Electrodialysis Stack Research Articles

Dual-purpose desalination-salt production electrodialysis

Electrodialysis (ED) is applied in Japan for seawater concentrating to ca. TDS 200 followed by thermal concentration and salt crystallization. A partially desalinated water (ED diluate) is obtained at the same time. Its further desalination for potable water production may be considered to decrease costs of ED by obtaining two valuable products instead of mere salt. Univalent ion permeable ion-exchange membranes are however applied in ED process and as a result the contents of calcium, magnesium and sulfate ions in diluate are relatively high (in relation to chloride), which makes any further processing difficult. Electrodialysis reversal (EDR) may overcome these shortcomings. The salinity of partially desalinated water is as high as TDS 22.9. If dual-purpose desalination-salt production electrodialysis concern is high degree of desalination, which is however necessary, this implicates many problems in the ED process that are the result of major differences in ED concentrate and diluate concentrations and the small conductivity of potable water grade diluate. Electrodialysis of seawater was investigated in a two-step stand with ED-MSF-Crystallization system being the first stage. Univalent ion permeable Tokuyama Co. ACS and CMS membranes were applied in the first step and normal grade CMV, AMV Asahi Glass membranes in the second step (for the first step diluate treating). ED stack developed by the author and equipped with 0.19 mm spacer was applied. A single-pass low residence time mode of operation was applied to avoid gypsum crystallization in the second step ED concentrate. The first step diluate containing, in g/L, Ca 2+—0.38, Mg 2+—1.26, Cl −—4.08, SO 4 2-—2.78 was treated by EDR yielding 90% recovery. Potable water grade EDR diluate was obtained. No gypsum crystallization in the EDR concentrate was observed despite its 360% saturation. The total water recovery of ED-MSF-crystallization system is equal to 90%. As there are two valuable products in the described process, the cost of each of them is obviously related to their selling price. IF $30 per one ton of salt is assumed, the potable water cost is estimated to be only $0.44/m 3.

Method of electrodialysis stack testing with the feed solution concentration regulation

A technique to test electrodialysis (ED) stacks designed to desalt dilute solutions is developed. The technique permits obtaining reproducible data on laboratory and medium-scale units useful for predicting mass transfer characteristics of large ED stacks. Quasi-steady state of desalination process is the main condition to obtain such data. This state can be attained by applying recycling hydraulic modes and special devices to maintain constant pH, or pH and concentration of the feed solution. To obtain data for scaling, it is necessary to apply a fixed voltage on the stack and to use a constant flow rate of the feed solution during the experiment, the feed solution concentration may vary with the time. Specifications of the stacks studied, rational hydraulic and electrical modes of testing are considered.

Deacidification of Mandarin Orange Juice by Electrodialysis Combined with Ultrafiltration

The effects of electrodialysis (ED) alone or ED plus ultrafiltration (UF) on deacidification of mandarin orange juice were studied by using a commercial ED stack with ion exchange membranes. ED processing, reduced the total acidity of the juices by 30% (0.6～0.7% as total acidity) after 50 min and by about 60～70% (0.23～0.4% as total acidity) after 100 min, as compared to the control juice. However, the acidity reduction after 50 min of ED was determined to be suitable, when considering total acidity (0.6～0.7%, w/w) and current efficiency. There was no color change in the juices following ED, and the pH and Brix were only slightly decreased. Furthermore, ascorbic acid and citric acid concentrations showed only minor decreases, and amino-N, free sugar, and flavonoid contents remained almost unchanged. Therefore, we concluded that the nutritional integrity of the juice was maintained. ED combined with UF may be effective, not only in preventing membrane fouling, but also in preserving the nutrients, such as ascorbic acid, in citrus juice.

Multistage electrodialysis for large-scale separation of racemic mixtures

A new counter-current process has been developed for large-scale enantiomer separations, based on a combination of enantioselective complexation in solution, counter-current fractionation and electrodialysis. The counter-current principle is obtained by the electrophoretic transport of the free enantiomer through size-selective membranes, whereas, the complexed enantiomer is retained by the membranes and is transported with the liquid flow in the opposite direction. This design has been validated in an electrodialysis stack containing 20 membrane compartments. A racemic mixture has been fed at one side of the closed system, simulating one half of a complete separation apparatus. Using this set-up the enantiomeric excess (e.e.) and the concentrations of both enantiomers have been determined, as a function of the current density at a constant liquid flow velocity. As a model system the separation of d, l-tryptophan in combination with α-cyclodextrin as the chiral selector is used. Despite the low selectivity of this selector (1.12), an e.e. difference of 14% has been obtained. Based on these experimental data, model calculations have shown that using an electrodialysis stack of 250 membrane compartments a complete separation (e.e.>99%) can be accomplished for this system. The proposed multi-stage electrodialysis separation principle has the advantage of using elements from existing analytical separation methods. As this method is suitable for low selective (1.1–2) chiral selectors, it provides a viable alternative for the current large-scale enantiomer separation processes.

