E. Brito-De La Fuente

Effect of nonsolvents on properties of spinning solutions and polyethersulfone hollow fiber ultrafiltration membranes

Abstract The relationship among the presence of nonsolvent additives, the rheological behavior of spinning solutions and properties of hollow fiber membranes was studied. The additives tested were water, polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), and the base mixture was polyethersulfone/N-methyl-2-pyrrolidone (PES–NMP). In addition the effect of combining water and PVP or PEG was also studied. Membranes were prepared using a spinneret having two concentric orifices. The internal coagulant used as well as the nonsolvent from the coagulation bath were both water at 28°C and 30°C, respectively. Rheological properties of polymer solutions were evaluated using a rheometer Haake RV 20. Changes on composition of spin-solutions were also evaluated in terms of membrane water permeability, solute rejection and membrane structure observed using scanning electron microscopy (SEM). Experimental results from this work showed that spinning solutions containing any of the three additives behave as Newtonian fluids in the range of shearing rates tested. The addition of water, PVP or PEG to the base PES–NMP solution increased its viscosity and this effect was independent of the type of additive used. A direct relation between viscosity of casting solutions and membrane thickness was found. However, rheological properties (viscosity and normal stress difference) could not be used to explain differences on membrane water flux (MWF) when using different additives at the same concentration. The addition of any of the three additives generally increased MWF. The extent of this increment seemed to be more related to changes on membrane porosity than changes on pore sizes induced by the nature and concentration of the additive used.

Mixing With Helical Ribbon Impellers

Batch mixing of rheological complex fluids using helical ribbon and helical ribbon screw impellers was investigated in the laminar mixing region. Eight models and one polysaccharide fermentation broth (gelkn) were used. These fluids exhibited different rheological properties: Newtonian viscosity, viscoelasticity, and pseudoplasti-city. The Ostwald-de Waele model (power law) described adequately the shear viscosity of pseudoplastic fluids, including the gellan broth ( n ranged from 0.14 to 0.65). Power consumption decreased as pseudoplasticity increased and a model was developed to predict the deviations from Newtonian power input due to pseudoplasticity. A dimensionless, unique representation of the power data using the predictions of the model, showed a shift of the upper limit for the laminar mixing region towards higher values of Re ( Re ≈ 100). The model predictions were found to be in good agreement with those obtained by using the classical approach of Metzner and Otto 1 . The constant of proportionality between the average shear rate and the impeller rotational speed, K s , was found to be a function of the power law index, especially for high shear thinning fluids. It was found that power consumption increased as the impeller blade pitch decreased or as the impeller blade width increased. Both impeller pitch and blade width do not have significant influence on K S .

Mixing Time in Rheologically Evolving Model Fluids by Hybrid Dual Mixing Systems

Mixing time experiments were performed using a hybrid dual mixing system, which included a helical ribbon impeller (HR) and either a Smith (ST) or Rushton turbine (RT). Xanthan gum solutions were used as rheologically evolving fluids to evaluate changes in mixing time under non-aerated and aerated conditions. The helical ribbon agitator and turbine of the hybrid dual mixing system was kept at a constant rotational speed ratio, N T /N HR =10. Experiments showed that performance of the hybrid mixing system was superior to that of the individual impellers. Flow properties and gassing conditions played an important role in mixing time. While mixing time was practically identical under non-gassed conditions for both the ST-HR and RT-HR mixing systems in low-viscosity fluids, differences up to 1000% were observed in high shear-thinning fluids. In these fluids, the RT-HR combination exhibited better performance than the ST-HR. In high-viscosity fluids, gassing enhanced mixing time particularly when the ST-HR hybrid system was used. Both mixing systems showed similar mixing times under the highest gassed condition evaluated in this work.

