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Mixing Time and Residence Time

Q: I'm going to calculate residence time in a static mixer, but I don't know what the exact difference is between mixing time and residence time. Are the baffles shape and their numbers and the Re number of the mixed fluid the only parameters that affect the residence time? What about the entrance distance of the mixer? Is that up to me, or it has a specified amount? If the mixed flow regime is laminar at the beginning, it must be Turbulent after passing the inserts? Should I consider this change of Re number through mixer for my calculation? About the mixer specification, I am going to use a helical static mixer for just mixing two different fluids (there is no suspension or bubble in the mixed flow.)

A:

The residence time in a static mixer is just the length of the mixer divided by the flow velocity.  Flow velocity is just volumetric flow rate divided by the cross-sectional area of the pipe.  You need to decide on those values.  Typical flow velocities are 4 to 10 feet/second, the lower end of that range is probably more typical of laminar flow applications, you can do the units conversion to whatever units you want to use.
 
Baffle shape and number will affect the mixing.  The Re number can be calculated and used to decide how many baffles (elements) are needed for good mixing.  If the flow entering the mixer is laminar, it will remain laminar throughout the mixer.  The Re number does not change.
 
I am not sure what you mean by entrance distance to the mixer.  The point where your two fluids should be only about a pipe length upstream from the mixer.  For best flow, the upstream and downstream pipe should be straight for at least 10 pipe diameters.  Static mixers are like a pipe section with some additional pressure drop that provides the energy for mixing to take place.

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