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

40 Reactors in Process Engineering

2. Classification result, the flow scheme in the reactor may differ depending
on the choice of attachment. Encapsulation allows shear
The most traditional application of the fermentor is in
at the surface of the support so that fluidization techniques
batch mode. In anaerobic fermentations the reactor looks
can be used. Attachment onto a surface usually limits the
like a normal batch reactor, since gas–liquid contact is
flow conditions to a packed-bed configuration.
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 2. Classification
bubbled through the media, and mass transfer between
An immobilized cell reactor is classified as a continu-
phases becomes one of the major design factors.
ous biological system that may follow either plug flow
theory or fluidized-bed theory depending on the mode of
3. Applications operation.
Since the characteristics of microbes lead to the batch
production of many products, examples of fermentors are 3. Applications
numerous. They include beer vats, wine casks, and cheese
The use of immobilized cell systems is applicable to all
crates as anaerobic food production equipment. The most
fermentation schemes and is being researched extensively
significant aerobic reactor is the penicillin fermentor.
for the production of alcohols, chemicals, and biological
products.
I. Gasifiers
1. Description
K. Jet Tube
A gasifier is used to produce synthesis gas from carbona-
1. Description
ceous material. The solid is packed in a column, and gas
is passed up through the bed, producing a mixture of com- For rapid exothermic reactions that require continuous
bustible products, primarily methane, hydrogen and car- stirred-tank operating conditions for good reaction con-
bon monoxide, with a low to medium BTU content. trol, a jet tube reactor can be used. This reactor gives thor-
ough mixing despite the extremely short residence times
involved in these reactions. One reactant is injected into
2. Classification the other through a jet, orifice, or venturi. This gives high
A gasifier is a continuous gas process in conjunction with turbulence to insure a well-mixed condition. Large-scale
either a batch of solids or continuous solids feed and prod- testingisneededtoselectthereactorconditionsaccurately,
uct removal. The gas phase passing up through the bed since minor errors in kinetic constants are magnified due
obeys plug flow behavior. In continuous solids handling, to the high activation energies of the reactions. Jets can
the bed is fed from the top and emptied from the bottom. handle both gas and liquid feed and are usually built in
These solids also obey plug flow assumptions with flow multiple jet configurations.
countercurrent to the gas phase.
2. Classification
3. Applications
Since reaction does not occur until one reactant is jetted
Coal gasifiers are used for the production of synthesis into the other, the actual jet does not become involved in
gas; however, any carbon source could be used. Biomass the kinetics, it is strictly a method for contacting reactants
is receiving attention as a carbon source. quickly. The actual reactor performance is based on CSTR
assumptions.
J. Immobilized Cell
1. Description 3. Applications
The washout problems associated with continuous fer- Oil burners are jet tube reactors. Jet washers are used for
mentation are eliminated by attaching the microbes or fast reactions such as acid–base reactions. An example is
enzymes to a solid support, preventing them from leav- the absorption of hydrochloric acid in sodium hydroxide–
ing the reactor. The attachment procedures vary, and as a sodium sulfite solutions.