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Process technology
B
atch reactors have been heavily used for fine chemical manufacture for centuries. However, while many processes work extremely well in batch mode, certain types of reactions are less successful. A few of these are difficult or dangerous to scale up. In those cases, fine chemical manufacturers have increasingly turned towards the use of continuous flow reactors.
A continuous flow reactor is in the order of 10,000
times smaller than a batch reactor with the same throughput performance. If the reaction and chemistry work for both types of reactors, a 10,000L batch reactor can be replaced by a 1L flow reactor. The resulting high surface-to-volume ratio is an important feature of flow reactors, along with their high heat and mass transfer capabilities. Another feature of flow reactors is the narrow and
constant residence time, which is especially beneficial for products that decompose when exposed for too long to the reaction conditions required to run the wanted reaction. The continuous nature of the processing further adds to the advantage because increasing batch size simply involves running a flow reactor for longer. It also reduces efforts in the long
run as sequential scale-ups to increasingly large batch reactors are not needed. Continuous flow reactors have been shown to
offer a range of advantages when applied in the right situations. These include better reproducibility, reduced development, improved safety, and even access to molecules that cannot otherwise be made on a large scale.
IMPROVED REPRODUCIBILITY One of the key advantages of a continuous flow process is that it often gives better reproducibility from run to run than from batch to batch in a traditional reactor; it is much easier to achieve a defined and homogenous temperature level with a flow reactor. This is particularly important for highly exothermic reactions. The heat of reaction is easier to dissipate with a small flow reactor than with a large batch reactor. A common problem of highly exothermic reactions run in batch reactors is non-uniform temperature distribution, so-called ‘hotspots’. This frequently leads to problems like reduced yields, more impurities during scale-up or variability in yield from batch to batch.
Chemistry&Industry • November 2013 21
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