Maria Norberta de Pinho

An ATR-FTIR study of water in cellulose acetate membranes prepared by phase inversion

Abstract The state of water in the active layer of a series of cellulose acetate asymmetric membranes, prepared by the phase inversion process, was investigated using attenuated total reflection infrared spectroscopy (ATR-FTIR). The relative amount of water present in the active layer depends on the inherent surface or skin layer morphology; the more permeable ultrafiltration asymmetric membranes contain more water compared to the less permeable nanofiltration / reverse osmosis asymmetric membranes studied. Lightly clustered water species, weakly hydrogen-bonded to the polymer hydroxyl groups, predominate in the skin layer of the reverse osmosis membranes whereas the ultrafiltration membranes contain predominantly bulk-like water cluster. During the rehydration of the dried membrane, either with liquid water or in a constant relative humidity environment, the size of the water clusters and the extent of the water-water interactions in these clusters increase as more water is sorbed by the active layer. This difference in the state of water (amount and type of water species) present in the skin layer evidences the heterogeneous morphology of the active layers for a series of cellulose acetate asymmetric membranes prepared by the phase inversion technique.

Atomic force microscopy of dense and asymmetric cellulose-based membranes

Abstract The surface structures of dense and integrally skinned cellulose acetate (CA) and cellulose acetate butyrate (CAB) membranes, prepared by phase inversion under different casting conditions, are investigated by tapping mode atomic force microscopy (TM AFM). The results obtained show that: (i) The top and bottom surfaces of the dense CA membrane were quite uniform in comparison with the corresponding faces of asymmetric CA and CAB membranes. Despite the casting conditions the active and support layers of the asymmetric membranes display large differences on the roughness parameters. (ii) The asymmetric membranes prepared with an organic system as a non-solvent pore-former (method IV) display smaller nodule aggregates and lower values of the roughness parameters than the ones prepared using an inorganic system as swelling agent (method I). This is more pronounced for the CA membranes than for the CAB membranes. (iii) In the active layer of asymmetric CA membranes casted at longer evaporation times, the measured values of surface roughness parameters tend to decrease. Also, for these CA membranes, as the evaporation time increases the average size of the depression areas observed on the surface decreases. The laboratory-made CA and CAB membranes display a wide range of nanofiltration and reverse osmosis permeation characteristics. These characteristics are correlated to surface roughness parameters of the active layers.

Design of polypropylene oxide/polybutadiene bi-soft segment urethane/urea polymer for pervaporation membranes

Novel cross-linked urethane/urea polymers with two soft segments, polypropylene oxide and polybutadiene, were prepared by extending polypropylene oxide-based isocyanate-terminated triol prepolymer with polybutadiene diol. Infra-red spectra indicated that with increasing polybutadiene content in the polymers, the hydrogen bonding of urethane/urea groups among hard segments became weaker. This was interpreted as an improvement of freedom of urethane/urea groups and their mixing with polypropylene oxide. The dynamic mechanical thermal analysis indicated that the two soft segments were highly dispersed, and could lead to a molecularly mixed phase. The swelling experiments with ethanol aqueous solutions indicated a preferential sorption of ethanol and a decrease of the swelling degree with increase of polybutadiene content in the polymer films. The preferential permeation of ethanol is confirmed by the pervaporation results.

The effect of the ladder-type spacers configuration in NF spiral-wound modules on the concentration boundary layers disruption☆

Abstract The laminar flow structure and concentration distribution in a narrow rectangular channel simulating the feed channel of nanofiltration (NF) spiral-wound modules are investigated. The continuity, Navier—Stokes equations and the solute continuity equation are solved by the control volume formulation. To validate the numerical predictions, NF permeation experiments with an aqueous solution of sodium chloride (2 g/l) at 25°C were performed in a laboratory cell with a rectangular feed channel (2 mm height × 30 mm width × 20 cm length) filled with a ladder-type spacer with a transverse inter-filament distance of 3.8 mm. The numerical results show that the average concentration polarization for the membrane wall with adjacent transverse filaments is independent of the distance to the channel inlet, while for the membrane wall without adjacent filaments the average concentration polarization increases with the channel length. This is due to the fact that in the first case the concentration boundary layer is periodically disrupted by the transverse filaments while in the second case the concentration boundary layer grows continuously along the channel length. The experimental results of the NaCl apparent rejection coefficients are compared to the model predictions, the agreement being good. These results clearly establish how crucial the spacers configuration is in the optimization of the spiral wound module efficiency.

