Electrodialysis Stack Research Articles

A new reverse electrodialysis design strategy which significantly reduces the levelized cost of electricity

We develop a framework for choosing the optimal load resistance, feed velocity and residence time for a reverse electrodialysis stack based on minimizing the levelized cost of electricity. The optimal load resistance maximizes the gross stack power density and results from a trade-off between stack voltage and stack current. The primary trade-off governing the optimal feed velocity is between stack pumping power losses, which reduce the net power density and concentration polarization losses, which reduce the gross stack power density. Lastly, the primary trade-off governing the optimal residence time is between the capital costs of the stack and pretreatment system. Implementing our strategy, we show that a smaller load resistance, a smaller feed velocity and a larger residence time than are currently proposed in the literature reduces costs by over 40%. Despite these reductions, reverse electrodialysis remains more expensive than other renewable technologies.

Electrodialytic upgrading of three different municipal solid waste incineration residue types with focus on Cr, Pb, Zn, Mn, Mo, Sb, Se, V, Cl and SO4

Handling of air pollution control (APC) residues from municipal solid waste incineration (MSWI) is a challenge due to its toxicity and high leaching of toxic elements and salts. Electrodialysis (ED) of the material has shown potential for reduction of leaching of toxic elements and salts to produce a material feasible for substitution of cement in mortar. In this work results of 23 pilot-scale experiments (5–8kg APC residue each) in electrodialysis stack designed to investigate the leaching properties as a function of time and current density for APC residue from semi-dry and wet flue-gas cleaning systems, as well as MSWI fly ash without flue-gas cleaning products are reported. Significant leaching reduction of the critical elements Pb, Zn and Cl was obtained. The final leaching, however, depended mostly on the initial leaching, thus as leaching from fly ash and residue of wet flue-gas cleaning was lower before treatment compared to residues from semidry flue-gas cleaning, both Pb and Zn leaching could be reduced to lower levels in those materials, and they therefore appear more suitable for use in construction materials. The leaching reduction of Zn and to some degree Pb decreased with longer retention times and higher current densities. Cr and SO4 leaching increased during ED treatment, with lower increase at higher current. Washing or carbonation in combination with ED significantly reduced leaching of Pb and Zn from semidry residue. An indication of a similar effect to carbonation by simultaneous aeration with ED was observed and should be investigated further. While Mn and Mo leaching did not, Se, V and Sb leaching exceeded threshold values in semidry residue. The leaching of V seemed to increase while Se and Sb remained more or less constant during ED treatment.

Hydrogen production from continuous flow, microbial reverse-electrodialysis electrolysis cells treating fermentation wastewater

A microbial reverse-electrodialysis electrolysis cell (MREC) was used to produce hydrogen gas from fermentation wastewater without the need for additional electrical energy. Increasing the number of cell pairs in the reverse electrodialysis stack from 5 to 10 doubled the maximum current produced from 60A/m3 to 120A/m3 using acetate. However, more rapid COD removal required a decrease in the anolyte hydraulic retention time (HRT) from 24 to 12h to stabilize anode potentials. Hydrogen production using a fermentation wastewater (10 cell pairs, HRT=8h) reached 0.9±0.1L H2/Lreactor/d (1.1±0.1LH2/g-COD), with 58±5% COD removal and a coulombic efficiency of 74±5%. These results demonstrated that consistent rates of hydrogen gas production could be achieved using an MREC if effluent anolyte COD concentrations are sufficient to produce stable anode potentials.

Increasing the power density and reducing the levelized cost of electricity of a reverse electrodialysis stack through blending

We increase the power density of a reverse electrodialysis (RED) stack by blending the low salinity feed with a higher salinity stream before the stack entrance. This lowers the capital cost of the system and the resulting levelized cost of electricity, enhancing the viability of RED renewable energy generation. Blending increases the power density by decreasing the dominating electrical resistance in the diluate channel as well as the effective resistance caused by concentration polarization, but not without sacrificing some driving potential. To quantify this trade-off and to evaluate the power density improvement blending can provide, a one-dimensional RED stack model is employed and validated with experimental results from the literature. For a typical stack configured with a feed velocity of 1cm/s, power density improvements of over 20% and levelized cost of energy reductions of over 40% are achievable, provided the salinity of the available river water is below 200ppm. Additional cost reductions are realized through back-end blending, whereby the diluate exit stream is used as the higher salinity blend stream. Also, improvements from blending increase for higher feed velocities, shorter stack lengths, and larger channel heights.

