Runaway temperature

The best fit, as measured by statistics, was achieved by one participant in the International Workshop on Kinetic Model Development (1989), who completely ignored all kinetic formalities and fitted the data by a third order spline function. While the data fit well, his model didn t predict temperature runaway at all. Many other formal models made qualitatively correct runaway predictions, some even very close when compared to the simulation using the true kinetics. 

The variations were mainly due to operating conditions very close to the parametrically sensitive region, i.e., to the incipient temperature runaway. Small errors in the estimation of temperature effects caused runaways and, consequently, large differences. 

To check the effect of integration, the following algorithms were tried Euler, explicit Runge-Kutta, semi-implicit and implicit Runge-Kutta with stepwise adjustment. All gave essentially identical results. In most cases, equations do not get stiff before the onset of temperature runaway. Above that, results are not interesting since tubular reactors should not be... 

The effeet of integration method and stepsize must be eheeked for every application where temperature runaway is possible. Those will be mostly oxidations, but other reaetions ean be very exothermie, too. During the 1973/74 oil erisis, when synthetie natural gas projeets were in vogue, one of the CO hydrogenation teehnologies was found to be very exothermie and prone to runaway also. 

The above statement is obvious. Almost as evident is the statement that since heat generation rate increases with temperature, heat removal rate should increase even faster. This would eliminate continued temperature increase and prevent temperature runaways. 

Now assume that U and S are not large and that AT = 20 K is needed to keep the heat balance. In this case when something causes the temperature to rise even for a very short time by one K, the reaction rate will increase, just as before, by 15%. The heat transfer rate will increase by a ratio of (21-20)720, that is by 5 %. The 5 % increase cannot restore the heat balance and the reaction temperature will keep rising. A temperature runaway will occur. 

In certain processes, decomposition reactions or temperature runaways may... 

Raghaven, K. V. (1992). Temperature Runaway in Fixed Bed Reactors Online and Offline Checks for Intrinsic Safety. Journal of Loss Prevention in the Process Industries 5, 3,153-59. 

In all these innovative processes, monoliths and monoUth-based filters are used. As temperature runaways can occur special materials are used. SiC appears to be the best choice because of its excellent heat conductivity. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

The formation of phthalic anhydride is highly exothermic, and even with the most careful design the heat removal from packed bed reactors can become uncheckable, leading to temperature runaways, meltdowns, and even explosions. If the chief engineer of those reactors had been required to sit on the reactor during start up, there would be fewer chief engineers about. ... 

If the rich gas from the CRG reactor is passed over another bed of high-nickel catalyst at a lower temperature, the equilibrium of the five components is reestablished. Carbon oxides react with hydrogen to form methane and the calorific value of the gas is increased. It should be noted that this methanation step differs from that encountered in ammonia synthesis gas production because of the high steam content the temperature rise is reduced and there is no possibility of temperature runaway as the... 

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the liquid or vapor phase. In the liquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature runaways and obtain high selectivity, the conversion per pass is limited in the liquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and liquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

The additive, prepared from iron sulfate [Fe2(S04)3], is used to promote hydrogenation and effectively eliminate coke formation. The effectiveness of the dual-role additive permits the use of operating temperatures that give high conversion in a single stage reactor. The process maximizes the use of reactor volume and provides a thermally stable operation with no possibility of temperature runaway. 

Several kinds of failures may compromise safety and productivity of industrial processes. Indeed, faults may affect the efficiency of the process (e.g., lower product quality) or, in the worst scenarios, could lead to fatal accidents (e.g., temperature runaway) with injuries to personnel, environmental pollution, and equipments damage. In the chemical process fault diagnosis area, the term fault is generally defined as a departure from an acceptable range of an observed variable or a parameter. Fault diagnosis (FD) consists of three main tasks fault detection, i.e., the detection of the occurrence of a fault, fault isolation, i.e., the determination of the type and/or the location of the fault, and fault identification, i.e., the determination of the fault profile. After a fault has been detected, controller reconfiguration for the self-correction of the fault effects (fault accommodation) can be achieved in some cases. 

In chemical processes, several kinds of failures may compromise safety and productivity. Indeed, the occurrence of faults may affect efficiency of the process (e.g., lower product quality) or, in the worst scenarios, could lead to fatal accidents (e.g., temperature runaway) with injuries to personnel, environmental pollution, equipments damage. 

There are several other aspects about CSTRs with exothermic reactions that should be mentioned at this point. The first involves the temperature of the feed. The colder the feed, the less heat must be transferred from the reactor. So control would be expected to be improved. However, as we will see in Chapter 3, a cold feed can produce some interesting dynamics for instance, an increase in feed flowrate initially decreases reactor temperature because of the sensible-heat effect. But as the reactant concentration in the reactor increases, the temperature eventually increases. A reactor temperature runaway can result if the cold feed quenches the reaction and reactant concentration builds to a very high level before the reaction lights off. ... 

K) is much higher. The second reactor in the series should be very controllable. It is the first reactor that could produce temperature runaways. It has all the ingredients irreversible exothermic reaction with sufficient reactant present to fuel the runaway because of the conversion is only 85% in the first reactor. 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Fig. 6.2 Energy-transfer curves for increasing applied fields, showing the onset of electron temperature runaway. After Frohlich (1947), by courtesy of the Royal Society.

A mullite green body < > = 0.7) in the form of an infinite plate of thickness 5 mm filled with polystyrene binder is fired in air at450°C. As a result, em exothermic reaction with a temperature runaway of 160°C inside a mullite green body occurs. Estimate of the surface stress. Assume that E = 144 GPa, = 5 x 10 /°C, and p = 0.25. 

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