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This Chapter treats the application of CFD technique in ED/RED processes. CFD simulations are able to predict flow and mass transfer in channels filled with spacers or membrane profiles, by providing dimensionless correlations useful for process models. At a larger scale, manifolds of the stack and inlet/ outlet zones of the channel can be simulated. Numerical models can include the computation of the electrical potential, thus characterizing the process performance. Direct numerical simulations can elucidate the complex phenomenology of overlimiting transport and of chaotic electrokinetic flows.

The BMED technology can be used for the production or the regeneration of acid and base solutions by water dissociation. BMED has practical applications in several fields including in chemical and biochemical industry, environmental protection and, recently, even for brine valorisation. Many are the irreversibility sources involved in the BMED process. Some of them are related to the non-ideal transport properties of the Ion Exchange Membranes causing undesired co-ion leakages and water flux. Furthermore, the presence of shunt currents through the manifolds (i.e. distributors and collectors of the solutions) decrease the current efficiency. The aim of this work is to give a quantitative description of the main phenomena involved in BMED processes by a multi-scale model. Four different dimensional scales were effectively combined within the gPROMS platform. The lowest scale simulates the channels and includes two sub-models. CFD simulations are devoted to the estimation of concentration polarization phenomena and pressure losses while the other channel sub-model evaluates the physical properties of the solutions. The middle-low level simulates the triplets, combining mass balance equations and fluxes through the membranes. Moreover, this model scale calculates electrical variables as the cell resistance and the electromotive force. Channel and triplet scales were implemented with 1-D discretization of the variables. The middle-high level is the stack model. It includes two sub-models: one predicts the shunt currents through the manifolds and the other one calculates the pressure losses in the whole stack. The highest level is the external circuit one, which computes the external pressure losses and the transient behaviour of closed-loop layouts. This multi-scale model was validated both by experiments with a laboratory test-rig unit and data already presented in literature. A sensitivity analysis was performed to gather useful information about the system performance by comparing current efficiency and net power density in different configurations. Simulation results show the effect of the operating variables (e.g. current density and mean flow velocity) and system geometry, highlighting the importance of the manifolds characteristics for the process efficiency. The thoroughness of this mathematical model provides a powerful tool for the design of BMED units.

Background Electro-membrane processes are gaining great interest in the field of desalination and brine valorisation. However, limiting current phenomena can be a bottleneck for their techno-economic performances. In the present work, the in-out distribution of current density is measured to elucidate the achievement of limiting conditions in real stacks. Materials and Methods A 10-cell pairs Electrodialysis stack (10×40 cm2 active area), equipped with four-segmented electrodes, was tested. NaCl solutions at an inlet concentration ranging from 0.5 to 60 g/l were fed at velocities of either 2 or 4 cm/s in parallel flow. Current density-voltage curves were built by applying equal increasing steps of voltage to each electrode. Outlet concentrations and current efficiency were investigated [1]. Results Figure 1 shows the current density-voltage curves for two couples of inlet concentration. Between the final tract of the ohmic region and the plateau region of the overall stack curve, the current density distribution at the four segments changes markedly. In fact, while at the first electrode the current density continues to increase, at the other three it reaches a maximum and decreases. Thus, as the voltage increases, the current concentrates in a shorter tract of the channel, while it reduces in the remaining part, becoming ineffective for desalination, due to its high resistance. This is caused by the high desalination rate in the first few centimetres, making the dilute conductivity much lower. Moreover, the longer tract of channels at high salinity gradient in the final part of the stack promotes larger diffusion, lowering the current efficiency [2]. Figure 1. Current density-voltage curves for tests at a) Cdil,IN=0.5 g/l and Cconc,IN=30 g/l and b) Cdil,IN=Cconc,IN=1 g/l. Conclusions The attainment of limiting conditions in electrodialysis stacks is strongly related to ohmic phenomena and to the distribution of current density, highlighting its importance in the design of efficient electro-membrane systems. Acknowledgments This work was supported by the SEARcularMINE (Circular Processing of Seawater Brines from Saltworks for Recovery of Valuable Raw Materials) project – Horizon 2020 programme, Grant Agreement no. 869467. The authors are grateful to REDstack B.V. and Fujifilm Manufacturing Europe B.V. for supplying stack and membranes, respectively.

