Reactive hazards from mixtures

Acetyl chloride CH3COCI Colourless, fuming, corrosive liquid Flash point 4°C When heated, emits phosgene Decomposes violently with water to produce heat and toxic fumes HCI 

Aluminium chloride (anhydrous) AICI3 Orange, yellow, grey or white powder which is a severe respiratory irritant and can cause skin/eye burns Reacts with air moisture to form corrosive HCI gas Violent reaction when a stream of water hits a large amount Do not use water in vicinity 

Benzoyl chloride CgHsCOCI Colourless, fuming, corrosive liquid with a strong odour Combustible flash point 72°C Generates phosgene gas when heated Reacts strongly with water or water vapour, producing heat and toxic/corrosive fumes Use of water must be considered carefully 

Calcium hypochlorite Ca(CIO)2 Water soluble white crystals or powder with strong chlorine odour Non-flammable but can evolve CI2 and O2 May undergo decomposition Water spray may be used but evolves CI2 gas freely at ordinary temperatures with moisture 

Sulphur monochloride S2CI2 Yellowish-red oily fuming liquid with a strong odour Combustible flash point 11 8°C Ignition temp. 233°C Liquid and vapours are irritating Decomposes when contacted by water, to produce heat and toxic/corrosive fumes Do not allow water to enter containers reaction can be violent Wash down spills with flooding amounts of water 

Many ehemieals are ineompatible beeause a violent reaetion may oeeur on mixing. This ean, in some eonditions, result in an explosion. Refer to Table 7.5. 

An appraisal is needed of all ehemieals whieh may be present, even if unintentionally (e.g. as intermediates, byproduets or wastes) and how they ean reaet under the most extreme eonditions 

Antimony hydride (Stibine) (SbH3) Rapid Very slow 

Arsenic hydride (Arsine) (ASH3) Moderate Very slow 

Calcium hydride (CaH2) Moderately fast Rapid 

In acid-base reactions, the heat of neutralization of aqueous acids and bases can be sufficient to cause spitting from containers when the concentrated reagents interact. This is also encountered when concentrated sulphuric acid is diluted (refer to Table 6.1) the acid should always be added cautiously to water and not vice versa. Eye protection is obligatory when using such reagents. 

---

Many of the hazards from the polymer industry arise from the monomers used as raw materials. Many monomers are reactive and flammable, with a tendency to form explosive vapor mixtures with air. All have a certain degree of toxicity vinyl chloride is a known human carcinogen. The combustion of many polymers may result in the evolution of toxic gases, such as hydrogen cyanide... 

In addition to the hazards due to the toxic effects of chemicals, hazards due to flammability, explosibility, and reactivity need to be considered in risk assessment. These hazards are described in detail in the following sections. Further information can be found in Bretherick s Handbook of Reactive Chemical Hazards (Bretherick, 1990), an extensive compendium that is the basis for the lists of incompatible chemicals included in various reference works. Bretherick describes computational protocols that consider thermodynamic and kinetic parameters of a system to arrive at quantitative measures such as the Reaction Hazard Index (RHI). So-called "reactive" hazards arise when the release of energy from a chemical reaction occurs in quantities or at rates too great for the energy to be absorbed by the immediate environment of the reacting system, and material damage results. In addition, the "Letters to the Editor" column of Chemical Engineering News routinely reports incidents with explosive reaction mixtures or conditions. 

Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson s method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997) describe the use of CART values in thermal hazard analysis. 

The extreme hazards involved in handling this highly reactive material are stressed. Freshly distilled material rapidly polymerises at ambient temperature to produce a gel and then a hard resin. These products can neither be distilled nor manipulated without explosions ranging from rapid decomposition to violent detonation. The hydrocarbon should be stored in the mixture with catalyst used to prepare it, and distilled out as required [1], The dangerously explosive gel is a peroxidic species not formed in absence of air, when some l,2-di(3-buten-l-ynyl)cyclobutane is produced by polymerisation [2], The dienyne reacts readily with atmospheric oxygen, forming an explosively unstable polymeric peroxide. Equipment used with it should be rinsed with a dilute solution of a polymerisation inhibitor to prevent formation of unstable residual films. Adequate shielding of operations is essential [3],... 

The material factor MF for the process unit is taken of the most hazardous substance present, which lead to the analysis of the worst case that could actually occur. MF is a value, which denotes the intensity of energy release from the most hazardous material or mixture of materials present in significant quantity in the process. MF is obtained from the flammability and reactivity of the substances. The process is divided into units. The material factor is calculated for each unit separately. Dow (1987) has listed a number of chemical compounds and materials with their MF s. 

There are four lists of hazardous wastes in the regulations wastes from nonspecific sources (F list), wastes from specific sources (K list), acutely toxic wastes (P list), and toxic wastes (U list) there are also the four characteristics mentioned before ignitability, corrosivity, reactivity, and extraction procedure toxicity. Certain waste materials are excluded from regulation under the RCRA. The various definitions and situations that allow waste to be exempted can be confusing and difficult to interpret. One such case is the interpretation of the mixture and derived-from rules. According to the mixture rule, mixtures of solid waste and listed hazardous wastes are, by definition, considered hazardous. Similarly, the derived-from rule defines solid waste resulting from the management of hazardous waste to be hazardous (40 CFR 261.3a and 40 CFR 261.1c). 

---