A mass transfer model for super-and near-critical CO₂ extraction of spearmint leaf oil

For the dynamic behavior in the super- or near-critical CO₂ extraction of essential oil components (carvone and limonene) from spearmint leaves, a mass transfer model was developed on the basis of intraparticle diffusion and external mass transfer. The effects of temperature, pressure, and CO₂ mass flow-rate on the model parameters which are the intraparticle effective diffusion and the external mass transfer coefficients, were investigated. Good agreement between the prediction by the model and the experimental data was obtained. When increasing the CO₂ pressure, the intraparticle effective diffusion coefficients mostly increased and the external mass transfer coefficients significantly increased by showing the enhanced mass transfer at an increased CO₂ density at a higher pressure. The effects of CO₂ flow rate on the extent of the extraction rate and external mass transfer coefficient were negligible. Also, the Biot numbers determined for the extraction runs were generally much larger than 5, which indicates the predominance of the internal mass transfer resistance over the external mass transfer resistance. The intraparticle effective diffusion coefficients estimated from the extraction data were markedly smaller than the molecular diffusivities determined by empirical estimation methods. Therefore, this study suggests the significance of the intraparticle mass transfer resistance in the CO₂ extraction of spearmint leaves, which implies that the complex matrix structure of the herbaceous spearmint leaf hinders the mass transfer inside leaf particles during the extraction process.