Types of Chemical Reactors and How They Work
Types of Chemical Reactors and How They Work
Textbook classification divides reactors into batch, semi-batch, CSTR and plug-flow. That classification is correct and almost useless at the specification stage, because a reactor that works in a reaction engineering diagram and a reactor that works on a factory floor are separated by something the diagram never shows: the viscosity curve of the batch as it cooks.
A resin, alkyd or epoxy cook starts at a few hundred centipoise and finishes somewhere between ten thousand and several hundred thousand. In that final stage the heat transfer coefficient collapses, the agitator torque rises, and the reaction mass stops behaving like a fluid the textbook equations were written for. So this guide covers the four types of reactors and their principle of operation, then goes to the part that actually decides whether the plant runs — the thermal structure, the impeller, and the shaft seal.
The four principle types are distinguished by how composition changes with time. In a batch reactor everything is charged at once and composition changes with time at a fixed location. In a semi-batch, one component is fed continuously to control rate or heat release. In a CSTR, feed and product flow continuously and the outlet has the same composition as the vessel contents. In a PFR, composition changes along the flow path with no back-mixing.
For high-viscosity polymerization and resin work — which is where jacketed agitated vessels are actually used — the operating selection usually comes down to batch or semi-batch, and the real engineering decisions move to heat transfer surface, impeller geometry and shaft sealing.
The Four Types Compared
| Type | Principle of operation | Where it fits | Hardware it needs |
|---|---|---|---|
| Batch | All charge loaded at once; composition varies with time, uniform in space | Multi-product plants, long cook cycles, resin and alkyd condensation | Agitated vessel, jacket or coil, reflux condenser, discharge valve |
| Semi-batch | One reactant fed progressively to hold rate, temperature or concentration in range | Exothermic reactions, controlled addition, emulsion and dispersion polymerization | Metered feed line, load cells or flow control, tighter temperature loop |
| CSTR | Continuous feed and overflow; outlet composition equals vessel composition at steady state | Single-product lines, high tonnage, reactions tolerant of back-mixing | Cascaded vessels, level control, continuous feed and transfer pumps |
| PFR | Composition changes along the flow path; essentially no back-mixing | Fast reactions in low-viscosity fluids, gas phase, high conversion per pass | Tubular or static-mixer reactor — outside the scope of an agitated vessel |
Note on scope: POLYC builds batch and semi-batch jacketed agitated reactors. A plug-flow reactor is listed above for completeness of the classification, not as a product offering — those duties need tubular equipment with a different mechanical design.
Batch and Semi-Batch: Where Agitated Vessels Live
In a batch reactor the whole charge is present from the start, so the driving force falls as reactants are consumed and the reaction slows toward the end. That is acceptable — sometimes desirable — for condensation polymerisation, where the final approach to the target acid value or viscosity needs time and tight temperature control rather than throughput.
A semi-batch converts the same vessel into a rate-controlled process by feeding one component. This is the standard answer to an exothermic reaction: instead of sizing the jacket for the total heat release of a full charge, the feed rate is set so the heat release never exceeds what the jacket can remove. The vessel becomes the heat sink and the feed becomes the control handle. Mechanically it is the same kettle with a metered feed line and a faster temperature loop.
The practical consequence is that a semi-batch specification is dominated by instrumentation rather than by vessel geometry. If the feed rate cannot be held, the temperature cannot be held, and no amount of jacket area compensates. This is why the same vessel can behave well on one product and badly on another with a sharper exotherm.
CSTR: What Changes When You Go Continuous
The defining feature of a continuous stirred-tank reactor is that the outlet stream has the same composition as the vessel contents. Feed enters, product overflows, and at steady state the vessel is a well-mixed pool at one composition. The immediate result is that conversion per vessel is limited: the reacting stream leaves as soon as it arrives, carrying unreacted material with it.
The usual engineering answer is a cascade — three to five vessels in series, where each one behaves as a stage and the series approaches plug-flow behavior overall. For high-viscosity products the cascade is often impractical, because transferring a 100,000 cP mass between vessels is a materials-handling problem before it is a reaction problem. That single constraint is why resin and adhesive plants stay batch while low-viscosity commodity chemistry goes continuous.
Thermal Structure: The Real Selection Decision
Three thermal structures cover most jacketed vessels, and the choice is driven by heat load and vessel size rather than by the reaction type. A conventional jacket is a second shell around the vessel. It is simple, it works, and it has a fixed and often modest heat transfer area per unit volume.
