development and application of a pilot scale facility for studing runaway exothermic reaction
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7/23/2019 Development and Application of a Pilot Scale Facility for Studing Runaway Exothermic Reaction
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Development and application of a pilot
scale facility for studying runaway
exothermic reactions
T J Snee and J A Hare
Health and Safety Executive Research and Laboratory Services Di vision Explosion
and F lame Laboratory Harpur Hil l Buxton Derbyshir e SK1 7 9JN UK
This paper describes a facility for investigating the control and stability of exothermic reactions
and the performance of relief systems in a pilot scale installation. A series of experiments is
reported in which a simple exothermic reaction is shown to proceed under isothermal conditions,
subcritical conditions (with some self-heating) and supercritical conditions leading to exothermic
runaway.
(Keywords: runaway reactions; thermal stability; pilot plant design)
It is well established that a runaway exothermic
reaction can occur if the rate of heat generation exceeds
the rate at which heat can be lost to the surroundings.
A number of theoretical models have been developed
to describe this phenomenon and to predict the critical
conditions that can lead to runawaylW4. Application of
the theoretical models requires data on the tem-
perature dependence of reaction rate and on the heat
transfer characteristics of the process vessel. Reaction
rate data are often obtained using small scale thermo-
analytical techniques such as differential scanning
calorimetry d.s.~.)~ and accelerating rate calorimetry
a.r.c.)6. The rate of heat loss as a function of excess
temperature can be determined from empirical cooling
curves or by calculation using geometrical considera-
tions and published data on heat transfer coefficients.
Theoretical models can then be used to indicate the
margins of safety that should be allowed between
process parameters and predicted critical temperatures,
pressures and concentrations. However, the interpreta-
tion of small scale thermoanalytical data can be difficult
and extrapolation to large scale industrial installations
can introduce significant uncertainties’. It is sometimes
possible to determine the critical conditions experi-
mentally using, for example, a Dewar flask to simulate
the heat transfer characteristics of a large process
vessel, but this does not allow extrapolation to vessels
of different sizes and fails to take full account of
complex scaling effects associated with agitation, and
heat and mass transfer in a large, jacketed chemical
reactor.
Thermal analysis and theoretical assessment can
be used to identify suitable process control measures
Received 1 March 1991
@
Crown Copyright
1992
0950 4230/92/010046 OS
0 1992
Butteworth-Heinemann Ltd
46
J. Loss Prev. Process Ind. 1992 Vol5 No
1
for exothermic reactions, but an emergency pressure
relief system ERS) is usually required as an additional
safety measure. The design of an ERS has to take
account of the complex fluid mechanics associated with
the relief process as well as the chemical thermokinetic
properties of the reacting medium. The operation of a
pressure relief device often leads to a two-phase
discharge from the reactor and there has been substan-
tial research effort, in recent years, in developing
models and computer codes to describe the phenome-
non so that safe and efficient relief systems can be
designed’. Small scale instruments have been deve-
loped to simulate venting of a large process vessel’,“,
and to provide source data for the theoretical models,
but few experimental studies have been performed to
test the validity of the various methodologies and
design criteria as applied to full scale industrial proces-
ses.
The critical conditions that can lead to a runaway
chemical reaction, the reliability of control measures,
and the performance of emergency relief systems
should be determined using vessel sizes and inventories
similar to normal process conditions. However, run-
away reaction experiments on this scale are both
hazardous and expensive, and only a very limited
number of studies of this kind have been performed to
dates.
This paper describes the development of a facility
for investigating the control and stability of exothermic
reactions and the performance of relief systems in a
pilot scale installation with many of the characteristics
of a normal chemical plant. A series of experiments are
reported in which a simple exothermic reaction is
shown to proceed under: isothermal conditions; sub-
critical conditions, but with some self-heating; super-
critical conditions leading to exothermic runaway.
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Development of pilot scale facil ity: T. J. Snee and J. A. Hare
These pilot scale experiments are related to small scale
thermoanalytical investigations and to measurements
of the cooling characteristics of the reactor.
The work reported here had the following broad
objectives.
