What advantages do dust-tight, explosion-pressure-resistant designs offer when processing starch powders?
Starch powders are among the combustible organic dusts and can form explosive dust-air mixtures under certain conditions. Dust-tight and explosion-pressure-resistant designs therefore offer important advantages for safety, plant availability and cleanliness, but they do not replace the full risk assessment and the explosion protection concept.
Safety advantages
A dust-tight design reduces the escape of fine dust from apparatus, vessels and pipework. This lowers dust deposition in the surroundings and thereby also the risk of secondary dust explosions, which can often be more severe than the primary explosion.
Explosion-pressure-resistant designs ensure that plant components withstand the expected explosion overpressure without bursting. Depending on the design, the explosion is controlled constructionally within the apparatus. This protects personnel and reduces consequential damage to adjacent plant components and buildings.
Operational and process advantages
A tight design makes inerting easier, because leakage losses are lower and the internal atmosphere can be kept more stable. This is particularly relevant in dusty processes with nitrogen or other protective gases.
Product losses, ingress of foreign matter and unwanted moisture uptake are also reduced. This improves product quality and lowers the risk of lump formation, caking and contamination. Cleaning can also be better controlled where dust does not escape in an uncontrolled manner.
Economic advantages
Less dust escape usually means lower cleaning effort and less soiling of peripherals, sensors and electrical components. This can reduce maintenance effort, downtime and cleaning intervals.
The tight design can also help to limit explosion protection in the surroundings, for example through smaller ex zones or less exposed peripherals. This applies, however, only within the framework of the specific risk assessment.
Even after an ignition event, a plant designed to be explosion-pressure-resistant may under certain circumstances be brought back into operation more quickly than a plant that has failed structurally.
How amixon® implements dust-tight, explosion-protected designs for starch powders
ATEX Zone 20 as standard, not as an option
At amixon®, the mixing chamber is designed for ATEX Zone 20 across all series, from the single-shaft mixer EM through vertical and conical mixers (VM, HM, AM) to mixing dryers and reactors (VMT, AMT), that is, for areas in which an explosive dust atmosphere is present continuously or for long periods. For fine starch dust with its low minimum ignition energy, that is the consistent design.
The mixers generally operate at low speed (approximately 0.8 to 3.5 m/s circumferential speed). Low mechanical energy inputs systematically reduce potential ignition sources such as friction and impact sparks, an advantage inherent in the principle over high-speed mixing systems.
Closed, dust-tight systems
amixon® mixers and reactors are closed apparatus: the mixing chamber and mixing tool are welded free of joints and ground, and inspection doors seal permanently with the OmgaSeal® seal, on request vacuum-tight or against overpressure. Fine starch dust does not escape into the installation room; the formation of dangerous dust layers as fuel for secondary explosions is prevented at source.
The mixing dryer reactors VMT and AMT are of highly gas-tight construction and are pressure- and vacuum-resistant (vacuum down to 5 mbar absolute). On request the mix can be inerted, in which case the explosive atmosphere is avoided from the outset through displacement of oxygen.
As a certified welding company with European, Japanese, Korean and American qualifications, amixon® manufactures the pressurised apparatus solidly and with the greatest depth of fabrication at the Paderborn works, including the choice of materials from mild steel S355J2+N through austenitic stainless steels to duplex steels.
Specifically for starch and starch derivatives
amixon® has extensive experience in starch extraction and processing: in conical reactors up to 50,000 litres, with all surfaces temperature-controllable, starch derivatives are mixed, thermally treated and modified. System pressure, temperature, sequential addition of substances (dry, wet or gaseous) and mixing intensity are freely adjustable as process parameters.
The final vacuum mixing drying takes place in the same apparatus. Every transfer step that is eliminated is at the same time an escape of dust that is eliminated; with combustible dusts, the closed process chain is the most effective explosion protection.
The dead-space-free mixing action ensures that processing succeeds reproducibly, irrespective of the rheological properties, from free-flowing dry starch to highly viscous, dilatant or shear-thinning states during modification.
Economic side effect
The dust-tight, dead-space-free design protects not only people and plant: it keeps the valuable product in the process, minimises cleaning effort caused by dust deposition in the surroundings and enables almost complete discharge of free-flowing materials, with amixon® conical mixers 99.997 % and better.