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Production of acidic and alkaline solutions via Electrodialysis with bipolar membranes from synthetic and real brines from saltworks
No full text1. Introduction
In recent decades, there has been a great deal of interest at both the industrial and academic levels in identifying unconventional sources for chemical production and raw material recovery. In this context, brine disposal, which was previously addressed as a priority to reduce environmental problems, is now seen as an opportunity to apply new or existing technologies in circular processes that allow for the valorization of previously considered waste solutions [1]. In this regard, Electrodialysis with bipolar membranes (EDBM) can be used to produce acidic and alkaline solutions from salty solutions. EDBM is an electro-membrane process distinguished by the alternated position of Ion Exchange Membranes (IEMs) for selective ion separation and the use of bipolar membranes that allow water dissociation [2]. In this work, a novel application of EDBM was proposed to valorize the remaining brine provided by the saltworks, which is typically returned to the sea despite being a highly concentrated solution (i.e., 20–40 times more concentrated than seawater). In more detail, an EDBM unit was tested for the first time with (i) synthetic and (ii) real brines from Trapani’s saltworks (Italy), which had previously been treated for mineral recovery.
2. Methods
A laboratory-scale EDBM unit, supplied by SUEZ-WTS France®, was equipped with 5 triplets of commercial IEMs (0.028 m2 of membrane active area). Closed-loop tests (see Figure 1) were carried out at constant current (typically 100-300 A m-2) and for a time duration sufficient to achieve a target NaOH concentration of 1M in the base compartment. Tests with various compositions were carried out in the salt compartment: (i) a reference case with pure NaCl solutions, (ii) synthetic brines containing NaCl, Na2SO4, and KCl, and (iii) real bitterns, containing traces of other minor elements. The initial composition of the feeds was assumed to be variable between several scenarios in the ranges reported in Table 1 in order to analyze the effect of different feeds in the process performance and acid and base solutions purities. Specific energy consumption, current efficiency, yield, and products purities were evaluated as performance indicators.
Table 1. Initial composition of the feeds for closed-loop experiments.
Salt compartment Base compartment Acid
compartment
Pure NaCl solutions Synthetic brines Real brines
NaCl 1-3 M 0.5-4 M 0.5-4 M 0-4 M 0-4 M
Na2SO4 - 0.1-1.2 M 0.1-1.2 M 0.1-1.2 0.1-1.2
KCl - 0.1-0.7 M 0.1-0.7 M 0.1-0.7 M 0.1-0.7 M
HCl - - - - 0-0.05 M
NaOH - - - 0-0.05 M -
Other elements - - traces - -
Figure 1. Closed-loop (batch) experimental configuration (adapted from [2]).
3. Results and discussion
For tests performed at the highest current density, the target concentration of 1M of NaOH for the alkaline solution was reached after ~45 minutes. Specific energy consumptions (SEC) of ~2.4 kWh kg-1NaOH were obtained at the target condition with pure NaCl solutions in the salt compartment at an initial concentration of 2M, whereas the use of a more concentrated synthetic brine reduced SECs to values less than 2 kWh kg-1NaOH. Current efficiency as a function of time showed a decreasing trend, but it remained relatively high (in the ~70-80% range) at the end of the test. In the case of ion mixtures, purities of ~90% were obtained in acid and base compartments, respectively.
4. Conclusions
A new application of EDBM for the valorization of highly concentrated brines from the saltworks process was proposed. The results obtained at laboratory-scale unit demonstrated the feasibility of the process for producing chemicals, specifically HCl and NaOH solutions, as an alternative to brine disposal. The process could then be tested at the pilot scale with long-run tests to determine its scalability at the industrial level
Limiting current phenomena in electro-membrane processes: local occurrence or stack-dependent one?
Full text linkBackground
Electro-membrane processes are gaining great interest in the field of desalination and brine valorisation. However, limiting current phenomena can be a bottleneck for their techno-economic performances. In the present work, the in-out distribution of current density is measured to elucidate the achievement of limiting conditions in real stacks.
Materials and Methods
A 10-cell pairs Electrodialysis stack (10×40 cm2 active area), equipped with four-segmented electrodes, was tested. NaCl solutions at an inlet concentration ranging from 0.5 to 60 g/l were fed at velocities of either 2 or 4 cm/s in parallel flow. Current density-voltage curves were built by applying equal increasing steps of voltage to each electrode. Outlet concentrations and current efficiency were investigated [1].
Results
Figure 1 shows the current density-voltage curves for two couples of inlet concentration. Between the final tract of the ohmic region and the plateau region of the overall stack curve, the current density distribution at the four segments changes markedly. In fact, while at the first electrode the current density continues to increase, at the other three it reaches a maximum and decreases. Thus, as the voltage increases, the current concentrates in a shorter tract of the channel, while it reduces in the remaining part, becoming ineffective for desalination, due to its high resistance. This is caused by the high desalination rate in the first few centimetres, making the dilute conductivity much lower. Moreover, the longer tract of channels at high salinity gradient in the final part of the stack promotes larger diffusion, lowering the current efficiency [2]. Figure 1. Current density-voltage curves for tests at a) Cdil,IN=0.5 g/l and Cconc,IN=30 g/l and b) Cdil,IN=Cconc,IN=1 g/l.
Conclusions
The attainment of limiting conditions in electrodialysis stacks is strongly related to ohmic phenomena and to the distribution of current density, highlighting its importance in the design of efficient electro-membrane systems.
