Reverse Osmosis Experiments Research Articles

Isotopic fractionation and overall permeation of lithium by a thin-film composite polyamide reverse osmosis membrane

A small, but significant fractionation of lithium isotopes was observed during reverse osmosis experiments performed with a thin-film composite polyamide membrane (FilmTec FT-30 membrane). Relative to the feed solution, the heavier lithium isotope 7Li was depleted in the permeate from 4.3 to 10.6 per mil (±3.3 per mil) during six experimental runs. The observed isotopic fractionation might be the result of the slightly different permeabilities of 6Li and 7Li across the FT-30 membrane. The heavier of two solute isotopes has a slightly lower mobility due to its slightly greater mass. Therefore, the heavy isotope permeates slightly more slowly inside the membrane than its isotopically lighter counterpart. As a result, more of the heavier isotope is retained on the high-pressure side of the membrane during each run. The magnitude of isotopic fractionation generally increased with increasing water permeation rates. Additionally, experimental evidence shows that the FT-30 membrane exhibits essentially identical solute rejection with NaCl and LiCl under the same operating conditions.

Studies on distribution and reverse osmosis properties of cellulose acetate derivatives for organic compounds

Particles of cyanoethyl cellulose acetate (CACN), hydroxypropyl cellulose acetate (CAHP) and cellulose acetate have been prepared from the same solutions as used for casting of reverse osmosis (RO) membranes. The distribution properties of these materials for alcohols have been studied by liquid chromatography. It was found that alcohols were attracted to the surfaces of these membrane materials. The K d values increased with increasing carbon numbers of n-alkyl alcohols. The order for alcohols with different structures was n-> iso-> sec-> tert-. The separation properties of the RO membranes cast from these materials for alcohols have also been investigated. It was found that the solute rejections for n-alkyl alcohols reached a maximum for alcohol molecules containing three carbon atoms. The order of solute rejection for alcohols with different structures was tert-> sec-∼ iso-> n-. The results from liquid chromatography and reverse osmosis experiments are discussed with respect to the chemical nature of the organics in terms of polar, nonpolar and steric effects estimated using the Taft number (σu*), the Small number ( s) and a steric parameter ( Es), respectively. The differences in performance of the solute between the liquid chromatography and the reverse osmosis processes are also discussed.

Characterization of the reverse osmosis membranes CA 80 and CA 95 by transport coefficients

For characterization of Bulgarian reverse osmosis membranes CA 80 and CA 95, the linear phenomenological relationships of the thermodynamics of irreversible processes are used. On the basis of the transport parameters l p, l π, R ∞ determined in reverse osmosis experiments, it is shown that the flux of the solute (NaCl) through the membrane is due to the diffusive and convective transport. The part of the latter increases with an increase of feed concentration and operating pressure. For the concentrating range investigated (0.03–0.3 mol/1) the slope of the straight lines 1 / R vs. 1 / q is approximately equal, i.e. it is nearly independent of feed concentration, which may be explained by non-electrochemical effects.

Theoretical and experimental analysis of diffusion and polarization phenomena in sugar solutions in spiral-wound modules

Reverse osmosis experiments have been carried out in a pilot plant equipped with a commercial high rejection spiral-wound module for glucose-water and sucrose-water systems at different feed concentrations in the range 0–14% and 0–25%, respectively, at pressures between 0 and 60 atm and at a temperature of 30°C. Rejections higher than 99% and fluxes of 30–40 l/m 2 h have been obtained at operating conditions of industrial interest. The data collected have been used to derive the characteristic parameters of the membrane and module, with special emphasis on polarization on the high pressure side and on diffusion through the membrane. Comparison of theoretical calculations with the experimental data indicates that a single set of parameters can be used over the whole range of concentrations and pressures for the two systems.

