Plug Flow Reactor (PFR): How It Works and How It Compares to Batch

A plug flow reactor is a tube that reaction fluid moves through in one direction, reacting as it travels, so position along the tube stands in for reaction time. Nothing back-mixes. A batch reactor does the same chemistry in one stirred vessel over the clock instead. We build the batch and jacketed side of that choice, not the tube.

How does a plug flow reactor actually work?

Picture a slice of fluid entering one end of the tube. It fills the cross section, it does not mix with the slice ahead or behind, and it travels the length of the tube while the reaction runs inside it. By the time it reaches the outlet it has been reacting for exactly as long as it took to travel. That travel time is the residence time, set by tube volume divided by volumetric flow rate.

Because slices do not exchange material, concentration changes with position. Feed composition sits at the inlet, product at the outlet, and every point between is a snapshot of the reaction partway through. The tube is a timeline you can touch.

Here is the part the textbooks derive and most equipment pages skip: that single slice behaves exactly like a small batch reactor. The ideal PFR design equation at residence time tau gives the same conversion a batch reactor gives at reaction time t = tau, for the same kinetics at the same temperature. A PFR is a batch reactor stretched along a pipe and set in motion. Prove the chemistry in a stirred flask over forty minutes and an ideal PFR at forty minutes of residence time gets you there too, provided the tube holds the temperature the flask held. That proviso is where the engineering lives.

What is “plug flow”, and when does the assumption break?

Plug flow is an assumption, not a property of pipes. It asks for a flat velocity profile across the tube, complete mixing radially, and zero mixing axially. Nothing in the real world delivers all three.

Run laminar and the profile is parabolic: fluid on the centreline moves faster than the average, fluid at the wall barely moves. Residence time is then not one number but a distribution. Some material leaves under-reacted, some sits at the wall long past its window. Static mixers, coiled tubes and gas-liquid slugs all exist to drag reality back toward the assumption.

The second break is mechanical, and it ends the conversation for a lot of processes. A PFR is a tube with a small cross section. Anything that precipitates, crystallises, gums or grows on a wall narrows that cross section, and a narrowing tube shifts flow, pressure drop and residence time at once. A stirred vessel is far less fussy. Our stainless reactors run a two-blade anchor type impeller sweeping close to the wall, a shape chosen precisely for viscous and depositing contents. A tube has no equivalent.

PFR, CSTR or batch: what actually separates them?

These three are the whole of ideal reactor theory, and what separates them is what each one holds constant.

The row worth staring at is the third. A CSTR runs the entire reaction at outlet concentration, which for most positive-order kinetics is the slowest place to be, and that is why a CSTR needs more volume than a PFR for the same conversion. A PFR keeps its inlet at feed concentration, so part of its length works at the fast end of the rate curve. Batch does the same, in time rather than in space.

The fourth row is where selectivity gets decided. If your reaction dumps its heat early, a batch vessel takes that peak through one jacket while a tube spreads it over metres of wall. If the peak is what cooks your product, the tube wins on chemistry, not on convenience.

What does the difference look like in real specs?

Reactor theory is dimensionless. Purchase orders are not. Here is what our jacketed batch hardware lists, straight from the parameter sheets, so you can hold it against a flow rig quote.

Two rows there decide more purchases than any of the theory above. The vacuum row: both sheets list vacuum to -0.095 MPa, and neither states a positive design pressure. The jacket row: temperature control is entirely external, sitting on 4 male thread circulation connections and whatever chiller or heater you hang off them. If your process must run above atmospheric inside a sealed tube, our batch vessels are the wrong shelf. If it needs a stirred, jacketed, vacuum-capable vessel you can watch and charge into, the BSF-20L with its 90 W gear motor and 60-600 rpm range covers bench and small pilot batches, and the BSF-100L, stepping up to a 250 W motor on the same speed range covers the tier above. The rest of the metal range sits in our stainless reactor category.

Glass is the other half of what we build, and its parameter sheet says something a PFR discussion needs to hear out loud.

Shared across the whole glass range: GG3.3 borosilicate, stirring 60-600 r/min, vacuum to -0.095 MPa, 220 V/50 Hz, and a pressure range the sheet states as normal or negative pressure, no positive pressure.

“No positive pressure” is printed on that sheet for a reason, and it is the sharpest line between our world and the PFR world. Glass buys you sight of the reaction and resistance to aggressive chemistry. It does not buy you pressure. The full range is on the double layer jacketed glass reactor page covering SF-10L through SF-100L, and if you are still weighing the two constructions, our write-up on choosing between a stainless steel reactor and a glass reactor goes through it property by property.

Why does batch still win most lab and pilot work?

Not inertia. Optionality.

Count the lid ports on the BSF-20L: a Φ50 mm flange port for stirring, KF25 for the constant-pressure funnel, KF40 for reflux and condensation, KF25 for a 350 mm thermometer sleeve, KF25 for the feeding valve, a Φ50 mm illuminated sight glass, and a KF40 solid charging port. Seven live openings on one lid, inside an ID Φ220 mm flange. Seven things you can do to a reaction while it runs: dose it, reflux it, probe it, watch it, throw a solid into it.

Now try to buy that on a tube. Each function becomes a designed-in fitting at a chosen position, and each position is a fixed point in the reaction timeline. Want to add a reagent later in the sequence? On the bench you turn a valve when you decide to. In a flow rig you cut the line and weld in a tee at the right length.

