RUNAWAY HAZARDOUS ASSESMENT BY

REACTION CALORIMETER

PREPARED BY: GUIDED BY: BHAVIN RANA Dr. K.S.AGRAWAL

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INTRODUCTION:

Thermal runaway occurs when the total rate of heat generation exceeds the rate of heat removal in a

reactive system.

Consequences of runaway chemical reactions can be devastating.(such as those at BHOPAL)

For addressing this issue requires to obtain process safety data for the Risk assessment.

The ultimate aim of such studies is to specify detailed basis of safety of personnel and plant from the

consequences of a runaway reaction.

Reaction Calorimetry is a scientific tool designed to measure the rate of heat evolution occurring in a

process.

It has been employed in process development, process safety and basic research over the last 30 years.

It allow us to check if a reaction will be controllable at full scale under normal running conditions.

Dr Stephen Rowe, D. A. (2007). chemical reaction hazards & thermally unstable substances. (Chilworth Technology Ltd.) Retrieved from www.chilworth.co.uk

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Reaction Calorimetry & Reaction Calorimeter

Reaction calorimetry is the measurement of energy consumption or evolution during a process (including

phase changes) which permit all pertinent rate processes (e.g., heat and mass transfer) to be measured

and studied.

Implicit to the concept of calorimetry (measurement of heat flow) is that both kinetics and

thermodynamics contribute to the measurement.

Kinetics emerge as rates of heat evolution or absorption, and thermodynamics as the integrated heats

(enthalpies) of reactions/phase changes.

Typical reaction calorimetry systems consist of a jacketed reaction vessel , a feed and often, a condenser

can be added to study the reaction under reflux and special reactors are available for reactions under high

pressure.

Usually consist of a stirred reaction vessel and offer the possibility of carrying out chemical reactions in

batch, semi-batch or – in the continuous mode.

Andreas Zogg, F. S. (2004). Isothermal reaction calorimetry as a tool for kinetic analysis. thermodynamic acta 419, 1-17.

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General application of reaction calorimeter:

Determination of calorimetric data for reactor and process design (e.g. overall heat release or supply

during chemical reactions, heat transfer coefficients, specific heat capacity, adiabatic temperature rise,

reactant accumulation.)

Study of the kinetic behaviour of chemical reactions and of physical changes.

On-line monitoring of heat release rate and other analytical parameters.(study of the influence of

mixing, errors in the charges or recipe, feed rate errors, improper feed sequences and the effect of

impurities temperature or concentration of reactants).

Development and optimization of chemical processes , for instance, an increase of yield, selectivity or

profitability.

Dr Stephen Rowe, D. A. (2007). chemical reaction hazards & thermally unstable substances. (Chilworth Technology Ltd.) Retrieved from www.chilworth.co.uk

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Devices using a jacketed vessel and control of the jacket temperature:

Types of the calorimetery calculations:

a) Heat balance calorimetry :Here, the reactor temperature is controlled by changing the

inlet temperature of the circulating fluid in the jacket.

The heat release rate due to chemical reactions, in the reactor can be calculated a

follows:

b) Heat flow (or heat flux) calorimetry :Here, the temperature of the jacket is kept essentially constant

at any time by ensuring a high mass flow rate of the fluid in the jacket.

R., D. N. (n.d.). THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS. Europe: The HarsNet Thematic network.

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c) Adiabatic calorimetry : Here, the jacket temperature is set close to the reactor temperature.

Extreme caution is advised when carrying out pseudo adiabatic experiments because of Violent

runaway reactions can occurred.

d) Temperature oscillation calorimetry : The principle of temperature oscillation calorimetry is based on

an oscillatory response that is evaluated during a reaction in the calorimeter.

It allows the on-line evaluation of the change in the heat transfer coefficient and the study of mixing in

high viscosity systems.

R., D. N. (n.d.). THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS. Europe: The HarsNet Thematic network.

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Devices using a constant surrounding temperature :

a) Isoperibolic calorimetry : This requires a constant surrounding temperature.

Maximum temperature difference between the reactor and the surroundings is limited to few Kelvin.

Therefore, mass and heat balances can be simplified and solved as independent equations.

b) Power compensation calorimetry : the reactor is maintained at constant temperature & the cooling jacket is

also held isothermally some 10 to 20 oC below the temperature of the reactor.

