Chemistry Reference
In-Depth Information
macrokinetic parameters of the process. At these rates, the surface temperatures of
decomposing compounds (such as composite solid propellants) are within the range
near to the upper limit of the temperature region that is characteristic of processes
studied by classical isothermal methods of chemical kinetics. However, due to an
extremely strong (Arrhenius) temperature dependence of the decomposition rate in
this region, the temperature of the heater should be kept strictly constant. In this
case, a thin metal plate cannot be used as a heater because of two reasons: it is
difficult to keep its temperature constant (this problem can be solved by using more
sophisticated thermostating systems; for example, proportional-plus-floating control
systems), and the spatially nonisothermal temperature distribution along the heating
plate surface. The latter is the more serious problem, and is caused by significantly
different heat exchange conditions for the parts that are in contact with the sample
compared to the parts that are not. To keep the temperature constant at all points
on the heater surface, one can use a bulky heat-conducting metal cylinder and press
the sample onto one of its ends. The cylinder is thermostated by a nichrome heating
coil wound around its lateral surface. The application of metal bodies with high heat
capacity and good heat conduction properties in contact with the sample was found
to be quite efficient for studying the macrokinetics of fast reactions in condensed
systems (Chaps. 4-8). A photo and a schematic of the LP-3 and LP-4 installations
with bulky heat-conducting heaters are shown in Figs. 2.5 and 2.6. When perform-
ing prolonged experiments on “slow” linear pyrolysis, four bulky aluminum heaters
are fixed onto a single base plate for simultaneous experiments with four samples
in installation LP-3. The heaters are equipped with autonomous systems of point-
to-point control (Fig. 2.6) with galvanometric null indicators (bench galvanometers
M-195 with built-in differential photocells).
Experimental data on the linear pyrolysis of the four samples are simultaneously
recorded by an EPP-09 eight-channel recorder with a set of four vertically mounted
photocells on its slide (Sect. 2.1). Preliminary alignment of the mirrors that are kine-
matically connected with the sample holders ensures that each light beam travels in
the plane of the corresponding photocell. The cells are connected to input terminals
1, 3, 5,
and
7
via an internal switch in the recorder, while input terminals
2, 4, 6,
and
8
are shorted. Thus the slide moves along the scale to a certain spot of reflected
light, where the digitized experimental point is plotted. Then the slide travels back
to the beginning of the scale to move again to the next spot of reflected light, etc.
Experiments on the linear pyrolysis of doubly thermostated samples (see “Lin-
ear pyrolysis of thin samples,” Sect. 1.5) were carried out using installation LP-4
(Fig. 2.6). The temperatures on the upper (
T
S
) and lower (
T
∞
) ends of a disk sample
(10 mm in diameter,
5 mm thick) were kept strictly constant due to their contacts
with a bulky heater (30 mm in diameter, 100 mm thick) and an aluminum container
(
8
) filled with a heat carrier (water or glycerin pumped through by the pumping
unit of a U-10 ultrathermostat), respectively. The container connected to the ultra-
thermostat with flexible tubing was fixed onto the upper part of the sliding cylin-
der, which moved on a fluoroplastic guide in the same way as in installation LP-1
(Fig. 2.1).
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