Effect of pulsed electric field on electrodialysis of a NaCl solution in sub-limiting current regime

In spite of growing applications of pulsed electric fields (PEF) in membrane separation processes, there are only few theoretical studies treating mass transfer. In this paper, we propose a 1D mathematical model, which adequately describes PEF mode electrodialysis (ED) with ion-exchange membranes at sub-limiting currents. The results of simulation are compared with experimental data obtained for a laboratory-scale electrodialysis stack. We show experimentally and theoretically that the average current density and mass transfer under PEF of a sufficiently high frequency (>1Hz in the used conditions) are higher than those in conventional steady state DС mode, if a same average voltage is applied. The advantage increases with frequency and reaches a maximum at about 100Hz. When applying a pulse after a pause, we benefit by low ohmic resistance and low diffusion potential drop caused by partial concentration restoration in close vicinity of the membrane. This allows passage of an instantaneous current of a high density, which can essentially exceed the limiting current density (jlim) in steady state DC conditions. However, at low frequencies this gain rapidly vanishes by increasing concentration polarization during the pulse, thereby the mass transfer in PEF mode is lower than that in DС mode. The gain is close to zero at low currents due to linearity of I–V curve and increases with increasing current. However, within the model, the average current density cannot be higher than ilim. The gain in mass transfer rate at high frequencies increases with decreasing duty cycle, but with this the energy consumption for electrodialysis desalination in PEF mode increases, it is always higher than the energy consumption in continuous DС mode.

C232 電気透析における流れ場の影響(一般講演(7))

Electrodialysis using ion-exchange membranes is a means of producing dilute and condensate under the influence of an applied electric potential difference of saltwater. In this study, we have investigated the transport process of ions in the electrodialysis stack under the influence of an applied electric field. Under a limiting current density, the ion concentration totally vanishes at the exit of the diluate channel so that the electric resistance goes infinity toward the diluate side of the cation exchange membrane, which is referred to as concentration polarization. The effect of the flow field on this limiting current density, which is one of the most important design parameters in the electrodialysis stacks, have been investigated by measurement of the electric resistance and concentration.

High power density of reverse electrodialysis with pore-filling ion exchange membranes and a high-open-area spacer

In the present study, a novel reverse electrodialysis (RED) stack with ultrathin lab-made pore-filling membranes and a high-open-area spacer was proposed to enhance the gross power density.

Influence of water content on properties of a heterogeneous bipolar membrane

The heterogeneous ion-exchange membranes consist of polymer matrix and ion-exchange resin powder. The goal of this work was to compare two types of the heterogeneous bipolar membranes made of powdered ion-exchange resin with different water content. The water content influences the resistance of layers and the kinetics of water dissociation. The properties of heterogeneous bipolar ion-exchange membranes were compared due to the current–voltage curve. The evaluated parameters are the limiting current density, the efficiency of the water dissociation, and the resistance below the potential for water splitting. Finally, the bipolar membranes were tested in the electrodialysis stack. The membranes with higher water content had about 25% higher performance at the stack.

Ion-exchange membrane reinforcing

Sufficient mechanical property of ion-exchange membrane is a key factor for its successful implementation to electrodialysis (ED) stack. Although the ED is not a pressure driven process, the membrane need to withstand some small overpressure and also manipulation during stack assembling. In this work, the use of short polypropylene and glass fibers as low cost alternative to common woven or nonwoven fabric was studied. We confirmed that short polypropylene fibers rapidly increase the mechanical strength in machine direction (MD) even at very low concentration.

