Chemical Reactivity Hazard Evaluation

 

1.0  Scope and Purpose

This document outlines the general approach undertaken to identify, assess and evaluate the chemical reaction hazards associated with chemical processes. It should be noted that this document is not intended to be a standard operating procedure, but an outline of the generalized working principles and documentation of the rationale used.

This guideline is directed to those personnel involved in research & development, process hazards analysis (Process safety lab), kilo lab and commercial plant operations. It is also useful to those involved in process hazard analysis and process safety management.

2.0  Objective

·         increase awareness of potential chemical reaction hazards associated with chemical manufacture in batch and semi-batch processes

·         help in the assessment of risks from chemical reactions, and advise on how to prevent and control these risks

·         provide a systematic approach for the design, operation and control of chemical reactions in batch and semi-batch processes

·         advise on safe management procedures and appropriate precautions to prevent or reduce injuries and damage caused to property or the environment associated with chemical manufacture; and

·         advise on maintenance, training and information needs to prevent and control chemical reaction hazards

3.0    Definition of terms

Activation energy: the constant Ea in the exponential part of the Arrhenius equation associated with the minimum energy difference between the reactants and an activated complex (transition state), which has a structure intermediate to those of the reactants and the products, or with the minimum collision energy between molecules that is required to enable a reaction to take place; it is a constant that defines the effect of temperature on reaction rate.

Adiabatic: a system condition in which no heat is exchanged between the system and its surroundings; in practice, near adiabatic conditions are reached through good insulation.

Adiabatic induction time: the delay time to an event (spontaneous ignition, explosion, etc.) under adiabatic conditions starting at operating conditions.

Adiabatic temperature rise: maximum temperature increase (readily calculated, that can be achieved) which would occur only when the substance or reaction mixture decomposes completely under adiabatic conditions.

Apparent activation energy: the constant Ea that defines the effect of temperature on the global reaction rate.

Autocatalytic reaction: reaction in which the rate is increased by the presence of one or more of its intermediates and/or products.

Batch reactor: reactor in which all reactants and solvents are introduced prior to setting the operating conditions (e.g., temperature and pressure).

Decomposition energy: the maximum amount of energy which can be released upon decomposition.

Decomposition temperature: temperature at which decomposition of a substance occurs in a designated system; it depends not only on the identity of the substance but also on the rate of heat gain or loss in the system.

Deflagration: a release of energy caused by a rapid chemical reaction in which the reaction front propagates by thermal energy transfer at subsonic speed

Detonation: a release of energy caused by an extremely rapid chemical reaction of a substance in which the reaction front propagates by a shock wave at supersonic speed.

Endothermic reaction: a reaction is endothermic if energy is absorbed; the enthalpy change for an endothermic reaction is a positive value.

Enthalpy of reaction: the net difference in the enthalpies of formation of all of the products and the enthalpies of all of the reactants; heat is released if the net difference is negative.

Exothermic reaction: a reaction is exothermic if energy is released; the enthalpy change for an exothermic reaction is a negative value.

Hazard: a chemical or physical condition that has the potential for causing harm or damage to people, property, or the environment.

Isothermal: a system condition in which the temperature remains constant; this implies that heat internally generated or absorbed is quickly compensated for by sufficient heat exchange with the surroundings of the system.

Onset temperature: temperature at which a detectable temperature increase is first observed due to a chemical reaction; it depends entirely on the detection sensitivity of the specific instrument involved; scale-up of onset temperatures and application of rules-of-thumb concerning onset temperatures are subject to many errors.

Phi-factor: a correction factor which is based on the ratio of the total heat capacity of a vessel and contents to the heat capacity of the contents; the Phi-factor approaches one for large vessels.

Quenching: Abruptly stopping a reaction by severe cooling or by catalyst inactivation in a very short time period; used to stop continuing reactions in a process thus preventing further decomposition or runaway.

Rate of reaction: technically, the rate at which conversion of the reactants takes place; the rate of reaction is a function of the concentrations and the reaction rate constant; in practical terms, it is an ambiguous expression that can describe the rate of disappearance of reactants, the rate of production of products, the rate of change of concentration of a component, or the rate of change of mass of a component.

Runaway: a thermally unstable reaction system which shows an accelerating increase of temperature and reaction rate which may result in an explosion.

Time to maximum reaction rate: the measured time to the maximum reaction rate during a runaway or rapid decomposition; the specific result is highly contingent on the test method used.

Venting: an emergency flow of vessel contents out of the vessel thus reducing the pressure and avoiding destruction of the unit from over-pressuring; the vent flow can be single or multiphase, each of which results in different flow and pressure characteristics.

1.0    Introduction

Any chemical process will involve chemicals and its interactions and equipment in which it is processed. This interaction of chemical within equipment possess some degree of hazard which needs to be understood to control or eliminate the risk. Hence it is important to understand what the parameters which determines safe chemical process are and what are chemical reaction hazards.

4.1    Parameters that determines the design of the safe chemical processes

Three parameters that is chemicals (its intrinsic properties), Reactions rates (Kinetics) and Hardware (equipment) determines the design of safe chemical process as depicted below.

                                        Fig 1: Parameters that determine safe chemical processes

·      Potential energy of chemicals involved: Design of a safe process requires an understanding of the inherent energy (exothermic release/endothermic absorption) during chemical reactions. This information can come from the literature, from thermochemical calculations, or from proper use of testing equipment and procedures. The potential pressure that may be developed in the process is also a very important design consideration.

·      Rates of the reaction/ decompositions: this depends upon temperature, pressure and concentration. Rates of reaction during the normal/ abnormal process conditions must be determined to design safer processes.

·      Plant process equipment and design:Any heat that is generated by the reaction must be removed adequately, and any gas production must be managed. The effects and requirements of scale-up (that is, the relation between bench-scale and plant equipment) must be considered.

4.2    Chemical Reaction Hazards

·      Chemical Reaction Hazards are the hazards that result from uncontrolled chemical reactions.

·      An uncontrolled chemical reaction can be defined as one in which the heat generated by the reaction is greater than the heat which can be removed to the surroundings (plant heat transfer systems)

This results in the temperature of the reaction mixture increasing, which results in an increase in the rate of reaction. This in turn leads to a further increase in the rate of heat generation. When the temperature reaches the boiling point (and/or) decomposition temperature, pressure generation rate can exceed the venting capacities of the plant system resulting in an explosion.

·      Runaway reactions can therefore start slowly but accelerate, until finally it can result in an explosion.

Chemical Reaction Hazards are the result of 3 main parameters:

Ø  The thermal instability of starting/raw materials, reaction mixtures and products.

Ø  Rapid exothermic reactions that raise the reaction temperature to decomposition or violent boiling.

Ø  Rapid gas evolution.

Fig 2: Overview of pathway of Chemical Reaction Hazards

Note: It is the pressure that results in a chemical reaction hazard incident and not the heat. However, the uncontrolled heat leads to pressure generation by the above scenarios.