Impact of osmotic pressure on the stability of Taylor vortices

We use linear stability analysis and direct numerical simulations to investigate the coupling between centrifugal instabilities, solute transport and osmotic pressure in a Taylor–Couette configuration that models rotating dynamic filtration devices. The geometry consists of a Taylor–Couette cell with a superimposed radial throughflow of solvent across two semi-permeable cylinders. Both cylinders totally reject the solute, inducing the build-up of a concentration boundary layer. The solute retroacts on the velocity field via the osmotic pressure associated with the concentration differences across the semi-permeable cylinders. Our results show that the presence of osmotic pressure strongly alters the dynamics of the centrifugal instabilities and substantially reduces the critical conditions above which Taylor vortices are observed. It is also found that this enhancement of the hydrodynamic instabilities eventually plateaus as the osmotic pressure is further increased. We propose a mechanism to explain how osmosis and instabilities cooperate and develop an analytical criterion to bound the parameter range for which osmosis fosters the hydrodynamic instabilities.

Evaluation of omphacite and iron-coated omphacite as a water filtration medium

In this study, omphacite media were modified by roasting at high temperature. The optimal preparation conditions were as follows: the pH value was 1, concentration of FeCl3 was 2 mol/L, roasting temperature was 450 °C and roasting time was 2 h. The specific surface area, scanning electron microscopy, and EDS analysis were used to compare the unmodified and modified omphacite, and a dynamic filtration experiment was performed to treat the slightly polluted water. The analysis of characterization results revealed that, the surface structure of the modified omphacite filter media has changed greatly. Its surface is rough and potholes have increased, and the specific surface area and adsorption capacity are significantly increased. Results of the dynamic filtration experiment revealed that the average removal efficiencies of organic matter, TOC, and turbidity by quartz sand were 21.17%, 2.2%, and 94.5%. The average removal efficiencies of organic matter, TOC, and turbidity by unmodified omphacite were 23.46%, 26.7%, and 95.2%. The average removal efficiencies of organic matter, TOC, and turbidity by modified omphacite were 50.35%, 45.5%, and 96.3%. On the whole, the filtration performance of the modified omphacite filter column is the best among three filter columns, and the recovery of the backwashing performance is also better. HIGHLIGHT At present, no application of omphacite in sewage has been found.The development of omphacite filtration technology can not only provide new market hot spots for minerals in the East China Sea area, but also have a very positive value for improving water treatment technology and enhancing water treatment efficiency.It is the intersection of mineral processing and environmental science.

Rheology and Dynamic Filtration of Foam Fracturing Fluid Enhanced by Cellulose Nanofibrils

Foam fracturing is an effective method for the development of unconventional reservoirs. However, due to lamellar film, high pressure differences within foam films, and the strong diffusivity of the internal phase, foam is prone to suffering from unstable phenomena such as rupture, drainage, disproportionation, etc., thus leading to uncontrollable foam flow behavior in the tube and formation. In this work, cellulose nanofibrils (CNFs) were used to enhance foam fracturing fluid. The target is not only to obtain a stable foam system, but also to control its rheology, proppant-carrying and leak-off behavior. The stability of the N2 foam fracturing fluid with CNFs was firstly explored via static tests by measuring its foam volume and liquid drainage. Then, the viscosity of foam fracturing fluids with different foam quality was measured using a tube viscometer under conditions of use, to evaluate the rheology of foam with CNFs. Subsequently, the proppant-carrying capacity was evaluated by observing suspension state of proppants in foam over time. The microscopic images of the foam with proppants were collected to analyze the interaction between bubbles and proppant. Finally, the dynamic filtration behavior and core damage of foam with CNFs were investigated by using a dynamic filtration apparatus. The results of the static tests showed that the stability of foam was significantly enhanced by the addition of CNFs, and the liquid drainage and gas diffusion could be effectively inhibited. Upon foam evolution, bare surfactant foam formed a polyhedral structure rapidly, while the CNFs enhanced foam maintained spherical and dense for a long time. The viscosity of foams with and without cellulose nanofibrils showed a shear thinning behavior. With the addition of CNFs, the viscosity of foam was improved by 3 - 6 times compared with bare surfactant foam and its value was increased with foam quality changing from 60% to 80%. The results of proppant-carrying tests indicated that the proppants suspension in foam was improved obviously as the cellulose nanofibrils were added. For CNFs-stabilized foam, the aqueous film of bubbles became thicker and the mechanical strength of foam structure was improved, thus enhancing the proppant suspension in the foams. Moreover, the filtration control performance of CNFs foam was also improved compared with bare surfactant foam. The filtration coefficient of CNFs foam fracturing fluid decreased with increasing CNFs concentration at a filtration pressure difference of 3 MPa, and core damage was maintained at a relatively low level. Additionally, the filtration coefficient of CNFs-stabilized foam and its core damage could be reduced with the increase of foam quality from 60% to 80%. The stability, rheology, proppant-carrying and dynamic filtration control of foam fracturing fluid enhanced by cellulose nanofibrils were explored in this work. The results show that the addition of CNFs effectively improves the stability of the foam, thus enabling the rheology, proppant-carrying and the dynamic filtration to be well controlled, which provides a high-performance and eco-friendly foam fracturing fluid.