The power supplied to an electrical heating element is continuously manipulated to maintain

the reactor at the desired temperature.

It has much faster response times so power compensation can give excellent temperature control,

particularly for reactions with fast kinetics.

But it may not be suitable for use with high viscosity systems or those where fouling of the electrical

heater

occurs.R., D. N. (n.d.). THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS. Europe: The HarsNet Thematic network.

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Thermal Hazard evaluation of Esterification of Methanol by RC:

The exothermic esterification reaction of acetic anhydride and methanol is:

(CH3CO)2O + CH3OH = CH3COOCH3 + CH3COOH

Reaction calorimeter: 2 dm3 glass reactor (RC1) comprises of a computer-controlled jacketed reactor

with an agitator, electrical immersion calibration heater, thermocouple, and controlled system. Silicone oil

is circulated through the jacket of the vessel.

Different quantities of methanol (16 g and 32 g) were added to the reactor containing a large quantity (756

g) of acetic anhydride. Isothermal testing was carried out at 30, 40, 50, 60, and 70°C by 0.47°C min-1

temperature scanning rate

Reaction heat can be calculated by the following equation:

The adiabatic temperature rise (∆Tad) can be calculated using the following equation provided that the

heat capacity (Cp) and reaction mass (m) of the reaction solution are known:

=

Yih-Shing Duh, C.-C. H.-S. (1996). Applications of reaction calorimetry in reaction kinetics & thermal hazard evaluation. thermochimica, 67-69.

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Results and discussion:

The adiabatic temperature rise (∆Tad ) was 28°C and the heat of reaction was calculated to be 70 kJ mol-1.

The critical heat-transfer parameter, (US/V), can be calculated from the following equation

with known values for the kinetic parameters and reaction enthalpy:

(U S /V)cr = exp{ ∆Hr ρ Ea A [exp (-Ea /R Ta) ]/ RTa2 }

Yih-Shing Duh, C.-C. H.-S. (1996). Applications of reaction calorimetry in reaction kinetics & thermal hazard evaluation. thermochimica, 67-69.

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Safe (thermal stability) and unsafe (thermal instability) regions can be determined in by the area either to

the left or right of the following critical heat transfer parameter curve.

The critical temperature (Tcr) was determined as shown in fig where Tcr is defined by the tangential contact of

the heat generation and heat removal curves.

Temperatures higher than Tcr cause an increase of temperature and an accelerating runaway will eventually

occur.

Yih-Shing Duh, C.-C. H.-S. (1996). Applications of reaction calorimetry in reaction kinetics & thermal hazard evaluation. thermochimica, 67-69.

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Advantages and disadvantages:

Calorimeter type and operation mode

Advantages Disadvantages

Heat balance calorimeter Isothermal TR = const. TJ = f(t)

Useful if the overall H.T.C. and the exchange area change strongly and non- linearly during the time of reaction

• Temperature difference in the jacket must be measured with high resolution

• Requires a feedback temperature control system

• Specific heat capacity of the reaction mixture must be known

Heat flow calorimeter Isothermal TR = const. TJ = f(t)

Useful if the overall H.T.C. and the exchange area doesn’t change strongly during the time of reaction

• Overall H.T.C. must be determined by calibration

• Requires a feedback temperature control system

• Specific heat capacity of the reaction mixture must be known

R., D. N. (n.d.). THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS. Europe: The HarsNet Thematic network.

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Calorimeter type and operation mode

Advantages Disadvantages

Power compensation Isothermal TR = const. TJ = const.

• Response time is faster• Experiments are much

quicker than heat flow experiment.

• Reaction power outputs up to 100 Watts per liter can be handled

• A large surface area heater should be used

• Fouling on the surface of the heater may occur

Isoperibolic TR = f(t) & TJ = const.

• Low cost equipment • Similar to an isothermal

calorimeter if H.T.C. is high enough

• The temperature in the reactor depends on the heat release rate

• No emergency cooling in the case of a “runaway” in the lab

Adiabatic TR = TJ very small heat losses

• Temperature is proportional to conversion

• Relatively small apparatus • Fast reactions can be studied

• Reaction rate depend on temperature

• Can be hazardous for exothermic reactions. Careful hazard identification and risk assessment required before experiments are started

R., D. N. (n.d.). THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS. Europe: The HarsNet Thematic network.