Chemical reaction engineering is the discipline of translating a reaction that works in a small laboratory flask into a reactor that runs safely and economically at industrial scale, and reactor design is where that translation actually happens. A reactor's shape, its internal mixing pattern, and specifically how long a given batch of reactant molecules actually spends inside it, its residence time, are all deliberate engineering choices, not incidental details, because the exact same chemical reaction can finish almost completely in one reactor design and barely get started in another running the very same feed.
Residence time determines how far a reaction actually gets to proceed
A reaction proceeds by converting reactant molecules into product over time, and the fraction of feed that's actually converted by the time it leaves the reactor depends directly on how long, on average, those molecules stayed inside reacting. A reactor built to move fluid through quickly gives reactant molecules relatively little time to convert before they exit, which might be exactly what a process needs if a fast intermediate reaction risks running further into unwanted side products, while a reactor built to hold fluid for a genuinely long residence time can push a slower reaction close to complete conversion. Choosing that residence time correctly is one of the reactor designer's central jobs, not an incidental side effect of choosing a vessel size.
The mixing pattern shapes exactly which molecules meet which, and when
Beyond simple residence time, a reactor's internal mixing pattern, whether fresh feed mixes instantly and thoroughly with everything already inside, as in a continuously stirred tank, or moves through in an orderly, largely unmixed stream, as in a tubular plug-flow reactor, changes the actual concentration each reacting molecule experiences at each stage of its trip through the vessel. Because reaction rate typically depends on concentration, this mixing pattern directly affects both how much product actually forms and, in reactions competing with unwanted side reactions, which product a reactor mostly produces, which is exactly why reactor design treats vessel geometry and mixing pattern as chemically consequential engineering choices, not just plumbing.
What we're still unsure about
That residence time and mixing pattern directly govern how far a given reaction actually proceeds are well established, extensively modelled principles within chemical reaction engineering, confirmed across a century of industrial reactor operation. What's more genuinely a matter of ongoing engineering judgement is exactly how faithfully a reactor's behaviour at large industrial scale can be predicted from small laboratory or pilot-scale experiments, since effects like imperfect mixing and uneven heat removal often behave quite differently as a reactor gets larger, and engineers continue to rely on careful, sometimes contested scale-up methods and safety margins rather than treating a laboratory result as a guaranteed preview of full-scale performance.
This sits inside Chemical Reaction Engineering & Reactor Design, one of eight topics in Chemical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.