Solar Desalination: a challenge for sustainable fresh water in the 21st century

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

Liquid viscosity and flow rate effects on interfacial area in packed columns

Abstract The influences of liquid kinematic viscosity and liquid flow rate on effective interfacial area for chemical absorption in a Raschig ring packed column were studied by absorbing carbon dioxide into potassium carbonate-bicarbonate-arsenite solutions. The viscosities of the absorbing liquid were varied by the addition of sugar. Analysis of the data shows that an increase in liquid flow rate increases the effective interfacial area over the whole viscosity range considered. The influence of liquid viscosity is more complex as the interfacial area increases with viscosity for low values, while decreasing at higher viscosities. Two correlations have been developed to express this behavi…

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

L'esperienza maturata in Sicilia nel campo della dissalazione

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

Photochemical Reactors Engineering Fundamentals

For the rapidly developing area of Photochemical Reactors Engineering an assessment of some fundamentals is reported. Topics surveyed are light sources, types of photoreactors and radiation modelling approaches. Some general remarks are also reported from which the need for further development of the general research area arises.

Analysis of bubbling fluidization dynamics

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

A phenomenological model for the quantitative interpretation of partial suspension conditions in stirred vessels

Publisher Summary Many important chemical engineering operations involve the suspension of solid particles in a liquid phase inside stirred tanks. Relevant examples include adsorption, crystallization, dissolution, leaching, precipitation, ion exchange and catalytic multiphase processes. This chapter describes a phenomenological model for the quantitative determination of the suspended solids mass fraction in stirred tanks operated at partial suspension conditions. Experimental data on fractional suspension, obtained by the “Pressure Gauge Technique” in vessels stirred by a downward-pumping pitched-blade-turbine, are presented. Furthermore, model predicted trends are compared with experimen…

Membrane distillation heat transfer enhancement by CFD analysis of internal module geometry

Module geometry optimisation can be a crucial matter in all separation processes using selective or hydrophobic membranes, e.g. reverse osmosis (RO) and membrane distillation (MD). In fact the choice of suitable channel shape and size can dramatically affect the performance of the process. With reference to the membrane distillation process, temperature polarization phenomena and pres¬sure drops along the channels largely affect the process efficiency (i.e. the efficient use of temperature driving force for the passage of vapour through the membrane) as well as pressure distribution, module mechanical resistance and pumping costs. Several works have been presented so far in literature on th…

Membrane Distillation efficiency increase by enhanced heat transfer

Solid-liquid mass transfer coefficients in gas-solid-liquid agitated vessels

The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid agitated systems. In this work a suitable dissolution technique was used to measure the solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical pow…

Le tecnologie utilizzate negli impianti di dissalazione

Kinetic parameter determination in monoculture and monosubstrate biological reactors

The kinetic parameters of a biological reactor, operating under monoculture and monosubstrate conditions, have been determined. Pure cultures of Nocardia corallina and Pseudomonas fluorescens species have been used with phenol as the only organic carbon source. The experimental runs have been carried out both in a batch and in a continuous stirred reactor. The batch results have been interpreted by zero order kinetics in phenol and first order kinetics in biomass. The kinetic-constants have also been calculated. The activation energy has been determined only for the Nocardia species. Using this strain, the continuous reactor, working without biomass recycle, has confirmed the first order ki…

Investigation of flashing phenomena in MSF chambers

The MSF process has covered in the last years more than 50% of world production of desalinated water from the sea. Although this process has been used for several decades, the large energy requirements force to search for new and feasible ways to improve the overall thermal efficiency. Few studies have bee carried out in lab-scale, pilot and industrial MSF units, underlining the complexity of flashing phenomena and the importance of operating parameters settings for optimal performance. In the present work a study on fundamental phenomena occurring inside MSF chambers has been carried out, focusing on the dependence of flashing efficiency on operating parameters and the effects of flashing …

Acqua dolce dal mare – L’esperienza di un trentennio di dissalazione in Sicilia

Quasi-isoactinic reactor for photocatalytic kinetics studies

Photochemical reactors characterized by almost uniform values of the local volumetric rate of photon absorption (LVRPA), i.e., quasi-isoactinic photoreactors, are particularly suitable for assessing the influence of radiant field intensity in kinetic studies. In this work, Monte Carlo simulations have been performed to obtain LVRPA values in a flat photoreactor irradiated on both sides. This configuration appears to be particularly suitable for obtaining quasi-isoactinic conditions. The influence of catalyst albedo and scattering phase function is assessed, and the conditions for obtaining iso-actinicity are discussed. Finally, these conditions are related to an easy-to-measure parameter, n…

