Hazards of Temperature

Upon completion of this chapter the student should be able to  

Extremes of either heat or cold can be more than uncomfortable—they can be dangerous. Heat stress, cold stress, and burns are major concerns of in the processing industry. Employees who work outside during the summer on surfaces that heat up and store heat will get very hot. They will be around metal equipment and vessels that are hot and radiate large amounts of heat. The opportunity for heat stress will be present. Also, winter conditions on a processing unit in Colorado or Alaska will present the opportunity for frostbite or hypothermia. The prudent process employee will seek to understand the types of heat and cold stress and how to avoid them. 

Heat is a form of energy indicated by temperature. Temperature extremes affect how well people work and how much work they can do. The human body is always producing heat and must remove excess heat in order to maintain body proper temperature. In the same way that excessive heat can affect process equipment and process safety, heat can also affect the human body and its proper functioning. During the summer, especially in geographic areas subject to hot summers, heat can become a serious safety hazard to a process technician. 

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The wound environment may be optimally maintained with a product that has the preferred performance parameters but, nevertheless, is disrupted during the dressing change. The hazards of temperature change and secondary infection may be accompanied by a secondary trauma caused by the dressing adhering to the wound and, on removal, stripping newly formed tissue. 

Example 9.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazards of operating with a liquid process inventory of 1000 kmol at 100 and 150°C based on the theoretical combustion energy resulting from catastrophic failure of the equipment. The normal boiling point of benzene is 80°C, the latent heat of vaporization is 31,000 kJ kmol the specific heat capacity is 150 kJkmoh °C , and the heat of combustion is 3.2 x 10 kJkmok. ... 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% hydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubiUty of hydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

In the fire codes, the atmospheric boiling point is an important physical property used to classify the degree of hazardousness of a Hquid. If a mixture of Hquids is heated, it starts to bod at some temperature but continues to rise ia temperature over a boiling temperature range. Because the mixture does not have a definite boiling poiat, the NFPA fire codes define a comparable value of boiling poiat for the purposes of classifying Hquids. For petroleum mixture, it is based on the 10% poiat of a distillation performed ia accordance with ASTM D86, Standard Method of Test for Distillation of Petroleum Products. 

Explosibility and Fire Control. As in the case of many other reactive chemicals, the fire and explosion hazards of ethylene oxide are system-dependent. Each system should be evaluated for its particular hazards including start-up, shut-down, and failure modes. Storage of more than a threshold quantity of 5000 lb (- 2300 kg) of the material makes ethylene oxide subject to the provisions of OSHA 29 CER 1910 for "Highly Hazardous Chemicals." Table 15 summarizes relevant fire and explosion data for ethylene oxide, which are at standard temperature and pressure (STP) conditions except where otherwise noted. 

Reactive System Screening Tool (RSST) The RSST is a calorimeter that quickly and safely determines reactive chemical hazards. It approaches the ease of use of the DSC with the accuracy of the VSP. The apparatus measures sample temperature and pressure within a sample containment vessel. Tne RSST determines the potential for runaway reactions and measures the rate of temperature and pressure rise (for gassy reactions) to allow determinations of the energy and gas release rates. This information can be combined with simplified methods to assess reac tor safety system relief vent reqiiire-ments. It is especially useful when there is a need to screen a large number of different chemicals and processes. 

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. 

Reactions in bulk are used commercially but careful control of temperature is required. Polymerisation in a suitable solvent will dilute the concentration of reacting material and this together with the capability for convective movement or stirring of the reactant reduces exotherm problems. There is now, however, the necessity to remove solvent and this leads to problems of solvent recovery. Fire and toxicity hazards may also be increased. 

Hazardous situations can develop with changes in volume witli temperature. Tlie effect of temperature on gases and liquids is mentioned on pages 45 and 65, respectively. 

The DTA or hot-stage microscope can be used under ignition conditions to obtain an ignition temperature. The nature of the decomposition can also be observed at a range of temperatures. Observations such as decomposition with evolution of gases prior to ignition are regarded as potentially hazardous. 

Fire Hazards - Flash Point (deg. F) 113 OC Flammable Limits in Air (%) Not pertinent Fire Extinguishing Agents Water, dry chemical, carbon dioxide Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Not pertinent Behavior in Fire May explode. Bums with accelerating intensity Ignition Temperature (deg. F) Explodes Electrical Hazard Data... 

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