fast chemical reactions, etc.

3 The Modern Stirred-Tank  Reactor

A modern STR is illustrated in Fig. 2 [12]. It consists of a vertical shaft, centrally mounted with one or more impellers driven through a gearbox by a variable-frequency drive electric motor inside a cylindrical vessel with dished ends, all designed to an appropriate pressure vessel code. For tur- bulent and high Re transitional flow conditions, four wall baffles, extending from the wall ~0.1T into the tank, are used to prevent gross vortexing and enable the desired flow pattern to be developed, radial as with Rushton turbines or axial, up or down, approximately parallel to the shaft. In addition, baffles are avoided in systems that are prone to fouling (e.g., polymerization) because of the risk of solid build-up; and where there is a perception that they are diffi-

cult to clean (e.g., pharmaceutical formulations and food processing), which results in relatively ineffective stirring. With glass lined STRs, the traditional baffle arrangement proved difficult so that a single baffle (often called a beaver tail because of its shape) is hung down through a top entry port into the liquid but without extending close to the base of the vessel. However, the effectiveness of stirring is again poor, similar to that found without any baffles. Recently, the ability to fabricate baffles made of suitable strength and corrosion resistant polymeric construction materials to pro- duce configurations similar to Fig. 4 has greatly reduced this problem [4]. Baffles are not needed at Re < ~200 where the high viscosity prevents vortexing. In addition, the flow from axial flow impellers perges, becoming increasingly radial as Re is reduced.

The volume of the STR is determined by the production rate of the process whether batch, semi-batch or continu- ous, but  different  aspect  ratios, AR (=  H/D) can  be   em-

ployed. Typically, ARs close to 1 are used but for processes

within some small percent (typically ± 5 %) of the final val- ue. It was established that in the turbulent region,

qmN  ¼ const: (10)

Another approach is to use an instantaneous chemical reaction involving a colour change. An excellent description of a modern use of both techniques along with power mea- surement is available [19]. For many years, it was consid- ered that low power number/high flow number stirrers, such as axial flow hydrofoils (Fig. 5), gave shorter mixing times at the same ¯eT (and the manufacturer’s websites still imply that concept, calling them high flow or high efficiency

impellers) compared to high power number impellers (such as the Rushton turbine, often called high shear) [20]. How- ever, by accurately measuring ¯eT and qm [19], it has been established experimentally that for square-batch cylindrical tanks (AR = ~1)

where the gas has to be dissolved, as with oxygen from  air

in fermentations. ARs up to ~3 are common to increase the gas residence time, with impellers placed ~2/3T to T apart. For vessels that are not subject to pressure, such as in stor- age vessels with a flat base, ARs < 1 are common with side entering impellers.

STRs often have to be heated or cooled to produce the desired temperature. For that purpose, a jacket is often used. However, the rate at which heat has to be transferred, defined by the overall heat transfer coefficient, is almost independent of the speed of the stirrer and is largely deter- mined by the heat transfer area. Since the load to be cooled

or heated scales with the volume of the vessel contents (aT3) whilst the area of the jacket scales with T2, it is often necessary to increase the area on scale up by adding internal coils. These coils come in a wide variety of shapes and in extremis for highly exothermic reactions and oxygen de- manding fermentations, the baffles can also be used to pro- vide more cooling area. Subsequent correction for an inad- equate heat transfer area is very expensive, requiring chilled water to be used and/or the feed rate (and hence productiv- ity) to be reduced.