Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis

DES-11947; No of Pages 11 Desalination xxx (2013) xxx–xxx

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Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis Mònica Reig a, Sandra Casas a,d, Carlos Aladjem b, César Valderrama a,⁎, Oriol Gibert a,d, Fernando Valero c, Carlos Miguel Centeno c, Enric Larrotcha d, José Luis Cortina a,d a

Chemical Engineering Dept. UPC-Barcelona TECH, Av. Diagonal 647, 08028 Barcelona, Spain SOLVAY Ibérica SL.C/Marie Curie 1-3-5, 08760 Martorell, Spain Aigües Ter LLobregat (ATLL) Sant Martí de l'Erm, 30. 08970 Sant Joan Despí, Barcelona, Spain d CETAQUA Carretera d'Esplugues, 75, 08940 Cornellà de Llobregat, Spain b c

H I G H L I G H T S • • • • •

IXM-ED pilot plant was used to evaluate the efficiency in concentrating a SWRO brine. NaCl concentration was evaluated covering the temperature range in a semiarid climate. Results show that IXM-ED can concentrate SWRO brines up to 245 g/L NaCl. Multivalent ions removal was achieved due to IXM selectivity and ion complexation. Energy consumption was 0.12 kWh/kg NaCl for 185 g NaCl/l at 27 °C and 0.35kA/m2.

a r t i c l e

i n f o

Article history: Received 3 August 2013 Received in revised form 17 December 2013 Accepted 18 December 2013 Available online xxxx Keywords: Ion exchange membranes Energy consumption Brine reuse Chlor-alkali industry

a b s t r a c t Currently, numerous studies are focused on the valorisation of seawater desalination reverse osmosis brines. Electrodialysis can be used to concentrate one of the primary components (NaCl) and obtain a suitable raw material for industrial applications, such as the chlor-alkali industry. An electrodialysis pilot plant was used to evaluate the efficiency of concentrating a seawater reverse osmosis (SWRO) brine under representative full-scale operational conditions covering the temperature range of a semiarid climate. The results indicate that electrodialysis is a technology that can concentrate SWRO brines from approximately 70 to 245 g/L NaCl, achieving an additional intrinsic purification of major multivalent ions (Ca2+, Mg2+, SO2− 4 ) due to the selectivity patterns of ion exchange membranes and the ion-complexation reactions in the concentrated brines. However, minor components, such as Ni and Cu, are concentrated due to the formation of Cu and Ni complexes with chloride ions to form monocharged species (e.g., NiCl+ and CuCl+). Energy consumption values of 0.12 kWh/kg NaCl for 185 g NaCl/l at 27 °C and 0.35 kA/m2 or 0.19 kWh/kg NaCl for 203 g NaCl/l at 27 °C and 0.50 kA/m2 were reached. These results were compared with the data obtained from the literature for salt production by electrodialysers. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The desalination of seawater and brackish water sources is a common method for providing fresh drinking water around the world. The use of reverse osmosis (RO) membranes is becoming an increasingly popular option for desalination, due to significant improvements in energy recovery systems and pre-treatment processes over the past couple of decades [1–3]. However, the disposal of the brines generated by the desalination process poses significant environmental issues, due to

⁎ Corresponding author at: Departament d’Enginyeria Química, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain. Tel.: + 34 93 4011818; fax: +34 93 401 58 14. E-mail address: [email protected] (C. Valderrama).

the high concentrations of salts and increases in the concentration of transition and heavy metals [4–8]. Traditional management of RO concentrates from desalination plants is mainly conditioned by the location of the plant. In the case of in-land plants, well injection into deep aquifers is one of the preferred options. In the last decade, new demonstration projects have been addressed to achieve an effluent volume reduction by either solar evaporation ponds or thermal evaporation [9,10]. Brine volume reduction by evaporation techniques results in a solid product that can more easily be disposed of compared to the original concentrate, whereas the low salinity effluent can be reused to increase the water production ratio or it can be directly discharged into surface or ground water bodies [11–13]. The normal management option in coastal areas is the direct discharge into the sea or, after appropriate dilution, with a secondary

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Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

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M. Reig et al. / Desalination xxx (2013) xxx–xxx