Investigation of the validity of current methods of
determining the thermokinetic parameters of an
exothermic reaction using techniques such as a.r.c.
and d.s.c.
Experimental study of the conditions that can lead
to a runaway reaction in an industrial reactor of a
standard design,
so that theoretical models for
predicting the critical conditions can be tested.
Development of techniques for controlling the
degree of supercriticality, using chemical systems
with well defined thermokinetic properties, so that
the performance of emergency pressure relief sys-
tems can be tested in order that reliable methods for
determining safe vent line diameters and relief set
pressures can be identified.
Pilot scale installation for studying runaway
reactions
The pilot plant was based around a 250 1 glass-lined
reactor in accordance with DIN Standard 28136. The
specifications for the reactor are listed in Table I. A
capacity of 250 1 was chosen in order to reproduce
conditions similar to those in a full size chemical reactor
2” NB SS
Catch tank
Pump
Figure 1
Diagram of pilot scale chemical plant
v3
PUCS
P
’ v3
1”
e
?
:illl
3” NB/SS
but on a scale which would allow experimental studies
to be performed safely and at reasonable cost.
The reactor was provided with two glass feed
vessels for charging reagents and was connected via a
bursting disc and an 80 mm diameter vent line to a
stainless steel catch tank. The specifications of the
catch tank are listed in Table 1. A pump was installed
below the reactor to allow transfer and recirculation of
reactor contents. The whole installation was mounted
on a transportable steel framework.
A diagram of the pilot plant is shown in Figure 1
and a piping diagram of the installation is shown in
Figure 2. The plant was designed to be readily
Table 1 Specifications for reactor and catch tank
Reactor
Catch tank
Vessel Jacket
Working pressure - 1- 6 -1-6 6
(bar g)
Design pressure - 1-6.6 -1-6.6 -
(bar g)
Test pressure 11.4 11.4
9
(bar g)
Temperature range -25-200
-
o-1 50
(“C)
Capacity 250 (nominal) 93
2500
0)
334 (total)
Material glass-lined
mild steel stainless
mild steel
steel
7 Return water (blue)
III} Temperature sensors
, 1 NB, PL/CS ) [II}Temperature sensors
i
1” NB/GAL
Feed water (black)
Vl
ar pipe
Reactor
Feed vessels
J. L oss Prev. Process Ind., 1992, Voi 5, No I
47
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Development of pilot sc ale facil i ty: T. J. Snee and J. A. Hare
ST/steel
r. gauge
2” dial dir
_I_____?_
filled
Enlargement of
top of ‘Richter’
bulls eye sight glass
O-160 p.s.i.
Thermocouole connection
block ’
Geared motor
/
T
ead vessels
500 gall - ST/steel receiver
Portobello Fab. Ltd - Sheffield
Deg No - 12239
Front elevation
Figure2 Front elevation of pilot scale installation
adaptable with the possibility of installing, for example,
reflux condensers, distillation columns and receiver
tanks. At present, the reactor is not provided with a
heating or refrigeration system. The initial experiments
required only the supply of cooling water to the reactor
jacket. Specifications for the heating system are cur-
rently in preparation.
Instrumentation and data acquisition
Plameproof motors and switchgear, and intrinsically
safe instrumentation have been used on the pilot plant.
The initial studies required measurement of the tem-
perature and pressure in the reaction vessel and
monitoring and control of the temperature and flow
rate of the cooling water supplied to the reactor jacket.
The speed of agitation can be monitored and con-
trolled. Sophisticated instrumentation for monitoring
two-phase discharges from the reactor has yet to be
installed.
Multipoint temperature and pressure measure-
ment was provided by a combination of thermocouples
and platinum resistance thermometers. These were
connected to intrinsically safe temperature transmitters
(Rosemount Ltd) which provided 4-20 ma signals for
transmission to potentiometric chart recorders and a
computer data acquisition system situated remotely
from the plant. The pressure in the reactor was
monitored using two intrinsically safe 4-20 ma pressure
transmitters (Sedeme) connected to the data acquisi-
tion system. The reactor was also provided with local
temperature and pressure indicators. The flow rate of
cooling water supplied to the reactor jacket was
monitored using a turbine flowmeter (Litre Meter Ltd).
