Lab Scale Mixing in Stirred Tanks

Small scale PMMA vessels up to about 400 mm can be manufactured by using the appropriate length of PMMA pipe to make the cylindrical section and attaching a base of PMMA that has been formed into the desired shape. Care should be taken to ensure that the base (whether it be flat or dished) is formed according to the relevant standards. A strengthening flange or lip should be added to the top rim of the vessel to increase rigidity. 

It may seem convenient to place an outlet and drain valve in the middle of the base of a vessel however, caution should be applied, as this can significantly affect any solid suspension measurements, and viscous materials added to the vessel may collect in the drain pipe or nozzle. In general, it is better that the vessel contents are removed by pumping through a piece of flexible hose, as this reduces the number of bosses and fittings on the vessel. 

Wherever possible, mixing vessels should be supported above the laboratory floor (or benchtop for small vessels), as this provides access for observations of the vessel base. In all situations, the vessel, drive, bounding box, and any other equipment should be mounted in a strong frame that provides support and prevents excessive vibration. 

Mixer drives are usually electric, although air motors are used in the presence of hazardous fluids. DC motors with variable speed and belt drive are popular, and direct-drive stepper motors are also recommended. It is generally simplest 

Care should always be taken to check that the system is running at a speed lower than the first critical oscillation speed of the mixer shaft. The critical speed can be calculated or measured directly (see Chapter 21). In situations where operation close to the critical speed of the unsupported shaft is required, a bottom bearing must be used. 

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