Under pressure

Moist activated carbon bursts locking ring

In a member company, activated carbon used to be moistened with water immediately before use. This procedure had been altered in that a supplier prepared the moistened carbon in sealed stainless steel drums on their premises prior to delivery.  At the end of 2005, an accident occurred. On a 100-litre stainless steel drum with an activated carbon/water mixture, a slight bulge was identified on the lid of the drum. The drum was closed with a sealed lid, locking ring and security seal. Upon attempting to carefully release the locking ring, the lid of the drum shot into the employee's face, causing extensive injuries.

Various experiments in the laboratory and on site demonstrated that, when moistening or wetting activated carbon with water, adsorbed gas is released. In this case, a pressure of around 0.8 bar had built up in the hermetically sealed drum.

All examined activated carbons adsorb gas such as air or carbon dioxide at their surface. By wetting the activated carbon with water or with solvents, this adsorbed gas can be released. The quantity of gas released is closely dependent on the initial water content of the carbon and the solvent. The moister the carbon, the less adsorbed gas is released. Organic solvents generally cause a greater gas release.

The quantities of gas released in this case were measured at 2.5 to 7.9 litres of gas per kilogramme of activated carbon. Gas is released immediately after adding water or an organic solvent to the carbon. This reaction is delayed when activated carbon is on top of the fluid.

In summary: when moistening or wetting activated carbon, it must always be assumed that adsorbed gas will be released. When filling closed containers, it must be ensured that these are or can be securely pressure-compensated. Alongside other measures, all seals were removed from the stainless steel drums used in the field in order to ensure that a dangerous build-up of pressure is no longer possible in this company.

See: Sichere Chemiearbeit 7/8-2006, page 68