Agitation three phase reactions

In a more recent work, Slesser et al.128 showed that small amounts of fine solid particles in an agitated three-phase slurry reaction can change the magnitude of the volumetric mass-transfer coefficient considerably. Chandrasekaran and Sharma14 reported a similar conclusion in the case of the oxidation of sodium sulfide in the presence of activated carbon. They argued that the presence of solids prevents bubble coalescence and thus increases the gas-liquid interfacial area. 

Fig. 5.10. Idealized representation of the effect of agitation rate on the rate of a heterogeneously catalyzed three phase reaction.

These multi-phase approaches, however, suffer from the drawback that some of the reaction media must be separated from the catalyst or the reaction products. Therefore, the conditions of catalyst separation may be very different from those of the reaction such that catalyst decomposition may still occur. Furthermore, most approaches have two or three phases during the reaction, which may cause problems in controlling the phase equilibria and in controlling the distribution of the reacting components and the catalytically active component between the phases. Most effective agitation of the reaction mixture is often required. 

The overall rate of reaction calculated for the three-phase fluidised-bed reactor above is approximately one tenth of the rate calculated for the agitated tank slurry reactor in Example 4.6. The main reasons are the very poor effectiveness factor and the relatively smaller external surface area for mass transfer caused by using the larger particles. Even the gas-liquid transfer resistance is greater for the three-phase fluidised-bed, in spite of the larger particles being able to produce relatively small bubbles these bubbles are not however as small as can be produced... 

Mechanically agitated slurry reactors are widely used in three-phase catalytic and noncatalytic reactions. In aerated slurry reactors, the three regimes outlined in Table V prevail. These regimes are schematically illustrated in Fig. 11. The gas flow rate and stirrer speed where the transition from regimes a to b or b to c with a Rushton turbine stirrer occurs can be estimated from the relationships described in Table VI. 

Polymer films can also be deposited on solid particles by vapor phase reaction or from a melt. The best example of vapor phase reaction is the deposition of Union Carbide s Parylene , a derivative of p-xylylene. In this process, di-p-xylylene, or more commonly a halogenated derivative of it, is vaporized in a vac and therm-mally dissociated into the very reactive monomer, a diradical. The monomer is allowed to condense on the surface of the particles to be coated, where it instantaneously polymerizes to form a high molecular weight, polymeric film (Ref 10). Less reactive, vaporizable or meltable polymers can be applied by hot spraying onto agitated particulates or by deposition in a fluidized bed or in liq suspension (Ref 2). Wax is a common example of wall material applied in all three ways... 

The ways in which reaction parameters affect a two phase batch reaction are similar to those considered above for the three phase systems. Since there is no gas phase, agitation only serves to keep the catalyst suspended making it more accessible to the dissolved reactants so it only has a secondary effect on mass transfer processes. Substrate concentration and catalyst quantity are the two most important reaction variables in such reactions since both have an influence on the rate of migration of the reactants through the liquid/solid interface. Also of significant importance are the factors involved in minimizing pore diffusion factors the size of the catalyst particles and their pore structure. 

Despite the experience with batch reactors it may be worthwhile to operate continuous reactors also for fine chemicals. Continuously operated reactors only demand for one start-up and one shut-down during the production series for one product. This increases the operating time efficiency and prevents the deactivation of dry catalysts this implies that the reactor volume can be much smaller than for batch reactors. As to the reactor type for three phase systems an agitated slurry tank reactor [5,6] is not advisable, because of the good mixing characteristics. Specially for consecutive reaction systems the yields to desired products and selectivities will be considerably lower than in plug flow type reactor. The cocurrent down flow trickle flow reactor... 

A number of studies have been reported on solid-liquid mass transfer in different kinds of contactors, and they have been periodically reviewed, for example Miller (1971), Nienow (1975), Wen and Fan (1975), Briens et al. (1993). Of these, only the mechanically agitated contactor is used with or without a gas phase. It is the only truly two-phase solid-liquid contactor. The other types of contactors, such as the bubble-column contactor (usually), the trickle-bed reactor, and the three-phase fluidized-bed reactor, all involve three phases and are considered in Chapter 17 on multiphase reactions. 

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