That is the real reason process development runs in stirred vessels. You do not know the recipe yet. Batch lets you not know it. A PFR asks you to have already decided.

When is a PFR the right answer and we are not your supplier?

Being plain about this is cheaper for both of us than a wasted enquiry. Go tubular when:

• The reaction is fast and violently exothermic. A tube has a large wall area per unit of contained volume, and only a small inventory reacts at any instant. A stirred vessel concentrates the same heat release into one jacket at one moment.
• The intermediate is something you do not want a hundred litres of. Flow keeps the hazardous inventory down to whatever is in the tube right now. That is a safety argument, not a throughput argument.
• You need positive pressure, a gas-phase feed, or a fixed catalyst bed. Our glass sheet rules out positive pressure outright, and the stainless sheets list a vacuum rating rather than a pressure rating. Buy the frame from a builder who rates it.
• The product is stable for seconds, not hours. Short, tight residence time is what a tube is for.
• You make one product, all year, at volume. Continuous plant amortises its rigidity across the run length. Campaign chemistry never gets to.

We build double-layer stainless and borosilicate glass jacketed batch vessels. We do not build PFRs, and no amount of configuration turns a 100 L stirred pot into one. If the list above describes your process, take it to a flow-chemistry specialist and keep the batch vessel for your workup, crystallisation and solvent recovery, which is where most flow plants still need one anyway.

Numbering up or scaling up?

This is the argument continuous people make best. Scale a PFR by running more tubes and the chemistry never notices, because the cross section never changed. Scale a batch vessel and everything changes at once.

They are right about the second half. Look at what moves between our 20 L and 100 L stainless units: volume goes up five times, the listed vessel diameter from Φ354 mm to Φ403 mm, the motor from 90 W to 250 W, and the working footprint barely at all, 780 x 650 x 1570 mm becoming 790 x 680 x 1680 mm. Five times the batch, on nearly the same floor tile, under a motor less than three times the size.

That is the trap in one line. Volume grew faster than the vessel surface did. Jacket area per litre is lower at 100 L than at 20 L, so the same recipe releasing the same heat per litre has less wall to lose it through. The stirring speed range did not change either, 60-600 rpm on both, so a given rpm buys a different tip speed and a different mixing intensity in the wider vessel. Nothing on the sheet warns you about this. It shows up as a longer cook and a worse impurity profile.

So the honest scale-up question is not which motor. It is how much heat your reaction releases and how fast, and whether the circulator on those 4 male thread jacket ports can pull it away at 100 L as well as it did at 20 L. We cannot answer that from a model number, because it depends on your exotherm. Our jacketed reactor selection guide walks through that confirmation list, and the broader equipment selection guide covers how this fits with the rest of the line.

Frequently asked questions

Is a plug flow reactor the same as a tubular reactor?

Not quite. “Tubular reactor” describes the hardware, a pipe you flow through. “Plug flow” describes an idealised behaviour inside it: flat velocity profile, mixing across the tube, none along it. Every PFR is a tubular reactor, but a tubular reactor only approaches plug flow if its flow regime, mixing and length-to-diameter ratio cooperate.

Can a batch reactor do the same chemistry as a PFR?

Usually yes, on conversion. The ideal PFR design equation matches the batch equation with residence time in place of reaction time, so the same kinetics give the same conversion. They part company on heat and selectivity: a batch vessel meets the whole exotherm through one jacket at one moment, a tube spreads it along its length. If your problem is the peak rather than the extent, the batch vessel is genuinely harder.

Do you supply plug flow reactors?

No. We build double-layer stainless steel reactors such as the BSF-20L and BSF-100L, and GG3.3 borosilicate glass jacketed reactors from SF-10L to SF-100L. If you need a tubular flow rig, we are not the supplier, and we will say so on the first email rather than the third.

Can your jacketed reactors run under pressure?

The glass sheet states normal or negative pressure with no positive pressure, and lists vacuum to -0.095 MPa. The stainless sheets for the BSF-20L and BSF-100L also carry a vacuum figure of -0.095 MPa rather than a positive design pressure. Treat both as vacuum and atmospheric vessels, and tell us your operating condition before you order, because pressure changes which family you should be looking at.

Glass or stainless for batch work?

Glass when you want to see the reaction and the chemistry attacks metal: GG3.3 borosilicate, 10 L to 100 L, no positive pressure. Stainless when you need the metal construction and the lid port set. Both run 60-600 rpm and both pull to -0.095 MPa, so the decision is material, not agitation.

What to confirm before you choose

If you came here to decide between a tube and a pot, the deciding facts are not on any spec sheet. They are: how much heat your reaction releases and over what fraction of the run; whether your product tolerates hours at temperature or only seconds; whether you make one thing forever or six things a quarter; and whether anything in the mixture settles, gums or crystallises. Answer those four and the reactor type answers itself.

If the answer lands on batch, what we need from you is short: working volume, material requirement, the lid ports you actually need, stirring speed, condenser and feeding configuration, and how you plan to connect the jacket. Skip the BSF-100L if your batch never exceeds 20 L, because the extra volume costs you jacket area per litre and buys nothing. Send the exotherm and the temperature window with the enquiry and we will size the vessel and the jacket duty against your process instead of against a catalogue.