The speed of agitation was controlled using a variable
frequency AC drive (I.M.O. Ltd) connected to the
agitator motor.
The computer control and data acquisition system
comprises a PC (IBM 386 AT compatible) provided
with analogue and digital input and output cards
(Burr-Brown Corp.). Control and data-logging soft-
ware (Lab. Tech. Corp.) was used to monitor and
record the temperature and pressure in the reactor and
the temperature at the inlet and outlet of the reactor
jacket. The software is capable of providing sophistic-
ated control functions which will be exploited when the
48 J. Los s Prev. Proc ess Ind., 7992, Vol5, No 7
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Development of pi lot scale faci l i ty: T. J. Snee and J. A. Hare
pilot scale installation is equipped with a heating system
and remote controlled valves.
Small scale thermal analysis
Considerable experimental effort was expended on the
selection of the chemical system suitable for pilot scale
experiments on runaway reactions. In particular, an
exothermic reaction with the following characteristics
was sought:
a simple reaction system that produced good
thermoanalytical data which would provide reliable
source terms for testing scaling criteria, theoretical
models and vent-sizing methodology;
moderate exothermicity with no possibility of pro-
ducing detonable or highly toxic products;
a reaction system where the physical properties of
the reagents and products were well established
with, for example, vapour pressure-temperature
dependencies that would lead to moderate pressure
during reaction to allow a realistic test of the
performance of the ERS.
a reaction with thermokinetic properties correspond-
ing to rates of heat generation that could lead to
critical runaway conditions over a temperature range
that is readily accessible experimentally.
a reaction system in which the rate of heat genera-
tion could be controlled by the addition of small
quantities of catalyst, without influencing the total
heat output, to allow a systematic study of the
dependence of criticality on reaction kinetics.
A wide variety of exothermic reactions was considered
and the more promising systems were selected for
thermal analysis using a.r.c. and d.s.c.
The esterification reaction between propionic an-
hydride and butan-2-01 was finally selected for the pilot
scale studies:
(CH$H&O),O + C2H5CH(OH)CH3 +
Propionic
anhydride
Butan-2-01
CH,CH2C02H +_ CH&H2COOCH(CH,)C2H,
Propionic
Butyl-Zpropionate
acid
The reaction is catalysed by the addition of small
quantities of sulphuric acid.
Figure 3
shows a series of
d.s.c. traces for the esterification reaction catalysed by
the addition of various concentrations of sulphuric acid.
The d.s.c. traces show an increase in reactivity with
increasing acid concentration. This is evident as a
progressive increase in the maximum rate of heat
generation, a progressive reduction in the onset tem-
perature at which exothermic reaction is first detected
and a progressive reduction in the temperature cor-
responding to the maximum rate of heat generation.
These parameters are listed as a function of acid
concentration in Table 2. The heat of reaction, evalu-
ated from the area of the d.s.c. peak, is also listed in
a
01 , , , , , , ) ,
40 GO 80 100 120 140 160 180 200
I I
I
I
I
1
I
40 60
80 100 120 140 160 180 200
503
0
1 I I I I I I
I
40 60 80 100 120 140 160 180 200
01 I
I I I I I I I
40 60 130
100 120
140 160
180 200
Temperature (“Cl
figure3 D.s.c. traces
for the esterification reaction between
propionic anhydride and butan-Z-01 catalysed by the addition of
various concentrations of sulphuric acid (expressed as a percen-
tage by mass of butan-2-01): a, 0.8%; b, 0.4%; c, 0.2%; d, 0.1%
Table 2 and can be seen to be approximately independ-
ent of the concentration of sulphuric acid.
Although it is possible, in principle, to use d.s.c.
data to determine the kinetic parameters of an exo-
thermic reaction ’ :
a.r.c. provides a more accurate
means of determmmg the temperature dependence of
reaction rate.
