Single Salt Solutions Research Articles

Integrated membrane distillation–crystallization: Process design and cost estimations for seawater treatment and fluxes of single salt solutions

The goal of this research is to design an integrated membrane distillation–crystallization (MDC) process for desalination of seawater with pure water and dry salts as the only products. The process is based on a combination of membrane distillation (MD) and osmotic distillation (OD) steps with integrated crystallization. In the first MD step seawater is concentrated up to the saturation point of CaCO3. In subsequent MD or OD steps, pure water and salts (a.o. CaCO3, NaCl, KCl) are separated until complete dewatering and crystallization. The equipment and energy costs for this process are approximately € 1.09/m3.Laboratory research with single salt solutions showed that after three days of MD operation with a saturated CaCO3 solution (77°C) a complete flux decline occurred. This could be prevented by adding CaCO3 crystals to the system to promote crystallization in the bulk liquid. The treatment of a saturated NaCl solution (80°C) with MD resulted in a complete flux decline within 1h. In an OD system NaCl crystallization (64°C) was more successful when NaCl seeds were added to the system and a draw solution was used at the condensate side. During 3h of operation the flux decreased from 3 to 1Lm−2h−1.

Analysis of lead(II) retention from single salt and binary aqueous solutions by a polyamide nanofiltration membrane: Experimental results and modelling

The present paper reports the performance of a polyamide thin-film composite NF membrane (AFC 80) in the removal of highly polluting and toxic Pb(II) ions from single salt and binary aqueous solutions simulating real wastewaters. The effect of the operating variables (pH, transmembrane pressure and feed solution concentration) on the separation process was investigated. It was observed that the rejection of lead ions slightly increases with increasing the transmembrane pressure and slightly decreases with increasing the metal concentration in feed at constant pH of aqueous solutions. The maximum rejection of lead ions in single salt solution was 99.44% for a feed solution containing 50mg Pb/L at pH 5.7. In binary Pb–Cd aqueous solutions the metals rejections are higher than 98.5% for concentrations in the 15–80mg/L range, and the rejection order follows the order of metals hydration energies. The optimum operating conditions selected were: transmembrane pressure in 10–30bars range, feed solutions (polluted wastewaters) at pH 5.7 containing 50–80mg metal/L. The Spiegler–Kedem model ensures a very good description and prediction of the NF processes under study for a wide concentrations range. The model parameters showed that the transport mechanism of the solute is mainly convective, being almost totally hindered by the size of the metal ions which finally determine the salts retention. The AFC 80 membrane proved to be able to perform an effective removal of lead, and also of cadmium and nitrate ions, in very mild and economically advantageous conditions, leading to wastewaters suitable to be discharged into the environment. The structural characteristics of the AFC 80 membrane (effective pore radius and the thickness to porosity ratio) were estimated from uncharged solutes rejection experiments. The charge properties of the membrane surface were assessed by measuring the tangential streaming potential. It was found that the membrane becomes positively charged in lead nitrate solution, and this behaviour was attributed to adsorption of lead ions on the membrane surface. The membrane characteristics thus determined were correlated with the results of lead separation experiments and with the Spiegler–Kedem model parameters.

Densities of SrCl2(aq), Na2SO4(aq), NaHCO3(aq), and Two Synthetic Reservoir Brines at Temperatures between (298 and 473) K, Pressures up to 68.5 MPa, and Molalities up to 3 mol·kg–1

We report the densities of SrCl2(aq), Na2SO4(aq), NaHCO3(aq), and the synthetic reservoir brine system [x1NaCl + x2KCl + x3MgCl2 + x4CaCl2 + x5SrCl2 + x6Na2SO4 + x7NaHCO3](aq), where x denotes the mole fraction of the different salts, at temperatures between (298 and 473) K and pressures up to 68.5 MPa. The molalities at which the single-salt solutions were studied are (1.022, 2.024, and 3.031) mol·kg–1 for SrCl2(aq), (0.783 and 1.502) mol·kg–1 for Na2SO4(aq), and (0.507 and 1.000) mol·kg–1 for NaHCO3(aq). The compositions of the synthetic reservoir brines studied are, first, x1 = 0.770, x2 = 0.022, x3 = 0.040, x4 = 0.105, x5 = 0.002, x6 = 0.044, and x7 = 0.018 with molality 0.359 mol·kg–1 and, second, x1 = 0.797, x2 = 0.006, x3 = 0.042, x4 = 0.145, x5 = 0.002, x6 = 0.006, and x7 = 0.002 with molality 1.900 mol·kg–1. The measurements were performed with a vibrating-tube densimeter calibrated under vacuum and with pure water over the full ranges of pressure and temperature investigated. An analysis of unce...

