100 Fermentation and Biochemical Engineering Handbook Not all of these options are practical because of shear, foaming and control devices 6.2 Fermenter Height The height-to-diameter(H/D)ratio of a fermenter is very important fo oxygen transfer efficiency. Tall, narrow tanks have three major advantages compared to short, squat fermenters. Bubble residence time is longer in taller vessels than shorter ones. The air pressure is greater at the sparger resulting in higher dissolved oxygen in taller vessels. The third advantage is shown in Table 4, namely that for a vessel of constant volume, as the Hd ratio increases, the volume of air required is reduced even though the superficial linear velocity remains constant. At the same time, bubble residence time and sparger air pressure increase. For larger volume fermenters, even greater vertical heights are used. the conclusion is that fermenter height is the most important geometrical factor in fermenter design. Conversely, shorter vessels need more air and/or more mechanical agitation to effect the same mass transfer rate of oxygen. The majority of industrial fermenters are in the h/d ange of 2-3. The largest sizes are about 10 liters It is thought that the cost of compressing air sufficient for air agitation alone is prohibitive. However, as seen in Table 5, if the fermenters are tall, the power consumption is less than for short squat tanks. Careful selection of compressors with high efficiencies will keep power costs at a minimum Table 4. Effect of Air Requirements on Geometric Fermenter Design Bubble parser H/D F D scfm Residence Time Pressure 227313.73,522 335811.92,683 160 43410.82,219 1.6 194 Constant: 30,000 gal tank; 24, 000 gal run vol: 0. 4 fU/sec SLV.100 Fermentation and Biocltemical Engineering Handbook Not all of these options are practical because of shear, foaming and control devices. 6.2 Fermenter Height The height-to-diameter (WD) ratio of a fermenter is very important for oxygen transfer efficiency. Tall, narrow tanks have three major advantages compared to short, squat fermenters. Bubble residence time is longer in taller vessels than shorter ones. The air pressure is greater at the sparger resulting in higher dissolved oxygen in taller vessels. The third advantage is shown in Table 4, namely that for a vessel of constant volume, as the WD ratio increases, the volume of air required is reduced even though the superficial linear velocity remains constant. At the same time, bubble residence time and sparger air pressure increase. For larger volume fermenters, even greater vertical heights are used. The conclusion is that fermenter height is the most important geometrical factor in fermenter design. Conversely, shorter vessels need more air and/or more mechanical agitation to effect the same mass transfer rate of oxygen. The majority of industrial fermenters are in the H/D range of 2-3. The largest sizes are about 10' liters. It is thought that the cost of compressing air sufficient for air agitation alone is prohibitive. However, as seen in Table 5, if the fermenters are tall, the power consumption is less than for short squat tanks. Carehl selection of compressors with high efficiencies will keep power costs at a minimum. Table 4. Effect of Air Requirements on Geometric Fermenter Design Bubble Sparger WDF D scfm Residence Time Pressure 2 27.3 13.7 3,522 1 12.3 3 35.8 11.9 2,683 1.3 16.0 4 43.4 10.8 2,219 1.6 19.4 Constant: 30,000 gal tank; 24,000 gal run vol; 0.4 ft/sec SLV