Safety runaway reactions

Clearly, the potential hazard from runaway reactions is reduced by reducing the inventory of material in the reactor. Batch operation requires a larger inventory than the corresponding continuous reactor. Thus there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be... 

R. D. Coffee, in H. H. Fawcett and W. S. Wood, Safety and Accident Prevention in Chemical Operations, 2nd ed., Wiley-Interscience, New York, 1982, p. 305 International Symposium on Runaway Reactions, Center for Chemical Process Safety, New York, 1989, pp. 140, 144,177, 234. 

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. 

Low temperatures can cause a phase separation in stabilized solutions in which case one phase can become deficient in stabilizer and subject to runaway reactions. Acrylic acid can crystallize out of stabilized solution, and subsequent thawing of these essentially pure acrylic acid crystals can initiate runaway reactions, often with severe consequences. Thawing of crystallized (frozen) materials needs to be accomplished using established procedures in thaw boxes or similar devices. If established procedures are not available, a safety review needs to be conducted and a procedure developed prior to thawing the material. 

The sudden mixing of large amounts of reactants under heating, instead of cooling, caused a runaway reaction. Once the pressure reached 16 bar pressure safety devices were actuated, the temperature at that point had reached 160°C,... 

In scale-up, runaway exothermic chemical reactions can be prevented by taking appropriate safety measures. The onset or critical temperature for a runaway reaction depends on the rate of heat generation and the rate of cooling, which are closely linked to the dimensions of the vessel. 

The first major objective for the inherent safety review is the development of a good understanding of the hazards involved in the process. Early understanding of these hazards provides time for the development team to implement recommendations of the inherent safety effort. Hazards associated with flammability, pressure, and temperature are relatively easy to identify. Reactive chemistry hazards are not. They are frequently difficult to identify and understand in the lab and pilot plant. Special calorimetry equipment and expertise are often necessary to fully characterize the hazards of runaway reactions and decompositions. Similarly, industrial hygiene and toxicology expertise is desirable to help define and understand health hazards associated with the chemicals employed. 

A chemist may think only of optimizing the route of synthesis to avoid a runaway reaction hazard and not consider the safety and design implications of flammable reaction by-products. 

Rogers, R. L., D. P. Mansfield, Y. Malmen, R. D. Turney, and M. Verwoerd (1995). The INSIDE Project Integrating Inherent Safety in Chemical Process Development and Plant Design. International Symposium on Runaway Reactions and Pressure Relief Design, August 2-4, 1995, Boston, MA, ed. G. A. Melhem and H. G. Fisher, 668-689. New York American Institute of Chemical Engineers. 

The information to be compiled about the chemicals, including process intermediates, needs to be comprehensive enough for an accurate assessment of the fire and explosion characteristics, reactivity hazards, the safety and health hazards to workers, and the corrosion and erosion effects on the process equipment and monitoring tools. Current material safety data sheet (MSDS) information can be used to help meet this requirement but must be supplemented with process chemistry information, including runaway reaction and over-pressure hazards, if applicable. 

The basic requirements of a reactor are 1) fissionable material in a geometry that inhibits the escape of neutrons, 2) a high likelihood that neutron capture causes fission, 3) control of the neutron production to prevent a runaway reaction, and 4) removal of the heat generated in operation and after shutdown. The inability to completely turnoff the heat evolution when the chain reaction stops is a safety problem that distinguishes a nuclear reactor from a fossil-fuel burning power plant. 

The research and technical evaluations have provided industry tvith extremely valuable information and design procedures, including, but not limited to, two-phase flow phenomena and runaway reactions during safety/over-pressure relief. 

Fauske, H. K., Scale-up for Safety Relief of Runaway Reactions, Plant/Operalions Progress, 3 (1), 7-11, January 1984. 

Proceedings of the International Symposium on Runaway Reactions, 1989 CCPS/AIChE Directory of Chemical Process Safety Services... 

A publication summarises all the then available technical evidence related to the Seveso accident, and recommends operational criteria to ensure safety in commercial processes to produce trichlorophenol [4], All the plant scale incidents were characterised [ 1 ] by the subsequent occurrence of chloracne arising from the extremely toxic and dermatitic compound 2,3,7,8-tetrachlorodibenzodioxin (structure IX, p. S-3), formed dining the thermal runaway reaction and dispersed in the ensuing explosion. It is also extremely resistant to normal chemical decontamination procedures, and after the 1968 explosion, further cases occurred after transient contact with plant... 

The event tree for the modified process is shown in Figure 11-11. The additional safety function provides a backup in the event that the high-temperature alarm fails or the operator fails to notice the high temperature. The runaway reaction is now estimated to occur 0.00025 time per year, or once every 400 years. This is a substantial improvement obtained by the addition of a simple redundant shutdown system. 

The root cause of this accident was poor operating procedures and poor process infoiv mation. The operating procedure, for example, did not cover the safety consequences of deviations from the normal operating conditions, such as the possibility of a runaway reaction and the specific steps to be taken to avoid or recover from such deviations. 

Techniques such as adiabatic calorimetry (Dewar calorimetry) were by then well established [2, 118, 119]. All these techniques can be used for obtaining data to design for the prevention of runaway reactions, that is, to design for inherent plant safety. 

Another approach for scale-up of safety relief for runaway reactions is shown in Figure 3.21 and is discussed by Fauske in [211]. The following sequence is used ... 

Frurip, D. J., E. Freedman, and G. R. Hertel, "A New Release of the ASTM CHET AH Program for Hazard Evaluation Versions for Mainframe and Personal Computer," in Proceedings of the International Symposium on Runaway Reactions, Center for Chemical Process Safety/AIChE, New York, NY (1989). 

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