Determination of the limiting current density in electrodialysis desalination as an empirical function of linear velocity

Electrodialysis is known to be a useful membrane process for water desalination. The limiting current density (LCD) in the electrodialysis process is an important parameter which determines the electrical resistance and the current utilization. Usually, LCD depends on membrane and solution properties as well as on the electrodialysis stack construction and various operational parameters such as the flow velocity of the diluate solution. Therefore, a reliable determination of LCD is required for designing an efficient electrodialysis plant. In this study, LCD was measured in an electrodialysis flow cell system of given geometry and spacer configuration as a function of the linear velocity. The coefficients a and b of an equation describing LCD as a function of the linear flow velocity of the diluate solution, i_lim=aCu^b, were determined from a plot of the measured LCD over the electrolyte concentration versus the linear velocity on a double logarithmic scale. It was found that the coefficient b was related to the hydrodynamic conditions, while the coefficient a was affected by the cell geometry, electrolyte concentration and the membrane properties.     

Desalination costs in Australia: A survey of operating plants

A survey of desalination costs in Australia was conducted using data obtained from plant operators, and is reported in second quarter 1986 A$. Unit water costs range from $0.76/kL (for a precursor to deionisation for boiler feed) to $14/kL (for emergency supplies for an island resort).

However, an average figure for desalination of brackish water is $3–$4/kL, and for seawater, $5–10/kL in medium-sized installations.

Capital costs for brackish water plants have been correlated with plant design capacity ranging from 10 to 3400 kL/d.

There is insufficient information to allow a proper comparison between reverse osmosis and electrodialysis for brackish water desalination.     

电驱动膜过程在氨基酸发酵液处理中的研究进展

在发酵法生产氨基酸的过程中,需要后续工艺对发酵液进行分离纯化以提取目标产物.电驱动膜过程正逐渐成为该领域研究与应用的热点.本文介绍了近年来国内外普通电渗析(ED)、双极膜电渗析(BMED)、离子取代电渗析(ISED)、电复分解反应器(BMT)等常见的电驱动膜过程在氨基酸发酵液处理中的研究进展,简述了常见的膜堆构型及其工作原理、特点与应用实例.分析表明电驱动膜过程可以实现混合氨基酸分离、无机盐脱除以及氨基酸的制备,膜堆结构、操作参数的优化以及新型分离膜的研究与应用可以提高过程性能.同时也指出目前该领域的研究尚处于实验室研究阶段,研究对象以模拟发酵液为主,规模化应用的报道还不多见.但是可以预见高效、环保的电驱动膜过程将会在氨基酸发酵液处理中发挥重要作用.     

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.     

Prozeßvergleich von Umkehrosmose und Elektrodialyse am Beispiel der Nitrat-Entfernung aus Grundwässern

Comparison of reverse osmosis and electrodialysis for removal of nitrate from groundwater . Reverse osmosis and electrodialysis are membrane processes suitable for desalination of aqueous solutions. Both processes were tested on a pilot scale in a waterworks for removal of nitrate from groundwater. A combination of the processes was also considered with a view to very high concentration of the residual water. The article shows how information gathered from long-term reverse osmosis trials and the experiments with the electrodialysis plant with industrial membrane dimensions can provide a basis for safe design of large plants. Treatment costs for large capacities are shown for the example of the above feed water. The advantages and disadvantages of reverse osmosis and electrodialysis are compared. This comparison demonstrates fitting applications of the two processes in other areas (chemical industry, sewage treatment).     

Optimal design of an electrodialysis brackish water desalination plant

This paper considers the optimal design and operation of electrodialysis (ED) desalination plants. In general an ED plant aims to produce potable water from a high salinity source, like brackish water or high salinity water. The system is modelled mathematically as mixed-integer non-linear programming (MINLP) optimization problem, determining the number of desalination stages, the membrane area, the total required energy so as to minimise the total annualised cost of the investment accounting for both infrastructure and operating costs. Two examples from the literature illustrate the applicability of the proposed approach and evaluate the quality of the results obtained.     

Desalting sea water to less than 4 ppm by electrodialysis

In designing a continuous (single pass) electrodialysis plant to desalt water from a sea water salinity of 35,000 ppm to less than 4 ppm, or approximately 10,000 fold reduction in concentration, the number and mix of hydraulic and electrical stages are key design variables to maximize the cut achieved by each electrodialysis stack and therefore to minimize the number of stacks in the plant. The total number of stages in the plant and the flow rate per stage determine the plant's specific production while the pressure drop per stage sets the requirement for interstage pumping. The number of electrical stages limits the total current density applied and also determines the specific energy consumption of the plant. The design analysis is applied, using experimental data, to a plant application requiring minimum investment and simplified plant operation.     

