Ignition-extinction behavior

As far as extinction/ignition behavior is concerned, oxidations in micro reactors can exhibit varied temperature profiles [19, 56, 57, 59-61]. As a consequence of their very distinct heat transfer characteristics, micro reactors can allow autofhermal operation at a different temperature level compared with processing in conventional reactors. As an example, this may raise the selectivity of value products. 

Oxidations of ammonia display ignition/extinction characteristics and auto-thermal reaction behavior. At low heat supply, only low conversion is observed and temperature remains nearly constant. With increasing heat supply and approaching a certain temperature, the reaction heat generated can no longer be transferred completely totally to the reactor construction material. At this stage, the reaction starts up . Suddenly, the temperature is raised by increased heat production until heat generation and removal are in balance. The reaction can now be carried out without a need for external heat supply, namely in autothermal mode. 

Flammability (extinction behavior under ambient conditions after removing an ignition source, reaction to a small flame)... 

In practical combustion systems, the predominant mode of heat transfer is usually not molecular conduction, but turbulent diffusion, except at the boundaries and the flame front. Conduction is the only mode of heat transfer through refractory walls, and it determines ignition and extinction behaviors of the flame. Turbulent diffusion, an apparent or pseudo conduction mechanism arising from turbulent eddy motions, will be discussed in Section 4.4. The relations from the theory of conduction heat transfer15-17 can be used to evaluate heat losses through furnace walls and load zones, and through the pipe walls inside boilers and heat exchangers, etc. 

The violation of condition (6) results in an oscillatory (limit cycle) behavior, and the violation of condition (7) results in the realization of the critical ignition-extinction conditions (generation of an instability of the saddle type). The critical temperatures for ignition and extinction are found from the condition A = 0 ... 

Puszynski, J. Hlavacek, V. Experimental study of ignition and extinction waves and oscillatory behavior of a tubular nonadiabatic fixed-bed reactor for the oxidation of carbon monoxide. Chem. Eng. Set 1984, 39, 681-692. 

More complex patterns of behavior can be found for finite transport resistances inside the porous catalyst [74]. Here, kinetic instabilities occur on two different length scales. On a first, macroscopic scale, multiplicity is induced by ignition and extinction of every single reactive column tray. On a second, microscopic scale, multiplicity comes from isothermal multiplicity of the single catalyst pellet, which is due to the finite transport resistance inside the catalyst. 

The third process can be circumvented by using more corrosion-resistant graphitized carbon supports. Other critical aspect is catalyst sensitivity to the dynamic behavior of the fuel cell, e.g., ditring the fuel cell ignition or extinction. 

In chemical reactors an enormous variety of possible regimes, both steady state and non-steady state (transient) can be observed. Steady-state reaction rates can be characterized by maxima and hystereses. Non-steady-state kinetic dependences may exhibit many phenomena of complex behavior, such as fast and slow domains, ignition and extinction, oscillations and chaotic behavior. These phenomena can be even more complex when taking into account transport processes in three-dimensional media. In this case, waves and different spatial structures can be generated. For explaining these features and applying this knowledge to industrial or biochemical processes, models of complex chemical processes have to be simplified. 

Principle of critical simplification. In accordance with this principle (Yablonsky et al., 2003), the behavior near critical points, for instance ignition or extinction points in catalytic combustion reactions, is governed by the kinetic parameters of only one reaction—adsorption for ignition and desorption for extinction— which is not necessarily the rate-limiting one. 

The generation of self-sustained oseillations is a particular case of bifurcation. The term bifurcation is often used in connection with the mathematical study of dynamical systems. It denotes a sudden qualitative ehange in the behavior of a system upon the smooth variation of a parameter, the so-eaUed bifureation parameter, and is applied to the point of the fundamental reeonstmetion of the phase portrait where the bifurcation parameter attains its critical value. The simplest examples of bifurcation are the appearance of a new rest point in the phase space, the loss of the rest-point stability, and the appearance of a new limit cycle. Bifurcation relates to physicochemical phenomena such as ignition and extinction, that is, a jump-like transition from one steady state to another one, the appearance of oscillations, or a chaotic regime, and so on. 

For exothermic reactions, the particle temperature may change drastically through small fluctuations of temperature or concentration. This ignition-extinction behavior can be analyzed for a porous particle by combining the steady-state heat balance [Eqs. (4.5.31) and (4.5.32)] ... 

Figure4.5.12 (a) Ignition-extinction behavior of a single catalytic particle (schematically, HR heat removal, HP heat production by chemical reaction) ...

Stability limits are provided in Fig. 6.6 for two inlet velocities, p = 5 bar and Tjj,j = 700 K, in terms of the critical heat transfer coefficient for extinction. For low thermal conductivities k < 2 W/mK), the reduced upstream heat transfer hinders catalytic ignition (light-off) and causes blowout. The stability limits at low fcs are narrower at higher inlet velocities (Fig. 6.6). In comparison to pure gas-phase combustion studies [18], there is a marked difference at the low behavior, which is discussed qualitatively (since the aforementioned work refers to different geometry and operating conditions). In gas-phase combustion, the blowout limits extend over a narrower range of ( 0.4-0.8 W/mK) and are nearly independent of h (the blowout limit line is almost parallel to the /i-axis). This is because low... 

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