Ozone absorption in aqueous phenol solutions

Abstract The rate of ozone absorption in aqueous solutions of phenol was measured in a wetted-wall laboratory absorber. The liquid and gas flow rates were fixed as well as the pH and temperature. The phenol concentration was varied in the range 2–300 ppm and the ozone partial pressure over an eight fold range. The gas phase resistance to mass transfer was determined by absorbing SO2 from an SO2N2 mixture in KOH aqueous solutions. The results have been interpreted by means of a simple kinetic assumption.

Acqua dolce dal mare. L’esperienza di un trentennio di dissalazione in Sicilia

Experimental analysis of bubble size distributions in 2D gas fluidized beds

Abstract The present work focuses on the development of a novel statistical approach for the analysis of the behavior of a 2D fluidized bed operating under bubbling and slugging conditions. Experimental data were collected by means of a purposely built lab-scale 2D fluidized bed and an original digital image analysis technique (DIAT), allowing the acquisition of a number of data sufficient to perform a statistical analysis of bubble dynamics. In particular, measurements of bubble size distributions (BSD) along the bed were performed. The analysis allowed to characterize the shape of BSD for different particle systems and fluidization gas velocities; interestingly a bimodal shape of BSD is o…

Estimating radiant fields in flat heterogeneous photoreactors by the six-flux model

Heterogeneous photoreactor modeling is a task complicated by the integro-differential nature of the Radiation Transfer Equation (RTE) when scattering phenomena are important. In the present work, a novel “Six Flux Model” (SFM) is proposed, which may be regarded as a step forward with respect to the previously proposed “Two Flux Model” (TFM). In order to validate the newly proposed model, Monte Carlo simulations of an indefinite plane-slab photoreactor have been performed. As no simplifying assumptions are involved in this case, the information obtained may be regarded as “pseudo-experimental,” and therefore compared with the predictions of both TFM and SFM models. Results show that the nove…

Seawater Desalination for Freshwater Production

In the last decades more and more countries have experienced water scarcity problems, thus pointing at alternative non-conventional sources of fresh water. Seawater desalination has proven to be a reliable and economically sustainable water resource since the second half of the 20th Century. A number of well proven technologies already exist, with advantages and disadvantages making each of them more suitable in specific sites. Moreover, quite recently, coupling the use of renewable energy to the production of fresh water from seawater results in novel technologies, able to minimise the environmental impact that desalination processes can create due to their intense energy consumptions.

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

Simplified Modeling of Radiant Fields in Heterogeneous Photoreactors. 2. Limiting “Two-Flux” Model for the Case of Reflectance Greater Than Zero

In the first part of this work a simple model for the description of the radiant field in heterogeneous photoreactors was developed, based on the assumption of zero reflectance of the particles. In this second part of the investigation a limiting model is developed in which the above hypothesis is removed. In this last model, scattering phenomena are dealt with in a very simple “two-flux” way, so that analytical solutions are obtained again. The case here developed is the same as in part 1, namely slab geometry with orthogonal parallel irradiation, giving special reference to the important case of semi-infinite reactor thickness. Both cases of solid catalysts with single particle size and w…

Misura di proprietà ottiche di fotocatalizzatori

A brine evaporative cooler/concentrator for autonomous thermal desalination units

In recent years growing attention has been paid to the problem of brine disposal due to the raising awareness of significant environmental issues related to the use of desalination processes for fresh water production. This is particularly relevant when desalination units are located in remote sites, characterised by major complexity in the construction and management of intake and outfall structures. In the present work a novel device, named brine evaporative cooler/concentrator (BECC, patent pending), has been developed for coupling with small-scale thermal desalination plants in order to reduce the problem of brine disposal. Such device fulfils two different functions: i) cooling of the …

Simplified Modeling of Radiant Fields in Heterogeneous Photoreactors. 1. Case of Zero Reflectance

