A hydrodynamic study of nickel suspension in stirred tanks

Many studies on solid-liquid mixing have been dedicated to low density particles at low solids concentrations. In the present work, computational fluid dynamics (CFD) simulation and experimental methods were employed to study suspension of high density particles (nickel) at high solids concentration in water. The work first focused on establishing the velocity field in a liquid-only system and then progressed to a solid-liquid system. In the liquid-only system, the influence of tank geometry and simulation strategies, including turbulence models, on fluid flow pattern and mixing was investigated in a tank stirred by a Rushton turbine. The standard k-f. model gave better overall predictions of mean velocity fields than the k-ro and RNG k-f. models. The CFD simulation and experimental results obtained with the laser Doppler velocimetry (LDV) method showed that mixing time and homogenization energy decreased with a decrease in the impeller bottom clearance. It was further shown that there is a bottom clearance range in which a draft tube can aid mixing in a tank stirred by the Rushton turbine. In the solid-liquid system, a hydrofoil impeller was used to investigate the influence of simulation strategies, particle properties and hydrodynamic operating conditions on mixing features such as the off-bottom solids suspension, cloud height, solids concentration distribution and local particle size distribution. The simulation results were compared with experimental ones, in which the off-bottom solids suspension was determined visually and an optical attenuation technique was employed to determine the cloud height and solids concentration distribution. The local particle size distribution (PSD) in the tank was measured by a laser diffraction method. A better agreement between the simulation and experimental results was obtained with drag models that account for the solids loading or free stream turbulence than those that do not. It was shown that the Stokes law applies up to a diameter of 150 ~m for the nickel particles. A CFD simulation strategy for studying mixing of high density solids is proposed and it is shown that a CFD simulation method can be used to develop empirical models that predict mixing features. A CFD simulation approach that takes particle size into account gives predictions that are more representative of practical applications than the mono-size particle simulation approach. Reactor configurations and hydrodynamic parameters that improve mixing were identified. These can also aid optimal design of mixing systems

Green economy in the wastewater treatment sector: Jobs, awareness, barriers, and opportunities in selected local governments in South Africa

The green economy (GE) has increasingly gained international focus, with new strategies aimed at restructuring the economy in an environmentally friendly manner. A study was carried out to analyse existing and potential green jobs and identify green qualifications and skills necessary for the development of the GE. The study was done within the context of the role of local governments in the adoption of green economy strategies in the wastewater treatment sector, in selected local governments across the northern provinces of South Africa. A descriptive survey method with qualitative and quantitative approaches was employed for data collection from twenty-four wastewater treatment plants in eleven local municipalities. The concept of the GE was foreign to several employees, who could not identify green jobs within most sectors of the economy. Only 13.5% of the employees of the surveyed plants were involved in directly green jobs. A further 36% were in green-related jobs while the remaining 50.5% were involved in non-green jobs. Barriers to the creation of green jobs and implementation of green practices were the shortage of employees with green and conventional wastewater treatment skills, and lack of training in green skills. Several opportunities for green jobs creation exist, such as the implementation of renewable energy, re-use of treated effluent, and processing of waste sludge into compost. To spur GE growth and create green jobs, the creation of awareness, development of skills and implementation of green technologies should be intensified

Green economy in the wastewater treatment sector: Jobs, awareness, barriers, and opportunities in selected local governments in South Africa

The green economy (GE) has increasingly gained international focus, with new strategies aimed at restructuring the economy in an environmentally friendly manner. A study was carried out to analyse existing and potential green jobs and identify green qualifications and skills necessary for the development of the GE. The study was done within the context of the role of local governments in the adoption of green economy strategies in the wastewater treatment sector, in selected local governments across the northern provinces of South Africa. A descriptive survey method with qualitative and quantitative approaches was employed for data collection from twenty-four wastewater treatment plants in eleven local municipalities. The concept of the GE was foreign to several employees, who could not identify green jobs within most sectors of the economy. Only 13.5% of the employees of the surveyed plants were involved in directly green jobs. A further 36% were in green-related jobs while the remaining 50.5% were involved in non-green jobs. Barriers to the creation of green jobs and implementation of green practices were the shortage of employees with green and conventional wastewater treatment skills, and lack of training in green skills. Several opportunities for green jobs creation exist, such as the implementation of renewable energy, re-use of treated effluent, and processing of waste sludge into compost. To spur GE growth and create green jobs, the creation of awareness, development of skills and implementation of green technologies should be intensified

Effective Microporosity for Enhanced Adsorption Capacity of Cr (VI) from Dilute Aqueous Solution: Isotherm and Kinetics

The adsorbent pore structure significant to enhanced adsorption capacity of Cr (VI) from dilute aqueous solution is evaluated. As reference, low-cost micro-mesoporous activated carbon (AC) of high basicity, mesoporosity centred about 2.4 nm, and effective microporosity centred about 0.9 nm was tested for removal of Cr (VI) from dilute aqueous solution in batch mode. At pH 2 the low-cost AC exhibited highly improved Langmuir Cr (VI) capacity of 115 mg/g which was competitive to high performance commercial AC. A Comparison with treated characterization results of literature adsorbents/ACs showed that moderate to high effective micropore volume of average pore-size about 0.9 ± 0.1 nm is critical for increased adsorption capacity of Cr (VI) from dilute aqueous solutions. The mesostructure of the tested low-cost AC was associated with rapid kinetics that was fitted by the Pseudo-second kinetics model. While Biot numbers suggested slight significant contribution of intraparticle diffusion. It is hoped that this study may be a useful contribution to development of effective adsorbents for the efficient abatement of toxic Cr (VI) from wastewater and water

