Optimum Shear Rate Research Articles

Hydrodynamics and Energy Analysis in Suspended Catalyst Photocatalytic Membrane Reactors

Photocatalysis is a non-selective advanced oxidation process (AOP) that can be effective for degradation of toxic compounds. Its main drawback however has been high processing costs compared to other AOPs. Suspended catalyst photocatalytic membrane reactors (SPMR) are promising design configuration for implementation of the technology. This work involves experimental and theoretically analysis of the hydrodynamics and energy consumption in a SPMR under crossflow and oscillatory shear conditions. An optimization approach is presented for intensification of the reactor performance that considers the coupling effects of the applied shear and the transmembrane pressure on the hydrodynamics and the specific energy consumption (kWh/m3). The results showed that the optimum shear rate decreases with increasing the membrane area due to the relatively high contribution of the permeate pumping power to the energy consumption which makes the advantages of operating at higher shear rate relevant. This trend changes for larger membranes areas where the energy consumption due to higher shear outweighs the advantage of its performance enhancement resulting in increasing the system specific energy consumption. In general, the latter was found to be higher for a crossflow system compared to an oscillatory membrane design, which in addition to its smaller footprints, can make such design an attractive option for confined and/or limited space applications.

Bimodal particle size distributions of fine-grained cohesive sediments in a settling column with oscillating grids

The time evolutions of local Particle Size Distributions (PSDs) of suspended kaolinite are explored under different hydrodynamic conditions. A setup that uses oscillating grids is adopted to generate a nearly isotropic turbulence in a settling column, and the results at the observation location show that the initial unimodal PSDs change to bimodal distributions due to flocculation processes. The first peak of the bimodal PSD, which represents primary particles (0.1–4 µm) and flocculi (4–20 µm), remains similar in size to the median size of the initial PSD. The second peak, which represents the flocs (> 50 µm), however, is size-varying under different experimental durations and shear rates. For low shear rates, the size of the second peak increases with time and reaches an equilibrium state; for high shear rates, it first increases and then decreases. Moreover, the two PSD peaks are reasonably simulated by a two-class population balance equation, with the experimental and predicted results comparing well. The results of this study also indicate that the optimum shear rate to boost flocculation is approximately 20 s−1.

Investigating the effects of shear rate on the collapse time in a gas–liquid system by lattice Boltzmann

In order to evaluate the effects of shear rate on the time of collapse process in a two-phase; one component system, a free energy lattice Boltzmann model is developed. Considering a system consists of two droplets in the surrounding vapor it is observed that, for various physical and geometrical conditions, imposing shear flow has similar effects on the collapse time. Actually, for each case there is an optimum shear rate of collapse that is corresponded to highest collapse rate and smallest time. In all test cases, for shear rates smaller than the optimum value, increasing the shear rate raise collapse rate and reduce its time. But for higher shear rates, increasing the shear rate has reverse effect and decreases the collapse rate. The results also show that, higher droplet radius ratio, viscosity ratio, surface tension and interface thickness lead to a higher optimum shear rate of collapse while higher density ratio of liquid and vapor decreases that.

A lattice Boltzmann model for the non-equilibrium flocculation of cohesive sediments in turbulent flow

Microflocs, which are the first-order aggregates of cohesive sediment, are formed during early-stage flocculation through random collisions in turbulent flow. This study proposes a numerical model to describe the non-equilibrium flocculation of cohesive sediments in homogeneous turbulent flows using the lattice Boltzmann method. The validity of the model is verified by analytical results. The influence of suspended sediment concentration and turbulence on the early-stage flocculation phenomena of cohesive sediments at the mesoscale is examined, and it is found that the number of microflocs increases with sediment concentration up to an optimum concentration of 2.0 kg/m3, after which it decreases with sediment concentration. The mean settling velocities of the suspension sediments in the computational domain first increase with increasing shear rate, then decrease, and the optimum shear rate is approximately 17.6 s−1. Additionally, the turbulence-induced flocculation process can have an influence on the turbulent flow.

Fabrication and characterization of integrally skinned‐oriented highly selective charged asymmetric low pressure poly(ether sulfone) membranes for nanofiltration