Electrodialysis with Bipolar Membranes: Principles, Optimization, and Applications

This paper is an overview of the bipolar membrane technology. The process of electrodialysis with bipolar membrane (EDBM) along with the different EDBM process configurations are presented. Some ways of optimization of both the bipolar membranes and the EDBM technology are envisaged. Most of the applications relate to the production or recovery of organic acids while the first plant has been commissioned by 1986 at Washington Steel (USA) for the recovery of HF/HNO3 from waste pickling liquors. In the last few years, there has been increasing interest in using bipolar electrodialysis stacks in chemical and agro industrial processes to directly acidify or basify process streams without the addition of chemicals. This attractive feature of this technology has contributed to the implementation of several plants in fermentation process for the production of organic and amino acids.

The effect of conducting spacers on transport properties of ion-exchange membranes in electrodriven separation

A conventional non-conducting spacer (interpolymer of polyethylene styrene-divinylbenzene copolymer) was transformed into an ion-conducting spacer by chemical modifications. Electrochemical characterization of cation-and anion-exchange membranes when it was alone or kept in between the non-, cation- or anion-conducting spacers was carried out by recording membrane potential and current-voltage curves in contact with NaCl solution of 220 ppm concentration at different flow rate of the solution. Transport parameters such as the counter-ion transport number, ionic permeability, membrane resistance and permselectivity were estimated for different experimental systems. Introduction of cation-conducting spacer near cation-exchange membrane and anion-conducting spacer near anion-exchange membrane led to a dramatic increase in counter-ion transport number and limiting current density along with the reduction in membrane resistance and thickness of diffusion boundary layer. On the basis of these studies, the position of conducting spacers (CS) in electrodialysis (ED) stack were optimized and packed. The performance of this ED stack has been tested with non-conducting and cation-conducting spacers in electrolytic solution of NaCl, NiCl 2 and CuCl 2 having concentration 500–1000ppm. The presence of conducting spacers not only suppress the concentration polarization but out put of the unit and its current efficiency also increases by about two times.

Production of High Resistivity Water by Electrodialysis. Influence of Ion-Exchange Textiles as Conducting Spacers

Production of high resistivity water was investigated by electrodialysis (ED) using either inert or conducting spacers. Ion-exchange textiles were used as conducting spacers. Experiments were performed on a preindustrial scale with a pilot consisting of nine two-compartment cells, each membrane having an effective area of 176 cm2. Three configurations of the ED stack were investigated for the dilution compartment: EDIT-(2) with a 2-mm thick ion-exchange textile, and ED-(2) or ED-(0.4), with a 0.4-mm thick inert spacer inserted between 2 or 0.4 mm thick dilution compartments, respectively. The textile induces a moderate increase in the pressure drop between the inlet and outlet of the stack. The performances of the different processes were compared under various experimental conditions of pH, nitrogen bubbling throughout the feed solution, flow rate, and current density. The results show that for an inlet conductivity of 10–15 μS·cm−1, a flow rate of of 2.2 × 10−5 m3·s−1, and an applied voltage of 80 V, an outlet conductivity of 0.4 μS·cm−1 was obtained with the EDIT process, while no value lower than 5 μS·cm−1 was obtained with the ED process using both stacks.

Electrodialysis of acetate fermentation broths.

Electrodialysis (ED) shows good potential for downstream processing of acetate fermentation broths, to separate acetic acid while unreacted glucose and other nutrients are partially recycled back to the fermenter. With conventional anion- and cation-exchange membranes, higher current increased acetate flux, water flux, and energy consumption. Multiple ED stacks connected in series with unequal initial volumes for a batch process maximized acetate concentration in the concentrating stream to 134 g/L calcium-magnesium acetate (CMA) in the fermentation broth at pH 6.8. Back-transport of acetate from the product into the feed stream and water transport limit the maximum concentration possible. Cost of ED is about 295 dollars/ton acetate for the CMA broth.

Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis

Cost-effective operations of a batch electrodialyzer for removal of salt from a single salt solution are investigated. It is desired to minimize the operating cost for a particular batch. The operating cost for an electrodialysis (ED) stack is comprised of cost related to energy consumption and cost of maintenance of the ED stack. In effective operations of an ED stack, the maintenance cost is a small fraction of the total operating cost. The bulk of the operating cost is therefore proportional to total energy consumption, which is the sum of the electrical energy needed for salt removal and the energy required to pump various solutions through the ED stack. For fixed feed composition and the desired percent salt recovery, the total energy required is influenced by trajectories of current flowing through and the voltage applied across the ED stack and the operating time. In this regard, the following operations are studied: (I) constant current operation, (II) constant voltage operation, (III) constant current operation followed by constant voltage operation, (IV) constant voltage operation followed by constant current operation, and (V) operation with time-variant current and voltage. For arbitrary relations among salt concentration, current utilization, and stack resistance, optimal current and voltage trajectories that lead to minimum energy requirement are identified for each of the five operations. It is established analytically that operation V is superior to operations III and IV, which in turn are superior to operations I and II. Numerical illustrations reveal that the performance differences in these operations are enhanced as the percent salt recovery is increased.

Competitive anion transport in desalting of mixtures of organic acids by batch electrodialysis

Desalting and separation of binary and quaternary acid mixtures via batch electrodialysis are investigated in this article. A monoselective cation exchange membrane and either a non-selective or a monoselective anion exchange membrane are employed in the electrodialysis stack. The effects of current density and composition of the initial feed of the electrodialysis stack (employing a non-selective anion exchange membrane) on its performance are studied in experiments involving mixtures of acetic and succinic acids. The effect of the type of the anion exchange membrane on the process performance is examined in desalting experiments involving a mixture of acetic, formic, lactic, and succinic acids. The trends observed in the experiments are interpreted in terms of species-specific parameters (such as molar concentration, charge on ionic species, molecular weight, degree of ionization, and ionic equivalent conductivity) and characteristics of anion exchange membrane used.

Extraction of amino acids from protein hydrolysates by electrodialysis

Protein hydrolysates were obtained by acid hydrolysis from animal or human residues, such as poultry feathers, ox blood and human hair. After neutralization and discolouration with active charcoal, the hydrolysates were treated by successive electrodialysis (ED) in order to extract amino acids into several fractions. The current density and pH were optimized for each ED operation performed with preindustrial pilot scale equipment. The first step was the demineralization of amino acid mixtures using an ED stack with two compartments. The salt removal was achieved with extraction degrees higher than 90% and current efficiencies of about 80%. In the most favourable case, the amino acid losses did not exceed 10%. The second step was the extraction of the charged amino acids using an ED stack with four compartments. Three fractions were obtained, corresponding to the acidic, basic and neutral amino acids. The extraction degrees varied from 80% to 100%. In the third step, the fractionation of basic amino acids on the one hand, and neutral amino acids on the other hand, was carried out with enrichment degrees varying from 50% to 80%. © 1998 SCI

Water Demineralization by Electrodeionization with Ion-Exchange Textiles. Comparison with Conventional Electrodialysis

ABSTRACT An electrodeionization process has been developed using ion-exchange textiles (EDIT) instead of ion-exchange resins as currently used for the production of ultra-pure water. The study was carried out with a laboratory pilot. Each membrane had an active area of 20 cm2. Experiments were performed with NaCl solutions at low concentration (from 3 to 300 mg-L%minus;1) under various experimental conditions of flow rate and current density. Different configurations of textile were tested: the diluate compartment of the electrodialysis stack was filled by either alternating layers of cation or anion-exchange textiles, or one layer of amphoteric textile or bipolar textile. The best results were obtained with the latter configuration. The study allowed comparison of the performances of EDIT with conventional electrodialysis (ED) in terms of current efficiency, demineralization ratio, and energy losses. A significant improvement was obtained by introducing the textile. At the lowest concentration (3 mg.L−1) and the highest flow rate (20·8 cm.s−1), the current and demineralization efficiencies were 150% higher with EDIT than with ED.

