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The increasing demand for environmentally friendly production of alkaline and acidic materials has boosted the advancement of electrodialysis with bipolar membranes (EDBM) as an efficient and sustainable technology. In this context, this study aims to optimize the EDBM system performance and minimize its environmental impact. Specifically, this work focuses on the use of concentrated brines in alkaline or acid channels as innovative EDBM schemes to reduce freshwater consumption. Two different commercial monopolar and bipolar membrane sets were tested (Fumatech and SUEZ) showing a difference in productivity ratios, between them, ranging from 9% to 40%. The use of brine in the compartments leads to a reduction in the water footprint and highlights potential pathways to enhance the overall EDBM system sustainability. Remarkably, introducing a brine in the alkaline channels does not affect energy performance while achieving 50% reduction in freshwater consumption. In contrast, feeding brine into acid channels leads to a performance decline. This research provides a holistic perspective on the interplay between technical and environmental performance of EDBM process, providing a valuable guide for future research and paving the way for novel membranes, long-term stability, and strategies to minimize energy consumption and environmental impact, toward sustainable industrial practices.

Magnesium hydroxide, Mg(OH)2, is an inorganic compound extensively employed in several industrial sectors. Nowadays, it is mostly produced from magnesium-rich minerals. Nevertheless, magnesium-rich solutions, such as natural and industrial brines, could prove to be a great treasure. In this work, synthetic magnesium chloride and sodium hydroxide (NaOH) solutions were used to recover Mg(OH)2by reactive crystallization. A detailed experimental campaign was conducted aiming at producing grown Mg(OH)2hexagonal platelets. Experiments were carried out in a stirred tank crystallizer operated in single- and double-feed configurations. In the single-feed configuration, globular and nanoflakes primary particles were obtained, as always reported in the literature when NaOH is used as a precipitant. However, these products are not complying with flame-retardant applications that require large hexagonal Mg(OH)2platelets. This work suggests an effective precipitation strategy to favor crystal growth while, at the same time, limiting the nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)2hexagonal platelets. The influence of reactant flow rates, reactant concentrations, and reaction temperature was analyzed. Scanning electron microscopy (SEM) pictures were also taken to investigate the morphology of Mg(OH)2crystals. The proposed precipitation strategy paves the road to satisfy flame-retardant market requirements.

Magnesium is a critical raw material and its recovery as Mg(OH)2from saltwork brines can be realized via precipitation. The effective design, optimization, and scale-up of such a process require the development of a computational model accounting for the effect of fluid dynamics, homogeneous and heterogeneous nucleation, molecular growth, and aggregation. The unknown kinetics parameters are inferred and validated in this work by using experimental data produced with a T2mm-mixer and a T3mm-mixer, guaranteeing fast and efficient mixing. The flow field in the T-mixers is fully characterized by using the k-εturbulence model implemented in the computational fluid dynamics (CFD) code OpenFOAM. The model is based on a simplified plug flow reactor model, instructed by detailed CFD simulations. It incorporates Bromley’s activity coefficient correction and a micro-mixing model for the calculation of the supersaturation ratio. The population balance equation is solved by exploiting the quadrature method of moments, and mass balances are used for updating the reactive ions concentrations, accounting for the precipitated solid. To avoid unphysical results, global constrained optimization is used for kinetics parameters identification, exploiting experimentally measured particle size distribution (PSD). The inferred kinetics set is validated by comparing PSDs at different operative conditions both in the T2mm-mixer and the T3mm-mixer. The developed computational model, including the kinetics parameters estimated for the first time in this work, will be used for the design of a prototype for the industrial precipitation of Mg(OH)2from saltwork brines in an industrial environment.

The continuous depletion of minerals caused by land mining and the increase in their demand have pushed the development of novel sustainable technological processes for mineral recovery from unconventional sources. In this context, magnesium (Mg) has gained considerable attention for its peculiar properties and high relevance of its compounds, such as magnesium hydroxide, Mg(OH)2. In the present work, the influence of several operating conditions on the Mg(OH)2precipitation process was thoroughly investigated by adopting a novel multiple feed-plug flow reactor. The influence of (i) initial Mg2+concentrations in the feed stream; (ii) brine and alkaline flow rates; and (iii) the product recycling strategy (seeded crystallization) was considered. The results marked the possibility of improving sedimentation and filterability properties of Mg(OH)2suspensions by adopting the recycling strategy to overcome industrial issues associated with the production of Mg(OH)2suspensions using NaOH solutions.

The implementation of effective storage systems is essential for a deeper market penetration of intermittent renewable sources. One promising, environmentally friendly energy storage technology is the Acid-Base Flow Battery (AB-FB). In the charge phase it stores electricity in the form of pH and salinity gradients via Bipolar Membrane Electrodialysis, while in the discharge phase it applies the reverse process for the opposite conversion. Despite the clear benefits over other osmotic batteries, the potential of the AB-FB has been poorly explored.

