Chemistry Reference
In-Depth Information
Fig. 1.5
Plot of the relative
rate of linear pyrolysis against
the dispersion degree for a
zero-order reaction (
1
)andfor
a first-order reaction (
2
)
U/U*
5
0.5
η
g
and first-order reactions against the dispersion degree are shown in Fig. 1.5 [
U
*is
expressed by relationships (1.1) and (1.37) for linear pyrolysis occurring without
dispersion in the case of zero-order reactions].
To conclude this section, the following general comment should be made: the
macrokinetics of decomposition characterized by a simple (not self-accelerating)
reaction mechanism and occurring during the “slow” linear pyrolysis of a sample
on a smooth solid metal heater can be described in terms of a pseudo-zero-order
(or similar) process. This can be explained by the continuous densification of the
surface layer of the substance due to the force pressing the sample onto the heater.
At high rates of decomposition (“fast” linear pyrolysis) accompanied by significant
dispersion of the substance, the pressure between the sample and the heater (in stan-
dard experiments, the applied pressure is usually in the range of one to a few atm;
see Chap. 3) almost does not influence processes in the surface layer. These factors
should be taken into account when estimating kinetic constants from experimen-
tal data on linear pyrolysis. Analysis of experimental data in the case of fast linear
pyrolysis with significant dispersion and minor densification of the surface layer by
using the Arrhenius anamorphosis approach can result in an apparent increase in the
kinetic constants
k
0
and
E
. The qualitative dependence of the linear pyrolysis rate
on the degree of dispersion corresponding to linear pyrolysis under real conditions
is shown in Fig. 1.5 as a dotted line.
1.7 Effect of the Gas-Film on Linear Pyrolysis: Method
for Calculating the Surface Temperature
One of the most challenging tasks one has to face when studying the kinetics of
the fast decomposition of condensed compounds is to determine the characteris-
tic sample temperature. The latter must be dramatically different from the tem-
perature of the thermostat used. For the kinetic analysis of slow linear pyrolysis
(
U
10
−
3
mm s
−
1
), the temperature drop across the heater-sample contact can be
neglected assuming that
T
S
=
T
0
. However, for fast process (for example, the linear
≤