1,3-丙二醇发酵液电渗析脱盐工艺的优化

研究了1,3-丙二醇发酵液电渗析脱盐过程中某些工艺参数,例如不同交联度离子交换膜的选择、不同碱离子发酵液以及原料液预处理方式等工艺对脱盐过程的影响。同时在前期实验的基础上,研究了淡浓室流速比、淡室流速和浓室初始浓度三因素对电渗析脱盐操作过程的影响,并利用正交试验对脱盐过程的工艺优化作了进一步的探讨。     

Status of reverse osmosis desalination technology

Reverse osmosis (RO), a relatively new technology, is gradually becoming an established and economical method for demineralization of saline waters. Over 50 commercial plants ranging in size from 50,000 gpd to 2 million gpd (2 mgd) are producing fresh quality water for municipal and industrial uses from brackish water sources. The U.S. Congress has authorized construction of a 100 mgd plant in Yuma, Arizona to demineralize otherwise unusable high salinity irrigation return flows as part of the Colorado River Salinity Control. Engineering design and operation details together with cost information on some commercial plants and the planned 100 mgd plant will be presented.A review of the plant operation data indicates that is imperative for the plant owners and equipment suppliers to place due emphasis on providing adequate feed water pretreatment facilities and trained plant operation personnel to ensure trouble-free operation and to achieve furthur economy in desalting costs.Significant advances have been made in the development of RO process for sea water desalination. Soaring energy costs are providing incentive for plant owners to prefer RO plants (up to 100,000 gpd) over vapor compression distillation hardware. Results of the Federal Government Desalting R & D Programs clearly indicate that RO desalting costs will be at least 20–30% lower than distillation.     

Process simulation of desalination by electrodialysis of an aqueous solution containing a neutral solute

A model was developed to describe the desalination process of an aqueous solution consisting of a salt and a neutral solute using electrodialysis (ED). Under the assumption of plug flow in compartments, the ED process was analyzed in two-dimensional directions of the electric field and flow to get the differential equations of mass balance in the flow length. Then the transport equations of solutes and water through the membrane were deduced by the irreversible thermodynamics approach. Under the limited condition of uniform current density, the model composed of a first-order differential equation set was developed. While the model parameters such as transport coefficients, dimensions of ED equipment, operation conditions and characteristics of solutes are given, the model was solved by the numerical method. The variations of current density, concentrations of solutes and velocities in dilute and concentrated compartments vs. flow length can be simulated by the model. While there was no neutral solute, the model was used to simulate the desalination process of a salt solution. By comparing the ED experiments to the simulations, it is shown that the model is well suited to describe the actual desalination process. The effects of the initial values of variables in the model on the desalination process were simulated to attempt to construct the actual ED process; and the general simulation of desalination process can be realized by the model. While the effect of concentration polarization on the desalination process is reflected by the variation of membrane conductivity, the model was verified to describe the ED process successfully under low velocity.     

Seawater desalination by reverse osmosis : Plant design,performance data,operation and maintenance (Tanajib,Arabian Gulf Coast)

A systematic interpretation of critical combinations of feed water, pretreatment and membranes and adequate conclusions can avoid setbacks of reverse osmosis processes. Reputed systems manufacturers will find the right approach if their tasks will be changed from a limited responsibility — for design, construction, installation and commissioning only — to an overall responsibility which includes operation and maintenance.In 1982/83 Preussag received from Aramco in Saudi Arabia the order for the first temporary reverse osmosis seawater desalination plant at Tanajib, Arabian Gulf Coast, with a total net capacity of 600,000 gpd. The orders were placed on a turnkey basis including operation and maintenance.The highlights of the RO seawater desalination plant are: shallow beach wells, ultrafiltration, vacuum deaeration, high-pressure multi-stage centrifugal pumps with energy recovery turbine, single stage reverse osmosis and free programmable logic and process controller.During the first more than one and a half years of operation, product quantity and quality has always met the requirements of Aramco's specification. The excellent experiences until today with the first temporary Tanajib reverse osmosis seawater desalination plant permit the conclusion that RO technology can avoid major setbacks and may face the competition with the distillation processes with confidence.     

Maintenance problems of the electrodialysis plants in Libya

Many electrodialysis plants have been installed in the last seven years in Libya. In this paper we shall analyse the operation features, the difficulties occurring in pretreatment and degasification, the locally adopted solutions to the maintenance problems encountered and last but not least why the operational costs are far beyond the tolerable allowance margin.The two plants studied extensively are Benina and Dahra—Benina because it was intended to be the world's largest ED plant with an installed capacity of 19,200m³/day (5.1mgd), and Dahra because of its unusually brackish water supply. Other electrodialysis plants are also considered and in each case the effects of the deteriorating input of brackish water, the pretreatment, the design gaps and sometimes faults are indicated together with the solutions developed on-site.Finally, using Libyan field experience with electrodialysis, some recommendations are presented for developing countries with ambitious desalination programs.     

Electrodialysis on the American continent

The Electrodialysis process has been used commercially in the United States to desalt brackish waters for over twenty years. The first municipal installation in Coalinga, California, was commissioned in 1959. The first plant continuously used to produce the total water supply for a community was installed in Buckeye, Arizona, in 1962.

