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

Download

Ezgi Altintas-Masters Thesis.pdf

Date

2021-6

Author

Altıntaş, Ezgi

Metadata

Show full item record

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Item Usage Stats

425
views

112
downloads

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.

Subject Keywords

Solids suspensions, Cloud height, High solids concentration, Tall tanks

URI

https://hdl.handle.net/11511/91333

Collections

Graduate School of Natural and Applied Sciences, Thesis

Suggestions

OpenMETU
Core

The Effect of particle shape on just-suspended speed of mixtures of solids at high concentrations Kütükçü, Başak; Ayrancı Tansık, İnci; Department of Chemical Engineering (2017) Solid-liquid mixing is a common application in the chemical industry. For solid-liquid mixing operations, it is important to determine the impeller speed which provides the maximum contact surface between the phases at minimum power consumption. This impeller speed is called just suspended speed, Njs. Njs can be determined experimentally by visual observation or pressure gauge measurement method. If the mixing system is at the design stage or if it is not possible to apply experimental methods, the correlat...
Analysis of velocity profiles in concentrated solid-liquid mixtures using ultrasound doppler velocimetry Taşkın, Ahmet Fırat; Ayrancı Tansık, İnci; Uludağ, Yusuf; Department of Chemical Engineering (2021-7) Stirred tanks are widely used for a variety of processes in industry such as solid dispersion, crystallization and dissolution processes. Several parameters are used to optimize these stirred tank processes such as impeller type, tank geometry and impeller speed. The flow field in the stirred tank is complex and varies depending on these parameters. In these processes diffusion and chemical reaction rate depend on the flow field and velocity fluctuations; therefore, information about the flow field is valua...
The formation of smooth, defect-free, stoichiometric silicon carbide films from a polymeric precursor Pitcher, Michael; Bianconi, Patricia (2006-04-20) Silicon carbide (SiC) materials, which are used in a variety of applications, are often produced using powder processing, sintering or bulk crystal growing techniques. The formation of silicon carbide films or shaped products, via these methods, is often extremely difficult and/or requires high temperatures. Here we report the synthesis and characterization of a polymeric precursor, Polymethylsilyne (PMSy), and it subsequent conversion to beta-SiC. The polymer is simple to synthesize and is easily manipulat...
The Effect of dispersed phase feed rate on the production of pickering emulsions Dönmez, Dila; Ayrancı Tansık, İnci; Department of Chemical Engineering (2018) Emulsions are fundamental in many applications such as food, petroleum, detergent, pharmaceutical and cosmetic industries. Pickering emulsions are defined as emulsions of any type, either oil-in water, water-in-oil or even multiple, stabilized by solid particles instead of surfactants. Solid particles may prevent the droplets from coalescing by getting adsorbed at the interface, and thus stabilizing the emulsion. Pickering emulsions have more advantages over emulsions stabilized by surfactants in terms of s...
The effect of Dilution, Aeration, and Agitation on Fungal Cellulase and Xylanase Production by DDGS-based Fermentation Media in Stirred Tank Bioreactors Iram, Attia; Çekmecelioğlu, Deniz; Demirci, Ali (2022-01-01) Fungal cellulases and xylanases are commonly used for biological pretreatment of lignocellulosic biomass into simple sugars, which can then be converted into many value-added products including bioethanol. However, these enzymes are costly to produce and need a feedstock that should be more economical and high in fiber and other nutrients. Distillers dried grains with solubles (DDGS) is the byproduct of corn ethanol production and can be considered an appropriate feedstock. Among various problems in adaptat...

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.