Efficient treatment of perfluorohexanoic acid by nanofiltration followed by electrochemical degradation of the NF concentrate

The present study was aimed at the development of a strategy for removing and degrading perfluorohexanoic acid (PFHxA) from industrial process waters at concentrations in the range 60–200 mg L-1. The treatment train consisted of nanofiltration (NF) separation followed by electrochemical degradation of the NF concentrate. Using a laboratory-scale system and working in the total recirculation mode, the DowFilm NF270 membrane provided PFHxA rejections that varied in the range 96.6–99.4% as the operating pressure was increased from 2.5 to 20 bar. The NF operation in concentration mode enabled a volume reduction factor of 5 and increased the PFHxA concentration in the retentate to 870 mg L-1. Results showed that the increase in PFHxA concentration and the presence of calcium sulfate salts did not induce irreversible membrane fouling. The NF retentate was treated in a commercial undivided electrochemical cell provided with two parallel flow-by compartments separated by bipolar boron doped diamond (BDD) electrode, BDD counter anode, and counter cathode. Current densities ranging from 20 to 100 A m-2 were examined. The electrochemical degradation rate of PFHxA reached 98% and was accompanied by its efficient mineralization, as the reduction of total organic carbon was higher than 95%. Energy consumption, which was 15.2 kWh m-3 of treated NF concentrate, was minimized by selecting operation at 50 A m-2. While most of the previous research on the treatment of perfluoroalkyl substances (PFASs) focused on the removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), these compounds have been phased out by chemical manufacturers. Our findings are relevant for the treatment of PFHxA, which appears to be one of the present alternatives to long-chain PFASs thanks to its lower bioaccumulative potential than PFOA and PFOS. However, PFHxA also behaves as a persistent pollutant. Moreover, our results highlight the potential of combining membrane separation and electrochemical oxidation for the efficient treatment of PFAS-impacted waters.Financial support from projects CTM2013-44081-R and CTM2016-75509-R (MINECO, SPAIN-FEDER 2014e2020) is gratefully acknowledged

Membrane preconcentration as an efficient tool to reduce the energy consumption of perfluorohexanoic acid electrochemical treatment

One of the key points for the large-scale implementation of electrochemical water treatment technologies lies in the need of reducing the energy consumption. The present work analyzes the removal of persistent perfluorohexanoic acid (PFHxA, 204 mg L−1) from industrial process waters using a strategy that combines membrane pre-concentration followed by electrooxidation of the concentrate. A mathematical model describing the nanofiltration (NF) system was developed and complemented with new and background experimental data of PFHxA and ion species rejections and total permeate flux through the NF270 and NF90 membranes. Similarly, the kinetics of PFHxA electrolysis on boron doped diamond anodes was determined at laboratory scale. Later, the model was used to simulate the NF-ELOX integrated process, where a commercial spiral wound unit (membrane area 7.6 m2) was implemented and the electrooxidation unit was scaled-up to pilot plant (anode area 1.05 m2). The obtained energy savings depended on a combination of the target PFHxA removal ratio at the end of the treatment train, the separation performance of the commercial membrane and the reduction of the electrolyte ohmic resistance in the electrooxidation stage, that was attained as a result of the increase of salts content in the concentrate. Only the tight NF90 membrane allowed to achieve high (99%) PFHxA removal ratios in the integrated NF-ELOX process, and the specific energy consumption was estimated at 11.6 kWh m−3, 59.2% less than when electrolysis alone was applied. Still, the electrolysis is the most energy demanding step, with 85.9% contribution to the total energy consumption. The strategy of combining membrane pre-concentration with electrochemical degradation could be extended to the treatment of other highly persistent organic compounds.Financial support from projects CTM2013-44081-R, CTM2016-75509-R and CTQ2016-75158-R (MINECO, SPAIN-FEDER 2014–2020) and to the Spanish Excellence Network E3TECH (CTQ2015-71650-RDT) is gratefully acknowledged

Thin-film composite matrimid-based hollow fiber membranes for oxygen/nitrogen separation by gas permeation