discharge (e.g., cooling streams of power plants) [14,15], although stringent regulations limit the development of desalination plants due to the restrictions and requirements on brine discharge. Environmentally sound disposal of seawater and industrial desalination brines will be a growing need in the next decades [16,17]. In the case of direct discharge, due to the adverse effects of brine discharge, together with its associated costs, research efforts in the last decades have been centred on reducing the impact of brines by reducing the volume and/or by diminishing the most critical pollution load [18,19]. However, the exponential increase of desalination potential, especially in the south of Europe, the Middle East and Australia, has promoted the concept of defining valorisation routes of brines or brines by-products. Conventionally, four components are extracted from seawater by evaporation: table salt (sodium chloride) and the by-products potassium chloride, magnesium salts and bromide salts [20–22]. Extraction processes for other components may be feasible because they are sufficiently valuable or rare on land, as proposed by Le Dirach et al. [23] who identified eight elements (sodium, magnesium, rubidium, potassium, phosphor, indium, caesium and germanium) that are potentially economically and technically viable. More recently, Jepessen et al. [24] examined the potential for economic extraction of rubidium and phosphorus and analysed the potential cost of potable water production for varying levels of extraction of sodium chloride, taking as reference for the calculations a typical, large RO plant (100,000 m3/d). These types of solutions are aligned with recent developments in zero liquid discharge desalination systems [25]. These entail further processing of the concentrate until dry salts are obtained, which can be disposed to landfills or used in other applications. These systems have multiple benefits: avoiding discharge to surface or ground waters, flexibility in site selection, and efficient reuse of water [26]. From an environmental perspective, such systems are desirable; however, further concentration of the brine can be achieved only by thermal processes, which adds significantly to the overall cost of desalination [27]. These problems can be overcome by developing 1) technologies that reduce brine volumes to increase salts concentrations and 2) separation technologies for the isolation of desired salts and purification, such as crystallisation processes [28]. The different options have been thoroughly reviewed [21,29]. Conventional treatments, such as concentration in evaporation ponds, have several disadvantages, such as extensive land use and low productivity [30,31]. Thus, investigation on new approaches to improve the valorisation options is a current demand. Membrane distillation (e.g., vacuum membrane distillation) has been studied as an alternative for the processing of highly concentrated aqueous solutions over conventional distillation processes because it operates in the range of 60–80 °C and provides a high contact area per unit of equipment volume, allowing very compact installations and a reduced footprint. However, development is still at the research level without successful industrial implementation, and the efforts are centred on the recovery of valuable by-products to reduce the high capital and operational expenditures and make it more economically attractive [29–31]. The major technical barrier to achieving zero liquid discharge systems has historically been the problem of scale formation (precipitation of alkaline earth metals such as CaCO3, Mg(OH)2 and CaSO4) in highly concentrated brines [32]. Recently, Kornold et al. [33] proposed electrodialysis (ED) to increase the brine concentration and thus reducing both the volume of the brine effluent and the cost of its disposal. The main drawback of this technique (membrane precipitation of CaSO4) is the continuous removal of gypsum from the brine using a separate precipitator, in which gypsum seeds precipitated the excess CaSO4 in the oversaturated solution. By applying ED to synthetic brine effluents, simulating effluents from the desalination of brackish and industrial waters, their salt concentration was increased from 0.2–2% to 12–20% with an energy consumption of 1.0–7.0 kW h/m3 in contrast to approximately 25 kW h/m3 needed by thermal evaporation [34]. In a second study

using RO brines saturated with CaSO4 and silica, brines were concentrated from 1.5% to approximately 10% with similar specific energy requirements (stack cell) and electrical efficiency as the previous study. Assuming that this process or similar techniques can be adopted, the production of highly concentrated brines that can be thermally treated to a dry product for the recovery of selected chemical resources can be performed. The use of SWRO brines as raw material for salt production was evaluated in the seventies by the Government Chemical Industrial Research Institute of Japan [35]. Seawater was desalinated by a flash evaporator (e.g., RO was not fully developed at this stage), and the discharged brine was concentrated by ED for further use in the production of chlorine and sodium chloride. Tanaka et al. [36] recently suggested that for areas with fresh water availability problems (e.g. Southeast Asia, the Middle and Near East), where desalination plants are installed, RO brine valorisation will be a need. Tanaka et al. [36] identified the possibility to obtain salt, which is not produced for industrial use in a manufacturing plant by means of ED technology. Moreover, industry in these countries is in development and the consumption of chlorine and sodium hydroxide is expected. In this study [36] was determined that the energy consumption in a salt manufacturing process (200,000 t NaCl/y as production capacity) using the brine discharged from an RO seawater desalination plant was 80% of the energy consumption in the process using seawater. It was also found that the optimum current density that minimised energy conditions was 0.3 kA/m2 for ED fed with either brine discharged from the RO desalination plant or seawater. Most of the research work has been based on the technical feasibility of isolating by-products, mainly as salts, where morphology and purity requirements need to be fulfilled. Analysis of the total amounts of potential raw materials present in the brines (NaCl, MgCl2, MgSO4, CaCO3, KCl, KBr, and CO2(g)) has opened the possibility of linking the seawater desalination plants to chemical production plants (e.g., chlor-alkali and fertiliser industries) [37–40] or even energy production (liquid fuels), as recently proposed by Eisaman et al. [41], with a process based on CO2(g) extraction from seawater using bipolar-electrodialysis. The present work reports the evaluation of ED as a technological solution to recover a by-product (NaCl) for the chlor-alkali industry from SWRO desalination brines. 2. Materials and Methods 2.1. Seawater reverse osmosis (SWRO) desalination brines The ED unit was fed with SWRO brine pumped directly from the brine deposits of the El Prat Seawater Desalination Plant (Aigues Ter Llobregat), Barcelona (Spain). The average composition is shown in Table 1. The brine was concentrated under different operation conditions to obtain the maximum NaCl concentration and the minimum energy consumption. Several experiments were conducted in a range of temperatures from 10 to 28 °C and currents densities from 0.30 to 0.60 kA/m2. The experimental programme included 24 months of operation to cover the lower seawater temperature of the Mediterranean

Table 1 Composition of seawater reverse osmosis desalination brine of El Prat Desalination Plant (Barcelona, Spain). Major components

Concentration (g/L)

Minor components

Concentration (mg/L)

Cl− Na+ SO2− 4 Mg2+ 2+ Ca K+ Br− Sr2+

38.8 20.8 5.41 2.64 0.83 0.75 0.13 0.016

SiO2 Al3+ Fe3+, Fe3+ Ba2+ Ni2+ Cu2+ Mn2+ Cr3+

b1 b0.5 b0.2 b0.2 0.07 ± 0.02 0.03 ± 0.01 0.01 ± 0.01 0.007 ± 0.003

± ± ± ± ± ± ± ±

0.4 0.3 0.2 0.2 0.04 0.05 0.06 0.003

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

M. Reig et al. / Desalination xxx (2013) xxx–xxx

Sea in winter time (10 °C) and the higher seawater temperatures of summer time (28 °C). As the brine was concentrated along the ED process, the potential precipitation of the brine components was estimated. The saturation index of minerals whose precipitation could limit the concentration step was calculated with the PHREEQC code [42] using the PITZER equilibrium data base. The saturation index of inlet brine and its evolution as a function of the concentration phenomena occurring on the concentrate circuit were calculated to determine the potential risks of precipitation.

