Liquid-phase reactants agitator

Liquid-solid mass transport (liquid reactant) Amount of catalyst Catalyst particle size Concentration of reactant in liquid phase Temperature Agitation rate Reactor design Viscosity Relative densities Concentration of gas-phase reactant Concentration of active components on catalyst... 

A new alternative approach for Stage I screening in liquid phase is the use of bubble column-type reactors. These parallel bubble columns can operate in batch and fed-batch mode regarding the reaction mixture, while a continuous stream of gas is used as reactant (H2, 02, or others) as well as for the intense agitation of the reaction mixture (Figure 11.39). 

In a liquid phase with mechanical agitation and reactant-gas bubbling, the operation is characterized by the following factors. 

The individual mass transfer and reaction steps outlined in Fig. 4.15 will now be described quantitatively. The aim will be firstly to obtain an expression for the overall rate of transformation of the reactant, and then to examine each term in this expression to see whether any one step contributes a disproportionate resistance to the overall rate. For simplicity we shall consider the gas to consist of just a pure reactant A, typically hydrogen, and assume the reaction which takes place on the interior surface of the catalyst particles to be first order with respect to this reactant only, i.e. the reaction is pseudo first-order with rate constant A ,. In an agitated tank suspended-bed reactor, as shown in Fig. 4.20, the gas is dispersed as bubbles, and it will be assumed that the liquid phase is well-mixed , i.e. the concentration CAL of dissolved A is uniform throughout, except in the liquid films immediately surrounding the bubbles and the particles. (It will be assumed also that the particles are not so extremely small that some are present just beneath the surface of the liquid within the diffusion film and are thus able to catalyse the reaction before A reaches the bulk of the liquid.)... 

Agitated Slurry Reactors The gas reactant and solid catalyst are dispersed in a continuous liquid phase by mechanical agitation using stirrers. Most issues associated with gas-liquid-solid stirred tanks are analogous to the gas-liquid systems. In addition to providing good... 

In a reaction between a liquid and the solids, the solids have to be agitated and distributed throughout the liquid either by agitators or by gas bubbles to avoid settling of the solids. Prior to entering the reactor the solids have to be milled to a suitable size so that good contact with the liquid reactants is maintained, but not so fine that it is difficult to remove unreacted solids from the liquid phase in the separator. 

Of all the reaction variables involved in a heterogeneously catalyzed reaction, the most important is the nature of the catalyst to be used. Factors associated with catalyst preparation and selection will be discussed in Sections II and III. The relative importance of the other reaction parameters will depend on a number of factors. Reactions that run in a continuous or flow system have different requirements from those run in a batch mode. Generally, parameters such as the quantity of catalyst, the size of the catalyst particles, the temperature of the system, the concentration of the substrate(s), and, when gaseous reactants are used, the reaction pressure, are important variables in heterogeneously catalyzed reactions. In flow reactions the catalyst substrate contact time can frequently have a significant impact on the outcome of the reaction. In liquid phase batch processes catalyst agitation can also play an important role. The one constant parameter in almost all liquid phase reactions is the presence of a solvent, the nature of which is an important factor in heterogeneously catalyzed liquid phase reactions. 

Increasing the pressure of the gaseous reactant not only increases the amount present in the gas phase but also increases gas/liquid transport and the solubility of the gas in the liquid phase. This, in turn, facilitates liquid/solid transport of this species. All of these factors increase the availability of the gaseous reagent to the catalyst. Fig. 5.11 shows a typical plot for the relationship between hydrogen pressure and the reaction rate at a fixed catalyst quantity and agitation rate.28 At lower values an increase in pressure promotes an increase in rate but above a given value further increases in pressure have little or no effect on the rate. In the... 

Agitation is a critical process parameter in mass transfer-limited processes, particularly ones such as this for which the reaction rates are purely gas transfer dependent. Moreover, control of reactant transfer from the gas phase to the liquid phase is vital to obtain consistent process performance. Thus, scale-up from the lab-scale to industrial production must focus on the critical parameters that preserve the desired mass transfer properties. 

The CSTR is used extensively in situations where intense agitation is required, such as the addition of a gaseous reactant to a liquid by transfer between the bubbles and the continuous liquid, and the suspension of a solid or second liquid within a continuous liquid phase. Polymerization reactions are sometimes conducted in CSTRs. It is common to employ a cascade or series of CSTRs in which the effluent from the first reactor is used as feed to the second and so forth down the cascade (Figure 1.4). The cascade permits one to realize high conversion of reactant, while minimizing total reactor volume. 

The second broad grouping of papers (Chapters 11—16) considers both the chemistry and physical transfer steps between phases which often occur during nitration. In aromatic nitrations using mixed acids, for example, the presence of two immiscible liquid phases complicates the nitration reaction. Agitation to emulsify the two phases is necessary to obtain adequate contact between the hydrocarbon and the nitrating species. Transfer of reactants and products, heat transfer, nature of emulsion, etc. are key factors. 

A liquid-phase oxidation is carried out in aqueous solution in an agitated, sparged reactor. Batch tests show that the absorption rate decreases as the conversion increases, judging from the change in exit gas composition. The dissolved reactant and the product are nonvolatile, and the vapor pressure of the solution is about 80% that of water. The solution density is 60 Ib/ft. ... 

Yang [234] has developed a theoretical model to investigate the effects of mass transfer and distribution of the catalyst within the third liquid phase and organic or aqueous phase on the overall reaction rate. The modeling considers the dispersed organic droplet surrounded by an interfacial catalyst layer under agitation conditions, as shown in Fig. 10. This type of droplet is similar to some oil/water emulsions in the presence of surfactants. The reactant... 

This problem has been adapted with permission from the late Professor C. N. Satterfield of MIT. R. H. Price and R. B. Schiewetz [Ind Eng. Chem., 49, 807 (1957)] studied the catalytic liquid-phase hydrogenation of cyclohexene in a laboratory-scale semibatch reactor. A supported platinum catalyst was suspended in a cyclohexene solution of the reactant by mechanical agitation of the solution. Hydrogen was bubbled through the solution continuously. The reactor is described in their words as follows ... 

All of the empirical parameters that affect the activity of polystyrene-supported onium ion catalysts in nucleophilic displacement reactions fit into a general mechanism. The reaction rates may be limited by 1) mass transfer of reactants from the bulk liquid phases to the surface of the catalyst, 2) diffusion of the reactants from the catalyst surface to the active site, and 3) intrinsic reactivity at the active site, as diagrammed in Figure 4. Mass transfer refers to transport of molecules to the catalyst surface first by agitation and diffusion in bulk liquid and then by film... 

The trickle-bed catalytic reactor shown in Fig. 6.8 utilizes product recycle to obtain satisfactory operating conditions for temperature and conversion. Use of a high recycle rate eliminates the need for mechanical agitation. Concentrations of the single reactant and the product are measured at a point in the recycle line where the product stream is removed, A liquid phase first-order reaction is involved. 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

A batch reactor is an agitated vessel in which the reactants are precharged and which is then emptied after the reaction is completed. More frequently for exothermic reactions, only part of the reactants are charged initially, and the remaining reactants and catalysts are fed on a controlled basis this is called a semi-batch operation. For highly exothermic reactions and for two-phase (gas-liquid) reactions, loop reactors with resultant smaller volumes can be used. 

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