Selectivity competitive-consecutive reaction

Chemical processes often involve multiple, competing reactions. A common situation is that of a competitive-consecutive reaction, such as that described in Section 1.1, where reactant A and the desired product R are competing for reactant B. The selectively for waste product S can be defined as... 

Competitive- Consecutive Reaction in a CSTR Micromixing Effects on Selectivity... 

Chemical reactions, which proceed extremely fast and without considerable heat of reaction, should not be carried out in stirred tanks, but in pipe reactors. This particularly applies for complex reactions of the type competitive-consecutive reactions, in which care must be taken, so that the desired product formed does not come into contact with the educt. Otherwise an undesired secondary reaction would take place, whereby the selectivity would be reduced. 

In this case, the desired reaction is an intramolecular reaction, and the undesired intermolecular reaction might be avoided under these high-dilution conditions. The high-dilution technique might also be effective for improving the selectivity of competitive consecutive reactions, although the overall reaction time should be greatly reduced in comparison with that required under normal conditions. 

In a previous section (Section 6.1.3), we discussed the problem of disguised chemical selectivity for extremely fast competitive consecutive reactions. This problem could be solved using micromixers, in which the mixing takes place in a very short period by virtue of a small diffusion path caused by the microstructure. Friedel-Crafts alkylation using N-acyliminium ion pools provides a nice example of the effectiveness of micromixing. 

In cases where reactions give multiple products, product selectivity is an important issue from a synthetic point of view. Usually much effort is made to increase the amount of a desired product and to decrease the amounts of undesired products. We have already discussed competitive consecutive reactions and competitive parallel reactions in Chapter 6. 

The effect of micromixing in the iodination reaction is smaller than that observed for the Friedel-Crafts alkylation using N-acyliminium ions. The smaller effect seems to be ascribed to the smaller rate of iodination, because computational fluid dynamics simulation indicated that the effect of the micromixing on the product selectivity of a competitive consecutive reaction increases with an increase in the reaction rate. Therefore, the electrochemically generated I seems to be less reactive than the N-acyliminium ions. 

Consider the series-parallel reaction scheme, also referred to as the competitive consecutive reaction scheme, discussed earlier. The desired product R continues to react with the initial reactant B to produce the undesired product S. Usually the reaction conditions (temperature or catalyst) are arranged such that is much faster than 2- The selectivity, is defined as... 

Nagaki, A., et al., Control of extremely fast competitive consecutive reactions using micromixing. Selective Friedel-Crafts aminoalkylation. Journal of the American Chemical Society, 2005, 127 11666-11675. 

When the kinetics of the desired and the undesired reactions are known, it is always possible to calculate the selectivity as a unique function of the degree of conversion of the reactant A, see eq. (3.8). We shall do so in the next sections for competitive and for consecutive reactions, for the cases of batch or plug flow reactors and for CSTR s. Calculations for competitive-consecutive reactions will be given for CSTR s only. 

Figure 3J2. The selectivity of a competitive-consecutive reaction pair such as in eqs. (355) and (3.67), versus the A-conversion, in a perfectly mixed CSTR a (left) for various values of the reactivity ratio and b (right) for = i and various values the relative B-excess p.

It should be stressed that the selectivity in competitive-consecutive reactions is very sensitive to incomplete micro-mixing. This will be discussed in section 5.2.2. 

For consecutive reactions batch and plug flow reactors will always give a higher selectvity than well mixed continuous reactors. For competitive-consecutive reactions the best selectivity can be found in semi-batch reactors. However, this type of reactions can in some cases be carried out with an acceptable selectivity in a continuous well mixed reactor, when sufficient excess of the "other reactant (that does not cause undesired reactions) is applied. 

Imperfect meso-mixing in turbulent flow can be important when several reactions take place. In particular, it can influence the selectivity in competitive and competitive-consecutive reactions. 

A somewhat similar situation may arise with competitive-consecutive reactions. Let us consider the example represented by eqs. (3.55) and (3.67) in section 3.4.4, for the situation that the undesired reaction is rapid, and the desired reaction is almost instantaneous (say 10Under conditions of perfect mixing the selectivity would be high. TTie undesired reaction will then mainly take place in the region where the ratio of P- and iB-concentrations is much higher than average. 

In many if not most organic syntheses the selectivity is important. Again, byproducts from competitive reactions may be formed specifically in the entrance zone, while products from competitive-consecutive reactions may form at the interface of the entrance zone and the mixed zone. In both cases, the selectivity will be increased by the same four measures as indicated on the preceeding page. For a given process, one may determine the minimum feed addition time that is necessary to obtain a desired selectivity. This problem has been studied extensively by Bourne (1991) and Baldyga and Bourne (1992, 1993). 

The selectivity for competitive-consecutive reactions is primarily determined by mixing of the incoming feed (containing A) with the internal circulation stream (containing B) (Note that Bourne et al. used these letters differently). Bourne suggested that the selectivity is determined by the ratio of the local molar fluxes of B and A. This ratio is equal to v c / v c = (p c / c (see eq. (4.13)). 

Figure 5A. The selectivity of a competitive-consecutive reaction pair, as a function of the ratio of diffusion time and reaction time. The parameter is the ratio tyt Diffusion time = 0.1 8 10...

The selectivity of rapid reactions may be sensitive to mixing conditions, particularly in the case of competitive and competitive-consecutive reactions. A detailed description of all the relevant phenomena may be quite complicated. However, we can see a priori that good mixing and well controlled feed rates will generally be favourable for the selectivity of semi-batch processes, hi section 3.4 the relation between the selectivity and the degree of conversion was calculated for a few types of reaction pairs, for batch (or plug-flow) reactors, and for CSTR s. In sections 5.2.2 and 5.2.3 the influence of incomplete micro-mixing on selectivity was briefly discussed, for turbulent and laminar flow, respectively. 

From this example of a fast, competitive consecutive reaction scheme we can see that nonideal mixing can cause a decrease in selectivity in both continuous and semibatch reactors. Residence time distribution issues can cause a reduction in yield and selectivity for both slow and fast reactions (see Chapter 1), but for fast reactions, the decrease in selectivity and yield due to inefficient local mixing can be greater than that caused by RTD issues alone. In semibatch reactors, poor bulk mixing can also cause these reductions (see Example 13-3). 

The effects of mixing on selectivity have been most carefully investigated for a competitive-consecutive reaction of the type... 

Selectivity (alternative) selectivity as used by Baldyga and Bourne for the competitive-consecutive reaction scheme described in Section 13-1.2, Xs = 2S/(2S-HR)... 

Goal determination of the cause of reduced selectivity in a manufacturing scale gas-liquid competitive-consecutive reaction and modification of the reactor to achieve target selectivity... 

This example presents reactor design problems experienced in the scale-up of a classical competitive-consecutive reaction from bench to manufacturing scale. Expected selectivity was not achieved initially, and a revised reactor was required. 

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. 

Mixing-Kinetic Problem. The reaction scheme that has received the most attention in both theoretical and experimental investigations of the effects of mixing on selectivity is the competitive-consecutive reaction. In addition, the parallel reaction system is receiving attention for its importance in reactions and pH adjustments. These systems are discussed in Chapter 13 and highlighted here because of their fundamental importance in the fine chemicals and pharmaceutical industries. The reaction scheme is as follows ... 

Fig. 7.9 CFD simulation of product selectivity of a competitive consecutive reaction...

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