AbstractBACKGROUND: The effects of shear rate induced molecular orientation in polymeric based asymmetric membranes on performance, structural details, key properties, morphologies and pore size distribution were studied. Asymmetric membranes fabricated at five different potential shear rates ranging from 93.33–466.67 s−1 were evaluated based on nanofiltration test. The use of Spiegler–Kedem, steric‐hindrance pore and Teorell–Meyers–Sievers models enables an assessment to be made of the relationship between shear rates and membranes properties.RESULTS: Experimental and modeling results show that there is a significant correlation between shear and membrane characteristics, whereby as shear rate increases, the separation performances of nanofiltration membranes increase until an optimum (critical) level of shear is achieved. Beyond the optimum shear, the deterioration in membrane performance suggests that there exists an optimum shear rate which produces optimal structural details, key properties, morphologies and pore size distributions.CONCLUSIONS: Results showed that a skinned‐oriented highly selective charged asymmetric low pressure nanofiltration (ALP‐NF) membrane was successfully developed. The optimum shear rate (critical shear) was found to be 233.33 s−1. At the optimum shear rate, the fabricated ALP‐NF membranes produced the finest properties, morphology and narrowest pore distributions. The positive improvement in performance properties of ALP‐NF membrane provides the potential for producing a highly selective NF membrane for different applications in the future. Copyright © 2011 Society of Chemical Industry

Dependence of membrane morphology and performance on preparation conditions: The shear rate effect in membrane casting

This study focuses on the effect of shear rates on molecular orientation and gas separation performance of polyimide/polyethersulfone (PI/PES)-zeolite 4A mixed matrix membrane. PI/PES-zeolite 4A mixed matrix membrane was prepared by a dry/wet phase inversion technique using a pneumatically controlled flat sheet membrane casting machine. Membranes were casted at different shear rates, and consequently its effects on molecular orientation and gas performance were studied. At low shear rates, the permeances of O 2 and N 2 decreased, while their selectivities increased with an increase in shear rates. In low shear rate region, the decrease in permeance or increase in selectivity with increasing shear arose from the better molecular orientation and chain-packing induced by shear. On the contrary, in the high shear region, the increase in permeance or decrease in selectivity was believed to be attributed by porous skins and fracture. Surprisingly, it was found that in high shear region the deterioration of molecular orientation had been detected and appeared to be contradicted from the previous studies. Based on the experimental results, there may have existed an optimum shear rate to yield optimal membrane morphology for gas separation. The selectivity of 581 s −1 sheared membrane had been further improved to 5.92 at 100 °C curing temperature and 72 h curing time.

The influential role of pulp chemistry, flocculant structure type and shear rate on dewaterability of kaolinite and smectite clay dispersions under couette Taylor flow conditions

Improving the dewatering behaviour of mineral waste tailings is an important issue central to sustainable hydrometallurgical processing of minerals worldwide. Chemical additive-mediated, flocculation processes are generally employed to amply sedimentation behaviour of colloidally stable mineral dispersions. In this paper, studies carried out to investigate mineral particle surface chemistry/structure, polymeric flocculant structure type, shear rate and their links with improved dewaterability of colloidal kaolinite and smectite dispersions at pH 7.5 are presented. The results show that, following orthokinetic flocculation under laminar couette Taylor flow conditions with high molecular weight, non-ionic polyacrylamide homopolymer (PAM N), anionic polyacrylamide copolymers: carboxylate substituted (PAM A) and sulphonate substituted (PAM S), and non-conventional non-ionic polyethylene oxide (PEO), distinct clay mineral specific, polymer type and shear dependent dewatering behaviour was displayed. Sedimentation rates were markedly higher for kaolinite pulps, however and the extents of pulp consolidation were substantially similar for both kaolinite and smectite pulps under common flocculation conditions. The pulps flocculated with PAM S polymer showed greatest dewatering behaviour, followed by PEO and then PAM A and PAM N. For each flocculated pulp, an optimum shear rate which led to maximum settling rate exists and some cases led to greater consolidation for PEO based pulps. The findings which are compared with those obtained by a 2-blade impeller sheared orthokinetic flocculation exemplify how judicious choice of polymeric flocculant and shear rate may be effectively used to significantly enhance the dewaterability of colloidally stable clay mineral dispersions.

Formation and characterization of asymmetric nanofiltration membrane: Effect of shear rate and polymer concentration

In this study, we report the effects of shear rates and polymer concentrations in the formation of asymmetric nanofiltration membrane using a simple dry/wet phase inversion technique. Employing the combination of irreversible thermodynamic model, solution-diffusion model (Spiegler–Kedem equation), steric-hindrance pore (SHP) model and Teorell–Meyers (TMS) model, the transport mechanisms and membrane structural properties were determined and have been characterized for different cases of those formation parameters. The experimental and modeling showed very promising results in terms of membrane performance with interesting structural details. The optimum shear rate (critical shear rate) was found to be at about 203.20 s −1 and the best polymer concentration toward the formation of high performance nanofiltration membrane is in the range of 19.60–23.10%. The modeling results suggested that the pore radius of the membranes produced lies within the range of pore radius of 29 commercial available membranes. This study also proposed that the electrolytes transport through nanofiltration membrane was dominated by a convection factor which accounted approximately 30% more than a diffusion factor. This study also indicated that shear rate and polymer concentration were found to affect the membrane performance and structural properties by providing, to a certain extent, an oriented membrane skin layer which in turn exhibiting an improvement in membrane separation ability.

