Vítor Geraldes

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.

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.

Process water recovery from pulp bleaching effluents by an NF/ED hybrid process

Abstract A membrane hybrid process of nanofiltration (NF) and electrodialysis (ED) is optimised to recover water from the E1 alkaline pulp bleaching effluent. This highly coloured effluent has a high content of organic and organochlorinated compounds and salts. NF is selected to remove organic compounds and undertake partial desalination. A high degree of water purity is further achieved using ED to reduce sodium chloride concentration to a level of 350 ppm. The effluent was processed by a NF pilot plant with 1.7 m2 spiral wound module. A model solution of NF permeate was used for ED pilot plant tests at room temperature. The effect of increasing temperature on ED performance was studied on the laboratory scale. A model including experimental data was used to simulate and optimise the hybrid process.

Measuring and modeling hemoglobin aggregation below the freezing temperature.

Freezing of protein solutions is required for many applications such as storage, transport, or lyophilization; however, freezing has inherent risks for protein integrity. It is difficult to study protein stability below the freezing temperature because phase separation constrains solute concentration in solution. In this work, we developed an isochoric method to study protein aggregation in solutions at -5, -10, -15, and -20 °C. Lowering the temperature below the freezing point in a fixed volume prevents the aqueous solution from freezing, as pressure rises until equilibrium (P,T) is reached. Aggregation rates of bovine hemoglobin (BHb) increased at lower temperature (-20 °C) and higher BHb concentration. However, the addition of sucrose substantially decreased the aggregation rate and prevented aggregation when the concentration reached 300 g/L. The unfolding thermodynamics of BHb was studied using fluorescence, and the fraction of unfolded protein as a function of temperature was determined. A mathematical model was applied to describe BHb aggregation below the freezing temperature. This model was able to predict the aggregation curves for various storage temperatures and initial concentrations of BHb. The aggregation mechanism was revealed to be mediated by an unfolded state, followed by a fast growth of aggregates that readily precipitate. The aggregation kinetics increased for lower temperature because of the higher fraction of unfolded BHb closer to the cold denaturation temperature. Overall, the results obtained herein suggest that the isochoric method could provide a relatively simple approach to obtain fundamental thermodynamic information about the protein and the aggregation mechanism, thus providing a new approach to developing accelerated formulation studies below the freezing temperature.

Numerical and experimental study of mass transfer in lysozyme ultrafiltration

This paper addresses protein ultrafiltration (UF) and its dependence on UF operating conditions. Cellulose acetate (CA) asymmetric membranes are laboratory made by the phase-inversion method and characterized in terms of pure water permeability, 8.8×10-12 m/s/Pa, and molecular weight cut-off (10000 Da for 98% of rejection). The important feature of the permeation cell is the slit feed channel of 200 mm×30 mm×1.2 mm that simulates the two-dimensional hydrodynamic flow conditions in a spiral wound membrane module. Permeation experiments were carried out for solutions of reference solutes in order to characterize the membranes and for lysozyme solutions under different operating conditions. The influence of the ionic strength in the permeation flux and protein rejection is studied by performing permeation tests with a solution of lysozyme (0.3 kg/m3) containing different NaCl concentrations. Experimentally was observed a decline in the permeate flux with increasing ionic strength. The membrane is almost completely retentive in relation to lysozyme, since the apparent rejection coefficient,f, for this protein is always higher than 95% (in almost all cases, higher than 98%). Two distinct sets of CFD simulations were performed. One to predict the permeation velocities, vp, and another to predict the lysozyme concentration polarization.

The importance of heat flow direction for reproducible and homogeneous freezing of bulk protein solutions

Freezing is an important operation in biotherapeutics industry. However, water crystallization in solution, containing electrolytes, sugars and proteins, is difficult to control and usually leads to substantial spatial solute heterogeneity. Herein, we address the influence of the geometry of freezing direction (axial or radial) on the heterogeneity of the frozen matrix, in terms of local concentration of solutes and thermal history. Solutions of hemoglobin were frozen radially and axially using small‐scale and pilot‐scale freezing systems. Concentration of hemoglobin, sucrose and pH values were measured by ice‐core sampling and temperature profiles were measured at several locations. The results showed that natural convection is the major source for the cryoconcentration heterogeneity of solutes over the geometry of the container. A significant improvement in this spatial heterogeneity was observed when the freezing geometry was nonconvective, i.e., the freezing front progression was unidirectional from bottom to top. Using this geometry, less than 10% variation in solutes concentration was obtained throughout the frozen solutions. This result was reproducible, even when the volume was increased by two orders of magnitude (from 30 mL to 3 L). The temperature profiles obtained for the nonconvective freezing geometry were predicted using a relatively simple computational fluid dynamics model. The reproducible solutes distribution, predictable temperature profiles, and scalability demonstrate that the bottom to top freezing geometry enables an extended control over the freezing process. This geometry has therefore shown the potential to contribute to a better understanding and control of the risks inherent to frozen storage.

Surface Characterization of Asymmetric Bi-Soft Segment Poly(ester urethane urea) Membranes for Blood-Oxygenation Medical Devices

Asymmetric bi-soft segment poly(ester urethane urea) (PEUU) membranes containing polycaprolactone (PCL) as a second soft segment are synthesized with PCL-diol ranging from 0% to 15% (w/w). Bulk and surface characteristics of the PEUU membranes were investigated by scanning electron microscopy (SEM), static water contact angles, and surface streaming potentials and were correlated to hemocompatibility properties, namely, hemolysis and thrombosis degrees. SEM analysis reveals PEUU membranes with asymmetric cross-sections and top dense surfaces with distinct morphologies. The increase in PCL-diol content yields PEUU membranes with blood-contacting surfaces that are smoother, more hydrophilic, and with higher maximum zeta potentials. The results obtained in this work give no evidence of a correlation between hydrophilicity/zeta potentials and the hemolysis/thrombosis degree of blood-contacting surfaces of the PEUU membranes. In contrast, other hemocompatibility aspects reveal that the more hydrophilic membranes are associated with lower platelet deposition and inhibition of extreme states of platelet activation.

Optimization of ladder-type spacers for nanofiltration and reverse osmosis spiral-wound modules by computational fluid dynamics

The velocity and solute concentration disitribution in the feed channel of a spiral wound module with ladder-type spacers is investigated by computational fluid dynamics (CFD) for laminar flow and permeation of sucrose aqueous solution. The spiral wound module feed channel was approximated by a rectangular channel filled with ladder-type spacers that have the transverse filaments adjacent to a single membrane wall. The momentum and mass transport equations together with the appropriate boundary conditions are solved numerically by the control volume formulation. The numerical predictions were performed for Reynolds numbers of Re = 50, 100 and 200, Pf ≡ pjh = 0.25, 0.5 and 0.75 and Lf≡ lf/h = 1.9, 3.8 and 5.7 — where h is the channel height, pf is the transverse filament height and lf is the distance between neighbor filaments axis. The numerical results show that the flow is dominated by a recirculation region downstream of each transverse filament and that there are no recirculation regions in the upper part of the channel. Three flow structures were identified: for low values of Re and Pf and high values of Lf the recirculation region does not fill the inter-filaments space; for high values of Re and Pfand low values of Lf the recirculation regions fills the inter-filaments space; and for Pf = 0.75, a secondary recirculation region can develop inside the main one. The numerical results show that the spacer optimal values of Pf = 0.25 and Lf= 5.7 minimize both the concentration polarization and the longitudinal pressure drop.