Runaway thermal

The maximum attainable production was sought that did not cause thermal runaway. By gradually increasing the temperature of the water, boiling under pressure in the reactor jacket, the condition was found for the incipient onset of thermal instability. Runaway set in at 485.2 to 485.5 K for the 12 m reactor and at 435.0 to 435.5 K for the shorter, 1.2 m reactor. The smaller reactor reached its maximum operation limit at 50 K lower than the larger reactor. The large reactor produced 33 times more methanol, instead of the 10 times more expected from the sizes. This... 

Mathematical models that ignored kinetic forms may fit the experimental results very well but fail to predict critical performance attributes. For example, neglecting the well known exponential form of the Arrhenius fianction made one, entirely mathematical, model fail in predicting the thermal runaway. 

Transient Studies in an Adiabatic Packed-Bed Reactor was the title of a publication by Berty et al (1972). This was in connection with thermal runaway of reactors. The pertinent subject will be discussed in a following chapter in which the interest is focused on how to avoid the onset of a runaway. Here the object of the experiment was to see what happens after a runaway has started. 

Heat exchanger-like, multi-tube reactors are used for both exothermic and endothermic reactions. Some have as much as 10,000 tubes in a shell installed between tube sheets on both ends. The tubes are filled with catalyst. The larger reactors are sensitive to transient thermal stresses that can develop during startup, thermal runaways and emergency shut downs. 

Coolant flow is set by the designed temperature increase of the fluid and needed mass velocity or Reynolds number to maintain a high heat transfer coefficient on the shell side. Smaller flows combined with more baffles results in higher temperature increase on the shell side. Reacting fluid flows upwards in the tubes. This is usually the best plan to even out temperature bumps in the tube side and to minimize temperature feedback to avoid thermal runaway of exothermic reactions. 

This result means that the reactor is insensitive if the temperature profile is concave toward the reactor length axis, and the inflection point is avoided. If the AT exceeds that permitted by the previous criterion—the limit set by RT /E— an inflection of the temperature vs., tube length will occur and thermal runaway will set in. Just before runway sets in the temperature at the hot spot can be 1.4 times higher than RT /E. 

In the last part of Chapter 7.4 (Transient Studies) the experimental work on ethylene oxidation was shown. There the interest was to investigate what occurs and how fast, after a thermal runaway started. The previous chapter discussed the criteria of how to design reactors for steady-state operation so that runaways can be avoided. One more subject that needs discussion is what transient changes can cause thermal runaways. 

The fundamental reason for runaway at transient changes is the large difference in the thermal capacity of the catalyst charge and the flowing fluid, especially if it is a gas-phase reaction. In these cases, if the reaction is running close to the runaway limit but still somewhat below it, sudden changes can start a thermal runaway. 

Adequate heat removal facilities are generally important when controlling the progress of exothermic chemical reactions. Common causes of thermal runaway in reactors or storage tanks are shown in Figure 7.4. A runaway reaction is most likely to occur if all the reactants are initially mixed together with any catalyst in a batch reactor where heat is supplied to start the reaction. 

Figure 7.4 Common causes of thermal runaway in reactors or storage tanks...

Controlling legionella m nursing and residential care homes Chemical reaction hazards and the risk of thermal runaway... 

Analysis of thermal runaway reactions is reviewed in Chapter 12. Table 4-6 gives various guidelines for the design of reactors and Figure 4-26 illustrates various reactor configurations. 

Thermal runaway is a partieular problem in unsteady state bateh reaetions, where the rate of reaetion and, therefore, the rate of heat produetion varies with time. The eonsequenees of thermal runaway are sometimes severe as in the ineidents at Seveso [3]. In this ease, a bursting disk ruptured on a reaetor. The reaetor was used to manu-faeture triehlorophenol at a temperature of 170-185°C and was heated... 

