YMC discusses the reasons why micro-reactors will often achieve better results than conventional reactors.
In order for a chemical reaction to run in the optimal manner, several prerequisites have to be met.
First, near-perfect mixing of the reactants has to be achieved, while ensuring a large phase transition area in reactions where more then one phase is present.
Next, the time needed for the reaction to take place has to be guaranteed by a residence time loop.
This is chosen to provide a residence time with as narrow as possible distribution.
In addition, any reaction heat needed or produced during the process has to be added or removed, respectively.
That this is possible has been shown in several different reactions.
Micro-mixers for liquids provide mixing times in the region of 1sec to 1msec, which are well below those of conventional mixers.
As a result, intensive mass transport is achieved, significantly lowering reaction times and increasing yield per volume and time.
At the same time the surface/volume ratio raises to several 10,000 compared to only about 1-10 for stirred tank reactors (STR).
If a micro-heat exchanger is integrated into the micro-reactor, both effects (high mass transfer and high surface/volume ratio) are combined with rapid heat transfer to or from the reactor.
In practice heavily exothermal reactions such as chlorination, oxidation, nitration and even fluorination with elemental fluorine (in metal microreactors) can be performed under almost isothermal conditions.
Process-safety micro-reactors, which are run in continuous mode, are also superior compared to STRs, which have to be run in discontinuous mode.
Continuous mode operation guarantees that no harmful side products can accumulate and they are also easier to control compared to discontinuous ones.
Finally, micro-reactors pose a much lower safety risk, even if a run away should occur, due to the low volumes if materials involved.
The above holds true for laboratory-scale systems, but before using micro-reactors for production scale, several other parameters have to be considered, such as the back pressure generated by the micro-structures involved.
These can result in a limitation of the throughput achievable with each micro-reactor module.
Micro-reactors are exceptionally well suited for the production of small to medium-sized quantities of product, such as those frequently required in the fields of active pharmaceutical ingredient or fine/speciality chemicals production.
In order to rate different reactions, process stages are normally categorised according to their reaction rate.
These requirements can be fulfilled by using micro-reactors and their use can optimise the performance of such reactions.
Reactions taking between 1sec and 10min may also benefit from the use of micro-reactors.
In this case, the overall reaction is no longer governed by the mixing, but via the kinetics of the reaction.
These cases still require optimal mass and heat transfer combined with improved control of the residence time to allow higher yields and better selectivity.
It is necessary to allow these slower reaction rates in the system by addition of residence time loops into the apparatus.
Slow reactions taking 10min and more are predestined to be performed in STRs.
Using micro-reactors for these reactions can lead to improved process safety, but would demand residence time modules that are so large that the process becomes inefficient.
Therefore, use of micro-reactors for such reactions is only viable in some exceptional cases.
