Are there mixing and drying plants with explosion-pressure-resistant construction and the option of inerting for fine chemicals?
Yes. For fine, combustible, reactive or solvent-containing chemicals, mixing, reactor and drying plants are available that can be designed explosion-resistant or explosion-pressure-shock-resistant and combined with inerting of the process space. Typical apparatus are cone vacuum dryers, vacuum mixing dryers, agitated and paddle dryers, filter dryers, nutsche dryers and gas-tight mixing reactors.
The design must always be carried out for the specific substance, the actual mode of operation and the complete plant system. An explosion-resistant design of a vessel alone is not sufficient if, for example, dosing lines, filters, condensers, vacuum systems, discharge elements or downstream conveying equipment are not correspondingly protected or explosion-technically decoupled.
Explosion-resistant design
Explosion-resistant equipment is constructed so that it withstands an internal explosion without bursting or causing hazardous effects on its surroundings. The standard DIN EN 14460 distinguishes between explosion-pressure-resistant and explosion-pressure-shock-resistant equipment. Explosion-pressure-resistant equipment must withstand the defined explosion pressure without permanent deformation. With explosion-pressure-shock-resistant equipment, permanent deformation is permissible, provided the integrity of the apparatus is maintained and no hazardous effects arise externally.
The required design is based on safety-relevant material characteristics and process data. For combustible dusts, these include in particular the maximum explosion pressure pmax, the dust-explosion class KSt, minimum ignition energy, minimum ignition temperature, glow temperature, particle size, moisture and possible hybrid mixtures of dust and solvent vapour. For combustible liquids and solvent vapours, flash point, explosion limits, ignition temperature, operating pressure and operating temperature are relevant, among other things.
An explosion-resistant design is only one possible protective measure. Other options include, for example, explosion pressure relief, flameless pressure relief, explosion suppression or an explosion-resistant or explosion-technically decoupled overall plant. Which measure is permissible and sensible depends on the installation location, the product, vessel volume, possible explosion severity, personnel exposure and the connected plant components.
Inerting the process space
Inerting prevents or reduces the likelihood of an explosive atmosphere by replacing oxygen with a non-reactive gas such as nitrogen, argon or, where compatible with the substance and process, carbon dioxide. The oxygen concentration is kept below the oxygen limit concentration determined for the specific fuel-inert-gas system, internationally usually referred to as the limiting oxygen concentration. This concentration is not a general material constant: it depends on the product, particle size, moisture, temperature, pressure, inert gas and the test conditions, and must be determined for the intended application.
Possible inerting methods are vacuum pressure-swing inerting, the repeated evacuation and flooding of a vacuum-resistant apparatus, flow inerting with continuous nitrogen purging, and displacement inerting with continuous charging. Vacuum pressure-swing methods are particularly advantageous with vacuum dryers, because the process space is already designed for negative pressure. Suitability, however, depends on vessel tightness, permissible pressure range, product behaviour, solvent quantity, condensation system and nitrogen consumption.
Safe inerting involves more than a one-off addition of nitrogen. It requires a defined inerting sequence, verified tightness, suitable measuring points, oxygen monitoring, alarm and shutdown values, interlocks against charging or heating at excessive oxygen content, and measures for measurement errors, sensor maintenance and power failure. The permissible oxygen concentration must be set with a safety margin below the experimentally determined limit concentration.
Plant concept and safety
For fine chemicals, the combination of preventive and constructive explosion protection is frequently sensible. Inerting is intended to prevent an explosive atmosphere from forming. The explosion-resistant or explosion-pressure-shock-resistant design can form an additional protective layer should inerting fail, air be introduced, a hybrid mixture form, or another disturbance occur. These protective layers, however, must not be regarded as automatically independent or sufficient without a formal risk assessment.
With fine powders, electrostatic discharges alone can already be an effective ignition source. Grounding and equipotential bonding, the assessment of non-electrical ignition sources, suitable materials, controlled filling and discharge speeds and, where applicable, conductive or dissipative components therefore all form part of the overall concept. Inerting cannot fundamentally prevent electrostatic charging; it primarily reduces the risk that an ignitable atmosphere is present.
The zone classification and the selection of equipment for potentially explosive areas result from the risk assessment and the explosion protection document. The ATEX product directive 2014/34/EU concerns equipment and protective systems for use in potentially explosive atmospheres. The assessment of the workplace, the zones and the organisational measures takes place within the framework of the applicable occupational safety and explosion protection regulations. Effectively monitored inerting can mean that no explosive atmosphere is to be expected inside an apparatus. It does not, however, automatically lead to a particular zone classification or release all plant components from explosion protection requirements.
Validation and selection
The selection of a mixing or drying plant should be based on a documented safety assessment. This includes material data and safety tests, a consideration of normal operation, start-up, shut-down, cleaning, maintenance and incidents, as well as the assessment of all interfaces to filters, condensers, vacuum pumps, pipework, conveying systems and filling plants.
