1,720,997 research outputs found
Recent developments in the preparation of improved nanofiltration membranes for extreme pH conditions
Full text linkConventional commercially available nanofiltration (NF) membranes show limited stability at extreme pH conditions, whereas commercially available NF membranes that can cope with these conditions often show (too) low permeance or are relatively open. Since NF applications in many industrial sectors require pH stable NF membranes with a range of molecular weight cut-offs (MWCO), including tight NF membranes, substantial research and development efforts are being devoted to the research and development of these membranes. This review discusses these developments as reported in open literature, with a focus on the period between 2016 and 2021. Most developments relate to the use of interfacial polymerization to produce thin film composite membranes. Polyamine and polyurea membranes with good chemical stability for extreme pH conditions have been prepared. For polysulfonamide membranes indications for good chemical stability at low pH have been shown. These membrane types show improved stability compared to developed poly(aryl cyanurate), polyesteramide, poly(amide-sulfonamide), and polyamide membranes, which are more susceptible to hydrolysis and therefore less chemically stable at extreme pH. Furthermore, layer-by-layer coating using strong cation - and anion polyelectrolytes has led to new pH stable NF membranes. Despite the extension of the pH stable NF membrane portfolio, most of the developed membranes still do not meet the performance characteristics of the best commercially available NF membranes applicable for the common pH range between 2 and 11. Only a few layer-by-layer coated polyelectrolyte membranes combine high permeance, relatively low MWCO and stability at extreme pH. However, these membranes are not yet commercially available for full-scale applications requiring high pressure operation. Consequently, there is still room for research and development to further improve pH stable NF membranes
Transport phenomena during nanofiltration of concentrated solutions
Full text linkIn most scientific studies on nanofiltration either the development of new membrane materials or the characterization of membranes is reported. In the latter case most studies use single solute salt or sugar solutions and/or investigate nanofiltration of solutions with mixtures of ions at low concentrations relative to solution concentrations often used in industrial applications. Furthermore, several of these studies have tried to predict retention performance of nanofiltration membranes for salt solutions containing two different salts, on the basis of these characterization experiments and derived model parameters, often with limited success. Only limited knowledge is available in open literature on the effect of salt ions in an aqueous feed solution on retention of neutral solutes such as glucose and vice versa. A better insight in these phenomena is needed, since several nanofiltration applications treat solutions containing a combination of salts and (neutral) components such as sugars, amino acids, peptides or proteins. In addition, there has been limited attention in open literature for nanofiltration membrane performance during treatment of more concentrated salt solutions, such as depleted brine in chlor/alkali production and saturated brines in the production of salt crystals, despite the fact that a substantial amount of (potential) nanofiltration applications deals with these types of solutions. A better understanding of the phenomena occurring during nanofiltration of these types of solutions is a pre-requisite for proper design of membrane units for these types of applications. These research questions form the basis for the work presented in this thesis
Creating saturated sodium chloride solutions through osmotically assisted reverse osmosis
Full text linkIncreasing sustainability awareness has created opportunities for the recovery and reuse of salt solutions in industrial processes. Osmotically assisted reverse osmosis (OARO) has the potential to concentrate these salt solutions. Until now OARO research focussed on the production of purified water yielding an unsaturated salt solution as retentate, whereas a saturated salt solution is often needed for reuse. OARO featuring recycling of part of the saturated salt solution to the permeate side of the membrane has now been investigated. The introduction of a saturated NaCl solution results in strongly changing osmotic pressure difference and flux profiles along the length of the modules, with a maximum osmotic pressure difference and a minimum flux close to the outlet of the OARO system. In contrast to the NaCl retention of the membrane, the NaCl feed concentration does not have an influence on the maximum osmotic pressure difference. The osmotic pressure difference and membrane flux are strongly dependent on the applied concentrate split factor, which has a strong effect on the required membrane area per feed flow supplied as well. The amount of surface area required per flow of saturated NaCl solution leaving the OARO unit has an optimum as function of the concentrate split factor. This is due to two counteracting effects, the lower osmotic pressure difference, and the lower fraction of saturated NaCl solution leaving the OARO for a higher concentrate split factor. Based on the modelling, essential knowledge has been generated for the further research and development of OARO
