Stirred-tank System Research Articles

Evaluation of Growth Kinetics of Antisolvent Crystallization of Paracetamol in an Oscillatory Baffled Crystallizer Utilizing Video Imaging

This work aims to show the potential of applying video imaging techniques to antisolvent crystallization of paracetamol in an oscillatory baffled crystallizer for the determination of crystal size distribution and mean crystal size with a high level of accuracy in comparison with the values measured with a Mastersizer. From these data, the effects of supersaturation, addition rate, and mixing on growth rate are examined and crystal growth kinetics extracted for various operational parameters. The kinetic data so generated are in good agreement with these from previous studies in stirred tank systems. It was found that the degree of supersaturation has the most significant effect on the overall growth rates, followed closely by the degree of mixing and then the rate of antisolvent addition. Both the kinetics and process data could be used as the tool for the design and operation of antisolvent crystallization.

Biodiversity and interactions of acidophiles: Key to understanding and optimizing microbial processing of ores and concentrates

Mining companies have become increasingly aware of the potential of microbiological approaches for recovering base and precious metals from low-grade ores, and for remediating acidic, metal-rich wastewaters that drain from both operating and abandoned mine sites. Biological systems offer a number of environmental and (sometimes) economical advantages over conventional approaches, such as pyrometallurgy, though their application is not appropriate in every situation. Mineral processing using micro-organisms has been exploited for extracting gold, copper, uranium and cobalt, and current developments are targeting other base metals. Recently, there has been a great increase in our knowledge and understanding of both the diversity of the microbiology of biomining environments, and of how the microorganisms interact with each other. The results from laboratory experiments which have simulated both stirred tank and heap bioreactor systems have shown that microbial consortia are more robust than pure cultures of mineral-oxidizing acidophiles, and also tend to be more effective at bioleaching and bio-oxidizing ores and concentrates. The paper presented a concise review of the nature and interactions of microbial consortia that are involved in the oxidation of sulfide minerals, and how these might be adapted to meet future challenges in biomining operations.

Effect of shaft eccentricity on the laminar mixing performance of a radial impeller

AbstractThe effects of impeller eccentricity and Reynolds number on the mixing performance were studied for Rushton turbine stirred tank systems operated in the laminar regime (Re < 10). A digital image analysis of an acid–base decolourization reaction was used to characterize the mixing efficiency. Results show that both parameters have remarkable effects on the destruction of the toroidal segregated regions surrounding the Rushton turbine in laminar flow. Criteria are given to prevent the formation of these segregated regions in a tank. It is also shown that shaft eccentricity does not impact on the impeller power consumption.

Hollow self-inducing impellers for gas–liquid–solid dispersion: Experimental and computational study

Abstract In this work, experimental and computational fluid dynamic studies have been carried out for hollow self-inducing three-phase stirred tank systems. The effect of different impeller designs (hollow self-inducing pitched blade down flow turbines of different blade angles (PBTD30, PBTD45, PBTD60), and self-inducing modified double disc (MDD) impeller) and operating conditions such as solid loading (0–7 wt. %) and solid particle size (125–700 μm) have been investigated on critical impeller speed for solid suspension ( N CS ), gas induction rate ( Q G ) and overall gas hold-up ( ∈ G ). Computational fluid dynamics model based on the Eulerian multi-fluid approach has been employed along with the standard k - ɛ turbulence model. A multiple reference frame (MRF) approach was used to model the impeller rotation. In this study, the multiphase flow has been simulated using a commercial CFD code, Fluent v6.2.16. A good agreement was found between the CFD predictions and the experimental values of N CS .

Design of Robust Quadratic-Optimal Controllers for Linear Multivariable Output Feedback PID Uncertain Control Systems

By complementarily fusing the robust regional eigenvalue-assignability condition, the orthogonal-functions approach (OFA) and the hybrid Taguchi-genetic algorithm (HTGA), an integrative method is proposed in this paper to design the robust optimal eigenvalue-assignable output feedback PID (proportional-integral-derivative) controller such that (i) the eigenvalues of a linear multivariable uncertain closed-loop system can be retained inside the same specified region as the nominal closed-loop system does, and (ii) a quadratic finite-hoziron integral performance index for the linear nominal multivariable control system can be minimized. A design example of the robust optimal eigenvalue-assignable output feedback PID controller for an uncertain stirred tank system is given to demonstrate the applicability of the proposed integrative approach.

