Short Notes #1

What is Runaway Reaction

A Runaway Reaction is any reaction system that displays an acceleration in the reaction rate at conditions so great that it becomes very difficult or impossible to control. The accompanying characteristics of a runaway reaction is that it occurs with a rapid increase of temperature and/or pressure.

Runaway reactions are generally extremely exothermic reactions meaning they give off heat to the surroundings as the reaction progresses. Thus, if the heat removal system is not able to remove as much of the heat produced during the reaction then the temperature of the reaction mixture starts increases. If the rapid temperature and/or pressure increase is not properly controlled then there is serious damage done to the equipment and may lead to loss of lives if there is any form of explosion or leak within the reactor.

Risk Assessment and Evaluation of Hazards

Any chemical reaction that has a chance of having a runaway under certain conditions needs to be studied properly and knowledge of the reaction mechanisms need to be properly researched and the risks assessed in order to design proper control systems to prevent any catastrophe. This can be attained by designing adequate hear removal systems or formulating appropriate operational conditions to avoid such conditions from occurring in the reaction i.e. prevention being better than cure.

The evaluation of any hazard that may occur in a chemical reaction requires adequate knowledge of chemistry and thermodynamics. The amount of reactant, products or intermediate getting decomposed and rate and quantity of gases and heat released during a runaway reaction need to be calculated.
The reaction rate is directly proportional to the volume of a reaction mixture. The rate at which heat is removed decreases with a decrease in the surface area. Thus, an increase in the volume to surface area ratio results in an initial the reaction rate and subsequently increases the difficulty in removing heat from the system. This point is vital when scaling the reaction system.

A full scale test of a runaway reaction requires large sums of money but the safety risks are too high, this means that calculations are done through research from literature, theoretical calculations as well as simulations. Small scale calorimetric studies can be done under controlled conditions to estimate sizes of a full operation. 

A risk assessment involves defining a process and its operating conditions, identifying the risks and assessing the necessary methods needed to deal with the risks. Choosing the best safety and protective measures throughout the operation of the plant is also vital. Safety measures are vital and needed from the planning step.

Safety, Protective and Control measures

The risk assessment helps decide which safety measures are needed. Process control can be used to prevent any hazard from occurring and also installing protective techniques to limit the effects of a runaway reaction.
It is important to try and reduce or eliminate the possibility of a runaway reaction occurring. This can be done by substituting the reactants with safer alternatives, if none are available then very low quantities of the reaction mixture should stay in the reactor through operating using a continuous process instead of a batch process. Semi batch process can be used if a continuous process is not possible as it is easier to control the heat liberated in an exothermic reaction through control of reactant amounts.

Process control procedures should be installed in order to prevent the runaway from occurring by using proper and reliable control systems, actuators, sensors and automatic systems to take actions when they predict the occurrence of such a hazard.

Protective measures are to be used at the time the hazard is occurring. Plant should be designed to contain the pressure generated during the runaway, emergency relief valves should be there to vent the gases produced to a safer place, the reaction should be stopped by either using a reaction inhibitor, quenching by crash cooling or by directly dumping the reaction mixture in to the quenching liquid itself.