Hazards Associated with Processing Particles 

Latest changes: 03Jun03 - created / 07Jul28 - reformat / 08Mar23 - reworded somewhat /

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What Sorts of Hazards are There? It is important to review all particulate processes for hazards so as to avoid a dangerous event and to minimize the consequences in case one occurs. Particulate systems may be generalized beyond powders to include systems with liquid droplets, dispersions of solids or gases in liquid, porous solids, and foams. Several of the most common hazards for systems involving particulate matter are: SUBMERSION: -- A) Dry Powder: People who fall into a bin of powder may sink into it and suffocate if loosely-packed powder forms of networks that collapse under the weight of the body. -- B) Sediment: Consolidation to a firm base may be significantly impaired by floc networks made up of large amounts of fine particless or by conditions and surfactants that create high surface charge densities on the particles. The result is a deep semi-fluid (boggy) layer into which a person may sink when the objective is to provide a firm foundation for human or vehicle traffic. EXPLOSION: Ignition of a flammable / explosive mixture of particulates in air or other reactive atmosphere due to a spark or corona discharge caused by -- A) Charge buildup due to contact electrification -- B) Milling (grinding, rubbing, high velocity impact) of metallic or ceramic particles RUNAWAY REACTION: A catalytic reaction in a fluidized bed may "run away" if the fines content or porosity of the particles increases due to a change in raw material source. This can rapidly overheat or over-pressurize the reactor. INHALATION, EYE IRRITATION: Particulates smaller than ten micrometers in diameter are often picked up by relatively gentle air currents, do not settle quickly, and can cause damage to eyes and lungs. This is a particular problem if the material is toxic, corrosive, radioactive, biologically active, or has pharmacological effects. INGREDIENT SEGREGATION: -- A) Foam may preferentially adsorb a key ingredient (such as a powder) from the bulk liquid and thus prevent proper mixing and reaction in the bulk liquid. -- B) Low-density powders or fine particles may be blown out the top of a fluidized bed, amaking them unavailable for reaction or catalysis within the bed. TANK OVERFLOW: -- A) Bubbles that take a long time to deflate or break due to stabilization by fine particles or excessive surfactant due to inadequate fines may lead to an unacceptable depth of foam or overflow of the foam out of the process equipment. -- B) Small bubbles take a longer time than large ones to rise to the top of the liquid layer (into the foam layer). The incease in the effective liquid depth (called "gas-swelling") and decrease in effective liquid density and viscosity may create operating, safety, and environmental problems. Prevent the Event Required use of filter masks and goggles reduces inhalation and eye contact with the powder. Guardrails prevent people from falling into a vat or silo. Lifelines aid workers in getting out of danger. Continuous Monitoring of particle size distribution helps keep the process within known-safe limits. Dust Collection Systems and Fully-enclosed Processes reduce areas where problems may arise. Re-direct Problems by quenching, rupture disks, and blowout panels. Magnetic separators eliminate many particles that might spark. Iinerting denies the mixture one reactant (oxygen) that is required for explosion. The rate of reaction and the surfactant adsorption capacity of small-size particulate systems are dominated by surface area. Variations in surface area per kilogram of material may occur due to batch-to-batch variations in raw material or changes in process conditions, especially during startup, power failure, other accidental upsets, and shutdown. Continuous monitoring of particle size disribution will help warn the operators of potentially dangerous changes in the process. DESIGN: When scaling-up a process be aware that the characteristics of some processes scale as length-cubed (volume) while others vary with length-squared (surface area) or with length (liquid depth) The designer must give primary consideration to factors that will optimize benefits (production rate, cost of production, quality), bearing im mind the factors that will minimize problems (safety, shutdowns, environment). These will determine the best shape for large-scale (commercial production) equipment that will behave in a way that corresponds well with laboratory and pilot runs made in small-scale equipment EXAMPLE: Pilot-scale runs of a continuous neutralization in a neutralization tank 60 cm deep with a diameter of 60 cm created a relatively persistent foam, stabilizing at a depth of 15 cm, if the liquid level was limited to 40 cm (thus 113 L of liquid, 42 L of foam). Stabilization here means that the rate of foam creation (bubble formation and rise to the bottom of the foam layer) was equal to the rate of foam dissipation (bubble breakage at the top of the foam layer). The objective was to design a process with 50 times the throughput (and same in-tank residence time). One approach is simply to scale-up each side of the tank by 501/3 = 3.7. However the foam layer that was manageable at 15 cm deep becomes unwieldy at 55 cm deep. It is better to use a tank with the same depth (60 cm) as the pilot-scale tank and to scale the diameter up by 501/2 = 7.1 to be 426 cm. This larger tank would look like a pancake and would cost more than a standard tank shape -- but it would have a manageable layer of foam.

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