Green Production of Ultrahigh-Basicity Polyaluminum Salts with Maximum Atomic Economy by Ultrafiltration and Electrodialysis with Bipolar Membranes

Ultrahigh-basicity polyaluminum, an important aluminum industrial product, is manufactured by the oxidation of metallic Al with acids. The process is not economically and chemically efficient because the aluminum atoms were reduced first from alumina to aluminum metal and then oxidized again to get the polyaluminum. In this work, an integrated synthesis system has been built to produce ultrahigh-basicity polyaluminum salts, including polyaluminum chloride, sulfate, and nitrate, from aluminum hydroxide and corresponding acids directly at normal temperature and pressure. Ultrafiltration and electrodialysis with bipolar membranes work in tandem during the synthesis. Aluminum salts are produced naturally from aluminum hydroxide solids and acids in a mixing tank, which are separated as pure solutions via ultrafiltration and then basified in an electrodialysis stack with bipolar membranes to get polyaluminum and regenerate acids as byproducts. These acids are sent back to the mixing tank for reuse again. This i...

Patterned ion exchange membranes for improved power production in microbial reverse-electrodialysis cells

Power production in microbial reverse-electrodialysis cells (MRCs) can be limited by the internal resistance of the reverse electrodialysis stack. Typical MRC stacks use non-conductive spacers that block ion transport by the so-called spacer shadow effect. These spacers can be relatively thick compared to the membrane, and thus they increase internal stack resistance due to high solution (ohmic) resistance associated with a thick spacer. New types of patterned anion and cation exchange membranes were developed by casting membranes to create hemispherical protrusions on the membranes, enabling fluid flow between the membranes without the need for a non-conductive spacer. The use of the patterned membrane decreased the MRC stack resistance by ∼22 Ω, resulting in a 38% increase in power density from 2.50 ± 0.04 W m−2 (non-patterned membrane with a non-conductive spacer) to 3.44 ± 0.02 W m−2 (patterned membrane). The COD removal rate, coulombic efficiency, and energy efficiency of the MRC also increased using the patterned membranes compared to the non-patterned membranes. These results demonstrate that these patterned ion exchange membranes can be used to improve performance of an MRC.

Methane production in microbial reverse-electrodialysis methanogenesis cells (MRMCs) using thermolytic solutions.

The utilization of bioelectrochemical systems for methane production has attracted increasing attention, but producing methane in these systems requires additional voltage to overcome large cathode overpotentials. To eliminate the need for electrical grid energy, we constructed a microbial reverse-electrodialysis methanogenesis cell (MRMC) by placing a reverse electrodialysis (RED) stack between an anode with exoelectrogenic microorganisms and a methanogenic biocathode. In the MRMC, renewable salinity gradient energy was converted to electrical energy, thus providing the added potential needed for methane evolution from the cathode. The feasibility of the MRMC was examined using three different cathode materials (stainless steel mesh coated with platinum, SS/Pt; carbon cloth coated with carbon black, CC/CB; or a plain graphite fiber brush, GFB) and a thermolytic solution (ammonium bicarbonate) in the RED stack. A maximum methane yield of 0.60 ± 0.01 mol-CH4/mol-acetate was obtained using the SS/Pt biocathode, with a Coulombic recovery of 75 ± 2% and energy efficiency of 7.0 ± 0.3%. The CC/CB biocathode MRMC had a lower methane yield of 0.55 ± 0.02 mol-CH4/mol-acetate, which was twice that of the GFB biocathode MRMC. COD removals (89-91%) and Coulombic efficiencies (74-81%) were similar for all cathode materials. Linear sweep voltammetry and electrochemical impedance spectroscopy tests demonstrated that cathodic microorganisms enhanced electron transfer from the cathode compared to abiotic controls. These results show that the MRMC has significant potential for production of nearly pure methane using low-grade waste heat and a source of waste organic matter at the anode.

The benefits of hybridising electrodialysis with reverse osmosis

A cost analysis reveals that hybridisation of electrodialysis with reverse osmosis is only justified if the cost of water from the reverse osmosis unit is less than 40% of that from a stand-alone electrodialysis system. In such cases the additional reverse osmosis costs justify the electrodialysis cost savings brought about by shifting salt removal to higher salinity, where current densities are higher and equipment costs lower. Furthermore, the analysis suggests that a simple hybrid configuration is more cost effective than a recirculated hybrid, a simple hybrid being one where the reverse osmosis concentrate is fed to the electrodialysis stack and the products from both units are blended, and a recirculated being one hybrid involving recirculation of the electrodialysis product back to the reverse osmosis unit. The underlying rationale is that simple hybridisation shifts salt removal away from the lowest salinity zone of operation, where salt removal is most expensive. Further shifts in the salinity at which salt is removed, brought about by recirculation, do not justify the associated increased costs of reverse osmosis.

