Showing papers on "Complete mixing published in 2002"

Reactive Transport in Porous Media: A Comparison of Model Prediction with Laboratory Visualization

Abstract: Groundwater transport models that accurately describe spreading of nonreactive solutes in an aquifer can poorly predict concentrations of reactive solutes. The dispersive term in the advection-dispersion equation can overpredict pore-scale mixing, and thereby overpredict homogeneous chemical reaction. We quantified this experimentally by imaging instantaneous colorimetric reactions between solutions of aqueous CuSO4 and EDTA4- within a 30-cm long translucent chamber packed with cryolite sand that closely matched the optical index of refraction of water. A charge-coupled device camera was used to quantify concentrations of blue CuEDTA2- within the chamber as it was produced by mixing of the two reactants at different flow rates. We compared these experimental results with a new analytic solution for instantaneous bimolecular reaction coupled with advection and dispersion of the product and reactants. For all flow rates, the concentrations of CuEDTA2- recorded in the experiments were about 20% less than predicted by the analytic solution, thereby demonstrating that models assuming complete mixing at the pore scale can overpredict reaction during transport.

Three‐dimensional simulation of grain mixing in three different rotating drum designs for solid‐state fermentation

Abstract: A previously published two-dimensional discrete particle simulation model for radial mixing behavior of various slowly rotating drums for solid-state fermentation (SSF) has been extended to a three-dimensional model that also predicts axial mixing. Radial and axial mixing characteristics were predicted for three different drum designs: (1) without baffles; (2) with straight baffles; and (3) with curved baffles. The axial mixing behavior was studied experimentally with video- and image-analysis techniques. In the drum without baffles and with curved baffles the predicted mixing behavior matched the observed behavior adequately. The predicted axial mixing behavior in the drum with straight baffles was predicted less accurately, and it appeared to be strongly dependent on particle rotation, which was in contrast to the other drum designs. In the drum with curved baffles complete mixing in the radial and axial direction was achieved much faster than in the other designs; that is, it was already achieved after three to four rotations. This drum design may therefore be very well suited to SSF. It is concluded that discrete particle simulations provide valuable detailed knowledge about particle transport processes, and this may help to understand and optimize related heat and mass transfer processes in SSF.

Study on flow and mixing characteristics of molten steel in RH and RH-KTB refining processes

Abstract: The flow and mixing characteristics of molten steel during the vacuum circulation refining, including RH(Ruhrstahl-Heraeus) and RH-KTB(Ruhrstahl-Heraeus-Kawasaki top blowing) processes, were investigated on a 1/5 linear scale water model of a 90 t multifunction RH degasser. The circulation rate was directly and more accurately determined, using a new method by which the more reliable results can be obtained. The fluid flow pattern and flow field in the ladle were demonstrated, observed and analyzed. The mixing time of liquid in the ladle was measured using electrical conductivity method. The residence time distribution in the RH model was obtained by tracer response technique. The influence of the main technological and geometric factors, including the gas top blowing (KTB) operation, was examined. The results indicated that the circulation rate of molten steel in the RH degasser can be fairly precisely calculated by the formula: Q lp=0.0333Q g 0.26 D u 0.69 D d 0.80 (t/min), where Q g - the lifting gas flow rate (NL/min); D u and D d - the inner diameters of the up and down-snorkels (cm), respectively. The maximum value of circulation rate of molten steel in the case of the 30 cm diameters either of the up-and down-snorkels for the RH degasser (the “saturated” rate) is approximately 31 t/min. The corresponding gas flow rate is 900 NL/min. Blowing gas into the vacuum chamber through the top lance like KTB operation does not markedly influence the circulatory flow and mixing characteristics of the RH process under the conditions of the present work. There exist a major loop and a large number of small vortices and eddies in the ladle during the RH refining process. A liquid-liquid two-phase flow is formed between the descending stream from the down-snorkel and the liquid around the stream. All of these flow situation and pattern will strongly influence and determine the mixing and mass transfer in the ladle during the refining. The correlation between the mixing time and the stirring energy density is τ m∞e −0.50 for the RH degasser. The mixing time rapidly shortens with an increase in the lifting gas flowrate. At a same gas flow rate, the mixing times with the up-and down-snorkel diameters either of 6 and 7 cm are essentially same. The 30 cm diameters either of the up-and down-snorkels for the RH degasser would be reasonable. The concentration-time curve showed that three circulation cycles are at least needed for complete mixing of the liquid steel in the RH degasser.

Mixing Performances of a New Mixing Equipment with Vibratory Fins

Abstract: Mixing performances of a new mixing equipment which has a vibrating motor with some fin oscillators on a pair of shafts were investigated. This mixing equipment, which is mainly used for industrial plating processes, was usually operated at the vibrating frequency from 30 Hz to 60 Hz with the amplitude from 1 mm to 10 mm.The mixing time and the circulation time were measured and the flow pattern and the mixing process were visualized. The mixing time and the circulation time took the optimum values (the minimum values) for the equipment design at the frequency range from 40 Hz to 50 Hz irrespective to the configuration of the fins. The complete mixing was achieved in three to four circulations of the liquid in the vessel. From the visualization of the flow pattern and the mixing process it was shown that a primary circulation flow passing through the fins was very important for the mixing operation.

