Chemical Engineering October 2012 - 46

Feature Report
Figure 5 illustrates different cooling
strategies for an exothermic reaction
in a 10-L reactor:
* In the first case, no cooling is applied,
and the tube diameter is 38
mm. The overall reactor length is
9 m, and the product temperature
rises to over 100°C
* In the second case, a single coolant
temperature of 0°C is used. The first
cooling stage (2 L) uses a 5-mm-dia.
tube, and the second stage (8 L) uses
a 38-mm-dia. tube. The overall reactor
length is 109 m and the process
temperature is held at 40-60°C
* In the third case, a single tube of
5-mm dia. is used. The first cooling
stage (3 L) uses a cooling jacket at
10°C, and the second stage (7 L)
uses a cooling jacket at 40°C. The
overall reactor length is 509 m and
the temperature is held at 40-60°C
* In the fourth case, a single tube of
5-mm dia. is used, and the cooling
jacket is at 10°C throughout. The
overall reactor length is 509 m, and
the process material is progressively
cooled to below 20°C
The first case is the simplest and
cheapest solution since the tube length
is short and no cooling system is required.
In the second and third cases,
the process temperature is controlled
within a range of 20°C but the second
case has the advantage of shorter tube
length. In the fourth case, the product
is subject to severe overcooling, which
will affect the reaction rate (and hence
loss of yield).
Common flow reactors
Most flow reactors fall into two broad
categories according to whether they
use static or dynamic mixing.
* Static flow reactors rely on fluid
movement through the reactor to
generate radial mixing and the
higher the axial velocity, the better
the mixing. Having no moving parts,
they can be fabricated with very
small channel diameters. They are
generally smaller reactors suited to
fast reactions with homogenous fluids
(although multiphase mixtures
can be handled in some cases)
* Dynamic flow reactors use mechanical
stirring to mix the product. The
mixing performance is independent
of fluid velocity through the reactor.
FIGURE 6. This
dynamically mixed
plug-fl ow reactor
uses transverse
mixing in tubes
These are generally larger systems
suited to slower reactions and multiphase
mixtures
Flow reactor types
This section summarizes common flow
reactor types. It has to be accepted that
the comments are generalizations and
there will be considerable overlap in
terms of capabilities between different
reactor types.
Microreactors (statically mixed):
These typically have channel diameters
of less than half a millimeter, although
channels of up to 1 mm or more
are often described as microreactors.
The flow channel may be a simple tube
or may include static mixing elements
to promote mixing. They operate under
laminar flow conditions with a Reynolds
number of less than 100 [4].
Advantages: Very good heat transfer;
Good plug flow at low velocities; Can
handle reaction times from less than
one second to many minutes within
short channels; Low startup and shutdown
losses; Can be used with very
little process knowhow
Disadvantages: Generally poor mixing
with two phase fluids, although this
can be addressed with static mixing elements;
Very high cost per unit volume;
Poor solids handling capabilities and
easily blocked; Difficult to clean other
than by flushing; High pressure drop
Typical applications: Ideal for R&D
where limited quantities of reagents
are available; Can be used for low
throughput product for reaction times
of a few seconds or less
Tubular flow reactors (statically
mixed): These are simple tubes ranging
from a few millimeters in diameter
to over 50 mm. They rely on turbulent
flow for effective mixing and
plug flow.
Advantages: Moderate to good heattransfer
capacity depending on tube
38 CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2012
diameter; Simple with low fabrication
cost; Low pressure drop in larger diameter
systems; Larger-diameter tubular
reactors are easy to clean
Disadvantages: High tube lengths
other than for short reaction times;
Poor to moderate mixing, which limits
performance with multiphase mixtures;
Tube lengths have to be varied
with reaction time; Poor to moderate
solids handling and easily blocked if
flow is interrupted; Startup and shutdown
losses can be high
Typical applications: Generally suitable
for small or large scale production
with low viscosity fluids and limited
mixing requirements; Generally
better for dedicated applications since
performance is sensitive to fluid velocity
(and therefore length has to suit
the reaction time)
Tubular flow reactors with baffles
or static mixing elements (statically
mixed): Baffles or static mixers
in tubes give better mixing and plug
flow than simple tubes, and this improves
with increased density of mixing
elements.
Advantages: Moderate to good heat
transfer depending on tube diameter;
Moderate to good mixing depending on
density of mixing elements and fluid
velocity; More flexible than simple
tubular reactors in terms of reaction
time for a given length; Can operate
effectively under laminar as well as
turbulent flow conditions subject to
adequate density of mixing elements
Disadvantages: Higher fabrication
cost than simple tubes; While performance
is improved with higher density
mixing elements this is achieved
at the cost of higher pressure drop and
increased difficulty of cleaning; Generally
poor solids handling capabilities
and easily blocked with high density
mixing elements; On large diameter
systems with high density mixing ele
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