Kinetic modeling and process optimization of Hibiscus rosa-sinensis flower oil extraction using microwave-assisted hydrodistillation

Microwave-assisted hydrodistillation (MAHD) has been considered as a green extraction process for the recovery of essential oils from medicinal and aromatic plants. The present study deals with extraction kinetics, mass-transfer behaviour, process optimisation and chemical characterisation of Hibiscus rosa-sinensis flower oil using MAHD. Response Surface Methodology (RSM) employing a Central Composite Design (CCD) was applied to investigate the effect of extraction time, microwave power and methanol-to-plant material ratio on the oil recovery. The best conditions for operation, determined by RSM optimisation were: microwave power 300 W, methanol to plant material ratio 8:1 and time of extraction 120 min, which gave oil yield of 1.25%. The kinetic studies were then prolonged up to 180 min under the optimised condition to further probe the extraction behaviour with the greatest cumulative oil yield of 1.48%. The extraction profile showed the typical three-step behaviour of a fast-washing step, a diffusion-controlled extraction step, and an equilibrium step. roughly 56.1% of maximum extraction yield was attained within the first 30 min whereas roughly 83.8% of final yield was reached within 120 min. Gas Chromatography–Mass Spectrometry (GC–MS) analysis showed a complex mixture of volatile chemicals where Ethyl ethanimidate was the major ingredient. The kinetic study demonstrated that the extraction rate was strongly influenced by the mass-transfer behaviour, and the observed extraction profile was consistent with the principles of Fick’s second law of diffusion. Among the kinetic models studied, the Peleg model had the best fit to the experimental data, highlighting the significance of mass-transfer processes during extraction. Microwave power was shown to have a substantial effect on the extraction efficiency, and the maximum oil recovery was achieved at 300 W, however the extraction performance decreased slightly at higher power levels. Similarly, the ratio of methanol to plant material was found to be very important in maximising the extraction yield and an optimum ratio of 8:1 was obtained. The results demonstrate the potential of MAHD for the extraction of Hibiscus rosa-sinensis flower oil under the investigated laboratory conditions and provide useful information for process optimization, kinetic modelling, mass-transfer interpretation, and future scale-up evaluation.