Multirate Constrained Predictive Control: Algorithm and Experimental Results

Control of multirate systems is a challenging problem due to several reasons such as increased complexity in the design with tighter performance specifications. In this work, an algorithm for multirate constrained predictive control (MCPC) is presented. The multirate predictive control system includes a multirate state estimator, which provides inter-sample estimates of state variables of the process from infrequent and slow measurements. Constraints are addressed rigorously in this framework. The proposed design method is verified via simulation as well as experimentation. The results of the multirate predictive control for temperature control of a stirred tank system are shown and compared with those of a proportional integral (PI) control system.

Design of a novel knowledge-based fault detection and isolation scheme.

In this paper, a real-time fault detection and isolation (FDI) scheme for dynamical systems is developed, by integrating the signal processing technique with neural network design. Wavelet analysis is applied to capture the fault-induced transients of the measured signals in real-time, and the decomposed signals are pre-processed to extract details about a fault. A Regional Self-Organizing feature Map (R-SOM) neural network is synthesized to classify the fault types. The R-SOM neural network adopts two regions adjustment in the learning algorithm, thus it has high precision in clustering and matching, especially when the noise, disturbance and other uncertainties exist in the systems. As a result, the proposed FDI scheme is robust and accurate. The design is implemented on a stirred tank system and satisfactory online testing results are obtained.

Laminar Mixing in Eccentric Stirred Tank Systems

AbstractLaminar flow structure and mixing patterns in stirred tanks with eccentrically located impellers is examined using tracer visualization techniques and laser induced fluorescence (LIF). The displacement of the shaft from the centerline of the tank has a remarkable effect on the manifold structure of the flow: segregated regions of regular motion observed in concentric systems are destroyed, impeller mid‐plane separatrices are eliminated, and the axial circulation is greatly improved even in systems agitated even by radial impellers.

Recovery of pectolytic enzymes produced by solid state culture of Aspergillus niger

Pectinases are enzymes with a wide range of applications in the food and drink industries. In the present work, the extraction of pectinases produced by Aspergillus niger in a solid state fermentation system was investigated. The purpose was to reduce enzyme losses in the fermented solids and at the same time obtain a crude extract as concentrated as possible. Initially the performances of stirred tank and fixed bed extractors were compared. Polygalacturonase activity and viscosity reducing capacity obtained in the stirred tank system were 105% and 15% superior, respectively. Repeated extractions and multiple stage countercurrent extraction were studied, employing stirred tanks. It was possible to observe that three stages were enough for total recovery of the enzymes contained in the solids. The final enzyme extract obtained by counter-current extraction with three stages showed a polygalacturonase activity 81% higher than the one obtained by one-stage extraction.

On the performance of a hollow-fiber bioreactor for acidolysis catalyzed by immobilized lipase

The present communication describes the chemical modification of anhydrous butterfat by interesterification with oleic acid catalyzed by a lipase of Mucor javanicus. Two reactor configurations were tested, a batch-stirred tank reactor containing suspended lipase and a batch-stirred tank reactor in combination with a hollow-fiber membrane module containing adsorbed lipase. The goal of this research was to assess the advantage of using a (hydrophobic) porous support to immobilize the lipase in attempts to engineer butterfat with increased levels of unsaturated fatty acid residues (oleic acid) at the expense of medium-to-long chain saturated fatty acids (myristic and palmitic acids). Reactions were carried out at 40 degrees C in the absence of solvent under controlled water activity, and were monitored by chromatographic assays for free fatty acids. The results obtained indicate that the rate of interesterification using the proposed reactor configuration is enhanced by a factor above 100 relative to that using suspended lipase, for the same protein mass basis. Although hydrolysis of butterfat occurred to some degree, the enzymatic process that uses the hollow-fiber reactor was technically superior to the stirred tank system. Copyright 1998 John Wiley & Sons, Inc.

Production of fumaric acid by immobilized rhizopus using rotary biofilm contactor

Rotary biofilm contactor (RBC) is a reactor consisting of plastic discs that act as supports for micro-organisms. The discs are mounted on a horizontal shaft and placed in a medium-containing vessel. During nitrogen-rich growth phase, mycelia of Rhizopus oryzae ATCC 20344 grew on and around the discs and formed the "biofilm" of self-immobilized cells on the surface of the plastic discs. During the fermentation phase, the discs are slowly rotated, and the biofilms are exposed to the medium and the air space, alternately. With RBC, in the presence of CaCO3,Rhizopus biofilm consumes glucose and produces fumaric acid with a volumetric productivity of 3.78 g/L/h within 24 h. The volumetric productivity is about threefolds higher with RBC than with a stirred-tank fermenter with CaCO3. Furthermore, the duration of fermentation is one-third of the stirred-tank system. The immobilized biofilm is active for over a 2-wk period with repetitive use without loss of activity.

