# Topic: Re: We would like to have a general idea of pumping number for our reactors. Can you help us with blend-time calculations?

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You seem to be making blend time calculations more complicated than they are or at least trying to describe your process results entirely on the basis of a blend time.&nbsp;The literature has blend time estimates with an expression much as you describe.&nbsp; Just remember that actual blend time, even with the same mixer, will vary from addition to addition by +/- 10% or more just because of turbulence and addition rate or location.&nbsp; The value of K in your expression is obviously just a function of uniformity and is typically assumed to follow an exponential decay.&nbsp; Thus the blend time for 90% uniformity is about 0.65 times the blend time for 99% uniformity.&nbsp; The blend time for 99.9% uniformity is about 1.5 time the blend time for 99% uniformity, etc.&nbsp;The values for "a" and "b" depend on the impeller type.&nbsp; For typical radial-flow, straight-blade impellers, a = 4.8 and b = 2.3.&nbsp; For typical axial-flow, pitched-blade impellers, a = 6.34 and b = 2.3.&nbsp; For typical three-blade hydrofoil impellers, a = 16.4 and b = 1.7.&nbsp;The idea of moving away from correlations for blend time and using pumping number to estimate blending performance introduces new problems.&nbsp; A typical use of pumping capacity to "estimate" blend time requires from 6 to 10 tank turnovers for blending uniformity.&nbsp; The range of tank turnovers and differences in pumping effects on blend time with different impellers makes those estimates even less certain.&nbsp;In most cases, blend time estimates are much shorter than the expected residence time in a stirred vessel.&nbsp; The only place that blending becomes critical is in chemical reactions.&nbsp; With parallel or sequential chemical reactions, the location and rate of reactant addition is more important than bulk blend times predicted by formulas or pumping calculations.&nbsp; If you are observing significant differences between reactors, your problems are more complicated than just blend times.&nbsp; Even large tanks will blend to uniformity in only a few minutes with moderate to intense agitation.&nbsp; You probably need to look beyond just blend time calculations to find differences in your process results.
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