(98a) Single Pass Drop Size Distribution Produced by an in-Line Rotor-Stator Mixer

Continuous or in-line high shear mixers are broadly employed in chemical processes to produce emulsions and liquid-liquid dispersions.  Despite their widespread use, there is little fundamental basis to theoretically predict or experimentally assess their performance.  As a result, process development, scale-up and operation are often by trial and error, leading to higher processing costs, start-up problems and lost time to market.  In order to develop a fundamental understanding, at least on a mechanistic basis, of how drop breakup affects liquid-liquid dispersion performance, we have measured power draw and single pass drop size distributions (DSD) exiting a IKA Labor Pilot 2000/4 in-line rotor-stator mixer.  This slot and tooth device can accommodate up to 3 stages of generators.  We have examined medium (2 rows of teeth per stage), fine (3 rows per stage) and ultra fine (4 rows per stage) generators.

A continuous, turbulent water phase is fed to the mixer via a progressive cavity pump.  At time zero, a single large oil drop is injected into the device and the resulting daughter DSD is measured at the exit using our previously reported Phase Doppler Anemometry (PDA) technique.  Since PDA yields the time history of the exiting drop population, or residence time distribution (RTD), bivariate statistics of drop size and residence time can be obtained from several parent drop trials.  Power draw is measured via a purpose build and fully integrated torque meter.  The background load needed to overcome bearing friction and other system losses is measured using “blank” stages and subtracted from the total load to determine the Power number due to rotor rotation.  Power due to flow losses is estimated from flow rate and pressure drop measurements.

Power draw has been measured for the various generators over a broad range of flow rate and rotor speed for 1, 2 1nd 3 stages.  Systematic experiments have been performed with a viscous drop phase to study the effect of flow rate, rotor speed generator geometry, and number of stages on the resulting DSD and RTD.  Results indicate that flow rate is not as influential as geometry and rotor speed, especially for multiple stages.  Power measurements are used to aid in interpretation of the DSD data.  The results will be discussed in detail, as well as implications to process performance.