Agriculture Reference
In-Depth Information
Critical
point
Supercritical
fluid
Solid
7.4
Triple
point
Liquid
5.1
Gas
-56.3
31.1
Temperature (°C)
Fig. 12.1
Phase diagram for CO
2
.
Table 12.2
Physical properties of gas, SCF and liquid CO
2
Property
Gas
SCF
Liquid
Density (g ml
−1
)
10
−3
0.1-0.9
1
Viscosity (Pa s)
10
−5
10
−4
10
−3
Diffusivity (cm
2
s
−1
)
0.1
10
−3
10
−5
to 10
−6
Source: Kerton (2009) .
good extraction solvent for natural products particularly where the extract or
residual products are ultimately to be used in consumer products. However,
supercritical fl uid extraction also has a number of disadvantages, which have had
a signifi cant impact on the adoption of this technique:
•
high capital installation costs;
•
relatively high operating costs;
•
requirement for technically skilled operators.
As a result, large-scale extraction using liquid or supercritical CO
2
has been
largely confi ned to the extraction of hops, the decaffeination of coffee and tea
(Bott and King 1993), and the production of edible oils such as sesame seed oil
(Döker
et al.
2010).
Industrial-scale supercritical CO
2
extractions are currently all carried out as a
batch process as shown in Fig. 12.2. The quick loading and unloading of materials
into the extractors minimises equipment downtime. An extractor with an internal
basket or hydraulically operated top and bottom lids allows for the gravity
discharge of materials into hoppers or conveyors, which can be used to aid the
loading and unloading process. A typical supercritical CO
2
extractor operates as a
pumped closed loop. Liquid CO
2
is taken from a storage vessel normally held
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