*4.1.3 Compressed air microwave distillation (CAMD)*

This method (**Figure 4**) uses the principle of steam entrainment, with compressed air instead of steam, to extract the essential oil. The extraction process consists of a compressor, a microwave oven and a refrigeration system. Compressed air is injected into the reactor, where the matrix is heated by microwaves and immersed in water. The steam, saturated with volatile molecules, is directed to a recovery container located outside the microwave oven and cooled by a refrigeration system. In just a few minutes, the water and aromatic molecules are condensed and recovered [30, 32].

A similar method using a condenser to cool the extraction gas (temperatures ranging from −20 to −15°C) has also been patented [33]. This extraction method is environmentally friendly, as no organic solvents or artificial chemical compounds are added.

**Figure 3.** *Experimental set-up for microwave solvent-assisted extraction.*

#### **Figure 4.**

*A schematic representation compressed air microwave distillation.*

### *4.1.4 Microwave hydrodistillation (MWHD)*

The MWHD (**Figure 5**) was developed by Stashenko et al., in 2004. It is based on the classic hydrodistillation principle. The process consists of a hydrodistillation unit placed inside a domestic microwave oven with a side port, through which an external glass condenser is connected to the round filter containing the matrix and water [34].

Microwave hydrodistillation is a widely used technique for extracting essential oils from various aromatic plants and spices, with examples such as *Thymus vulgaris* L., *Zataria multiflora* Boiss., and *Satureja montana*.

**Figure 5.** *Microwave hydrodistillation.*

### *Essential Oil Extraction Process DOI: http://dx.doi.org/10.5772/intechopen.113311*

An improved version of this technique was developed in 2007, which involves introducing a microwave coaxial antenna insulated inside a glass flask containing the matrix and water [35]. This in situ microwave heating offers advantages in terms of time and energy savings and can be useful for industrial applications.

Microwave Steam Distillation (MSD) (**Figure 6**) is another innovative technique that was developed. It is based on the conventional steam distillation principle and has been successfully used for the extraction of essential oil from Lavender flowers [30].

### *4.1.5 Solvent-free microwave extraction (SFME)*

This is one of the most recent techniques for the microwave-assisted extraction of essential oils, without solvents and using water at atmospheric pressure. The SFME process consists mainly of four parts: a reactor where the matrix to be treated is placed, a microwave oven, a cooling system, and an essential oil container where the oil is collected (**Figure 7**).

The process is based on a relatively simple principle, described as microwaveassisted dry distillation; the fresh matrix is placed in a microwave reactor without the addition of water or organic solvent. Heating the raw material with water breaks down the glands containing the essential oil. This phase releases the essential oil, which is then carried away by the steam produced by the water in the matrix. A cooling system located outside the microwave oven enables continuous condensation of the distillate, composed of water and essential oil, and the return of excess water

**Figure 6.** *Microwave steam distillation (MSD).*

**Figure 7.** *Improved solvent-free microwave extraction (improved SFME).*

to the bottle, thus maintaining the appropriate moisture content of the matrix. For example, Milestone's "DryDist" laboratory microwave oven makes it easy and efficient to extract high-quality essential oils.

Wang et al. in 2006 proposed an improved SFME extraction method. The method is based on the addition and mixing of carbonylated iron powder with the dry matrix. Spherical particles of carbonylated iron are capable of absorbing part of the energy emitted by microwaves and returning it to the medium in the form of heat. In this way, the matrix can be heated by simple conduction without any auxiliary energy. Various types of materials such as activated carbon, graphite powders, and ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate) can absorb microwave radiation [36].