In recent years, the need to reduce energy consumption worldwide to move towards sustainable development has led many of the conventional technologies used in the industry to evolve or to be replaced by new alternatives. Oxygen is a compound with diverse industrial and medical applications. For this reason, obtaining it from air is one of the most interesting separations, traditionally performed by cryogenic distillation and pressure swing adsorption, two techniques which are very energetically expensive. In this sense, the implementation of membranes in a hollow fiber configuration is presented as a much more efficient alternative to carry out this separation. The aim of this work is to develop cost-effective multilayer hollow fiber composite membranes made of Matrimid and polydimethylsiloxane (PDMS) for the separation of oxygen and nitrogen from air. PDMS is used as a cover layer but can also enhance the performance of the membrane. In order to compare these two materials, three different configurations are studied. First, integral asymmetric Matrimid hollow fiber membranes were produced using the spinning method. Secondly, by using dip-coating method, a PDMS dense selective layer was deposited on a self-made polyvinylidene fluoride (PVDF) hollow fiber support. Finally, the performance of a dual-layer hollow fiber membrane of Matrimid and PDMS was studied. Membrane morphology was characterized by SEM and separation performance of the membranes was evaluated by mixed-gas permeation experiments. The novelty presented in this work is the manufacture of hollow fiber membranes and the way Matrimid is treated. This makes it possible to develop much thinner dense layers than in the case of flat-sheet membranes, which leads to higher permeance values. This is a key factor when implementing this technology on an industrial scale. Membranes prepared in this work were compared to the current state of the art, reporting quite good performance for the dual-layer membrane, reaching O2 permeance of 30.8 GPU and O2/N2 selectivity of 4.7, with a thickness of about 5–10 μm (counting both selective layers). In addition, the effect of operating temperature on the membrane permeances has been studied experimentally; we analyze its influence on the selectivity of the separation process.This work was supported by the Spanish AEI through the project PID2019-104369RB-I00 and the European Union through the projects “HYLANTIC”-EAPA_204/2016, which is co-financed by the European Regional Development Fund in the framework of the INTERREG Atlantic program, and the Project ENERGY PUSH SOE3/P3/E0865, which is co-financed by the European Regional Development Fund (ERPF) in the framework of the INTERREG SUDOE Programme

Exploring the potential application of Matrimid® and ZIFs-based membranes for hydrogen recovery: a review

Hydrogen recovery is at the center of the energy transition guidelines promoted by governments, owing to its applicability as an energy resource, but calls for energetically nonintensive recovery methods. The employment of polymeric membranes in selective gas separations has arisen as a potential alternative, as its established commercial availability demonstrates. However, enhanced features need to be developed to achieve adequate mechanical properties and the membrane performance that allows the obtention of hydrogen with the required industrial purity. Matrimid®, as a polyimide, is an attractive material providing relatively good performance to selectively recover hydrogen. As a consequence, this review aims to study and summarize the main results, mechanisms involved and advances in the use of Matrimid® as a selective material for hydrogen separation to date, delving into membrane fabrication procedures that increase the effectiveness of hydrogen recovery, i.e., the addition of fillers (within which ZIFs have acquired extraordinary importance), chemical crosslinking or polymeric blending, among others.This research was funded by Agencia Estatal de Investigación (PID2019-104369RB-I00/ AEI/10.13039/501100011033). This work was also partially funded by European Regional Development Fund (“HYLANTIC”-EAPA_204/2016) in the framework of the Interreg Atlantic program

Polyether-block-amide thin-film composite hollow fiber membranes for the recovery of butanol from ABE process by pervaporation

This work reports the continuation of previous efforts to recover butanol from the ABE (acetone-butanol-ethanol) fermentation process by pervaporation (PV). A key aspect to improve the efficiency of the technology is the membrane used to perform the selective butanol separation; hence, this study focuses on the implementation of hollow fiber (HF) membrane configuration for the ABE separation by PV as opposed to flat sheet membrane configuration. The HF membrane preparation was done by dip coating, a frequently used process for the production of HF membranes, which involves the deposition of a thin film of a coating solution. Different thicknesses of the active layer were obtained by modifying the polymer content in the coating solution, allowing later to evaluate the influence of the thickness on the separation performance. This study includes a description of the procedure to prepare selective membranes, its characterization and an analysis of the influence of operating conditions on membrane separation performance. SEM and water contact angle were used to characterize the produced membranes. The mass transport phenomena in the pervaporation process were characterized using a resistances-in-series model. The results allow to adopt a criterion to select the most suitable thickness for the membrane active layer, which allows to achieve an adequate separation performance, and reveal the importance in the choice of the membrane support material. Finally, a comparative analysis of the self-made hollow fiber membranes performance in terms of flux, separation factor and PSI with respect to those found in the literature is presented.This research is being supported by the Spanish AEI under the projects PID2019-104369RB-I00 and RTI2018-093310-B-I00, and by the Project ENERGY PUSH SOE3/P3/E0865, which is co-financed by the European Regional Development Fund (ERPF) in the framework of the INTERREG SUDOE Programme. Carla Arregoitia also thanks for a FPI research scholarship (BES-2017-081708)

Facilitated transport of propylene through composite polymer-ionic liquid membranes. Mass transfer analysis

Separation of light gaseous olefins from paraffin’s of the refinery process off-gasses has been traditionally performed by cryogenic distillation, which is a highly capital and energy intensive operation. This handicap creates an incentive for the investigation of alternative olefin/paraffin separation technologies. In this regard, membrane technology supposes a potential solution for process intensification. Previous works of our research group reported the use of facilitated transport composite membranes integrating the use of PVDF-HFP polymer, BMImBF4 ionic liquid and AgBF4 silver salt. In this type of membranes, the silver cations react selectively and reversibly with the olefin, allowing the separation via mobile and fixed carrier mechanisms. Ionic liquids were selected as membrane additives because in addition to their negligible vapor pressure that avoids solvent losses by evaporation, they provide stability to the metallic cation dissolved inside, and modify the structure improving the facilitated transport. This technology offers a commercial attractive separation alternative thanks to their modular form of operation, high values of selectivity and permeability and low operational costs. In the present work, propane/propylene permeation experiments involving the use ionic liquids and different membrane compositions were performed. Moreover, basing on the transport and equilibrium parameters previously obtained, a mathematical model description of the system will be proposed fitting the remaining parameters and allowing the design and optimization of the propane/propylene separation process at industrial levels.This research was supported by the Spanish Ministry under the projects CTQ2012-31639 (MINECO, SPAIN-FEDER 2007–2013) and (CTM2013-44081-R)

