SOLID-LIQUID SUSPENSIONS IN UNBAFFLED TANKS

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled tanks. However, they might provide significant advantages in a wide range of applications like crystallization processes as well as for food and pharmaceutical industries, where the presence of baffles is often undesirable. In the present work solid-liquid suspension in an unbaffled stirred tank was investigated. A novel experimental method (steady cone radius method) was devised to ease the evaluation of the minimum impeller speed for complete particle suspension (Njs). Experiments encompassed a quite wide range of particle sizes, densities and solids concentration. The Njs…

Digital Image Analysis Technique for mixing pattern measurements in bi-dispersed 2D gas fluidized beds

This work concerns a new experimental technique based on Digital Image Analysis aimed at the measurement of the mixing behaviour of the granular phase in a 2D fluidized bed. In particular, the preliminary results obtained about the mixing pattern of a bi-disperse 2D gas fluidized bed are reported. Two different kind of particles were fluidized having the same density but different size. The technique is based on suitable colour-field decomposition of the fluidized bed, in which particles of different colours are employed. The technique here proposed can effectively measure the concentration field in the whole bed during operation, thus obtaining data that can be reliably used for CFD simula…

Transient behaviour of a MSF desalination unit: modelling and experiments

SIMULAZIONI CFD DEL CAMPO DI MOTO A BASSO NUMERO DI REYNOLDS IN BIOREATTORI AGITATI CON E SENZA SETTI FRANGIVORTICE

I recipienti meccanicamente agitati sono apparecchiature frequenti dell’industria chimica di processo. Questi sono tipicamente dotati di setti frangivortice (recipienti baffled) atti a rompere il moto prettamente tangenziale tipico dei recipienti sprovvisti di setti (recipienti unbaffled) e convertirlo in moto assiale e radiale. La presenza dei setti evita inoltre nella zona centrale del sistema la formazione del vortice d’aria che può talvolta essere indesiderato o in taluni casi creare instabilità una volta arrivato alla girante. I recipienti unbaffled sono considerati quindi dei miscelatori meno efficienti rispetto a quelli provvisti di setti. Tali sistemi sono pertanto ad oggi impiegati…

DYNAMIC MODELLING OF A DESALINATION MSF UNIT AND COMPARISON WITH EXPERIMENTAL DATA

Particle distribution in unbaffled stirred vessels

The present work is devoted to providing an insight into the solid-particle distribution within top-covered unbaffled stirred tanks via purposely collected local experimental data. Experiments were carried out on a lab- scale unbaffled stirred tank by making use of a recently introduced technique named Laser Sheet Image Analysis (LSIA). In its original formulation, the technique includes an image post-processing procedure to delete reflection effects on results. In the framework of the present work, a method combining the use of purposely produced fluorescent particles and a suitable camera high pass filter was devised and presented. Results collected with (new method) and without (old fash…

Large Eddy Simulation of Unbaffled Stirred Tanks

CFD MODELLING OF PARTICLE SUSPENSION IN STIRRED TANKS

Mixing of solid particles into liquids in mechanically agitated vessels is a topic of primary importance for several industrial applications. A great deal of research efforts has been devoted so far to the assessment of the minimum impeller speed (Njs) able to guarantee that all particles are suspended. Conversely, only little attention has been paid to the evaluation of the amount of solid particles that are suspended at impeller speeds N lower than Njs, despite the fact that in a number of industrial applications agitation speeds smaller than Njs are actually adopted [1,2]. The present work deals with dense solid-liquid partial suspensions in baffled stirred tanks and particularly focuses…

CFD modelling of profiled membranes channels for reverse electrodialysis

Reverse electrodialysis (RE) is a promising technology for electric power generation from controlled mixing of two differently concentrated salt solutions, where ion-exchange membranes are adopted for the generation of ionic currents within the system. Channel geometry strongly influences fluid flow and thus crucial phenomena such as pressure drop and concentration polarization. Profiled membranes are an alternative to the more commonly adopted net spacers and offer a number of advantages: avoiding the use of non-conductive and relatively expensive materials, reducing hydraulic losses and increasing the active membrane area. In this work, Computational Fluid Dynamic simulations were perform…

A continuous plug flow reactor for magnesium recovery from concentrated brine

Magnesium is one of the eight most abundant elements on the world and the third most abun- dant in the sea, buc practically only two countries (Russia and China) control the world market for this compound. Product purity, Mg recovery, crystal morphology and granulometric distribution of precipitated particles were examined through careful laboratory analysis and results from different tests were compared, in order to identify the effect of allcaline species’ nature and concentration on precipitation phenomena.

