Explosion dryer

Relief venting Relief vents that are properly sized relieve and direct dryer explosions to protect the dryer and personnel if an explosion does occur. Normally they are simple pop-out panels with a minimum length of ducting to direct the explosion away from personnel or other equipment. 

FIGURE 56.7 Apparatus for testing dryer explosion characteristics for organic powders, (a) Hartmann bomb apparatus, (b) Chemical igniter. 

FIGURE 48.6 Apparatus for testing dryer explosion characteristics for organic powders. 

Through-ckculation compartments employ perforated or screen bottom trays and suitable flow baffles so gas is forced through the material. If material is not inherently pervious to gas flow, it may be mechanically shaped iato noodles, pellets, or briquettes. These dryers are used ia small-scale operations to dry explosives, foods, and pigments. Dryer efficiency is 50—70%. Based on tray area, water vaporization rates are 1—10 kg/(h-m ). 

For example, in rotary vacuum dryers it is possible to prevent the formation of explosible dust-air mixtures by setting and monitoring a certain partial vacuum (negative pressure). This pressure value must be determined by experiment for each type of dust. With pressures of less than O.I bar, in general, hazardous effects of dust explosions need not be anticipated. If the vacuum system malfunctions, the partial vacuum must be released by inert gas and the instaUation shut down. 

Containment (Explosion-Pressure-Resistant Design for Maximum Explosion Overpressure) An explosion-resistant construction is understood to mean the possibihty of designing vessels and equipment for the full maximum explosion ove (pressure, which is generally of the order P = 9 bar. The explosion-resistant vessel can then be designed as explosion pressure resistant or explosion pressure shock resistant. This protective measure is generally employed when small vessel volumes need to be protected, such as small filter units, fluidized-bed dryers, cyclones, rotaiy valves, or mill housings. 

An operator had tested dryer samples on a number of occasions. After the last sampling, he closed the manhole cover, put the dryer under vacuum, and started rotation of the dryer. A few minutes later an explosion and flash fire occurred, which self-extinguished. No one was injured. Investigations revealed that after... 

Premature shutdown of fans/venti-lation system immediately following shutdown of heat input (prior to sufficient cooling) resulting in hot spots and flammable pockets (dryers, carbon beds, and thermal oxidizers). Possibility of subsequent ignition resulting in fire or explosion. 

Batch dryer operation resulting in a high peak evaporation rate of flammable solvent causing buildup of flammables. Possibility of fire/explosion. 

Institution of Chemical Engineers (1990) Prevention of fires and Explosions in Dryers A User Guide, 2nd edn, London. 

Dust explosions usually occur in two stages a primary explosion which disturbs deposited dust followed by the second, severe, explosion of the dust thrown into the atmosphere. Any finely divided combustible solid is a potential explosion hazard. Particular care must be taken in the design of dryers, conveyors, cyclones, and storage hoppers for polymers and other combustible products or intermediates. The extensive literature on the hazard and control of dust explosions should be consulted before designing powder handling systems Field (1982), Cross and Farrer (1982), Barton (2001), and Eckhoff (2003). 

After two minor dust explosions in an industrial adipic acid dryer, evidence was obtained that adipic acid forms an iron complex capable of both decarboxyla-tion/dehydration of adipic acid to cyclopentanone and of catalysing air oxidation, giving exotherms from as low as 135°C. 

Reay, D., "Fire and Explosion Hazards in Dryers," Loss Prevention Bulletin, 025,1. 

A 5A molecular sieve, not previously soaked in dilute ethylene, was used to dry compressed ethylene gas in a flow system. An exothermic reaction attained red heat and caused explosive failure of the dryer. The smaller-pored 3A sieve is not catalytically active towards ethylene. 

Solid materials are often dried or heated using combustion gas exhaust from a fired heater as the material is conveyed through a hot combustion gas zone. Solids handling dryers may take a number of forms, e.g., a rotary kiln. Losses involving dryers usually involve internal fires or explosions. 

Steam snuffing in solids dryers can also provide effective fire protection. In some cases, the major concern for a fired product dryer may be explosion involving product dust. Where there is a possibility of an internal dust explosion in a product dryer, a fast response explosion suppression system should be considered in addition to fire protection. 

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