Preventing Runaway Reactions Department Editor: Rebekkah Marshall Chemical Engineering®

General considerations⁽¹⁾

A process is considered to be thermally safe only if the reactions can easily be controlled, and if the raw material, the products, the intermediates and the reaction masses are thermally stable under the considered process conditions. Check into the process equipment, its design, its sequence of operation and the control strategies. In addition to the engineering aspects, get detailed information on thermodynamic and kinetic properties of the substances involved, such as the reaction rates or heat-release rates as a function of process conditions. Determine the physical and chemical properties, as well.

Understanding of thermal-hazard potential requires knowledge of various skills and disciplines⁽³⁾. These include:

Operating mode: the mode of operation is an important factor. For instance, a batch reaction, where all the reactants are charged initially, is more difficult to control than a semi-batch operation in which one of the reactants is charged progressively as the reaction proceeds (for more, see Design Options).

Engineering: design and layout of the plant and equipment and its built-in controls impact the entire process. The capacity of the heating or cooling system is important in this context. Process engineering is used to understand the control of the chemical processes on a plant scale. It determines which equipment should be used and how the chemical processes should be performed. In addition, take into account technical failure of equipment, human errors (deviations from operating instructions), unclear operating instructions, interruption of energy supply, and external influences, such as frost or rain (for more, see Design Options).

Chemistry: the nature of the process and the behavior of products must be known, not only under reaction conditions, but also in case of unexpected deviations (for example, side reactions, instability of intermediates). Chemistry is used to gain information regarding the reaction pathways that the materials in question follow.

Physical chemistry and reaction kinetics: the thermophysical properties of the reaction masses and the kinetics of the chemical reaction are of primary importance.

Physical chemistry is used to describe the reaction pathways quantitatively.

Data collection

The following data are especially relevant in avoiding runaway reactions:

• Physical and chemical properties, ignition and burning behavior, electrostatic properties, explosion behavior and properties, and drying, milling, and toxicological properties
• Interactions among the chemicals
• Interactions between the chemicals and the materials of construction
• Thermal data for reactions and decomposition reactions
• Cooling-failure scenarios

Design Options⁽²⁾

If a reaction is has the potential for runaway, the following design changes should be considered:

• Batch to continuous. Batch reactors require a larger inventory of reactants than continuous reactors do, so the potential for runaway in continuous systems is less by comparison

• Batch to semi-batch. In a semi-batch reaction, one or more of the reactants is added over a period of time. Therefore, in the event of a temperature or pressure excursion, the feed can be switched off, thereby minimizing the chemical energy stored up for a subsequent exothermic release

• Continuous, well-mixed reactors to plug flow designs. Plug-flow reactors require comparatively smaller volumes and therefore smaller (less dangerous) inventories for the same conversion

• Reduction of reaction inventory via increased temperature or pressure, changing catalyst or better mixing. A very small reactor operating at a high temperature and pressure may be inherently safer than one operating as less extreme conditions because it contains a much lower inventory⁽³⁾. Note that while extreme conditions often result in improved reaction rates, they also present their own safety challenges. Meanwhile, a compromise solution employing moderate pressure and temperature and medium inventory may combine the worst features of the extremes⁽³⁾.

• Less-hazardous solvent

• Externally heated or cooled to internally heated or cooled

Thermal Stability Criteria ^{(1, 4)}

As a guideline, three levels are sufficient to characterize the severity and probability of a runaway reaction, as shown in the Table.

Adiabatic temperature rise

The adiabatic temperature rise is calculated by dividing the energy of reaction by the specific heat capacity as shown in Equation(a).

ΔT_ad = 1,000Qr/Cp ............(a)

where:
ΔTad = adiabatic temperature rise, K
Qr = energy of reaction, kJ/kg
Cp = heat capacity, J/(kg)(K)

Time to maximum rate (TMR)

TMRad (the time to maximum rate, adiabatic) is a semiquantitative indicator of the probability
of a runaway reaction. Equation (b), defining TMRad in hours, is derived for zero-order
reaction kinetics:

TMR_ad = CpRTo2/3,600qoEa .............(b)

where:
R = gas constant, 8.314 J/molK
To = absolute initial temperature, K
qo = specific heat output at To, W/kg
Ea = activation energy, J/mol

The TMR value provides operating personnel with a measure of response time. Knowledge of the TMR allows decisions to be based on an understanding of the time-frame available for corrective measures in case heat transfer is lost during processing.

References
(1). Venugopal, Bob, Avoiding Runaway Reactions, Chem. Eng., June 2002, pp. 54–58.
(2). Smith, Robin, ”Chemical Process Design”, McGraw-Hill, New York, 1995.
(3). Kletz, T. A., “Cheaper, Safer Plants” IChemE Hazard Workshop, 2d., IChemE, Rugby, U.K., 1984.
(4). Gygax, R., Reaction Engineering Safety, Chem. Eng. Sci., 43, 8, pp. 1759–71, August 1998.