Numerical modelling of surface aeration and N2O emission in biological water resource recovery

Biokinetic modelling of N2O production and emission has been extensively studied in the past fifteen years. In contrast, the physical-chemical hydrodynamics of activated sludge reactor design and operation, and their impact on N2O emission, is less well understood. This study addresses knowledge gaps related to the systematic identification and calibration of computational fluid dynamic (CFD) simulation models. Additionally, factors influencing reliable prediction of aeration and N2O emission in surface aerated oxidation ditch-type reactor types are evaluated. The calibrated model accurately predicts liquid sensor measurements obtained in the Lynetten Water Resource Recovery Facility (WRRF), Denmark. Results highlight the equal importance of design and operational boundary conditions, alongside biokinetic parameters, in predicting N2O emission. Insights into the limitations of calibrating gas mass-transfer processes in two-phase CFD models of surface aeration systems are evaluated.

On the use of deep learning and computational fluid dynamics for the estimation of uniform momentum source components of propellers

This article proposes a novel method based on Deep Learning for the resolution of uniform momentum source terms in the Reynolds-Averaged Navier-Stokes equations. These source terms can represent several industrial devices (propellers, wind turbines, and so forth) in Computational Fluid Dynamics simulations. Current simulation methods require huge computational power, rely on strong assumptions or need additional information about the device that is being simulated. In this first approach to the new method, a Deep Learning system is trained with hundreds of Computational Fluid Dynamics simulations with uniform momemtum sources so that it can compute the one representing a given propeller from a reduced set of flow velocity measurements near it. Results show an overall relative error below the 5 % for momentum sources for uniform sources and a moderate error when describing real propellers. This work will allow to simulate more accurately industrial devices with less computational cost.

Gas–liquid mixing in the stirred tank equipped with semi-circular tube baffles

Despite extensive contributions have been made over the past several decades, there are still ongoing challenges in improving gas dispersion performance in stirred tanks. To that end, a stirred tank equipped with four semi-circular (SC) tube baffles was designed. Hydrodynamics in the stirred tank before and after aeration were studied. The turbulent fluid flow was simulated using the standard k-ε turbulence model. The gas–liquid two-phase flow was simulated by the Eulerian–Eulerian multiphase model and the k-ε dispersed turbulence model. The impeller rotation was modeled with the multiple reference frame (MRF) approach. Firstly, the grid independence test was made. By comparing the distributions of gas holdup in the flat-plate (FP) baffled stirred tank with literature results, the reliability of the numerical model and simulation method was verified. Subsequently, the flow field, gas holdup and power consumption of the SC and FP configurations were studied, respectively. Results show that the former can increase the fluid velocities and promote the gas holdup dispersion. Besides, it is energy-saving and has a higher relative power demand (RPD). The findings obtained here lay a preliminary foundation for the potential application of the SC configuration in the process industry.

Experimental and simulation study on mixing time and suspension quality of liquid-solid flow field in stirred reactor with draft tube

The solid-liquid flow field is established in the stirred reactor with baffles and draft tube. Mixing time and suspension quality of the flow field are studied. Based on the Eulerian multiphase flow model and RNG k−ε turbulence model, the CFD model is established for the simulation research. By comparing the experimental data of solid concentration distribution and mixing time with the simulated values, it is proved that the CFD model established can be used to study the flow field in the stirred reactor. The effects of stirring speed, liquid viscosity and solid particle size on mixing time and suspension quality are considered. With the increase of stirring speed, mixing time decreases, mixing uniformity is improved. Mixing time decreases first, then increases slightly with the increase of liquid viscosity, and the suspension quality is improved. When the solid particle size increases, mixing time increases rapidly, but the mixing uniformity becomes worse.

CFD Simulation of the Particle Dispersion Behavior and Mass Transfer–Reaction Kinetics in non-Newton Fluid with High Viscosity

Solid particle dispersion and chemical reactions in high-viscosity non-Newtonian fluid are commonly encountered in polymerization systems. In this study, an interphase mass transfer model and a finite-rate/eddy-dissipation formulation were integrated into a computational fluid dynamics model to simulate the dispersion behavior of particles and the mass transfer–reaction kinetics in a condensation polymerization-stirred tank reactor. Turbulence fields were obtained using the standard k–ε model and employed to calculate the mixing rate. Cross model was used to characterize the rheological property of the non-Newton fluid. The proposed model was first validated by experimental data in terms of input power. Then, several key operating variables (i.e. agitation speed, viscosity, and particle size) were investigated to evaluate the dispersive mixing performance of the stirred vessel. Simulation showed that a high agitation speed and a low fluid viscosity favored particle dispersions. This study provided useful guidelines for industrial-scale high-viscosity polymerization reactors.