Fractioning electrodialysis: a current induced ion exchange process

In desalination often multi ionic compositions are encountered. A preferential removal of multivalent ions over monovalent ions can be of interest to prevent scaling in the desalination process. Recently, a novel fractionating electrodialysis stack is described by Zhang et al., 2012 (in Sep. purify. Technol. 88). In the present work a small modification to such a stack was made, to create a current induced ion exchange process, in which no longer desalination occurs. This was done by building a membrane stack in which monovalent-selective ion-exchange membranes and standard grade ion-exchange membranes, with similar charge sign (so either anion or cation exchange groups), were placed alternatingly between an anode and a cathode to form a membrane stack. A proof of principle of the fractioning electrodialysis technology is given. Ternary mixtures, with a divalent-monovalent ion ratio similar to seawater, were used as feed water. For a cation and an anion fractioning stack, maximum fractionations of divalent ions were obtained of approximately 90 and 60%. At higher applied current density, ions can be fractionated to a larger extent than at lower applied current density. For both stacks the water recovery was ∼50%. Coulombic efficiency of both processes decrease rapidly after the start of the experiment. This leads to relatively large volumetric energy consumptions.

Production of Lithium Hydroxide from Lake Brines through Electro–Electrodialysis with Bipolar Membranes (EEDBM)

To investigate the feasibility of a novel method to produce LiOH from lithium contained brine, A lab-scale electro–electrodialysis with bipolar membrane (EEDBM) was installed with an arrangement of bipolar membrane–cation exchange membrane–bipolar membrane–cation exchange membrane in series. Conventional electrodialysis (CED) stack configured with repeat-arranged five cation exchange membranes and four anion exchange membranes was installed as a pretreatment process. After preconcentrating and precipitating brine with CED and Na2CO3, a high purity of ca. 98% Li2CO3 powder was obtained. The influence of current density and feed concentration on the production of LiOH was investigated. EEDBM Process cost is estimated to 2.59 $/kg at current density of 30 mA/cm2 and feed concentration of 0.18 M. It can be inferred that lower energy consumption would be obtained at the case of scaling up. Considering the environmental aspects, the corporate process for LiOH production is also mutually beneficial.

Fumaric acid separation from fermentation broth using nanofiltration (NF) and bipolar electrodialysis (EDBM)

The suitability of two techniques: nanofiltration (NF) with ceramic membrane and bipolar electrodialysis (EDBM), in separation and concentration of fumaric acid from fermentation broth obtained during biotechnological conversion of glycerol, was investigated. The influence of composition, concentration and pH of fumaric acid model solutions on the efficiency of nanofiltration process was studied. It was found that the retention of fumaric salts increased strongly with increasing pH of the initial solution, while the retention of glycerol was lower than 6%, irrespective of pH. In the second step, the influence of electrodialysis stack configuration, current density, pH and concentration of fumaric acid in model solutions on the efficiency of bipolar electrodialysis was analysed. Then, after NF pretreatment, EDBM process of simulated solutions as well as fermentation broth was performed. The current density equal to 50A/m2 was found sufficient to achieve a 53% desalination of fumaric acid from fermentation broth. Moreover, the results obtained showed that bipolar electrodialysis stack, consisting of anion-exchange and bipolar membranes, allowed selective separation of fumaric acid from the broth.The results suggest that NF with ceramic membrane as well as EDBM can be effectively applied as a removal step of fumaric acid from fermentation broth.

