Estimation of Blend Time

The dimensionless blend time, NO, is a constant for a helical ribbon operating in the laminar regime (see, e.g., Hoogendoom and den Hartog, 1967 Johnson, 1967 Rieger et al., 1986). This means that the blend time is independent of Reynolds number and the fluid viscosity, so that even if the fluid is shear-thinning the blend time will not be affected by the rheological behavior. This is not true for visco-elastic behavior. 

Grenville et al. (2001) took the data of Rieger et al. (1986) and found that the dimensionless blend time could be correlated with the constant Kp by 

The higher the value of Kp, the lower the dimensionless blend time. Grenville et al. (2001) also found that an impeller with a high Kp value was the most energy efficient geometry (i.e., gave the shortest blend time for a given power input). 

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Other blend time correlations were presented by Penney (1971), Khang and Levenspiel (1976), and Fasano and Penney (1991). Use of these correlation equations allows the estimation of blending times, which can be compared to molecular reaction times for all the reactions in the reactor. Even though local mixing time is the critical time for determining apparent reaction rate, blend time can be used in an approximate manner. If the characteristic molecular reaction time (e.g., the half-life) is much greater than the characteristic blend time... 

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