What is residence time and why is it important in flow reactors?
We discuss the tangible benefits that understanding and controlling residence time can bring.
What is residence time distribution?
The residence time distribution of a reactor is an abstract concept to most chemists. In batch chemistry, residence time is simple – you add reactants and remove products. The time chemicals spent in the reactor is the residence time.
In a continuous flow system, however, there is rarely a single “time” the molecules spend in a reactor. Each molecule spends a slightly different time flowing through the reactor and this leads to a distribution of residence times.
Case Study # 01, rev 13,
2 Aug 2021, By Dr Samuel Adams
The mean residence time is the reactor volume divided by the volumetric flow rate. We can change the residence time by either changing the volume of the reactor or the flow rate.
Molecules do not spend this exact time in the reactor.
In a tube, for example, fluid close to the walls moves slower due to friction, and diffusion in the liquid can both cause the molecules to jump forwards or backwards within the flow. Therefore, chemicals that enter the reactor at one moment spend various amounts of time inside.
One way to think of this is as many parallel batch reactors that work for various amounts of time. The number of reactors (Y axis) and time on stream (X axis) is, in effect, the residence time distribution curve.
Batch reactors also suffer from variability in “reaction times” for other reasons. Particularly at large scale, heating, charging, quenching, cooling down, and emptying processes also determine the amount of time that reactants and products spend in the reactor.
Why residence time is important: Product quality
Residence time is vital for product quality. If you have a selective process that must be stopped at a precise moment, then the residence time distribution is critical.
If chemicals spend less time reacting, this has the effect of decreased conversion and a significant amount of reactants at the outlet. If the chemicals spend more time, over-reaction and decreased selectivity result in an increase in by-products at the outlet.
Even in seemingly non-selective reactions, a substantial impurity formation could be observed; ppm-level quantities of impurities may quickly rise to unacceptable levels. Hence, knowing residence time distribution is crucial for impurity control.
Why residence time is important: Reactor throughput
The residence time distribution also defines throughput (how many kilos of product per hour are obtained). Imagine a process where the reaction proceeds to completion in 2 hours. In a continuous reactor, all of the molecules need to spend at least 2 hours in the reactor for the outlet stream to give complete conversion.
In a batch reactor, we often stop the reaction well after 2 hours to obtain the products; then we spend hours on cleaning, validating, re-charging, and heating.
We could use the same reactor as a continuously stirred tank reactor (CSTR, also called semi-batch) constantly adding reactants and withdrawing products. No more wasted efforts in charging! But it does not work because of residence time distribution.
In a CSTR, you can see many molecules spend less than 2 hours in the reactor. These molecules are not reacted and you see low conversion at the outlet.
On the other hand, many reactants spend much longer in the reactor, without any benefit in product formation but a possibly of side-reactions.
With a broad residence time distribution, you are therefore fighting a losing battle for high conversion and are forced to use much longer mean residence times to achieve acceptable conversion.
In the SABRe reactor, we use a series of 10 CSTRs with a narrow residence time distribution. Because of this, we can use the 10-fold lower mean residence time compared with a single CSTR of the same volume. For example, a 1 kilo a day production in a single CSTR could be intensified to 10 kilo a day under the same conditions in SABRe. This means that SABRe shows a 10-fold higher throughput (kg/h) compared to a CSTR of the same volume.
Residence time distribution is vital for product quality and throughput. Combined with the intrinsic chemistry, heat and mass transfer, the residence time distribution defines the reactor performance.
The SABRe system (available in steel, Hastelloy or glass) is suitable for a wide range of chemical applications. Combining simplicity with superb reaction control, SABRe is the best choice for simple, safe and cost effective chemistry.
What can the SABRe do for you today? Get in touch and arrange a trial.
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The SABRe provides precise residnece time control.