Study on Flow and Mixing Characteristics of Molten Steel in RH and RH-KTB Refining Processes

Abstract: The flow and mixing characteristics of molten steel during the vacuum circulation refining, including RH(Ruhrstahl Heraeus) and RH KTB(Ruhrstahl Heraeus Kawasaki top blowing) processes, were investigated on a 1/5 linear scale water model of a 90 t multifunction RH degasser. The circulation rate was directly and more accurately determined, using a new method by which the more reliable results can be obtained. The fluid flow pattern and flow field in the ladle were demonstrated, observed and analyzed. The mixing time of liquid in the ladle was measured using electrical conductivity method. The residence time distribution in the RH model was obtained by tracer response technique. The influence of the main technological and geometric factors, including the gas top blowing (KTB) operation, was examined. The results indicated that the circulation rate of molten steel in the RH degasser can be fairly precisely calculated by the formula: Q lp =0.0333 Q 0.26 g D 0.69 u D 0.80 d(t/min), where Q g-the lifting gas flow rate (NL/min); D u and D d-the inner diameters of the up and down snorkels (cm), respectively. The maximum value of circulation rate of molten steel in the case of the 30 cm diameters either of the up and down snorkels for the RH degasser (the “saturated” rate) is approximately 31 t/min. The corresponding gas flow rate is 900 NL/min. Blowing gas into the vacuum chamber through the top lance like KTB operation does not markedly influence the circulatory flow and mixing characteristics of the RH process under the conditions of the present work. There exist a major loop and a large number of small vortices and eddies in the ladle during the RH refining process. A liquid liquid two phase flow is formed between the descending stream from the down snorkel and the liquid around the stream. All of these flow situation and pattern will strongly influence and determine the mixing and mass transfer in the ladle during the refining. The correlation between the mixing time and the stirring energy density is τ m∝e -0.50 for the RH degasser. The mixing time rapidly shortens with an increase in the lifting gas flowrate. At a same gas flow rate, the mixing times with the up and down snorkel diameters either of 6 and 7 cm are essentially same. The 30 cm diameters either of the up and down snorkels for the RH degasser would be reasonable. The concentration time curve showed that three circulation cycles are at least needed for complete mixing of the liquid steel in the RH degasser.

The Experimental and Residence Time Distribution Estimation of the Decomposition of Hydrogen Peroxide within a Hydrodynamic Vortex Separator

Abstract: The performance of any device in which a kinetic process occurs e.g. disinfection, coagulation and chemical reactions is dependent on the mixing regime for a given set of operating conditions. Subsequently by characterising the mixing regime within a device and conducting batch reactor experiments to obtain specific kinetic process parameters it is possible to determine the system’s efficiency. This has been achieved for a hydrodynamic vortex separator (HDVS) operating with no baseflow component by undertaking residence time distribution (RTD) investigations. The kinetic process investigated at both batch scale and within the continuously operated HDVS to provide comparative data was the first–order decomposition (conversion) of hydrogen peroxide (H2O2) by an enzyme catalase. The HDVS is a high rate vortex process predominantly used for the separation of solids from an incoming feed. The HDVS is typically used as a combined sewer overflow (CSO) in the drainage system and at wastewater treatment works (WWTW). However this research is part of a long-term study to investigate the potential of the HDVS for kinetic process applications. The kinetic process efficiency of the HDVS using RTD and batch reactor data was determined using a range of flow models. This includes the upper and lower efficiency models namely the plug-flow and complete mixing models respectively and the axial dispersion, tanks-in-series and complete segregation model used for intermediate mixing regimes. The RTD results showed that the HDVS has an imperfect plug-flow mixing regime and shortcircuiting is present depending on the inlet flow rate. The RTD results justified conducting further kinetic process investigations as if the HDVS mixing regime conformed to a theoretical mixing regime of either plug-flow or complete mixing then these models could be used with confidence to obtain the efficiency of the HDVS for kinetic process applications. Investigation of the H2O2 conversion kinetics showed that the RTD and batch reactor data could be used to predict the efficiency of the continuously operated HDVS for kinetic process applications. The experimental results are between the theoretical conversion boundaries i.e. plug-flow and complete mixing. These results provide confidence in the experimental data i.e. RTD curve, reaction rate constant (k) and the H2O2 conversion. The flow models that describe a well-mixed system generally provide a better estimation of the experimental H2O2 conversion and this supports previous descriptions of the HDVS’s mixing regime using RTD analysis. To minimise any limitations in data analysis techniques the use of the complete

The floating separation method of contaminant from waste water by adding chemicals

Abstract: PURPOSE: A method for separating contaminants such as suspended solid and dissolved solid from wastewater by an air flotation process that is combined with chemical injection is provided, which is more economical in comparison with conventional flotation processes such as dissolved air flotation and vacuum flotation because it does not require complex facilities to dissolve compressed air into a wastewater to be treated. CONSTITUTION: The method includes the steps of (i) injection of 30-40% Fe2Cl3 solution(or Al2(SO4)3 solution) into wastewater in an amount of 2 to 20ml/L· wastewater, followed by complete mixing within a period of up to 20 seconds; (ii) injection of a solution dissolved with a polymer at a mixing ratio of 2 to 4g/L·distilled water into wastewater in an amount of 5 to 30ml/L·wastewater, followed by complete mixing within a period of up to 20 seconds.