Reverse Electrodialysis Unit Research Articles

Performance improvement of LiBr-H2O reverse electrodialysis unit for heat to power conversion with finite solution flowrate and large concentration change

The reverse electrodialysis heat engine is an emerging technology that converts low-grade heat into electric power. However, the conventional reverse electrodialysis unit, developed based on the “infinite” flowrate assumption, fails to effectively utilize salinity gradient power produced by the regeneration unit that operates under a certain concentration gradient. In this study, a novel LiBr-H2O reverse electrodialysis unit based on the “finite” flowrate assumption is investigated to match the regeneration unit better. The resistance-distributed mathematical model is built and experimentally validated, considering that the resistance along the flow channel shows more significant variation under the condition of finite flowrate and large concentration change. Results show that the finite flowrate assumption is more suitable for the reverse electrodialysis heat engine application, and the utilization rate of Gibbs free energy can reach 99.8 %, indicating that the novel reverse electrodialysis unit is well-matched with the regeneration unit. The valuable salinity gradient power is nearly completely recovered. As a result, the maximum overall efficiency of the novel reverse electrodialysis unit is 28.6 %, which is superior to that of the reverse electrodialysis unit with a recirculating arrangement and comparable with the reverse electrodialysis unit with serious membrane stacks. Moreover, the technical economy and the exergy efficiency of the novel reverse electrodialysis unit are better than the rest. This work proves the capacity of a LiBr-H2O reverse electrodialysis unit based on a “finite” flowrate assumption as a more adaptable arrangement for the REDHE applied for efficient capture of the widespread low-grade energy.

Photothermal Sweeping Gas Membrane Distillation and Reverse Electrodialysis for light-to-heat-to-power conversion

Water and energy are two intimately interconnected issues of strategic relevance for a sustainable industrial development. Herein, we integrated light-harvesting/self-heating membranes and salinity gradient technology with the aim to implement the innovative concept of light-to-heat-to-power conversion.Novel photothermal membranes, prepared by immobilizing silver nanoparticles (AgNPs) on the top layer of microporous polyvinylidene fluoride (PVDF) matrix, were tested – for the first time – in a Sweep Gas Membrane Distillation (SGMD) unit applied to the desalination of synthetic seawater solution (0.5M NaCl). As a result of the ability of noble metal nanofillers to act as localized thermoplasmonic nano-heaters at membrane-feed interface for efficient water evaporation, an increase of transmembrane flux under UV radiation by about 10-fold with respect to unloaded PVDF membrane was observed.The SGMD retentate, consisting in hypersaline brine (progressively concentrated up to 4M NaCl and rejected at about 40°C) was fed to a Reverse Electrodialysis unit with the aim to harvest electrochemical energy. The maximum power density, measured for a retentate concentration increasing from 1M to 4M, raised from 0.13 to 0.9 W/m2MP (MP: RED membrane pair). Overall, the proposed integrated membrane system allowed to extract about 10% of the energy not employed for water evaporation.

The first operating thermolytic reverse electrodialysis heat engine

Thermolytic reverse electrodialysis heat engine (t-RED HE) has been recently proposed as a technology for converting low-temperature waste heat into electricity. The construction and operation of the first world lab-scale prototype unit are reported. The system consists of: (i) a reverse electrodialysis unit where, the concentration gradient between two solutions of thermolytic salts is converted into electricity and (ii) a thermally-driven regeneration unit where low-temperature heat is used to restore the initial conditions of the two feed streams. Regeneration is based on a degradation process of salts into gaseous ammonia and carbon dioxide, which can be removed almost entirely from the exhausted dilute solution by vapour stripping and, subsequently, reabsorbed into the exhausted concentrate solution, thus restoring the initial salinity gradient of the two streams. For the first time, the feasibility of the process was demonstrated through an experimental campaign to evaluate the system performance via long-run tests.

Thermolytic reverse electrodialysis heat engine: model development, integration and performance analysis

Salinity gradient heat engines represent an innovative and promising way to convert low-grade heat into electricity by employing salinity gradient technology in a closed-loop configuration. Among the aqueous solutions which can be used as working fluid, ammonium bicarbonate-water solutions appear very promising due to their capability to decompose at low temperature. In this work, an experimentally validated model for a reverse electrodialysis heat engine fed with ammonium bicarbonate-water solutions was developed. The model consists of two validated sub-models purposely integrated, one for the reverse electrodialysis unit and the other for the stripping/absorption regeneration unit. The impact of using current commercial membranes and future enhanced membranes on the efficiency of the system was evaluated, along with the effect of operating and design parameters through sensitivity analyses. Results indicated that exergy efficiency up to 8.5% may be obtained by considering enhanced future membranes and multi-column regeneration units.

