As mentioned in the previous section, the volumetric behavior of a real substance rarely conforms to the ideal gas law. This nonideal behavior requires developing an accurate EOS for determining thermodynamic property changes in processes.
Most of the process streams in practice typically consist of mixtures. Even pure product streams typically contain (tolerable) levels of impurities. The complexity of behavior of mixtures and resultant deviation from the idealized behavior increases with the increasing complexity of interactions among the mixture constituents. The extensive properties are rarely additive; for example, the total volume of a liquid mixture is invariably less than the sum of the individual volumes mixed together. It follows that the intensive properties will also differ significantly from what can be estimated from mole- or mass-fraction weighted individual intensive properties. This results in nonideal behavior of the system, which has a significant impact both on energy-work interconversions and equilibrium conditions. The mathematical treatment of nonideality, particularly with respect to equilibrium problems, involves refining fugacity computations through determination of fugacity coefficients (ϕi) or activity coefficients (γi). This complex treatment is discussed in chemical engineering thermodynamics courses.
The next section provides an introduction to some of the computational problems in chemical engineering thermodynamics.
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