Dr Sam Adams from Stoli Catalysts discusses why the batch technology dominates, introduce the flow chemistry, and discuss problems & opportunities in switching to flow.
Flow chemistry is not new
In fact it has been around for over 400 years.
This picture comes from a book published in 1556.
It is a continuous gold-processing plant. Here you can see gold ore is crushed in a mill, before being mixed with water and fed into these tubs here. Each tub is partially filled with mercury and stirred, acting like a separating funnel to attract the pure gold into the mercury layer, and washing the impurities away. The tubs here act a lot like a series of continuously stirred tank reactors, and you can find more about this type of reactor by watching our CSTR video.
You see familiar pictures of continuously stirred tank reactors –
one of the main continuous flow reactors.
You can find more about this type of reactor by watching our CSTR video.
Therefore, neither flow nor batch is new.
Flow chemists talk about moving away from “outdated” batch technology, but I think it’s very important to appreciate how useful batch actually is.
For all synthetic chemists, batch is the bread and butter of what we do,
it is what we grew up on.
Whether it is making milligrams of a pharmaceutical in a research lab, or scaling up to tons, the general idea is exactly the same.
Mix some components together, stir, heat if necessary and then remove from the vessel for purification.
It is a tried and tested method. And it works.
You can do a vast array of things with it without having to learn multiple new techniques to get it to work.
But, in a world of tiny profit margins and environmental concerns, it can be quite inefficient. So can you do better with flow?
The easiest way to answer this question is to look at how flow chemistry actually works.
In flow, on the face of it, things seem complicated because you are constantly feeding new material into the reactor and taking out different material at the other end.
In fact, it quite resembles conventional batch chemistry. In batch, you mix reactants, and it takes sooooome time for them to react and obtain products.
The arrow in the equation corresponds to the time it takes for the reaction.
In flow, we have the same idea, only the arrow corresponds to the space, not time.
If everything is set right, the start of your flow reactor corresponds to the left side of your reaction scheme; and the exit can be the right side.
If you imagine a single point in the reactor, you see an effective snapshot of your reaction coordinate. It is a batch reactor equivalent from, say, 7 to 8 minutes reaction time. The numbers here are of course, arbitrary.
The actual number is the time required for materials to reach and leave a particular reactor point. See our other videos on residence time for more details. Simplifying a bit, you just calculate the linear fluid velocity dividing the volumetric flow by the reactor cross-section.
Of course, different types of flow reactors have their own eccentricities and you can find out all the types and how they work by clicking the links on screen.
The great thing about flow reactors is being able control of mixing, heat and mass transfer rates much better than in flow.
For heat transfer, a round bottom flask exchanges heat with its external area. The volume to surface area is high, so heat transfer is limited.
In flow, your reactors are smaller; their volume to surface area is small and you have very good heat transfer.
The same is also true for mixing. In smaller volumes, reactants have less of a distance to travel to find one another and initiate a reaction. When you combine these advantages, it is much easier to fine control reaction parameters. This means you improve reaction and impurity profiles.
As heat transfer is better, it also means that less energy needs to be pumped in or taken away from the system in order to keep the reaction going. This not only improves energy efficiency, but it also improves safety, as the potential for thermal runaway or other dangerous occurrences is removed.
This in turn aids scale-up, as reactor volumes can be kept relatively small compared with batch technology.
The breadth of design in flow reactors is quite large, with many different available systems such as CSTRs, as well as plate-based microreactors or trickle bed reactors.
Such diversity has both advantages and disadvantages when you compare against batch.
On one hand, specialised flow reactors are often more efficient than batch. Product quality could be improved and production simplified. Yet, each step often requires a different reactor – a cost and scale-up challenge.
On the other hand, batch technology is simple and multifunctional. Process development work from lab to production scale happens in the same batch reactor concept that accelerates development and cuts costs.
So, what is better – batch or flow?
There is not a one-size-fits-all flow solution to synthetic problems.
Batch technology is still the most economical way to run many chemical processes. It is simple, well-known and multifunctional.
Flow chemistry does bring enormous benefits to many chemical transformations. Flow can be greener and more sustainable than batch, flow can give you better control of your process. It could save both time and money in running processes. Yet, almost any flow reactor has a rather limited application range.
At Stoli we develop novel flow reactor that combines simplicity, scalability, and multifunctionality.
We have developed a monolith-based fuel cell catalyst for sustainable remote energy generation.
Stoli cascaded imine formation and hydrogenation; intensified process to maximise rate, and catalyst utilisation.
Short residence time and high temperature – an impossible combination for batch – allowed increasing specific reaction rates 8-fold in flow with the same product quality.