Cleaner production of essential oils by steam distillation
Introduction
Although essential oils are produced by different methods such as solvent extraction, expression and critical fluid extraction, most are produced by steam distillation [1]. The proportion of different essential oils extracted by steam distillation is 93% and the remaining 7% is extracted by the other methods [7]. Essential oils are multi-component chemicals. The mixture of oil compounds that constitute the essential oil comprises polar and non-polar compounds. Some of the compounds in the composite oil are lost in the wastewaters, as shown in the work by Fleisher and Fliesher [2] and Bohra et al. [3]. In the case where the vegetable material and water are mixed in the still, the oil is lost in the water in the still as well as in the aqueous phase of the distillate. During steam distillation of essential oils, the recovery of all organic constituents as the product depends on their partition between water and oil phases of the distillate. In the majority of cases the oil is less dense than the water and so forms the top layer of the distillate. The very important compounds that make up the chemicals usually referred to, as the top notes, in the fragrance industry are the polar compounds [4], [5]. This polarity makes the compounds soluble in water and this solubility is a function of the physical properties of the system such as pressure, temperature and chemical potential. In many steam distillation processes, vegetable material is mixed with water and the system is brought to a boil, a process commonly referred to as hydrodistillation [6]. The vapour is collected and condensed in order to separate the water from the oil fraction. However, the residual oil dissolved in the water usually causes odour nuisance and is also a waste of the valuable product in the water stream. Studies have been done to quantify and qualify these water-soluble compounds in distillation wastewater [6], [7].
In order to optimise the recovery of essential oils, the loss of some of the oil components such as the polar components in both the aqueous fraction of the distillate and the water in the still, the water has to be redistilled, a process called cohobation. Redistilling to process wastewater in order to recover the dissolved oil components results in increased utility cost, mainly heating or energy costs [8], [9], [10].
Section snippets
Experimentation—new process design
A batch of 750 g each of artemisia leaves and lavender flowers was packed in 0.002 m3 extractor at a time. The raw material forms the packed bed. The packed bed can theoretically be broken into small elements of volume from bottom to top. As steam is passed through the packed bed, it condenses in the first element before going to the next and so on, where it releases its enthalpy of vaporisation and the heat raises the temperature of solids to operation temperature. The solid vegetable particles
Results
The calibration graphs for the two major components, one for each of the two types of oils are given in Fig. 3, Fig. 4.
The chromatograms shown in Fig. 9, Fig. 10 are those of the water-soluble compounds in artemisia and lavender oil, respectively. In order to determine the dissolved polar compounds in the aqueous phase of the distillate, their maximum possible content in water can be determined from the calibration curves. In order to simplify the calculations not all the minor compounds in
Discussion
The optimum insulation thickness on the exterior circumference of the packed bed column has to be calculated so that it is exact. This eliminates excessive condensation in the packed bed. Consequently, the packed biomass bed operates as an isothermal system i.e. a constant temperature column. The high temperature that is maintained during the entire distillation period ensures that the water retained in the vegetable mass in the packed column is that which is lost when the steam loses its
Conclusions
The adoption of the new still with steam jacket design and operation of steam distillation processes result in improved recovery of valuable essential oils (Table 1, Table 2), energy saving and environmental improvement. The steam jacket design of the packed bed column provides a steady-high-temperature distillation process. Towards the end of the distillation cycle the yield drops so low that it is uneconomical to continue distilling. At that stage, relatively more oil is lost to the aqueous
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