Lipid Identification and Extraction Techniques - Biotechnological Applications of Microalgae - page 94

The solvent extraction process occurs when a solvent comes into contact with
microalgae to release lipids, solvate the lipids in solvent, and separate the oil from
miscella by distillation of the solvent. The drawbacks of solvent-based oil extraction
are that (1) the solvent is highly inflammable, (2) the energy requirements are high,
and (3) the process requires high capital investments.
A solvent extraction plant consists of extractors, desolventizers, evaporators, strip-
ping towers, and condensers in the extractor. Solvent is sprayed over the oil-bearing
materials and solvent penetrates the biomass, targeting the soluble compounds. The
mixture of algae lipid dissolved in the solvent is called a miscella and is sent to col-
lection tanks. The algal oil is separated from the solvent using evaporators and strip-
ping towers. The oil coming out of these units is first cooled, and then filtered and
sent to storage tanks. The oil cake coming out of the extractor unit may contain some
residual amounts of solvent. The residual solvent present in the oil cake is removed in
the desolventizer unit, and recovered solvent can be reused in the extractor.
The various organic chemical solvents employed for oil extraction include ben-
zene, hexane, cyclohexane, acetone, chloroform, ethanol (96%), and hexane-ethanol
(96%) combinations. It is possible to extract up to 98% quantitative of purified fatty
acids (Richmond, 2004). Among these solvents, hexane is most commonly used in
the food industry. Hexane meets many of the requirements of an ideal oil solvent
(Johnson and Lusas, 1983) due to it having a high extraction efficiency, a low viscos-
ity, and a low boiling point, and being a nonpolar solvent, is easily miscible with oil
and inexpensive.
7.3.2.2 Supercritical Fluid Extraction (SFE)
When a fluid is subjected to temperatures and pressures above its critical temperature
and pressure, it becomes a supercritical fluid. Supercritical fluid extraction (SFE) is
the process of extracting oil from oil-containing materials using a supercritical fluid
as the extraction solvent. The advantage of supercritical fluids used in oil extraction
is their increased solvating power (Mercer and Armenta, 2011). Factors to consider
when selecting an SFE solvent include that the solvent is nonflammable, nontoxic,
has low critical parameters, good solvating properties, is easily separated from prod-
uct, and is environmentally friendly and inexpensive. The added advantages of SFE
over conventional solvent extraction are that it provides simple and flexible process
control of temperature, shorter extraction times, low cost, and solvent-free product.
The SFE consists of an SFE solvent tank, solvent and feed pumps, a high-pressure
pump, extraction vessels, and restrictor and absorbent vessels. Carbon dioxide (CO 2 )
is widely used as a solvent in SFE due to its moderate critical temperature (31.1°C)
and pressure (72.9 atm) (Cooney et al., 2009). In this method, CO 2 is used as the
extracting solvent when it is in a supercritical state (i.e., it has both gas and liquid
properties). The supercritical state of CO 2 can be achieved by liquefying CO 2 under
higher pressure and heating to a particular temperature. The important operating
parameters considered for optimizing the extraction efficiency of this method are
operating temperature and pressure, quantity of CO 2 supplied, feed particle size,
and residence time. Dried algae paste must be used for supercritical extraction; this
helps in increasing the contact time between the SFE solvent and the algae paste.
CO 2 acts as a gas in air at ambient temperature, and can be removed after the