Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

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8 MICRODISTILLATION,THERMOMICRODISTILLATION AND MOLECULAR DISTILLATION TECHNIQUES Figure 2: Flowsheet of microwave-assisted extraction (courtesy, Radient Technologies Inc). 8.4 Microwave-assisted Hydrodistillation The ability of microwave radiation to heat solid material effectively can also be used for obtaining essential oils. Thus, the herb is placed in a microwave cavity and irradiated with microwaves. This process yields essential oils consisting of relatively low volatile fractions as compared to hydrodistillation. For instance, in coriander oil, the percentage of tetradecanoic and hexadecanoic acid increased whereas that of linalool decreased. This is possibly due to the poor stability of linalool, a tertiary alcohol. Dill seed oil obtained by microwave-assisted hydrodistillation (MWAHD) contained greater quantities of compounds with higher boiling points and lesser quantities of compounds of lower stability. These and other fi ndings indicate that MWAHD is better for extracting stable, high-boiling point components, whereas it is not suitable for recovering chemically unstable compounds. 136
EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS 8.5 Molecular Distillation or Short Path Distillation Molecular distillation (MD), also known as short path distillation, is a fairly well established technique. In view of this, the discussion of MD is restricted to its principle, advantages and applications in the processing of MAPs. 8.5.1 Principle of MD The term MD refers to a non-equilibrium process. The still used has an evaporating surface very close to a condensing surface. Under very low pressures, this results in a situation where the distance traveled by the evaporating molecules is comparable to the mean free path of the molecules. The nomenclature MD is derived from this particular condition under which the so-called distillation is carried out. 8.5.2 Advantages of MD 1. Operating pressures as low as 0.001 mbar can yield relatively low processing temperatures, thereby reducing thermal degradation. 2. Agitated fi lm MD units can process high viscosity feeds with very good turndown. 3. Combination of low pressures and high temperatures (up to 300° C) allows processing of extremely high-boiling materials without degradation. 4. Short exposure to high temperature (low residence time) prevents degradation. 5. Very low liquid hold-up allowing use in applications involving low volume, high value materials. 6. Available in low (laboratory scale) to high heat transfer areas to suit the requirements. 8.5.3 Separation Efficiency of MD For high viscosity liquid fi lms falling under gravity, agitated fi lm MD units perform far better than those without agitation of the fi lm. This is due to the fact that, particularly for high viscosity liquids, the agitation of the fi lm renews the surface more frequently than when there is no agitation. The surface renewal model is useful for predicting the effi ciency η of MD stills without mechanical control. 137
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