Digester Tank Design.docx

Fermenter Tank Design A basic stirred tank design is shown in Fig. 1. Height/diameter ratio is H/D = 1 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube can enhance vertical circulation. The vapor space is about 20 percent of the total volume. More intensive dispersion and redispersion are obtained by mechanical agitation. Power inputs of 0.6 to 2.0 kW/m3 (3.05 to 10.15 hp/1000 gal) are suitable (Perry, 2008). A hollow shaft and impeller increase liquid circulation. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous liquids. When the ratio of liquid height to diameter is H/D ≤1, a single impeller suffices, and in the range 1 ≤ H/D ≤ 1.8, two are needed (Perry, 2008).

FIG. 1: Basic Stirred Tank Design (Perry, 2008)

The figure above shows a basic stirred tank design, not to scale, showing a lower radial impeller and an upper axial impeller housed in a draft tube. Four equally spaced baffles are standard where H =height of liquid level, Dt =tank diameter, and d =impeller diameter. For radial impellers, 0.3 ≤ d/Dt ≤ 0.6 (Perry, 2008).