Stirred vessels baffles

Fig. 10. Comparison of stirred vessels with and without baffles Reference floe diameter dpv in dependency on specific impeller power P/V H/D = 1 D = 0.4 m...

Fig. 16. Comparison of floe destruction in stirred vessels with baffles, bubble columns and viscosimeters...

Fig. 17. Comparison of floe destruction stirred vessels with baffles and bubble columns...

Fig. 18. Comparison of results from various particle systems for stirred vessel with baffles and bubble columns Activity a/ao of Acylase resin after t = 300 h, equilibrium drop diameter dg of silicon oil-water-surfactant emulsion and reference floe diameter dpv of floe system in dependency on specific power P/V H/D = 1 D = 0.15 m 0.4 m...

It could be shown (see Sect. 6) that in stirred vessels with baffles and under the condition of fully developed turbulence, particle stress can be described by Eqs. (2) and (4) alone. The turbulent eddys in the dissipation range are decisive for the model particle systems used here and many biological particle systems (see Fig. 2), so that the following equation applies to effective stress ... 

Fig. 1. Phase-averaged plots of the anisotropy distanced in a plane midway between two baffles in a stirred vessel provided with a Rushton turbine, as obtained by means of LES, with two different SGS models (a) the Smagorinsky model (b) the Voke model. Reproduced with permission from Hartmann et al. (2004a).

A first-order liquid-phase reaction takes place in a baffled stirred vessel of 2 volume under conditions when the flow rate is constant at 605 dm min and the reaction rate coefficient is 2.723 min the conversion of species A is 98%. Verify that this performance lies between that expected from either a PFR or a CSTR. Tracer impulse response tests are conducted on the reactor and the data in Table 6 recorded. Fit the tanks-in-series model to this data by (A) matching the moments, and (B) evaluating N from the time at which the maximum tracer response is observed. Give conversion predictions from the tanks-in-series model in each case. 

In Figure 11.2 a schematic view of a stirred vessel is given. The vessel is cyhndrical with a height (m) and a diameter T (m). Usually is equal to or greater than 2 T. It is equipped with a stirrer in the lower compartment. TTiis stirrer is mounted near the bottom, usually at a distance equal to the stirrer diameter. At a lower position the stirrer and bottom interact, leading to a decrease in power consumption. At a higher position hquid circulation problems can occur because, at increased gas flow rate in case of aeration, the bubbles will not be recirculated in the lower compartment. Sometimes the upper compartment (s) are also equipped with a stirrer. The vessel is equipped with baffles to prevent rotation of the contents as a whole. For aeration an air sparger is mounted below the stirrer. For mass transfer... 

Stirred vessel Figure 2.5(a) (Cylindrical flat bottom vessel, four baffles). Impeller six-flat blade turbine type. 

In recent years attempts have been made to improve the gas-liquid mass transfer by changing the design of the mechanically agitated vessel. Mann et al. (1989) evaluated the use of horizontal baffles mounted near the gas-liquid surface. Horizontal baffles prevent vortex formation, generate less shear than standard baffles, increase gas holdup, and improve gas-liquid mass transfer. The latter two results are due to the rotational flow below the baffles, which causes gas bubbles to move upward in a spiral trajectory and induces surface aeration. For a 12-inch i.d. and 18-inch-tall stirred vessel, they showed kLat to be improved by a factor of 1.6 to 2.3 with 30 to 50% lower agitation power compared to the standard vessel. 

A flow configuration of particular interest to the chemical engineer is the (baffled) stirred vessel and very significant efforts have been documented in the literature (Ranade and Joshi, 1990a,b), Kresta and Wood, 1991) to compute the flow patterns inside the vessel using CFD models. Here models with a varying degree of sophistication both from a physical and numerical point of view have been... 

Flow in baffled stirred reactors has been modeled by employing several different approaches which can be classified into four types, and are shown schematically in Fig. 10.3. Most flow simulations of stirred vessels published before 1995 were based on steady-state analyses (reviewed by Ranade, 1995) using the black box approach. This approach requires boundary conditions (mean velocity and turbulence characteristics) on the impeller swept surface, which need to be determined experimentally. Although this approach is reasonably successful in predicting the flow characteristics in the bulk of the vessel, its usefulness is inherently limited by the availability of data. Extension of such an approach to multiphase flows and to industrial-scale reactors is not feasible because it is virtually impossible to obtain (from experiments) accurate... 

Brucato, A., Ciofalo, M, Grisafi, F. and Micale, G. (1994), Complete numerical simulations of flow fields in baffled stirred vessels the inner-outer approach, I ChemE Symposium Series no. 136, 155. 

Ranade, V.V. and Dommeti, S. (1996b), Computational snapshot of flow generated by axial impellers in baffled stirred vessels. Presented at AIChE Annual Meeting, Chicago, November. 

Ranade, V.V., Tayalia, Y. and Krishnan, H. (2001a), CFD predictions of flow near impeller blades in baffled stirred vessels, accepted for publication to Chem. Eng. Commun. 

A stirred vessel crystallizer is shown in Fig. 6-4. Included are a dual-impeller pitched-blade turbine with a tickler blade (see Section 6.6.1.6), a subsurface addition line, baffles, and a ram-type bottom outlet valve to aid in discharge of slurries. 

Fukuda T., Idogawa K., Ikeda K., Endoh K., Volumetric gas-phase mass transfer co cient in baffled horizontal stirred vessels, J. Chem. Eng. Japan 13 (1980) 4, p. 298-303... 

Gotz S., Sperling R, Liepe F., Jembere S., Numerical determination of the three-dimensional velocity distribution in a baffled pitched blade impeller stirred vessel, Chem. Eng. Technol. 20 (1997), p. 596-605... 

The basic idea is to describe a snapshot of the flow in a stirred vessel with a fixed relative position of blades and baffles. It is assumed that the main flow characteristics of a stirred vessel at the particular time instant in question can be captured approximately from the solution of the steady-state equations, provided that artificial cell volnme adjustments and momentum sources are implemented to represent the effect of the impeller rotation. 

Luo JY, Gosman AD, Issa RI, Middleton JC, Fitzgerald MK (1994) Full flow field computation of mixing in baffled stirred vessels. Trans IChemE 71(A) 342-344... 

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