Maximum allowable retention for low-salt-rejection reverse osmosis membranes and its effect on concentrating undersaturated NaCl solutions to saturation
Full text linkFor conventional reverse osmosis (RO), the osmotic pressure difference limits the allowable concentration factor for concentrating NaCl solutions. The use of low-salt-rejection RO (LSRRO) membranes could allow much higher concentration factors, provided that these membranes show a sufficient decrease in NaCl retention with increasing NaCl concentration in the retentate. This work quantifies allowable NaCl retentions for LSRRO as function of the retentate concentration for different pressures. For a pressure of 70 bar allowable NaCl retentions start to deviate from retentions reported for commercial RO membranes at NaCl weight fractions below 0.1. NaCl retentions for commercial nanofiltration (NF) membranes are much lower than allowed for LSRRO, leading to high NaCl losses in permeate. The required NaCl retention for LSRRO as function of the NaCl retentate concentration therefore falls in between those reported for commercial RO and NF membranes, implying that LSRRO membrane development is required. Based on the obtained relations required concentration factors for the concentration of purified seawater RO retentate and depleted brine from chlor/alkali production have been determined. The concentration factor for LSRRO for concentrating solutions with a NaCl concentration of either 0.07 kg·kg−1 or 0.18 kg·kg−1 to saturation at 120 bar pressure is 4 – 5 times higher than for a case where full retention would have been allowed. By optimizing the unit lay-out to minimize NaCl loss in permeate, approximately 1.2 m2·kg−1·h membrane area per saturated NaCl solution flow produced would be required for concentrating a 0.18 kg·kg−1 NaCl solution to saturation, assuming a permeance of 0.36 kg·m−2·h−1·bar−1.</p
Electro-membrane filtration for the selective isolation of bioactive peptides from an αs2-casein hydrolysate
No full textet al.For the isolation of the ingredients required for functional foods and nutraceuticals generally membrane filtration has too low a selectivity and chromatography is (too) expensive. Electro-membrane filtration (EMF) seems to be a breakthrough technology for the isolation of charged nutraceutical ingredients from natural sources. EMF combines the separation mechanisms of membrane filtration and electrophoresis. In this study, positively charged peptides with antimicrobial activity were isolated from an alpha(s2)-casein hydrolysate using batch-wise EMF. alpha(s2)-Casein f(183-207), a peptide with strong antimicrobial activity, predominated in the isolated product and was enriched from 7.5% of the total protein components in the feed to 25% in the permeate product. With conventional membrane diafiltration using the same membrane (GR60PP), isolation of this and other charged bioactive peptides could not be achieved. The economics of EMF are mainly governed by the energy costs and the capital investment, which is affected by the flux of the desired peptide. A maximum average transport rate of alpha(s2)-casein f(183-207) during batch-wise EMF of 1.2 g/m2. h was achieved. Results indicate that an increase in the hydrolysate (feed) concentration, the applied potential difference and the conductivity of the permeate and electrode solutions, and a reduction in the conductivity of the feed result in a higher transport rate of alpha(s2)-casein f(183-207). This is in line with the expectation that the transport rate is improved when the concentration, the electrical field strength, or the electrophoretic mobility is increased, provided that the electrophoretic transport predominates. The expected energy consumption of the EMF process per gram of peptide transported was reduced by approximately 50% by applying a low overall potential difference and by processing desalinated hydrolysate. Considerable improvements in transport rate, energy efficiency, and process economics seem to be attainable by additional optimization of the process parameters and the EMF module design.Contract grant sponsor: Dutch Programme on Ecology, Economy and Technology (E.E.T.)Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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