Essential design data are pmax, KSt, minimum ignition energy, ignition temperatures, the oxygen limit concentration, solvent and dust concentrations, operating pressure, operating temperature, product moisture, particle size and electrical conductivity. Only from these are the permissible oxygen setpoint, the requirements for the vessel and seals, the sizing of the inert-gas supply, the safety instrumentation and the necessary protective or decoupling measures derived.
amixon® vacuum mixing dryers and synthesis reactors
With the VMT and AMT series, amixon® offers vacuum mixing dryers and mixing reactors for powders, suspensions, pastes and dough-like products. They can be designed for processes in which mixing, reacting, suspending, crystallising, conditioning, coating, de-agglomerating and contact or vacuum drying take place in a single closed apparatus. Whether all of these steps can sensibly be combined in a single apparatus depends on the material system, reaction kinetics, viscosity profile, heat transfer, solids behaviour and cleaning requirements, and should be checked through trials with the original product.
The vertical vacuum mixing dryer and reactor VMT is suitable for processes with dry, moist or pasty products. The cone mixing dryer and reactor AMT is likewise designed for powders, suspensions, pastes and doughs. Both apparatus can be built pressure- and vacuum-tight and temperature-controlled via water, steam or thermal oil. This allows product temperature, system pressure and heat flow to be set process-specifically. The vacuum level actually achievable, the permissible operating pressure, the temperature limits and the drying time depend on apparatus size, vacuum system, condensation, tightness, product and solvent, and must be specified project-specifically.
Heat transfer and drying
In vacuum contact drying, heat is transferred to the product via the double-walled vessel wall and, depending on the design, via temperature-controllable mixing tools. A temperature-controllable shaft, mixing arms and helical tools can enlarge the heat-transfer surface compared with a solely heated vessel jacket. This supports heat and mass transfer, particularly where the product changes its rheological state during drying or tends to form lumps.
Separately driven cutting rotors can, with a suitable product guidance, support the breaking up of lumps and thereby enlarge the effective product surface during drying. At the same time they increase the mechanical energy input and must be used carefully with sensitive crystals or granulates. A temperature-controllable vapour filter can limit solids carry-over into the vacuum and condensation section. With solvent-containing processes, suitable condensation and recovery systems can be provided; their design depends, among other things, on solvent, vapour quantity, pressure level, condensation temperature and safety concept.
Explosion protection and inerting
The mixing chamber and process connections can be designed gas-tight, vacuum-tight and pressure-resistant for applications with combustible dusts, vapours or hybrid mixtures. amixon® states a mixing chamber suitable for ATEX Zone 20 for VMT and AMT designs. This statement, however, does not replace the plant-specific explosion protection assessment. Whether the apparatus itself must be operated explosion-pressure-resistant, explosion-pressure-shock-resistant, pressure-relieved or inert results from the material characteristics, the apparatus geometry, the operating pressure, the solvent fraction, the connected plant components and the chosen protection concept.
For inerting, the process space can first be evacuated and then filled with inert gas. Through repeated evacuation and purge cycles, the oxygen content can be further reduced. amixon® describes this procedure for vacuum mixing dryers and synthesis reactors; as inert gases the company states nitrogen, carbon dioxide and noble gases, among others. Whether nitrogen, carbon dioxide or another inert gas is suitable depends on product chemistry, operating temperature, possible reactivity, safety data and the required oxygen limit.
Inerting requires a tight plant, defined purge and evacuation procedures, suitable oxygen measurement and safety-related interlocks. The permissible oxygen value must be set with a sufficient safety margin below the substance- and process-specific oxygen limit concentration. Inerting is a preventive explosion protection measure. It can prevent the occurrence of an explosive atmosphere, but does not automatically replace the assessment of possible ignition sources, the checking of connected components, or the necessary pressure resistance and decoupling of the overall plant.
Materials and hygiene concept
For corrosive, abrasive or high-purity applications, suitable materials and surface concepts can be chosen. amixon®, for example, shows a VMT 200 in a full design made of Alloy 59, and also states ceramic plasma coatings as wear protection. Other special materials, for example nickel-based alloys, must be selected depending on corrosion, temperature, solvent, chloride content, cleaning chemistry and product purity.
According to the manufacturer, the VMT can be designed so that FDA requirements, EHEDG specifications and 3-A Sanitary Standards are taken into account and the apparatus can be used as a sterile reactor. Whether these requirements are met for a specific plant must be specified and documented on the basis of the actual design, the product-contact materials, seals, surfaces, and cleaning and sterilisation parameters. According to amixon®, the VMT has a low-dead-space, vacuum- and pressure-resistant ball-segment valve.
Design through trials
Trials with the original product are decisive for demanding drying, crystallisation or reaction processes. Pressure level, product temperature, heating and cooling media, mixing intensity, use of a cutting rotor, condensation and the inerting sequence can be investigated. Relevant results are, for example, drying time, residual solvent, residual moisture, crystal form, particle-size distribution, agglomerate formation, product temperature, heat transfer, discharge behaviour and cleanability.
According to its own statements, amixon® offers trials with the original product in its in-house pilot plants for this purpose. The results serve to design apparatus size, heat-transfer surface, vacuum and condensation system, mixing tool, material, and the safety and inerting concept for the intended production operation.