Figure 4 shows the a.r.c. plots of
log (self-heat rate) versus reciprocal temperature for
J. Los s Prev. Proc ess Ind., 1992, Vol5, No 1 49
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Development of pi lot s cale faci l i ty: T. J. Snee and J. A. Hare
Table 2 Summary of d.s.c. data for the esterification reaction
between propionic anhydride and butan-2-01 catalysed by the
addition of sulphuric acid (scan rate 8 “C min-‘1
Sulphuric acid Onset
Peak Heat of reaction
concentration temperature’
temperature
(J g-‘1
f%)
(K (“C)) (K (“0)
0
366 (92) 411 (138) -196
0.025
350 (77) 389(116) -201
0.05
341 (68) 378 (105) -190
0.1
340 (67) 372 (99) -217
0.2
336 (63) 364 (91) -259
0.4
320 (47) 351 (78) -232
0.8
316 (43) 345 (72) -256
“Onset temperature: temperature at which a deflection from the
baseline is first observed
the esterification reaction. The plots show the same
dependence of reactivity on sulphuric acid concentra-
tion as observed using d.s.c.. An exothermic reaction is
detected by a.r.c. at an onset temperature at which the
rate of self-heating, under adiabatic conditions, ex-
ceeds 0.02 KS-‘. The increase in reactivity with in-
creasing acid concentration is seen, in F i gu r e 4 , as
progressive reduction in onset temperature and at
concentrations greater than 0.05 H2S04, as an
increase in the rate of self-heating at the initial sample
temperature.
The initial section of the a.r.c. self-heat rate plots
are approximately linear, suggesting that the rate of
heat generation can be described by an Arrhenius type
temperature dependence of the form:
qr =
QVPA exp -EDT)
1)
The
initial gradient of the plot corresponding to no
added HzSO, is significantly less than the initial
gradients of the plots for the catalysed compositions.
Autocatalysis due to propionic acid produced in the
reaction would be expected to produce an increase in
the curvature of the a.r.c. self-heat rate versus tem-
perature plot. The effect o autocatalysis is unlikely to
102
I
lo-33
-3.50 -3.40 -3.30 -3.20 -3.10 -3.M -z.m -2.80 --2.,cl -2.60 -2.60
-1000/T K-l,
Figure 4 A.r.c. plots of In (self-heat rate) versus reciprocal ab-
solute temperature for the esterification reaction between
propionic anhydride and butan-2-01 catalysed by the addition of
various concentrations of sulphuric acid (expressed as a percen-
tage by mass of butan-2-01): A, 0.4%; B, 0.2%; C, 0.1%; D,
0.05%; E. 0.025%; F, 0%
be apparent when significant quantities of sulphuric
acid are present.
At concentrations greater than 0.05 H2S04 the
self-heat rate plots show good initial linearity. Evalu-
ation of the initial gradients yields a value for the
activation energy E = 95.37 X lo3 J mol-’ which,
within experimental error, is independent of the
concentration of sulphuric acid. This suggests that the
change in reactivity can be represented as a change in
pre-exponential factor A . Pre-exponential factors,
evaluated by regression analysis of self-heat rate plots
over the first 20 K of self-heating, are plotted against
acid concentration in F i gu r e 5 . Good linear correlation
was obtained suggesting that, over this range of
concentrations, the effect of sulphuric acid on the
pre-exponential factor can be represented by the
equation:
A = 4.50 x 10’*(x) + 1.72 x lOlo
(2)
Equation (2) can be used to predict the reactivity of
mixtures containing more than 0.4 HsSO,.
Heat transfer characteristics of the reactor
Initial studies of the heat transfer characteristics of the
reactor were performed with water in the vessel and
with water as the heat transfer fluid circulating through
the reactor jacket. An electrical immersion heater was
used to raise the temperature of the water in the vessel
to around 50°C. The immersion heater was then
withdrawn and the temperature of the contents and the
temperature at the inlet and outlet of the reactor jacket
were recorded as the contents cooled with water
circulating through the reactor jacket at around 12°C.
