Which safety concepts exist for inerted processes when mixing reactive chemicals?
Objective and principle of inerting
When mixing reactive chemicals, an inert gas atmosphere is frequently used in order to prevent the formation of ignitable atmospheres from combustible vapours, gases or dusts. The core objective is to lower the oxygen concentration in the gas space of the mixing vessel below the limiting oxygen concentration (LOC), plus a defined safety margin (e.g. approximately 2 vol.-% in accordance with EN 1127-1), so that an explosion is no longer physically possible.
Typical inert gases are:
- Nitrogen
- Carbon dioxide
- Noble gases such as argon
The selection is made according to chemical compatibility with the substances used as well as process engineering framework conditions.
Inerting procedures
In practice, various technical inerting concepts are used.
Displacement inerting
Inert gas is introduced into the vessel in order to purge air and oxygen by displacement; suitable for simple vessel geometries.
Pressure blanketing
The vessel is operated under a constant inert gas overpressure; a pressure drop can indicate leaks or the ingress of air.
Continuous oxygen measurement
Use of paramagnetic, electrochemical or zirconium-oxide-based sensors for real-time measurement of the O₂ concentration. In highly reactive systems this is the central monitoring instrument, often with redundant sensors and safety-related evaluation.
Safety when charging solids
Charging solids into inerted reactors is critical, since oxygen can inevitably enter when manholes or filling nozzles are opened. The following measures are used to minimise this risk.
Inert gas locks
Solids are fed in via double flap valve systems or rotary valves; the intermediate space is evacuated or purged with inert gas before every opening, in order to prevent the ingress of oxygen.
Inert gas curtains
A permanent stream of nitrogen at the filling nozzle forms a barrier that reduces exchange with the ambient air.
Vacuum/inert gas cycling
Repeated evacuation and inerting of the entire vessel before the start of the process in order to reduce residual oxygen, particularly in porous or adsorptive solids.
Constructional safety measures
In parallel with inerting, constructional explosion protection measures are used in order to limit damage to the plant should ignition nevertheless occur:
- Explosion-pressure-shock-resistant or pressure-resistant design of the apparatus up to the maximum explosion pressure.
- Bursting discs or safety valves with defined pressure relief into closed systems (e.g. flare, scrubber).
- Flame arresters in gas and exhaust air lines to prevent flame propagation.
- Earthing and equipotential bonding of all conductive parts of the plant for the dissipation of electrostatic charges in accordance with TRGS 727.
How amixon® safeguards inerted processes when mixing reactive chemicals
Inerting by means of vacuum technology
When mixing, drying and synthesising reactive chemicals, inerting the process chamber is an essential protection principle. amixon® mixers, vacuum mixers and mixer-dryer reactors can be built vacuum-tight. The atmosphere present in the mixing chamber can thereby first be evacuated and then replaced in a targeted manner by an inert gas.
Depending on the product, the process and the safety concept, nitrogen, carbon dioxide or noble gases, for example, are possible inert gases. The aim is to lower the oxygen content in the process chamber permanently below the limiting oxygen concentration relevant for the particular substance system. Below this concentration no explosible atmosphere of fuel, oxygen and inert gas can form. The limiting oxygen concentration is not, however, a universally valid substance constant: it has to be determined for the specific combination of product, any solvent vapour and the inert gas used, under defined test conditions.
Inerting as an overall system
The effectiveness of inerting does not depend on the choice of inert gas alone. Decisive factors are the explosion-protection characteristic data of the input materials, the tightness of the entire apparatus including feeding, discharge and connected periphery, the mode of operation and suitable measurement, control and monitoring technology.
amixon® apparatus can be built as closed process systems. This includes mixing chambers welded free of joints, mixing tools supported at the top without a lower shaft passage, and inspection and maintenance doors with a static OmgaSeal® seal. Depending on the task, these connections can be built gas-tight, vacuum-tight and pressure-resistant or pressure-shock-resistant. amixon®’s long-standing experience in processing demanding powders, reaction masses and pasty products supports the development of application-specific apparatus and safety concepts. Inerting is not regarded as an isolated measure but as part of a closed overall system coordinated in safety terms.
Safe inerting also requires a coordinated approach to feeding, evacuation, inert gas admission, pressure equalisation, discharge and cleaning. The oxygen content has to be recorded and monitored reliably at a suitable point. In addition, a sufficient safety margin is required between the oxygen concentration measured in operation and the experimentally determined limiting oxygen concentration.
Assessing hybrid mixtures reliably
So-called hybrid mixtures require particular attention. They arise when combustible dust and combustible gases or solvent vapours are present in the process chamber at the same time. Such mixtures can display considerably higher ignition sensitivity and explosion violence than the dust or the gas on its own. They can even be explosible although both the dust concentration and the gas or vapour concentration lie below the respective lower explosion limit.
For hybrid mixtures, generally valid statements are only possible to a limited extent. The safety assessment must therefore be based on the actual input materials, concentration ranges, temperatures, pressures and operating states. Considerably less validated data is available in the publicly accessible technical literature for hybrid mixtures than for pure dusts or pure gas-air mixtures.
Depending on the task, targeted explosion tests may therefore be necessary. Among other things, the maximum explosion pressure pmax, the maximum rate of pressure rise and equivalent KSt values for the hybrid mixture can be determined. These characteristic values are important foundations for assessing hazard potentials and for correctly dimensioning measures such as pressure-resistant design, pressure relief or explosion-proof decoupling. For pure dust explosions, pmax and KSt are determined in standardised laboratory procedures as a measure of explosion violence; with hybrid mixtures, product-specific testing is especially important.
Avoiding ignition sources and limiting consequences
Alongside the avoidance of explosible atmospheres through inerting, the avoidance of effective ignition sources forms part of the protection concept. amixon® mixing tools can be operated at low rotational speeds. If the process concept requires it, the circumferential speed can be limited to less than 1 m/s. This reduces mechanical stresses and can lower the risk of ignition-effective friction, impact or spark events. The specific assessment of ignition sources always remains application-related. It covers in particular possible electrostatic charging, the handling of foreign bodies, the design of bearings and seals, temperature monitoring and the selection and execution of electrical and mechanical components.
If an explosible atmosphere cannot be reliably excluded even with a preventive protection concept, design explosion protection may become necessary. Depending on the task, amixon® apparatus can be realised pressure-resistant or pressure-shock-resistant. The apparatus design is based on the determined characteristic data of the real substance system, in particular on pmax and KSt or the corresponding characteristic values of the hybrid mixture. Explosion pressure relief only makes sense as part of a holistically coordinated concept. Among the aspects to be taken into account are the safe discharge of pressure and flames, possible dust or product emissions, the endangerment of personnel, the installation situation and the interactions with filters, pipework, feeding and discharge equipment.
Safe trial operation at amixon®
Before trials with hazardous substances, a joint safety discussion takes place at amixon®. Safety data sheets, available explosion-protection characteristic data, possible hazards and additional customer information are examined. Hazardous substances may only be sent to amixon® after written release.
The amixon® pilot plant makes it possible to prepare and carry out mixing trials, vacuum drying and synthesis reactions with original products under realistic conditions. Relevant process variables can be recorded systematically and process-engineering relationships investigated at an early stage. All important process data are recorded digitally and documented in the trial report. The trial results form a robust basis for the design of the production apparatus, the definition of safe operating limits, the cost calculation and the later industrial process management.