Food Lipids: Chemistry, Nutrition, and Biotechnology

in successive passes through the bed, while the solvent spray and the bed move in
opposite directions to each other. Percolation extractors also have larger extraction
capacities in less space, and fewer operating problems are associated with these
devices than with immersion extractors.
Most modern extractors are of the percolation type (Figs. 9, 10). The extraction
principles employed by most extractors are the same, but there are different methods
of achieving countercurrent flow of solvent to flakes. The shallow-bed chain extractor
(Fig. 9) is one of today’s popular extractors and resembles a full-loop conveyor [30].
Flakes fed into an inlet hopper are conveyed down the first leg of the loop, where
they are washed with moderately dilute miscella to extract surface oil and penetrate
the cells. As the flake bed moves into the bottom horizontal section, full miscella is
recycled through the bed for filtering, and then to a liquid cyclone for removing
fines and, finally, to the evaporation system. Flakes are conveyed counterclockwise,
through a progressively more dilute miscella washes, until a final wash with fresh
solvent is used in the top horizontal section of the loop. The latter half of the top
loop is used for drainage, after which solvent-laden spent flakes pass to the desolventizer.
The following advantages are claimed: the shallow bed (usually about 1 m)
promotes drainage and, thus, low solvent carryover to the meal desolventizer and
uniform contact of the flakes with solvent, since the bed is turned over while going
up the right vertical leg.
Today hexanes (a blend of about 60% n-hexane with other hexane isomers) are
the solvent of choice, although many other solvents were used during development
of solvent extraction technologies. Alternative extraction solvents were extensively
reviewed [31,32], but no suitable alternative has been developed. The primary disadvantage
of hexane is its flammability and a price structure and supply that are tied
to petroleum prices.
Flakes are conveyed to the extractor, where they are extracted for 30–60
minutes. Generally, less than 1% residual oil in the extracted material is achieved,
and the amount is lower for soybeans (about 0.5%). The miscella contains 22–30%
oil, and the solvent is separated from the crude oil by distillation and stripping
columns. The miscella is heated under vacuum to evaporate the solvent, which is
usually done in two stages. The first-stage evaporator concentrates the oil to about
90% and uses reclaimed heat from heated solvent vapors from meal desolventization.
Steam is used to heat the second-stage evaporator, where the oil is concentrated to
>99%. Most of the remaining solvent is removed in a disc-and-doughnut stripping
column where evaporation is promoted by means of heat, vacuum (450–500 mm
Hg absolute pressure), and steam sparging. Crude oil leaves the stripping column
with less than 0.15% moisture and hexane [33]. Trading specifications require the
oil to have a flash point greater than 250�C, which is equivalent to no more than
800 ppm of hexane.
The marc generally contains 30–32% solvent holdup, which must be recovered
and recycled. Heat must be used to evaporate the solvent holdup from the meal. Live
steam is also injected to aid heat transfer and to provide moisture vapor to strip the
solvent. Regardless of the type of extractor used, the extracted flakes (spent flakes)
must be drained of the solvent held by the material. The solvent that will not drain
is referred to as solvent holdup, the solvent-laden flakes are called marc. Solvent
holdup should be minimized because this solvent must be removed by evaporation
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