A. Tamburini

Modelling the Reverse ElectroDialysis process with seawater and concentrated brines

Abstract Technologies for the exploitation of renewable energies have been dramatically increasing in number, complexity and type of source adopted. Among the others, the use of saline gradient power is one of the latest emerging possibilities, related to the use of the osmotic/chemical potential energy of concentrated saline solutions. Nowadays, the fate of this renewable energy source is intrinsically linked to the development of the pressure retarded osmosis and reverse electrodialysis technologies. In the latter, the different concentrations of two saline solutions is used as a driving force for the direct production of electricity within a stack very similar to the conventional electrodialysis ones. In the present work, carried out in the EU-FP7 funded REAPower project, a multi-scale mathematical model for the Salinity Gradient Power Reverse Electrodialysis (SGP-RE) process with seawater and concentrated brines has been developed. The model is based on mass balance and constitutive equations collecte...

CFD analysis of the fluid flow behavior in a reverse electrodialysis stack

Abstract Salinity Gradient Power by Reverse Electrodialysis (SGP-RE) technology allows the production of electricity from the different chemical potentials of two differently concentrated salty solutions flowing in alternate channels suitably separated by selective ion exchange membranes. In SGP-RE, as well as in conventional ElectroDialysis (ED) technology, the process performance dramatically depends on the stack geometry and the internal fluid dynamics conditions: optimizing the system geometry in order to guarantee lower pressure drops (ΔP) and uniform flow rates distribution within the channels is a topic of primary importance. Although literature studies on Computational Fluid Dynamics (CFD) analysis and optimization of spacer-filled channels have been recently increasing in number and range of applications, only a few efforts have been focused on the analysis of the overall performance of the process. In particular, the proper attention should be devoted to verify whether the spacer geometry optimi...

REAPower: use of desalination brine for power production through reverse electrodialysis

AbstractSalinity gradient power (SGP) represents a viable renewable energy source associated with the mixing of two solutions of different salinities. Reverse electrodialysis (SGP-RE or RED) is a promising technology to exploit this energy source and directly generate electricity. However, although the principle of this technology is well known since several years, further R&D efforts are still necessary in order to explore the real potential of the SGP-RE process. With this regard, the aim of the REAPower project (www.reapower.eu) is the development of an innovative system for power production by SGP-RE process, using sea (or brackish) water as a diluted solution and brine as a concentrate. The use of sea or brackish water (instead of fresh water) as diluate allows reducing the electrical resistance of the diluate compartment and increasing the achievable output power. This work presents the R&D activities carried out so far within the REAPower project, particularly focusing on the relevant progresses in...

CFD simulation of channels for direct and reverse electrodialysis

Abstract Flows within very thin channels, typically filled with spacers, can be often encountered in many processes such as electrodialysis (ED) and reverse electrodialysis (RED). Although the ED and the RED processes have been studied for a long time, the optimization of the fluid dynamics within the channels is still an open problem. In the present work, realized within the EU-FP7 funded REAPower project, computational fluid dynamics simulations were carried out in order to predict the fluid flow field inside a single ED/RED channel. Some different configurations were tested which includes: an empty channel, a channel provided with a spacer, and a channel filled with a purposely manufactured fiber porous medium. Two types of spacers were investigated: (1) a commercial type made of woven perpendicular filaments and (2) an overlapped perpendicular filament spacer. A sensitivity analysis concerning computational grid size and topology was performed. For the cases investigated, adopting the hybrid grids mai...

CFD modelling of profiled-membrane channels for reverse electrodialysis

AbstractReverse 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 performed to predict the fluid flow and mass transfer behaviour in channels with profiled membranes for RE applications. In particular, channels equipped with pillars were simulated. The influence of channel geometry on fluid flow and concentration polarization was assessed by means of a parametric analysis for differe...

