Flow Reaction Technology

 

The Chemist’s Round Bottom Flasks of the 21-st Century

 

Flow reaction or microreactor  technology opens up a new way of chemical synthesis in a highly controlled way. Due to their improved mass and heat transfer, these devices are well suited, e.g., for mixing-sensitivy, fast and highly exothermic reactions.  Endothermic reactions profit as well from the improved thermal management. This leads to a better exploitation of resources and decrease of energy consumption of chemical processes and will hereby increase their costing and co-efficiency. As an additional interesting benefit it enables to run reactions in a safe way under unconventional process parameters known as new process windows. This includes processing at unusually high temperatures and pressures – high-p-T processing.

 

 

 

In the last years, great efforts have been undergone to carry out reactions at small scale. These undertakings led to the field of Flow Reaction Technology, which includes the concept of doing chemistry a different way by improving and intensifying existing processes. Microreactors are typically continuous systems and microstructure dimensions have to be small enough to provide the necessary heat transport and mass transfer. The prefix “micro” relates to the inner life of the reactor and not to the components that are build around.

The main argument for using flow reaction technology in a chemical process is process intensification. This subsumes first of all order-of-magnitude changes in conversion and selectivity or increased space-time yields, but is more and more used in a broader definition adressing also waste reduction, energy savings and better safety. Totally, this means having much increased performance at the same reactor footprint or shrinking down the reactor dimensions. The preferential way for the usage of such microstructured components is to implement them into a running process at the point where they are needed or where their benefits can intensify the course.

 

There are several benefits of microreactors:

 

 

  • Fast and reproducible mixing under laminar flow conditions while the Reynolds Number stays below 2300.

  • Excellent heat exchange/transfer resulting from a high contact surface of the micro reactors.

 

  • Microreactors typically have a heat exchange coefficient of at least 1 megawatt per cubic meter per Kelvin, up to 500 MW/m3K  vs. a few kilowatts in conventional glassware (1 l flask ~10 kW/m3K).

 

  • Ratio of contact surface to volume of the reacting substance in microreactors is up to 500 times more than of the conventional batch reactors.

 

  • Industrial batch reactors typically have a surface/volume ratio of some 10 m2/m3 up to 100 m2/m3. Conventional lab-scale apparatus normally comprise a few 100 m2/m3. Opposed to this, the surface/volume ratio of microstructured elements is very large up to 50000 m2/m3 and higher.

 

  • Faster, more precise and efficient temperature control. No temperature gradient in the micro reactors.

 

  • Hot spot temperature as well as the duration of high temperature exposition due to exothermicity decreases remarkably. Thus, microreactors may allow better kinetic investigations because local temperature gradients affecting reaction rates are much smaller than in any batch vessel.

 

  • Works under highest pressure and supercritical conditions.

 

  • Pressurisation of materials within the microreactors is generally easier than with traditional batch reactors. This allows reactions to be increased in rate by raising the temperature beyond the boiling point of the solvent. This is more easily facilitated in microreactors and should be considered a key advantage. Pressurisation may also allow dissolution of reactant gasses within the flow stream.

 

  • Shorter reaction time trough higher temperature and pressure.

 

  • Better selectivity – reactions without by-products trough homogenous temperature and accurate mixing.

 

  • Microreactors are normally operated continuously. This allows a subsequent processing of unstable intermediates and avoids typical batch workup delays. Especially low temperature chemistry with reaction times that are within the range of a millisecond to a second are no longer stored for hours until the dosing of reagents is finished and the next reaction step may be performed. This rapid work up avoids a decay of precious intermediates and often allows better selectivities.

 

  • Exothermic/ endothermic reactions perform safely because of a small reaction volume of the micro reactors and a small amount of hazardous reagents.

 

  • Microreactors can remove heat much more efficiently than vessels and even critical reactions such as nitrations can be performed safely at high temperatures. Many low temperature reactions as organo-metal chemistry can be performed in microreactors at temperatures of −10 °C rather than −50 °C to −78 °C as in laboratory glassware equipment.

 

  • Reduced scale-up issues.

  • Easy automation – completely automated micro reaction systems with online analytics working 24 h/day.