Page 71 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: GRB Final Pages
Encyclopedia of Physical Science and Technology EN002G-67 May 25, 2001 20:8

Bioreactors 253

lized enzyme reactors and “biofilters” for the treatment of
gaseous pollutants. Such reactors are particularly attrac-
tive for product inhibited reactions: the product concen-
tration varies from a low value at the inlet of the bed to
a high value at the exit; thus, only a part of the biocat-
alyst is exposed to high inhibitory levels of the product.
In contrast, if the catalyst particles were suspended in a
well mixed stirred vessel, all the catalyst will experience
the same inhibitory product concentration as in the fluid
stream that leaves the reactor.

6. Photobioreactors

Photobioreactors are used for photosynthetic culture of
cyanobacteria, microalgae, and to a much lesser extent,
cells of macroalgae (seaweeds) and plants. Photosynthe-
sis requires light and light stimulates some cultures in
ways not seen in purely heterotrophic growth. Because of
the need to provide light, photobioreactors must have a
high surface-to-volume ratio and this greatly affects the
design of bioreactor. The demand for light is reduced in
photomixotrophic culture where an organic compound is
the major source of carbon for the cells and only a lim-
ited amount of photosynthesis (i.e., the fixation of carbon
dioxide in presence of light) takes place.
Onlyafewphototrophicmicroorganismsmainlycyano-
bacteria and microalgae are cultured on large scale. This
kind of mass culture is carried out in photobioreactors
FIGURE 8 A packed bed bioreactor. open to atmosphere, e.g., in ponds, lagoons, and “race-
way” channels (Fig. 9). The latter are widely used and
fluidization velocity or the bed will fluidize. The depth of
the bed is limited by several factors, including the density
and the compressibility of the solids, the need to maintain
a certain minimal level of a critical nutrient such as oxygen
through the entire depth, and considerations of the max-
imum acceptable pressure drop. For a given voidage—or
solids-free volume fraction of the bed—the gravity driven
flow rate through the bed declines if the depth of the bed
is increased. Nutrients and substrates are depleted as the
fluid moves down the bed. Conversely, concentrations of
metabolites and products increase. Thus, the environment
of a packed bed is nonhomogeneous, but concentration
variations along the depth can be reduced by increasing
the flow rate. Gradients of pH may occur if the reaction
consumes or produces the H ion. Because of poor mix-
+
ing, pH control by addition of acid and alkali is nearly
impossible. Beds with greater voidage permit greater flow
velocities through them, but the concentration of the bio-
catalyst in a given bed volume declines as the voidage is
increased. If the packing—i.e., the biocatalyst-supporting
solids—is compressible, its weight may compresses the
bed unless the packing height is kept low. Flow is difficult
through a compressed bed because of a reduced voidage. FIGURE 9 A closed-loop raceway channel for outdoor culture of
Packed beds are used especially commonly as immobi- photosynthetic microorganisms.