2 Industrial Plant Oil Extraction
59
2.4 Solvent Extraction
After leaving the preparation process, the flakes are delivered to the solvent extraction
operation. Extraction of the oil from the seed by solvent that leaves behind the nonoil constituents, such as protein and fibre, is practically the only method to achieve
an almost complete recovery of oil. Hexane is widely accepted as the most effective
solvent used today; there are concerns about its flammability, exposure, and environmental impacts. As this process typically uses a flammable solvent, the operation
is usually somewhat removed from other facilities, and access to the controlled area
is restricted. Research has focused on various alternative solvents in the hopes of
finding one with acceptable performance while providing greater safety. Alternative
solvents that have received some attention include isopropyl alcohol, supercritical
carbon dioxide, and other fluids. However, no economical alternative to n-hexane
has been accepted at this point.
The typical unit operations are associated with solvent extraction, which include
extraction, solvent distillation, and liquid-phase recovery. Upon discharge from the
extractor, solid-phase extracted material is desolventized, toasted, dried, and cooled
prior to meal finishing. And so, the solvent extraction plant is usually consisted of
three parts. The main equipment is the extractor, in which the oilseed, after pretreatment and, if applicable after pre-expelling, makes contact with the solvent. The other
two basic sections involve the removal of the solvent from the extracted meal and
from the miscella, or solvent–oil mixture.
In the extractor, which is a countercurrent flow device, the solid oil-bearing material is conveyed in an opposite direction to the solvent. The extracted meal leaves at
one end, and the miscella at the other end. As the oil-bearing material to be extracted
enters the unit, it is contacted with the miscella at nearly full oil concentration. After
this first wash, the miscella, containing around 25%–30% oil, leaves the extractor for
solvent distillation and recovery. After passing through the various washing stages,
finally being contacted with fresh solvent and allowed to drain for a brief period, the
extracted material, commonly known as white flakes, is removed from the extractor
and is conveyed to the desolventizing process.
The vast majority of extractors in operation are based on a continuous conveyor
on which the solid prepared seed is carried from the feed end to the discharge end.
The conveying element may be an endless belt or chain carrying buckets. The belt,
or the bottom of the buckets, consists of sieves, or perforated material, that allow
free drainage of the solvent from the oilseeds. Fresh solvent is sprayed over the oilmeal from near the meal discharge end of the extractor; it is enriched with oil as it
percolates through the oil-meal and is collected in a series of troughs.
A countercurrent movement of the solvent is achieved in stages by a series of
pumps that spray the miscella over the part of the conveyor near the entry of the
oilseed. At the final stage, the miscella percolates through the oil-rich feed and
leaves the extractor containing the maximum concentration of oil.
59
2.4 Solvent Extraction
After leaving the preparation process, the flakes are delivered to the solvent extraction
operation. Extraction of the oil from the seed by solvent that leaves behind the nonoil constituents, such as protein and fibre, is practically the only method to achieve
an almost complete recovery of oil. Hexane is widely accepted as the most effective
solvent used today; there are concerns about its flammability, exposure, and environmental impacts. As this process typically uses a flammable solvent, the operation
is usually somewhat removed from other facilities, and access to the controlled area
is restricted. Research has focused on various alternative solvents in the hopes of
finding one with acceptable performance while providing greater safety. Alternative
solvents that have received some attention include isopropyl alcohol, supercritical
carbon dioxide, and other fluids. However, no economical alternative to n-hexane
has been accepted at this point.
The typical unit operations are associated with solvent extraction, which include
extraction, solvent distillation, and liquid-phase recovery. Upon discharge from the
extractor, solid-phase extracted material is desolventized, toasted, dried, and cooled
prior to meal finishing. And so, the solvent extraction plant is usually consisted of
three parts. The main equipment is the extractor, in which the oilseed, after pretreatment and, if applicable after pre-expelling, makes contact with the solvent. The other
two basic sections involve the removal of the solvent from the extracted meal and
from the miscella, or solvent–oil mixture.
In the extractor, which is a countercurrent flow device, the solid oil-bearing material is conveyed in an opposite direction to the solvent. The extracted meal leaves at
one end, and the miscella at the other end. As the oil-bearing material to be extracted
enters the unit, it is contacted with the miscella at nearly full oil concentration. After
this first wash, the miscella, containing around 25%–30% oil, leaves the extractor for
solvent distillation and recovery. After passing through the various washing stages,
finally being contacted with fresh solvent and allowed to drain for a brief period, the
extracted material, commonly known as white flakes, is removed from the extractor
and is conveyed to the desolventizing process.
The vast majority of extractors in operation are based on a continuous conveyor
on which the solid prepared seed is carried from the feed end to the discharge end.
The conveying element may be an endless belt or chain carrying buckets. The belt,
or the bottom of the buckets, consists of sieves, or perforated material, that allow
free drainage of the solvent from the oilseeds. Fresh solvent is sprayed over the oilmeal from near the meal discharge end of the extractor; it is enriched with oil as it
percolates through the oil-meal and is collected in a series of troughs.
A countercurrent movement of the solvent is achieved in stages by a series of
pumps that spray the miscella over the part of the conveyor near the entry of the
oilseed. At the final stage, the miscella percolates through the oil-rich feed and
leaves the extractor containing the maximum concentration of oil.