Pulse testing of an agitated vessel

The dynamic study of an agitated insulated vessel was performed using the pulse test method. Adequate pulse characteristics which will disturb a first- and a second-order system in a single pulse test were found by analysing the effects of pulse height and duration, integration step size in computation and the type of approximation used on the accuracy of the results. The theoretical multivariable matrix for the system was found and the best manipulated-controlled variable couples were determined by Bristol's technique.     

Dissolution mass transfer in a turbine agitated baffled vessel

Mass transfer coefficients were measured for the dissolution of crystalline electrolytes suspended in a baffled vessel by means of disc turbines. A 3-fold range of density differences, a 5-fold range of diffusivities and a 50-fold range of particle size were covered and the effect of impeller speed, size and clearance from the base was studied. The increase in coefficent with speed is both a function of the clearance and solid density and size and is difficult to quantify. However, in the most economic configuration of small clearance and large impeller, this is not of great importance since the coefficient increases but slowly with speed and the minimum suspended coefficient can be calculated. This is done by means of a slip velocity theory which has been extended to include larger particles, which allows for the effect of concentration dependent physical properties and which is based on the impeller speed at which particle suspension occurs.     

PTA打浆釜搅拌传动装置扩能的计算

对PTA打浆釜的搅拌能力进行分析,校核各传动部件的使用余量,确认其传动部件工作能力可以在目前工作状态下,满足投料量提升1倍的使用要求,从而节约提产改造的费用。     

Investigation of feedpipe backmixing in an agitated vessel

Feedpipe backmixing in an agitated vessel was investigated using a newly developed conductivity technique. By this technique, the onset of feedpipe backmixing could be detected and the penetration depth of the vessel fluid into a feedpipe was determined. For a given feedpipe flowrate. critical agitator speeds to eliminate feedpipe backmixing were determined using Rushton six-bladcd disk turbine impeller (6BD) and high efficiency, axial-flow type 3-bladed impeller (HE-3) of 8.89 and 12.70 cm diameters in 11.2 liter reactor. The ratio of the feedpipe velocity to the critical agitator speed (v _f /v _i ) was determined as a function of feedpipe Reynolds number (N _R,T ). The conductivity technique was successful either in the laminar regime, the transitional regime, or in the turbulent regime in the feedpipe.     

Local drop size variation in an agitated vessel

The local droplet diameters were measured at two positions inside the circulation region of a stirred tank. The Sauter mean diameter near the top of the vessel was always 1.4 times greater than the one just outside the impeller region for rotational speeds of 250, 300, 350 and 400 rpm and for hold-up fractions of 0.01, 0.025 and 0.05. The drop size distributions at a hold-up fraction 0.01 had one mode for all impeller speeds studied and at both positions. At higher hold-up fractions the distributions usually were bimodal.     

Design of a laboratory experiment on heat transfer in an agitated vessel

A novel teaching laboratory experiment is described, which demonstrates heat transfer under agitation. The experiment involves a simple and inexpensive apparatus with necessary basic components. The laboratory deals with the experimental determination of the heat transfer rates and the overall heat transfer coefficient between steam and water in an agitated vessel. The apparatus can be operated under both static and flow conditions, which affords the student an improved understanding of heat transfer during transient and steady-state modes. Further, student is trained to study the effect of the impeller speed and water flow rate on the rate of heat transfer. By performing the experiment, the student is able to determine the overall heat transfer coefficients experimentally, and compare the results with those obtained from theoretical calculations using correlations available in the literature. In addition, while working in groups, students develop team work and technical writing skills in preparing a comprehensive laboratory report.     

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, , in the unbaffled tank that can be utilised without severe air incorporation is compared to with baffles. However, at this lower , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε_T gave values of at every feed position where measurement was undertaken. Since the spatially averaged value of φ=1, this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε_T at the feed point may not be valid for such swirling flows.     