The sustainable production of freshwater from seawater desalination is receiving increasing attention. Recently, some desalination technologies are taking advantage from the coupling with renewable resources; among them, membrane distillation (MD) is one of the most promising since it can be easily powered by low-grade thermal energy. MD being an emerging technology, efforts are required to optimize geometry and operating conditions of real units in order to reduce the unitary freshwater production cost. In particular, temperature polarization is a well-known detrimental effect for the process driving force; spacers are traditionally used to enhance mixing and make temperature boundary layers thinner, at the cost of higher pressure losses. The present work is devoted to testing and comparing the performance of two different two-layer net spacers: overlapped and woven. Investigations were carried out both by experiments and by computational fluid dynamics (CFD) at different Reynolds numbers, ranging from creeping flow to turbulent flow regimes. Experiments (for intermediate to high Re) made use of thermochromic liquid crystals along with digital image processing. Computational results (for low to intermediate Re) were obtained via steady state (low Re) or direct numerical simulations (intermediate Re) along with the unit cell approach. A good agreement between experiments and CFD results was obtained in the range of superposition. Results showed that woven spacers guarantee a better mixing than overlapped ones, especially in the low to intermediate Re range, thus resulting in Nusselt numbers 2.5–3 times higher. On the other hand, the less disturbed flow field induced by overlapped spacers was found to yield friction coefficients up to 4 times lower, thus allowing lower pumping costs. The choice between the two configurations depends crucially on the relative importance attributed to savings in membrane surface area and in pumping energy for any specific application.

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 different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

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 different concentrations and flow rates for the feed streams has been considered, as well as different aspect ratios in asymmetric stacks. Moreover, the influence of the scaling-up a RED unit was investigated, starting from a 22 × 22 cm2100 cell pairs lab-stack, and simulating the performance of larger stacks up to a 44 × 88 cm2500 cell pairs unit. Finally, different scenarios are proposed for a prototype-scale RED plant, providing useful indications for the technology scale-up towards 1 kW of power production, relevant to the installation of a real prototype plant in Trapani (Italy) being the final objective of the R&D activities of the REAPower project.

AbstractThis work presents an overlook on a new model that simulates the physical operation in steady state of a multi-effect distillation (MED) plant with parallel-feed (P) configuration. This model includes the consumption of steam with steam ejectors, and its validation was done using data from a real MED industrial plant using a thermal vapor compressor (TVC) operating in Italy, in the Sicilian city of Trapani. Results show that the MED model returns accurate predictions of the plant behavior, very useful for a first analysis on such type of investments. This MED model was also integrated into the system advisor model developed by the US National Renewable Energy Laboratory. Simulations with this new tool were run using the location of Trapani as case study for a concentrating solar power (CSP) plant working in cogeneration with a low-temperature MED-P plant vs. other cooling options available for CSP plants (wet cooling, dry cooling, and a once through seawater cooling circuit). These results were compared with the existing TVC-MED plant, and indicate that CSP+MED has the potential to be economically attractive.

This work presents an overlook on a new model that simulates the physical operation in steady state of a multi-effect distillation (MED) plant with parallel-feed (P) configuration. This model includes the consumption of steam with steam ejectors, and its validation was done using data from a real MED industrial plant using a thermal vapor compressor (TVC) operating in Italy, in the Sicilian city of Trapani. Results show that the MED model returns accurate predictions of the plant behavior, very useful for a first analysis on such type of investments. This MED model was also integrated into the system advisor model developed by the US National Renewable Energy Laboratory. Simulations with this new tool were run using the location of Trapani as case study for a concentrating solar power (CSP) plant working in cogeneration with a low-temperature MED-P plant vs. other cooling options available for CSP plants (wet cooling, dry cooling, and a once through seawater cooling circuit). These results were compared with the existing TVC-MED plant, and indicate that CSP+MED has the potential to be economically attractive.

The 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 different concentrations and flow rates for the feed streams has been considered, as well as different aspect ratios in asymmetric stacks. Moreover, the influence of the scaling-up a RED unit was investigated, starting from a 22 × 22 cm2100 cell pairs lab-stack, and simulating the performance of larger stacks up to a 44 × 88 cm2500 cell pairs unit. Finally, different scenarios are proposed for a prototype-scale RED plant, providing useful indications for the technology scale-up towards 1 kW of power production, relevant to the installation of a real prototype plant in Trapani (Italy) being the final objective of the R&D activities of the REAPower project.

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 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 different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

AbstractSeawater desalination is becoming an important source of fresh water in several countries all around the world. One of the main drawbacks of desalination processes, however, is related to the disposal of large quantities of concentrated brine, which is an always-present by-product of the process. An integrated production of fresh water and salts may be achieved using the discharge brine from a desalination plant as a feed for conventional salt ponds, with the advantages of using brine more concentrated than sea water and, in the case of thermal desalination plants, warmer than sea water. By doing so, the process is faster as a consequence of the enhancement of evaporation rate on the surface of ponds. The above concept has been proposed already several years ago, but only rare examples exist of real applications. A pilot test has been performed in the last 4 years in Trapani (Italy), where a 36,000-m3/d multiple effects desalination with thermal vapour compression plant is operating very close to a traditional salt pond normally fed with sea water. Furthermore, the use of fractionated crystallisation process, typically adopted in conventional salt ponds, allows for the easy separation of salts like calcium carbonates and sulphates, sodium chloride and a final saturated brine which is extremely rich in magnesium as a sole bivalent cation. Thus, the possibility of a further exploitation of such saturated brine has been experimentally analysed by laboratory tests in order to produce high-purity magnesium to be commercialised in the pharmaceutical, food and metal industries. Results have shown a very promising enhancement of the salt pond production capacity, keeping at the same time the very high quality standards required for the production of food-grade salt from sea water. On the other side, laboratory experiments indicated the actual possibility of producing high-purity magnesium salts, thus encouraging towards further investigation for the development of a pilot process development and installation.