A strongly simplified model is proposed for the estimation of the radiant flow field, and related quantities such as the local volumetric rate of energy absorption, in heterogeneous photoreactors. The model is based on the assumption that a photon carrying energy greater than that of the catalyst band gap, when interacting with a catalyst particle, is invariably absorbed. The model equations allow, for simple geometries, analytical integral solutions to be obtained. These have the advantage of giving an immediate grasp on how the main physical parameters affect the radiation field:  an important feature especially for engineering purposes. The case developed here is that of a plane slab, fo…

CFD prediction of bubble behavior in two-dimensional gas-solid fluidized beds

This work focuses on the computational fluid dynamics (CFD) simulation of a laboratory-scale, two-dimensional fluidized bed and the relevant experiments in order to validate the prediction capability of the adopted codes and models. Both experimental and computational quantitative data were analyzed by means of an original digital image analysis technique, allowing for coherent comparison of computational and experimental results. In particular, this work analyzes the capability of the CFD simulations in predicting the fluctuating behavior of bubbling fluidized beds by means of frequency analysis of bubble-related phenomena.

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…

The absorption of light energy in flat fluidized photoreactors

Abstract A flat fluidized bed with transparent walls is an ideal reactor for gas-solid photocatalytic or photoassisted reactions. This paper presents the relati

SOLAR DESALINATION: A CHALLENGE FOR SUSTAINABLE FRESH WATER IN THE 21ST CENTURY

generate a sustainable source of fresh water as well as energy. This combination is highly valued as it limits and reduces air pollutant emissions and green house gases generated by combustion of fossil fuels. Increase in energy demand during the first half of this century is expected to continue, which makes the cost of renewable solar energy highly competitive against fossil fuels. Desalination has been relied on to provide fresh water for large cities and countries across the world. The desalination industry continues to grow in countries in arid regions. Various aspects of solar desalination processes, solar energy, and conventional desalination are discussed. Scenarios for combination …

The Six Flux Model for the modelling of radiant fields in heterogeneous photoreactors

Experimental investigation of fluidization dynamics at various flow regimes

MODELING OF A TiO2-COATED QUARTZ -WOOL PACKED-BED PHOTOCATALYTIC REACTOR

A fixed-bed, photocatalytic laboratory reactor aimed to degrade pollutants from water streams was designed and built. Quartz wool coated with a thin film of TiO2 was employed as the reactor filling. The photocatalyst was placed in the reactor forming a loose packing to guarantee the intimate contact among reactants, photons, and the photocatalytic surface. This reactor was employed to study the photocatalytic decomposition of a model pollutant (formic acid). A reactor–radiation–reaction model was developed, which was comprised of the reactor mass balance, radiation model, and kinetic model for the degradation of formic acid. The local superficial rate of photon absorption, which was necessa…

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

Studio di cinetiche di reazione in un reattore fotocatalitico quasi-isoattinico

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

CFD Simulation of Particle Suspension Height in Stirred Vessels

Computational fluid dynamics (CFD) simulation capabilities for stirred solid–liquid dense systems are explored. These systems may give rise to the formation of a thick and well defined clear liquid layer in the upper part of the vessel, whose extension progressively reduces with increasing impeller speed. Experimental measurements of the suspension height (the height of the particle laden layer) were carried out at various agitation speeds for a variety of solid–liquid systems in a fully baffled transparent tank. A clear layer of liquid was actually observed in all runs, with the suspension height almost linearly dependent on agitation speed. CFD simulations of the above described systems w…

The measurement of light transmission through an irradiated fluidised bed

Abstract The transmission of light through an irradiated fluidised bed with vertical flat walls containing catalytic particles has been experimentally studied. The amount of light transmitted through the reactor was measured at different levels by a photocell. The experimental variables studied included: (i) gas flow rate, varying from 2 to more than 10 times that required for minimum fluidisation; (ii) mean particle size, ranging from 0.138 to 0.275 mm; and (iii) internal bed thickness from 2.1 to 4.9 mm. The effect of the reflectance of the particles was also examined. Results show that the amount of light transmitted through a flat fluidised bed is low. The average transmitted light has …