Membrane separation processes for the clean production of xanthates

The performance of pervaporation for the purification of an amyl alcohol—water mixture that was recycled to the reactor in the potassium amyl xanthate production was investigated. The purification consists of the reduction of the water content from a saturation concentration of 7.4% (w/w) to a concentration of 3.5% (w/w) and leads to higher reaction yields. Pervaporation experiments were performed with poly(vinyl alcohol)/polyacrylonitrile membranes. Selectivity and flux data were obtained for the concentration range of interest. Based on these data, mass balance calculations were carried out. The same calculations were performed for the purification by distillation. Since phase separation occurs, the cleaning of a water rich phase through the use of distillation and reverse osmosis was also considered. Reverse osmosis experiments were performed with polyamide/polyethersulfone membranes.

Reverse osmosis of single and mixed electrolytes with charged membranes: Experiment and analysis.

Reverse osmosis experiments using single electrolytes, mixed electrolytes having common cations (sodium chloride/sodium sulfate and sodium hydroxide/sodium sulfate) and mixed electrolytes having common anions (sodium chloride/magnesium chloride and hydrochloric acid/magnesium chloride) were carried out with negatively charged membranes. Rejection occurred only due to the charge effect and was dependent upon volume flux, the ratio of the membrane charge density to feed concentration and the mole fraction in mixtures. Mono- and divalent anions in mixed electrolytes having common cations were found to be separated effectively. In particular, monovalent anion showed negative rejection. Mono- and divalent cations could not be separated as efficiently as mono- and divalent anions.Experimental results for single electrolyte solutions were well explained by applying the extended Nernst–Planck equation, and two transport parameters (effective charge density and structural parameters) were obtained. Rejection of ions in mixtures was well predicted by the extended Nernst–Planck equation with the transport parameters obtained from the single-electrolyte experiments.

A comparative study of fouling-index and fouling-potential of waters to be treated by reverse osmosis

Fouling-index (FI) measurement is widely used as a means of predicting fouling potential of waters to be treated by reverse osmosis (RO). However, it has been found that even low FI waters may cause severe fouling under practical conditions. In this work FI measurements of solutions containing inorganic or organic solutes have been compared to RO experiments with a flat channel test module measuring flux decline and salt rejection behaviour with time. Predictions of high fouling potential according to FI values could only partly be verified by RO experiments. Conclusively fouling potential of waters to be treated by RO should be determined by carrying out experiments with a test module under quasi-real RO conditions.

Interactions in reverse osmosis of electrolyte/ charged surfactant solutions through porous membranes

Reverse osmosis experiments have been carried out using very weakly negatively charged cellulose acetate membranes and membranes made from sulfonated polysulfone which have cation—exchange properties. Experimental solutions consisted of hydrophobic charged surfactants and inorganic electrolytes. The rejection of the surfactants decreases markedly as the concentration of electrolytes increases and reaches negative values at low pressures. The volume flow in the presence of the surfactants also decreases. A membrane polymer/substrate interaction, e.g. adsorption, is proposed to account for this effect which is more pronounced with the ion—exchange membranes. However, at c.m.c. levels in the solution, the surfactant rejection increases and it appears to be related to a change in the activity.

Study on the interaction between membranes and organic solutes by the HPLC method

Interactions between three membrane materials and about 90 organic solutes have been studied by the high-performance liquid chormatography method. Three relationships characterizing the interactions, which are related to membrane performace, have been obtained by means of both thermodynamics and reverse osmosis experiments. The relatinship between interaction and structure of polymer and organic solutes is discussed based on the partition coefficient and Gibbs free energy obtained from high-performance liquid chromatography experiments. The results may be useful for the molecular design of membrane materials.

Interactions in reverse osmosis of electrolyte/ charged surfactant solutions through porous membranes

Reverse osmosis experiments have been carried out using very weakly negatively charged cellulose acetate membranes and membranes made from sulfonated polysulfone which have cation—exchange properties. Experimental solutions consisted of hydrophobic charged surfactants and inorganic electrolytes. The rejection of the surfactants decreases markedly as the concentration of electrolytes increases and reaches negative values at low pressures. The volume flow in the presence of the surfactants also decreases. A membrane polymer/substrate interaction, e.g. adsorption, is proposed to account for this effect which is more pronounced with the ion—exchange membranes. However, at c.m.c. levels in the solution, the surfactant rejection increases and it appears to be related to a change in the activity.