2.2. Analytical methodology and chemical analysis Samples were taken from the feed tank, the inlet brine, and the diluate and concentrate flows leaving the stack every 2 hours. First, the pH was measured in every sample using a glass electrode. The NaCl, 2+ SO2− and Mg2+ levels in the samples were analysed using differ4 , Ca ent analytical methods. Cl−was determined potentiometrically through precipitation with AgNO3 and an AgCl electrode in a METHROM 721 instrument. Ca2+ and Mg2+ were determined by atomic absorption spectrophotometry using a Perkin Elmer Analyst 300. SO2− 4 was determined by ionic chromatography using a Methrom 761 Compact IC equipped with an Anion Dual 2–6.1006.100 column. Finally, NaCl was determined by electrically balancing the major ions of the solution. The conductivity and temperature of the diluate and concentrate streams were also monitored during operation to ensure the concentration process was successful. Minor and trace elements of the brines (Sr, Cu, Ni, Cd, Hg, and Pb) were analysed by ICP-OES, Variant 725 at the end of each experiment. Along the experimental programme, the membrane stack was open for membrane maintenance when needed. Membrane surfaces, especially those close to both electrodes, were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) using a JEOL 3400® scanning electron microscope with an energy dispersive system.

2.4. Limiting current density (LCD) The maximum operating current intensity using SWRO brine was established at the ED pilot plant using a procedure described previously [43]. In the voltage range studied with the SWRO brine, thanks to the single-pass configuration and the high concentration of the solution, no increase of resistance was observed, and the LCD could not be determined. Although the maximum nominal voltage that could be applied to the stack was 65 V, which corresponds to a maximum current density of 0.65 kA/m2, the pilot plant operated at current densities not higher than 0.60 kA/m2 to avoid the formation of H2 bubbles inside the stack. To reduce this problem, the cell was placed vertically and a second gasket in the cathode circuit was added to help with gas removal. 2.5. Evaluation of the brine concentration data analysis: concentration factors (CF), membrane selectivity (SAB ) and energy consumption (ENaCl) The concentration factors (CF) were calculated according to Eq. (1): C FA ¼

C A ðt Þ C A;0

ð1Þ

where CA,0 and CA(t) are initial concentration and the concentration at a given time t of component A in the concentrated loop, respectively. The membrane selectivity (SAB) was calculated as proposed by Zhang et al. [44] according to Eq. (2): A

SB ¼

C F A −C F B ð1−C F A Þ þ ð1−C F B Þ

ð2Þ

where CFA and CFB are the concentration factors of components A and B, respectively. The energy consumption (E) for the NaCl concentration was calculated in two different ways: a) In a batch process, EB(NaCl) ((kWh/kg NaCl)), as the energy necessary to increase the NaCl concentration in the tank, was calculated according to Eq. (3):

2.3. ED pilot plant description and operation The ED pilot plant has been described elsewhere [43]. The ionexchange membranes stack was an EURODIA AQUALIZER SV-10 with 50 cell pairs made up of Neosepta cation-exchange membranes (CIMS) and anion-exchange membranes (ACS) (1000 cm2 active surface area per membrane). The stack dimensions were 620x450x313 mm. The intermembrane distance was 0.43 mm, whereas linear flow velocity at the inlet of desalting and concentrating cells was around 10.8 cm/s. The brine flow rate through the ion-exchange membranes stack was 0.50 m3/h in both the concentrate and the diluate compartments, and 0.15 m3/h in the electrolyte chambers. The diluate and electrolyte circuits had a single-pass design to operate at higher current densities and minimise the problems of the increase of temperature in the cell and the precipitation of CaSO4 in the diluate compartments. The concentrate flow was recirculated until the maximum concentration was reached. Generated H2 accumulation in the catholyte chamber was prevented by ventilation, whereas Cl2 formed in the anolyte chamber was neutralised with sodium bisulphite. The volume of the feed (SWRO brine) and concentrate containers were 1 m3 and 0.25 m3, respectively. Hydrochloric acid was added to keep the pH of the cathodic circuit below 3, below 7 and 5.5 in the diluate and the concentrate stream, respectively. The current densities were varied between 0.30 and 0.60 kA/m2. The inlet and outlet temperatures were monitored during all the experiments. The brine concentration process was monitored by in-line measurements of conductivity, temperature, pH, flow-rate, pressure, current intensity and voltage.