Electrodialysis (ED) is an electro-driven process that makes use of ion exchange membranes (IEMs) under an applied electric field. The main application of ED is the desalination for drinking water production. A transmembrane pressure (TMP) distribution may arise in ED stacks due to an uneven pressure distribution in the two fluid channels, thus causing membrane/channel deformation and flow redistribution. This can occur in large-scale non-parallel configurations, e.g. crossflow arrangements. Detrimental effects of membrane deformation have widely been studied with reference to several membrane processes. However, this aspect has been neglected in ED applications. In this work, a novel process model of ED units including the effects of membrane deformation is presented. The model was developed with a multi-scale architecture. The model simulates the fluid-structure interaction (i.e. membrane deformation and flow redistribution) in a cell pair based on correlations from small-scale numerical simulations (structural mechanics and computational fluid dynamics). Then, transport and electrochemical phenomena occurring in ED systems are simulated. Cross-flow ED units were simulated in two-dimensions (length and width). Results showed that mild deformations have a negligible impact on the ED process performance. However, configurations prone to larger deformations (e.g., thin membranes) exhibited more significant effects, with an increase in the specific energy consumption. The same approach can be used for other configurations (e.g. counter-flow and asymmetrical channels in parallel flow) and for reverse electrodialysis.

In the membrane processes, a trans-membrane pressure (TMP) may arise due to design features or operating conditions. In most applications, stacks for electrodialysis (ED) or reverse electrodialysis (RED) operate at low TMP (&lt, 0.1 bar), however, large stacks with non-parallel flow patterns and/or asymmetric configurations can exhibit higher TMP values, causing membrane deformations and changes in fluid dynamics and transport phenomena. In this work, integrated mechanical and fluid dynamics simulations were performed to investigate the TMP effects on deformation, flow and mass transfer for a profiled membrane-fluid channel system with geometrical and mechanical features and fluid velocities representative of ED/RED conditions. First, a conservatively high value of TMP was assumed, and mechanical simulations were conducted to identify the geometry with the largest pitch to height ratio still able to bear this load without exhibiting a contact between opposite membranes. The selected geometry was then investigated under expansion and compression conditions in a TMP range encompassing most practical applications. Finally, friction and mass transfer coefficients in the deformed channel were predicted by computational fluid dynamics. Significant effects of membrane deformation were observed: friction and mass transfer coefficients increased in the compressed channel, while they decreased (though to a lesser extent) in the expanded channel.

The need for unconventional sources of fresh water is pushing a fast development of desalination technologies, which proved to be able to face and solve the problem of water scarcity in many dry areas of the planet. Membrane desalination technologies are nowadays leading the market and, among these, electrodialysis (ED) plays an important role, especially for brackish water desalination, thanks to its robustness, extreme flexibility and broad range of applications. In fact, many ED-related processes have been presented, based on the use of Ion Exchange Membranes (IEMs), which are significantly boosting the development of ED-related technologies. This paper presents the fundamentals of the ED process and its main developments. An important outlook is given to operational aspects, hydrodynamics and mass transport phenomena, with an extensive review of literature studies focusing on theoretical or experimental characterization of the complex phenomena occurring in electromembrane processes and of proposed strategies for process performance enhancement. An overview of process modelling tools is provided, pointing out capabilities and limitations of the different approaches and their possible application to optimisation analysis and perspective developments of ED technology. Finally, the most recent applications of ED-related processes are presented, highlighting limitations and potentialities in the water and energy industry.

