Molecular Thermodynamics for Chemical Process Design

Thermodynamic properties are essential for quantitative process design to produce chemical products. Caloric properties are required for heat balances, but these properties are usually available or estimated easily. More important—and often much more difficult to estimate—are the chemical potentials of components in mixtures; it is these potentials which determine phase equilibria, as required for separation operations, and chemical equilibria, as required for chemical reactors and for separation operations based on chemical reactions. Molecular thermodynamics is an engineering‐oriented science for calculating the desired chemical potentials from a minimum of experimental data. This applied science, based on classical and statistical thermodynamics, yields chemical potentials through models that are based on molecular physics and physical chemistry. Selected examples are cited to illustrate the applicability of molecular thermodynamics: group‐contribution methods for obtaining chemical potentials in highly nonideal mixtures as required for distillation‐column and process‐safety design; equation of state for precipitation of uniform‐sized crystals from supercritical fluids; molecular‐orbital calculations to guide process development for alternatives to environmentally dangerous chlorofluorohydrocarbons; molecular‐simulation calculations for separation of gas mixtures with porous adsorbents; equilibria in two‐phase aqueous systems for separation of protein mixtures; and, finally, extended polymer‐solution thermodynamics to guide synthesis of hydrogels suitable for protein recovery from soybeans and for novel drug‐delivery devices.