Seawater desalination : state of the art on conventional technologies

CFD prediction of bubbles local gas hold-up in 2-dimensional gas-solid fluidized beds

Analysis of bubbling fluidization dynamics

Multi Stage Flash Desalination with Direct Mixing Condensation

Multi Stage Flash desalination technology has been a leading technology in the desalination market for the last four decades. However as a result of the continuous increase of costs for the special materials required for the tube bundles the share of MSF technology in the market is nowadays steadily shrinking. Aim of the present work is to present a novel configuration for the MSF process. In the proposed arrangement evaporation occurs in a multi stage vertical column filled with packing material to enhance the achievement of flashing equilibrium in each stage, while condensation occurs in a second column through direct mixing with a colder water stream. Thermal integration for the recovery…

Experimental Analysis of bubbling dynamics in bidisperse gas-solid 2D fluidized beds

L’utilizzo del carbone come combustibile economico e di limitato impatto ambientale

Membrane Distillation efficiency increase by enhanced heat transfer

Apparato di sintesi su letto catalitico e separazione di fasi liquido-gas

Apparato di sintesi su letto catalitico e di separazione dei prodotti di sintesi comprendenti una fase gassosa e due fasi liquide, una fase liquida più pesante e l’altra fase liquida più leggera, l’apparato comprendente - un modulo di sintesi (M1) comprendente un primo tubo (1) provvisto ad una prima estremità di un’apertura e chiuso ad una seconda estremità da una griglia (7), il primo tubo (1) essendo atto a contenere al suo interno un letto catalitico (6); - ed un modulo di separazione (M2) per separare le due fasi liquide e la fase gassosa provenienti dal modulo di sintesi (M1), comprendente un secondo tubo (1’) disposto adiacente alla seconda estremità del primo tubo (1), comunicante c…

CFD prediction of solid particle distribution in baffled stirred vessels under partial to complete suspension conditions

Solid-liquid mixing within tanks agitated by stirrers can be easily encountered in many industrial processes. It is common to find an industrial tank operating at an impeller speed N lower than the minimum agitation speed for the suspension of solid particles: under such conditions the distribution of solid-particles is very far from being homogeneous and very significant concentration gradients exist. The present work evaluates the capability of a Computational Fluid Dynamics (CFD) model to reliably predict the particle distribution throughout the tank under either partial or complete suspension conditions. A flat bottomed baffled tank stirred by a Rushton turbine was investigated. Both tr…

ASSESSMENT OF THE MINIMUM POWER REQUIREMENTS FOR COMPLETE SUSPENSION IN TOP-COVERED UNBAFFLED STIRRED TANKS

On the Simulation of Solid Particle Distribution in Multiple Impeller Agitated Tanks via Computational Fluid Dynamics

Operation and perspectives of the first reverse electrodialysis pilot plant fed with brackish water and brines

This work describes the performance of the first RED prototype plant fed with real brackish water and saltworks brine

Numerical Simulation of Low Reynolds Number Flow Fields in Unbaffled Stirred Vessels

Multi-physical modelling of Reverse ElectroDialysis

Energy extraction from salinity gradients (salinity gradient power, SGP) represents a novel and valuable renewable energy source. Among the existing SGP technologies, reverse electrodialysis (RED) is the oldest and one of the most promising. RED is a membrane-based electrochemical process that directly converts the salinity gradient energy into electric current. More precisely, in a RED unit two solutions at different concentration flow in two series of alternated channels, which are formed by piling two alternated series of cation and anion exchange membranes (CEMs and AEMs, respectively). The chemical potential difference between the two solutions generates an electric potential differenc…

