Filipe Gaspar

Modelling the extraction of essential oils with compressed carbon dioxide

Abstract Three models were used to describe the extraction of essential oils from oregano bracts using compressed carbon dioxide. They were developed on the basis of a plate-like geometry of the particles and were tested experimentally using various bract pre-treatments, pressures, temperatures and solvent flow rates. The two particle phase derived models applied, the Single Plate model (SP model) and the Simple Single Plate model (SSP model), differ only by the allowance or not for a film coefficient (kf) and have as the only adjustable parameter the matrix diffusivity (Dm). The third model, the Fluid Phase/Simple Single Plate model (FP/SSP model), considers both the particle mass balance and a detailed description of the fluid mass balance. In addition to the matrix diffusivity the FP/SSP model may require the adjustment of the fraction of oil leached by the solvent during the pressurisation procedure (f0). All models gave a good fit to the experimental data though the FP/SSP model gave the best fit. However, the matrix diffusivities found correlated very poorly with the estimated diffusivity of essential oils in carbon dioxide (D12), which indicates a complex intraparticle transport.

Disruption of glandular trichomes with compressed CO2: alternative matrix pre-treatment for CO2 extraction of essential oils

Abstract The disruption of essential oils glandular trichomes by contact with compressed carbon dioxide followed by rapid decompression was studied at isothermal condition (310 K). The pre- and post-expansion pressures, the exposure time to the pre-expansion pressure and the rate of decompression were all found to have a significant effect on the efficiency of the disruption process. The efficiency of the disruption process (percentage of glands disrupted during the fast decompression) was deduced from the results of subsequent extraction tests using compressed CO 2 under standard conditions. The damage to the glands was observed by SEM microscopy. The efficiency of the disruption process seems to be closely related to the amount of gas dissolved within the glands, the rate of decompression of the bed and the permeability of the glands. The experimental results revealed that for an exposure time of 60 min and a rate of decompression of 2 kg m −3 s −1 , the decompression from 70 barg to atmosphere led to a maximum efficiency of the disruption process. At these conditions, the improvement in the extractability of essential oils by compressed CO 2 was similar to that obtained by cryogenic comminution of the matrix and better than that obtained by ambient comminution. One advantage of the fast decompression treatment method is that it virtually eliminates the losses of essential oils observed in mechanical treatments. A further advantage is that it selectively liberates the essential oils with respect to higher molecular weight material. This results in a final extract with a higher content of essential oils.

Modelling the disruption of essential oils glandular trichomes with compressed CO2

Abstract A theoretical model is proposed for the evaluation of the disruption efficiency of essential oils glandular trichomes (glands) with compressed CO 2 . The disruption of glands occurs during the fast decompression of the bed of herbaceous material. The glands are described as closed structures slightly permeable to CO 2 . When exposed to compressed CO 2 , the gas slowly penetrates the glands and dissolves in the intraglandular oil until the solubility limit is reached. During the fast decompression of the bed, the dissolved gas is desorbed from the oil phase and discharged to the bulk solvent. The inability of the glands to discharge the gas, at a rate dictated by the loss of solubility in the oil with the decompression of the bed, generates a pressure gradient across the glands that may lead to its rupture. In the present model, the excess pressure is described by an equation similar to Hagen–Poiseuilles formula for viscous flow due to a pressure gradient. The maximum pressure gradient across the glands during the fast decompression of the bed is then used to calculate the percentage of glands disrupted (efficiency of disruption) assuming a normal distribution of the bursting pressures of the glands. The model was applied to experimental results where the effects of pre- and post-expansion pressure, exposure time to pre-expansion pressure and the rate of decompression were investigated. Predictions of the model are also presented.

Comparison Between Compressed CO2 Extracts and Hydrodistilled Essential Oil

Abstract The yields and compositions of volatile concentrates and essential oils obtained from compressed CO2 extraction and hydrodistillation of a model herb, respectively, are compared. The experimental tests were undertaken on Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart using both intact and communited bracts. The yields and compositions obtained from the communited bract sizes compared well for both extraction techniques. This is an indication that degradation of the oil during the distillation process was minimal. However, when the intact bracts were used, the compressed CO2 extraction yield was significantly lower than that achieved by hydrodistillation. This stresses the need for an efficient pre-treatment step for the CO2 extraction of volatile concentrates.

Essential Oil from Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart: Content, Composition and Distribution Within the Bracts

Abstract The yield and composition of the essential oils from different size bracts of Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart were measured. The bracts from the herb were separated in six different sizes by a sieving process. The essential oil content was determined by hydrodistillation for each sieving class. The smaller bracts were found to have a much higher content of oils. The three smallest classes of bracts, representing 43% of the total mass of bracts, contained 75% of the oils. Moreover, with the increase of bract size a decrease in the concentration of the monoterpene hydrocarbons, p-cymene and γ-terpinene, was observed; in contrast an increase of the oxygenated monoterpenes, linalool, α-terpineol and the phenol derivative thymol, was observed.

Effect of Flow Rate on the Extraction of Volatile Concentrates and Resinoid Compounds from Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart using Compressed CO2

Abstract The effect of CO2 flow rate on the extraction of the volatile concentrate and resinoid compounds from Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart was studied at liquid (300 K, 70 bar) and supercritical (310 K, 100 bar) conditions. The mass flow rate of CO2 was varied between 0.3 kg/h to 0.9 kg/h with 0.2 kg/h increments. The degree and rate of extraction of the volatile concentrate was little dependent on CO2 flow rate and was controlled by intraparticle resistances. An extract slightly richer in volatile concentrate was collected at liquid CO2 conditions. The extraction of the resinoid compounds was mostly dependent on the flow rate of CO2 and was limited by their solubility in compressed CO2.

The Effect of Water on the Solubilities of Essential Oils in Dense CO2

Abstract The solubility of carvacrol and limonene were measured in the presence of water-saturated CO2 between pressures of 80-300 bar and temperatures of 40°C and 50°C. A decrease in the solubilities of both components was noted in comparison to tests with dry CO2. This behavior was attributed to the low ability of carvacrol and the inability of limonene to form hydrogen bonds and the lack of dipole interactions.