The effect of solids concentration and particle properties on cloud height in tall stirred tanks

Altıntaş, Ezgi
Solid-liquid mixing is one of the most commonly used unit operations in industries such as petrochemicals, polymer processing, biotechnology, pharmaceuticals, and mineral processing. There are two focuses in solid-liquid mixing operations: solids suspension and solids distribution. The key design parameter for solids suspension is Njs. In most solids suspensions, the main objective is to provide maximum contact between solid and liquid phases with minimum power consumption, and this can be achieved by setting the impeller speed (N) to Njs. The key design parameter for solids distribution is cloud height. At high solids concentrations (XV) the solids can reach a level beyond which the concentration of the solids dramatically drops. This level appears like an interface between solid-rich and liquid-rich parts of the tank. The height of this interface is known as cloud height. Due to the fluctuating nature of cloud height its measurement is not straightforward. Despite this, there is no clearly defined measurement point of cloud height in literature. Besides, the current definition of cloud height does not involve any limitations on XV and N, which are two parameters that significantly affect hydrodynamics in a stirred tank, and thus the cloud height. To obtain meaningful cloud height data, a measurement point of cloud height should be determined; likewise, limitations on XV and N should be identified. This study aims to propose a clarified definition of cloud height that takes XV and N into account, to investigate the effects of XV and particle properties on cloud height and propose a correlation to predict cloud height. A flat-bottomed tank in which liquid level was equal to 1.5 tank diameter (H=1.5T) was used with four equally spaced baffles. A 45° pitched blade turbine (PBT) was used as an impeller. Six different particles were used in the experiments. According to observations, in the tank, efficient solid-liquid mixing takes place until the maximum point that solids can reach in the axial direction. Beyond this, only rare bursts of a small portion of solids were observed. The measurement point of cloud height, therefore, was determined as the maximum level that solids can reach. This corresponds to the front of the baffle for the configuration tested. The limitations on XV and N for the measurement of cloud height were identified using this measurement point. According to findings, to observe a meaningful cloud height, XV should be at and above 2 vol% and N should be at and above 0.32Njs. For a H=1.5T tank, a meaningful cloud height can be observed until N is equal to 1.45Njs, at which all solids are distributed throughout the tank and no interface can be observed. It was also found that beyond 9 vol% cloud height remains constant. After limitations of XV and N on cloud height were determined, the effect of XV, particle properties and off-bottom clearance (C/T) on cloud height was investigated. The results showed that cloud height is a strong function of XV and C/T; but not a strong function of particle properties. Based on the findings, a purely empirical model that predicts cloud height as a function of XV, particle properties and C/T was proposed.
Citation Formats
E. Altıntaş, “The effect of solids concentration and particle properties on cloud height in tall stirred tanks,” M.S. - Master of Science, Middle East Technical University, 2021.