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Flow reactors bring a wealth of benefits in the synthesis of chemicals. Many of these advantages derive from the small size of the reactor, allowing rate promoting and elevated conditions to be safely applied – the enhancement then brings about the productivity required to run a reaction in the small reactor.
Flow is an area of chemistry that may previously have been considered inaccessible to many laboratories, particularly teaching labs, due to potentially prohibitive costs of apparatus and what could be considered as a fear of the unknown. The Asynt fReactor platform has changed that. This fully accessible benchtop Flow Chemistry platform brings a range of intuitive and flexible Flow reactors and photochemical add-ons capable of operating with single and multiphasic reaction systems to support the development of materials and synthesis routes.
Reactions can be run under high pressures, which in turn allows the temperature to be safely increased above the normal (atmospheric) boiling point of the solvent. This allows significant rate enhancement. Since the volume within the reactor (the hold-up volume) is small, then, subject to proper management of risks, it can become feasible to carry this out in a laboratory where a batch process may not be possible. Additionally, Flow reactors can help to generate hazardous and unstable reagents in situ meaning that relocation/transportation of them is unnecessary, and they can be used in the location of manufacture.
By using continuous stirred tank reactors (CSTR), extended reaction times are possible – enabling greater productivity with reduced operator-time. CSTRs are a mainstay in the batch manufacture of chemical products due to their flexibility in dealing with a wide range of materials; they span single and multi-phase chemistry, fermentation requiring gas sparging, crystallisations, viscous materials and many more.
CSTRs come in a range of sizes and process chemists have access to considerable data and a wide range of correlations with which to specify equipment (e.g., mixer type) and operating parameters for specific functions (e.g. suspension of particles, mixing of phases). There is also a great volume of documented information covering scaling the process (e.g., from small development scale to manufacture scale) with CSTRs.
With batch reactors, the composition of the reaction changes with time until a steady state is reached. With flow reactors, once these have been running for long enough, the composition of the outgoing product is constant. During the initial phase of running a flow reactor, the conditions take some time to equilibrate and for this steady state to be reached. A useful parameter in estimating the time it takes to reach steady state is to express the cumulative flow through the reactor in terms of (flowed) reactor volumes.
(Flowed) reactor volumes (RV) = Total flowrate of incoming streams (ml∕min) Reactor volume (ml) × time (min)
By monitoring the output of the reactor and plotting an indicator of the reaction (for example the yield or concentration of one of the products) as a function of reactor volumes allows the time taken to reach steady state to be established and confirmation that the reactor and reaction is running in this way.
Flow Chemistry enables scientists to take absolute control of their reaction parameters, with automated additions and extractions and fully repeatable conditions. Whereas syringes may be automated for additions, there is a limit to the volume of product this allows. Utilisation of Flow Chemistry apparatus enables greater control of supply to the reaction and reduces the capacity for human error.
A single well mixed reactor (a continuous stirred tank reactor – CSTR) means an incoming ‘element’ of fluid is instantaneously mixed into the reaction mixture within the reactor. Since there is a steady flow, then for every incoming ‘element’ there must be an outgoing ‘element’. With good mixing, important for uniform reaction conditions, there is chance that the incoming ‘element’ almost immediately leaves in the outgoing stream. Equally, the ‘element’ may spend a long time in the reactor. It is just like a bag of balls where you drop a ball in, shake it and immediately select one at random to be the one that leaves. Some balls spend only a small amount of time in there whilst some will be spending a long time – it really is just a game of chance and probability.
But by cascading the reactors together minimises the odds of our ‘element’ passing straight through the cascade of reactors. There are well established models that predict this (and the fReactors have been shown to fit this model in this paper ).
One particularly noteworthy feature is the ability to run Flow Chemistry reactions under high pressures, which in turn allows the temperature to be increased above the normal (atmospheric) boiling point of the solvent. This allows significant rate enhancement. Since the volume within the reactor (the hold-up volume) is small, then, subject to proper management of risks, it can become feasible to carry this out in a laboratory where a batch process may not be possible.
Flow Chemistry enables scientists to automate entire reactions, ensuring accuracy and repeatability of all essential parameters. This in terms reduces waste and increases productivity both in material supplies and energy usage but also in operational input.
Inherent to laboratory based and industrial manufacture is multiphasic systems to, for example, transfer reaction products between aqueous-organic phases, in crystallisation, catalysis or gas-liquid reactions. The mainstay of batch processes is a stirred tank reactor. Here active mixing is provided through agitators capable of suspending solids, creating, and maintaining droplet suspensions or in dispersing gas as small bubbles. The energy input effects contact between the phases. There are different ways of effecting contact between two or more phases – one strategy is to flow separate phases into the reactor and promote interaction between these through mixing; active mixing includes stirrers whilst passive mixing includes static mixing elements which promotes mixing through twisting flow elements together at the expense of a larger pressure drop.
Flow chemistry systems perform reactions on a small scale for optimization but once the preferred reaction conditions are ascertained, the flow rate can be increased to produce far larger volumes of product.
The fReactor modular Flow Chemistry reactor system
fReactor photochemistry 3 The fReactor Photo Flow photochemistry modules
The fundamental principal of “Greening” in the laboratory (a key consideration to the Asynt chemists in all product development) is to minimize the use and generation of hazardous substances. Flow chemistry enables fast and straight-forward optimization of reactions which reduces the quantity of waste products produced whilst achieving this state. The use of catalysts can become more efficient in flow chemistry. In a continuous process, the catalyst can be fixed, and the reaction mixture set to flow over it. This reduces the need for extraction of the catalyst from the final product. It is also possible that the catalyst may last longer due to decreased exposure to the environment.
By utilising systems such as the Asynt fReactor in conjunction with Photo-flow modules scientists are able to carry out Photochemical reactions in conjunction with Flow reactions.
The Photo-Flow modules are an easy-to-use, powerful light source designed to integrate seamlessly with the fReactor platform. Each universal module sits directly on the fReactor and internal lensing ensure that the maximum power is delivered from the single wavelength LED to the reaction mixture.
The excellent stirring within the fReactor means that the photo-active zone near the window is constantly replenished with reactants giving fast reactions. This stirring means the full range of multi-phasic reactions can be carried out allowing novel reaction schemes to be established. Of course, single phase flows are easily accommodated as well. The modules have a light-tight fit onto the fReactors and switch off instantly when removed from the reactor, minimizing risk of exposure to the powerful light source.
Asynt bring you interactive apps to support your understanding of flow chemistry – giving you the confidence in this technique alongside the ways to properly describe your reactions.
https://www.freactor.com/index.html where you will find a library of relevant published papers, set-up guides, and much more.
On demand webinar: For those wishing to further understand how Flow Chemistry can benefit their work in the laboratory, we offer a FREE on-demand webinar: An Introduction to Flow Chemistry
For further technical support or pricing: Please visit https://www.asynt.com/products/flow-chemistry/freactor/ or contact Asynt via email at enquiries@asynt.com.
