Runaway reactions characterization

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. 

Fauske, H. K., Clare, G. H. and Creed, M. J., Laboratory Tool for Characterizing Chemical Systems, Proceedings of the International Symposium on Runaway Reactions, CCPS, Cambridge, MA, March 1989. 

Several studies have been published to assess the kinetics of polymerization reactions at high temperatures. (irZ) However, most of these studies only describe experiments conducted at isothermal conditions. Only a few papers are based on adiabatic runaways. This paper is one of the first studies based on "first principles" characterizing adiabatic runaway reactions. 

Several commercial calorimeters are available to characterize runaway reactions. These include the accelerating rate calorimeter (ARC), the reactive system screening tool (RSST), the automatic pressure-tracking adiabatic calorimeter (APTAC), and the vent sizing package (VSP). Each calorimeter has a different sample size, container design, data acquisition hardware, and data sensitivity. 

Pressure resistant construction is characterized by a design pressure of a vessel or equipment that is higher than the pressure that can be reached in case of an explosion or runaway reaction. When decomposition of condensed materials need to be considered then it is usually very expensive to realize pressure resistant construction because of the high pressures that can be expected. 

Adiabatic conditions may be achieved either by a thermal insulation or by an active compensation of heat losses. Examples are the Dewar calorimeter, achieving a thermal insulation [2-4] or the Accelerating Rate Calorimeter (ARC) [5] or the Phitec [6], using a compensation heater avoiding the heat flow from the sample to the surroundings. These calorimeters are especially useful for the characterization of runaway reactions. 

Decomposition reactions are often involved in thermal explosions or runaway reactions, in certain cases as a direct cause, in others indirectly as they are triggered by a desired synthesis reaction that goes out of control. A statistical survey from Great Britain [1, 2] revealed that out of 48 runaway reactions, 32 were directly caused by secondary reactions, whereas in the other cases, secondary reactions were probably involved too, but are not explicitly mentioned (Figure 11.1). Therefore, characterizing secondary decomposition reactions is of primary importance when assessing the thermal hazards of a process. 

Figure 11.2 Characterization of thermal risks linked with decomposition reactions the temperature increase is a measure of the severity and the time-scale gives a measure of the probability of triggering a runaway reaction.

Boyle [15] and Huff [16] first accounted for two-phase flow with relief system design for runaway chemical reactions. A computer simulation approach to vent sizing involves extensive thermokinetic and thermophysical characterization of the reaction system. Fisher [17] has provided an excellent review of emergency relief system design involving runaway reactions in reactors and vessels. Fauske [18] has developed a simplified chart to the two-phase calculation. He expressed the relief area as ... 

The information required to characterize a runaway reaction or decomposition is ... 

The data required for characterizing the consequences of a runaway reaction... 

In common with using prevention as a basis of safety, it is essential that a full evaluation of the hazards of the process is carried out, before the type of protective measure is chosen and designed. The identification and definition of the worst case scenario is particularly important as, in contrast to prevention, any protective measure has to be able to cope with this worst case runaway reaction. In addition, the course of the runaway reaction has to be fully characterized and evaluated using the techniques described in Chapters 3 and 4. 

While ACOMP gives the comonomer conversions, which those techniques also do, ACOMP additionally and simultaneously evolution of weight average molecular weight and [p], average polymer properties of critical importance in the nltimate characterization, and utilization of the polymers. Additionally, ACOMP can provide immediate detection of nnforeseen or unwanted phenomena such as microgelation, runaway reaction, dead end reaction, onset of tnrbidity, and so on. 

Runaway Reactions Experimental Characterization and Vent Sizing This instruction module describes the ARSST and its operation, and illustrates how this instrument can easily be used to experimentally determine the transient characteristics of mnaway reactions, and how the resulting data can be analyzed and used to size the relief vent for such systems. Rupture of a Nitwaniline Reactor This case study demonstrates the concept of runaway reactions and how they are characterized and controlled to prevent major losses. 

The self-discharge reaction of calcium with calcium chromate is highly exothermic, forming complex chromium(m) oxides. Above about 600°C the selfdischarge reaction accelerates, probably due to the markedly increasing solubility of the chromate in the chloride electrolyte. This acceleration increases the rate of fonnation of calcium-lithium alloy. The resulting thermal runaway is characterized by short battery lives, overheating and cell step-outs, shorts, and noise characteristics of excess alloy. 

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

Water could additionally be injected via a third micro mixer in the ethanol/cata-lyst solution mixture [51], This served for heat transfer characterization, adjustment of temperature before reaction and most prominently dilution of the reaction mixture. By the last step, runaway situations occurring during the reaction can be managed. 

H. K. Fauske and J. C. Leung, New Experimental Technique for Characterizing Runaway Chemical Reactions, Chemical Engineering Progress (August 1985). 

This chapter describes a runaway scenario. The first section presents a general review of the decomposition reaction characteristics. The second section is devoted to the energy release that defines the consequences of a runaway. The third section deals with triggering conditions of undesired reactions, based on the concept of TMRld. The next section reviews some important aspects for the experimental characterization of decomposition reactions. Finally, the last section gives some examples stemming from industrial practice. 

However, several exothermic reactions are characterized by moderate or low values of the B number here, the transition stages from safe to runaway conditions may cover a quite wide range of the parameter values, and the choice of the boundaries for the safe region is very discretional. Hence, not surprisingly, the main discrepancies among the different criteria are found at low B numbers [14, 15]. Moreover, in this case, runaway is a less dramatic phenomenon posing the problem to decide whether a bland explosion still represents a safety issue. In this case, an effective runaway criterion should be more properly determined on the basis of the actual ability of the system to comply with certain levels of temperature and pressure. 

---