Rejection of Alkyl Phenols by Reverse Osmosis Membranes

The removal of different alkyl phenols by two commercially available reverse osmosis membranes, one cellulose acetate and one aromatic polyamide both with very high rejection of NaCℓ was investigated. The polyamide membrane rejected phenol very effectively while the cellulose acetate membrane showed virtually no phenol rejection. In order to explain these differences a study was made of the mechanisms of phenol rejection by the two membranes. Transient diffusion tests as well as partitioning tests were conducted in addition to the reverse osmosis experiments. From the results it is concluded that phenol is sorbed in both membranes and that the forces of attraction between phenol and cellulose acetate are relatively weak compared to the attraction between phenol and polyamide resulting in a faster rate of diffusion in cellulose acetate and hence poor rejection.

Calcium sulphate fouling of reverse osmosis membranes: Flux decline mechanism

The cake filtration fouling model for reverse osmosis (RO) was examined and found to be physically unrealistic for flux decline due to calcium sulphate precipitation fouling. Continuous flow RO experiments on a saturated solution of calcium sulphate were performed to elucidate the flux decline mechanism. The calcium sulphate deposit on the membrane was characterized by SEM and computer-aided image analysis. It is shown that flux decline is due to blockage of the membrane surface by lateral growth of the deposit rather than the hydraulic resistance of a cake building up at the membrane surface. The importance of deposit characterization for the verification of fouling mechanisms is emphasized.

The influence of membrane heterogeneity on the solute retention

A Spiegler—Kedem type expression for the retention R as a function of the volume flow J ν is derived for heterogeneous membranes. Linearization of the expression yields a well practicable equation by which reverse osmosis experiments can be simply analysed and the reflection coefficient and the solute permeability or their ratio can be determined. A discussion is presented on the meaning of the slope and the intercept of the linear plot of 1/ R against 1/ J ν. The results are specified for membranes of known simple membrane structures. Special attention is paid to the Loeb—Sourirajan type membrane. Finally the proposed linear equation is compared with other known linear equations and some reverse osmosis experiments are reanalysed.v

Separation of aromatic substances from aqueous solutions using a reverse osmosis technique with thin, dense cellulose acetate membranes

Investigations were made of the water flux rate and rejection characteristics of aromatic substances in aqueous solutions using a thin, dense cellulose acetate membrane in reverse osmosis experiments. The aromatic substances used were phenol, aniline, hydroquinone and p-chlorophenol. The permeate became more enriched in aromatic compounds as compared to the feed solution as the water content of the membrane increased. By considering both the effects of pressure on the chemical potential of a component and the contribution of viscous flow to the overall transport of that component in the hydrated membrane, a theoretical relationship was developed to predict the negative solute rejection of the membrane. Based on this proposed theory, the permeability coefficients of water and organic solute were estimated from experimental solute rejection data, including negative values. The permeability coefficients of components were in good agreement with previously established correlations in measurements of partition and diffusion coefficients.

Precise measurement of membrane constants of cellulose acetate membranes by direct osmosis tests

A method for the determination of membrane constants by direct osmosis tests was studied using cellulose acetate membranes. A countercurrent type osmosis cell was designed and made for this study, and a method for precise measurements of permeated water and solutes through the membrane was established. Based on the membrane constants derived from direct osmosis tests, membrane performances of cellulose acetate membranes under pressure of 40 atm were predicted. The predicted values were in good agreement with the observed values in reverse osmosis experiments and it was confirmed that membrane performances under pressure could be predicted by the direct osmosis with considerably good accuracy.

Selective transport mechanism through the reverse osmosis membrane.

