Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 57

saturation (with respect to air-saturated culture) (Chisti, 2007). A build-up of O 2 in
the reactor can cause the key carbon-fixing enzyme RuBisCO to bind oxygen instead
of carbon dioxide, leading to photorespiration instead of photosynthesis (Dennis
et al., 1998). High oxygen concentrations, in addition to intense light, lead to the
formation of oxygen radicals that have toxic effects on cells due to membrane dam-
age (Molina Grima et al., 2001; Pulz, 2001). Many algal strains cannot survive in
O 2 over-saturated conditions for more than 2 to 3 hours. High temperatures and light
intensify the damage (Pulz, 2001). Oxygen build-up limits the maximum length of a
closed tubular reactor. Typically, a continuous tube should not exceed 80 m (Molina
Grima et al., 2001), although the exact length depends on biomass concentration,
light intensity, liquid velocity, and initial O 2 concentration. In a closed reactor,
culture must continuously return to a degassing zone, where it is bubbled with air to
strip the O 2 . The degassing zone is typically optically deep compared with the solar
collector, and hence poorly illuminated; thus its volume should be small relative to
the solar collector (Chisti, 2007).
In high-density algal cultures, the key challenges in nutrient provision are in mass
transfer of CO 2 to cells and O 2 away from cells. Efficient mixing and aeration, with-
out inducing shear stress and requiring excessive energy input, are important param-
eters. Bubbling of gas through cultures can be used to simultaneously introduce CO 2 ,
strip O 2 , and mix the culture broth (e.g., bubble columns and airlift reactors). The
overall mass transfer coefficient (kLa) of the reactor is an important parameter in
determining the carbon supply. The kLa depends on reactor geometry, agitation rate,
sparger type, temperature, mixing time, liquid velocity, gas bubble velocity, and gas
holdup (Ugwu et al., 2008).
5.2.4 M ixinG
Mixing is of paramount importance in microalgal culture systems as it is directly
linked to other key parameters such as light provision, gas transfer, and nutrient
provision. Good mixing keeps the cells in suspension, eliminates thermal stratifica-
tion, determines the light-dark regime by moving cells through an optical gradient,
ensures efficient distribution of nutrients, improves gas exchange, reduces mutual
shading at the center of the reactor, and decreases photo-inhibition at the surface
(Ugwu et al., 2008). Mixing affects the mass transfer rates of dissolved nutrients and
gases by reducing the boundary layer between the surface of cells and gas particles
and the bulk liquid (Grobbelaar, 2009). The synergistic effect on several parameters
at once means that mixing efficiency has a strong effect on growth rate.
One of the major differences between open and closed reactors is the degree of tur-
bulence achieved. Higher turbulences are more easily achieved in closed PBRs with
narrow tubes or plates. Mixing in open ponds is typically provided by a paddlewheel
or rotating arm. In closed reactors, mixing can be achieved mechanically (by pumping
or stirring) or by aeration via a variety of gas transfer systems (e.g., bubble diffusers,
pipes, blades, propellers, jet aerators, or aspirators). Stirring is efficient but incurs high
mechanical stress. Mixing by gas injection is relatively gentle and efficient, but may
require energy intensive gas pressurization. Gas introduced into reactors can serve
a number of purposes, including supply of nutrients, control of pH, stripping of O 2 ,