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Observations of Slurry Concentration Uniformity on Draining a Mechanically Agitated Vessel

Matthew Hunnemeder1, Robert P. Hesketh1, Bethany Schmid1, YeJI Shin1, and Arthur W. Etchells III2. (1) Chemical Engineering, Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028, (2) Industrial Mixing, Philadelphia, PA 19106

Draining slurries containing settling solids is common in industrial operations. In many situations the delivery of slurry with a uniform concentration is desired. This slurry delivery configuration is a complex unsteady state multi-phase problem and has not been the subject of intense research. A laboratory scale reaction vessel drained by siphon and mixed with a high-efficiency A310 impeller was used in this study. In order to determine the effect of agitation on the system, impeller speeds were investigated on a range of 0-600 RPM for a preliminary model using just water. For the range investigated, it was found that agitator speed has no effect on gravity flow out of the vessel as long as no air is entrained. Following this, tests were conducted in which the outlet concentration of sand-water slurry was measured as a function of draining time and slurry level in the stirred tank. Impeller speeds of 900-1500 RPM were used for the slurry delivery system charged with 10% solids by mass. A Mettler-Toledo Lasentec probe was used to determine the solids distribution in a stirred tank. From this, it was concluded that using a higher agitation speed increases the uniformity of the slurry efflux concentration. Impeller speeds of 1200-1500 RPM delivered more uniform slurry. With the Lasentec data it was determined that there was a maximum slurry concentration in a region above the impeller. This region was approximately 0.7 impeller diameters measured vertically from the bottom of the impeller. It was also observed that this high slurry concentration is independent of the liquid height in the tank.