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Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

Reactors in Process Engineering 39

fermentations, oxygen or air is bubbled through the tank Friction in the extruder produces high temperatures and
to allow oxygen mass transfer into the media, resulting in pressures, and the product is forced out dies at the end of
a three-phase reactor. the extrusion tube. This type of extruder is referred to as a
dry extruder. If steam is injected along the extrusion tube,
the reactor is referred to as a wet extruder.
3. Applications
Continuous fermentation processes are primarily used
2. Classification
in the research and development stage. However, more
chemostat operations are being used at the production The extruder is essentially a plug flow reactor. Although
level as the understanding of this reactor increases. Ex- the material is being well mixed, this mixing is primarily
amples include ethanol fermentation for the production of in the radial and circumferential directions rather than ax-
fuel grade ethanol and single-cell protein production from ially. Due to the extreme conditions in the extruder, solids
methanol substrates. can liquefy, resulting in heterogeneous operation.

E. Digestor
3. Applications
1. Description
The extruder is used extensively in the food processing
The digestor is a biological reactor used mainly for the industry. Grains and starches can be hydrolyzed easily.
treatment of municipal and industrial wastes. Wastes are
fed continuously to the digestor, where some solids settle
to the bottom of the tank, and other solids are matted G. Falling Film
and lifted to the surface by the gases produced during the
1. Description
fermentation. In an aerobic digestor the mat is broken and
mixed by gas circulation. The solid sludge in the bottom Falling-film reactors have a liquid reactant flowing down
of the tank is raked down a conical bottom and pumped the walls of a tube with a gaseous reactant flowing up or
from the tank. A fraction of the sludge is recycled back to down (usually countercurrent). This reactor is particularly
the digestor to maintain a steady microbial population. advantageous when the heat of reaction is high. The re-
action surface area is minimal, and the total reaction heat
generated can be controlled.
2. Classification
Thedigestorisclassifiedasacontinuousbiologicalhetero-
2. Classification
geneous reactor. Liquid flow through the digestor roughly
follows the CSTR assumptions. Digestion of the solids This reactor may follow the plug flow assumptions, or it
is a complex mechanism that requires empirical design may be equilibrium limited depending on the operating
equations to describe. conditions.

3. Applications
3. Applications
The digestor is mainly restricted to the treatment of munic-
An example of a reaction performed in a falling-film re-
ipal and industrial wastes. Substantial research has been
actor is the sulfonation of dodecyl benzene.
done on using anaerobic digestion of biomass for the pro-
duction of methane gas. These systems are limited to
small-scale applications where alternative energy sources H. Fermentor
are inadequate. Some current anaerobic digestors use the
methane produced as a by-product to supply heat for op- 1. Description
eration of the digestor.
The term fermentation is used to describe the biological
transformation of chemicals. In its most generic applica-
F. Extruder tion, a fermentor may be batch, continuous-stirred tank
(chemostat), or continuous plug flow (immobilized cell).
1. Description
Most industrial fermentors are batch. Several configura-
For reactions that require high temperature and pressure tions exist for these batch reactors to facilitate aeration.
for short periods of time, the extruder is ideal. The reactant These include sparged tanks, horizontal fermentors, and
is fed to a screw type device that narrows toward the exit. biological towers.