Chemical Engineering October 2012 - 43

FIGURE 2. In plug fl ow, the fl uid velocity across the face of a channel is uniform
good understanding of the underlying
process characteristics and a broad
knowledge of reactor types to address
the specific needs.
Reasons for continuous
The benefits of flow reactors vary according
to the nature of the process, but
they typically include the following:
* Improved yield and quality - For
competitive and consecutive reactions,
flow reactors can deliver
significant improvements to yield
and purity
* Reduced solvent use - For heattransfer
limited reactions, flow reactors
can operate with leaner reaction
mixtures and therefore with
less solvent. Similarly, continuous
countercurrent processes for operations,
such as extraction, yield better
separation with less solvent
* Capital cost - For mixing or heattransfer
limited processes, flow reactors
are smaller and often significantly
smaller than batch reactors.
Smaller physical size contributes
to lower equipment costs as well as
smaller utilities and buildings. Capital
expenditure reductions on building
costs alone for a continuous plant
can amount to 50% or more [1]
* Utility costs - Over 50% of the energy
used in a batch reactor is wasted
on the hardware, cross mixing between
the heating and cooling fluids
and uneven utility loads. Flow reactors
can reduce this waste by 90% or
more by virtue of reduced physical
size and steady state operation
* Plant flexibility - Batch reactors
have good flexibility in terms of the
unit operations they can perform,
but very limited flexibility in terms
of working capacity. A flow reactor
is more specialized in terms of unit
operations it can perform, but the
capacity per cycle can be varied by
orders of magnitude by changing the
cycle time. This can contribute to
greater plant flexibility and therefore
less process equipment
* Safety costs - For hazardous processes,
the cost of managing safety
in small flow reactors is inherently
cheaper than large batch reactors.
Small pressure vessels are cheaper
to fabricate and require smaller
emergency-relief systems
The extent to which these benefits
apply depends on factors such as process
type, product value and throughput.
It also depends on whether the
application is for an existing facility
or a new one.
Manufacturers from the chemical
process industries (CPI) often view
the batch versus continuous question
as an either/or option, and reject
flow reactors on the grounds that the
batch vessels are still required for
work-up operations. While this may
be true in many cases, the commercial
advantages of using flow reactors
within existing batch processes
should not be underestimated. Flow
reactors generally occupy small footprints
and can be integrated within
existing batch plants. Where flow reactors
can make a material difference
to yield or reduction in batch failures,
the commercial case for using
them within existing batch processes
can be self-evident. There are also
other reasons for using flow reactors
within existing batch processes, such
as debottlenecking, reducing energy
waste, more efficient use of solvents,
improved safety, reduced problems of
bursting disc failure (which can disable
an entire production facility)
and increasing plant flexibility.
Understanding the process
While microreactors can be used successfully
with very little process information,
the same does not apply to
larger flow reactors. Scaling up flow
reactors by trial-and-error is difficult
and potentially dangerous. A good understanding
of the process is required
before trials are undertaken.
A variety of analytical devices can be
used to study flow processes, but one
of the most useful tools for generating
design data is a reaction calorimeter,
since this provides information about
both kinetics and heat of reaction. It
is also worth noting that the differences
between batch and flow reactors
relate to scale (there is no flow effect)
and a small reaction calorimeter with
similar heat transfer and mixing characteristics
(to the proposed flow reactor)
can provide reliable design data
for scaleup.
Even where processes are relatively
well understood however, there is no
substitute for testing under flow conditions.
The equipment used for scaleup
development should be as similar
as possible to the full sized reactor.
This generally favors the use of large
flow reactors for scaleup work rather
than microreactors (other than where
microreactors can be used at the production
scale).
Reactor capacity
The volumetric capacity of a flow reactor
should be as small as possible.
Apart from the obvious benefits of
small footprint and higher performance,
small flow reactors have
proportionately lower startup and
shutdown losses. The physical size is
calculated from the relationship:
Reactor size (L) = Throughput (L/s) ×
Reaction time (s)
Reactor sizes for a range of throughputs
and reaction times are shown in
Figure 1.
A flow reactor can be scaled up by
a process of numbering up (or scaling
out). For this, the channel size is kept
constant and capacity is increased by
using multiple parallel channels of
similar size. The size of the scaled up
system can be predicted accurately
when numbering up, since the functional
capabilities of the channels remain
unchanged.
The reactor can also be scaled up
by increasing the channel length, although
scope for this is limited by
pressure drop. In some cases, longer
channels will give better performance
(due to higher velocities) and therefore
require smaller working capacities
than predicted.
Increasing the tube diameter is often
the only practical way of scaling up,
especially where high reactor volumes
are required. Increasing the diameter,
however, often reduces performance,
and therefore the scaled system may
have to be larger than predicted to
compensate for the difference.
When scaling up, it is preferable to
use the maximum tube diameter possible,
subject to satisfactory performance.
Short, large-diameter tubes
have lower fabrication costs, a reduced
tendency to block and a lower pressure
drop. They are also easier to clean.
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