Date

2017

Author

Kütükçü, Başak

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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 correlations in literature are used to predict Njs. Most of the studies on this subject have been done with unimodal slurries of spherical particles at low solids concentrations, and some with mixed slurries of spherical particles at high concentrations. But industrial applications generally consist of mixtures containing many solid phases and irregularly shaped particles at high concentrations. In this study, the aim was to test the applicability of the correlations in literature for mixed slurries that have non-spherical particles at high concentrations and to test the applicability of pressure gauge measurement method at conditions that are different than literature conditions. First, the suspension behaviour of mixed slurries with non-spherical particles was investigated. The results showed that spherical particles are harder to suspend. To investigate whether a correction is required in correlations, the Njs predictions of Ayranci’s correlation were compared for the unimodal slurries with spherical and non-spherical particles. Ayranci’s correlation was chosen because the most accurate Njs predictions were obtained with this correlation compared to the other correlations in literature without any shape consideration. It was seen that the predictions with Ayranci’s correlation are at almost the same accuracy for both spherical and non-spherical particles. Nevertheless, to obtain more accurate results for non-spherical particles, the performance of aspect ratio, circularity, convexity, Wadell’s sphericity and Corey’s shape factor was compared to incorporate particle shape mathematically. It was decided that Wadell’s sphericity is the most convenient factor to represent the effect of particle shape on solids suspension in stirred tanks. With this, the standard deviations in the predictions decreased. The mixed slurry Njs was measured experimentally and these results were compared with the predictions using the power model. A correction on the power model was recommended for the slurries that show networking effect. This correction allowed for the prediction data to follow the experimental data more closely. When the correction for the particle shape in Ayranci’s correlation was introduces, the standard deviations in the prediction of mixed slurries decreased. Finally, the applicability of pressure gauge measurement method was tested at conditions that are beyond the limits that it was developed initially. The problems encountered with collection and analysis of data at these out-of-limit applications and the solutions to overcome them are reported. Also, the visual observation method and the pressure gauge measurement method were compared with a large data set for the first time, in this study.

Subject Keywords

Solution (Chemistry)., Solid-liquid equilibrim., Solid-liquid interfaces., Suspensions (Chemistry)., Chemistry, Physical and theoretical.

URI

http://etd.lib.metu.edu.tr/upload/12621088/index.pdf
https://hdl.handle.net/11511/26567

Collections

Graduate School of Natural and Applied Sciences, Thesis