A series of experiments was performed over a
range of cooling water flow rates and a range of speeds
of agitation. The rate of heat loss from the reactor to its
surroundings, assuming Newtonian cooling, is:
q,=SX T -- T,)= v,cg
(3)
5.0 -
-7
4.5 -
v, 4.0 -
N
F
x 35-
T
_ 3.O-
5 25-
.?
.
s 2.0-
z
g
1.5 -
2 l.O-
p 0.5 -
b
0
1 I I I 1 I I 1 I
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Sulphuric acid concentration (%1
Figure5 Dependence of pre-exponential factor (A) on sulphuric
acid concentration evaluated using a.r.c. data for the esterifica-
tion reaction
50
J. Los s Prev. Proc ess Ind., 7992, Vol5, No I
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Development of pilot scale facil i ty: T. J. Snee and J. A. Hare
or, by integration:
t/PC
- In ( T - T,) + constant
sx
where
Vpc/Sx =
Newtonian cooling time = tN.
A value for Newtonian cooling time was obtained
as the gradient of the t versus In T - T,) graph. As
would be expected, the experiments showed increased
rates of cooling (i.e. lower Newtonian cooling’ time)
with increasing agitator speed and increasing cooling
water flow rates, see Ta b le 3 .
pilot reactor have been calculated for a range of
sulphuric acid concentrations using thermokinetic para-
meters from Equation (1) and Newtonian cooling times
from Ta b le 3 . Th e results are plotted in Figure 6. The
results suggest that with water circulating through the
reactor jacket at 12°C (mains water temperature) a
runaway exothermic reaction will occur for acid con-
centrations in excess of 0.65%.
Detailed analysis of the cooling curves was not
undertaken as part of the present work. The main
objective of the experiments was to determine the
cooling characteristics of the vessel with sufficient
accuracy to allow approximate prediction of the critical
conditions for the esterification reaction so that the
pilot scale runaway reaction experiments could be
defined.
These calculations were used to specify a series of
pilot scale experiments designed to investigate the
exothermic reaction under subcritical and supercritical
conditions.
Pilot scale experiments
The main objective of the first pilot scale experiments
was to record temperature-time histories for the
reactor contents and jacket during exothermic reaction
under subcritical and supercritical conditions so that
the results could be compared with detailed theoretical
models of various degrees of complexity and sophistica-
tion. The experiments would provide some indication
of the validity of the Semenov model as applied to the
pilot scale reaction and test the interpretation of the
thermoanalytical data by determining, at a fixed jacket
temperature, the minimum sulphuric acid concentra-
tion which could lead to exothermic runaway. Experi-
mental determination of the conditions which could
lead to runaway would facilitate the design of future
experiments on venting phenomena at varying degrees
of supercriticality.
‘Estimate of critical conditions
A range of theoretical models have been developed for
predicting the conditions that can lead to a runaway
exothermic reactionle4. Relatively simple treatments,
such as those of Semenov’ and Frank Kemenetskii3 can
be used to predict the critical ambient temperature that
can lead to exothermic runaway, but detailed analysis
of the temperature, time and concentration dependen-
cies usually requires sophisticated computer models.
The conditions in a well stirred, jacketed, batch
reactor correspond closely to the Semenov model in
which reactant temperature is assumed to be uniform
with resistance to heat flow only at the interface
between the reactants and the surroundings. This
model assumes an Arrhenius type temperature depend-
ence for the rate of heat generation, and the rate of
heat loss is assumed to be Newtonian. Under these
conditions, the critical jacket temperature can be
calculated using the expression:
1
-=*(&)exp(g)
e
I
(5)
The Semenov criterion assumes that the critical state,
when the rate of temperature rise begins to increase, is
reached before reactant depletion causes significant
reduction in rate of heat generation. This assumption is
valid for reactions of at least moderate exothermicity.
Critical jacket temperature for the reaction be-
tween propionic anhydride and butan-2-01 in the 250 1
Table3 Newtonian cooling times for 200 I of water (heated
electrically in the reactor to 50°C) at various speeds of agitation
and flow rates of cooling water (at 12 “C)
Cooling water
flow rate
(I min-‘)
Stirring rate
(rev min-I)
Newtonian cooling
time
W
10
69
5630
10
139
20
69 2:
20
139
4465
Pilot scale experiments were performed using
equimolar mixtures of propionic anhydride and butan-
2-01 with sulphuric acid in concentrations of 0.1 ,
0.4% and 0.8% in a batch size of approximately 200 1.
