Route C Equation of State

As discussed in Chapter 2, all the information about the nonideality can be derived from an equation of state. Similarly to Route A, the ideal gas at standard conditions is taken as the reference state  

The enthalpy of the ideal gas mixture is calculated using Eq. (6.57). To obtain the enthalpy for a real phase, h-h ) must be added, which is calculated by an equation of state valid both for the liquid and for the vapor phase  

For the Soave-Redlich-Kwong and the Peng-Robinson equation, expressions for (h-fi ) are given in Eq. (6.53) and Eq. (6.54), respectively. The relationship for 

For cp, the same problem arises as for Route A. Analogously, it can be overcome by fitting the a-function both to vapor pressure and cp data. If a generalized equation of state like the Peng-Robinson or the Soave-Redlich-Kwong equation of state is used, there is no way to overcome this difficulty. 

For the handling of enthalpies in a process simulation program, the change of a model between two blocks is often critical. This problem has much to do with the enthalpy description. Between the two blocks, the simulation program hands over the values for P and h to describe the state of the stream. According to the particular models used in the two blocks, the stream is assigned with two different temperatures that may differ significantly. 

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