Chemistry Reference
In-Depth Information
V
V
b
Fig. 4.1
Schematic of the high-temperature block calorimeter: asbestos cement casing (
1
), external
heater (
2
); resistance thermometers (
3
); inner heater (
4
), quartz Dewar vessel (
5
), metal cylinder
(duralumin or copper) (
6
), thermocouples (
7
), photoamplifier (
8
), recorder (
9
), control relay (
10
)
the difference between the values of the sample weight before and after the exper-
iment). An additional internal heater on the lateral surface of the block is used for
calorimeter standardization (determining the heat exchange parameters in the De-
war vessel at high temperatures and measuring the effective heat capacity of the
block calorimeter). The expression for calculating
Q
∑
from the experimental data
was found by solving the problem of nonstationary heat conduction in the block.
Two process stages were considered: linear pyrolysis and the subsequent thermal
relaxation. The solution obtained for the first stage was used as the initial condition
for the solution of the problem related to the second stage. In order to simplify
the resulting analytical expression the following assumptions were made (detailed
information regarding the analytical method can be found in [2]):
a) The model used is one-dimensional (any cross-sectional temperature difference
is much lower than that longitudinally)
b) The linear pyrolysis rate is constant
c) The rates of heat dissipation from the block top end before and after linear py-
rolysis are equal
The assumptions are quite reasonable due to a big (
25-fold) difference in the cor-
responding Fourier numbers (relates to assumption a) and to an insignificant vari-
ation in the block end temperature during linear pyrolysis (
∼
10-15
◦
C) (relates to
assumptions b, c). Typical time dependencies for the block temperature during the
linear pyrolysis of PMMA are shown in Fig. 4.2. The maximum temperature change
in the top section of the block is equal to 11
◦
C (from 493 to 482
◦
C). Shortly after
linear pyrolysis, the thermal relaxation of the block was found by numerical analysis
∼