3

EBðNaClÞ ¼

V cell  I·t v tank  C NaCl

ð3Þ

where Vcell is the mean membrane stack potential (V), I is the applied current intensity (A), t is the operation t (h) and vtank is the volume of the concentrate tank (L). b) In a continuous process, EC(NaCl) (kWh/kg NaCl), as the energy necessary to generate the product overflow in the concentrated brine tank, calculated according to Eq. (4) [43]: EC ðNaClÞ ¼

V cell  I P NaCl overflow

ð4Þ

where PNaCl_ overflow is the concentrate brine production overflow (g NaCl/h), and Vcell is defined as [45]: V cell ¼ ðr mem þ r dc þ r cc ÞI þ V mem

ð5Þ

where rmem, rdc, and rcc describe the ion exchange membrane, the diluate circuit and the concentrate circuit resistances, and Vmem is the average membrane potential [36]. Vcell was experimentally measured. 3. Results and discussion 3.1. Evaluation of the NaCl concentration. The evolution of the NaCl concentration results for different current densities (from 0.30kA/m2 to 0.6 kA/m2) at three different temperature

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

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ranges: a) T N 25 °C, b) 25 °C ≥ T ≥ 20 °C and c) T b 20 °C, are plotted in Fig. 1. A summary of the concentration factors achieved for the more relevant experiments is given in Table 2. For the three temperature ranges, the concentration of NaCl in the concentrate tank increased gradually with time (from an initial 65 g NaCl/L) until a plateau was reached after several hours at concentrations between 210 mg/L and 265 mg/L, depending on the current density applied: the higher the intensity applied, the higher the final concentration of NaCl reached (Fig. 1). In the first step, an increase of the electrical potential is applied, favouring two main mass transport phenomena: a) the transfer of charged species in the solution to the cathode or anode through the ion-exchange membranes (ion migration flux) and b) the flux of water (as hydrated water to ions) due to ion migration, through the ion-exchange membranes (electro-osmosis flux). However, the concentration gradient between the diluate and the concentrate circuits increased from 0 up to 90 to 160 g/L NaCl, depending

on the intensity and temperature conditions. A gradient of NaCl concentration appears and two mass transport phenomena occur: diffusion of NaCl from the concentrate compartment to the diluate compartment (NaCl back diffusion) and osmotic flux. In this case, water is transported from the diluate compartment to the concentrate one. These two mass transfer phenomena diminish the desired objective of increasing the NaCl concentration on the concentrate tank and increase the energy requirements of the process. As seen in Fig. 2, for current intensities of 0.3kA/m2 (Fig. 2A) and 0.4kA/m2 (Fig. 2B) at the low temperature range (b20 °C), NaCl concentrations of 220 and 250 g/L, respectively, were achieved after 25 hours of operation, whereas only 170 and 200 g/L were achieved at the high temperature range (N25 °C). Lengthening the experiment time did not provide any substantial increase in the NaCl concentration (increase differences less than approximately 2 g/L) after 15 hours of additional concentration. Although the production of concentrate brines close to NaCl saturation could be desirable

Fig. 1. NaCl concentration vs. time at various current densities and different temperatures applied A) T N 25 °C, B) 25 °C ≥ T ≥ 20 °C, and C) T b 20 °C.

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

M. Reig et al. / Desalination xxx (2013) xxx–xxx

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Table 2 Composition of concentrated brines, concentration factors (CF) and time necessary to reach the maximum NaCl concentration for experiments at 30A (0.3 kA/m2) and 40A (0.4 kA/m2) at 14 °C and 10 °C, and at 35A (0.35 kA/m2), 50A (0.5 kA/m2) and 60A (0.6 kA/m2) at 28° inlet SWRO brine. 0.30 kA/m2 (14 °C)

0.40 kA/m2 (10 °C)

0.35 kA/m2 (28 °C)

0.50 kA/m2 (28 °C)

0.60 kA/m2 (28 °C)

g/L NaCl K+ Mg2+ Ca2+ SO2− 4

C0 71.5 0.89 2.5 0.8 4.7

Cf 239 3.33 1.3 0.7 1.4

CF 3.3 3.7 0.5 0.9 0.3

C0 57.7 0.68 2.5 0.8 4.9

Cf 264 3.55 1.2 0.7 1.0

CF 4.5 5.2 0.5 0.9 0.2

C0 65 0.70 2.2 0.6 5.4

Cf 210 3.10 1.1 0.4 1.7

CF 3.2 4.4 0.5 0.6 0.3

C0 65 0.77 2.3 0.7

Cf 244 3.2 1.3 0.3

CF 3.8 4.2 0.6 0.4

C0 65.1 0.74 2.3 0.7

Cf 261 3.50 1.2 0.3

CF 4 4.7 0.5 0.4

mg/L Al3+ Ni2+ Sr2+ Cu2+ Time (h)

0.05 0.06 15.0 0.02 35

DL 0.14 12.7 0.03

– 2.3 0.9 1.5

0.03 0.05 14.7 0.03 38

DL 0.12 10.7 0.04

– 2.4 0.7 1.3

0.28 0.06 13.2 0.02 15

0.17 0.11 13.2 0.2

0.6 1.8 1.0 10

0.11 0.06 14.0 0.03 14

0.10 0.20 14.0 0.2

1 3.3 1.0 6.6

0.08 0.06 13.6 0.02 13

DL 0.12 13.0 0.13

– 2.0 0.9 6.5

DL: Detection limit.

