7.3.2.1 Solvent Extraction

The process of extracting oil from oil-containing materials using a suitable solvent is called solvent extraction. It is well suited for lipid recovery from materials with low oil content, and it produces oil cake with low residual oil content (<1% by weight) (Erickson et al., 1984; Hamm and Hamilton, 2000). Algae cell walls are made of multiple layers and they are more recalcitrant than those of other microorganisms (Sander and Murthy, 2009). Some species having an additional trilaminar sheath (TLS) containing an algaenan component are resistant to degradation (Allard et al., 2002; Versteegh and Blokker, 2004).

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, strip­ping 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 col­lection tanks. The algal oil is separated from the solvent using evaporators and strip­ping 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 ben­zene, 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 viscos­ity, and a low boiling point, and being a nonpolar solvent, is easily miscible with oil and inexpensive.