Exothermic reaction temperature oscillations

Exothermic reaction parameters of, 27 63 temperature oscillations, 27 65-67 Explosion, petovskite preparation, 36 250 Extended Hiickel treatment, 34 136, 147, 154, 156, 166, 173... 

The specific models we will analyse in this section are an isothermal autocatalytic scheme due to Hudson and Rossler (1984), a non-isothermal CSTR in which two exothermic reactions are taking place, and, briefly, an extension of the model of chapter 2, in which autocatalysis and temperature effects contribute together. In the first of these, chaotic behaviour has been designed in much the same way that oscillations were obtained from multiplicity with the heterogeneous catalysis model of 12.5.2. In the second, the analysis is firmly based on the critical Floquet multiplier as described above, and complex periodic and aperiodic responses are observed about a unique (and unstable) stationary state. The third scheme has coexisting multiple stationary states and higher-order periodicities. 

In the case of the nonisothermal first-order exothermic reaction heat is auto catalytic, for it raises the temperature and provokes an increase of reaction rate, yet is itself a product of the reaction. In the Gray-Scott scheme, B is plainly autocatalytic and its degeneration by the second reaction plays the role of the direct cooling in the non-isothermal case. This reaction appears in the chemical engineering literature in 1983,16 and is the keynote reaction in Gray and Scott s 1990 monograph on Chemical Oscillations and Instabilities.17 A justification of the autocatalytic mechanism in terms of successive bimolecular reactions is the subject of Chapter 12. 

P, I, D system (Figure 9.10). This type of temperature control requires careful tuning of the control parameters, in order to avoid oscillations, which may lead to loss of control of reactor temperatures in cases where an exothermal reaction is carried out. The main advantage of the isothermal control is to give a smooth and reproducible reaction course, as long as the controller is well tuned. 

This type of periodic operation allows for conversion improvement in reversible exothermic reactions [9], A cycle average inlet temperature for the conditions of continuous temperature oscillation can be substantially lower than the inlet temperature under steady-state conditions. This leads to a lower outlet temperature and higher equilibrium conversion for a reversible reaction. Better performance is achieved if temperature oscillations attenuate sufficiently during the passage through the catalyst bed [9]. 

The above example of the determination of the change in Cp as the epoxy resin underwent crosslinking is an example of a reversible thermal event Cp decreases on vitrification) convoluted by a non-reversible chemical change due to the formation of the network by the ring-opening reaction of the epoxy resin (which produced a large exotherm). Experimentally this separation of the events is achieved by applying a small temperature oscillation on top of the isothermal temperature. The conditions are thus quasi-isothermal. The condition may be described (Jones et al, 1997) by... 

The autocatalator model is in many ways closely related to the FONI system, which has a single first-order exothermic reaction step obeying an Arrhenius temperature dependence and for which the role of the autocatalyst is taken by the temperature of the system. An extension of this is the Sal nikov model which supports thermokinetic oscillations in combustion-like systems [48]. This has the form ... 

Oscillatory States in the CSTR limit Cycles.— The nature of the diemically open system makes it an ideal vehicle for studying reactions which odiibit chemical oscillations. The continuous supply of reactants diminates damping from reactant depletion inevitable in closed systems and permits the experimental establishment of true limit-cycle behaviour. However, not all oscillations in the CSTR need be kinetically interesting in their origin (e.g. the periodic variations in temperature and concentrations in reactors run with feedback control More importantly from the combustion researcher s viewpoint, oscillations may arise between multiple stable steady states of any normal exothermic reaction because of restric-... 

Between 350°C and 420°C, the DTA-signal exhibited strong oscillations similar to those detected by IMR-MS in the reactor experiment (see Fig. 1). Weight changes were not detected by TG analysis. The oscillations in the DTA thus seem to arise from the highly exothermic catalytic reaction. A relation between the oscillations frequencies and the reaction temperature could not be derived. 

In the present chapter, steady state, self-oscillating and chaotic behavior of an exothermic CSTR without control and with PI control is considered. The mathematical models have been explained in part one, so it is possible to use a simplified model and a more complex model taking into account the presence of inert. When the reactor works without any control system, and with a simple first order irreversible reaction, it will be shown that there are intervals of the inlet flow temperature and concentration from which a small region or lobe can appears. This lobe is not a basin of attraction or a strange attractor. It represents a zone in the parameters-plane inlet stream flow temperature-concentration where the reactor has self-oscillating behavior, without any periodic external disturbance. 

From the study presented in this chapter, it has been demonstrated that a CSTR in which an exothermic first order irreversible reaction takes place, can work with steady-state, self-oscillating or chaotic dynamic. By using dimensionless variables, and taking into account an external periodic disturbance in the inlet stream temperature and coolant flow rate, it has been shown that chaotic dynamic may appear. This behavior has been analyzed from the Lyapunov exponents and the power spectrum. 

Undamped oscillations had been reported by Adlhoch et al. (160) for a Pt ribbon operated in the 10 5 torr range and by Schiith and Wicke (124) for a supported Pt catalyst working near atmospheric presure. An estimate for the former case yields temperature variations of the order of 10 K due to the exothermicity of the reaction in the latter case even periodic changes by 25 K were measured—quite obviously heat conductance is efficient enough to synchronize the oscillatory behavior of these systems. 

A further observation was that the optical absorption by the N2O increased somewhat during the induction period. At the same time additional measurements of the IR emission at 4.75 pm showed a growth in the radiation from the oscillators CO, N2 0(1 3) and 02(1 3) before appreciable decomposition of the N2O had occurred. Both these effects can be associated with vibrational disequilibrium in the system (the absorption coefficient of the N2O being directly related to its vibrational temperature), and Zaslonko et al. use this evidence to support the view that the overall CO + N2O reaction occurs even at lower temperatures via a chain process initiated by the unimolecular dissociation of NjO- The overall CO + N2O reaction is strongly exothermic (AH — —87... 

Even though the exothermicity has been increased significantly the start-up behaviour has improved to quite some extent. The first overshoot in temperature is still there, but by far not as pronounced as it was observed in the first case. This is especially to be appreciated in view of the absolute value of adiabatic temperature increase, amounting to 210 K. All in all, the start-up phase proceeds as a damped oscillation. The most pronounced first oscillation is caused by the comparably low initial temperature resulting in a low reaction rate and consequently in an accumulation of educts. This effect can be mitigated by increasing the initial temperature (c.f. Figure 4-21). 

The component reactions of oscillatory reaction can be exothermic or endothermic. This aspect is expected to influence oscillatory characteristics. In this case of B-Z reaction involving malonic acid as substrate, it has been found that the rate of temperature rise oscillates rather than the temperature of the reaction mixture [19]. All the component reactions are found to be exothermic. Typical results are given in Fig. 9.7. Temperature begins to fall after IV2 hr. 

The mixture placed in a crucible is heated with a 15 kW HF-generator (oscillation frequency 3-9 Mhz). Tungsten is oxidized and, because the oxidation reaction is exothermal, the reaction mixture reached temperatures sufficient for the complete fusion and oxidation of the zirconium sample. A 4 1/min oxygen flow carries the gases from the reaction mixture through the following devices ... 

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