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

36 Reactors in Process Engineering

equations for a CSTR and a PFR can be modified to reflect make the batch reactor particularly attractive in industrial
this quantity. applications.
The batch reactor also has disadvantages. These include
C A0 X A
CSTR, τ = high labor cost, manual control, poor heat transfer con-
−r A
ditions, and mixing problems. Poor heat transfer results
X A from relatively low area-to-volume ratios. This can be

dX A
PFR, τ = C A0 avoided with the use of internal coils or external recycle
0 −r A
heat exchangers. Batch reactors are generally not suitable
For the special cases of constant density, these equations
for highly endothermic or highly exothermic reactions.
simplify to
These heat effects can be partially avoided by running in
C A0 − C A a semibatch operation.
CSTR, τ =
−r A Good mixing is required for approaching theoretical
[constant volume or density] conversion. Depending on impeller design, a power of
3
0.5–1.0 kW/m produces 90% of the calculated theoretical
C A

dC A conversion. Care must be taken to design batch reactors
PFR, τ =−
−r A with a height-to-diameter ratio close to one. For larger
C A0
ratios, pump circulation or baffling is required. For high-
[constant volume or density].
pressure reactions, sealing problems may be encountered
on the agitator shaft.
5. Transient Stirred-Tank Reactors Perhaps the biggest disadvantage of a batch reactor is
the difficulty encountered for isolation of intermediates.
Design equations for unsteady-state operation are needed
For series reactions such as A → B → C, where B is the
for start-up of CSTRs or for semibatch operation. These
desired product, it is difficult to stop the reaction (quench)
equations must have the ability to predict accurately the
without overshooting.
concentration or conversion changes before steady-state
Continuous tubular flow reactors are most commonly
flow is obtained. Starting with the general design equation,
used for large quantity items such as chemicals manu-
and assuming perfect mixing, we obtain
factured in the petroleum industry. There are many ad-
t dN i vantages of continuous tubular flow reactors. Labor costs
˙ n i0 − ˙ n i + r i V = .
dt are very low, and automatic control is easy to implement.
Since Liquid- or gas-phase homogeneous reactions are routine
for all temperature and pressure ranges. Heterogeneous
˙
˙ n i0 = C i0 V 0
reactions, such as solid-catalyzed reactions, are easily run
˙ n i = C i V ˙ in packed beds or packed tube reactors. Intermediates are
easy to isolate for any desired conversion, since the re-
t
N i = C i V
actor length can be adjusted. Heat transfer is relatively
and good with large area-to-volume ratios and can be made
as large as required by using smaller tubes. For large heat
t
t
dN i ≡ V dC i + C i dV
effects, the reactor can be designed as a counter-current
upon substitution the resulting equation is heat exchanger or as a single jacketed reactor. For highly
endothermic reactions, the reactor tubes can be placed in a
˙ t ˙
dC i C i (V + dV /dt) − C i0 V 0
+ − r i = 0. furnace and heated radiantly or with hot combustion gases.
dt V t Tubullar flow reactors are usually inflexible. Normally
they are designed and dedicated to a single process. They
C. Design Considerations are typically hard to clean and maintain, have high capital
costs, and depending on materials and geometry, are rarely
1. Batch Versus Flow Reactors
stock items.
Commercial-scale batch reactors are generally used for To achieve desired conversions predicted by ideal de-
small-lot or specialty items. This includes chemicals such sign equations, plug flow is required. This implies turbu-
as paints, dyes, and pharmaceuticals. Batch reactors are lent flow and higher energy costs if packing is used. Mass
very simple and flexible. Vessels used to make one com- transfer can also be a problem. Axial diffusion or disper-
pound can be washed and reused to make other products. sion tends to decrease residence time in the reactor. High
The ease of cleaning and maintaining batch reactors along values of the length-to-diameter ratios (L/D > 100) tend
with low capital investment and low instrumental costs to minimize this problem and also help heat transfer.