Biomedical Engineering Reference
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might be the channels through which oil flows out of the seed during expression (Diekert
and Diekert, 1976). Microstructure studies of rapeseed and cashew using transmission
electron microscopy confirmed the porous structure of oilseed cell walls, indicating that oil
expression could occur through a porous microstructure without cell wall rupture (Mrema
and McNult, 1984). A mathematical model based on the latter theory was developed to
describe the mechanical oil expression process (Mrema and McNulty, 1985). It was assumed
that the process involved three phases: (1) oil flow through the cell wall pores, (2) oil flow
in the inter-kernel voids and (3) consolidation of the oilseed cake (reduction of cake volume).
Three fundamental equations - the Hagen Poiseulle equation for flow of fluids in pipes to
describe the flow of oil through the pores on the cell wall; Darcy's law of fluid flow through
porous media to describe the flow of oil through the inter-kernel voids; and a modified form
of Terzaghi's equation for the consolidation of saturated soils - were used to develop a
general equation to describe the oil expression process. The general equation was solved for
two loading regimes: a linearly increasing load regime to simulate the load application
phase in hydraulic presses and the overall screw expeller process; and a constant load to
simulate the hydraulic pressing. Experimental and predicted data obtained from the model
were in good agreement. The flow of oil across cell walls in the seed was the rate determining
step (Mrema and McNulty, 1985).
Screw presses, roll presses and mills, collapsible-plate and frame-filter presses, disc
mills, rack and cloth presses and hydraulic presses are examples of the wide variety of
equipment available for expression processing (Khan and Hanna, 1983; Bredeson, 1977;
Bockisch, 1998). Lever and wedge presses were the early mechanical oil expression
equipment used for oilseed processing. The first cottonseed oil mill built in the United States
in the 1920s utilized a hydraulic press. Seeds in filter bags were manually loaded into
perforated, horizontal boxes between the head block and the ram of the press. Boxes were
pressed together by applying hydraulic pressure on the ram. Oil was pressed through the
filter bag. Then the filter bag containing spent cake was manually removed from the
hydraulic press (Khan and Hanna, 1983). Later versions of the hydraulic presses used cages
instead of filter bags. Hydraulic presses were in use until the 1950s. They were replaced
with continuous screw presses and continuous solvent extraction plants, which are less labor
intensive. The olive oil industry still utilizes hydraulic presses today.
The mechanical screw press consists of a vertical feeder and a horizontal screw with
increasing body diameter to exert pressure on the oilseeds as it advances along the length of
the press. The barrel surrounding the screw has slots along its length to allow it to expel air
and then drain the oil. Oil is collected in a trough under the screw and the de-oiled cake
discharged at the end of the screw. The main advantages of the screw press are that large
quantities of oilseeds can be processed with minimal labor and it allows continuous oil
extraction (Ward, 1976). The maintenance of screw presses is challenging. The moving
parts and high pressure and temperature generated in the press increase wear. The primary
improvement in mechanical screw press design has been developing materials of construction
that extend the life of the parts that wear the most. Equipment parts that used to last three
months before requiring replacement may now last up to two years. Today, much larger
screw presses have been built. Former capacities of five tonnes per day are now up to over
100 tonnes per day for full pressing and over 800 tonnes per day for pre-pressing applications.
A growing number of mini crushing mills, which employ extruding-expelling (E-E)
technology, have been built as farmer-owned cooperatives or as on-farm operations to
process locally produced soybeans or other oilseeds. E-E uses a dry extruder in which heat
is generated internally by friction and dissipation of mechanical energy prior to screw
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