Anaerobic bioprocessing of organic wastes

Anaerobic digestion of dissolved, suspended and solid organics has rapidly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial conversions and adhesion are addressed initially. It is argued in the light of ΔG-values of reactions, and in view of the minimum energy quantum per mol, that anaerobic syntrophs must have special survival strategies in order to support their existence: redistributing the available energy between the partners, reduced end-product fermentation reactions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrate thresholds are strongly related to minimum ΔG-values. These new fundamental insights open perspectives for efficient design and operation of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last decade; the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physiologically well-organized way. The microbial principles of such granular sludge growth are presented. Using a thermodynamic approach, the formation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Due to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degrading xenobiotics. Operating granular sludge reactors at high upflow velocities (5-6 m/h) in expanded granular sludge bed (EGSB) systems enlarges the application field to very low strength wastewaters (chemical oxygen demand < 1 g/l) and psychrophilic temperatures (10°C). For the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank system to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumetric loading rates and at shorter residence times. With respect to organic solids, the recent trend in society towards source separated collection of biowaste has opened a broad range of new application areas for solid state anaerobic fermentation.

Preparation of monodisperse microspheres using the Shirasu porous glass emulsification technique

Abstract Relatively uniform polymeric microspheres, with diameters ranging from 2.5 to 60 μm, and relative standard deviations of diameters close to 10%, were prepared by the ordinary suspension polymerization of either styrene(hydrophobic) or methyl methacrylate (MMA)-(more hydrophilic) based monomers. Unlike the conventional stirred-tank system, a particular microporous glass membrane (Shirasu porous glass; SPG) provided uniform monomer droplets continuously when monomer was allowed to permeate through the micropores under a carefully controlled nitrogen atmosphere. The monomer droplets, a mixture of monomers, diluents, oil-soluble initiator as well as water-insoluble reagent, were then suspended in the aqueous solution containing stabilizing agents, transferred to a stirred vessel, and polymerized. In the case of hydrophilic MMA spheres, the size distribution tends to become broader because hydrophilic substances easily wet the surface of SPG, leading to a permeation process which is uncontrollable. This difficulty was overcome by adopting the droplet swelling technique, in which the secondary emulsion droplets containing hydrophilic MMA were absorbed in the primary emulsion droplets consisting of hydrophobic components on the principle of the degradative diffusion process. Uniformity of the initial droplets is preserved during the swelling step, and subsequent polymerization. Applications of crosslinked microporous spheres were promising as packing beads for gel permeation chromatography and as carriers for enzyme immobilizations.

Adsorbents for the removal of humic acid from surface water based on modified porous zirconia and silica

Abstract In these investigations porous zirconia and silica particles have been modified with anion-exchanger functionalities of different ligand characteristics. Three different modifications were examined. Two modifications were based on polystyrene-coated particles with introduced quaternary amino groups, namely trimethylphenylammonium chloride and triethylphenylammonium chloride as the functional groups. The third modification was achieved by immobilising 4-amino-4′,4″-bisdimethylaminotriphenylcarbinol, which is the 4-amino derivative of malachite green. These new high-performance ion-exchange adsorbents were then used to remove humic acid substances from river water, sampled at the well known wine growing district of the Barossa Valley, South Australia. Adsorption studies were performed in stirred tank systems with the adsorption behaviour monitored from the change in absorbance at 254 nm as function of time. After reaching saturation with the humic acid substances, the ion-exchange modified zirconia and silica particles could be readily regenerated with concentrated sodium chloride solutions.