Thermal runaway reactions are the results of chemical reactions in batch or semi-batch reactors. A thermal runaway commences when the heat generated by a chemical reaction exceeds the heat that can be removed to the surroundings as shown in Figure 12-5. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase, and subsequently accelerates the rate of heat production. Thermal runaway occurs as follows as the temperature rises, the rate of heat loss to the surroundings increases approximately linearly with temperature. However, the rate of reaction, and thus the... 

The released energy might result from the wanted reaction or from the reaction mass if the materials involved are thermodynamically unstable. The accumulation of the starting materials or intermediate products is an initial stage of a runaway reaction. Figure 12-6 illustrates the common causes of reactant accumulation. The energy release with the reactant accumulation can cause the batch temperature to rise to a critical level thereby triggering the secondary (unwanted) reactions. Thermal runaway starts slowly and then accelerates until finally it may lead to an explosion. 

The induetion period of a thermal runaway reaetion will be inereased eompared to the plant. 

Figure 12-7. Simplified scenario of a thermal runaway. (Source T. Hoppe and B. Grob, Heat flow calorimetry as a testing method for preventing runaway reactions," Int. Symp. on Runaway Reactions, OCRS, AlChE, March 7-9, 1989.)...

The ARC analysis has been extended to determine the eonditions that may lead to thermal runaway in reaetors or storage vessels [10]. The equipment timeline is defined by the ratio of the heat eapaeity of the reaetor and its eontents to the reaetor heat transfer area and heat transfer eoeffieient. This is expressed by... 

Barton, J. A. and Nolan, P. F. Incidents in the Chemical Industry due to Thermal-runaway Chemical Reactions, Hazards X Process... 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

The level of mean stress also has an effect on the occurrence of thermal failures. Typically, for any particular sUess amplitude the stable temperature rise will increase as the mean stress increases. This may be to the extent that a stress amplitude which causes a stable temperature rise when the mean stress is zero, can result in a thermal runaway failure if a mean stress is superimposed. 

Controlling temperature is easier and the risk of thermal runaway is reduced. Greater heat transfer surface per unit of reactor volume is provided by a smaller reactor. 

Similar approaches are applicable in the chemical industry. For example, maleic anhydride is manufactured by partial oxidation of benzene in a fixed catalyst bed tubular reactor. There is a potential for extremely high temperatures due to thermal runaway if feed ratios are not maintained within safe limits. Catalyst geometry, heat capacity, and partial catalyst deactivation have been used to create a self-regulatory mechanism to prevent excessive temperature (Raghaven, 1992). 

Gygax, R. W., Scaleup Principles for /Vssessing Thermal Runaway Risks, Chem. Eng. Prog, V. 86, No. 2, 1990. 

Leung, J. C., Fauske, H. K and Fisher, H. G., Thermal Runaway Reactions In A Low Thermal Inertia Apparatus, Ther-mochimica Acta, 104, 13-29, 1986. 

The system can prevent explosion, fire, and venting with fire under conditions of abuse. These batteries have a unique battery chemistry based on LiAsF6/l,3-di-oxolane/tributylamine electrolyte solutions which provide internal safety mechanism that protect the batteries from short-circuit, overcharge and thermal runaway upon heating to 135 °C. This behavior is due to the fact that the electrolyte solution is stable at low-to-medium temperatures but polymerizes at a temperature over 125 °C... 

Possible Haystack-Type Reaction Associated with Thermal Runaway in a Closed Reaction Vessel 329... 

Imagine a closed reaction vessel in which an exothermic reaction proceeds at room temperature at a finite rate. Although the temperature in the reaction vessel is initially the same as room temperature, it rises gradually until the rate of heat generation due to the exothermic chemical reaction is equal to the rate of heat escape from the reaction vessel surface. However, if a thermal balance is not established for such a chemical reaction, the reaction rate is accelerated by self-heating as the temperature rises, leading to thermal runaway. The temperature change in a reaction vessel is represented by Eq. (1),... 

Another problem is the delayed action involved in thermal runaway, which makes... 

Figure 8. Clock or haystack-type reaction associated with thermal runaway derived from self-heating due to exothermic reactions in a closed vessel. Parameters ...

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