Understanding the Software Options

Temperature, °C
Self-heating test with a constant rate
of temperature rise, 0.5°C/min
450
400
350
Oven
75% height
300
50% height
250
200
150
100
50
25% height
12.5% height
0
0 100 200 300 400 500 600 700
Time, min
FIGURE 7. In this screening test, the oven temperature was
increased from 20 to 400°C at a rate of 0.5°C/min. The exothermic
onset temperature was identi�ed to be 166°C, which was lower
than the hot air temperature of the dryer, indicating that selfheating
was the most probable ignition source for this incident
isted in the dryer before the incident.
An typical powder sample was placed
in a temperature-programmed oven
and the temperature of the oven was
increased from 20 to 400°C at a rate
of 0.5°C/min (Figure 7). The exothermic
onset temperature was identified
to be 166°C, which was lower than the
hot air temperature of the dryer. The
result indicated that self-heating was
the most probable ignition source for
this incident.
This test provides a useful tool for a
quick identification of the self-ignition
hazard of materials and should be conducted
for materials whose thermal
stability characteristics are not known.
However, to be of any practical value,
the test results obtained in a laboratory-scale
apparatus have to be scaled
up to plant-size process. In order to
establish the relationship between
the exothermic onset temperature and
powder layer thickness with the aid of
thermal explosion theory, the sample of
the powder that was being dried in the
same incident batch was tested at three
different layer thicknesses. In each
trial, the powder sample was exposed
to a constant temperature to test if
self-heating would actually take place.
The highest temperature at which selfheating
did not occur and the lowest
temperature at which self-heating did
occur were determined. The average
value of these two temperatures was
taken as the exothermic onset temperature
for each powder layer.
The exothermic onset temperatures
were used to determine the
unknown constants of the following
equation, which expresses the relationship
between the exothermic
onset temperature and the thickness
of the powder layer:
Onset temperature of self-heating, °C
180
170
160
150
140
130
(1)
Where
Ta = Exothermic onset temperature
for a powder layer, K
r = One half of the powder layer
thickness, m
δc = Frank-Kamenetskii-parameter,
dimensionless
M, N = Constants determined by properties
of the powder material
Using Equation (1), the exothermic
onset temperatures of layers of the
powder at different thicknesses were
calculated and the results are plotted
(Figure 8). As the thickness of the
powder layer was increased from 1 to
12 in., the exothermic onset temperature
decreased by 48°C.
The powder layer in the dryer before
the incident was on the order of 4–8
in. This suggests that the exothermic
onset temperature for the powder layer
was well below the hot air temperature.
Ignition would occur if the heating time
exceeded the induction time.
Concluding remarks
The self-heating hazard of solid materials
to be dried should be determined.
Depending on the drying process, the
solid materials can be tested in different
shape, heating environment, with
or without an airflow through the material
and an airflow at the surface of
the material. The exothermic onset
temperatures can be used to determine
safe drying temperatures using
sufficient safety margins. However, the
relationship between the exothermic
onset temperature and the dimension
of solid bulk is often needed in order to
design a safe drying process. This rela-
Dependence of self-heating onset
on the thickness of powder layer
120
0 2 4 6 8 10 12
Thickness of powder layer, in
FIGURE 8. Using Equation (1), the exothermic onset temperatures
of layers of the powder at different thicknesses were
calculated and the results plotted here. As the thickness of the
powder layer was increased from 1 to 12 in., the exothermic
onset temperature decreased by 48°C
tionship can be established from selfheating
experiments based on thermal
explosion theories, as demonstrated by
the example introduced above. ■
Edited by Rebekkah Marshall
Suggested reading
J.A. Abbott (technical ed.), “Prevention of Fire &
Explosions in Dryers”, 2nd ed., The Institution of
Chemical Engineers, Rugby U.K. 1990.
Authors
Vahid Ebadat is the CEO
of Chilworth North America
(Chilworth Global, Princeton,
NJ; Phone: 609-799-
4449; Email: safety-usa@
chilworthglobal.com; Website:
www.chilworth.com). He holds
a B.S. in electrical engineering
and a Ph.D. from Southampton
University. He has worked
extensively as a process and
operational hazards consultant
for the chemical, pharmaceutical and food
industries. Ebadat is a regular speaker at training
courses on gas and vapor flammability, dust
explosions, and controlling electrostatic hazards.
He is a member of NFPA 77 Technical Committee
on Static Electricity; NFPA 654 Standard for
the Prevention of Fire and Dust Explosions from
the Manufacturing, Processing, and Handling of
Combustible Particular Solids; and ASTM E27
Committee on Hazard Potential of Chemicals.
Ebadat’s research has culminated in the publication
of numerous technical articles and papers.
Pieter Zeeuwen, is a senior
process safety specialist at
Chilworth Global. He holds
a M.Sc. in applied physics
from Eindhoven University
of Technology and has more
than 30 years experience
in the gas and dust explosion
fields, including materials
testing, small and large
scale explosion research, and
consultancy for industry and
government agencies in a number of countries.
His areas of expertise include gas and dust explosion
hazard assessment, gas and dust explosion
prevention and protection, electrostatic hazard
assessment, hazardous area classification, and
gas cloud explosions as well as incident investigations.
Over the years, Zeeuwen has served on
many working groups including various standards
committees, both nationally and internationally,
for instance, most recently CEN (European
Standards Committee) working groups on explosion
protection methods and on test methods. He
regularly lectures on various aspects of explosion
safety and acts as seminar chairman and course
director. Zeeuwen has published numerous articles
in scientific journals and presented many
papers at international conferences.
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 47