Experimental conditions and critical temperatures for
these concentrations (calculated using the Semenov
model and heat transfer data from the experiments
using water) are summarized in Ta b le 4 . Butan-2-01,
containing the appropriate concentration of sulphuric
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Sulphuric acid concentration (%)
Figure6 The dependence of critical jacket temperature (Seme-
nov criteria) on sulphuric acid concentration calculated using
extrapolated a.r.c. data and empirically determined Newtonian
cooling constant for 250 I reactor
J . Loss Prev. Process Ind., 7992, Vol5, No 1
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Development of pilot scale facil i ty: T. J. Snee and J. A. Hare
Table 4 Critical reactor jacket temperatures (Semenov criterion)
calculated for the reaction between propionic anhydride and
butan-2-01 using a.r.c. kinetic data, with the Newtonian cooling
times evaluated from: (i) cooling rates measured with water in
the reactor; (ii) cooling rate of the contents of the reactor after a
runaway esterification reaction
Critical reactor jacket temperature
Sulphuric acid Heat transfer data Heat transfer data
concentration for water
for reaction products
(%)
W (“C))
(K W))
0.1
298.7 (25.5)
296.8 (23.7)
0.4
288.5 (15.4)
286.7 (13.8)
0.8 284.5 (11. 4)
282.8 (9.7)
acid, was added to the reactor and allowed to reach
thermal equilibrium with the reactor jacket. Propionic
anhydride was pumped into one of the feed vessels and
allowed to reach thermal equilibrium before being
added to the reactor. Ambient temperature and the
temperature of the water circulating through the
reactor jacket differed by no more than 2 K. Hence a
similar temperature difference would have existed
between the initial temperatures of the reagents.
The experimental records of temperature of the
reaction mixture and the water temperature at the inlet
and outlet of the reactor jacket are shown in Figure 7.
Figure 7u shows the temperature-time histories
for the mixture containing 0.1% HzSO.,. The initial
part of the experimental record shows a drop in
temperature of approximately 10 K due to endothermic
mixing of the two reagents. The temperature at the
outlet to the reactor jacket becomes less than that at the
inlet as the temperature of the contents of the reactor is
gradually returned to that of the cooling water. The
temperature in the reactor then remains constant, and
equal to that of the water circulating through the
jacket, over a period of more than 12 h. Gas chromato-
graphic analysis of a sample taken from the reactor
after 12 h, showed the presence of butyl-2-propionate
and propionic acid, indicating that the reaction had
proceeded isothermally to completion.
Temperature records for the mixture containing
0.4% HZS04 are shown in Figure 7b. In this case,
endothermic mixing is followed by self-heating due to
exothermic reaction. The temperature at the outlet to
the jacket is initially less than the inlet temperature and
then begins to exceed the inlet temperature as the
direction of heat transfer is reversed. A maximum
temperature excess of 12.4 K between reactor contents
and the jacket is recorded after approximately 9 h. This
represents a substantial proportion of the maximum
subcritical temperature excess predicted by Semenov,
suggesting that at this acid concentration the reaction
was close to runaway conditions.
A runaway exothermic reaction was observed with
the mixture containing 0.8% H2S04 as seen in Figure
7~. Very rapid rates of temperature rise were observed
some 3 h after the reagents were mixed. At this point, a
difference of 20 K was recorded between the tem-
52 J. Los s Prev. Proc ess Ind., 1992, Vol5, No 1
a
90
80 - b
-
E 600 -
$ 50 -
2 40-
0 10 20 30 40 50 60
I
--
80
-
E 70- I
C
E 60 -
$ 50 -
$ 40 -
5
I-
30-
20-
10* ’
C
OO 10 20 30 40 50 60
Time (s x 103)
Figure7 Temperature records during the reaction between
propionic anhydride and butan-2-01 in the 250 I reactor, cata-
lysed by various concentrations of sulphuric acid: a, 0.1%; b,
0.4%; c, 0.8%. Curve A. reactorcontents; curve 0. cooling water
out; curve C, cooling water in
perature at the inlet and the outlet to the reactor jacket,
indicating very substantial rates of heat transfer
(> 6 kW), but this is insufficient to prevent an acccler-
ating rate of temperature rise for the reactor contents,
which reached a maximum temperature of 335 K.
