Agitation 213 2「T=le"2=18 6FBT C=6 CENTER INLET 1上F=o8FT/sEc 02 204.080|O 020 HP/I000 GALS. GASSED ire 32. Effect of horsepower and impeller diameter on mass transfer coefficient at 0.08 T=8"z=|8" 6FBT C=6 CENTER INLET F= 13 FT/SEC 04 02 20 4.0 8.0|o HP/1OOO GALS GASSED Figure 33. Effect ofhorsepower and impeller diameter on mass transfer coefficient for gas velocity of0. 13 ft/sec
Agitation 213 J v) J 0 I m J - 0 l- 2 .08 a Y h I- v)L 02 .04 2- J, h mg CENTER INLET .08 - .04 .02 - - CENTER INLET A - F=.O8 FT/SEC .08 - .O 4 .o 2 I/' "'l'o i.0 410 8.b ;O 2b HP/ 1000 GALS. GASSED Figure 32. Effect ofhorsepower and impeller diameter on mass transfer coefficient at 0.08 Wsec gas velocity., ."I 1.0 2.0 4.0 8.0 IO HP/1000 GALS. GASSED Figure 33. Effect ofhorsepower and impeller diameter on mass transfer coefficient for gas velocity of 0.13 Wsec
214 Fermentation and Biochemical Engineering Handbook At the far right of Fig. 29 is shown high mixer power levels relative to e gas rate, and it can be seen that d/t makes no difference to the mass transfer. This occurs in some types of hydrogenation, carbonation, and chlorination. In those cases, the power level is so high relative to the amount of gas added to the tank that flow to shear ratio is of no importance. In Figs. 30 through 33, the gas rate is successively increased in each of the four figures. At the low gas rate, the 4-inch impeller is more effective than the 6 or 8-inch impeller under all power levels. At the higher gas rates the larger impellers become more effective at the lower gas rates, while the smaller impellers are more effective at the higher power levels, fitting generally into the scheme shown in Fig. 29 A sparge ring about 80% ofthe impeller diameter is more effective than an open pipe beneath the impeller or sparge rings larger than the impeller Figure 34 shows this effect and indicates that the desired entry point for the gas is where it can pass initially through the high shear zone around impeller This has led to the common practice today of using the distribution of power in a three- impeller system, for example, 40%to the lower impeller and 30%to each of the two upper impellers, Fig. 35 T:460 mm 2=460mm 150 mm -6FBT C: 150m 007 m/sec CENTER INLET KW/cu METER GASSED Figure 34. Gas-liquid mass transfer data for 150 mm turbine in 460 mm tank at 0.07 m/ sec superficial gas velocity
214 Fermentation and Biochemical Engineering Handbook At the far right of Fig. 29 is shown high mixer power levels relative to the gas rate, and it can be seen that DlT makes no difference to the mass transfer. This occurs in some types of hydrogenation, carbonation, and chlorination. In those cases, the power level is so high relative to the amount of gas added to the tank that flow to shear ratio is of no importance. In Figs. 30 through 33, the gas rate is successively increased in each of the four figures. At the low gas rate, the 4-inch impeller is more effective than the 6 or 8-inch impeller under all power levels. At the higher gas rates, the larger impellers become more effective at the lower gas rates, while the smaller impellers are more effective at the higher power levels, fitting generally into the scheme shown in Fig. 29. A sparge ring about 80% ofthe impeller diameter is more effective than an open pipe beneath the impeller or sparge rings larger than the impeller. Figure 34 shows this effect and indicates that the desired entry point for the gas is where it can pass initially through the high shear zone around the impeller. This has led to the common practice today of using the distribution of power in a three-impeller system, for example, 40% to the lower impeller and 30% to each of the two upper impellers, Fig. 35. (P 0 Y T=460mm Z=460mm 1- 0.1 I I 11 I .2 .4 .8 1.0 2.0 KWI cu METER GASSED Figure 34. Gas-liquid mass transfer data for 150 mm turbine in 460 mm tank at 0.07 m/ sec superficial gas velocity
Agitation 215 30% CE 30% Figure 35. Typical power consumption relations for triple impeller installation, giving higher horsepower in proportion to the lower impeller In regard to tank shape, it has turned out over the years that about the biggest tank that can be shop-fabricated and shipped to the plant site over the highways is about 14 ft (4.3 m)in diameter. As fermentation volumes have gone from 10,000 gallons(38 m)to 50 or 60 thousand gallons, tank shapes have tended to get very tall and narrow, resulting in Z/Tratios of 2: 1, 3: 1, 4: 1 or even higher on occasion. This tall tank shape has some advantages and disadvantages, but tank shape is normally a design variable to be looked at in terms of optimizing the overall plant process design This leads to the concept of mass transfer calculation techniques in scaleup. Figure 36 shows the concept of mass transfer from the gas-liquid step as well as the mass transfer step to liquid-solid and/or a chemical reaction. Inherent in all these mass transfer calculations is the concept of dissolved oxygen level and the driving force between the phases. In aerobic fermentation, it is normally the case that the gas-liquid mass transfer step from gas to liquid is themost important. Usually the gas-liquid mass transfer rates measured, a driving force between the gas and the liquid calculated, and the mass transfer coefficient, Kg a or K a obtained. Correlation techniques use the data shown in Fig. 37 as typical in which KGa is correlated versus power level and gas rate for the particular system studied
Agitation 21 5 f 30% 30% 40% Figure 35. Typical power consumption relations for triple impeller installation, giving higher horsepower in proportion to the lower impeller. In regard to tank shape, it has turned out over the years that about the biggest tank that can be shop-fabricated and shipped to the plant site over the highways is about 14 ft (4.3 m) in diameter. As fermentation volumes have gone from 10,000 gallons (38 m3) to 50 or 60 thousand gallons, tank shapes have tended to get very tall and narrow, resulting in Z/Tratios of 2: 1,3 : 1,4: 1, or even higher on occasion. This tall tank shape has some advantages and disadvantages, but tank shape is normally a design variable to be looked at in terms of optimizing the overall plant process design. This leads to the concept of mass transfer calculation techniques in scaleup. Figure 36 shows the concept of mass transfer from the gas-liquid step as well as the mass transfer step to liquid-solid andor a chemical reaction. Inherent in all these mass transfer calculations is the concept of dissolved oxygen level and the driving force between the phases. In aerobic fermentation, it is normally the case that the gas-liquid mass transfer step from gas to liquid is the most important. Usually the gas-liquid mass transfer rate is measured, adriving force between the gas and the liquid calculated, and the mass transfer coefficient, &a or &a obtained. Correlation techniques use the data shown in Fig. 37 as typical in which &a is correlated versus power level and gas rate for the particular system studied