The determination of transport parameters used for inorganic solutes was extended to various organic solutes including those which show negative rejection. The physical meaning of the three transport parameters (hydraulic permeability, solute permeability and reflection coefficient) was studied. The solute permeability obtained both from diffusion and reverse osmosis experiments using a symmetric membrane coincided well, and it could be split into diffusion coefficient and partition coefficient by the former experiments. It was also found that results for symmetric and asymmetric membranes, which have the same reflection coefficient, could be transformed only by correction of the thickness.

Effect of Polymer Membrane Material on Reverse Osmosis Separations of Free Radicals in Aqueous Solutions

Abstract The reverse osmosis separations of four different piperidinooxy free radicals, one undissociated piperidine solute, and one dissociated hydrochloride solute of similar molecular structure were studied by using cellulose acetate (CA-398) and aromatic polyamidohydrazide (PPPH 8273) membranes of different surface porosities under different operating pressures. Liquid chromatography experiments were conducted parallel to the reverse osmosis experiments, and the order of the preferential sorption among solute compounds and solvent water was found to be, ionic solutes < D2O < radical solutes < nonionized solute in the case of cellulose acetate material, and ionic solutes < radical solutes < D2O < nonionized solute in the case of aromatic polyamidohydrazide, with the one exception of the piperidinooxy free radical containing an amino group which was slightly more strongly adsorbed to the latter polymeric material than D2O. The above order of preferential adsorption is reflected in the reverse osmosis se...

Reverse Osmosis Separations of Free Radicals in Aqueous Solutions

Abstract The reverse osmosis separations of two piperidinooxy radicals from aqueous solutions were studied at operating pressures of 1724 to 6895 kPag. Both radicals are stable during the reverse osmosis experiment and positively separated by using cellulose acetate membranes. The liquid chromatography data indicates that piperidinooxy radicals are weakly adsorbed to cellulose acetate polymer, while their electrolyte analogues are rejected, and undissociated piperidine solute (also a structural analogue of the radicals) is strongly adsorbed.

Bentonite-stabilized CDA/CTA membranes: II. Determination of the transport coefficients lep and lp

Improved integral-asymmetric CDA/CTA blend membranes developed at the GKSS Research Center Geesthacht were formulated with various bentonite dopes to investigate their influence on membrane properties. The characterization of the membrane transport behavior was determined by the hydrodynamic permeability, l p , and the electromechanical coupling coefficient, l ep , as functions of the external NaCl-concentrations, c s . The membranes tested were prepressurized in reverse osmosis experiments for 1 hour and 200 hours, respectively, at the experimental conditions of 0.6 mol/l NaCl-solution, 25°C, 80 bar. The results obtained were as follows: The transport coefficients l ep and l p were considerably changed after the long-term RO test run. For all salt concentrations, the permeability l p was lowered by about 20%. But at the highest salt concentration of c s = 0.2 mol/l, the minimum permeability changes were observed for the membranes with 700 ppm and without bentonite addition. Moreover, both these membrane types showed the smallest reduction of thickness after the 200 hours test run. The alterations of the electro-mechanical coupling coefficient l ep were interpreted — applying the finely porous membrane model to the two-layer membranes — as an increase of the diffusion potential and a decrease of the streaming potential. The salt diffusion coefficient was analysed to be diminished by about 10% for all membrane types. The lowest performance changes of the membranes with about 1000 ppm bentonite dope, support the experimental findings of flux stabilization of the bentonite-containing CDA/CTA membranes, published previously.

On the nature of free and bound water in water permeation of cellulose acetate membranes in reverse osmosis

Cellulose acetate membranes were prepared from several Manjikian-type casting solutions, whose formamide content varied. Their phenomenological coefficients (σ, L p , P, and L p π ) were obtained by reverse osmosis experiment and the states of water in these membranes were investigated by means of differential scanning calorimetry. It was stated that the coupling coefficient was concerned with the bound water content in membrane and the hydraulic permeability, with the total water content. The contribution of both bound and free water to water permeation was discussed.