Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 62

Scale-up of any PBR design is challenging due to the difficulty in maintaining
optimum light, temperature, mixing, and mass transfer in large volumes (Ugwu et al.,
2008). Large-scale closed systems will likely be based on the integration of multiple
units rather than increasing the size of a single reactor (Brennan and Owende, 2010).
5.3.2.1 Tubular Reactors
Tubular reactors are characterized by very high area-to-volume ratios (dependent
on tube diameter) but poor mass transfer, leading to O 2 build-up and CO 2 depletion
(resulting in photorespiration, oxidative damage, and pH gradients) over the length
of closed tubing. Tubes are generally manufactured from polyethylene or glass. The
most important criteria for construction material are transparency, to allow good
light penetration, and low cost (Ugwu et al., 2008). Additional challenges include
photo-inhibition, temperature control, and fouling due to cells adhering to the inside
of tube walls, leading to decreased light penetration (Ugwu et al., 2008). Narrow-
diameter tubes can present a challenge to clean.
Most tubular reactors can be categorized into one of three types:
1. Vertical airlifts or bubble columns consisting of a clear vertical tube mixed
by gas sparging from the bottom
2. Horizontal tubular systems with clear, thin-diameter tubing lying or stacked
horizontally, usually connected to a gas transfer system
3. Helical tubular reactors consisting of thin, flexible tubing coiled around a
circular framework
Airlift and bubble column reactors (Figures 5.2a and b) are examples of vertical
tubular reactors. Air, or air enriched with CO 2 , is bubbled into the bottom, providing
efficient mixing and gas transfer throughout the reactor. The simplest form of bubble
column reactor is a hanging polyethylene bag, and these have frequently been used
as a low-cost option. Plastic bags have a high transparency, good sterility at start-up
(due to the high temperatures used in plastic extrusion), and are readily replace-
able. Concentrations three times that of open ponds were obtained by culturing
Porphyridium in 25 L hanging bags (Cohen and Arad, 1989). Other researchers have
also found that 40 to 50 L bags are practical (Trotta, 1981; Martínez-Jerónimo and
Espinosa-Chávez, 1994).
Although cultivation in plastic bags is simple, cheap, and widely employed, par-
ticularly in the production of microalgae as feed for aquaculture hatcheries, scale-up
is limited by the fragility of cheap plastic and light penetration, as increases in bag
volume lead to decreased productivity due to mutual shading (Martínez-Jerónimo
and Espinosa-Chávez, 1994). Rigid vertical tubes have also been frequently used
(Carvalho et al., 2006). In an airlift reactor, an inner tube called the riser directs air
bubbles up the center of the reactor and then down the outer region, called the down-
comer (Figure 5.2b). This provides effective, gentle mixing and produces regular
light-dark cycles.
Vertical reactors are compact, low cost, and easy to clean and keep sterile (Ugwu
et al., 2008); however, their size is limited by the surface-to-volume ratio. The scale-
up of any tank, container, or hanging bag becomes limited by light penetration at a