Lactic acid production by electrodialysis Part II: Modelling

One interesting alternative to the usual end-of-pipe processes for treating waste water produced during cheese-making involves its fermentation to sodium lactate. Then, electrodialysis can be used to recover free lactic acid from the fermentation broth. The design and optimization of such a process is aided by a suitable mathematical model. This work presents a model of the performance of an electrodialysis stack to be used in such a process. Its reliability has been assessed by comparison with experimental data for free lactic acid recovery from sodium lactate solutions. A suitable procedure for estimating some process-dependent parameters is also discussed.

Lactic acid production by electrodialysis Part I: Experimental tests

Lactic acid, the production of which from cheap raw materials is attractive due to its great potential in the production of biodegradable polymers, can be obtained from fermentation broths using electrodialysis. This work discusses the feasibility of this process using commercial membranes, together with the influence of several operating parameters on the electrodialysis stack performance. It was found that it is possible to operate with high current efficiency values, while the overall recovery of sodium lactate depends on the current density. Electroosmotic water transport limits the maximum concentration value achievable using this technique.

Improved electrodialysis stacks

Improved electrodialysis stacks

Effects of spacer geometry on pressure drop, mass transfer, mixing behavior, and residence time distribution

Net-type turbulence promotors or grids, so-called spacers, play an important role in membrane systems referring to problems of mass transfer, homogenizing and mixing behavior. In spiral wound modules and electrodialysis stacks spacers have several functions as supporting nets (stabilisators) and as turbulence promotors to increase mass transfer rates and reduce fouling layers. Spacers show a number of geometric and fluid-dynamic parameters like, e.g., the distance between two grid rods (wavelength), the inclination angle of the grid rods and the Reynolds number. The effects of these parameters on the local flow and mass transport phenomena in narrow rectangular ducts including different spacers are investigated in gas flows in a wind tunnel. For this purpose spacers were enlarged geometrically similar and a fundamental study of the geometric parameters, based on the results of Schock [1], led to a new definition of the hydraulic diameter of spacers.

Spirally wound electrodialysis (SpED) modules

Development of spirally wound electrodialysis modules is presented, and their advantages and limitations are discussed. The single start parallel flow spirally wound electrodialysis modules were designed, fabricated and operated successfully. The construction of the modules is shown, and their typical desalination experimental results are presented and compared with those from a traditional electrodialysis stack.

A new type of electrodialyzer

A new type of electrodialyzer consists of both ion exchange spacer-membranes and electrodes. The electrodialysis stack arrangement has 10 cell pairs (cation and anion) spacer-membranes and an effective cross-sectional area of 156 cm 2. The spacer-member is out of level on the surface. It used a desalting test for NACl saline water with different concentrations ranging from about 2000 ppm to 500 ppm down to 150 ppm. We found by experiment that the device equipped with the stack of ion exchange spacer-membranes can decrease direct current energy consumption by more than 20% and the desalting rate obviously increased comparing with the stack of common heterogeneous membranes under the same operating conditions.

Application of Electrodialysis in Purification and Reuse of Spent Alkaline Process Streams

Experiments using electrodialysis (ED) units for purification and recovering of spent alkaline process streams, which contain potassium hydroxide, were carried out. According to the experimental results, the spent alkaline process streams, which were produced in production of alkaline storage batteries, can be completely recovered and directly reused as the alkaline process streams. A great number of benefits has been obtained to recover the spent alkaline process streams by using the ED units. The process of the ED unit and the structure of the ED stack are discussed. In this case, optimal current density (170-200 mA/cm2) and current efficiency (75-80%) were observed.

Desalination of brackish water of higher salinity by electrodialysis

A laboratory model electrodialysis(ED) stack containing 35 cell pairs of cation and anion exchange membranes of 80 cm 2 effective cross-sectional area is used to study the desalination of brackish water having total dissolved solids (TDS) of 6000 – 10250 ppm. For brackish water containing TDS of 12000 – 27170 ppm, two ED stacks and for TDS of 27100 – 36000 ppm, three ED stacks containing different cell pairs of membranes having same effective cross-sectional area are used. Parallel-cum-series flow system is employed in all the stacks. The reduction in anions like Cl −, NO 3 − along with total hardness and TDS have been reported. The energy requirements and current efficiencies are also given based on the experimental data. Optimum flow rates and energy consumptions are reported for obtaining potable water having TDS of ⋍ 800 ppm from different brackish water. The results will be useful for design and operation of different capacity of electrodialysis plants for desalting brackish water of different salinity.