Measurement and modelling of urea solubility in supercritical CO 2 and CO 2 + ethanol mixtures

Abstract

The solubility of urea in supercritical CO 2 and CO 2 + ethanol was measured over the pressure and temperature ranges 100–300 bar and 313–373 K, respectively, and ethanol concentrations of 0–25 mass% (urea free basis). The solubility in CO 2 was measured by a once-through packed bed gravimetric method at a laboratory and pilot scale. The solubility in CO 2 + ethanol was measured using two different methods: antisolvent precipitation and a gravimetric packed bed method. The solubility of urea in pure CO 2 is very low, ranging from 1.1 × 10 −6 mol fraction at 100 bar, 333 K to 3.1 × 10 −5 at 300 bar, 353 K. The solubility increases exponentially with a linear increase in ethanol content. The highest solubility measured was ∼1 × 10 −2 mol fraction at 333 K, 150 bar and 26% ethanol. At high ethanol contents, the solubility was almost independent of pressure at a fixed temperature. The solubility was correlated by the Peng–Robinson equation of state with standard mixing rules for the repulsive and attractive parameters; and mixing rules that incorporated the formation of a complex between urea and ethanol. The solubility of urea in ethanol, and ethanol in CO 2 was also modelled to obtain interaction parameters for the model. It was not possible to find pairs of interaction parameters that modelled both the solubility in pure CO 2, and CO 2 + ethanol using the standard Peng–Robinson mixing rules. However, the solubility of urea in pure CO 2, and CO 2 + ethanol, was well modelled using the Peng–Robinson EOS that incorporated complex formation between urea and ethanol. Assuming an ethanol/urea complex stoichiometry of 2:1 gave a good fit between model predictions and experimental results.