(290 g/L) under the studied conditions (current densities and temperature), those values could not be reached. An extra evaporation or the addition of a solid salt might be required, depending on the application, e.g., raw material for the chlor-alkali industry. The concentration factors (CF) achieved were higher than those for monovalent ions (Na+, K+ and Cl−). In Table 3, CF and the selectivity of the ACS and CIMS membranes operated at 0.3 kA/m2 with SWRO brines are compared with published data on brine concentrations under similar concentration ranges. The most similar studies are the use of CLS-25 T and AVS-4 T Neosepta membranes for salt production at 0.2 kA/m2 [46], Selemion ASA and CMA membranes at 0.3 kA/m2 [47], Selemion CMV and AMV [48] and Selemion CMT and AMT Ashahi Glass [33]. CFs were higher in the reviewed studies because the initial concentrations were lower. CFs obtained by Turek et al. [48] were especially high because the brine was treated using batch ED. The comparison

of these results is valuable for the identification of energy requirements ranges as a function of the brine source, although, there is a limitation due to the scarce published data available to compare. The temperature influence on the concentration process in ED is more complex than that exerted by the current intensity because the different mass transfer processes are temperature dependent. Most of the published data are at laboratory scale and typically at room temperature (20–25 °C). Temperature influences the main transport processes affecting the ionic species and water and also affects the properties of the brines (e.g., density and electrical conductivity), the properties of the solvent (diffusivity) and the membrane properties (electrical conductivity and membrane resistance). Only Tanaka et al. [49–51] reported on the concentration of seawater with ED with similar membranes and a similar tendency of the salt concentrations was identified. The approach developed by Tanaka for the concentration of seawater to

Fig. 2. NaCl concentration vs. time at various temperatures and different electric current applied A) 0.3kA/m2 and B) 0.4kA/m2.

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

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Table 3 Concentration factors (CF) and selectivity obtained at 0.3 kA/m2 and 14 °C with SWRO brines and CIMS/AMS membranes compared with the data from different previous experiences reported in the literature.

Feed inlet Na+ Ca2+ Mg2+ Cl− SO2− 4 S(Na/Ca) S(Na/Mg) S(Cl/SO4)

CIMS//ACS

CLS-25 T/AVS-4 T [46]

ASA/CMA [47]

CMV/AMV [48]

CMT/AMT [33,34]

SWD-RO brine 3.2 0.8 0.5 3.2 0.3 −1.2 −1.6 −1.9

SW 5.7 7.6 6.0 5.7 0.6 0.2 0.0 −1.2

SW 9.7 6.3 2.7 11.5 0.2 −0.2 −0.7 −1.2

BWRO brine 11.1 10.4 10.8 11.1 10.3 0.0 0.0 0.0

BWRO brine – 1.1 1.8 1.4 1.3 – – −0.1

produce salt water was derived of experimental equations from a high number of experimental assays of ion transport through the ion exchange membranes in addition to the correction of temperature for physical parameters, such as density, electrical conductivity and membrane resistance. A model describing the concentration process has been published recently [52,53]. Membrane pair characteristics of three different IX membranes were measured as a function of the current density and seawater temperature allowed for the determination of the hydraulic permeabilities and the cell electric resistances by using the same empirical functions of temperature. Temperature influence on the performance of the electrodialyser to increase the NaCl concentration in the brine and the energy consumption implies an increase of consumption with a decrease of temperature. A decrease of the temperature from 60 °C to 25 °C is accompanied by an increase of the

Fig. 3. Divalent ion concentration vs. time at different electric current applied. A) Ca2+ high temperature, B) Mg2+ high temperature, and C)SO2− low temperature. 4

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

M. Reig et al. / Desalination xxx (2013) xxx–xxx

energy consumption up to 20%; however, the increase of temperature limits the concentration factor, which could be reduced by up to 30%. The direct current electric resistance of a membrane pair is calculated, and it is predominant over that of a desalting cell and a concentrating cell. It is necessary to decrease the electric resistance of an ionexchange membrane for reducing energy consumption in a salt manufacturing process. 3.2. Evaluation of the behaviour of minor and trace species The ion exchange membranes used in this study (ACS and CIMS) proved to be highly selective to mono-charged ions (e.g., Na+, K+ and Cl−), as the selectivity factor was the highest in all cases reviewed. The influence of the brine composition on the speciation of the main components of the SWRO brine (Cl−, Na+ and K+) as a function of pH was determined with the Hydra and Medusa codes [54]. The results indicated that the main species present in the brine solution were Cl− (I) as Cl− (N 98%); Na(I) as Na+ (90%), NaClaq (8%) and NaSO− 4 (2%); and K(I) as K+ (87%), KCl(9%) and KSO−4 (4%). The evolution of the concentration of Ca2 +, Mg2 + and SO24 − as a function of time for different current intensities is shown in Fig. 3. For

1.0

0.6 0.4

Cu 2+

0.6 0.4 CuCl +

0.2

0.2 MgCl

Cu(OH) 2

A

0.8

Mg 2+

Fraction

Fraction

0.8

minor components (Ca2 +, Mg2 + and SO24 −), the concentrations decreased in the concentration tank, reaching a CF close to or below 1 as a consequence of the concentration reduction from the initial values of the SWRO brines. The sulphate, calcium and magnesium concentrations in the diluate were similar to the inlet brine composition, given that the circuit had a single-pass design. This confirmed the fact that multi-charged ionic species (Ca2+, Mg2+ and SO2− 4 ) in the concentrate tank were mainly diluted by water transport (osmotic water ion flux) inside the cell and that the migration of these ions (ion flux) was not significant because the membranes are selective to monovalent species. For this reason, multivalent ions concentration plunged and as the time increases, a reduction of the slope of this function is observed; obtaining a value close to zero, and the diffusion process, back diffusion and osmotic flux are the dominant phenomena. The measured values indicated that CF was lower for sulphate (0.2– 0.3) than for Ca (0.4–0.9) and Mg (0.5–0.6) ions. In addition to the influence of the dilution effect due to the electro-osmotic flow, other factors are affecting ions selectivity with the ion exchange membranes or the potential effect of the ions speciation. Literature review shows scarce data on the ion exchange selectivity coefficients for chloride/sulphate with ACS and Na/Ca and Na/Mg with CIMS. Only CFs could be found on the application of ED for the concentration of seawater or on the

