Page 531 - Dust Explosions in the Process Industries

498 Dust Explosions in the Process Industries

7.7.2.6
Other Methods

Standard instruments for differential thermal analysis (DTA) have been used for fast
screening of self-heating and self-ignition properties of dusts and powders.
Bowes (1984) wrote an updated, comprehensive account of the state of the art on the-
ories and experiments on self-heating in powder deposits by 1983-1984. Since that date,
further development has taken place, and new instruments for detailed studies of the rate
of reaction as a function of temperature, under adiabatic conditions, are commercially
available. As described by Townsend and Tou (1980), such “accelerating rate calorime-
ters” (ARCS) are essentially highly computerized adiabatic calorimeters. During an ARC
experiment, the sample is maintained in a near-to-perfect adiabatic condition, while
time, temperature, and pressure data are automatically collected and stored. The data can
then be processed by computers. An ARC is illustrated in Figure 7.23.

TOP ZONE THERMOCOUPLE

BOMB
THERMOCOUPLE
JACKET
THERMOCOUPLE

JACKET

INSIDE THERMOCOUPLE
I ’ \? 1 SPHERICAL BOMB

EOTTOMZONE
BOTTOM ZONE THERMOCOUPLE
Figure 7.23 Accelerating rate calorimeter for assessing exothermal behavior of a substance, includ-
ing powders and dusts, under adiabatic conditions (From Townsend and Tou, 1980).

In addition to ARC, differential scanning calorimetry (DSC) is in use, as discussed by
Snee (1987). DSC implies measurements of the rate at which heat must be transferred
to or from the test sample to maintain it at the same temperature as an inert reference
sample. The reference sample temperature is usually increased at a predetermined linear
rate (constant temperature rise per unit time).
Grewer et al. (1989) and Racke (1989) gave comprehensive reviews of instrumenta-
tion and procedures in use for assessing the exothermal behavior of reactive chemicals,
including powders and dusts.