Feasibility studies on arsenic removal from aqueous solutions by electrodialysis

The effectiveness of electrodialysis (ED) in removing inorganic arsenic (As) from aqueous solution was investigated. A tailor-made ED stack was used to perform current-voltage and optimization experiments in a recirculating batch mode. Samples were pre-oxidized with NaClO using 1:2 sample to oxidant weight ratio (RS:O) to transform 100% of As(III) to As(V) in 180 seconds. A high feed water conductivity of 1500 μS/cm and a low feed water conductivity of 800μS/cm had limiting currents of 595 mA and 525 mA, respectively. Optimum experimental conditions that provided maximum As separation were applied potential (E) of 12 V, feed flow rate (Q) of 0.033 L/s, feed concentration (C) of 662.0 μg L−1, and operating time (t) of 45 min, the most significant ones were applied potential, feed concentration and operating time. Model confirmation experiments showed a good agreement with experimental results with only 0.031% error. The total As in the diluate stream was 4.0 μg L−1, consisting of an average of 3.0 μg L−1 As(V) and 1.0 μg L−1 As(III).

Seawater electrodialysis with preferential removal of divalent ions

In this work desalination of a ternary salt mixture and artificial North Sea water is studied with a lab scale electrodialysis stack, which was used in a recycling batch mode. During desalination samples were taken and the ionic composition of the dilute stream was determined. The effect of applied current density (10–300A/m2) on this composition was investigated.A clear effect of applied current density was observed. A lower applied current density leads to a more complete reduction in concentration of divalent ions, in an earlier extent of desalination. This influence of the applied current density could be related to the concentration polarization effects that occur in the diffusional boundary layer and are explained with a model based on the Nernst–Planck flux equation. It was found that the lower initial ion concentration of Ca2+, Mg2+, but also of K+, and SO42− compared to respectively Na+ and Cl−, leads to stronger depletion of these ions in the transport layer adjacent the membrane. These boundary layer effects are more pronounced at higher applied current densities, resulting in reduced transport of ions with a low initial concentration. High monovalent over divalent ion ratios can be found at low applied current.

Salt Concentration Differences Alter Membrane Resistance in Reverse Electrodialysis Stacks

Membrane ionic resistance is usually measured by immersing the membrane in a salt solution at a single, fixed concentration. While salt concentration is known to affect membrane resistance when the same concentration is used on both sides of the membrane, little is known about membrane resistance when the membrane is placed between solutions of different concentrations, such as in a reverse electrodialysis (RED) stack. Ionic resistance measurements obtained using Selemion CMV and AMV that separated sodium chloride and ammonium bicarbonate solutions of different concentrations were greater than those measured using only the high-concentration solution. Measured RED stack resistances showed good agreement with resistances calculated using an equivalent series resistance model, where the membranes accounted for 46% of the total stack resistance. The high area resistance of the membranes separating different salt concentration solutions has implications for modeling and optimizing membranes used in RED systems.

Evolution of anion-exchange membrane properties in a full scale electrodialysis stack

In this paper, the physicochemical, structural and mechanical properties of an anion-exchange membrane were investigated, during its lifetime in industrial electrodialysis for whey demineralization. Several analytical techniques permitted to disclose the membrane structure and to describe the evolution with time of the membrane properties. The homogeneous anion-exchange membrane, AMX-SB, is constituted of a semi-interpenetrating polymer network of poly(vinyl chloride) and functional poly(styrene-co-divinylbenzene). No significant loss of the ion-exchange capacity or degradation of the functional poly(styrene-co-divinylbenzene) chains were detected. However, fouling caused a decrease in the counter-ion mobility within the membrane, which produced a reduction of the electrical conductivity. A progressive loss of poly(vinyl chloride), which was degraded or washed out from the membrane during the cleaning-in-place process, was clearly evidenced. This led to the formation of non-charged pores available for electroneutral electrolyte solution and large molecules, such as lactose and proteins. The occurrence of such pores resulted in an increase in electrolyte permeability through the membrane and a rebound of the conductivity at the last stage of electrodialysis. The microheterogeneous model was applied to well account for these changes. By means of tensile strength tests, it was possible to investigate the mechanical properties of the membrane, which turned gradually from a rigid and tough material, to a rigid and brittle one, due to the loss of poly(vinyl chloride), leading to the end of the membrane lifetime.