Scale reaction, competitive-consecutive

Once the reaction is established, three key factors must be addressed for successful development and scale-up of homogenous reactions 1) the effect of concentration on yield for competitive-consecutive reactions 2) the effect of feed rate or addition time on yield and 3) feed pipe backmixing. 

Figure 13-1 Diffusion and chanical reaction at an A-B mixing snrface. In this competitive-consecutive reaction, the first reaction, which forms the desired prodnct (R), is fast, and the consecutive reaction step, forming the nndesired by-product (S), is slower. Local mixing conditions at the molecular scale determine the amonnt of nndesired by-product (S) formed.

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. 

Fig. 1 illustrates the mixing problem for the competitive-consecutive case. R is the desired product and S is the undesired overreaction product. During the time from when the reactants are first contacted to when they are completely mixed on the molecular scale, reaction of A with B to form the desired product R occurs along with the undesired reaction of R with B to form S. When A and B are well mixed at the molecular scale, mainly R is formed, but when there is a boundary between A and B, a significant amount of undesired S appears. Competitive-parallel reactions can be subject to similar mixing effects where the first reaction is the desired one and the second is a simultaneous decomposition of A to form the undesired U. While these two reaction systems have received the most attention, the course of any reaction that is... 

The complex interaction between mass transfer and reaction kinetics requires determination of mixing sensitivity for virtually all heterogeneous reactions for which competitive and/or consecutive reactions are possible to be sure of successful scale-up. This requirement is prompted by issues of both 1) overall reaction completion time and 2) undesired reactions in the films around the discontinuous phase(s). The following set of guidelines may be useful in evaluating mixing sensitivity and for scale-up. 

Industrially relevant consecutive-competitive reaction schemes on metal catalysts were considered hydrogenation of citral, xylose and lactose. The first case study is relevant for perfumery industry, while the latter ones are used for the production of sweeteners. The catalysts deactivate during the process. The yields of the desired products are steered by mass transfer conditions and the concentration fronts move inside the particles due to catalyst deactivation. The reaction-deactivation-diffusion model was solved and the model was used to predict the behaviours of semi-batch reactors. Depending on the hydrogen concentration level on the catalyst surface, the product distribution can be steered towards isomerization or hydrogenation products. The tool developed in this work can be used for simulation and optimization of stirred tanks in laboratory and industrial scale. 

In dealing with chemical process engineering, conducting chemical reactions in a tubular reactor and in a packed bed reactor (solid-catalyzed reactions) is discussed. In consecutive-competitive reactions between two liquid partners, a maximum possible selectivity is only achievable in a tubular reactor under the condition that back-mixing of educts and products is completely prevented. The scale-up for such a process is presented. Finally, the dimensional-analytical framework is presented for the reaction rate of a fast chemical reaction in the gas/liquid system, which is to a certain degree limited by mass transfer. 

Practically any experimental kinetic curve can be reproduced using a model with a few parallel (competitive) or consecutive surface reactions or a more complicated network of chemical reactions (Fig. 4.70) with properly fitted forward and backward rate constants. For example, Hachiya et al. used a model with two parallel reactions when they were unable to reproduce their experimental curves using a model with one reaction. In view of the discussed above results, such models are likely to represent the actual sorption mechanism on time scale of a fraction of one second (with exception of some adsorbates, e.g, Cr that exchange their ligands very slowly). Nevertheless, models based on kinetic equations of chemical reactions were also used to model slow processes. For example, the kinetic model proposed by Araacher et al. [768] for sorption of multivalent cations and anions by soils involves several types of surface sites, which differ in rate constants of forward and backward reaction. These hypothetical reactions are consecutive or concurrent, some reactions are also irreversible. Model parameters were calculated for two and three... 

Viewed from the perspective of ethylene oxide, these reactions are competitive by contrast, from the perspective of the amines, they are consecutive. Consider a research scale batch reactor operating at 60°C and 20 bar to maintain all species in the liquid phase. Actual production of these commodity products on a large scale would be conducted in flow reactors, as described in Illustration 9.5. The rate laws are of the mixed second-order form (first-order in each reactant), with hypothetical rate constants ki, k2, and equal to 1,0.4, and 0.1 L-moCV min, respectively. MEA and DEA are both high-volume chemicals, while TEA is less in demand. The distribution of alkanolamine products obtained under the specified conditions can be influenced by controlling the initial mole ratio of EO to A and the time of reaction. 

Viewed from the perspective of ethylene oxide, these reactions are competitive by contrast, from the perspective of the amines, they are consecutive. An analysis to determine the concentrations of the indicated alkanolamines as functions of time in a pressurized batch reactor was considered in Illustration 5.6. However, MEA, DEA, and TEA are conomodity chemicals that are produced on an industrial scale in continuous flow reactors. 

These results continue to indicate mixing sensitivity, indicating that extreme caution must be taken on scale-up to manufacturing. The effect of addition time is not as expected for a classic consecutive-competitive reaction system, suggesting that the reaction pathway contains a step that requires maintaining short addition time on scale-up. 

Yield and/or selectivity of homogeneous consecutive-competitive reactions that are subject to mixing effects can be lower on scale-up if proper precautions are not taken for mixing the reagents—mesomixing problems get worse on scale-up and blend times increase. 

Feeding at the impeller, or the region of most intense turbulence, is recommended for consecutive or competitive reactions to avoid reduced yield/selectivity on scale-up. It is better to feed at the impeller when this is not actually required than to feed on the surface when subsurface feed was in fact necessary. 

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