Reverse Electrodialysis with seawater and concentrated brine: a comprehensive process modelling

EXPERIMENTAL INVESTIGATION OF DILUTE SOLID-LIQUID SUSPENSION IN AN UNBAFFLED STIRRED VESSELS BY A NOVEL PULSED LASER BASED IMAGE ANALYSIS TECHNIQUE

The availability of experimental information on solid distribution inside stirred tanks is a topic of great importance in several industrial applications. The measurement of solid particle distribution in turbulent multiphase flow is not simple and the development of suitable measurement techniques is still in progress. In this work a novel non-intrusive technique for measuring particle concentration fields in solid-liquid systems is employed. The technique makes use of a laser sheet, a high sensitivity digital camera for image acquisition and a Matlab procedure for post-processing the acquired images. Experimental data are here obtained for the case of an unbaffled stirred tank. Stable tor…

Lagrangian simulation of bubbling dynamics in a lab-scale 2D fluidized bed

The present work focuses on the development of a novel computational code able to predict with a reasonable level of accuracy the bubble behavior in gas fluidized beds with minimum computational demands. The code simulates the bubble chaotic rise motion and coalescence along bed height via simple lagrangian tracking of bubbles. An original empirical model for the assessment of bubble-bubble interactions is developed. The code is used to simulate a lab-scale unit in bubbling and slugging mode. On this basis, fast simulations are performed to successfully predict bubble population and fluxes within the bed. The main aim of this code is to be embedded within CAPE codes for the process simulati…

Exergy analysis and thermoeconomic cost accounting of a CHP steam cycle integrated with MED-TVC desalination

In this paper exergy analysis and thermoeconomic cost accounting of a CHP steam cycle integrated with MED-TVC plant is performed; the purpose is to show how this methodology provides a rational criterion to allocate production costs on electricity and freshwater. After a brief overview on methodology and a description of reference plant, exergy analysis is carried out in order to calculate exergy flows and plant exergetic performance. A detailed description of the adopted thermoeconomic model is given. Cost accounting is performed considering two scenarios: in the first, the concentrated brine is disposed back to sea, thus being its exergy content definitively wasted; in the second, convers…

CFD simulation of solid-liquid suspension startup in a stirred tank

A Digital Image Analysis Technique for 2-D Gas-Solid Fluidized Beds

NUMERICAL SIMULATION OF SEDIMENT RESUSPENSION IN MECHANICALLY STIRRED VESSEL

MISURA DELLA CONCENTRAZIONE DI SOLIDI PARTICELLARI IN SISTEMI SOLIDO-LIQUIDO AGITATI MEDIANTE LASER PULSATO E ANALISI DI IMMAGINI

A novel TLC based technique for temperature field investigation in MD channel

Computational modelling of flow and turbulence in submerged dense jets

DYNAMIC MODELLING TOOLS FOR SOLAR POWERED DESALINATION PROCESSES DURING TRANSIENT OPERATIONS

CFD simulation of dense plumes in an atmospheric wind tunnel

CFD analysis of mass transfer in spacer-filled channels for reverse electrodialysis

Reverse electrodialysis (RE) is a promising technology for electric power generation by converting the chemical potential difference of a salinity gradient, within a stack equipped by selective ion-exchange membranes. Concentration polarization phenomena and pressure drop affect strongly the power output obtainable; therefore the channel geometry is a fundamental operating parameter for the optimization of the system. In this work, Computational Fluid Dynamic simulations were performed to predict fluid flow and mass transfer in spacer-filled channels for RE applications. A parametric analysis for different spacer geometries was carried out; in particular, woven and non woven spacers were si…

Top-covered unbaffled stirred tanks: experiments and numerical simulations

In this work computational fluid dynamics was employed to predict the flow field of an unbaffled stirred tank from steady to turbulent conditions

Analysis of the bubbling behaviour of 2-D gas solid fluidized beds part II: Comparison between experiments and numerical simulations via Digital Image Analysis Technique