The present work describes the development of the first dynamic model for CED

In this work, we developed an advanced model useful for the design of profiled IEMs, based on the coupled simulation of local mechanical deformations and of fluid dynamics and associated mass transport phenomena within deformed channels

The present work aimed to verify the feasibility of producing polyhydroxyalkanoates (PHAs) at high concentrations from an alternative and cheap carbon source such as the carboxylic acids (VFAs) obtained by the anaerobic fermentation of cheese whey (CW). An electrodialysis (ED) step was proposed for the obtainment of a concentrated acidic stream, suitable for being employed as feeding solution in the consecutive fed-batch culture system for PHAs production. Experiments conducted in duplicate shown that a packed bed anaerobic bioreactor resulted a robust and high repeatable culture system for a high performing continuous production of VFAs from CW. The acidogenic effluent contained 12.55 ± 1.10 g L−1 of carboxylic acids (ca. 90% of the effluent soluble COD), including the hexanoic (4.13 ± 0.56 g L−1) and octanoic (3.12 ± 0.94 g L−1) acids. The application of 7 sequential batch ED processes allowed to obtain a carboxylic acids concentrated stream (ca. 63 g L−1); by the achievement of high molar flow. Finally, this stream was utilized as the feeding solution in a fed-batch fermentation aimed to produce PHAs. The attained PHAs yield (ca. 0.60 gPHAs gVFAs−1) was comparable to that obtained in a parallel test accomplished with a VFAs-water simulating solution and also to those previously reported from pure VFAs; i.e. no inhibition effects due to the employment of an actual biowaste as the feedstock were detected. As a whole, the results allow to conclude that the proposed integrated valorisation scheme fed with CW for the production of PHAs by an ED-concentrated carboxylic acids solution is technically feasible and robust.

In recent years, water and energy supply issues have boosted a noticeable interest in the scientific community on electromembrane processes such as electrodialysis and reverse electrodialysis. In order to gain an important place in the industrial market, technological challenges on various aspects are involved for the optimization of these processes. In this context, profiled membranes exhibit interesting performances and offer countless geometric alternatives. However, the mechanical behavior of the membranes and its interaction with fluid dynamics has been poorly investigated so far. In membrane-based processes, a trans-membrane pressure (Ptm) between the different solutions flowing through a module may be a design feature or may arise for various reasons, including flow arrangement and differences in physical properties, flow rate or friction coefficient. This leads to local deformations of membranes and channels, affecting flow and mass transfer characteristics, thus causing uneven distributions of flow and mass fluxes, which worsen the process performance. In this work, we developed an integrated model for the numerical simulation of local mechanical deformations and of fluid dynamics and associated mass transport phenomena inside deformed channels. Two diverse profiled membrane types (“overlapped cross filaments”, OCF, and “round pillars”, RP) were simulated under conditions representative of (reverse) electrodialysis and under the assumption of perfectly elastic behaviour. 3-D simulations of a couple of membranes and of the interposed fluid were conducted by the unit cell approach (periodic domain). The Ansys Mechanical 18 (Workbench) and the Ansys CFX 18 software was used. The selected geometries were simulated under Ptm ranging from -0.4 to +0.4 bar, computing expanded and compressed configurations. Then, CFD simulations of the deformed channels were performed, showing significant effects of the deformation on fluid flow and mass transfer. The influence of Ptm was to increase friction under compression conditions (up to ∼2.2-2.5 times) and to reduce it under expansion conditions (but to a lesser extent, i.e. up to ∼50-60%). Overall, compression enhanced mass transfer and expansion reduced it, but with smaller and more complex effects than on friction. The influence of the flow attack angle was negligible for friction, but more significant for mass transfer. In future works the same simulation approach will be adopted in order to compute also the Ohmic resistance in deformed configurations. The simulation results will be implemented in the form of correlations into higher-scale models, in order to study distributions of flow, mass transfer and Ohmic resistance in whole channels. The method proposed can be extended to other membrane applications with minor modifications.

The future energetic supply based on the massive use of renewable sources poses issues linked to fluctuations of power produced and consumed, thus requiring the use of energy storage systems.