Simulation of deep-bed filtration of a gasoline particulate filter with inhomogeneous wall structure under different particle size distributions

In order to describe the microstructure of the porous wall of a gasoline particulate filter (GPF), a pore size distribution based on a probability density function (PDF) and a non-uniform porosity distribution are introduced. The dynamic process of deep-bed filtration in GPF with inhomogeneous wall structure is studied, considering different particle size distributions (PSDs). The results show that most of the particles are captured in the top region of the porous wall, in which the porosity and permeability reduce more obviously during dynamic filtration, and the bottom of the porous wall contributes little to the overall filtration. As time increases, the filtration efficiency of the porous wall for each particle size increases, and the most penetrating particle’s diameter becomes smaller gradually. The dynamic evolution of characteristic parameters of the porous wall, the most efficient filtration region, the pressure drop and the duration of deep-bed filtration are strongly influenced by PSD. This research illustrates the necessity to consider difference of PSDs when working on the filtration process of GPFs.

Rapid photocatalytic degradation of phenol from water using composite nanofibers under UV

Background The removal of phenol from aqueous solution via photocatalytic degradation has been recognized as an environmentally friendly technique for generating clean water. The composite nanofibers containing PAN polymer, CNT, and TiO2 NPs were successfully prepared via electrospinning method. The prepared photocatalyst is characterized by SEM, XRD, and Raman spectroscopy. Different parameters are studied such as catalyst amount, the effect of pH, phenol concentration, photodegradation mechanism, flow rate, and stability of the composite nanofiber to evaluate the highest efficiency of the photocatalyst. Results The composite nanofibers showed the highest photodegradation performance for the removal of phenol using UV light within 7 min. The pH has a major effect on the photodegradation of phenol with its maximum performance being at pH 5. Conclusions Given the stability and flexibility of the composite nanofibers, their use in a dynamic filtration is possible and can be even reused after several cycles.

Single-step dynamic dewatering of microalgae from dilute suspensions using flocculant assisted filtration

Background Dewatering constitutes a major challenge to the production of microalgae, accounting for 20–30% of the product cost. This presents a setback for the applicability of microalgae in the development of several sustainable products. This study presents an investigation into the dynamic dewatering of microalgae in a combined flocculation-filtration process. The effect of process conditions on the performance of 12 flocculants and their mixtures was assessed. Results The mechanism of flocculation via the electrostatic path was dominated by charge neutralization and subsequently followed bridging in a ‘sweep flocculation’ process. Cationic polyacrylamide (CPAM) based flocculants recorded the highest biomass retention with PAM1 and PAM2 attaining 99 and 98% retention with flocculant dosages of 10 and 15 mg/L respectively. Polyvinylamine (PVAM) was also found to improve system stability across the pH range 4–10. Alum was observed to be only effective in charge neutralization, bringing the system close to its isoelectric point (IEP). Chemometric analysis using the multi-criteria decision methods, PROMETHEE and GAIA, was applied to provide a sequential performance ranking based on the net outranking flow (ф) from 207 observations. A graphical exploration of the flocculant performance pattern, grouping the observations into clusters in relation to the decision axis ($$\pi$$ π ), which indicated the weighted resultant of most favorable performance for all criteria was explored. Conclusion CPAM based flocculants and their mixtures demonstrated superior performance due to their viscoelastic behaviour under turbulence. The use of PVAM or alum in mixtures with CPAM reduced the required doses of both flocculants, which will provide beneficial financial impact for largescale microalgae dewatering in a flocculant assisted dynamic filtration process. Chemometric analysis based on the physico-chemical properties of the system provides a time saving assessment of performance across several criteria. The study findings provide an important foundation for flocculant assisted dynamic filtration processes.

Filtration characteristics of garnet media before and after modification

High-temperature calcination was used to modify garnet media. Brunauer–Emmett–Teller (BET) measurements, X-ray diffractometer (XRD), scanning electron microscopy (SEM), zeta potential analysis, and a static adsorption experiment on humic acid removal were carried out to compare unmodified garnet and traditional quartz sand. Fitting adsorption isotherm of the media before and after modification was conducted to determine the adsorption type, and a dynamic filtration experiment was performed to treat micro-polluted water. Results of the characterization analysis and the static adsorption experiment revealed that, compared with the smooth surface of unmodified garnet, the surface of modified garnet media was covered with Fe2O3, which showed a rough concave-convex structure with a specific area that was 2.44 times larger than that of unmodified garnet. The removal efficiency of organic matter after modification increased from 2.5–4.5% to 51.7–63.1%, and the adsorption capacity increased 11–24 times. The adsorption type of the modified garnet media belongs to the Langmuir and Freundlich adsorption mode, while that of the original media belongs to the Freundlich adsorption mode. Results of the dynamic filtration experiment revealed that the effect of modified garnet media on turbidity, CODMn, and UV254 removal was better than that of unmodified garnet and traditional quartz sand.