Effect of some parameters on the performance of eletrodialysis using new type of PVDF–SiO2 ion-exchange membranes with single salt solution

The effect of operating condition on the performance of new type of PVDF–SiO2 organic–inorganic ion-exchange membranes used in electrodialysis process was investigated with single salt solution in comparison to PVDF membranes. The experimental results indicated that better transport properties of PVDF–SiO2 membranes may be responsible for the limited current density (LCD) of the membranes higher than that of PVDF membranes. The LCD values of PVDF–SiO2 and PVDF membranes at NaCl solution of 5000mg/L were 1.43mA/cm2 and 1.13mA/cm2, respectively. In addition, desalination ratio increased with increasing current density and feed flow rate. It was also found that at the optimum operating condition, the removal ratios of Na+, Ca2+, Cl− and SO42− by electrodialysis employing PVDF–SiO2 membranes were 97.7%, 93.5%, 96.5% and 88.5%, respectively, and the removal ratios of the ions in the presence of PVDF membranes were 91.4%, 88.4%, 92.5% and 85.7%, respectively. It can be concluded that these PVDF–SiO2 organic–inorganic ion-exchange membranes with better membrane properties can be used in electrodialysis process.

Desalination of aqueous solutions by LTA and MFI zeolite membranes using pervaporation method

LTA and MFI zeolite membranes were hydrothermally grown on the surface of an α-alumina porous support. The synthesized membranes were used for removal of cationic and anionic species from aqueous solutions by the pervaporation method. The perfection of the membranes was improved by employing the multi-stage synthesis method. The membranes were characterized by XRD, SEM and IR methods. The membranes were initially evaluated by the pervaporation separation of water from aqueous 2-propanol mixtures. The separation factors obtained were 7081 and 105 for NaA and ZSM-5 membranes, respectively. The ability of membranes for desalination of some aqueous solutions containing I-, Cs+ and Sr2+ ions was examined in various conditions. These ionic species were chosen because of their importance in the nuclear sciences. Both membranes effectively removed (more than 99 wt%) I-, Cs+ and Sr2+ from their singlesalt solutions (0.001mol dm-3) over a temperature range of 298-338 K. The effects of parameters such as time and temperature on the separation factors and fluxes were investigated. This work shows that, due to their excellent chemical, thermal and mechanical stability, the zeolitic membranes are useful for desalination of aqueous solutions and treating saline wastewaters by pervaporation. Therefore, this method has the ability to desalinate harsh environment solutions involving strong solvent and radioactive components.

Preparation and characterization of negatively charged hollow fiber nanofiltration membrane by plasma-induced graft polymerization

A new negatively charged nanofiltration (NF) membrane has been prepared by grafting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the polysulfone (PSf) hollow fiber ultrafiltration membrane using two-step plasma method. First, PSf substrate was pretreated by plasma. Then, the pretreated membrane was soaked in AMPS solution and the loaded monomer was grafted on the membrane by plasma-induced. ATR-FTIR, FESEM, water contact angle (WCA) and breaking strength were employed to characterize the structural and morphological changes of membranes. The effects of pretreatment time, AMPS concentration and grafting reaction time on the membrane performances were studied. In addition, the rejections to single salt solutions and mixed salt solutions were examined. The results indicated that the pretreatment of PSf substrate played an important role in the preparation of this NF membrane. The NF membrane, pretreated for 30 s and grafted for 80 s with 5% AMPS solution, showed a rejection to 1 g/L Na 2SO 4 of 80.8 ± 1.2% with a permeate flux of 6.4 ± 0.2 L m − 2 h − 1 at 0.2 MPa and at 23 ± 1 °C. The rejection sequence of single salts was Na 2SO 4 > MgSO 4 > NaCl > MgCl 2. And the negatively charged NF membrane showed a better selectivity to SO 4 2 − than Cl − in a mixed salt solution.