The REvivED water project aims to contribute to overcoming the drinking water challenge through desalination technology. The goal is to produce safe and affordable drinking water with a significantly reduced energy consumption compared to the current state-of-the-art technology. The overall project comprises several systems and applications with twelve pilots in total, ranging from Electrodialysis (ED) small systems for brackish water desalination to larger scale hybrid (RED/ED-RO) systems for sea water desalination. Specific attention is devoted to develop energy efficient and robust brackish water desalination systems for application in developing countries. To reduce the energy consumption of the desalination process, several design parameters have been optimized, e.g., membranes and spacers characteristics, stack configurations, flow path length. To assure the system robustness, several concepts have been investigated, including pre-treatment options, capacitive electrodes, one pass flow as well as gravity driven flow. Intensive modelling and experimental validation are being carried out for the process and system design. The first brackish water desalination results demonstrate that up to a conductivity of ~4.000 µs/cm the project energy consumption target of 0.4 kWh/m3 can be achieved.

In electro-membrane processes, a pressure difference may arise between solutions flowing in alternate channels. This transmembrane pressure (TMP) causes a deformation of the membranes and of the fluid compartments. This, in turn, affects pressure losses and mass transfer rates with respect to undeformed conditions and may result in uneven flow rate and mass flux distributions. These phenomena were analyzed here for round pillar-type profiled membranes by integrated mechanical and fluid dynamics simulations. The analysis involved three steps: (1) A conservatively large value of TMP was imposed, and mechanical simulations were performed to identify the geometry with the minimum pillar density still able to withstand this TMP without collapsing (i.e., without exhibiting contacts between opposite membranes), (2) the geometry thus identified was subject to expansion and compression conditions in a TMP interval including the values expected in practical applications, and for each TMP, the corresponding deformed configuration was predicted, and (3) for each computed deformed configuration, flow and mass transfer were predicted by computational fluid dynamics. Membrane deformation was found to have important effects, friction and mass transfer coefficients generally increased in compressed channels and decreased in expanded channels, while a more complex behavior was obtained for mass transfer coefficients.

The performances of reverse electrodialysis depend on several factors, including the nature of the electrode material and of the redox processes adopted to make possible the conversion between chemical potential and electric power. In this paper the possible utilization of various redox processes (reduction/oxidation of iron species, oxidation and reduction of water, oxidation of chlorine and reduction of water) was studied in a stack equipped with 10–50 cell pairs and by focused electrolyses in a three compartment cell. The effect of selected redox processes on power density output and eventual contamination of saline solutions flowing in the stack was evaluated in detail. The effect of the number of cell pairs and of the concentration of saline solutions was also investigated.

In recent years, thanks to the development of ion exchange membranes (IEMs) manufacturing industry, Electrodialysis (ED) is spreading as a viable alternative to the more common membrane desalination processes. Therefore, many research efforts have been recently devoted to studying this process both via experimental and modelling activities. In the present work a novel mathematical model for ED was developed using a multi-scale approach. This method allows to build a hierarchical simulation tool that is able to gauge the impact of all the phenomena involved in the process. The lower-hierarchy model describes the behaviour of the elementary unit of an ED stack, namely cell pair. This model is based on differential mass balance equations and accounts for transport phenomena including salt migration and diffusion as well as water osmosis and electro-osmosis. In addition, Kirchhoff’s law together with the Nernst’s law for the non-Ohmic voltage drop was used to determine the electrical behaviour of the equivalent circuit. Interestingly, the model makes also use of CFD correlations from a lower scale as input data in order to predict the effect of concentration polarization. In the higher-hierarchy model the whole stack was described, allowing the simulation of multiple cell-pairs together with the end electrode compartments. Again, CFD correlations were used to include the contribution of pumping loss in the overall energy consumption. The model was implemented in PSE gPROMS Modelbuilder. Several simulations were carried out by changing flow arrangement (co-current, counter-current and cross-flow), operating conditions and stack features. Results showed that the model is able to reliably predict the effect of these variables on the performance of the ED unit investigated. In particular, the model was found capable of estimating the distribution of current density/voltage and concentration along the channels. In addition, stack resistance, overall and specific energy requirements were computed. Collected findings suggest that the model proposed might be a powerful tool to improve and optimise ED process.

