With no antifoam added With antifoam added 泡泡較多且較小 泡泡較少但較大 1 • Because antifoams accumulate around bubbles, they block the movement of oxygen through the interfacial boundary. 2 Gas hold up • Gas hold up and Bubble residence time • Bubble size and gas hold up 3 Gas hold up and bubble residence time • The gas hold up is defined as: • For a cylindrical tank, the gas hold up is equal to: 4 • Gas holdup increases with increasing air flow rate • 導致氣泡滯留時間增加有以下原因 – 高黏度的培養基 – 氣泡過小 – 反應器高度過高 5 With large bubbles, the bubbles contain sufficient oxygen at the surface to ensure that oxygen will be transferred out of the bubble and CO2 will be move into the bubble 6 Small bubbles rapidy become saturated with CO2 and release the CO2 into the medium under the low pressure conditions at the surface 7 Draft tubes • The insertion of a draft tube into a fermenter causes the liquid inside the draft tube to flow uniformly upwards • This leads to reduced bubble coalescence and to more energy efficient mixing of the fermenter contents. 8 Stirred tank bioreactor with draft tube (Imhoff fermenter) Bubble colum bioreactor with draft tube 9 (Airlift fermenter) 謝 謝 大 家 10