Data on the agitation of a viscous Newtonian fluid by radial impellers in a cylindrical tank

In this paper, the data assembled concerning the agitation of a Newtonian fluid in a cylindrical vessel is disclosed. The stirred vessel is not provided with baffles and has a flat-bottom. The data presents some information on the characteristics of two impellers: a six-blade Rushton turbine and a six-blade paddle impeller. The flow patterns generated by both impellers are depicted and compared. Also, the power required when changing the impeller rotational speed is given. The data summarized here via three-dimensional calculations of velocities and viscous dissipation in the whole volume of the tank provides additional knowledge for the best choice of impellers for each industrial process.

Radiotracer technology in mixing processes for industrial applications

Many problems associated with the mixing process remain unsolved and result in poor mixing performance. The residence time distribution (RTD) and the mixing time are the most important parameters that determine the homogenisation that is achieved in the mixing vessel and are discussed in detail in this paper. In addition, this paper reviews the current problems associated with conventional tracers, mathematical models, and computational fluid dynamics simulations involved in radiotracer experiments and hybrid of radiotracer.

Computational Fluid Dynamics Simulation of Flows in an Oxidation Ditch Driven by a New Surface Aerator

In this article, we present a newly designed inverse umbrella surface aerator, and tested its performance in driving flow of an oxidation ditch. Results show that it has a better performance in driving the oxidation ditch than the original one with higher average velocity and more uniform flow field. We also present a computational fluid dynamics model for predicting the flow field in an oxidation ditch driven by a surface aerator. The improved momentum source term approach to simulate the flow field of the oxidation ditch driven by an inverse umbrella surface aerator was developed and validated through experiments. Four kinds of turbulent models were investigated with the approach, including the standard k-ɛ model, RNG k-ɛ model, realizable k-ɛ model, and Reynolds stress model, and the predicted data were compared with those calculated with the multiple rotating reference frame approach (MRF) and sliding mesh approach (SM). Results of the momentum source term approach are in good agreement with the experimental data, and its prediction accuracy is better than MRF, close to SM. It is also found that the momentum source term approach has lower computational expenses, is simpler to preprocess, and is easier to use.

Experimental and computational determination of blend time in USP Dissolution Testing Apparatus II

Blend time, the time to achieve a predefined level of homogeneity of a tracer in a mixing vessel, is an important parameter to evaluate the mixing efficiency of mixing devices. In this work, the blend time required to homogenize the liquid content of a USP Dissolution Testing Apparatus II under a number of operating conditions was obtained using two different experimental methods (tracer detection via colorimetric and conductivity measurements), a computational approach (computational fluid dynamics (CFD)), and a semi-theoretical analysis of the phenomenon. Under the standard geometric and operating conditions in which the USP Apparatus II is typically used (N = 50 rpm) the experimental blend time to achieve a 92.74% uniformity level was found to be between 27.5 and 33.3 s, depending on the location of the injection point and monitoring point for the tracer. These values were in close agreement with those obtained from CFD simulations. Changing the impeller vertical position (+/-2 mm) had only a limited effect. The CFD predictions also indicated that blend time is inversely proportional to the agitation speed. This conclusion is in agreement with previous reports and equations for blend time in mixing vessels. The blend times obtained in this work appear to be some two orders of magnitude smaller than the time usually required for appreciable tablet dissolution during the typical dissolution test, implying that the liquid contents of the USP Apparatus II can be considered to be relatively well mixed during the typical dissolution test.

Hydrodynamic Investigation of USP Dissolution Test Apparatus II

The USP Apparatus II is the device commonly used to conduct dissolution testing in the pharmaceutical industry. Despite its widespread use, dissolution testing remains susceptible to significant error and test failures, and limited information is available on the hydrodynamics of this apparatus. In this work, laser-Doppler velocimetry (LDV) and computational fluid dynamics (CFD) were used, respectively, to experimentally map and computationally predict the velocity distribution inside a standard USP Apparatus II under the typical operating conditions mandated by the dissolution test procedure. The flow in the apparatus is strongly dominated by the tangential component of the velocity. Secondary flows consist of an upper and lower recirculation loop in the vertical plane, above and below the impeller, respectively. A low recirculation zone was observed in the lower part of the hemispherical vessel bottom where the tablet dissolution process takes place. The radial and axial velocities in the region just below the impeller were found to be very small. This is the most critical region of the apparatus since the dissolving tablet will likely be at this location during the dissolution test. The velocities in this region change significantly over short distances along the vessel bottom. This implies that small variations in the location of the tablet on the vessel bottom caused by the randomness of the tablet descent through the liquid are likely to result in significantly different velocities and velocity gradients near the tablet. This is likely to introduce variability in the test.