Experimental analysis of a continuously operated reverse electrodialysis unit fed with wastewaters

Reverse Electrodialysis (RED) is one of the most promising technologies to convert salinity gradient chemical energy into electricity. RED units are traditionally operated with natural streams as river water and seawater thereby limiting the spread of the technology in sites far from coastal areas. Aim of the present work is that of exploring and expanding feed possibilities for RED systems by employing waste streams. Thus, an experimental study was performed by testing, for the first time, a Reverse Electrodialysis (RED) unit fed with a high salinity wastewater originated in a fish canning factory, and a low salinity wastewater from a sewage treatment plant. Uninterrupted, long duration experiments were carried out to evaluate time-evolution of the key electrical parameters. Power output was monitored over time along with pumping losses and electrical resistance. The effect of chemical backwashing was analysed in order to assess process maintenance. Results showed that fouling issues have a crucial impact on process performance and should be properly addressed before operating RED units fed by waste streams.

Exergy analysis and thermoeconomic cost accounting of a Combined Heat and Power steam cycle integrated with a Multi Effect Distillation-Thermal Vapour Compression desalination plant

In this paper an exergy analysis and thermoeconomic cost accounting of a Combined Heat and Power steam cycle integrated with Multi Effect Distillation-Thermal Vapour Compression plant is performed; the goal of the study is to show how these methodologies provide a rational criterion to allocate production costs on electricity and freshwater in such a dual purpose system. After a brief overview on the methodology and a description of reference plant, exergy analysis is carried out to calculate exergy flows and exergy efficiencies at component level. A detailed description of the adopted thermoeconomic model is given. In a first scenario, cost accounting is performed assuming that the concentrated brine is disposed back to sea, thus being its exergy content definitively wasted; furthermore, a sensitivity analysis is carried out in order to assess the changes in the unit cost of electricity and freshwater with several design and operation parameters. In a second scenario, conversely, part of brine exergy is used in a Reverse Electrodialysis unit to produce further electricity. In both cases results show that high unit costs are obtained for the material streams or energy flows which involve major exergy destruction along their production process, particularly freshwater in the former configuration and Reverse Electrodialysis electric output in the latter one.

Changes in the microstructure and operational characteristics of the MK-40 sulfocation-exchange membrane during the electrodialysis of natural waters

Structural changes in MK-40 sulfocation-exchange membrane material are estimated after the electrodialysis of natural waters. The reasons for a deterioration of its operational properties under the influence of various factors are revealed. Differences in the surface and bulk microstructures of membrane samples, which arise after long-term operation in electrodialysis apparatuses of various types, are visualized via scanning electron microscopy. The membrane extracted from an electrodialyzer-desalinator is characterized by an increase in macroporosity. The membrane from the near-electrode section of a reverse electrodialysis unit is distinguished by the fact that slightly soluble compounds are formed both on the membrane surface and in its bulk (salt contamination). It is established that morphological changes and sedimentation caused by the electrodialysis of natural waters affect the electrochemical and physical-chemical properties of the MK-40 sulfocation-exchange membrane. The growth in membrane macroporosity in the case of the longterm desalination of natural waters is the primary reason for an increase both in the electroconductivity against the background of losses in the exchange capacity and selectivity and in the water content and diffusion permeability. It is ascertained that the membrane transport properties deteriorate due to sedimentation which affects not only the membrane surface but also its bulk.

A feasibility study of industrial wastewater recovery using electrodialysis reversal

A wastewater reclamation and reuse “mini-plant” was established at the site of China Steel Corporation (CSC) wastewater treatment plant. The purpose of the mini-plant is to demonstrate the technical and economical feasibility of electrodialysis reversal (EDR) process. The mini-plant includes a sand filtration unit and an EDR unit which has a daily treatment capacity of 350 CMD. Raw wastewater had an averaged conductivity of 3860 μS/cm, pH between 7.7 and 8.3, and COD between 30 and 70 mg/L. When the EDR unit operates at a 75% of water reclamation rate, 92% of desalination rate, 98% of chlorine ion removal, 80% of sulfate removal, 99% of calcium ion removal, and 51% of COD removal rate can be achieved. The water quality of product stream is 305 μS/cm for conductivity, 13 mg/L for chlorine ion, 86 mg/L for sulfate, 1.4 mg/L for calcium ion, and of 20 mg/L for COD concentration. Operation cost of the EDR mini-plant was also estimated in this project. The electricity required for electrodes power supply and pumping is 0.85 kW h per cubic meter of treated water. Total operation cost including electricity fee and chemical cost is about $0.146 per cubic meter of treated water.

