Phase Equilibrium Engineering of Supercritical Reactors

S. PEREDA; L.J. ROVETTO; S.B. BOTTINI; E.A. BRIGNOLE

Versailles, France

Simposio; 6th International Symposium on Supercritical Fluids; 2003

International Society for Advancement of Supercritical Fluids (ISASF)

The phase behavior of supercritical hydrogenation mixtures can be quite complex. Drastic changes in density and solubility with temperature, pressure and composition can be expected, due to the presence of a near-critical or supercritical solvent in a mixture with a permanent gas (hydrogen) and heavy liquid components. The large difference in molecular size and volatility between these components is likely to give rise to liquid phase unstability and multiphase behavior. The conceptual design of process conditions is discussed and procedures are presented to determine the range of single-phase operating conditions for two reaction problems: the hydrogenation of vegetable oils and the hydrogenolysis of fatty acid methyl esters. This is a typical problem of phase equilibrium engineering; i.e. the systematic study and application of phase equilibria to the development of chemical processes. For each process there will be a set of specifications, which represent problem restrictions. In the case of catalytic hydrogenation of liquid substrates these specifications are: a) components of the reactive mixture, i.e. reactants and products; b) operating temperature, given by the reaction kinetics and catalyst; c) degree of conversion, fixed by the hydrogenation goal. By applying phenomenological and modeling phase equilibrium engineering tools it is possible to find the reactor operating conditions which guarantee the existence of a single fluid phase, at any reaction time or reactor location, subject to the above specifications. The solution to this problem requires the determination of the following process variables: a) supercritical solvent; b) operating pressure; c) solvent/feed ratio. A group contribution equation of state is applied to predict the required phase equilibrium scenarios for solvent containing reactive mixtures. Model predictions are compared with recent experimental data. This complex behavior makes the phase equilibrium engineering of supercritical hydrogenation reactors a challenging problem from both, an experimental and modeling point of view.