Power consumption of a dual turbine-helical ribbon impeller mixer in ungassed conditions

Abstract Ungassed power measurements in a dual coaxial mixer composed of a helical ribbon and a Rushton turbine were carried out in laminar mixing conditions for Newtonian and non-Newtonian shear thinning fluids. For the Newtonian case, the power draw constant Kp for the hybrid geometry was not the sum of the individual impellers. This was explained by considering the radial discharge flow in the turbine region as well as the top-to-bottom circulation pattern of the helical ribbon impeller. For the non-Newtonian fluids, the results showed that, at a given Reynolds number, power consumption decreases as the shear thinning behaviour increases. A dimensionless and unique representation of the power draw data was obtained by shifting the non-Newtonian power draw results to the Newtonian curve. This was carried out with a Ks function defined from the Kp(n) data. The predictions for Ks were found to be in good agreement with those obtained using the classical method of A.B. Metzner and R.E. Otto (AIChE J., 3 (1957) 3–10). It was observed that pseudoplasticity tends to shift the upper limit of the laminar region toward Reynolds numbers higher than 10.

Flow patterns in rheologically evolving model fluids produced by hybrid dual mixing systems

The flow patterns produced by two dual mixing systems composed of independently driven impellers were studied. The dual impellers included a turbine rotating at high speed (Rushton or Smith) and a slowly rotating helical ribbon agitator (HR). Visualizations and power input were used to evaluate mixing performance. The influence of the rotational speed ratio on the flow patterns was evaluated. For high shear-thinning fluids, N T /N HR modifies the flow patterns considerably. Three typical behaviors were found with shear thinning fluids: segregation of two principal flow patterns (N T /N HR 10), and a well-distributed flow pattern throughout the tank (N T /N HR =10). For low-viscosity fluids, the motionless HR reduced the vortex length and the T-HR systems eliminated vortex when the impellers rotated in opposite directions at N T /N HR =10. Finally, a relationship between the dimensionless vortex length and the Froude number is proposed for individual turbines as well as for the turbine-motionless HR systems.

Forward deformable roll coating at high speed with Newtonian fluids

The flow of Newtonian fluids through the deformable nip of a high speed roll coater was experimentally studied. A digital video system was used for the observation of the formation, elongation and break-up of fluid filaments at the exit of the nip and to investigate how misting droplets are ejected to the surroundings. The pressure profile in the nip was obtained by a miniaturized piezoelectric transducer mounted on one of the coater rolls. Experimental results show that both film splitting and air entrainment into the nip can be responsible for misting occurrence when a low load is applied between the rolls. When a high load is applied, misting can be eliminated and a periodic flow is generated in the nip gap.

Gas Dispersion in Rheologically‐Evolving Model Fluids by Hybrid Dual Mixing Systems

Gas dispersion experiments (0.18≤Fr≤0.71, 0.02≤Fl≤0.09) were carried out using a hybrid dual mixing system, which included a helical ribbon impeller and either a Smith or a Rushton turbine. Newtonian and non-Newtonian model fluids were used as rheologically-evolving fluids to evaluate changes in gas dispersion performance. A motionless helical ribbon agitator was used as a baffle in low-viscosity Newtonian fluids. Both Smith and Rushton turbines produced a vortex, which was eliminated by the motionless helical ribbon impeller. Gas dispersion in low-viscosity fluids was enhanced when the helical ribbon agitator and turbine of the dual hybrid mixing system was kept at a rotational speed ratio of 10 (N T /N HR =10), which allowed dispersion at a lower Fr than the turbine alone. For moderate-viscosity Newtonian fluids, gas dispersion was achieved at Fr≤0.71 and Fl≤0.05. Flow properties of non-Newtonian fluids played an important role in gas dispersion; transition from dispersing to flooding stages was observed for the fluids that were more shear-thinning (n≤0.38).

POWER CONSUMPTION WITH ANCHOR MIXERS EFFECT OF BOTTOM CLEARANCE

The effect of bottom clearance on power draw for anchors is investigated. The results suggest that power input decreases as the bottom clearance ratio b/D increases. A new correlation is proposed in which both bottom and wall clearance effects on power input are considered. The dependence of power input on these effects is additive rather than multiplicative. Thus, the total power consumption is the sum of the power inputs generated by the wall and bottom clearance. Power results from 3D-CFD numerical simulations are also presented. These results confirm fairly well the experimental points.