Activated Carbon

Activated carbon is one of the few bulk materials that changes the atmosphere around it.

Oxygen depletion

Fresh carbon adsorbs oxygen from the air in a silo, hopper or container, and the headspace can fall well below a survivable concentration.

Entry requires measurement, forced ventilation and a permit; the hazard is invisible and gives no warning.

Self-heating

Carbon loaded with solvents or ketones can oxidise exothermically, and a hot spot in a deep bed is difficult to detect and harder to extinguish.

Temperature monitoring, limits on storage time for loaded carbon and a defined response plan belong in the operating procedure.

Attrition and dust

Granular carbon is friable, and high velocity conveying turns a specified particle size into fines that blind the bed and escape as black dust.

• Dense phase or vacuum conveying at modest velocity.
• Minimised drop heights into vessels.
• Screening before use where the specification depends on grain size.

Powdered carbon dosing

Powdered activated carbon is cohesive and extremely light, so dosing into a flue gas or water stream needs a feeder that tolerates both bridging and flooding.

Loss-in-weight feeding with a conditioned inlet and contained big bag discharge keeps rate stable and the room clean.

What to establish before buying equipment

• Entry procedure and gas monitoring for every enclosed vessel.
• Whether loaded carbon will be stored, and for how long.
• Attrition tolerance and the measured fines increase across the line.
• Dosing accuracy required at the process.

Activated carbon adsorbs continuously, and that is the storage problem rather than a side effect.

A full silo takes up oxygen from the air between the particles and gives off heat while it does so. In a closed vessel this produces two conditions at once: an atmosphere that can be well below breathable oxygen, and a bed that is warmer in the middle than at the wall.

• Treat every storage vessel as a confined space with a depleted atmosphere until it is measured.
• Watch the bed temperature in large volumes; a rising trend is the early signal of self-heating.
• Keep solvent-laden carbon separate from fresh carbon. Loaded carbon has already released its adsorption heat and behaves differently.

Moisture adds a second effect. Wet carbon is heavier, more cohesive and slower to discharge, and the water occupies pore volume that the process expected to be free.

Two properties drive the selection: the dust is hard, and the product is porous.

Hardness means abrasion. Conveying velocity sets the wear rate on bends and valve seats, so a dense phase line or a vacuum system at modest velocity outlasts a dilute phase line at full speed, often by years rather than months.

Porosity means attrition. Every impact breaks granules into fines, and fines are the part that dusts, blinds filters and changes the pressure drop in a filter bed downstream. A conveying system that halves the particle size has changed the product, not just moved it.

• Prefer vacuum or dense phase over high-velocity dilute phase.
• Use long-radius or wear-backed bends, and expect to inspect them.
• Specify filters for an abrasive, fine dust rather than a general-purpose one.

Carbon dust is combustible, and activated carbon brings a second hazard that ordinary combustible dust does not: it can heat itself.

The adsorption that makes the product useful releases heat, and in a large insulated volume that heat has nowhere to go. Smouldering fires in carbon silos develop slowly, deep inside the bed, and are not visible from an inspection hatch.

• Dust explosion protection follows the measured values for the specific grade.
• Entry into a vessel requires atmosphere testing for oxygen, not only for toxic gases.
• Carbon loaded with solvent is a different material from fresh carbon and is assessed separately.

Water is not always the right response to a carbon fire, because a wet bed can continue to heat internally. The fire plan belongs in the design, agreed with the supplier.