Ignition/extinction behaviour

The observed phenomenon is the so-called ignition-extinction-behaviour. A close examination of Figure 4-11 will explain this terminology. If one follows the curve of possible steady-state operating points, starting at low reference temperature values, a point is reached at which ... 

Veser and Schmidt studied the catalytic and homogeneous ignition and the homogeneous extinction behaviour of lower alkanes and alkenes ]428]. Platinum foil was used as the catalyst The surface ignition temperature decreased with increasing number of carbon atoms in the fuel and with increasing modified equivalence ratio 0, which was derived from the equivalence ratio. The latter is defined as the ratio of vol.% air to vol.% fuel in the feed normalised by the ratio required for total combustion ... 

Experimental data describing the dynamics of ignition and extinction will be reported in detail, and the principal factors determining the behaviour of the different reactants will be... 

It is important to note that the behaviour of these steady states is not identical with respect to inherent disturbances in operating conditions, as for example feed reactor temperature, composition or flow rate of the feed, or temperature and flow rate of the cooling agent, etc. Some are insensitive to such variations, in the sense that after the disturbance vanishes the system comes back naturally in the original state. These are stable stationary states, as the points A and C in Fig. 8.17. In the case of the point B the situation is essentially different. This is an unstable stationary state because in the absence of a control action small disturbances will move the system either to the high conversion state (reaction ignition), or to the low conversion state (reaction extinction). This type of behaviour is dangerous for operation and must be avoided. 

A similar behaviour is found for the case of an increase in the ammonia contraits at the inlet of the converter. In fact, when the molar fiaction of ammonia in the feed stream increases, the equilibrium shifts to the reactants and the heat generation rate decreases. If the reactor is being operated under open loop, an extinction phenomenon rqipears due to the autothermal operation of the converter (Fig. 11). Under closed loop operation, the control action leads to a decrease in the cold by-pass fraction (Fig. 12). As a result, the reactor remains at the upper branch of the curve shown in Fig. 2 (ignited steady-state) and the outlet conversion drops slightly. 

The prototype, cubic autocatalytic reaction (A + 2B 3B) forms the basis of a simple homogeneous system displaying a rich variety of complex behaviour. Even under well-stirred, isothermal open conditions (the CSTR) we may find multi stability, hysteresis, extinction and ignition. Allowing for the finite lifetime of the catalyst (B inert products) adds another dimension. The dependence of the stationary-states on residence-time now yields isolas and mushrooms. Sustained oscillations (stable limit cycles) are also possible. There are strong analogies between this simple system and the exothermic, first-order reaction in a CSTR. 

They are of great value, illustrating all aspects of the stationary-state behaviour unique and multiple solution, hysteresis and jumps between different branches (ignition and extinction or washout), and the effects of reversibility and of non-zero inlet concentration of the autocatalyst. The algebraic analyses are, by comparison, far less transparent, although their forms can also be expressive. 

This analysis, based on the single variable T, shows that multiple critical transitions ("ignition and extinction) and regions of bistability are possible in non-isothermal reaction when complex Kinetics occur. It cannot reveal the types of behaviour associated with the singularities of the Gray and Yang scheme, but the proper two-dimensional (T-[x]) stability analysis establishes the following ... 

---