Hazard rating chemical reactions

EXPLOSION and FIRE CONCERNS moderate fire hazard in form of dust and powder, when exposed to flame or by spontaneous chemical reaction NFPA rating Health 4, Flammability 1, Reactivity 1 slight explosion hazard in the form of powder or dust mixtures of the powder with carbon tetrachloride or trichloroethylene will flash or spark on impact reacts incandescently with fluorine or chlorine decomposition emits very toxic fumes of Beryllium oxide incompatible with acids, caustics, strong oxidizers, chlorinated hydrocarbons, and molten lithium use dry sand, dry clay, dry ground limestone, or other methods for firefighting purposes. 

EXPLOSION and FIRE CONCERNS not combustible flammable by chemical reaction NFPA rating Health 3, Flammability 0, Reactivity 2 contact with water will cause violent spattering and formation of toxic hydrogen chloride gas and phosphoric acid mist elevated temperatures may cause containers to burst pressure will develop in closed containers when exposed to moisture reacts explosively with chlorine dioxide and chlorine, sodium, and urea and heat ignites on contact with fluorine forms explosive products with carbamates and 3 -methyl-2-nitrobenzanilide reacts violently with water, acids, alkalies, alkali metals, alcohols, amines and organic acids incompatible with aluminum chlorine dioxide, chlorine, diphosphorus trioxide, fluorine, hydroxylamine, magnesium oxide, nitrobenzene, sodium, urea, and water hazardous... 

Chemical reaction hazards are associated with thermal runaway, which begins when the heat generated by a reaction exceeds the heat which can be removed to the surroundings. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase and in turn accelerates the rate of heat production. Thermal runaway occurs because, as the temperature rises, the rate of heat loss to the surroundings increases linearly with temperature, but the rate of reaction — and so the rate of heat generation — increases exponentially. Therefore, thermal runaway can start slowly but then accelerate, until eventually it can lead to an explosion. 

Chapter 3 describes how calculations and calorimetry provide the raw data — the temperatures, rate and quantity of heat and gas evolution — from which chemical reaction hazards can be assessed. This chapter describes how to interpret these raw data for application to plant and operating procedures. 

Materials which (in themselves) are readily capable of detonation or of explosive decomposition or explosive reaction at normal temperatures and pressures. Includes materials which are sensitive to mechanical or loceilized thermal shock. If a chemical with this hazard rating is in an advanced or massive fire, the area should be evacuated. 

Chemical processing under "extreme conditions" of high temperatures and pressures requires more tliorough analysis and extra safeguards. As discussed in Chapter 7, e.xplosions at liigher initial temperatures and pressures are much more severe. Therefore, chemical processes under extreme conditions require specialized equipment design and fabrication. Otlier factors tlrat should be considered when evaluating a chemical process are rate and order of the reaction, stability of the reaction, and tlie healtli hazards of the raw materials used. 

Figure 5.4-57. Programmable DTA plot.s for variou.s heating rates 4-nitrobenzoic acid (reprinted from Grewer (1994), Thermal hazards of chemical reactions , Copyright (1994) with permission from Elsevier Science). 

The rate of an exothermic chemical reaction determines the rate of energy release, so factors which affect reaction kinetics are important in relation to possible reaction hazards. The effects of proportions and concentrations of reactants upon reaction rate are governed by the Law of Mass Action, and there are many examples where changes in proportion and/or concentration of reagents have transformed an... 

D. R. Stull11 developed a rating system to establish the relative potential hazards of specific chemicals the rating is called the reaction hazard index (RHI). The RHI is related to the maximum adiabatic temperature reached by the products of a decomposition reaction. It is defined as... 

Hazardous chemical reactivity is any chemical reaction with the potential to exhibit rates of increase in temperature and/or pressure too high to be absorbed by the environment surrounding the system. Included are both reactive materials (those which enter into a chemical reaction with other stable or unstable materials) arid unstable materials (those which in a pure state or as normally produced decompose or undergo violent changes). 

Understand the rate of aU chemical reactions. Thermal hazard calorimetry testing can provide useful kinetic data. 

There are chemical reactions between the released contaminant and ambient air or surfaces. If the released contaminant reacts, any reacted material can no longer be considered airborne (although the reaction products may also be hazardous), and so chemical reactions effectively reduce the rate or amount of airborne contaminant. Some reactions can be characterized as dry or wet deposition. 

When the NFPA diamond is used for container or vessel labeling, and the white (bottom) quadrant contains the W symbol, the material will react violently or explosively with water, and a chemical reactivity hazard obviously exists. However, if the W symbol is not present, the material may still be water reactive, but at a slower rate, since the pur-pose of the NFPA symbol is to alert emergency responders to significant, immediate water reactivity n. hazards. Water reactivity is often very rapid, but can j also be slow. The reaction may generate sufficient gas Twy to rupture a closed container or vessel. The reaction of f an organic material with water may be delayed due to reaction only occurring at the interface. 

Physical or chemical processes involving chemical reactivity hazards require carefully determined, facility-specific operating limits, which may go well beyond temperature control. Limits may need to be specified for addition quantities, rates and sequences agitation pH conductivity concentration pressure and other variables that either keep an undesired chemical reaction from starting or control a desired chemical reaction. Determination of these limits is outside the scope of this publication references such as Barton and Rogers (1997), CCPS (1995a) and HSE (2000) can be consulted for further information. 

The potential hazards of such chemical reactions are primarily determined by the quantity of energy or gas that is generated and released and/or by the type and quantity of the materials involved. These hazards are the result of the interaction between the properties of individual components and mixtures, the process and equipment parameters and possible failure values. Since this interaction can be influenced by reaction rates it is necessary to consider the conversion of the reactant(s) over time. 

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