Process safety runaway reactions

Polymerization processes are carried out in many different variations. Process selection depends on the monomer and, also, strongly on the desired product properties. Selection of the right polymerization process is also very important with regard to process safety. Polymerization reactions belong to the most dangerous processes in the chemical industry with respect to the risk of an uncontrollable thermal runaway in the reactor. The reasons for the special hazard potential caused by polymerization reactions are threefold ... 

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. 

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. 

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 number of reactions that can run away is enormous, Bretherick s Handbook of Reactive Chemical Hazards [1] lists about 4,700 chemicals that have been involved in hazardous reactions of one sort or another, and there are more than 20,000 cross-references to entries involving more than one chemical. It is an essential work of reference for the chemist, the process engineer, and everyone involved in process safety. All I can do here is give a few examples to illustrate the reasons why runaways occur. 

Th is reached and decomposition can be triggered. Process safety depends on both rates, Qit.hi, and Qd mp-Decomposition will be triggered and 7), reached during a runaway of the decomposition reaction. The rate Qoer.hp determines the thermal safety of the process. 

Safety. The MR is much safer than the MASR. (1) The reaction zone contains a much smaller amount of the reaction mixture (hazardous material), which always enhances process safety. (2) In case of pump failure, the reaction automatically stops since the liquid falls down from the reaction zone. (3) There is no need to filter the monolithic catalyst after the reaction has been completed. Filtration of the fine catalysts particles used in slurry reactors is a troublesome and time-consuming operation. Moreover, metallic catalysts used in fine chemicals manufacture are pyrophoric, which makes this operation risky. In a slurry reactor there is a risk of thermal runaways. (4) If the cooling capacity is insufficient (e.g. by a mechanical failure) a temperature increase can lead to an increase in reaction, and thus heat generation rate. 

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. 

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). 

Hoppe, T., and R. Bruderer, "Thermal Analytical Investigation into a Runaway Reaction," in Proceedings of the 6th Annual Symposium on Loss Prevention and Safety Promotion in the Process Industries, p. 37, Oslo,... 

Gustin, J. L., "Thermal Stability Screening and Reaction Calorimetry—Application to Runaway Hazard Assessment and Process Safety Management," /. Loss Prev. Proc. Ind., 6,275 (1993). 

Historical perspective An analysis of thermal runaways in the United Kingdom (Barton and Nolan, Incidents in the Chemical Industry due to Thermal Bunaway Chemical Reactions, Hazards X Process Safety in Fine and Specialty Chemical Plants, IChem 115 3-18) indicated that such incidents occur because of the following general causes ... 

The flame came from a process vessel, the "60 still base," used for the batch distillation of organics, which was being raked out to remove semisolid residues, or sludge. Prior to this, heat had been applied to the residue for three hours through an internal steam coil. The UK Health and Safety Executive (HSE) investigation concluded that this had started self-heating of the residue and that the resultant runaway reaction led to ignition of evolved vapors and to the jet flame. 

De Faveri, D. M., C. Zonato, C. Pagella, and A. Vidili (1989). "Runaway Reaction and Safety Measures in Organic Processes." International Symposium on Runaway Reactions, March 7-9, 1989, Cambridge, MA, 155-175. New York American Institute of Chemical Engineers. 

---