Agitated reactors mixing

Mixing Models. The assumption of perfect or micro-mixing is frequently made for continuous stirred tank reactors and the ensuing reactor model used for design and optimization studies. For well-agitated reactors with moderate reaction rates and for reaction media which are not too viscous, this model is often justified. Micro-mixed reactors are characterized by uniform concentrations throughout the reactor and an exponential residence time distribution function. 

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. 

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

If the process is continuous and in the complete mixed-flow mode, for both the gas and slurry phases, the equations derived for agitated sluny reactors are valid (see Section 3.5.1) (Ramachandran and Chaudhari, 1980) by simply applying the appropriate mass transfer coefficients. Note that in sluiiy-agitated reactors, the material balances are based on the volume of the bubble-free liquid. Furthermore, in reactions of the form aA(g) + B(l) — products, if gas phase concentration of A is constant, the same treatment holds for the plug flow of the gas phase. 

Tlie requirement for mechanical agitation can be avoided by using a fluidized bed reactor. In this type of reactor, the agitation and mixing are achieved by means of the moving liquid that carries the solids through the reactor or mixes with the particle phase. Thus, high heat and mass transfer rates are assured. 

Bubble slurry column reactors (BSCR) and mechanically stirred slurry reactors (MSSR) are particular types of slurry catalytic reactors (Fig. 5.3-1), where the fine particles of solid catalyst are suspended in the liquid phase by a gas dispersed in the form of bubbles or by the agitator. The mixing of the slurry phase (solid and liquid) is also due to the gas flow. BSCR may be operated in batch or continuous modes. In contrast, MSSR are operated batchwise with gas recirculation. 

The simplest reactor configuration for any enzyme reaction is the batch mode. A batch enzyme reactor is normally equipped with an agitator to mix the reactant, and the pH of the reactant is maintained by employing either a buffer solution or a pH controller. An ideal batch reactor is assumed to be well mixed so that the contents are uniform in composition at all times. 

Figure 1.5 illustrates a flow reactor in which the contents are mechanically agitated. If mixing caused by the agitator is sufficiently fast, the entering feed will be quickly dispersed throughout the vessel and the composition at any point will approximate the average composition. Thus, the reaction rate at any point will be approximately the same. Also, the outlet concentration will be identical to the internal composition, a= a. 

Although, as shown in this monograph, mechanical agitation is provided in a number of different ways, the most common method is by a stirrer in a standard vessel. In many mechanically agitated reactors, the vessel contains internals such as baffles (particularly for low-viscosity fluids), feed and drain pipes, heat transfer coils, and probes (e.g., thermometers or thermocouples, pressure transducers, level indicators). The degree of mixing and power requirement depend on the nature of the internals present in the vessel. 

The liquid-phase mixing in a multistage mechanically agitated reactor is best correlated by Eq. (2.31) in the absence of gas flow and by Eq. (2.32) in the presence of gas flow. The mixing time can be estimated from the study of Paca et al. (1976). Experimental work is needed to estimate gas-phase back-mixing. The use of Eq. (2.36) for the calculation of the gas-liquid volumetric mass transfer coefficient in a multistage mechanically agitated column is recommended. 

In previous sections, we examined the design parameters for gas-liquid, gas-solid, liquid-liquid, gas-liquid-solid, biological polymerization, and special types of mechanically agitated reactors. In this section we present a brief review on available techniques for the measurement of various mixing and transport parameters for a mechanically agitated vessel. Both physical and chemical techniques are examined. 

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