MODELING OF PHASE EQUILIBRIA FOR AQUEOUS SOLUTIONS OF SUGARS USING A CUBIC EQUATION OF STATE

Abstract

ABSTRACTPRSV, which is the Peng‐Robinson equation of state improved by Stryjek and Vera in 1986, was used to model and predict the thermodynamic properties of water‐sucrose, water‐glucose, water‐fructose, water‐maltose and water‐xylose solutions. Binary interaction parameters were obtained by fitting the model to binary vapor‐liquid equilibrium data expressed as water activity. The water activity of synthetic honey and the boiling point temperature of pure sucrose solutions were calculated and compared with the experimental data. The fugacities of glucose and xylose were also estimated in aqueous saturated solutions at equilibrium with the solid phase, and these values were compared with data from the extrapolated sublimation pressure. The results showed that this method was able to estimate the water activity of sugar mixtures and the order of magnitude of the vapor pressure of sugars.PRACTICAL APPLICATIONSNowadays there is a tendency to employ high‐pressure processes, but activity coefficient models of the type UNIQUAC and UNIFAC are not appropriate for modeling phase equilibria. In these cases, the model employing the equation of state has been recommended to predict the thermodynamic properties. However, a simple cubic equation of state such as the original Peng‐Robinson equation of state (PR‐EOS) does not accurately predict the vapor pressure of pure water, which is critical for the calculation of equilibrium when the water molar fraction xw→1, so the temperature dependence of the attractive term of PR‐EOS was calculated from the PRSV‐EOS, the analytical form of the 1986 Stryjek and Vera equation, which can predict the vapor pressure of water. The main purpose of the present study was to model sugar‐water systems at low pressure, which should be useful for high‐pressure processes.