Development of the Electrodialysis Reversal (EDR) process in the early 1970's resulted in a system featuring reduced operation and maintenance due to the elimination of the need to continuously feed acids and chemicals. This process is in use in typical municipal applications in Dell City, Texas, and Coupeville, Washington, along with industrial applications in electronics manufacture. Safe drinking waters are produced by EDR units at a number of highway rest and service areas.

Research and development in electrodialysis is sponsored by Corporate and Government funding. New membrane systems, membrane stack design and applications are under study. High temperature membranes and spacers have been developed for use in a 100,000 gpd seawater desalting system now being assembled for test. The combination of solar electric power and electrodialysis is also under study for use in purifying water in remote locations.

The application of electrodialysis to the treatment of brackish water has grown significantly over the past seven years. Improvements in system reliability, combined with a lowering of operating and maintenance costs resulting from new developments in equipment and technology, should enable electrodialysis applications to continue to grow in the future     

电渗析苦咸水淡化技术研究进展

电渗析苦咸水淡化技术具有脱盐效果好、成本较低、绿色环保等优点,但存在制膜工艺繁琐、传质模型不够精确、能效有待提升等问题。本文首先分析了苦咸水电渗析用离子交换膜的制备及改性方法,对膜材料存在的问题进行了探讨。综述对比了苦咸水电渗析在简化模型、理论模型、半经验模型方面的原理及最新进展,系统总结了常规苦咸水电渗析过程的运行方式和工艺优化策略,并进一步介绍了以新型电去离子、冲击电渗析、可再生能源驱动电渗析为代表的新型电渗析过程在苦咸水淡化方面的原理及应用。在此基础上,提出了今后的研究方向集中于降低制膜成本、优化传质模型、探究集成膜法淡化工艺以及新型电渗析过程等方面。     

Chapter 1.1 The technical and economic implications of measurement and control errors in water desalination

Dependable control systems are essential for the successful operation of a desalination plant. A well-operated process will prolong plant life, minimize maintenance costs, reduce energy consumption, and achieve the design output of desalted water. Poor process control, on the other hand, will result in scaling, corrosion, and reduced water output. In both thermal and membrane processes, temperatures, pressures, flow rates, and the concentration of additives must be carefully maintained at or near design values. Reliable pH control is essential to minimize corrosion and scale formation.This paper discusses the predominant desalination processes and, for each process, analyzes the influence of deviations in the operating parameters. Where possible, operating limits are indicated. The basic principles outlined herein are equally applicable to the newer variants of the basic desalination plant designs and to the combination processes now appearing on the market.     

Chloride behaviour in electromembrane treatment of brine issued from desalination plants

In this work, the chloride behavior through an electrochemical treatment of brines is examined using ion exchange membranes like in electrodialysis. All experiments have been performed using solutions of NaCl before an application on real brines issued from an Algerian desalination plant. After checking oxidation parameters of chloride oxidation by electrolysis, ion exchange membrane have been introduced to control both the pH and the species migrated. The effect of current density and the membrane nature has been studied. The electrochemical treatment described in this work allows transforming the brines in useful products as NaOH, HCl and Cl₂. The pH and the salt concentration are varied and the products obtained at the electrodes were identified and analyzed. It was shown that we can get chlorates according to the current density applied and the fixed pH. This fact gives rise to an economical process where valuable products can be obtained using only the chloride oxidation current. Results were linked to the Pourbaix diagram and allow the prediction of the process efficiency.     

双极膜电渗析法处理天然碱卤水的工艺条件优化

采用阴离子交换膜和双极膜组成的两隔室双极膜电渗析处理天然碱卤水,考察了膜堆电压、物料流量、温度和进料浓度对脱盐率、电流效率以及耗电量的影响.通过单因素实验和正交实验确定两隔室双极膜电渗析脱盐的最佳工艺条件为:膜堆电压24 V、物料流量5.0L·h-1、温度40℃、进料浓度2.0 mol·L-1,在此条件下,脱盐率、电流效率较高,耗电量较低,脱盐效果最好.     

On the experimental verification of an electrodialysis simulation model for optimal stack configuration design through solver software

Electrodialysis (ED) is well known as a desalination method. In the ED process, anions and cations are transported respectively through anion and cation conductive membranes, while applying an electric field. Specific process membrane stack parameters are determining the ion transport, making ED a relatively complex process. Mathematical modeling is reported in literature, enhancing the insight in the phenomena and allowing the optimization of the stack design and process conditions for a specific application. A comprehensible software based design procedure, using such a mathematical model, is therefore interesting for the industrial ED user. As a result, a selected model was implemented in a solver software. An experimental verification of the software was then performed, using an ED pilot with an industrial type of stack and sodium chloride model solutions. Prior to this, limiting current densities were measured in a small stack. The large stack was used to verify the ohmic region and to obtain basic stack model parameters. In a next step, limiting current density conditions in the large stack were theoretically evaluated in the model calculations for design purposes and correlated with the experimental results. These verifications showed that the software approach is relevant for the estimation of an optimal stack geometry. A number of model restrictions are however indicated as well and some recommendations are made with respect to the expansion and fine tuning of the selected model.