Topic: Can I adjust the standard mixing time equation to account for differences in tank geometry?

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Can I adjust the standard mixing time equation to account for differences in tank geometry? Or is there another relation that will allow me to do this? I would like to quantitatively assess the mixing efficiency of two different tanks with bottom mounted impellers, one of which is cubical and the other is cylindrical. I will be using fluid which can assumed to be water. Could I use the hydraulic diameter of each shape (such as with the Reynolds number calculation for flow in a pipe)?
• Dave Dickey Forum Moderator 324 Posts

Re: Can I adjust the standard mixing time equation to account for differences in tank geometry?

Last edited: 16 July 2014 at 8:27am

An answer to this question involves several general and specific unknowns.  The blend times for the geometries described are not available in the literature.  Specific information about blend times for bottom mounted impellers is not available, especially because several impeller geometries are available.  This description does not say whether these tanks are have high-shear center-mounted mixers or off-set magnetic drive mixers.

The best and most direct answer to the question about the difference between a cylindrical and cubic tanks really depends on baffles, provided the mixer speeds and impeller geometries are the same.  A cylindrical tank without baffles may have uncontrolled swirling, which can cause a deep vortex that may reach the impeller.  A vortex reaching the impeller will introduce air and drastically reduce liquid blending.  A cylindrical tank with baffles to control swirling and limit any vortex depth to less than half the liquid depth should blend quickly.  The corners of a cubic tank will provide a limited, but fixed, baffle effect.  If the mixer speed and liquid level of either tank is controlled so the vortex does not reach the impeller, the blend time should be so similar as to be indistinguishable.  Some variation in blend time is expected because of random flow patterns and turbulent mixing.

In any case, the blend time for multi-component formulation should be sufficiently short with a water-like liquid to show little if any difference between the tanks.  Blend time differences will only be a factor for complicated chemical reactions with multiple steps and reaction rates, which will also be affected by the location of reactant introduction.

• Chris Chuckran Community Member 1 Post

Re: Can I adjust the standard mixing time equation to account for differences in tank geometry?

Thank you for the quick reply! In the case of a centered low-shear magnetic drive impeller, operating within the laminar range, would the same effects be expected?