CFD-based process optimization of a dissolved air flotation system for drinking water production

Dissolved air flotation (DAF) has received more attention recently as a separation technique in both drinking water as well as wastewater treatment. However, the process as well as the preceding flocculation step is complex and not completely understood. Given the multiphase nature of the process, fluid dynamics studies are important to understand and optimize the DAF system in terms of operation and design. The present study is intended towards a comprehensive computational analysis for design optimization of the treatment plant in Kluizen, Belgium. Setting up the modelling framework involving the multiphase flow problem is briefly discussed. 3D numerical simulations on a scaled down model of the DAF design were analysed. The flow features give better confidence, but the flocs escape through the outlet still prevails which is averse to the system performance. In order to improve the performance and ease of maintenance, design modifications have been proposed by using a perforated tube for water extraction and are found to be satisfactory. The discussion is further reinforced through validating the numerical model against the experimental findings for stratified flow conditions

Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

Image Analysis and Multiphase Bioreactors

The applications of visualisation and image analysis to bioreactors can be found in two main areas: the characterisation of biomass (fungi, bacteria, yeasts, animal and plant cells, etc), in terms of size, morphology and physiology, that is the far most developed, and the characterisation of the multiphase behaviour of the reactors (flow patterns, velocity fields, bubble size and shape distribution, foaming), that may require sophisticated visualisation techniques

Annual report 2006 // Institute of Safety Research

[no abstract available

Multiphase contacting in PGM hydrometallurgy

This thesis describes hydrodynamic studies of the leach and solvent extraction stages of a Platinum Group Metal (PGM) hydrometallurgical flowsheet. The studies were motivated by the need to increase PGM throughput in Johnson Matthey’s PGM refining business. In the leach stage, key components in the feed are selectively dissolved using acids in a stirred tank before they are recovered by liquid-liquid (L-L) solvent extraction and finally purified. The work described in this thesis tackles four main areas: hydrodynamic studies of L-L PGM solvent extraction in both mixer and settler stages, whilst for the leach stage, studies of particle behaviour in gas evolving solid-liquid (S-L) reactions and gas-liquid-solid (GLS) characterisation by a novel Electrical Resistance Tomography (ERT) technique are performed. In the mixer-settler, the effects of impeller diameter, D, to vessel diameter, T, ratio (D/T), the phase flow ratio, cφ/dφ; (where cφis the continuous phase flow fraction and dφ is the dispersed phase flow fraction) and the specific power input,Tε, upon the droplet size distribution in a L-L system and their phase separation were investigated. Changing a smaller D/T impeller for a larger D/T impeller at constant P/V and cφ/dφincreased droplet size because the maximum shear rate decreased as a result of increasing ratio of impeller pumping capacity (Q) with tip speed (Utip). Changing a larger cφ/dφfor smaller cφ/dφat a fixed P/V and D/T impeller increased droplet size because turbulent dampening increased since the average density, ρ ∝ dφ. Meanwhile, Kolmogoroff-Hinze’s theory was shown to apply for the measured relationship between Tε and droplet size. A settler design criterion, which relates the dispersed phase concentration (Ca) in the dispersion band to the dispersed phase throughput (Qd/A) agreed with the model by Ryon et al. (1959). Ca was significantly dependent on P/V and Qd/A, whilst the effects of Qc/Qd (where Qc is the continuous phase flowrate and Qd is the dispersed phase flowrate) and D/T were minimal. Droplet size analysis of the sedimenting region of the dispersion band and dense packed layer revealed a transitional distribution of droplet sizes due to the counteracting effects of droplet sedimentation, hindered settling and droplet-droplet coalescence. Particle behaviour in gas evolving S-L systems were quantified using the Zwietering ‘just-suspended’ impeller speed (Njs) condition in a sponge nickel® and sodium hypochlorite system. The presence of gas caused Njs to increase, however a coherent relationship between Njs in an ungassed and gassed system 3 could not be easily ascertained. Further work with Positron Emission Particle Tracking (PEPT) was advised to quantify the relationship. A well-known electrical concept called skin effect, which describes how the effective resistance of an electrical conductor varies as the frequency of an alternating current (AC) increases and decreases, was used to investigate GLS behaviour via a novel ERT spectroscopic technique. The process relies on the change in effective resistance of conducting objects with changing AC frequency to selectively detect different phases. The concept was initially validated with static phantoms of a stainless steel and plume of gas before being applied to dispersible stainless steel particles and gas. ERT spectroscopy showed that two AC frequencies (0.3 kHz and 9.6 kHz) could successfully isolate and simultaneously detect the gas and solid phases at a fixed current. By subtracting solids and gas conductivity, the change in solids and gas holdup were obtained

Experimental and CFD–PBM investigation of an agitated bioreactor using a dual helical ribbon impeller

