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Evaluate reactivity with limited resources

By W. J. Rogers, C. Wei and M. S. Mannan, Mary Kay O’Connor Process Safety Center

ChemicalProcessing.com

Keywords: reactivity, design and simulation

The initial critical steps of reactivity risk reduction need not be expensive, time consuming and require extensive expertise.

Highly reactive industrial chemicals have made available an enormous variety of useful products that we often take for granted. However, chemical reactivity can present significant hazards under certain conditions, and the control of chemical energy continues to be a significant concern for facilities that process, handle, transport or store chemicals. The potential to inflict harm or damage exists because reaction energy can cause over-pressurization of process equipment and subsequent release of toxic or flammable materials. To control reaction energy and manage reactive hazards and risk, it is necessary to characterize chemical reactivity that can appear under normal and likely upset conditions. Certainly, general hazard-assessment methods can be used, but they usually require extensive resources. This article describes how to adequately evaluate reactivity with limited resources.

The U.S. Chemical Safety and Hazard Investigation Board (CSB) in 2002 released a report on 167 reactive-chemical incidents that occurred in the U.S. between January 1980 and June 2001 [1]. More than 50% of these incidents involved chemicals that are not covered by U.S. Occupational Safety and Health Administration (OSHA) or U.S. Environmental Protection Agency (EPA) regulations. Existing OSHA and EPA regulations are based on lists of individual chemicals that are considered inherently unstable or toxic. However, reactive hazards are associated with an enormous range of chemical reactivity behavior based on wide ranges of process and upset conditions that challenge attempts at regulation.

A complex issue
The intrinsic properties of a chemical are based on the molecular electronic structure as represented by its molecular formula. Although each chemical has unique properties that affect reactive behavior, observed reactivity and, therefore, reactivity hazard is not an inherent property. In fact, the observed reactivity results from the interaction of the chemical in contact with other chemicals and materials at the conditions of the interaction.

Because there are endless combinations of chemicals and conditions, no simple test, indicator or assessment can fully characterize a reactivity hazard. Therefore, lists of reactive chemicals are futile and misleading because they can suggest that an unlisted chemical is never hazardous. A listed reactive chemical can be benign under some conditions, while a normally benign compound might not be, as was the case with water, which in the presence of methyl isocyanate, led to the Bhopal tragedy. So, it is not surprising that a thorough reactive hazard assessment can be expensive, time consuming and require extensive expertise. Because of limited resources, a sufficient reactivity assessment often is not performed to characterize hazards or to determine whether the process must be altered to decrease the risk to acceptable levels. However, the initial steps of a reactivity assessment to identify basic hazards are critically important to reduce risk, and these steps require minimal resources.
Reactivity hazard assessment is best accomplished using a multi-leveled approach [2] that begins with available information about the chemicals, including records of previous incidents, and proceeds with computational and experimental tests (Table 1), which indicates relative experience, time and cost, and gives examples for the various levels.

<Production dept., place table close to here>

Level 1 — prescreening
Level 1 prescreening assembles information needed for a hazard assessment and anticipates potential hazards. Material Safety Data Sheets (MSDSs) are always available but often are incomplete and should be supplemented with other sources to obtain reliable information, such as physical and thermodynamic properties, reactivity, flammability and toxicity, on each chemical. Also important is compatibility information, which is available from a variety of sources; for instance, the U.S. National Oceanic and Atmospheric Administration offers a Chemical Reactivity Worksheet (CRW) program for each chemical pair of  more than 6,000 chemicals. (http://response.restoration.noaa.gov/chemaids/react.html). Many reference books and literature articles provide free of charge MSDSs as well as other Level 1 information. “Bretherick’s Handbook of Reactive Chemical Hazards[3]”, for example, is available in libraries and includes numerous records of incidents that are valuable for anticipating potential reactivity hazards.

Chemical bonds and structures. As part of prescreening, a look at the molecular structure, the fundamental basis for intrinsic properties, is useful for anticipating potential hazards. Some chemical bonds and structures, such as unsaturated bonds (acetylene, butadiene) that are not in benzene rings, are more likely than others to exhibit high reactivity or instability. Other examples include certain compounds containing nitrogen-nitrogen bonds (azo compounds, azides) or oxygen-oxygen bonds (peroxides, peroxyacids) and certain structures containing metals (metal fulminates, arylmetals) or halogens (halamides, halogen azides). Compounds with strained rings, such as ethylene oxide, are less stable and have a greater tendency to react to a more stable state with a release of energy. Extensive examples of bonds and structures are provided in Bretherick’s [3], the Center for Chemical Process Safety (CCPS) Guidelines [4] and Lees’ Loss Prevention [5].

Chemical reaction types. It also is important to gather information on the types of expected reactions and their behavior, which include oxidation, oxidation-reduction, polymerization, combustion, decomposition, rearrangement and autocatalytic [4, 6].