Investigation on Preparation and Performance of Self-Assembly Nanofiltration Based on the Coordination

Layer-by-layer self-assembly technology has been widely used and developed recently. However, most assembly was directly carried out using polyamions and polycations which is simply based on electrostatic force. In this work, a coordination multilayer with PS ultrafiltration (UF) membrane as substrate was explored by the alternate deposition of a transition-metal-containing polyelectrolyte and a ligand-containing polymer via the formation of complexes. Desalination experiments showed that for single salt solution filtration, the prepared metal-ligand-coordinated multilayer membrane has a 85% rejection of Na2SO4 and a solution flux of 60 L/m2h at 1.0MPa, while the rejection of NaCl was only 20% and the solution flux was about 85 L/m2h at the same pressure. Continuous test indicated that the performance of the membrane is basically unchanged during the two weeks, so the prepared membrane has excellent stability and resistance to swelling.

Use of Mordenite Surface Acidity Properties for the Selective Separation of Halide Salts: Modification of Dielectric Effects

The Mordenite membrane was synthesized onto commercial ceramic tubular support (α-alumina), and surface characterizations were carried out on Mordenite powder. The surface charge density remains stable when Mordenite was in contact with monovalent or divalent salts. Pore diameter was estimated in the range of 8−10 nm. Filtrations tests were performed using the Mordenite membrane with single monovalent halide salt solutions. No rejection was observed. Then, the Mordenite membrane was treated with a divalent anionic salt (Na2CO3). Weak rejections were observed according to the order of hydration energy (NaF > NaCl > NaBr > NaI). After an acid cleaning, the membrane recovered its initial properties. A new estimation of average pore radius proved that steric effects were not responsible for rejection themselves. A new filtration test of mixed monovalent salt solution was carried out. The fluoride ion was retained by a majority. A preferential transfer was observed for chloride, bromide, and iodide ions.

Ability of multicellular salt glands in Tamarix species to secrete Na+ and K+ selectively

The present study aimed to determine the mechanism of cation-selective secretion by multicellular salt glands. Using a hydroponic culture system, the secretion and accumulation of Na(+) and K(+) in Tamarix ramosissima and T. laxa under different salt stresses (NaCl, KCl and NaCl+KCl) were studied. Additionally, the effects of salt gland inhibitors (orthovanadate, Ba(2+), ouabain, tetraethylammonium (TEA) and verapamil) on Na(+) and K(+) secretion and accumulation were examined. Treatment with NaCl (at 0-200 mmol L(-1) levels) significantly increased Na(+) secretion, whereas KCl treatment (at 0-200 mmol L(-1) levels) significantly increased K(+) secretion. The ratio of secretion to accumulation of Na(+) was higher than that of K(+). The changes in Na(+) and K(+) secretion differed after adding different ions into the single-salt solutions. Addition of NaCl to the KCl solution (at 100 mmol L(-1) level, respectively) led to a significant decrease in K(+) secretion rate, whereas addition of KCl to the NaCl solution (at 100 mmol L(-1) level, respectively) had little impact on the Na(+) secretion rate. These results indicated that Na+ secretion in Tamarix was highly selective. In addition, Na(+) secretion was significantly inhibited by orthovanadate, ouabain, TEA and verapamil, and K(+) secretion was significantly inhibited by ouabain, TEA and verapamil. The different impacts of orthovanadate on Na(+) and K(+) secretion might be the primary cause for the different Na(+) and K(+) secretion abilities of multicellular salt glands in Tamarix.

