Supercritical compound

Brunner, G. Selectivity of supercritical compounds and entrainers with respect to model substances. Fluid Phase Equilib. 1983, 10, 289-298. 

Mathias, P. M. and J. P. O Connell. 1981. Molecular thermodynamics of liquids containing supercritical compounds. Chemical Engineering Science. 36, 1123. 

In Approach A, the fugacity coefficients of the liquid (pf and vapor phase are needed. They describe the deviation from ideal gas behavior and can be calculated with the help of equations of state, for example, cubic equations of state and reliable mixing rules. In Approach B, besides the activity coefficients s value for the standard fugacity is required. In the case ofVLE usually the fugacity of the pure liquid at system temperature and system pressure is used as standard fugacity. For the calculation of the solubilities of supercritical compounds Henry constants are often applied as standard fugacity (see Section 5.7). 

As mentioned before, Approach A (also called phase equilibrium behavior various other properties such as densities, enthalpies including enthalpies of vaporization, heat capacities and a large number of other important thermodynamic properties can be calculated via residual functions for the pure compounds and their mbctures. For the calculation besides the critical data and the acentric factor for the equation of state and reliable mixing rules, only the ideal gas heat capacities of the pure compounds as a function of temperature are additionally required. A perfect equation of state with perfect mixing rules would provide perfect results. This is the reason why after the development of the van der Waals equation of state in 1873 an enormous number of different equations of state have been suggested. 

While there is no difference for the calculation of gas solubilities in comparison to VLE in the case of the equation of state (approach A), for approach B, there is the problem that the standard state (pure liquid at system temperature and system pressure) used for VLE calculations cannot be used anymore, since supercritical compounds are not existent as liquid. This means an alternative standard fugacity is required for the y —y)-approach. 

As can be recognized from the results shown before, modified UNI FAC is a very powerful predictive model for the development and design of chemical processes, in particular separation processes. However, modified UNIFAC is a g -model. This means that it cannot handle supercritical compounds. For supercritical compounds either Henry constants have to be introduced or Approach A has to be used. In the latter case, an equation of state is required, which is able to describe the PvT behavior of both the vapor (gas) and the liquid phase. 

In the case of the group contribution equation of state VTPR, instead of temperature-independent group interaction parameters from original UNIFAC, temperature-dependent group interaction parameters as in modified UNIFAC are used. As for modified UNIFAC, the required temperature-dependent group interaction parameters of VTPR are fitted simultaneously to a comprehensive data base. Besides VLE data for systems with sub and supercritical compounds, gas... 

In Section 11.4, it was shown how suitable solvents can be selected with the help of powerful predictive thermodynamic models or direct access to the DDB using a sophisticated software package. A similar procedure for the selection of suitable solvents was also realized for other separation processes, such as physical absorption, extraction, solution crystallization, supercritical extraction, and so on. In the case of absorption processes or supercritical extraction instead of a g -model, for example, modified UNIFAC, of course an equation of state such as PSRK or VTPR has to be used. For the separation processes mentioned above instead of azeotropic data or activity coefficients at infinite dilution, now gas solubility data, liquid-liquid equilibrium data, distribution coefficients, solid-liquid equilibrium data or VLE data with supercritical compounds are required and can be accessed from the DDB. 

In the case of the ammonia synthesis, the enthalpy of ammonia decreases since attractive forces dominate, whereas for both highly supercritical compounds N2 and H2, the forces are mainly of repulsive nature. 

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