Throughout past decades, the management of solid waste by producing methane gas, as a renewable source of energy, has featured as an important research objective. Anaerobic digesters are widely used in countries with environmental initiatives and green approaches, where biogas produced from a bioreactor is a carbon neutral source of energy. Biogas contains 70% methane, 30% CO2 and some other gases. The by-product of an anaerobic digester is solid sludge that can be used as either fertilizer or compost. Anaerobic digestion biogas plants can benefit industries by adding value to solid organic waste, reducing fossil fuel usage, eliminating solid waste disposal costs, in addition to generating power. Setting up an anaerobic digestion biogas plant is a green investment for industries interested in environmentally friendly biological processes. A variety of organic solid waste including municipal, industrial, livestock, poultry, meat, and food waste can be digested in an anaerobic system. To treat the large volume of waste generated by industries and urban sewerage systems, more efficient digesters and a continuous improvement of digestion processes are required. To accomplish these objectives, crucial factors including the size, design, and shape of a bioreactor, its working temperature, pH and the hydrodynamics of a system need to be studied. A considerable amount of literature has been published regarding the hydrodynamics of anaerobic digesters. Further, several studies have explored the factors thought to influence the hydrodynamics of anaerobic digesters. These studies have identified that the hydrodynamics of a system could be influenced by the rheological characteristics of sludge, as well as mixer type and shape. Inadequate and poor mixing in a digester can cause the failure of a reactor, non-uniform distribution of mass and heat, imbalanced microbial activity, as well as formation of sediment and scum. Although studies have successfully demonstrated that close-clearance mixers (screw, helical, anchor impellers) increase biogas production, the information about hydrodynamic characteristics and flow field generated by these types of agitators is inadequate. Although hydrodynamics and the rheology of sludge have been studied in the past, more research is required to address these gaps. The application of visual and measuring instruments could facilitate further research on sludge behaviour in an agitated anaerobic digester, but this type of study is not possible due to the opaque nature of real sludge. The main objectives of this project are (i) to find a safe, cheap, clear and stable material that can emulate digested sludge rheological characteristics in a laboratory; (ii) to study and optimize the mixing performance of a dual helical ribbon as an efficient impeller to create an ideal mixing pattern (iii) to investigate the flow pattern and hydrodynamics of a shear thinning fluid in a batch gas-liquid reactor using a combination of a computational fluid dynamics (CFD) simulation and a population balance model (PBM). Study 1 has analysed and compared the Zeta potential, pH resistance, flow curve, viscoelasticity, and thixotropy of four popular model fluids reported previously as ideal simulant of primary, activated, and digested sludge. The results of the correlational analysis indicate that xanthan gum is the best simulant to mimic the rheological characteristics of activated sludge that is sheared less than 100 S-1. There are similarities between the viscosity and flow curve of activated sludge and xanthan gum which can be described by its internal network and molecular structure. This study also compares rheological properties of 2% NaCMC solution and digested sludge containing 3.23% solid sheared between 10-300 S-1, concluding that they behave in an essentially identical manner. The findings from this study provide several contributions towards selecting and applying a clear and safe polymer that emulates the rheological behaviour of sludge. Study 2 has evaluated the performance of a dual helical ribbon impeller in agitating shear thinning fluid. The effects of impeller rotational speed, gas flow rate, clearance to the bottom, and viscosity on power uptake and mixing time have been studied. This study suggests that determining optimum operating conditions can minimize power consumption and time required to achieve the maximum volume of uniformity in reactor. Although the study successfully reports a significant positive correlation between the rotational speed of the impeller and the performance of mixing, there is still a threshold limit for rotational speed. Experimental data shows that power consumption would increase with rotational speed however increasing the rotational speed beyond the certain level does not affect the mixing time significantly. This study suggests two practical equations to estimate power consumption and mixing time under specific operating conditions by applying an ANOVA method. To cover some of the limitations related to the experimental study of hydrodynamics of gasliquid systems, a combination of computational fluid dynamics (CFD) simulation and population balance model (PBM) has been used in the third study. The main purpose of this work is to evaluate the impacts of using a dual helical ribbon on the hydrodynamics of a multiphase reactor. The governing equations and turbulent model of agitated bubbly flow have been solved through a standard k-e model and Eulerian-Eulerian (E-E) multiphase approach. Following grid sensitivity analyses, findings through simulation have been verified by PIV measuring tests. Further, the PBM model has been discretized into five bubble size groups. The results show a positive relationship between rotational speed and bubble breakage. The comparative study indicates an increase in the likelihood of bubble channeling when the rotational speed is insufficient to break the gel-like structure of the liquid. By increasing rotational speed, the bubble hits the blades, breaks, and disperses, leading to improved interfacial area between phases. Further, rotating mechanical blades induce shear stress to bulk of liquid, resulting in a significant drop in viscosity and diminishing the stagnant regions

Annual report 2006 // Institute of Safety Research

[no abstract available

Industrial Radiotracer Technology for Process Optimizations in Chemical Industries – A Review

Radioisotope techniques are constantly and extensively used all over the world as a method to identify process systems malfunctions in various industries without requiring the shut down of the processing plant thus leading to high economical benefits to the plant owner. Different aspects of industrial radiotracer technology for troubleshooting, process control and optimization are evaluated through an exhaustive literature survey. The review covers the advantages of radiotracers, most commonly used radiotracers in industry for specific studies, applications of radiotracer techniques in various chemical industries, the design of radiotracer technology experiments, radiation detection and data acquisition in radiotracer technology as well as radiological safety aspects. Two industrial radiotracer techniques of residence time distribution (RTD) measurements and radioactive particle tracking (RPT) are discussed. The design of radiotracer technology experiments are also divided into two categories - radioactive particle tracking applications and residence time distribution applications