Page 256 - Modeling of Chemical Kinetics and Reactor Design

226 Modeling of Chemical Kinetics and Reactor Design

• Reaction of a gas with a liquid where the gas bubbles through
the liquid.
• Reaction of a highly reactive substance with a relatively inert
substance where the reactive substance, if present in large amounts
at the beginning of the reaction, either polymerize or decompose.

A semi-batch reactor has the same disadvantages as the batch
reactor. However, it has the advantages of good temperature control
and the capability of minimizing unwanted side reactions by maintain-
ing a low concentration of one of the reactants. Semi-batch reactors
are also of value when parallel reactions of different orders occur,
where it may be more profitable to use semi-batch rather than batch
operations. In many applications semi-batch reactors involve a sub-
stantial increase in the volume of reaction mixture during a processing
cycle (i.e., emulsion polymerization).

CONTINUOUS FLOW ISOTHERMAL PERFECTLY
STIRRED TANK REACTOR

A continuous flow stirred tank reactor (CFSTR) differs from the
batch reactor in that the feed mixture continuously enters and the outlet
mixture is continuously withdrawn. There is intense mixing in the
reactor to destroy any concentration and temperature differences. Heat
transfer must be extremely efficient to keep the temperature of the
reaction mixture equal to the temperature of the heat transfer medium.
The CFSTR can either be used alone or as part of a series of battery
CFSTRs as shown in Figure 4-5. If several vessels are used in series,
the net effect is partial backmixing.
It is easy to maintain good temperature control with a CFSTR.
However, a disadvantage is that the conversion of reactant per volume
of reactor is the smallest of the flow reactors. Therefore, very large
reactors in series are needed to achieve high conversions. For example,
the first reactor could be run to give a 50% conversion, yielding a
high rate of reaction and subsequently reducing the total reactor
volume. The next reactor might run from 50%–80% conversion and
the third from 80%–90% until the desired conversion is reached. The
effect of this process is a continuous reaction system that has a much
lower volume, but has more equipment items because of the reactor
vessels required.
Industrial reactors operate in the steady state with the volume,
concentration, and temperature of the reaction mixture being constant