Polyelectrolyte modification of nanofiltration membrane for selective removal of monovalent anions

Nanofiltration (NF) membranes operate with higher flux and lower energy requirements than reverse osmosis (RO) membranes, and exhibit relatively low rejection and selectivity for monovalent ions. In cases where high flux, high selectivity and high monovalent ion rejection are desired, an ideal membrane would exhibit the flux behavior of NF membranes and rejection behavior of RO membranes. Thus, a commercially available NF membrane was modified by layer-by-layer (LbL) assembly of alternating polyelectrolyte thin films in order to increase rejection and selectivity towards monovalent ions, namely F − and Cl −. Poly (styrene sulfonate) (PSS) was the anionic polyelectrolyte and poly (diallyldimethyl ammonium) chloride (PDADMAC) was the cationic polyelectrolyte. Thin (0.5–8.5) PDADMAC/PSS bilayers were deposited on the substrate membrane. NF experiments were performed in a cross-flow cell with feed solutions of (a) a ternary mixture of sulfate, fluoride and chloride at 100, 500 and 1000 mg/L concentrations and (b) single salt solutions of Cl − or F −. In the ternary mixture experiment, fluoride rejection increased from 40% (unmodified) to 70% (8-bilayer modified), whereas chloride rejection was reduced from 55% (unmodified) to 20% (8-bilayer modified). The selectivity for chloride over fluoride was 2.7 for an 8-bilayer modified membrane compared to 1 for unmodified membrane. In single salt filtration of fluoride there was no appreciable change in the rejection for the modified membranes, whereas in the filtration of chloride the rejection increased from 30 to 91% upon modification (8-bilayer). The flux decreased by 30% with 8-bilayer modification for all feed solutions. The performance (flux, rejection, selectivity) of the modified membranes was compared to a BW30 RO membrane; under identical operating conditions, the modified membranes exhibit higher selectivity and flux than the commercial RO membrane.

10.2478/s11814-009-0286-4

In this work an equation of state applicable to the system containing electrolytes has been developed by coupling the perturbed chain statistical associating fluid theory (PC-SAFT) with the primitive mean spherical approximation. The resulting electrolyte equation of state is characterized by 4 ion parameters for each of the cation and anion contained in aqueous solutions, and 4 ion specific parameters for each of six cations (Li+, Na+, K+, Rb+, Mg2+ and Ca2+) and six anions (Cl−, Br−, I−, HCO 3 − , NO 3 − and SO 4 2− ) were estimated, based upon the individual ion approach, from the fitting of experimental densities and mean ionic activity coefficients of 26 aqueous single-salt solutions at 298.15 K and 1 bar. The present equation of state with the estimated individual ion parameters has been found to satisfactorily describe not only the densities and mean ionic activity coefficients, but also osmotic coefficients and water activities of single-salt aqueous solutions. Furthermore, the present model was extended to two-salt aqueous solutions, and it has been found that thermodynamic properties such as mentioned above, of two-salt solutions, can be well predicted with the present model, without any additional adjustable parameters.

Determining the Dielectric Constant inside Pores of Nanofiltration Membranes from Membrane Potential Measurements

The membrane potential technique was applied to a nanofiltration polyamide membrane to determine its mean pore radius and the dielectric constant of electrolyte solutions inside pores. To our knowledge, this is the first attempt to assess these features from membrane potential measurements. Membrane potential data were analyzed by means of the SEDE (steric electric and dielectric exclusion) transport model. Experiments were conducted with single-salt solutions of NaCl and CaCl(2) and mixed-salt solutions of NaCl and CaCl(2) at various concentrations. It was shown that the pore-size values deduced from the high-concentration limit of the membrane potential measured with the two single-salt solutions are in good agreement. With this parameter being known, the membrane potential measured at high salt concentration with electrolyte mixtures was further used to compute the dielectric constant inside pores. The latter was found to be smaller than its bulk value and to decrease when sodium ions were replaced by calcium ions.

The effect of feed pH on the performance of a reverse osmosis membrane

Membranes are predominantly characterized using single salts at one pH. In this paper, a more comprehensive characterization of a reverse osmosis membrane is provided. The zeta potential, flux and rejection behaviour of a brackish water membrane is examined over a wide pH range. Performance in both single salt solutions (sodium chloride, calcium chloride and sodium sulfate) and a binary solution of calcium and sodium chloride are considered. Results are consistent with the Donnan exclusion theory. That is, larger, more highly charged ions are rejected more strongly than smaller or less strongly charged species. In particular, this behaviour can lead to hydronium ion rejections of up to −300% as this ion permeates preferentially over other cations. Membrane flux is relatively constant with respect to pH although there is some evidence of an increase in flux in doubly charged salt solutions at high pH.

