Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

8 MICRODISTILLATION,THERMOMICRODISTILLATION AND MOLECULAR DISTILLATION TECHNIQUES
dividual δ h , δ d and δ p values for compounds not listed in the tabulation can
be obtained using the group contributions due to various different groups
given by Grulke. When the individual δ i (solvent) and δ i (solute) values are
very close, a high solubility of the solute in the solvent is obtained. For
instance, it is well known that non-polar solvents dissolve terpene fractions
more than oxygenated compounds because both are non-polar. On
the other hand, mixed solvents of polar and non-polar compounds can yield
better results for oxygenated compounds. Bio-ethanol is a good solvent for
such oxygenated compounds on two accounts: (i) it is natural, and (ii) it is
“green” (renewable). However, most MAPs contain water and the complete
miscibility of ethanol with water implies dilution of the solvent after each
use. This is further complicated by the fact that ethanol forms an azeotrope
at high concentration (~95 wt%). As a result, ingress of small quantities of
water is suffi cient to reach the azeotropic composition. Implementation of
the Montreal Protocol, the Clean Air Act, and the Pollution Prevention Act of
1990 has resulted in increased awareness of organic solvent use in chemical
processing.
8.3.2 Solid-liquid Mass Transfer
The MAPs to be processed are in solid form. Solid-liquid extraction
is a typical heterogeneous mass transfer process. In such processes,
the rate of extraction depends upon: (i) the interface area, and (ii) the mass
transfer coeffi cient. Both should be high. High effective interface area can
be obtained by comminuting the solid material to be processed. During comminution,
the ensuing friction can increase the temperature of the solid
and thereby possibly lead to degradation of thermally labile components. To
avoid this, special water-cooled roll crushers are used.
The mass transfer coeffi cient depends on the diffusivity of the
solute in the solid matrix (main resistance) and the level of turbulence in
the extractor. Traditional extraction has relied upon percolation or extraction
in stirred vessels. In the case of percolation, the solid is packed in a vessel
which is fi lled with solvent. The latter is allowed to percolate in the solid matrix
under stagnant conditions. In the case of extraction in stirred vessels,
different types of agitators are used to suspend the solid in the solvent and
accelerate the mass transfer process. In both percolation and extraction in
stirred vessels, the solvent is fi rst sorbed by the matrix of the solid. This
sorption, which causes swelling of the matrix, is a relatively slow process.
However, once the matrix is swollen, the diffusion coeffi cient increases several
fold or even by an order of magnitude as compared to the dry matrix.
Evidently the controlling step is the diffusion of the solute through the
solid matrix to the surface of the solid. Once the solute is available at the
surface, the solvent can dissolve it depending upon the rate of transport
from the solid surface into the bulk of the solvent. In percolation vessels,
this latter transport is predominantly by molecular diffusion and hence is
slow, although not as slow as the transport through the solid matrix. The
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