Biomedical Engineering Reference
In-Depth Information
Conditioning is done in rotating drums with indirect steam heating. Tempering improves
flaking performance and extraction efficiency.
4.3 OIL EXTRACTION
Two common oilseed extraction processes are solvent extraction and mechanical extraction
using a screw press. Solvent extraction with hexane is the standard practice in today's
modern oilseed-processing facilities. Solvent extraction plant capacities range from 100 to
9000 metric tonnes/day. High oil content seeds, that is canola and sunflower, are processed
by using a combination of mechanical pressing and solvent extraction. Seeds are first
pressed to reduce the oil content to about 20% and the remaining cake is then solvent
extracted. Use of the screw press without solvent extraction is preferred by small processors
because of its low capital cost. Supercritical fluid, water and enzyme-aided water extraction
processes are of interest for specialty and gourmet oils production.
4.3.1 Solvent extraction
Solvent extraction is a separation process by which solutes are transferred from a solid or
liquid mixture into a solvent. Oilseed extraction refers to preferential dissolution of oil by
contacting oilseeds with a liquid solvent. Solvent extraction is the most efficient technique
to recover oil from oilseeds. The process efficiency depends on the oilseed preparation prior
to extraction, temperature, mode of operation (batch vs. continuous and co-current vs. coun-
tercurrent operations), equipment design and, most importantly, solvent type. Residual oil in
the meal is expected to be less than 1% after commercial solvent extraction.
4.3.1.1
Solvent selection
There is no ideal solvent but there are numerous physical and chemical properties that have
to be considered while choosing a solvent for oil extraction from oilseeds: availability, cost,
oil solubility, toxicity, flammability, selectivity for the desired oil components, specific heat,
latent heat of vaporization, boiling point, viscosity, specific gravity, stability to heat and
light, inertness to oil, meal and equipment (Johnson and Lusas, 1983). Light paraffinic
petroleum fractions - pentane (boiling point 31-36 °C), hexane (boiling point 63-69 °C),
heptanes (boiling point 90-99 °C) and octane (boiling point 102-129 °C) - can be used for
oil extraction. Currently hexane is the solvent of choice for commodity vegetable oil
extraction. Toxicological and environmental concerns, several tragic explosions and fires at
hexane extraction plants have motivated searches for alternative solvents. In 1990, the US
Environmental Protection Agency Clean Air Act listed hexane as a hazardous air pollutant
(EPA, 1990). Flammability of hexane has been a major issue, limiting its use in small
processing plants. Only large centralized facilities can afford the capital investment and the
highly skilled labor required to operate a hexane extraction plant.
A number of alternative solvents have been examined to replace hexane. For example,
trichloroethylene, which is a non-flammable and non-explosive compound, received some
attention as an alternative solvent to hexane during the period 1930-1955. Trichloroethylene
has a high solvent capacity (oil solubility) but it is less selective and extracts more pigments
than hexane. Yet, oil can be bleached without excessive losses during refining (Duncan,
1948). Oilseed extraction plants using trichloroethylene were converted to hexane after only
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