In the field of gas–solid fluidization, bubbles, and all features regarding them, have a very great importance, as they significantly affect the process performance. Numerous experimental studies on bubbles, and their formation, evolution, and properties, have been performed in the past. These investigations appear particularly difficult, due to the nature of these systems, since the gas phase is distributed in both the bubble and the emulsion phase. Several experimental approaches have been developed to tackle this study. Among these, the Digital Image Analysis Technique purposely developed in Part I of the present work, based on the use of a video camera for monitoring the phenomenon coup…

CFD PREDICTION OF BUBBLES BEHAVIOUR IN 2-DIMENSIONAL GAS-SOLID FLUIDIZED BEDS

In recent years the use of Computational Fluid Dynamics (CFD) is significantly increasing to simulate multi-phase flows. It is invariably emphasized that a necessary step towards the development of reliable fully predictive CFD models is an extensive experimental validation of the simulation results. This work in particular focuses on the CFD simulation of a lab-scale 2D fluidized bed and the relevant experiments, in order to validate the prediction capability of the used codes and models. It must be emphasized that both experimental and computational quantitative data have been obtained by means of an original Digital Image Analysis Technique, that allows coherent comparison of computation…

An Ion Exchange Membrane Crystallisation reactor for Magnesium recovery from brines

CrIEM technology is a novel ion exchange application that allows reactive crystallization for separation of valuable species (e.g. Mg from brines) with a large flexibility in the choice of reactants.

Temperature-responsive cellulose-based hydrogels as draw agents in Forward Osmosis

Forward Osmosis is a powerful tool to produce clean water at a low cost if appropriate draw agents and regeneration methods are employed (1). In this frame, stimuli-responsive hydrogels are interesting candidates due to their ability to uptake water and release it under physical stimuli, such as hydrostatic pressure and temperature variations. In particular, temperature-responsive hydrogels that are able to absorb large amount of water at room temperature and release it upon a slight temperature increase are very appealing candidates because they can be regenerated easily and with very low energy consumption (2). Chemical stability and biocompatibility are two further requirements that hydr…

A multi-scale model for the reverse electrodialysis process with seawater and concentrated brines

Complete Numerical Simulation of Flow Fields in Baffled Stirred Vessels: The Inner-Outer Approach

Particle Distribution in an Unbaffled Stirred Vessel by a Novel laser Sheet Image Analysis Technique

The availability of experimental information on solid distribution inside stirred tanks is a topic of great importance in several industrial applications. The measurement of solid particle distribution in turbulent multiphase flow is not simple and the development of suitable measurement techniques is still in progress. In this work a novel non-invasive technique for measuring particle concentration fields in solid-liquid systems is presented and employed. The technique makes use of a laser sheet, a high sensitivity digital camera for image acquisition and a Matlab procedure for post-processing the acquired images. Experimental data are here obtained for the case of an unbaffled stirred tan…

CFD analysis of concentration polarization phenomena in spacer-filled channels for Reverse Electro-Dialysis

In this work, carried out within the EU-FP7 funded REAPower project, CFD simulations were carried out in order to study the fluid flow behaviour and mass transport phenomena within spacer-filled channels.

Experimental investigation of fluidization dynamics at various flow regimes

EXPERIMENTAL INVESTIGATION OF THE FLASHING PHENOMENON INSIDE MSF DESALINATION UNITS; EFFECT OF EFFICIENCY PROMOTERS

Computational modelling of submerged dense jets

CFD simulations of spacer-filled channels for pressure retarded osmosis applications

Different spacer features and operating conditions were investigated in order to identify a good compromise between concentration polarization and pumping power reductions.