Numerical simulation of turbulent batch mixing in a vessel agitated by a Rushton turbine

In this study, computational fluid dynamics (CFD) modelling of turbulent batch mixing of an inert tracer in a baffled vessel agitated by a six-bladed Rushton turbine has been carried out using the proprietary code FLUENT. The study is intended to evaluate the CFD predictions of key properties related to the mixing against measurements and to provide a detailed insight into the process. Three-dimensional, time-dependent flow and mixing calculations have been performed using the fully predictive sliding-mesh technique for the impeller/tank geometry employed by Distelhoff et al. [M.F.W. Distelhoff, A.J. Marquis, J.M. Nouri, J.H. Whitelaw, Scalar mixing measurements in batch operated stirred tanks, Can. J. Chem. Eng. 75 (1997) 641–652] for mixing studies using a laser induced fluorescence technique. Complementary validation of hydrodynamic predictions in a geometrically similar tank was carried out against the experimental data obtained by Hockey [R.M. Hockey, Turbulent Newtonian and non-Newtonian flows in a stirred reactor, Ph.D. Thesis, Imperial College, London, 1990]. The predicted mean velocity components in the bulk regions of the tank above and below the impeller compare well with the experimental data. However, the turbulent kinetic energy is significantly underestimated in these areas. The predicted tracer concentration variations with time at different locations in the tank, in common with measurements, show initial fluctuations, which eventually approach the fully mixed concentration. However, the time required for the appearance of first peak in the concentration–time plot, peak value of the tracer concentration and the time required for the local tracer concentration to attain the final value depend on the position in the tank. The CFD predicted mixing times at different locations in the tank as well as the overall mixing time show reasonably good agreement with the measured data and with those calculated from published experimental correlations.     

Transport phenomena under unsteady conditions in an agitated vessel

An experimental study of transport phenomena at the wall of an agitated vessel with a propeller impeller under unsteady conditions is reported. The transient in heat transfer at the wall is caused by a step change in the speed of agitation from an initial static condition. The results are expressed in terms of a rise in modified “j-factor” above the initial free convection value and the number of revolutions taken by the impeller from the start of agitation. Also, an attempt has been made to find an analogy beween heat and momentum transfer. Results are given for the power in terms of Euler number plotted against the impeller Reynolds number. An empirical expression is also given for the transient heat transfer considering the system as a pure first order one with time delay. A “time-factor” constant which gives the number of impeller revolutions for 63.2% of the change from the initial to a final steady state is expressed as a function of impeller Reynolds number (10² to 10⁶) and fluid Prandtl number (6 to 10⁶).     

Concentrated slurry formation via drawdown and incorporation of wettable solids in a mechanically agitated vessel

This article describes the effect of vessel configurations on the drawdown and incorporation of floating solids to prepare concentrated alumina slurries in stirred tanks. The impeller speed and power draw required to incorporate all dry powder within four seconds, N_JI and P_JI, are used to evaluate incorporation performance. The effect of impeller type is assessed, with pitched blade impellers proving to be the most effective across the full range of solid contents considered. At higher solids content the energy demand is shown to increase dramatically, with a 100‐fold increase in energy required to add 1% w/w more solid at 50% by weight compared to 1% by weight. Analysis of impeller power numbers show this coincides with a transition from constant power number to a region where power number increases linearly with decreasing Reynolds number. Contrary to studies at low solids content, the presence of baffles is shown to inhibit drawdown. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1885–1895, 2018     

Mixing studies of non-Newtonian fluids in an anchor agitated vessel

The success of any mixing operation involving liquid–liquid, gas–liquid and gas–liquid–solid systems depends mainly on the geometry of the vessel and impeller, operating conditions and properties of the system. Transformation of laboratory results to commercial scale unit is very difficult due to the complexity of flow phenomena and the scale up is being done by adopting a conservative approach which is based on the geometric, kinematic and dynamic similarities. This approach does not take into account the non-ideal flow behavior of the fluid and the design of commercial unit will be more rational if this information is included in the design of the unit.     

