Agitated Slurry Reactors

The trickle-bed reactor (TBR) and slurry reactor (SR) are the most commonly used for multiphase reactions in the chemical industries. A new reactor type, the monolithic reactor (MR), offers many advantages. Therefore, these three types of reactors are discussed below in more detail. Their general characteristics are given in Table 5.4-44. With respect to slurry reactors, the focus will be on mechanically agitated slurry reactors (MASR) because these are more widely used in fine chemicals manufacture than column slurry reactors. 

Agitated slurry reactor (suspended fine solids)... 

Agitated slurry reactor (ASR) This is a mechanically agitated gas-liquid-solid reactor (Figure 3.13). The liquid is agitated by a mechanical apparatus (impeller). The fine solid particles are suspended in the liquid phase by means of agitation. Gas is sparged into the liquid phase, entering at the bottom of the tank, normally just under the impeller. This reactor can also be of continuous type or of semibatch type. This type is used only in catalysis. 

There are two types of slurry reactors slurry bubble column reactor (SBCR, Figure 3.25) and agitated slurry reactor (ASR, Figure 3.26). These reactors differ in that the solid... 

AGITATED SLURRY REACTORS 3.5.1 Agitated slurry reactor models... 

The proposed correlation for agitated slurry reactors is that of Sano et al. (1974 Ramachandran and Chaudhari, 1984 Koide, 1996) ... 

The removal of sulfur dioxide from air has been studied by Komiyama and Smith (1975) in an agitated slurry reactor using activated carbon particles as catalyst at 25 °C and 1 atm. The gas stream consisted of 2.3% S02and 21% oxygen, and pure water was used as liquid phase. Activated carbon catalyses the oxidation of S02 to S03, which dissolves in the water to produce H2S04 ... 

In the following we will describe mechanically agitated slurry reactors and slurry bubble columns. 

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... 

A comparison between a liquid entrained slurry reactor and a mechanically agitated slurry reactor for methanol synthesis was made by Vijayaraghavan et al. [11]. 

Mechanically agitated slurry reactors have a wide range of applications in the chemical, biochemical, and pharmaceutical industries and, in particular, catalytic processes (see Table IV). They are commonly used for catalytic hydrogenation, oxidation, halogenation, or polymerization reactions such as... 

Some Applications of Mechanically Agitated Slurry Reactors... 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

Shinn (1982) developed a mechanically agitated slurry reactor with induction heaters for coal liquefaction. While the induction heaters required large power input, they allowed the slurry to heat up to 400-450°C in few minutes, thus cutting down the heat-up period. In the use of such reactors, the effects of induction heating on the metal degradation and failure need to be carefully considered. Except for the induction heating system, the rest of the reactor was a conventional slurry reactor. The concept of induction heating is more practical for smaller-size reactors. 

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. 

Agitated slurry reactor Catalytic and noncatalytic operations Transport rates are high Catalyst erosion, sintering... 

In the case of a stationary basket-type reactor, kLaL correlations proposed for conventional agitated slurry reactors by Ramachandran and Choudhari (1983) can be used. No experimental data for liquid-solid systems are, however, available. 

Since mixing and good heat transfer are of vital importance in viscous polymerization reactions, a mechanically agitated continuous stirred-tank reactor is widely used in polymerization processes. Solution polymerization, emulsion polymerization, and solid-catalyzed olefin polymerization are all carried out in a mechanically agitated slurry reactor. 

Figure 1-3 Schematic diagrams of some gas-liquid-suspended-solid reactors, (a) Agitated slurry reactor, ( >) nonagitated slurry reactor, (c) fluidized-bed reactor.

It allows more flexibility for mixing, e.g.. agitated slurry reactor. 

An interesting study of the gas liquid mass transfer in a three-phase agitated slurry reactor was recently reported by Joosten et al.51 They showed that in the absence of solids, the volumetric mass-transfer coefficient can be well correlated to total power (power dissipated by stirrer + gas) per unit volume, but poorly correlated to the power dissipated by the stirrer only, as done in Fig. 9-14. Their data were well correlated by the correlation of Van Dierendock.23... 

Any form of convection, of course, increases the value of Ks. In slurry operation with no liquid flow, gas flow induces convection. In an agitated slurry reactor, stirring causes convection. In a pulsating slurry reactor, pulsation of the slurry induces convection and in a three-phase fluidized bed, the movements of both gas and liquid phases cause convection. Any one or more modes of convection will increase the value of the solid-liquid mass-transfer coefficient. In broad terms, the convective liquid-solid mass-transfer coefficient is correlated by-two steady state theories. Here we briefly review and compare them. 

The fundamental objection to the above relations is that they are derived assuming steady-state flow. In practice, the intensity of turbulence in agitated slurry reactors is time dependent. Also, an accurate estimate of the relative velocity between the liquid and solid is often difficult.45,12S The relative velocity has been related to various system parameters by Kuboi et al.67,68... 

KolmogorofTs theory Brian et al.,9 Elenkov et al.,26 and Middleman84 used Kolmogoroff s theory of local isotropic turbulence in an attempt to correlate the effective relative velocity with some macroscopic variables, such as stirrer speed and particle diameter. From the dimensionless analysis of agitated slurry reactors,45,84 they suggested a correlation... 

Cybulski, A., Stankiewicz, A., Edvinsson Albers, R.K. and Moulijn, J.A. (1999), Monolithic reactors for fine chemicals industries A comparative analysis of a monolithic reactor and a mechanically agitated slurry reactor, Chem. Eng. Sci., 54, 2351-2358. 

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