Studies in the development of a continuous process for the extraction of essential oils using microwaves

Resumo

The industry of the extraction of essential oils has used basically the same process for more than 100 years. Steam distillation is a slow and energy consuming process. But there are alternative technologies such as microwave energy that can improve efficiency and energy consumption of this process. But in order to develop a process based on the use of this energy, it is first necessary to understand the behavior of its main variables, their relationship and their evolution during the process. Thus, the aim of this work is to propose tools that allow this. These include experimental, analytical and numerical tools. Thus, the dielectric properties of the plant material treated are measured in order to characterize the interaction of the electromagnetic energy with the material. They were measured in low moisture rosemary, essential oil, and dried material. Dielectric properties are reported for these materials at one atmosphere and from 20ºC to 160ºC. The frequencies used were 2450MHz and 910 MHz. A new approach to the dielectric properties estimation of low moisture plant materials is proposed. In this approach the inert plant material is considered as a homogeneous phase in the mixing models presented in literature. The heat and mass transfer during the extraction of essential oils was studied experimentally, analytically and numerically. During experiments hot spots were detected when the sample was placed in the center of the oven. Thus a rotating system and a semicontinuous system were used to analyze the evolution of the system. Extraction of essential oils with microwaves is influenced by the power applied, the moisture content and the equilibrium between surface oil and vapor passing over. During microwave distillation when more power was applied the yield was increased. When different moisture contents are compared, an important diminish can be seen in yield obtained with microwaves. The proposed analytical model fitted well the experimental values and can be used to study the behavior of the SFME extraction as a function of the power applied. The microwave heating is underpredicted in the area closer to the walls, by the numerical model, thus the evaporation resulting in the simulations is lower than the measured one. Nevertheless, the results show a behavior very similar to the experimental values and the model can be used to simulate the microwave heating during SFME.