Mg(OH) 2 (c)

A

1.0

7

MgSO

+

4

Cu(OH) 4 2Cu(OH) 3-

CuSO 4

CuCl 2

0.0

0.0 2

4

6

8

10

2

12

4

6

pH 1.0

1.0

B Ca 2+

10

12

Ni(OH) 2 (c)

B

0.8

Fraction

Fraction

0.8

8

pH

0.6 0.4

0.6 Ni 2+

0.4 NiCl 2

0.2

0.2

NiCl +

CaSO4

CaCl +

0.0

0.0 2

4

6

8

10

2

12

4

6

1.0

1.0

C

8

10

12

pH

pH

C

SO 4 2-

Sr 2+

0.8

Fraction

0.8

Fraction

Ni(OH) 3-

NiSO 4

0.6 0.4 MgSO

0.2

4

0.6 0.4 0.2

SrSO 4

HSO4-

SrCl

CaSO 4

0.0

0.0 2

4

6

8

10

+

12

pH Fig. 4. Speciation of Mg2+, Ca2+ and SO2− (fraction percentage) in the SWRO brine as a 4 function of the pH. Calculations performed with the Hydra and Medusa codes [54].

2

4

6

8

10

12

pH Fig. 5. Speciation of Cu2+, Ni2+ and Sr2+ (fraction percentage) in the SWRO brine as a function of the pH. Calculations performed with the Hydra and Medusa codes [54].

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

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M. Reig et al. / Desalination xxx (2013) xxx–xxx

treatment of seawater RO desalination brines, where pre-concentrations 2+ factors were from 0.3 to 10 for SO2− and from 0.5 4 , from 0.8 to 10 for Ca 2+ to 11 for Mg [34,49–51]. A second effect to be taken into account to explain the differences in behaviour between the three ions is an indication that the selectivity of the cation and the anion membranes for Ca2+, Mg2+ and SO2− could be influenced by the properties of the involved 4 species in solution. The speciation diagrams as a function of the pH of Ca2+, Mg2+ and SO2− in the treated brines are plotted in Fig. 4. In the 4 pH working range of 5 to 7, the high concentration values of chloride (1.1 M) is promoting the formation of Ca and Mg chloride complexes (CaCl+ and MgCl+), and the strong complexing properties of sulphate ions with Ca2+ and Mg2+ promote the formation of the non-charged complexes (MgSO4(aq) and CaSO4(aq)). In the treated brines, the three species are present in solution as mixtures of bi-charged, monocharged and non-charged species with the following distribution (%): Mg: Mg2+ (80%), MgSO4aq (12%) and MgCl+ (8%); Ca: Ca2+ (82%), CaSO4aq (10%) and CaCl+ (8%); and sulphate: SO24 − (72%), MgSO4aq (22%) and CaSO4aq (6%). The lower concentration factors for sulphate demonstrate the higher selectivity of the anion exchange membrane for chloride than sulphate, with the transport potentially enhanced due to the presence of two non-charged species that suffer from electrical exclusion on the anion exchange membranes. In the case of the Ca and Mg species, the selectivity of the cation exchange membranes for Na over Ca and Mg is lower than the selectivity value for chloride/sulphate, and an additional 8% of the Ca and Mg species are in the form of monovalent species and could more easily be transported through the cation exchange membranes. The cation concentration order of Ca(II) N Mg(II) is in agreement with the fact that the hydrated ionic radii of Ca2+ is lower than for Mg2+ (stokes radius of 0.349 over 0.429 nm), and a slightly higher CF for calcium was obtained compared to magnesium. However, other minor components present in the brines, (Ni(II) and Cu(II)), did not follow the general trend; their concentrations increased in all the experiments. Fig. 5 shows that in the pH range studied (5–7), the formation of mono-charged species of nickel and copper are NiCl+ and CuCl+, and their transport through the membrane is enhanced. The concentration factors of copper are 2 to 3 times higher than the concentration factors of nickel, depending on the temperature and the current density. The speciation diagrams of Fig. 5 show that the fraction of CuCl+ in the brine is up to 2 times greater than the fraction of NiCl+. Consequently, it can obtain higher concentration factors because more copper is available to be transported. Contrary to the case of Sr(II), CF between 0.7 and 1 are described for Ca(II), along with similar complexation behaviour with the formation of SrCl+. For the trace metals present in the brines, the concentration order was Cu (II) N Ni (II) N Sr (II). Diluate production from the cell was constant in all experiments at approximately 50–52 g NaCl/L, which means that only 6–8 g/L NaCl was removed in each pass, depending on the current density applied. The current efficiency showed that chloride transported more than 80% of the current in the stack for the experiments at low temperature. When the inlet temperatures were high, this value was reduced due to a