High efficiency electrodialysis reversal of concentrated calcium sulfate and calcium carbonate solutions

Electrodialysis reversal (EDR) of high calcium sulfate/carbonate containent low-salinity waters was examined in the laboratory. The previously developed ED stack, equipped with a 0.19-mm-thin spacer, was modified to achieve low mean residence time and low residence time distribution. As a consequence of the above modification, higher than reported for stable EDR unit operation gypsum supersaturation levels (up to 368%) and a longer polarity cycle time (up to 22 min) were observed. At the same time the increase in stable operation LSI from 1.92 to 2.12 was observed. It was also shown that controlled gypsum crystallization from the concentrate stream resulted in an 80–83% reduction in calcium content and simultaneously a 57–60% reduction in sulfate content. This should enable concentrate re-circulation and high overall desalted water recovery using a modified EDR unit.

Electrodialysis reversal in high CaSO 4 supersaturation mode

In the case of the most troubling saline waters with CaSO 4 content close to saturation level, electrodialysis reversal (EDR) was found to be serviceable as a desalination method since its laboratory and commercial applications report stable long-term operation at high CaSO 4 concentrate supersaturation level. The available literature data exhibit stable long-term operation even at CaSO 4 supersaturation degree close to 175%. Previously the authors found that EDR of concentrated calcium sulfate solution might be carried out without scaling risk even at a higher CaSO 4 supersaturation degree, but a special effort must be addressed to lower the mean value and variance of concentrate channel residence time distribution. Also, special care must be taken with respect to CaSO 4 nucleation kinetics. Presently the authors have attempted to examine the previously observed relationship between EDR concentrate chamber residence time and calcium sulfate nucleation time. At first the maximum CaSO 4 supersaturation level was measured during long-term operation of a previously designed, low mean residence time EDR unit. These values were compared with those previously obtained from non-modified EDR units. It was found that the mean residence time reduction enables stable EDR long-term operation at high calcium sulfate saturation levels (326–364%) at a polarity cycle time prolonged from 17 to 30 min. The above data were then analyzed with respect to CaSO 4 nucleation kinetics.

Electrodialysis desalination designed for off-grid wind energy

An electrodialysis desalination plant has been set up and tested to treat brackish water while driven from an off grid wind energy system. The tests were carried out in the framework of a wider scope project, located on Gran Canaria Island (Spain). The main goal of this project was to test and identify the most suitable desalination systems for connection to the above-mentioned medium-scale off grid wind farm. After having previously analysed the behaviour of the system on-grid, the following stage was to develop an operational envelope for the electrodialysis reversal (EDR) unit while operating off grid, i.e., only coupled to the wind farm. The unit included power converters for the membrane stacks (DC-drivers) and variable frequency drivers (VFD) for the feed pumps. The tests were carried out to establish the power intervals for the EDR unit depending on the product flow rate specified as well as water quality. Product flow rate between 3 and 8.5 m 3/h, power requirements between 4 and 19 kW, while product water conductivity ranged between 200 and 500 μS/cm were recorded. The desalination unit showed good flexibility, adapting smoothly to variations in wind power, even when sudden drops or rises occurred. The control system, slightly modified from a standard design, can cope with such sudden variations. Good agreement between performance predicted with software and the actual operating performance was observed. The presence of harmonics in the electric system due to DC drivers and VFD became harmful for the control and electric system, and care must be taken through appropriate mitigating measures.

Electrodialysis reversal of calcium sulphate and calcium carbonate supersaturated solution

Electrodialysis reversal (EDR) in single-pass mode was investigated to coal-mine water desalination. Water containing (mg/L): Ca 2+ - 284, Mg 2+ - 130, SO 4 2− - 1066, Cl − - 359 and HCO 3 − - 297 was desalinated in EDR laboratory unit with 0.19 mm intermembrane distance. The electrodes polarity was changed each 17 min. 90.1 and 90.6% recoveries were achieved applying diluate velocities 8.35 and 8.14 cm/s respectively, while concentrate velocities were 0.86 and 0.90 cm/s. Mean residence time of concentrate in EDR unit was equal to 352 s. The percent of calcium sulphate saturation in EDR concentrates was equal to 368 and 362% while Langelier Saturation Index (LSI) to 1.92 and 1.60 respectively. Voltage drops vs. EDR time show no increasing tendency, which would be the result of calcium sulphate or calcium carbonate scaling in the EDR unit. It was then demonstrated that EDR in the presented mode could operate at such a high supersaturation level, which has been achieved up to now, only with chemical addition to the concentrate stream.