Discussion
Detailed interpretation of the temperature-time his-
tories by integration of the equations governing reac-
tion kinetics and the rate of heat transfer will be
reported later. The present discussion will be restricted
to a qualitative appreciation of some of the features of
exothermic runaway, and simple analytical interpreta-
tion using classical thermal explosion theory. Some
shortcomings of this simple interpretation are identified
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Development of pilot s cale facil i ty: T. J. Snee and J. A. Hare
and the direction of the future experimental pro-
gramme is outlined.
Semenov theory provides a prediction of the
critical ambient temperature beyond which it becomes
impossible for stationary states to exist , i.e. states in
which the rate of heat generation is balanced by the
rate of heat loss to the surroundings. The Semenov
model does not take account of, and cannot be used to
describe, the following features of the experimental
results.
The time dependence of the rate of heat generation
and, in particular, the effect of reactant consumption
on reaction rate.
Temporal variation in the effective ambient tem-
perature, as evidenced by the increase in the jacket
outlet temperature as the rate of self-heating in-
creases.
Variations in physical properties such as heat capa-
city, density and viscosity of the contents of the
reactor as the reaction proceeds, and the effect of
these on the agitation and heat transfer.
Physical processes, such as endothermic mixing or
heat loss by evaporation, occurring at the same time
as exothermic reaction.
Despite these shortcomings the predicted critical
temperatures listed in Tab l e 4 are in reasonable accord
with the experimental results. The critical temperature
calculated for the mixture containing 0.8% acid indic-
ates that, at a jacket temperature of 285 K, conditions
should be marginally supercritical. A runaway exo-
thermic reaction was observed for this composition, but
the absence of any pronounced inflexion in the tem-
perature record during the induction period suggests
that conditions were significantly supercritical. A small
error in the determination of the kinetic parameters for
the reaction from a.r.c. data would readily account for
this discrepancy. An additional source of error occurs
in the assessment of the heat transfer characteristics of
the reactor using water. Standard correlations can be
used to calculate, from the results for water, coeffici-
ents for heat transfer between an organic liquid in the
reactor and a water-cooled jacket. Alternatively, an
overall Newtonian cooling constant can be determined
from the post-reaction cooling curve. A cooling con-
stant determined in this way would be constrained to
take partial account of changes in the average jacket
temperature as the temperature of the contents of the
reactor increases. Revised values for the critical tem-
peratures calculated on this basis are listed in Tab l e 4
Th ese values suggest that, under the experimental
conditions, the mixture containing 0.8% acid would
have been more supercritical than previously calcul-
ated.
Some areas where improvements can be made to
the interpretation of the experimental results have been
identified in the above discussion, particularly in the
following areas:
improvements in the quality and interpretation of
the thermoanalytical data, and correlation with
chemical analysis, should provide a more complete
understanding of the temperature and concentration
dependence of the rate of heat generation;
the application of chemical engineering methods to
provide a detailed description of the complex heat
transfer in the agitated, jacketed reactor;
refinement of the theoretical model for predicting
criticality and application of numerical methods so as
to predict temperature-time histories under subcrit-
ical and supercritical conditions.
Improvements in the interpretation and theoretical
description of runaway reaction phenomena in a batch
reactor can be compared with the existing data, but
more experimental results are required in order that
the validity of these refinements can be properly tested.
