Modeling phase behavior of multicomponent mixtures of wood preservatives in supercritical carbon dioxide with cosolvents

Significant efforts have been devoted to development of methods for using supercritical fluids, primarily carbon dioxide, to impregnate solid wood and wood composites with biocides for improved resistance to decay. Studies of phase behavior at both supercritical and subcritical (with respect to pure carbon dioxide) temperatures and pressures is an essential part of this development process since they can be used to optimize treatment cycles and help to explain variations in results. Compositions of coexisting phases were measured for a CO2/acetone/TCMTB (2-(thiocyanomethylthio) benzothiazole) system at three sets of temperature and pressure for TCMTB at two levels of purity using a stoichiometric method. The phase behavior of complex SCF solutions is essential for reliable scale-up of these processes, however, such measurements are very time-consuming. Mathematical models were developed in an effort to reduce the number of required experimental studies. This paper presents results of efforts to combine existing simple phase models to predict phase compositions for four different ternary systems including the simplest realistic mixture expected in the wood treatment process: carbon dioxide, a cosolvent and a biocide. The objective of this research was to develop and analyze a simple, theoretically-based model with a minimum number of fitting parameters. To sec if such a simple model provided useful results, capabilities of different models were examined and compared for predicting vapor-liquid-liquid equilibria (VLLE) of ternary systems. Two different equations of state and three different mixing rules were used. For the CO2/acetone/TCMTB mixture, models were only successful at conditions far from the critical point. (C) 2001 Elsevier Science B.V. All rights reserved.