Tower bubble area, increase

Figure 12-9 Bubble column and spray tower reactors. Large drop or bubble areas increase reactant mass transfer,...

Increasing Tower Bubble Area to Reduce Jet Flood... 

Fractional hole area. Efficiency increases with a reduction in fractional hole area (23,28,110,144,186). Yan and Sakata (23), experimenting with commerdal-scale towers, show a 10 to 15 percent increase in tray efficiency when firactional hole area was lowered from 14 to 8 percent of the bubbling area (Fig. 7.10a). Kreis and Raab (28j show an identical increase for N2/O2 separation, and an even larger increase (20 to 25 percent) when fractional hole area was lowered from 8 to 5 percent of the bubbling area. Prado and Fair (110,144) showed an efficiency increase of the Older of 5 percent as fractional hole area was reduced from 11 to 6 percent of the bubbling area in humidification and stripping tests. The above data were collected both in the froth and spray regimes. 

It is industry-accepted practice to slojje the downcomer, so that the bottom edge of the downcomer is only 60 percent of the top area. This increases the bubble area by maybe 5 to 7 percent. 1 never do this on new towers. It s an unnecessary risk and installation complication. However, for retrofits, it s not a bad idea. 

It is well known that fluid dynamics influence the process performance. Therefore, bubble velocity and gas/liquid interfacial area were monitored during the cultivation of E. coli. The effect of an AFA on the bubbles was determined by monitoring the bubble velocity with an ultrasound Doppler velo-cimeter (UDV) in situ [48,49]. By adding an AFA to the cultivation medium, the mean bubble velocity instantaneously increased by a factor of about two in the airlift tower loop reactor during the cultivation of E. coli [50] (Fig. la). 

In Fig. 2 the key parameters are presented for recombinant E. coli batch cultivation in a 60-1 working volume airUft tower loop reactor at constant aeration rate up to 16 h, whereupon the temperature was increased from 30 to 42 °C and gene expression was induced. At the same time concentrated Luxia-Bertani (LB) medium was added to the reactor. To avoid oxygen limitation, the aeration rate was increased (Fig. 2 a). At 12 h the foaming increased and SE9 was added to the medium. The bubble velocities (Fig. 2b) and the specific gas/liquid interfacial area (Fig. 2 c) quickly increased and passed a narrow maximum, but kLa dropped and the OTR was not influenced (Fig. 2d). After the induction of the gene expression by a temperature increase and medium supplement the dissolved oxygen concentration with respect to the saturation increased due to the elevation of the aeration rate (Fig. 2 a) the mean bubble velocity (Fig. 2 b) and specific interfacial area (Fig. 2 c) decreased, OTR increased and kLa remained at low values (Fig. 2d). The mass transfer coefficient with respect to the liquid phase kL dropped from about 1.67 to 0.67 ms after the addition of SE9 to the medium [51]. 

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