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

Study of bubbling fluidization dynamics via Digital Image Analysis Technique

Analysis of the bubbling behaviour of 2D gas solid fluidized beds

Abstract 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 phenom…

Modelling of light transmittance and reflectance in flat fluidized photoreactors

In recent years several photon induced chemical reactions involving the use of a solid catalyst have been studied. Flat fluidized beds have proved to be promising devices for carrying out these photocatalytic reactions. There is therefore a need for the modelling of light interactions within such kinds of photoreactors. In this paper, a phenomenological model is proposed for simulating the light transmission and reflection in flat fluidized gas-solid photoreactors. A detailed description of the model is presented. The model predictions were compared with experimental data and a satisfactory agreement was found. Correlations were also suggested for the estimation of model parameters. Depuis …

A critical assessment of desalination operations in Sicily

In this paper a brief history of desalination plants in Sicily is presented together with an in-depth description of processes and operating data, relevant to the plants actually operating on the island. 5 multi stage flash (MSF) units and one reverse osmosis (RO) plant are operating in Gela (total nominal production of about 80,000 m 3 /d), thus providing drinking water for more than 300,000 people living in the southern coast of the island. A thermal vapour compression multiple effect distillation (TVC-MED) plant (36,000 m 3 /d) is operating in Trapani and a small mechanical vapour compression (MVC) unit (5000 m 3 /d) is located in Porto Empedocle. In the oldest MSF units in Gela, several…

Analysis of bulk evaporation phenomena in Multi Stage Flash desalination units

A Six Flux Model for the estimation of radiant fields in flat heterogeneous photoreactors

Gli aspetti economici e gestionali della dissalazione

I processi disponibili per la dissalazione di acque marine e salmastre

Bubble formation at single nozzles

Misure di proprietà ottiche di fotocatalizzatori

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

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 …

CFD simulation of a membrane distillation module channel

The interest towards the use of membrane distillation (MD) processes for seawater desalination has been rising recently due to the ease of coupling MD with waste and/or solar thermal energy. Notwithstanding the flexibility of the process and its potential for further developments in membrane performances, one of the main drawbacks is the thermal efficiency reduction caused by temperature polarization. Because of such phenomenon, only a small amount of the driving force potentially available for the separation process, i.e. the temperature difference between evaporating and condensing fluids, is actually used for the separation. In order to reduce temperature polarization a study on the effe…

Radiation model of a TiO2-coated, quartz wool, packed-bed photocatalytic reactor

The radiation field of a packed-bed photocatalytic reactor filled with quartz wool coated with titanium dioxide was modeled using the Monte Carlo technique and the following information: the radiation flux emitted by the lamps, the diameter size distribution of the quartz fiber cloth, the mass of quartz fibers and of TiO2 that was immobilized on the fiber surface as well as the refractive index and the spectral absorption coefficient of the materials of the system. Modeling predictions were validated with radiometer measurements of the transmitted radiation through the reactor, the root mean square error being < 9.7%. Finally, by means of a parametric study, the validated model was used to …

Diffusional kinetics of metalliding zinc into solid copper

The process of incorporation of zinc into a copper cathode has been experimentally studied in a molten salt system at 381±2° C and at various current densities. The process is shown to be kinetically controlled by the diffusion of Zn into the solid matrix. A galvanostatic pulse titration technique has been used to determine the chemical diffusion coefficient at various alloy compositions, and an exponential relationship has been found between the diffusivity and the third power of the zinc concentration in the alloy. This relationship was then used in the diffusion equation within the solid matrix and a numerical integration was performed. Very good agreement was found between the calculate…

Experimental investigation of water desalination in a MSF pilot unit

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

RAFFREDDATORE-CONCENTRATORE EVAPORATIVO DI SALAMOIA

Linear stability analysis of gas-fluidized beds for the prediction of incipient bubbling conditions

Abstract This work focuses on the development of a novel linear stability criterion for the state of homogeneous fluidization regime, based on a new mathematical model for gas-fluidized beds. The model is developed starting from the well-known particle bed model. A mono-dimensional momentum balance is derived leading to a set of equations which explicitly include voidage-gradient dependent terms (elastic force) for both solid and fluid phases. A fully predictive criterion for the stability of homogeneous fluidization state is here proposed, based on the well-known Wallis’ linear stability analysis. The criterion requires the choice of an appropriate averaging distance, which in the present …

Mathematical Model and Stability analysis of Multiphase Flow in Fluidized Beds