A suitable agitator for this application partly depends on the availability of impellers or custom fabricators.  The application is difficult, because the actual range of viscosities probably is from 1 cp to 8,000 cp.  At 1 cp, the mixing is turbulent and not difficult with standard impellers.  At 8,000 cp, the mixing is definitely laminar and difficult even with a close-clearance, helical ribbon impeller.  The combination of a small vessel and high viscosity puts design out of the range of some commercially available laboratory mixers.
 
Because of the tall vessel (70 cm) with a small diameter (30 cm), multiple impellers will be necessary.  Hydrofoil impellers are not recommended for the high viscosity.  Better impellers are 45-degree, pitched-blade impellers.  To provide good mixing over the entire height of the vessel, three 4-blade impellers or four 2-blade impellers are needed.  The 2-blade impellers should be staggered at 90 degrees to each other.  Fewer impellers might be appropriate for lower liquid levels.  Impeller spacing should be about 3/4 to 1 times the impeller diameter.  Whether the blades of the 4-blade impellers are staggered or aligned should make little difference.  The diameter of the impellers should be from 12cm to 15 cm, 0.4 to 0.5 times the vessel diameter.  Rotational speeds from 100 to 350 rpm might be needed.

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