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.



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



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



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



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



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

51



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



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



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

Documents

Development and Application of a Pilot Scale Facility for Studing Runaway Exothermic Reaction