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 gross and net power density. The influence of various site-dependent parameters are investigated, identifying the optimal operating conditions for different feed streams. The model shows that different optimal operating conditions can be identified, according to the specific conditions of the feeds and the target of the process. In particular, lower fluid velocities are preferable to maximize the net power density, while larger stack size leads to higher process yields. Finally, optimal stack designs are discussed, showing the effect of different technological strategies on the large-scale power production.

Adsorption, chromatography-based techniques, nanofiltration (NF), and electrodialysis (ED) are con- solidated operations that could be applied in the fourth stage of the Universal Recovery Process (Chap- ter 3). Since engineering aspects of such conventional techniques are well known, they could be applied with a large range of raw materials, depending on the chemical/physical compositions of such matri- ces. Recovery yields generally depend on the concentration of target molecules and on their relative amounts with respect to those compounds having similar features (size, charge, chemical structure, etc.). The latter compounds compete with the former ones within the recovery processes. This chapter is dedicated to briefly describing the main features of the above-mentioned technologies and proposes practical suggestions for the development of processes aimed at the selective recovery of chemicals from wastes. Furthermore, a number of case studies and recent works are presented.

As resources of both water and traditional fuels are becoming increasingly limited, the need for desalination technologies that can operate using sustainable energy is growing. This chapter looks at the major ways in which solar energy, which is particularly promising in this respect, can be integrated with desalination technologies. The chapter first examines the tested desalination techniques of reverse osmosis and (reverse) electrodialysis, which can be powered using photovoltaic systems, and discusses their mechanisms as well as their advantages and disadvantages. The second main focus is on types of solar thermal collectors and their use with membrane distillation technology, which is a less well established but very promising desalination method. © 2014 Woodhead Publishing Limited All rights reserved.

The performances of electrodialysis (ED) and reverse electrodialysis (RED) processes depend on several factors, including the nature of the electrode material and of the redox couple adopted to make possible the conversion between electric power and chemical potential. In this paper, the possible utilization of iron-based redox couples (FeCl3/FeCl2, hexacyanoferrate(III)/hexacyanoferrate(II) and Fe(III)-EDTA/Fe(II)-EDTA) on graphite and DSA electrodes for RED processes was studied by a detailed experimental investigation. The hexacyanoferrate(III)/hexacyanoferrate(II) system was stable for lonf time (more than 12 days) in the absence of light and oxygen at high redox couple concentrations and low current densities both at compact graphite and DSA electrodes. Perfluorinated Nafion cationic membranes were found to be impermeable to the components of the redox couple. Fe(II)-EDTA exhibited a limited electrochemical stability in long term electrolyses at all adopted operative conditions, which discourages the use of the Fe(III)-EDTA/Fe(II)-EDTA for RED applications. The FeCl3/FeCl2 system was, on the other hand, stable for long times (more than 12 days) at acidic pH at compact graphite electrodes. Selemion anionic membranes allowed to confine the redox couple in the electrode compartments with very slow passage of protons to the side compartment (“dilute or concentrated compartment”).

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 mainly composed of hexahedral volumes was found to be more reliable and less computational demanding than tetrahedral grids. As concerns the dependence of the pressure drops on the flow rate, the empty channel was found to guarantee the lowest pressure drops at a given fluid flow rate, as expected. Conversely, the woven spacer filled channel was found to provide larger pumping costs. The pressure drops along the channel filled with a porous medium even at low flow rate were very high thus suggesting that this specific configuration may be unsuitable and that further investigations should be devoted to this topic.