Hydrodynamics and concentration polarization in NF/RO spiral-wound modules with ladder-type spacers

Abstract The hydrodynamics and concentration polarization in the feed-channel of a NFIRO spiral-wound module with ladder-type spacers were investigated by computational fluid dynamics. The momentum and mass transport equations together with the appropriate boundary conditions were solved numerically by the control volume formulation for stable two-dimensional laminar flow. Permeation experiments with aqueous solutions of sodium chloride at feed concentration of 2 gll and 25°C were performed in a laboratory NF cell with a spacer-filled channel (2 mm height × 30 min width × 20 cm length) in order to validate the numerical model. The tested spacer had a set of transverse filaments with a diameter of 1.0 mm, equally spaced and connected by two longitudinal filaments, with a distance between the axis oftwo consecutive filaments of 3.8 mm, forming a ladder-type structure. A thin-film composite nanofiltration membrane from Separem (Italy) was used. The numerical results show that the concentration polarization index exhibits strong local variations, and its profile is correlated with the flow pattern. It was also found that the increase of the Reynolds number is not by itself a sufficient condition to enhance the hydrodynamic conditions over the whole membrane: an adequate control of the flow structure through the careful selection of the cross section of the filaments is also indispensable. Good agreement was observed between the predicted and experimental values of the apparent rejection coefficient for all the range of operating conditions tested.

Structure of water in asymmetric cellulose ester membranes — and ATR-FTIR study

The structure of water in the active layer of cellulose acetate and cellulose acetate butyrate asymmetric membranes of nanofiltration/reverse osmosis is studied and correlated with the permeation properties. Membranes are prepared by phase inversion, varying the casting solution and the casting conditions. Their preferential permeation performance is tested with pure water and a model solution of NaCl and the water structure in the active layer is investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). ATR-spectra of cellulose acetate and cellulose acetate butyrate membranes show bands due to OH stretching centred in the 3370–3460 cm−1 range, their position depending on the polymer, casting solution and casting conditions. The casting conditions, namely the evaporation time, have a strong effect on the structure of the active layer: the spectra of cellulose acetate membranes prepared using lower evaporation time show lower values of νoh this band shifting from 3372 to 3458 cm−1 for samples prepared using evaporation time of 1 and 10 min, respectively. ATR-FTIR spectra of the support show lower values of νoh than those of the active layer which is in agreement with the presence of larger pores and larger water clusters in the sublayer. The presence of a larger and more hydrophobic substitute, such as butyryl, has a strong influence on the permeation performance, leading to higher values of f. The higher values of νoh observed in spectra of cellulose acetate butyrate membranes have been associated to the presence of weakly H-bonded water clusters. For both cellulose acetate and cellulose acetate butyrate membranes a relation between the permeation properties and the structure of the water in the active layer was found, lower values of apparent rejection being obtained for increasing size of the water clusters.

