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

Reactors in Process Engineering 37

Continuous-stirred tank reactors lie somewhere be- conversion and capacity can be achieved by running ten
tween tubular and batch reactors. Mixing and heat transfer 2-m reactors in series or ten 2-m reactors in parallel. The
problems are similar to those of batch reactors. However, split of the feed in the parallel case must be one tenth of the
many of the stirred-tank reactors have benefits of the tubu- total to keep the same space time. In industrial applications
lar flow reactors. These include isolation of intermediates, the geometry chosen is a function of cost of construc-
automatic control, and low labor costs. tion, ease of operation, and pressure drop. Parallel oper-
ation is normally preferred to keep the pressure drop at a
minimum.
2. Heat Effects
ii. Stirred-tank reactors in series and parallel.
Most reactors used in industrial operations run isother- Stirred-tank reactors behave somewhat differently from
mally. For adiabatic operation, principles of thermody- plug flow reactors. Operation of CSTRs in parallel, as-
namics are combined with reactor design equations to suming equal space time per reactor, gives the same con-
predict conversion with changing temperature. Rates of version as a single reactor but increases the throughput or
reaction normally increase with temperature, but chemi- capacity proportional to the number of reactors.
cal equilibrium must be checked to determine ultimate lev- This is not the case for multiple CSTRs in series. CSTRs
els of conversion. The search for an optimum isothermal operated in series approach plug flow and therefore give
temperature is common for series or parallel reactions, much higher levels of throughput for the same conver-
since the rate constants change differently for each reac- sion. When we have two reactors of unequal size in series,
tion. Special operating conditions must be considered for highest conversion is achieved by keeping the intermedi-
any highly endothermic or exothermic reaction. ate concentration as high as possible. This implies putting
the small CSTR before the large CSTR.
3. Design for Multiple Reactors
c. Plug flow and stirred-tank reactors in series.
Common design problems encountered in industrial oper- When two reactors, a plug flow and a stirred tank are
ations include size comparisons for single reactors, mul- operated in series, which one should go first for maxi-
tiple reactor systems, and recycle reactors. mum conversion? To solve this problem the intermediate
conversion is calculated, the outlet conversions are de-
a. Size comparisons of single isothermal flow termined, and the best arrangement chosen. Keeping the
reactors. The rate of reaction of a CSTR is always fixed intermediate conversion as high as possible results in the
by the outlet concentrations. Since the rate is constant maximum conversion. Concentration levels in the feed do
(first- or second-order, etc.), a large volume is required not affect the results of this analysis as long as we have
to provide enough time for high conversion. In general, equal molar feed.
a plug flow reactor is much more efficient and requires
less volume than a stirred-tank reactor to achieve the same
level of conversion. In a plug flow reactor, the rate changes 4. Recycle Reactors
down the length of the reactor due to changes in reactant
In a recycle reactor, part of the exit stream is recycled back
concentrations. High initial rates prevail in the front of the
to the inlet of the reactor. For a stirred-tank reactor, recycle
reactor with decreasing rates near the end. The overall in-
has no effect on conversion, since we are essentially just
tegration of these rates is much higher than the fixed rate
mixing a mixed reactor. For a plug flow reactor, the effect
in a CSTR of equal volume. For complex kinetics such as
of recycle is to approach the performance of a CSTR. This
autocatalytic reactions, where the concentrations of both
is advantageous for certain applications such as autocat-
reactants and products increase the forward rate of reac-
alytic reactions and multiple reaction situations where we
tion, stirred-tank reactors are preferred and require less
have a PFR but really require a CSTR.
volume. Under most common kinetics, a series of three
or four stirred-tank reactors of equal volume in series ap-
proaches the performance of a plug flow reactor.
IV. SPECIAL REACTOR
b. Reactors in series and parallel. CONFIGURATIONS
i. Plug flow reactors. Plug flow reactors are unique
in the sense that operation in parallel or series give the Additional reactors exist that are either completely or par-
same conversion if the space time is held constant. This tially based on the five primary reactor types discussed in
implies, for example, that if a 20-m reactor of fixed diam- Section II. They receive special attention due to specific
eter is required to achieve a given conversion, the same applications and/or unique mass transfer characteristics.