Oscillatory Flow Reactor

What it is

An oscillatory flow reactor (or oscillatory baffled reactor) drives one reagent through a tube while a second is injected, and an oscillating piston sloshes the flow back and forth through mixing internals. They are used to make pharmaceuticals and fine chemicals, from paracetamol to cancer medication.

Net flowReagent A is pumped through the tube.
OscillationA piston sloshes the flow back and forth through the insert.
Phase-gated feedReagent B enters only inside a chosen part of each cycle, at the same average dose as a continuous feed.
The cartridge mixesPrintable helical elements ending in a smooth constriction.

Fig. 2. The reactor, schematic; proportions illustrative.

Timing the feed

The oscillation itself is unchanged. Only the feed of reagent B is timed to it, at the same average dose as a continuous feed. In practice it is a pump programme and a sync signal.

A passive, electronics-free version, using a pressure-actuated one-way element at the feed port, is also part of the patent application (not yet modelled or built).

Fig. 3. Feed rate of reagent B over two oscillation cycles, schematic. The shaded areas are equal, so the dose per cycle is the same.

The commercial case

Higher selectivity

In simulation, for competing reactions, the redesign favours the desired reaction path, which can mean more product or lower purification costs.

Retrofit

In simulation the dosing gain held with a plain baffle string (Fig. 4), and the printable cartridge is designed to slide into an existing tube.

Matched to each reaction

The best operating point and cartridge depend on the reaction, so we choose them for each one.

Results

In simulation, phase-gated dosing gave higher selectivity than continuous feed of the same dose at every operating point and internals tested, and the margin grew as the simulation grid was refined.

The absolute levels are indicative (see Status).

Three internals at ψ 7.8

Fig. 4. Selectivity at ψ 7.8, as a share of the instantaneous-mixing ceiling (the selectivity if the two feeds mixed instantly). Coarse grid, in simulation; dose-matched pairs.

Blade insert, ψ 2.6 to 40

Fig. 5. One model reaction scheme (see Status). Coarse grid, in simulation; dose-matched pairs.

All three internals across ψ

Selectivity, % of the instantaneous-mixing ceiling. Coarse grid, in simulation; dose-matched pairs. – = no run at that ψ.
ψ	blade		helicoid		baffle string
ψ	cont.	gated	cont.	gated	cont.	gated
2.6	56.0	81.5	53.1	81.0	–	–
5.2	60.2	85.6	–	–	–	–
7.8	66.0	87.0	60.4	85.8	62.1	85.1
10.4	69.5	87.6	–	–	–	–
15.6	72.9	88.7	68.2	86.8	68.6	86.6
25	76.4	90.5	–	–	–	–
40	79.1	91.0	–	–	–	–

Operating window

In simulation, raising ψ, the oscillation velocity over the net flow velocity, improves mixing and plug flow together, at essentially unchanged dissipation.

The downside is throughput: a higher ψ means a lower net flow, so less product out.

Plug-flow quality rises with ψ

Fig. 6. Blade insert, continuous feed, in simulation, to ψ 10.4 only. Pe_ax above about 100 is commonly taken as near plug flow. Not yet measured on a bench.

Two reaction families

Selectivity reactions

A + B → R, then R + B → S (or R + A → T). High local concentrations of one reagent, from poor mixing, are the main cause of the unwanted step.

Reactions of this kind include nitrations, halogenations, diazotisations and azo couplings, organometallic additions and fast ionic polymerisations. Only the model scheme in Status has been simulated.

Crystallisation and precipitation (under study)

The main current industrial use of oscillatory reactors. It needs the right supersaturation with low peak shear, as do flocs, cells and emulsions.

That calls for a different insert from the selectivity one. The work is at the modelling stage and waits on bench kinetics.

The cartridge

A printable cartridge of same-handed helical elements in series, separated by small gaps and ending in a smooth converging–diverging constriction with no sharp edges (Fig. 1). It is designed to slide into an existing tube. For selectivity reactions the insert is built to mix. For crystallisation it has a smooth, edge-free profile intended to lower peak strain (not yet measured).

1helical elements, all the same hand
2small gap at each junction
3smooth converging–diverging constriction

Fig. 7. The cartridge from the side, inside a conduit. Schematic; proportions illustrative, as in the 3D view (Fig. 1).

Status and evidence

Method: Time-resolved 3D flow simulation of one model reaction scheme: A + B → R, R + B → S, equal rate constants, Re_o ≈ 700, Da ≈ 20.
Comparison: Every phase-gated case is dose-matched to the continuous case beside it.
Grid: The grid cannot resolve the finest mixing scales, so selectivity levels are indicative; the gated-against-continuous comparison is the robust result.
Crystallisation: No crystallisation benefit is claimed yet.
Bench: There is no bench data yet. An experimental programme is the next step, and we are looking for a process partner to run it with.
Patent: The method and insert are the subject of a filed Australian provisional patent application (patent pending).

Talk to us about a pilot

If you have a selectivity-limited reaction, especially one already running in an oscillatory or baffled reactor, we'd like to hear about it. Technical briefings available under NDA. Contact details.