Limiting current density in the electrodialysis of multi-ionic solutions

The aim of this work was the development of an explicit model to estimate the limiting current density in the electrodialysis of dilute multi-ionic solutions. The model assumes that the ionic transport occurs in a film layer adjacent to an ion exchange membrane and it is quantified by a linearized form of the Nernst–Planck (NP) equations together with the electroneutrality requirement at the solution/ion exchange membrane interface. An explicit expression for the limiting current density of a dilute multi-ionic solution was derived, involving a mass-transfer coefficient based on an effective diffusivity of the multi-ionic solution. The model further assumes that the steric exclusion of the ions by the membrane is negligible, and thereby the limiting current density is attained when the concentration of each and every ion is null at the solution/membrane interface. The model predictions for the limiting current density were compared with experimental data of single salt solutions (MgCl 2) and multi-ionic solutions (MgSO 4 + MgCl 2) in a bench-scale electrodialysis unit (EUR2C-7P18, Eurodia, France) for various Reynolds numbers and salts concentrations. The average relative deviations between the model predictions and the experimental data were lower than 13% for MgCl 2 solutions (10, 20 equiv./m 3) and solutions of MgSO 4 + MgCl 2 (5 + 5, 10 + 10 equiv./m 3). The dimensionless limiting current density and the counterions transport numbers predicted by the linearized and non-linear Nernst–Planck equations were also compared for a broad range of dimensionless operating parameters and a fair to good agreement was observed between the two approaches.

Multilayered PEC nanofiltration membranes based on SPEEK/PDDA for anion separation

Layer-by-layer (LBL) deposition of polyelectrolytes (PEs) on porous substrates has emerged as a versatile and inexpensive method to form high flux nanofiltration (NF) membranes. PE complexes (PECs) were prepared by alternative deposition of poly(diallyldimethylammonium) chloride (PDDA) and sulfonated poly(ether ether ketone) (SPEEK) on hydrolyzed polyacrylonitrile (PAN-H) supports. PDDA/SPEEK multilayers consisting of 5, 10, 15 and 20 bilayers were deposited from solutions with different NaCl concentrations. Film thickness was studied by AFM. Thicker films were formed with increasing the bilayer number and the salt concentration. For the first time, multilayered PDDA/SPEEK PEC membranes were applied as NF membrane in an anion separation application. The NF performance of PDDA/SPEEK membranes was studied for single salt solutions (chloride, sulfate and phosphate) as well as mixed-salt (binary and ternary). Membranes showed different retentions and fluxes for anions in single and mixed-salt solutions. When using mixed-salt solutions, the retentions were found to be higher compared to the single salt NF experiment results. HPO 4 2− ions showed a remarkable influence on the performance of the PEC membranes via formation of intermolecular bonds within the polymer chains. In the best case, membranes showed Cl −/SO 4 2− and Cl −/HPO 4 2− selectivities of 20 in the separation of mixed-salt solutions.

Nanofiltration of single and mixed salt solutions: Analysis of results using principal component analysis (PCA)

The retention of single (NaCl) and mixed (Na 2SO 4 + NaCl) salt solutions by nanofiltration membranes was analysed using principal component analysis (PCA). Seven variables—five different membranes (Desal-5DK, NF 270, NF Filmtec, NF 20 and ESNA-1-LF), flux and salt concentration—were taken into account in the analysis. The dependences of NaCl retention on these variables for single and mixed solutions were evaluated by PCA. Differences and similarities between the obtained results and those from a previous study based on a univariate method were observed. The PCA results from single and mixed salt solution experiments indicated that the NF 20 membrane differs from the others; a finding that is in accordance with earlier observations. The difference could be explained by the fact that the NF 20 membrane was not charged at the filtration pH (6) and by its bigger pore size. For single salt solutions, the results for the NF Filmtec and NF 270 membranes (manufactured by Filmtec) resembled each other when using the PCA method; a finding which could not be shown using a univariate method. These membranes did not resemble each other when filtering mixed salt solutions. Analysis of the results when using the PCA method for the single and mixed salt solutions indicated that the retention of the ESNA-1-LF membrane behaved differently from all the other tested membranes. While a negative retention for NaCl in a mixed solution was observed with the other membranes, the ESNA membrane showed a positive retention which was close to the retention of a reverse osmosis membrane (above 80%). The results showed that both salt concentration and permeate flux had major effects on the salt retentions of the NF 20, NF Filmtec, NF 270 and Desal-5DK membranes.

