Progress in flow chemistry

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.

Flow reactors (Figure 2) operate continuously, processing a small fraction of the total lot quantity at any time. Residence time in flow is dictated by reaction time. If this is 6 minutes, material remains in a flow reactor for 6 minutes. Shorter residence times, better mixing, and reduced contact between reacted and unreacted material contribute to improved quality and yield (2, 3). For a 6 minute residence time, the productive capacity in flow is 10,000 litres per hour per cubic metre of reactor capacity. This falls to 1,000 for a one hour reaction and approaches parity with large batch at reaction times above 10 hours. Higher productivity contributes to substantial reductions in plant size, operating costs, and energy use.