CFD prediction of scalar transport in thin channels for reverse electrodialysis

AbstractReverse electrodialysis (RED) is a very promising technology allowing the electrochemical potential difference of a salinity gradient to be directly converted into electric energy. The fluid dynamics optimization of the thin channels used in RED is still an open problem. The present preliminary work focuses on the computational fluid dynamics simulation of the flow and concentration fields in these channels. In particular, three different configurations were investigated: a channel unprovided with a spacer (empty channel) and two channels filled with spacers, one made of overlapped filaments and the other of woven filaments. The transport of two passive scalars, representative of the ions present in the solution, was simulated in order to evaluate concentration polarization phenomena. Computational domain effects were also addressed. Results show that: (i) the adoption of a computational domain limited to a single unit cell along with periodic boundary conditions provides results very close to tho...

Analysis and simulation of scale-up potentials in reverse electrodialysis

AbstractThe reverse electrodialysis (RED) process has been widely accepted as a viable and promising technology to produce electric energy from salinity difference (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 RED 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 laboratory-scale unit, providing a new modelling tool for process optimisation. In this work, performed within the REAPower project (www.reapower.eu), a process simulator previously proposed by the same authors has been modified in order to predict the behaviour of a cross-flow RED unit. The model was then adopted to investigate the influence of the most important variables (i.e. solution properties and stack geometry) on the overall process performance. In particular, the use of diff...

CFD Predictions of Sufficient Suspension Conditions in Solid-Liquid Agitated Tanks

Abstract Most research efforts on mechanically agitated solid-liquid contactors have been devoted to the assessment of the minimum impeller speed for complete off-bottom suspension, Njs. Actually, many industrial vessels are operated at impeller speeds slightly lower than Njs (Oldshue, 1983; Rieger et al., 1988). This suggests that the sufficient suspension condition, which is quantitatively specified in this paper by introducing a suitably defined quantity Nss, may represent a valid alternative to that of complete suspension. In the present work time-dependent RANS simulations were carried out with the aim of predicting the achievement of sufficient suspension conditions. The Eulerian-Eulerian Multi Fluid Model along with the Sliding Grid algorithm were employed. The Unsuspended Solids Criterion (USC) (Tamburini et al., 2011a) was adopted to distinguish the suspended particles from the unsuspended ones. The experimental suspension curves (mass fraction of suspended particles versus impeller speed, i.e. xsusp vs N) collected by Tamburini et al. (2011a) and the relevant Nss values were predicted by the present CFD simulations and a very good agreement was found, with maximum discrepancies of about 4%. Moreover, CFD results were used to test the ability of some Njs assessment methods proposed in the literature to predict sufficient suspension conditions.

Dense Solid-liquid Suspensions in Top-covered Unbaffled Stirred Vessels

In the present work solids suspension is investigated in an unbaffled tank stirred by a Rushton turbine and provided with a top-cover in order to avoid the formation of the well known central vortex. The data obtained are compared with those pertaining baffled stirred tanks via comparison with the well known Zwietering’s correlation. The dependence of Njs on particle concentration is found to be similar to that well established for baffled vessels, while, as a difference from the latter, Njs is found to decrease when liquid viscosity increases. Results also show that Njs is substantially independent of particle size, a feature that may advice the adoption of unbaffled tanks when large heavy particles are to be dealt with. On the other hand, a smaller exponent for the scale-up rule is found with respect to baffled tanks, which implies the need for larger specific power consumptions the larger the vessel size and may limit useful applications to small to medium plant sizes. Finally, a correlation akin to Zwietering’s correlation is proposed for top-covered unbaffled stirred tanks.

Performance of a RED system with ammonium hydrogen carbonate solutions

AbstractThe use of closed-loop salinity gradient power (SGP) technologies has been recently presented as a viable option to generate power using low-grade heat, by coupling a SGP unit with a thermally-driven regeneration process in a closed loop where artificial solutions can be adopted for the conversion of heat into power. Among these, the closed-loop reverse electrodialysis (RED) process presents a number of advantages such as the direct production of electricity, the extreme flexibility in operating conditions and the recently demonstrated large potentials for industrial scale-up. Ammonium hydrogen carbonate (NH4HCO3) is a salt suitable for such closed-loop RED process thanks to its particular properties. At temperatures above 40–45°C, it decomposes into a gaseous phase containing NH3, CO2 and water. Thus, the use of NH4HCO3 solutions for feeding a RED unit would allow their easy regeneration (after the power generation step) just using low-temperature waste heat in a purposely designed regeneration u...