Iowa's first electrodialysis reversal water treatment plant

In 1979 the City of Washington was notified by the Iowa Department of Natural Resources (IDNR) that the City was in violation of the radium standard for drinking water. The City of Washington authorized an engineering study to determine the most cost-effective and practical way to remove radium and, at the same time, improve overall water quality. Several possible treatment alternatives were evaluated. It was finally decided to utilize electrodialysis reversal (EDR). Washington obtains its water from three deep wells ranging in capacity from 600–780 gpm. The untreated water withdrawn from the wells first passes through the EDR units. There are three EDR units, each able to produce 285 gpm of finished water. In the future, another EDR unit can be easily added to the other three units, since the new plant was built and plumbed for an additional EDR unit if water demand increased. The Jordan aquifer supply is adequate for current and future needs. The average daily water usage in 1993 was 818,000 gal/d. In order to meet peak flows, it is possible to bypass the EDR units with part of the untreated water and then blend treated and untreated water. The treated water meets IDNR standards of 5.0 pC/L. After the EDR units, the water flows through an aerator where odor-causing gases and carbon dioxide are removed. Aeration reduces the amount of caustic soda and chlorine used in the finished water. The hydrogen sulfide gas leaves the water as it passes through the aerator, and this loss of gas creates less chlorine demand. Total and free chlorine residuals are now detected in every water main of the town, whereas before, the residuals would not be detected in certain area of Washington. Phosphates have been cut back from 7 pounds per day to one pound per day. Better water quality is now being achieved with fewer chemicals added to the finished water. Washington's water treatment plant is the first municipal EDR plant in the State of Iowa and one of the largest municipal installations in the United States.

Investigations with Electrodialysis Reversal for the Treatment of Surface Water to Make-Up Water

Abstract In the Dutch power stations, boiler make-up water is nowadays prepared by treating drinking water with ion exchangers. A problem, however, is that drinking water is expensive in certain areas of the Netherlands. Therefore it seems to be attractive to prepare make-up water out of surface water with aid of electrodialysis reversal (EDR). The experiments with an EDR installation were carried out with different types of surface water. The process consists of three main stages: pretreatment of the surface water to meet the requirements set for feed water for the EDR unit desalination in the EDR unit post-treatment of product water (with ion exchangers) from the EDR unit to comply with the guidelines for make-up water. From experiments on pilot plant scale it could be concluded that a simple filtration step to remove suspended matter was sufficient to comply with the requirements set for feed water for the EDR unit. Further it was possible to desalt the surface water to a large extend (about 90% or mor...

Parametric tests of an electrodialysis reversal. (EDR) system with aliphatic anion membranes

During 1980, parametric studies were performed at the U.S. Department of the Interior, Roswell Test Facility, for the purpose of comparing the performance of aliphatic anion membranes to standard anion membranes in an electrodialysis reversal (EDR) system. An Ionics AQUAMITE × EDR unit was used to desalt a high sodium chloride, 4100 mg/S, TDS feed water. During these tests, the product salinity averaged 388 mg/l at an average production rate of 25,800 USGPD. The use of aliphatic membranes reduced the desalting energy consumption from 8.1 kwh/KGal with standard membranes to 6.8 kwh/kGal with aliphatic membranes at 73% recovery and, at 92% recovery, from 14.6 kwh/kGal for standard membranes to 11.0 kwh/kGal for the aliphatics. Slightly higher water recoveries were also possible with the aliphatic membranes as they exhibited less water transfer. A maximum recovery of 94% was obtained without operational problems despite calcium sulfate (gypsum) supersaturation levels of up to 165% in the waste Stream. Additional tests were conducted to evaluate the performance using a separate brine reed of inferior quality water (14,000 mg/l TDS) in order to obtain nearly complete recovery of the better quality brackish feed water. The separate brine feed allowed very high recoveries of the simulated Virginia Beach feed water while maintaining moderate brine stream concentrations. As a result, energy consumption was reduced from 11.9 kwh/kGal, for the maximum recovery case without separate brine feed, to 7.5-8.1 kwh/kGal using the separate brine feed approach. During these tests, extension of the time period between polarity reversal from 20 to 30 minutes was also investigated. This was found to improve power consumption slightly and increased the production by as much as 7.6% at very high water recoveries. It was concluded, as a result of these tests, that the following characteristics would be desirable for an EDR plant to treat a high sodium chloride 4100 ppm water: 1. Aliphatic anion membranes for reduced' energy consumption 2. Separate feed for brine and electrode flows for nearly complete recovery of a limited higher quality brackish water resources 3. 30-minute reversal cycle to improve production and increase recovery.