device that narrows toward the exit.
Friction in the extruder produces high
temperatures and pressures, and the product is forced out dies at the end of 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.
CLASSIFICATION. The extruder is essentially a plug flow reactor. Although
the material is being well mixed, this mixing is primarily in the radial and
circumferential directions rather than axially. Due to the extreme conditions in
the extruder, solids can liquefy, resulting in heterogeneous operation.
APPLICATIONS. The extruder is used extensively in the food processing
industry. Grains and starches can be hydrolyzed easily.
G. FALLING FILM
DESCRIPTION. Falling-film reactors have a liquid reactant flowing down the
walls of a tube with a gaseous reactant flowing up or down (usually
countercurrent). This reactor is particularly advantageous when the heat of
reaction is high. The reaction surface area is minimal, and the total reaction heat
generated can be controlled.
CLASSIFICATION. This reactor may follow the plug flow assumptions, or it
may be equilibrium limited depending on the operating conditions.
APPLICATIONS. An example of a reaction performed in a falling-film reactor
is the sulfonation of dodecyl benzene.
H. FERMENTOR
DESCRIPTION. The term fermentation is used to describe the biological
transformation of chemicals. In its most generic application, a fermentor may be
batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized
cell). Most industrial fermentors are batch. Several configurations exist for these
batch reactors to facilitate aeration. These include sparged tanks, horizontal
fermentors, and biological towers.
CLASSIFICATION. The most traditional application of the fermentor is in
batch mode. In anaerobic fermentations the reactor looks like a normal batch
reactor, since gas-liquid contact is not an important design consideration.
Depending on the reaction, the microbes may or may not be considered as a
separate phase. For aerobic fermentations, oxygen is bubbled through the media,
and mass transfer between phases becomes one of the major design factors.
APPLICATIONS. Since the characteristics of microbes lead to the batch
production of many products, examples of fermentors are numerous. They
include beer vats, wine casks, and cheese crates as anaerobic food production
equipment. The most significant aerobic reactor is the penicillin fermentor.
I. GASIFIERS
DESCRIPTION. A gasifier is used to produce synthesis gas from carbonaceous
material. The solid is packed in a column, and gas is passed up through the bed,
producing a mixture of combustible products, primarily methane, hydrogen
and carbon monoxide, with a low to medium BTU content.
CLASSIFICATION. A gasifier is a continuous gas process in conjunction with
either a batch of solids or continuous solids feed and product removal. The gas
phase passing up through the bed obeys plug flow behavior. In continuous solids
handling, the bed is fed from the top and emptied from the bottom. These solids
also obey plug flow assumptions with flow countercurrent to the gas phase.
APPLICATIONS. Coal gasifiers are used for the production of synthesis gas;
however, any carbon source could be used. Biomass is receiving attention as a
carbon source.
J. IMMOBILIZED CELL
DESCRIPTION. The washout problems associated with continuous
fermentation are eliminated by attaching the microbes or enzymes to a solid
support, preventing them from leaving the reactor. The attachment procedures
vary, and as a result, the flow scheme in the reactor may differ depending on the
choice of attachment. Encapsulation allows shear at the surface of the support so
that fluidization techniques can be used. Attachment onto a surface usually limits
the flow conditions to a packed-bed configuration.
CLASSIFICATION. An immobilized cell reactor is classified as a continuous
biological system that may follow either plug flow theory or fluidized-bed theory
depending on the mode of operation.
APPLICATIONS. The use of immobilizedcell systems is applicable to all
fermentation schemes and is being researched extensively for the production of
alcohols, chemicals, and biological products.
K. JET TUBE
DESCRIPTION. For rapid exothermic reactions that require continuous stirred-
tank operating conditions for good reaction control, a jet tube reactor can be used.
This reactor gives thorough mixing despite the extremely short residence times
involved in these reactions. One reactant is injected into the other through a jet,
orifice, or venturi. This gives high turbulence to insure a well-mixed condition.
Large-scale testing is needed to select the reactor conditions accurately, since
minor errors in kinetic constants are magnified due to the high activation energies
of the reactions. Jets can handle both gas and liquid feed and are usually built in
multiple jet configurations.
CLASSIFICATION. Since reaction does not occur until one reactant is jetted
into the other, the actual jet does not become involved in the kinetics, it is strictly
a method for contacting reactants quickly. The actual reactor performance is
based on CSTR assumptions.
