Estimation of enthalpy changes

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. 

It is necessary to evaluate enthalpy changes for the following elementary processes  

Only the first of these will be elaborated on in this section—for details of (ii) to (v) see references 2-5. 

1 Use of heat capacities. The change in enthalpy at constant pressure due to a change in temperature from To to Tj is given by 

Determine the heat load required to heat a stream of nitrogen, flowing at 50molmin , from 20°C to 100°C. 

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For approximate estimation of enthalpy changes during reactions, use can be made of empirical bond energies (Table 6-7) which represent the approximate enthalpy changes (-AH°) for formation of compounds in a gaseous state from atoms in the gas phase. Other more comprehensive methods of approximation have been developed 49/50... 

An estimation of enthalpy changes for the formation of (M — 2) ions in pentenes [132] indicates that the reaction is definitely exothermic for normal-chain isomers, whereas it is definitely endothermic or only slightly exothermic for branched pentenes. This may explain the non-occurrence of the parent-minus-two reactions in branched pentenes. 

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