Acknowledgments
This work was supported by the SEARcularMINE (Circular Processing of Seawater Brines from Saltworks for Recovery of Valuable Raw Materials) project – Horizon 2020 programme, Grant Agreement no. 869467. The authors are grateful to REDstack B.V. and Fujifilm Manufacturing Europe B.V. for supplying stack and membranes, respectively
Testing of Electrodialysis with bipolar membrane at semi-industrial scale for in-situ reactant production
No full textIntroduction. Electrodialysis with bipolar membranes (EDBM) has been gaining attention for the production of chemicals, such as acids and bases, for direct use in situ within circular and integrated processes, thus avoiding transport and handling of highly concentrated and dangerous reagents. Although this process has been explored in recent years at laboratory scale, few studies aimed at the development of pilot scale units. This work aims to fill this gap by scaling up and testing an EDBM unit at semi-industrial scale.
Materials and methods. The pilot unit consists of 50 triplets of 0.32 m2 of active membrane area. The setup includes measurement instruments to monitor flowrates, conductivity, pressure, etc. Tests were performed in batch mode with synthetic solutions at initial concentration of 1.0 mol/L NaCl in the saline channels and 0.05 mol/L HCl and NaOH in the acid and base ones, respectively. The effect of a different initial volumes in the compartments was investigated by comparing performance indicators as current efficiency (CE), specific energy consumption (SEC) and specific productivity (SP).
Results and discussion. Results showed that, when doubling the volume of acid, a SEC of 1.05 kWh/kgNaOH and CE of 95% were reached at 0.5 mol/L of OH-. Moreover, OH- concentration above 1.0 mol/L could also be reached (with corresponding SEC and CE of 1.05 kWh/kgNaOH and 76%, respectively). Indeed, lowering the acid concentration (by increasing the acid tank volume) reduced undesired effect of diffusion towards the saline channel and partial neutralization of acid and base. This work demonstrated the scalability of the EDBM process and highlighted some practical operating conditions to optimize the EDBM performance indicators
Semi-industrial Electrodialysis with bipolar membranes (EDBM) unit for harvesting valuable acid and base from exhausted bitterns
No full textAcid and alkaline production from multi-ionic brines via Electrodialysis with Bipolar Membranes
No full textIn recent years, a great attention has grown towards the brine valorization through chemicals production and mineral recovery as an alternative to conventional disposal. Electrodialysis with Bipolar Membranes (EDBM) is an emerging process that can be used for the production of alkaline and acidic solutions from salt solutions. Within the SEArcularMINE project framework, the exploitation of saltworks bitterns (highly concentrated solutions generated during the sea-salt production process) is proposed for minerals recovery and NaOH and HCl solutions production.
In this work, an EDBM unit, equipped with commercial ion exchange membranes, was tested for the first time in closed-loop mode with: (i) single NaCl solutions as reference case (ii) synthetic brines including NaCl, Na2SO4, and KCl salts, and (iii) real saltwork brines, which contains traces of other minor elements. Different scenarios were assumed in terms of feed compositions to study the effect on process performance parameters as specific energy consumption (SEC), acid and base solutions purities, current efficiency, and yield.
Main results at laboratory-scale unit highlights the process feasibility to produce alkaline solutions at target concentrations exceeding 1M NaOH equivalent, while maintaining high product purities and current efficiencies
Advances in two-compartment electrodialysis configurations with bipolar membranes for chemicals production
No full textAssisted-Reverse Electrodialysis for the remineralization of RO permeate through recovery of minerals from brines
No full textAssisted-Reverse Electrodialysis: a novel method to remineralize surface RO permeate by recovering minerals from its brine
No full textValorization of surface-water RO brines via Assisted-Reverse Electrodialysis for minerals recovery: performance analysis and scale-up perspectives
No full textReverse Osmosis (RO) plays a key role in seawater and brackish water desalination to fulfill the growing demand for fresh water. In recent years, RO has also been more and more adopted for the treatment and potabilization of surface waters, leading to two main problems: (i) the depletion in minerals of the product water, making it aggressive and unsuitable for drinking purposes and (ii) the production of a concentrated brine requiring proper disposal. Permeate remineralization post-treatments include pH adjustment and addition of minerals, such as bicarbonates, calcium and magnesium, which are essential for human health and required to meet drinking water guidelines. However, such solutions are exposed to critical issues related to supply, cost as well as extraction and transportation of chemicals.
In this work, we investigate, via modelling tools, the use of Assisted-Reverse Electrodialysis (A-RED) to remineralize a surface water RO permeate stream by recovering minerals from its corresponding brine itself. This concept (patent pending [1]) was explored at the bench scale [2] and the subsequent experimental data used to validate the multi-scale model used in this study.
The validated model was implemented to perform a process parametric analysis aiming at the design and optimization of a pilot-scale plant. Sensitivity analysis was performed considering the use of different stacks in series and hybrid configurations, including feed & bleed loops for one or both compartments and permeate by-pass.
Moreover, the use of simplified techno-economic analysis tools allowed to identify the most promising configurations, which reduce the main cost items relevant to the industrial scale-up of the technology. More than 150 simulations were performed to compare different scenarios on the basis of the main performance parameters characterizing electro-membrane processes such as energy consumption, apparent flux and remineralization capacity. Results have shown that energy consumption can be reduced to less than 0.1 kWh/kg of salt transported to the permeate, in the case of permeate flow rates up to ~2 m3/h, while apparent fluxes can rise to ~170 l/(h*m2) for larger bypass flow rates, resulting in lower capital and overall costs
Investigation of two-channel Electrodialysis with Bipolar Membrane process to in-situ produce chemicals
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