Academic literature on the topic 'Agitated'

In agitated dryers the motion of the impeller leads to shear deformation within the bed, which causes attrition. The attrition caused by the prevailing stresses and strains in a particle bed exposed to simple, well defined shear deformation in an annular shear cell has been extensively studied in the literature, however little effort has been made to better understand more complex shearing flows. A method of predicting particle breakage in agitated dryers is introduced here. The method uses models based on the Distinct Element Method (DEM), validated by torque and Positron Emission Particle Tracking (PEPT) measurements, to estimate the stress and strain rate distributions throughout the dryer bed. The attrition of Paracetamol and Aspirin under shear deformation in an annular shear cell is assessed and relationships of attrition to stress and strain developed. These relationships are combined with the stress and strain rate distributions obtained from the DEM to predict the attrition in a small-scale dryer. The attrition of Paracetamol and Aspirin in the small-scale dryer is measured experimentally and compared to the prediction. The relationship of Paracetamol attrition to stress and strain correlates well with the shear cell results; this is not the case for Aspirin. The prediction of Paracetamol attrition in the dryer therefore compares well to that measured experimentally, whereas the prediction of Aspirin attrition is less successful as the attrition resulting from the prevailing stresses and strains is not adequately described. The methodology developed here is generic and can be applied to a wide range of processes, such as dense pneumatic conveying, milling and grinding etc. III

L’argomento trattato è lo studio di due liquidi immiscibili all’interno di un recipiente agitato. Una nuova tecnica di misura delle dimensioni delle particelle viene presentata, la tecnica sperimentata è non invasiva in quanto tutti gli strumenti di misura sono stati posizionati esternamente al vessel. Il recipiente conteneva una dispersione di olio siliconico in acqua, i test sono stati condotti in assenza di coalescenza. Il sistema è agitato in un primo test da una girante Rushton e in un secondo da una girante con denti; esso consiste in un recipiente cilindrico dal diametro T=300mm in vetro, questo vessel è stato inserito in un secondo recipiente, anch’esso in vetro ma dalla geometria cubica, riempito di acqua per ridurre problemi legati alla distorsione ottica dovuta alla cilindricità delle pareti del vessel agitato. Il recipiente è stato posto tra una fotocamera ad alta velocità e una lampada avente lo scopo di illuminare la dispersione. Sono state quindi relazionate le reali dimensioni in mm delle gocce, con i pixel della fotocamera nella fase di calibrazione; la taratura è stata effettuata tramite l’utilizzo di speciali sfere solide monodimensionali. L’analisi della dispersione in esame consisteva nella cattura di più set di immagini ad intervalli di tempo prestabiliti, solo dopo che la dispersione fosse arrivata all’equilibrio. La foto sono state quindi salvate in stack ed analizzate da un apposito codice che è stato scritto per il programma di analisi di immagini utilizzato: ImageJ. La possibilità di implementare macro in ImageJ rende molto flessibile questo programma, caratteristica fondamentale in questo lavoro in quanto lo studio di questi liquidi ha richiesto un notevole numero di test per ottenere una corretta interpretazione delle dimensioni delle gocce. Segue infine l’analisi dei dati ottenuti, alcune correlazioni riportate in letteratura sono state verificate statisticamente a partire dai risultati ottenuti.

Experimental data are reported for solids suspension and distribution in four geometrically similar vessels with diameters equal to 0.31, 0.61, 1.83 and 2.67 m. Agitation was provided by a series of pitched blade turbines with impeller to vessel diameter ratios from 0.3 to 0.6 and pitched angles between 30° and 90°. The effect of impeller clearance on solids suspension was examined for a clearance range of T/4 to T/8. Dual impeller systems were also studied, covering two combinations (dual pitched and flat/pitched) and impeller spacing of half to two diameters apart. The majority of the experiments were carried out with 150-210 μm round-grained sand (density: 2630 kg ‭m-3 and settling velocity: 0.015 m s-1) and tap water. Solids concentration was varied between 0.1 to 40% by weight. Four parameters were measured; impeller speed, using an optical tachometer, power input, calculated from the shaft torque given by strain gauges, just suspension speed, ascertained both visually and by use of an ultrasonic Doppler flowmetering (UDF) technique and the local solids concentration, measured by a in-house solids concentration probe. In addition extensive flow visualisations were made with the 0.61 m vessel in order to establish both liquid and particles flow patterns during the experiments. Results from this study were compared with previous publications in order to examine the effects of some of the important geometrical variables on solids suspension and distribution. This work revealed that for the range of parameters covered, the smallest (D/T=0.3) and the largest (D/T=0.6) impellers are the most and least efficient ones for solids suspension. Distribution tests with the three geometrically similar impellers show that the results are neither correlated in terms of tip speed nor power input but are best described by the thrust force generated by the impellers. In general, dual impeller systems improve solids distribution but require more power to just suspend solids compared with a single impeller. The scaling effect proposed by Zwietering (1958) for solids suspension has been confirmed by this study for vessel up to 2.67 m in diameter. The constant tip speed rule for solids distribution, which is based on one-dimensional dispersion models was found to underestimate the power requirement in large scale applications. This study indicates that equal power per unit volume is required to achieve the same degree of homogeneity.

