Bulk Powder (diffusion cell)

Prevention of Fires and Explosions in Dryers – John Abbott

The bulk powder diffusion cell test is conducted to establish the onset temperature of exothermic activity of a material when exposed to elevated atmospheric temperatures. It replicates small bulk deposits in any drying situation where air is naturally available.

Using an open cylindrical glass test cell with sintered glass bottom, the sample is placed inside the cell and thermocouples inserted through its wall into the material at four locations from the bottom upwards. This assembly is placed into a digitally controlled laboratory oven and either of the following tests performed using a data acquisition system to monitor, detect and record any exothermic activity.

SCREENING TEST

The environmental temperature is digitally ramped from ambient conditions up to 400°C at 0.5°C.min-1. This test configuration is used for estimating exothermic activity onset temperature.

ISOTHERMAL TEST

The loaded test cell is placed into a heated oven, set to a specified temperature for a duration of 24hr. The temperature selected can either be based on working or proposed process conditions (plus a factor of safety) or full determination can be undertaken, conducting several isothermal tests to establish a definitive ignition temperature.

Not only is the bulk powder diffusion cell test a necessity in evaluating whether current or proposed drying and process temperatures are safe, it can also confirm the possibility of using a higher temperature to increase productivity.