An external half-pipe coil welded to the shell takes thermal fluid at higher pressure and higher velocity through a smaller channel, which raises the shell-side film coefficient while keeping the wall thinner than a pressure jacket needs to be. On large vessels this is the difference between a workable heat-up time and an unworkable one. An internal coil places the surface directly in the product, giving the most area in the smallest vessel, but it obstructs the agitator, traps material and is harder to clean between products.
| Thermal structure | Strength | Limitation |
|---|---|---|
| Conventional jacket | Simple, robust, easy to clean externally, low cost | Limited area per unit volume; slow on large vessels |
| External half-pipe (limpet) coil | Higher fluid pressure and velocity, thinner wall, better heat-up on big vessels | More fabrication; outside surface harder to insulate cleanly |
| Internal coil | Most area in the smallest envelope; fast response | Obstructs agitation, traps product, cleaning between products is harder |
Heating source is a separate decision: thermal oil circulation, plant steam, electric heating elements or zero-distance induction. Steam gives high film coefficients but a fixed temperature set by pressure. Thermal oil allows higher temperatures at low pressure and is the usual choice when the process runs near 300 °C. Electric heating suits small vessels and sites without a thermal fluid system.
Impeller Selection Follows the Viscosity Curve
Reactors are often specified by volume and jacket area, with the agitator treated as an accessory. In practice the impeller decides whether the batch is uniform, whether the wall fouls, and whether the motor has enough torque at the end of the cook when the product is thickest.
- Anchor with PTFE wall scrapers — the high-viscosity workhorse. It sweeps the wall, so it removes the stagnant film that otherwise insulates the jacket from the product and cuts heat transfer exactly when heat transfer matters most.
- Multi-stage Rushton turbine — radial flow and high shear for low-viscosity duties and gas-liquid dispersion. Poor choice once the batch thickens.
- Pitched paddle — axial flow, good bulk turnover at medium viscosity, gentler on shear-sensitive products.
- Helical ribbon — for very high viscosity, where the whole mass must be moved rather than circulated.
The transmission is part of this decision, not a purchased detail. A heavy reducer and an appropriately sized motor must carry peak torque in the final stage of the cook, which can be many times the starting torque. A motor sized on the initial charge will stall or trip on the last hour of the batch.
Vacuum, Pressure and Shaft Sealing
Where solvent is boiled overhead and condensed back, the vessel runs under vacuum. POLYC resin reactors are built for full vacuum to -0.098 MPa and positive pressure to 1.6 MPa, with process temperature up to 300 °C. The vacuum is not a convenience — it lowers the boiling point so the solvent can be stripped without cooking the product, and it is what makes the reflux ratio controllable.
The seal is where vacuum and solvent meet. A liquid-cooled double mechanical seal with its own barrier fluid circuit is the standard answer, because it keeps the sealing faces cool and provides a buffer between process and atmosphere. A cartridge seal simplifies replacement. A magnetic drive removes the penetration entirely and is chosen where the solvent is toxic or highly volatile. Packing glands remain in service on older or lower-duty vessels, and they leak by design.
Reference Specifications
The table below shows the standard build range. Agitator power scales with volume, while speed drops on the largest sizes to keep tip speed and torque within the drive's capability.
| Volume (L) | Agitator power (kW) | Agitator speed (rpm) | Heating area (m²) | Design pressure (MPa) |
|---|---|---|---|---|
| 500 | 4.0 | 60 – 85 | 2.2 | -0.1 to 0.4 |
| 1,000 | 5.5 | 60 – 85 | 3.8 | -0.1 to 0.4 |
| 2,000 | 7.5 | 60 – 85 | 5.6 | -0.1 to 0.4 |
| 3,000 | 11.0 | 60 – 85 | 7.2 | -0.1 to 0.4 |
| 5,000 | 15.0 | 60 – 85 | 10.5 | -0.1 to 0.4 |
| 10,000 | 22.0 | 45 – 60 | 16.8 | -0.1 to 0.4 |
| 20,000 | 37.0 | 45 – 60 | 28.5 | -0.1 to 0.4 |
Standard build range. Capacity, wetted metallurgy (SUS304 / SUS316L), thermal structure, impeller type and shaft seal are configured per duty. Higher design pressures are available; explosion-proof configurations are available upon request, confirmed against your hazardous-area classification.