On the improved absorption of carbon monoxide in the ionic liquid 1-hexyl-3-methylimidazolium chlorocuprate

This study is focused on the design of a liquid phase system to be used in facilitated transport-supported ionic liquid membranes (SILMs) for the recovery of carbon monoxide from gaseous streams based on the ability of CO molecules to form "pi" complexation bounds with Cu+ ion. As liquid phase we propose the use of the ionic liquid 1-hexyl-3-methyl-imidazolium chlorocuprate prepared by the direct mixture of copper(I) chloride (CuCl) with 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]). A comprehensive look at the reaction mechanism and the equilibrium parameters obtained from the experimental characterization of the physical and chemical solubility of carbon monoxide in pure [hmim][Cl], and in mixtures CuCl/[hmim][Cl] is presented. The gas equilibrium solubility experimental work was carried out in the CuCl/[hmim][Cl] molar ratio range from 0 to 0.75, temperature from 273.15 to 303.15 K and pressures up to 20 bar. The values of the Henry's law constant for the physical solubility of CO in [hmim][Cl] changed from 15.3 × 10-3 to 2.7 × 10-3 mol kg-1 bar-1 as the temperature increased from 273.15 to 293.15 K. The chemical solubility of CO in the reactive ionic liquid media increased with the increase of the concentration of CuCl, with the increase of pressure and as temperature decreases. In the operation range of variables the maximum absorption of CO was of 2.26 mol kg-1 that was reached working at 20 bar, at CuCl/[hmim][Cl] molar ratio of 0.75 and 273.15 K.Financial support from the Spanish Ministry of Education and Science under the projects: ENE2010-15585 and CTQ 2008-00690/PPQ is gratefully acknowledged. Oana C. David and Gabriel Z. also thank the Ministry of Education for a FPU and a FPI, respectively, postgraduate research grant

A practical approach to fixed-site-carrier facilitated transport modeling for the separation of propylene/propane mixtures through silver-containing polymeric membranes

In this work, a new consistent mathematical model for the description of the olefin flux through Ag+-containing polymeric dense membranes is proposed. A fixed site carrier "hopping" parameter acting as an effective permeability for this specific transport phenomenon is defined and calculated for the first time. This study reports a simple and versatile approach that can be incorporated into future models to simulate the more complex mobile/fixed hybrid mechanism acting in composite membranes. Furthermore, in order to validate the model, the proof of concept has been carried out with PVDF-HFP/AgBF4 facilitated transport membranes. The experimental analysis has been performed by the continuous flow permeation method through flat membranes containing increasing silver loads, from 17 to 38% w/w at olefin partial pressures ranging from 0.5 to 1.5 bar and temperatures of 293 and 303 K. These membranes showed a promising performance, reaching values of propylene permeability up to 1800 Barrer and very high propylene/propane selectivities. The reported model constitutes a very useful tool for process optimisation and scale-up.Financial support from the Spanish Ministry of Science under the projects CTQ2015-66078-R and CTQ2016-75158-R (MINECO, Spain-FEDER 2014–2020) is gratefully acknowledged. Raúl Zarca also thanks the Universidad de Cantabria for a postgraduate fellowship

Optimization of multistage olefin/paraffin membrane separation processes through rigorous modeling

In this work, we explore the capabilities of an NLP optimization model to determine the viability of facilitated transport membrane processes intended to replace traditional distillation currently employed for propane/propylene separation. An NLP optimization model for multistage membrane processes has been formulated, introducing the mathematical description of the facilitated transport mechanisms in the PVDF‐HFP/BMImBF4/AgBF4 membranes previously developed by our research group. For the first time, a simultaneous optimization of the process and the membrane material (i.e., carrier concentration) has been performed, thanks to the implementation of the governing equations for the fixed site and mobile carrier mechanisms. Once the model is solved in GAMS it returns the optimal membrane area, carrier loading and permeate pressure of each stage based on Net Present Value Cost (NPVC) minimization. Different process flow sheets were evaluated and the results show prominent reductions on NPVC for facilitated transport multistage processes when compared to distillation.Financial support from the Spanish Ministry of Science under the pro-jects CTQ2015-66078-R and CTQ2016-75158-R (MINECO, Spain-FEDER 2014–2020) is gratefully acknowledged. Raúl Zarca also thanks the Universidad de Cantabria for the postgraduate fellowship