and a correction term ... 

Cmpd. no. Name Formula CAS no. Mol wt Ideal gas enthalpy of formation, J/kmol X lE-07 Ideal gas Gihhs energy of formation, J/kmol X lE-07 Ideal gas entropy, J/(kmol-K) X lE-05 Standard net enthalpy of combustion, J/kmol X lE-09... 

One problem remains. The reference enthalpy must be defined at temperature T and pressure P0. The reference state for enthalpy can be taken as an ideal gas. At zero pressure, fluids are in their ideal gaseous state and the enthalpy is independent of pressure. The ideal gas enthalpy can be calculated from ideal gas heat capacity data3 ... 

The ideal gas enthalpy can be correlated as a function of temperature, for example3 ... 

To calculate the enthalpy of liquid or gas at temperature T and pressure P, the enthalpy departure function (Equation 4.78) is evaluated from an equation of state2. The ideal gas enthalpy is calculated at temperature T from Equation 4.81. The enthalpy departure is then added to the ideal gas enthalpy to obtain the required enthalpy. Note that the enthalpy departure function calculated from Equation 4.78 will have a negative value. This is illustrated in Figure 4.9. The calculations are complex and usually carried out using physical property or simulation software packages. However, it is important to understand the basis of the calculations and their limitations. 

Both enthalpy and entropy can be calculated from an equation of state to predict the deviation from ideal gas behavior. Having calculated the ideal gas enthalpy or entropy from experimentally correlated data, the enthalpy or entropy departure function from the reference state can then be calculated from an equation of state. 

Now calculate the molar enthalpies of the vapor and liquid streams. Enthalpies were calculated here from ideal gas enthalpy data corrected using the Peng-Robinson Equation of State (see Chapter 4) ... 

Figure 9 provides a comparison of the predictions of empirical methods with Wormald s data for a 50/50 mole percent mixture of steam and methane. As can be seen, the frequently used artifices of calculating mixture enthalpies by blending the pure component enthalpies at either total or partial pressures are very inaccurate. Likewise, the assumption of ideal gas enthalpy for the real gas mixture, equivalent to a zero enthalpy departure on the diagram, is an equally poor method. 

Steele, W.V., Chirico, R.D., Cowell, A.B., Knipmeyer, S.E., and Nguyen, A. Thermodynamic properties and ideal-gas enthalpies of formation for 2-aminoisobutyric acid (2-methylalanine), acetic acid, ( -5-ethylidene-2-norbornene, mesityl oxide (4-methyl-3-penten-2-one), 4-methylpent-l-ene, 2,2 -bis(phenylthio)propane, and glycidyl phenyl ether (1,2-epoxy-3-phenox3q>ropane), / Chem. Eng. Data, 42(6) 1053-1066, 1997. 

The enthalpy models are actually for the enthalpy departure function. Normally, the molar enthalpy of a phase is found from the ideal gas enthalpy and the enthalpy departure... 

H°j Molar ideal gas enthalpy of vapor phase, stage j. 

Compute the ideal-gas enthalpy change for p-xylene between 289 and 811 K (61 and 1000°F), assuming that the ideal-gas heat-capacity equation is (with T in kelvins)... 

Compute the ideal-gas enthalpy difference. The ideal-gas enthalpy difference (//2° — H ) is obtained by integrating the C° equation between two temperature intervals ... 

In an ideal gas, enthalpy is a function of temperature only, and temperature remains constant. 

Example Estimate the standard and ideal gas enthalpies of formation of o-toluidine. 

Example Estimate the standard ideal gas enthalpy of formation of... 

Since U, P, and V are state functions, enthalpy is also a state function. Like internal energy, for an ideal gas enthalpy depends only on temperature. Enthalpy is an extensive property. 

Chapters 2-5 deal with chemical engineering problems that are expressed as algebraic equations - usually sets of nonlinear equations, perhaps thousands of them to be solved together. In Chapter 2 you can study equations of state that are more complicated than the perfect gas law. This is especially important because the equation of state provides the thermodynamic basis for not only volume, but also fugacity (phase equilibrium) and enthalpy (departure from ideal gas enthalpy). Chapter 3 covers vapor-liquid equilibrium, and Chapter 4 covers chemical reaction equilibrium. All these topics are combined in simple process simulation in Chapter 5. This means that you must solve many equations together. These four chapters make extensive use of programming languages in Excel and MATE AB. 

Steele, W. V. Chirico, R. D. Cowell, A. B. Knipmeyer, S. E. Nguyen, A. (2002). Thermodynamic properties and ideal-gas enthalpies of formation for /rans-methyl cinnamate, a-Methyl cinnamaldehyde, methyl methacrylate, 1-nonyne, trimethylacetic acid, trimethylacetic anhydride, and ethyl trimethylacetate, J. Chem. Eng. Data, 47, pp. 700-714. 

For gases at relatively low pressures, the ideal-gas law is often sufficient to calculate the density. The ideal-gas heat capacity (and its integrated form, the ideal-gas enthalpy) are used not only for calculating the properties of gases assumed to be ideal, but also as a starting point in many calculations of the heat capacity and enthalpy of nonideal fluids. 

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