Solvents such as ethanol, hexane, or a mixture of hexane-ethanol are generally
used for extraction of lipids to be used in biodiesel production. Sometimes methanol
can be also used which can serve both the purpose of extraction and as a reactant in
the subsequent transesterification reaction. The extraction process is limited by mass
transfer and hence the use of process intensification approaches can be very beneficial. Patil et al. (2011a, b) performed microwave-assisted direct transesterification
of the microalgae using methanol and reported yield of up to 77% with process
optimization. Wahlen et al. (2011) studied the comparison of the use of wet and dry
algae biomass in direct transesterification and also the effect of water content on
FAME yield. It was reported that 30 mg FAME can be produced from 100 mg of
wet algae sample via this process of direct transesterification as compared to the
conventional process which gave only 27 mg FAME from 100 mg. Study also
concluded that the wet biomass of algae can be effectively utilized for biodiesel
production based on nullifying the effects of water content by the addition of higher
amount of methanol, also giving advantages of elimination of processing step.
Super critical extraction is also one of the techniques efficiently used for
intensified extraction of lipids from microalgae. Many process intensification
benefits have been reported with the use of super critical conditions as mentioned in
Table 2. Similarly, ultrasound has also been applied in few studies to give intensified extraction. Ferreira et al. (2016) performed a study using low-frequency
ultrasound with pure solvent (n-hexane and ethanol) and binary mixture of solvents
(chloroform:ethanol; chloroform:isopropanol; chloroform:methanol; n-hexane:
methanol; nhexane: isopropanol; n-hexane:ethanol, and n-hexane:2-butanol). It was
reported that the frequency of 50/60 kHz with binary solvent of nhexane: isopropanol in 2:1 ratio was the most effective. It was also reported that the energy
requirements were lesser as compared to conventional Soxhlet extraction and super
critical extraction (SRE).
Table 2 Different super critical extraction (SRE) processes for microalgae processing with
process parameters and yields (Lee et al. 2014)
Specie
Solvent/co-solvent
Temperature (°C)/
pressure (MPa)
Time
(min)
Yield
(%)
Chlorella vulgaris
Ethanol (6.6 ethanol/solids
mass ratio)
H 2 O (10.1 wt%)
325
120
100
Chlorella vulgaris
Methanol (4 mL/g)
H 2 O (80 wt%)
175/2.2
240
89.7
Nannochloropsis
(CCMP1776)
Methanol (9.0 mL/g)
H 2 O (ratio not mentioned)
255/8.27
25
84.1
Nannochloropsis
salina
Ethanol (9 mL/g)
H 2 O (60 wt%)
265/8.27–9.30
20
67
Nannochloropsis
salina
Ethanol (9 mL/g)
H 2 O (ratio not mentioned)
260/8.0
25
30.9
Chlorella
protothecoids
Ethanol (20:1 ethanol/fatty
acid molar ratio)
275/20.0
180
89
66
S. Joshi and P. Gogate
used for extraction of lipids to be used in biodiesel production. Sometimes methanol
can be also used which can serve both the purpose of extraction and as a reactant in
the subsequent transesterification reaction. The extraction process is limited by mass
transfer and hence the use of process intensification approaches can be very beneficial. Patil et al. (2011a, b) performed microwave-assisted direct transesterification
of the microalgae using methanol and reported yield of up to 77% with process
optimization. Wahlen et al. (2011) studied the comparison of the use of wet and dry
algae biomass in direct transesterification and also the effect of water content on
FAME yield. It was reported that 30 mg FAME can be produced from 100 mg of
wet algae sample via this process of direct transesterification as compared to the
conventional process which gave only 27 mg FAME from 100 mg. Study also
concluded that the wet biomass of algae can be effectively utilized for biodiesel
production based on nullifying the effects of water content by the addition of higher
amount of methanol, also giving advantages of elimination of processing step.
Super critical extraction is also one of the techniques efficiently used for
intensified extraction of lipids from microalgae. Many process intensification
benefits have been reported with the use of super critical conditions as mentioned in
Table 2. Similarly, ultrasound has also been applied in few studies to give intensified extraction. Ferreira et al. (2016) performed a study using low-frequency
ultrasound with pure solvent (n-hexane and ethanol) and binary mixture of solvents
(chloroform:ethanol; chloroform:isopropanol; chloroform:methanol; n-hexane:
methanol; nhexane: isopropanol; n-hexane:ethanol, and n-hexane:2-butanol). It was
reported that the frequency of 50/60 kHz with binary solvent of nhexane: isopropanol in 2:1 ratio was the most effective. It was also reported that the energy
requirements were lesser as compared to conventional Soxhlet extraction and super
critical extraction (SRE).
Table 2 Different super critical extraction (SRE) processes for microalgae processing with
process parameters and yields (Lee et al. 2014)
Specie
Solvent/co-solvent
Temperature (°C)/
pressure (MPa)
Time
(min)
Yield
(%)
Chlorella vulgaris
Ethanol (6.6 ethanol/solids
mass ratio)
H 2 O (10.1 wt%)
325
120
100
Chlorella vulgaris
Methanol (4 mL/g)
H 2 O (80 wt%)
175/2.2
240
89.7
Nannochloropsis
(CCMP1776)
Methanol (9.0 mL/g)
H 2 O (ratio not mentioned)
255/8.27
25
84.1
Nannochloropsis
salina
Ethanol (9 mL/g)
H 2 O (60 wt%)
265/8.27–9.30
20
67
Nannochloropsis
salina
Ethanol (9 mL/g)
H 2 O (ratio not mentioned)
260/8.0
25
30.9
Chlorella
protothecoids
Ethanol (20:1 ethanol/fatty
acid molar ratio)
275/20.0
180
89
66
S. Joshi and P. Gogate