In this dissertation the power draw, the mass transfer and the liquid mixing behaviour of boiling and hot sparged stirred tank reactors (STRs) were investigated. The power draw characteristics of six different impellers were studied under varying operating conditions and expressed in terms of the relative power draw, RPD. Generally the impellers showed considerably higher power draw in hot gassed than in comparable cold conditions. The main phenomenon in hot systems, the evaporation of liquid into a gas bubble suddenly exposed to it, was investigated experimentally. The growth of nitrogen and air bubbles in hot water was found to be very fast and be completed typically within milliseconds after their exposure to the liquid. This was confirmed by studies applying acoustic bubble sizing techniques which were carried out together with Richard Manasseh from CSIRO, Melbourne, Australia. Experimental studies of the gas-liquid mass transfer in hot sparged STRs were carried out using a tank of 450 mm in diameter agitated by a 180 mm Rushton turbine. Experiments involving the temperature kinetics in an air-water system and absorption and desorption of ammonia were used to determine gas and also some liquid side mass transfer coefficients. The gas side coefficients kga were found to be typically in the order of 0.01 s-1. An enhancement of the liquid side mass transfer was also observed. Both Fick and Stefan-Maxwell models were employed to express gas side mass transfer coefficients. The models predict there will be a decrease of the coefficients at high concentrations. In the hot sparged tank the temperature is essentially uniform throughout the STR. Liquid mixing times have been measured for various boil-off and sparging rates. No significant deviations from the single phase or the cold gassed mixing times in this equipment were recorded.

Liquid-liquid extraction has long been known as a unit operation that plays an important role in industry. This process is well known for its complexity and sensitivity to operation conditions. This thesis presents an attempt to explore the dynamics and control of this process using a systematic approach and state of the art control system design techniques. The process was studied first experimentally under carefully selected. operation conditions, which resembles the ranges employed practically under stable and efficient conditions. Data were collected at steady state conditions using adequate sampling techniques' for the dispersed and continuous phases as well as during the transients of the column with the aid of a computer-based online data logging system and online concentration analysis. A stagewise single stage backflow model was improved to mimic the dynamic op.eration of the column. The developed model accounts for the variation in hydrodynamics, mass transfer, and physical properties throughout the length of the column. End effects were treated by addition of stages at the column entrances. Two parameters were incorporated in the model namely; mass transfer weight factor to correct for the assumption of no mass transfer in the. settling zones at each stage and the backmixing coefficients to handle the axial dispersion phenomena encountered in the course of column operation. The parameters were estimate.d by .. minimizing the differences between the experimental and the model predicted concentration profiles at steady state conditions using non-linear optimisation technique. The estimated values were then correlated as functions of operating parameters and were incorporated in·the .. model equations. The model equations comprise a stiff differential~algebraic system. This system was solved using the GEAR ODE solver. The calculated concentration profiles were compared to those experimentally measured. A very good agreement of the two profi1es was' achieved within a percent relative error of ±2.S%.The developed rigorous dynamic model of the extraction column was. \ls~d to derive linear time-invariant reduced-order models that relate the input variables (agitatorsjJeed, . solvent feed flowrate and concentration, feed concentration andflowrate) to the output variables (raffinate concentration and extract concentration) using the asymptotic method of system identification. The reduced-order models were shown to be accurate in capturing the dynamic behaviour of the process with a maximum modelling prediction error of I %. The simplicity and accuracy of the derived reduced-order models allow for control system design and analysis of such complicated processes. The extraction column is a typical multi variable process with agitator speed and solvent feed flowrate considered as manipulative variables; raffinate concentration and extract concentration as controlled variables and the feeds concentration and feed flowrate as disturbance variables. The control system design of the extraction process was tackled as multi-loop decentralised S1S0 (Single Input Single Output) as well as centralised MIMO (Multi-Input Multi-Output) system using both conventional and model-based control techniques such as IMC (Internal Model Control) and MPC (Model Predictive Control). Control performance of each control scheme was. studied in terms oJ stahilit~, speed of response, sensitivity to modelling errors (robustness), setpoint tracking capabilities ahd load rejection. For decentralised control, multiple loops were assigned to pair .each manipulated variable with each controlled variable according to the interaction analysis and other pairing criteria such as relative gain array (RGA), singular value analysis (SVD) and Jacobi eigenvalue criterion. Loops namely Rotor speed-Raffinate concentration and Solvent flowrateExtract concentration showed weak interaction. Multivariable MPC has shown more effective performance compared to other conventional techniques since it accounts for loops interaction, time delays, and input-output variables constraints