(428d) Solid Suspension in Unbaffled Vessels Using Vertical Off-Center Agitators

Solids suspension via mechanical agitation has been studied extensively, with most of these studies being performed in baffled vessels. Baffling promotes axial flow that provides the desired performance with minimized capital and operating costs. While most studies focus on fully-baffled conditions, there are solids suspension studies that consider reduced baffling. An example is the work of Drewer et al. (1994) who reported advantages to using shorter baffles that do not extend to the tank base when agitating high concentration solid-liquid suspensions. Similarly, Myers et al. (2005) found that hydrofoil impellers can generate top-to-bottom motion with limited baffling, providing the opportunity to achieve solids suspension without full baffling. Baffling is not used in some applications, often motivated by the desire to promote cleanliness and avoid the potential for fouling. Very limited information is available concerning solids suspension in unbaffled vessels. In their study of solids suspension using angle-mounted agitators in unbaffled vessels, Myers et al. (2011) noted that the just-suspended speed in this configuration is typically seventy-five percent higher than with a vertically-mounted agitator in a fully-baffled vessel. Angle mounting is not always an acceptable approach, particularly for larger agitators whose weight leads to large forces on the agitator shaft and mounting structure. In these instances, vertical off-center agitator mounting may be used in an unbaffled vessel. Moving the agitator off the vessel centerline reduces swirl that leads to near solid-body rotation that often makes meeting agitation objectives challenging. Little information has been published about the performance of vertical off-center agitators in unbaffled vessels, and the study described in this presentation characterizes the effect of impeller type, size, and off-center location on the speed, torque, and power requirements for solids suspension in this configuration.

References

• Drewer, G. R., N. Ahmed, and G. J. Jameson, “Suspension of High Concentration Solids in Mechanically Stirred Vessels”, Proceedings of the Eighth European Conference on Mixing, pages 41-48 (1994).
• Myers, K. J., E. E. Janz, and J. B. Fasano, “Baffling Approaches for Modern Axial-Flow Impellers”, paper 335h, 2005 Annual AIChE Meeting.
• Myers, K. J., J. P. Herr, and E. E. Janz, “Solids Suspension with Angle-Mounted Agitators in Unbaffled Vessels”, Canadian Journal of Chemical Engineering, Volume 89, pages 940-947 (2011).