Research Article pubs.acs.org/journal/ascecg
Process Economic Evaluation of Resource Valorization of Seawater Concentrate by Membrane Technology Wei Zhang,† Mengjie Miao,† Jiefeng Pan,† Arcadio Sotto,‡ Jiangnan Shen,*,† Congjie Gao,† and Bart Van der Bruggen§ †
Center for Membrane Separation and Water Science & Technology, Zhejiang University of Technology, Hangzhou, 310014, China Department of Chemical and Environmental Technology, Rey Juan Carlos University, 28933 Móstoles, Madrid, Spain § Department of Chemical Engineering, KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium ‡
ABSTRACT: In this study, a process design consisting of chemical precipitation, electrodialysis with monovalent-selective membranes, and bipolar membrane electrodialysis (BMED) is proposed to valorize seawater concentrate discharged from an RO (reverse osmosis) plant for the production of acid/base and coarse salt with high purity. After pre-precipitation and electrodialysis with monovalent-selective membranes, a high purity of coarse salt (∼92%) was obtained. Furthermore, the effect of current density and feed concentration of the BMED process on the production of acid/base with high purity was investigated. It is acceptable to attain acid/base with a purity of ∼95%/∼85% when operating at a current density of 10 mA/cm2 and a feed conductivity of 100 mS/cm by applying the screened BMED stack. Finally, the total process cost for the acid/base production was estimated at $0.50/kg at the current density of 10 mA/cm2, which is appropriate and competitive for industrial application. KEYWORDS: Electrodialysis, Monovalent-selective ion-exchange membrane, Seawater concentrate, Bipolar membrane electrodialysis, Coarse salt, Acid−base production, Purity, Valorization, Estimation
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INTRODUCTION Nowadays, water scarcity has become a global issue because of the rapid development of the worldwide population. Seawater reverse osmosis (SWRO) as the most optimized technology is applied worldwide on a large scale.1 However, for the application of SWRO technology, one of the drawbacks is the disposal of the SWRO concentrate as a byproduct generated from the desalination process. Normally, this brine is directly discharged back into the sea,2 resulting in an environmental problem for marine ecosystems.3 However, seawater concentrate contains many useful elements. If the constituents of seawater concentrate can be recycled effectively, natural resources can be preserved. Therefore, it should be beneficial to investigate how to reuse and valorize the components of seawater concentrate from RO plants, in view of ecological environmental protection and resource recovery. Electrodialysis may play a role in this. However, pretreatment is usually necessary prior to the application of electrodialysis with monovalent-selective ionexchange membranes and bipolar membrane electrodialysis in order to obtain a brine that is abundant in NaCl.4 Electrodialysis is a mature technology that has been widely used for the treatment of brackish water sources.5 For instance, Reig et al.6 concentrated NaCl from RO brines by electrodialysis to use as a raw material in the chlor-alkali industry. Jiang et al.7 have investigated the feasibility of ED to produce © 2017 American Chemical Society
coarse salt and freshwater from RO effluent. Furthermore, Zhang et al.8,9 considered a techno-economic analysis in order to reduce the operational cost, and also investigated the feasibility to improve the water recovery with the treatment of RO concentrate through ED. However, the conventional ED technology has limitations for ions with the same size and charge sign. ED stacks with monovalent-selective ion-exchange membranes can be used to separate monovalent ions from multivalent ions. Several studies have been carried out on this technology (i.e., electrodialysis with monovalent-selective ionexchange membranes). Zhang et al.10 have investigated the separation efficiency of monovalent/multivalent anions from RO concentrate by monovalent-selective anion membranes used in ED. Interestingly, they found that lowering the current density can increase the separation efficiency of monovalent/ multivalent anions with monovalent-selective membranes. Furthermore, Nie et al.11 proved that ED using monovalentselective membranes is technically and economically feasible to separate lithium from salt lake brine with a high Mg/Li ratio. Ghyselbrecht et al.12 utilized monovalent-selective anion- and cation-exchange membranes to remove the divalent ions from Received: February 22, 2017 Revised: May 19, 2017 Published: May 26, 2017 5820
DOI: 10.1021/acssuschemeng.7b00555 ACS Sustainable Chem. Eng. 2017, 5, 5820−5830
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic diagrams and configuration of ED stack with monovalent-selective ion-exchange membranes, BMED stack, and pellet reactor.
