In brief
In batch reactors, most serious incidents start at four moments: charging done wrongly or in the wrong order, an exothermic reaction that cooling cannot keep up with, pressure relief that cannot do its job, and sampling done before pressure or temperature has fallen. The operator must monitor the rate of temperature rise, dosing and the agitator, must not restart a stopped agitator without a written instruction, and must never disable the safety valve. Safety valves are part of the annual periodic inspection of pressure vessels.
Reactor safety: the four critical moments of a batch process
Most plants producing resins, paints, adhesives and textile auxiliaries work with batch reactors: raw materials are charged in sequence, heated or cooled, the reaction is completed, a sample is taken and the product is discharged. Every batch repeats the same steps; this repetition makes the most dangerous steps feel routine. Reactor safety relies on a set of continuously monitored parameters and on the operator responding correctly at critical moments.
| Critical moment | What can go wrong? | Operator's focus |
|---|---|---|
| Charging | Wrong substance, excess quantity, wrong order, dust and vapour from an open manway | Recipe and label verification, inerting, earthing |
| Reaction | Heat generation exceeding cooling, accumulated reactant reacting suddenly | Rate of temperature rise, dosing, agitator, cooling |
| Pressure relief | Valve not working, relief discharging into the work area | Valve status, destination of the relief line |
| Sampling and discharge | Opening under pressure or temperature, splashing, vapour | Closed sampling system, PPE, depressurising |
For businesses in the OIZs (organised industrial zones) of Bursa and the surrounding provinces that produce chemicals for the textile, automotive and furniture industries, these four moments are the main headings of the risk assessment. For similar process risks, also see our article on paint and varnish production plants.
Reactor charging: the right substance, the right quantity, the right order
A charging error can cause the reaction to get out of control before it has even started. During reactor charging, keep the following steps written in the instruction and mandatory in practice:
- Verify the recipe: The substance name, quantity and order on the batch card are checked by a second person before charging.
- Compare the label and lot: The label of the drum, bag or IBC must match the substance in the recipe exactly. Raw materials with similar names are kept in separate locations in the store.
- Check temperature and agitator: The reactor must be at the starting temperature in the recipe and the agitator must be running.
- Watch out for dust and vapour when charging solids: Charging powder through an open manway into a reactor containing solvent can create an explosive atmosphere where flammable vapour and dust come together. Nitrogen inerting, closed charging systems and extraction reduce this risk.
- Manage static electricity: The reactor, charging equipment, drums and operator are earthed; non-conductive bags and hoses are assessed. For details, see our article on the ATEX and explosion protection document.
- Limit the liquid dosing rate: The dosing pump or valve is set so as not to exceed the rate in the recipe; the rate limit is not increased arbitrarily.
Exothermic reactions and runaway reactions
An exothermic reaction generates heat. In normal operation, this heat is removed by jacket or coil cooling. A runaway reaction starts at the point where heat generation exceeds cooling capacity: as the temperature rises, the reaction speeds up, and the faster reaction generates more heat. Within minutes, this cycle can push the pressure above the capacity of the relief valve.
Typical starting causes of runaway reactions are:
- Accumulation: A reactant dosed while the temperature is lower than in the recipe does not react immediately and accumulates in the reactor. When the temperature rises, the accumulated quantity reacts all at once.
- Loss of cooling: A cooling water pump failure, a valve left closed or a fouled heat exchanger.
- Agitator failure: When agitation stops, reactants can separate into layers; when the agitator restarts, a sudden reaction can occur.
- Wrong charging: Excess catalyst, the wrong substance or accelerated dosing.
The parameters the operator should monitor and the first responses to a deviation can be summarised as follows:
| Parameter | Why is it important? | First response to a deviation |
|---|---|---|
| Reactor temperature and rate of rise | An accelerating rise is the first sign of a runaway | Stop dosing, maximise cooling, inform the person in charge |
| Jacket inlet and outlet temperature | Shows whether cooling is really working | Check the cooling circuit |
| Reactor pressure | Gas evolution or rising vapour pressure | Apply the emergency steps in the instruction |
| Dosing quantity and rate | Preventing accumulation | Correct an off-recipe rate; stop dosing if accumulation is suspected |
| Agitator current and speed | Proof that agitation is continuing | On failure, stop dosing; follow the instruction for restarting |
Pressure relief valves and bursting discs
The safety valve and bursting disc protect the reactor when all other measures fail. What operators and the maintenance team need to know about the pressure relief valve:
- Never disabled: Fitting a blind flange in front of the valve, forgetting to open the isolation valve between the relief valve and the reactor, or removing the valve because "it's leaking" means removing the protection entirely. If there is a valve between the relief valve and the vessel, it is locked in the open position.
