Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 212

Fig. 4.37 Dependence of the
pressure term in Eq. 4.87 on
the gaseous product pressure.
The vertical lines denote the
values of the equilibrium
pressure

3

3DX

runs. The situation is illustrated in Fig. 4.38 for decomposition of calcium carbon-
ate under ambient pressure of carbon dioxide, i.e., 40 Pa. When the temperature
rises, so does the equilibrium pressure in accord with Eq. 4.77. As illustrated in
Fig. 4.34 , decomposition would start when temperature rises above 535 ﾰC and the
equilibrium pressure above 40 Pa. Because at this point P 0 is yet very close to P , the
pressure term in Eq. 4.87 is large so that the effective activation energy at the initial
stages of decomposition can also be expected to be unusually large. However, as
temperature continues to rise causing continuous increase of the equilibrium pres-
sure, the pressure term becomes progressively smaller. This means that at some
later stages of decomposition the pressure dependence of the effective activation
energy should vanish. As mentioned before, the pressure dependence practically
vanishes when the equilibrium pressure is at least ten times larger than the gaseous
product pressure. Figure 4.38 shows that this is accomplished at temperatures above

Fig. 4.38 Decrease of the
pressure term ( solid line )
with increasing temperature
at constant pressure (40 Pa)
of the gaseous product. The
equilibrium pressure ( dashed
dotted line ) increases with
temperature in accord with
Eq. 4.77