Measurement of rheologically induced molecular orientation using attenuated total reflection infrared dichroism in reverse osmosis hollow fiber cellulose acetate membranes and influence on separation performance

The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s −1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.

Rheological characterization of nephila spidroin solution.

We report the results of an investigation into the rheology of solutions of natural spider silk dope (spinning solution). We demonstrate that dilute dope solutions showed only shear thinning as the shear rate increased while more concentrated solutions showed an initial shear thinning followed by a shear thickening and a subsequent decline in viscosity. The critical shear rate for shear thickening depended on dope concentration and was very low in concentrated solutions. This helps to explain how spiders are able to spin silk at very low draw rates and why they use a very concentrated dope solution. We also show that the optimum shear rate for shear thickening in moderately concentrated solutions occurred at pH 6.3 close to the observed pH at the distal end of the spider's spinning duct. Finally, we report that the addition of K(+) ions to dilute dope solutions produced a spontaneous formation of nanofibrils that subsequently aggregated and precipitated. This change was not seen after the addition of other common cations. Taken together, these observations support the hypothesis that the secretion of H(+) and K(+) by the spider's duct together with moderate strain rates produced during spinning induce a phase separation in the silk dope in which the silk protein (spidroin) molecules are converted into insoluble nanofibrils.

Study of shear rate influence on the performance of cellulose acetate reverse osmosis hollow fiber membranes

The effect of shear rate on the separation performance of reverse osmosis hollow fiber membrane is discussed. Experiments involving six different dope extrusion rates (DERs) (ranging 2.5–5 ml/min) are performed with the other process factors set at the optimum conditions determined by the Taguchi analysis. This will enable an assessment to be made on the relationship between the DER and the rejection rate. The regression method is used to analyse the experimental results and an empirical model has been developed. Simultaneously, it is found that there is a fairly strong correlation between extrusion shear rate and the rejection rate of the membranes, whereby as the shear rate increases, the rejection rate increases until a critical level of shear is achieved, beyond which reverse osmosis membrane performance deteriorates, suggesting that there exists an optimum shear rate which yields optimal membrane morphology for reverse osmosis hollow fiber membranes.

The effect of shear rates on gas separation performance of 6FDA-durene polyimide hollow fibers

We have determined the effect of shear rates within a spinneret on gas separation performance of asymmetric 6FDA-durene polyimide hollow fiber membranes. We purposely wet-spun the hollow fibers at different dope flow rates without drawing and with a constant ratio of bore fluid flow rate to dope flow rate in order to study the effect of shear within a spinneret during hollow fiber spinning on fiber morphology and gas separation performance. For the first time, we found there is a V (down and up) pattern for permeance versus shear rate relationship, while a Λ (up and down) pattern for selectivity versus shear rate relationship. At low shear rates, the permeances of non-polar molecules such as H 2, O 2, N 2, and CH 4 decrease, while their relative selectivities increase with an increase in shear rates. Once a certain shear rate is reached, all permeances increase, while their selectivities decrease with an increase in shear rates. In low shear rate regions, the decrease in permeance or increase in selectivity with increasing shear rates arises from the better molecular orientation and chain packing induced by shear. With increasing shear in high shear rate regions, the increase in permeance or decrease in selectivity is mainly attributed to relatively porous skin structures induced by the low viscosity nature of a power-law spinning fluid at high shear rates, fracture, and modified thermodynamics and kinetics of phase inversion process. This work suggests there may exist an optimum shear rate to yield optimal membrane morphology for gas separation. To our surprise, an increase in CO 2 permeance with increasing shear rates are possibly due to enhanced coupling effect between CO 2 and the highly oriented and closely packed fluoropolyimide molecular chains induced by shear.

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The objective of this study is to investigate the pure influence of shear and its capacity to increase the separation performance of cellulose acetate reverse osmosis hollow fiber membranes, produced using a dry/wet spinning process with forced convection in the dry gap. In order to achieve this, the experiments are designed to decouple the effect of extrusion shear from forced convection residence time by fixing the residence time at 0.615 s. RO hollow fibers were spun at various dope extrusion shear rates ranging from 3.0 to 4.5 ml/min corresponding to shear rates of around 13,479–20,219 s−1, respectively, at the outer spinneret wall. Fourier transform attenuated total reflection (ATR) also known as internal reflection spectroscopy was used to probe the membrane active layer so as to examine if high shear rates may induce anisotropy at the molecular level in the active layer. The rejection rates of the membranes were evaluated using salt water. Results revealed that: (i) ATR can be used to determine qualitatively and quantitatively—the degree of molecular orientation in the sheared membranes; (ii) molecular orientation is enhanced as the dope extrusion rate increases; and (iii) there exist an optimum shear rate that can induce a certain degree of molecular orientation to yield membrane morphology with optimum separation performance.