- Correct setting: Under Annex III of the Regulation on Work Equipment, in pressure equipment with a defined operating pressure, the safety valve and similar safety devices are set to the operating pressure, or if none is defined, to the maximum pressure on the nameplate.
- Where does the relief go? The relief line is directed not to the work area but to a catch tank, a scrubber or a safe point. It is checked that the line is not blocked or frozen.
- Bursting disc: A burst disc must be replaced with a new one of the same specification and the event recorded; a disc burst is investigated as a near miss.
Under Annex III, pressure vessels and installations undergo periodic inspection at least once a year unless the standard or the manufacturer's instructions specify a shorter period. The Regulation also states explicitly that the inability to stop the process cannot justify using another method instead of a hydrostatic test. Chemical engineers are among the professions that may inspect pressure vessels and installations. For the general rules, see our article on pressure vessels and compressor tanks.
Sampling and discharge
A sample taken at the end of a batch or during the reaction is the step that brings the operator into the closest contact with the reactor contents.
- Use a closed sampling system: The sample is taken without opening the reactor manway, using a closed sampling valve or device.
- Check pressure and temperature: Before the sampling valve is opened, the reactor pressure and the temperature of the sampling line must be within the limits in the instruction.
- Correct PPE: A face shield together with chemical goggles, gloves suited to the SDS (safety data sheet) and, if necessary, respiratory protection.
- If the manway must be opened: The manway is opened only after the pressure has been released and, if necessary, after purging with inert gas; the worker does not put their face over the opening.
- Discharge: Hose connections are checked before discharge; the suitability and earthing of the containers receiving hot product are verified.
For what to do in the event of a spill, see our article on responding to chemical spills. If the reactor is to be entered for cleaning or maintenance, the work is done under a confined space permit after all feeds have been physically isolated and the agitator locked out; see working in confined spaces.
Process safety operator training and management of change
Process safety is not a burden for the operator to carry alone; but they are the last line of defence on site. The following arrangements need to be in place alongside the operator:
- Written instructions: For each product, the charging order, temperature and pressure limits, what to do in the event of a deviation and the emergency shutdown steps.
- Shift handover: The stage the reactor is at, the quantity dosed and any deviations are handed over in writing.
- Management of change: A change of raw material supplier, recipe, equipment or capacity is not implemented without a written assessment. Under the Regulation on OHS Risk Assessment, changes in technology, substances or equipment and changes in the production method are among the situations that require the risk assessment to be renewed regardless of its validity period.
- Analysis method: The legislation does not mandate a specific method; methods such as HAZOP and FMEA are common in practice for reactor processes. See FMEA risk analysis.
- Vocational training: Annex 1 of the vocational training regulation lists many jobs in the chemical industry among those requiring vocational training; match operators' certificates to their duties.
If hazardous substances above certain thresholds are present at the site, the Regulation on the Prevention and Mitigation of the Effects of Major Industrial Accidents also applies. This Regulation divides establishments into lower-tier and upper-tier; it requires a major accident scenario document (Art. 8), a major accident prevention policy document for lower-tier establishments (Art. 10), a safety report for upper-tier establishments (Art. 11) and an internal emergency plan (Art. 13).
As Bursa Sistemer, we address reactor processes individually in our risk assessment work at chemical businesses in Bursa and the surrounding provinces, and monitor the on-site application of instructions and checks as part of OHS specialist services. We also plan together the incorporation of runaway reaction and chemical release scenarios into the emergency plan. For the general risks of the sector, see our chemicals and plastics page, and for similar heat risks in rubber processes, our article on rubber compounding and vulcanisation.
Frequently Asked Questions
What are the first signs of a runaway reaction?
The reactor temperature rising faster than expected in the recipe, the rise not stopping despite jacket cooling, and increasing pressure. In this case, the operator stops dosing, maximises cooling, informs the person in charge and applies the written emergency instruction.
How often is a safety valve inspected?
Safety valves are part of the periodic inspection of the pressure vessels and installations they are connected to; Annex III of the Regulation on Work Equipment requires inspection at least once a year unless the standard or manufacturer specifies a shorter period. At the start of each shift, it is checked that the valve in front of the safety valve is open and locked.
Why is static electricity important when charging a reactor?
At a reactor opening where solvent vapour and dust are present, a spark from a static discharge can cause ignition. The reactor, charging equipment, containers and operator are earthed; where possible, nitrogen inerting and a closed charging system are used.
Can a reactor be entered for maintenance?
Yes, but under a confined space work permit. All feeds are physically isolated, the agitator is locked out, the reactor is emptied and cleaned and the atmosphere is measured. During entry, there is an attendant outside and a rescue plan ready.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.