Effect of spacer geometry on the process performance of pervaporation membrane modules

Numerical simulations of bubbling fluidized beds

Lagrangian simulation of bubbling behaviour in gas fluidized beds and validation with experimental data

Investigation of Reverse ElectroDialysis Units by Multi-Physical Modelling

Reverse electrodialysis (RED) is an electrochemical membrane process that converts the salinity gradient energy between two solutions into electric current, by using ion exchange membranes. A novel multi-physical approach for RED modelling is proposed. 2-D simulations of one cell pair with tertiary current distribution (Nernst–Plank equation and local electroneutrality) were performed. Moreover, the Donnan exclusion theory was implemented for simulating double layer phenomena. Transport phenomena and electrochemical behavior were well described. The influence of membrane/channel configuration, dilute concentration and feeds velocity on the process performance was assessed. For a dilute conc…

Pressure drop in woven-spacer-filled channels for reverse electrodialysis: CFD prediction and experimental validation

Reverse electrodialysis (RED) is a promising technology for electric power generation by the chemical potential difference of two salt solutions within a stack equipped by selective ionexchange membranes (salinity gradient energy). Mechanical energy is required for pumping the feed solutions, which can reduce dramatically the net power output. In this work Computational Fluid Dynamics (CFD) simulations of spacer-filled channels for RED were carried out in parallel with an experimental campaign focused on the collection of data for model validation.

Comparison of Agitators Performance for Particle Suspension in Top-Covered Unbaffled Vessels

Power savings is a problem of crucial importance nowadays. In process industry, suspension of solid particles into liquids is usually obtained by employing stirred tanks, which often are very power demanding. Notwithstanding tanks provided with baffles are traditionally adopted for this task, recent studies have shown that power reductions can be obtained in top-covered unbaffled vessels. In the present work experiments were carried out in a top-covered unbaffled vessel with a diameter T=0.19m and filled with distilled water and silica particles. Two different turbines were tested: a standard six-bladed Rushton Turbine (RT) and a 45° four bladed Pitched Blade Turbine (PBT). For the case of …

Geometrical characterization of reverse electrodialysis stack via CFD simulations

In the presente work, CFD simulations were carried out in order to assess the effects of different parameters on the global process efficiency, such as the choice of spacer material and morphology, and the optimisation of feed and blowdown distribution systems

Modelling the transient behaviour of a MED-TVC plant for coupling with CSP

In this paper a dynamic model for the MED-TVC process has been developed and valided by a comparison with data from a real plant situated in Trapani, Italy, composed by 12 effects with parallel-feed configuration and equipped with a medium-pressure steam vapour ejector.

How spacer geometry can affect the Thermo-Fluid Dynamics of a Membrane Distillation module

Measurements of temperature polarization phenomena in membrane distillation channels by a thermographic technique

In the present work, a recently presented experimental technique, based on the use of thermochromic liquid crystals (TLCs) and digital image processing, has been employed in order to measure the temperature and local heat transfer coefficient distribution on the membrane surface in a MD spacer-filler channel

Study of bubbling fluidization dynamics via Digital Image Analysis Technique

Oxygen Transfer Performances of Unbaffled Bio_Reactors with Various Aspect Ratios

Cultivation of microorganisms, plants or animal cells requires liquid agitation in order to ensure oxygen and nutrient transfer and to maintain cell suspension. Many studies on animal cell damage due to mechanical agitation and sparging aeration have shown that mechanical damage of freely suspended animal cells is in most cases associated with bursting bubbles at the air–liquid interface (Barrett et al., 2010). Gas bubbles are usually generated by direct air sparging aimed at supplying oxygen to the culture medium. Mechanical agitation may also introduce gas bubbles in the culture medium via vortexing entrainment from the free surface. In this work oxygen transfer performance of an unbaffle…

CFD simulations of the clear liquid layer formation in dense solid-liquid suspension

Analysis of bulk evaporation phenomena in Multi Stage Flash desalination units

Characterization of membrane-spacer industrial distillers using thermochromic liquid crystals

Reverse Electrodialysis Process: Analysis of Optimal Conditions for Process Scale-up

Reverse Electrodialysis (SGP-RE or RED) process has been widely accepted as a viable and promising technology to produce electric energy from salinity difference (i.e salinity gradient power - e.g. using river water/seawater, or seawater and concentrated brines). Recent R&D efforts demonstrated how an appropriate design of the SGP-RE unit and a suitable selection of process conditions may crucially enhance the process performance. With this regard, a process simulator was developed and validated with experimental data collected on a lab-scale unit, providing a new modelling tool for process optimisation. In this work, performed within the REAPower project (www.reapower.eu), the process simu…

