Agitation energy efficiency in gas-solid-liquid stirred vessels operating at ultra-high solids concentrations

In this work, suspension of high-concentration slurry in the presence of gas was investigated using different baffling configurations, particle sizes, gas flow rates, and impeller types with the goal of improving the agitation energy efficiency. A term defined as power efficiency factor epsilon(-1)(jsg) (kg/W) was used as an indication of the amount of solid particles that could be suspended per unit of power consumed by an impeller. By varying operational parameters like solids concentration and gas flow rate, and design parameters like impeller type and baffling condition, it was found that the epsilon(-1)(jsg) values can be maximised by operating the tank with an optimum range of solids concentration, which is around 0.2-0.3 (v/v) for the systems studied in this work. Increase in particle size resulted in lower epsilon(-1)(jsg) values and this phenomenon was more prominent in unbaffled tanks. Another term, known as baffling efficiency factor, 'R-epsilon', was used to study how baffle removal influences the energy efficiency of impellers in three-phase systems. It was observed that the absence of baffles could lead to decreased energy efficiency for axial- and mixed-flow impellers at particular operating conditions. The investigations also included the effect of baffle removal on solids dispersion, and results of this investigation indicate that it is possible to obtain complete solids dispersion with reduced power input, whilst the solids were suspended from the tank bottom. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.