Effects of geometry and Reynolds number on the production of essential oil were investigated. Experiments, performed on an oregano packed bed, were designed to reproduce expected plant operating conditions. A diffusive model was found to apply to oil extraction and effective diffusion coefficients were calculated as a function of the bed L/D ratio and Reynolds number. Effective diffusivities were found to be of the same order of magnitude as those for drying of similar materials (congruent to 10(-9) m(2)/s). Statistical analysis showed that RE effects are more important, but that extraction is also affected by LID variations. Application of this information to select the optimum operation parameters needs additional plant-specific, cost-related data. (C) 2000