Ind. Eng. Chem. Res. 2006, 45, 4849
4849
CORRESPONDENCE Comments to “Effect of Internals on the Flow Pattern and Mixing in Stirred Tanks” Ivan Forˇ t* Department of Process Engineering, Czech Technical UniVersity, Technicka´ 4, 166 07 Prague 6, Czech Republic Sir: Kumaresan et al.1 published a comprehensive study on influence of internals (radial baffles or draft tube) on velocity distribution in a pilot plant stirred vessel with standard 45° pitched-blade down pumping turbine (45° PBTD) as well as the main integral characteristics of the investigated system: impeller power input P and its pumping capacity Qp. The former quantity plays an important role in many processes taking place in a stirred vessel, e.g., at blending of miscible liquids, suspension of solid particles in liquid, heat transfer between vessel and liquid, etc. Fort and Medek2 proved that, for the fully turbulent regime of flow of an agitated liquid, the impeller energetic efficiency
E ) NQp3/Np
(1)
is directly proportional to its hydraulic efficiency under the assumption that the increase of the potential energy when liquid is passing through the cylindrical body of the rotating impeller is negligible with respect to the increase of its kinetic energy. So the ratio of the cube of impeller flow rate number NQp and the power number Np expressed an ability of the impeller to convert its power input to the convective flow of the agitated liquid. Table 1 consists of dependence of the impeller energetic efficiency of 45° PBTD on the number of baffles NB and the baffle clearance BC, and for the hydrofoil impeller with three blades HF4, there is a dependence of the quantity E on the length of the draft tube Ldr. All the data in the table are taken from the article by Kumaresan et al.1 It follows from the table that number of baffles significantly increases the impeller energetic efficiency. This finding is in accordance with the results of Medek and Fort,3 who investigated the influence of the number of * E-mail:
[email protected].
Table 1. Effect of Configuration on Impeller Energetic Efficiency (W/D ) 0.3, C/T ) 0.33, H/T ) 1, D/T ) 0.33, and P/V ) 1 kW/m3) Nb
2
4
6
2
4
6
4
4
BC (m) Ldr (m) E
0.00
0.00
0.00
0.05
0.05
0.05
0.284
0.435
0.617
0.179
0.159
0.174
0.00 0.125 0.230
0.00 0.167 0.784
baffles on the quantity E in the system with 45° PBTD, as well. Moreover, it follows from the cited paper1 that the higher number of baffles reduces the tangential component of the mean velocity in the space around the rotating impeller and increases its axial component. As far as the baffle clearance BC, it considerably contributes to the reduction of the impeller energetic efficiency, although it may have some significance for crystallizers or bioreactors. Introduction of the draft tube significantly affects/increases the impeller energetic efficiency and is proportional to its length Ldr. These results correspond to the finding published by Fort4 for the more sophisticated design of this internal-conical draft tube with a lower cylindrical part consisting of a rotating impeller. So it is possible to conclude that proper design of internals in an agitated system can significantly affect an optimization of its operational conditions as well as its production capacity. Literature Cited (1) Kumaresan, T.; Nere, N. K.; Joshi, J. B. Effect of Internals on the Flow Pattern and Mixing in Stirred Tanks. Ind. Eng. Chem. Res. 2005, 44, 9951-9961. (2) Fort, I.; Medek, J. Hydraulic and Energetic Efficiency of Impeller with Inclined Blades. Proc. 6th Eur. Conf. Mixing (PaVia, Italy) 1988, 5156. (3) Medek, J.; Fort, I. Mixing in Vessel with Eccentrical Mixer. Proc. 5th Eur. Conf. Mixing (Wuerzburg, Germany) 1985, 263-268. (4) Fort, I. Flow and Turbulence in Vessels with Axial Impellers. In Mixing: Theory and Practice, Vol. III.; Uhl, V. W., Gray, J. B., Eds.; 1986; pp 249-256.
IE060404L
10.1021/ie060404l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
