Enthalpy estimation methods

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

The program sets four criteria, leading to a three-level qualitative classification low risk, medium, high for each of them. Each criterion quantifies an aspect of the decomposition risk. So these four classifications need to be taken into account to arrive at a final estimation. Someworkers have tried to use a sole criterion, which mathematically combines the four criteria, but failed. Three out of these four criteria involve calculating the enthalpies of decomposition and combustion of the particular compound. In order to do so it is necessary to know the enthalpies of formation of the compound and of the decomposition and combustion products. A lot of these values are inevitably absent in Part Three, so it was thought necessary to include estimation methods for enthaipies of formation as weil as for enthalpies of vapourisation/condensation, since in many cases there is only available the value for the physical state of the compound that is not always appropriate. 

Enthalpy 2900-3800 n/a n/a n/a Forest et al. 1995 Gregory and McIntosh 1996 Wolfe et al. 1998 Included for comparison with above estimates. Method is not based on lapse rates... 

A pure component constant that is occasionally used in property correlations is the enthalpy of vaporization at the normal boiling point, AHvbp. In addition, several special estimation methods are suggested. The Chen equation (15) gives a relation among the enthalpy of vaporization, the reduced vapor pressure, and the reduced temperature. When applied to the normal boiling point, Eq. 2 is obtained, with an average error of 2% ... 

In view of the above survey, neither correlation nor estimation method has been developed to determine the enthalpy of formation and the absolute entropy of coal-derived liquids. 

Derived using equation 15. When available, enthalpy of hydrogenation A + H2 AH2, is taken from direct measurements (27c) or differences between measured gas-phase enthalpies of formation (27a) otherwise, estimation methods (1,7b) are employed. Except where noted, bond strengths are derived from recently recommended enthalpy of formation at 298 K of benzyl radicals (47.0 kcal/mol) (Ilf). 

This chapter has emphasized again the advantage of having A, G ° for an enzyme-catalyzed reaction as a function of temperature, pH, and ionic strength. If magnesium ions or other ions are bound by reactants, the free concentrations of more ions can be included as independent variables. This chapter has also emphasized the value of calorimetric data. More standard transformed enthalpies of reaction need to be measured so that temperature effeets can be calculated for more reactions. The database can also be extended by use of reliable estimation methods based on species properties. This may be especially useful with larger biochemical reactants where reactive sites are nearly independent. 

Stolten has developed a semi-empirical estimation method to obtain enthalpies of formation of carbides, nitrides and oxides in metastable crystallographic structures. The calculations involved are based on the classical alloying criteria of atomic size, valence and electronegativity, and incorporate a correction factor obtained... 

The Kesler-Lee correlations for liquid and vapour phase heat capacities of petroleum fluids are used for estimating the respective enthalpies at temperatures of interest. The Lee-Kesler corresponding-states method is used for obtaining estimates of the heats of vaporization and for developing the saturation envelope enthalpies. This method uses the Curl and Pitzer approach and calculates various thermodynamic properties by representing the compressibility factor of any fluid in terms of a simple fluid and a reference fluid as follows ... 

Enthalpy of Fusion The enthalpy (heat) of fusion AH is the difference between the molar enthalpies of the equilibrium liquid and solid at the melting temperature and 1.0 atm pressure. There is no generally applicable, high-accuracy estimation method for AH , but the GC method of Chickos can be used to obtain approximate results if the melting temperature is known. 

Table 5.5 Comparison in accuracy between different methods for enthalpy estimation ...

This study reports bond energies, enthalpy, entropy, heat capacity, internal rotation potential, and structure data for a series of unsaturated peroxides. Thermochemical property groups are developed as well for future use in group additivity estimation methods. 

Smith et al. [64] reviewed kinetics of the WGSR and proposed micro- and macro-kinetic models. The micro-kinetic method is based on the knowledge about the elementary steps that are involved in the reaction and its energetics. This method explores the detailed chemistry of the reaction. Using this method it is possible to estimate the smface coverage, reaction order and activation enthalpy. This method provides the accm-ate pathway and prediction of the reac-ti(Mi, but is computationally intensive. On the other hand, the empirical models are based on the experimental results and are typically expressed in the Arrhenius model and provide an easy and computationally lighter way to predict the rate of reactiOTi. 

To apply equation 7.3.3 the enthalpy of inlet and outlet streams must be obtained. For a few common substances tables of thermodynamic properties are available (see reference 3 for a reasonably up-to-date list). In most cases a correlation or estimation method has to be used. The thermodynamic background is outside the scope of this chapter but is well covered in standard texts. Reid et alf have produced a compendium of correlation/estimation methods for a wide range of thermophysical properties. Simple correlations for use in energy balances have been compiled by Himmelblau. Most of the data used in this section are taken from that source. 

However, especially the experimental determination of critical constants usually requires a high experimental effort. Moreover, a lot of substances used in chemical industry decompose at high temperatures before the critical point is reached. Thus, these quantities are often not available from experiment. Therefore, reliable estimation methods are necessary, as the estimation of many other properties (density, thermal conductivity, surface tension, heat capacity, vapor pressure, and enthalpy of vaporization) is based on the critical data and needs reliable input data. In the following section, the currently most frequently used estimation methods for T,-, Pc, and at are introduced, as well as the ones for the normal boiling point, critical volume, and melting point. 

The problem of differences between large numbers has always to be taken into account. Relatively small errors in determining AH and especiallyA can lead to significant errors in estimating the enthalpy of reaction or the equilibrium constant, respectively. Therefore, the results of estimation methods should be handled with care. The following example gives an impression about the sensitivity of Ah and Agf... 

The significance of this equation is that it provides an estimation method for the activation energy of a reaction based on a reference system and the difference in reaction enthalpies between both, a is positive and normally in the range 0 1 (often 0.5). Its value can be determined for a family of reactions if kinetic data for two reactions of different exo-thermicity are available. From equation (137) it is clear that the barrier for a reaction within a reaction family increases with decreasing exo-thermicity. 

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 ... 

General Properties of Computerized Physical Property System. Flow-sheeting calculations tend to have voracious appetites for physical property estimations. To model a distillation column one may request estimates for chemical potential (or fugacity) and for enthalpies 10,000 or more times. Depending on the complexity of the property methods used, these calculations could represent 80% or more of the computer time requited to do a simulation. The design of the physical property estimation system must therefore be done with extreme care. 

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). 

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