Tubular PBR The most widely used PBR is of tubular design which is composed
of an array of straight glass or plastic tubes (Chisti 2007). These tubes are generally
0.1 m or less in diameter and can be aligned horizontally, vertically, inclined, or as a
helix. These tubes allow the sufficient sunlight penetration for photosynthesis. The
culture is circulated through the tubes and then returns to a reservoir. Re-circulation
of algal biomass as turbulent flows is maintained through airlift pump inside the
reactor. Excess amount of dissolved O 2 in the medium is reduced through agitation
and mixing by using gas exchange chambers.
Helical PBR Helical PBRs are made up of a cylinder and coiled parallel transparent
tubes. Increased surface area of helical PBR is allowing sufficient sunlight to reach,
thus increased productivity attained as compared to tubular PBR (Carvalho 2006).
However, its unique shape and increased cost did not make it as popular.
Airlift PBR An airlift PBR is composed of a simple vertical cylinder made out of
transparent glass or plastic. There is an air inlet on the bottom of the tube to create air
bubbles through the column for sufficient mixing and gas exchange. These systems
provide aerial productivity of algae as compared to tubular reactor.
Flat Panel (Flat Plate) PBR Flat panels, also known as flat plate PBRs, are
consisting of rectangular boxes made up of transparent materials for maximum
solar energy capture. A thin layer of algal culture flows across the flat plate. Air
inlet on the bottom provides bubbles which provide sufficient mixing and gas
transfer. It is the earliest form of closed system which has received research attention
toward mass cultures of algae due to large surface area exposed to illumination and
low accumulation of dissolved oxygen. Because of the increased surface area for
light, higher algae productivity is achieved in flat plate PBR as compared to open
pond system. For example, in one study of algae S. platensis; the flat panel PBR
produced 2.15 g L
À1 day
À1 of algal biomass and an open pond produced
0.15 g L
À1 day
À1 of algal biomass (Carvalho 2006).
1.3.2 Harvesting Methods
1.3.2.1 Flocculation
Flocculation is the first stage of harvesting process that is intended to agglomeration
of algae cells in order to increase the effective “particle” size. Flocculation is a
preparatory step prior to other harvesting methods (filtration, flotation, or gravity
sedimentation) (Golueke and Oswald 1965; Molina et al. 2003). The addition of
flocculants (multivalent cations and cationic polymers) neutralizes the negative
charge of microalgae cells and causes cells to form large clumps. These clumps
can be easily fall out of suspension. Different methods of flocculation have been
introduced for algal cell suspension.
1 Algal Biofuel: A Sustainable Approach for Fuel of Future Generation
9
of an array of straight glass or plastic tubes (Chisti 2007). These tubes are generally
0.1 m or less in diameter and can be aligned horizontally, vertically, inclined, or as a
helix. These tubes allow the sufficient sunlight penetration for photosynthesis. The
culture is circulated through the tubes and then returns to a reservoir. Re-circulation
of algal biomass as turbulent flows is maintained through airlift pump inside the
reactor. Excess amount of dissolved O 2 in the medium is reduced through agitation
and mixing by using gas exchange chambers.
Helical PBR Helical PBRs are made up of a cylinder and coiled parallel transparent
tubes. Increased surface area of helical PBR is allowing sufficient sunlight to reach,
thus increased productivity attained as compared to tubular PBR (Carvalho 2006).
However, its unique shape and increased cost did not make it as popular.
Airlift PBR An airlift PBR is composed of a simple vertical cylinder made out of
transparent glass or plastic. There is an air inlet on the bottom of the tube to create air
bubbles through the column for sufficient mixing and gas exchange. These systems
provide aerial productivity of algae as compared to tubular reactor.
Flat Panel (Flat Plate) PBR Flat panels, also known as flat plate PBRs, are
consisting of rectangular boxes made up of transparent materials for maximum
solar energy capture. A thin layer of algal culture flows across the flat plate. Air
inlet on the bottom provides bubbles which provide sufficient mixing and gas
transfer. It is the earliest form of closed system which has received research attention
toward mass cultures of algae due to large surface area exposed to illumination and
low accumulation of dissolved oxygen. Because of the increased surface area for
light, higher algae productivity is achieved in flat plate PBR as compared to open
pond system. For example, in one study of algae S. platensis; the flat panel PBR
produced 2.15 g L
À1 day
À1 of algal biomass and an open pond produced
0.15 g L
À1 day
À1 of algal biomass (Carvalho 2006).
1.3.2 Harvesting Methods
1.3.2.1 Flocculation
Flocculation is the first stage of harvesting process that is intended to agglomeration
of algae cells in order to increase the effective “particle” size. Flocculation is a
preparatory step prior to other harvesting methods (filtration, flotation, or gravity
sedimentation) (Golueke and Oswald 1965; Molina et al. 2003). The addition of
flocculants (multivalent cations and cationic polymers) neutralizes the negative
charge of microalgae cells and causes cells to form large clumps. These clumps
can be easily fall out of suspension. Different methods of flocculation have been
introduced for algal cell suspension.
1 Algal Biofuel: A Sustainable Approach for Fuel of Future Generation
9