higher water flux in the stack, reaching approximately 50% of the transport. The current efficiency of the chloride ion was improved after cleanings of the membranes, reaching more than 92% of the current transport for the lower inlet temperatures. In Table 4, the brine concentration obtained at the end of the experiments is compared to the membrane electrolysis requirements in the chlorine production industry. The brine needs to have higher NaCl concentration and impurities content must be lower than the achieved values to avoid precipitation on the membrane surface or to increase the efficiency of the process. These requirements are specified in the last column of Table 4. The elements that do not meet the electrolysis requirements are Ca, Mg, Sr, Cu and Ni. In all cases, the brine should be further concentrated to reach the saturation limit. This could be achieved by adding a small amount of solid salt (between 90 and 40 g/L NaCl). Moreover, the brine would also need a purification step to remove calcium, magnesium, copper, nickel and strontium in all cases studied. After ten months of operation, the stack was opened to analyse the membrane integrity. The membranes were examined by SEM-EDS, and no precipitates were observed on their surfaces. The absence of precipitates of magnesium, calcium and carbonates supports the deduction provided in the analysis of the brine saturation indexes. The initial inlet brines were oversaturated with calcium carbonate, magnesium carbonate and calcium magnesium carbonate. Additionally, the evolution of the saturation index for NaCl, KCl, MgCl2•6H2O, CaSO4•2H2O, SrSO4, CaCO3, MgCO3 and CaMg(CO3)2 as a function of the concentration process indicates that the brine could be concentrated up to 4 times (approximately 280 g/L NaCl) without risk of NaCl precipitation. Because the brines contained the antiscalant from the RO process, precipitation of carbonate and sulphate salts did not occur easily, despite their oversaturation, thus benefiting the ED process. The antiscalants used in the RO process (1–2.5 mg/L of PC-1020) were mainly super-threshold agents that were able to stabilise supersaturated salt solutions to prevent precipitation of carbonate and sulphate salts. Moreover, HCl was added at different points of the concentrate to maintain a constant pH of 5.5 and to ensure that most of the inorganic carbon was present as bicarbonate, which does not cause scaling. At the end of the two years of the research evaluation programme, the stack was open again and a membrane autopsy was completed. In addition to the visual inspection of the anion and cation exchange membranes, those identified in this step as potentially more affected were analysed by SEM-EDAX, FTIR and XPS. The SEM-EDAX analysis showed the scattered presence of some precipitates, in most cases, crystalline forms containing typical (O and Si), and also the presence of non-crystalline precipitates rich in O, Fe, Al and in minor cases, Mn. Samples were also analysed by XRD, and no crystalline compounds were detected, or their size was so small that there was no diffraction. The membrane surfaces were analysed, and the main elements detected in the cationic membrane were C, O, S, Na, Cl, and K, and the main elements detected in the anionic membrane were C,O, N and Cl. The FTIR analysis did not detect the presence of mineral forms of carbon, such as Ca and Mg carbonates, and the

Table 4 Comparison of concentrated brines obtained with ED and membrane cell requirements. 0.30 kA/m2 (14 °C)

0.40 kA/m2 (10 °C)

0.35 kA/m2 (28 °C)

0.50 kA/m2 (28 °C)

0.60 kA/m2 (28 °C)

Membrane cell requirements

g/L NaCl K+ Ca2+ Mg2+ SO2− 4

239 3.33 0.6 1.3 1.4

264 3.55 0.7 1.2 1.0

210 3.10 0.4 1.1 1.7

244 3.20 0.3 1.3

261 3.50 0.3 1.2

300 – 20 μg/L

mg/L Al3+ Ni2+ Sr2+ Cu2+

DL 0.14 12.7 0.03

DL 0.12 10.7 0.04

0.17 0.11 13.2 0.2

0.1 0.2 14.0 0.2

8

DL 0.12 13.0 0.1

0.1 0.01 0.4 0.01

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

M. Reig et al. / Desalination xxx (2013) xxx–xxx

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Fig. 6. Energy consumption working under batch configuration as a function of NaCl concentration in the tank and current density applied for different inlet temperatures.

peak assignments were in agreement with the main functional groups of the anionic and cationic ion exchange resins. Finally, the results of the XPS analysis of both cationic and anionic membranes detected C, O, S, Na and Cl for the cationic membranes and C, O, N, Na and Cl in the anionic membranes. Only Si was detected as a foreign element in both membranes, and Ca and Mg were not detected. 3.3. Energy consumption on the concentration of NaCl (ENaCl) from seawater reverse osmosis desalination brines The energy consumption increases needed to increase the NaCl concentration in the tank (batch operation (EB(NaCl))) over the concentration reached at different current densities are shown in Fig. 6. EB(NaCl) rises gradually in all the experiments until the concentration in the tank reaches approximately 210 g/L NaCl. The energy consumption increased dramatically because osmosis and ion diffusion fluxes inside the stack reach a maximum due to the concentration gradient between the diluate and the concentrate compartments. The NaCl concentration in the tank increased very slowly from this point. Then, the energy applied was used to slowly increase the NaCl concentration of the tank, and the specific energy consumption increased. If the process needs to be operated in a batch mode, the optimum energy consumption concentration would be obtained approximately 200 g/L NaCl at 0.2–0.3 kWh/kg NaCl in the range of 0.3–0.4 kA/m2, depending on the current density applied and the feed inlet temperature. A summary of the energy consumption for continuous operation (EC(NaCl)) for different operation conditions is shown in Table 5. Although