Transport of NaCl, MgSO_4, MgCl_2 and Na_2SO_4 across DL type nanofiltration membrane

The separation of inorganic ions by membrane filtration of aqueous solutions through NF-membranes was investigated. The single and binary salt solutions of NaCl, MgCl 2 , Na 2 SO 4 and MgSO 4 were used in this study. These salts are the most commonly found in natural water resources. From the research literature it was stated that NaCl has a very low rejection range with NF membranes while MgCl 2 is moderately rejected, Na 2 SO 4 and MgSO 4 are very highly rejected salts. Thus, these four salts provide a full range of rejection behavior for the NF system and hence constitute an ideal system. In addition, these salts have a very high solubility in water so there would not be any cake layer formed due to precipitation of salts at the surface of the membrane where a significantly higher concentration of salt can be developed due to concentration polarization.

Comparison of activity coefficient models for electrolyte systems

AbstractThree activity coefficient models for electrolyte solutions were evaluated and compared. The activity coefficient models are: The electrolyte NRTL model (ElecNRTL) by Aspentech, the mixed solvent electrolyte model (MSE) by OLI Systems, and the Extended UNIQUAC model from the Technical University of Denmark (DTU). Test systems containing a single salt (NaCl), multiple salts, and mixed solvent aqueous electrolyte solutions were chosen. The performance of the activity coefficient models were compared regarding the accuracy of solid–liquid and vapor–liquid equilibrium calculations for the test systems. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Membrane-based CO 2 absorption into blended amine solutions

A blended solution based on amino acid salt was presented as a CO 2 absorbent. Coupling process of a membrane contactor and the blended solution was investigated. The performances of the blended solution were experimentally compared with the single amino acid salt solution. Effects of various factors such as gas and liquid flow rates on CO 2 concentration at gas and liquid outlets, capture efficiency, and overall mass transfer coefficient were discussed. A resistance in series model was developed to predict the overall mass transfer coefficients. Results show that performance of the blended solution is evidently better than that of the single solution. The blended solution can give lower CO 2 concentration at gas outlet and higher CO 2 concentration at liquid outlet as well as higher capture efficiency when compared with the single solution. The overall mass transfer coefficient with the blended solution is much higher than that with the single solution. The blended solution is confirmed to be an efficient absorbent for CO 2 capture in membrane gas absorption. Model values are in good agreement with the experimental values.

Modeling of aqueous electrolyte solutions based on perturbed-chain statistical associating fluid theory incorporated with primitive mean spherical approximation

In this work an equation of state applicable to the system containing electrolytes has been developed by coupling the perturbed chain statistical associating fluid theory (PC-SAFT) with the primitive mean spherical approximation. The resulting electrolyte equation of state is characterized by 4 ion parameters for each of the cation and anion contained in aqueous solutions, and 4 ion specific parameters for each of six cations (Li+, Na+, K+, Rb+, Mg2+ and Ca2+) and six anions (Cl−, Br−, I−, HCO3−, NO3− and SO42−) were estimated, based upon the individual ion approach, from the fitting of experimental densities and mean ionic activity coefficients of 26 aqueous single-salt solutions at 298.15 K and 1 bar. The present equation of state with the estimated individual ion parameters has been found to satisfactorily describe not only the densities and mean ionic activity coefficients, but also osmotic coefficients and water activities of single-salt aqueous solutions. Furthermore, the present model was extended to two-salt aqueous solutions, and it has been found that thermodynamic properties such as mentioned above, of two-salt solutions, can be well predicted with the present model, without any additional adjustable parameters.