Hazards thermal instability

Mix, K. K., "The Use of Advanced DTA Methods for the Evaluation of Thermal Instability, Hazard Evaluation, and Process Design," in Proceedings Runaway Chemical Reaction Hazards Symposium, IBC, London, England (1987). 

Follow-up Tests. In our follow-up tests we want to better define at what temperature we can detect the thermal instability, and to gain some knowledge about how much of a hazard the instability represents. The majority of this work is done with an instrument called a Sikarex Safety Calorimeter (3,4) xt consists of a sample oven, a control and measurement module, and a recorder. In the sample oven is placed 10-30 g of sample in either an open glass tube, a closed glass tube with a capillary bleed, or a stainless steel autoclave. The control and measurement module controls the oven temperature and measures the sample and oven jacket temperatures. 

Reaction conditions which allowed for the large scale nitration of 5-Chloro-l,3-dimethy1-lH-pyrazole were developed which minimized the hazards generally associated with nitration reactions. Dilution with sulfuric acid decreased the risk of thermal instability. Using ordinary laboratory equipment, the experimental heat of reaction was determined to be -12,5 Kcal/mole. Likewise, the adiabatic temperature rise was found to be about 20°C, An exotherm was found to initiate at 100°C, The thermal stability and shock sensitivity of the product, 5-Chloro-l,3-dimethy1-4-nitro-lH-pyrazole, was investigated using simple tests. 

Atmospheres, various, thermal instability data, 65t Avoidance of hazards in... 

Second, the preparation of new chemicals for new pharmaceutical products, synthetic materials and foods could add to the hazards which workers and customers face. Thermal instability and explosive behaviour can be extremely destructive and costly events. Reaction calorimetry and similar techniques can help to predict the likely behaviour of chemicals when reactions, transport and storage are concerned. Physiological behaviour may vary with the nature and form of a drug, and the nature and interconversion of these forms is often studied by thermal and calorimetric methods. 

Chemical reaction hazards resulting from exothermic chemical reaction or thermal instability of reactants, reaction mixture or product are the prime concern of this guide. Chapter 2 describes a procedure for the assessment of these hazards. 

Small-scale screening tests such as DSC, 10 g tube, lET (Sections 3.3.2, 3.3.3 and 3.3.5, pages 28-30) and so on enable the rapid testing of a large number of samples. The data obtained provide a preliminary indication of potential chemical reaction hazards, both thermal instability and high heats of reaction. However, great care must be taken in interpreting information from such tests. 

Schulz, N., Pilz, V. and Schacke, H.. 1983. Controlling thermal instabilities in chemical process. Loss Prevention and Safety Promotion in the Process litdustries. Volume 3 — Chemical Process Hazards. Symposium Series No. 82. B1 (IChemE, Rugby, UK). 

Chilworth reqitires yon to emoll to gain access to their library. Their library has a large mtmber of technical papers addressing electrostatic technology chemical process evaliration industrial explosion hazards (Dust) thermal instability hazards and other topics. See http //www.chilworth.com/about-us/ for details. 

The feedback effect of heat evolution on the rate of exothermic reactions may cause thermal runaway. This is a major issue in the operation of industrial reactors, as the loss of control of a chemical reactor constitutes a serious hazard everybody has in mind the SEVESO accident or those which occured recently in the Swiss industry. Thermal instability is due to the irreducible coupling between heat accumulation and the quasi-exponential increase of reaction rate as a function of temperature accounted for by Arrhenius equation. This problem can be studied by the methods of non linear dynamics. Here again, characteristic times make it possible to establish simple criteria which give at least an order of magnitude for dangerous and safe ranges of operation. 

No process can be made one hundrend percent safe. However, one can identify hazards and, if the hazard is severe, avoid such a chemical or process, or if the hazard is moderate, take precautions that minimize the risk. Safety hazards can be divided into four categories thermal instability, toxicity, flammability, and explosiveness. Typically the in-house safety laboratory measures the decomposition temperature of all reagents, intermediates, solvents, distillation residues and evaporation residues, and any exotherms associated with the decomposition, so that one stays well below these temperatures in the process. The heat of reactions is measured to ensure adequate cooling capacity of the reactor before scaling up a reaction. This minimizes the risk of runaway reactions. 

Chemical reactivity has many different names, such as reactive materials, runaway reaction hazards, instability, thermal sensitivity, and incompatibility. Flammability, toxicity, and corrosion are also forms of reactivity. Since these topics are addressed elsewhere, our focus here will be on those reactions that fall outside the normal definitions of flammable or toxic and that generally occur far more rapidly than corrosion. 

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