4. Air-lift bioreactors are similar to bubble column reactors, but differ by the fact that they contain a draft tube. The draft tube is always an inner tube (this kind of air-lift bioreactor is called "airlift bioreactor with an internal loop), fig. 4 illustrates the basic structure of an air-lift bioreactor with an internal loop. The circulation of the fermentation broth (average density: 1 and average viscosity: 1 ) is being maintained by injecting compressed air (pressure: Pair and volume flow rate: Qair ) via a perforated plate (radius of openings: r1 ), located just at the bottom of the central draft tube. As a potential Chemical Engineer you are asked to calculate the rate of circulation (volume flow rate) of the fermentation broth. State your assumptions clearly and assume any missing data if necessary