the electrode potentials are not considered in industrial cells because the voltage produced is low compared to that produced by the total number of cells, for pilot plants, the electrode potential significantly increases the energy consumption and must be taken into account. The energy consumption sharply increased from 0.5 to 0.6 kA/m2 (28 °C), thus, 0.5 kA/m2 was considered the maximum operable point at higher inlet temperatures. The temperature affects the product flow rate, and lower flow rates were obtained at higher temperatures. A comparison of the performance of an industrial electrodialyser to obtain solid salt from seawater and the results obtained in the pilot plant in Barcelona in continuous operation and batch mode can be found in Table 5. The values obtained in the pilot plant were within the range of the industrial process in all cases studied. Higher concentrations were reached in the pilot plant with slightly higher current densities and energy consumptions. For laboratory scale studies [55], the energy consumption with NaCl solutions up to approximately 100 g/L ranged between 0.18 and 0.33 kWh/kg with 80% solute recovery and current densities of 0.20 to 0.30 kA/m2 were reported. Furthermore, Turek et al. [56] worked at laboratory scale obtaining more than 300 g/L NaCl in the concentrate stream with a current density of 0.6kA/m2, nevertheless, the energy consumption reported was 0.28 kWh/kg NaCl transported. Technical targets when operating electrodialyzers to produce solid salt from seawater in Japan are obtaining a concentration higher than 200 g/L NaCl with energy consumption lower than 0.12 kWh/kg NaCl [57–59]. This value was nearly reached 0.12 kWh/kgNaCl when operating the plant in a continuous mode at 27 °C at 0.35 kA/m2. Also, for 203 g/L NaCl at 27 °C with and at 0.50 kA/m2 an ENaCl of 0.19 kWh/kg was

Table 5 Comparison of the performance of an industrial electrodialyzer using seawater in Japan and the pilot plant using SWRO brine in Barcelona.

Industrial cell [45] Batch mode

Continuous mode

Current density (kA/m2)

Temperature (°C)

Energy consumption (kWh/kg NaCl)

NaCl concentration (g NaCl/L)

0.27 0.30 0.40 0.35 0.50 0.60 0.30 0.30 0.35 0.40 0.45 0.50 0.50 0.60 0.60

23.5 14.0 10.0 28.0 28.0 28.0 16.0 18.0 27.0 17.0 18.0 20.0 27.0 20.0 27.0

0.160 0.20 0.30 0.29 0.35 0.38 0.16 0.15 0.12 0.20 0.26 0.26 0.19 0.30 0.24

174 199 209 208 186 208 176 176 185 178 219 246 203 245 244

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

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M. Reig et al. / Desalination xxx (2013) xxx–xxx

Fig. 7. Relationship between the energy consumption working under continuous configuration and the current density for experiments at low and high temperature range for the IXM-ED.

obtained. However, in order to enhance the performance of the ED, it is expected to increase the NaCl in the brine and decrease the energy consumption necessary to produce one tonne of NaCl (ENaCl (kWh/KgNaCl)). The actual technical target for strengthening the competitive position of an electrodialysis system in the salt market was discussed by Tanaka [60] to be CNaCl N 200 g NaCl/L and ENaCl b 120 (kWh/t NaCl). Fig. 7 shows the relationship between CNaCl and EC(NaCl) obtained in the electrodyalisis experiments performed under the different conditions of intensity and temperature, and Fig. 8 compares the EC(NaCl) data to achieve a desired CNaCl showing that as the target indicates a reduction in energy consumption is desirable. EC(NaCl) depends on the membrane cell potential (Vcell) which includes the ohmic term (rmem, rdc and rcc) and membrane potential term (see Eq. (5)). As for the electric resistances in the ohmic term (rmem, rdc and rcc) the membrane resistance (rmem) is predominant over rdc and rcc [52] so reduction of energy consumption is associated to the reduction of the membrane resistance. However, this situation is accompanied by a decrease in CNaCl, because of the increase in density. Then rationale proposed [52] for the reduction of the membrane electric resistance by diminishing the membrane thickness is the development of a double-layered membrane consisting of a fine porous thinner functional layer and porous reinforced layer. 4. Conclusions Electrodialysis has proved to be a feasible technology for SWRO brine concentration. ED performance was highly dependent on inlet

temperature and current densities used. At higher inlet temperatures lower concentrations were obtained, but higher production flows and lower energy consumptions could be reached. At higher current densities applied, higher production flows and concentrations could be obtained but higher energy consumptions were recorded. ED pilot plant was designed in one single pass for diluate, which allowed to work with higher current densities and to get higher concentrations. ED intrinsically purified polyvalent ions, which were diluted due to osmosis and electro-osmosis phenomena. Nickel and copper were an exception and were concentrated in the brine produced. In the operating pH, these ions were in the form of univalent compounds and so, could pass through the membranes. The concentration order of divalent and trivalent ions was highly dependent on electric charge and ionic radii. Polyvalent ions with lower electric charge got more concentrated in all the experiments. Migration of these ions was not a primary phenomenon and their final composition was mainly determined by the osmosis and electro-osmosis fluxes. The optimal ED operation point was determined when working in a continuous mode at 0.35 kA/m2 of 185 g/L NaCl with 0.12 kWh/kg NaCl energy consumption were obtained. This energy consumption values were in the range of the industrial cells used in Japan to produce solid salt from seawater although higher concentrations can be obtained with SWRO brine. Further optimization will be needed to reach the technical target to get brines with NaCl contents higher than 200 g/L with less than 0.12 kWh/kg. As can be seen, the energy consumption values obtained were in the range of those reported in the literature,

Fig. 8. Relationship between the NaCl concentration and the continuous energy consumption necessary to produce one tonne of NaCl.

Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021

M. Reig et al. / Desalination xxx (2013) xxx–xxx

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Please cite this article as: M. Reig, et al., Concentration of NaCl from seawater reverse osmosis brines for the chlor-alkali industry by electrodialysis, Desalination (2013), http://dx.doi.org/10.1016/j.desal.2013.12.021