Chemistry Reference
In-Depth Information
where
,
c
,
V
are the density, heat capacity and volume of the liquid respectively, and
T
0
is the
ambient (thermostat) temperature.
The first term on the right hand side of Eq. (10.9) referring to the kinetics of heat
evolution is quite complex. As the temperature grows, its value first increases (not
influenced by evaporation) and reaches its maximum value, and then it decreases
due to evaporation-related heat absorption. Equation (10.9) can be rewritten as
α
is the heat emission coefficient,
S
is the heat-exchange surface area,
ρ
=
Qk
0
exp
Lk
0
exp
b
a
dT
dt
E
R
T
E
R
T
P
0
exp(
−
L
/
R
T
)
L
/
R
T
)
−
α
S
V
c
ρ
−
−
−
(
T
−
T
0
)
P
∞
−
P
0
exp(
−
(I)
(II)
(III)
(IV)
so that it can be considered a heat-balance equation for a process with volumetric
(term III) and surface (through the vessel walls, term IV) heat emission, similar to
chain termination in the volume and on the walls in the case of an explosive chain
reaction.
Let us consider possible stationary modes and critical conditions for this process.
The best way to visualize the analysis is to use the Semenov diagram for the case
under discussion (Fig. 10.2). The curves corresponding to heat evolution for the pro-
cess with (
2
) and without (
1
) volumetric evaporation coincide at low temperatures
(when the liquid vapor pressure is much lower than the ambient pressure). In this
case, the limiting explosion temperature,
T
∗
, exceeds the temperature characteris-
tic of the process without evaporation,
T
i
. One can see from Fig. 10.2 that, if all
other conditions are the same, the critical heat emission, (
S
/
V
)
∗
, corresponding to
the explosion initiation limit for a volatile liquid is lower than that for a nonvolatile
compound (characterized by the absence of volumetric heat absorption). The pro-
cess characteristics corresponding to the region below the explosion initiation limit
α
R
R
R
ad
b
st
0
Fig. 10.2
The Semenov diagram for the thermal explosion of nonvolatile (
1
) and volatile (
2
)ex-
plosives. Straight line
3
corresponds to heat removal in the region above the explosion initiation
limit, determined only by the condensed-phase reaction (without taking evaporation into account)
