Mechanically Stirred Vessels

Processing with mechanical mixers occurs under either laminar or turbulent flow conditions, depending on the impeller Reynolds number, defined as Re = pND / x. For Reynolds numbers below about 10, the process is laminar, also called creeping flow. Fully turbulent conditions are achieved at Reynolds numbers higher than about 10 , and the flow is considered transitional between these two regimes. 

Fluid mixing is carried out in mechanically stirred vessels for a variety of objectives, including for homogenizing single or multiple phases in terms of concentration of components, physical properties, and temperature. The fundamental mechanism involves physical movement of material between various parts of the entire mass using rotating impeller blades. Over 50% of the world s 

Handbook of Industrial Mixing Science and Practice, Edited by Edward L. Paul, 

An optimum approach to designing these mixing systems consists of the following steps  

To design an effective stirred tank, an efficient impeller should be chosen for the process duty. More than one impeller may be needed for tanks with high aspect ratio (Z/T 1.5). Sizing of the impeller is done in conjunction with mixer speed to achieve the desired process result. The appropriate size and type of wall baffles must be selected to create an effective flow pattern. The mixer power is then estimated from available data on impeller characteristics, and the drive size is determined. The mixer design is finalized with mechanical design of the shaft, impeller blade thickness, baffle thickness and supports, inlet/outlet nozzles, bearings, seals, gearbox, and support structures. 

---

The uncertainty involved in the calculation of the energy dissipation rate makes it difficult to compare experimental results reported by different researchers. For the same reasons, so far it has proved difficult to assess flow induced effects in different items of process equipment using the common basis of equal energy dissipation rate. For example. Fig. 14 shows the biological response of (SF-9) insect cells as a function of the energy dissipation rates in a capillary tube and in a mechanically stirred vessel [99]. In these plots the calcula-... 

Stirred tanks. These are mechanically stirred vessels, which are advantageous when absorption is accompanied by a slow liquid-phase chemical reaction. As discussed earlier (Section II), this application is considered a chemical reactor rather than an absorber. Stirred tanks provide high liquid residence times but are limited to low gas flow rates. 

Hemrajani, R.R. Tatterson, G.B. Mechanically stirred vessels. In Handbook of Industrial Mixing Science and Practice Paul, E.L., Atiemo-Obeng, V.A., Kresta, S.M., Eds. John Wiley Sons New York, 2004 345-390. 

Sfatic mixers offer certain advantages over dynamic in-line mixers and continuous stirred tanks. Table 9.18 is a summary of the characteristics of a static mixer compared with a conventional mechanically stirred vessel. 

Hemrajani RR, Tatterson GB. (2004) Mechanically stirred vessels chapter 6. In Paul EL, Atiemo-Obeng VA, Kresta SM, editors. Handbook of industrial mixing science and practice. John Wiley Sons, Inc., Hoboken, NJ, USA. 

Abdullah B, Dave C, Nguyen TH, Cooper CG, Adesina AA. (2011) Electrical resistance tomography-assisted analysis of dispersed phase hold-up in a gas-inducing mechanically stirred vessel. Chem. Eng. Sci., 66 5648-5662. 

Mechanically stirred vessels Mixing is promoted by mechanical stirring (occasionally magnetic in bench-scale vessels) using different impeller types. In addition, aeration is also employed to augment mixing. In this most commonly used type of bioreactor, the tank content is perfectly mixed and hence uniform in composition. 

Fig. 10.32 Power curves for some typical impellers. (From R. Hemrajani, G.B. Tatterson, Mechanically stirred vessels, in E. L Paul, V.A. Atiemo-Obeng, S. M. Kresta (Eds.), Handbook of Industrial Mixing Science and Practice, DOI 10.1002/0471451452.ch6, (Chapter 6). Copyright 2004 Wiley).

---