Level 2 — screening
After Level 1 information is gathered, experimental and computational screening tests should be performed to identify and characterize basic hazards, including self-reactivity or thermal stability, oxidizers, peroxide formation sensitivity, spontaneous combustibility, reactivity with air, shock sensitivity and incompatibility [7]. A useful guide for identifying and managing basic hazards [8] is available free from the OSHA website on reactive chemical hazards (http://www.osha.gov/SLTC/reactivechemicals/). Even though the necessary tests incur more costs than Level 1 information searches, they still only require limited resources as part of hazard assessment to design and implement reliable chemical processes.

Included in a hazard assessment are scenarios or chains of events, developed to determine how the identified hazards can threaten process equipment and personnel. For this analysis, sensitivities of the chemicals to likely contaminants and conditions, as measured in screening tests, together with lessons learned from investigations of previous incidents involving the chemicals are extremely valuable. For each developed scenario, the potential consequence severity and the likelihood of each event are analyzed as needed to manage risk.

Experimental screening. Many calorimetric devices are available for testing reaction behavior at process and likely upset conditions. A Differential Scanning Calorimeter (DSC) is useful for examining a few milligrams of sample over wide ranges of temperature to identify exothermic processes and estimate evolved heat [4]. Other small calorimeters, such as the ARSST [9], test a few grams of sample to estimate reaction behavior, including heat of reaction, and pressure change.

Computational screening. Although accurate measurements of reaction energies or heat are expensive, reaction energy can be calculated by subtracting the heats of formation of the products from those of the reactants. Experimental heats of formation are available for many molecules from the National Institute of Science and Technology (NIST) Webbook (http://webbook.nist.gov). Reaction energy can be used to develop parameters for reactivity prediction methods. An example reactivity parameter is the Computed Adiabatic Reaction Temperature (CART), which is the maximum temperature under adiabatic conditions from the heat of reaction and heat capacity of reaction components [10].

Especially for molecules with unusual functional groups and reactive intermediates, a calculation method is a convenient way to determine heats of formation when experimental values are not available. High-level quantum chemical calculations can rival experimental values in accuracy but can be time consuming and expensive <em dash>— and often are not required. As screening tools, however, empirical and semi-empirical desktop software methods with usually acceptable accuracy are available at low cost.

The CHETAH computer program, which employs the Benson group additive method, is useful for estimating gas-phase heats of formation [11]. MOPAC within the ChemOffice Ultra 2004 programs (http://www.cambridgesoft.com/) also calculates gas-phase heats of formation. MOPAC includes semi-empirical quantum methods, in which many of the electron interaction terms are neglected or replaced by empirical parameters based on measured data. These widely applicable methods are inexpensive and require only about a minute on a personal computer to calculate a heat of formation [7].

Employing a multi-leveled approach to hazard assessment, information gathering and hazard identification is the critical preliminary to hazard characterization. Each step along this assessment route should lower the overall risk. Examples of basic reactive chemical hazards from the CSB 167 incidents [1], summarized below, demonstrate that relatively low-cost Level 1 and 2 methods could have identified, and semi-quantitatively characterized, basic hazards that were involved in each incident.
CSB Incident No. 4, spontaneous combustibility. A fire appeared in a scrubber tank that contained mineral oil and alkyl aluminums, which are pyrophoric and ignite on contact with air or water. Although it could have been determined from a Level 1 search, the propensity of water-contaminated alkyl aluminums to self ignite and be an ignition source for mineral oil was not identified, so personnel had not prepared procedures for safe handling and use of these chemicals.

CSB Incident No. 167, oxidizer. Personnel were unclogging a dust collector that removed combustible dust, including ammonium perchlorate, a powerful oxidizing agent. The personnel were not aware of the dust-explosion initiation potential of ammonium perchlorate, which could have been discovered from a Level 1 search.

CSB Incident No. 11, thermal runaway polymerization, shock sensitivity. An overheated tank containing 80,000 gallons of acrylic acid required extensive spraying with water to prevent explosion. Acrylic acid is chemically stable under normal storage and handling conditions, but is sensitive to peroxide formation and exothermic polymerization in the presence of heat or contaminants that can deactivate peroxide formation inhibitors. Organic peroxides’ sensitivity to heat, shock and friction has resulted in many unexpected explosions. Also, with its relatively high freezing point of 14°C, acrylic acid often partly solidifies, with the peroxide formation inhibitor remaining in the liquid phase. Uninhibited acrylic acid polymerizes exothermically at ambient temperature and may accelerate to an explosive state. This basic information about acrylic acid, organic peroxides and potential runaway behavior could have been discovered from a Level 1 search that included records of previous incidents. Level 2 screening or higher-level tests could characterize the potential hazards and further reduce risk.