Global and Local Liquid Velocity

In fermentation processes like those found in airlift reactors, the difference in riser and downcomer gas holdup creates a hydrostatic pressure difference between the bottom of the riser and the bottom of the downcomer, which in turn acts as the driving force for hquid circulation. A mean circulation velocity is defined as (Blenke, 1979) 

The superficial liquid velocity cannot be directly measured and is usually determined from the knowledge of the linear liquid velocity (Vl) and gas holdup. In airlift reactors, and Vl have both riser and downcomer components, yet the riser superficial liquid velocity (I/lj) is the parameter of greatest interest and the one commonly reported in the literatnre. 

The determination of riser and downcomer [/l is often accomplished using a tracer technique or specially calibrated flow meters and mathematical relationships to convert the measurable Vl to I/l- The tracer techniques commonly used to determine Vl are based on determining the time it takes for a given tracer to travel a set distance. For example, a potassium chloride salt tracer and conductivity electrodes are commonly used to measure the time it takes an injection of the salt solution to travel past two fixed locations from which Vl is calculated (Bello et al.. 

1984 Chisti, 1989 Jones, 2007 Van t Riet and Tramper, 1991). Knowing the superficial liquid velocity is determined from Chisti (1989) 

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