Enthalpy Description in Process Simulation Programs

The enthalpy difference at a given temperature from the ideal gas state at P 0 and a state at an arbitrary pressure in the gas or liquid phase can be calculated with an equation of state. For nonassociating substances, the cubic equations are often used in process simulations. The corresponding expressions for the pure component enthalpy departure for the most often applied equations are  

For the specific volume, both the liquid and the vapor can be inserted, depending on the phase that is regarded. 

In process simulation, it is necessary that the enthalpy can be continuously described in the liquid as well as in the vapor phase. With this purpose, the problem arises that the particular quantities that contribute to the enthalpy are not independent from each other. For any calculation strategy (route), there will be one quantity left which can be calculated using the others. The three most widely used routes are discussed below. 

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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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