Anoxic pilot plant studies for biological hydrolysis of urea in nitrogeneous fertilizer wastewater using a continuous flow stirred tank reactor

A continuous stirred tank system was used for pilot plant studies under anoxic biological conditions for biological hydrolysis of urea. It has been found that nearly all urea can be hydrolysed at a loading rate of 1.8 kg urea/kgMLSS x d which corresponds to 0.12 kg NH3‐N/kgMLSS x d. The application of air stripping technique for removal of secondary treated effluent removed ammonia which is generated during hydrolysis of urea within the prescribed limits of pollution control authorities. Methanol as carbon source has been found to be effective and, the methanol requirement in kg per kg of urea was found to be around 0.24. The biological sludge settled well with an SVI of about 90 to 110. The methanol concentration in the treated effluent was almost non‐detectable in all experimental runs.

Synthesis of polymeric microspheres employing SPG emulsification technique

AbstractRelatively uniform polymeric microspheres, the coefficients of variation being close to 10%, were obtained by the BPO‐initiated suspension polymerization of styrenic monomers. Unlike the conventional stirred‐tank system, a particular microporous glass membrane (SPG) provided uniform monomer droplets continuously when monomer was allowed to permeate through the micropores. The monomer droplets were suspended in an aqueous solution containing the stabilizing agents, transferred to a stirred vessel, and polymerized. Up to 10μm spheres, of a far narrower size distribution than those obtained by conventional microsuspension polymerization spheres, were obtained. The initial droplet size and distribution were retained with the successful suppression of secondary particle nucleation by the addition of hydroquinone in the aueous phase. Crosslinked polystyrene spheres were also synthesized in the presence of various low‐molecular‐weight diluents. While a good solvent, toluene, was not so effective; poor solvents, n‐heptane and n‐heptane, easily yielded the microporous structure, the specific surface area being as high as 160 m2/g. © 1994 John Wiley &amp; Sons, Inc.

Effect of mechanical arrangements on the performance of stirred tank systems

ABSTRACT Computational modelling approach can provide very valuable insight as to the relevance of the various mechanisms influencing the performance of process systems. It can also reveal the interaction between process and mechanical designs. In the case of stirred tank devices, it indicates how factors arising from different operating and configuration conditions affect the transfer processes, the mixing and the associated power requirements. This contribution highlights how computational techniques can facilitate validated design information.

Modeling chemical process systems via neural computation

The use of neural nets for modeling nonlinear chemical systems is discussed. Three cases are considered: a steady-state reactor, a dynamic pH stirred tank system, and interpretation of biosensor data. In all cases, a back-propagation net is used successfully to model the system. One advantage of neural nets is that they are inherently parallel and, as a result, can solve problems much faster than a serial digit computer. Furthermore, neural nets have the ability to learn. Rather than programming neural computers, one presents them with a series of examples, and from these examples the nets learn the governing relationships involved in the training database.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Adsorption of Phenol and p-Nitrophenol on Activated Carbon: Determination of Effective Diffusion Coefficients

Adsorption of phenol and p-nitrophenol in aqueous solution on granular activated carbon has been studied in a stirred-tank system. Equilibrium isotherms and concentration decay curves were obtained experimentally for each adsorbate in the finite batch adsorber, for three temperatures in the range 274-313 K. Effective internal diffusion coefficients, Di, of the two adsorbates were calculated by means of a mathematical model of diffusion-adsorption that takes into account ordinary liquid phase diffusion and surface diffusion mechanisms. Surface diffusion coefficients, DS, were also calculated and correlated with temperature and surface coverage. Isosteric heat of adsorption and activation energy for the adsorption of p-nitrophenol were calculated; values obtained are between those of characteristic physical and chemical adsorption.

Studies in the selective flocculation of hematite from gangue using high molecular weight polymers. Part 2: Physical factors

Having previously established a system of hematite and silica which can be separated effectively using long-chain polyacrylamide flocculants (Part 1) a stirred tank system was chosen to enable physical variables of flocculant concentration, impeller speed, time of flocculation, slurry concentration, ratio of hematite to quartz in the feed and particle size distribution to be studied. It was concluded that the time of flocculation allowed should be kept short and that there is an optimum of impeller speed though not a sharp one. Slurry concentration must be kept to 1% w/w to prevent serious performance deterioration. A clear optimum flocculant concentration was not found under present circumstances though very low concentrations (< 1 ppm) gave poor results while high concentrations gave no improvement on the results at about 2 ppm. Comparison between an impeller and a propeller of the same diameter and location in the tank pointed to the possibility of critical agitation configurations and the need for caution in scaling up results. The grade of material settling from a selective aggregation process is heavily dependent on the particle size distributions of the mineral species. Larger flocculated minerals produce richer flocs.