How the Vessel Gets Built
Reactor fabrication, jacket welding, impeller assembly and seal fitting are done in the 9,000 m² facility in Wuxi, Jiangsu. That matters for reactors more than for most equipment, because a vessel is a one-off — jacket geometry, impeller clearance and seal arrangement are set by the specific product and duty, and a specification error is not corrected by a catalog substitution.
A resin reactor is delivered as a package: vessel, overhead condenser, decanter and receiver sized together so reflux ratio and solvent split are settled before shipment. Standard builds ship in 30 to 45 days. Every unit carries a 3-Year Global Warranty.
Choosing Between Them: A Short Checklist
- Start from the viscosity curve, not the reaction class. Give the supplier starting and final viscosity at process temperature. Most reactor sizing failures are viscosity failures in the final stage.
- Decide batch or semi-batch on the exotherm. If total heat release over the cycle exceeds what the available surface can remove, feed control is the answer, not a bigger jacket.
- Size the thermal structure for heat-up time, not just for steady state. A vessel that holds temperature well may still take too long to reach it, and the limitation usually appears at the largest sizes.
- Treat the agitator and its transmission as one item. Peak torque at the end of the cook governs the reducer and motor, not average torque.
- Match the seal to the solvent, not to the pressure alone. Toxic or highly volatile solvents justify a magnetic drive or a double seal with a barrier circuit.
- Confirm the scope boundary early. Vessel, condenser train and receiver are one thermal system; specifying them separately is how reflux ratios end up wrong.
Send Us the Reaction and the Viscosity Curve
Tell us the batch size, starting and final viscosity, process temperature and solvent. We respond within 24 hours with the thermal structure, impeller type and seal arrangement that fit the duty, plus the condenser and receiver sizing that goes with them.
Direct email: export@polycmill.com
Frequently Asked Questions
What are the four main types of chemical reactors?
Batch, semi-batch, continuous stirred-tank (CSTR) and plug-flow (PFR). They are distinguished by how composition changes with time and position: batch varies with time at one location, semi-batch adds one component progressively, CSTR holds one composition throughout the vessel, and PFR varies along the flow path with no back-mixing.
Why is a batch reactor used for resin and adhesive production?
Because viscosity climbs by one to two orders of magnitude during the cook. Moving a very high-viscosity mass between vessels is a materials-handling problem before it is a reaction problem, so a cascade of continuous vessels becomes impractical. Batch operation also suits multi-product plants where the same vessel runs different formulations.
When should a jacket be replaced by a half-pipe coil?
When heat-up time or heat removal becomes the bottleneck, which usually happens on large vessels or with high-pressure thermal fluid. An external half-pipe coil takes fluid at higher pressure and velocity through a smaller channel, raising the shell-side coefficient while allowing a thinner wall than a pressure jacket requires.
Which impeller suits a reaction that thickens during the batch?
An anchor with PTFE wall scrapers, because it sweeps the wall and removes the stagnant film that otherwise insulates the jacket. A Rushton turbine that works well on the initial low-viscosity charge loses effectiveness once the mass thickens. The reducer and motor must be sized on peak torque at the end of the cook, not on the starting charge.
Why do reactors run under vacuum during solvent processing?
Vacuum lowers the boiling point so solvent can be stripped at a temperature that does not degrade the product, and it makes reflux ratio controllable. POLYC resin reactors are built for full vacuum to -0.098 MPa and positive pressure to 1.6 MPa. The shaft seal is the critical component in that duty.
Which shaft seal should be used with volatile solvents?
A double mechanical seal with its own barrier fluid circuit keeps the faces cool and buffers process from atmosphere; a magnetic drive removes the shaft penetration altogether and is the usual choice where the solvent is toxic or highly volatile. Packing glands still appear on lower-duty vessels but leak by design and are not suitable for vacuum service with hazardous solvents.
Explore Related Resources
Jacketed reaction kettle from 200 L to 20,000 L+, vacuum to -0.098 MPa, process temperature to 300 °C.
Metallurgy, thermal structure and seal options laid out, plus what drives the cost of a jacketed vessel.
Reactor plus fractionating column, condenser and receiver — how the four pieces are sized as one system.
How to compare suppliers on fabrication, seal supply and package scope rather than on price alone.
