Reactor performance parallel reactions

So far, consideration has been limited to chemistry physical constraints such as heat transfer may also dictate the way in which reactions are performed. Oxidation reactions are highly exothermic and effectively there are only two types of reactor in which selective oxidation can be achieved on a practical scale multitubular fixed bed reactors with fused salt cooling on the outside of the tubes and fluid bed reactors. Each has its own characteristics and constraints. Multitubular reactors have an effective upper size limit and if a plant is required which is too large to allow the use of a single reactor, two reactors must be used in parallel. 

The present book is devoted to both the experimentally tested micro reactors and micro reaction systems described in current scientific literature as well as the corresponding processes. It will become apparent that many micro reactors at first sight simply consist of a multitude of parallel channels. However, a closer look reveals that the details of fluid dynamics or heat and mass transfer often determine their performance. For this reason, besides the description of the equipment and processes referred to above, this book contains a separate chapter on modeling and simulation of transport phenomena in micro reactors. 

As demonstrated in the previous sections of this review, microwave-assisted reactions allow rapid product generation in high yield under uniform conditions. Therefore, they should be ideally suited for parallel synthesis and/or combinatorial chemistry applications. The first example of parallel reactions performed under micro-wave irradiation conditions involved the nucleophilic substitution of an alkyl iodide with 60 diverse piperidine or piperazine derivatives (Scheme 12.22) [71]. Reactions were performed in a multimode microwave reactor in individual sealed polypropy-... 

The dimensions of these vials were 40 x 13 mm, and the dimensions of the magnetic stir bar were 3.0 x 6.5 mm. Four of these vials fit exactly into the Premex HPM-005 50 mL reactor. Identical results were obtained using this parallel reactor compared with reactions performed in isolated vessels. 

Fig. 1.24. Performance of cross-flow reactors with five equidistant feed points Parallel reactions A + B -> P > kPCACB...

Microreactors (flow reactors with micrometer scale) were first employed in organic synthesis to perform chemical reactions in flow processes. The small dimensions of microreactors allow the use of minimal amounts of reagent under precisely controlled conditions, and the rapid screening of reaction conditions with improved overall safety of the process. To obtain synthetically useful amounts of material, either the microreactors are simply allowed to run for a longer period of time ( scale-out ), or several reactors are placed in parallel ( numbering up ) [29],... 

It is observed, even in the partial solution to the problem, that realistic models of the droplet coalescence and breakage processes as discussed in Section V,D,2 have yet to be employed. A parallel development has occurred. The work is currently at the point where the realistic model of the droplet dynamics can be applied to the pertinent problems of extent of reaction and solute depletion in dispersions. The success of this effort would permit the researcher and designer to predict dispersed-phase reactor performance from fundamental properties of the dispersion, operation conditions of the vessel, and knowledge of the intrinsic kinetics. 

A stirred tank may give better or worse selectivities than a tubular-flow unit in multiple-reaction systems. As usual, the key point is the relative values of the activation energies for the reactions. In particular, for a set of parallel reactions, where the desired product is formed by the reaction with the higher activation energy, the stirred tank is advantageous. The production of allyl chloride considered in Example 5-2 is a case in point. The performance of a stirred-tank reactor for this system is discussed next, and the results are compared with the performance of the tubular-flow reactor. 

Next, we consider parallel chemical reactions. These are reactions in which a reactant of the desirable reaction also reacts in another reaction to form undesirable species. In such cases, it is important to consider not only the conversion of the reactant but also the amount of desirable and undesirable products formed. The yield of the desirable product provides a measure of the reactor performance. 

The kinetic measurements were performed by monitoring the gas phase composition along the length of a fixed bed of catalyst. The reactor was treated as an isothermal plug flow system. The reaction kinetics can be described with a simple triangle network consisting of the main reaction (aldehyde to carboxylic acid), a consecutive reaction (carboxylic acid to byproducts) and a parallel reaction (aldehyde to by-products). 

This approach can also be used for multichannel MSRs. Because of the small volumes of the individual channels, many channels have to be used in parallel to produce substantial quantities of product. A uniform distribution of the reaction mixture over thousands of microchannels is usually necessary to obtain adequate performance of a multichannel MSR [76]. Flow maldistribution will broaden the RTD in such a multitubular reactor and will lead to a reduced reactor performance, meaning reduced product yield and selectivity at a given space velocity [75,77]. Therefore, several authors have presented design studies of flow distribution manifolds [71,78-80]. 

Riser technology appears to be quite versatile. Patience and Mills [33] investigated propylene oxidation into acrolein and found that this technique has a potential for the commercial scale production of acrolein. Their kinetic model was based on a simplified single site redox mechanism involving consecutive-parallel reactions for the partial and complete oxidation of propylene. Its predictions of the performance of the reactor gave correct trends. 

Owing to the small volume of a single channel, many channels have to be used in parallel to obtain a sufficient reactor performance. A uniform distribution of the reaction mixture over thousands of microchannels is necessary. Elow... 

EP.IO The parallel reactions are irreversible and the A- 2R and A- S product rates are of second order with respect to reactants. Reaction was performed in a PFR reactor. Feed composition was 50% for A and 50% for Inert. The molecular weights are 40 and 20, respectively. The exit flow rate of product R is 6 mol/h. The total flow rate is 1000 mL/min at 10 atm and constant temperature 400° C. Assume ideal gases. The reactor volume is 2 L and the selectivity relative to R is 85%. Calculate constants ki and k2-... 

In the case of known formal kinetics, the reactor performance can be determined directly from the RTD. We can imagine, for example, that the RTD in the reactor under consideration can be represented by a series of ideal plug flow reactors of different lengths arranged in parallel through which the reaction mass flows at equal rates (see Figure 3.17). 

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