Consecutive Reaction Systems

In dealing with systems of consecutive reactions we can classify the components of such a system into three groups. 

Initiation Reactions. These are the first reactions responsible for the production of the intermediates which give rise to the subsequent reactions. 

Propagation Reactions. These are all the reactions in which intermediate products react to produce further intermediates and usually result in removal of reactant. 

Termination Reactions, These are the reactions in the sequence which result in the destruction of the intermediates. 

Thus in a typical scheme of second-order reactions 

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The chemistry considered up to this point in this section has two simultaneous reactions in which the undesirable product depends on the concentration of one of the reactants. The methods discussed can be applied to other chemistry. Suppose that the reactions are such that the desired component can react further to form an undesirable product in a consecutive reaction system ... 

The following competitive-consecutive reaction system was studied ... 

For the fluids and the monoliths considered in this comparison, this limit is approximately 0.50 m/sec (depending on the catalyst load), which is in good agreement with the value found experimentally by Irandoust et al. [12]. If the liquid load is less than this limit, gas will be sucked in as well. Hence the sum of the linear velocities will tend to be close to the maximum flow rate of liquid alone. Frictional pressure drop in the MR is up to two orders of magnitude lower than in the TBR. Consequently, for the MR we may consider very high flow rates and high columns, higher than appears to be of practical interest, before the pressure drop becomes a restriction with the physical properties considered. For practical reasons an upper limit of 20 m for Lp was taken. In order to make a comparison between the MR and the TBR, some more restrictions were imposed. In a consecutive-reaction system like the one considered, selectivities must be compared at the same conversion, and results discussed below are for a 50% conversion of reactant A. 

Change the reaction system to reduce k2, thereby increasing the yield of R and decreasing S. Reacting systems were found that achieved reduced 2 at a favorable rate constant ratio. However, the primary reaction rate was also reduced, as is common for competitive-consecutive reaction systems, and was too slow for manufacturing purposes. 

When multiple reactions are involved, the yield and selectivity are important as well as the conversion. The following example illustrates the method of solving Eq. (4-4) for both single and consecutive reaction systems. The procedure is essentially the reverse of that for interpreting laboratory data on integral reactors (see Sec. 4-3). 

Despite the experience with batch reactors it may be worthwhile to operate continuous reactors also for fine chemicals. Continuously operated reactors only demand for one start-up and one shut-down during the production series for one product. This increases the operating time efficiency and prevents the deactivation of dry catalysts this implies that the reactor volume can be much smaller than for batch reactors. As to the reactor type for three phase systems an agitated slurry tank reactor [5,6] is not advisable, because of the good mixing characteristics. Specially for consecutive reaction systems the yields to desired products and selectivities will be considerably lower than in plug flow type reactor. The cocurrent down flow trickle flow reactor... 

We also should mention some drawbacks of the proposed reactor. Axial mbdng is not fully suppressed, so additional bed height is required to compensate for it. Further addition bed height is required for the production of intermediate products in a consecutive reaction system to ensure the hydrogen is almost completely consumed. Further we have to realize that the evaporation of solvent or reactant reduces the partial pressure of hydrogen, above all in the upper part of the reactor. Also this aspect demands for ditional catalyst. As a consequence the productivity of the reactor per unit of catalyst bed will be only a fraction of a reactor with cooling coils or catalyst in wall-cooled, small diameter tubes without evaporation. However, at the expense of productivity the reactor has a simple construction and does not need a hydrogen recycle compressor. We therefore expect it to be also a very cheap if not the cheapest reactor. 

A classic example of the product distribution in a consecutive reaction system is the study of benzene chlorination by MacMullin [1]. He measured the concentration of mono-, di-, and trichlorobenzene produced in a batch chlorination and determined the relative rate constants. He also showed that batch chlorination gave a higher yield of monochlorobenzene than a single-stage continuous-flow reactor. 

Consider a consecutive reaction system where the intermediate C is the desired product ... 

Table 1 Application of the Himmelblau-Jones-Bischpff method to estimation of rate coefficients in a simple consecutive reaction system... 

Figure 11.7.C-1 shows the results obtained for an inlet temperature of 357°C. The bulk mean conversion and temperature profile is shown. The conversion to phthalic anhydride tends to a maximum, as is typical for consecutive reaction systems, but which is not shown on the figure. Also typical for exothermic systems, as we have seen already, is the hot spot, where T equals about 30 C Even for this case, which is not particularly drastic, and with a small tube diameter of only 2.54 cm, the radial temperature gradients are severe, as seen from Fig. 11.7.C-2. The temperature in the axis is well above the mean.

The use of a single reaction requires the online measurement of the local species concentration along the flow. With such systems, one experiences the main drawback of physical methods with the local measurement and the influence of the probe size on the mixing quality estimation. For that reason, the so-called test reactions are very attractive. Two main systems, based on competitive chemical reactions, have been proposed for the investigation of mixing effects, that is, the competitive consecutive reaction system (Scheme 6.1) and the competitive parallel reaction system (Scheme 6.2). Let us consider the following simplest reactions schemes which do not exactly match the published real systems, but which facilitate the comparison ... 

Goal development of a solid-liquid competitive-consecutive reaction system... 

Note This protocol is focused on mixing effects for the classic competitive-consecutive reaction system. Reaction systems may also include parallel reactions in which A, B, or R are reacting to form unwanted products that are not represented by the consecutive-competitive system as used to derive eq. (13-5). To keep these reactions from making more unwanted products on scale-up, the overall reaction (addition) time may have to be held constant. In this case, the mesomixing issue for the primary reactions, A - - B R and R - - B S, would predict that more S would be formed. These issues may require selection of an alternative reactor, such as an in-line mixer, for successful scale-up. 

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