Progress in flow chemistry


Consultant, Buckinghamshire, Unite Kingdom


The inventory in a flow reactor at any point is a small fraction of the total material being processed. This is an inherently better solution than batch methods in virtually all respects. Capability, however, is dictated by the flow reactor type and size. Current devices generally fall into three areas of use: micro reactors for research, specialised reactors for difficult or dangerous chemistry and large dedicated flow reactors for bulk chemicals. Despite progress, over 90% of industrial chemicals by type are still made in batch reactors. Large parts of the process industries rely on high volume reactors capable of handling different operations. The underlying principles for equivalent capability in flow are well established but as yet, underexploited.

Batch reactors (Figure 1) are stirred vessels used in the process industries for mixing, synthesis, and separation. Most have external jackets for heating and cooling. At the industrial scale, they typically have volumetric capacities of between one and ten cubic metres. Their value lies in the ability to process thousands of litres in a day for a broad range of applications. Despite these benefits, large vessels have significant limitations. To compensate for the low heat transfer area per unit volume, measures such as increased dilution, reduced operating temperatures, and semi batch addition are common practice. These prolong the process cycle and increase contact between reactants and product. Slow cooling leaves materials exposed to elevated temperatures for long periods after the reaction is complete. Mixing times (1) are poor and shear is localised. These factors variously impact yield and quality. A typical batch cycle involves inertion, filling, heating, addition, cooling, discharge, and cleaning. These activities reduce productive capacity to a value in the region of 100 litres per hour per cubic metre of reactor volume. The number falls with increasing reactor size.