Stirred tank reactors baffles

In a stirred tank reactor, these low-pressure regions are behind the impeller blades, in the trailing vortices leaving the impeller blades, behind the baffles, and at the center of the large turbulent eddies. 

The presented results for systematic studies on hydrodynamic stress in shake flasks, baffled stirred tanks, reactors in which boundary layer flow predominates (e.g. stirred tank with a smooth disc or unbaffled stirred tank), viscosi-... 

The classical CRE model for a perfectly macromixed reactor is the continuous stirred tank reactor (CSTR). Thus, to fix our ideas, let us consider a stirred tank with two inlet streams and one outlet stream. The CFD model for this system would compute the flow field inside of the stirred tank given the inlet flow velocities and concentrations, the geometry of the reactor (including baffles and impellers), and the angular velocity of the stirrer. For liquid-phase flow with uniform density, the CFD model for the flow field can be developed independently from the mixing model. For simplicity, we will consider this case. Nevertheless, the SGS models are easily extendable to flows with variable density. 

Correlations are available for mixing times in stirred-tank reactors with several types of stirrers. One of these, for the standard Rushton turbine with baffles [13], is shown in Figure 7.9, in which the product of the stirrer speed N (s ) and the mixing time t (s) is plotted against the Reynolds number on log-log coordinates. For (Re) above approximately 5000, the product N t (-) approaches a constant value of about 30. 

Figure 17.8. Typical proportions of a stirred tank reactor with radial and axial impellers, four baffles, and a sparger feed inlet.

Stirred tank reactors are often equipped with baffles in order to obtain an optimal effect of the action of the stirrer and to distribute turbulence as homogeneously as possible. As a consequence, however, this may introduce some dead corners in the wake of these baffles. 

A study carried out at the Lawrence Radiation Laboratory of the University of California by Vanderveen (Vl) strongly suggests another type of interaction model see also Vermeulen (V5). Vanderveen measured the drop size at different distances from the impeller of a baffled, stirred tank reactor in which two immiscible liquid phases were contacted, and found that a very substantial increase in drop size occurs at remote distances. The increase, which was attributed to coalescence, appeared to be dependent on the physical properties of the phase system. 

As iB well known, in baffled, stirred tank reactors there is a strong circulation pattern, as shown in Fig. 22. The dispersed drops will move with this... 

Formulation of multi-component emulsions and mixtures are of interest in chemical and industrial processes (Vilar, 2008 Vilar et al., 2008). Standard stirred tank reactors (STR) and oscillatory baffled reactors (OBR) are traditional methods for the formulation of liquid-liquid mixtures and liquid-solid emulsions. Compared with STR, oscillatory baffled reactors provide more homogeneous conditions and uniform mixing with a relatively lower shear rate (Gaidhani et al., 2005 Harrison and Mackley, 1992 Ni et al., 2000). Figure 17 is a sketch of a typical oscillatory baffled reactor. It consists of the reactor vessel, orifice plate baffles, and an oscillatory movement part. The orifice plate baffles play an important role in the OBR for the vertex generation in the flow vessels as well as the radial velocities of the emulsions and mixtures. They are equally spaced in the vessel with a free area in the center of each baffle... 

The continuous-stirred tank reactor (CSTR) has continuous input and output of material. The CSTR is well mixed with no dead zones or bypasses in ideal operation. It may or may not include baffling. The assumptions made for the ideal CSTR are (1) composition and temperature are uniform everywhere in the tank, (2) the effluent composition is the same as that in the tank, and (3) the tank operates at steady state. 

Few comprehensive studies have appeared concerning the influence of solids on k a in stirred tank reactors. Oguz et al. [87] measured k a in various slurries in a baffled 14.5 cm diameter stirred-tank reactor. Water and three organic liquids ( -butanol, 1-tetradecene and 1,2,4-trimcthylbenzcnc) were used as liquid phases. The particles applied varied in density from 2070 to 4720 kg m 3 and average particle sizes from 80 pm to below 1 pm. The observed cflccts of the particles on k a appear at first sight to be confusingly different for different slurry systems (sec Fig. 9). However, if the increase in apparent slurry viscosity due to the presence... 

A stirred tank reactor is a cylindrical container with a height-to-diameter ratio of 1 3, and it is often provided with baffles in order to avoid the formation of vortices during agitation. In general, four baffles with a width of 0.1 times the reactor diameter are arranged symmetrically with respect to the stirrer shaft. A sparger is provided at the bottom to introduce gas to the liquid. 

Determine the reactor volume required for one reactor and that for two equal-sized reactors in series for 80 percent conversion of A. And if the capital cost of a continuous-flow stirred-tank reactor unit is given by 200,000(17/100)° 6 (where V is reactor volume in m3), the life is 20 years with no salvage value, and power costs 3 cents per kilowatt-hour, determine which system has the economic advantage. Assume that overhead, personnel, and other operating costs, except agitation, are constant. The operating year is 340 days. Each reactor is baffled (with a baffle width to tank diameter of 1/12) and equipped with an impeller whose diameter is one-third the tank diameter. The impeller is a six-bladed turbine having a width-to-diameter ratio of 1 /5. The impeller is located at one-third the liquid depth from the bottom. The tank liquid-depth-to-diameter ratio is unity. 

Chapter 7, Reactor Design, discusses continuous and batch stirred-tank reactors and die packed-bed catalytic reactor, which are frequently used. Heat exchangers for stirred-tank reactors described are the simple jacket, simple jacket with a spiral baffle, simple jacket with agitation nozzles, partial pipe-coil jacket, dimple jacket, and the internal pipe coil. The amount of heat removed or added determines what jacket is selected. Other topics discussed are jacket pressure drop and mechanical considerations. Chapter 7 also describes methods for removing or adding heat in packed-bed catalytic reactors. Also considered are flow distribution methods to approach plug flow in packed beds. 

The operating mode of a stirred-tank reactor may be either continuous or batch. A STR consists of a vessel to contain the reactants, a heat exchanger, a mixer, and baffles to prevent vortex formation and to increase turbulence, enhancing mixing. 

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). 

Jenne, M. and Reuss, M. (1999), A critical assessment on the use of k-e turbulence model for simulation of the turbulent liquid flow induced by a Rushton turbine in baffled stirred tank reactor, Chem. Eng. ScL, 54, 3921-3941. 

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