Liquid reactors

Mixing of Two Miscible Liquids 1. Correlation of Mixing Time 

In the development of the above equation, it is assumed that the number of stirrer beams increases with vessel height in accordance with Fig. 21a. Homogenization and power characteristics for enamel-coated stirrers with rounded edges are described by Gramlich and Lamade (1973) a number of other mixer types and insert configurations are treated by Henzler (1978). 

This relationship is valid for 10 Re 10s and 102 Ar 1011. Similar relationships for NO for other configurations would also depend on both Reynolds and Archimedes numbers. 

The mixing time for viscous liquids was examined by Hoogendoorn and Den Hartog (1967). The types of mixers examined in this study are illustrated in Fig. 23. The mixing time was measured by a decoloration and a thermal response technique (see Section IX). In truly viscous flow, the mixing time was inversely proportional to the stirrer speed. The performance of the various mixers were compared using the two dimensionless correlations 02P/(dfp) and pdf/(fid). The turbine and anchor mixers were found to be unsatisfactory for viscous mixing. 

The power required for a given stirrer type and associated vessel configuration depends on the speed of rotation N, the stirrer diameter du the density p, and the kinematic viscosity v of the medium. In vessels without baffles, the liquid vortex, and therefore the acceleration due to gravity, g, is immaterial, as long as no gas is entrained in the liquid. Thus, P = f(N, dt,p, v), and in the dimensionless form, Ne = /(Re), a relationship generally known as the power characteristics of the stirrer. Here, Ne = P/(pN3df) is the Newton or Power number, and Re s Ndf/v the Reynolds number. This relationship was described in Sections II and III for gas-liquid and gas-liquid-solid systems. 

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Figure 8.12 Gas-liquid reactor model (Yagi, 19H6) where...

Figure 8.33 Schematic development of an industrial gas-liquid reactor (Wachi and Jones, 1994)...

The interfacial area AtV usually excludes the contact area between the vapor space and the liquid at the top of the reactor. The justification for this is that most gas-liquid reactors have gas bubbles as a dispersed phase. This gives a much larger interfacial area than the nominal contact area at the top of the reactor. There are exceptions—e.g., polyester reactors where by-product water is removed only through the nominal interface at the top of the reactor— but these are old and inefficient designs. This nominal area scales as while the contact area with a dispersed phase can scale as S. 

TABLE 11.4 Typical Flow and Mixing Regimes for Gas-Liquid Reactors... 

Mass transfer in a gas-liquid or a liquid-liquid reactor is mainly determined by the size of the fluid particles and the interfacial area. The diffusivity in gas phase is high, and usually no concentration gradients are observed in a bubble, whereas large concentration gradients are observed in drops. An internal circulation enhances the mass transfer in a drop, but it is still the molecular diffusion in the drop that limits the mass transfer. An estimation, from the time constant, of the time it wiU take to empty a 5-mm drop is given by Td = d /4D = (10 ) /4 x 10 = 6000s. The diffusion timescale varies with the square of the diameter of the drop, so... 

An effective hquid-liquid reactor may be designed to obtain drops that continuously break up and coalesce, or it may be designed to obtain very small drops that have very efficient mass transfer and follow the continuous phase with a low rate of coalescence. The former will require a much larger reactor, but the separation of the phases after reaction is simpler. 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

Table 5.2 Comparison of reaction conditions and results for hydrogenation of methyl (Z)-a-acetamidocinnamate in mini batch, micro liquid/liquid and micro gas/liquid reactors [70. ...

A survey of the mathematical models for typical chemical reactors and reactions shows that several hydrodynamic and transfer coefficients (model parameters) must be known to simulate reactor behaviour. These model parameters are listed in Table 5.4-6 (see also Table 5.4-1 in Section 5.4.1). Regions of interfacial surface area for various gas-liquid reactors are shown in Fig. 5.4-15. Many correlations for transfer coefficients have been published in the literature (see the list of books and review papers at the beginning of this section). The coefficients can be evaluated from those correlations within an average accuracy of about 25%. This is usually sufficient for modelling of chemical reactors. Mathematical models of reactors arc often more sensitive to kinetic parameters. Experimental methods and procedures for parameters estimation are discussed in the subsequent section. 

Mass and heat balance equations for typical gas-liquid reactors in heterogeneous systems at steady state... 

Requirements regarding laboratory liquid-liquid reactors are very similar to those for gas-liquid reactors. To interpret laboratory data properly, knowledge of the interfacial area, mass-transfer coefficients, effect of contaminants on mass-transport processes, ionic characteristics of the system, etc. is needed. Commonly used liquid-liquid reactors have been discussed by Doraiswamy and Sharma (1984). 

Shah, Y.T., 1979, Gus-Solid-Liquid Reactor Design , McGraw-Hill Publishing Co., New York. Shah, Y.T., 1991, Ac/v. Chem. Eng. 17, 1. 

Two separate 2.1 L reservoirs contain the catalyst and product phases and the contents are fed into the reactor through a standard liquid mass flow controller. The contents of the reactor can be sampled from a pressure fed sample tube. The pressurized liquid reactor products exit the reactor through a pressure control valve, which reduces the pressure to atmospheric, and the liquid contents are delivered to a continuous decanter where the phases separate. The catalyst phase then settles to the bottom where it is drained for recycle and reuse, while the product phase is collected into a 4.2 L reservoir. 

Gas-liquid reactors. Gas-liquid reactors are quite common. Gas-phase components will normally have a small molar mass. Consider the interface between a gas and a liquid that is assumed to have a flow pattern giving a stagnant film in the liquid and the gas on each side of the interface, as illustrated in Figure 7.2. The bulk of the gas and the liquid are assumed to have a uniform concentration. It will be assumed here that Reactant A must transfer from the gas to the liquid for the reaction to occur. There is diffusional resistance in the gas film and the liquid film. 

Liquid-liquid reactors. Examples of liquid-liquid reactions are the nitration and sulfonation of organic liquids. Much of the discussion for gas-liquid reactions also applies to liquid-liquid reactions. In liquid-liquid reactions, mass needs to be transferred between two immiscible liquids for the reaction to take place. However, rather than gas-and liquid-film resistance as shown in Figure 7.2, there are two liquid-film resistances. The reaction may occur in one phase or both phases simultaneously. Generally, the solubility relationships are such that the extent of the reactions in one of the phases is so small that it can be neglected. 

Figure 7.4 illustrates some of the arrangements that can be used for liquid-liquid reactors. The first arrangement... 

Figure 7.4 Contacting patterns for liquid-liquid reactors.

Figure 7.4c shows an in-line static mixer. Dispersion is usually promoted by repeatedly changing the direction of flow locally within the mixing device as the liquids are pumped through. This will give a good approximation to plug-flow in both phases in cocurrent flow. As with gas-liquid reactors, static mixers are particularly suitable when a short residence time is required. 

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