A crucial parameter for the design and operation of electrodialysis (ED) units is the limiting current density (LCD). This is often identified with the diffusion-limited current density, which corresponds to the complete solute depletion in the layer adjacent to the membrane. Current-voltage curves obtained from measurements with electrodes in contact with the solution (i.e. without membranes) are consistent with this interpretation and exhibit a horizontal plateau identifying LCD. However, real ED systems show more complex behaviours, with a reduced-slope tract instead of a plateau and a third region in which the current increases more markedly (overlimiting current). The phenomena involved in the limiting region are not yet totally characterized and the determination of LCD in ED units is still ambiguous. In the present work, we explore the issues related to the identification of LCD, by measurements on ED units, assessing the influence of operating conditions and validating a simplified process simulator. A new method to determine LCD, based on the current efficiency, is proposed and compared with other methods presented in the literature. A second limiting quantity is also identified, i.e. the critical current density, below which diffusion phenomena prevail on migration and a method for its assessment is proposed.

In recent years, water and energy supply issues have drawn the attention of the scientific community to electromembrane processes. Electrodialysis (ED) and Reverse Electrodialysis (RED) are two of the most attractive electromembrane technologies for water desalination and electric energy production from salinity gradients, respectively. In order to gain an important place in the industrial market, technological challenges on various aspects are involved in the optimization of these processes. In this context, profiled membranes exhibit interesting performance. However, the mechanical behavior of the membranes and its interaction with fluid dynamics has been poorly investigated so far. In membrane-based processes, a trans-membrane pressure (TMP) between the different solutions flowing through a module may be a design feature or may arise for various reasons (e.g. flow arrangement, differences in physical properties). This may lead to a local deflection of membranes due to their low mechanical stiffness. As a result, the channel geometry may be modified affecting flow and mass transfer characteristics. In this work, we developed an integrated model for the numerical simulation of local mechanical deformation and of fluid dynamics and associated mass transport phenomena inside deformed channels. Profiled membranes with Round Pillars were simulated under typical REDED working conditions. Membranes were assumed to be with perfectly linear elastic behaviour. 3-D simulations of a couple of membranes and of the interposed fluid were conducted by the unit cell approach (periodic domain). The Ansys Mechanical 18 (Workbench) and the Ansys CFX 18 software was used. The profiled membranes were investigated in both expanded and compressed configurations by varying the TMP of 0.1 bar steps from -0.4 to +0.4 bar. Then, CFD simulations of the deformed channels were performed. The influence of TMP was to increase friction under compression conditions (up to 2.2 times) and to reduce it under expansion conditions (i.e. up to 60%). Overall, compression enhanced mass transfer and expansion reduced it, but with smaller and more complex effects than on friction. Channel deformations affect largely fluid dynamics and mass transfer characteristics. Even mild trans-membrane pressures may produce significant variations in the overall process performance. Results will be implemented in a novel higher-scale simulation tool for studying the distribution of flow in whole channels.

In water desalination by electrodialysis, the current density i cannot exceed specific constraints, notably the diffusion limit. Working at higher i (overlimiting current regime) would make higher desalination rates possible. The main phenomenon allowing overlimiting current densities is the electrokinetic instability that arises when a sufficiently intense electric potential gradient is imposed, and leads to electroconvective mixing in the near-wall layer. In this study, these phenomena were investigated by CFD. The governing equations were the Nernst-Planck transport equations for anions and cations, the Poisson equation for the electrical potential and the Navier-Stokes and continuity equations for fluid motion (NPP-NS approach). Time-dependent simulations were conducted both in 2-D and 3-D domains for different imposed potential gradients. Computational stability was obtained only by using very small time steps, typically 10-8~10-7 s. Starting from rest, statistically stationary conditions, characterized by intense vortices a few mm in size, were attained after times of the order of milliseconds. In 3-D simulations, coherent structures reminiscent of Rayleigh-Bénard convection were predicted. Current densities well above the diffusion limit were achieved.