Page 231 - Advances in Eco-Fuels for a Sustainable Environment

Advances in bio-oil extraction from nonedible oil seeds and algal biomass 195

7.6 Extraction of oil from algal source

Cell disruption is generally required for the efficient extraction of materials that are
inside the cells. The disruption of cells is mostly carried out in a concentrated cell,
considering the cost and energy involved. The main challenge lies in the minimization
of energy with the rapid disruption method. The factors to be considered before the
disruption process are the durability of the cell and the significance of the products
to be extracted [47a]. Cell disruption is a critical parameter in determining the yield
and can be done chemically (by treating with acids, alkalis, and surfactants), mechan-
ically (by physical force), or biologically (by enzymes) [19].
Algae such as diatoms produce more oil than other edible and nonedible crops. The
lipid extracted from algae is used for the conversion of biofuel by transesterfication,
pyrolysis, hydrothermal liquefaction, and catalytic hydrodeoxygenation. In case of
algae, pretreatment of the biomass is a necessary step as they have thick rigid cell walls
that are a main constraint for high lipid extraction. The dry and wet biomass play a sig-
nificantroleduringtheextractionoflipids.Dryingofalgalbiomassisoneoftheprimary
stepscarriedouttoreduce thecost and energy involved during theextraction of thelipid
content. Dewatering of algal biomass is an energy-intensive process and the main dis-
advantage of using wet biomass with low extraction yield is the immiscibility of water
with organic solvents. Traditional lipid extraction methods developed by Folch and
Bligh [47b] and Dyer [47c] are used for the extraction of lipids from dry biomass. These
traditional extraction methods were not feasible for wet biomass as the cells remain in
the water phase, preventing its contact from the organic phase. Hence, various extrac-
tion methods have been developed for the efficient extraction of lipids from the cell.
Prior to the extraction of oil from nonedible sources, the seeds have to be dried in an
ovenat105°C.Kernelsare mainly usedforchemicalextractionwhereasformechanical
expellers, a combination of both whole seeds and kernels is used [32]. The different
types of extraction techniques and their extraction parameters are given in Table 7.1.

7.6.1 Harvesting of microalgae
The typical tiny size of a microalgal cell (less than 10μm in diameter) in a diluted
culture medium (less than 2g/L) with a density similar to that of water makes micro-
algal harvesting a the crucial process for the production of biodiesel from microalgae.
The negatively charged surfaces of the microalgae prevent these organisms from eas-
ily settling by gravity. The best way to harvest various microalgal species has not yet
been determined [24, 25]. The following points should be considered when designing
an efficient harvesting strategy.
i. The choice of harvesting technique depends on the characteristics of the microalgae
species and the type of desired product.
ii. A combination of different harvesting techniques can be adopted for a synergistic effect on
the harvesting process.
iii. It is necessary to develop a process that achieves complete cell separation in a dilute
suspension and is advantageous to recycle the water for further growth.