The effect on mass transfer of momentum and concentration boundary layers at the entrance region of a slit with a nanofiltration membrane wall

Abstract An integrated model based on the finite volume formulation to numerically simulate the fluid flow of the feed phase in nanofiltration systems is presented. This model accounts for the transport phenomena occurring inside the membrane through the use of appropriate boundary conditions. It allows for predictions of developing laminar flows hydrodynamics and mass transfer of aqueous solutions in slits with permeable walls. The experimental cell (200 mm ×30 mm ×2 mm ) simulates the two-dimensional conditions of flows in spiral-wound modules channels. Experimental data validate predictions of apparent rejection coefficients and permeate fluxes. Correlations for the concentration/hydrodynamic boundary layers thickness ratio, δ ω / δ u =2.92 Sc −0.37 Re p −0.17 [( x / h ) Re −1 +0.013 Sc 0.45 Re p 0.75 ] 0.5 , and for the permeation Stanton number, St =1.3( l / h ) −0.2 Re 0.05 Re p −0.4 Sc −0.1 (1− f ′)/ f ′, are proposed, in the range 250 0.02 p and 570 sC

Numerical modelling of mass transfer in slits with semi‐permeable membrane walls

A mathematical model to predict the concentration polarisation in nanofiltration/reverse osmosis is described. It incorporates physical modelling for mass transfer, laminar hydrodynamics and the membrane rejection coefficient. The SIMPLE algorithm solves the discretised equations derived from the governing differential equations. The convection and diffusive terms of those equations are discretised by the upwind, the hybrid and the exponential schemes for comparison purposes. The hybrid scheme appears as the most suitable one for the type of flows studied herein. The model is first applied to predict the concentration polarisation in a slit, for which mathematical solutions for velocities and concentrations exist. Different grids are used within the hybrid scheme to evaluate the model sensitivity to the grid refinement. The 55×25 grid results agree excellently for engineering purposes with the known solutions. The model, incorporating a variation law for the membrane intrinsic rejection coefficient, was also applied to the predictions of a laboratory slit where experiments are performed and reported, yielding excellent results when compared with the experiments.

Ultrafiltration for colour removal of tannery dyeing wastewaters

Investigation of ultrafiltration (UF) performance for the purpose of removing the colour of three different effluents coming from the dyeing cycle of a tannery industry was carried out using four polyethersulphone (PES) membranes with molecular weight cut-offs (MWCO) of 40, 10, 5 and 3 kDa. A closed-loop recycle (CLR) and batch (B) modes of operation were used. For the first mode of operation, the evolution of permeate flux and colour rejection vs. time of operation was evaluated. For the batch concentration experiments, attention was focused upon the evolution of the rejection and/or concentration of colour with the volume reduction factor (VRF) and on the decrease of the permeate flux as a function of VRF for all the membranes under study. All experiments showed the occurrence of concentration polarization and adsorptive fouling with a pronounced effect for the membranes with higher MWCO. Due to that, the 40 and 10 kDa membranes did not offer a significant regaining of permeate productivity. High-colour and low-solids rejections were observed for all the experiments, showing a marked dependence on the type of effluent used.

The role of ultrafiltration and nanofiltration on the minimisation of the environmental impact of bleached pulp effluents

Abstract Today the bleaching stages of the pulp industry are faced with an enormous challenge concerning the minimisation of the environmental impact of its effluents. Organochlorinated compounds (TOX) and mainly the low-molecular-weight fraction of the extractable compounds (EOX) are major pollutants because of their lipophilicity and bioaccumulation ability. This work evaluates the performance of ultrafiltration (UF) and nanofiltration (NF) on the reduction of organochlorinated matter and colour in the effluents from the first alkaline extraction (E1) of two different bleaching sequences, one with chlorine/chlorine dioxide and the other only with chlorine dioxide. Three types of membranes were used. Ultrafiltration membranes, CA-400 and ETNA01A, and nanofiltration membranes, CDNF50. The substitution of chlorine/chlorine dioxide by chlorine dioxide in the first bleaching stage reduces 84% of EOX, 82% of TOX and 72% of colour in the E1 effluent. The highest removal of TOX and colour achieved by ultrafiltration is 72 and 92%, respectively. The total removal of colour and more than 90% removal of TOX is achieved by nanofiltration. The two operations, by themselves or integrated, can serve the regulatory and/or mill demands.