Limitations of Equilibrium Thermodynamics

In process simulation, one must be aware that equilibrium thermodynamics is not the only decisive issue. Often mass transfer or reaction kinetics must be taken into account to get results that ensure an appropriate equipment design. Special literature is available in these areas [3, 4]. 

Heat and mass transfer have an influence on each other, which can be taken into account by the Ackermann [5] correction factor E for the heat transfer stream 

Correlations for the mass transfer coefficient P can be obtained from the corresponding correlations for the heat transfer coefficients, taken from the literature for the particular geometrical constraints [10]. The diffusion coefficients necessary for the determination of P can be calculated as described in Section 3.3.6. 

Beyond the mass transfer problem, it has to be considered if aerosol formation plays a major role [11]. 

It is assumed that the mass tiansfer occurs in the layers on both sides of the phase boundary. According to the model, the mass transfer is caused by molecular diffusion. Therefore, a linear concentration profile is built up in the boundary layers. At the phase boundary itself, thermodynamic equilibrium is reached. More or less empirical correlations for the mass transfer coefficients /I and for the effective phase interface area are usually provided by the simulation program for different column internals (random packings, structured packings, and trays). The necessary diffusion coefficients can again be calculated according to Section 3.3.6. 

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We next begin our development of Onsager s theory, which transcends some of the limitations of equilibrium thermodynamics. 

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