Single zone reaction

There are two broad classes of reactions that involve a single zone  

This category gathers together all the heterogeneous reactions that can be divided into two classes  

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Figure 4.2 Autoclave reactor single zone reaction.

Let us reconsider reaction [l.Rl] in gas phase. It is a single zone reaction because the entire reaction can take place at any point of the reactor and the reactor is the reaction zone. 

This distinction is historic but does not appear to be very usefiil. In fact, for kinetic studies, experience shows us that it is more appropriate to separate single zone reactions from multizone reactions. We will see that differences appear at this level. Thus, in our previous examples we will classify reaction [1.R1] in gas phase and the catalytic reaction in Figure 1.1a in the same category of single zone reactions while the reaction presented in Figure 1.1b will be a multizone reaction. In addition to this, it should be noted that the same chemical reaction might occur with a different number of zones depending on the conditions of the experiment. 

The volumetric speed of a single-zone reaction is the speed of the reaction per unit of volume (the areal speed will be used for 2D zones). We emphasize that this notion is only defined for reactions with a single reaction zone, which includes all homogeneous reactions, in other words that occur entirely in one phase ... 

If our reaction is a single-zone reaction, taking into account [1.12] yields ... 

This reactor comprises a single-channel reaction zone followed by a quenching (cooling) zone [72], Gases, pre-heated in a separate zone, are contacted in an T-junction and mixed in a short passage thereafter. Such mixed gases enter the above-mentioned reaction zone. 

Autoclave reactors differ from tubular reactors in two respects. Autoclave reactors have much smaller length-to-diameter ratios (2-4 for single-zone and up to 15-18 for multizone reactors) and operate at a much narrower reaction temperature range. 

Fig. 1 Problem schematic for burning homogeneous propellant showing condensed phase (surface) reaction zone, gas phase reaction zone and corresponding steady-state temperature profiles. Propellant is fed from left at surface regression rate r. Simplified kinetics description has propellant (A) decomposing in condensed phase to intermediate species (B) via zero-order, high activation energy, irreversible single-step reaction, and (B) reacting to (C) in gas phase via second-order (overall), irreversible single-step reaction.

The study of turbulent mixing in a single-phase reaction mixtnre has shown that the radial inpnt of reactants and conical widening at the input zone of a reactor can decrease diffnsion limitations for fast liqnid-phase reactions. At the same time, the size of dispersed phase particles in a two-phase reaction mixture flow is determined by the hydrodynamic inflnx (shear pressnre valne) in a dispersion medium, which increases with the growth of the intensity of the tnrbulent pulsations. 

For a single catalytic reaction, a generic cross sectional average species mole balance for component A in the bubble zone gas can be written as ... 

Remark.- a homogenous reaction proceeds completely in a single zone, which is the reaction volume. This explains why the concept of zone is not introduced in homogenous kinetics. 

Speeds and reactivities of reactions taking place in only a single zone... 

Consider a reaction that proceeds in only a single zone with the form ... 

Theorem.- For a reaction proceeding in only a single zone of diffusion, the reactance at any time is put in the form of the pixKluct of two functions reactivity, a function of only the intensive properties at this time, and the space function, a function of only the shapes and sizes, that is, only morphology of the zone at the considered time. 

According to the first condition of their macroscopic definition (see section 4.1), the elementary reactions proceed in a single zone only, and by applying to them all that has come before and by using the second condition of section 7.4 and relation [7.3], their reactivity can be given as ... 

The pseudo-steady state modes for a reaction, if the whole of the reaction is held in only a single zone or several zones whose dimensions always remain equal to each other, even if these sizes vary with time (we will encounter this case in the process of nucleation of a new solid phase on the surface of an initial solid phase, see chapter 8, the mode being a pseudo-steady state one). 

A reaction such as the one shown in Figure 1.1a that we have called a single zone is actually a heterogeneous reaction with several zones rendered to single zone behavior by the experimental conditions (agitation, solid non-porous). 

Each elementary step occurs in a single zone. In fact, if it involves two zones, then one part of the reaction will occur in one zone and the second part in the other. In this case, the reaction would no longer be elementary since it would occur in two steps and at least one intermediate species would be formed between both steps. 

In an elementaiy reaction, which occurs in a single zone, the space function of this elementary step is the ratio of the volume (or area) of the reaction zone to the reference amount chosen to define the fractional extent ... 

The rate of an elementary reaction (with a single zone) is the product of its reactivity by its space function. This relation is always tme, whether we are dealing with a volitme zone or a strrface zone. 

Regarding reaction zones, nucleation is a single-area reaction the surface of solid v4 is not yet covered by B whereas growth involves several areas, as discussed in section 14.4.1. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

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