Contacting, multiphase reactions

In all these reactors gravity plays an important role. To obtain good contact between phases, we need to overcome the separations that will be driven by gravity whenever the phases have different densities. This requires that the flow conditions and drop, bubble, or particle sizes are properly chosen. It is also important that the phases be separated after the reactor, and mists, emulsions, and dust in separation units can cause major problems in design of these multiphase reactors. Reactor orientation plays an obvious role in any multiphase reaction processes. 

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

CSTR may also be used for liquid-phase reactions. However, there can be severe problems with heat and mass transfer in such systems, and sampling of products can cause difficulties. For multiphase reactions it is necessary to ensure good contact between the gas and the liquid as well as between the liquid and the catalyst. 

Multiphase reactions can be significantly affected by how well mixed the system is and how intimately dispersed the phases are. The reason for this is easy to explain, but more difficult to quantify although the course of any reaction is determined exclusively by the local concentrations of the reactants and the intrinsic reaction kinetic rates, in any real reactive system, the local reactant concentrations depend not only on how fast the reactants are depleted by the reaction, but also on how fast they are locally replenished from the bulk of the phases in which they initially reside. The latter phenomenon is directly related to the existence of a mass transfer step (in series with the reaction step), which determines the rate at which the reactants in different phases are brought in contact with each other. In many cases, especially if the rate of reaction is fast with respect to the mass transfer rate, the latter mechanism can become controlling over the former, and the overall reaction process is dominated by mass transfer and, hence, multiphase mixing. 

For multiphase reactions, the contacting patterns are used as a basis for classifying the reactors. Common configurations include ... 

There are of course a number of well established alternatives to the stirred tank used in the large scale chemical industries, and strategies have been proposed for selection of the best reactor for a given multiphase reaction The nature of flow and contacting in these reactors differs significantly. Industrial applications are largely dominated however by a small number of these ... 

Effect of Agitation. Since the reduction process under consideration is a multiphase reaction, it is clear that the best results are obtainable only when the nitro compound, iron, and water-soluble catalyst are in intimate contact. A stirrer that merely pushes the iron around the bottom of the vessel and permits the charge to separate out into layers does not function efli-ciently. It is apparent, therefore, that, a sturdy sleeve-and-propelier or double-impeller type of stirrer will in some cases be superior to the slow-moving plow type, speeding up the reaction considerably. 

The phenomenon of "mass transfer with chemical reaction" takes place v/henever one phase is brought into contact with one or more other phases not in chemical equilibrium with it. This phenomenon has industrial, biological and physiological importance, in chemical process engineering, it is encountered in both separation processes and reaction engineering. In some cases, a chemical reaction may deliberately be employed for speeding up the rate of mass transfer and/or for increasing the capacity of the solvent in other cases the multiphase reaction system is a part of the process with the specific aim of nroduct formation. 

Of course, not all multiphase microstructured reactors are presented in Table 9.1. Either because they have attracted (too ) little interest, because they may have been qualified as microreactors in spite of their overall size but caimot be considered as microstmctured , or because they combine several contacting principles. Examples are a reactor developed by Jensen s group featuring a chaimel equipped with posts or pillars, thus resembling more a packed bed but with a wall-coated layer of catalyst [20], and a string catalytic reactor proposed by Kiwi-Minsker and Renken [21], that may applied to multiphase reactions. 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

A spinning basket reactor has been developed for studying multiphase reactions at high pressure. Studies have been performed on the hydrodesulfurization of various petroleum fractions and a model sulfur compound. Hydrogen and liquid feed are continuously fed to the reactor and contacted with the catalyst which is held in a rotating annular basket. The... 

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

Let us consider a simple example of multiphase reactors where the reaction A B occurs in a CSTR, but now A enters the reactor in phase a but does not react until it enters phase P, where the homogeneous rate is itC. As an example A could be an ester in an oil phase O, which will hydrolyze into an alcohol and an acid when it comes in contact with a water... 

The term "heterogeneous" as applied to the atmosphere refers to chemistry that occurs in or on ambient condensed phases that are in contact with the gas phase aerosols, clouds, surface waters, etc. It is important to distinguish between heterogeneous processes that occur on the surface of the solid, and multiphase chemical reactions that take place in the bulk of the liquid medium. In the latter case, it is assumed that the reaction takes place after the molecule has been incorporated in the bulk liquid medium, such as occurs by wet deposition, where a species is ultimately removed from the atmosphere, especially in the troposphere. 

Membrane bioreactors have been reported for the production of diltiazem chiral intermediate with a multiphase/extractive enzyme membrane reactor [15, 16]. The reaction was carried out in a two-separate phase reactor. Here, the membrane had the double role of confining the enzyme and keeping the two phases in contact while maintaining them in two different compartments. This is the case of the multiphase/ extractive membrane reactor developed on a productive scale for the production of a chiral intermediate of diltiazem ((2R,3S)-methylmethoxyphenylglycidate), a drug used in the treatment of hypertension and angina [15]. The principle is illustrated in... 

The thickness of the AnBq layer as well as the AtB layer is thus determined by the rate of occurrence of two partial chemical reactions. The thickness of the ArBs layer located between them depends on the rate of four partial chemical reactions. The same applies to any other compound layer of a multiphase system, having no direct contact with either of initial phases, if the number of compounds on the phase diagram exceeds three. 

Multiphase reactors are reactors in which two or more phases are necessary to carry out the reaction. The majority of multiphase reactors involve gas and liquid phases which contact a solid. In the case of the slurry and trickle bed reactors, the reaction between the gas and the liquid takes place on a solid catalyst Sluface (see Table 12-2). However, in some reactors the liquid phase is an inert medium for the gas to contact the solid catalyst. The latter situation arises when a large heat sink is required for highly exothermic reactions. In many cases the catalyst life is extended by these milder operating conditions. 

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