runaway hazardous assesment by reaction calorimeter
DESCRIPTION
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).TRANSCRIPT
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.