AbstractTechnologies 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 collected from relevant scientific literature for the simulation of the process under extreme conditions of solutions concentration. A multi-scale structure allows the simulation of the single cell pair and the entire SGP-RE stack. The first can be seen as the elementary repeating unit constituted by cationic and anionic membrane and the relevant two channels where dilute and concentrate streams flow. The reverse electro-dialysis stack is constituted by a number of cell pairs, the electrode compartments and the feed streams distribution system. The model has been implemented using gPROMS®, a powerful dynamic modelling process simulator. Experimental information, collected from the FUJIFILM laboratories in Tilburg (the Netherlands), has been used to perform the tuning of model formulation and eventually to validate model predictions under different operating conditions. Finally, the model has been used to simulate different possible scenarios and perform a preliminary analysis of the influence of some process operating conditions on the final stack performance.

Seawater desalination is becoming an important source of fresh water in several countries all around the world. One of the main drawbacks of desalination processes, however, is related to the disposal of large quantities of concentrated brine, which is an always-present by-product of the process. An integrated production of fresh water and salts may be achieved using the discharge brine from a desalination plant as a feed for conventional salt ponds, with the advantages of using brine more concentrated than sea water and, in the case of thermal desalination plants, warmer than sea water. By doing so, the process is faster as a consequence of the enhancement of evaporation rate on the surface of ponds. The above concept has been proposed already several years ago, but only rare examples exist of real applications. A pilot test has been performed in the last 4 years in Trapani (Italy), where a 36,000-m3/d multiple effects desalination with thermal vapour compression plant is operating very close to a traditional salt pond normally fed with sea water. Furthermore, the use of fractionated crystallisation process, typically adopted in conventional salt ponds, allows for the easy separation of salts like calcium carbonates and sulphates, sodium chloride and a final saturated brine which is extremely rich in magnesium as a sole bivalent cation. Thus, the possibility of a further exploitation of such saturated brine has been experimentally analysed by laboratory tests in order to produce high-purity magnesium to be commercialised in the pharmaceutical, food and metal industries. Results have shown a very promising enhancement of the salt pond production capacity, keeping at the same time the very high quality standards required for the production of food-grade salt from sea water. On the other side, laboratory experiments indicated the actual possibility of producing high-purity magnesium salts, thus encouraging towards further investigation for the development of a pilot process development and installation.

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 collected from relevant scientific literature for the simulation of the process under extreme conditions of solutions concentration. A multi-scale structure allows the simulation of the single cell pair and the entire SGP-RE stack. The first can be seen as the elementary repeating unit constituted by cationic and anionic membrane and the relevant two channels where dilute and concentrate streams flow. The reverse electro-dialysis stack is constituted by a number of cell pairs, the electrode compartments and the feed streams distribution system. The model has been implemented using gPROMS®, a powerful dynamic modelling process simulator. Experimental information, collected from the FUJIFILM laboratories in Tilburg (the Netherlands), has been used to perform the tuning of model formulation and eventually to validate model predictions under different operating conditions. Finally, the model has been used to simulate different possible scenarios and perform a preliminary analysis of the influence of some process operating conditions on the final stack performance.

AbstractSalinity 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 optimization really represents the main factor affecting the overall process performance. In the present work, realized within the EU-FP7 funded REAPower project, 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 optimization of feed and blowdown distribution systems. Spacer material and morphology can affect the fluid dynamics inside each channel. In particular, the appropriate choice of net spacer material can influence the slip/no-slip condition of the flow on the spacer wires, thus significantly affecting the channel fluid dynamics in terms of pressure drops. A Unit Cell approachwas adopted to investigate the effect of the different choices on the fluid flow along the channel. Also, the possibility of choosing a porous mediumto substitute the net spacer was theoretically addressed. Such investigation focused on the porosity and the fiber radius required to respect the process constrains of pressure drops and mechanical stability. On the other hand, the overall pressure drops of a SGP-RE or ED stack can be considered as resulting from different contributions: the pressure drop relevant to the feed distributor, the pressure drop inside the channel, and the pressure drop in the discharging collector. The choice of the optimal stack geometry is, therefore, strongly related to the need of both minimizing each of the above terms and obtaining the most uniform feed streams distribution among the stack channels. In order to investigate such aspects, simulations were performed on a simplified ideal planar stack with either 50 spacer-less or 50 spacer-filled channels. The effect of the distribution/collector channel thickness and geometry on single-channel flow rates and overall pressure drops in the system was analyzed and a significant influence of distributor layout and size on the overall process performance was found.