Study of bubble size distribution in bubbling fluidized beds via Digital Image Analysis Technique

Optimisation analysis of Reverse Electrodialysis systems for power production from concentrated brines

Reverse Electrodialysis (RED) is rapidly growing as technology to produce electric energy by mixing saline solutions with different salinity. Recent developments have shown promising results on real site installations, demonstrating the feasibility of the RED process on the pilot scale. Therefore, further modelling efforts are now needed to optimise the technology, in order to enhance the performance. In this work, an optimisation study for the RED process is presented, taking into account saline waters and concentrated brine as feed streams. The model, which is developed within GAMS environment, predicts the optimal set of process variables that maximise the process yield, as well as the g…

Pressure Retarded Osmosis: a Membrane Process for Environmental Sustainability

Salinity Gradient Power (SGP) based on the controlled mixing between two solutions at different salinities is a viable alternative to produce power from renewable sources. Pressure Retarded Osmosis (PRO) is one of the most promising technologies proposed so far for the exploitation of such energy source. Apart from the typical source of salinity gradients, namely river water and seawater, more and more interest has been raised recently towards the use of non-conventional saline solutions. In this work, water originating from a sewage treatment plant is used as the dilute solution (feed solution), while brine exiting from a desalination plant is used as the concentrate (draw solution), thus …

Analysis of the bubbling behaviour of 2-D gas solid fluidized beds - Part I Digital Image Analysis Technique

Bubble characteristics are very important in the design of fluidized beds because they govern hydrodynamics and efficiency of the operation for which the bed is used. In this work, a digital image analysis technique has been developed to study the fluidization dynamics of a lab-scale two-dimensional bubbling bed. Digital image analysis may supply a great quantity of information; it is non-intrusive, capable of securing several properties simultaneously and cost effective. The image analysis method here developed allows for the simultaneous measurements of various significant bubble properties, i.e. bubble size and bubble velocity distributions, bed height and bubble-phase hold-up, by means …

Reverse electrodialysis: Applications

Reverse electrodialysis (RED) technology has grown significantly in the last decade, gaining a fast increase in its technology readiness level and presenting some interesting examples of RED pilot systems operating under very different real environments. In this chapter, an overview of technological developments and piloting examples are reported. In particular, a short introduction is given on the historical trend of RED technology growth, followed by a careful analysis of which feed solutions can be adopted and how these can affect the process performance, potentials, and applications. Most prominent fluid dynamics aspects for the RED process are presented, highlighting how these can infl…

Coupling sustainable energy with membrane distillation processes for seawater desalination

Desalination technologies nowadays is a very interesting and emerging alternative to conventional water sources; their high energy consumption, however, generates some problems in adopting them, especially in remote areas where electric grid is not available and fuel transportation is not always sustainable. In this paper a short overview of possible couplings between the membrane distillation technology for desalination and the use of sustainable thermal energy sources is presented. Two case studies of ongoing research projects at Università di Palermo are also presented.

CFD modelling of thin channels for direct and reverse electrodialysis

Membrane Distillation for a solar powered desalination pilot unit

A tool for modelling spacer-filled MD channels based on open source CFD code

Experimental investigation of water desalination in a MSF pilot unit

Experimental investigation of the effects of efficiency promoters in a MSF lab-scale unit

REATTORE E PROCEDIMENTO PER EFFETTUARE REAZIONI SELETTIVE

La presente invenzione riguarda un reattore e un procedimento per eseguire processi di cristallizzazione reattiva con elevata selettività, consentendo di evitare la miscelazione diretta tra le due soluzioni contenenti i reagenti della reazione e di minimizzare l’effetto di reazioni secondarie indesiderate. Per brevità, nel testo seguente tale reattore e procedimento verrà indicato con l’acronimo CrIEM (Crystalliser with Ion Exchange Membrane).

RAFFREDDATORE-CONCENTRATORE EVAPORATIVO DI SALAMOIA

Mathematical Model and Stability analysis of Multiphase Flow in Fluidized Beds