membranes is feasible and effective for separating monovalent ions from multivalent ions. Hence, in this paper, electrodialysis with monovalent-selective ion-exchange membranes was selected for application in the pretreatment stage of seawater concentrate in order to reduce the multivalent ion content. Electrodialysis in combination with bipolar membrane has also been applied for the treatment of RO concentrate, since BMED can be used to split an aqueous saline stream into its
industrial saline water to avoid the formation of CaSO4 scaling during the ED process. Moreover, Reig et al.13 concluded that it is possible to achieve Cl−/SO42− separation by selectrodialysis (SED) with monovalent-selective anion-exchange membranes, and then pure NaOH and a mixture of HCl and H2SO4 can be attained by bipolar membrane electrodialysis (BMED) for the treatment of effluent after SED. In conclusion, these studies indicate that electrodialysis with monovalent-selective ion-exchange 5821
DOI: 10.1021/acssuschemeng.7b00555 ACS Sustainable Chem. Eng. 2017, 5, 5820−5830
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Types of Monopolar Membranes and Their Propertiesa membrane type NEOSEPTA NEOSEPTA NEOSEPTA NEOSEPTA
CIMS ACS CMX AMX
thickness (μm)
burst strength (MPa)
area resistance (Ω cm2)
150 130 170 140
≥0.10 ≥0.15 ≥0.40 ≥0.25
1.8b 3.8b 3.0b 2.4b
transport number (%)
temp (°C)
pH
>96 >96
≤40 ≤40 ≤40 ≤40
0−10 0−8 0−10 0−8
a
The data were collected from the product brochure provided by manufacturers. bThe ion-exchange membrane area resistance was measured with a 0.5 N NaCl solution, at 25 °C.
of the experimental process was investigated to evaluate the feasibility of the new process design. The energy consumption, current efficiency, and process economics were calculated as well.
corresponding acid and base without any addition of chemicals. The elementary principle of this technology is that the water molecules at the interphase of the bipolar membrane are split into hydrogen ions and hydroxide ions with application of a direct electrical potential, and the hydrogen ions combine with anions migrating from the feed/salt compartment through the anion-exchange membrane (AEM) to generate acid in the acid compartment. The hydroxide ions combine with cations migrating from the feed/salt compartment through the cationexchange membrane (CEM) to produce the corresponding base in the base compartment.14 Furthermore, Badruzzaman et al.15 conducted BMED experiments for treating RO concentrate to produce mixed acids and mixed bases with concentrations as high as 0.2 N. Yang et al.16 proved that BMED for mixed acid and sodium hydroxide production with pretreated RO concentrate is suitable for a long-term run. Moreover, Ghyselbrecht et al.17 indicated that BMED is a promising and attractive technology in the area of saline effluent reclamation and reuse. Ibáñez et al.18 also proved that it is a technically feasible option for the conversion of RO concentrates into acid and base products by BMED. Furthermore, Wang et al.19 confirmed that a continuous BMED experiment was suitable to reclaim products of brine generated from the desalination process with an acceptable current efficiency and energy consumption. Furthermore, the production of acids and bases that can be used for ion-exchange media regeneration from dilute salt solutions was carried out with BMED by Davis et al.20 Reig et al.21,22 have valorized RO brine to produce acid and base with high concentration by BMED. In addition, BMED has also been applied successfully in the chemical and biochemical process industry, food and drug industry, and waste management and reclamation.23−30 Nevertheless, although some studies about the application of BMED to produce acid and base from RO concentrate have shown promising results, the valorization of real seawater concentrate from an RO plant has rarely been investigated. In this work, an ED stack with monovalent-selective ionexchange membranes was applied to achieve the aim of (a) concentration and purification of NaCl solution from seawater concentrate to avoid the formation of scaling (Mg(OH)2, CaCO3) in the subsequent BMED experiments; then, a BMED stack was integrated for (b) in situ production of acid and base with high purity. (c) The NaCl abundant brine attained by ED can be used to produce coarse salt with high purity after posttreatment. A part of acid and base produced by BMED was also applied to the pretreatment of raw solution (i.e., seawater concentrate) and the post-treatment of brines in the pellet reactor for reducing the divalent metal ions content (Ca2+, Mg2+), which is shown in Figure 1. Therefore, this study provides a useful reference for valorizing seawater concentrate to obtain a series of target products (coarse salt, acid and base) by using a process design that is different from that in previous work. Furthermore, the effect of current density and feed concentration
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MATERIALS AND METHODS
Materials. The reagents such as Na2SO4, NaOH, Na2CO3, H2SO4, and HCl were of analytical grade, and deionized water was used throughout. The monovalent-selective ion-exchange membranes used for the ED stack were NEOSEPTA CIMS (monovalent-selective cation-exchange membranes, from ASTOM Co., Japan) and NEOSEPTA ACS (monovalent-selective anion-exchange membranes, from ASTOM Co., Japan). Conventional ion-exchange membranes (NEOSEPTA CMX/NEOSEPTA AMX from ASTOM Co., Japan) and bipolar membranes (NEOSEPTA BP-1 from ASTOM Co., Japan or FBM from Fuma-Tech Co., Germany) were used in the BMED stack. The main properties of these membranes are listed in Tables 1 and 2. The composition of the seawater concentrate is shown in Table 3.
Table 2. Types of Bipolar Membranes and Their Propertiesa membrane type
thickness (μm)
NEOSEPTA BP-1 FBM
200−350
burst strength (MPa)
200
area resistance (Ω cm2)
efficiency (%) >98
b
