Stirred tank reactors multiphase

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

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. 

Real kinetics data To date, almost all the kinetics data on reaction systems in liquid phase or multiphase with liquid as the continuous phase have been measured in traditional stirred tank reactors. From the results reported in this chapter, it is likely that significant deviations exist in the existing kinetics data. On the other hand, the LIS device cannot yet be considered as absolutely ideal for kinetics investigation, not least because its micromixing time, tM, is not zero. What then is the ideal equipment and conditions for obtaining real kinetics data ... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Example 4.8 Chemical reactions and reacting flows The extension of the theory of linear nonequilibrium thermodynamics to nonlinear systems can describe systems far from equilibrium, such as open chemical reactions. Some chemical reactions may include multiple stationary states, periodic and nonperiodic oscillations, chemical waves, and spatial patterns. The determination of entropy of stationary states in a continuously stirred tank reactor may provide insight into the thermodynamics of open nonlinear systems and the optimum operating conditions of multiphase combustion. These conditions may be achieved by minimizing entropy production and the lost available work, which may lead to the maximum net energy output per unit mass of the flow at the reactor exit. 

FIGURE 10.1 Types of stirred tank reactor, (a) Multiphase stirred reactor. I impeller, 2 baffles, 3 cooling coils, 4 gas sparger, (b) Stirred reactor with gas-inducing impeller (dead-end type), (c) Stirred reactor with helical ribbon impeller (used with or without a draft tube). 

In general, it may be concluded that the computational snapshot approach or other equivalent, state of the art CFD models can capture the key features of flow in stirred tank reactors and can be used to make either quantitative (for single-phase or pseudo-homogeneous applications) or semi-quantitative (for complex, multiphase applications) predictions. Possible applications to reactor engineering are discussed below. 

Complex multiphase flows (e.g., slurry bubble columns and stirred tank reactors i.e., bubbly flows in slurries three phase fluidized beds, i.e., liquid droplets and particles in continuous gas) where many phases interact simultaneously. 

RGibbs - rigorous equilibrium and/or multiphase Gibbs free energy minimization RCSTR - continuous stirred-tank reactor, specify volume... 

Multiphase mass transfer is a vast topic. A general reference for gas-liquid and solid-liquid mass transfer in stirred tank reactors is ... 

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

It is rather difficult to establish a reasonably accurate mathematical model of a gas-liquid-liquid three-phase reactor for biphasic hydroformylation, because of the complexity in formulating all the necessary mechanisms such as phase dispersion and distribution, multiphase flow, interphase mass transfer, micromixing, and the hydroformylation reaction. Besides, the task is further complicated by turbulence in multiphase flow and the complex domain of stirred-tank reactors. 

Chemical engineers intuitively work under the assumption that to improve the mass-transfer characteristics of a gas-liquid reactor, more energy must be dissipated in the fluids to effect a more vigorous contacting of the fluids. For singlephase flow in turbulent systems, this concept has become known as the Chilton-Colburn analogy. Another example of the coupling of hydrodynamics and mass transfer in multiphase systems is the common correlation of the mass transfer and the power input for stirred-tank reactors. 

Extent of reaction specified Two-phase, chemical equilibrium Multiphase, chemical equilibrium Continuous-stirred tank reactor Plug-flow tubular reactor Pump or hydraulic turbine Compressor or turbine Pressure drop in a pipe Stream multiplier Stream duplicator... 

In addition to processes involving gas-liquid reactions, stirred-tank reactors can also be used for single (liquid)-phase reactions. Moreover, their operation is not limited to the continuous mode, and they can be easily adapted for use in semibatch and batch modes. The absence of a gas phase does not pose important structural and operational differences from those stated earlier for multiphase systems. However, in the case of single-phase operation, the aspect ratio is usually kept lower ( 1) to ensure well mixing of the reactive liquid. Regardless of the number of phases involved, stirred-tank reactors can approach their ideal states if perfect mixing is established. Under such conditions, it is assumed that reaction takes place immediately just... 

By using CFD, the fluid flows can be taken into closer examination. Rigorous submodels can be implemented into commercial CFD codes to calculate local two-phase properties. These models are Population balance equations for bubble/droplet size distribution, mass transfer calculation, chemical kinetics and thermodynamics. Simulation of a two-phase stirred tank reactor proved to be a reasonable task. The results revealed details of the reactor operation that cannot be observed directly. It is clear that this methodology is applicable also for other multiphase process equipment than reactors. 

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