Future work
The experiments reported here have demonstrated that
the pilot scale facility, based around the 250 1 glass-
lined reactor, can be used to investigate the salient
features of exothermic batch reaction proceeding under
subcritical and supercritical conditions. It has been
shown that the degree of supercriticality can be
predicted and controlled. The initial experiments have
established the feasibility for future study of the
thermal stability of exothermic reactions and the
performance of emergency pressure relief systems. The
following provisional experimental programme is envis-
aged:
determination of the minimum sulphuric acid con-
centration that can lead to runaway exothermic
reaction with cooling water supplied to the reactor
jacket at a fixed temperature, and investigation of
the effect of cooling water flow rate, agitator speed
and batch size;
design and installation of a heating system for the
reactor so that the experiments defined above can be
repeated at a series of jacket temperatures;
at elevated jacket temperatures, criticality will occur
for the less reactive concentrations and exothermic
runaway will lead to higher temperatures than those
reported here. As a result, substantial vapour
pressures will be developed in the reactor and this
will allow a study of venting phenomena and the
performance of emergency pressure relief systems at
various degrees of supercriticality;
the lower molecular weight homologues of propionic
anhydride and butan-2-01 are both more reactive and
produce higher vapour pressures. Investigation of
the reaction of these materials will allow the study of
exothermic runaway and emergency pressure relief
under progressively more stringent conditions.
Depending on the resources available, the pilot scale
facility may be used in the future for more extensive
studies including: (a) substances and chemical systems
where there has been a history of incidents involving
J. Los s Prev. Proc ess Ind., 7992, Vol
5,
No
1
53
7/23/2019 Development and Application of a Pilot Scale Facility for Studing Runaway Exothermic Reaction
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Development of pilot scale facil i ty: T. J. Snee and J. A.
Hare
runaway reaction; (b) investigation of distillation reac-
tions or reactions under reflux; (c) comparison of the
critical conditions and safety criteria for batch and
semibatch reactions” ; (d) investigation of the design
parameters for catch tanks and other methods of
containing or treating relieved fluids13.
References
1 Gray, P. and Lee, P. R. ‘Thermal Explosion Theory’, Oxidation
and Combustion Reviews, Vol. 2,.Elsevier, Amsterdam, 1967
2 Semenov, N. N. Z. Physik 1928,4&571
3 Frank Kamenetskii, D.A. Zuhr. Fiz. Khi m. 1939 13 738
4 Thomas, P. H. Trans. Far aday Sm. 1958 54 60
5 Rogers, R. N. and Smith, L .C. Thermochim. Acra 1970 1 1
6 Townsend, D. I. andTou, .I. C. Thermochim. Acta 1980 37 1
7 Gygax, R. in Proceedings of International Symposium on Run-
away Reactions, AIChE, 1989, p. 52
8 AIChE, 19th Annual Loss Prevention Symposium at AIChE 1985
Spring National Meeting, Houston, Texas, March 1985, Sessions
55 and 56
9 Fauske, H. K. and Leung, J. C. C/rem. Eng. Prog. 1985,81,39
10 Singh, J. in Proceedings of International Symposium on Runaway
Reactions, AIChE, 1989, p. 313
11 American Society for Testing Materials, ‘Arrhenius kinetic
constants for thermally unstable materials’, ASTM E698-79,
Committee E-27, SCE27.02,1979
12 Hugo, P. German Chem. Eng. 1981,4,161
13 Grossel, S. S. Plant Oper. Prog. 1986,5 (3), 129
Nomenclature
A
c
E
e
Q
41
4r
R
s
T
T
T
tN
V
x
P
X
Pre-exponential factor (frequency factor) (s-l)
Reaction mixture specific heat capacity (_I kg-1 K-l)
Activation energy (J mol-i)
Exponential constant
Reaction exothermicity (J kg-l)
Rate of heat loss from reactor (W)
Rate of heat generation by reaction (W)
Universal gas constant (J mol-i K-t)
Surface area (m2)
Reactant temperature (K)
Ambient temperature (K)
Critical ambient temperature (K)
Time (s)
Newtonian cooling time (s)
Reaction mixture volume (ms)
Sulphuric acid concentration ( by mass)
Reaction mixture density (kg mm3)
Heat transfer coefficient (W m-* K-t)
54 J. Los s Prev. Proc ess Ind., 7992, \/o/5, No 1