Over the last two decades, continuous flow chemistry has received intense interest and development as applied toward the transformation of organic chemicals. Significant pharmaceutical industry adoption has followed because micro- and meso-scale reactors provide precise control of reaction parameters. This control is of particular interest to the process chemist whose goal is to safely produce syntheses of the highest reliability and limit impurities of an active ingredient.
Many of the advantages of flow stem from the small features of the reaction system, principle of which is a high surface area to volume ratio. The micro-features allow:
Excellent heat transfer
Large interfacial area and high mass transfer
High and uniform light efficiency
Rapid heat transfer is an enabling attribute which distinguishes flow chemistry from batch. The rate at which heat is added to or removed from a system can influence regio, chemo, and stereoselectivity, reaction yield and the quantity of byproducts formed. It can also enable otherwise prohibitive highly exothermic reactions to be run in a safe and controlled manner and to run reactions at higher temperatures and concentrations.
A visual example illustrating heat transfer efficiency enhancement under flow conditions is shown in Figure 1, considering two containers with different geometry – flask or tubing – containing 1 L of water. Batch analogy would be using a cube (simplified flask) and flow a thin tube. In the case of batch, 1 L of water fits in a 10 cm edge cube which corresponds to 600 cm2 surface area. In flow, using a 0.1 cm edge square tube (simplified tubing), the same amount of water fills 1 km of tubing which corresponds to 40000 cm2. Intuitively, one can see that heat transfer efficiency in the tube is much higher than in the cube due to a greater interaction between the fluid and the container walls (~ 67 times larger).