Cobalt removal from wastewater using pine sawdust

Agricultural wastes can cause environmental problems if not well managed, but there is a lot of potential to use these wastes as raw material in other processes. In this investigation, pine sawdust was evaluated as an adsorbent in the treatment of wastewater containing cobalt ions. A two-level three-factor full-factorial experimental design with centre points was used to study the interactive effect of the operating parameters in order to achieve the best conditions for the batch adsorption of cobalt ions. A response surface analysis was also conducted to further understand the interactions amongst the factors such as adsorbent dose, solution pH and initial concentration. In addition, adsorption isotherms, namely the Freundlich and Langmuir, were used to characterize the removal of cobalt from the wastewater. It was observed that the combined effect of low adsorbent dose, high pH and high initial concentration of wastewater resulted in the highest adsorption capacity. The Freundlich isotherm provided a better fit to the experimental data than the Langmuir isotherm. Moreover, pine sawdust showed adsorption capabilities for cobalt, and hence it could be an option in the quest to use waste to treat wastewater

CFD simulation of solids suspension in stirred tanks: Review

Many chemical reactions are carried out using stirred tanks, and the efficiency of such systems depends on the quality of mixing, which has been a subject of research for many years. For solid-liquid mixing, traditionally the research efforts were geared towards determining mixing features such as off-bottom solid suspension using experimental techniques. In a few studies that focused on the determination of solids concentration distribution, some methods that have been used have not been accurate enough to account for some small scale flow mal-distribution such as the existence of dead zones. The present review shows that computational fluid dynamic (CFD) techniques can be used to simulate mixing features such as solids off-bottom suspension, solids concentration and particle size distribution and cloud height. Information on the effects of particle size and particle size distribution on the solids concentration distribution is still scarce. Advancement of the CFD modeling is towards coupling the physical and kinetic data to capture mixing and reaction at meso- and micro-scales. Solids residence time distribution is important for the design; however, the current CFD models do not predict this parameter. Some advances have been made in recent years to apply CFD simulation to systems that involve fermentation and anaerobic processes. In these systems, complex interaction between the biochemical process and the hydrodynamics is still not well understood. This is one of the areas that still need more attention

Cobalt(II) removal from synthetic wastewater by adsorption on South African coal fly ash

Advanced wastewater-treatment techniques such as adsorption are essential in the removal of non- biodegradable toxic wastes from water. In this study, the use of South African coal fly ash, an industrial byproduct, has been investigated as a potential replacement for the current costly adsorbents used for removing heavy metals from wastewater. We utilised coal fly ash for the adsorption of cobalt(II) ions from synthetic petrochemical wastewater and characterised its performance. A two-level three-factor full-factorial design was successfully employed for experimental design and analysis of the results. The combined effects of pH, initial concentration and adsorbent dose on cobalt(II) removal were assessed using response surface methodology. Although the focus was on removal of cobalt(II), the adsorption was carried out in the presence of phenol and other heavy metal ions using the batch technique. The applicability of the Freundlich and Langmuir models to the equilibrium data was tested. Consequently, the equilibrium data was found to conform more favourably to the Freundlich isotherm than to the Langmuir isotherm; in this case, the coal fly ash had a maximum adsorption capacity of 0.401 mg/g for cobalt(II). We conclude that South African coal fly ash, as a natural, abundant and low-cost adsorbent, might be a suitable local alternative for elimination of cobalt(II) from aqueous solutions

Simulation models for food separation by adsorption process

Separation of simulated industrial food products, by method of adsorption, has been studied. A thermodynamic approach has been applied to study the liquid adsorption where benzene and cyclohexane have been used to simulate edible oils in a system that employs silica gel as the adsorbent. Different models suggested in the literature have been tested with experimental data in order to characterize properties of the system, such as the tendency to ideality, for both adsorbed and bulk liquid phases. The methods of determining thermodynamic properties such as free energy of adsorption, adsorption capacity and activity coefficients in the adsorbed phase have been studied. It was observed that a model based on the assumption that adsorbed phase, but not liquid phase, is ideal gave a good for that may suffice for engineering design of liquid adsorption systems that may be applied in food processing, such as decolourising crude sugar syrup, recovery of polypeptides and extraction of flavouring. The Journal of Food Technology in Africa Volume 6 Number 2 (April-June 2001), pp. 63-67 KEY WORDS: Separation, food, model, adsorption, simulation and surface excess

Solar Photocatalysis of Methyl Orange Using Multi-Ion Doped TiO2 Catalysts

Solar-light activated titanium dioxide photocatalysts were prepared by hydrolysis of titanium (IV) isopropoxide with thiourea, followed by calcinations at 450 °C. The experiments demonstrated that methyl orange in aqueous solutions were successfully degraded under solar light using doped TiO2. The photocatalytic oxidation of a mono azo methyl-orange dye has been investigated in multi ion doped TiO2 and solar light. Solutions were irradiated by solar-light until high removal was achieved. It was found that there was no degradation of methyl orange in the dark and in the absence of TiO2. Varieties of laboratory prepared TiO2 catalysts both un-doped and doped using titanium (IV) isopropoxide and thiourea as a dopant were tested in order to compare their photoreactivity. As a result, it was found that the efficiency of the process strongly depends on the working conditions. The highest degradation rate of methyl orange was obtained at optimum dosage using commercially produced TiO2. Our work focused on laboratory synthesized catalyst and the maximum methyl orange removal was achieved at 81% with catalyst loading of 0.04 g/L, initial pH of 3 and methyl orange concentration of 0.005 g/L using multi-ion doped catalyst. The kinetics of photocatalytic methyl orange dye stuff degradation was found to follow a pseudo-first-order rate law. The presence of the multi-ion dopant (thiourea) enhanced the photoefficiency of the titanium dioxide catalyst