Trouble Shooting of Agitation in an Oxidation Ditch : Applicability of Hydraulic Modeling

Abstract

Hydrodynamics in an oxidation ditch is a main key of an optimal treatment. Mixing and aeration are closely linked to biological performances. One parameter considered to be essential for correct design and operation is the horizontal velocity without aeration. A value of 0.3-0.35 m.s -1 for this parameter is regarded as a prerequisite (Da Silva, 1994). Hence its correct prediction is high importance. Different CFD modelling approaches have been applied and evaluated with regard to their performance in modelling the agitation in an oxidation ditch and consequently assessing the horizontal velocity. The two approaches used differ in the way they take the agitation, i.e. the impeller, into account in the model set-up : a) the "Fixed Values" approach, imposing a local axial velocity on cylinders which represent the impellers; b) the "Multiple Reference Frame" approach (MRF), using the exact geometry of the impellers. Theses approaches are compared with experimental data, i.e. with the average horizontal velocity as well as with velocity profiles. The first CFD agitation model ("Fixed Values") is highly dependant on the quality of the information provided by the supplier of the impellers. This is due to the fact that the supplier presents the value of the imposed axial velocity for a given impeller. Hence this whole modelling approach depends on the accuracy of the value provided. However, once applying the correct axial velocity imposed, this approach can gain quite some relevance as it is less computational intensive than the MRF approach . Experimental measurements have in fact revealed a too low horizontal velocity in the oxidation ditch of concern. Thanks to the CFD simulations, presented in this study, explanations of the low horizontal velocity could be put forward. This illustrates their potential for advanced troubleshooting.

CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch

“…The computational challenges make considerable simplifications necessary for geometries and operating parameters such as impeller modelling and using a single bubble mean diameter, respectively. The modelling of submerged agitators is considered a reliable method and it is experimentally validated by comparing the axial liquid velocity of single-phase flow in a full-scale aeration tank [10]. The same method was also used to investigate the hydrodynamics of two-phase flow in a full-scale study, but the difference between experimental and numerical data was observed.…”

In situ characterization of local hydrodynamic parameters in closed-loop aeration tanks

“…In addition, computational fluid dynamics (CFD) is more and more used to interpret the obtained results [3][4][5][6]. However, no set of data including all the parameters required to understand and to simulate the phenomena (bubble size as input data; gas hold-up, axial liquid velocity and oxygen transfer coefficients as validation data) has ever been proposed for full-scale closed-loop aeration tanks.…”

CFD Modeling: An efficient tool to optimize the design and operation of aeration systems

“…(ii) Mixers' thrust velocity (U p ): This velocity corresponds to the velocity passing through the mixers. Its value is generally provided by manufacturers or could be determined using CFD (Vermande et al, 2003).…”

“…To optimize full-scale aeration systems, models coupling computational fluid dynamics (CFD) and mass transfer equations have been in development since a few years (Vermande et al 2003; The main advantage of such procedures lies in their capacity to ignore all scale effects, which constitute the principal difficulties to apply recommendations resulting from experimental studies, especially performed on lab scale pilots. However, the reliability of the results strongly depends on the precise knowledge of input data, i.e.…”