- Title
- Oxygen mass transfer in solid-laden liquids
- Creator
- Jalayer, Alireza
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Masters Research - Master of Philosophy (MPhil)
- Description
- In this study a falling film apparatus was used to aerate a continuous water stream containing solids. Specifically, the aim of the research was to investigate the performance of the apparatus, in terms of the oxygen mass transfer rate, overall volumetric mass transfer coefficient, interfacial area and alpha factor, as a function of water volumetric flow rate, initial dissolved oxygen concentration, solids concentration, and apparatus geometry. The falling film apparatus was comprised of a vertical array of horizontal wooden cylinders (0.035 m diameter and 0.150 m length); where the number of cylinders and vertical spacing between each cylinder in the array was between 5-11 and 0-6 mm, respectively. The cylinder array was housed inside an enclosed chamber, of volume approximately 33 L, which was filled with a pure oxygen atmosphere during the experiments. The tap water feed with volumetric flow rate in the range 0.7-4.5 L/min and corresponding to film Reynolds numbers between 142-1135 was introduced via an inlet at the top of the chamber. The feed was then directed uniformly along the entire width of the uppermost cylinder through a distribution nozzle; and then cascaded over the cylinders below before being discharged through an exit at the bottom of the enclosed chamber. A pressure manometer and dissolved oxygen (DO) meters were used to monitor the chamber pressure and DO of the inlet and outlet streams, respectively. Experiments were performed for feed water solids (MLSS) concentrations of between 0-12820 mg/L. The solids used in the study were activated sludge residue that had been stabilised: (1) biologically, with at least 1 year of residence time in a sludge lagoon that was either submerged or had been drained for dewatering purposes; and (2) thermally, in a drying oven maintained at 90ºC for at least 12 hours. The expectation was that there would be no biological uptake during the mass transfer experiments. Whilst this was the case, chemisorption of the dissolved oxygen onto the heat treated activated (carbon) sludge was found to take place and was accounted for quantitatively when determining the oxygen mass transfer coefficient from the mass transfer experiments. The mass transfer experiments involved filling the aeration chamber with pure oxygen and recording the inlet and outlet DO concentrations of the downflowing water stream both without and with MLSS solids present. The mass of oxygen transferred over the experiment duration was then inputted in Fick’s Law to determine the mass transfer coefficient and alpha value based on measured/computed average falling film thickness and velocity. For experiment with no biosolids and zero vertical spacing between adjacent cylinders, it was found that the oxygen mass transfer rate increased with increasing volumetric feed flow rate and increasing number of cylinders in the vertical array—which corresponded to an increase in the gas-liquid contact time. The mass transfer rate could be adequately fitted to a power law relationship for the volumetric feed rate. Similarly, the computed mass transfer coefficient, using the wetted surface area of the cylinders as the mass transfer area, could be reasonably modelled by a power law where the fitted volumetric feed rate exponent of 0.82 was independent of the number cylinders in the array. The exponent value of 0.82 was consistent with published literature values where wavy motion were present on the surface of the falling film—as was observed in this study. Average film thickness and water mean velocity were measured experimentally for a water-only feed and 8-cylinder array with zero spacing. The measurements were able to be adequately described by existing analysis for vertical falling films as well as the correlation by Rogers and Goindy (1989). A limited number of experiments were performed for a vertical spacing of 3 mm. It was found that there was an increase in the oxygen mass transfer rate when compared with that of zero spacing. However, once the resultant increase in mass transfer area and contact time, due to the increased distance the feed travels whilst the velocity remained relatively unchanged, it was found that there was no statistical difference to that of the mass transfer coefficient for zero spacing. For vertical spacing beyond 5mm, the integrity of the liquid film was not able to be maintained and not considered further as part of this study. Mass transfer experiments were performed for MLSS in the range 740-12820 mg/L for an 8-cylinder array with 3mm vertical separation. It was found that the mass transfer coefficient could be fitted to the volumetric feed rate to a power law with the same exponent of 0.82 as that obtained for the water only experiments. The experimental mass transfer coefficient versus MLSS concentration was compared with the predictions from correlations of Al-Malah (2013) and Seader and Henley (2006) for vertical falling films. In order to apply the correlations the presence of solids on the apparent viscosity was taken into account using the expression of Mooney (1950) for non-aggregating, spherical particles. Good agreement between measurement and predictions from Seader and Henley (2006) was obtained for experimental film Reynolds number average of 865. For experimentally determined mass transfer coefficient values normalised to a film Reynolds number of 25, and corresponding to a smooth film surface, it was found that the model of Al-Malah (2013) provided better agreement, which is not surprising given the analysis was based on a smooth, laminar film. At ReF of 25, the Seader and Henley (2006) correlation over-predicted the kL value by approximately 500 percent. Finally, the computed alpha followed an exponential decay with increasing MLSS concentration, with an exponential decay coefficient of -0.3, which compared very favourably with results with all other aeration devices (see Figure 4.11) considered in this study. The finding supports the original hypothesis that the negative impact of the presence of solids on the mass transfer performance, i.e. reduction in alpha value, commonly experienced in bubble-type aeration devices due to coalescence can be avoided by not having the gas as the dispersed phase. The falling film device has demonstrated that mass transfer is much less sensitive to the presence of solids, and for this reason offers an alternative aeration methodology.
- Subject
- oxygen mass; sold-laden liquids; water; oxygen mass transfer; solids; oxygen concentration; water volumetric flow rate; solids concentration; apparatus geometry
- Identifier
- http://hdl.handle.net/1959.13/1310002
- Identifier
- uon:21971
- Rights
- Copyright 2015 Alireza Jalayer
- Language
- eng
- Full Text
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