An experimental and computational study of the vortex shape in a partially baffled agitated vessel

The vortex shape in a non-standard partially baffled agitated vessel in the form of a glass-lined, under-baffled stirred vessel has been investigated using both experimental and numerical approaches for an air/water system for different rotation speeds of the agitator. A simple and flexible experimental strategy was developed for determination of the time-averaged location of the unstable free surface using a process involving superimposition of images. CFD simulations were made to predict the vortex shape by using an Eulerian-Eulerian multiphase model coupled with a homogenous turbulence model. The simplifying assumptions of a constant bubble size, a constant drag coefficient and use of the k-ε turbulence model were made. An assessment of the capability of the numerical method to predict the vortex shape was carried out through comparison between experimental data and numerical results. Considering for comparison purposes a water isosurface volume fraction equal to 0.9, to account for the existence of air/water mixture present at the interface in the experiments, instead of the classical value of 0.5, gave very good agreement with the experimental data.     

运用粒子图像测速仪研究双层Rushton桨搅拌槽内湍流特性

Particle Image Velocimetry （PIV） has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height （ H = 1.4T ） of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter （ D = 0.4T ）. The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liq- uid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns.
Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different lo- cations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.     

Desorption of carbon dioxide from supersaturated water in an agitated vessel

The rates of desorption of carbon dioxide from supersaturated water solutions into pure carbon dioxide or nitrogen gas stream were measured at 15,25, and 35°C in a baffled agitated vessel with a flat gas-liquid interface operated in a continuous manner. The volumetric liquid-phase mass transfer coefficients for the bubbles generated in the agitated liquid and the enhancement factors of the volumetric liquid-phase mass transfer coefficient for the free liquid surface due to the bubbling were calculated from the measured desorption rates and correlated as functions of the relative supersaturation of the solution and the liquid-phase Reynolds number.     

Turbulent flow field in a stirred vessel agitated by an Impeller with flexible blades

In order to reveal the effect of the blades normal vibration on flow turbulence in the stirred vessel, we designed three kinds of blades: the flexible, flat‐rigid and curved‐rigid blades. The flow fields produced by the impellers with these three kinds of blades were measured by two‐dimensional particle image velocimetry. The results showed that the calculated turbulent kinetic energy (TKE) based on the pseudo‐isotropic assumption is slightly higher than that by the three fluctuating velocities for the flexible and curved‐rigid impellers, and the difference between above two calculations is smaller for the former impeller. For the flexible blades, the trailing vortices slightly move outwards in radial direction than those for the curved‐rigid blades, enhancing TKE transport from the blade to the bulk region of the vessel. For the flexible impeller, the phase‐averaged TKE differs slightly from that for the flat‐rigid impeller, but is higher than that for the curved‐rigid impeller. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4148–4161, 2018     

Some characteristics of a sparged agitated vessel under foaming — Mechanical control of foaming —

Foam-breaking with a rotating disk mechanical foam-breaker (MFRD) was studied for a foaming system containing a diluted detergent solution in a laboratory sparged agitated vessel. The change of the foam-breaking capacity of the MFRD by varying the air sparge rate, the working volume and the impeller speed or the disk diameter was measured. Based on these measurements, empirical equations are presented for predicting the critical foam-breaking regions of the MFRD fitted with the agitated vessel. The operational range where foam-breaking with the MFRD can be carried out effectively and economically is also discussed.     

A coalescence redispersion model for drop-size distributions in an agitated vessel

A population balance model has been developed accounting for coalescence and redispersion of drops in an agitated liquid—liquid dispersion. It is assumed that drop coalescence is followed by immediate redispersion into two drops whose sizes are distributed according to a uniform distribution in the ratio of the daughter drop-volume to the parent drop-volume. If it is assumed that the coalescence frequency is independent of drop-size, the resulting non-linear integral equation admits an exponential solution for the equilibrium drop-volume distribution in a batch vessel. On comparing experimental data obtained from several sources with the prediction of this model, it is found that for flat-blade turbine agitated systems, the agreement is very good; in fact, the sign test ranks the model as better fitting than empirical expressions for drop-sizes proposed by other workers. For circulation stirring systems, e.g. those having Waldhof agitator, the model predictions are substantially improved by assuming that the coalescence frequency is proportional to the diameters of the interacting drops.